Films were prepared by the Xe-dilution method in an attempt to mimic the microstructure found in low-photodegradation deuterated films which show a greater number of clustered deuterium and microvoids when compared to conventional hydrogenated films. Common features were found for the light soaking behaviors in the film properties characterized by the electron spin resonance, gas thermal evolution, Raman spectroscopy, and infrared spectroscopy among deuterated, conventional hydrogenated, and Xe-diluted films.

The kinetics of light-induced defect generation in a-Si:H was investigated over a wide range of illumination intensities and temperatures. The defect density around 1016cm-3 exhibits a power-law time dependence Ns ∼ G2εfε with ε = 0.2 to 0.3, where G is the photo-carrier generation rate. A model for the kinetics of defect generation is proposed based on the existence of an exponential distribution of defect formation energies in the amorphous network, associated with the valence band tail states. The model reproduces the observed time dependence of the defect density with an exponent e determined by the exponential width of the valence band tail. The temperature dependence of the defect generation rate is well-reproduced by the model, which provides a connection between the Stabler-Wronski effect and the weak-bond model.

The introduction of several new principles into the analysis of transition kinetics of metastable defects in a-Si:H has produced substantially improved rate equation for the density of defects as functions of time, light intensity, and temperature. The solution of this equation is stretched exponential (SE) having properties that explain in unifying way many observations of defect properties, including generation and anneal of the defect density in homogeneous films and degradation and anneal of solar cells. Major consequences are found for both the steady-state and transient properties of the defect density and for interpretations of microscopic models of the defects. These properties are also shown to be analogous to those of metastable centers in other materials, particularly the metastable DX center in AlGaAs which offers rare insight into the microscopic origins of stretched exponentials that can be applied to a-Si:H in ways that provide new perspectives on effects of alloying and hydrogen on stability.

We report new experimental data on the light-soaking of a-Si:H with a Kr-ion laser at an optical generation rate G of at least 4×1021 to 3×1022 cm-3s1. We studied the temperature and intensity dependence of the saturation of the defect density and found that the saturation value of light-induced defects (Nsat) is insensitive to temperature and light intensity below about 90°C. Above 90°C Nsat drops with increasing temperature. This behavior can be explained within the defect pool model by a limited number of defect sites coupled with an annealing process.

Electron drift mobilities μe were measured in undoped hydrogenated amorphous silicon specimens for different light soaking states. Standard transient photocurrent and time-of-flight techniques were employed; the light soaking state was monitored using the electron deep-trapping mobility-lifetime product μτe,t. Two different measurement series are reported. The first series measured a specimen from the reactor at Syracuse at room-temperature. No significant effects of light soaking on μe were found. The second series measured a solar-optimized specimen from Energy Conversion Devices, Inc. below 200°K. In these measurements we found a decrease in μe of about a factor 3.

We report evidence for a deep defect exhibiting very large lattice relaxation in n-type a-Si:H. In light-soaked partially-annealed samples with low phosphorus doping levels one obtains large controlled variation of the Fermi energy position in the mobility gap. Examination by photocapacitance spectroscopy of a sample having a Fermi energy near the minimum in the density of states shows dramatic change in the shape of the photocapacitance spectra. We interpret this as strong evidence for lattice relaxation of deep defects.

The effects of positive and negative bias stress on hydrogenated amorphous silicon nitride / crystalline silicon and hydrogenated amorphous silicon nitride / hydrogenated amorphous silicon (a-Si:H) structures are investigated as a function of stress time, stress temperature and stress bias. It is shown that in both structures bias stress induces a parallel shift of the C-V (capacitance-voltage) characteristics. For a given stress bias the direction of the C-V shift depends on the sign of the applied stress voltage, while the magnitude of the C-V shift depends on stress time and temperature. In addition, it is shown that positive bias stress slightly increases the number of localized states in the a-Si:H mobility gap, but negative bias stress does not. These results lead us to conclude that the C-V shift is not induced by dangling bond defects in a-Si:H but rather by carrier trapping in the insulator.

Light exposure of multilayers consisting of alternating 2.5 nm thick a-Si:H and about 40 nm thick a-SiNx:H produces a large metastable excess dark conductance. This appears to be due to metastable defects in a-SiNx:H produced by hopping injection of photocarriers from the a-Si:H. They anneal at an equilibration temperature TE∼460°K. These metastable defects are created with the same efficiency between 4K and 300K. The efficiency rapidly falls above 350K. The metastable defects have the same anneal temperature regardless of creation temperature. The mechanism of defect creation will be compared with that in a-Si:H.

To first order, the relaxation kinetics of thermally generated defects with spin observed in two differently prepared, 60-μm-thick undoped hydrogenated amorphous silicon (a-Si:H) films are consistent with a two-level system having a formation energy of 0.35 eV and an anneal barrier of 2.1 eV. However, closer examination of how relaxation depends on thermal treatments reveals the complexity that might be expected from a disordered material. For example, the stabilization of many spins quenched in from 260°C can be increased by annealing at an intermediate temperature: It appears that, some 260°C defects equilibrating further at 205°C will relax and become more locked-in configurationally than defects simply equilibrated at 260°C. Crossover of annealing data is the result. Crossover cannot be explained with two-level system approaches. Models in which a spin can be stabilized with alternate structural configurations must be invoked.

