Study on structural and optical properties of Mn-doped ZnO thin films by sol-gel method

Study on structural and optical properties of Mn-doped ZnO thin films by sol-gel method

Optical Materials 100 (2020) 109657 Contents lists available at ScienceDirect Optical Materials journal homepage: http://www.elsevier.com/locate/opt...

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Optical Materials 100 (2020) 109657

Contents lists available at ScienceDirect

Optical Materials journal homepage: http://www.elsevier.com/locate/optmat

Study on structural and optical properties of Mn-doped ZnO thin films by sol-gel method Xu Li a, Xinghua Zhu a, b, Kangxin Jin a, Dingyu yang a, * a b

College of Optoelectronic Technology, Chengdu University of Information Technology, Chengdu, 610225, China Sichuan University of Arts and Science, Dazhou, 635000, China

A R T I C L E I N F O

A B S T R A C T

Keywords: ZnO thin films Manganese doping Structural and optical properties Doping concentration and thickness

For ZnO, doping transition metal is one of the powerful tools to improve its photoelectric performance. In this study, Mn-doped ZnO thin films with various thickness and different doping concentration were synthesized by low cost sol-gel method. We systematically investigated structural and optical properties of Mn-doped ZnO with different concentrations and film thickness. Research showed that with the increase of Mn doping concentration, the crystallization of ZnO decreased, and the preferred orientation was weakened. Moreover, the ultra­ violet–visible (UV–Vis) absorption coefficient rose as the energy gap (Eg) decreases and Urbach energy (EU) increases). In the intrinsic emission region below 380 nm and in the defect emission region of 440–600 nm, Mndoped 1.5% ZnO showed the strongest emission, then the excessive doping would reduce photoluminescence of ZnO. However, in the defect emissions region of 380–440 nm, pure ZnO showed highest emission that decreased gradually with the increase doping concentration. Meanwhile, as the increase of film thickness, the crystalli­ zation properties of ZnO were improved, but there were more cracks which resulted in the decrease of UV–vis absorption coefficient, the decrease of Eg and the increase of EU). Meanwhile, the intrinsic emission enhanced and the defect emissions decreased.

1. Introduction Recent research indicates that the application prospects of ZnO are expanding and in some areas can even replace SiO2 and GaN semi­ conductor materials [1]. As a new type of direct wide band gap II-VI metal semiconductor material, ZnO has a hexagonal closest densely packed wurtzite structure with a band gap value of about 3.37 eV, which corresponds to ultraviolet range and is higher than the energy of visible light. ZnO thin films have a high transparency and a low absorption coefficient in the visible light region, the exciton binding energy is about 60 meV, which can be used as a blue or ultraviolet laser material [2,3]. Divalent cations to change the forbidden bandwidth and optical prop­ erties of ZnO [4]. Many studies have reported the use of elements such as Al, In, Sm, Mg, Li or co-doped ZnO [5–8]. Mn is one of commonly used metallic dopants in ZnO thin films, many publications have dealt with the study on some physical properties of Mn-doped ZnO thin films synthesized under various experimental conditions [9,10]. In order to improve research further, we have adopted good control and low cost sol-gel method with simple process, to grow Mn-doped ZnO with different concentrations and different film thickness on glass substrate.

The effect mechanisms of doping concentration and film thickness on the structure, morphology and optics properties of ZnO films were investigated. 2. Experimental details 2.1. Sample preparation Zinc acetate dihydrate powder as the metal precursor was weighed on the analytical balance with the four parts of the same quality mass, and then crystalline manganese chloride tetrahydrate as the doping agent was mixed into them. Mn ion doping atomic mole ratios (Mn/Zn) were 0%, 1.5%, 3%, and 4.5% respectively. These four mixtures were dissolved in four amounts of anhydrous ethanol respectively, while ethanolamine was used as stabilizer. In a magnetic stirrer, these mix­ tures were stirred for 1 h in a water bath heated at 60 � C to form initial sol, and aged them at room temperature 20 � C for 24 h to form a uniform transparent wet gel to be spin coated. Use the spin coater, and set the parameter to low speed of 1000 rad/min for 10 s and high speed of 3000 rad/min for 30 s, then the coating can be started. After coating, it is

* Corresponding author. E-mail address: [email protected] (D. yang). https://doi.org/10.1016/j.optmat.2020.109657 Received 26 August 2019; Received in revised form 6 December 2019; Accepted 1 January 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved.

