As a potential method for controlling the surface reaction of plasma CVD, mechanical vibration generated by a piezoelectric device was applied on the substrate during glow discharge decomposition of SiH4. The application of 2MHz vibration dramatically improved the photoconductivity of a-Si:H films prepared at a substrate temperature of 120°C. IR and CPM spectra for the films revealed that the piezoelectric vibration remarkably modulated the amorphous network structure and reduced the defect density to 4.3x1014cm −3 which is the best among the values ever reported.

We report the electronic properties, stability and microstructure of a-Si:H films grown at very high substrate temperature (Ts = 320∼425°C) by DC reactive magnetron sputtering (RMS). The partial pressures of Ar and H2 are fixed at 1.5 and 0.8 mT, respectively, during the deposition. The initial defect state density, determined by the constant photocurrent method (CPM), varies from 2∼5×l015 cm−3with H content changing from 15–10 at.% as Ts increases from 320–375°C. For 100 hrs white light exposure at lW/cm2, a heavily degraded state was obtained with mid–gap state density in the range 2-3×l016cm−3 over this Ts range. These are among the lowest values reported for intrinsic a–Si:H.

A low temperature process for laser dehydrogenation and crystallization of hydrogenated amorphous silicon has been studied. The key feature of this process is the removal of hydrogen from the amorphous silicon thin films while crystallizing the films at the same time. Studies of transient phenomena, hydrogen loss, and crystallinity, using transient reflectivity analyses, transmission electron microscopy and quadrupole mass spectrometry, find that hydrogen out-diffusion depends strongly on film structure and the melt duration controlled by the laser energy density. Utilizing this process, for which the maximum temperature is 350 °C, both high quality polycrystalline and amorphous silicon TFT's have been fabricated on the same Corning 7059 glass substrate.

We give a brief description of the various classes of molecular dynamics simulations and then describe what we have learned from our simulations recently. This includes information on the nature of defects in a-Si:H including positions in the energy gap and localization, relaxation and rearrangement effects, light induced defects, and the motion of H atoms and Si dangling bonds.

Electronic structure and dynamics of defects in hydrogenated amorphous silicon (a-Si:H) are investigated based upon ab–initio molecular–dynamics simulations. It is shown that (i) the hydrogen–passivated dangling bond (Si-H), (ii) the positively-ionized three–centered bond (Si– H+–Si), (iii) the negatively–ionized three–coordinated dangling bond (D−) and (iv) the five- coordinated floating bond (F5) are the intrinsic defects in a–Si:H. Based upon the calculated result, we discuss the role of hydrogen and the origin of the photo–induced defect in a-Si:H.

The atomic and electronic structures of tilt and twist boundaries in Si have been calculated by using the transferable semi-empirical tight-binding (SETB) method, and the relations between the local structural disorder and the electronic properties of Si have been obtained clearly. The odd-membered rings and the four-membered rings induce the changes of the shape of the local density of states (LDOS). The bond distortions generate the peaks at the band edges in the LDOS, and greatly distorted bonds induce the weak-bond states inside the band gap. The three-coordinated defect generates a deep state in the band gap, which is much localized at the three-coordinated atom. The five-coordinated defect generates both deep and shallow states. The deep state is localized in the neighboring atoms except the five-coordinated atom, although the shallow states exist among the five-coordinated atom and the neighboring atoms. Configurations of boundaries are very effective in order to clarify the effects of the local structural disorder in amorphous SI.

Using transient capacitance and transient spin techniques, we have the determined the manner in which the mobility gap energy of the D defect is altered following a change in its charge state. This relaxation process gives rise to a power law rather an exponential thermal release of defect electrons with time and also causes the charge emission and spin transients to obey a scaling law. We also deduce that the D°/D+ transition rate depends on the tenure of the proceeding D-/D° transition. This last aspect of the D defect emission behavior implies that it must be treated as a non-Markovian process. Such relaxation dynamics have profound consequences for the steady state distribution of D defect energies. Using the relaxation parameters determined by the transient measurements we have been able to solve a set of coupled differential equations under steady-state conditions to provide the energy distributions of both the D° and DD+ defect sub-bands. The results of these calculations agree remarkably well with the experimental distributions determined by modulated photocurrent and steady-state capacitance measurements. This implies that the statistical variations in the occupation history of the defect may be the dominant factor determining both distributions.

