Charge transport in organic nanocrystal diodes based on rolled-up robust nanomembrane contacts

The investigation of charge transport in organic nanocrystals is essential to understand nanoscale physical properties of organic systems and the development of novel organic nanodevices. In this work, we fabricate organic nanocrystal diodes contacted by rolled-up robust nanomembranes. The organic nanocrystals consist of vanadyl phthalocyanine and copper hexadecafluorophthalocyanine heterojunctions. The temperature dependent charge transport through organic nanocrystals was investigated to reveal the transport properties of ohmic and space-charge-limited current under different conditions, for instance, temperature and bia

Charge transport in organic nanocrystal diodes based on rolled-up robust nanomembrane contacts

The investigation of charge transport in organic nanocrystals is essential to understand nanoscale physical properties of organic systems and the development of novel organic nanodevices. In this work, we fabricate organic nanocrystal diodes contacted by rolled-up robust nanomembranes. The organic nanocrystals consist of vanadyl phthalocyanine and copper hexadecafluorophthalocyanine heterojunctions. The temperature dependent charge transport through organic nanocrystals was investigated to reveal the transport properties of ohmic and space-charge-limited current under different conditions, for instance, temperature and bias

Quantum transport in topological surface states of Bi2_2Te3_3 nanoribbons

Quasi-1D nanowires of topological insulators are emerging candidate structures in superconductor hybrid architectures for the realization of Majorana fermion based quantum computation schemes. It is however technically difficult to both fabricate as well as identify the 1D limit of topological insulator nanowires. Here, we investigated selectively-grown Bi$_2$Te$_3$ topological insulator nanoribbons and nano Hall bars at cryogenic temperatures for their topological properties. The Hall bars are defined in deep-etched Si$_3$N$_4$/SiO$_2$ nano-trenches on a silicon (111) substrate followed by a selective area growth process via molecular beam epitaxy. The selective area growth is beneficial to the device quality, as no subsequent fabrication needs to be performed to shape the nanoribbons. Transmission line measurements are performed to evaluate contact resistances of Ti/Au contacts applied as well as the specific resistance of the Bi$_2$Te$_3$ binary topological insulator. In the diffusive transport regime of these unintentionally $n$-doped Bi$_2$Te$_3$ topological insulator nano Hall bars, we identify distinguishable electron trajectories by analyzing angle-dependent universal conductance fluctuation spectra. When the sample is tilted from a perpendicular to a parallel magnetic field orientation, these high frequent universal conductance fluctuations merge with low frequent Aharonov-Bohm type oscillations originating from the topologically protected surface states encircling the nanoribbon cross section. For 500 nm wide Hall bars we also identify low frequent Shubnikov-de Haas oscillations in the perpendicular field orientation, that reveal a topological high-mobility 2D transport channel, partially decoupled from the bulk of the material.Comment: 13 pages, 5 figures, 6 pages supplementary information, 5 supplementary figure

Engineering topological superlattices and their epitaxial integration in selectively grown hybrid nanostructures via MBE

