Growth of ultra small self-assembled InGaN nanotips

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1 Journal of Crystal Growth 263 (2004) Growth of ultra small self-assembled InGaN nanotips L.W. Ji a, *, Y.K. Su a, S.J. Chang a, T.H. Fang b, T.C. Wen a, S.C. Hung a a Institute of Microelectronics & Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan 701, ROC b Department of Mechanical Engineering, Southern Taiwan University of Technology, Yong-Kan, Taiwan 710, ROC Received 3 October 2003; accepted 7 November 2003 Communicated by M. Schieber Abstract Vertical self-organized nanotips were grown on InGaN film via metal-organic chemical vapor deposition (MOCVD) and thermal annealing. It was found that typical height of these nanotips is 20 nm with an average width of 1 nm. It was also found that the local density of the vertically grown self-assembled InGaN nanotips could reach cm 2. Furthermore, it was found that height and width of the nanotips both distributed uniformly. The possible formation mechanism of self-assembled nanotips has been also discussed in this work. Such a small size of vertical nanotips could show strong quantum localization effects and have potential applications in field emission devices, near-field microscopy, and blue photonics. r 2003 Elsevier B.V. All rights reserved. PACS: Lp; Ps; Hb; La; Hc; Dn; Ya Keywords: A1. Atomic force microscopy; A1. Nanotips; A3. Metalorganic chemical vapor deposition; B1. Nitrides 1. Introduction III V nitride semiconductor materials have a wurtzite crystal structure and a direct energy band gap. At room temperature, the band gap energy of AlInGaN varies from 0.7 to 6.2 ev depends on its composition. Therefore, III V nitride semiconductors are particularly useful for light-emitting and laser diodes in this wavelength range [1 4]. Lowdimensional nanostructures, such as quantum wires (or nanotips) and dots (or islands), are quite attractive for application to high performance *Corresponding author. Tel.: ; fax: address: lwji@seed.net.tw (L.W. Ji). electronic and optical devices. Among these nanostructures, one-dimensional (1D) nanomaterials are expected to play a key role in future nanotechnology. These 1D nanomaterials could also provide model systems to demonstrate quantum size effects. It has been demonstrated that these 1D nanotips can be used in scanning nearfield optical microscopy (SNOM) and field emission display (FED) [5,6]. For FED applications, we can apply a high electric field between the conductive nanotips and the anodes so that electrons are emitted from the cathode, which consists thousands of conductive nanotips. To achieve a high performance, we need to use nanotips with a proper electron affinity so that electrons can be emitted easily. On the other hand, /$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi: /j.jcrysgro

2 64 L.W. Ji et al. / Journal of Crystal Growth 263 (2004) these nanotips should be hard enough so that wearing out of the tips due to radiation damage will not occur [6,7]. It is known that nitride-based materials are hard and chemically stable. With wide bandgap energy and low work function, nitride nanotips are potentially useful in FED applications. Indeed, 1D GaN nanotips have attracted much attention recently. Many research groups succeed in the fabrication of nitride nanotips using various methods, such as carbon nanotube confined reaction [8], arc discharge [9], reactive ion etching [6,10], chemical vapor deposition [11,12] and molecular beam epitaxy (MBE) [13]. However, these methods are all very complicated. Some methods might even result in severe damage on the surface of the fabricated nitride nanotips. In this work, we report the fabrication of ultra small self-organized nitride nanotips. The fabrication procedure and the properties of the fabricated nitride nanotops will be discussed. 2. Experiment Samples used in this study were all grown on ( )-oriented sapphire (Al 2 O 3 ) substrates in a vertical low-pressure metal-organic chemical vapor deposition (MOCVD) reactor with a highspeed rotation disk [14 18]. The gallium, indium and nitrogen sources were trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH 3 ), respectively. The MOCVD growth procedure is as follows. After a 30-nm-thick lowtemperature GaN nucleation layer was deposited onto the sapphire substrate at 500 C, the temperature was raised to 1000 C to grow a 2-mmthick undoped GaN buffer layer. The growth temperature was then reduced to 730 Ctogrowa 3-nm-thick InGaN layer on top of the undoped GaN buffer layer with a growth rate of 0.05 nm/s. Hydrogen (H 2 ) and nitrogen (N 2 ) were used as carrier gases for GaN and InGaN, respectively. It should be noted that the nominal indium content in the InGaN layer is around 0.3. As-grown samples were then thermally annealed in a quartz tube furnace at 740 C for 20 min. Room temperature surface morphologies of the as-grown and annealed samples were then ex situ characterized by an atomic force microscopy (AFM) system (Veeco/TM CP-R SPM) with tapping mode. Photoluminescence (PL) was also used to study the optical properties of these samples at room temperature. During PL measurements, a 325 nm He Cd laser was used as the excitation source. The collected luminescence signal was dispersed by a monochromator and detected by a photomultiplier tube. 3. Results and discussions Fig. 1 shows a two-dimensional (2D) AFM image for the surface morphology of the as-grown sample without thermal annealing. It was found that there were quite a few three-dimensional (3D) islands or quantum dots (QDs) on the sample surface. It was also found that size fluctuation of these QDs was large. These nanometer-sized QDs were formed by the strain-induced Stranski Krastanov (S-K) growth mode due to the lattice mismatch between InGaN and the underneath GaN. To obtain such QDs, it is necessary to precisely control the MOCVD growth conditions. Similar self-organized QDs have also been observed previously by other research groups [19 21]. The formation of QDs or islands in the asgrown sample can be explained in terms of thermal Fig. 1. 2D AFM image for the surface morphology of the asgrown sample without thermal annealing.

