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氮化镓基材料体系（如AlGaN/GaN异质结构）的表面/界面特性，以提高高频和大功率晶体管的稳定性。得到的主要结果如下：(1)处理后的AlGaN表面出现了严重的化学计量紊乱和缺氮现象。这导致了与N空位相关的局部深施主能级（V_N）的形成，在AlGaN/GaN异质结构场效应晶体管中导致了AlGaN Schottky界面的过量泄漏电流和严重的漏极电流崩溃。(2)提出了一种基于al_2o_3的表面钝化方案，包括n_2 -等离子体表面处理，并应用于绝缘栅型HFET。在Al_2O_3/Al_<0.3>Ga_<0.7>N界面上实现了2.1 eV的大导带偏移。在漏极应力和栅极应力下，制备的Al_2O_3绝缘栅极hfet均未观察到电流坍塌。(3)通过详细的温度相关电流-电压（I-V-T）测量，我们讨论了漏电流通过GaN和AlGaN肖特基界面的机理。将实验结果与考虑表面缺陷影响的基于薄表面势垒模型的计算结果进行了比较。我们的模拟结果表明，无意表面缺陷供体引起的屏障变薄增强了漏斗输运过程，导致通过GaN和AlGaN肖特基界面的大泄漏电流。

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.