Carrier-induced changes in the gap states of plasma-enhanced-chemical-vapor-deposited (PECVD) undoped a-Si:H films are systematically studied using isothermal capacitance transient spectroscopy (ICTS) method via the novel structure we proposed previously. The density-of-state distribution g (E) and the energy dependence of electron-capture cross section σn (E) of gap states in undoped a-Si:H films before and after injection of electrons are directly measured for the first time. Experimental results show that the density of g (E) increases but the σn (E) has no obvious change after the electron injection. These indicate that the defects created during the electron injection are the same in type as those of the as-deposited films. In addition, it is found the shape of g (E) distorts after the injection, implying that the gap states with different energy levels are associated with distinct types of defects.

Recombination-driven mechanisms for defect formation do not account well for several experimental facts, and indeed conflict with some facts. To overcome these problems, an alternative mechanism is proposed in which latent defect centers are converted to metastable defects (dangling bonds) without a recombination event. The transitions are driven simply by electron capture at the center with no energy barrier. A new configuration-coordinate diagram expressing this mechanism includes essential configuration-induced changes in the energies of band edges, and accounts for several other previously problematic observations. This carrier-induced mechanism (CIM) is consistent with defect formation in forward bias and defect inhibition in reverse bias, light-enhanced annealing, as well as the usual light-induced and beam-induced formation. For low orħħωthe process starts by ionization of an existing defect.

Despite its importance for technological applications, the behavior of hydrogen in amorphous silicon is not fully understood. In particular, the anomalously low activation energy (1.5 eV) for hydrogen diffusion has remained unexplained. We investigate the interaction of hydrogen with dangling bonds using first-principles pseudopotential-density-functional calculations. Our analysis shows that the diffusion activation energy should be measured from the hydrogen chemical potential, and that this level should be identified with the formation energy of Si-H bonds. A quantitative identification of the energy levels with experimental observables is then possible.

Light-induced rearrangements in a-Si:H,D during and after illumination have been studied by deuteron magnetic resonance (DMR). A film sample was deposited on a 230°C Al substrate by plasma decomposition of S1H4 and D2. The high quality film contained 4 at.% H and 4 at. % D. DMR spectra of fast-relaxing components were examined at 700 msec or less after radio frequency saturation. At 30 K these showed a broad central signal (primarily nanovoid-trapped HD and D2), part of which shifted paramagnetically during illumination. No comparable shifts were shown by spectral components from Si-bonded hydrogen or from microvoid-contained dense fluid hydrogen. The observed post-illumination shifts vanished upon 150°C dark anneal and reflected light-induced metastable magnetic changes in the surroundings of hydrogen trapped in nanovoids.

Time traces of conductance fluctuations in the co-planar current of hydrogenated amorphous silicon (a-Si:H) display sharp jumps between discrete resistance levels, termed random telegraph switching noise (RTSN). Measurements of the temperature dependence and effects of light soaking of the RTSN in n-type doped a-Si:H are reported. The rise times between the two level fluctuators yield activation energies and attempt to hop frequencies for microscopic hydrogen motion which agree with those obtained from NMR experiments. Computer simulations of a dynamical percolation random resistor network support the suggestion that the RTSN arises from local diffusion processes altering the conductance of inhomogeneous current filaments.

Molecular dynamics with the tight-binding approach are utilized to examine the fundamental process of dangling bond creation via the rebonding of H from Si-H bonds to weak Si-Si bonds. The defect formation energy is found to strongly correlate with the bond-length of the weak Si-Si bond, indicating that the distribution of weak Si-Si bonds controls the total defect density. Rate equations for thermally generated and light-induced defects are developed and utilized to calculate the equilibrium and saturated defect density. The results agree well with experimental data.

We present the results of H effusion studies on device-quality glow discharge deposited hydrogenated amorphous silicon (a-Si:H) films. We measure the decrease in the amount of Si-H infrared absorption as pieces of an a-Si:H sample are annealed isothermally at temperatures from 425°C to 500°C, until more than 95% of the initial H is evolved. We use the rate equation for loss of H due to annealing to calculate the activation energy for H effusion. For anneals below 500°C we observe two distinct rate processes, a fast process corresponding to release of ∼ 70% of the total H (∼10 at. %) contained in the sample, and a slower process corresponding to effusion of the remainder. The fast loss process yields an activation energy of ∼1.4 eV; this is the energy level widely observed in diffusion coefficient measurements. A similar analysis for the slow loss process yields an activation energy of ∼2.1 eV and a diffusion constant prefactor higher than that for the fast loss process. We suggest that this slow component represents the first determination of the energy depth of the “isolated” H component commonly observed in nuclear magnetic resonance experiments.

