Formation of Excitons in Semiconductor Nanostructures in the Presence of Electron–Hole Plasma V. A. Tekkozyan * , K. Li, A. Zh. Babajanyan, and Kh. V. Nerkararyan Yerevan State University, Yerevan, Armenia * vahant@mail.ru Received December 20, 2013 Abstract⎯We propose a mechanism of increase in the binding energy of an exciton in wide band-gap semiconductors in the presence of optically pumped electron–hole plasma. These excitons with relatively high binding energy (>150 meV) can exist at room temperature when the dielectric constant of semiconductor in the infrared region of spectrum approaches zero. Calculations for CdS show that the density of electron–hole plasma should be higher than 10 19 cm −3 for formation of such excitons. We show that there exist a considerable number of close-lying energy levels of excitons with high binding energy in the forbidden band of the semiconductor. We guess that these excitons participate in the process of laser generation in optically pumped semiconductor nanocrystals. DOI: 10.3103/S1068337214030074 Keywords: semiconductor, nanostructure, electron–hole plasma, excitons, binding energy 1. INTRODUCTION Recent achievements in the field of technologies of fabrication of wide band-gap semiconductor nanostructures allowed performing a number of experiments, which are unique because of possibility of localization of photons and electrons in one or two dimensions. Laser influence in semiconductor nanocrystals can generate localized strong monochromatic light whose geometry is perfectly suitable for interaction with such nanophotonic devices as quantum dots, metal nanoparticles, plasmon waveguides, and biological samples. These nanolasers may become very important components in study and elaboration of new nanosize photonic elements. Along with well-studied nanolasers based on ZnO [1–5], GaN [6–8], and CdS [9–11], the number of nanocrystals exhibiting laser generation at room temperature, increases [12–15]. In such laser generation, the main role is frequently ascribed to excitons [16, 17]. This is, however, not obvious. Binding energy of excitons in CdS and GaN is lower than 30 meV, therefore they cannot exist sufficiently long at room temperature. Binding energy of excitons in ZnO is relatively high (60 meV), but the concentration of charge carriers needed for laser generation exceeds the Mott concentration where electron–hole plasma is formed. Therefore works [16, 17] proposed an alternative interpretation of emission in ZnO nanostructures, based on laser generation of electron–hole plasma at room temperature. Nevertheless, the arguments given in these works are not sufficient for refusing the exciton mechanism of emission. We believe that emission in semiconductor nanocrystals is caused by combination of the two above- mentioned processes. In the present work we consider the possibility of formation of high-binding-energy excitons in conditions where the dielectric constant of the semiconductor approaches zero in the infrared range of spectrum, due to presence of optically pumped electron–hole plasma. 2. THEORY We describe the possibility of exciton formation in broadband semiconductors based on hydrogen-like model. Let E s be the binding energy of the ground state and ε s the dielectric constant of the semiconductor in the absence of the electron–hole plasma. In the framework of hydrogen-like model we have 2 s s E − ∝ε ISSN 1068–3372, Journal of Contemporary Physics (Armenian Academy of Sciences), 2014, Vol. 49, No. 3, pp. 123–126. © Allerton Press, Inc., 2014. Original Russian Text © V.A. Tekkozyan, K. Li, A.Zh. Babajanyan, Kh.V. Nerkararyan, 2014, published in Izvestiya NAN Armenii, Fizika, 2014, Vol. 49, No. 3, pp. 196–201. 123