Recently, Caroline B Lim and her team published a paper in Japanese Journal of Applied Physics, in which the nonpolar m-oriented GaN:Si/Al(Ga)N heterostructures grown on free-standing GaN for intersubband optoelectronics in the short-wavelength, mid- and far-infrared ranges Were assessed
To determine the accessible spectral ranges for ISB absorption in SWIR, MIR, and FIR spectral windows, they designed three series of m-GaN/AlGaN MQWs with different QW thicknesses and Al compositions for comparison. The structural analysis showed that the decrease of Al composition of the barriers below 10% led to an improved flatness and regularity of the layers and a reduced dislocation density. Optically, ISB absorption was observed in the 1.5–5.8 μm (827–214 meV) range with the upper limitation being set by the second order of the GaN Reststrahlen band. By increasing the QW width and reducing the Al composition in the barriers, it is possible to shift the ISB absorption to the FIR range, from 1.5 to 9 THz (6.3 to 37.4 meV), which demonstrates that it is possible for GaN to cover the 7–10 THz band, forbidding to GaAs-based technologies. However, the high doping density adapted to ISB absorption in the SWIR and MIR regions (high-energy transitions 200–800 meV) leads to broadband ISB absorption in the FIR range (low-energy transitions ≈30 meV). The decrease of doping level by one order of magnitude leads to a significant reduction of the absorption line width.
The free-standing GaN semi-insulating m-GaN substrates used in their work were supplied by Suzhou Nanowin Science and Technology Co.,Ltd. This kind substrates has very high quality with low dislocation density (less than 5*10-5cm-2) , which is very suitable to be used in exploring and making advanced optoelectronic devices .
So far, most studies on ISB transitions in multi-quantum-wells (MQWs) for group-III–nitride have focused on c-plane polar structures. However，in this crystallographic orientation, the polarization-induced internal electric field makes ISB transition energies become more sensitive to the strain state of the quantum wells (QWs). As a result, it hampers the extension of ISB transitions towards far-infrared wavelengths. Although the internal electric field can be partially compensated by the implementation of multi-layer QW architectures, it is still a major hurdle for device design. It is well known that the use of nonpolar crystallographic orientations can avoid the polarization-induced field in GaN/AlGaN heterostructures, and facilitate device design while maintaining the benefits of GaN materials.
Obviously, GaN/AlGaN nanostructures are promising for new intersubband (ISB) devices with the potential to cover the whole infrared spectrum. In the short-wavelength infrared (SWIR), the large conduction band offsets and sub-picosecond ISB relaxation times make them appealing for ultrafast photonics devices for telecommunications. On the other side of the infrared spectrum, the development of compact solid-state THz sources is strongly motivated by its applications in biological and medical sciences, industrial and pharmaceutical quality control, security screening and communication.