Long-wavelength (1.3-1.55 um) vertical-cavity surface-emitting lasers (VCSELs) lattice-matched to InP need thick epitaxial distributed Bragg reflectors (DBRs), and AlAsSb-based mirrors offer high index contrast but suffer a large p-type mirror voltage caused by valence-band offsets. In 1999 Hall, Kroemer, and Coldren demonstrated Be-doped InAlGaAs/AlAsSb DBRs on InP with about 99% reflectivity and a p-type voltage drop of 92 meV per mirror pair at 1 kA/cm2, far below the roughly 240 meV/pair of earlier AlGaAsSb/AlAsSb mirrors; the same work further proposes that adding antimony to the InAlGaAs high-index layers - approximately 10% Sb - would flatten the valence band against AlAsSb and reduce the mirror voltage further. Neither that flat-valence-band prediction nor the use of InAlGaAs/AlAsSb mirrors inside an actual electrically-pumped VCSEL has since been demonstrated: the follow-on device program used AlGaAsSb/AlAsSb mirrors and routed current around them via a tunnel junction and double intracavity contacts; later mirror research changed the alloy system (AlGaAsSb/InP, AlGaPSb/InP) or targeted the n-type side; and no long-term reliability data exist for any Sb-based-DBR VCSEL. The open problem is therefore twofold: (i) grow and characterize p-type quinary InAlGaAsSb/AlAsSb DBRs lattice-matched to InP, verifying or refuting the predicted valence-band flattening and its mirror-voltage reduction below 92 meV/pair; and (ii) demonstrate an electrically-pumped 1.3-1.55 um VCSEL whose p-type current path traverses an InAlGaAs(As)Sb/AlAsSb DBR, with its threshold current and output power reported.

Background

A vertical-cavity surface-emitting laser (VCSEL) is a semiconductor laser whose optical cavity is vertical, so that light exits the wafer surface; it requires two mirrors with very high reflectivity (typically above 99%) because the gain region is extremely thin. The mirrors are distributed Bragg reflectors (DBRs): alternating quarter-wavelength dielectric layers of high and low refractive index, whose reflectivity grows with the number of pairs and with the index contrast between the two layer materials. For telecommunication wavelengths (1.3-1.55 um) the natural substrate is InP, and the conventional InGaAsP/InP material system has such a small index contrast that roughly 40-50 mirror pairs are needed, making monolithic InP-based VCSELs electrically resistive and thermally insulating. Antimony-containing alloys grown lattice-matched to InP (same lattice constant, so no dislocations) changed this: AlAsSb has an unusually low refractive index, so pairing it with a higher-index alloy gives large contrast with far fewer pairs. However, AlAsSb creates a large valence-band offset (a step in the energy of the valence band edge between adjacent layers) of roughly 350-400 meV relative to GaAsSb or AlGaAsSb, so in a p-type (hole-conducting) mirror the holes see an electrostatic barrier at every interface. This 'mirror voltage' - the accumulated voltage drop across the mirror stack at operating current density - wastes drive voltage and dissipates heat, and was one of the main obstacles to electrically-pumped long-wavelength VCSELs around 1999. The paper attacked this by using InAlGaAs as the high-index layer: because the InGaAs/AlAsSb band alignment is of type II (the valence-band edge of AlAsSb lies above that of the InGaAs alloy), the valence-band offset between InAlGaAs and AlAsSb is much smaller, and the authors measured 92 meV/pair for their Be-doped p-type InAlGaAs/AlAsSb mirror at 1 kA/cm2 (grown by molecular beam epitaxy, MBE, an ultrahigh-vacuum layer-by-layer crystal growth technique), compared with roughly 240 meV/pair for earlier Be-doped AlGaAsSb/AlAsSb mirrors, while achieving about 99% reflectivity and roughly 30 cm-1 optical loss. The paper further predicts that adding a small amount (approximately 10%) of antimony to the InAlGaAs layers - which would create a five-component (quinary) InAlGaAsSb alloy, still lattice-matched to InP - should raise the InAlGaAs valence-band edge to 'flat-band' alignment against AlAsSb, eliminating the residual hole barriers and reducing the mirror voltage further. The physical logic is sound: Sb incorporation raises the valence-band edge of an III-V alloy, moving it toward that of AlAsSb. Two things remain untested. First, no one has grown the quinary InAlGaAsSb alloy as a DBR high-index layer on InP or measured its valence-band offset against AlAsSb, so the flat-band prediction and the further voltage reduction have never been verified experimentally (a 2026 demonstration of the related quinary AlGaInAsSb alloy lattice-matched on InP exists, but only as an avalanche-photodiode multiplication layer, leaving mirrors and band offsets against AlAsSb unaddressed). Second, no electrically-pumped long-wavelength VCSEL has ever been built whose p-type current path actually traverses an InAlGaAs/AlAsSb (or Sb-added quinary) DBR. The working Sb-mirror VCSELs that the same group demonstrated from 1999 to 2003 used AlGaAsSb/AlAsSb mirrors and a double-intracavity-contact architecture with a tunnel junction (a thin, heavily doped p+-n+ junction that converts holes to electrons) so that the current bypassed both mirrors entirely; their early air-post devices, in which current did flow through the Sb-based mirrors, showed threshold voltages near 7 V and never operated continuously. The problem asks for the mirror itself to carry the current, which is what makes it a materials/band-engineering question rather than an architecture question.

