ON A METASTABLE CONFIGURATION AT THE H-TERMINATED (100) Si SURFACE

1. ON A METASTABLE CONFIGURATION AT THE H-TERMINATED (100) Si SURFACE Giacomo Giorgi Dipartimento di Chimica, Università di Perugia

2. -After etching in HFaq and rinsing in O2-free water processes • Exp & Theor works have shown through AR-XPS and MIR-IR spectroscopy • the chemical heterogeneity of the native (1 0 0) Si surface Nevertheless… … even if the so-prepared (100) Si surface is mainly characterized by H-terminations and siloxo groups, IR and XPS are not completely able to interpretate some anomalous features. Neither IR nor XPS could be decomposed in terms of known chemical configurations

3. XPS Si 2p spectrum Si(- ) shifted by –(0.27±0.04) eV with respect to Si IR spectrum  in the 3100-3650 cm-1 region, peaks of ambiguous attribution Si features with negative chemical shift (-0.5 eV with respect to Si(0))are observed at the surface of the clean, 2 x 1 reconstructed Si(100) surface, due to the Si2 dimers Species too reactive to survive after exposure to air XPS peak with negative chemical shift  Reduced silicon (?metal contaminant bonded to Si?) Metal presence should be accessible to XPS (no evidence in our samples)

4. 3290 & 3440 cm-1: O-H stretching of interacting silanols and water. Tentative attribution: 3200 cm-1: O-H stretching modes in H2O+ 3115 cm-1:O-H stretching in water molecules as acceptors in complexes HF2¯···H2O 3620 cm-1: F-H stretching mode with HF as acceptor in H2O···HF pair None of these attributions seems to be fully satisfactory due to the low probabilty of surviving of the ionic species and to the high strength of the neutral species mode if compared with XPS evidence

5. Unexplained XPS & IR signals: feature Si(-) for the peaks at about 3115, 3250, 3600 cm-1 “Numquam ponenda est pluralitas sine necessitate” William Ockham (c. 1285–1349) Search for a unique feature: Lewis adduct Si(3)O(3) - Divalent Si datively stabilized by lone e- pair donation from H2O molecule -The partial positive charge on Si, due to c(Si-O) is more than counterbalanced by the formal unit negative charge Si(3)O(3) explains both Si shift towards lower binding E in XPS and O-H stretching depression

6. CLUSTER MODELS FOR THE SUPERFICIAL SITES H-terminated Si atoms forming two fused cyclohexasilenes sharing 3 adjacent Si atoms. In particular a. Corners opposed to the one formed by the 3 shared Si atoms and H2-terminated  1 x 1 (100) SiH2 surface

7. b. Corners H-terminated and mutually bonded 2 x 1 (100) (SiH)2 surface c. Corners bonded each other by a double bond 2 x 1 (100) Si2 surface

8. Computational details • DFT calculations performed with ADF package (Amsterdam Density Functional) • MO expanded in a basis set of Slater type orbitals (STOs) with frozen core approx • Full geometry optimizations within spin restricted and unrestricted (i.e. singlet & triplet) approaches including Becke’s and Perdew’s gradient corrections to exchange and correlation part of the potential, respectively. • Si, H, and O MO expanded in a triple-ζ STO (2p STO for H and 3d + 4f STO for Si & O, as polarization functions) • -No symmetry imposed; stability checked through a normal mode analysis (All νvib> 0) • -VDD (Voronoi Deformation Density)  charge calculations on the clusters

9. To understand the IR evidence we have considered the following molecules: Most stable species in presence of H2O. Too high Ea (0.95 eV)ignored: can not occur at room T

10. Ragavachari et al.  H2O + H2Si: exothermal by 0.56 eV (vs.“our” 0.15 eV for 4s+H2O) -The second H2O molecule stabilizes the complex much more than the first H2O DE= -0.49 eV (i.e. condensation enthalpy of H2O) -Assuming that the formation of dried silylene takes place through the water evaporation the equilibrium coverage can be described in terms of Langmuir isotherm Θ= (p2 = Peqof H2O dimers, pL = (2πm2 kBT)1/2 ν2 /σ2 , DE=0.49 eV (i.e. 6 4s + (H2O)2) - In our exp. Conditions (IR) only dimers occur in appreciable amounts p2 p2 + pL exp(-ΔE /kBT) IR: Silylene centres are highly covered by H2O molecules. 2 strong vibration modes at 3193 & 3687 cm-1 for centre 5 HYDRATED :SiH2RESPONSIBLE OF IR PEAKS OF UNCERTAIN ATTRIBUTION

