Electronic Structure Studies of Semiconductor Surface Chemistry

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