Elsevier

Thin Solid Films

Volume 519, Issue 19, 29 July 2011, Pages 6381-6387
Thin Solid Films

Modification of AlN thin films morphology and structure by temporally shaping of fs laser pulses used for deposition

https://doi.org/10.1016/j.tsf.2011.04.065Get rights and content

Abstract

We studied the effect of temporally pulse shaping upon the properties of thin layers synthesized by pulsed laser deposition with fs laser pulses generated by a Ti-sapphire laser source. We showed that the film morphology and structure can be gradually modified when applying mono-pulses of different duration or passing to a sequence of two pulses of different intensities.

Introduction

Laser pulses of tens or hundreds of fs duration have been applied for the deposition of thin films of a number of materials. Several examples include diamond-like carbon (DLC) [1], some nitrides [2], [3], [4], [5], oxides [6], carbides [7] or metals [8]. It was shown that many parameters have to be controlled in order to get thin films with the desired quality. They are, but not limited to: the laser intensity distribution, scanning speed of the laser focal spot across the target surface, energy of the pre-pulse (in case of Ti-sapphire lasers) or post-pulse (for excimer lasers), pressure and nature of the gas in the reaction chamber, and so on.

We previously observed [9], [10] that high intensity fs laser deposition produces amorphous structures with a prevalent content of particulates. This seems to be the consequence of coupling features of common fs laser pulses to solid targets. The presence of particulates can hamper the extension of the structures obtained to some key technological domains, like optics, opto-electronics, hard coatings, and electrical or magnetic applications. On the other hand, for some critical applications, such as medicine for implants [11], chemical catalysis [12] or sensing [13], [14], rough layers are preferable and the presence of particulates should be promoted and controlled.

As shown in Ref. [15], the particular type of processes induced by irradiation with ultrashort laser depends mainly on the material properties. For example, the morphology of AlN structures obtained with laser sources generating short or ultra-short pulses consisted of conglomerates of various size and shape [15], [16]. Our suggestion was that the observed conglomerates, usually called particulates, were forming by clustering of ablated material due to very intense collisions between species in the plasma.

Many attempts have been made to control the density of particulates. We thus mention the fragmentation of particulates by a delayed laser beam propagating parallel to the substrate surface. The delayed laser pulse can be obtained by turning the beam polarization with the aid of a half-wave plate and splitting the beam by a polarizing prism [17]. The delay between the two laser pulses could be varied under control by adjusting the length of one beam path. This technique proved rather efficient, but met however difficulties with a delicate alignment and/or synchronization.

To surpass this constraint, we considered the possibility of detaching from the “main” pulse a first signal with an intensity in excess of plasma ignition threshold. One then expects that by a proper tailoring of the 2 pulses, the second pulse intercepts and overheats the particulates generated by the pre-pulse causing their partial or total elimination. Ultimately, the deposition of a film can become possible with a controlled particulate density along with a significant improved crystalline status.

The pulse shaping technique has already been applied to the study of plasma plumes [18], [19], [20], [21], controlling of two-photon photoemission [22], or coherent control experiments in the UV where many organic molecules have strong absorption bands [23].

Pulsed laser deposition using femtosecond temporally shaped pulses was first applied for the synthesis of cubic SiC [24] and DLC films [25]. No detailed study was, however carried out in correlation between the pulse shapes and the morphology and composition of the synthesized structures. Double laser pulses were also shown to be advantageous in laser-induced breakdown spectroscopy [26], since they allow for the increase of both ion production and ion energy.

There are several consistent attempts in the literature for describing the interaction of ultrashort laser pulses with metallic materials [27], [28], [29], [30]. Conversely, there are only a few that deal with the interaction of ultrashort pulses with wide band-gap (dielectric, insulator and/or transparent) materials. As known, wide band-gap materials normally present a rather limited density of conduction electrons. Under intense (ultrashort) laser irradiation, the generation of a high number of conduction electrons is initiated, strongly influencing the electron interactions, and eventually determining the structural modifications of the material. Itina and Shcheblanov [31] recently proposed a model based on simplified rate equations [32], [33] instead of the Boltzmann equation to predict excitation by ultrashort laser pulses of conduction electrons in wide band-gap materials, the next evolution of the surface reflectivity and the deposition rate. The model accounts for the multiphoton ionization process, for the electron tunneling effect, the electron-impact (or avalanche) ionization, as well as for the formation of self-trapped excitons and plasma relaxation processes [34], [35], [36]. The analysis was extended from single to double and multipulse irradiation. They predicted that under optimum conditions the laser absorption can become smoother so that both excessive photothermal and photomechanical effects accompanying ultrashort laser interactions can be reduced. On the other hand, temporally asymmetric pulses were shown to significantly affect the ionization process [37], [38].

We report herewith the gradual modification of the thin film morphology and structure, with special emphasis upon the presence of particulates, when shaping the fs laser pulses. We conducted this study for the case of a wide band-gap semiconductor, AlN (Eg = 6.2 eV), because of the interest for this structure in key applications in crucial technological sectors, from acoustic wave devices on Si, optical coatings for spacecraft components, electroluminescent devices in the wavelength range from 215 nm to the blue end of the optical spectrum, as well as heat sinks in electronic packaging applications, where films with suitable surface finishing (roughness) are requested. We studied the effects of ultrashort laser pulses, but also of a double pulse with asymmetric peaks to compare with theoretical predictions.

Section snippets

Materials and methods

The laser system was a Spectra Physics Tsunami with a BM Industries amplifier generating 200 fs pulses of 400 mW at 1 kHz and 800 nm wavelength, and a CRI Spatial Light Modulator (SLM) in a 4-f configuration. The system was operated in a phase only configuration so that the energy of the laser was independent of the pulse shape. The general layout can be seen in Ref. [39]. We selected a generation regime where the pulse has the typical shape shown in Fig. 1. The pulses were temporally

Results and discussion

SEM observations (Fig. 2) revealed similar surface morphologies in the three cases. The layers had rather rough surfaces due to the presence of micronic particulates partially embedded into the deposited films.

For samples AlN-1 (Fig. 2a), three classes of surface particulates could be distinguished: particulates smaller than 100 nm, medium sized particulates up to 1 μm and large particulates, up to 2 μm. The large particulates were rather rare. The typical surfaces of the AlN-2 film (Fig. 2b) also

Conclusions

It was shown by SEM, TEM and XRD that the AlN films deposited under the action of temporally shaped or unshaped fs laser pulses consisted of a mixture of crystalline phases characterized by the prevalent presence of hexagonal AlN and existence of metallic Al traces. SEM and TEM investigations showed that when using shaped pulses the number of large crystal grains in the films was increasing. On the other hand, the average grain size decreased by about a half as an effect of shaping. When using

Acknowledgments

Experiments were carried out at the Ultraviolet Laser Facility operating at IESL-FORTH and supported by the EU through the Research Infrastructures activity of FP6 (Project: Laserlab-Europe; Contract No: RII3-CT-2003-506350). CR, GS and INM acknowledge the partial support of the UEFISCSU under the contract 547/2009.

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