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J. Phys. Chem. C 2008, 112, 820-824
Luminescent Properties of ZnO Nanorod Arrays Grown on Al:ZnO Buffer Layer Ting Fung Chung, Juan Antonio Zapien, and Shuit-Tong Lee* Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China ReceiVed: August 17, 2007; In Final Form: October 18, 2007
Well-aligned ZnO nanorod arrays were synthesized noncatalytically on a conducting aluminum-doped zinc oxide (AZO) buffer layer. The synthesis uses no metal catalyst and is cost-effective. The buffer layer facilitates the growth of vertically aligned nanorods; furthermore, we demonstrate that the optical properties of the resulting nanorods are closely correlated to the crystallographic quality of the buffer layer. Spatially resolved cathodoluminescence microscopy measurements show that for nonideal buffer layers the optical quality of nanorods improves along the [0001] direction leading to a lower defect incorporation at the nanorod tip. We have minimized defect incorporation by proper adjustment of the buffer layer growth. In addition, the ZnO nanorods grown from our best AZO buffer layer show improved alignment perpendicular to the substrate and a narrower diameter distribution. Our results suggest that the quality of the buffer layer and the growth conditions are key parameters to optimize the nanorods’ optical properties.
1. Introduction One-dimensional ZnO semiconductor nanostructures, such as nanowires (NWs) and nanoribbons (NRs), are promising candidates to realize a variety of applications arising from their unique properties.1 Vertical ZnO NW arrays are readily prepared via various methods including thermal evaporation, solution method, and epitaxy techniques.2-4 ZnO NW arrays have been fabricated into heterojunction light-emitting diodes (LEDs),5 nanogenerators,6 vertical field-effect transistors (FETs),7 and sensors.8 While sapphire is the most frequently used substrate, it is not the best option to prepare ZnO NW devices because it is insulating and expensive. The use of a lattice-matched and conducting buffer layer may circumvent the problem and lead to potential integration with silicon microelectronics.9-11 Indeed, well-aligned ZnO NW arrays have been formed on GaN, AlN, Al1-xGaxN, 6H-SiC, and ZnO buffer layers.12-14 The optical properties of NWs grown on buffer layers have been scarcely investigated, especially with respect to impurity and defect distribution; this can hinder the applications of NW arrays. In this article we employ spatially resolved cathodoluminescence (CL) microscopy to investigate the optical properties of ZnO nanorods on an aluminum-doped zinc oxide (AZO) buffer layer. 2. Experimental Details The ZnO nanorods were grown on AZO by chemical vapor transport and condensation technique. The AZO polycrystalline layer (∼2.8 at. % of Al) was deposited onto a (100) Si wafer at ∼500 °C using radio frequency magnetron sputtering and a 2 wt % Al2O3:ZnO target. The Si substrate was cleaned and degreased prior to film deposition. Then ZnO nanorod arrays were synthesized in a small quartz tube system.15 A mixture of ZnO (99.9%, Aldrich) and graphite powder was heated to 950 °C and kept at this temperature for 15-20 min. The chamber pressure was maintained constant by a continuous flow of pure argon gas (99.995%). A small amount of oxygen gas (99.99%) * To whom correspondence should be addressed. Email: apannale@ cityu.edu.hk.
