Abstract:
A graphene field effect transistor includes a gate stack, the gate stack including a seed layer, a gate oxide formed over the seed layer, and a gate metal formed over the gate oxide; an insulating layer; and a graphene sheet displaced between the seed layer and the insulating layer.

Description:
[0001]    This invention was made with Government support under FA8650-08-C-7838 awarded by DARPA. The Government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to transistors, and more specifically, to field effect transistors formed with graphene channels. 
         [0003]    Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It can be viewed as an atomic-scale chicken wire made of carbon atoms and their bonds. 
         [0004]    Graphene possesses great potential for high-speed electronics because of its high carrier mobility and the ultra-thin single atom body thickness. In addition to the carrier mobility, the performance of a transistor also depends on the parasitic capacitance and resistance associated with its terminals. The impact of these parasitic capacitances on device performance becomes particularly important with the shrinking of device dimensions. In order to minimize the parasitic capacitance between source/drain and gate electrodes, it is necessary to maintain a device structure where the gate electrode lies between the source/drain electrodes without overlaps. However, this gap (the un-gated region between source/drain and the gate) gives rise to an access resistance, and contributes to the overall drain-to-channel and source-to-channel parasitic resistances. Therefore, the most desirable transistor structure is a self-aligned device where the gate and the source/drain electrodes align to each other without overlaps and with minimum gaps. 
         [0005]    In silicon (Si) MOSFETs, self-aligned FETs are usually realized by forming an oxide side-wall surrounding the gate stack before the formation of source/drain contacts. However, this technique cannot be directly applied in the case of graphene due to the differences in material processing requirements between Si and carbon. Currently, there is no scheme known to fabricate such self-aligned graphene transistors. 
       SUMMARY 
       [0006]    According to one embodiment of the present invention, a graphene field effect transistor is disclosed. The field effect transistor of this embodiment includes a gate stack that includes a seed layer, a gate oxide formed over the seed layer, and a gate metal formed over the gate oxide. The field effect transistor of this embodiment also includes an insulating layer and a graphene sheet displaced between the seed layer and the insulating layer. 
         [0007]    According to another embodiment of the present invention, a method of forming a graphene field effect transistor is disclosed. The method of this embodiment includes providing an insulating layer; forming a graphene sheet on the insulating layer; forming a seed layer on the graphene sheet; forming a gate dielectric layer over the seed layer; forming a gate on top of the gate dielectric layer; encapsulating the seed layer, the gate dielectric and the gate in a spacer material to form an encapsulated gate stack; forming a source contact on the graphene sheet on a first side of the encapsulated gate stack; and forming a drain contact on the graphene sheet on a second side of the encapsulated gate stack. 
         [0008]    Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0010]      FIG. 1  shows a first processing stage of forming a FET according to one embodiment of the present invention; 
           [0011]      FIG. 2  shows the structure of  FIG. 1  after a seed layer has been added; 
           [0012]      FIG. 3  shows the structure of  FIG. 2  after an oxide layer has been formed on the seed layer; 
           [0013]      FIG. 4  shows the structure of  FIG. 3  after a gate has been formed over the oxide layer; 
           [0014]      FIG. 5  shows the structure of  FIG. 4  after the portions of the seed and oxide layers not covered by the gate have been removed; 
           [0015]      FIG. 6   a  shows the structure of  FIG. 4  after the gate, seed layer and oxide layer have been encapsulated by a spacer material; 
           [0016]      FIG. 6   b  shows the structure of  FIG. 6   a  in three dimensions; 
           [0017]      FIG. 7  shows the structure of  FIG. 6   b  after portions of the spacer material have been removed; and 
           [0018]      FIG. 8  shows the structure of  FIG. 7  after source and drain contacts have been added. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    One embodiment of the present invention is directed to a self-aligned, top-gated grapheme field effect transistor (FET) device. In one embodiment, the gate electrode aligns the source/drain electrodes without resorting to any lithographic alignment procedures. In particular, the surface properties of graphene are utilized to form an oxide side-wall around the gate electrode, as described below, thus enabling a self-aligned gate similar to that achieved in state-of-the-art Si MOSFETs through different methods. 
