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Influence of Mg content on tailoring optical bandgap of Mg-doped ZnO thin film prepared by sol-gel method
Accepted Manuscript Influence of Mg content on tailoring optical bandgap of Mg-doped ZnO thin film prepared by sol-gel method Md. Nasrul Haque Mia, Md. Firoz Pervez, Md. Khalid Hossain, Mohammad Reefaz Rahman, M. Jalal Uddin, Md. Abdullah Al Mashud, Himangshu Kumar Ghosh, Mahbubul Hoq PII: DOI: Reference:
S2211-3797(17)30602-2 http://dx.doi.org/10.1016/j.rinp.2017.07.047 RINP 820
To appear in:
Results in Physics
Received Date: Revised Date: Accepted Date:
9 April 2017 20 July 2017 20 July 2017
Please cite this article as: Mia, d.N.H., Pervez, d.F., Hossain, d.K., Rahman, M.R., Uddin, M.J., Mashud, d.A.A., Ghosh, H.K., Hoq, M., Influence of Mg content on tailoring optical bandgap of Mg-doped ZnO thin film prepared by sol-gel method, Results in Physics (2017), doi: http://dx.doi.org/10.1016/j.rinp.2017.07.047
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Influence of Mg content on tailoring optical bandgap of Mg-doped ZnO thin film prepared by sol-gel method Md. Nasrul Haque Mia1, Md. Firoz Pervez1, Md. Khalid Hossain1, Mohammad Reefaz Rahman2, M. Jalal Uddin3, Md. Abdullah Al Mashud3, Himangshu Kumar Ghosh1, Mahbubul Hoq1 1
Institute of Electronics, Atomic Energy Research Establishment, Bangladesh Atomic Energy Commission, Savar, Dhaka, Bangladesh. 2 University of Liberal Arts Bangladesh (ULAB) 3 Dept. of Applied Physics, Electronics & Communication Engineering, Islamic University, Kushtia, Bangladesh. Abstract Tailoring optical bandgap of ZnO nanostructured thin films doped with different elements facilitates potential material for photonic applications. Different methods of fabrication process result in different optical and structural properties for the same amount of Mg content. Therefore, details investigation of structural and optical parameters, and their correlation need to be revealed to utilize the fabricated thin films. In this work, Mg-doped ZnO thin film of 200 nm thickness was fabricated by sol-gel spin coating method on a glass substrate for four different Mg content levels. Multiple layers were deposited by a spin coater to increase the film thickness. The prepared thin films were characterized by SEM, XRD, EDS, and UV-vis spectroscopy. The spectroscopic analysis showed a uniform crystalline nanostructured surface with less structural defects, enhanced transmittance, and higher optical bandgap than that of pure ZnO nanostructured thin film. Change of Mg content from 2% to 8% facilitated tuning of bandgap in the range of 3.30 eV to 3.39 eV. Changing trend of structural and optical parameters with Mg content showed non-linear, non-monotonic relation. Indepth analysis of structural and optical properties provides crucial information to form a better view about bandgap dependency on structural parameters.
Keywords: Mg-doped ZnO; MgZnO; Bandgap tuning; sol-gel; spin coating.
1. Introduction Zinc Oxide (ZnO) is a compound II–VI n- type semiconductor exhibiting high exciton binding energy (~60meV), wide direct bandgap (~3.36eV), high redox potential, high resistance to radiation at room temperature. It already found enormous attention for practical application in photonics, optoelectronics, and spintronics as a promising material [1–9]. Though optical bandgap of ZnO is wide enough for solar cell window layer, liquid crystal display, heat mirrors and other optoelectronic devices applications, however, white LED, UV photo-detector, UV LED, etc. short-wavelength applications require higher bandgap than that of ZnO. Furthermore, tailoring bandgap is necessary to realize barrier layer, cladding layer, quantum well structures, etc. in heterostructures optoelectronic and photonic devices [4,8–11]. A feasible way of altering bandgap of ZnO is doping that shifts fermi level creating impurity states [12]. Thus, bandgap of zinc oxide can be changed by alloying it with group II and III elements, e.g. Be, Mg, Ca, Cd [5][6][10]. While dopant element is selected, the radius of dopant element need to be Page 1 of 14
considered since dopant atoms of similar radius are recommendable to produce compound material with less lattice distortion. Mg2+ ion radius (0.57 Å), which has comparable radius to Zn2+ ion radius (0.60 Å), makes Mg a suitable dopant which can replace Zn atom in the lattice. Moreover, when Mg is alloyed to form a solid solution of MgxZn1−xO, the higher bandgap of MgO (~7.5eV) facilitates widening the bandgap in the deeper UV region, making useful for various applications. Thus, varying the content of Mg in ZnO lattice provides a way of tuning bandgap [7][8][13]. Doping changes lattice properties along with optical bandgap since ZnO and MgO have different lattice structures due to electronegativity and subtle ionic radius difference between Zn and Mg ion. If the Mg content is large, it creates phase separation [7][14]. For this reason, it is difficult to obtain single phase pure hexagonal wurtzite MgZnO when Mg content is increased beyond a certain level [8][9]. Besides, solubility of MgO in MgZnO depends on fabrication process and other growth conditions affecting crystallinity of MgZnO alloy [6][10][14] [15]. Moreover, interstitial atoms caused by sub-lattice localization influence the structural, electronic and optical properties of MgZnO alloy [14]. As optical characteristics also influence by structural parameters, therefore, in-depth investigation of structural and optical parameters is required to obtain certain features of thin film desirable for photonic and optoelectronics devices [13]. Many reports already have been published in this regard. However, some previous study gives controversial results of the nonlinear trend of lattice parameters with the increase of Mg concentration [8][16][17]. There are many different techniques to synthesize Mg-doped ZnO thin film such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), plasma enhanced chemical deposition (PECVD), r.f magnetron sputtering, spray pyrolysis, atomic layer deposition (ALD), pulsed laser deposition (PLD), electron beam evaporation, solgel method etc. [5][10]. Each synthesized technique has unique advantages compared to others. Among these, Sol-gel method facilitates several distinct advantages such as the use of cheap and simple equipment with easy composition control [5][6][10][18]. However, various factors e.g. growth technique, precursors, temperature, amount of dopant, etc. influence structural and optical properties of the thin film. In this work, we fabricated MgxZn1−xO thin film with four different concentrations of Mg content on glass substrates by sol-gel method. Then, we investigated the structural and optical properties of Mgdoped ZnO for different Mg content using SEM, XRD, EDS, and UV-vis spectroscopy. Non-linear structural parameters with different doping content and its influence on optical properties has been explored in details.
