Crystal growth of functional materials by using csvs and mocvdthe aiimnbvi and ii-oxides case

Abstract

This thesis presents an in-depth study of the crystal growth and characterization of some functional materials of the II-VI family. The studied structural, morphological, optical and electrical properties have been correlated with the crystal growth methodology and growth conditions used. Among the variety of II-VI materials, 2 groups of semiconductors have been chosen due to their unique properties. The first includes oxides of II-group, with elements such as Zinc and Cadmium, with high transparency in the visible optical range. These compounds can have applications in optoelectronics and can be used as transparent conductive oxides (TCOs). The other group of studied materials is some diluted magnetic semiconductors (DMS) based on Manganese and the II-VI semiconductors ZnS and ZnTe. In addition to the usual semiconductor properties, the Mn alloyed materials can have interesting magnetic properties. In order to grow the binary and ternary compounds studied in this thesis, two growth methods have been used, the more accurate, multiparameter-controlled Metal-Organic Chemical Vapor Deposition (MOCVD) method and the simpler Close Space Vacuum Sublimation (CSVS). The characterization of the samples has been done using the following techniques: atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), energy-dispersive X-ray spectroscopy (EDX), particle-induced X-ray emission (PIXE) and optical reflectance and transmission measurements. Thus, ZnO thin films were grown using the MOCVD method on the crystal planes C-, A-, M- and R- of sapphire substrates. The ZnO direct bandgap of ~ 3.4 eV provides a good degree of optical transparency in the visible region of the electromagnetic spectrum, and its piezoelectric properties are ideal for application on surface acoustic waves (SAWs) filters. Moreover, ZnO has shown its potentiality in fields such as photonics and microelectronics. In this regard, obtaining layers of nanometric thickness, while maintaining a good morphological and structural quality, is both a requirement and a challenge. In this thesis, a systematic study of the thickness and roughness decrease of the ZnO layers has been undertaken as a function of the growth conditions (precursor flux, growth temperature and time). In a complementary manner, a chemical treatment of the substrate has been carried out, in order to increase the nucleation points. This systematical study has resulted in the attainment and characterization of high-quality layers with thicknesses of about 34 nm and roughness of about 2 nm. Taking into account the possibility of changing the properties of binary compounds (such as the carrier density or bandgap energy) by alloying the material with another element, the growth of the CdZnO alloy was carried out. One of the main difficulties in obtaining this ternary compound with desirable properties and good crystal quality is due to the difference in crystal structures between ZnO (wurtzite) and CdO (cubic). Most of the works that can be found in the literature are devoted to the ternary with high content of zinc or cadmium, while the study of the transition between the cubic and wurtzite phases has received less attention. In this thesis, we have delved into the growth of the cadmium rich region of the alloy on R-sapphire, and we have found the solubility limit of Zn under our experimental conditions, and how the transition from the cubic phase to a mixed one affects the structural properties of this ternary compound. Additionally, the influence of the carrier gas on some structural and optical features of the CdZnO thin layers has been analyzed. In the same frame of studying the II-VI family, CdTe stands out as one of the most significant of this family. CdTe is often used as an absorber layer in CdTe/CdS heterostructures. Nevertheless, heterostructures with TCO, in particular the p-n-heterojunction CdTe/CdO, have been less studied. The complexity in obtaining this structure is due to, among other factors, the lattice mismatch between CdO and CdTe. In this thesis, the growth parameters were chosen in such a way that the stresses between the layers of the CdTe/CdO/R-sapphire heterostructure grown by the MOCVD method are minimized. The study of the Contact potential difference (CPD) and the change in the Surface photovoltage (SPV) as a function of the applied power of the incident laser have been analyzed in correlation with the growth temperature. On the other hand, DMS, as said before, combine elements of the physics of semiconductors and magnetism, which is a unique opportunity for research and technology. Unlike in classical semiconductor alloys, the random distribution of magnetic ions leads to the appearance and development of individual magnetic phases. Among alloying elements, the inclusion of Mn atoms in the II–VI lattice is particularly interesting since it leads to phenomena such as negative magnetoresistance, giant Faraday rotation and spin-glass behavior. We have studied the growth and characterization of the ZnS and ZnTe alloying with Mn, due to the wide bandgap and optical features that these alloys can present, which make them attractive for photovoltaic applications. Thus, ZnMnTe and ZnMnS thin films were obtained on glass substrates by the CSVS technique. The study has shown the Mn solubility limit under our experimental conditions, and how the incorporation of manganese affects the structural, substructural, and optical characteristics of the ternary compounds.