Research On Monolithic Integrated GaN Based Ultraviolet Device

GaN-based materials have excellent physical and chemical properties such as direct band gap,wide forbidden bandwidth,large light absorption coefficient,radiation resistance and high temperature resistance,so they are very suitable for making high-performance photodetectors or electronic devices.With the change of Al composition,the band gap of Al GaN material can change from 3.4 e V to 6.2 e V,which just covers the ultraviolet(UV)band in the range of 200 nm-400 nm,so Al GaN material has great application value in the field of ultraviolet detection(especially the solar-blind ultraviolet region).With the demand for researching ultraviolet focal plane detectors in aerospace,military and scientific research,relevant research reports emerge endlessly.At present,the ultraviolet focal plane detectors are mainly hybrid focal plane detectors,which are the detectors that ultraviolet photodetectors and silicon readout circuits interconnected with Indium column.However,as the scale of the focal plane increases,this working way also has some shortcomings,such as:the signal generated by the photosensitive element is relatively weak,and it is susceptible to noise signal;the input capacitance and parasitic capacitance affects the output voltage range and charge conversion gain;the size of Indium column needs to become smaller as the focal plane pixel pitch decreases,which places higher requirements on the interconnection process.Based on this,this article studies the monolithic integrated GaN-based ultraviolet detector,which can realize the photo-generated charge on-chip processing function,that is,the photodetector and the device with charge processing capability are made on the same GaN-based epitaxial material.Focusing on the goal of making a monolithic integrated GaN-based UV detector,this article sequentially carried out the Al GaN material transmission spectrum fitting and extraction of material optical parameters,the monolithic integrated GaN-based UV detector device simulation design,GaN-based UV device and GaN-based HEMT device preparation,test analysis and other related research.Using the principle of multilayer film system and transfer matrix,the transmission spectra of Al_0.65Ga_0.35N thin film material grown on sapphire substrate is fitted,and the film thickness,surface roughness,refraction index and absorption coefficient of the thin film material are extracted.The difference between the fitted thickness of the film and the design thickness of the material is less than 4%,and the surface roughness is in good agreement with the AFM(Atomic Force Microscope)test results.The extracted refractive index and absorption coefficient of the Al GaN material provide further research on the response spectrum of the Al GaN UV detector with accurate experimental data.In the design of monolithic integrated GaN-based UV devices,this article mainly studies the model and theoretical knowledge of traditional silicon-based four Transistors APS(Active Pixel Sensor)in photogenerated charge generation and processing,using TCAD(Technology Computer Aided Design)tools to design the monolithic integrated GaN-based ultraviolet detector with functions such as photogenerated charge storage,transfer and conversing the photogenerated charge into voltage output.The device has been optimized in terms of device structure,the length of pinned photodetectors'N region,and a monolithic integrated GaN-based ultraviolet device with less residual photogenerated charge and fast photogenerated charge transfer speed is discussed.At the same time,for GaN-based APS device in simulation design,the process flow of GaN-based ultraviolet detectors as photodiodes and HEMT(High Electron Mobility Transistors)devices as transfer and reset transistors are designed,and corresponding devices have been fabricated.In terms of GaN-based UV devices,GaN UV devices with n-on-p structure was designed and fabricated.The peak responsivity of the detector is 0.073 A/W at 348 nm.In order to verify the experiment of annealing the buried p-layer GaN to activate the in-situ doped Mg,the effect of different annealing conditions on the ohmic contact quality of the p-type GaN material was explored.Finally,GaN-based HEMT devices of depletion mode without etched trench gate and enhancement mode with etched trench gate were prepared;and the ohmic contact performance of source and drain electrodes under annealing conditions of 750°C for30s and 850°C for 30s in nitrogen were compared.The test results show that GaN-based HEMT devices can form good and stable ohmic contacts under nitrogen annealing at750°C for 30s.