Jiho Chang

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.

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...

Optical Resonant Cavity in a Nanotaper

The present study describes an optical resonant cavity in a nanotaper with scale reduction from micro to several nanometers. Both experimental results and a finite-difference time-domain (FDTD)-based simulation suggested that the nanometer-scale taper with a diameter similar to the wavelength of light acted as a mirror, which facilitated the formation of a laser cavity and caused lasing in ZnO nanotapers. As the light inside the nanotaper propagated toward the apex, the lateral mode was reduced and reflection occurred. This report suggests that use of the resonant optical cavities in nanotapers might result in novel active and passive optical components, which will broaden the horizons of photonic technology.

Cathodoluminescence study of dislocation-related luminescence from small-angle grain boundaries in multicrystalline silicon

Dislocation-related luminescence from small-angle grain boundaries (SA-GBs) in multicrystalline Si was investigated by cathodoluminescence. D3 and D4 emissions were detected at SA-GBs with a misorientation angle of around 1°–1.5°, and D1 and D2 at SA-GBs with a misorientation angle of around 2°–2.5°. Electron beam-induced current investigations indicate that the former SA-GBs possess only shallow energy levels, while the latter possess both deep and shallow levels. The origins of D-line luminescence at SA-GBs are discussed in terms of dislocation structures.

Effects of controlled ambidirectional nucleation on the heteroepitaxial growth of m-GaN on m-sapphire

We found that m-plane GaN grown on m-plane sapphire nucleates in ambidirections at the initial growth stage, which seriously degrades the surface morphology and the crystallinity of m-GaN films. To avoid the ambidirectional islanding of m-plane GaN, off-cut m-plane sapphire (toward to the [112¯0] direction) substrates were introduced. When the off-angle was small as 1°–2°, the surface step determined the epitaxial orientation of m-GaN islands. Hence, an m-GaN film with a smooth surface and a low-dislocation density was obtained. However, the dislocation component of m-GaN film was increased with increasing of the off angle due to step bunches.

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...

Deep-level-transient spectroscopy of heavily Al-doped ZnSe layers grown by molecular-beam epitaxy

Using deep-level-transient spectroscopy, we have investigated deep levels in heavily Al-doped ZnSe layers grown by molecular-beam epitaxy. The Al concentration of the ZnSe layers lies in the range of 5×1018–9×1018cm−3. The ZnSe:Al layers exhibit two electron-trap centers with the thermal activation energies of 0.16eV (ND1) and 0.80eV (ND2). ND2 is a dominant trap center with a trap density of 3×1016cm−3, while the trap density of ND1 is estimated to be 2×1015cm−3. However, ND2 shows anomalous behaviors, different from isolated point defects, in the following points: (1) the emission peak of ND2 moves to the low temperature side with increasing filling pulse duration; (2) the emission peak of ND2 is broader than theoretically calculated one for an isolated point defect; and (3) the capacitance-transient curve is nonexponential. It is observed by high-resolution x-ray diffraction that heavy Al doping results in the relaxation and plastic deformation of the ZnSe lattice. These behaviors can be ascribed to ex...

Influence of Isoelectronic Te Doping on the Physical Properties of ZnO Films Grown by Molecular-Beam Epitaxy

In molecular-beam epitaxy, isoelectronic Te doping induces the serious change of the physical properties of ZnO films: i) Incorporated Te concentration is proportional to the 2.2th power of injected Te flux in the logarithmic scale; ii) Te-doped ZnO lattices are dominated by the relaxation mechanism of compressive strain; iii) incorporated Te atoms substitute to the O sites of ZnO lattices; iv) the low-level injection of Te atoms below ~1019 cm-3 improves the crystalline quality in the ZnO films; and v) isoelectronic Te centers act as donor impurities, resulting in the increase of electrons in the ZnO films.

Spontaneous transition in preferred orientation of GaN domains grown on r-plane sapphire substrate from [112¯0] to [0001]

GaN films were grown at 550 °C and subsequently at 1040 °C on sapphire (101¯2) (r-plane) substrates by using hydride vapor phase epitaxy with different layer thicknesses. As the thickness of a low-temperature-grown GaN layer was increased, a preferred orientation of GaN grown at 1040 °C changed from [112¯0] to [0001]. A detailed atomistic model reveals that this spontaneous transition in preferred orientation is due to the formation of inversion domain boundaries and stacking faults. This result has a significant implication that tailoring film characteristics in terms of controllability of preferred orientation may be possible independent of substrate orientation.

Development of waterborne oil spill sensor based on printed ITO nanocrystals.

Oil spill accidents occasionally occur in coastal and ocean environments, and cause critical environmental damage, spoiling the marine habitats and ecosystems. To mitigate the damages, the species and amount of spilled oil should be monitored. In this study, we developed a waterborne oil spill sensor using a printed ITO layer. ITO is a compatible material for salty environments such as oceans because ITO is strong against corrosion. The fabricated sensor was tested using three oils, gasoline, lubricant and diesel, and different oil thicknesses of 0, 5, 10, and 15mm. The results showed that the resistance of the sensor clearly increased with the oil thickness and its electrical resistance. For sustainable sensing applications in marine environments, XRD patterns confirmed that the crystal structure of the ITO sensor did not change and FE-SEM images showed that the surface was clearly maintained after tests.