R. Miotto

III–N(110) surface relaxation and its dependence on the chemical bonding

Abstract Using existing and new results obtained from first-principles pseudopotential calculations, we have studied the atomic relaxation and its dependence on the chemical bonding, on III–N(110) surfaces. It is found that the characteristics of III–N(110) surfaces differ from other III–V(110) and II–VI(110) surfaces in two important aspects: a significantly reduced surface bond rotation, and a much lower binding energy of the highest occupied surface state. Furthermore the results obtained for AlN and InN corroborate our previously proposed model of a linear relationship between the vertical buckling of the top layer and the relaxed surface bond length not only for III–N, but also for III–V and II–VI semiconductors in general.

First-principles study of the adsorption of PH3 on Ge(001) and Si(001) surfaces

Using a first-principles pseudopotential method we have compared the adsorption and dissociation of the common n-type dopant molecule PH3 on the Si(001)-(21) and Ge(001){(21) surfaces. We find that the dissociated state is energetically more favourable than the molecular state by 1.70(0.81) eV, whereas the latter is 0.58(0.25) eV more stable than the system composed of the free silicon(germanium) surface and PH3(g). The chemisorbed system is characterised by elongated Si{Si(Ge{Ge) dimers that are symmetric in the dissociative case and asymmetric in the molecular case and by the fact that the Si(Ge){PH2 as well as the PH3(ads) groups retain the pyramidal geometry of the phosphine molecule. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental works.

Maleic anhydride adsorption on silicon (001)

The adsorption of maleic anhydride on the Si(001) surface has been investigated using the first-principles pseudopotential formalism. Our total-energy calculations suggest that maleic anhydride (C2H2-C2O3) adsorbs preferentially through a [2+2] cycloaddition of the C=C bond ([2+2]) with an adsorption energy of around 42 kcal/mol. Besides the [2+2] configuration we have also considered other possible coverages and adsorption models, including the adsorption on inter-row and intrarow dimer sites. Based on the analysis of the relative stability of different adsorption models, we propose the formation of mixed domains, containing the [2+2] unit and an interdimer unit. The comparison of our calculated electronic band structure, vibrational modes, and scanning tunneling microscopy images for the [2+2] and the favored interdimer adsorbed structures corroborate our proposed mixed domain model.

Phonons on GaN(110)

We present results of adiabatic bond-charge model calculations for the vibrational properties of the GaN(110) surface using electronic and structural data obtained from a first-principles pseudopotential method. It is found that in order to relate the energy locations of optical phonon modes on this surface with corresponding modes on nonnitride III–V(110) and II–VI(110) surfaces, it is necessary to consider scaling of results with the lattice constant in addition to the reduced mass.

Zn-induced features at the GaAs(110) surface and its importance in the growth of ZnSe on GaAs(110)

A possible model for the ZnSe growth on GaAs(110) is proposed based on a first-principles pseudopotential method. Our calculations suggest that ZnSe growth on GaAs(110) could be understood in a two-step process: (i) Zn atoms will be adsorbed over Ga and As sites of the GaAs(110) surface, and (ii) the Zn atom over the Ga site will be replaced by a Se atom, followed by layer-by-layer ZnSe growth. We have also investigated Zn-induced features at the GaAs(110) surface, during the initial Zn interaction with the surface. Zn was found to adsorb preferentially at Ga substitutional sites at the subsurface layer and over Ga and As surface atoms. Theoretical STM images show the presence of bright features related to the Zn at Ga substitutional sites in the subsurface layers in agreement with recent experimental works.