Seunghwan Park

Lattice relaxation mechanism of ZnO thin films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy

We report on the lattice relaxation mechanism of ZnO films grown on c-Al2O3 substrates by plasma-assisted molecular-beam epitaxy. The lattice relaxation of ZnO films with various thicknesses up to 2000nm is investigated by using both in situ time-resolved reflection high energy electron diffraction observation during the initial growth and absolute lattice constant measurements (Bond method) for grown films. The residual strain in the films is explained in terms of lattice misfit relaxation (compression) at the growth temperature and thermal stress (tension) due to the difference of growth and measurement temperatures. In thick films (>1μm), the residual tensile strain begins to relax by bending and microcrack formation.

Polarity control of ZnO films on (0001) Al2O3 by Cr-compound intermediate layers

This letter presents a reliable and very easy method for selective growth of polarity controlled ZnO films on (0001) Al2O3 substrates by plasma-assisted molecular-beam epitaxy. Cr-compound intermediate layers are used to control the crystal polarity of ZnO films on (0001) Al2O3. ZnO films grown on rocksalt structure CrN/(0001) Al2O3 shows Zn polarity, while those grown on rhombohedral Cr2O3∕(0001) Al2O3 shows O polarity. Possible interface atomic arrangements for both heterostructures are proposed.

Comparative study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks

The authors have an extensive study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks. In the photoluminescence (PL) spectra at 10 K, Zn-polar and O-polar faces show a common emission feature: neutral donor-bound excitons and their longitudinal-optical (LO) phonon replicas are strong and free excitons are very weak. However, in the PL spectra at room temperature (RT), Zn-polar and O-polar faces show extremely different emission characteristics: the emission intensity of Zn-polar face is 30 times larger than that of O-polar face and the band edge of Zn-polar face is 33 meV redshifted from that of O-polar face. The temperature dependence of photoluminescence indicates that the PL spectra at RT are closely associated with free excitons and their phonon-assisted annihilation processes. As a result, it is found that the RT PL spectra of Zn-polar face are dominated by the first-order LO phonon replica of A free excitons, while that of O-polar face is determined by A free excit...

Effects of interfacial layer structures on crystal structural properties of ZnO films

Single crystalline ZnO films were grown on Cr compound buffer layers on (0001) Al2O3 substrates by plasma assisted molecular beam epitaxy. In terms of lattice misfit reduction between ZnO and substrate, the CrN and Cr2O3/CrN buffers are investigated. The structural and optical qualities of ZnO films suggest the feasibility of Cr compound buffers for high-quality ZnO films growth on (0001) Al2O3 substrates. Moreover, the effects of interfacial structures on selective growth of different polar ZnO films are investigated. Zn-polar ZnO films are grown on the rocksalt CrN buffer and the formation of rhombohedral Cr2O3 results in the growth of O-polar films. The possible mechanism of polarity conversion is proposed. By employing the simple patterning and regrowth procedures, a periodical polarity converted structure in lateral is fabricated. The periodical change of the polarity is clearly confirmed by the polarity sensitive piezo response microscope images and the opposite hysteretic characteristic of the piez...

High-Quality p-Type ZnO Films Grown by Co-Doping of N and Te on Zn-Face ZnO Substrates

This article will report the epitaxial growth of high-quality p-type ZnO layers on Zn-face ZnO substrates by nitrogen and tellurium (N+Te) co-doping. ZnO:[N+Te] films show p-type conductivity with a hole concentration of 4×1016 cm-3, while ZnO:N shows n-type conduction. The photoluminescence of ZnO:N shows broad bound exciton emission lines. Meanwhile, ZnO:[N+Te] layers show dominant A0X emission line at 3.359 eV, with a linewidth as narrow as 1.2 meV. Its X-ray linewidth shows narrower line width of 30 arcsec. Detailed investigation of photoluminescence properties of (N+Te) codoped ZnO layers suggest that the binding energy of N acceptors lies in a range of 121–157 meV.

