In-situ NC-AFM measurements of high quality AlN(0001) layers grown at low growth rate on 4H-SiC(0001) and Si(111) substrates using ammonia molecular beam epitaxy

Abstract

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.

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

“…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)] .…”

“…The study of the AlN surface in real space can be done by scanning tunneling microscopy (STM), when the surface or the layer is conductive, or by atomic force microscopy (AFM) when they are insulating, which is the case in this study. 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 …”

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

“…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.…”

“…AlN films with a thickness between 100 and 150 nm are grown on 4H-SiCð0001Þ substrates in the presence of 2 × 10 −5 Torr of NH 3 and under an Al flux at a substrate temperature of 990 AE 30°C [37]. The experimental conditions needed to stabilize the ð2 × 2Þ N ad reconstruction are discussed in Ref.…”

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

“…The growth of AlN samples was carried out in a MBE chamber equipped with a RHEED gun working at 15 keV. The AlN layer is grown on a 4H-SiC(0001) substrate following a recipe described elsewhere [33,44]. Two kinds of samples were considered with growth speeds of 100 and 10 nm/h, corresponding to a measured beam equivalent pressure (BEP) for Al of 3 × 10 −8 and 3 × 10 −9 Torr, respectively.…”