         [0020]    It has been known that ALD (atomic layer deposition) oxide cannot be directly deposited on a clean graphene surface. In order to form a uniform oxide layer on graphene using the ALD process, the graphene surface has to be first functionalized or covered with some kind of seed layer. Embodiments of the present invention may take advantage of this unique ability to selectively deposit ALD oxide on graphene depending on the surface configuration. In addition, a method to fabricate top-gated graphene FET with self-aligned source/drain and gate terminals is disclosed. 
         [0021]    In general, in one embodiment, a top gate stack consisting of a layer of ALD oxide and metal electrode is formed. The gate metal electrode is then used as the etch mask to remove the oxide and seed layer. This results in a clean graphene surface except for the regions under the gate stack. In a subsequent ALD process, a thin layer of oxide is deposited on the gate stack, leaving the rest of the graphene surface void of any oxide. Source and drain contacts are finally formed on the “clean” graphene region. 
         [0022]    It will be understood, that the second oxide coating on the gate stack serves as the spacer between the source/drain and gate electrodes. This spacer creates a minimum un-gated region. 
         [0023]    Some of the advantages of the present invention include, but are not limited to, parasitic resistance and capacitance minimization by the self-aligned gating, and hence improving the high frequency performance of the device. In addition, the method disclosed herein takes advantage of the unique surface properties of graphene and is highly manufacturable and scalable. The method does not rely on any lithographic alignment processes to achieve the alignment of source/drain and gate electrodes. High-k ALD oxide may be used as the gate dielectric, which allows for the ultimately nano-scaled graphene FET. 
         [0024]    With reference now to  FIG. 1 , an example of a structure in the production process of a FET according to one embodiment of the present invention is shown. The structure includes a substrate  102 . The substrate  102  may be formed of any material but, in one embodiment, is formed of silicon. It shall be understood that the substrate  102  may be omitted in some embodiments but may be useful in the event that the grapheme FET is to be integrated with other elements. 
         [0025]    An insulating layer  104  is disposed on top of the substrate  102 . The insulating layer  104  may be formed of any electrically insulating material. In one embodiment, the insulating layer  104  is formed of an oxide. In another embodiment, the insulating layer  104  is formed of an undoped silicon carbide (SiC) material. 
         [0026]    A graphene layer  106  is disposed on top of the insulating layer  104 . The graphene layer  106  will ultimately form the source, drain and channel of a FET according to one embodiment. In order to increase transistor switching speed, both the channel length and height (or depth) need to be minimized. Utilizing a grapheme channel provides the minimal (one atom thick) channel height. Utilizing the self-alignment disclosed herein may help reduce the channel length. 
         [0027]    The graphene layer  106  may be deposited on the insulating layer  104 . In the event the insulating layer  104  is formed of SiC, the graphene layer  106  may be grown directly on the insulating layer. Regardless of how formed, the graphene layer  106  may have a height of only one atom in one embodiment. Of course, the graphene layer  106  could also consist of mono, bi-, or few layer graphene in some embodiments. 
         [0028]    In order to form an oxide (gate oxide layer) on the graphene layer  106 , the graphene layer must be functionalized. Functionalization shall refer to a process that makes it possible for an oxide to be deposited by ALD on top of the graphene layer  106 . One way in which this may be done is to deposit a layer of seed layer by direct physical vapor deposition, such as Al 2 O 3  formed by the natural oxidation of Al. In the following description, such an additional layer shall be referred to as a seed layer. 
         [0029]    As shown in  FIG. 2 , a seed layer  202  is formed on top of the graphene layer  106  and contacts at least a portion of the insulating layer  104 . The seed layer  202  may be formed in a variety of manners. For example, the seed layer  202  may be formed by depositing NO 2  or spin-coating various polymers onto the graphene layer  106 . In another embodiment, a metal may be deposited over the graphene layer  106  by physical vapor deposition (PVD) and then oxidized. For instance, the metal may be aluminum (Al) which, when oxidized, forms aluminum oxide (Al 2 O 3 ). 
         [0030]    An intrinsic property of graphene is that an oxide cannot be directly formed on graphene. Such a limitation applies to ALD deposition as well. Accordingly, the seed layer  202  allows for an oxide (in this case, a so-called “gate dielectric”) to be formed over the graphene layer  106 . The process of adding a seed layer  202  shall be referred to herein as “functionalizing” the graphene layer  106 . Of course, other methods of functionalizing the graphene layer  106  may be utilized. 
