Ya-Tung Cherng

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.

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.

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.

Photo-enhanced native oxidation process for Hg/sub 0.8/Cd/sub 0.2/Te photoconductors

Proposes an easy and reproducible vapor-phase photo surface treatment method to improve the device performance of the Hg/sub 0.8/Cd/sub 0.2/Te photoconductive detector. We explore the effect of surface passivation on the electrical and optical properties of the HgCdTe photoconductor. Experimental results, including surface mobility, surface carrier concentration, metal-insulator-semiconductor leakage current, 1/f noise voltage spectrum, the 1/f knee frequency, responsivity R/sub /spl lambda//, and specific detectivity D* for stacked photo surface treatment and ZnS or CdTe passivation layers are presented. These data are all directly related to the quality of the interface between the passivation layer and the HgCdTe substrate. We found that, by inserting a photo native oxide layer, we can shift the 1/f knee frequency, reduce the noise power spectrum, and achieve a lower surface recombination velocity S. A higher D* can also be achieved. It was also found that HgCdTe photoconductors passivated with stacked layers show improved interface properties compared to the photoconductors passivated only with a single ZnS or CdTe layer.

Dark Currents in HgCdTe Photodiodes Passivated with ZnS/CdS

Experimental and theoretical results are presented for current-voltage and dynamic resistance-voltage characteristics of Hg 1-x Cd x Te ion-implanted p-n junction photodiodes with x 0.22 passivated with ZnS/CdS layers. By measuring the temperature dependence of the dc characteristics in the temperature range 25-140 K, the dark current mechanisms are studied and the validity of the modeling is confirmed. It was found that the dark currents can be represented with three current components over a broad range of voltage and temperature. At high temperature (>90 K) and in low reverse bias region, the diffusion current dominates. On the other hand, at medium temperature (40-80 K) and medium reverse bias (< -0.15 V), trap-assisted tunneling plays an important role. At low temperature (<40 K) and in the medium reverse bias region (< -0.15 V), band-to-band tunneling is the key leakage current source. However, when the temperature is further lowered to 25 K and the applied reverse bias is very small (-0.15 to 0 V), the band-to-band tunneling current will be ruled out and the trap-assisted tunneling mechanism dominates again.

1/f noise and specific detectivity of HgCdTe photodiodes passivated with ZnS-CdS films

1/f noise in HgCdTe photodiodes has been measured as a function of temperature, diode bias, and dark current. The dependence of 1/f noise on dark current was measured over a wide temperature range. At low temperatures, where surface generation and leakage current were predominant, a linear relationship between 1/f noise and dark current was observed. At higher temperatures, where diffusion current is predominant, the correlation no longer holds. The temperature dependence of 1/f noise was also determined. The temperature dependence of the 1/f noise was found to be the same as that for the surface generation and leakage currents. All the data obtained in these experiments could be fit with theoretical predictions by a simple relationship between 1/f noise and dark current. The 1/f noise in the HgCdTe photodiode varies with diode bias, temperature, and dark current only through the dependence of the surface current on these devices. The maximum specific detectivity (D*) value and the maximum signal-to noise ratio are approximately 3.51/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W and 5096 at 50 mV reverse bias, respectively.

High optical-gain AlGaN/GaN 2 dimensional electron gas photodetectors

Nitride-based AlGaN/GaN heterostructure two dimensional electron gas (2DEG) photodetectors have been successfully fabricated. By using such an AlGaN/GaN heterostructure, we could significantly reduce the recombination of photogenerated carriers and thus achieve an extremely high photodetector responsiveness. At an incident light wavelength of 240 nm, it was found that the responsiveness could reach 5.2×109 A/W.

Two-step epitaxial lateral overgrowth of GaN

A two-step epitaxial lateral overgrowth (ELO) method was proposed to improve the quality of GaN-based epitaxial layers. In the first step, we grew a three-dimensional GaN low temperature buffer layer at 520 °C for different amount of time, and the second step is similar to conventional ELO. For two-step ELO GaN samples on SiO2 stripes along 〈1120〉 direction, it was found that an 8 min first step growth time could provide us the highest lateral to vertical growth rate ratio and the largest angle between sidewalls and basal plane. Under such growth conditions, we could achieve a surface root-mean-square (rms) roughness of 0.480 nm and an etch pits density (EPD) of 1.6×107 cm−2 in the stripe regions. The surface morphology of the two-step ELO GaN sample is also much better than that of the one-step ELO GaN sample.

Nitride-based devices fabricated by wet etching

Photo-enhanced chemical (PEC) wet etching technology was used to etch GaN and AlGaN epitaxial layers. Figure 1 shows PEC etch rate for the GaN and Al/sub x/Ga/sub 1-x/N epitaxial layers in aqueous KOH and H/sub 3/PO/sub 4/ solutions. It was found that the maximum etch rates were 510 nm/min, 1960 nm/min, 300 nm/min and 0 nm/min for GaN, Al/sub 0.175/Ga/sub 0.825/N, Al/sub 0.23/Ga/sub 0.77/N and Al/sub 0.4/Ga/sub 0.6/N, respectively. Nitride-based Schottky diodes and heterostructure field effect transistors (HFETs) were also fabricated by PEC wet etching. As shown in figures 2, 3 and 4, it was found that we could achieve a saturated I/sub D/ larger than 850 mA/mm and a maximum g/sub m/ about 163 mS/mm from PEC wet etched HFET with a 0.5/spl mu/m gate length. Compared with dry etched devices, the leakage currents observed from the PEC wet etched devices were also found to be smaller.