2. Experimental Details In this experiment, Zinc acetate dehydrate (Zn(CH3COO)2.2H2O), 2-Methoxy ethanol (C3H8O2) and Mono ethanolamine (C2H7NO) were used as a precursor, solvent, and solution stabilizer respectively. In addition, Magnesium acetate 4 hydrate ((CH3COO)2Mg.4H2O) was used as the doping elements. A 0.3M ZnO solution was prepared by dissolving 1.31706 gm of zinc acetate in 20ml of 2methoxyethanol. Then, different amount of doping elements (Magnesium acetate) were added to the solutions with different doping concentrations. The doping concentration of Mg in the ZnO solution was kept as 2 at%, 4 at%, 6 at% and 8 at%. A magnetic stirrer on a hot plate was used to stir the solution rigorously at 75 degrees Celsius for 30 minutes during which the solution was observed and found to turn cloudy. Maintaining a molar ratio of 1:1 for zinc acetate to monoethanoloamine, dropwise addition of monoethanoloamine to the solution clears the cloudy solution to homogeneously transparent. The solution was further stirred for 2 hours and aged for 24 hours at room temperature. Microscopic glass slides, pre-cleaned in an ultrasonic washer, were used to develop the doped ZnO Page 2 of 14
thin films by solgel technique. The deposition of the film was performed by a conventional spin coater rotated at 2500 rpm. The prepared samples were placed on a hot plate at 200 degrees Celsius for 10 minutes for preheat treatment. Film thickness was raised to 200 nm by repeating the process of spin coating and preheat treatment 8 times. The films were finally annealed at 500 degrees Celsius for 2 hours in a conventional muffle furnace.
3. Result and discussion 3.1 Scanning Electron Microscopy (SEM) Fig. 1 shows the top view of SEM images of magnesium doped ZnO nanostructured thin film on glass substrates synthesized from different concentration solgel precursor. All Mg-doped ZnO films possess the crystalline microstructures of nanometer order with uniform and dense distribution. Concentration of precursor solution exhibits subtle effect on porous microstructure surface. Uniformity and roughness of the surfaces are seen to be decreased with the incorporation of Mg [10][19].
Fig. 1. Scanning Electron Microscopy of MgZnO thin film surface.
3.2 Energy-dispersive X-ray spectroscopy (EDS) As a small amount of Mg content provides a subtle shift in XRD diffraction pattern, it is necessary to characterize the sample using EDS to be certain of incorporation of Mg in the ZnO thin Film. Fig. 2 shows the EDS spectra steamed from of a magnesium-doped ZnO films sample. The peak at 1.25 KeV indicates magnesium. The two peaks at 1.012 keV and 0.525 keV validate the presence of zinc Page 3 of 14
and oxygen respectively. Another peak at around 1.74 keV suggests the existence of silicon since the films were developed on a substrate made with silicon [6].
Fig. 2. EDS spectra of a Mg Doped ZnO sample.
3.3 X-ray diffractometry (XRD) Crystalline structures of the developed thin film samples were investigated by X-ray diffraction technique. Fig. 3 shows the XRD curve for different concentration of Mg doped ZnO thin films developed on glass substrates. The X-ray diffraction technique was performed from 0° to 70° at 2θ position. By comparing with JCPDS data sheet, it is observed that three main distinct XRD peaks for each sample around 31.7, 34.5 and 36.2 degrees were originated from (100), (002), and (101) planes respectively. Strong narrow diffracted peaks from (002) plane suggest polycrystalline nature of the synthesized thin film of all the samples [20]. Though peak intensity of diffracted peaks changes erratically (Fig. 3), it is seen form Table 1 and Fig. 4 that FWHM gradually decreases with Mg content. The variation of peak intensity of diffracted peak (002) originated from lower structure factor of (002) plane. Higher peak intensity indicates higher number of (002) plane oriented perpendicular to X-ray flux. As FWHM is directly related to grain size and other lattice parameters, in the following segment these parameter’s values were estimated and summarized in the Table 1 and Table 2. Grain size of Mg-doped ZnO thin films was obtained by using Debye-Scherrer formula [5,21]:
Where λ, θ and β are the X-ray wavelength, Bragg diffractive angle, and instrument-corrected fullwidth at half maximum (FWHM) of the diffraction peak of Mg-doped ZnO thin film respectively.
