The terahertz (THz, 30 µm – 300 µm) spectral region covers various relevant and significant real-world applications like spectral imaging , medical diagnostics  and trace gas spectroscopy . But due to the lack of high-performance laser sources, it is also referred to as the “THz-gap” in the electromagnetic spectrum. To close this gap, room temperature THz emitting laser sources are needed. Up to now, GaAs-based quantum cascade lasers (QCLs)  are the most promising candidates to fulfil this task but they lack significant improvements within recent years concerning their maximum operating temperature and are still limited to operation at cryogenic temperatures around 200K. They are fundamentally limited by the parasitic, non-optical LO-phonon transitions (36meV in GaAs), being on the same order as the thermal energy at room temperature (kT = 26meV). Therefore, other semiconductor materials might solve this problem, including new material systems like ZnO or GaN with their larger LO-phonon energy (ELO,ZnO = 72meV, ELO,GaN = 91meV). While GaN has recently already been investigated in detail without final breakthrough concerning GaN-based QC devices, ZnO is new to this field of application. Consequently, the design and fabrication of ZnO-based QC structures has to be mastered, including a high quality epitaxial growth and well-controlled fabrication processing.
© 2019 IEEEPDF Article