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Islands in the form of truncated triangular pyramids on the surface of an epitaxial Ge3Sb2Te6 layer grown on a Si(111) substrate are identified by scanning electron microscopy. It is established that sides of their bases are oriented along the Si〈110〉 directions, and sizes vary from several tens to several hundred nanometers. It is shown by high-resolution electron microscopy of cross-sectional specimens that the (111) planes of the cubic phase or (0001) planes of the hexagonal phase of Ge3Sb2Te6 in the epitaxial layer are oriented parallel to Si(111) and, in surface domains, can make an angle of 70.5° with the layer–substrate interface. It is established, using an atomic structure analysis and energy-dispersive X-ray microanalysis, that the composition in islands changes from Ge3Sb2Te6 to GeSb2Te4 along the growth direction.

We study the cross-sectional shape of GaN nanowires (NWs) by transmission electron microscopy. The shape is examined at different heights of long NWs, as well as at the same height for NWs of different lengths. Two distinct trends in the evolution of the cross-sectional shape along the NW length are observed. At the top, merging NWs develop common {11̄00} side facets. At the bottom, the NWs acquire roundish shapes. This observation is explained by the entirely different NW environments at the top and the bottom of the NWs. At the top, NWs are exposed to the Ga and N atomic fluxes giving rise to axial growth, resulting in the equilibrium growth shape with zero growth rate at the {11̄00} facets. At the bottom, NWs are shadowed from the impinging fluxes and are only annealed, allowing them to eventually approach the equilibrium crystal shape. The study of identical samples by grazing incidence small-angle X-ray scattering independently confirms these trends in the shape evolution of the sidewall facets.

Combining graphene and the insulating hexagonal boron nitride (h-BN) into two-dimensional heterostructures is promising for novel, atomically thin electronic nanodevices. A heteroepitaxial growth, in which these materials are grown on top of each other, will be crucial for their scalable device integration. However, during this so-called van der Waals epitaxy, not only the atomically thin substrate itself must be considered but also the influences from the supporting substrate below it. Here, we report not only a substantial difference between the formation of h-BN on single- (SLG) and on bi-layer epitaxial graphene (BLG) on SiC, but also vice versa, that the van der Waals epitaxy of h-BN at growth temperatures well below 1000 °C affects the varying number of graphene layers differently. Our results clearly demonstrate that the additional graphene layer in BLG enhances the distance to the corrugated, carbon-rich interface of the supporting SiC substrate and thereby diminishes its influence on the van der Waals epitaxy, leading to a homogeneous formation of a smooth, atomically thin heterostructure, which will be required for a scalable device integration of 2D heterostructures.

Hexagonal boron nitride has emerged as a fundamental building block for atomically thin electronic devices. This contribution investigates its formation on graphene via molecular beam epitaxy at temperatures well below 1300°C, where it is considered metastable as a bulk material. However, it is shown that even at lower temperatures single- to few-layer hexagonal boron nitride still nucleates on graphene, and that the formation of the sp2- and sp3-bonded phases of BN can be manipulated via an in-situ N 2 -plasma pretreatment of the substrates. Here the experimental discovery of a novel, metastable, sp3-bonded phase, which so far has been only predicted theoretically, demonstrates a rich polymorphism in BN at the nanoscale.

GeSbTe (GST) phase-change alloys feature layered crystalline structures made of lamellae separated by van der Waals (vdW) gaps. This work sheds light on the dependence of interlamellae interactions at the vdW gap on film thickness of GST alloys as probed by vibrational spectroscopy. Molecular beam epitaxy is used for designing GST layers down to a single lamella. By combining density-functional theory and Raman spectroscopy, a direct and simple method is demonstrated to identify the thickness of the GST film. The shift of the vibrational modes is studied as a function of the layer size, and the low-frequency range opens up a new route to probe the number of lamellae for different GST compositions. Comparison between experimental and theoretical Raman spectra highlights the precision growth control obtained by the epitaxial technique.

Molecular dynamics based on the empirical Tersoff potential was used to simulate the deposition of amorphous silicon and germanium on silicon(111) at various deposition rates and temperatures. The resulting films were analyzed quantitatively by comparing one-dimensional atomic density profiles to experimental measurements. It is found that the simulations are able to capture well the structural features of the deposited films, which exhibit a gradual loss of crystalline order over several monolayers. A simple mechanistic model is used to demonstrate that the simulation temperature may be used to effectively accelerate the surface relaxation processes during deposition, leading to films that are consistent with experimental samples grown at deposition rates many orders-of-magnitude slower than possible in a molecular dynamics simulation.

A study of the composition and structure of large single crystal inclusions formed inside cavities in pyroboroncarbon by means of energy dispersive X-ray microanalysis and electron diffraction analysis shows that they correspond to one phase of rhombohedral boron carbide. Bright-field images obtained by transmission electron microscopy show that their growth continues throughout the technological cycle of material growth due to the diffusion of boron atoms from the pyrocarbon phase of pyroboroncarbon adjacent to the cavities.