Research On AlGaN-based Wide Badgap Semiconductor Ultraviolet Photodetector

Ultraviolet（UV）photodetectors（PDs）have attracted considerable attention due to their important applications in frontier science,national economy and national defense construction.Since the solar-blind UV PDs detect the solar blind UV signal produced by unnatural factors without the interference of solar background noise,it can achieve no interference,more sensitive and more accurate detection.Thus,it has become a research hotspot today.The wide bandgap semiconductor AlGaN possesses intrinsic solar-blind characteristic,high electron mobility,high temperature resistance,and excellent chemical and physical stability.Therefore,AlGaN is an ideal material choice for high performance UV PDs.So far,the performance of AlGaN-based UV PDs has been greatly improved but failed to reach the level of commercialization.Particularly,the 14th Five-Year Plan is a crucial stage for the Chinese third-generation semiconductor industry to achieve rapid development and level improvement independently.Therefore,considering the great strategic needs of AlGaN-based UV PDs,it is most important to develop an independent innovative high-performance device with high responsivity,low noise,high speed and low energy consumption in high temperature and harsh environment.However,the main difficulties which restrict the development and application of AlGaN-based UV PDs are the dark current caused by the poor quality of high Al-content AlGaN and the fabrication of high-performance structure devices.In this dissertation,we investigated the solar-blind UV photodetection performance of high Al-content AlGaN-based metal-semiconductor-metal（MSM）,Schottky barrier and p-i-n UV PDs.Besides,the relationship between the dark current and the dislocation of AlGaN was explored.Furthermore,the physical mechanisms of dark current transport were analyzed to demonstrate the reliability of solar blind UV detection in high temperature environment.Since the traditional structure cannot achieve high gain,a symmetrical interdigitated AlGaN/Ga N two-dimensional electron gas（2DEG）based UV PD has been proposed by improving the device structure.To overcome the high energy consumption of MSM UV PD with a horizontal structure,a self-powered AlGaN-based solar-blind UV PD with asymmetric energy band structure has been developed.The specific research contents are as follows:（1）The dislocation density of AlGaN with Al-content more than 0.4 and device performance were characterized.The high Al-content AlGaN-based MSM,Schottky barrier and p-i-n structure PDs were successfully fabricated.The low screw dislocation density of AlGaN was demonstrated by transmission electron microscopy,which was beneficial to obtain a low dark current and better photodetection performance.The Al_0.6Ga_0.4N-based MSM UV PD obtained a dark current of less than 0.33 n A at 5 V bias,a responsivity of 1.53 A/W,and a quantum efficiency of 825%,demonstrating a photoconductivity gain.The dark current of Al_0.58Ga_0.42N Schottky barrier UV PD was only on the order of p A at low bias.Besides,the device obtained a responsivity of 0.005 A/W at 0 V.The Al_0.42Ga_0.58N-based p-i-n UV PD has an ultra-low dark current with a dark current density<1.50×10^-11 A/cm² at-5 V.In addition,the p-i-n UV PD exhibited a wide peak response characteristic in the range of 240-285 nm.The peak responsivity and corresponding external quantum efficiency were 0.10 A/W and 45%respectively at 0 V,which can realize self-power characteristic.（2）Study on high temperature dark current transport mechanism of AlGaN UV PDs with high Al-content.Based on the theory and experiment,the I-V-T characteristics of solar-blind AlGaN-based UV PDs were studied in depth,and the dark current transport physical mechanism was discussed.Moreover,the physical parameters were obtained.The I-V-T characteristics of MSM and Schottky barrier UV PDs were investigated.It is shown that the dark current of the device increases exponentially with the increase of temperature and bias voltage under high temperature environment.The thermal field emission plays a dominant role in the low dark current at low bias.Additionally,the Poole-Frenkel emission（PFE）model can accurately describe the I-V-T characteristics at high bias.Accordingly,the dark current transport process at high temperature was determined to be the carrier from the trap state to the conductive dislocation-related continuous state.Besides,the I-V-T characteristics of p-i-n UV PD under forward and reverse bias were researched.The results indicate that the contribution of tunneling current to forward current was proved,and two models that hopping conduction and PFE were proposed to dominate the reverse dark current transport.（3）Study on the high gain Al_0.18Ga_0.82N/Ga N 2DEG-based UV PD with a symmetrical interfinger structure.The device was successfully fabricated with AlGaN fingers instead of metal electrodes of a traditional MSM structure.It solves the problem that the traditional structure mentioned above had low responsivity.The device achieved a wide peak response of300-360 nm,a high responsivity of 800 A/W,a high gain of 3.33×10³ and a response speed of21 ms.It was found that the high conductive 2DEG and valence band shift were the main reasons for the high gain of the device.Based on the current transport mechanism at high temperature,it is shown that the dark current transport was owing to thermal emission and PFE.（4）Study on the self-powered Al_0.4Ga_0.6N-based MSM UV PD with an asymmetric band structure.Based on the polarization effect of Al_0.55Ga_0.45N/Al_0.4Ga_0.6N/Al_0.65Ga_0.35N heterojunction structure,the device with asymmetric structure was fabricated by etching Al_0.55Ga_0.45N layer.It solves the high energy consumption of symmetric MSM structure and achieves self-powered characteristic.As a consequence,the device presents a responsivity of0.04 A/W,corresponding to an external quantum efficiency of 19%,and a cutoff wavelength of285 nm.In addition,the internal physical mechanism of asymmetric structure and symmetric structure was compared by the simulation to elucidate the principle of self-powered nature.To form a band with asymmetric,the Al_0.55Ga_0.45N barrier layer was inserted only on one side of the metal/Al_0.4Ga_0.6N Schottky contact,which constituted an asymmetric polarization Al_0.55Ga_0.45N/Al_0.4Ga_0.6N heterostructure.Thus,different barrier heights were formed at schottky contacts to induce band bending and form asymmetric bands.In this dissertation,the high temperature dark current transport mechanisms and high gain mechanisms of AlGaN-based UV PD are clarified.It provides a new structure design and improvement method for the development of high responsivity,low energy consumption and self-driven detector.Furthermore,it can offer a theoretical basis and new ideas for the development of UV photodetection technology.