成長於氧化鋅緩衝層的氮化鎵，會因鋅離子的擴散，自發性地成為p型半導體。此自發性的p型氮化鎵，有助於形成p-side-down的發光二極體。由於內建電場反向的關係，p-side-down的發光二極體在理論上有更低的驅動電壓、更高的內部量子效率。然而，在磊晶的過程中，鎂離子(氮化物最常見的p型摻雜)極易殘留在反應爐內，造成n型氮化物的汙染問題。因此，目前的氮化物磊晶技術難以實現p-side-down的元件結構。 在本研究中，我們利用光致激發光譜與電容-電壓曲線圖，確認成長於氧化鋅緩衝層的氮化鎵為自發性的p型半導體，並比較薄膜式及奈米陣列式的緩衝層效果，同時也探討退火溫度與時間所造成的差異。為了評估氮化鎵表面下的電洞濃度，我們將氮化鎵磊晶層製成蕭特基二極體，並且利用電容-電壓的量測結果，換算出電洞濃度在磊晶層中的縱深分佈。量測結果顯示:850 ºC以上的退火溫度能使鋅離子(與電洞)往磊晶層表面擴散。此外，我們也以二次離子質譜儀分析不同退火條件下的鋅離子縱深分佈，質譜儀顯示的離子縱深分佈與電容-電壓曲線所推算的結果一致。這些成果為p-side-down發光二極體提供一個新的研究方向。 ;The spontaneous p-type GaN grown on a ZnO buffer holds great promise for the realization of p-side-down light emitting diodes (LEDs), which are not achievable with convention growth techniques because of the memory effect of Mg ions (the most commonly used acceptor for p-type III-nitrides). In light of the reverse junction field, LEDs with the p-side-down structure can in principle exhibit higher internal quantum efficiencies and lower turn-on voltages, compared to their p-side-up counterparts. In this study, the p-type behavior of the GaN/ZnO structure is confirmed with photoluminescence (PL) spectroscopy and capacitance-voltage (C-V) measurements. The effects of buffer geometry (ZnO film vs. ZnO nanorods) and annealing conditions on p-type doping are also investigated. In order to evaluate the hole concentrations buried under the surface, we fabricated GaN Schottky contacts, through which the depth profiles of hole concentration can be estimated using the C-V results. It is found that the annealing temperature above 850 °C increases the diffusion of Zn acceptors toward the epitaxial surface. The depth profiles of hole concentration are compared with those of Zn ions obtained with secondary ion mass spectroscopy (SIMS). Consistent results of these two characterization techniques are obtained. These studies open a new pathway for the research on p-side-down LEDs.
