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After exposure of amorphous selenium islands to luminous ux of 1¢10 6 lx, they do not become crystalline, but remain amorphous. However, light stimulates polymerization of molecules in those islands. For this reason, heating of a layer that is exposed to light does not cause a decrease of the mean thickness of the layer, but heating of the layer in the dark causes a decrease of its mean thickness. Therefore, the mean thickness of the illuminated layer is 3 to 4 times greater than the mean thickness of the layer in the dark. In addition, the fraction of the substrate covered by the islands is 1.5 to 2 times larger in the exposed layer than in the unexposed layer. However, illumination of a continuous layer speeds up its transformation into an island-type layer. Therefore, in order to obtain an extremely thin continuous layer of amorphous selenium, during growth of the layer it must be exposed to light only until it becomes continuous. Starting from that moment, the light must be switched off.

It is shown that the parameters and characteristics of a dielectric layer can be determined by the periodic dosed deposition of a charge on the layer surface combined with the synchronous measurement of the accumulated charge and surface potential, as well as by measuring the surface potential when the layer is both freely discharged and discharged when exposed to light. The measurements data are used to plot the surface potential of the layer versus the charge density, as well as to find the dependences of the effective capacitance, differential capacitance, and leakage resistance on the surface potential. It is demonstrated that the characteristics thus obtained may provide an insight into the physics of processes occurring in the layer; determine time parameters, such as the half-value periods of the potential in the dark and under exposure to light; and find the capacitance, limiting value of the surface potential, permittivity, leakage resistance, and photosensitivity of the layer.

A method of determination of charge carrier lifetime from experimental data of small charge current transient under constant electric field strength is discussed. On the basis of experimental results on transient of small charge photocurrent of holes and electrons which were obtained at different electric field strengths, an analysis of charge carrier lifetimes in a-Se layer has been performed. For interpretation of experimental results the approximation, according to which photogenerated electrons or holes can be trapped in levels of three types, has been used. We found that the process of localized charge carrier release is influenced by the electric field strength and that in a-Se layer, in the vicinity of the substrate, there is a narrow region where the density of localized states exceeds its mean in a layer.

Single and double photoinjections prove to be a useful tool for investigations of recombination mechanisms. The photodischarge of selenium electrophotographic layers due to double and single photo-injections has been investigated. Experimental results show that every photoproduced pair of free carriers either recombines independently of the other free carriers or takes part in the photodischarge without subsequent recombination. This recombination explains energy-dependent quantum efficiency in the 2.5-eV (and greater) spectral region.

This chapter discusses the results of the change of electrical features in the electric fields of amorphous selenium (a-Se) and noncrystalline semiconductor As–Se and Ga (or In)–Te-based structures. The features of these compounds are exposed under the influence of various parameters, including the chemical composition of noncrystalline materials, lattice temperature, the regime of heat treatment, and doping impurity. Emphasis is on the multiplication effect, nonactivated conductivity, formation of space charge, and influence of potential barriers on the strong-field conductivity. Before proceeding to the detailed description of the physical phenomena, the main features of the materials under investigation are presented in the chapter. The chapter concludes that in the field and temperature dependences of conductivity of noncrystalline films of Ga-Te systems in high (pre-breakdown) fields, there appears the features of the Poole-Frenkel effect in the case of isolated and screened impurity centers, as well as in the delocalization of shallow local states that are typical of structures based on chalcogenides glasses. There are also signs of multi-phonon tunneling under conditions of nonuniform field distribution.

The absorption of light, the luminescence, the spectral distribution of the photoconductivity and the drift mobility of charge carriers in a new organic semiconductor poly-N-epoxypropylcarbazole have been investigated.

The breakdown of a selenium electrophotographic layer (EPL) often occurs during its charging process. The amount of surface charge flowing down a conductive filament may be determined from the current kinetics of layer charging. The active area of the conductive breakdown filament has been determined from the potential distribution and the spots of the developed image. Using an electron microscopy method the investigations of thin selenium sections have shown that under the influence of local Joule's heating selenium in the conductive filament melts; and immediately after the charging it cools off transforming into the vitreous phase. Numerous breakdown at the same place lead to the formation of a crystalline filament. It has been established that under influence of the tangential component of electric field, surface conductive filaments are formed around the conductive bulk filament: this is the main difference between breakdown in a system of free surfaces and breakdown or switching in a system with electrodes.

Using electron microscopy methods it has been determined that there are numerous crystalline inclusions in evaporated layers of amorphous selenium. The density, dimensions, modification and distribution in the cross section of a layer are essentially dependent upon the method of layer preparation. The maximum micro-inclusion density is observed in layers evaporated at a substrate temperature ( t s ) between 8 to 30°C. These layers also possess crystallites of monoclinic modification. In layers evaporated at t s ⩾ 50°C inclusions of trigonal modification prevail. The parameters of the deep trapping levels for holes and electrons have been determined from photodischarge kinetics. The origin of these levels is found to be related to the phase transition. The hole traps are caused by the crystalline formation of trigonal modification, and the electron traps by crystallites of monoclinic modification.

