Principles of electrical behavior of amorphous semiconductor alloys

A new approach to understanding amorphous semiconductors is provided by a model whose novel concepts are necessary consequences of transport and recombination fundamentals. The amorphous semiconductors are examples of the ‘relaxation semiconductor’: When dielectric relaxation is slow compared with recombination, familiar transport behavior is drastically altered and the recombination characterized by cross sections for carrier capture which are extremely large. At thermal equilibrium, ‘equality recombination’ obtains, with which local space-charge neutrality is maintained through equal total capture rates of electrons and holes per unit volume. In the alloys, such recombination after single free paths gives equal electron and hole conductivities, and thus pins the Fermi level at the position of minimum conductivity for a given band gap. Both mean free paths are equal and of the order of the electron wavelength. They determine the concentrations of centers positively and negatively ionized, which are equal and large compared with mobile carrier concentrations. Nearly all the centers are neutral, are donors near the valence band and acceptors near the conduction band, and may be in fairly short band tails. The recombination is mainly at the deeper levels of the acceptor centers, which act also as shallow electron traps. On a simple and unified electronic basis, this model accounts in detail for the various aspects of observed behavior. The threshold switching is due to characteristics ‘recombinative injection’ of minority electrons. This produces a region of injected space charge in which the net recombination rate is zero, and puts at the anode an adjoining ‘recombination front’ of a type in which carriers are activated by field equal to the threshold field. Other aspects treated include, for example: the small negative Hall coefficient; the positive thermoelectric power; preswitching; delay time; integrated current or total charge during preswitching in its quantitative dependence on delay time; the ‘blocked ON state’; and the field effect, analysis of which gives the ionized concentration. The model has further implications for theory of the band structure.     

Angular dependence of low-field microwave absorption in Co-rich amorphous alloys

Microwave power absorption measurements at 9.4 GHz (X-band) were carried out on as-cast amorphous ribbons of nominal composition Co₆₆Fe₄B₁₂Si₁₃Nb₄Cu, prepared by melt spinning. The angular dependence of low-field absorption (LFA) was investigated from θ = 0° to 180° in two orientations. In both cases the ribbon axis was orientated parallel to the DC magnetic field. In orientation 1, the ribbon plane was parallel to the AC microwave field. In orientation 2, it was perpendicular to the AC field. LFA spectra showed an antisymmetric shape (a minimum and a maximum), whose separation increased as a function of the angle between the DC field and the ribbon axis. A similar angular behavior was observed for the resonance field of the ferromagnetic resonance (FMR). For the orientation 1, the resonance field increased as a function of the angle, which can be explained in terms of the contribution of the shape anisotropy to the total anisotropy field. A similar behavior was observed in LFA, which suggests that the shape anisotropy field can be observed in the angular dependence of LFA. In the orientation 2, the observed changes occur within the plane associated with the induced anisotropy due to the rapid solidification process. The LFA signal is capable therefore to detect different contributions to the total anisotropy.     

The electrical and optical properties of amorphous germanium-gallium alloys

The paper extends an earlier study of the far infrared properties of a series of evaporated amorphous GeGa alloys. The results of temperature dependent dc resistivity and near infrared and visible optical experiments are presented. Since the properties of amorphous films are dependent on the preparation conditions, a technique has been developed in which four samples of varying composition are prepared in the same evaporation. The far infrared data indicate that a significant fraction of Ga atoms dissolve in a-Ge with four-fold coordination. However, there is also indirect evidence of mixed valence.     

Impact of electrical resistance and TEP in layered SnSe crystals under high pressure

SnSe crystals belong to IV–VI layered binary semiconducting compound category. These layered compounds have generated a great deal of interest due to their interesting electronic properties. These electronic materials are useful because of their applications in holographic‐recording systems, optoelectronics and memory switching. Earlier, several investigators have studied and reported the influence of temperature and pressure over the phase transitions in SnS and SnSe semiconductors. They also utilized Mossbauer Spectroscopy and Hydrostatic Pressure techniques to examine the bonding of the Sn atoms in SnS and SnSe. It is therefore thought worthwhile to study the effect of pressure on the thermoelectric power and the electrical resistance of SnSe crystals synthesized by a modified direct vapour transport technique. The electrical resistance has been found to be pressure dependent. The transition in electrical resistance behaviour, observed at 65 Kbar, has been explained on the basis of transition from a predominantly two‐dimensional material to a more three‐dimensional one. Similar to the resistance behaviour, the thermoelectric power is also found to be pressure dependent. The increase in thermoelectric power with pressure at the transition pressure of 65 Kbar has been explained. The implications are discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the compositional dependence of the optical gap in amorphous semiconducting alloys

It is found that the optical gap E_AB for amorphous A-B alloys can be determined by the energy gap E_A for element A and E_AB for the element B in the equation E_AB(Y) = YE_A + (1?Y) E_B where Y is the volume fraction of element A. Calculations based on a random bond network agree with experiments for SiGe, SbSe, and AsTe films (class A). Calculations based on a chemically ordered bond network which tends to form microscopic molecular species gree with experimental results for the AsSe, AsS, GeTe and Sb₂Se₃As₂Se₃ systems (class B). In contrast to the above systems, agreement with experiment is not obtained for the TeSe, As₂Te₃As₂Se₃ and GeTe₂GeSe₂ systems which contain atoms of both Te and Se (class C). The classification into three types (classes A, B and C) is consistent with the calculation based on effective medium percolation theory which interprets the compositional dependence of the conductivity of chalcogenide glasses.     

