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Fabrication of two domain Cu2O(0 1 1) films on MgO(0 0 1) by pulsed laser deposition
Applied Surface Science 273 (2013) 19–23
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of two domain Cu2 O(0 1 1) films on MgO(0 0 1) by pulsed laser deposition Yajun Fu a,b , Hongwei Lei b , Xuemin Wang b,∗ , Dawei Yan b , Linhong Cao a , Gang Yao a,b , Changle Shen b , Liping Peng b , Yan Zhao b , Yuying Wang b , Weidong Wu b a State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, People’s Republic of China b Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyan, 621900, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 September 2012 Received in revised form 18 December 2012 Accepted 6 January 2013 Available online 11 January 2013 Keywords: Cu2 O MgO PLD Orientation relationships
a b s t r a c t Single oriented Cu2 O(0 1 1) films were fabricated on MgO(0 0 1) substrates at temperature from 450 to 600 ◦ C by pulsed laser deposition. In situ X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy showed that the oxidation state of Cu should be Cu1+ . In situ reflection high-energy electron diffraction, ex situ X-ray diffraction and cross-sectional transmission electron microscopy have been used to characterize the structural properties of deposited Cu2 O films. The coexistence of two kinds of domains in the Cu2 O films was observed. The in-plane orientation relationships for the two kinds of domains are Cu2 O[1 0 0]||MgO[1 1 0] and Cu2 O[1 0 0]||MgO[1 −1 0], respectively. The formed rotation domains, which perpendicular to each other, can be attributed to the mismatch of rotational symmetry at interface between Cu2 O and MgO. The strip-like surface morphology of Cu2 O films was investigated by atomic force microscopy and scanning electron microscopy. The band gap of Cu2 O films was determined by using the linear extrapolation method. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Increasing attention has been drawn on harvesting solar power as it is the most abundant, clean and inexhaustible form of renewable energy. P-type and n-type silicon (Si) homojunction structures were used for the first generation solar cell devices. The second generation photovoltaic devices using thin films as an essential alternative and Si-based devices have shown remarkable promise [1]. However, the poor absorption efficiencies and relatively high cost of Si-based solar cells make it necessary to investigate new materials for thin film solar cells [2]. Cu2 O is regarded as one of the promising materials for photovoltaic application due to its low cost, nontoxicity, and high absorption coefficient in the visible region [3]. Many kinds of Cu2 O-based solar cells have been investigated during the past three decades, such as metal/Cu2 O Schoktty junctions [4], p–n heterojunctions include n-ZnO/p-Cu2 O [5,6], n-CdO/p-Cu2 O [7] and n-ITO/p-Cu2 O [8] have been fabricated. Furthermore, p–n homojunctions of Cu2 O have been used to increase the efficiency of Cu2 O-based solar cells [9–11]. Until now, the highest conversion efficiency with Cu2 O as the active layer is around 2% with a
∗ Corresponding author. Tel.: +86 816 2480830; fax: +86 816 2480830. E-mail address: [email protected] (X. Wang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.019
p–n heterojunction structure [8], while the theoretical efficiency for Cu2 O solar cells is about 20% [12]. Due to the film orientation and microstructure were not well controlled in most of the previous investigations of Cu2 O thin film growth, Cu2 O-based p–n heterojunctions have not demonstrated good photovoltaic performance. To improve the performance of Cu2 O based p–n heterojunctions, it is essential to get a high quality Cu2 O film. Several techniques for achieving Cu2 O thin films have been reported. Darvish et al. [13] fabricated Cu2 O(0 0 1) films on MgO(0 0 1) substrate by molecular beam epitaxy (MBE). Magnetron sputtering was also been used to deposit Cu2 O films [14], the morphology and grains shape were heavily dependent on the deposition rate. Thus, it is difficult to control the film quality. The electrodeposition method was widely used in Cu2 O deposition [9–11,15,16]. However, the morphology will be different for the films grown at different pH, and the films usually have a rough surface. As a pulsed laser thin film deposition technique, pulsed laser deposition (PLD) is a relatively new growth technique that has been applied to a wide range of materials [17]. For PLD, the growth rate can be changed via adjusting the repetition rate of the laser, which is useful for both atomic level investigations and thick layer growth. The relatively high kinetic energy of the adsorbed species in PLD may lead to smoother film morphology at low substrate temperatures [18]. But up to now, only a few studies have been focused
20
Y. Fu et al. / Applied Surface Science 273 (2013) 19–23
on the growth of Cu2 O films with pulsed laser deposition technique [19]. In this work, Cu2 O films were grown on MgO(0 0 1) by PLD. The chemical valence state, crystallographic orientation, and optical property of Cu2 O films were characterized and discussed.
