Micro- and Nano-Scale Electrical Characterization of Epitaxial Graphene on Off-Axis 4H-SiC (0001)

F. Giannazzo 1,a,*, C. Bongiorno 1,b, S. Di Franco1,c, E. Rimini1,d, V. Raineri1,e 1CNR-IMM, Strada VIII 5, 95121 Catania, Italy.

a filippo.giannazzo@imm.cnr.it, b corrado.bongiorno@imm.cnr.it, csalvatore.difranco@imm.cnr.it, demanuele.rimini@imm.cnr.it, evito.raineri@imm.cnr.it, *corresponding author

Keywords: Epitaxial graphene, specific contact resistance, mobility, electron mean free path

Abstract. The electrical properties of epitaxial graphene (EG) grown on 8° off-axis 4H-SiC (0001)

by annealing at 1600 °C in inert gas ambient (Ar) were studied. The sheet resistance of the EG

layers (Rsh=740±50Ω/sq) and the specific contact resistance of Ni contacts to EG (ρc≈6×10-5

Ωcm2)

were evaluated on micrometer scale by measurements on transmission line model (TLM) structures.

Si3N4 was evaluated as a gate dielectric, showing excellent coverage to EG and a limited effect on

its conductivity. The high n-type doping (∼1013

cm-2

) of EG, as well as the field effect mobility (µ)

dependence on n were determined using top gated field effect transistors (FETs) with Si3N4 gate

dielectric. Electron mean free path (lloc) and mobility (µloc) were also locally determined, on

submicrometer scale, by scanning probe microscopy, showing a broad distribution of µloc values,

with the most probable value very similar to the macroscopic carrier mobility µ.

Introduction

Epitaxial graphene (EG) obtained by high temperature decomposition of hexagonal SiC (h-SiC)

represents a promising material for the development of future high frequency electronics [1]. The

electronic properties of this material have been mainly studied on EG grown on on-axis semi-

insulating h-SiC, Si face [2,3] , due to the fact that single or few layers of graphene can be obtained

in a controlled way by properly choosing the process temperature and the inert gas pressure [3].

However, in the perspective of future industrial applications, the growth of EG on off-axis 4H-SiC

(0001) has been recently investigated, and the number of graphene layers as a function of the

annealing temperature was determined in a wide temperature range, from 1600 to 2000 °C [4].

In this paper, a study of the electronic properties of EG grown on 8° off-axis 4H-SiC (0001) at

1600 °C is reported. The sheet resistance of EG and the specific contact resistance of Ni contacts to

EG were evaluated on micrometer scale by transmission line model (TLM) measurements. The

carrier density (n) as a function of the gate bias and the field effect mobility dependence on n were

determined on top gated FETs with Si3N4 gate dielectric. Furthermore, electron mean free path and

mobility were locally determined, on submicrometer scale, by scanning probe microscopy, showing

a broad distribution with the most probable value very similar to the macroscopic carrier mobility.

Experimental Details

Highly doped 8° off-axis 4H-SiC (0001) with low doped (~1014

cm−3

) epi-layers on top were used

as the substrates for graphene growth. Thermal decomposition of SiC was carried out in argon (Ar)

ambient in an industrial furnace by Centrotherm Thermal Solutions at 1600 °C. After growth,

samples for device fabrication were preliminarily characterized by nondestructive methods, i.e.

Raman spectroscopy and tapping-mode atomic force microscopy (AFM) [5], to check graphene

formation and its uniformity. AFM measurements of the step height in selectively etched EG at

different sample positions, as well as high resolution transmission electron microscopy (HRTEM)

on other SiC samples annealed in the same run, were carried out to determine the number of layers

in EG [4]. The cross-comparison of all of these methods indicated that EG grown at 1600 °C is

made up of three graphene layers on most of the area. TLM and long channel top gated FETs were

fabricated as follows. The first step of device fabrication was lateral insulation of individual test

Materials Science Forum Vols. 717-720 (2012) pp 637-640Online available since 2012/May/14 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.717-720.637

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.250.144.144, University of Melbourne, Melbourne, Australia-12/09/14,21:34:43)

structures by selective etching of EG around the effective area by O2 plasma. Afterwards, metal

pads of TLM structures, as well as source and drain contacts of FETs were obtained by lift-off of a

Ni/Au bilayer. At this stage of device fabrication, the sheet resistance of EG and the specific contact

resistance of Ni/EG interface were determined by TLM measurements. After that, the Si3N4 gate

dielectric, ∼40 nm thick, was deposited by plasma-enhanced chemical vapor deposition (PECVD).

