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

为了实现具有原子突变界面的薄层结构，我们研究了等离子体辅助分子束外延(PA-MBE)在GaN模板衬底上生长的GaN、GaCrN和AlN薄层的表面形貌随V/III比的变化。在化学计量比V/III区生长的GaN和GaCrN层都显示出原子平坦的表面，由单原子台阶和100 nm宽的阶梯组成，这表明GaCrN和GaN的生长是以逐层方式进行的。V/III比值的降低(增加)导致了Ga液滴(坑)的形成。我们提出了一种在化学计量比或富铝V/III区生长AlN过程中的铝通量调制技术，以消除Al液滴。得到的AlN层具有良好的表面形貌(无Al滴的阶梯状结构)。我们尝试用磁力显微镜和扫描隧道显微镜来表征GaCrN层中的磁畴结构。然而，覆盖着硬磁材料的探测器产生的杂散磁场使我们无法观察到磁畴结构。或者，我们提出了使用原子力显微镜和非磁性探针来绘制隧道磁阻图谱。作为该方法的第一步，我们研究了在上述条件下用PA-MBE生长的不同铬含量的GaCrN/AlN/GaCrN三层结构。即使在室温下，样品在磁化强度与磁场的关系曲线上也表现出明确的磁滞曲线。电流-电压特性表明，AlN层在GaCrN/AlN/GaCrN结构中表现为隧道势垒。当电流垂直于结平面，磁场平行于结平面时，在77K下观察到了平坦的AlN(3 Nm)隧道势垒结的隧道磁阻效应。77K时，隧道磁电阻率约为0.1%。

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.