Defect generation in deep-UV AlGaN-based LEDs investigated by electrical and spectroscopic characterisation

AlGaN-based UV-B LEDs are promising candidates to replace conventional ultraviolet sources, thanks to their low operation voltage and to the possibility of tuning the emission wavelength by changing the Al content in the alloy. For this reason, these devices are expected to have wide spectrum of application. However, UV-B LEDs can still show a relatively fast degradation during operation, mostly affecting the optical power and the electrical characteristics. Moreover, stress may induce an increase in point defect density, with subsequent increase in the Shockley-Read-Hall recombination components. The aim of this paper is to describe an extensive analysis of the degradation of (InAlGa) Nbased UV-B LEDs submitted to constant current stress, and to investigate the impact of defects in changing the electrooptical performance of the devices. The study is based on combined electrical and optical characterization, deep-level transient spectroscopy (DLTS) and photocurrent spectroscopy. The results of this analysis demonstrate that UV-B LEDs show that the decrease of the optical power during the stress is stronger at low measuring current levels, indicating that the degradation is related to the increase of Shockley-Read-Hall (SRH) recombination. DLTS measurements allowed the identification of three defects responsible for the degradation of the optical and electrical characteristics, for example, one associated to the decrease of the drive voltage and ascribed to the presence of Mg-related acceptor traps. By means of photocurrent spectroscopy, it was possible to localize the defects behaving as non-radiative recombination centers close to the midgap, 2-2.5 eV below the conduction band.