A comparative study of AlGaN- and GaN-based lasing structures for near- and deep-UV applications
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A comparative study of AlGaN- and GaN-based lasing structures for near- and deep-UV applications Sergiy Bidnyk, Jack B. Lam, Gordon G. Gainer, Brian D. Little, Yong-Hwan Kwon, JinJoo Song, Gary E. Bulman1, and Hua-Shuang Kong1 Center for Laser and Photonics Research and Department of Physics, Oklahoma State University, Stillwater, OK 74078, USA 1 Cree Research, Inc., Durham, NC 27713, USA ABSTRACT We report a comprehensive study on the optical properties of GaN- and AlGaN-based lasing structures at high-levels of optical excitation (carrier densities of 1017-1020 cm-3) and identify critical issues necessary for the development of near- and deep-UV light emitting devices. We successfully achieved room temperature stimulated emission (SE) with emission wavelengths ranging from 351 nm to 373 nm in a variety of samples. Through an analysis of the temperature-dependent lasing characteristics, combined with absorption and time-resolved photoluminescence measurements, we estimated the carrier density required to achieve the SE threshold in GaN epilayers. We found that in AlGaN epilayers, the onset of SE (~1019 cm-3) occurs at carrier densities one order of magnitude higher than in thick GaN epilayers, indicating that an electron-hole plasma is the dominant gain mechanism over the entire temperature range studied (10 K to 300 K). A remarkably low lasing threshold was observed in GaN/AlGaN heterostructures over the temperature range of 10 K to 300 K. Our experimental results indicate that GaN/AlGaN heterostructures could be used to efficiently generate laser emission with wavelengths shorter than 373 nm. The implications of this study on the development of UV laser diodes is discussed. INTRODUCTION Success in the fabrication of InGaN-based laser diodes (LD) [1] was largely due to the realization that the incorporation of indium into GaN drastically lowers the lasing threshold [2], enhances emission intensity [3], and improves temperature characteristics [4]. However, due to a reduction of the fundamental bandgap of GaN with increasing indium incorporation, most of the InGaN-based LDs reported thus far have emission wavelength lying between 376 nm [5] and 440 nm [6]. In order to develop LDs with emission wavelength shorter than 370 nm, one has to consider structures with an active medium different from InGaN. Recently, efficient ultraviolet lasing was reported in GaN/AlGaN separate confinement heterostructures (SCHs) [7] and deepUV stimulated emission (SE) was observed in AlGaN epilayers [8]. In the present study, we perform a comparison between AlGaN- and GaN-active medium lasing structures through highdensity optical pumping experiments combined with picosecond time-resolved experiments. We measured the carrier lifetime and estimated the carrier densities needed to reach the SE/lasing threshold in our structures. We also discuss the critical issues necessary for the development of UV laser diodes.
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EXPERIMENTAL DETAILS All samples used in this study were grown by low-pressure metalorganic chemical vapor deposition
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