Structural and Electronic Disorder in Boron, Nitrogen and Phosphorus Doped 50kHz PECVD a-Si:H
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STRUCTURAL AND ELECTRONIC DISORDER IN BORON, NITROGEN AND PHOSPHORUS DOPED 50kHz PECVD a-Si:H ETIENNE BUSTARRET, FREDERIC VAILLANT* AND BERNARD HEPP** * L.E.P.E.S./ C.N.R.S., B.P.166X, 38042 Grenoble, France **L.C.R. Thomson C.S.F., Corbeville, B.P. 10, 91401 Orsay, France
ABSTRACT The Urbach energy E0 of undoped, Boron-, Nitrogen- and Phosphorus-doped a-Si:H deposited at 320'C from hydrogen-diluted silane mixtures has been measured by PDS. Its variations with impurity concentrations were related to those of the optical gap, of the defect density and of the width of the Raman TO peak. Taking into account the change in Hydrogen bonding in these materials, the physicochemical nature of the Valence Band Tail (VBT) states is discussed. A formal correlation between the dispersion of the Si-Si bond angle distribution and the width of the VBT is proposed within the Keating formalism. INTRODUCTION: DOPING REGIME AND ALLOYS The apparent contradiction between the production of hydrogenated Silicon-based amorphous alloys where the valencies of all elements are satisfied at all compositions and the possibility of doping electrically these materials in situ has been resolved a few years ago by the "modified 8-N rule" approach [1]. Unlike most studies devoted to doped materials, we shall here focus on the overall response of the matrix to the incorporation of all impurity atoms rather than on the small fraction which are efficient for the Fermi level (EF) shifts and the gap DOS modifications. Photothermal deflection, polarized Raman [2] and optical transmission spectroscopies have been applied at room temperature to the same one micron thick samples. Mean energy values (extrapolated optical gap EG and TO mode peak frequency) as well as dispersion parameters (E0 or upper half width at half maximum 17/2 of the TO Raman peak) have thus been determined, while the solid phase impurity contents were measured by infrared spectroscopy (H) or by SIMS (B,N,P) in layers deposited simultaneously on other substrates than fused silica. Because the subgap optical absorption is not very sensitive to EF shifts, the density NR of optically active deep defects was deduced from the PDS spectra [3]. As seen on the log-log plot of fig. 1, the solid phase incorporation of Phosphorus is slightly supralinear at all measured gas phase ratios, while that of Boron (resp. Nitrogen) shifted from this to a square root dependence above a diborane (resp. ammonia) to silane ratio of 100ppm (resp. l%).Such a square root dependence has been observed [4] in our nitrides up to N/Si values of 5 and indicates that a chemical reaction of the type AB >A+B is limiting the growth of the alloy. On the other hand, the nearly linear behavior observed at lower gas ratios is expected for our low frequency discharge, where the ionic bombardment is rather intense. The H/Si ratio schematically represented in fig. 1 decreases from 21 to 18% with Phosphorus content, and increases with both Boron and Nitrogen concentrations at gas ratios greater than 10ppm. Our distinction of two r
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