Abstract:
A field effect transistor includes a substrate, a first graphene (Gr) layer on the substrate, a second graphene (Gr) layer on the substrate, a fluorographene (GrF) layer on the substrate and between the first and second graphene layers, a first ohmic contact on the first graphene layer, a second ohmic contact on the second graphene layer, a gate aligned over the fluorographene layer, and a gate dielectric between the gate and the fluorographene layer and between the gate and the first and second ohmic contacts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims the benefit of U.S. Provisional Patent Application No. 61/767,922, filed Feb. 22, 2013, which is incorporated herein as though set forth in full. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to graphene transistors. 
     BACKGROUND 
     Several alternative forms of graphene transistors have been described utilizing wideband gap materials. 
     L. Britnell et al., “Field-effect Tunneling transistor based on vertical graphene heterostructures”, Science, vol. 335, p. 947, 2012 describe utilizing hexagonal boron nitride (h-BN) or MoS 2  as a wideband gap tunnel barrier for both vertical geometry field effect transistors (FETs) and vertical graphene heterostructure tunnel field effect transistors (FETs). The vertical geometry field effect transistors (FETs) demonstrated an on/off ratio of ˜50 (h-BN). The vertical graphene heterostructure tunnel field effect transistors (FETs) demonstrated an on/off ratio of ˜10 4  (MoS 2 ). 
     H. Yang, J. Heo, S. Park, H. J. Song, D. H. Seo, K. E. Byun, P. Kim, I. Yoo, H. J. Chung, and K. Kim, “Graphene Barristor, a triode device with a gate-controlled Schottky barrier”, Science, vol. 336, p. 6085, 2012 describe graphene/Si vertical barristors with gate control of the graphene/Si Schottky barrier height. 
     W. Mehr, J. Dabrowski, J. C. Scheytt, G. Lippert, Y.-H. Xie, M. C. Lemme, M. Ostling, and G. Lupina, “Vertical Graphene Base Transistor”, IEEE Electron Dev. Lett., vol. 33, pp. 691, 2012 describe a vertical graphene base transistor. 
     What is needed are improved graphene transistors. The embodiments of the present disclosure answer these and other needs. 
     SUMMARY 
     In a first embodiment disclosed herein, a field effect transistor comprises a substrate, a first graphene (Gr) layer on the substrate, a second graphene (Gr) layer on the substrate, a fluorographene (GrF) layer on the substrate and between the first and second graphene layers, a first ohmic contact on the first graphene layer, a second ohmic contact on the second graphene layer, a gate aligned over the fluorographene layer, and a gate dielectric between the gate and the fluorographene layer and between the gate and the first and second ohmic contacts. 
     In another embodiment disclosed herein, an integrated circuit comprises a substrate, a first heterostructure field effect transistor comprising a first graphene (Gr) layer on the substrate, a second graphene (Gr) layer on the substrate, a first fluorographene (GrF) layer on the substrate and between the first and second graphene layers, a first ohmic contact on the first graphene layer, a second ohmic contact on the second graphene layer, a first gate aligned over the first fluorographene layer, and a first gate dielectric between the first gate and the first fluorographene layer and between the first gate and the first and second ohmic contacts, wherein the first and second graphene (Gr) layers are n-type, and the first and second ohmic contacts are n-type, and a second heterostructure field effect transistor comprising a third graphene (Gr) layer on the substrate, a fourth graphene (Gr) layer on the substrate, a second fluorographene (GrF) layer on the substrate and between the third and fourth graphene layers, a third ohmic contact on the third graphene layer, a fourth ohmic contact on the fourth graphene layer, a second gate aligned over the second fluorographene layer, and a second gate dielectric between the second gate and the second fluorographene layer and between the second gate and the third and fourth ohmic contacts, wherein the first and second graphene (Gr) layers are n-type, and the first and second ohmic contacts are n-type. 
