“X” on Insulator (XOI)Ali JaveyUC BerkeleyElectrical Engineering and Computer Sciences

Materials Sciences DivisionLawrence Berkeley National Laboratory

Planar MOSFETs

• As length scales down, competition between gate and drain causes reduction in gate control

• Leakage path cannot be eliminated, even with infinite gate oxide capacitance

Chenming Hu (2011)

Structure innovation for future FETs

• Materials miniaturization at extreme (For LG ~ 5 nm, T ~ 1-2 nm)

• Questions to be addressed with material miniaturization:The effect of quantum confinementThe effect of surface roughness

3

T < ~ 1/3 LGT < ~ 2/3 LG

S

D

G

Tsi

Lg

C. Hu, et al.

Device Advantages of XOI

Enhanced electrostatics arising from the fully depleted bodyReduced leakage currents Compatibility with CMOS/SOI processing Generic device architecture for different material systems

Fabrication Features for XOI

III‐V integration on Si/SiO2 substrates –Wafer Bonding / Epilayer Transfer Nanoscale doping of contacts –Monolayer Surface Doping High quality interfaces –Surface passivation & treatment

Si

SiO2

Xn+ n+

Gate Stack

X on Insulator (XOI)

XOI – Compound Semiconductors on Insulator

H. Ko, K. Takei, R. Kapadia et al, Nature, 2010.

X: Ultrathin III-V (down to 3 nm) Layered Semiconductors (down to a

monolayer)

III-V XOI achieved by layer transfer5

H. Ko, K. Takei, R. Kapadia, S. Chuang, H. Fang et al., Nature 468 (2010), 286-289

Ultrathin InAs XOI – HRTEM analysis

7 nmInAs

SiO2

Si

SiO2

Ni

InAsZrO2

50 nm

2D XOI of various III-Vs7

InAs

5 nm

ZrO2

SiO2

H. Ko, K. Takei, R. Kapadia, S. Chuang, H. Fang et al., Nature 468 (2010), 286-289

InAs XOI

5 nm

7 nm InAs0.7Sb0.3

ZrO2

SiO2

InAsSb XOI

H. Fang et al., IEEE Electron Device Letters, 33(4), 504-506, 2012.

InGaSb XOI

K. Takei, M. Madsen, H. Fang et al., Nano Letters, 12, 2060-2066, 2012.

Spatial confinement8

])([2

222*

2

zyxn

Lnkk

mE

kx

ky

E

X

x

zLz Insulator

Insulator

Material Si Ge GaAs InAs InSb

Bohr radius (nm)

4.9 17.7 14 35 69

Rule of Thumb: quantum confinement is observed when a material dimension is less than the excitonic Bohr radius

How do we probe 2D sub-bands?

Probing 2D sub-bands with FTIR

• FTIR + Microscope• InAs micro-ribbon arrays on optically transparent

substrate, CaF2 (transfer with PR as buffer)

9

Micro-FTIR

H. Fang et al., Nano Letters, under review.

Direct Bandgap Measurement of InAs XOI:InAs on CaF2 substrate

5 10 15 200.4

0.6

0.8

1.0

Ban

dgap

(eV)

Thickness (nm)

ExperimentalTheoretical

Key Features of Absorption:

The onset of shifts to higher energies with thickness reductionBandgap increases with downscaling of the thickness due to quantum confinement

Clear steps in absorption depict the optical transitions between 2D subbandsThe spacing between the subbands increases with thickness reduction

Quantum unit of absorptance in 2D semiconductorsAbsorptance due to interband transitions between the 2-D subbands is independent of thickness and band structure details. Simply, it is AQ=8πα/(3nr) ~ 1.7%, where α is the fine-structure constant and nr is the refractive index. Total absorptance is then A=M.AQ .

Hui Fang, et al, submitted, 2012.

InAs XOI – Materials and Device Performance

0 2000 4000 6000 8000Field effect (cm2/Vs)

02468

10121416

Num

ber

18 nm InAs

Parameter InAs XOI FET

Si MOSFET Si MOSFET

Gate Length 500 nm 500 nm 65 nm

ION 1.4 mA/μm 0.32 mA/μm 1.75 mA/μm

VDD 1 V 3.3 V 1.2 V

ION/IOFF 104 107 104

H. Ko, K. Takei, R. Kapadia, et al., Nature 2010

InAs XOI nFETs – Effect of Thickness Scaling

InAs Thickness 13 nm 8 nm 5 nmGate Length 230 nm 205 nm 207 nm

ION 1.35 mA/μm 0.6 mA/μm 0.2 mA/μm

ION/IOFF 5 x 102 6 x 103 2 x 104

Peak gm 1.7 mS/μm 0.9 mS/μm 0.6 mS/μm

SS (mV/dec) 180 ~125 115

Eg ~0.44 eV ~0.5 eV ~0.6 eV

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

I DS

(A/µ

m)

