M. A. Vidal

Dislocation densities in MBE grown ZnSe epitaxial layers on GaAs by HRXRD

Abstract In this work, we report on dislocation densities in MBE grown ZnSe/GaAs/GaAs heterostructures of different epilayer thickness. We examined the full-width at half-maximum of the reflection peaks obtained by high-resolution X-ray diffraction. We observed three regimes of dislocation generation. The first regime is present for samples of subcritical thickness, where only stacking faults are present and their formation depends on the conditions of preparation of the substrate and the initial conditions of growth. The second regime exists for samples with layer thickness greater than the critical thickness and thinner than a threshold thickness h t ≅0.3xa0μm, where a large amount of misfit and threading dislocations are generated. The third regime starts for layer thickness greater than h t . In this regime the formation of dislocations is substantially slowed down and the dislocation density follows a 1/ h dependence as predicted by the glide model. Comparing dislocation densities obtained by X-ray diffractometry with the pit density revealed by chemical etching of one sample and those observed by transmission electron microscopy of two samples, agreement exists among them within the same order of magnitude.

Determination of the Thermal Expansion Coefficient of Single-Wall Carbon Nanotubes by Raman Spectroscopy

We have examined the effect of high temperature on single-wall carbon nanotubes under air and nitrogen ambient by Raman spectroscopy. We observe the temperature dependence of the radial breathing mode and the G-band modes. The thermal expansion coefficient (β) of the bundled nanotubes is obtained experimentally using the estimated volume from Raman scattering. β behaves linearly with temperature from 0.33 × 10−5 K−1 to 0.28 × 10−5 K−1 in air and from 0.58 × 10−5 K−1 to 0.47 × 10−5 K−1 in nitrogen ambient, respectively. The temperature dependence of the radial breathing mode Raman frequencies is consistent with a pure temperature effect.

Structural study of ZnSe films grown on substrate with InxGa1-xAs and Al1-xGaxAs buffer layers: strain, relaxation and lattice parameter

ZnSe layers of various thickness were grown on (001) GaAs substrates, using InxGa1-xAs or Al1-xGaxAs as buffer layers by molecular beam epitaxy and were studied by high-resolution x-ray diffraction. The principal structural characteristics of ZnSe layer and buffer layer were determined using several reflections, such as (004) and two pairs of coupled asymmetric reflections, namely (224), (-2-24) and (115) (-1-15). In order to evaluate their validity, the experimental data obtained from these reflections were handled by means of two known expressions found in the literature. We have found the relaxation process of ZnSe layers is well described by a geometrical model including the thermal strain and small strain due to work hardening. The relaxation process is faster for ZnSe grown on ternary buffer layers despite the fact that, some buffer layers are pseudomorphically grown to the substrate; therefore we conclude that not only the lattice mismatches have effect on the relaxation process but also the surface state of the buffer layer has an influence in this process.

Excitonic transitions in (GaAs)1−x(Ge2)x/GaAs multilayers grown by magnetron sputtering

We report on excitonic transitions observed at room temperature in multilayers of (GaAs)1−x(Ge2)x/GaAs grown by magnetron sputtering. We attribute the observation of the exciton absorption at room temperature to confinement effects on the bound electron–hole pairs inside the Ge-rich layers of the (GaAs)1−x(Ge2)x alloys. From secondary ions mass spectroscopy these layers are measured to be 50, 33, and 30 nm in full width at half maximum (FWHM), for three different samples. These layers alternate with (GaAs)1−x(Ge2)x alloy layers of very low Ge concentration that can be considered of plain GaAs, ∼360u2009nm in thickness for two samples and 240 nm for a third one. The observed transition energies of the exciton are very close to ≈1u2009eV both at 300 and 4 K. A calculation of exciton energy-level spacing and transition probabilities provides acceptable values for the exciton transition energies observed, considering a triangular quantum well as an approximation to the Ge profile in the confinement layers. The observ...

Strain in GaAs at the heterointerface of ZnSe/GaAs/GaAs

GaAs at interfaces of molecular beam epitaxy (MBE) grown ZnSe/GaAs/GaAs films with ZnSe layers of different thicknesses is studied by photoreflectance (PR) spectroscopy. We can separate two different near-band-edge optical features originating from two different regions of the heterostructure by using in-phase and out-phase PR measurements as well as the results of two different wavelength pumping lasers. One of the transitions is a bulk-like signal as for bare GaAs and another signal is attributed to a strained region adjacent to the ZnSe/GaAs interface. The bulk-like signal originates in a region that encompasses the buffer layer/substrate GaAs interface, as is also revealed by the observation of Franz--Keldysh oscillations in the transitions from this region, which manifest the existence of an electric field within it. An electric field which is larger in magnitude is also visible in the PR signal from the heterointerface. Results for the second derivative of reflectance difference spectra (SDRD) further supports the existence of two spatially separated regions in the GaAs that produces two independent overlapping optical modulated signals in these heterostructures. From the theory of PR we estimate that the observed compressive strain giving rise to the second component has a value , which is independent of the thickness of the ZnSe epilayer. Atomic force microscopy (AFM) measurements were carried out on the GaAs epilayer prior to ZnSe growth revealing an almost uniform density of pits for all samples observed. These have irregular cross section profiles, a situation that tends to preclude coherent growth between the ZnSe and the GaAs. We calculate that a strain in the upper atomic layers of the GaAs has to be present due to the incoherent growth of ZnSe inside the GaAs pits and to the difference in thermal expansion coefficients between the GaAs and the ZnSe. Both phenomena are expected to produce a total strain of the same magnitude as that observed by PR.