TEM study of the explosive crystallization process in perovskite PZT ‐based heterostructures

Abstract

Explosive crystallization (EC) of materials has been extensively studied both theoretically and experimentally in the end of 20 th century. Now EC can be used for investigations of fast phase transitions [1] and controlled fabrication of heterostructures, for example, by the laser annealing [2]. In this case, location of the heat source depends on an absorption coefficient of an annealing substance. In transparent amorphous material high energy, laser pulses give rise to ionization due to multiphoton absorption, which initiates melting with further crystallization. The power density to start the process falls in the range of tenths TW/cm 2 [3]. The use of ultrashort pulses provides locality of the process seeding while the surrounding layers accept almost unperturbed even if ablation of material accrues [4]. In this work we have studied of the EC process in ferroelectric precursor films after multipulse femtosecond laser annealing. For the study we used films of lead zirconate‐titanate (PZT) with the thickness of 700nm deposited on a platinized silicon substrate (Pt (80 nm)/SiO 2 (300nm)/Si (300μm)) by RF magnetron sputtering. Wavelength of the laser radiation was set at 800nm.The laser pulses have duration of 100fs, and the repetition rate of 80MHz. To anneal the ferroelectric film the latter was exposed to the laser radiation for the period τ A from 0.1s to 1.2s. The power density at the annealed spot ranges from 1.0 to 2.0 MW/cm 2 . Cross‐sections for TEM were prepared by focused ion beam (FIB) in a FEI Helios. TEM investigations were carried out in a Tecnai G 2 30ST equipped by a HAADF detector for STEM mode and an EDX detector at accelerating voltage of 300kV and Tecnai Osiris equipped by Bruker Super‐X system at accelerating voltage of 200kV. General TEM views of the cross‐sectional selected annealed microstructures are shown in Fig.1. The marked areas are semicircles with the center at the surface of the film. In the annealed areas, the structure is changed to a granulated one. The size of grains varies from 10 to 200nm. The most unexpected peculiarity of these images is the semispherical shape of crystallized areas. Although the heat source is located at the bottom PZT/Pt interface, the center of semi‐spheres is located at the top interface (surface) of the PZT film. Calculations of the interplanar distances showed that the perovskite in this area has an increased tetragonality (1.357±0.05) compared to the tabulated value of 1.02. Cross‐sectional transmission electron microscopy (TEM) images obtained for different crystallization times allow us to consider the crystallization propagation within the film. Although the femtosecond pulses are practically not absorbed by the film and result in an ultrafast laser induced heating of Pt‐layer, the crystallization is seeded at the surface of the film and propagates to the heat source at the film/Pt interface (Fig. 2.). The source of the heat is localized at the bottom interface. However, the heat propagates very fast and the temperature of both the top and the bottom interfaces are almost equal due to a very small thickness of the layer. At the same time, the film at the bottom interface undergoes high strain due to the difference in thermal expansion of Pt and PZT layers. The strain increase results in an increase of activation energy, which suppresses crystallization at the bottom interface. As a result, crystallization starts from the top interface i.e. the surface of the film. This work was performed using the equipment of the Shared Research Center IC RAS and particularly supported by the Ministry of Education and Science of Russian Federation (State task no. 11.144.2014) and Grant no.14.Z50.31.0034, p220.