Kai-Hsuan Lee

Graphene as tunable transparent electrode material on GaN: Layer-number-dependent optical and electrical properties

Graphene has been regarded as a prospective transparent electrode for a GaN-based light-emitting diode (LED), but fundamental knowledge about the intrinsic properties of the graphene/GaN contact is lacking. We have studied the optical and electronic properties of graphene exfoliated on an n-type GaN surface. The graphene visibility was simulated based on Fresnels law and confirmed with an optical microscope and micro-Raman spectra. The interfacial electronic property was studied with a scanning photoelectron microscope. We found that the Schottky barrier height of the graphene/n-GaN is decreased with decreasing graphene number of layers, yielding an improved GaN-based LED performance.

Synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy of n-ZnO:Al/p-GaN:Mg heterojunction

Al-doped ZnO (AZO) deposited by radio frequency co-sputtering is formed on epitaxial Mg-doped GaN template at room temperature to achieve n-AZO/p-GaN heterojunction. Alignment of AZO and GaN bands is investigated using synchrotron radiation based cross-sectional scanning photoelectron microscopy and spectroscopy on the nonpolar side-facet of a vertically c-axis aligned heterostructure. It shows type-II band configuration with valence band offset of 1.63 ± 0.1 eV and conduction band offset of 1.61 ± 0.1 eV, respectively. Rectification behavior is clearly observed, with a ratio of forward-to-reverse current up to six orders of magnitude when the bias is applied across the p-n junction.

InGaN Metal-Semiconductor-Metal Photodetectors With Aluminum Nitride Cap Layers

We report on the fabrication and characterization of InGaN metal-semiconductor-metal photodetectors (PDs) by using triethylgallium gallium source for the growth of InGaN active layers and in-situ aluminum nitride as cap layers. Improved characteristics such as reduced dark leakage current, large ultra violet (UV)-to-visible rejection ratio, low noise level, and high detectivity can be achieved in our devices. Current transport mechanisms in InGaN PDs were also investigated. Fowler-Nordheim mechanism associated with the defect-related tunneling should be included besides the traditional thermionic emission model.

Effect of surface modification by self-assembled monolayer on the ZnO film ultraviolet sensor

A simple surface modification process to reduce the negative influences of oxygen vacancies/surface contamination and promote the performance of ZnO ultraviolet (UV) sensor is reported. ZnO film was self-assembled by 3-aminopropyltrimethoxysilane (APTMS) molecules for passivating its surface defects. APTMS molecules are hydrolyzed at the methoxy end, and then condensed with the substrate hydroxyl groups to produce siloxanes. Compared with the conventional ZnO UV sensor, in which the reproducibility and homogeneity of device performance strongly suffer from a difficulty in controlling the ZnO surface state conditions, the APTMS molecules leads to a reduction of the dark leakage current by more than 2 orders of magnitude.

Simulated-stop tribological behavior of pitch—resin—CVI carbon—carbon composite

Abstract This work studies the braking (simulated-stop) behavior of a mesophase pitch fiber-reinforced phenolic resin plus chemical vapor infiltrated (CVI) hybrid matrix carbon—carbon (C-C) composite under different surface conditions. The results indicate that friction and wear behavior is sensitive to sliding surface condition, and the initial surface condition has a significant effect on tribological behavior of the composite. AP/1400, the only condition that does not experience a transition, shows much lower friction coefficient and wear than the other conditions. Under the same test conditions, BI specimens exhibit higher friction coefficients and wear than those of AP specimens. Of all the conditions, BI/2000 induces the highest temperature rise. Specimens braked from higher speed always suffer higher wear, but the stopping time is dependent more on the initial surface condition than the initial speed. As soon as a type I-to-type II transition occurs, the friction and wear rise abruptly. A V-shaped variation is exhibited in post-transitional friction coefficient of the composite. The severe structural damage during the final stage is responsible for the final increase in friction coefficient. Collection of large wear particles is consistent with the observed heavily delaminated surface film during this stage.

In situ grown AlN/AlGaN/GaN heterostructure field-effect transistor

In situ grown AlN/AlGaN/GaN metal-insulator-semiconductor (MIS) heterostructure field-effect transistor (HFET) is promising for high power applications due to the reduced gate leakage current together with the successful surface passivation. In this study, we present a novel AlGaN/GaN MIS-HFET using in situ AlN as a gate insulator. The AlN is formed subsequently after the epitaxial growth in the same reactor without any exposure in the air. This “earlier” passivation by in situ AlN protects the surface during processing and neutralizes the charges at the top of AlGaN interface, which leads to a higher electron density of channel and superior device characteristics. The unity gain cutoff frequency (fT) and maximum frequency of oscillation (fmax) are 14.5 and 24.9 GHz, respectively. The fabricated AlN/AlGaN/GaN MIS-HFET at 2.4 GHz delivers 2.3 W/mm output power density and 21.3% peak power added efficiency. It demonstrates a great potential to the next-generation power transistor.