was admitted after the system reached the target temperature. Three samples were prepared with the conditions listed in Table 1. The morphology and structure of the samples were characterized by field-emission-type scanning electron microscopy (FEGSEM, Philips XL 30), transmission electron microscopy (TEM, Philips CM20, operated at 200 kV), and X-ray diffraction (XRD, Philips X’Pert MRD with Cu KR radiation, 40 kV, 30 mA). CL studies were performed in a FEG-SEM equipped with an Oxford Instruments MonoCL2 spectrometer at variable temperatures between 80 and 300 K. The focused electron beam was scanned over the surface, and the emitted light was collected with a parabolic aluminum mirror and guided to the slit of a grating monochromator with a focal length of 30 cm equipped with a 1200 lines/mm grating blazed at 500 nm. The experimentally determined spectral resolution of this system is ∼3 meV at 510 nm (2.04 eV). A photomultiplier tube (PMT) was employed as a detector. Spectrum line scan, local spot-mode CL spectrum, pan- and monochromatic mappings are available using this instrument. All measurements were conducted at an accelerating voltage of 5 kV and probe current of 98 pA. 3. Results and Discussion A. Morphology and Structural Characterization. Figure 1 shows the SEM images of three ZnO nanorod arrays synthesized using buffer layers of different thicknesses prepared at different temperatures and pressures. The surface morphology of sample A (Figure 1a) is quasialigned nanorod arrays, whereas vertically well-aligned nanorod arrays are observed in the other two samples (parts b and c of Figure 1). Most of the nanorods in these samples are nearly perpendicular to the AZO buffer layer substrate and have a hexagonal cross section. The diameters of the nanorods range from 150 to 700 nm in samples A (Figure 1a) and B (Figure 1b), whereas those from sample C (Figure 1c) fall in a narrower range of 60-350 nm. Figure 1d illustrates the TEM image and selected area electron diffraction (SAED) pattern of a single ZnO nanorod, where clear and bright spots on the SAED pattern indicate the nanorod is single crystalline and grows in the [0001] direction and {10-10} side
10.1021/jp076618d CCC: $40.75 © 2008 American Chemical Society Published on Web 01/03/2008
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TABLE 1: Thickness of the ZnO:Al Buffer Layer and Growth Conditions of the ZnO Nanorod Arrays
sample A sample B sample C
buffer layer thickness (nm)
substrate temperature (°C)
pressure (mbar)
100 800 800
700 700 750
300 300 120
surface. We also notice that the Al content, if any, is below the detection limit of energy dispersive X-ray (EDX) measurements. This suggests that Al dopant in the underlying AZO buffer layer is not involved in the nucleation and growth of ZnO nanorods and that the growth is a catalyst-free process. The θ-2θ XRD data of the as-grown nanorod samples (parts a-c of Figure 2) clearly show the (002) and (004) peaks of ZnO. The measured c-axis lattice parameter of the nanorods is 0.52 nm, which is comparable to that of the ZnO thin film (0.5207 nm).16 Moreover, the spectra contain only intense (002) and (004) peaks, which are attributed to the polar direction of wurtzite-structured ZnO. The sole presence of an intense (002) line in the XRD of sample B and C is in accordance with the highly aligned morphology of the nanorods. In contrast to samples B and C, the weaker (002) signal and the presence of the additional peak of (103) reflection reveal the poorer oriented morphology of sample A. The narrow full widths at halfmaximum (fwhm) of the rocking curve of (002) plane shown in the insets of Figure 2 further confirms the presence of highly aligned nanorod arrays grown in the [0001] direction and a significant improvement in the alignment of the nanorods of sample C. Under identical synthesis conditions, the difference in morphology and XRD spectrum between samples A and B indicates the significant influence of buffer layer quality. Parts d and e of Figure 2 depict the XRD spectra and corresponding rocking curves of two different buffer layers (thicknesses of 100 and 800 nm, respectively), which show that the degree of c-axis preferential growth increases with increasing AZO film thickness. In AZO films deposited on Si, a preferred c-axis orientation arises from the relatively fast growth rate of the (0001) plane.17 As a result, a thicker buffer layer, made of stacking many preferred c-axis grains perpendicular to the substrate, shows a stronger (002) peak in the XRD spectrum and a narrower fwhm (4.31°) in the rocking curve. In contrast, a thin polycrystalline AZO film yields a larger proportion of inclined grains, which gives rise to a weaker (002) peak and a wider fwhm (4.9°) in the XRD spectrum and rocking curve, respectively. Appropriate thickness of buffer layer is essential to attain better-aligned ZnO NW arrays.18 Likewise, the ZnO nanorods show the narrowest diameter distribution (∼180 ( 45 nm) in sample C, whereas the corresponding values for samples A and B are ∼340 ( 139 nm and ∼360 ( 132 nm, respectively. These distributions indicate that the nanorod diameters are strongly dependent on growth conditions, such as temperature and pressure, but only slightly dependent on buffer layer quality. B. CL Properties. To investigate the optical properties of ZnO nanorods, CL measurements were performed taking advantage of the luminescence mapping ability and good sensitivity to defect and impurity light emission. The CL spectra of nanorod samples recorded at room temperature (298 K) were conducted by using an electron probe current of ∼100 pA and an accelerating voltage of 5 kV. All three nanorod samples (Figure 3a) show a strong band-edge emission (UV range) at ∼3.26 eV and a green-band emission of different intensities at ∼2.5 eV, presumably due to oxygen vacancies.19 Sample A, which has the poorest nanorod alignment, shows the strongest
green-band luminescence, with an intensity comparable to that of the band-edge emission. Conversely, weak green-band luminescence is found on sample C, which has well-aligned and the smallest diameter nanorods. Temperature-dependent near band-edge emission spectra of nanorod arrays are shown in parts b-d of Figure 3. The UV luminescence of all three samples exhibits a similar line shape; the assignment of the distinct peaks is illustrated in Figure 4. Neutral donor-bound exciton (D°X), free exciton (FX), and its phonon replica (FX-xLO) transitions dominate at low and high temperatures, respectively. This is in accordance with previous reports on ZnO nanorods.20,21 FX are formed by the dissociation of D°X as the delocalization energy of D°X is lower than thermal energy. While samples A and B exhibit a similar UV line shape evolution (parts b and c of Figure 3), as they are prepared under the same growth conditions, sample C (Figure 3d) shows a slightly different line shape evolution with a much higher intensity ratio of D°X to FX1LO at low temperature. This may be due to the more regular structure and a lower density of defects in sample C, thus providing evidence that the growth conditions used in this case result in additional nanorod quality (given that sample B is also grown on a high quality buffer layer, see Table 1). Since sample C nanorod arrays have the best optical quality, we focus on the distinct peaks of its band-edge emission. Figure 4 depicts the temperature evolution of the ZnO band-edge emission of sample C in the 81-143 K temperature range; the spectra have been vertically displaced for clarity. A series of CL emission features can be clearly observed between 3.103.40 eV; the spectra are similar to the commonly reported PL spectra of ZnO at low temperatures.20 At 80 K, the near bandedge emission is dominated by D°X at 3.356 eV, while the FX can be observed as a high energy shoulder at 3.372 eV. Two additional peaks can be observed on the lower-energy side and separated in almost equal spacing from the FX peak with a spacing close to the theoretical value of 72 meV of the ZnO longitudinal optical (LO) phonon replica energy.22 Therefore, these peaks can be assigned to the first- and second-order LO phonon replicas of the FX peak as depicted in Figure 4. A slight peak energy deviation of the FX-xLO phonon replica is observed when it is compared with the calculated phonon-assisted transition. The peak energy shift of FX-xLO is temperature dependent, and the shift of the line maximum should be from the exciton-phonon coupling mechanism. According to Permogorov’s model, which successfully describes the optical emission due to FX scattering by LO phonons in semiconductors, the line shifts for 1LO and 2LO are 3/2kBT and 1/2kBT, respectively.23 The temperature-dependent peak positions of FX, D°X, and FX-1LO are illustrated in inset. The open triangles are LO-phonon replica plotted with the equation EFX-mLO ) EFX - mpωLO + (5/2 - m)kBT, where EFX-mLO denotes the position of the energy peak, EFX denotes the position of FX emission, pωLO is the ZnO phonon energy, m denotes the number of phonons, and kB and T are the Boltzmann constant and temperature, respectively.22 We observe a good match between experimental and calculated data points. As seen in Figure 4, the main D°X line quenches with increasing temperature, whereas the intensity of FX-1LO transition increases with increasing temperature. The asymmetric peak at room temperature should be considered as a combination of multiple peaks including FX and associated LO phonon replica transitions. Finally, we fit the temperature variation of the FX transition energy of ZnO using the Varshni’s formula,24Eg(T) ) E0(0) RT2/(β + T), where Eg is the band gap energy and R and β are
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Figure 1. SEM image of ZnO nanorod arrays morphology: (a) sample A, (b) sample B, and (c) sample C. The insets illustrate the diameter distribution of 50 nanorods. (d) Representative TEM image of a ZnO nanorod. The insets show the corresponding SAED taken along the [2-1-10] zone axes and EDX spectrum, in which Cu signal arises from a copper grid.
