氮化镓高压高重复频率新型光电导功率开关的结构设计及其关键技术研究

To a wide-bandgap photoconductive semiconductor switch (PCSS), it is only based on Fe-doping highly-compensated GaN semi-insulating crystal (GaN:Fe) that has hope to simultaneously obtain the two expected performances, ≥100 kV dc withstand voltage and 100 MHz repetition frequency, when triggering with laser pulses of visible-spectrum wavelength (i.e., the photon energy is lower than the band gap of the wide-bandgap material). However, the traditional structures of PCSS are not fit for the ultra-high-voltage demand of GaN:Fe PCSSs, due to the intolerable leakage current. Therefore, the physical mechanisms of the leakage current in traditional PCSS structures are analyzed and then a novel structure, named "Insulated-Gate PCSS", is presented to restrain the leakage current due to the Ohmic resistance and the positive-biased high-low junction effects. Compared with the traditional PCSS structure of L⊥(N//E), there is a U-shape n-channel MISFET cells structure in the novel Insulated-Gate PCSS. We will research the key technology to build the device physical model and the fabrication process model of the novel GaN:Fe-based Insulated-Gate PCSS devices, optimize the design details, fabricate a few samples with the dc withstand voltage of more than 20 kV, and then test the samples with high-repetition-frequency laser pulses. It has been proved via our pre-research simulation work that, the reverse-biased p-n junction introduced to the novel PCSS structure not only can share the bias dc voltage with the semi-insulating photoconductive area to decrease the leakage current down to 1/100 of that in a comparable traditional PCSS, but also can transfer itself voltage to the photoconductive area if triggered with the gate electrode integrated in the p-n junction area. The extra transient voltage transferred across the photoconductive area yields higher photoelectric conversion efficiency, because the photon-generated carriers drift in the higher electrical field than the dc withstand one.

掺铁高度补偿的半绝缘氮化镓（GaN:Fe），是宽禁带光电导开关能实现用亚带隙（即可见光波段）激光脉冲触发得到十万伏以上高压、百兆高重复频率电脉冲序列的必选材料，然而传统光电导开关结构漏电流严重已不适用于GaN:Fe光电导开关的超高压需求，因此针对传统结构产生漏电流的两种主要物理机制，提出“绝缘栅型光电导开关”新结构，即在传统L⊥(N//E)光电导开关结构基础上引入U形n沟道MISFET多胞结构来抑制欧姆电流和高低结正偏漂移电流。本申请将研究GaN:Fe基的绝缘栅型光电导开关器件物理建模和工艺建模关键技术，优化设计最终制造出直流耐压20kV以上的样品并进行高压高频测试。预研已证明，新结构中的反偏pn结区不但能与半绝缘光触发区静态分压，使漏电流减小2个数量级；而且栅极触发后pn结区高压会被转移到光触发区，使随后产生的光生载流子漂移在高于直流额定的瞬态高场，提高光电转换效率。

掺铁高度补偿的半绝缘氮化镓（GaN:Fe），是宽禁带光电导开关能实现用亚带隙（即可见光波段）激光脉冲触发得到十万伏以上高压、百兆高重复频率电脉冲序列的必选材料，然而传统光电导开关结构漏电流严重已不适用于GaN:Fe光电导开关的超高压需求。 因此，本项目首先基于第一性原理理论和拉曼散射实验分析了GaN材料特性，然后提出了三种GaN:Fe基的光电导功率开关结构，并建立了它们的器件仿真模型，分别是耐压大于20kV的U沟道绝缘栅型、耐压大于10kV的纵向双扩散绝缘栅型和耐压10kV的 AlGaN/GaN光电导开关模型。这三种引入栅控反偏pn结的结构基于空间电荷区载流子耗尽机理使漏电流不随电压线性增长，因此在10kV量级高压时比传统光电导开关的漏电流低2个数量级；而且栅压触发后pn结区高压被快速转移到光触发区，使随后产生的光生载流子漂移在高于直流额定的瞬态高场，因此绝缘栅型光电导开关的光电转换效率高于传统光电导开关。最后，对比分析了这三种结构在耐压、通流和工艺复杂性上的优缺点，其中耐压潜力最高的是U沟道绝缘栅型光电导开关结构。. 针对U沟道绝缘栅型光电导开关的关键制造工艺，主要开展了4个方面的研究：1）用SIMS方法研究GaN:Fe衬底和非故意掺杂n型外延层之间界面Si富集现象，以合理修正器件结构参数；2）为实现深刻蚀槽而进行了C轴GaN的Ga面干刻实验；3）基于GaN六方晶系的特性，提出一种基于红外激光束烧蚀预处理的KOH湿法刻蚀方法，实验证明该方法不需光刻版和掩膜层，就能获得蜂窝状排布的六边形深槽，具有较高的性价比；4）为解决大功率激光束自动精准定焦的难题，提出了一种基于紫外辐射探测的强激光束自动寻焦方法并制造出样机。