Article pubs.acs.org/JPCC
Interplay between Occupation Sites of (Co, Cu) Codopants and Crystal Orientation of ZnO Matrix Shibao Zhang,† Fengchun Hu,† Jingfu He, Weiren Cheng, Qinghua Liu,* Yong Jiang, Zhiyun Pan, Wensheng Yan, Zhihu Sun,* and Shiqiang Wei National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China ABSTRACT: Elemental codoping has been an effective way to modify the structural and electronic properties of semiconductors. By use of X-ray diffraction and X-ray absorption fine structure spectroscopy, we show that the spatial occupations of Co and Cu codopants in ZnO thin films could be regulated by the crystal orientation of the matrix prepared by pulsed laser deposition at different temperatures. At deposition temperatures lower than 300 °C, the ZnO matrix was grown along the [201] preferential orientation, with the substitutional incorporation of Co dopants and formation of metallic Cu nanoclusters. Increasing the growth temperature to 300 °C or higher drives a transform of the ZnO film to the c-axial [002] preferred orientation. Consequently, the Cu atoms aggregate into larger fccstructured Cu nanocrystals that attract part of the Co dopants to precipitate out as metallic clusters. The relation between the growth orientation of ZnO thin films and the occupation positions of Co/Cu codopants is discussed in terms of the temperature-dependent mobility and formation energy of the doping atoms.
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INTRODUCTION Zinc oxide (ZnO)-based semiconductors have been under intensive investigation due to their potential applications in the fields of photoelectronics, blue/UV light luminescence, and biomedical diagnosis.1−4 To maximize the functionality of ZnO, doping of impurity atoms into ZnO was widely used.2,5,6 Through elemental doping, impurity energy states within the energy gap were created and the semiconductor band gap was narrowed, thus extending the usage of ZnO semiconductors toward red shift of photoluminescence, visible-light photoconversion, and spin functional electronics.7−10 For the practical use of these doped semiconductors in photoelectronics and spintronic devices, high carriers density and/ or efficient electron transportation are required, which were subsequently proposed to be achieved by codoping of another donor/acceptor dopant into the single-doped systems.11−16 In spite of the realization of various codoped ZnO systems, the effects of codoping on the atomic and electronic structures of the semiconductor materials are still quite in debate, all of which need in-depth investigations. It has been widely accepted that the microstructures of the codopants are intimately related with the sample growth/posttreatment conditions. By means of Raman spectroscopy, Zhong et al.17 reported that the local structure of Er ions in ZnO nanowire films was influenced by the post heat treatment. Via elevating the annealing temperature from 600 to 1000 K, the Er ions were forced to diffuse toward the surface of ZnO nanowires, forming the ErO6−xNx octahedrons that enhanced the photoluminescence intensity of the red emission at around 660 nm. Chakraborti et al.16 grew (Co, Cu):ZnO thin films on sapphire c-plane by pulsed-laser deposition (PLD) under © 2013 American Chemical Society
different growth temperatures. By using the X-ray diffraction and photoelectron spectroscopy, they found that the films deposited on 500 °C showed segregation of Cu nanoparticles within the ZnO matrix in comparison with the films grown on 600 °C, responsible for the decrease of magnetization of the samples. Whereas, Larde et al.18 have reported that depending on the substrate temperatures, Co- or Cu-related clusters or secondary phases could be formed in ZnO, which strengthened the optical, electronic, and magnetic interactions of ZnO semiconductors.19,20 All of these studies clearly suggest that the atomic occupation and electronic properties of doped ZnO systems are sensitive to the growth temperature. To further effectively govern the structure and performance of ZnO-based semiconductors, it is thus of particular interest to investigate the atomic and electronic interactions of the codopants in ZnO thin films under different growth temperatures. In this work, we have prepared a series of (Co, Cu)-codoped ZnO thin films with the same composition under different substrate temperatures by pulsed-laser deposition method. The X-ray absorption fine structure (XAFS) spectroscopy as a sensitive local structural probe is used to determine the atomic and electronic structures of the (Co, Cu) codopants in ZnO. Together with the X-ray diffraction (XRD) results, it is shown that the Co/Cu atomic structures are strongly dependent on the crystal orientation of the ZnO matrix, which could be driven to transform by the increase of the growth temperature. In combination with the first-principles total energy and Received: September 6, 2013 Revised: October 22, 2013 Published: October 31, 2013 24913
dx.doi.org/10.1021/jp408928q | J. Phys. Chem. C 2013, 117, 24913−24919
The Journal of Physical Chemistry C
Article
Figure 1. (a) XRD patterns, (b) Co K-edge XANES spectra, and (c) Cu K-edge XANES spectra of the Zn0.90Co0.05Cu0.05O thin films grown at various temperatures and Co and Cu foils as a reference. The inset in (a) shows the original XRD patterns.
