Preliminary results on infrared and luminescence measurements of tritium incorporated amorphous silicon are reported. Tritium is an unstable isotope that readily substitutes hydrogen in the amorphous silicon network. Due to its greater mass, bonded tritium is found to introduce new stretching modes in the infrared spectrum. Inelastic collisions between the beta particles, produced as a result of tritium decay, and the amorphous silicon network, results in the generation of excess electron-hole pairs. Radiative recombination of these carriers is observed.

Further details are given of a unified model of the hydrogen density of states and defect pool of a-Si:H. The model is compared to other defect models and extended to describe a-Si alloys and the creation of valence band tail states during growth.

We report on extensive molecular dynamics (MD) simulations on a-Si:H for up to 5 Ps using the ab initio code of Sankey and Drabold. The supercells contain about 70 atoms and only one defect in order to minimize defect-defect interaction. Simulations on supercell samples that originally contain one bond centered (BC) H in an otherwise defect free sample exhibit BC to BC diffusion as in c-Si. However, we also observe localized motion of defects and H atoms on a very fast time scale that probably has been observed in several experiments.

Hydrogen transport in polycrystalline silicon was investigated by deuterium diffusion experiments. D was introduced either from a remote plasma or a solid-state source. The data can be explained by a two-level model used to explain diffusion in amorphous silicon. The energy difference between transport level and deuterium chemical potential was found to be 1.3 eV. A band of shallow levels for hydrogen trapping is located about 0.6 eV below the transport level, while deep levels are about 1.7 eV below.

A new model to explain stretched exponential relaxation in hydrogenated amorphous silicon is presented. The model does not invoke statistical distributions; rather, it is based on a careful treatment of diffusion, including retrapping. Excellent fits to a variety of experimental data are obtained.

To understand the electrical properties and behavior of amorphous silicon, it is imperative to understand the behavior of hydrogen in the amorphous silicon lattice. Although considerable effort has been extended to study hydrogen trapping in a-Si:H, the energetics and populations of the traps remain unresolved. We have developed a reaction diffusion model which includes multiple bulk trapping mechanisms, bulk transport, and realistic surface processes. The model results are compared with existing hydrogen temperature programmed evolution data, isothermal hydrogen evolution experiments, and thermal quenching experiments. We find consistently good fits for all three types of experiments with a shallow trap depth of 1.5 eV and deep trap depth of 1.8–1.9 eV below the mobility level, with 20–30% of hydrogen present in the deep trap. The difference of 0.3–0.4 eV between the two trap depths is similar to the defect formation activation energy and suggests a defect formation mechanism which involves movement of hydrogen from a deep to a shallow trap.

The role that disorder plays in shaping the form of the optical absorption spectrum of hydrogenated amorphous silicon is investigated. Disorder leads to a redistribution of states, which both reduces the ‘Tauc’ gap and broadens the absorption tail. The observed relationship between the ‘Tauc’ gap and the breadth of the absorption tail is thus explained.

Two major problems in the field of a-Si:H and related alloy films such as a-Sil-xNx:H are addressed in this contribution, namely, the photocreation of neutral Si dangling bonds and the origin of the components of the light-induced ESR (LESR). We have proposed a new model for the photocreation of neutral Si dangling bonds in a-Si:H based on the presence of floating bonds. The model can explain the metastabilization of the broken weak bonds without the movement of H atoms. The broad, the narrow, and the dangling bond components of the LESR signal have been attributed to holes captured at negatively charged floating bonds, electrons trapped at antibonding states of weak bonds and photoexcited carriers captured at charged dangling bonds, respectively.

The photoconductivities ap of glow discharge deposited a-Ge:H and amorphous Si-Ge alloys prepared in different laboratories were measured between 4.2K and 300K and compared with σp(T) of intrinsic and compensated a-Si:H films. The alloys as well as a-Ge:H do not exhibit thermal quenching of σp(T) at elevated temperatures which suggests that the valence and conduction band tails have similar widths. Remarkable is the near absence of light-induced metastable defects in the alloys as well as in a-Ge:H even after prolonged exposures at low temperatures.