3. Results and discussion Fig. 1(a) shows the Cu 2p core-level of Cu2 O film deposited at different substrate temperatures (Ts = 450–600 ◦ C), Cu 2p3/2 and 2p1/2 peaks are observed at 932.4 and 952.3 eV, respectively. The weak peaks at the lower binding energy side of the main peaks (marked by solid squares) are 2p3/2 and 2p1/2 levels excited by satellite lines of the Mg K␣ X-ray radiation. The absence of characteristic shakeup satellites of Cu2+ , which should be observed at binding energy about 9 eV higher than the main peaks, indicates the oxidation state of Cu should be Cu1+ . This conclusion is further supported by the appearance of a weak hump at around 965 eV (marked by an arrow), which is in good agreement with the satellite peak position of Cu2 O [22]. The major Cu L3 VV Auger peak with kinetic energy of 916.3 eV is presented in Fig. 1(b). It has been reported that the Cu0 (metal–Cu) Auger spectrum has a distinct satellite feature at kinetic energy of ∼2.5 eV higher than the main peak [20–22]. The absence of the satellite peaks in the presented curves implies the Auger peaks are associated with Cu1+ , i.e. Cu2 O is formed. Fig. 2(a) shows a typical RHEED pattern of Cu2 O layer grown on MgO(0 0 1) substrate at 550 ◦ C. A similar pattern will emerge via rotate the sample ∼20◦ from the initial azimuth, which indicates some kinds of symmetry of the films. The same characteristics were observed on RHEED patterns of Cu2 O films deposited at Ts of 450 and 600 ◦ C. The RHEED pattern is consist of two sets of diffraction spots, as shown in Fig. 2(b). According to the miller indices labeled in the schematic diagram, it is found that the films surface is Cu2 O(0 1 1) plane instead of Cu2 O(0 0 1) plane. The incident
Intensity(a.u.)
Cu2 O thin films were deposited by PLD with a KrF-excimer laser of 248 nm wavelength and 20 ns pulse duration. A ceramic CuO target with purity of 99.9% was used for ablation. The MgO(0 0 1) substrates were annealed for 30 min at 650 ◦ C under background vacuum before film deposition. The lattice mismatch of only 1.2% ´˚ and MgO(a = 0.422 A) ´˚ facilitated the epibetween Cu2 O(a = 0.427 A) taxial growth. The substrates were mounted 5 cm away from the target in the PLD chamber with a base pressure maintained at ∼2 × 10−7 Pa. Laser energy density and laser frequency were kept as ∼1 J/cm2 and 2 Hz, respectively. For Cu2 O film deposition, the substrate temperature (Ts ) ranged from 450 to 600 ◦ C. The deposition process of Cu2 O layers grown on MgO(0 0 1) substrates was monitored by in situ reflection high-energy electron diffraction (RHEED). The incident angle of the 25 keV electrons beam was 1–3◦ . In situ X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES) were used to analyze the chemical composition and valence state. The unmonochromatized Mg K␣ (1253.6 eV) radiation was used; the base pressure of UHV chamber was ∼5 × 10−8 Pa during the test. The binding energy is calibrated by the Cu 2p3/2 peak position at 932.4 eV [20,21]. The crystal structure and crystallographic orientation of the films were determined by means of X-ray diffraction (XRD) with Cu K␣ radiation. The cross section structure of the sample was studied by cross-sectional transmission electron microscopy (TEM). The atomic force microscopy (AFM) and field emission scanning electron microscopy (SEM) was used to investigate the surface morphology characterization. The optical transmission spectra were detected by a double beam UV–visible spectrophotometer.
Cu 2P3/2 Cu 2P1/2
600 C 550 C 450 C
980
970
960
950
940
930
920
Binding Energy (eV)
(b) Cu L3VV Intensity(a.u.)
2. Experiment
(a)
600 C 550 C 450 C
905
910 915 920 Kinetic Energy (eV)
Fig. 1. In situ XPS measurement of the as-grown Cu2 O films deposited at different substrate temperatures: (a) Cu 2p core-level XPS spectrum; (b) XAES spectrum of Cu L3 VV.
electron beam azimuth for the two sets of diffraction spots are along Cu2 O[2 1 −1] and Cu2 O[1 1 −1], respectively. The two sets of diffraction spots imply two kinds of domains coexist in the Cu2 O(0 1 1) films. Fig. 3(a) shows the XRD –2 pattern of Cu2 O(0 1 1) films grown on MgO(0 0 1) substrates at different substrate temperatures. The lower curve belongs to MgO(0 0 1) substrate and sample stage, while the upper three curves indicate that only Cu2 O(0 1 1) and (0 2 2) reflection can be observed for Ts = 450–600 ◦ C. No other oriented grains were detected indicates that the out-of-plane orientation relationship is Cu2 O(0 1 1)||MgO(0 0 1), which confirms the result derived from RHEED pattern. A typical Cu2 O(4 0 0) scan image of Cu2 O(0 1 1) film (Ts = 600 ◦ C) is shown in Fig. 3(b). The four Cu2 O(4 0 0) peaks with 90◦ intervals suggest that the Cu2 O(0 1 1) plane is fourfold symmetric. Therefore, it can be conclude that the Cu2 O(0 1 1) films grown on MgO(0 0 1) substrates are consist of two kinds of domains since the rectangular surface cell of Cu2 O(0 1 1) plane is only twofold symmetry. It is also clear that the two kinds of domains are perpendicular to each other. Compare the peaks position of Cu2 O(4 0 0) with MgO(2 2 0) in scanning plot, the 45◦ intervals of these peaks indicate the in-plane orientation relationships for the two kinds of domains are Cu2 O[1 0 0]||MgO[1 1 0] and Cu2 O[1 0 0]||MgO[1 −1 0], respectively. According to the RHEED and XRD results, the in-plane epitaxial relationship is illustrated in Fig. 4(a) by schematic diagrams. Two different kinds of domains orthogonal to each other as characterized by dashed line. The sides of Cu2 O(0 1 1) rectangular surface cell, in both of the two kinds of domains, are parallel to the diagonals
Y. Fu et al. / Applied Surface Science 273 (2013) 19–23
21
(a) [011]
[100]
[01-1]
aCu2O
2 aCu2O
19
B
.47
[11-1]
A
(b) [21-1]
[21-1] / [11-1]
Fig. 4. Schematic diagrams of Cu2 O(0 1 1) films on MgO(0 0 1) substrates: (a) the orientation relationship of Cu2 O(0 1 1) on MgO(0 0 1) and (b) the rectangular surface cell of Cu2 O(0 1 1) plane.
