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Impact of carrier-density screening on Urbach-tail energies and optical polarization in (Al,Ga)N quantum well systems

Robert Finn1,*, Michael O’Donovan2, Thomas Koprucki2, and Stefan Schulz1,3

  • *Contact author: robert.finn@tyndall.ie
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Phys. Rev. Applied 24, 044084 – Published 27 October, 2025

DOI: https://doi.org/10.1103/x5d6-w3cp

Abstract

Aluminum gallium nitride [(Al,Ga)N] presents an ideal platform for designing ultraviolet (UV) light emitters across the entire UV spectral range. However, in the deep-UV spectral range (<280 nm) these emitters exhibit very low quantum efficiencies, which in part is linked to the light-polarization characteristics of (Al,Ga)N quantum wells (QWs). In this study, we provide insight into the degree of optical polarization of (Al,Ga)N QW systems operating across the UV-C spectral range by means of an atomistic multiband electronic structure model. Our model not only captures the difference in valence-band ordering in AlN and GaN but it also accounts for alloy-disorder-induced band-mixing effects originating from random alloy fluctuations in (Al,Ga)N QWs. The latter aspect is often not captured in widely employed continuum-based models. The impact of alloy disorder on the electronic structure is studied in terms of Urbach-tail energies, which reflect the broadening of the valence-band density of states due to carrier-localization effects. We find that especially in wider wells, Urbach-tail energies are reduced with increasing carrier densities in the well, highlighting that alloy-disorder-induced carrier-localization effects in (Al,Ga)N QWs are also tightly linked to electrostatic built-in fields. Our calculations show that for QWs designed to emit at the longer-wavelength end of the UV-C spectrum, carrier density and well width are of secondary importance for their light-emission properties, meaning that one observes mainly transverse electrical polarization. However, for (Al,Ga)N QWs with high Al contents, we find that both well width and carrier density will impact the degree of optical polarization. Our calculations suggest that wider wells will increase the degree of optical polarization and may therefore be an option to improve the light-extraction efficiency in deep-UV light emitters.

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References (68)

  1. H. Amano et al., The 2020 UV emitter roadmap, J. Phys. D: Appl. Phys. 53, 503001 (2020).
  2. M. Kneissl, T.-Y. Seong, J. Han, and H. Amano, The emergence and prospects of deep-ultraviolet light-emitting diode technologies, Nat. Photonics 13, 233 (2019).
  3. S. L. Chuang and C. S. Chang, k⋅p method for strained wurtzite semiconductors, Phys. Rev. B 54, 2491 (1996).
  4. M. A. Caro, S. Schulz, S. B. Healy, and E. O’Reilly, Built-in field control in alloyed c-plane III-N quantum dots and wells, J. Appl. Phys. 109, 084110 (2011).
  5. O. Ambacher, J. Majewski, C. Miskys, A. Link, M. Hermann, M. Eickhoff, M. Stutzmann, F. Bernardini, V. Fiorentini, V. Tilak et al., Pyroelectric properties of Al(In)GaN/GaN hetero- and quantum well structures, J. Phys.: Condens. Matter 14, 3399 (2002).
  6. F. Bernardini, V. Fiorentini, and D. Vanderbilt, Spontaneous polarization and piezoelectric constants of III–V nitrides, Phys. Rev. B 56, R10024(R) (1997).
  7. C. X. Ren, Polarisation fields in III-nitrides: Effects and control, Mater. Sci. Technol. 32, 418 (2016).
  8. R. Goldhahn, C. Buchheim, P. Schley, A. Winzer, and H. Wenzel, in Nitride Semiconductor Devices: Principles and Simulation (John Wiley & Sons, Weinheim, 2007), p. 95.
  9. E. Sakalauskas, H. Behmenburg, C. Hums, P. Schley, G. Rossbach, C. Giesen, M. Heuken, H. Kalisch, R. Jansen, J. Bläsing et al., Dielectric function and optical properties of Al-rich AlInN alloys pseudomorphically grown on GaN, J. Phys. D: Appl. Phys. 43, 365102 (2010).
  10. R. G. Banal, M. Funato, and Y. Kawakami, Optical anisotropy in [0001]-oriented AlxGa1−xN/AlN quantum wells (x>0.69), Phys. Rev. B 79, 121308(R) (2009).
