The DFT calculations show that the AlN(0001) surface has a charge equal to one fourth of a volume N atomic plane which compensates the potential electrostatic divergence arising from the wurtzite structure. The phase diagram calculations in function of the substrate temperature and the Al and NH3 beam fluxes suggest that the surface obtained at a growth rate of 10 nm/h is mainly (2x2)-Nad reconstructed in concordance with the in-situ RHEED and NC-AFM observations.
The aluminum nitride (AlN) semiconductor has a large gap of 6.2 eV, and its (0001) surface was chosen for molecules deposition within the framework of researches on molecular electronics within the Nanosciences Group. To be used, this surface should be perfectly known and controlled at the atomic scale. An ultra high vacuum (UHV) equipment was specially designed and assembled in the CEMES since 2007 to allow the growth of thin layers of AlN by molecular beam epitaxy (MBE), and the observation of the surface by atomic force microscopy in the non-contact mode (NC-AFM). The first studies of AlN(0001) surface obtained at a growth rate of 100 nm/h (G1 growth condition) showed a very strong disorder at the atomic scale. By decreasing the growth rate to 10 nm/h (G2 growth condition), the surface presents a (2x2) reconstruction observed by in-situ RHEED during growth, and by room temperature NC-AFM after UHV transfer of the samples into the microscope (figure (b)). In order to determine the atomic structure of this (2x2) reconstruction, we have done firstly DFT calculations on different atomic models of (2x2) reconstruction. These calculations allowed us to demonstrate that the reconstructions fulfilled the electrostatic stability condition emitted by Goniakowsky et al. Indeed the hexagonal AlN has the wurtzite structure and its (0001) surface is electrostatically unstable. This electrostatic unstability is cancelled if the surface layer has a net charge equal to -1/4 of the charge of an Al bulk atomic plane, which was verified by analyzing the Bader charges calculated from the DFT results. We also calculated the phase diagram in function of the substrate temperature, ammonia (NH3) pressure and Al beam flux. At a substrate temperature of 1040°C, we observe on the diagram (figure (c)) a transition between a disordered surface obtained with the growth conditions G1 and a surface 90% reconstructed (2x2) type VI Nad for the G2 conditions. This reconstruction has the particularity to be not hydrogenated, with the presence of one additional nitrogen atom highly reactive from a chemical point of view.
Noncontact atomic force microscopy and density functional theory studies of the (2×2) reconstructions of the polar AlN(0001) surface, Florian Chaumeton, Roberto Robles, Miguel Pruneda, Nicolás Lorente, Benoit Eydoux, Xavier Bouju, Sébastien Gauthier, and David Martrou
Phys. Rev. B 94, 165305 – Published 17 October 2016
David Martrou (dmartrou at cemes.fr)