SummaryThe adsorption on KBr(001) of a specially designed molecule, consisting of a flat aromatic triphenylene core equipped with six flexible propyl chains ending with polar cyano groups, is investigated by using atomic force microscopy in the noncontact mode (NC-AFM) coupled to Kelvin probe force microscopy (KPFM) in ultrahigh vacuum at room temperature. Two types of monolayers are identified, one in which the molecules lie flat on the surface (MLh) and another in which they stand approximately upright (MLv). The Kelvin voltage on these two structures is negatively shifted relative to that of the clean KBr surface, revealing the presence of surface dipoles with a component pointing along the normal to the surface. These findings are interpreted with the help of numerical simulations. It is shown that the surface–molecule interaction is dominated by the electrostatic interaction of the cyano groups with the K+ ions of the substrate. The molecule is strongly adsorbed in the MLh structure with an adsorption energy of 1.8 eV. In the MLv layer, the molecules form π-stacked rows aligned along the polar directions of the KBr surface. In these rows, the molecules are less strongly bound to the substrate, but the structure is stabilized by the strong intermolecular interaction due to π-stacking.

Nitride wide-band-gap semiconductors are used to make high power electronic devices or efficient light sources. The performance of GaN-based devices is directly linked to the initial AlN buffer layer. During the last twenty years of research on nitride growth, only few information on the AlN surface quality have been obtained, mainly by ex-situ characterization techniques. Thanks to a Non Contact Atomic Force Microscope (NC-AFM) connected under ultra high vacuum (UHV) to a dedicated molecular beam epitaxy (MBE) chamber, the surface of AlN(0001) thin films grown on Si(111) and 4H-SiC(0001) substrates has been characterized. These experiments give access to a quantitative determination of the density of screw and edge dislocations at the surface. The layers were also characterized by ex-situ SEM to observe the largest defects such as relaxation dislocations and hillocks. The influence of the growth parameters (substrate temperature, growth speed, III/V ratio) and of the initial substrate preparation on the dislocation density was also investigated. On Si(111), the large in-plane lattice mismatch with AlN(0001) (19%) induces a high dislocation density ranging from 6 to 12×1010/cm2 depending on the growth conditions. On 4H-SiC(0001) (1% mismatch with AlN(0001)), the dislocation density decreases to less than 1010/cm2, but hillocks appear, depending on the initial SiC(0001) reconstruction. The use of a very low growth rate of 10 nm/h at the beginning of the growth process allows to decrease the dislocation density below 2 × 109/cm2.

Combined experimental and theoretical studies permit us to determine new protocols for growing by molecular beam epitaxy the technologically interesting N-rich aluminum nitride (AlN) surfaces. This is achieved by dosing the precursor gases at unusually low rates. With the help of calculated structures by using density functional theory and Boltzmann distribution of the reconstructed cells, we proposed to assign the measured surface obtained with a growth rate of 10 nm/h to a (2 × 2) reconstructed surface involving one additional N atom per unit cell. These N-rich AlN surfaces could open new routes to dope AlN layers with important implications in high-power and temperature technological applications. DOI: 10.1103/PhysRevB.94.165305 High-power electronic devices require materials with large electron mobilities and densities and large band gaps. Group-III nitride semiconductors are ideal candidates for these applications [1]. Among these materials, aluminum nitride (AlN) has the largest band gap [2]. It also has unique properties such as small density, large stiffness, large piezoelectric constant [3], large fracture resistivity, and chemical inertness [4]. Recently, the two-dimensional electron gases appearing at the interface of a strained GaN quantum well sandwiched between relaxed AlN layers have permitted the realization of field effect transistors with a high cut-off frequency of 104 GHz [5]. Unfortunately, defects and interface states seriously compromise devices based on these materials and there is an urgent need for high-quality interfaces and surfaces. For these reasons, its surface reconstructions have received a lot of attention theoretically [6][7][8][9][10][11]. Furthermore, due to its high ionicity, AlN crystallizes in the wurtzite structure and its (0001) growth surface is polar, like other zinc-blende (001) semiconductor surfaces [12]. The consequence of this polarity is that the crystal should be stabilized by the formation of surface charges that can be generated by different mechanisms like surface reconstructions (see the review article by Noguera [13], and references therein).Experimentally, due to the large gap of AlN (6.2 eV) it is not possible to observe its surface by scanning tunneling microscopy (STM) except for the Al rich phase as explored by Lee et al. [14]. One effective way to get information at the atomic scale is to use atomic-force microscopy in the noncontact mode (NC-AFM), as developed by Albrecht et al. in 1991 [15]. NC-AFM allows the observation of surfaces with atomic resolution of some ionic [16][17][18][19] . All these substrates can be prepared by cleavage or ionic bombardment followed by a soft annealing. In the case of nitride semiconductors, the layers should be grown under ultrahigh vacuum (UHV) and then transferred into an AFM chamber under UHV, since their surfaces are not stable in air. We were able to realize the NC-AFM study of AlN(0001) using custom-made equipment [30] where the AlN layer is grown by molecular beam epitaxy (MBE) using ammonia (NH 3 ) as n...