Effects of Mn and Cu doping on the microstructures and optical properties of sol–gel derived ZnO thin films

Optical Materials 29 (2007) 672–678 www.elsevier.com/locate/optmat

Effects of Mn and Cu doping on the microstructures and optical properties of sol–gel derived ZnO thin films Z. Banu Bahsßi, A. Yavuz Oral

*

Department of Materials Science and Engineering, Gebze Institute of Technology, Gebze 41400, Turkey Received 5 May 2005; accepted 26 November 2005 Available online 18 January 2006

Abstract Both doped (Cu or Mn) and undoped zinc oxide thin films were deposited on glass substrates by a sol–gel technique. Zinc acetate, copper acetate and manganese nitrate were used as metal sources. A homogeneous and stable solution was prepared by dissolving ZnO acetate (ZnAc) in the solution of 2-propanol and ethanolamine (EA) followed by mixing with the doping solutions. ZnO:(Cu or Mn) thin films were obtained after preheating the spin coated films at 250 C for 1 min after each coating. A post-annealing at 550 C was applied to all films for 1 h after the deposition of the last layer. XRD analysis revealed that all films consist of single phase ZnO with zincite structure (Card no: 36-1451). While undoped films showed the strongest orientation, c-axis grain orientation was apparent in all films. TGA analysis of the undoped dried gel showed that weight loss continued until 400 C. Compared to the undoped film, grain size of the films decreased by Mn doping and increased by Cu doping. All films had a very smooth surface with RMS surface roughness values between 0.23 and 1.15 nm and surface roughness increased by doping. Both Mn and Cu doping resulted in a slight decrease in the optical band gap of the films. The largest width of band tail was measured in Mn-doped film.  2005 Elsevier B.V. All rights reserved. PACS: 78.20.Ci; 81.20.Fw; 87.64.Dz Keywords: Doped zinc oxide; Sol–gel; Thin film; Transparent coating

1. Introduction ZnO is an inexpensive, n-type, wide band gap semiconductor with optical transparency in the visible range. It crystallizes in a hexagonal wurtzite structure (zincite) with ˚ , a = 3.249 A ˚ ) [1]. The the lattice parameters of (c = 5.205 A n-type semiconductor behavior is originated by the ionization of excess zinc atoms at interstitial positions and the oxygen vacancies [2]. The resistivity values of ZnO films may be adjusted between 104 and 1012 X cm by doping and changing the annealing conditions [3]. Both doped and undoped ZnO thin films are promising materials for the development of gas sensors [4,5], solar cell windows,

*

Corresponding author. Tel.: +90 262 653 8497; fax: +90 262 653 8490. E-mail address: [email protected] (A.Y. Oral).

0925-3467/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.11.016

[6] and transparent electrodes [7]. Furthermore, since the grain growth of ZnO films shows preferential orientation along the c-axis, they are useful in optical wave-guides, and surface acoustic wave (SAW) and acoustic-optic devices [8]. ZnO thin films have been prepared by various techniques such as rf sputtering [9], spray pyrolysis [10], chemical vapor deposition (CVD) [11,12], pulsed laser deposition [13], and sol–gel processing [14,15]. One of the most important advantages of sol–gel processing, over conventional thin film deposition techniques, is the ease of chemical composition control. This advantage makes sol–gel processing a very attractive method especially for doped ZnO thin film fabrication. Doping of ZnO with Ib and IIb transition elements is relatively less common [16] compared to IIIb elements such as Al. Mn-doped ZnO films have been studied to evaluate

Z. Banu Bahsßi, A.Y. Oral / Optical Materials 29 (2007) 672–678

electro-optic [16] and (anti)ferromagnetic properties [17], and photoluminance [14,15]. There have been different approaches to the defect chemistry of Mn-doped ZnO. Cao et al. [18] assumed that doping ZnO with manganese dominantly forms Mn2+ with a small amount of Mn4+. According to their model, when Mn4+ ions substitute Zn2+ ions, they act as donor atoms generating two free electrons while Mn2+ ions only generate oxygen vacancies. Han et al. [19] observed that Mn doping makes ZnO more resistive at room temperature and highly conductive at high temperatures and proposed a different model where manganese acts as a deep donor in ZnO: Mnx ! Mn þ e0 ð1Þ Zn

