Direct fabrication of record low specific resistivity metal contacts for n-type Al<sub>x</sub>Ga<sub>1−x</sub>N (x ≥ 0.8)

Tingang Liu; Haicheng Cao; Mingtao Nong; Zhiyuan Liu; Zixian Jiang; Kexin Ren; Glen Isaac; Maciel Garcia; Xiaohang Li

doi:10.1088/1674-4926/25120008

J. Semicond. > Just Accepted

ARTICLES

Direct fabrication of record low specific resistivity metal contacts for n-type Al_xGa_1−xN (x ≥ 0.8)

Tingang Liu, Haicheng Cao, Mingtao Nong, Zhiyuan Liu, Zixian Jiang, Kexin Ren, Glen Isaac, Maciel Garcia and Xiaohang Li

+ Author Affiliations

Corresponding author: Xiaohang Li, xiaohang.li@kaust.edu.sa

DOI: 10.1088/1674-4926/25120008CSTR: 32376.14.1674-4926.25120008

Abstract: Al-rich Al_xGa_1−xN (x ≥ 0.8) is promising for power and deep-ultraviolet (DUV) optoelectronic applications, owing to its ultra-wide bandgap and excellent thermal stability. However, forming low-resistivity contacts on n-type Al-rich AlGaN remains a significant challenge. In this work, we utilized an Au-free Ti/Al/Ti metal stack contact on n-type Al-rich AlGaN without graded layers. Record-low contact resistivities were achieved after annealing: 1.52×10⁻⁶ Ω·cm² for n-Al_0.8Ga_0.2N, 3.56×10⁻⁶ Ω·cm² for n-Al_0.86Ga_0.14N, and 5.79×10⁻⁵ Ω·cm² for n-Al_0.9Ga_0.1N. These results demonstrate a significant advancement in forming low-resistance contacts directly on Al-rich n-AlGaN, offering a viable path forward for next-generation power electronics and DUV optoelectronic devices.