     In still another embodiment disclosed herein, a method of fabricating a heterostructure FET comprises forming graphene on a substrate, forming a graphene mesa by dry etching, forming a first ohmic contact for a source on one end of the graphene mesa, forming a second ohmic contact for a drain on an opposite end of the graphene mesa, forming an opening by masking and etching to expose a portion of the graphene between the first and second ohmic contacts, doping an exposed portion of the graphene with Fluorine, annealing to reduce defects, depositing a gate dielectric over the opening, and forming a gate over the gate dielectric. 
     These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a diagram of an n-type graphene (Gr), fluorographene (GrF), and graphene (Gr) heterostructure FET (HFET) in accordance with the present disclosure; 
         FIG. 1B  shows a band alignment diagram at zero source-drain bias for a Gr/GrF/Gr FET in accordance with the present disclosure; 
         FIG. 1C  shows a graph of a measured Gr/GrF/Gr diode current vs. diode voltage for two different devices with source-drain spacing of 150 nm and 250 nm, respectively, in accordance with the present disclosure; 
         FIG. 1D  shows a graph of a measured graphene HFET channel resistance as a function of gate voltage in accordance with the present disclosure; 
         FIG. 2A  shows a band alignment diagram at finite source-drain bias for a Gr/GrF/Gr FET in accordance with the present disclosure; 
         FIG. 2B  shows a measured direct current (DC) source-drain current-voltage characteristic graph in accordance with the present disclosure; 
         FIG. 2C  shows a current transfer curve graph as a function of gate voltage at room temperature in accordance with the present disclosure; 
         FIG. 3A  shows an equivalent circuit for a graphene HFET as a series resistor network in accordance with the present disclosure; 
         FIG. 3B  shows a channel current vs. Vds graph at room temperature for Lds=0.25 μm and Lds=0.5 μm, respectively, in accordance with the present disclosure; 
         FIG. 3C  shows a graph of the calculated on-state current with respect to GrF barrier thickness in accordance with the present disclosure; 
         FIG. 4A  shows a diagram of a p-type Gr/GrF/Gr HFET in accordance with the present disclosure; 
         FIG. 4B  shows integration of n- and p-type graphene HFETs on a common substrate in accordance with the present disclosure; and 
         FIG. 5  shows a method of fabricating a lateral graphene heterostructure FET in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention. 
     Referring now to  FIG. 1A  a lateral graphene HFET  10  is shown that has a lateral graphene heterostructure channel, which has a graphene (Gr) layer  14 , a fluorographene (GrF) layer  16 , and a graphene (Gr) layer  18  arranged laterally on a substrate  12 , which may be Si, SiO 2 , SiC, glass, or pyrex among other suitable substrate materials. The fluorographene (GrF) layer  16  is between the graphene (Gr) layer  14  and the graphene (Gr) layer  18 . Ohmic contacts  24  and  26  are on the graphene (Gr) layer  14  and graphene (Gr) layer  18 , respectively, for source and drain contacts. A gate  20 , which may be any metal, such as gold, is aligned with the fluorographene (GrF) layer  16  and insulated from the fluorographene (GrF) layer  16  by gate dielectric layer  22 , which may be silicon nitride (SiN), silicon dioxide (SiO 2 ), Boron Nitride (BN), aluminum oxide (Al 2 O 3 ), or hafnium oxide (HfO 2 ), among other suitable insulators. As shown in  FIG. 1A , the gate  20  is aligned with the fluorographene (GrF) layer  16  and the gate  20  has dimensions such that the gate  20  overlaps the fluorographene (GrF) layer  16  arranged laterally between the graphene (Gr) layer  14  and the graphene (Gr) layer  18 . The edges  21  on either side of the fluorographene (GrF) layer  16  are under and overlapped by gate  20 . In order to overlap the fluorographene (GrF) layer  16 , the gate  20  has a greater lateral dimension than the fluorographene (GrF) layer  16 . 