constants. A good agreement with the measured data is observed in the inset of Figure 4. The top view SEM and CL images of the three samples taken at room temperature are shown in Figure 5. Four images are shown for each sample corresponding to the secondary electron (SE), CL panchromatic (Pan), CL-UV (UV), and CL-visible (Vis) images, respectively. For simplicity the images are labeled (sample, img) where “sample” corresponds to one of the preparation conditions described in Table 1 (A, B, or C), and “img” denotes the corresponding imaging mode (SE, Pan, Vis, or UV). Whereas the nanorods sides are distinguishable in sample A due to their poor vertical alignment, regions of samples B and C were purposefully chosen close to an area were the nanorods have been mechanically detached from the substrate in order to reveal their side characteristics. The panchromatic images of the nanorod arrays depicted in Figure 5 (A-C, Pan) show that the strongest variation in the light intensity along the nanorod growth axis is observed in samples A and B. Interestingly, the distribution of bright regions among the samples is different; samples B and C show the bright regions are near the nanorod tip, whereas in sample A the brightest CL intensity is near the root of nanorods (closest to the buffer layer). Further insight into the nature of this behavior is found from the monochromatic images in the UV and Vis regions. Figure 5 (A-C, UV) shows the CL image recorded at a UV wavelength of 380 nm; clearly, the near band-edge UV emission appears strongest at the rod tips and is slightly quenched along the rod with the strongest effect in samples A and B. In comparison, the CL images of the green luminescence ≈ 490 nm in Figure 5 (A-C, Vis) show clearly that the visible emission is the strongest at the root of the nanorod for samples A and B, whereas it is beyond the detection limit for sample C. The intense band-edge emission at the tip position is associated with less defective sites and the formation of exciton polaritons.25 Diminishing defect emission along the nanorod growth direction indicates a nonuniform distribution of defect sites along the nanorod. We suggest that a higher concentration of defect sites found at the bottom of the nanorods may be due to grain boundary and residual strain of the buffer layer. A thicker buffer
Figure 2. θ-2θ XRD data of the ZnO nanorod samples in Figure 1. (a) sample A, (b) sample B, and (c) sample C. The intensity axes of graphs are in a 1:1 ratio. θ-2θ XRD data AZO buffer layer of two different thicknesses: (d) 100 nm (20× magnifications) and (e) 800 nm. The insets show the θ rocking curve of the (002) peak for each sample.
layer provides a substrate of less residual strain for ZnO nanorods to grow on. Furthermore, growth conditions also influence defect concentration; a higher growth temperature facilitates stress release and incorporation of incoming atom thus resulting in the low Vis emission found in sample C. The CL results shown in Figures 6 and 7, obtained from the cross-sectional ZnO nanorod arrays for samples A and C, respectively, further confirm the findings of Figure 5. As shown in Figures 6b and 7b, an intense band-edge emission is usually found at the tip of nanorods; while visible emission intensity increases towards the bottom of the nanorods. The visible emission obtained from sample A (Figure 6c) is clearly stronger than that from sample C (Figure 7c). These results are consistent with the investigations from the top view of the nanorod shown in Figure 5. From the cross-sectional view of the nanorod arrays, the influence on the structural and optical properties by the AZO buffer layer can be revealed. As seen in Figures 6a and 7a, the alignment of nanorod arrays in sample C is better than that in sample A, which has nanorods perpendicular to the substrate surface. It is also clear that the nanorods of sample A exhibit a larger CL variation with many of them presenting poor CL
Luminescent Properties of ZnO Nanorod Arrays
Figure 3. Normalized CL spectra of ZnO nanorod arrays. (a) At room temperature (∼300 K). The letters A, B, and C correspond to samples A, B and C, respectively. The temperature dependence of the CL spectra of (b) sample A, (c) sample B, and (d) sample C in the temperature ranges of 103-253 K.