Comparing the photoconductivity σp of undoped samples with different native defect concentrations we find that light-induced metastable defects decrease the electron lifetime more strongly than the native defects. We discuss the differences between native and metastable dangling bond defects. We find that the constant photocurrent method underestimates the defect concentration in undoped a-Si:H.

We discuss the possible existence of a considerable density of charged dangling bond defects in device-quality hydrogenated amorphous silicon, which for example has been postulated by recent thermal equilibrium models for the density-of-states distribution. Based on a quantitative analysis of spin resonance and light-induced spin resonance data at different temperatures as well as on subgap absorption measurements, we conclude that intrinsic a-Si:H only has a small density of charged defects caused by unintentional impurity doping. The same conclusion also holds for light-soaked a-Si:H and for samples which are dehydrogenated by annealing at high temperatures.

Photoelectron Yield Spectroscopy is one of the most direct methods to study the density of gap states of amorphous semiconductors. We have made use of it to investigate the transient changes of the density of gap states of a-Si:H upon illumination with above-band-gap light. An excitation density of 50 mW/cm2 of 532 nm light from a frequency doubled Nd-YAG laser was modulated with frequencies between 0.1 Hz and 10 KHz and the transient response of the photoelectron yield signal was monitored by gated electron counting or a lock-in amplifier. From this we deduce a reversible increase of the occupied density of defect states under illumination with a maximum Δg ∼ 1017 cm3 eV-1 at about 0.3 eV below the Fermi energy and a decrease below EF− 0.75 eV, i. e. in the region of deep tail states. The effect exhibits a wide spectrum of time constants involved. Based on additional transient surface photo voltage measurements as well as numerical simulations we argue that the observed redistribution of states is not due to a light induced change in surface band bending or due to a mere excitation of electrons from occupied to empty states. Instead, we interpret the observed effect as a conversion of weak to dangling bonds, i.e. a precursor of the Staebler-Wronski Effect, lacking however the final step of hydrogen stabilisation necessary for the creation of metastable defects.

We compare the electron spin resonance (ESR) signal of the dangling bond in porous silicon films, produced by electrochemical etching, to the ESR signal from hydrogenated amorphous Si (a-Si:H). The anisotropy of the ESR signal in porous Si showed g values varying as for the Pb Si/SiO2 interface dangling bond. The g value varies from g|| − 2.0020 to gL − 2.0080 with an inhomogeneously broadened line width increasing from 1.8 to 3.8 G. A porous Si ESR powder line, with superhyperfine and strain broadening intrinsic to porous Si, is compared to the a−Si:H dangling bond line. The result is more inhomogeneous broadening of line widths parallel and perpendicular to the dangling bond axis in a-Si:H, and less anisotropy in g|| − gL- No evidence was seen for light-induced metastability on a H-passivated porous Si film.

We have studied the influence of the photon energy of the exciting light on the distribution of the carriers in the gap of a-Si:H by light-induced electron spin resonance (LESR) and photoluminescence (PL). In the LESR spectra the number of spins in the broad line (h) is larger than in the narrow line (e or e+db) by a factor of 2-3. For excitation in the bulk of the sample this asymmetry does not depend on the light intensity and photon energy, which suggests that it is not possible to relate the asymmetry to a specific defect structure. LESR and PL show that both bandtails can be populated by absorption of IR light (1060 nm) by optical transitions from occupied and into empty defect states. The recombination kinetics suggest that these absorption processes occur as a two-step process at the same defect

Microvoids have been observed in a-Si:H as demonstrated by small angle X-ray scattering. We have studied the structural properties of these microvoids with molecular dynamics simulations. Using molecular dynamics simulations with classical potentials, we have created microvoids by removing Si and H atoms from a computer generated a-Si:H network. The internal surfaces of the microvoids were passivated with additional H atoms and the microvoids were fully relaxed. Microvoids over a limited range of sizes (5-90 missing atoms) were examined. We obtained a relaxed microvoid structure with no dangling bonds for a microvoid with 17 missing atoms, whereas other sizes examined produced less relaxed models with short H-H distances at the microvoid surface. The strains near the microvoid surface are described. The microvoid model was stable to local excitations on weak bonds in the vicinity of the microvoid.

The effect of microvoids on optical properties of a-Si:H deposited by PECVD of SiH4 has been investigated in the deposition temperature range between 143°C and 266°C. Microvoids seem not to affect the density up to H concentrations CH of 15%. The density is decreased only by H incorporation, which, at the same time, influences directly the optical energy gap Eg Above CH=15%, microvoids can easily accommodate H and no further variation either of Eg or the index of refraction n can be appreciated. Microvoids are proven to be affected not only by deposition temperature, but also by deposition time and by a further annealing step, even if at a relative extent.

