INDIANA UNIVERSITY DEPARTMENT OF CHEMISTRY Electronic Structure Studies of Semiconductor Surface Chemistry using Cluster Models Krishnan Raghavachari Indiana University Bloomington, IN 47405

Computational Chemistry Conference – Kentucky – 2003 Outline Quantum Chemistry of Materials – Cluster Approach Wet oxidation of silicon (100) ALD growth of Al 2 O 3 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

Computational Chemistry Conference – Kentucky – 2003 Collaborators Mat Halls Theory Boris StefanovPost-Docs Yves ChabalExperiment Marcus WeldonAFM, IR on silica Kate QueeneyInfrared on Si Olivier PlucheryInfrared on InP Martin Frank ALD of Al 2 O 3 on H/Si Bob Hicks (UCLA)IR, STM Gangyi ChenInP surface chemistry Julia Hsu, Loo, Lang, Rogersmolecular electronics

Computational Chemistry Conference – Kentucky – 2003 Quantum Chemistry of Materials Cluster Approach Truncate back-bonds with H Describe the local region of interaction Appropriate for localized bonding (e.g., Si, SiO 2 )

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/ SiO 2 Interface Structure  Mechanistic problems  HF etching of silicon surfaces  Oxidation of Si(100)  ALD growth of Al 2 O 3 on Si  CVD growth of InP

Computational Chemistry Conference – Kentucky – 2003 Dimerized Si(100) Surface

Computational Chemistry Conference – Kentucky – 2003 Si 9 H 14 Si 15 H 20 Si 21 H 28 H/Si(100) Surface Models

Computational Chemistry Conference – Kentucky – 2003 Embedded H/Si(111) Surface Models Si 10 H 16 Si 43 H 46

Computational Chemistry Conference – Kentucky – 2003 Outline Quantum Chemistry of Materials – Cluster Approach Wet oxidation of silicon (100) ALD growth of Al 2 O 3 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

Computational Chemistry Conference – Kentucky – Frequency (cm -1 ) Absorbance 2 ×  (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 Si  O Si  H O  H 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 (10  4 Å, 1 cm  1 ) Determine trends in frequencies (e.g.) Si  H 2085 cm  1 OSi  H 2110 cm  1 O 2 Si  H 2165 cm  1 O 3 Si  H 2250 cm  1 Trends in intensities, Isotope effects, H vs. D, 16 O vs. 18 O Determine small number of correction factors ~ 100 cm  1 for Si  H stretch ~ 20 cm  1 for Si  O  Si

Computational Chemistry Conference – Kentucky – 2003 Structures assigned at 400 °C

Computational Chemistry Conference – Kentucky – 2003 Outline Quantum Chemistry of Materials – Cluster Approach Wet oxidation of silicon (100) ALD growth of Al 2 O 3 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

Computational Chemistry Conference – Kentucky – 2003 As device dimensions shrink, there is a need to replace SiO 2 with alternative dielectric materials Al 2 O 3 growth on Si is an active topic: Al 2 O 3 vs. SiO 2 (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(CH 3 ) 3 (TMA) and H 2 O are commonly used ALD of Al 2 O 3 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 H 2 O deposition of Al species with TMA no reactivity observed for H 2 O –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 Si 9 H 14 Si 15 H 20

Computational Chemistry Conference – Kentucky – 2003 H/Si + H 2 O → Si  OH + H  0.15 eV 1.58  H 2 O + H/Si(100) Rxns

Computational Chemistry Conference – Kentucky – 2003 H/Si + Al(CH 3 ) 3 → Si  Al(CH 3 ) 2 + CH 4 TMA + H/Si(100) Rxns 0.0  0.02 eV 1.22 

Computational Chemistry Conference – Kentucky – 2003 H 2 O and TMA + H/Si(100)-2×1 Rxns H 2 O 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 H 2 O

Computational Chemistry Conference – Kentucky – 2003 TMA vs. H 2 O

Computational Chemistry Conference – Kentucky – 2003 TMA vs. H 2 O TMA barrier is 0.3 eV lower than H 2 O barrier TMA reaction ~ 10 3 faster than H 2 O reaction Consistent with the experimental observations no reaction with H 2 O at 300°C reactive products seen with TMA

Computational Chemistry Conference – Kentucky – 2003 H/Si(111) Surface Models Si 10 H 16 Si 43 H 46

