ZnSe nanowires by molecular beam epitaxy: growth mechanisms and properties

2013/2014This thesis is devoted to the study of the growth of Zinc Selenide (ZnSe) nanowires (NWs) by Au-assisted molecular beam epitaxy (MBE). The growth process consists in many steps which were individually investigated by means of in-situ and ex-situ spectroscopic and microscopic techniques. First, the formation of nanoparticles upon annealing of a thin Au film deposited on different substrates was studied by X-ray photoemission spectroscopy, grazing incident X-ray diffraction and scanning electron microscopy. The nanoparticles were used as seeds for the 1-dimensional ZnSe crystal growth by MBE through evaporation of Zn and Se from elemental solid sources. The obtained NWs were characterized by scanning and transmission electron microscopy and their optical properties were assessed by means of photoluminescence and cathodoluminescence measurements. This systematic investigation approach allowed us to understand the NWs growth mechanism and, as a consequence, to obtain the control over the NWs properties. Indeed, it was found that an interplay between substrate, seed particles and beam fluxes takes place and strongly affects the NWs growth mode. In particular, a chemical interaction between substrate and Au may occur during the annealing, changing chemical composition and physical state of the nanoparticles before the NWs growth. The vapour composition, i.e. the Zn-to-Se beam pressure ratio, can also modify the nanoparticles composition and the NWs growth mechanism. Therefore, by changing the growth conditions, it was possible to grow ZnSe NWs through different mechanisms, with important consequences on their properties, in terms of morphology, crystal quality and optical properties. Understanding the growth mechanism and its effects on the wires properties allowed us to achieve the control over the growth process and the selective growth of ZnSe nanowires with the desired properties. Vertically oriented ZnSe NWs with a defect-free hexagonal crystal structure were obtained on GaAs(111)B substrates, having either Au-Ga alloy nanoparticles or Au nanocrystals on their tips. Uniformly thin and straight blue-emitting ZnSe NWs were also grown on various substrates, after optimizing gold film thickness, annealing and growth temperature. The possible integration of such nanostructores in novel nanodevices was proposed and preliminary demonstrated.XXVII Ciclo198

Surface Nano-Patterning for the Bottom-Up Growth of III-V Semiconductor Nanowire Ordered Arrays

Ordered arrays of vertically aligned semiconductor nanowires are regarded as promising candidates for the realization of all-dielectric metamaterials, artificial electromagnetic materials, whose properties can be engineered to enable new functions and enhanced device performances with respect to naturally existing materials. In this review we account for the recent progresses in substrate nanopatterning methods, strategies and approaches that overall constitute the preliminary step towards the bottom-up growth of arrays of vertically aligned semiconductor nanowires with a controlled location, size and morphology of each nanowire. While we focus specifically on III-V semiconductor nanowires, several concepts, mechanisms and conclusions reported in the manuscript can be invoked and are valid also for different nanowire materials

A Double Quantum Dot Spin Valve

A most fundamental and longstanding goal in spintronics is to electrically tune highly efficient spin injectors and detectors, preferably compatible with nanoscale electronics. Here, we demonstrate all these points using semiconductor quantum dots (QDs), individually spin-polarized by ferromagnetic split-gates (FSGs). As a proof of principle, we fabricated a double QD spin valve consisting of two weakly coupled semiconducting QDs in an InAs nanowire (NW), each with independent FSGs that can be magnetized in parallel or anti-parallel. In tunneling magnetoresistance (TMR) experiments at zero external magnetic field, we find a strongly reduced spin valve conductance for the two anti-parallel configurations, with a single QD polarization of $\sim 27\%$. The TMR can be significantly improved by a small external field and optimized gate voltages, which results in a continuously electrically tunable TMR between $+80\%$ and $-90\%$. A simple model quantitatively reproduces all our findings, suggesting a gate tunable QD polarization of $\pm 80\%$. Such versatile spin-polarized QDs are suitable for various applications, for example in spin projection and correlation experiments in a large variety of nanoelectronics experiments

Understanding the Morphological Evolution of InSb Nanoflags Synthesized in Regular Arrays by Chemical Beam Epitaxy

InSb nanoflags are grown by chemical beam epitaxy in regular arrays on top of Au-catalyzed InP nanowires synthesized on patterned SiO2/InP(111)B substrates. Two-dimensional geometry of the nanoflags is achieved by stopping the substrate rotation in the step of the InSb growth. Evolution of the nanoflag length, thickness and width with the growth time is studied for different pitches (distances in one of the two directions of the substrate plane). A model is presented which explains the observed non-linear time dependence of the nanoflag length, saturation of their thickness and gradual increase in the width by the shadowing effect for re-emitted Sb flux. These results might be useful for morphological control of InSb and other III-V nanoflags grown in regular arrays

InAs nanowire superconducting tunnel junctions: spectroscopy, thermometry and nanorefrigeration

We demonstrate an original method -- based on controlled oxidation -- to create high-quality tunnel junctions between superconducting Al reservoirs and InAs semiconductor nanowires. We show clean tunnel characteristics with a current suppression by over $4$ orders of magnitude for a junction bias well below the Al gap $\Delta_0 \approx 200\,\mu {\rm eV}$. The experimental data are in close agreement with the BCS theoretical expectations of a superconducting tunnel junction. The studied devices combine small-scale tunnel contacts working as thermometers as well as larger electrodes that provide a proof-of-principle active {\em cooling} of the electron distribution in the nanowire. A peak refrigeration of about $\delta T = 10\,{\rm mK}$ is achieved at a bath temperature $T_{bath}\approx250-350\,{\rm mK}$ in our prototype devices. This method opens important perspectives for the investigation of thermoelectric effects in semiconductor nanostructures and for nanoscale refrigeration.Comment: 6 pages, 4 color figure