     The GrF  16  is a wide bandgap material, providing a potential barrier to carrier transport between the graphene (Gr) layer  14  and the graphene (Gr) layer  18 . The potential barrier of the GrF  16  is a function of the bias on gate  20 . With no gate  20  bias, the graphene HFET  10  is normally-off. 
       FIG. 1B  shows a schematic sketch of the band alignment at zero source-drain bias for the graphene (Gr)  14 , fluorographene (GrF)  16 , and graphene (Gr)  18  HFET  10 . The conduction and valence band offsets are defined by a work function difference ΔΦ=Φ Gr −Φ GrF .  FIG. 2A  shows a simplified schematic sketch of the Gr/GrF/Gr band alignment at a finite source-drain bias. 
     The current vs. voltage characteristics were measured for lateral Gr/GrF/Gr diodes with lateral channel lengths (Lds) of 150 nm and 250 nm, as shown in  FIG. 1C . The diodes showed highly nonlinear I-V curves with turn-on characteristics that indicate the formation of a potential barrier in the GrF layer  16  in the channel. 
       FIG. 1D  shows a graph of measured graphene HFET channel resistance as a function of gate voltage Vgs, which shows the Gr  14 /GrF  16 /Gr  18  channel resistance varying from 3 GΩ to 30 kΩ, which is an on-off resistance ratio of 10 5 , at room temperature with a gate bias applied from 0 V to +5 V. By contrast, the on-off resistance ratio due to modulation of a graphene only channel is less than 10. 
     Lateral graphene HFETs according to the present disclosure have a stable operation up to 100° C. 
     The heterostructure graphene HFET  10 , as shown in  FIG. 1A , is an n-type graphene HFET. In an n-type graphene HFET, the graphene (Gr)  14  and the graphene (Gr)  18  are n-type, and the ohmic contacts  24  and  26  are n type ohmic contacts, which may be titanium (Ti) based. 
     The heterostructure graphene HFETs were characterized at room temperature in terms of direct current (DC) source-drain current-voltage characteristics and transfer curves, as shown in  FIGS. 2B and 2C . As shown in  FIGS. 2B and 2C , current pinch-off is observed until the gate is biased to a sufficient positive voltage. The off-state current (Ioff) was measured to be 0.7 μA/mm at Vds=1 V and Vgs=0 V, which shows that lateral graphene HFETS have a normally-off and enhancement-mode operation as a consequence of the GrF barrier  16  in the channel. For Vgs&gt;2 V, the graphene HFETs turned-on and a saturated on-state current (Ion) of 5.2 mA/mm for Vds=1 V was measured. The resulting Ion/Ioff ratio at Vds=1 V was measured to be 7,000. The measured I-V curves, as shown in  FIG. 2B , also show excellent saturation behavior of the source-drain current.  FIG. 2B  shows I-V curves for Vgs=+4V, reference  30 ; Vgs=+3.5V, reference  32 ; Vgs=+3V, reference  34 ; Vgs=+2.5V, reference  36 ; Vgs=+2V, reference  38 ; and Vgs=0-2V, reference  40 . 
       FIG. 3A  shows an equivalent circuit for a graphene HFET as series resistor network, showing ohmic contact  24  resistance  50 , graphene layer  14  resistance  52 , GrF  16  resistance  54 , graphene layer  18  resistance  56 , and ohmic contact  26  resistance  58 . 
       FIG. 3B  shows a graph of the channel current versus Vds at room temperature for channel length Lds=0.25 μm and Lds=0.5 μm, respectively. The channel current density is 2.5 A/mm for Lds=0.5 μm and 3.1 A/mm for Lds=0.25 μm at Vds=1 V, as shown in  FIG. 3B . 
       FIG. 3C  is a graph of on-state current with respect to the GrF  16  barrier thickness.  FIG. 3C  shows a calculated on-state current density for Gr/GrF/Gr FETs with two different mean free paths of 8 nm and 50 nm, with the current density of 3.0 A/mm as I ballistic . In the case of λ=50 nm, 1000 mA/mm of on-state current can be achieved with ˜50 nm of GrF barrier thickness. 
