Keith R. Evans

Publications (2)3.19 Total impact

• Article: In Situ Raman Analysis of a Bulk GaN-Based Schottky Rectifier Under Operation

  Hui Xu, Siddharth Alur, Yaqi Wang, An-Jen Cheng, Kilho Kang, Yogeshkumar Sharma, Minseo Park, Claude Ahyi, John Williams, Chaokang Gu, Andrew Hanser, Tanya Paskova, Edward A. Preble, Keith R. Evans, Yi Zhou
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  ABSTRACT: We have fabricated vertical Schottky rectifiers based on a free-standing GaN substrate and have measured the temperature of the device under operation insitu using micro-Raman spectroscopy. The n-type bulk GaN wafer with 500μm thickness was prepared using hydride vapor-phase epitaxy. The carrier concentration of the wafer was ~2.4×1016cm−3. Semitransparent Ni and multilayered Ti/Al/Pt/Au were used to make a Schottky and a full backside ohmic contact, respectively. In this investigation, Raman spectra were collected as a function of the forward power applied to the Schottky diode. A systematic shift and broadening of the Raman E 2 peak were observed as a function of increasing bias. This was caused by device heating due to the increase in current as the forward bias was increased. It was demonstrated that micro-Raman spectroscopy can serve as an excellent insitu diagnostic tool for analyzing thermal characteristics of the GaN Schottky diode. Moreover, the strain caused by the piezoelectric effect was calculated to lead to a shift of the Raman peak at the level of 0.001cm−1. This confirmed that the observed Raman peak shift was predominantly produced by a thermal not piezoelectric effect. KeywordsGaN-Schottky rectifier-Raman spectroscopy
  Journal of Electronic Materials 04/2012; 39(10):2237-2242. · 1.47 Impact Factor
• Article: Ultra-low leakage and high breakdown Schottky diodes fabricated on free-standing GaN substrate

  Yaqi Wang, Siddharth Alur, Yogesh Sharma, Fei Tong, Resham Thapa, Patrick Gartland, Tamara Issacs-Smith, Claude Ahyi, John Williams, Minseo Park, Mark Johnson, Tanya Paskova, Edward A Preble, Keith R Evans
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  ABSTRACT: Vertical Schottky diodes were fabricated on the bulk GaN substrate with decreasing impurity concentration from N-face to Ga-face. An array of circular Pt Schottky contacts and a full backside Ti/Al/Ni/Au ohmic contact were prepared on the Ga-face and the N-face of the n-GaN substrate, respectively. The Schottky diode exhibits a minimum specific on-state resistance of 1.3 mΩ cm2 and a maximum breakdown voltage of 600 V, resulting in a figure-of- merit of 275 MW cm−2. An ultra-low reverse leakage current density of 3.7 × 10−4 A cm−2 at reverse bias of 400 V was observed. Temperature-dependent I–V measurements were also carried out to study the forward and reverse transportation mechanisms.
  Semiconductor Science and Technology 12/2010; 26(2):022002. · 1.72 Impact Factor