Amorphous Gallium Arsenide Research Articles

Magnetoelectric effect in layered structures of amorphous ferromagnetic alloy and gallium arsenide

A paper devotes to theoretical and experimental studying the magnetoelectric interaction in layered structures of amorphous ferromagnetic alloy and single- crystal gallium arsenide.The authors investigated the magnetoelectric effect in the (100) plane of gallium arsenide in the electromechanical resonance range of 200–240kHz and obtained maximal ME voltage coefficient of 120V/A at bias field equaled 3.6kA/m for the direction parallel to the [011] axis. Also the magnetoelectric effect in the (110) and (111) planes is discussed. The results can be used for design of new electronic devices based on the magnetostrictive-semiconductor materials.

Optical absorption enhancement in a hybrid system photonic crystal – thin substrate for photovoltaic applications

A hybrid approach for light trapping using photonic crystal nanostructures (nanorods, nanopillars or nanoholes) on top of an ultra thin film as a substrate is presented. The combination of a nanopatterned layer with a thin substrate shows an enhanced optical absorption than equivalent films without patterning and can compete in performance with nanostructured systems without a substrate. The designs are tested in four relevant materials: amorphous silicon (a-Si), crystalline silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP). A consistent enhancement is observed for all of the materials when using a thin hybrid system (300 nm) even compared to the non patterned thin film with an anti-reflective coating (ARC). A realistic solar cell structure composed of a hybrid system with a layer of indium tin oxide (ITO) an ARC and a back metal layer is performed, showing an 18% of improvement for the nanostructured device.

Hydrogen influence on gallium arsenide thin films prepared by rf-magnetron sputtering technique

We investigate the effect of the hydrogen intentional incorporation on the structural properties of the amorphous gallium arsenide prepared by rf-magnetron sputtering technique. The properties of the non-hydrogenated films are: band gap of 1.4 eV (E04), Urbach energy of 110 meV, stoichiometric composition ([As]/[Ga] = 0.50), and dark conductivity of about 3.2 x 10-5 (W.cm)-1. Hydrogen was incorporated in the films by the introduction of an electronically controlled H2 flux during deposition, keeping constant the other deposition parameters. It was observed that small hydrogen incorporation produces a great change in the structural properties of the films. The main changes result from the formation of GaAs nanocrystals with mean sizes of about 7 nm into the amorphous network.

Thermal annealing effect on the electrical properties and structural defects density of non-stoichiometric a-GaAs films

Amorphous gallium arsenide (a-GaAs) films were prepared from their constituents using the co-evaporation technique. The temperature dependence of DC conductivity for the as-deposited (fresh) As-rich and that of thermally annealed films over the temperature range 373–553K is measured. Results of the characteristic electrical parameters indicate non-monotonic behavior during the thermal annealing range 373–423K and a monotonic one in the range 423–553K. Results of the density of structural defect states, using the space-charge-limited current model, are given and discussed in relation to the nature and distribution of defect states regarding their dependence on the thermal treatment.

Structural and electrical properties of amorphous GaAs sputtered at high substrate temperature (220 and 400 °C)

The influence of deposition conditions on structural and electrical properties of amorphous gallium arsenide (a-GaAs) thin films, deposited by RF sputtering at two substrate temperatures T S = 220 ° C (glass substrates) and T S = 400 ° C (Mo and ITO/glass substrates), is studied by both X-ray diffraction and electrical dark conductivity σ measurements, in the range (−150 °C to +150 °C) of temperature. The increase of sputtering argon pressure, P Ar, leads to the growth of an amorphous structure and reduces the room-temperature dark conductivity, σ RT , which suggests a reduction in the density of defects. Similar results are obtained with a decrease of the dc self-bias voltage, V p , of RF electrodes. These results suggest that the density of defects is reduced with the energy decrease of the bombarding species while increasing P Ar or decreasing V p . The product of the target-to-substrate distance by the argon pressure, ( d× P Ar), representing the number of particles free path, confirms some results of the literature which indicate that the density of defects can be reduced significantly by reducing the energy of the bombarding species while increasing P Ar or decreasing V p . Materials of great resistivity (≈10 8 Ω cm) are obtained either for low V p or for great d× P Ar. In addition, the thermal annealing reduces considerably the room-temperature dark conductivity.

