Shiue-Lung Chen

Evidence of oxygen vacancy enhanced room-temperature ferromagnetism in Co-doped ZnO

The annealing effects on structure and magnetism for Co-doped ZnO films under air, Ar, and Ar∕H2 atmospheres at 250°C have been systematically investigated. Room-temperature ferromagnetism has been observed for the as-deposited and annealed films. However, the saturation magnetization (Ms) varied drastically for different annealing processes with Ms∼0.5, 0.2, 0.9, and 1.5μB∕Co for the as-deposited, air-annealed, Ar-annealed, and Ar∕H2-annealed films, respectively. The x-ray absorption spectra indicate all these samples show good diluted magnetic semiconductor structures. By comparison of the x-ray near edge spectra with the simulation on Zn K edge, an additional preedge peak appears due likely to the formation of oxygen vacancies. The results show that enhancement (suppression) of ferromagnetism is strongly correlated with the increase (decrease) of oxygen vacancies in ZnO. The upper limit of the oxygen vacancy density of the Ar∕H2-annealed film can be estimated by simulation to be about 1×1021cm−3.

Use of patterned laser liftoff process and electroplating nickel layer for the fabrication of vertical-structured GaN-based light-emitting diodes

The fabrication process and performance characteristics of a vertical-structured GaN-based light-emitting diode (VM-LED) employing nickel electroplating and patterned laser liftoff techniques are presented. As compared to regular LED, the forward voltage drop of the VM-LED at 20–80 mA is about 10%–21% lower, while the light output power (Lop) is more than twice in magnitude. Especially, the Lop exhibits no saturation or degradation at an injection current up to 520 mA which is about 4.3 times higher than that of the regular one. Substantial improvements in the VM-LEDs performances are mainly attributed to the use of metallic substrate which results in less current crowding, larger effective area, and higher thermal conductivity.

Role of grain boundary and grain defects on ferromagnetism in Co:ZnO films

The annealing effects on magnetism, structure, and ac transport for Co:ZnO films have been systematically investigated. The room temperature saturation magnetization (Ms) varies drastically with Ar or Ar∕H2 annealing processes. By using the impedance spectra, the change in grain boundary and grain defects of these films can be analyzed. The results demonstrate that Ar annealing produces mainly the grain boundary defects which cause the enhancement of Ms. Ar∕H2-annealing creates not only grain boundary defects but also the grain defects, resulting in the stronger enhancement of Ms. Ferromagnetism for Co:ZnO films is influenced by both grain boundaries and grain defects.

Effect of Surface Treatment on the Performance of Vertical-Structure GaN-Based High-Power Light-Emitting Diodes with Electroplated Metallic Substrates

Large-area (0.6 ×0.6 and 1 ×1 mm2) highly-efficient GaN-based light-emitting diodes (LEDs) with a vertical-conducting structure (VM-LEDs), using a patterned laser lift-off technique and a Ni electroplating process as well as a surface treatment of the top n-GaN epilayer by plasma and chemical etching, were successfully fabricated and investigated. Compared to regular LEDs of the same size, both the forward voltage drop and the light output power (Lop) of the VM-LED were substantially improved. With inductively coupled plasma (ICP) etching followed by an additional KOH etching and an HF/HCl treatment on the n-GaN layer, an increase in Lop by 227% (195%) at 350 (800) mA has been achieved for the (1 ×1 mm2)-sized VM-LEDs.

Enhanced ferromagnetism in grain boundary of Co-doped ZnO films: A magnetic force microscopy study

Microscopic grain structures and magnetic properties of Co-doped ZnO-patterned films were studied by combinatorial atomic force microscopy and magnetic force microscopy (MFM) imaging. Whereas Co-doped ZnO was not uniformly magnetized, Co ions were homogeneously doped in the films and substitute from the Zn sites. Most Co:ZnO grains were weakly ferromagnetic (FM) and surrounded by strong FM foamlike grain boundaries networks. No MFM contrast was observed in pure ZnO films grown under similar conditions. The findings clearly demonstrate that defective grain boundaries and magnetic doping have key roles FM properties of diluted magnetic oxides.