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baked at 200 � C for 8 min and cooled to room temperature 20 � C. The coating processes were repeated for 3–7 times. Finally, all the samples were annealed at 450 � C for 1.5 h. The temperature was raised to 450 � C at the rate of 5 � C/min and then naturally cooled to 20 � C at room temperature.

results of which are shown in Fig. 1(b). Fig. 1(b) shows that as the number of layers increases, the intensity of the diffraction peaks in­ creases, indicating its crystallinity becomes better. The grain sizes calculated according to the Scherrer formula are 35 nm, 44 nm, and 48 nm, respectively. Lattice parameters (a and c) of all samples were calculated using the following formula [9]:

2.2. Characterization techniques Using Fang yuan DX-2700 X-ray diffractometer with Cu Kα radiation (λ ¼ 1.54056 Å) for varying angles from 20� to 70� to produce X-ray diffraction (XRD) patterns of samples. Using scanning electronic mi­ croscope (Zeiss ULTRA 55 SEM) observed the morphology of samples. The ultraviolet–visible (UV–Vis) transmittance properties of samples were measured by Shimadzu UV-2550 spectrometer in 300–800 nm wavelength range. To further analyze the optical properties of the samples, Shimadzu RF-5301PC fluorescence spectrometer was a tool to record photoluminescence spectrum (PL).

λ sinðθÞð002Þ

(3a)

Where λ ¼ 0.154 nm is the X-ray wavelength for CuKα radiation, and θ is the Bragg’s diffraction angle. The values of a, c and the FWHM of the peak (002) are gathered in Table 1. The doping of foreign atoms, the difference in defects and the ionic radius can all change the size of the lattice parameters. Due to the difference in the radius of manganese ions and zinc ions, the values of a and c have increased, and the values of FWHM changed. Previous studies have reported that ZnO films do not easily grow well when the film thickness is thin, because the crystal lattice of ZnO does not match the substrate. As the film thickness increases, the mismatch­ ing effect between the glass substrate and the ZnO lattice reduces [13]. Hence, the crystallization of ZnO increases.

3.1. Structural analysis Fig. 1(a) shows the XRD diffraction patterns of different concentra­ tions of Mn-doped ZnO thin films (Please note that the ZnO:Mn means Mn-doped ZnO), and all films are single-phase. Compared with powder diffraction standard card JCPDS No.36–1451, the (100), (002) and (101) planes diffraction peaks prove that it has a hexagonal wurtzite structure [11,12]. The pure ZnO film has a clear preferential growth trend to­ wards the (002) direction. But the addition of manganese ions limits the crystal preferred orientation, the intensity of the visible diffraction peak decreases sharply, and the average grain size decreases. The average size of the crystal grain can be calculated by the Scherrer formula. The for­ mula can be written as formula (1): kλ F � cos θ

(2a)



3. Results and discussions



λ a ¼ pffiffi 3 sinðθÞð100Þ

3.2. Surface morphology The surface morphology of ZnO films was characterized by scanning electron microscopy (SEM). As shown in Fig. 2(a–c), the surface of pure ZnO shows a dense grains distribution, and obvious hexagonal nanorodshaped grains were grown in the direction of (002) plane. J. Li et al.

(1)

Table 1 The lattice parameters, FWHM for (002) peak, and 2θ values obtained from XRD curves for ZnO and MZO thin films.

