INDIANAUNIVERSITY

DEPARTMENT OF CHEMISTRY

Electronic Structure Studies of Semiconductor Surface Chemistry using Cluster Models

Krishnan Raghavachari

Indiana UniversityBloomington, IN 47405

Computational Chemistry Conference – Kentucky – 2003

Outline

• Quantum Chemistry of Materials – Cluster Approach • Wet oxidation of silicon (100)

• ALD growth of Al2O3 on H/Si Initial reaction mechanism

• Indium Phosphide Surface Chemistry H on P-rich InP(100)

H on In-rich InP(100)

• Semiconductor – molecule – metal systemGaAs – alkanedithiol – Gold

Computational Chemistry Conference – Kentucky – 2003

Collaborators

Mat Halls Theory

Boris Stefanov Post-Docs

Yves Chabal Experiment

Marcus Weldon AFM, IR on silica

Kate Queeney Infrared on Si

Olivier Pluchery Infrared on InP

Martin Frank ALD of Al2O3 on H/Si

Bob Hicks (UCLA) IR, STM

Gangyi Chen InP surface chemistry

Julia Hsu, Loo, Lang, Rogers molecular electronics

Computational Chemistry Conference – Kentucky – 2003

Quantum Chemistry of MaterialsCluster Approach

• Truncate back-bonds with H

• Describe the local region of interaction

• Appropriate for localized bonding (e.g., Si, SiO2)

Computational Chemistry Conference – Kentucky – 2003

Cluster approach - Questions

• Cluster size dependence

• Embedded cluster approaches

• Cluster termination

• Cluster constraints

Cluster approach vs. Slab approach

Computational Chemistry Conference – Kentucky – 2003

Cluster models for Si, InP

Vibrational problems

Accurately describe vibrations above the phonons ( 500 cm-1) Hydrogen vibrations on Si, InP

Oxidation of Si(100)

InP oxides

Photoemission

Si/ SiO2 Interface Structure

Mechanistic problems

HF etching of silicon surfaces

Oxidation of Si(100)

ALD growth of Al2O3 on Si

CVD growth of InP

Computational Chemistry Conference – Kentucky – 2003

Dimerized Si(100) Surface

Computational Chemistry Conference – Kentucky – 2003

Si9H14Si15H20

Si21H28

H/Si(100) Surface Models

Computational Chemistry Conference – Kentucky – 2003

Embedded H/Si(111) Surface Models

Si10H16 Si43H46

a b

Computational Chemistry Conference – Kentucky – 2003

Outline

• Quantum Chemistry of Materials – Cluster Approach • Wet oxidation of silicon (100)

• ALD growth of Al2O3 on H/Si Initial reaction mechanism

• Indium Phosphide Surface Chemistry H on P-rich InP(100)

H on In-rich InP(100)

• Semiconductor – molecule – metal systemGaAs – alkanedithiol – Gold

Computational Chemistry Conference – Kentucky – 2003

500 3500250015001000 2000 3000 4000Frequency (cm-1)

Abs

orba

nce 2 × 10-4(HOH)

(HOH)

(SiH)

(SiH)

(OH)

(Si-OH)

Water dissociation on Si(100)-2x1

Room temperature

Computational Chemistry Conference – Kentucky – 2003

Infrared spectra at 400 °C

3 6 7 53 6 5 7

3 6 3 8

2 0 8 5

6 7 06 3 2

6 2 2

7 9 97 7 9

7 5 77 4 3

9 9 01 0 1 1

1 0 3 8

6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 2 0 5 0 2 2 5 02 1 5 0 3 5 0 0 3 6 0 0 3 7 0 0 3 8 0 0

F re q u e n c y (c m - 1 )

Abs

orba

nce

3 7 3 83 6 8 63 6 7 5

2 1 1 8

2 1 6 5

2 2 6 1

2 1 0 92 0 9 9

2 0 8 8

6 0 5

6 3 0

6 1 9

7 9 0

8 2 5

8 1 2

4 × 1 0 -4

( a )

2 × 1 0 -4 2 × 1 0 -4

2 × 1 0 -4

2 × 1 0 -4

2 × 1 0 -4

( c )( b )

( f )( e )( d )

