D. C. Oh

Origin of forward leakage current in GaN-based light-emitting devices

The authors fabricated GaN-based light-emitting diodes (LEDs) on two different GaN templates with the same LED structure. One on thin GaN template (∼2μm) with high dislocation density [low (109cm−2)] grown by metal-organic vapor-phase epitaxy (sample A) and the other on thick GaN template (∼20μm) with comparatively low dislocation density [high (108cm−2)] by hydride vapor-phase epitaxy (sample B). In order to understand the mechanism of leakage current in LEDs, the correlation between current-voltage characteristics and etch pit density of LEDs was studied.

Lattice relaxation mechanism of ZnO thin films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy

We report on the lattice relaxation mechanism of ZnO films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy. The lattice relaxation of ZnO films with various thicknesses up to 2000nm is investigated by using both in situ time-resolved reflection high energy electron diffraction observation during the initial growth and absolute lattice constant measurements (Bond method) for grown films. The residual strain in the films is explained in terms of lattice misfit relaxation (compression) at the growth temperature and thermal stress (tension) due to the difference of growth and measurement temperatures. In thick films (>1μm), the residual tensile strain begins to relax by bending and microcrack formation.

Comparative study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks

The authors have an extensive study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks. In the photoluminescence (PL) spectra at 10 K, Zn-polar and O-polar faces show a common emission feature: neutral donor-bound excitons and their longitudinal-optical (LO) phonon replicas are strong and free excitons are very weak. However, in the PL spectra at room temperature (RT), Zn-polar and O-polar faces show extremely different emission characteristics: the emission intensity of Zn-polar face is 30 times larger than that of O-polar face and the band edge of Zn-polar face is 33 meV redshifted from that of O-polar face. The temperature dependence of photoluminescence indicates that the PL spectra at RT are closely associated with free excitons and their phonon-assisted annihilation processes. As a result, it is found that the RT PL spectra of Zn-polar face are dominated by the first-order LO phonon replica of A free excitons, while that of O-polar face is determined by A free excit...

Optimization of ZnSe growth on miscut GaAs substrates by molecular beam epitaxy

Abstract We report growth optimization of molecular beam epitaxy (MBE) grown ZnSe on GaAs (0xa00xa01) substrate tilted by 15° toward [1xa01xa00] in terms of beam equivalent pressure (BEP) ratio and growth temperature. A LT-ZnSe buffer was grown to reduce the formation of Ga–Se bonding, a well-known source of defect generation, due to interdiffusion through the heterointerface in the initial stage of growth. The ZnSe layer was further optimized by low-temperature-grown (LT-ZnSe) buffer. The optical and structural properties of the ZnSe film with LT-ZnSe and GaAs buffer are also analyzed by photoluminescence spectroscopy (PL), X-ray diffraction (XRD), and secondary ion mass spectroscopy (SIMS), which show very large intensity ratio of near-band-edge emission to deep level emission, narrow XRD peak width of (0xa00xa04) rocking curves, and abrupt ZnSe/GaAs heterointerface under the optimum growth condition, respectively. The optimum growth conditions are BEP ratio ( P Se / P Zn ) of 3 and growth temperature of 310°C with an LT-ZnSe buffer grown at 250°C.

Impact of V/III ratio on electrical properties of GaN thick films grown by hydride vapor-phase epitaxy

Impact of V/III ratio on electrical properties of GaN thick films are investigated, which are grown by hydride vapor-phase epitaxy. The authors note that the electron concentration of GaN films decreases with the increase of V/III ratio, while their electrical resistivity and electron mobility increase simultaneously. These indicate that enhancing V/III ratio suppresses electron-feeding sources in GaN films, which is not by generating electron-trapping centers but by reducing donor-type defects. On the other hand, it is shown that the linewidth of x-ray rocking curves in GaN films decreases and the near-band edge emission intensity of 10K photoluminescence spectra increases as V/III ratio increases. These mean that higher V/III ratio condition helps for reducing crystalline point defects in GaN films. In terms of theoretical fitting into the temperature-dependence curves of electron mobilities, it is found that the electron transport of GaN films grown in lower V/III ratio condition is more hampered by de...

