Pulsed-Laser Crystallization of Ferroelectric/Piezoelectric Oxide Thin Films

Open Access
Author:
Rajashekhar, Adarsh
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 17, 2016
Committee Members:
  • Susan E Trolier-Mckinstry, Dissertation Advisor
  • Susan E Trolier-Mckinstry, Committee Chair
  • Clive A Randall, Committee Member
  • Long-Qing Chen, Committee Member
  • Srinivas A Tadigadapa, Outside Member
Keywords:
  • Ferroelectric Thin Films
  • Piezoelectric Thin Films
  • Laser Annealing
  • Pulsed-Laser Heating
  • Pulsed Laser Deposition
  • Laser Crystallization
  • Integration with polymeric substrates
  • Low Substrate Temperature Processing
  • In situ Laser Annealing
  • PZT Thin Films
Abstract:
Integration of ferroelectric/piezoelectric thin films, such as those of lead zirconate titanate (PZT), with temperature sensitive substrates (complementary metal oxide semiconductors (CMOS), or polymers) would benefit from growth at substrate temperatures below 400˚C. However, high temperatures are usually required for obtaining good quality PZT films via conventional routes like rapid thermal processing (>550°C). Those conditions are not compatible either with polymer substrates or completed CMOS circuits and dictate exploration of alternative methods to realize integration with such substrates. In part of this work, factors influencing KrF excimer laser induced crystallization of amorphous sputtered Pb(Zr0.30Ti0.70)O3 thin films at substrate temperatures <~215°C were investigated. (111) Pt/Si substrates were utilized to understand the process window. Laser energy densities studied were in the range 35 – 85 mJ/cm2. The Pb content in the films was varied via the Ar gas pressure (in the range 5 mTorr – 9 mTorr) during sputtering of amorphous films. It was seen that a higher Pb content in the as-deposited films aided nucleation of the perovskite phase. Ozone-containing ambients (10% O3/90% O2) during the annealing promoted the formation of the metastable Pb-rich pyrochlore/fluorite phase, while annealing in pure oxygen produced the perovskite phase at relatively lower annealing laser energy densities. Heterogeneous nucleation from the substrate is favored on utilizing a layer-by-layer growth and crystallization process. Films were also grown on polymers using this method. Ferroelectric switching was demonstrated, but extensive process optimization would be needed to reduce leakage and porosity. Real time laser annealing during growth allows for scaling of the layer-by-layer growth process. A pulsed laser deposition system with in situ laser annealing was thus designed, built, and utilized to grow Pb(Zr0.52Ti0.48)O3 thin films on a laser crystallized Pb(Zr0.20Ti0.80)O3 seed layer, at a temperature of ~370˚C. Polycrystalline 1.1 μm thick films exhibited columnar grains with small grain sizes (~30 nm). The films showed well-saturated hysteresis loops (with a remanent polarization of ~25 μC/cm2, and a coercive field of ~50 kV/cm) and exhibited loss tangents <2.5% with a permittivity of ~730. Film orientation could be controlled via the substrate choice; {111} Pb(Zr0.52Ti0.48)O3 films were grown on oriented (111) Pb(Zr0.30Ti0.70)O3 sol-gel seed layers, while epitaxial {001} films were prepared on (100) SrTiO3 single crystals. In order to study the microstructure evolution in these films, in situ pulsed-laser annealing was used to grow crystalline lead zirconate titanate (PbZr0.52Ti0.48O3) thin films at a substrate temperature of ~370˚C on PbZr0.30Ti0.70O3-buffered platinized silicon substrates. Transmission electron microscopy (TEM) analysis indicated that the films were well crystallized into columnar grains, but with pores segregated at the grain boundaries. Lateral densification of the grain columns was significantly improved by reducing the partial pressure of oxygen from 120 mTorr to 50 mTorr, presumably due to enhanced adatom mobility at the surface accompanying increased bombardment. It was found that varying the fractional annealing duration with respect to the deposition duration produced little effect on lateral grain growth. However, increasing the fractional annealing duration led to shift of 111 PZT X-ray diffraction peaks to higher 2θ values, suggesting residual in-plane tensile stresses in the films. Thermal simulations were used to understand the annealing process. Evolution of the film microstructure is described in terms of transient heating from the pulsed laser determining the nucleation events, while the energy of the arriving species dictates grain growth/coarsening.