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Polarity-controlled ZnO films with an MgO buffer layer were grown on c -plane sapphire by plasma-assisted molecular beam epitaxy (MBE). The effect of the thickness of the MgO buffer layer on the MBE growth of ZnO was investigated. The growth rate of ZnO increased from 250 to 500 nm/h when the MgO layer thickness was increased from 1 to 3 nm. Convergent beam electron diffraction results showed that Zn-polarity (+ c ) growth occurred when the MgO layer was thicker than 3 nm, whereas O-polarity (− c ) growth occurred when the layer was less than 2 nm. Polarity conversion apparently occurred due to the difference in atomic structure between the MgO wetting layer and MgO islands. The minimum line-widths of (0 0 0 2) and (1 0 1¯ 0) ω -rocking curves were 70 and 722 arcsec, respectively, for O-polar ZnO, and 289 and 851 arcsec, respectively, for Zn-polar ZnO. The O-polar ZnO with a 1-nm-thick MgO buffer layer had a residual carrier concentration of 4.6×10 16 cm −3 with an electron mobility of 155 cm 2 /V s, whereas Zn-polar ZnO with an MgO layer thicker than 3 nm showed semi-insulating properties. In conclusion, control of the polarity of ZnO films grown on c -plane sapphire substrates by plasma-assisted MBE was achieved by inserting an MgO buffer layer of specific thickness between a ZnO film and a nonpolar substrate.

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.

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.

Both the growth mechanism and material properties of ZnO epilayers are found to be strongly dependent on slight misorientation of GaN templates. The observation by in situ reflection high-energy electron diffraction and atomic force microscopy reveals that the growth mode changes from a three-dimensional to a two-dimensional growth mechanism, as the misorientation of GaN templates is varied from [0001] just to 0.2 degree toward the a axis. However, ZnO grows under a three-dimensional mode for misorientation toward the m axis. The observed changes in the growth mechanism are discussed in terms of the different surface migration length along the m and a axis, which is caused by the different surface atomic arrangement along the a and m axis. With an increase in the misorientation angle from 0 to 0.2° toward the a axis, the linewidth of (0002) x-ray rocking curve shows a dramatic narrowing from 1768 to 277 arcsec, while the photoluminescence intensity of exciton emission shows a remarkable enhancement by mor...

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.

We have studied the optical orientation and polarization relaxation dynamics of electron spins in Ga‐doped and undoped ZnO films by time‐resolved Kerr rotation (TRKR). In the Ga‐doped film, we have observed a long‐lived spin signal more than 10 ns when the pump‐probe energy is resonant to the neutral donor‐bound excitons (D° X). It is not observed in the undoped film. Meanwhile, when the energy is resonant to free excitons, faster spin relaxation of about 2 ns is observed for itinerant electrons in both the films. The optical spin orientation is always parallel to the c‐axis in the ZnO films, and independent of the pump incidence angle. This contrasts with the observation in crystals with the zincblende structure.