AVS 51st International Symposium
    Surface Science Thursday Sessions
       Session SS+EM+SC-ThA

Paper SS+EM+SC-ThA6
First Scanning Tunneling Microscopy and Spectroscopy Study of c-GaN(001)-4x1 Tetramer Structure and SIESTA Surface Simulation

Thursday, November 18, 2004, 3:40 pm, Room 210C

Session: Compound Semiconductor Growth and Surface Structure
Presenter: H.A. Al-Brithen, Ohio University
Authors: H.A. Al-Brithen, Ohio University
M.B. Haider, Ohio University
N. Sandler, Ohio University
A.R. Smith, Ohio University
P. Ordejon, Institudo de Cienias de Materiales, Spain
Correspondent: Click to Email
Although early papers of the surface structure of c-GaN(001) reported 2x2 and c(2x2) reconstructions,[a] it was later shown both experimentally[b] and theoretically[c] that the intrinsic reconstruction is 4x1. However, until now, this 4x1 reconstruction has never been reportedly observed in real space. We have grown c-GaN on MgO(001) using radio frequency nitrogen plasma molecular beam epitaxy under Ga-rich conditions. RHEED patterns show that GaN(001) clearly exhibits 1x1 reconstruction during and after the growth; in fact, after cooling to ~ 200 °C a reversible disorder-order transition from 1x1 to 2x occurs, which may have been neglected or confused in earlier experiments with 2x2. This 2x is imaged using STM, finding that it is actually c(4x16) for Ga-rich growth and c(4x20) for more Ga-rich growth; STS spectra suggest that GaN(001)-c(4x16) is metallic. Annealing the film at T@sub s@ ~ 700-800 °C leads to the 4x1 reconstruction, as indicated by RHEED. STM performed on c-GaN(001)-4x1 shows that the surface consists of rows aligned along [110] with row spacing of 12.8 Å. Dual-bias STM images show a 180@super o@ phase shift of the filled and empty states profiles, as the sample bias changes from -1.2 V to +1.2 V, consistent with our recent simulated STM images, calculated using SIESTA code based on the tetramer model, showing that the filled state peak centered on the tetramer corresponds to the empty state minimum. STS acquired on the tetramer surface agrees with the semiconducting nature of 4x1, having a surface gap of 1.3 eV. In fact, the 4x1 tetramer structure was also predicted for c-AlN(001)[d], which widens the importance of understanding this reconstruction. Work is supported by NSF. @FootnoteText@ [a] Brandt et al., Phys. Rev. B R2253 (1995). [b] Feuillet et al., Appl. Phys. Lett. 70(8) (1997). [c] Neugebauer et al., Phys. Rev. Lett. 80(14) (1998). [d] Felice et al., Appl. Phys. Lett. 74(15) (1999).