Semiconductor Quantum Dots
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Semiconductor Quantum Dots Alex Zunger, Guest Editor Semiconductor "quantum dots" refer to nanometer-sized, giant (103-105 atoms) molecules made from ordinary inorganic semiconductor materials such as Si, InP, CdSe, etc. They are larger than the traditional "molecular clusters" (~1 nanometer containing £100 atoms) common in chemistry yet smaller than the structures of the order of a micron, manufactured by current electronic-industry lithographic techniques. Quantum dots can be made by colloidal chemistry techniques (see the articles by Alivisatos and by Nozik and Micic in this issue), by controlled coarsening during epitaxial growth (see the article by Bimberg et al. in this issue), by size fluctuations in conventional quantum wells (see the article by Gammon in this issue), or via nanofabrication (see the article by Tarucha in this issue). Colloidal dots are "freestanding" in that they are not buried inside another semiconductor. Thus, they are strain-free. They are coated with organic ligand molecules used to prevent the coagulation of small dots during growth. The size of these molecules can be controlled during growth, and their shape is nearly spherical. Colloidal techniques have been perfected mostly for ionic II-VI systems (CdS, CdSe) and more recently for III-V semiconductors (InP, GaP, InAs). Because of the excellent size uniformity, high-resolution spectroscopic studies are available. These have revealed new physical effects, including dramatic enhancement of the electron-hole exchange interaction relative to the corresponding bulk solids, charge transfer in the excited state, unusual behavior (relative to bulk) under pressure (e.g., delayed phase transitions), and the detection of up to 10 excited electron-hole exciton states. It is now possible to replace the organic passivation shell around these dots by inor-
ganic semiconductors—for example, CdSe (ZnS)—thus producing "coreshell" structures. Arrays of colloidal dots have also been made. Futhermore gate structures permitting the "loading" of colloidal quantum dots by carriers have recently become possible for a 60-A dot. Controlled coarsening of a film of mate-
rial A grown on a substrate made of material B produces islands of A, provided that A and B have a sufficiently different atomic size. (Otherwise one forms films of A on B.) Examples of A/B pairs include InAs/GaAs and InP/GalnP. If one stops the metalorganic-chemical-vapordeposition or the molecular-beamepitaxy growth just before the islands coalesce, one gets a surprisingly uniform set of dots of material A. Capping by B then produces "buried" dots. Because a difference in A versus B atomic size is needed to produce those islands in the first place, capping produces—by necessity—strain. Thus the electronic structure of those dots is controlled not only by size effects but also by (inhomogeneous) strain. Capping by a native semiconductor also produces excellent electronic passivation. The shapes of these dots are rather controversial. They start out as pyramids, but capping changes the shape
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