  We report that an ultra-thin, post-oxidized aluminum epilayer grown on the
AlGaAs surface works as a high-quality tunnel barrier for spin injection from a
ferromagnetic metal to a semiconductor. One of the key points of the present
oxidation method is the formation of the crystalline AlOx template layer
without oxidizing the AlGaAs region near the Al/AlGaAs interface. The oxidized
Al layer is not amorphous but show well-defined single crystalline feature
reminiscent of the spinel gamma-AlOx phase. A spin-LED consisting of an Fe
layer, a crystalline AlOx barrier layer, and an AlGaAs-InGaAs double
hetero-structure has exhibited circularly polarized electroluminescence with
circular polarization of P_{EL} = 0.145 at the remnant magnetization state of
the Fe layer, indicating the relatively high spin injection efficiency (epsilon
= 2P_{EL} / P_{Fe}) of 0.63.