Fig. 2. RHEED pattern of Cu2 O(0 1 1) films grown on MgO(0 0 1) substrate at 550 ◦ C. (a) Electron diffraction pattern of as grown film; (b) schematic diagram of the Cu2 O[2 1 −1] (hollow circle) and Cu2 O[1 1 −1] (solid circle) zone-axis patterns.
of MgO(0 0 1) foursquare surface cell. Fig. 4(b) shows the sketch of rectangular surface cell of Cu2 O(0 1 1) plane. According to the schematic drawings, the [1 1 −1] orientation of domain A is just at the same direction with [2 1 −1] orientation of domain B due to the orthogonal relationship, and vice versa. Thus, the diffraction spots derived from Cu2 O[1 1 −1] and Cu2 O[2 1 −1] zone axis can coexist in one RHEED pattern, as shown in Fig. 2(a). The separation angle between Cu2 O[1 1 −1] and Cu2 O[2 1 −1] crystallographic ◦ ◦ orientation √ in one domain is 19.47 (calculated by 90 -2arctan (aCu2 O / 2aCu2 O )). Therefore, an identical RHEED pattern of Cu2 O layers can be observed when rotating the sample about 20◦ from the initial azimuth. Hereby, the Cu2 O(0 1 1) films are fabricated on MgO(0 0 1) by PLD with a CuO target. The pressure was on the order of 10−5 Pa during the film growth. The growth temperatures varied from 450 to 650 ◦ C. In these cases Cu2 O is the thermal equilibrium phase according to the phase diagram of Cu–O system [23]. It is interesting that the orientation relationship is Cu2 O(0 1 1)||MgO(0 0 1) rather than Cu2 O(0 0 1)||MgO(0 0 1), although the cubic lattice constants are so close. Firstly, the lower surface energy of Cu2 O(0 1 1) plane contrast with (0 0 1) plane may lead the epitaxial process to grow along (0 1 1) plane [24]. Besides, the mismatch between Cu √2 O[1 1 0] orientation and MgO[1 1 0] orientation (expressed by (( 2aCu2 O − √ √ 2aMgO )/ 2aMgO ) is just the same as mismatch between aCu2 O and aMgO , 1.2%. These two factors induce the out-plane orientation along Cu2 O(0 1 1) plane much more favorable. Marius Grundmann’s theory can be applied to describe the two rotation domains [25]. Here, the mismatch of rotational symmetry at the interface between films and substrates leads to rotation domains as a consequential defect. The number of equivalent rotation domains NRD can be expressed as follows: NRD = 1 cm(n, m)/m
Fig. 3. XRD patterns of Cu2 O(0 1 1) films on MgO(0 0 1) substrates at different growth temperatures: (a) –2 scans; (b) scans of Cu2 O film grown at 600 ◦ C.
(1)
The substrate surface and epilayer should have Cn and Cm symmetry with regard to the surface normal, respectively, possible values being n, m ∈{1, 2, 3, 4, 6}; 1 cm (n, m) being the least common multiple of n and m. For cubic Cu2 O structure, its (0 1 1) plane has
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Y. Fu et al. / Applied Surface Science 273 (2013) 19–23
Fig. 5. Surface morphology of the film deposited at 550 ◦ C: (a) SEM image and (b) AFM image.
twofold symmetry (C2 ), while MgO(0 0 1) plane has fourfold symmetry (C4 ). According to Eq. (1), n = 4 and m = 2 leading to NRD = 2, that is to say the mismatch of rotational symmetry in this case leads to two Cu2 O rotation domains. The prediction matches experimental results very well. Thus, the mismatch of rotational symmetry between Cu2 O(0 1 1) and MgO(0 0 1) plane leads to two domains in the epilayer, rotated 90◦ against each other. The surface morphology of the films was investigated by scanning electron microscopy (SEM) and AFM. The typical SEM and AFM images of Cu2 O thin films grown at 550 ◦ C are shown in Fig. 5(a) and (b), respectively. The scanning areas exhibit a strip-like morphology with an average length of 300 nm. Each strip-like region consists of a Cu2 O domain, which is arranged along two mutually perpendicular directions. The root mean square (RMS) surface roughness of the film obtained from AFM image is about 12 nm. The relatively large RMS may be partially caused by the presence of rotation domains. The cross section structure of Cu2 O/MgO interface was studied by cross-sectional transmission electron microscopy (TEM). Fig. 6(a) shows a bright field cross-sectional TEM image from a sample grown at 450 ◦ C. The grain contrast can be seen in the micrograph, and the interface is obviously and quite abrupt.
Fig. 6. Cross-sectional TEM images of the Cu2 O film grown on MgO substrate at 450 ◦ C: (a) TEM bright-field image and (b) high-resolution TEM image of the Cu2O/MgO interface.