The lattice strain of all the thin films were calculated by using the tangent formula [10]:
The distance between planes was found by the following formula as [22]: d= Where, d is the interplanar distance. Moreover, the values of lattice constants were calculated by the following formula as [23]:
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Where, a and c are lattice constants. The strain towards c-axis and towards a- axis were calculated from the formula [24][25]:
Fig. 3. XRD Spectra of Mg-doped ZnO thin films of different concentrations.
and Where
and
are strain towards c-axis and a-axis respectively.
Dislocation density was calculated by the following formula as [26]:
Bond length was calculated by the relation as [27]:
Where a and c is lattice parameters and u is defined as positional parameter of the wurtzite structure which is given by [26]:
The nearest neighbor bond length towards c-direction (termed as b) and off c-axis (termed as b1) were calculated by the relations as [28] Bond angles
and
are given by
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FWHM, average grain size, lattice strain, and lattice constant a and c, Interplanar distance, Micro strain, Lattice strain, and Dislocation density of the thin films obtained from the XRD spectra [22–28] are listed in Table-1. The calculated positional parameter, bond angles α, β and c/a ratio are in the Table-2. Table 1: Grain size, FWHM, micro strain, lattice constants, interplanar distance, and lattice strain of the Mg-doped ZnO thin films estimated from (002) diffraction peak %Mg FWHM (degree)
2 4 6 8
0.44453 0.29206 0.2110 0.1336
Grain Size (nm) 17.0477 25.9517 35.9255 56.7031
Lattice constant, a (nm) 0.3282 0.3291 0.3276 0.3272
Lattice constant, c (nm) 0.5134 0.5143 0.5148 0.5131
Micro strain for a
Micro strain for c
0.0102 0.0127 0.0084 0.0071
-0.0138 -0.0122 -0.0111 -0.0144
Interplanar Lattice distance d strain (nm) 0.2567 0.2571 0.2574 0.2565
0.006166 0.004059 0.002936 0.002035
Table 2: Positional parameter, bond angles α, β and c/a ratio % Mg
Positional parameter, u
2 4 6 8
0.386264 0.386487 0.385006 0.385573
Bond angle, α (degree) 107.1246 107.0798 107.3784 107.2637
Bond angle, β (degree) 111.7123 111.7530 111.4804 111.5855
Ratio c/a 1.564044 1.562766 1.571314 1.568023
The changing trends of several structural parameters are shown in Fig. 4-6. The value of the lattice constant c for Mg-doped ZnO thin films is smaller than that of the ZnO thin films, indicating that Mgdoped ZnO thin films have tensile forces towards c-axis [10]. C-axis compression is observed as predicted by Vegard’s law [9]. The length of c-axis deviates from the linear fit, as shown in Fig. 4, which is attributed to the difference in the chemical bonding property between MgO and ZnO, presence of compensated defects, residual strain, interstitial incorporation of Mg atoms in the Zn1−xMgxO matrix, and non-solubility limit of the Mg-precursor [9][16][29]. Moreover, the FWHM and lattice strain of Mg-doped ZnO thin films decrease with increasing Mg doping content up to 8%, indicating that Mg-doped ZnO thin films have good crystalline quality. The grain size of the Mgdoped ZnO thin films changes from 17 nm to 56 nm for varying Mg content from 2% to 8%. However, lattice strain decreases with Mg concentration (Fig. 5) suggesting better crystalline quality with increase of Mg content [5][10]. From Fig. 6, it is also observed that positional parameter (u) and bond angle follow similar trend whereas ratio c/a and bond angle follow opposite similar trend. The values of positional parameter (u) are larger than theoretical value (0.375) of ideal ZnO hexagonal wurtzite phase. The non-linear varying trend of u with Mg content suggests non-linear variation of spontaneous polarization [16]. The values of ratio c/a vary slightly below than ideal ZnO c/a ratio (1.633). It is also observed that the values of bond angle are slightly less than the ideal value (109.47 degree) whereas the values of the bond angle are slightly higher than ideal value ((109.47 degree) [3] . This deviation occurs mainly due to ionic radius and electronegativity difference between Mg and Zn atom.
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Fig. 4. FWHM, Grain Size, and Lattice constant changing trend with %Mg content.
Fig. 5. Micro strain, lattice strain, and interplanar distance changing trend with %Mg content.
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Fig. 6. Positional parameter, Ratio c/a, and Bond angle changing pattern with %Mg content.