Key words: Al-rich AlGaN, ohmic contact, contact resistivity

References

[1]	Baca A G, Armstrong A M, Klein B A, et al. Al-rich AlGaN based transistors. J Vac Sci Technol A Vac Surf Films, 2020, 38(2): 020803 doi: 10.1116/1.5129803
[2]	Hirayama H, Fujikawa S, Kamata N. Recent progress in AlGaN-based deep-UV LEDs. Elect Comm Japan, 2015, 98(5): 1 doi: 10.1364/soled.2014.dw2c.1
[3]	Zhang Y N, Zhang J C, Liu Z H, et al. Demonstration of a 2 kV Al_0.85Ga_0.15N Schottky barrier diode with improved on-current and ideality factor. IEEE Electron Device Lett, 2020, 41(3): 457 doi: 10.1109/LED.2020.2967895
[4]	Baca A G, Armstrong A M, Allerman A A, et al. An AlN/Al_0.85Ga_0.15N high electron mobility transistor. Appl Phys Lett, 2016, 109(3): 033509 doi: 10.1063/1.4959179
[5]	Douglas E A, Reza S, Sanchez C, et al. Ohmic contacts to Al-rich AlGaN heterostructures. Phys Status Solidi A, 2017, 214(8): 1600842 doi: 10.1002/pssa.201600842
[6]	Peng J L, Jiang K, Zhang S L, et al. Reducing specific contact resistivity of V/Al/Ti/Au n-electrode on n-AlGaN with Al content over 80% for far-UVC LEDs. J Semicond, 2025, 46(9): 092501 doi: 10.1088/1674-4926/25010026
[7]	Readinger E D, Mohney S E, Pribicko T G, et al. Ohmic contacts to Al-rich n-AlGaN. Electron Lett, 2002, 38(20): 1230 doi: 10.1049/el:20020800
[8]	Cao H C, Nong M T, Li J Q, et al. Low contact resistivity at the 10–4 Ω cm² level fabricated directly on n-type AlN. Appl Phys Lett, 2024, 125(8): 081602 doi: 10.1063/5.0215744
[9]	Razzak T, Hwang S, Coleman A, et al. Design of compositionally graded contact layers for MOCVD grown high Al-content AlGaN transistors. Appl Phys Lett, 2019, 115(4): 043502 doi: 10.1063/1.5108529
[10]	Cho H K, Mogilatenko A, Susilo N, et al. Electrical properties and microstructure of V/Al/Ni/Au contacts on n-Al_0.65Ga_0.35N: Si with different Au thicknesses and annealing temperatures. Semicond Sci Technol, 2022, 37(10): 105016 doi: 10.1088/1361-6641/ac8e8f
[11]	Sulmoni L, Mehnke F, Mogilatenko A, et al. Electrical properties and microstructure formation of V/Al-based n-contacts on high Al mole fraction n-AlGaN layers. Photon Res, 2020, 8(8): 1381 doi: 10.1364/PRJ.391075
[12]	Chettri D, Mainali G, Cao H C, et al. Demonstration of aluminum nitride metal oxide semiconductor field effect transistor on sapphire substrate. J Phys D: Appl Phys, 2025, 58(3): 035104 doi: 10.1088/1361-6463/ad8759
[13]	Miller M A, Lin S K, Mohney S E. V/Al/V/Ag contacts to n-GaN and n-AlGaN. J Appl Phys, 2008, 104(6): 064508 doi: 10.1063/1.2980038
[14]	Nagata N, Senga T, Iwaya M, et al. Reduction of contact resistance in V-based electrode for high AlN molar fraction n-type AlGaN by using thin SiN_x intermediate layer. Phys Status Solidi C, 2017, 14(8): 1600243 doi: 10.1002/pssc.201600243
[15]	Nong M T, Liao C-H, Tang X, et al. Epitaxial AlN film with improved quality on Si (111) substrates realized by boron pretreatment via MOCVD. Appl Phys Lett, 2024, 124(17): 172107 doi: 10.1063/5.0207884
[16]	Sarkar B, Washiyama S, Breckenridge M H, et al. N- and P- type doping in Al-rich AlGaN and AlN. ECS Trans, 2018, 86(12): 25
[17]	Wang J M, Xu F J, Zhang L S, et al. Progress in efficient doping of Al-rich AlGaN. J Semicond, 2024, 45(2): 021501 doi: 10.1088/1674-4926/45/2/021501
[18]	Chua E K, Zhao R, Shi L P, et al. Effect of metals and annealing on specific contact resistivity of GeTe/metal contacts. Appl Phys Lett, 2012, 101: 012107 doi: 10.1063/1.4732787
[19]	Herath Mudiyanselage D, Wang D W, Da B C, et al. Over 600 V lateral AlN-on-AlN Schottky barrier diodes with ultra-low ideality factor. Appl Phys Express, 2024, 17(7): 074001 doi: 10.35848/1882-0786/ad5e5a
[20]	Maeda T, Page R, Nomoto K, et al. AlN quasi-vertical Schottky barrier diode on AlN bulk substrate using Al_0.9Ga_0.1N current spreading layer. Appl Phys Express, 2022, 15(6): 061007 doi: 10.35848/1882-0786/ac702e
[21]	Cho H K, Ostermay I, Kolbe T, et al. Au-free V/Al/Pt contacts on n-Al_0.85Ga_0.15N: Si surfaces of far-UVC LEDs. ECS J Solid State Sci Technol, 2024, 13(9): 093009 doi: 10.1149/2162-8777/ad78ff
[22]	France R, Xu T, Chen P P, et al. Vanadium-based Ohmic contacts to n-AlGaN in the entire alloy composition. Appl Phys Lett, 2007, 90(6): 062115 doi: 10.1063/1.2458399
[23]	Ji J L, Liu T, He Z K, et al. Semipolar (11–22) AlN films grown by hydride vapor phase epitaxy for Schottky barrier diodes. Cryst Growth Des, 2024, 24(9): 3960 doi: 10.1021/acs.cgd.4c00292
[24]	Yang Y, Xiong F B, Lin H Y, et al. Evaluation of Ti/Al/Ni/Au ohmic contact to n-AlGaN with different Ti/Al thickness for deep ultraviolet light emitting diode. Solid State Electron, 2023, 208: 108752 doi: 10.1016/j.sse.2023.108752
[25]	Cho H K, Rass J, Mogilatenko A, et al. Impact of plasma treatment of n-Al_0.87Ga_0.13N: Si surfaces on V/Al/Ni/Au contacts in far-UVC LEDs. IEEE Photonics Technol Lett, 2023, 35(17): 915 doi: 10.1109/LPT.2023.3288216
[26]	Zollner C J, Yao Y F, Wang M, et al. Highly conductive n-Al_0.65Ga_0.35N grown by MOCVD using low V/III ratio. Crystals, 2021, 11(8): 1006 doi: 10.3390/cryst11081006
[27]	Lapeyrade M, Alamé S, Glaab J, et al. Effect of Cl₂ plasma treatment and annealing on vanadium based metal contacts to Si-doped Al_0.75Ga_0.25N. J Appl Phys, 2017, 122(12): 125701 doi: 10.1063/1.4993447
[28]	Mori K, Takeda K, Kusafuka T, et al. Low-ohmic-contact-resistance V-based electrode for n-type AlGaN with high AlN molar fraction. Jpn J Appl Phys, 2016, 55(5S): 05FL03 doi: 10.7567/JJAP.55.05FL03
[29]	Srivastava S, Hwang S M, Islam M, et al. Ohmic contact to high-aluminum-content AlGaN epilayers. J Electron Mater, 2009, 38(11): 2348 doi: 10.1007/s11664-009-0924-y

