Optical Properties of Novel Vibronic Bands in Electron-Irradiated Tin Doped Silicon
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OPTICAL PROPERTIES OF NOVEL VIBRONIC BANDS INELECTRONIRRADIATED TIN DOPED SILICON J. H. SVENSSON, B. MONEMAR, B. G. SVENSSON* Department of Physics and Measurement Technology, Institute of Technology, Link6ping University, S-581 83 Link6ping, Sweden *Department of Solid State Electronics, The Royal Institute of Technology, P. 0. Box 1298, S-164 28 Kista, Sweden ABSTRACT The optical absorption of two new electronic transitions in silicon doped with tin has been investigated. At low temperatures two no-phonon lines are observed at 2755.3 and 4112.2 cm-1, each with strong coupling to a single quasi-localized vibration in the excited electronic state. These vibrations have quantum energies of 69.6 and 70.2 cm- 1, respectively. At higher temperatures coupling to thermally excited vibrational states in the ground electronic states is observed for both lines. The transition with the no-phonon line at 4112.2 cm- 1 has been studied in detail and is found to be well described using the adiabatic and Condon approximations. The optical properties of the two transitions are found to be quite similar. Moreover the relative intensities of the two lines are found to be dependent on the optical excitation conditions. INTRODUCTION It has been demonstrated [1] that vacancies created by electron-irradiation of silicon at room temperature are highly mobile in the lattice. Impurity elements such as tin and germanium have been found to trap vacancies efficiently in silicon. The tin-vacancy defect has been found to be stable at room temperature [2,3], whereas
the germanium-vacancy complex dissociates at approximately 200 K [4]. The electronic ground state of the tin-vacancy defect has been studied directly with Electron Paramagnetic Resonance (EPR) [3]. Moreover the defect has been found to be stable at temperatures up to approximately 470 K,observed both directly via the decrease in the EPR signal, and indirectly as the cause of an increase in the
intensity of infrared absorption bands due to other vacancy related defects [5]. In this work we present results from infrared absorption studies of electronirradiated silicon doped with tin. The ordinary tin-vacancy defect is found to be inactive with respect to infrared absorption, but during isochronal annealing of the samples previously unreported transitions are observed. The transitions emerge at annealing temperatures higher than 470 K, and are found to disappear at temperatures exceeding 580 K. We have focused on the optical properties of these transitions. EXPERIMENTAL The samples are Czochralski-grown phosphorus-doped silicon with a concentration of tin atoms of approximately 5x10 18 cm- 3 . Irradiation with 2 MeV
electrons with a dose of 2x10
18
e-/cm 2 was done at room temperature. All
absorption measurements were performed with a BOMEM DA3.26 Fourier Transform Infrared Spectrometer (FTIR) fitted with a liquid nitrogen cooled indium-
antimonide detector. A halogen lightbulb source was filtered with silicon and germanium wafers held at room temperature. Mat. Res. Soc. Symp. Pr
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