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Figure 5. CL images from samples A-C. For clarity the images are labeled (sample, img), where “sample” corresponds to one of the preparation conditions described in Table 1 (A, B, or C) and where “img” denotes the imaging mode. There are four different imaging modes, and they are denoted by the following notations: “SE” represents the SEM mode, “Pan” represents the pan-chromatic mode, “UV” and “Vis” represent the monochromatic mode collected at wavelengths of 380 nm and ∼490 nm, respectively.
Figure 4. Normalized CL spectra of sample C at four temperatures from 81 to 143 K. The emission lines involving FX, D°X, and xLO are marked in the graph. The inset shows the temperature dependence of transition energy of FX (dots), D°X (squares), and FX-1LO (triangles) emission from the CL peaks. The solid line is the corresponding fitting curve derived from the empirical Varshni’s formula.24 The open triangles are the fitting points derived from the theoretical phonon replica of FX with thermal shift consideration.22
Figure 6. CL images and selective spectra recorded from the crosssectional view of sample A nanorod arrays. (a) SEM image of the examined area. Monochromatic CL images centered at (b) 3.265 eV (UV emission) and (c) 2.516 eV (Visible emission). (d) Normalized CL spectra taken from several positions on nanorod indicated in part a, including S1, S2, and S3.
efficiency. Taking advantage of the high spatial resolution of CL, local CL spectra at different positions of samples A and C
are obtained as shown in Figures 6d and 7d, respectively. Figure 6d illustrates the normalized CL spectra taken at positions S1, S2, and S3 as labeled in Figure 6a. Spectrum S3 obtained at
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Chung et al. properties of the nanorods are influenced by the properties of the buffer layer, such as thickness, growth orientation, and surface morphology. Band-edge luminescence intensity tends to increase from the root to tip of nanorods, revealing inhomogeneous distribution of defects along the nanorod. Our findings show that a high quality buffer layer and optimal growth conditions are two significant parameters for improving the optical properties of self-assembled ZnO nanorods. Acknowledgment. The work is supported by the Research Grants Council of Hong Kong SAR, China (No. CityU 125/05 and CityU 115905) and the National Basic Research Program of China (No. 2006CB933000). The authors thank Mr. T. F. Hung for assistance with the TEM measurements. References and Notes
Figure 7. CL images and selective spectra recorded from the crosssectional view of sample C nanorod arrays. (a) SEM image of the examined area. Monochromatic CL images centered at (b) 3.265 eV (UV emission) and (c) 2.481 eV (visible emission). (d) Normalized CL spectra taken from positions S1, S2, and S3 indicated in part a.
the bottom of the nanorod exhibits a substantially stronger green luminescence (center at ≈ 2.5 eV). The green emission quenches along the direction of growth. Spectrum S1 collected at the tip of nanorod possesses the strongest band-edge UV emission and the weakest green band emission along the nanorod. Together with the Vis quenching along the nanorod axis, there is a red shift of the band-edge UV emission of ∼30 and 27 meV for samples A and C, respectively. Such red shift could be attributed to the presence of strain near the interface26 or a change in the concentrations of native defects.15 Moreover, charge-compensating defects induced by Al dopant may contribute to the luminescence at the bottom of nanorods because Al atomic diffusion adjacent to the boundary between nanorods and buffer layer is mildly active at high temperatures (∼700 °C, which is the preparation temperature). Spectra collected from sample C (Figure 7d) show the same trend in the band-edge emission but with much lower intensity of defect related luminescence. Inhomogeneous distribution of luminescence along ZnO nanorods indicates that more defect sites are located at the bottom of the nanorods and decreases along the [0001] growth direction. The phenomenon may be due to the influence of the buffer layer, for instance, residual strain, surface roughness, and orientation of preferred c-axis grain. 4. Conclusion We demonstrate catalyst-free growth of well-aligned ZnO nanorod arrays on AZO buffer layers. The alignment and optical
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