Ga-rich Solutions Research Articles

Single crystal growth in the Ga–Pd system

Single crystal growth of the intermetallic compounds GaPd2, GaPd, and Ga7Pd3 is described for the first time. These phases have recently been explored as highly selective heterogeneous catalysts in the semi-hydrogenation of acetylene. Well-oriented single-crystalline surfaces are needed to perform fundamental studies on catalytic processes. The three Ga–Pd phases were grown using the Czochralski method from Ga-rich solutions. Thermodynamic properties of the Ga–Pd system determine very different growth temperatures reaching from less than 460°C for Ga7Pd3 to about 1200°C for GaPd2. Avoiding mother liquid inclusion formation proved to be the key problem that could be solved by pulling rates sometimes as low as 25µm/h and forced convection by high crystal rotation rates. In the last grown part of GaPd2 crystals decomposition into GaPd2/Ga3Pd5 lamellas occurred which can be explained either by spinodal decomposition or by nucleation caused by the GaPd2 stability region that becomes narrower with decreasing temperatures.

Growth of large PdGa single crystals from the melt

Single crystal growth of PdGa which is a highly selective catalyst for the semi-hydrogenation of acetylene is reported for the first time. Bridgman as well as Czochralski methods were used starting from off-stoichiometric or congruent melting compositions. While Bridgman growth always resulted in polycrystalline ingots with individual grains restricted to a few millimeters in size, using the Czochralski technique from Ga-rich solutions allowed the growth of PdGa single crystals of several cubic centimeters.

Recovery of 65Zn from waste solutions from gallium targets at Brookhaven Linac Isotope Producer

A procedure for recovery of 65Zn from acidic Ga-rich solutions generated in 68Ge production process was developed and implemented in a “hot cell” environment. A one step simple separation was carried out on anion exchange column filled with Biorad AG1-X8 resin. With 65Zn recovery yields of more than 95% this procedure is currently routinely used at Brookhaven Linac Isotope Producer.

Investigations on liquid phase electroepitaxial growth kinetics of GaAs

This paper presents a model based on solving a two-dimensional diffusion equation incorporating the electromigration effect by numerical simulation method corresponding to liquid phase electroepitaxial (LPEE) growth of GaAs, whose growth is limited by diffusion and electro migration of solute species. Using the numerical simulation method, the concentration profiles of As in Ga rich solution during the electroepitaxial growth of GaAs have been constructed in front of the growing crystal interface. Using the concentration gradient at the interface, the growth rate and thickness of the epitaxial layer of GaAs have been determined for different experimental growth conditions. The proposed model is based on the assumption that there is no convection in the solution. The results are discussed in detail.

STUDY OF Al0.065Ga0.935Sb AVALANCHE PHOTODETECTORS FOR 1.55 μm OF WAVELENGTH BY LIQUID PHASE EPITAXY AT LOW TEMPERATURE

We report the results of a study of Al 0.065 Ga 0.935 Sb avalanche photodetectors grown on n-GaSb substrates. The devices have been fabricated from layers with the structure p-Al 0.13 Ga 0.87 Sb/n-Al 0.065 Ga 0.935 Sb:Te/n-GaSb (substrate) grown by liquid phase epitaxy (LPE) with a composition matched to detect light of 1.55 μm. The heterostructures were grown from Ga-rich solutions at 400°C. Just after their growth, the structures were subjected to baking processes inside the growth chamber. The baking time was varied and its influence on the breakdown voltages of the junctions was observed. Breakdown voltages up to 12 V, very low net donor concentration in the active layer (1E15 cm -3), and avalanche multiplication factors of around 50 have been obtained. The carrier concentration was determined by the C–V method. Photoluminescence and X-ray diffraction measurements were carried out to investigate the properties of the LPE-grown ternary layers, to determine the band gap and to estimate the quality of epitaxial layers. The photoresponses of the detectors are also presented.

Electrical properties and deep levels in bulk solution grown GaAs crystal

Electrical and photoelectrical properties, deep levels spectra and microcathodoluminescence spectra were measured for bulk high-resistivity GaAs samples grown from Ga-rich solution by a synthesis solute diffusion technique. It is shown that the main portion of the grown crystal is high-resistivity p-type with electrical properties determined by deep hole traps with the level near 0.43 eV from the valence band edge. The density of these 0.43 eV hole traps was shown to decrease with increasing distance from the crystallization front and the traps were associated with the deep hole traps observed earlier in Ga-rich liquid-phase-epitaxy-grown films. The single crystalline end portion of the crystal was semi-insulating n-type with a very low (some 10 14 cm −3) concentration of midgap EL2 donors. This end portion of the crystal was characterized by a very high photosensitivity. Possible advantages of the use of such material in radiation detectors are briefly discussed.

Simulation studies on the liquid phase electroepitaxial growth of III–V compound semiconductors

In this paper, the growth kinetics of liquid phase electroepitaxy of III–V binary and ternary compound semiconductors have been studied. Concentration profiles of solute atoms such as As, P, Sb in front of a crystal (GaAs, InP, InAs, GaSb) interface growing in a In- or Ga-rich solution for the case of growth of binary compounds and for the case of ternary compounds (InGaAs, AlGaSb, InAsP) have been constructed using a computer simulation technique. The effect due to Peltier heating or cooling and electromigration during the growth have been incorporated to simulate the concentration profiles. The growth velocity in the absence and presence of convection, due to the Peltier effect and electromigration are calculated under different conditions. The results are discussed in detail.

