Study of Vacancy and Impurity Complexes in Si Solid-Phase Epitaxial Crystallization with Positron Annihilation Spectrosc
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Study of Vacancy and Impurity Complexes in Si Solid-Phase Epitaxial Crystallization with Positron Annihilation Spectroscopy Claudine M. Chen, Stefano Rassiga1, Marc H. Weber1, Mihail P. Petkov1, Kelvin G. Lynn1 and Harry A. Atwater Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125 1 Dept of Physics, Washington State University, Pullman, WA 99164 ABSTRACT We investigate the residual vacancy defect species after crystallization of amorphous Si (a-Si) by solid phase epitaxy (SPE). To this end, we correlate the total and electronically-active doping concentrations measured by secondary mass spectrometry and spreading resistance analysis, and data from positron annihilation spectroscopy (PAS), which is sensitive to openvolume defects. Float-zone silicon substrates were implanted with boron, phosphorus and both phosphorus and boron ions to create nonuniform doping profiles at degenerate doping levels, after an amorphization step by 29Si+ ions. Samples were vacuum annealed at 600°C to induce SPE, and the SPE rate was measured by time-resolved reflectivity. PAS was used for identification of the impurity-defect complexes. Momentum-resolved PAS measurements enable the detection of phosphorus-vacancy (P-V) and oxygen-vacancy (O-V) complexes. INTRODUCTION Solid phase epitaxy (SPE) [1] is a useful technique for low temperature processing of photovoltaics, and for activation of ion-implanted dopants in submicron microelectronic devices. The study of various point defects formed in this process is important for optimizing device performance. Positron annihilation spectroscopy (PAS) [2], with its high sensitivity to openvolume defects, is uniquely suited to investigate defects too small to see by electron microscopy, such as vacancies and vacancy-impurity complexes. EXPERIMENT Samples with an amorphous silicon (a-Si) layer atop a crystalline silicon (c-Si) layer were made by amorphizing float-zone (FZ) Si wafers (, p-type, 200-300 Ω -cm) by ion implantation of 29Si at l-N2 temperatures. Two implantations, with energies of 70 keV (dose, 2×1015 at/cm2) and 200 keV (dose, 6×1015 at/cm2), amorphized Si to a depth of ~300 nm below the surface; 29Si was used to prevent CO and N2 (e/m=28) contamination during implantation. Undoped Si, P-doped Si, B-doped Si, and compensated P&B-doped Si cases were investigated. Dopants were implanted into the amorphized layer such that the peak of the profile occurred at a depth of approximately 300 nm, thus the a-Si/c-Si interface encountered a decreasing doping concentration as crystal growth proceeded. TRIM (Transport of Ions in Matter) simulations were used to select the implant energies. The P-doped samples were generated by phosphorus implantation at an energy of 200 keV with a dose of 1.3×1014 at/cm2 (peak concentration, 7×1018 at/cm3). The B-doped samples were generated by boron implantation at an energy of 72 keV
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with a dose of 1.2×1014 at/cm2 (peak concentration, 7×1018 at/cm3). The compensated samples were generated by both phosph
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