InAs/GaAs superlattices with ultra-thin InAs (few monolayer) were grown on GaAs substrates. Nucleation of InAs occurs in a two-dimensional or a three-dimensional way depending on the growth conditions. The physical properties: x ray, transmission electron microscopy, and photoluminescence were used to characterize the different growth processes.

We present optical absorption experiments in strained In"Gal "As-GaAs superlattices grown by molecular-beam epitaxy. From our data, we show that ground heavy-and light-hole states are confined in the In"Ga& "As and in the GaAs layers, respectively. We also use a new method to observe the optical selection rules in such two-dimensional structures.Among semiconductor superlattices, which are twodimensional systems first proposed by Esaki and Tsu' in 1970, the GaAs-Ga"All As structures have been the most thoroughly studied. This system is usually said to be of type I: Owing to the different band gaps of the host semiconductors, a superpotential is created in the indirection perpendicular to the plane of the layers and tends to confine both electrons and holes in the smaller gap material, i.e. , in the GaAs layers. 2 On the other hand, in InAs-GaSb superlattices, the top of the GaSb valence band lies at higher energy than the bottom of the InAs conduction band and, as a result, electrons are mainly localized in the InAs layers while holes are confined in the GaSb ones:3 Such a system, where electrons and holes are spatially separated, is said to be of type II. In this work, we show for the first time that an alternative situation can be reached in strainedIn"Gal "As-GaAs multiquantum wells or superlattices.Indeed, because of the strain, which is due to the lattice mismatch between the involved semiconductors, such a structure is, at the same time, of type I for heavy holes and of type II for light holes.The potential interest in these strained semiconductor heterostructures has already been demonstrated. 4 The possibility of getting rid of the drastic lattice-matching condition in the heteroepitaxy of sufficiently thin'layers' enlarges the choice of materials that can be grown on a given substrate. In the considered system, for an In content of 15%, the ternary layer thickness should be smaller than 120A to obtain good-quality layers where the lattice mismatch is totally accommodated by elastic strain. In this elastic limit, in an In"Gal As/GaAs heterostructure grown on a GaAs substrate, only the In"Gal "As layers are strained, and the strain amplitude is independent of their thicknesses. Accordingly, the valence-band degeneracy should be lifted. In such a system, the relative positions of the bands in the In"Gal "As wells and the GaAs barriers can lead to several plausible configurations of the quantizing superpotentials, which are shown in Fig. 1. By analyzing optical data obtained on a series of In"Gal "As/GaAs multi-quantum-well structures with x =0.15, we show that, around this composition, the GaAs valence band actually lies between the In"Gal "As heavy-and light-hole bands. Figure 2 shows absorption spectra obtained at 77 K on three samples consisting of 10 periods of alternate layers of GaAs (thickness 200 A) and In"Ga~"As (thickness 50, 100, 120 A, x =0.15) grown by molecular-beam epitaxy on a (100) GaAs substrate. They have steplike shapes and exGaAs GaAs CB« InGa As InGaAs VS~v h 'f~v LH (b) LH FIG. 1. Pos...

A bevel has been etched in a GaAs epitaxial film grown on Si substrate, so that Raman spectrum of GaAs layers can be measured as a function of distance from the GaAs/Si interface. The amount

Time-resolved photoluminescence (PL) spectroscopy has been performed on erbium-doped Ga0.55Al0.45As. We have investigated the 4I13/2→4I15/2 optical transition. The measured fluorescence lifetime is around 1.2 ms which is similar to values found for other erbium-doped III-V compounds. Studies as a function of temperature indicate nonradiative transitions characterized by thermal activation of 40 meV. With the help of two beam experiments we have confirmed the existence of Auger losses during the PL excitation and a model of the excitation process is proposed that accounts for the experimental results.

We analyze in this paper the effect of thermal treatment on structural properties of GaAs/InGaAs strained-layer superlattices grown by molecular beam epitaxy. The superlattices are analyzed using double-crystal x-ray rocking curves. In order to model the satellites intensity variation as a function of the heat treatment time at a temperature of 850 °C, we have calculated the structure factors of the superlattices, taking into account both composition and lattice spacing modulation. The latter is found to be more influent in the calculation in this particular case. The deduced values of the diffusion coefficient, about 2×10−18 cm2/s, is discussed and compared to those determined on GaAs/AlAs structures.