Pradissitto, Jasmine; (1996) Rare earth doping of silicon. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Glasses have long been successfully doped with erbium ions for amplification at 1.53μm. There is also, however, great interest in doping silicon with erbium for the purpose of integration of optical and electronic devices. However, from the work carried out to date, it is clear that the erbium ion does not interact significantly with the host. 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 emission cross-section of the ion must be increased or the concentration of active erbium ions be in excess of 1020 ions/cm3. 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 the erbium ion is forced into a 'mixed valence' state. This thesis presents the results of an investigation into the possibility of increasing the radiative cross-section (and therefore, the electrical activation efficiency) in erbium doped silicon. The energy levels of the isolated erbium ion have been theoretically predicted by employing the Thomas-Fermi method. The behaviour of these levels in silicon was then investigated using a Kronig-Penney approach. The results show that fluorine, in addition to erbium in silicon, doubles the radiative cross-section of the rare earth ion by enhancing the mixing of the 4f and 5d levels of erbium. Photoluminescence spectra of erbium in silicon doped with fluorine, also demonstrate that the introduction of a co-dopant enhances erbium luminescence by approximately an order of magnitude. This can be further increased by post- amorphisation, solid phase epitaxial re growth and the correct choice of substrate (i.e. in terms of electrical dopants). Erbium photoluminescence in silicon exhibits a stronger quenching behaviour as the temperature is increased, thus, room temperature luminescence was not observed. In addition, diffusion of erbium into silicon was successfully attempted and photoluminescence was observed.

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