- Physical Review Journals
All JournalsPhysics Magazine
Access Provided by Xinjiang University

Physical Review Materials

Export citation

Export citation

Choose format for download:

Download Citation
  • Access by Xinjiang University

Quantitative analysis of heterogeneous microstructures in compositionally graded (0001) AlGaN grown by ammonia molecular beam epitaxy

Ashley E. Wissel-Garcia1,*, Feng Wu1, Yinxuan Zhu2, Siddharth Rajan2, and James S. Speck1

  • *Contact author: awissel@ucsb.edu
PDF

Phys. Rev. Materials 10, 024605 – Published 11 February, 2026

DOI: https://doi.org/10.1103/1kcg-cjns

Abstract

Future high-power, high-frequency transistors may make use of high-Al-content AlGaN due to its large critical electric field and resulting potential for highly scaled devices. Since AlN is the most likely substrate for these devices, there is a significant lattice mismatch between the AlN substrate and lower-Al alloy layers needed for barriers, channels, and contacts. This work demonstrates fully coherent graded AlGaN heterostructures based on AlN templates, which could be implemented into future devices. We show the importance of well-controlled growth conditions, AlGaN layer thicknesses, and composition steps through transmission electron microscopy, atomic force microscopy, and high-resolution x-ray diffraction measurements to guide the epitaxial design of future generations of AlGaN-based transistors. We also emphasize the importance of careful analysis of high-resolution x-ray diffraction data in the context of a heterogeneous microstructure, such as one exhibiting crosshatch, in which there are simultaneously heavily dislocated and coherent crystalline regions in a thin layer.

View figure in article

Physics Subject Headings (PhySH)

Article Text

References (52)