Dependence of the as-grown defect concentration in PECVD undoped a-Si:H on the deposition parameters, i.e., substrate temperature and precursor density in the plasma produced by H2 dilution of SiH4 gas, is investigated. It is found that the defect density behaves similarly to the concentration of SiH2 configuration in the films deposited at substrate temperatures below 300 °C. The defect concentration, however, varies proportionally to about the 3rd to 4th power of the SiH2 concentration depending on the deposition condition. Based on the surface reaction mechanism proposed in our previous paper, a large enthalpy change is expected to accompany the bimolecular reaction of the adsorbed SiH3 radicals, which incorporates the SiH2 configuration into the network. Characteristics of the defect formation caused by tile breaking Si-Si bond near the growing surface owing to this energy is compared with those of the Staebler-Wronski effect. Both similarity in dependence on II atom concentration and difference in annealing temperature are discussed. It is concluded that the as-grown defects are most likely formed during deposition owing to the large enthalpy change accompanying the bimolecular surface reaction.

We report new results from subgap absorption and transport measurements on pulse and continuous (cw) light-soaked a-Si:H which include that (1) the mobility-lifetime product of electrons (μeτe) shows the same dependence on defect subgap absorption irrespective of whether the sample was soaked by pulse or continuous lights, but that (2) the dependence of μeτe on defect subgap absorption is not linear. These results are compared with annealing energy distribution of light-induced defects, and discussed in terms of (1) annealing energy (Eann) distribution of light-induced defects (LIDs), and of (2) photo-induced structural change.

We have characterized the defect state structure in a series of device quality glow discharge produced a-Si,Ge:H alloys with Ge content ranging from 30 at.% to 50 at.% using capacitance profiling, modulated photocurrent, transient junction photocurrent and photocapacitance measurements. As previously reported, these methods have allowed us to identify two types thermally induced defect transitions plus two types of optical transitions from deep defects. In the current study we have examined the changes in these defects, along with the changes in the hole mobility-time products, that result from prolonged light exposure. By comparing these changes in the annealed state and light soaked state of the same sample, we attempt to correlate the changes in defects with the hole mobility-time product. In general, although all of the defect bands are found to increase after light soaking, the relative factor is found to be different for the various defect transitions within the same sample. We also try to identify a defect bands may be acting as a “safe electron trap”, enhancing the lifetime of the minority carriers. We propose that the observed decrease of this defect band relative to the midgap defect band with light soaking could be a significant factor in determining the degradation of these a-Si, Ge:H alloys in the device performance.

Photogenerated carrier dynamics in undoped a-Si:H have been studied by photoinduced absorption (PA), photoluminescence (PL) and their respective optically detected magnetic resonances: PADMiR and PLDMR. We have detected for the first time the “g = 4” resonance in PADMR, in addition to the previously measured narrow and broad resonances at g≈2. We compare the PADMR and PLDMR resonances over a broad spectral range of detection energies and conclude from the similarities that they share a common underlying mechanism. The PADMIR spectra of the narrow and the broad resonances at g≈2, measured in the spectral range of 0.7 to 1.7 eV, suggest that a correlation exists between the two resonances and PA.

High rate deposition of a-Si:H films has become one of the key techniques for low-cost, large-scale production of thin film devices. Hydrogenated amorphous silicon films were fabricated with a thermocatalytic PCVD method of which the deposition rate was up to 1.5 nm/sec. The Heterojunction Monitored Capacitance method was employed to determine the midgap-state densities in the undoped semiconductor film from high frequency C-V characteristics. Experimental results showed that the thermocatalytic PCVD method is an effective way to produce high-rate deposited a-Si:H films.

Photoluminescence (PL) has been measured at 80 K in a series of samples of sulfur doped a- Si:H using above- and below-bandgap excitation energies (2.4 and 1.38 eV). In addition, the absorption coefficients at room temperature have been obtained using photothermal deflection spectroscopy (PDS). At light sulfur doping levels (S/Si < 10-2), the Urbach slopes of the absorption coefficients on a semilog plot and the optical energy gaps, as measured by the points at which ca =104 cm-1, are independent of sulfur concentration. The slopes decrease and optical gaps increase with increasing doping level for doping levels above 10-2. At light sulfur doping levels the PL spectra excited with both above- and below-gap light are independent of sulfur concentration. For larger sulfur concentrations the shapes of the PL spectra vary. In particular, for S/Si > 10-2the peak of the PL spectrum shifts to below 0.8 eV using below-gap excitation at 1.38 eV, and the defect PL band dominates. Comparing the PL spectra of sulfur- and phosphorus-doped samples, the PL spectra change for sulfur doping above 10-2 and for phosphorus doping above 1 ppm. This trend is consistent with inefficient sulfur doping.

