Fabrication, Doping And Controlled Preferred Growth Orientation Mechanism Studies Of ZnO Thin Films By Chmical Solution Methods

Zinc Oxide (ZnO), as an important inorganic compound has been the part of human history since centuries. The new technologies of the twentieth century facilitated to discover revolutionary aspects and potentials of this material in material sciences. Since last couple of decades, it has been explored vigorously for its potential applications in today’s and future nanotech industry. 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 to optoelectronic ability, electrical and photochemical properties of ZnO have been used as gas sensors, transparent electrodes, piezoelectric devices, solar cells and UV-detection devices. Furthermore, inexpensive and abundant raw material availability, bio-compatibility, non-toxicity, good chemical and thermal stability, and strong radio resistance ability has made ZnO a material of choice for future applications.High quality ZnO thin films have been found to be an efficient way to utilize its vital properties at nano scale which has also lead to the integration and miniaturization of modern day devices. The developments in thin film fabrication techniques has provided more control to exploit the properties of ZnO and paved the way for its industrial applications. The low-cost and environment friendly chemical solution film grown methods are always more attractive because of their flexibility as they provide more optimization space than their rival sophisticated physical methods, but the quality of thin films, their controlled growth, doping and ambiguity in growth mechanism are the intractable problems still associated with solution based methods.In this thesis, we employed simple chemical solution methods to fabricate doped and undoped ZnO thin films to optimize the doping conditions and to exclusively address the bottleneck problem of controlled growth for ZnO thin films. In this regard, some novel approaches were devised to modulate the chemical solution methods. Based on experimental results and analysis growth mechanisms were proposed and explained comprehensively.Al-doped zinc oxide (AZO) thin films with Al doping content ranging from 1-10 at.% were fabricated on amorphous glass substrates by sol-gel method under different annealing temperatures. XRD, SEM and band gap measurements revealed that the thin film doped with Al 4 at.% content has the highest doping level under our preparation conditions. The annealing temperature has no apparent effect on the doping level of films but the crystallinity and transparency of the films was increased with annealing temperature. Films doped at relatively higher doping content (e.g.10 at.%) were severely cracked irrespective of annealing temperature which was attributed to the presence of excessive amount of amorphous Al2O3, owing to undoped Al contents, in the film structure. The sol aging time was discovered to be affecting the doping level of the dopant in the film, whereas the optical transparency of the thin films was enhanced with sol-aging time. Through XRD, XPS and UV-Vis spectroscopy analysis,20-24 h sol aging time was proved to be optimal to achieve the saturation of dopant atoms in the AZO films and the prepared films with such sol exhibit low surface roughness and superior quality. It was proposed that precursor solution aging for 20-24 h would provide high quality doped films due to an ideal stability of solution system.A novel approach was used to control the growth orientation of ZnO thin films in a facile two step solution method. ZnO seeds grown by hydrothermal method using sol/reverse-microemulsion solution system were deposited on Si substrates which were etched in "V" shaped grooves by customized wet etching technique. Using a suitable solution and growth conditions of a chemical bath deposition system, non-polar m-plane (1010) ZnO thin films were successfully grown on the seeded etched Si substrate. The substrate surface modification, ZnO seeds characteristics and CBD solution chemistry were optimized to achieve the controlled growth orientation. XRD analysis confirmed preferential (1010) non-polar growth orientation while FESEM characterization revealed high density nano-plate structured film morphology. Through time-resolved growth reaction FESEM analysis, a three step growth process viz. dissolution, recrystallization & primary growth and in-situ nucleation/crystallization and secondary growth was proposed and explained. PL analysis affirmed the high optical quality defects free films owing to pure and high crystalline film structure.Highly transparent and conductive non-polar a-plane (1120) ZnO thin films were successfully grown on amorphous glass substrates. ZnO seed layers characteristics were manipulated to control the growth of ZnO thin films. The results proved that the presence of IPA in aqueous CBD solution inhibits the film growth along (0002) polar plane due to selective adsorption on polar crystal faces. In the absence of (0002) growth, seed layer grains directed the growth along (1120) non-polar crystal plane due to its relatively high surface energy and fast growth velocity. A competitive growth mechanism of crystal planes discovered by TEM was proved by time-resolved CBD growth and based on results a growth model was proposed. Non-polar film showed a transmittance around 95%, which is quite striking for an undoped ZnO film. PL analysis confirmed the high optical quality non-polar films with very low level of structural defects. Temperature dependent resistivity measurements confirmed that non-polar film has room temperature resistivity around ～9.3 Q.cm, which is quite comparable and even better than many of solution grown polar ZnO films or films deposited by physical methods. Surface wettability measurement showed that a non-polar film has good hydrophobic character in comparison to polar film.Relying on a proposed growth model for the growth of organic crystals, we successfully grew ZnO thin films with controlled crystal growth orientations by manipulating the solution/solvent polarity and the seed layers characteristics on amorphous glass substrates. Through this model high quality ZnO thin films with major polar i.e. (002), (101), (112) and non-polar (100), (110) crystal planes were successfully grown. XRD, FESEM, TEM and AFM characterization was employed to study the structural and morphological characteristics of grown films and based on these results a potential growth mechanism was explained. Experimental results proved a growth phenomenon where solution/solvent polarity induced and seed layers characteristics directed and fine tuned the crystal growth orientation for ZnO films. The grown films were highly transparent in visible spectrum range, with a transparency level over 80%. PL analysis showed a possible difference in defects concentration and type for films with polar and non-polar crystal growth orientation. The surface wetting properties of the CBD grown films revealed that wettability of films surface was polarity dependent, as relative hydrophilicity of films decreased with changing thin film crystal orientation from strongly polar to non-polar. The surface of (002) polar film showed good hydrophilicity, while non-polar film with (110) surface was found to be relatively hydrophobic in nature.