Characterization of GaN:ZnO p-n junctions

Abstract

In this master thesis, thin lms of ZnO doped with various cobalt concentra-tions have been grown using Pulsed Laser Deposition (PLD). This growthtechnique is preferred because it is able to grow thin lms from a targetmaterial without changing the elemental ratios from the target. The targetsusing during deposition are sintered pellets of ZnO doped with cobalt, withdoping concentrations of 10, 20 and 30 %.The material was chosen because of its potential for Intermediate BandSolar Cells (IBSC). IBSC is a new concept which aims to create solar cellswhich have higher eciencies than the solar cells available today. From de-tailed balance theory, the concept of intermediate band solar cells shows apotential of a 86 % eciency limit under perfect conditions, which is remark-able compared to the conventional silicon solar cell devices on the markettoday with a eciency limit of 41 %.A series of ZnO thin lms of 10, 20 and 30 % cobalt concentration weredeposited by PLD on a sapphire substrate. These lms were, together with asimilar series containing 1,2 and 5 % cobalt concentration deposited on bothsilicon and sapphire substrates, characterized using Scanning Electron Mi-croscopy (SEM), Energy Dispersive X-ray (EDX) Microanalysis, X-ray Pho-toelectron Spectroscopy (XPS), Hall measurements, X-ray diraction (XRD)and optical transmission spectroscopy.In addition, an attempt to make Schottky diodes of the cobalt dopedZnO thin lms and a p-IB-n junction using GaN:Mg and ZnO:Al as the p-and n- emitter was done. These intermediate band solar cell devices weregoing to be studied by recording dark and illuminated current-voltage (I-V)characteristics.The focus in the study has been to determine which cobalt doping con-centration gives the desired properties for use of ZnO:Co in an intermediateband solar cell device, and to study how a solar cell device with an inter-mediate band material can be realized. The experiments were carried outat NTNU NanoLab, NTNU Department of Physics, NTNU Department ofElectronics and Telecommunications and SINTEF Oslo.Optical transmission spectroscopy and XPS showed that the cobalt inthe ZnO:Co lms were Co2+. This indicates that cobalt is substituting forzink in the ZnO lattice. XRD showed the crystal structure to be c-orientedwith reasonable order. The carrier concentration in the ZnO:Co lms were inthe order of 1018- 1019, decreasing with increasing cobalt concentration. Theresistivity in the samples increased with increasing doping concentration.Investigation of the GaN:Mg wafer showed that it did not have the desiredproperties for device fabrication as it showed n-type conductivity insteadof p-type. The Schottky diodes made of Au-ZnO:Co and Pt-ZnO:Co did not show diode behavior because of surface states in ZnO:Co making ohmiccontacts.