Zn

where superscript ‘‘x’’ represents no effective charge. In this model, the energy level of Mn donor was estimated to be 2.0 eV below the conduction band. ZnO:Cu films have been usually fabricated for their electrical and ferromagnetic properties [17]. In addition, they have potential in surface acoustic wave device applications [20]. Due to its similar electronic shell structure, Cu has many physical and chemical properties similar to those of Zn [21]. The solubility of Cu in ZnO lattice is estimated to be around 1.0 mol% Cu [22]. It is well known when ZnO is doped by Cu atoms, Zn2+ ions are substituted by Cu1+ ions in the ZnO lattice. Hall coefficient measurements shown that number of carriers is reduced by Cu doping at room temperature since some of the n-type ZnO electrons occupies empty lower energy 3d Cu states leading to Cu1+ ions [17]. An interesting associate donor–acceptor model for Cuzn was proposed [23] where x Cu þ Zn ! ½Cu þ Zn  ð2Þ Zn

i

Zn

i

The ionization of these deep neutral defects requires high energy (3.0 eV) and the presence of this type of complex defects compensates for the n-type of conductivity of ZnO. The aim of this work is to evaluate the effect of doping (Mn or Cu) on the microstructure, optical properties, and the grain orientation of ZnO thin films prepared by sol– gel spin-coating. 2. Experimental The basic solution was prepared by partially dissolving ZnAc (Zn(CH3COO)2 Æ 2H2O) in 2-propanol and then adding ethanolamine (C2H7NO) (EA:ZnAc = 1:1) to increase solubility. The mixture was stirred by a magnetic stirrer at 50 C until a clear solution formed. Afterwards, water (H2O:ZnAc = 1:2) was slowly added to obtain optimum wettability between the precursor film and the substrate. Finally, a solution with a concentration of 0.4 mol/l was obtained. Cu doping solution with a concentration of 0.2 mol/l was prepared the same way as ZnAc solution by dissolving copper acetate monohydrate (Cu(CH3COO)2 Æ H2O, CuAc). Manganese chloride solution (0.05 M) was prepared by dissolving manganese chloride dihydrate (MnCl2 Æ 2H2O) in 2-propanol. Proportional amounts of

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doping solutions were added to ZnAc solutions in such a way that final Zn:(Mn or Cu) ratio was 99:1, respectively in each solutions. Three types of samples, undoped, ZnO:Cu and ZnO:Mn, were prepared. The microscope glass substrates were cleaned with HCl and distilled water. Deposition was carried out in air at three steps with different spinning speeds, which are 2000 rpm for 30 s, 4000 rpm for 30 s, and 6000 rpm for 60 s. A precursor film formed following the spin coating process. The film was then dried at 250 C for 1 min on a hot plate. Since the boiling point of EA is 217 C and the thermal decomposition temperature of ZnAc is 240 C [24], the preheat treatment temperature of 250 C is required for the complete evaporation of organics and the initiation of formation and crystallization of the ZnO film. After the deposition of the tenth layer, the resulting films were annealed in air at 550 C for 1 h. The thickness of the films was approximately 0.75 lm and the area of the films was 1.5 cm · 1.5 cm. The crystal structure and grain orientation of ZnO films were determined by X-ray diffraction (XRD, Rigaku Dmax 2200) with CuKa radiation. The surface morphology of the films was characterized by scanning electron microscope (SEM, Philips 30XL SFEG) and atomic force microscope (AFM, Digital Instrument Nanoscope-IV). Optical transmission and absorbance spectra of the films were analyzed by using UV–Visible spectrophotometer (Shimadzu2101PC). The chemistry of undoped precursor solution is thermally analyzed with DSC and TGA (Mettler Toledo Star System). 3. Results and discussion 3.1. Thermal analysis of the dried undoped ZnO gel Thermal analysis of the dried gels obtained from the undoped solution showed three endothermic and one exothermic reactions in the DSC graph (Fig. 1(a)) and three clearly distinguishable weight loss steps in the TGA graph (Fig. 1(b)). In the first step (RT-150 C), solvents and polyethylene glycol evaporated. Furthermore, the endothermic reaction in DSC graph with the peak around 120 C (Fig. 1(a)), corresponding to a slope increase in weight loss (Fig. 1(b)), is attributed to evaporation of water that is chemically bonded to ZnAc. In the second step (215–263 C), EA boils at lower temperatures (BPEA = 217 C). Then, ZnAc melts (MPZnAc = 237 C) with an endothermic peak around 249 C which is also accompanied by a weight loss peak at 238 C. In the third step (299–403 C), ZnO is crystallized with a very small exothermic peak around 350 C and residual organics are simultaneously evaporated. 3.2. The grain orientation in the ZnO films XRD analyses showed that all samples were polycrystalline and consist of single phase ZnO with zincite structure. The diffraction peaks belonging to (1 0 0), (0 0 2), and (1 0 1)

