The (11-22) GaN layer used for SATE was grown on m-plane sapphire using our high temperature AlN buffer technique by metal organic chemical vapor deposition (MOCVD). The substrate is initially subjected to thermal cleaning in flowing H2, and an atomically flat AlN layer with a thickness of 220 nm is then grown at 1170 °C, followed by the growth of a single GaN with a thickness of 1.3 μm at 1100 °C. 

SiNx mask were fabricated on 2-inch c-plane GaN and (11-22) semi-polar GaN template. 30 nm SiNx was first deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD). A ~ 270 nm bottom antireflective coating (BARC) (Wide 30W – Brewer Science) layer and a ~ 360 nm high-contrast positive resist (Dow® Ultra-i 123 diluted with Dow® EC11 solvent) were spin-coated at 3000 rpm. Circular openings were defined in the resist via Displacement Talbot Lithography (PhableR 100, Eulitha) using a coherent 375 nm light source, an energy density of 1 mW.cm-2 and a gap of 150 μm. An integration time comprised between 210 and 270 sec and a Talbot length of 8.81 μm was employed for the 1.5 μm pitch amplitude mask with 800 nm diameter circular opening while an integration time of 60 sec and a Talbot length of 750 nm was employed for the 500 nm pitch phase mask with 300 nm diameter circular opening. After a post-bake at 120°C for 1 min 30 sec, the resist was developed in MF-CD-26 for 90 sec. The circular openings were then transferred into the BARC and SiNx layers via an inductively coupled plasma dry etch system (Oxford Instruments System 100 Cobra) with the following parameters: a CHF3 chemistry of 25 sccm, a temperature set to 20 °C, a pressure of 8 mTorr, 50 RF power and 300 W ICP source power. Finally, the wafer was cleaned in a piranha solution (3:1) to remove the resist and BARC, an oxygen plasma to remove any residue and deep in BOE 100:1 for 10 sec. 

SATE was carried out in a 1 x 2” horizontal Aixtron MOVPE reactor.