We have studied the photoluminescence and photoexcitation spectra of ultrasmall structures, of approximately 500 Å in dimension, which we refer to as quantum ribbons and quantum disks. These are fabricated from GaAs-AlGaAs quantum wells grown by molecular beam epitaxy and patterned by electron beam lithography. Contrary to our expectation, photoluminescence from these structures is extremely efficient. The excitation spectra of the two types of small structures differ greatly from each other and from that of the as-grown quantum wells. These differences may be a result of the confinement of the carriers in these small structures.

We report the observation of surface-enhanced Raman scattering (SERS) from a chemically etched ZnSe surface using 4-mercaptopyridine (4-MPy) as probe molecules. A thin film of ZnSe is grown by molecular beam epitaxy (MBE) and then etched using a strong acid. Protrusions of hemiellipsoidal nanoparticles are observed on the surface. Using the results of the Mie theory, we controlled the size of the nanoparticles to overlap significantly with maximum efficiency of near-field plasmon enhancement. In the Raman spectrum, we observe large enhancements of the a1, b1, and b2 modes when 4-MPy molecules are adsorbed on the surface using a 514.5 nm laser for excitation, indicating strong charge-transfer contributions. An enhancement factor of (2 × 106) is observed comparable to that of silver nanoparticles. We believe this large enhancement factor is an indication of the coupled contribution of several resonances. We propose that some combination of surface plasmon, charge transfer, and band-gap resonances is most likely the contributing factor in the observed Raman signal enhancement because all three of these resonances lie close to the excitation wavelength.

Using surface enhanced Raman spectroscopy (SERS), we observed Raman enhancements (104−105) for pyridine molecules adsorbed on II−VI semiconductor quantum dots on (uncapped CdSe/ZnBeSe) produced by molecular beam epitaxy. When a monolayer of pyridine is adsorbed on these structures, excitation at 488 nm produces intense Raman spectra, a very large enhancement of the a1, b1, and b2 modes. This indicates the presence of charge transfer as a contributor to the enhancement. Furthermore, the excitation wavelength is in the vicinity of several interband transitions located both in the quantum dots and the wetting layer, and it is likely that these resonances also contribute to the enhancement factor.

We present Raman scattering data from GaAs samples whose surfaces had been treated with thin films of sodium sulfide nonahydride (Na2S⋅9H2O). Raman scattering provides a quantitative, contactless means of measuring the reduced barrier height associated with decreased density of GaAs surface states. For GaAs samples doped at levels of n≊1018 cm−3, the barrier height is reduced to 0.48±0.10 eV.

GaAs/AlGaAs superlattice heterostructures with layer thicknesses ≲100 Å were grown by molecular beam epitaxy on nonplanar GaAs substrates. The resulting superlattices exhibit different periods, depending on the crystal plane on which they grow. Period variation of more than 50%, from 180 to 80 Å, was obtained for adjacent superlattice sections. The transition between regions of different periodicity was mostly smooth and occurred within lateral dimensions ≲100 Å. Our results suggest that molecular beam epitaxy of superlattice heterostructures on patterned substrates provides a method for obtaining controllable lateral variations in physical properties which depend on the superlattice period. In particular, by growing quantum well heterostructures on nonplanar substrates, it might be possible to utilize the strong dependence of the carrier confinement energy on the well thickness in order to achieve lateral carrier confinement.