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Fig. 1. (a) DSC, (b) TGA graphs of dried-gels obtained from undoped solution.

500 400

(002)

(101)

300 200

(100)

Intensity (a.u.)

planes were observed in all films prepared. Compared to powder diffraction data of zincite structure (JCPDS 361451), the XRD patterns of all the samples indicated enhanced intensities for the peaks corresponding to (0 0 2) plane indicating a preferential orientation along the c-axis. Grain orientation behavior was significantly affected by doping type (Fig. 2). The highest level of c-orientation was obtained in the undoped film and both Mn and Cu doping decreased the extent of c-orientation (Table 1). Previously, the effect of Cu doping on c-orientation of ZnO films has been inconsistently reported. Gonzalez [16] also detected a decrease in the extent of c-orientation while Lee et al. [20] observed an increase in the extent of c-orientation by Cu doping. There are numerous ideas about the basis of the c-orientation in ZnO thin films. Bao et al. [3] believed that the preferential orientation is caused by the minimization of internal stress and surface energy. Amirhaghi et al. [25] reported that c-orientation may be

100 0

ZnO:Cu ZnO:Mn undoped ZnO 30

31

32

33

34

35

36

37

38

39

2θ Fig. 2. XRD patterns of undoped, Cu-doped, and Mn-doped ZnO films.

Z. Banu Bahsßi, A.Y. Oral / Optical Materials 29 (2007) 672–678 Table 1 Lattice parameters and relative intensities of (1 0 0) and (0 0 2) peaks of doped ZnO films Doping type

a

c

I ð100Þ I ð100Þ þI ð002Þ þI ð101Þ

I ð002Þ I ð100Þ þI ð002Þ þI ð101Þ

Undoped Cu Mn JCPDS 36-1451

3.245 3.268 3.287 3.244

5.194 5.159 5.265 5.205

0.071 0.205 0.128 0.283

0.837 0.408 0.583 0.219

resulted from facilitated growth along c-axis due to highest atomic density found along (0 0 2) plane. Lee et al. [26] produced ZnO films by sol–gel processing and stated that strong preferred orientation is resulted by solvents with high boiling points since structural relaxation of the film

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before crystallization was more easily accepted by EA and the resulting organics at higher temperatures. The lattice parameters of the undoped film are very close to the parameters obtained from zincite structure (JCPDS 36-1451). Mn doping slightly increased both a and c parameters while Cu doping increased a and decreased c (Table 1). The enlargement in the c-axis lattice parameter by Mn doping has been previously reported by Posada et al. [27] and Cao et al. [18] and it is probably caused by larger Mn2+ ions (0.066 nm) substituting Zn2+ ions (0.060 nm). In that case, majority of the doped manganese in the film must be in the ionic state of Mn2+ rather than any other ionic state with smaller ionic radii. The ionic radii of Cu2+ (0.057 nm) and Cu1+ (0.060 nm) are the same

Fig. 3. SEM micrographs of (a) undoped ZnO film at high magnification, (b) undoped ZnO film at low magnification, (c) Cu-doped ZnO film at high magnification, (d) Cu-doped ZnO film at low magnification, (e) Mn-doped ZnO film at high magnification, (f) Mn-doped at low magnification.