It was selectively etched by CHF3 plasma on the TLM metal pads and on the source and drain

contacts of FETs. Top gate contacts were finally obtained by Pt/Au lift-off.

Micro-scale Electrical Characterization

The sheet resistance Rsh of graphene layers as well as the specific contact resistance (ρc) of Ni

contacts to EG was determined by measurements on TLM structures, consisting of a set of Ni pads

having identical geometry (width W=200 µm and length L=100 µm) and different spacing d

deposited on a laterally insulated rectangular graphene area. Fig.1(a) shows an optical microscopy

image of two TLM structures. In Fig.1(b), the I-V curves measured between pads with different

separations are reported, showing the Ohmic behavior of Ni/EG contacts. Finally, in Fig.1(c) the

resistance RT between two adjacent metal pads is reported as a function of the distance d.

Rsh=740±50Ω/ as well as the contact resistance RC=14±5Ω were determined by fitting of the data

with the linear relation RT=2RC+Rshd/W. The specific contact resistance ρc (independent of pad

geometry) can be obtained as ρc= RCWLT, with LT being the transfer length, i.e. the effective length

of graphene under the pads involved in current transfer from the contact to graphene and from

graphene to the contact. According to the TLM theory, LT2=(ρc/Rsh’), where Rsh’ is the sheet

resistance of graphene under the metal [6]. Due to the charge transfer from metal to the few layers

of graphene [7], Rsh’ can be very different than Rsh in EG between the pads. As discussed in the

following, EG exhibits an overall n-type doping (≈1013

cm-2

) induced by the presence of a buffer

layer at the interface with SiC [5]. This doping is spread between the 3 graphene layers (higher in

the layer closest to the buffer layer, lower in the topmost one). However, in graphene regions coated

by Ni, the work-function difference between the metal and graphene (WNi-WGr≈1eV) causes a

positive charge transfer to EG [8], leading to a counter doping effect, especially in the topmost

graphene layer. This results in an increase of the sheet resistance of EG under the Ni contact.

Taking into account these issues, Rsh’=1160±50Ω/, LT=2.4±0.8µm and ρc=(6.7±4.8)×10-5

Ωcm2

were estimated.

Fig.1 (a) Optical microscopy of two TLM structures. (b) I-V curves measured between pads

with different spacings d. (c) Linear fit of the resistance RT between two adjacent metal pads

as a function of d, yielding the sheet resistance of EG and the contact resistance.

d (µm)

80

100

d (µm)

20

40

60

-30 -15 0 15 30-0.10

-0.05

0.00

0.05

0.10

V(mV)

I(mA)

0 20 40 60 80 1000

100

200

300

400

500R

sh=740±50 Ω/sq

RC=14±5 Ω

RT (

Ω)

d (µm)(b) (c)

200 µm (a)

d (µm)

80

100

d (µm)

20

40

60

-30 -15 0 15 30-0.10

-0.05

0.00

0.05

0.10

V(mV)

I(mA)

0 20 40 60 80 1000

100

200

300

400

500R

sh=740±50 Ω/sq

RC=14±5 Ω

RT (

Ω)

d (µm)(b) (c)

200 µm200 µm200 µm (a)

638 Silicon Carbide and Related Materials 2011

Fig.2(a) shows measurements on TLM structures with EG between metal pads coated by Si3N4.

