Abstract:
A method for fabricating multi-component nanowires is disclosed, which can make multi-component nanowires used to realize a nanowire-based memory device by an electroplating process using a multi-component solution. The method for fabricating multi-component nanowires in accordance with the present invention includes the steps of: (a) preparing an anodized aluminum oxide nanotemplate having a plurality of pores; (b) forming an electrode layer on one surface of the anodized aluminum oxide nanotemplate; (c) injecting the anodized aluminum oxide nanotemplate in a predetermined multi-component solution and then growing multi-component nanowires through the pores of the anodized aluminum oxide nanotemplate by an electroplating process in which the anodized aluminum oxide nanotemplate is used as a cathode; and (d) removing the anodized aluminum oxide nanotemplate.

Description:
PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. §119(a) to an application filed in the Korean Intellectual Property Office on Dec. 31, 2008 and assigned Korean Patent Application No. 10-2008-0137859, the contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to a method for fabricating nanowires, and in particular, to a method for fabricating multi-component nanowires used to implement a nanowire-based memory device by an electroplating process using a multi-component solution. 
       BACKGROUND OF THE INVENTION 
       [0003]    A phase change random access memory (PRAM) or a resistive random access memory (ReRAM), which is one of non-volatile memories, is attracting attention as a leading next generation memory device which can replace a dynamic random access memory (DRAM), which is a mainstream memory at present, owing to the advantages such as high storage speed and density, non-volatility, small size, low-power consumption and low-production cost, and so on, and thus many studies have been conducted on the PRAM and the ReRAM. 
         [0004]    The PRAM basically operates based on a different principle from that of a general semiconductor memory, and writes data using a phenomenon that the crystalline state of phase change materials based on chalcogenide compounds is changed by electrical switching. 
         [0005]    To realize a memory device of a very small size, nanowires have been suggested as an alternative for a thin film as a memory layer in the PRAM. 
         [0006]    Conventional nanowire fabrication methods include a vapor-liquid-solid (VLS) process (that is, an epitaxial process), a chemical vapor deposition (CVD) process, and a physical vapor deposition (PVD) process. 
         [0007]    However, these conventional methods has a problem that the fabrication cost is high because the price of equipment used to fabricate nanowires is high or the process takes a long time and is complicated. That is, the conventional methods have been carried out usually by sputtering, E-beam evaporation or plasma enhanced chemical vapor deposition (PECVD), but have a problem that the fabrication cost is high and the fabrication process is complicated because high-priced equipment is used and the fabrication is performed in high temperature and vacuum. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a method for fabricating multi-component nanowires, which makes the fabrication process easier and offers low fabrication cost by enabling it to fabricate multi-component nanowires in a nanotemplate having a plurality of pores by an electroplating process with the use of a multi-component solution. 
         [0009]    In accordance with one aspect of the present invention, there is provided a method for fabricating multi-component nanowires, including the steps of: (a) preparing an anodized aluminum oxide nanotemplate having a plurality of pores; (b) forming an electrode layer on one surface of the anodized aluminum oxide nanotemplate; (c) injecting the anodized aluminum oxide nanotemplate in a predetermined multi-component solution and then growing multi-component nanowires through the pores of the anodized aluminum oxide nanotemplate by an electroplating process in which the anodized aluminum oxide nanotemplate is used as a cathode; and (d) removing the anodized aluminum oxide nanotemplate. 
         [0010]    The diameter of the pores can be several tens to several hundreds nanometers. The electrode layer can comprise gold (Au). If the multi-component solution is an Ag—Se solution, Ag—Se nanowires can grow. The Ag—Se solution can include Ag and Se as precursors and nitric acid and ethylglycerol as solvents. If the multi-component solution is a Ge—Sb—Te solution, Ge—Sb—Te nanowires can grow. The Ge—Sb—Te solution can include GeO 2 , SbO 2 , and TeO 2  as precursors and hydrochloric acid and ethylglycerol as solvents. In the step (c), a platinum electrode can be used as an anode. In the step (c), a composition ratio of the multi-component nanowires can be determined depending on the change of at least one of a mixing ratio of precursors in the multi-component solution and a current density applied during the electroplating process. In the step (d), the anodized aluminum oxide nanotemplate can be removed with a NaOH solution. 
