Kazuhiro Miyamoto

Homoepitaxial growth of high-quality zn-polar ZnO films by plasma-assisted molecular beam epitaxy

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.

Effect of O/Zn Flux Ratio on Crystalline Quality of ZnO Films Grown by Plasma-Assisted Molecular Beam Epitaxy

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.

Effects of ZnO/MgO Double Buffer Layers on Structural Quality and Electron Mobility of ZnO Epitaxial Films Grown on c-Plane Sapphire

High-electron-mobility ZnO epilayers were grown on c-plane sapphire with ZnO/MgO double-buffer layers by plasma-assisted molecular beam epitaxy. Precisely controlled low growth rate of the double-buffer layers was crucial to the improvement of electrical properties. Both X-ray diffraction ω rocking curve measurement and calculated electron mobilities revealed that the improvement of electron mobility of ZnO films is due to a decrease in dislocation density. The highest electron mobility of 137 cm2V-1 s-1 in as-grown ZnO film was achieved at room temperature.

ZnO Thin Films Prepared by Ion-Assisted Deposition Methods

Transparent conducting ZnO films were prepared by an ion-assisted deposition method. The glass substrate was irradiated with Ar ions during deposition of amorphous films and the c-axis-oriented ZnO film with the lowest resistivity is obtained when the growing film is irradiated with Ar plasma. Optical emission lines due to Ar, Zn2, Zn and O of neutral species were detected. In particular, the ratio of emission line intensity O(777 nm)/Zn(635 nm) is closely related with film properties. The control of plasma conditions by the check of the emission line intensity provides a method of obtaining a high-quality transparent conducting ZnO film.

The Role of Surface Chemistry in Growth and Material Properties of ZnO Epitaxial Layers Grown on a-Plane Sapphire Substrates by Plasma-Assisted Molecular Beam Epitaxy

The role of surface chemistry in the growth and material properties of ZnO epilayers grown on a-plane sapphire by plasma-assisted molecular beam epitaxy is investigated. The surface chemistry of a-plane sapphire is controlled from O-rich to Al-rich by changing the pregrowth treatment from oxygen plasma to atomic hydrogen. Such a change in surface treatment causes a significant difference in growth mode presumably due to a difference in the surface migration of adatoms: two-dimensional growth is more favorable on an atomic-H-treated surface. Accordingly, ZnO layers grown on an atomic-H-treated surface show a smoother surface morphology consisting of larger hexagonal islands with a typical size of 2.5 µm, which should be compared with an island size of 0.2 µm on an O-plasma-treated surface. The observed surface morphology is found to be consistent with the result of X-ray diffraction analysis that shows a larger coherent length for ZnO films with an atomic-H pretreatment. Accordingly, the ZnO films with an atomic-H treatment show stronger excitonic emission with weaker deep-level emission than those on an O-treated surface. High-quality undoped ZnO epilayers with an electron mobility as high as 130 cm2V-1s-1 and an electron concentration of 1.4×1017 cm-3 are grown with good reproducibility.