Zexuan Zhang1,Yusuke Hayashi2,Vladimir Protasenko1,Jashan Singhal1,Hideto Miyake3,Huili Xing1,Debdeep Jena1,Yongjin Cho1
Cornell University1,Osaka University2,Mie University3
Zexuan Zhang1,Yusuke Hayashi2,Vladimir Protasenko1,Jashan Singhal1,Hideto Miyake3,Huili Xing1,Debdeep Jena1,Yongjin Cho1
Cornell University1,Osaka University2,Mie University3
N-polar aluminum nitride (AlN) is an important building block for next-generation high-power RF electronics due to its highest electrical resistivity and thermal conductivity in the nitride semiconductor family.[1] For example, the current state-of-the-art performance achieved in N-polar GaN/AlGaN high electron mobility transistors (HEMTs) can potentially be further boosted by replacing AlGaN buffer with AlN.[2]<br/>Although N-polar AlN has been synthesized on different <i>foreign</i> substrates such as Si, SiC and sapphire,[3-5] the development of homoepitaxial growth technique is highly desired to maximize the performance of N-polar AlN-based devices. However, to the best of the authors’ knowledge, successful MBE homoepitaxy of N-polar AlN has not been reported yet.<br/>In this work, we report successful homoepitaxial growth of N-polar AlN by MBE on large-area cost-effective sputtered face-to-face annealed N-polar AlN/<i>c</i>-plane sapphire templates.[5] Direct growth leads to film with inverted Al-polarity. But with <i>in-situ</i> Al-assisted cleaning [6,7] before growth, the epilayer is found to maintain the N-polarity. The MBE-grown N-polar AlN epilayer is electrically insulating and has pit-free smooth surface with a root-mean-square (rms) roughness of 0.3 nm in a 10x10 μm<sup>2 </sup>area with parallel atomic steps. Narrow X-ray rocking curves (XRCs) with full width at half of maximums (FWHMs) of 14/380 arcsec across AlN (0002)/(10-12) reflections indicate high structural quality of the N-polar AlN epilayer. Moreover, clear near band-edge photoluminescence (PL) emission peaks are observed on samples with MBE-grown N-polar AlN epilayers at room temperature. This peak is absent on the bare AlN substrates, implying the suppression of non-radiative recombination centers in the MBE-grown epitaxial N-polar AlN layers. These results suggest the significant potential of MBE homoepitaxy for preparation of electronics-grade N-polar AlN and are important milestones towards N-polar RF electronics based on AlN platform.<br/>The authors at Cornell University acknowledge financial support from the Cornell Center for Materials Research (CCMR)—a NSF MRSEC program (No. DMR-1719875); ULTRA, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No. DE-SC0021230; and AFOSR Grant No. FA9550-20-1-0148. This work uses the CESI Shared Facilities partly sponsored by NSF No. MRI DMR-1631282 and Kavli Institute at Cornell (KIC).<br/>References:<br/>[1] A. L. Hickman, R. Chaudhuri, S. J. Bader, K. Nomoto, L. Li, J. C. Hwang, H. G. Xing, and D. Jena, <i>Semiconductor Science and Technology</i> <b>36</b>, 044001 (2021).<br/>[2] J. Lemettinen, H. Okumura, T. Palacios, and S. Suihkonen, <i>Applied Physics Express</i> <b>11</b>, 101002 (2018).<br/>[3] S. Dasgupta, F. Wu, J. Speck, and U. Mishra, <i>Applied Physics Letters</i> <b>94</b>, 151906 (2009).<br/>[4] J. Lemettinen, H. Okumura, I. Kim, C. Kauppinen, T. Palacios, and S. Suihkonen, <i>Journal of Crystal Growth</i> <b>487</b>, 12 (2018).<br/>[5] K. Shojiki, K. Uesugi, S. Kuboya, and H. Miyake, <i>Journal of Crystal Growth</i> <b>574</b>, 126309 (2021).<br/>[6] Y. Cho, C. S. Chang, K. Lee, M. Gong, K. Nomoto, M. Toita, L. Schowalter, D. Muller, D. Jena, and H. G. Xing, <i>Applied Physics Letters</i> <b>116</b>, 172106 (2020).<br/>[7] K. Lee, Y. Cho, L. J. Schowalter, M. Toita, H. G. Xing, and D. Jena, <i>Applied Physics Letters</i> <b>116</b>, 262102 (2020).