On‐Chip Andreev Devices: Hard Superconducting Gap and Quantum Transport in Ballistic Nb–In0.75Ga0.25As‐Quantum‐Well–Nb Josephson Junctions

A superconducting hard gap in hybrid superconductor–semiconductor devices has been found to be necessary to access topological superconductivity that hosts Majorana modes (non‐Abelian excitation). This requires the formation of homogeneous and barrier‐free interfaces between the superconductor and semiconductor. Here, a new platform is reported for topological superconductivity based on hybrid Nb–In_0.75Ga_0.25As‐quantum‐well–Nb that results in hard superconducting gap detection in symmetric, planar, and ballistic Josephson junctions. It is shown that with careful etching, sputtered Nb films can make high‐quality and transparent contacts to the In_0.75Ga_0.25As quantum well, and the differential resistance and critical current measurements of these devices are discussed as a function of temperature and magnetic field. It is demonstrated that proximity‐induced superconductivity in the In_0.75Ga_0.25As‐quantum‐well 2D electron gas results in the detection of a hard gap in four out of seven junctions on a chip with critical current values of up to 0.2 µA and transmission probabilities of >0.96. The results, together with the large g ‐factor and Rashba spin–orbit coupling in In_0.75Ga_0.25As quantum wells, which indeed can be tuned by the indium composition, suggest that the Nb–In_0.75Ga_0.25As–Nb system can be an excellent candidate to achieve topological phase and to realize hybrid topological superconducting devices.     

Control of electron concentration in liquid-phase epitaxial layers of Pb1−xSnxTe by indium doping

Indium-doped n-type Pb_1?xSn_xTe, 0?x?0.2, epilayers were grown by liquid-phase epitaxy (LPE) using metal-rich solutions containing from 0.002–10 at. % indium. Doping characteristics of indium and electrical properties of the epilayers were studied by Hall and resistivity measurements at 77K. The dependence of electron concentration and mobility of the Pb_1?xSn_xTe epilayers on indium doping is given for various alloy compositions and LPE growth temperatures. Doping coefficients of ～0.05 and ～0.03 are found for PbTe and Pb_0.8Sn_0.2Te, respectively, grown at ～450°C. For medium to high indium doping, the electron concentration saturates to a constant value independent of doping and LPE growth temperature. The saturation values decrease substantially with increasing x. This work presents the first guide for producing n-type liquid-phase epitaxial Pb_1?xSn_xTe layers with controlled carrier concentration.     

Fabrication of Pb1‐xSnxTe/PbTe heterojunctions by liquid phase epitaxy

Abstract

Pb_1‐x Sn _x Te epitaxial layers were successfully grown on PbTe substrates by liquid phase epitaxy (LPE) technique. The compositions and surface morphology of the epitaxial Pb_1‐x Sn _x Te layers were controlled in the LPE growth, and p‐Pb_1‐x Sn _x Te/n‐PbTe heterodiodes with a good junction‐perfection factor were made.     

Pressure-induced band cross-over in Pb1−x Sn x Te

Resistivity and thermoelectric power studies have been carried out on two semiconductor alloy systems viz Pb_0·8Sn_0·2Te and Pb_0·6Sn_0·4Te up to 35 kbar pressure. Thermoelectric power and resistivity data on Pb_0·8Sn_0·2Te indicate that the energy gapE _g=E _{L −} ⁶ −E _{L 6} ⁺ decreases with pressure resulting in a zero gap state near 35 kbar pressure. TEP studies on the alloy system Pb_0·6Sn_0·4Te provide direct evidence for a pressure induced L ₆ ⁻ →L ₆ ⁺ cross over transition.     

2.22 Electrical and piezoresistive properties of Pb1−xSnxTe thin films

Some of the problems of producing Pb_1?xSn_xTe films by hot-wall technique are discussed in relation to the design of the deposition conditions. Electrical and piezoresistive properties of the p- and n-type films were measured. Temperature dependence of the resistivity, Hall constant, Hall mobility and gauge factor of the films were measured and discussed. Dependence of the conductivity on applied strain to the p-type and n-type films were measured. A probable explanation of the mechanism of the physical phenomena in strained Pb_1?xSn_xTe films is described.     

