Amorphous semiconductors studies

The few surface studies reported suggest the presence of a considerable density of surface and gap states in the amorphous semiconductors studied. Kastner and Fritzsche (1970) found that one monolayer of H2O adsorbed on a 1000 A thick film of chalcogenide alloy increases its conductance by less than one percent. Amorphous Ge evaporated at room temperature is porous to H2 0 so that a large area of internal surfaces can be covered with water. A density of 5 X 10 H2 0/cm absorbed in a 1000 A thick Ge film produced an increase in conductance by only 10 percent. These small changes contrast strongly with the behavior of crystalline semiconductors. They suggest a large density of surface and gap states. 

This volume reviews experimental and theoretical studies that have been performed primarily during the last decade on the effects of hydrogen in both crystalline and amorphous semiconductors. The authors of the individual chapters were encouraged to adopt a tutorial format in order to make the volume as useful as possible, both to graduate students and to scientists from other disciplines, as well as to the active participants in this exciting arena of semiconductor research. We anticipate that this volume will prove to be an important and timely contribution to the semiconductor literature. 

A turning point in the study of amorphous semiconductors was reached with the discovery that the addition of hydrogen to amorphous silicon could dramatically improve the material s optical and electrical properties. Unlike pure amorphous silicon, which is not photoconductive and cannot be readily doped, hydrogenated amorphous silicon (a-Si H) displays a photoconductive gain of over six orders of magnitude and its dark conductivity can be changed by over ten orders of magnitude by n-type or p-type... 

In crystalline semiconductors, the most common technique for the measurement of carrier mobility involves the Hall effect. However, in noncrystalline materials, experimental data are both fragmentary and anomalous (see, for example. Ref. [5]). Measured HaU mobility is typically of the order of 10 - 10 cm A /s and is frequently found to exhibit an anomalous sign reversal with respect to other properties providing information concerning the dominant charge carrier. Thus, apart from some theoretical interest, the Hall effect measurements are of minimal value in the study of macroscopic transport in amorphous semiconductors. 

The most familiar application of amorphous semiconductors will, for many readers, be in the field of replication of printed matter. The xerography process, npon which many modem photocopiers are based, involves the ability of an electrostatically charged plate of amorphous chalcogenide (or similar material) to discharge under illn-mination. Residual charging of illuminated areas is employed in the transfer of ink onto the duplicator paper. Naturally, the mobility of photoinduced carriers in the amorphons semiconductor photoreceptor is of central importance in the validity of the process, and considerable commercial effort has been (and is being) devoted to the study of transport in disordered materials suitable for the process. 

The charge transport in amorphous selenium (a-Se) and Se-based alloys has been the subject of much interest and research inasmuch as it produces charge-carrier drift mobility and the trapping time (or lifetime) usually termed as the range of the carriers, which determine the xerographic performance of a photoreceptor. The nature of charge transport in a-Se alloys has been extensively studied by the TOF transient photoconductivity technique (see, for example. Refs. [1-5] and references cited). This technique currently attracts considerable scientific interest when researchers try to perform such experiments on high-resistivity solids, particularly on commercially important amorphous semiconductors such as a-Si and on a variety of other materials... 

In this section, we propose that the transit time of transport photocarriers can be obtained by an analysis of transient current in a double layer that consists of a thin chalcogenide nnder test and another material with higher mobility snch as a-Se. Figure 4.19(a) shows the type of carrier transit pulse that is frequently enconntered in the study of amorphous semiconductors. 

The rate of decay and the temperature dependence of the saturated voltage can be used to obtain the concentration and energy distribution of the deep traps responsible for the residual potential. Thus, provides a useful means of studying the nature of deep traps in amorphous semiconductors and has been successfully used to derive the energy distribution of deep localized states in the mobility gap of both a-Se and a-Si H [10,18],... 

Spectroscopic Studies of Gap States and Laser-Induced Structural Transformations in Se-Based As-Free Amorphous Semiconductors... 

G. Miller, S. Kalbitzer, and G. N. Greaves, Ion Beams in Amorphous Semiconductor Research J. Boussey-Said, Sheet and Spreading Resistance Analysis of Ion Implanted and Annealed Semiconductors M. L. Polignano and G. Queirolo, Studies of die Stripping Hall Effect in Ion-Implanted Silicon J. Sroemenos, Transmission Electron Microscopy Analyses... 

The critical value of VJB for complete localization is about three. Since the band widths are of order 5 eV, a very large disorder potential is needed to localize all the electronic states. It was apparent from early studies of amorphous semiconductors that the Anderson criterion for localization is not met. Amorphous semiconductors have a smaller disorder potential because the short range order restricts the distortions of the bonds. However, even when the disorder of an amorphous semiconductor is insufficient to meet the Anderson criterion, some of the states are localized and these lie at the band edges. The center of the band comprises extended states at which there is strong scattering and... 

The existence of localized states was predicted early on in the studies of amorphous semiconductors by the Anderson localization theory (Section 1.2.5) and their presence is well established ex-... 

The growth of a multilayer structure creates a new material with an imposed periodicity of the layer spacing. Such structures are familiar in crystalline semiconductors GaAs/GaAlAs multilayers have been particularly widely studied. Both the electronic and vibrational states of the material are inhuenced by the multilayer structure. The quantum conhnement of the electrons and holes in the narrow wells of the multilayer is of special interest because it is not obvious that such effects can occur in amorphous semiconductors. Quantum effects require that the coherence length is larger than the size of the confining well. The short mean free path of the carriers in a-Si H implies that quantmn effects are observed only in very narrow wells. 

The threshold switching in melanins and melanosomes, a rather exotic property of amorphous semiconductors, was studied by McGuiness et al.,... 

Hall effect is another important transport phenomenon and has been extensively studied in amorphous semiconductors. The Hall effect studies also assumed importance because of an anomaly observed between the sign of the charge carriers indicated by Hall coefficient and S in amorphous semiconductors. The Hall coefficient Rh is given by. 

In the following sections we shall describe the field-effect technique in some detail and present a sample of the experimentally determined densities of states. Some of the issues related to the overall sensitivity and reliability of the FE method to determine a bulk density of states in a-Si H will be examined. We shall also mention some of the recent applications of the FE technique to the study of a great variety of phenomena in a-Si H and related materials. Indeed, in spite of their shortcomings, FE measurements continue to be widely applied in the study of amorphous semiconductors and hence still qualify as one of the primary techniques to determine g(E) in these materials. 

In the following discussion we shall examine some of the various ways in which junction capacitance measurements for an amorphous semiconductor may be interpreted. Experimental results for a-Si H are presented for several representative studies mentioned above. We shall then discuss the limitations of capacitance techniques to deduce densities of states and also mention some of the capacitance profiling techniques that have been used to aid in the interpretation of capacitance measurements. 

Advantages and Limitations of DLTS in the Study OF Amorphous Semiconductors... 

Abstract SIESTA was developed as an approach to compute the electronic properties and perform atomistic simulations of complex materials from first principles. Very large systems, with an unprecedented number of atoms, can be studied while keeping the computational cost at a reasonable level. The SIESTA code is fi-eely available for the academic community (http //www.uam.es/siesta), and this has made it a widely used tool for the study of materials. It has been applied to a large variety of systems including surfaces, adsorbates, nanotubes, nanoclusters, biological molecules, amorphous semiconductors, ferroelectric films, low-dimensional metals, etc. Here we present a thorough review of the applications in materials science to date. 

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