Growth monitoring with sub-monolayer sensitivity via real time thermal conductance measurements

Growth monitoring during the early stages of film formation is of prime importance to understand the growth process, the microstructure and thus the overall layer properties. In this work, we demonstrate that phonons can be used as sensitive probes to monitor real time evolution of film microstructure during growth, from incipient clustering to continuous film formation. For that purpose, a silicon nitride membrane-based sensor has been fabricated to measure in-plane thermal conductivity of thin film samples. Operating with the 3{\omega}-V\"olklein method at low frequencies, the sensor shows an exceptional resolution down to {\Delta}({\kappa}*t)=0.065 nm*W/(m*K), enabling accurate measurements. Validation of the sensor performance is done with organic and metallic thin films. In both cases, at early stages of growth, we observe an initial reduction of the effective thermal conductance of the supporting amorphous membrane, K, related with the surface phonon scattering enhanced by the incipient nanoclusters formation. As clusters develop, K reaches a minimum at the percolation threshold. Subsequent island percolation produces a sharp increase of the conductance and once the surface coverage is completed K increases linearly with thickness The thermal conductivity of the deposited films is obtained from the variation of K with thickness

The 60 GHz IMPATT diode development

The objective is to develop 60 GHz IMPATT diodes suitable for communications applications. The performance goals of the 60 GHz IMPATT is 1W CW output power with a conversion efficiency of 15 percent and 10-year lifetime. The final design of the 60 GHz IMPATT structure evolved from computer simulations performed at the University of Michigan. The initial doping profile, involving a hybrid double-drift (HDD) design, was derived from a drift-diffusion model that used the static velocity-field characteristics for GaAs. Unfortunately, the model did not consider the effects of velocity undershoot and delay of the avalanche process due to energy relaxation. Consequently, the initial devices were oscillating at a much lower frequency than anticipated. With a revised simulation program that included the two effects given above, a second HDD profile was generated and was used as a basis for fabrication efforts. In the area of device fabrication, significant progress was made in epitaxial growth and characterization, wafer processing, and die assembly. The organo-metallic chemical vapor deposition (OMCVD) was used. Starting with a baseline X-Band IMPATT technology, appropriate processing steps were modified to satisfy the device requirements at V-Band. In terms of efficiency and reliability, the device requirements dictate a reduction in its series resistance and thermal resistance values. Qualitatively, researchers were able to reduce the diodes' series resistance by reducing the thickness of the N+ GaAs substrate used in its fabrication

Quantum properties of atomic-sized conductors

Using remarkably simple experimental techniques it is possible to gently break a metallic contact and thus form conducting nanowires. During the last stages of the pulling a neck-shaped wire connects the two electrodes, the diameter of which is reduced to single atom upon further stretching. For some metals it is even possible to form a chain of individual atoms in this fashion. Although the atomic structure of contacts can be quite complicated, as soon as the weakest point is reduced to just a single atom the complexity is removed. The properties of the contact are then dominantly determined by the nature of this atom. This has allowed for quantitative comparison of theory and experiment for many properties, and atomic contacts have proven to form a rich test-bed for concepts from mesoscopic physics. Properties investigated include multiple Andreev reflection, shot noise, conductance quantization, conductance fluctuations, and dynamical Coulomb blockade. In addition, pronounced quantum effects show up in the mechanical properties of the contacts, as seen in the force and cohesion energy of the nanowires. We review this reseach, which has been performed mainly during the past decade, and we discuss the results in the context of related developments.Comment: Review, 120 pages, 98 figures. In view of the file size figures have been compressed. A higher-resolution version can be found at: http://lions1.leidenuniv.nl/wwwhome/ruitenbe/review/QPASC-hr-ps-v2.zip (5.6MB zip PostScript

Crossing the ballistic-ohmic transition via high energy electron irradiation

P.H.M. and M.D.B. received PhD studentship support from the UK Engineering and Physical Science Research Council via Grant No. EP/L015110/1. C.P. and P.J.W.M. are supported by the European Research Council under the European Union's Horizon 2020 research and innovation programme (Microstructured Topological Materials Grant No. 715730). E. Z. acknowledges support from the International Max Planck Research School for Chemistry and Physics of Quantum Materials (IMPRS-CPQM). Irradiation experiments performed on the SIRIUS platform were supported by the French National Network of Accelerators for Irradiation and Analysis of Molecules and Materials (EMIR&A) under Project No. EMIR 2019 18-7099.The delafossite metal PtCoO2 is among the highest-purity materials known, with low-temperature mean free path up to 5 μm in the best as-grown single crystals. It exhibits a strongly faceted, nearly hexagonal Fermi surface. This property has profound consequences for nonlocal transport within this material, such as in the classic ballistic-regime measurement of bend resistance in mesoscopic squares. Here, we report the results of experiments in which high-energy electron irradiation was used to introduce pointlike disorder into such squares, reducing the mean free path and therefore the strength of the ballistic-regime transport phenomena. We demonstrate that high-energy electron irradiation is a well-controlled technique to cross from nonlocal to local transport behavior and therefore determine the nature and extent of unconventional transport regimes. Using this technique, we confirm the origins of the directional ballistic effects observed in delafossite metals and demonstrate how the strongly faceted Fermi surface both leads to unconventional transport behavior and enhances the length scale over which such effects are important. © 2023 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Publisher PDFPeer reviewe

