Photofacilitated Controllable Growth of ZnO Films Using Photoassisted

Junyan Jiang , Yuantao Zhang , Chen Chi , Yan Long , Xu Han , Bin Wu , Baolin Zhang ... Ni Wu , Chengwu Shi , Nannan Li , Yanqing Wang , Mingqian Li...
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Photofacilitated Controllable Growth of ZnO Films Using Photoassisted Metal Organic Chemical Vapor Deposition Zhifeng Shi,† Bin Wu,† Xupu Cai,† Xiaochuan Xia,‡ Shikai Zhang,† Wei Yin,† Hui Wang,† Jin Wang,† Xin Dong,† Yuantao Zhang,*,† Baolin Zhang,† and Guotong Du†,‡ †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China ‡ School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116023, China S Supporting Information *

ABSTRACT: High-quality ZnO thin films were deposited on sapphire substrates using a photoassisted metal−organic chemical vapor deposition (PA-MOCVD) system. A controllable morphology evolution with varying degrees of crystallinity was observed. The microstructure of the film changes from a three-dimensional nanorod form to two-dimensional continuous dense form as the light irradiation intensity increases. A possible photofacilitated nucleation mechanism with a higher nucleation rate and density was proposed to explain the variation of the resulting ZnO formation. In this case, the crystallinity and optical properties of the epitaxial ZnO were also optimized, such that a low full width at half-maximum (0.079°) of the rocking curve could be obtained, similar to that of single-crystal ZnO. It was also found that the tensile strain in the films could be availably relaxed by added thermal energy and energetic photons provided by light irradiation. Additionally, temperature-dependent photoluminescence results behaving as strong exciton effects confirmed the relatively excellent quality of the obtained ZnO films.

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directional.15,16 As a whole, optimizing growth parameters in an attempt to obtain well-oriented dense ZnO thin films still requires systematic investigation in the progression toward efficient ZnO-based nanodevices. For ZnO films growth, numerous deposition techniques including sputtering, pulsed laser deposition, MOCVD, and molecular beam epitaxy have been frequently employed.5,8,12,13 Among them, MOCVD provides the advantages of growing high-quality ZnO films due to its versatility in controlling the various thermodynamic interactions. Besides, it offers a high growth rate, large area uniformity, and different doping processes. In this article, we have studied the effects of light irradiation on epi-ZnO films produced by a PA-MOCVD system. The advantages of light irradiation were utilized to raise the substrate temperature by the absorption of light, especially the infrared photons, and simultaneously promote the thermal dissociation efficiency of the source precursor in the substrate surface with a positive role of energetic photons.12,17 As expected, a previously unreported phenomenon of controllable morphological evolution was observed, the microstructure of the film changed from a nanorod form to a continuous dense form with enhancement of the light irradiation intensity. It is demonstrated that the crystallinity and optical properties of the

INTRODUCTION Recently, substantive research efforts have been made on the short-wavelength exciton-based optoelectronic device of ZnO emitters because of its wide bandgap (3.37 eV) and larger exciton binding energy of 60 meV,1,2 much higher than that of other semiconductor materials such as GaN (25 meV) and ZnSe (22 meV), as well as a high optical gain of 300 cm−1 (100 cm −1 for GaN).3,4 However, the far-ranging practical applications on ZnO-based functional devices, such as ultraviolet light-emitting diodes (UV-LEDs),5,6 laser diodes (LDs),7,8 photodetectors,9 and chemical sensors,10 have been delayed by the difficulties in acquiring high-quality ZnO films. This issue is still not resolved effectively up to now although numerous efforts have been paid, which seriously defers ZnO's practical applications in the optoelectronic field. Although the quality of ZnO films has been greatly improved with the continual renovations of superior thin film epitaxial growth technology, it has been observed that the epitaxial ZnO layer often suffers the formation of three-dimensional (3D) rodlike microstructures instead of well-aligned two-dimensional (2D) continuous thin films in virtue of its inherent c-axis orientation growth feature.11−14 Consequently, many grain boundaries are inevitably observed inside the resulting films, which is unfavorable for the performance of the subsequent ZnObased devices. It can be accepted that the ZnO crystals possess different growth rates and surface energies on different crystal facets; therefore, its growth process is generally highly © 2012 American Chemical Society

