Study On The Properties Of Indium Doped Cadmium Oxide Transparent Conductive Films Prepared By Filtered Cathodic ARC Deposition

Cadmium oxide (CdO) is an important transparent conductive oxide (TCO)material. However, the optical bandgap of undoped CdO is relatively small inducingthe narrow transparent range, adding with the relatively low visible transparency,which strictly limits its wide applications as a TCO material. Doping cansignificantly widen the optical bandgap, and can also improve the conductivity andthe transparency effectively. Nevertheless, research on doped CdO films isinsufficient. Much controversy on its fundamental properties exists and systematicstudy on its optical and electrical interactions as well as conduction band dispersionsis seriously lack.To solve the aforementioned problems in CdO films, high quality indium dopedcadmium oxide (CdO:In) films were deposited by filtered cathodic arc deposition(FCAD) in this study.(1) The effects of indium doping concentration, substratetemperature, oxygen partial pressure, and substrate material on film structural,electrical, and optical properties were systematically studied.(2) Subsequently, theimprovements of electrical and optical properties of CdO:In films via annealing indifferent atmospheres were also investigated.(3) Finally, the electrical and opticalinteractions, as well as the conduction band dispersions, were studied by usingdifferent theoretical models.It is observed that all of the CdO:In films are phase-pure with featuresassignable to cubic CdO. No In2O3, CdIn2O4or other phases is observed. Thecrystalline structure of CdO:In films is dependent on the growth parameters:(1) Thevariation of In concentration and oxygen pressure induces the change of thepreferred growth orientations in CdO:In films.(2) Increasing substrate temperaturecan not only increase the grain size, but also can improve the alignment of grains.(3)The MgO buffer layers on glass substrates provide the template for CdO growth,which induces the diverse growth orientations of CdO. On sapphire substrates, theAl atoms in the substrates diffuse into the MgO buffer layers and new MgAl2O4structure forms, resulting in the improvement of crystal quality of CdO:In films. TheCd and In atoms in the films are oxidized and a small content of oxygen vacanciesexists in the CdO:In films.CdO:In films show excellent electrical conductivity and optical transparency.The conductivity is closely related to the growth parameters:(1) With increasing Inconcentration, the resistivity decreases at first and then increases slightly, and theelectron mobility increases at first and subsequently decreases.(2) Increasing oxygen pressure reduces the content of oxygen vacancies in the films, resulting inthe decrease of carrier concentration, which leads to the increase of electronmobility.(3) As the substrate temperature increases, the grain boundary scatteringweakens, which results in the increase of electron mobility.(4) The MgO bufferlayers on glass substrates reduce the resistivity of CdO:In films and improve themobility effectively. On the contrary, on sapphire substrates, the MgO buffer layersincrease the resistivity and cause the decrease of electron mobility. CdO:In filmsexhibit excellent optical transmittance and wide transparent wavelength range. Meantransmittance of230nm thick CdO:In films is over80%in500~1250nmwavelength range. By using the derivative of transmittance, the optical bandgaps ofCdO:In films are about2.5eV~3.1eV. It has been observed that optimized CdO:Infilms can be obtained at230℃and7mTorr with1.2at.%In doping by filteredcathodic arc deposition.The electrical properties of CdO:In films can be improved significantly byannealing. After annealing, the grains of undoped films grow up and its opticaltransmittance improves. For CdO:In films, the crystal length of films decreases andthe absorption edge blue shifts. After annealing in air, the resistivity in the lowdoping level films decreases, while in high doping level films it increases. Whenannealing in nitrogen, the resistivity of CdO:In films decreases and the Hall mobilityincreases.The valence and conduction bands of CdO:In show evident nonparabolicity.The extensively misused Tauc relation to obtain optical bandgap is based onassuming parabolic valence and conduction bands. For CdO:In films, of whichobvious nonparabolic valence and conduction bands are already observed, the Taucrelation overestimates the optical bandgaps. The band filling effect in these CdO:Infilms is about0.5~1.2eV, while the bandgap renormalization is about0.1~0.3eV.The dispersion of conduction bands and the Fermi energy in CdO:In films calculatedby using Kane’s two bands perturbation theory agree well with the experimentalresults. Appropriate random phase theory can describe the bandgap renormalizationeffectively in these arc-grown CdO:In films. Simply using the effective mass at theFermi level to calculate the nonparabolic Fermi energy would underestimate theresults for CdO:In films.These indium doped cadmium oxide films deposited by filtered cathodic arcdeposition show excellent electrical conductivity and optical transparency. It isimportant to note that the transparent wavelength region of these indium dopedcadmium oxide films agrees well with the solar spectra. Therefore, these highquality indium doped cadmium oxide films are potentially suitable for high effeciency multijunction solar cells and other photovoltaic devices.