Undoped ZnO crystals exhibit a large variety of bound-exciton luminescence lines, due to the efficient exciton binding to chemically and structurally different neutral impurity centers. This offers an excellent possibility to identify excited states and excitation channels of these systems by comparing various bound-exciton complexes. For this reason, we performed intensity-dependent high-resolution excitation spectroscopy in magnetic fields up to 15 T. The analysis of the obtained data enables us to distinguish states of neutral-acceptor-exciton complexes with holes of the lower-lying B valence band from excited electronic states where one particle within the complex is in an excited orbital. Furthermore, some evidence is found that states due to vibronic or rotational excitation seem to occur in all acceptor-exciton systems. The highly resolved fine structure of all states and the comparison with model calculations allow for a detailed determination of the term sequence of acceptor-exciton complexes in ZnO and the interdependences of their excitation mechanisms.

• Received 20 May 1988

DOI:https://doi.org/10.1103/PhysRevB.38.9746