Reseaech On 4H-SiC Photoconductive Switch Based On AlN Antirefletion Coatings

High power pulse technology is widely used in the fields of national defense and military,aerospace,heavy industry,and so on.Power switch plays an important role in the power pulse system.Compared with other types of power switches,the photoconductive switch has the characteristics of peak power,high frequency,low trigger jitter,and fast response speed.It is one of the best choices of switching devices in ultra-high and fast power pulse systems.In recent years,the third generation semiconductor silicon carbide(SiC)materials have been widely used.Compared with traditional semiconductor materials,SiC materials have the advantages of a wide band gap,large dielectric constant,high breakdown field strength and high thermal conductivity.It has great advantages in high frequency and high power switching applications and has become the preferred material for the preparation of the photoconductive switches.Photoconductive switch based on semi-insulating SiC has become an important research direction of photoconductive switch at home and abroad because of its simple structure,low process difficulty,easy integration,and simple peripheral circuit.In this paper,a transverse structure 4H-SiC photoconductive switch with aluminum nitride(Al N)antireflective film is proposed to solve the problems of surface flashover breakdown and low efficiency of laser utilization.Al N layer is deposited on the back of the photoconductive switch as an antireflection film on the laser incident surface.The Al N layer is deposited between cathode and anode in front of the photoconductive switch as a passive layer,to improve the optical utilization and reliability of the device.In this paper,the transient,dark state and breakdown characteristics of the photoconductive switch based on Al N antireflection film are mainly studied,and the fabrication process of 4H-SiC photoconductive switch device is designed,and the related experimental research is carried out.(1)Firstly,the research status and principle of the photoconductive switch are analyzed,and the transverse structure photoconductive switch based on semi-insulating 4H-SiC is selected as the research object.A 4H-SiC photoconductive switch with Al N antireflective film and passivation layer is designed to solve the problems of flashover breakdown on the surface of traditional photoconductive switch and low laser utilization efficiency.(2)The semiconductor process and device simulation software SILVACO-TCAD is used to simplify the structure of the photoconductive switch.Atlas simulator is used to program mesh generation and material parameter settings,and the simulation model of the photoconductive switch is established.The transient,dark state,and breakdown characteristics of the photoconductive switch are simulated and analyzed.Firstly,the influence of different vanadium compensation impurity concentrations in the substrate on the dark state and transient characteristics of 4H-SiC photoconductive switch is simulated and analyzed.The simulation results show that when the concentration of vanadium doping is 1×1016 cm-3 the dark state resistivity of the switch device reaches 1012 Ω·cm,and the transient response time is fast,but the transient current is small.Considering the dark state and transient characteristics,the vanadium doping concentration of 1×1016 cm-3 is selected in this paper.Then,the effects of different substrate thickness on transient,minimum on-resistance and breakdown characteristics are analyzed.According to the simulation results,it is determined that the thickness of 4H-SiC photoconductive switch substrate is 350 μm,and the breakdown of the switch device reaches 80 k V.Finally,the influence of Al N antireflection coating on 4H-SiC photoconductive switch is simulated.Under the conditions of 532 nm wavelength,3000 MW/cm2 optical power density,and 2 k V bias voltage,the maximum transient current of the switch is increased from 9.96 A to 11.49 A,which is about 15.4 % higher.The optical utilization ratio of the maximum transient current and laser energy is significantly improved.(3)The fabrication process of 4H-SiC photoconductive switch device was designed.The preparation process of Al N antireflective film and multilayer electrode of 4H SiC photoconductive switch was studied.Al N thin films were prepared on 4H SiC substrates by RF magnetron sputtering system.The results show that the Al N films have(002)crystal orientation,sharp XRD diffraction peaks,and high intensity;the optical properties of the films are good,and the antireflection rate of Al N films is about 15%.The surface quality of the Al N films meets the requirements of this paper.To reduce the contact resistance between the metal electrode and 4H SiC substrate,Ni / Ti / Pt / Au multilayer metal electrode was prepared by magnetron sputtering system.Finally,according to the simulation results,the performance of the 4H SiC photoconductive switch device designed in this paper is summarized,and the optimization and improvement methods for the deficiencies of the preparation and testing of the photoconductive switch device are proposed.