A. B. M. Almamun Ashrafi

Growth and characterization of hypothetical zinc-blende ZnO films on GaAs(001) substrates with ZnS buffer layers

A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.

Nitrogen-Doped p-Type ZnO Layers Prepared with H2O Vapor-Assisted Metalorganic Molecular-Beam Epitaxy

Nitrogen (N) doping in ZnO is studied to realize reproducible p-type conductivity. Undoped ZnO layers prepared on a-face of sapphire substrates with H2O vapor-assisted growth showed n-type conductivity. However, N-doped ZnO (ZnO:N) layers grown in the similar manner showed the type conversion to p-type conductivity. As-grown p-type ZnO:N layers showed low net acceptor concentrations (NA–ND) of ~ 1014 cm-3, but thermal annealing of the N-doped ZnO samples as well as the optimization of growth parameters increased the NA–ND up to ~ 5×1016 cm-3. Photoluminescence measurements showed consistent spectra with the electrical properties by a clear conversion from neutral donor-bound exciton emission in n-ZnO to neutral acceptor-bound exciton emission in the p-ZnO layers.

Single-crystalline rocksalt CdO layers grown on GaAs (001) substrates by metalorganic molecular-beam epitaxy

In this letter, we report the growth of single-crystalline rocksalt CdO layers on (001) GaAs substrates using ZnS buffer layers. The growth processes of CdO layers were studied by reflection high-energy electron diffraction (RHEED), and the grown CdO layers were evaluated with atomic force microscopy (AFM), and x-ray diffraction (XRD) measurements. After an initial growth delay, the formation of polycrystalline CdO was observed in RHEED measurements during the initial growth of very thin CdO layers. With the increase of the CdO layer thicknesses, streaky RHEED patterns were observed, which indicate the formation of single-crystalline cubic-phase CdO layers. Surface morphology of the CdO layers observed by AFM was atomically flat with root-mean-square roughness of ∼1 nm. The crystalline structures were elucidated from XRD measurements by the determination of the lattice constant to be 4.686±0.001 A, indicating the single-phase rocksalt CdO structure.

H2O-Vapor-Activated ZnO Growth on a-Face Sapphire Substrates by Metalorganic Molecular-Beam Epitaxy

ZnO layers have been grown on a-face sapphire substrates using H2O vapor and O2 plasma as oxygen sources. The growth rate of H2O-vapor-assisted growth of ZnO (v-ZnO) is ~3 times higher than that of plasma-assisted growth of ZnO (p-ZnO) for the same DEZn flux. This report addresses the higher growth activation of ZnO layers with H2O vapor than with O2 plasma at the same substrate temperature. A sharp and intense photoluminescence (PL) spectrum is observed in v-ZnO at the neutral donor-bound exciton energy of 3.368 eV at 16 K. The PL excitation spectrum measurement revealed A and B free exciton energies of 3.382 and 3.388 eV, respectively. On the other hand, the p-ZnO showed the band-edge emission energy of 3.373 eV but with a very weak PL intensity and broader half width. The PL intensity from v-ZnO was ~104 times brighter than that of p-ZnO and the integrated PL intensity measured at room temperature was kept to ~1/8 of that measured at 16 K.

CdO epitaxial layers grown on (0 0 1) GaAs surfaces by metalorganic molecular-beam epitaxy

Atomically flat CdO thin layers were grown on GaAs(00 1) substrates with ZnS buffer layers. The grown layers were evaluated by reflection high-energy election diffraction (RHEED). atomic force microscopy (AFM). X-ray rocking curve (XRC), and transmission electron diffraction (TED) measurements. The RHEED observations of CdO thin layers showed two-dimensional growth mode revealed by streaky patterns with excellent homogeneity. through [110] and [I I 0] directions. This streaky pattern is the reflection of the flat surface morphology in atomic level assessed by AFM measurements with the surface root mean square roughness of 1 nm. The crystalline structure as well as quality of the thin CdO layers was investigated by X-ray diffraction and XRC measurements which indicated the single-crystalline rocksalt CdO thin layers with minimum full-width at half maximum under the optimal growth condition. The TED observation showed the epitaxial relationship between the rocksalt CdO and zincblende GaAs substrate as [1 1 0] CdO ∥[1 1 0] GaAs , [1 1 0] Cao ∥[1 1 0] GaAs , and [0 0 1] CaO ∥[001] GaAs .