We report on the effect of electron injection on the magneto-optical properties of MgZnO/ZnO(5nm)/MgZnO single quantum wells (SQWs) by embedding a Ga delta-doped layer in one of the MgZnO barriers. Approaching the delta-doped layer to the SQW resulted in a photoluminescence (PL) blueshift due to the screening of internal polarization field by the injected electrons. Circular polarization in PL was enhanced by a magnetic field up to 50 T with a Faraday configuration. The observed Zeeman splitting and the degree of circular polarization in the magnetic field revealed that the PL originates from the negatively charged excitons X-.