A. Szczerbakow

Topological crystalline insulator states in Pb1−xSnxSe

Topological insulators are a class of quantum materials in which time-reversal symmetry, relativistic effects and an inverted band structure result in the occurrence of electronic metallic states on the surfaces of insulating bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical results have suggested the existence of topological crystalline insulators (TCIs), a class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection. In this study we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a TCI for x  =  0.23. Temperature-dependent angle-resolved photoelectron spectroscopy demonstrates that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a TCI. These experimental findings add a new class to the family of topological insulators, and we anticipate that they will lead to a considerable body of further research as well as detailed studies of topological phase transitions.

Spin-polarized (001) surface states of the topological crystalline insulator Pb0.73Sn0.27Se

We study the nature of (001) surface states in Pb0.73Sn0.27Se in the newly discovered topological-crystalline-insulator (TCI) phase as well as the corresponding topologically trivial state above th ...

Observation of topological crystalline insulator surface states on (111)-oriented Pb1-xSnxSe films

We present angle-resolved photoemission spectroscopy measurements of the surface states on in-situ grown (111) oriented films of Pb1-xSnxSe, a three-dimensional topological crystalline insulator. We observe surface states with Dirac-like dispersion at (Gamma) over bar and (M) over bar in the surface Brillouin zone, supporting recent theoretical predictions for this family of materials. We study the parallel dispersion isotropy and Dirac-point binding energy of the surface states, and perform tight-binding calculations to support our findings. The relative simplicity of the growth technique is encouraging, and suggests a clear path for future investigations into the role of strain, vicinality, and alternative surface orientations in (Pb,Sn)Se solid solutions.

Recrystallization prospects for freestanding low-temperature GaN grown using ZnO buffer layers

Remote plasma enhanced-laser-induced chemical vapor deposition was used to grow gallium nitride films on zinc oxide buffer layers deposited by atomic layer epitaxy on soda lime glass. Freestanding layers of gallium nitride were processed by etching away the substrate and ZnO buffer layer. The n-type carrier mobility for the GaN on ZnO/soda lime glass was found to be similar to the highest values achieved on pure silica, and was accompanied by high carrier concentration. As-grown polycrystalline materials were recrystallized at low temperature (below the 570°C gallium nitride growth temperature). This recrystallization process greatly improved the film structure with a self-assembled multilayer structure evident in the oxygen-rich surface layer of the films that had undergone the process.

Monocrystalline ZnS-sphalerite films grown by atomic layer epitaxy in a gas flow system

Abstract Monocrystalline films of cubic ZnS have been produced from zinc chloride and hydrogen sulphide by atomic layer epitaxy ALE in a gas flow system. “Rocking curve” measurement gives the half-width of 1500 arcsec for a sample of approximately 460 nm thick. Tetragonal distortion of the unit cell is observed.

Investigation of the composition of vapour-grown Pb1-xSnxSe crystals ( x ≤ 0.4) by means of lattice parameter measurements

Abstract Experiments on the crystal growth of pseudobinary lead-tin selenide from the vapour were performed under near-equilibrium conditions for SnSe molar fractions of 0 ≤ x ≤ 0.4, where the rocksalt structure exists. Precise X-ray lattice parameter measurements were carried out on single crystals by means of the Soller-slit method. A difference of 0.01 in x between the crystal and the remainders of the source was found for average x = 0.35, but for x = 0.15 it was barely significant. The compositional uniformity allowed applying the single crystals for determination of a linear dependence of the lattice parameter on the SnSe molar fraction. The dependence is valid for Pb 1- x Sn x Se solid solutions displaying metal excess (n-type conductivity).

Influence of thermal radiation on crystal growth by sublimation of AIVBVI solid solutions on source material

The generation of radial temperature differences by thermal radiation and their influence on the mass transfer by sublimation have been studied. Based on this, some practical conclusions are formulated. Several examples of application to the crystal growth of A IV B VI solid solutions are presented.

Robust spin-polarized midgap states at step edges of topological crystalline insulators

An edge that is hard to get rid of A distinguishing characteristic of topological insulators (TIs) is that they have conducting states on their boundary—a surface for a three-dimensional (3D) TI or a line edge for a 2D TI. Sessi et al. used scanning tunneling spectroscopy to discover unusual 1D states in a 3D crystalline TI. The states appeared on the edge of a particular kind of step in the crystal and survived large magnetic fields and increased temperatures. This robustness bodes well for the potential use of these states in practical applications. Science, this issue p. 1269 Scanning tunneling spectroscopy is used to uncover a one-dimensional protected state in (Pb,Sn)Se. Topological crystalline insulators are materials in which the crystalline symmetry leads to topologically protected surface states with a chiral spin texture, rendering them potential candidates for spintronics applications. Using scanning tunneling spectroscopy, we uncover the existence of one-dimensional (1D) midgap states at odd-atomic surface step edges of the three-dimensional topological crystalline insulator (Pb,Sn)Se. A minimal toy model and realistic tight-binding calculations identify them as spin-polarized flat bands connecting two Dirac points. This nontrivial origin provides the 1D midgap states with inherent stability and protects them from backscattering. We experimentally show that this stability results in a striking robustness to defects, strong magnetic fields, and elevated temperature.

Methods of dislocation distribution analysis and inclusion identification with application to CdTe and (Cd, Zn)Te

Although the density of dislocations is used as an indicator of the quality of semiconductor wafers, it is also necessary to provide a description of the dislocation distribution for those materials prone to polygonization (namely the formation of sub-grain boundaries). In this work it is shown that visual inspection of etch pit distributions is insufficient and that quantitative comparison with random distributions is more informative. This was achieved in two ways: (i) by comparison with a Poisson distribution and (ii) by using normalized radial dislocation density plots, the former yielding the type of distribution and the latter the characteristic sub-grain tilting angle for the sample. Although these methods are general, examples for melt- and vapour-grown CdTe and (Cd, Zn)Te are given. Also reported upon in this work is the use of etching solutions to reveal inclusions in CdTe and (Cd, Zn)Te and hence permit their chemical identification by SEM/EDAX.

Structural defects and compositional uniformity in CdTe and Cd1-xZnxTe crystals grown by a vapour transport technique

Abstract In this work the principles of self-selecting vapour growth (SSVG) are described and the advantage of the method in allowing continuous refinement of a crystalline mass is explained. The methods use of low supersaturation and its ability to grow crystals free from the walls of a capsule make the SSVG principle an attractive one for the growth of CdTe and Cd1−xZnxTe boules. Here a new vertical SSVG apparatus was used to grow small crystals of these two materials and the defects in them were characterised by etching and HRXRD. Compositional uniformity of the Cd1−xZnxTe grown was found by PL and HRXRD to vary by not more than 0.0014 in x (i.e. 0.14%). This compares favourably with other vapour growth studies in which low undercooling (ΔT∼1–2°C) was used.