Maura Pavesi

Accelerated Life Test of High Brightness Light Emitting Diodes

Short-term accelerated life test activity on high brightness light emitting diodes is reported. Two families of 1-W light-emitting diodes (LEDs) from different manufacturers were submitted to distinct stress conditions: high temperature storage without bias and high dc current test. During aging, degradation mechanisms like light output decay and electrical property worsening were detected. In particular, the degradation in light efficiency induced by thermal storage was found to follow an exponential law, and the activation energy of the process was extrapolated. Aged devices exhibited a modification of the package epoxy color from white to brown. The instability of the package contributes to the overall degradation in terms of optical and spectral properties. In addition, an increase in thermal resistance was detected on one family of LEDs. This increase induces higher junction temperature levels during operative conditions. In order to correlate the degradation mechanisms and kinetics found during thermal stress, a high dc current stress was performed. Results from this comparative analysis showed similar behavior, implying that the degradation process of dc current aged devices is thermal activated due to high temperatures reached by the junction during stress. Finally, the different effects of the stress on two families of LEDs were taken into account in order to identify the impact of aging on device structure.

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.

Optical evidence of an electrothermal degradation of InGaN-based light-emitting diodes during electrical stress

The optical properties of blue InGaN-based light-emitting diodes aged at high current levels have been studied by electroluminescence and cathodoluminescence. The onset of a broad optical band peaked at about 3.1 eV in devices aged without a heat sink (junction temperature higher than 300 °C) has been correlated to an electrothermal threshold effect. The band is attributed to the dissociation of Mg–H complexes inside the p-type layers and to the consequent formation of Mg-related metastable complexes acting as acceptors. Subsequent electron-beam irradiation determines the almost complete quenching of the band.

Study of Surface Treatment Effects on the Metal-CdZnTe Interface

The quality of a CdZnTe-based X-ray detector is highly related to the interface between semiconductor and metal contact. One of the factors that increase leakage currents in CdZnTe based X-ray detectors is the presence of a conductive surface layer. In this paper the result of the passivation of the CdZnTe surface by means of an aqueous solution of NH4F/H2O2 is studied by optical ellipsometry and by the current-voltage characteristics of gold contacts deposited on the oxidized surface. Collected data show that leakage currents can be reduced and contact stability improved by the combined use of the passivation layer and a guard ring.

Analysis of hot carrier transport in AlGaAs/InGaAs pseudomorphic HEMTs by means of electroluminescence

The carrier transport phenomena occurring in pseudomorphic AlGaAs/InGaAs HEMTs biased in the on-state impact-ionization regime is analyzed in this paper. We confirm the presence, in the electroluminescence spectra of pseudomorphic HEMTs, of a dominant contribution due to electron-hole recombination and we identify a composite peak due to recombination of cold carriers. We analyze the recombination peak using a high-resolution monochromator, which reveals the fine structure due to transitions between electron and hole subbands in the channel quantum well, thus providing useful data concerning the properties of the InGaAs HEMT channel. We also demonstrate that recombination between nonenergetic electrons and holes occurs in the gate-source region, as already observed in InAlAs/InGaAs HEMTs on InP. This recombination emission is superimposed to a less intense contribution mostly coming from the gate drain region. This contribution has a nearly Maxwellian distribution which extends to fairly high energies (>3 eV) and has equivalent temperatures in the 1000-3000 K range. Finally we show evidence of recombination in the AlGaAs layers (observed at high electric field), which demonstrates, in these devices, real space transfer of both electrons and holes.

Growth and Characterization of CZT Crystals by the Vertical Bridgman Method for X-Ray Detector Applications

CdZnTe crystals were grown by the vertical Bridgman method in closed quartz ampoules. The crystalline quality and the impurity content of these crystals were studied. Several X-ray detectors were cut out of these crystals. The resistivity, emission spectra, μτ product, and spectroscopic characteristics of these detectors were extensively measured and compared with the characteristics of detectors obtained from CdZnTe crystals grown by the boron oxide encapsulated vertical Bridgman technique. The detectors prepared from crystals grown without boron oxide show good μτ value, spectroscopic resolution, and higher reproducibility. The influence of growth method on impurity content and on detector response was discussed.