Heat-Driven Iontronic Nanotransistors

Thermoelectric polyelectrolytes are emerging as ideal material platform for self-powered bio-compatible electronic devices and sensors. However, despite the nanoscale nature of the ionic thermodiffusion processes underlying thermoelectric efficiency boost in polyelectrolytes, to date no evidence for direct probing of ionic diffusion on its relevant length and time scale has been reported. This gap is bridged by developing heat-driven hybrid nanotransistors based on InAs nanowires embedded in thermally biased Na+-functionalized (poly)ethyleneoxide, where the semiconducting nanostructure acts as a nanoscale probe sensitive to the local arrangement of the ionic species. The impact of ionic thermoelectric gating on the nanodevice electrical response is addressed, investigating the effect of device architecture, bias configuration and frequency of the heat stimulus, and inferring optimal conditions for the heat-driven nanotransistor operation. Microscopic quantities of the polyelectrolyte such as the ionic diffusion coefficient are extracted from the analysis of hysteretic behaviors rising in the nanodevices. The reported experimental platform enables simultaneously the ionic thermodiffusion and nanoscale resolution, providing a framework for direct estimation of polyelectrolytes microscopic parameters. This may open new routes for heat-driven nanoelectronic applications and boost the rational design of next-generation polymer-based thermoelectric materials

Gate-controlled supercurrent in ballistic InSb nanoflag Josephson junctions

High-quality III-V narrow bandgap semiconductor materials with strong spin-orbit coupling and large Landé g-factor provide a promising platform for next-generation applications in the field of high-speed electronics, spintronics, and quantum computing. Indium antimonide (InSb) offers a narrow bandgap, high carrier mobility, and small effective mass and, thus, is very appealing in this context. In fact, this material has attracted tremendous attention in recent years for the implementation of topological superconducting states supporting Majorana zero modes. However, high-quality heteroepitaxial two-dimensional (2D) InSb layers are very difficult to realize owing to the large lattice mismatch with all commonly available semiconductor substrates. An alternative pathway is the growth of free-standing single-crystalline 2D InSb nanostructures, the so-called nanoflags. Here, we demonstrate fabrication of ballistic Josephson-junction devices based on InSb nanoflags with Ti/Nb contacts that show a gate-tunable proximity-induced supercurrent up to 50 nA at 250 mK and a sizable excess current. The devices show clear signatures of subharmonic gap structures, indicating phase-coherent transport in the junction and a high transparency of the interfaces. This places InSb nanoflags in the spotlight as a versatile and convenient 2D platform for advanced quantum technologies

Temperature behavior and logic circuit applications of InAs nanowire-based field-effect transistors

InAs nanowire-based back-gated field-effect transistors realized starting from individual InAs nanowires are investigated at different temperatures and as building blocks of inverter circuits for logic applications. The nanodevices show n-type behavior with a carrier concentration up to 8.0 × 1017 cm−3 and corresponding electron mobility exceeding 1590 and 1940 cm2 V−1 s−1 at room temperature and 200 K, respectively. The investigation over a wide temperature range indicates no Schottky barrier at source/drain electrodes, where Ohmic contacts are formed with the Cr adhesion layer. The switching characteristics of the devices improve with decreasing temperature and a subthreshold swing less than 1 V/decade is achieved at 200 K, suggesting the occurrence of a trap population with density around 4 × 108 cm−1 eV−1. Besides, the nanodevices are exploited in single-transistor circuits with a resistive load. As an inverter, the circuit shows 30 % and 24 % of the voltage supply noise margins for the high and low states, respectively; as a low signal amplifier, it shows a gain that is weakly dependent on temperature. The present study highlights the impact of temperature on the operation of InAs nanowire-based back-gated transistors and evidences their potential applications in logic circuits including inverters and low-signal amplifiers

Highly symmetric and tunable tunnel couplings in InAs/InP nanowire heterostructure quantum dots

We present a comprehensive electrical characterization of an InAs/InP nanowire heterostructure, comprising two InP barriers forming a quantum dot (QD), two adjacent lead segments (LSs) and two metallic contacts, and demonstrate how to extract valuable quantitative information of the QD. The QD shows very regular Coulomb blockade (CB) resonances over a large gate voltage range. By analyzing the resonance line shapes, we map the evolution of the tunnel couplings from the few to the many electron regime, with electrically tunable tunnel couplings from 600 ${\mu}$eV, and a transition from the temperature to the lifetime broadened regime. The InP segments form tunnel barriers with almost fully symmetric tunnel couplings and a barrier height of ~350 meV. All of these findings can be understood in great detail based on the deterministic material composition and geometry. Our results demonstrate that integrated InAs/InP QDs provide a promising platform for electron tunneling spectroscopy in InAs nanowires, which can readily be contacted by a variety of superconducting materials to investigate subgap states in proximitized NW regions, or be used to characterize thermoelectric nanoscale devices in the quantum regime.Comment: 8 pages, 4 figure