<title>Self-assembled visible-bandgap II-VI quantum dots</title>

Abstract

This paper reports on the self-assembled growth of Il-VI semiconductor quantum dots by molecular beam epitaxy. The dots are formed in a highly-strained (Zn,Cd)Se film of only a few monolayer width grown on ZnSe. The formation sets on when the CD mole fraction exceeds 30 %. We present data on the recombination and relaxation of carriers and excitons in these zero-dimensional structures as well as their interaction with phonons.keywords: quantum dots, (Zn,Cd)Se, ZnSe, luminescence, phonons, confined excitons, MBE growth 1. ENTRODUCTION Wide-gap 11-Vt heterostructures are promising candidates for the fabrication of visible-wavelength opto-electronic and photomc devices. In recent years, semiconductor quantum dots have attracted particular interest. There are various predictions that these zero-dimensional structures exhibit a number of extraordinary properties, making them especially useful for light emitting devices, modulators, or switches. However, despite of extensive efforts, many of these predictions have not been confirmed experimentally so far. This is partly caused by an insufficient sample quality (large homogeneous and itthomogeneous broadening, surface states, low radiative yield,...) and partly by inappropriate theoretical approaches.Recent work on 111-V materials, e.g. (In,Ga)As, has demonstrated that quantum dots can be directly fabricated by molecular beam epitaxy utilizing self-organization [1]. One example is the Stranski-Krastanov mechanism, where strain relaxation turns the growth from an initially two-dimensional into a three-dimensional one. This allows to apply purity and quality standards inherent in epitaxial growth. By definition, in a "true" quantum dot, the carriers (electron and holes) are confined in a volume that is comparable with the excitonic Bohr radius. In this situation, concepts successfully used to understand the physics of bulk or quantum well devices, are no longer valid. Ultimately, the electron-hole Coulomb interaction will play a key role. The laser action of quantum dots is one example, where this is readily seen. Instead of an electron-hole plasma, bi-excitons are carrying the optical gain [2][3][4]. It is obvious from Table 1, comparing exciton and bi-exciton binding energies for (Zn,Cd)Se and (In,Ga)As, that Il-VI quantum dots will provide much more direct experimental access to the relevant processes (though the underlying physics in Ill-V structures is qualitatively the same). Therefore, in addition to their visible band-gap, Il-VI quantum dots are also ideal candidates to study various fundamental aspects of zero-dimensional semiconductor structures. SPIE Vol. 2994 • 0277-786X197/$1 0.00 43 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/27/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx