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Effect of Seed Layer on Structural Properties of ZnO Nanorod Arrays Grown by Vapor-Phase Transport Chun Li,†,‡ Guojia Fang,*,†,‡ Jun Li,† Lei Ai,† Binzhong Dong,† and Xingzhong Zhao† Key Laboratory of Acoustic and Photonic Materials and DeVices of Ministry of Education, Department of Electronic Science and Technology, School of Physical Science and Technology, Wuhan UniVersity, Wuhan, 430072, P. R. China, and State Key Laboratory of Transducer Technology, Chinese Academy of Sciences, Shanghai, 200050, P. R. China ReceiVed: September 5, 2007; In Final Form: NoVember 7, 2007
By using different types of ZnO thin films as seed layers, we systematically investigated the effects of seed layers on the morphologies and growth behaviors of ZnO nanorods by the vapor-phase transport method. It was found that ZnO nanorods can be grown on a variety of polycrystalline ZnO seed layers. The formation of a network wetting layer facilitates the uniform growth of an aligned ZnO nanorod. However, once a continuous ZnO network layer is formed, the alignment will be independent of seed layer thickness and surface roughness. Our results indicate that a ZnO seed layer with a high c-axis orientation, good crystallinity, and less lattice stress leads to the growth of well-aligned ZnO nanorod arrays with uniform diameters and narrow density distributions.
1. Introduction Zinc oxide (ZnO), which is a wide band gap (3.37 eV) semiconductor material with a large exciton binding energy (60 meV), has multiple properties such as excellent chemical and thermal stability, piezoelectricity, and biocompatibility.1 One-dimensional (1D) ZnO nanostructures are considered to be one of the most important semiconductor nanomaterials for fabricating nanodevices.2 From an application point of view, the device performance will be reinforced if the nanostructures can be shape, size, density, and alignment-controllably fabricated because it is directly related to how the nanostructures interact with each other optically, electronically, and mechanically.3 Therefore, great efforts have been devoted to control the morphology and alignment of 1D ZnO nanostructures. Aligned ZnO nanorods with growth directions perpendicular to the substrate have been demonstrated as ultraviolet nanolasers,4 photonic crystals,5 field emitters,6 and dye-sensitized solar cells.7 Until now, a considerable number of approaches, including vapor-phase transport (VPT),8-10 metal-organic chemical vapor deposition (MOCVD),11-13 electrochemical deposition,14 and hydrothermal deposition techniques,15,16 have been employed to synthesize vertically well-aligned ZnO nanorod arrays. Among them, VPT based on either the vapor-liquid-solid (VLS) or the vapor-solid (VS) process is considered to be the most widely used technique for controllable synthesis of aligned ZnO nanorods because of its fast growth process, simple apparatus requirement, and large-scale production.17 Generally, in the VLS synthesis process, a metal catalyst thin film is used to facilitate the nucleation and oriented growth of aligned ZnO nanorods on substrates with little lattice mismatch such as GaN (1.8%), SiC (3.5%), and sapphire (18.4%, [011h0]ZnO|[112h0]sapphire), and even on a Si substrate that shows a large lattice mismatch of 40.1%.1,18 The length, diameter, and * To whom correspondence should be addressed. Tel: +86 27 87642784. Fax: +86 27 68752569. E-mail:
[email protected]. † Wuhan University. ‡ Chinese Academy of Sciences.
density of ZnO nanorods are usually dominated by the size of catalyst particle and the thickness of the metal catalyst film.3,19 Recently, in order to achieve catalyst-free growth of aligned ZnO nanorods on a Si substrate, a thin ZnO film served as a seed layer that was deposited onto a Si substrate not only to facilitate the nucleation of ZnO nuclei but also to decrease the lattice mismatch between the Si substrate and the ZnO nanorods. The growth process can be explained with the VS mechanism. Compared with the metal catalyst film, the ZnO seed layer has more complex structure characteristics. The growth of ZnO nanorods will be rationally influenced by the crystallinity, orientation, surface roughness, and thickness of the ZnO seed layer. Using MOCVD and sapphire substrate, Kim et al. reported the effects of the ZnO buffer layer thickness on the growth and optical properties of ZnO nanorods,11 and Park et al. reported the modulation of the growth direction and morphology of ZnO nanorods by changing the growth temperature of ZnO buffer layers deposited previously.12 Song et al. reported the effect of seed layer on the growth rate, diameter, and density of ZnO nanorods grown via aqueous solution routes.16 Zhao et al. reported the different nucleation mechanisms in the VPT growth process of vertically aligned ZnO nanorods on Si by comparing the Au catalyst with a ZnO thin buffer layer.20 However, no systematical studies on the effect of the ZnO seed layer on the growth of ZnO nanorods by the VPT method have been reported. Previously, using a highly c-oriented pulsed laser deposited ZnO film as a seed layer, we reported the effect of substrate temperature on the synthesis of ZnO nanorods via VPT.21 In this paper, we systematically explore the effect of ZnO seed layer preparation conditions on the growth of ZnO nanorod arrays on a Si substrate. The possible dominating factors that determine the growth rate, diameter, density, and alignment were also discussed. Our investigation on seed layer effect will shed light on the controllable synthesis of ZnO nanorod arrays simply by choosing the proper seed layer.
