Adsorbate related electronic and vibrational states on surfaces

Summary of work done under EPSRC grant no. GR/R41231/01
Principal Investigator Professor G P Srivastava, School of Physics, University of Exeter

With the financial help provided by this Overseas Travel Grant I was able to: present an invited talk on 'Surface Passivation by Dissociative Molecular Adsorption on Si(001)' at the Second International Seminar on Surface Passivation, held at Ustron, Poland, September 2001; present results of surface phonon calculations at the 10th International Conference on Phonon Scattering in Solids (PHONONS 2001), Darthmouth College, Hanover, NH, USA, August 2001; participation at the Conference in the capacity of International Programme Committee member; and present results on electronic and vibrational properties of semiconductor surfaces at the 20th European Conference on Surface Science (ECOSS20), Krakow, Poland, September 2001.

A summary of the results obtained follows:

1. It is shown that the application of an adiabatic bond charge model produces excellent lattice dynamical results for III-nitride materials in the zincblende and wurtzite phases. Our work presented a discussion on the angle dependence of the zone-centre optical modes, the effect of internal parameter u on the A1(LO) and E1(LO) modes, and the anti-crossing effect between the A1 and the E2 optical phonon modes for the wurtzite phase. It is further shown that isotopic considerations for N change all the zone-centre optical modes, except for B₁¹.

2. Application of the adiabatic bond charge model, in conjunction with the relaxed atomic and electronic structural data obtained from an ab-initio pseudopotential method, produces lattice dynamical results for the As:InP(110) and Sb:InP(110) systems which compare very well with ab-inito results. Our calculations reveal that the deposition of Sb and As on InP(110) lead to several characteristic new phonon modes in the bulk acoustic-optical gap region. While there are two competing geometries, an exchange-reacted geometry and the epitaxially continued layer geometry, the former is characterised by a clear gap of 10 meV in the density of states for As:InP(110).

3. Ab initio calculations reveal that for H passivation of Ge/Si(001) systems there is no significant energy difference between the segregated and non-segregated structures. For Cl passivation, the non-segregated surface structure is energetically more favourable than the segregated structure, by 0.23 eV for the (2x1) reconstruction and by 0.48 eV for the (3x1) reconstruction.

4. We have presented a more detailed ab-initio explanation of the atomic geometry, electronic states, and orbital nature for the In adsobed Si(001) surface, than is available in the liturature.

5. The adsorption and desorption of Se on the Si(001) surface has been investigated from first-principles, and recent experimental observations have been supported. Our results suggest that there is a possible semiconducting phase transition as the coverage of Se changes from a half-manolayer to the full monolayer.

6. Ab-inito investigations of the half-monolayer adsorption of C2H4 on the Si(001) and Ge(001) surfaces reveal that the di-sigma bond structure is the most stable structure at low temperatures. The adsorption of the molecule does not provide electronic or chemical passivation of the surfaces. The calculated vibrational modes for the adsorption model agree well with recent HREELS experimental data.

7. It has been found that one monolayer of Bi deposition on Si(111) can be explained by the milk-stool model. Reducing the Bi coverage to 1/3 monolayer, the adatoms chemisorb in the T4 sites. However, there is no stable configuration for the 2/3 monolayer coverage of Bi. The observed STM images are explained in terms of the milk-stood model for occupied states (negative bias) and the T4 sites for the honey-comb picture for the images of unoccupied states (positive bias).

8. It is shown that self-organised Bi wires on the Si(001) surface are formed by Bi-dimers parallel to the surrounding Si dimers, with a missing dimer row between the Bi-dimers. Our simulated STM images suggest a low density of states close to the valence band maximum, localised on the Bi-lines, supporting a recently proposed model of quantum antiwire systems.