3.4 UV-Vis Spectroscopy 3.4.1 Transmittance, Absorption coefficient, and Reflectance Optical transmittance, absorbance, and reflectance measurements were performed from 275 nm to 800 nm wavelength range (Fig. 7). Absorption coefficient (Fig. 7b) which determines the light absorbing property of a material is related to the characteristics of the respective materials and wavelength of light. Absorption coefficient α can be determined by the relation [30] as: α= (2.303 A)⁄t Where, A and t stand for absorbance and thickness of a film respectively. All samples exhibit an enhanced higher uniform transmittance level (about 89%~98%) within the visible spectrum (Fig. 7a). The transmittance characteristic is attributed to less defect and more smooth surface due to Mg incorporation [19]. Moreover, all the samples encounter a sharp cut-off at ultraviolet (UV) portion of around 375 nm wavelength, demonstrating high UV light absorption that leads to the possible use of Mg-doped ZnO in UV detection efficiently [31][32]. The sharp absorption edge, which also indicates a direct bandgap material, exhibits blue shifts with the increase of Mg content. The phenomenon of absorption edge shifting can be attributed to the Burstein-Moss effect [10][15][32][33]. However, the shift of absorption edge is not changed monotonically due to the possible interstitial incorporation of Mg in the lattice and vacancy in the surface. Fig. 7c shows the percentage of reflectance which swings in the range of 6% to 18% with wavelength.
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Fig. 7. (a) Optical transmittance (b) absorption coefficient (c) reflectance of Mg Doped ZnO for different Mg content. 3.4.2 Optical bandgap and Urbach energy Optical bandgap of the samples has been estimated from Tauc’s relationship: , where , , , and represent absorption coefficient, plank’s constant, frequency of radiation, and energy bandgap respectively; and n are constant. Considering Mg doped ZnO as a direct transition material, value of n was taken 2 to draw the graph vs as shown. in Fig. 8a. Afterward, by extrapolating the linear portion of the non-linear curve to the x-axis, optical bandgap of different samples prepared from different Mg concentrations 2%, 4%, 6%, and 8% were estimated as 3.30 eV, 3.32 eV, 3.33 eV, and 3.39 eV respectively [6,34]. Electrons near band edges at the uppermost position of the valence band get excited absorbing photon and enter the lowermost position of the conduction band [11]. These electrons likely experience disorder caused by amorphous solids or defect centers or thermal vibration in crystals during this event. As a result, the density states of electrons along both conduction band and valence band edges induce tails towards the energy bandgap which is termed as Urbach Tail. The energy associated with the Urbach Tail is known as Urbach Energy [35]. Urbach energy can be calculated by the formula as [36],
Where,
is absorption co-efficient, Eu is Urbach energy, Page 9 of 14
is constant and
is photon energy.
Fig. 8b presents the graph of ln(α) against photon energy (hν) of the four samples. By extrapolating the linear segment of the curve, and then taking the reciprocal value of the slope of the linear part gives the Urbach energy. The estimated Urbach Energies are 75 meV, 77 meV, 87 meV and 65 meV for the samples of Mg 2%, 4%, 6%, and 8% respectively.
Fig. 8. Graph (a)
vs
(b) ln(α) against photon energy (hν).
Fig. 9 shows that bandgap is increased nonlinearly and monotonically with the increase of Mg content. However, urbach energy exhibits non-linear, non-monotonic function with Mg content. In addition, we observe that there is no direct relationship with bandgap and urbach energy. The possible reason for non-monotonic relationship is the interstitial Mg at the sites and at the grain boundaries [14][15][37]. Fig. 10 presents the variation of lattice constants, FWHM and grain size against bandgap. Hybridization of a localized zinc 3d electron with an oxygen 2p electron is attributed to band gap formation in ZnO [14]. The Burstein–Moss shift and bandgap narrowing phenomenon, the dominant mechanisms, together determine the bandgap [33]. When Mg2+ is incorporated in ZnO to widen bandgap, it elevates the conduction-band potential and lowers the valence-band potential at certain ratios [32]. Structural deformation, piezoelectric polarization and spontaneous polarization, and alloy mismatch influence the bandgap as well [20]. The structural disorder results in generation of local electric field caused by piezoelectric and spontaneous polarization along the c-axis. These phenomena lead to band bending at the crystallite boundaries influencing energy band gap [15][37]. Many published articles claim that MgZnO is a polar material and build in spontaneous polarization is major contributor on bandgap shifting [20][38]. Thus, polarization probably increases with bandgap as well. However, spontaneous polarization is originated from ionic displacement, which is related to lattice constant. Fig. 10 and Fig. 4 show that lattice constant a and c as well as positional parameter u (Fig. 6) do not follow linear relationship with Mg content and bandgap. As many other factors influence band gap shifting, further investigation about the role of spontaneous polarization on bandgap shifting need to be investigated.
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Fig. 9. Correlation of bandgap and Urbach energy with Mg content.
Fig. 10. Correlation of grain size, FWHM, and lattice constant with Bandgap.
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4. Conclusion In this study, we developed 200 nm Mg-doped ZnO thin film by sol-gel spin coating method where multiple times coating process performed to increase the film thickness. The increase of grain size and the decrease of FWHM suggest that incorporation of Mg enhances crystalline quality of the film. The increase of Mg content changes crystal structure by deforming a and c axis. These structural deformations produce polarization which influences optical bandgap as well. Interstitial Mg and oxygen vacancies contribute to the non-linear relationship between structural and optical parameters. According to the experimental results and analysis, it is observed that optical bandgap become larger than pure nanostructured ZnO thin film prepared by solgel method after Mg doping. The change of bandgap occurs in the range of 3.30 to 3.39 eV by modulating Mg content from 2% to 8%, with acceptable crystal deformation, urbach energy, and excellent optical properties. The enhancement of structural and optical properties holds promise for many practical photonic applications.
