Browsing by Author "Zheng, Xia-Xi"

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Analysis of Leakage Current Mechanism for Ni/Au Schottky Contact on InAlGaN/GaN HEMT
(2018-06-06)
Lumbantoruan, Franky
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Wu, Chia-Hsun
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Zheng, Xia-Xi
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Singh, Sankalp K.
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Dee, Chang-Fu
The gate leakage mechanism for InAlGaN/GaN high electron mobility transistors (HEMTs) is systematically studied using temperature-dependent gate current-voltage characteristics. The electric field across the barrier layer is calculated through the extracted polarization charge and dielectric constant of the InAlGaN/GaN HEMT. The gate current of the InAlGaN/GaN HEMT is analyzed by fitting the experimental data using Themionic Emission (TE), Poole-Frenkel (PF), and Fowler-Nordheim (FN) tunneling. The results show that 1) reverse leakage current in the thin InAlGaN barrier layer is dominated by FN tunneling due to triangular barrier formation; 2) for thicker barrier layer, the reverse leakage current at low electric field (<2.23MVcm(-1)) is dominated by PF emission; 3) at high electric field near the threshold voltage the FN tunneling dominates. Extraction of effective barrier height by fitting the experimental data with models leads to the suggestion for the improvement of the device leakage current.
Effects of the growth temperature on structural and electrical properties of AlN/GaN heterostructures grown by metal organic chemical vapor deposition
(2020-09-01)
Zheng, Xia-Xi
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Lin, Chun-Hsiung
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Ueda, Daisuke
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Chang, Edward-Yi
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交大名義發表
In this paper, we present the dependence of structural and electrical properties of AlN/GaN heterostructure on the growth temperature on sapphire substrates by metal organic chemical vapor deposition. The results revealed that higher Ga incorporation (similar to 47%) and higher trench density on the surface of AlN barrier layer when grown at 1125 degrees C. However, further decreasing the AlN growth temperature to 500 degrees C results in the lower Ga incorporation (similar to 5%), higher dislocation density, 3D island growth and larger tensile strain of AlN barrier layer. Degradation of structural properties and surface morphologies AlN barrier layer results in the higher resistivity of 2-dimensional electron gas transport properties. Low sheet resistance (255.45 Omega/sq), high free carrier electron density (2.86 x 10(13) cm(-2)), simultaneously maintaining low surface roughness and high crystal quality of AlN barrier were achieved at optimized growth temperature around 800 degrees C.
Structural and electrical properties analysis of InAlGaN/GaN heterostructures grown at elevated temperatures by MOCVD
(2018-11-01)
Lumbantoruan, Franky
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Zheng, Xia-Xi
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Huang, Jian-Hao
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Huang, Ren-Yao
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Mangasa, Firman
We report the effects of the growth temperature on the structural and electrical properties of the InAlGaN/GaN heterostructures grown on c-plane sapphire substrates. X-ray Photoelectron Spectroscopy and Atomic Force Microscopy measurements results indicate that the InAlGaN layer properties is strongly dependent on the growth temperature. It is observed that the Gallium incorporation increases with the increase of the growth temperature. Meanwhile, the surface roughness decreases from 0.49 nm to 0.34 nm with the increase of growth temperature. The variation of structural properties, composition and surface morphology influences the transport properties of the InAlGaN/GaN heterostructures. High 2DEG electron density, low sheet resistance and good C-V response with a steep slope for InAlGaN/GaN HEMT were achieved at an optimized growth temperature window between 900 degrees C and 950 degrees C.
Tunable Spectrum Selectivity for Multiphoton Absorption with Enhanced Visible Light Trapping in ZnO Nanorods
(2018-05-17)
Tan, Kok Hong
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Lim, Fang Sheng
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Toh, Alfred Zhen Yang
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Zheng, Xia-Xi
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Dee, Chang Fu
Observation of visible light trapping in zinc oxide (ZnO) nanorods (NRs) correlated to the optical and photoelectrochemical properties is reported. In this study, ZnO NR diameter and c-axis length respond primarily at two different regions, UV and visible light, respectively. ZnO NR diameter exhibits UV absorption where large ZnO NR diameter area increases light absorption ability leading to high efficient electron-hole pair separation. On the other hand, ZnO NR c-axis length has a dominant effect in visible light resulting from a multiphoton absorption mechanism due to light reflection and trapping behavior in the free space between adjacent ZnO NRs. Furthermore, oxygen vacancies and defects in ZnO NRs are associated with the broad visible emission band of different energy levels also highlighting the possibility of the multiphoton absorption mechanism. It is demonstrated that the minimum average of ZnO NR c-axis length must satisfy the linear regression model of Z(p,min) = 6.31d to initiate the multiphoton absorption mechanism under visible light. This work indicates the broadening of absorption spectrum from UV to visible light region by incorporating a controllable diameter and c-axis length on vertically aligned ZnO NRs, which is important in optimizing the design and functionality of electronic devices based on light absorption mechanism.