Prof. Xie's Group
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Semiconductor Materials Research Lab

Figure 1. Energy band gap and lattice constant of various semiconductors at room temperature.


  1. T. C. Lu, S. W. Chen, T. T. Wu, P. M. Tu, C. K. Chen, C. H. Chen, Z. Y. Li, H. C. Kuo, and S. C. Wang: Appl. Phys. Lett. 97 (2010) 071114.
  2. S. Nakamura, N. Iwasa, M. Senoh, S. Nagahama, T. Yamada, and T. Mukai: Jpn. J. Appl. Phys. 34 (1995) L1332.
  3. E. F. Schubert and J. K. Kim: Science 308 (2005) 1274.
  4. T. Paskova and K. R. Evans "GaN substrates-Progress, status and prospects",  IEEE J. Sel. Topics Quantum Electron., vol. 15, no. 4, pp.1041-1052 2009.
  5. S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho: Appl. Phys. Lett. 72 (1998) 211.
  6. N. G. Weimann, L. F. Eastman, D. Dharanipal, H. M. Ng, and T. D. Moustakas: J. Appl. Phys. 83 (1998) 3656.
  7. U.S. Patent Number 6,633,056 B2, October 14, 2003: "Hetero-integration of Dissimilar Semiconductor Materials," Ya-Hong Xie;
  8. See, for example, “Normally Off n-Channel GaN MOSFETs on Si Substrates Using an SAG Technique and Ion Implantation”, Hiroshi Kambayashi, Yuki Niiyama, Shinya Ootomo, Takehiko Nomura, Masayuki Iwami, Yoshihiro Satoh, Sadahiro Kato, and Seikoh Yoshida, IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 12, DECEMBER 2007 1077.
  9.  “Defect Reduction via Selective Lateral Epitaxy of GaN on an Innovative Masked Structure with Serpentine Channels”, Lei Li, Justin P. C. Liu, Lei Liu, Ding Li, Lei Wang, Chenghao Wan, Weihua Chen, Zhijian Yang, Yahong Xie, Xiaodong Hu, and Guoyi Zhang, Appl. Phys. Express v.5, 051001 (2012).

Semiconductor Materials Research Laboratory, Department of Materials Science and Engineering, University of California, Los Angeles

Box 951595, Los Angeles, CA 90095-1595 (Tel) +1 310 825 2971 UCLA SMRL © 2013 | All Rights Reserved







Group III-nitrides, including GaN, AlN, and InN and their alloys, represent a unique sub-group within the III-V compound semiconductor family characterized by their smaller lattice constant comparing to most other III-V materials with the energy band gap ranging from 0.6 eV to over 6 eV (covering the entire visible spectrum and beyond). Semiconductors with large band gaps (Eg>1.5 eV) are of particular interests in such applications as high brightness LEDs, UV laser diodes, and high-power transistors [1-3]. The small lattice constants, on the other hand, is the reason for the challenge in the epitaxial growths of group III-nitride films, namely the lack of lattice-matched substrates [4] (Fig.1). A direct consequence of the large lattice mismatch is the high density of dislocations. Although GaN light emitting diodes have been shown to be surprisingly immune to the degradation by dislocations, the performance in demanding applications (including high brightness LEDs, laser diodes, and high power transistors) is limited by the high density of dislocations [5-6] with typical value of >10^8 cm-2. The bottom line is that defect-free crystals are always better for device applications, although sometimes the advantages become obvious only for the more demanding applications.

Our group investigates the epitaxial growths of dislocation-free III-nitride. One of our approaches involves an innovative dislocation filtration approach [7] (Fig.2). We conduct research aimed at gaining understanding of dislocation dynamics in group III-nitrides for the purpose of reducing dislocation density. We further strive to design, fabricate and characterize new devices such as enhancement mode III-nitride MOSFETs for automobile applications [8] leveraging the intrinsic materials capability in dislocation-free III-nitrides.

The research consists of two components: epitaxial growths and devices. Studies to date include theoretical studies of selective nucleation and growth of GaN on our innovative masked substrate with serpentine channels (Fig.3), MOCVD growth of GaN using this technique, characterization including CL and STEM, etc [9] (Fig. 4, 5).

We work closely with our collaborations including Prof. Y. K. Su and Prof. S. J. Chang in the Department of Electrical and Electronic Engineering of National Cheng Kung University (NCKU) in Taiwan and Prof. XiaoDong Hu’s group in the School of Physics in Peking University (PKU) in China.

NCKU group websites:

PKU group website:


Towards Dislocation-free III-nitrides: Selective epitacy of GaN











Figure 2. Scheme of the innovative dislocation filtering approach and the unique mask structure.

Figure 3. The result of finite element method simulation using COMSOL that helped our understanding of the experimentally observed perfect selectivity during MOCVD growths of GaN on masked substrates and the associated proximity effect.

Figure 4. The plan-view cathodoluminescence (CL) image at 360 nm wavelength of a-GaN stripe grown out of the serpentine channel structure.


Figure 5. Cross-section Scanning Transmission Electron Microscope (STEM) image of GaN grown on masked r-plane sapphire substrate. Dislocations are effectively blocked by the serpentine shaped channel of the SiO2/Si3N4 mask.