Seok Kyu Han

Anisotropic optical properties of free and bound excitons in highly strained A-plane ZnO investigated with polarized photoreflectance and photoluminescence spectroscopy

We have investigated the polarization dependence of the near-band-edge photoluminescence and photoreflectance spectra in nonpolar (A-plane) ZnO films under strong biaxial compressive strain. We show that anisotropic strain and the orientation of the nonpolar plane play an important role in determining the polarization selectivity and properties of excitonic transitions. We identified four distinct band-edge transitions at 3.449, 3.420, 3.386, and 3.326eV. They were identified as E2 and E1 free excitons, E1 excitons bound to a donor, and free-electron-to-bound-hole transition, respectively. Unlike previously reported results on relatively thick nonpolar films, the E1 exciton (lowest energy) was mainly polarized to E⊥c and weakly polarized to E∥c under strong biaxial compressive strain in the 100nm thick film. The E2 exciton (next higher energy) was exclusively polarized to E∥c. The localization energy of DX is 34meV, which is much larger than that in polar ZnO, and the DX was not thermally delocalized even...

ZnO nanowires prepared by hydrothermal growth followed by chemical vapor deposition for gas sensors

Vertical ZnO nanowires with a very high aspect ratio of more than 250 were synthesized by thermal chemical vapor deposition (CVD) on vertical nanowires grown by hydrothermal growth (HG) without using the metal catalyst. A ZnO seed layer grown by sputtering was crucial to synthesize the vertical nanowires in both the CVD and the HG processes. The underlying vertical nanowires grown by the HG process (HG-nanowire) enhanced nucleation of ZnO during the followed ZnO synthesis by CVD. Hydrogen-gas sensitivity was measured from the synthesized ZnO nanowires. High sensitivity of 72% and response time as fast as 30s was observed at 200°C from the vertical ZnO nanowires grown by CVD on the HG-nanowire, which is much higher than the sensitivity value of 50% from the vertical HG-nanowire and the value of 35% from the vertical CVD nanowires without the HG-nanowire. The results indicate that the method synthesizing the vertical ZnO nanowires by combining the HG process and thermal CVD is a very promising way to fabric...

Growth and structural properties of ZnO films on (10−10) m-plane sapphire substrates by plasma-assisted molecular beam epitaxy

Growth and structural properties of ZnO films on (10−10) m-plane sapphire substrates by plasma-assisted molecular beam epitaxy were studied. The ZnO films were grown at a wide range of growth temperature of 100–800°C. From the x-ray diffraction θ-2θ measurements, overwhelming ZnO (10−10) reflection was confirmed in addition to the very weak (0002) and (10−13) reflections. The sample grown at 500°C showed the highest purity with the very weak (10−13) and the negligible (0002) reflections. The full widths at half maximum (FWHMs) for the (10−10) x-ray rocking curves (XRCs) with ϕ=0° is much higher than those for with ϕ=90°, which indicates that the films have an anisotropic structural property. The FWHMs for the off-axis (10−11) XRCs were larger than by about 0.1° than those for the (10−10) XRCs with ϕ=90°. Two distinct features of the striated surface morphology with the grains along the ZnO ⟨0001⟩ direction and the inhomogeneity with the appearance of islands on the surface were observed. Detailed investigation on the microstructure by transmission electron microscopy revealed that ZnO (10−13) inclusion was present above the ZnO∕Al2O3 interface not at the interface. The ZnO (10−13) inclusion did reach to the top surface, which is believed to the cause for the extra spots in the reflection high energy electron diffraction patterns and the inhomogeneous islands in the atomic force microscope images.Growth and structural properties of ZnO films on (10−10) m-plane sapphire substrates by plasma-assisted molecular beam epitaxy were studied. The ZnO films were grown at a wide range of growth temperature of 100–800°C. From the x-ray diffraction θ-2θ measurements, overwhelming ZnO (10−10) reflection was confirmed in addition to the very weak (0002) and (10−13) reflections. The sample grown at 500°C showed the highest purity with the very weak (10−13) and the negligible (0002) reflections. The full widths at half maximum (FWHMs) for the (10−10) x-ray rocking curves (XRCs) with ϕ=0° is much higher than those for with ϕ=90°, which indicates that the films have an anisotropic structural property. The FWHMs for the off-axis (10−11) XRCs were larger than by about 0.1° than those for the (10−10) XRCs with ϕ=90°. Two distinct features of the striated surface morphology with the grains along the ZnO ⟨0001⟩ direction and the inhomogeneity with the appearance of islands on the surface were observed. Detailed investig...

