Over the last years, important efforts have been done in order to understand the degradation mechanisms of GaN-based
LEDs submitted to forward-bias stress tests. On the other hand, only little work has been done to understand the
degradation of LEDs submitted to reverse-bias stress. However, this topic is of high interest, since (i) the reverse-bias
robustness of the LEDs is strongly correlated to their stability under Electrostatic Discharge (ESD) events and (ii) the
analysis of the reverse-bias degradation can provide important information on the role of high electric fields and reverse
current in limiting the reliability of the LEDs.
Therefore the aim of this paper is to describe a detailed investigation on the reverse-bias degradation of GaN-based
LEDs. The results described in this paper indicate that: (i) under reverse bias, LEDs can show a weak luminescence
signal, due to the recombination of carriers injected in the quantum-wells; (ii) reverse-bias stress can induce the
degradation of the electrical characteristics of the LEDs (increase in reverse-current, decrease in breakdown voltage),
due to the generation of point defects in proximity of pre-existing defective regions. (iii) Furthermore, our tests indicate
that the defective regions responsible for reverse-current conduction can constitute weak points with respect to ESD
events: ESD failures are determined by the shortening of the junction in proximity of one of the defective sites
responsible for reverse-current conduction.
With the new Generation of InGaN-based thinfilm Chips efficacies of 110/lm/W and output power of 32 mW at 20 mA
(5 mm Radial lamp, 438nm, chip-size 255&mgr;m x 460&mgr;m) are reached. Due to the scalability of the ThinGaN concept chip
brightness and efficiency are scalable to larger chip sizes: the brightness achieved for a 1 mm2 ThinGaN Power chip at
350 mA were 495mW (445nm) and 202mW or 100 lm (527nm). White LEDs with phosphorus achieved 102 lm at
350mA, mounted in an OSTAR module with six LED chips 1200 lm were demonstrated at 1000 mA driving current.
White emitting automotive headlamp modules with 620lm (5x 1mm2 chip at 700mA) and 41 MCd/m2 as well as green
emitting projection modules with 57 MCd/m2 at 2A/mm2 drive current and 12mm2 chip area are realized. These
technological improvements demonstrate the straight way of GaInN-LEDs for Solid State lighting.
In this paper we present a combined current-voltage, capacitance-voltage, Deep Level Transient Spectroscopy and electroluminescence study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. In the early stages of the aging tests at low forward current levels (15-20 mA), LEDs present a decrease in optical power, which stabilizes within the first 50 hours and never exceeds 10% (measured at 20 mA). The spectral distribution of the electroluminescence intensity does not change with stress, while <i>C-V</i> profiles detect changes consisting in apparent doping and/or charge concentration increase within quantum wells. This increase is correlated with the decrease in optical power. Capacitance Deep Level Transient Spectroscopy has been carried out to clarify the DC aging induced generation/modification of the energy levels present in the devices. Remarkable changes occur after the stress, which can be related to the doping/charge variation and thus to the efficiency loss.
An additional approach to further improve the reliability of ZnSe based devices is to use beryllium containing II-VI compounds. BeS, BeSe and BeTe are characterized by a considerable amount of covalent bonding and a high bond energy. This distinguishes these materials from the conventional ionic wide gap II-VI semiconductors like ZnSe, ZnTe or CdTe. Recently, thin film structures using Be- compounds have been fabricated and characterized. It became clear that--besides the application aspects--these materials are also very interesting from a more fundamental point of view. Using Be-containing II-VI compounds, ionic and covalent lattice matched II-VI materials can be combined in quantum well structures. The type II band alignment of BeTe and ZnSe gives additional freedom in the band gap engineering, and it is possible to grow lattice method quaternaries of low polarity onto silicon. Here, basic principles of Be containing II-VI compounds will be described, and the potential of these novel materials will be discussed.