David Flötotto

Effect of adatom surface diffusivity on microstructure and intrinsic stress evolutions during Ag film growth

The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO2 (a-SiO2) and amorphous Ge (a-Ge) substrates, with high and low Ag adatom surface diffusivity, respectively. The surface diffusivity of the deposited Ag adatoms on the a-Ge substrate is suppressed also after coalescence of Ag islands due to the continuous (re)segregation of Ge at the surface of the growing film as evidenced by in-situ XPS. An assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth. As demonstrated by ex-situ TEM and ex-situ XRD, the Ag films grown on the a-SiO2 and a-Ge substrates possess strikingly different microstructures in terms of grain shape, grain size, and crystallographic texture. Nevertheless, the real-time in-situ stress measurements revealed a compressive → tensile → ...

Kinetics and magnitude of the reversible stress evolution during polycrystalline film growth interruptions

During the deposition of polycrystalline thin films, often intrinsic compressive stresses develop, which reversibly change in tensile direction once the deposition process is interrupted. Up to date, the underlying mechanism of such reversible stress changes during growth interruptions have been controversially discussed, mainly because the correlations between the growth conditions, the developing film microstructure and the reversible stress change were still largely unclear. The present study has experimentally established the separate effects of the pre-interruption deposition rate and the average lateral film grain size on both the magnitude and the kinetics of the reversible tensile stress rise during polycrystalline film growth interruption. To this end, real-time in situ substrate-curvature measurements were performed during polycrystalline Ag growth and upon subsequent growth interruptions for well-defined and controlled adjusted microstructures. It is shown that the magnitude of the reversible tensile stress rise during growth interruption is predominantly governed by the grain-boundary density, while the rate of the tensile stress rise during growth interruption increases with increasing pre-interruption deposition rate and increasing (lateral) Ag grain size. These phenomena can be rationalized by taking deposition-rate and lateral-grain-size dependent surface morphological developments into account.

Gapped electronic structure of epitaxial stanene on InSb(111)

Stanene, a single tin atomic layer akin to graphene, is a quantum spin Hall insulator. Its spin-polarized edge states within the gap would be well suited for spintronic applications, but this attractive property has not been realized because the substrate for supporting stanene in prior experiments leads to a metallic contact that fills the band gap and shorts out the quantum spin Hall channels. By judiciously selecting InSb(111) as the substrate, the resulting system shows a large gap of 0.44 eV well suited for room-temperature device operations. Stanene on InSb(111) is thus a strong contender for next-generation spintronic technology.

Intrinsic stress evolution during amorphous oxide film growth on Al surfaces

The intrinsic stress evolution during formation of ultrathin amorphous oxide films on Al(111) and Al(100) surfaces by thermal oxidation at room temperature was investigated in real-time by in-situ substrate curvature measurements and detailed atomic-scale microstructural analyses. During thickening of the oxide a considerable amount of growth stresses is generated in, remarkably even amorphous, ultrathin Al2O3 films. The surface orientation-dependent stress evolutions during O adsorption on the bare Al surfaces and during subsequent oxide-film growth can be interpreted as a result of (i) adsorption-induced surface stress changes and (ii) competing processes of free volume generation and structural relaxation, respectively.

Interdiffusion and stress development in single-crystalline Pd/Ag bilayers

Interdiffusion and stress evolution in single-crystalline Pd/single-crystalline Ag thin films were investigated by Auger electron spectroscopy sputter-depth profiling and in-situ X-ray diffraction, respectively. The concentration-dependent chemical diffusion coefficient, as well as the impurity diffusion coefficient of Ag in Pd could be determined in the low temperature range of 356 °C–455 °C. As a consequence of the similarity of the strong concentration-dependences of the intrinsic diffusion coefficients, the chemical diffusion coefficient varies only over three orders of magnitude over the whole composition range, despite the large difference of six orders of magnitude of the self-diffusion coefficients of Ag in Ag and Pd in Pd. It is shown that the Darken-Manning treatment should be adopted for interpretation of the experimental data; the Nernst-Planck treatment yielded physically unreasonable results. Apart from the development of compressive thermal stress, the development of stress in both sublayer...

