Controlling Hot Charge Carrier Transfer in Monolithic AlSiAl Heterostructures for Plasmonic On-Chip Energy Harvesting.

The generation of hot carriers by Landau damping or chemical interface damping of plasmons is of particular interest to the fundamental aspects of extreme light-matter interactions. Hot charge carriers can be transferred to an attached acceptor for photochemical or photovoltaic energy conversion. However, these lose their excess energy and relax to thermal equilibrium within picoseconds and it is difficult to extract useful work thereof with thermodynamic efficiencies that are of interest for practical devices. Without a detailed understanding of the underlying plasmon decay processes and transfer mechanisms, proper material matching and design considerations for novel plasmonic devices are extremely challenging. Here, a multifunctional AlSiAl heterostructure device with tunable Schottky barriers is presented to control plasmon-induced hot carrier injection at an abrupt metal-semiconductor interface. Light absorption, surface plasmon generation, and separation of hot carriers arising from the non-radiative decay of surface plasmons are realized in a monolithic Schottky barrier field effect transistor. Aside from barrier modulation, a virtual p-n junction can be emulated in the semiconductor channel with the distinct merit that carrier concentration and polarity are tunable by electrostatic gating. The investigations are carried out with a view to possible use for CMOS-compatible plasmonic photovoltaics, with versatile implementations for autonomous nanosystems.

Synthesis of Novel Phases in Si Nanowires Using Diamond Anvil Cells at High Pressures and Temperatures.

Silicon has several technologically promising allotropes that are formed via high-pressure synthesis. One of these phases (hd) has been predicted to have a direct band gap under tensile strain, whereas other (r8 and bc8) phases are predicted to have narrow band gaps and good absorption across the solar spectrum. Pure volumes of these phases cannot be made using conventional nanowire growth techniques. In this work, Si nanowires were compressed up to ∼20 GPa and then decompressed using a diamond anvil cell in the temperature range of 25-165 °C. It was found that at intermediate temperatures, near-phase-pure bc8-Si nanowires were produced, whereas amorphous Si (a-Si) dominated at lower temperatures, and a direct transformation to the diamond cubic phase (dc-Si) occurred at higher temperatures under compression. Thus this study has opened up a new pressure-temperature pathway for the synthesis of novel Si nanowires consisting of designed phase components with transformative properties.

Optical Signatures of Dirac Electrodynamics for hBN-Passivated Silicene on Au(111).

The allotropic affinity for bulk silicon and unique electronic and optical properties make silicene a promising candidate for future high-performance devices compatible with mature complementary metal-oxide-semiconductor technology. However, silicene's outstanding properties are not preserved on its most prominent growth templates, due to strong substrate interactions and hybridization effects. In this letter, we report the optical properties of silicene epitaxially grown on Au(111). A novel in situ passivation methodology with few-layer hexagonal boron nitride enables detailed ex situ characterization at ambient conditions via µ-Raman spectroscopy and reflectance measurements. The optical properties of silicene on Au(111) appeared to be in accordance with the characteristics predicted theoretically for freestanding silicene, allowing the conclusion that its prominent electronic properties are preserved. The absorption features are, however, modified by many-body effects induced by the Au substrate due to an increased screening of electron-hole interactions.

Plasmon-assisted polarization-sensitive photodetection with tunable polarity for integrated silicon photonic communication systems.

To establish high-bandwidth chip-to-chip interconnects in optoelectronic integrated circuits, requires high-performance photon emitters and signal receiving components. Regarding the photodetector, fast device concepts like Schottky junction devices, large carrier mobility materials and shrinking the channel length will enable higher operation speed. However, integrating photodetectors in highly scaled ICs technologies is challenging due to the efficiency-speed trade-off. Here, we report a scalable and CMOS-compatible approach for an ultra-scaled germanium (Ge) based photodetector with tunable polarity. The photodetector is composed of a Ge Schottky barrier field effect transistor with monolithic aluminum (Al) source/drain contacts, offering plasmon assisted and polarization-resolved photodetection. The ultra-scaled Ge photodetector with a channel length of only 200 nm shows high responsivity of aboutR = 424 A W-1and a maximum polarization sensitivity ratio of TM/TE = 11.

In Situ Transmission Electron Microscopy Analysis of Copper-Germanium Nanowire Solid-State Reaction.

