First Principles Study Of The Optoelectronic Properties Of Doped Two-dimensional GaN And Its Heterojunction

As the representative of the third generation semiconductor,GaN has a very low heat generation rate and high breakdown voltage compared with the first and second generation semiconductor materials,and is an ideal material for making high-frequency microwave devices and high-temperature power devices.With the application of MBE technology in GaN material synthesis and the breakthrough of film growth technology,two-dimensional GaN structures and various heterostructures have been successfully grown.Based on GaN materials,metal field effect transistor(MESFET),heterojunction field effect transistor(HFET),modulation doped field effect transistor(MODFET)and other new devices have been prepared.The modulated doped GaN/AlN structure has high electron mobility(2000cm~2/v·s)and high saturation velocity(1×10~7cm/s),low dielectric constant,is an ideal material for making microwave devices;At the same time,GaN is an ideal material for preparing short wavelength light emitting devices.The light absorption characteristics of GaN and its alloys cover the spectral range from the extreme ultraviolet band to the far infrared band.GaN is generally grown on sapphire and other substrates.Because of its good heat dissipation,it is beneficial for the device to work at high power.Since the two-dimensional GaN was produced in the experiment,the band gap of the two-dimensional GaN material will increase due to its thinner thickness due to the quantum confinement effect.Relevant research indicates that the two-dimensional GaN structure has a larger band gap width than the bulk GaN.Based on the excellent performance of two-dimensional materials,it is of great significance to deeply explore the photoelectric properties of defects,doping and heterojunction for promoting the development of new generation of photoelectric devices and power devices.Based on the density functional theory,this paper calculates and studies the effects of different concentrations of intrinsic defects,different concentrations of boron,thallium,phosphorus and bismuth substitutional doping on two-dimensional GaN materials,and the effects of boron,thallium,phosphorus and bismuth doped single-layer and double-layer GaN/AlN heterostructures on GaN/AlN heterostructures through first-principles calculations.The research contents and results are as follows:(1)The structure and photoelectric properties of two-dimensional GaN crystals with intrinsic defects and different concentrations of intrinsic defects are calculated by first principles.The calculation results show that the intrinsic two-dimensional GaN is a p-type semiconductor with a band gap of 1.70 e V,which indicates that the conduction type of two-dimensional GaN is different from that of bulk GaN.The bandgap widths of two-dimensional GaN with Ga vacancy defects of 2.78%,5.56%,8.83%and 11.1%concentration are 1.78 e V,1.75 e V,1.28 e V and 0.62 e V respectively;The band gap widths of nitrogen vacancy defects at the concentrations of 2.78%,5.56%,8.83%and 11.1%are1.63 e V,1.45 e V,1.20 e V and 1.00 e V,respectively.The results show that different concentrations of vacancy defects have different effects on the band gap width of two-dimensional GaN.The nitrogen defect changes the type of band gap of two-dimensional GaN,and its conduction band bottom and valence band top are at different high points,changing from direct band gap semiconductor to indirect band gap semiconductor;Its Fermi energy level moves from the top of the valence band to the bottom of the conduction band,changing the intrinsic two-dimensional GaN conduction type from p-type semiconductor to n-type semiconductor,while the intrinsic defect structure of gallium atom has not changed,the type of the band gap is direct band gap,and the conduction type is still p-type semiconductor.The results show that the introduction of different intrinsic defects has different effects on the band gap type and conduction type of intrinsic two-dimensional GaN.According to the density of states diagram,in the energy range near the bottom of the conduction band and the top of the valence band,the electronic states mainly come from the p-orbital contributions of N,and a small part from the s-orbital and p-orbital contributions of Ga.With the increase of defect concentration,the absorption peak of nitrogen defect structure has almost no emission change.Both gallium defect structure and nitrogen defect structure have expanded the optical absorption capacity in the mid-infrared and far-infrared bands,in which the gallium vacancy defect structure with a defect concentration of 5.56%has the strongest absorption capacity in this band,and the absorption edge has expanded from 1137 nm of the intrinsic structure to 7560 nm,greatly expanding the optical absorption capacity of two-dimensional GaN in the far-infrared band.(2)The photoelectric properties of group III elements(boron,thallium)and group V elements(phosphorus,bismuth)doped two-dimensional GaN materials are calculated.The results show that with the doping concentration increasing from 1%to 4%,the band gap width of boron doped structure increases,while the band gap width of thallium phosphorus and bismuth doped structure decreases,which provides a basis for regulating the band gap width of two-dimensional GaN.Bismuth doping changed the type of semiconductor band gap from direct band gap to indirect band gap;At the same time,the impurity level of bismuth doped structure is located near the Fermi level,which is far away from the bottom of the conduction band and the top of the valence band.It belongs to the deep level impurity,also known as the composite center of semiconductors.With the increase of doping amount,the absorption peak of boron doped system gradually shifts blue with the increase of doping amount;The absorption peaks of thallium,phosphorus and bismuth doped systems are gradually red-shifted with the increase of doping amount,and the red-shifted range of bismuth doped structure with 4%doping concentration is the most prominent.(3)The crystal structure and photoelectric properties of single layer GaN/AlN and double layer GaN/AlN heterostructures doped with boron,thallium,phosphorus and bismuth are calculated by first principles.The results show that the band gap widths of boron,thallium,phosphorus and bismuth doped single-layer GaN/AlN structures are 2.784 e V,2.022 e V,2.106 e V and 0.896 e V respectively,and the band gap widths of doped double-layer GaN/AlN structures are 2.577 e V,2.644 e V,1.980 e V and 0.645 e V respectively.The bismuth doped structure changes the type of GaN/AlN band gap from direct band gap semiconductor to indirect band gap semiconductor,and the impurity energy level of bismuth doped structure is near the Fermi energy level,belonging to the composite center.In the doping structure of the four elements,only the boron doped system has a blue shift,and the optical absorption peaks of the thallium,phosphorus and bismuth doped system have a red shift.