Capacitance-voltage characteristics of ZnO∕GaN heterostructures

We have investigated the electrical properties of ZnO∕GaN heterostructures by capacitance-voltage (C-V) measurements. ZnO∕GaN heterostructures are fabricated on Ga-polar GaN templates by plasma-assisted molecular-beam epitaxy. The ZnO∕GaN heterostructures exhibit a plateau region of 6.5V in the C-V curves measured at 10kHz and room temperature. Moreover, it is found that a large electron density is accumulated at the interface of ZnO∕GaN, where the concentration approaches ∼1018cm−3. The distinct C-V characteristics are ascribed to large conduction-band discontinuity at the ZnO∕GaN heterointerface. It is suggested that the ZnO∕GaN heterostructure is a very promising material for the application to heterojunction transistors.

Structure and magnetic properties of Cr-doped GaN

Structure and magnetic properties of the Ga1−xCrxN (x=0.013, 0.063, and 0.101) have been investigated. The lattice constant of the c axis is systematically decreased with increasing Cr content. The local structure around a Cr atom maintains tetrahedral symmetry up to x=0.101 the same as the local symmetry of GaN by x-ray absorption fine structure analysis. The analysis on x=0.013 indicated that the second nearest neighbor around the absorbing Cr atoms consists of only 12Ga atoms. However, for x=0.063, the second nearest neighbor around the absorber consists of Ga and Cr atoms with an unexpectedly high ratio of about Ga:Cr=2:1. In CrK-edge x-ray absorption near edge structure, we observed the oxidation state of Cr ion increases with increasing Cr content. Ferromagnetic behavior was observed in all Ga1−xCrxN films. However, the paramagnetic component also coexists with the ferromagnetic component. Total effective magnetic moment per Cr atom decreased from 3.17μB∕atom for x=0.013 to 1.05 and 0.79μB∕atom for ...

Influence of heat treatments on electrical properties of ZnO films grown by molecular-beam epitaxy

We report on the influence of heat treatments on the electrical properties of ZnO films grown by molecular-beam epitaxy. We note that the electrical resistance of the ZnO films is significantly changed by the heat treatments: the electrical resistance increases with the increase of ambient temperature, but above a critical temperature the resistance decreases with the increase of temperature, irrespective of ambient gases. On the other hand, it is found that the large amount of photocurrent is generated in the ZnO films, exposed to white sources: the photocurrent decreases with the increase of the obtained resistance, and the current increases with the decrease of the resistance. Also, it is shown that the x-ray diffraction linewidth of the ZnO films is significantly decreased by the heat treatments. These indicate that the increase/decrease of the electrical resistance is ascribed to the annihilation/formation of the residual donor-type defects in the ZnO films by the heat treatments. It is suggested tha...

Electrical properties of heavily Al-doped ZnSe grown by molecular beam epitaxy

Abstract We study the electrical properties of heavily Al-doped ZnSe grown by molecular beam epitaxy, whose electron concentration is saturated. Hall measurements show that electron mobility decreases with the increase of Al cell temperature, while it is almost independent of measurement temperature. These phenomena can be explained in terms of scattering mechanisms by ionized impurities and point defects. The point defects, which are induced by the incorporation of excess Al atoms, result in carrier compensation in heavily Al-doped ZnSe and decrease the electron mobility. As Al concentration increases, deep level emission around 2.25xa0eV, associated with a point defect such as a self-activated center [Al Zn –V Zn ], develops in photoluminescence spectrum. The line width of (0xa00xa04) X-ray rocking curve is broadened due to degradation of crystal quality, which is induced by the increase of point defects with an increase of Al concentration. As a result, it is suggested that the carrier compensation in highly Al-doped ZnSe is dominated by generation of complex point defects such as self-activated centers [Al Zn –V Zn ].

High quality ZnTe heteroepitaxy layers using low-temperature buffer layers

Abstract Low dislocation density ZnTe heteroepitaxial layers are achieved by inserting a thin ZnTe buffer layer at low growth temperature (LT-buffer) followed by annealing. The ZnTe heteroepitaxy layer with a LT-buffer shows a narrow line width of X-ray rocking curve as narrow as 42xa0arcsec in (0xa00xa04) reflection and the dislocation density is estimated as 1.3×10 6 cm −2 . A cross-sectional view of transmission electron microscopy shows high structural quality of ZnTe layer with LT-buffers.