Kwan Soo Chung

Electrical and Optical Properties of ZnO Films Grown on GaAs Substrates

Undoped ZnO films were deposited on GaAs substrates by conventional rf magnetron sputtering technique. After thermal annealing at temperatures of 500°C and higher for 20 min, the Hall coefficient of ZnO films on GaAs substrate becomes positive. The long-time annealing of 550 min at a temperature of 400°C also converts the sign of the Hall coefficient to positive. X-ray microanalysis shows that the diffusion of Zn atoms into the GaAs substrate and Ga atoms from the GaAs substrate into the ZnO film during thermal annealing occurs. The results of Hall measurements were analyzed by using the two-layer model of conductivity. It was shown that the positive sign of the Hall coefficient for the annealed ZnO film on the GaAs substrate is due to p-type conductivity of the GaAs substrate as a result of the diffusion of the Zn atoms from ZnO film into the GaAs substrate. With increasing annealing temperature or annealing time the ZnO films become more n-type due to the diffusion of Ga atoms from the GaAs substrate into the ZnO film.

Dependence of the Au/Ni/Si/Ni contact properties on the si-layer thickness and the annealing temperature in p-type GaN epilayers

The dependences of the properties of Au/Ni/Si/Ni contacts, deposited on p -GaN epilayers by using electron-beam evaporation, on the Si layer thickness and the annealing temperature were investigated with the goal of producing contacts with low specific resistances. The results of the current–voltage ( I–V ) curves showed that the lowest specific contact resistance obtained for the Au/Ni/Si/Ni contact with a 1200-A- thick Si layer on p -type GaN annealed at 700 °C for 1 min in a nitrogen atmosphere was 8.49 × 10 -4 Ω cm 2 . The x-ray diffraction (XRD) measurements on the annealed Au/Ni/Si/Ni/ p -GaN/sapphire heterostructure showed that Ni 3 Si, GaAu, and NiGa layers were formed at the Au/Ni/Si/Ni/ p -GaN interfaces. While the intensities corresponding to the Ni 3 Si layer decreased with increasing annealing temperature above 700 °C, those related to the GaAu and the NiGa layers increased with increasing temperature. These results indicate that the Au/Ni/Si/Ni contacts with 1200-A-thick Si layers annealed at 700 °C hold promise for potential applications in p-GaN-based optoelectronic devices.

Electro-Mechanical Properties of Metal–Insulator–Metal Device Fabricated on Polymer Substrate Using Low-Temperature Process

High-performance metal–insulator–metal (MIM) devices on flexible polymer substrates were successfully fabricated without any defects such as cracks, delamination and blistering. This work examines the mechanical and electrical properties of MIM devices constructed using anodic Ta2O5 films. Using newly developed methods including stepped heating process and low-temperature post-annealing below 180°C, we obtained high-performances MIM devices on polymer substrates. Here, we propose the use of stacked bottom electrode and water barrier layer in order to enhance the ductility of the Ta electrode and to prevent blistering problems, respectively. Rutherford backscattering spectroscopy (RBS), auger electron spectroscopy (AES) and transmission electronic microscope (TEM) observations were performed for the structural investigation of the MIM devices on polymer substrates. Electrical measurements were also carried out for as-deposited and thermally treated MIM devices including Al/Ta/Ta2O5/Cr or Ti structures. They exhibit a low leakage current (below 10-7 A/cm2 at 2 MV) and reasonable breakdown voltage (5–7 MV/cm) with a uniformity of 92%. Finally, under low-temperature post-annealing conditions, The Current–Voltage (I–V) behaviors and conduction mechanisms of MIM devices on polymer substrates are discussed based on the results of electrical measurements, structural investigations and conduction band modeling.

A Study of Photoelectrochemical Oxidation of GaN Epilayers by Extrinsic Photoconductivity

GaN epilayers grown on sapphire substrates by molecular-beam epitaxy were oxidized by photoelectrochemical (PEC) treatment in KOH. The effect of the oxidation on the GaN surface was investigated by extrinsic photoconductivity. The PEC-treated samples show decreased photoresponse and relaxation time compared with those of the as-grown sample. The concentration of deep states in the epilayers as determined from the light-intensity dependence of rise and decay times is reduced after PEC oxidation. The results suggest that the defective bonds at the surface are effectively saturated by oxygen coverage during PEC growth of the oxide layer. Results of temperature-dependent photoconductivity measurement reveal that shallow levels are also passivated within a short time of PEC treatment.

Annealing Effect on Passivated Deep Levels in GaN Epilayers

Transient extrinsic photoconductivity (PC) measurements on as-grown, hydrogenated, and hydrogenated and annealed GaN epitaxial layers grown on sapphire substrates using plasma-assisted molecular-beam epitaxy were performed to investigate the annealing effect on passivated deep levels in a GaN epilayer. A PC spectrum at room temperature showed that the deep-level state in an as-grown GaN epilayer was located 2.9 eV below the conduction-band minimum of the GaN epilayer. The photoionization cross section and the concentration of the deep levels for the as-grown GaN epilayers, determined from the dependence of the rise and decay times of the extrinsic PC response on the illumination intensity of the blue light-emitting diode with 2.9 eV, were 9.7×10-15 cm2 and 4.5×1014 cm-3, respectively. The concentration of the as-grown deep levels in the GaN epilayer was significantly reduced by the hydrogenation treatment, and this passivation effect was still effective after thermal annealing at a temperature up to 700°C. The activation energy of the deep levels and the dissociation energy of the complexes between the deep levels and hydrogen atoms were 0.55 eV and 3.0 eV, respectively. These results provide important information for improving the quality of devices utilizing GaN epilayers.

Activation Energy, Capture Cross Section, and Emission Frequency of the Trap Level in Unintentionally Doped n-Type GaN Epilayers Grown on Sapphire Substrates in a Nitrogen-Rich Atmosphere.

Thermally stimulated current (TSC) measurements of unintentionally doped n-type GaN epilayers grown on sapphire substrates in a nitrogen rich atmosphere using plasma-assisted molecular beam epitaxy were carried out to investigate the physical properties of trap levels in GaN films. The TSC spectra showed one dominant trap level at 150 K, and the activation energy, the capture cross section, and the emission frequency of the trap level were 0.265 eV, 2.58×10-21 cm2, and 3.17×105 s-1, respectively. These results can help clarify the potential applications of GaN epilayers in optoelectronic devices in the blue region of the spectrum.

InGaN‐Based Nanorod Array Light Emitting Diodes

We demonstrate the realization of the high‐brightness and high‐efficiency light emitting diodes (LEDs) using dislocation‐free indium gallium nitride (InGaN)/gallium nitride (GaN) multi‐quantum‐well (MQW) nanorod (NR) arrays by metal organic‐hydride vapor phase epitaxy (MO‐HVPE). MQW NR arrays (NRAs) on sapphire substrate are buried in silicon dioxide (SiO2) to isolating individual NRs and to bring p‐type NRs in contact with p‐type electrodes. The MQW NRA LEDs have similar electrical characteristics to conventional broad area (BA) LEDs. However, due to the lack of dislocations and the large surface areas provided by the sidewalls of NRs, both internal and extraction efficiencies are significantly enhanced. At 20 mA dc current, the MQW NRA LEDs emit about 4.3 times more light than the conventional BA LEDs, even though overall active volume of the MQW NRA LEDs is much smaller than conventional LEDs.