-1.0 -0.5 0.0 0.5 1.0VGS (V)

Exp. Sim. VDS=500 mV VDS=50 mV

TInAs=5 nmLG~207 nm

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

I DS

(A/µ

m)

-1.0 -0.5 0.0 0.5 1.0VGS (V)

TInAs=13 nmLG=230 nm

Exp. Sim. VDS=500 mV VDS=50 mV

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

I DS

(A/µ

m)

-1.0 -0.5 0.0 0.5 1.0VGS (V)

TInAs=8 nmLG~200 nm

Exp. Sim. VDS=500 mV VDS=50 mV

Kuni Takei, et al, APL, 99, 103507, 2011.

Thinner body results in reduced mobility

0.5 µm

MBE InAs

Surface roughness

Atomic Force Microscope Image

103

2

4

68104

2

4

68105

Mob

ility

(cm

2 /Vs)

5040302010

Thickness (nm)

, Field-effect, experimental , Field-effect, simulated , Phonon, calculated

10-10

10-9

10-8

10-7

10-6

10-5

10-4 I D

S (A

/µm

)

1050-5-10VGS (V)

8nm (sim) 8nm (exp)

13nm (sim) 13nm (exp)

18nm (sim) 18nm (exp)

48nm (sim) 48nm (exp)

Interface trapsSurface scatteringField-induced scattering

Vds=0.1VL~5 µmtox=50nm

Layered semiconductors: new opportunities

c/2

A

B

AXM

-9

-8

-7

-6

-5

-4

-3

-2

E re

lativ

e to

Vac

uum

(eV)

MoS2 MoTe2WSe2 SnS2 SnSe2

• Elements (eg. C), TMDC (eg. WSe2, MoS2), III-VI (eg. GaSe)• Offering new 2-D semiconductors

MoS2 MOSFETs

• Single Layer MoS2: true ‘2-D’ channel• The limit for electrostatic control over channel• ION/Ioff > 108

• Mobility ~200 cm2/V-sA. Kis, Nature Nano (2011)

2-D WSe2

Monolayer WSe2

H. Fang et al., Nano Lett. (2012)

-5.0 -2.5 0.0 2.5 5.01E-13

1E-11

1E-9

1E-7

1E-5

-I DS (A

/m

)

VGS (V)

VDS=-1 VL ~ 7.6 µm~ 7 MLs

WSe2

Key: Carrier Injection

• Schottky barrier height control enables on current• Can width also be easily controlled?

Ti Ti

Pd

ФBp

ФBp

Pd

-5.0 -2.5 0.0 2.5 5.01E-13

1E-11

1E-9

1E-7

1E-5

-I DS (A

/m

)

VGS (V)

VDS=-1 VL ~ 7.6 µm

Pd contacts

Ti contacts

~ 7 MLs

H. Fang et al., Nano Lett. (2012)

Key: Carrier Injection

• Doping level: n2D~2.2×1012 cm-2 (n3D~3.1×1019 cm-3)• EF: ~16 meV below the valence band edge (EV)

H. Fang et al., Nano Lett. (2012)

-10 -5 0 5 101E-12

1E-10

1E-8

1E-6

1E-4

-I DS (A

/m

)VGS (V)

No NO2

After NO2

VDS=-1 V L ~ 8 µmMonolayer WSe2

NO2SeW

Key: Carrier Injection

• Mobility: ~250 cm2/V.s

• Subthreshold Swing (SS): 60 mV/dec

H. Fang et al., Nano Lett. (2012)

-1.0 -0.5 0.00

2

4

6

8

10

12

-I DS (

A/

m)

VDS (V)

VGS= -1.7 V

-1.4 V

-1.1 V

-0.8 V -0.5 V

-1.5 -1.0 -0.5 0.0 0.5

1E-11

1E-9

1E-7

1E-5

-I DS (A

/m

)

VGS (V)

60 mV/dec limit

— VDS=-1 V— VDS=-0.05 V

L ~ 9.4 µm

Peak μeff~ 250 cm2/V.s

Conclusions

• XOI presents a new platform for exploring news material systems on Si.

• High performance n- and p- FETs have already been demonstrated using XOI geometry.

• A number of fundamental material/device questions related still need to be addressed.

• Manufacturing issues still need to be addressed.