Research On Key Technology Of U-trench Insulated Gate Photoconductive Semiconductor Switch

A U-trench insulated-gate photoconductive semiconductor switch(U-IGPCSS)is of a new type structure of photoconductive semiconductor power switches.The dark-state leakage current of the U-IGPCSS is much lower than that of a traditional PCSS made of the same wide-bandgap crystal.It means that,the leakage current problem of GaN photoconductive switches is solved for the coming application of high-voltage pulsed power technology and the photoconductive semiconductor switch advantages of ultra-fast opening,ultra-low triggering jitter and high repetition frequency are still retained.It is a complex process flow to fabricate a U-IGPCSS device,and the GaN process level is not enough mature today.Therefore,in this thesis,the U-IGPCSS structure parameters were designed after the current level boundedness on GaN-based device processes was investigated,and the key processes were studied,including the expitaxy and the deep etching.Firstly,the current international and domestic research progress on the low-defect-density free-standing GaN substrate and the epitaxy-layer growing technologies were investigated,a U-IGPCSS was designed according to the limitations of processing parameters,and then it is verified in performance with the relevant device simulation model built in this thesis.Secondly,the Ga-face deep-groove etching of a c-axis free-standing GaN crystal was studied as the most difficult and key process,considering that the Ga-face is regard as the fittest face for epitaxy growing but the most difficult face for being etched than any other faces of a GaN crystal.The process experiment results show that,the etching depth of more than 5 ?m is achieved using the two-step double-mask-layer dry etching method,with a satisfied steep degree of the sidewall.Thirdly,a laser-pretreatment wet etching method is proposed,since the slow etching velocity and the lattice damage restoration lead to a high cost problem to the Ga-face dry etching.The process experiment is designed with the method using a kind of hot KOH solution.It is preliminarily verified that,the array of hexagon-shaped deep grooves needed for the U-1GPCSS can be obtained on the Ga-face without any alkali-resistant protection layer.Finally,the Raman spectrum characteristics of the region near the halo defects on the Ga-face of a typical GaN wafer were experimentally studied,and the variations of Raman frequency shift values of the E2 mode and the A1(LO)mode were found for estimating the effect radius of the halo defects.Based on the effect radius estimation method,a scribing scheme of GaN wafer was proposed to improve the yield rate of GaN dies.