Abstract
Through comparing ZnO directly grown on the substrates of a-plane, c-plane, and (11-22) plane GaN and AlxGa1−xN (0.06 ≤ x ≤ 1), the roles of different factors that may influence growth have been studied. Seeded by surface pits, ZnO nanowire (NW) preferentially grew along the polarized direction on top of the nonpolar GaN (laterally aligned), polar GaN and AlGaN (vertically aligned), and semipolar GaN (obliquely upward aligned). Nanosheets were easily formed when the polarized surface of the AlGaN film was not intact. The kinetic effect of polarization must be considered to explain the high aspect ratio of NWs along the polarized direction. It was found that dislocation affected NW growth through the surface pits, which provided excellent nucleation sites. If the surface pits on GaN could be controlled to distribute uniformly, self-organized ZnO NW array could be controllably and directly grown on GaN. Moreover, surface pits could also seed for nanosheet growth in AlN, since Al(OH)4− would presumably bind to the Zn2+ terminated surface and suppress the kinetic effects of polarization.
Highlights
Over the past decades, wide bandgap semiconductors based on AlGaN, GaN, and ZnO have attracted considerable attention for applications in optoelectronics and microelectronics
Controlling the formation of ZnO nanowire arrays on GaN substrates is crucial for their efficient integration into such nanoscale devices mentioned above, because the ZnO/GaN heterojunction, which is quite important to its device performance, depends highly on how ZnO NWs are grown on
Large triangular surface pits are the typical surface morphology for a-plane GaN [23], which wereLarge produced by incomplete nucleation islands coalescence
Summary
Wide bandgap semiconductors based on AlGaN, GaN, and ZnO have attracted considerable attention for applications in optoelectronics and microelectronics. ZnO is well known for diverse nanostructures [1,2] with distinct properties, such as carrier confinement and the high surface-to-volume ratio [3,4], and GaN is known to be superior in its high stability and epitaxial film quality. ZnO nanostructures combined with GaN-based film are a promising structural candidate to design many devices, such as LEDs [5,6,7], pressure sensors [4,8], UV sensors [9,10,11,12], gas sensors [13], etc. Related to UV sensors, so group [14] demonstrated that a ZnO nanowire array on GaN possessed a much higher switch ratio than bare GaN. Controlling the formation of ZnO nanowire arrays on GaN substrates is crucial for their efficient integration into such nanoscale devices mentioned above, because the ZnO/GaN heterojunction, which is quite important to its device performance, depends highly on how ZnO NWs are grown on
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