Passivation of GaAs Surface using SAM of Redox-Active Ruthenium-based OrganicMolecules in a Matrix of Non Redox Active OPE1 Molecules

*Rand K. Jean, *Dmitry Zemlianov, *David B. Janes, **Bin Xi, and **Tong Ren

*Birck Nanotechnology Center, Purdue University

**Department of Chemistry, Purdue University

2

Outline Motivation for use of molecules; redox-active and spin

selective Molecular Deposition Surface Analysis:

Cyclic Voltammetry (CV) Fourier Transform Infra-red Spectroscopy (FTIR) X-ray Photoemission Spectroscopy (XPS)

Conclusion

3

Metal

Semiconductor

Molecule

Metal Molecule Semiconductor Metal Redox-Active Molecule Semiconductor

Molecular functionality; possible use in sensing, memory etc.

CV indicate E-levels close to Ef

Non-resonant tunneling

E-levels 1-3 eV apart

Resonant tunneling

Molecular level close to Ef Molecules in net charge state

LUMO

LUMO

HOMO

HOMO

Ef Ef

Ev

Ec

Ev

Ec

-2.5-2-1.5-1-0.500.51

-1/-2

+1/00/-1

E(V), vs Ag/AgCl

* Cyclic Voltammogram of Redox active Molecule

*

Motivation for Redox-active Structures

T. Ren et. al Journal of Organometallic Chemistry 690 (2005) 4734-4739

4

Spin Properties

Increased spin selectivity; spin valve effect

Possible use in spin-based transistor, and memory based device

L. Bogani and W. Wernsdorfer, Nature Materials , Vol. 7, MARCH 2008

*

*

Molecular Spin Valve MR Effect

Spin Transistor

5

Ru Redox Properties

Ef(Au) = 5.1

0.13

0.5

5.23

4.6

GV Analysis

Evac

0.13

0.5

4.97

5.6

ENHE= 4.43

EAg/AgCl= 0.197

+

-

CV0~4.63

0.4

0.9

2.0

5.0

3.73

2.6

CV Analysis

EHOMO

ELUMO(1)

ELUMO(2)

Emol

Au EF (5.1)

-2.5-2-1.5-1-0.500.51

-1/-2

+1/00/-1

E(V), vs Ag/AgClCyclic voltammogram of Ru molecule

Molecular functionality

Possible use in sensing, memory etc.

CV indicate E-levels close to Au Ef

Ren et. al. J Orgn. Chem. 690, 4734, 2005.

6

Other Molecular properties Ru complex

Possibility of net spin in Ru-Ru core Thiol end group allows easy attachment to substrates

OPE1 Morphologically similar to Ru anchor end, thus forming good

undergrowth to fully passivate semiconductor surface

7

Molecules Deposited on (100) GaAs

H

1. 2.3.

Diruthenium (III) tetra-2-anilinopyridinate-2-(Trimethylsilyl)ethyl-4-(ethynyl)phenyl Sulfide (Ru-complex)

Length: 16 Å Coverage: ? 8 hrs in 0.5mM in tetrahydrofuran (THF), 60 C

2-(Trimethylsilyl)ethyl-4-phenlSulfide (OPE1)

Length: 14Å

Coverage: 3.04 e14 cm-2

1-24 hrs hrs in 0.5mM in

THF, 60 C

SS SS SS S SSS

Octadecanethiol (ODT)

Length: 23 Å

Coverage: 4.5e14 cm-2

8 hrs in 0.5mM in

ethanol, 60 C

8

“Size Matters”

HGa GaAs

Ga GaAs

Ga GaAs

Ga GaAs

Ga GaAs

Ga GaAs

Ga GaAs

GaAs

HGa GaAs

Ga GaAs

Ga GaAs

GaAs

Ru Complex: Maximum density limited by size of molecule

Leaves relatively high density of unterminated surface atoms (and defect states)

9

Simple Aromatic

HGa GaAs

Ga GaAs

Ga GaAs

Ga GaAs

Ga GaAs

Ga GaAs

Ga GaAs

GaAs

HGa GaAs

Ga GaAs

Ga GaAs

GaAs

Maximum packing density ~ 75% of a monolayer for simple alkanes, slightly lower for small aromatics.

Local reconstruction of surface allows relatively low interface state density

10

Mixed Monolayer Deposition

OPE1 deposition, 3 hours

1-24 hrs 0.5 mM Ru in THF solution, 60 C

TBAF cleaves off protective end group, exchange interaction of OPE1 and Ru complex molecules to form Mixed Monolayer,

Mixed Monolayer has more coverage of surface than Ru SAM

4.

11

Characterization Techniques

• Fourier Transform Infrared Spectroscopy (FTIR)

Vibrational modes of molecule

• Cyclic Voltammetry with ferrocene solution –

Molecular layer blocks redox process of ferrocene

• X-Ray Photoemission Spectroscopy – surface chemistry and relative surface coverage

12

FTIR Characterization of Ru InsertionC=C IR signature (1506 cm-1) with increase in Ru depostion

time

1.00E-04

1.00E-03

1.00E-02

0 5 10 15 20 25 30

Time (hours)

IR a

bso

rban

ce

C=C presence is 9x greater in Ru than in OPE1; Ru insertion monitored by increase in C=C symmetric stretching signature

Rapid insertion of Ru complex molecules for first 6 hours

Saturation after 6 hours; decrease in wavenumber indicates ordering of SAM

13

CV Measurement Set-up

P. Carpenter, Metal/Molecule/Semiconductor Devices, MS thesis Purdue University 2005

14

Insulation Properties of SAMs

• Scan rate at 100 mv/s

• Current limit of measurement system was 1e-9A; OPE1 2 hr to 18 hr registered sub nano-ampere currents were given values of zero

-4.00E-05

-2.00E-05

0.00E+00

2.00E-05

4.00E-05

6.00E-05

8.00E-05

1.00E-04

-1 -0.5 0 0.5 1

Voltage (V) vs. Ag/AgCl

Cu

rren

t (A

)

Control

ODT

OPE1 1 hr

OPE1 2hr

OPE1 3hr

OPE1 6 hr

OPE1 18 hr

OPE1 42 hr

OPE1 1 hr indicative of incomplete monolayer; growth stage of pattern

OPE1 2 hr to 18 hr completely insulates the substrate; saturation part of growth pattern.