Characterization of Bulk and Surface Transport Mechanisms by Means of the Photocurrent Technique

The transport properties of ternary alloys, as CdZnTe, affect heavily the features required in X and Gamma spectroscopic detectors. The density and the nature of bulk defects and the lacking quality of contacts, characterized by high recombination center densities near the surface, damage strongly the quality of these materials. The photocurrent technique is a powerful tool to study the bulk properties and to investigate contact and surface quality. From steady-state photocurrent spectra information about the bulk trap defects and about the kinds of surface states far and near the contacts, has been obtained. By varying the bias at fixed wavelength, instead, it can be calculated the transport parameters like the product mobility-lifetime ¿¿ and the ratio s/¿, this last one related to the surface underneath the metal contacts. This paper gives some contribution in the interpretation of high energy region of the spectra. The authors correlate the response of samples illuminated with energies above the band gap to the surface states and show the different effects of this states on the electron and the hole transport case. Moreover in this work the authors compare the photocurrent measurement with the X ray spectroscopy and also discuss several phenomena like the shape difference between the spectra taken at different photon flux and bias polarities. Also the band edge shift and the modification of S/¿ parameter observed on varying the bias polarity have been discussed giving new a possible interpretations.

Reliability of Deep-UV Light-Emitting Diodes

This paper analyzes the performance and reliability of deep-ultraviolet light-emitting diodes (LEDs) on AlGaN emitting at 280 and 295 nm. By means of detailed electroluminescence characterization, we show that the optical properties of the LEDs are strongly influenced by the presence of deep-level-related radiative transitions, and we separately evaluate the contribution of each of these recombination mechanisms on the overall light emission. The reliability analysis presented in this paper shows that stress determines the gradual decrease of the output power of the LEDs, which is more prominent at low measuring current levels. Degradation is attributed to the increase of the nonradiative recombination rate. By means of C-V analysis, we give evidence of modifications of the charge distribution in the active layer, taking place as a consequence of stress: this mechanism is considered to be related to the generation of new defect states near/within the active region.

HOT CARRIER EFFECTS IN ALGAAS/INGAAS HIGH ELECTRON MOBILITY TRANSISTORS : FAILURE MECHANISMS INDUCED BY HOT CARRIER TESTING

A new failure mechanism of AlGaAs/InGaAs pseudomorphic high electron mobility transistors has been observed after hot carrier dc accelerated testing. Hot carrier effects have been characterized by means of gate current measurements and electroluminescence spectroscopy. After accelerated testing, a permanent degradation has been found, consisting of the decrease of drain current ID, and of the absolute value of the pinch-off voltage Vp, at low drain-source voltage VDS, resulting in the development of a remarkable “kink” in the output characteristics. Direct current, pulsed, and low-frequency ac measurements demonstrate that the failure mechanism consists of the creation of deep levels under the gate which act as electron traps at low gate-to-drain electric fields. Deep level transient spectroscopy and photoinjection measurements reveal the presence of two levels at 0.77 eV and 1.22 eV. The intensity of the 1.22 eV peak is correlated with the degradation observed in stressed devices.

Study of surface treatment effects on the metal-CdZnTe interface

The quality of a CdZnTe-based X-ray detector is highly related to the interface between semiconductor and metal contact. One of the factors that increase leakage currents in CdZnTe based X-Ray detectors is the presence of a conductive surface layer. In this paper the result of the passivation of the CdZnTe surface by means of an aqueous etching solution of NH4F/H2O2 is studied by optical ellipsometry and by the current-voltage characteristics of gold contacts deposited on the etched surface. Collected data show that leakage currents can be reduced and contact stability improved by the combined use of the passivation layer and a guard ring.