| 摘要: | 在最近幾年中，氮化鎵(GaN)系列材料在光電和電子元件的商業應用上，有相當大量的成長。以氮化鎵系列為材料的電晶體中，如氮化鋁鎵-氮化鎵(AlGaN/GaN)的高速電子遷移率電晶體，可以在惡劣和高溫度環境之下工作並提供高功率密度操作特性。然而，製作出以氮化鎵系列為材料的雙載子電晶體卻是非常困難的。這是因為以氮化鎵系列為材料的雙載子電晶體會有：(1)如同蕭基特性的基極接點；(2)低的基極傳導特性；(3)長晶缺陷和製程中所造成的高漏電流。因此，氮化鋁鎵-氮化鎵(AlGaN/GaN)的異質接面雙載子電晶體的研究是一項非常有挑戰性的研究題目。世界上少數著名的研究團隊，已經利用不同方法做出 n-p-n 或 p-n-p 的氮化鎵系列為材料的雙載子電晶體。提出如直接台面蝕刻法、基極重新成長法、射極重新成長法，和基極使用 p型的氮化銦鎵等方法，來製作氮化鎵系列為材料的雙載子電晶體。而本論文主要是在探討與發展氮化鋁鎵-氮化鎵的異質接面雙載子電晶體的製造技術和與它有關的議題。本論文的目標是利用直接台面蝕刻法，製造出氮化鋁鎵-氮化鎵的異質接面雙載子電晶體。 在第2章中，首先提出欲製作之氮化鋁鎵-氮化鎵(AlGaN/GaN)的異質接面雙載子電晶體結構。晶片是利用有機化學汽相沈積法(MOCVD)成長，使用的長晶基板是藍寶石，而射極鋁含量為0.17。在本章也介紹了利用直接台面蝕刻製程方法所製作出的氮化鋁鎵-氮化鎵異質接面雙載子電晶體之製程步驟，製作的電晶體之射極面積為75?75 ?m-2 。並對幾項重要的關鍵製程和會遇到的問題，做詳細的說明與解釋，其中的乾式蝕刻條件將在第3章報告。 在第3章中，首先研究p型氮化鎵的乾式蝕刻條件，作為製作氮化鋁鎵-氮化鎵的異質接面雙載子電晶體所用。此實驗是利用氯氣-氬氣混合氣體，研究在乾性蝕刻製程中對p型氮化鎵的影響。表面粗糙度之方均根和深度分佈比例被用來討論氮化鎵表面之特性。電流-電壓特性分析則是利用鎳(20奈米) / 金(20奈米) 之金屬，鍍在蝕刻後的p型氮化鎵上來量測。實驗結果說明了：在固定的蝕刻功率和腔體壓力之下，蝕刻速率不會隨著增加氯氣的流量而增加。由分析的資料看來，深度分佈比例對蝕刻條件的相關性，比表面粗糙度之方均根對蝕刻條件的相關性來的明顯。吾人利用表面粗糙度之方均根和深度分佈比例這兩種依據，可以找到最理想的蝕刻條件，來作為製造氮化鋁鎵-氮化鎵的異質接面雙載子電晶體的製程條件。 為了得到好的基極金屬接觸，一些發表之論文在製作氮化鋁鎵-氮化鎵的異質接面雙載子電晶體時，使用了基極重新成長的方法。在第4章中，做了相關的基極重新成長之實驗研究。利用p型氮化鋁銦鎵(AlInGaN)和氮化銦鎵(InGaN)作為重新成長之材料，重長在蝕刻過後的p型氮化鎵之上，並鍍上鎳(20奈米) / 金(20奈米)，作為量測電流-電壓特性之金屬。p型氮化鋁銦鎵和氮化銦鎵是利用有機化學汽相沈積法重新成長的，成長的厚度是100奈米。從蕭基位障的特性來看，蝕刻後的p型氮化鎵蕭基位障是0.65 eV，重新成長p型氮化鋁銦鎵和p型氮化銦鎵之後，蕭基位障改善為0.56 eV和0.58 eV。除了蕭基位障分析之外，表面形態學和x光的頻譜分析在本章都有詳細之研究討論。 在第5章中，首先針對第2章提出的氮化鋁鎵-氮化鎵異質接面雙載子電晶體做相關之材料成長與物理特性分析，再對製作元件的特性做電性分析。首先，利用x光的分析頻譜與二次離子光譜對成長的材料進行物理分析。在x光的分析頻譜的交互空間分析中，可以證實此長晶的結構在晶格常數上是一致的，而不是一個疏鬆的長晶結構。雖然如此，由穿透式電子顯微鏡 (TEM)和蝕刻缺陷密度的測量，證實了線缺陷存在於此長晶晶片中。蝕刻缺陷密度的平均值是3.38 ? 108 cm-2。此外，在本章也介紹了利用直接台面蝕刻製程方法所製作出的氮化鋁鎵-氮化鎵的異質接面雙載子電晶體之元件特性分析，製作的電晶體之射極面積為75?75 ?m-2 。在Gummel Plot 特性曲線圖中，量測之電流增益高達1 ? 103以上，此不正常的高電流增益並非本質元件的特性，而是因為高的基極電阻與漏電流所造成的。電晶體在射極接地(common-emitter)的電流-電壓特性中，有較小的的位移電壓為2.04 V，其直流增益則是1.22，而調變直流增益則是1.32。最後，利用外加寄生元件的元件模型可以解釋分析，Gummel Plots 特性曲線圖所得到較高的電流增益是高的基極電阻與漏電流所造成的。 在最後結論中，整理了本論文所有的結果，並提供一些對未來研究有所幫助的建議，包含了：氮化銦鎵為基極之電晶體、平面結構的氮化鎵的接面雙載子電晶體和重新成長氧化鋅(ZnO)做射極之技術。 In recently years, the commercial outlook for GaN optoelectronic and electronic devices has grown considerably. GaN-based transistors, such as AlGaN/GaN high electron mobility transistors, are capable of delivering the high-power density under the harsh and high-temperature environments. However, it is difficult to fabricate working GaN-based HBTs due to the Schottky-like ohmic contacts on p-GaN, low base conductivity, and high leakage paths resulting from dislocations in materials and processing. Therefore, the research on an AlGaN/GaN HBT is one of the challenging research subjects. Different approaches have been utilized to obtain the n-p-n and p-n-p GaN-based HBTs such as direct mesa etch, base regrowth, emitter regrowth, and p-InGaN base. This dissertation is focused on efforts to develop fabrication technology for the GaN-based HBT and its related issues. The primary propose of this dissertation is to fabricate working AlGaN/GaN HBTs using the double mesa etching process. In chapter 2, we present the design and growth of an AlGaN/GaN HBT. The proposed Al0.17Ga0.83N/GaN heterojunction bipolar transistor (HBT) was grown on c-face sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Additionally, the fabricated Al0.17Ga0.83N/GaN n-p-n HBT with 75 x 75 um2 emitter area was demonstrated HBTs by direct mesa etching process. The details of the Al0.17Ga0.83N/GaN HBT