Growth of MgO thin films with

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Thin Solid Films 515 (2007) 6433 – 6437 www.elsevier.com/locate/tsf

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Growth of MgO thin films with subsequent fabrication of ZnO rods: Structural and photoluminescence properties

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Hyoun Woo Kim ⁎, Seung Hyun Shim, Jong Woo Lee

School of Materials Science and Engineering, Inha University, Incheon 402-751, South Korea Available online 10 January 2007

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Abstract

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Keywords: Thermal evaporation; MgO; Atomic layer deposition; ZnO

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We have grown magnesium oxide (MgO) films by the simple evaporation of MgB2 powders. The subsequent deposition of ZnO by using an atomic layer deposition (ALD) technique generated the ZnO rods on MgO films, realizing the first production of rod-like structures using ALD. We have employed X-ray diffraction, scanning electron microscopy, transmission electron microscopy and photoluminescence (PL) spectroscopy to characterize the samples. PL of MgO films exhibited two emission bands peaked in the blue and blue-green region, respectively. The deposition of ZnO rods changed the shape of the PL spectrum. © 2006 Elsevier B.V. All rights reserved.

1. Introduction

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Not only that the MgO is a typical wide bandgap insulator with its electronic and optical properties well investigated [1–3] but also that it has been widely used as a transition layer for growing various kinds of materials including thin films with its excellent thermodynamic stability, low dielectric constant, and low refractive index. The MgO thin films have been produced by various synthetic methods including laser ablation [4], pulsed laser deposition [5], e-beam evaporation [6], atomic layer deposition [7], metalorganic chemical vapor deposition [8,9], rf magnetron sputtering [10], and spray pyrolysis [11]. Although thermal evaporation is a simple, low-cost and environment-friendly method which guarantees high-quality and reproducible product, there has been not many compound that can be deposited by direct evaporation, due to a serious limitation of thin-film techniques and especially of the evaporation of materials. To the best of our knowledge, the preparation of MgO films by the thermal evaporation has not been reported. In this paper, we demonstrate the growing of MgO films using a simple evaporation of MgB2 powders. In addition, in order to investigate the property of the asgrown MgO films as a substrate material, we have grown the ⁎ Corresponding author. Tel.: +82 32 860 7544; fax: +82 32 862 5546. E-mail address: [email protected] (H.W. Kim). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.071

one-dimensional (1D) structures of ZnO using the atomic layer deposition (ALD) technique. ZnO has been well known for its attractive optical properties based on wide energy bandgap with large exciton binding energy and is one of the most promising material for application in the fields of short wavelength optoelectronics, gas sensors, acoustic devices and solar cells. Particularly, 1D ZnO have received much attention due to their potential applications in constructing nanoscale electronic and optoelectronic devices [12]. Fabricating the ZnO/MgO heterostructures in the present work, we investigate the effect of ZnO coating on the PL properties. Up to the present, the production of 1D structures of any material using the ALD method has never been reported. Owing to its self-limiting growth mechanism, the ALD technique allows atomic-scale control and low-temperature process, finding useful applications in flat panel displays, photovoltaics, and catalysis, etc. Furthermore, ALD is considered as a remarkable tool for advanced basic studies of surface chemistry [13] and reaction mechanisms [14]. Therefore, the first deposition of 1D rods using the ALD will contribute not only to finely controlled growth of 1D structures but also to deep understanding of 1D growth mechanisms. 2. Experimental The experimental setup employed in this research was previously described [15]. An alumina boat was filled with

H.W. Kim et al. / Thin Solid Films 515 (2007) 6433–6437

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Fig. 1. (a) SEM cross-section and (b) SEM top view of MgO films. (c) TEM–SAED pattern recorded from the MgO film ([011¯] zone axis). (d) XRD glancing-angle pattern recorded from the MgO films.

