Comparative study for seed layer solvent effects on structural and optical properties of MgZnO thin films deposited by chemical bath deposition technique

C-axis oriented Magnesium doped Zinc Oxide (MgZnO) thin films are promising candidates for variety of applications especially optoelectronic devices. In this work, the effects of seeding solvents on structural and optical properties of MgZnO films were analyzed. The comparison was performed by varying the seed layer solvent i.e. 2-Methoxyethanol, 2-Propanol, Ethanol and Methanol. MgZnO thin films were synthesized on different seed layered glass substrates using Chemical Bath Deposition (CBD) method. The XRD results revealed the hexagonal wurtzite structure for all samples. However, the film with seed layer 2-Methoxyethanol solvent had preferred c-axis orientation and better crystallanity. For 2-Methoxyethanol solvent based seed layer: the morphological study showed wrinkled free, unfractured and highly aligned MgZnO nanorods grown perpendicular to the substrate. Energy dispersive x-ray (EDS) spectroscopy confirmed the presence of magnesium in ZnO film. Highest average transmittance of 80 % and band gap value of 3.3 eV was recorded through UV–vis spectroscopy. The optical constants (refractive index and extinction coefficient) were determined by reflectance and absorbance data. Furthermore complex dielectric constant and energy loss functions were estimated.


Introduction
Zinc Oxide (ZnO) is a semiconductor material of group II-IV. It has a wide band gap of 3.3 eV. ZnO has numerous applications in many fields like solar cells, optoelectronic devices, gas sensors, field effect transistors, nano-generators etc [1,2]. Zinc Oxide thin films gained great attention for optical properties by using different coating techniques [3][4][5]. In many cases, well-aligned ZnO nanorods showed better performance in terms of high visible transmittance [6,7]. Therefore high quality c-axis growth of ZnO thin films are considered essential for potential applications like optoelectronic devices. The optical and structural properties of ZnO films can be altered by doping them with transition, rare earth and alkaline elements. It has been reported that optical properties of ZnO can significantly be improved by magnesium doping while keeping the distortion in the host lattice to a minimum due to comparable ionic radii of Mg 57 pm ( ) and Zn 60 pm ( ) [8]. The researcher recently reported magnesium compound with oxygen (MgO thin film) application as super capacitor for space applications [9].
There exist a number of different methods to synthesize the MgZnO films to improve the optical properties [10,11]. In order to achieve controlled ZnO morphological growth, methods like chemical vapor deposition (CVD) [12], pulse laser deposition (PLD) [13], spray pyrolysis [14], sol-gel method [15] and chemical bath deposition (CBD) [16,17] are already in use by researchers. Among these, CBD method is a cost effective and largely used because of its minimal thermal budget requirement. Recently, report on possibility of ZnO thin films synthesis at a temperature of  50 C has been appeared in literature [18]. Synthesis of Zinc Oxide thin films through CBD method can result into different morphological structures. There are many control parameters, such as precursor, solvent, substrate, bath temperature, bath time and seed layer, the combined effects of which determine the properties of Zinc Oxide nanostructures. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Seed layers are used to reduce the lattice mismatch between film and substrate. It has been established that seed layer morphology directly affects the structural properties of Zinc Oxide nanowires and nanorods [19,20] thus making it the most important control parameter before CBD process. Many aspects of the seed layer growth process and its effects on ZnO thin films have been well explored. The wrinkled seed layer structure produces roughness in the ZnO thin films and affects the optical properties [21]. Boiling temperature of solvent directly affects the morphology of ZnO thin film. For smooth ZnO layers, the solvent having higher boiling temperature are favorable [22]. The drying temperature introduces residual stresses in seed layer which ultimately affects the optical properties and controlled growth of doped ZnO nanorods. However, the influence of solvents, used in preparation of seed layer, on ZnO thin films has rarely been published. The seed layer solvent remarkably affects the structural and optical properties of ZnO nanorods grown on ZnO seed layers [23]. Mingsong wang et al synthesized ZnO nanorods by using mixture of different solvents in the reaction bath solution. They also explained the smooth seed layer plays a key role on controlled growth of nanostructure [24]. Kai Loong Foo et al have grown ZnO nano rods by hydrothermal method using different solvent for seed layer [23].
In this work, the effects of seed layers grown using different solvents on MgZnO thin films are presented. Solgel method was used to deposit seed layer of constant molar concentration by using four different solvents [25,26]. In order to achieve better optical results, the molar concentration was fixed at 0.5 M for all the solvents as suggested in literature [25,27]. The mechanism of quality seed layer produced by the solvent to grow well aligned MgZnO grains was discussed. The magnesium doping percentage was fixed at 5% for all samples in order to: (i) get better optical result [28] (ii) avoid phase segregation (MgO) at higher doping level [29] and (iii) eliminate the effects of doping variations. The best solvent for the synthesis of quality seed layer for MgZnO thin films and optical properties by CBD method was revealed. The seed layer effects on structural, morphological and optical properties of MgZnO thin films prepared by CBD method was studied by XRD, SEM and UV-vis. Additionally EDS was used for elemental analysis. The optical constants were explored by optical data.

