  We report on the photoluminescence (PL) properties of MgZnO/ZnO
heterojunctions grown by plasma-assisted molecular-beam epitaxy. Influence of
the applied magnetic field (B) on the radiative recombination of the
two-dimensional electron gas (2DEG) is investigated up to 54 T. An increase in
magnetic field in the range of B <= 20 T results in a redshift in the PL.
Abrupt lineshape changes in the PL spectra are observed at higher magnetic
fields, in correlation with the integer quantum Hall states. We attempt to
interpret these features using the conventional model for the 2DEG-related PL
based on the transition between the 2DEG and a hole as well as a model taking a
bound state effect into account, i.e., a charged exciton. The comparison about
the adequateness of these models was made, being in favor of the charged
exciton model.
