Ion beam induced surface and interface engineering

Abstract

The injection of material into a target specimen in the form of an accelerated ion beam offers a most valuable tool for altering its physical, chemical, structural, surface and interface properties in a controlled manner and tailoring new materials for basic and applied research for science and technology. The present review describes experimental, theoretical and recent aspects of ion beam modifications at various solids, thin films, and multilayered systems covering wider energy ranges including the older basic concepts which are now of interest. These results reveal that the ion–solid interaction physics provides a unique way for controlling the produced defects of the desired type at a desired location. These interests have been stimulated by the possibilities of synthesizing novel materials with potential applications in the field of thin films, surfaces and interface science. Many applications of ion induced engineering are being developed for various sciences of high technological interest for future aspects.

Introduction

Many interdisciplinary areas of physics, chemistry, material science etc. which are developing so rapidly, ion induced modifications of solids, thin films, surface and interfaces is one such field which has emerged as an areas, to quantify the properties of solids up to a shallow depth. The subject has remained on the sidelines for a long time, although the tools for its exploration existed and such considerations it received was from the viewpoint that, ionic damage in the materials of which nuclear reactors are constructed, has led to a considerable investigation of atomic collision processes and the behaviour of defects introduced by radiation.

In recent years, however, has come the realization that the injection of material into a target specimen in the form of an accelerated ion beam offers a most valuable tool for altering its physical properties in a controlled manner. This is particularly the case in such substances as semiconductors, in which the electrical behaviour is determined by extremely small concentrations of certain impurities. The ability to eliminate or at least minimize the otherwise overwhelming effects of the damage induced by ion bombardment leaves the possibility of controlling reproducibly the doping effects of the implanted ions.

Since radiation damage by fast neutrons in reactors takes place principally by the elastic recoil of atoms with energies of few tens of keV. Since early application of ion bombardment also involved low particle energies. The bulk of earlier work was mainly concentrated in the energy region well below 500 keV. The requirement of an adequate penetration to the depths of the order of 1–50 μm in the variety of solids is now demanding a better understanding of phenomena induced by faster ions, with energies from 1 MeV to multiple hundred of MeV.

In addition, the earlier work could be accommodated by beams of a limited number of ion species, such as inert gases and a few metals, it is now desirable to have available a very wide range of ion beams. Other practical requirements in the handling of these beams and in the manipulation of target specimens have arisen for the first time, and they require consideration for the exploration of ion–solid interaction process.

The present review attempts to deal with the new aspects of ion bombardment at various thin film systems, and to cover the wider energy range which is now of interest. For completeness a number of older basic concepts are outlined. For instance, the first section summarizes the ideas regarding the ion beam induced terminologies such as ion beam, its interaction with solid, ion irradiation and its range inside solid. In the next section we consider modes of energy loss of ions injected into solids. Ion beam induced mixing process including low energy and high energy ions also receive detailed consideration. Since they must be understood and controlled if there is to be any useful application of the technique of IBM. Some recent work on IBM at various interfaces and surfaces is reviewed next; here again the recent studies have concentrated upon the influence of irradiation over structural, elemental, optical, morphological, electrical properties of solids and some striking effects have been observed which have important bearing on the ideas discussed elsewhere. Most of the work to be reviewed here has been on the Metal/Si, Metal/Metal, Metal-oxide/Si, Metal/Ceramic systems, which are, of course of great technological importance. In the final section we turn to the practical aspects of using this technique and conclude the review.

Throughout the review, attention will be drawn to further outstanding problem, since there are many areas of incomplete understanding. The physical problems to be discussed range over Solid-State Physics and the techniques employed include many which have been the realm of Nuclear physics and sophisticated electron microscopy. The results are being applied firstly in microelectronics. It will be more interesting to observe the interplay of ideas from all these disciplines in the progress of this subject, and it is hoped that this review may excite the interest of those in related fields who may realize the ion beam irradiation for their own studies, and who will in turn be able to throw new light upon its problems.

The focus of this review is to describe the recent progress in understanding the energetic beam induced modifications of well-defined solid materials. In spite of these achievements the review represents the basic idea of ion beam kinematics, ion–solid interaction kinematics in terms of implantation and irradiation. This apparently simple subject has received considerable attraction using many probing surface and interface techniques, such as GIXRD, RBS, AFM, SIMS, ERDA, XTEM, SIMS, Raman Spectroscopy, HTIR etc. Apart from direct role of ion induced phenomenon, ion–solid interaction plays an intrinsic role in the description of various surfaces and interfaces and in understanding the synthesis and intermixing of thin films while industrial concerns underline the increasing importance of the electronic submicron devices and interest in nanotechnology. As well as ion induced modifications acts as a useful model system to reveal the effect of solid reaction of films under high temperature to form compound or alloys at surfaces or interfaces. It provides a convenient environment in which ion–solid interaction can be explore at nanolevel. Energetic ions are able to produce effects at various interfaces of metals, semiconductors, insulators, polymers and biological surfaces, both in bulk and thin film structures, but all of them require a clear understanding of how the surface and interface influences by the impact of ions and behaviour of atoms under such conditions.

Despite the general importance, ion bombardment and the disordering nature of host material complicated this study. Progress has been made by comparing new, detailed experimental data with the theoretical models, for the stoichiometry of solid atoms. Theoretical calculations must be capable of reproducing the correct solid geometries and experimental studies must deal with similar parameters and be capable of providing a clear network between theses two. Even though the area of uncertainty, understanding and discussion remain, a more qualitative picture of ion beam interaction to solid is emerging for several systems and the results are beginning to inform the description of such interaction induced modifications.

The main issues in ion beam processing of materials can be grouped into three major issues as shown in Fig. 1.1. Although some overlaps are unavoidable, they can be addressed reasonably well in terms of ion beam modification and the commonly available ion beam analysis techniques like RBS, ERDA, NRA, and PIXE.

The literature is witnessed that the ion induced radiation damage studies were started 50 years ago with the advent of nuclear energy. Most of interest in this early period had been devoted to the modification of metallic compounds under neutron irradiation [1]. Later the use of implantation for doping semiconductors extended massively [2]. Finally the formation of new phases by ion irradiation, ion implantation, atomic mixing of multilayers and by dynamic mixing during implantation or deposition, constituted an active research area.

This review provides a clear summary of the established intermixing models. This work makes no attempts to replace these earlier literature and reviews. The established picture of ion interaction with metals, semiconductors, polymers, nitrides, carbides and biological samples etc. the damage and consequently the development of such materials studies. Low energy ion beams, particularly involves the build-up of a basic understanding of radiation damage processes, especially those due to the elastic collision effects. Recent literature and examples have shown how one can in this way go so far as to design systems with interesting physical properties which may lead to applications in microelectronics/nanotechnology [3], [4], [5], [6]. Ion beam impact can serve all classes of materials, ranging from metal to living cells. It is interesting to use ion accelerators to simulate the radiation damage induced by different radiation field. It is significant to simulate the wide spectrum of irradiation at which electronic devices are submitted in space and to predict their behaviour. To conclude, it is necessary to understand ion beam induced effects both on basic and technological ground.

The review will start by making a brief discussion about the use of ion beam technology, in material science with respect to past, present and future trends along with ion beam accelerators. The terms ion implantation and irradiation are well-defined. The basic mechanism of ion–solid interaction and induced effects are well pronounced within Section 1. Section 2 deals with general kinetics of ion beam in bulk and thin films and its preparations using ion beam technology Section 3, discusses the experimental techniques used to investigate ion beam induced effects, those have been important in recent work and the information they provide. In Section 4 we present a detailed discussion about qualitative analysis using various models which are applicable to such phenomenon. Section 5 discusses the implantation within semiconductors while the Section 6 categories in two major fields, low and high energy ion beam irradiation to form metastable and stable systems at various surfaces and interfaces including ion beam application in Science and Nanotechnology for future scope. Finally in Section 7 highlights some issues that remain to be clarified, particularly where either theoretical or experimental advances are needed.

Modern material research has been enriched by energetic ion beams from various types of accelerators were ion beams are used for material’s surfaces modification as well as for characterization. The first particle accelerator was planned with an objective to split the atom in 1932 with an achievement through Cockroft–Walton particle accelerator when Li atom was split with 400 keV protons. At same age Van de Graaff invented the electronic generator which provided an excellent tool for probing the nucleus which led to the enhance need of higher energy and associated advances in accelerator development. Besides atomic, nuclear and particle physics, the impacts of accelerator developments have been felt in astrophysics, material science, radiobiology, medical diagnosis and therapies and industry [7]. Nuclear accelerators originally planned for basic nuclear and atomic physics research invariably open up completely new perspectives in various fields, particularly in material science and device technology. Low energy accelerators now become standard tools and their exploitation in engineering materials of all kinds of interest provide very fruitful activity. With the advances in technologies ion beams of almost all the elements across the periodic table are becoming accessible over a very wide range of heavy ion beams of energies up to 100s of MeV are beginning to be exploited in almost all the emerging new areas in material science [8] and the present review discuss only about the early accelerators in India which may be similar to any other country.

Accelerator technology in India has started in 1940s and could run in 1950s. Initially first Cyclotron was installed in Saha Institute of Nuclear Physics (SINP), Kolkata and 1 MeV Cockroft–Walton Accelerator was commissioned in Tata Institute of Fundamental Research (TIFR), Mumbai in 1953. Additionally, in Bose Institute Kolkata, Aligarh Muslim University, Aligarh and Andhra University, Waltair, 400 keV neutron generators were installed. In early sixties one 5.5 MV Van de Graaff accelerator of High Voltage Engineering Corporation (HVEC), was installed at the Bhabha Atomic Research Centre (BARC), Mumbai which provided much needed fillip in accelerator based research in the country. Large tandem accelerator was established in late sixties and would have made a significant impact to nuclear physics community in the country. Cyclotron became incorporation in late 70s. During this period small Van de Graaff accelerators (400 keV) were installed in the Universities at Varanasi and Patiala, and a 2 MV Van de Graaff accelerator at Indian Institute of Technology, Kanpur. The variable energy cyclotron at Kolkata (VECC) is providing proton beams and functioning as a user facility.

In early 1980s there was a boost for accelerator development program through the indigenous development of 2 MV tandem Van de Graaff accelerator in BARC Mumbai and establishment of the Centre for Advanced Technology (CAT) at Indore, for the development of technologies associated with accelerators, lasers and related systems. In 1988 there was a considerable research work in the field of heavy ion nuclear reaction, spectroscopy, nuclear fission, atomic physics using 14 MV Pelletron (Joint BARC–TIFR facility) located at TIFR campus, Mumbai.

