Deep levels in annealed low-temperature molecular beam epitaxial layers were characterized by using an n-i-n structure. It was found that annealing at 620C resulted in the highest possible resistivity of such layers. However, deep-level transient spectroscopy and photo-capacitance quenching measurements showed no signal from the mid-gap EL2 level. Only a so-called shallower EL3-like level was observed in the high-resistivity layer. The fact that no mid-gap level was detected in the layer with the highest resistivity implied that the compensation mechanism did not play an important role in controlling the semi-insulating properties of low-temperature layers. A mid-gap level appeared upon annealing above 620C, but the resistivity of low-temperature layers decreased by more than 5 orders of magnitude. Photo-quenching measurements suggested that this level was identical to the EL2 level. The results supported the so-called buried Schottky barrier model.

T.C.Lin, T.Okumura: Japanese Journal of Applied Physics, 1996, 35[1-3], 1630-6