Experimental procedure Preparation of seed layer
The procedure adopted to grow seed layer was the sol-gel spin coating technique [21,30]. Soda lime glass slides were used as a substrate. All the substrates were washed with DI water followed by ultrasonic cleaning using acetone. Zinc Acetate dihydrate Zn CH3COO 2.2H2O ( ( ) )was used as a host precursor to prepare ZnO seed layer. 2-Methoxyethanol, 2-propanol, Ethanol and Methanol were used as solvents and Monoethanolamine (MEA) as a stabilizing agent. The ZnO solution concentration for seeding was fixed at 0.5 M. Same procedure was adopted to prepare solution for all solvents. First, required quantity of precursor salt was dissolved in each solvent in a separate beaker and stirred for ten minutes by using magnetic stirrer at 300 rpm. After that, MEA poured drop wise in this milky solution to get a clear homogeneous solution with a constant stirring of one hour. The MEA and Zn molar ratio was kept constant at 1: 1. The final prepared solution was stored for aging of 24 hours at room temperature. The seed layer was then spin coated on the substrates at the speed of 2000 rpm for thirty seconds. After each coating all wet substrates were dried at  200 C for thirty minutes in oven. This procedure was repeated seven times.
Preparation of magnesium doped zinc oxide thin films by CBD In CBD, same precursor (Zinc Acetate dehydrate) was used for solution preparation. Magnesium acetate tetrahydrate Mg CH COO 2.4H O 3 2 ( ( ) )was used as a dopant reagent. The doping percentage was fixed at 5 at%. The precursor and dopant with required quantity was dissolved in 125 ml DI water to make 25 mM solution. At the same time another solution of hexamine was made at 20 mM concentration in DI water. Both solutions were separately stirred for fifteen minutes. The prepared solutions were added in CBD double wall beaker, placed on the hot plate magnetic stirrer, to grow MgZnO thin film. When temperature of CBD equipment reached at  85 C, the seeded glass substrates were immersed vertically in the double wall beaker and kept in this solution for three hours. At the end, films were washed with DI water, dried in air and annealed at  450 C for two hours. Assigned names for samples were: 2ME for 2-methoxyethanol ZnO seed layer and MgZnO by CBD; 2 P for 2-Propanol ZnO seed layer and MgZnO by CBD; E for Ethanol ZnO seed layer and MgZnO by CBD; M for Methanol ZnO seed layer and MgZnO by CBD.
Orientation and crystallanity of all the films were studied by x-ray diffraction spectroscopy (XRD; PANalytical X'pert Pro) using a CuK source l =  1.54056 A .
( ) The morphology of thin films was observed by SEM (JEOL, JSM-6480LV) with 10 K magnification. The energy dispersive x-ray spectroscopy (EDS, JEE-420) attached with SEM was used for elemental analysis. UV-vis spectrophotometer (Model: USB4000 oceanoptic) was used for optical measurements with the wavelength range of 300-800 nm.

Results and discussions XRD analysis
The XRD spectra of MgZnO thin films by CBD using different seed layer solvents along with ZnO seed layer spectra showed in figures 1(a)-(d). The crystal structure had been observed between  25 to  50 diffraction angles. The major peaks detected were at (100), (002) and (101) plane which showed a good agreement with JCPDS card (no. 36-1451) [10]. The MgZnO Films synthesized by CBD possessed hexagonal wurtzite structure and polycrystalline nature. The ZnO seed layer spectra by different solvents showed low intensity peaks of (100), (002) and (101) plane. 2ME seed layer showed sharp peaks with low intensity and (002) plane dominance which exhibited good crystal structure. 2P seed layer showed minor intensity peaks. E seed layer did not showed any peak even with low intensity. M seed layer showed broader peaks due to low crystal quality appeared at preheating (drying) state.
The MgZnO film pattern for 2ME sample revealed (002) plane dominance which indicated the alignment of nanorods along c-axis perpendicular to the substrate. 2ME sample seed layer and MgZnO film both have (002) plane dominance (c-axis) which indicated better optical results. The MgZnO pattern for 2P sample showed (100) plane dominance. However E and M sample presented (101) plane dominance. In all the MgZnO samples there was no peak related to impurity phase of MgO which indicated that Mg ion successfully occupied the lattice site.
Scherrer's formula was used to calculate average crystallite size for preferred crystal plane [31].
( ) is the constant, l is the x-ray wavelength q is the Bragg's angle and b is the full width half maximum for dominating plane peak.
The dislocation density d ( ) and lattice strain e ( ) were calculated by using the following relations [32,33].
The lattice constants were calculated by the relations given below [25].   network (fiber) type structure and wrinkled ZnO seed layer using Methanol solvent respectively (sample M). This clarified that the morphology of MgZnO films changed with the seed layer surface morphology. The wrinkles produced in seed layer may be attributed to the solvent boiling temperature, as Ethanol and Methanol have low boiling temperatures i.e.  78 C and  64.7 C respectively. Since drying temperature was  200 C, therefore solvent with lower boiling temperature abruptly evaporates which produce stresses (wrinkles) in seed layer [22]. Also, the substrate temperature increases during drying process which forces the solvent to evaporate more rapidly and produce stresses between seed layer and substrate. The wrinkled surface was formed by the evaporation rate and solvent with higher boiling temperature has slower evaporation rate than the solvents with lower boiling temperature. For smooth ZnO surface, slower evaporation rate favors the relaxation of mechanical stress. Low drying temperature [21] or close to the evaporation temperature can produce transparent films. This process directly influences the growth and alignment of upper nanorods. The seed layer morphology greatly affects the growth morphology of nanorods [19,20].

