Composition Dependent Electrical Transport in Si_1−xGe_xNanosheets with Monolithic Single‐Elementary Al Contacts

Abstract

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.

Electrical and Structural Properties of Si1−xGex Nanowires Prepared from a Single-Source Precursor

“…Applying a bias voltage of V D = 250 mV, a constant transfer characteristic was observed for the NWs with an Au shell, which screens the electric field of the back-gate, resulting in no gate tunability. On the other hand, the NWs without an Au shell showed typical charge carrier modulation capabilities, i.e., a slightly ambipolar characteristic with dominant hole conduction, as expected for an intrinsic Si 1− x Ge x semiconductor with Al contacts [ 15 , 69 ]. To further map the gate-tunable transport through the proposed NWs, Figure 4 c,d show color plots of the recorded current density depending on the applied bias- and gate-voltage.…”

Reconfigurable Field‐Effect Transistor Technology via Heterogeneous Integration of SiGe with Crystalline Al Contacts

“…This channel itself is terminated by two {111} facets bordering the Si interlayer. [ 31 ] We have to note that the metal‐semiconductor exchange dynamics leading to the formation of the Al‐Si‐SiGe multi‐heterojunction is not entirely clarified. However, the remarkably differing diffusion coefficients of Si and Ge in Al might play an important role as the Ge diffusion is significantly faster, which might promote the formation of the Si pile‐up region.…”

“…Based on a systematic analysis of Si x Ge 1 −x with varying stoichiometric compositions carried out in Ref. [31], the ideal Ge content for delivering the envisioned equal injection capabilities for both electrons and holes was estimated to be 33% in this work. Importantly, using the proposed Si to Ge ratio, the relevant advantages of Ge (i.e., higher on‐currents and higher switching speeds) are still accessible.…”

“…This observation is a further indication of tunneling through a thin barrier, which is determining the transport, rather than thermionic emission. [31] Such highly transparent junctions are only seldomly observed in Schottky contacts with intrinsic or lowly doped semiconductors, for example, in carbon nanotubes with high work-function Pd contacts [43,44] or for the efficient use of thin Si 3 N 4 interface layers to Si with Fermi-level depinning properties. [45,46] Importantly, the fact that the proposed SiGebased RFET shows stable unipolar n-and p-type operation up to temperatures of T = 400 K implies that static n-and p-type FETs as conventionally used in a CMOS circuit can be replaced, principally enabling deliberate logic functions in a complementary design.…”

Atomistic Insights into Ultrafast SiGe Nanoprocessing

“…In this way, it not only overcomes the limits of purely continuum-based tools but also overcomes those of other hybrid FEM-KMC approaches, which either lack self-consistent information exchange between the two frameworks or are limited to defect-free LA simulations of silicon without any superlattice formulation . In particular, we demonstrate the method by focusing on ultraviolet nanosecond-pulsed-LA processes of SiGe, an alloy with composition-dependent electronic and optical properties − increasingly relevant to future nanoelectronic, ,,− thermoelectronic, optoelectronic, ,, and quantum technologies. ,− The multiscale methodology provides unique atomistic insights into the complex and ultrafast morphological, compositional, and structural transformations of SiGe during laser irradiation, ,,, giving invaluable support to process engineers aiming at the exploitation of this material’s full potential.…”