Fig. 1. (Color online) 3D cross-section view of MOCVD grown (a) n-Al_0.8Ga_0.2N, (b) n-Al_0.86Ga_0.14N, and (c) n-Al_0.9Ga_0.1N with AlN regrowth buffer layer on commercial AlN template. (d) Schematic of the CTLM pattern and fabrication process, featuring contact pads with a diameter of 700 μm and gap spacings ranging from 5 to 50 μm.

DownLoad: Full-Size Img PowerPoint

Fig. 2. (Color online) AFM surface morphology images of (a) n-Al_0.8Ga_0.2N, (b) n-Al_0.86Ga_0.14N, and (c) n-Al_0.9Ga_0.1N, respectively. (d) HR-XRD 2θ-ω scan results of the AlGaN samples, with sapphire substrate peaks used for calibration. (e) X-ray rocking curves of the AlGaN samples for the symmetric (002) and asymmetric (102) reflections.

DownLoad: Full-Size Img PowerPoint

Fig. 3. (Color online) CTLM current-voltage (I-V) characteristics of (a) reference, (b) n-Al_0.8Ga_0.2N, (c)n-Al_0.86Ga_0.14N, and (d) n-Al_0.9Ga_0.1N. Specific contact resistivity extraction based on the linear fitting of total resistance R_t versus ln(R_n/R₀) for (e) reference, (f) n-Al_0.8Ga_0.2N, (g) n-Al_0.86Ga_0.14N, and (h) n-Al_0.9Ga_0.1N.

DownLoad: Full-Size Img PowerPoint

Fig. 4. (Color online) (a) Measured specific contact resistivity of the samples, with error bars indicating measurement variation. All contact resistivity results are presented in Supplementary Figs. S1−S3. (b) Benchmark of specific contact resistivity for directly fabricated contacts on n-type Al-rich AlGaN and AlN. Red stars indicate the best results achieved in this work.

DownLoad: Full-Size Img PowerPoint

Fig. 5. (Color online) SEM surface morphologies the Ti/Al/Ti/Au contact (a, b) and of the Ti/Al/Ti contact (f, g). TEM cross-section images of Ti/Al/Ti/Au metal contact (c−e) and Ti/Al/Ti metal contact (h−j)

DownLoad: Full-Size Img PowerPoint

Table 1. FWHM, dislocation densities, and hall-effect measurement results of MOCVD grown n-AlGaN, including carrier concentration n mobility μ, and sheet resistance R_s at 300K.

