**6. Development of a band structure for the MQWs**

Quantum confinement-induced blue shifts have also been observed as the diameter of InAs NWs is reduced [14]; however, in the case of 100-nm diameter wires, the shift is rather small ~5 meV. Hence, the emission observed from the InAs NWs grown in this work is in good agreement with earlier reports. We envisage modest size-related confinement effects in the InAsSb MQW NWs. The PL emission energy from the InAs NWs is blue-shifted by 21 meV over the range of pump powers used in our experiments, which is similar to that obtained by others [14] and is associated with band filling. The commensurate shift for the MQW NWs is much larger, at 45 meV (see **Figure 6**). The majority of this blue shift occurs at low pump powers before the dependence becomes similar to that for pure InAs wires at higher powers. The blue shift in the MQW nanowires arises due to band-bending effects characteristic of type II QWs

64 Nanowires - Synthesis, Properties and Applications

**Figure 7.** Determination of the flat-band transition energy at the QW. (a) PL peak energy versus cube root of integrated PL intensity, elucidating the charging of the type II QW and allowing the flat-band transition energy to be identified and (b) the calculated band diagram for the InAsSb/InAs QW and photon energy, showing the band bending and triangular

well formation. Figure obtained with permission from authors [38].

Developing a band diagram for the InAs/InAsSb MQW NWs is not straightforward, due to limited data describing band gaps and alignments for the Wurtzite phases. In particular, pure InAs1−xSb<sup>x</sup> nanowires take on a predominately zinc-blende structure for x > 4%, precluding the measurement of Wurtzite band gaps at higher antimony fractions. By contrast, the InAs1−xSb<sup>x</sup> growth in this work maintains its Wurtzite structure due to the growth being on the (10¯ 11) facets of the Wurtzite InAs wire. To approximate the band diagram in this absence of reported parameters, we start with the band alignment for a comparable zinc-blende structure, calculated taking account of the Sb fraction and the strain within a (111) orientated nanowire. The band gap of Wurtzite InAs is taken to be 60 meV greater than that for InAs zinc-blende (EGwz), with the noted caveat that there is a lack of consensus in the study. The value of the band gap for the wurtzite InAs1−xSb<sup>x</sup> can be calculated using the quadratic approximation:

$$\mathrm{E}\_{\mathrm{g}}\text{(InAs}\_{1\text{-x}}\,\mathrm{Sb}\_{\mathrm{x}}\text{)}=\mathrm{xE}\_{\mathrm{gWZ}}\mathrm{(InAs)}+(1-\mathrm{x})\mathrm{E}\_{\mathrm{gWZ}}\mathrm{(InSb)}+\mathrm{x}(1-\mathrm{x})0.67\tag{1}$$

where 0.67 is the bowing parameter for zinc-blende InAsSb and EgWZ (InSb) is the band gap of wurtzite InSb taken as 0.287 eV [52]. This gives Eg (InAs0.93Sb0.07) = 0.424 eV which is 63 meV greater than the known value for the zinc-blende phase. This also agrees with results by Farrell et al. who showed that for x = 3.9%, the band gap of Wurtzite InAs1−xSb<sup>x</sup> was also ~60 meV greater than the known value for the zinc-blende phase.

The confined hole states were calculated using a six-band k.p. solver in Nextnano. The first confined heavy hole state is calculated to be 8 meV above the band edge, as shown in the schematic energy band diagram in **Figure 7(b)** corresponding to a flat-band recombination transition energy, E<sup>t</sup> , of 0.443 eV.
