Showing papers on "Wurtzite crystal structure published in 1992"

GaN, AlN, and InN: A review

Abstract: The status of research on both wurtzite and zinc‐blende GaN, AlN, and InN and their alloys is reviewed including exciting recent results. Attention is paid to the crystal growth techniques, structural, optical, and electrical properties of GaN, AlN, InN, and their alloys. The various theoretical results for each material are summarized. We also describe the performance of several device structures which have been demonstrated in these materials. Near‐term goals and critical areas in need of further research in the III–V nitride material system are identified.

Luminescence properties of CdSe quantum crystallites: Resonance between interior and surface localized states

Abstract: We use time‐, wavelength‐, temperature‐, polarization‐resolved luminescence to elucidate the nature of the absorbing and ‘‘band edge’’ luminescing states in 32 A diameter wurtzite CdSe quantum crystallites. Time‐resolved emission following picosecond size‐selective resonant excitation of the lowest excited state shows two components—a temperature insensitive 100 ps component and a microsecond, temperature sensitive component. The emission spectrum, showing optic phonon vibrational structure, develops a ∼70 wave number red shift as the fast component decays. Photoselection shows the slow component to be reverse polarized at 10 K, indicating this component to be the result of a hole radiationless transition. The 100 ps emitting state is repopulated thermally as temperature increases from 10 to 50 K. All available data are interpreted by postulating strong resonant mixing between a standing wave molecular orbital delocalized inside the crystallite and intrinsic surface Se lone pair states. The apparent excit...

Relaxation Process of the Thermal Strain in the GaN/α-Al2O3 Heterostructure and Determination of the Intrinsic Lattice Constants of GaN Free from the Strain

Abstract: The relaxation process of the thermal strain in a GaN film due to the thermal expansion coefficient difference in the GaN(0001)/α-Al2O3(0001) heterostructure is studied by varying the film thickness of GaN in a wide range from 1 to 1200 µm. The lattice constant c has a large value of 5.191 A at a film thickness less than a few microns, while it decreases to about 150 µm, and becomes constant above 150 µm, indicating that the strain is almost completely relaxed. The intrinsic lattice constants of wurtzite GaN free from the strain, a0 and c0, are determined to be 3.1892±0.0009 and 5.1850±0.0005 A, respectively.

X-ray observation of the structural phase transition of aluminum nitride under high pressure.

Abstract: The III-V compound AlN has been studied under pressure to 30 GPa by x-ray diffraction in a diamond-anvil cell. At 22.9 GPa the wurtzite-to-rocksalt phase transition takes place, accompanied by a volume reduction of 17.9%. The structure of the high-pressure phase and the amount of volume reduction are in agreement with a recent calculation of Gorczyca et al. [Solid State Commun. 79, 1033 (1991)]. On the other hand, the bulk modulus of the wurtzite phase (${\mathit{B}}_{0}$=207.9\ifmmode\pm\else\textpm\fi{}6.2 GPa) agrees well with a calculation of Ching and Harman [Phys. Rev. B 34, 5305 (1986)]. The axial ratio c/a in the wurtzite phase monotonically decreases wtih increasing pressure from the atmospheric value of 1.60.

Luminescent properties of ZnS:Mn films deposited by spray pyrolysis

Abstract: The room temperature, photoluminescent properties of manganese‐doped zinc sulfide films deposited by spray pyrolysis are reported. These films were deposited on Pyrex glass substrates at atmospheric pressure using air as a carrier gas. All films were polycrystalline with a wurtzite (hexagonal) structure. The manganese doping was achieved by mixing MnCl3 with the starting solution to deposit ZnS. The photoluminescence spectra was measured at room temperature as a function of the different deposition parameters and the Mn concentration. Besides the characteristic light emission associated with Mn impurities in a ZnS matrix, a peak associated with the self‐activated emission was also observed under certain deposition conditions (low substrate temperatures and/or long deposition times). The presence of chlorine impurities in the films is suggested to be associated with this emission. The Mn luminescence presents a quenching effect with the Mn concentration. This quenching effect is similar to the one reported on films deposited by other techniques. The light emission at this center has an activation energy of 0.71±0.05 eV with the deposition temperature. This energy is proposed to be related with the energy required by the Mn atoms to find a proper site during the growth process to form a Mn2+ center.

