Michihiro Sano

Correlation between grain size and optical properties in zinc oxide thin films

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.

Growth and characterization of Ga-doped ZnO layers on a-plane sapphire substrates grown by molecular beam epitaxy

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.

Polarity control of ZnO on sapphire by varying the MgO buffer layer thickness

Polarity-controlled ZnO films with an MgO buffer layer were grown on c-plane sapphire by plasma-assisted molecular-beam epitaxy. 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. Reflection high-energy electron diffraction results revealed that MgO growth was Stranski–Krastanov mode, and that the growth mode transition from two- to three-dimensional occurred when the layer was thicker than 1 nm. In conclusion, polarity conversion apparently occurs due to the different atomic structure between the wetting layer and islands of MgO.

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.

Characteristics of dislocations in ZnO layers grown by plasma-assisted molecular beam epitaxy under different Zn∕O flux ratios

We have investigated the characteristic of the dislocations in the ZnO layers grown on c sapphire by the plasma-assisted molecular beam epitaxy under the different Zn∕O flux ratios. The ZnO layers were characterized by the transmission electron microscopy (TEM) and the high-resolution x-ray diffraction (HRXRD). The TEM and HRXRD experiments revealed that the major threading dislocations (TDs) in the ZnO layers are the edge dislocations running along the c axis with Burgers vector of 1∕3⟨11–20⟩. The TD densities are determined to be 6.9×109, 2.8×109, and 2.7×109cm−2, for O-rich, stoichiometric, and Zn-rich grown ZnO, respectively. Different from the O-rich grown ZnO where the dislocations run along the c-axis, several dislocations in the stoichiometric and the Zn-rich grown ZnO are inclined to 20°∼30° from the c-axis. By considering the slip system in the wurtzite-structure ZnO, the glide planes of the dislocations are close to (10-10) for the O-rich grown ZnO and close to (10-11) for the stoichiometric an...

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.

Role of hydrogen in molecular beam epitaxy of ZnO

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...

Impact of Mixture Gas Plasma of N2 and O2 as the N Source on ZnO-Based Ultraviolet Light-Emitting Diodes Fabricated by Molecular Beam Epitaxy

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.

Effects of slight misorientation of GaN templates on molecular-beam-epitaxy growth of ZnO

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...