A. R. Middya

Improvement of microstructure of amorphous silicon–germanium alloys by hydrogen dilution

The microstructures of two sets of hydrogenated amorphous silicon–germanium (a‐Si1−xGex:H) alloys prepared by the plasma‐enhanced, chemical‐vapor‐deposition technique with and without hydrogen dilution of the source gases (silane and germane) have been analyzed by small‐angle x‐ray scattering (SAXS), infrared vibrational spectroscopy, and flotation density measurements. Optoelectronic properties of codeposited films have also been characterized. Hydrogen dilution suppresses dihydride/polyhydride formation, reduces bonded H content, and reduces the SAXS‐detected microstructure for x≳0. Studies of anisotropy in the SAXS intensity indicate an increased amount of oriented microstructure as Ge is added, consistent with a trend toward columnarlike growth in both undiluted and hydrogen‐diluted films, but the diluted films have a significantly reduced degree of such oriented microstructure. The improvement in the microstructure of a‐Si1−xGex:H films by H2 dilution correlates with concomitant improvement of optoel...

Nanostructures and defects in silicon–hydrogen alloys prepared by argon dilution

Abstract Nanostructural heterogeneity of silicon–hydrogen (Si:H) alloy films deposited in a conventional radio frequency plasma-enhanced chemical vapour deposition unit from silane argon mixture has been studied by small-angle X-ray scattering (SAXS). The densities of defect states of Si:H films have been estimated by dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS) and the modulated photocurrent (MPC) method. From the structural and defect studies we identify two regions of Ar dilution, where the structure of the films are distinctly different. Up to 90% Ar dilution, the nanostructural as well as the large-scale (>30 nm) structural heterogeneities in the amorphous Si:H (a-Si:H) network decrease. A lowering of the bulk defect density has also been observed in this Ar dilution region. Increasing Ar dilution to greater than 90% of the mixture, the a-Si:H films show some dense regions embedded in the amorphous matrix. The high- and low-density amorphous structures within the films can explain the experimental results obtained from SAXS, DBP and PDS. A negligible conduction band tail, as observed from MPC result, suggests the formation of high degree of crystallinity in the film deposited with 99% Ar dilution and higher rf power density (80 mW/cm 2 ).

Growth of device quality amorphous SiGe:H alloys with high deposition rate under helium dilution

Highly photoconductive amorphous silicon‐germanium alloy (a‐SiGe:H) films have been developed by plasma‐enhanced chemical‐vapor deposition using helium dilution (HeD) of the process gases (silane and germane). On comparison with high‐quality a‐SiGe:H alloys prepared under hydrogen dilution it has been observed that HeD films have higher deposition rates as well as higher mobility lifetime products ημτ throughout the alloy range; however, midgap defect densities and Urbach energy values of the two types of materials are nearly the same. Improvement in ημτ values of HeD films are found to be consistent with the reduction of microstructural defects in the films. Device potential of the helium‐diluted a‐SiGe:H films has also been investigated.

Improvement in the properties of a-SiGe:H films : roles of deposition rate and hydrogen dilution

Device quality a‐SiGe:H thin films have been deposited by radio‐frequency plasma‐assisted decomposition of silane and germane diluted with and without hydrogen. Improvement in structural and electronic properties have been achieved employing low deposition rate and high hydrogen dilution. It has been observed that low deposition rate can reduce the preferential attachment of H to silicon throughout the alloy region while the beneficial effect of hydrogen dilution is more effective in a low band gap region (Eg≤1.40 eV). The photoconductivities of the good quality a‐SiGe:H alloy films under white light illumination (50 mW/cm2) are 1.34×10−4 S cm−1 and 1.9×10−5 S cm−1 at the optical gaps of 1.51 and 1.35 eV, respectively. The changes in midgap defect density and tail states have been correlated with the photoconductivities of the samples deposited under different conditions. An attempt has been made to explain the results from the growth kinetics of the films.

Highly photosensitive helium diluted amorphous silicon 1.5 eV band gap: Role of pressure

Highly photosensitive low band gap (≥1.5 eV) hydrogenated amorphous silicon (a‐Si:H) films have been developed by plasma enhanced chemical vapor deposition using helium dilution of the process gas, silane at substrate temperature 210 °C. Low band gap of a‐Si:H films is achieved by reducing bonded hydrogen content and hence by increasing compactness of the films. At the optical gap ∼1.5 eV, a‐Si:H film has high mobility lifetime product, ημτ (8×10−5 cm−2 V−1) and photosensitivity (≳7×104) values. The defect density of the material is as low as 7.8×1015 cm−3 eV−1 and hydrogen content is 4.54 at. %. Low band gap amorphous silicon germanium (a‐SiGe:H) films reported so far do not have such high ημτ and photosensitivity values at 1.5 eV optical gap. Thus, this low band gap a‐Si:H films can be used as intrinsic layer in the bottom cell of a multijunction solar cell replacing a‐SiGe:H alloy films.

