Hoch-k/Metall-Gate Stacks auf Silicene (HKMG-Silicene)

After the breakthrough of the High-k + Metal Gate (HKMG) technology, the continuous scaling in Si based MOSFET devices suffers from insufficient electrostatic control. In the present 14 nm FinFET device node, the channel thickness of one Fin has been already reduced to 8 nm. It is not clear if for future generations beyond the 7 nm node the channel mobility will be limited due to scattering from imperfect Si surface boundaries, thickness non-uniformities, or if quantum confinements unfavourably increase the effective band-gap of the ultra-thin Si channel. The key idea here is to reach the ultimate limit of channel thickness by using a monolayer of Si atoms. While in the nearer term new semiconductors like Ge and GaAs will be integrated into high performance and low power consumption MOSFET channels, one of the most interesting materials in the longer term is silicene, which belongs to a new type of artificial 2D monolayer crystals with outstanding electronic properties. It has been shown that in silicene the Si atoms are arranged in a graphene-like honeycomb monolayer structure offering very high charge carrier mobilities comparable to those of graphene with the crucial advantage that there is evidence of a tunable bandgap. The project aims at a better understanding of interface layer formation in high-k oxide/silicene heterojunctions and to provide information about the stability of capped silicene undergoing a thermal or plasma assisted Atomic Layer Deposition (ALD) process. The main objectives of this project include i) the development of a stable process for the growth of silicene on single crystalline substrates, ii) the study of interactions of thin oxidizing metal capping layers with silicene, which are intended to stabilize the unique properties of silicene in a subsequent ALD process for high-k gate oxide deposition, and iii) the generation of key information on the electrical, physical, and chemical properties of the HKMG stack on silicene system by means of capacitors, field-effect devices, and various analyses techniques. Synthesis, stability, and interfacial reactions of silicene are planned to be characterized primarily in situ by means of advanced physical, chemical and electrical characterization. For this purpose a vacuum cluster tool will be used incorporating the surface analysis techniques Low Energy Electron Diffraction (LEED), X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), and Angle Resolved Photoelectron Spectroscopy (ARPES), as well as the deposition techniques ALD and electron beam (e-beam) evaporation. The use of ex situ Raman Spectroscopy and Scanning Capacitance Microscopy (SCM) is also projected in order to provide information on the thermal stability of silicene. In addition, Scanning Tunneling Microscopy (STM), Time-of-Flight Secondary Ion mass Spectroscopy (ToFSIMS), Grazing Incidence X-ray Diffraction (GIXRD), X-ray Diffraction (XRD), Atomic Force Microscopy (AFM), and High-Resolution Transmission Electron Microscopy (HR-TEM) are planned to be employed for further characterization. The simulation of interactions of silicene with atoms of oxide, metal, and nitride layers are also aspired in this project by using the Vienna ab-initio simulation package (VASP) providing valuable theoretical support.

The ongoing downscaling of the device dimensions in MOSFETs is aiming at cost reduction and simultaneous increase of device performance. After implementation of several breakthrough technologies, these days the Si channel material is considered as a limiting factor due to only fair electron and hole mobilities. The allotropic affinity for silicon and unique electronic properties make silicene a promising candidate for future high-performance devices compatible with mature CMOS technology. In silicene, the Si atoms are arranged in a graphene-like honeycomb structure offering very high charge carrier mobilities with the crucial advantage that there is evidence of a tunable bandgap. We investigated the properties of silicene grown via Molecular Beam Epitaxy on metallic substrates. Foremost we studied the stability of encapsulated layers since silicene instantly oxidizes when handled under atmospheric conditions and passivation is a prerequisite for integration into devices. We achieved the synthesis of various single and multilayer silicene superstructures on different substrates. For the optimization of synthesis and to investigate stability, silicene has been characterized primarily in-situ by means of advanced physicochemical analyzing techniques. For this purpose a vacuum cluster tool was adapted where the samples after MBE growth can be transferred to several surface analysis techniques without breaking the vacuum. In addition, we have developed a very innovative technique for in-situ passivation with few layers graphene/hBN enabling ex-vacuo Raman spectroscopy in order to provide information about the thermal stability of silicene. A further issue is that silicene's outstanding properties may not be preserved on particular growth templates, due to hybridization effects. Using the above mentioned in-situ passivation methodology with hBN enables detailed ex-situ characterization at ambient conditions via reflectance measurements. The optical properties of silicene on Au(111) appeared to be in accordance with the characteristics predicted theoretically for freestanding silicene. In order to take full advantage of the outstanding properties of silicene, it was further necessary to explore a scalable approach for an insulating layer that can be interfaced directly to silicene without perturbing its bidimensional nature. This layer should exhibit low leakage currents even when highly scaled, to fully exploit the advantages of using a 2D material at the core of e.g a MOSFET. CaF2 is known to form a quasi van der Waals interface with 2D materials, as well as to maintain its insulating properties even at ultrathin scales. We demonstrated that CaF2 grows epitaxially on silicene/Ag(111), with its domains fully aligned to the 2D lattice. Further in-situ XPS analysis evidenced that no changes in the chemical state of the silicon atoms can be detected upon CaF2 deposition. Finally polarized Raman analysis showed that silicene undergoes a structural change upon interaction with CaF2, however retaining the bidimensional character and without transitioning to sp3-hybridized, bulk-like silicon.