Furthermore, a high-resolution TEM image in Fig. 6(b) taken around the Cu2 O/MgO interfacial region. It is clear that the film on MgO has an atomically sharp interface without any amorphous interlayer. To determine the band gap of a material, the relationship [26] ˛E = A(E − Eg )n
(2)
is widely used. Where ˛ is the absorption coefficient, E is the energy of the photon and Eg is the band gap. The value n = 0.5 is used for a direct band gap material and n = 2 for an indirect band gap material. The absorption coefficient can be calculated from the Beer–Lambert’s attenuation equation. The band gap of the Cu2 O film deposited at 450 ◦ C is obtained from the (˛E)2 versus photon energy plot since it is a direct band gap material, as shown in Fig. 7. The insert shows a plot of optical transmission spectrum for the Cu2 O thin film. Therefore, the extrapolation of the plots yields a Cu2 O band gap of 2.46 eV, which is consistent with other reported values [27].
Y. Fu et al. / Applied Surface Science 273 (2013) 19–23
2
6
80
Transmittance (%)
9
5
( h) (10 eV/cm)
2
12
60 40 20 0
500
600
700
Wavelength (nm)
3 0 1.6
800
Eg=2.46 eV 1.8
2.0
2.2
2.4
2.6
2.8
Photon Energy (eV) Fig. 7. Plot of (˛h)2 versus photon energy for Cu2 O(0 1 1) film deposited at 450 ◦ C. The extrapolation gives the band gap for a direct gap material. The inset shows the corresponding optical transmission spectrum.
4. Conclusions In summary, Cu2 O(0 1 1) films have been demonstrated on MgO(0 0 1) substrates by PLD. The formation of Cu2 O(0 1 1) surface rather than expected (0 0 1) surface was ascribed to the lower surface free energy in (0 1 1) plane. In the Cu2 O(0 1 1) thin films, two types of domains were formed and perpendicular to each other. The in-plane orientation relationships for the two kinds of domains were Cu2 O[1 0 0]||MgO[1 1 0] and Cu2 O[1 0 0]||MgO[1 −1 0], respectively. The appearance of rotation domains can be explained by the mismatch of rotational symmetry between films and substrates at interface. The band gap of 2.46 eV for the Cu2 O film was obtained from the optical transmission spectrum. Acknowledgement We acknowledge financial support from the Ministry of National Science and Technology Major Instrumentation Special of China (Grant No. 2011YQ130018). References [1] S. Hegedus, Thin film solar modules: the low cost, high throughput and versatile alternative to Si wafers, Progress in Photovoltaics 14 (2006) 393–411. [2] M.A. Green, Consolidation of thin-film photovoltaic technology: the coming decade of opportunity, Progress in Photovoltaics 14 (2006) 383–392. [3] A.E. Rakhshani, Preparation, characteristics and photovoltaic properties of cuprous oxide – a review, Solid State Electronics 29 (1986) 7–17.
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[4] L.C. Olsen, R.C. Bohara, M.W. Urie, Explanation for low-efficiency Cu2 O Schottky-barrier solarcells, Applied Physics Letters 34 (1979) 47–49. [5] J. Herion, E.A. Niekisch, G. Scharl, Investigation of metal oxide/cuprousoxide heterojunction solarcells, Solar Energy Materials 4 (1980) 101–112. [6] J. Katayama, K. Ito, M. Matsuoka, J. Tamaki, Performance of Cu2 O/ZnO solar cell prepared by two-step electrodeposition, Journal of Applied Electrochemistry 34 (2004) 687–692. [7] L. Papadimitriou, N.A. Economou, D. Trivich, Heterojunction solar cells on cuprous oxide, Solar Cells 3 (1981) 73–80. [8] A. Mittiga, E. Salza, F. Sarto, M. Tucci, R. Vasanthi, Heterojunction solar cell with 2% efficiency based on a Cu2 O substrate, Applied Physics Letters 88 (2006) 163502. [9] L.C. Wang, M. Tao, Fabrication, Characterization of p–n homojunctions in cuprous oxide by electrochemical deposition, Electrochemical and Solid-State Letters 10 (2007) H248–H250. [10] K. Han, M. Tao, Electrochemically deposited p–n homojunction cuprous oxide solar cells, Solar Energy Materials and Solar Cells 93 (2009) 153–157. [11] C.M. McShane, W.P. Siripala, K.S. Choi, Effect of junction morphology on the performance of polycrystalline Cu2 O homojunction solar cells, Journal of Physical Chemistry Letters 1 (2010) 2666–2670. [12] L.C. Olsen, F.W. Addis, W. Miller, Experimental and theoretical studies of Cu2 O solar cells, Solar Cells 7 (1982) 247–249. [13] D.S. Darvish, H.A. Atwater, Epitaxial growth of Cu2 O and ZnO/Cu2 O thin films on MgO by plasma-assisted molecular beam epitaxy, Journal of Crystal Growth 319 (2011) 39–43. [14] Z.G. Yin, H.T. Zhang, D.M. Goodner, M.J. Bedzyk, R.P.H. Chang, Y. Sun, J.B. Ketterson, Two-dimensional growth of continuous Cu2 O thin films by magnetron sputtering, Applied Physics Letters 86 (2005) 061901. [15] R. Liu, E.W. Bohannan, J.A. Switzera, Epitaxial electrodeposition of Cu2 O films onto InP (0 0 1), Applied Physics Letters 83 (2003) 1944–1946. [16] B.M. Farizaz, J. Sasano, T. Shinagawa, H. Nakano, Electrochemical growth of (0 0 0 1)-n-ZnO film on (1 1 1)-p-Cu2 O film and the characterization of the heterojunction diode, Journal of the Electrochemical Society 158 (2011) D621–D625. [17] D.P. Norton, Pulsed laser deposition of thin films: applications-led growth of functional materials, in: R. Eason (Ed.), Pulsed Laser Deposition of Complex Materials: