An ohmic contact method that deserves serious consideration, but this research did not have enough time to investigate, is using a Ga-doped ZnO (ZnO:Ga) ohmic layer instead of applying plasma treatments. Previous research has shown that ZnO:Ga grown by PLD in pure Ar demonstrates high conductivity [33]. Essentially, this was the goal of the plasma treatment in this thesis, but instead of treating the contact regions with plasma, ZnO:Ga is deposited. To fabricate this ZnO:Ga ohmic contact, the same TFT fabrication process should be used as in this thesis, but instead of the plasma treatments, ZnO:Ga should be deposited immediately after deposition of the ZnO active layer as shown in Figure 41.
Figure 41. Cross-sectional representation of the Ga-doped ohmic layer deposited di-rectly after the ZnO layer.
After the ZnO:Ga is deposited, the fabrication process is identical to the process in this thesis until after the W is cleared in the channel region of the TFT, shown in Figure 42.
Figure 42. Cross-sectional representation of the Ga-doped ohmic layer ZnO TFT just after the W is cleared from the channel region.
Finally, with the ZnO:Ga expose in the channel region, a ZnO:Ga etch should be accomplished. The ZnO:Ga must be cleared to prevent shorting between the source and drain contacts. Therefore an over-etch must be accomplished. The over-etch will etch partially into the ZnO active layer, so in order to avoid hurting the performance TFT, a thicker ZnO active layer should be deposited. The ZnO TFT with the ZnO:Ga cleared out of the channel region is shown in Figure 43.
Figure 43. Cross-sectional representation of the Ga-doped Ohmic layer deposited di-rectly after the ZnO layer.
The ZnO:Ga is cleared from the channel region of the ZnO TFT, but remains under the W contacts. The highly-conductive ZnO:Ga ohmic layer allows for negligible contact resistance with the W contacts. More experimentation will be needed to determine how thick to make the ZnO:Ga ohmic layer; the etch method
and etch rate for clearing the ZnO:Ga in the channel region; and finally, how thick to make the ZnO active layer to account for the ZnO:Ga over-etch.
Bibliography
1. A. Janotti and C. G. Van de Walle, “Fundamentals of zinc oxide as a
semiconductor,” Reports on Progress in Physics, vol. 72, no. 12, p. 126501, dec 2009. [Online]. Available: http://stacks.iop.org/0034-4885/72/i=12/a=126501?
key=crossref.a4a9ab118b46f216f40add61af7ba458
2. M. L. Herold, “SELECTIVE DRY ETCH FOR DEFINING OHMIC
CONTACTS FOR HIGH PERFORMANCE ZnO TFTs.” [Online]. Available:
http://www.dtic.mil/dtic/tr/fulltext/u2/a602454.pdf
3. S. Lee, S. Bang, J. Park, S. Park, W. Jeong, and H. Jeon, “The effect of oxygen remote plasma treatment on ZnO TFTs fabricated by atomic layer deposition,”
physica status solidi (a), vol. 207, no. 8, pp. 1845–1849, aug 2010. [Online].
Available: http://doi.wiley.com/10.1002/pssa.200925514
4. N. Bouhssira, S. Abed, E. Tomasella, J. Cellier, a. Mosbah, M. S. Aida, and M. Jacquet, “Influence of annealing temperature on the properties of ZnO thin films deposited by thermal evaporation,” Applied Surface Science, vol. 252, no. 15, pp. 5594–5597, 2006.
