indiana university department of chemistry electronic structure studies of semiconductor surface...

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

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

Quantum Chemistry of MaterialsCluster Approach

• Truncate back-bonds with H

• Describe the local region of interaction

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

Cluster approach - Questions

• Cluster size dependence

• Embedded cluster approaches

• Cluster termination

• Cluster constraints

Cluster approach vs. Slab approach

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

Dimerized Si(100) Surface

Si9H14Si15H20

Si21H28

H/Si(100) Surface Models

Embedded H/Si(111) Surface Models

Si10H16 Si43H46

Outline

500 3500250015001000 2000 3000 4000Frequency (cm-1)

nce 2 × 10-4(HOH)

(Si-OH)

Water dissociation on Si(100)-2x1

Room temperature

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

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 )

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

4 × 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

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

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

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

7 9 9 9 9 01 0 1 1

1 0 4 07 4 5

9 4 29 2 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 )

Outline

• 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

• 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

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

Si9H14Si15H20

H/Si + H2O → SiOH + H2

0.00.15

H2O + H/Si(100) Rxns

H/Si + Al(CH3)3 → SiAl(CH3)2 + CH4

TMA + H/Si(100) Rxns

0.00.02

1.5 8 e V 1 .5 7 e V

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

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

Si10H16 Si43H46

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

Outline

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?

Polarized Spectra (PH region)

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

Vibrational spectrum (PH region)

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

• Neglecting the truncated dative bonds leads to

unphysical structures - with bridging hydrogens

Complications for InP

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

Single dimer model for InP(001)-21

• 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

Hydrogenated structures –InP(001)-21

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

Polarized Spectra (InH, PH region)

Hydrogen Adsorption onIn-rich InP (24)

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

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

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?

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

Plasma Grown Oxide: FTIR Analysis

Referenced to HCl etched surface

IR Transmission spectra

5x10-3

1600140012001000800

Wavenumber /cm-1

•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)

Cluster model for InPO4

970 - 980 cm1 (w)

1015-1020 cm1 (vw)

1090-1110 cm1 (s)

Larger Cluster model for InPO4

995 - 1000 cm1 (w)

1045 cm1 (vw)

1095-1135 cm1 (s)

Outline

Nanotransfer Printing (nTP)

(a) Etch oxide; deposit dithiol monolayer

(b) Bring stamp into contact with substrate

(c) Remove stamp; complete nTP

(CH2)x

PDMS stamp 20 nm Au

(CH2)x

JVST B20, 2853 (2002)

Hsu, LooLang, Rogers

0.6 0.8 1.0 1.2 1.4 1.6

controlevaporatednTP

A(E-)2

C*exp(E/E0)

Ephoton (eV)

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)

C9 43.5

C10 37Au n+ GaAs

EgGaAs

Au n+ GaAs

GaAs Eg