5-1 Conclusions
High-quality ZnO epitaxial films have been successfully grown by pulse laser deposition on Si(111) substrates with a thin oxide γ-Al2O3 buffer layer. There are two (111)-oriented γ-Al2O3 domains rotated 60 degrees from each other relative to the surface normal. The in-plane epitaxial relationship between the wurtzite ZnO, cubic
γ-Al2O3 and cubic Si follows {1010}_ ZnO || defect structures in ZnO films are edge TDs. According to the XRD, TEM and LT-PL results, we established the correlation between influence of different types of TDs and optical properties of the ZnO epi-films. Our results demonstrated that the ratio (IDLE/INBE) is mainly affected by edge TDs and the FWHM of NBE is dominantly influenced by screw TDs. The reduction of the dislocation density is one of the most
important factors to improve the ZnO-based optical device.
5-2 Perspective
In our research, we have derived the relationship between the photoluminescence and structural properties of ZnO films. It is always speculated that the DLE is coupled with the point defects such as O vacancies and Zn interstitials and aging in air can
improve the optical properties of ZnO layer has been reported [46]. We therefore performed post growth thermal annealing in an oxygen ambient (800 ºC in 1 atm O2 for 100 min.) and the optical performance of all the samples exhibited evident improvement.
Take a 270 nm thick sample grown at 300ºC as an example, its PL spectra measured before and after thermal annealing are displayed in Fig. 5-1(a) and (b), respectively;
significant reduction of the DLE signals and narrower FWHM of NBE were observed after the annealing. We also annealed a similarly prepared sample in N2 atmosphere;
the PL spectrum was much worse than the one before annealing especially a huge broad peak appeared in the DLE region. These observations supported the argument that the oxygen vacancies played the most important role in the native defects in ZnO thin films deposited by PLD [46].
It is known that ZnO epi-layers with larger thickness usually have better structural characteristics and optical properties. We thus grew a series of samples with various thickness to examine if thermal annealing has the same effect on them. Figures 5-2(a), (b) and (c) are PL spectra of samples before annealing with different thickness
~37, 165 and 300 nm, respectively. The corresponding PL spectra of these samples after annealing are (a’), (b’) and (c’). We can clearly observe the reduced DLE and better NBE performance after annealing. Figures 5-3(a) and (b) depict the tilt and twist angles, respectively, as a function of ZnO film thickness before and after thermal
annealing. We can clearly tell for films of thickness less than ~200 nm their structural properties improve after annealing and the thinner the ZnO layer the better is the structural improvement. On the other hand, for the ZnO film of thickness ~300 nm, the structural property even became worse after annealing but the undesirable DLE was reduced in the PL measurement indicating the improvement in the optical performance.
The inconsistency in structural and optical variation may be attributed to the different probing depth of XRD and PL as well as the non-uniformity of the annealing effect to the thick films. Figure 5-4(a) is the x-ray reflectivity curve of a ~300 nm thick ZnO film prior thermal annealing. Only a periodic feature yielding a thickness of ~10 nm, which is consistent with the thickness of the γ-Al2O3 buffer layer, can be identified.
After annealing, additional oscillations with a smaller periodicity appeared in the reflectivity curve, as illustrated in Fig. 5-4(b), and all the oscillations became more pronounced indicating smoother interfaces. The presence of the finer fringes revealed the appearance of an extra layer of thickness ~98 nm near the surface after annealing.
This observation suggests that under the employed annealing treatment, the ZnO film was split into two parts and the oxygen atoms can penetrate into the ZnO film as far as
~98 nm in depth to form the upper layer. Thus, the upper layer was annealed under an oxygen rich circumstance and most defect structure was eliminated. The defect structure in the lower layer was not improved or even got worse due to the oxygen
deficient annealing environment. Typically, the PL measurement can only probe a region about 100 nm thick but X-ray can easily penetrate into the sample over several hundreds of nanometers. Therefore, for the thinnest film (~37 nm thick), the whole ZnO layer was nicely annealed in an oxygen rich environment and both XRD and PL were sampling this layer with improved structural and optical properties. In contrast, for the thickest film (~300 nm thick), PL signals were predominantly coming from the
~100 nm thick upper layer but XRD data were an average over the upper layer and the twice thick bottom layer which had worse characteristics in structure and optical performance. As to the sample with medium thickness (~165 nm thick), the PL spectrum is similar to that of the other two samples but only mild improvement in structure was observed as a result of an ensemble average between the more perfect upper layer and the worse bottom layer. That adapted thermal treatment only improves film structure over a finite thickness provides a reasonable explanation to the inconsistency trend found in structural and optical properties as a function of film thickness.
According to this phenomenon, we supposed that we could calculate the diffusion velocity of oxygen in ZnO film by implementing time dependent annealing process.
