Spectroscopic evidence for the interaction between shallow hydrogenic donors in GaAs, InP and CdTe

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Abstract
Spectroscopic studies of the shallow donor impurities in compensated samples of n-GaAs, InP and CdTe are made using the technique of photothermal excitation of photoconductivity. In addition to the usual 1s-2p line of the isolated donors, other features are observed which connot be explained in terms of the hydrogenic model of isolated impurities. The most prominent line consists of a relatively sharp peak observed at 60 per cent of the impurity Rydberg in all three materials. The position of this line is independent of impurity concentration but the line increases in intensity with increasing doping. Several models suggesting an explanation involving interacting donor impurities are considered. The most plausible explanation appears to be an analogue of the ionized hydrogen molecule in which the line arises from electronic transitions between the 2p sigma u and 3p sigma g states where the transition energy is almost independent of internuclear separation for a range from six to thirteen Bohr radii.