Synthesis and characterisation of hexagonal-2H germanium in nanowires

International audience; Our goal is to synthesize the hexagonal crystal phase Ge-2H which may offer a possible direct band-gap optical transition. We investigate two approaches. 1) Stress induced phase transformation. Ge nanowires with standard diamond structure (3C) undergo plastic deformation under external shear stress leading to a phase transformation toward the 2H-hexagonal phase and resulting in a quasi-periodic 3C/2H heterostructures along the -oriented NW axis. The transformation is dependent upon various key parameters: the diameter of the nanowires and the crystallographic direction, the stress that can be viewed as the driving force for the phase transformation, and the temperature. 2) Growth of GaAs/Ge core/shell structures. GaAs nanowires are used as template to epitaxially transfer the wurtzite structure to the Ge shell. The GaAs-w and Ge-2H structures take advantage to present almost the same lattice constants. GaAs nanowires were grown directly in a E-TEM microscope (NANOMAX) at 400°C using TMGa and TBAs precursors. By changing the III/V flux ratio, we manage to monitor in-situ the wurtzite/zing-blende polytytpism of the GaAs nanowire. Lateral overgrowth is then performed with Ge 2 H 6. We observe, in real time at the atomic scale, the epitaxial growth of Ge-2H. The dependence of the formation of intrinsic stacking faults on the growth conditions can be discussed. Finally, those heterostructured allotrope nanowires were characterized by various complementary methods such as Raman, NIR absorption, thermal conductivity measurements to gain insight into the basic fundamental properties of this promising Ge-2H structure.