Bandgap engineering, by means of alloying or inserting nanostructures, is the bedrock of high efficiency photovoltaics. III-V quaternary alloys in particular enable bandgap tailoring of a multi-junction subcell while conserving a single lattice parameter. Among the possible candidates, AlInAsSb could in theory reach the widest range of bandgap energies while being lattice-matched to InP or GaSb. Although these material systems are still emerging photovoltaic segments, they do offer advantages for multi-junction design. GaSbbased structures in particular can make use of highly efficient GaSb/InAs tunnel junctions to connect the subcells. There has been only little information concerning GaSb-lattice matched AlInAsSb in the literature. The alloy’s miscibility gap can be circumvented by the use of non-equilibrium techniques. Nevertheless, appropriate growth conditions remain to be found in order to produce a stable alloy. Furthermore, the abnormally low bandgap energies reported for the material need to be confirmed and interpreted with a multi-junction perspective. In this work, we propose a tandem structure made of an AlInAsSb top cell and a GaSb bottom cell. An epitaxy study of the AlInAsSb alloy lattice-matched to GaSb was first performed. The subcells were then grown and processed. The GaSb subcell yielded an efficiency of 5.9% under 1 sun and the tandem cell is under optimization. Preliminary results are presented in this document.
We present in this paper measurements made by quartz enhanced photoacoustic spectroscopy (QEPAS) technique with antimonide laser diodes emitting at 2.3 μm and 3.3 μm. These measurements dedicated to environmental purposes allow us sensitive detection of ethylene and methane. Two experimental setups are reported: a laboratory and brand new compact benches. The detection limits are mentioned.
Development of a reliable, selective, sensitive, technique for atmospheric trace gas concentrations monitoring is a critical challenge in science and engineering. Tunable single-frequency laser in the 2.3 to 3.3µm wavelength range, working in a continuous regime at room temperature can be used for absorption spectroscopy to identify and quantify several gases such as methane (greenhouse gases) and ethylene (food-processing) which are studied in the IES. We report here on the design and fabrication of 1st to 4th order distributed-feedback (DFB) antimonide-lasers diodes in the 2.3 to 3.3µm wavelength range. This process is applied to all studied structures grown by molecular beam epitaxy (MBE) on GaSb substrate.
Electromagnetic modeling helps us to determine the Bragg grating period as well as the global geometry of the structure in order to optimize both modal discrimination and optical power of the lasing mode. The grating is defined by holographic lithography.
Two DFB laser diode designs are proposed and investigated in parallel:
-Side wall corrugation DFB: A corrugation on the lateral sides of the ridge waveguide is transferred by both wet and dry on a hard mask followed by a Cl2/N2 dry etching in the III-V heterostructure.
-Buried DFB: The MBE growth is stopped at the top of the active region. Then the Bragg grating is etched by Ar sputtering . A MBE regrowth process is performed allowing the growth of the upper cladding layer. Next chemical etching of the mesa is done with fluoro-chromic acid.
Si3N4 isolation and evaporation of ohmic contacts ends those processes.
Finally we will show the results on the fabrication and characterization of the devices.
This work is supported by the ANR NexCILAS international project, ANR MIDAS project, NUMEV labex and RENATECH national Network.
There are reviewed the optical properties of two kind of active regions of mid infrared laser devices both grown on GaSb
substrates: GaInAsSb/AlGaInAsSb type I QWs for laser diodes and InAs/GaInAsSb type II QWs for interband cascade
lasers. There are presented their crucial optical properties and the related current challenges with respect to the device
performances. This covers such issues as spectral tenability of the emission via the structure parameters, the band gap
discontinuities, carrier loss mechanisms and oscillator strengths. For that, spectroscopic techniques have been used
(photoluminescence and its temperature dependence, and photoreflectance) and combined with the energy level
calculations based on effective mass approximation and kp theory. Eventually, the potential for further material
optimization and prospects for the improved device performances are also discussed.
The 3 to 4 μm range had long appeared inaccessible to quantum well lasers made on GaSb. Despite having excellent
performance in the 2 to 3 μm range, GaInAsSb/AlGaAsSb quantum well lasers rapidly show their limits when crossing
the 3 μm barrier (the highest wavelength reached with such a device was 3.04 μm under cw operation at 20°C). This
situation was all the more regrettable because several gases have their strongest absorption lines in the 3 to 4 μm range:
methane, for example, has a peak of absorption at 3.26 μm overhanging a weaker peak at 2.31μm by a factor 40.
Works carried out in the University of Munich in 2005 gave new hopes to the world of laser diode spectroscopy. By
replacing the quaternary AlGaAsSb barrier by a quinary AlGaInAsSb barrier, researchers were able to reach laser
operation at 3.26 μm and room temperature in the pulsed mode. Since then, several teams have engaged in the objective
of reaching cw operation at room temperature with such structures. We will give an insight into the phenomena
responsible for the increase of threshold current with growing wavelength. Finally, we will present results obtained with
a monomode DFB laser diode emitting at 3.37 μm having a threshold current of 140 mA at 18°C.
