8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Structural, Optical and Electrical properties of MOCVD grown highly aligned ZnO nanorods
S. Majumdar, S. Bhunia1, P. Banerji*
Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India
1Surface Physics Division, Saha Institute of Nuclear Physics,
1/AF Saltlake, Sector – I, Kolkata 700064, India
∗Corresponding author Phone: +91-3222-283984, Fax: +91-03222-55303
E-mail: pallab@matsc.iitkgp.ernet.in
Abstract: Highly aligned ZnO nanorods were grown by metalorganic chemical vapor deposition (MOCVD) on r-plane [(1, 0,-1,2)] sapphire using a horizontal atmospheric pressure reactor at 4000C without employing any metal catalyst. We had used DEZn as organmetallic precursor for Zn while tertiary-butanol for oxygen source. The MOCVD grown nanorods were characterized by XRD and SEM. UV and PL studies were performed to determine the bandgap and to study the other optical characteristics. The electrical study was done to obtain resistivity, Hall mobility and carrier concentration of the MOCVD grown nanorods.
1. Introduction
ZnO has hexagonal crystal (wurtzite structure) having voids at tetrahedral and octahedral position [1]. It is a wide bandgap ( Eg = 3.37 eV ) II – VI semiconductor having large exciton binding energy of 60 meV [2]. Consequently it can lase even at temperature higher than the room temperature. It is a naturally grown n-type direct bangap nonstochiometric semiconducting oxide with 63% ionicity. However, the major difficulty in achieving ZnO based device is the p-type doping. The advantage of MOCVD technique is the growth over large surface area with almost uniformity, not available in other methods. Precise control of the thickness and doping are also possible by MOCVD. The mobility of a film not only depends upon carrier concentration but also on method of growth such as MOCVD, PLD etc. and also on temperature and most importantly on VI/II ratio [3]. In this paper, we report the MOCVD growth of ZnO and its
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characterization to assess the optical, structural and electrical quality of the grown samples
2. Experimental Procedure
The MOCVD unit consists of horizontal reactor within which a stainless steel sample holder is mounted with the sample upon it. Diethyl zinc and oxygen were used as precursor for group II and VI respectively while nitrogen was used as carrier gas. The temperatures of the bubblers containing DEZn and tertiary butanol were maintained at temperature 25ºC and 30ºC respectively. The flow rate of DEZn and tertiary butanol were kept at 20 sccm and 6 sccm respectively to achieve a VI/II ratio of 25.6. The growth was carried out for 1 hour while the growth temperature was maintained at 4000C. The growth rate was found to be 1 nm/min.
3. Results and discussion:
3.1. XRD Result
The X-ray diffraction of MOCVD grown samples was done by using Cu Kα radiation operating at 50 KV, 40mA with normal (θ-2θ) scanning. The lattice parameters a and c of the ZnO nanorods were evaluated from x-ray diffraction analysis and were found to be 3.2370 Å and 5.1840 Å, respectively, giving the c/a ratio of 1.6014, which was within the reported range of 1.593 to 1.6035 in the literature [4]. The sharp peak from XRD indicated highly crystalline hexagonal ZnO that was deposited on the sapphire shown in the figure 1.
3. 2. UV Study
The bandgap of the MOCVD grown ZnO was evaluated using optical absorption in transmission mode. It was found that there was a sharp absorption at around 3.3 eV (from figure 2) which was the bandgap of the grown sample.
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3. 3 SEM Studies
Scanning electron microscopy (SEM) studies of ZnO films showed highly aligned nanorods throughout the substrate at growth temperature 4000C as shown in figure 3. The rods had symmetrical hexagonal facets on the sidewalls and very sharp tips, characteristics of (100) growth of wurtzitic ZnO.
3.4. PL Study
The photoluminescence studies showed strongest band edge related transition at around 380 nm, in addition to many other peaks due to impurities as shown in figure 4. This corresponds to the near band edge emission from ZnO. A peak observed at 575 nm corresponds to yellow band emission. A peak around 527 nm corresponds to green emission which occurred due to oxygen vacancies in ZnO. A weak blue band emission peak around 450-490 nm was observed.
4. Electrical Study
The resistivity of MOCVD grown ZnO samples on r-sapphire was determined to be 3.744 X 10-3 Ω-cm at room temperature. The carrier concentration and Hall mobility were found to be 1.07 X 1019 cm-3 and 156 cm2/V-sec, respectively. The negative Hall coefficient confirmed the ZnO nanorods was n-type in nature.
5. Conclusion
We have grown ZnO by MOCVD and its structural property reveals that at a growth temperature of 4000C, highly oriented nanorods were formed even without any metal catalyst on r-sapphire. The bandgap was measurd by absorption spectroscopy and was found to be 3.3 eV. The PL studies showed blue, green and yellow band emissions due to various impurities and oxygen vacancy in the samples. The electrical study showed a good conductive sample with high electron mobility on r-sapphire substrate. More studies are required for standardization of the growth parameters.
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Reference:
[1] G. S. Wu, T. Xie, X.Y. Yuan, Y. Li, L. Yang, Y. H Xiao, L.D. Zhang, Solid State Comm. 134 (2005) 458.
[2] Changsong Liu, Yoshitake Masuda, Yunying Wu, Osamu Takia, Thin Solid Films 503 (2006) 110.
[3]. O. Pani, N. N. Somhlahlo, C. Weichsel, A. W. R. Leitch, Physics B 367-377 (2006) 749.
[4] Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho and H. Morkoç, J. Appl. Phys. 98 (2005) 41301.
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