J. H. Chang

Low stacking-fault density in ZnSe epilayers directly grown on epi-ready GaAs substrates without GaAs buffer layers

We report a remarkably low stacking-fault density in ZnSe epilayers directly grown on commercial epi-ready GaAs (001) substrates without GaAs buffer layer growth. It is found that proper pregrowth treatments on epi-ready GaAs (001) substrates to obtain clean surfaces are crucial for two-dimensional layer-by-layer growth and suppression of stacking fault generation. Chemical etching using a NH4OH-based solution is found to reduce not only the thickness of the oxide layers but also the ratio of Ga2O3 to As2O3 to about half of that before etching. A clean GaAs (001) surface characterized by a (4×1) reconstruction in the present case is obtained after thermal cleaning followed by Zn pre-exposure. Reflection high-energy electron diffraction intensity oscillations with more than 50 periods are observed even from the very beginning of ZnSe growth on GaAs substrates cleaned as such. The stacking fault density in such a ZnSe layer is in the low-105u200acm−2 range.

Growth and characterization of beryllium-based II-VI compounds

We report on the growth and characterization of beryllium–chalcogenide layers prepared on GaAs (100) by molecular beam epitaxy. Be- and Te-terminated BeTe surfaces show (4×1) and (2×1) reconstructions, respectively. The stability of each surface is investigated by reflection high energy electron diffraction as a function of substrate temperature. The dependence of growth rate of BeTe on growth temperature and Be cell temperature is investigated. The best full width at half maximum (FWHM) of a (400) x-ray rocking curve of BeTe is 78 arcsec. The dependence of the ZnBeSe energy gap on Be composition is obtained by four-crystal x-ray diffraction (XRD) and low temperature photoluminescence measurements. The energy gap of Zn1−xBexSe varies as Eg=0.0107x+2.790u2009(eV) for small Be composition (x<0.25) at 77 K. Lattice-matched ZnBeSe (Eg=2.82u2009eV) and ZnMgBeSe (Eg=2.975u2009eV) layers show narrower XRD peaks, the FWHM values of which are 64 and 21 arcsec, respectively. The variation of FWHM of x-ray rocking curve due to ...

Effects of interfacial layer structures on crystal structural properties of ZnO films

Single crystalline ZnO films were grown on Cr compound buffer layers on (0001) Al2O3 substrates by plasma assisted molecular beam epitaxy. In terms of lattice misfit reduction between ZnO and substrate, the CrN and Cr2O3/CrN buffers are investigated. The structural and optical qualities of ZnO films suggest the feasibility of Cr compound buffers for high-quality ZnO films growth on (0001) Al2O3 substrates. Moreover, the effects of interfacial structures on selective growth of different polar ZnO films are investigated. Zn-polar ZnO films are grown on the rocksalt CrN buffer and the formation of rhombohedral Cr2O3 results in the growth of O-polar films. The possible mechanism of polarity conversion is proposed. By employing the simple patterning and regrowth procedures, a periodical polarity converted structure in lateral is fabricated. The periodical change of the polarity is clearly confirmed by the polarity sensitive piezo response microscope images and the opposite hysteretic characteristic of the piez...

Growth mechanism of ZnO low-temperature homoepitaxy

The authors report on the growth mechanism of ZnO homoepitaxy at the low-temperature range of 500u2009°C, which is unavailable to obtain high-quality ZnO films in heteroepitaxy. One typical set of ZnO films were grown on (0001) ZnO substrates by molecular-beam epitaxy: a standard structure without buffer and two buffered structures with high-temperature (HT) homobuffer and low-temperature (LT) homobuffer. As a result, the LT homobuffered structure had the outstanding material properties: the surface roughness is 0.9 nm, the full width at half maximum of x-ray rocking curve is 13 arcsec, and the emission linewidth of donor-bound excitons is 2.4 meV. In terms of the theoretical interpretation of the experimentally obtained electron mobilities, it was found that the LT homobuffered structure suffers less from the dislocation scattering and the ionized-impurity scattering compared to the HT homobuffered structure. It is proposed that, in the ZnO low-temperature homoepitaxy, the LT homobuffer plays a key role in i...