There are two particularly promising approaches to reach laser emission in the 3.0 - 3.5 μm wavelength range with
application grade performance; GaSb based laser structures using GaInAsSb / AlGaInAsSb type-I quantum well (QW)
active region, as well as type-II interband cascade (IC) material have been investigated and corresponding results are
discussed in this paper. We also present different techniques for the fabrication of spectrally monomode distributed
feedback (DFB) lasers for sensing applications in the targeted wavelength range. Based on the different waveguide
designs of the two material approaches, different concepts to achieve monomode emission were applied: lateral metal
gratings were used for type-I laser structures, vertical sidewall gratings for ICL designs. The fabrication procedure,
including growth of the laser structures by molecular beam epitaxy, device processing and characterization, are described
in the following. DFB emission under continuous wave (cw) operation was achieved up to room temperature (RT) in the
target wavelength range. Sidemode suppression ratios (SMSRs) exceed 30dB for the fabricated devices and mode-hop
free monomode tuning ranges of several nanometers are demonstrated.
Laser diodes emitting in the mid-infrared 2-2.7 μm range are of particular interest for spectroscopic applications and
especially for trace gas detection due to the presence of strong absorption bands of several species  of pollutants.
These applications require single-frequency, wavelength tunable lasers. In this perspective, we have studied designs
made of two coupled cavities (C2), coupled by an intracavity Photonic Crystal (PhC) mirror as proposed by Happ et al.
. The first devices of this type have recently been proposed on GaSb, emitting at 1.9 μm .
In this paper, we demonstrate the first coupled cavity (C2) PhC devices operating above 2.3 μm.
We have made quantum wells laser diodes by Molecular Beam Epitaxy with emission wavelengths from 2.3 &mgr;m to 3.1
&mgr;m. With growing wavelength, threshold current densities increase almost exponentially. We obtained threshold values
as low as 65 A/cm2 at 2.3 &mgr;m and 156 A/cm2 at 2.62 &mgr;m. At the same time, the valence-band offset decrease from 132
meV (at 2.3 &mgr;m) to 78 meV (at 2.6 &mgr;m). A threshold current density study shows that Auger effect is not the only
responsible for the augmentation of Jth. The reduction of internal efficiency ηi has a greater impact on the increase of Jth. The diminution of the holes confinement is incriminated for the degradation of ηi with growing wavelength. Therefore, to improve Jth at higher wavelengths another kind of barrier has to be utilized (for example, thanks to the use of the quinary material AlGaInAsSb).
DFB lasers, microcavity and External-cavity VCSELs exhibit narrow single-frequency operation and wide mode-hop-free tuning range, especially well adapted for gas spectroscopy application in the 2-2.7μm window. We will present a review of the results achieved and a systematic comparison, with such Sb-based lasers emitting near 2.3μm. These sources operate in CW above 300K, with up to 5mW output power in a single transverse mode and linear light polarization. Diode-pumped V(E)CSELs and electrically-pumped DFB lasers were designed, grown, processed, and the spectral, spatial, thermal properties characterized. These sources are now being applied in high sensitivity spectroscopy instruments for in-situ measurements.
Specific developments on technological steps involved in the fabrication of antimony based diode lasers have been carried out. Evident improvements of the performances have been achieved. High power, low threshold, room temperature (RT) and continuous wave (CW) operation laser diodes emitting around 2.4μm, based on the InGaAsSb / AlGaAsSb materials system, have been realised. with a 100μm wide and 1mm long cavity laser, a maximum output power of 720mW has been obtained at 285K.
We review here our results concerning laser diodes emitting at 2.38 µm and 2.60 μm. We present an original method allowing to determine the monomolecular, radiative and Auger recombination coefficients A, B and C, as well as the transparency carrier density Ntr, the internal loss αi and the gain coefficient go from the differential efficiency and the threshold current density obtained with different laser diodes. We show how these parameters can be used to optimize the number of quantum wells and explain the differences existing between laser diodes emitting at 2.38 and 2.60 μm. At 2.38 μm, we obtained a threshold current density of 76 A/cm2 with a single quantum well laser diode and at 2.60 μm, a Jth of 152 A/cm2 with a double quantum well laser diode. These threshold current densities can be compared favorably to the best reported values in the 0.85-3.0 mu;m range.
We report GaInAsSb/GaSb multiple quantum well lasers with type-II band alignment operating at room temperature. Basic properties of GaInAsSb/GaSb system in presence of strains are presented. Room temperature lasing has been achieved at wavelengths up to 2.65 micrometer. For the first time, stimulated emission has been obtained from a type-III quantum well structure at room temperature at 1.98 micrometer and 2.32 micrometer for the structures with 6- and 12-angstrom-thick InAs quantum wells, respectively. Modification of the band structure near interfaces of the type-II quantum wells due to carrier injection is shown to be a decisive factor allowing to obtain low threshold lasing in quantum well structures with indirect radiative recombination.