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o m , K d Department of Electronic Materials Engineering, Silla University, Gwaebeop-dong, Sasang-gu, Busan 617-736, Republic of Korea e Department of Nano Engineering, Dong-Eui University, 995 Eomgwangno, Busanjin-gu, Busan, 614-714, Republic of Korea f School of Advanced Materials Eng., Kookmin University, 861-1, Jeongneung-dong, Seongbuk-gu, Seoul, 136-702, Republic of Korea g Institute of Microelectronics, 11 Science Park Road, Science Park II, Singapore 117685, Singapore h School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore a r t i c l e i n f o Article history: Received 7 September 2008 Accepted 16 September 2008 by P. Sheng Available online 24 September 2008 PACS: 81.05.Dz 81.40.Ef Keywords: A. ZnO B. Atomic layer deposition B. Annealing a b s t r a c t ZnO thin film was deposited on Si substrate with the insertion of ZnO buffer layer, which was annealed at various temperatures between 600 and 900 ◦C. ZnO thin film was grown by Atomic layer deposition (ALD) technique and the ZnO/ZnO-buffer/Si films have been further annealed for 30 min in N2 ambient. High quality ZnO thin films were obtained on the annealed ZnO-buffer/Si layer. In particular, the ZnO thin film that is grown on 750 ◦C-annealed ZnO buffer exhibits a smoother surface, and enhanced near- band-edge emission compared to those grown on the as-prepared ZnO buffer layer. In the meantime, ZnO with 900 ◦C-annealed ZnO buffer layer shows the narrowest and strongest (002) peak from the XRD measurement. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction ZnO is one of the attractive II–VI semiconductors due to its wide direct band gap (3.37 eV at room temperature) and large exciton binding energy (60 meV), which is larger than GaN (28 meV) and ZnSe (19 meV). As a result, ZnO has been intensively investigated especially in the application of light-emitting devices, photodetectors, and transparent thin-film transistors [1–3]. So far, ZnO thin films have been grown by various deposition techniques, such as pulse laser deposition, molecular beam epitaxy, sputtering, chemical vapor deposition (CVD) and atomic layer deposition (ALD) [4–7]. Among these, ALD has been proposed as a promising method to deposit ZnO thin film. ALD growth is similar to CVD growth. However, the method of injection of precursors is different for these two growth techniques. In ALD growth, precursors are injected into the reactor ∗ Corresponding author. Tel.: +82 55 320 3874; fax: +82 55 320 3631. E-mail address: hhryu@inje.ac.kr (H. Ryu). as pulses, while the precursors are introduced simultaneously into the chamber for CVD growth. Since the precursors are injected alternatively into the chamber, chemical reaction is confined to the surface of substrate. Therefore, it is possible to deposit one monolayer at one cycle by the chemisorption and desorption of precursors. By controlling the injection of precursors and the number of deposition cycles, high quality atomic-scale thin film can be realized by ALD technique. More importantly, due to the self-limiting process of the precursors, ALD technique could deposit thin film at relatively low temperature comparedwith CVD technique. However, for low deposition temperature, there are possibili- ties that all the chemical reactions do not occur ideally, precursors do not decompose completely and atoms fail to move to equilib- rium atomic sites [8]. Annealing treatment is one of the most com- monmethods to reduce the defects and improve the structural and optical qualities of as-grown thin film [9]. Even if low temperature growth of ZnO thin film is available in ALDprocess, annealing treat- ment is necessary to further improve its quality. Silicon has been widely used as a substrate for ZnO deposition due to many advantages such as large wafer size, low cost and Solid State Communicati Contents lists availa Solid State Com journal homepage: www Effects of annealing temperature of buffe properties of ZnO thin film grown by ato C.R. Kim a, J.Y. Lee a, C.M. Shin a, J.Y. Leem a, H. Ryu a,∗ W.G. Jung f, S.T. Tan g, J.L. Zhao h, X.W. Sun g,h a Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje Universit bMajor of Nano Semiconductor, Korea Maritime University, #1 Dongsam-dong, Yeongdo- c Department of Mechatronics Engineering, Korea Maritime University, #1 Dongsam-dong 0038-1098/$ – see front matter© 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2008.09.034 ns 148 (2008) 395–398 ble