A promising approach of making high quality contacts on semiconductors is a silicidation (for silicon) or germanidation (for germanium) annealing process, where the metal enters the semiconductor and creates a low resistance intermetallic phase. In a nanowire, this process allows one to fabricate axial heterostructures with dimensions depending only on the control and understanding of the thermally induced solid-state reaction. In this work, we present the first observation of both germanium and copper diffusion in opposite directions during the solid-state reaction of Cu contacts on Ge nanowires using in situ Joule heating in a transmission electron microscope. The in situ observations allow us to follow the reaction in real time with nanometer spatial resolution. We follow the advancement of the reaction interface over time, which gives precious information on the kinetics of this reaction. We combine the kinetic study with ex situ characterization using model-based energy dispersive X-ray spectroscopy (EDX) indicating that both Ge and Cu diffuse at the surface of the created Cu3Ge segment and the reaction rate is limited by Ge surface diffusion at temperatures between 360 and 600 °C. During the reaction, germanide crystals typically protrude from the reacted NW part. However, their formation can be avoided using a shell around the initial Ge NW. Ha direct Joule heating experiments show slower reaction speeds indicating that the reaction can be initiated at lower temperatures. Moreover, they allow combining electrical measurements and heating in a single contacting scheme, rendering the Cu-Ge NW system promising for applications where very abrupt contacts and a perfectly controlled size of the semiconducting region is required. Clearly, in situ TEM is a powerful technique to better understand the reaction kinetics and mechanism of metal-semiconductor phase formation.

In Situ Transmission Electron Microscopy Analysis of Aluminum-Germanium Nanowire Solid-State Reaction.

To fully exploit the potential of semiconducting nanowires for devices, high quality electrical contacts are of paramount importance. This work presents a detailed in situ transmission electron microscopy (TEM) study of a very promising type of NW contact where aluminum metal enters the germanium semiconducting nanowire to form an extremely abrupt and clean axial metal-semiconductor interface. We study this solid-state reaction between the aluminum contact and germanium nanowire in situ in the TEM using two different local heating methods. Following the reaction interface of the intrusion of Al in the Ge nanowire shows that at temperatures between 250 and 330 °C the position of the interface as a function of time is well fitted by a square root function, indicating that the reaction rate is limited by a diffusion process. Combining both chemical analysis and electron diffraction we find that the Ge of the nanowire core is completely exchanged by the entering Al atoms that form a monocrystalline nanowire with the usual face-centered cubic structure of Al, where the nanowire dimensions are inherited from the initial Ge nanowire. Model-based chemical mapping by energy dispersive X-ray spectroscopy (EDX) characterization reveals the three-dimensional chemical cross-section of the transformed nanowire with an Al core, surrounded by a thin pure Ge (∼2 nm), Al2O3 (∼3 nm), and Ge containing Al2O3 (∼1 nm) layer, respectively. The presence of Ge containing shells around the Al core indicates that Ge diffuses back into the metal reservoir by surface diffusion, which was confirmed by the detection of Ge atoms in the Al metal line by EDX analysis. Fitting a diffusion equation to the kinetic data allows the extraction of the diffusion coefficient at two different temperatures, which shows a good agreement with diffusion coefficients from literature for self-diffusion of Al.

Strain engineering of core-shell silicon carbide nanowires for mechanical and piezoresistive characterizations.

This study evaluated the mechanical properties and piezoresistivity of core-shell silicon carbide nanowires (C/S-SiCNWs) synthesized by a vapor-liquid-solid technique, which are a promising material for harsh environmental micro electromechanical systems (MEMS) applications. The C/S-SiCNWs were composed of a crystalline cubic (3C) SiC core wrapped by an amorphous silicon dioxide (SiO x ) shell; however, TEM observations of the NWs showed that hexagonal polytypes (2H, 4H , and 6H) were partially induced in the core by a stacking fault owing to a Shockley partial dislocation. The stress-strain relationship of the C/S-SiCNWs and SiC cores without an SiO x shell was examined using MEMS-based nanotensile tests. The tensile strengths of the C/S-SiCNWs and SiC cores were 7.0 GPa and 22.4 GPa on average, respectively. The lower strength of the C/S-SiCNWs could be attributed to the SiO x shell with the surface roughness as the breaking point. The Young's modulus of the C/S-SiCNWs was 247.2 GPa on average, whereas that of the SiC cores had a large value with scatter data ranging from 450 to 580 GPa. The geometrical model of the SiC core based on the transmission electron microscopy observations rationalized this scatter data by the volume content of the stacking fault in the core. The piezoresistive effects of the C/S-SiCNW and SiC core were also evaluated from the I-V characteristics under uniaxial tensile strain. The gauge factor of -30.7 at 0.008 ε for the C/S-SiCNW was approximately two-times larger than that of -15.8 at 0.01 ε for the SiC core. This could be caused by an increase of the surface state density at the SiO x /SiC interface owing to the positive fixed oxide charge of the SiO x shell.

