Study on Characteristics of Gallium Oxide Material and Photodetectors

Author:Dong Lin Peng

Supervisor:yang lin an


Degree Year:2019





The ultra-wide bandgap oxide semiconductorβ-Ga2O3 has been widely used in high-power electronic devices,energy catalysis,gas sensors,and optoelectronic devices due to its excellent physical and chemical properties,low-cost,and convenient preparation process.Focusing on the basic characteristics and problems ofβ-Ga2O3,this work investigated the oxygen vacancies and p-type doping ofβ-Ga2O3 both theoretically and experimentally.Besides,the magnetron sputtering and pulsed laser deposition(PLD)methods were used to prepareβ-Ga2O3 films,and the growth parameters were further optimized.Finally,the application ofβ-Ga2O3 in the solar-blind photodetectors was studied.The main details,results and conclusions are as follows:1)Based on DFT,the DFT+U calculation method was used to obtain a reasonable bandgap value(4.9 eV)ofβ-Ga2O3 by adjusting the parameter of Hubbard U value.Based on this method,we systematically studied the O vacancies which are easily appearing inβ-Ga2O3,the formation energies,transition levels,and the influence on the structure as well as optical properties ofβ-Ga2O3 were calculated and analyzed.According to the calculated results,the formation energies of the three oxygen vacancies are negative under both O-rich and O-poor atmosphere conditions at absolute zero temperature,thus is highly prone to occur inβ-Ga2O3.The thermodynamic defect transition levelε(+2/0)of the three vacancies are 3.2,3.7 and 3.9 eV measured from VBM,respectively.The distance of the three vacancies fromε(+2/0)to CBM are all greater than 1.0 eV,and the oxygen vacancies are believed to act as deep donor impurities inβ-Ga2O3.When the oxygen vacancy occurs,the value of the static dielectric constantε0 is slightly reduced due to the influence of the impurity level,the absorption ofβ-Ga2O3 in the visible region is enhanced,and the absorption coefficient is increased.For V0OI,V0 OIIand V0OIII,the center of the new absorption peaks are located at 3.80,3.52 and 3.37 eV,respectively.2)The DFT+U method was used to study the most promising dopant N(nitrogen)for the realization of p-typeβ-Ga2O3,and the presence of intrinsic defects in N-dopedβ-Ga2O3were also investigated.For N-dopedβ-Ga2O3,the energy band calculation results indicate that the N impurity introduces a deep acceptor impurity level at a distance of 1.33 eV above the VBM,and the N impurity doping is less soluble due to its higher formation energy.As a result,N cannot be used as an effective p-type dopant forβ-Ga2O3.When the intrinsic defects are considered,the formation energies of O vacancies and Ga interstitial defects in N-dopedβ-Ga2O3 are close to the value of the defect-free N-dopedβ-Ga2O3under Ga-rich conditions,thus are easily present in N-dopedβ-Ga2O3.The band structures,density of states and differential charge density distributions indicate that the O vacancies and Ga interstitial defects tend to transform the N-dopedβ-Ga2O3 to n-type conductivity.In addition,the thermodynamic transition levelsε(0/-1)of NGa2O3,NGa2O3VO,and NGa2O3Gai are 3.37,3.34,and 5.27 eV,respectively.The O vacancies and the Ga interstitial defects make the doped N atoms more difficult to ionize,thereby reducing the efficiency of N as an acceptor impurity.Finally,we briefly discuss the absorption coefficient of the N-dopedβ-Ga2O3 structure with intrinsic defects as a function of incident photon energy.3)The Ga2O3 films were prepared by magnetron sputtering method,the effects of oxygen concentration and the subsequent annealing temperature on the growth rate,crystal quality,surface morphology and optical properties of the films were investigated.XRD,SEM,AFM and Raman test results indicate that the film has the best crystal quality when1%oxygen is introduced into the film deposition process and annealed at 1000°C for 1hour.Compared to the directly deposited film N-N,the optical properties of the films prepared under 1%oxygen are significantly improved,and the sample O1-T1000 has the highest optical transmittance in the ultraviolet-visible range.By fitting the absorption coefficient spectra through Tauc scheme,the optical bandgaps of the samples N-N,N-T1000 and O1-T1000 are 4.70,4.80 and 4.87 eV,respectively.XPS and PL spectroscopy test results indicate that the introduction of oxygen during film growth can effectively inhibit the presence of oxygen vacancies in the films,thus weaken the intensity of PL luminescence peaks associated with oxygen vacancies.These experimental results are also highly consistent with the DFT calculated results.4)Theβ-Ga2O3/SiC heterojunctions were prepared on the 4H-SiC substrates by PLD method.The growth temperature and annealing temperature of the films were briefly studied,and the solar-blind photodetectors based onβ-Ga2O3/SiC heterojunctions were studied.The photoelectric characteristics of the device were characterized.The prepared photodetectors H800,H900,and H1000 all exhibited good solar-blind characteristic.Due to the uniform and denseβ-Ga2O3 film and excellent interface quality,the H1000 has the lowest dark-state leakage current and the largest switch ratio.Under 254 nm wavelength illumination,the light/dark current ratio,R?,EQE,D*and LDR of the detector H1000were 384.6,67.83 mA/W,33.1%,82.3×1010 Jones,and 51.7 dB,respectively.The H1000exhibits excellent switching characteristic with the rise/fall time constants of 0.18 and 0.31s,respectively.The band offset and energy band diagram ofβ-Ga2O3/SiC heterojunction were analyzed by XPS results,and the working mechanism of heterojunction photodetectors were analyzed by energy band diagram.5)Self-powered solar-blind photodetectors with MSM(Metal-Semiconductor-Metal)structure based onβ-Ga2O3 single crystal were studied.The self-powered property of the device was realized by a simple one-step deposition of structurally asymmetric interdigital electrodes.The prepared detectors R1,R2,R3,R4,and R5 all exhibit good solar-blind characteristic,and the self-powered characteristic are enhanced with the symmetry of the electrode structure degenerate.For R5 with the worst symmetrical electrode,it exhibits an excellent switching characteristic and stability under zero bias.The rise/fall time constants are 0.03 and 0.08 s,respectively.Under the illumination with 254 nm wavelength,the photocurrent of R5 is 0.16 nA,R?,EQE,D*and LDR are 1.28 mA/W,0.63%,17.7×1010Jones,23.5 dB,respectively.Finally,we explained the origin of the self-powered characteristic through the energy band diagram.The reason can be attributed to the difference of the effective Schottky barrier height between the asymmetric interdigital electrode,which is related with surface defects under electrodes.