Surface/interface control of high-frequency and high-power transistors based on GaN materials

本研究的目的是表征和控制GaN基材料系统的表面/界面特性，如AlGaN/GaN异质结构，以提高高频和大功率晶体管的稳定性。主要研究结果如下：（1）在AlGaN表面处理过程中发现了严重的化学计量无序和氮缺陷（N缺陷）等劣化现象。在AlGaN/GaN异质结场效应晶体管中，由于N空位（VN）的存在，形成了局域深施主能级，导致AlGaN肖特基界面处漏电流过大，漏电流严重崩塌。(2)An提出了一种包括N_2等离子体表面处理的Al_2O_3基表面钝化方案，并将其应用于绝缘栅型HFET。在Al_2O_3/Al_ Ga_ N界面处获得了2.1eV的大导带偏移<0.3><0.7>。在漏极应力和栅极应力作用下，Al_2O_3绝缘栅HFET均未出现电流崩塌现象。（3）通过详细的温度相关的电流-电压（I-V-T）测量，我们讨论了GaN和AlGaN肖特基界面漏电流的产生机理。将实验结果与考虑表面缺陷影响的薄表面势垒模型计算结果进行了比较。我们的模拟结果表明，无意的表面缺陷施主所造成的势垒变薄增强了势垒输运过程，导致通过GaN和AlGaN肖特基接口的大的漏电流。

The purpose of this research was to characterize and control surface/interface properties of GaN-based material systems such as AlGaN/GaN hetrostrcutures for the stability improvement of high-frequency and high-power transistors. The main results obtained are listed below :(1)Serious deterioration such as stoichiometry disorder and nitrogen deficiency (N deficiency) was found at the processed AlGaN surfaces. This resulted in formation of a localized deep donor level related to N vacancy (V_N), causing excess leakage currents at the AlGaN Schottky interface and serious drain current collapse in AlGaN/GaN heterostructure field effect transistors.(2)An Al_2O_3-based surface passivation scheme including the N_2-plasma surface treatment was proposed and applied to an insulated-gate type HFET. A large conduction-band offset of 2.1 eV was achieved at the Al_2O_3/Al_<0.3>Ga_<0.7>N interface. No current collapse was observed in the fabricated Al_2O_3 insulated-gate HFETs under both drain stress and gate stress.(3)From the detailed temperature-dependent current-voltage (I-V-T) measurements, we discussed the mechanism of leakage currents through GaN and AlGaN Schottky interfaces. The experiments were compared to the calculations based on thin surface barrier model in which the effects of surface defects were taken into account. Our simulation results indicates that the barrier thinning caused by unintentional surface-defect donors enhances the funneling transport processes, leading to large leakage currents through GaN and AlGaN Schottky interfaces.