Noncontact atomic force microscopy and density functional theory studies of the (2×2) reconstructions of the polar AlN(0001) surface

Abstract

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...

Morphology and Work Function of In, Ag, Mg, and Au Nano‐Islands Grown on AlN(0001) Surface

“…In a previous work, we have characterized the surface of AlN(0001) layers grown by NH3‐MBE either on Si(111) or nH‐SiC(0001) substrates by non‐contact atomic force microscopy (NC‐AFM) under UHV. We found a much better quality for the films grown on SiC, allowing the determination of the atomic structure of the (2 × 2) reconstruction observed by RHEED and NC‐AFM on the AlN(0001) surface grown at a very low growth rate of 10 nm h −1 thanks to ab initio DFT calculations coupled to thermodynamic models …”

“…In particular such features are an advantage for UV optoelectronic and high‐power devices . Many studies have investigated crystalline quality of AlN layers for different epitaxy techniques such as metal‐organic chemical vapour deposition (MOCVD), hybrid vapour phase epitaxy (HVPE), plasma assisted MBE or ammonia (NH3) molecular beam epitaxy (MBE) . Usually three substrates are preferred for AlN growth which are silicon [Si(111)], silicon carbide [nH‐SiC(0001)] and sapphire [Al 2 O 3 (0001)] .…”

Stabilization of Au Monatomic-High Islands on the (2×2)−Nad Reconstructed Surface of Wurtzite AlN(0001)

“…We recently elaborated and characterized AlN(0001) thick films, whose surface is polar. We identified the mechanism of charge compensation for one of its (2 × 2) reconstructions [34]: According to the ð2 × 2Þ-N ad , the surface charge is mostly localized on one extra N adatom per (2 × 2) surface unit cell, satisfying quantitatively the electrostatic stability criterion predicted by classical electrostatic theory [27]. Other recent calculations confirmed the stability of the AlNð0001Þð2 × 2Þ-N ad reconstruction [35].…”

“…The experimental conditions needed to stabilize the ð2 × 2Þ N ad reconstruction are discussed in Ref. [34]. After the formation of the ð2 × 2Þ N ad reconstructed surface, the substrate temperature is decreased to 500°C and gold is deposited with a beam equivalent pressure of 5 × 10 −9 Torr during one to two minutes.…”

“…Under these conditions no tunneling current can be detected through the sample in the [−10 V, 10 V] accessible bias range, confirming the truly insulating character of the AlN substrate. The AlN surface is characterized by triangular terraces limited by h1010i-oriented step edges [34,37]. The Au deposit appears as large disconnected islands around 3-Å high, indicating a monatomic thickness.…”