T. W. Kang

Optical and magnetic measurements of p-type GaN epilayers implanted with Mn+ ions

The p-type GaN epilayers were prepared by metalorganic chemical vapor deposition and subsequently Mn+ ions implanted. The properties of Mn+ ions-implanted GaN epilayers were investigated by optical and magnetic measurements. The results of photoluminescence measurement show that optical transitions related to Mn apparently appear at 2.5 eV and around 3.0 eV. It is confirmed that the photoluminescence peak at 2.5 eV is a donor–Mn acceptor transition. Ferromagnetic hysteresis loop was observed, and the temperature-dependent magnetization displayed a ferromagnetic behavior persisting up to ∼270 K.

Photoluminescence studies of GaN nanorods on Si (111) substrates grown by molecular-beam epitaxy

We have investigated the optical properties of dislocation-free vertical GaN nanorods grown on (111) Si substrates by radio-frequency plasma-assisted molecular-beam epitaxy. The hexagonal shape nanorods with lateral diameters from 80 to 190 nm are obtained. They are fully relaxed and have a very good crystal quality characterized by extremely strong and narrow photoluminescence excitonic lines near 3.47 eV. Three distinct features are observed in photoluminescence. First, free exciton transition is observed at 3.477 eV for GaN nanorods of decreased diameter. Second, the photoluminescence spectra show an abnormal behavior with increasing temperature. The third feature is the size effect in that the PL peak energies are blueshifted with decreasing diameter of the GaN nanorod. The activation energy of the free exciton for the GaN nanorods with different diameters was evaluated.

Postgrowth annealing effect on structural and optical properties of ZnO films grown on GaAs substrates by the radio frequency magnetron sputtering technique

High-resolution scanning electron microscopy and cathodoluminescence spectroscopy measurements were performed to study the effect of postgrowth annealing on properties of ZnO films grown on GaAs substrates by rf sputtering. The films annealed at 550 °C show a well-oriented columnar structure and strong exciton emission at room temperature. Outdiffusion of gallium and arsenic from substrate into a ZnO film has been found to result in a different secondary electron dopant contrast, measured by the through-the lens secondary electron detector. Extended structural defects such as subgrain boundaries in ZnO assist Ga outdiffusion from the GaAs substrate and show a reduced secondary electron (SE) emission after annealing, while As doped ZnO adjacent to the ZnO/GaAs interface demonstrates an enhanced SE emission and the enhanced luminescence associated with donor–acceptor pairs and exciton bound to acceptors.

Characteristics of TiN barrier layer against Cu diffusion

Abstract Thin films of TiN interposed between Cu and SiO2 layers have been examined as a diffusion barrier as well as an adhesion-promoting layer for Cu metallization. Reliability of TiN barrier layer has been investigated by studying the penetration of copper into thermal oxide as a function of temperature. We have used X-ray diffraction, Auger electron spectroscopy and capacitance-voltage(C–V) measurements in order to investigate the Cu/TiN/SiO2/Si metallization system. The characteristics of metal/oxide/silicon capacitors employing this barrier layer were not significantly affected by a temperature stress of up to 600°C for 1 h.

Memory effects related to deep levels in metal–oxide–semiconductor structure with nanocrystalline Si

Nanocrystalline(nc)-Si was grown on SiO2 by rapid thermal chemical vapor deposition. The tunneling oxide layer of a thickness of 4 nm was formed on p-type Si(100) by rapid thermal oxidation at 1050 °C for 30 s. Metal–oxide–semiconductor (MOS) structures were fabricated and capacitance–voltage characterization was carried out to study the memory effects of the nc-Si embedded in the MOS structure. We found the memory effect to be dominantly related to hydrogen-related traps, in addition to being influenced by the three-dimensional quantum confinement and Coulomb charge effects. Deep level transient spectroscopy reveal that the activation energies of the hydrogen-related traps are Ev+0.29 eV (H1) and Ev+0.42 eV (H2), and the capture cross sections are 4.70×10−16 cm2 and 1.44×10−15 cm2, respectively. The presence of Si–H and Si–H2 bonds was confirmed by Fourier transform infrared spectroscopy.Nanocrystalline(nc)-Si was grown on SiO2 by rapid thermal chemical vapor deposition. The tunneling oxide layer of a thickness of 4 nm was formed on p-type Si(100) by rapid thermal oxidation at 1050 °C for 30 s. Metal–oxide–semiconductor (MOS) structures were fabricated and capacitance–voltage characterization was carried out to study the memory effects of the nc-Si embedded in the MOS structure. We found the memory effect to be dominantly related to hydrogen-related traps, in addition to being influenced by the three-dimensional quantum confinement and Coulomb charge effects. Deep level transient spectroscopy reveal that the activation energies of the hydrogen-related traps are Ev+0.29 eV (H1) and Ev+0.42 eV (H2), and the capture cross sections are 4.70×10−16 cm2 and 1.44×10−15 cm2, respectively. The presence of Si–H and Si–H2 bonds was confirmed by Fourier transform infrared spectroscopy.