Ultrascaled Germanium Nanowires for Highly Sensitive Photodetection at the Quantum Ballistic Limit.

We report an experimental study on quasi-one-dimensional Al-Ge-Al nanowire (NW) heterostructures featuring unmatched photoconductive gains exceeding 107 and responsivities as high as 10 A/µW in the visible wavelength regime. Our observations are attributed to the presence of GeO x related hole-trapping states at the NW surface and can be described by a photogating effect in accordance with previous studies on low-dimensional nanostructures. Utilizing an ultrascaled photodetector device operating in the quantum ballistic transport regime at room temperature we demonstrate for the first time that individual current channels can be addressed directly by laser irradiation. The resulting quantization of the photocurrent represents the ultimate limit of photodetectors, allowing for advanced concepts including highly resolved imaging, light effect transistors and single photon detectors with practically zero off-state current.

Room-Temperature Quantum Ballistic Transport in Monolithic Ultrascaled Al-Ge-Al Nanowire Heterostructures.

Conductance quantization at room temperature is a key requirement for the utilizing of ballistic transport for, e.g., high-performance, low-power dissipating transistors operating at the upper limit of "on"-state conductance or multivalued logic gates. So far, studying conductance quantization has been restricted to high-mobility materials at ultralow temperatures and requires sophisticated nanostructure formation techniques and precise lithography for contact formation. Utilizing a thermally induced exchange reaction between single-crystalline Ge nanowires and Al pads, we achieved monolithic Al-Ge-Al NW heterostructures with ultrasmall Ge segments contacted by self-aligned quasi one-dimensional crystalline Al leads. By integration in electrostatically modulated back-gated field-effect transistors, we demonstrate the first experimental observation of room temperature quantum ballistic transport in Ge, favorable for integration in complementary metal-oxide-semiconductor platform technology.

Dynamical Tuning of Nanowire Lasing Spectra.

Realizing visionary concepts of integrated photonic circuits, nanospectroscopy, and nanosensing will tremendously benefit from dynamically tunable coherent light sources with lateral dimensions on the subwavelength scale. Therefore, we demonstrate an individual nanowire laser based device which can be gradually tuned by reversible length changes of the nanowire such that uniaxial tensile stress is applied to the respective semiconductor gain material. By straining the device, the spontaneous excitonic emission of the nanowire shifts to lower energies caused by the bandgap reduction of the semiconductor. Moreover, the optical gain spectrum of the nanolaser can be precisely strain-tuned in the high excitation regime. The tuning of the emission does not affect the laser threshold of the device, which is very beneficial for practical applications. The applied length change furthermore adjusts the laser resonances inducing a redshift of the longitudinal modes. Thus, this concept of gradually and dynamically tunable nanolasers enables controlling and modulating the coherent emission on the nanoscale without changing macroscopic ambient conditions. This concept holds therefore huge impact on nanophotonic switches and photonic circuit technology.

Synthesis, Morphological, and Electro-optical Characterizations of Metal/Semiconductor Nanowire Heterostructures.

In this letter, we demonstrate the formation of unique Ga/GaAs/Si nanowire heterostructures, which were successfully implemented in nanoscale light-emitting devices with visible room temperature electroluminescence. Based on our recent approach for the integration of InAs/Si heterostructures into Si nanowires by ion implantation and flash lamp annealing, we developed a routine that has proven to be suitable for the monolithic integration of GaAs nanocrystallite segments into the core of silicon nanowires. The formation of a Ga segment adjacent to longer GaAs nanocrystallites resulted in Schottky-diode-like I/V characteristics with distinct electroluminescence originating from the GaAs nanocrystallite for the nanowire device operated in the reverse breakdown regime. The observed electroluminescence was ascribed to radiative band-to-band recombinations resulting in distinct emission peaks and a low contribution due to intraband transition, which were also observed under forward bias. Simulations of the obtained nanowire heterostructure confirmed the proposed impact ionization process responsible for hot carrier luminescence. This approach may enable a new route for on-chip photonic devices used for light emission or detection purposes.

Electrical transport properties of single-crystal Al nanowires.

