Voids in amorphous semiconductors

Small angle X-ray scattering (SAXS) has been used to investigate density fluctuations occurring in amorphous semiconductors prepared by sputtering, evaporation and electrodeposition. Correlations of the total number of dangling bonds determined by SAXS, optical absorption, and ESR signals have been made. The density deficits from the density of the FC-2 crystal are in some cases accounted for by the voids. It is argued that models based on domains 10–15 Å in size are not supported by the SAXS data.     

Switching in amorphous semiconductors

This article presents experimental data on switching and other related properties of (1?x)As₂Se₃·xSb₂Se₃ materials. These data indicate that the threshold switching mechanism is electronic in thin devices (<10 μm) and thermal in thick devices. In addition, it raises some questions about the crystallization/phase separation model for memory switching.     

Optical properties of amorphous semiconductors

The density of states and the dielectric constant of electrons in crystalline solids can be discussed easily by using the band structure scheme. This is due to the fact that a crystal is defined by both a short range order and a long range order of atoms. If the long range order is relaxed this procedure no longer works and one has to calculate the measurable quantities directly, which involves some configurational averaging process.     

Avalanche ionization in amorphous semiconductors

Using the results of a previous treatment of hot electrons in amorphous semiconductors, the avalance ionization coefficient, α, has been calculated as a function of the electric field. The Shockley hot-electron model was used. Because there is no conservation of crystal momentum, the threshold for electron-hole pair production is lower than it would be in a crystal of the same energy gap, and above the threshold the probability per unit time for pair production is quadratic in the energy. The results for α has been used to calculate the current-voltage relation for avalanche injection, and to study the avalanche process as a mechanism for switching.     

Thermoelectric power of amorphous compound semiconductors

Thermoelectric power measurements as a function of temperature have been made on four V–VI amorphous chalcogenides, ten IV–VI amorphous chalcogenides of the Ge_xTe_1?x system, and five amorphous IV–V materials. In the IV–V and the intermediate composition IV–VI materials, the charge transport cannot be described on the basis of conduction at only one energy level. The former exhibit characteristics of conduction both in extended states and in localized states at the Fermi level. Transport in the V–VI materials can be formally described in terms of conduction at one level in either the chaotic band or the small polaron models, but the use of a two-channel model with transport simultaneously in both p-type extended and localized states seems the most promising.     

Pressure-induced structural transformations in amorphous semiconductors

Pressure-induced amorphous-to-crystalline transformations in Ge, As and Se have been studied thermodynamically and microscopically. A thermodynamic analysis based on experimental data suggests that the structural transformations in Ge and Se can be regarded as a phase transition and a structural relaxation, and that the transformation in As may be relaxational. There is a correlation between the transformation pressure and the covalent bond energy, but such a correlation cannot be seen in the thermal crystallization temperature.     

The structure of tetrahedrally coordinated amorphous semiconductors

The diffracted X-ray intensities, F(k)'s, and radial distribution functions of Ge, some III–V compounds, and Ge_0.5Sn_0.5 are presented. It is shown that the radial distribution functions of the III–V compounds are similar to those of Ge. If the continuous random network, as developed for amorphous Ge, is also applicable to the III–V's, then several bonds between like atoms (wrong bonds) must be present. It is argued that the energy of wrong bonds may not be sufficiently high to prohibit their formation.     

High E-field microwave properties of bulk amorphous semiconductors

The effects of high microwave electric fields on bulk amorphous semiconductors were studied at room temperature. Samples of eight different glasses were mounted in the center of a reduced height, X-band waveguide in a sandwich geometry parallel to the electric field of the dominant mode. Samples were 90 mil (2.286 mm) square by 8.5 mil (0.2159 mm) thick with thin-film CrAu electrodes. The microwave conductivity remained constant at field strengths below a critical value near 10⁴V/cm. Above this value, which depends upon the microwave pulse length, switching to a low-resistance state was observed with the thermal breakdown as the switching initiation mechanism.     

Experimental evidence of energy-controlled switching in amorphous semiconductors

The switching delay time and transition time, and threshold voltage for the onset of switching in the amorphous semiconductor Si₁₂Ge₁₀As₃₀Te₄₈ have been measured under various conditions using rectangular voltage pulses. The results show that both the threshold voltage and the delay time decrease, but the transition time increases with increasing temperature; and that these switching properties are strongly dependent on the width and the repetition rate of applied pulses. It is proposed that the delay time is associated with the time required for the formation of a filament to cause switching, and that the transition time is associated with the transit time of a carrier across the switching filament. All the experimental phenomena indicate clearly that the switching process is energy-controlled.     

