Spin-Dependent Optical Processes in II-VI Diluted Magnetic Semiconductor Nanostructures
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Spin-Dependent Optical Processes in II-VI Diluted Magnetic Semiconductor Nanostructures Yasuo Oka, Kentaro Kayanuma, Mio Sakuma, Ayahito Uetake, Izuru Souma, Zhanghai Chen, and Akihiro Murayama Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan ABSTRACT Nanostructures of diluted magnetic semiconductors have potential possibility for the spin electronics application due to their large magneto-optical effects and the quantum confinement effects for band electrons. We have fabricated quantum wells and quantum dots by II-VI diluted magnetic semiconductors of Cd1-xMnxTe, Zn1-xMnxTe and Zn1-x-yCdyMnxSe. These nanostructures showed spin injection, spin switching and ultrafast spin relaxation processes derived in the confined nanostructures. Hybrid structures consisting of diluted magnetic semiconductor and Co showed Mn spin polarizations induced by the Co wire shape. INTRODUCTION Diluted magnetic semiconductors (DMSs) show marked spin-dependent properties due to the s (p)-d exchange interaction of band-electrons (-holes) with contained magnetic ions [1,2]. A number of works have been made recently on both III-V and II-VI DMS materials aiming application to spin electronics [3]. However, DMS nanostructures still need basic understanding on the quantum confinement effects, the spin interaction between carriers and magnetic ions, and the spin dynamics. We describe our recent studies on spin-dependent optical processes and the dynamics in nanostructure II-VI DMSs [4]. Spin injection and transport of carriers have been reported in magnetic semiconductor quantum wells (QWs) [5-11]. Details of the spin injection process, however, have not been disclosed up to now. A double quantum well (DQW) system with Cd0.9Mn0.1Te and CdTe wells is investigated by transient circularly polarized photoluminescence (PL) spectroscopy. The ultrafast spin injection and relaxation processes are clarified. In a QW system of DMS (a type of so called spin-superlattices), modification of the carrier-spin distribution in the QWs by magnetic field is possible and the spatial spin distribution can be controlled by the giant Zeeman effect of the DMS wells [12-14]. We study field-induced switching of the spin-injection of excitons in DQWs composed of a DMS magnetic well (MW) and a non-magnetic well (NW) [15,16]. Transient optical phenomena of electron-hole (e-h) plasma, excitons and magnetic ions in DMS QWs involve basic dynamics of their spin systems [17-19]. To clarify these ultrafast spin dynamics, transient pump-probe spectroscopy in magnetic fields has been made. The measurement presents detailed dynamics on creation and relaxation of the e-h plasma, excitons and subsequent spin alignment of magnetic ions [20,21]. Quantum dots (QDs) composed of DMSs are expected to show efficient magneto-optical functionalities due to the strong confinement effect and the high quantum yield for luminescence [22-25]. We can design the dot dimensions by fabricating with electron-beam lit
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