We report on the first demonstration of quantum cascade detectors based on ZnO/ZnMgO quantum wells grown by molecular beam epitaxy on an m-plane ZnO substrate. The sample is processed in the form of square mesas with special attention paid to the passivation of the side facets. Photocurrent spectroscopy reveals a resonance at 2.8 μm wavelength slightly blue-shifted with respect to the intersubband absorption peak at 3 μm wavelength. The photocurrent persists up to room temperature. The peak responsivity amounts to 0.15 mA/W under irradiation at Brewster’s angle of incidence of the top surface of the mesas.
Mid-infrared (MIR) spectroscopy is a reliable tool for the identification of gaseous and liquid mixtures due to their unique and inherent absorption spectra. Quantum Cascade (QC) Lasers and Interband Cascade Lasers are modern reliable sources to penetrate the MIR spectral range.
To increase the functionality of QC devices we designed and optimized a QC material that can be used as a QC laser and as a QC detector for the very same MIR wavelength, respectively. Switching from laser to detector is achieved by biasing the semiconductor (lasing mode) or operate it without any electric field applied (detecting mode), respectively. Due to this functionality increase the on-chip integration of a designable QC light source, an interaction zone and a QC detector is now feasible and has been demonstrated recently.
In this talk we present improved bi-functional QC material for the integration and further development of sensor systems, as well as different cavity concepts for gas and liquid sensing scenarios. Proof of concept sensing examples to demonstrate the integrated sensor systems will be given. Multi mode and single mode lasers made from bi-functional materials show comparable performance to regular state of the art QC lasers and no performance drop due to the additional detection functionality.
While QC lasers are already accepted within the scientific community, QC detectors still need to be further promoted. Thus, in addition to the improvement of the bi-functional QC material, we demonstrated a single period quantum cascade photo-detector with a responsivity of up to 1.3 A/W.
We review recent advances in chemical sensing applications based on surface emitting ring quantum cascade lasers (QCLs). Such lasers can be implemented in monolithically integrated on-chip laser/detector devices forming compact gas sensors, which are based on direct absorption spectroscopy according to the Beer–Lambert law. Furthermore, we present experimental results on radio frequency modulation up to 150 MHz of surface emitting ring QCLs. This technique provides detailed insight into the modulation characteristics of such lasers. The gained knowledge facilitates the utilization of ring QCLs in combination with spectroscopic techniques, such as heterodyne phase-sensitive dispersion spectroscopy for gas detection and analysis.
The development of Zinc Oxide (ZnO)-based applications have been strongly limited due to the lack of reproducible p-type doping. Here we present novel opportunities in the field of unipolar oxide wide band gap semiconductors. First we have developed the growth of nonpolar ZnO/ZnMgO multiple quantum wells (MQWs) and then we demonstrate that the structural and optical properties of the MQWs are reaching the required level for intersubband devices in terms of defects, surface and interface roughness and doping. We will show and discuss the most recent results as, for instance, intersubband transitions which have been observed in such structures.
This "Zoterac" project has received funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No 665107
The terahertz (THz) spectral range (lambda ~ 30µm – 300µm) is also known as the “THz-gap” because of the lack of compact semiconductor devices. Various real-world applications would strongly benefit from such sources like trace-gas spectroscopy or security-screening. A crucial step is the operation of THz-emitting lasers at room temperature. But this seems out of reach with current devices, of which GaAs-based quantum cascade lasers (QCLs) seem to be the most promising ones. They are 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). This can be solved by using larger LO-phonon materials like ZnO (E_LO = 72meV). But to master the fabrication of ZnO-based QC structures, a high quality epitaxial growth is crucial followed by a well-controlled fabrication process including ZnO/ZnMgO etching.
We use devices grown on m-plane ZnO-substrate by molecular beam epitaxy. They are patterned by reactive ion etching in a CH4-based chemistry (CH4:H2:Ar/30:3:3 sccm) into 50μm to 150μm square mesas. Resonant tunneling diode structures are investigated in this geometry and are presented including different barrier- and well-configurations. We extract contact resistances of 8e-5 Omega cm^2 for un-annealed Ti/Au contacts and an electron mobility of above 130cm^2/Vs, both in good agreement with literature. Proving that resonant electron tunneling can be achieved in ZnO is one of the crucial building blocks of a QCL.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 665107.