Molecular Beam Homoepitaxy of N-Polar AlN on Bulk AlN Substrates

High quality N-polar AlN epilayers were grown and characterized on N-face bulk AlN substrates by plasma assisted molecular beam epitaxy. This is the first step towards achieving N-polar AlN based high-power RF electronics. While the successful molecular beam homoepitaxy of metal polar AlN on single-crystal AlN substrates has been recently demonstrated [1,2] there has been no work reported thus far on the homoepitaxy of N-polar AlN on bulk substrates owing to the challenges of surface cleaning of the highly chemically reactive N-face of Aluminum Nitride. Due to the ultrawide-bandgap, direct bandgap and excellent photonic properties, high thermal conductivity and high piezoelectricity of AlN, it provides a promising platform for next generation nitride electronics and photonics. [3]<br/>High-structural-quality N-polar AlN (000-1) bulk substrates with a dislocation density of 10⁴ cm^–2produced by Asahi Kasei were used as the substrates in this study. With a combination of in situ thermal deoxidation and Al-assisted thermal desorption at ≈ 1000C in a high vacuum environment monitored with reflection high-energy electron diffraction (RHEED), native surface oxides and impurities were removed from the N-face of the AlN surface prior to the homoepitaxy. Subsequent epitaxial growth of 1 μm thick AlN layer on the in situ cleaned substrates exhibited smooth surface morphologies measured by atomic force microscopy (AFM) to be of rms roughness &lt;0.8 nm over a 20 μm x 20 μm area with clean and wide atomic terraces on the surface.<br/><i>In situ</i> RHEED reconstructions and KOH etching studies were used to study the polarity of the as-grown samples. N-polarity was confirmed by observing strong c(6x12) RHEED reconstructions on a thin 6-7 nm GaN epilayer grown over the 1 m AlN epitaxial layer. Homoepitaxial grown 1 μm thick AlN samples were etched in 2% KOH solution for 10s at 80C, the surface was vigorously etched. Cross sectional scanning electron microscopy scan of the etched surface revealed triangular shaped pyramids over a large coverage area confirming the N-polarity. Some small regions were unetched which were attributed to inversion domains arising from metal polar nucleation due to defects on the native substrate itself. In addition, the incorporation of chemical impurities in the homoepitaxially grown AlN was studied by SIMS and the impurity concentrations which revealed that Si, O, H and C concentrations are very low, all near the detection limit throughout the epitaxial layer away from the nucleating interface.<br/>In conclusion, these results indicate that with the help of in situ Al assisted cleaning procedure we have achieved the epitaxial growth of excellent structural quality N-polar AlN on single crystal AlN substrates with very few inversion domain incorporations. Thus, this serves as a good starting point for epitaxial growth of device layers for N-polar heterostructures devices and an important first step in achieving N-polar RF electronics on AlN platform.<br/>References:<br/>[1] Y. Cho, C. S. Chang, K. Lee, M. Gong, K. Nomoto, M. Toita, L. Schowalter, D. Muller, D. Jena, and H. G. Xing, Applied Physics Letters 116, 172106 (2020).<br/>[2] K. Lee, Y. Cho, L. J. Schowalter, M. Toita, H. G. Xing, and D. Jena, <i>Applied Physics Letters</i> 116, 262102 (2020).<br/>[3] 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> 36, 044001 (2021).