Deuk Young Kim

Growth and characterization of single-crystal GaN nanorods by hydride vapor phase epitaxy

Single-crystalline GaN nanorods are formed on a sapphire substrate by hydride vapor phase epitaxy (HVPE). Their structural and optical properties are investigated by x-ray diffraction, scanning and transmission electron microscopy, and cathodoluminescence (CL) techniques. The high density of straight and well-aligned nanorods with a diameter of 80–120 nm formed uniformly over the entire 2 in. sapphire substrate. The x-ray diffraction patterns and transmission electron microscopic images indicate that the formed GaN nanorods are a pure single crystal and preferentially oriented in the c-axis direction. We observed a higher CL peak position of individual GaN nanorods than that of bulk GaN as well as a blueshift of CL peak position with decreasing the diameter of GaN nanorods, which are attributed to quantum confinement effect in one-dimensional GaN nanorods. We demonstrate that the well-aligned, single-crystalline GaN nanorods with high density, high crystal quality, and good spatial uniformity are formed b...

Magnetic Characteristic of Mn+ Ion Implanted GaN Epilayer

Nanoscale ferromagnets (GaMn) with the implantation of Mn+ ions have been incorporated into unintentionally doped (n-type) GaN epilayers grown on sapphire substrate by molecular beam epitaxy (MBE). Energy dispersive X-ray (EDX) spectrometry, atomic force and magnetic force microscopy (AFM and MFM) are used to characterize the GaMn precipitates which form within the GaN epilayer. MFM images reveal nanoscale ferromagnets (GaMn), and a small magnetic hysteresis loop indicates that there are ferromagnetic particles in our GaN:Mn layer involving the paramagnetic property and is measured by superconducting quantum interference device (SQUID) magnetometer.

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.

Improved ferromagnetism of (Zn0.93Mn0.07)O through rapid thermal annealing

After annealing at 900°C, the ferromagnetic properties of (Zn0.93Mn0.07)O thin films were dramatically improved. The resultant remanent magnetization (Mr) and Curie temperature (TC) were 1.17μB∕Mn and 83K. The improvement of ferromagnetism was confirmed to as resulting from the enhancement of magnetic anisotropy. This result is attributed to the improvement of crystallinity and the stabilization of unstably bonded Mn2+ ions by thermal treatments. These results suggest that ferromagnetism of (Zn1−xMnx)O thin films can be improved by modifying the crystal magnetic anisotropy through postgrowth thermal treatments.

Structural, optical, and magnetic properties of As-doped (Zn0.93Mn0.07)O thin films

The As-doped (Zn0.93Mn0.07)O thin film prepared by As+ ion implantation showed a clear peak of (A0,X) having acceptor binding energy of 181meV. The sample showed high TC ferromagnetism persisting up to 285K. The contribution of magnetization from Mn ion at 280K was determined to be 0.13μB∕Mn. The improved ferromagnetism is expected to be originated from hole-induced ferromagnetism and enhanced magnetic anisotropy because crystallographically improved sample showed p-type conductivity with hole concentration of 4.8×1018cm−3 and hole mobility of 11.8cm2V−1s−1. These results suggest that high TC ferromagnetism can be realized by codoping the acceptor dopant and improving the magnetic anisotropy.

Enhanced ferromagnetism in H2O2-treated p-(Zn0.93Mn0.07)O layer

Enhanced ferromagnetism was observed from the H2O2-treated p-type (Zn0.93Mn0.07)O:As layer. Compared with the untreated sample, the H2O2-treated sample showed the enlarged ferromagnetic hysteresis loop with approximately two-times-increased spontaneous magnetization. And also, in comparison with the untreated sample (TC∼280 K), the H2O2-treated sample exhibited to have the increased TC persisting up to above 350 K. These results were confirmed to originate from the enhanced p-d hybridization due to the decrease in negatively charged residual background carriers. This is because the increased effective g-factor resulting from the decrease in oxygen-related defects acting as native deep donors was observed from the H2O2-treated sample.

Impact of defect distribution on transport properties for Au/ZnO Schottky contacts formed with H2O2-treated unintentionally doped n-type ZnO epilayers

The Au/ZnO Schottky contacts fabricated using H2O2-treated unintentionally doped ZnO epilayers showed an abnormal behavior in their transport properties; i.e., the background carrier density-dependent trade-off relation between the barrier height and the ideality factor was observed. This result is attributed to the difference in carrier transport mechanisms for each sample fabricated using ZnO epilayers with different background carrier concentrations; namely, the observed trade-off relation originates from a result that the difference in the distribution of oxygen vacancies near the surface and depletion regions, which depends on the initial background carrier concentration of each sample, causes the different carrier transport mechanism.

Back-gate tuning of Schottky barrier height in graphene/zinc-oxide photodiodes

We demonstrate back-gate-tuning of the Schottky barrier height in graphene/zinc oxide photodiodes that are devised by a selective sputter-growth of ZnO on pre-patterned single-layer graphene sheets. The devices show a clear rectifying behavior (e.g., Schottky barrier height ∼0.65 eV and ideality factor ∼1.15) and an improvement in the photo-response via application of a back-gate voltage. The back-gate bias tunes the effective Schottky barrier-height and also promotes the activation of photo-excited carriers, which leads to an enhancement in the thermionic emission process.

Dependence of ferromagnetic properties on conductivity for As-doped p-type (Zn0.93Mn0.07)O layers

The As-doped p-type (Zn0.93Mn0.07)O layers show a strong dependence of their ferromagnetic properties on the hole conductivity that were controlled through the modification of negative background-charge density by changing the oxygen partial pressure during the initial growth stage before As doping. Curie temperature and spontaneous magnetization were observed to be increased as the hole conductivity increases. This result was confirmed to originate from stabilizations of incorporated Mn2+ ions and doped As acceptors, which can give rise to long-range ferromagnetic coupling.

Optical and Electrical Properties of Si Nanocrystals Embedded in SiO2 Layers

Si nanocrystals were formed using a Pt nanoscale island etching mask. A high-resolution transmission electron microscopy image showed that the Si nanocrystals were created inside a SiO2 layer, and the cathodoluminescence spectrum showed that the luminescence peak is related to the Si nanocrystals. Capacitance-voltage measurements on Al/SiO2/nanocrystalline Si/SiO2/p-Si structures at 300 K showed that the value of the flatband voltage shift increased parabolically with increasing Si nanocrystal size. These results indicate that Si nanocrystals can be formed using a Pt island etching mask and that the magnitude of the flatband voltage shift for the Al/SiO2/nanocrystalline Si/SiO2/p-Si structures is significantly affected by the size of the Si nanocrystals embedded in the SiO2 layer.