Scaling of Al2O3 dielectric for graphene field-effect transistors

Fallahazad, Babak; Lee, K.; Lian, Guoda; Kim, Seyoung; Corbet, Chris M.; Ferrer, Domingo; Colombo, Luigi; Tutuc, Emanuel

doi:10.1063/1.3689785

Applied Physics Letters

2012

DOI: 10.1063/1.3689785

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Scaling of Al2O3 dielectric for graphene field-effect transistors

Babak Fallahazad

K. Lee

Guoda Lian

et al.

Abstract: We investigate the scaling of Al2O3 dielectric on graphene by atomic layer deposition (ALD) using ultra-thin, oxidized Ti and Al films as nucleation layers. We show that the nucleation layer significantly impacts the dielectric constant (k) and morphology of the ALD Al2O3, yielding k = 5.5 and k = 12.7 for Al and Ti nucleation layers, respectively. Transmission electron microscopy shows that Al2O3 grown using the Ti interface is partially crystalline, while Al2O3 grown on Al is amorphous. Using a spatially uni… Show more

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Cited by 115 publications

(102 citation statements)

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“…To be accurate, a stack fabricated with the same process was used to determine the dielectric constant by using a dual gate measurement. 31 As a result, Fermi level shifts (E f ) are found to be 0.33 eV (Al 2 O 3 top gate) and 0.30 eV (HfO 2 top gate) above the Dirac point in graphene when there is no applied gate bias or V G = 0.…”

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confidence: 99%

Direct Measurement of Dirac Point Energy at the Graphene/Oxide Interface

Zeng

Zhang

et al. 2012

Nano Lett.

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ABSTRACT:We report the direct measurement of the Dirac point, the Fermi level, and the work function of graphene by performing internal photoemission measurements on a graphene/SiO 2 /Si structure with a unique optical-cavity enhanced test structure. A complete electronic band alignment at the graphene/SiO 2 /Si interfaces is accurately established. The observation of enhanced photoemission from a one-atom thick graphene layer was possible by taking advantage of the constructive optical interference in the SiO 2 cavity. The photoemission yield was found to follow the well-known linear density-of-states dispersion in the vicinity of the Dirac point. At the flat band condition, the Fermi level was extracted and found to reside 3.3 eV ± 0.05 eV below the bottom of the SiO 2 conduction band. When combined with the shift of the Fermi level from the Dirac point, we are able to ascertain the position of the Dirac point at 3.6 eV ± 0.05 eV with respect to the bottom of the SiO 2 conduction band edge, yielding a work function of 4.5 eV ± 0.05 eV which is in an excellent agreement with theory. The accurate determination of the work function of graphene is of significant importance to the engineering of graphene-based devices, and the measurement technique we have advanced in this Letter will have significant impact on numerous applications for emerging graphene-like 2-dimensional material systems. KEYWORDS: Graphene, work function, internal photoemission, band alignment, graphene−insulator−semiconductor S ince the pioneering work of Novoselov et al. in 2004, 1 graphene has attracted an immense amount of interest from many related disciplines.2,3 Fundamental knowledge of the physical properties of graphene and the physical mechanisms governing the electrical operation of graphenebased devices has grown dramatically. 4 With the recent success of large area chemical vapor deposition (CVD) growth of graphene, 5 industrial applications such as transparent electrodes, 6 field-effect transistors (FETs), 7 and quantum well devices 8 are becoming more promising. Many studies have been conducted to characterize the various physical properties of graphene, including the work function, which is one of the most important electronic parameters. Among the numerous investigations by techniques such as Kelvin probe measurements, 9−11 ab initio calculations, 9,12 and recently by capacitance−voltage measurements, 13 the values of work function scatter in a rather wide range from 4.2 to 5.0 eV. Experimentally, the work function may be found to vary depending on the type of metal contact due to interactions between the graphene and the metal, which may result in pinning of the work function.9−13 Surprisingly, there is little information on the intrinsic electronic band alignment of the graphene/oxide interface to date, despite its important role in the design, fabrication, and characterization of graphene-based devices. For example, the accurate band alignment between graphene and another material determines how effectively to turn on an...

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confidence: 99%

Direct Measurement of Dirac Point Energy at the Graphene/Oxide Interface

Zeng

Zhang

et al. 2012

Nano Lett.

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“…This has been attributed to the roughening of the graphene surface. However, using atomic layer deposition (ALD), Fallahazad and co-workers [5,8] have been able to deposit ultrathin high-κ dielectric materials on SLG with much less roughness and observe a significant improvement in the carrier mobility. Hollander and co-workers were also able to see an increase of the Hall mobility in epitaxial graphene with thinner top gate dielectrics [15].…”

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confidence: 99%

“…We consider three top gate dielectrics, SiO 2 (κ=3.9), Al 2 O 3 (κ=12.5) [8] and HfO 2 (κ=22.0) [5]. The bottom dielectric is assumed to be SiO 2 .…”

mentioning

confidence: 99%

“…However, their theory is limited to the simple geometry of SLG supported by a substrate. In more realistic graphene heterostructures, the SLG is encapsulated between the substrate and the top gate [5,8,9], an arrangement which offers better local electrostatic control than the bottom gate and is probably required for large scale integration. On the other hand, top gates modify the dielectric environment of the SLG by introducing image charges along the various interfaces.…”

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confidence: 99%

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Charged impurity scattering in top-gated graphene nanostructures

Ong

Fischetti

2012

Phys. Rev. B

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We study charged impurity scattering and static screening in a top-gated substrate-supported graphene nanostructure. Our model describes how boundary conditions can be incorporated into scattering, sheds light on the dielectric response of these nanostructures, provides insights into the effect of the top gate on impurity scattering, and predicts that the carrier mobility in such graphene heterostructures decreases with increasing top dielectric thickness and higher carrier density. An increase of up to almost 60 percent in carrier mobility in ultrathin top-gated graphene is predicted.