Graphene's Latest Cousin: Plumbene Epitaxial Growth on a “Nano WaterCube”

While theoretical studies predicted the stability and exotic properties of plumbene, the last group‐14 cousin of graphene, its realization has remained a challenging quest. Here, it is shown with compelling evidence that plumbene is epitaxially grown by segregation on a Pd_1?xPb_x(111) alloy surface. In scanning tunneling microscopy (STM), it exhibits a unique surface morphology resembling the famous Weaire–Phelan bubble structure of the Olympic “WaterCube” in Beijing. The “soap bubbles” of this “Nano WaterCube” are adjustable with their average sizes (in‐between 15 and 80 nm) related to the Pb concentration (x < 0.2) dependence of the lattice parameter of the Pd_1?xPb_x(111) alloy surface. Angle‐resolved core‐level measurements demonstrate that a lead sheet overlays the Pd_1?xPb_x(111) alloy. Atomic‐scale STM images of this Pb sheet show a planar honeycomb structure with a unit cell ranging from 0.48 to 0.49 nm corresponding to that of the standalone 2D topological insulator plumbene.     

Thallium and indium doped Pb1−xSnxTe by liquid phase epitaxy

Doped Pb_1?xSn_xTe epilayers have been grown on PbTe substrates by liquid phase epitaxy using lead-rich melts containing thallium or indium. Thallium is a p-type dopant; indium is an n-type dopant. It is also demonstrated that the doped layers can be annealed under conditions that convert undoped material from n to p-type, or vice-versa, without themselves being converted. This is the first application of doping to lead-tin salt LPE and significantly increases the flexibility of this technique for fabricating detector and laser heterostructures.     

Phase relations and stoichiometry of superconducting Pbx Mo6 S8−y

The composition of Pb_xMo₆S_8?y can vary between 0.85 < x < 1.05 and 0.6 < y < 1.2 at 1100 °C and is shifted to lower Pb content at higher temperatures. The phase decomposes peritectically at about 1530 °C. At the approximate composition line Pb_xMo₆S₇, the rhombohedral lattice constant and the transition temperature T_c for superconductivity show maxima while the rhombohedral angle has a minimum. Small single crystals (～0.2 mm) of Pb_xMo₆S_8?y and larger crystals (～3 mm) of Cu_xMo₆S_8?y, Sn_xMo₆Se_8?y can be obtained in sealed Mo-crucibles     

Effect of copper content on glass formation and superconductivity in the Bi-Pb-Sr-Ca-Cu-O system

High-T _c superconducting ceramics of formula Bi_0.8Pb_0.2SrCaCu _x O _y (x=1.5, 1.8 and 2.0) were prepared by using the melt-quenching method, and the effect of copper content on glass formation and superconductivity was examined. It was found that the composition withx=1.5 had a tendency to form a glass and Bi₂(Sr, Ca)₂CuO_y crystals tended to precipitate easily during the rapid quenching of melts in the compositions withx=1.8 and 2.0. It was found from the temperature dependence of a.c. complex susceptibility that the intergrain coupling of superconducting crystals in the samples obtained was weak, but the weak coupling was improved by increasing the annealing time. The superconducting glass-ceramics Bi_0.8Pb_0.2SrCaCu_1.5O_y exhibited superconductivity with aT _c (zero) of 106 K and aJ _c of 250 Acm^–2.     

Band Structure Perfection and Superconductivity in Type‐II Dirac Semimetal Ir1−xPtxTe2

The discovery of a new type‐II Dirac semimetal in Ir_1?xPt_xTe₂ with optimized band structure is described. Pt dopants protect the crystal structure holding the Dirac cones and tune the Fermi level close to the Dirac point. The type‐II Dirac dispersion in Ir_1?xPt_xTe₂ is confirmed by angle‐resolved photoemission spectroscopy and first‐principles calculations. Superconductivity is also observed and persists when the Fermi level aligns with the Dirac points. Ir_1?xPt_xTe₂ is an ideal platform for further studies on the exotic properties and potential applications of type‐II DSMs, and opens up a new route for the investigation of the possible topological superconductivity and Majorana physics.     

