Effect of boric acid content on the structural and optical properties of ZnO films prepared by spray pyrolysis technique

Zehra Kaya and Mustafa Öztas

University of Yalova, Chemical and Process Engineering, Yalova/TURKEY

Abstract

Boron doped ZnO films were prepared by spray pyrolysis technique at 450 °C substrate temperature, which is a low cost and large area technique to be well-suited for the manufacture of solar cells, using boric acid (H3BO3) as dopant source, and their properties were investigated as a function of doping concentration. X-ray analysis showed that the films were polycrystalline fitting well with a hexagonal structure and have preferred orientation in (002) direction. And optical band gap of the undoped and B-doped ZnO films were found to vary from 3.46 to 3.29 eV. The changes observed in the energy band gap and structural properties of the films related to the boric acid concentration are discussed in detail.

Key words: spray pyrolysis, ZnO, Boron, optical properties

  1. Introduction

ZnO is a very interesting material for many different applications in both microelectronic and optoelectronic devices. It is a wide-bandgap oxide semiconductor with a direct energy gap of about 3.37eV. As a consequence, ZnO absorbs UV radiation due to band-to-band transitions. However, it can be used as transparent conductive oxide (TCO) thin films, mainly for applications such as solar cells, liquid crystal displays and heat mirrors [1], [2], [3]and[4]. Furthermore, ZnO is used as semiconducting multilayers, photothermal conversion system, gas sensors, and optical position sensors [5]. Among all the oxide materials studied, in the last years, zinc oxide (ZnO) has emerged as one of the most promising materials due to its optical and electrical properties, high chemical and mechanical stability, together with its abundance in nature, which makes it a lower cost material when compared to the most currently used transparent conductive oxide materials (ITO and SnO2). In order to improve the properties of the films, several techniques such as sputtering [6]and[7], thermal evaporation [8] and spray pyrolysis [9] have been applied for the production of ZnO. Spray pyrolysis technique is preferred among these techniques since it is cheaper, simpler and more versatile than the others, which allows the possibility of obtaining films with the required properties for different applications and also when large areas of the films are needed. Also, it is known that metals group I (Ag, Cu) are fast-diffusing impurities in compound semiconductors. Therefore, the interdiffusion of components of Cu–Zn bilayer structures and particularly the diffusion penetration of copper into ZnO can cause changes in the physical properties of the near-interface region of ZnO and thereby in characteristics of its structures. The Cu impurity penetrates through the lattice during deposition process and thus results in the formation of complex centers (CuZn, Cui). The diffusion of Cu into ZnO grains occurs by a dissocative mechanism which is a combination of the interstitial and vacancy mechanism. It is also possible that copper ions from the spray deposition can replace either substitutional or interstitial zinc ions in the ZnO lattice creating the structural deformation. In this work, the influence of boric acid concentration on the structural and optical properties of sprayed ZnO:B films are reported.

2. Experimental details

Spray pyrolysis is basically a chemical process, which consists of a solution that is sprayed into a substrate held at high temperature, where the solution reacts forming the desired thin film. In this technique, the ZnO:B films were deposited on heated microscope glass (Objekttrager, 1cm×1cm) substrates by spraying an aqueous solution in air atmosphere. The spray solutions are comprised of zinc chloride (0.5M, Merck, ≥99%) and boric acid (H3BO3) as dopant source in the deionized water. The substrate temperatures 450o was used and the temperature was controlled within ±5°C through a chromel–alumel thermocouple as a sensor for the temperature controller. Before deposition, the glass substrates were ultrasonically cleaned in acetone solution, and then rinsed in de-ionized water. The distance between nozzle and substrate was about 35cm. The prepared solution is sprayed (5ml/min) onto the clean glass substrates with a deposition time ranged between 0.5 and 2.5h. The atomic percentage of dopants in (B/Zn) solution were 1.0, 1.5, and 3.0 at.% in spray solution. In this procedure, compressed air was used to atomize the solution containing the precursor compounds through a spray nozzle over the heated substrate; air is compressed from the atmosphere. The precursor is pyrolyzed on the heated substrate. The solution was pumped into the air stream in the spray nozzle by means of a syringe pump. The structural characterization of deposited films was made by X-ray diffraction (XRD) technique on Braker AXS D5005 diffractometer (monochromatic CuKα radiation, λ=1.54056A). Perkim Elmar Lambda 25 UV–VIS–NIR was used to determine the optical absorbance of the films as a function of wavelength at room temperature. The optical bandgap energy Eg was determined by extrapolating the high absorption region of the curve to the energy axis [10].