Single-crystal Al nanowires (NWs) were fabricated by thermally induced substitution of vapor-liquid-solid grown Ge NWs by Al. The resistivity of the crystalline Al (c-Al) NWs was determined to be ρ = (131 ± 27) × 10(-9) Ω m, i.e. approximately five times higher than for bulk Al, but they withstand remarkably high current densities of up to 1.78 × 10(12) A m(-2) before they ultimately melt due to Joule heating. The maximum current density before failure correlates with the NW diameter, with thinner NWs tolerating significantly higher current densities due to efficient heat dissipation and the reduced lattice heating in structures smaller than the electron-phonon scattering length. The outstanding current-carrying capacity of the c-Al NWs clearly exceeds those of common conductors and surpasses requirements for metallization of future high-performance devices. The linear temperature coefficient of the resistance of c-Al NWs appeared to be lower than for bulk Al and a transition to a superconducting state in c-Al NWs was observed at a temperature of 1.46 K.

Strain distribution in single, suspended germanium nanowires studied using nanofocused x-rays.

Within the quest for direct band-gap group IV materials, strain engineering in germanium is one promising route. We present a study of the strain distribution in single, suspended germanium nanowires using nanofocused synchrotron radiation. Evaluating the probed Bragg reflection for different illumination positions along the nanowire length results in corresponding strain components as well as the nanowire's tilting and bending. By using these findings we determined the complete strain state with the help of finite element modelling. The resulting information provides us with the possibility of evaluating the validity of the strain investigations following from Raman scattering experiments which are based on the assumption of purely uniaxial strain.

Origin of anomalous piezoresistive effects in VLS grown Si nanowires.

Although the various effects of strain on silicon are subject of intensive research since the 1950s the physical background of anomalous piezoresistive effects in Si nanowires (NWs) is still under debate. Recent investigations concur in that due to the high surface-to-volume ratio extrinsic surface related effects superimpose the intrinsic piezoresistive properties of nanostructures. To clarify this interplay of piezoresistive effects and stress related surface potential modifications, we explored a particular tensile straining device (TSD) with a monolithic embedded vapor-liquid-solid (VLS) grown Si NW. Integrating the suspended NW in a gate all around (GAA) field effect transistor (FET) configuration with a transparent gate stack enables optical and field modulated electrical characterization under high uniaxial tensile strain applied along the ⟨111⟩ Si NW growth direction. A model based on stress-induced carrier mobility change and surface charge modulation is proposed to interpret the actual piezoresistive behavior of Si NWs. By controlling the nature and density of surface states via passivation the "true" piezoresistance of the NWs is found to be comparable with that of bulk Si. This demonstrates the indispensability of application-specific NW surface conditioning and the modulation capability of Si NWs properties for sensor applications.

Gate-Tunable Electron Transport Phenomena in Al-Ge⟨111⟩-Al Nanowire Heterostructures.

Electrostatically tunable negative differential resistance (NDR) is demonstrated in monolithic metal-semiconductor-metal (Al-Ge-Al) nanowire (NW) heterostructures integrated in back-gated field-effect transistors (FETs). Unambiguous signatures of NDR even at room temperature are attributed to intervalley electron transfer. At yet higher electric fields, impact ionization leads to an exponential increase of the current in the ⟨111⟩ oriented Ge NW segments. Modulation of the transfer rates, manifested as a large tunability of the peak-to-valley ratio (PVR) and the onset of impact ionization is achieved by the combined influences of electrostatic gating, geometric confinement, and heterojunction shape on hot electron transfer and by electron-electron scattering rates that can be altered by varying the charge carrier concentration in the NW FETs.

Anomalous Plastic Deformation and Sputtering of Ion Irradiated Silicon Nanowires.

Silicon nanowires of various diameters were irradiated with 100 keV and 300 keV Ar(+) ions on a rotatable and heatable stage. Irradiation at elevated temperatures above 300 °C retains the geometry of the nanostructure and sputtering can be gauged accurately. The diameter dependence of the sputtering shows a maximum if the ion range matches the nanowire diameter, which is in good agreement with Monte Carlo simulations based on binary collisions. Nanowires irradiated at room temperature, however, amorphize and deform plastically. So far, plastic deformation has not been observed in bulk silicon at such low ion energies. The magnitude and direction of the deformation is independent of the ion-beam direction and cannot be explained with mass-transport in a binary collision cascade but only by collective movement of atoms in the collision cascade with the given boundary conditions of a high surface to volume ratio.