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“…Recently, graphene FETs have been reported with ALD Al2O3 topgate insulators as thin as 2.6 nm. 9 Insulators fabricated by PVD and ALD, however, suffer from the dielectric breakdown at the voltage much lower than the complete breakdown voltage due to the fragility of dielectrics accumulated by the low-field leakage during the iterative measurements. Typical electrical field for this low-field dielectric breakdown is ~0.2 V/nm.…”

mentioning

confidence: 99%

Large Fermi energy modulation in graphene transistors with high-pressure O₂-annealed Y₂O₃topgate insulators

et al. 2014

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We demonstrate a considerable suppression of the low-field leakage through a Y2O3 topgate insulator on graphene by applying high-pressure O2 at 100 atm during post-deposition annealing (HP-PDA). Consequently, the quantum capacitance measurement for the monolayer graphene reveals the largest Fermi energy modulation (EF = 0.52 eV, i.e., the carrier density of 2×10 13 cm -2 ) in the solid-state topgate insulators reported so far. HP-PDA is the robust method to improve the electrical quality of high-k insulators on graphene.The deposition of ultrathin and reliable high-k dielectrics on graphene is required to realize the high transconductance in graphene field-effect transistors (FETs). Generally, two kinds of deposition methods are used: one is physical vapor deposition 1-4 (PVD) with low particle energies to avoid the defect introduction in graphene, 5 the other is atomic layer deposition (ALD) with buffer layers 6-8 to overcome the chemically inert surface of graphene. Recently, graphene FETs have been reported with ALD Al2O3 topgate insulators as thin as 2.6 nm. 9 Insulators fabricated by PVD and ALD, however, suffer from the dielectric breakdown at the voltage much lower than the complete breakdown voltage due to the fragility of dielectrics accumulated by the low-field leakage during the iterative measurements. Typical electrical field for this low-field dielectric breakdown is ~0.2 V/nm. This is a critical issue for the reliability of the topgate insulators. A common limitation for the insulators on graphene fabricated by both PVD and ALD is the lack of a robust methodology for post-deposition annealing (PDA) in an O2 atmosphere. Although PDA at a high temperature (e.g., ~500 C) is known to improve the electrical quality of the insulator considerably, 10 it introduces many defects in graphene by oxidation. 11 Here, from a thermodynamic viewpoint, 12 let's consider the Gibbs free energy change (G = G -RTlnPO2) for the oxidation reaction of M + O2 = MO2, where G is the standard Gibbs free energy change for the oxidation reaction, R is gas constant, PO2 is the oxygen partial pressure and M is the metal. G should be negative to facilitate the oxidation. The first term for G can be used for material selection. An appropriate rare-earth element for use in high-k insulators should be highly susceptible to oxidation. Figure 1(a) shows G of rare-earth, transition and representative elements at 300 C calculated using a thermodynamic database. 13 The G for yttrium is negatively largest among all of the metal oxides, even smaller than that of carbon. Therefore, Y2O3 can be obtained at relatively low oxidation temperatures and is thermodynamically stable on graphene. The band gap for Y2O3 is ~5.5 eV, which is almost identical to that of h-BN. 14 The dielectric constant of Y2O3 is ~12, 15 while it is ~3-4 for h-BN. 16 The second term for G is the process condition, which is derived from the oxygen potential. G should be further decreased to reduce the defects in the insulator such as oxygen vacancy. This c...

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Scaling of Al2O3 dielectric for graphene field-effect transistors

Cited by 115 publications

References 19 publications

Direct Measurement of Dirac Point Energy at the Graphene/Oxide Interface

Direct Measurement of Dirac Point Energy at the Graphene/Oxide Interface

Charged impurity scattering in top-gated graphene nanostructures

Large Fermi energy modulation in graphene transistors with high-pressure O₂-annealed Y₂O₃topgate insulators

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Scaling of Al2O3 dielectric for graphene field-effect transistors

Cited by 115 publications

References 19 publications

Direct Measurement of Dirac Point Energy at the Graphene/Oxide Interface

Direct Measurement of Dirac Point Energy at the Graphene/Oxide Interface

Charged impurity scattering in top-gated graphene nanostructures

Large Fermi energy modulation in graphene transistors with high-pressure O2-annealed Y2O3topgate insulators

Contact Info

Product

Resources

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Large Fermi energy modulation in graphene transistors with high-pressure O₂-annealed Y₂O₃topgate insulators