Nano-characterization of diluted magnetic semiconductors by scanning probe microscopy and spin polarized scanning tunneling microscopy

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To realize thin-layered structures with atomically abrupt interfaces, we have investigated surface morphological changes of GaN, GaCrN, and AlN thin layers grown on GaN-template substrates by plasma-assisted molecular beam epitaxy (PA-MBE) as a function of V/III ratio. Both GaN and GaCrN layers grown under stoichiometric V/III regions display atomically flat surfaces consisting of monatomic steps and 100 nm-wide terraces, indicating that growth of GaCrN as well as GaN proceeds as a layer-by-layer fashion. Decrease (increase) in V/III ratio results in the formation of Ga droplets (pits). We have proposed an Al flux modulation technique during growth of AlN under stoichiometric or Al-rich V/III regions to eliminate Al droplets. Resultant AlN layers have good surface morphologies (step-terrace structures) without Al droplets.We have tried to characterize magnetic domain structures in GaCrN layers by magnetic force microscopy and scanning tunneling microscopy. Stray magnetic field from probes covered with hard magnetic materials, however, prevents us from observing magnetic domain structures. Alternatively, we have proposed tunnel magnetoresistance mapping by using atomic force microscopy with non-magnetic probes. For the first step in this approach, GaCrN/AlN/GaCrN trilayer structures with different Cr concentrations grown by PA-MBE under above-mentioned conditions have been investigated. The samples show a well-defined hysteresis loop in magnetization vs. magnetic field curves even at room temperature. Current vs. voltage characteristics show that the AlN layer behaves as a tunnel barrier in the GaCrN/AlN/GaCrN structures. The tunnel magnetoresistance effect was clearly observed at 77 K for the junction with a flat AlN (3 nm) tunnel barrier when the current flow was perpendicular to the junction plane and the magnetic field was applied parallel to the junction plane. The tunnel magnetoresistance ratio was about 0.1 % at 77 K.