N. Otsuka

Heteroepitaxial evolution of AlN on GaN Grown by metal-organic chemical vapor deposition

We have investigated the morphological evolution during heteroepitaxial growth of AlN on GaN by metal-organic chemical vapor deposition at different V/III ratios. Two-dimensional layer–by–layer and step flow growth modes, combined with strain-induced cracking, are observed at low and intermediate V/III ratios, while nitrogen-rich conditions yield three-dimensional domain-like growth due to limited Al adatom diffusion. Samples grown at the metal-rich conditions exhibit a crosshatch pattern of surface undulations possibly related to the presence of misfit dislocations that form at the early stages of nucleation. Our observations show that the local stoichiometry at the vapor-solid interface strongly influences the adatom kinetics during the growth, thereby affecting the nature of strain relaxation and growth mode.

Electrical properties of nearly stoichiometric GaAs grown by molecular beam epitaxy at low temperature

Nearly stoichiometeric GaAs epilayers doped with Si or Be were grown by molecular beam epitaxy at a low temperature with closely controlled fluxes near the stoichiometric conditions. Excess As point defects in the epilayers which acted as trap sites of free carriers were studied by means of the Hall effect measurements. Under the same As/Ga flux condition, concentrations of excess As point defects which trap free carriers increase with increase of concentrations of Si or Be. With the same concentration of the dopants, on the other hand, concentrations of excess As point defects increase linearly with the As/Ga flux ratio. The dependence of carrier mobility on the trapped carrier concentrations indicates that free carriers are trapped by neutral point defects and change them into ionized ones. In the epilayers which are very close to the stoichiometric composition, Si exhibits its amphoteric nature, occupying both donor and acceptor sites and results in the self-compensation.

Photoluminescence of nearly stoichiometric LT-GaAs and LT-GaAs/AlAs MQW

Optical properties of nearly stoichiometric GaAs epilayers and GaAs/AlAs multiple quantum wells which were grown by molecular beam epitaxy at low temperatures were studied by utilizing photoluminescence measurements. Bright near-band edge emissions are observed from these low-temperature grown structures. GaAs epilayers exhibit strong carbon acceptor related emission and weak exciton related emission. The intensities of these near-band edge emissions are found to be highly dependent on the As/Ga flux ratio utilized for the growth. The low-temperature grown multiple quantum wells exhibit strong emission at 4.5 K whose intensities are comparable to those of multiple quantum wells grown at normal high temperatures. The emissions of the low-temperature grown samples are observed even at room temperature indicating effective confinement of excitons in the wells, but their intensities become weaker than those of the high-temperature grown samples by two order of magnitudes.

Large anomalous Hall resistance of pair δ-doped GaAs structures grown by molecular-beam epitaxy

Beryllium/silicon pair δ-doped GaAs structures grown by molecular-beam epitaxy exhibit a Hall resistance which has a nonlinear dependence on the applied magnetic field and which is strongly correlated to the negative magnetoresistance observed under the applied magnetic field parallel to the δ-doped layers. Dependence of the occurrence of the nonlinear Hall resistance on the growth condition is investigated. A significantly large increase in both the magnitude and the nonlinearity of the Hall resistance is observed from samples whose GaAs buffer layers are grown under the condition of a low As∕Ga flux ratio. Reflection high energy electron diffraction and electron microscope observations show that a faceted surface develops with the growth and postgrowth annealing of a GaAs buffer layer under the condition of a low As flux. From samples which have only Si δ-doped layers and exhibit the n-type conduction, such nonlinear Hall resistance is not observed. The nonlinearity of the Hall resistance of Be∕Si pair ...

Raman scattering study of GaAs crystalline layers grown by molecular beam epitaxy at low temperature

Raman scattering, x-ray diffraction, and transmission electron microscopy (TEM) were used to study GaAs layers grown by molecular beam epitaxy at low substrate temperatures (LT-GaAs). The intensity of forbidden Raman scattering of longitudinal optical and transverse optical phonons linearly increases as a function of the concentration of excess As in the range of [AsGa]=0.04×1020–1.175×1020 cm−3. Concentrations of excess As in LT-GaAs layers were estimated from the lattice spacings measured with an x-ray diffractometer. No obvious defect was seen in cross-sectional TEM images of these nonstoichiometric As-rich GaAs layers. The origin of the forbidden Raman scattering of the nonstoichiometric LT-GaAs layers is explained as the strain induced by AsGa (As antisite)-related defects with low structural symmetry.