OPE1 42 hr indicates monolayer is leaky again, indicating decline part of growth pattern

15

XPS Wide Scans of SAMs

C and S shows presence of monolayers

Ga and As are the substrate signatures

O indicates the presence of an oxide layer

16

As 3d Composition

17

As 3d Composition

% Concentration

SAM As 3d 5/2 As 3d 5/2 As-S As-O

OPE1 1hr 80.27 15.25 4.48 -

OPE1 3hr 81.92 14.13 3.95 -

OPE1 6 hr 83.13 13.70 3.57 -

OPE1 24 hr 81.49 13.95 3.25 1.31

ODT 79.68 14.80 5.52 -

Ru 77.43 12.95 2.65 6.97

MM 70.47 12.57 3.37 13.59

O signature only present in OPE1 24 hr, Ru and Mixed Monolayer sample

Optimum deposition time for OPE1 should be less than 24 hours.

Strong As-S presence in SAMs

18

Ga 3d Composition

19

Ga 3d Composition

% Concentration

SAM Ga 3d 5/2 Ga 3d 3/2 Ga-S Ga-O Ga-S/Ga-O

OPE1 1 hr 81.97 11.61 - - 6.42

OPE1 3 hr 82.06 8.73 7.59 1.62 -

OPE1 6 hr 84.65 11.13 - - 4.22

OPE1 24 hr 85.24 10.45 - - 4.31

ODT 86.66 10.48 2.86 - -

Ru 68.70 26.51 - - 4.79

MM 70.90 26.76 - - 2.34

GaO/GaS signature decreases overall with deposition time

OPE1 3 hr has small GaO signature; ODT has no GaO signature

20

Percentage Composition of SAMs* TOA = 0 ˚

ODT most passivating SAM, Ru is least passivating

Optimal deposition time for OPE1 is around 3 hours

Mixed Monolayer shows increase in O, S and C signatures over Ru

SAM Percentage Composition (%)

O N C S As Ga

OPE1 1hr 9 - 32.3 2.1 26.8 29.8

OPE1 3 hr 7.9 - 34.1 2.3 26.9 28.8

OPE1 6 hr 9 - 33.5 1.9 26.3 29.3

OPE1 24 hr

8.2 - 34.9 2.1 26.2 28.6

ODT 3.1 - 51.5 2.2 21.1 22.1

Ru 14.1 5.7 32.2 0.87 22.13 25

Mixed Monolayer

16.2 7.8 33.9 1.2 19 21.9

21

Dependence of Elemental Conc. with TOA, OPE1 and MM

OPE1 3 hr Mixed Monolayer

C increase with TOA shows upright orientation for OPE1 3 hr and Mixed monolayer

O signature is inconclusive in OPE1 but decreasing in Mixed monolayer

All other species are at substrate-molecule interface as expected

0

5

10

15

20

25

30

0.40.50.60.70.80.911.1

cos TOA

atom

ic c

once

ntra

tion

%

0

10

20

30

40

50

60

C a

tom

ic c

once

ntra

tion

%

O

S

As

Ga

C

0

5

10

15

20

25

0.40.50.60.70.80.911.1

cos TOA

ato

mic

co

nce

ntr

atio

n %

0

10

20

30

40

50

60

C a

tom

ic c

on

cen

trat

ion

%

O

N

S

As

Ga

C

22

Dependence of Elemental Conc. with TOA, ODT and Ru

ODT Ru

C signature increasing with TOA shows upright orientation for ODT and Ru

N increasing with TOA in Ru, indicative of upright orientation

All other species are at substrate molecule interface

0

5

10

15

20

25

0.40.50.60.70.80.911.1

cos TOA

atom

ic c

once

ntra

tion

%

0

10

20

30

40

50

60

70

80

C a

tom

ic c

once

ntra

tion

%

O

S

As

Ga

C

0

5

10

15

20

25

30

0.40.50.60.70.80.911.1

cos TOA

Ato

mic

co

nce

ntr

atio

n %

0

10

20

30

40

50

60

C A

tom

ic c

on

cen

trat

ion

% O

N

S

As

Ga

C

23

Percentage Coverage on GaAsSAM Deposition Time (hrs) Coverage (%)

ODT 8 68

Ru 24 24.8

Mixed Monolayer 32

(24 hr OPE1 + 8 hr Ru) 34.8

OPE1 1 hr 1 49.2

OPE1 3 hr 3 56.3

OPE1 6 hr 6 45.4

OPE1 24 hr 24 53.1

ODT has best coverage on GaAs substrate while Ru has least coverage

OPE1 3 hrs has optimal coverage and thus deposition time

Mixed Monolayer increases coverage from solely Ru by 10%

24

Conclusion OPE1 undergoes a growth saturation and decline pattern of

SAM coverage with 3 hours being the optimal deposition time

Ru insertion into OPE1 matrix also exhibits a saturation curve, where optimal deposition time is around 6 hours

The heterogeneous SAM or mixed monolayer improves coverage by 10% over the Ru SAM

The most passivating SAM was ODT followed by OPE1 (at 3 hours), then Mixed Monolayer and lastly Ru

Funding: National Science Foundation