ZnO thin films as equivalent as possible, first, the same Zn0.90Co0.05Cu0.05O target prepared by a solid-state reaction method was used during the pulsed-laser deposition processes of all samples. Moreover, all the PLD environmental factors except the substrate temperature were fixed in the process of sample deposition. The structural properties of the Co/Cu-codoped ZnO thin films were first characterized by using X-ray diffractometer with Cu Kα (λ = 0.154 nm) in the 2θ range from 20° to 80°. To detect the local environment around the Co and Cu ions, the Co and Cu K-edge XAFS spectra were measured in the fluorescence mode at the U7C beamline in National Synchrotron Radiation Laboratory (NSRL) and at the BL14W1 beamline in Shanghai Synchrotron Radiation Facility (SSRF), China. The storage ring of NSRL was working at the energy of 0.8 GeV and a maximum electron current of 250 mA. The Si(111) double-crystal monochromator was used to monochromize the X-rays, and the fluorescence signal was collected by a 7-element high-purity Ge solid-state detector. The XAFS data were analyzed by the Athena soft-package based on the standard data analyzed procedures.
electronic structure calculations, the mechanism of interactions between the occupation sites of (Co, Cu) codopants and the growth orientation of ZnO thin films is discussed. These results broad our understanding on the modulation of codopants for modifying the atomic and electronic properties of ZnO-based thin films.
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EXPERIMENTS The Zn0.90Co0.05Cu0.05O thin films were fabricated on Si (100) substrates by pulsed laser deposition (PLD) under various substrate temperatures of room temperature (RT), 200, 300, 500, 600, and 650 °C. More detailed procedures were described in our previous work.21 The films were prepared by pulsed laser beam (248 nm wavelength, 5 Hz repetition frequency) with the laser energy density of 150 mJ/pulse. The horizontal distance of the target to the substrate was fixed at 5 cm, and the chamber was evacuated to a base pressure of 1 × 10−4 Pa by a turbomolecular pump. In order to obtain flat and homogeneous surfaces, both the targets and the substrates were rotating in the process of deposition. The deposition time lasted for 1 h, yielding films of about several hundred nanometers. To keep the concentrations of Co/Cu codopants in all the codoped 24914
dx.doi.org/10.1021/jp408928q | J. Phys. Chem. C 2013, 117, 24913−24919
The Journal of Physical Chemistry C
Article
Figure 2. (a) Co K-edge EXAFS oscillation functions k3χ(k) (black lines) and (b) FT spectra of k3χ(k) (black lines) of Zn0.90Co0.05Cu0.05O thin films compared with the simulating results (color lines) and (c) Cu K-edge EXAFS oscillation functions χ(k) and (d) FT spectra of k3χ(k) for Zn0.90Co0.05Cu0.05O thin films deposited at different temperatures of RT, 200, 300, 500, 600, and 650 °C.
