Measurements of the conductance fluctuations and thermal equilibration of the dark conductivity of a series of undoped hydrogenated amorphous silicon thin films synthesized by Hot-Wire Chemical Vapor Deposition (HWCVD), with hydrogen contents varying from less than one to twelve atomic percent are reported. The spectral density of the conductance fluctuations varies inversely with frequency f and is dependent upon hydrogen concentration; the 1/f noise statistics are non- Gaussian, indicating correlated fluctuators as is observed in PECVD a-Si:H. These results indicate that aspects of electronic transport and defect dynamics in HWCVD films are similar to those in PECVD a-Si:H films.

In plasma-deposited a-Si:H films, the increase of the metastable defect density produced by high-intensity illumination usually follows a stretched-exponential time-dependence, with a characteristic time τSE and a steady-state value Nss. For a wide range of deposition conditions, we have observed that both parameters depend on the material properties. The strong correlation between Nss and the monohydride [SiH]2000 density, reported previously, has been interpreted as due to the trapping of metastable H atoms at specific sites.

In this study of the kinetics of defect equilibration under high-intensity illumination, we find two groups of a-Si:H films with fast and slow kinetics, respectively. These two groups display a very different dependence of the defect creation rate as a function of the optical gap. For the fast kinetics films, we emphasize the critical influence of the Urbach energy Eu deduced from the exponential optical absorption edge (1/τSE increases as a function of Eu). The slow kinetics films are characterized by a high nanovoid density evidenced by their SiHx infrared signature at 2090 cm-1. The results are discussed in relation to the medium-range H motion.

Hydrogenated amorphous silicon was implanted with phosphorus ions to a uniform concentration of 3×1020 cm-3 and defect saturation. The implants were annealed isochronally up to 400°C in the dark or under additional illumination. This illumination had no effect on recovery. The Urbach energy remains higher than that of silicon-implants. The midgap defect density anneals to ˜ 1018 cm-3, typical of gas-phase doped samples. The dark conductvity remains lower and its thermal activation energy higher than in gas-phase doped samples. We surmise that the Si-Si network absorbs some of the donor electron-induced defect density by forming strained Si-Si bonds. These strained bonds widen the band tails, and thus reduce the effective electron mobility and pin the Fermi level.

Concomitant studies of kinetics of defects ND(t), photoconductivity σph(t), electron and hole diffision lengths Le(t), Lh(t) during light soaking have been carried out. The data have been fitted by stretched exponential expressions and characteristic parameters of kinetics have been determined. Correlation between the kinetics is discussed. In contrast to Nd(t), σph(t) and Le(t), the hole diffusion length Lh(t) was observed to remain constant during initial time (˜ 103s) and then decreased with characteri stic time ˜ 104s.

Random fluctuations of localized state energies will result in thermal release of carriers trapped in those states at shorter times than would be observed from a stationary distribution of the same energies. An experimentally observed distribution of activation energies will hence differ from the distribution of average energies of the states involved. It will also be temperature-dependent. In a-Si:H, low-frequency fluctuations with a spectrum comparable to the one of 1/f noise, can account for the measured temperature dependence of the distribution. They also explain the apparent shift in localized-state energy under steady-state illumination.

Tight-binding total energy calculations are used to describe the changes in local structure following either electron or hole capture by a neutral dangling bond in computer generated a-Si:H models. After the change in charge state, the structure is allowed to relax by a steepest descent energy minimization procedure. Generally the local bond angles increase (decrease) rapidly by 3- 100 in transitions from the D0 to the D+ (D-) configurations. The displacement of nearest neighbor atoms and nearby H atoms is large (more than 0.2 Å), but displacement of distant atoms is generally much smaller. Calculated optical transition levels have the D- level below D0 and the D+ level above D0. The fast relaxation of the charged defect configurations suggest a smooth energy surface for the relaxation.

