Extremely high-density doping of magnetic impurity in magnetic semiconductors and surface nano-structure preparation by controlling fundamental surface process during MBE growth

为了开发新型的自旋电子学材料和器件，我们探索了由半导体和磁性材料组成的纳米结构的制备工艺和材料性质。主要结果如下。(1)We研究了GaAs（001）c（4×4）表面低温MBE生长过程中RHEED强度振荡的重入行为，发现其起因是衬底温度为400℃时生长初期的异常表面粗糙化。(2)We用扫描隧道显微镜和I-FED-IV技术研究了GaAs（001）c（4×4）α和c（4×4）β的再构，确定了它们的详细原子排列，观察到了As_4通量下的相互可逆相变。(3)We研究了MnAs在GaAs（001）c（4×4）a衬底上的初始生长过程，发现在MnAs生长的早期就发生了c（4×4）a → c（4×4）β的相变，导致了明显的反常表面覆盖。(4)We已经通过使用弹道电子发射光谱（BEES）研究了在GaAs（001）上生长的Fe层中的弹道电子运动，并且发现具有1.1至1.6eV能量范围的弹道电子的穿透深度随着它们的能量而减小，并且为2.7至1.5nm。(5)我们在GaMnAs/GaAs/GaMnAs双势垒磁性隧道结中，在最低阈值电流密度为3 × 10 ~ 4A/cm ~ 2时，观察到了电流注入引起的磁化方向反转。

We have explored preparation process and materials properties of nanostructures consisted of semiconductors and magnetic materials in order to develop novel spin-electronics materials and devices. Major results obtained are as follows.(1)We have investigated reentrant behavior of RHEED intensity oscillation during low temperature MBE growth of GaAs on GaAs (001) c (4×4) surface,s and found that the origin is due to anomalous surface roughening, which takes place at very early stage of the growth at a substiate temperature of 400℃.(2)We have explored GaAs (001) c (4×4) α and c (4×4) β reconstructions by using scanning tunneling microscopy and I FED-IV technique, and determined their detailed atomic arrangement, and observed mutual reversible phase transition under As _4 flux.(3)We have investigated initial growth process of MnAs on GaAs (001) c (4×4) a, and found that phase transition from c (4×4) α to c (4×4) β took place very early stage of MnAs growth, which lead to apparent anomalous surface coverage.(4)We have investigated ballistic electron motion in Fe-layers grown on GaAs (001) by using ballistic electron emission spectroscopy (BEES), and found that penetration depth of ballistic electrons having energy ranging from 1.1 to 1.6 eV is decreasing with their energy and to be 2.7-1.5 nm.(5) We have observed magnetization orie ntation reversal induced by current injection at lowest threshold current density as low as 3x10^4A/cm^2 in GaMnAs/GaAs/GaMnAs double barrier magnetic tunneling junctions, so far.