Microscopic Identification of Defects in Semiconductors by Electron-Spin-Resonance and Related Techniques

Abstract

This talk will review and compare very basic intrinsic and extrinsic paramagnetic defect structures which are characteristic for group IIB-VI, III-V and elemental semiconductors. The case of cation vacancy related defects in ZnS and ZnSe, that of antisite defects in GaP and GaAs and that of deep impurity donors in IIB-VI semiconductors and silicon will be treated as representative examples, thus illustrating the possibilities (and limitations) of the ESR technique. THE NUCLEAR LABELElectron-spin-resonance (ESR) can be one of the most efficient techniques to unravel the electronic structure of paramagnetic defects in solids. In semiconductors, such defects are first of all formed by shallow donors and acceptors which enable controlled n-type or p-type electrical conductivity of the material. ESR of shallow donors was first reported in 1954, for phosphorous and arsenic doped silicon [1]. A characteristic hyperfine (hf) splitting could be resolved in the ESR-spectra which arises from the coupling o-ft-hedonor electron spin S to that of the donor nucleus, I, see Fig. 1. This nuclear signature (when resolved in an ESR-spectrum) is a strong hint for the chemical identity of an impurity, since it reveals the magnitude of its nuclear spin. If also the associated nuclear magnetic moment can be determined, as by the auxiliary electron-nuclear double resonance (ENDOR) technique, the chemical identification of the defect becomes unambiguous. Alternatively, several hyperfine structure patterns can appear in the ESR-spectrum if the impurity element happens to occur in more than one isotopic species carrying nuclear spin. From the relative splittings and intensities of the individual hyperfine structure patterns the chemical identity of the defect can also be definitely established. Examples for this situation are found for the shallow antimony donors (1 2 1 Sb, 57 2 % I = 5/2; 1 2 3 Sb, 42.8 % I = 7/2), and for the deep tellurium double donors (125Te, 7.0 %, I = 1/2; 1•3Te, 0.9 %, I = 1/2) in silicon.Additional hyperfine couplings exist if also the ligand nuclei of the host carry nuclear spin and moment. Observation of the ligand hyperfine structure splittings (and their anisotropies) may reveal the microscopic symmetry of the lattice site occupied by the defect in question. On the other hand, ligand hf-coupling can also lead to (inhomogeneous) broadening of individual lines in the ESR-spectrum, which may mask microscopic information otherwise apparent.Silicon and gallium arsenide are illustrative examples for such limiting situations: In Si only one, rather low abundant isotope exists which carries nuclear spin: 29 Si, 4.7 %, I = 1/2. Consequently, the ESR-linewidth in this semiconductor may be quite narrow, typically of the order of a few gauss (at sufficiently low temperature), and 2 9 Si ligand hyperfine structure is usually resolved in the ESR-spectra. In contrast, in GaAs all isotopes, 6 9 Ga, 7 1 Ga and 7 5 As, carry nuclear spin, I = 3/2, leading to very broad ESR signals, often far in excess of 100 gauss. Thus, helpfu...