M. Gendry

Role of buffer surface morphology and alloying effects on the properties of InAs nanostructures grown on InP(001)

We show the role played by the buffer surface morphology and by alloying effects on the size, shape and lateral distribution of InAs nanostructures grown on InP(001) substrates by molecular beam epitaxy. Three buffers, viz., In0.53Ga0.47As, In0.52Al0.48As, and InP lattice matched on InP have been studied. Differences in nanostructure morphology and in carrier confinement have been evaluated by atomic force microscopy and by low-temperature photoluminescence measurements, respectively. Alongside the classical relaxation mode through two-dimensional/three-dimensional surface morphology change, a chemical relaxation mode has to be introduced as a competitive mode of relaxation of strained layers. This chemical relaxation mode, due to alloying between the InAs deposit and the buffer, is thought to be responsible for most of the observed differences in the InAs nanostructure properties.

Surface effects on shape, self-organization and photoluminescence of InAs islands grown on InAlAs/InP(001)

InAs nanostructures were grown on In0.52Al0.48As alloy lattice matched on InP(001) substrates by molecular beam epitaxy using specific growth parameters in order to improve island self-organization. We show how the change in InAs surface reconstruction via growth temperature from (2×4) to (2×1) and/or the use of InAlAs initial buffer surface treatments improve the island shape homogeneity (either as quantum wires or as quantum dots). Differences in island shape and in carrier confinement are shown by atomic force microscopy and by photoluminescence measurements, respectively. We point out that such shape amendments induce drastic improvements to island size distribution and discernible changes in photoluminescence properties, in particular concerning polarization.

Strong normal-incidence infrared absorption in self-organized InAs/InAlAs quantum dots grown on InP(001)

InAs self-assembled quantum dots in InAlAs matrix grown on InP(001) substrates have been fabricated using Stranski–Krastanov growth mode. A strong in-plane polarized intraband absorption in the 10.6–20 μm wavelength region has been observed and ascribed to a transition from the ground electron state to an excited state confined in the layer plane along the [110] direction. The absorption at normal-incidence reaches 7.8% for ten layers of n-doped quantum dots. The oscillator strength of the intraband transition is comparable to that achieved in quantum wells for a conduction band intersubband transition. The dependence of the intraband absorption on carrier concentration and temperature suggests a quantum-wire type confinement potential.

From large to low height dispersion for self-organized InAs quantum sticks emitting at 1.55 μm on InP (001)

We show how the height dispersion of self-organized InAs/InP(001) quantum islands emitting at 1.55 μm was reduced by optimizing the epitaxial growth parameters. Low height dispersion was obtained when the InAs deposit thickness was much greater than the critical thickness for two-dimensional/three-dimensional growth mode transition, and when adatom surface diffusion was favored by increasing the growth temperature or reducing the arsenic pressure during the InAs growth. When these growth conditions are not respected, the multicomponent photoluminescence spectrum obtained is explained through the common interpretation of island height varying with monolayer fluctuation. In optimized growth conditions, the multicomponent spectrum obtained is interpreted as emission from fundamental and excited levels of InAs islands with low height dispersion. Transmission electron microscopy (TEM) imaging shows that these InAs islands are stick-like, 50–100 nm in length and 22±1.2 nm in width. Cross-sectional TEM reveals f...

Structural properties of epitaxial SrTiO3 thin films grown by molecular beam epitaxy on Si(001)

This work reports on the structural properties of an epitaxial SrTiO3 (STO) layer grown by molecular beam epitaxy on a Si(001) substrate with a two step process. The study, which includes a complete characterization of large scale plane-view images of the STO layer, is based on a careful analysis of x-ray spectra and transmission electron microscopy images. The STO layer presents a good crystalline quality and a slight texturation related to the presence of extended defects. A thin Ti-rich amorphous silicate layer (thickness ≈1.3nm) is formed at the interface between the STO and the Si substrate, evidencing the thermodynamic instability of the STO/Si interface. The difference between the thermal expansion coefficients of Si and STO is shown to be at the origin of an increased in-plane lattice parameter (3.927A) of the STO layer as compared to its bulk value (3.905A). This effect of differential thermal expansion is expected to be responsible for the formation of at least part of the extended defects of th...