Received: May 6, 2012 Revised: July 12, 2012 Published: July 17, 2012 4417

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Figure 1. (a) Schematic illustration of the PA-MOCVD chamber used in this work. (b) Photograph of the chamber under light irradiation. (c and d) The light source using five parallel tungsten−halogen lamps with and without electric drive.

epi-ZnO films also can be tuned to achieve the best values at the same time, with a minimum full width at half-maximum (fwhm) value (0.079°) of the rocking curves and a maximum IUV/Ivis value of the photoluminescence (PL) performance at a high light irradiation intensity (2000 W). Additionally, the formation mechanisms of ZnO nanorods arrays and dense film with entirely different photofacilitation were studied in detail using nucleation theory including nucleation rate and density. It is believed that the promising experimental results obtained here are a significant step toward the realization of large-area epitaxy of single-crystal ZnO films and novel high-efficiency ZnO-based optoelectronic devices.

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(XRD) with Cu Kα radiation was employed to study the crystallinity and crystallographic orientation of ZnO films. The morphology of the films was examined by a field emission scanning electron microscope (FE-SEM), and the PL spectra were obtained at room temperature (RT) and 10 K using a closed-cycle liquid helium cryogenerator. A He−Cd laser with a wavelength of 325 nm and power of 30 mW was utilized as an excitation light source.

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RESULTS AND DISCUSSION The variation of film morphology with light irradiation intensity was examined using FE-SEM. As presented in Figure 2, some distinct trends were revealed. Films deposited without light irradiation show a well-aligned nanorod arrays with high density oriented normal to the substrate (Figure 2a,b), the column width distributes in a narrow range of 35−45 nm. The inset in Figure 2a shows the end planes of the nanorods with smooth hexagons, corresponding to the ZnO (0001) crystal plane. It is observed in Figure 2c,d, as the light irradiation is applied (500 W), that sample B displays a rough surface with many irregular shapes as well as some pores that exist in the film. Note that these resulting pores do not penetrate throughout the entire film; instead, they only extend a certain depth during the growth process (Figure 2d). The reason for the formation of microcracks is primarily the residual stresses within the (0001) planes that resulted from the mismatch in the coefficients of the thermal expansion and lattice parameters between ZnO and Al2O3, which produce in-plane tensile stresses in ZnO.21 The situation is further modified as the input power is increased to 1000 W; it is found that grain consolidation plays a primary role, and a dense film with less grain boundaries inside the layer has been acquired, although there are still many irregular bumps and a low density of microholes on the surface, as seen in the inset of Figure 2e. In the case of a higher light irradiation (2000 W), a mirrorlike surface without any trace of grain boundary is observed with low root-mean-square (rms) surface roughness of about 1.47 nm (not shown here), which is typical

EXPERIMENTAL SECTION

The ZnO films were prepared on (0001)-oriented sapphire substrates by a PA-MOCVD system using diethylzinc (DEZn) and ultrahigh purity oxygen as the precursors. Figure 1a shows a schematic diagram of the reactor, and details about the reactor have been reported elsewhere.18 During the epitaxial growth process, a homemade optical equipment was used as the light source to assist the MOCVD growth, as presented in Figure 1c,d. The input power of the optical system can be up to 7500 W, which is equipped with five parallel tungsten− halogen lamps that are located in the chamber away from the susceptor of about 10 cm. It is worth mentioning that the continuous blackbody radiation spectra of the tungsten−halogen lamp cover a broad band from near UV to the infrared (IR) range, of which the UV cutoff wavelength is ∼200 nm (6.2 eV), far higher than the dissociation energy of DEZn (2.26 eV).19,20 For each experiment, the light irradiation intensity can be controlled flexibly by modulating the input power of lamps, allowing comparison of the layer properties. For the films prepared with different input electrical power (0, 500, 1000, and 2000 W), the samples were denoted as A, B, C, and D. During growth, the operating pressure of the reactor was fixed at 12.78 Torr, and the mol flux ratio of Zn to O was kept at 1:600, one of the optimum growth conditions developed in our previous work. To study the effects of light irradiation on the properties of the prepared ZnO films, a Bruker D8 high-resolution X-ray diffractometer 4418

