Reversible Al Propagation in Si x Ge 1-x Nanowires

While reversibility is a fundamental concept in thermodynamics, most reactions are not readily reversible, especially in solid state physics. For example, thermal diffusion is a widely known concept, used among others to inject dopant atoms into the substitutional positions in the matrix and improve the device properties. Typically, such a diffusion process will create a concentration gradient extending over increasingly large regions, without possibility to reverse this effect. On the other hand, while the bottom up growth of semiconducting nanowires is interesting, it can still be difficult to fabricate axial heterostructures with high control. In this paper, we report a reversible thermal diffusion process occurring in the solid-state exchange reaction between an Al metal pad and a SixGe1-x alloy nanowire observed by in-situ transmission electron microscopy. The thermally assisted reaction results in the creation of a Si-rich region sandwiched between the reacted Al and unreacted SixGe1-x part, forming an axial Al/Si/SixGe1-x heterostructure. Upon heating or (slow) cooling, the Al metal can repeatably move in and out of the SixGe1-x alloy nanowire while maintaining the rod-like geometry and crystallinity, allowing to fabricate and contact nanowire heterostructures in a reversible way in a single process step, compatible with current Si based technology. This interesting system is promising for various applications, such as phase change memories in an all crystalline system with integrated contacts, as well as Si/SixGe1-x/Si heterostructures for near-infrared sensing applications

Ballistische Transportphänomene in Al-Ge-Al NW Heterostrukturen

Zusammenfassung in deutscher SpracheAbweichender Titel nach Übersetzung der Verfasserin/des VerfassersSo far, continuous miniaturization of classical planar electronic devices has been the main driving force behind the advancement of modern integrated circuit technology. However, due to physical limits and dramatic repercussions of short channel effects, scaling becomes increasingly difficult. Hence, a shift towards the adoption of new materials and novel design architectures is predicted to insure further improvement of integration densities, power dissipation and performance. Semiconductor nanowires (NWs) are predicted to be one of the most promising building blocks for future ultra-scaled high-speed microelectronics. Out of the wide range of NWs, germanium (Ge) occupies an exceptional position, because it combines a high carrier mobility, enabling high performance devices, with a more than five times larger exciton Bohr radius compared to silicon (Si). Hence, Ge is especially interesting for the development of novel quantum devices. Dedicated to the small feature sizes required, until now it was not possible to show ballistic transport in group IV semiconductor NW based devices at temperatures above 20mK. The scope of the diploma thesis at hand was to synthesize axial Al-Ge-Al NW heterostructures with abrupt interfaces and monocrystalline aluminum (Al) leads. This was achieved by using a thermally initiated exchange reaction between vapor-liquid-solid (VLS) grown single-crystalline Ge NWs and Al contact pads. Applying rapid thermal annealing (RTA) for the formation of Al-Ge-Al NW heterostructures is one of the key advantages of the fabrication strategy, because it enables the formation of a Ge nanodot without requiring precise lithographic alignment of the contacts, which is one of the most challenging issues of fabricating nanodot based devices. The capability to control the size of the Ge segment connected by two Schottky tunnel barriers was achieved by fine tuning of the process parameters. Thus, it was possible to fabricate Al-Ge-Al NW heterostructures featuring ultrashort Ge segments down to 10 nm, which can be operated as back-gated field-effect transistors (FETs). Based on NW heterostructures with ultrasmall Ge segments, a systematic investigation of ballistic transport phenomena was carried out by electrical characterizations at room temperature as well as cryogenic temperatures down to 5K. In order to allow interpretation and to gain a better insight into the measurement results of the conducted transport measurements, the diameter dependence of quantum confinement effects in Ge NWs was investigated by simulations based on the 2D Schrödinger equation. By comparing the experimental data and simulation results, evidence of ballistic transport for Al-Ge-Al NW heterostructures with Ge segment lengths varying between 10nm and 30nm at room-temperature is presented.9

Elektrische Transportuntersuchungen an monolithischen Al-Ge-Al Nanodrahtheterostrukturen

