We examine the energy and time scales of configurational relaxation around the dangling bond defect, D, in hydrogenated Amorphous silicon (a-Si:H). After D captures or emits charge, its bond angle, electron energy eigenvalues and local structural environment all change. This determines the measured electronic energy levels; we use previous theoretical results and experimental data to estimate the density of gap states in the different atomic configurations of D. We also describe D relaxation effects observed in experiments, including the very slow relaxations found in recent transient capacitance measurements. To explain the unusual T-independent kinetics of transient capacitance carrier emission, we propose a model of “structural memory” in a-Si:H. After carrier capture, neighbors of D retain memory of their pre-capture configuration for seconds at 300K. The rate-limiting step to carrier emission is an effectively one-dimensional random walk of these neighbors through their configuration space and back to the pre-capture configuration. The final, activated, step of emission is very rapid. We describe analytic and monte Carlo calculations that support the structural memory Model and propose possible microscopic Mechanisms.

The defect-related photoluminescence (0.8–0.9eV) of a-Si:H is studied as a function of the defect density ND and the temperature using subgap excitation of Ex= 1.16eV. Electron bombardment, light exposure and phosphorus doping were employed to create defects. The defect density ND was infered from CPM- and PDS-spectroscopy. We conclude from the measured density-of-states distributions of doped films that the radiative process is tunneling of bandtail carriers (Majority carriers) into the defect states. The dependence of the emission intensity on ND suggests that only a subgroup of the defects acts as radiative center. We propose that the local structure of the defect determines whether it acts as radiative or nonradiative center.

We derive the hydrogen density of states for hydrogenated Amorphous silicon (a-Si:H), which is completely consistent with the electronic density of states for the defect pool model. If silicon dangling bond energies are distributed in energy, as in a defect pool model, then the hydrogen density of states becomes quite complex, with the hydrogen binding energy dependent on the Fermi level and dangling bond transition energy. We demonstrate that the electronic density of states for dangling bonds is almost identical to our previous defect pool model, while the hydrogen density of states can account for the results of hydrogenation-dehydrogenation experiments. The effective hydrogen correlation energy is variable, being negative for most hydrogen binding sites, but positive for most defect sites.

We examine the treatment of the hydrogenated Amorphous silicon (a-Si:H) dangling-bond defect used in the Analysis of microelectronic and Photonic Structures (AMPS) computer model and in other models of a-Si:H semiconductor devices. The dangling bond defect (D) is trivalent, with two correlated electronic levels in the gap. However, Modelers typically employ two uncorrelated bivalent levels to represent D; this introduces fictitious neutral dangling bond (D°) levels whenever D is charged. We find that the bivalent (AMPS) representation captures the important aspects of D physics and introduces only small errors into simulations. To reach this conclusion, we examine charge density and recombination and compare the trivalent D defect and its bivalent representation in both thermal equilibrium and non-equilibrium (illuminated) cases. The charge density is identical, implying that band bending computed by the simulation is accurate. We find errors in the recombination rate for the bivalent representation are normally less than or equal to σ0/σch, where σ° is the capture cross section of D0 and σch is the capture cross section of a charged state of D. Typically Modelers use σ0/σch of 1:10 to 1:100 yielding insignificant errors in the recombination rate with the uncorrelated bivalent representation.

We show that the meyer-Neldel Rule (or compensation law) arises from the entropy effect of accumulating several excitations in order to surmount an activation barrier. This explanation applies both to annealing and to conductivity phenomena in a-Si:H. We consider three consequences of this theory for a-Si:H : 1) MN processes in a-Si:H involve coupling to a bath of “optical” phonons; 2) dc conductivity involves strongly electron-phonon coupled states rather than extended electrons and a fixed density of states; and 3) time-of-flight exponents depend upon the MN temperature.

In this paper we develop a model of the defect kinetics in hydrogenated Amorphous silicon (a:Si:H) with the goal of predicting the density of defect states g (E) established by any given light intensity I, for arbitrary times t and temperatures T. While we build on widely accepted expressions for the the rates of light-induced and thermal annealing, we examine in more detail the light induced annealing (LIA) term. The model shows that the LIA process can be described with the thermal annealing term if a suitable reduction to the annealing energy is introduced. This reduction depends on the light intensity such as to suggest a relation to the shift of the electron quasi-Fermi level under illumination.

