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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 4, FEBRUARY 15, 2014
Performance Characteristics of UV-C AlGaN-Based Lasers Grown on Sapphire and Bulk AlN Substrates Martin Martens, Frank Mehnke, Christian Kuhn, Chirstoph Reich, Viola Kueller, Arne Knauer, Carsten Netzel, Carsten Hartmann, Juergen Wollweber, Jens Rass, Tim Wernicke, Matthias Bickermann, Markus Weyers, and Michael Kneissl Abstract— The performance characteristics of optically pumped laser heterostructures emitting in the UV-C spectral range between 272 and 279 nm are investigated. The laser heterostructures were grown by metal–organic vapor phase epitaxy on (0001) planar AlN/sapphire, epitaxially laterally overgrown (ELO) AlN/sapphire, and bulk AlN substrates with threading dislocation densities ranging from 2×1010 to 104 cm−2 . We found that the defect density strongly affects the laser performance. The lowest pulse threshold energy density of 50 mJ/cm2 under resonant optical pumping condition was obtained for an AlGaN multiple quantum well laser grown pseudomorphically on low defect density bulk AlN substrate. Lasing was also observed for AlGaN MQW heterostructures grown on ELO AlN/sapphire templates. The laser emission in all lasers was TE polarized. However, no lasing was observed for heterostructures grown on high defect density AlN/sapphire. Index Terms— AlGaN, AlN substrates, epitaxial lateral overgrowth, lasers, polarization, sapphire substrates, ultraviolet.
I. I NTRODUCTION LTRAVIOLET (UV) semiconductor laser diodes are of great interest for a wide range of applications including medical diagnostics, gas sensing, biochemical agent detection and materials processing [1]–[4]. The AlGaN alloy system is ideally suited for the fabrication of UV light emitters since its band gap energy covers almost the entire UV-A to UV-C spectrum. However, a number of material challenges limit the realization of short wavelength lasers in the UV-C spectral range (λ ≤ 280 nm). For example, the large lattice mismatch between commonly used sapphire substrates and the epitaxial layers lead to high dislocation densities resulting in low internal quantum efficiencies (IQE) [5]. Although sapphire
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Manuscript received October 10, 2013; revised November 13, 2013; accepted November 27, 2013. Date of publication December 3, 2013; date of current version January 21, 2014. This work was supported in part by the German Federal Ministry of Education and Research within the WideBaSe Project under Contract 03WKBT03C and in part by the Deutsche Forschungsgemeinschaft within the Collaborative Research Center Semiconductor Nanophotonics under Grant SFB 787. M. Martens, F. Mehnke, C. Kuhn, C. Reich, J. Rass, and T. Wernicke are with the Institut für Festkörperphysik, Technische Universität Berlin, Berlin 10623, Germany (e-mail:
[email protected]). V. Kueller, A. Knauer, C. Netzel, and M. Weyers are with the FerdinandBraun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin 12489, Germany. C. Hartmann, J. Wollweber, and M. Bickermann are with the Leibniz-Institut für Kristallzüchtung, Berlin 12489, Germany. M. Kneissl is with the Institut für Festkörperphysik, Technische Universität Berlin, Berlin 10623, and also with the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin 12489, Germany. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2293611
combines the advantages of lower price and better availabilty compared to AlN or SiC substrates, so far no UV-C lasers have been demonstrated on sapphire substrates. The shortest optically pumped laser emission wavelengths were obtained by AlGaN based structures grown on bulk AlN at 237 nm [6] and 4H-SiC at 241 nm [7]. In addition, longer wavelength emission in the UV-C range was reported for AlGaN based structures grown on SiC and bulk AlN [2]–[4], [8], [9]. So far sapphire substrates seemed to be unsuitable for the growth of UV-C lasers due to the high defect densities in the heterostructures [10]. However, recently epitaxially laterally overgrown (ELO) AlN/sapphire has proven to have a low enough defect density to be suitable for the growth of efficient UV LEDs [11]. In this letter we compare optically pumped AlGaN multiple quantum well (MQW) laser heterostructures emitting in the wavelength range between 272 nm and 279 nm grown on planar AlN/sapphire, ELO AlN/sapphire, and bulk AlN substrates. II. E XPERIMENT For the planar AlN/sapphire templates, an 800 nm thick AlN layer was grown on a 2-inch sapphire substrate exhibiting threading dislocation densities of 2 × 1010 cm−2 in the AlN layer as determined by high resolution X-ray diffraction (HRXRD) measurements. For the ELO templates the AlN was grown on patterned 2-inch AlN/sapphire templates [10]. Thereby the dislocation density could be reduced to 5 × 108 cm−2 [12]. Bulk AlN substrates used for heterostructure growth exhibit an even lower dislocation density of