  1. J. Y. Tsao, S. Chowdhury, M. A. Hollis, D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar, S. Rajan, C. G. Van de Walle, E. Bellotti, et al., Ultrawide-bandgap semiconductors: Research opportunities and challenges, Adv. Electron. Mater. 4, 1600501 (2018).
  2. A. M. Armstrong, B. A. Klein, A. Colon, A. A. Allerman, E. A. Douglas, A. G. Baca, T. R. Fortune, V. M. Abate, S. Bajaj, and S. Rajan, Ultra-wide band gap AlGaN polarization-doped field effect transistor, Jpn. J. Appl. Phys. 57, 074103 (2018).
  3. J. Lemettinen, N. Chowdhury, H. Okumura, I. Kim, S. Suihkonen, and T. Palacios, Nitrogen-polar polarization-doped field-effect transistor based on Al0.8Ga0.2N/AlN on SiC with drain current over 100 mA/mm, IEEE Electron Device Lett. 40, 1245 (2019).
  4. A. M. Armstrong, B. A. Klein, A. G. Baca, A. A. Allerman, E. A. Douglas, A. Colon, V. M. Abate, and T. R. Fortune, AlGaN polarization-doped field effect transistor with compositionally graded channel from Al0.6Ga0.4N to AlN, Appl. Phys. Lett. 114, 052103 (2019).
  5. T. Razzak, S. Hwang, A. Coleman, S. Bajaj, H. Xue, Y. Zhang, Z. Jamal-Eddine, S. H. Sohel, W. Lu, A. Khan, and S. Rajan, RF operation in graded AlxGa1−xN (x=0.65 to x=0.82) channel transistors, Electron. Lett 54, 1351 (2018).
  6. P. H. Carey IV, F. Ren, A. M. Armstrong, B. A. Klein, A. A. Allerman, E. A. Douglas, A. G. Baca, and S. J. Pearton, High temperature operation to 500 °C of AlGaN graded polarization-doped field-effect transistors, J. Vac. Sci. Technol., B 38, 033202 (2020).
  7. Y. Zhu, A. A. Allerman, C. Joishi, J. Pratt, A. M. Dominic Merwin Xavier, G. Calderon Ortiz, B. A. Klein, A. Armstrong, J. Hwang, and S. Rajan, Heterostructure and interfacial engineering for low-resistance contacts to ultra-wide bandgap AlGaN, Appl. Phys. Lett. 126, 062107 (2025).
  8. A. Schmid, Ch. Schroeter, R. Otto, M. Schuster, V. Klemm, D. Rafaja, and J. Heitmann, Microstructure of V-based ohmic contacts to AlGaN/GaN heterostructures at a reduced annealing temperature, Appl. Phys. Lett. 106, 053509 (2015).
  9. R. France, T. Xu, P. Chen, R. Chandrasekaran, and T. D. Moustakas, Vanadium-based Ohmic contacts to n-AlGaN in the entire alloy composition, Appl. Phys. Lett. 90, 062115 (2007).
  10. J. H. Wang, S. E. Mohney, S. H. Wang, U. Chowdhury, and R. D. Dupuis, Vanadium-based ohmic contacts to n-type Al0.6Ga0.4N, J. Electron. Mater. 33, 418 (2004).
  11. J. W. Chung, W. E. Hoke, E. M. Chumbes, and T. Palacios, AlGaN/GaN HEMT With 300-GHz fmax, IEEE Electron Device Lett. 31, 195 (2010).
  12. W. Li, B. Romanczyk, M. Guidry, E. Akso, N. Hatui, C. Wurm, W. Liu, P. Shrestha, H. Collins, C. Clymore, et al., Record RF power performance at 94 GHz from millimeter-wave N-Polar GaN-on-sapphire deep-recess HEMTs, IEEE Trans. Electron Devices 70, 2075 (2023).
  13. T. Razzak, S. Hwang, A. Coleman, H. Xue, S. H. Sohel, S. Bajaj, Y. Zhang, W. Lu, A. Khan, and S. Rajan, Design of compositionally graded contact layers for MOCVD grown high Al-content AlGaN transistors, Appl. Phys. Lett. 115, 043502 (2019).
  14. S. Bajaj, F. Akyol, S. Krishnamoorthy, Y. Zhang, and S. Rajan, AlGaN channel field effect transistors with graded heterostructure ohmic contacts, Appl. Phys. Lett. 109, 133508 (2016).
  15. J. W. Matthews and A. E. Blakeslee, Defects in epitaxial multilayers: I. Misfit dislocations, J. Cryst. Growth 27, 118 (1974).
  16. Y. Taniyasu, M. Kasu, and T. Makimoto, Increased electron mobility in n-type Si-doped AlN by reducing dislocation density, Appl. Phys. Lett. 89, 182112 (2006).
  17. T. J. Anderson, M. J. Tadjer, J. K. Hite, J. D. Greenlee, A. D. Koehler, K. D. Hobart, and F. J. Kub, Effect of reduced extended defect density in MOCVD grown AlGaN/GaN HEMTs on native GaN substrates, IEEE Electron Device Lett. 37, 28 (2016).
