**3. Materials used and growth methods**

The first QCL in 1994 used InAlAs as the cladding layers due to its low refractive index of around 3.20. The core region, which includes active and injector regions, usually has 500 stacking layers consisting of alternative InGaAs and InAlAs layers with total thickness about 1.5–2.5 μm. The average refractive index in this region can be calculated according to the volume fraction of these two constituent materials and is often around 3.35, which is clearly higher than the cladding layers [16]. The confinement factor with typical Np of approximately 30 is usually around 0.5. To reduce the optical loss, the cladding layers are usually doped with a low concentration of 5 × 1016 cm−3 and the separate confinement heterostructures (SCHs) are implemented with InGaAs of high refractive index to increase the optical confinement factors. In later designs, the InAlAs cladding layers were replaced by InP because it has a lower refractive index of around 3.10 and a higher thermal conductivity for better heat treatment, which is a critical step for the device performance [16].

So far, the emission wavelength of QCLs has been extended from the near infrared (around 100 THz) to terahertz regimes. While the longest demonstrated wavelength is 1.6–1.8 THz with GaAs/Al0.1Ga0.9As system at 80 K under continuous-wave (CW) operation [17], the shortest wavelength has been extended to 3 μm with In0.53Ga0.47As/AlAs0.56Sb0.44 system at 300 K under pulsed operation [18]. The highest output power of short-wavelength QCL (4.6 μm) under CW operation at room temperature has been demonstrated (100 mW and maximum temperature 105°C). All of these state-of-the-art QCLs have been grown by MBE so far, but for industry MOCVD is preferable for mass production. As a result, attempts have been made to grow QCLs by MOCVD. Until now, there are only three groups successfully reporting the growth of QCLs by MOCVD: an InP-based QCL (*λ* ≈ 8.5 μm) operating in pulsed mode at room temperature, with low-temperature threshold current density in the region of 1500 A/cm2 [19], an In0.53Ga0.47As/In52Al0.48As QCL (*λ* ≈ 8.5) operated in continuous-wave operation at room temperature with an output power of 5.3 mW [20], and Diehl et al. [21] managed to grow a QCL working in continuous-wave mode above 370 K, with an optical output power of 312 mW at room temperature and an emission wavelength of 5.29 mm. More effort is still required for MOCVD growers to develop more advanced techniques to compete with MBE. Having said that, room-temperature operation in InAs/AlSb QCLs has been achieved at 4.5 μm [22]; more recently, MBE-grown InAs/AlSb on n-InAs (100) substrate QCL operating at 8.9 μm has been reported, with a maximum operating temperature of 305 K [23].

QCLs provide one possible method of realizing high-efficiency light emitters in indirect band gap materials such as silicon. Electroluminescence at terahertz frequencies from Si/SiGe intrasubband transitions has been demonstrated [24], and recently silicide low-loss (down to 2 cm−1) waveguides were designed [25, 26]. **Figure 4** shows a transmission electron microscopy (TEM) image from a Si/SiGe QCL.

wavelength have been reported [14]. These lasers were pulsed, continuously tunable singlemode emission and were achieved from 90 to 300 K with a tuning range of 65 nm and a peak output power of approximately 100 mW at room temperature—so the lasers described by Pellandini et al. [12], Gmachl et al. [13] and Köhler et al. [14] were laser sources for the midinfrared region. In order to realize near-infrared QCLs, optical non-linearity in intersubband lasers has been used to design such lasers emitting at 4.76 μm, with third harmonic and second

The first QCL in 1994 used InAlAs as the cladding layers due to its low refractive index of around 3.20. The core region, which includes active and injector regions, usually has 500 stacking layers consisting of alternative InGaAs and InAlAs layers with total thickness about 1.5–2.5 μm. The average refractive index in this region can be calculated according to the volume fraction of these two constituent materials and is often around 3.35, which is clearly higher than the cladding layers [16]. The confinement factor with typical Np of approximately 30 is usually around 0.5. To reduce the optical loss, the cladding layers are usually doped with a low concentration of 5 × 1016 cm−3 and the separate confinement heterostructures (SCHs) are implemented with InGaAs of high refractive index to increase the optical confinement factors. In later designs, the InAlAs cladding layers were replaced by InP because it has a lower refractive index of around 3.10 and a higher thermal conductivity for better heat treatment,

