Deposition of SiO2 Layers on GaN by Photochemical Vapor Deposition

Journal of The Electrochemical Society, 150 共2兲 C77-C80 共2003兲

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0013-4651/2003/150共2兲/C77/4/$7.00 © The Electrochemical Society, Inc.

Deposition of SiO2 Layers on GaN by Photochemical Vapor Deposition Shoou-Jinn Chang,a,z Yan-Kuin Su,a Yu-Zung Chiou,a Jung-Ran Chiou,b Bohr-Ran Huang,b Chia-Sheng Chang,a and Jone F. Chena a

Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan 70101 Institute of Electronics and Information Engineering and Department of Electronic Engineering, National Yunlin University of Science and Technology, Touliu, Taiwan 640

b

SiO2 insulating layers were first deposited onto GaN by photochemical vapor deposition 共photo-CVD兲 technology using a deuterium (D2 ) lamp as the excitation source. Physical, chemical, and electrical characteristics of the Al/SiO2 /GaN metalinsulator-semiconductor 共MIS兲 capacitors are reported for the first time. It was also found that the limiting factor of SiO2 growth rate was the number of SiH4 and O2 molecules available to provide excited Si and O atoms. Furthermore, it was found from high-frequency capacitance-voltage measurements that the photo-CVD SiO2 /n-GaN interface state density, D it , was estimated to be 8.4 ⫻ 1011 cm⫺2 eV⫺1 for the photo-CVD SiO2 layers prepared at 300°C. With an applied field of 4 MV/cm, the oxide leakage current density was found to be only 6.6 ⫻ 10⫺7 A/cm2 . © 2003 The Electrochemical Society. 关DOI: 10.1149/1.1534598兴 All rights reserved. Manuscript submitted February 26, 2002; revised manuscript received May 20, 2002. Available electronically January 6, 2003.

The excellent physical and electrical properties of GaN have made it a good candidate in high-temperature, high-power, and highfrequency applications.1 High quality III-V ternaries such as Alx Ga1⫺x N and Iny Ga1⫺y N were also demonstrated for heterojunction field effect transistors 共HFETs兲,2,3 bipolar junction transistors 共BJTs兲,4 and light-emitting diodes 共LEDs兲.5 Nitride-based blue and green LEDs6 are already commercially available. The high dislocation density measured in these blue/green LEDs has led to the conclusion that dislocations in group III nitrides are not efficient recombination centers. Such a dislocation density also suggested that the number of surface states is small in GaN.7,8 Recently, many researches were focused on the fabrication of GaN-based metalinsulator-semiconductor 共MIS兲 capacitors with SiO2 , 9,10 Si3 N4 , and Ga2 O3 11,12 as the insulating material. It has been reported that SiO2 layers can be deposited onto GaN by plasma-enhanced chemical vapor deposition9,10 and liquid-phase deposition.13 Previously, it was shown that photochemical vapor deposition 共photo-CVD兲 can also be used to grow high quality SiO2 layers14-18 on various semiconductor substrates. In using photo-CVD to grow thin films, selecting the proper light source with a radiation spectrum matching the absorption spectra of the reactance gases is very important. In this study, we used a deuterium (D2 ) lamp as the excitation source. It is known that D2 lamp emits strong ultraviolet 共UV兲 and vacuum ultraviolet 共VUV兲, which can effectively decompose SiH4 and O2 , since O2 could absorb photons in the wavelength region from 133 to 175 nm and SiH4 could absorb photons in the wavelength region below 147 nm.14-18 Thus, energy can be directly transferred from the D2 lamp to the excited Si and O atoms. In addition, such a photoCVD system offers better control in the oxide region and selective growth is possible. The quality of oxide layers grown by such a photo-CVD system is close to that grown by thermal oxidation, and the electrical properties of the photo-CVD grown oxide are acceptable for device applications.14-18 In this paper, the deposition of SiO2 layers on GaN and the properties of Al/photo-CVD SiO2 /n-GaN MIS capacitors are reported for the first time.

The 50 nm thick SiO2 films were subsequently deposited onto the GaN epitaxial layer by 150 W D2 lamp photo-CVD under different process pressures and different substrate temperatures. The gas ratio was fixed at SiH4 /O2 ⫽ 0.055.10-14 For comparison, SiO2 films with the same thickness were also deposited on the same GaN epitaxial layer by rf magnetron sputtering. Atomic force microscopy 共AFM兲 and Auger electron spectroscopy 共AES兲 were then used to characterize the deposited SiO2 films. Al/SiO2 /GaN MIS capacitors were subsequently prepared by etching and metal evaporation. The capacitance-voltage 共C-V兲 and current-voltage (I-V) characteristics of these fabricated MIS capacitors were then measured by an HP 4284B LCR meter and an HP 4156B semiconductor parameter analyzer, respectively. Results and Discussion Figure 1 shows SiO2 growth rate as a function of process pressure for the photo-CVD SiO2 layers grown at different substrate temperatures. It was found that the SiO2 growth rate increases as the substrate temperature increases. It was also found that the SiO2 growth rate increases linearly as the process pressure increases. Such a linear increase suggests that the limiting factor for growth rate is the amount of SiH4 and O2 molecules available to provide excited Si and O atoms. At higher process pressure, the number of