     A heterostructure graphene HFET  60 , as shown in  FIG. 4A , may be a p-type graphene HFET. In a p-type graphene HFET  60 , the lateral graphene heterostructure channel has a p-type graphene (Gr) layer  62 , a fluorographene (GrF) layer  64 , and a p-type graphene (Gr) layer  66  arranged laterally on a substrate  12 . Ohmic contacts  68  and  69  are on the graphene (Gr) layer  62  and graphene (Gr) layer  66 , respectively, for source and drain contacts. The ohmic contacts  68  and  69  are p type ohmic contacts and may be palladium (Pd) based. A gate  70  is aligned with the fluorographene (GrF) layer  64  and insulated from the fluorographene (GrF) layer  64  by gate dielectric layer  72 . As shown in  FIG. 4A , the gate  70  is aligned with the fluorographene (GrF) layer  64  and the gate  70  has dimensions such that the gate  70  overlaps the fluorographene (GrF) layer  64  arranged laterally between the graphene (Gr) layer  62  and the graphene (Gr) layer  66 . In order to overlap the fluorographene (GrF) layer  64 , the gate  70  has a greater lateral dimension than the fluorographene (GrF) layer  64 . 
     The n-type graphene HFET  10 , shown in  FIG. 1A , and the p-type graphene HFET  60 , shown in  FIG. 4A , may be integrated on a common substrate  12 , as shown in  FIG. 4B . 
     As shown in the flow diagram of  FIG. 5 , a method of fabricating a lateral graphene heterostructure FET includes the following steps: in step  100  forming graphene on a substrate by chemical vapor deposition (CVD) or epitaxial processes, in step  102  forming a graphene mesa by dry etching, in step  104  forming ohmic contacts on opposite ends of the graphene mesa for source and drain contact. The ohmic contacts may be either n-type or p-type, in step  106  forming an opening by masking and etching to expose a portion of the graphene between the source and drain, in step  108  doping the exposed portion of the graphene with Fluorine doping using either XeF2 or other gas chemistry or gas plasma chemistry, in step  110  annealing to reduce defects, in step  112  depositing a gate dielectric in the opening, and in step  114  forming a metal gate over the gate dielectric. This can be followed by a metal2 layer process to connect with other circuitry. 
     The step of forming graphene on a substrate or wafer may be use normal by epitaxial processes. Alternatively, the step of forming graphene on a substrate or wafer may be performed by using chemical vapor deposition (CVD) to deposit a graphene layer on a metal foil and then transferring the graphene layer to a substrate. 
     As discussed above, the substrate  12  may be Si, SiO 2 , SiC, glass, or pyrex among other suitable substrate materials. The fluorographene (GrF) layer  16  formed between the graphene (Gr) layer  14  and the graphene (Gr) layer  18 . The ohmic contacts  24  and  26  are formed on the graphene (Gr) layer  14  and graphene (Gr) layer  18 , respectively, for source and drain contacts. The gate  20  is aligned with the fluorographene (GrF) layer  16  and insulated from the fluorographene (GrF) layer  16  by gate dielectric layer  22 , which may be silicon nitride (SiN), silicon dioxide (SiO 2 ), Boron Nitride (BN), aluminum oxide (Al 2 O 3 ), or hafnium oxide (HfO 2 ), among other suitable insulators. 
     In an n-type graphene HFET, the graphene (Gr)  14  and the graphene (Gr)  18  are n-type, and the ohmic contacts  24  and  26  are n type ohmic contacts, which may be titanium (Ti) based. In a p-type graphene HFET  60 , the lateral graphene heterostructure channel has a p-type graphene (Gr) layer  62  and a p-type graphene (Gr) layer  66 , and the ohmic contacts  68  and  69  are p type ohmic contacts and may be palladium (Pd) based. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein. 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . ”