In this thesis, spin polarization of light emission arising from self-assembled ZnO/SrRuO3 nanocomposites, with heterojunctions of magnetic materials and semiconductors, have been investigated. Instead of spin injection by optical or electrical method, the population imbalance between spin-up and spin-down electrons in semiconductors can be achieved by intrinsic distinct band alignments of electrons with different spin orientations at the interface of materials. The energy-band structure induces the selective charge transfer of electrons with opposite spins and generates spin accumulation at the boundary inside ZnO, which then results in circularly polarized photoluminescence. With this mechanism, we believe that self-polarization of electron spin in semiconductors can be achieved and may have a variety of application in spintronics.