Efforts have been made by academic committee to have a front grade accelerator based research facility for teaching institutions, which is finally succeeded in 1984 with the establishment of Nuclear Science Centre (NSC) which is presently known as Inter University Accelerator Centre (IUAC) and under the aegis of University Grants Commission (UGC), New Delhi, India.

A 15 MV Pelletron accelerator was made operation in 1989 and started opening as a user facility in 1990 with experimental facilities such as Heavy Ion Reaction Analyzer (HIRA), Gamma Detector Array (GDA), facilities for on-line/in-situ materials science with Swift heavy ion beams; with the addition of beam lines for atomic physics and radiobiology. Commissioning of a 3 MV Pelletron in Institute of Physics, Bhubaneshwar provided support for increasing demand for materials science and characterization with ion beams, and initiation of Accelerator Mass Spectroscopy (AMS) program. ECR development project in IUAC has result a low energy ion beam facility (LEIBF) to provide high flux of highly charged ions.

In the present review most of the work reported using low energy facility and high energy Pelletron accelerators.

Ion beam induced irradiation concerns all classes of materials ranging from metals to living cells. Present section is therefore closely related and devoted to the impact of nuclear science on biology (and therapy) and on energy. Within this latter field, it is of prime importance to use ion accelerators to simulate the radiation damage induced by different radiation at which electronic devices are submitted in space and to predict their behaviour. To conclude, it is necessary to understand irradiation induced effects both on the basic and technological grounds.

Low and high energy ion beams have various uses mainly in material science and biological sciences. Ion beam techniques are very powerful tools in thin film technology due to their high flexibility and the possibility to control exactly technological important parameters like energy, fluence, beam size and position. According to the effect responsible for the change of surface properties, the modification of materials by energetic ions may be classified in the following way:

Doping: Implantation of impurities at low doses (mainly in semiconductors)

Ion beam synthesis: Formation of buried layers or precipitates of new, also metastable compounds by implantation of doses exceeding the solubility limit

Ion beam mixing: Mixing of interfaces, layer systems or growing layers by

Ion beam assisted deposition: Implantation of inert or reactive ions

Use of radiation defects: Recrystallization, amorphization, sputtering, particle track effects

Ion beam irradiation: Industrial, technological and medical applications for the compound formation, nanostructure, thin films, biological diagnosis etc.

According to different applications of ion beams, they are useful and applied for most of technology and research such as:

Engineering of materials

Using heavy ions of high energetic beam, one can modify materials so that they can acquire desired optical, electrical and mechanical properties. Heavy ion induced electronic energy loss in the materials can be varied from the level of eV/Å up to a very high value, say 10 keV/Å by choosing approximate ions and their energies. This remarkable flexibility provides unique opportunities to engineer properties of material.

Ion beam induced adhesion and stitching

A tribological coating depends greatly on the adhesion of film on substrates. Ion beam interactions can produce remarkable changes in the mechanical properties of thin films on various types of substrates. It is clear that interface structure disorder is caused by both ionizing and ballistic nuclear collisions. Hardness study of Cr coatings with 75 MeV Ni beam indicated noteworthy increase, when the ion range is many times higher the film thickness [9]. Stitching of Al films on SiO₂ substrates was enhanced by 70 keV Al⁺ ions. The adhesion was determined by a scratch test. It has shown that the film adhesion has markedly improved [10].

Ion beam mixing and multilayer

Thin film compounds formation such as nitrides, carbides, silicides etc. using ion beam mixing method is an ideal way. By using the kinetic energy of ion beams to mix deposited layers together or to mix them with substrate material, it is possible to achieve such kind of mixing. Having such energetic beams, many results have been published and known for their application in tailoring or modifying materials used in Science and Technology, such as Ni/C, Pt/C, Fe/Si multilayers were studied with 100–200 MeV Ag ions [11]. The observed modifications were at much below the predicted threshold values of defect production in these materials which requires proper understanding of physics. Suggestion stands that this may be due to small thickness and/or discontinuous nature of material layers, which may decrease mobility of the conduction electrons. This mobility change can be due to interface scattering resulting in confinement of the deposited energy to a smaller region thus raising its temperature and enhancing the interdiffusion.

Study of magnetic and magnetotransport properties of Fe/Tb and Fe/Cr multilayers with 80 MeV Si ions indicated that interface roughness increases without intermixing while intermixing at the interface accompanied by a large decrease in perpendicular magnetic anisotropy has been found using 150 MeV Ag ions [12]. Atomic mixing has been showed in metallic bilayers of Ni/Ti irradiated with Pb, Ta and U ions [13]. Magnetic multilayers shows there dependence upon ion mass, energy and dose stress relaxation, interface roughness and intermixing at the interface is produced [14]. Ag nanoclusters embedded in amorphous silica (a-SiO₂) matrices through low energy ion (105 keV Ar) beam mixing followed by thermal annealing, have been studied by means of optical absorption [15].

Electronic devices

Heavy ions can anneal existing defects in the materials under certain conditions and of course, produce defects after the threshold in electronic energy loss is exceeded. This has applications in deep implantation associated devices. It is observed that strains in the crystal lattice can be relieved by electronic energy loss. The characteristics of the devices can be profitably modified. Example: For Si surfaces, where the electronic energy loss threshold is very large, ions produced mainly primary defects (point defects, interstitials etc.) and defect complexes [16]. Extended defects in elemental semiconductors are not observed even with GeV U ions. In general, irradiation induces deep level defects, which act as trapping centres, lead to decreased mobility and carrier concentration. These can be controlled by varying electronic energy loss and thus the defects can be engineered.

It has been demonstrated that the minority carrier life time can be reduced significantly by introducing deep level defects in Si and it is possible to improve the switching characteristics of the diodes [17].

Radiation can improve the switching characteristics of diodes but the defects increase the forward voltage drop significantly. The studies were made by introducing into the n-region of Si junction diode with 100 MeV Si, 70–80 MeV O, 65 MeV B and 35 MeV Li ions. In this way, defects could be induced at different locations in the n-side covering a range from 35 to 170 μm away from the contact of n-side. It has been observed that the defects produced near the junction are more effective in reducing the turn-off time. Further, it was found that B and Li ions are more effective because while decreasing the turn-off time, the increase in forward voltage drop is small. The increase in the forward voltage drop can be recovered, partially by annealing the diodes at 450 °C [18].

High $T_c$ temperature superconductors

It has been demonstrated that columnar defects in high $T_c$ superconductors provide the pinning of vertices that improves the critical current and associated properties of materials. In superconducting microwave devices subject to magnetic field, the columnar defects help to control dissipation due to the motion of vortices [19]. Microwave absorption in YBCO thin films with columnar defects was studied [20] by measuring the angular dependence of microwave magneto-absorption of both pristine and 270 MeV Ag ion irradiated YBCO film on LaAlO₃ substrates. Results revealed that microwave loss in sample was reduced by a factor of more than 2 as compared to those of pristine samples. The effect of Swift Heavy Ions (SHI) induced structural strain in thin films of Y Ba₂Cu₃O₇-y, found that around the columnar amorphization structural strain is produced [21]. Change of resistivity and the critical temperature has been investigated by several groups and interpreted by considering the modification due to microstructural changes, defect generation and effect of SHI in the grain boundaries in sintered materials and thick films [22], [23], [24].

Colossal magneto-resistance (CMR) in perovskites occur only at large (several Tesla) magnetic field and low temperature which restricts the applicability of these materials considerably. It is required to achieve large magneto-resistance (MR) at low fields and at room temperature. Attempts are being made to engineer the appropriate material to achieve this objective. Few studies are being made on superlattices involving magnetic layers, polycrystalline samples etc. [25], [26]. It is found that low energy (keV) ion implantation causes significant changes in the transport behaviour of manganites but since the ion remains in the film it has non-uniform depth profile in the film [27], [28]. High energy oxygen ion irradiation of epitaxial LCMO thin film has been used to study the structural magnetization and magnetotransport properties. The Oxygen ions have low threshold value which produces point defects. SHI produced columnar defects generated compressive stress in thin film [29].

Polymers

Ion beams can modify in a controlled way molecular structure in polymers leading to changes in their chemical, electronic, electrical, tribological and optical properties. Ionization trail produced by SHI causes bond cleavages and free radicals produced at one site react in a molecular site of a different type from their original site. These are responsible for most of the chemical transformations observed in polymer films: chain scission, cross-linking and double and triple bond formation. Irreversible cleavages of bonds within macromolecule produce volatile species.

The track diameter is a quantity of interest for the understanding of basic ion insulator interaction. These have been measured by scanning force microscopy and other surface morphology techniques. A novel approach [30] has been used to determine the track diameters in polymers by on-line measurement of hydrogen loss during ion irradiation by on-line ERDA. Hydrogen atoms liberated by the breaking of the bonds combine to form hydrogen molecule, and being lighter gaseous molecule having high diffusivity, escape from the polymer causing reduction in H content. Thus incident ion along its path releases H from the cylindrical zone of damaged polymer. The track radius has been estimated by using the relation

$$H\left(\varphi \right)=H_{in}exp(-\rho \varphi )\text{,}$$

where, $H_{in}$ represents initial content of H in the sample; $\rho$, the cross-section of release of H and $\varphi$, the ion fluence. The initial slope gives the cross-section of H release, which is equivalent to $\pi r^2$ with $r$ as the track radius. Evolution of gases from mylar was studied with an on-line quadrupole mass analyzer during irradiation with 180 MeV Ag ions. The gases evolved due to breakup of bonds were H₂, Co and C₂H₄. The track dimensions were evaluated by estimating the cross-section of release of these gases. All the three gases are evolved from the region within a region of 6 nm. The outer region between 6 and 8 nm correspond to H₂ emission [31].

What happens in the material inside the track is a matter of great interest. It critically depends on the primary parameter, Se. There are clusters formed, C₆₀ gets synthesized in the column, possibly nanotubes are created etc. SHI increases reflective index to permit optical guidance. It is seen that variation in reflective index depends on the parameters Se and fluence and not on doping effect. This provides new possibilities for optimizing and controlling the loss in optical wave guides and channeling. Particle track membrane (PTM) which provides cylindrical pores of uniform size can be modified to possess characteristics which depend on physical and/or chemical properties, according to environmental factors such as temperature, pH value, electric field etc. This provides [32] possibilities of making sensitive sensors with SHI modified polymer materials.