  18. M. Ťapajna, S. W. Kaun, M. H. Wong, F. Gao, T. Palacios, U. K. Mishra, J. S. Speck, and M. Kuball, Influence of threading dislocation density on early degradation in AlGaN/GaN high electron mobility transistors, Appl. Phys. Lett. 99, 223501 (2011).
  19. R. Jaszek, Carrier scattering by dislocations in semiconductors, J. Mater. Sci. Mater. Electron. 12, 1 (2001).
  20. A. E. Wissel-Garcia, F. Wu, and J. S. Speck, Growth regimes of (0001) AlN by ammonia molecular beam epitaxy, J. Vac. Sci. Technol. A 43, 032706 (2025).
  21. A. L. Corrion, F. Wu, and J. S. Speck, Growth regimes during homoepitaxial growth of GaN by ammonia molecular beam epitaxy, J. Appl. Phys. 112, 054903 (2012).
  22. V. Srikant, J. S. Speck, and D. R. Clarke, Mosaic structure in epitaxial thin films having large lattice mismatch, J. Appl. Phys. 82, 4286 (1997).
  23. Q. S. Paduano, A. J. Drehman, D. W. Weyburne, J. Kozlowski, J. Serafinczuk, J. Jasinski, and Z. Liliental-Weber, X-ray characterization of high quality AlN epitaxial layers: Effect of growth condition on layer structural properties, Phys. Status Solidi C n/a, 2014 (2003).
  24. R. Chierchia, T. Böttcher, H. Heinke, S. Einfeldt, S. Figge, and D. Hommel, Microstructure of heteroepitaxial GaN revealed by x-ray diffraction, J. Appl. Phys. 93, 8918 (2003).
  25. A. M. Andrews, J. S. Speck, A. E. Romanov, M. Bobeth, and W. Pompe, Modeling cross-hatch surface morphology in growing mismatched layers, J. Appl. Phys. 91, 1933 (2002).
  26. M. Wang, F. Wu, Y. Yao, C. Zollner, M. Iza, M. Lam, S. P. DenBaars, S. Nakamura, and J. S. Speck, 10.6% external quantum efficiency germicidal UV LEDs grown on thin highly conductive n-AlGaN, Appl. Phys. Lett. 123, 231101 (2023).
  27. C.-Y. Huang, K.-S. Chang, C.-Y. Huang, Y.-H. Lin, W.-C. Peng, H.-W. Yen, R.-M. Lin, and H.-C. Kuo, The origin and mitigation of volcano-like morphologies in micron-thick AlGaN/AlN heteroepitaxy, Appl. Phys. Lett. 111, 072110 (2017).
  28. S. Rathkanthiwar, J. Houston Dycus, S. Mita, R. Kirste, J. Tweedie, R. Collazo, and Z. Sitar, Pseudomorphic growth of thick Al0.6Ga0.4N epilayers on AlN substrates, Appl. Phys. Lett. 120, 202105 (2022).
  29. R. W. Enck, N. Woodward, C. Gallinat, G. Metcalfe, A. V. Sampath, H. Shen, and M. Wraback, Plasma-assisted molecular beam epitaxy of strain-compensated a-plane InGaN/AlGaN superlattices, Phys. Status Solidi C 12, 434 (2015).
  30. K. Lee, R. Page, V. Protasenko, L. J. Schowalter, M. Toita, H. G. Xing, and D. Jena, MBE growth and donor doping of coherent ultrawide bandgap AlGaN alloy layers on single-crystal AlN substrates, Appl. Phys. Lett. 118, 092101 (2021).
  31. P. Sohi, D. Martin, and N. Grandjean, Critical thickness of GaN on AlN: Impact of growth temperature and dislocation density, Semicond. Sci. Technol. 32, 075010 (2017).
  32. M. E. Rudinsky, A. V. Lobanova, S. Yu. Karpov, and R. A. Talalaev, Impact of metalorganic vapor phase epitaxy growth conditions on compressive strain relaxation in polar III-nitride heterostructures, Jpn. J. Appl. Phys. 58, SC1017 (2019).
  33. D. J. DUNSTAN, Strain and strain relaxation in semiconductors, J. Mater. Sci. Mater. Electron. 8, 337 (1997).
  34. T. Wang, S. Liu, X. Zheng, P. Wang, D. Wang, Z. Chen, J. Wei, X. Rong, R. Tao, S. Guo, et al., Microstructure and dislocation evolution in composition gradient AlGaN grown by MOCVD, Superlattices Microstruct. 152, 106842 (2021).
  35. A. M. Andrews, R. LeSar, M. A. Kerner, J. S. Speck, A. E. Romanov, A. L. Kolesnikova, M. Bobeth, and W. Pompe, Modeling crosshatch surface morphology in growing mismatched layers. Part II: Periodic boundary conditions and dislocation groups, J. Appl. Phys. 95, 6032 (2004).
