Transport properties in a Sb-Te binary topological-insulator system

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Single crystal growth and ferromagnetism of Cr-doped Sb4Te3

Here we report the single crystal growth, magnetic and transport properties of Cr-doped Sb₄Te₃, (Sb_1-x Cr _x )₄Te₃, with doping concentrations x  =  0.25%, 0.5%, 0.75%, and 1%. The samples with lower doping concentrations are paramagnetic, while ferromagnetism appears in higher doped samples with the highest Curie temperature of 7 K when x  =  1%. Anomalous Hall effect with clear hysteresis loop is observed in the samples with x  =  1%, indicating the intrinsic ferromagnetism in the system. Hall resistivity measurements show the dominant charge carriers are holes and the density of holes increases with the doping concentration. This work provides a possible single-crystalline platform for further experimental researches on the nontrivial band topology in Sb₄Te₃, and enriches the ferromagnetic members in the transition metal doped (Sb₂) _m -Sb₂Te₃ topological material series.

High-Density Sb2 Te3 Nanopillars Arrays by Templated, Bottom-Up MOCVD Growth

Sb₂ Te₃ exhibits several technologically relevant properties, such as high thermoelectric efficiency, topological insulator character, and phase change memory behavior. Improved performances are observed and novel effects are predicted for this and other chalcogenide alloys when synthetized in the form of high-aspect-ratio nanostructures. The ability to grow chalcogenide nanowires and nanopillars (NPs) with high crystal quality in a controlled fashion, in terms of their size and position, can boost the realization of novel thermoelectric, spintronic, and memory devices. Here, it is shown that highly dense arrays of ultrascaled Sb₂ Te₃ NPs can be grown by metal organic chemical vapor deposition (MOCVD) on patterned substrates. In particular, crystalline Sb₂ Te₃ NPs with a diameter of 20 nm and a height of 200 nm are obtained in Au-functionalized, anodized aluminum oxide (AAO) templates with a pore density of ≈5 × 10¹⁰ cm^-2 . Also, MOCVD growth of Sb₂ Te₃ can be followed either by mechanical polishing and chemical etching to produce Sb₂ Te₃ NPs arrays with planar surfaces or by chemical dissolution of the AAO templates to obtain freestanding Sb₂ Te₃ NPs forests. The illustrated growth method can be further scaled to smaller pore sizes and employed for other MOCVD-grown chalcogenide alloys and patterned substrates.

Electrical and optical properties of epitaxial binary and ternary GeTe-Sb2Te3 alloys

Phase change materials such as pseudobinary GeTe-Sb₂Te₃ (GST) alloys are an essential part of existing and emerging technologies. Here, we investigate the electrical and optical properties of epitaxial phase change materials: α-GeTe, Ge₂Sb₂Te5 (GST225), and Sb₂Te₃. Temperature-dependent Hall measurements reveal a reduction of the hole concentration with increasing temperature in Sb₂Te₃ that is attributed to lattice expansion, resulting in a non-linear increase of the resistivity that is also observed in GST225. Fourier transform infrared spectroscopy at room temperature demonstrates the presence of electronic states within the energy gap for α-GeTe and GST225. We conclude that these electronic states are due to vacancy clusters inside these two materials. The obtained results shed new light on the fundamental properties of phase change materials such as the high dielectric constant and persistent photoconductivity and have the potential to be included in device simulations.

Topological states and phase transitions in Sb2Te3-GeTe multilayers

Topological insulators (TIs) are bulk insulators with exotic 'topologically protected' surface conducting modes. It has recently been pointed out that when stacked together, interactions between surface modes can induce diverse phases including the TI, Dirac semimetal, and Weyl semimetal. However, currently a full experimental understanding of the conditions under which topological modes interact is lacking. Here, working with multilayers of the TI Sb2Te3 and the band insulator GeTe, we provide experimental evidence of multiple topological modes in a single Sb2Te3-GeTe-Sb2Te3 structure. Furthermore, we show that reducing the thickness of the GeTe layer induces a phase transition from a Dirac-like phase to a gapped phase. By comparing different multilayer structures we demonstrate that this transition occurs due to the hybridisation of states associated with different TI films. Our results demonstrate that the Sb2Te3-GeTe system offers strong potential towards manipulating topological states as well as towards controlledly inducing various topological phases.

Ferromagnetism in Mn-doped Sb(2)Te

We report that Sb2Te, a natural superlattice phase consisting of two elemental Sb2 layers interleaved with single Sb2Te3 layers, becomes ferromagnetic at low temperatures on doping with small percentages of Mn. Ferromagnetism appears for Mn concentrations as low as Sb1.98Mn0.02Te, where a ferromagnetic Tc of ~8.6 K is observed. Tc decreases with increasing Mn content in the stoichiometric materials but increases with increasing Te excess in materials of the type Sb1.93-yMn0.07Te1+y, starting at ~3 K at y = 0 and reaching a Tc of ~8.9 K at y = 0.06.

Multiple state transport deduced by weak antilocalization and electron-electron interaction effects in Sb(x)Te(1-x) layers

Quantum corrections to the conductivity due to the weak antilocalization (WAL) and electron-electron interaction (EEI) effects are investigated in Sb-Te layers to evaluate the number of independent conduction channels in the topological insulator system. We separate the two contributions in the logarithmic temperature dependence of conductivity relying on their distinct response to a magnetic field. For the WAL effect, the amplitude parameter α being -1 observed in magnetoconductivity is confirmed. The magnitude of the EEI contribution is too large to be produced by one transport channel. The mixing between the surface and bulk states is thus indicated to be weak in the Sb-Te system. In addition, the disorder scattering appears to be less influential for the EEI effect than for the WAL effect.