Majority Carrier Transport Across Semiconductor Grain Boundaries
- PDF / 393,053 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 41 Downloads / 268 Views
MAJORITY CARRIER TRANSPORT ACROSS SEMICONDUCTOR GRAIN BOUNDARIES S.F. NELSON*, P.V. EVANS", S.L. SASS- AND D.A. SMITH** *Dept. of Materials Science, Cornell University, Ithaca, NY 14853 **IBM T.J. Watson Research Center, PO Box 218, Yorktown Heights, NY 10598 Abstract Majority carrier transport measurements were made across the potential barriers at (100) twist boundaries in silicon. The bicrystals were prepared by hot-pressing single crystals of lightly doped float-zone material, under ultra-high vacuum conditions. The current - voltage measurements were analyzed using combined drift-diffusion and thermionic emission transport mechanisms, and incorporating some inhomogeneity in the charge distribution at the boundary. Evidence has been found for a small, Nd = 3 x 1010 cm-2 , density of mono-energetic defect states near nmidgap, in bicrystals characterized by a variety of misorientation angles. This density is too small to result from the intrinsic structure of the boundary. In addition, no dependence was found on misorientation angle. Introduction A fundamental question in the study of semiconductor grain boundaries is how the intrinsic structure of a boundary influences its electrical properties. An electrically active grain boundary presents a, potential barrier to majority carrier current flow, resulting from majority carriers trapped in localized defect states at the boundary. The origin of the defect states could be intrinsic - "dangling" bonds, or distorted bonds resulting from reconstructions at the boundary; alternatively, the origin could be extrinsic, resulting from impurities or extrinsic dislocations in the boundary plane. This question has frequently been addressed, both theoretically and experimentally, but primarily for symmetrical tilt boundaries. Evidence has mounted suggesting that these boundaries reconstruct with tetrahedral bonding throughout [1,2], and thuis may not be intrinsically active. Electrical activity has been correlated instead with impurity effects, suggesting that the primary role of the boundary structure is in determining the degree of impurity segregation for a given misorientation[3]. The situation is not necessarily the same for twist boundaries, or for random combinations of tilts and twists. Indeed, total energy calculations suggest that the degree of disorder at large angle twist misorientations result in a number of three-fold and five-fold coordinated atoms, and electronic energy levels in the ga.p[4]. In the present study, the electrical activity of a series of twist boundaries in silicon was investigated. Current-voltage (J-V) mneasurements were made, from which could be derived the grain boundary defect state density, Following the methods of Pike and Seager[5]. However, because of the light doping density of our starting material, n-type with 3 Nd = 2 x 1013 cm- , transport across the boundary potential barrier could not be assumed to be dominated by thermionic emission. A treatment combining drift-diffusion transport in the space-charge region and thermionic emissio
Data Loading...
