Fabrication of continuous ultrathin ferroelectric films by chemical solution deposition methods

Abstract

The integration of ferroelectrics in nanodevices requires firstly the preparation of high-quality ultrathin films. Chemical solution deposition is considered a rapid and cost-effective technique for preparing high-quality oxide films, but one that has traditionally been regarded as unsuitable, or at least challenging, for fabricating films with good properties and thickness below 100 nm. In the present work we explore the deposition of highly diluted solutions of pure and Ca-modified lead titanates to prepare ultrathin ferroelectric films, the thickness of which is controlled by the concentration of the precursor solution. The results show that we are able to obtain single crystalline phase continuous films down to 18 nm thickness, one of the lowest reported using these methods. Below that thickness, the films start to be discontinuous, which is attributed to a microstructural instability that can be controlled by an adequate tailoring of the processing conditions. The effect of the reduction of thickness on the piezoelectric behavior is studied by piezoresponse force microscopy. The results indicate that films retain a significant piezoelectric activity regardless of their low thickness, which is promising for their eventual integration in nanodevices, for example, as transducer elements in nanoelectromechanical systems.

Use of the solvent chemistry for the control of the critical thickness of PbTiO3 ultrathin films

“…Most of the reflections come from the perovskite phase, except the 111 from the Pt bottom electrode. Similarly to our previous reported results on ultrathin films, 19,20 001 and 002 reflections are not observed. This may be caused by a preferential crystallographic orientation present in the film.…”

“…9 However, CSD has been regarded traditionally as unsuitable for the deposition of ultrathin films (below 100 nm thickness), and other deposition techniques as RF magnetron sputtering, pulsed laser deposition or chemical vapor deposition are often used to prepare them instead. [8][9][10][11][12] Regardless of that, ultrathin films have successfully been obtained by CSD methods: both of nonferroelectric 13,14 and ferroelectric [15][16][17][18][19][20] compositions, with thickness as low as 9 nm, as was reported recently. 21 However, there is a serious problem concerning the loss of continuity in films with reduced thickness.…”

“…Figure 1 shows the evolution of the film thickness with the concentration of the precursor solution for PbTiO 3 -based films. Together with results previously reported of films for which precursor solutions are diluted either in diol or water, 19,20 we include a new series obtained from solutions diluted in 2ethyl-1hexanol. We observe the same trend in all of them: a correlation between the thickness and the molar concentration that is lost in the thinnest films.…”

“…[23][24][25][26][27] Similarly, for a series of films with decreasing thickness we can define a critical thickness below which a complete coating of the substrate is not achieved. Figure 1 shows the results of our previous works on the preparation of ultrathin films by chemical solution deposition methods, 19,20 and the critical thickness deduced from the analysis carried out on the films. The more diluted is the precursor solution, the thinner is the resulting film.…”

Processing Technologies for High‐Permittivity Thin Films in Capacitor Applications

“…For this reason, simply diluting the solution enables the fabrication of ultrathin layers that are electrically functional across macroscopic (mm 2 ) areas. Other groups have also had success spin coating ultrathin dielectric films from chemical solution 87,116 …”