Research on III-nitride intersubband (ISB) transitions in the THz spectral range is motivated by the large LO-phonon energy of GaN, which should permit device operation with limited thermal interference, and at infrared wavelengths inaccessible to other III-V compounds due to Reststrahlen absorption. A main challenge to extend the polar GaN-ISB technology towards the THz region comes from the polarization-induced internal electric field, which imposes an additional confinement that increases the energetic distance between the electronic levels. In order to surmount this constraint, we propose alternative multi-layer quantum well designs that create a pseudo-square potential profile with symmetric wavefunctions . The robustness of these designs and their integration in device architectures requiring tunneling transport will be discussed.
An alternative approach to obtain square potential profiles is the use of nonpolar crystallographic orientations. In this contribution, we present an experimental study of THz ISB transitions in m-plane GaN/AlGaN quantum wells grown on free-standing m-GaN . For Al contents below 15%, such structures can be grown without epitaxially-induced extended defects. We demonstrate nonpolar quantum wells which display ISB transitions in the 7-10 THz band, and we will discuss the effect of the doping density in the quantum wells on the transition energy and line width. Finally, we will present a comparative study using silicon and germanium as n-type dopants.
 M. Beeler, et al., Appl. Phys. Lett. 105, 131106 (2014)
 C.B. Lim, et al., Nanotechnology 26, 435201 (2015); Nanotechnology 27, 145201 (2016).
Caroline B. Lim, Akhil Ajay, Catherine Bougerol, Jörg Schörmann, David A. Browne, Mark Beeler, and Eva Monroy, "Intersubband transitions in the THz using GaN quantum wells (Conference Presentation)," Proc. SPIE 10104, Gallium Nitride Materials and Devices XII, 101040C (Presented at SPIE OPTO: January 31, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2252455.5391659229001.
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