The comparison with measurements on structures without the Si3N4 layer demonstrates only a

limited increase of EG sheet resistance (15-20% with respect to bare EG) due to dielectric

deposition. Raman spectra collected in the same regions before and after Si3N4 deposition (Fig.2(b))

do not show a significant increase of the intensity in the defect related D band, confirming no

structural degradation of EG. Carrier mobility in the EG layer was evaluated using top gated FETs

with ∼40 nm thick Si3N4 gate dielectric. Long channel devices (gate length LG from 5 to 20 µm)

were used to maximize the weight of the channel resistance with respect to contact resistance on the

source-drain current ID. A cross-section schematic of a FET is shown in Fig.3(a), and a top-view

optical micrograph of a device with LG=10 µm and W=100 µm in Fig.3(b). The drain current vs

top-gate bias (ID-Vtg) characteristic (Fig.3(c)) exhibit an ambipolar behavior. The minimum

conductivity (neutrality point) is shifted to negative gate bias (VNP≈-10 V), indicating n-type doping

of EG. The carrier density (n) as a function of the gate bias (upper scale in Fig.3(c)) was calculated

as n=ε0ε(Vtg-VNP)/(qt), where ε0 and ε are the vacuum and Si3N4 relative dielectric constants,

respectively, q the electron charge and t the Si3N4 film thickness. EG is highly n-type doped (n≈1013

cm-2

) at Vtg=0 V. This result is consistent with the commonly reported doping values in EG grown

on the Si face of SiC and is due to the presence of a positively charged buffer layer at the interface

with SiC [5]. The mobility µ vs carrier density in the electron conduction branch (Fig.3(d)) was

obtained from the conductivity σ and the carrier density n in Fig.3(c), as µ=σ/(qn). µ≈700 cm2V

-1s

-1

was obtained at Vtg=0V, whereas µ≈2000 cm2V

-1s

-1 was obtained near the neutrality point.

Fig.2 Measurements on

TLM structures where the

graphene region between

metal pads is coated by 40

nm Si3N4 (squares) or not

coated (dots) (a). Raman

spectra collected in the

same regions before and

after Si3N4 deposition (b). 1500 2000 2500 3000

0

50

100

150

with Si3N

4

D

D

without Si3N

4

Counts (a.u.)

Raman shift (cm-1)

0 20 40 60 80 1000

100

200

300

400

500 With Si3N

4

Rsh=900±50 Ω/sq

RC=12.8±4.7 Ω

No Si3N

4

Rsh=740±50 Ω/sq

RC=14±5 Ω

RT (

Ω)

d (nm)

(a) (b)

1500 2000 2500 30000

50

100

150

with Si3N

4

D

D

without Si3N

4

Counts (a.u.)

Raman shift (cm-1)

0 20 40 60 80 1000

100

200

300

400

500 With Si3N

4

Rsh=900±50 Ω/sq

RC=12.8±4.7 Ω

No Si3N

4

Rsh=740±50 Ω/sq

RC=14±5 Ω

RT (

Ω)

d (nm)

(a) (b)

Fig.3 (a) Schematic of the cross-section and (b) optical micrograph of a FET with LG=10 µm and

W=100 µm. (c) Drain current vs top gate bias and carrier density. (d) Calculated mobility µ in the

electron branch.

20 µm

(b)

0 1 2 3500

1000

1500

2000

2500

n (1013 cm

-2)

µ (cm

2V

-1s-1) (d)

Gate

DrainSource

EG

Si3N

4

4H-SiC (0001), 8 ° off, 1018 cm-3

1014 cm-3

(a)

-20 -10 0 10 200

10

20

30

Vtg (V)

I d (mA)

0 1 2

0

1

2

3

n (1013 cm

-2)

σ (mS)

(c)

VNP

20 µm 20 µm 20 µm

(b)

0 1 2 3500

1000

1500

2000

2500

n (1013 cm

-2)

µ (cm

2V

-1s-1) (d)

Gate

DrainSource

EG

Si3N

4

4H-SiC (0001), 8 ° off, 1018 cm-3

1014 cm-3

Gate

DrainSource

EG

Si3N

4

4H-SiC (0001), 8 ° off, 1018 cm-3

1014 cm-3

(a)

-20 -10 0 10 200

10

20

30

Vtg (V)

I d (mA)

0 1 2

0

1

2

3

n (1013 cm

-2)

σ (mS)

(c)

VNP

Materials Science Forum Vols. 717-720 639

Nano-scale Electrical Characterization

The mobility values extracted from measurements on

FETs are average values from several µm2 areas and are

the result of the combination of different scattering

sources inhomogeneously distributed on EG. To get a

deeper understanding of the lateral homogeneity of the

carrier mobility, the electron mean free path in EG was

locally probed by a scanning probe method based on

scanning capacitance microscopy [9,10,11].

