Study On AlGaN-based Solar-blind Ultraviolet Phototransistor Detector

Photoelectric detectors working in the solar-blind ultraviolet band have important application prospects and market demands in national defense and civil fields such as missile approach warning,extra-high voltage and ultra-high voltage transmission line leakage corona detection.As a typical representative of the third generation of semiconductors,the ternary alloy AlGaN has a direct wide band gap,whose forbidden band width can be adjusted continuously from 3.4 e V to 6.2 e V by adjusting the Al composition,and the intrinsic cut-off wavelength covers 200 nm-365nm ultraviolet band.As well as the advantages of high inter-band transition probability,stable physical and chemical properties,strong anti-irradiation ability make it an ideal material for the development of intrinsic cut-off blind ultraviolet photodetectors.This thesis focuses on the development direction of high-gain,low-noise solar-blind ultraviolet photodetectors which is required for weak solar-blind ultraviolet signals detection in the military and civilian fields.Because of the current gain-type AlGaN-based solar-blind ultraviolet photodetectors,the photoconductive structure has conflicts in response speed and gain,and the avalanche diode structure amplifies noise while amplifying the signal.We proposed to carry out research on AlGaN-based solar-blind ultraviolet phototransistor detectors,and had achieved the following main research results:1.Focusing on solving the problem of lacking high-quality growth substrates for the growth of high-Al content AlGaN materials,on the basis of the traditional"two-step growth method",a method of medium temperature Al N thin insertion layer combined with high temperature Al N growth rate regulation was adopted.The Al N template with low dislocation density was prepared,which provided a good growth substrate for the growth of high Al composition AlGaN transistor structure materials.2.Facing the difficulty of doping with high carrier concentration for high-Al composition AlGaN p-type materials,a doping method was proposed to combine uniform Mg doping with periodic high-Al content AlGaN materials,ultra-thin Ga N materials with Mg doping and high Mg concentration?doping,and vacuum thermal annealing was carried out at the end.A superlattice structure p-type AlGaN material with a carrier concentration as high as 2.16×10¹⁸ cm^-3 was obtained.3.In order to improve the light absorption capacity and photoelectric conversion efficiency of AlGaN phototransistors,a growth technology combining Ga source periodic modulation and growth temperature control was proposed to prepare high-quality solar-blind ultraviolet band Al_0.45Ga_0.55N/Al_0.5Ga_0.5N multiple quantum well active layer as the light absorption layer.4.Based on the above research,an AlGaN-based solar-blind ultraviolet phototransistor structure material was epitaxially grown.Then through optoelectronic processing technology,a back-illuminated NPN bipolar transistor structure solar-blind ultraviolet detector was prepared.Under the applied 12 V bias voltage,the peak responsivity is 4.56 A/W at 267 nm,and corresponding the external quantum efficiency of the device reached 2115.19%.The ultraviolet/visible light suppression ratio is 5.05×10⁴,and the dark current of the device is on the order of 10^-10A.