脉冲功率半导体开关关键技术研究

Photoconductive semiconductor switches (PCSSs) are widely used in ultra wideband (UWB) pulse source, THz electromagnetic wave emission source, high current ignition device, and dielectric wall accelerator (DWA). However, PCSSs develop very slowly due to their low hold-off voltages and short working lifetime. Taking the advantages of high critical breakdown electric field and high thermal conductivity of silicon carbide (SiC) into consideration, this project takes the SiC PCSSs as the research object. The switches use vertical structure with mesa contoured electrodes, which can greatly reduce the field enhancement near the electrode/semiconductor interfaces. The Ohmic contact is prepared by tunneling mechanism, in which the heavy doping layer is formed by ion implantation. The heavily doped layer not only effectively inhibits filamentary current formation, reduces current crowding, and prolongs the switch lifetime, but also shifts the cut-off point of filamentary current from metal/semiconductor contact (M/S) interface to semiconductor/semiconductor contact (S/S) interface to improve hold-off voltage. The physical model of the switches is established through the device simulation. By studying the physical nature of the switch breakdown caused by the field enhancement, the solutions to reduce the field enhancement are put forward. Through understanding of electrode damage mechanism, the optimal design scheme of Ohmic contact is given, and the Ohmic contact electrode with long time, high temperature stability, uniform and low contact resistance is prepared. The successful implementation of this project not only plays a positive role in promoting the development of DWA in China, but also can be used as a reference for the failure mechanism analysis of high power devices.

光导开关在超宽带脉冲源、THz电磁波发射源、大电流点火装置、介质壁加速器等领域有广泛应用，然而受制于其较低的耐压和较短的寿命，发展较为缓慢。基于碳化硅高临界击穿电场和高热导率的优势，本项目以碳化硅开关为研究对象，采用台面型垂直结构，通过离子注入形成重掺杂层，利用隧穿机理制备欧姆接触。台面型电极最大程度地减弱了边缘场强集中；重掺杂层不仅有效抑制丝状电流形成，减轻电流拥挤，延长开关寿命，而且能把丝状电流的截止点从金属/半导体接触面移到半导体/半导体接触面，提高开关耐压。通过器件仿真，建立开关工作的物理模型；通过电极边缘场强集中引起开关击穿的物理本质研究，提出减轻场强集中的措施；通过欧姆接触电极损伤机理研究，给出欧姆接触的优化设计方案，制备出长时间高温稳定的、均匀的、比接触电阻低的欧姆接触电极。本项目的实施不仅对我国正在研制中的DWA起到积极的推动作用，对大功率器件失效机制分析也有借鉴意义。

本项目主要从制备高质量的欧姆接触和降低开关电极边缘电场集中这两方面开展了研究工作。为了制备金属/半导体界面均匀、热稳定性高、比接触电阻率低的欧姆接触，首先研究了Ni/碳化硅（SiC）势垒不均匀性，发现高温退火后的SiC一侧因局部反应不均匀产生六边形凹坑，进一步研究发现凹坑的存在不仅导致界面粗糙和局部电场增强，而且其引起的局部界面成分不均匀，这是降低势垒的主要原因。其次，基于上述结果，提出了一个制备高质量欧姆接触的方案即W/Ni双金属层体系。经研究发现W/Ni/SiC 欧姆接触的比接触电阻率可降低到3.2E-5 Ω·cm2，老化研究表明与Ni/SiC欧姆接触相比，W/Ni双金属层体系SiC欧姆接触在空气氛围下300°C老化950 h后其比接触电阻率提高了一个数量级。显然该体系具有良好的热稳定性，这归因于W的存在在老化过程中起到了耗氧作用，减少了氧原子向界面的扩散。这为制备耐高压和使用寿命长的SiC光导开关提供了行之有效的方法。为了降低边缘电场集中，在实验方面，结合实验结果和数值计算成功建立了纳秒激光烧蚀SiC 物理模型，这为激光加工台面型垂直结构奠定了理论基础；激光波长和激光功率对半绝缘SiC的LO声子拉曼位移的影响的研究结果发现在激光辐照期间，钒补偿半绝缘SiC光激发载流子复合以SRH复合为主，然而高纯半绝缘SiC光激发载流子复合以SRH复合和陷阱辅助Auger复合为主。这为光导开关仿真提供了合理可靠的物理模型。在器件仿真方面，模拟仿真了不同串联结构的SiC光导开关以及相关参数对其耐压性能和导通能力的影响，得到由9层6 μm高反膜加3 μm掺杂层(掺杂浓度为1×1019 cm-3)构成的组合式串联垂直结构的光导开关在偏压为45 kV时其峰值电场强度能达到1.69 MV/cm，电流最大值可以达到1531 A。该结构既达到提高耐压的目的也保证了光导开关的导通能力，基本满足DWA的要求。本项目研究工作均在实验和理论方面为提高SiC 光导开关的耐压性能和寿命提供了新的思路和方法。