MECHANICAL BEHAVIOUR OF DOPED AND UNDOPED ZnO THIN FILMS

Abstract

Fabrication of nanostructured thin films for numerous functional applications has been emerging rapidly in recent times due to their excellent mechanical, optical, and magnetic properties. Nanostructured thin films are rigorously explored in various disciplines such as material science, engineering applications, physical, chemical and biological fields owing to their multi-functional properties. Multi-functionality and diverse properties of ZnO thin films have led to continuous development and renewed interests among researchers. ZnO exhibits wide direct band gap, high exciton binding energy, high piezoelectric coefficient, easy to fabricate nanostructure, etc. However, nanostructure of ZnO offers remarkable stability and easily optimized by tailoring microstructure, defect morphology and crystallinity using synthesis route and thermal treatments. These properties of ZnO can be exploited for fabrication of piezoelectronics devices, UV-photo detectors, gas sensors, and dye sensitized solar cells etc. Also, incorporation of different dopants in ZnO thin films could manipulate its microstructure and crystalline properties, used to tailor the optical, gas sensing and piezoelectric properties. Commercialization of ZnO based nanostructured devices is heavily dependent on its structural integrity along with other functional properties. This thesis is focused to investigate the mechanical and tribological properties of ZnO nanostructured thin films in a combined framework, using experimental and simulation techniques. Rare earth ions as the active constituents of ZnO, create distinct crystalline behavior after doping that could serve in many applications. ZnO thin films are doped with rare earth ions such as Yttrium and Praseodymium using direct current (DC) sputtering technique and their doping profile was studied by X-Ray photoelectron spectroscopy and Energy Dispersive X- Ray spectroscopy (EDS). The influence of deposition parameters on the structural and mechanical properties of thin films has been explained. The main objective of the present work is to, i) Synthesize pure ZnO thin films on glass substrate and fused quartz substrate using DC sputtering technique, and further to investigate the effect of sputtering process parameters on mechanical and tribological properties, ii) Study the effect of dopants e.g. (Yttrium, Praseodymium) on micro-structure and mechanical properties of ZnO thin films. iii) Develop a finite element model using nanoindentation parameters and to study the effect of friction, yield stress, substrate elastic-plastic properties on simulated load-displacement curve. The key results obtained are described as follows. Abstract ii | P a g e The nano-mechanical properties of pure ZnO thin films deposited at different substrate temperature such as (RT) 250C, 1000C, 2000C, and 3000C using DC sputtering on glass substrate were investigated. The ZnO thin films are found to be predominately c-axis (002) oriented and sensitive to increasing substrate temperature, new crystal planes become visible at 3000C as thin films become highly polycrystalline. The presence of (103) crystal plane is more pronounced with the increasing substrate temperature. High crystallinity and peak intensity ratio I(002)/I(103) (counts) is highest for thin films deposited at 1000C, attributed for high hardness and better adhesive properties observed for ZnO thin films. No major sudden burst of displacement ‘pop-in’ event in loaddisplacement curve of thin films was observed during indentation, indicating the films are dense with low defects and adhered strongly to the substrate. Subsequently when underlying substrate has been changed from glass to fused quartz and ZnO thin films deposited at different sputtering deposition pressures (5, 10, 15, and 20 mTorr) using DC sputtering. The crystallinity and microstructure revealed a marked influence on the mechanical properties of ZnO thin films. The structural evolution of the thin films is in (002) crystal plane and influenced by deposition pressure variations. The intensity of (002) peak of the films rises initially and decreases with further increasing deposition pressure. The mechanical properties and coefficient of friction of ZnO thin films were measured using three-sided pyramidal Berkovich nanoindentation. The strength of thin films was measured by using scratch test under ramp up loading. Load– displacement profile of thin films at continuous indentation cycle was observed without any discontinuity revealed no fracture, cracking event, and defects, which is a consequence of dense microstructure and good adherence of films to the substrate. The mechanical properties of Y (yttrium) doped ZnO (YZO) thin films deposited on glass substrates