Materials Chemistry and Physics 96 (2006) 326–330
Mn doped and undoped ZnO films: A comparative structural, optical and electrical properties study V.R. Shinde a , T.P. Gujar a , C.D. Lokhande a , R.S. Mane b , Sung-Hwan Han b,∗ a
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur-416004 (M.S.), India b Inorganic Nano-Materials, Department of Chemistry, Hanyang University, Sungdong-Ku, Haengdang-dong 17, Seoul 133-791, Republic of Korea Received 16 March 2005; received in revised form 7 June 2005; accepted 15 July 2005
Abstract Undoped and manganese doped zinc oxide (ZnO) thin films were prepared by pyrolytic decomposition of aqueous solution onto glass substrates. The structural properties studied using X-ray diffraction showed that the undoped ZnO films exhibit hexagonal wurtzite structure with strong c-axis orientation, however Mn doped ZnO films were polycrystalline. The surface morphological studies from SEM depicted the formation of clusters like structure of undoped ZnO while the Mn doped film showed the nanocrystalline grains on the surface. From the optical studies, the transmittance in the wavelength range 350–850 nm was found to be decreased after doping of Mn. The optical band gap was found to be 3.3 eV for undoped ZnO film and 3.10 eV for Mn doped films. From the electrical resistivity measurement, it is found that the Mn doping significantly caused to increase the room temperature resistivity from 104 to 106 cm. © 2005 Elsevier B.V. All rights reserved. Keywords: Mn doped and undoped ZnO films; Spray pyrolysis; Electrical; Optical; Structural properties
1. Introduction Zinc oxide is one of the versatile and important oxide material because of its typical properties such as resistivity control over the range 10−3 –105 cm, transparency in the visible range, high electrochemical stability, direct band gap (3.37 eV), absence of toxicity, abundance in nature, etc. [1]. It crystallizes in a wurtzite structure and exhibits n-type conductivity [2]. ZnO thin films have been used in varistors [3], gas sensors [4], solar cell transparent contact fabrications [5], surface acoustic wave systems [6], UV laser [7] etc. Another interesting application of ZnO is in the field of spintronics [8,9]. Spintronics is an emerging field in physics focused on spin-dependent phenomena applied to modern electronic devices. Diluted magnetic semiconductors (DMS), which combine the two interesting properties: semiconducting and magnetic, are considered as an ideal system for spintronics [10,11]. Zinc oxide is one of oxide DMS, to which addition of ∗
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[email protected] (S.-H. Han).
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Mn causes to turn it into ferromagnetic material with Tc above ˚ is room temperature [12]. The ionic radius of Mn+2 (0.66 A) ˚ relatively close to that for Zn (0.60 A), suggesting moderate solid solubility without phase segregation [13]. Furthermore, Mn was assumed to be a deep donor in ZnO with the energy levels at 0.45, 0.7, or 2.0 eV below the conduction band edge at room temperature [14]. Many researchers have worked on Mn doped ZnO in bulk and thin film form, keeping different views. Kim et al. [10] have studied the change in microstructure and growth behavior of Mn doped ZnO films epitaxially grown on Al2 O3 (0 0 0 1) substrates by pulsed-laser deposition. Jin et al. [15] have fabricated epitaxial Zn1−x Mnx O (x < 0.22) thin films by combinatorial laser molecular-beam epitaxy method to study the blue and ultraviolet luminescence from this system. Fukumura et al. [16,17] have obtained epitaxial thin films of Mn doped ZnO by pulsed-laser deposition, with Mn substitution as high as 35%, while maintaining the wurtzite structure. Sharma et al. [18] have observed ferromagnetism above room temperature in both bulk and thin film forms of Mn doped ZnO.
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Some studies have been carried out to realize Mn doped ZnO as a DMS, however studies to understand the other properties of Mn doped ZnO are also necessary. Also, as far as the importance of Mn doped ZnO is considered, it is necessary to obtain Mn doped ZnO films by inexpensive technique. In the present work, for the first time, using the simple and inexpensive chemical spray deposition technique, Mn doped ZnO films have been deposited and the structural, surface morphological, optical and electrical properties of ZnO and Mn doped ZnO have been investigated. Since most of the results [18–20] show that Mn doped ZnO has shown ferromagnetism for doping concentration up to 5%, hence results of 5% Mn doped ZnO have been reported here.
