Investigations on HVPE of non-and semipolar GaN

作为课题组“氮化物光发射器中偏振场控制”的一部分，本项目旨在研究高质量厚的非极性和半极性GaN层的外延生长，然后将其用作后续异质结构沉积的衬底。为此，我们将把金属有机气相外延用于优化和减少这些层中缺陷的高度灵活性与氢化物气相外延用于生长几个100μm厚的层的高生长速度相结合。在项目的第一阶段，我们开发了一种方法，可以利用c平面生长方向来获得半极平面曲面。这是通过在特定取向的蓝宝石晶片中刻蚀具有c面状侧壁的沟槽来完成的，随后的外延生长从这些特定取向开始。在第二阶段，我们将对这种方法进行优化，特别是针对(11_23)(n面)蓝宝石衬底上生长的GaN的(10_11)面，以实现大面积低缺陷密度的GaN半极面。此外，我们还将研究这一过程是否也适用于其他半极曲面，例如(20_21)曲面。随后，这些层将被高压聚乙烯覆盖生长，以获得几个100μm厚的层，最终可以从蓝宝石衬底上分离出来，得到独立的半极性GaN晶片。通过使用标志层，可以对生长过程有一个深刻的了解。除了稍微改变衬底的取向偏差以改善条纹的结合外，我们还将研究用高压聚乙烯直接生长这种薄膜的方法。与我们研究小组合作伙伴的密切合作将有助于了解这种层的微观结构，并最终减少这种非c面生长过程中通常涉及的晶体缺陷。

As a part of the research group proposal "Polarization field control in nitride light emitters", the current project aims to investigate the epitaxial growth of high quality thick non- and semipolar GaN layers which then can be used as substrates for a subsequent heterostructure deposition. To this end, we will combine the high flexibility of metalorganic vapor phase epitaxy (MOVPE) for the optimization and defect reduction in such layers with the high growth rate of hydride vapor phase epitaxy (HVPE) for the growth of several 100μm thick layers. In the first phase of the project, we have developed a method which allows to achieve semipolar planar surfaces by making use of the c-plane growth direction. This is done by etching grooves with c-plane like side walls into sapphire wafers of specific orientations from which the subsequent epitaxial growth starts. In the second phase, we will optimize this method in particular for the (10_11) plane of GaN grown on (11_23) (n-plane) sapphire wafers to realize large area semipolar planes of GaN with low defect density. Additionally, we will investigate whether this procedure is also applicable to other semipolar surfaces like, e.g. the (20_21) surface. Subsequently, these layers will be overgrown by HVPE to achieve several 100μm thick layers, which eventually can be separated from the sapphire substrates to get free-standing semipolar GaN wafers. By using marker layers, a profound understanding of the growth process can be obtained. Besides varying slightly the substrate misorientation for an improved coalescence of the stripes, we will also study approaches to grow such layers directly by HVPE. The close collaboration with the partners of our research group will help to understand the microstructure of such layers and eventually reduce the crystalline defects typically involved in such non-c-plane growth procedures.