Electrical and Optical Properties of Titanium, Vanadium, Molybdenum, and Tungsten Related Defects in Silicon
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ELECTRICAL AND OPTICAL PROPERTIES OF TITANIUM, VANADIUM, MOLYBDENUM, AND TUNGSTEN RELATED DEFECTS IN SILICON K.Schmalz*, H. G. Grmmeiss*,**, H.Pettersson*, and L. Tilly** * Institute of Semiconductor Physics, POB 409, 0-1200 Frankfurt (Oder), Federal Republic of Germany; **Department of Solid State Physics, University of Lund, Box 118, S-22100 Lund, Sweden ABSTRACT Recent studies of the deep transition-metal centers Ti, V, Mo and W in silicon are discussed, which have been investigated using junction-space charge techniques. The changes in the Gibbs free energy, enthalpy, and entropy due to electron or hole excitation of these transition metal related levels are presented. The good agreement of the Gibbs free energies with the optical threshold energies of the corresponding photoionization cross sections suggests negligible lattice relaxation. For the Ti-doped samples three energy levels at EC-0.065 eV, Ec-0.295 eV and Ev+0.255 eV at 80 K were observed. For Mo and W the energy levels are located at Ev+0.298 eV and Ev+0.379 eV, respectively, at 80 K.Three energy levels of the V-related centers exist at EC-0.207 eV, EC-0.483 eV and Ev+0.356 eV at 0 K. A structure in the low-energy part of the spectral distribution of the level EC-0.483 eV is assumed to be due to excited states and shown to be in good agreement with effective-mass theory. Previous assignments of the energy levels should be reconsidered.
INTRODUCTION Transition metals (TM) in silicon are considered to be of great technological interest since their electronic properties influence the function of silicon devices. Although the electronic properties of several TM, such as gold, platinum and iron have been studied in detail to obtain the energy levels in the forbidden gap as well as their charge states [1], the electronic properties of several TM in Si are yet not completely known. In particular, the electronic properties of the TM of the 3d-, 4d- and 5d-series are not fully understood. The possible charge states and energy levels of 3d- and 4dTM were obtained by theoretical studies performing spin-unrestricted ab initio LMTO Green-function calculations [2,3]. For interstitial V and Ti the theory predicts a lowspin ground state. EPR data were measured only in the case of single positive charge state of Ti [4] and the double positive charge state of V [5], but for these charge states high-spin and low-spin ordering coincide. The good agreement between the calculated donor- and acceptor levels for interstitial 3d-TM in Si in comparison with experimental data has been considered Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society
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as a strong argument for Ti and V having low-spin ground states. Theory predicts that the 4d- transition metals are preferentially dissolved in Si on substitutional sites, and that according to the switch between the interstitial Cr and V a change from high- to low-spin ground states occurs between Cr and Mo [3]. In this paper we will therefore focus our interest to the transition metals Ti, V, Mo and W. Sinc
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