Effect of AIN buffer layer properties on the morphology and polarity of GaN nanowires grown by molecular beam epitaxy

摘要

Low-temperature AIN buffer layers grown via plasma-assisted molecular beam epitaxy on Si (111) were found to significantly affect the subsequent growth morphology of GaN nanowires. The AIN buffer layers exhibited nanowire-like columnar protrusions, with their size, shape, and tilt determined by the AIN Ⅴ/Ⅲ flux ratio. GaN nanowires were frequently observed to adopt the structural characteristics of the underlying AIN columns, including the size and the degree of tilt. Piezores-ponse force microscopy and polarity-sensitive etching indicate that the AIN films and the protruding columns have a mixed crystallographic polarity. Convergent beam electron diffraction indicates that GaN nanowires are Ga-polar, suggesting that Al-polar columns are nanowire nuclea-tion sites for Ga-polar nanowires. GaN nanowires of low density could be grown on AIN buffers that were predominantly N-polar with isolated Al-polar columns, indicating a high growth rate for Ga-polar nanowires and suppressed growth of N-polar nanowires under typical growth conditions. AIN buffer layers grown under slightly N-rich conditions (Ⅴ/Ⅲ flux ratio = 1.0 to 1.3) were found to provide a favorable growth surface for low-density, coalescence-free nanowires.