Relationship between the optical gap and the optical-absorption tail breadth in amorphous GaAs

We study the relationship between the optical gap and the optical-absorption tail breadth for the case of amorphous gallium arsenide (a-GaAs). In particular, we analyze the optical-absorption spectra corresponding to some recently prepared a-GaAs samples. The optical gap and the optical-absorption tail breadth corresponding to each sample is determined. Plotting the optical gap as a function of the corresponding optical-absorption tail breadth, we note that a trend, similar to that found for the cases of the hydrogenated amorphous silicon and hydrogenated amorphous germanium, is also found for the case of a-GaAs. The impact of alloying on the optical-absorption spectrum associated with a-GaAs is also briefly examined.

Influence of the argon flow rate on the arsenic fraction of a-Ga xAs 1−x sputtered at high substrate temperature

The experimental study of the chemical composition of amorphous gallium arsenide (a-Ga x As 1− x ) versus argon flow rate, Q, by rf sputtering, shows that the As fraction of sputtered films is controlled by the argon flow rate. At the substrate temperature, T s =500 °C, the films are stoichiometric when deposited under the argon flow rate between 8 and 22 sccm. These observations indicate that at low argon flow rate the As fraction of films is governed only by the preferential re-sputtering of As during the film growth. In addition, a correlation between the deposition rate R, and chemical composition x was deduced from these results.

Near K-edge linear attenuation coefficients for amorphous and crystalline GaAs

We present tabulated near K-edge linear attenuation coefficients for stoichiometric crystalline and amorphous gallium arsenide. The coefficients were derived from total electron yield photocurrent measurements at the Daresbury Synchrotron Radiation Source and show considerable near-edge structure when compared to curves generated from standard atomic data tables. However, very little difference is found between the crystalline and amorphous samples, suggesting that the bulk of the structure arises from the local coordination environment.

Carrier Transport Mechanisms of a-GaAs/ n-Si Heterojunctions

Heterojunctions have been fabricated of p-type amorphous gallium arsenide (a-GaAs) thin films onto n-type silicon (n-Si) single crystals using thermal evaporation method. Current density-voltage and capacitance-voltage measurements have been performed to determine the electrical properties of the structures. Rectifying current involves tunneling and is explained by a multi-tunneling capture-emission model. The reverse current is limited by the carrier generation process. The capacitance-voltage behavior indicates an abrupt interface with a main-band discontinuity of 0.25 eV occurs in the valance band.

Topology of amorphous gallium arsenide on intermediate length scales: A molecular dynamics study

Structural correlations in amorphous gallium arsenide are investigated with molecular-dynamics simulations using a new interatomic potential function. The calculated static structure factor, in particular the height and width of the first peak which is a signature of the intermediate-range correlations, is in excellent agreement with x-ray diffraction experiments. Atomistic topology on intermediate length scales is elucidated through the analyses of shortest-path rings, partial static structure factors, and bond-angle distributions. The calculated energy difference between crystalline and amorphous systems is also in good agreement with electronic-structure calculations.

Optical absorption, disorder, and the disorderless limit in amorphous semiconductors

We study the role that disorder plays in shaping the functional form of the optical absorption spectra of amorphous semiconductors. We find, for hydrogenated amorphous silicon, amorphous germanium, amorphous gallium arsenide, and amorphous indium phosphide, that, while the breadth of the absorption tail is a strong function of disorder, the mean energy gap is insensitive to the amount of disorder. As the disorder is decreased, the optical absorption spectra associated with these amorphous semiconductors approach well defined disorderless limits, the energy gaps associated with these limits being greater than the corresponding crystalline gaps. The physical implications of these results are discussed.