Surface reconstruction and crystal structure of MgSe films grown on ZnTe substrates by MBE

Abstract MgSe is an important component in Mg-based ternary or quaternary alloys, but is less understood due to its hygroscopy and unstable zinc blende structure. In order to get a clear knowledge of this material, we have investigated the growth of MgSe films of ZB,(zinc blende), structure on ZnTe substrates by MBE (molecular beam epitaxy). The surface reconstruction of MgSe under different flux ratios and growth temperatures have been studied systematically by RHEED (reflected high-energy electron diffraction). (4×1) reconstruction is observed under VI/II flux ratios of 16–30, while (2×1) reconstruction appears at higher VI/II ratios. The oscillation of RHEED intensity at the first stage of MgSe growth indicated that 2D growth was achieved, and thereafter the growth mode changed to 3D growth due to the lattice mismatch. The structure of the MgSe layer is further confirmed by XRD (X-ray diffraction). The XRD results indicated the growth of a MgSe film having a ZB structure for a layer below 2000xa0A thick. No diffraction is observed from 2000 to 4000xa0A thick MgSe films. Thicker samples show the formation of MgSe layers having a rock salt structure.

Structural and optical investigations of periodically polarity inverted ZnO heterostructures on (0001) Al2O3

We report the structural and optical properties of one-dimensional grating of ZnO consisting of periodically polarity inverted structures on (0001) Al2O3 substrates. The inversion domain boundaries (IDBs) between the Zn- and the O-polar ZnO regions were clearly observed by transmission electronic microscopy. The investigation of spatially resolved local photoluminescence (PL) revealed strong excitonic emission at the interfacial region including the IDBs. The possible mechanism of strong PL has been discussed by the consideration of atomic configuration and carrier collection including its lifetime and diffusion process in Zn- and O-polar regions. Therefore the authors conclude that the IDBs can be active for the strong emission not a nonradiative center.

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.

Optical properties of ZnMgSeTe quaternary alloys grown on ZnTe substrates by molecular-beam epitaxy

Zn1−xMgxSeyTe1−y quaternary alloys are grown on ZnTe(001) substrates for the first time. The photoluminescence properties of ZnMgSeTe are characterized by strong excitonic luminescence intensity with a narrow linewidth that indicate the high crystal quality and the composition homogeneity. It is deduced that the composition homogeneity of the alloy is originated from the stable sticking coefficient of group-VI species and the exclusion of impurity diffusion from the substrate. Type-I band lineup in the ZnTe/ZnMgSeTe single quantum well is estimated from the narrow and strong exciton luminescence line of the quantum well, and the band offset in this heterostructure is estimated based on the photoluminesence, photoluminescence excitation, and reflectance results as 20δEc:80δEv.

Characteristics of BeTe films grown by molecular beam epitaxy

Systematic investigations are performed on the surface morphology, structural properties, and p-type dopability of BeTe epilayers grown on GaAs (001) by molecular beam epitaxy under various growth temperatures and p(Te)/p(Be) flux ratios. A phase diagram for surface reconstruction is obtained with regard to the BEP (beam equilibrium pressure) ratio of p(Te)/p(Be) and growth temperature. Irrespective of growth temperature (300–500u200a°C), a surface phase transition from weak (4×1) to (2×1) reconstruction occurs near p(Te)/p(Be)∼10 as the p(Te)/p(Be) value increases. Epilayers grown at a low Te pressure of p(Te)/p(Be)⩽10 show Be droplets on the surface as observed by atomic force microscopy, which eventually leads to degraded crystallinity. It is noted that such growth features are different from conventional II–VI compounds. The differences in growth features are discussed in terms of the relationships between the equilibrium vapor pressures of the compound and constituent elements. By optimizing growth condi...

Energy band gap of the alloy Zn1−xMgxSeyTe1−y lattice matched to ZnTe, InAs and InP

Abstract The correlated function expansion (CFE) methodology has been used to estimate the energy band gap and the alloy bond length of Zn1−xMgxSeyTe1−y over the entire composition space (x , y) . The lattice matching condition was obtained by optimizing the alloy bond length to the bond length of the substrates (ZnTe, InAs and InP), and the corresponding band gap and its bowing effect were identified. Excellent agreement was obtained between the CFE-estimated band gap and the recent photoluminescence data for the alloy grown on ZnTe and GaAs. As the CFE only requires ternary input data, it can accelerate efforts to find desirable quaternary alloy compositions.