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Figure 2. Top view and cross-sectional SEM images of ZnO films prepared with different lamps input power of (a, b) 0, (c, d) 500, (e, f) 1000, and (g, h) 2000 W.

in the case of heteroepitaxial ZnO single-crystal films. The corresponding cross-section image (Figure 2h) indicates that the prepared films are dense enough, and the ZnO is formed in terms of a layer-by-layer growth mode. Apparently, the striking changes of the morphology for the resulting ZnO films should be because of the increased light irradiation intensity. However, as deduced from the foregone research on electron beam or laser-assisted semiconductor growth, one possibility of increased nucleation rate and density induced by the photodissociation action ought to bear mostly liability in controllable ZnO morphology evolution. Figure 3 is a schematic illustration demonstrating the formation mechanism of ZnO nanorod arrays and dense films with entirely different photofacilitation. As shown (Figure 3a,b with and without light irradiation), the nucleation rate and

corresponding nucleation number density of ZnO at the initial stage increase significantly in the case of an appropriate light irradiation intensity, which is a reasonable assumption considering their dissimilar nucleation mechanism. For the growth without light irradiation, we therefore suggest that an uniformly distributed crystal nucleus with a low distribution density results in the formation of vertically aligned ZnO nanorods. Herein, a low distribution density of the crystal nucleus means that spacing exists between neighboring nuclei, as presented in Figure 3a. Thus, the subsequent main film growth is largely based on the grain enhancement of individual ZnO nanorods developed from the already existed nuclei. Consequently, a 3D growth mode with well-aligned nanorods arrays could be expected with the increase of film thickness. As an appropriate light irradiation is applied, as shown in Figure 4419

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Figure 3. Schematic illustration of the formation mechanism of (a) ZnO nanorods without light irradiation and (b) dense films with light irradiation.

3b, a densely packed ZnO film with a 2D layer-by-layer growth mode can be achieved. Theoretically, the nucleation rate of ZnO at the initial stage is promoted because of the thermal and photostimulation of such chemical and physical related nucleation mechanisms. Accordingly, the nucleus number density also increases, implying the reduced spacing between adjacent nuclei. Thus, a coalescence process for the neighboring grains developed from such high-density nuclei may take place. In this case, the facet growth of ZnO proceeds simultaneously along with the c-axis growth, as described in Figure 3b. The continuous dense ZnO film form without any trace of grain boundary is acquired finally. As well-known, the key for the explanation of the distinct growth model lies in their internal different nucleus density. Here, a schematic diagram of photodissociation of MO molecules in the adsorption phase, shown in the upper-right corner of Figure 3b, is demonstrated for further understanding of the increased nucleus density under light irradiation, where photofacilitated growth with photon energies greater than the dissociation energy of DEZn could result in a promotion of the nucleus processing with higher activation energies and diffusion ability. As expected, energetic photons adequately excite DEZn molecules from the ground state to the excited electronic state, adsorbed ethyl radicals are desorbed in degree, and the dissociated zinc atoms adsorb to the substrate and further incorporate into the crystalline lattice.22 Most importantly, surface migration of adatoms is enhanced on the sapphire substrate in virtue of an increased thermal energy induced by photostimulation, leading to an increased nucleation rate at more regions. Thus, a high nucleus density can be achieved naturally. X-ray diffraction measurements were performed to examine the variation of the structural properties of the ZnO films grown under various light irradiation intensity. As shown in Figure 4 a, all of the samples displayed two diffraction peaks corresponding to ZnO (002) and (004) planes only, and no other peaks were detected except for the phases from the Al2O3 substrate. This observation indicates a pure hexagonal wurtzite ZnO phase with a preferred c-axis orientation for four ZnO samples. As seen in the magnified image of ZnO (002) peaks, an ever-reduced fwhm value and continuously increased diffraction peak intensity were observed as the light irradiation intensity increased gradually. These results suggest that the crystallinity of ZnO films could be improved by light irradiation

