Infrared Materials
The Gen930 MBE system here at UNSW is set up to be able to grow so called infrared materials, these are semiconductors with narrow band gaps meaning they emit light in the infra-red part of the spectrum. Of course if a semiconductor emits in the infra-red it will absorb there as well. So these types of materials are of interest for many applications where there is longer wavelength light or radiant heat involved. So they are critical for sensing applications in defence settings as well as monitoring.
Thermoradiative Diodes
Here at UNSW as part of the QPV Group I am heading the growth of structures in InAs and GaSb based devices for use in a novel power generation device, the thermoradaitve diode (TRD). A TRD relies on the natural tendency of materials to emit light when they are facing a colder surface. We see this day to day with a cold wall making us feel colder than if there is a warm wall. This concept of pointing a material at a cold environment is adapted for power generation in the case of a TRD. Having a TRD at say 40 C and facing the cold background of space induces the device to emit light by excited electrons relaxing from the conduction band down to the valence band. These type of temperatures mean band gaps corresponding to emission wavelengths in the range 3-10 micrometres - hence the need for infra-red materials. This depletes the devices of carriers and so more charge carriers flow in to replenish and this means there is a flow of current from the TRD to an external load. What happens is that the TRD runs in reverse bias with a current running to an external load - in other words electrical power generation [1].
Related to operation as a TRD is the concept of negative luminescence (NL), where applying a negative bias on the structure in question reduces the light emission below that for thermal equilibrium. Applying a positive bias still sees a boost in emission, but running in reverse makes the device dark compared to the environment. The presence of NL is a key fingerprint of good TRD operation, so characterising NL and exploring what enhances this effect is complementary to TRD research. It also allows for covert communications as recently pointed out in a recent article [2].
Finally, whilst most of the reported results are from HgCdTe structures (typical for the wavelengths in question) a recent report shows that operating HgCdTe as a TRD at elevated temperatures likely brings irreversible damage pointing to a clear advantage of using III-V materials like InAs and GaSb [3]. We are looking these materials for realising TRDs.
References
[1] Semiconductor thermoradiative power conversion
M. P. Nielsen, A. Pusch, P. M. Pearce, M. H. Sazzad, P. J. Reece, M. A. Green, N. J. Ekins-Daukes
Nature Photonics, 18(11), 1137-1146, 2024.
[2] Balancing positive and negative luminescence for thermoradiative signatureless communications
M. P. Nielsen, S. A. Maier, M. S. Fuhrer, N. J. Ekins-Daukes
Light: Science & Applications, 15(1), 148, 2026.
[3] Temperature-dependent power generation from HgCdTe and III–V thermoradiative diodes
V. Radchenkov, J. A. Harrison, M. H. Sazzad, P. M. Pearce, A. Pusch, S. P. Bremner, P. J. Reece, N. J. Ekins-Daukes, M. P. Nielsen
Applied Physics Letters, 126(17), 173901, 2025.
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