Biomedical

Materials used in medicine should fulfill several strict conditions primarily imposed by biological restrictions: absence of harmful interactions with body cells, good compatibility with organs and, finally, suitable mechanical properties. These conditions are often contradictory; hence, only exceptionally single-phased materials can provide the best choice and the appropriate surface treatment appears thus as a mandatory condition for designing of advanced devices. Ion implantation offers an unrivalled tool for tailoring of the surface properties, making possible to design new biocompatible materials characterized by superior mechanical or biological properties. Such properties as: cell adhesion, wettability, hardness, wear resistance and friction are intensively investigated [33]. Such characteristic allows one to design materials combining very different properties like high and low wettability, locally increased cell adhesion, bone compatibility or low friction properties. Finally, the implanted layer is the integral part of material structure; thus, the implanted layers simply cannot delaminate as-deposited layers reducing the risk of accidental failure of the implant. A classical example of the possibilities offered by this technology is hip joint prostheses made of titanium alloy: carbon and nitrogen implantation into their femoral heads resulted in order-of-magnitude reduction of wear [34], whereas the contact with the bone of the stem part can be improved by oxygen implantation [35]. The working conditions of human implants are well different from those typical for most of mechanical parts. Firstly, the implants are designed to long-lasting operation what imposes very low wear rates. Secondly, the mechanical contact takes place in the synovial fluids. The presence of the body fluid in the contact area appears as a mandatory condition for correct operation of natural hip joints and their prostheses. Appropriate wettability is important for good cell adhesion, contact with the body tissue and lubrication of the contact area. The possibility of tailoring of surface wettability appears thus as particularly important characteristic of the method critical for cell adhesion and friction properties. The modification of both metallic/ceramic and polymer parts of the prosthesis aiming the formation of hydrophobic/hydrophilic system should ensure good lubrication and reduced friction coefficient of the whole system. The main drawbacks of ion implantation are shallow penetration depth and relatively high cost. These limitations are of lesser importance in biomedical applications. The costs of the treatment are insignificant when compared to the costs of the whole treatment of the patient. All these arguments point to a final conclusion: ion implantation offers a powerful tool of surface engineering of biomaterials that merits the wider use in the practice.

Ion beams in science and technology continue to provide new possibilities in sample characterization, analysis, development of materials, modification of the selective properties etc. Ion beam methods provide a highly controllable set of tools which can be used for investigation systematically at the atomic level and ultimately controlled and exploited at the macroscopic level. It is observed that $S_e$ leads to various different types of effects depending on the nature of the target. Ion beams can produce defects of different types, can partially anneal defects originally present in the material, can create phase transformation (amorphization or crystallization), anisotropic growth of the material etc. Atomic size point defects can be produced if $S_e$ is below threshold value $S_t$ and microstructure can be modified. The effects depend not only on the magnitude of $S_e$ but also on fluence $\Phi$. Further, being the dynamic phenomena, effects produced depend on flux rate besides the most crucial factor which is the physics of the material. In some targets $S_e$ does not seem to play any role in damage creation. There is a need to have more dynamic studies of the $S_e$ induced effects. Since defect creation and annealing are transient processes probing of these phenomena needs on-line monitoring of the transient response of the system during irradiation or implantation under different conditions. Penetration of ion beam in the material involves charge exchanges between the projectile ion and the target. One has to understand transient strongly ionized cylinder produced by the ions passage. The high energy ionoluminescence has the potential to provide information on material structure during formation and evolution of radiation damages.

An ion beam is a type of particle beam consisting of ions. Ion beams have many uses in electronics manufacturing (principally ion implantation) and other industries. A variety of ion beam sources exist. One type of ion beam source is the duoplasmatron. Ion beams can be used for sputtering, ion beam etching, for ion beam analysis and most important for implantation and irradiation depending on their uses in industrial and living science. High energy ion beams produced by particle accelerators are used in atomic physics, nuclear physics and particle physics.

There are a number of accelerator facilities spread over worldwide. We are discussing here about a unique low energy ion beam facility situated at Inter University Accelerator Centre (IUAC), New Delhi, India, which provides low and medium energy ions for atomic physics and materials science research. The important feature of this facility is the availability of large currents of multiply charged ions from an electron cyclotron resonance (ECR) ion source placed entirely on a high voltage platform. All the electronic control devices of the ECR source including high power UHF transmitter placed on the high voltage platform are controlled through optical fiber communication in multiplexed mode. Some details of the source performance are described. The ECR source can be used to produce a wide variety of beams. The following techniques are available:

• Gas inlet: This is the simplest and most robust way to produce ions. The elements whose ions are required are injected into the ECR source in the form of a gas. This is straight forward for the noble gasses and elements like oxygen, nitrogen, hydrogen etc.

• Micro-oven: For elements that cannot be obtained in gaseous form, a micro-oven is available. The oven is inserted into the plasma chamber, and can be heated up to a 1000 °C. Volatile compounds can therefore be loaded into the micro-oven and be used to produce ions.

• Sputtering: A mechanized arrangement to insert a long thin wire (1 mm or 0.5 mm thick) wire of any element may be introduced into the source plasma volume. The high energy electrons can sputter out elements present in the wire, ionize them and produce the required beam.

• MIVOC: For metal ions, the technique of Metal Ions from Volatile Organic Compounds has been indigenously implemented. Organic compounds that have a metal atom, and are volatile are sealed into a stainless steel chamber and then allowed to permeate the ECR source volume. This allows us to extract metal ion beams that are difficult to produce by other techniques.

The schematic of the low energy ion implantation is shown in Fig. 1.2. The ion beams extracted from the ECR source are charge and mass analyzed by a dipole magnet and injected into one of two available beam lines (one at 90° and one at 15°). This figure shows the dipole magnet which switches the beam between the two experimental beam lines. Detailed information is referred to few of references [36], [37]. This facility is very useful for various exciting experimental research programs in materials science, e.g. ion beam induced epitaxial recrystallization, hetero- and nanostructure formations in semiconductors, modification of surface and near-surface properties of materials, rare earth doping of semiconductors and interactions of highly charged ions with solid surfaces.

The Pelletron accelerator

A high energy Pelletron accelerator, tandem Van de Graaff type accelerator, for basic and applied research in nuclear physics, atomic physics, materials science, biosciences and other allied fields is running at IUAC. This Linear accelerator is planned as a booster accelerator. A 15UD Pelletron accelerator is capable of delivering any ion from proton to uranium up to energy of a few hundred MeV depending upon the nature of ion. It has been installed at IUAC, New Delhi by the Electrostatic International Inc., USA. This is a tandem Van de Graaff accelerator, in which the charge carrier belt is replaced by a chain of pellets. The digit 15 stands for 15 MV terminal voltage and UD stands for Unit Double. The whole machine is mounted vertically; a schematic of the machine is shown in Fig. 1.3.

Fig. 1.3 shows the schematic of Pelletron accelerator, has an insulating steel tank of height 26.57 and width 5.5 m. In order to attain insulation (to prevent sparking/discharging) the tank is filled with sulfur hexafluoride (SF₆) gas at a pressure of 4.0 Torr. The SNICS (Source of Negative Ion by Cesium Sputtering) ion source acts as a source of negative ions that are momentum analyzed by the injector magnets.

A high voltage terminal with 1.52 mm diameter and 3.81 mm length at the middle of the tank can be charged by a high potential varied from 4 to 16 MV using an electrostatic charge transfer device. This terminal is connected to the tank vertically through ceramic titanium tubes known as the accelerating tubes. A potential gradient is maintained with the help of these tubes. Negative ions from the ion source are injected towards the terminal and are stripped off a few electrons through stripper foils. The yield is converted into positive ions. These ions are further accelerated as they proceed to the bottom of the tank at ground potential. As a result the ions from the accelerator gain energy, given by Eq. (1.1) as,

$$E=V_{\pi }(q+1)\mathrm{MeV}$$

where $V_{\pi }$ is terminal potential and $q$ is the number of positive charges (charge states) on the ions after stripping.

Thus a heavy ion of charge state $q$ will attain a final kinetic energy equal to (q+1)×16 MeV. Thus protons accelerated to a full terminal voltage would have energy of 32 MeV. By using appropriate magnets with respect to the charge states and energies, the high energetic ions are analyzed and are bent at 90° with respect to vertical position by using analyzer magnet. These redirected ions are directed to the desired experimental area in the beam hall with the help of multi-port switching magnet. This switching magnet can redirect the beam to any one of the seven beam lines as shown in Fig. 1.4.

Irradiation procedure

The irradiation process for materials generally carried out at the material science beam line facility of IUAC New Delhi. This beam line is at 15° angle with respect to the direction of the unswitched direct beam. The beam line is maintained at ultra-low pressure of the order of 10⁻⁹ Torr and the irradiation is carried out in high vacuum target chamber (HVC). It is fixed in Material Science beam line of Pelletron. It has arrangement of temperature control from low temperature to high temperature, dose control which includes positive bias to the target for secondary electron suppression (Faraday cup) and proper mechanical support and alignment. The vacuum in the target chamber is generally maintained below 10⁻⁶ Torr. A large number of samples can be mounted on all the four sides of a specially designed ladder, which is 10 cm long copper block of rectangular cross-section. Each sample was fixed on the ladder with the help of silver paste. Conducting path was provided by using a line of silver paste from the top surface of the sample to the copper block. The target ladder is inserted in the HVC from the top. Keeping beam current constant, irradiation is carried out at room temperature. By using magnetic quadruple and a steerer the beam is focused on the target. For attaining uniform irradiation the beam is scanned in a desired cm² area with the help of a magnetic scanner. The ion fluence is estimated using the ladder current and the current integrator.

Ion beam loses energy by means of both nuclear and electronic interactions with the substrate atoms. The former interaction consists of individual elastic collisions between the ion and target atom nuclei, whereas the electronic interactions can be viewed more as a continuous viscous drag phenomenon between the injected ion and the sea of electrons surrounding the target nuclei. For the energy regime normally used in heavy ion implantation (10–100 keV), the nuclear contribution to the stopping process normally dominates, which will be reflected in the particular ion trajectories as the ion comes to rest within the solid.

In Fig. 1.5 we see a two-dimensional schematic view of an individual ion’s path in the ion implantation process as it comes to rest in a material. As this figure shows, the ion does not travel in a straight path to its resting place due to collisions with target atoms. The actual integrated distance traveled by the ion is called the range, R. The ion’s net penetration into the material, measured along the vector of the ion’s incident trajectory, which is perpendicular to the surface in this example, is called the projected range, $R_p$.

In Fig. 1.6 more general three-dimensional presentation of a projectile into a solid is shown. In this schematic, an energetic projectile enters the sample surface at the point (0, 0, 0), at an angle $\alpha$ to the surface normal. The projectile is stopped at the point ($x_s$, $y_s$, $z_s$). For this presentation of an ion’s penetration into a solid, we define the range, $R$, and the projected range, $R_p$, similar to the definitions used in Fig. 1.5. However, since the ion is not incident parallel to the surface normal, the depth of penetration, $x_s$, which is defined as the perpendicular distance below the surface that the projectile comes to rest, is not equal to the projected range. If $\alpha =0$, these two quantities would be equal. The radial range, $R_r$, is the distance from the surface at the point of entrance, (0, 0, 0) to the point where the projectile comes to rest, ($x_s$, $y_s$, $z_s$). The spreading range, $R_s$, is the distance between the point where the projectile enters the surface and the projection of projectile’s final resting place onto the surface plane. The transverse projected range, $R_p^t$ is the vector connecting the radial range and the projected range.

