What hexagonal-2H Si and Ge can offer? – HEXSIGE

HEXSIGE project departs from our pioneered observation of shear-induced phase transformation in Si and Ge nanowires (NWs). The transformation results in quasi-periodic heterostructures constituted of standard cubic-diamond-3C and hexagonal-2H allotropes along the nanowire. This unprecedented nanostructure may open new properties and functionalities to be explored.

HEXSIGE project is a fundamental scientific project with a twofold objective:
(1) Investigating the size-related mechanical properties, identifying the driving force of the transformation and understanding the mechanisms of phase transformation in Si and Ge NWs. This study must enable the optimisation of the process for the synthesis of 3C/2H heterostructures.
(2) Providing an exhaustive knowledge of the basic physical properties of Ge-2H and Si-2H structures and on the resulting heterostructured 3C/2H NWs in order to identify their potential applications. All the aspects of electronic, optical and vibrational properties will be investigated by means of a thorough experimental approach associated to theoretical calculations. Among other parameters, particular attention will be paid to the determination of the band gap of the 2H phases still lacking in literature. We also intend to assess the data of electronic and thermal conductivities in heterostructured NWs.

The project benefits from a well-established know-how on the growth and processing of NWs at the C2N and on a recognized expertise of the five partners (C2N, SOLEIL-synchrotron, LPS, INL, LOMA) in nanomaterial characterizations based on the development of next-generation tools. The objectives and the success of the project rely on the complementarity and the specificity of a wide range of advanced characterization methods.
IR spectroscopy will allow to probe the optical and electronic properties of Ge-2H and Si-2H (by measuring the free carrier absorption of light in the mid and near IR) and the vibrational properties (by measuring phonon absorption in the far-IR). The future EQUIPEX CHROMATEM microscope will make possible the measurement in a nanostructure of the expected bad gap values by electron energy loss spectrometry (EELS) with the required energy and spatial resolutions and the determination of the direct/indirect character of the gaps. Finally, low and room temperature photoluminescence will be used to study the conduction band offset.
We will deploy electrical, thermal and thermo-electrical characterizations to evaluate the relationship between structural and transport properties of the heterostructured NWs. For the thermoelectric measurements, a unique tool will be developed to measure the thermoelectric properties of the NWs. This development meets the needs of the thermoelectric community and is likely to be very useful in a wide range of applications in nanoelectronics and organic electronics.
Our pioneered discovery of phase transformation in Si and Ge NWs has opened an exciting field of research. HEXSIGE project is intended to develop further this thematic in France to maintain its leadership in this field. The novel heterostructured 2H/3CNWs are expected to offer a broad range of scientific and nanotechnological spin-offs. We aim at demonstrating the functionalities of these NWs for more than Moore applications.