Where D means average grain size, F is full width at half maximum (FWHM) of diffraction peaks. k ¼ 0.9 is a constant, λ ¼ 0.154 nm is the Xray wavelength, θ in the above formula represents Bragg angle of diffraction peak. Based on the Scherrer formula, the crystal sizes are about 50 nm, 45 nm, 39 nm, 35 nm with the Mn-content at 0%, 1.5%, 3%, 4.5% respectively. The results show that the addition of manganese destroys the crystallization. Previous study has shown the increase of film thickness could improve the crystallization [13]. Therefore, in order to improve the crystalline quality of Mn-doped ZnO thin films at 4.5%, we further prepared 5 layers and 7 layers of film thickness, and the

Sample

ZnO ZnO:Mn 1.5% ZnO:Mn 3% ZnO:Mn 4.5% ZnO:Mn 5 layer ZnO:Mn 7 layers

Lattice Constants (nm)

2θ(deg)

FWHM(deg)

a

c

(100)

(002)

0.3189 0.3201 0.3212 0.3222 0.3209 0.3199

0.5032 0.5046 0.5061 0.5077 0.5067 0.5050

32.38 32.26 32.15 32.04 32.18 32.28

35.650 35.548 35.442 35.322 35.40 35.52

0.3236 0.2025 0.1898 0.1903 0.2034 0.2048

Fig. 1. (a) XRD patterns of ZnO thin films with different doping Mn concentration; (b) XRD patterns with various layers for thin film doped at 4.5%. 2

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reported that the formation of nanorods on thin films could be due to the uneven distribution of growth energy [14]. Then, with the increase of doping concentration to 4.5%, grain size decreases gradually and hex­ agonal nanorods disappear (Fig. 2(d)). Further, as the film thickness increases, the grain size rises(increase) gradually, as shown in Fig. 2 (e–f). All SEM images reveal a good agreement with the results of XRD patterns. Nevertheless, it can be seen from Fig. 2(d–f) that the increase of film thickness induces some cracks. These cracks in SEM images have the darker color. Thus, we can enhance the contrast between black and white, and then the cracks will show obvious black in SEM images, which can makes a good distinguishing between grains and cracks. As shown in Fig. 2(d-1, e 1 & f-1), the number of cracks increases with the increase of thickness, which will cause a decrease in light absorption (this will be proven in the next 3.3 section). As the thin film thickness increases, the formation of cracks can be attributed to that the insuffi­ cient constraint of substrate to the thicker film. The weakening substrate restraint is insufficient to restrain stress and strain in the films, resulting in cracks.

nm, 1190 nm and 1415 nm for 3 layers, 5 layers and 7 layers respec­ tively. From Fig. 3(a–b), we can obtain that transmittance of pure ZnO film is approximately 80% in the range of 500–700 nm, while the transmittance reduces to only 20% upon doping concentration to 4.5%. The reduced transmittance of the film means an effectively increase in the absorption of photon energy. The transmittance of the ultraviolet and visible light regions of the film is greatly reduced, indicating that the doping effect can effectively improve the optical absorption character­ istics. The observed decrease in transmittance of the film may be due to the increase of the defect concentration. As shown in Fig. 3(c–d), as the film thickness increases, the thicker film causing the decrease in trans­ mittance seems to show an enhanced absorption property (Fig. 3(c)), however the absorption coefficient decreases actually due to light loss caused by films cracks (Fig. 3(d)). 3.3.2. Optical band gap and Urbach energies It can be seen from Fig. 3(b) and Fig. 3(d), with the increase of film thickness and Mn doping concentration, the absorption edge of the films begins to move, meaning a change of energy gap Eg Ref. [15]. In order to calculate the optical energy gap (Eg) of the thin films, formula (4) can be utilized: � (4) ðαhvÞn ¼ C0 � hv Eg

3.3. Optical properties 3.3.1. Transmisttance and absorption spectra The optical properties of ZnO thin films are closely related to its structure, especially for its transmittance and absorption properties. The transmittance (T) and absorption coefficient of the films can be expressed by the following formula (2) and (3): T ¼ eð

