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 midinfrared 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. Nontrivial 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 stressinduced band gap tuning in the functional nanoheterostructures.
One of the promising areas of research applicable in astrophysics is the study of the properties of quantum dots (QDs) and quantum wells (QWs) in the IR region of the spectrum. The IR region of the spectrum contains a huge amount of astronomical information. Devices based on GaAs/AlGaAs QWs and Ge/Si QDs can serve as detectors and sources of IR radiation. In the current work, the photoluminescence, absorption, and transmission spectra of these nanoheterostructures were obtained in the near, mid, and far IR ranges using Fourier spectroscopy. Thus, the studied optical properties of QWs and QDs make it possible to use these structures in astrophysics as sources and detectors of IR radiation.
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