O. Mauguin

Optical spectroscopy of two-dimensional layered (C6H5C2H4-NH3)2-PbI4 perovskite

We report on optical spectroscopy (photoluminescence and photoluminescence excitation) on two-dimensional self-organized layers of (C(6)H(5)C(2)H(4)-NH(3))(2)-PbI(4) perovskite. Temperature and excitation power dependance of the optical spectra gives a new insight into the excitonic and the phononic properties of this hybrid organic/inorganic semiconductor. In particular, exciton-phonon interaction is found to be more than one order of magnitude higher than in GaAs QWs. As a result, photoluminescence emission lines have to be interpreted in the framework of a polaron model.

High quality tensile-strained n-doped germanium thin films grown on InGaAs buffer layers by metal-organic chemical vapor deposition

We show that high quality tensile-strained n-doped germanium films can be obtained on InGaAs buffer layers using metal-organic chemical vapor deposition with isobutyl germane as germanium precursor. A tensile strain up to 0.5% is achieved, simultaneously measured by x-ray diffraction and Raman spectroscopy. The effect of tensile strain on band gap energy is directly observed by room temperature direct band gap photoluminescence.

Quantum-well saturable absorber at 1.55μm on GaAs substrate with a fast recombination rate

We propose and realize a structure designed for fast saturable absorber devices grown on GaAs substrate. The active region consists of a 1.55μm absorbing GaInNAsSb quantum well (QW) surrounded by two narrow QWs of GaAsN with a N concentration up to 13%. Photoexcited carriers in the GaInNAsSb QW are expected to recombine by tunneling into the wide distribution of subband gap states created in the GaAsN QW. An absorption study shows that edge energy and excitonic peak intensity of the GaInNAsSb QW are not affected by the proximity of the GaAsN QWs. Pump-probe measurements provide information on the carrier relaxation dynamics which is dependent on spacer thickness, as expected for a tunneling process. We show that this process can be enhanced by increasing the N content in the GaAsN layers. Using this design, we have realized a monolithic GaAs-based saturable absorber microcavity with a 1∕e recovery time of 12ps.

Magnetic properties and domain structure of (Ga,Mn)As films with perpendicular anisotropy

The ferromagnetism of a thin GaMnAs layer with a perpendicular easy anisotropy axis is investigated by means of several techniques, that yield a consistent set of data on the magnetic properties and the domain structure of this diluted ferromagnetic semiconductor. The magnetic layer was grown under tensile strain on a relaxed GaInAs buffer layer using a procedure that limits the density of threading dislocations. Magnetometry, magneto-transport and polar magneto-optical Kerr effect (PMOKE) measurements reveal the high quality of this layer, in particular through its high Curie temperature (130 K) and well-defined magnetic anisotropy. We show that magnetization reversal is initiated from a limited number of nucleation centers and develops by easy domain wall propagation. Furthermore, MOKE microscopy allowed us to characterize in detail the magnetic domain structure. In particular we show that domain shape and wall motion are very sensitive to some defects, which prevents a periodic arrangement of the domains. We ascribed these defects to threading dislocations emerging in the magnetic layer, inherent to the growth mode on a relaxed buffer.

Strain control of the magnetic anisotropy in (Ga,Mn) (As,P) ferromagnetic semiconductor layers

A small fraction of phosphorus (up to 10%) was incorporated in ferromagnetic (Ga,Mn)As epilayers grown on a GaAs substrate. P incorporation allows reducing the epitaxial strain or even change its sign, resulting in strong modifications of the magnetic anisotropy. In particular a reorientation of the easy axis toward the growth direction is observed for high P concentration. It offers an interesting alternative to the metamorphic approach, in particular for magnetization reversal experiments where epitaxial defects strongly affect the domain wall propagation.