         [0011]    In accordance with another aspect of the present invention, there are provided multi-component nanowires fabricated by the method disclosed above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The above and other objects and features of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
           [0013]      FIGS. 1 to 4  are views showing the configuration of a method for fabricating multi-component nanowires according to the present invention; 
           [0014]      FIG. 5  is a photograph of Ag—Se nanowires taken by scanning electron microscopy; 
           [0015]      FIG. 6  is a photograph of Ag—Se nanowires taken by transmission electron microscopy; 
           [0016]      FIG. 7  is a graph showing the results of analysis of Ag—Se nanowires obtained by energy dispersive X-ray spectroscopy; 
           [0017]      FIG. 8  is a photograph of Ag—Se thin film taken by a scanning electron microscopy; 
           [0018]      FIG. 9  is a graph showing the results of X-ray diffraction analysis of Ag—Se nanowires; 
           [0019]      FIG. 10  is a photograph of Ge—Sb—Te nanowires taken by scanning electron microscopy, which shows the results of analysis of Ge—Sb—Te nanowires obtained by energy dispersive X-ray spectroscopy; 
           [0020]      FIG. 11  is a view of Ge—Sb—Te nanowires taken by transmission electron microscopy; 
           [0021]      FIG. 12  is a graph showing current changes (I-V) of Ag—Se nanowires; and 
           [0022]      FIG. 13  is a graph showing current changes (I-V) of Ag—Se thin film. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    In the following detailed description, reference is made to the accompanying drawings that show, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that the various embodiments of the invention, although different from one another, are not necessarily mutually exclusive. For example, a particular feature, structure, or characteristic described herein in connection with one embodiment may be implemented within other embodiments without departing from the spirit and scope of the present invention. Also, it should be understood that the positions or arrangements of individual elements in the embodiment may be changed without separating the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims that should be appropriately interpreted along with the full range of equivalents to which the claims are entitled. In the drawings, like reference numerals denote like or similar elements or functions through the several views. 
         [0024]    Hereinafter, the configuration of the present invention will be described in detail with reference to the accompanying drawings. 
         [0025]      FIGS. 1 to 4  show the configuration of a method for fabricating multi-component nanowires in accordance with one embodiment of the present invention. 
         [0026]    First of all, referring to  FIG. 1 , an anodized aluminum oxide nanotemplate  10  is prepared. 
         [0027]    The anodized aluminum oxide nanotemplate  10  having a plurality of pores  12  formed thereon can be fabricated as follows. 
         [0028]    First, an aluminum having a purity of 99.999% is electrochemically etched to planarize a surface thereof, and then the aluminum is anodized in oxalic acid at a voltage of 40 V and a temperature of 4° C. 
         [0029]    When the anodization is carried out once, the uniformity of the pores  12  is poor, and therefore it is preferable to carry out the anodization twice in order to improve the uniformity of the pores  12 . To this end, a firstly-formed anodized aluminum oxide layer is removed with chromic acid at a temperature of 65° C. The second anodization is carried out under the same condition as the first anodization. 
         [0030]    Next, the aluminum is removed with a silver nitrate aqueous solution in order to leave only an anodized aluminum oxide layer formed by carrying out anodization twice. 
         [0031]    Lastly, the pores  12  are expanded by using a phosphoric acid aqueous solution, thereby improving the uniformity of the pores  12  of the anodized aluminum oxide nanotemplate  10 . Thus, finally, the preparation of the anodized aluminum oxide nanotemplate  10  having the plurality of the pores  12  is completed (see  FIG. 1 ). 
