Growth and Polarity Control of GaN and AlN on Carbon-face SiC by Metalorganic Vapor Phase Epitaxy
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Growth and Polarity Control of GaN and AlN on Carbon-face SiC by Metalorganic Vapor Phase Epitaxy Yi Fu1, Xianfeng Ni2, Jingqiao Xie2, N Biyikli2, Qian Fan2, S Chevtchenko2, Ü Özgür2, Hadis Morkoç2, You Ke3, Robert Devaty3, W. J. Choyke3, C. K. Inoki4, and T. S. Kuan4 1 Dept of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA, 23284 2 Virginia Commonwealth University, Richmond, VA, 23284 3 University of Pittsburgh, Pittsburgh, PA, 15260 4 State University of New York, Albany, NY, 12222 ABSTRACT Growth and polarity control of GaN and AlN on carbon-face SiC (C-SiC) by metalorganic vapor phase epitaxy (MOVPE) are reported. The polarities of GaN and AlN layers were found to be strongly dependent on the pre-growth treatment of C-SiC substrates. A pre-flow of trimethyaluminum (TMAl) or a very low NH3/TMAl ratio results in Al(Ga)-polarity layers on C-SiC. Otherwise, N-polarity layers resulted. The polarities of AlN and GaN layers were conveniently determined by their etching rate in KOH or H3PO4, a method reported earlier. We suggest that the Al adatoms, which have a high sticking coefficient on SiC, form several Al adlayers on C-SiC and change the incorporation sequence of Ga(Al) and N leading to metal polarity surface. In addition, the hexagonal pyramids, typical on N-polarity GaN surface, are absent on N-polarity GaN on off-axis C-SiC owing to high density of terraces on off-axis C-SiC. The properties of GaN layers grown on C-SiC are studied by X-ray diffraction. INTRODUCTION As substrate for GaN epitaxy, SiC has polar surfaces along the +c (Si-terminated face) and –c (C-terminated face) directions. Growth of III-nitrides on SiC is almost exclusively performed on Si-terminated SiC (Si-SiC) which leads to metal polar GaN. Only a few studies have been reported on the growth of III-nitirde layers on C-terminated SiC (CSiC), with conflicting experimental results. In this paper, we report an investigation on the growth and polarity control of GaN and AlN on C-SiC by MOVPE. It is found that a pre-growth treatment of C-SiC with TMAl, or a very low NH3/TMAl ratio at initial growth stage of AlN can pave the way for Al-polarity AlN or Ga-polarity GaN. In addition, the hexagonal pyramids, characteristic of N-polarity GaN on on-axis C-SiC, are absent on the N-polarity GaN grown on off-axis C-SiC substrates. EXPERIMENTS Two types of 4H C-SiC were used as substrates in this study, including 1) 4º off c-axis ntype SiC, and 2) on c-axis n-type SiC. The growths of GaN and AlN were carried out in a custom-designed, vertical chamber equipped with Emcore D125 gas control panel. Trimethylgallium (TMGa), trimethyaluminum (TMAl), ammonia (NH3) were used as Ga, Al and N sources, respectively. Purified H2 was used as carrier gas. Two types of nucleation layers were studied for GaN growth of C-SiC, including 1) 100 nm thick GaN grown at 850 ºC under 30 Torr and 2) 100 nm thick AlN grown at 1070 ºC under 30 Torr. After annealing of GaN NLs, the GaN epilayers were grown at 1030ºC with a chamber
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