We demonstrate that the exisiting ambiguity between the Nuclear Magnetic Resonance (NMR) and the infra-red (IR) interpretation of the density of clustered hydrogen can be removed by monitoring the effective charge of the silicon hydrogen vibrations in a-Si:H versus the hydrogen content. By monitoring the peak position of the low frequency component (2000 cm−1 mode) of the stretching doublet, we conclude that the 2000 cm−1 and the 2100 cm−1 modes can be uniquely attributed to the monohydride and multihydride bonding configurations, respectively.

We show that the parameters describing the IR spectrum depend on the hydrogen content only and do not depend on the deposition technique, deposition conditions or post-deposition annealing treatment.

We compare measurements in a-Si:H of ambipolar diffusion length Lamb (from steady-state photocarrier gratings (SSPG)) and hole drift Χ (t) (from time-of-flight (TOF)). Using the response time tR from small-signal photocurrent decay measurements, we find that the equation L2amb = 2 (kT/e) Χ (tR) /E is consistent with the measurements, where E is the electric field inducing hole drift in TOF. Several samples under different temperature and light intensity levels have been studied. This equation has several implications. Under the usual SSPG illumination conditions, electron-hole recombination occurs while holes are still occupying valence bandtail states; hence SSPG is not sensitive to hole capture by deep levels. Furthermore, the experiments show that the Einstein relation is valid for holes in a-Si:H. We are unaware of prior direct tests of this relation in an amorphous semiconductor.

We have measured the electron drift mobility in a-Si:H prepared by hot wire (HW) deposition using photocarrier time-of-flight. Initial work has shown that light-soaked HW material can have much better ambipolar diffusion lengths than the plasma-deposited material following extended light soaking. In a sample with about 2% H-concentration in the intrinsic layer, we find that the electron drift mobility is quite different from that of a-Si:H alloys prepared by normal glow-discharge CVD, even allowing for the reduced bandgap of the hot Wire material. This result challenges the principle that the bandgap of optimized amorphous silicon based material is sufficient to predict the electron drift mobility.

A full and detailed transient space-charge-limited current (T-SCLC) study of a-Si:H p-i-n diodes has been carried out in the time range from 108s to 10s. In the short-time regime, general features of T-SCLC such as the current cusp and the carrier extraction period were observed, and related transport parameters were deduced. Electron emission from deep states was studied by measuring the current transients well beyond the extraction time. The emission time is thermally activated at temperatures higher than 250K and levels off at lower temperatures. The high temperature behaviour places the upper edge of the deep states at 0.42–0.52eV below the conduction band edge, and the attempt-to-escape frequency in the range of 1011-1013Hz. An observed shift of emission time with light intensity is attributed to defect relaxation.

Amorphous hydrogenated p-i-n diodes submitted to a high reverse bias for a long period of time undergo a metastable evolution which tends to improve their properties. A forming procedure based on this effect has been developed and leads to a significant decrease of the reverse current and increase of the breakdown voltage. In this paper, the mechanisms underlying this forming process have been investigated using the Constant Photocurrent Method (CPM), (I-V) characteristics and Spectral Response under reverse bias. The effects of annealing and current induced defect creation have been studied. The forming process is found to be most consistent with a metastable increase in the dopant activation of the p-layer and a small decrease of the density of states in the i-layer.

Electrically detected magnetic resonance (EDMR) - also often referred to as spin-dependent photoconductivity - is a particularly attractive technique for the investigation of the electronic properties of semiconductor devices. This method detects the paramagnetic states involved in transport and recombination by recording changes of the sample current induced by a resonant enhancement of the recombination rates. This is in contrast to ESR which monitors the resonant enhancement of microwave absorption. Therefore, EDMR is more sensitive by many orders of magnitude than conventional ESR and can be used to investigate a-Si:H solar cell devices under standard operating conditions.

In this report we review the EDMR technique and discuss, in detail, the spin-dependent processes controlling dark and photocurrent of standard a-Si:H pin solar cells. In particular, we show that EDMR is sensitive to changes in the internal field of the i layer. We compare the experimental results to numerical calculations of the potential profiles and of EDMR-related quantities. We arrive at the conclusion that degradation by both high forward current and illumination increases the defect density mainly in the i layer, rather than at the pi interface.