First principles molecular dynamics methods are used to construct supercells for diamondlike Amorphous carbon, to study its properties, and to compare and contrast it with a-Si. As with recent lab fabricated material, these supercells contain no hydrogen. Several structural models are introduced and the topological, electronic, and vibrational properties are discussed. In particular, in spite of the presence of 3-fold coordinated sites and no hydrogenation, we have obtained a supercell sample that has a gap of about 2.5 eV containing no defect states. To our knowledge, ours is the only theoretical work that agrees with recent experiments in this respect. We explore the nature of defects in the Amorphous network and, in particular, the atomistic origin of the clean gap in unhydrogenated C but not in unhydrogenated Si.

A Molecular dynamics method with a Many-body Tersoff-type interatomic potential has been being applied to analyses of hydrogenated Amorphous silicon (a-Si:H) thin-film growth processes. As a first step toward film growth simulations, Molecular dynamics simulations of SiH3 radical, which would be a significant precursor for the a-Si:H thin-film growth processes, and a-Si:H formation with a rapid quenching method have been performed by developing new Tersoff-type interatomic potential between Si and H in this study. Visualization of SiH3 radical dynamics by computer graphics has made it possible to observe the inversion and rotation of SiH3 radical, which had been predicted by infrared diode-laser spectroscopie measurement in other group. In addition, visualization of the a-Si:H sample has helped us to find that there are some microcavities in the sample and that there are two kinds of hydrogen in the sample, gathering closely together while lying scattered, which had been predicted in IR absorption experimental results.

The elastic properties of hydrogenated silicon nitride (a-SiNx:H) films, with x=1, have been investigated by means of the Brillouin light scattering technique. The films were deposited on an opaque substrate (silicon) in order to improve the scattering efficiency from acoustic phonons in the film. Measurement of the frequency shift of Brillouin peaks relative to both the surface Rayleigh Mode and the longitudinal bulk waves enabled us to determine the two independent elastic constants c11 and c44.

Frequency-resolved photoluminescence spectroscopy (FRS) is used to study non-radiative recombination in a-Si:H using generation rates sufficiently small to garantee geminate recombination at low temperature. The quenching of the photoluminescence by a higher defect density ND and an increase of temperature influences the QFRS spectra differently: Whereas for increasing ND the quenching of the signal is more pronounced on the low frequency side raising temperature leads to a uniform decrease in the entire frequency range. The dependence of the lifetime distribution on ND is quantitatively explained in a model where radiative recombination competes with non-radiative tunneling into defect states.

In an electrophotographic experiment, surface voltage is measured for a-Si1-xCx:H films of different thicknesses. It is observed that the thickness normalised dark decay rate (TNDDR) at longer times is smaller for thicker than for thinner specimens. This phenomenon has also been reported for other materials. In order to get a better understanding of the dark discharge process, the monte Carlo technique is used to model electrophotographic dark discharge in materials of which a-Si1-xCx:H is typical. The present study considers every carrier within the model after each increment of a short time step. This then allows bimolecular effects and the presence of space charge to be accounted for.

The study concentrates on two different discharge mechanisms and the effects they have on the TNDDR for varying specimen thicknesses. For the first of these, a negative charge is deposited on the surface of the sample and drifts through the bulk under the influence of the local internal field (Surface Injection model). In the second, electron/hole pairs are generated within the bulk with both carriers being mobile through the specimen (Bulk Generation model). The subsequent surface decay is due to the recombination of bulk generated holes and trapped surface electrons. Results from these models are compared with those found experimentally.

In this paper we show that the usual behaviour of forward bias C-V-f measurements performed on p-i-n structure, Modifies at the lowest test frequencies when the measurement is performed at high temperature (120°C), or if the cell under test is at a certain stage of the degradation process. We relate the effect to the relaxation of charged defects toward dangling bonds in the i-layer of a p-i-n structure. In capacitance measurements under forward bias, the applied sinusoidal test voltage periodically changes the density of injected charge and the defect occupancy in the intrinsic layer. We show that this effect may Modulate the tendency of defects to relax to lower energy configuration. Relaxation changes the time scale of the subsequent thermal emission of the trapped charge; this reflects on the measured capacitance which drops at low frequency. We also demonstrate the possibility to obtain the relaxation parameters from the frequency dependence of the measured capacitance.

We have carried out a series of charge transient measurements on a-Si:H in which we insert a double temperature step during the period when electrons are being emitted from deep defects. The behavior of this emitted defect charge is completely inconsistent with any density of states that remains static during the emission; that is, defect relaxation must be invoked. Such measurements allow us to separate the temperature dependence of relaxation from that of thermal emission. In particular, we demonstrate that the emission itself exhibits thermally activated behavior in spite of the ongoing relaxation processes.

