Ultrathin continuous undoped diamond films: Investigation of nanoscale conduction properties

doi:10.1016/j.diamond.2005.01.043

Diamond and Related Materials

Volume 14, Issues 3–7, March–July 2005, Pages 272-278

https://doi.org/10.1016/j.diamond.2005.01.043 Get rights and content

Abstract

Using a specialized ultrasonic seeding technique to produce very high diamond nucleation density (>10¹⁰ cm⁻²), ultrathin, continuous, polycrystalline films of thickness from 70 to 100 nm were deposited onto silicon substrates. Raman spectroscopy revealed these films to be of high quality, with a clear peak at 1332 cm⁻¹ and also a peak at 1140 cm⁻¹. These ultrathin films display excellent electron emission properties. The nanoscale electrical properties of the films were investigated as a function of film thickness and temperature by conducting probe atomic force microscopy (CP-AFM) to reveal the nature of the electrically conducting pathways in the films. Images of current distribution of the films show that the conductive properties of the films are not homogeneous on the nanoscale, with regions of varying electrical conductivity. The local and averaged I–V characteristics of the films shows a higher conductivity when electrons are being injected from the tip into the diamond sample than the reverse. The 100-nm-thick continuous diamond film, in which the crystallites just touch one another, is the most conductive due to the existence of conductive pathways along the diamond grains. For thicker films, the conductive pathways appear to be on the surface; but in this case, some facets of the diamond crystallites in the films appear to be more electrically active than others. The enhanced conductivity of the 100-nm-thick diamond film can explain its enhanced electron emission properties.

Introduction

CVD diamond films are considered to be good candidates for cold-cathode electron field emitters for applications in field emission flat panel displays and different vacuum microelectronic devices [1], [2]. This is primarily attributed to the negative electron affinity of diamond surface [2] and to electron states within the band gap induced by surface states [3], defects [4], [5], [6], [7] and impurities [8]. In addition, stable intense electron emission from the diamond surface requires an effective electron transport mechanism from the bulk to the surface. Indeed, it has been reported, that B-doped diamond films with higher conductivity exhibit also a lower turn-on field for field emission [9], [10]. The undoped diamond films with higher sp² bonded carbon content [11] and smaller crystalline size [1], [12], [13] are better emitters due to the efficient electron transport. A strong correlation between highly conductive sites and highly emitting sites on the diamond surface was also observed [9], [14], [15], [16]. It has been therefore suggested that conducting pathways can be important for effective field emission, especially at low applied fields.

Using a specialized ultrasonic seeding technique [17], [18], [19], it is possible to achieve high nucleation density of ∼5×10¹⁰ cm⁻² on Si substrates and to obtain an ultrathin, continuous, polycrystalline diamond films of thickness from 70 to 100 nm. Such sub-micrometer-thick diamond films display excellent electron emission properties. The threshold field values of ≤10 V/μm (for emission current of 10 μA/cm²) were measured for diamond films of thickness up to 300 nm. Also, an enhanced quantum photo-yield (QPY) of 14–15% (at photon energy of 140 nm) was measured for film thickness of ∼100 nm [20], as compared to the thicker films, for which values of 11–12% were obtained.

In order to understand the electron emission and electrical properties of these ultrathin diamond films, it is desirable to reveal the nature, at the nanometer scale, of the electrically conducting pathways in the films. In this study, systematic measurements of the nanoscale conductivity properties were carried out for different film thicknesses by conducting probe atomic force microscopy (CP-AFM). This technique provides information on the distribution of local conductivity and is very powerful for studying electrical transport in nanoparticles assembles and can be applied to highly resistive samples [21]. CP-AFM utilizes a conducting tip in the contact mode to measure simultaneously the surface topography and spatial distribution of local I–V characteristics. The local conductivity is measured independently of the topography. The temperature dependence of the nanoscale conductivity was also evaluated. The surface morphology of the as-deposited diamond films was examined by high-resolution scanning electron microscopy (HR-SEM) and atomic force microscopy (AFM). The different carbon phase content was studied by micro-Raman spectroscopy.

Section snippets

Experimental details

Diamond polycrystalline films of different thicknesses from 100 nm up to 2050 nm were deposited onto p-type Si(100) substrates by a standard hot-filament chemical vapor deposition (HF-CVD) system using a gas mixture of 1% CH₄ and 99% H₂ at total flow of 100 sccm and system pressure of 50 Torr. The gas is activated by two rhenium filaments heated to 2000 °C positioned 8 mm above the substrate. The temperature of the substrate was maintained at 800 °C during the deposition. Prior to deposition,

Results and discussion

Fig. 1 shows that the expected linear I–V curve for tip-graphite contact, recorded with bias, varied between −50 and +50 mV. A mirror current image of diamond film in Fig. 2 shows that the tip is stable and the four images, scanned in four directions, are consistent.

Fig. 3 shows typical HR-SEM plane (a) and cross section (b) view of 100-nm- and 2050-nm-thick diamond films deposited onto silicon substrates pretreated with mixed diamond +Al₂O₃ slurry. The thickness of diamond films was measured

Summary

The nanometer-scale electronic properties of the diamond films with ultra-high nucleation density were investigated as a function of film thickness and temperature by CP-AFM technique at elevated temperature. The experimental observations can be summarized as follows: (1) the I–V characteristics show that electrons are being injected from the tip into the diamond sample; (2) the 100-nm-thick continuous diamond film, in which the crystallites just touch one another, is the most conductive due to

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Ultrathin continuous undoped diamond films: Investigation of nanoscale conduction properties

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Summary

Solid State Electron.

Diamond Relat. Mater.

Diamond Relat. Mater.

Diamond Relat. Mater.

Diamond Relat. Mater.

J. Non-Cryst. Solids

Diamond Relat. Mater.

Diamond Relat. Mater.

Science

J. Vac. Sci. Technol., B

J. Vac. Sci. Technol.

J. Appl. Phys.

Appl. Phys. Lett.