Effects of two-step growth by employing Zn-rich and O-rich growth conditions on properties of (112¯0) ZnO films grown by plasma-assisted molecular beam epitaxy on sapphire

The authors report properties of a-plane ZnO films on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy in which two-step growth is employed. They show that the two-step growth is effective in improving structural and optical properties of a-plane ZnO films. Here, the two-step growth is preceded by growing the first layer under Zn-rich (O-rich) conditions and growing the second layer under O-rich (Zn-rich) conditions. All the grown samples show striated anisotropic morphology. The samples with the first, thin, O-rich layer plus the second, thick, Zn-rich layer show smaller root-mean-square (rms) roughness than those with the first, thin, Zn-rich layer plus the second, thick, O-rich layer. The sample with the 20-nm-thick first layer grown under O-rich condition shows the smallest rms roughness of 1.06nm, which is a smaller rms value than that of the sample grown under the single-step, stoichiometric condition. This sample shows the highest intensity of DX0 emission at 3.392eV and small ...

Effects of gallium doping on properties of a-plane ZnO films on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy

The authors report on the structural, optical, and electrical properties of Ga-doped a-plane (112¯0) ZnO films grown by plasma-assisted molecular beam epitaxy. Ga doping level was controlled by changing the Ga cell temperatures from 350 to 470 °C with an interval of 30 °C. With up to Ga cell temperatures of 440 °C, single crystalline Ga-doped a-plane ZnO films were grown; however, the sample with a Ga cell temperature of 470 °C showed polycrystalline features. The typical striated surface morphology normally observed from undoped ZnO films disappeared with Ga doping. ZnO films doped with Ga cell temperatures up to 440 °C did not show a significant change in full width at half maximum (FWHM) values of (112¯0) x-ray rocking curves by doping. The smallest FWHM values were 0.433° (ϕ=90°) and 0.522° (ϕ=0°) for the sample with a Ga cell temperature of 350 °C. The polycrystalline ZnO film with excessive Ga doping at the Ga cell temperature of 470 °C showed significantly increased FWHM values. Hall measurements a...

Microstructural characterization of high indium-composition InXGa₁-XN epilayers grown on c-plane sapphire substrates.

The growth of high-quality indium (In)-rich In(X)Ga(1-X)N alloys is technologically important for applications to attain highly efficient green light-emitting diodes and solar cells. However, phase separation and composition modulation in In-rich In(X )Ga(1-X)N alloys are inevitable phenomena that degrade the crystal quality of In-rich In(X)Ga(1-X)N layers. Composition modulations were observed in the In-rich In(X)Ga(1-X)N layers with various In compositions. The In composition modulation in the In X Ga1-X N alloys formed in samples with In compositions exceeding 47%. The misfit strain between the InGaN layer and the GaN buffer retarded the composition modulation, which resulted in the formation of modulated regions 100 nm above the In(0.67)Ga(0.33)N/GaN interface. The composition modulations were formed on the specific crystallographic planes of both the {0001} and {0114} planes of InGaN.

Growth Characteristics of AlN by Plasma-Assisted Molecular Beam Epitaxy with Different Al Flux

We have grown AlN nanorods and AlN films using plasma-assisted molecular beam epitaxy by changing the Al source flux. Plasma-assisted molecular beam epitaxy of AlN was performed on c-plane substrates with different levels of aluminum (Al) flux but with the same nitrogen flux. Growth behavior of AlN was strongly affected by Al flux, as determined by in-situ reflection high energy electron diffraction. Prior to the growth, nitridation of the substrate was performed and a two-dimensionally grown AlN layer was formed by the nitridation process, in which the epitaxial relationship was determined to be [11-20]AlN//[10-10], and [10-10]AlN//[11-20]. In the growth of AlN films after nitridation, vertically aligned nanorod-structured AlN was grown with a growth rate of , in which the growth direction was , for low Al flux. However, with high Al flux, Al droplets with diameters of about were found, which implies an Al-rich growth environment. With moderate Al flux conditions, epitaxial AlN films were grown. Growth was maintained in two-dimensional or three-dimensional growth mode depending on the Al flux during the growth; however, final growth occurred in three-dimensional growth mode. A lowest root mean square roughness of 0.6 nm (for area) was obtained, which indicates a very flat surface.

Growth and optical properties of ZnO nanorods prepared through hydrothermal growth followed by chemical vapor deposition

Very well aligned vertical ZnO nanorods with high optical quality were successfully synthesized by sequentially employing an aqueous solution method on a ZnO seed layer and a thermal chemical vapor deposition (CVD). The process is growing the upper ZnO nanorods by using CVD on top of the bottom nanorods synthesized by using an aqueous solution method on a Si substrate with a ZnO seed layer. These double-structured-nanorods showed a significant improvement of optical quality. The CVD re-grown upper nanorods showed orders-of-magnitude enhancement of the band edge emission and complete compression of the deep level emissions, compared with the solution-grown bottom nanorods. By comparing emission spectra from the series of samples with and without the seed layer or the second CVD growth, we can conclude that the improvement can only be achieved by combining the CVD, the solution-based synthesis technique, and the seed layer.