3 L.W. Ji et al. / Journal of Crystal Growth 263 (2004) equilibrium. Although 3D islands have a larger surface than a 2D film, the elastic relaxation energy (possible in an island) could overcompensate the increased surface energy. Thus, islands are formed instead of a strained epitaxial film for the as-grown sample [22]. Instead of on top of the substrate directly, these islands are often formed on top of the wetting layer. Here, we can neglect the effects of surface stress and assume that total energy change from 2D growth to 3D island growth is the sum of changes in surface and strain energy, i.e. DE total ¼ DE surf +DE elast [23]. It should be noted that DE surf > 0 and DE elast o0: Thus, if 9DE surf 9o9DE elast 9; we can see clearly that DE total o0; which indicates that 3D island growth is the favorable state in the system. As a result, we could observe the self-assembled QDs shown in Fig. 1. Figs. 2(a) and (b) show the nm 2 2D AFM picture and the height profile, respectively, of the thermally annealed sample. It was found that self-organized nanotips were formed vertically protruding above the sample after annealing. Furthermore, it was found that these nanotips were Fig. 2. (a) nm 2 2D AFM picture and (b) height profile of the thermally annealed sample.

4 66 L.W. Ji et al. / Journal of Crystal Growth 263 (2004) assembled on top of the QDs, as shown in Fig. 2(b). The exact mechanisms for the formation of these nitride nanotips after thermal annealing are not yet clear. We believe the mechanisms to be similar to the formation of self-assembles QDs during MOCVD growth. It should be noted that these self-organized nanotips were all grown on the islands. It should also be noted that the thermal annealing temperature (i.e. 740 C) was higher than the MOCVD growth temperature of InGaN layer (i.e. 730 C). It is possible that the sample could gain enough energy during annealing so as to drive the growth of these self-organized nanotips on the surface of islands. On the other hand, no nanotips could be found in the non-island areas. In order to gain more information on these nanotips, we focused our study on one randomly picked QD and took AFM pictures with higher resolutions. Figs. 3(a c) show the nm 2 3D AFM image, the nm 2 2D AFM image and the height profile, respectively, of the thermally annealed sample. It was found that typical height of these nanotips is 20 nm with an average width of 1 nm. It was also found that the local density of these vertically grown self-organized InGaN nanotips could reach cm 2.Furthermore,it was found that these nanotips were all grown along the [ ] direction with a coherent orientation. It can also be seen from these figures that height and width of these nanotips both distributed uniformly. Fig. 3. (a) nm 2 3D AFM image, (b) nm 2 2D AFM image and (c) height profile of the thermally annealed sample.