Synthesis and Transport Properties of Topological Crystalline Insulator SnTe Nanowires

Over the last decade, significant progress has been made in studying topological materials whose wavefunctions possess a distinct topological invariant signature barring adiabatic deformation from a trivial phase to a non-trivial phase. There has been mounting experimental evidence for the presence of topological nature in nanomaterials due to their favorable surface-to-volume ratio and phase-coherent confinement. Considering that the material synthesis and transport measurement challenges must be overcome before topological nanomaterials can be used in next-generation electronic devices, in my dissertation, I focus on improving crystal quality and controlling dimensions of topological crystalline insulator SnTe in nanoscale as it provides a rich playground to examine interactions of correlated electronic states, such as ferroelectricity, topological surface states, and superconductivity. To develop facile strategies to suppress surface defects during chemical vapor deposition growth of SnTe nanostructures, we systematically investigate the origin and evolution of three-dimensional surface defects commonly observed on SnTe microcrystals and nanostructures. By employing alloy nanoparticles as growth catalyst, SnTe nanowires are synthesized with reduced diameters and high crystalline quality, such that one-dimensional confinement and phase coherence of the topological surface electrons can be ensured to probe the topological surface states. To further alleviate the high carrier density inside the bulk of SnTe nanowires and surface degradation, surface passivation of SnTe nanowires using in situ Te deposition during growth process is investigated. The material improvement approach in this dissertation aims to facilitate future research on understanding the extent of scattering of topological surface states due to crystalline defects, impurities, and coupling to bulk electron states

Biasing Plasmonic Nanocavities

Molecular electronics promises a new generation of ultralow-energy information technologies, based around functional molecular junctions where two electrodes are bridged by just a few molecules. How molecules exactly behave in a real junction however is still not well understood, since interactions with the electrode materials and neighbouring molecules at the nanoscale is difficult to model and probe experimentally. Many studies so far have characterised in detail the electrical response of molecular junctions by statistical analysis of many repeated measurements, but without monitoring in real time the behaviour of individual devices. The main difficulty of in situ measurements is the absence of characterisation tools that can provide direct access to the dynamics of nm-sized junctions during device operation. This thesis explores two novel approaches used to create optically accessible nanoscale junctions. One method is based on graphene electrodes, used to fabricate extended junctions with a metal oxide spacer. These graphene junctions are found to behave as memristive devices, where a solid state redox reaction releases gas from the oxide spacer that remains trapped under the graphene, resulting in an actuating mechanism. Displacement of the surface layers can be optically tracked in real time using metallic nanoparticles deposited on the sample, whose plasmonic coupling with the bottom electrode is modulated by the actuation mechanism. The second method used to construct nanoscale junctions is more appropriate for molecular junctions, and is based on electrical contacting of individual metallic nanoparticles with a conductive transparent cantilever. Nanoparticles are deposited on a molecular monolayer and represent one electrode of a molecular junction, and at the same time allow to optically probe the junction itself by enabling plasmonic confinement of light to volumes <100nm$^3$. Darkfield and Raman spectroscopy are performed on single nanoparticle junctions in real time while voltage is applied, and a modulation of the optical response with voltage is observed, revealing that molecules undergo conformational changes during device operation

Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials

Confining materials to two-dimensional forms changes the behavior of electrons and enables new devices. However, most materials are challenging to produce as uniform thin crystals. Here, we present a new synthesis approach where crystals are grown in a nanoscale mold defined by atomically-flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mold made of hexagonal boron nitride (hBN), we grow ultraflat bismuth crystals less than 10 nanometers thick. Due to quantum confinement, the bismuth bulk states are gapped, isolating intrinsic Rashba surface states for transport studies. The vdW-molded bismuth shows exceptional electronic transport, enabling the observation of Shubnikov-de Haas quantum oscillations originating from the (111) surface state Landau levels, which have eluded previous studies. By measuring the gate-dependent magnetoresistance, we observe multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. Our vdW-mold growth technique establishes a platform for electronic studies and control of bismuth's Rashba surface states and topological boundary modes. Beyond bismuth, the vdW-molding approach provides a low-cost way to synthesize ultrathin crystals and directly integrate them into a vdW heterostructure