This paper reviews the growth of power-grade GaN crystals by the near-equilibrium ammonothermal (NEAT) method and reports the evaluation of a 4" bulk GaN crystal. Ammonothermal growth is a proven method of producing low-dislocation, GaN crystals with practical productivity. [1, 2] By choosing a near-equilibrium growth condition, consistent growth with maintained crystal quality has been achieved. [3] As the development of GaN-based power devices shifts from GaN-on-Si horizontal architecture to GaN-on-GaN vertical architecture, the development of low-cost, large-area, low-dislocation, pit-free GaN substrates has become an urgent agenda. SixPoint started the development of 4" power-grade GaN substrates with reduced dislocation density to meet the increasing demand of GaN substrates for the development of power devices.
Bulk GaN crystals are grown on HVPE-grown GaN substrates (HVPE seeds) by the NEAT method. The Ga-face of HVPE seed is covered so that growth only occurs on the N-face. In one batch of growth, many seed crystals, including 2" and 4" sizes, are loaded in a high-pressure autoclave. The details of the growth conditions are found elsewhere [4]. After growth, the crystals are sliced into wafers and polished to be "epi-ready" substrates.
A 4" bulk GaN was grown on a 4" HVPE seed. The grown crystal was evaluated with Nomarski microscopy, mapping of full-width half maximums (FWHMs) of 002 and 201 X-ray diffraction rocking curves (X-ray mapping), and X-ray topography. To evaluate the dislocation density of the grown bulk crystal, X-ray topography was measured on the top as-grown N-face and the exposed Ga-face of the HVPE-seed (i.e., the backside of the bulk crystal).
Both X-ray mapping and X-ray topography showed improved microstructure quality. The dislocation density measured on the as-grown N-face was 2-5 x 105 cm-2, whereas that measured on the Ga-face of the HVPE seed was too high to be separated in the topography image. The estimated dislocation density of the HVPE seed was at the order of 106 cm-2. In addition, the grown 4" bulk crystal does not have any macro defects (i.e., pits and holes). The observed reduction of dislocation density in the growth of the 4" crystal was similar to what was seen in the growth of 2" crystals. A typical dislocation density of 2" NEAT GaN substrates is low-105 cm-2. The result was quite encouraging to apply the NEAT method to the production of large-are power-grade GaN substrates.
References
[1] E. Letts, Y. Sun, D. Key, B. Jordan, and T. Hashimoto, J. Cryst. Growth 501 (2018) 13.
[2] R. Dwilinski, R. Doradzinski, J. Garczynski, L. Sierzputowski, R. Kucharski, M. Zajac, M. Rudzinski, R. Kudrawiec, W. Strupinski, and J. Misiewicz, Phys. Stat. Sol. A 208 (2011) 1489.
[3] T. Hashimoto, E.R. Letts, D. Key, and B. Jordan, Jpn. J. Appl. Phys 58 (2019) SC1005.
[4] T. Hashimoto, E. Letts, M. Ikari, and Y. Nojima, J. Cryst. Growth 312 (2010) 2503.
Acknowledgement
This work was supported by U.S. Department of Energy (DOE), the Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office (AMO), Small Business Innovation Research (SBIR) program (DE-SC0013791), US DOE EERE AMO FY18/FY19 Lab Call (DE-LC-000L059, Program Manager: A. Hefner), US DOE Advanced Research Program Agency, Energy (ARPA-E) OPEN 2018 program (DEAR0001036, Program Manager: I. Kizilyalli), and the Office of Naval Research (N00014-19-1-2069, Program Manager: L. Petersen).The synchrotron radiation experiments were performed at the BL16B2 of Spring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2021B5370). The authors appreciate Prof. Masakazu Kanechika at Nagoya University, and Satoshi Yamaguchi, Kosuke Kitazumi, and Yasuji Kimoto at Toyota Central R&D Labs., Inc. for the measurement of X-ray topography.