We have carried out capacitance transient measurements on lightly PH3 doped a-Si:H films incorporating both ohmic and blocking contacts. We find that the previously reported anomalous variation of emission time with filling pulse duration is completely independent of whether one uses an n+ c-Si substrate (providing a reasonably ohmic back contact) or a p+ c-Si substrate (giving a blocking back contact). Quite similar behavior is observed on a sample which includes an n+ a-Si:H layer at the back contact, exhibiting a variation of more than 2 orders of magnitude change in emission time when the filling pulse duration is decreased from Is to 100μs. Computer simulations are used to demonstrate that for a fixed (non-relaxing) deep state distribution, a filling problem in general cannot account for the observed experimental behavior. Finally, we report measurements that demonstrate that the filling of defects too close to the barrier interface can create difficulties observing the relaxation effects when an n+ a-Si:H contacting layer is used.

Using junction capacitance methods, we describe the effect of contacts on charge emission transients in n-type hydrogenated amorphous silicon. The results demonstrate some of the difficulties encountered in observing and interpreting anomalous temperature independent emission transients(“slow relaxation”).

We investigate capacitance transients under various experimental conditions on a number of different amorphous silicon Schottky barrier device structures. The samples differ mainly in their back contact configuration, which is non-ohmic for one type of sample and ohmic for the second type of samples. Anomalous transients can be observed for all samples regardless of the back contact if the depletion region is only partially collapsed by partial voltage pulsing. We find that samples without an ohmic back contact show a slowly rising capacitance during the filling pulse. These samples show anomalous capacitance transients for short filling pulses.

In order to fill the gap of little knowledge about their details, the density of states distributions (DOS) in the upper half of the band gap were determined for a series of wellcharacterised amorphous hydrogenated silicon carbide samples with Tauc gaps between 1.78 and 1.94 eV. A DOS spectroscopic technique, based on the Fourier transform of time-sampled transient photocurrents, allowed the DOS determination on an absolute scale for a wide energy range. The DOS increases in the band tail region with carbon content. It exhibits a minimum at about 0.5 eV which is followed by a defect structure at deeper energies, the density of which also increases with C-content. We find a decreasing time-dependent drift mobility for larger C-content consistent with the lower transit time-determined drift mobility in time-of-flight.

We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurrent method. We find that for typical monochromator probe beam intensities the measurement is not significantly influenced by lifetime changes. When the measurement is performed under light bias, an apparent increase in the defect absorption coefficient is observed, but no change in the photoexcitation threshold or spectral shape of the absorption band is seen. We show that this increase is likely due to a bias-light amplification of spectrally dependent lifetime changes. Our measurements suggest a larger electron capture cross section of positive valence band tail states compared to neutral dangling bonds.

In standard, isothermal capacitance transient measurements, configurational changes in defect states are normally very difficult to identify because, depending on the relative rates of thermal emission to the configurational relaxation, charge will be emitted predominantly from only one configuration. We have found, however, that employing a thermal step during the emission phase of the transient enhances the effect of defect relaxation; one can then observe the resultant switch between distinct configurations. We have applied this method quite successfully to lightly doped n-type a-Si:H samples by varying the overall temperature (between 270K and 350K) and the magnitude of the temperature switch (from 20K to 35K). For the smallest temperature steps, the resultant transients suggest two distinct configurations that, we believe, reflect only a fraction of the many latent configurations that account for the full range of relaxation possible.

CPM and PDS spectra of a-Si:H yield identical shape of the Urbach tail, while the defect absorption measured by PDS differs significantly from CPM. In this work an analysis of CPM and PDS spectra of annealed and degraded films is presented. Numerical simulations of CPM and PDS data, taking into account optical transitions, capture and emission processes as well as the Fermi level, yield information on the energy distribution and the charge state of the defects. The simulations reveal the coexistence of defects in the D−, D+ and D0 states. The defect distribution is dominated by charged states as predicted by the defect-pool model. Good agreement between measured and simulated PDS and CPM spectra can be obtained in the case of a homogeneous defect density. It is shown that differences between CPM and PDS are due to different sensitivities of the techniques to charged and neutral defect states. Microscopic inhomogeneities may cause significant additional differences.

Transient grating (TG) experiments were performed to study carrier diffusion and recombination in amorphous silicon films (a-Si:H) at high light intensities using 8 ns pulses from a frequency-doubled Nd:YAG laser. The ambipolar diffusion coefficients reached about 10−2cm2/s, which is 2 orders of magnitude larger than the steady-state value. Similar results were obtained in intrinsic, p-, and n-doped a-Si:H films, indicating that the diffusion coefficients in all cases reflect the near band edge mobility of the slower carriers, that is holes. In particular, the p-type sample shows an initially fast, then a slow grating efficiency decay, consistent with dispersive transport.

