IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 53, NO. 6, DECEMBER 2017 2100208 High Reﬂectivity Hybrid AlGaN/Silver Distributed Bragg Reﬂectors for Use in the UV-Visible Spectrum Karan Mehta , Theeradetch Detchprohm, Young Jae Park, Yuh-Shiuan Liu, Oliver Moreno, Shanthan Reddy Alugubelli, Shuo Wang, Fernando A. Ponce, Shyh-Chiang Shen, Senior Member, IEEE, Russell D. Dupuis, Fellow, IEEE , and P. Douglas Yoder, Senior Member, IEEE Abstract— Indium-free AlGaN-based distributed Bragg reﬂec- tors (DBRs) in the UV spectrum are known to have very low reﬂectivities due both to the low refractive index contrast as well as limitations imposed by the critical thickness of AlGaN alloys (tensile strain of ∼2.41% for AlN on GaN). Near-bandedge excitonic resonances inﬂuence the real part of AlGaN’s dielectric function, which sharply increases its refractive index as the photon energy approaches the bandgap. Furthermore, heavy doping (Si: 10 20 cm −3 ) can modify the plasma frequency of AlGaN, leading to a reduction in its refractive index. Hence, judiciously choosing the high index material to exploit excitonic resonances and using heavy doping to reduce the refractive index of the low index material can enhance the index contrast and enable growth of epitaxial DBRs with higher reﬂectivities. We have demonstrated this technique both experimentally and by simulations for wavelengths ranging from 240 to 370 nm. Typically, over 50 epitaxial pairs are needed to achieve a mirror whose reﬂectivity exceeds 99%, but this can be shrunk down to 20–30 epitaxial pairs by depositing silver/aluminum underneath the epitaxial DBR stack. Silver and aluminum exhibit >90% reﬂectivity at the AlGaN/metal interface between wavelengths ranging from >360 to 180–670 nm, respectively. A thinner DBR stack also reduces the thermal resistance, which would allow the VCSEL to achieve higher peak output powers, and simultaneously reduce overall tensile strain. Index Terms— Distributed Bragg reﬂectors, vertical cavity surface emitting laser, ultraviolet laser diode, mirror. I. I NTRODUCTION E LECTRICALLY-PUMPED III-nitride based blue-violet vertical-cavity surface-emitting lasers (VCSELs) have been gaining in popularity since being ﬁrst demonstrated in 2008 [1], [2]. VCSELs have some advantages over edge- emitting laser diodes such as circular and low-divergence Manuscript received August 24, 2017; revised September 29, 2017; accepted October 20, 2017. Date of publication October 25, 2017; date of current version November 6, 2017. This work was supported by the Defense Advanced Research Projects Agency under Grant W911NF-15-1-0026. (Corresponding author: Karan Mehta.) K. Mehta, T. Detchprohm, Y. J. Park, Y.-S. Liu, O. Moreno, S.-C. Shen, R. D. Dupuis, and P. D. Yoder are with the School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: karan.mehta@gatech.edu). S. R. Alugubelli, S. Wang, and F. A. Ponce are with the Department of Physics, Arizona State University, Tempe, AZ 85287-1504 USA. This paper has supplementary downloadable material available at http://ieeexplore.ieee.org, provided by the author. Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/JQE.2017.2766288 output beams, lower threshold currents and single longitudinal mode operation. Typically, VCSELs have a lower round-trip gain compared to long edge-emitting laser diodes. This low round-trip gain necessitates high reﬂectivity mirrors for the optical gain to equal the loss. In our experience, the greatest barrier to realizing III-N VCSELs is in obtaining a bottom distributed Bragg reﬂector (DBR) whose reﬂectivity exceeds 99%. There are two DBR conﬁgurations commonly used in III-N VCSELs: VCSELs with double dielectric DBRs (dielectric DBRs on the n- and p-sides) and VCSELs with hybrid DBRs (dielectric DBR on the p-side and an epitaxial DBR on the n-side). The p-side mirror is always a high reﬂectivity dielectric DBR stack of Ta 2 O 5 /SiO 2 [2] or Nb 2 O 5 /SiO 2 [1], [3], [4] or ZrO 2 /SiO 2 [5] for blue VCSELs and HfO 2 /SiO 2 [6], [7] for UV light emitters. There have been two main approaches to obtaining high reﬂectivity bottom mirrors: epitaxial AlInGaN DBRs and dielectric DBRs. VCSELs grown on GaN substrates with a dielectric DBR on the bottom can be realized primarily by two methods: substrate thinning by chemical mechanical polish- ing (CMP) [8]–[12] or by epitaxial lateral overgrowth (ELOG) [4], [13]. After bonding the VCSEL to a Si support substrate, CMP can be used to thin the substrate, followed by a dielectric DBR deposition. The primary disadvantages of this method are the difﬁculty in precisely controlling the cavity length since polishing lacks the precision of epitaxy by MOCVD or MBE, and the necessity of obtaining a smooth surface after polishing in order to minimize scattering losses at the cavity and dielec- tric DBR interface. The ELOG method has been effectively used by Sony to obtain milliwatt class blue VCSELs [4], [13]. In this process, the SiO 2 /SiN x bottom dielectric DBR stack is embedded in n-GaN grown by ELOG [14]. This method allows more precise cavity length control than that obtained by CMP, thus increasing the yield. The advantages of using double dielectric DBRs over hybrid DBRs include higher reﬂectivity and wider stopbands obtained due to the increased refractive index contrast between the two dielectric materials. This leads to lower threshold currents (due to higher reﬂectivity), and the wider stopband improves the VCSEL’s robustness against ﬂuctuations in material composition and thickness by ensuring that the lasing wavelength is always within the DBR’s stop- band, while simultaneously extending the VCSEL’s operating temperature range. 0018-9197 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.