Applied Surface Science 255 (2008) 746–748
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Application of spectral reflectance to the monitoring of ZnO nanorod growth T.H. Ghong a, Y.D. Kim a,*, E. Ahn b, E. Yoon b, S.J. An c, G.-C. Yi c a
Nano Optical Property Lab and Department of Physics, Kyung Hee University, Seoul 130-701, Republic of Korea Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea c National CRI center for Semiconductor Nanorods and Department of Materials Science and Engineering, POSTECH, Pohang 790-784, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history:
By using spectral reflectance (SR), we report on the in situ monitoring of ZnO nanorod growth on Si and sapphire substrates by catalyst-free, low pressure metalorganic chemical vapor deposition. Initially, the SR signals showed the same behavior at various wavelengths but at some point they began to strongly interfere and oscillate. This is interpreted as the starting point of the growth of nanorods. Simulation results using a multilayer model confirmed our analysis. ß 2008 Elsevier B.V. All rights reserved.
Available online 10 July 2008 PACS: 68.55. a 68.65.+g 81.15.Kk 81.70.Fy Keywords: ZnO nanorod Silicon Spectral reflectance In situ monitoring
1. Introduction
2. Experiment
With a direct bandgap slightly lower than that of GaN (Eg,ZnO 3.3 eV at 300 K) [1], ZnO has the potential for short wavelength applications. The important advantage of ZnO is a large free excition binding energy of about 60 meV (21–25 meV for GaN), which makes ZnO a very promising material for use in a lowthreshold laser [2]. Recently, ZnO nanorods grown by catalyst-free metal organic vapor phase epitaxy (MOVPE) were reported [3,4] to have superior properties by comparison with those grown using a conventional vapor–liquid–solid method. For example, they have excellent optical properties due to a low defect concentration and low growth temperature. Moreover, these nanorods are vertically well aligned and have a uniform diameter and length, which is useful for device applications based on bottom-up fabrication [5]. However, the growth process and mechanisms of ZnO nanorods grown without catalysis are yet to be fully understood. In this work, we monitored ZnO nanorod growth using in situ spectral reflectance (SR) [6], which is well known for its anisotropic surface sensitivity. So far it has been used for analyzing the structure of zincblende materials. To our knowledge, this measurement is the first in situ SR monitoring of the growth of ZnO material and its nanorod structure.
ZnO nanorods were grown on Si (0 0 1) and sapphire (0 0 0 1) substrates using a prototype low pressure MOVPE with a gas mixture of diethylzinc (DEZn) and O2 as precursors at a pressure of 1 Torr and temperature of 500 8C for 1 h. Field emission scanning electron microscopy (FE-SEM) and X-ray diffractometry were used to observe the morphology and crystal structure of the ZnO nanorods, respectively. Crystal growth was monitored using in situ SR and the sample was irradiated with light at or near the Brewster angle (708). Fig. 1 (a) and (b) show SEM images of ZnO nanorods on Si and on sapphire substrates, respectively [4,7]. The nanorods were well aligned vertically, showing uniformity in their diameters, lengths, and densities. The ZnO nanorods grown on Si substrates had sharp needle like tips and mean diameters and lengths of 40 and 740 nm, respectively. By comparison, those on sapphire had blunt tips with mean diameters and lengths of 35 and 800 nm, respectively. Details of the growth were reported elsewhere [7]. Fig. 2(a) shows time-transient SR signals of ZnO nanorods on Si substrates at selected wavelengths. Since the refractive index generally increases with temperature in this wavelength range [8], the initial increase in the SR signals at all wavelengths is due to the initial increase in growth temperature. After 14–15 min of ZnO growth, marked by the upward pointing arrow in Fig. 2(a), the SR signal suddenly begins to oscillate as if it were due to a transparent two dimensional layer with interference effects. From SEM
* Corresponding author. Tel.: +82 2 9610525; fax: +82 2 9578408. E-mail address:
[email protected] (Y.D. Kim). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.048
T.H. Ghong et al. / Applied Surface Science 255 (2008) 746–748
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Fig. 1. SEM images of ZnO nanorods on (a) Si (0 0 1) and (b) sapphire (0 0 0 1) substrates, respectively.
analysis, this signal transition point coincides with the transition from 2D layer growth to 3D nanorod growth. This transition point is observed more clearly at the downward pointing arrow in Fig. 2(b). At the end of ZnO growth in Fig. 2(a) indicated by the vertical line at 4300 s, the SR signal does not continue to oscillate but remains constant. 3. Simulation
Fig. 2. Time-transient SR signals at selected wavelengths produced by ZnO nanorods on (a) Si (0 0 1) substrates and (b) ZnO/Si (0 0 1) buffer layer.
