Self-annihilation of antiphase boundaries in GaAs epilayers on Ge substrates grown by metal-organic vapor-phase epitaxy

The self-annihilation of antiphase boundaries (APBs)in GaAs epitaxial layers grown by low-pressure metal-organic vapor-phase epitaxy on Ge substrates is studied by several characterization techniques. Cross-sectional transmission electron microscopy shows that antiphase domain free GaAs growth on Ge was possible due to the proper selection of the growth parameters. The antiphase boundaries annihilate with each other after a thick 3 mm layer of GaAs growth on a Ge substrate as observed by scanning electron microscopy studies. Double crystal x-ray diffraction data shows a slight compression of GaAs on Ge, and the full width at half maximum decreases with increasing growth temperatures. This confirms that the APBs annihilate inside the GaAs epitaxial films. Low temperature photoluminescence measurements confirm the self-annihilation of the APBs at low temperature growth and the generation of APBs at higher growth temperatures

Interface characterization of GaAs/Ge heterostructure grown by metalorganic vapor phase epitaxy

GaAs/Ge heterostructures having abrupt interfaces were grown under different growth conditions and investigated by atomic force microscopy (AFM), cross sectional high resolution transmission electron microscopy (HRTEM), low temperature photoluminescence (LTPL) spectroscopy, electrochemical capacitance voltage (ECV) profiling and current/voltage (I/V) characteristics. Our results indicate that a 6° off cut Ge substrate coupled with a growth temperature of ~675° C, growth rate of ~3 μm/hr and a V/III ratio of ~88 is an optimum growth condition for the buffer layer growth of GaAs/Ge heterostructure solar cells. The surface morphology was found to be very good on 6° off oriented Ge substrate and rms roughness was ~30.8 nm over 10 × 10 μm2 area scan over 2° and 9° off oriented Ge substrates. The lattice indexing of HRTEM exhibited an excellent lattice line matching between GaAs and Ge substrates. The ECV profiler shows an excellent abruptness between the film/substrate interface of GaAs/Ge and also between various layers of the complete solar cell structures. Finally, the I/V characteristics of GaAs/Ge solar cells were analysed under AM0 conditio

Interface states density distribution in Au/n-GaAs Schottky diodes on n-Ge and n-GaAs substrates

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of Au/n-GaAs Schottky diodes on n-Ge substrates are investigated and compared with characteristics of diodes on GaAs substrates. The diodes show the non-ideal behavior of I–V characteristics with an ideality factor of 1.13 and barrier height of 0.735 eV. The forward bias saturation current was found to be large $(3 \times 10^{-10} A$ vs. $4.32 \times 10^{-12} A)$ in the GaAs/Ge Schottky diodes compared with the GaAs/GaAs diodes. The energy distribution of interface states was determined from the forward bias I–V characteristics by taking into account the bias dependence of the effective barrier height, though it is small. The interface states density was found to be large in the Au/n-GaAs/n-Ge structure compared with the $Au/n-GaAs/n^+$-GaAs structure. The possible explanation for the increase in the interface states density in the former structure was highlighted

Optimization of off-oriented Ge substrates for MOVPE-grown GaAs solar cells

GaAs/Ge heterostructures having abrupt interfaces were grown on 2degrees, 6degrees, and 9degrees off-cut Ge substrates and investigated by cross-sectional high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy, photoluminescence spectroscopy and electrochemical capacitance voltage (ECV) profiler. The GaAs films were grown on off-oriented Ge substrates with growth temperature in the range of 600-700degreesC, growth rate of 3-12 mum/hr and a V/III ratio of 29-88. The lattice indexing of HRTEM exhibits an excellent lattice line matching between GaAs and Ge substrate. The PL spectra from GaAs layer on 6degrees off-cut Ge substrate shows the higher excitonic peak compared with 2degrees and 9degrees off-cut Ge substrates. In addition, the luminescence intensity from the GaAs solar cell grown on 6degrees off-cut is higher than on 9degrees off-cut Ge substrates and signifies the potential use of 6degrees off-cut Ge substrate in the GaAs solar cells industry. The ECV profiling shows an abrupt film/substrate interface as well as between various layers of the solar cell structures

Si incorporation and Burstein-Moss shift in n-type GaAs

Silane (SiH4) was used as an n-type dopant in GaAs grown by low pressure metalorganic vapor phase epitaxy using trimethylgallium (TMGa) and arsine (AsH3) as source materials. The electron carrier concentrations and silicon (Si) incorporation efficiency are studied by using Hall effect, electrochemical capacitance voltage profiler and low temperature photoluminescence (LTPL) spectroscopy. The influence of growth parameters, such as SiH4 mole fraction, growth temperature, TMGa and AsH3 mole fractions on the Si incorporation efficiency have been studied. The electron concentration increases with increasing SIH4 mole fraction, growth temperature, and decreases with increasing TMGa and AsH3 mole fractions. The decrease in electron concentration with increasing TMGa can be explained by vacancy control model. The PL experiments were carried out as a function of electron concentration (10(17) - 1.5 x 10(18) cm(-3)). The PL main peak shifts to higher energy and the full width at half maximum (FWHM) increases with increasing electron concentrations. We have obtained an empirical relation for FWHM of PL, Delta E(n) (eV) = 1.4 x 10(-8) n(1/3). We also obtained an empirical relation for the band gap shrinkage, Delta E-g in Si-doped GaAs as a function of electron concentration. The value of Delta E-g (eV) = -2.75 x 10(-8) n(1/3), indicates a significant band gap shrinkage at high doping levels. These relations are considered to provide a useful tool to determine the electron concentration in Si-doped GaAs by low temperature PL measurement. The electron concentration decreases with increasing TMGa and AsH3 mole fractions and the main peak shifts to the lower energy side. The peak shifts towards the lower energy side with increasing TMGa variation can also be explained by vacancy control model. (C) 1999 Elsevier Science S.A. All rights reserved

