Oxygen diffusion barriers for epitaxial thin-film heterostructures with highly conducting SrMoO3 electrodes

Patrick Salg,Lukas Zeinar,Dominik Walk,Lambert Alff,Aldin Radetinac,Holger Maune,Rolf Jakoby,Philipp Komissinskiy

doi:10.1063/1.5129767

Abstract

Transition metal perovskite oxide SrMoO3 with a Mo4+ 4d2 electronic configuration exhibits a room-temperature resistivity of 5.1 μΩcm in a single-crystal form and, therefore, is considered a prominent conducting electrode material for all-oxide microelectronic devices. Stabilization of the unfavorable Mo4+ valence state in SrMoO3 thin films necessitates reductive growth conditions that are often incompatible with a highly oxidative environment necessary to grow epitaxial heterostructures with fully oxygenated functional layers (e.g., tunable dielectric BaxSr1−xTiO3). Interestingly, only a few unit cells of the perovskite titanate capping layers SrTiO3, BaTiO3, and Ba0.5Sr0.5TiO3 act as an efficient oxygen barrier and minimize SrMoO3 oxidation into electrically insulating SrMoO4 in the broad range of the thin-film growth parameters. The Mo valence state in SrMoO3, determined by x-ray photoelectron spectroscopy, is used to analyze oxygen diffusion through the capping layers. The lowest level of oxygen diffusion is observed in Ba0.5Sr0.5TiO3. A Ba0.5Sr0.5TiO3 film with a thickness of only 6 unit cells preserves the Mo4+ oxidation state in the SrMoO3 underlayer up to the oxygen partial pressure of 8 mTorr at the temperature of 630 °C. Results, therefore, indicate that SrMoO3 films covered with atomically thin Ba0.5Sr0.5TiO3 remain conducting in an oxygen environment and can be integrated into all-oxide thin-film heterostructures with other functional materials.

Highlights

Ferroelectic varactors with voltage tunable dielectric permittivity are considered for integration in modern microwave microelectronic devices as they deliver fast and flexible impedance matching and frequency tuning.[1,2] Varactors are often manufactured in a metal–insulator–metal (MIM) stack geometry with a metal bottom electrode (e.g., Pt).[1]
The period of the oscillations yields 33 + 2 nm SrMoO3 film thickness, which is independently confirmed by the reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and x-ray reflectivity (XRR) data [see Fig. 1 and the inset in Fig. 2(a), respectively]
The XRD pattern of the BaTiO3-capped sample shows an asymmetric 002 SrMoO3 peak shape with a higher intensity shifted toward the FIG. 2. 2θ–θ XRD patterns around the 002 reflections of SrMoO3 for the (a) as-grown heterostructures (b) after their annealing in oxygen for 2 min at 630 C and 10 mTorr

Summary

INTRODUCTION

Ferroelectic varactors (tunable capacitors) with voltage tunable dielectric permittivity are considered for integration in modern microwave microelectronic devices as they deliver fast and flexible impedance matching and frequency tuning.[1,2] Varactors are often manufactured in a metal–insulator–metal (MIM) stack geometry with a metal bottom electrode (e.g., Pt).[1]. Perovskite, a highly conducting oxide SrMoO3, with a low room-temperature resistivity of only 5.1 μΩ cm (as reported for SrMoO3 single crystal)[6] is an advantageous substitution material for metal bottom electrodes of the varactors as it enables epitaxial growth of the commonly used tunable dielectric Ba0:5Sr0:5TiO3 (BST). The obstacle can be surmounted if the SrMoO3 film is capped by a thin layer of a material that minimizes oxygen diffusion between BST and SrMoO3 and, prevents Mo4þ ! For heterostructures with SrMoO3, oxygen diffusion through the capping layers can be indirectly, yet still accurately, analyzed by monitoring the Mo valence state, using x-ray photoelectron spectroscopy (XPS).[17]. We utilize a similar measurement route and monitor the Mo oxidation state in SrMoO3 by XPS to compare oxygen diffusion through the Ba0:5Sr0:5TiO3 and two other capping layers, namely, SrTiO3 (STO) and BaTiO3 (BTO). An oxidation limit for the growth of functional BST on top of SrMoO3 is determined from the rise of the Mo6þ oxidation state for each capping material

EXPERIMENTAL DETAILS

RESULTS AND DISCUSSION

CONCLUSION