Hall effect and resistivity measurements were performed, as a function of hydrostatic pressure, on molecular beam epitaxially grown In-doped layers. The data suggested that the In introduced a localized resonant state that lay at about 0.130eV above the bottom of the conduction band, and had a DX-like nature. The data were analyzed by using both positive-U and negative-U models for the In charge state, and both models were found to give a reasonable reproduction of the experimental behavior.

D.Wasik, J.Przybytek, M.Baj, G.Karczewski, T.Wojtowicz, J.Kossut: Journal of Crystal Growth, 1996, 159, 392-6