Strain-dependence of χ(2) in thin film barium strontium titanate

Natalie D Foster,David J Smith,Michael C Downer,Bryce I Edmondson,John G Ekerdt

doi:10.1063/1.5055731

Natalie D Foster, David J Smith + Show 3 more

Open Access

https://doi.org/10.1063/1.5055731

Copy DOI

Abstract

Optical second-harmonic generation (SHG) in reflection characterized variations in the second-order nonlinear optical susceptibility χ(2) of Ba1−xSrxTiO3 (BSTO) alloy films grown epitaxially on SrTiO3(001) (STO) as Sr molar ratio x changed from 0 to 1 and thickness from 10 to 50 nm. X-ray diffraction showed that BSTO films with x ≳ 0.1 were highly crystalline and compressively strained with out-of-plane (in-plane) lattice constant 4.09 &gt; c &gt; 3.97 (3.903 &lt; a &lt; 3.907 Å, pinned to substrate) for 0 &lt; x &lt; 1. Spectroscopic ellipsometry showed that the BSTO dielectric function was nearly x-invariant in the range of fundamental (780 nm) and SH (390 nm) wavelengths. SHG intensity peaked at x ∼ 0.3 for a given film thickness, suggesting that composition control can optimize ferroelectric and electro-optic properties of BSTO films grown on STO buffer layers.

Highlights

The first successful growth of a ferroelectric oxide on silicon while maintaining its polarization reversibility and avoiding deleterious chemical reactions1 opened opportunities for ferroelectric field-effect transistors2 and non-volatile memories3 that consume less power than standard metal-oxidesemiconductor (MOS) devices
second-harmonic generation (SHG) intensity peaked at x ∼ 0.3 for a given film thickness, suggesting that composition control can optimize ferroelectric and electro-optic properties of BSTO films grown on STO buffer layers
We evaluate crystallinity of MBE-grown BSTO films of thickness 10 < d < 50 nm using X-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy (HAADF-scanning transmission electron microscope (STEM)), and their linear optical properties using spectroscopic ellipsometry (SE)

Summary

INTRODUCTION

The first successful growth of a ferroelectric oxide on silicon while maintaining its polarization reversibility and avoiding deleterious chemical reactions opened opportunities for ferroelectric field-effect transistors and non-volatile memories that consume less power than standard metal-oxidesemiconductor (MOS) devices. Compressive strain at the Si(001) interface induced ferroelectricity over a strain relaxation thickness of only ∼ 30 Å in an epitaxial film of strontium titanate (SrTiO3, or STO), which lacks bulk ferroelectricity at room temperature (RT). Such a thin active layer had limited device applications. The interplay of lattice-mismatch strain and bulk structural variation with x creates a rich test study in χ(2) engineering, analogous to past studies of other BSTO properties.28–31 To highlight these two influences on χ(2), we study the BSTO/STO system in isolation from a semiconductor substrate. Similar methods can be employed in future studies of semiconductor/STO/BSTO structures to diagnose the additional influence of thermal expansion mismatch strain on BSTO properties

FILM FABRICATION AND CHARACTERIZATION

SHG MEASUREMENTS

RESULTS AND DISCUSSION

CONCLUSION