| dc.description.abstract | p-n homo- and heterojunctions are the cornerstones and defining technology of our society. Even though decades of knowledge have brought several improvements upon the transistors, LED diodes and other devices that build upon these junctions, the issue of overheating and long-term stability is not yet fully solved. The existence of thermodynamically stable heterojunctions, especially undoped, is believed to be a solution to these issues. The search for suitable p- and n-type semiconductors that can exist in contact over large temperature ranges, that are eternally stable at equilibrium, and display promising junction characteristics is the overall aim of the CoExist project. The goal of this thesis is to study the junction characteristics and thermal stability of the La2CuO4(LCO)-Nd2CuO4(NCO) p-n metal-semiconductor system. LCO and NCO are reported to exhibit p-type and n-type conductivity, respectively, and share a phase diagram. In this respect a mixed-phase composite of the materials was synthesised, and electrical measurements were conducted on the single materials, p-n junctions and p-c-n junctions. The objective is to decide whether the system is coexistent, and if the composite is beneficial to the properties of a junction of the system. The pure materials, LCO and NCO, were synthesised by the ceramic method and the composites were synthesised by the sol-gel method. All precursors were pressed to dense pellets and sintered. Characterisation by X-ray diffraction (XRD) revealed single phases corresponding to an orthorhombic structure of LCO and a tetragonal structure of NCO. The composite containing 57.5 % NCO and 42.5 % LCO, L0.43N0.57CO, conformed to a tetragonal single-phase solid-solution, corresponding to LCO in NCO. The opposite composite of 57.5 % LCO and 42.5 % NCO, L0.57N0.43CO, conformed to a mixed-phase solid-solution of a tetragonal LCO and orthorhombic structure LaNdCuO4. A third phase corresponding to monoclinic CuO was also observed. Pellets of LCO, NCO and composites were used to measure the temperature dependencies of the single materials and stacks, corresponding to a p-n junction setup and a p-c-n junction setup. Pt electrodes were painted on both side of the single pellets, while the p-n and p-c-n junction setups were first polished to ensure intimate contact and received painted electrodes only on the outward-facing surfaces. The van der Pauw method was employed to measure the resistivity of the single samples up to 800 °C in ambient conditions. Potentiostatic Electrochemical Impedance Spectroscopy (EIS) was used to study the junction setups with a 2-point probe setup, as well as Single Frequency impedance analysis to monitor the impedance. A single semi-circle was observed at low temperatures for some of the samples and fitted to a simple Randles type equivalent circuit with the high frequency range assigned to the bulk contribution. All junction setups were also monitored by Linear Sweep Voltammetry (LSV) to obtain the I-V curves of the junctions. The temperature dependencies were carried out in ambient conditions between room temperature and 800 °C. The results revealed that LCO follows a metallic-type conductivity, while NCO follows the behaviour of a semiconductor, as expected. The conductivity measured for L0.43N0.57CO is staggeringly high, 7000 S/m at 800 °C, while L0.57N0.43CO follows the same trend, reaching 1500 S/m at 600 °C. The conductivities of the p-n and p-c-n setups were lower, but showed a 10-fold increase between NCO, the p-n junction and the p-c-n junction. This increase in conductivity is assumed to arise from device limitations and a mistake in the measurement setup. Activation energies were calculated for all samples fitted to a small polaron hopping mechanism for NCO and the composites at high temperatures and compared to a regular fitting in the linear regions of the Arrhenius plots. From this, NCO follows a small polaron hopping mechanism (0.541 ± 0.009 eV), which is lower than the calculated values for L0.43N0.57CO (0.68 ± 0.04 eV) and L0.57N0.43CO (0.71 ± 0.03 eV) fitted to the same model. The low-temperature region is observed to follow a linear trend, comparable to LCO. Leading to believe that the composites follow a metallic-like conduction mechanism in the low-temperature range and transitioning to a small polaron hopping mechanism in the high-temperature range. For the p-n and p-c-n junctions lower activation energies were observed. Linear fitting to the Arrhenius plot of the temperature dependent conductivities were compared to the Arrhenius plot of the temperature dependent current-voltage curves. A drop in the activation energy occurs when placing either of the composites in the p-n junction. The greatest difference was observed for p-L0.43N0.57CO-n, by 0.142 eV. Ohmic characteristic is observed for all junctions after heat-up, and a slight increase in the current produced through the sample was observed for both p-c-n junctions compared to the p-n junction. Interestingly, a rectifying curve was measured for both p-c-n junctions before heat-treatment. These observations lead to the conclusion that both composite samples placed between the p- and n-type LCO and NCO, can indeed enhance the junction properties, by allowing more current to pass through the p-c-n junction, and significantly decreasing the activation energy related to the processes behind the conduction in these setups. The study gave also an insight into the composites themselves, exhibiting exceedingly high conductivities with ohmic characteristics in a junction. | eng |