**7. PL temperature dependence**

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

**Figure 8.** Temperature dependence of the PL emission spectra obtained from the InAsSb MQW NWs. Emission spectra measured over the range of 4–300 K using high excitation (2.6 × 10<sup>4</sup> W cm−2), showing the room-temperature emission required for future practical NW infrared emitters. The dotted lines indicate Gaussian fits used to extract the peak emission wavelength. Atmospheric water vapour absorption is again evident in all the spectra. Figure obtained with permission from authors [38].

to be accumulated due to Fermi level pinning, resulting in a low efficiency for radiative emission [53]. In our case, the quantum confinement of the MQWs allows room-temperature emission to be observed without passivation. The PL spectra are inhomogeneously broadened due to length variations in the NWs which also result in thickness variations in

**Figure 10.** Temperature-dependent analysis of PL peak from the InAsSb/InAs MQWs, InAsSb and InAs NW samples. The temperature dependence of the PL peak emissions (points) was used to fit the Varshni relationships (solid lines) and extract the associated coefficients, for the three NW samples. Figure obtained with permission from authors [38].

**Figure 9.** Temperature-dependent analysis of PL data from the InAsSb/InAs MQWs, InAsSb and InAs NW samples; the graph demonstrates quenching behaviour of the three NW sample-integrated PL intensities with increasing temperature.

Nanowires for Room-Temperature Mid-Infrared Emission

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

67

Figure obtained with permission from authors [38].

**Figure 9.** Temperature-dependent analysis of PL data from the InAsSb/InAs MQWs, InAsSb and InAs NW samples; the graph demonstrates quenching behaviour of the three NW sample-integrated PL intensities with increasing temperature. Figure obtained with permission from authors [38].

**Figure 10.** Temperature-dependent analysis of PL peak from the InAsSb/InAs MQWs, InAsSb and InAs NW samples. The temperature dependence of the PL peak emissions (points) was used to fit the Varshni relationships (solid lines) and extract the associated coefficients, for the three NW samples. Figure obtained with permission from authors [38].

to be accumulated due to Fermi level pinning, resulting in a low efficiency for radiative emission [53]. In our case, the quantum confinement of the MQWs allows room-temperature emission to be observed without passivation. The PL spectra are inhomogeneously broadened due to length variations in the NWs which also result in thickness variations in

**Figure 8.** Temperature dependence of the PL emission spectra obtained from the InAsSb MQW NWs. Emission spectra

required for future practical NW infrared emitters. The dotted lines indicate Gaussian fits used to extract the peak emission wavelength. Atmospheric water vapour absorption is again evident in all the spectra. Figure obtained with

W cm−2), showing the room-temperature emission

measured over the range of 4–300 K using high excitation (2.6 × 10<sup>4</sup>

permission from authors [38].

66 Nanowires - Synthesis, Properties and Applications

the QWs. The atmospheric water vapour absorption is again evident near 2.7 μm (0.459 eV) in all the spectra.

**Figure 9** presents a comparison of the temperature quenching of the NW samples, where the superior performance of the InAsSb MQW NWs is clearly evident due to the increase in the radiative emission rate and suppression of Auger recombination.

The temperature dependence of the peak energies for the NWs is shown in **Figure 10**. The dotted lines represent fitting of the results using the empirical Varshni equation. The values obtained for the fitting parameters (Eg (0), α and β) are given in **Table 1** along with reference values for bulk InAs; both the InAs and the InAsSb MQW NWs have a WZ crystal structure and consequently have a weaker dependence of band gap on temperature (lower value of *α*) than the corresponding bulk ZB materials.

An Arrhenius plot is shown in **Figure 11** for the InAsSb MQW NWs from which an activation energy of 49 meV was obtained from the high-temperature region and ~5 meV for the low-temperature region. This is in approximate agreement with the confinement energy for thermal excitation of holes out of the QW (hole localisation energy of 34 meV) and electrons from the interface triangular QWs (<13 meV localisation energy), respectively. The activation energy of ~49 meV obtained from the Arrhenius plot is consistent with quenching due to carriers escaping confinement rather than Auger recombination and so provides indirect evidence for Auger suppression.

> In type II QWs, the Auger rate is determined by the overlap integral (between initial and final electron states at small transferred momentum) which is a minimum when the valence band offset is about three times the conduction band offset. Meanwhile, the radiative rate does not depend on the final state of any excited carrier, since it is a two-body process, so the radiative

> **Figure 11.** Temperature-dependent analysis of PL emission intensity from the InAsSb/InAs MQWs sample shows an Arrhenius plot of integrated PL intensity as a function of inverse temperature for the InAsSb/InAs MQW NWs. The curve was used to extract activation energies for the mechanisms that drive thermal quenching. Figure obtained with

Nanowires for Room-Temperature Mid-Infrared Emission

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

69

The development of quantum structures in NWs systems has shown the potential to extend the concept of band-gap engineering to optimise the building blocks of such systems, as well as allowing integrating these systems into the leading CMOS technology, providing a promising future for nanotechnology in optics and electronics. This chapter has presented InAsSb MQWs heterostructures within InAs NWs which exhibit mid-infrared emission at room temperature. The type II QWs provided quantum confinement and spatial localisation of the carriers combined with a suppression of Auger recombination, resulting in enhanced PL emission with respect to the bulk InAs NWs. Furthermore, having characterised the effect of charging on the type II QW, the flat-band transition energy was found to be in good agreement with calculations for both samples. These new quantum-structured NWs will allow novel nano-photonic and quantum light sources to be developed for the technologically important mid-infrared spectral range. They can exploit both the general advantages of site-controlled NWs, such as integration with silicon substrates, and also enhance light-matter coupling based on their dimensions and geometry, opening the way

rate can remain comparable with that in a type I QW systems [55].

**8. Conclusion**

permission from authors [38].

for a wide range of applications.


**Table 1.** Comparison of Varshni parameters. The fitted Varshni parameters E0 , α and β for the NW samples, compared with published parameters for bulk InAs and InSb [54].

**Figure 11.** Temperature-dependent analysis of PL emission intensity from the InAsSb/InAs MQWs sample shows an Arrhenius plot of integrated PL intensity as a function of inverse temperature for the InAsSb/InAs MQW NWs. The curve was used to extract activation energies for the mechanisms that drive thermal quenching. Figure obtained with permission from authors [38].

In type II QWs, the Auger rate is determined by the overlap integral (between initial and final electron states at small transferred momentum) which is a minimum when the valence band offset is about three times the conduction band offset. Meanwhile, the radiative rate does not depend on the final state of any excited carrier, since it is a two-body process, so the radiative rate can remain comparable with that in a type I QW systems [55].
