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 J max = (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 nextgeneration reconfigurable electronics relying on reproducible nanojunctions.

The functional diversification and adaptability of the elementary switching units of computational circuits are disruptive approaches for advancing electronics beyond the static capabilities of conventional complementary metal-oxide-semiconductor-based architectures. Thereto, in this work the onedimensional nature of monocrystalline and monolithic Al−Gebased nanowire heterostructures is exploited to deliver charge carrier polarity control and furthermore to enable distinct programmable negative differential resistance at runtime. The fusion of electron and hole conduction together with negative differential resistance in a universal adaptive transistor may enable energy-efficient reconfigurable circuits with multivalued operability that are inherent components of emerging artificial intelligence electronics.

and drain electrodes in highly scaled p-channel field-effect transistors (FETs) for the realization of very-large-scale integration (VLSI) systems. [1] Despite these efforts, the continuous scaling of metaloxide-semiconductor field-effect transistors (MOSFETs) is approaching physical limits where the nature of deterministic charge carrier separation between source and drain by an energy barrier is not applicable anymore. [2,3] In the quest of overcoming scaling limitations, new lines of research arose. Device research has shifted toward new architectures, materials, and technologies to enable "More than Moore" paradigms, [4] extending the mature Si complementary metal-oxidesemiconductor (CMOS) platform. [5] In this regard, Si 1−x Ge x and Ge active regions integrated on Si platforms are promising candidates for future optoelectronic devices [6] and the realization of energy efficient steep subthreshold switches such as band-to-band tunneling transistors (TFETs), [7,8] negative capacitance Ge nanowire FETs, [9,10] and positive feedback FETs. [11] Conventionally, degenerately doped semiconductor regions in combination with thin layers made of transition-metal semiconductor alloys, such as metalsilicides [12] and metal-germanides, [13] have been used to obtain ohmic contacts to most Si 1−x Ge x and Ge based devices. Toward the achievement of ohmic contacts, pinning-free metal semiconductor contacts have been explored in Si and Ge through Si 1−x Ge x is a key material in modern complementary metal-oxide-semiconductor and bipolar devices. However, despite considerable efforts in metal-silicide and -germanide compound material systems, reliability concerns have so far hindered the implementation of metal-Si 1−x Ge x junctions that are vital for diverse emerging "More than Moore" and quantum computing paradigms. In this respect, the systematic structural and electronic properties of Al-Si 1−x Ge x heterostructures, obtained from a thermally induced exchange between ultrathin Si 1−x Ge x nanosheets and Al layers are reported. Remarkably, no intermetallic phases are found after the exchange process. Instead, abrupt, flat, and void-free junctions of high structural quality can be obtained. Interestingly, ultra-thin interfacial Si layers are formed between the metal and Si 1−x Ge x segments, explaining the morphologic stability. Integrated into omega-gated Schottky barrier transistors with the channel length being defined by the selective transformation of Si 1−x Ge x into single-elementary Al leads, a detailed analysis of the transport is conducted. In this respect, a report on a highly versatile platform with Si 1−x Ge x composition-dependent properties ranging from highly transparent contacts to distinct Schottky barriers is provided. Most notably, the presented abrupt, robust, and reliable metal-Si 1−x Ge x junctions can open up new device implementations for different types of emerging nanoelectronic, optoelectronic, and quantum devices.

In the quest to push the contemporary scientific boundaries in nanoelectronics, Ge is considered a key building block extending device performances, delivering enhanced functionalities. In this work, a quasi‐1D monocrystalline and monolithic Al–Ge–Al nanowire heterostructure are embedded into a novel field‐effect transistor architecture capable of combining Ge based electronics with an electrostatically tunable negative differential resistance (NDR) distinctly observable at room temperature. In this regard, a detailed study of the key metrics of NDR in Ge is presented. Most notably, a highly efficient and low‐footprint platform is demonstrated, paving the way for potential applications such as fast switching multi‐valued logic devices, static memory cells, or high‐frequency oscillators, all implemented in one fully complementary metal–oxide–semiconductor compatible Al‐Ge based device platform.

No abstract