The slow relaxation of the persistent photoconductivity (PPC) effect in sulfur-doped hydrogenated amorphous silicon (a-Si:H) has been measured as a function of temperature and illumination time. The relaxation is found to be thermally activated, with an activation energy which varies with sulfur concentration, while illuminating the film for a longer time leads to a longer relaxation time. A correlation is observed between changes of the photoconductivity during illumination and the magnitude of the PPC effect following illumination. These effects are also observed in compensated a-Si:H, suggesting that the mechanism for the PPC effect is the same in both sulfur-doped a-Si:H and compensated a-Si:H. The presence of donor and compensating acceptor states in sulfur-doped a-Si:H could arise from valence alternation pair sulfur atom defects.

The LESR signals are observed for the a-Si1-xNx:H samples over a wide range of nitrogen content at 77K. By decomposing the LESR signals using three components, i.e. the neutral Si dangling bond (Do) component, the broad component with a linewidth of 16 G, and the narrow component with a linewidth of 5 G, we find that the three components are observed only in the Si-rich samples. The density of light-induced Do increases with x, whereas the densities of both the narrow and the broad components increase in the range of x≦0.32 and are almost independent of x for larger x. The g-value of the broad components falls down sharply from 2.01 to 2.006 with an increase in x, while the g-value of the narrow component is unchanged and it is around 2.004. All of the three components can be annihilated by sub-gap illumination. The origins of the broad and the narrow components are discussed.

A model of metastable defect formation via H-rebonding in hydrogenated amorphous silicon is developed where the defect density and defect formation energy are controlled by the bond-length disorder of the material. Dangling bond defects are created by H motion from SiH bonds to weak Si-Si bonds. The model predicts formation energies for thermal and light-induced defects in good agreement with experiment. The relaxation of thermal equilibrium defects is stretched exponential, with stretch parameters varying approximately linearly with temperature and relaxation times that are thermally activated- in good agreement with experiment. The annealing of light-induced defect densities also shows relaxation behavior. The model accounts for barriers of ≈ 1.5 eV for H diffusion. The energetics of the H*2 complex will also be discussed. The rms bond-length deviation is a new parameter that controls the quality of the material.

A mechanism for charged-carrier-trapping-inducedde fect metastability in hydrogenated amorphous silicon (a-Si:H) and in hydrogenated amorphous silicon alloys containing relatively high concentrations of oxygen and/or nitrogen atoms (a-Si:X:H, X = O or N) is described. The experimental results that identified this defect metastability mechanism were i) differences in the Staebler-Wronski effect in a-Si:H and a-Si:N:H alloys prepared from N2 and NH3 source gases by remote plasma-enhanced chemical-vapor deposition, and ii) differences in defect generation at N-atom terminated Si-SiO2 interfaces prepared from NH3 and N2O.

The comparative study of relaxation processes in amorphous hydrogenated (a- Si:H) and porous silicon (PS) by use of differential scanning calorimetry (DSC) measurements is presented. Films of a-Si:H were deposited by RF glow discharge of two gas mixtures (10% SiH4+ 90% H2) and (5% SiH4 + 95% He). PS films have been prepared by electrochemical etching of 1Ω-cm (p- PS) and of 0.01Ω-cm (p+ PS) ptype Si substrates. The DSC traces were recorded during the heating of samples at a constant rate of 10°C/min from 20 to 5700C in an Ar atmosphere. All investigated samples present a low temperature exothermic effect with a maxima within 120-2800C. At higher temperatures (T>3000C) a second exothermic effect is observed for a-Si:H films prepared from both hydrogen and helium diluted silane mixtures while two endothermic effects are observed for PS samples. Analysis of the low temperature exothermic effects has been performed, and focussed on the relaxation of weak Si-Si bonds which are the features of both amorphous hydrogenated and porous silicon. It was shown that the endothermic effect connected with hydrogen effusion from PS at higher temperatures is compensated by exothermic structural rearrangement in the case of a-Si:H.