A note on the temperature dependence of the electrical conductivity in amorphous semiconductors

The conductivity in many amorphous materials obeys the intrinsic semiconductor relation σ = σ₀ exp( − E_σ/ kT). This note discusses the effects that various assumptions about the temperature dependence of carrier mobility have on the relationship between the experimentally-derived value of σ₀ and the carrier mobility at a given temperature.     

Effect of metalloidal elements on corrosion resistance of amorphous iron-chromium alloys

Corrosion and electrochemical behaviours of amorphous iron-chromium alloys containing different sets of glass-forming metalloidal elements, boron, carbon, silicon and phosphorus, have been investigated. The corrosion resistance in 0.1N H₂SO₄ increases in the order of alloys containing silicon, boron, carbon and phosphorus, and the corrosion resistance in 3% NaCl increases in the order of alloys containing boron, silicon or carbon, and phosphorus. Difference in the corrosion resistance of amorphous alloys containing various metalloids has been interpreted in terms of the difference in the formation rate of the chromium-enriched protective film on the surface of alloys.     

Threefold bonding of tellurium in liquid semiconductor alloys

Bond orbital considerations show that a third covalent band in tellurium is unstable unless the concentration of threefold bonded (3F) tellurium atoms is smaller than the concentration of dangling bonds. At low hole concentrations, the formation of 3F atoms is inhibited further because of non-linear screening of their positive charge, which results in a large electrostatic energy. Consequently 3F bonding becomes important only in liquids whose electrical conductivity is higher than an estimated value $\text{≈1500 Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$.     

Structural behavior of amorphous and liquid metallic alloys at elevated temperatures

The thermal behavior of the short-range order of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glasses has been investigated in situ by means of high-temperature X-ray synchrotron diffraction. The dependence of the X-ray structure factor S(q) of the glassy state on temperature follows the Debye theory up to the glass transition. Above the glass transition temperature T_g, the temperature dependence of S(q) is altered toward a continuous development of structural changes in the liquid state with temperature. The behavior of the structure factor during heating and cooling through the glass transition gives experimental evidence for melting the glass, and for freezing the liquid, respectively at the caloric glass temperature.     

Kinetics of crystallization of titanium based binary and ternary amorphous alloys

Metallic glasses are kinetically metastable materials. These amorphous materials can be transformed into a crystalline state by both isothermal and isochronal methods. The study of this transformation, and hence the thermal stability of metallic glasses, are important from an application view-point. In the present work, the non-isothermal crystallization kinetics of two titanium-based amorphous alloys namely, Cu₅₀Ti₅₀ and Ti₅₀Ni₃₀Cu₂₀, are reported. The activation energies for crystallization, E_c for both the systems have been evaluated using different non-isothermal methods viz. derived through Kissinger, Augis and Bennet and Ozawa. The values of E_c obtained using these methods are consistent for both the metallic glasses and it is found that E_c for the ternary metallic glass is considerably higher than the binary metallic glass. The increase in the activation energy on the substitution of Ni in the Cu–Ti metallic glass suggests the increase in the thermal stability.     

Enhancement in electrical and stability properties of amorphous polymer based nanocomposite electrolyte

A novel combination of dispersed phase polymer nanocomposite electrolyte (PNCE) series based on an amorphous polymer host (PMMA)₄–LiClO₄ complex dispersed with nanocrystalline CeO₂ is reported. XRD analysis has confirmed the dispersed phase nanocomposite formation. Effect of nano CeO₂ dispersion on ion–ion and ion–polymer interactions has been analyzed. A drastic enhancement in electrical conductivity, by 2 orders of magnitude at 30 °C and 5 orders of magnitude at 100 °C, has occurred on nano CeO₂ dispersion when compared with room temperature conductivity of undispersed PS film. An excellent correlation between variation of d.c. conductivity and free mobile charge carriers has been observed. An ion conduction model is proposed. Strength of the model lies in the experimental evidences from FTIR, conductivity and TEM analyses. Thermal analysis indicates a strong dependence of thermodynamical parameters, e.g., glass transition temperature (T_g), crystalline melting temperature (T_m), enthalpy etc. on filler addition. Substantial improvement in voltage stability (~ 4.4 V), thermal stability and ion transport properties has been noticed on nano CeO₂ dispersion.     

