Self-Assembled AlGaN Nanostructures Design, Modelling, Opto-Electronic Properties for UV Emitters – NANOGANUV

Gallium Nitride (GaN) based materials have emerged as the leading technology for a wide range of optoelectronic applications in the visible range. In particular, blue emitting lasers and light emitting diodes (LEDs) have led to the development of widely used “white LEDs” and “Blu-Ray” technologies. A next step, which represents one of the most important challenges for nitride LEDs, is to replicate in the ultra-violet (UV) range, i.e. below 360 nm, the performances obtained in the blue. Indeed, AlxGa1-xN materials, which allow covering an emission spectrum between 360 nm and 200 nm, are well adapted to become the next technology in the UV, in replacement of the mercury-vapour lamps, which are hindered by environmental issues (toxicity, hazardous waste) and technological limitations (large size equipments, low efficiency and lifetime, etc.).
However, the internal quantum efficiency (IQE) of UV LEDs – which is given by the product of the radiative recombination efficiency (RE) and the injection efficiency (IE) - is rapidly decreasing when going towards shorter wavelengths. Therefore, the general objective of NANOGANUV project is to develop alternative solutions to the current technologies and designs, adopted by most R&D laboratories, by focusing on two major key elements of AlxGa1-xN LED structures:
- 1) the active region (on which depends the RE);
- 2) the p-type region (on which depends the IE).
In order to address these locks, different approaches will be developed. Presently, RE and EI are limited in AlxGa1-xN materials by high defect densities and an impediment to the doping, respectively. Regarding the doping issue, which is due to the dopant ionization energy increase with Al content, the main lock concerns the case of p-type doping.
Three paths will be followed to improve the IQE:
- 1) the use of quantum dots (QDs), grown by Molecular Beam Epitaxy (MBE, the most mature epitaxial technique for QD fabrication). Indeed, owing to the spatial confinement of carriers in 3 dimensions (D) instead of the 1D spatial confinement in the case of quantum wells, QDs strongly improve the RE by reducing the influence of defects;
- 2) the optimization of Mg doping in AlxGa1-xN layers by MBE, for which p-type GaN doped layers have been demonstrated with the largest hole concentrations (close to 10^19 cm^-3). In order to reach the level of optimized doping conditions and associated electrical characteristics, the fabrication of a high-stability dopant evaporation cell dedicated to Mg will also be developed;
- 3) the use of AlN bulk substrates and the development of a specific high-temperature growth furnace to improve the structural quality of AlxGa1-xN. These two approaches will allow determining the potential of MBE to reach high-quality AlxGa1-xN layers.
The overall ambition is to develop a novel route towards the fabrication of efficient UV LEDs, by using alternative scientific and technical solutions at both the nanoscale level, i.e. involving QD modelling, fabrication and quantum engineering, and the micro/macroscopic level i.e. investigating optical and transport properties, to identify, design and assemble the building blocks for the fabrication of QD-based UV sources.
The final targets are to design and fabricate UV LEDs operating in the 260 – 360 nm spectral region. This large UV range (from UV-A to UV-C regions) should allow addressing a wide range of applications, from UV curing and counterfeit analysis to medical phototherapy, water and air purification. At the end of the project, devices presenting the best performances will be further processed in LED packages with the aim of performing a series of tests on experimental workbenches by companies specialized in LED testing for UV applications (which will be defined by the specific UV region covered by the LED prototypes).