Optical Properties Of Sn Doped And Ta Doped Gallium Oxide Single Crystals Before And After Annealing

In recent years,?-Ga₂O₃ has attracted much attention as a new generation of semiconductor materials.It has ultra-wide band gap of 4.5?4.9 e V,a breakdown field strength of up to 8 MV/cm,and has the advantages of high withstand voltage and high melting point.It has broad application prospects in the fields of high-power devices,solar-blind ultraviolet detection devices,and X-ray photoelectric detection.Unintentional doping?-Ga₂O₃ is a weakly n-type conducting material.In order to improve the carrier concentration in the material,n-type doping is always the focus of research and the need of device development.At present,most of the reported n-type doping are Sn⁴⁺and Si⁴⁺.As a group V element,Ta has 5valence electrons in its outer layer,and its ion radius is very similar to that of Ga³⁺.It is an excellent new donor element doped with high carrier concentration.The annealing process can not only improve the crystal quality,but also change the internal defect of the crystal.Therefore,it becomes very important to study the properties and defects of the crystal after annealing and doping.This paper uses a series of characterization techniques to study the optical properties and defects changes of Sn-doped and Ta-doped gallium oxide before and after annealing.The main contents of this paper are as follows:(1)Sn:?-Ga₂O₃ and Ta:?-Ga₂O₃ single crystals were grown by edge-defined film-fed growth method and floating zone method,respectively.And the crystal quality of the samples was analyzed by X-ray diffraction spectroscopy(XRD)and Raman scattering spectroscopy(Raman).The study found that after annealing,the full width at half maximum of the double crystal rocking curve of the two crystals are narrowed,and the quality of the crystals are improved to a certain extent,and the small grain boundary phenomenon in the Ta:?-Ga₂O₃ single crystal was also improved to a certain extent.After annealing,the intensities of the wavenumber phonon modes A_g⁴ and A_g⁷ in the Raman scattering spectra of the two crystals are significantly increased,indicating that annealing improves the deformation of the Ga O₆ octahedron and Ga O₄ tetrahedron of gallium oxide.(2)From the photoluminescence emission spectrum(PL),it is found that for Sn:?-Ga₂O₃ crystals,annealing in an oxygen-rich environment greatly enhances the green light emission of the crystal.The reason is that the oxygen-rich environment produces a large number of V_Ga vacancy defects inside the crystal,and the transition of electrons to V_Ga vacancies is promoted.In an air annealing environment,Ta:?-Ga₂O₃ crystal does not produce green light emission.By analyzing the temperature-dependent PL spectra of Ta:?-Ga₂O₃,the thermal quenching equation was used to fit the activation energy of light emission in the blue band(DAP mechanism emission)at 40 me V,which was attributed to the shallow impurity donor level formed by Ta.Through the luminescence excitation spectrum(PLE)of light with temperature-dependent of Ta:?-Ga₂O₃,the property of its band gap changing with temperature is obtained,which satisfies the Varshni equation,and the self-trapped holes(STHs)position is roughly determined from the low-temperature excitation spectra.(3)Through the transmission spectra,it is found that nitrogen annealing conditions basically do not change the optical band gap of Sn-doped?-Ga₂O₃single crystal,and oxygen atmosphere annealing reduces the optical band gap of Sn:?-Ga₂O₃ single crystal slightly.From the temperature-dependent transmission spectrum of Ta:?-Ga₂O₃ single crystal,the relationship between the band gap and the temperature is obtained,and the relationship function is analyzed and fitted by the equation including the Bose-Einstein statistical factor and the thermal vibration of phonons.From the Hall effect and the infrared region of the transmission spectrum,it can be found that Ta⁵⁺doping can increase the carrier concentration in gallium oxide crystals more than Sn⁴⁺doping.(4)X-ray photoelectron spectroscopy(XPS)shows that the Ga/O element ratios of gallium oxide single crystals before and after air annealing were 2:2.54and 2:2.46,respectively,indicating that not only V_Ga vacancies are generated in Ta:?-Ga₂O₃ single crystals at high temperature,but also a large number of V_Ovacancies are generated.Ultraviolet photoelectron spectroscopy(UPS)shows that the Fermi level of Ta-doped Ga O crystals decreases after air annealing,and the calculated work function of the crystals increases slightly after air annealing.