Besides, by implementing the (short)-time dependent annealing of ZnO film with defect rich, we could get the rough information about how depth the PL measurement can
detect and reflect a measurable signal of DLE.
In the future, we could also work on the ZnO based LED as we can successfully grow P-type ZnO film on silicon based system.
2.0 2.5 3.0 3.5
@15K
Intensity (a.u.)
grown at 300
oC
after 800
oC 100 min annealing
Photon Energy (eV)
7.3 meV (b)
1.5 2.0 2.5 3.0 3.5
grown at 300
oC before annealing
(a)
In te n s ity (a.u .)
Photon energy (eV)
@15K
9.61meV
Fig. 5-1 PL spectra measured at 15K of ZnO film grown at 300oC before annealing (a) and after annealing (b). The DLE is obviously reduced and the FWHM of NBE becomes narrower.
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
Fig. 5-2 PL spectra measured at 15K of ZnO film grown at 300oC with different thickness before annealing: (a) 37 nm, (b) 165 nm, and (c) 300 nm. The right plots (a’), (b’), and (c’) are samples after annealing and are correspond to (a), (b) and (c),
0 50 100 150 200 250 300
Fig. 5-3 Twist (a) and tilt (b) angles of ZnO films grown with different thickness. The black curves are ZnO films before annealing while the red ones are after annealing.
0.0 0.1 0.2 0.3 0.4 0.5
Intensity (a.u.)
q
~10 nm (from buffer) (a)
(Å
-1)
0.0 0.1 0.2 0.3 0.4 0.5
(b)
Intnesit y ( a .u.)
q
~ 98nm
(from ZnO, appears after annealing)
~10nm (from buffer)
(Å
-1)
Fig. 5-4 X-ray reflectivity curves of samples (a) before and (b) after annealing. We can clearly observe the presence of extra fringes after annealing in (b). The extra fringes yields a thickness of ~98nm. In both (a) and (b), there always exist fringes, enclosed by rectangle, which corresponds to a layer of thickness ~ 10 nm. This is originated from buffer layer.
References
[1] Z. K. Tang, G. K. L. Wang, and P. Yu, Appl. Phys. Lett. 72, 3270, 1998.
[2] Yefan Chen, D. M. Bahnall, Hang-jun Koh, Ki-tae Park, Kenji Hiraga, Ziqiang Zhu, and Takafumi Yao, J. App Phy. 84, 7, 1998.
[3] D. M. Bagnall, Y. F. Chen, and Z. Zhu, Appl. Phys. Lett. 70, 17, 1997.
[4] M. Kawasaki, and Y. Segawa, Phys. Stat. Sol (b) 202, 669, 1997.
[5] Y. Chen, H. Ko, S. Hong, and T. Yao, Appl. Phys. Lett. 76, 559, 2000.
[6] K. Ogata, T. Kawanishi, K. Maejima, K. Sakurai, Shizuo Fujita and Shigeo Fujita, Jpn. J. Appl. Phys. 40, L657, 2001.
[7] I. Yamauchi, S. Ueyama and I. Ohnaka, Mater. Sci. Eng. A 208, 108, 1996.
[8] Y. Z. Yoo, T. Sekiguchi, T. Chikyow, M. Kawasaki, T. Onuma, S. F. Chichibu, J. H.
Song, and H. Koinuma, Appl. Phys. Lett. 84, 502, 2004.
[9] C.C. Lin, S. Y. Chen, S. Y. Cheng, and H. Y. Lee, Appl. Phys. Lett. 84, 5040, 2004.
[10] H. M. Cheng, H. C. Hsu, S. Yang, C. Y. Wu, Y. C. Lee, L. J. Lin, and W. F. Hsieh, Nanotechnology 16, 2882, 2005.
[11] M. Fujita, N. Kawamoto, M. Sasajima, and Y. Horikoshi, J. Vac. Sci. Technol. B 22, 1484, 2004.
[12] X. N. Wang, Appl. Phys. Lett. 90, 151912, 2007.
[13] R. D. Vispute, Appl. Phys. Lett. 73, 348, 1998.
[14] A. Nahhas, H. K. Kim, and J. Blachere J, Appl. Phys. Lett. 78, 1511, 2001.
[15] Y. Oyama, J. Nishizawa, T. Kimura, and T. Tanno, Phys. Rev. B 74, 235210, 2006.
[16] U. Bangert, A. J. Harvey, R. Jones, C. J. Fall, A. Blumenau, R. Briddon, M.
Schreck, and F. Hormann, New J. Phys. 6, 184, 2004.
[17] H. M. Ng, D. Doppalapudi, T. D. Moustakas, N. G. Weimann, and L. F. Eastman, Appl. Phys. Lett. 73, 821, 1998.
