Emerging Nanoelectronic Devices

About the lab

In the quest to push the contemporary scientific boundaries in nanoelectronics, the Weber group is focusing on a "More than Moore" approach extending device performances beyond the limits imposed by transistor miniaturization, enabling next generation energy efficient reconfigurable integrated circuits, targeting low supply voltages and a reduction of transistor count. Moreover, novel devices that fuse computing with non-volatile memory functionality are being conceived and advanced towards circuit enablement.

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Featured projects (3)

The scope of this project is to study photonic and plasmonic effects of monolithic Al-Ge based metal-semiconductor heterostructures.
The scope of this project is to study electrical transport phenomena in Ge nanostructures embedded in metal-semiconductor heterostructures.
The goal of this project is the systematic investigation of metal-semiconductor-metal heterostructures for reconfigurable transistors. Further, following a “More than Moore" approach extending device performances beyond the limits imposed by transistor miniaturization, these new reconfigurable transistors will be investigated and benchmarked for next generation energy efficient reconfigurable integrated circuits.

Featured research (11)

Overcoming the difficulty in the precise definition of the metal phase of metal−Si heterostructures is among the key prerequisites to enable reproducible next-generation nanoelectronic, optoelectronic, and quantum devices. Here, we report on the formation of monolithic Al−Si heterostructures obtained from both bottom-up and top-down fabricated Si nanostructures and Al contacts. This is enabled by a thermally induced Al−Si exchange reaction, which forms abrupt and void-free metal−semiconductor interfaces in contrast to their bulk counterparts. The selective and controllable transformation of Si NWs into Al provides a nanodevice fabrication platform with high-quality monolithic and single-crystalline Al contacts, revealing resistivities as low as ρ = (6.31 ± 1.17) × 10^−8 Ω m and breakdown current densities of Jmax = (1 ± 0.13) × 10^12 Ω m^−2. Combining transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the composition as well as the crystalline nature of the presented Al−Si−Al heterostructures, with no intermetallic phases formed during the exchange process in contrast to state-of-the-art metal silicides. The thereof formed single-element Al contacts explain the robustness and reproducibility of the junctions. Detailed and systematic electrical characterizations carried out on back-and top-gated heterostructure devices revealed symmetric effective Schottky barriers for electrons and holes. Most importantly, fulfilling compatibility with modern complementary metal−oxide semiconductor fabrication, the proposed thermally induced Al−Si exchange reaction may give rise to the development of next-generation reconfigurable electronics relying on reproducible nanojunctions.
Group-IV based nanodevices are an active area of research for CMOS-compatible photonic components in the visible and near-infrared region, covering the C-band optical communication range. Especially, nanowires (NWs) have gained significant interest, due to their inherent nanocylinder resonator shape, allowing light trapping in circulating orbits by multiple total internal reflections from the periphery. Importantly, leaky resonant modes in NWs provide an antenna functionality that enhances their performance, especially when embedded in metal-semiconductor heterostructures. Here, we advance a fundamental step beyond those concepts and demonstrate a Ge NW photodetector with switchable photo-conductance, effective dark-current suppression and remarkably high polarization sensitivity. In particular, our highly sensitive Ge NW photodetector is capable of suppressing the dark current by a paramount factor of 100 by unprecedentedly using the negative differential resistance (NDR) electronic transport regime, significantly enhancing the performance of Ge photodetectors. Most notably, the NDR regime further enables a bias-switchable positive (PPC) and negative (NPC) photo-conductance with relative symmetric photoconductive gains of gNPC = -1.7 x 10^5 and gPPC = 4.4 x 10^5 at λ = 532 nm. Further, investigating the polarization sensitivity in the valley region of the NDR, the Ge photodetector revealed a remarkably high TM/TE ratio of 33. Importantly, utilizing our Al-Ge-Al heterostructures with reliable and reproducible contacts as an advanced material system with full CMOS compatibility for photonic applications, the proposed bias-switchable Ge photodetector platform may pave the way for innovative optoelectronic devices including compact light tunable memories, or light effect transistors.