GaN metal–oxide–semiconductor structures using Ga-oxide dielectrics formed by photoelectrochemical oxidation

GaN metal–oxide–semiconductor (MOS) capacitors were fabricated by using Ga oxide formed by photoelectrochemical oxidation of GaN. The electrical properties of the MOS structures as characterized by capacitance–voltage measurement were found to be dependent on the oxidation time and posttreatment. Positive flatband voltage was observed in devices with thin oxide layers indicating the existence of negative oxide charge. Very thin oxide exhibits high capacitance and reverse leakage, which can be reduced by rapid thermal annealing (RTA). Passivation of the interface by RTA is partially responsible for the improvement. Thicker oxide layers exhibit improved electrical properties. Low density of interface states (∼1011 eV−1 cm−2) was obtained in the Ga-oxide/GaN structure grown under optimized conditions.

Effect of additional nonmagnetic acceptor doping on the resistivity peak and the Curie temperature of Ga1-xMnxAs epitaxial layers

We have investigated the effect of additional doping by Be on the properties of Ga1−xMnxAs (x=0.03). For this relatively low value of x, the Curie temperature is observed to increase with increasing Be concentration. We show that the temperature dependence of the resistivity at zero magnetic field, including the resistivity maximum near the Curie temperature, can be successfully described by the magnetoimpurity scattering model proposed by Nagaev [Phys. Rep. 346, 387 (2001)] in both the paramagnetic and the ferromagnetic temperature regions. Quantitative analysis of the data in terms of this model yields the value of the p–d exchange energy |N0β|≈1.6 eV for Ga0.97Mn0.03As.

Mo- and N-doped BiNbO4 for photocatalysis applications

The electronic structure of pure BiNbO4 has been calculated and their electronic band positions have been aligned with respect to the water oxidation/reduction potential. The effect of cationic (Mo), anionic (N), and co-doping (Mo-N) on BiNbO4 has been studied and discussed with respect to the standard redox potential levels. Our results show that co-doping of Mo and N in BiNbO4 reduces the band gap up to 31.8%, thus making it a potential candidate for the photocatalysis of water for hydrogen production. The relative stability between the mono- and co-doped BiNbO4 materials show that co-doped material is more stable and feasible in comparison to the mono-doped materials.

Band gap engineering in BiNbO4 for visible-light photocatalysis

We have investigated the electronic structure of anionic mono- (S, N, and C) and co-doping (N-N, C-N, S-C, and S-N) on BiNbO4 for the visible-light photocatalysis. The maximum band gap reduction of pure BiNbO4 is possible with the (C-S) co-doping and minimum with N mono-doping. The calculated binding energies show that the co-doped systems are more stable than their mono-doped counterparts. Our optical absorption curves indicate that the mono- (C) and co-anionic doped (N-N and C-S) BiNbO4 systems are promising materials for visible light photocatalysis.

Enhanced positive magnetoresistance effect in GaAs with nanoscale magnetic clusters

The enhanced positive magnetoresistance effect has been observed in GaAs containing nanoscale magnetic clusters. The ferromagnetic metallic clusters were embedded into GaAs by Mn ion implantation and rapid thermal annealing. Positive magnetoresistance in these structures has been observed and attributed to the enhanced geometric magnetoresistance effect in inhomogeneous semiconductors with metallic inclusions. The additional enhancement of positive magnetoresistance under light illumination is due to the higher mobility of photoexcited electrons in comparison with the mobility of holes in p-type GaAs prepared by Mn ion implantation.