Research On 4H-SiC Photoconductive Switches Based On Tilted AlN Nanopillars Antireflection Coatings

Based on the development status of photoconductive switches,compared and analyzed the advantages and disadvantages of three generations of semiconductor materials as substrates for photoconductive switches in the dissertation.Then,from the perspective of laser utilization,it is discussed whether an anti-reflection film can be added to the laser trigger surface of the SiC photoconductive switch to improve the transmittance of light inside the SiC substrate material,thereby further improving the performance of the photoconductive switch.For the electrodes of the switch,the electrode structures of multilayer metal,circular negative electrode and circular positive electrode are selected,and on this basis,the electrode size is simulated and designed.Finally,4H-SiC-based photoconductive switching devices were fabricated.The specific research contents are as follows:(1)First,after analyzing the research status and principle of photoconductive switches,the 4HSiC-based photoconductive switch with lateral structure is determined as the research object of the thesis.Second,in order to improve the utilization rate of light by the photoconductive switch,it is planned to add an anti-reflection coating on the laser trigger surface of the photoconductive switch.(2)The design of the anti-reflection coating of the light guide switch.This dissertation mainly analyzes several common antireflection coating materials from the aspects of band gap,thermal expansion coefficient and lattice mismatch degree.For aluminum nitride(AlN)materials with a smaller degree of coordination and the thermal expansion coefficient of the same order of magnitude,the lattice mismatch between the two is about 1.2%.According to the thin film interference theory,when the incident light wavelength is 532 nm,the theoretical refractive index of the optimal antireflection coating is 1.63,while the refractive index of the dense AlN film prepared by the traditional method is 2.16.In order to solve this problem,the dissertation proposed to use the inclined angle deposition technique(GLAD)to prepare the AlN film,which has been proved to be able to control the refractive index of the film by increasing the porosity inside the film.(3)Simulation design of the photoconductive switch.This dissertation uses FDTD Solution and Silvaco TCAD software to carry out simulation experiments respectively.Firstly,three simulation models of4H-SiC material without antireflection coating,dense AlN thin film and inclined AlN nanopillar thin film as antireflection coating of 4H-SiC material were established by FDTD software respectively.Transmission inside the 4H-SiC material.The simulation results show that the addition of AlN film significantly increases the transmittance of light inside the 4H-SiC material.Compared with the dense AlN film,the inclined AlN nanopillar film is used as the antireflection coating of the 4H-SiC material,so that the light can be transmitted in the 4H-SiC material.-SiC material has higher transmittance,and its transmittance reaches 97.3% at a wavelength of 532 nm;secondly,three main parameters of AlN nanopillar thin film are analyzed: the change of tilt angle,height and diameter on 4H-Influence of internal transmittance of SiC material.According to the simulation results,it is found that the AlN film with a thickness of 260 nm,an inclination angle of nanopillars of 10°,and a diameter of 80 nm has the best antireflection effect on 4H-SiC material,and the transmittance of light in 4H-SiC material is 98.5%.Compared with the 4H-SiC material without antireflection and the 4H-SiC material with a dense AlN film as the anti-reflection coating,the transmittance is increased by 23.8% and 13.7%,respectively.After the structure of the optimal antireflection coating was determined,the device simulation software Silvaco TCAD was used to establish three photoconductive switch models with different structures,and the effects of antireflection coatings with different structures on the transient current of the 4H-SiC-based photoconductive switch were analyzed.After analyzing the output results,it is found that compared with the 4H-SiC-based photoconductive switch without anti-reflection and the 4H-SiC-based photoconductive switch with the dense AlN film as the anti-reflection coating,the thickness of 260 nm and the inclination angle of AlN nanopillars are When the AlN film with a diameter of10° and a diameter of 80 nm is used as an antireflection film for a 4H-SiC-based photoconductive switch,the switching transient current can reach 12.05 A,with a year-on-year increase rate of 20.7% and 10.6%.Finally,the Silvaco TCAD software is used to simulate and analyze the electrode size of the switch.According to the breakdown voltage of the switch under different electrode sizes,the electrode size with better withstand voltage is selected to prepare for the preparation of the switch electrode later.(4)Preparation of 4H-SiC-based photoconductive switching devices.In this dissertation,inclined AlN nanopillar thin films were prepared by three-target-magnetron sputtering deposition equipment using inclined angle deposition technology.The films were characterized by scanning electron microscopy(SEM),X-ray diffractometer(XRD),atomic force microscopy(AFM)and UV-Vis spectrophotometer.The results show that the columnar growth inside the film is obvious.The measured average inclination angle of the nanopillars is about 9°,the thickness is 272 nm,and the diameter is about 76 nm,which basically meets the requirements of the simulation results.The intensity(002)crystallographic diffraction peak is the preferred orientation of high C-axis,and the average roughness of the film surface is 6.52.The photometer results show that the addition of the inclined AlN nanopillar film makes the transmittance of the trigger laser significantly enhanced inside the 4H-SiC substrate material.Finally,the Ni metal electrode was fabricated with the aid of a three-target-magnetron sputtering deposition equipment using a mask process.