Size effects in ultrathin epitaxial ferroelectric heterostructures

Abstract

In this letter we report on the effect of thickness scaling in model PbZr 0.2 Ti 0.8 O 3 ͑PZT͒ / SrRuO 3 heterostructures. Although theoretical models for thickness scaling have been widely reported, direct quantitative experimental data for ultrathin perovskite ͑Ͻ10 nm͒ films in the presence of real electrodes have still not been reported. In this letter we show a systematic quantitative experimental study of the thickness dependence of switched polarization in (001) epitaxial PZT films, 4 to 80 nm thick. A preliminary model based on a modified Landau Ginzburg approach suggests that the nature of the electrostatics at the ferroelectric-electrode interface plays a significant role in the scaling of ferroelectric thin films. The effect of thickness scaling in ferroelectrics has recently been of immense interest. [1][2][3][4][5][6][7][8][9][10][11][12] As the dimensions (both lateral and vertical direction) of the perovskite layer decreases, the fundamental question of size dependence becomes crucial. From a theoretical point of view, two models have been traditionally used to describe size effects, namely an "intrinsic" effect 9,10,13 and a "depoling field" effect. 4,5,14,15 Experimentally Tybell et al. 8 qualitatively showed that even a 4-nm-thick epitaxial PbZr 0.2 Ti 0.8 O 3 (PZT) film on Nb:STO is ferroelectric. However a direct experimental quantification of the ferroelectric polarization, particularly for films in the sub-10 nm thickness range has not been reported. For such ultrathin films direct experimental quantification of size effects are complicated by extrinsic effects such as leakage and therefore methods other than the traditional P -E hysteresis loop have been reported to characterize the stability of the polar state. 1,7 This letter presents experimental measurements of the switched polarization in PZT films of thickness down to 4 nm, in the presence of real electrodes.A 70-nm-thick SRO layer was grown on STO at 650°C followed by the PZT layer via pulsed laser deposition. The deposition was carried out at 100 mTorr of oxygen and the sample was cooled down from growth temperature 1 atm of oxygen. In order to avoid complications from 90°domain formation the PZT films were grown in a thickness range from 4 to 80 nm, for which they are entirely c-axis oriented. The switchable polarization was measured using a Radiant Technologies Precision Premier system at 16 kHz (hysteresis loops) and an AFM based pulsed probing technique with conductive Pt-Ir tips was employed to measure the pulsed polarization. 16 We focused on the PZT ͑0/20/80͒ composition, since it has an in-plane lattice parameter of 3.94 Å, 17 which is closely lattice matched to SRO͑3.93 Å͒. Figure 1(a) is a low magnification TEM image of a 4 nm (nominal) thick film; the interfaces between SRO and PZT are sharp, identified as dashed lines in the high resolution image, Fig. 1(b). The electrode-ferroelectric interface shows a significantly reduced dislocation density, attributed to the small lattice mismatch (0.7%) at growth temperature of 600...

Electroresistance effects in ferroelectric tunnel barriers

“…1 We did not include the thickness dependence in our analysis. However, since the polarization increases with increasing thickness, this would add to the trend of increasing ER for increasing thickness [ Fig.…”

Scaling of structure and electrical properties in ultrathin epitaxial ferroelectric heterostructures

“…Details of the growth are given elsewhere. 13 The maximum thickness of the PZT film presented in this study was limited to 500 Å in order to maintain fully c-axis oriented films and avoid the formation of ferroelastic a-axis domains ͑90°domains͒. The microstructure and local distortion in ultrathin films were investigated by means of highresolution transmission electron microscopy ͑HRTEM͒.…”

“…In our previous report we presented cross-sectional HRTEM data on a film that was 40 Å thick. 13 In that case, the average lattice tetragonality ͑c / a ratio͒ was calculated based on the power spectrum of a lattice image to be 1.05. In the present report we show a detailed analysis of the local lattice strain of the film by lattice mapping directly on the image using a numerical center-of-mass approach.…”

“…1,2 One of the most crucial questions regarding the densities and performance that can ultimately be achieved is that of the size effect [3][4][5] on the magnitude and stability of uniform switchable polarization. In the past few years, careful measurements on high-quality ultrathin films [6][7][8][9][10][11][12][13][14] and related theoretical models [15][16][17][18] for thin films have shown that a ferroelectric instability can be exhibited by perovskite oxide films as thin as a few lattice constants. This has led to renewed interest in understanding the scaling of ferroelectric properties with thickness in the ultrathin regime.…”

Future Memory Technology and Ferroelectric Memory as an Ultimate Memory Solution

“…There has been a trend in recent literature to use the term "size effect" relating to the stability of spontaneous polarization to specifically describe the manner in which reduced size leads to progressive collapse of ferroelectricity (Saad et al, 2006). Finding the point at which this size-driven phase transition occurs is obviously interesting and fundamentally important, and thus various groups have done excellent works to elucidate, via both theory (Li et al, 2996;Junquera & Ghosez, 2003) and experiment (Streiffer et al, 2002;Tybell et al, 1999;Nagarajan et al, 2004), the dimensions at which ferroelectricity is lost. In that sense, one of the most critical quantities in ferroelectrics is remanent polarization P r , which can be expressed as below:…”