SiO SiH OH

400 °C

25 °C

Computational Chemistry Conference – Kentucky – 2003

Theoretical Strategy

• Errors are similar in related systems, Use exactly similar models• Tight convergence, precise calculations (104 Å, 1 cm1)• Determine trends in frequencies

(e.g.) SiH 2085 cm1

OSiH 2110 cm1

O2SiH 2165 cm1

O3SiH 2250 cm1

• Trends in intensities, Isotope effects, H vs. D, 16O vs. 18O• Determine small number of correction factors

~ 100 cm1 for SiH stretch

~ 20 cm1 for SiOSi

Computational Chemistry Conference – Kentucky – 2003

Structures assigned at 400 °C

6 0 0 8 0 0 1 0 0 0 2 0 0 0 2 2 0 0 3 6 0 0 3 8 0 0

4 x 1 0 - 4

Abs

orba

nce 2 x 1 0 - 4 2 x 1 0 - 4

( S i - O H )

( S i - H )

( O - H )

( S i - H )

( O - H )

F r e q u e n c y ( c m - 1 )

2 2 6 02 1 6 5

2 1 1 6

2 1 0 82 0 9 9

2 0 8 8

6 7 0

7 9 9 9 9 01 0 1 1

1 0 4 07 4 5

7 5 7

6 3 4

9 4 29 2 0

8 4 0

6 0 0 8 0 0 1 0 0 0 2 0 0 0 2 2 0 0

F r e q u e n c y ( c m - 1 )

Computational Chemistry Conference – Kentucky – 2003

Outline

• Quantum Chemistry of Materials – Cluster Approach • Wet oxidation of silicon (100)

• ALD growth of Al2O3 on H/Si Initial reaction mechanism

• Indium Phosphide Surface Chemistry H on P-rich InP(100)

H on In-rich InP(100)

• Semiconductor – molecule – metal systemGaAs – alkanedithiol – Gold

Computational Chemistry Conference – Kentucky – 2003

• As device dimensions shrink, there is a need to replace SiO2 with

alternative dielectric materials

• Al2O3 growth on Si is an active topic: Al2O3 vs. SiO2

(ε = 9.8 vs. 3.9 ); thermodynamically stable interface in contact with

Si

• Atomic layer deposition provides a mechanism to have controlled

growth

• Involves two self-terminating half-steps, one involving the metal and

the other involving the oxide

• Al(CH3)3 (TMA) and H2O are commonly used

ALD of Al2O3 on H-passivated Silicon

Computational Chemistry Conference – Kentucky – 2003

Experimental Motivation

• Frank, Chabal and Wilk (APL, 2003)

– 300° C exposure of H/Si substrates to TMA or H2O

• deposition of Al species with TMA

• no reactivity observed for H2O

– Surprising observation: Metal precursor (TMA) controls

nucleation on H-passivated silicon

Theoretical focus

The initial surface reactions between ALD precursors

and H-passivated silicon surfaces

Computational Chemistry Conference – Kentucky – 2003

H/Si(100) Surface Models

Si9H14Si15H20

Computational Chemistry Conference – Kentucky – 2003

H/Si + H2O → SiOH + H2

0.00.15

eV

1.58

0.75

+

H2O + H/Si(100) Rxns

Computational Chemistry Conference – Kentucky – 2003

H/Si + Al(CH3)3 → SiAl(CH3)2 + CH4

TMA + H/Si(100) Rxns

0.00.02

eV

1.22

0.31

+

Computational Chemistry Conference – Kentucky – 2003

a b

1.5 8 e V 1 .5 7 e V

c d

1.2 2 e V 1 .2 5 e V

H2O and TMA + H/Si(100)-2×1 Rxns

• H2O and TMA activation

energies and overall enthalpy

are similar with single-dimer

and double-dimer

H/Si(100) models

• Barrier for TMA lower than

the barrier for H2O

Computational Chemistry Conference – Kentucky – 2003

TMA vs. H2O

Computational Chemistry Conference – Kentucky – 2003

TMA vs. H2O

• TMA barrier is 0.3 eV lower than H2O barrier

• TMA reaction ~ 103 faster than H2O reaction

• Consistent with the experimental observations

no reaction with H2O at 300°C

reactive products seen with TMA

Computational Chemistry Conference – Kentucky – 2003

H/Si(111) Surface Models

Si10H16 Si43H46

Computational Chemistry Conference – Kentucky – 2003

H2O and TMA + H/Si(111) Rxns

1.6 4 e V 1 .6 4 e V

1 .2 3 e V 1 .4 5 eV

• H2O activation energies and

overall enthalpy are conserved

with Si10 and Si43

• TMA energetics are dramatically

different – indicating significant

steric interactions

Computational Chemistry Conference – Kentucky – 2003

Outline

• Quantum Chemistry of Materials – Cluster Approach • Wet oxidation of silicon (100)

