Kwang Gug Yim

Effects of post-heated ZnO seed layers on structural and optical properties of ZnO nanostructures grown by hydrothermal method

ZnO nanostructures were grown by the hydrothermal method on ZnO seed layers post-heated in the range 350°C–500°C. The effects of the post-heated ZnO seed layers on the structural and optical properties of the ZnO nanostructures were investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD) spectroscopy, and photoluminescence (PL) spectroscopy. The average grain sizes in the ZnO seed layers increased with increasing post-heating temperature, and nano-fibrous structures were observed on the surface of the ZnO seed layers post-heated at 450°C. The ZnO seed layers post-heated in the range 350°C–500°C affected the residual stress, lattice distortion in the ZnO nanostructures and the intensity, positions, and full widths at half maximum of 2θ and PL peaks in the XRD and PL spectra for the ZnO nanostructures.

Growth and characterization of seed layer-free ZnO thin films deposited on porous silicon by hydrothermal method

Catalyst- and seed layer-free zinc oxide (ZnO) thin films were grown on porous silicon (PS) by a hydrothermal method. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and photoluminescence (PL) were carried out to investigate the structural and optical properties of the PS and the ZnO thin films. The ZnO thin films have an extraordinary tendency to grow along the a-axis with a hexagonal wurtzite structure. The growth rate of the ZnO thin films was increased with the increase in the precursor concentration. The crystal quality of the ZnO thin films was improved, and the residual stress was decreased as their thickness increased. Monochromatic indigo and red light emission peaks were observed from the ZnO thin films and the PS, respectively. At an excessively high precursor concentration, a green light emission peak was also observed in the ZnO thin films. The luminescent efficiency of the indigo light emission peak was enhanced with the increase in the precursor concentration.

Photoluminescence Studies of Porous ZnO Nanorods

Porous ZnO nanorods were prepared by thermal annealing at 700 °C in argon atmosphere. Temperature-dependent photoluminescence (PL) was carried out to investigate the optical properties of the porous ZnO nanorods. The average diameter and density of the pores formed on the surface of the porous ZnO nanorods are 25 nm and 1.2×1010 cm-2, respectively. Six peaks at 3.380, 3.366, 3.338, 3.267, 3.227, and 3.198 eV, corresponding to free exciton (FX), neutral donor bound exciton (D0X), first-order longitudinal optical phonon replica of free exciton (FX-1LO), FX-2LO, FX-3LO, and FX-4LO emissions, are obtained at low temperature (10 K). The FX emission peak energy of the porous ZnO nanorods exhibits an anomalous behavior as the temperature is increased to 100 K. The activation energy of the porous ZnO nanorods is found to be about 8.6 meV, and the values of fitting parameters to Varshnis empirical equation are α= 6×10-4 eV/K and β= 680 K.

Optical Properties of ZnO Soccer-Ball Structures Grown by Vapor Phase Transport

ZnO soccer balls were grown on an Au-catalyzed Si(100) substrate by vapor phase transport (VPT) with a mixture of zinc oxide and graphite powders. Temperature-dependent PL was carried out to investigate the mechanism governing the quenching behavior of the PL spectra. From the PL spectra of the ZnO soccer balls at 10 K, several PL peaks were observed at 3.365, 3.318, 3.249, and 3.183 eV corresponding to excitons bound to neutral donors (DoX), a donor–acceptor pair (DAP), first-order longitudinal optical phonon replica of donor–acceptor pair (DAP-1LO), and DAP-2LO, respectively. The mixed system composed of the free exciton (FX) and DoX and the DAP radiative lifetimes were estimated with a theoretical relation between the lifetime and the spectral width. The exciton radiative lifetimes were observed to increase linearly with temperature.

Effects of Cadmium Content on Optical Parameters of CdxZn1-xO Thin Films Prepared by Sol–Gel Method

CdxZn1-xO thin films with different Cd contents ranging from 0 to 0.75 were prepared by the sol–gel spin-coating method. The optical parameters, such as optical band gap, absorption coefficient, Urbach energy, refractive index, dispersion parameter, and optical conductivity, were studied in order to investigate the effects of Cd content on the optical properties of the CdxZn1-xO thin films. The optical band gap and Urbach energy decreased as Cd content increased. The dispersion energy, single-oscillator energy, average oscillator wavelength, average oscillator strength, and refractive index at infinite wavelength of the CdxZn1-xO thin films were affected by Cd incorporation. The optical conductivity of the CdxZn1-xO thin films also increased with increasing photon energy.