For a single projectile coming to rest at the point ($x_s$, $y_s$, $z_s$), we have the following mathematical descriptions for the quantities defined in Fig. 1.6:

• the range spread

  $$R_s={(y_s^2+z_s^2)}^{1/2}$$

• the radial range

  $$R_r={(x_s^2+y_s^2+z_s^2)}^{1/2}$$

• the transverse projected range

  $$R_p^t={[{(x_{s}sin\alpha -y_{s}cos\alpha )}^2+z_s^2]}^{1/2}$$

• the longitudinal projected range

  $$R_p={[{\left(R_r\right)}^2+{\left(R_p^t\right)}^2]}^{1/2}\text{.}$$

For normal incidence projectiles, the range spreading is equal to the transverse projected range.

Ion penetration inside a solid undergoes a series of collisions with the atoms and electrons in the target. In these collisions the incident particle loses energy at a rate of $dE/dx$ of a few to 100 eV/nm, depending upon the energy and mass of the ion as well as on the substrate material. We are concerned here with the penetration depth, or range $R$, of the ions (Fig. 1.5). The range $R$ is determined by the rate of energy loss along the ion path,

$$R={\int }_{E_0}^0\frac{1}{dE/dx}dE$$

where $E_0$ is the incident energy of the ion as it penetrates the solid. The sign of $dE/dx$ is negative, as it represents the energy loss per increment of path, although tabulated values are given as positive. The main parameters governing the range or energy loss rate are the energy $E_0$ and atomic number $Z_1$ of the ion and atomic number $Z_2$ of the substrate if we exclude the effect of orientation of the crystal lattice. As the incident ion penetrates the solid undergoing collisions with atoms and electrons, the distance traveled between collisions and the amount of energy lost per collision are random processes. Hence all ions of a given type and incident energy do not have the same range. Instead there is a broad distribution in the depths to which individual ion penetrates. The distribution in ranges is referred to as the range distribution or range straggling. Further in ion implantation it is not the total distance $R$ traveled by the ion that is of interest but the projection of $R$ normal to the surface (i.e. the penetration depth of projected range $R_p$) (Fig. 1.6). A detailed discussion has been represented by Nastasi et al. [38].

Ion implantation is the process whereby controlled amounts of chosen foreign species can be introduced into near-surface regions of a material in the form of accelerated beam of ions. The energies used normally lie between 10 and 500 keV, with corresponding penetrations ranging from 100 Å to 1 μm, depending also on the target material. The ions impinge on the substrate with kinetic energies 4–5 orders of magnitude greater than the binding energy of the solid substrate and form an alloy with the surface upon impact. Virtually any element can be injected into the near-surface region of any solid substrate. Commonly implanted substrates include metals, ceramics, and polymers. The most commonly implanted metals include steels, titanium alloys, and some refractory metals. The advantage of the process over other surface modification techniques are that it is independent of temperature and solubility limits, requires no refinishing or heat treatment of the part after treatment, and is well controlled and universal process.

During the conventional Ion Implantation Process, a beam of positively charged ions of the desired element (either a gas such as nitrogen or a metal such as boron) is formed. Beam formation of a gas (e.g., nitrogen, oxygen, carbon, and inert gases) occurs by feeding a gas into an ion source. In the ion source, electrons, emitted from a hot filament, ionize the gas to form plasma. Ionization of the element is performed for the purpose of acceleration. Incorporation of an electrostatic field results in the acceleration of the positive ions at high energies under high vacuum. Forming a beam of a solid element (e.g. metals, metalloids, and certain non-metals) can occur by one of given methods. The first method is commonly used in the semiconductor industry, which requires extremely high-purity beams. In this method, a reactive gas, such as chlorine, is used to form the plasma. A metal chloride is generated as the chlorine ions chemically react with the metal walls of the ion source. The metal chloride then is ionized to form plasma of metal and chlorine ions. An analyzing magnet is used to separate the chlorine ions from the desired metal ion beam. A schematic of ion implanter is shown in Fig. 1.7.

The second method employs sputtering to generate metal ions. In this method, inert argon gas is ionized. The positively charged ions are attracted to a negatively biased metal target. As the argon ions strike the target, pure metal atoms and ions are dislodged from the target. The metal ions are extracted, focused into a beam, and directed toward the part to be implanted. These methods of generating beams of solids do not require the costly analyzing magnets and provide very high ion currents.

A newer form of ion implantation involves using plasma within the chamber from which gaseous ions are extracted. Similar to the beam line method, the gas is excited to form plasma, typically through the use of an RF antenna. The positively charged gas ions are accelerated towards the substrate by subjecting the substrate to high voltage pulsed biasing. This method of implantation is referred to as plasma source ion implantation (PSII) and circumvents some of the line-of-sight issues associated with conventional beam line methods.

The technique has proved highly successful for the fabrication of semiconductor devices, in spite of being in competition with well-establishes processes of thermal diffusion or epitaxy, and requiring a relatively expensive accelerator and vacuum equipment. More than 200 accelerators are now in operation throughout the world and this number is expected to rise 10 fold — far in excess of the number used for nuclear research. There are several reasons for the success of ion implantation. Among the most important are the control and resolution offered by this technique. Both the total amount and the purity of the implanted material can be accurately controlled and monitored, in a way that is impossible, for example, with diffusion, in which surface phenomena determine the dose which diffuses in. Furthermore, the concentration of impurities as a function of depth can be controlled by means of the ion energy, so that it is feasible to implant a buried layer of dopant. But for practical purposes, a crucial feature of the technique means that the well directed beam entering the surface produces a doped region which can be given a very high lateral resolution using conventional masking techniques.

Physical process

When ions enter a solid, they lose energy through elastic collisions and through electron excitations. A reasonably accurate theory of the process was developed by Lindhard et al. [39]. Most semiconductors are covalently bonded materials, with rigidly defined bond lengths and bond angles. Under ion bombardment this structure is vulnerable to disordering and in most cases it is necessary to restore crystallinity by thermal annealing at 800–1200 K for about 20 min. Otherwise, electrically active defects will dominate the effects of the atoms introduced. Fortunately, the electron transport properties of the semiconductor are also very good and the carrier mobility in the implanted zone may approach that of bulk material at the same impurity concentration. Moreover, the carrier density will (in silicon) correspond to the number of phosphorus or arsenic atoms introduced, although for boron the degree of electrical activity is commonly less than 100% because the formation of complexes as SiB₆. In few applications the electrical effects of defects introduced by bombardment can be put to good use. Proton bombardment raises the resistivity of GaAs by seven orders of magnitude [40] and is increasingly adopted for producing the isolation between neighboring devices.

At each age of research, ion implantation has become attractive for world wide researchers because of its applicability in various fields such as LSI (Large Scale Integration) device fabrication, powder surface modification, nanoparticle formation in insulators for quantum devices, catalysis and LE (light emitting) devices, and biocompatibility control for nerve cell patterning and nervous system repair [41], [42], [43], [44], [45], [46], [47], [48], [49].

Application of ion implantation technique

Powder surface modification

Powder surface modification by ion implantation, such as ceramic and polymer micrometer-sized particles, is greatly desired for the applications in medical and catalytic fields. When the powder particles are implanted with positive ions, they are scattered by a Coulomb repulsion force due to their high surface charging voltage, so the implantation with a sufficient fluence is very difficult. Fig. 1.8 [50] shows the threshold charging voltages obtained in positive Ar ion implantation for silica microbeads and for soda-lime glass beads, including a theoretical charging voltage curve for silica microbeads.

Nanoparticle formation

Insulators including metal nanoparticles will offer a possibility of implantation for various kinds of devices such as quantum devices (single electron devices or memory devices), nonlinear optical devices, photocatalysts with high performance, and LE devices. Metal nanoparticle formation in insulators by ion implantation is a preferable method because ion implantation can precisely control the depth and fluence of implanted atoms by the ion energy and current, respectively.

a. Quantum devices

In order to realize quantum devices such as a single electron device, the formation of thin layered nanoparticles in a thin insulator film with a thickness of less than 10 nm is required. Synthesis of gold nanoclusters in sapphire, using Ar ion implantation has been presented [51]. Unlike the conventional method of Au implantation followed by thermal annealing, Au was deposited on the surface of m- and a-cut sapphire single crystal samples including those pre-implanted with Ar ions. Au atoms were brought into the substrate by subsequent implantation of Ar ions to form Au nanoparticles. Samples were finally annealed step wisely in air at temperatures ranging from 400 to 800 °C. Evidence of the formation Au nanoparticles in sapphire obtained using transmission electron microscopy as shown in image Fig. 1.9 [51]. TEM results indicate that the specimen orientations and pre-implantation also influence the size and the volume fraction of Au nanoparticles formed. In another study [52] Au nanoparticles were fabricated by 3.0 Mev Au⁺ ions implantation into SiO₂ at fluences below the threshold for spontaneous cluster formation. By sequential heat treatment at different temperatures characterize the surface plasmon resonance of gold nanoparticles. The average particle radius of Au colloid particles is estimated to be about 4.2 nm.

b. Catalysis [50]

In order to improve photocatalytic efficiency of rutile Ti, implantation of Ag or Cu negative ions is performed into rutile titanium to form metal nanoparticles. The photocatalytic efficiencies have been evaluated by decolorization of methylene-blue solution by irradiation with fluorescent light.

Fig. 1.10 shows the relative efficiency as a function of the annealing temperature for silver negative ion implanted rutile titanium samples. Photocatalytic efficiency was improved by about 2 times due to the metal nanoparticle formation with a suitable annealing process. This would be due to the third harmonic wave generation caused by nonlinear effects with metal nanoparticles and/or due to the efficient charge separation by electron trap with metal nanoparticles.

