Kuo-Hua Chang

The amorphous Si/SiC heterojunction color-sensitive phototransistor

An amorphous Si/SiC heterojunction color-sensitive phototransistor was successfully fabricated by plasma-enhanced chemical vapor deposition. The structure is glass/ITO/a-Si(n+-i)/a-SiC(p+-i-n+)/Al. The device is a bulk barrier transistor with wide-bandgap amorphous SiC emitter and base. The phototransistor revealed a very high optical gain of 40 and a response speed of 10 µs at an input light power of 5 µW and a collector current of 0.12 mA at a voltage of 14 V. The peak response occurs at 610 nm under 1-V bias and changes to 420 and 540 nm under 7- and 13-V biases, respectively.

Demonstration of GaN-Based Solar Cells With GaN/InGaN Superlattice Absorption Layers

In this letter, we display InGaN/GaN-based photovoltaic (PV) devices with active layers in absorbing the solar spectrum around blue regions. The GaN/In0.25Ga0.75 N superlattice layers grown by metalorganic vapor-phase epitaxy are designed as the absorption layers with the same total thickness. The PV effect is almost absent when the In0.25Ga0.75N single layer is used as the absorption layer. This could be due to the large leakage current caused by the poor material quality and the relatively small shunt resistance. Devices with superlattice structure illuminated under a one-sun air-mass 1.5-G condition exhibit an open-circuit voltage of around 1.4 V and a short-circuit current density of around 0.8 mA/cm2 corresponding to a conversion efficiency of around 0.58%.

Inverted Al0.25Ga0.75N/GaN ultraviolet p-i-n photodiodes formed on p-GaN template layer grown by metalorganic vapor phase epitaxy

Inverted Al0.25Ga0.75N/GaN ultraviolet (UV) p-i-n photodiodes (PDs) were grown by selective-area regrowth on p-GaN template. The inverted devices with low-resistivity n-type AlGaN top-contact layers exhibited a typical zero-bias peak responsivity of 66.7 mA/W at 310 nm corresponding to the external quantum efficiency of 26.6%. The typical UV-to-visible (310/400 nm) spectral rejection ratio at zero-bias was over three orders of magnitude. The differential resistance and detectivity were obtained at approximately 6.2×1012 Ω and 3.4×1013 cm Hz1/2 W−1, respectively. Compared with conventional AlGaN/GaN-based UV p-i-n PDs, the proposed device structure can potentially achieve solar-blind AlGaN/GaN-based p-i-n PDs with low-aluminum content or aluminum-free p-contact layer and reduce excessive tensile strain due to the lattice mismatch between AlGaN and GaN layers.

Ultraviolet band-pass photodetectors formed by Ga-doped ZnO contacts to n-GaN

In this study, Ga-doped ZnO (GZO) films prepared by cosputtering were deposited onto n-GaN films with a low-temperature-grown GaN cap layer to form Schottky barrier photodetectors (PDs). The ultraviolet (UV) PDs exhibited a narrow band-pass spectral response ranging from 340to390nm. The short-wavelength cutoff at around 340nm can be attributed to the marked absorption of the GZO contact layer. With a zero-biased condition, the UV PDs exhibited a typical peak responsivity of around 0.10A∕W at 365nm, which corresponds to the quantum efficiency of around 34%. When the reverse biases were below 10V, the dark currents of the PDs were well below 30pA.

The Structure of GaN-Based Transverse Junction Blue LED Array for Uniform Distribution of Injected Current/Carriers

In this study, we demonstrate a GaN-based transverse junction blue LED array. This device was realized by the regrowth of n-type GaN layers on the sidewall of p-type GaN and undoped multiple quantum wells (MQWs). Due to the transverse flow of injection carriers, problems related to nonuniform current distribution, nonuniform carrier distribution among different MQWs, and bias-dependent shape of the electroluminescence spectra such as that occurring in traditional GaN-based blue LEDs with vertical p-n junctions and large active area (>1 mm2) are all greatly minimized in our structure.

