Optoelectronic Devices Based on ZnO/ZnMgO

Summary form only given. The terahertz (THz, 30 pm - 300 pm) 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 (E
_LO,ZnO
= 72meV, E
_LO,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. The devices we investigate in this work are grown on m-plane [10-10] ZnO-substrate by molecular beam epitaxy (MBE). Their active layer typically consists of ZnO quantum wells, sandwiched between ZnMg0 barriers, while above and below highly doped ZnO-layers ensure good electrical contacts. They are patterned by (dry) plasma etching in a CH4-based chemistry into square MESAs. The CH4-process protects the mask by an amorphous carbon-layer increasing the selectivity of the etching. Resonant tunneling diodes and similar structures are then investigated in this geometry including different barrier-and well-configurations, for a typical RTD structure including AFM surface roughness image). The devices have typical resistances of -mid lo -5 1 cm 2 to -mid 10'1 cm 2 for the top and bottom contact, respectively, for not annealed Ti/Au contacts and an electron mobility of above 130cm 2 Ns. Those values are in good agreement with typical values from literature. We will further show latest results of RTD structures including typical IV-characteristics.