Erbium in Semiconductors: Where are we coming from; Where are we going?
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Erbium in Semiconductors: Where are we coming from; Where are we going? A. R. Peaker The University of Manchester, School of Electrical & Electronic Engineering, Manchester, M60 1QD, UK. ABSTRACT It is one of the curious twists of technology that transitions which are parity forbidden in the free ions of rare earths should have become of immense importance in solids used in fluorescent lighting, cathode ray tubes and optical amplifiers. It is not an unreasonable expectation that having achieved such success with excitation from photons and accelerated electrons that junction electroluminescence should also be important. Since Ennen demonstrated good low temperature electroluminescence in silicon in the early 80’s, a formidable amount of work has been done to try to understand the excitation and quenching mechanisms in common semiconductor hosts such as silicon and gallium arsenide. Although some remarkable experimental results have been obtained for erbium in nanostructures, insulators and wide bandgap materials the performance in bulk silicon and silicon germanium is disappointing. More importantly we still have not achieved a comprehensive, detailed understanding of the processes of non-radiative competition to the rare earth emission. In this paper the key steps that have been made over the last twenty years towards our present day knowledge of erbium luminescence in semiconducting hosts are reviewed and an assessment made of what remains to be done. INTRODUCTION Although light emission from semiconductors is now a central part of our everyday technological infrastructure there are many applications in which it is believed rare earths could improve performance or even open new horizons. This is particularly true for the case of indirect gap materials. Indeed achieving optical functionality in silicon, particularly emission has been referred to as the holy grail1 of semiconductor science. The internal transitions of the rare earths have attracted much attention from this viewpoint, in particular the energy between the 4I13/2 to 4I15/2 excited states of the 3+ charge state of erbium, produces emission at around 1.54μm. This is the wavelength at which third and fourth generation optical communication systems are centred. The motivation for choosing this wavelength for third generation systems was that silica based fibres have minimum attenuation at this wavelength and can be made to have near zero dispersion. In the case of fourth generation (wavelength division multiplexed) systems the motivation is that “all optical” erbium fibres amplifiers can be used to boost the signals over wavelength channels around 1.5μm at extremely high data rates (10 to 40Gbit). Although only the high data rate issue is particularly relevant to short range communication such as might be needed for inter and intra chip it has provided direction in this area of research with the result that in the past a disproportionate amount of work has focused on erbium in silicon. The rare earths are the lanthanide sequence of elements and pre
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