Characteristic Of ?-Ga₂O₃ Solar-blind Photodetectors

?-Ga₂O₃ has the characteristics of wide bandgap,excellent optical properties,and stable physical and chemical properties.The transmittance of light at wavelengths longer than300 nm is greater than 80%,?-Ga₂O₃ is one of the promising candidates for the solar-blind UV detectors.In this paper,we fabricated the well-behaved Ga₂O₃ photodetectors on bulk substrate and sapphire.Comparing the performance of Ga₂O₃ photodetectors on bulk substrate and sapphire were carried out,and the effect of oxygen pressure on Ga₂O₃photodetectors performance was also studied.Get the following two conclusions:First,the bulk and Ga₂O₃ on sapphire were grown by using the edge-defined film-fed growth technique and PLD,respectively.The TEM test results show that the several dislocation defects exist in the sapphire epitaxial Ga₂O₃ film,while the bulk Ga₂O₃ has the smooth surface and defect-free crystal.This is due to the lattice mismatch between Ga₂O₃and sapphire,resulting in the residual strain that cannot be released at the Ga₂O₃/sapphire interface.We noted that the XRD peaks of Ga₂O₃ on sapphire move to the low-angle side with the different degrees of shift,which indicates that there is an out-of-plane tensile strain in the Ga₂O₃ film.The magnitude of strain is calculated to be 0.40%according to the?603?peak positions.AFM measurements demonstrate that the bulk and Ga₂O₃ on sapphire samples have the RMS surface roughness values of 1.3 and 7.5 nm,respectively.The significantly enhanced photocurrent,dark current,PDCR,and responsivity performance are achieved in bulk Ga₂O₃ compared with the device on sapphire thanks to the higher crystal quality in bulk Ga₂O_3.Based on the measured responsivity characteristics,the direct and indirect E_G of bulk Ga₂O₃ are 4.78 and 4.59 eV,respectively.The Ga₂O₃photodetector on sapphire only exhibits a maximum cutoff wavelength at 253 nm,corresponding to the direct E_G of 4.90 eV.We speculate that Ga₂O₃ epitaxially grown on sapphire has more traps and defects,which result in the significant degradation of indirect absorption intensity.A 120 meV direct E_G increment is obtained in Ga₂O₃ on sapphire over the bulk Ga₂O₃ due to the out-of-plane tensile strain in the epitaxial film.The time-dependent photoresponse measurement for the photodetectors illustrates that the defect band might participate in the optical absorption and carrier annihilation processes,which degrades the photoresponse speed of the devices.Second,the Ga₂O₃ films were epitaxially grown on sapphire substrate using laser PLD with three different oxygen pressures,0.01,0.05,and 0.09 mbar,denoted as sample A,B,and C,respectively.The XRD test results show that Sample A,B,and C are all ?-Ga₂O₃ films grown along?201?direction.According to the AFM measurements,it can be seen that as the oxygen pressure increases,the surface roughness of the epitaxial Ga₂O₃ on sapphire becomes better and the island size of the surface becomes smaller and smaller.According to XPS,the O/Ga ratios of Sample A,B,and C were calculated to be 1.03,1.08,and 1.09,respectively.And the decrease in the fitted peak area of the assorted gallium suboxides and the increase in binding energy of Ga2p3/2 peak,indicating that the oxygen vacancy in the film decreases and Ga-O binding structure increases with the increment of the oxygen pressure.Compared to Sample A and C,Sample B achieves the improved photocurrent,PDCR,and responsivity.It is speculated that this is due to the low oxygen pressure during the growth of sample A,so that sample A film has more oxygen vacancies than sample B.However,as the oxygen partial pressure increases,a large number of gallium vacancies and V_O-V_Gaa complexes will be generated in Sample C.These point defects will act as a trapping center to capture photogenerated carriers,leading to a decline in the performance of the device.According to the responsivity results,the cut-off wavelength of the three samples was at253 nm,corresponding to the direct bandgap of 4.90 eV.The transient response fitting results show that the shorter slow response time in the decay process of sample B is achieved,further indicating that there are more oxygen vacancies and deep level defects in the sample A and C Ga₂O₃ films.