Laser Diode Facet Degradation Study
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Laser Diode Facet Degradation Study Ulrich T. Schwarz1, Thomas Schoedl, Angewandte und Experimentelle Physik, Universität Regensburg, 93040 Regensburg, Germany V. Kümmler, A. Lell, V. Härle OSRAM Opto Semiconductors GmbH, Wernerwerkstr. 2, 93049 Regensburg, Germany ABSTRACT We study the degradation behaviour of GaN gain guided laser diodes (LDs) on SiC substrates with cleaved facets and reflective coatings on none, one, or both facets. This allows us to demonstrate that in addition to volume effects there is a contribution of the laser facets to laser degradation. We observe that for the uncoated LDs the threshold current density is increasing considerably faster compared to LDs with mirror coatings. Degradation is observed during operation but not during storage at ambient conditions and thus expected to be photon or current induced. Operation of the uncoated laser in a nitrogen atmosphere reduces the degradation rate with respect to operation in air. INTRODUCTION Blue emitting laser diodes based on GaN and its alloys InxGa1-xN and AlxGa1-xN have been demonstrated in many research laboratories and are now commercially available. While LD lifetimes of over 10.000 h have been reported, degradation is still a major issue. We investigated the influence of facet preparation on degradation. Facet degradation is well known for InGaAs/GaAs [1] and (AlGa)As [4] but there have been no reports on facet degradation in InGaN/GaN multiple quantum well lasers. We report here degradation results for coated and uncoated LDs and in different atmospheres. The degradation process can be switched from a fast diffusion limited oxidation in air to a slow reaction limited oxidation in an N2 atmosphere. A protective coating on the laser facet reduces degradation. These observations are consistent with the oxidation of laser facets reported for other III-V semiconductor systems [1,2]. EXPERIMENTAL DETAILS The LDs are grown on SiC substrate without defect reduction. We achieve continuous wave (cw) operation times of over 140 hours at 1 mW optical output power [3]. The facets are cleaved along the common (1100 ) plane of SiC and GaN. They are atomically flat with a RMS roughness lower than 1nm. The LDs were processed as gain guided LDs with a 5 µm × 600 µm p-contact. The LDs have a backside n-contact on the SiC substrate, and are attached n-side down to a copper heat sink stabilized at 25°C. The whole setup is placed in an acrylic glass box, which can be flushed with nitrogen. The LDs were aged using a pulsed power source. The current through the LD was determined by measuring the voltage drop at a known resistor.
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Ith (A)
0.9 0.8 aging current
0.7 0.6 0.0
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1.0
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eff. operation time (min.) Figure 1: Threshold current Ith over effective operation time for two uncoated LDs from different growth runs with high aging current. The horizontal lines represent the aging current for each LD. Usually the duty cycle was set to 1% and the pulses had a duration of τp =50
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