5. C. R. Kagan and P. Andry, Thin-film transistors. CRC Press, 2003.
6. N. Stavitski, J. H. Klootwijk, H. W. van Zeijl, A. Y. Kovalgin, and R. A. M.
Wolters, “Cross-Bridge Kelvin Resistor Structures for Reliable Measurement of Low Contact Resistances and Contact Interface Characterization,” IEEE Transactions on Semiconductor Manufacturing, vol. 22, no. 1, pp. 146–152, feb 2009. [Online]. Available: http://ieeexplore.ieee.org/document/4773494/
7. B. Bayraktaroglu, K. Leedy, and R. Neidhard, “Microwave ZnO Thin-Film Transistors,” IEEE Electron Device Letters, vol. 29, no. 9, pp. 1024–1026, sep 2008. [Online]. Available: http://ieeexplore.ieee.org/document/4604848/
8. M. L. Schuette, A. J. Green, K. Leedy, A. Crespo, S. E. Tetlak, K. A. Sutherlin, and G. H. Jessen, “Ionic Metal-Oxide TFTs for Integrated Switching
Applications,” IEEE Transactions on Electron Devices, vol. 63, no. 5, pp.
1921–1927, 2016.
9. B. Bayraktaroglu, K. Leedy, and R. Neidhard, “Nanocrystalline ZnO microwave thin film transistors,” T. George, M. S. Islam, and A. K. Dutta, Eds., vol. 7679.
International Society for Optics and Photonics, apr 2010, p. 767904. [Online].
Available: http:
//proceedings.spiedigitallibrary.org/proceeding.aspx?doi=10.1117/12.849666 10. C. G. Van De Walle, “Hydrogen as a Cause of Doping in Zinc Oxide,” Physics
Review Letters, vol. 85, no. 5, p. 1012, 2000. [Online]. Available:
https://journals-aps-org.afit.idm.oclc.org/prl/pdf/10.1103/PhysRevLett.85.1012
11. K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono,
“Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature, vol. 432, no. 7016, pp. 488–492, nov 2004. [Online]. Available: http://www.nature.com/articles/nature03090
12. D. C. Look, G. C. Farlow, P. Reunchan, S. Limpijumnong, S. B. Zhang, and K. Nordlund, “Evidence for native-defect donors in n-type ZnO,” Physical Review Letters, vol. 95, no. 22, pp. 1–4, 2005.
13. A. Janotti and C. G. Van de Walle, “Oxygen vacancies in ZnO,” Applied Physics Letters, vol. 87, no. 12, p. 122102, sep 2005. [Online]. Available:
http://aip.scitation.org/doi/10.1063/1.2053360
14. H. Qi, Q. Li, C. Wang, L. Zhang, and L. Lv, “Effects of oxygen pressure on n-ZnO/p-Si heterojunctions fabricated using pulsed laser deposition,” Vacuum, vol. 81, no. 8, pp. 943–946, mar 2007.
15. A. Janotti and C. G. Van de Walle, “Native point defects in ZnO,” Physical Review B, vol. 76, no. 16, p. 165202, oct 2007. [Online]. Available:
https://link.aps.org/doi/10.1103/PhysRevB.76.165202
16. A. Janotti and C. G. Van de Walle, “New insights into the role of native point defects in ZnO,” Journal of Crystal Growth, vol. 287, no. 1, pp. 58–65, jan 2006.
[Online]. Available:
https://www.sciencedirect.com/science/article/pii/S0022024805011656
17. L. J. Brillson and Y. Lu, “ZnO Schottky barriers and Ohmic contacts,” Journal of Applied Physics, vol. 109, no. 12, p. 121301, jun 2011. [Online]. Available:
http://aip.scitation.org/doi/10.1063/1.3581173
18. A. Y. Polyakov, N. B. Smirnov, A. V. Govorkov, K. Ip, M. E. Overberg, Y. W.
Heo, D. P. Norton, S. J. Pearton, B. Luo, F. Ren, and J. M. Zavada, “Hydrogen plasma treatment effects on electrical and optical properties of <i>n</i>
-ZnO,” Journal of Applied Physics, vol. 94, no. 1, pp. 400–406, jul 2003.