Multimode silicon nanowire transistors.

The combined capabilities of both a nonplanar design and nonconventional carrier injection mechanisms are subject to recent scientific investigations to overcome the limitations of silicon metal oxide semiconductor field effect transistors. In this Letter, we present a multimode field effect transistors device using silicon nanowires that feature an axial n-type/intrinsic doping junction. A heterostructural device design is achieved by employing a self-aligned nickel-silicide source contact. The polymorph operation of the dual-gate device enabling the configuration of one p- and two n-type transistor modes is demonstrated. Not only the type but also the carrier injection mode can be altered by appropriate biasing of the two gate terminals or by inverting the drain bias. With a combined band-to-band and Schottky tunneling mechanism, in p-type mode a subthreshold swing as low as 143 mV/dec and an ON/OFF ratio of up to 10(4) is found. As the device operates in forward bias, a nonconventional tunneling transistor is realized, enabling an effective suppression of ambipolarity. Depending on the drain bias, two different n-type modes are distinguishable. The carrier injection is dominated by thermionic emission in forward bias with a maximum ON/OFF ratio of up to 10(7) whereas in reverse bias a Schottky tunneling mechanism dominates the carrier transport.

Electrostatic actuated strain engineering in monolithically integrated VLS grown silicon nanowires.

In this paper we demonstrate the fabrication and application of an electrostatic actuated tensile straining test (EATEST) device enabling strain engineering in individual suspended nanowires (NWs). Contrary to previously reported approaches, this special setup guarantees the application of pure uniaxial tensile strain with no shear component of the stress while e.g. simultaneously measuring the resistance change of the NW. To demonstrate the potential of this approach we investigated the piezoresistivity of about 3 µm long and 100 nm thick SiNWs but in the same way one can think about the application of such a device on other geometries, other materials beyond Si as well as the use of other characterization techniques beyond electrical measurements. Therefore single-crystal SiNWs were monolithically integrated in a comb drive actuated MEMS device based on a silicon-on-insulator (SOI) wafer using the vapor-liquid-solid (VLS) growth technique. Strain values were verified by a precise measurement of the NW elongation with scanning electron microscopy (SEM). Further we employed confocal µ-Raman microscopy for in situ, high spatial resolution measurements of the strain in individual SiNWs during electrical characterization. A giant piezoresistive effect was observed, resulting in a fivefold increase in conductivity for 3% uniaxially strained SiNWs. As the EATEST approach can be easily integrated into an existing Si technology platform this architecture may pave the way toward a new generation of nonconventional devices by leveraging the strain degree of freedom.

Peculiarities of temperature dependent ion beam sputtering and channeling of crystalline bismuth.

In this paper, we report on the surface evolution of focused ion beam treated single crystalline Bi(001) with respect to different beam incidence angles and channeling effects. 'Erosive' sputtering appears to be the dominant mechanism at room temperature (RT) and diffusion processes during sputtering seem to play only a minor role for the surface evolution of Bi. The sputtering yield of Bi(001) shows anomalous behavior when increasing the beam incidence angle along particular azimuthal angles of the specimen. The behavior of the sputtering yield could be related to channeling effects and the relevant channeling directions are identified. Dynamic annealing processes during ion irradiation retain the crystalline quality of the Bi specimen allowing ion channeling at RT. Lowering the specimen temperature to T = -188 °C reduces dynamic annealing processes and thereby disables channeling effects. Furthermore unexpected features are observed at normal beam incidence angle. Spike-like features appear during the ion beam induced erosion, whose growth directions are not determined by the ion beam but by the channeling directions of the Bi specimen.

Electronic Transport Modulation in Ultrastrained Silicon Nanowire Devices.

In this work, we explore the effect of ultrahigh tensile strain on electrical transport properties of silicon. By integrating vapor-liquid-solid-grown nanowires into a micromechanical straining device, we demonstrate uniaxial tensile strain levels up to 9.5%. Thereby the triply degenerated phonon dispersion relation at the Γ-point of silicon disentangle and the longitudinal phonon modes are used to precisely determine the extent of mechanical strain. Simultaneous electrical transport measurements showed a significant enhancement in the electrical conductance. Aside from considerable reduction of the Si bulk resistivity due to strain-induced band gap narrowing, comparison with quasi-particle GW calculations further reveals that the effective Schottky barrier height at the electrical contacts undergoes a substantial reduction. For these reasons, nanowire devices with ultrastrained channels may be promising candidates for future applications of high-performance silicon-based devices.