Wen-How Lan

Effect of thermal annealing of Ni/Au ohmic contact on the leakage current of GaN based light emitting diodes

The effect of thermal annealing on current–voltage properties of GaN light emitting diodes (LEDs) has been studied. At annealing temperatures above 700 °C, the p–n junction of the diodes became very leaky and Ga-contained metallic bubbles were observed on the surface of Ni/Au p-ohmic contact. Transmission electron microscopy and energy dispersive x-ray spectrometer studies revealed that these metallic bubbles resided directly on top of the threading dislocations in GaN and both Ni and Au were indiffused into the LED structure along the cores of the TDs. The conducting paths formed by the metal containing dislocation cores are believed to be the cause for the observed short circuit behavior of p–n junctions at high annealing temperatures.

Photo-enhanced chemical wet etching of GaN

Abstract In this paper, we report a photo-enhanced chemical etch rate study on two GaN samples of differing structural and electrical quality as a function of the KOH or H3PO4 etch solution molarity. The etch rate of KOH was observed to be higher than that of H3PO4. This was found to be result from the effects of surface band bending, and surface pinning on the chemical etching and photo-assisted etching of the layers. It was also found that the optimal etch rate occurred at different values of molarity for the two samples and that very different morphologies were observed after etching.

InGaN/GaN light emitting diodes with a p-down structure

Nitride-based p-down blue light emitting diodes (LEDs) were successfully fabricated. It was found that we could improve the crystal quality of these nitride-based p-down LEDs by inserting a codoped interlayer between the p-type cladding layer and MQW active layers. It was also found that the turn-on voltage could be reduced from 15 V to less than 5 V for the p-down LED with codoped layer and tunnel layer. The 20 mA output power was 1 mW for the p-down LED with an Mg+Si codoped interlayer and a rough p-tunnel layer.

High responsivity of GaN p - i - n photodiode by using low-temperature interlayer

Gallium nitride p-i-n ultraviolet photodiodes with low-temperature (LT)-GaN interlayer have been fabricated. It was found that the dark current of photodiode with LT-GaN interlayer is as small as 143pA at 5V reverse bias. It was also found that the responsivity of the photodiode with LT-GaN interlayer can be enhanced at a small electric field (∼0.4MV∕cm) due to the carrier multiplication effect. The UV photocurrent gain of 13 and large ionization coefficient (α=3.1×105cm−1) were also observed in the detector with LT-GaN interlayer. Furthermore, we can achieve a large peak responsivity of 2.27A∕W from the photodiode with LT-GaN interlayer.

P-Down InGaN/GaN Multiple Quantum Wells Light-Emitting Diode Structure Grown by Metal-Organic Vapor-Phase Epitaxy.

An inverted or p-down InGaN/GaN multiple quantum wells (MQW) light-emitting diode (LED) structure is studied. The crystalline quality of the quantum wells is comparable to that of the n-down structure by using a Si or In co-doped GaN:Mg layer underneath the active layer. It was found that I–V characteristics can be improved by insertion of a tunnel layer, either a 3D growth GaN:Mg layer or an AlGaN/GaN superlattice layer. The feasibility of such a structure for practical application is also evaluated by luminescence measurement.

ZnSTeSe metal-semiconductor-metal photodetectors

High-quality quaternary ZnSTeSe epitaxial layers were successfully grown by molecular beam epitaxy (MBE). It was found that a ZnS/sub 0.18/Se/sub 0.82/ layer was automatically formed in between the ZnSe buffer layer, and the ZnS/sub 0.17/Te/sub 0.08/Se/sub 0.75/ epitaxial layer, when we increased the ZnS cell temperature. ZnSTeSe metal-semiconductor-metal (MSM) photodetectors were also fabricated for the first time. It was found that we could achieve a photocurrent to dark current contrast higher than five orders of magnitude by applying a 10-V reverse bias. We also found that the maximum photoresponsivity is about 0.4 A/W under a 10-V reverse bias. Such a value suggests that the ZnSTeSe MSM photodetector is potentially useful in the blue-UV spectral region.

On the Carrier Concentration and Hall Mobility in GaN Epilayers

The dependence of Hall mobility and carrier concentration in GaN epilayers on light illumination was examined. It was found that Hall mobility and electron concentration both increased after illumination with red laser (632.8 nm) and green laser (530 nm). However, no changes in Hall mobility and carrier concentration were found, if IR-laser (850 nm) was used. The results reveal that deep-level defects were excited and hence extra carriers were generated by light illumination. The influence is more pronounced for thinner films. These observations indicate that donor-like defect-related states were located 1.48 to 2.33 eV below the conduction band edge.

Improved Output Power of Nitride-Based Light-Emitting Diodes With Convex-Patterned Sapphire Substrates

Nitride-based light-emitting diodes (LEDs) grown on different convex-patterned sapphire substrates are proposed and fabricated. The electrical and optical properties of these LEDs are discussed in detail. It is found that the LED with a ball-shape patterned sapphire substrate has the best crystalline quality and I-V characteristic. On the other hand, on reducing total internal reflection, the LED with hexagonal-shape patterned sapphire substrate (HPSS) has the best performance on output power and external quantum efficiency. In comparison to the LED without patterned substrate, the LED with HPSS has 175% and 165% enhancement on output power and external quantum efficiency, respectively.

UV enhanced oxygen response resistance ratio of ZnO prepared by thermally oxidized Zn on sapphire substrate

ZnO thin film was fabricated by thermally oxidized Zn at 600°C for 1 h. A surface containing nanostructured dumbbell and lines was observed by scanning electron microscope (SEM). The ZnO resistor device was formed after the following Ti/Au metallization. The device resistance was characterized at different oxygen pressure environment in the dark and under ultraviolet (UV) light illumination coming from the mercury lamp with a short pass filter. The resistance increases with the increase of oxygen pressure. The resistance decreases and response increases with the increase of light intensity. Models considering the barrier height variation caused by the adsorbed oxygen related species were used to explain these results. The UV light illumination technology shows an effective method to enhance the detection response for this ZnO resistor oxygen sensor.

ZnCdSeTe-based orange light-emitting diode

We report the fabrication of orange light-emitting diodes (LEDs) by introducing Te into the well layers of ZnCdSe-ZnSSe multiple quantum-well active region. It was found that a 15% Te can result in a 91 nm photoluminescence (PL) red-shift at 16 K and a 81 nm PL red-shift at room temperature. Such a ZnSe-based orange LED can be integrated easily with conventional ZnSe-based blue-green LED on the same GaAs substrate. Thus, such a ZnSe-based orange LED is potentially useful in realizing a new one-chip type white LED without the use of phosphor.