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?

YEAR TRANSISTORS

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 N Nano anoT Tubes and is without the helicity ubes and is without the helicity 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 cm2/ V•s, Si: 1360 cm2/ 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 periodicity is 6 3 x 6 3 x 6  3 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

X X-

• ray reflectivity measures

ray reflectivity measures vertical density gradients vertical density gradients

Surface X Surface X-

• ray Diffraction

ray Diffraction

 

 ∝

• j

Z iq j j r q i j

j Z j

e q f e q f q I ) ( ) ( ) (

 

q

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 i
• substrate. Dashed lines are the bulk SiC lattice planes before interface

nterface relaxation ( relaxation (¢ ¢'s). The 5th plane of atoms (adatom) is displaced 's). The 5th plane of atoms (adatom) is displaced dad dad from the 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
• atoms. The shaded circles in the interface ("layer-
• 0") can be either carbon or

0") can be either carbon or 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 "layer-

• 1",

1", -

• 2,

2, -

• 3,

3,

l (r.l.u.)

1 2 3 4 5 6 7 8 9 10

Integrated Intensity (arb. units)

10-1 100 101 102 103 104 105 106 107 108 109 1010 1011 1012 relaxed bulk adatom model

4H-SiC (00l ) rod

data

relaxed bulk

D0

Graphene

adatom model

D0

Graphene

Si-Face SiC Bragg Peaks Graphite Bragg Peaks

• F. Varchon, et al., condmat (2008). Si-face

Graphene/SiC Interface Graphene/SiC Interface

“ “buffer layer buffer layer” ”

Dangling bond states Dirac cone

Gap 2.3Å Si-Face 0.12nm 0.17nm

Graphene modulation Graphene modulation

l (r.l.u.)

5 6 7 8 9 10

Integrated Intensity (arb. units)

10-1 100 101 102 103 104 105 106 107 108 109

4H-SiC (00l ) rod

σG= 0.30Å σG= 0.00Å Si-adatom model, σG= 0.16Å

l (r.l.u.)

5 6 7 8 9 10

Integrated Intensity (arb. units)

100 101 102 103 104 105 106 107 108 σG = 0.25A σG = 0.00A

Si Si-

• face

face C C-

• face

face

σG = 0.16Å σG < 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 computed all the delta’ ’s s

• 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-02 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 1.E+10 1.E+12 2 4 6 8 10 12 l (r.l.u) Integrated Intensity (arb. units) intfinaldat ck582_a ck582_b CK582_c CK582_d

Fits for Reflectivity data Fits for Reflectivity data… …

Si Face INTFINAL

1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10 1.E+11 2 4 6 8 10 12 l (r.l.u) I n t e g r a t e d I n t e n s i t y ( a r b . u n i t s ) 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

• f C
• f C –

– face graphene grown by UHV face graphene grown by UHV method method

• Properties of interface layers of furnace

Properties of interface layers of furnace grown Si grown Si – – face face

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