| AVS 61st International Symposium & Exhibition | |
| Surface Science | Monday Sessions |
| Session SS+EN-MoM |
| Session: | Photocatalysis and Photochemistry at Surfaces |
| Presenter: | Sergei Novikov, University of Nottingham, UK |
| Authors: | S.V. Novikov, University of Nottingham, UK K.M. Yu, Lawrence Berkeley National Laboratory W.L. Sarney, US Army Research Laboratory Z. Liliental-Weber, Lawrence Berkeley National Laboratory R.W. Martin, University of Strathclyde, UK S.P. Svensson, US Army Research Laboratory W. Walukiewicz, Lawrence Berkeley National Laboratory C.T. Foxon, University of Nottingham, UK |
| Correspondent: | Click to Email |
We have grown GaN layers alloyed with GaAs, GaSb and GaBi compounds using plasma-assisted molecular beam epitaxy (PA-MBE) and extensively characterized their structural, optical and electrical properties.
Electronic band structures of these so-called highly mismatched alloys (HMAs) are described by the band anticrossing (BAC) model which predicts that the alloys should exhibit a wide range of direct energy gaps. We have shown previously that the energy gap of GaN1-xAsx alloys varies from 0.7eV to 3.4eV. An even larger modification of the band structures is anticipated for more extremely mismatched GaN1-xSbx and GaN1-xBix alloys. The large band gap range and controllable conduction and valence band edge positions makes the HMAs promising materials for efficient solar energy conversion devices. For example, these HMAs may be suitable for solar water splitting applications for hydrogen production. As efficient photoelectrodes, the bandgap of the semiconductor must be >2 eV to induce electrochemical decomposition of water but still small enough to absorb a significant portion of the solar spectrum. In addition the band edges must also straddle the H2O redox potentials.
At dilute doping levels, substitutions of As, Sb and Bi into the N sublattice results in formation of localized energy levels above the valence band in GaN. Our measurements on GaN doped with As and Sb have demonstrated that the As and the Sb impurity levels lie at about 0.7eV and 1.2eV above the valence band edge of GaN, respectively.
The BAC model predicts that at a higher concentration of the group V elements the interaction of the impurity levels with the extended states of the valence band leads to formation of an impurity-derived, fully occupied narrow band that plays a role of the new valence band edge. This results in an abrupt upward shift of the valence band edge and a reduction of the optical gap of the HMAs. We have achieved the enhanced incorporation of As, Sb and Bi by growing the layers at extremely low temperatures (down to about 100oC). Although the layers become amorphous for high As, Sb and Bi content, the measured composition dependence of the optical absorption edge are consistent with the predictions of the BAC model, indicating that the amorphous HMAs samples have a short-range order resembling random crystalline alloys. The large band gap range and controllable positions of the conduction and valence bands make these HMAs promising materials for efficient solar energy conversion devices.