| 摘要: | 我們利用有機金屬氣相沉積法在矽基板上成長氮化鎵，並使用氧化鋅奈米線陣列當作緩衝層。氮化鎵與氧化鋅，晶格差異只有1.85%，而氧化鋅奈米陣列能有效減少氮化鎵磊晶層的應力能量。本研究的動機在實現低成本、大面積、高品質的新世代氮化鎵晶體。然而，以氧化鋅奈米線陣列在矽基板上成長氮化鎵仍有很多挑戰，例如：氫氣蝕刻氧化鋅、氧化鋅在高溫下的不穩定性、氮化鎵磊晶層癒合困難.....等。
氧化鋅奈米線製造過程主要有兩個步驟：(1)氧化鋅薄膜的沉積。(2)水熱法合成。我們利用磁控濺鍍機，濺鍍有特定結構的氧化鋅薄膜在矽基板上，此氧化鋅薄膜對奈米線的幾何型態相當重要。我們發現，氧化鋅薄膜的退火條件，可以控制氧化鋅奈米線的型態，例如：直徑、密度、形狀、垂直排列。退火後的氧化鋅薄膜具有較一致的C軸取向，以及較低的表面粗糙度，這將導致氧化鋅奈米線陣列的成長，具有高密度、高垂直的特性。
在氮化鎵的磊晶過程中，我們先在低溫(< 600 °C)、氮氣的環境中，成長氮化鎵磊晶層來包覆氧化鋅奈米柱，以預防氫氣的蝕刻。接著，再將氮化鎵的成長溫度拉高至950 °C，以橫向連接每根奈米柱，並進一步保護氧化鋅奈米柱。最後，將溫度提升至1120 °C，並在氫氣的環境中成長高品質的氮化鎵單晶。在成長高溫的氮化鎵前，我們先利用一層氮化鋁薄膜來幫助氮化鎵的癒合。雖然高溫的氮化鎵磊晶層厚度可達2 μm，且不會有剝離、龜裂的現象，但氮化鎵磊晶層還是無法完全癒合。未來，我們將繼續優化各磊晶層的成長條件，例如：成長時間、五三比、腔體壓力等等，希望能進一步提升氮化鎵磊晶層的表面平整度。
GaN was grown on Si with the buffer layer of ZnO nanorods arrays (ZnO NRAs) by metalorganic chemical vapor deposition (MOCVD). The lattice mismatch between GaN and ZnO is only 1.85% and the ZnO NRAs buffer is employed to reduce strain energy in the epilayer. We aim to achieve the next-generation GaN crystal with advantages of low cost, large scale, and high qualities. Nevertheless, many challenges remain to be overcome, such as H2 back-etching of ZnO NRAs, thermal decomposition of ZnO, the difficult coalesce of GaN, etc.
The major fabrication procedure of ZnO NRAs consists of two steps: (1) ZnO seed layer deposition (2) hydrothermal process (HTP). The textured ZnO film was deposited on Silicon (100) substrates by radio-frequency sputter (RF-Sputter). Post-annealing of the sputtered ZnO layer is found be vitally important to the morphology of ZnO NRAs, such as diameters, densities, shapes, and tilting angles.
In the growth of GaN, low-temperature (< 600 °C) GaN (LT-GaN) in N2 ambiance is adopted to form the ZnO-GaN core-shell structure in order to prevent the back etching of NRAs at high temperatures. The growth temperature of GaN is then raised to 950 °C to horizontally connect every nanorod and to provide further protection of ZnO. Finally, high-quality GaN is grown at 1120 °C in H2. Before the growth of high-temperature GaN (HT-GaN), a thin layer of high-temperature (also at 1120 °C) AlN (HT-AlN) is used to assist the coalescence of GaN. Although the HT-GaN epilayer reaches the thickness of 2 μm without showing peeling-off and cracks, the crystal still remains uncoalescent. In the Future, optimization of the growth parameters for each layer, including duration, V/III ratios and reactor pressure, should be performed in order to improve the surface morphology of GaN grown with this novel technology. |