Atom scattering from atomic surfactants: Collisions of argon with a dilute Bi:Ga solution

Gas–liquid scattering experiments are used to determine the ways in which surfactant bismuth atoms alter energy flow in collisions of argon atoms with a liquid solution of 0.02% bismuth: 99.98% gallium. We find that argon–alloy energy exchange and argon scattering intensities are governed by the surface coverage of Bi atoms. A single Bi monolayer masks the underlying Ga atoms, causing the Ga-rich solution to behave like pure liquid Bi.

Concentration profile surfaces and contour studies of GaSb by liquid phase epitaxy using a simulation technique

A two-dimensional numerical simulation technique has been employed to construct the concentration profile surfaces and the concentration contours of antimony (Sb) in a gallium (Ga)-rich solution in front of the growing crystal interface during the liquid phase epitaxial growth of gallium antimonide (GaSb). This has been achieved by solving a two-dimensional diffusion equation with appropriate boundary conditions. The concentration profile surfaces and the concentration contours have been constructed for the antimony (Sb) atoms in the Ga-rich solution for different temperatures and cooling rates. The temperature contours in the melt have also been determined by solving an appropriate two-dimensional heat equation for different cooling rates and growth temperatures. Using the concentration gradient that exists at the interface, the growth rate and the growth thickness of gallium antimonide (GaSb) have been determined for different growth parameters, such as time and temperature, at different cooling rates. Our theoretical findings have been compared with the experimentally reported values.

Application of the shear cell technique to diffusivity measurements in melts of semiconducting compounds: Ga–Sb

We have developed a shear cell technique for the measurement of diffusivities in molten semiconducting materials. We applied the technique to the Ga–Sb system over a wide range of relative concentrations. In Ga-rich solutions, the diffusion coefficients were found to depend on concentration. Furthermore, we found evidence for an apparent enhancement of the diffusion caused by the initial shearing process. We show that this additional transport can be corrected for.

GaSb crystals pulled from non-stoichiometric solution at constant temperature

In order to obtain GaSb bulk crystals with a controlled residual acceptor concentration, the solute-feeding Czochralski method was applied to the pulling from Ga-rich and Sb-rich solutions at various constant temperatures. Photoluminescence and electrical properties were homogeneous throughout the whole of the grown crystal, and varied from sample to sample according to the Sb content of the growth solution, impling native-defect dependence.

Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from theC-V characteristics. Layers grown from antimony rich melts always exhibitp-type conductivity. In contrast, a type conversion fromp- ton- was observed in layers grown from gallium rich melts below 400 C. The electron mobility of undopedn-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated.

Photoluminescent properties of undoped and Pr-doped GaAs epilayers

The various degrees of incorporation of praseodymium (Pr) into GaAs during liquid-phase epitaxy (LPE) growth from a Ga-rich solution were investigated by low temperature photoluminescence (PL) measurements. The PL spectrum of the undoped (background) GaAs epilayer shows four different peaks which are considered to be due to an intense transition between exciton and neutral donor (DX), an intense transition between exciton and neutral acceptor (AX), a transition of electron-acceptor (eA) recombination and a weak transition from recombination of donor-acceptor (DA), respectively. Variations of the different peaks with amount of Pr doping are detailed in the text. Generally we found that the PL spectra of the low Pr-doped GaAs epilayer demonstrate an obvious gettering effect on the donor-related peaks. As to the spectra of high Pr-doped epilayers, a defect-related (dX) emission peak was produced. Thus, for the purpose of purification, low Pr-doped ( X Pr ratio less than 0.05%) is suggested. Examination of electrical properties illustrates that all the Pr-doped samples exhibit p-type conduction. Depending on the extend of Pr doping in the growth melts, GaAs epilayers have hole concentrations ranging from 10 15 to 10 16 cm −3 and with corresponding hole mobilities varying from 380 to 50 cm 2/V · s. The electrical data also reveal that the undesired donor impurities are gettered by Pr ions during LPE growth, but the Pr-related defects formed in high Pr-doped melts may act as scattering centers and cause their mobilities to decrease.

Concentration profiles of As in a Ga rich solution during electroepitaxy of GaAs using a computer simulation technique

Concentration profiles of As in front of a GaAs crystal interface growing in a Ga rich solution have been determined during the electroepitaxial growth of GaAs layers using a computer simulation technique. The effect due to Peltier heating or cooling and electromigration during growth have been incorporated to simulate the concentration profiles. The growth velocity in the absence and presence of convection, due to Peltier effect and electromigration, are calculated under different conditions. It is observed that there is a transition in the movement of arsenic atoms towards the GaAs crystal interface from smooth and orderly to turbulent and wavy as the intensity of electric field increases during the electroepitaxial growth of GaAs.

