Study On Controlled Preferred Growth Orientated Pure And Al-doped ZnO Thin Films By Chemical Solution Methods

Zinc Oxide(ZnO),as an important inorganic compound has been the part of human history since centuries.Since last couple of decades,it has been explored vigorously for its potential applications in current and future nanotech industry,such as optoelectronic devices,light emitting diodes,photodetectors,gas sensors and so on.ZnO has been regarded as third generation semiconductor material due to its relatively wide band gap(3.37 eV)and large exciton binding energy(60 meV),which provides it a high potential for room temperature light emission.In addition,cheap and abundant raw materials,good biocompatibility,non-toxicity,good chemical and thermal stability and strong radio resistance ability have made ZnO a material of choice for future applications.At present,researches on ZnO thin films with preferential growth orientation focuses on physical methods,such as molecular beam epitaxy,RF magnetron sputtering,pulsed laser deposition,etc.,but expensive equipment,high cost,complex operation and high vacuum conditions restrict the widespread use of these methods.In this paper,cheap and highly optical transparent amorphous glasses were choosen as substrates,seed layers were deposited in advance on these substrates by Sol-Gel method,then thin films with preferential growth orientation were grown in a controlled way by chemical bath deposition method by using different kind of alcohols and content of non-polar auxiliary agent(cyclohexane).Different alcohols were adsorbed on certain ZnO planes in order to restrict or slow down the growth of these crystal planes.Isopropanol was found to be the strongest non-polar solvent among the four kinds of solvent used,which may be adsorbed on(002)and(101)crystal plane and favoured the growth of(100)and(110)crystal planes.The contents of cyclohexane as non-polar auxiliary agent mainly adjusts the polarity of solution.The content of the non-polar adjuvant(cyclohexane)can adjust preferential growth orientation of ZnO thin films,when the polarity of the solvent and certain ZnO lattice planes is close,it will more beneficial to the growth along these lattice planes and lead to the growth of non-polar ZnO thin films.If the concentration of cyclohexane is too small,the preferential growth orientation may lead to poor effect,on the other hand if the concentration of cyclohexane is too high,it may impede the Zn2+ movement towards substrates surface resulting in poor quality of ZnO thin films.In this paper,XRD,AFM and SEM were used to investigate the crystal structure and surface morphology of ZnO thin films,the results revealed that using isopropanol as solvent and cyclohexane with the content ratio of 10%successessfully optimize the growth orientation of non-polar ZnO thin film,and the thin films grown under these growth parameters show good crystallinity.The conductivity of Nonpolar ZnO thin films is worse in cmparision to pure ZnO films,which hinders its photovoltaic applications.Al doped ZnO thin films are the potential candidate to increase the conductivity of films,which are also a good alternate of ITO conductive thin films.In this paper,the Al doped ZnO films were also studied,the results revealed that when the ZnO thin films doped with Al 1.5 at%content has the highest doping level,the largest crystal size,smallest resistivity and large optical band gap.When the content of Al increase continuously,the atoms may form a state of amorphous Al2O3 and pin the movement of ZnO grain boundary,which lead leads to the decrease of grain size.Through the UV-Vis and PL spectroscopy analysis,it was found that with the increase of Al doing content,the optical band gap of ZnO thin film shows a gradual blue shift.But the maximum of the blue shift was not occured for the Al content of 1.5 at%.With the increase of Al content,the optical band gap increased contiuously,which could be attributed to quantum size effect.The maximum optical band gap for grown doped films was obtained for the Al doping content of 5at%.Photluminescence analysis results showed the green light emissions for the Al doped non-polar ZnO thin films,which may broaden the scope of their practical applications.