Experimental Prior to the deposition of SiO2 layers, an n-type GaN epitaxial layer was grown on 共0001兲 sapphire substrates by metallorganic chemical vapor deposition 共MOCVD兲.19-34 The electron concentration of the n-type GaN epitaxial layer was about 5 ⫻ 1017 cm⫺3 .

z

E-mail: [email protected]

Figure 1. SiO2 growth rate as a function of process pressure for samples grown at different substrate temperatures. The refractive index of the 300°C grown SiO2 layer is also shown.

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Figure 3. AES spectra of SiO2 films prepared by photo-CVD and sputtering.

condensed again due to the increased number of excited Si and O atoms. It should be noted that the refractive index of the 300°C photo-CVD grown SiO2 layer prepared at 0.9 Torr is very close to that of the thermally grown SiO2 layer prepared on top of Si substrates. Figures 2a, b, and c show the AFM images of bare GaN without SiO2 , photo-CVD SiO2 grown at 300°C and sputtered SiO2 , respectively. It was found that the root mean square 共rms兲 roughness was 1.85, 2.8, and 1.3 nm for bare GaN without SiO2 , photo-CVD SiO2 grown at 300°C, and sputtered SiO2 , respectively. The smooth surface observed from the sputtered SiO2 was probably due to its amorphous nature. Figures 3a and b show the AES depth profiles of sputtered SiO2 and photo-CVD SiO2 grown at 300°C, respectively. It could be seen from Fig. 3b that the depth profile was uniform in the insulating layer with an O/Si ratio almost equal to 2 for the photo-CVD SiO2 grown at 300°C. In contrast, although the depth profile was also uniform in the insulating layer for the sputtered SiO2 , its O/Si ratio was only 1.5. Such a result suggests that the composition of the sputtered insulating layer is SiO1.5 , instead of SiO2 . Such a result also suggests that there exist a large number of dangling bounds in the sputtered SiO2 due to the lack of oxygen. Figure 4 shows the C-V characteristics 共1 MHz兲 of the photoCVD SiO2 grown at different temperatures. The ideal C-V curve

Figure 2. AFM images of SiO2 films prepared by photo-CVD and sputter.

excited Si and O atoms will increase so as to result in a higher SiO2 growth rate. The refractive index of the 300°C photo-CVD grown SiO2 layer prepared at different process pressure is also shown in Fig. 1. It can be seen that the refractive index also increases as the process pressure increases. Such an observation is probably due to the fact that the SiO2 layer prepared at high process pressure is more

Figure 4. High frequency 共1 MHz兲 C-V characteristics of Al/SiO2 /GaN MIS capacitors.



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Conclusions In summary, Al/photo-CVD SiO2 /n-GaN MIS structures were fabricated by photo-CVD technique using a deuterium (D2 ) lamp as the excitation source. It was found that the limiting factor for the SiO2 growth rate is the number of SiH4 and O2 molecules available to provide excited Si and O atoms. It was also found that we could achieve a low interface state density, of 8.4 ⫻ 1011 cm⫺2 eV⫺1 . Furthermore, it was found that the leakage current was only 6.6 ⫻ 10⫺7 A/cm2 with an applied field of 4 MV/cm for the 300°C photo-CVD grown Al/SiO2 /GaN MIS capacitor.

Acknowledgments The authors acknowledge the financial support from the National Science Council for their research grant of NSC 90-2215-E-008-043 and NSC 90-2112-M-008-046. Figure 5. The I-V characteristics of the Al/SiO2 /GaN MIS capacitors.