Evolution of surface stress during oxygen exposure of clean Si(111), Si(100), and amorphous Si surfaces

The evolutions of the surface stress of Si(111)-7 × 7, Si(100)-2 × 1, and a-Si surfaces upon oxygen exposure at pO2 = 1 × 10−4 Pa and room temperature have been investigated in a comparative manner using a specimen-curvature based technique. To this end, a generally applicable, dedicated set of experiments has been devised and performed to deduce and correct for the surface stress change owing to oxygen reaction(s) at the (poorly-defined) back face of the specimen only. On this basis, it could be demonstrated that exposure of clean Si(111)-7 × 7, Si(100)-2 × 1 and a-Si surfaces to pure oxygen gas results in compressive surface stress changes for all three surfaces due to the incorporation of oxygen into Si backbonds. The measured surface stress change decreases with decreasing atomic packing density at the clean Si surfaces, which complies well with the less-densily packed Si surface regions containing more free volume for the accommodation of adsorbed O atoms.

Stress originating from nanovoids in hydrogenated amorphous semiconductors

Structural inhomogeneities in the form of voids of nanometer sizes (nanovoids) have long been known to be present in hydrogenated amorphous semiconductors (Si, Ge). The physical and electrical properties of hydrogenated amorphous semiconductors can be pronouncedly influenced by the presence and characteristics of such nanovoids. In this work, by measuring in situ the intrinsic stress developments during deposition of pure, amorphous and of hydrogenated amorphous semiconductor (Si, Ge) thin films, under the same conditions in ultrahigh vacuum and on a comparative basis, a major source of tensile stress development could be ascribed to the occurrence of nanovoids in a-Si:H and a-Ge:H. The measurements allowed a quantitative evaluation of the surface stress acting along the surface of the nanovoids: 1.1–1.9 N/m for a-Si:H and 0.9–1.9 N/m for a-Ge:H.

Concentration-dependent self-diffusion coefficients in amorphous Si1−xGex solid solutions: An interdiffusion study

Self-diffusion coefficients of Si and Ge in amorphous Si1−xGex (a-Si1−xGex) solid solutions were determined quantitatively in the temperature range of 440 °C – 460 °C by the investigation of interdiffusion in amorphous Si/Si0.52Ge0.48 multilayers using Auger electron spectroscopy sputter-depth profiling. The determined concentration dependent self-diffusion coefficients of Si and Ge in a-Si1−xGex with 0 ≤ x ≤ 0.48 at. % Ge are about ten orders of magnitude larger than in the corresponding crystalline phases, due to the inherent, excess free volume in the amorphous phase. The self-diffusion coefficient of Si (or Ge) in a-Si1−xGex increases in association with a decreasing activation enthalpy with increasing Ge concentration. This concentration dependence has been related to an overall decrease of the average bond strength with increasing Ge concentration.

Superconducting pairing of topological surface states in bismuth selenide films on niobium

Bismuth selenide becomes superconducting upon coupling to metallic niobium, and its topological states pair up to form a gap. A topological insulator film coupled to a simple isotropic s-wave superconductor substrate can foster helical pairing of the Dirac fermions associated with the topological surface states. Experimental realization of such a system is exceedingly difficult, however using a novel “flip-chip” technique, we have prepared single-crystalline Bi2Se3 films with predetermined thicknesses in terms of quintuple layers (QLs) on top of Nb substrates fresh from in situ cleavage. Our angle-resolved photoemission spectroscopy (ARPES) measurements of the film surface disclose superconducting gaps and coherence peaks of similar magnitude for both the topological surface states and bulk states. The ARPES spectral map as a function of temperature and film thickness up to 10 QLs reveals key characteristics relevant to the mechanism of coupling between the topological surface states and the superconducting Nb substrate; the effective coupling length is found to be much larger than the decay length of the topological surface states.

Microstructure and intrinsic stress evolution during epitaxial film growth of an Ag0.93Al0.07 solid solution on Si(111); excessive planar faulting due to quantum confinement

The correlation of microstructural development and the kinetics of film growth has been investigated during the epitaxial film growth of an ultrathin binary Ag0.93Al0.07 solid solution on a Si(111)-7×7 surface at 300 K by the combination of high-resolution transmission electron microscopy, X-ray diffraction, scanning tunneling microscopy, low energy electron diffraction, and real-time in-situ stress measurements. Up to a film thickness of 6 ± 2 nm, epitaxial Ag0.93Al0.07 film growth is characterized by the strikingly extensive formation of planar faults parallel to the film/substrate interface, while at larger thickness the film grows practically defect-free. As revealed by real-time in-situ stress measurements, the extensive formation of planar faults at the very initial stage of growth is not driven by the reduction of the systems elastic strain energy but is rather caused by a striking thickness-dependence of the stacking-fault energy owing to a quantum size effect of the ultrathin metal alloy film, r...