The interface structure of The interface structure of few layer - PowerPoint PPT Presentation

The interface structure of The interface structure of few layer epitaxial graphene few layer epitaxial graphene grown on 4H_SiC(0001) grown on 4H_SiC(0001) C.Krishnasamy C.Krishnasamy Duluth High School Duluth High School STEP_UP 2008 STEP_UP 2008

Why graphene? Why graphene? TRANSISTORS YEAR

Why graphene? Why graphene? • Is a single layer of graphite Is a single layer of graphite • • Electrical properties as good as Electrical properties as good as C Carbon arbon • Nano anoT Tubes and is without the helicity ubes and is without the helicity N problem problem • Flat graphene sheets have zero energy Flat graphene sheets have zero energy • gap so a viable candidate for post CMOS gap so a viable candidate for post CMOS electronics electronics • Demonstrates 2D properties Demonstrates 2D properties E • k

Single layer graphene • Ballistic transport  highest mobilities ever observed GRAPHENE: 250,000 cm 2 / V•s, Si: 1360 cm 2 / V•s • Micron scale electron coherence lengths • Essentially no electron-phonon coupling ITRS 2007 “emerging research devices”

Graphene in GaTech.. Graphene in GaTech.. • Grown by UHV method Grown by UHV method • • Si Face Si Face - - slow growth ,1 slow growth ,1- -5 layers thick, 5 layers thick, • periodicity is 6  3 x 6  3 periodicity is 6  3 x 6  3 • Electron mobility and coherence length are Electron mobility and coherence length are • a magnitude smaller than C face graphene a magnitude smaller than C face graphene • Using furnace C Face Using furnace C Face – – fast growth, more fast growth, more • than 5 layers of thickness, has high than 5 layers of thickness, has high concentration of nanocaps and defects concentration of nanocaps and defects

Research Goal Research Goal ! To study the structure of interface graphene To study the structure of interface graphene ! layer grown on SiC layer grown on SiC ! Learn about the charge transfer between Learn about the charge transfer between ! substrate and the graphene film substrate and the graphene film ! My role is to see whether small changes in the My role is to see whether small changes in the ! parameters of the model will provide a better parameters of the model will provide a better fit for the reflectivity data fit for the reflectivity data

X ray diffraction X ray diffraction • Crystallography software used to model Crystallography software used to model • diffraction patterns based on assumed diffraction patterns based on assumed structure structure • Model compared to experiment to Model compared to experiment to • characterize the interface structure characterize the interface structure

Surface X- -ray Diffraction ray Diffraction Surface X      i q r iq Z ( ) ( ) ( ) • I q f q e j f q e Z j ∝ j j j j q X- -ray reflectivity measures ray reflectivity measures X vertical density gradients vertical density gradients

Models of graphene Models of graphene FIG. 1: A schematic model of multi FIG. 1: A schematic model of multi- -layer graphene grown on the 4H layer graphene grown on the 4H- -SiC(0001) SiC(0001) substrate. Dashed lines are the bulk SiC lattice planes before interface nterface substrate. Dashed lines are the bulk SiC lattice planes before i relaxation (¢ ¢'s). The 5th plane of atoms (adatom) is displaced 's). The 5th plane of atoms (adatom) is displaced dad from the relaxation ( dad from the topmost atom plane in the interface. ( topmost atom plane in the interface. ( ² ² ) are carbon atoms and ( ) are carbon atoms and ( ± ± ) are silicon ) are silicon atoms. The shaded circles in the interface ("layer- -0") can be either carbon or 0") can be either carbon or atoms. The shaded circles in the interface ("layer silicon atoms. The graphene layers above the interface layer are silicon atoms. The graphene layers above the interface layer are referred to as referred to as "layer- -1", 1", - -2, 2, - -3, 3, "layer

Si-Face 10 12 adatom model 4H-SiC 10 11 relaxed bulk (00 l ) rod Integrated Intensity (arb. units) 10 10 data SiC Bragg Peaks 10 9 10 8 10 7 10 6 10 5 10 4 10 3 Graphite Bragg 10 2 Peaks 10 1 10 0 10 -1 0 1 2 3 4 5 6 7 8 9 10 l (r.l.u.) Graphene Graphene D 0 D 0 adatom model relaxed bulk

Graphene/SiC Interface Graphene/SiC Interface F. Varchon, et al., condmat (2008). Si-face 0.12nm “buffer layer buffer layer” ” “ 2.3Å Si-Face Gap 0.17nm Dangling bond states Dirac cone

Graphene modulation Graphene modulation Si- -face face C- -face face Si C 10 9 10 8 Si-adatom model, 4H-SiC 10 8 σ G = 0.25A σ G = 0.16Å Integrated Intensity (arb. units) 10 7 (00 l ) rod Integrated Intensity (arb. units) 10 7 σ G = 0.00A σ G = 0.30Å 10 6 σ G = 0.00Å 10 6 10 5 10 5 10 4 10 4 10 3 10 3 10 2 10 2 10 1 10 1 10 0 10 -1 10 0 5 6 7 8 9 10 5 6 7 8 9 10 l (r.l.u.) l (r.l.u.) σ G = 0.16Å σ G < 0.05Å

Proposed model Proposed model

My part My part • Created an excel file for data and Created an excel file for data and • computed all the delta’ ’s s computed all the delta • Modified the C code to accommodate the Modified the C code to accommodate the • corrugated graphene layer contribution to corrugated graphene layer contribution to the intensity of the scattered electron the intensity of the scattered electron • Do various fits on the reflectivity data by Do various fits on the reflectivity data by • changing a few parameters changing a few parameters

Fits for Reflectivity data… … Fits for Reflectivity data Si Face INTFINAL 1.E+12 Integrated Intensity (arb. units) 1.E+10 1.E+08 1.E+06 1.E+04 1.E+02 1.E+00 0 2 4 6 8 10 12 1.E-02 l (r.l.u) intfinaldat ck582_a ck582_b CK582_c CK582_d

Fits for Reflectivity data… … Fits for Reflectivity data Si Face INTFINAL 1.E+11 1.E+10 1.E+09 1.E+08 ) s t i n u 1.E+07 . b r a 1.E+06 ( y t i s n 1.E+05 e t n I 1.E+04 d e t a r 1.E+03 g e t n I 1.E+02 1.E+01 1.E+00 0 2 4 6 8 10 12 1.E-01 l (r.l.u) intfinaldat ck582_e.fit ck582_i.fit ck582_f.fit ck582_g.fit ck582_h.fit

Results Results The purple line is the best fit so far. The purple line is the best fit so far. • • Assuming that there is some corrugation Assuming that there is some corrugation • • present in graphene layers seems valid . present in graphene layers seems valid . The interface is not composed of a simple • graphene-like layer above a relaxed SiC bilayer instead, the interface reconstruction is more complicated and extends deeper into the bulk

Future Research Future Research • Studying the properties of interface layers Studying the properties of interface layers • of C – – face graphene grown by UHV face graphene grown by UHV of C method method • Properties of interface layers of furnace Properties of interface layers of furnace • grown Si – – face face grown Si

Sincere Thanks Sincere Thanks • Dr. Ed Conrad Dr. Ed Conrad • • Dr. Leyla Conrad Dr. Leyla Conrad • • J.E. Millan J.E. Millan- -Otoya Otoya • • Kevin Kubista Kevin Kubista • • Johanna Haas Johanna Haas • • Fellow step Fellow step- -up participants up participants •