Acknowledgement The authors wish to acknowledge the kind support from Department of Glass and Ceramic Engineering (GCE), BUET for SEM images and EDS; and Institute of Fuel Research & Development (IFRD), BCSIR for XRD.
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S2211-3797(17)30602-2 http://dx.doi.org/10.1016/j.rinp.2017.07.047 RINP 820
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Results in Physics
Received Date: Revised Date: Accepted Date:
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Please cite this article as: Mia, d.N.H., Pervez, d.F., Hossain, d.K., Rahman, M.R., Uddin, M.J., Mashud, d.A.A., Ghosh, H.K., Hoq, M., Influence of Mg content on tailoring optical bandgap of Mg-doped ZnO thin film prepared by sol-gel method, Results in Physics (2017), doi: http://dx.doi.org/10.1016/j.rinp.2017.07.047
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Influence of Mg content on tailoring optical bandgap of Mg-doped ZnO thin film prepared by sol-gel method Md. Nasrul Haque Mia1, Md. Firoz Pervez1, Md. Khalid Hossain1, Mohammad Reefaz Rahman2, M. Jalal Uddin3, Md. Abdullah Al Mashud3, Himangshu Kumar Ghosh1, Mahbubul Hoq1 1
Institute of Electronics, Atomic Energy Research Establishment, Bangladesh Atomic Energy Commission, Savar, Dhaka, Bangladesh. 2 University of Liberal Arts Bangladesh (ULAB) 3 Dept. of Applied Physics, Electronics & Communication Engineering, Islamic University, Kushtia, Bangladesh. Abstract Tailoring optical bandgap of ZnO nanostructured thin films doped with different elements facilitates potential material for photonic applications. Different methods of fabrication process result in different optical and structural properties for the same amount of Mg content. Therefore, details investigation of structural and optical parameters, and their correlation need to be revealed to utilize the fabricated thin films. In this work, Mg-doped ZnO thin film of 200 nm thickness was fabricated by sol-gel spin coating method on a glass substrate for four different Mg content levels. Multiple layers were deposited by a spin coater to increase the film thickness. The prepared thin films were characterized by SEM, XRD, EDS, and UV-vis spectroscopy. The spectroscopic analysis showed a uniform crystalline nanostructured surface with less structural defects, enhanced transmittance, and higher optical bandgap than that of pure ZnO nanostructured thin film. Change of Mg content from 2% to 8% facilitated tuning of bandgap in the range of 3.30 eV to 3.39 eV. Changing trend of structural and optical parameters with Mg content showed non-linear, non-monotonic relation. Indepth analysis of structural and optical properties provides crucial information to form a better view about bandgap dependency on structural parameters.
Keywords: Mg-doped ZnO; MgZnO; Bandgap tuning; sol-gel; spin coating.
1. Introduction Zinc Oxide (ZnO) is a compound II–VI n- type semiconductor exhibiting high exciton binding energy (~60meV), wide direct bandgap (~3.36eV), high redox potential, high resistance to radiation at room temperature. It already found enormous attention for practical application in photonics, optoelectronics, and spintronics as a promising material [1–9]. Though optical bandgap of ZnO is wide enough for solar cell window layer, liquid crystal display, heat mirrors and other optoelectronic devices applications, however, white LED, UV photo-detector, UV LED, etc. short-wavelength applications require higher bandgap than that of ZnO. Furthermore, tailoring bandgap is necessary to realize barrier layer, cladding layer, quantum well structures, etc. in heterostructures optoelectronic and photonic devices [4,8–11]. A feasible way of altering bandgap of ZnO is doping that shifts fermi level creating impurity states [12]. Thus, bandgap of zinc oxide can be changed by alloying it with group II and III elements, e.g. Be, Mg, Ca, Cd [5][6][10]. While dopant element is selected, the radius of dopant element need to be Page 1 of 14
considered since dopant atoms of similar radius are recommendable to produce compound material with less lattice distortion. Mg2+ ion radius (0.57 Å), which has comparable radius to Zn2+ ion radius (0.60 Å), makes Mg a suitable dopant which can replace Zn atom in the lattice. Moreover, when Mg is alloyed to form a solid solution of MgxZn1−xO, the higher bandgap of MgO (~7.5eV) facilitates widening the bandgap in the deeper UV region, making useful for various applications. Thus, varying the content of Mg in ZnO lattice provides a way of tuning bandgap [7][8][13]. Doping changes lattice properties along with optical bandgap since ZnO and MgO have different lattice structures due to electronegativity and subtle ionic radius difference between Zn and Mg ion. If the Mg content is large, it creates phase separation [7][14]. For this reason, it is difficult to obtain single phase pure hexagonal wurtzite MgZnO when Mg content is increased beyond a certain level [8][9]. Besides, solubility of MgO in MgZnO depends on fabrication process and other growth conditions affecting crystallinity of MgZnO alloy [6][10][14] [15]. Moreover, interstitial atoms caused by sub-lattice localization influence the structural, electronic and optical properties of MgZnO alloy [14]. As optical characteristics also influence by structural parameters, therefore, in-depth investigation of structural and optical parameters is required to obtain certain features of thin film desirable for photonic and optoelectronics devices [13]. Many reports already have been published in this regard. However, some previous study gives controversial results of the nonlinear trend of lattice parameters with the increase of Mg concentration [8][16][17]. There are many different techniques to synthesize Mg-doped ZnO thin film such as molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), plasma enhanced chemical deposition (PECVD), r.f magnetron sputtering, spray pyrolysis, atomic layer deposition (ALD), pulsed laser deposition (PLD), electron beam evaporation, solgel method etc. [5][10]. Each synthesized technique has unique advantages compared to others. Among these, Sol-gel method facilitates several distinct advantages such as the use of cheap and simple equipment with easy composition control [5][6][10][18]. However, various factors e.g. growth technique, precursors, temperature, amount of dopant, etc. influence structural and optical properties of the thin film. In this work, we fabricated MgxZn1−xO thin film with four different concentrations of Mg content on glass substrates by sol-gel method. Then, we investigated the structural and optical properties of Mgdoped ZnO for different Mg content using SEM, XRD, EDS, and UV-vis spectroscopy. Non-linear structural parameters with different doping content and its influence on optical properties has been explored in details.
