The Effect of Co-Dopants on the Photoluminescence of Er 3+ in Silicon
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W.P. GILLIN, A.G. JAMES, AND R.J. WILSON Department of Electronic and Electrical Engineering, University of Surrey, Guildford, GU2 5XH
Abstract This paper presents the results of our investigation into the possibility of increasing both the radiative cross-section and the electrical activation efficiency in erbium (Er 3 +) doped silicon (Si). The energy levels of the isolated Er 3 + have been theoretically predicted, employing the Thomas-Fermi method. The behaviour of these levels in Si was then investigated using a KronigPenney approach. Initial theoretical results imply that fluorine (F), in addition to Er 3 + in Si, increases the radiative cross-section of Er 3 + by at least an order of magnitude, and that co-doping appears to enhance the mixing of the 4f and 5d levels and causes the Er 3+ energy levels to overlap 3
with those of the host. Photoluminescence spectra of Er + in Si co-doped with F also indicate an
interaction with the host lattice which appears to be dependent on its electrical characteristics.
Introduction Glasses have been successfully doped with Er 3 + and other rare earth ions for the amplification of infrared signals. The emission arises from transitions occurring between levels of the 4f band. Since these levels are shielded from their environment, the emission and absorption spectra are largely unaffected by the host material. The emission cross-section is different from zero for the following two reasons: a magnetic dipole contribution and mixing between the 4f and the 5d levels. There is great interest in doping semiconductors with rare earth elements for the purpose of integration of optical and electronic devices [1,2]. However, from the work carried out to date, it is clear that the Er3 + ion does not interact significantly with the host [3]. This implies that the emission cross-section therefore remains unchanged at approximately 10-21 cm 2 resulting in a small gain coefficient in comparison to gains obtained from band to band transitions in III-V's. Thus, if doped material is to provide significant gain and be electrically activated, either the 217 Mat. Res. Soc. Symp. Proc. Vol. 392 ©1995 Materials Research Society
3 emission cross-section of the ion must be increased, or the concentration of active Er + ions be in 3 excess of 1020 ions/cm . An enhancement of the emission cross-section may be achieved by increasing the degree of mixing between the 4f and 5d levels and electrical activation becomes
feasible if Er 3 + is forced into a 'mixed valence' state. This can be attained by an overlap between the 4f levels and semiconductor bands [4].
Modelling In order to study the behaviour of the radiative cross-section, we have developed a model [5] of the Er 3+ energy levels. This was based upon the Thomas-Fermi approximation for the radial component of the 4f wave function [6] and on Symmetry group theory [7] for the angular part. The crystal potential expanded in spherical harmonics was taken as
V = YAla1,mY1,m(O,(O)rl
(1)
i,m
where the coefficients aim are determined by the s
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