Problem Statement

The open problem has two linked parts, both without a published experimental answer. (1) Flat-valence-band quinary mirror: grow p-type (Be-doped) distributed Bragg reflectors on InP whose high-index layers are the antimony-added quinary InAlGaAsSb - the InAlGaAs of the 1999 demonstration with antimony added (approximately 10% Sb), lattice-matched to InP - paired with AlAsSb low-index layers, and determine experimentally whether the valence-band discontinuity against AlAsSb is flattened as predicted and whether the p-type mirror voltage per pair falls below the demonstrated Sb-free value of 92 meV/pair at 1 kA/cm2. This requires measuring the InAlGaAsSb/AlAsSb valence-band offset, together with the mirror's reflectivity and optical loss at 1.3-1.55 um. (2) Current-conducting mirror in a device: demonstrate an electrically pumped 1.3-1.55 um VCSEL on InP in which the p-type current path traverses an InAlGaAs/AlAsSb (or the Sb-added quinary) distributed Bragg reflector rather than bypassing it through a tunnel junction and intracavity contacts, and report the device's threshold current and output power. Both parts serve one scientific objective - making the low-voltage p-type AlAsSb-based mirror itself carry current in a practical long-wavelength VCSEL - and existing results (AlGaAsSb/AlAsSb mirrors with current routed around them) answer neither.

References

  1. E. Hall, H. Kroemer, L. A. Coldren, 'AlAsSb-based distributed Bragg reflectors using InAlGaAs as high-index layer', Electronics Letters 35(5), 425-427 (1999). DOI: 10.1049/el:19990293, https://doi.org/10.1049/el:19990293
  2. O. Blum, M. J. Hafich, J. F. Klem, K. L. Lear, S. N. G. Chu, 'Electrical and optical characteristics of AlAsSb/GaAsSb distributed Bragg reflectors for surface emitting lasers', Applied Physics Letters 67, 3233-3235 (1995). DOI: 10.1063/1.114882, https://doi.org/10.1063/1.114882
  3. O. Blum, I. J. Fritz, L. R. Dawson, A. J. Howard, T. J. Headley, J. F. Klem, T. J. Drummond, 'Highly reflective, long wavelength AlAsSb/GaAsSb distributed Bragg reflector grown by molecular beam epitaxy on InP substrates', Applied Physics Letters 66, 329-331 (1995). DOI: 10.1063/1.114202, https://doi.org/10.1063/1.114202
  4. O. Blum, J. F. Klem, K. L. Lear, G. A. Vawter, S. R. Kurtz, 'Optically pumped, monolithic, all-epitaxial 1.56 um vertical cavity surface emitting laser using Sb-based reflectors', Electronics Letters 33, 1878-1880 (1997). DOI: 10.1049/el:19971284, https://doi.org/10.1049/el:19971284
  5. G. Ungaro, J. C. Harmand, I. Sagnes, B. Sermage, J. P. Debray, C. Meriadec, T. Rivera, J. L. Oudar, R. Raj, 'Room-temperature continuous-wave operation VCSEL at 1.48 um with Sb-based Bragg reflector', Electronics Letters 34, 1402-1404 (1998). DOI: 10.1049/el:19980980, https://doi.org/10.1049/el:19980980
  6. M. J. Mondry, D. I. Babic, J. E. Bowers, L. A. Coldren, 'Refractive indexes of (Al,Ga,In)As epilayers on InP for optoelectronic applications', IEEE Photonics Technology Letters 4(6), 627-630 (1992). DOI: 10.1109/68.141990, https://doi.org/10.1109/68.141990