  11. S.-H. Wei and A. Zunger, Valence band splittings and band offsets of AlN, GaN, and InN, Appl. Phys. Lett. 69, 2719 (1996).
  12. J. Pastrňák and L. Roskovcova, Optical absorption edge of AlN single crystals, Phys. Status Solidi (b) 26, 591 (1968).
  13. J. L. Birman, Some selection rules for band-band transitions in wurtzite structure, Phys. Rev. 114, 1490 (1959).
  14. M. V. Durnev and S. Y. Karpov, Polarization phenomena in light emission from C-plane Al(In)GaN heterostructures, Phys. Status Solidi (b) 250, 180 (2013).
  15. J. Li, K.-B. Nam, M. L. Nakarmi, J.-Y. Lin, H. Jiang, P. Carrier, and S.-H. Wei, Band structure and fundamental optical transitions in wurtzite AlN, Appl. Phys. Lett. 83, 5163 (2003).
  16. J. Rass and N. Lobo-Ploch, in III-Nitride Ultraviolet Emitters: Technology and Applications (Springer International Publishing, Switzerland, 2016), p. 137.
  17. H.-T. Shen, Y.-C. Chang, and Y.-R. Wu, Analysis of light-emission polarization ratio in deep-ultraviolet light-emitting diodes by considering random alloy fluctuations with the 3D k⋅p method, Phys. Status Solidi 16, 2100498 (2022).
  18. M. Kneissl, T. Kolbe, C. Chua, V. Kueller, N. Lobo, J. Stellmach, A. Knauer, H. Rodiguez, S. Einfeldt, Z. Yang, N. M. Johnson, and M. Weyers, Advances in group III-nitride-based deep UV light-emitting diode technology, Semicond. Sci. Technol. 26, 014036 (2011).
  19. J. E. Northrup, C. L. Chua, Z. Yang, T. Wunderer, M. Kneissl, N. M. Johnson, and T. Kolbe, Effect of strain and barrier composition on the polarization of light emission from AlGaN/AlN quantum wells, Appl. Phys. Lett. 100, 021101 (2012).
  20. H. Ryu, I. Choi, H. Choi, and J. Shim, Investigation of light extraction efficiency in AlGaN deep-ultraviolet light-emitting diodes, Appl. Phys. Express 6, 062101 (2013).
  21. H. Ryu, Large enhancement of light extraction efficiency in AlGaN-based nanorod ultraviolet light-emitting diode structures, Nanoscale Res. Lett. 9, 58 (2014).
  22. C. Frankerl, F. Nippert, M. P. Hoffmann, H. Wang, C. Brandl, H.-J. Lugauer, R. Zeisel, A. Hoffmann, and M. J. Davies, Strongly localized carriers in Al-rich AlGaN/AlN single quantum wells grown on sapphire substrates, J. Appl. Phys. 127, 095701 (2020).
  23. C. Frankerl, F. Nippert, M. P. Hoffmann, C. Brandl, H.-J. Lugauer, R. Zeisel, A. Hoffmann, and M. J. Davies, Carrier dynamics in Al-rich AlGaN/AlN quantum well structures governed by carrier localization, Phys. Status Solidi (b) 257, 2000242 (2020).
  24. R. Finn and S. Schulz, Impact of random alloy fluctuations on the electronic and optical properties of (Al, Ga)N quantum wells: Insights from tight-binding calculations, J. Chem. Phys. 157, 244705 (2022).
  25. S. Kölle, F. Römer, G. Cardinali, A. Schulz, N. Susilo, D. H. Vidal, T. Wernicke, M. Kneissl, and B. Witzigmann, Optical gain in AlGaN quantum wells: Impact of higher energy states, IEEE Photonics J. 16, 1 (2024).
  26. M. Piccardo, C.-K. Li, Y.-R. Wu, J. S. Speck, B. Bonef, R. M. Farrell, M. Filoche, L. Martinelli, J. Peretti, and C. Weisbuch, Localization landscape theory of disorder in semiconductors. II. Urbach tails of disordered quantum well layers, Phys. Rev. B 95, 144205 (2017).
