Materials Science and Engineering B82 (2001) 245 – 247 Patterned dielectric mirrors for lateral overgrowth of GaN-based lasers. Taek Kim a , R.W. Martin b, *, I.M. Watson a , M.D. Dawson a , T.F. Krauss c , J.H. Marsh c , R.M. De La Rue c a Institute of Photonics, Uniersity of Strathclyde, Glasgow G40NW, UK b Department of Physics, Uniersity of Strathclyde, Glasgow G40NG, UK c Department of Electronics & Electrical Engineering, Glasgow Uniersity, Glasgow G12 8LT, UK Abstract The performance of GaN-based surface-emitting lasers may be greatly improved by the use of highly-reflecting SiO 2 /ZrO 2 multilayers for both cavity mirrors. We consider some of the limitations of GaN/Al(Ga)N multilayer mirrors and discuss alternative routes for incorporating dielectric multilayers within InGaN/GaN quantum well surface-emitting devices, using lateral epitaxial overgrowth. The use of lateral overgrowth techniques promise the benefit of reduced dislocation densities within the active region. The use of single layer lift-off techniques for the fabrication of patterned mirror templates suitable for overgrowth on GaN-on-sapphire is described. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nitride semiconductors; VCSELs; Microcavities; Oxide mirrors www.elsevier.com/locate/mseb The impressive development of violet and blue edge- emitting laser diodes with InGaN/GaN active regions has been well documented [1] and promises to impact a wide range of important areas. Vertical cavity surface emitting lasers (VCSELs) constructed from III-nitride materials should possess a number of additional advan- tages over these edge-emitters, including lower thresholds, improved mode quality and possibilities for planar arrays of large numbers of devices. Further- more, the need to fabricate cavity mirrors by etching (due to the difficulties of producing cleaved mirror facets) is avoided by the VCSEL design, with its cavity formed by two distributed Bragg reflectors (DBRs). Recent reports of optically-pumped surface emitted las- ing [2 – 4] have indicated good progress towards short wavelength VCSELs containing InGaN/GaN active regions. The fabrication of high performance DBRs remains one of the major hurdles (along with the achievement of good hole injection and current spreading) to over- come for the realisation of viable III-nitride VCSELs. The first choice, compatible with one-step epitaxial growth, would be for multilayers of GaN and Al x Ga 1 -x N but leads to a number of difficulties, which are more extreme than in comparable III-arsenide or III-phosphide structures. Most significant of these are the limited bandwidth of the mirrors and the crystalline quality of material grown epitaxially above the mirrors. The first is a result of the small difference in refrac- tive index [5], which even for the limiting binary – binary pairing is only 0.15 and 0.10 at wavelengths of 410 and 445 nm, respectively. To achieve mirror reflec- tivities high enough (ideally 99%) for satisfactory VCSEL operation requires large numbers of Ga(Al)N layers. Calculated reflectance spectra for 10 and 50 period ‘blue’ GaN/AlN DBRs (centre wavelength 445 nm) are shown in Fig. 1, giving a peak reflectivity of 97.5% and a very narrow high-reflectivity bandwidth. The peak reflectivity increases for shorter wavelength devices, as shown in the right panel of Fig. 1, but the limited bandwidth remains an issue — with the reflec- tivity exceeding 99% for only 5 nm on either side of the 50 period ‘violet’ mirror (centre wavelength 405 nm). In addition to demanding an extremely high level of control of the luminescence wavelength of the laser active region this will also probably lead to poor per- * Corresponding author. Tel.: +44-141-5483466; fax: +44-141- 5522891. E-mail address: r.w.martin@strath.ac.uk (R.W. Martin). 0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0921-5107(00)00782-0