Optical and structural properties of semi-insulating polycrystalline silicon thin films
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S. Lombardo, R. Reitano, and S.U. Campisano Dipartimento di Fisica dell'Universita' di Catania, Corso Italia 57, 95129 Catania, Italy (Received 11 April 1994; accepted 13 October 1994)
Optical properties of semi-insulating polycrystalline silicon (SIPOS) thin films containing 30 at. % oxygen atoms are investigated in the near ultraviolet, visible and infrared region to improve knowledge on the structure and chemical bonding of these mixtures. An effective medium approximation model is used for a microscopic investigation of the oxide species involved as a function of the annealing temperature (600-1200 °C). The results are compared with other optical spectroscopies (infrared and Raman) and with transmission electron microscopy to give a selected picture of the pure and oxide components.
I. INTRODUCTION Semi-insulating polycrystalline silicon (SIPOS) is widely used as a passivation layer and as a resistive field shield in high voltage Si devices. 12 Its inhomogeneous nature gave rise to a large variety of investigations to establish both the microstructure and the conduction mechanisms. 3 ^ Despite the large number of studies on SIPOS, there are still basic aspects with respect to structural and electric transport properties to be clarified. One reason for this is that the microstructure of CVDdeposited SIPOS depends sensitively on the preparation parameters, i.e., the deposition and annealing temperature. Transmission electron microscopy (TEM) of plan view samples showed that the as-deposited material at 620 °C consists of an amorphous single phase, while material deposited at 660 °C shows a crystalline or amorphous structure, depending on composition.7 Further investigations showed a large variety of local Si-Si and S i - 0 bonds whose amounts depend on the composition and on the post-deposition annealing temperature.8 We would like to focus our investigation on some samples deposited in such a way as to obtain ~ 3 0 at. % O. Two opposite pictures were drawn for the material annealed above 900 °C: one describes SIPOS as a continuous mixture of crystalline and amorphous silicon containing small clusters of SiO2, while the other picture describes it as small silicon grains completely surrounded by SiOx shells. Recent electrical measurements suggested that 30 at. % O SIPOS samples annealed at 900 °C are characterized by a shell structure in which each Si grain is surrounded by a SiOx shell approximately two monolayers thick.9 At high annealing temperature the SiO^ shell collapsed into small clusters, and the material showed a mosaic microJ. Mater. Res., Vol. 10, No. 4, Apr 1995
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structure, characterized by large Si grains in direct contact and SiO2 islands. Scientific interest in SIPOS is growing rapidly since it has been shown that this material, doped with Er by ion implantation, shows strong luminescence at 1.5 fim wavelength even at room temperature when it is pumped with an Ar + ion laser (0.514 /xm). 10 For this reason it became interesting to investigat
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