AlN/GaN 异质结场效应晶体管中与应变分布相关的载流子散射机理研究

Nowadays, AlN/GaN heterostructure field-effect transistors (HFET) have been the un-to-date subject of intense investigation due to their high frequency and compatible E/D mode applications. The soure/drain and gate metals together effect the strain of the AlN barrier layer of AlN/GaN HFET, thus, the polarization Coulomb field scattering related to irregularly distribution of polarization charges is formed attributed to partial strain relaxation in AlN layer. However, little attention has been paid to the theoretical research of the polarization Coulomb field scattering, especially the role that the polarization Coulomb field scattering plays in the up-to-date AlN/GaN HFET devices. The influence of soure/drain and gate metals on the strain of the AlN barrier layer is more obviously due to the thinner barrier layer. The goal of this Research Project is to investigate the influence of material structure (different thickness of AlN barrier layer and GaN cap layer) and device process (different soure-to-drain length, gate length and passivation) on the strain of the AlN barrier layer of AlN/GaN HFET, and to analyze the role that the polarization Coulomb field scattering plays in the AlN/GaN HFET devices. Further, the wave function of the two-dimensional electron gas (2DEG) in the AlN/GaN HFET and the matrix element of electron transitions are calculated by self-consistently solving Schrodinger's and Poisson's equations. Finally, the equation of the polarization Coulomb field scattering function is obtained based on the polarization-distribution model and calculated 2DEG scattering probability. Systemic investigation of polarization Coulomb field scattering is of great importance to optimize the material/device structure and device process for further improving the frequency of AlN/GaN HFET.

由于卓越的高频性能和E/D模兼容特性， AlN/GaN 异质结场效应晶体管（HFET）已成为国内外研究热点。AlN/GaN HFET中AlN势垒层在源、漏和栅金属作用下会出现应变梯度分布，从而对沟道载流子造成弹性散射，即极化梯度库仑场散射。目前，该散射机制的理论研究还远不够深入，特别是该散射机制对AlN/GaN HFET器件特性的影响知之甚少。由于AlN/GaN HFET势垒层很薄，源、漏和栅金属对势垒层应变的影响将更加显著。本项目拟研究材料结构（不同厚度的势垒层及GaN盖帽层）和器件工艺（不同源漏间距、栅长及钝化工艺）对AlN/GaN HFET 中AlN势垒层应变的影响，分析应变极化梯度库仑场散射对AlN/GaN HFET中沟道载流子迁移率的影响，结合薛定谔方程和泊松方程自洽计算，确立该散射机制的解析表达式，并用于指导材料器件设计，改善AlN/GaN HFET器件的频率特性。

本项目重点研究了材料结构和器件工艺对AlN/GaN HFET器件中势垒层应变和沟道迁移率的影响，系统分析了AlN/GaN HFET中沟道载流子的输运特性以及与迁移率相关的散射机理，建立了应变极化梯度库仑场散射的理论模型，确立了应变极化梯度库仑场散射和电子迁移率的解析表达式，并指导AlN/GaN HFET材料器件设计，提升了 AlN/GaN HFET器件的频率特性，圆满完成了计划书规定的研究目标。.取得的标志性成果或主要结论为：（1）研究发现在超薄AlN/GaN异质结材料（AlN势垒层为3nm）中纵光学波声子散射占据主导地位，但完成器件工艺后，由于欧姆退火、肖特基金属以及电场的作用，与应变不均匀分布相关的应变极化梯度库仑场散射成为主要散射机理，而AlN势垒层厚度增加后，势垒层中的部分应变驰豫导致缺陷增加，导致电子受界面粗糙散射的影响更强，同时通过变温实验和改变器件工艺，证实了极化梯度库仑场散射是由栅下与栅源（栅漏）的极化电荷分布不均引起的；（2）研究了GaN盖帽层对AlN/GaN异质结材料的影响，发现GaN盖帽层太薄无法有效保护AlN势垒层，但GaN盖帽层太厚， AlN势垒层的张应变能也越强，因此，界面粗糙强度和位错密度也就越大。发现势垒层为3 nm的AlN/GaN异质结材料中最优盖帽层厚度在4 nm左右；（3）研究发现SiN钝化减弱了AlN/GaN HFET中应变极化梯度库仑场散射对沟道电子迁移率的影响，同时证实SiN钝化后沟道二维电子气密度增多主要是因为SiN钝化影响了器件的表面态，而非AlN势垒层的应力 ；（4）理论分析了 AlN/GaN HFET器件中三角形势阱电子波函数形式，计算应变极化梯度库仑场散射势函数的跃迁矩阵元和散射几率，建立应变极化梯度库仑场散射的理论模型，最终确立了应变极化梯度库仑场散射和2DEG迁移率的解析表达式；（5）突破100 nm T型栅工艺，实现AlN/GaN HFET高频器件，器件电流增益截止频率达（fT）达到119GHz，最大振荡频率(fmax)达到155 GHz，为国内AlN/GaN HFET器件较高频率指标。.本研究成果对AlN/GaN HFET高频器件的设计有重要指导意义，在该项目资助下共发表SCI或EI收录的相关学术文章13篇。