[Online]. Available: http://aip.scitation.org/doi/10.1063/1.1579114
19. H. Akazawa, “Argon Plasma Treatment of Transparent Conductive ZnO Films,”
Housei Akazawa Appl. Phys. Express, vol. 2, 2009. [Online]. Available:
http://iopscience.iop.org.afit.idm.oclc.org/article/10.1143/APEX.2.081601/pdf 20. J.-H. Bang, H.-S. Uhm, W. Kim, and J.-S. Park, “Effects of additive gases and
plasma post-treatment on electrical properties and optical transmittance of ZnO thin films,” Thin Solid Films, vol. 519, pp. 1568–1572, 2010. [Online]. Available:
https://ac-els-cdn-com.afit.idm.oclc.org/S0040609010012034/1-s2.
0-S0040609010012034-main.pdf?{\ }tid=
de35b7fe-ed3c-49b1-b0f0-08918df21618{\&}acdnat=1524336233{\ } 6831e0a8a713576f28bbb8eb1eff16b3
21. J.-M. Lee, K.-K. Kim, S.-J. Park, and W.-K. Choi, “Low-resistance and nonalloyed ohmic contacts to plasma treated ZnO,” Applied Physics Letters, vol. 78, no. 24, pp. 3842–3844, jun 2001. [Online]. Available:
http://aip.scitation.org/doi/10.1063/1.1379061
22. M. G. Wardle, J. P. Goss, and P. R. Briddon, “First-Principles Study of the Diffusion of Hydrogen in ZnO.” [Online]. Available: https:
//journals-aps-org.afit.idm.oclc.org/prl/pdf/10.1103/PhysRevLett.96.205504 23. Y. M. Strzhemechny, H. L. Mosbacker, D. C. Look, D. C. Reynolds, C. W.
Litton, N. Y. Garces, N. C. Giles, L. E. Halliburton, S. Niki, and L. J. Brillson,
“Remote hydrogen plasma doping of single crystal ZnO,” Applied Physics Letters, vol. 84, no. 14, pp. 2545–2547, apr 2004. [Online]. Available:
http://aip.scitation.org/doi/10.1063/1.1695440
24. B. Du Ahn, H. S. Shin, H. J. Kim, J.-S. Park, and J. K. Jeong, “Comparison of the effects of Ar and H2 plasmas on the performance of homojunctioned
amorphous indium gallium zinc oxide thin film transistors,” Applied Physics Letters, vol. 93, no. 20, p. 203506, nov 2008. [Online]. Available:
http://aip.scitation.org/doi/10.1063/1.3028340
25. C.-F. Hu, J.-Y. Feng, J. Zhou, and X.-P. Qu, “Investigation of oxygen and argon plasma treatment on Mg-doped InZnO thin film transistors,” Applied Physics A, vol. 122, no. 11, p. 941, nov 2016. [Online]. Available:
http://link.springer.com/10.1007/s00339-016-0475-z
26. F.-L. Kuo, Y. Li, M. Solomon, J. Du, and N. D. Shepherd, “Workfunction tuning of zinc oxide films by argon sputtering and oxygen plasma: an
experimental and computational study,” Journal of Physics D: Applied Physics, vol. 45, no. 6, p. 065301, feb 2012. [Online]. Available: http://stacks.iop.org/
0022-3727/45/i=6/a=065301?key=crossref.26021a22e4f536364d469b62d6da1edc 27. J. Zhao, L. Hu, Z. Wang, Y. Zhao, X. Liang, and M. Wang, “High-quality ZnO
thin films prepared by low temperature oxidation of metallic Zn,” Applied Surface Science, vol. 229, no. 1-4, pp. 311–315, may 2004. [Online]. Available:
https://www.sciencedirect.com/science/article/pii/S0169433204001102 28. W. Gao, Z. Li, R. Harikisun, and S.-S. Chang, “Zinc oxide films formed by
oxidation of zinc under low partial pressure of oxygen,” Materials Letters, vol. 57, no. 8, pp. 1435–1440, feb 2003. [Online]. Available:
https://www.sciencedirect.com/science/article/pii/S0167577X02010030
29. K. H. Ji, J.-I. Kim, H. Y. Jung, S. Y. Park, R. Choi, U. K. Kim, C. S. Hwang, D. Lee, H. Hwang, and J. K. Jeong, “Effect of high-pressure oxygen annealing on negative bias illumination stress-induced instability of InGaZnO thin film transistors,” Applied Physics Letters, vol. 98, no. 10, p. 103509, mar 2011.
