Epitaxial n-GaAs Layers Research Articles

Self-powered photo diodes based on Ga-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-/n-GaAs structures

The electrical and photovoltaic characteristics of the Ga2O3/n-GaAs structures have been studied. A gallium oxide film was obtained by HF magnetron sputtering on n-GaAs epitaxial layers with concentration of N_d=9.5·1014 cm-3. The thickness of the oxide film was 120 nm. Measurements at a frequency of 106 Hz have shown that the capacitance-voltage and conductance-voltage dependences are described by curves characteristic of metal-insulator-semiconductor structures and exhibit low sensitivity to radiation with λ=254 nm. When operating on a constant signal, the samples exhibit the properties of a photodiode and are able to work offline. The photoelectric characteristics of the detectors during continuous exposure to radiation with λ=254 nm are determined by the high density of traps at the Ga2O3/GaAs interface and in the oxide film. Keywords: MIS-structures, capacitance-voltage characteristics, volt-siemens characteristics, photocurrent, trap density.

Фотодиоды на основе структур Ga-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-/n-GaAs, способные работать в автономном режиме

The electrical and photoelectric characteristics of the Ga2O3/n-GaAs structures have been studied. A gallium oxide film was obtained by HF magnetron sputtering on n-GaAs epitaxial layers with concentration of Nd = 9.5ˑ1014 cm–3 concentration. The thickness of the oxide film was 120 nm. Measurements at a frequency of 106 Hz have shown that the capacitance-voltage and conductance-voltage dependences are described by curves characteristic of metal-insulator-semiconductor structures and exhibit low sensitivity to radiation with λ = 254 nm. When operating on a constant signal, the samples exhibit the properties of a photodiode and are able to work offline. The photoelectric characteristics of the detectors during continuous exposure to radiation with λ = 254 nm are determined by the high density of traps at the Ga2O3/GaAs interface and in the oxide film.

Temperature dependent spin Hall conductivity in n-GaAs epitaxial layers measured by inverse spin Hall effect

The photo-induced inverse spin Hall effect (ISHE) experiments are conducted in heavily doped n-GaAs epitaxial layers by measuring the transverse electric current generated through the diffusion of optically injected spin orientations over a temperature range of 10–300 K. ISHE origin of the measured signal is confirmed through meticulous checks including the characteristic dependence of magnitude of signal on the angle of incidence. The measured value of ISHE current (IISHE) is observed to fall with the increase in temperature. Furthermore, the value of spin current density is theoretically estimated by solving the spin diffusion equation with appropriate boundary conditions for an epitaxial layer. It is shown that by near resonant excitation and subsequent solution of diffusion equation, the spin Hall angle (γ) and spin Hall conductivity (σSH) can be estimated, provided the effective life time of spin polarized electrons(τS) is known independently. By using the numerically calculated value of τS, the proposed method is implemented to estimate the values of γ and σSH. It is found that the fall in the values of IISHE at high temperatures is not governed by σSH, rather by a rapid decrease in the values of τS. In fact, σSH is seen to increase with the temperature, which is compared with existing literature. The present work provides the necessary insight into material parameters which are essential for the development of advanced spin-photonic semiconductor devices.

A model for arsenic anti-site incorporation in GaAs grown by hydride vapor phase epitaxy

GaAs growth by hydride vapor phase epitaxy (HVPE) has regained interest as a potential route to low cost, high efficiency thin film photovoltaics. In order to attain the highest efficiencies, deep level defect incorporation in these materials must be understood and controlled. The arsenic anti-site defect, AsGa or EL2, is the predominant deep level defect in HVPE-grown GaAs. In the present study, the relationships between HVPE growth conditions and incorporation of EL2 in GaAs epilayers were determined. Epitaxial n-GaAs layers were grown under a wide range of deposition temperatures (TD) and gallium chloride partial pressures (PGaCl), and the EL2 concentration, [EL2], was determined by deep level transient spectroscopy. [EL2] agreed with equilibrium thermodynamic predictions in layers grown under conditions in which the growth rate, RG, was controlled by conditions near thermodynamic equilibrium. [EL2] fell below equilibrium levels when RG was controlled by surface kinetic processes, with the disparity increasing as RG decreased. The surface chemical composition during growth was determined to have a strong influence on EL2 incorporation. Under thermodynamically limited growth conditions, e.g., high TD and/or low PGaCl, the surface vacancy concentration was high and the bulk crystal was close to equilibrium with the vapor phase. Under kinetically limited growth conditions, e.g., low TD and/or high PGaCl, the surface attained a high GaCl coverage, blocking As adsorption. This competitive adsorption process reduced the growth rate and also limited the amount of arsenic that incorporated as AsGa. A defect incorporation model which accounted for the surface concentration of arsenic as a function of the growth conditions, was developed. This model was used to identify optimal growth parameters for the growth of thin films for photovoltaics, conditions in which a high growth rate and low [EL2] could be attained.