High-quality electrical contacts are of utmost importance for nanoscale devices as they have a large impact on their electrical performance, reliability and reproducibility. Nanowires (NWs) are of particular interest to ultra-scaled transistors because of their enhanced suppression of short channel effects. Especially, a deterministic top-down NW processing has brought inroads towards implementation into mature CMOS technology. In this regard, we investigate contact formation in SixGe1-x NWs with different Ge contents and further systematically compare results from both top-down [1] and bottom-up processed NWs, the former are formed by patterning epitaxial grown SixGe1-x layers on SOI and the latter by VLS growth. Specifically, we apply the thermally induced Al-SiGe exchange in SixGe1-x nanostructures, resulting in monolithic metal semiconductor heterostructures with highly transparent Al-SixGe1-x interfaces [2], without the formation of intermetallic phases. By implementing SixGe1-x NWs with different stochiometric compositions Schottky barrier field effect transistors (SBFETs) with tunable band structure as well as contact properties are studied. The corresponding electrical behavior of the fabricated devices, such as Schottky barrier height, on/off currents, subthreshold slope and electrostatic gating capabilities can be extracted and optimized for specific applications. Further, pulsed I-V measurements were conducted to investigate dynamic effects of charge carrier trapping at different time scales. TEM and EDX are performed for a detailed study of the interface configuration and elemental composition of the formed metal-semiconductor heterostructure after the exchange reaction and are correlated to electrical data. To complement the results parallel arrays of epitaxial SixGe1-x NWs embedded in metal-semiconductor-metal heterostructures (Fig. 1) are used to enable reconfigurable field-effect transistors (RFETs) to deliver controllable charge carrier polarity at runtime [3]. Figure 1 (left)
Information and communication technology has become ubiquitous in everyday life. Emerging distributed computing paradigms such as the Internet of Things are demanding the implementation of novel electronic device functionalities that go beyond the capabilities of conventional field effect transistors. In this context, nanometer scale Ge departs from its bulk counterpart and delivers unique electronic transport mechanisms that can be exploited at the device level. Thereto, a highly interesting transport mechanism is the transferred electron effect, enabling negative differential resistance (NDR). This effect is triggered by the application of high electric fields forcing a scattering of electrons from the energetically favorable conduction band valley, characterized by a low effective mass, to a heavy mass valley nearby. Despite a vast body of pioneering work, the practical use of NDR is still restricted to expensive GaAs and GaN semiconductors. Here, we exploit the nanometer scale properties of Ge nanowires with unique monocrystalline Al contacts to deliver a strong and reproducible NDR effect at room temperature. Our monolithic Al-Ge-Al nanowire heterostructures embedded into field-effect transistor architectures are capable of combining doping-free Ge based electronics with an electrostatically tunable NDR. In this regard, we support our results with a detailed study of the key parameters of NDR. Most notably, we demonstrate a highly efficient and low-footprint platform paving the way for potential applications such as fast switching multi-valued logic devices, static memory cells, and high-frequency oscillators all implemented in one fully CMOS compatible Al-Ge based device platform. We believe that our investigations provide a significant step towards a beyond CMOS approach enabling functional diversification and alternative computing for the post-Si era by exploiting the unique band structure of nanometer scale Ge.
Metal-semiconductor-metal heterostructures are an attractive platform for both fundamental studies of low-dimensional nanostructures as well as future high-performance low power dissipating nanoelectronic and quantum devices. Most notably, they provide enormous potential for a vast array of key components for quantum computing such as SQUIDs, oscillators, mixers and amplifiers. Related to its inherently strong spin-orbit coupling and the ability to host superconducting pairing correlations, Ge is emerging as a versatile material to realize devices capable of encoding, processing, or transmitting quantum information. Here, we demonstrate 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 these metal−semiconductor nanowire heterostructures. Integrating such 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 channels. Conducting low temperature (400 mK) DC spectroscopy measurements, we report highly gate-tunable hole transport from a completely insulating regime, through a low conductive regime, that exhibits properties of a single-hole transistor, to a superconducting regime, resembling a Josephson field-effect transistor. The experimental proof of exchanging cooper-pairs between superconducting Al leads through a gate-tunable Ge channel, resulting from the superconducting proximity effect, is the first demonstration of superconductivity induced in an intrinsic Ge channel. The realization of a Josephson field-effect transistor with a high junction transparency in the supercurrent regime allows us to study the sub-gap transport mediated by Andreev states. The presented results establish Ge quantum dots monolithically embedded in Al-Ge-Al nanowire heterostructures as a highly promising platform for hybrid superconductor-semiconductor devices for the study of Majorana zero modes and key components of quantum computing such as gatemons or gate tunable SQUIDS.

Lab head

Walter M. Weber
  • Institute of Solid State Electronics
About Walter M. Weber
  • In the quest to push the contemporary scientific boundaries in nanoelectronics, the Weber group is focusing on a "More than Moore" approach extending device performances beyond the limits imposed by transistor miniaturization, enabling next generation energy efficient reconfigurable integrated circuits, targeting low supply voltages and a reduction of transistor count. Moreover, novel devices that fuse computing with non-volatile memory functionality are being conceived.