  36. E. A. Fitzgerald, Y.-H. Xie, D. Monroe, P. J. Silverman, J. M. Kuo, A. R. Kortan, F. A. Thiel, and B. E. Weir, Relaxed GexSi1−x structures for III–V integration with Si and high mobility two-dimensional electron gases in Si, J. Vac. Sci. Technol. B Microelectron. Nanometer Struct. Process. Meas. Phenom. 10, 1807 (1992).
  37. J. Aubin, J. M. Hartmann, A. Gassenq, J. L. Rouviere, E. Robin, V. Delaye, D. Cooper, N. Mollard, V. Reboud, and V. Calvo, Growth and structural properties of step-graded, high Sn content GeSn layers on Ge, Semicond. Sci. Technol. 32, 094006 (2017).
  38. S. Srinivasan, L. Geng, R. Liu, F. A. Ponce, Y. Narukawa, and S. Tanaka, Slip systems and misfit dislocations in InGaN epilayers, Appl. Phys. Lett. 83, 5187 (2003).
  39. J. A. Floro, D. M. Follstaedt, P. Provencio, S. J. Hearne, and S. R. Lee, Misfit dislocation formation in the AlGaN/GaN heterointerface, J. Appl. Phys. 96, 7087 (2004).
  40. C. Bazioti, E. Papadomanolaki, Th. Kehagias, T. Walther, J. Smalc-Koziorowska, E. Pavlidou, Ph. Komninou, Th. Karakostas, E. Iliopoulos, and G. P. Dimitrakopulos, Defects, strain relaxation, and compositional grading in high indium content InGaN epilayers grown by molecular beam epitaxy, J. Appl. Phys. 118, 155301 (2015).
  41. D. Jena and U. K. Mishra, Effect of scattering by strain fields surrounding edge dislocations on electron transport in two-dimensional electron gases, Appl. Phys. Lett. 80, 64 (2002).
  42. M. N. Gurusinghe, S. K. Davidsson, and T. G. Andersson, Two-dimensional electron mobility limitation mechanisms in AlxGa1−xN/GaN heterostructures, Phys. Rev. B 72, 045316 (2005).
  43. G. Liu, J. Wu, G. Zhao, S. Liu, W. Mao, Y. Hao, C. Liu, S. Yang, X. Liu, Q. Zhu, and Z. Wang, Impact of the misfit dislocations on two-dimensional electron gas mobility in semi-polar AlGaN/GaN heterostructures, Appl. Phys. Lett. 100, 082101 (2012).
  44. D.-D. Jin, L. Wang, S.-Y. Yang, L.-W. Zhang, H. Li, H. Zhang, J. Wang, R. Xiang, H. Wei, C. Jiao, et al., Anisotropic scattering effect of the inclined misfit dislocation on the two-dimensional electron gas in Al(In)GaN/GaN heterostructures, J. Appl. Phys. 115, 043702 (2014).
  45. D. Holec, Y. Zhang, D. V. S. Rao, M. J. Kappers, C. McAleese, and C. J. Humphreys, Equilibrium critical thickness for misfit dislocations in III-nitrides, J. Appl. Phys. 104, 123514 (2008).
  46. A. M. Smirnov, E. C. Young, V. E. Bougrov, J. S. Speck, and A. E. Romanov, Critical thickness for the formation of misfit dislocations originating from prismatic slip in semipolar and nonpolar III-nitride heterostructures, APL Mater. 4, 016105 (2016).
  47. H. Ye, P. Lu, Z. Yu, Z. Chen, B. Jia, H. Feng, and Y. Liu, Equilibrium critical thickness for a wurtzite InGaN/GaN heterostructure, Superlattices Microstruct. 48, 58 (2010).
  48. L. B. Freud, The stress distribution and curvature of a general compositionally graded semiconductor layer, J. Cryst. Growth 132, 341 (1993).
  49. J. E. Ayers, Critical layer thickness in compositionally-graded semiconductor layers with non-zero interfacial mismatch, Semicond. Sci. Technol. 23, 045018 (2008).
  50. C. A. B. Ball and C. Laird, A calculation of the energy of misfit dislocations and the critical thickness in graded epitaxial layers, Thin. Solid. Films 41, 307 (1977).
  51. T.-Y. Zhang, A dislocation in a compositionally graded epilayer, Phys. Status Solidi A 148, 175 (1995).
  52. A. E. Romanov and J. S. Speck, Stress relaxation in mismatched layers due to threading dislocation inclination, Appl. Phys. Lett. 83, 2569 (2003).

Outline

Information

Phys. Rev. Materials 10, 024605– Published 11 February, 2026
  • Received 19 August 2025
  • Accepted 14 January 2026
Article part of CHORUS
Crossmark - Check for updates button

©2026 American Physical Society

Sign In to Your Journals Account

Filter

Filter

Article Lookup

Enter a citation