The realization of advanced spintronics applications including the topological quantum computation, spin manipulation for data storage, dissipation less ballistic transport for ultra-fast quantum devices and topological switching for low energy memory applications etc. became more feasible with the experimental discovery of 3D topological insulators (TIs). The incorporation of exotic spin-momentum locked Dirac surface states (of 3D TIs) into these futuristic complex quantum devices requires not only the growth of high crystal quality epilayers but also the fabrication of pristine nanostructures, topological band engineering, ultra-smooth and defect-free surfaces, and atomically transparent epitaxial interfaces. This work deals with a systematic study of epitaxial growth of convention 3D TIs via molecular beam epitaxy(MBE) and atomic-scale structural characterization via scanning transmission electron microscope (STEM)to explore the above mentioned requirements. At first, the relation between the growth parameters and the defect density in the Van-der-Waals (VdW) based layered structures is investigated. The optimum growth parameters are extracted and the defect-free epilayers are prepared. Later, the technique of selective area epitaxy (SAE) is explored to develop a platform to achieve a scalable nano-architecture. Utilizing CMOS compatible fabrication technology, Si (111) substrates with crystalline and amorphous combinational surfaces are prepared. The precisely controlled growth parameters facilitated the realization of selectively grown topological structure. Based on statistical analysis, a generalized growth model is established that provided control over structural defects through the effective growth rate at the nanoscale and assisted in achieving high quality nanostructures. Based on conventional 3D TIs, the capabilities of VdW epitaxy are exploited further with the growth of topological-trivial hetero structures. The stoichiometric adjustment in these hetero structures is utilized as a tool to control the strength of spin-orbit coupling (SOC) and to engineer the topological band structure. Two such systems are explored including BixTey = (Bi2)m(Bi2Te3)n and GST/GBT = (GeTe)n(Sb2Te3/Bi2Te3)m. With the continuous addition of Bi2 bilayers and GeTe (materials that exhibit trivial phase) into 3D TIs, the stoichiometric modulations are achieved. Moreover, the modification of growth parameters is conducted to incorporate these stoichiometries with the pre-patterned substrates and selectively grown nanostructures of the corresponding alloys are prepared. Assisted by the atomic-scale structural characterizations, the phenomenon of VdW reconfiguration is explored to observe the transformation of layer architecture; the key mechanism in the evolution of interfacial phase change materials (IPCMs).Moreover, the systematic alterations in the atomic interaction and resulting changes in bond lengths within a pristine and hybrid VdW stacks are investigated. The focus is then shifted towards surfaces where the stability (inertness) of TI epilayers in the ambient conditions via structural and compositional investigations, is analyzed. An undeniable evidence of the aging effect in all material systems is obtained where a non-saturating oxidation process at the (0001) surfaces with a continually decreasing oxidation rate is witnessed. Using the in situ thin film deposition of Al (2 nm),the top surfaces are passivated and the aging effect is neutralized. The phenomenon of charge transfer due to band alignment at the Si (111) - TI bottom surface is investigated with a comparative growth, structural and transport analysis of TI epilayer prepared on HfO2 substrate. Finally, the interfaces between TIs and various s-wave superconductors (SCs) are explored. The challenges to achieve the induced superconductivity in TI-SC hybrid junction and highly transparent interfaces are addressed. The issue of metal diffusion into the TI epilayer and the resulting formation of Schottky-like barriers is avoided with the introduction of a thin metallic film as a diffusion barrier. Using the natural tendency of transition metals to transform into their corresponding di-chalcogenides (TMDCs) at the exposure to TI surfaces, atomically well-defined and VdW assisted epitaxial interfaces are engineered. The newly evolved interfaces assisted in achieving the induced superconductivity that was a huge limitation in realizing the complex functional devices

Atomic Diffusion-Induced Polarization and Superconductivity in Topological Insulator-Based Heterostructures

The proximity effect at a highly transparent interface of an s-wave superconductor (S) and a topological insulator (TI) provides a promising platform to create Majorana zero modes in artificially designed heterostructures. However, structural and chemical issues pertinent to such interfaces have been poorly explored so far. Here, we report the discovery of Pd diffusion-induced polarization at interfaces between superconductive Pd1+x(Bi0.4Te0.6)2 (xPBT, 0 ≤ x ≤ 1) and Pd-intercalated Bi2Te3 by using atomic-resolution scanning transmission electron microscopy. Our quantitative image analysis reveals that nanoscale lattice strain and QL polarity synergistically suppress and promote Pd diffusion at the normal and parallel interfaces, formed between Te–Pd–Bi triple layers (TLs) and Te–Bi–Te–Bi–Te quintuple layers (QLs), respectively. Further, our first-principles calculations unveil that the superconductivity of the xPBT phase and topological nature of the Pd-intercalated Bi2Te3 phase are robust against the broken inversion symmetry. These findings point out the necessity of considering the coexistence of electric polarization with superconductivity and topology in such S–TI systems

Pd-doping of Bi₂Te₃ and superconductivity of Pd(Bi,Te)x from density functional theory