Enhanced Performance of Vertical GaN-Based Light-Emitting Diodes with a Current-Blocking Layer and Electroplated Nickel Substrate

The performance of vertical-structure metallic-substrate GaN-based light-emitting diodes (VM-LEDs) with a patterned SiO2 film as the current-blocking layer (CBL) was investigated. From theoretical calculations of current and light distributions and experimental results on current–voltage (I–V) and light output power–current (L–I) characteristics, we found that SiO2 CBL inserted under the n-pad electrode increases light output power by 35.4% at 20 mA as compared with VM-LEDs without CBL. Such an improvement is attributed to the insulated CBL structure, which provides better current spreading and less photon absorption and/or reflection at the n-electrode.

High-Power GaN-Based Light-Emitting Diodes with Transparent Indium Zinc Oxide Films

Large-area (0.6 mm×0.6 mm–1.5 mm×1.5 mm), high-power GaN-based blue-light-emitting diodes (LEDs) with an indium zinc oxide (IZO) overlay as a transparent conduction layer (TCL) and the effects of the overlayer on light output power (Lop) improvement are investigated. Experimental results show that sputter-deposited IZO TCLs with thicknesses in the range of 100–500 nm have a low resistivity ranging from 12.1–3.1×10-4 Ω-cm and a transparency ≥80% in the visible light range. The benefit obtained from the use of an IZO TCL is much more profound in LEDs of larger chip size; in addition, the optimum IZO TCL thickness is approximately 300 nm. As compared to the case without an IZO layer, under an injection current of 60–1000 mA, a 39–90% improvement in Lop has been achieved from LEDs (1.5 mm×1.5 mm) with a 300-nm-thick IZO TCL.

Fabrication of Dicing-Free Vertical-Structured High-Power GaN-Based Light-Emitting Diodes With Selective Nickel Electroplating and Patterned Laser Liftoff Techniques

Through the use of selective nickel (Ni) electroplating, patterned laser liftoff technique, and surface roughing of the top n-GaN epilayer, a novel process for the fabrication of vertical-structured metal-substrate GaN-based light-emitting diodes (VM-LEDs) to avoid difficulties in Ni substrate dicing and improve device yield was proposed and demonstrated. In conjunction with a sidewall passivation with SiO2 and keeping the size of epilayer smaller than that of Ni island, a considerable improvement in yield and device performance were shown. As compared to conventional lateral-structured GaN-based LEDs, VM-LEDs show an increase in light output power about 174% at 350 mA with a significant decrease in forward voltage from 3.5 to 3.17 V

The use of transparent conducting indium-zinc oxide film as a current spreading layer for vertical-structured high-power GaN-based light-emitting diodes

In this study, the performance of vertical-structured high-power GaN-based light-emitting diodes (VM-LEDs) with a transparent and low-resistant indium-zinc oxide (IZO) film as a current spreading layer (CSL) was investigated. Nickel electroplating and patterned laser liftoff techniques were employed for the transfer of sapphire substrate to nickel substrate. The novel IZO CSL atop n-side-up VM-LEDs offering benefits of superior current spreading ability, larger extraction efficiency, and lower forward voltage drop was demonstrated. As compared to the regular LED without IZO CSL, the use of an IZO CSL with an optimum thickness of around 300 nm leads to an increase in light output power by 97.1 (67.8)% and a decrease in forward voltage drop by 4.9 (15.5)% under an injection current of 350 (800) mA

Use of Elastic Conductive Adhesive as the Bonding Agent for the Fabrication of Vertical Structure GaN-Based LEDs on Flexible Metal Substrate

Through the use of elastic conductive adhesive (ECA) as the bonding agent and patterned laser lift-off technology, a flexible metal substrate technology for the fabrication of vertical structured GaN-based light-emitting diodes (flex-LEDs) was proposed and demonstrated. It showed that the flex-LEDs have negligible changes in dominant wavelength-current and light output intensity-current-voltage characteristics when subjected to an external bending stress, indicating that the ECA used in the present technology performed well as a buffer to external stresses. As compared with conventional sapphire substrate GaN-based LEDs, Flex-LEDs with a chip size of 600 x 600 mum2 showed an increase in light output intensity (power) about 216% (80%) at 120 mA with an essential decrease in forward voltage from 3.51 to 3.3 V.