Local triboelectrification of an n-GaAs surface using the tip of an atomic-force microscope

The method of scanning Kelvin-probe microscopy is used to show that the effect of triboelectrification is observed when the tip of an atomic-force microscope interacts with the surface of n-GaAs epitaxial layers. The sign of the change in the potential indicates that the sample surface after triboelectrification becomes more negative. The observed specific features of the phenomena can be attributed to the thermally activated generation of point defects in the vicinity of the sample surface due to deformation caused by the tip.

Schottky contacts to high-resistivity epitaxial GaAs layers for detectors of particles and X- or γ-ray photons

The electrical characteristics of Ti and Pt Schottky contacts to epitaxial n-GaAs layers with the charge-carrier concentration <1012 cm−3 for detectors of particles and X- or γ-ray photons are studied. It is shown that it is preferable to use a diffusion-based theory of charge transport in calculations of the parameters of Schottky contacts to thick high-resistivity lightly compensated GaAs layers. The calculated barrier heights were 0.84 and 0.87 eV for the Ti and Pt contacts, respectively. The fabricated samples of the surface-barrier detectors featured a linear response in the studied range of energies from 6 to 140 keV for γ-ray photons and from 4 to 8 MeV for α particles; the efficiency of charge collection was close to 100% and the energy resolution was high at room temperature.

The effect of different treatments on fractal forms of gallium arsenide surface

The surface of epitaxial gallium arsenide (as-grown and subjected to standard chemical treatments for producing metal-GaAs electrical contacts) has been studied by atomic force microscopy. It was shown that the surface relief can be characterized by fractal relief in the local approximation. As-grown epitaxial n-GaAs layers have lower values of the mean relief irregularity and roughness and reveal high surface homogeneity. The most marked relief changes are observed due to plasmachemical deposition of silicon dioxide and its removal in a buffered etch and deionized water rinse. The following finishing treatments make the surface more smooth and homogeneous, especially in ammonia-water mixture, which gives almost an ideal (Gaussian) distribution of irregularities and phase contrast in most cases. The fractal dimensionality of the studied surfaces changes in the relatively narrow limits D f = 2.5−2.8. The values of the local approximation limit L determining the boundary of the applicability of the fractal approach when describing the surfaces change from 0.039 to 10.99 μm depending on the kind of treatment.

Photomagnetic effect in In(Ga)As/GaAs heteronanostructures with quantum dots and wells

The influence of quantum-size layers (InAs quantum dots, In0.2Ga0.8As quantum wells, and combined quantum-well/quantum-dot layers) and heteroepitaxial passivation of surface by an In0.5Ga0.5P layer on the photomagnetic effect in epitaxial n-GaAs layers has been investigated.

Determination of the fractal dimension for the epitaxial n-GaAs surface in the local limit

Atomic-force microscopy studies of epitaxial n-GaAs surfaces prepared to deposit barrier contacts showed that major relief for such surfaces is characterized by a roughness within 3–15 nm, although “surges” up to 30–70 nm are observed. Using three independent methods for determining the spatial dimension of the surface, based on the fractal analysis for the surface (triangulation method), its section contours in the horizontal plane, and the vertical section (surface profile), it was shown that the active surface for epitaxial n-GaAs obeys all main features of behavior for fractal Brownian surfaces and, in the local approximation, can be characterized by the fractal dimension D f slightly differing for various measuring scales. The most accurate triangulation method showed that the fractal dimensions for the studied surface of epitaxial n-GaAs for measurement scales from 0.692 to 0.0186 μm are in the range D f = 2.490−2.664. The real surface area S real for n-GaAs epitaxial layers was estimated using a graphical method in the approximation δ → 0 δ is the measurement scale parameter). It was shown that the real surface area for epitaxial n-GaAs can significantly (ten times and more) exceed the area of the visible contact window.

