Graphene Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109 Single atomic layer of graphite 1 I. Graphene Electronic Properties (isolated graphene sheets) II. Graphene Formation—Growth on SiC III.Graphene Growth on BN, Co3O4, etc. 2 Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109 3 Graphene’s band structure yields unusual properties Castro Neto EF The velocity of an electron at the Fermi level (vF) Is inversely related to meff Effective mass (m*) ~ [dE2/dk2]-1 Most semiconductors, 0.1 m0 < m* < 1 me Graphene, m* < 0.01 m0 (depending on number of carriers) Therefore, expect VERY high mobility in graphene Both holes and electrons can be carriers 4 Effective mass for graphene does get very small as n~ 1012 Castro-Neto, et al. Rev. Mod. Phys. 81 (2009) 109 5 6 The Big Problem with graphene: an imagined conversation: A. OK: Graphene is great, lots of interesting properties for devices! B. How do you make a device? A. You need a sheet of graphene! B. OK, how do you get a sheet of graphene? A. HOPG, scotch tape, and tweezers! B. [email protected]#$%% 7 How do you “grow” graphene? You can evaporate Si from SiC(0001) (either face) Popularized by the de Heer group at Georgia Tech. 8 Can grow multilayer films of graphene on SiC (azimuthally rotated from each other—electronically decoupled!) Anneal at 1350 C Interfacial layer (anneal at 1150 C) SiC Auger, graphene growth on SiC, deHeer et al. 9 Inverse photoemission and LEED (Forbeaux, et al, PRB, 58 (1998) 16396) Growth of graphite on SiC(0001) π* feature 10 Angle resolved UPS (Emtsev, et al, PRB 77(2008) 155303) shows transition to graphene band structure 11 Adjacent layers on graphene /SiC are decoupled from each other, Due to azimuthal rotation 12 Graphene on SiC(0001) Not uniform on an atomic level, different regions due to different #s of layers, orientations M B 13 Graphene/SiC photoemission: varying hv can vary the sampling depth (Emtsev, et al, PRB 77 (2008) 155303 14 The covalently bound stretched graphene (CSG model) Emtsev, et al., PRB 77 (2008) 155303 15 Pertinent Questions: How do Adjacent Graphene Sheets couple electronically? Single layer Graphene (good) Many layerGraphite (meh!?) When/how this transition occurs is very pertinent to devices Answer: On SiC, Adjacent Sheets apparently not coupled due to azimuthal rotation 16 Core (left) and valence band (right) PES graphene growth on SiC (Emtsev, et al) Explain the implications of this for graphene coupling between layers 17 Motivation: Direct Growth on Dielectric Substrates: Toward Industrially Practical, Scalable Graphene—Based Devices Graphene Growth: Conventional Approaches CVD graphene monolayer transfer SiO2 Metal or HOPG SiC(0001) Our Focus: Direct CVD, PVD or MBE On Dielectrics Si Si evaporation > 1500 K SiC(0001) Charge-based devices Result: graphene monolayer, interfacial inhomogeneities Result: graphene monolayer or multilayer on SiC(0001) FET: Band gap n Spintronics graphene MgO(111) Si(100) graphene Coherent-Spin FET: Top Gate Co3O4(111) Multi-functional, nonvolatile devices 18 Direct Growth of Graphene on Dielectric Substrates: Summary Substrate Growth Temperature Method MgO(111) ~ 1000 K L. Kong, et al. J. Phys. Chem. C. 114 (2010) 21618 Co3O4(111) 1000 K Mica ~1000 K Al2O3(0001) 1800 K CVD, PVD Interfacial reaction, band gap MBE Incommensurate interface, Ferromagnetism1 MBE Oxidation at C(111) edge sites? CVD High temp. required for