Abstract We investigated the influence of the V/III ratio on the surface morphologies of N-polar GaN epilayers during the stage of high-temperature (HT) GaN growth. The epilayers were grown on 4°-offcut sapphire substrates by metal-organic vapor phase epitaxy. We observed prominent undulations on the GaN surface when a low V/III ratio was used, and ridged hillocks on the GaN surface when a high V/III ratio was used. With the V/III ratio increased from 128 to 2237, the density of ridged hillocks increased but the surface roughness was significantly reduced from 13.8 nm to 1.7 nm in 25 × 25 μm2 AFM images. The results differ from the existing reports in which a smoother N-polar GaN surface morphology was usually obtained with a lower V/III ratio. Non-uniform island formation in the initial growth of HT-GaN and enhanced step-bunching effect under a low V/III ratio were blamed for the large surface roughness of the N-polar GaN film. The increasing density of ridged hillocks on the N-polar surface grown under a high V/III ratio was attributed to the reduced diffusion rate of Ga adatoms due to excessive N. Completely hillock-free smooth N-polar GaN surfaces were achieved by further increasing the growth temperature of the GaN buffer layer.

中文翻译：

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V/III 比对切割蓝宝石衬底上生长的 N 极性 GaN 薄膜表面形貌的影响

摘要 我们研究了高温 (HT) GaN 生长阶段 V/III 比对 N 极性 GaN 外延层表面形貌的影响。通过金属有机气相外延在 4° 切割蓝宝石衬底上生长外延层。当使用低 V/III 比时，我们在 GaN 表面观察到明显的起伏，而当使用高 V/III 比时，GaN 表面上会出现脊状小丘。随着 V/III 比从 128 增加到 2237，在 25 × 25 μm2 AFM 图像中，脊状小丘的密度增加，但表面粗糙度从 13.8 nm 显着降低到 1.7 nm。结果不同于现有的报告，其中通常以较低的 V/III 比获得更光滑的 N 极性 GaN 表面形态。HT-GaN 初始生长中的非均匀岛形成和低 V/III 比下增强的阶跃效应被归咎于 N 极性 GaN 膜的大表面粗糙度。在高 V/III 比下生长的 N 极性表面上脊状小丘的密度增加归因于由于过量的 N 导致 Ga 吸附原子的扩散速率降低。通过进一步增加实现完全无小丘的光滑 N 极性 GaN 表面GaN缓冲层的生长温度。