X-ray diffraction analysis of the structure of antisite arsenic point defects in low-temperature-grown GaAs layer

A method for the structure analysis of point defects in a semiconductor layer is developed by combining x-ray diffraction and growth of a superlattice where the concentration of point defects is periodically varied in the growth direction. Intensities of satellite reflections from the superlattice depend predominantly on the atomic structure of point defects, and hence this method can be applicable to the case of a low concentration of point defects. By using this method, the atomic structure of antisite As point defects in GaAs layers grown by molecular-beam epitaxy at low temperatures has been analyzed. Measured intensity ratios of the first-order satellite reflection in the lower angle side to that in the higher angle side for a number of (hkl) reflections are compared with those calculated based on structure models. The analysis has shown that experimental intensity ratios cannot be reproduced by models which include only a uniform tetragonal lattice distortion and local atomic displacements around an...

Relaxation process of photoexcited carriers in GaAs structures with low-temperature-grown layers

Spatial relaxation processes of photoexcited carriers in GaAs structures are studied by means of photoluminescence spectroscopy. A single GaAs∕Al0.3Ga0.7As quantum well and a low-temperature-grown GaAs (LT-GaAs) layer containing a high concentration of excess arsenic are placed in a GaAs structure as optical markers; the former serves as the radiative recombination site, while the latter as the trapping site of photoexcited carriers. The photoluminescence intensity from the quantum well is significantly reduced by the presence of a LT-GaAs layer immediately next to a barrier layer. The effect of the LT-GaAs layer is exponentially enhanced as a thickness of the barrier layer decreases. The results suggest that once an excess As point defect is placed within an extent of a wave function of a photoexcited carrier, trapping of the photoexcited carrier occurs at an extremely fast rate. In a structure where a LT-GaAs is placed at a distant location from the quantum well, the photoluminescence intensity from the...

RAMAN SPECTRA OF TITANIUM DI-, TRI-, AND TETRA-CHLORIDES

We have obtained the Raman spectra of TiCln (n= 2, 3, and 4). Assignments of the observed Raman bands were made by a normal mode analysis. The force constants were determined from the observed Raman band frequencies. We have found that the Ti-Cl stretching force constant increases as the oxidation number of the Ti species increases.

X-ray diffraction analysis of LT-GaAs multilayer structures

Multilayer structures of low-temperature-grown GaAs(LT-GaAs) into which ultra-thin layers containing excess As are periodically introduced are grown by molecular beam epitaxy. The concentration of excess As in the ultra-thin layers is determined by the analysis of the intensity of a satellite reflection of the multilayer structure along with a peak shift of the 400 Bragg reflection from that of the GaAs substrate. The analysis shows that the concentration of excess As in the ultra-thin layers is significantly higher than those of thick LT-GaAs layers grown under the identical condition of the fluxes and the substrate temperature.

Photoluminescence study on heavily donor and acceptor impurity doped GaAs layers grown by molecular-beam epitaxy

Gallium arsenide layers doped with high concentrations of Be and Si by molecular-beam epitaxy are studied by photoluminescence (PL) spectroscopy. PL peaks from doped layers are observed at energies significantly lower than the band-gap of GaAs. The growth and doping conditions suggest that the origin of these peaks is different from that of low energy PL peaks, which were observed in earlier studies and attributed to impurity-vacancy complexes. The dependence of the peak energy on the temperature and the annealing is found to differ from that of the peaks attributed to impurity-vacancy complexes. On the basis of these observations, it is suggested that the low energy peaks are attributed to short range ordered arrangements of impurity ions. This possibility is examined by calculations of the PL spectra with models of pairs of acceptor and donor delta-doped layers and PL experiments of a superlattice of pairs of Be and Si delta-doped layers.