Patent Application: US-201414188736-A

Abstract:
a coherent spin field effect transistor is provided by depositing a ferromagnetic base like cobalt on a substrate . a magnetic oxide layer is formed on the cobalt by annealing at temperatures on the order of 1000 ° k to provide a few monolayer thick layer . where the gate is cobalt , the resulting magnetic oxide is co 3 o 4 . other magnetic materials and oxides may be employed . a few ml field of graphene is deposited on the cobalt oxide by molecular beam epitaxy , and a source and drain are deposited of base material . the resulting device is scalable , provides high on / off rates , is stable and operable at room temperature and easily fabricated with existing technology .

Description:
referring initially to the schematic illustration in fig1 , and recognizing that this is a “ conventional representation ” and that structure and dimensions will be subject to modification depending on the ultimate application envisioned , the formation of each layer of the coherent spin - fet of the invention is described . this is best begun with a discussion of the field itself , as shown . graphene — this film must consist of ( 111 )- oriented , sp 2 carbon ( graphene ), as either a single layer or several layers , as desired to control potential oxide / graphene interactions such as charge transfer [ 10 , 11 ]. this layer can be deposited by molecular beam epitaxy , or possibly by chemical or physical vapor deposition . the deposition of graphene on a substrate has been described at some length in u . s . patent application ser . no . 12 / 543 , 053 and u . s . patent application ser . no . 12 / 980 , 763 , both of which are included herein - by - reference . additional advances in the control over few layer graphene deposition are provided in u . s . provisional patent application ser . nos . 61 / 490 , 650 and 61 / 497 , 971 , both of which are incorporated herein - by - reference . the controlled direct growth of graphene by mbe ( layer - by - layer growth of macroscopically continuous graphene sheets on co 3 o 4 ( 111 ) at 1000 k by carbon molecular beam epitaxy ( mbe ) from a graphite rod source ) is described in detail in u . s . provisional patent application ser . no . 61 / 522 , 600 . the disclosure of this pending application is incorporated herein - by - reference . any of the methods described in the incorporated applications can be used to form the graphene field , with a preference for controlled molecular beam epitaxy . magnetic oxide , source and drain : this material electrically isolates the graphene from the ferromagnetic gate layer , and allows polarization of the graphene valence / conduction electrons via polarization of the cations in the magnetic oxide . potential candidates include co 3 o 4 ( 111 ), fe 3 o 4 ( 111 ), nio ( 111 ), and potentially spinels such as cofe 2 o 4 ( 111 ), as well as cr 2 o 3 ( 111 ), bafe 2 o 4 . a critical feature is the polarization within the ion layer adjacent to the graphene ( fig1 ). the polarization of the ions must be ferromagnetic within each layer , even if adjacent ion layers in the oxide are polarized antiferromagnetically to each other . a uniform ferromagnetic polarization within the surface layer is needed to polarize the graphene electrons . further , the direction of polarization is important . if the oxide ions are polarized in a direction parallel to the surface plane , then the graphene will be similarly polarized , and so must the source and drain . in that case , appropriate source / drain materials could be co , ni , fe , or various alloys . however , if the oxide cations are polarized perpendicularly to the surface plane , then the source and drain should have easy axes of magnetization perpendicular to the plane , and should be made of materials such as permalloy , co — pd alloys and multilayers , co — pt alloys and multilayers , fe — pt alloys and multilayers , some llo ferromagnetic compounds , etc . similar nickel - iron alloys , and triblends , such as molybdenum permalloy , may also be used . note , since nio ( 111 ) has the same rocksalt structure as mgo ( 111 ), deposition of graphene may result in oxide reconstruction , destroying the chemical equivalence of graphene a sites and b sites [ 12 ], removing the homo / lumo equivalence at the dirac point and opening a band gap , as set forth in u . s . patent application ser . no . 12 / 980 , 763 . development of bandgap potential in graphene bearing materials may provide important electronic advantages , in addition to preserving spintronic adaptability . ferromagnetic gate : this layer should have its axis of magnetic polarization that is easily switchable and ideally has a low coercive field for multifunction logic gates , and be ferromagnetic at room temperature . appropriate materials include co , ni , or fe . in order to prepare a coherent spin - fet of the invention , a sapphire ( aluminum oxide ( 0001 )) substrate is prepared for deposition . an electron beam evaporator may be used to reduce movement between chambers and improve productivity , switching in various targets for deposition . thus , a fifty angstrom layer of cobalt may be deposited under conventional conditions on the substrate at 750 ° k in uhv . this deposition is followed by an oxidation anneal at 1000 ° k which results in surface segregation of dissolved oxygen and the development of a thin layer of co 3 o 4 ( 111 ) ( may be 2 - 5 ml thick ). graphene ( 2 or 3 ml ) is deposited on the co 3 o 4 using molecular beam epitaxy at 1000 ° k , yielding a macroscopically continuous graphene film of approximately 3 ml thickness . graphene may also be deposited via cvd and pvd processes , as disclosed , but mbe is preferred , not only because of the fine control and developed information for this method , but because it is compatible with the other process steps in the formation of the coherent spin - fet of the invention . this leads to high productivity . in this example , the coherent spin - fet is finished with the application of co source and drain , and connected in the device for use . on magnetization , it is stable and exhibits a very high on / off rate with low power consumption at temperatures substantially above room temperature . thus , referring to fig1 , the coherent spin - fet prepared has a gate of cobalt , with an insulating layer of cobalt oxide ( co 3 o 4 ). a few ml layer of graphene is deposited over the magnetic oxide , with co source and drain . application of a field to the device gives a durable , low power , high on / off rate spin - fet that is operable at room temperature . the resulting coherent spin - fet gives on / off rates ( or switching rates ) of at least 10 7 - 10 8 per second depending on the materials and conditions selected . 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