ZnO thin films deposition by spray pyrolysis: Influence of precursor solution properties
Highlights
► The solution surface tension and dissociation enthalpy of the precursor alter the droplet dynamic. ► A linear relationship between the solution dissociation energy and the growth rate activation energy was found. ► Both precursor surface tension and dissociation enthalpy control film microstructure. ► SEM image reveals that film deposited with zinc chloride is composed with separated hexagonal ZnO columns. ► Films deposited with acetate and nitride precursors have a dense microstructure.
Introduction
Spray deposition is well established and widely used technique for thin film processing. It is a chemical vapour deposition that has been succefully used for the deposition of various oxides based materials [1], [2], [3], [4], [5]. Two major interests in this method are the operating at atmospheric pressure and the deposition on a large surface. It is a quite simple method due to the use of air without vacuum system which is mostly present in several deposition techniques (sputtering, CVD, evaporation, ablation laser, etc…). The spray deposition is based on the mechanical transformation of the solution to a droplets stream by using compressed gas or ultrasonic waves. The droplets are then sprayed onto a heated surface where they react to form a solid film. Thereafter, the properties of the used solution and the thermodynamics at the interface droplet/substrate are key parameters controlling the spray deposition processes and the deposited films properties.
Many applications of droplet system such as spray deposition, painting, ink, jet printing surface cooling involves droplets spray toward a heated solid surface. The droplets stream or the molten solid particles strike the solid surface where they undergo a rapid deformation and form a disk like splat or break up. In these applications the droplet impact on the heated surface is important, the performances of these applications depend strongly on the rate of mass, momentum and heat transfer, the liquid droplet flatten and dissociation.
A large number of thin films material were deposited by spray pyrolisis namely SnO2, ITO, ZnO ZnS, CuInSe2, CdTe, CdS. Investigations of deposition parameters influence on ZnO thin films properties have been intensively carried. Parameters such as: substrate temperature [5], [6], [7], [8], [9], [10], [11], solution molarity [12], [13], substrate nature [7], flow rate [14], [15], annealing [4], [16], [17], [18] and doping [4], [19], [20], [21], [22], [23] were carried in the literature. The used precursor solutions are generally zinc acetate [5], [6], [7], [8], [9], [10], [11], [12], [13], [17], [18], [24], zinc nitrate [25] and zinc chloride [14], [26]; while the mostly used one is the zinc acetate. To our knowledge, there is no study devoted to the correlation between the starting solution properties and the ZnO thin prepared by spray pyrolisis.
In the present paper we address a comprehensive study of the influence of the solution nature and properties on the characteristics of ZnO thin films deposited by spray pyrolysis technique.
Section snippets
Experimental details
In Fig. 1 we have reported a schematic illustration of the used spray deposition system. A filtered air under 1.5 Bar pressure is injected in the precursor solution chamber. The droplets are born and collected at the top of the vessel and sprayed onto the heated substrate through a nozzle fixed at 5 cm from the substrate holder. Three different solutions, with 0.1 molarity, were prepared by mixing zinc salts with distilled and de-ionized water. The used salts are zinc acetate, zinc nitrate and
Droplet impact
In Fig. 2 we have reported the surface micrograph of films deposited with zinc acetate (Fig. 2.a) and zinc chloride (Fig. 2.b). As can be seen, droplet impacts are visible, their shape and size depend on the nature of the starting precursor. The droplet impacts have a disk form; their diameters are reported in the Table 1. The larger diameter is measured in the chloride solution (up to 580 μm). However for acetate and nitride solutions, the droplet impacts have a diameter 340 and 380 μm
Conclusion
In the present work we have studied the influence of solution properties on ZnO thin films deposition by spray pyrolysis. Three Zn salts were investigated. The obtained results indicated that surface tension and dissociation enthalpy of the solution play an important role in the droplet dynamic and in its final impact shape. The growth rate activation energy varies linearly with the dissociation energy of the used solution. Both surface tension and dissociation enthalpy of the precursor control
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