10.1021/jp077133s CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008
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2. Experimental Methods A. Seed Layer Film Deposition Procedures. Three groups of ZnO films deposited on an n-type Si (100) wafer were designed to evaluate the seed layer effect. Group I: ZnO films were prepared by different film deposition techniques including thermal oxidation of metallic Zn film (Type A), direct-current (dc) reactive magnetron sputtering deposition (Type B), radio frequency (rf) magnetron sputtering deposition (Type C), and pulsed laser deposition (PLD, Type D). The metallic Zn film was deposited by dc sputtering deposition using metallic Zn (99.99%) as the target and high purity Ar (99.999%) gas as the sputtering gas. Then the Zn film was annealed at 600 °C under atmospheric oxygen pressures for 1 h. The dc reactively sputtered ZnO film was deposited at a substrate temperature of 300 °C in an Ar/O2 gas mixture with a dc power of 150 W. The total pressure during sputtering was 5 Pa, and a 20% oxygen composition was used. Films (Type D) were also deposited by PLD with the purity of ZnO ceramic target, laser wavelength, laser frequency, and oxygen pressure of 99.99%, 248 nm, 5 Hz, and 0.02 Pa, respectively. The details of film deposition conditions are summarized in the Supporting Information Table S1. Group II: For comparison of the crystallinity effect, ZnO films were deposited by PLD at substrate temperatures of 100, 300, 500, and 700 °C with thicknesses of 210, 240, 200, and 180 nm, respectively. Group III: For further comparison of the seed layer thickness effect, ZnO films with thicknesses of 5, 20, 90, 200, and 260 nm were deposited by PLD at a substrate temperature of 500 °C and the film thickness was controlled by different laser irradiation durations. B. Nanorods Growth Procedures. ZnO nanorods were fabricated by a simple thermal evaporation and VPT method in a horizontal quartz tube furnace. The details of the nanorod synthesis process have been reported by our group previously.21 Briefly, a powder mixture of ZnO (99.99%) and graphite (99.9%) with a molar ratio of 1:1 was placed in the closed end of a one-end sealed small quartz tube. All of the ZnO-coated Si substrates for the comparison of the seed layer effect were cut into 5 × 3 mm2 size and placed onto a quartz plate (20 × 20 mm2), which has a definite distance away from the evaporation source in the small quartz tube. Then, the small tube was pushed into the large quartz tube with the source positioned at the center of the furnace and the open end toward the gas flow. The furnace was heated at a rate of 25 °C/min under a constant flow of 100 sccm Ar gas and held at 950 °C for 5 min. The local temperature of the quartz plate was about 850 ( 10 °C, and the variation of the quartz plate temperature can be ignored. During whole synthesis process, the pressure in the tube was kept at about 200 Pa. C. Film and Nanorod Characterization Methods. Film thickness was determined by the surface profilometer (Talysurf Series II). Before being used for nanorod growth, the surface morphologies of ZnO buffer layers were observed by atomic force microscopy (AFM, Shimadzu SPM-9500J3). The morphology and crystal structure of ZnO nanorods were characterized with field emission scanning electron microscopy (FE-SEM, Philips FEI XL30) and X-ray diffraction spectroscopy (XRD, Bruker D8 Advance), respectively. 3. Results and Discussion A. Effect of Seed Layer Deposited by Different Techniques. Currently, it is technique-difficult to realize the realtime observation of nanorod growth on seed layer. Because of the short time duration of temperature elevating and the fast nanorod growth process, the annealing effect of the seed layer
Figure 1. XRD patterns of seed layers deposited by different film deposition techniques.