The process of sensitivity reduction (fatigue) in amorphous selenium layers irradiated by X-rays has been considered taking into account carrier photogeneration, trapping, and recombination. Analytical expressions describing the kinetics of carrier trapping at the levels responsible for a fatigue have been obtained. Formulas to determine a sensitivity of both fatigued and unfatigued layers have been derived. Computed and experimental dependences of sensitivity upon the layer thickness and the hardness of X-rays are in a good qualitative agreement. Theoretical dependences of fatigue vs. irradiation dose, layer thickness, electric field, and density of traps correctly reflect analogous experimental regularities. It has been established that the fatigue dependence on the irradiation dose rate is determined by the carrier lifetime at the traps responsible for a fatigue process. Comparing theoretical results with experimental data on fatigue, the density of traps responsible for a fatigue process have been determined as well as values of recombination and trapping coefficients in electroradiographic selenium layers. Es wird der Prozes der Abnahme der Photoempfindlichkeit (Ermudung) von mit Rontgenstrahlen behandelten amorphen Selenschichten untersucht, wobei Ladungstragererzeugung, Trapping und Rekombination berucksichtigt werden. Analytische Ausdrucke werden erhalten, die die Kinetik des Anhaftens der Ladungstrager an den fur die Ermudung verantwortlichen Niveaus beschreiben und es werden Formeln abgeleitet, die die Empfindlichkeit sowohl von behandelten als auch von unbehandelten Schichten bestimmen. Die berechneten und gemessenen Abhangigkeiten der Empfindlichkeit von der Schichtdicke und der Harte der Rontgenstrahlen befinden sich in guter qualitativer Ubereinstimmung. Die theoretische Abhangigkeit der Ermudung von der Bestrahlungsdosis, Schichtdicke, dem elektrischen Feld und der Trapdichte geben die experimentellen Beobachtungen richtig wieder. Es wird gefunden, das die Ermudungsabhangigkeit von der Bestrahlungsdosis durch die Lebensdauer der Ladungstrager an den fur den Ermudungsprozes verantwortlichen Haftstellen bestimmt wird. Durch den Vergleich der theoretischen Ergebnisse mit experimentellen Werten uber die Ermudung werden sowohl die Dichte der fur den Ermudungsprozes verantwortlichen Haftstellen als auch Werte des Rekombinations- und Trappingkoeffizienten in elektroradiographischen Selenschichten bestimmt.

Equations of the longitudinal photocurrent in a layer having arbitrary distributed trapping levels of one type and non-uniform field are solved. Expressions for the time dependence of the photocurrent in the form of formulae and curves are presented. The procedure for the determination of the dependence of the charge carrier lifetime and the field strength on the coordinate from experimental data of the drift current of a small charge is described. The lifetime of holes and electrons in selenium amorphous layers has been experimentally found to depend substantially on the manufacturing process: increasing the substrate temperature during vacuum evaporation the lifetime of charge carriers increases. As the concentration of As in amorphous As-Se layers is increased the lifetime of charge carriers rises to several ms. The lifetime of charge carriers in amorphous selenium layers has been found to increase with the distance from the layer surface. In As2Se3 layers a negative space charge is formed on applying the voltage, and the density of the charge formed substantially increased towards the anode. Gleichungen des longitudinalen Photostromes in einer Schicht mit willkurlich verteilten Haftstelleniveaus einer Art und nichtgleichformigem Feld werden gelost. Ausdrucke fur die Zeitabhangigkeit des Photostromes in Form von Formeln und Kurven werden angegeben. Das Verfahren fur die Bestimmung der Abhangigkeit der Ladungstragerlebens-dauer und der Feldstarke von den Koordinaten aus den experimentellen Werten des Driftstromes einer kleinen Ladung wird beschrieben. Es wird gefunden. das die Lebensdauer der Locher und Elektronen in amorphen Selenschichten wesentlich vom Herstellungsprozes abhangen: eine Steigerung des Substrattemperatur wahrend der Vakuumverdampfung erhoht die Lebensdauer der Ladungstrager. Wenn die Konzentration von As in amorphen As-Se-Schichten erhoht wird, steigt die Lebensdauer der Ladungstrager auf einige ms. Es wird gefunden, das die Ladungstragerlebensdauer in amorphen Selenschichten von der Oberflache nach innen zunimmt. In As2Se3-Schichten wird eine negative Raumladung bei Anlegen der Spannung gebildet, die Dichte der gebildeten Ladung nimmt zur Anode hin wesentlich zu.

The distribution of both the amorphous and crystalline phase has been studied in the cross-section of electrophotographic and electroradiographic layers of Se by the method of electron microscopy and microelectronography. The dependence of the amount of the crystalline phase, its modification and morphology on the substrate temperature during the evaporation of electrophotographic and electroradiographic layers has been investigated. Es werden elektronenmikroskopische und mikroelektronographische Untersuchungen der Verteilung von amorphen und kristallinen Phasen des Selens im Querschnitt von im Vakuum aufgedampften elektrophotographischen und elektrorontgenographischen Selenschichten durchgefuhrt. Es wird der Anteil der kristallinen Phase, ihre Modifikation und Morphologie und ihre Verteilungen im Querschnitt dieser Schichten in Abhangigkeit von der Substrattemperatur untersucht.

Single and double photoinjections prove to be a useful tool for investigations of recombination mechanisms. The photodischarge of selenium electrophotographic layers due to double and single photo-injections has been investigated. Experimental results show that every photoproduced pair of free carriers either recombines independently of the other free carriers or takes part in the photodischarge without subsequent recombination. This recombination explains energy-dependent quantum efficiency in the 2.5-eV (and greater) spectral region.