• ALD growth of Al2O3 on H/Si Initial reaction mechanism

• Indium Phosphide Surface Chemistry H on P-rich InP(100)

H on In-rich InP(100)

• Semiconductor – molecule – metal systemGaAs – alkanedithiol – Gold

Computational Chemistry Conference – Kentucky – 2003

III-V Materials - InP• important for lasers and high-speed electronics• Surface structure and chemistry poorly understood

• Difficult to prepare surfaces (requires MOVPE)• High quality experimental data (Hicks)• Vibrational spectra (complicated)

• Band structure methods – difficult for vibrations• Cluster models - difficult to formulate

• Can models similar to that used for silicon be

successfully used for InP, GaAs, ...?• How accurate are theoretical calculations for InP?

Computational Chemistry Conference – Kentucky – 2003

Polarized Spectra (PH region)

Hydrogen Adsorption onP-rich InP(100)-(21)

Computational Chemistry Conference – Kentucky – 2003

Vibrational spectrum (PH region)

Computational Chemistry Conference – Kentucky – 2003

Complications for InP

• Bonding has covalent and dative contributions

• On average, there are three covalent and one

dative bond around each element

• Terminating all back bonds with hydrogens

leads to unphysical structures

• Hydrogen atoms can be used to terminate

truncated covalent bonds but cannot form

dative bonds

Computational Chemistry Conference – Kentucky – 2003

• Neglecting the truncated dative bonds leads to

unphysical structures - with bridging hydrogens

Complications for InP

Computational Chemistry Conference – Kentucky – 2003

Cluster model for InP(001)-21

• Terminate truncated covalent bonds with H

• Terminate truncated dative bonds with PH3

• Two such dative groups are sufficient to define

a physically reasonable charge-neutral cluster

with all atoms being tetracoordinated

Computational Chemistry Conference – Kentucky – 2003

Single dimer model for InP(001)-21

Computational Chemistry Conference – Kentucky – 2003

• Unit cell has two surface P and two second-layer In• Two surface P atoms contribute 10 e- (2x5)• Second layer In atoms contribute half their

valence electrons - 3e-• Total electrons - 13• Bonds formed 5 (1 dimer + 4 back bonds) - uses 10 e-• The remaining 3 electrons are distributed

between the two lone-pair dangling bonds per dimer

Electron count forP-rich InP(001) dimer

Computational Chemistry Conference – Kentucky – 2003

Hydrogenated structures –InP(001)-21

1 2 3

Computational Chemistry Conference – Kentucky – 2003

Vibrational Frequencies

Cluster Assignment Theory Experiment

1 PH 2302 2301

2 HPPH (as) 2256 2265

2 HPPH (s) 2260 2265

3 PH 2238 2225

3 HPH (s) 2319 2317

3 HPH (as) 2339 2338

Computational Chemistry Conference – Kentucky – 2003

Polarized Spectra (InH, PH region)

Hydrogen Adsorption onIn-rich InP (24)

Computational Chemistry Conference – Kentucky – 2003

Electron count forIn-rich InP(001) dimer

• Unit cell has two surface In and two second-layer P• Two surface In atoms contribute 6 e- (2x3)• Second layer In atoms contribute half their

valence electrons - 5e-• Total electrons - 11• Bonds formed 5 (1 dimer + 4 back bonds) - uses 10 e-• The remaining 1 electron is distributed

between the two In atoms of the dimer

Computational Chemistry Conference – Kentucky – 2003

H-adsorption onIn-rich InP (2x4) surface

• Surface has 4 In dimers in the unit cell• There is 1 In-P mixed dimer as well

Computational Chemistry Conference – Kentucky – 2003

Two dimer model with terminaland bridging H

Theory:Terminal H - 1659, 1675 cm1

Bridged H - 1348, 1384

Expt: 1660, 1682 cm1

1350 (broad) 1150 (broad)

Terminal and bridged In hydrides can be clearly assignedWhat is the band at 1150 cm1?

