GaN THz optoelectronics – OptoTeraGaN

The terahertz (THz) frequency domain, between infrared and microwave spectral range, is referred as the « THz gap » because of the lack of compact and efficient semiconductor devices. Exploration of this spectral region is considerably developing due to the wide range of applications: medical diagnostic, security, detection of molecules, astronomy, non-destructive control of material, high data rate secured communications in open space… One pre-requisite for scientific applications like spectroscopy, astrophysics, space imaging, is the availability of compact, fast and low-noise detectors. A necessary large detectivity imposes the use of cryogenic cooling for the detector and the main challenge is to increase the working temperature. However, for a widespread use of the THz technology, compact sources working at non-cryogenic temperatures are necessary.

The OptoTeraGaN project is intended to tackle both issues, namely the development of high-detectivity THz quantum detectors with increased operating temperature with respect to existing technologies as well as THz light emitters, both based on the quantum cascade (QC) concept and on high-quality polar and semipolar GaN/AlGaN semiconductors. We intend to benefit from the large energy of optical phonons in GaN materials for demonstrating QC devices in a much broader spectral range (1-15 THz) and in particular in the 5-12 THz range, which cannot be covered with other III-V semiconductors. The large optical phonon energy is also one key point for increasing the operating temperature of THz QCD and for achieving room temperature operation of THz QC lasers, which appears to be out of range of current GaAs-based QC laser technology.

The first objective of the project is to demonstrate THz quantum cascade detectors (QCD). QCDs are formed by the repetition of active and extractor quantum well (QW) regions and rely on intersubband (ISB) absorption in the active QW and photo-excited electron transfer through the extractor from one period to the other. In contrast to existing THz quantum detectors such as GaAs QWIPs, these photovoltaic devices operate under zero bias and do not suffer from any dark current, which is one main advantage for increasing the operation temperature while benefiting from the maximum detectivity limited by the background (BLIP). Our target is to demonstrate THz QCDs with a responsivity larger than 100 mA/W and a BLIP temperature of 77 K at 12 THz.

A second related objective of the project is to make significant progress towards THz QC lasers in the GaN/AlGaN material system. We will make use of the advanced know-how acquired on the design, growth and processing of GaN-based THz QCD devices to develop electroluminescent sources. One first goal is to develop spectrally narrow THz light emitting devices at room temperature based on in-plane transport, which can find a number of applications because of their fast modulation capabilities. Our final target is to demonstrate stimulated gain under vertical transport using plasmonic waveguide resonators and lasing at cryogenic temperatures.

The consortium, which regroups teams with a world-class expertise on GaN-based material growth by MBE and MOVPE, GaN ISB devices, as well as on QCL/QCD and THz technology and characterization, has been specifically assembled to meet the objectives of this basic research project. The recent demonstration by members of the consortium of GaN-based ultrafast QCDs in the mid-IR spectral range as well as the first observation of reproducible resonant tunnelling and THz ISB electroluminescence from GaN/AlGaN QWs are preliminary results, which constitute the major building blocks required for the present project.

The project OptoTeraGaN is of major technologic and scientific impact in agreement with “défi n°7 société de l’information et de la communication: micro et nanotechnologie, optoélectronique”.