Technological Developments in Pursuit of Recess-Free Normally-OFF AlGaN/GaN HEMTs

免刻蚀的常关型氮化镓高电子迁移率晶体管技术开发

郑韦志

11751021

博士

Electronic and Computer Engineering

Doctor of Philosophy

于洪宇

2021-08-12

2021-12-31

香港科技大学

香港

Thanks to GaN's high breakdown electric field, high electron mobility and saturation velocity, GaN-based devices are considered promising candidates for power switching and RF power amplifier applications. Among them, the highly conductive two-dimensional electron-gas (2DEG) at the (In,Al,Ga)N/GaN heterostructure enables the power-efficient operation of high-electron-mobility transistors (HEMTs). However, the inherent channel makes GaN HEMTs normally-ON, while the Normally-OFF operation of GaN HEMTs is preferred for various applications. Recess processes for fabricating normally-OFF devices generally compromise devices’ performance. In this thesis, technologies for realizing recess-free normally-OFF GaN HEMTs are developed. Due to the piezoelectric nature of GaN, strain engineering can modulate the electric performance of GaN HEMTs. The local strain engineering based on the SiNx stress liner is developed. The SiNx-introduced compression in the gate region increases the threshold voltage (VTH) and reduces the Schottky gate leakage. As a result, the ION/IOFF ratio of the Schottky gate HEMTs with strain engineering reaches 108. Furthermore, the normally-OFF GaN HEMT using strain engineering is realized, showing the potential application on class-AB or class-B RF power amplifier applications.Schottky gate is inappropriate for normally-OFF GaN HEMTs due to its severe leakage under forward bias. Therefore, as the second part of the thesis, the ozone-based deposition of HfO2 for viable MIS gate structure is proposed. Replacing the conventional water oxidant with ozone reduces the oxide bulk's oxygen vacancy-related defects, resulting in smaller oxide leakage and higher critical electric field. Furthermore, the more abrupt HfO2/GaN interface and amorphous HfO2 bulk form on the ozone pre-oxidized GaN surface. The improved interface translates to the satisfactory electric performance of MIS HEMTs. The high-k nature is also evidently helpful to develop the normally-OFF HEMTs, supported by an analytical model for VTH of MIS HEMTs. The proposed HfO2 gate oxide scheme is also integrated with an ultra-thin barrier heterostructure. With the optimized annealing condition, the normally-OFF operation of MIS HEMTs is realized. Also, the electron mobility in the gate region is enhanced after annealing. The high electron mobility in the gate region together with the successful 2DEG recovery by the SiNx passivation enable the excellent output performance, benchmarked against recess-based devices.

由于GaN(氮化镓)的高击穿电场、高电子迁移率和饱和速度，GaN电子元件被认为是下世代电源开关和射频功率放大器应用的选择之一。其中，(In,Al,Ga)N/GaN异质接面的二维电子气(2DEG)通道实现了高电子迁移率电晶体 (HEMT)。然而，元件却因该通道具有常开特性，不利于各项应用，而制备常关元件的技术却又通常会损害元件的性能。本论文开发了实现免蚀刻常关GaN HEMT的技术。由于 GaN 的压电特性，GaN HEMT的电特性可藉由应力工程调整。首先，本论文开发了基于SiNx应力膜层的局部应力工程。在闸极区域中，SiNx引入的压应力，增加了元件开启电压并减少了肖特基闸极漏电。因此，采用应力工程的肖特基闸极电晶体的开关电流比达到108。此外，本工作演示了利用应力工程实现的常关 GaN HEMT，该常关元件于 AB 类或 B 类射频功率放大器具有潜在应用价值。肖特基闸极不适用于常关型 GaN HEMT，因为肖特基接面在正向偏压下会出现严重漏电。因此，作为论文的第二部分，我们提出了基于臭氧的 HfO2 沉积技术，以应用于高质量的金属-氧化物-半导体(MOS)闸极结构。用臭氧代替传统的水作为氧化剂减少了氧化物本体的氧空位缺陷，抑制氧化物漏电和提高氧化物的崩溃电场。此外，我们发现在臭氧预氧化的GaN表面可形成更锐利的HfO2/GaN界面和非晶态的HfO2层。增进的氧化物/半导体界面和氧化物质量成功反应在MOS HEMT的优异电性能。在元件开启电压的模型的支持下，HfO2的高介电常数也有助于开发常关型元件。本论文提出的闸极HfO2方案还与超薄势垒异质结构结合制成MOS HEMT元件。通过优化的退火条件，实现了常关型元件。此外，退火提升了闸极区中的电子迁移率。闸极区域的高电子迁移率以及经过SiNx钝化，成功恢复的二维电子气实现了出色的元件输出特性。

GaN HEMT Enhancement-mode

氮化镓 电晶体 常关型

英语

联合培养

南方科技大学

Zheng WZ. Technological Developments in Pursuit of Recess-Free Normally-OFF AlGaN/GaN HEMTs[D]. 香港. 香港科技大学,2021.