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MgB2 powders and placed in the middle of the quartz tube contained in a furnace. We employed Au (about 3 nm)-coated Si substrates. On top of the alumina boat, a piece of the substrate was placed with the Au-coated side downwards. During the experiment, the furnace was heated to 900 °C under a flowing atmosphere (∼ 1.5% O2 in a balance of argon) at 2 Torr. These conditions were maintained for 2 h, after which the substrate was cooled down and was transferred to an ALD chamber. Subsequently, we have carried out the ZnO coating experiments on the as-grown MgO films. For comparison study, we have also employed (001) MgO single-crystal substrates (crystal orientation accuracy of 0.2°, EPI polished to surface roughness (Ra) b 1.0 nm; MTI Co.) for the same ZnO coating experiments. The schematic diagram of the ALD system was previously reported [16]. Diethylzinc (DEZn) and H2O were kept in bubblers at 10 °C. These source gases were alternately fed into the chamber though separate inlet lines and nozzles. The typical pulse lengths were 0.2 s for DEZn, 0.2 s for the H2O and 2 s for purging the reactants. The substrate temperature and pressure in the chamber were set to 150 °C and 0.3 Torr, respectively. The structural properties of the as-grown products were investigated using X-ray diffraction (XRD, X'pert MRD-Philips) with Cu Kα1 radiation (λ = 0.154056 nm), scanning electron microscopy (SEM, Hitachi S-4200), and transmission electron microscopy (TEM, Philips CM-200). For XRD analysis, we have utilized both the conventional diffraction geometry (θ–2θ scanning) and the glancing angle diffraction geometry (incident

angle = 0.5°). PL measurements were carried out by using a He– Cd laser line (325 nm, 55 mW) as the excitation source at room temperature. 3. Results and discussion Fig. 1a and b show cross-sectional and plan-view SEM images of MgO films, respectively. The cross-sectional SEM image confirms that thin film has indeed been deposited on the substrates. The plan-view SEM image indicates that the grainlike structures are found on the surface of the product. Although the SEM images show the formation of irregular structures, some square faces presumably representing the cubic crystallites are clearly visible. We performed a TEM analysis for further characterization of the MgO films. Fig. 1c shows a selected area electron diffraction (SAED) pattern of the film when the electron beam is focused along [011¯] direction, and the diffusion rings from inside to outside belong to (200), (211), and (222) planes of cubic MgO, respectively. The XRD pattern shown in Fig. 1d reveals the overall crystal structure of the MgO films. Miller indices are indicated on each diffraction peak. The diffraction peaks of (111), (200), (220), (311), and (222) correspond to the cubic MgO structure with lattice constants of a = 0.421 nm (JCPDS: 04-0829). Additionally, (111) and (200) diffraction peaks of Au from the substrate were detected. XRD and SAED analyses in the present study suggest that the MgO film is close to polycrystalline.


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With the as-synthesized MgO films, we have performed ZnO coating experiments using ALD. The cross-sectional SEM image of ZnO-deposited sample is shown in Fig. 2a, indicating that 1D structures are grown on the predeposited MgO films. Close examination reveals that the cross-section of the rods is close to circular-shaped. The rods have an almost straight-line morphology along the length direction. Statistical analysis of many SEM images shows that the rods have average diameters ranging from 2.0 to 2.5 μm. Fig. 2b is a typical plan-view SEM image of the product, showing an agglomeration of solid rods over the substrate surface. It is noteworthy that the wrinkled surface is clearly observed not only from the ZnO rods but also from the substrate surfaces between the rods.

Fig. 2. (a) SEM cross-section and (b) SEM top view of ZnO-deposited MgO films. (c) XRD glancing-angle and (d) XRD conventional pattern of the ZnOdeposited MgO films.

Fig. 3. (a) SEM cross-section and (b) SEM top view images showing the ZnO deposits on the flat MgO(001) substrates. (c) XRD conventional pattern of the ZnO-deposited MgO(001) substrates.

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When the ZnO coating was performed in the experiment under the same condition but with a flat MgO(001) substrate, we obtained the film-like structures without any 1D structure (Fig. 3a and b). The associated XRD pattern shown in Fig. 3c reveals that the film-like structures correspond to ZnO. By comparing Fig. 3 with Fig. 2, we propose that the generation of rods on the MgO films (Ra N 500 nm) is mainly due to their surface roughness, because no rods were observed on the smooth MgO(001) substrate (Ra b 1 nm). To our knowledge, this is the first report on the growth of rodlike structures by employing the ALD technique for all materials systems and the formation mechanism of such a rod is not well understood. Although further investigation is necessary, homogeneous distribution of the active species was not attainable for some unknown reason, particularly when the rough substrate has been used. Fig. 4a and b illustrate the growth models of ZnO deposits on rough and flat MgO substrates, respectively. Since DEZn and H2O species are alternately fed into the chamber, the solid circles represent DEZn or H2O species. We surmise that the concentrated deposition of active species onto the favorable site throughout the rough surface results in the anisotropic or uneven growth, whereas the flat substrate surface favored the twodimensional growth. Similarly, Chou and Ting previously reported that a rough substrate surface favored the formation of naorods in the sputter deposition of ZnO [17]. Once the

Fig. 4. Schematic diagram of the proposed growth mechanism of ZnO deposits formed on (a) rough and (b) flat MgO substrates. Black circles represent DEZn or H2O species.

