TY - JOUR
T1 - High Reflectivity Hybrid AlGaN/Silver Distributed Bragg Reflectors for Use in the UV-Visible Spectrum
AU - Mehta, Karan
AU - Detchprohm, Theeradetch
AU - Park, Young Jae
AU - Liu, Yuh Shiuan
AU - Moreno, Oliver
AU - Alugubelli, Shanthan Reddy
AU - Wang, Shuo
AU - Ponce, Fernando
AU - Shen, Shyh Chiang
AU - Dupuis, Russell D.
AU - Yoder, P. Douglas
N1 - Funding Information:
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).
Publisher Copyright:
© 2017 IEEE.
PY - 2017/12
Y1 - 2017/12
N2 - Indium-free AlGaN-based distributed Bragg reflectors (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.
AB - Indium-free AlGaN-based distributed Bragg reflectors (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.
KW - Distributed Bragg reflectors
KW - mirror
KW - ultraviolet laser diode
KW - vertical cavity surface emitting laser
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U2 - 10.1109/JQE.2017.2766288
DO - 10.1109/JQE.2017.2766288
M3 - Article
AN - SCOPUS:85032455246
SN - 0018-9197
VL - 53
JO - IEEE Journal of Quantum Electronics
JF - IEEE Journal of Quantum Electronics
IS - 6
M1 - 8082794
ER -