Magnetoreflection of Ion-Implanted Bismuth
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MAGNETOREFLECTION OF ION-IMPLANTED BISMUTH E. M. KUNOFF,* B. S. ELMAN,* M. S. DRESSELHAUS*+ Department of Physics; +Department of Electrical Engineering and Computer Science; Massachusetts Institute of Technology, Cambridge, MA 02139 USA
ABSTRACT Bi single crystals have been implanted with isoelectronic ions (As, Sb and Bi) and the electronic structure of these implanted materials has been studied using the magnetoreflection technique. Since the ion penetration depth and optical skin depths are of roughly the same magnitude, this technique provides a sensitive test for implantation-induced changes in the electronic structure. Explicitly, the magnetoreflection spectra show changes in lineshape, resonant frequency and in some cases the introduction of Landau level transitions forbidden in unimplanted bismuth. In particular, implantation-induced changes in the resonance lineshapes indicate an increase in plasma frequency as either the fluence of the implants or the ion size is increased. Further analysis of the data shows that the Lax model, which accounts for the magnetoreflection spectra of unimplanted bismuth, is equally applicable to bismuth implanted with isoelectronic ions. Our results yield measurable changes in the L-point band gap and smaller relative changes in the band parameter combination E /m . The mechanism responsible for these changes in the efectronic structure of bismuth is suggested.
INTRODUCTION The magnetoreflection technique has been used successfully to study changes in the electronic structure of bismuth induced by pressure [1] and doping [2]. Since ion implantation strains the lattice by introducing a foreign species into the crystal, we expect this experiment to give similar information for the case of ion implanted bismuth. In this paper, we discuss the results obtained using the magnetoreflection technique to study the effect of ion implantation on the electronic properties of bismuth. We have chosen isoelectronic implants, As, Sb and Bi, to isolate one of the possible modifications due to the presence of impurity atoms in the lattice; we consider the effect of strain and not charge transfer. Fluence was also varied to allow study of the effect of different concentrations of implanted atoms in the lattice. Results show that increasing both the size and fluence of the implanted atoms causes a small decrease in the band gap at the L-point, slightly increases the momentum matrix element coupling the valence and conduction bands, and significantly increases the number of carriers. Some of the theory necessary to analyze our results is presented in section II. A description of the experimental details comprises section III and we present our experimental results and analysis in section IV. The implications of those results are discussed in section V, especially in relation to the model for the bismuth electronic band structure proposed by Abrikosov and Falkovskii [3] and Abrikosov [4] and modified by McClure [5] which is based on the deviation of the bismuth lattice from cubic symmetry.
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