Fabrication and characterization of small magnetic tunnel junction by atomic force microscope for observation of Coulomb blockade

通过在导电原子力显微镜（AFM）悬臂梁上施加负偏压，可以在金属薄膜上制备具有良好定义的氧化物纳米结构。利用该技术制备了平面型磁隧道结。这种结构的结面积可以比传统的多层型结小得多。这有利于制造具有库仑封锁特性的单电子器件。采用射频磁控溅射技术在SiO2/Si衬底上沉积了8 ~ 20 nm的Ni薄膜。通过光刻法和干蚀刻法将薄膜制成条纹形状。然后采用原子力显微镜纳米氧化技术制备了氧化物纳米线，成功地获得了平面型磁性隧道结。在17-300K温度范围内的电流-电压测量中观察到二极管的特性。发现所制备的氧化物纳米线可以作为隧道势垒。表面上屏障的宽度较厚，但利用原子力显微镜操作的攻丝方式进行原子力显微镜纳米氧化后，屏障的宽度较薄。利用Simmons公式拟合得到势垒高度为1.5 eV，势垒宽度为1.7 nm。这些结果对具有库仑阻塞特性的铁磁单电子晶体管是有希望的。

Well-defined nanostructures of oxide can be fabricated on metal thin films by applying a negative bias on a conductive atomic force microscopy (AFM) cantilever. With using this technique, planar-type magnetic tunnel junctions have been fabricated. The junction area of this structure can be much smaller than that of a conventional multilayer-type junction. This is advantageous for fabrication of single electron devices exhibiting Coulomb blockade. Ni thin film of 8-20 nm were deposited on SiO2/Si substrates by rf magnetron sputtering. The films were patterned to stripe shapes by photolithography and dry etching. Then nanowires of oxide were fabricated by the AFM nano-oxidation technique, and planar-type magnetic tunnel junctions were successfully obtained.A diode characteristic was observed in current-voltage measurement at the temperature range of 17-300K. It was found that the fabricated nanowires of oxide could be a tunnel barrier. Formally the width of the barrier was thick but the AFM nano-oxidation using tapping mode of the AFM operation realized a thinner width. The barrier height of 1.5 eV and barrier width of 1.7 nm were derived from a calculation using the Simmons formula fitting. There results are promising for ferromagnetic single electron transistors exhibiting Coulomb blockade.