Formation of multiple tunneling junctions of quantum dots using self-organized nanoholes and its application to spintronics

针对新型自旋电子学器件，我们研究了纳米铁磁材料在多孔氧化铝纳米孔阵列中形成的铁磁性多隧道结及其导电特性。为了在纳米孔中填充铁磁性材料，我们采用了脉冲直流电镀的方法，在铝的阳极氧化过程中，通过改变阳极电压在20-5V之间，可以改变铁磁柱的直径在10-50 nm之间。隧道结形成的意想不到的问题是，纳米孔之间的氧化铝薄壁(即纳米磁铁之间)即使在退火后也太过泄漏。氧化铝层内的导电机制是由氧化铝中的空位引起的Pool-Frenkel导电。因此，提高隧道势垒层的质量是必要的，进一步的研究，如等离子体或自由基氧化将是重要的。为了改善铁磁纳米点的开关性能，我们试图改善电沉积材料的晶体结构，因为它们是多晶的，很难取向。然后，我们尝试去除存在于纳米孔底的底层非晶氧化铝层，并成功地在Si(100)衬底上外延生长了金属纳米点阵列，并在真空中充分清洁了Si表面。从理论上推导了CSET器件的隧道导电电流的解析公式，CSET器件是控制库仑阻塞的基本结构，考虑了共隧道效应。此外，我们扩展了这一理论，以便用非平衡格林函数来处理CSET和电极系统，并成功地发展了一种估计提高隧道磁阻(TMR)比的最佳条件的方法。

We studied formation of ferromagnetic multiple tunneling junction using nano-ferromagnetic materials buried in porous alumina nanohole array and its conduction properties in aiming at new spintronics devices. For filling of ferromagnetic materials in nanoholes, we used pulsed-DC electroplating of Ni or Co, and found that ferromagnetic column diameter could be changed between 10 to 50nm by changing anodic voltage between 20 and 5V during anodic oxidation of aluminum. The unexpected problem for tunneling junction formation was that alumina thin wall between nanoholes (i.e. between nanomagnets) are too leaky even after annealing. The conduction mechanisms within alumina layer was Pool-Frenkel conduction derived from vacancies in alumina. Thus it is essential to improve quality of tunneling barrier layer, and further study such as plasma or radical oxidation would important. In order to improve switching properties of ferromagnetic nanodots, we tried to improve crystalline structure of electrodeposited materials, because they were polycrystalline and difficult to align crystalline orientation. Then we tried to remove bottom amorphous alumina layer that existed at the nanohole bottom, and succeeded in epitaxial growth of metal nanodot array on Si (100) substrate with adequate cleaning of Si surface after annealing in vacuum.From theoretical approach, we derived analytical formalisms of tunneling conduction currents of CSET devices that was a fundamental structure to control Coulomb blockade, by taking co-tunneling effect into account. Furthermore, we expanded this formalism in order to treat with CSET and electrode systems altogether using non-equilibrium Green functions, and succeeded in developing the method to estimate an optimum condition to increase TMR (tunneling magnetoresistance) ratio.