Non-Uniformly Strained Core-Shell InAs/InP Nanowires for Mid-Infrared Photonic Applications

Crystal phase and strain engineering in epitaxial nanowire (NW) heterostructures provide a widely tunable functionality for future nanoscale light emitters and photodetectors. Thus, InAs band gaps can be finely tuned in the mid -infrared range by introducing a mechanical strain through lattice mismatch in core-shell NWs. Here, we demonstrate that the inhomogeneity of the InP shell thickness leads to a non-uniform stress field and a local variation in the degree of surface passivation along the InAs NW length which both have a strong effect on the recombination mechanisms. Catalyst-free InAs NWs with a coherent nanometer thick InP shell were grown on Si(111) by means of molecular beam epitaxy. Temperature-dependent photoluminescence (PL) studies (5-150 K) allow us to distinguish the surface passivation and strain-induced effects. Non-trivial temperature dependence in contrast to the expected monotonic band gap shrinkage with temperature was found. At high temperatures (100-150 K), radiative recombination predominantly occurs in the NW regions, which have a thicker InP shell and, as a result, are free from surface states. In turn, the coherently grown InP shell induces a tensile strain in the InAs NW core and leads to a blue shift in an emission energy by a 45-50 meV at 100 K. In contrast, at low temperatures (<100 K), the PL band undergoes a red shift with decreasing temperature since photogenerated carriers are able to radiatively recombine at band gap energy minima at the unstrained NW regions. According to the performed ab initio calculations, observed emission is attributed to the interband optical transitions of the hexagonal InAs polytype with an intermediate band gap energy (435 meV at 5 K) lying between the zinc-blende and wurtzite values. Current observations are in a great demand for stress -induced band in the functional nanoheterostructures.