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RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of (Co, Cu)-codoped ZnO thin films deposited at various substrate temperatures. To highlight the dopant-related weak peaks, the peak intensity is shown at a logarithmic scale. Strikingly, the XRD patterns show a strong relevance to the substrate temperature, with an orientation transform of ZnO crystals occurring at the deposition temperature of 300 °C. Specifically, for the films deposited at temperatures lower than 300 °C, only a (201) diffraction peak of wurtzite ZnO located at 69°could be observed. Increasing the substrate temperature to 300 °C, there appear two additional peaks corresponding to the (002) and (004) planes of ZnO. Furthermore, it is found that increasing the substrate temperature gradually intensifies these two peaks but weakens the (201) peak. Finally, at a substrate temperature of 650 °C, the (201) peak disappears completely. These results clearly suggest that the codoped ZnO films preferably grew
along the [201] direction at low substrate temperatures but transformed to grow along the c-axial orientation at the substrate temperature Ts ≥ 300 °C. Moreover, for the films deposited at Ts ≥ 300 °C, we also note that a small peak at 43° corresponding to metallic Cu appears,22 but no peak ascribed to any Co- or Cu-related secondary phases is observed in the whole substrate temperature region under investigation. To determine the presence form of Co and Cu dopants, we used a local-structure sensitive probe of XAFS to examine the atomic and electronic structures of the samples. First, the normalized X-ray absorption near-edge structure (XANES) spectra at Co and Cu K-edges for various (Co, Cu)-codoped ZnO thin films are shown in Figures 1b and 1c, respectively. For the Co K-edge XANES spectra, it is clear that there are three characteristic peaks of A, B, and C for all the (Co,Cu):ZnO thin films, of which the features for the postedge peaks B and C are close to that of ZnO. As for the Cu K-edge 24915
dx.doi.org/10.1021/jp408928q | J. Phys. Chem. C 2013, 117, 24913−24919
The Journal of Physical Chemistry C
Article
Table 1. Structure Parameters around Zn Atoms in ZnO and Co Atoms in Zn0.90Co0.05Cu0.05O Thin Films with the Increased Temperatures and Co Foil sample ZnO Ts = RT Ts = 200 °C Ts = 300 °C Ts = 500 °C
Ts = 600 °C
Ts = 650 °C
Co foil
bond type Zn−O Zn−Zn Co−O Co−Zn Co−O Co−Zn Co−O Co−Zn (Zn,Co)O Co−O Co−Zn metallic Co Co−Co (Zn,Co)O Co−O Co−Zn metallic Co Co−Co (Zn,Co)O Co−O Co−Zn metallic Co Co−Co Co−Co
a/b (%)
N
R (Å) ± ± ± ± ± ± ± ±
0.02 0.01 0.02 0.01 0.02 0.01 0.02 0.01
σ2 (Å2) 0.0040 0.0075 0.0062 0.0152 0.0071 0.0154 0.0058 0.0113
± ± ± ± ± ± ± ±
4 12 4 12 4 12 4 12
1.97 3.21 1.96 3.21 1.97 3.23 1.97 3.24
0.0002 0.0005 0.0004 0.0002 0.0006 0.0003 0.0003 0.0005
4 12
1.98 ± 0.02 3.24 ± 0.03
0.0065 ± 0.0003 0.0110 ± 0.0004
12
2.53 ± 0.02
0.0083 ± 0.0007
4 12
1.99 ± 0.02 3.25 ± 0.03
0.0056 ± 0.0007 0.0095 ± 0.0008
12
2.53 ± 0.02
0.0085 ± 0.0007
4 12
2.00 ± 0.02 3.26 ± 0.02
0.0067 ± 0.0005 0.0072 ± 0.0008
12 12
2.54 ± 0.02 2.49 ± 0.02
0.0096 ± 0.0007 0.0065 ± 0.0002
78
22 65
35 39
61 100
local structures of Cu dopants in samples deposited at low and high substrate temperatures. It should be remarked that the presence of Cu and Co clusters could be detected by XAFS, but could not by XRD, possibly due to their too small sizes. To quantitatively clarify the dopant local structures in these codoped thin films, we fitted the Co K-edge EXAFS spectra using the method in ref 26. In order to obtained the proportion of the metallic Co species in total Co dopants, we considered the separate EXAFS functions contribution of the substitutional Co (χsubstitution) and Co clusters (χcluster) to the total EXAFS function χ(k) for the films. The mathematical expression is χ(k) = aχsubstitution(k) + bχcluster(k), where a and b are adjustable parameters. After carefully simulating the experimental curves using this equation, a set of proportions of metallic Co for different films were obtained. For the Cu K-edge EXAFS spectra, we fitted the main FT peak of the first-nearest Cu−Cu coordination shell. The extracted quantitative structural parameters in Table 1 show that metallic Co species emerges at the substrate temperature of 500 °C, accounting for 22% of the total Co dopants content. At the growth temperature of 650 °C, the proportion of the metallic Co species increases to as high as 61%. For the samples deposited at the lower temperatures (
