Growth of Ca$_2$MnO$_4$ Ruddlesden-Popper structured thin films using Combinatorial Substrate Epitaxy

TL;DR

This study investigates the epitaxial growth of Ca$_2$MnO$_4$ thin films on polycrystalline substrates using combinatorial substrate epitaxy, revealing dominant orientation relationships and demonstrating the method's potential for complex oxide synthesis.

Contribution

It identifies primary orientation relationships in RP heteroepitaxy and showcases the effectiveness of combinatorial substrate epitaxy for high-throughput oxide film growth.

Findings

Three main orientation relationships describe nearly all grain pairs.

Optimal growth temperature is around 750°C based on diffraction quality.

Few orientation relationships are needed to characterize RP heteroepitaxy.

Abstract

The local epitaxial growth of pulsed laser deposited Ca$_2$MnO$_4$ films on polycrystalline spark plasma sintered Sr$_2$TiO$_4$ substrates was investigated to determine phase formation and preferred epitaxial orientation relationships ($ORs$) for isostructural Ruddlesden-Popper (RP) heteroepitaxy, further developing the high-throughput synthetic approach called Combinatorial Substrate Epitaxy (CSE). Both grazing incidence X-ray diffraction and electron backscatter diffraction (EBSD) patterns of the film and substrate were indexable as single-phase RP-structured compounds. The optimal growth temperature (between 650 $^{\circ}$C and 800 $^{\circ}$C) was found to be 750 $^{\circ}$C using the maximum value of the average image quality (IQ) of the backscattered diffraction patterns. Films grew in a grain-over-grain pattern such that each Ca$_2$MnO$_4$ grain had a single $OR$ with the Sr$_2$TiO$_4$ grain on which it grew. Three primary $ORs$ described 47 out of 49 grain pairs that covered nearly all of RP orientation space. The first $OR$, found for 20 of the 49, was the expected RP unit-cell over RP unit-cell $OR$, expressed as [100][001]$_{film}$||[100][001]$_{sub}$. The other two $ORs$ were essentially rotated from the first by 90$^{\circ}$, with one (observed for 17 of 49 pairs) being rotated about the [100] and the other (observed for 10 of 49 pairs) being rotated about the [110] (and not exactly by 90$^{\circ}$). These results indicate that only a small number of $ORs$ are needed to describe isostructural RP heteroepitaxy and further demonstrate the potential of CSE in the design and growth of a wide range of complex functional oxides.