         [0031]    After the graphene layer  106  has been functionalized, and as shown in  FIG. 4 , an oxide layer  302  is formed over the seed layer  202  so that it overlays at least a portion of the graphene layer  106 . In one embodiment, the oxide layer  302  is deposited by ALD. The oxide layer  302  may form the dielectric layer of gate. To that end, the oxide layer  302  may be formed of a dielectric material. In one embodiment, the oxide layer  302  may be formed of a high-k dielectric material. The oxide layer  302  may have a thickness ranging from 1 to 20 nm. Of course, this range is not required. The oxide layer  302  may be formed, for example, of halfnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ), or Al 2 O 3 . 
         [0032]      FIG. 4  shows the structure of  FIG. 3  after a gate electrode  402  has been formed. The gate electrode  402  may be formed of a metal in one embodiment. In one embodiment, metal is deposited as the gate electrode  402  by lithography and lift-off. Of course, other methods of forming the gate may be utilized. 
         [0033]      FIG. 5  shows the structure of  FIG. 4  after portions of the oxide layer  302  and seed layer  202  not covered by the gate electrode  402  have been removed. The remaining portion of the oxide layer  302  forms a gate dielectric  502  under which is a thin seed layer portion  504 . The oxide layer may be removed by etching and the seed layer may be removed in solvents or chemicals. In one embodiment, the thin seed layer portion  504  is thin enough that it will not interfere with the operation of the graphene layer  106  when a gate voltage is applied to the gate electrode  402 . 
         [0034]      FIG. 6   a  shows the structure of  FIG. 5  after a gate spacer  602  has been formed around the gate electrode  402 , the gate dielectric  502  and the thin seed layer portion  504  (collectively, the gate stack). To form the spacer  602  on the gate stack, an ALD process is again employed to deposit a layer of thin oxide surrounding the gate stack. Note that since the seed layer is removed everywhere else on the substrate, no oxide is deposited on the graphene layer  106 . That is, the spacer  602  does not adhere to the graphene layer  106 . This allows for the source and drain contacts to be made in the subsequent processes without having to perform any patterning or etching. Thus, the length of the channel is defined by the length of the gate stack. The distance between the gate stack and the source and drain is defined by the thickness of the spacer  602 . 
         [0035]      FIG. 6   b  is a three dimensional depiction of the structure shown in  FIG. 6   a . For purposes of  FIG. 6   b  it is assumed that the spacer  602  is deposited substantially only in the area of the gate stack. Of course, the spacer  602  could be deposited on other portions of the insulating layer  104  as well. It should be noted that the spacer  602  is not present over the graphene layer  106 . As discussed above, most materials will not stick to graphene. As the exposed portions of the graphene layer  106  will form the source and drain, the gate stack aligns the location thereof. Accordingly, the source and drain are self-aligned with the gate. 
         [0036]      FIG. 7  shows the structure of  FIG. 6   b  after a portion of the gate spacer  602  has been removed to expose the gate electrode  402 . The oxide that forms the gate spacer  602  may be removed, for example by lithography and etching. The gate spacer  602  is removed to allow for electrical contact with the gate electrode  402 . 
         [0037]      FIG. 8  shows the structure of  FIG. 7  after the gate stack and graphene layer  106  have been covered with an electrode material. The electrode material is shown by electodes  802 ,  804  and  806 . Electrodes  802  and  804  may form source and drain contacts of a FET. The ungated area (defined as the portion of the graphene layer  106  under the spacer  602 ) has minimal dimensions. This allows for a FET having a channel length that where a majority of the length is comprised by the gate. That is, the only limit on reducing the channel length is the minimal gate length required. 
         [0038]    The source and drain contacts ( 802 / 804 ) to the graphene layer  106  are made by depositing a thin layer of metal on the “clean” graphene layer  106 , as shown in  FIG. 9 . Unlike oxides, metal electrodes do not require a seed layer or functionalization agents to adhere to graphene. Note that the oxide spacer  602  ensures that the source, drain and gate contacts are not electrically shorted. The final structure is a self-aligned, top-gated graphene FET where the gate is separated from the source/drain region by a side-wall, the thickness of which is smaller than the alignment resolution that can be achieved by any state-of-the-art lithography. 
         [0039]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
         [0040]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated 
         [0041]    The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
         [0042]    While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.