| β-Ga2O3 is a typical representative of the ultra-wide band gap semiconductors.Compared with the third generation of wide band gap semiconductors silicon carbide and gallium nitride,β-Ga2O3 has the advantages of larger band gap width(～4.9 eV),higher breakdown field intensity(8 MV/cm),higher Baliga’s figure of merit value(3444)and deeper absorption cutoff wavelength(～ 254 nm),and has great advantages in the fields of ultra-high-voltage and highpower devices and ultraviolet photodetectors.The absorption cutoff wavelength of β-Ga2O3 is just in the solar blind UV region,and β-Ga2O3 has high anti-radiation ability,high chemical and thermal stability,and the ability to work in extreme environments.Therefore,β-Ga2O3 has great advantages in the applications including all-weather anti-strong interference missile warning,shipboard communication,fire detection,high voltage line maintenance,etc.In this thesis work,we use pulsed laser deposition(PLD)to grow β-Ga2O3 thin films on c-plane sapphire substrate heteroepitaxial with small lattice mismatch constant.We have systematically investigated the effects of process conditions,including growth temperature,oxygen partial pressure,and Si-doping concentration of the Ga2O3 target,on the film crystalline quality,composition,and the performances of the Ga2O3 based Ti/β-Ga2O3/Ti metalsemiconductor-metal(MSM)photodetectors,by X-ray diffraction(XRD),scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS),and UV-Vis transmittance characterizations.The main contents are as follows:(1)The influences of the growth temperature in range of 550～750℃ on the film and device performances.The XRD and SEM results indicate that with the growth temperature of 600℃,(-201)oriented β-Ga2O3 with the largest grain size and highest crystalline quality can be realized.The XPS and UV-Vis transmittance results reveal that then the growth temperature reaches or even over 700℃,the sapphire substrate will contribute Al-doping.The MSM device performance study shows that with the growth temperature of 600℃,the device shows the highest photodetection performance due to the highest crystalline quality.(2)The influences of oxygen partial pressure in range of 7.5～45 mTorr on the film and device performances.The XRD and SEM results indicate that with the smallest oxygen partial pressure of 7.5 mTorr,(-201)oriented β-Ga2O3 with the largest grain size and highest crystalline quality can be realized.The XPS results reveal that with increased oxygen partial pressure,the oxygen vacancy density decreases.The MSM device performance study shows that with the smallest oxygen partial pressure of 7.5 mTorr,the device shows the highest photodetection performance,due to the highest crystalline quality and the highest oxygen vacancy concentration.(3)The influences of the Si-doping concentrations of Ga2O3 target in range of 0～3wt%on the film and device performances.The XRD and SEM results indicate that with the appropriate Si-doping concentration of 1.5wt%,(-201)oriented β-Ga2O3 with the largest grain size and highest crystalline quality can be realized.The MSM device with the same Si-doping concentration also shows the highest photodetection performance,because of the highest crystalline quality and the high activated electron doping concentration.(4)Based on the above studies,the β-Ga2O3 MSM photodetectors were prepared at the optimized conditions including 600℃ growth temperature,7.5 mTorr oxygen partial pressure,and 1.5wt%Si-doping concentration have the best crystal quality.The photodetector realize low dark current(Idark)of 9.9 pA at 5 V bias,and high light current(Ilight)of 2.7 μA at 254-nm light illumination,with the high PDCR of 2.8×10-5,R of 69.8 A/W.D*of 9.3×1014 Jones,and EQE of 34,206%.Some of the performance parameters of this device are better than those of the previously reported β-Ga2O3 MSM photodetectors prepared by PLD method. |
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