Variations in the Refractive Index of the Near-Surface Region of Low Energy Hydrogen Ion Bombarded Silicon
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VARIATIONS IN THE REFRACTIVE INDEX OF THE NEAR-SURFACE REGION OF LOW ENERGY HYDROGEN ION BOMBARDED SILICON* R. B. PETTIT AND J. K. G. PANITZ Sandia National Laboratories, P. 0. Box 5800, Albuquerque,
NM87185
ABSTRACT Using ellipsometric analysis, the complex index of refraction of lowenergy, hydrogen ion bombarded, [100] single-crystal silicon was measured as a function of distance from the bombarded surface. The bombardment conditions were a 1600 eV hydrogen beam produced by a Kaufman ion source, 1.4 mA/cm 2 flux, 2 x 1018 ions/cm2 fluence and 275 0 C bulk silicon temperature. These conditions are comparable to the conditions generally reported to result in a substantial increase in the electrical conductivity of polycrystalline silicon solar cell material. The results of this study indicate that the real and imaginary parts of the refractive index of the ion bombarded surface region approach that of the unbombarded substrate at a depth of 50 nm. The refractive index of about the first 10 nm of ion bombarded material is strongly dependent on the bombardment conditions. Variations in the imaginary part of the refractive index indicate that approximately 10% of incident radiation is absorbed by the first 50 nm of modified material. INTRODUCTION Bombarding polycrystalline silicon n/p junctions with hydrogen ions presently appears to be an attractive technique for fabricating costcompetitive solar cell structures. Hydrogen can diffuse from the surface along grain boundaries to depths of 70 to 200 um and passivate dangling silicon bonds which are associated with undesirably high electrical resistivities.[1-4] However, the microstructure of the near-surface region (20-100 nm) of hydrogen-ion-bombarded silicon is extensively modified so that the refractive index of this region could be expected to vary substantially from that of bulk silicon.[5) Information about the refractive index is crucial for specifying an antireflection (AR) coating, which is necessary to optimize the performance of a silicon solar cell, and for determining the amount of solar radiation which can be absorbed in the near-surface region. We have measured the changes in the refractive index through the near surface region of hydrogen-bombarded, [100] single-crystal silicon by measuring the change in ellipsometric parameters as small increments of the surface region were successively removed. This was done by anodizing bombarded samples, etching away the anodic layer, and then evaluating the optical properties of the remaining material. The amount of surface material removed was determined using interferometry. By appropriately modeling these results, the complex index of refraction as a function of depth was determined. The results of this study can be directly compared with the results of earlier studies of the electrical properties [1,2,4] and the microstructure [5] performed at this laboratory with the same ion bombardment system and conditions.
*This work performed at Sandia National Laboratories supported by the U. S. Department of Energy und
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