Sample ID	FWHM (002) (arcsec)	FWHM (002) (arcsec)	Screw dislocation ρ_s (cm⁻²)	Edge dislocation ρ_e (cm⁻²)	Carrier concentration ${n} $ (cm⁻³)	Mobility ${\mu } $ [cm²/(V·s)]	Sheet resistance $ {{R}}_{{s}} $ (kΩ per sq.)
Al_0.8Ga_0.2N	200.48	342.12	8.60 × 10⁷	6.46 × 10⁸	1.54 × 10¹⁸	10.2	39.8
Al_0.86Ga_0.14N	134.38	295.16	3.88 × 10⁷	4.82 × 10⁸	4.85 × 10¹⁶	15.2	845
Al_0.9Ga_0.1N	148.13	234.50	4.73 × 10⁷	3.05 × 10⁸	0.69 × 10¹⁶	39.7	2280

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[1]	Baca A G, Armstrong A M, Klein B A, et al. Al-rich AlGaN based transistors. J Vac Sci Technol A Vac Surf Films, 2020, 38(2): 020803 doi: 10.1116/1.5129803
[2]	Hirayama H, Fujikawa S, Kamata N. Recent progress in AlGaN-based deep-UV LEDs. Elect Comm Japan, 2015, 98(5): 1 doi: 10.1364/soled.2014.dw2c.1
[3]	Zhang Y N, Zhang J C, Liu Z H, et al. Demonstration of a 2 kV Al_0.85Ga_0.15N Schottky barrier diode with improved on-current and ideality factor. IEEE Electron Device Lett, 2020, 41(3): 457 doi: 10.1109/LED.2020.2967895
[4]	Baca A G, Armstrong A M, Allerman A A, et al. An AlN/Al_0.85Ga_0.15N high electron mobility transistor. Appl Phys Lett, 2016, 109(3): 033509 doi: 10.1063/1.4959179
[5]	Douglas E A, Reza S, Sanchez C, et al. Ohmic contacts to Al-rich AlGaN heterostructures. Phys Status Solidi A, 2017, 214(8): 1600842 doi: 10.1002/pssa.201600842
[6]	Peng J L, Jiang K, Zhang S L, et al. Reducing specific contact resistivity of V/Al/Ti/Au n-electrode on n-AlGaN with Al content over 80% for far-UVC LEDs. J Semicond, 2025, 46(9): 092501 doi: 10.1088/1674-4926/25010026
[7]	Readinger E D, Mohney S E, Pribicko T G, et al. Ohmic contacts to Al-rich n-AlGaN. Electron Lett, 2002, 38(20): 1230 doi: 10.1049/el:20020800
[8]	Cao H C, Nong M T, Li J Q, et al. Low contact resistivity at the 10–4 Ω cm² level fabricated directly on n-type AlN. Appl Phys Lett, 2024, 125(8): 081602 doi: 10.1063/5.0215744
[9]	Razzak T, Hwang S, Coleman A, et al. Design of compositionally graded contact layers for MOCVD grown high Al-content AlGaN transistors. Appl Phys Lett, 2019, 115(4): 043502 doi: 10.1063/1.5108529
[10]	Cho H K, Mogilatenko A, Susilo N, et al. Electrical properties and microstructure of V/Al/Ni/Au contacts on n-Al_0.65Ga_0.35N: Si with different Au thicknesses and annealing temperatures. Semicond Sci Technol, 2022, 37(10): 105016 doi: 10.1088/1361-6641/ac8e8f
[11]	Sulmoni L, Mehnke F, Mogilatenko A, et al. Electrical properties and microstructure formation of V/Al-based n-contacts on high Al mole fraction n-AlGaN layers. Photon Res, 2020, 8(8): 1381 doi: 10.1364/PRJ.391075
[12]	Chettri D, Mainali G, Cao H C, et al. Demonstration of aluminum nitride metal oxide semiconductor field effect transistor on sapphire substrate. J Phys D: Appl Phys, 2025, 58(3): 035104 doi: 10.1088/1361-6463/ad8759
[13]	Miller M A, Lin S K, Mohney S E. V/Al/V/Ag contacts to n-GaN and n-AlGaN. J Appl Phys, 2008, 104(6): 064508 doi: 10.1063/1.2980038