Synthesis and Superconductivity of New Cuprates (Pb0.8W0.2)Sr2(Nd1 − x Ca x )Cu2O7 − δ

New cuprates with nominal composition (Pb_0.8W_0.2)Sr₂(Nd_{1 ? x} Ca _x )Cu₂O_{7 ? δ}(0≤x≤1) were synthesized by solid-state reaction in N₂. The crystal structure was characterized by X-ray powder diffraction (XRD) as tetragonal. Direct current electrical resistance measurements were applied to check the existence of superconductivity in these cuprates. Superconductivity with T _c (onset) up to 82 K is observed when x = 0.6. Synthesis in N₂ is necessary in obtaining superconductivity in these cuprates. Preparation in air or post-treatment in flowing oxygen destroys superconductivity. A comparison is made with previous Pb-based 1212 superconducting oxides. The valence of Pb and the possible position of W in the lattice are discussed.     

Segregation in Pb1−xSnx Te during solidification by the Bridgman technique

The influence of the growth parameters on the concentration gradient of Pb_1–xSn_xTe monocrystals grown by the vertical Bridgman technique is described. The concentration profiles of all three constituents (Pb, Sn, Te) have been analysed in ingots having different Pb-Sn ratios and grown under different conditions. The presumed role played by free convection in the Pb-Sn ratio is shown. Te was found to be constant in all ingots and not dependent upon the growth conditions.     

Molecular beam epitaxy of Pb1 ? xEuxTe and Pb1 ? xSnxTe layers and related periodic structures

Layers and periodic Bragg structures based on Pb_{1 − x} Eu _x Te (0 < x < 1) and Pb_{1 − x} Sn _x Te (0 < x < 0.1) ternary solid solutions have been grown on (111) BaF₂ and (111) Si substrates by molecular beam epitaxy. X-ray diffraction measurements show that the layers in three-period EuTe/Pb_0.94Eu_0.06Te structures maintain the [111] crystallographic orientation normal to the substrate plane. In the interface plane, the [110] directions of the layers and substrate are parallel to each other. The full width at half maximum of the rocking curve of each layer of both the binary and ternary compounds is about 20′. The large optical contrast (40%) in such structures allowed us to reach 99.9% mid-IR reflectivity.     

Charge Disproportionation and Superconductivity in Tl- and In-doped Lead Telluride

The “negative-U model” is analyzed and applied to Tl- and In-doped lead telluride, Pb_1−x Tl _x Te, and Pb_1−x In _x Te, respectively. The former, but not the latter, is superconducting (T _c<1.5 K) in a certain concentration range (x _c>0.003). At very low concentrations, carriers are created in the valence band and the conductivity is metallic, of the ordinary kind. Matsushita et al. and Erickson et al. explain this as due to disproportionation 2Tl²⁺→Tl⁺ and Tl³⁺. The latter authors are of the opinion that indium also disproportionates in the same way as thallium. If this would be the case, it would be hard to explain the absence of superconductivity at indium doping. In this paper it is shown that In²⁺ does not disproportionate under the given circumstances. The Hubbard model, extended and generalized, is therefore consistent with all experimental results.     

Component distribution in A Pb1−xSnx Te seed grown by directional solidification

Anomalous behaviour of components in the first-to-freeze solid of Pb_1?xSn_x Te grown by vertical Bridgman technique has been observed. The segregation coefficient, which takes into account this anomalous behaviour, is calculated by X-Ray microanalysis measurements.     

Growth and applications of GeSn-related group-IV semiconductor materials

Abstract

We review the technology of Ge_1?xSn_x-related group-IV semiconductor materials for developing Si-based nanoelectronics. Ge_1?xSn_x-related materials provide novel engineering of the crystal growth, strain structure, and energy band alignment for realising various applications not only in electronics, but also in optoelectronics. We introduce our recent achievements in the crystal growth of Ge_1?xSn_x-related material thin films and the studies of the electronic properties of thin films, metals/Ge_1?xSn_x, and insulators/Ge_1?xSn_x interfaces. We also review recent studies related to the crystal growth, energy band engineering, and device applications of Ge_1?xSn_x-related materials, as well as the reported performances of electronic devices using Ge_1?xSn_x related materials.     

