Reliability and efficiency-limiting mechanisms in III-N LEDs: an experimental analysis assisted by numerical simulations

This thesis reports a deep investigation on light-emitting diodes (LEDs) at the current state-of-art in both visible and UV ranges of emission and focusses on the defect- and temperature-related loss processes that affect these devices limiting their efficiency and reliability. The analysis studies LED samples fabricated in collaboration with the École Polytechnique Fédérale de Lausanne (EPFL) and the Technische Universität Berlin (TUB). In particular this research activity includes (a) an experimental characterization of the crystallographic defects present in the devices by means of deep level optical-spectroscopy and Light capacitance-voltage measurements; (b) the evaluation of the device reliability through accelerated stress test and the identification of the mechanisms involved in the degradation; (c) a modeling of the electrical characteristics of LEDs based on the experimental results and of their behavior during ageing and (d) the design of a set-up for fast electro-optical characterization of high-power devices in order to measure the self-heating transients and evaluate the impact of heating in the LED performance. In particular this study demonstrates that in InGaN/GaN LEDs: I) The defects are preferentially incorporated in indium-containing layers such as under layers and quantum wells. Their density is proportional to In-content and, if incorporated within the active region, they negatively affect the reliability of the devices contributing to their optical degradation. II) The superlattice underlayer effectively contributes to reduce the defectiveness of the active region, partially preventing the defect propagation toward the QW. III) It is possible to model the electrical characteristic starting from the defect characterization and considering their correlated leakage mechanisms such as trap-assisted tunneling. Furthermore, the model can reproduce the behavior of the device under ageing contributing to the identification of the degradation physical mechanisms. IV) An increase in defects at the QW interfaces influence the optical behavior, alternating the injection of the carriers into the QW. Whereas, for UV-C AlGaN-based LEDs: I) An increase in the defect density has been identified during LED ageing. This increase that affects the electrical characteristic mainly involves the region from the electron-blocking layer (EBL) toward the active region. A possible cause of degrade is migration of impurities, such as hydrogen, as a result of the high p-doping density in the EBL. II) In the region adjacent to the contact, a doping passivation process would generate a loss in the injection efficiency and consequently, a shift of the turn-on voltage during the stress test. Finally, the designed setup for the self-heating characterization of high-power LEDs leads to the following advantages: I) The circuit allows the measurement of the self-heating transients of voltage and optical power at high currents (up to 2A) from 6 µs with a high stability (<0.1%) and accuracy. It is thus possible to analyze the variation in optical emission and extrapolate the value of the thermal impedance; II) The measurement of the heating transients is carried out directly at high current, rather than at low current recording the cooling transients as performed by the traditional technique; III) The pulsed electro-optical and spectra measurements allow us to extract information on the LED performance before the self-heating transient or at a specified junction temperature; IV) The spectrum of the device can be monitored during all the heating transient starting from about 10 µs. This allows a correct estimation of the decrease in optical power and the analysis of the phosphor behavior during the heating.