2. Experimental Undoped and manganese doped zinc oxide films were deposited by spray pyrolysis technique. Aqueous zinc nitrate (0.1 M) was used to obtain undoped ZnO films and manganese chloride (0.1 M) [10:2] was used to obtain Mn doped films. The solution was sprayed through a glass nozzle onto the ultrasonically cleaned glass substrates kept at 623 K. The spray rate of 3 mL min−1 was maintained using air as a carrier gas. The temperature was controlled using electronic temperature controller. Hazardous fumes that evolved during the thermal decomposition of initial ingredient were expelled out. The nozzle to substrate distance was 28 cm. The thickness of undoped and doped ZnO films was measured by weight difference method using a sensitive microbalance. To study the structural properties of ZnO and Mn doped ZnO films, X-ray diffraction analyses were performed on a Philips (PW 3710) diffractometer ˚ Microstructural studwith chromium target (λ = 2.2896 A). ies were carried out using scanning electron microscopy (SEM). For this, the films were coated with gold–palladium (Au–Pd) using polaron SEM sputter coating with E-2500. The SEM micrographs were obtained with Cambridge stereoscan 250 MK-3 assembly. The optical absorption and transmission spectra were investigated within the wavelength range 350–850 nm using a Systronic spetrophotometer-119. A two-probe method was used for electrical resistivity measurement.
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as follows:
2Zn(NO3 )2 6H2 O + O2 −→2ZnO ↓ + 4NO2 ↑ + 2O2 ↑ +12H2 O
(1)
Adherent zinc oxide thin films were obtained on the glass substrate. As-deposited films were transparent with faint white in color. The manganese-doped films were slightly brownish in appearance. The zinc oxide and Mn doped ZnO thin films were used for the further characterization. 3.2. Structural studies Fig. 1(a) and (b) shows the X-ray diffraction patterns for the undoped and Mn doped ZnO films. The ‘d’ values of the both undoped and Mn doped thin films were in good agreement with those reported in the PDF for ZnO [PDF 79˚ and c = 5.2065 A], ˚ possessing hexagonal 206, a = 3.2499 A wurtzite structure. It is further seen that the undoped films
3. Results and discussion 3.1. Film formation Aqueous solution of zinc nitrate, when sprayed over the hot substrates, fine droplets of solution thermally decompose after falling over the hot surface of substrates. This results in the formation of well adherent and uniform zinc oxide film. The possible chemical reaction that takes place is
Fig. 1. The XRD patterns of undoped and Mn doped zinc oxide thin films.
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exhibit a strong orientation along c-axis (0 0 2). Other orientations corresponding to (1 0 0) and (1 0 1) are present with low relative intensities (4 and 13%, respectively) as compared to that of (0 0 2) plane. The XRD pattern of Mn doped ZnO shows hexagonal wurtzite structure along with no additional peaks corresponding to manganese oxide. However, XRD pattern of Mn doped ZnO film shows the polycrystalline nature. The relative intensities of the planes (1 0 0), (0 0 2) and (1 0 1) are 84, 62 and 100%, respectively, which indicates the orientation of undoped ZnO along (0 0 2) has disturbed after Mn doping. Here during deposition of ZnO, Mn atoms may act as obstacle for the growth of ZnO along (0 0 2). 3.3. Surface morphological studies Surface morphological studies of the undoped ZnO and Mn doped ZnO films have been carried out from scanning electron micrographs. Fig. 2(a) and (b) shows the SEMs
of undoped and Mn doped ZnO films, respectively. From the micrographs one can see the total coverage of the substrate with the clusters of ZnO. The micrographs indicate that, during deposition of ZnO by spraying the solution onto heated substrates, the growth has taken place by nucleation and coalescence process. Randomly distributed (0 0 2) oriented nuclei may have first formed and these nuclei then have grown to form observable islands. As islands increase their size by further deposition and come closer to each other, the larger ones appeared to grow by coalescence of smaller one. Flattening of islands to give increased surface coverage of the clusters follows this step. However, the microstructure of ZnO contains some pits and nanoholes. These might be occurred during the deposition through which the gases have escaped due to the compact nature of ZnO clusters. The microstructure of Mn doped ZnO film has drastically changed. The Mn doped ZnO film shows the formation of nanosize smooth grains all over the surface. The Mn atoms may have disturbed the growth process resulting into formation of nanosize grains. Inset of Fig. 2(b) shows the nanocrystalline grains from the shaded region. The average grain size of Mn doped ZnO was found to be 40 nm from the micrograph. 3.4. Optical properties The optical absorption and transmission spectra of undoped and Mn doped ZnO films of thickness 0.56 m are shown in Fig. 3. The undoped ZnO shows the transmittance of the order of 85% while that of Mn doped ZnO film was of the order of 80%. It is noticed that the undoped film has lower absorbance in visible range of spectrum. The absorbance has slightly increased after Mn doping. This behavior may be due to the introduction of the Mn defect states within the forbidden band, which may lead to absorption of incident
Fig. 2. Scanning Electron micrographs (SEM) of (a) undoped (10,000×) and (b) Mn doped zinc oxide (40,000×) thin films.
