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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.

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...

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.

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.

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.

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.

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.

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.

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.

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.

Experiments which led to the growth of a ZnSe layer suitable for a blue‐light‐emitting diode are presented. The ZnSe epitaxial layer was obtained by the solution growth method and, in the process, the electrical and optical properties of the ZnSe layers doped with various impurities were investigated. Through research it was concluded that the group‐VII elements (Cl,Br,I) were much more suitable than the group‐III elements (Al,Ga,In). It has also been proven that the Hall mobility and the intensity ratio of the deep to edge emission in the photoluminescence spectrum depended on the Se pressure applied during the annealing of the ZnSe source crystal. The deep emission in the photoluminescence spectrum was suppressed by using a ZnSe source material annealed prior to growth at a Se pressure of 8 atm. The maximum Hall mobility attained was 380 cm2/V s at a carrier density of 1.6×1017 cm−3.

We investigate post-annealing effects using an epi-GaN substrates for ZnO thin film growth by pulsed laser deposition (PLD). The growth of ZnO nanorods on a Si(100) substrate through a two-step process, annealing and off-axis PLD, without a metal catalyst is demonstrated as well. The as-grown films were annealed for one hour under atmospheric pressure air. ZnO morphologies after annealing were measured and the post-annealed ZnO films grown at T g = 700 o C had very smooth surfaces and the rms roughness was about 0.5 nm. Finally, ZnO post-annealed buffer layer was inserted between ZnO epi-layer and GaN/sapphire substrates. It was evident by AFM that growth temperature of 700 o C helps the films grow in a step-flow growth mode. It was confirmed by cathode luminescence (CL) spectrum that the ZnO film grown at 700 o C had very low visible luminescence, resulting in a decrease of the deep level defects. In the case of ZnO nanorods, controlling growth parameters during deposition enabled the adjustment of the dimensions of nanorods. The diameters of the grown nanorods ranged from 50 to 700 nm and the lengths are from 2 to 10 μm. The CL spectra were used to evaluate the states of defects within the ZnO nanorods. According to the CL results, the thinnest nanorod arrays were found to have fewer defects, while more defects were introduced as nanorods became thicker.

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.