Progress Towards Applications, John Wiley & Sons, Inc., New Jersey, 2007, pp. 1–28. [18] M.E. Taylor, H.A. Atwater, Monte Carlo simulations of epitaxial growth: comparison of pulsed laser deposition and molecular beam epitaxy, Applied Surface Science 127–129 (1998) 159–163. [19] A. Chen, H. Long, X. Li, Controlled growth and characteristics of single-phase Cu2 O and CuO films by pulsed laser deposition, Vacuum 83 (2009) 927–930. [20] S. Poulston, P.M. Parlett, P. Stone, M. Bowker, Surface oxidation and reduction of CuO and Cu2 O studied using XPS and XAES, Surface and Interface Analysis 24 (1996) 811–820. [21] J.P. Tobin, W. Hirschwald, J. Cunningham, XPS and XAES studies of transient enhancement of Cu1 at CuO surfaces during vacuum outgassing, Applied Surface Science 16 (1983) 441–452. [22] S.R. Barman, D.D. Sarma, Investigation of the L3 -M45 M45 Auger spectra of Cu, Cu2 O and CuO, Journal of Physics: Condensed Matter 4 (1992) 7607–7616. [23] R.D. Schmidt-Whitley, M. Martinez-Clemente, A. Revcolevschi, Growth and microstructural control of single crystal cuprous oxide Cu2 O, Journal of Crystal Growth 23 (1974) 113–120. [24] A. Soon, M. Todorova, B. Delley, C. Stampfl, Thermodynamic stability and structure of copper oxide surfaces: a first-principles investigation, Physical Review B 75 (2007) 125420. [25] M. Grundmann, T. Böntgen, M. Lorenz, Occurrence of rotation domains in heteroepitaxy, Physical Review Letters 105 (2010) 146102. [26] V. Srikant, D.R. Clarkea, Optical absorption edge of ZnO thin films: the effect of substrate, Journal of Applied Physics 81 (1997) 6357–6364. [27] L.M. Wong, S.Y. Chiam, J.Q. Huang, Growth of Cu2 O on Ga-doped ZnO and their interface energy alignment for thin film solar cells, Journal of Applied Physics 108 (2010) 033702.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Fabrication of two domain Cu2 O(0 1 1) films on MgO(0 0 1) by pulsed laser deposition Yajun Fu a,b , Hongwei Lei b , Xuemin Wang b,∗ , Dawei Yan b , Linhong Cao a , Gang Yao a,b , Changle Shen b , Liping Peng b , Yan Zhao b , Yuying Wang b , Weidong Wu b a State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, People’s Republic of China b Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyan, 621900, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 September 2012 Received in revised form 18 December 2012 Accepted 6 January 2013 Available online 11 January 2013 Keywords: Cu2 O MgO PLD Orientation relationships
a b s t r a c t Single oriented Cu2 O(0 1 1) films were fabricated on MgO(0 0 1) substrates at temperature from 450 to 600 ◦ C by pulsed laser deposition. In situ X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopy showed that the oxidation state of Cu should be Cu1+ . In situ reflection high-energy electron diffraction, ex situ X-ray diffraction and cross-sectional transmission electron microscopy have been used to characterize the structural properties of deposited Cu2 O films. The coexistence of two kinds of domains in the Cu2 O films was observed. The in-plane orientation relationships for the two kinds of domains are Cu2 O[1 0 0]||MgO[1 1 0] and Cu2 O[1 0 0]||MgO[1 −1 0], respectively. The formed rotation domains, which perpendicular to each other, can be attributed to the mismatch of rotational symmetry at interface between Cu2 O and MgO. The strip-like surface morphology of Cu2 O films was investigated by atomic force microscopy and scanning electron microscopy. The band gap of Cu2 O films was determined by using the linear extrapolation method. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Increasing attention has been drawn on harvesting solar power as it is the most abundant, clean and inexhaustible form of renewable energy. P-type and n-type silicon (Si) homojunction structures were used for the first generation solar cell devices. The second generation photovoltaic devices using thin films as an essential alternative and Si-based devices have shown remarkable promise [1]. However, the poor absorption efficiencies and relatively high cost of Si-based solar cells make it necessary to investigate new materials for thin film solar cells [2]. Cu2 O is regarded as one of the promising materials for photovoltaic application due to its low cost, nontoxicity, and high absorption coefficient in the visible region [3]. Many kinds of Cu2 O-based solar cells have been investigated during the past three decades, such as metal/Cu2 O Schoktty junctions [4], p–n heterojunctions include n-ZnO/p-Cu2 O [5,6], n-CdO/p-Cu2 O [7] and n-ITO/p-Cu2 O [8] have been fabricated. Furthermore, p–n homojunctions of Cu2 O have been used to increase the efficiency of Cu2 O-based solar cells [9–11]. Until now, the highest conversion efficiency with Cu2 O as the active layer is around 2% with a
∗ Corresponding author. Tel.: +86 816 2480830; fax: +86 816 2480830. E-mail address: [email protected] (X. Wang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.019