Fig. 3. Variation of absorption (αt) and transmittance (%T) with wavelength (λ) of (a) undoped and (b) Mn doped zinc oxide thin films.
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photons. The absorption edge of the Mn doped ZnO film appeared to shift towards the longer wavelength side. Similar type of behavior has been reported in case of Co doped ZnO films by Kim et al. [21], which is because absorption higher energy photons causes activation of ‘spd’ exchange interactions and typical ‘dd’ transitions, which has led to the enhancement of ferromagnetic properties. Similar activations may be expected here for spray deposited Mn doped ZnO film. On the other hand, Tiwari et al. [20] have observed blue shift of band gap of Mn doped ZnO films deposited epitaxially on sapphire substrates, along with no evidence of ferromagnetism. The theory of optical absorption gives the relationship between the absorption coefficient α and the photon energy hν for direct allowed transition as a = (hν − Eg )1/2 / hν
(2)
This equation gives the band gap (Eg ) when straight portion of (αhν)2 against hν plot is extrapolated to the point α = 0. The plots of (αhν)2 versus hν of the undoped and Mn doped ZnO films are shown in Fig. 4. From Fig. 4 the band gap of undoped ZnO was found to be 3.3 eV. The band gap was found to be decreased to 3.1 eV for Mn doped ZnO. Kim et al [21] observed the band gap shift from 3.25 to 3.13 eV for Co doped ZnO film (8% doping concentration). 3.5. Electrical properties The two point D.C. probe method of dark electrical resistivity measurement was used to study the variation of electrical resistivity (ρ) with temperature. The variation of log ρ with reciprocal temperature (K−1 ) is depicted in Fig. 5. The electrical resistivity at room temperature of Mn doped
Fig. 5. Variation of dark resistivity (log ρ) with reciprocal of temperature of undoped and Mn doped zinc oxide thin films. Encircled part in fig shows metallic behavior.
ZnO was increased by two orders of magnitude than that of undoped ZnO film. The increased room temperature resistivity may be due nanocrystalline nature of the film, which has been evidenced from SEM [22]. The increase in resistivity is consistent with the argument of Han et al. [14], that a small amount of Mn below the solubility limit in ZnO makes ZnO resistive material at room temperature, because it depresses the carrier concentration. The resistivity curve exhibits a hump around T = 373 K at which the resistivity shows the metallic behavior from 353 K. This critical behavior of resistivity is commonly observed in magnetic semiconductors and is known to be due to the scattering of carriers by magnetic spin fluctuation via exchange interaction [23]. Further, it can be seen that electrical resistivity of Mn doped ZnO has two linear portions; first in the lower temperature range (300–350 K), is characterized by a small slope. In the higher temperature range (350–500 K), it is characterized by large slope. The activation energies in these two regions were calculated using the relation ρ = ρ0 exp (Ea /kT )
Fig. 4. Plot of (αhν)2 against h of undoped and Mn doped zinc oxide thin films (derived from Fig. 3).
(3)
where, ρ is the resistivity at temperature T, ρ0 is constant, k is Boltzmann constant, T the absolute temperature and Ea is the activation energy. The activation energy represents the location of trap levels below the conduction band. From Fig. 5, for Mn doped ZnO film, one can observe two distinct activation energy regimes. This clearly indicates that different scattering mechanisms are operative in the two regimes. The activation energy is 0.29 eV, at low temperature regime and at high temperature activation energy is 0.80 eV, which may correspond to deep donor states introduced by Mn doping. In case of undoped ZnO film, the activation energy 0.18 eV was obtained.
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4. Conclusions Mn doped ZnO and pure ZnO films have been prepared using simple and relatively economic spray technique. The XRD studies depicted that c-axis (0 0 2) oriented ZnO films were converted to polycrystalline after Mn doping. The surface morphological studies revealed that nanocrystalline grains with total coverage over the substrate were obtained by Mn doping which were initially microcrystalline clusters including pits and nanoholes. The optical band gap was found to be decreased from 3.3 to 3.1 eV by Mn doping. The resistivity measurement showed normal semiconducting behavior of pure ZnO with room temperature resistivity of the order of 104 cm; however, resistivity of Mn doped ZnO showed the critical behavior as magnetic semiconductor with increased room temperature resistivity.
Acknowledgements Authors are grateful to the UGC, New Delhi, for providing financial support through the DRS (II) phase. One of the author VRS is very much thankful to the Shivaji University, Kolhapur for the award of Departmental Research Fellowship (DRF). One of the authors (RSM) wishes to thank Brain Korea 21 for the award of post-doc fellowship. This work is partially supported by the Korean Science and Engineering Foundation (ABRL R14-2003-014-01001-0). References [1] B. Ismail, M. Abaab, B. Rezig, Thin Solid Films 383 (2001) 92.
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