Resonant Bragg structures based on III-nitrides
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We demonstrate a resonant Bragg structure formed by quasi-two-dimensional excitons in periodic systems of InGaN quantum wells (QWs) separated by GaN barriers. When the Bragg resonance and exciton–polariton resonance are tuned to each other, the medium exhibits an exciton-mediated resonantly enhanced optical Bragg reflection. The enhancement factor appeared to be largest for the system of 60 QWs. Owing to a high binding energy and oscillator strength of the excitons in InGaN QWs, the resonant enhancement was achieved at room temperature. The samples were grown by the metal–organic vapor-phase epitaxy (MOVPE) on GaN-on-sapphire templates. The most important technological problem of the developed structures is inhomogeneous broadening of the excitonic states due to nonuniform chemical composition of the QWs driven by InN–GaN phase separation trend. We addressed this problem by variation of the vapor pressure, growth rate, growth interactions, and admixing of hydrogen during the MOVPE. The lowest width of 74 meV at room temperature and 41 meV at 77 K was achieved for the excitonic emission line from a single InGaN QW.
I. INTRODUCTION
A resonant Bragg structure (RBS) is a kind of onedimensional photonic crystal where the periodic perturbation in the dielectric susceptibility is provided by the quasi-2D excitons in a system of quantum wells (QWs). The electro-magnetic coupling of the individual excitons leads to the formation of a superradiant optical mode.1 The exciton binding energy and the strength of its coupling with light are the most important physical parameters in this phenomenon. A sufficiently large value of the exciton binding energy is necessary for a stable excitonic state existing at room temperature. Exciton binding energy (exciton Rydberg) is proportional to the electron and hole effective masses and inversely proportional to the squared dielectric constant. It is known that larger band gap materials have larger effective masses and smaller dielectric constant. Therefore, the wide-band-gap III-nitrides should be the materials of choice, if we consider possible applications of such excitonic systems in photonic devices.2,3 Technological development of GaN-based heterostructures with multiple InGaN QWs faces two major problems. One of them is a significant density of dislocations due to a large difference between the lattice
Contributing Editor: Winston Schoenfeld a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2014.397 J. Mater. Res., Vol. 30, No. 5, Mar 14, 2015
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parameters and thermal expansion coefficients of GaN and sapphire substrate.4 The dislocations affect the excitonic states of interest in many aspects, including inhomogeneous broadening and enhanced nonradiative decay (see, for instance, Ref. 5). The other problem is a phase separation in the InGaN QWs due to a difference between the lattice parameters of InN and GaN and nonequilibrium g
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