| 摘要: | 氮化鎵(GaN)材料於近十年來應用於光電與電子元件的發展突飛猛進，產品種類也不斷推陳出新。以電子元件為例，氮化鋁鎵/氮化鎵(AlGaN/GaN)高速電子遷移率電晶體由於氮化鎵本身的材料特性，使其可操作在高溫的環境下，也可比其他材料製作的電晶體提供較高的功率輸出。因此，本論文研究的目標為製作出氮化鎵系列的異質接面雙極性電晶體。然而，有兩個關鍵因素使得氮化鎵系列的異質接面雙極性電晶體難以製作出來：(1)經由乾蝕刻造成p型氮化鎵表面破壞，進而形成蕭基特性的基極接點；(2)長晶缺陷(線缺陷)所產生的高漏電流路徑。因此，以氮化鎵為基材來製作異質接面雙載子電晶體是非常有挑戰性的研究。本論文的目標是利用不同方法，包括台面直接蝕刻法及射極重新成長法來製作氮化鎵系列的異質接面雙極性電晶體。 p型氮化鎵在經由乾蝕刻後會造成表面破壞，使得其金屬歐姆接觸難以形成。第二章介紹兩種方法來研究p型氮化鎵金屬歐姆接觸特性，並找出適合於製作氮化鋁鎵/氮化鎵異質接面雙載子電晶體的條件。一為蝕刻後的p型氮化鎵在鹽酸與王水溶液經由表面處理後，蒸鍍鎳/金，鉑/金與鉻/金三種不同金屬，採用電流-電壓特性分析表面處理與不同金屬對金屬歐姆接觸的影響。另一方法為使用鋅(Zn)擴散在蝕刻後之p型氮化鎵，電流-電壓特性分析研究不同鋅擴散時間對金屬接觸的影響。表面粗糙度之方均根和縱深成分分析被用來討論氮化鎵表面之特性。實驗結果顯示，王水能有效去除表面生成之氧化物，鎳/金比其他金屬得到較好的金屬接觸特性。因此，採用此條件製作氮化鋁鎵/氮化鎵異質接面雙載子電晶體。另一研究結果說明鋅擴散似乎能填補蝕刻後粗糙的表面，但金屬接觸特性卻反而變差。 利用台面直接蝕刻法來製作氮化鋁鎵/氮化鎵異質接面雙載子電晶體的研究內容在第三章中討論，其結構是利用有機化學汽相沈積法(MOCVD)成長，長晶基板為藍寶石，而射極鋁含量為0.17。製作出來的電晶體射極面積為110?110 ?m2，在Gummel plot特性曲線圖中，當VBE = 0.53 V時，得到電流增益為564；在射極接地電流-電壓特性(common-emitter I-V)中，當IB ＝ 10 nA時，電流增益為126。由於長晶產生的線缺陷造成的漏電流路徑與製程產生的高基極電阻，使得在低電流密度下的Gummel plot特性曲線圖中，得到異常高之電流增益。因此要確認電晶體是否真正製作成功，射極接地電流-電壓特性曲線是比較正確的判斷方法。另外由元素縱深分佈發現，p型氮化鎵中的鎂摻雜，在經過高溫成長後，擴散到射極區，進而使電晶體特性變差。因此為避免基極區p型氮化鎵乾蝕刻所造成的表面損傷，以及長晶過程中鎂擴散的影響。接下來利用射極重新成長的方式，製造出氮化鎵系列異質接面雙載子電晶體。 氧化鋅(ZnO)為光電產業中常被使用之透明導電膜，其為寬能隙材料，且晶格常數與氮化鎵相近，因此在第四章選擇氧化鋅當作射極重新成長的材料。晶片結構使用有機化學汽相沈積法成長，重新成長氧化鋅是利用濺鍍(sputter)法。首先x光的分析頻譜(XRD)與光激發螢光頻譜(PL)對重新成長的氧化鋅材料進行物理分析。在x光的分析頻譜分析中，可以證實此重新成長的氧化鋅在退火後，晶格常數與結構都有獲得改善。Gummel plot特性與射極接地電流-電壓特性用來分析射極面積為120?120 ?m2的氧化鋅/氮化鎵異質接面雙載子電晶體，另外也探討不同溫度下電晶體特性的變化。在Gummel plot量測特性中，在溫度為300K且VBE = 2.1 V時，最大的電流增益為168；在溫度為100K且VBE = 2.35 V時，量測到最大的電流增益為1117。由實驗結果顯示，集極電流隨著溫度降低而減少，這是因為載子的活動力在低溫下變低，使得入射效率也變差所導致。然而在低溫下，基極電流減少的幅度比集極電流大，所以發生電流增益變大的現象。 在第五章中，氧化鋁鋅(AZO)取代氧化鋅來當作重新成長的射極層，因為在第四章中，發現氮化鎵/氧化鋅接面特性需要改善，於是採用較高濃度的氧化鋁鋅材料來製作氮化鎵系列異質接面雙載子電晶體。氧化鋁鋅與氧化鋅的結構都與氮化鎵相同，也都具有與氮化鎵相似的晶格常數與寬能隙特性。電晶體結構採用有機化學汽相沈積法成長，氧化鋁鋅射極區還是利用濺鍍法重新成長。先使用x光分析頻譜與霍爾量測(Hall measurement)對成長的氧化鋁鋅進行材料分析。以Gummel plot量測法分析氧化鋁鋅/氮化鎵異質接面雙載子電晶體，在偏壓2V下，得到電流增益為120。從射極接地電流-電壓特性曲線顯示，當基極電流為5 ?A時，電流增益為1.2，且有較小的的位移電壓0.3 V。另外使用將製作出來的電晶體經過硫化處理後，在Gummel plot特性曲線中發現集極電流輕微上升且基極電流些微下降，這顯示硫化處理能有效的去除表面氧化物，導致電流增益從5.8上升到9.4，當偏壓為3 V時。 在最後結論中，整理了本論文所有的結果，並提供一些對製作氮化鎵異質接面雙極性電晶體有所幫助的建議。如以台面直接蝕刻法製作電晶體時，必定面臨p型氮化鎵在蝕刻後所造成的表面破壞，使得歐姆接觸難以製作。可試著利用變溫蝕刻法、濕蝕刻法及提高鎂在p型氮化鎵的濃度來解決。另外，在基極層使用氮化銦鎵可以提高鎂在p型氮化鎵的濃度，進而提升基極的電洞濃度。在緩衝層可採用氮化鋁/低溫成長之氮化鋁來取代原本的氮化鎵，降低晶格不匹配而產生的線缺陷，進而減少漏電流路徑。 In the recent years, the GaN-based electronic devices have attracted attention for high power microwave application, such as AlGaN/GaN high electron mobility transistors (HEMT). However, it is difficult to fabricate working GaN-based HBTs due to the plasma-induced damages on p-GaN, low base conductivity, and high leakage paths resulting from threading dislocations (TDs) in materials and processing. Therefore, this dissertation presents two technologies to fabricate the GaN-base heterojunction bipolar transistors (HBT). In Chapter 2, we study the p-GaN after dry etching and then treat with different solutions, HCl and aqua regia. Next, different metal contacts on p-GaN were formed for Ohmic contact study. The details of the process flows are presented and related issues are discussed in this chapter. The transmission line method (TLM) measurement is used to characterize the electrical characteristics. The extracted Schottky