No abstract
In 0.53 Ga 0.47 As/InP quantum wires have been investigated by cw high-excitation luminescence spectroscopy. Quantum-wire states up to the edge of the InP conduction band have been populated in the dense neutral electron-hole plasma. Up to three lateral subband transitions are clearly observed in the luminescence spectra. By using calculated line shapes, the temperature and the density of the e-h plasma as well as the band-gap reduction of the different lateral subbands have been determined. We observe a clear dependence of the renormalization on the subband index, which is traced to the density dependence of the exchange energy.
A free-standing lateral nanostructure based on GaAs͓001͔ containing a Ga 0.97 In 0.03 As single quantum well and similar structures after the overgrowth with GaAs and AlAs, respectively, have been investigated by high-resolution x-ray grazing incidence diffraction ͑GID͒ and conventional x-ray diffraction ͑HRXRD͒. The wire shape of the freestanding structure and the lateral density variation in the overgrown samples, were determined by running scans with constant length of the scattering vector ͑transverse scans͒ across the grating truncation rods ͑GTR's͒ close to the (2 20) reflection. The in-plane strain distribution became available crossing the (220) GTR's by a scan in the longitudinal direction. Exploiting the capability of GID for depth resolution, the in-plane strain distribution was analyzed for different values of depth below the sample surface. The strain analysis was completed by HRXRD measurements close to the ͑004͒ reflection. The x-ray measurements were interpreted in terms of the distorted wave Born approximation applied for GID geometry. The strain distribution is determined by comparing the measured GTR intensities with the corresponding simulations containing the displacement fields obtained from finite-element calculations. At the freestanding wire structure we find laterally compressive strain of about ⌬a/a ʈ ϭϪ2ϫ10 Ϫ3 at the single quantum well ͑SQW͒ with a steep strain gradient close to the wire side walls. Both overgrown samples show pronounced lateral strain variation within the overgrown layer, which still appears up to the completely planar surface. Within the SQW the in-plane strain is still compressive after GaAs overgrowth and of similar amount compared to the freestanding grating. The strain is increased by about 30% after overgrowth with AlAs. For both overgrown samples the strain gradient near the wire side walls is reduced, but reaches a maximum close to the SQW. Accompanied by the defect passivation, these findings explain the difference in the energy shift of the photoluminescence line between freestanding and overgrown lateral nanostructures. ͓S0163-1829͑99͒08047-9͔
The vertical variation of in-plane strain induced by an In0.1Ga0.9As single quantum well (SQW) embedded in a free-standing wire structure on GaAs[001] has been investigated by depth resolved x-ray grazing incidence diffraction. If the wires are oriented along the [110] direction both the shape and strain influence on the x-ray intensity distribution can be separated by running transverse or longitudinal scans across the grating truncation rods (GTRs) close to the (2̄20) and (2̄2̄0) in-plane Bragg reflection, respectively. The GTRs themselves are modulated due to the vertical layering of the wires. The vertical strain variation in the vicinity of SQW is particularly inspected at the weak (200) Bragg reflection which is most sensitive to the scattering density difference between the SQW and GaAs. The theoretical analysis is based on the distorted wave Born approximation for grazing incidence geometry. The structural parameters of the surface nanostructure were determined with high accuracy by fitting of the complete set of experimental GTRs simultaneously. In agreement with finite-element calculations we find a maximum in-plane lattice displacement within the SQW of (Δa‖/a≈3.5×10−4) with respect to the substrate. It induces dilative in-plane strain in the GaAs confinement layers decreasing towards the upper free surface and the bulk, respectively. The evaluated in-plane strain within the SQW is used for estimating the strain induced redshift of the photoluminescence wavelength of the respective optical device.
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