fabrication are described in this chapter. Some of the important issues related to base layer and contact are discussed and discussed in detailed in the following chapters. In chapter 3, the optimized etching process condition is studied for AlGaN/GaN HBTs. This chapter investigates the effect of Cl2/Ar dry etching on p-GaN. The root-mean-square (RMS) surface roughness is measured and depth display (Bearing analysis) is monitored. The current-voltage (I-V) characteristics of etched p-GaN with Ni (20 nm)/Au (20 nm) metallization are studied. Experimental results indicate that the etching rate does not increase significantly with the Cl2 flow rate at a constant power or chamber pressure. The Bearing ratio data exhibit a much stronger variation with etch conditions, the RMS displays the same trend but to a lesser extent. By the analysis of the RMS and the Bearing ratio, the optimal etching recipe is obtained and applied to the etching process of AlGaN/GaN HBTs. In order to improve the base contacts after dry etching, base regrowth technique has been applied to fabricate GaN-based HBTs. In chapter 4, we use the regrown p-type AlInGaN and InGaN to decrease the base damage from the dry etching process. The p-type AlInGaN and InGaN contact layers are regrown on the etched p-GaN to study the Ni (20 nm)/Au (20 nm) contact current-voltage (I-V) characteristics. The thickness of the contact layer is 100 nm and regrown by metalorganic chemical vapor deposition. By using the regrown contact layer on etched p-GaN, Schottky barrier height (SBH) from the I-V characterization is reduced. The SBH of 0.65 eV from the contact to the etched p-GaN is reduced to 0.56 eV and 0.58 eV, respectively, after the AlInGaN and InGaN contact layers were formed. In addition to the I-V characterization of Ni/Au contacts, surface morphology and x-ray analysis are studied. In chapter 5, the material properties of Al0.17Ga0.83N/GaN HBT presented in chapter 2 is investigated first. The epi-taxial layers are analyzed by x-ray diffraction pattern and secondary ion mass spectroscopy. The reciprocal space map verifies that the Al-content layer in emitter is a coherently strained structure. The threading dislocations are revealed by transmission electron microscopy (TEM) and etch pit density (EPD) measurement. The average value of the EPD is 3.38 x 108 cm-2. Additionally, the fabricated HBT with 75 x 75 um2 emitter area is demonstrated by direct mesa etching process. The measured gain exceeds 103 over the wide range of collector current in Gummel plots characteristic. In addition, the fabricated HBT with 75 x 75 um2 emitter area demonstrates a low offset voltage of 2.04 V and dc current gain of 1.22. The differential current gain is 1.32 in the common-emitter I-V characteristics. To analyze the device characteristics, a VBIC model for the intrinsic device and parasitic elements are used to implement an equivalent circuit model. From simulated results and fitting comparison, the high current gain in the Gummel plots is due to the extrinsic leakage current and poor base contact. Finally, we summarize the results obtained in this dissertation and present some suggestions for further studies. The suggestions to fabricate GaN based HBT include the p-type InGaN base, the planar GaN bipolar transistors and the ZnO/InGaN HBTs using emitter regrowth. |