2. Experimental Details In this experiment, Zinc acetate dehydrate (Zn(CH3COO)2.2H2O), 2-Methoxy ethanol (C3H8O2) and Mono ethanolamine (C2H7NO) were used as a precursor, solvent, and solution stabilizer respectively. In addition, Magnesium acetate 4 hydrate ((CH3COO)2Mg.4H2O) was used as the doping elements. A 0.3M ZnO solution was prepared by dissolving 1.31706 gm of zinc acetate in 20ml of 2methoxyethanol. Then, different amount of doping elements (Magnesium acetate) were added to the solutions with different doping concentrations. The doping concentration of Mg in the ZnO solution was kept as 2 at%, 4 at%, 6 at% and 8 at%. A magnetic stirrer on a hot plate was used to stir the solution rigorously at 75 degrees Celsius for 30 minutes during which the solution was observed and found to turn cloudy. Maintaining a molar ratio of 1:1 for zinc acetate to monoethanoloamine, dropwise addition of monoethanoloamine to the solution clears the cloudy solution to homogeneously transparent. The solution was further stirred for 2 hours and aged for 24 hours at room temperature. Microscopic glass slides, pre-cleaned in an ultrasonic washer, were used to develop the doped ZnO Page 2 of 14
thin films by solgel technique. The deposition of the film was performed by a conventional spin coater rotated at 2500 rpm. The prepared samples were placed on a hot plate at 200 degrees Celsius for 10 minutes for preheat treatment. Film thickness was raised to 200 nm by repeating the process of spin coating and preheat treatment 8 times. The films were finally annealed at 500 degrees Celsius for 2 hours in a conventional muffle furnace.
3. Result and discussion 3.1 Scanning Electron Microscopy (SEM) Fig. 1 shows the top view of SEM images of magnesium doped ZnO nanostructured thin film on glass substrates synthesized from different concentration solgel precursor. All Mg-doped ZnO films possess the crystalline microstructures of nanometer order with uniform and dense distribution. Concentration of precursor solution exhibits subtle effect on porous microstructure surface. Uniformity and roughness of the surfaces are seen to be decreased with the incorporation of Mg [10][19].
Fig. 1. Scanning Electron Microscopy of MgZnO thin film surface.
3.2 Energy-dispersive X-ray spectroscopy (EDS) As a small amount of Mg content provides a subtle shift in XRD diffraction pattern, it is necessary to characterize the sample using EDS to be certain of incorporation of Mg in the ZnO thin Film. Fig. 2 shows the EDS spectra steamed from of a magnesium-doped ZnO films sample. The peak at 1.25 KeV indicates magnesium. The two peaks at 1.012 keV and 0.525 keV validate the presence of zinc Page 3 of 14
and oxygen respectively. Another peak at around 1.74 keV suggests the existence of silicon since the films were developed on a substrate made with silicon [6].
Fig. 2. EDS spectra of a Mg Doped ZnO sample.
3.3 X-ray diffractometry (XRD) Crystalline structures of the developed thin film samples were investigated by X-ray diffraction technique. Fig. 3 shows the XRD curve for different concentration of Mg doped ZnO thin films developed on glass substrates. The X-ray diffraction technique was performed from 0° to 70° at 2θ position. By comparing with JCPDS data sheet, it is observed that three main distinct XRD peaks for each sample around 31.7, 34.5 and 36.2 degrees were originated from (100), (002), and (101) planes respectively. Strong narrow diffracted peaks from (002) plane suggest polycrystalline nature of the synthesized thin film of all the samples [20]. Though peak intensity of diffracted peaks changes erratically (Fig. 3), it is seen form Table 1 and Fig. 4 that FWHM gradually decreases with Mg content. The variation of peak intensity of diffracted peak (002) originated from lower structure factor of (002) plane. Higher peak intensity indicates higher number of (002) plane oriented perpendicular to X-ray flux. As FWHM is directly related to grain size and other lattice parameters, in the following segment these parameter’s values were estimated and summarized in the Table 1 and Table 2. Grain size of Mg-doped ZnO thin films was obtained by using Debye-Scherrer formula [5,21]:
Where λ, θ and β are the X-ray wavelength, Bragg diffractive angle, and instrument-corrected fullwidth at half maximum (FWHM) of the diffraction peak of Mg-doped ZnO thin film respectively.