A Short Theoretical Note on Optical Absorption of Amorphous Gallium Arsenide in the Infrared Range

An approach to determine the optical absorption coefficient of amorphous GaAs in the far infrared range is developed. Results from this approach are compared with experiment and with other theoretical results. In our formulation, contributions corresponding to both dynamical and structural disorders are taken into consideration.

Structural, electronic, and dynamical properties of amorphous gallium arsenide: A comparison between two topological models

We present a detailed study of the effect of local chemical ordering on the structural, electronic, and dynamical properties of amorphous gallium arsenide. Using the recently-proposed ``activation-relaxation technique'' and empirical potentials, we have constructed two 216-atom tetrahedral continuous random networks with different topological properties, which were further relaxed using tight-binding molecular dynamics. The first network corresponds to the traditional, amorphous, Polk-type, network, randomly decorated with Ga and As atoms. The second is an amorphous structure with a minimum of wrong (homopolar) bonds, and therefore a minimum of odd-membered atomic rings, and thus corresponds to the Connell-Temkin model. By comparing the structural, electronic, and dynamical properties of these two models, we show that the Connell-Temkin network is energetically favored over Polk, but that most properties are little affected by the differences in topology. We conclude that most indirect experimental evidence for the presence (or absence) of wrong bonds is much weaker than previously believed and that only direct structural measurements, i.e., of such quantities as partial radial distribution functions, can provide quantitative information on these defects in a-GaAs.

Study of Amorphous Gallium Arsenide Films Using a Novel Evaporator with Nondestructive Pickup Image and Electron Diffraction Pattern Investigations.

In order to investigate the structure of amorphous gallium arsenide (a-GaAs) targets we developed a novel evaporator attached to an inspection chamber wherein an electron beam serves as both a television (TV) scanning beam and an electron diffraction beam. GaAs targets fabricated by a flash evaporation method at a substrate temperature of above 220°C consist of polycrystalline GaAs (poly-GaAs), the grain size of which is 75A, and have an electron mobility of 125180 cm2/ Vs. GaAs targets fabricated at a substrate temperature below 200°C are amorphous. We identified two bond lengths, 2.48 A and 4.00 A, in radial distribution function (RDF) analyses. These values indicate the interatomic bond lengths of Ga-As and Ga-Ga or As-As, respectively. The including angle of an isosceles triangular molecule of As-Ga-As or Ga-As-Ga is 107.5°. The standard deviations of the bond lengths are 0.145 A for the 2.48 A peak and 0.348 A for the 4.00 A peak. The electron mobility of a-GaAs is 0.170.45 cm2/ Vs. The resistivities of poly-GaAs and a-GaAs targets are 102Ωcm and 107 Ωcm, respectively. Moreover, we achieved a high resolution of pick-up images on an a-Se/ a-GaAs target.

Photoconductivity of amorphous GaAs

Abstract The spectral response of photoconductivity has been measured in samples of amorphous gallium arsenide deposited by rf sputtering of monocrystalline GaAs targets with and without hydrogen. The maximum of the normalized spectral photoconductivity (hν ≅ 2.0 eV) as a function of hydrogen pressure, pH2, increases when pH2 ≤ 0.1 Pa and then tends to a constant value. The values of the Tauc optical gap, Eg, and those of the photoconductive threshold, Et,ph are similar. Their difference for the same sample tends to decrease with increasing pH2. The same trend is shown by the difference between the Urbach energy computed either by photothermal deflection spectroscopy, E0, or by photoconductivity absorption coefficient data, E0,ph. These results and the dependence of other quantities, such as the activation energy of the conductivity, Eσ, on pH2, are interpreted in terms of the possible effects that hydrogenation can induce in a compound semiconductor. They are: a less disordered network reducing the band tail depth; decrease of the bond-angle fluctuation and ineffectiveness of hydrogen on the density of wrong bonds. The first two contributions are strictly linked to the deposition conditions of the films which determine, for example, the deposition rate of the film.