Computational Chemistry Conference – Kentucky – 2003 H 2 O and TMA + H/Si(111) Rxns H 2 O activation energies and overall enthalpy are conserved with Si 10 and Si 43 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 Al 2 O 3 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

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 (P  H region) Hydrogen Adsorption on P-rich InP(100)-(2  1)

Computational Chemistry Conference – Kentucky – 2003 Vibrational spectrum (P  H 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)-2  1 Terminate truncated covalent bonds with H Terminate truncated dative bonds with PH 3 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)-2  1

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 for P-rich InP(001) dimer

Computational Chemistry Conference – Kentucky – 2003 Hydrogenated structures – InP(001)-2 

Computational Chemistry Conference – Kentucky – 2003 Vibrational Frequencies Cluster Assignment Theory Experiment 1 P  H H  P  P  H (as) H  P  P  H (s) P  H H  P  H (s) H  P  H (as)

Computational Chemistry Conference – Kentucky – 2003 Polarized Spectra (In  H, P  H region) Hydrogen Adsorption on In-rich InP  (2  4)

Computational Chemistry Conference – Kentucky – 2003 Electron count for In-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 on In-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 terminal and bridging H Theory: Terminal H , 1675 cm  1 Bridged H , 1384 Expt: 1660, 1682 cm  (broad) 1150 (broad) Terminal and bridged In hydrides can be clearly assigned What is the band at 1150 cm  1 ?

Computational Chemistry Conference – Kentucky – 2003 Coupled bridging hydrogens – “Butterfly” Isomer Terminal H , 1660 cm  1 Bridged H (w), 1142(s) Consistent with the broad band observed at 1150 cm  1

Computational Chemistry Conference – Kentucky – 2003 Plasma Grown Oxide: FTIR Analysis Referenced to HCl etched surface IR Transmission spectra p-pol s-pol vibrational modes at: 1076 cm -1 (s) 1010 (vw) 932 (w) assigned to phosphate compounds (In 2 O 3 has no mode in the cm -1 region) s-pol  p-pol  oxide is dense (LO-TO splitting  100 cm -1 )

Computational Chemistry Conference – Kentucky – 2003 Cluster model for InPO cm  1 (w) cm  1 (vw) cm  1 (s)

Computational Chemistry Conference – Kentucky – 2003 Larger Cluster model for InPO cm  1 (w) 1045 cm  1 (vw) cm  1 (s)

Computational Chemistry Conference – Kentucky – 2003 Outline Quantum Chemistry of Materials – Cluster Approach Wet oxidation of silicon (100) ALD growth of Al 2 O 3 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

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 PDMS stamp 20 nm Au GaAs JVST B20, 2853 (2002) Hsu, Loo Lang, Rogers

Computational Chemistry Conference – Kentucky – 2003 E photon (eV) Photoresponse yield 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 E 0 (meV) C8 50 C C10 37 Au n + GaAs  E EvEv EcEc EFEF E g GaAs Au n + GaAs E EvEv EcEc EFEF GaAs E g

Computational Chemistry Conference – Kentucky – 2003 Ga 4 As 5 H 10 -SC 8 H 16 S-Au 5 (B3-LYP/6-31+G*)

Computational Chemistry Conference – Kentucky – 2003 HOMO -6.1 eV O-245

Computational Chemistry Conference – Kentucky – 2003 LUMO -3.2 eV V-246

Computational Chemistry Conference – Kentucky – 2003 Au-S-Alkyl -8.0 eV O-226

Computational Chemistry Conference – Kentucky – 2003 Au-S-Alkyl -6.5 eV O-242

Computational Chemistry Conference – Kentucky – 2003 GaAs-S-Alkyl -7.4 eV O-237

Computational Chemistry Conference – Kentucky – 2003 GaAs-S-Alkyl -6.4 eV O-243

Computational Chemistry Conference – Kentucky – 2003 GaAs-S-Alkyl -6.3 eV O-244

Computational Chemistry Conference – Kentucky – 2003 S-Alkyl-S 0.07 eV V-269

Computational Chemistry Conference – Kentucky – 2003 GaAsAudithiol EvEv EcEc EFEF HOMO Band Alignment & Transport Mechanism Au-S GaAs-S E<E g E>E g