Computational Chemistry Conference – Kentucky – 2003

Coupled bridging hydrogens –“Butterfly” Isomer

Terminal H - 1659, 1660 cm1

Bridged H - 1117(w), 1142(s)

Consistent with the broad band observed at 1150 cm1

Computational Chemistry Conference – Kentucky – 2003

Plasma Grown Oxide: FTIR Analysis

Referenced to HCl etched surface

IR Transmission spectra

5x10-3

4

3

2

1

0

Ab

sorb

ance

1600140012001000800

Wavenumber /cm-1

p-pol

s-pol

1010

932

1076

•3 vibrational modes at:1076 cm-1 (s)1010 (vw) 932 (w)

•assigned to phosphate compounds (In2O3 has no mode in the 650-4000cm-1 region)

•s-pol p-pol oxide is dense (LO-TO splitting 100 cm-1)

Computational Chemistry Conference – Kentucky – 2003

Cluster model for InPO4

970 - 980 cm1 (w)

1015-1020 cm1 (vw)

1090-1110 cm1 (s)

Computational Chemistry Conference – Kentucky – 2003

Larger Cluster model for InPO4

995 - 1000 cm1 (w)

1045 cm1 (vw)

1095-1135 cm1 (s)

Computational Chemistry Conference – Kentucky – 2003

Outline

• Quantum Chemistry of Materials – Cluster Approach • Wet oxidation of silicon (100)

• ALD growth of Al2O3 on H/Si Initial reaction mechanism

• Indium Phosphide Surface Chemistry H on P-rich InP(100)

H on In-rich InP(100)

• Semiconductor – molecule – metal systemGaAs – alkanedithiol – Gold

Computational Chemistry Conference – Kentucky – 2003

Nanotransfer Printing (nTP)

(a) Etch oxide; deposit dithiol monolayer

(b) Bring stamp into contact with substrate

(c) Remove stamp; complete nTP

GaAs

SH

(CH2)x

S

SH

(CH2)x

S

SH

(CH2)x

S

SH

(CH2)x

S

PDMS stamp 20 nm Au

GaAs

SH

(CH2)x

S

SH

(CH2)x

S

S

(CH2)x

S

Au

S

(CH2)x

S

Au

JVST B20, 2853 (2002)

Hsu, LooLang, Rogers

Computational Chemistry Conference – Kentucky – 2003

10-6

10-5

10-4

10-3

10-2

10-1

100

101

0.6 0.8 1.0 1.2 1.4 1.6

controlevaporatednTP

A(E-)2

C*exp(E/E0)

Ephoton (eV)

Pho

tore

spon

se y

ield

Photoresponse

• nTP diodes do not show Au/GaAs Schottky characteristics• Exp E reflects the exponential distribution of electronic states in the emitter Longer

molecules: better ordered monolayer, lower fields• Origin: molecular occupied levels, interfacial GaAs-S states

E0 (meV)

C8 50

C9 43.5

C10 37Au n+ GaAs

E

Ev

EcEF

EgGaAs

Au n+ GaAs

E

Ev

EcEF

GaAs Eg

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

Ga4As5H10-SC8H16S-Au5 (B3-LYP/6-31+G*)

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

HOMO -6.1 eV

O-245

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

LUMO -3.2 eV

V-246

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

Au-S-Alkyl -8.0 eV

O-226

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

Au-S-Alkyl -6.5 eV

O-242

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

GaAs-S-Alkyl -7.4 eV

O-237

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

GaAs-S-Alkyl -6.4 eV

O-243

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

GaAs-S-Alkyl -6.3 eV

O-244

Computational Chemistry Conference – Kentucky – 2003

-10 -8 -6 -4 -2 0 2 4Energy (eV)

DO

S

S-Alkyl-S 0.07 eV

V-269

Computational Chemistry Conference – Kentucky – 2003

GaAsAu dithiol

Ev

Ec

EF

HOMO

Band Alignment & Transport Mechanism

Au-S

GaAs-S E<Eg

E>Eg