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Fig. 2c indicates the glancing angle XRD pattern of the ZnOcoated sample with the contribution from the substrate minimized. Since we did not observe XRD peaks in Fig. 2c, we believe that the ZnO deposits on MgO to be amorphous. This result stands in contrast with the result from Fig. 1d, in which we found MgO to be oriented, as observed in the sharp XRD peaks. Fig. 2d shows the corresponding conventional XRD pattern. The spectrum exhibits the (200), (220), (311), and (222) peaks of MgO from the underlying film. Close examination of the diffractogram reveals that the lines observed at 31.7°, 34.4°, 36.2°, 47.5°, and 56.6° correspond to the (100), (002), (101), (102), and (110) peaks of hexagonal ZnO (JCPDS 05-0664). The existence of ZnO-related peaks confirms the production of ZnO deposits. From Fig. 2c and d, we suggest that although the ZnO rod is mainly amorphous, the ZnO crystallites may have been formed locally near the substrate surface in the ZnO-coated samples.

Fig. 5. PL spectrum of MgO films (a) without and (b) with the ZnO coating.


light emissions, originating from the defects in MgO. The ZnOdeposited MgO films showed similar PL spectrum to the MgO films with the relative intensity of blue-green emission being increased by ZnO coating. Further TEM work is ongoing to investigate ZnO rods/MgO films' structures in detail.

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amorphous 1D structures start to grow, the turbulent flow between the 1D structures results in adsorption of fresh species mainly on the rod tips. Since more favorable sites may exist on the rod tip, the growth rate is higher on the rod tips than on the sidewalls. Also, we reveal that the average thickness of the ZnO-coated film between the rods and the substrate (Fig. 2a; about 2.4 μm) is almost equal to the thickness of the as-grown MgO films (Fig. 1a), indicating that the ZnO layer conformally coating the MgO films surface is quite thin, though the layer can be close to crystalline from the XRD analysis (Fig. 2d). Fig. 5a and b show the room temperature PL spectra of the MgO films without and with the subsequent ZnO coating, respectively. The PL spectrum of the uncoated product is mainly located in the visible region. After multi-peak Gaussian fitting to two major peaks of the PL spectrum, we found that the Gaussian curves fit the original curves almost perfectly. Therefore, the PL spectrum mainly consists of two bands, which peaks at approximately 465 nm in blue region and 531 nm in bluegreen region, respectively. The similar blue [18–20] and bluegreen emission [21] have been previously observed from the MgO nanostructures. Both blue and blue-green light emissions may originate from the defects in MgO, such as oxygen vacancy [22], Mg vacancies, and interstitials, presumably being generated during the high-temperature evaporation process. We found that the shape of normalized PL spectrum was changed by the ZnO coating. Being similar to the spectrum of the uncoated MgO films, the Gaussian fitting analysis indicated that the broad emission band was a superimposition of two major peaks at 465 and 528 nm, respectively. The blue-green emission at about 528 nm in case of ZnO-coated MgO films may originate from MgO and/or ZnO [23–25], being known to be related to the defects such as oxygen vacancies in ZnO [23,24] as well as above-mentioned MgO-related defects, whereas the blue emission at 465 nm should come from the underlying MgO films. Since the relative intensity of the bluegreen peak was increased by the ZnO doping, we suggest that the increased intensity of the blue-green peak is attributed to the ZnO rods. Additionally, close examination (upper left inset in Fig. 5b) reveals that there exists a weak UV emission band around the wavelength of 365 nm, presumably resulting from the emission mechanism associated with excitons in ZnO rods [26–28]. Several researchers have previously reported the UV emission from ZnO regardless of its crystallinity [29–31]. This result will contribute to the potential applications of various kinds of heterostructures to optoelectronic devices. 4. Conclusions

We have demonstrated the first deposition of MgO thin films by using the thermal evaporation technique. XRD and EDX analyses indicate that the product is polycrystalline with a cubic MgO structure. The subsequent deposition of ZnO by using an ALD technique generated the ZnO rods on the predeposited MgO films, while no rods are obtained on the flat MgO(001) substrate. We have discussed the possible growth mechanism of ZnO rods, corresponding to the first production of 1D structures by using ALD. MgO films exhibited both blue and blue-green

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