[14]	Nagata N, Senga T, Iwaya M, et al. Reduction of contact resistance in V-based electrode for high AlN molar fraction n-type AlGaN by using thin SiN_x intermediate layer. Phys Status Solidi C, 2017, 14(8): 1600243 doi: 10.1002/pssc.201600243
[15]	Nong M T, Liao C-H, Tang X, et al. Epitaxial AlN film with improved quality on Si (111) substrates realized by boron pretreatment via MOCVD. Appl Phys Lett, 2024, 124(17): 172107 doi: 10.1063/5.0207884
[16]	Sarkar B, Washiyama S, Breckenridge M H, et al. N- and P- type doping in Al-rich AlGaN and AlN. ECS Trans, 2018, 86(12): 25
[17]	Wang J M, Xu F J, Zhang L S, et al. Progress in efficient doping of Al-rich AlGaN. J Semicond, 2024, 45(2): 021501 doi: 10.1088/1674-4926/45/2/021501
[18]	Chua E K, Zhao R, Shi L P, et al. Effect of metals and annealing on specific contact resistivity of GeTe/metal contacts. Appl Phys Lett, 2012, 101: 012107 doi: 10.1063/1.4732787
[19]	Herath Mudiyanselage D, Wang D W, Da B C, et al. Over 600 V lateral AlN-on-AlN Schottky barrier diodes with ultra-low ideality factor. Appl Phys Express, 2024, 17(7): 074001 doi: 10.35848/1882-0786/ad5e5a
[20]	Maeda T, Page R, Nomoto K, et al. AlN quasi-vertical Schottky barrier diode on AlN bulk substrate using Al_0.9Ga_0.1N current spreading layer. Appl Phys Express, 2022, 15(6): 061007 doi: 10.35848/1882-0786/ac702e
[21]	Cho H K, Ostermay I, Kolbe T, et al. Au-free V/Al/Pt contacts on n-Al_0.85Ga_0.15N: Si surfaces of far-UVC LEDs. ECS J Solid State Sci Technol, 2024, 13(9): 093009 doi: 10.1149/2162-8777/ad78ff
[22]	France R, Xu T, Chen P P, et al. Vanadium-based Ohmic contacts to n-AlGaN in the entire alloy composition. Appl Phys Lett, 2007, 90(6): 062115 doi: 10.1063/1.2458399
[23]	Ji J L, Liu T, He Z K, et al. Semipolar (11–22) AlN films grown by hydride vapor phase epitaxy for Schottky barrier diodes. Cryst Growth Des, 2024, 24(9): 3960 doi: 10.1021/acs.cgd.4c00292
[24]	Yang Y, Xiong F B, Lin H Y, et al. Evaluation of Ti/Al/Ni/Au ohmic contact to n-AlGaN with different Ti/Al thickness for deep ultraviolet light emitting diode. Solid State Electron, 2023, 208: 108752 doi: 10.1016/j.sse.2023.108752
[25]	Cho H K, Rass J, Mogilatenko A, et al. Impact of plasma treatment of n-Al_0.87Ga_0.13N: Si surfaces on V/Al/Ni/Au contacts in far-UVC LEDs. IEEE Photonics Technol Lett, 2023, 35(17): 915 doi: 10.1109/LPT.2023.3288216
[26]	Zollner C J, Yao Y F, Wang M, et al. Highly conductive n-Al_0.65Ga_0.35N grown by MOCVD using low V/III ratio. Crystals, 2021, 11(8): 1006 doi: 10.3390/cryst11081006
[27]	Lapeyrade M, Alamé S, Glaab J, et al. Effect of Cl₂ plasma treatment and annealing on vanadium based metal contacts to Si-doped Al_0.75Ga_0.25N. J Appl Phys, 2017, 122(12): 125701 doi: 10.1063/1.4993447
[28]	Mori K, Takeda K, Kusafuka T, et al. Low-ohmic-contact-resistance V-based electrode for n-type AlGaN with high AlN molar fraction. Jpn J Appl Phys, 2016, 55(5S): 05FL03 doi: 10.7567/JJAP.55.05FL03
[29]	Srivastava S, Hwang S M, Islam M, et al. Ohmic contact to high-aluminum-content AlGaN epilayers. J Electron Mater, 2009, 38(11): 2348 doi: 10.1007/s11664-009-0924-y