Enhanced output power of GaN-based LEDs with embedded AlGaN pyramidal shells

In this article, the characteristics of GaN-based LEDs grown on Ar-implanted GaN templates to form inverted Al0.27Ga0.83N pyramidal shells beneath an active layer were investigated. GaN-based epitaxial layers grown on the selective Ar-implanted regions had lower growth rates compared with those grown on the implantation-free regions. This resulted in selective growth, and formation of V-shaped concaves in the epitaxial layers. Accordingly, the inverted Al0.27Ga0.83N pyramidal shells were formed after the Al0.27Ga0.83N and GaN layers were subsequently grown on the V-shaped concaves. The experimental results indicate that the light-output power of LEDs with inverted AlGaN pyramidal shells was higher than those of conventional LEDs. With a 20 mA current injection, the output power was enhanced by 10% when the LEDs were embedded with inverted Al0.27Ga0.83N pyramidal shells. The enhancement in output power was primarily due to the light scattering at the Al0.27Ga0.83N/GaN interface, which leads to a higher escape probability for the photons, that is, light-extraction efficiency. Based on the ray tracing simulation, the output power of LEDs grown on Ar-implanted GaN templates can be enhanced by over 20% compared with the LEDs without the embedded AlGaN pyramidal shells, if the AlGaN layers were replaced by Al0.5Ga0.5N layers.

InGaN light-emitting diodes with oblique sidewall facets formed by selective growth on SiO 2 patterned GaN film

In this study, GaN-based light-emitting diodes (LEDs) with naturally formed oblique sidewall facets (OSFs) were fabricated through a selective regrowth process. The SiO₂ mask layer was patterned on a heavily doped n-GaN template layer rather than on a sapphire substrate. As a result, the periphery of the LED included several OSFs around the regrown GaN mesa. While processing the device, dry etching was unnecessary for exposing the n-GaN underlying layer in order to form the n-type Ohmic contacts. This could be attributed to the fact that the n-GaN template layer with an electron concentration of around 8 × 10¹⁸/cm³ was exposed after the removal of the SiO₂ mask layer. With an injection current of 20 mA, GaN-based LEDs with OSFs exhibited a 21% enhancement in light output compared with those that have vertical sidewall facets. The enhancement is attributed to the fact that photons extracted from OSFs can reduce internal absorption loss.

Optical and Electrical Properties of µ-Slice InGaN/GaN Light Emitting Diodes Shaped by Focused Ion Beam Process

In this study, slice-type GaN-based light-emitting diodes (µ-slice LEDs) scaled down to a few micrometers using focused ion beam (FIB) process were demonstrated. Electroluminescence peak wavelengths (λp) of the single µ-slice LEDs were nearly independent of driving current density due to the fact that the effects of joule heating and band filling on the shift of λp compete with each other. The smaller full width of half maximum of electroluminescence peaks observed from the µ-slice LEDs could result from the effect of strain release after the FIB process and thermal annealing during the FIB process on sliced structures.

Ga-Doped ZnO/GaN Schottky Barrier UV Band-Pass Photodetector with a Low-Temperature-Grown GaN Cap Layer

Ga-doped ZnO (GZO) films were deposited onto low-temperature-grown (LTG) GaN/i-GaN (PD-I) and i-GaN (PD-II) epitaxy layers to form Schottky barrier UV band-pass photodetectors (PDs). The UV PDs exhibited a narrow band-pass spectral response ranging from 330 to 380 nm. It was also found that by using an LTG GaN layer on top of conventional nitride-based UV PDs, the leakage current was significantly reduced and a much larger photocurrent-to-dark-current contrast ratio was achieved. The short-wavelength cutoff at around 330 nm can be attributed to the marked absorption of the GZO top contact layer. The zero-bias peak responsivities were estimated to be 0.13 and 0.08 A/W at 360 nm for PD-I and PD-II, respectively. When the reverse bias was below -10 V, the dark current of PD-I was considerably below 20 pA.

Characteristics of InGaN/sapphire-based photovoltaic devices with different superlattice absorption layers and buffer layers

In this study, hetero-structure p-i-n type epitaxy wafers were deposited on the GaN/sapphire templates with different buffer layers by the MOVPE system. The absorption layers sandwiched in top p-GaN and bottom n+-GaN layers were designed into different short-period InGaN/GaN superlattice structures with specific pair numbers to maintain a total absorption thickness of 200 nm. As the buffer layer was properly adjusted, the VOC and JSC were enhanced by 35% and 95%, respectively. In addition to material qualities, the thickness of GaN buffer layers and piezoelectric-induced stain in the InGaN film itself also influenced the PV device performance.