First principles molecular dynamics methods are used to study the motion of defects including dangling bonds and H. We study motion among H passivating dangling bonds, bond centered H, and tetrahedral H. We find that relaxation effects can reduce activation energies by up to one eV. Our studies also include the actual simulation of the time evolution of an a-Si:H system for several picosec. Here we observe the motion of a dangling bond over several hops and the motion of an H atom by means dangling bond exchange.

A new tight-binding molecular dynamics approach for Si-H systems is developed using the valence orbitale of Si and H for calculation of atomic forces. Previous tight-binding models are not able to describe formation energies of different charge states of H in c-Si and new physics is introduced in our model to describe both effects of charge transfer and varying atomic environments. The Si-H Model was developed by fitting to silane, and ensuring that the formation energies of different charge states of H in c-Si were correctly described. This new model also describes well vibrational properties of SiHn configurations, and the structural and electronic properties of a-Si:H Models. The new molecular dynamics utilizes quantum mechanical forces, incorporating important electronic effects, and is robust enough to simulate hundreds of atoms as would be needed in realistic a-Si:H systems.

We compared the effect of light soaking on the photoresponse and defect concentration ND of samples prepared by normal glow discharge, by remote plasma discharge, by the heated mesh and by the hot wire deposition methods. After exposure to 4×1027 cm−3 absorbed photons all samples have the nearly the same Np and photoresponse. At low temperatures additional defects with small anneal energies are created. Defects created at low temperatures were found to relax between 100K and 300K before they anneal. These new results cannot be explained by present models of defect creation. The kinetics of defect creation at low temperatures is discussed.

High quality a-Si:H was fabricated from SiH2Cl2 by ECR hydrogen plasma at high growth rate (>15 A/s). After sticking the precursors, SiHnClm (n+m≤3), stable Si-network was formed by chemical reactions enhanced on the heated substrate due to strong interaction between Cl and H bonded with Si. The defect densities of 3×1015 and 3×1016 cm3 were obtained for the films in the as-grown and the saturated states after light soaking, respectively. In these stable films, a surprisingly large amount of voids (2–10 vol%) was found by the simulation of the spectra of pseudo-dielectric constant measured by spectral ellipsometry.

We propose a rate law that unifies the dark and light-induced annealing of metastable defects in hydrogenated Amorphous silicon (a-Si:H). Its form is dN/dt ∼ - C Naf (T), where N is the defect density, C the concentration of free electrons (holes), f (T) a dispersive function, t the time, and T the temperature. Special dark and light-annealing experiments, designed to keep the free electron density constant, were conducted to eliminate C from the rate law. We find that f (T) for the dark annealing process differs from that for light-annealing. We calculated the thermal activation energies for the decay of N to 1/e of its initial value. They are 1.56±0.2 eV for dark annealing and 0.91±0.2 eV for light-induced annealing.

Light induced degradation of intrinsic Amorphous silicon (a-Si:H) is investigated as a function of temperature. Previous work described an equilibrium framework for the high temperature behavior of dangling bonds defects (DB) 11]; and the temperature dependence of the annealed state photo, σPH, and dark, σD, conductivities of a series of intrinsic a-Si:H Materials deposited over a range of substrate temperatures, 200°C < Ts < 380°C [2]. These results are extended to the light degraded state where elevated temperatures provide for equilibration of the free carrier and DB concentrations. For the equilibrium, light degraded state, both σD and σPH, decrease compared to the annealed state while the ratio, σD/σPH remains unchanged. Relationships between the ratio [DB+]/[DB] and the Fermi level are derived from the equilibrium framework.

The temperature dependence of the dark conductivity, σD, of unhydrogenated and hydrogen passivated polycrystalline silicon (poly-Si) films was Measured. While σD of unhydrogenated poly-Si did not exhibit any influence of thermal treatment prior to the measurement, striking effects were observed in hydrogenated poly-Si films. Below 268 K a cooling-rate dependent metastable change of σD is observed. The dark conductivity increases by more than 8 orders of magnitude. This frozen-in state is metastable: Annealing and a slow cool restore the temperature dependence of the relaxed state. The time and temperature dependence of the relaxation reveal that this process is thermally activated with 0.74 eV. The lack of the quenching metastability in unhydrogenated poly-Si is direct evidence that the metastable changes in σD are due to the formation and dissociation of an electrically active hydrogen complex, in the grain-boundary regions.