p–n heterojunction structure [8], while the theoretical efficiency for Cu2 O solar cells is about 20% [12]. Due to the film orientation and microstructure were not well controlled in most of the previous investigations of Cu2 O thin film growth, Cu2 O-based p–n heterojunctions have not demonstrated good photovoltaic performance. To improve the performance of Cu2 O based p–n heterojunctions, it is essential to get a high quality Cu2 O film. Several techniques for achieving Cu2 O thin films have been reported. Darvish et al. [13] fabricated Cu2 O(0 0 1) films on MgO(0 0 1) substrate by molecular beam epitaxy (MBE). Magnetron sputtering was also been used to deposit Cu2 O films [14], the morphology and grains shape were heavily dependent on the deposition rate. Thus, it is difficult to control the film quality. The electrodeposition method was widely used in Cu2 O deposition [9–11,15,16]. However, the morphology will be different for the films grown at different pH, and the films usually have a rough surface. As a pulsed laser thin film deposition technique, pulsed laser deposition (PLD) is a relatively new growth technique that has been applied to a wide range of materials [17]. For PLD, the growth rate can be changed via adjusting the repetition rate of the laser, which is useful for both atomic level investigations and thick layer growth. The relatively high kinetic energy of the adsorbed species in PLD may lead to smoother film morphology at low substrate temperatures [18]. But up to now, only a few studies have been focused
20
Y. Fu et al. / Applied Surface Science 273 (2013) 19–23
on the growth of Cu2 O films with pulsed laser deposition technique [19]. In this work, Cu2 O films were grown on MgO(0 0 1) by PLD. The chemical valence state, crystallographic orientation, and optical property of Cu2 O films were characterized and discussed.
3. Results and discussion Fig. 1(a) shows the Cu 2p core-level of Cu2 O film deposited at different substrate temperatures (Ts = 450–600 ◦ C), Cu 2p3/2 and 2p1/2 peaks are observed at 932.4 and 952.3 eV, respectively. The weak peaks at the lower binding energy side of the main peaks (marked by solid squares) are 2p3/2 and 2p1/2 levels excited by satellite lines of the Mg K␣ X-ray radiation. The absence of characteristic shakeup satellites of Cu2+ , which should be observed at binding energy about 9 eV higher than the main peaks, indicates the oxidation state of Cu should be Cu1+ . This conclusion is further supported by the appearance of a weak hump at around 965 eV (marked by an arrow), which is in good agreement with the satellite peak position of Cu2 O [22]. The major Cu L3 VV Auger peak with kinetic energy of 916.3 eV is presented in Fig. 1(b). It has been reported that the Cu0 (metal–Cu) Auger spectrum has a distinct satellite feature at kinetic energy of ∼2.5 eV higher than the main peak [20–22]. The absence of the satellite peaks in the presented curves implies the Auger peaks are associated with Cu1+ , i.e. Cu2 O is formed. Fig. 2(a) shows a typical RHEED pattern of Cu2 O layer grown on MgO(0 0 1) substrate at 550 ◦ C. A similar pattern will emerge via rotate the sample ∼20◦ from the initial azimuth, which indicates some kinds of symmetry of the films. The same characteristics were observed on RHEED patterns of Cu2 O films deposited at Ts of 450 and 600 ◦ C. The RHEED pattern is consist of two sets of diffraction spots, as shown in Fig. 2(b). According to the miller indices labeled in the schematic diagram, it is found that the films surface is Cu2 O(0 1 1) plane instead of Cu2 O(0 0 1) plane. The incident
Intensity(a.u.)
Cu2 O thin films were deposited by PLD with a KrF-excimer laser of 248 nm wavelength and 20 ns pulse duration. A ceramic CuO target with purity of 99.9% was used for ablation. The MgO(0 0 1) substrates were annealed for 30 min at 650 ◦ C under background vacuum before film deposition. The lattice mismatch of only 1.2% ´˚ and MgO(a = 0.422 A) ´˚ facilitated the epibetween Cu2 O(a = 0.427 A) taxial growth. The substrates were mounted 5 cm away from the target in the PLD chamber with a base pressure maintained at ∼2 × 10−7 Pa. Laser energy density and laser frequency were kept as ∼1 J/cm2 and 2 Hz, respectively. For Cu2 O film deposition, the substrate temperature (Ts ) ranged from 450 to 600 ◦ C. The deposition process of Cu2 O layers grown on MgO(0 0 1) substrates was monitored by in situ reflection high-energy electron diffraction (RHEED). The incident angle of the 25 keV electrons beam was 1–3◦ . In situ X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (XAES) were used to analyze the chemical composition and valence state. The unmonochromatized Mg K␣ (1253.6 eV) radiation was used; the base pressure of UHV chamber was ∼5 × 10−8 Pa during the test. The binding energy is calibrated by the Cu 2p3/2 peak position at 932.4 eV [20,21]. The crystal structure and crystallographic orientation of the films were determined by means of X-ray diffraction (XRD) with Cu K␣ radiation. The cross section structure of the sample was studied by cross-sectional transmission electron microscopy (TEM). The atomic force microscopy (AFM) and field emission scanning electron microscopy (SEM) was used to investigate the surface morphology characterization. The optical transmission spectra were detected by a double beam UV–visible spectrophotometer.