The subgap absorption αcpm measured by the constant photocurrent method (CPM) was studied between 4.2K and 300K for different dopings and defect concentrations N in hydrogenated amorphous silicon (a-Si:H). We found that αcpm is reduced by up to a factor 100 between 50K and 200K due to infrared selfquenching which changes the charge state of the defects and enhances recombination. The effect is diminished at high N and large doping. The relative change Δα/α reflects ΔN/N both at very low and high temperatures. For undoped samples αcpm is about four times larger at 4.2K than at 300K. For doped samples the two values are essentially the same.

The room temperature hole drift mobility measured by the ‘time-of-flight’ technique can be increased to 0.5 cm2/Vs with a valence band tail width of 25meV by optimizing the ion impingement energy during film growth using a triode configured PECVD reactor. The reverse bias dependence of the carrier collection in Au Schottky diodes formed on these specimens show significantly improved carrier collection which can be approximately quantified using the C-V measured voltage dependent depletion widths to yield μτ≈10−6cm2V−1.

Hydrogenated amorphous silicon (a-Si:H) films prepared by the glow discharge (GD) technique show superior optoelectronic properties over those prepared by rf sputtering (RFS). To find out whether this is associated to structural differences in the amorphous network, we have carried out a comprehensive comparison of the Raman spectra of the two types of films grown at different substrate temperatures. The use of two properly chosen excitation radiations allowed the observation of the Raman spectra from the near surface versus that from the bulk of the films. The results show that the short-range order in the bulk of GD films is close to that of the ideal tetrahedral network, having an rms bond angle deviation (Δθ) of ≈9°. In contrast, the smallest value of Δθfound in the RFS films was ≈15°. There is also a short-range order inhomogeneity in both sets of materials that can be reduced significantly by selecting the appropriate substrate temperature. The intermediate-range disorder is relatively small and uniform in GD films, while large differences exist in this parameter between the surface and bulk of RFS films. In general, the results indicate that the short-range order and the inhomogeneity in intermediate-range order present in the RFS films cannot be improved to equal those of GD materials by annealing at temperatures low enough that no substantial hydrogen effusion occurs. These structural differences are argued to be the reasons for the superior phototransport properties of GD over RFS materials and are interpreted in terms of the differences between the two deposition processes.

Dual beam photoconductivity with bandgap primary light and hv = 0.4- 0.6eV infrared light steps was measured with Ims time resolution in hydrogenated amorphous silicon (a-Si:H) at 4.2K. The results can be described by assuming that the photocurrent transients are due to energy-loss hopping of photocarriers and that the infrared light promotes recombination by reexciting photocarriers thereby enhancing the probability of tunneling recombination.

The characteristics of a-Si:H based switches, as used in liquid crystal displays, degrade as a result of current injection. To assess this degradation we investigated the effects of current injection on the electron mobility, lifetime and the built-in electric field profile in a-Si:H Schottky diodes.

We present steady-state current-voltage and transient photocurrent (TP) measurements on Mo and Pd a-Si:H Schottky diodes. The mobility, lifetime and electric field profile were determined from TP measurements. The electric field profile was calculated by fitting simulations from a non-linear model of the measurement to the measured TP currents.

The electron drift mobility is not affected by current stressing, the electron lifetime reduces slightly, the built-in voltage decreases significantly. The widths of the built-in fields of Schottky and ohmic back contact are reduced with approximately the same factor, corresponding with a spatially homogeneous increase of the defect density.

The application of time-of-flight (TOF) in the space charge limited current (SCLC) mode to a-Si:H pin-diodes has led to the discovery of new features in the current transients. An alternative to analytic models, our numerical modelling describes the specific nature of amorphous semiconductors by taking into account the interaction of free carriers with tail states and the contribution of trapped carriers to the space charge. Typical features of experimental SCLC-TOF-currents such as a cusp in the transient can be reproduced. We confirm the concept of ‘effective thickness’ for the determination of the transit time. At high injection intensity only a small fraction of the charge can be collected. Bimolecular recombination via tail states and surface recombination is identified as a loss mechanism that competes with recombination via dangling bonds. The post-transit behaviour of electron-TOF is dominated by holes emitted from the valence band (vb) -tail. The density of states distribution from post-transit-spectroscopy (PTS) shows features of the vb-tail. Finally, hole SCLC-TOF shows a different behaviour from electron SCLC-TOF.

Transient photoconductive response of undoped a-Si:H has been studied; the changes were analysed between two slightly different steady-state illumination conditions, at room temperature. A theoretical model is developed to describe transient photoconductivity; it yields good agreement with the measured curves for a whole range of light intensities. Numerical evaluations allows one to extract the recombination time of electrons. Comparison with steady-state photoconductivity yields a band mobility of free electrons between 0.1 and 6 cm2V−1s−1, depending upon sample quality.