Photoluminescence of geminate and non-geminate pairs in amorphous semiconductors

Rates of radiative recombination of geminate and non-geminate pairs and the corresponding lifetimes are calculated at thermal equilibrium. It is shown that two kinds of geminate recombinations occur in amorphous semiconductors: (I) through the recombination of the usual geminate pairs (non-excitonic) and (II) through the recombination of those excitons that have relaxed to the tail states. The latter geminate pairs can exist in singlet and triplet spin states, only singlet is considered here, whereas former ones do not have spin dependence. Examples of recent observations and their comparison with the theoretical results are presented.     

Topological and electronic structure in group IV amorphous semiconductors

The effect of topological structure on the electronic structure of silicon is examined by obtaining the valence-band densities of states of four topologically distinct tetrahedrally bonded networks. The gross differences in the experimentally observed electronic structure of crystalline and amorphous Si can be explained in terms of the ring statistics of the topological structure, without reference to the angular dependence of the local interactions between bonds.     

Chemical bond approach to the electronic structure of amorphous semiconductors

The electronic structure of amorphous semiconductors with various compositions is described by the assembly of bonding, antibonding and lone-pair orbitals. Energy levels for these orbitals are calculated by extended Hückel theory. Broadening of the levels due to the interaction among chemical bonds and/or lone-pairs is determined empirically. The present calculation is free from detailed atomic structures. The changes of densities of valence states, average energy gaps and optical gaps with composition are calculated for various amorphous semiconductors. The results agree rather well with the observed ones.     

On the character of local electric fields in amorphous semiconductors

On the basis of the agreement of calculated and measured parameters of some amorphous semiconductors, an argument is stated in favour of the central-symmetric type of random local electric fields. A possible mechanism of the transport of electrons is discussed.     

Xerographic spectroscopy of gap states in Se-rich amorphous semiconductors review

One of the most important parameters which determine the performance of many modern devices based on amorphous semiconductors is the drift mobility-lifetime product, μτ. There has been much interest in determination of charge-carrier ranges in amorphous semiconductors by various measurement techniques. Although the mobility, μ, can be measured by the conventional time-of-flight transient photoconductivity technique, the determination of the lifetime, τ, is often complicated by both experimental and theoretical limitations. The present article provides an overview of xerographic measurements as a tool for studying the electrical properties of amorphous semiconductors. First, details of the experimental set-up are discussed. Thereafter, the analysis and interpretation of dark discharge, the first cycle residual potential, cycled-up saturated residual potential are considered. It is shown that from such measurements the charge-carrier lifetime, τ, the range of the carriers, μτ, and the integrated concentration of deep traps in the mobility gap can be readily and accurately determined. Xerographic measurements on Se-rich amorphous photoconductors have indicated the presence of relatively narrow distribution of deep hole traps with integrated density of about 10¹³ cm^? 3. These states are located at ~ 0.85 eV from the valence band. A good correlation was observed between residual potential and the hole range, in agreement with the simple Warter expression. The capture radius is estimated to be r_c = 2–3 Å. Since r_c for pure a-Se and a-As_xSe_1 ? x is comparable to the Se–Se interatomic bond length in a-Se, it can be suggested that deep hole trapping centers in these chalcogenide semiconductors are neutral-looking defects, possibly of intimate valence-alternation pair (IVAP) in nature. The absence of any electron spin resonance signal (ESR) at room temperature seemed to be a strong argument in favor of this suggestion. Finally, photoinduced effects on xerographic parameters are discussed. It has been shown that photoexcitation of a-As_xSe_1 ? x amorphous films with band-gap light alters deep hole and electron states. During room-temperature annealing photosensitized states relax to equilibrium. Recovery process becomes slower with increasing As content. Qualitative explanation of the observed behavior may be based on associating the deep states with C₃⁺ and C₁^?intimate-valence-alternation–pair (IVAP) centers.     

Some predicted properties of amorphous semiconductors with undulatorily-graded band edges

After a brief review of the theory proposed by Inglis and Williams¹), some properties predictable from the model are discussed. The probable origin of the graded compositional or structural inhomogeneities is considered. The temperature- and field-dependent conductivity is formulated. The application of the theory is outlined for several types of amorphous semiconductors, for an associated liquid, and for materials unidirectionally-graded undulatorily.