Light emitting devices

Above all, the progress theory of defect engineering for semiconductor devices technology (particularly in light emitting devices) using implantation has also been established [53]. Erbium implantation in silicon has been done to tailor the optical properties of Si towards the achievement of a light emission at 1.54 μm [54]. In particular a detailed investigation of the non-radiative processes, competing with the radiative emission of Er in Si has been presented. Among these processes, an Auger de-excitation with the energy released to free carriers has been found to be extremely efficient. Using the knowledge on the material properties, an efficient Er implanted light emitting diode has been fabricated. It is shown that by exciting Er within the depletion region of reverse biased $p^{+}$–$n^{+}$ Si diodes in the breakdown regime it is possible to avoid Auger quenching and to achieve high efficiency. Moreover, at the switch off of the diode, when the depletion region shrinks, the excited Er ions become suddenly embedded within the neutral heavily doped region of the device. In this region Auger de-excitation with free carriers sets in and quenches the luminescence rapidly. This allows modulation of the light emitting devices at frequencies as high as a few MHz. Time decay curves, measured at 15 K with a 200 mW laser beam, for the sample co-implanted with 1×10 ¹⁹ Er cm⁻³ and 1×10 ²⁰ O cm⁻³ is shown in Fig. 1.11.

Biocompatibility control

Nerve cell engineering is a promising technique for the future. This method can be applied to the following developments: (1) artificial nerve networks with living cells, for realizing a bio-interface between nerve system and external electronic circuit, and (2) guide tubes for nerve regeneration, i.e., tubulation. Good nerve cell affinity (nerve cell attachment and neurite outgrowth properties) on the surface of polymers (such as polystyrene and silicone rubber) just on a micrometer-sized patterned modified region is important. Then, negative ion implantation, with the ability of precise control of ion beam trajectory is preferred.

A new method to control the position of neuron cell attachment and extension region of neural outgrowth has been developed by using a pattering ion implantation with silver negative ions into polystyrene dishes [55]. This technique offers a promising method to form an artificially designed neural network in cell culture in vitro. Silver negative ions were implanted into non-treated polystyrene dishes (NTPS) at conditions of 20 keV and 3×10¹⁵ ions/cm² through a pattering mask. For cell culture in vitro, nerve cells of PC-12h (rat adrenal pheochromocytoma) were used because they respond to a nerve growth factor (NGF). In the first 2 days in culture without NGF, it is observed that there is a selective cell attachment only to the ion implanted region in patterning Ag⁻ implanted polystyrene sample (p-Ag/NTPS). In another 2 days in culture with NGF, the nerve cells expanded neurites only over the ion implanted region. For collagen-coated p-Ag/NTPS sample of which collagen was coated after the ion implantation (Collagen/p-Ag/NTPS), most nerve cells were also attached on the ion implanted region. However, neurites expanded in both ion implanted and unimplanted regions. The contact angle of NTPS decreased after the ion implantation from 86° to 74°. The region selectivity of neuron attachment and neurite extension is considered to be due to contact angle lowering by the ion implantation as radiation effect on the surface.

Energetic ion beams have been exploited by researchers in different ways in the field of materials science. Ion beam effect on the materials depends on the ion energy, fluence and ion species. The interaction of the ion with material is the deciding factor in the ion beam induced material modification. Depending on the energy density, the melting point of the materials can be exceeded in a region around the ion track. As a consequence crystalline material can be transferred to the amorphous state and vice versa.

Now a days both low energy and heavy ions are able to produced using accelerator facilities all over the world for their various applications. Swift Heavy Ions (SHI) can in a controlled manner produce changes at the bulk, thin film surfaces, interfaces and is becoming increasingly important in basic and applied research. The interaction of ions with surface and interface results in the modifications of structural characteristics and applied properties and can tailor new materials. Ion irradiation being a thermal process therefore properties of nonmaterial could be tailored which are otherwise difficult or not feasible by conventional methods. Ion beam synthesis and processing of materials in nanometer scale offers both appealing advantages and new challenges in various areas of modern science and technology. We will discuss about the tremendous applications of irradiation process in later Section 6.

In the last few decades, ion–solid interaction has become advanced as to become a broad science and technological field in its own right. Many experimental effects involving radiation and solids are listed, which in a sense were merely simple individual initiatives and that was essentially academic research and now it is recognized for many preparatory and varied analytic techniques in material science. Ion–solid interaction is one of the key foundations, for example, of the new nanotechnology for which, in the beginning of this 21st century, we are witnessing exponential growth. Perhaps it is a consequence of the swiftness of these advances and still the present day state of the art is yet to be explored. It follows that now there exists many challenges both for well-known practitioners in the field and for newcomers alike.

Ion beam applications being for a long time the domain of low energy accelerators have started to conquer medium to high energy machines. Some pioneering activities which began one or two decades ago have by now become standard tools of ion beam applications. Energetic ions penetrate more deeply into matter. In one way or the other this is the most important reason to utilize them.

We have three main categories of applications:

• Materials analysis,

• Materials modification, and

• Medical applications.

The interaction of ion beam with a solid is a non-equilibrium process. Energetic ions lose energy during passage through materials mainly by two processes:

• Inelastic collisions of the highly charged energetic ions with the atomic electrons of the matter known as electronic energy loss ${(dE/dx)}_e$ dominant at low energies in keV range and

• Inelastic scattering from nuclei of atoms of the matter termed as nuclear energy loss ${(dE/dx)}_n$ dominant at high energies in MeV range.

Low energy ions up to a few hundred keV used implantation in semiconductors, which modify the surface and interface of materials [56]. They can also used to alter the surface and interface properties after ion irradiation of solid bulk and thin films as well as applicable in biological and medicine field.

Fundamentals of ion–solid interaction

(i) Ion sources

The basic requirement in this field is the availability of source of energetic ions. There are many small accelerators available used exclusively in ion implantation research (energies up to 2–3 MeV). Swift heavy accelerated ions (up to GeV) depending on the material irradiated, being the capacity to access a large range of lower energies using ‘absorbing’ foils of different thickness and different material. Typical of the high energy machines is the heavy ion accelerator such as 15UD Pelletron accelerator at Inter University Accelerator Centre, New Delhi, India. Another accelerator named IonenStrahl-Labor at the Hahn-Meitner Institute in Berlin which combines several accelerating units to one machine. Highly charged ions are produced in different electron cyclotron resonance sources, and then accelerated in a Van de Graaff accelerator or, alternatively, in a radiofrequency linear accelerator. This particle beam is then fed into a cyclotron where the particle energy is enhanced by a factor 17. Thus, ion beams can be obtained with atomic masses $M$ ranging from hydrogen to gold, and with maximum energies of 70 MeV for hydrogen and 400 MeV for gold. The maximum effective ionic charge $Z_{eff}$ of the ions, which both fluctuates and falls during energy dissipation, is of the order of 26⁺, while the typical ion fluences which can be provided are 10⁹–10¹⁰ ions/cm² s. In one of the irradiation facilities, the particle beam can be scanned one dimensionally over 5 cm width or two dimensionally over a maximum area of 3 cm×3 cm, and targets can be moved across this spot both horizontally and vertically, thereby permitting in-situ irradiation of maximum target areas of 900 cm² in 30 cm×30 cm sheets. In another facility, presently devoted to the commercial irradiation of polymer foils, the ion beam can be scanned horizontally up to ±15 cm, whereas the target foil is fed vertically through the scanning beam. In this way, foils with lengths of hundreds of meters can be irradiated in a single experimental run. In order to ensure good homogeneity of irradiation, the incident ion current can be stabilized within ±1%.

(ii) Stopping power

Once an energetic particle beam strikes matter it immediately begins to transfer energy to the target system. The energy deposition process is commonly described by the ‘stopping power’ $(-dE/dx)$ the energy transfer per unit path length of an ion on its trajectory. It is also convenient to split up the particle stopping into two basic and dominant energy transfer mechanisms. One arises from ‘ballistic’ billiard ball type atomic collisions with the target atoms (‘nuclear’ energy transfer) and the other from excitation and ionization of the target electrons (‘electronic’ energy transfer). The total stopping power is then the sum of both components whose reciprocal integral defines the total projectile range. Both stopping powers increase with increasing energy, reach a maximum and thereafter fall away as shown in Fig. 1.12. The accumulated electronic stopping power, however, reaches its maximum–commonly referred to as the ‘Bragg peak’–at energies which are orders of magnitude higher than that for nuclear stopping.

Light ions of any energy, and energetic heavy ions with stopping powers in keV/Å range, deliver much more energy via electronic excitation than by nuclear collisions. For low energy heavy ions with stopping powers in eV/Å range, the nuclear energy transfer dominates. Primary nuclear interactions lead to transfer of kinetic energy from the ion to the target atoms, which can be displaced themselves from their original positions if the transferred energy exceeds the target atomic-mass-dependent and angular-dependent displacement energy $E_d$. If the energy transferred to the primary atom ‘knocked on’ is sufficiently high, there can then be secondary, tertiary and higher order atomic knock-ons all of which, if sufficiently localized, comprise an energetic atomic cascade in the target as shown in Fig. 1.13.

In crystals stable or metastable ‘point’ defects are then inevitable, Frenkel interstitial vacancy pairs in metals, alloys and semiconductors and Schottky disorder in insulators. Subsequent point defect motion can also lead to the formation of ‘extended’ defects—dislocations lines and especially loops, planar and tetrahedral stacking faults, coherency strain fields, etc. In the case of molecular targets, atomic displacements can lead to permanent radiochemical destruction, particularly in carbonaceous targets [57].

Condensed matter phase transitions can also be a consequence of ion beam irradiations. In metallic targets, this usually requires extreme electronic energy transfer by heavy ions, and new crystallographic phases, or crystallographic voids, sometimes arranged on superlattices can be created. For relatively small electronic energy transfers, metal and metallic alloy targets show no substantial permanent effects. On the contrary electronic stopping most frequently plays a dominant role in semiconductors and insulators, where it may lead to the formation of structural defects e.g. F-centres, V-centres, colloids and voids, etc., chemical radical formation, bond breaking, or the formation of novel bonds. This constitutes the base of radiochemistry with energetic ions.

Analytical theories for electronic stopping are well established but are generally limited to some extent by fundamental assumptions which often cannot be sustained. For more than two decades, however, a number of computer codes for the numerical simulation of three-dimensional range and damage distributions in solids have been developed. The best known and most used and given some care, the most helpful, is undoubtedly the transport and range of ions in matter (TRIM) code due to Biersack and Haggmark [58]. Absolutely, these codes have an accuracy of a few percent at best, but they are extremely useful in generating and presenting quite reasonable overall pictures of the complex comparative stopping processes and range evaluations in both elemental and compound targets. They generally draw on the Bethe–Bloch analytical theory for electronic stopping and for compound targets the ‘Bragg rule of additivity’ from individual constituent atoms is assumed. Also with TRIM, sometimes dominant secondary effects are not accounted for, including damage annealing and epitaxy, radiochemical changes, and defect diffusion and agglomeration.

In particular, it is important to be aware that the simulated target is fundamentally a ‘random’ distribution of atoms; though the real macroscopic mass density is retained. The absence of any dependence on angle of incidence of ion beam, free surface, crystallinity and therefore target-dependent directional effects, and of no real point or extended defects, means that a good deal of caution and understanding is called for in the more complex projectile/target interactions.

