Untersuchung von Degradationsmechanismen an (Al/In)GaN-basierenden Laserdioden (English shop)

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This book examines the analysis of various degradation mechanisms of blue-violet III-V semiconductor laser diodes. These edge-emitting laser diodes are fabricated by MOVPE from aluminium, indium, gallium and nitrogen (AlInGaN) by OSRAM Opto Semiconductors GmbH in Regensburg.

This dissertation presents (Al/In)GaN-based laser diodes emitting at a wavelength of around 405 nm. Best values achieved include a threshold of 30 mA, a slope efficiency of 1.5 W/A, a maximum output power of 3.4 W from one facet at a pulse length of 200 ns, corresponding to a COD of 71 MW/cm2, as well as a lifetime of more than 1300 h at an optical output power of 10 mW.

The first part of the work discusses the material system of the group III nitrides in more detail, with a particular focus on the choice of a suitable substrate and the resulting density of threading dislocations in the epitaxial layers.
This is followed by a brief description of the vertical layer structure, the lateral patterning and the mounting of the laser diodes. In order to better interpret the measured light-current characteristics of the laser diodes, the fundamental equations of edge-emitting semiconductor lasers are derived and the required physical quantities for the GaN system are given. Furthermore, the equivalent circuit diagram of a lossy pn junction is presented, with the help of which real low-current characteristics can be modelled.

The second part of this book presents several measurement methods and the corresponding measurement setups for characterising the electrical and optical properties as well as the lifetime of laser diodes.

In the third part of the dissertation, the measurements, analyses and results are presented in three chapters.
On the basis of low-current characteristics, in which the current is measured as a function of the voltage also below the band gap, electrical defects of the laser diodes can, for example, be detected and analysed. By investigating low-current characteristics during the ageing of laser diodes on SiC and GaN substrates, conclusions can be drawn about the formation of non-radiative recombination centres (NRC).

The phenomenon of correlated voltage and light ageing is analysed in great detail. In this ageing mechanism, the operating voltage increases while at the same time the optical output power of the laser diode decreases. A partially degrading p-contact, which leads to inhomogeneous current injection and thus raises the laser threshold, was identified as the cause of this.

Attempts are made to improve the epitaxy ageing by means of design variants. However, the analysis is complicated by the formation of higher-order optical modes and filaments in the laser diodes, which cause kinks in the light-current characteristics and likewise have a strong influence on the course of the ageing.