Cu 2P3/2 Cu 2P1/2
600 C 550 C 450 C
980
970
960
950
940
930
920
Binding Energy (eV)
(b) Cu L3VV Intensity(a.u.)
2. Experiment
(a)
600 C 550 C 450 C
905
910 915 920 Kinetic Energy (eV)
Fig. 1. In situ XPS measurement of the as-grown Cu2 O films deposited at different substrate temperatures: (a) Cu 2p core-level XPS spectrum; (b) XAES spectrum of Cu L3 VV.
electron beam azimuth for the two sets of diffraction spots are along Cu2 O[2 1 −1] and Cu2 O[1 1 −1], respectively. The two sets of diffraction spots imply two kinds of domains coexist in the Cu2 O(0 1 1) films. Fig. 3(a) shows the XRD –2 pattern of Cu2 O(0 1 1) films grown on MgO(0 0 1) substrates at different substrate temperatures. The lower curve belongs to MgO(0 0 1) substrate and sample stage, while the upper three curves indicate that only Cu2 O(0 1 1) and (0 2 2) reflection can be observed for Ts = 450–600 ◦ C. No other oriented grains were detected indicates that the out-of-plane orientation relationship is Cu2 O(0 1 1)||MgO(0 0 1), which confirms the result derived from RHEED pattern. A typical Cu2 O(4 0 0) scan image of Cu2 O(0 1 1) film (Ts = 600 ◦ C) is shown in Fig. 3(b). The four Cu2 O(4 0 0) peaks with 90◦ intervals suggest that the Cu2 O(0 1 1) plane is fourfold symmetric. Therefore, it can be conclude that the Cu2 O(0 1 1) films grown on MgO(0 0 1) substrates are consist of two kinds of domains since the rectangular surface cell of Cu2 O(0 1 1) plane is only twofold symmetry. It is also clear that the two kinds of domains are perpendicular to each other. Compare the peaks position of Cu2 O(4 0 0) with MgO(2 2 0) in scanning plot, the 45◦ intervals of these peaks indicate the in-plane orientation relationships for the two kinds of domains are Cu2 O[1 0 0]||MgO[1 1 0] and Cu2 O[1 0 0]||MgO[1 −1 0], respectively. According to the RHEED and XRD results, the in-plane epitaxial relationship is illustrated in Fig. 4(a) by schematic diagrams. Two different kinds of domains orthogonal to each other as characterized by dashed line. The sides of Cu2 O(0 1 1) rectangular surface cell, in both of the two kinds of domains, are parallel to the diagonals
Y. Fu et al. / Applied Surface Science 273 (2013) 19–23
21
(a) [011]
[100]
[01-1]
aCu2O
2 aCu2O
19
B
.47
[11-1]
A
(b) [21-1]
[21-1] / [11-1]
Fig. 4. Schematic diagrams of Cu2 O(0 1 1) films on MgO(0 0 1) substrates: (a) the orientation relationship of Cu2 O(0 1 1) on MgO(0 0 1) and (b) the rectangular surface cell of Cu2 O(0 1 1) plane.
Fig. 2. RHEED pattern of Cu2 O(0 1 1) films grown on MgO(0 0 1) substrate at 550 ◦ C. (a) Electron diffraction pattern of as grown film; (b) schematic diagram of the Cu2 O[2 1 −1] (hollow circle) and Cu2 O[1 1 −1] (solid circle) zone-axis patterns.
of MgO(0 0 1) foursquare surface cell. Fig. 4(b) shows the sketch of rectangular surface cell of Cu2 O(0 1 1) plane. According to the schematic drawings, the [1 1 −1] orientation of domain A is just at the same direction with [2 1 −1] orientation of domain B due to the orthogonal relationship, and vice versa. Thus, the diffraction spots derived from Cu2 O[1 1 −1] and Cu2 O[2 1 −1] zone axis can coexist in one RHEED pattern, as shown in Fig. 2(a). The separation angle between Cu2 O[1 1 −1] and Cu2 O[2 1 −1] crystallographic ◦ ◦ orientation √ in one domain is 19.47 (calculated by 90 -2arctan (aCu2 O / 2aCu2 O )). Therefore, an identical RHEED pattern of Cu2 O layers can be observed when rotating the sample about 20◦ from the initial azimuth. Hereby, the Cu2 O(0 1 1) films are fabricated on MgO(0 0 1) by PLD with a CuO target. The pressure was on the order of 10−5 Pa during the film growth. The growth temperatures varied from 450 to 650 ◦ C. In these cases Cu2 O is the thermal equilibrium phase according to the phase diagram of Cu–O system [23]. It is interesting that the orientation relationship is Cu2 O(0 1 1)||MgO(0 0 1) rather than Cu2 O(0 0 1)||MgO(0 0 1), although the cubic lattice constants are so close. Firstly, the lower surface energy of Cu2 O(0 1 1) plane contrast with (0 0 1) plane may lead the epitaxial process to grow along (0 1 1) plane [24]. Besides, the mismatch between Cu √2 O[1 1 0] orientation and MgO[1 1 0] orientation (expressed by (( 2aCu2 O − √ √ 2aMgO )/ 2aMgO ) is just the same as mismatch between aCu2 O and aMgO , 1.2%. These two factors induce the out-plane orientation along Cu2 O(0 1 1) plane much more favorable. Marius Grundmann’s theory can be applied to describe the two rotation domains [25]. Here, the mismatch of rotational symmetry at the interface between films and substrates leads to rotation domains as a consequential defect. The number of equivalent rotation domains NRD can be expressed as follows: NRD = 1 cm(n, m)/m
Fig. 3. XRD patterns of Cu2 O(0 1 1) films on MgO(0 0 1) substrates at different growth temperatures: (a) –2 scans; (b) scans of Cu2 O film grown at 600 ◦ C.