Luminescence properties of thin zinc oxide membranes prepared by the sol-gel technique: change in visible luminescence during firing

Abstract: Thin inc oxide (ZnO) membrane were prepared by the sol-gel technique using ZnO colloids composed of nano-sized wurtzite crystals and supported on either quartz slides or silicon wafers. The change in visible luminescence intensity during firing of the membranes was examined from the standpoint of the change in surface properties of the ZnO crystals. Membranes fired at temperatures lower than 180 o C exhibited strong visible luminescence, but the intensity decreased markedly when the membranes were fired at temperatures above 180 o C

Crystal Structure Change of GaAs and InAs Whiskers from Zinc-Blende to Wurtzite Type

Abstract: Crystal structures of GaAs and InAs whiskers grown by metalorganic vapor phase epitaxy are evaluated by means of a transmission electron microscope. The whiskers are grown epitaxially on GaAs substrates with diameters of 20-100 nm and lengths of 1-5 µm. They have the following characteristics. 1) GaAs whiskers have layered structures with 2-30 nm period, that are the 111 rotating twins of the zinc-blende type. 2) InAs whiskers also have layered structures which consist of wurtzite and zinc-blende type crystals. The wurtzite type InAs is observed for the first time in this study. The volume ratio of these two types strongly depends on the growth conditions, such as substrate temperature and material gas pressure. This suggests that defect-free whiskers with a single phase that are useful for quantum wire devices can be grown by controlling the growth conditions.

First-principles total-energy calculation of gallium nitride.

Abstract: A first-principles total-energy calculation is performed on gallium nitride (GaN). The equilibrium lattice parameters, the bulk modulus, and the cohesive energy of GaN in the wurtzite structure is calculated and compared with experimental values. In our calculation, the ground state of GaN is a zinc-blende structure, and the difference between these two phases is around 1.4 mRy.

CdS/CdSe intrinsic Stark superlattices

Abstract: Strained layer superlattices of wurtzite CdS/CdSe have been grown on (111)A GaAs substrates by metalorganic chemical vapor deposition and their optical properties studied by photoluminescence spectroscopy. It is shown that the superlattice layers contain giant strain‐induced piezoelectric fields exceeding 2×108 V m−1. These fields are similar to those reported for (111) orientated III–V superlattices, but an order of magnitude greater. The recombination energies from a series of samples provide evidence for a type II conduction band offset of 0.23±0.10 eV (the electron wells being in the CdS), with the band structure heavily modified by the internal electric fields. In addition, the photoluminescence peak emission energy shows a strong dependence on the excitation power. This is interpreted as further evidence for the effect of internal fields. We conclude that this system shows new effects not previously observed in II–VI compound superlattices. The large band‐gap tunability and the space‐charge effects ...

Preparation of quantum-size CdS-doped Na2OB2O3SiO2 glasses with high non-linearity

Abstract: A series of optically transparent quantum-size CdS crystal doped Na 2 O B 2 O 3 SiO 2 glasses have been prepared from NaOAc, B(OC 2 H 5 ) 3 , Si(OCH 3 ) 4 and Cd(OAc) 2 ·2H 2 O by a sol-gel process. The glasses with CdS microcrystallites of wurtzite structure were developed by exposing CdO in the pre-heated gel to H 2 S gas and reheating to 590°C to densify the gel. Fully densified glasses containing 0.5–8.0 wt% CdS exhibit third-order non-linear susceptibility, χ (3) , of (1.2–6.3) × 10 −7 esu for degenerate four-wave mixing (DFWM) with short (7 ns) laser pulses at wavelengths 450 and 460 nm. The average microcrystallite diameter observed by transmission-electron microscopy was from 3.5 to 6.0 nm.

Surface atomic and electronic structure of ZnO polymorphs.

Abstract: The surface atomic and electronic structures of the zinc-blende-, wurtzite-, and rocksalt-structure polymorphs of ZnO are investigated within a unified tight-binding (TB) model. For the fourfold-coordinated wurtzite and zinc-blende structures, large relaxations ({similar to}0.54 A) of the surface atoms are observed as a consequence of their ability to undergo approximately bond-length-conserving motions. For the sixfold-coordinated rocksalt structure, these motions are prohibited by the close-packed surface topology. Thus, only small movements of the cation and anion are expected in the direction normal to the surface. Within the resolution of this TB model ({similar to}0.01 A), we observe no surface relaxation of the rocksalt structure.

