Nanocharacterization and modeling of interfaces and high strain in group III - antimonide systems – NAIADE

NAIADE is a fundamental and interdisciplinary project which applies to antimonide nanostructures for middle-IR photonics devices. The mid-infrared spectral range covering wavelengths between 2 and 5 µm is very important for different applications based on molecular spectroscopy because it contains strong absorption lines of many hazardous and industrial gases. High performance mid-infrared photonic devices are required for developing systems for environmental monitoring, medical diagnosis, and also for laser surgery, free space communications etc... The antimony based semiconductor compounds, GaSb, AlSb, InSb and their narrow gap alloys are well suited for the development of middle-IR photonic devices but the technology of nanostructures on their base is not yet matured.
The goal of NAIADE is to study the properties of interfaces in antimonide nanostructures in order to master their formation and properties in novel photonic devices based on Sb rich quantum dots (QD) and overstrained InAs(Sb) quantum wells grown on InP or short wavelength InAs/AlSb quantum cascade lasers (QCLs).

These systems are submitted to huge epitaxial stresses (a misfit up to 10% is expected for InSb on InP) and the influence of interfaces on properties of antimonide nanostructures is extremely strong. In InAs/AlSb QCLs, because of the absence of common atoms, the length of atomic bonds at interfaces can be varied in the range of +/- 7 % depending on growth conditions. This strong perturbation affects both crystalline quality of the structure and its electronic properties. In some cases, like in QCLs, the individual layers in the structure can be just several angstroms thick.

For these reasons the knowledge of the strain fields at an atomic scale is essential. Our ambitions are to answer some important issues identified in this context: What are the « elastic » properties of an interface? Is it possible to tune these properties during the epitaxial formation of interfaces? What is the impact of interfacial strains on device properties? Is the continuous description (elasticity) relevant for ultra stressed nano-objects?

To answer these questions, NAIADE aims at developing specific tools of nanocharacterization and modeling adapted to the high strain level and to an atomic scale. Molecular beam epitaxy (MBE) will allow us to control the growth at the atomic level.
Advanced techniques will be combined to analyze the strain and chemical profiles at the subnanometric scale. They include transmission electron microscopy (TEM), high resolution electron microscopy (HREM), electron energy loss spectroscopy (EELS), high angle annular dark field (HAADF), dark field holography, reflexion high energy electron diffraction (RHEED)… State of the art transmission electron microscopes will be used. Numerical modeling will be developed at different scales
- using continuous elasticity (finite element) from nm to micrometers
- at nanoscopic scale (1-10 nm) using atomistic semi-empiric potentials
- at the atomic scale (a few atomic layers) using first principles (ab initio – DFT).

Modeling will be an essential tool, first to support interpretation of TEM and HREM experiments. Modeling will also provide a fundamental understanding of the elastic properties; in particular, the limit of validity of continuous elasticity which is not yet established in these systems. Optical and electronic properties will be analyzed taking into account the interfacial layers and the associated strain in the modeling of band structures.

The NAIADE consortium will associate 3 complementary partners: CEMES (CNRS Toulouse), a laboratory expert in structural analysis and modeling; IES (University of Montpellier), international leader in antimonide based lasers for middle infra–red emission, and FOTON (INSA Rennes), expert in growth of quantum dots compatible with the well mature InP technology.