Calculation of electric-field strength profiles in GaAs:Cr epitaxial-diffusion structures

The electric-field strength profiles are calculated for the structures used in X-ray detectors produced by diffusion of the deep acceptor Cr impurity into an n-GaAs epitaxial layer. The effects of diffusion temperature, external bias, and the ratio of electron and hole emission coefficients on the distribution of electric-field strength in the structure are examined.

Current Oscillations under Lateral Transport in GaAs∕InGaAs Quantum Well Heterostructures

The current-voltage (I–V) characteristics were measured and the current-pulse oscilloscope patterns were obtained for multilayer n-InGaAs/GaAs quantum-well heterostructures and n-GaAs epitaxial layers with various doping levels. It is shown that the I–Vcharacteristics flatten at low doping levels in fields of 300–400 V/cm. In more heavily doped samples, current oscillations are initiated with a period corresponding to a drift velocity of (3–3.5)×105 cm/s at E ‖ [110] and are approximately larger at E ‖ [100] by a factor of 1.5. The results obtained are attributed to the formation of, respectively, stationary and moving acoustoelectric domains in the structures. In fields above 1.5 kV/cm, high-frequency Gunn oscillations corresponding to a drift velocity of 1.5 × 107 cm/s were observed.

Correlation between the electrical and luminescent properties of high-purity gallium arsenide

Epitaxial n-GaAs layers with a background impurity concentration of ND-NA<1015 cm−3, grown by chloride vapor phase epitaxy in an open system, exhibit correlation between the electrical properties and the long persistence of the edge photoluminescence lines D0x and D0h related to the hole trapping centers. An increase in the concentration of such trapping centers in n-GaAs leads to a decrease in the mobility of free charge carriers.

In situ resistivity studies of 200 MeV 107Ag 14+-ion irradiated n-GaAs epitaxial layers

The effect of 200 MeV 107Ag 14+-ion irradiation on the electrical transport behavior of Si-doped n-GaAs epitaxial layers has been investigated employing in situ resistivity measurements. The n-GaAs epitaxial layers grown by metallorganic chemical vapor deposition method have carrier concentrations of 3×10 15, 3×10 16 and 3×10 17 cm −3. The fluence of 200 MeV 107Ag 14+ ion is varied from 2×10 8 to 5×10 11 ions cm −2. It is observed that in all the three epitaxial layers, resistivity initially remains constant up to a critical fluence. After this critical fluence, which depends on the initial carrier concentration in the layer, there is a rapid increase in resistivity with fluence. The swift heavy-ion irradiation-induced deep-level defects, which trap the free carriers and causes the mobility degradation, are responsible for enhancement in resistivity. The swift heavy ion traversing through the semiconductor, transfers its energy via nuclear and electronic energy-loss mechanisms. The implications of these energy-loss processes inside n-GaAs have been discussed.

Experimental investigation of 200 MeV 107Ag 14+ ion induced modifications in n-GaAs epitaxial layer by in situ resistivity and Hall measurements

Gallium arsenide (GaAs) grown by metallorganic chemical vapor deposition (MOCVD) method and n-doped with silicon to a concentration of 4×10 17 cm −3 was exposed to 200 MeV 107Ag 14+ ion irradiation. The ion fluence was varied from 2×10 8 to 5×10 11 ions cm −2. The modifications in electrical transport behavior of n-GaAs epitaxial layer was studied by employing in situ resistivity and Hall measurements. The resistivity exhibited a slow increase upto a fluence of 1×10 10 ions cm −2, and then increased in a quasi-exponential manner with respect to fluence. The free carrier concentration in the epitaxial layer, as calculated from Hall measurements, initially showed a slow decrease until a fluence of 1×10 10 ions cm −2 is reached. After this fluence, the free carrier concentration decreased quickly with increase in fluence. The carrier mobility also exhibited a similar kind of trend. The observed modifications in transport properties have been explained on the basis of production of defects in n-GaAs epitaxial layer by swift heavy ion irradiation and the resultant trapping of free carriers.

