Artificial materials: photonics and electronics properties engineering

Electromagnetics modelling:

 
Unfolded band structure of  a  mesoscopic self-collimating photonic structure: reduce frequency increasing from blue to red with null curvature regions superimposed in blue. In these regions, light propagates without any lateral spreading.
 
  

Over the last 10 years, the Photonic group has developed expertise in many complementary modelling methods for electromagnetic simulations of sub-wavelength structures including plane wave expansion method (PWEM), finite difference in time domain (FDTD), rigorously coupled wave analysis (RCWA) and coupled-mode theory. The group has focussed on free open source softwares that are easily deployed on large clusters to provide high computational capabilities at low cost. These have opened the way to systematic studies and optimization of complex photonic structures that were not accessible otherwise. Several in-house ad-hoc models have also been developed to study new photonic functionalities like beam propagation in mesoscopic self-collimating structures, or reflectivity of cavity resonant grating filters for external cavity laser diodes.

Related publications:

III-V hetero-epitaxy:

Growth of III-V quantum wells on (111)B oriented GaAs:

 
Kerr rotation dynamics at T  50K for NIP GaAs-GaAlAs MQW samples grown on (111)B substrates with different applied voltages. Inset: dependence of the electron spin lifetime with the applied voltage.
 
  

We have grown (111)B-oriented GaAs-GaAlAs PIN multi-quantum well structures, beneficiating from our know-how acquired in the past (GHISO FP5 project). These structures have allowed our LPCNO partner to study their spin dynamics behaviour by means of Time Resolved Kerr Rotation experiments. They have found that spin relaxation time is significantly increased by applying an external electric field for III-V QW structures so-oriented. For these structures, they also measured for the first time the spin diffusion length increase under reverse bias and determine the Dresselhaus and Rashba coefficients which are the key parameters to control the carrier spins in semiconductors.

Related publications:

Regrowth on patterned surface :

 
AFM image (3µmx3µm) of InAs quantum dots/10nm-thick GaInAs quantum well grown on a nano-patterned GaAs substrate with an array of 120nm-wide and 20nm-deep stripes.
 
  

Growth of InAs quantum dots (QD) on nano-patterned GaAs surfaces is studied with a view  integrating them into advanced laser diodes. We aim to localize periodicically these nanostructures while improving their uniformity and preserving them a high density. Patterns with 250 to 300nm-period stripes have been fabricated by e-beam lithography, and transfered  into GaAs by wet etching. Such a process leads to shallow patterns (15-25nm deep and 120-100nm wide), with low angle facets. Under these conditions QD localization has been successfully achieved with an underlying strained Ga0.82In0.18As QW introduced to flatten the patterned surface and get a modulated surface strain. The QD density is of the order of 1010/cm2 as required for active devices. This regrowth process will be also exploited to realize buried submicrometric diffraction gratings.

 

 

 

 

 

Emerging GaAsBi alloys:

 
Spectres de photoluminescence de couches minces de GaAsBi montrant la diminution de l’énergie d’émission avec le contenu de Bi (collaboration LPCNO).
 
  

Adding bismuth to GaAs efficiently decreases the bandgap energy of this semiconductor through a change in its valence band properties, and an increase of the spin-orbit coupling. GaAsBi/GaAs quantum wells (QWs) are of interest to fabricate laser diodes which could benefit from the latter properties, in particular from the higher spin-orbit splitting expected to lower the non-radiative carrier recombination due to Auger mechanisms. Besides, the GaAsBi alloys are promising candidates for intermediate bandgap solar cells. GaAsBi quantum wells were grown by molecular beam epitaxy with a Bi content up to 7.5%, emitting at 1.23µm at room temperature. These QWs are entirely elastically strained, exhibit good structural properties with a uniform thickness, sharp interfaces, and an absence of extended defects. Furthermore, interesting spin properties have been observed by our partner LPCNO on a series of GaAsBi thin layers grown at LAAS with a bismuth content ranging from 1 to 5.5%.

Related projects:

AO Université Paul Sabatier (2011), AO TECH INTER-LABS INSA (2013).

Related publications:

3D electro-optical confinements in planar photonic devices:

 Structure test du capteur de CH4 composée de guides optiques diffractifs juxtaposés (le pas des réseaux de diffraction est égal à 1,28 µm). 
AFM image of the surface after lateral oxidation of a buried AlAs layer and epitaxial regrowth
 
  

To improve the performance of current photonic devices, as well as to increase their functionalities, technological issues such as the non uniformity of the electrical injection, or the fine and flexible control of the active region size and shape by AlOx technique, must be addressed. To that extent, we have developped a new technological process of planar oxidation, making possible a sophisticated structuration of the refractive indices around the active region. This allows to treat separately electrical and optical properties of optoelectronic devices, opening new developments of innovative structures with better transverse control.

We have recently demonstrated a technological approach for forming a flexible and versatile confinement scheme based on oxidation of AlGaAs buried layers combined to an epitaxial regrowth. This method improves the electrical and optical confinements compared to the standard lateral oxidation, since it allows to define confinement areas from a planar surface. This technique is suitable for the realization of advanced integrated photonic components arrays with close device-to-device spacing such as two-dimensional arrays of vertical-cavity surface-emitting lasers. Our results prove that the oxidation and epitaxial regrowth can be sequenced in a process flow, leading to viable confinement while preserving good radiative properties.