Photoluminescence of liquid-phase epitaxial Te-doped GaSb

The photoluminescence (PL) spectra of Te-doped GaSb epitaxial layers grown from Ga-rich solutions by liquid-phase epitaxy have been studied in the electron concentrations from 8×1015 to 4×1018 cm−3. The electron concentration can be accurately controlled by varying the growth temperature and adding the polycrystalline Te-doped GaSb to replace half or all the undoped GaSb starting material in the growth solution. The dependence of line position, line intensity, spectral shape, and broadening on the doping level, power excitation, and temperature has been investigated in detail. At concentrations as low as 1×1016 cm−3, the GaSb sample has become degenerate because of the small effective mass of electrons and the broad band consisting of five partially resolved line dominates the low-temperature PL spectra. At concentrations above 1×1018 cm−3, the 19 K PL spectra is mainly dominated by the sub-band-gap, substrate-induced line A′ at 775.8 meV which is enhanced by the scattering of light off the back surface. This line A′ is direct evidence for the band-gap shrinkage at high doping level. This is the first report to present the detailed luminescence lines in the PL spectra of the Te-doped GaSb samples.

Growth of Al xGa 1− xSb and GaSb bulk crystals with liquid phase electro-epitaxy (LPEE)

A vertical LPEE system was developed for growing thick layers from a source material. Al xGa 1− x Sb and GaSb layers with a thickness of more than 3 mm and a diameter of 15 mm were successfully grown on GaSb substrates from Ga-rich solutions. The growth temperature was in the range of 550–575°C and the current density varied from 5to 10 A/cm 2. It was proven that the growth velocity is a linear function of the current density.

Photoluminescence of high-quality GaSb grown from Ga- and Sb-rich solutions by liquid-phase epitaxy

The temperature dependence of photoluminescence from the high-quality unintentionally doped GaSb layers grown by liquid-phase epitaxy has been studied. The epitaxial layers grown at temperatures above 590 °C, from the Ga- or Sb-rich solutions, reproducibly have a low-carrier concentration of 6–8×1015 cm−3 and exhibit p-type conduction. But it gives n type as grown from the Ga-rich solution at 360 °C. For the samples grown from the Ga-rich solutions, the 16 K photoluminescence spectrum is dominated by the partially resolved lines related to the transitions of excitons bound to donors and neutral acceptors. The acceptor-related band (777.8 meV) which is always presented in the GaSb material due to the native lattice defects has been much reduced as compared to the exciton-related lines. Especially for the n-type samples grown at low temperatures (360 °C), the ratio of the emission intensity from the exciton-related lines to that from the acceptor-related band is 10. For the samples grown from the Sb-rich solutions, the 16 K photoluminescence spectrum is also dominated by a bound exciton-related line with a full width at half maximum of 4.2 meV. In addition, the free-exciton transition with a full width at half maximum of 0.4 meV is also observed. As the temperature is increased, the intensity of the lines associated with the bound excitonic transitions for all the GaSb samples rapidly quenches off and the free-electron-to-free-hole transition becomes dominant. The temperature dependent band gap in GaSb layers determined from the photoluminescence peak energy varies as 0.813–[1.08×10−4 T2/(T−10.3)] eV.

Liquid phase epitaxial growth of AlxGa1− xSb from Sb-rich solution

This is, to our knowledge, the first precise report on the liquid phase epitaxial growth of Al x Ga 1− x Sb from a Sb-rich solution. The phase diagram of Al-Ga-Sb at the Sb-rich corner was determined. Epitaxial layers with low background carrier concentration and good surface morphology can be obtained from Sb-rich solutions, when compared to LPE growth from Ga-rich solutions.

The liquid-phase epitaxial growth of low net donor concentration (5 × 10<sup>14</sup>-5 × 10<sup>15</sup>/cm<sup>3</sup>) GaSb for detector applications in the 1.3 - 1.6 µm region

We report on the liquid-phase epitaxial (LPE) growth of low doped n-GaSb for long wavelength photodiodes. Very low net donor concentration epilayers ( 5 \times 10^{14}-5 \times 10^{15} /cm3) were grown both from undoped Ga rich solutions in the 300-375\deg C range and by compensation using Ga-rich solutions at \sim500\deg C with intentionally added Te. The preparation of Au-GaSb Schottky diodes for C-V profiling is also discussed.

Solubility and point defect-dopant interactions in gallium arsenide—III: Germanium-doping

The form of the solubility curves for VPE and LPE grown material are rationalised using a thermodynamic model which predicts that compensation of donors in VPE material is due to the donor-gallium vacancy complex Ge Ga V − ga whereas, in LPE material, the dominant acceptor making the material p type is the arsenic substituted state Ge − As. Mass action constants for the incorporation reactions for these entities and for the donor Ge + Ga are obtained. The compensation ratio in crystals grown from a congruent melt is predicted to be N D / N A ~ 5.9, and the transition to p type conductivity as one grows from Ga-rich solutions at progressively lower temperature is shown to occur at less than 100°C below the congruent melting point so that all LPE material should be p type. The superlinear behaviour of the total-germanium solubility curve at very high doping levels is postulated to be due to the formation of neutral Ge GaGe As pairs.