A. Riposan

Surface reconstructions of In-enriched InGaAs alloys

The atomic structure of In0.81Ga0.19As/InP alloy layers was examined using in situ scanning tunneling microscopy. The (2×3) reconstruction observed during growth by reflection high-energy electron diffraction represents a combination of surface structures, including a β2(2×4) commonly observed on GaAs(001) and InAs(001) surfaces, and a disordered (4×3) that is unique to alloy systems. The proposed (4×3) structure is comprised of both anion and cation dimers. Empty and filled states images show that the features reverse contrast with sample bias, in agreement with the model.

The effect of island density on pit nucleation in In0.27Ga0.73As/GaAs films

Pit nucleation during the growth of In 0.27 Ga 0.73 As/GaAs compressively strained films was investigated under various growth conditions. In this system, pit nucleation occurs after the nucleation of three-dimensional islands, but prior to the formation of surface ripples. Island and pit size, density, area coverage and average separation were measured at various stages during growth. Pits form in regions of high local island density, and significant pit formation is correlated with a critical island separation. These results are interpreted taking into account the adatom concentration on the surface during island growth. It is postulated that adatom depletion in regions where the local island density is high, combined with local strain effects, make areas of high island density favorable sites for pit nucleation.

Resonant coherent Bragg rod analysis of strained epitaxial heterostructures

The resonant response of the complex x-ray scattering factor has been used in conjunction with the coherent Bragg rod analysis phase-retrieval algorithm to determine the composition and strain profiles of ultrathin layers of GaAs grown on InGaAs buffers. The buffer layers are nominally latticed matched with the InP substrate and the subsequent GaAs growth is compared at two different temperatures: 480 and 520°C. We show that electron density maps extracted from Bragg rod scans measured close to the Ga and As K-edges can be used to deconvolute roughness and intermixing. It is found that indium incorporation and roughening lead to a significant reduction of the strain in this system.

Critical film thickness dependence on As flux in In0.27Ga0.73As∕GaAs(001) films

The transition between planar and nonplanar growth is examined for compressively strained In0.27Ga0.73As∕GaAs(001) films using reflection high energy electron diffraction, atomic force microscopy, and scanning tunneling microscopy (STM). For a narrow range of temperature and composition, the critical thickness (tSK) is strongly dependent on As flux. For high values of As flux, tSK increases by more than a factor of 2. The morphology of three-dimensional islands formed during the initial stages of nonplanar growth is also characterized by high resolution STM.

Strain mediated reconstructions and indium segregation on InGaAs∕GaAs(001) alloy surfaces at intermediate lattice mismatch

In vacuo scanning tunneling microscopy is used to investigate the surface reconstructions of pseudomorphic InGaAs alloys at intermediate values of compressive strain. The coverage of different reconstructions varies with film thickness, concomitant with changes in composition and strain at the surface arising from In segregation and changes in surface morphology. Thin samples exhibit mainly disordered (1×3) reconstructions along with small regions of incommensurate (1×2). With increasing thickness, the (1×3) transforms into more regular (4×3) or c(4×6), whose coverage mirrors the increase and saturation of In surface composition. Regions of α2(2×4) reconstructions are also present, and their coverage initially increases with In surface composition, but later decreases upon saturation of In at the surface. This decrease is concurrent with the onset of surface roughening, suggesting that the α2(2×4) reconstruction is strain stabilized.

Step instability and island formation during annealing of pseudomorphic InGaAs/GaAs layers

The morphological stability of compressively strained In0.27Ga0.73As/GaAs pseudomorphic layers has been investigated during annealing. Large three-dimensional islands form at the beginning of annealing on initially flat surfaces, likely to relieve strain energy. The islands disappear with increasing annealing, being reabsorbed into the terraces. At the same time, the step line destabilizes forming cusps that inject two-dimensional vacancy islands into the terrace. At high temperatures, this process leads to a severe deterioration of the morphology that is not due to decomposition. The island dissolution and the development of the step instability are likely alternative paths towards the reduction of surface energy.

Strain and composition mapping of epitaxial nanostructures

We have used surface x-ray diffraction and a direct method of phase reconstruction to obtain subangstrom resolution maps of an ion-beam milled In0.27Ga0.73As∕GaAs(001) thin film exhibiting three-dimensional (3D) epitaxial nanostructures. The 3D electron density was calculated based on the diffraction pattern along the Bragg rods measured with synchrotron radiation, from which the chemical composition, strain profile, and average nanostructure shape were extracted. The film maintained a wetting layer exhibiting a sharp strain gradient, which extended into the substrate. Atop the wetting layer, the ion-beam milled islands possessed an apical shape and were depleted in indium.

Room temperature spin dependent current modulation in an InGaAs-based spin transistor with ferromagnetic contact

We have observed clear current oscillations when the electrodes were magnetized along the channel current at room temperature. The drain current oscillation dependence on gate voltage agreed with the estimation. We believe that this is the first observation of spin transistor operation.

Inverse Stranski-Krastanov growth in InGaAs/InP

Thin films of InxGa1−xAs are observed to develop either islands or pits on the surface to relieve lattice mismatch strain after some critical thickness depending on the composition of the film. The composition is thought to alter either the surface energy or the equilibrium adatom concentration of the growing film, thus changing which strain relieving features, islands, or pits nucleate on the surface first. Once pits form, their behavior is similar to that of islanding, resulting in an “inverse” Stranski–Krastanov growth mode. Furthermore, the atomic surface structure near the pits is different than away from the pits and is correlated with island formation.