The validity of the Brodsky, Cardona and Cuomo (BCC) [1] and the Connell and Lewis (CL) [2] methods to analyze infrared transmission data of hydrogenated amorphous silicon (a-Si:H) was examined using computer simulations. Transmission spectra for a-Si:H films 0-5¼m thick and containing up to 30 atomic% hydrogen were simulated assuming coherent reflections in the film and incoherent reflections in the c-Si substrate. Analysis of the simulated data for the 640cm−1 Si-H wagging mode shows that the BCC and CL techniques systematically overestimate the absorption coefficeint, α, and hence hydrogen content, CH, when the film thickness, d, is less than ∼l¼m. The error is nearly independent of CH and is as large as 80% in the limit d→0. On this basis, previously reported experimental evidence for the dependence of CH on d is shown to be an analysis artifact. A simple method to correct the hydrogen content determined by the BCC or CL analysis using only the film thickness is presented.

The results of a transmission electron microscopy (TEM) study of excimer laser crystallized a-Si:H thin films are described in this paper. High resolution TEM has shown the crystallization threshold of 10−2 P a-Si:H, grown by PECVD, to be 85 mj/cm2. TEM reveals that PECVD a-Si:H films crystallize in a stratified structure at laser energy densities greater than 120 mj/cm2. This stratified structure consists of a polycrystalline Si region near the surface, a microcrystalline region below the polycrystalline region and uncrystallized a-Si near the substrate. The depth of the crystallized region increases as the laser energy density increases. Spherical voids are also present in the film in the microcrystalline region. These voids are formed by H2 evolving from the film during crystallization. TEM also reveals that thermal annealing a 50 nm film of a-Si:H followed by laser crystallization produces 100 nm Si grains with low defect density.

The results of secondary ion mass spectrometry and IR studies of hydrogen diffusion in a-Si:H and a-Ge:H are reviewed and discussed. In a-Si:H, the diffusion is significantly slower at low total H content. The exponent α of the power-law time dependent diffusion constant D(t) = Doo(ωt)−α does not decrease with temperature as 1-T/To. D(tL), for constant diffusion length L, thus deviates from an Arrhenius behavior. The “apparent” activation energy Ea and prefactor Do derived from lnD(tL) vs 1/T yield a Meyer-Neldel relation Do = Aooexp(Ea/kToo) irrespective of L, H content, or microstructure. Aoo ≅ 3.3 × 10−14cm2/s and Too ≅ 730K for 2.5 × 10−5 ≤ Do ≤ 102cm2/s and 1.3 ≤ Ea ≤ 2.2eV. The preliminary results on a-Ge:H are similar, with Aoo ≅ 1.3 × 10−15cm2/s and Too = 530K. It is suspected that deviations from an exponential distribution of H site energies, structural relaxation, and deep H trapping sites related to microvoids may all contribute to the deviation of α from a 1-T/To behavior. Measurements following prolonged annealing suggest structural relaxation that affects H-site energies.

The thermal stability of hydrogen in hydrogenated amorphous germanium films was studied by hydrogen evolution and by deuterium and hydrogen inter-diffusion experiments. Similar to a-Si:H, the hydrogen stability in a-Ge:H is found to depend strongly on the film structure and on the position of the Fermi level.

Thermally-induced metastable phenomena in amorphous silicon-germanium alloys were studied by conductivity and ESR measurements. Fast cooling from 250 °C reduced both dark- and photo-conductivities by a factor of 3–4 while the neutral defect density remained unchanged. Thermally-induced change in conductivity relaxed towards equilibrium with a stretched exponential form. The thermal equilibrium temperature was found to be roughly proportional to the optical gap for a–Si:H, a–Sii−xCx:H, a–Si1−xNx:H and a–Si1−xGex:H:F.

We report the effect of thermal equilibration on the dark- and photo- conductivities of an un-doped a-Si,Ge:H,F with optical gap of 1.47 eV. Annealing at high temperature and subsequent quenching can freeze in the equilibrium configuration at the annealing temperature. The characteristic glass-like transition behavior of the conductivities was observed and used to estimate a freeze-in temperature of about 140°C. As the annealing temperature increases above the freeze-in temperature, the frozen-in dark- and photo- conductivities decrease, the photo- to dark- conductivity ratio increases, and the photoconductivity-generation rate exponent increases. These changes in conductivities are explained by a model calculation, which assumes that the quenching introduces new defect states to die lower energy flanks of the Gaussian defect distributions.

We have investigated the metastable states produced in n-type a-Si:H samples by 3 kinds of treatments: light soaking followed by partial annealing, fast (quench) cooling, and bias-annealing. We employed voltage pulse photocapacitance to examine the deep defect state distribution, ND, and drive level capacitance profiling to determine the occupied conduction bandtail states NBT. Our results indicate that: (1) Although the most heavily doped films exhibit a larger increase in ND with light soaking than the corresponding decrease in NBT (consistent with some proposed defect creation reactions), in more lightly doped films we observe significant changes in NBT for much smaller corresponding differences in ND; (2) for quench cooling we observe increases by factors of up to over 2 in NBT without any measurable changes in ND for all samples; and (3) we observe increased NBT in bias annealed samples with no changes in ND. All of these results are inconsistent with a purely silicon bond breaking mechanism for metastable defect creation. We consider a significant contribution from a dopant activation mechanism which does not involve changes in ND).