  27. J. M. McMahon, D. S. P. Tanner, E. Kioupakis, and S. Schulz, Atomistic analysis of radiative recombination rate, Stokes shift, and density of states in c-plane InGaN/GaN quantum wells, Appl. Phys. Lett. 116, 181104 (2020).
  28. F. Römer, M. Guttmann, T. Wernicke, M. Kneissl, and B. Witzigmann, Effect of inhomogeneous broadening in ultraviolet III-nitride light-emitting diodes, Materials 14, 7890 (2021).
  29. K. B. Nam, J. Li, M. L. Nakarmi, J. Y. Lin, and H. X. Jiang, Unique optical properties of AlGaN alloys and related ultraviolet emitters, Appl. Phys. Lett 84, 5264 (2004).
  30. B. Janjua, H. Sun, C. Zhao, D. H. Anjum, D. Priante, A. A. Alhamoud, F. Wu, X. Li, A. M. Albadri, A. Y. Alyamani, M. M. El-Desouki, T. K. Ng, and B. S. Ooi, Droop-free Al1−xGa1−xN/AlyGa1−yN quantum-disks-in-nanowires ultraviolet led emitting at 337 nm on metal/silicon substrates, Opt. Express 25, 1381 (2017).
  31. G. Hofmann, A. Muhin, N. Susilo, F. Römer, T. Wernicke, M. Kneissl, and B. Witzigmann, Simulation of carrier injection efficiency in AlGaN-based UV-light-emitting diodes, IEEE Photonics J. 16, 4 (2024).
  32. T.-J. Yang, R. Shivaraman, J. S. Speck, and Y.-R. Wu, The influence of random indium alloy fluctuations in indium gallium nitride quantum wells on the device behaviour, J. Appl. Phys. 116, 113104 (2014).
  33. D. Watson-Parris, M. J. Godfrey, P. Dawson, R. A. Oliver, M. J. Galtrey, M. J. Kappers, and C. J. Humphreys, Carrier localization mechanisms in InxGa1−xN/GaN quantum wells, Phys. Rev. B 83, 115321 (2011).
  34. D. Chaudhuri, M. O’Donovan, T. Streckenbach, O. Marquardt, P. Farrell, S. K. Patra, T. Koprucki, and S. Schulz, Multiscale simulations of the electronic structure of III-nitride quantum wells with varied indium content: Connecting atomistic and continuum-based models, J. Appl. Phys. 129, 073104 (2021).
  35. C. Broderick, E. P. O’Reilly, and S. Schulz, Perspective: Theory and simulation of highly mismatched semiconductor alloys using the tight-binding method, J. Appl. Phys. 135, 100902 (2024).
  36. M. A. Caro, S. Schulz, and E. P. O’Reilly, Hybrid functional study of the elastic and structural properties of wurtzite and zinc-blende group-III nitrides, Phys. Rev. B 86, 014117 (2012).
  37. C. Coughlan, S. Schulz, M. A. Caro, and E. P. O’Reilly, Band gap bowing and optical polarization switching in Al1−xGaxN alloys, Phys. Status Solidi B 252, 879 (2015).
  38. N. Susilo, E. Ziffer, S. Hagedorn, L. Cancellara, C. Netzel, N. L. Ploch, S. Wu, J. Rass, S. Walde, L. Sulmoni, M. Guttmann, T. Wernicke, M. Albrecht, M. Weyers, and M. Kneissl, Improved performance of UVC-LEDs by combination of high-temperature annealing and epitaxially laterally overgrown AlN/sapphire, Photonics Res. 8, 589 (2020).
  39. C. Reich, M. Guttmann, M. Feneberg, T. Wernicke, F. Mehnke, C. Kuhn, J. Rass, M. Lapeyrade, S. Einfeldt, A. Knauer et al., Strongly transverse-electric-polarized emission from deep ultraviolet AlGaN quantum well light emitting diodes, Appl. Phys. Lett. 107, 142101 (2015).
  40. M. Guttmann, F. Mehnke, B. Belde, F. Wolf, C. Reich, L. Sulmoni, T. Wernicke, and M. Kneissl, Optical light polarization and light extraction efficiency of AlGaN-based LEDs emitting between 264 and 220 nm, Jpn. J. Appl. Phys. 58, SCCB20 (2019).