  1. Results and discussion

3.1 Structural studies

Fig. 1 shows the X-ray diffraction patterns of B-doped ZnO films with different boron concentrations deposited at substrate temperature 450 °C.Film was characterized with the (002) preferential planes. The preferential orientation of all films is found to be along (002) crystal plane (crystallites are oriented with the [002] direction perpendicular to the substrate, with a slight shift of the maximum towards higher 2θ values). This indicates that the ZnO:B films prepared by spray pyrolysis method are polycrystalline with the hexagonal structure and show a good c-axis orientation perpendicular to the substrate. The intensity of (002) peak is increased with increasing B concentration up to 1.0 at.% and then the intensity of (002) peak is decreased. This behaviour can be understood by two competing processes; the increase of boron doping improves the stoichiometry of the films and the crystal quality. This indicates that boron ions are substituted at manganese ions sites up to 1.0 at.% after that B-B intragrain cluster is evaluated. XRD pattern of highly doped (1.5 and 3.0 at.%) ZnO:B is shown in Fig. 1 and the inset shows the B-B cluster intragrain.It is observed that the XRD intensity depends strongly on the boron concentrations. The maximum of the XRD intensity is illustrated by the pronounced peak at a boron concentration of 1.0 at.%. Thus, increasing the dopant level results in a change in preferred growth (002) direction. The spray deposition of 1.0 at.% boron-doped ZnO deposited at a substrate temperature of 450 °C is found to be optimum for the deposition of good quality B–ZnO films at the specified spray conditions. It is shown that there is a critical doping value in the starting solution for which the characteristics of the ZnO:B films has a minimum value, corresponding to the maximum crystal grain size value measured for these films. Consequently, the characteristics of B–ZnO films prepared by the spray pyrolysis process depend strongly on the boron incorporation at the films, as similar behavior has been observed by Pawar et al.[11]. It is also shown in Fig. 1 that the initial increase in the XRD peaks can be explained by the creation of new nucleating centers due to the B dopant atoms. The subsequent decrease of the XRD peaks for the high doping level could be explained by two factors; firstly, by the saturation of the newer nucleating centers and secondly, due to the change of the energy absorption at the time of collision, and of the physical and chemical interaction between ad-atoms and the film. For B-doped films, the grain size initially increases with an increase in dopant contents (up to 1.0%). Then, there is a decrease in the crystal grain size with the increase in the boron concentrations time may be due to the sufficient increase in supply of thermal energy for recrystallization. This trend suggests that boron dopant creates newer nucleation centers, which in turn, would change the nucleation type from homogeneous to heterogeneous, and deteriorate the crystalline structure at high doping level.

The grain size of ZnO and the boron doped ZnO films were estimated for the (002) plane by using the Scherrer formula [12]

