A. C. Ferraz

A comparative study of dissociative adsorption of NH3, PH3, and AsH3 on Si(001)–(2×1)

Using a first-principles pseudopotential method we have studied the adsorption and dissociation of NH3, PH3, and AsH3 on the Si(001)–(2×1) surface. Apart from the existence of a barrier for the adsorption of the precursor state for arsine, we observe that the global behavior for the chemisorption of the XH3 molecules considered in this work is as follows: the gas phase XH3 adsorbs molecularly to the electrophilic surface Si atom and then dissociates into XH2 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The energy barrier, corresponding to a thermal activation, is much smaller than the usual growth temperature, indicating that all three molecules will be observed in their dissociated states at room temperature. All adsorbed systems are characterized by elongated Si–Si dimers that are (almost) symmetric in the dissociative case but asymmetric in the molecular case. According to our first-principles calculations, all XH3 and XH2 systems retain the pyramidal g...

Surface Properties of CdS Nanoparticles

With a view to contribute to the understanding the surface effects on optical properties process, and its hole in the electronic properties of the nanoparticles, CdS based nanoparticles are characterised by different experimental techniques and the experimental results compared to density functional theory calculations. Our results indicate that cubic CdS nanoparticles present a strong structural deformation, hexagonal reconstructed structures preserve their lattice behaviour. Both cubic and hexagonal CdS nanoparticles are S-terminated after relaxation, even when mildly Cd-rich nanoparticles are considered. A broad peak observed in our PL measurements is interpreted as an experimental evidence of the surface related peak observed around 1.8 eV in our calculated DOS for the hexagonal relaxed structure.

Oxygen adsorption on CdTe(1 1 0)

Using a first-principles pseudopotential method we have investigated the adsorption of oxygen atoms over the CdTe(110) surface. Our calculations suggest that the oxidation process of the CdTe(110) surface could be understood as follows: (i) an oxygen atom is adsorbed over Cd and Te sites in a bridge configuration; (ii) a second oxygen atom is adsorbed over this system forming an oxygen complex composed of Cd–O2 and Te–O bonds; and (iii) a third oxygen atom is adsorbed over this system forming a new complex with Te–O2–O and Cd–O–O bonds. The final adsorbed complex is characterised by the breaking of the Cd–Te bond and the formation of Te–O, Cd–O and O–O bonds, explaining why apparent contradicting experimental observations are reported by different groups. Our calculated electronic structure for the adsorbed complex suggest that the so expected passivation effect does not occur. In addition we present calculated vibrational modes for the adsorbed system in order to contribute to further experimental investigation.

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.

Ab initio study of substitutional nitrogen in GaAs

We investigate the atomic geometry, formation energies, and electronic structure of nitrogen occupying both arsenic and gallium sites in GaAs (NAs and NGa) using first-principles total-energy calculations. We find that both neutral defects induce impurity-like empty levels in the band gap acting as acceptors. While NAs shows a s-like a1 level in the middle of the band gap, NGa shows a p-like t2 level close to the bottom of the conduction band. The gap level of NAs gives theoretical support for the experimentally observed band-edge redshift on the GaAsN alloy for a N concentration ∼3%. Strong inward relaxations preserving the Td symmetry characterize the NAs equilibrium geometry in all the charge states investigated. In contrast, NGa exhibits a structural metastability in neutral charge state and Jahn–Teller off-center distortions in negative charge states forming a negative-U center. Formation energies of competing NAs and NGa defects are also discussed.

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.

Stability and electronic structure of hydrogen–nitrogen complexes in GaAs

We investigate the stability and electronic properties of defects formed by a substitutional nitrogen in GaAs (NAs) plus interstitial hydrogen atoms using first-principles total-energy calculations. We find the formation of strong NAs–H bond when a single H atom is incorporated in the lowest-energy bond centered (BC) position. This defect induces an electrically active level in the GaAs band gap. When two H atoms are incorporated, we find the stable NAs–H2* complex as the lowest-energy configuration, with one H atom at the BC position and the second H atom at an antibonding position. The electronic structure of this complex shows the passivation of the gap level restoring the GaAs band gap.

Atomic geometry and electronic structure of SInP(001)

Abstract We have performed ab initio pseudopotential calculations of the atomic geometry and electronic states for a monolayer S-covered (001) surface of InP. It is found that for the 1 × 1 periodicity with S in the bridge site, the overlayer-substrate distance between S and In is 1.3 A and the InSIn angle is 113.8°. While the calculated InSIn angle is close to the experimentally deduced value, we find that our prediction of overlayer-substrate distance is smaller than the value obtained from LEED analysis. We also discuss the energetics of formation of long and short dimer bonds for the 1 × 2, 2 × 1 and 2 × 2 reconstructions of the surface.

The Oxidation Mechanism of CdTe (110) Surface

Using a first-principles pseudopotential technique within a generalized gradient approximation of the density functional theory, we have investigated the mechanism of adsorption of molecular oxygen on the CdTe(110) surface. The determination of the more favorables structures and their formation through the activation barrier analysis indicates that the formation can occur in the regimes of low and high temperatures. According to our calculations it was verified that in the regime of low temperatures the oxygen molecule binds exclusively to Cd at the topmost site through Cd-O-O or Cd-O2 bonds. The electronic band structure presents a characteristic state like a double acceptor defect. In the high temperature regime the molecule adsorbs between Cd of the surface and Te of the subsurface, breaking Cd-Te bond and causing an upward dislocation of Cd from the subsurface. In this regime of high temperatures the complex presents Cd-O, Cd-O2, Te-O and O-O bonds and the electronic band structure is almost free of gap of states.

Adsorption and dimer exchange processes in surfactant-mediated epitaxial growth of GaAs/InAs

Abstract We investigate, using first-principles total energy calculations, the initial steps of adsorption and dimer-exchange mechanisms in the surfactant-mediated epitaxial growth of InAs over GaAs(001) surface with Te as a surfactant. We find that on the Ga-terminated GaAs(001) surface, covered by Te atoms, the dimer exchange between the layer of Te atoms and an overlayer of As atoms, based on a first neighbour exchange process, is only partial, in agreement with recent experimental results. On the other hand, if the exchange process between As and Te atoms occurs after the deposition of In and As atoms, in a second neighbour exchange process, our results indicate a complete segregation of Te atoms, reducing the surface total energy. The Te atoms also prevent the adsorption of As 2 molecule on neighbour sites, avoiding the formation of As clusters.