αdÞ

� � � � 1 1 � ln α¼ d T

Where C0 is a constant, hv means photon energy, Eg is optical energy gap, and n ¼ 2 for the direct band gap semiconductor. As shown in Fig. 4 (a) and Fig. 4(b), the Eg decreases from 3.287 eV to 3.259 eV as doping amount rises, and the Eg decreases from 3.259 eV to 3.163 eV as the thickness increases. Previous studies reported that the sp-d spin ex­ change interaction between the band electrons and localized spin of the transition metal ions might contribute the decrease in Eg of ZnO [16]. Nevertheless, most reports believe that the increase of defects amount and disorder is still the main origin of Eg decrease [17–19]. Indeed, with the increase of defect, the electronic transitions occur from the filled

(2b) (3b)

Where α is absorption coefficient. d represents the thickness of the film, In this experiment, the measured thicknesses of thin films are about 913

Fig. 2. SEM images of different concentrations of Mn doped ZnO and different film thickness; (a) Mn-doped 0%, (b) Mn-doped 1.5%, (c) Mn-doped 3%, (d) Mn-doped 4.5%, (e) Mn-doped 4.5% with 5 layers, (f) Mn-doped 4.5% with 7 layers. (d-1, e 1, f-1) As enhancing contrast between black and white colors, corresponding to Mndoped 4.5% with 3 layers, 5 layers and 7 layers, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 3

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Fig. 3. (a) Transmittance of undoped and Mn doped ZnO; (b) absorption coefficient of undoped and Mn doped ZnO; (c) transmittance of various layers of ZnO at 4.5% Mn doping concentration; (d) absorption coefficient of various layers of ZnO at 4.5% Mn doping concentration.

valence band to defects energy levels, instead of those commonly happening from the filled valence band to the empty conduction one [17]. Both the defect state and disorder produce band tails and nar­ rowing Eg Ref. [18]. Usually, Urbach energy (EU) can be used to analyze the disorder degree and defect concentrations in the material semi-quantitatively as formula (5) [19]: lnðαÞ ¼ C þ ðhv

E0 Þ = EU

decreases with the increase of doping concentration. Fig. 5(b) shows PL of ZnO thin films with different thicknesses at 4.5% doping concentra­ tion. Since photoluminescence mainly occurs on the surface of the film, the PL intensity is closely related to defect concentrations of film surface not that of whole films. Due to the increase of crystallization, the defects on film surface are reduced as the film thickness increases, so that Fig. 5 (b) exhibits a stronger intrinsic emission peak in the ultraviolet 360 nm (3.44 eV) and the weaker defect emissions in the visible 390–600 nm (2.06–3.17 eV). In addition, the morphology and size of the samples also affect its PL properties, PL spectrum shows that the doping only changes the intensity of the PL spectrum and does not change its emission wavelength [20]. In order to better analyze photoluminescence mechanism, the emissions in visible region are fitted using Gauss mode as shown in Fig. 5 (c). The visible emissions between 390 nm and 600 nm (2.06–3.17 eV) are considered to come from intrinsic point defects [21]. Among the intrinsic defects of the ZnO thin film, point defects such as interstitial zinc, zinc vacancy and oxygen vacancy are most easily formed due to the lowest energy [22,23]. In the range of 380–440 nm, two main emissions located at 390 nm and 424 nm can be derived from the near band edge transition of electrons [23]. As the Mn doping concentration increases, some non-radiative centers may be formed in near band edge so that these emissions in the range of 380–440 nm are weakened (Fig. 5(a)). Since the energy level of the interstitial zinc is close to the conduction band, and that of zinc vacancy is close to the valence band, we take up the position that the light emissions at 451 nm, 470 nm and 472 nm are caused by the electrons transition from the interstitial zinc level to the valence band according to the literature [14,24]. Meanwhile, due to that

(5)

