The problems of defining an electronic density of states for amorphous semiconductors are summarized. A new equilibrium statistical-mechanical model for the structure and electronic properties of disordered systems is discussed and applied to the case of hydrogenated amorphous silicon (a-Si:H). The difficulty in treating non-equilibrium effects is briefly analyzed, and some of the unresolved controversies with regard to the density of states in a-Si:H are enumerated.

This paper reviews various experiments that give information about the density of states in hydrogenated amorphous silicon (a-Si:H). The data are used to construct the density of states distribution N(E), and its dependence on doping. The structural origin of the different groups of states is discussed, and it is also shown that N(E) depends on the thermal history of the samples.

The electron time-of-flight characteristics of un-alloyed a-Si:H and of a-Si,Ge:H,F alloys were studied. The log I -log t characteristics of the alloys are highly dispersive and include anomalous features. The characteristics depend strongly on measurement temperature, becoming more regular as the temperature increases. Substantial differences between low-gap alloys with comparable optical gaps are observed. Computer simulation suggests that an extra density of deep tail states, or the midgap states, are responsible for the anomalous characteristics.

Electron spin resonance (ESR) is an important tool for providing detailed microscopic information about electronic states located within the energy gap in hydrogeated amorphous silicon (a-Si:H). There always exists an ESR signal in a-Si:H on the order of 1015 spins cm-3 which is attributed to silicon dangling bonds. After optical excitation with band-gap light, a metastable increase in the dangling bond ESR signal is observed. After rapid quenching of a sample from -200°C to room temperature, there is also an increase in the ESR signal with time (without optical excitation) as the sample approaches thermal equilibrium.

We present experimental results on the transient response of i/n/i hydrogenated amorphous silicon (a-Si:H) structures as a function of voltage. Comparison with M/i/n devices and results from a novel double-pulse experiment show that the transient current is initially space-charge limited and later limited by emission from deep states in the injecting n layer. Theoretical analysis of these results suggests the existence of a peak in the density of states of As-doped a-Si:H located within 0.3 eV of the conduction-band mobility edge.

Transient photocurrent techniques for the characterization of amorphous semiconductors are reviewed; the techniques are based on the assumption that the photocarrier dynamics have the multiple-trapping form. The application of these techniques to five properties of defects in amorphous hydrogenated silicon (a-Si:H) are reviewed. The five properties are: the attempt-to-escape frequency, the photocarrier diffusion length, the total trap density, the capture coefficient-diffusion constant ratio, and the density of states. A discussion of the role of time-of-flight determinations in transient photocurrent characterization is given.

The distribution of deep states in undoped a-Si:H is investigated. For this purpose, a new experimental technique is introduced which is based on the principle of thermally-stimulated space-charge relaxation. A peak in the density of states is resolved at - 0.6 eV below the mobility edge of the conduction band. The energy position and magnitude of this peak are found to be consistent with space-charge-limited current measurements.

Space charge limited current measurement has emerged as a simple dependable spectroscopy to study the gap states in a-Si:H. In this paper we discuss this measurement along with its sensitivity, advantages and disadvantages.

We have studied the effect of light and electron irradiation on the density of states (DOS) of intrinsic a-Si:H films prepared by magnetron sputtering. The DOS of the films prepared under optimum conditions was obtained by space-charge-limited current method. The generation of metastable defects by light soaking and KeV electron irradiation has been directly observed by the change in the DOS. The defect creation by electrons is a factor of 3300 higher than that of light for the same deposition energy. Also the generation process is found to be different.

We briefly review dispersive transport and treat in more detail the multiple-trapping mechanism. We consider the problem of transient current spectroscopy: Given the experimental current I(t) how does one extract a distribution of rates? Extensive new measurements were made in a well studied polymeric material. Photocurrents in polyvinylcarbarzole were studied over 10 decades in time. An analytical approach is proposed for extracting rate distributions from the measured photocurrents. We find that the trapping-rate distribution is not exponential but "flat" and that it shows a cutoff at low rates. This distribution gives rise to the novel feature of a gradual transition from dispersive to nondispersive charge transport during a single transit. The temperature and field dependence of the cutoff rate rc was studied in the framework of a Poole-Frenkel model.

A discrete version of the multiple trapping transport model for amorphous semiconductors is presented and its application to transient photocurrents demonstrated. The relationship between the average energy of a thermalizing carrier distribution and the demarcation energy of the TROK formalism is discussed in connection with structured density of states distributions. A novel way to calculate the transit time in a time-of-flight simulation is introduced and subsequently used to examine drift mobilities for various a-Si:H inspired density of states models.

Several time-resolved experimental techniques have been applied to a-Si:H based thin films and diodes to determine carrier ranges, junction field profiles and charge storaae times. The results on p-i-n type diodes are compared to a theoretical model of a-Si:H so-lar cells. Electrochemical surface treatment of Schottky-barrier type devices indicates that improved carrier transport is obtained for interfaces from which oxide related defects are removed. For the case of thin a-SiC:H alloy films it is shown that deep trapping gives rise to significant space charge build-up which can be used for charge storage applications.

