Ulrich Zehnder

A combined electro-optical method for the determination of the recombination parameters in InGaN-based light-emitting diodes

We present an electro-optical method for the extrapolation of the nonradiative and Auger recombination coefficients in InGaN/GaN Light-emitting diodes (LEDs). The method has the advantage of permitting the extrapolation of the recombination parameters of packaged devices, contrary to conventional techniques based on the analysis of quasibulk structures. For the analyzed devices, the average values of the nonradiative and Auger recombination coefficients have been determined to be equal to 2.3×107 s−1 and 1.0×10−30 cm6 s−1, respectively. These results are consistent with previous reports based on the analysis of quasibulk structures and on theoretical simulations. The method described in this paper constitutes an efficient tool for the analysis of the recombination dynamics in GaN-based LEDs. The results obtained within this work support the hypothesis on the importance of Auger recombination in determining the so-called efficiency droop in LED structures.

Influence of short-term low current dc aging on the electrical and optical properties of InGaN blue light-emitting diodes

This work describes an experiment on degradation mechanisms of InGaN light-emitting diode (LED) test structures which do not fulfill the requirements of longlife products. We present a combined capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), electroluminescence (EL), and cathodoluminescence (CL) study of short-term instabilities of InGaN∕GaN LEDs submitted to low current aging tests at room temperature. In the early stages of the aging tests, the EL and CL characterizations showed an optical power decrease, more prominent at low current levels. The C-V profiles indicated that the stress induced an apparent charge increase, well related to the deep level changes detected by DLTS and to the optical power decrease. It is supposed that the main cause of the degradation is the generation of nonradiative paths, due to the generation/propagation of defects activated by carrier transport.

Reversible Degradation of Ohmic Contacts on p-GaN for Application in High-Brightness LEDs

This paper analyzes the high-temperature long-term stability of ohmic contacts on p-type gallium nitride (p-GaN). The contributions of the ohmic contacts and semiconductor material degradation are separated by adopting the transmission line method (TLM). Before stress, the current-voltage (I-V) curves measured at the pads of the TLMs showed a linear shape, indicating a good ohmic behavior of the contacts. Thermal treatment at 250degC was found to induce the worsening of the electrical characteristics of the contacts: identified degradation modes consist of a shift of the I-V curves toward higher voltages and strong nonlinearity of the characteristics around zero. This paper shows that the high-temperature instabilities of ohmic contacts on p-GaN are related to the interaction between the device surface and the plasma-enhanced chemical vapor deposition SiN passivation layer. Hydrogen contained in the passivation layer is supposed to play an important role in the degradation process: the interaction with the acceptor dopant at the metal/semiconductor interface induces the decrease of the effective acceptor concentration. As a consequence, both the ohmic contact characteristics and the semiconductor sheet resistance are worsened.

Leakage current and reverse-bias luminescence in InGaN-based light-emitting diodes

This paper reports an electro-optical analysis of the correlation between reverse-bias leakage current and luminescence in light-emitting diodes based on InGaN. The results of the analysis suggest that (i) the main mechanism responsible for leakage current conduction is tunneling, (ii) leakage current is correlated with the presence of reverse-bias luminescence, (iii) leakage current flows through preferential paths, that can be identified by means of emission microscopy, and (iv) reverse-bias luminescence could be ascribed to the recombination of electron-hole pairs in the quantum well region.

Degradation of High-Brightness Green LEDs Submitted to Reverse Electrical Stress

This letter describes an extensive analysis of the reverse-bias degradation of green light-emitting diodes. The analysis consists in a wide set of stress tests carried out under different negative-bias levels. The results presented in this letter indicate the following: 1) Leakage current is strongly correlated to the presence of reverse-bias luminescence; 2) reverse current flows through preferential leakage paths and is due to a soft-breakdown mechanism that is possibly correlated to the presence of structural defects; 3) reverse-bias stress can induce an increase in the leakage current, with a corresponding decrease in the breakdown voltage of the samples; and 4) the degradation rate has a linear dependence on the (reverse) stress-current level, suggesting that degradation is induced by hot carriers. On the basis of the evidence collected in this letter, degradation can be ascribed to the generation/propagation of point defects due to the injection of highly accelerated carriers.

Homogeneous linewidth of the direct exciton in a type-II ZnSe/BeTe quantum well

Abstract We present an optical study of novel ZnSe/BeTe heterostructures with a type-II band alignment based on beryllium chalcogenides. A strong exciton transition involving a confined electron and a quasibound hole state (both in the ZnSe layer) is observed in photoluminescence, photoluminescence excitation and reflectivity spectra. This exciton state is drastically broadened by increasing temperature due to enhanced exciton—acoustic phonon interaction. Pronounced features related to the quasibound hole states in the ZnSe layer (upto n = 4) are detected.

Failure mechanisms of gallium nitride LEDs related with passivation

This paper analyzes the thermally-activated failure mechanisms of GaN LEDs under thermal overstress related with the presence of a PECVD SiN passivation layer. It is shown that the properties of the passivation layer can remarkably affect devices reliability during high-temperature stress: degradation mechanisms identified consist in emission crowding and series resistance increase, attributed to the thermally-activated indiffusion of hydrogen from the passivation to the p-layer, and subsequent p-doping compensation

Field dependence of the carrier injection mechanisms in InGaN Quantum wells: Its effect on the luminescence properties of blue light emitting diodes

Carrier injection and radiative recombination processes in InGaN∕GaN blue light emitting diodes are investigated by bias-dependent cathodoluminescence. The samples are designed with a single-quantum-well (SQW) light emitter and an adjacent multi-quantum-well (MQW) carrier injector of lower In content. In unbiased samples, the MQW emission dominates over the SQW at high temperatures (T>160K) or low beam currents (IB<5nA). This is ascribed to changes in the device energy band diagram, dependent on the field in the p-n junction and on the level of electron beam induced excitation of excess carriers. A field screening, resulting in a forward biasing of the devices, is highlighted. A maximum value of +2.65V is reached at T=100K for a carrier generation rate G0∼5.4×1013s−1. At a fixed electron beam power, the effects of an external applied field on the emitter efficiency are considered. The SQW emission is promoted in the forward-current regime, while in the reverse-current regime (−1⩽V<2.65 at T=100K) it under...

A study of the failure of GaN-based LEDs submitted to reverse-bias stress and ESD events

This paper describes an extensive analysis of the degradation of InGaN-based LEDs submitted to reverse-bias stress and Electrostatic Discharge events. Results described within the paper indicate that: (i) reverse-bias current flows through localized leakage paths, related to the presence of structural defects; (ii) the position of these paths can be identified by means of emission microscopy; (iii) reverse-bias stress can induce a degradation of the electrical characteristics of the devices (decrease in breakdown voltage); (iv) degradation is due to the injection of highly accelerated carriers through the active region of the LEDs, with the subsequent generation/propagation of point defects; (v) the localized leakage paths responsible for reverse-current conduction can constitute weak regions with respect to reverse-bias ESD events.

Temperature dependence of the electrical activity of localized defects in InGaN-based light emitting diodes

Traps govern the temperature dependence of current in III-nitride quantum heterostructures, but frequently electrical measurements result unable to identify how many and what kind of defects take part in the conduction. The present work shows how a combined electrical and optical characterization in temperature can detect localized defects involved in injection mechanisms in InGaN∕AlGaN∕GaN blue light lmitting diodes. At least two different traps assisting the carrier injection by tunneling and playing an active role below and above 175K, respectively, are identified in or nearby the active layers.