Two-dimensional hexagonal-shaped microcavities formed in a two-dimensional photonic crystal on an InP membrane

We present simulation and experimental results on hexagonal-shaped microcavities formed in two-dimensional (2D) photonic crystals (PC’s). The PC structures, realized with InP-based materials, are studied in two configurations : Air-suspended membranes (A type) and membranes supported by silica (S type). The optical properties of these microcavities are analyzed through photoluminescence experiments. Plane-wave expansion method calculations provide simulation results that are consistent with experimental data. The influence on spectral properties of various parameters, such as cavity size or air filling factor (f), is thoroughly analyzed, and their effect on resonator loss mechanisms is extracted, to give guidance for further PC laser improvement, e.g., threshold reduction.

Staggered vertical self-organization of stacked InAs/InAlAs quantum wires on InP(001)

Abstract Using atomic force microscopy (AFM) imaging, transmission electron microscopy (TEM) and photoluminescence (PL), we have studied InAs stacked islands on InP(001) versus the InAlAs spacer layer thickness (SLT). We have found that first wire-like island shape is strongly favored by such a stacking process and second in the 10–25 nm SLT range, the wire size and height are dependent on the SLT. TEM images show off a new surprising staggered vertical island organization that can be explained by the phase separation appearing in the InAlAs spacer layers.

Optical properties of self-assembled InAs quantum islands grown on InP(001) vicinal substrates

We have investigated the effect of misorientated InP(001) substrates on the optical properties of InAs quantum islands (QIs) grown by molecular-beam epitaxy in the Stranski–Krastanow regime. Detailed temperature-dependent photoluminescence (PL), excitation density PL, and polarization of photoluminescence (PPL) are studied. PPL shows a high degree of linear polarization (near 40%) for the nominally oriented substrate n and for the substrate with 2° off miscut angle toward the [110] direction (2° F), while it is near 15% for the substrate with 2° off miscut angle towards [010] direction (2° B), indicating the growth of InAs quantum wires on nominal and 2° F substrates and of InAs quantum dots on 2° B substrate. These island shapes are confirmed by morphological investigations performed by atomic force microscopy. The integrated PL intensity remains very strong at room temperature, as much as 36% of that at 8 K, indicating a strong spatial localization of the carriers in the InAs QIs grown on InP(001).

Tensile and compressive strain relief in InxGa1−xAs epilayers grown on InP probed by Raman scattering

Strain relaxation has been investigated by means of Raman scattering in strained InxGa1−xAs layers (with x ranging from 0 to 1) grown on In0.53Ga0.47As/InP (001). The epilayers are either under tensile (x 0.53) strain. Relaxation coefficients have been deduced from the frequency shifts of the GaAs-like optical phonons. A marked dissymmetry in strain relief is found over the whole composition range between equivalent tensile and compressive misfits. Disorder activated Raman scattering features have been analyzed and correlated to the structural defects resulting from the strain relief in the two and three-dimensional growth modes. Strain inhomogeneities resulting from surface corrugation are evidenced by micro-Raman measurements on layers with tensile misfits.

IMPROVED ELECTRONIC PROPERTIES OF GAAS SURFACES STABILIZED WITH PHOSPHORUS

We report on a new passivation procedure of the GaAs surface based on a thermal treatment under a PH3 overpressure. This treatment results, by As/P exchange, in the formation of a thin superficial GaP layer which prevents the formation of an arsenic oxide, as observed by x‐ray photoelectron spectroscopy analysis. Subsequent increase of the photoluminescence signal indicates improved electronic properties of GaAs surfaces as a result of this passivation procedure.