To understand the change of the SR signals we used a multilayer model to simulate the reflection of p-polarized light on ZnO surfaces with an Effective Medium Approximation (EMA) [9] to model the refractive index of the nanorod layer. We note that the ZnO crystal has a wurtzite structure, so their principle axes are not perpendicular to each other. Therefore, the traditional subtraction method of the SR and its time-transient analysis along a certain crystal direction is not applicable to this structure. Currently, the application of SR measurements to wurtzite structures is limited to the analysis of bulk material, such as the thickness of the growing layer and the change of growth mode (e.g., nanorod growth). We, therefore, used several approximations. The 3D layer which is composed of the ZnO nanorods and the empty space between them was assumed to act like a uniform 2D layer with an effective refractive index, neff, as shown in Fig. 3. From the reported ZnO crystal dielectric function [10] we modified the sharp peaks into a broad structure by increasing the linewidth values to model amorphous ZnO, whose experimental data is not available yet. We also increased the baseline of the dielectric function spectrum to approximate high temperature values by following the case of GaAs [8]. Theoretical SR signal data from the multilayer model of p-polarized light is shown in Fig. 4, demonstrating significant similarity to experimental data (Fig. 2(a)). Therefore, we believe
Fig. 3. Approximate multilayer model based on an effective reflective index.
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reflectance in order to investigate the suitability of SR to the in situ monitoring of epitaxial layers with wurtzite structures. The timetransient SR signals of ZnO nanorod layers were observed to strongly interfere producing oscillatory behavior. We found that this oscillation can be explained using a p-polarized light reflection from the multilayer structure composed of nanorod layer having an effective refractive index. Moreover, the transition point from 2D layer to 3D nanorod layer growth coincides with the starting point of the SR signal oscillation at various wavelengths. Acknowledgments
Fig. 4. Simulation of time-transient SR signals at selected ZnO nanorod wavelengths on Si substrates.
that SR can also be successfully applied to non-cubic material systems growth monitoring. Furthermore, we concluded that the starting point of the strong oscillation at different wavelengths is due to the onset of 3D nanorod layer growth. Further investigations into ZnO on LT–ZnO buffer layers on Si (Fig. 2(b)) and also on sapphire are in progress; preliminary results indicate similar behavior and the same conclusion. 4. Conclusion We monitored the growth of ZnO buffer layers and nanorods on Si (0 0 1) and sapphire (0 0 0 1) substrates using spectral
This work was supported by the NRL Fund through NOPL and also by the KOSEF through the q-Psi. The work at SNU was supported by the SRC/ERC program of MOST/KOSEF (R11-2005048-00000-0) and the Ministry of Science & Technology through NRL Program. The work at POSTECH was supported by the National Creative Research Initiative Project of the KOSEF. References [1] D.C. Look, Mater. Sci. Eng., B 80 (2001) 383. [2] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, App. Phys. Lett. 73 (1998) 1038. [3] M. He, I. Minus, P. Zhou, S.N. Mohammed, J.B. Halpern, R. Jacobs, W.L. Sarney, L. Salamanca-Riba, R.D. Vispute, Appl. Phys. Lett. 77 (2000) 3731. [4] W.I. Park, D.H. Kim, S.-W. Jung, Gyu-Chul Yi, Appl. Phys. Lett. 80 (2002) 4232. [5] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [6] N. Kobayashi, Y. Kobayashi, K. Uwai, J. Cryst. Growth 170 (1997) 225. [7] W.I. Park, G.-C. Yi, M. Kim, S.J. Pennycook, Adv. Mater. 14 (2002) 1841. [8] H. Yao, P.G. Snyder, J.A. Woollam, J. Appl. Phys. 70 (6) (1991) 3261. [9] D.E. Aspnes, Thin Solid Films 89 (1982) 249. [10] H. Yashikawa, S. Adachi, Jpn. J. Appl. Phys. 36 (1997) 6237.