Atomic-plane-thick reconstruction across the interface during heteroepitaxial bonding of InP-clad quantum wells on silicon

Monolithic integration of InP-based materials on Si will allow lasers as well as optical amplifiers operating at 1.55μm to be efficiently included in photonic integrated circuits. We demonstrate here oxide-free heteroepitaxial bonding of InP-clad GaInAs quantum wells to Si, with an atomic-plane-thick reconstruction across the InP-Si interface. The wells photoluminescence emitted-wavelength demonstrates no shift after bonding. Several InP surface activation procedures have been investigated. This bonding approach is compatible with guiding designs including a nanostructuration, thus enabling specific designs associated to desirable optical functions.

Nanometer-scale, quantitative composition mappings of InGaN layers from a combination of scanning transmission electron microscopy and energy dispersive x-ray spectroscopy

Using elastic scattering theory we show that a small set of energy dispersive x-ray spectroscopy (EDX) measurements is sufficient to experimentally evaluate the scattering function of electrons in high-angle annular dark field scanning transmission microscopy (HAADF-STEM). We then demonstrate how to use this function to transform qualitative HAADF-STEM images of InGaN layers into precise, quantitative chemical maps of the indium composition. The maps obtained in this way combine the resolution of HAADF-STEM and the chemical precision of EDX. We illustrate the potential of such chemical maps by using them to investigate nanometer-scale fluctuations in the indium composition and their impact on the growth of epitaxial InGaN layers.

Effect of increasing thickness on tensile-strained germanium grown on InGaAs buffer layers

We have investigated the optical properties of tensile-strained germanium grown on InGaAs buffer layers as a function of film thickness and buffer layer composition. We study the dependence of the photoluminescence as a function of the strain amplitude and degree of relaxation which are also monitored by X-ray diffraction and Raman spectroscopy. We show that 0.75% biaxially strained germanium can be obtained up to a thickness of 150 nm, a value sufficiently high to allow confinement of the spontaneous emission in a guiding structure. For large thicknesses (>200 nm) and large indium content in the buffer layer, a partial relaxation of the film is observed characterized by a large in-plane anisotropy of the germanium lattice. In this case, a difference of strain magnitude deduced either by microphotoluminescence spectra or by X-ray or Raman measurements is reported. We explain this difference by the sensitivity of microphotoluminescence to the local properties of the material. This study provides guidelines...

Metamorphic approach to single quantum dot emission at 1.55μm on GaAs substrate

We report on the fabrication and the characterization of InAs quantum dots (QDs) embedded in an indium rich In0.42Ga0.58As metamorphic matrix grown on a GaAs substrate. Growth conditions were chosen so as to minimize the number of threading dislocations and other defects produced during the plastic relaxation. Sharp and bright lines, originating from the emission of a few isolated single quantum dots, were observed in microphotoluminescence around 1.55 μm at 5 K. They exhibit, in particular, a characteristic exciton/biexciton behavior. These QDs could offer an interesting alternative to other approaches as InAs/InP QDs for the realization of single photon emitters at telecom wavelengths.

Domain structure and magnetic anisotropy fluctuations in (Ga,Mn)As: Effect of annealing

We investigate the effect of post-growth annealing on the magnetic domain structure and magnetization reversal process of (Ga,Mn)As epilayers grown with tensile strain on a (Ga,In)As buffer. In the case of perpendicular magnetic easy-axis, annealing drastically changes the domain structure observed at magnetization reversal. In as-grown samples, strongly anisotropic domain growth is observed. Dendritic-like domain expansion with guided branching along the directions results in a grid-like pattern. This is tentatively attributed to spatial fluctuations of the uniaxial anisotropy constant, correlated with the cross-hatch pattern. In annealed samples, domain wall motion is much more isotropic, which likely results from a decrease of the relative amplitude of the uniaxial anisotropy fluctuations with increasing carrier density. However domain wall motion is impeded by linear or slightly curved defects, hundreds of micrometers long, and point-like pinning centers. The density of nucleation centers for magnetization reversal strongly decreases upon annealing.