Lattice Location of Rare Earth Ions in Semiconductors: Interpretation and Limitations of using g values
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Lattice Location of Rare Earth Ions in Semiconductors: Interpretation and Limitations of using g values David Carey Nanoelectronics Centre, Advanced Technology Institute, University of Surrey, Guildford, GU2 7XH, United Kingdom. Email: [email protected]
ABSTRACT The g values of rare earth ions obtained from either paramagnetic resonance or Zeeman measurements are often used to interpret the location and/or environment surrounding rare earth ions. In the case of centres with cubic symmetry the g value can be used to distinguish between substitutional and interstitial sites. For centres with less than cubic symmetry the average g value, taken as 1/3 trace of the g tensor, is often used as an indication of the lattice location and/or a measure of the strength of the local crystal field. This approach is widely used but is based on the assumption that the non-cubic terms in the total crystal field potential are small compared with the cubic crystal field. In this paper we have explored this assumption by calculating the principal g values in axial crystal fields for the Er3+ ion. We examine the limits over which the average g value approach is valid. Comparison is made with published results.
INTRODUCTION Light emission from rare earth (RE) ions is now well established with emission from Er3+ ions being of particular importance since the transition between the two lowest spin-orbit energy levels occurs at the technologically important wavelength of 1.55 µm. Efficient luminescence can be assisted by maximising the concentration of optically active centres. It is now generally established that this is not the same as maximising the RE ion concentration since precipitation and/or up-conversion effects can dominate at higher RE ion concentrations [1]. Incorporation of light atoms, such as oxygen and carbon, have been shown to increase the solubility limit of Er in silicon through the formation of erbium impurity complexes [2]. The most appropriate method that can be used to elucidate the lattice location of the optically active RE ion is a crystal field analysis of high resolution photoluminescence (PL) spectra. In addition, the lattice location and/or coordination of RE ions (which includes both optically active and inactive ions) can be obtained from extended X-ray absorption fine structure [3], Zeeman measurements [4] and electron paramagnetic resonance measurements (EPR) [5]. The results from the latter two experiments are usually discussed in terms of g values. In the case of Zeeman measurements, g values are extracted on the basis of fitting PL peak positions to a suitable spin Hamiltonian. For EPR, the variation of the resonance magnetic field, as a function of orientation between the Zeeman applied field and the crystal axes is measured. Again by fitting the observed variation to a spin Hamiltonian, g values can be obtained which in turn can be interpreted. For systems with less than cubic symmetry various approaches and simplifications are used. It is the aim of this paper to test some of the commo
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