El óxido de zinccrecimiento cristalino mediante transporte en fase gaseosa y caracterización de propiedades físicas.

Laburpena

Due to its high melting point (~2000ºC), is difficult to grow ZnO from the liquid phase. That is why hydrothermal and vapour transport methods have been mainly implemented so far for the growth of bulk ZnO crystals. Crystal growth from the vapour phase appears as an attractive alternative. Nevertheless, it has been well established that the growth rate of ZnO in closed ampoules at moderate temperatures (~1000ºC) and temperature gradients is very low if the process takes place in absence of any additional species. Higher growth rates are obtained if some particular additional species are introduced in the ampoule In this work, we will try to gain a further insight into the vapour generation and transport mechanisms involved in the ZnO growth using some additional species. The knowledge of these mechanisms can be very useful to optimise the growth rate and it could allow to control the gas composition inside the growth ampoule. This fact would allow to act on the off-stoichiometry degree of the ZnO crystals and, therefore, on their physical properties. Besides, we will analyse some ways to localise and control the onset of nucleation that can improve the structural properties and the crystal size. On the other hand, some of the grown ZnO crystals will be annealed at temperatures in the range 900-1200 ºC in vacuum, as well as in oxygen and zinc atmospheres. These annealing processes improve the structural quality and will be used as a tool to obtain some information about the initial intrinsic defects present in as-grown crystals. Different fundamental characterisation techniques as X-ray diffraction, Raman spectroscopy, optical transmission and Hall effect measurements will be used to study the physical properties of the as-grown and annealed ZnO crystals. In addition, we will study the application of two p-i-n heterostructures based on a ZnO electrodeposited layer, which shows nanocolumnar morphology, as eta (extremely thin absorber)-solar cells. The ZnO and the successively built up layers will be analysed by scanning electron microscopy, X-ray diffraction and optical transmission and reflectance. Finally, the i-V curves and the energy conversion efficiency of the device will be measured.