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersMetall-Halbleiter-Metall Heterostrukturen sind für grundlegende Untersuchungen von niedrigdimensionalen Nanostrukturen und der Erforschung zukünftiger hochleistungsfähiger Nanoelektronik- und Quantenbauelemente attraktiv. Insbesondere bieten sie ein enormes Potenzial für eine Vielzahl von Schlüsselkomponenten von Quantencomputern wie beispielsweise SQUIDs, Oszillatoren, Mischern und Verstärkern. In diesem Zusammenhang nimmt Ge, durch die Kombination hoher Ladungsträgermobilitäten und ausgeprägterer Quanten-Confinement-Effekte, eine Sonderstellung für die Entwicklung neuartiger Quantenbauteile in der Post-Si-Ära ein. In der vorliegenden Arbeit wird gezeigt, dass unter Verwendung einer thermisch induzierten Austauschreaktion zwischen einkristallinen Ge-Nanodrähten und Al-Kontakten, Al-Ge-Al Nanodraht-Heterostrukturen mit ultra-kurzen Ge-Segmenten und kristallinen quasi-1D Al-Zuleitungen, ohne lithografische Einschränkungen, hergestellt werden können. Hochauflösende Transmissionselektronenmikroskopie und energiedispersive Röntgenspektroskopie belegten dabei die perfekte Kristallinität aller Komponenten der Nanodraht-Heterostruktur. Durch die Integration ultrakurzer Ge Segmente als aktive Kanäle in elektrostatisch gesteuerten Feldeffekttransistoren wurde eine Plattform für die systematische Untersuchung elektrischer Transportmechanismen in ultra-skalierten Ge-Nanodrähten geschaffen. Im Gegensatz zu herkömmlichen Kurzkanalbauelementen verhindert die quasi-1D Metall-Halbleiter-Metall Architektur eine Abschirmung des elektrischen Feldes der Gate-Elektrode durch die Anschlusskontakte und ermöglicht somit eine perfekte elektrostatische Steuerung von ultra-skalierten Ge-Kanälen. Basierend auf diesen Strukturen wurden ballistische und quantenballistische Transportphänomene in Abhängigkeit der Kanallänge und dem Nanodrahtdurchmesser untersucht. Temperaturabhängige Messungen des spezifischen Widerstands und Gate-abhängige Leitfähigkeitsmessungen im Bereich zwischen 5K und 300K, haben einen quantisierten Stromtransport durch einzelne quasi-1D Subbänder nachgewiesen. Darüber hinaus, wurden die Transporteffekte im Temperaturbereich von 10K bis 2K untersucht. Hier war es möglich zu zeigen, dass ein kurzer Ge-Kanal die Charakteristik eines Single-Hole Transistors aufweist. Systematische Untersuchungen des Tunnelns einzelner Löcher ergaben ein Multi-Quantum-Dot System innerhalb des Ge-Segments, welches sich über die Gate-Spannung in ein Single-Quantum-Dot System überführen ließ. Des Weiteren, wurde das Temperaturregime von 1.5K bis 400mK untersucht, in dem ein ultra-skalierter Ge-Kanal, der an supraleitende Al-Zuleitungen gekoppelt ist, einem Josephson-Feldeffekttransistor entspricht. Hier konnte der experimentelle Nachweis für den Austauschs von Cooper-Paaren zwischen den supraleitenden Al-Kontakten durch den Gate-kontrollierbaren Ge-Kanal basierend auf dem supraleitenden Proximity-Effekt erbracht werden. Die Transportmessungen ergaben dabei einen einstellbaren kritischen Superstrom im Ge-Quantenpunkt bis zu ca. 20 nA.Metal-semiconductor-metal heterostructures are attractive for both fundamental studies of low-dimensional nanostructures as well as for future high-performance low power dissipating nanoelectronic and quantum devices. Most notably, they bear enormous potential for a vast array of key components for quantum computing such as SQUIDs, oscillators, mixers and amplifiers. In this context, combining high carrier mobilities and leveraging strong quantum confinement effects due to a more than five times larger exciton Bohr radius compared to Si, Ge occupies an exceptional position for the development of novel quantum devices in the post Si era. Within this thesis, it is shown that utilizing a thermally induced exchange reaction between single-crystalline Ge nanowires and Al pads, monolithic Al-Ge-Al nanowire heterostructures with ultra-small Ge segments contacted by self-aligned, quasi-1D, crystalline Al leads can be fabricated without lithographic constraints. High-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy proved the composition and perfect crystallinity of the entire nanowire heterostructure. Integrating such Al-Ge-Al nanowire heterostructures as active channels in electrostatically gated field-effect transistor devices, provides a platform for the systematic investigation of electrical transport mechanisms in ultra-scaled Ge nanowires. In contrast to common short channel devices, the 1D monolithic metal-semiconductormetal architecture effectively prevents the screening of the gate electric field by lithographically defined contacts and thus enables perfect electrostatic control of ultra-scaled Ge channels. Based on these structures, ballistic transport as well as quantum ballistic transport phenomena were systematically investigated depending on the Ge channel length and nanowire diameter. Resistivity and gate-dependent conductance measurements as well as detailed bias spectroscopy studies in the temperature range between 5K and 300K revealed a current transport through spindegenerate 1D sub-bands in ultra-scaled Ge channels up to room temperature. The second part of the thesis is dedicated to the transport effects in the temperature range from 10K to 2K, where a small Ge channel reveals the characteristic of a single hole-transistor. Systematic investigations of single-hole tunnelling through Ge quantum dots revealed a complex charge trapping related multi-dot system near the pinch-off gate-voltage that evolves into a single-dot regime. The third set of experiments investigated the temperature regime from 1.5K to 400mK, where an ultra-scaled Ge channel coupled to superconducting Al leads, reassembles a Josephson field-effect transistor. The experimental proof of exchanging Cooper pairs between the superconducting Al leads and a gate-tunable Ge channel, mediated by the superconducting proximity effect enabled the first demonstration of superconductivity induced in a pure Ge channel. Gate-dependent transport measurements revealed a tunable critical supercurrent in the Ge quantum dot from zero to approximately 20 nA.12