An equilibrium framework for the high temperature behavior of dangling bond defects in amorphous materials is developed. With this framework it is possible to relate the thermal formation of defects directly with those created by high temperature illumination or current injection. It is found that the free energy change associated with dangling bond formation is negative. The negative free energy change means that if one considered only the structural changes, the lowest energy state for the system is with the weak bonds split into neutral and charged dangling bonds!

We measured the freeze-in temperature of the dangling-bond density in a-Si:H in nine samples with hydrogen concentrations ranging from 7.0 to 31 at.%. The measurements were made by determining the defect density of samples quenched from successively higher temperature. We determined the defect densities with the constant photoconductivity method. The freeze-in temperature is 211±10 °C, and is independent of hydrogen concentration.

Secondary ion mass spectrometry and IR studies of long-range hydrogen motion in undoped a-Si:H and a-Ge:H of varying H content and microstructure are reviewed and discussed. In particular, their relation to the multiple trapping (MT) model, the role of microvoids, the significance of the Meyer-Neldel relation (MNR), and the nature of H sites is addressed. It is suggested that while the MT mechanism may be significant in a-Si:H of low H content Cfj, it is largely marginal in films where CH ≥ 10 at.% H and in a-Ge:H. Mono Si-H bonds on microvoid surfaces are apparently deep H trapping sites up to ∼ 400°C, but H is desorbed from such sites in a-Ge:H above 180°C. The MNR between the diffusional activation energy and prefactor is observed among the various a-Si:H and a-Ge:H, but its significance is questionable, and may be due to the MT mechanism only in low H content a-Si:H. The nature of the distribution of H sites is also discussed.

The thermal stability of hydrogen in amorphous silicon-based alloy films was studied by deuterium/hydrogen interdiffusion and hydrogen effusion experiments. Depending on the film structure, hydrogen stability is limited by hydrogen surface desorption or hydrogen diffusion. The hydrogen surface desorption energy is found to decrease with rising germanium content and to increase with rising nitrogen and carbon content. At T = 400°C, hydrogen diffusion is found to proceed in the germanium subnetwork for a-SiGe alloys and in the silicon subnetwork for a-SiN and a-SiC alloys.

Thermal quenching (TQ) of photoconductivity σp occurs when the demarkation level of minority carriers passes through recombination centers having small capture cross section for majority carriers compared to other centers present but normal cross section for minority carriers. The photoconductivity becomes superlinear with light intensity at the temperature of maximum TQ. We discovered TQ not only in n-type but also p-type a-Si:H. This cannot happen with the same centers unless the sign of the majority carriers changes. We present evidence that in p-type and undoped films majority carriers are electrons at T below TQ and holes above TQ. The nature of these special centers will be discussed.

The saturated light-induced defect density Nsat in a-Si:H was studied as a function of deposition temperature and of pre-light-soaking anneals. The deposition temperature and annealing affect the microstructure, which we characterize by the Si-H and Si-H2 infrared absorption intensities. We find correlations between Nsat and the initial defect density, the Urbach energy, the hydrogen concentration and the fraction of H in SiH4 groups. Annealing before light-soaking can reduce the saturated defect density substantially.

The mechanisms of the Staebler-Wronski effect are investigated by examining the stability of computer-generated amorphous hydrogenated silicon networks with a molecular dynamics approach. Models with both monohydride and dihydride species are examined. A new Si-H interatomic potential is utilized for the simulations. A localized excitation is used to model the non-radiative transfer of photo-excited carrier energy to the lattice. The a-Si:H model with only monohydride species is stable to bond-breaking excitations. The a-Si:H model with both monohydride and dihydride species is less stable and exhibits, after local excitations, higher energy dangling bond states that can however be easily annealed away.

Studies of r.f. sputter deposited hydrogenated amorphous silicon (a-Si:H) find that the light induced decrease in the dark conductivity and photoconductivity (the Staebler-Wronski effect) is reduced when the r.f. power used during deposition is increased. The slower Staebler-Wronski effect is not due to an increase in the initial defect density in the high r.f. power samples, but may result from either the lower hydrogen content or the smaller optical gap found in these films.

We present a study of depletion width effects on the photocarrier collection efficiency in reactive magnetron sputtered a-Si:H films. Results are presented for as-deposited and light soaked 1.75 eV optical gap samples, and an as-deposited 1.60 eV gap film. The depletion width behavior with reverse bias is inferred from capacitance measurements. Comparison with photocurrent collection versus reverse bias voltage suggests that space charge effects can have an important role in the interpretation of collection efficiency on Schottky barriers.

The effects of excitation rate and temperature on the kinetics and steady-state behavior of metastable defect formation in hydrogenated amorphous silicon (a-Si:H) have been studied. The dependences on temperature of the lifetime, τ, and stretching parameter, β, from a stretched exponential description of the kinetics were measured for one sample. We do not see a linear dependence of β on temperature over die entire temperature range studied (270K–370K), and τ increases monotonically with decreasing temperature. Steady-state results show defect density to be dependent on bodi temperature and excitation rate over the ranges measured (from 395K to 470K and from 6 × 1020 to 2 × 1022 s-l cm-3). The gradual change in temperature dependence is explained by a distribution of barrier heights between the ground and metastable states.

Thermally activated conductivity of a—Si:H at a slow cooling rate of 0.3 K/min is connected with temperature dependent changes of the mobility gap states. By means of the Fermi—level shift calculated from these data and a density of states model it is possible to determine this dependence. The results mainly reveal a decrease of the defect state density by about a tenth between 375 K and 400 K.