  7. D. I. Babic, J. Piprek, K. Streubel, R. P. Mirin, N. M. Margalit, D. E. Mars, J. E. Bowers, E. L. Hu, 'Design and analysis of double-fused 1.55-um vertical-cavity lasers', IEEE Journal of Quantum Electronics 33(7), 1369-1383 (1997). DOI: 10.1109/3.605560, https://doi.org/10.1109/3.605560
  8. E. Hall, G. Almuneau, J. K. Kim, O. Sjolund, H. Kroemer, L. A. Coldren, 'Electrically-pumped, single-epitaxial VCSELs at 1.55 um with Sb-based mirrors', Electronics Letters 35(16), 1337-1338 (1999). DOI: 10.1049/el:19990965, https://doi.org/10.1049/el:19990965
  9. G. Almuneau, E. Hall, T. Mukaihara, S. Nakagawa, C. Luo, D. R. Clarke, L. A. Coldren, 'Improved electrical and thermal properties of InP-AlGaAsSb Bragg mirrors for long-wavelength vertical-cavity lasers', IEEE Photonics Technology Letters 12(10), 1322-1324 (2000). DOI: 10.1109/68.883817, https://doi.org/10.1109/68.883817
  10. G. Almuneau, E. Hall, S. Nakagawa, J. K. Kim, D. Lofgreen, O. Sjolund, C. Luo, D. R. Clarke, J. H. English, L. A. Coldren, 'Molecular beam epitaxial growth of monolithic 1.55 um vertical cavity surface emitting lasers with AlGaAsSb/AlAsSb Bragg mirrors', Journal of Vacuum Science & Technology B 18(3), 1601-1604 (2000). DOI: 10.1116/1.591436, https://doi.org/10.1116/1.591436
  11. G. Almuneau, E. Hall, S. Nakagawa, J. K. Kim, L. A. Coldren, 'Long-wavelength VCSELs with AlGaAsSb/AlAsSb Bragg mirrors lattice-matched on InP substrates', Proceedings of SPIE 3946, 48-56 (2000). DOI: 10.1117/12.384360, https://doi.org/10.1117/12.384360
  12. E. Hall, S. Nakagawa, G. Almuneau, J. K. Kim, L. A. Coldren, 'Room-temperature, CW operation of lattice-matched long-wavelength VCSELs', Electronics Letters 36(17), 1465-1467 (2000). DOI: 10.1049/el:20001070, https://doi.org/10.1049/el:20001070
  13. S. Nakagawa, E. Hall, G. Almuneau, J. K. Kim, D. A. Buell, H. Kroemer, L. A. Coldren, '88 °C, continuous-wave operation of apertured, intracavity contacted, 1.55 um vertical-cavity surface-emitting lasers', Applied Physics Letters 78(10), 1337-1339 (2001). DOI: 10.1063/1.1352668, https://doi.org/10.1063/1.1352668
  14. S. Nakagawa, E. Hall, G. Almuneau, J. K. Kim, D. A. Buell, H. Kroemer, L. A. Coldren, '1.55-um InP-lattice-matched VCSELs with AlGaAsSb-AlAsSb DBRs', IEEE Journal on Selected Topics in Quantum Electronics 7(2), 224-230 (2001). DOI: 10.1109/2944.954134, https://doi.org/10.1109/2944.954134
  15. S. Nakagawa, E. Hall, L. A. Coldren, 'Long-wavelength VCSELs with AlGaAsSb DBRs', Proceedings of SPIE 4533 (Semiconductor Lasers for Lightwave Communication Systems), 24 (2001). DOI: 10.1117/12.447761, https://doi.org/10.1117/12.447761
  16. T. Asano, D. Feezell, R. Koda, M. H. M. Reddy, D. A. Buell, A. S. Huntington, E. Hall, S. Nakagawa, L. A. Coldren, 'InP-based all-epitaxial 1.3-um VCSELs with selectively etched AlInAs apertures and Sb-based DBRs', IEEE Photonics Technology Letters 15(10), 1333-1335 (2003). DOI: 10.1109/LPT.2003.817987, https://doi.org/10.1109/LPT.2003.817987