The lattice strain of all the thin films were calculated by using the tangent formula [10]:
The distance between planes was found by the following formula as [22]: d= Where, d is the interplanar distance. Moreover, the values of lattice constants were calculated by the following formula as [23]:
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Where, a and c are lattice constants. The strain towards c-axis and towards a- axis were calculated from the formula [24][25]:
Fig. 3. XRD Spectra of Mg-doped ZnO thin films of different concentrations.
and Where
and
are strain towards c-axis and a-axis respectively.
Dislocation density was calculated by the following formula as [26]:
Bond length was calculated by the relation as [27]:
Where a and c is lattice parameters and u is defined as positional parameter of the wurtzite structure which is given by [26]:
The nearest neighbor bond length towards c-direction (termed as b) and off c-axis (termed as b1) were calculated by the relations as [28] Bond angles
and
are given by
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FWHM, average grain size, lattice strain, and lattice constant a and c, Interplanar distance, Micro strain, Lattice strain, and Dislocation density of the thin films obtained from the XRD spectra [22–28] are listed in Table-1. The calculated positional parameter, bond angles α, β and c/a ratio are in the Table-2. Table 1: Grain size, FWHM, micro strain, lattice constants, interplanar distance, and lattice strain of the Mg-doped ZnO thin films estimated from (002) diffraction peak %Mg FWHM (degree)
2 4 6 8
0.44453 0.29206 0.2110 0.1336
Grain Size (nm) 17.0477 25.9517 35.9255 56.7031
Lattice constant, a (nm) 0.3282 0.3291 0.3276 0.3272
Lattice constant, c (nm) 0.5134 0.5143 0.5148 0.5131
Micro strain for a
Micro strain for c
0.0102 0.0127 0.0084 0.0071
-0.0138 -0.0122 -0.0111 -0.0144
Interplanar Lattice distance d strain (nm) 0.2567 0.2571 0.2574 0.2565
0.006166 0.004059 0.002936 0.002035
Table 2: Positional parameter, bond angles α, β and c/a ratio % Mg
Positional parameter, u
2 4 6 8
0.386264 0.386487 0.385006 0.385573
Bond angle, α (degree) 107.1246 107.0798 107.3784 107.2637
Bond angle, β (degree) 111.7123 111.7530 111.4804 111.5855
Ratio c/a 1.564044 1.562766 1.571314 1.568023
The changing trends of several structural parameters are shown in Fig. 4-6. The value of the lattice constant c for Mg-doped ZnO thin films is smaller than that of the ZnO thin films, indicating that Mgdoped ZnO thin films have tensile forces towards c-axis [10]. C-axis compression is observed as predicted by Vegard’s law [9]. The length of c-axis deviates from the linear fit, as shown in Fig. 4, which is attributed to the difference in the chemical bonding property between MgO and ZnO, presence of compensated defects, residual strain, interstitial incorporation of Mg atoms in the Zn1−xMgxO matrix, and non-solubility limit of the Mg-precursor [9][16][29]. Moreover, the FWHM and lattice strain of Mg-doped ZnO thin films decrease with increasing Mg doping content up to 8%, indicating that Mg-doped ZnO thin films have good crystalline quality. The grain size of the Mgdoped ZnO thin films changes from 17 nm to 56 nm for varying Mg content from 2% to 8%. However, lattice strain decreases with Mg concentration (Fig. 5) suggesting better crystalline quality with increase of Mg content [5][10]. From Fig. 6, it is also observed that positional parameter (u) and bond angle follow similar trend whereas ratio c/a and bond angle follow opposite similar trend. The values of positional parameter (u) are larger than theoretical value (0.375) of ideal ZnO hexagonal wurtzite phase. The non-linear varying trend of u with Mg content suggests non-linear variation of spontaneous polarization [16]. The values of ratio c/a vary slightly below than ideal ZnO c/a ratio (1.633). It is also observed that the values of bond angle are slightly less than the ideal value (109.47 degree) whereas the values of the bond angle are slightly higher than ideal value ((109.47 degree) [3] . This deviation occurs mainly due to ionic radius and electronegativity difference between Mg and Zn atom.
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Fig. 4. FWHM, Grain Size, and Lattice constant changing trend with %Mg content.
Fig. 5. Micro strain, lattice strain, and interplanar distance changing trend with %Mg content.
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Fig. 6. Positional parameter, Ratio c/a, and Bond angle changing pattern with %Mg content.
3.4 UV-Vis Spectroscopy 3.4.1 Transmittance, Absorption coefficient, and Reflectance Optical transmittance, absorbance, and reflectance measurements were performed from 275 nm to 800 nm wavelength range (Fig. 7). Absorption coefficient (Fig. 7b) which determines the light absorbing property of a material is related to the characteristics of the respective materials and wavelength of light. Absorption coefficient α can be determined by the relation [30] as: α= (2.303 A)⁄t Where, A and t stand for absorbance and thickness of a film respectively. All samples exhibit an enhanced higher uniform transmittance level (about 89%~98%) within the visible spectrum (Fig. 7a). The transmittance characteristic is attributed to less defect and more smooth surface due to Mg incorporation [19]. Moreover, all the samples encounter a sharp cut-off at ultraviolet (UV) portion of around 375 nm wavelength, demonstrating high UV light absorption that leads to the possible use of Mg-doped ZnO in UV detection efficiently [31][32]. The sharp absorption edge, which also indicates a direct bandgap material, exhibits blue shifts with the increase of Mg content. The phenomenon of absorption edge shifting can be attributed to the Burstein-Moss effect [10][15][32][33]. However, the shift of absorption edge is not changed monotonically due to the possible interstitial incorporation of Mg in the lattice and vacancy in the surface. Fig. 7c shows the percentage of reflectance which swings in the range of 6% to 18% with wavelength.
