Electronic Structure Studies of Semiconductor Surface Chemistry using Cluster Models

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Electronic Structure Studies of Semiconductor
Surface Chemistry using Cluster Models
Krishnan Raghavachari Indiana UniversityBlooming
ton, IN 47405
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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 system
• GaAs alkanedithiol Gold

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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

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Quantum Chemistry of MaterialsCluster Approach

• Describe the local region of interaction

• Truncate back-bonds with H

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

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Cluster approach - Questions

• Cluster size dependence
• Embedded cluster approaches
• Cluster termination
• Cluster constraints
• Cluster approach vs. Slab approach

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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

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Dimerized Si(100) Surface
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H/Si(100) Surface Models
Si15H20
Si9H14
Si21H28
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Embedded H/Si(111) Surface Models
Si10H16
Si43H46
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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 system
• GaAs alkanedithiol Gold

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Water dissociation on Si(100)-2x1
Room temperature
d(HOH)
d(SiH)
n(HOH)
n(Si-OH)
n(SiH)
n(OH)
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Infrared spectra at 400 C
Si?O Si?H O?H
400 C 25 C
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Theoretical Strategy

• Errors are similar in related systems, Use
  exactly similar models
• Tight convergence, precise calculations (10?4 Å,
  1 cm?1)
• Determine trends in frequencies
• (e.g.) Si?H 2085 cm?1
• OSi?H 2110 cm?1
• O2Si?H 2165 cm?1
• O3Si?H 2250 cm?1
• Trends in intensities, Isotope effects, H vs. D,
  16O vs. 18O
• Determine small number of correction factors
• 100 cm?1 for
  Si?H stretch
• 20 cm?1 for
  Si?O?Si

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Structures assigned at 400 C
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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 system
• GaAs alkanedithiol Gold

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ALD of Al2O3 on H-passivated Silicon

• 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(e 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

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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

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H/Si(100) Surface Models
Si15H20
Si9H14
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H2O H/Si(100) Rxns

• H/Si H2O ? Si?OH H2

1.58

0.0
?0.15
?0.75
eV
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TMA H/Si(100) Rxns
H/Si Al(CH3)3 ? Si?Al(CH3)2 CH4

1.22
0.0
?0.02
?0.31
eV
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H2O and TMA H/Si(100)-21 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

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TMA vs. H2O
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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 300C
• reactive products seen with TMA

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H/Si(111) Surface Models
Si10H16
Si43H46
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H2O and TMA H/Si(111) Rxns

• H2O activation energies and overall enthalpy are
  conserved with Si10 and Si43
• TMA energetics are dramatically different
  indicating significant steric interactions

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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 system
• GaAs alkanedithiol Gold

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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?

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Polarized Spectra (P?H region)
Hydrogen Adsorption onP-rich InP(100)-(2?1)
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Vibrational spectrum (P?H region)
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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

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Complications for InP

• Neglecting the truncated dative bonds leads to
• unphysical structures - with bridging hydrogens

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Cluster model for InP(001)-2?1

• 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

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Single dimer model for InP(001)-2?1
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Electron count forP-rich InP(001) dimer

• 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

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Hydrogenated structures InP(001)-2?1
1 2 3
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Vibrational Frequencies
Cluster Assignment Theory
Experiment 1 P?H 2302
2301 2 H?P?P?H (as) 2256
2265 2 H?P?P?H (s)
2260 2265 3 P?H
2238 2225 3
H?P?H (s) 2319 2317 3
H?P?H (as) 2339 2338
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Polarized Spectra (In?H, P?H region)
Hydrogen Adsorption onIn-rich InP ?(2?4)
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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

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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

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Two dimer model with terminaland bridging H
Expt 1660, 1682 cm?1 1350 (broad)
1150 (broad)
Theory Terminal H - 1659, 1675 cm?1 Bridged H
- 1348, 1384
Terminal and bridged In hydrides can be clearly
assigned What is the band at 1150 cm?1?
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Coupled bridging hydrogens Butterfly Isomer
Terminal H - 1659, 1660 cm?1 Bridged H -
1117(w), 1142(s)
Consistent with the broad band observed at 1150
cm?1
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Plasma Grown Oxide FTIR Analysis
IR Transmission spectra

• 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)

p-pol
1076
s-pol
932
1010
Referenced to HCl etched surface
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Cluster model for InPO4
970 - 980 cm?1 (w) 1015-1020 cm?1 (vw) 1090-1110
cm?1 (s)
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Larger Cluster model for InPO4
995 - 1000 cm?1 (w) 1045 cm?1 (vw) 1095-1135 cm?1
(s)
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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 system
• GaAs alkanedithiol Gold

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Nanotransfer Printing (nTP)
Hsu, Loo Lang, Rogers
JVST B20, 2853 (2002)
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Photoresponse
E0 (meV)
C8 50
C9 43.5
C10 37

• 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

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Ga4As5H10-SC8H16S-Au5 (B3-LYP/6-31G)
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HOMO -6.1 eV
O-245
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LUMO -3.2 eV
V-246
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Au-S-Alkyl -8.0 eV
O-226
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Au-S-Alkyl -6.5 eV
O-242
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GaAs-S-Alkyl -7.4 eV
O-237
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GaAs-S-Alkyl -6.4 eV
O-243
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GaAs-S-Alkyl -6.3 eV
O-244
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S-Alkyl-S 0.07 eV
V-269
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Band Alignment Transport Mechanism
EF
Ec
Ev
HOMO
GaAs
Au
dithiol