Impact of V/III ratio on electrical properties of GaN thick films grown by hydride vapor-phase epitaxy

Impact of V/III ratio on electrical properties of GaN thick films are investigated, which are grown by hydride vapor-phase epitaxy. The authors note that the electron concentration of GaN films decreases with the increase of V/III ratio, while their electrical resistivity and electron mobility increase simultaneously. These indicate that enhancing V/III ratio suppresses electron-feeding sources in GaN films, which is not by generating electron-trapping centers but by reducing donor-type defects. On the other hand, it is shown that the linewidth of x-ray rocking curves in GaN films decreases and the near-band edge emission intensity of 10K photoluminescence spectra increases as V/III ratio increases. These mean that higher V/III ratio condition helps for reducing crystalline point defects in GaN films. In terms of theoretical fitting into the temperature-dependence curves of electron mobilities, it is found that the electron transport of GaN films grown in lower V/III ratio condition is more hampered by de...

Investigation of the crystallinity of N and Te codoped Zn-polar ZnO films grown by plasma-assisted molecular-beam epitaxy

We report on the crystallinity, N incorporation efficiency, optical properties, and electrical properties of N and Te codoped ZnO films grown by plasma-assisted molecular-beam epitaxy. Te improves the surface morphology and roughness of ZnO films in terms of both streak reflection high energy electron diffraction pattern and atomic force microscopy observations. Also, N and Te codoping is helpful to improve the crystallinity and N incorporation efficiency simultaneously. We found that; (a) narrower x-ray linewidth and higher N concentration were obtained by codoping. (b) Nitrogen related emission lines including donor-acceptor pair and acceptor-bound exciton dominantly emerged in photoluminescence spectra. (c) Codoping enhanced the carrier compensation of native donors in ZnO films and suppressed the dislocation scattering. As a consequence, we concluded that N and Te codoping is very effective for the growth of reliable p-type ZnO films which fulfill the controversial requirements; high N concentration a...

Growth mechanism of ZnO low-temperature homoepitaxy

The authors report on the growth mechanism of ZnO homoepitaxy at the low-temperature range of 500 °C, which is unavailable to obtain high-quality ZnO films in heteroepitaxy. One typical set of ZnO films were grown on (0001) ZnO substrates by molecular-beam epitaxy: a standard structure without buffer and two buffered structures with high-temperature (HT) homobuffer and low-temperature (LT) homobuffer. As a result, the LT homobuffered structure had the outstanding material properties: the surface roughness is 0.9 nm, the full width at half maximum of x-ray rocking curve is 13 arcsec, and the emission linewidth of donor-bound excitons is 2.4 meV. In terms of the theoretical interpretation of the experimentally obtained electron mobilities, it was found that the LT homobuffered structure suffers less from the dislocation scattering and the ionized-impurity scattering compared to the HT homobuffered structure. It is proposed that, in the ZnO low-temperature homoepitaxy, the LT homobuffer plays a key role in i...

Influence of heat treatments on electrical properties of ZnO films grown by molecular-beam epitaxy

We report on the influence of heat treatments on the electrical properties of ZnO films grown by molecular-beam epitaxy. We note that the electrical resistance of the ZnO films is significantly changed by the heat treatments: the electrical resistance increases with the increase of ambient temperature, but above a critical temperature the resistance decreases with the increase of temperature, irrespective of ambient gases. On the other hand, it is found that the large amount of photocurrent is generated in the ZnO films, exposed to white sources: the photocurrent decreases with the increase of the obtained resistance, and the current increases with the decrease of the resistance. Also, it is shown that the x-ray diffraction linewidth of the ZnO films is significantly decreased by the heat treatments. These indicate that the increase/decrease of the electrical resistance is ascribed to the annihilation/formation of the residual donor-type defects in the ZnO films by the heat treatments. It is suggested tha...

Effect of niobium doping on the optical and electrical properties in titanium dioxide grown by pulsed laser deposition

The influence of niobium (Nb) on both the electrical and optical properties in titanium dioxide (TiO2) grown by pulsed laser deposition was investigated. Nb atoms with a critical doping ratio of 8 at. % were carefully controlled in a 100 nm-thick TiO2 layer (Nb:TiO2). The Hall effect results revealed that the electrical resistivity and electron concentration of the resulting Nb:TiO2 film were 30 times lower and 2 orders of magnitude higher than those of un-doped TiO2 (u-TiO2), respectively, leading to a slight degradation of optical transmittance in the visible region. The room temperature photoluminescence results showed that the emission intensity at the near band of Nb:TiO2 was greatly enhanced, while, that at the impurity band created by oxygen vacancy (Vo) was almost constant. Detailed investigation of the T-dependent conductivity indicated that the Nb:TiO2 film possessed an activation energy as shallow as 20.8 meV, which suggested that the electrical properties were dominantly determined by Nb shall...