Radiation resistance of GaAs–AlGaAs heterostructures doped with isovalent and rare-earth elements

When GaAs–Si and GaAs–AlGaAs heterostructures are exposed to γ-quanta, radiation stimulated ordering is observed. However, the gettering efficiency in such systems falls for layer widths more than 1 μm. For this reason we seek effective methods of radiation resistance improvement of materials in which one would expect point radiation defects to be gettered not only at defect boundaries, but also in the active layer volume. S.i.GaAs–s.i.Al x Ga 1− x As– nGaAs : Te heterostructures are presented with epitaxial layers (doped with Yb or undoped), obtained by means of LPE (liquid-phase epitaxy). The electron concentration in nGaAs was found to be (1–3)×10 18 cm −3 for widths 1–3 μm. The samples were exposed to 60Co γ-quanta with doses of 10 5–10 7 rad. Investigations of irradiated samples by means of low-temperature (4.2 K) photoluminescence have shown considerable decrease of exciton halfwidth in the boundary spectra of nGaAs : Te : Yb epitaxial layers in comparison with nGaAs : Te layer spectra. This is caused by background impurity gettering which happens on the s.i.Al x Ga 1− x As– nGaAs heteroboundaries as well as in deformed regions in the epitaxial layer volume. Formation of such regions is caused by the difference between the covalent radii of Yb atoms and GaAs lattice atoms. The maximum effect of radiation stimulated gettering of dopants in nGaAs epitaxial layers is observed for Yb concentrations which are equal to 10 −4–10 −5 atomic fractions in a solution-melt. It is determined that the deformed regions in epitaxial layer volumes and heteroboundaries could be efficient drains for point radiation defects which form under radiation exposure. The investigations carried out showed that the doping of an epitaxial layers by rare-earth impurities provides considerable improvement in forming radiation resistant III–V materials.

Physicochemical properties of the surface and near-surface region of epitaxial n-GaAs layers modified by atomic hydrogen

Changes in the static electrical parameters of the Au-GaAs Schottky barriers in the (n-n+)-GaAs structures treated with atomic hydrogen are closely related to modification of the chemical properties of the surface layers in these structures, including changes in the rate of n-GaAs etching in a DMF-monoethanol amine (1:3) solution, in the electrochemical deposition rate and the structure of the resulting Au layer, and in the degree of passivation of linear defects emerging on the surface. The unprotected surface of epitaxial n-GaAs(100) layer exhibited virtually no etch pits upon the treatment in atomic hydrogen at 100°C. For n-GaAs protected with a 50-A-thick SiO2 film, a drop in the etching rate and a considerable decrease in the number of etch pits, as well as a decrease in the thickness of electrochemically deposited gold layer and a change in is structure, were observed for the samples treated in atomic hydrogen at all temperatures in the range studied (100–400°C).

Temperature dependence of current—voltage characteristics of Au/n-GaAs epitaxial Schottky diode

The influence of temperature on current-voltage (I-V) characteristics of Au/n-GaAs Schottky diode formed on n-GaAs epitaxial layer grown by metal organic chemical vapour deposition technique has been investigated. The dopant concentration in the epitaxial layer is 1 X 1016 cm-3. The change in various parameters of the diode like Schottky barrier height (SBH), ideality factor and reverse breakdown voltage as a function of temperature in the range 80–300 K is presented. The variation of apparent Schottky barrier height and ideality factor with temperature has been explained considering lateral inhomogeneities in the Schottky barrier height in nanometer scale lengths at the metal-semiconductor interface

Current filamentation in point contact geometry and its 2D stationary model

Two-dimensional reconstructions of the filamentary current flow are shown, obtained by the spatially resolved photoluminescence of a sample of n-GaAs epitaxial layer with two point contacts. A mathematical model is constructed, based on a bistability of the local conductivity controlled by the local electric field strength. The interface between the high and low conductive states is assumed to occur always at a constant critical value of the field strength. Corresponding mathematical description of the model leads to the nonlinear Laplace free boundary problem. Its two-dimensional numerical solution is shown to satisfactorily reproduce the main characteristics of the filament observed in the experiment.

Self-organized current-density patterns and bifurcations in n-GaAs with a circular symmetry of contacts

Self-organized nucleation of current-density filaments has been investigated in an n-GaAs epitaxial layer with circular contact symmetry. The first filament arises from a spatially uniform state, further filaments are generated by filament division processes. Filament growth, filament division and rearrangement of filaments reveal hysteretic behavior accompanied with subcritical bifurcations.

Self-Organized Current-Density Patterns and Bifurcations in n-GaAs with Concentric Contact Geometry

Self-organized nucleation of current-density filaments has been investigated in n-GaAs epitaxial layers with different concentric contact geometries by measuring the photoluminescence spatially resolved. While the first filament arises from a spatially uniform state, further filaments are generated by filament division processes. Filament growth and filament division of filaments reveal hysteretic behavior. In a magnetic field filament bending or destabilization of stationary patterns in favor of oscillating filamentary patterns occur.