  41. F. Römer, G. Hofmann, J. Höpfner, M. Schilling, A. Muhin, T. Wernicke, M. Kneissl, and B. Witzigmann, Carrier transport in a deep ultraviolet mixed quantum well light emitting diode, IEEE Photonics J. 16, 1 (2024).
  42. J. Höpfner, F. Kühl, M. Schilling, A. Muhin, M. Guttmann, G. Hofmann, F. Römer, T. Wernicke, B. Witzigmann, and M. Kneissl, Unraveling carrier distribution in far-UVC LEDs by temperature-dependent electroluminescence measurements, Appl. Phys. Lett. 125, 071109 (2024).
  43. A. Ibanez, M. Leroux, N. Nikitskiy, W. Desrat, M. Moret, P. Valvin, G. Cassabois, J. Brault, B. Gil, F. Chugenji, K. Taiga, M. A. Khan, and H. Hirayama, The influence of alloy disorder effects on the anisotropy of emission diagrams in (Al, Ga)N quantum wells embedded into AlN barriers, Phys. Status Solidi B ,.
  44. G. Cardinali, S. Kölle, A. Schulz, N. Susilo, D. H. Vidal, M. Guttmann, M. A. Blonski, F. Römer, B. Witzigmann, F. Nippert, M. R. Wagner, T. Wernicke, and M. Kneissl, Gain characteristics of optically pumped UVC lasers with wide AlGaN single-quantum-well active regions, Phys. Status Solidi (a) 221, 2400067 (2024).
  45. H. Hirayama, S. Fujikawa, N. Noguchi, J. Norimatsu, T. Takano, K. Tsubaki, and N. Kamata, 222–282 nm AlGaN and InAlGaN-based deep-UV LEDs fabricated on high-quality AlN on sapphire, Phys. Status Solidi (a) 206, 1176 (2009).
  46. M. Winkelnkemper, A. Schliwa, and D. Bimberg, Interrelation of structural and electronic properties of InGaN/GaN quantum dots using an eight-band k⋅p model, Phys. Rev. B 74, 155322 (2006).
  47. Q. Yan, P. Rinke, M. Winkelnkemper, A. Qteish, D. Bimberg, M. Scheffler, and C. G. V. de Walle, Band parameters and strain effects in ZnO and group-III nitrides, Semicond. Sci. Technol. 26, 014037 (2010).
  48. P. Rinke, M. Winkelnkemper, A. Qteish, D. Bimberg, J. Neugebauer, and M. Scheffler, Consistent set of band parameters for the group-III nitrides AlN, GaN, and InN, Phys. Rev. B 77, 075202 (2008).
  49. I. Vurgaftman and J. R. Meyer, Band parameters for nitrogen-containing semiconductors, J. Appl. Phys. 94, 3675 (2003).
  50. I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, Band parameters for III–V compound semiconductors and their alloys, J. Appl. Phys. 89, 5815 (2001).
  51. C. Frankerl, F. Nippert, A. Gomez-Iglesias, M. P. Hoffmann, C. Brandl, H.-J. Lugauer, R. Zeisel, A. Hoffmann, and M. J. Davies, Origin of carrier localization in AlGaN-based quantum well structures and implications for efficiency droop, Appl. Phys. Lett. 117, 102107 (2020).
  52. E. P. O’Reilly, A. Lindsay, S. Tomic, and M. Kamal-Saadi, Tight-binding and k⋅p models for the electronic structure of Ga(In)NAs and related alloys, Semicond. Sci. Technol. 17, 870 (2002).
  53. Z. Q. Li and W. Pots, Electronic density of states of semiconductor alloys from lattice-mismatched isovalent binary constituents, Phys. Rev. B 46, 2109 (1992).
  54. T. B. Boykin, N. Kharache, G. Klimeck, and M. Korkusinski, Approximate bandstructures of semiconductor alloys from tight-binding supercell calculations, J. Phys.: Condens. Matter 19, 036203 (2007).
  55. P. G. Moses, M. Miao, Q. Yan, and C. G. Van de Walle, Hybrid functional investigations of band gaps and band alignments for AlN, GaN, InN, and InGaN, J. Chem. Phys. 134, 084703 (2011).
