Studies On The Structural And Photoelectric Properties Of Ga₂O₃ Thin Films With Ultra-wide Band Gap

As one of group III-oxide semiconductor materials,monoclinic gallium oxide(?-Ga₂O₃)has attracted considerable attention due to its wide band-gap of?4.9 e V at room temperature(RT),good transparency from deep ultraviolet to infrared region,high thermal and chemical stabilities,high dielectric breakdown field value,high mechanical strength,and intrinsic oxygen vacancy.Owing to these excellent properties,?-Ga₂O₃ has been of great interest for applications in high-power and high-temperature electronics,solar-blind UV detectors,transparent conducting films,photocatalyst,solar cells,lasers liquid crystal displays,and chemical sensors.Comprehensive structural,electrical and optical studies are performed on a series of gallium oxide(Ga₂O₃)ultrathin films grown on sapphire with different growth temperatures(400-1000?)by pulsed laser deposition,via X-ray diffraction(XRD),optical transmittance(OT),spectroscopic ellipsometry(SE),X-ray Reflectometry(XRR),X-ray absorption spectroscopy(XAS),Raman scattering(RS)and X-ray photoelectron spectroscopy(XPS).The following conclusions are drawn:(1)The phase was transformed from amorphous to polycrystalline?-Ga₂O₃structure with increasing growth temperature.The optical properties including refractive index,absorption coefficient and band gap of the films were obtained,along with surface roughness and actual film thickness.Our findings indicate that,refractive indexes increase with increasing thickness or growth temperature of the films in the transparent area,where the effect of film thickness is much more significant than that of growth temperature.The Urbach tail observed in absorption edge combined with the transmittance spectra and the XRD intensities illustrate that the higher growth temperature or thicker thickness can improve the crystal quality of films.The band gap of Ga₂O₃ films decrease with increasing growth film thickness and elevated experimental temperature(11–873 K),and the quantitative analysis of temperature-dependent band gap was conducted by using the Varshni equation.(2)For samples grown at different temperatures,the XAS results showed the coordination numbers of the materials varying,while their bond lengths remained nearly similar value.The RS revealed a low frequency vibration translation tetrahedral-octahedral mode(202 cm^-1)of Ga O₄ and a mid-frequency deformation octahedral mode(346 cm^-1)of Ga O₆ for films grown at higher temperatures.XPS analyses suggested the surface of samples composed of Ga-O bonds with binding energy decreasing as the growth temperature increased.The?-Ga₂O₃/sapphire heterojunction is identified with the staggered-gap(type II)structure,and the valence-band offset(VBO)is found between(-0.52)–(-0.74)e V while conduction-band offset(CBO)from(-2.32)–(-3.32)e V.The band gap of the?-Ga₂O₃ was deduced from the energy loss signals for O 1s photoelectrons.With the increase of the growth temperature,the band gap increases and both the VBO and CBO decrease.The identification of band alignment for heterojunction may facilitate interests in designing advanced opto-electronic devices.