A.K. Dua

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.

Structural aspects of PbO–P2O5 glasses containing ThO2

Abstract PbO–P 2 O 5 glasses in which part of PbO/P 2 O 5 was replaced by ThO 2 have been prepared by the conventional melt quench method and their structural aspects have been studied by 31 P MAS NMR, Raman and FTIR techniques. Binary PbO–P 2 O 5 glass is characterized by mainly Q 2 and Q 1 structural units of P (Q n represents the phosphorus structural units having ‘ n ’ number of bridging oxygen atoms). When part of the PbO has been replaced by ThO 2 , the phosphorus structural units are not significantly affected, indicating that Th 4+ replaces Pb 2+ at the interstitial positions in the glass network. Unlike this when part of P 2 O 5 has been replaced by ThO 2 extensive depolymerisation of the phosphate network resulted in the conversion of Q 2 to Q 1 structural units of P. Above 10 mol% of ThO 2 replacement in either case resulted in the partial devitrification of the glass.

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.

Thermal conductivity of U3O8 from 300 to 1100 K

The thermal conductivity of orthorhombic α-U3O8 has been measured in air from 300 to 1100 K using an axial heat flow comparative set-up. The results show that the conductivity decreases monotonically with increasing temperature. The observed conductivity can be explained in terms of the phonon-defects and phonon–phonon interaction processes. It is also shown that the intrinsic thermal resistivity can be quantitatively explained by the modified Leibfried–Schlomann (LS) equation proposed by G.A. Slack (in: H. Ehrenreich, F. Seitz, D. Turnbull (Eds.), Solid State Physics, vol. 34, Academic Press, New York, 1979, p. 1).

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.

Charge deep-level transient spectroscopy of Al/intrinsic diamond/p + -Si Schottky diodes

Charge deep-level transient spectroscopy (Q-DLTS) and the feedback charge capacitance method (FCM) were applied to Al/intrinsic diamond/p-Si diodes, using the top Al electrode as a Schottky gate. The Q-DLTS data recorded over the temperature range of 90-450 K could be split into two components: (i) a peak of the signal due to a discrete energy level, the hole emission of which is thermally activated by �E = 0.29 ± 0.02 eV; (ii) a broad spectrum of relaxation times manifesting itself as a signal that increases continuously on heating the diodes toward the ultimate temperature. To exclude any effects caused by the silicon back contact (Al), Q-DLTS spectra taken at different polarities of the filling pulse were compared. The related FCM capacitance-voltage measurements at the uppermost temperature revealed no changes in the capacitance with bias if sampling the time domain capacitance in the microsecond region. Over a longer timescale an excess capacitance was detected as expected on the basis of the complementary Q-DLTS data. Taking into account the absence of any capacitance changes at lower temperatures and/or short observation times (excitation), it is concluded that the defects reside in the polycrystalline diamond layer rather than at the diamond/silicon interface.

Improving the performance of the feedback charge capacitance–voltage method

The feedback charge capacitance–voltage method (FCM) (Mego 1986 Rev. Sci. Instrum. 58 2798) is a pure time-domain technique originally used for measuring the quasi-static capacitance of semiconductor devices. The method is based on processing the output of a charge-to-voltage converter in response to a double-step (pulse) excitation of the device. Using a transient voltage processor comprising three gated integrators connected to a mixing unit, steady-state (leakage) current may be the source of severe experimental error in capacitance. First, there is a parasitic charge during the pulse that causes an error in sampling the baseline while activating the first channel for an aperture of finite duration Δt. Second, any uncompensated leakage present after the pulse leads to a charge increment representing what is called conduction loss in the frequency domain. Measures to be taken towards minimizing both errors are provided, based on a simultaneous action of active leakage current compensation and second-order filtering. A simple hardware solution for optimizing the dynamic range of the FCM under the leakage is provided. A non-instrumental FCM error connected with the constant dielectric loss in diamond thin films, due to the anomalous kinetics j(t) ∝ t−1 of the transient current, is analysed. The latter causes inaccuracy in assessing the instantaneous (geometrical) capacitance of diamond-based Schottky diodes. A proper gating of the charge-to-voltage converter is suggested for removal of the predicted error.

Diamond nucleation and growth on zeolites

Abstract In this work, we report the use of zeolites as substrates for the deposition of porous diamond films. Films were deposited in a hot-filament chemical vapor deposition (HFCVD) apparatus. The HFCVD system was fed with a mixture of methane (0.8%) with the balance being hydrogen. A series of depositions were done in the pressure range 20–120 Torr and at substrate temperature 880 °C. The morphologies of the as-deposited films were analyzed by scanning electron microscopy and show isolated diamond grains in the initial nucleation stages, which develop into a microporous film in the next stage and form a continuous film after long time deposition. Raman spectroscopy was used to investigate the crystal morphology, structure and non-diamond impurities in the films deposited at various growth conditions. The nature of the hydrogen bonding with sp 3 and sp 2 network and the quantitative analysis were done by Fourier transform infrared spectroscopy.