Nanoscale physics

Properties of spectral parameters of multicascade nanostructure being a model of quantum cascade detector
Seti Ju., Vereshko E., Voitsekhivska O., Tkach M., Trysyruka Y.
Department of Theoretical Physics and Computer Simulation, Yuriy Fedkovych Chernivtsi National University, 2, Kotsyubinsky Str., 58012, Chernivtsi, Ukraine,
E-mail: vereshko.yevheniia@chnu.edu.ua
Since the appearance of the first quantum cascade detector (QCD) in 2004 [1], intensive applied and fundamental research has resulted in QCD devices [2, 3] that cover the entire infrared frequency range. It is obvious that for the successful improvement of their spectral characteristics the theory based on realistic models, which take into account the main mechanisms of their functioning, is required. In the majority of papers the theory of QCD functioning was based on single-cascade models, where the optical properties of a separate cascade were studied.
In this paper, we propose the results of developed theory and comparative analysis of an electron energy spectrum in a multicascade closed nanostructure and a Kronig-Penny-type nanostructure, being the two models for typical QCD. The spectral parameters of nanostructures are studied as functions of number of cascades (N). It is established that in both models the bigger number of N brings to the splitting of the energies of the single-cascade structure into groups of levels, which in the limit case at N→∞, in agreement with physical considerations, form continuous energy bands. The bandwidths in the closed structure increase for bigger N and asymptotically coincide with the independent of N bandwidths in the Kronig-Penny-type structure. It is shown that the number of levels in each band of closed nanostructure is the same as the number of cascades, while in the Kronig-Penny-type structure, it depends on the parity of N, being equal to (N+1)/2 at odd N and N/2 at even N. It is revealed that in the model of closed multicascade QCD structure the "near-surface" states are observed. They appear due to the localization of electron in the outer potential wells of the first and N-th cascade. The energies of these states do not change if the number of cascades varies.

1.	Gendron L., Carras M., Huynh A., Ortiz V., Koeniguer C., Berger V. Quantum cascade photodetector // Appl. Phys. Lett.-2004.-85.-2824.
2.	Lei W., Jagadish C. Lasers and photodetectors for mid-infrared 2–3 μm applications // J. Appl. Phys.-2008.- 104.- 091101.
3.	M Beeler et al. III-nitride semiconductors for intersubband optoelectronics: a review // Semicond. Sci. Technol. – 2013. –28. –074022.