Characterization of Dielectric Layers on Hydrogen Passivated Si Surfaces

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C-IARACTERIZATION OF DIELECTRIC LAYERS ON HYDROGEN PASSIVATED SI SURFACES J.A. Gregory, C.E. Dub6, J.I. Hanoka, Z.Y. Vayman Mobil Solar EnergyCorporation, 16 Hickory Drive, Waltham, Massachusetts 02254 ABSTRACT We have Investigated the layer formed on Si surfaces passivated by a Chemical, Kaufman Ion source using mixtures of hydrogen and hydrocarbons. optical, and electronic techniques were employed to characterize this layer The evidence Indicates and Its effect on passivation of defects in the Si. (H). It that the layer Is a dielectric and probably has the formula Si C does not appear to Impede the passivation of polycrystalline S0,(%lf)it does affect the reflectivity of the Si material, and also the barrier height In Schottky junctions. We suggest that a similar layer may be formed during other hydrogen/hydrocarbon plasma treatments of SI. INTRODUCTION Hydrogen passivation has been shown to be an effective means of increasing the bulk minority carrier lifetime [I] and decreasing the barrier height between adjacent grains in polycrystalline sil icon [2]. These changes In electronic properties have been particularly striking in their effect on The means by which the efficiency of polycrystalline solar cells [3,4]. hydrogen passivates defects in solar cells, i.e., by elimination of recombination at grain boundaries and dislocation arrays has become clearer [5] but the question of the Interaction of hydrogen with the surface of the Si Is still poorly understood. The work reported here was undertaken to learn more about the effect of hydrogen and hydrocarbon Ion beams on the chemical and physical properties of SI surfaces. Hydrogen and hydrocarbon Ions were generated using a Kaufman ion Techniques used to characterize the surface of the Ion bombarded Si source. Included ESCA, ellipsometry, reflectivity from the near-UV through the IR, and Schottky junction barrier height measurements. This combination of chemIcal, optical, and electronic measurement techniques provided details about changes In the structure of the SI surface and permitted studying these changes as a function of variations In both the electrical parameters of the Ion source and the composition of the feed gas. The minority carrier diffusion length of polycrystalline blanks that had been subjected to Ion beam treatment was used to determine If the degree of passivation of defects depended on the hydrocarbon content of the beam. The analyses have Indicated that the Si surface has a disordered layer of silicon carbide and that the extent of this layer depends on the particular beam and gas conditions. The Inclusion of small amounts of hydrocarbons In the hydrogen Ion beam has a negligible effect on the Increase In the minority carrier diffusion length of polycrystalline blanks and so does not seem to Interfere with the bulk passivation. EXPERIMENTAL The SI substrates used in this work were either Wacker CZ pieces with (110) orientation or EFG nonagon blanks [6], with both groups doped p-type to a level of 3-5 Q-cm. The SI pieces were etched In 4-0-1 for

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