**5. PL power dependence**

PL emission characteristics of the NWs in a number of ways. A comparison of the spectra measured at 4 K from the InAsSb MQW NWs, bulk alloy InAsSb NWs and InAs NW samples

**Figure 4(a)** and **(b)** shows the PL emission from the InAsSb MQW NWs at high and low excitations, respectively. The emission from the bulk InAsSb NWs is shown in **Figure 4(c)** and are deconvoluted into peaks at 0.380 eV corresponding to 6% Sb with a dominant ZB phase in agreement with previous work [46], and a shoulder on the main peak originating from ZB InAs which appears as the dominant phase in the early stage of all the NW growths [47].

The emission from the InAs NWs peaks at 0.482 eV is shown in **Figure 4(d)**, demonstrating a dominant WZ phase [3, 33]. The PL emission of the InAsSb MQW NWs collected at high and low excitations exhibits a clear increase in peak emission energy with respect to the bulk InAsSb NWs. This is due to the strong carrier confinement within the quantum wells. In addition, the InAsSb MQW NWs also exhibit an increased emission intensity and a superior temperaturequenching behaviour compared with the bulk InAsSb NWs as expected. Most notably, the emission intensity is enhanced at all temperatures, due to the quantum confinement of electrons and holes. PL originates from type II spatially indirect recombination of electrons in the InAs layers with confined hole states in the InAsSb QWs, where the spatial separation helps in reducing non-radiative Auger recombination with a corresponding increase in radiative emission [48]. Single Gaussian fitting to the spectra reveals interference from characteristic atmospheric absorption by water vapour between 0.445 and 0.485 eV [10]. The spectra can be scaled to account for the reduced cross-sectional area of the nanowire samples, where only 7% of the surface area is covered by the NWs, assuming a 100% nucleation yield in the mask sites. Accounting for this reduced active area allows the most direct comparison of emission intensity.

**Figure 5.** Comparison of emission intensities. PL spectra, under 3.2 × 10<sup>4</sup>

from authors [38].

intensities for MQW NWs, InAs NWs and an InAs bulk sample, scaled by active area. Figure obtained with permission

W cm−2 excitation, showing the relative emission

is shown in **Figure 4**.

62 Nanowires - Synthesis, Properties and Applications

Under low excitation conditions, bulk ZB InAs at 4 K normally exhibit characteristic emission from bound exciton and donor-acceptor transitions around 0.417 and 0.374 eV, respectively [51]. The high excitation intensity (~ 0<sup>4</sup> W cm−2) in our micro-PL experiments results in state filling such that a single InAs peak is observed at 0.425 eV. In the present case, the InAs NW emission is further blue-shifted with respect to the bulk ZB reference sample, due to the WZ crystal structure of the NW, with a peak emission energy ranging from 0.469 eV under low excitation, to 0.485 eV under high excitation, see **Figure 6**. The band gap for WZ InAs is known to be higher than that of ZB InAs, and our result is consistent with earlier studies of WZ InAs NWs which reported band gaps in the range of 0.477–0.540 eV [12, 32].

**Figure 6.** Power dependence of PL emission. The dependence of the peak emission energy on the power of the pump laser incident on the sample, for InAs and InAsSb/InAs MQW NWs, showing the difference in the blue shift between the pure InAs NW, with minimal quantum confinement effects and the MQW NW, with a strong quantum confinement and charging effects. Figure obtained with permission from authors [38].

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 and originates from Coulombic attraction between localised holes in the InAsSb quantum well

An increase in excitation intensity will raise the steepness of the confining potential and consequently the electron quantisation energy E, with a typical ∆E~L1/3 behaviour. Accordingly, the flat-band transition energy of the InAsSb/InAs MQW can be extracted from the intercept of **Figure 7(a)**—the PL peak position versus L1/3. Consequently, the flatband transition energy for the InAs/InAsSb MQW NWs is obtained as 0.438 eV which is in good agreement with the calculated transition energy of 0.443 eV, as shown in the sche-

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

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¯

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

Eg(InAs1−<sup>x</sup> Sbx) = xEgWZ(InAs) + (1 − x) EgWZ(InSb) + x(1 − x)0.67 (1)

where 0.67 is the bowing parameter for zinc-blende InAsSb and EgWZ (InSb) is the band gap of

greater than the known value for the zinc-blende phase. This also agrees with results by

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

PL spectra obtained at different temperatures for the InAsSb MQW NWs are shown in **Figure 8**. Although the wires are not capped or passivated, they exhibit strong PL emission which persists up to room temperature. This indicates that radiative recombination occurs primarily in the MQW away from the near surface regions, which in InAs NWs are known

Farrell et al. who showed that for x = 3.9%, the band gap of Wurtzite InAs1−xSb<sup>x</sup>

nanowires take on a predominately zinc-blende structure for x > 4%, precluding the

can be calculated using the quadratic approximation:

(InAs0.93Sb0.07) = 0.424 eV which is 63 meV

Nanowires for Room-Temperature Mid-Infrared Emission

http://dx.doi.org/10.5772/intechopen.79463

11)

65

was also

attracting electrons from the adjacent InAs barrier forming triangular quantum wells.

matic band diagram in **Figure 7(b)**.

InAs1−xSb<sup>x</sup>

for the wurtzite InAs1−xSb<sup>x</sup>

transition energy, E<sup>t</sup>

wurtzite InSb taken as 0.287 eV [52]. This gives Eg

, of 0.443 eV.

**7. PL temperature dependence**

~60 meV greater than the known value for the zinc-blende phase.

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

**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].

and originates from Coulombic attraction between localised holes in the InAsSb quantum well attracting electrons from the adjacent InAs barrier forming triangular quantum wells.

An increase in excitation intensity will raise the steepness of the confining potential and consequently the electron quantisation energy E, with a typical ∆E~L1/3 behaviour. Accordingly, the flat-band transition energy of the InAsSb/InAs MQW can be extracted from the intercept of **Figure 7(a)**—the PL peak position versus L1/3. Consequently, the flatband transition energy for the InAs/InAsSb MQW NWs is obtained as 0.438 eV which is in good agreement with the calculated transition energy of 0.443 eV, as shown in the schematic band diagram in **Figure 7(b)**.
