氫氣蝕刻氮化鎵之原理與應用

Abstract

氫氣對氮化鎵具有蝕刻效果是為人所熟知的現象。然而一般文獻中在探討氫氣對氮化鎵的蝕刻時，都是以氫氣在氮化鎵磊晶過程中所扮演之載氣的角色為研究的主題；幾乎沒有任何的文獻針對氫氣對氮化鎵蝕刻所產生的特殊形貌做分析。 本論文以氫氣對氮化鎵的蝕刻進行研究，發現在不同的蝕刻條件下氮化鎵表面會產生不同的形貌。蝕刻條件以溫度和壓力對形貌的影響最鉅。在相同溫度下，高壓的蝕刻會讓氮化鎵表面佈滿類似繫船柱的氮化鎵柱子結構，其中底切是這種形貌的主要特徵；而低壓的蝕刻則會造成佈滿深洞的表面形貌。溫度和壓力的效果一樣，只是趨勢相反；在高溫時會是深洞的形貌而在低溫時是繫船柱的形貌。我們對這些不同形貌產生的原因提出一氫氣蝕刻的機制，並加以驗證。在我們所提的蝕刻機制中，氫氣從線缺陷開始蝕刻是主要的論點；由陰極發光光譜與穿透式電子顯微鏡的分析結果可作為驗證。而不同形貌產生的原因則是因為各個晶面在不同的條件下有不同的穩定度所致；這部分由化學反應速率限制步驟的實驗可作為佐證。 除了研究氫氣蝕刻的原理外，我們亦將氫氣蝕刻分別應用在氮化鎵厚膜磊晶與增加發光二極體內部量子效率上。在厚膜磊晶中我們藉由在成長厚膜之前，先在氮化鎵樣板上做氫氣蝕刻後再磊晶，這樣可以減少厚膜因為與藍寶石基板晶格之不匹配所造成的應力。若使用不同的氫氣蝕刻條件相互搭配，可製造出更多種類的圖樣。例如先做低壓蝕刻再做高壓蝕刻可產生表面平整但底部掏空的樣板；後續磊晶厚膜並降溫之後，便產生了厚膜自我分離的效果。在增加元件的內部量子效率方面，由於氫氣是選擇性對線缺陷蝕刻的，因此可用來消除磊晶層中已存在的線缺陷。當發光二極體中的線缺陷被去除後，實驗證實內部量子效率可有效的被提升；然而相對的代價是由於線缺陷被蝕刻後會留下許多孔洞，這些孔洞的出現位置乃是隨機分佈，因此電極的製作將會變得困難。

Although it is a well-known effect that hydrogen can increase the decomposition rate of gallium nitride (GaN), most studies focus on the role of it as a carrier gas during epitaxial growth. Studies of hydrogen etch are therefore scarce. Moreover, there is almost no research investigating on the specific surface morphology produced by hydrogen. In this work, we systematically analyzed the etching behavior of hydrogen on GaN, and found that it can produce various surface structures depending on the conditions. Among the etching parameters, temperature and pressure are the most dominative factors determining the morphology, but their tendencies are opposite. If the temperature is fixed, a high-pressure hydrogen etch will produce a surface decorated by mooring-post-like GaN columns; in contrast, a low-pressure etch will produce a cavity-ridden surface. On the other hand, modulating temperature can also obtain similar results. We also proposed a mechanism to elaborate on the evolution of the surfaces and performed some examinations to verify it. Besides studying the mechanism, we also applied hydrogen etch to thick-film overgrowth and the enhancement of internal quantum efficiency (IQE) of a light-emitting diode (LED). In the thick-film application, by inserting a hydrogen etching step to a GaN template before overgrowth, the stress in the film can be relieved. Furthermore, by combining different etching conditions, such as a two-step etch, we can obtain a "stilt GaN template," which can assist in self-separation of a thick film grown on it. In the IQE application, given that dislocation sites are the prior locations to be etched, hydrogen can therefore be used to remove the existing dislocations in an LED; consequentially, IQE can be elevated. PL measurement authenticated that after hydrogen etch, the IQE of an LED is enhanced; this can probably be accredited to the removal of dislocations. Nevertheless, the drawback is the difficulty of electrode deposition.