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Z. Banu Bahsßi, A.Y. Oral / Optical Materials 29 (2007) 672–678

or smaller than ionic radii of Zn2+ (0.060 nm). Therefore, the increase in the lattice parameter a cannot be solely explained by ionic radii difference. The complex defect formation (Eq. (2)) may be responsible for the enlargement in a in ZnO:Cu films. 3.3. The morphology of the ZnO films Fig. 3 shows the SEM micrographs of doped and undoped ZnO films. The average grain size of undoped and Mn-doped films were below 50 nm. Cu doping

increased the average grain size to above 50 nm. Low magnification micrographs showed that the film density decreased as the grain size increased and Mn-doped film with the smallest grain size had the highest density. Atomic force microscope (AFM) was used to measure the surface roughness of the films over a 5 · 5 lm area by contact mode. Compared to undoped films, it was clearly visible that both Mn and Cu doping increased the surface roughness (Fig. 4). Undoped films had a very smooth surface with a root mean square (RMS) surface roughness of 0.23 nm (Fig. 4(a), Table 2). This value increased to

Fig. 4. AFM micrographs of (a) undoped, (b) Mn-doped, and (c) Cu-doped ZnO films.

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Undoped Cu Mn

0.232 1.149 0.998

0.99 nm for Mn-doped films and 1.15 nm for Cu-doped films (Table 2). 3D AFM micrographs indicated that the increase in the surface roughness in Mn doped film was caused by cluster formation of smaller grains (Fig. 4(b)). In the Cu-doped film, both surface defects (mainly nanopores) and the increase in the grain size resulted in the higher surface roughness (Fig. 4(c)). 3.4. Optical properties of the ZnO films The optical band gap of the ZnO thin films was estimated by extrapolation of the linear portion of a2 versus hm plots using the relation ahm = A(hm  Eg)n/2 where a is the absorption coefficient, hm is the photon energy and Eg is the optical band gap. For different n values, a good linearity was observed at n = 1 (direct allowed transition) which was found to give the best fit for these films. Optical band of intrinsic ZnO shows variations depending on its fabrication process. In general, polycrystalline thin films possess a higher band gap due to an electric field forming at the grain boundaries or imperfections caused by the potential barriers of free carrier concentration gradient [7]. The measured optical band gap values of the films were between 3.26 (Cu and Mn-doped) and 3.28 eV (undoped) (Table 3), which were in the range of band gap values of intrinsic ZnO (3.2–3.3 eV) measured by previous researchers at room temperature [1,14]. Both doping elements (Mn or Cu) resulted in a slight decrease in the band gap values. Assuming doping levels are well below Motts critical density, the change in the band gap values can be explained by Burstein–Moss effect. At high doping concentrations, Fermi level lifts into the conduction band. Consequently, due to the filling of the conduction band, absorption transitions occur between valence band and Fermi level in the conduction band, instead of valence band and the bottom of the conduction band. This change in the transition levels shifts the absorption edge to higher energies (blue shift) and leads to the energy band broadening (DEg) which can be calculated by the following equation [26]:  2=3 h2 3 DEg ¼ ne2=3 ð3Þ 8m p

