An official website of the United States government
Bi 2 Se 3 is a widely studied 3D topological insulator having potential applications in optics, electronics, and spintronics. When the thickness of these films decreases to less than approximately 6 nm, the top and bottom surface states couple, resulting in the opening of a small gap at the Dirac point. In the 2D limit, Bi2Se3 may exhibit quantum spin Hall states. However, growing coalesced ultrathin Bi2Se3 films with a controllable thickness and typical triangular domain morphology in the few nanometer range is challenging. Here, we explore the growth of Bi2Se3 films having thicknesses down to 4 nm on sapphire substrates using molecular beam epitaxy that were then characterized with Hall measurements, atomic force microscopy, and Raman imaging. We find that substrate pretreatment—growing and decomposing a few layers of Bi2Se3 before the actual deposition—is critical to obtaining a completely coalesced film. In addition, higher growth rates and lower substrate temperatures led to improvement in surface roughness, in contrast to what is observed for conventional epitaxy. Overall, coalesced ultrathin Bi2Se3 films with lower surface roughness enable thickness-dependent studies across the transition from a 3D-topological insulator to one with gapped surface states in the 2D regime.
more »
« less
Quantum magnetoconductivity characterization of interface disorder in indium-tin-oxide films on fused silica
Abstract Disorder arising from random locations of charged donors and acceptors introduces localization and diffusive motion that can lead to constructive electron interference and positive magnetoconductivity. At very low temperatures, 3D theory predicts that the magnetoconductivity is independent of temperature or material properties, as verified for many combinations of thin-films and substrates. Here, we find that this prediction is apparently violated if the film thickness d is less than about 300 nm. To investigate the origin of this apparent violation, the magnetoconductivity was measured at temperatures T = 15 – 150 K in ten, Sn-doped In 2 O 3 films with d = 13 – 292 nm, grown by pulsed laser deposition on fused silica. We observe a very strong thickness dependence which we explain by introducing a theory that postulates a second source of disorder, namely, non-uniform interface-induced defects whose number decreases exponentially with the interface distance. This theory obeys the 3D limit for the thickest samples and yields a natural figure of merit for interface disorder. It can be applied to any degenerate semiconductor film on any semi-insulating substrate.
more »
« less
- Award ID(s):
- 1800130
- PAR ID:
- 10289012
- Date Published:
- Journal Name:
- Communications Materials
- Volume:
- 2
- Issue:
- 1
- ISSN:
- 2662-4443
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Bismuth vanadate (BiVO4) is promising for solar-assisted water splitting. The performance of BiVO4 is limited by charge separa- tion for > 70 nm films or by light harvesting for < 700 nm films. To resolve this mismatch, host–guest architectures use thin film coatings on 3D scaffolds. Recombination, however, is exacerbated at the extended host–guest interface. Underlayers are used to limit this recombination with a host-underlayer- guest series. Such underlayers consume precious pore volume where typical SnO2 underlayers are optimized with 65–80 nm. In this study, conformal and ultrathin SnO2 underlayers with low defect density are produced by atomic layer deposition (ALD). This shifts the optimized thickness to just 8 nm with sig- nificantly improved space efficiency. The materials chemistry thus determines the dimension optimization. Lastly, host– guest architectures are shown to have an applied bias photon- to-charge efficiency of 0.71%, a new record for a photoanode absorber prepared by ALD.more » « less
-
GdFe₀.₅Cr₀.₅O₃ (GFCO) is a single-phase magnetoelectric multiferroic at temperatures close to ambient. Epitaxial thin films of this orthorhombic perovskite would offer the possibility of tuning its electrical and magnetic properties through control of strain and interface effects. Here, 200 nm thick GFCO thin films have been grown on (001) SrTiO3 substrates by solution synthesis and the microstructures have been investigated by cross-sectional transmission electron microscopy. The GFCO films are epitaxial but exhibit a mixture of three different orientation relationships in the form of domains ≈50 nm in diameter. Geometric analyses of the lattice matching show that the misfits for these domains would be tensile with magnitudes of less than 2 %. Pockets of a SrCrO4 reaction product form at the film/substrate interface and do not exhibit any simple orientation with the adjacent phases. The product morphology indicates that the outward diffusion of Sr is more rapid than the inward diffusion of Cr, and this is related to the microstructures of the surrounding phases. These data show that epitaxial films of GFCO can be obtained via this route, but careful control of process parameters would be required to produce single-domain films, and alternate substrates or buffer layers would be needed to inhibit SrCrO4 formation.more » « less
-
Films formed by dip coating brass wires with dilute and semi-dilute solutions of polyvinyl butyral in benzyl alcohol were studied in their liquid and solid states. While dilute and semi-dilute solutions behaved as Maxwell viscoelastic fluids, the thickness of the liquid films followed the Landau-Levich-Derjaguin prediction for Newtonian fluids. At a very slow rate of coating, the film thickness was difficult to evaluate. Therefore, the dynamic contact angle was studied in detail. We discovered that polymer additives preserve the advancing contact angle at its static value while the receding contact angle follows the Cox–Voinov theory. In contrast, the thickness of solid films does not correlate with the Landau-Levich-Derjaguin predictions. Only solutions of high-molecular-weight polymers form smooth solid films. Solutions of low-molecular-weight polymers may form either solid films with an inhomogeneous roughness or solid polymer domains separated by the dry substrate. In technological applications, very dilute polymer solutions of high-molecular-weight polymers can be used to avoid inhomogeneities in solid films. These solutions form smooth solid films, and the film thickness can be controlled by the experimental coating conditions.more » « less
-
Two distinct ultra-thin Ge1−xSnx (x ≤ 0.1) epilayers were deposited on (001) Si substrates at 457 and 313 °C through remote plasma-enhanced chemical vapor deposition. These films are considered potential initiation layers for synthesizing thick epitaxial GeSn films. The GeSn film deposited at 313 °C has a thickness of 10 nm and exhibits a highly epitaxial continuous structure with its lattice being compressed along the interface plane to coherently match Si without mismatch dislocations. The GeSn film deposited at 457 °C exhibits a discrete epitaxial island-like morphology with a peak height of ∼30 nm and full-width half maximum (FWHM) varying from 20 to 100 nm. GeSn islands with an FWHM smaller than 20 nm are defect free, whereas those exceeding 25 nm encompass nanotwins and/or stacking faults. The GeSn islands form two-dimensional modulated superlattice structures at the interface with Si. The GeSn film deposited at 457 °C possesses a lower Sn content compared to the one deposited at lower temperature. The potential impact of using these two distinct ultra-thin layers as initiation layers for the direct growth of thicker GeSn epitaxial films on (001) Si substrates is discussed.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s43246-021-00137-y
