1063-7826/03/3706- $24.00 © 2003 MAIK “Nauka/Interperiodica” 0699 Semiconductors, Vol. 37, No. 6, 2003, pp. 699–704. Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 6, 2003, pp. 724–729. Original Russian Text Copyright © 2003 by Valakh, Vuychik, Strelchuk, Sorokin, Shubina, Ivanov, Kop’ev. 1. INTRODUCTION Semiconductor quantum dots (QDs) on the basis of II–VI and III–V compounds in semiconductor matrices hold promise for modern optoelectronic applications, in particular, for the development of highly efficient light-emitting diodes and lasers with a high quantum efficiency and extremely low pump intensity. A considerable number of investigations in this field are concerned with the self-organized growth of QDs. For example, TEM studies of CdSe/ZnSe heterosys- tems [1] showed that the deposition of CdSe layers with a nominal thickness of 0.5–3.0 monolayers (MLs) at a temperature ≤280°C results in the formation of a two- dimensional (2D) Zn 1 – x Cd x Se layer with mixed com- position and a thickness of ~10–11 ML (one ML corre- sponds to 0.283 nm). This 2D layer contains two pre- vailing types of coherently stressed islands: cadmium- enriched small islands (≤10 nm) with a high density (~10 11 cm –2 ) and large islands (30–130 nm) with a com- parably lower density (~10 10 cm –2 ). These intralayer 2D islands may be considered as forerunners of three- dimensional (3D) islands. Therefore, the realization of a 2D–3D morphological transition can be assumed as smooth in contrast to the Stranski–Krastanov mecha- nism, which has a pronounced threshold and is typical of most III–V systems, where the 2D wetting layer abruptly transforms into 3D islands. 1 An appreciable thickening of the 2D layer in comparison with the nom- inal thickness of the deposited CdSe layer may be due to the interdiffusion and segregation of Cd/Zn at the heterointerface [1]. Highly efficient mixing of the bar- rier and quantum-well materials in the CdSe/ZnSe het- 1 According to [2], the formation of 3D islands in II–VI nanostruc- tures can be stimulated in a special mode of thermal activation. erostructure was previously observed both for an indi- vidual quantum well and for superlattices [3–5]. An increase in the diffusion length of Cd atoms in this sys- tem is attributed to nonequilibrium growth conditions during molecular-beam epitaxy, which leads to the for- mation of cation vacancies with a high density. The life- time of carriers captured by defects may be fairly long. In the presence of such defects and sufficiently fast radiative-recombination centers in the steady-state con- dition, the excitation by photons with a lower energy may cause the emission of photons with a higher energy (anti-Stokes emission). Anti-Stokes photoluminescence (APL) was observed in crystalline semiconductors [6, 7], porous silicon [8], colloidal semiconductor QDs [9], semiconductor het- erojunctions and quantum wells (QWs) [10–13], as well as in InAs QDs [14] and InP QDs [15]. In most of these studies, interpreting the physical mechanism behind the ALP process involved the Auger recombina- tion of carriers [12, 13] and nonlinear two-step two- photon absorption via the real intermediate states [6–11, 14, 15]. For bulk GaAs [6], epitaxial GaAs and GaAlAs layers [7], and InP QDs [15], APL was interpreted as the process of a two-step optical excitation of an elec- tron–hole pair via the intermediate deep levels of defects. Note that, in the case of semiconductor hetero- junctions and nanostructures with QWs and QDs, the two-step photoexcitation of highly efficient APL both via the deep levels of defects and via the size quantiza- tion levels was observed in the region of wide gap mate- rial. In this study, we investigate the phenomenon of anti-Stokes luminescence in CdSe/ZnSe nanostructures emitting from the region of CdSe insert, which is a nar- rower (in comparison to ZnSe) energy-gap component Low-Temperature Anti-Stokes Photoluminescence in CdSe/ZnSe Nanostructures M. Ya. Valakh*, N. V. Vuychik*, V. V. Strelchuk*, S. V. Sorokin**, T. V. Shubina**, S. V. Ivanov**, and P. S. Kop’ev** *Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kiev, 252028 Ukraine e-mail: valakh@isp.kiev.ua **Ioffe Physicotechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia Submitted October 16, 2002; accepted for publication October 28, 2002 Abstract—Intense anti-Stokes photoluminescence was observed at low temperatures in CdSe/ZnSe nanostruc- tures with separate CdSe inserts in a ZnSe matrix; the nominal thickness of these inserts amounted to 1.5 and 0.6 monolayers. It is shown that the intensity of an anti-Stokes band excited by the photons of energies consid- erably lower than the band’s peak is quadratic in the excitation power; in the case of resonance excitation, a weaker dependence is obtained. A mechanism behind the excitation of anti-Stokes photoluminescence is sug- gested on the basis of nonlinear two-step two-photon absorption via the deep states of the defect centers includ- ing cation vacancies localized at the barrier–nanoisland interface. © 2003 MAIK “Nauka/Interperiodica”. LOW-DIMENSIONAL SYSTEMS