It has been reported that nanostructures of oxide can be fabricated on surfaces of metal and semiconductor by applying a negative bias on a conductive tip of scanning probe microscope. With using this technique, planar-type magnetic tunnel junctions have been fabricated [1]. The planar structure has an advantage of its small junction area comparing to conventional multilayer-type junctions. Ni strip films of 50-200 nm width and 10 nm thickness were patterned by electron beam lithography. Nanowires of Ni-oxide were fabricated by the nano-oxidation using atomic force microscope (AFM). A diode characteristic was observed in current-voltage measurement at the temperature range of 17-300K.
In fabrication of planer-type tunnel junctions by the AFM nano-oxidation, nanostructures of oxide are preferably narrow and uniform in width. Also the oxidation should reach to the bottom of the films. In order to enhance an aspect ratio of height/width in nanostructures of oxides, the AFM nano-oxidation with a modulated applied voltage has been studied by Perez-Murano et al [2]. Recently, nano-oxidation of ferromagnetic metals covered with insulating layers has been investigated in order to obtain the nano-oxides with a high aspect ratio and a uniformity in their width [3]. Insulator/metal double-layer films were prepared on thermally oxidized Si substrates. After depositing Ni thin films on the substrates, Al layers were capped on the Ni films. Then the Al layers were oxidized to Al2O3 by a natural oxidation process in air at room temperature. The thickness of Al layers was 1 nm, which corresponded to Al2O3 thickness of 1.3 nm. Then nanowires of Ni-oxide were fabricated by the AFM nano-oxidation. The planar-type magnetic tunnel junction covered with the insulating layer of Al2O3 was successfully fabricated. The surface of this device structure prevents from exposure to air and humidity which degrade the device performance.
References:
[1] J. Shirakashi and Y. Takemura, IEEE Trans. Magn. 40, 2640, 2004.
[2] F. Perez-Murano, K. Birkelund, K. Morimoto and J.A. Dagata, Appl. Phys. Lett. 75, 199, 1999.
[2] K. Watanabe, T. Tamura, Y. Takemura, Y. Shimazu and J. Shirakashi, Nanotechnology, 15, S566, 2004.
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