Tamiazzo, Gianluca (2008) Loss Mechanisms in InGaN/GaN Quantum Well Structures. [Tesi di dottorato]
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The study of the electro-optical properties of semiconductors has represented one of the major topics in the research of physicists and material scientists of the last century. Extensive efforts have been especially put in the last four decades into the search for materials emitting in the visible region, with the aim of developing Light Emitting Diodes (LEDs) for general outdoor lighting as well as for new applications, where micro-size and device integration are necessary requirements.
GaN/InGaN/AlGaN-based LEDs lead nowadays the solid-state lighting market, which is in continuous expansion and finds always new applications, such as display back-lighting and projection, full color displays and automotive lighting. Due to the increasing market competitiveness, further improvement of LED brightness and long term reliability, together with the reduction of production costs, are of primary concern for each LED supplier. The enhancement of device effciency and reliability requires the understanding of the physics of these devices and the optimization of the LED design.
Despite of the high long-term stability reached, degradation mechanisms could still potentially occur. In this work, loss mechanism causing aging are investigated for GaN-based LEDs. Operation-induced degradation of high-brightness LEDs internal quantum e’±ciency is analyzed both experimentally and theoretically. The dynamics of the aging process are studied by means of electroluminescence relative intensity
measurements in dependence of time. A model able to interpret this behavior is proposed. In particular, a thermally activated mechanism, likely correspondent to the p-type-dopant diffusion process, is believed to be responsible for long-term DC aging.
In section 1, properties of Gallium Nitride material are analyzed in detail. Growing and doping methods, employed substrates and fabrication of contacts are briefly described. A description of related GaN alloys is then given. Information wrapped in this chapter will be frequently mentioned in the entire work.
In section 2, an overview of the Light Emitting Diode structure is given. Topics as radiative recombination theory, non radiative recombination mechanisms and LED theory basics are examined. Finally, recently developed LED improvement techniques are shortly described.
In section 3, LED degradation theory is analyzed. Principal degradation modes are first briefly traced. Then, samples used in this study are described together with measuring methods and general dynamics of aging. Acceleration concept isfinally depicted.
In section 4, a new model based on dopant diffusion is developed. The model is able to interpret most of the degradation phenomena experimentally encountered and gives a first complete description of GaN/InGaN/AlGaN-based LEDs aging. A general introduction to diffusion theory is first given, together with literature results. The model is then mathematically described. An experimental application explaining temperature dependence of LED samples is presented. Finally, the model is developed from a physical point of view, and a derivation of the acceleration factor is given.
In section 5, a description of an experiment based on EL and PL analysis over double and multi quantum well structures is given. The aim of the experiment is to monitor the behavior of a single QW in terms of effciency and reliability in dependence of its positioning.
In section 6, current density impact over the structures is analyzed. The diffusion model is systematically applied to find experimental indications to the effects of a current over-stress on GaN-based LEDs. The current-induced phenomena are reduced to simple thermal degradation.
In section 7, LED aging characteristics are monitored in dependence of device wavelength. A set of samples emitting at different wavelengths is compared in terms of aging properties.
In section 8, some diffusion evidences are considered. Diffusion is proved to be actual already during sample growth. Moreover, defect density impact is taken into consideration, in particular related to the initial aging phase. Finally, a theoretical interpretation is given for a contactless case of degradation named photoaging.
Results of the study are summarized in the last section.
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