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Fig. 7. (a) Optical transmittance (b) absorption coefficient (c) reflectance of Mg Doped ZnO for different Mg content. 3.4.2 Optical bandgap and Urbach energy Optical bandgap of the samples has been estimated from Tauc’s relationship: , where , , , and represent absorption coefficient, plank’s constant, frequency of radiation, and energy bandgap respectively; and n are constant. Considering Mg doped ZnO as a direct transition material, value of n was taken 2 to draw the graph vs as shown. in Fig. 8a. Afterward, by extrapolating the linear portion of the non-linear curve to the x-axis, optical bandgap of different samples prepared from different Mg concentrations 2%, 4%, 6%, and 8% were estimated as 3.30 eV, 3.32 eV, 3.33 eV, and 3.39 eV respectively [6,34]. Electrons near band edges at the uppermost position of the valence band get excited absorbing photon and enter the lowermost position of the conduction band [11]. These electrons likely experience disorder caused by amorphous solids or defect centers or thermal vibration in crystals during this event. As a result, the density states of electrons along both conduction band and valence band edges induce tails towards the energy bandgap which is termed as Urbach Tail. The energy associated with the Urbach Tail is known as Urbach Energy [35]. Urbach energy can be calculated by the formula as [36],
Where,
is absorption co-efficient, Eu is Urbach energy, Page 9 of 14
is constant and
is photon energy.
Fig. 8b presents the graph of ln(α) against photon energy (hν) of the four samples. By extrapolating the linear segment of the curve, and then taking the reciprocal value of the slope of the linear part gives the Urbach energy. The estimated Urbach Energies are 75 meV, 77 meV, 87 meV and 65 meV for the samples of Mg 2%, 4%, 6%, and 8% respectively.
Fig. 8. Graph (a)
vs
(b) ln(α) against photon energy (hν).
Fig. 9 shows that bandgap is increased nonlinearly and monotonically with the increase of Mg content. However, urbach energy exhibits non-linear, non-monotonic function with Mg content. In addition, we observe that there is no direct relationship with bandgap and urbach energy. The possible reason for non-monotonic relationship is the interstitial Mg at the sites and at the grain boundaries [14][15][37]. Fig. 10 presents the variation of lattice constants, FWHM and grain size against bandgap. Hybridization of a localized zinc 3d electron with an oxygen 2p electron is attributed to band gap formation in ZnO [14]. The Burstein–Moss shift and bandgap narrowing phenomenon, the dominant mechanisms, together determine the bandgap [33]. When Mg2+ is incorporated in ZnO to widen bandgap, it elevates the conduction-band potential and lowers the valence-band potential at certain ratios [32]. Structural deformation, piezoelectric polarization and spontaneous polarization, and alloy mismatch influence the bandgap as well [20]. The structural disorder results in generation of local electric field caused by piezoelectric and spontaneous polarization along the c-axis. These phenomena lead to band bending at the crystallite boundaries influencing energy band gap [15][37]. Many published articles claim that MgZnO is a polar material and build in spontaneous polarization is major contributor on bandgap shifting [20][38]. Thus, polarization probably increases with bandgap as well. However, spontaneous polarization is originated from ionic displacement, which is related to lattice constant. Fig. 10 and Fig. 4 show that lattice constant a and c as well as positional parameter u (Fig. 6) do not follow linear relationship with Mg content and bandgap. As many other factors influence band gap shifting, further investigation about the role of spontaneous polarization on bandgap shifting need to be investigated.
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Fig. 9. Correlation of bandgap and Urbach energy with Mg content.
Fig. 10. Correlation of grain size, FWHM, and lattice constant with Bandgap.
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4. Conclusion In this study, we developed 200 nm Mg-doped ZnO thin film by sol-gel spin coating method where multiple times coating process performed to increase the film thickness. The increase of grain size and the decrease of FWHM suggest that incorporation of Mg enhances crystalline quality of the film. The increase of Mg content changes crystal structure by deforming a and c axis. These structural deformations produce polarization which influences optical bandgap as well. Interstitial Mg and oxygen vacancies contribute to the non-linear relationship between structural and optical parameters. According to the experimental results and analysis, it is observed that optical bandgap become larger than pure nanostructured ZnO thin film prepared by solgel method after Mg doping. The change of bandgap occurs in the range of 3.30 to 3.39 eV by modulating Mg content from 2% to 8%, with acceptable crystal deformation, urbach energy, and excellent optical properties. The enhancement of structural and optical properties holds promise for many practical photonic applications.
Acknowledgement The authors wish to acknowledge the kind support from Department of Glass and Ceramic Engineering (GCE), BUET for SEM images and EDS; and Institute of Fuel Research & Development (IFRD), BCSIR for XRD.
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