Intermixing during Epitaxial Growth of van der Waals Bonded Nominal GeTe/Sb₂Te₃ Superlattices

In the present work, GeTe and Sb₂Te₃ van der Waals bonded superlattices epitaxially grown by molecular beam epitaxy are investigated. These structures are grown on passivated Si substrates, resulting in one single epitaxial domain and its twinned domain, both sharing the same out-of-plane orientation. Supported by X-ray diffraction and Raman spectroscopy, attention is called to the thermodynamically driven tendency of GeTe and Sb₂Te₃ to intermix into a Ge–Sb–Te (GST) alloy at the interfaces. A growth model is proposed to explain how these GST structures are formed

Consequences of High Adatom Energy during Pulsed Laser Deposition of La_0.7Sr_0.3MnO₃

The impact of the adatom energy on the stoichiometry, surface morphology, and crystalline twinning during pulsed laser deposition of La_0.7Sr_0.3MnO₃ is studied. We show that although nonthermal growth using highly energetic adatoms results in very smooth ultrathin films, it also causes preferential resputtering of Mn and a surface roughening transition with increasing film thickness. This can be circumvented by carefully tuning the adatom energy into thermal growth, resulting in more Mn rich samples and a delayed roughening transition. Furthermore, we demonstrate that the crystalline twinning can be controlled by controlling the adatom energy. Hence, a detailed control of the adatom energy during growth opens for better stoichiometry control as well as surface quality

Toward Truly Single Crystalline GeTe Films: The Relevance of the Substrate Surface

The growth of GeTe thin films on a Si(111)-(√3 × √3)­R30°-Sb surface is reported. At growth onset, the rapid formation of fully relaxed crystalline GeTe(0001)-(1 × 1) is observed. During growth, a GeTe(0001)-(√3 × √3)­R30° surface reconstruction is also detected. Indeed, density functional theory (DFT) simulations indicate that the reconstructed GeTe(0001)-(√3 × √3)­R30° structure is energetically competing with the GeTe(0001)-(1 × 1) reconstruction. The out-of-plane α-GeTe<0001>||Si<111> and in-plane α-GeTe<−1010>||Si<−211> epitaxial relationships are confirmed by X-ray diffraction (XRD). Suppression of rotational twist and reduction of twinned domains are achieved. The formation of rotational domains in GeTe grown on Si(111)-(7 × 7) is explained by domain matched coincidence lattice formation with the Si(111)-(1 × 1) surface. Atomic force microscopy (AFM) images show the coalescence of well-oriented islands with subnanometer roughness on their top part. van der Pauw measurements are performed to verify the electric properties of the films. The quality of epitaxial GeTe thin film is discussed and related to the crystalline structure of GeTe and its rhombohedrally distorted resonant bonds

Surface Reconstruction-Induced Coincidence Lattice Formation Between Two-Dimensionally Bonded Materials and a Three-Dimensionally Bonded Substrate

Sb₂Te₃ films are used for studying the epitaxial registry between two-dimensionally bonded (2D) materials and three-dimensional bonded (3D) substrates. In contrast to the growth of 3D materials, it is found that the formation of coincidence lattices between Sb₂Te₃ and Si(111) depends on the geometry and dangling bonds of the reconstructed substrate surface. Furthermore, we show that the epitaxial registry can be influenced by controlling the Si(111) surface reconstruction and confirm the results for ultrathin films

Effect of Polar (111)-Oriented SrTiO₃ on Initial Perovskite Growth

In crystalline thin film growth a prerequisite is substrate surfaces with a stable and uniform structure and chemical composition. Various substrate treatments were used to obtain atomically smooth, step-and-terrace (111)-oriented SrTiO₃ with uniform cation layers at the surface, i.e., single termination. The surface control enables subsequent layer-by-layer epitaxial growth of perovskite thin films of La_0.7Sr_0.3MnO₃, LaFeO₃, and BaTiO_3. Reflection high-energy electron diffraction and electron energy loss spectroscopy revealed that a single chemically intermixed (A,A′)­BO₃ perovskite layer formed at the interface. As the terminating layer of (111) SrTiO₃ is polar, a surface reconstruction consisting of TiO_<i>x</i> surface layers is expected, and the intermixing at the interface can be understood as A′-cations from the film material compensating an A-cation deficient substrate surface during initial growth. This finding has important consequences for engineered interfaces between perovskite thin films and polar substrate facets