Measurements were performed in a bare EG region of

1µm2 size located between two TLM pads. On that area

the tip is displaced on a regular array of 10×10 positions

spaced by ~100 nm and for each position the local electron

mean free path (lloc) was measured. In Fig.4, lower scale,

the histogram of the probed lloc values is reported. A broad

distribution, ranging from ∼10 to ∼120 nm can be

observed. Assuming an average carrier concentration n≈1013

cm-2

, the distribution of the local

carrier mobility (µloc) values (upper scale) is calculated from the distribution of lloc values, using the

relation µloc=qlloc/ħ(πn)-1/2

between mobility and mean free path. It is worth comparing the average

µ extracted from long-channel FETs with this distribution, to understand how µlocal values combine

to yield the macroscopic µ. Although this subject deserves further modeling, it can be observed that

the most probable µlocal value (700±50 cm-2

V-1

s-1

) is very similar to the macroscopic carrier

mobility obtained on a FET at Vtg=0 V.

Conclusion

Macroscopic measurements on TLM structures were used to determine the sheet resistance of EG

on 4H-SiC (0001) and its modification after deposition of a dielectric overlayer. Si3N4 was

evaluated as a gate dielectric for EG, showing excellent coverage to EG and a limited effect on its

conductivity. The specific contact resistance of Ni contacts to EG was evaluated, taking into

account the effect of charge transfer from metal to EG. The mobility dependence on the carrier

density n in EG was obtained using long channel top gated FETs. Finally, electron mean free path

and mobility were locally determined on submicrometer scale by scanning probe microscopy,

showing a broad distribution with the most probable value very similar to the macroscopic mobility.

References

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Fig.4 Histogram of the locally

probed values of electron mean free

path (lower horizontal scale) and

mobility (upper horizontal scale) on

10×10 sample positions in 1µm2

area on bare EG.

0

20

40

60

80

1000 1000 2000 3000

Counts (%)

µloc (cm

2V

-1s-1)

0 40 80 120

lloc (nm)

640 Silicon Carbide and Related Materials 2011

Silicon Carbide and Related Materials 2011 10.4028/www.scientific.net/MSF.717-720 Micro- and Nano-Scale Electrical Characterization of Epitaxial Graphene on Off-Axis 4H-SiC (0001) 10.4028/www.scientific.net/MSF.717-720.637

DOI References

[1] Y. -M. Lin, et al., Science 327 (2010) 662.

http://dx.doi.org/10.1126/science.1184289 [2] C. Virojanadara, et al., Phys. Rev. B 78 (2008) 245403.

http://dx.doi.org/10.1103/PhysRevB.78.245403 [3] K. V. Emtsev, et al., Nature Mater. 8 (2009) 203.

http://dx.doi.org/10.1038/nmat2382 [4] C. Vecchio, S. Sonde, C. Bongiorno, M. Rambach, R. Yakimova, E. Rimini, V. Raineri, F. Giannazzo,

Nanoscale Research Letters 6 (2011) 269.

http://dx.doi.org/10.1186/1556-276X-6-269 [5] S. Sonde, F. Giannazzo, V. Raineri, R. Yakimova, J. -R. Huntzinger, A. Tiberj, J. Camassel, Phys. Rev. B

80 (2009) 241406(R).

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http://dx.doi.org/10.1103/PhysRevB.79.195425 [9] F. Giannazzo, S. Sonde, V. Raineri, E. Rimini, Nano Lett. 9 (2009) 23-29.

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http://dx.doi.org/10.1063/1.3280860