Optisch gepumpte blau-violette Halbleiterscheibenlaser auf der Basis von GaN
Zusammenfassung der Projektergebnisse
Systematic theoretical studies for different designs of GaN-based semiconductor disk laser structures were carried out at IFSW. The optical gain budget of several active region designs with varying numbers of quantum wells and with or without resonant periodic gain structures were evaluated. In addition, several pumping schemes were considered that include direct pumping of the quantum wells and pumping of the surrounding GaN layers with cw or pulsed sources. As a result of these simulations a disk laser with 10 (or more) quantum wells in a resonant periodic gain configuration was selected as the most promising design. Initially, optical pumping was only considered for pulsed laser sources to eliminate heating issues. Compared to conventional semiconductor lasers GaN-based disk lasers differ particularly with respect to the large numbers of quantum wells within the active region. In order to further improve the laser parameter performance - in particular to achieve a reasonable low laser threshold - the defect density had to be minimized as well. During the project an epitaxial growth process was established at FBH for the growth of lnGaN/(AI)GaN multi quantum well structures with up to 20 quantum wells arranged in a resonant periodic gain (RPG) structure. Parallel to the theoretical modeling at IFSW and development of the growth process at FBH an RF-sputtering processes for the deposition of highly reflecting DBR mirrors based on Ta2O5/SiO2 quarter wavelength stacks was developed at TUB and FBH. For efficient heat management an excimer laser lift-off technique has been developed for separating the epitaxial InGaN/GaN heterestructure from the sapphire substrate. This is required also for the development of new techniques for mounting the semiconductor disk laser onto a submount. In a separate study different solder materials have been investigated and mounting technologies with SnAu solder as well as InPd and Auin eutectics were successfully implemented. In addition, a polishing technique was developed for backside polishing of GaN substrates. The remaining surface roughness was low enough so that the surface can be considered as optically flat. With minor modifications these process would be also applicable for the complete removal or thinning of the entire GaN substrate. Growth on low defect density GaN substrate would provide lower threshold current densities and possibly allow cw operation of GaN-based SCDL. First InGaN DQW RPG lasers have been grown and fabricated on low defect density GaN substrate at the end of this project. Finally, several InGaN DQW disk lasers with emission wavelengths between 395 nm and 420 nm have been demonstrated. The measurements using an especially developed setup with a pulsed nitrogen laser as a pumping source at IFSW showed peak output powers up to 300 Watt, which is the highest reported output power for a GaN-based VECSEL or VCSEL. The measurement also yielded record slope efficiencies for SCDL of 3.5% at a threshold power density of 700 kW/cm2. Furthermore, experiments with an InGaN DQW VCSEL structures at TUB showed that an order of magnitude reduction in threshold power densities can be achieved by resonant excitation of the InGaN quantum well and barriers with a pulsed dye laser source emitting near 370 nm. The long term target is the realization of a compact, fully semiconductor based semiconductor disk laser system, utilizing GaN high power laser diodes as a pumping source, that allows continuous wave (cw) operation in the blue spectral region (> 420 nm) and to possibly realize efficient frequencydoubled SCDL in the UV spectral range (200 - 250 nm). Overall, GaN based semiconductor disk lasers could provide access to a wide wavelength spectrum ranging from the blue-violet to the deep ultraviolet. Possible applications are high power blue and green laser sources for RGB projection displays and frequency-doubled short wavelength (i.e. UV) lasers for medical applications and materials processing. Although the epitaxial growth and fabrication technologies for GaN-based SCDL have shown tremendous progress during the course of this project, further advances will be required to realize external vertical cavity surface emitting laser structures with lower threshold power densities, improved slope efficiencies, and higher output powers. This will require e.g. further improvement in the structural and optical properties of the InGaN quantum wells gain region, in terms of design and epitaxial growth processes. For example additional efforts should concentrate on the reduction of the defect density in the InGaN QW, e.g. by growth on low defect density bulk GaN substrates. Further advances in the design and thermal management of the SCDL will be also required in order to operate these devices in continuous wave mode. Currently the applicable pump sources are restricted to short wavelength high power gas lasers like nitrogen lasers or frequency doubled/tripled solid state lasers. GaN-based violet laser diodes with sufficient output power would be a further next step towards compact high efficiency GaN-based disk laser systems. In order to utilise GaN-based violet laser diodes as pump sources, novel pumping configurations (e.g. radial arrangement of multiple laser diodes) have to be developed.
Projektbezogene Publikationen (Auswahl)
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"Development of InGaN-based thin disk lasers". DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM), Dresden, 22.3 - 27.3.2009
R. Debusmann, V. Hoffmann, W. John, O. Krüger, P. Vogt, M.Kneissl, and M. Weyers
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„Design and Characterization of InGaN-based vertical external cavity surface emitting lasers". DPG-Frühjahrstagung der Sektion Kondensierte Materie (SKM), Regensburg, 21.3 - 26.3.2009
R. Debusmann, N. Dhidah, V. Hoffmann, L. Weixelbaum, U. Brauch, M. Weyers, M. Kneissl, Th. Graf, P. Vogt
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„Substrate dependent wafer curvature during MOVPE growth of GaN-based laser diode structures". 24th DGKK-Workshop on Epitaxy of III/V-Semiconductors, 10.-11.12.2009, Berlin
V. Hoffmann, A. Knauer, F. Brunner, S. Einfeldt, M. Weyers, G. Tränkle, T. Schenk, J.-T. Zettler, M. Kneissl
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"440 nm laser heterostructures: Investigations of active region deteriorations through p-side overgrowth". 25th DGKK-Workshop on Epitaxy of III/V-Semiconductors, 09.12. - 01.12.2010, Aachen
V. Hoffmann, A. Knauer, U. Zeimer, L. Radaelli, S. Einfeldt, M. Weyers, G. Tränkle, M. Kneissl
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"Effects of low charge carrier wave function overlap on internal quantum efficiency in GaInN quantum wells". physica status solidi (c), Vol.7, No. 7-8, pp 1872, (2010)
C. Netzel, V. Hoffmann, T. Wernicke, A. Knauer, M. Weyers, M. Kneissl
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"Optical and Structural Properties of In0.08GaN/In0.02GaN Multiple Quantum Wells Grown at Different Temperatures and with Different Indium Supplies". Journal of Electronic Materials, 39, pp 677, (2010)
U. Zeimer, U. Jahn; V. Hoffmann; M. Weyers, M. Kneissl
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„InGaN-GaN Disk Laser for Blue-Violet Emission Wavelengths". IEEE Photonics Technology Letters, Vol. 22, No. 9, pp 652 (2010)
R. Debusmann, N. Dhidah, V. Hoffmann, L. Weixelbaum, U. Brauch, T. Graf, M. Weyers, and M. Kneissl
