Erbium Related Defects in Gallium Arsenide

This paper reviews our recent experimental work on erbium incorporated into gallium arsenide during MBE growth. The electrical behaviour of the erbium is considered in relation to its interaction with intentionally added dopants (silicon, selenium and berylium) and the effect of the erbium on the deep state population is discussed. At higher concentrations the morphology of phase separation is considered and results are presented showing the controlled precipitation of erbium arsenide as quantum structures. The magnetic properties of these dots and wires are reported. Introduction We have studied the behaviour of erbium in gallium arsenide and silicon over the concentration range 10 cm to 10 cm. In this paper the work on gallium arsenide is presented but comparisons are made with the silicon work which will be published in detail elsewhere. In order to interpret the behaviour of the erbium related defects and link it to previously published work, three regimes are considered. At low concentrations the behaviour is dominated by complexes formed as a result of the reaction of the erbium with impurities in the semiconductor host. At higher concentrations the behaviour is much more dependent on the interactions of erbium with the host itself and at very high concentrations precipitation, or phase separation, occurs in which the properties are dominated by the presence of erbium arsenide in the gallium arsenide host or erbium silicide in silicon. The emission wavelength of erbium (and other rare earths) is almost independent of the host because of the strong screening due to the outer electron shells, in the case of erbium the 5s and 5p shells. However, subtle changes in the optical absorption and emission spectra are attributable to crystal field splitting from the host lattice. Information on the siting symmetry can be derived from an analysis of these splittings provided the erbium siting is limited to one or two structural locations. In other cases the complexity and broadening of the lines contribute to reduced confidence in the interpretation. In an insulating host, the luminescence efficiency can be very high but when the rare earths are incorporated into semiconductors, mechanisms come into effect which compete with the overall luminescence process, generally resulting in a reduction in the overall luminescent efficiency. This is particularly marked for semiconductors with “non-wide” bandgaps (eg, Si and GaAs) at temperatures >100K. In recent years effort has been directed towards the electrical excitation of rare earths in semiconductors and hence on understanding the electrical states of the rare earths in various semiconductors. This contrasts with the majority of earlier studies which have focused on optical excitation (which is now the basis for fibre lasers, fluorescent tube phosphors and electron beam/hot electron excitation which is used in the cathodo luminescent materials).