Low temperature photoluminescence properties of Zn-doped GaAs

Dimethylzinc (DMZn) was used as a p-type dopant in GaAs grown by low pressure metalorganic chemical vapor deposition (MOCVD). The influence of growth parameters, such as, DMZn mole fractions, growth temperature, trimethylgallium (TMGa) mole fractions, substrate surfaces on the Zn incorporation have been studied. The surface morphology of the layers was measured by scanning electron microscopy (SEM). The hole concentrations and zinc (Zn) incorporation efficiency are studied by using Hall effect, electrochemical capacitance voltage (ECV) profiler, and low temperature photoluminescence (LTPL) spectroscopy as functions of hole concentration $(10^{17}-1.5x10^{20} cm^{-3})$ and experimental temperatures (4.2–300 K). The hole concentration increases with increasing DMZn and TMGa mole fractions and decreases linearly with increasing growth temperature. The main PL peak shifted to lower energy and the full width at half maximum (FWHM) increased with increasing hole concentration. An empirical relation for FWHM, $\Delta Ep$, band gap, Eg, and band gap shrinkage, $\Delta Eg$ in Zn doped GaAs as a function of hole concentration were obtained. These relations are considered a useful tool to determine the hole concentration in Zn doped GaAs by low temperature PL measurement. The hole concentration increases with increasing TMGa mole fraction and the main peak is shifted to lower energy side

Anomalous current transport in Au/low-doped n-GaAs Schottky barrier diodes at low temperatures

The current-voltage characteristics of Au/low-doped n-GaAs Schottky diodes were determined at various temperatures in the range of 77-300 K, The estimated zero-bias barrier height and the ideality factor assuming thermionic emission (TE) show a temperature dependence of these parameters. While the ideality factor was found to show the $T_o$ effect, the zero-bias barrier height was found to exhibit two different trends in the temperature ranges of 77-160 K and 160-300 K, The variation in the Bat-band barrier height with temperature was found to be - (4.7 +/- 0.2) $210^4$ $eVK^1$, approximately equal to that of the energy band gap, The value of the Richardson constant, A**, was found to be 0.27 A cm^-^2 K^-^2 after considering the temperature dependence of the barrier height. The estimated value of this constant suggested the possibility of an interfacial oxide between the metal and the semiconductor. Investigations suggested the possibility of a thermionic field- emission-dominated current transport with a higher characteristic energy than that predicted by the theory, The observed variation in the zero-bias barrier height and the ideality factor could be explained in terms of barrier height inhomogenities in the Schottky diode

Comparative studies of Si-doped n-type MOVPE GaAs on Ge and GaAs substrates

Comparative studies of silicon (Si) incorporation in GaAs on both polar GaAs and nonpolar Ge substrates by low temperature photoluminescence (LTPL) spectroscopy were carried out. The PL spectrum shifts towards higher energy with growth temperature, arsine $(AsH_3)$ and trimethylgallium (TMGa) mole fractions on Ge substrates; whereas the PL spectrum shifts towards higher energy with growth temperature and shifts to lower energy with $AsH_3$ and TMGa mole fractions on GaAs substrates. The shift in PL peak energy towards the higher energy is due to the increase in electron concentration The full width at half maximum (FWHM) increases with increasing growth temperature, $AsH_3$ and TMGa mole fractions on Ge substrates. But the FWHM increases with increasing growth temperature and decreases with increasing AsH3 and TMGa mole fractions on GaAs substrates. A vacancy control model may explain the PL peak shift towards higher energy with increasing $AsH_3$ mole fraction on Ge substrates and with increasing TMGa mole fraction on GaAs substrates. The experimental results of the studies of the effect of TMGa mole fraction variation on zinc (Zn)-doped GaAs on both GaAs and Ge substrates were presented for better understanding of the growth process

Photoluminescence of Zn- and Si-doped GaAs epitaxial layers grown by MOCVD

Low temperature photoluminescence spectroscopy was used to study the band gap shrinkage in Zn and Si doped GaAs films grown by MOCVD technique. The PL experiments were carried out as a function of hole concentration $(10^{17}-1.5x^{20} cm^{-3})$ and electron concentration $(10^{17}-1.5x10^{18} cm^{-3})$. The main peak shifted to lower energy and the full width at half maximum (FWHM) increases with increasing hole concentrations. But in Si doped films the main peak shifted to higher energy and the FWHM increases with increasing electron concentrations. We have obtained an empirical relation for FWHM of PL, \Delta E(p) (eV) = $1.15x10^{-8} p^{1/3}$and for Si doped films \Delta E(n) (ev) = $1.4x10^{-8} n^{1/3}$. We also obtained an empirical relation for the band gap shrinkage, \Delta Eg(eV) = $-2.75x10^{-8} p^{1/3}$ in Zn doped GaAs as a function of hole concentration and \Delta Eg (eV) =$-1.45x10^{-8} n^{1/3}$ in Si doped GaAs as a function of electron concentration. These values indicates a significant hand gap shrinkage at high doping levels. These relations are considered to provide a useful tool to determine the hole/electron concentration in Zn/Si doped GaAs by low temperature PL measurement, respectively