Page - 156 - in Photovoltaic Materials and Electronic Devices

The results show a direct correlation between the oxygen concentration and the optical transmittances of the films and an inverse relationship with the electrical conductivity of the ITO films. These results are in agreement with observation from previous studies [47,48]. Figure 4A for 8 min etch, and Figure 4C for 5 and 8 min etch showed a transmittance cut-off wavelength around 350 nm indicating the absorptionedge. 4. Discussion The results presented provide further insight on the interaction of the most common fabrication variables that influence the electrical and optical properties of ITO films for PV and other opto-electronic applications. There is evidence of a strong correlation between oxygen concentration and both the resistivity and transmittancesofRFsputterdepositedITOfilms. Theprocessingconditionshave a strong bearing on the structure of the ITO films. The (222) lattice planes are preferred in films grown in oxygen rich ambient whilst more lattice planes; (211), (222), (400), and (440) are observed in films grown under less oxygen or oxygen deficient conditions. Wan et al. have reported the (211), (400) and (440) planes reflections as being associated with ITO films processed under high RF power in an oxygen deficient atmosphere [35]. From the structural analysis, the exhibited (222) reflection clearly indicatedthe cubic indiumoxide formation. Thefilm growth rate decreased with increased O2 concentration resulting in a much thicker ITO critical thickness (amorphoustopolycrystalline transitionthickness) for the0sccm RF sputtered films. The overall film is mixed phase crystalline and amorphous in nature. Thenoise isdueto theultra-thinporousandamorphousfilmleftonce the top crystalline film is etched off. This is not observed in the 0.4 and 1 sccm films because the increased oxygen composition results in reduced growth rates giving filmsthataremorecrystalline innaturewithamuchthinnercritical thickness. Thelatticeparametersandlatticedistortionareseentovarycloselywithoxygen concentration in the sputter chamber and the length of the etching process. The differentoxygen–argonratiossputteredITOfilmshavedifferentetchingbehaviors, whichtheneffectedtheirelectricalandopticalproperties. Duringtheetchingprocess, the crystal lattice of ITO films is degraded due to the exchange of bonds between indium oxide with HCl and HNO3. This means that the indium oxide In–O and H–Cl bonds are substituted by In–Cl, In-(NO3)3 and O–H in the ITO surfaces [49]. Thesekindsofreactionsmayreducetheoxygenconcentrationsanddistort structural long range order of the ITO films. This was reflected in the variation of electrical and opticalpropertiesof the ITOfilmswithetchingtime. Asnoevidenceof tinphases were detected, it can be concluded the reactions involving tin phases have negligible effectontheoverall etchratesdescribed in this study. 156