Highly Strained III–V–V Coaxial Nanowire Quantum Wells with Strong Carrier Confinement

Coaxial quantum wells (QWs) are ideal candidates for nanowire (NW) lasers, providing strong carrier confinement and allowing close matching of the cavity mode and gain medium. We report a detailed structural and optical study and the observation of lasing for a mixed group-V GaAsP NW with GaAs QWs. This system offers a number of potential advantages in comparison to previously studied common group-V structures (e.g., AlGaAs/GaAs) including highly strained binary GaAs QWs, the absence of a lower band gap core region, and deep carrier potential wells. Despite the large lattice mismatch (∼1.7%), it is possible to grow defect-free GaAs coaxial QWs with high optical quality. The large band gap difference results in strong carrier confinement, and the ability to apply a high degree of compressive strain to the GaAs QWs is also expected to be beneficial for laser performance. For a non-fully optimized structure containing three QWs, we achieve low-temperature lasing with a low external (internal) threshold of 20 (0.9) μJ/cm2/pulse. In addition, a very narrow lasing line width of ∼0.15 nm is observed. These results extend the NW laser structure to coaxial III–V–V QWs, which are highly suitable as the platform for NW emitters.

. Band structure and carrier dynamics schematic of NW QWs composed of (a) AlGaAs and (b) GaAsP material systems.
As can be seen in Figure S1a, the AlGaAs/GaAs quantum well (QW) is built on a GaAs core nanowire (NW). The GaAs core forms a potential minimum and can therefore trap a large number of carriers at the centre of the NW. As a result, these carriers cannot contribute to the quantum well emission. In contrast, the GaAsP/GaAs QW is built on a GaAsP core NW and there is no potential minimum at the centre of the NW ( Figure   S1b). A higher fraction of injected carriers are hence captured by the quantum well and contribute to its emission.  As shown in Figure S2a, the bottom of the NW has a uniform contrast and no stackingfault related contrast difference is observed, indicating a high-quality, single crystal structure. The middle part has a slight contrast difference ( Figure S2b), which indicates the generation of stacking faults. The tip has an irregular morphology which accompanies a higher-density of stacking faults and threading dislocations ( Figure   S2c).    We have modelled energy band edge profiles and calculated the lowest confined electron and heavy-hole energies using NextNano software. 5 This uses an 8-band k.p envelope function approximation to solve the Schrödinger equation for the single GaAs-GaAsP NW QW structure in one dimension along a line joining two opposite NW facets. The simulation includes pseudomorphic strain, with the 46% phosphorous outer shells assumed to be unstrained as these represent the largest volume of material.
The calculated QW emission wavelength as a function of QW width is shown in Figure   S5   The lifetimes of the QW confined carriers were measured for a range of energies across the PL emission ( Figure S6a) and for a sample temperature of 6K. Figure S6b shows typical PL transients. The lifetime is longer for carriers with a lower energy (longer emission wavelength) and exhibits an initial plateau-like behaviour consistent with the transfer of carriers from high to low energy states within the QW.
S6: Temperature dependence of the photoluminescence Figure S7. Arrhenius fitting of temperature-dependent PL data.
The temperature dependence of the photoluminescence (PL) intensity was measured for an ensemble of NWs. The integrated intensity, normalised to the low temperature value, is shown in Figure S7. The temperature-dependent PL quenching processes can be analyzed by fitting the data using a modified Arrhenius equation, 6 where I 0 is the low temperature intensity, E 1 and E 2 are activation energies for processes that remove carriers from the optically active state, the QW in this case, and A 1 and A 2 characterize the strengths of the two processes and are determined in part, by the number of states to which carrier loss can occur. The fitting procedure gives a value for E 1 of 151 meV; this process is associated with non-radiative recombination centres at Page 8 of 10 the GaAsP surface. 6 E 2 is 12512 meV.
The ratio A 1 /A 2 for the current structure (8x10 -4 ) is much smaller than previously reported values for related structures, for example 2x10 -2 for a GaAs/GaAs 0.85 P 0.15 coaxial structure without a QW 6 and infinity for pure GaAs NWs. This result suggests that carrier loss to surface non-radiative recombination is greatly reduced in the present structure. For example, carriers on the inner side of the QW are inhibited from reaching the surface due to the deep QW. The influence of the QW on the PL emission of the NW is further studied by comparing the optical properties of core-shell GaAsP NWs grown with and without a GaAs QW. Figure S8a shows 10K macro-PL spectra recorded from ensembles of the as-grown NWs which, for the GaAsP/GaAs NW QWs, probe both the defect-free lower and defective tip regions. The spectrum of the NW QW sample comprises two peaks, representing emission from the defective tip at ~765nm and from the defect-free region