We have studied the dependence of the photoconductivity σp on photocarrier generation rate G in intrinsic a-Si:H at 300K between G=1012cm−3s−1 and 1028cm−3s−1. Below a certain value Go, we find σo =AGγ with γ=0.9±0.05 and the values of A vary considerably with defect concentration Nd which signifies monomolecular recombination through defects. Above Go the recombination is bimolecular, γ=0.5±0.02 and A=(6±3)×10−15 Ω−1cm1/2s1/2 is indpendent of Nd. The transition value Go is about 3×1020cm−3s−1 for high quality annealed a-Si:H and increases with Nd. A simulation of σp(G) assuming conduction in and recombination from extended states fits our experiments within a capture coefficient Ct=(6±2)×10−9cm3s−1 of carriers to their opposite tail states. Our Ct is close to the value (5±2)×10−9cm3s−1 obtained from optical measurements but higher than (0.5±0.1)×10−9cm3s−1 determined from photoelectric studies. Below T=150K our model calculations overestimate σp because the tunneling transitions, becoming important for recombination and conduction, are neglected.

The long range fluctuation model has been widely used to account for the difference in activation energies seen experimentally in dark conductivity and thermopower measurements in hydrogenated amorphous silicon. We report on a test of this model using measurements of the conductivity and thermoelectric effects carried out in both open and short circuit configurations. While the thermopower activation energy is less than that of the dark conductivity, the short circuit Seebeck conductivity is found to be nearly identical to the dark conductivity in both activation energy and magnitude, consistent with the long range fluctuation model.

Quantum confinement effects in the transmission spectrum of thin amorphous silicon, a-Si:H, films require a coherence length comparable to the film thickness, as well as good film homogeneity. After a careful investigation, we conclude that there is no quantum confinement in single layer a-Si:H films at room temperature.

We present a systematic experimental study of low frequency noise behavior in Mo/a-Si:H Schottky diodes under reverse bias operation. The noise in the Schottky diode was found to increase with increasing reverse current and with an approximate 1/f behavior at low bias voltages, yielding a Hooge parameter in the range (2 to 3) × 10−4. At high reverse voltages, due to electrical stressing and hence, bias-induced material instability, a significant deviation from the 1/f behavior was observed.

The rise and decay of the photoconductivity σp of intrinsic and strongly p-type hydrogenated amorphous silicon (a-Si:H) samples was studied as a function of photocarrier generation rate G between 4.2K and 300K. In intrinsic samples the temperature regime T<60K of energy-loss hopping is clearly distinguishable from T>80K where thermal re-excitation of photocarriers is possible. For the p-type sample the temperature dividing the two regimes is near 170K. In intrinsic samples the rise of σp is faster when residual photocarriers exist from previous light exposures than when the samples are in the electronic dark equilibrium. This indicates that geminate recombination decreases with increasing photocarrier concentration. In the p-type sample we observe an up to 20 percent overshoot in the rise of σp before σp settles down to its steady state value. This overshoot increases with G. We interpret the overshoot as an interplay of two recombination channels with different time scales.

Multi-trapping transient currents through pure exponential band tails are calculated by Monte Carlo simulations of unprecedented accuracy. The time and energy dependent carrier distributions, the current and the charge signals are obtained. The transit time tT and the dispersion parameters α1, and α2, before and after the transit time, are extracted from the simulated currents. Results are presented for the expected temperature dependence of α1 and α2 and of the drift mobility μD for electrons and holes in a-Si:H. At low temperature, α1 = α2 = T/To is found, but above T ≈ 0.6To, α1, bends downwards to saturate at 1 at high temperature while α2 bends upwards, clearly improving the agreement with the experimental data. The temperature dependence of the drift mobility μD is well reproduced for holes with a microscopic mobility of μo = 0.5 cm2V−1s−1. For electrons, the agreement is also excellent but the surprisingly low value of μo = 4 cm2V−1s−1 is found.