Raman Studies of Heavily Doped Polycrystalline Si Films Prepared by Excimer-Laser-Annealing of Doped a-Si:H
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RAMAN STUDIES OF HEAVILY DOPED POLYCRYSTALLINE Si FILMS PREPARED BY EXCIMER-LASER-ANNEALING OF DOPED a-Si:H
M.E. SAVAGE,* U. JAYAMAHA,* A. COMPAAN,* A. AYDINLI,** & DASHEN SHEN*** *Department of Physics and Astronomy, The University of Toledo, Toledo, OH 43606 "**Departmentof Physics, Bilkent University, Bilkent, Ankara, 06533 Turkey ***Dept. of Electrical and Computer Engineering, University of Alabama, Huntsville, AL 35899
ABSTRACT Extremely heavily doped polycrystalline silicon films were prepared by multiple-pulse XeCI excimer laser annealing of hydrogenated amorphous silicon films. The as-grown films used here included five types: intrinsic, boron-doped, phosphorous-doped, and carbon alloyed with and without boron doping. Raman studies reveal that the annealed films prepared from the boron or phosphorous doped a-Si:H have high carrier activation and display the interference lineshape (Breit-Wigner-Fano) of the discrete phonons (Si-Si and Si-B modes) interacting with the continuum of the single particle electronic Raman scattering. The Raman lineshapes indicate concentrations of ~I x 102 1 cm-3 . This is confirmed by dark conductivities exceeding 100 S/cm in the annealed boron-doped and phosphorous-doped layers. INTRODUCTION Polycrystalline silicon (poly-Si) prepared by pulsed laser annealing or rapid thermal annealing of hydrogenated amorphous silicon has provided interesting material for studies of optical and electronic properties.1,2,3 Poly-Si has key applications in crystalline Si electronic devices, but also for thin-film transistors, 4 and for devices based on a-Si:H such as solar cells. 3' 5 Pulsed laser annealed and rapid thermally annealed epitaxial silicon previously amorphized by ion implantation has provided convenient material for the study of ultraheavy doping 6 and its influence on electronic 7 and vibrational 8 properties. Polycrystalline silicon prepared by laser annealing thin films of a-Si:H grown by glow discharge also can yield heavily doped Si but it arrives at its final state by a route very much different from that of ion-implantation amorphized Si. In this paper we examine by Raman scattering the vibrational properties of a range of XeCI excimer-laser-annealed poly-Si films. The amorphous silicon films used as starting materials include five types: intrinsic, boron-doped, phosphorous-doped, and carbon alloyed films both undoped and doped with boron. The Raman scattering provides information both on the degree of crystallinity and on the degree of electrical activation of the boron and phosphorous dopants in much the same way as Raman scattering in heavily doped crystalline Si.8' 9 Electrical measurements were made on the as-grown and laser-annealed films to confirm general trends in the carrier concentrations. We find that the electrically active dopant concentrations in the polySi may exceed by a substantial amount the solubility limits of the phosphorous and boron dopants in crystalline Si. 10 SAMPLE PREPARATION The films used in this study were prepared in a double chamber, turbom
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