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H-bond mediated dissociation of ammonia on Si(001)

The modification of Si surfaces with N atoms or N-carrying ligands is an effective way to expand the Si chemical functionalities in microelectronics, energy conversion and gas sensing. NH3 is the most common and most widely investigated Si nitridation agent.

At about or below room temperature, NH3 chemisorbs dissociatively on Si(001) surfaces with the NH2 and H fragments bonded to two Si atoms. This process is characterized by an energy barrier of ~1 eV that could stabilize, to some extent, the chemisorbed NH3 in a metastable configuration (Fig. 1a). At low temperature, indeed, this energy barrier can be overcome only if the internal energy of the adsorbing NH3 molecule is totally spent for the dissociation rather than being released to the substrate phonons. Conversely, if the energy is dissipated through the substrate, the chemisorbed NH3 molecules are supposed to be stable on the Si surface. As a matter of fact, intact NH3 molecules have been observed on the Si(001)-2x1 surface but exclusively after the exposure to very low ammonia doses, whereas only NH2 and H fragments are revealed on the surfaces dosed at saturation.

In order to explain the disappearance of the chemisorbed NH3 molecules for coverage close to saturation, we performed energetics and structural calculations to single out a dissociation process involving a lower energy barrier.
We have identified a novel route to the dissociation by investigating the stability of an isolated chemisorbed ammonia molecule (NH3c) on the Si(001)-2x1, reached by a gas phase NH3 molecule (NH3g) (Fig. 1b). Following the formation of a H-bond between the NH3c and NH3g, a reaction intermediate (NH4+) is formed, opening an alternative path for the transfer of a H atom from the NH3c to the Si dangling bond nearby. In the final configuration (Fig. 1b) the initially chemisorbed NH3c is dissociated into NH2 and H while the NH3g molecule remains physisorbed being H-bonded both to the amino group and to the H atom. This H bond mediated reaction path is characterised by an energy barrier of about 0.5 eV, less than half the barrier attributed to the non-mediated process (Fig. 1a). The whole energy balance, including the energy of NH3g, makes this process active even at temperatures as low as 150 K.

Figure 1:  (a) Potential energy curves relative to (a) the dissociative adsorption of the NH3 on Si(001)-2x1, along the reaction coordinate R, i.e., the distance between the N and the migrating H atom. Ead indicates the adsorption energy; (b) the ‘‘H-bond mediated’’ reaction, along the reaction coordinates R1 and R2, corresponding to the distances between the H of the chemisorbed NH3c and the N of the incoming NH3g molecule, and between the N of the ammonium ion and migrating H, respectively. De represents the energy required to dissociate the H-bond between NH3c and NH3g. H (white), Si (yellow) and N (blue).

 

In order to prove this, we have studied the adsorption of NH3 on Si(001)-2x1 at 150 K by using fast core-level photoelectron spectroscopy as a function of the ammonia dose, at the SuperESCA beamline of Elettra. As seen in Fig. 2, up to a dose of 0.3 L the N1s spectra show the component Nd at 399 eV binding energy (BE) attributed to the chemisorbed NH2 and the Nc component at 402.0 eV, never been recorded so far, assigned to the NH3c molecules on the basis of the close agreement between the measured (+3.0 eV) and the calculated (+3.2 eV) BE shift with respect to Nc. At doses higher than 0.3 L the component Np appears at 400.6 eV, being due to the condensed NH3 molecules starting to pile up at the interface.
Our results show that undissociated molecules are observed exclusively for low doses and never on the surface dosed at saturation due to the balance between the chemisorption rate and the H-bonding mediated dissociation rate. At late adsorption stage, the rate of dissociation prevails and the surface is covered only by NH2 and H fragments. By facing adsorbate-gas phase interactions, which are usually left out in the modelling of the ammonia dissociation on Si surface, we elucidate the microscopic aspect of the initial Si nitridation.


Figure 2: NH3 adsorption on the Si(100)-2x1 surface followed by fast XPS. a) Selected N 1s spectra measured at different NH3 doses. Nd, Nc, and Np represent the chemisorbed NH2 amino groups, the chemisorbed and the physisorbed NH3 molecules, respectively. b) Intensity of the Nd, Nc, and Np components as a function of the NH3 dose.


 

This research was conducted by the research team of the SuperESCA beamline of the Elettra Laboratory, in collaboration with researchers of the CNR-ISMN, CNR-IMIP, CNR-ISC, University of Trieste and IOM-CNR.

  • Mauro Satta, CNR-ISMN, Istituto per lo Studio dei Materiali Nanostrutturati, Dip. Chim., Universita` ‘‘La Sapienza’’, Roma, Italy
  • Roberto Flammini, CNR-IMIP Istituto di Metodologie Inorganiche e Plasmi, Monterotondo Scalo, Italy
  • Andrea Goldoni and Silvano Lizzit, Sincrotrone Trieste S.C.p.A., Trieste, Italy
  • Alessandro Baraldi, Physics Department, University of Trieste, Trieste, Italy and IOM CNR, Laboratorio TASC, Trieste, Italy
  • Rosanna Larciprete, CNR-ISC, Istituto dei Sistemi Complessi, Roma, Italy

Reference

Mauro Satta, Roberto Flammini, Andrea Goldoni, Alessandro Baraldi, Silvano Lizzit and Rosanna Larciprete, Fundamental Role of the H-Bond Interaction in the Dissociation of NH3 on Si(001)-(2×1), Phys. Rev. Lett. 109, 036102 (2012); DOI: 10.1103/PhysRevLett.109.036102

Last Updated on Thursday, 11 October 2012 10:04