Band edge motion in quantizing magnetic field and nonequilibrium states in Pb1−x Sn x Te alloys doped with In

Galvanomagnetic and oscillation effects in Pb_1–x Sn _x Te single crystals doped with 0.5 at % In have been studied in magnetic fields up to 60 kOe at temperatures from 4.2 to 30 K under hydrostatic pressure up to 18 kbar. Beyond the ultraquantum magnetic field limit (H _uql) for the metallic state of Pb_1–x Sn _x Te(In) alloys, Fermi level pinning by high-density quasilocal states takes place. In a strong fieldH>H _uql the equationE _F = const is valid instead of the equationn = const which is usual for degenerate semiconductors (E _F is the electron or hole Fermi energy, andn is their concentration). This makes it possible to determine the direction of the band edge motion in the Pb_1–x Sn _x Te energy spectrum in a quantizing magnetic field in the direct and inverse spectral regions. It is found that the charge carrier transitions between quasilocal and band states are of anomalously long duration (10⁵ sec atT=4.2 K). By the application of a quantizing magnetic field we obtained a nonequilibrium metallic state of the system with a frozen or slowly diminishing Fermi surface. The characteristic time of the transition was found as a function of temperature and pressure. The relaxation kinetics of the nonequilibrium states induced by a quantizing magnetic field and infrared irradiation is discussed.     

Lead germanium telluride: a mechanically robust infrared high-index layer

A mechanically robust infrared high-index coating material is essential to the infrared interference coatings. Lead germanium telluride (Pb_1−x Ge _x Te) is a pseudo-binary alloy of IV–VI narrow gap semiconductors of PbTe and GeTe. In our investigation, the hardness and Young’s modulus of thin films of Pb_1−x Ge _x Te, which were deposited on silicon substrates using electron beam evaporation, were identified by means of nanoindentation measurement. It is demonstrated that layers of Pb_1−x Ge _x Te have greater hardness and Young’s modulus compared with those of PbTe. These mechanical behaviors of layers can be linked to a ferroelectric phase transition from a cubic paraelectric phase to a rhombohedral, ferroelectric phase. Moreover, the strength loss in the layers of Pb_1−x Ge _x Te can be also explained in light of strong localized elastic-strain fields in concentrated solid solutions. In addition, it is observed that layers of Pb_1−x Ge _x Te are highly transparent and refractive in the mid- and long-wave infrared spectral range (~3–40 μm). A conclusion can be drawn that a mechanically robust infrared high-index layer can be obtained using Pb_1−x Ge _x Te as starting materials.     

Pb and In Codeposition in the Vacuum Growth of PbTe<In> Films on Si Substrates

Pb_{1 – x} In _x melts were proposed to be used as In vapor sources, in combination with separate Pb and Te sources, in depositing PbTe films onto Si substrates by a modified hot-wall method. Under the assumption that the presence of Pb in Pb_{1 – x} In _x melts may raise the In partial pressure, the vaporization behavior of Pb_{1 – x} In _x (0.05 x 0.70) was studied between 900 and 1200 K in the reaction chamber of the deposition unit. Using electron probe x-ray microanalysis and x-ray diffraction, all the deposited films were shown to contain In. The In content of the Pb_{1 – y} In _y deposits varied in the range 0.002 <y < 0.07 and increased with increasing In concentration in the Pb_{1 – x} In _x melt and with increasing vapor source temperature. The vapor over molten Pb_{1 – x} In _x was shown to exhibit a positive deviation from ideality.     

Crystal pulling and constitution in Pb1−xSnxTe

Single crystals of Pb_1–x Sn _x Te have been pulled from the melt using the liquid encapsulation technique and the onset of constitutional supercooling as a function of growth rate has been investigated. From the analysis of crystals and their corresponding melts, a drastically revised phase diagram of the PbTe/SnTe pseudo-binary system has been constructed.Address: Chancery House, Chancery Lane, London WC2, UK