Spin transistor using tunneling barrier

采用磁控溅射系统，在真空系统中改变电极的金属掩模，用5个靶制备了自旋隧道器件。磁性层由多晶NiFe层改为非晶CoFeB层，磁电阻率由原来的几个百分点提高到20%以上，并提高了隧道结的再现性。这是由于没有多晶晶粒的结构改善了界面的平整度。采用直流等离子体和射频等离子体对纯氧气中铝层的氧化过程进行了研究。在直流等离子体的情况下，隧穿器件表现出大的MR比与自旋阀型MR曲线的范围内的Al厚度在1.3 nm和1.6 nm之间，MR比显示在1.4 nm处的最大值为30%。同一工艺制备的7个样品的MR比的分散变得很小。在射频等离子体的情况下，虽然MR比在Al厚度为1.6nm时为15%，但它随着Al厚度的减小而减小，并且在1.3nm时变得小于1%。样品间MR比的离散度大于直流等离子体情况。这些结果表明，直流等离子体比射频等离子体更适合于铝层的氧化。然而，直流等离子体情况下的MR比下降更陡峭的增加电压偏置相比，射频等离子体的情况下，这意味着直流等离子体的氧化过程也应该得到改善。分析认为，这是由于铝层氧化程度和隧穿势垒界面粗糙度不同造成的。隧道器件表现出良好的热稳定性，在真空中退火低于300 ℃时MR比不会发生变化。

Spin tunneling devices were fabricated by the magnetron sputtering system with 5 targets, where the metal masks for forming the electrodes were changed in vacuum system. Magnetoresistance(MR) ratio increased from few % to over 20% and the reproductivity of tunneling junction was improved by changing the magnetic layers from polycrystalline NiFe to amorphous CoFeB layer. This is due to improvement of the flatness of interface because of the structure without polycrystalline grains. Oxidization process of Al layer were investigated by dc plasma and rf plasma discharge in pure oxygen. In case of dc plasma, the tunneling devices exhibit large MR ratio with spin valve type MR curves in the range of the Al thickness between 1.3 nm and 1.6 nm and MR ratio shows maximum value of 30% at 1.4 nm. The dispersion of MR ratio between seven samples prepared at same process becomes very small. In case of rf plasma, although the MR ratio is 15% at Al thickness of 1.6nm, it decreases with decreasing Al thickness and becomes less than 1% at 1.3 nm. The dispersion of MR ratio between samples is larger than that for dc plasma case. These results show that the dc plasma is more suitable for oxidization of Al layer compared with rf plasma. However, the MR ratio for dc plasma case decreases more steeply with increasing voltage bias compared with rf plasma case, which implies that the oxidization process by dc plasma also should be improved. It is considered that these results are due to the deference in the degree of oxidization of Al layer and interface roughness of tunneling barrier. The tunneling devises show good thermal stability, where MR ratio does not change below 300 C annealing in the vacuum.