Figure 4. (a) XRD pattern of ZnO films grown at the lamps input power ranging from 0 to 2000 W. Note the shift in position to higher angles with decreasing light irradiation intensity. (b) (002) ω-rocking curves of ZnO films grown with various light irradiation intensity.

due to photogenerated electron/hole pairs that are able to assist in the desorption of the adsorbed ethyl radicals and the zinc atoms could incorporate into the crystalline lattice. More specifically, the energetic photon is conducive to atomic rearrangement into their required crystallographic positions.12,23 The apparent evolution of the crystal quality suggests that the crystallographic mosaicity of the ZnO films can be 4420

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known, D is also an important parameter for characterization of material quality; the higher the D value, the better the ZnO crystal quality will be. The different angular dependencies of the domain size and the inhomogeneous strain allow them to be separately determined from the measurements of ZnO (002) and (004) planes. Specific calculated values for D and εin are presented in Table 1, and the gradually increased average dominant size and ever-reduced inhomogeneous strain demonstrate that the light irradiation process can promote ZnO material quality. The variation in optical properties for ZnO films grown with different light irradiation intensity was investigated using PL measurements. Here, all of the positions of the emission peak have been calibrated by the laser line (He−Cd laser of 325 nm). As displayed in Figure 5, four ZnO samples presented

significantly influenced by the growth parameters. At low light irradiation, the thermal energy for adatoms to migrate on the sapphire substrate surface and crystallize may not be enough. As a result, the crystalline grains can stabilize with their orientations slightly tilted from the substrate, leading to poor structural characteristics. In the case of a high light irradiation level, the added thermal energy and energetic photons together play a positive role, the surface migration of adatoms is enhanced to nucleate into thermodynamically stable crystalline, and thus, the crystalline microstructures of ZnO films are improved. From the inset of Figure 4a, a phenomenon of the shift in (002) peak position to lower angles as a function of light irradiation intensity was observed. On the basis of the Bragg formula, the calculated lattice constants (c) were 0.51810, 0.51906, 0.52005, and 0.52034 nm, respectively. As compared to the value of 0.52069 nm for bulk ZnO,24 our prepared samples share a slightly small c, which manifests themselves by the existence of tensile stress in the films, especially in the films deposited with low light irradiation intensity. The above analyses can be confirmed by the following equation: ε = (c − cbulk)/cbulk, where ε is strain and cbulk is the unstrained lattice constant measured from bulk ZnO. The corresponding calculated results are presented in Table 1, from which the Table 1. Variation of XRD Patterns (2θ), c, Strain (ε and εin), and Crystallite Size (D) as a Function of Light Irradiation Intensity for ZnO Films 2θ sample no.