(1)
The substrate surface and epilayer should have Cn and Cm symmetry with regard to the surface normal, respectively, possible values being n, m ∈{1, 2, 3, 4, 6}; 1 cm (n, m) being the least common multiple of n and m. For cubic Cu2 O structure, its (0 1 1) plane has
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Y. Fu et al. / Applied Surface Science 273 (2013) 19–23
Fig. 5. Surface morphology of the film deposited at 550 ◦ C: (a) SEM image and (b) AFM image.
twofold symmetry (C2 ), while MgO(0 0 1) plane has fourfold symmetry (C4 ). According to Eq. (1), n = 4 and m = 2 leading to NRD = 2, that is to say the mismatch of rotational symmetry in this case leads to two Cu2 O rotation domains. The prediction matches experimental results very well. Thus, the mismatch of rotational symmetry between Cu2 O(0 1 1) and MgO(0 0 1) plane leads to two domains in the epilayer, rotated 90◦ against each other. The surface morphology of the films was investigated by scanning electron microscopy (SEM) and AFM. The typical SEM and AFM images of Cu2 O thin films grown at 550 ◦ C are shown in Fig. 5(a) and (b), respectively. The scanning areas exhibit a strip-like morphology with an average length of 300 nm. Each strip-like region consists of a Cu2 O domain, which is arranged along two mutually perpendicular directions. The root mean square (RMS) surface roughness of the film obtained from AFM image is about 12 nm. The relatively large RMS may be partially caused by the presence of rotation domains. The cross section structure of Cu2 O/MgO interface was studied by cross-sectional transmission electron microscopy (TEM). Fig. 6(a) shows a bright field cross-sectional TEM image from a sample grown at 450 ◦ C. The grain contrast can be seen in the micrograph, and the interface is obviously and quite abrupt.
Fig. 6. Cross-sectional TEM images of the Cu2 O film grown on MgO substrate at 450 ◦ C: (a) TEM bright-field image and (b) high-resolution TEM image of the Cu2O/MgO interface.
Furthermore, a high-resolution TEM image in Fig. 6(b) taken around the Cu2 O/MgO interfacial region. It is clear that the film on MgO has an atomically sharp interface without any amorphous interlayer. To determine the band gap of a material, the relationship [26] ˛E = A(E − Eg )n
(2)
is widely used. Where ˛ is the absorption coefficient, E is the energy of the photon and Eg is the band gap. The value n = 0.5 is used for a direct band gap material and n = 2 for an indirect band gap material. The absorption coefficient can be calculated from the Beer–Lambert’s attenuation equation. The band gap of the Cu2 O film deposited at 450 ◦ C is obtained from the (˛E)2 versus photon energy plot since it is a direct band gap material, as shown in Fig. 7. The insert shows a plot of optical transmission spectrum for the Cu2 O thin film. Therefore, the extrapolation of the plots yields a Cu2 O band gap of 2.46 eV, which is consistent with other reported values [27].
Y. Fu et al. / Applied Surface Science 273 (2013) 19–23
2
6
80
Transmittance (%)
9
5
( h) (10 eV/cm)
2
12
60 40 20 0
500
600
700
Wavelength (nm)
3 0 1.6
800
Eg=2.46 eV 1.8
2.0
2.2
2.4
2.6
2.8
Photon Energy (eV) Fig. 7. Plot of (˛h)2 versus photon energy for Cu2 O(0 1 1) film deposited at 450 ◦ C. The extrapolation gives the band gap for a direct gap material. The inset shows the corresponding optical transmission spectrum.
4. Conclusions In summary, Cu2 O(0 1 1) films have been demonstrated on MgO(0 0 1) substrates by PLD. The formation of Cu2 O(0 1 1) surface rather than expected (0 0 1) surface was ascribed to the lower surface free energy in (0 1 1) plane. In the Cu2 O(0 1 1) thin films, two types of domains were formed and perpendicular to each other. The in-plane orientation relationships for the two kinds of domains were Cu2 O[1 0 0]||MgO[1 1 0] and Cu2 O[1 0 0]||MgO[1 −1 0], respectively. The appearance of rotation domains can be explained by the mismatch of rotational symmetry between films and substrates at interface. The band gap of 2.46 eV for the Cu2 O film was obtained from the optical transmission spectrum. Acknowledgement We acknowledge financial support from the Ministry of National Science and Technology Major Instrumentation Special of China (Grant No. 2011YQ130018). References [1] S. Hegedus, Thin film solar modules: the low cost, high throughput and versatile alternative to Si wafers, Progress in Photovoltaics 14 (2006) 393–411. [2] M.A. Green, Consolidation of thin-film photovoltaic technology: the coming decade of opportunity, Progress in Photovoltaics 14 (2006) 383–392. [3] A.E. Rakhshani, Preparation, characteristics and photovoltaic properties of cuprous oxide – a review, Solid State Electronics 29 (1986) 7–17.
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