The Origin of the Structural Phase Transition in the Growth of Ultra-Thin Bi Films on Si(111) (original) (raw)

Epitaxial Growth of Bi(111) on Si(001)

e-Journal of Surface Science and Nanotechnology, 2009

Despite the large lattice misfit and different lattice symmetry, it is possible to grow smooth and almost defectfree bismuth (Bi) films on a Si(001) substrate. High resolution low-energy electron diffraction measurements have confirmed that the (111) orientation is the preferred direction of the growth. However, at low temperature and low coverage regime, rotationally disordered crystallites of (110) orientation are also observed. After the formation of a continuous layer at 5.6 bilayer (2.2 nm), the growth occurs in a bilayer-by-bilayer fashion at 150 K. The remaining lattice mismatch of 2.3 % is accommodated by a periodic array of interfacial misfit dislocations, which gives rise to a periodic surface height undulation with sub-ångström amplitude. Additional growth to the desired thickness caps the height undulation resulting in an atomically smooth surface (terrace size > 100 nm). The Bi(111) film is relaxed to bulk lattice constant and shows excellent crystalline quality with an abrupt interface to the Si substrate.

Controlling the growth of Bi(110) and Bi(111) films on an insulating substrate

Nanotechnology, 2017

Here we demonstrate the controlled growth of Bi(110) and Bi(111) films on an (insulating) α-Al 2 O 3 (0001) substrate by surface X-ray diffraction and X-ray reflectivity using synchrotron radiation. At temperatures as low as 40 K, unanticipated pseudo-cubic Bi(110) films are grown having a thickness ranging from a few to tens of nanometers. The roughness at the film-vacuum as well as at the film-substrate interface, can be reduced by mild heating, where a crystallographic orientation transition of Bi(110) towards Bi(111) is observed at 400 K. From 450 K onwards high quality and ultrasmooth Bi(111) films are formed. Growth around the transition temperature results in the growth of competing Bi(110) and Bi(111) thin film domains.

Homoepitaxial growth of Bi(111)

Physical Review B, 2008

Homoepitaxial growth of Bi͑111͒ at temperatures between 80-300 K has been studied using spot profile analyzing low-energy electron diffraction ͑SPA-LEED͒ and scanning tunneling microscopy ͑STM͒. From the intensity oscillations of the ͑00͒-spot with Bi coverage and the STM topography of two-dimensional ͑2D͒ islands at low coverage, a pure 2D nucleation followed by a quasi bilayer-by-bilayer growth mode has been confirmed. The oscillation amplitude decays slowly with coverage, indicating a slow kinetic roughening due to a weak Ehrlich-Schwoebel step edge barrier. From the Arrhenius behavior of the average island separation an intraterrace diffusion barrier of E d = 0.135 eV is estimated. Regularly ordered quasidendritic shape islands reflect an asymmetry in adatom diffusion along the steps and the corners of the islands.

Stable Nontrivial Z_ {2} Topology in Ultrathin Bi (111) Films: A First-Principles Study

2011

Recently, there have been intense efforts in searching for new topological insulator (TI) materials. Based on first-principles calculations, we find that all the ultrathin Bi (111) films are characterized by a nontrivial Z2 number independent of the film thickness, without the odd-even oscillation of topological triviality as commonly perceived. The stable nontrivial Z2 topology is retained by the concurrent band gap inversions at multiple time-reversal-invariant k-points and associated with the intermediate inter-bilayer coupling of the multi-bilayer Bi film. Our calculations further indicate that the presence of metallic surface states in thick Bi(111) films can be effectively removed by surface adsorption.

Role of Quantum and Surface-State Effects in the Bulk Fermi-Level Position of Ultrathin Bi Films

Physical Review Letters, 2015

We performed high-resolution photon-energy and polarization-dependent ARPES measurements on ultrathin Bi(111) films [6-180 bilayers (BL), 2.5-70 nm thick] formed on Si(111). In addition to the extensively studied surface states (SSs), the edge of the bulk valence band was clearly measured by using S-polarized light. We found direct evidence that this valence band edge, which forms a hole pocket in the bulk Bi crystal, does not cross the Fermi level for the 180 BL thick film. This is consistent with the predicted semimetal-to-semiconductor transition due to the quantum-size effect [V.B. Sandomirskii, Sov. Phys. JETP 25, 101 (1967)]. However, it became metallic again when the film thickness was decreased (below 30 BL). A plausible explanation for this phenomenon is the modification of the charge neutrality condition due to the size effect of the SSs.

High-quality epitaxial Bi (111) films on Si (111) by isochronal annealing

2012

Bi(111) films grown on Si(111) at room temperature show a significantly higher roughness compared to Bi films grown on Si(100) utilizing a kinetic pathway based on a low-temperature process. Isochronal annealing steps of 3 min duration each with temperatures up to 200°C cause a relaxation of the Bi films' lattice parameter toward the Bi bulk value and yield an atomically flat Bi surface. Driving force for the relaxation and surface reordering is the magic mismatch of 11 Bi atoms to 13 Si atoms that emerges at annealing temperatures above 150°C and reduces the remaining strain to less than 0.2%.

Nanofilm Allotrope and Phase Transformation of Ultrathin Bi Film on Si 111†-7 7

Our scanning tunneling microscopy and electron diffraction experiments revealed that a new two-dimensional allotrope of Bi forms on the Si 111†-7 7 surface. This pseudocubic f012g-oriented allotrope is stable up to four atomic layers at room temperature. Above this critical thickness, the entire volume of the film starts to transform into a bulk single-crystal (001) phase, as the bulk contribution in the cohesion becomes dominant. Based on ab initio calculations, we propose that the new allotrope consists of black phosphorus-like puckered layers stabilized by saturating all the p z dangling bonds in the film.

Composition-dependent structural transition in epitaxial Bi1−xSbx thin films on Si(111)

Physical Review Materials, 2019

Bismuth-antimony alloys (Bi 1−x Sb x) are topological insulators between 7 and 22% Sb in bulk crystals, with an unusually high conductivity suitable for spin-orbit torque applications. Reducing the thickness of epitaxial Bi 1−x Sb x films is expected to increase the maximum band gap through quantum confinement, which may improve isolation of topological surface-state transport. Like Bi(001) on Si(111), Bi 1−x Sb x has been predicted to form a black phosphoruslike allotrope with unique electronic properties in nanoscale films; however, the impact of Sb alloying on both the bulklike and nanoscale crystal structures on Si(111) is currently unknown. Here we demonstrate that the allotropic transition in ultrathin epitaxial Bi 1−x Sb x films on Si(111) is suppressed above 8-9% Sb, resulting in an unexpected (012) orientation within the topologically insulating regime. The metallic temperature-dependent conductivity associated with surface states in Bi(001) was not observed in the Bi 1−x Sb x (012) films, suggesting that the (012) orientation may significantly reduce surface-state transport. Growth on a Bi(001) buffer layer may prevent this orientation transition. Finally, we demonstrate that Sb alloying improves the continuity and quality of nanoscale Bi 1−x Sb x (012) films in the thickness regime expected for the black phosphorus allotrope, suggesting a promising route to large-area growth of puckered-layer two-dimensional Bi 1−x Sb x , which will be necessary to harness its unique electronic properties in practical applications.