(002) (°)

(004) (°)

c (nm)

ε (×10−3)

εin (×10−3)

D (nm)

A B C D

34.591 34.527 34.450 34.432

72.74 72.67 72.62 72.58

0.5181 0.5191 0.5201 0.5203

4.974 3.130 1.229 0.672

1.50 0.53 0.31 0.29

89.37 95.63 110.3 232.1

Figure 5. PL spectra of ZnO films deposited at different light irradiations. The inset shows the intensity ratio of IUV/Ivis and the fwhm of the UV peak vs sample number.

excellent optical quality with dominant near-band-edge (NBE) emission around 378 nm as well as rather weak defect-related deep level emission (DLE) in the visible range. As seen, the emission intensity of DLE decreases with increasing light irradiation, while the opposite is the case for the NBE emission. The diminishing DLE means a decrease of the grain boundary in ZnO, where impurities and/or defects have a preferential accumulation according to the result of Grovenor et al.26 One way to evaluate the concentration of the structural defects in ZnO was to compare the PL intensity ratio of the UV emission to visible emission. As shown in the inset of Figure 5, large ratios were obtained as enhancing input power of lamps to a high level. Besides, the fwhm of UV emission of ZnO samples was also plotted in the inset for a better observation. A diminishing value of fwhm versus sample number also indicates a high UV emission efficiency for the ZnO films with higher light irradiation intensity. The changes in the fwhm and IUV/Ivis values for four samples are well consistent with the results of XRD performance. It should be noted that the defect density in ZnO films could be reduced but not completely eliminated despite the facilitation of photoassistance. One possible explanation is that oxygen adsorption and desorption exist simultaneously during the high-temperature growth process, and it is likely that the adsorbed oxygen is prone to desorb as the photon energies produced by the photoirradiation are larger than the band gap of the epilayers.12 In this course, much more oxygen defects are generated, which are responsible for the

ZnO sample with the highest light irradiation intensity possesses the smallest stress due to its slightly bigger c value. Consequently, our results approve that the tensile stress in the films could be availably released by regulating light irradiation in a suitable range. Commonly, to clarify the crystallinity of ZnO films, ωrocking curve measurements are also performed to quantify the crystallinity using the fwhm values of rocking curves. As displayed in Figure 4b and also summarized in Table 1, a gradual reduction trend is demonstrated with enhancing the light irradiation intensity. When the input power of the tungsten−halogen lamps increases to 2000 W, the fwhm of ZnO (002) diffraction peak is as small as 0.079° (284 arcsecond), indicating excellent ordering along the growth direction. For the samples prepared at low light irradiation, larger fwhm values are observed, as we expected. In general, the broadening effect of the Bragg peak in a ω scan can be investigated by the following equation:25 ⎧ λ + εin tan θ002 ⎪ β002 = 2D cos θ002 ⎪ ⎨ λ ⎪ ⎪ β004 = 2D cos θ + εin tan θ004 ⎩ 004

(1)

Here, β is the fwhm of the diffraction peak, D is the finite domain size, and εin is the inhomogeneous strain. As well4421

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DLE performance in the PL. This explanation may be one of the possibilities for the DLE in photoinduced samples. In addition, another interesting phenomenon was observed, as shown in Figure 6; the UV peak position shifted to a smaller

Figure 6. UV peak position shift for the samples at different light irradiation intensities.

photon energy with the increase of light irradiation intensity. In the case of the sample A, the photon energy of UV luminescence is 3.29 eV (376.5 nm), and it gradually reduces to 3.26 eV (380.5 nm) as the input power reaches 2000 W. It is well-known that the shift of the peak position is closely related to the structural property of ZnO.27 It is because the as-grown ZnO thin films have a tensile built-in strain, which has already been confirmed by the XRD results, and the tensile strain in the films could be availably relaxed by added thermal energy and energetic photon provided by the light irradiation. If the tensile strain is relaxed, the band energy of ZnO is decreased consequently. Temperature-dependent PL spectra were also employed from 10 (LT) to 300 K (RT) to inspect the optical transition mechanism and performance of the ZnO films prepared by photoirradiation. Figure 7a showed the PL spectra of sample D (2000 W) measured at 10 K, strong NBE emission including several exciton-related peaks was observed, and the DLE relating to structural defects was almost negligible. Both rather weak DLE and the narrow and sharp NBE emission are signatures of excellent optical properties of photoinduced ZnO films with a low defect density. As shown in Figure 7b, a plot containing a series of PL spectra was presented. The dominant peak at 3.361 eV and the shoulder on the high-energy side, 3.378 eV, are generally assigned to neutral donor-bound exciton (D0X) and the free-exciton (FX) transition. As compared with the peak position of D0X (3.364 eV) from a high-quality singlecrystal line ZnO wafer,28 a slight difference with ∼3 meV redshift is presumably due to the tensile strain induced from the lattice mismatches between the ZnO films and the Al2O3 substrate, well consistent with the XRD results. Additionally, a distinctive feature of the PL spectra is the two sets of equally distanced peaks. According to their energy values and the separation, the significant exciton features of NBE emission are related to D0X, FX, and their longitudinal optical (LO) photon replicas. In our case here, two peaks at 3.290 and 3.218 eV are observed at the low-energy side, which can be attributed to the LO-photon replicas of D0X. Meanwhile, the FX-nLO replicas for n = 1, 2, 3, 4, and 5 are also observed at 3.310, 3.239, 3.168, 3.096, and 3.025 eV, respectively. The clear observations of FX