National Cheng Kung University assisted in meeting the publication costs of this article.

was also plotted in the same figure. It was found that no significant hysteresis was observed as the gate voltage varied at 0.1 V/s from ⫹5 V to ⫺20 V and then back to ⫹5 V for all three samples. The lack of hysteresis in these C-V curves indicated that the number of mobile ions in SiO2 layer is negligibly small. At high frequency measurement 共1 MHz兲, there is not enough response time for minority carriers 共holes for n-GaN兲 to be generated in n-GaN, so that significant inversion characteristics could not be observed from this figure. Similar results were also observed by Casey et al.13 Using the standard high frequency capacitance method,35 we can thus calculate the interface state density from these C-V curves D it ⫽

C ox q

冋冉冊 d⌿ s dV g

⫺1

册

⫺1 ⫺

Cs q

关1兴

where C ox and C s were the oxide and depletion capacitance, respectively, ⌿ s was the band bending, and V g was the gate voltage. From Eq. 1, it was found that D it equals 1.2 ⫻ 1012 and 8.4 ⫻ 1011 cm⫺2 eV⫺1 for photo-CVD SiO2 layers on GaN prepared at 150 and 300°C, respectively. The smaller D it for the photo-CVD SiO2 layer deposited at 300°C could be attributed to the fact that a higher substrate temperature can significantly improve the SiO2 /GaN interfacial properties probably through supplying thermal energy to the Si and O atoms. Compared to Ga2 O3 on GaN reported by Fu et al., the D it of our photo-CVD SiO2 prepared at 300°C on GaN was one order of magnitude smaller. The D it observed from our photo-CVD SiO2 /n-GaN interface was also comparable to those observed from PECVD SiO2 /n-GaN interfaces reported by Arulkumaran et al.10 However, D it was increased to 6.4 ⫻ 1012 cm⫺2 eV⫺1 when the substrate temperature was increased to 500°C. The exact reason for the increase in D it is not clear yet. Possible reasons for such degradation in oxide quality include too fast an oxide growth rate and/or some interface reactions, which occur at high temperatures. Figure 5 shows the I-V characteristics of the four fabricated Al/photo-CVD SiO2 /n-GaN MIS capacitors. During I-V measurement, all capacitors were biased in accumulation. It was found that the leakage currents of the photo-CVD grown SiO2 layers were all much smaller than that of the sputtered SiO2 layer. This could be understood by the poor quality of the sputtered SiO2 layer. Furthermore, it was found that photo-CVD SiO2 deposited at 300°C exhibits the smallest leakage current among the four samples. Such a result also agrees well with the C-V data shown in Fig. 4. With an applied field of 4 MV/cm, it was found that the leakage current was only 6.6 ⫻ 10⫺7 A/cm2 for the photo-CVD SiO2 layer deposited at 300°C, as shown in Fig. 5.

