Defect States in Boron and Phosphorus Doped a-Si:Ge:H Alloys
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DEFECT STATES IN BORON AND PHOSPHORUS DOPED a-Si:Ge:H ALLOYS F. FINGER and W. FUHS Philipps-Universitat Marburg, Renthof 5, D-3550 Marburg, F.R.G. ABSTRACT Paramagnetic defects in the mobility gap of doped a-Si:Ge:H alloys with a Ge-content of 20% and 30% are studied by electron spin resonance (ESR and LESR). The singly occupied states of Si- and Ge-dangling bonds are found to be centered around midgap with no detectable difference in energy position. The ESR-spectra exhibit at least two paramagnetic resonances in both n- and p-type samples which are attributable to singly occupied tail states. At a given distance of the Fermi level from the mobility edge of the conduction band the density of states is much higher in the alloys than in a-Si:H. In phosphorus doped material hyperfine structure is observed which arises from two different phosphorus states: fourfold coordinated phosphorus (PO).and twofold coordinated phosphorus (PO). INTRODUCTION In recent years hydrogenated silicon-germanium alloys, a-Sil.xGe,:H, have attracted much attention due to their potential application in photovoltaics as low band gap material. In general the photoelectric properties of these alloys are inferior to those of a-Si:H [1-3]. One of the reasons probably is an increase in micro-structure with increasing content of Ge [4] and indeed the higher photoconductivity of fluorinated alloys has been attributed to reduced microstructure [5]. It is accepted that in addition the density of gap states is strongly enhanced as one adds Ge to a-Si:H which strongly affects the lifetime of the carriers [6,7]. In a-Si:H [8] and a-Ge:H [9] valuable information on the nature and distribution of gap states has been obtained in studies of electron spin resonance (ESR) on a series of samples where the Fermi level was shifted by doping. A similar study on Si/Ge-alloys with x = 0.3 has been published recently by Stutzmann et al. [10]. In this work we report on ESR and light-induced ESR (LESR) in two series of specimens with x = 0.2 and 0.3 where the Fermi level is shifted by doping and also by defect creation and annealing. The purpose of this study was on the one hand to gain information on the energy distribution of paramagnetic states in the gap of the alloys and on the other hand to investigate the influence of alloying on the tail states. EXPERIMENTAL DETAILS The samples were deposited in a capacitively coupled glow discharge system from gas mixtures of silane and germane diluted in hydrogen or helium at a substrate temperature of 490 K. Doping was achieved by adding small amounts of phosphine or diborane to the discharge with concentrations up to 1%. The films were studied by Auger electron spectroscopy, hydrogen evolution, optical absorption and electronic transport experiments for compositionalstructural and electronic characterization [4,7]. For transport- and optical experiments the films were grown on quartz substrates. The electrical conductivity was measured in gap cell configuration using chromium contacts and the position of the Fermi level wa
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