Layers which had been grown by molecular beam epitaxy, and which were doped with ZnO, were characterized by using low-temperature photoluminescence measurements as a function of excitation level, temperature, and laser energy (selectively excited donor-acceptor pair luminescence), together with reflectance measurements. An O-related donor-to-acceptor (D0-A0) pair band was observed in all of the ZnO-doped layers, and its position varied from 2.7196 to 2.7304eV; depending upon the excitation level. The same peak occurred in various undoped, As-doped, or Ga-doped molecular beam epitaxial samples, thus showing that O could occur as a residual impurity. Temperature-dependent measurements revealed the existence of a corresponding conduction band-to-acceptor (e-A0) peak at 2.7372eV (39.8K); thus confirming the existence of the acceptor level. The binding energy of this acceptor was about 0.084eV. This was 0.027eV shallower than that of N. The selectively excited donor-acceptor pair luminescence measurements revealed 4 excited states of the shallow acceptor level. These were separated from the 1s¾ ground state by 0.0482, 0.0571, 0.0643 and 0.0677eV. These energies agreed well with conventional effective mass theory; thus demonstrating that this O-related acceptor level was effective mass-like. Luminescence and secondary ion mass spectrometry showed that ZnO doping introduced shallow donor impurities as well as O acceptors.

J.Chen, Y.Zhang, B.J.Skromme, K.Akimoto, S.J.Pachuta: Journal of Applied Physics, 1995, 78[8], 5109-19