Origin of dual epitaxy in the growth of CdTe on (211) GaAs

Abstract: The atomic structure of the (133)CdTe/(211)GaAs interface is analyzed by high resolution transmission electron microscopy in order to elucidate the origin of the dual epitaxy in the growth of CdTe on (211)GaAs by molecular beam epitaxy. The analysis shows that the lattice mismatch at the interface is accommodated by a novel mechanism, which occurs with the combination of the 14.6% lattice mismatch between CdTe and GaAs and one wurtzite type bond sequence on the (211) substrate surface.

Magnetoreflectance and magnetization of the Co-based wurtzite-structure diluted magnetic semiconductor Cd1-xCoxSe.

Abstract: Magnetoreflectance measurements of excitonic interband transitions in the wurtzite-structure compounds Cd 1 − x Co S e with various directions of applied magnetic field combined with magnetization measurements, allowed us to determine the ion-carrier exchange parameters for the conduction and valence bands (N 0 α=258±10 meV, N 0 β=−1883±30 meV), without making any assumption concerning the band parameters. The magnitude of these exchange parameters, compared with the values previously obtained for Cd 1−x Fe x Se and Cd 1−x Mn x Se, indicates a small variation of the conduction-band exchange parameter, but an important enhancement of the valence-band exchange parameter. This large variation of the hybridization-induced exchange is explained within a generalized Schrieffer-Wolff formula

Photoluminescence characterization of AlGaN-GaN pseudomorphic quantum wells and calculation of strain induced bandgap shifts

Abstract: The low temperature (77 K) photoluminescence characteristics of Al x Ga1-x N-GaN strained layer quantum wells with differentx values grown by metalorganic chemical vapor deposition (MOCVD) were investigated. The photoluminescence spectra were useful in analyzing both quantum confinement effects and strain induced energy shifts. The strain induced shifts were found to be a strong function of aluminum compositionx. A model was developed to calculate the strain induced bandgap shifts atk = 0. The values predicted by this model which took into account the wurtzite crystal structure of the material system, were in good agreement with (i.e. within 2 meV of) the experimentally measured shifts.

A Comparison of the Wurtzite and Zincblende Band Structures for SiC, AlN and GaN

Abstract: Total energy and band structure results of linear-muffin-tin orbital calculations within the atoiuic-spliere-approxiiuation are presented for SiC, GaN and A1N in the zincblende and the wurtzite structure. We discuss the origin of the directness or indirectness and the relative magnitudes of the band gaps.

A Comparative Study of GaN Epitaxy on Si(001) and SI(111) Substrates

Abstract: Epitaxial GaN films were grown on Si(001) and (111) substrates, using a two-step process. The films on Si(001) are single crystalline having the zincblende strucutrue, while those on Si(111) have the wurtzite structure. The crystalline qualities of the films were studied by X-ray diffraction. While the zincblende GaN has a perfect cubic structure, the wurtzitic GaN on Si(111) has a considerable amount of stacking faults along (0002) direction, which gives rise to significant zincblende component with (111) orientation. Room temperature resistivity for both type of GaN films was found to be larger than 100 Ω· cm. The temperature dependence of th resistivity gives a defect level at HOmeV for wurtzitic GaN and 80meV for cubic GaN. Optical studies show that GaN on Si(001) has a gap 3.2eV, and GaN on Si(111) has a gap of 3.4eV at room temperature.

Habit and structure of Cd(S, Se) quantum dots in silicate glasses: High-resolution transmission electron microscopy

Abstract: Cd(S, Se) nanocrystals were developed in silicate glass by heat treatments and were characterized by high-resolution transmission electron microscopy. They are crystallized in the hexagonal structure of wurtzite type. Their average size increases with increasing heat-treatment time from 5 to 13 nm at 700°C and from 5 to 10 nm at 675°C. The crystal morphology depends on the size. Below a critical size sc, the shape is close to either a sphere or a hexagonal prism with nearly equivalent dimensions in every direction. Above sc, the crystallites definitely adopt a prismatic habit, growing longer in the direction of the c axis. The critical size depends on the temperature of the heat treatment: At 700°C, s c is about 10nm and, at 675°C, it is around 6–7 nm. The basal faces of the prism present an asymmetry, indicating different growth rates in opposite directions of the c axis, in relation to the polarity of the wurtzite structure.