The isothermal dark decay of open circuit surface voltage measured on a composition series (0-14 at % Te) of capacitively charged glassy Se:Te alloy films is analyzed and compared with similar measurements on halogen and alkali doped a-Se films. It is demonstrated that in both materials systems dark decay is controlled by thermal emission of one carrier type from states (emission centers) deep in the mobility gap leading to the progressive buildup of a uniform deeply trapped space charge of opposite sign -a process called depletion discharge. From analysis of the parameters of the depletion discharge process compositionally induced changes in dark decay can be related directly to changes in the density of states (DOS) controlling emission. Whereas alloying of Se with Te is thus found to induce major alteration of the DOS wrt Se the doping of Se operates to specifically alter only the relative populations of certain discrete states near midgap. The experiments further indicate that the mechanism of doping involves the interaction of dopant atoms with native alternatively charged and diamagnetic defect pairs.

Hole transport in polyvinylcarbazole (PVK) has been the subject of many previous investigations. Earlier workers have reported significant differences between their measurements and those predicted for the continuous time random walk model (CTRW) of dispersive transport in amorphous systems. The experimental results reported here are, contrary to these earlier experimental findings, in good agreement with dispersive tranport theory. This good agreement is only found, however, using exciting light intensities two orders of magnitude below values conventionally used. In contrast to earlier results we find a single temperature independent vaule of a equal to 0.66 (±0.05). This temperature independence of a implies that hole transport is determined solely by the geometry of the amorphous polymer and not by a Boltzmann equilibrium among energy levels of the disordered system.

Polysilylenes, organic polymers with silicon backbone, represent a new class of dielectric materials capable of transporting injected holes. Charge transport in poly(methylphenylsilylene), a typical representative of these polymers, is thermally activated and relatively non-dispersive over a wide range of temperatures. The carrier drift mobility is high for organic glasses, near 10-4 cm2N.s. at E = 105 V/cm, and field dependent at T < Tg. The charge transport in polysilylenes has most of the characteristics of hopping transport among discrete states, but it is insensitive to the nature of the side groups (aromatic or aliphatic) and the molecular weight of the polymer. The proposed mechanism of transport is that of hopping among rather short segments of the silicon chain.

We have developed a computer model to describe the steady-state behaviour of a range of amorphous silicon devices. It is based on the complete set of transport equations and takes into account the important role played by the continuous distribution of localized states in the mobility gap of amorpl iL,us silicon. Using one set of parameters we have been able to self-consistently simulate the current-voltage characteristics of p-i-n ( or n-i-p) solar cells under illumination, the dark behaviour of field-effect transistors, p-i-n diodes and n-i-n diodes in both the ohmic and space-charge limited regimes. This model also desribes the steady-state photoconductivity of amorphous silicon, in particular, its dependence on temperature, doping and illumination intensity.

A detailed numerical model incorporating exponential tail states and Gaussian-distributed dangling bond states and doping states in the gap is used to compute the performance characteristics of thin film Si:H solar cells. A one-to-one relationship between four-fold coordinated doping atoms and dangling bonds is included. A good match to experiment occurs if the front contact is treated as non-ohmic (i.e. with a non-infinite surface recombination velocity) and a critical capture cross section is taken much smaller than expected. The numerical model is also used to compute the dependence of dark conductivity on doping, which exhibits saturation at high doping levels as observed.

Recent advances in physics for submicron, bipolar-crystalline devices suggest principles that are valid when modeling bipolar devices with noncrystalline regions such as those with polysilicon, polycrystalline silicon, and hydrogenated amorphous silicon emitters. These principles from crystalline device physics are summarized, and their implications for the noncrystalline regions of bipolar devices are given.

A simplified analytical model that describes the forward-bias current flow in recombination-limited p/i/n structures is described and applied to amorphous silicon alloys. The results obtained from this model reproduce those from more exact computer simulations. The analytic solution, however, elucidates the general form of the dependence of the current upon such parameters as the density of states and the recombination kinetics. We show that the current depends primarily on the ratio of the free-carrier mobilities to the recombination and trapping rate constants and on the energy dependence of the density of states.

a-Si:H TFT technology has been used in large-area linear array applications. Using large-area photolithography, these arrays can be batch produced. Applications to image scanners and electronic printers will be described. Generic processing issues and future development trends will be discussed.

Performance of tandem and triple multijunction solar cells fabricated from 1.7 eV a-Si:F:H alloy as well as in conjunction with 1.5 eV a-Si:Ge:F:H alloy will be presented. Using the 1.7 eV a-Si:F:H alloy, we have achieved a 12.07 conversion efficiency for the tandem and triple structures. Spectrum splitting dual band gap tandem and triple structures incorporating both the 1.7 eV and 1.5 eV materials give rise to efficiencies of 12.5% and 13.0%, respectively. Stability data for the tandem and triple configurations will be presented.