CRYSTAL GROWTH : GROWTH AND CHARACTERISATION OF KDP SINGLE CRYSTALS AND YBCO SUPERCONDUCTING THIN FILMS

Abstract

Crystal growth is an interdisciplinary subject in the sense that contributions are continuously made by scientists and engineers from many professional fields. Fundamentally, crystals are grown from three different ambient phases (solutions, melts and vapour) in useful forms of bulk crystals or epitaxy films. As the phenomenon coming under the topic of crystallisation are extremely diverse, this present work aims to provide a two-portioned framework, essentially for the understanding of crystal growth mechanisms and their relationships with growth conditions. Part I is devoted to the study of crystal growth from solution. This simple, convenient and economical technique is ideal for the growth of KDP single crystals. Chapter I provides the theoretical background of KDP crystallisation from aqueous solutions. Chapter II reports on the experimental details and results of the relationship between the crystallisation phenomena and the presence of organic additives (OA) in crystallising KDP media. A comprehensive perception of the role of OA on growth processes was proposed considering their abundance in the natural environments should affect crystal growth. Moreover, relevant literature are sporadic and only less unequivocal results are presented. Then, Chapters III & IV gives a brief account of Laser Light Scattering Technique (LST) working principles and experimental results of its effectiveness as a characterisation tool for detection of crystals' internal defects respectively. LST was specially implemented to promote significant in-situ observations on the development of liquid inclusions with changes in growth conditions. Part II concentrates on crystal growth from vapour phase. YBa2Cu307-x (YBCO) superconducting thin films were deposited by an evaporation method called Pulsed Laser Ablation (PLA). The simplicity, high deposition rates and versatility of PLA has led to overwhelming response from scientists since its discovery. Our prioritised concern emphasises on better understanding of the ablation and deposition processes in order to achieve better control of the synthesis of high quality films that are meant for fabrication of highly efficient devices. In Chapter V, the relationship between film quality and growth parameters; the characteristics, advantages and disadvantages of PLA are introduced. Chapter VI describes two basic techniques for measuring superconducting properties of thin films, namely four-point probe and magnetic induction methods. Chapter VII contains the experimental details and results of the effective application of on-axis PLA for depositing single-sided YBCO thin films. The desirable film crystalline texture and morphology which determines the success of patterning process on devices, can be controlled by optimising the substrate temperatures, Ts. Thus series of films were deposited on single-sided polished YSZ substrates at Ts ranging from 600°C to 800°C while keeping other parameters constant. Films with the best texture and quality were produced around Ts = 700°C. In Chapter VIII, details of the successful application of off-axis PLA for depositing good quality double-sided YBCO thin films on polished (100) LaAlO3 substrate were given. Direct thermal contact with the heater was featured to improve the film quality as a result of induced recrystallisation. (Note: Other characterisation techniques include XRD, SEM, AFM and Micro- Raman Spectroscopy.] Finally, future undertakings to acquire more information on vapour growth mechanisms by LST application is suggested.