RF-sputtered Ge–ITO nanocomposite thin ﬁlms for photovoltaic applications
Authors : G.H. Shihn, C.G. Allen, B.G. Potter Jr.
Nanocomposite thin ﬁlms, composed of a germanium nanocrystalline phase embedded within a tindoped indium oxide (ITO) matrix, were produced using a multisource, sequential, RF-magnetron sputter deposition technique. The inﬂuence of nanocomposite structure on the resulting optical absorption and carrier transport properties was investigated in the context of the use of such materials as functional elements in thin ﬁlm photovoltaic architectures. Deposition controls and post-deposition thermal anneals were successful in modifying the phase assembly of the nanocomposites, enabling the manipulation of Ge volume fraction, nanocrystallite size and morphology, and spatial distribution within the ITO embedding phase. Modiﬁcations in semiconductor nanostructure were correlated with changes in nanocomposite spectral absorption that were consistent with quantum-size-induced variation in Ge absorption onset energy, despite the close agreement in electron afﬁnity between the Ge and ITO components. This suggests the formation of a high band-gap (low electron afﬁnity) interfacial phase between the Ge and ITO components of the nanocomposite. Increased free-carrier (n-type) densities and spectrally resolved photoconductivity were also associated with the presence of the Ge phase. These results emphasize the impact of local and extended length scale structure on properties of importance to photovoltaic performance in semiconductor-based nanocomposites and the utility of the sequential sputter deposition method as a means to manipulate nanocomposite structure.