Microstructural and optical characterization of germanium:indium tin oxide (Ge:ITO) nanocomposite films

• Published in: Materials science & engineering. B, Solid-state materials for advanced technology Solid-state materials for advanced technology, 2010-11, Vol.175 (2), p.150-158
• Main Authors: Allen, C.G., Shih, G.H., Potter, B.G.
• Format: Article
• Language: English
• Subjects: Annealing; Carrier density; Germanium; Indium oxide; Indium tin oxide; Metal–oxide–semiconductor structures; Nanocomposites; Nanomaterials; Nanostructure; Quantum structures; Semiconductors; Thin films
• ISSN: 0921-5107; 1873-4944
• DOI: 10.1016/j.mseb.2010.07.018

The nanophase assembly and resulting optical and electronic properties of Ge:ITO composite thin films, produced by a sequential RF-sputtering deposition approach, were manipulated via deposition conditions and subsequent isochronal thermal anneals. The study examined the combined influences of thermally induced changes in phase crystallinity, semiconductor-phase morphology, and Ge–ITO interfacial structure on properties of relevance to photovoltaic function. A range of Ge-phase spatial distributions within the ITO embedded phase were produced, including isolated Ge nanocrystals and two-dimensional-extended semiconductor structures, as evaluated using cross-sectional transmission electron microscopy. The magnitude of a quantum-confinement induced blue-shift in the Ge absorption onset increased monotonically with increased isochronal anneal temperature and was concomitant to the decrease in connectivity of the as-deposited Ge-phase assembly. Raman spectroscopy, over the range of nanocomposite structures examined, confirmed the evolution of a germanium oxide interfacial structure anticipated to affect both carrier confinement within the Ge and long-range charge transport in the nanocomposite. Shifts in the near-infrared transmission edge with anneal temperature were further correlated, using Hall-effect measurements, with a thermally equilibrated free carrier population. An increased free carrier density in composite films, over that of similarly treated single-phase ITO, was attributed to the presence of the Ge semiconductor-phase. While a general reduction in carrier mobility accompanied the increased carrier density, resistivities of the composite films were found to be largely insensitive to the nanostructure morphology changes and, moreover, were comparable to that of single-phase ITO films produced under similar preparation conditions. Finally, optical excitation at energies resonant with the Ge absorption onset, but below the band-gap of the ITO, resulted in a photoconductive response attributed to photoelectron transfer from the Ge-phase to the ITO.