Figure 7. (a) PL spectra measured at 10 K with a dominant NBE emission. (b) The evolution of PL spectra for sample D measured at different temperatures (from 10 to 300 K).

peak and its LO-photon replicas (up to five) further indicate that the photoinduced ZnO films are of high optical quality. To our knowledge, it is the first report with the maximum LOphoton replicas of FX in ZnO films. The mechanism responsible for the temperature quenching of the luminescence is important for the determination of the performance of the perfection and the quality of the ZnO films. As shown in Figure 7b, from 10 to 300 K, an obvious temperature-dependent redshift of D0X and FX was observed due to energy gap shrinkage. As the temperature increases, the intensity of FX emission peak increases accordingly, whereas the intensity of bound exciton peak gradually gets weaker and broadened and finally merges into the low-energy tail of the FX peak at temperatures above 100 K. The opposite trend can be ascribed to the thermal occupation of the free exciton states at the cost of bound exciton states at high temperature for its smaller binding energy of D0X.29 The integrated PL intensity for D0X as an inverse function of temperature is shown in Figure 8a. The nearly exponential increase of PL intensity is assigned to thermal ionization of excitons and thermally activated nonradiative recombination mechanisms. This overall temperature quenching behavior can be described by the equation:30 I0 I (T ) = 1 + A exp( −Ea /KT ) (2) where I0 is the emission intensity at 0 K, A is a proportional constant, and Ea is the donor bonding energy of the thermal quenching process. The best fit, as shown in Figure 8a, gives the value of Ea as 16.4 meV. Therefore, the Ea of D0X is in good agreement with the exciton-to-defect binding energy since it has been reported that the localization energy of the exciton to the defect-pair complexes ranges from 10 to 20 meV.31 Figure 8b showed the peak energies of FX and D0X versus temperature, and the PL peak energies displayed monotonic 4422

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Figure 8. (a) Integrated PL intensity of the D0X as a function of inverse temperature. (b) PL peak energy of FX and D0X vs temperature. The inset shows the energy separation between FX and D0X (FX − D0X).

redshift with increasing temperature. Varshni's empirical formula can be fitted with the data to find the temperature dependence of the free and bound energies. Therefore, the formula was used E(T ) = E(0) −

αT 2 (β + T )

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CONCLUSION

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ASSOCIATED CONTENT

In conclusion, a controllable process route using photoassistance for growing dense ZnO single-crystal thin films has been developed. The light irradiation facilitates the initial nucleation and promotes the subsequent growth of ZnO layer, thus resulting in the formation of densely packed ZnO film with relatively high crystalline quality. A possible photofacilitated nucleation mechanism with higher nucleation rate and density was proposed to explain the variation of the resulting ZnO formation. In addition, temperature-dependent PL results behaving with strong exciton effects show that the resulting ZnO thin films grown with light irradiation were indeed of high crystalline and optical quality with low defect density. This research provides valuable information for the fabrication of single-crystal ZnO films that can be employed in reliable ZnObased optoelectronic devices, such as UV-LEDs and LDs.