Individual successive nuclear collisions lead to an accumulated deviation of the projectile flight direction from the original, and the more so as the kinetic energy is attenuated. Projectile ions for which nuclear energy transfer dominates, therefore, describe an increasingly zig-zag motion until they finally come to rest. The consequence is a spatially extended damage distribution. In contrast, incident ions with dominant electronic energy transfer (light ions at any energy, or very swift heavy ions) follow a relatively straight flight path, apart from very occasional high-angle, low impact parameter, nuclear scattering events. The stable or metastable damaged zone around and along such an ion trajectory is usually referred to as a ‘latent ion track’ created by what is more loosely referred to as an energy ‘spike’. Typical track lengths for SHIs are in the order of 30–130 μm, and the central zone of maximum damage along each track, so-called ‘core’ typically has a radial extent of 3–8 nm. The latter is surrounded, however, by a three-dimensional zone of defective crystal which falls away with distance, sometimes slowly, sometimes quickly, depending on the macroscopic physical properties of the target crystal, and on the target-dependent diffusive behaviour of characteristic point defects. It follows that very frequently the ‘radius’ of a track is much more difficult to define than the contemporary scientific literature would have us believe. Also important in this penumbral ‘haze’ of defects is the nature and volume concentration of pre-existing and conglomerating trapping centres (nucleation, saturable, unsaturable, impurity traps, etc.). Similarly, when extended defects such as dislocations, stacking faults, grain boundaries and free surfaces are present, and close enough, the penumbral haze about a latent track core can be drastically modified due to dominant diffusion of radiation induced defect to these well-known ‘sinks’. Finally, for increasing ion fluences, where the spatial hazes of some neighboring latent ion tracks begin to overlap, as a prelude to damage saturation, we find from direct TEM observations that the rather ill defined outward reach of a typical haze can sometimes be up to 100 nm or more.

(iii) Ion radiochemistry of organic matter

It appears that different transferred electronic energy densities has lead to different types of defects, those formed after low energy ion impact being less stable than those introduced by more energetic ions. Consequently, a major fraction of the electronic damage induced by low energy projectiles at ambient temperature often anneals after the irradiation, so that eventually the more stable but originally much less abundant nuclear defects finally dominate, in spite of the 10 to 1000 fold more overall energy spent for the electronic energy transfer. This effect vanishes, however, for more energetic projectiles, apparently due to the dominating influence of more stable types which electronic excitation gives rise to.

Specifically for polymeric targets, there are chemical bond breakage and bond formation effects, usually described as ‘chain scission’ and ‘cross-linking’ respectively, to be considered [59]. Of special importance is the fact that ion induced chain scission of polymers upon energetic ion impact leads to an enhanced chemical etchability of many ion irradiated synthetic polymers. In other words, etchant attack of such an irradiated polymer foil leads to the formation of nano- or micropores the so-called ‘etched tracks’. These pores can have different shapes, cylindrical, conical, funnel-like, etc., so that to some extent they can be tailored by judicial choice of the various parameters, such as projectile energy transfer, polymer type, etchant efficiency, etc.

(iv) Surface effects [60]

Both the projectiles and the knock-on target atoms give rise to a number of effects at the surface and in the subsurface region of an irradiated target which can be exploited in experimental methods for material analysis. If a projectile ion entering a solid is backscattered from a near-surface atom in a large angle collision, its direction-dependent energy loss sensitively depends on both the depth of collision and the target atom mass. Therefore, the ‘Rutherford backscattering’ technique, in which protons or more frequently $\alpha$-particles comprise the analyzing ion beam ‘probe’, has become one of the favored analytical methods for determining the composition with the depth of solid targets and the depth distributions of radiation damage impurity atoms, defect aggregates, etc. This technique holds, however, only for heavy target atoms. Alternatively, for the depth distribution of light atoms, it is preferable to knock-on these atoms in such a way that they escape from the target through the surface in a forward direction, which implies that the projectiles must hit the target surface at an oblique angle. This gives rise to ‘elastic recoil detection’ analysis.

The backward ejective release of surface and near-surface atoms by the dynamic impact of an energetic ion is generally referred to as ‘sputtering’ as shown in Fig. 1.14. Here, one must distinguish between nuclear sputtering, the atomic yield of which scales with the nuclear energy transfer for any target and is reciprocally related to the surface binding energy $U$, and electronic sputtering which takes place above some threshold energy upon the impact of very energetic heavy ions onto, for example, metallic targets or upon the impact of an ion of any energy onto insulators.

The theory of electronic sputtering is not yet well developed, though there is a general understanding that the strong ionization implies that the physics of ‘Coulomb explosion’ plays a major part. In either case, the mass distribution of sputtered atoms and the dependence of the amount of removed surface material, also as a function of location on the sample surface, can be determined by ‘secondary ion mass spectrometry’. By these means, it is possible to carry out three-dimensional mapping of the surface and immediate subsurface atomic layer compositions. The impact of highly energetic ions onto molecular insulators releases not only small fragments from the very point of impact in a jet-like manner, which may be theoretically accessed using conventional classical hydrodynamics, but also larger fragments and even intact molecules from more distant points on the surface due to shock-wave propagation mechanisms. The physics of these sputtering processes is sufficiently reproducible and understood. It has lead to a well-established technique for the analysis of complex organic matter namely plasma desorption mass spectrometry.

With all the experiments done so far, it is concluded that particle–solid interactions has become a very promising probing tool in particle physics, astrophysics, and cosmology. As a conclusion, one might say that the field of ion–solid interaction, a former spin-off from nuclear science, is now a well-established science with good fundamentals and useful applications emerging. However, due to the relatively short life time of that science, many details are still unknown. Therefore, ion–solid interaction is still now regarded as a challenging scientific field that offers still lots of promising applications, and it justifies especially the invitation to interested scientists from other fields to contribute to this discipline with their great specific expertise.

When an energetic ion passes through a solid, it loses energy through elastic and inelastic scattering processes and as a result, the following interesting aspects can be observed in the Fig. 1.15.

The interaction of ions with any material is a deciding factor in the ion beam material modification. The ions lose energy during their passage through the material, which is spent in either displacing atoms by elastic collisions (nuclear stopping) or exciting the atoms by inelastic collisions (electronic stopping). The energy lost due to nuclear stopping is called nuclear energy loss and one due to electronic loss is known as electronic energy loss. Apart from these two events, another mechanism which takes place during slowing down of incident ions is charge exchange process between the ion and the atoms of the solid. This can be expressed as

$${\left(\frac{dE}{dX}\right)}_{loss}={\left(\frac{dE}{dX}\right)}_{Elect}+{\left(\frac{dE}{dX}\right)}_{Nucl}+{\left(\frac{dE}{dX}\right)}_{ChargeExchange}$$

where ${\left(\frac{dE}{dX}\right)}_{Elect}$ is loss due to electron, ${\left(\frac{dE}{dX}\right)}_{Nucl}$ is loss due to neutron and ${\left(\frac{dE}{dX}\right)}_{ChargeExchange}$ is loss due to charge exchange. Since charge exchange loss represents a small fraction of total energy loss, which can be neglected.

A general way to treat the slowing down of an ion in matter is through stopping power $(dE/dX)$ defined as the energy $dE$ lost by an ion for traversing a distance $dX$. Thus the total stopping power can be written as

$$\left(\frac{dE}{dX}\right)={\left(\frac{dE}{dX}\right)}_e+{\left(\frac{dE}{dX}\right)}_n\cong S_e+S_n\text{.}$$

The electronic stopping can be divided into two regions separated by the velocity ${\upsilon }_0{Z}_i^{2/3}$, where ${\upsilon }_0$ is the Bohr velocity. When the ion velocity ${\upsilon }_i$ is in the range of $0.1{\upsilon }_0$ to ${\upsilon }_0{Z}_i^{2/3}$, the electronic energy loss is approximately proportional to ${\upsilon }_i$ (or $1/E_i^{1/2}$) while for ${\upsilon }_i>{\upsilon }_0{Z}_i^{2/3}$ the electronic energy loss is proportional to $1/{\upsilon }_i^2$ (or $1/E_i$). Nuclear and electronic energy losses are schematically shown in Fig. 1.12.

Nuclear energy loss is due to elastic binary collision between a projectile ion and target atoms. This is based on the Screened coulomb potential and impulse approximation. The interaction potential, $V\left(r\right)$, between two atoms $Z_1$ and $Z_2$ can be written in the form of a screened potential using $\chi$ as the screening function,

$$V\left(r\right)=\frac{Z_1{Z}_2{e}^2}{r^2}\chi \left(\frac{r}{a}\right)$$

where $a$ is the Thomas–Fermi screening radius for collision and is given by

$$a=\frac{0.885a_0}{{(Z_1^{1/2}+Z_2^{1/2})}^{1/2}}$$

where $a_0$ is the Bohr radius and value lie between 0.1 and 0.2 Å for most interactions. Like Thomas–Fermi potential, Lenz–Jensen, Moliere and Bohr potentials are also used to calculate $S_n$. The expression for the $S_n$ is given as

$$S_n=-{\left(\frac{dE}{dX}\right)}_n=N_2{\int }_0^{T_{\max }}Td{\sigma }_n(E,T)$$

where, $N_2$ is the atomic density of the target, $t$ the energy transferred from incident ion to a target atom. $T_{\max }$ is the maximum value of $T$, and $d{\sigma }_n$ is the differential cross-section. Using an appropriate screening potential and impulse approximation, the final expression for $S_n$ can be derived as

$$S_n=-{\left(\frac{dE}{dX}\right)}_n=N\frac{{\pi }^2}{2}{Z}_1{Z}_2{e}^{2}a\frac{M_1}{M_1+M_2}\text{.}$$

Though this equation gives the correct order of magnitude for $S_n$, the lacuna is that it deviates considerably in energy dependence.

In addition to this a more detailed theoretical approach has been studied to propose a self-consistent nonlinear method to calculate the energy loss of heavy ions on a wide range of velocities. The model is based on the transport cross-section approach and on a previous extension of the Friedel sum rule for moving ions. The purpose is to develop a nonlinear stopping power evaluation method that could be applied at finite ion velocities [61].

Energetic ions entering a solid immediately interact with many electrons simultaneously. In such encounter, the electron experiences an impulse from the attractive Coulomb force as the projectile ion passes its area. Sometimes this impulse may be sufficient either for excitation or for ionization. The energy, which is transferred to the electron, comes from the energetic ion. So, the velocity of the ion will be decreased as a result of the encounter. At any given time the ion interacts with many electrons, so the net effect is to decrease its velocity continuously until it is stopped. The swift heavy ions can move a few microns to tens of microns in the target because a single encounter of ions with an electron does not deflect its path. So, these particles pass a definite range in a given material.

