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

为了开发新型自旋电子材料和器件，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特性。获得的主要结果如下。(1)研究了GaAs在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，发现这是由于在400℃的基体温度下，GaAs在生长的早期阶段就发生了异常的表面粗化。(2)利用扫描隧道显微镜和I - FED-IV技术对GaAs (001) c (4×4) α和c (4×4) β进行了重构，确定了它们的详细原子排布，并观察了As _4通量下的相互可逆相变。(3)我们研究了MnAs在GaAs (001) c (4×4) a上的初始生长过程，发现从c (4×4) α到c (4×4) β的相变发生在MnAs生长的早期阶段，导致表面覆盖明显异常。(4)利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，发现能量在1.1 ~ 1.6 eV之间的弹道电子穿透深度随着能量的增加而减小，为2.7 ~ 1.5 nm。(5)在GaMnAs/GaAs/GaMnAs双势垒磁隧结中，我们已经观察到在最低阈值电流密度低至3 × 10^4A/cm^2的情况下，电流注入诱导磁化方向反转。

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.