Guilhem Almuneau

Real-time in situ monitoring of wet thermal oxidation for precise confinement in VCSELs

A new imaging method is presented enabling the monitoring of the lateral wet thermal oxidation of a thin Al-containing layer embedded in a vertical cavity lasers (VCSEL) structure. This method is based on the measurement of the modification of the VCSEL reflectivity spectrum inherent to the aperture layer refractive index change, with an observation window restricted to the wavelength ranges for which this reflectivity variation is maximal. The main purpose is the accurate control of the buried confinement aperture, and, thereby, that of the electro-optical characteristics of the laser device. The kinetics of the lateral oxidation has been studied for small-size aperture VCSEL (3–10 µm) and for long-range oxidation depths. This straightforward method based on an optical imaging system will enable robust improvement of the production yield of this multifactor-dependent technological process.

Oxide confinement and high contrast grating mirrors for Mid-infrared VCSELs

A new mid-infrared (MIR) Vertical Cavity Surface Emitting Laser (VCSEL) structure is proposed. We have integrated to the VCSEL structure both an oxide aperture for lateral confinement, and a sub-wavelength high-contrast-grating top mirror. Upon the GaSb-based half-VCSEL, we have grown a metamorphic AlGaAs heterostructure to enable thermal oxidation and grating mirror fabrication steps. A methodology based on optimization and anti-optimization methods has been used to design the optical grating, with improved parameter tolerances regarding processing errors. Finally, we show the complete fabrication of an electrically-pumped MIR monolithic VCSEL structure implementing both oxide confinement and a subwavelength grating top mirror.

Efficient lateral confinement by an oxide aperture in a mid-infrared GaSb-based vertical light-emitting source

The use of lateral oxidation for electrical and optical confinement on a GaSb-based mid-infrared vertical light-emitting diode is demonstrated in this paper. The metamorphic growth of (Al)GaAs above a tunnel junction grown on the GaSb-based resonant-cavity light-emitting diode enables good structural quality of the As-based layers and oxidation of the AlAs embedded film. Oxide-confined devices with emission wavelength around 2.6 µm are demonstrated, with no noticeable degradation from the oxidation thermal treatment. Such an efficient oxide confinement scheme can be applied for the realization of high-performance mid-infrared vertical-cavity lasers.

Optimal control of AlAs oxidation via digital alloy heterostructure compositions

A thorough study of wet thermal oxidation in AlAs/AlxGa1−xAs superlattices is presented. The results shown here demonstrate that the final oxidation depth can be finely tuned via the composition and thickness of AlxGa1−xAs into the digital alloy. A complete model of oxidation in these structures is proposed, relying on diffusion through the AlAs layer, its oxidation and an additional effect due to the AlxGa1−xAs intermediate barriers. This barrier contribution is shown to further improve the control of the oxidation rate, and thereby fabrication of sophisticated AlOx/GaAs integrated optoelectronic devices.

Modelling of interband transitions in GaAs tunnel diode

In this paper, an improved model for non-local band-to-band tunneling carrier transport is presented and compared to experimental measurement from GaAs tunnel junctions devices. By carefully taking into account the coupling between the conduction band and the light holes valence band, the model is able to predict, with realistic material parameters, the amplitude of the current density throughout the whole tunneling regime. The model suggests that elastic band-to-band tunneling instead of trap-assisted-tunneling is the predominant mechanism in GaAs tunnel junctions, which is of great interest for better understanding and improving III–V multi-junction solar cells.

Scanning microwave microscopy applied to semiconducting GaAs structures

A calibration algorithm based on one-port vector network analyzer (VNA) calibration for scanning microwave microscopes (SMMs) is presented and used to extract quantitative carrier densities from a semiconducting n-doped GaAs multilayer sample. This robust and versatile algorithm is instrument and frequency independent, as we demonstrate by analyzing experimental data from two different, cantilever- and tuning fork-based, microscope setups operating in a wide frequency range up to 27.5 GHz. To benchmark the SMM results, comparison with secondary ion mass spectrometry is undertaken. Furthermore, we show SMM data on a GaAs p-n junction distinguishing p- and n-doped layers.

Vertical integration of an electro-absorption modulator within a VCSEL device

The huge increase of datacom traffic requires laser sources of ever-widening modulation bandwidth. Vertical-Cavity Surface-Emitting Lasers (VCSEL) are strategically relevant given that their wide use for short communication links such as in datacenters and are, in light of recent developments, good candidates to address such demands. We propose to increase the modulation bandwidth by vertically integrating a continuous-wave VCSEL with a high-speed electroabsorption modulator (EAM-VCSEL). We will present our studies on the electrical and optical designs, on the development of optimized fabrication steps and on the microwave characterizations leading to the realization of integrated EAM-VCSEL devices.

Anisotropic oxidation of circular mesas for complex confinement in photonic devices: Experiments and modelling

In many different AlGaAs-based photonic and optical devices, the selective oxidation of an Al-rich layer is a very efficient way to create a lateral electrical and optical confinement. The degree of lateral confinement can thus be adjusted with the depth of the oxide within the structure. It is then of primary importance to control the lateral spreading of the oxidation reaction and that in all the crystallographic directions in order to master the waveguide properties in the 3 directions. Thanks to the epitaxial structure the vertical confinement can be designed with the index profile of the epitaxial multilayers, but in the lateral directions (in the plane of the epilayers) only the kinetics of the selective oxidation steers the waveguide dimensions.

Type II heterojunction tunnel diodes based on GaAs for multi-junction solar cells: Fabrication, characterization and simulation

In this work, Molecular Beam Epitaxy (MBE) grown tunnel junctions (TJs) based on GaAs(Sb)(In) materials are experimentally and numerically studied. From simple GaAs TJs grown with various n-doping levels, we develop a semi-classical interband tunneling model able to quantify the magnitude of the tunneling current density, which shows that direct interband tunneling is the predominant tunneling mechanism in GaAs tunnel junctions instead of trap-assisted-tunneling mechanisms. Numerical simulations based on non equilibrium perturbation theory through Non Equilibrium Greens Functions (NEGF) and a multi-band kp hamiltonian that includes both gamma and L valleys were performed by the IM2NP (Marseille) and confirmed this result. In order to further improve the performance of the TJs, we are fabricating a type II tunnel heterojunction based on GaAsSb and InGaAs materials.

Three dimensional confinement technology based on buried patterned AlOx layers: Potentials and applications for VCSEL arrays

A new planar technology for flexible and versatile confinement design schemes based on patterned oxide layers is presented. This method of electrical and optical confinement is an improvement over the conventional and widely used lateral oxidation, since it allows to define, from a planar surface, the confined areas. This new technique is particularly suitable for the realization of integrated photonic components arrays such as for VCSELs. First demonstrations show that the oxidation and epitaxial regrowth can be sequenced in a device process flow, leading to successful confinement while preserving good radiative properties.