Pulsed laser deposition of heteroepitaxial ferroelectric thin film capacitor

Abstract

In the present studies, high quality conducting SrVO3 (SVO) and YBa2Cu3Or7-x (YBCO) oxide thin films were grown by Pulsed Laser Deposition (PLD) method. It has been demonstrated that SVO films can be epitaxially grown on Si at a processing temperature as low as 5500C and heteroepitaxial relationship of (100)SVO (100)TiN (100)Si has been obtained. Our SVO films are of good metallic properties and a very low resistivity of 3 u -cm at 78 K has been recorded. The high temperature superconducting YBCO films integrating on Si through the STO/TiN buffer layers also show good metallic property and have good electrical conduction of 95uQ-cm at room temperature. Its transition temperature (Tc) is about 88 K. However, from the scanning electron microscopic studies of the YBCO/STO/TiN/Si heterostructure, microcracks were observed. The microcracks may arise from the large difference in thermal expansion mismatch between YBCO and STO and the oxidation of TiN layer. In view of this, magnesium oxide (MgO), a buffer material with better thermal matching with YBCO and a good oxygen diffusion barrier for protecting TiN from oxidation during the growth of YBCO was used. From the scanning electron microscopic studies of the YBCO/MgO/TiN/Si heterostructure, a crack-free and a much better surface morphologies has been observed. In addition, it did allow a better crystal growth of PZT films on YBCO bottom electrode, i.e. PZT/YBCO/MgO/TiN/Si. Both the structure and electrical properties of the Au/PZT/YBCO/MgO/TiN/Si integrated ferroelectric capacitors are comparable to those grown on single crystal LaAlO3 (LAO) substrates. The Au/PZT/YBCO/MgO/TiN/Si integrated ferroelectric capacitor shows good ferroelectric properties such as high remnant polarization, low coercive field and high dielectric constant with low dielectric loss values. No polarization fatigue was observed at bi-polar switching up to io7 cycles. The leakage current of such capacitor is in the order of 10-7 A/cm2. In short, SVO films of good crystal quality has been fabricated on Si substrates through TiN buffered layer and the processing temperature can be as low as 550oC. Besides, a very low resistivity of SVO films has been obtained. In addition, the merits of using MgO/TiN buffer layers over that of STO/TiN has been demonstrated. Up till now, there are many choices of buffer layers for growing perovskites films. It is our believe that the discovery of the advantage of using MgO/TiN buffer Layers will have a great influence on people's decision when they choose the materials for buffer layers. Finally, as an example, excellent quality of the heteroepitaxial Au/PZT/YBCO/MgO/TiN/Si integrated ferroelectric capacitors for potential non-volatile memory devices were fabricated.