Strain engineered ferroelectricity in barium strontium titanate grown on niobium-doped strontium titanate via pulsed laser deposition

Caitlin Beth Wildeson, Purdue University

Abstract

Ferroelectric thin films are interesting materials because of their possible application in ferroelectric memory devices (FE-RAM and FE-FETs), and as high-κ dielectrics. However, many ferroelectric films contain lead, which is both volatile and toxic. In this work, lead-free and typically paraelectric Ba0.5Sr0.5TiO3 thin films have been strain engineered to become ferroelectric by applying in-plane strain via epitaxial growth. A niobium-doped SrTiO3 substrate, which has a smaller lattice parameter than that of Ba0.5Sr0.5TiO3, is used to apply bi-axial compressive stress on the film. The films are grown using pulsed laser deposition. The optimized growth conditions included a laser fluence of 6.8 J/cm2 with a pulse rate of 5 Hz, a substrate temperature of 514°C, and an oxygen pressure of 150 mTorr. The critical thickness for dislocation nucleation for this material system is expected to be near 50 nm. However, ferroelectric films incorporated into capacitors are required to be several hundred nanometers thick. This presents a challenge because it is desirable to maintain compressively strained films throughout the thickness in order to achieve the best possible ferroelectric properties. In this work, Ba0.5Sr0.5TiO3 films were compressively strained well above their critical thickness by growing under conditions that kinetically restricted the films from relaxing into their cubic structure. Specifically, the effects of temperature, pressure and laser fluence were all investigated for their influence on growing strained films. Asymmetric coplanar reciprocal space mapping, an x-ray diffraction technique, indicates that strained films as thick as 800 nm have been attained. Ferroelectric behavior of the strained films is confirmed by piezoresponse force microscope. Ferroelectric films with paraelectric interlayers have also been fabricated. These superlattice structures have been confirmed to be fully strained in-plane by asymmetric reciprocal space mapping, with a small degree of mosaic spread. Such films may be used in non-volatile memory devices as their properties suggest that they may have long retention lifetimes. They may also exhibit highly square hysteresis loops due to the improved domain alignment, which may allow these structures to become the ferroelectric component for low power negative capacitance devices.

Degree

M.S.M.S.E.

Advisors

Appenzeller, Purdue University.

Subject Area

Materials science

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