InGaAs/AlInGaAs THz quantum cascade lasers operating up to 195 K in strong magnetic field

Terahertz quantum cascade lasers based on InGaAswells and quaternary AlInGaAs barriers were measured inmagneticfield. This studywas carried out on a four-quantum-well active-region design with photon energy of 14.3meVprocessedwith bothAu andCuwaveguides. The heterostructure operates up to 148K at B= 0T in aCuwaveguide. The completemagneto-spectroscopic study allowed the comparison of emission and transport data. Increasing themagneticfield, the low effectivemass of the InGaAswells allowed us to reach the very strong confinement regime. At B= 12T,where the cyclotron transition is almost resonantwith the LO-phonon, we recorded amaximumoperating temperature of 195K for the devices withCuwaveguide. Additional lasing at 5.9meVwas detected for magneticfields between 7.3 and 7.7 T. The terahertz (THz) spectral range is regardedwith ever-increasing interest for sensing, imaging, and spectroscopy applications and quantum cascade lasers (QCLs) represent a primary semiconductor-based, electrically pumped source that can cover this range [1–6]. THzQCLs in theGaAs/AlGaAsmaterial system saw big improvements both in frequency coverage and operating temperature since their first demonstration, but room temperature operation is stillmissing. In order to increase the gain, and therefore the operating temperature of THz-QCLs, InGaAs-based active regionswere investigated, first in combinationwith InAlAs barriers reaching 122K [7] and,more recently, withGaAsSb reaching 142K [8]. Such expectations stem from the beneficial lower electron effectivemasses of thesematerials with respect to theGaAs/AlGaAsmaterial system, thus allowing higher oscillator strength and gain. On the other hand, one of themost relevant limiting factors, especially for InGaAs/GaAsSbQCLs, is the interface asymmetry that causes strong elastic electron scattering [9]. Furthermore both ternary compounds are latticematched to InP for a single stoichiometry that has a high conduction band offset that results in very thin barriers and is very sensitive to inherent thickness fluctuations from the growth process. An alternative option comes fromQCLs based on quaternary AlInGaAs barriermaterial [10, 11]. InGaAs/AlInGaAsQCLswere recently demonstrated, reporting amaximumpower = P 35 max mWat 3.8 THz at 10 K and amaximumoperating temperature = T 130 max K [12]. Thismaterial systemmaintains the beneficial low electron effectivemasses while presenting amore symmetric interface and a lower,more suitable conduction band offset that can be adjusted by the composition for the quaternary barrier material. The application of amagnetic field along the growth axis is a very useful tool to investigate theQCL operation, to identify the different scatteringmechanisms [7, 13, 14], and can enhance the gain because of the selective closing of electronic channels [15], allowing a higher operating temperature. Such kind of studywas performed for the differentmaterial systemswhere THzQCLs have been realised. TheGaAs/AlGaAsQCL reached 225K at 19.3 T [16], while the InGaAs/GaAsSbQCL showed laser action up to 190K at 11 T [17]. The low effectivemass of InGaAs allows and fosters these studies that can be performedwith lab-size superconductingmagnets, contrary to the high-field facilities required forGaAs-based devices. Here, we present a study in amagnetic field for InGaAs/AlInGaAs THzQCLs.Wewill also show that, due tomagnetic field enhanced gain, theQCLprocessed in aCu–Cuwaveguide reaches amaximum temperature of 195K for an OPEN ACCESS