V.C. George

Bias enhanced deposition of highly oriented β-SiC thin films using low pressure hot filament chemical vapour deposition technique

Abstract Highly oriented cubic silicon carbide (β-SiC) thin films are deposited on Si(111) substrates using bias assisted low-pressure hot filament chemical vapour deposition technique. Methane (CH 4 ) is used as the source for carbon, while the substrate itself acts as the source for silicon. The technique is quite simple, cheap, has one step, and requires no stringent reaction conditions; the substrate temperature used being in the range of ∼750 °C and the chamber pressure ∼1 torr. The films have been characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy. Bombardment of negatively biased substrate by high-energy positive ions under relatively low chamber pressure is believed to facilitate the growth of highly oriented SiC films.

Detection of nanophase at the surface of HFCVD grown diamond films using surface enhanced Raman spectroscopic technique

Abstract Diamond films prepared by hot filament chemical vapour deposition technique were characterised using X-ray diffraction, scanning electron microscopy and surface enhanced Raman spectroscopic technique. Silver was deposited on to diamond thin film under ultra high vacuum conditions and macro-Raman spectra were recorded during the deposition. An asymmetric broadening of the Raman peak at 1332 cm−1 and gradual emergence of a new peak at 1240 cm−1 with increasing thickness of the silver layer were observed. These observations were explained on the basis of phonon confinement in nanometer-sized crystals. It is proposed that the detected nanophase is present only at the surface and is not a bulk property. The average particle size of the film was estimated from X-ray analysis and also from the symmetric broadening of the 1332 cm−1 line in the Raman spectra of the bare sample. The presence of the nanophase was further evidenced by high resolution scanning electron microscopy. The probable mechanism of formation was briefly explored.

Large area diamond deposition in HFCVD technique employing convective flow of gases

Abstract Hot filament chemical vapour deposition technique is widely used for the synthesis of diamond at sub-atmospheric pressure. The normal procedure involves diffusional flow of the diamond precursor species and the atomic hydrogen, produced by the input gases at the heated filament. The active species are short lived and can therefore travel relatively small distances, in their lifetime, in an intrinsically slow diffusion controlled process. This restricts the distance between the filament and the substrate to ≲ 1 cm. In this communication, it is brought out that this shortcoming can be overcome, to a large extent, by using convective flow of gases. With minor modifications in the standard experimental set up, consisting of a single filament-jet assembly, it has been possible to deposit good quality diamond on zirconium substrate, placed at a distance ≈ 5 cms from the filament and over an area ≈ 7 cm2. Besides deposition on larger area, this concept minimises the problems associated with the close proximity of an intense localized heat source to the substrate.

Growth of high quality silicon carbide films by bias enhanced low-pressure HFCVD using methane

Highly oriented, β-SiC films have been grown on Si substrates by a bias enhanced low-pressure hot filament chemical vapour deposition (LP-HFCVD) technique. The films have been characterised using X-ray diffraction (XRD), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The application of bias to the substrate is found to induce crystallinity, increase particle size as well as the roughness of the deposit on the Si surfaces. The orientation of the film is found to depend on the orientation of the substrate.

In-depth analysis of impurities in HFCVD diamond thin films grown from acetone/hydrogen mixtures

Abstract Diamond films have been deposited on silicon single crystals from CH 3 COCH 3 –H 2 mixtures by hot filament chemical vapor deposition (HFCVD) method and are subsequently analyzed for the presence of impurities using a number of analytical techniques. The bulk concentration for O 2 is found to be ∼1 at.% and that for silicon and hydrogen is ∼0.07 at.% and ∼3 at.%, respectively. Nitrogen level remains below the detection limit for both Secondary ion mass spectrometry (SIMS) and Ion beam analysis (IBA) techniques and could only be detected from micro-photoluminescence studies. Concentration levels of these impurities as a function of depth within the film have been investigated. Their possible configurations and the different modes of their incorporation into the films have been explored in the light of diamond deposition mechanism.

Formation of self-supporting hollow diamond helix and diamond sieve using “jet-flow” HFCVD

Abstract This paper describes the methodology for the preparation of self- supporting hollow diamond helix and diamond sieve in addition to diamond microtubes. The key idea is to deposit thick, compact and sufficiently strong coating on a suitable mandrel and then dissolve the latter so as to form the required shape. For this purpose we have made use of ‘jet-flow’ HFCVD which allows diamond deposition over larger area on three dimensional substrates both inside as well as outside. X-ray diffraction, laser macro-Raman spectroscopy and scanning electron microscopy have been used to characterise shape, morphology and the phase purity of the material.

The origin of charge transients in Al/undoped diamond/p-Si diodes

An experimental study of relaxation behaviour of Alyundoped diamondyp-Si diodes was conducted using charge deep-level transient spectroscopy (QDLTS) and feedback charge capacitance method (FCM) in the time domain. Prior to vacuum deposition of Al onto 5-mm-thick diamond films, the latter were exposed to a hydrogen discharge at 150 8C for 30 min. When measuring capacitance at delays of a few microseconds with respect to the trailing edge of the probing pulse DU at ambient temperatures, only the instantaneous capacitance C(0) corresponding to the thickness of the diamond film was detected. After shifting the excitation and processing of the charge transients to the region of tens of milliseconds, excess capacitance (charge) is observed, its value depending on temperature. Both isothermal and thermal-scan QDLTS measurements revealed a broad spectrum of relaxation times indicating a Debye-type dielectric relaxation, i.e. the amplitude of the QDLTS peak is a linear function of DU. The thermal activation energy DEf0.13 eV of the spectral components corresponding to the maximum of the signal matches perfectly the activation energy of dc conductivity at equivalent temperatures. The origin of the dielectric relaxation is explained in terms of a Maxwell–Wagner polarisation due to a conducting phase embedded in the semi-insulating matrix of the diamond films, the width of the spectrum of relaxation times reflecting the spread of the sizes of microcrystallites. The results may also be useful for finding the demarcation temperature of detecting response from deep traps located within the depletion region at the Alydiamond interface. 2002 Elsevier Science B.V. All rights reserved.

Hydrogen flow rate dependence of diamond crystallite size, density and quality in “jet flow” HFCVD employing CH4H2O2 gaseous mixture

Abstract The effect of variation of hydrogen flow rate (300–2000 scc min−1) on the crystallite size, crystallite density and the quality of diamond deposited on tungsten and silicon substrates has been investigated in “jet-flow” HFCVD technique. In these studies, fixed flow rates of methane (6 scc min−1) and oxygen (0 or 2 scc min−1) have been employed. Scanning electron microscopy and laser Raman spectroscopy have been utilised for the characterisation of the deposit. Intermediate hydrogen flow rates are found to give maximum crystallite size, crystallite density and better quality of diamond deposit. The presence of oxygen does not affect the trend but it increases the deposition rate and improves the quality of the deposit. It has been brought out that atomic hydrogen besides etching also plays a role in the formation of diamond nuclei and their growth.