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High-quality Zn-polar ZnO films were grown on metal-organic chemical-vapor deposition (MOCVD) grown ZnO/a-sapphire templates by plasma-assisted molecular beam epitaxy (MBE). Annealing of the MOCVD-ZnO layer in an O 2 atmosphere improved the surface roughness and crystallinity of the template. Reflection high-energy electron diffraction and atomic force microscopy observations revealed that MBE-ZnO maintained its Zn-polarity, which is the polarity of the underlying MOCVD-ZnO, and that the optimized growth condition is the O-rich flux regime. Structural and optical properties of the MBE-ZnO films were significantly improved by growth under O-rich flux condition and introducing a low-temperature ZnO buffer. The line widths of (0002) and (1010) X-ray ω-rocking curves for MBE-ZnO films were 533 and 582 arcsec, respectively. The ground and excited (n = 2) states of A-exciton were clearly evident at 3.377 and 3.423 eV in low-temperature (4.2 K) photoluminescence of the Zn-polar ZnO film.

The role of hydrogen in the growth and material properties of ZnO films grown at temperature as low as 300 °C by plasma-assisted molecular beam epitaxy with and without hydrogen irradiation was investigated. Results showed that during growth, the surface morphology changed from small hemispherical grains (10 nm) to large hexagonal islands (100 nm) by molecular hydrogen irradiation. The observed changes in the surface morphology correlated with the surface migration length of Zn adatoms on the surface with or without H termination. X-ray diffraction and photoluminescence measurements showed that the structural and optical properties of ZnO films were significantly improved by H2 irradiation during growth and that the ZnO films grown here at low temperature (300 °C) by H2 irradiation had crystalline quality as high as that of ZnO grown at higher temperature (600 °C) without H2 irradiation. Secondary ion mass spectrometry and Hall effect measurements indicated that most of hydrogen incorporated in the ZnO fi...

High-quality ZnO epilayers have been grown on Zn-face ZnO substrates by plasma-assisted molecular beam epitaxy. With increasing O/Zn flux ratio from the stoichiometric to the O-rich, the growth mode changed from three- to two-dimensional growth and the line widths of (0 0 0 2) and (1 0 1 0) ω -rocking curves showed dramatic narrowing from 471 to 42 arcsec and from 1635 to 46 arcsec, respectively. These values are narrower compared to those of ZnO on sapphire and also those of device-grade MOCVD-grown GaN. Moreover, A-, B-excitons (FE A , FE B ), and the n =2 state of FE A at 3.378, 3.393, and 3.424 eV, respectively, were clearly observed in the low-temperature (4.2 K) photoluminescence spectrum of ZnO grown under O-rich flux conditions. Our results show that growth under high O-rich flux conditions is required to produce high-quality Zn-polar ZnO films.

High-electron-mobility ZnO epilayers are grown on c -plane sapphire with ZnO/MgO double-buffer layers by plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction and transmission electron microscopy analysis showed the growth mode of ZnO buffer layers (LT-ZnO) grown at low temperature significantly affected the structural properties of the ZnO epilayers grown at high temperature, thereby affecting the electrical properties of the epilayers. When LT-ZnO was grown at a high-growth-rate, three-dimensional growth dominated and threading dislocation (TD) density was as high as ca. 1×10 10 cm −2 . By using the low growth rate of LT-ZnO, two-dimensional growth dominated and TD density was reduced by one order of magnitude, down to ca. 2×10 9 cm −2 , yielding significantly improved electrical properties of the ZnO epilayers. The highest electron mobility in as-grown undoped ZnO film, 145 cm 2 V −1 s −1 , was achieved at room temperature, comparable to the mobility previously reported for high-quality bulk ZnO.

High-quality ZnO epilayers have been grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy. With increasing O/Zn flux ratio from the stoichiometric to the O-rich, the growth mode changed from three- to two-dimensional growth and the line widths of (0002) and (10-10) ω-rocking curves showed dramatic narrowing from 471 to 42 arsec and from 1635 to 46 arcsec, respectively. These values are narrower compared to those of ZnO on sapphire and also those of device-grade MOCVD-grown GaN. Moreover, A-, B-excitons (FE A , FE B ), and the n = 2 state of FE A at 3.378, 3.393, and 3.424 eV, respectively, were clearly observed in the low-temperature (4.2 K) photoluminescence spectrum of ZnO grown under O-rich flux conditions. Our results show that growth under high O-rich flux conditions is required to produce high-quality Zn-polar ZnO films.