Materials that can host Majorana zero modes gained a lot of attention in recent years due to the possibility to engineer topologically protected quantum computing platforms. Promising candidates are heterostructures of topological insulators and superconductors. Here we present density-functional-theory-based calculations for Pd-doped Bi₂Te₃ and Pd(Bi,Te)x (x=1,2) in order to shed light on the superconducting properties in the self-formed superconducting phase when Pd is deposited on top of the topological insulator Bi₂Te₃.This dataset accompanies a joint experiment/theory publication and publishes the related density functional theory calculations for:- relaxed geometries for Pd intercalation in the Bi₂Te₃ vdW gap- electronic structure of PdTe and PdTe₂ compared to alloy phases of Pd(Bi,Te) and Pd(Bi,Te)₂, collectively referred to as "xPBT"- calculations for the superconducting state of xPBT phases within the Kohn-Sham Bogoliubov-de Gennes metho

Atomic diffusion-induced polarization and superconductivity in topological insulator-based heterostructures

The proximity effect at a highly transparent interface of an s-wave superconductor (S) and a topological insulator (TI) provides a promising platform to create Majorana zero modes in artificially designed heterostructures. However, structural and chemical issues pertinent to such interfaces are poorly explored so far. Here, we report the discovery of Pd diffusion induced polarization at interfaces between superconductive Pd$_{1+x}$(Bi$_{0.4}$Te$_{0.6}$)$_2$ (xPBT, $0\le x \le 1$) and Pd-intercalated Bi$_2$Te$_3$ by using atomic-resolution scanning transmission electron microscopy. Our quantitative image analysis reveals that nanoscale lattice strain and QL polarity synergistically suppress and promote the Pd diffusion at the normal and parallel interfaces, formed between Te-Pd-Bi triple layers (TLs) and Te-Bi-Te-Bi-Te quintuple layers (QLs), respectively. Further, our first-principles calculations unveil that the superconductivity of xPBT phase and topological nature of Pd-intercalated Bi$_2$Te$_3$ phase are robust against the broken inversion symmetry. These findings point out the necessity of considering coexistence of electric polarization with superconductivity and topology in such S-TI systems

Phase-Selective Epitaxy of Trigonal and Orthorhombic Bismuth Thin Films on Si (111)

Over the past three decades, the growth of Bi thin films has been extensively explored due to their potential applications in various fields such as thermoelectrics, ferroelectrics, and recently for topological and neuromorphic applications, too. Despite significant research efforts in these areas, achieving reliable and controllable growth of high-quality Bi thin-film allotropes has remained a challenge. Previous studies have reported the growth of trigonal and orthorhombic phases on various substrates yielding low-quality epilayers characterized by surface morphology. In this study, we present a systematic growth investigation, enabling the high-quality growth of Bi epilayers on Bi-terminated Si (111) 1 × 1 surfaces using molecular beam epitaxy. Our work yields a phase map that demonstrates the realization of trigonal, orthorhombic, and pseudocubic thin-film allotropes of Bi. In-depth characterization through X-ray diffraction (XRD) techniques and scanning transmission electron microscopy (STEM) analysis provides a comprehensive understanding of phase segregation, phase stability, phase transformation, and phase-dependent thickness limitations in various Bi thin-film allotropes. Our study provides recipes for the realization of high-quality Bi thin films with desired phases, offering opportunities for the scalable refinement of Bi into quantum and neuromorphic devices and for revisiting technological proposals for this versatile material platform from the past 30 years

Room temperature in-situ measurement of the spin voltage of a BiSbTe3 thin film

One of the hallmarks of topological insulators (TIs), the intrinsic spin polarisation in the topologically protected surface states, is investigated at room temperature in-situ by means of four-probe scanning tunnelling microscopy (STM) for a BiSbTe3 thin film. To achieve the required precision of tip positions for measuring a spin signal, a precise positioning method employing STM scans of the local topography with each individual tip is demonstrated. From the transport measurements, the spin polarisation in the topological surface states (TSS) is estimated as p ~ 0.3 – 0.6, which is close to the theoretical limit