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Electronic Structure Studies of Semiconductor Surface Chemistry using Cluster Models

Electronic Structure Studies of Semiconductor Surface Chemistry using Cluster Models. Krishnan Raghavachari Indiana University Bloomington, IN 47405. Outline. Quantum Chemistry of Materials – Cluster Approach Wet oxidation of silicon (100) ALD growth of Al 2 O 3 on H/Si

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Electronic Structure Studies of Semiconductor Surface Chemistry using Cluster Models

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  1. Electronic Structure Studies of Semiconductor Surface Chemistry using Cluster Models Krishnan Raghavachari Indiana UniversityBloomington, IN 47405

  2. 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

  3. 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

  4. Quantum Chemistry of MaterialsCluster Approach • Describe the local region of interaction • Truncate back-bonds with H • Appropriate for localized bonding (e.g., Si, SiO2)

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

  6. 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

  7. Dimerized Si(100) Surface

  8. H/Si(100) Surface Models Si15H20 Si9H14 Si21H28

  9. Embedded H/Si(111) Surface Models Si10H16 Si43H46

  10. 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

  11. 2 × 10-4 Absorbance 500 1000 1500 2000 2500 3000 3500 4000 Frequency (cm-1) Water dissociation on Si(100)-2x1 Room temperature d(HOH) d(SiH) n(HOH) n(Si-OH) n(SiH) n(OH)

  12. Infrared spectra at 400 °C SiO SiH OH 400 °C 25 °C

  13. 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

  14. Structures assigned at 400 °C

  15. 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

  16. ALD of Al2O3 on H-passivated Silicon • As device dimensions shrink, there is a need to replace SiO2 with alternative dielectricmaterials • 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

  17. 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

  18. H/Si(100) Surface Models Si15H20 Si9H14

  19. H2O + H/Si(100) Rxns H/Si + H2O → SiOH + H2 1.58 + 0.0 0.15 0.75 eV

  20. TMA + H/Si(100) Rxns H/Si + Al(CH3)3→ SiAl(CH3)2 + CH4 + 1.22 0.0 0.02 0.31 eV

  21. 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

  22. TMA vs. H2O

  23. 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

  24. H/Si(111) Surface Models Si10H16 Si43H46

  25. 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

  26. 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

  27. 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?

  28. Hydrogen Adsorption onP-rich InP(100)-(21) Polarized Spectra (PH region)

  29. Vibrational spectrum (PH region)

  30. 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

  31. Complications for InP • Neglecting the truncated dative bonds leads to • unphysical structures - with bridging hydrogens

  32. 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

  33. Single dimer model for InP(001)-21

  34. 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

  35. Hydrogenated structures –InP(001)-21 1 2 3

  36. 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

  37. Hydrogen Adsorption onIn-rich InP (24) Polarized Spectra (InH, PH region)

  38. 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

  39. 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

  40. 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?

  41. 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

  42. 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

  43. Cluster model for InPO4 970 - 980 cm1 (w) 1015-1020 cm1 (vw) 1090-1110 cm1 (s)

  44. Larger Cluster model for InPO4 995 - 1000 cm1 (w) 1045 cm1 (vw) 1095-1135 cm1 (s)

  45. 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

  46. (a) Etch oxide; deposit dithiol monolayer GaAs PDMS stamp 20 nm Au (b) Bring stamp into contact with substrate GaAs (c) Remove stamp; complete nTP Nanotransfer Printing (nTP) Hsu, Loo Lang, Rogers JVST B20, 2853 (2002)

  47. E Ec EF EgGaAs Ev n+ GaAs Au GaAs Eg Photoresponse yield E f EF Ec Ev n+ GaAs Au 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

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

  49. HOMO -6.1 eV O-245

  50. LUMO -3.2 eV V-246

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