[18] J. Y. Shi, L. P. Yu, Y. Z. Wang, G. Y. Zhang, and H. Zhang, Appl. Phys. Lett. 80, 2293, 2002.
[19] B. D. Cullity, Element of Xray diffraction, 2nd ed, (Addison Wesley, Cabada, 1978) [20] Callister, D. Jr. William, "Fundamentals of Materials Science and Engineering."
John Wiley & Sons, Inc. Danvers, MA. (2005)
[21] B. Heying, X. H. Wu, S. Keller, Y. Li, D. Kapolnek, B. P. Keller, S. P. DenBaars, and J. S. Speck, Appl. Phys. Lett. 68, 643, 1996.
[22] F. Vigué, P. Vennéguès, S. Vézian, M. Laügt, and J. P. Faürie, Appl. Phys. Lett. 79, 194, 2001.
[23] V. Strikant, J. S. Speck, and D. R. Clarke, J. Appl. Phys. 82, 4286, 2002.
[24] G. K. Williamson, and W. H. Hall, Acta Metall. 1, 22, 1953.
[25] P. Gay, P. B. Hirsch, and A. Kelly, Acta Metall. 1, 315, 1953.
[26] H. J. Hordon, and B. L. Averbach, Acta Met. 9, 237, 1961.
[27] B. Fultz, J. Howe, Transmission Electron Microscopy and Diffractometry of Materials, springer, 2000.
[28] D. B. Williams, and C. B. Carter, Transmission Electron Microscopy, 1997:
Plenum.
[29] S. Y. Wu, M. Hong, A. R. Kortan, J. Kwo, J. P. Mannaerts, W. C. Lee, and Y. L.
Huang (2005). Appl. Phys. Lett. 87, 091908.
[30] M. Hong, M. Passlack, J. P. Mannaerts, J. Kwo, S. N. G. Chu, N. Moriya, S. Y. Hou, and V. J. Fratello, J. Vac. Sci. Technol. B 14, 2297, 1996.
[31] W. Kollenberg, and J. Margalit, J. Mater. Sci. Lett. 11, 991, 1992.
[32] R. T. Tung, and J. L. Batstone, Appl. Phys. Lett. 52, 1611, 1988.
[33] Y. Z. Yoo, T. Sekiguchi, T. Chikyow, M. Kawasaki, J. H. Song, and H. Koinuma, Appl. Phys. Lett. 84, 502, 2004.
[34] Y. Chen, F. Jiang, L. Wang, C. Zheng, J. Dai, Y. Pu, and W. Fang, J. Cryst.
Growth 275, 486, 2005.
[35] J. Zhu, B. Lin, X. Sun, R.Yao, C. Shi, and Z. Fu, Thin Solid Films 478, 218 2005.
[36] W. R. Liu, W. F. Hsieh, C. H. Hsu, Keng S. Liang and F. S. S. Chien, J. Appl.
Cryst. 40, 924, 2007.
[37] F. A. Ponce, Group Nitride Semiconductor Compounds: Physics and Ⅲ Applications, edited by B. Gil, 123 Oxford: Clarendon, 1998.
[38] F. Vigué, P. Vennéguès, S. Vézian, M. Laügt, and J.-P. Faurie, Appl. Phys. Lett.
79, 194, 2001.
[39] R. Chierchia, T. Böttcher, H. Heinke, S. Einfeldt, S. Figge, and D. Hommel, J. Appl.
Phys. 93, 8918, 2003.
[40] V. Srikant, J. S. Speck, and D. R. Clarke, J. Appl. Phys. 82, 4286, 1997.
[41] T. Metzger, R. Hopler, E. Born, O. Ambacher, M. Stutzmann, R. Stommer, M.
Schuster, H. Gobel, S. Christiansen, M. Albrecht, and H.P. Strunk, Philos. Mag. A, 77, 1013, 1998.
[42] A. Teke, Ü. Özgür, S. Dogan, X. Gu, H. Morkoç B. Nemeth, J. Nause, and H. O.
Everitt, Phys. Rev. B 70, 195207, 2004.
[43] Y. F. Chen, D. M. Bagnall, H. J. Koh, K. T. Park, K. Hiraga, Z. Zhu, and T. Yao, J.
Appl. Phys. 84, 3912, 1998.
[44] A. Teke, U. Ozgur, S. Dogan, X. Gu, H. Morkoc, B. Nemeth, J. Nause, and H. O. Everitt, Phys. Rev. B 70, 195207, 2004.
[45] W. J. Shen, J. Wang, Q. Y. Wang, Y. Duan, and Y. P. Zeng, J. Phys. D: Appl. Phys.
39, 269, 2006.
[46] F. K. Shan, G. X. Liu, W. J. Lee, and B. C. Shin, Appl. Phys. Lett. 101, 053106, 2007.