The energy loss per unit path length of the ion is known as Specific Energy Loss. This is also termed as stopping power or linear transfer. In 1913, Bohr first proposed the theory of electronic energy loss $S_e$ of energetic ions in solids [62]. He also derived an expression for the $S_e$. He considered that the target as a collection of harmonic oscillators whose frequency was determined by optical absorption data. Bethe and Bloch [63], [64] extended this work for the relativistic ions and solved the problem quantum mechanically in the first Born approximation. The electronic energy loss $S_e$ of highly energetic ion in solid is stated as follows

$$S_e=-{\left(\frac{dE}{dX}\right)}_e=\frac{4\pi e^4{Z}_p^2{Z}_t{N}_t}{m_e{v}^2}[ln\left(\frac{2m_e{v}^2}{I}\right)-ln(1-\frac{v^2}{c^2})-\frac{v^2}{c^2}]$$

where, $\nu$ and $Z_{p}e$ are the velocity and charge of the projectile ions, $Z_t$ and $N_t$ are the atomic number and number density of the target atoms, $m_e$ the electron rest mass and $e$ the electronic charge. The parameter $I$ is the average excitation and ionization potential of the target. For non-relativistic projectile ions the term $ln\left(\frac{2m_e{v}^2}{I}\right)$ is significant. Eq. (1.13) is valid only when the velocity of ions is larger when compared to the velocities of the orbital electrons in the target. From Eq. (1.13) it is seen that for a non-relativistic ion, $S_e$ varies inversely with ion energy. This is because if the velocity of the ion is low, it spends more time in the electron area and transfers greater impulse, and imparts larger energy to the electron.

Studies involving ion–solid interactions in the range of a few eV to a few keV that are intermediate between thermal gas–surface interactions and high energy bulk implantation are collectively examined in this section. The interactive process is traced in a phenomenological manner by providing a simple treatment of the different physical and chemical phenomena involved.

The kinematics of noble gas ion as well as atomic and molecular reactive ion interactions with surfaces are treated in terms of classical dynamics and experiments are described [65] that probe the nature of ion–solid interaction potentials by examining the scattering of ions from surfaces and the charge exchange processes operative during scattering. Molecular ion scattering is investigated and the behaviour interpreted in terms of partial dissociation that results in both atomic and molecular scattering. A neutralization model is described that considers the interaction domain to be divided into three steps: the incoming trajectory, the close encounter, and the outgoing trajectory. A spectrum of possible results from the interaction process is described; these include reaction and chemical alteration of the target surface, desorption of adsorbate atoms by hyperthermal physical/chemical interaction, and accumulation of a high surface concentration of the projectile species, viz. film deposition. These studies investigated the basic science of a number of technological phenomena and represent an elegant approach to the simulation of a number of these phenomena in the controlled environment of ultra-high vacuum. Qualitative and quantitative arguments are made for a number of the experimental results and cross-examination of data from different laboratories is made in terms of simple kinetic and thermodynamic concepts to illustrate fundamental similarities in the various studies.

Applications

a. Ion beam implantation

Although implantation has been well-defined in one of previous section, therefore we will represent only a brief and important outlook of this technique. Negative ion implantation has been applied to fabricate metal nanocomposites of metals and polymers which are promising for nonlinear electronic, optical and biomedical applications. Nanoparticle formation processes of Ag and their effects on surface properties of polymers have been recently studied, in comparison with those of Cu [66]. Polycarbonate (PC) substrates were irradiated by negative Ag and Cu ions of 60 keV to various doses up to 3×10¹⁷ ions/cm². A surface-plasmon-resonance peak of Ag around 2 eV appears in the optical spectra of PC implanted to a dose of 1×10¹⁷ ions/cm², whereas their defect absorption is not significant. The TEM and RBS measurements show that well-defined implantation is capable at this fairly low energy, though surface recession takes place because of ion sputtering. The structure of PC is stable enough for formation of Ag nanocomposites. Depth distribution of Ag nanoparticles significantly shifts towards the surface, differently from the case of Cu nanoparticles. The near-surface localization of Ag nanoparticles was caused by the sputtering effects as well as higher mobility and reactivity of Ag atoms. In this way negative ion implantation provides nanoparticle formation for surface modification of polymers, with understanding the ion induced precipitation processes in the polymers.

It is well known that surface properties at nanoscale are determinant in a number of applications, such as sensors, biomedical and optical devices. Nevertheless, relations between surface treatment parameters and their effects on topography at the nanoscale, surface energy or light reflectivity are often poorly understood. In a study [67], a non-fluorescent glass material (Knittel) was selected and subjected to ion implantation treatments with different parameters and species, including Ar, Ne, C, N, CO and NH₂. AFM results showed remarkable differences in surface nano-topographies and contact angles (from 15° to 70°) that were obtained. Furthermore, the effects of ion implantation parameters had also very significant consequences on background noise effects, of great importance for optical properties. It was found that the best implantation treatment corresponded to N₂⁺ ions implanted to a dose of 3×10¹⁷ ions/cm² at energy of 30 keV. This treatment resulted in an adequate contact angle, producing a nano-textured surface with potential features for a good attachment and orientation of deposited biomolecules, and very low background fluorescence, hence allowing a high degree of scanning sensitivity, for application on DNA micro-arrays. The study has shown that ion implantation represents a powerful tool for modifying key properties on surfaces that play an important role in the response elicited on living tissue and biomolecules, which is notoriously relevant for the application as bio-sensors.

Avalanche silicon photodiodes have potential applications to detect low energy single ions for counting single ion impacts in shallow implant depths for the deterministic doping of nanoscale electronic devices. An investigation is reported for avalanche photodiode detectors in the linear operation mode for detection of 0.5–2 MeV He ions [68]. The charge gain was found to saturate at a level that correlated with the ion stopping depth in silicon. The measured charge gain for energetic ions, which have a well-defined depth in a silicon substrate for the deposition of ionization energy, is compared with that of X-rays and photons, which deposit the ionization energy over a wider range of depth. This allowed the identification of a suitable structure for an avalanche photodiode optimized for the detection of sub-10 keV heavy ions with an internal charge gain above 10 achievable. This offers significant advantages over conventional PIN devices where the signals from such ions would be lost in the noise.

Recently very interesting results of magnetic nano-patterning of perpendicular hard disk media with perpendicular anisotropy, but preserving disk surface planarity, are presented by one group as shown in Fig. 1.16 [69]. They have been used reactive ion implantation to locally modify the chemical composition (hence the magnetization and magnetic anisotropy) of the Co/Pd multilayer in irradiated areas. The procedure involves low energy, chemically reactive ion irradiation through a resist mask. They found that among N, P and As ions, P ions are to be most adequate to obtain optimum bit density and topography flatness for industrial Co/Pd multilayer media. The effect of this ion contributes to isolate perpendicular bits by destroying both anisotropy and magnetic exchange in the irradiated areas. Low ion fluences are effective due to the stabilization of atomic displacement levels by the chemical effect of covalent impurities. Low energy chemically reactive ion implantation modifies the structural and the magnetic properties of magnetic multilayers with perpendicular anisotropy (e.g. Co/Pd multilayers). For N ions, interfacial disorder is generated, giving rise to a strong reduction of the local magnetic anisotropy and exchange coupling. A stronger reduction is observed for P and As ions when a chemical bonding with Co is established. By using proper masks and flux densities, magnetic recording media with perpendicular anisotropy and magnetically hard bits in a non-magnetic matrix can be fabricated. Heavier ions like P are a better choice in order to optimize damage depth, to reduce polymer mask thickness and to limit lateral ion straggling. In principle, this technique could be a feasible industrial production method for nano-patterning hard disk magnetic media with perpendicular magnetization while preserving surface flatness.

b. Ion beam irradiation

Low energy ion beam irradiation may have various applications and invariably open up completely new perspectives in various fields, particularly in materials science and device technology. Low energy accelerators are now standard tools and their exploitation in engineering materials of all kinds of interest: electronic, tribological, and metallurgical etc. provide very fruitful activity.

Low energy ion beam bombardment of ethylenetetrafluoroethylene (ETFE) modifies the physical and chemical properties of the polymer surface in ways that enhance or compromise applications in the technological and medical physics fields [70]. When a material is exposed to ionizing radiation, its changes depend on the type, energy and intensity of the applied radiation. In order to determine the nature of the induced radiation changes, ETFE films were bombarded with fluences from 10¹² up to 10¹⁵ ions/cm² of keV N and protons. It has been examined, by residual gas analysis, the emission products from ETFE films during bombardment by protons and nitrogen. HF and $H_2$ are the dominant emitted entities. FTIR and Raman spectroscopic analysis of the films after bombardment do not detect the formation of CC double bonds, but transmission measurements of UV–visible–near IR light indicate absorption of blue light perhaps by these bonds. The RGA analyses indicate that C–F and C–H bonds are broken allowing hydrogen and hydrogen fluoride to escape. Beyond the penetration depth of keV protons and nitrogen ions, there is no detectable damage or structural change.

Energetic ion beams, when penetrate through the interface of different materials, produce massive atomic transport across the interface which results in many stable, unstable or even thermodynamically non-equilibrium phase formation around the interface. Due to the improved electrical, chemical or optical properties of the ion irradiated materials, ion beam induced mixing of metal/metal or metal/semiconductor bilayer systems occupy a major share of the accelerator based material research. Metal silicides, produced by ion beam mixing of metal/semiconductor, have found extensive applications in microelectronic devices as contact materials to the source/drain areas to control the Schottky barrier height due to their low electrical resistivity, low consumption of Si and good thermal stability.

Traditionally, silicides using such technique are TiSi₂,CoSi₂,NiSi₂,FeSi₂ and few more transition metal silicides such as Ta and V silicides [71], [72], [73], [74], [75], [76], [77], [78], makes themselves attractive candidate materials for use in future complementary metal-oxide-semiconductor (CMOS) device generations. Ion irradiation is a well-known technique by which metallic silicides can be formed at the interfaces of metal/semiconductors in a very controlled fashion.

Polymers are a class of materials widely used for a broad field of applications. Ion irradiation ranging from several eV to keV is a quite efficient tool to modify the properties of polymers like wettability, optical properties, adhesion between metal and polymer surfaces. In the field of optical telecommunication, polymers are discussed as a new class of materials for the fabrication of passive optical devices. Ion irradiation is a promising method to generate structures with a modified index of refraction, which is necessary for the guidance of light with different wavelengths in optical devices. In an experiment, Poly-Allyl-Diglycol-Carbonate (CR-39) polymer samples are used to bombard with 320 keV Ar and 130 keV He ions, at different ion fluences [79]. Effects of ion bombardment on the optical properties of CR-39 have been investigated. UV–Vis spectra of irradiated samples reveal that the optical band gap decreases with increasing the ion fluence for both Ar and He ions. The decrease in the PL intensity with the increase in the ion fluence is attributed to ion bombardment induced defects and clusters in the CR-39 which serves as non-radiative centres.

