Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

crystal, indicating lattice-matched epitaxial growth of the shell onto the core.The ZnCdSe 2 shell passivates the surface of the InP core. Hence, whereas bareInP cores with diameters of 22 and 42 A˚ exhibited no photoluminescence,these cores capped with a 5-Å ZnCdSe 2 shell show PL quantum yields of 5–10% at room temperature (see Fig. 4). The absorption and emission spectrashow a red shift of the core-shell QD compared to the core alone. The red shiftwas measured as a function of ZnCdSe 2 shell thickness (up to 50 A˚ ) for a corediameter of 30 A˚ and increased with increasing shell thickness. This red shiftwas not as large as that between a 30-A˚ InP core and a larger InP QDconsisting of the 30-A˚ InP core plus InP shells of equivalent thickness to the(InP)ZnCdSe 2 QDs. High-level calculations of the electronic structure of thecore-shell (InP)ZnCdSe 2 and bare InP QDs were made using both selfconsistentfield and tight-binding methods [34]. The wave functions andelectron radial probability density distributions were calculated, and thetheoretical red shifts calculated from these functions were consistent withexperiment.B. Quantum Dots Grown via Vapor-Phase DepositionSemiconductor QDs can also be formed via deposition from the vapor phaseonto appropriate substrates in MBE or MOCVD reactors [36,37]. There aretwo modes of formation. In one, termed Stranski–Krastinow (S-K) growth,nanometer-sized islands can form when several monolayers (about 3–10) ofone semiconductor are deposited upon another and there is large latticemismatch(several percent) between the two semiconductor materials; this hasbeen demonstrated for Ge/Si [38,39], InGaAs/GaAs [40–42], InP/GaInP [43],and InP/A1GaAs [44,45]. For these highly strained systems, epitaxial growthinitiates in a layer-by-layer fashion and transforms to 3D island growth abovefour monolayers to minimize the strain energy contained in the film (seeFig. 5). The islands then grow coherently on the substrate without generationof misfit dislocations until a certain critical strain energy density, correspondingto a critical size, is exceeded [38,40]. Beyond the critical size, thestrain of the film-substrate system is partially relieved by the formation ofdislocations near the edges of the islands [40]. Coherent S-K islands can beovergrown with a passivating and carrier-confining epitaxial layer to produceQDs with good luminescence efficiency. The optical quality of such overgrownQD samples depends on the growth conditions of the capping layer.The second approach is to first produce a near-surface quantum well(formed from 2D quantum films) and then deposit coherent S-K islands ontop of the outer barrier layer of the QW that have a large lattice mismatch withthe barrier that subsequently produces a compressive strain in the island[46,47]. The large resultant strain field can extend down into the QW structureCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.