fewdefect films BN(0001) 1000 K CVD C. Bjelkevig, et al., J. Phys.: Cond. Matt. 22 (2010) 302002 1 Remarks Monolayer BN by ALD, strong BN graph charge transfer References M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201 G. Lippert, et al. Phys. Stat. Sol. B. 248 (2011) 2619 M. Fanton,et al., Conf. Abstract (Graphene 2011, Bilbao, Spain) Unpublished result 19 Gate valves BCl3 NH3 Butterfly valve Turbo MBE Intro/ transfer deposition Sample heating to 1000 K @ 1 Torr Auger Graphene/Co3O4 STM Graphene/MgO(111) UHV chamber, 10-11 Torr LEED I(V) ALD or PVD Free radical source Hemispheri cal analyzer (XPS) LEED Graphene growth & characterization without ambient exposure Sample Intro chamber P = 103 Sample processing P = 10-9 Torr – -10-3 Torr UHV Analysis 10-6 Torr Chamber 20 P ~ 5 x 10-10 Torr Graphene/BN/Ru(0001): Bjelkevig, et al LEED shows BN and Graphene NOT azimuthally rotated! Orbital hybridization with Ru 3d! 21 Gr/BN/Ru(0001): Inverse photoemission. π* not observed! BN layer does NOT screen graphene from orbital hybridization and charge transfer from Ru! 22 Graphene on Co3O4(111): Molecular Beam Epitaxy Substrate Preparation Evaporator P~ 10-8 Torr 750 K Co(111)+ dissolved O Sapphire(0001) Sapphire(0001) 1000 K/UHV ~3 ML Co3O4(111) Co(111) O segregation Sapphire(0001) 23 Graphene growth on Co3O4(111)/Co(0001) MBE (graphite source)@1000 K: Layer-by-layer growth 1st ML 3 ML 2nd ML 0.4 ML M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201 24 Graphene Domain Sized (from FWHM) (c) 65eV ~1800 Å (comp. to HOPG) Oxide spots attenuated with increasing Carbon coverage (b) G1 40000 G2 graphene 35000 0.4 ML Intensity 30000 Co3O4(111) O1 25000 O2 20000 15000 10000 5000 400 300 200 100 0 Pixel Position (d) 65 eV beam energy 40000 3 ML G1 35000 Intensity LEED: (a) 65eV Oxide/Carbon Interface is incommensurate: Different than graphene on SiC or BN! G2 30000 2.5 Å 25000 O1 20000 2.8 Å O2 15000 10000 5000 400 300 200 100 0 Pixel Position M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201 2.8 Å O-O surface repeat distance on Co3O4(111) W. Meyer, et al. JPCM 20 (2008) 265011 25 XPS: C(1s) Shows π system: Binding Energy indicates graphene oxide charge transfer XPS Intensity (CPS) 8000 XPS (separate chamber): Al Kα source C(1s) 284.9(±0.1) eV binding energy: Interfacial polarization/charge transfer to oxide π→π* No C-O bond formation x75 300 297 294 291 288 0 300 297 294 291 288 285 282 Binding Energy (eV) 279 276 26 M. Zhou, et al., J. Phys.: Cond. Matt. 24 (2012) 072201 Directly grown graphene/metals and dielectrics: Inverse photoemission and charge transfer n-type Ef charge transfer p-type Position of * (relative to EF) indicates direction of interfacial charge transfer (Kong, et al., J.Phys. Chem. C. 114 (2010) 21618 Forbeaux, et al. Multilayers 27 Generalization, Directly Grown Graphene and Charge Transfer: Oxides (p-type) vs. Metals (n-type) e- graphene Transition metals (Ru, Ni, Cu, Ir…) e- graphene Oxides, SiC EF n-type; metal to graphene charge transfer p-type; graphene to substrate charge transfer EF 28 Suspended graphene Graphene (few layer) on Co3O4: Much more conductive than suspeneded graphene Why?? •Significant doping????? •High mobility (How high)????? 29 Conclusion: Graphene: Large area growth on practical substrates critical for device development. Interactions with substrates and (maybe) other graphene layers are critical to device properties 30