So far, the emission wavelength of QCLs has been extended from the near infrared (around 100 THz) to terahertz regimes. While the longest demonstrated wavelength is 1.6–1.8 THz with GaAs/Al0.1Ga0.9As system at 80 K under continuous-wave (CW) operation [17], the shortest wavelength has been extended to 3 μm with In0.53Ga0.47As/AlAs0.56Sb0.44 system at 300 K under pulsed operation [18]. The highest output power of short-wavelength QCL (4.6 μm) under CW operation at room temperature has been demonstrated (100 mW and maximum temperature 105°C). All of these state-of-the-art QCLs have been grown by MBE so far, but for industry MOCVD is preferable for mass production. As a result, attempts have been made to grow QCLs by MOCVD. Until now, there are only three groups successfully reporting the growth of QCLs by MOCVD: an InP-based QCL (*λ* ≈ 8.5 μm) operating in pulsed mode at room temperature,

with low-temperature threshold current density in the region of 1500 A/cm2

reported, with a maximum operating temperature of 305 K [23].

In0.53Ga0.47As/In52Al0.48As QCL (*λ* ≈ 8.5) operated in continuous-wave operation at room temperature with an output power of 5.3 mW [20], and Diehl et al. [21] managed to grow a QCL working in continuous-wave mode above 370 K, with an optical output power of 312 mW at room temperature and an emission wavelength of 5.29 mm. More effort is still required for MOCVD growers to develop more advanced techniques to compete with MBE. Having said that, room-temperature operation in InAs/AlSb QCLs has been achieved at 4.5 μm [22]; more recently, MBE-grown InAs/AlSb on n-InAs (100) substrate QCL operating at 8.9 μm has been

[19], an

harmonic generation at 1.59 and 2.38 μm, respectively [15].

**3. Materials used and growth methods**

8 Quantum Cascade Lasers

which is a critical step for the device performance [16].

**Figure 4.** TEM image from a Si/SiGe quantum cascade structure, consisting of 600 periods of 6.5 nm Si 0.7Ge0.3 quantum wells with 2-nm strained-Si barriers, all grown on top of a Si0.8Ge0.2 virtual substrate. The quantum cascade laser has a total thickness of 5 mm.

Although GaN-based materials have not been employed to fabricate QCLs, they are also promising materials to be used as such devices. III–V nitrides are known in their wurtzite structure to possess a large spontaneous polarization and piezoelectric constants. As a result, two-dimensional (2D) charges build up at nitride heterointerfaces where the polarization discontinuities occur, causing strong built-in electric fields [27]. Furthermore, GaN is a material with large LO-phonon energy, leading to a thermal population reduction of the lower laser state, a feature desirable for high-temperature operation of terahertz QCLs, as proposed by Diehl et al. [21]. On the other hand, ultrafast LO-phonon scattering in GAN/AlGaN QWs can be useful in order to rapidly depopulate the lower laser state [28, 29]. Lastly, the large LOphonon energy can also increase the lifetime of the upper laser state by reducing the relaxation of electrons with higher in-plane kinetic energy via emission of a LO phonon. Using lowpressure MOCVD, GaN/AlGaN active regions for QCLs have been grown by Huang et al. [30]. GaN/AlGaN active layers are depicted in **Figure 5**.

Finally, typical phonon frequency redshift is a key indicator of good periodicity of MOVPEgrown GaN/AlGaN QCL structures, 822 cm−1 in the superlattice has been measured, indicating a redshift with respect to the single AlGaN layer [31].

AlN/GaN compound is another possible material to be used to fabricate quantum cascade structures. A suitable method of fabrication is hot-wall epitaxy (HWE) [32], a low-cost, convenient and scalable technique, where the epitaxial layers are grown under conditions as near as possible to thermodynamic equilibrium, allowing a minimum material loss. Inoue et al. [33] grew short-period superlattices consisting of five periods of GaN wells of 10 (or nine) molecular layers (MLs) each with 1 ML AlN barriers, which was designed to emit photons at a wavelength within the mid-infrared range (around 5 μm) [34, 35].

**Figure 5.** Schematic diagram of the AlGaN QCL active layer structure (left) and cross-sectional TEM (right) of the 20 period GaN/AlGaN MQW MQL structure.