High-quality ZnO films have been grown on Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy. With increasing O/Zn ratio from the stoichiometric to the O-rich flux condition, the growth mode and the surface morphology changed from three-dimensional growth with a rough surface to two-dimensional growth with a smooth surface. The minimum linewidth from the (1010) ω-rocking curve was 100 arcsec, and the n = 2 state of A-exciton was clearly observed in the photoluminescence at 4.2 K. Due to the reduction in the edge-type threading dislocation density, the residual carrier concentration in these homoepitaxial ZnO films was as low as 2.2×1016 cm-3, which is one order of magnitude lower than that previously reported for heteroepitaxial ZnO films.

The effect of O/Zn flux ratio on the crystalline quality of ZnO films grown at 700°C by plasma-assisted molecular beam epitaxy was investigated. Zinc beam flux (FZn) was varied from 2.2≤FZn≤8.3 A/s with an O2 flow rate of 3 sccm and RF power of 300 W. The surface morphology of the ZnO layers strongly depended on FZn. ZnO epilayers grown under stoichiometric flux conditions (i.e., FZn=5.1 A/s) had high crystalline quality, as was confirmed by using X-ray diffraction, photoluminescence (PL), and Hall-effect measurements: the full width at half maximum (FWHM) of a skew symmetric (1010) X-ray rocking curve was 720 arcsec; the dominant neutral donor bound exciton emission intensity in the PL spectra became maximum with the narrowest FWHM; the electron mobility was a maximum of 130 cm2V-1s-1; and a residual carrier concentration of 1.2×1017 cm-3 was achieved. We demonstrated that stoichiometric ZnO films have the lowest dislocation density and the highest electron mobility compared with ZnO films grown under nonstoichiometric flux conditions.

Gallium-doped ZnO epitaxial layers were grown on a-plane sapphire substrates by molecular beam epitaxy (MBE) at various Ga cell temperatures from 350°C to 450°C. The ZnO layers grown on a-plane sapphire were c-oriented without any trace of the 30° rotation domains often observed in ZnO on c-plane sapphire. The Ga concentration in Ga-doped ZnO increased from 4 x 10 16 to 7 × 10 18 cm -3 with increasing Ga cell temperature. The activation ratio of Ga was about unity when the Ga concentration exceeded 3 x 10 17 cm -3 . The photoluminescence (PL) spectra of Ga-doped ZnO were dominated by an emission at 3.362eV which can be assigned to emission of exciton bound to Ga-related neutral donors. The intensity of this emission was maximum when the Ga concentration was 2 × 10 18 cm -3 . The high crystalline quality of the Ga-doped ZnO epilayers was confirmed by X-ray diffraction (XRD). Hall effect measurement and Rutherford backscattering spectrometry. Our results show that high-quality Ga-doped n-type ZnO can be grown on a-plane sapphire substrates by using MBE.

The stoichiometry of grown ZnSe crystals was controlled by using the source crystal pre-annealed under an optimum Zn vapor pressure for Se Te solvent, or an optimum Se vapor pressure for Zn solvent. The full width at half maximum (FWHM) value of the X-ray rocking curve of the crystal grown from the Se Te solvent by using a source annealed under an optimum Zn pressure was three times narrower than that of the one grown from an unannealed source. In the crystal grown from a Zn solvent, the Hall mobility of the crystal obtained from the pre-annealed source under the optimum Se pressure was one and a half times as high as that of the crystal using an unannealed source. The original idea of a stoichiometry control of the II–VI compound semiconductors using a pre-annealed source is proposed.

Photoluminescence (PL) and time-resolved PL spectra of zinc oxide (ZnO) films were investigated as a function of the grain size of the microcrystals. Correlation was found between the grain size and the optical properties—in the bound exciton states, both the PL intensity and PL decay time increased with increasing the grain size. This correlation can be well explained by the existence of nonradiative-surface and/or -interface states in the grain boundaries of ZnO microcrystals.

Bulklike ZnSe crystal with crystallographic quality the same as commercial GaAs can be grown at a growth temperature of 950 °C by the solution growth method. The growth rate of the crystal depends on the temperature gradient in the solvent, the length of the heat sink and the mol % of Se in the solvent. The full width at half maximum (FWHM) of x‐ray rocking curve of the ZnSe crystal grown by this method is nearly the same as that of GaAs. The crystal grown from a Te/Se solvent of Te≤70 mol % can be regarded as essentially ZnSe from the cathodoluminescence measurement.

ZnO-based double-heterostructure ultraviolet light-emitting diodes (LEDs) were fabricated on n-type Zn-polar ZnO substrates by plasma-assisted molecular beam epitaxy employing mixture gas plasma of N2 and O2 as the N source. By using mixture gas plasma of N2 with a little added O2, the number of N atoms increased and N2 molecules decreased, as confirmed by optical emission spectrometry. The fabricated LEDs had an ultraviolet near-band-emission of around 380 nm. The integrated electroluminescence intensity of the LED fabricated using N2 and O2 plasma was more than 10 times higher than that of an LED fabricated using N2 plasma.