  17. D. O. Toginho Filho, I. F. L. Dias, J. L. Duarte, S. A. Lourenco, L. C. Pocas, E. Laureto, J. C. Harmand, 'A comparative study of AlGaAsSb/AlAsSb distributed Bragg mirrors on InP', Superlattices and Microstructures 31(6), 277-283 (2002). DOI: 10.1006/spmi.2002.1046, https://doi.org/10.1006/spmi.2002.1046
  18. T. C. Lu, J. Y. Tsai, J. T. Chu, Y. S. Chang, S. C. Wang, 'InP/InGaAlAs distributed Bragg reflectors grown by low-pressure metal organic chemical vapor deposition', Journal of Crystal Growth 250(3-4), 305-312 (2003). DOI: 10.1016/S0022-0248(02)02409-0, https://doi.org/10.1016/S0022-0248(02)02409-0
  19. T. C. Lu, J. Y. Tsai, H. C. Kuo, S. C. Wang, 'Comparisons of InP/InGaAlAs and InAlAs/InGaAlAs distributed Bragg reflectors grown by metalorganic chemical vapor deposition', Materials Science and Engineering B 107(1), 66-69 (2004). DOI: 10.1016/j.mseb.2003.10.067, https://doi.org/10.1016/j.mseb.2003.10.067
  20. J. F. Klem, D. K. Serkland, J. Kim, T.-Y. Wang, 'Molecular beam epitaxial growth of AlGaPSb and AlGaPSb/InP distributed Bragg reflectors on InP', Journal of Vacuum Science & Technology B 22(3), 1468-1471 (2004). DOI: 10.1116/1.1669600, https://doi.org/10.1116/1.1669600
  21. O. Ostinelli, G. Almuneau, M. Ebnother, E. Gini, M. Haiml, W. Baechtold, 'MOVPE growth of long wavelength AlGaAsSb/InP Bragg mirrors', Electronics Letters 40(15), 940-942 (2004). DOI: 10.1049/el:20045373, https://doi.org/10.1049/el:20045373
  22. G. Almuneau, 'Les lasers à cavité verticale infrarouge : d'une approche matériau à des géométries de composant innovantes' ('Vertical cavity lasers: from materials towards innovative geometries for devices'), Habilitation à diriger des recherches, Université Paul Sabatier - Toulouse III (2010). HAL: tel-00473597, https://theses.hal.science/tel-00473597
  23. A. Islam, S. I. Islam, S. Islam, 'Designing an all epitaxial 1,550 nm intra-cavity VCSEL using GaInAsN/AlGaInAs in the active region and AlGaAsSb/AlAsSb in top and bottom DBRs', Optical and Quantum Electronics 45(12), 1199-1212 (2013). DOI: 10.1007/s11082-013-9736-7, https://doi.org/10.1007/s11082-013-9736-7
  24. K. Iga, 'Vertical-Cavity Surface-Emitting Laser: Its Conception and Evolution', Japanese Journal of Applied Physics 47(1R), 1-10 (2008). DOI: 10.1143/JJAP.47.1, https://doi.org/10.1143/JJAP.47.1
  25. Ž. Gačević, N. Vukmirović, 'Effective Refractive-Index Approximation: A Link between Structural and Optical Disorder of Planar Resonant Optical Structures', Physical Review Applied 9, 064041 (2018). DOI: 10.1103/PhysRevApplied.9.064041, https://doi.org/10.1103/PhysRevApplied.9.064041
  26. J. Radadiya, A. Rahman, Y. Zhao, S. Zhao, H. Jung, J. P. R. David, S. Lee, 'Low-excess-noise quinary AlGaInAsSb avalanche photodiodes on InP', Applied Physics Letters 128 (2026). DOI: 10.1063/5.0335508, https://doi.org/10.1063/5.0335508