J. M. Perz

Disorder and optical absorption in amorphous silicon and amorphous germanium

Abstract The role that disorder plays in shaping the functional form of the optical absorption spectra of both amorphous silicon and amorphous germanium is investigated. Disorder leads to a redistribution of states, which both reduces the empirical optical energy gap and broadens the optical absorption tail. The relationship between the optical gap and the breadth of the absorption tail observed in amorphous semiconductors is thus explained.

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.

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.

HYDROGEN-INDUCED QUANTUM CONFINEMENT IN AMORPHOUS SILICON

We study how hydrogen‐induced quantum confinement in hydrogenated amorphous silicon influences the distribution of tail states. To do this, the potential structure of this semiconductor is treated as being comprised of an ensemble of potential wells, these wells corresponding to unhydrogenated regions enveloped by hydrogenated regions. To evaluate the distribution of states, we determine the ground state associated with each well, and then average over the distribution of wells. We find that our calculated distribution of tail states exhibits an essentially exponential functional dependence, over several decades, and that this tail of states shifts toward the band edge as the hydrogen content is increased. This shift toward the band edge is suggested to be one of the factors responsible for the observed increase in energy gap with higher hydrogen content.

Photoluminescence study of radiative recombination in porous silicon

Photoluminescence in porous Si films has been studied in the temperature range from 15 to 250 K. The luminescence peak is found to shift to higher frequency with increasing temperature. Above 100 K the luminescence intensity shows strong thermal quenching with an activation energy of 60 meV. Below 100 K photoluminescence decay data obtained using quadrature frequency resolved spectroscopy are characterized by a single lifetime of about 300 μs. At 250 K several time constants are seen to contribute to the luminescence decay. We attribute the very intense low‐temperature photoluminescence to recombination at localized extrinsic centers.

An effective‐mass model of hydrogenated amorphous silicon: A tail state analysis

An effective‐mass model of hydrogenated amorphous silicon (a‐Si:H) is proposed. This model focuses upon the impact of hydrogen on the electronic structure of the tail states. Tail state density‐of‐states calculations are performed in the high hydrogen concentration limit, and the results are found to be in general agreement with present experimental data. These results are shown to be quite robust to variations in the modeling parameters.

Atmospheric aging and thermal annealing effects in a-C:H thin films

Abstract Atmospheric aging and thermal annealing effects have been studied in hydrogenated amorphous carbon (a-C:H) thin films deposited using a dc saddle field glow discharge technique with different ion energies (85–225 eV) during deposition. The a-C:H films grown with low ion energies showed aging effects when they were exposed to the ambient atmosphere. Infrared (IR) absorption due to O–H and CO vibration modes increased while the IR absorption due to C–H vibrations decreased with aging. The absence of absorption due to O–D vibrations in deuterated films indicates that the films reacted with water rather than oxygen when exposed to the atmosphere. The C–H and O–H bond concentrations decreased when the a-C:H films which had been exposed to the atmosphere for a one month period were thermally annealed. The a-C:H films grown with high ion energies exhibited neither atmospheric aging nor thermal annealing effects.

The dependence of the dark conductivity of hydrogenated amorphous silicon films on the hydride content

Abstract Dark conductivity and photoconductivity measurements on hydrogenated amorphous silicon (a-Si:H) films, prepared by saddle-field glow discharge and plasma-enhanced reactive evaporation, are correlated with the hydrogen content of these films. In particular, the dependence of the dark conductivity on the monohydride, polyhydride, and total bonded hydrogen content is studied. In addition, the dependence of the monohydride and polyhydride contents on the total bonded hydrogen content is investigated. The results are consistent with the hypothesis that it is the total amount of bonded hydrogen that determines the conductive properties of these films, rather than the proportion of polyhydride to monohydride sites. The monohydride and polyhydride contents of the films are compared with the predictions of a statistical model which appears to establish theoretical limits on the functional dependencies of these contents with respect to the total bonded hydrogen content.

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.