         [0032]    In the above procedure, the diameter of the pores  12  formed on the anodized aluminum oxide nanotemplate  10  is preferably several tens to several hundreds nanometers (nm). 
         [0033]    Next, referring to  FIG. 2 , an electrode layer  20  is formed on one surface of the anodized aluminum oxide nanotemplate  10 . 
         [0034]    Preferably, the electrode layer  20  is formed at a thickness of 300 to 400 nm by E-beam evaporation process on one surface of the anodized aluminum oxide nanotemplate  10  after completion of the formation of the anodized aluminum oxide nanotemplate  10 . Here, the material of the electrode layer  20  may comprise gold (Au). 
         [0035]    Next, referring to  FIG. 3 , multi-component nanowires  30  are grown through the pores  12  of the anodized aluminum oxide nanotemplate  10  by using an electroplating process. 
         [0036]    That is, the anodized aluminum oxide nanotemplate  10  having the plurality of pores  12  is injected in a multi-component solution, and then the electroplating process is carried out by using the anodized aluminum oxide nanotemplate  10  as a cathode. 
         [0037]    The multi-component solution used at this time may be an Ag—Se or Ge—Sb—Te solution for growing Ag—Se nanowires or Ge—Sb—Te nanowires that constitute the memory layer of the PRAM. 
         [0038]    In the electroplating, a platinum electrode may be used as an opposite electrode of the anodized aluminum oxide nanotemplate  10  which is used as the cathode, i.e., an anode. Here, the purity of the platinum is preferably 99.99%. 
         [0039]    If the multi-component solution is Ag—Se solution, the precursors may include Ag and Se and the solvents may include nitric acid and ethylglycerol. In this embodiment, in case of producing a total of 500 ml of Ag—Se solution, 0.1 mol of Ag, 0.05 mol of Se, 10 ml of nitric acid, and 100 ml of ethylglycerol were used. 
         [0040]    If the multi-component solution is Ge—Sb—Te solution, the precursors may include GeO 2 , SbO 2 , and TeO 2  and the solvents may include hydrochloric acid and ethylglycerol. In this embodiment, in case of producing a total of 500 ml of Ge—Sb—Te solution, 0.0428 mol of GeO 2 , 0.0028 mol of SbO 2 , 0.007 mol of TeO 2 , 10 ml of hydrochloric acid, and 100 ml of ethylglycerol were used. 
         [0041]    In the electroplating process of growing multi-component nanowires, a composition ratio of the multi-component nanowires can be determined depending on the change of at least one of a mixing ratio of the precursors in a multi-component solution and an applied current density. 
         [0042]    In this embodiment, if the multi-component solution is Ag—Se solution, the current density is preferably 2 mA/cm 2 , and if the multi-component solution is Ge—Sb—Te solution, the current density is preferably 1 mA/cm 2 . 
         [0043]    Lastly, referring to  FIG. 4 , the anodized aluminum oxide nanotemplate  10  is removed, thereby finally obtaining multi-component nanowires  30 . At this time, the anodized aluminum oxide nanotemplate  10  is preferably removed with NaOH (sodium hydroxide) solution. Afterwards, it is preferred that the multi-component nanowires  30  are washed with ultrapure water and ethanol. 
         [0044]      FIG. 5  is a photograph of Ag—Se nanowires taken by scanning electron microscopy (SEM). It can be seen that Ag—Se nanowires are grown uniformly in the anodized aluminum oxide nanotemplate. 
         [0045]      FIG. 6  is a photograph of Ag—Se nanowires taken by transmission electron microscopy (TEM). It can also be seen that Ag—Se nanowires are grown uniformly in the anodized aluminum oxide nanotemplate. 