11. UHV (i.e. p2 =0)  silylene CAN NOT remain hydrated 4T  H strongly symmetric 4S H strongly asymmetric (diborane-like) LEWIS FORMULA Silylenic Si  LEWIS ACID VDD analysis sustains the silanic H “positivization”  inverted hydrogen bond Explaination for the low adsorption energy of water (“our” 0.15 vs 0.56 eV of Ragavachari) The net charge < 0 on silylene might explain the presence of a superficial XPS feature with chemical shift < 0 with respect to elemental Si on H-terminated Si

12. Process thermodynamically favored but …. The lifetime of 4Scan be sufficiently high to survive on the lab time scale Suggested mechanism: the decay 4S2 requires the excitation to a triplet state followed by H-abstraction from –SiH2 and Si-Si bond formation Energy considerations: ES-T=0.16 eV (excitation of 4T to a singlet state) DE4T-4S(=0.84eV) < Ea 4S 4T < DE4T-4S + ES-T (=1.00 eV)

13. If reaction 4S  4T is thermally activated the lifetime t is expressed as: • t=t0 exp (E*/ kBT) (for t0 = 10-13 s, 40 x 104 s < t < 2.4 x 10 4 s) 4S stabilty on the lab time scale is guaranteed

14. Surface modellization of the clusters (SIESTA package) • 8 layers of Si, each formed by 12 Si atoms, PBC; the first one is terminated with • the H atoms required to attain the wanted termination and reconstruction • -NAO/GGA/Perdew-Burke-Enzerhof • -Basis set: DZP for Si, SZ for H • -Core electrons: Norm-conserving pseudopotentials in the TM form • -k-point sampling: Monkhorst-Pack 3x2x2 • - Mesh cutoff 200. Ry • -Cell-size: 11.5 Å x 14.7 Å x 16.5 Å, dihydrogenated, monohydrogenated, clean

16. a. 2 SiH2 (SiH)2 + H2DE=-0.09 eV 1 x 1 (100) 2 x 1 (100) 1  2 + H2DE= 0.20 eV b. (SiH2)  Si2 + H2DE=1.89 eV 2 x 1 (100) 2 x 1 (100) 2  3 + H2DE= 2.00 eV

17. Conclusions • We have considered the hypothesis that the HFaq etching of the (100) Si surface gives rise to a quantitative amount of silylene groups • -Quantum mechanical modelling of the surface has revealed that: • a. In the absence of external bases  silylene stabilization by the formation • of an inverted H-bond with a neighbouring silanic group • b. In the presence of a moisture dative bond via the donation of electron pair • by oxo-oxygen. • In the silylene-H metastable complex Si is negatively charged  responsible for the Si(-) in XPS • The silylene-H2O pairs  responsible for few IR peaks of uncerted attribution

18. Dr. Gianfranco Cerofolini STMicroelectronics Agrate (Milan) Prof. Paola Belanzoni Chemistry Dept. Perugia Prof. Antonio Sgamellotti Chemistry Dept. Perugia

20. The results: XPS detects Si, O, C (adventitious), F (traces) AR-XPS + MIR-IR: surface with prevailing H-terminations AR-XPS : high % of H-termination is oxidized and covered with -OH groups (MIR-IR) AR-XPS  Evidence for a broad superficial signal in the region of the H-terminated Si MIR-IR  Consistent with AR-XPS providing evidence for a very broad band (3100-3650 cm-1), decomposed in terms of silanols, H2O, and HF vibrational modes

21. Voronoi Deformation Density (VDD) method • “Usual” Mulliken  scarce reliability due to heavy basis set dependency • (50%-50% sharing of the overlap populations no diff. in χ between atoms) • Bader & NPA  Not REALISTIC (too much ionic character also for covalent bonds) • On the opposite: • VDD & Hirshfeld give rise to chemically meaningful charges • (NOT amount of charge in a volume, BUT flow of charge from one atom to another)

22. ρpromolecule( r ) = ∑ ρB ( r ) B ∫ QAVDD = - [ ρ( r ) - ∑ ρmolecule( r )]d r Voronoi Cell of A VDD is based on • Direct Spatial Integration of the e- density function over an atomic domain • Atomic domain = Voronoi polyhedron of the atom • Use of the deformation density ρdef(r) = ρ(r) - ρpromolecule(r)  • focus on the density variation from the superposition of atomic densities • to the final molecular density