R.V. Kruzelecky

Dependence of optical gap in a-Si:H on bonded hydrogen concentration

Abstract An analysis of new and existing data over a wide range of hydrogen concentrations, from 0.3 to 50 at%, shows that the optical gap of hydrogenated amorphous silicon prepared by various techniques varies linearly with the bonded hydrogen density per unit length. The extrapolated gap for zero hydrogen concentration, (1.23±0.05) eV, is close to the minimum (indirect) gap in crystalline silicon, as would be expected if the electron energy levels in amorphous silicon are determined primarily by the near-neighbour environment.

Interband optical absorption in amorphous silicon

Abstract The interband optical absorption characteristics of amorphous silicon films prepared by various techniques have been investigated. Above the main absorption edge, the absorption coefficient α can be fitted to the “Tauc” model ( α h ω) 1 2 = C 0 1 2 ( h ω − E G ) . The experimental ( α h ω) 1 2 versus ħω plots are generally piecewise linear, with an increase in slope above an energy u. Structure in the “Tauc” plots is correlated with preparation conditions; the experimental results are consistent with a broadening of the density of states distribution at the band edges within a gap defined by Eu. The incorporation of bonded hydrogen into the a-Si network results in compositional disorder and deeper potential fluctuations that widen Eu. The bonded hydrogen increases both extrapolated energy gaps; the blue shift is proportional to the line density of hydrogen atoms over a wide range of bonded hydrogen concentrations. A unified model for the interband absorption edge in a-Si and a-Si : H is presented.

Luminescence in hydrogenated amorphous carbon films grown by dc saddle‐field glow‐discharge decomposition of methane

Photoluminescence in hydrogenated amorphous carbon thin films deposited using the dc saddle‐field glow‐discharge technique onto glass and single‐crystal silicon substrates was studied. Samples prepared using positive substrate bias exhibited strong broad‐band photoluminescence at room temperature. The luminescence spectrum had a major peak at 1.9 eV and two smaller peaks at 2.3 and 2.6 eV. Samples prepared using grounded or negatively biased substrates exhibited only weak photoluminescence near 2.6 eV. Infrared spectroscopy indicates that the luminescence at 1.9 and 2.3 eV is related to the presence of C–OH bonds, whereas the photoluminescence at 2.6 eV appears to be an intrinsic property of the a‐C:H films.

The preparation of hydrogenated amorphous silicon by plasma-enhanced reactive evaporation

Abstract A novel technique, plasma-enhanced reactive evaporation (PERE), has been developed for the preparation of a-Si:H. Silicon is evaporated from a molten source and deposited onto a substrate in the presence of a dc glow discharge in hydrogen or silane at pressures from about 0.01 to 0.1 Torr. The PERE method facilitates relatively high deposition rates, direct control of bonded hydrogen incorporation and uniform hydrogenation of large substrate areas. The dark electrical transport characteristics of PERE a-Si:H films are comparable to those of discharge-deposited films containing considerably more bonded hydrogen. Furthermore, the results suggest that the presence of SiHn radicals is required during film growth to obtain highly photoconductive films.

In situ work function measurements in evaporated amorphous silicon

Abstract In situ contact potential difference measurements have been used to determine the work function of evaporated amorphous silicon (a-Si). The work function of unhydrogenated a-Si is found to be (4.66±0.02) eV. It is increased by 0.1 eV in hydrogenated amorphous silicon (a-Si:H). The work function is insensitive to variations in substrate temperature between 60 and 250° C. The results are correlated with optical absorption and conductivity data. Limitations of the contact potential difference methods of monitoring changes in the bulk Fermi level are discussed.

The effect of preparation conditions on the morphology of low-temperature silicon films

Abstract The structure of UHV-evaporated a-Si and reactively-evaporated a-Si:H films, as observed by TEM, is correlated with preparation conditions. The results suggest that a-Si growth by various vapour-deposition techniques can be interpreted in terms of the effect of the deposition parameters on nucleation and coalescence. UHV-evaporated films contain a large volume fraction of surface-like tissue. Higher substrate temperatures enhance agglomeration, resulting in a two-phase structure consisting of island clusters interconnected by a porous amorphous tissue. “Activated” hydrogen increases the nucleation density, resulting in a more homogeneous fine-grained structure, while silicon hydrides further enhance coalescence between the island structures. The application of a positive substrate bias during film growth promotes microcrystallite formation in both a-Si and a-Si:H films.

The effects of the glass substrate on the properties of rf glow discharge amorphous Si:H thin films

The quality of a rf glow discharge deposited a‐Si:H thin film is affected by the substrate material. As a result of ion bombardment, some of the reactive species from the glass substrate are introduced into the deposition chamber by desorption. These species are then adsorbed back into the surface with the decomposed gas and form impurities in the film. The presence of Na‐Si and K‐Si impurities in the a‐Si:H film lowers the electrical conductivity and decreases the film stability.

Doping of a-Si:H using BF3

Abstract P-type dc-glow-discharge a-Si films have been prepared using boron trifluoride as the doping agent. Film growth on various cathodic substrates is examined. The effect of BF 3 -doping on hydrogen incorporation into the a-Si network is investigated using the 15 N profiling technique and FIR measurements of Si-H vibrational modes. The concentration of boron and fluorine that is incorporated into BF 3 -doped films is profiled using SIMS. Data on the dark conductivity of BF 3 -doped films is presented.

Morphology and electronic properties of phosphorus-doped hydrogenated amorphous and microcrystalline silicon deposited by DC discharge in SiH4/PH3

Abstract n-Type Si: H films have been prepared by DC discharge decomposition of SiH 4 /PH 3 gas mixtures to investigate the effects of the discharge current density ( J DC ) on film structure and dark electrical transport characteristics. Examination of the resulting Si : H films by transmission electron microscopy (TEM) indicates a discernable microstructure that depends on the discharge parameters. Film growth can be interpreted in terms of the effect of the deposition parameters on nucleation and surface diffusion. Increasing V DC enhances the nucleation density, resulting in a homogeneous, fine-grained structure that is largely amorphous. In contrast, increasing J DC enhances surface mass transport, resulting in a two-phase structure consisting of crystallites, as large as 1000 A in diameter, interconnected by an amorphous tissue. For a given impurity level, the resulting electrical transport characteristics are correlated with microstructure; higher dark conductivities are associated with deposition conditions that enhance microcrystallization.

Oxygen traps in evaporated hydrogenated amorphous silicon

Abstract The release of carriers from traps in evaporated hydrogenated amorphous silicon has been studied by photocurrent transient measurements. The traps are found to have field-enhanced escape probabilities. The capture cross section at room temperature is estimated to be of the order of 10 −22 cm 2 . A model involving oxygen atoms is proposed for the traps.