Description of chemical ordering in amorphous alloys

Partial coordination numbers $\text{Z}{}_{\mathrm{ij}}{}^1$ for a chemically disordered binary amorphous alloy A_XAB_XB are obtained in terms of total coordination numbers Z_i and average coordination numbers 〈Z〉 $\text{Z}{}_{\mathrm{ij}}{}^1\text{=x}{}_{\mathrm{j}}\text{Z}{}_{\mathrm{i}}\text{Z}{}_{\mathrm{j}}\text{/〈Z〉}$. Departures from complete chemical disorder are characterized by a short-range order coefficient with η_AB^max=x_BZ_B/x_AZ_A for X_BZ_B<x_AZ_A and η_AB^max=x_AZ_A/x_BZ_B for x_BZ_B>x_AZ_A.For complete chemical disorder ν_AB⁰ = 0; for complete chemical order, i.e. with chemical preference for AB nearest neighbor pairs, ν_AB⁰ = 1. For departures from chemical disorder associated with clustering, i.e. chemical preference against AB nearest neighbor pairs, ν_AB⁰ < 0. Experimental results for several amorphous alloys are used to demonstrate the usefulness of these results. A more precise calculation of η_AB^max is necessary to compare the degree of ordering in the metal-metalloid and metal-metal alloys.     

The influence of solute distribution on the high nucleation density of Al crystals in amorphous aluminum alloys

An extension of a previous model that describes the role of solute atoms on glass formability leads to the conclusion that solute distribution plays an important role in the formation and stability of amorphous metals. A random distribution of solutes is shown to produce local solute-depleted regions (on the size scale of the mean inter-solute spacing) that provide preferred sites for the formation of crystalline nuclei. The possibility that these solute-depleted regions are responsible for the exceptionally high number density of critical Al nuclei is explored for three Al-Y binary alloys using a computer simulation. Up to 10⁷ Y atoms were placed at random locations in the system, and the number of solute-free regions were counted as a function of the size of these regions. The experimentally observed number density of critical nuclei (∼3 × 10²¹ m⁻³) is reproduced for a critical nucleus about 5 Al atoms in diameter, containing ∼60 Al atoms in an fcc array. Good agreement with previous suggestions of the size of a critical nucleus (about 6 atoms in diameter, containing about 100 atoms) support the conclusion that the current model provides a reasonable physical explanation for the quenched-in features responsible for the exceptionally high nucleation density in some amorphous Al alloys.     

Rapid recrystallization of amorphous silicon utilizing the plasma jet at atmospheric pressure

The rapid recrystallization of amorphous silicon utilizing the very high frequency (VHF) plasma jet of argon at atmospheric pressure is presented. Highly crystallized polycrystalline Si film was synthesized by optimizing the translating velocity of the substrate stage and flow rate of argon. The temperature of the plasma exposure area reached at 1300 °C and the recrystallization of a-Si proceeded with a time constant of 10–50 ms. The effects of the translating velocity of the substrate stage and flow rate of argon on the rapid recrystallization of a-Si are demonstrated along with its mechanism.     

Densification of alkali silicate glasses at high pressure

The densification behavior of a number of alkali silicate glasses has been investigated using a Bridgman anvil high pressure device. The pressure range of the investigation was 10 to 60 kilobars; the temperature range was 100 to 400°C. In all cases, the densification was found to increase with increasing temperature and pressure. In the two systems where compositional variations were explored, Na₂OSiO₂ and K₂OSiO₂, a pronounced maximum in densification in the vicinity of 10 mole % alkali oxide was observed (for a given temperature and pressure). These maxima are taken to reflect a competition between two processes, one of which — the variation of molecular mobility with composition — should lead to increasing densification with increasing alkali concentration. Several possibilities are discussed for the decrease in densification with alkali oxide concentrations greater than about 10% (to values smaller than that of SiO₂ in some cases). The present results are related to those obtained in previous investigations of the densification of oxide and polymeric glasses.     

Raman scattering and electrical characterizations studies of hydrogenated amorphous silicon-germanium alloys prepared by 40 MHz plasma-enhanced CVD

This paper presents results on Raman scattering and electrical characterizations in a-SiGe:H films prepared by 40 MHz very-high frequency plasma-enhance chemical vapor deposition (VHF-PECVD) technique from various gas mixtures of silane and germane. We found that when GeH₄/SiH₄ + GeH₄ ratio increases, Raman spectrum results observed that the Si-Si peaks intensity decreases and the Ge-Ge peaks intensity increase, respectively. This can be attributed to incorporation of Ge, an increase disorder in a-SiGe:H film. The conductivity characterizations were shown that when GeH₄/SiH₄ + GeH₄ ratio increases, the deterioration of a-SiGe:H films also increases, which is in agreement with Raman spectrum analysis results.     

Spectral dependence of transient photocurrent quantum efficiency in amorphous arsenic triselenide

The relative magnitude of the quantum efficiency for photogeneration of mobile carriers in a-As₂Se₃ is measured using transient photocurrent. The quantum efficiency is quite constant for photon energies above 1.4eV, but falls approximately exponentially with decreasing photon energy for lower energies.