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

Abstract

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.

Growth, Crystal Structure, and Photoluminescent Properties of Dilute Nitride InAsN Nanowires on Silicon for Infrared Optoelectronics

“…As reported in refs , , self-induced grown InAs NWs possess a high stacking fault density along the (0001) growth direction. High-resolution transmission electron microscopy (HRTEM) studies of individual NWs revealed that the presence of a nitrogen flux during NW MBE growth does not significantly alter the density or type of the observed defects.…”

“…As shown in refs , , a random distribution of stacking faults can result in a red shift of the PL emission compared to the WZ band gap value (see Section S4 in the Supporting Information). Consequently, the PL emission maxima of the pristine InAs NWs at 437 meV lie between the theoretical band gap values of ZB (415 meV) and WZ (477 meV) InAs phases (marked by red and blue vertical lines, as shown in Figure a, respectively).…”

“…According to the structural characterization, both pristine InAs NW and InAs/InAsN NW heterostructures possess a high stacking fault density and, therefore, cannot be considered as having a pure wurtzite structure. As shown in ref , the band gap value of the WZ polytype with a random stacking sequence should lie between the band gap energy values of ZB and WZ phases. Thus, we expect that at low nitrogen concentrations (<0.78%), the band gap shrinkage value for InAsN NWs lies between 55 and 77 meV/%N.…”

“…The chosen growth conditions allow the acquisition of arrays of vertically oriented NWs with a high surface density in the range from 5 to 10 μm –2 . Details on the InAs NW epitaxial growth are presented in the Materials and Methods Section and ref . For producing an active nitrogen flux of 40–50 nm/h in GaN-equivalent growth rate units, we employed a radio frequency (RF)-plasma source operated at a nitrogen flow rate of 0.9 sccm and a RF power of 350 W. The additional InAsN test sample was grown under identical conditions, but with a restrictor diaphragm installed in front of the nitrogen source, resulting in a reduction of the active nitrogen flux by half (further denoted as low-N InAsN NWs).…”

“…The Γ-centered MP grid with a density of 10 points in Å –1 and a plane-wave cutoff of 700 eV was used in meta-GGA calculations. The free parameters of the Tran and Blaha model were adjusted (α = −0.012 and β = 0.976 Bohr 0.5 ) to match the existing experimental and theoretical data that band gaps of the (ZB, 6h, 4h, and WZ) polytypes of InAs were equal to 417, 436, 448, and 477 meV, respectively, , and the band gap of WZ InN was equal to 716 meV. , …”