Heteroepitaxy in III-nitride semiconductors by means of control of surface structures in atomistic scale.

降低III族氮化物薄膜中的缺陷密度(特别是线位错)对于实现高性能的光电器件具有重要意义。由于缺少体相GaN衬底，它不得不使用异质材料作为衬底，例如蓝宝石和碳化硅，因此III-氮化物的生长是通过异质外延完成的。在III-氮化物薄膜生长中降低偏转密度的许多潜在问题中，原子尺度的衬底表面结构和形貌是重要的课题。我们特别注意到了碳化硅表面，在正常情况下，它的表面表现出相当粗糙的形态。我们初步发现，高温(~1400℃)氢气(含~14%HCl)刻蚀可以改善仅6H-SiC0001表面的这种表面特征，从而获得原子光滑的表面。事实上，这一过程影响了在AlN和GaN异质薄膜中引入的结构偏转。邻近的衬底，4度偏置的6H-碳化硅(0001)和8度偏置的4H-碳化硅(0001)也被刻蚀，以观察是否有类似的平滑效果。两个相邻的碳化硅表面都显示出纳米尺度的规则排列的阶梯/阶梯结构。研究了这种有序结构的起源，发现表面由纳米面组成；(0001)和(11-2n)，这是由于表面相分离使表面自由能最小。在量子阶跃聚束和小平面-小平面相互作用下，考虑了这种纳米小平面对的有序性。重要的是，由于碳化硅的多晶型性质，前者在很大程度上归因于与传统的立方半导体(硅和砷化镓)相比具有更好的周期性。我们研究了GaN在具有周期性纳米面的邻近碳化硅表面上的成核和生长，发现了各向异性的结构特征，从而意味着缺陷密度的降低。

Reduction of defect density (especially threading dislocations) in group III-nitride thin films is of great importance to achieve high performance optical and electrical devise. Due to the lack of bulk GaN substrate it is obliged to use foreign materials as a substrate, e.g.sapphire and SiC, and thus growth of III-nitrides is done by a heteroepitaxy. Among many potential problems to solve for the reduced deflect density in III-nitride thin film growth substrate surface structures and morphologies in atomistic scale are essentially important topics. We especially noticed SiC surfaces, which normally show rather rough morphology at the as-received condition. We initially found that such surface features on just 6H-SiC(0001) were improved by high temperature (〜1400℃) hydrogen gas (with 〜14% HCl) etching, resulting in atomically smooth surfaces. Indeed, this process affects the introduction of structural deflects in AIN and GaN hetero-thin films. Vicinal substrates, 4-degree-off 6H-SiC(0001) and 8-degree-off 4H-SiC(0001), were also etched to see if similar smoothening effects were observed. Both vicinal SiC surfaces indicated regularly spaced step/terrace structures in nanometer scale. The origins of such ordering structures were investigated and found that the surfaces consist of nanofacets; (0001) and (11-2n), due to surface phase separation in minimizing surface free energies. The ordering of such nanofacet pairs is considered with quantum step bunching and facet-facet interactions. Importantly the former is greatly attributed to superior periodicity compared to conventional cubic semiconductors (Si and GaAs), owing to polytymorpholic nature of SiC. We investigated GaN nucleation and growth on vicinal SiC surfaces with periodic nanofacets and showed anisotropic structural characteristics and thus implication of reduction of the defect density.