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20THz-I-3
THz quantum cascade lasers: Materials evaluation and optimization
H. Detz12
1TU Wien, Center fur Micro- and Nanostructures, Wien, Austria
2Brno University of Technology, Central European Institute of Technology Brno, Czech Republic
Intersubband transitions enable compact coherent sources for THz frequencies by circumventing bandgap-related restrictions of conventional diode lasers. While superlattice-based quantum cascade laser (QCL) active regions provide some freedom regarding the selection of quantum well and barrier material, the interplay of beneficial material parameters and technical maturity leads to a complex situation question regarding the optimum material system. In contrast to mid-infrared QCLs, THz devices have been realized mostly based on GaAs/AlGaAs heterostructures due to their technological maturity. A lack of progress regarding the maximum operating temperature (Tmax) led to a search for alternative material combinations. Nevertheless, recent improvements allowed to push Tmax of GaAs/AlGaAs THz QCLs beyond 200 K and enabled thermo-electric cooling as a major step towards system integration [1,2].
The lower effective mass (m*) of InGaAs quantum wells leads to a larger optical gain [3]. The combination with InAlAs barriers, lattice-matched or strain-balanced on InP substrates also reached a high level of maturity. For THz QCL active regions, this material system remains challenging as precise lattice-matching is required over the whole active region thickness, typically around 10 - 15 pm. As a side effect of the higher conduction band offset (CBO), thinner barrier layers are required for emission energies in the THz range, which leads to larger relative thickness errors. Alternatively, GaAsSb was used as barrier material, which leads to slightly thicker and therefore less critical layer thicknesses due to its moderate CBO of 0.36 eV. While the electronic properties are clearly advantageous, the growth of mixed-anion alloys like GaAsSb, particularly including group V switching, still requires optimization [4]. Both InGaAs-based material systems were shown to be interesting candidates for improved future device generations, presently reaching Tmax of 155 K with InAlAs barriers and 142 K using GaAsSb barriers.
Recently, InAs-based THz QCL active regions were studied to exploit the even lower m*. Lattice- matched AlAsSb barriers exhibit a CBO of 1.6 eV, which leads to layer thicknesses approaching the single monolayer level, which still need to be grown reproducibly over several pm active region thickness [3]. While this material combination is clearly not as mature, already first-generation devices were operational. We will review the perspectives of different THz QCL material systems. Though not yet competitive to GaAs-based devices, the potential of the low m* heterostructures is underlined by state-ofthe- art operational devices. Further improvement of the growth conditions is expected to boost the performance devices based on mixed-anion barriers.
References:
[1] L. Bosco et al., Appl. Phys. Lett. 115, 010601 (2019).
[2] M.A. Kainz et al., Opt. Express 27, 20688 (2019).
[3] H. Detz et al., Phys. Stat. Solidi A 216, 1800504 (2019).
[4] T. Zederbauer et al., APL Mater. 5, 035501 (2017).