We describe a technique for measuring localization of holes in mid-gap states in high quality a-Si:H devices. The localization of holes is determined by measuring quantum efficiency of a-Si:H devices as a function of reverse bias voltage and wavelength of light. It is shown that the QE of localized holes increases significantly upon application of high electric fields, whereas the QE of de-localized holes does not show such a behavior. The voltage-induced increase in QE is explained using a Frenkel-Poole tunneling model. It is also shown that the density of mid-gap states (states in which holes are localized) increases significantly upon light soaking, and that a major consequence of this increase in mid-gap density is a decrease in the electric field in the device. The decrease in electric field is experimentally estimated by fitting the increased current due to tunneling to the expression for Frenkel-Poole tunneling.

The effect of trace amounts of impurities such as carbon and nitrogen on the density of states in undoped a-Si:H has been investigated using drive-level capacitance profiling and transient photocurrent methods. The results indicate a significant correlation between impurity levels and the susceptibility to metastable state creation after light-soaking. We found this correlation to be independent of bandtail broadening and hydrogen content in the material.

Electron spin resonance spectroscopy is used to monitor the light-induced, paramagnetic (neutral) defect density in gate-quality nitrogen-rich hydrogenated amorphous silicon nitride (a-SiN1.6:H) thin films. A new phenomenon has been observed in which the light-induced electron spin resonance (LESR) signal can be reduced (photo-bleached) by re-illuminating with monochromatic light. The extent to which the LESR signal is bleached depends strongly upon the incident photon energy used for re-illumination. Photo-bleaching as well as the photo-production of the LESR signal follow a stretched exponential dependence upon illumination time.

We report, for the first time, on the stretched-exponential time dependence of the generation and bleaching of positive charge in gate-quality nitrogen-rich amorphous silicon nitride films subjected to the sub-bandgap illumination at room temperature in vacuum. We also propose a mechanism which we believe is responsible for the generation and bleaching of the positive charge in the nitride films.

In order to improve our understanding of the motion of hydrogen and the mechanism by which structural changes occurr, two sets of a-SiC:H samples obtained by glow-discharge at different substrate temperatures and power densities, respectively without and with hydrogen flow, were analyzed.

Results from structural, optical and electrical measurements show that annealing and hydrogen diffusion allow for a possible reorganization of the a-SiC:H amorphous network.

We present a detailed study of the effects of a negative bias applied to the substrate on the electronic properties of a-Si:H films deposited by r.f. glow discharge. Two series of samples deposited at 30 and 100 mTorr respectively have been studied. For each series the negative d.c. bias applied to the substrate was decreased from 0 to −100 V in steps of 25 V. We observe for both series of samples an improvement of the electronic properties of the films as we decrease the substrate bias (increase the ion energy) down to − 50 V. We have found a clear correlation between the negative bias applied to the substrate and the subgap absorption, the valence band tail slope and the electron and hole μτ products.

Using a Kr ion laser (λ = 647.1 nm) to produce a carrier generation rate G of 3 × 1020 cm−3s−1, we have saturated the light-induced defect generation in hydrogenated (and fluorinated) amorphous silicon (a-Si:H(F)), within a few hours near room temperature. While the defect generation rate scales roughly with 1/G2, the saturation defect densities Ns,sat are essentially independent of G. The saturation is not due to thermal annealing. We have further measured Ns,sat m 37 a-Si:H(F) films grown in six different reactors under different conditions. The results show that Ns,sat lies between 5 × 1016 and 2 × 1017 cm−3, that Ns,sat drops with decreasing optical gap and hydrogen content, and that Ns,sat is not correlated with the initial defect density or with the Urbach energy.

The occupied density of states distributions in doped and undoped a-Si:H are investigated. Results reveal the presence of a distinct defect sub-band at 0.6 eV above the valence band edge Ey in n-type and in undoped films. In p-type films, this band is absent. Instead, a new band centered at 0.8 eV above Ev develops. This band is normally unoccupied and is therefore only seen under non-equilibrium conditions. A sub-band at this energy is also discovered in undoped a-Si:H films. The effects of temperature and light bias on these defect sub-bands are discussed.

A simple recombination model is proposed in order to explain Fermi level effects on steady-state photoconductivity in a-Si:H. The model assumes positively correlated dangling-bonds as the only localized states in the gap and takes into account the occupation statistics of correlated defect centers in the dark and under illumination. The steady-state photoconductivity and its dependence on the light intensity are investigated as a function of the Fermi level position in the gap. The results show a good agreement with experimental data from undoped, phosphorus and boron doped a-Si:H material. The quenching of the g≈2.0055 Electron Spin Resonance signal upon illumination in undoped samples and its enhancement in doped ones are also reproduced by the model.

We report on light induced metastable conductivity changes in a-Ge:H thin films deposited by rf-sputtering. The dark- and the photoconductivity of the samples decrease after light soaking. The changes are metastable and both the defect formation and the defect annealing processes are temperature activated. As in the case of a-Si:H films, the time evolution of the conductivity changes is well represented by a stretched exponential type of decay, with activated time constants.

We have prepared unhydrogenated and hydrogenated ([H] = 14 at.%) amorphous silicon thin films using magnetron sputtering with substrate temperature, TS = 40°C. After deposition, the films were annealed at temperatures between 150 and 200°C and conductivity was measured as a function of anneal time. We find that for that for both materials, the conductivity changes non-exponentially with annealing time. The characteristic time constant for annealing at 175°C is approximately the same in unhydrogenated films (τ≈100min) as found in films containing 14 atomic % hydrogen (τ≈200min).

Quantum molecular dynamics (QMD) simulations provide the real-time dynamics of electrons and ions through numerical solutions of the time-dependent Schrodinger and Newton equations, respectively. Using the QMD approach we have investigated the localization behavior of an excess electron in amorphous silicon at finite temperatures. For time scales on the order of a few picoseconds, we find the excess electron is localized inside a void of radius ∼ 3 Å at finite temperatures.