We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

The authors have designed, fabricated, and tested vertical-cavity surface-emitting lasers (VCSEL) with diameters ranging from 0.5 mu m to>50 mu m. Design issues, molecular beam epitaxial growth, fabrication, and lasing characteristics are discussed. The topics considered in fabrication of VCSELs are microlaser geometries; ion implementation and masks; ion beam etching packaging and arrays, and ultrasmall devices. >

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Vertical-cavity surface-emitting lasers: Design, growth, fabrication, characterization

A nanocrystal capable of light emission includes a nanoparticle having photoluminescence having quantum yields of greater than 30%.

Highly luminescent color-selective nanocrystalline materials

A semiconductor nanocrystal compound and probe are described. The compound is capable of linking to one or more affinity molecules. The compound comprises (1) one or more semiconductor nanocrystals capable of, in response to exposure to a first energy, providing a second energy, and (2) one or more linking agents, having a first portion linked to the one or more semiconductor nanocrystals and a second portion capable of linking to one or more affinity molecules. One or more semiconductor nanocrystal compounds are linked to one or more affinity molecules to form a semiconductor nanocrystal probe capable of bonding with one or more detectable substances in a material being analyzed, and capable of, in response to exposure to a first energy, providing a second energy. Also described are processes for respectively: making the semiconductor nanocrystal compound; making the semiconductor nanocrystal probe; and treating materials with the probe.

Semiconductor nanocrystal probes for biological applications and process for making and using such probes

Surface Chemistry of Prototypical Bulk II-VI and III-V Semiconductors and Implications for Chemical Sensing.

X‐ray photoelectron spectroscopy reveals that the remarkable electronic quality of GaAs/sulfide interfaces can be ascribed to the formation of AsxSy phases which grow on an oxide‐free GaAs surface. While one of these phases is akin to As2S3, another shows significant in‐plane S—S bonding. Raman experiments indicate that the band bending on this disulfide‐ terminated surface has been reduced to 0.12 eV.

Electronic passivation of GaAs surfaces through the formation of arsenic—sulfur bonds

We describe chemical modifications of GaAs surfaces which produce robust selenium layers that significantly enhance the electronic properties of the surface. The terminating layers are produced by depositing elemental selenium on GaAs surfaces under alkaline conditions followed by conversion to selenide and selenate using sodium sulfide. These selenium phases are more stable against photo‐oxidation than their sulfide counterparts. On the chemically modified surface, photoluminescence is enhanced 400× and Raman spectroscopy indicates that surface band bending has been reduced to ∼0.1 eV. X‐ray photoelectron spectroscopy reveals significant AsSe bond formation at the surface and a complicated interfacial structure with selenium present in oxidation states varying from 2− to 4+.

Enhanced electronic properties of GaAs surfaces chemically passivated by selenium reactions

Molecular beam epitaxy has been used to grow microcrystalline clusters of gallium arsenide (GaAs) in the size range from 2.5 to 60 nanometers on high-purity, amorphous silica supports. High-resolution transmission electron microscopy reveals that clusters as small as 3.5 nanometers have good crystalline order with a lattice constant equal to that of bulk GaAs. Study of the microcrystallite surfaces by x-ray photoelectron spectroscopy shows that they are covered with a shell (1.0 to 1.5 nanometers thick) of native oxides of gallium and arsenic (Ga2O3 and As2O3), whose presence could explain the low luminescence efficiency of the clusters. Optical absorption spectra of the supported GaAs are consistent with the blue-shifted band edge expected for semiconductor microcrystallites in the quantum size regime.

http://science.sciencemag.org/content/sci/245/4916/391.full.pdf

M. S. Hegde

Papers

Electronic passivation of GaAs surfaces through the formation of arsenic—sulfur bonds

Enhanced electronic properties of GaAs surfaces chemically passivated by selenium reactions

GaAs Clusters in the Quantum Size Regime: Growth on High Surface Area Silica by Molecular Beam Epitaxy.