where d is the grain size, λ is the X-ray wavelength used, D is the angular line width of the half maximum intensity and θ is the Bragg angle. Table 1 shows the various grain parameters of the undoped and boron doped ZnO films which is associated to the (002) peak. From these results, we can see that the fundamental effect of the boron is related to an increase in the size of the crystallites and a decrease in the l the bandgap energy. As the boron concentration increases the intensity of ZnO (002) peak increases and this peak becomes narrower indicating an improvement of the crystallinity. This means that the grain size of the films increases with increasing up to 1.0 % (see Table 1). The XRD intensity depends strongly the boron concentraition permitted for maximum XRD intensity is illustrated by the pronounced peak with dopant contents (up to 1.0%). It shows that ZnO films the (002) preferred growth is maintained up to 1.0% boron contents. It can also be observed, with the addition of boron contents for larger boron contents, that the FWHM increases due to the destruction of the crystal structure and reduction of the grain size. It may be possible that this drastic change in grain size is due to the large difference in ionic radius of zinc and boron.The decrease in the grain size is correlated with the broadening of the XRD peak. Smaller crystallite size results in a higher density of grain boundaries, which behaves as barriers for carrier transport and traps for free carrier. Hence, a decrease of crystallite size can cause an increase of grain boundary scattering [13].

It is concluded that the bandgap decrease with increase in grain size, which indicates a lower number of lattice imperfections. This may be due to a decrease in the occurrence of grain boundaries because of an increase on the grain size of the film with increase of boron concentration up to 1.0%. These parameters indicate the formation of high quality B-ZnO films deposited on the well cleaned glass substrate by spraying pyrolysis methodwith dopant contents (up to 1.0%).

3.2 Optical Studies

Undoped and boron doped ZnO films are a direct transition semiconductor and its absorption coefficient (α) and optical band gap energy (Eg) are interrelated [14]. The band gap energy of the films was calculated from the (αhν)2 versus hν (energy) which is plotted. The plots are parabolic in nature and the number of inflexions reveals the number of transitions. At inflexion, the tangential extrapolation to X-axis (energy) gives the band gap energy in Table 1.It is seen from Table 1 that the decreases in the optical band gap of the films with with an increase in dopant contents (up to 1.0%) can be attributed to the increase in the grain size. Another reason could be the improving crystallinity with increasing grain size and this may be due to the extension of electronic states of the impurity phase, precipitates and clusters, into the band gap of ZnO. It is seen at higher doping levels that the band gap was increased, and the grain size of ZnO:B films were decreased. The change in Eg with the atomic percentage of dopants in (B/Zn) solution and grain size was observed in ZnO:B films. Therefore, the band gap energy shift of ZnO:B films in our study can be attributed to the quantum size effect. The change in Eg with atomic percentage of dopants in (B/Zn) solution can be understood by the quantum size effect observed in the films of semiconductors. Therefore, the optical band gap (Eopt) of doped ZnO is broader than that of undoped zinc oxide films. It may be also another reason that defects are accumulated at the grain boundaries. Smaller grain size results in a tensile strain arising from thermal mismatch between the MnS film and the substrate. This indicates that the presence of large number of grain boundaries increases the defects in the film.

Conclusion

The study of the structural and optical properties of undoped and doped ZnO films obtained by the spray pyrolysis technique shows that they arestrongly dependent on the boron concentration. Particularly,it is observed that the best crystallinity of ZnO films isobtained at the atomic percentage of dopants in (B/Zn) solution at %1.0. The films have polycrystalline structures and show a preferential orientationalong (002) with well-defined microstructures. B incorporation (at 1.0%) caused the crystallinity levels to increase.Thecrystallizationlevelis low at higher doping level (1.5% and 3.0%) due to the increasing grain boundaries which behave as defects in the structure affecting the structural properties of the films. From the XRD analyses it was concluded that B incorporation plays a significant role in the crystalline and structural properties of the ZnO films, and 1.0% B incorporations are the most suitable B amounts.

References

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Table Captions:

Table 1. Various grain parameters of the undoped and boron doped ZnO films

Figure Captions:

Figure 1. XRD patterns of undoped and boron doped ZnO films.with the atomic percentage of dopants in (B/Zn) solution; (a) %0.0, (b) %1.0, (c) %1.5, (d) %3.0.

Sample / Substrate / % Borik asid / Grain size (nm) / Energy gap
temperature (oC) / (eV)
ZnO / 450 / 0 / 152 / 3,38
1 / 175,0 / 3,29
3 / 155,0 / 3,35
5 / 135,0 / 3,42

Table 1

.

Figure 1