利用脈衝雷射沈積法成長不同應力氮化鎵薄膜及應用

Abstract

本論文主要研究為利用實驗室自行架設之脈衝雷射沈積系統搭配後退火處理，在藍寶石基板上成長出極平整、高品質、無應力之氮化鎵薄膜。在論文中，我們首先探討以脈衝雷射沈積法成長高品質氮化鎵薄膜之關鍵參數—溫度、腔體壓力以及雷射能量，藉由X光繞射儀(XRD)來判斷成長之氮化鎵薄膜是否具有良好的c軸取向。透過將成長參數最佳化，我們發現最佳生長條件為800°C、10 mtorr及350 mJ/pulse 的入射能量。在該條件下所生長之氮化鎵薄膜具有相當好的c軸取向，且其表面非常平整，表面粗糙度小於0.93 nm；和目前已知文獻脈衝雷射沈積系統相比，其平整度是最小的，且和一般利用MOCVD所成長的氮化鎵薄膜相當接近；另外，藉由EDS分析成分組成可知其氮鎵比為1:1。 然而，我們成長出來的氮化鎵薄膜和一般磊晶所得之氮化鎵薄膜仍不盡相同，其一是我們使用脈衝雷射法所成長之氮化鎵薄膜是不透明的；其二是當我們利用拉曼光譜儀分析時，看不到氮化鎵薄膜纖鋅礦結構之訊號。經進一步分析，我們發現以脈衝雷射沈積法所成長之氮化鎵薄膜含有氮化鎵的不穩定相-岩鹽結構。為了將不穩定相去除，我們利用高溫爐，對氮化鎵薄膜進行後退火處理；透過後退火處理，我們發現以脈衝雷射沈積法成長之氮化鎵薄膜總共會經歷三個階段—相轉變階段、應力轉變階段以及熱分解階段。首先，氮化鎵在退火溫度900°C之前會由岩鹽結構轉變為纖鋅礦結構，同時，因轉變成纖鋅礦薄膜，薄膜本身會變得透明，且纖鋅礦結構之拉曼訊號會變得相當明顯。接著，當退火溫度在900°C到1000°C時，為應力轉變階段，此時薄膜殘餘應力會由壓縮應力變成拉伸應力，最後，當退火溫度超過1000°C以上時，氮化鎵薄膜會開始熱分解，並使薄膜變得非常粗糙。藉由拉曼光譜儀分析薄膜殘餘應力，我們發現當退火溫度在950°C下，氮化鎵薄膜是沒有殘餘應力的，因此藉由控制退火溫度，我們能得到高品質、高平整性且無應力之氮化鎵薄膜；同時也可藉由控制退火溫度來調變氮化鎵薄膜之應力。 最後，我們製作出Top-gate結構的氮化鎵薄膜電晶體，並探討應力對元件效能之影響；由Id-Vd圖可知其汲極電流在高電壓時呈現飽和狀態，表示氮化鎵薄膜電晶體具有場效電晶體之特性，由Id-Vg圖可知on/off ratio之數量級為103， 符合目前氮化鎵薄膜電晶體on/off ratio之數量級，接著我們從Id-Vg圖萃取出氮化鎵薄膜電晶體之場效載子遷移率；藉由計算出來之場效載子遷移率，無應力狀態之氮化鎵薄膜電晶體其場效載子遷移率為39.4 cm2/V-s，此值已遠高於一般氮化鎵薄膜電晶體；且和MOVCD所製作之氮化鎵金氧半場效電晶體場效載子遷移率相當。而當氮化鎵薄膜承受拉伸應力時會使元件的場效載子遷移率變差，而壓縮應力則可使氮化鎵薄膜電晶體之場效載子遷移率大幅提升。當氮化鎵薄膜承受1.55GPa之壓縮應力時，其場效載子遷移率高達961 cm2/V-s，此場效載子遷移率已和氮化鎵高電子遷移率電晶體的非常接近。因此藉由將氮化鎵薄膜電晶體控制在壓縮應力之狀態，我們可獲得高場效載子遷移率之氮化鎵薄膜電晶體。

The study of this thesis is to use the pulsed laser deposition (PLD) system and post-annealing process to fabricate the extremely smooth, high quality, and strain-free GaN film. In this thesis, we first investigate the most important growth parameters of PLD: the growth temperature, the chamber pressure, and the incident laser energy. By using the X-ray diffraction to confirm whether GaN thin film has c-plane orientation, we find out that the best growth condition lies at 10 mtorr, 350 mJ/pulse, and 800 °C. The surface roughness is as small as 0.93 nm that is the smoothest one by PLD system. The crystallinity of GaN thin film under this condition is also the best. The composition of GaN is 1:1. However, the PLD growth GaN film is still different from metal-organic chemical vapor deposition (MOCVD) growth GaN film. First, our PLD growth GaN film is not transparent. Second, by using Raman spectroscopy, we find out that there is no Raman peaks of Wurtzite structure GaN. To further investigate the reason, we find out that the PLD growth GaN contains unstable phase--rock salt structure GaN. To eliminate the rock salt phase GaN, we use the post-annealing treatment to anneal the PLD growth GaN film. By annealing GaN film at different temperature, we find out that the PLD growth GaN thin film undergoes three stages: phase transition, stress-alteration, and thermal decomposition stage. At low annealing temperature, the GaN film transfers from rock salt to Wurtzite phase. The rock salt GaN diminishes with increasing annealing temperature. At medium annealing temperature, the residual stress of the film changes significantly from compressive strain to tensile strain. As the annealing temperature further increases, the GaN undergoes thermal decomposition and the surface becomes granular. By exploring the annealing temperature effects and controlling the optimized annealing temperature of the GaN thin film, we are able to obtain highly crystalline and strain-free GaN thin film by pulsed laser deposition. Finally, we fabricate the top-gate GaN thin film transistor (TFT) and investigate the stress effect on device performance. From Id-Vd curve, the GaN TFT follows the field effect transistor characteristic since the drain current saturates at high drain voltage. From Id-Vg curve, we can obtain the on/off ratio. The on/off ratio is in the order of 103, which is comparable to other GaN transistor. Then, we extract the mobility of GaN TFT from Id-Vg curve. The calculated field effect mobility of strain-free GaN TFT is 39.4 cm2/V-s. Comparing with other devices, it is much greater than typical GaN TFT and is comparable to GaN metal oxide semiconductor field effect transistor (MOSFET) with the GaN grown by MOCVD. When the GaN film is subjected to tensile stress, the mobility of GaN TFT largely decreases. However, for the compressive stress GaN TFT, the mobility increases significantly. When the compressive stress of GaN is 1.55 GPa, the mobility is 961 cm2/V-s. The value is comparable to the AlGaN/GaN high electron mobility transistor. As a consequence, by controlling the stress in GaN film, we can obtain high mobility GaN TFT.