at ScienceDirect munications .elsevier.com/locate/ssc r layer on structural and optical ic layer deposition J.H. Chang b, H.C. Lee c, C.S. Son d, W.J. Lee e, y, Obang-dong, Gimhae, Gyeongnam 621-749, Republic of Korea u, Busan 606-791, Republic of Korea , Yeongdo-Ku, Busan 606-791, Republic of Korea Administrator 高亮 2. Experimental procedure ZnO buffer on Si was prepared by CVD technique and the detail experiment can be found elsewhere [3]. Prior to the ALD deposition of ZnO thin films, the ZnO-buffer/Si were annealed in a temperature range from 600◦C to 900 ◦C for 1 h in N2 ambient. After that, ZnO thin films were deposited on the ZnO- buffer/Si at 150 ◦C. Diethylzinc (carried by 6N high purity N2) and oxygen plasma gas (99.995%) were used as the precursors. These precursors were alternately fed into a deposition chamber with a typical pulse duration of 3 s and 5 s for diethylzinc and oxygen, respectively. The N2 purge time between the precursors injection was controlled at 6 s. After the ALD deposition of ZnO thin films on ZnO-buffer/Si, ZnO/ZnO-buffer/Si were further annealed at 900 ◦C for 30 min to further improve the film quality. The schematic diagram of ZnO/ZnO-buffer/Si is shown in Fig. 1. Surface condition Fig. 2. Surface and cross-section SEM images of the as-grown and a flattening at higher annealing temperature (900 ◦C) could be due to the thermal evaporation of ZnO during the annealing. As a result, the thickness of the ZnO buffer layer annealed at 900 ◦C decreased, as shown in the cross-section SEM in Fig. 2. By adopting the as-grown and annealed ZnO buffer/Si samples above, ZnO thin films were grown by ALD technique. Fig. 3 shows the AFM images with the corresponding RMS value of the ZnO thin films grown on as-grown and annealed ZnO-buffer/Si. For the ZnO thin film that was grown on as-grown ZnO-buffer/Si, it has the roughest surface, with the RMS roughness of 32.32 nm. On the other hand, the RMS roughness of the ZnO thin films grown on annealed ZnO-buffer/Si decreased significantly, as shown in Fig. 3. Generally, rough surface of buffer layer facilitates the growth of ZnO nuclei on top of the highly c-oriented columnar structure. The high density of nucleation site on the buffer layer facilitates the growth of smooth ZnO thin films on ZnO-buffer/Si [10]. As a result, the ZnO thin film that grown on 750 ◦C-annealed ZnO-buffer/Si has the smoothest surface, with a RMS roughness of 18.62 nm. Besides, 396 C.R. Kim et al. / Solid State Communications 148 (2008) 395–398 Fig. 1. The schematic diagram of ZnO/ZnO-buffer/Si. easy transformation into electronic device. However, direct growth of ZnO films on Si substrate exhibit poor quality due to the easy oxidation of Si substrates and large mismatch of lattice constant between Si and ZnO. In order to overcome these problems, buffer layer has been adopted to improve the quality of ZnO films. Many reports have studied the various types of buffer layer, the effects of thickness, growth temperature of buffer layer on structural and optical properties of ZnO film [10,11]. But the report on the effects of annealing temperature of ZnO buffer layer is rare. In this paper, ZnO films were grown by remote plasma ALD on ZnO buffer/Si annealed at various temperatures. The effects of annealing temperature of buffer layer on optical and structural properties of ZnO/ZnO buffer/Si films have been investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), photoluminescence (PL), and X-ray diffraction (XRD). of ZnO-buffer/Si, structural and optical properties of ZnO/ZnO- buffer/Si were characterized by scanning electron microscopy (SEM), atomic forcemicroscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL) using He-Cd lasers (λ = 325 nm). 3. Results and discussion Fig. 2 shows the surface and cross-section SEM images of the as-grown and annealed ZnO buffer/Si at 600, 750 and 900 ◦C, respectively. It is seen that the surface morphology of the as- grown ZnO buffer layer is in granular form and comparatively flat. After annealing at 600 and 750 ◦C, the ZnO buffer layers are roughening with ‘voids’ appearing in between the grain as shown in Fig. 2. With the increase of annealing temperature, the atoms gain sufficient energy to initiate the coalescence process [9]. The surface roughening of the ZnO thin films upon annealing is governed by the well-known Ostwald ripening. Oswald ripening is a coalescence process that involving the growth of the larger crystallites at the expense of