Electrical characteristics of amorphous GaAs-n-crystalline Si heterojunctions

Capacitance-voltage ( C- V) characteristics and temperature dependence of current density-voltage ( J- V) characteristics of as-grown amorphous gallium arsenide (a-GaAs) thin film heterojunctions formed on n-type crystalline silicon (c-Si(n)) have been measured. It has been found that the depletion layer of a-GaAs-c-Si(n) junction spreads in the c-Si side. The forward current, for bias voltages less than 0.4 V, shows voltage and temperature dependence expressed as exp ( − E af/k T) exp ( AV), where E af and A are constants independent of voltage and temperature. This current may be ascribed to a multi-tunnelling capture-emission phenomena. The reverse current is proportional to exp( −E ar / kT)(V) 1 2 , where E ar is a constant. This current is probably limited by a generation process.

High-temperature sputtered amorphous GaAs

Amorphous gallium arsenide films have been deposited on polycrystalline molybdenum and amorphous glass substrates heated at temperatures as high as 600°C using rf sputtering. This has been achieved by operating at lower self-bias voltages when the substrate temperature is increased. The effect of the self-bias voltage on the crystallographic structure of the films is discussed in terms of the kinetic energy of the particles bombarding the growing film. Measurements of the chemical composition of the films using electron microprobe analysis show that films deposited on molybdenum substrates are stoichiometric at a substrate temperature of 400°C.

Electrical properties of a-GaAs/c-Si (p) heterojunctions

Heterojunctions have been fabricated by deposition of amorphous gallium arsenide on p-type crystalline silicon. C( V) and J( V) measurements were performed to determine electrical properties of these structures. Rectifying properties have been obtained, and capacitance-voltage behavior indicates an abrupt interface with a main discontinuity in the valence bands. The forward current involves tunneling and is successfully explained by a multi-tunneling capture-emission model. The reverse current is probably limited by the generation process.

Boson peak in the Raman spectra of amorphous gallium arsenide: Generalization to amorphous tetrahedral semiconductors.

We report on the observation of the boson peak in amorphous (a) GaAs formed by high-energy ion bombardment of the crystalline lattice. The experimental data are analyzed according to the theory of inelastic light scattering from fractons. The correlation length \ensuremath{\xi} and the spectral dimension d\ifmmode \tilde{}\else \~{}\fi{} of the fractal are determined. In comparison to a-Si:H the crossover frequency from the phonon to the fracton scattering regime, ${\mathrm{\ensuremath{\omega}}}_{\mathrm{co}1}$, is lower and scales according to mass law, confirming the vibrational character of the boson peak. The origin of the fractals in tetrahedral amorphous semiconductors is discussed in terms of strained nanometer blobs of host atoms whose overcoordination is relaxed through bond percolation. The intensity increase of the boson peak relative to the amorphous component during the process of amorphization of GaAs, and the increase in the carbon content in a-Si:H, shows a composite structure that consists of a strained fractal region in the relaxed network. Some experimentally observed anomalies such as low values of the sound velocity in tetrahedral amorphous semiconductors, and transformation of the vibrational spectrum by quenching in ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$ are qualitatively explained by the fractal model. On the basis of maximum positions of the boson peak and the first sharp diffraction peak observed in the structure factor of inelastic x-ray or neutron scattering of amorphous semiconductors, the correlation lengths of medium-range order (MRO) are determined and compared for different tetrahedral and vitreous amorphous semiconductors. The observed three to four times shorter value of MRO in tetrahedral relative to vitreous amorphous semiconductors is used to explain a number of differences between their properties.

Structure and properties of amorphous gallium arsenide by tight-binding molecular dynamics

We present and discuss the room-temperature structural, vibrational, and electronic properties of a-GaAs, as obtained by a tight-binding molecular-dynamics quenching of the liquid from 1600 K. An interesting comparison is also made with the overcooled liquid at 800 K, where solidlike and liquidlike features do coexist. At variance with previous ab initio calculations, we have been able to produce a good amorphous sample without introducing ad hoc moves of atoms, favoring a preselected distribution of point defects. Our results are in rather good agreement with the overall structural properties of a-GaAs, as deduced from experiments, and provide a detailed microscopic characterization of such a material.