The Minamata Convention entered into force on 16 August 2017, with the purpose of progressively banning mercury use and mercury-using devices. This directly concerns mercury lamps, which are the current source of UV light for a wide range of applications in the field of UV curing, counterfeit detection, medical and instrumentation applications, air purification. Specifically concerning food and water disinfection applications, some of current applications rely on applying bactericidal substances such as chlorine to drinking water or using antibiotics which can have unwanted side effects. Also the contact of disinfectants with food is detrimental, which requires an ecologically and economically efficient alternative. This political/societal context should boost the rapidly growing market of environment friendly UV light emitting diodes (LEDs) in the 260-280 nm range and stimulate even more the active research and development activities in the field.
AlGaN materials are ideally suited for the realization of UV LEDs since they are applicable throughout the UV-B (320–280 nm) ranges and even access a large segment of the UV-C (280–200 nm) spectral range. Despite the excellent external quantum efficiency (EQE) that have been obtained for LEDs in the near UV and visible spectral range, the EQE of AlGaN-based UV LEDs with emission wavelength shorter than 365 nm is at least one order of magnitude below the best devices in the near UV and violet wavelength range. Currently the best LEDs in the UV-B and UV-C spectral ranges exhibit EQE around 10 %. A number of factors contribute to limit the efficiencies of such conventional, layer-structured nitride based UV LEDs, resulting in this overall small value. These factors include the high density of extended defects in layers, the limited efficiency of AlGaN p-type doping associated with a limited current injection and the light extraction issue.
All these limitations can be overcome to a wide extent by using nanowires (NWs). As a matter of fact, the absence of extended defects in NWs, the higher limit solubility of both Si and Mg electrical dopants, the eased light extraction intrinsically related to the large “roughness” of an ensemble of NWs make them particularly favorable to the realization of efficient LEDs.
Indeed, the CEA-INAC group in collaboration with Néel Institute has demonstrated the realization of AlN NW-based UV LEDs by taking advantage of a high p-type doping level achieved by Mg-In codoping. This pioneering work has opened the path to a new concept of UV LEDs made of AlN- and AlGaN-based NW heterostructures.
It will be the goal of this PhD subject to amplify the preliminary results already obtained, with the purpose of obtaining an optimized, pre-industrial demonstrator. The growth of the structures will be performed by plasma-assisted molecular beam epitaxy in CEA-Grenoble INAC/PHELIQS-NPSC, the electrical characterization being made in collaboration with Institut Néel. The process of the LEDs, their light emission properties characterization and efficiency measurements will be achieved in CEA-LETI DOPT, all three partners being integrated in a feedback loop to improve the optimization process.
The candidate should have a master 2 in Nanosciences or equivalent, with a marked interest in experimental physics, material growth and characterization.