Very large dielectric response from ferroelectric nanocapacitor films due to collective surface and strain relaxation effects

Abstract

Dependence of the dielectric response of ferroelectrics on defect types, particularly those with long range strain fields in confined geometries have been often mentioned, especially in interpreting experimental results in films. However, detailed discussions on the mechanisms with which defects alter properties, particularly in the presence of interfaces imposing certain boundary conditions, are seldom made. Here, we studied the thickness dependence of transition temperatures and dielectric response of Metal/BaTiO3/Metal ferroelectric nanocapacitor structures grown on SrTiO3 using a phenomenological approach accounting for the equations of electrostatics and semiconductors. Relaxation of the misfit strain via misfit dislocations amplify the surface effects in films below a critical thickness and favor electrical domains leading to very large dielectric responses in this regime. Thin film structures with relaxed misfit strain in this work are fully depleted in the presence of moderate densities of impurities ( $ 10 25 m À3 ). This is due to the reduction of polarization amplitude parallel to the film normal and its mplications for near-micron thick films are discussed. Consequently, the misfit dislocation sites have nearly no free carrier localization, making the role of these sites on leakage currents highly questionable. Dielectric response of intrinsic thicker films ( > 40 nm) is mostly under the influence of strain relaxation only with minimal interface impact in the limit of ideal electrodes. Our results point out that control of the dislocation density can lead to non-conventional functionalities in ferroelectric thin film capacitors via electromechanical coupling of properties to structure and domain stabilization. V C 2013 AIP Publishing LLC.

Carrier accumulation near electrodes in ferroelectric films due to polarization boundary conditions

“…Films with 10 20 m À3 impurity density behave identical to the case of 10 24 m À3 for both small and large k. Infinite values of k cause full carrier depletion for the range of impurity densities, here, and does not lead to any interesting or different behaviors other than what we previously reported. 26,27,51 Figure 5 reveals that, in all cases, in this work, for k ¼ 0.4 nm and 2 nm, p-type carrier domination is observed despite the fact that the films are n-type FE: this is due to the depletion of the films of their electrons. That there is a possibility of a p-type Ohmic-like interface formation in a FE with n-type impurities is quite interesting for an interface potential barrier at the order of 0.7 eV for q ffi 10 24 m À3 at RT, which is close to values in experiments for oxide FE-noble metal couples.…”

“…10,11,13,22 A number of other works adopt thermodynamic approaches coupled with electrostatics and semiconductor equations for a given FE-electrode couple. [23][24][25][26][27][28][29][30] While the importance of electrical and polarization BCs on properties of FE films is very well anticipated, only a handful of relatively recent studies have seriously tried to address their impact on the properties. 27,[31][32][33][34][35][36][37][38] Given the great importance of the BCs and the theoretically proven sensitivity of FE films to interface characteristics in sandwich type FE thin film capacitor structures, observing hysteresis loops and butterfly type C-V curves in these systems is a routine practice but is also quite contraversial.…”

“…[23][24][25][26][27][28][29][30] While the importance of electrical and polarization BCs on properties of FE films is very well anticipated, only a handful of relatively recent studies have seriously tried to address their impact on the properties. 27,[31][32][33][34][35][36][37][38] Given the great importance of the BCs and the theoretically proven sensitivity of FE films to interface characteristics in sandwich type FE thin film capacitor structures, observing hysteresis loops and butterfly type C-V curves in these systems is a routine practice but is also quite contraversial. The reason behind this thought is that the electrodes used to contact the FE film often have finite screening lengths 39,40 and that thermodynamic analysis has shown the ultimate stability of multidomain (MD) state in these systems.…”

Comprehensive study of high pressure annealing on the ferroelectric properties of Hf0.5Zr0.5O2 thin films

“…Though the phenomenon of NC is associated with an unstable state, dielectric capacitance in a series may stabilize the NC of FE, similar to an Esaki diode, whose negative differential resistance is stabilized by connecting another component with series resistance [20][21][22]. In fact, despite the fact that the NC effect of FE materials has already been studied using the Landau model [1,[24][25][26] followed by direct measurements, as demonstrated in the reported literature [27,28] using different materials, a systematic study demonstrating the impact of the high pressure annealing (HPA) temperature on Hf 0.5 Zr 0.5 O 2 has not yet been reported. The post-metal annealing (PMA) process is the governing mechanism that controls the phase transition related to the formation of the orthorhombic phase within the thin layer of Hf 0.5 Zr 0.5 O 2 , which plays a significant role in driving the FE behavior.…”

Layer thickness and period as design parameters to tailor pyroelectric properties in ferroelectric superlattices

“…The local internal electric fields that emanate from imperfections forming during processing impact P-E hysteresis loops, fatigue behavior, and loss and leakage characteristics of FE films. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Despite the fact that such internal fields tend to destabilize the FE state, they may lead to interesting and highly non-linear properties in thin films. Indeed, a number of studies have focused on finding ways to tailor the changes in the phase transition characteristics of epitaxial FE films where misfit strain imposed by the substrate was used as a design parameter to bring T C to around room temperature (RT) at which most IR devices operate.…”