Bismuth Magnesium Titante - Lead Titanate Thin FIlms for High Temperature Ferroelectric Memory

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Author:
Morandi, Carl Sebastian
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 08, 2018
Committee Members:
  • Susan E Trolier-Mckinstry, Dissertation Advisor
  • Susan E Trolier-Mckinstry, Committee Chair
  • Clive A Randall, Committee Member
  • Thomas R Shrout, Committee Member
  • Douglas Edward Wolfe, Outside Member
Keywords:
  • Bismuth Magnesium Titanate - Lead Titanate
  • Ferroelectric Random Access Memory
  • FeRAM
  • Pulsed Laser Deposition
  • BiMT-PT
  • High Temperature Ferroelectric Thin Films
Abstract:
This work provides a comprehensive study on the development of 35Bi(Mg1/2Ti1/2)O3-65PbTiO3 thin films deposited by pulsed laser deposition, detailing the effects of processing variables on the film composition and resulting electrical properties. The primary application focus is on a potential high temperature ferroelectric layer for ferroelectric random access memory (FeRAM) devices. To assess whether 35BiMT-65PT thin films are of interest for this application, the films were assessed by determining the switchable polarization, the dielectric properties and their thickness dependence, the high temperature polarization and dielectric properties, and the retention characteristics. For pulsed-laser deposited 35BiMT-65PT films on 36 nm thick PbTiO3 or 16 nm thick 5 mol% La-doped PbTiO3 seed layers, the processing window for developing a perovskite structure was found to be wide. Within the detection limits of x-ray diffraction, scanning electron microscopy and transmission electron microscopy, the films were found to be phase pure perovskite for growth temperatures from 600 ˚C to 700 ˚C at O2(90%)/O3(10%) background pressures of 100-300 mTorr. Target compositions from 10 mol% to 85 mol% excess Pb produced phase pure perovskite films when depositing at 700 ˚C and a 10 Hz laser repetition rate. The film composition was found to vary as a function of processing parameters. As the chamber pressure during deposition decreased, the film Mg and Pb concentration decreased, while the concentration of Bi increases. For films on 36 nm PbTiO3 seed layers, the remanent polarization, Pr, increased 64% to ≈ 21 µC/cm2 and the polarization – electric field loops rotated counterclockwise as the deposition pressure increased from 60 mTorr to 340 mTorr. Decreasing the seed layer thickness from 36 to 16 nm led to a decrease in Pr to ≈ 14 µC/cm2. Adjusting the target composition allowed the deposition of films which had near-stoichiometric Bi and Mg concentrations, but in all cases, the grown films were lead deficient. These films had remanent polarizations of 18 to 20 µC/cm2. If the lead content of the target was increased too far, the remanent polarization decreased, possibly due to the need to evolve more PbO from defective growth layers. Finally, the deposition rate showed no substantial effect on the film composition, but did have a significant impact on the ferroelectric properties. As the deposition rate decreased, the Pr increased to ≈ 22 µC/cm2 presumably due to enhanced crystalline quality and time for atomic rearrangement. At laser frequencies of 5 Hz, a Mg-rich pyrochlore phase begins to form and films showed a maximum Pr ≈ 22 µC/cm2. The optimal processing window for the maximum in ferroelectric properties shifts to higher PbO excess contents in the target as the deposition rate decreased. A film deposited at 5 Hz with a 65 mol% Pb, 20 mol% Bi, and 10 mol% Mg excess target had the maximum Pr ≈ 25 µC/cm2. The processing-composition behavior is explained via preferential adsorption of Bi on the A-site, which results in lead vacancies. The dielectric, polarization and leakage properties of 35BiMT-65PT thin films with varying levels of A-site deficiency were investigated as a function of thickness and/or temperature. PbTiO3 seed layers utilized to nucleate the perovskite phase in A-site deficient films induced a thickness dependence to the ferroelectric hysteresis and dielectric permittivity. Adjusting for this, the dielectric response of the 35BiMT-65PT films is ≈960. The dielectric permittivity maximum was 430 ˚C at 1 MHz for A-site deficient films. The transition temperature is independent of film thickness to 85 nm. Tan(δ) remains less than 15% at 1 MHz, regardless of film thickness and temperature up to 585 ˚C. High temperature polarization-electric field hysteresis measurements show charge injection with is exacerbated on increasing temperature, while PUND measurements show little temperature dependence of Pr up to temperatures of 200 ˚C. Poole-Frenkel emission dominated the high field leakage behavior. The refractive index measured by ellipsometry is 2.58 at 633 nm. All samples show significant retention loss. As the stoichiometry improves, retention improves such that >40% of the initial ΔP is retained over ≈280 min. at room temperature. To remove the influence of the PbTiO3 seed layers on the apparent thickness dependence of 35BiMT-65PT, films without a PbTiO3 seed layer were deposited. Similar to their seed layer counterparts, seedless films show phase purity within the same target composition range investigated for films on seed layers. Phase pure films show somewhat reduced thickness dependent dielectric properties with respect to seeded films. Pr measured at 10 kHz for seedless films was ≈22-25 μC/cm2 until thicknesses of ≈200 nm and decreased to 15.5 μC/cm2 for films that were ≈100 nm thick. I-V measurements of seedless films deposited under nominally the same conditions as seeded films resulted in higher leakage compared to seeded films. Deposition under an Ar/O2 atmosphere decreased the leakage behavior by up to five times. Films grown under the optimum 50%O2/50%Ar atmosphere demonstrate little temperature dependence in Pr up to 200 ˚C. As temperature increases, the DC dielectric breakdown of strength of seedless films decreases faster compared to seeded films. Retention analysis shows similar behavior to that of seeded films.