Diamond-Like Carbon Films Prepared by Photochemical Vapour Deposition

Diamond-like carbon films have been successfully grown at comparatively low substrate temperature (250°C) on single-crystal silicon substrates by the photochemical vapour deposition technique. Raman spectroscopy and scanning electron microscopy studies have shown the formation of diamond particles embedded in the diamond-like carbon film. However, some amount of graphitic carbon is present in the material. The presence of sp3 as well as sp2 hybridised carbon atoms is also confirmed by infrared vibrational spectroscopy. Extremely high transparency (>90%) for 500 A film from the near-infrared to visible regions is an indication of the high optical gap of the film.

The effect of variation in hydrogen dilution and RF power density on the properties of a-SiGe:H and related solar cells

The effect of systematic variation in hydrogen dilution and RF power density on the different properties (such as optoelectronic and structural properties and defect density) of hydrogenated amorphous silicon germanium alloy (a-SiGe:H) films, prepared at ultra-high vacuum () in a plasma-enhanced chemical vapour deposition multichamber system with a load - lock facility has been studied. Proper structural relaxation of the alloy matrix is controlled by the growth zone reactions. These grown zone reactions depend on the type and number density of reactive radicals and on their energy and momentum. All these factors, hence growth zone reactions, may be changed by variation of the interlinked parameters hydrogen dilution and RF power density. Another noteworthy point is the investigation of the dependence of the improvement in characteristics of single junction solar cells having a-SiGe:H intrinsic layers on that of material quality. Improvement of material quality may or may not be reflected in cell performances.

Influence of deposition parameters on defect formation in a-SiGe:H alloys

Abstract Highly photoconducting (ημτ) films of a-SiGe: H with very low defect densities have been produced by a plasma-enhanced chemical vapour deposition technique under low flow rate (2·5 sccm) of source gases (silane and germane) and hydrogen dilution. The defect states have been characterized by photothermal deflection spectroscopy, the constant photocurrent method dual-beam photoconductivity and electron spin resonance. A low deposition rate is necessary to obtain high-quality alloy materials with comparatively high band gaps, i.e., 1·7eV>E g> 1·5 eV, as manifested by a small disorder (lower Urbach parameter E 0) and defect density, N D (mainly reduced Ge dangling-bond density), whereas additional hydrogen dilution of the source gases is necessary to reduce N D in the low band-gap region. Hydrogen dilution improves homogeneity throughout the band-gap region with a corresponding decrease of E 0 values which has no correlation with the reduction of N D. Studies of the temperature dependence of the phot...

INFLUENCE OF DEPOSITION RATE AND HYDROGEN/HELIUM DILUTION ON THE STRUCTURAL RELAXATION OF A-SIGE:H NETWORK PREPARED AT LOW SUBSTRATE TEMPERATURE

Device quality a‐SiGe:H alloy has been developed at low substrate temperature (Ts∼180 °C), suitable for solar‐cell fabrication using low flow rate of silane and germane and strong hydrogen dilution. The optoelectronic properties of a‐SiGe:H films prepared with different substrate temperature and under different deposition conditions, i.e., low flow rate of source gases and with/without hydrogen dilution, high flow rate and with hydrogen dilution, low flow rate and helium dilution, are compared. It is found that both low deposition rate and strong hydrogen/helium dilution are essential to develop a‐SiGe:H material with low defect density. Structural properties, midgap defect densities, and Urbach energies of the optimized and unoptimized materials are correlated with their optoelectronic properties. The results very well support the idea that there should be a fine balance between deposition rate and substrate temperature in developing low defect density materials. Quantum efficiencies of three Schottky ba...

Low defect density amorphous silicon germanium alloy (1.5 eV) deposited at high growth rate under helium dilution in RF-PECVD method

Abstract Optimized deposition parameters for helium diluted a-SiGe: H material differ from those for hydrogen diluted a-SiGe: H material with the same optical gap (∼ 1.5 eV). Deposition rate of optimized helium diluted film is 3.3 times greater than that of hydrogen diluted film, however the photoconductivities of both type of materials are the same. Defect density (6.72 ± 1 × 10 15 cm −3 eV −1 ) and Urbach energy (46 ± 1.5 meV) of helium diluted optimized film is less than corresponding values (8.45 ± 1 × 10 15 cm −3 eV −1 , 49 ± 1.5 meV) of hydrogen diluted alloy materials. Schottky barrier solar cells have been fabricated using both optimized materials. Blue response is greater for the optimized helium diluted a-SiGe:H material than that of optimized hydrogen diluted alloy film. Red response is also greater in the first case in comparison to that in the second case.