barrier height (SBH) of Ni/Au, Pt/Au, and Cr/Au were obtained from the current-voltage characteristics to be 0.71, 0.75, and 0.88 eV, respectively. This indicates surface treatment with aqua regia prior to metal deposition can effectively removes the contamination and the Ni/Au metal contact on etched p-GaN shows better contact performance. Additionally, we study the p-GaN after dry etching and then treat with Zinc- (Zn) diffusion in the furnace. Both TLM measurement and surface morphology including scanning electron microscope (SEM) and atomic force microscope (AFM) are presented. We also investigate the elements distribution on the samples by second ion mass spectroscopy (SIMS) measurement. The measured I-V characteristics of etched p-GaN show the higher specific contact resistance and sheet resistance by surface Zn-diffusion as diffusion time increases. The extracted SBH on etched p-GaN also increases with diffusion time. In Chapter 3, the fabricated Al0.17Ga0.83N/GaN npn HBT with 110?110 ?m2 emitter area was demonstrated by direct mesa etching process. SIMS analysis for Mg, Si, Ga and Al elements confirms the location of the base-emitter junction. The details of the Al0.17Ga0.83N/GaN HBT fabrication are also described in this chapter. The DC measurement of the Al0.17Ga0.83N/GaN HBT includes the junction, Gummel plot and common-emitter I-V characteristics (CE-IV) are presented. Finally, the current gain of the transistor in collector-up Gummel plot is 4.3 with the VBE ＝0.9 V, while the CE-IV demonstrates the current gain of 126. Experimental results indicate that the differential current gain was obtained in the Gummel plot and CE-IV characteristics, which is because the leakage currents result in abnormal high gain form Gummel plot measurement. In order to obtain good base contacts, some of the publications had demonstrated the base- or emitter-regrowth on fabricated GaN-based HBTs. In chapter 4, we used the ZnO film to be the regrown layer by sputter to decrease the base damage from the dry etching process. The SEM, Hall measurement, Photoluminescence (PL) and the X-ray diffraction (XRD) are used to characterize the n-ZnO films. The details of the ZnO/GaN HBT fabrication are also described in this chapter. Both Gummel plot and CE-IV characteristics are measured to verify the fabricated ZnO/GaN HBTs. The measured maximum current gain from Gummel plot is 168 at 300K, while the CE-IV demonstrates the current gain of 41. Moreover, the temperature dependence of DC characteristic is studied in this section. In chapter 5, an npn AZO/GaN HBT by using emitter re-deposited is presented. The utilization of AZO is similar to the ZnO in the previous chapter. Except for the similar lattice constant with GaN, AZO shows a higher doping concentration and higher bandage than ZnO. The characteristics of carrier concentration, electrical resistivity and Hall mobility of AZO films were studied. The CE-IV characterization shows DC current gain of 1.2. In addition, improved device characteristics are observed after the P2S5/(NH4)2S treatment. Finally, in chapter 6, we summarized the results obtained in this dissertation and presented some suggestions for further studies. |
