Quantum dots formed by ultrathin insertions in wide-gap matrices

We report on experimental and theoretical studies on a new type of quantum- dot (QD) structures obtained using ultrathin, i.e. below the critical thick ness for 2D-3D transition, strained narrow gap insertions in wide bandgap m atrices. We concentrate on submonolayer (SML) or slightly above 1 hn CdSe i nsertions in a wide-gap Il-VI matrices and give the first results on ultrat hin InGaN insertions in a GaN matrix. A discussion on detailed comparison o f our original results with the results of other authors is presented. The formation of dense arrays (up to 10(12) cm(-2)) of nanoscale two-dimensiona l (2D) islands is revealed in processed high-resolution transmission electr on microscopy images. In the case of stacked sheets of SML insertions, the islands in the neighboring sheets are formed predominantly in correlated or anticorrelated way for thinner and thicker spacer layers, respectively. Di fferent polarization of photoluminescence (PL) emission recorded in edge ge ometry for vertically-uncoupled and coupled QDs confirms the QD nature of e xcitons. By monitoring of sharp lines due to single QDs using cathodolumine scence the 3D confinement is manifested. We demonstrate significant squeezi ng of the QD exciton wavefunction in the lateral direction using magneto-op tical experiments. We point to complete suppression of lateral motion of ex citons bound to islands in case of wide-gap (ZnMgSSe) matrices, as follows from PL excitation studies. A resonant (0-phonon) lasing is observed in ult rathin CdSe insertions and proves the lifting of the k-selection rule for Q D excitons. We show that lack of exciton screening in QDs up to high excita tion densities enables strong resonant modulation of the refractive index i n stacked ultrathin insertions and allows realization of resonant (excitoni c) waveguiding and lasing. This enables the realization of a new type of he terostructure laser operating without external optical confinement by layer s having lower average refractive indices. Ultrahigh QD excitonic gain in d ense arrays of stacked QDs allows a new type of surface-emitting laser. (C) 2000 Elsevier Science S.A. All rights reserved.