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Received: 03 December 2025 Revised: 06 February 2026 Online: Accepted Manuscript: 15 March 2026

Article Navigation > Journal of Semiconductors > 2026 > Accepted Manuscript

Tingang Liu, Haicheng Cao, Mingtao Nong, Zhiyuan Liu, Zixian Jiang, Kexin Ren, Glen Isaac, Maciel Garcia, Xiaohang Li. Direct fabrication of record low specific resistivity metal contacts for n-type AlxGa1−xN (x ≥ 0.8)[J]. Journal of Semiconductors, 2026, In Press. doi: 10.1088/1674-4926/25120008 ****T N G Liu, H C Cao, M T Nong, Z Y Liu, Z X Jiang, K X Ren, G Isaac, M Garcia, and X H Li, Direct fabrication of record low specific resistivity metal contacts for n-type AlxGa1−xN (x ≥ 0.8)[J]. J. Semicond., 2026, accepted doi: 10.1088/1674-4926/25120008

Citation:

T N G Liu, H C Cao, M T Nong, Z Y Liu, Z X Jiang, K X Ren, G Isaac, M Garcia, and X H Li, Direct fabrication of record low specific resistivity metal contacts for n-type Al_xGa_1−xN (x ≥ 0.8)[J]. J. Semicond., 2026, accepted doi: 10.1088/1674-4926/25120008

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Direct fabrication of record low specific resistivity metal contacts for n-type Al_xGa_1−xN (x ≥ 0.8)

DOI: 10.1088/1674-4926/25120008

CSTR: 32376.14.1674-4926.25120008

Advanced Semiconductor Laboratory, Electrical and Computer Engineering Program, CEMSE Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia

More Information

Tingang Liu got his bachelor’s degree in 2022 from University of Electronics Science and Technology of China and his master’s degree in 2023 from King Abdullah University of Science and Technology. Now he is a doctoral student at King Abdullah University of Science and Technology under the supervision of Prof. Xiaohang Li. His research focuses on ultra-wide bandgap Al-rich AlGaN based power devices
Professor Xiaohang Li received his Bachelor degree in Applied Physics from Huazhong University of Science and Technology, China, his Master's degree in Electrical Engineering from Lehigh University, U.S., and his Ph.D. degree in Electrical Engineering from Georgia Institute of Technology, U.S. Prof. Li has extensive research experience in III-nitride and III-oxide (ultra)wide bandgap semiconductors
Corresponding author: xiaohang.li@kaust.edu.sa
Received Date: 2025-12-03
Revised Date: 2026-02-06

Available Online: 2026-03-15

Abstract

Abstract

Al-rich Al_xGa_1−xN (x ≥ 0.8) is promising for power and deep-ultraviolet (DUV) optoelectronic applications, owing to its ultra-wide bandgap and excellent thermal stability. However, forming low-resistivity contacts on n-type Al-rich AlGaN remains a significant challenge. In this work, we utilized an Au-free Ti/Al/Ti metal stack contact on n-type Al-rich AlGaN without graded layers. Record-low contact resistivities were achieved after annealing: 1.52×10⁻⁶ Ω·cm² for n-Al_0.8Ga_0.2N, 3.56×10⁻⁶ Ω·cm² for n-Al_0.86Ga_0.14N, and 5.79×10⁻⁵ Ω·cm² for n-Al_0.9Ga_0.1N. These results demonstrate a significant advancement in forming low-resistance contacts directly on Al-rich n-AlGaN, offering a viable path forward for next-generation power electronics and DUV optoelectronic devices.
- Al-rich AlGaN,
- ohmic contact,
- contact resistivity

Supplementary materials to this article can be found online at https://doi.org/10.1088/1674-4926/25120008.

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