Effects of Growth Conditions on Properties of ZnO Nanostructures Grown by Hydrothermal Method

ZnO nanostructures were grown on an Au seed layer by a hydrothermal method. The Au seed layer was deposited by ion sputter on a Si (100) substrate, and then the ZnO nanostructures were grown with different precursor concentrations ranging from 0.01 M to 0.3M at and different growth temperatures ranging from to with 0.3 M of precursor concentration. FE-SEM (field-emission scanning electron microscopy), XRD (X-ray diffraction), and PL (photoluminescence) were carried out to investigate the structural and optical properties of the ZnO nanostructures. The different morphologies are shown with different growth conditions by FE-SEM images. The density of the ZnO nanostructures changed significantly as the growth conditions changed. The density increased as the precursor concentration increased. The ZnO nanostructures are barely grown at and the ZnO nanostructure grown at has the highest density. The XRD pattern shows the ZnO (100), ZnO (002), ZnO (101) peaks, which indicated the ZnO structure has a wurtzite structure. The higher intensity and lower FWHM (full width at half maximum) of the ZnO peaks were observed at a growth temperature of , which indicated higher crystal quality. A near band edge emission (NBE) and a deep level emission (DLE) were observed at the PL spectra and the intensity of the DLE increased as the density of the ZnO nanostructures increased.

Post‐Annealing Effects on Structural and Optical Properties of a‐axis Oriented ZnO Nanorods

ZnO nanorods with a‐axis orientation on Si (100) substrate were grown by hydrothermal method and then post‐annealing process was carried out at various temperatures ranging from 500 to 900 °C in vacuum. Atomic force microscopy (AFM), scanning electron microscopy (SEM), X‐ray diffraction (XRD), and photoluminescence (PL) were carried out to investigate the structural and optical properties of the ZnO nanorods. XRD pattern of the ZnO nanorods exhibits three diffraction peaks a 31.84°, 34.48°, and 66.43°, corresponding to ZnO (100), (002), and (200), respectively. The texture coefficient (TC) ratio of a‐axis to c‐axis is increased as the annealing temperature is increased. The residual tensile stress of the ZnO nanorods is increased and the bond length is slightly decreased with increase in the annealing temperature. The near‐band‐edge emission (NBE) of the ZnO nanorods is blue‐ shifted by the annealing process.

ZnO Thin Films Grown on Porous Silicon by Plasma-Assisted Molecular Beam Epitaxy

ZnO thin films were grown on porous silicon by plasma-assisted molecular beam epitaxy. Thermal annealing was then carried out at various temperatures in the range from 500 to 700 °C for 10 min. Atomic force microscopy, scanning electron microscopy, X-ray diffraction, and photoluminescence were carried out to investigate the effects of the annealing temperature on the properties of the ZnO thin films. The ZnO thin films exhibit a mountain-range-like surface. With increasing annealing temperature, the grain size increased and the residual stress decreased. The activation energy of the free exciton (FX) emission from the ZnO thin films was found to be about 32 meV and the fitting parameters in Varshnis empirical equation were α= 2×10-3 eV/K and β= 1200 K. The photoluminescence (PL) intensity ratio of near-band-edge emission to deep-level emission of the ZnO thin films increased with increasing annealing temperature.

GaAs Epilayer Growth on Si(100) Substrates Cleaned by As/Ga Beam and Its RHEED Patterns

The GaAs epitaxial layers were grown on Si(100) substrates by molecular beam epitaxy(MBE) using the two-step method. The Si(100) substrates were cleaned with different surface cleaning method of vacuum heating, As-beam, and Ga-beam at the substrate temperature of . Growth temperature and thickness of the GaAs epitaxial layer were and 1 , respectively. The surface structure and epitaxial growth were observed by reflection high-energy electron diffraction(RHEED) and scanning electron microscope(SEM). Just surface structure of the Si(100) substrate cleaned by Ga-beam at shows double domain (). RHEED patterns of the GaAs epitaxial layers grown on Si(100) substrates with cleaning method of vacuum heating, As-beam, and Ga-beam show spot-like, () with spot, and clear (). From SEM, it is found that the GaAs epitaxial layers grown on Si(100) substrates with Ga-beam cleaning has a high quality.