Growth And Simulation Studies Of Ga-Cd-O Thin Films

Gallium oxide（Ga₂O₃）semiconductors are a new type of broadband semiconductor material with a band gap as wide as 4.9 e V corresponding to a wavelength of 250 nm in the UV region,making it a natural material for day-blind detectors.However,its band gap width is too large and corresponds to a small range of UV detection wavelengths,thus limiting the application prospects of Ga₂O₃.Cadmium oxide（Cd O）is a transparent conducting oxide material with high electrical conductivity and electron mobility.However,its narrow optical band gap（2.2 e V）leads to premature absorption in the short wavelength direction of visible light,making its application limiting.The band gap can be adjusted by doping Cd O with Ga₂O₃,and the electrical properties of Ga₂O₃can also be improved.For this reason,the theoretical and experimental studies of Cd-doped Ga₂O₃are carried out in this paper.Theoretical aspects:In this paper,the important effect of Cd doping on the change of the photoelectric properties ofβ-Ga₂O₃crystals has been verified by using the density flooding theory and first-principles calculations.It is found that the ionic radius of doped Cd²⁺is larger than that of Ga³⁺,which leads to a significant increase in the cell volume of dopedβ-Ga₂O₃and an increase in the total energy of the system.At the same time,Cd doping can effectively adjust the forbidden band width ofβ-Ga₂O₃,making the band gap of(Cd_xGa_1-x)₂O₃system show a trend of first decreasing and then increasing;and with different amounts of Cd doping（12.5%,25%,50%）,the material properties of the doped system(Cd_xGa_1-x)₂O₃change,from the indirect band gap material ofβ-Ga₂O₃first to a direct bandgap material（at 12.5%,25%Cd concentration）and then back to an indirect bandgap material（at 50%Cd concentration）.In addition,the doping of Cd reduces the absorption coefficient and reflectivity of the system in a certain energy range（6～25 e V）,thus improving the light transmission.Experimental aspects:In this paper,(Cd_xGa_1-x)₂O₃（x=40%,53%,68%,85%）films with various ratios of concentration were prepared by rotational sputtering of Cd O targets and Ga₂O₃targets using RF magnetron sputtering method.The photovoltaic properties,surface morphology and crystal structure of the films were then characterised in detail and comprehensively analysed.（1）The(Cd_xGa_1-x)₂O₃（x=40%,53%,68%,85%）films prepared at room temperature were amorphous with smooth and flat surfaces,and the optical band gap decreased as the percentage of Cd elements within the films increased;and Cd was able to increase the carrier concentration and electron mobility of the(Cd_xGa_1-x)₂O₃films.（2）Subsequent post-annealing treatments of(Cd_xGa_1-x)₂O₃（x=40%,53%,85%）sample films were carried out at 0℃,300℃,500℃,700℃and 900℃in an oxygen atmosphere.For the(Cd_0.4Ga_0.6)₂O₃films with a Cd content of approximately x=40%the Cd content within the films decreased after annealing and the films showed a significant increase in transmittance and progressively better crystalline quality.When the annealing temperature is 900℃,the crystalline phase of(Cd_0.4Ga_0.6)₂O₃films appears as a cubic phase,and after annealing,the surface of the sample is relatively dense and flat,and the thickness of the film is uniform.For(Cd_0.53Ga_0.47)₂O₃films with Cd content x=53%,the optical band gap becomes larger and the resistivity decreases sharply after annealing.The EDS results indicate that the Cd concentration within the films is basically constant from 0to 700℃,and the percentage of Cd elements decreases at 900℃.In addition,the(Cd_0.53Ga_0.47)₂O₃films are in cubic phase at 900℃,and their chemical composition of Cd Ga₂O₄,while a monoclinic phase of gallium oxide peak sites is present.For(Cd_0.85Ga_0.15)₂O₃films with Cd content x=85%,the band gap of the film becomes larger after annealing.The electron mobility of the films increased with increasing annealing temperature.（3）Finally,the effect of Cd content x on(Cd_xGa_1-x)₂O₃（x=40%,53%,68%,85%）films was investigated at 900℃.EDS tests revealed a clear trend of decreasing Cd content within the films at 900℃.As x decreases,the crystalline quality of the films increases and grain characteristics become apparent,while the optical absorption edge moves towards shorter wavelengths.