can be ignored. Therefore, we can investigate the seed layer effect by evaluating the structure transition from thin film to nanorod. Figure 1 shows the XRD patterns of seed layers deposited by different film deposition techniques. Except for the thermal oxidation of the Zn film, which exhibits a polycrystalline structure with weak crystallinity, the other three types of ZnO seed layers show a strong c-axis-preferred growth orientation. The distinct difference of the (0002) peak positions indicates the existence of residual stress in the ZnO seed layers, which may due to the different thermal expansion coefficients between the ZnO film and the Si substrate induced during different film deposition processes.22 The AFM images of seed layers and top-down and crosssectional view SEM images of nanorods grown on corresponding seed layers are shown in Figure 2. Nanorods can be grown on all types of ZnO seed layers, even on relatively poor crystalline ZnO films thermally oxidized from metallic Zn films with no preferred orientation growth. However, ZnO nanorods grown on different seed layers exhibit obvious differences in growth rate, diameter, density, and alignment. XRD results reveal that all nanorod arrays show a strong c-axis-preferred growth orientation. The full width at half-maximum (FWHM) of (0002) ω-rocking curve can be used to evaluate the alignment degree of the nanorod arrays. The corresponding results are summarized in Table 1. Nanorods grown on the pulsed laser deposited film show the largest density of 12 ( 2 per µm2, best alignment of 1.4°, and lowest growth rate of 80 nm/min (Figure 2d, also see the enlarged SEM images in Figure 4b). Nanorods grown on the dc reactive sputtered seed layer have smaller diameters and larger densities than those of nanorods grown on rf sputtered seed layers (Figure 2b and c). The reason will be explained in the next section. Usually, in the vaporphase nanomaterial synthesize process, the nanorod growth rate is dominated by the local temperature.17,21 However, our results show that the seed layer fabrication methods also have a strong influence on the nanorod growth rate. Because ZnO film deposition methods are diverse and the physical properties are strongly deposition-condition-dependent, in the following two sections we focus on the seed layers deposited by PLD in order to obtain well-aligned nanorod arrays.
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Figure 2. (a-d) AFM images of seed layer film (the left column) and SEM images of as-grown ZnO nanorod arrays on Type A, B, C, D seed layers, respectively.
TABLE 1: Main Experimental Results of Nanorod Arrays Grown on Different Types of ZnO Seed Layers seed layer density FWHM of RC-curve growth rate type (per µm2) of (0002) peak (deg) (nm/min) Type A Type B Type C Type D
6(3 4(2 5(2 12 ( 2
6.2 5.8 10.5 1.4
400 600 400 80
mean diameter (nm) 100 ( 50 80 ( 20 200 ( 50 100 ( 10
B. Effect of Seed Layer Deposition Temperature. It is known that the deposition temperature of the substrate is a key factor that affects the crystallization of ZnO films. To investigate the effect of ZnO seed layer crystallinity on nanorod growth, we grew ZnO nanorods on ZnO thin films deposited by PLD at different substrate temperatures. All of the seed layers showed a c-axis-preferred growth orientation, as shown by the XRD patterns in Figure 3. The values of FWHM of the (0002) peaks decrease as the substrate temperature increases from 100 to 500 °C. When the substrate temperature increases to 700 °C, the crystallinity of the seed layer decreases due to the decomposition of ZnO at high temperature.22 The AFM images of the seed layer and SEM images of nanorods grown on a corresponding seed layer with a different crystallinity were displayed in Figure 4. The FWHM of ZnO nanorod (0002) ω-rocking curve, nanorod growth rate, and density with different seed layer deposition temperatures are shown in Figure 5. Clearly, the surface roughness of the seed layer and the corresponding nanorod density increase with the increase of seed layer deposition temperature. For a relatively
Figure 3. XRD patterns of pulsed laser deposited ZnO seed layer films at different substrate temperatures.
poor crystallinity seed layer (deposited at 100 °C), the nanorod can only be grown on some randomly distributed active sites where the ZnO grains on the seed layer surface are lattice-
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Figure 4. (a-d) AFM images of seed layer films (the left column) and SEM images of as-grown ZnO nanorod arrays on pulsed laser deposited ZnO seed layer films deposited at substrate temperatures of 100, 300, 500, and 700 °C, respectively. The wetting layer (between the two dotted lines) is indicated by an arrow.
Figure 5. FWHM of ω-rocking curve of thr nanorod (0002) peak, nanorod growth rate, and density with different seed layer deposition temperatures.
matched with the ZnO nucleus, resulting in nanorod sparsity and poor alignment. In addition, the lower seed layer deposition temperature usually leads to large residual stress, which is detrimental to the nanorod epitaxial growth. Therefore, although a seed layer deposited at 100 °C shows a stronger c-axis oritentaion, relatively poor aligned nanorods are exhibited compared with that of Type A, even with Types B and C. For nanorods grown on a seed layer deposited at substrate temperatures from 300 to 500 °C, a continuous ZnO network was observed at the interface between the seed layer and ZnO
nanorods prior to the growth of the nanorods, which has also been found in ZnO nanowires grown on the Al0.5Ga0.5N epitaxial layer in the VLS process.3 Zn/ZnOx particles first form a wetting layer on the ZnO seed layer film, and then the nanorods grow at the junction of combined ZnO grains (ZnO network). Therefore, the nanorods show lower growth rates but better uniformity in length and diameter. This is consistent with the result in the comparison of seed layers deposited by different techniques because a continuous ZnO network cannot be formed on the relatively poor crystalline seed layer due to limited nuclei sites. However, for the nanorods grown on a seed layer deposited at substrate temperature of 700 °C, no obvious continuous network ZnO film was found. In the PLD film deposition process, a high substrate temperature can offer more kinetic energy for higher mobility for particles on the surface, resulting in lower lattice stress caused by lattice mismatch between the seed layer and the Si substrate.23 More nuclei sites with good lattice match between the nuclei and ZnO grains on the surface of the seed layer film were achieved. In addition, the relatively higher surface roughness also limits the migration of the nuclei. Once a Zn/ZnOx particle is nucleant on the seed layer surface, a nanorod will simultaneously grow on it. If the adjacent two nanorods are too close, then they will combine together and a nanorod with a larger diameter will occur. Therefore, the nanorods show faster growth rates and higher densities but relatively poor uniformity of diameter distribution. The better the seed layer crystalline, the higher the obtained nanorod
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Figure 7. FWHM of ω-rocking curve of the nanorod (0002) peak and nanorod density with different seed layer thicknesses.
