Pulsed Laser Ablation Deposited (Pb, La) Tio3 Thin films for Pyroelectric applications

Abstract

The present study was focused on the growth of lanthanum-modified lead titanate (PLT) thin films by pulsed laser ablation technique and characterization of the thin films in order to examine their suitability for pyroelectric applications. PLT thin films with different mole percentages of La (ranging up to 30%) were grown, and the Rapid Thermal Annealing (RTA) process was used to induce crystallinity in some films, while in other cases, crystallinity was achieved during growth by maintaining a higher substrate temperature. Different growth parameters were used to optimize and understand the growth process–induced effects. The relaxor-like properties of PLT thin films, the charge transport phenomenon in insulators under both AC and DC electric fields, the microstructure dependence of pyroelectric properties, and the dependence of capacitance on voltage were studied. The effect of particulate irradiation on the properties of PLT thin films was also investigated.

Chapter 1 of this thesis gives a brief introduction to ferroelectrics, the basic principles of pyroelectricity, the importance of lanthanum-doped lead titanate thin films, and the process of pulsed laser ablation technique.

In Chapter 2, the experimental details are described, including the laser setup and rapid thermal annealing process. The characterization techniques were briefly explained to describe the environment to which the film was subjected during characterization.

Chapter 3 outlines the processing of PLT targets, and the growth and optimization of PLT thin films on Pt-coated Si substrates using pulsed excimer laser ablation technique. The growth parameters were varied during deposition. The crystalline phase evolution with RTA process has been discussed. It was observed that the in-situ grown films showed a higher degree of crystallinity. The effects of different processing parameters, such as energy fluence, substrate temperature, operating oxygen partial pressure, and the mode of crystallization, on the evolution of final crystallization and microstructural changes are discussed.

Chapter 4 describes the study of AC electrical properties, including dielectric response and AC conductivity using dielectric and impedance spectroscopy techniques for in-situ grown PLT thin films. Maxwell-Wagner relaxation might be responsible for the frequency dispersion of dielectric constant. The frequency dispersion was more prominent at low frequencies, and as the temperature increased, it extended to higher frequencies. The AC conductivity also exhibited a power-law dependence on frequency, confirming the validity of the universal frequency dispersion relation. The activation energy of the AC conductivity indicates a shallow trap-controlled conduction mechanism. The Cole-Cole plot of the electric impedance, as well as the double peaks in electric modulus plots, indicated the presence of two dissimilar regions in the sample, identified as the grain and the grain boundary, respectively. The relaxation time decreased with increasing temperature. The obtained activation energy from impedance spectroscopy shows that oxygen vacancy motion plays a dominant role in conduction mechanisms at high temperatures.

Chapter 5 describes the analysis of leakage current conduction phenomena in in-situ grown PLT 15 thin films and the thickness dependence of RTA annealed PLT 28 thin films. At lower voltages, the leakage follows ohmic conduction. The Schottky plots show linear curves with a barrier height of 0.93 eV at the gold electrode–film interface and 1.03 eV at the Pt–film interface. The variation of activation energy with voltage shows the presence of different current conduction mechanisms in different voltage regimes. Modified Schottky and Poole-Frenkel mechanisms were studied. At higher voltages, space-charge-limited conduction plays a dominant role, and the estimation of traps responsible for space charge has been carried out. Space-charge-modified Schottky conduction appears as the leakage phenomenon at higher voltages. The thickness dependence of leakage phenomenon was studied and attributed to space-charge-dominated conduction.

In Chapter 6, the relaxor-like behavior of PLT thin films is presented. PLT 25 thin films exhibit a broad, diffused phase transition with the shifting of the temperature corresponding to the maximum dielectric constant to higher frequencies. Frequency dispersion in dielectric constant is observed in the ferroelectric phase, whereas in the paraelectric phase no dispersion occurs. With increasing applied AC field, frequency dispersion in dielectric constant appears in the ferroelectric regime, and the Tm decreases with increasing field. This behavior is explained based on existing models. The effect of DC bias on phase transition was also investigated. The phase transition temperature increased with the application of DC bias, and tan ? values were reduced. This is attributed to poling and coalescing of nanodomains into microdomains. The Curie-Weiss behavior was not followed; instead, the phase transition temperature followed the Vogel-Fulcher relation, a characteristic feature of relaxors, in PLT 25 and PLT 30 thin films. With increasing La doping, the creation of defects increases, causing a local distribution of random fields and disrupting ferroelectric order. At higher La doping, PLT thin films exhibited relaxor-like behavior.

Chapter 7 presents the results of capacitance-voltage measurements, polarization-field hysteresis, and the effect of sub-switching fields on reversible and irreversible polarizations. From the C–V characteristics of PLT 28 thin films, the charge storage density was calculated and its variation with temperature and frequency explained. The surface trap density of 1 × 10¹³ /cm²/eV was estimated. The effect of La doping on capacitance was studied. Using the Schottky–Mott relation, the carrier concentration, and consequently the trap concentration, was calculated. The polarization hysteresis loops confirmed the existence of ferroelectricity, and the hysteresis in C–V curves supported this. Under sub-switching fields, the domain wall contribution to irreversible polarization was analyzed using Rayleigh’s law. The linear increase of permittivity with applied AC field is attributed to the depinning of domain walls from randomly distributed defects acting as pinning centers.

Chapter 8 describes the pyroelectric properties of PLT 20 thin films. Using Byer-Roundy technique, pyroelectric current was measured for both RTA-annealed and in-situ grown films. The microstructure dependence of pyroelectric properties was studied. The pyroelectric coefficient and figures of merit for current, voltage, and detectivity of in-situ grown films were lower compared to RTA-annealed films. This was attributed to the presence of columnar grain structure with highly oriented crystallites. Self-polarization and internal biasing fields also contribute to pyroelectric properties. With increased La doping, the pyroelectric current and coefficients decreased, confirming a reduction in ferroelectric nature. Variation of pyroelectric coefficient with DC bias was also studied.

Chapter 9 deals with the results of 70 MeV oxygen ion–irradiated PLT thin films. High-energy ions cause damage to the films, resulting in decreased properties. The degraded AC, DC, and polarization properties were explained based on existing mechanisms.

Chapter 10 presents the main findings, conclusions, and future suggestions for further research.