and fused quartz at different substrate temperature (Room temperature 250C, 1000C) using DC sputtering were studied. It is observed that growth of crystalline nature and micro-structural features of thin films are sensitive to different substrate material and substrate temperature as indicated by X-Ray diffraction. YZO thin films have emerged in (002) diffraction plane and improves peak intensity with higher substrate temperature at glass substrate. The doping profile of Y (yttrium) in ZnO thin films and micro-structural features were characterized by X-Ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM) and Field effect scanning electron microscopy (FESEM). The scratch resistances in terms of lateral and normal force curve, Abstract iii | P a g e hardness, Young’s modulus of the films were measured using Berkovich tip nanoindentation and nanoscratch. YZO thin films have shown better mechanical properties (hardness 5.06±0.70 GPa, Young’s modulus 166.81±16.39 GPa) on fused quartz substrate compared to glass substrate. XPS and EDS mapping of the films confirm Y3+ presence as well as uniform distributions throughout thin films. The mechanical properties of YZO thin films deposited at room temperature observed to be poor. It is due to the inadequate thermal energy at lower substrate temperature is not able to facilitate the formation of defect free and crystalline thin films. The denser microstructure and sharp morphology of the films observed at high substrate temperature have improved strength in terms of critical load 1210.6 μN of the films required for scratch resistance. Furthermore, mechanical properties of Pr (Praseodymium) doped ZnO thin films deposited on glass substrates and fused quartz at different sputtering deposition pressure (5mTorr, 10mTorr) using DC sputtering were studied. Growth of crystalline phase in Pr doped ZnO thin films particularly in (002) diffraction plane is more pronounced and it improves with higher peak intensity at 10mTorr sputtering pressure as observed by X-Ray diffraction. However, the lower sputtering deposition pressure evoked deposition rates to the formation of poly-crystalline films emerged in several crystal planes. The presence of Pr ions incorporated ZnO host lattice as well as morphology was examined by XPS, AFM and FESEM. XPS spectroscopy revealed the presence of Pr3+, Pr4+ at ZnO top surface layer and it was in tandem with EDS mapping. The thickness of thin films was found to be varying from 220 nm to 310 nm with dense morphology and uniform growth throughout on the deposited area. Nanoindentation prior to scratch testing (ramp up load condition) of Pr doped ZnO thin films was performed to understand its deformation characteristics. The three sided Berkovich indenter tip is used for the measurements and the films deposited on glass substrate have shown hardness (9.89±0.14 GPa), Young’s modulus (112.12±3.45 GPa) as compared to films deposited on fused quartz substrate, hardness (9.83±0.24 GPa), Young’s modulus 110.9±3.7 GPa), at similar synthesis conditions. The Nano-scratch tests yield lower critical load Lc1, 2250.5 μN for the crack initiation and upper critical load Lc2, 2754.5 μN for the complete failure. The crack propagation resistance parameter (CPRS) of the films was evaluated using initial critical load Lc1 and upper critical load Lc2 for film failure. The better crack propagation resistance was observed for films deposited at 10mTorr sputtering pressure on both substrates, attributed to better crystalline nature of the films. Finite element (FE) modeling and simulation of nano-mechanical behavior of ZnO thin films were performed. Experimental results obtained from nanoindentation, are used to develop the FE Abstract iv | P a g e model. The FE calculation was performed as two dimensional model, assuming Berkovich tip as rigid material. FE analysis was used to elucidate the phenomenon of force-penetration depth by independently considering the variations in yield stress, friction coefficient and substrate effect of the crystalline ZnO thin films. The FE analysis was adopted in such a way to provide a best fit to experimental load displacement curve and investigated equivalency of force-indentation depth curves by Berkovich, conical or spherical tip with same radius. However, ZnO films are assumed as elastic-plastic material and perfectly bonded to substrate, so there is no slippage or delamination at the interface. The deformation in substrate depends on the difference in mechanical properties with coating and increase in penetration depth is observed when substrate yields before coating. FE simulation coupled with experimental process of indentation provides a better insight and optimization of the mechanical response for predicting the deformation behavior of ZnO thin films