alignment of ZnO nanorods. Our previous research shows that during the PLD process, for thicker ZnO seed layers, the later grown layer can form a high-quality texture easier on the former ZnO polycrystalline layer; thus, the lattice stress in the crystal grain is reduced.24,25 Meanwhile, the crystal quality of the thin film will be improved. The relatively strong stress in the very thin ZnO film limited the mobility of the ZnO nuclei, and a continuous network layer cannot be formed. Therefore, poorly aligned nanorod arrays with lower densities were obtained. If the continuous network layer is prior to deposit on the seed layer, then the alignment will be independent of the seed layer thickness. 4. Conclusions Figure 6. (a, c, e, and g) AFM images of seed layer films with thicknesses of 5, 20, 90, and 260 nm. Their corresponding ZnO nanorod SEM images are showing in b, d, f, and h, respectively. The insets in d, f, and h show the corresponding cross-sectional view images.
density and degree of alignment. This trend has also been found in rf sputtered ZnO seed layers (see Supporting Information S2 and S3). C. Effect of Seed Layer Thickness. To investigate the effect of seed layer thickness on the density and alignment of the nanorods, we fabricated the seed layers at the same deposition condition except laser irradiation duration. Figure 6 shows the typical SEM images of the ZnO nanorods deposited on seed layers with thicknesses of 5, 20, 90, and 260 nm. (For the 200 nm seed layer, see Figure 4c.) Clearly, the variation of the seed layer thickness results in a significant change in the alignment of ZnO nanorods. With the increase of the seed layer thickness, the nanorod density increases and the degree of vertical alignment becomes better, as shown in the inset of Figure 6. The thinnest seed layer (5 nm) results in no nanorod growth. Vertically well-aligned nanorod arrays were formed on a substrate with a seed layer thickness of 200 nm (Figure 4c). No significant density or alignment changes were found after further increasing the seed layer thickness, as shown in Figures 6h and 7. By using MOCVD, Kim et al. reported that the use of a thick buffer layer resulted in the formation of inclined nanorods because inclined nanorods grew along the c-axis of tilted grains on the rugged faceted surface of the thick buffer layer.10 However, in our case, although the surface roughness increases with the increase of seed layer thickness, a significant alignment improvement for thicker buffer layers was found. This means that surface roughness is not a key factor to influence the
By XRD, AFM, and SEM measurements, it was found that the growth methods, deposition temperature, and thickness of ZnO seed layer have a great effect on the growth of ZnO nanorods in the vapor-phase synthesized process. ZnO nanorods can be grown on a variety of polycrystalline ZnO seed layers. Prior to nanorod growth, the self-assembled growth of a network layer formed on a highly c-axis-oriented seed layer is found favorable for the achievement of high-quality nanorod arrays. The crystal quality of the ZnO seed layer is the dominating factor that influences nanorod growth, and seed layers with thicknesses above 200 nm deposited at 500 °C by PLD give rise to high-quality arrays with narrower diameter distributions, larger nanorod densities, and better alignments. Our results indicate that ZnO seed layers with high c-axis orientations, good crystallinities, less lattice stress, and smoother surfaces lead to well-aligned ZnO nanorod arrays with uniform diameters and narrow density distributions. Acknowledgment. We thank Dr. Sheng Xu for his help in AFM measurement. This research was supported by the Natural Science Foundation of Hubei Province under Grant No. 2006ABA215 and by the Special Fund of Ministry of Education for Doctor’s Conferment Post under Grant No. 20070486015. Supporting Information Available: Deposition conditions, XRD patterns, and SEM images. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O ¨ zgu¨r, U ¨ .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogˇan, S.; Avrutin, V.; Cho, S. J.; Morkoc¸ , H. J. Appl. Phys. 2005, 98, 041301.
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