ON A METASTABLE CONFIGURATION AT THE H-

TERMINATED (100) Si SURFACE

Giacomo Giorgi

Dipartimento di Chimica,Università di Perugia

-Exp & Theor works have shown through AR-XPS and MIR-IR spectroscopy the chemical heterogeneity of the native (1 0 0) Si surface

-After etching in HFaq and rinsing in O2-free water processes

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

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)

3290 & 3440 cm-1:O-H stretching of interactingsilanols and water.

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 ofsurviving of the ionic species and to the high strength of the neutral species mode if

compared with XPS evidence

Tentative attribution:

“Numquam ponenda est pluralitas sine necessitate” William Ockham (c. 1285–1349)

Si O

H

H

Si O

H

H

Si O

H

H

Si

O H

H

+

- Divalent Si datively stabilized by lone e- pair donation from H2O molecule

Unexplained XPS & IR signals: feature Si(-) for the peaks at about 3115, 3250, 3600 cm-1

Search for a unique feature: Lewis adduct Si(3)O(3)

-The partial positive charge on Si, due to 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

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

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

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

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

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 H2OE= -0.49 eV (i.e. condensation enthalpy of H2O)

-Assuming that the formation of dried silylene takes place through the water evaporationthe equilibrium coverage can be described in terms of Langmuir isotherm

Θ=

(p2 = Peq of H2O dimers, pL = (2πm2 kBT)1/2 ν2 /σ2 , E=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

UHV (i.e. p2 =0) silylene CAN NOT remain hydrated

4T H strongly symmetric 4S H strongly asymmetric (diborane-like)

LEWISFORMULA

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 XPSfeature with chemical shift < 0 with respect to elemental Si on H-terminated Si

Process thermodynamically favoredbut ….

The lifetime of 4S can 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) E4T-4S(=0.84eV) < Ea 4S 4T < E4T-4S + ES-T (=1.00 eV)

- If reaction 4S 4T is thermally activated the lifetimeis expressed as:

=0 exp (E*/ kBT)

(for0 = 10-13 s, 40 x 104 s < < 2.4 x 10 4 s)

4S stabilty on the lab time scale is guaranteed

0.84 eV

0.16 eV

4S

4T

E (eV)

TS

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

2 SiH2 (SiH)2 + H2 E=-0.09 eV

1 x 1 (100) 2 x 1 (100)

1 2 + H2 E= 0.20 eV

(SiH2) Si2 + H2 E=1.89 eV

2 x 1 (100) 2 x 1 (100)

2 3 + H2 E= 2.00 eV

a.

b.

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

Prof. Antonio SgamellottiChemistry Dept. Perugia

Prof. Paola BelanzoniChemistry Dept.

Perugia

Dr. Gianfranco Cerofolini

STMicroelectronicsAgrate (Milan)

The results:XPS detects Si, O, C (adventitious), F (traces)AR-XPS + MIR-IR: surface with prevailing H-terminationsAR-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

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)

QAVDD = - [ ρ ( r ) - ∑ ρmolecule ( r )] d r ∫

Voronoi

Cell of A

VDD is based on

ρpromolecule ( r ) = ∑ ρB ( r )B

1. Direct Spatial Integration of the e- density function over an atomic domain

2. Atomic domain = Voronoi polyhedron of the atom

3. 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

Reaction E (eV)

2H2O (H2O)2 -0.19

1 2 + H2 0.20

2 3 + H2 2.00

4S 4T 0.84

4S 2 -1.47

4S + H2O 5 -0.15

4S + (H2O)2 6 -0.49

4S + H2O 7 -2.14