Reliably straining suspended van der Waals heterostructures

2D materials provide a rapidly expanding platform for the observation of novel physical phenomena and for the realization of cuttingedge optoelectronic devices. In addition to their peculiar individual characteristics, 2D materials can be stacked into complex van der Waals heterostructures, greatly expanding their potential. Moreover, thanks to their excellent stretchability, strain can be used as a powerful control knob to tune or boost many of their properties. Here, we present a novel method to reliably and repeatedly apply a high uniaxial tensile strain to suspended van der Waals heterostructures. The reported device is engineered starting from a silicon-on-insulator substrate, allowing for the realization of suspended silicon beams that can amplify the applied strain. The strain module functionality is demonstrated using singleand double-layer graphene layers stacked with a multilayered hexagonal boron nitride flake. The heterostructures can be uniaxially strained, respectively, up to ∼1.2% and ∼1.8%

Nanometer-Scale Ge-Based Adaptable Transistors Providing Programmable Negative Differential Resistance Enabling Multivalued Logic

International audienc

Solution-based low-temperature synthesis of germanium nanorods and nanowires

The Ga-assisted formation of Ge nanorods and nanowires in solution has been demonstrated and a catalytic activity of the Ga seeds was observed. The synthesis of anisotropic single-crystalline Ge nanostructures was achieved at temperatures as low as 170 °C. Gallium not only serves as nucleation seed but is also incorporated in the Ge nanowires in higher concentrations than its thermodynamic solubility limit

Drastic Changes in Material Composition and Electrical Properties of Gallium-Seeded Germanium Nanowires

The final publication is available via https://doi.org/10.1021/acs.cgd.9b00210.Varying the growth conditions of gallium-seeded germanium nanostructures leads to significant variations in morphology, and particularly of the electronic properties inducing a transition from the hyperdoping regime to intrinsic germanium crystal formation. The consumption of the growth seed leads to cone-type nanostructures with incorporation of ∼3.8 ± 0.7 at% Ga in Ge at temperatures below 350 °C, while a high density of Ge nanowires with constant diameter can be obtained at higher temperatures. The high-temperature Ga-seeded Ge nanowires exhibit electronic properties of intrinsic Ge nanowires.Austrian Science Funds (FWF)Deutsche Forschungsgemeinschaft (DFG