Chemically pure (a-Si) and hydrogenated amorphous silicon (a-Si:H) are metasta- ble materials which are thermodynamically unstable with respect to crystalline silicon (c-Si). In both materials, however, partial thermal equilibria can be established between certain structural, configurational and electronic degrees of freedom. The present paper discusses experiments on both amorphous (a-) materials showing that two kinds of structural change can take place within random Si networks: structural relaxation and configurational equilibration. The first process can be observed in both materials indicating that it is supported by intrinsic degrees of freedom of the random Si networks. During these changes partial thermal equilibria between distorted and broken bonds are established via irreversible and relatively long-range relaxation processes. The second kind of change can only be observed in a-Si:H, indicating that it is H-related. The H-related degrees of freedom support reversible valence alternation reactions in which the local bonding configuration of the dopant and defect sites is changed and in which their charge states are altered. These latter interactions establish a strong coupling between the electronic system and the configurational degrees of freedom of the random Si networks. Formally, these latter changes bear strong similarity to the electrochemical processes that take place in liquid electrolytes.

The kinetics of small changes from thermal equilibrium are measured and simulated for thermally induced metastable changes. We find that only a distribution of rate constants is consistent with the data, and therefore we can eliminate certain types of microscopic models for metastability which depend on a single variable such as the carrier density.

The thermally induced metastability was observed in unhydrogenated doped amorphous silicon prepred by Low Pressure Chemical Vapour Deposition (LPCVD). The equilibration temperature (T*) was found to be 380°± 20°C. The preliminary study of the influence of hydrogen content on thermally induced metastability in amorphous silicon is reported. In the hydrogenated doped films the equilibration temperature changes from 380° ± 20°C to 170° ± 10°C, as H content increases to 12 at.%.

Hydrogenated amorphous silicon films were deuterated through a sequence of 1h exposures to a remote deuterium plasma at 350°C. The concentration profiles of hydrogen and deuterium were determined by SIMS at various times during the exposure sequence. The defect density in state A, after deuteration and after illumination with white light were determined using CPM measurements following each 1h exposure sequence. We find that post-deuteration does not alter the defect density in state A, change the Urbach edge, nor significantly alter metastable defect formation. Intense light soaking increases the defect density by about 5 × 10l6cm−3 independent of the total H + D concentration. These results suggest that D always enters the sample in pairs pinning the hydrogen chemical potential which supports the idea of a negative U system for hydrogen and deuterium. Despite an increase of Si-H bonds by as much as 3 × 1021cm−3, the annealed dangling bond density and the weak Si-Si bond density did not change.This suggests that the density of weak Si-Si bonds as well as the dangling bond density is determined by equilibration with strong Si-Si bonds through the interchange of H. The implications of these results for H bonding will be discussed.

The interaction between electronic carriers and hydrogen migration in a-Si:H was investigated by diffusion experiments in the intrinsic (i−) layer of p-i-n a-Si:H photo-diodes. The carrier concentration in the i-layer was controlled by varying the bias applied to the devices. Hydrogen migration (i) is enhanced when the carrier population is increased by illumination and (ii) is suppressed when it is reduced below the thermal equilibrium value by the application of a reverse bias to the diodes. The effect is attributed to the dependence on carrier density of the dissociation rate of hydrogen from Si-H bonds into the diffusion path consisting of interstitial sites. In addition, the migration length in the diffusion path increases under reverse bias. The enhanced migration is associated with a decrease in the effective density of traps for hydrogen in a carrier-depleted layer. Possible mechanisms for the interaction between hydrogen migration, carriers and defects are discussed.

We compare experimental diffusion studies to the results of a theoretical study of diffusion controlled by a single deep trap level. Analytic solutions for the D profiles after annealing depend on the characteristic H release time, τ, from the deep trap. At times much shorter than τ, the D profile develops exponential wings whose decay length is the mean D displacement between trapping events. The long-time D profile is a solution to the ideal diffusion equation, but with an effective diffusion coefficient that can be calculated from features of the early-time profiles. New experimental data establish the validity of the model at a range of anneal times and temperatures. We also find that the mean displacement of free H before retrapping decreases with both increased illumination and increasing anneal temperature.