3=2

aðhmÞ ¼ AE0 expðhm=E0 Þ

where E0 is the parameter describing the width of the localized states in the band gap, a is the absorption coefficient, A is a constant, hm is the energy of light, and Eg is the band gap [28]. Since absorption coefficient is the cumulative effect of all defects (i.e. point, planer etc.), it is difficult to pinpoint the nature of variation in E0. The undoped film with the smoothest surface had the smallest width of the band tail and both type of doping increased the widths of the tails (Table 3). All films showed high transparency in the visible range regardless of doping type (Fig. 5). In the near IR range, the energy of the light is well below band gap of the films and the wavelength of radiation is much above the grain size, pore size or mean surface roughness value of the films. Therefore, in this range there is no scattering in the films and the loss of transparency is mainly by the reflection of undoped ZnO:Mn ZnO:Cu

100 90 80

1 .0 x1 0

9

70

8 .0 x1 0

8

6 .0 x1 0

8

4 .0 x1 0

8

2 .0 x1 0

8

Z n O :C u a n d Z n O :M n (o ve rla p p e d ) E g = 3 .2 6 e V

60 50 40 30

undoped ZnO

20 Table 3 The optical band gaps and band-tails of doped ZnO thin films Doping type Undoped Cu Mn

Calculated optical band gap (eV) 3.25 3.27 3.28

Band-tail energy (meV) 62 74 80

ð4Þ

for hm < Eg

2

Surface roughness RMS (nm)

(αhν)

Doping type

where h is the Planck’s constant, m* is the electron effective mass in conduction band, and ne is the negative carrier concentration. Both type of doping (Mn or Cu) caused a reduction in carrier concentration donated by interstitial zinc atoms or oxygen vacancies at room temperature. Therefore, the Fermi level shifted to lower energies. Consequently, the band gap values of doped ZnO films decreased due to the decrease in Burstein–Moss shift. Band tailing can be explained with local perturbations of the band edges by the effect of impurities or any other defect. Density-of-states distribution integrates the number of states at each energy inside the whole volume and reveals the existence of conduction band states at relatively low potentials and valence band states in high potential regions (band tails) due to local perturbations [1]. Band tails cause absorption below energy gap and alter the absorption edge from a steeply rising one to exponentially increasing one. The width of the band tail can be calculated by Pankove’s [1] expression,

Transmission (%)

Table 2 The effect of doping on surface roughness

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0 .0 3 .2

10 0 300

400

500

E g = 3 .2 8 e V 3 .3

E n e rg y (e V )

600

700

3 .4

800

Wavelength (nm) Fig. 5. Transmittance spectra of undoped, Cu-doped, and Mn-doped ZnO films.

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the light. However, the UV transparencies of all the films were much lower due to electron transitions between the valence and conduction bands. Furthermore, since UV radiation has shorter wavelength, it can be scattered by smaller defects. The substantial decrease in UV transparency in doped films must be caused by the scattering from pores and other defects heavily present in doped films. 4. Conclusion Both undoped and doped (Mn or Cu) ZnO films were prepared by a sol–gel method. The films were transparent and consisted of single phase ZnO with zincite structure. All the films showed preferred orientation in c-axis and both Mn and Cu doping decreased the extent of orientation. Cu doping increased the grain size of the films while Mn doping slightly decreased it. Both doping elements increased the surface roughness of the ZnO films and the width of the band tails, and decreased the band gaps. References [1] J.I. Pankove, Optical Progress in Semiconductors, Dover Publications, New York, 1975. [2] J.H. Lee, K.H. Ko, B.O. Park, J. Cryst. Growth 247 (2003) 125. [3] D. Bao, H. Gu, A. Kuang, Thin Solid Films 312 (1998) 37. [4] P. Mitra, A.P. Chatterjee, H.S. Maiti, Mater. Lett. 35 (1998) 33. [5] B.B. Rao, Mater. Chem. Phys. 64 (2000) 62. [6] R.W. Birkmire, E. Eser, Annu. Rev. Mater. Sci. 27 (1997) 625. [7] S. Jager, B. Szyszka, J. Szczyrbowski, G. Brauer, Surf. Coat. Technol. 98 (1998) 1304. [8] Y. Yoshio, T. Makino, Y. Katayama, T. Hata, Vacuum 59 (2000) 538.

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