Recently biotechnological and biomedical applications of polymer materials have considerably increased. One of the most versatile polymer is UHMWPE (ultra-high molecular weight polyethylene) being used in different fields such as engineering, microelectronics, but also medicine and biology. The development of biological and biomedical application of this polymer material is accompanied by permanently increasing requirements concerning their biocompatibility. One of the most promising ways to improve biocompatibility and bacteriostaticity is ion bombardment of polymer surface. He, N, Ar and Ag ions of energy 65–150 keV were applied to this polymer [80]. Surface layer oxidation, graphitization and changes to the surface geometry lead to increase of the surface energy. Modified surface exhibits bacteriostatic properties particularly for higher ion fluences. Aggregation of blood platelets on polymer surface subjected to ion bombardment is limited.

There are several experiments performed using low energy ion beam which all cannot be included in such a brief section. We have tried to note out few more of them in later sections.

In the inelastic collision (cross-section 10⁻¹⁶ cm²) energy is transferred from the projectile to atoms of the matter through excitation and ionization of their surrounding electrons. The amount of electronic energy loss in each collision varies from a few eV/Å to a few keV/Å. For a swift heavy ion (SHI) moving at a velocity comparable to the Bohr velocity of electron, this is the dominant mechanism for transfer of energy to the material causing the modification of its properties. SHI, having energy of 1 MeV/nucleon or more, transfers its energy to the atoms in a solid, predominantly through inelastic scattering producing a trail of excited/ionized atoms. During passage of SHI through the materials, cylinders containing highly charged ions and electrons are formed causing modifications. After the passage of the SHI, the solid returns to its equilibrium state leaving behind bulk and surface modifications. The nature of modification depends on the electrical, thermal and structural properties of the target material, the mass of the projectile ion and irradiation parameters. In insulating and semi-insulating materials, damaged tracks along the trajectory of the ion beam may be formed. In high temperature superconducting materials columnar defects are formed along the ion trajectory which acts as flux pinning centres [81], [82]. In polymers, gaseous species evolve from the ion tracks because of chain scission and cross linkage [83], [84]. In colossal magneto-resistance (CMR) materials, electrical transport and noise measurements studies give the basic information of defects created by SHI [85]. Large electronic excitations can cause mixing at the interface of metal film on Si [86], [87], [88].

MeV ion bombardment in high $T_c$ superconductors [89] causes several changes in their properties, in particular increase in the normal state resistance, a progressive metal to insulator transition and normally lowering of their superconducting transition temperature ($T_c$). Sputtered TiN coatings on stainless steel were irradiated with 210 MeV Ag ions and coatings were studied [90] before and after irradiation using scratch adhesion test. A significant improvement in tribological properties, indicated by the average load at which various scratch induced failure modes commence for TiN coating on SS, was observed in irradiated films. Clearly fast moving ions can produce complex changes resulting in an improvement in the adhesion, in spite of the fact that the nuclear energy loss is extremely small. Defect distribution in semiconductors with MeV ions has been studied [91] in Si (100) crystal with 100 MeV Ti ions using X-ray topographic techniques. Controlled surface etching and subsequent topography is performed to extract information about the distribution of the strain in the depth of the crystal.

Diamond-like carbon (DLC) thin films have been the subject of considerable attention because of their unique combination of properties; extreme hardness, optical transparency, high electrical resistivity and chemical inertness. Energetic heavy ions can provide a handle for controlling or engineering these properties [92] over a wide range by judicious control of film growth parameters, such as the deposition temperature, nature of source gas, hydrocarbon vs. hydrogen gas ratio etc.

The main difference of materials modification by ion implantation and swift heavy ion irradiation is that in ion implantation the incident low energy ions get embedded in the material cause changes whereas in swift heavy ion irradiation the impinging ions do not get embedded in the materials due to their larger range.

Ion bombardment of Si monocrystals with amorphous layers (a-layers) under certain implant conditions may lead to ion beam induced epitaxial crystallization (IBIEC) of the layers. One of the advantages of IBIEC over direct thermal epitaxial regrowth is a lower annealing temperature, which depends on implant conditions. Although a Si crystal after IBIEC remains defective, such reduction of the annealing temperature gives an opportunity to decrease undesired diffusion of dopants during annealing and to avoid the temperature ranges of formation of the electrically active defect structures, such as low temperature thermal donors, detrimental for Si devices. Moreover, IBIEC affords selective recrystallization of chosen parts of the a-layer and, therefore, seems to be applicable for the formation of crystalline nanostructures within an amorphous matrix. A feature of IBIEC is that under quite similar conditions when the substrate temperature is decreased lower than some critical value $Tr$, called the reversal temperature, which is a function of implant conditions, the sign of the regrowth rate $Vr=-dh/d\Phi$ (where $h$ is the a-layer thickness, and $\Phi$ is the delivered dose) changes, and ion beam induced interface amorphization (IBIIA) takes place. This transition from the crystallization to amorphization regime can also be caused by increasing the beam flux $F$, and the reversal flux $Fr$, also dependent on irradiation conditions, may be introduced in addition to the reversal temperature $Tr$. It is also notable that the reversal temperature, as well as the regrowth rate $Vr$, depends on the amorphous/crystalline (a/c) interface position because the damage generation is a function of depth.

It has been established that the IBIEC/IBIIA kinetics intricately depends on the following parameters: the substrate temperature ($T$), dose $\left(\Phi \right)$, beam flux ($F$), ion species and energy, dopant type and concentration, and a/c interface orientation with respect to the crystallographic axes. Furthermore, the regrowth rate dependence on a given parameter may exhibit diverse behaviours for different values of the other parameters controlling the process. For instance, the dependence of the regrowth rate on the beam flux differs for light and heavy ion bombardment, for substrate temperatures near and far from the reversal temperature $Tr$, for the different a/c interface positions regarding the damage generation profile [93], [94]. A review of the main experimental data on IBIEC and IBIIA can be found in [95].

It has been experimentally ascertained that IBIEC and IBIIA are caused by an interaction of some defects with the a/c interface. Nonetheless, the nature of the defects responsible for crystallization and amorphization remains unclear.

Au implantation at 32 keV into Si has been used to produce a gold-rich damaged Si layer of thickness around 30 nm. Local recrystallization of this layer with a low energy Au irradiation has been found to result in Si nanocrystal (NC) formation of 4 nm [96]. IBIEC of silicon implanted with 100 keV Te⁺, Pb⁺ and Bi⁺ ions at liquid nitrogen temperature has been studied. During IBIEC of the implanted amorphous layer as a function of annealing dose, dopants used are with very low solid solubility and low diffusivity in Si. Behaviour of such elements was investigated when they are swept by the moving crystal/amorphous interface. IBIEC of the implanted amorphous silicon was induced at 400 °C using a 3 MeV Si beam. The evolution of Te, Pb and Bi species, implanted into Si during the pure thermal and ion beam induced crystallization of the amorphized substrates has been analyzed using RBS. The presence of Pb inhibits the SPEG at temperature 525 °C. Massive redistribution of the initially formed profiles of Pb and Bi was observed during IBIEC. The maximum concentration of Te incorporated in substitutional sites to the crystallized Si increased continuously with the beam annealing dose, while for Pb and Bi substitutional fractions reach saturation [97].

The effect of SHI irradiation on recrystallization of silicon-on-insulator SOI structure was thoroughly investigated by Virdi et al. [98]. The nitrogen profile is modified after SHI irradiation at the interface resulting in better stoichiometry of the Si₃N₄ layers and creation of abrupt Si₃N₄–Si substrate interfaces in the SOI structure. Recrystallization of the amorphized layers produced by high dose of nitrogen implantation was confirms by ESR analysis. The improvement in the electrical breakdown strength of the synthesized buried silicon nitride layers in SOI structure due to irradiation of SHIs are attributed to the removal of polycrystalline inclusions from the Si₃N₄ layer near the buried Si₃N₄ substrate interface. It also illustrate that the post-annealing after SHI irradiation of the synthesized SOI structure by high dose nitrogen implantation is a better choice as far as the improvement the quality of these SOI structure is concerned. The improvement in the stoichiometry, dielectric breakdown field strength, dielectric integrity of the buried silicon nitride layer and the reduction in the fixed insulator charge density as well as the interface state density at the buried Si₃N₄–Si substrate interface in these SOI structure is explain on the mechanism of the enhancement by inelastic electronic scattering. Singh et al. studied the swift heavy ion induced recrystallization of top silicon layer by AFM [99].

Formation of stationary amorphous layers during high-temperature ion irradiation of semiconductors is considered in the case of 5 keV Ar⁺ ion bombardment of germanium. The dependence of their thickness on substrate temperature and ion flux is studied by Belyakov et al. [100]. It is shown that the thickness of the amorphous layer does not depend on the initial structural state of a sample. Local critical temperatures for ion beam induced crystallization and amorphization are introduced and their dependence on depth for the given conditions of ion bombardment is obtained [101], [102]. These two parameters are shown to coincide. Structural phase transformations in gallium arsenide implanted with phosphorus and argon ions of energy 140 keV. Under the action of the ion beam a transition from amorphization to restoration of the irradiated layer was found to take place in GaAs at 470–650 K. The amorphization-to-crystallization transition took place at the target temperature of 470 K. The first cycle of the phase transitions in the small fluence region occurs in the thin near-surface layer, and its mechanism is unclear. The second cycle of the phase transitions is associated with the phenomena of ion induced amorphization and crystallization and occurs in the layer approximately $R_p$ thick.

Recently, a growing activity is observed in the research of ceramic materials for inert matrices. An important requirement is the stability of the proposed ceramics against the impact of fission fragments (ff), which can induce tracks (amorphous, recrystallized) and swelling. Simultaneously, high stresses can evolve, which affect the mechanical stability. Irradiation with high fluences may induce complete amorphization. In insulators, the threshold electronic stopping power for track formation $S_{et}$ is the main parameter characterizing the amorphization. The correct $S_{et}$ values can be reliably estimated from the position of the amorphous–crystalline interface in ion irradiation experiments with high fluences. The analysis of ion induced amorphization data on Al₂O₃, MgAl₂O₄ and UO₂ ceramics confirmed the reliability of the predictions of $S_{et}$ based on thermal spike model. Prediction of the room temperature value of $S_{et}$ for ff energies was made also for Si₃N₄, CeO₂, pure ZrO₂, and ZrSiO₄ [103].