[Online]. Available: http://aip.scitation.org/doi/10.1063/1.3564882
30. S. Baier, M. Shur, K. Lee, N. Cirillo, and S. Hanka, “FET Characterization using gated-TLM structure,” IEEE Transactions on Electron Devices, vol. 32, no. 12, pp. 2824–2829, dec 1985. [Online]. Available:
http://ieeexplore.ieee.org/document/1485168/
31. J. Park, C. Kim, S. Kim, I. Song, S. Kim, D. Kang, H. Lim, H. Yin, R. Jung, E. Lee, J. Lee, K. W. Kwon, and Y. Park, “Source/drain series-resistance effects in amorphous gallium-indium zinc-oxide thin film transistors,” IEEE Electron Device Letters, vol. 29, no. 8, pp. 879–881, 2008.
32. D. C. Montgomery, Design and Analysis of Experiments, 2012, vol. 2. [Online].
Available: http://cataleg.uab.cat/record=b1764873{∼}S1*cat
33. R. C. Scott, K. D. Leedy, B. Bayraktaroglu, D. C. Look, and Y.-H. Zhang,
“Highly conductive ZnO grown by pulsed laser deposition in pure Ar,” Applied Physics Letters, vol. 97, no. 7, p. 072113, aug 2010. [Online]. Available:
http://aip.scitation.org/doi/10.1063/1.3481372
Vita
Second Lieutenant Blaine Z. Underwood graduated from Niceville High School, Niceville, Florida in 2012. He entered undergraduate studies at The University of West Florida where he graduated magna cum laude with a Bachelor of Science degree in Electrical Engineering in 2017. He commissioned through the Air Force Reserve Officer Training Corps Detachment 014. In August 2017, he started working toward his Master of Science degree in Electrical Engineering at the Air Force Institute of Technology. Upon graduation, he will be assigned to the Information Directorate, Air Force Research Laboratory, Rome, NY.
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21-03-2019 Master's Thesis August 2017 - March 2019
Plasma Treatment Method for Ohmic Contacts on Zinc Oxide Thin Film Transistors
Underwood, Blaine Z, 2d Lt, USAF
Air Force Institute of Technology
Graduate School of Engineering and Management (AFIT/EN) 2950 Hobson Way
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This work is declared a work of the U.S. Government and is not subject to copyright protection in the United States.
This research utilizes plasma treatments as a method to decrease the contact resistance on zinc oxide (ZnO) thin film transistor (TFT). In recent years, researchers have achieved gigahertz RF switch cutoff frequency with ZnO TFTs. To further increase the cutoff frequency, the total resistance of this device must be minimized. This research developed a method to integrate plasma treatments into the fabrication of ZnO TFT to decrease the contact resistance, thus increasing the cutoff frequency of the device.
Three plasma treatment methods were used, including Ar plasma generated in an inductively coupled reactive-ion etch (ICP-RIE) chamber, Ar plasma applied in-situ in a metal-sputter chamber before sputtering tungsten (W) contacts, and by remotely generated hydrogen plasma. The plasma treatments were implemented after the deposition of ZnO and prior to the deposition of sputtered tungsten (W) contacts. The plasma treatments increased the conductivity of the ZnO, which allowed for the formation of ohmic contacts on the ZnO TFT. The contact resistance of the untreated ZnO TFT sample was 1.86 ohm-mm, while the ICP-RIE Ar plasma treatment in situ Ar plasma treatment and H plasma treatment demonstrated minimum contact resistances of 1 18 1 03 and Contact Resistance, Plasma Treatment, RF Switch, Thin Film Transistor, ZnO
U U U UU 83
Maj Tod Laurvick, AFIT/ENG
(312) 785-3636 x4382 [email protected]