SEM and EDS analysis
In the CBD process there are two main steps, nucleation and growth. The seeding process creates the ZnO layer which serves as a ZnO nuclei and it directly influence the MgZnO nanorods formation. The orientation of nanorods is determined by the orientation of seed layer. The seed layer shown in figure 2(a2) for sample 2ME was relatively unstressed compared to other wrinkled seed layer surface samples and had c-axis (002) orientation dominance as revealed by XRD study. Therefore the MgZnO nanorods for sample 2ME also had c-axis orientation due to homogeneous nucleation growth. The MgZnO films for other samples had random orientations of nanorods due to the stressed structure of seed layer and random alignment of ZnO seed nuclei [35].  Figure 3 represented the optical absorbance, transmittance, reflectance and tauc's relation for all MgZnO films. The results revealed that the lowest absorbance was achieved by 2ME sample. The average transmittance of nearly 80% had been observed in visible region. The reason was wrinkled free seed layer surface with c-axis preferred orientation, better optical quality and c-axis orientation of MgZnO nanorods as revealed by XRD and SEM study. The c-axis orientation is useful for optical properties [11,21]. The transmittance for 2ME sample attained maximum value of 90% near IR region indicated that this sample film is useful for many optoelectronic applications. However reflectance rapidly reduced near UV and in visible region. The lowest transmittance value mentioned in table 3 produced by sample M was due to dense, stressed seed layer opaque surface and optical loss by upper fiber type nanostructure as revealed by SEM analysis.

UV Analysis
The band gap values were calculated by the relation given below [32,36] and it ranged from 3.15 eV to 3.3 eV. The maximum band gap value among all samples had been achieved by sample 2ME. Here u h ( ) is the photon energy, a is the absorption coefficient, B is a constant and Eg is the optical energy gap. The Zinc oxide relates to the direct band gap system and the plot between a u h 2 ( ) and u h ( ) is supposed to have linear behavior. This corresponds to the strong absorption presence close to absorption edge. Hence extrapolating the linear portion also gives optical energy gap of the films.

Optical constants analysis
The optical parameters like refractive index n ( ) and extinction coefficient k ( ) for MgZnO films by CBD were calculated by the relations taken from [10,37,38] and are given below.
Here R is the reflectance and l is the incident beam wavelength. Figure 4(a) explained that the refractive index continuously decreased in visible region and lowest value found for sample 2ME as mentioned in table 3. This verified the transparency of the film. Figure 4(b) showed the extinction coefficient values for all samples and revealed that 2ME sample had lowest value compared to others due to little optical loss.
The complex dielectric constant e ( ) defines the phonons excitation and optical transition in the material. The real and imaginary parts of e can be calculated by the following relations [37,39].
e e e = + r i 13 ( ) e = + r n k 2 2 e = i 2nk  These values describe light dispersion in materials and energy absorption by dipole motion. Figure 5 revealed the real and imaginary parts produced enhance optical response than optical constants. Furthermore, the two important parameters that are related to real and imaginary parts i.e. volume energy loss VELF ( ) and surface energy loss SELF ( ) can be estimated by the following relation [37,40]. (( ) ) ( ) / Figure 6 explained that VELF values were higher than SELF. The energy loss occurred in sample film interior by electronic transition than surface and found identical results for both VELF and SELF. All the results from figures 4-6 verified that the MgZnO film by CBD using 2-Methoxyethanol solvent for seed layer produced better optical response than others. Hence 2-Methoxyethanol solvent is suitable for seed layer synthesis before CBD method for optoelectronic devices.

Conclusion
Mg doped ZnO nanostructures were well synthesized by CBD on ZnO seed layers prepared by different solvents using sol-gel spin coating method. The following conclusions were made by this research. 1. The XRD pattern revealed the hexagonal wurtzite structure with c-axis preferred orientation both for MgZnO nanorods and seed layer using 2-Methoxyethanol solvent.
3. The SEM analysis revealed the vertically aligned nanorods produced for sample 2ME.
4. Complete incorporation of Mg in ZnO was elucidated by EDS analysis.
6. The optical constants n k and ( ) and complex dielectric constant values were found to be decreased for sample 2ME. This verified film transparency. 7. From above all results it has finally concluded that 2-Methoxyethanol solvent is suitable for: (i) The synthesis of wrinkled free, unstressed, unfractured and better quality seed layers.
(ii) The optical properties of MgZnO nanorods for optoelectronic devices.