Sub-gap Photoconductivity in Germanium-silicon Films Deposited by Low Frequency Plasma
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Sub-gap photoconductivity in Germanium-Silicon films deposited by low frequency plasma. Andrey Kosarev and Francisco Avila Electronics Department, Institute National for Astrophysics, Optics and Electronics, Puebla, 72840, Mexico. ABSTRACT (GexSi1-x:H) films are of much interest for many device applications because of narrow band gap and compatibility with films deposited by plasma. However, electronic properties of GexSi1-x:H films for high Ge content x > 0.5 have been studied less than those of Si films. In this work, we present a study of sub-gap photoconductivity (σpc) in GexSi1-x:H films for x = 1 and x = 0.97 deposited by low frequency plasma enhanced chemical vapor deposition (LF PECVD) with both various H-dilution (RH) during growth (non-doped films) and boron (B) incorporation in the films. Spectra of sub-gap photoconductivity σpc(hν) were measured in the photon energy range of hν = 0.6 to 1.8 eV. σpc(hν) spectra were normalized to constant intensity. For hν < Eg two regions in σpc(hν) can be distinguished: “A”, where σpc is related to transitions between tail and extended states, and “B”, where photoconductivity is due to defect states. σpc(hν) in ”A” region showed exponential behavior that could be described by some characteristic energy EUPC similar to Urbach energy EU in spectral dependence of optical absorption. EUPC > EU was observed in all the films studied. This together with higher relative values (i.e. normalized by the maximum value at hν = Eg) for photoconductivity comparing with those for α means that mobility-lifetime product (µτ) depends on photon energy µτ = f(hν) that was determined from α(hν)and σpc(hν). µτ(hν) increases by factor of 20 to 40 depending on the sample with reducing hν from 1.1 to 0.7 eV. In some samples, this dependence was monotonous, while in others demonstrated maxima related to both interference and density of states. Effects of both RH and boron incorporation have been found and are discussed. INTRODUCTION Hydrogenated germanium film (Ge:H) deposited by plasma enhanced chemical vapor deposition (PE CVD) are of much interest because of possible applications in devices such as long wavelength part of tandem solar cells, photo-detectors suitable for optical communication, thermo-voltaic devices, IR micro-bolometers etc. Some devices with Ge:H films have been reported [1, 2], where Ge:H was used as the intrinsic layer in p-i-n structures, however, doped nand p-layers were fabricated of silicon. Dalal et al. [3] have reported the important role of ion bombardment during growth from plasma for high quality Ge:H films. In this respect low frequency (LF) PECVD with its inherent higher ion bombardment than that in conventional RF plasma is very attractive. In our previous studies [4,5] we have demonstrated a LF PECVD fabrication of Ge:H and GexSi1-x:H films with both low tail and deep localized states. It has been also demonstrated that GexSi1-x:H films with x>0.95 have shown superior electronic properties in comparison with those in Ge:H films having practically th
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