         [0046]      FIG. 7  is a graph showing the results of analysis of Ag—Se nanowires obtained by energy dispersive X-ray spectroscopy (EDS). It can be seen that the composition ratio of Ag and Se constituting Ag—Se nanowires is 2:1, which is the same as the mixing ratio of the precursors (that is, Ag and Se) in the multi-component (Ag—Se) solution. 
         [0047]      FIGS. 8-9  show the properties of Ag—Se thin film formed on a silicon wafer by an electroplating process under the same environment as the multi-component (Ag—Se) solution in which Ag—Se nanowires are grown, which is a comparative example of  FIGS. 5-7 . To this end, gold (Au) is deposited at a thickness of 300˜400 nm on the silicon wafer by E-beam evaporation process, and then Ag—Se thin film is formed thereon. 
         [0048]      FIG. 8  is a photograph of Ag—Se nanowires taken by scanning electron microscopy, and  FIG. 9  is a graph showing the results of X-ray diffraction analysis of Ag—Se thin film. 
         [0049]    In  FIG. 9 , the upper peak represents an X-ray diffraction peak of Ag—Se thin film grown by the electroplating process in this embodiment, and the lower peak represents an X-ray diffraction peak of a typical silver selenide (Ag 2 Se). As shown therein, considering that the upper and lower peaks match each other, it can be seen that the composition ratio of Ag—Se nanowires grown by electroplating in this embodiment is 2:1, which is the mixing ratio of Ag and Se serving as the precursors in the multi-component (Ag—Se) solution. In view of this, according to the method for fabricating multi-component nanowires of the present invention, the composition ratio of final multi-component nanowires can be controlled based on the mixing ratio of the precursors in the multi-component solution. 
         [0050]      FIG. 10  is a photograph of Ge—Sb—Te nanowires taken by scanning electron microscopy, which also shows the results of analysis of GST nanowires obtained by energy dispersive X-ray spectroscopy (EDS). It can be seen that Ge—Sb—Te nanowires are grown uniformly in an anodized aluminum oxide nanotemplate and the composition of the Ge—Sb—Te nanowires is 1:6:6.5. From this, according to the method for fabricating multi-component nanowires of the present invention, even if there are three or more types of precursors in a multi-component solution, multi-component nanowires containing all of these precursors can be fabricated. 
         [0051]      FIG. 11  is a view of Ge—Sb—Te nanowires taken by transmission electron microscopy. Also, it can be found that Ge—Sb—Te nanowires are grown uniformly in an anodized aluminum oxide nanotemplate. 
         [0052]      FIGS. 12 and 13  are graphs showing current changes (I-V) of Ag—Se nanowires and Ag—Se thin film, which are the results measured by using conductive atomic force microscopy (C-AFM) and 4-probe station, respectively. 
         [0053]    As shown therein, both Ag—Se nanowires and Ag—Se thin film undergo an abrupt change in resistance at a certain voltage. Hence, this embodiment shows that Ag—Se nanowires grown by the electroplating process are sufficiently applicable to a nanowire-based PRAM device. 
         [0054]    As described above, the method for fabricating multi-component nanowires in accordance with the present invention can fabricate multi-component nanowires in a nanotemplate having a plurality of pores by electroplating with the use of a multi-component solution, thereby making the nanowires fabrication process easier and thus offering low fabrication cost. 
         [0055]    Although the method for fabricating multi-component nanowires in accordance with the present invention has been described with respect to Ag—Se nanowires and Ge—Sb—Te nanowires that are applicable to PRAM devices, the present invention is not limited thereto but may also be applied to Ag—Se nanowire-based, Ge—Sb—Te nanowire-based ReRAM devices, and In—Se nanowire-based, Te—Se nanowire-based, and In—Ag—Se nanowire-based PRAM and ReRAM devices. 
         [0056]    Further, the method for fabricating multi-component nanowires in accordance with the present invention may also be utilized in the fields of nanobiosensors, fuel cells, and biological cell isolation, in addition to the memory devices including PRAM and ReRAM. 
         [0057]    While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.