(3)

where E(0) is the band gap at an absolute temperature T = 0 K and α and β are the Varshni thermal coefficients. Given the binding energies of the FX and D0X in ZnO being nearly independent of temperature, the constant parameters E(0), α, and β are found to be 3.385 eV, 4.3 meV/K, and 4210 K for FX. As for D0X, the best-fitted values are 3.3683 eV, 6.4 meV/K, and 3912 K, respectively. These values are comparable to the reported results in the literature for ZnO.32 Note that the peak positions of FX and D0X exhibit the same trends, which generally decrease with increasing temperature. However, the inclinations of the FX and D0X lines are completely independent, which acknowledges the contribution of the origin of different peaks. On the basis of the careful analysis of peak positions, the energy separation between FX and D0X (FX-D0X) is calibrated and plotted in the inset of Figure 8b. It should be stated that an approximately regular change with increasing temperature for the value of FX-D0X is observed, which gradually decreases as the temperature continues increasing. On the basis of the results and the works of Reynolds et al. and Ashrafi et al.,33,34 the reduction of energy separation (FX-D0X) can be attributed to the various unintentional donors and/or donorlike defects. We can infer that, with increasing temperature, the thermal dissociation of the bound excitons with the activation energy of about 16.4 meV results in the creation of free excitons and a neutral donorlike defect pair complex, which contributes to the forming of shallower donor-bound excitons states with comparatively less donor bonding energy. Consequently, the filling of such donor-related energy levels gives rise to the decrease of the value of FX-D0X.

S Supporting Information *

AFM image of the ZnO sample with a light irradiation intensity of 2000 W and XRD-Φ scans of ZnO(10-11) and substrate Al2O3(11-23) of sample D. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was supported by the “973” program (2011CB302005), the National Natural Science Foundation of China (Nos. 61006006, 61076045, 61106003, and 60976010), and the Fundamental Research Funds for the Central Universities [dut11rc(3)45]. 4423

