Simulation of GaN-based LEDs and solar cells: correlating quantum-corrected models and experimental characterizations

Nitride alloys have emerged as the most promising semiconductor compounds for ultraviolet (UV) applications. GaN-based LEDs are of particular interest in the medical environment, the use of UV light for disinfection in clinical settings and for catalyzing chemical reactions is well known. Color-coded LED microscopy (cLEDscope) is a novel computational microscopy technique capable of multi-contrast and quantitative phase imaging of biological specimens using color-multiplexed illumination. GaN-based solar cells find their main application when incorporated in multi-junction photovoltaic structures which can provide high theoretical conversion efficiencies. In addition, it has been demonstrated that these solar cells exhibit excellent thermal stability and high-energy-proton radiation resistance which makes them suitable to be used in harsh environments such as concentrated sunlight and space applications. However, most of the reported InGaN-based solar cells are not as efficient as expected, mostly due to the difficulty of growing high crystalline quality InGaN alloys. It has been observed that multi-quantum-well (MQW) active regions show an obvious advantage in terms of structural quality, in fact, InGaN is typically grown on GaN layers and the lattice mismatch strain limits the indium composition and layer thickness. This thesis investigates the performances of state-of-the-art GaN-based light emitting diodes (LEDs) and solar cells through experimental characterizations and simulations. The experimental measurements performed on the LED samples and solar cells have been provided by the Department of Information Engineering (DIE) of the University of Padova, courtesy of Nicola Renso, Alessandro Caria, Matteo Meneghini, Carlo De Santi et al.. The experimental results reported in the thesis include current-voltage (IV) characteristics, electroluminescence (EL) spectra and light-current (LI) measurements. Moreover, the thermal behaviour of the LED samples is analyzed through IV and EL measurements from cryogenic temperatures to 350K. The experimental results are interpreted through the comparison with the results of a campaign of parametric simulations based on a quantum-corrected drift-diffusion electro-optical model. The investigated opto-electronic devices are based on InGaN/GaN MQW active regions and AlGaN electron blocking layers (EBLs). The InGaN band gap engineering, performed by varying the In molar fraction, permits the light emission and absorption in the near UV range. The role of the EBL is investigated in detail, demonstrating how it can enhance the spontaneous emission in the active region of a GaN-based LED or improve the extraction of the photogenerated carriers in a GaN-based solar cell. The main purpose of this thesis is to achieve a deep understanding of the physical behaviour of GaN-based devices. In addition to effects appearing in typical III-V optoelectronic devices such as band gap renormalization due to coulombian interactions in the QWs, current crowding, or the uncertainty given by the composition fluctuations inside the wells, a proper description of nitride-based devices requires to account for physical effects peculiar of these material systems. Among these, the most noticeable is the presence of polarization fields at the GaN/InGaN heterointerfaces, related to the spontaneous polarization of III-N wurzite materials and induced by the lattice mismatch at the interfaces.