Injection Electroluminescence from Thin Film p-i-n Structures made from Nanocrystalline Hydrogenated Silicon

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A.A. ANDREEV*, B.Y. AVERBOUCH*, P. MAVLYANOV*, S.B. ALDABERGENOVA**, M. ALBRECHT**, D. STENKAMP**, and H.P. STRUNK** * A.F.loffe Physical-Technical Institute, 194021, St. Petersburg, Russia "**Universitat Erlangen-Nimberg, Institut ffr Werkstoffwissenschaften, Mikrocharakterisierung, D91058 Erlangen, Germany ABSTRACT Nanocrystalline silicon films are prepared by plasma enhanced chemical vapour deposition of silane under the conditions of high hydrogen dilution (3:100). The film structure consists of nanoclusters 0.8 to 5 nm in size (volume fraction 30%) embedded in an amorphous matrix. The Tauc gap of the amorphous matrix is 1.95 to 2.05 eV depending on deposition parameter. These films are characterized as regards photoluminescence (PL) and, prepared to p-i-n structures, electroluminescence (EL). The PL and EL agree in (i) luminescence peak at 1.9 eV, i.e. small Stokes shift, (ii)almost no temperature dependence between 77 K and 293 K, (iii) fast kinetics with time constant of a few 108 s. These data can be understood in terms of quantum confinement in Si nanocrystallites smaller than around 2 nm. The EL in addition exhibits a luminescence band extending up to 3 eV, which can be interpreted by interband transition due the hot carriers. INTRODUCTION Because of the indirect fundamental bandgap monocrystalline Si shows only weak band-toband emission in the near infrared. The recent developments in the field of microscopically structured silicon in the form of porous silicon [1,2] have opened a new way to obtain bright interband emission. This way is based on the quantum confinement effect for small silicon clusters, which leads to change in nature of the optical transitions. Therefore, it seems to be quite attractive to study other possible approximations to produce microstructured silicon. One of the most promising technological direction is the preparation of microcrystalline or nanocrystalline silicon (diameter in the order of nanometer) in a matrix of amorphous hydrogenated silicon. The main experimental effect of the quantum confinement is the blue-shift of the absorption edge. Theoretical models based on the effective mass approximation have established that for crystallites with of 3-5 nm phonon assisted transitions are dominant and for smaller crystallites direct optical transitions become allowed [3]. According to [4], emission in the energy range of 1.8 2.8 eV could be possible only in very small two or three-dimensional structures and fast optical properties in the" nanosecond range are only possible for crystallites or wires with diameters smaller than around 1 nm. Thus, the size of 1 nm seems to be a critical value for nanocrystalline Si, where one would expect unique properties. In this paper, we present experimental results on microstructure, optical absorption and luminescent properties of Si:H films prepared under conditions of high hydrogen dilution of silane. This material exhibits an optical gap up to 2.05 eV as is also discussed in the literature [5,6]. This new modification of amorphous wide gap

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