Where both C and E0 are constants. As shown in Fig. 4(c), the EU in­ creases from 112 meV to 142 meV with doping. As this energy is asso­ ciated to the lattice disorder, it can be suggested that the incorporation of manganese ions leads to an increase of disorder in the ZnO. Mean­ while, as shown in Fig. 4(d) the local cracking of the thick films increases the overall defect concentrations of the films, so the EU rises from 142 meV to 206 meV with the thickness increases. 3.3.3. Photoluminescence (PL) performance As shown in Fig. 5, the excitation wavelength is 320 nm. Fig. 5(a) shows the photoluminescence spectra of ZnO films doped with different concentrations. The samples have obvious intrinsic and defect emission peaks in the ultraviolet and visible regions, respectively. In the ultra­ violet region below 380 nm and in the visible region of 440–600 nm (2.06–2.81 eV), the strongest PL emissions are 1.5% Mn-doped thin films. As the doping concentration increases, the PL intensity gradually decreases. Even, upon doping to 4.5%, PL intensity begins to be lower than that of pure ZnO. However, between 380 nm and 440 nm (2.81–3.26 eV), the emission intensity of pure ZnO is the highest and 4

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Fig. 4. (a) Plot of (αhν) 2 vs. hν for various Mn concentration; (b) plot of (αhν) concentration; (d) plot of ln(α) vs. hν for various for various thickness.

2

the oxygen vacancy defect has a deep donor level, the light emissions at 520 nm (2.38 eV) may come from the oxygen vacancy related electrons transition [14,24]. At doping 1.5%, Fig. 5(a) shows that those emissions after 440 nm are immediately enhanced, which can be attributed to the increase of defect concentration. But the intrinsic emission is also enhanced, which is possibly due to the quantum size effect and the sudden decrease in specific surface area. And then all the light emissions are weakened as the doping concentration increases to 4.5%. This trend is clearly inconsistent with the increase in defects. Some people assumed that the doping concentrations might be too high making the fluores­ cence quench [25]. If this assumption is right, the detect emissions are desired to increase slightly by appropriately reducing defect concen­ trations. However, comparing the Mn-doped 4.5% thin films that have better crystal quality and lower surface defect concentration with rising thickness, it can be found from Fig. 5(b) that these thin films show the stronger the intrinsic emission is, the weaker the defect emission is, which is a normal trend, and the desired defect emission enhancement does not occur. Thus, such the defect concentration is insufficient to cause fluorescence quenching. On the other hand, comparing XRD again, it can be found that the preferred orientation of the film gradually weakens and suddenly disappears at a concentration of 4.5%, which is very related to the luminescence tendency, hence the weakened lumi­ nescence of thin films after 450 nm with the doping concentration rises from 1.5 to 4.5% might be due to the decrease in preferred orientation growth instead of fluorescence quenching.

vs. hν for various for various thickness; (c) plot of ln(α) vs. hν for various Mn

4. Conclusions Mn-doped ZnO films with different doping concentration and different thickness were prepared on glass substrates by low-cost sol-gel method. XRD and SEM characterization show that all samples have a hexagonal wurtzite structure. The crystal quality of the ZnO thin film decreases with the introduction of the Mn impurity, and the growth trend of preferred orientation weakens. Further, as the thickness of the film increases, the crystallization of sample can be improved while more cracks occur. As the doping concentration increases, the ultra­ violet–visible light transmittance of the film decreases and the absorp­ tion coefficient increases, which is due to the increase of the defect concentration. As the film thickness increases, the absorption coefficient decreases, which is due to the increase of the number of cracks. In addition, the 1.5% Mn-doped ZnO film has excellent photoluminescence (PL) properties, but the PL intensity of thin films decreases gradually as the doping concentration increases from 1.5% to 4.5%. With the film thickness increases, due to the improved surface crystallization and the declined surface defect concentration, thin films have the higher intrinsic emission and the lower defect emissions. Generally speaking, this work provides a further reference for the study on structural and optical properties of Mn-doped ZnO thin films. Authors’ contributions Xu Li conceived of the study, designed the study and collected the 5

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Fig. 5. PL spectra of ZnO thin films; (a) with different doping concentrations, (b) with various thickness of ZnO at doping Mn 4.5% concentration, and (c) PL fitting chart of ZnO.

data. All authors analysed the data and were involved in writing the manuscript.

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