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REFERENCES

(1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (2) Wong, E. M.; Searson, P. C. Appl. Phys. Lett. 1999, 74, 2939. (3) Guo, Z.; Zhao, D. X.; Shen, D. Z.; Fang, F.; Zhang, J. Y.; Li, B. H. Cryst. Growth Des. 2007, 7, 2294. (4) Masuda, Y.; Kato, K. Cryst. Growth Des. 2008, 8, 275. (5) Zhu, H.; Shan, C. X.; Li, B. H.; Zhang, J. Y.; Yao, B.; Zhang, Z. Z.; Zhao, D. X.; Shen, D. Z.; Fan, X. W. J. Phys. Chem. C 2009, 113, 2980. (6) Shi, Z. F.; Xia, X. C.; Yin, W.; Zhang, S. K.; Wang, H.; Wang, J.; Zhao, L.; Dong, X.; Zhang, B. L.; Du, G. T. Appl. Phys. Lett. 2012, 100, 101112. (7) Leong, E. S. P.; Yu, S. F. Adv. Mater. 2006, 18, 1685. (8) Zhang, Y.; Russo, R. E.; Mao, S. S. Appl. Phys. Lett. 2005, 87, 043106. (9) Park, W. I.; Jun, Y. H.; Jung, S. W.; Yi, G. C. Appl. Phys. Lett. 2003, 82, 964. (10) Fan, Z. Y.; Wang, D. W.; Chang, P. C.; Tseng, W. Y.; Lu, J. G. Appl. Phys. Lett. 2004, 85, 5923. (11) Lee, S. D.; Kim, Y. S.; Yi, M. S.; Choi, J. Y.; Kim, S. W. J. Phys. Chem. C 2009, 113, 8954. (12) Li, X. P.; Zhang, B. L.; Zhu, H. C.; Dong, X.; Xia, X. C.; Cui, Y. G.; Huang., K. K.; Du, G. T. Appl. Surf. Sci. 2008, 254, 2081. (13) Nicolay, S.; Fay, S.; Ballif, C. Cryst. Growth Des. 2009, 9, 4957. (14) Sun, Y.; Fox, N. A.; Fuge, G. M.; Ashfold, M. N. R. J. Phys. Chem. C 2010, 114, 21338. (15) Wang, D.; Song, C. J. Phys. Chem. B 2005, 109, 12697. (16) Li, W. J.; Shi, E. W.; Zhong, W. Z.; Yin, Z. W. J. Cryst. Growth 1999, 203, 186. (17) Zhong, Q.; Chou, P. C.; Li, Q. L.; Taraldsen, G. S.; Ignatiev, A. Phys. C 1995, 246, 288. (18) Du, G. T.; Ma, Y.; Zhang, Y. T.; Yang, T. P. Appl. Phys. Lett. 2005, 87, 213103. (19) Rabek, J. F. Experimental Methods in Photochemistry and Photophysics; John Wiley and Sons: New York, 1982. (20) Yatsui, T.; Kawazoe, T.; Ueda, M.; Yamamoto, Y.; Kourogi, M.; Ohtsu, M. Appl. Phys. Lett. 2002, 81, 19. (21) Liu, K. P.; Yen, K. Y.; Lin, P. Y.; Gong, Y. R.; Wu, K. D.; Chen, W. L. J. Vac. Sci. Technol., A 2011, 29, 03A101. (22) Ho, E.; Coronodo, C. A.; Kolodziejski, L. A. J. Electron. Mater. 1993, 22, 473. (23) Somgj, R. J. Appl. Phys. 1998, 63, R59. (24) Pearton, S. J.; Norton, D. P.; Ip, K.; Heo, Y. W.; Steiner, T. Prog. Mater. Sci. 2005, 50, 293. (25) Lei, T.; Ludwig, K. F.; Moustakas, T. D. J. Appl. Phys. 1993, 74, 4430. (26) Grovenor, C. R. M. J. Phys. C: Solid State Phys. 1985, 18, 4079. (27) Ohtomo, A.; Tamura, K.; Saikusa, K.; Takahashi, K.; Makino, T.; Segawa, Y.; Koinuma, H.; Kawasaki, M. Appl. Phys. Lett. 1999, 75, 2635. (28) Teke, A.; Ö zgür, U.; Doğan, S.; Gu, X.; Morkoç, H.; Nemeth, B.; Nause, J.; Everitt, H. O. Phys. Rev. B 2004, 70, 195207. (29) Pan, X. H.; Guo, W.; Ye, Z. Z.; Liu, B.; Che, Y.; Tian, W.; Schlom, D. G.; Pan, X. Q. Appl. Phys. Lett. 2009, 95, 152105. (30) Jiang, D. S.; Jung, H.; Ploog, K. J. Appl. Phys. 1988, 64, 1371. (31) Hamby, D. W.; Lucca, D. A.; Klopfstein, M. J.; Cantwell, G. J. Appl. Phys. 2003, 93, 3214. (32) Li, C.; Fang, G. J.; Liu, N. S.; Li, J.; Liao, L.; Su, F. H.; Li, G. H.; Wu, X. G.; Zhao, X. Z. J. Phys. Chem. C 2007, 111, 12566. (33) Reynolds, D. C.; Look, D. C.; Jogai, B.; Cantwell, C. W.; Collins, T. C.; Harsch, W.; Cantwell, G. Phys. Rev. B 1998, 57, 12151. (34) Ashrafi, A. B. M. A.; Binh, N. T.; Zhang, B. P.; Segawa, Y. J. Appl. Phys. 2004, 95, 7739.

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dx.doi.org/10.1021/cg300609f | Cryst. Growth Des. 2012, 12, 4417−4424