5 L.W. Ji et al. / Journal of Crystal Growth 263 (2004) from Center for Nanotechnology of STUT. This work was financially supported by the National Science Council of Taiwan (Project No. NSC E ). References Fig. 4. Room temperature PL spectrum observed from the thermally annealed sample. Fig. 4 shows room temperature PL spectrum observed from the thermally annealed sample. It was found that PL peak located at 457 nm with a narrow full width at half maximum (FWHM) of around 21 nm. The narrow PL FWHM also suggests that the height and width of the nanotips formed in this study are both reasonably uniform. Such uniform size distributions are extremely important for practical FED applications. 4. Conclusion In summary, it has been demonstrated that selforganized InGaN nanotips can be vertically grown via MOCVD and thermal annealing. It was found that typical height of these nanotips is 20 nm with an average width of 1 nm. It was also found that the local density of the vertically grown selforganized InGaN nanotips could reach cm 2. Furthermore, it was found that height and width of the nanotips both distributed uniformly. Acknowledgements The authors would like to thank S. Z. Liao for his great aid in AFM measurement and the help [1] S. Nakamura, T. Mukai, M. Senoh, J. Appl. Phys. 76 (1994) [2] F.A. Ponce, D.P. Bour, Nature 386 (1997) 351. [3] H. Akasaki, Amano, J. Crystal Growth 175/176 (1997) 29. [4] S. Nakamura, Science 281 (1998) 956. [6] S. Khalfallah, C. Gorecki, J. Podlecku, M. Nishioka, H. Kawakatsu, Y. Arakawa, Appl. Phys. A 71 (2000) 223. [5] Y. Terada, H. Yoshida, T. Urushido, H. Miyake, K. Hiramatsu, Jpn. J. Appl. Phys. 41 (2002) L1194. [7] S. Muthukumar, H. Sheng, J. Zhong, Z. Zhang, N.W. Emanetoglu, Y. Lu, IEEE Tran. Nanotechnol. 2 (2003) 50. [8] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997) [9] W. Han, P. Redlich, F. Ernst, M. R.uhle, Appl. Phys. Lett. 76 (2000) 652. [10] H. Yoshida, T. Urushido, H. Miyake, K. Hiramatsu, Jpn. J. Appl. Phys. 40 (2001) L1301. [11] C.C. Tang, S.S. Fang, H.Y. Dang, P. Li, Y.M. Liu, Appl. Phys. Lett. 77 (2000) [12] S.Y. Bae, H.W. Seo, J. Park, H. Yang, H. Kim, S. Kim, Appl. Phys. Lett. 82 (2003) [13] L.W. Tu, C.L. Hsiao, T.W. Chi, I. Lo, Appl. Phys. Lett. 82 (2003) [14] S.J. Chang, M.L. Lee, J.K. Sheu, W.C. Lai, Y.K. Su, C.S. Chang, C.J. Kao, G.C. Chi, J.M. Tsai, IEEE Electron. Dev. Lett. 24 (2003) 212. [15] S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, J.K. Sheu, T.C. Wen, W.C. Lai, J.F. Chen, J.M. Tsai, IEEE J. Sel. Top. Quan. Electron. 8 (2002) 744. [16] S.J. Chang, W.C. Lai, Y.K. Su, J.F. Chen, C.H. Liu, U.H. Liaw, IEEE J. Sel. Top. Quan. Electron. 8 (2002) 278. [17] W.C. Lai, S.J. Chang, M. Yokoyama, J.K. Sheu, J.F. Chen, IEEE Photon. Technol. Lett. 13 (2001) 559. [18] S. Tanaka, S. Iwai, Y. Aoyagi, Appl. Phys. Lett. 69 (1996) [19] K. Tachibana, T. Someya, Y. Arakawa, Appl. Phys. Lett. 74 (1999) 383. [20] C. Adelmann, J. Simon, G. Feuillet, N.T. Pelekanos, B. Daudin, Appl. Phys. Lett. 76 (2000) [21] L.W. Ji, Y.K. Su, S.J. Chang, L.W. Wu, T.H. Fang, J.F. Chen, T.Y. Tsai, Q.K. Xue, S.C. Chen, J. Crystal Growth 149 (2003) 144. [22] N. Moll, M. Scheffler, E. Pehlke, Phys. Rev. B 58 (1998) [23] V.A. Shchukin, D. Bimberg, T.P. Munt, D.E. Jesson, Phys. Rev. Lett. 90 (2003)