References 1. T. Tojyo, T. Asano, M. Takeya, T. Hino, S. Kijima, S. Goto, S. Uchida, and M. Ikeda, Jpn. J. Appl. Phys., Part 1, 40, 3206 共2001兲. 2. J. B. Webb, H. Tang, J. A. Bardwell, and P. Coleridge, Appl. Phys. Lett., 78, 3845 共2001兲. 3. A. Tarakji, G. Simin, N. Ilinskaya, X. Hu, A. Kumar, A. Koudymov, J. Yang, M. Asif Khan, M. S. Shur, and R. Gaska, Appl. Phys. Lett., 78, 2169 共2001兲. 4. S. Yoshida and J. Suzuki, Jpn. J. Appl. Phys., Part 2, 38, L851 共1999兲. 5. S. Nakamura, T. Mukai, and M. Senoh, J. Appl. Phys., 76, 81989 共1994兲. 6. T. Mukai, M. Yamada, and S. Nakamura, Jpn. J. Appl. Phys., Part 1, 38, 3976 共1999兲. 7. S. D. Lester, F. A. Ponce, M. G. Craford, and D. A. Steigerwald, Appl. Phys. Lett., 66, 1249 共1995兲. 8. Y. K. Su, Y. Z. Chiou, F. S. Juang, S. J. Chang, and J. K. She, Jpn. J. Appl. Phys., Part 1, 40, 2996 共2001兲. 9. H. C. Casey, Jr., G. G. Fountain, R. G. Alley, B. P. Keller, and Steven P. DenBaars, Appl. Phys. Lett., 68, 1850 共1996兲. 10. S. Arulkumaran, T. Egawa, H. Ishikawa, T. Jimbo, and M. Umeno, Appl. Phys. Lett., 73, 809 共1998兲. 11. D. J. Fu, Y. H. Kwon, T. W. Kang, C. J. Park, K. H. Baek, H. Y. Cho, D. H. Shin, C. H. Lee, and K. S. Chung, Appl. Phys. Lett., 80, 446 共2002兲. 12. L. H. Peng, C. H. Liao, Y. C. Hsu, C. S. Jong, C. N. Huang, J. K. Ho, C. C. Chiu, and C. Y. Chen, Appl. Phys. Lett., 76, 511 共2000兲. 13. H. C. Casey, Jr., G. G. Fountain, and R. G. Alley, B. P. Keller, and S. P. DenBaars, Appl. Phys. Lett., 68, 13 共1996兲. 14. C. J. Huang and Y. K. Su, J. Appl. Phys., 67, 3350 共1990兲. 15. S. J. Chang, Y. K. Su, F. S. Juang, C. T. Lin, C. D. Chiang, and Y. T. Cherng, IEEE J. Quantum Electron., 36, 583 共2000兲. 16. C. T. Lin, Y. K. Su, S. J. Chang, H. T. Huang, S. M. Chang, and T. P. Sun, IEEE Photonics Technol. Lett., 9, 232 共1997兲. 17. C. T. Lin, Y. K. Su, H. T. Huang, S. J. Chang, G. S. Chen, T. P. Sun, and J. J. Luo, IEEE Photonics Technol. Lett., 8, 676 共1996兲. 18. C. T. Lin, S. J. Chang, D. K. Nayak, and Y. Shiraki, Jpn. J. Appl. Phys., Part 1, 34, 72 共1995兲. 19. J. K. Sheu, J. M. Tsai, S. C. Shei, W. C. Lai, T. C. Wen, C. H. Kou, Y. K. Su, S. J. Chang, and G. C. Chi, IEEE Electron Device Lett., 22, 460 共2001兲. 20. C. H. Chen, S. J. Chang, Y. K. Su, G. C. Chi, J. Y. Chi, C. A. Chang, J. K. Sheu, and J. F. Chen, IEEE Photonics Technol. Lett., 13, 848 共2001兲. 21. W. C. Lai, S. J. Chang, M. Yokoyama, J. K. Sheu, and J. F. Chen, IEEE Photonics Technol. Lett., 13, 559 共2001兲. 22. Y. K. Su, Y. Z. Chiou, F. S. Juang, S. J. Chang, and J. K. Sheu, Jpn. J. Appl. Phys., Part 1, 40, 2996 共2001兲. 23. C. H. Chen, S. J. Chang, Y. K. Su, G. C. Chi, J. K. Sheu, and I. C. Lin, Jpn. J. Appl. Phys., Part 1, 40, 2762 共2001兲. 24. L. W. Wu, S. J. Chang, T. C. Wen, Y. K. Su, W. C. Lai, C. H. Kuo, C. H. Chen, and J. K. Sheu, IEEE J. Quantum Electron., 38, 446 共2002兲. 25. C. H. Kuo, S. J. Chang, Y. K. Su, J. F. Chen, L. W. Wu, J. K. Sheu, C. H. Chen, and G. C. Chi, IEEE Electron Device Lett., 23, 240 共2002兲. 26. C. H. Ko, Y. K. Su, S. J. Chang, T. M. Kuan, C. I. Chiang, W. H. Lan, W. J. Lin, and J. Webb, Jpn. J. Appl. Phys., Part 1, 41, 2489 共2002兲. 27. J. K. Sheu, C. J. Pan, G. C. Chi, C. H. Kuo, L. W. Wu, C. H. Chen, S. J. Chang, and Y. K. Su, IEEE Photonics Technol. Lett., 14, 450 共2002兲. 28. S. J. Chang, W. C. Lai, Y. K. Su, J. F. Chen, C. H. Liu, and U. H. Liaw, IEEE J. Sel. Top. Quantum Electron., 8, 278 共2002兲. 29. C. H. Chen, S. J. Chang, Y. K. Su, G. C. Chi, J. K. Sheu, and J. F. Chen, IEEE J. Sel. Top. Quantum Electron., 8, 284 共2002兲.


C80


30. C. H. Chen, Y. K. Su, S. J. Chang, G. C. Chi, J. K. Sheu, J. F. Chen, C. H. Liu, and U. H. Liaw, IEEE Electron Device Lett., 23, 130 共2002兲. 31. C. H. Ko, S. J. Chang, Y. K. Su, W. H. Lan, J. F. Chen, T. M. Kuan, Y. C. Huang,C. I. Chiang, J. Webb, and W. J. Lin, Jpn. J. Appl. Phys., Part 2, 41, L226 共2002兲. 32. S. J. Chang, C. H. Kuo, Y. K. Su, L. W. Wu, J. K. Sheu, T. C. Wen, W. C. Lai, J. F. Chen, and J. M. Tsai, IEEE J. Sel. Top. Quan. Electron., 8, 744 共2002兲.

33. J. K. Sheu, C. J. Tun, M. S. Tsai, C. C. Lee, G. C. Chi, S. J. Chang, and Y. K. Su, J. Appl. Phys., 91, 1845 共2002兲. 34. C. H. Kuo, S. J. Chang, Y. K. Su, L. W. Wu, J. K. Sheu, C. H. Chen, and G. C. Chi, Jpn. J. Appl. Phys., Part 2, 41, L112 共2002兲. 35. E. H. Nicollian and J. R. Brews, MOS (Metal Oxide Semiconductor) Physics and Technology, p. 96, Wiley, New York, 共1982兲.
