基于选区外延p型栅的增强型GaN基HEMT研究

GaN-based high electron mobility transistors (HEMTs) have low switching loss, high power density and high voltage, which can improve the conversion efficiency of power electronic system. However, usually as-grown GaN-based HEMTs are depleted, which need a complex circuit, enhancement-mode(E-mode) HEMTs are indispensable to simplify the circuit and to realize failsafe system.Utilization of p-type cap layer under the gate is an effective method for achieving E-mode HEMTs. However, because of several questions, such as current leakage induced by dry-etching damages and low threshold voltage caused by low hole concentration, the development of E-mode HEMTs with p-type cap layer is slow..This project aims to prepare large gate width p-type gated E-mode HEMT devices. In order to solve the dry-etching damages, the p-type cap layer will be grown by selective area re-growth by metalorganic chemical vapor deposition (MOCVD). The growth parameters will be studied for achieving small area and high quality uniform p-type layer. In addition, the mask making process and surface treatment process before re-growth also will be researched to suppress the interface defects, and to improve the quality of the interface between p-type gate and AlGaN barrier. For enhancing the threshold voltage, polarization-doping method will be used to increase hole concentration by growing graded p-AlGaN and graded p-InGaN cap layer. Moreover, based on the characteristics of the selective area regrowth, the p-type gated HEMTs assisted by recess gate will be developed. The group will explore the repair mechanism of the high temperature gas treatment on the dry-etching damage of the recess in the MOCVD. Based on the above research, and the development of the large gate width process, the group will prepare a large gate width E-mode HEMT with a p-type cap layer.

GaN基HEMT器件的开关损耗低且功率密度高，可提高电力电子系统的转换效率，满足当前节能减排的时代要求。通常外延所得HEMT均为耗尽型，而为了简化电路，提高电路安全性，应用中更希望使用增强型HEMT。p型栅有利于实现商品化的增强型HEMT，但受困于刻蚀损伤导致表面漏电大，进而降低器件击穿电压、空穴浓度不足导致阈值电压低等问题，性能尚远低于预期。本项目以制备大栅宽p型栅增强型HEMT为目标，拟采用选区二次外延技术生长p型栅，以避免刻蚀损伤问题。研究小面积选区二次外延的生长条件，进而获得均匀高质量的大栅宽p型栅。研究掩膜制备工艺及表面处理工艺，抑制二次外延的界面缺陷，提高界面质量。为提高阈值电压，拟采用极化掺杂技术提高p型栅的空穴浓度，并结合二次外延技术特点，开发凹槽辅助p型栅的增强型HEMT器件。基于以上研究内容，并开发大栅宽器件工艺，最终研制出大栅宽p型栅增强型HEMT器件。

GaN HEMT具有功率密度高等优势，特别适用于电力电子系统。通常GaN HEMT为常开型，即它的关断电压为负压。为了提高电路的安全性和降低成本，需要增强型HEMT。p型栅是实现增强型HEMT的主流技术之一，先整片外延p-GaN，然后刻蚀非栅区域形成p-GaN栅，但刻蚀可能会带来晶格损伤。本项目利用MOCVD选区外延技术制备p-GaN栅，实现增强型HEMT。此方法可以避免刻蚀p型栅技术带来的2DEG损伤。首先对选区外延气流模式进行分析，主要包含表面原子迁移和气相扩散两种气流模式。随着选区面积的减小，生长速度显著提高，且选区外延速率从掩膜边缘向内部逐渐减小。通过优化外延工艺和掩膜占空比，获得了均匀的选区外延速率，实现了表面光滑且无V坑的p-GaN栅。开展了选区外延界面及GaN内部杂质的抑制研究。通过表面处理工艺有效祛除了界面的C、O、Si杂质。但选区外延中，SiO_2掩膜高温降解使得Si杂质扩散至选区外延材料中。生长时间越长、选区面积越小则Si杂质浓度越高。开发了金属W作为选区外延掩膜，解决了Si杂质问题，获得了空穴浓度为3.56×10^17 /cm^3，迁移率为 9.55 cm^2/V·s 的p-GaN。本项目组还针对Mg-H络合物的问题，开展了退火激活Mg的研究。制备了p-GaN栅HEMT器件。与同片制备的常开型 HEMT的阈值电压（-3.8 V）相比，p-GaN 栅 HEMT的阈值电压为-0.5 V，正向移动了 3.3 V。