the smaller crystallites. Through the atom migration, small crystallites migrate to larger crystallites, resulting in large volume of ‘voids’. However, the surface of the ZnO buffer becomes flat upon annealing at 900 ◦C. The surface nnealed ZnO buffer/Si at different annealing temperatures for 1 h. Administrator 高亮 Administrator 删除线 Administrator 高亮 Administrator 下划线 Administrator 下划线 Administrator 高亮 Administrator 高亮 Fig. 3. AFM images of ZnO thin films grown on as-grown and annealed ZnO buffer/Si at different annealing temperatures. Fig. 4. XRD patterns of ZnO thin films grown on as-grown and annealed ZnO buffer/Si at different annealing temperatures. it can be seen from Fig. 3 that the ZnO thin films grown on annealed ZnO-buffer/Si have larger grains compared to those grown on as- grown ZnO-buffer/Si. The effect of the buffer layer on structural quality of ZnO film was studied by XRD measurement, as shown in Fig. 4. All samples show strong (002) and two weak (100), (101) diffraction peaks. The (002) peaks are detected at around 34.4◦, indicating the c- axis oriented films are grown with hexagonal wurtzite structure. It can be seen from Fig. 4 that the ZnO thin films grown on (a) Photon energy (eV). (b Fig. 5. (a) PL spectra of ZnO thin films grown on as-grown and annealed ZnO buffer/S function of annealing temperature of buffer layer. should be exciton-related near-band-edge emission (NBE) and the emission band peaking at 2.3 eV is defect-related deep-level emission (DLE). DLE peak at around 2.3 eV (530 nm) is a green emission which is normally found in ZnO [6,8]. The origin of the green emission is still an open and controversial question. However, it is commonly suggested to be related to oxygen vacancies in ZnO [12]. The fullwidth at halfmaximum (FWHM) and the intensity of NBE peak of the ZnO thin films grown on as-grown and annealed ZnO-buffer/Si was plotted in Fig. 5(b) for a better observation. It is seen that with the increase of the buffer layer annealing temperature, the NBE increases significantly especially for the sample with buffer layer annealed at 750 ◦C. However, the NBE for the sample with buffer layer annealed at 900 ◦C decreased abruptly. The deterioration of the optical properties might be due to the degradation of the film at high-temperature annealing. Other than the NBE intensity, the FWHM of the NBE decreases with the increase of annealing temperature up to 750 ◦C as well. 4. Conclusions In summary, the effect of annealing temperature of ZnO buffer layer on the ZnO films grown byALDwas investigated. It was found that the pre-growth annealing treatment on the ZnO buffer layer improved the structural and optical properties of the further ALD- grown ZnO thin films. In particularly, the ZnO thin film that grown on the 750 ◦C-annealed ZnO buffer layer exhibits good surface morphology and optical properties. The roughening of the ZnO buffer layer annealed at 750 ◦C was found to be beneficial for the growth of ZnO thin films. In the meantime, ZnO with 900 ◦C- annealed ZnO buffer layer shows the narrowest and strongest (002) peak from the XRD measurement. Acknowledgment This work was supported by the 2009 Inje University research grant. ) Annealing Temperature (◦C). C.R. Kim et al. / Solid State Communications 148 (2008) 395–398 397 annealed ZnO-buffer/Si have better crystallinity than those grown on as-grown ZnO-buffer/Si. The mean grain sizes that estimated by the Scherrer formula are 59.05, 59.11, 63.82 and 71.92 nm for ZnO thin films grown on as-grown, ZnO-buffer/Si annealed at 600, 750 and 900 ◦C, respectively. The increase in grain size and hence improvement on crystallinity is consistent with the AFM results. Fig. 5(a) shows the PL spectra of ZnO thin films grown on as-grown and annealed ZnO-buffer/Si at 600, 750, and 900 ◦C, respectively. In Fig. 5(a), two typical emissions of ZnO at around 3.242 eV and 2.3 eV are observed. The emission at 3.242 eV i at different annealing temperatures. (b) FWHM and intensity value of NBE peak as a Administrator 高亮 Administrator 高亮 Administrator 高亮 Administrator 高亮 398 C.R. 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Kusumori, H. Minoura, H. Muto, Thin Solid Films 411 (2002) 60. [12] A.B. Djurisic, Y.H. Leung, Small 2 (2006) 944. Effects of annealing temperature of buffer layer on structural and optical properties of ZnO thin film grown by atomic layer deposition Introduction Experimental procedure Results and discussion Conclusions Acknowledgment References