  56. W. Mönch, Empirical tight-binding calculation of the branch-point energy of the continuum of interface-induced gap states, J. Appl. Phys. 80, 5076 (1996).
  57. M. A. Caro, S. Schulz, and E. P. O’Reilly, Theory of local electric polarization and its relation to internal strain: Impact on the polarization potential and electronic properties of group-III nitrides, Phys. Rev. B 88, 214103 (2013).
  58. N. Pant, Z. Deng, and E. Kioupakis, High electron mobility of AlxGa1−xN evaluated by unfolding the DFT band structure, Appl. Phys. Lett. 117, 242105 (2020).
  59. J. M. McMahon, E. Kioupakis, and S. Schulz, Carrier density dependant Auger recombination in c-plane (In, Ga)N/GaN quantum wells: Insights from atomistic calculations, J. Phys. D: Appl Phys. 57, 125102 (2024).
  60. N. Pant, K. Bushick, A. McAllister, W. Lee, C. G. V. de Walle, and E. Kioupakis, Carrier confinement and alloy disorder exacerbate Auger-Meitner recombination in AlGaN ultraviolet light-emitting diodes, Appl. Phys. Lett. 125, 021109 (2024).
  61. R. M. Barrett, J. M. McMahon, R. Ahumada-Lazo, J. A. Alanis, P. Parkinson, S. Schulz, M. J. Kappers, R. A. Oliver, and D. Binks, Disentangling the impact of point defect density and carrier localization-enhanced auger recombination on efficiency droop in (In, Ga)N/GaN quantum wells, ACS Photonics 10, 2632 (2023).
  62. E. Kioupakis, P. Rinke, K. T. Delaney, and C. G. V. de Walle, Indirect Auger recombination as a cause of efficiency droop in nitride light-emitting diodes, Appl. Phys. Lett. 98, 161107 (2011).
  63. M. O’Donovan, M. Luisier, E. P. O’Reilly, and S. Schulz, Impact of random alloy fluctuations on inter-well transport in InGaN/GaN multi-quantum well systems: An atomistic non-equilibrium Green’s function study, J. Phys.: Condens. Matter 33, 045302 (2021).
  64. S. Schulz, D. P. Tanner, E. P. O’Reilly, M. A. Caro, T. L. Martin, P. A. J. Bagot, M. P. Moody, F. Tang, J. T. Griffiths, F. Oehler, M. J. Kappers, R. A. Oliver, C. J. Humphreys, D. Sutherland, M. J. Davies, and P. Dawson, Structural, electronic, and optical properties of m-plane InGaN/GaN quantum wells: Insights from experiment and atomistic theory, Phys. Rev. B 92, 235419 (2015).
  65. G. Muziol, H. Turski, M. Siekacz, K. Szkudlarek, L. Janicki, M. Baranowski, S. Zolud, R. Kudrawiec, T. Suski, and C. Skierbiszewski, Beyond quantum efficiency limitations originating from the piezoelectric polarization in light-emitting devices, ACS Photonics 6, 1963 (2019).
  66. C. Liu, J. Liu, Y. Huang, X. Sun, Q. Sun, M. Feng, Q. Xu, Y. Shen, and H. Yang, Toward high-performance AlGaN-based UV-B LEDs: Engineering of the strain relaxation process, Cryst. Growth Des. 24, 3672 (2024).
  67. Y. Lin, Y. Zhang, Z. Liu, L. Su, J. Zhang, T. Wei, and Z. Chen, Interplay of point defects, extended defects, and carrier localization in the efficiency droop of InGaN quantum wells light-emitting diodes investigated using spatially resolved electroluminescence and photoluminescence, J. Appl. Phys. 115, 023103 (2014).
  68. R. Finn et al., Dataset for “Impact of carrier density screening on Urbach-tail energies and optical polarization in (Al,Ga)N quantum well systems”, https://doi.org/10.5281/zenodo.17226581 (2025).

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Phys. Rev. Applied 24, 044084– Published 27 October, 2025
  • Received 20 November 2024
  • Revised 12 June 2025
  • Accepted 4 September 2025
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