Epitaxy of Hexagonal Ge-2H : Lessons from in Situ TEM Observations

Silicon and Germanium crystallize in the cubic diamond structure 3C with which they dominate definitely the electronics. Playing on crystal phases in semiconductors occurs to be a valuable mean of electronic band engineering. Remarkably, the hexagonal 2H phase of SiGe turns to get a direct band gap with light emission capabilities within a specific composition range (Si<35%). This material holds the promise to fill the gap between electronics and photonics industry using group IV semiconductors. We use GaAs NWs with the wurtzite structure as a template to create both (1) core/shell and (2) trunk/nanobranches heterostructures [2,3]. The GaAs-wurtzite is an ideal template to copy the structure by epitaxy forcing the Ge to adopt the hexagonal crystal phase and in turn it is used here as a textbook case The growths are followed using the in situ TEM NANOMAX. This unique microscope can be implemented either with molecular beam epitaxy (MBE) sources or with a gas injector for chemical vapor deposition (CVD). Real time TEM observations at the atomic scale show the fundamental aspects of the epitaxy and the formation of growth-related staking faults in Ge-2H. 1) On core/shell configuration [1] , depending on growth conditions, different growth regimes highly impact the crystal quality of the of Ge-2H shell. On {1-100} prismatic surfaces, a regular step flow supports a flat surface and a perfect replication of the hexagonal structure However, when the step flow is destabilized, original intrinsic I 3 basal stacking faults (BSF) are formed during growth. We evidence their correlation with the growth modes related to surface diffusion. Understanding the nucleation of these defects is necessary to prevent their formation during epitaxy. Possible scenarios of I 3 BSF formation are discussed. Theses defects show a faulted stacking ABACABAB with only one faulted basal plane bounded by a pair of partial dislocations along <11-20> with opposite Burger vectors b=+/-1/3<1-100>. Thermal annealing induces a motion of the dislocation and an expansion of the I 3 BSF resulting in a 4H stacking in the core/shell structure. 2) On wurtzite GaAs nanowires, Au catalysts are deposited on the sidewalls. Nanobranches grow with an axial direction along <1-100> and exhibit a hexagonal crystal structure [2] . With in situ observations, we study the VLS and VSS growths of Au catalyzed Ge-2H branches depending on the growth temperature. [1] L. Vincent et al. Adv. Mat. Inter. 9-16 (2022) 2102340, doi.org/10.1002/admi.202102340 [2] A Li et al. Nanotechnology 34 (2023) 015601 doi: 10.1088/1361-6528/ac9317 Figure 1