Modelling of Pyroelectric Response in Inhomogeneous Ferroelectric-Semiconductor Films

We have modified Landau-Khalatnikov approach and shown that the pyroelectric response of inhomogeneous ferroelectric-semiconductor films can be described by using six coupled equations for six order parameters: average displacement, its mean-square fluctuation and correlation with charge defects density fluctuations, average pyroelectric coefficient, its fluctuation and correlation with charge defects density fluctuations. Coupled equations demonstrate the inhomogeneous reversal of pyroelectric response in contrast to the equations of Landau-Khalatnikov type, which describe the homogeneous reversal with the sharp pyroelectric coefficient peak near the thermodynamic coercive field value. Within the framework of our model pyroelectric hysteresis loop becomes much smoother, thinner and lower as well as pyroelectric coefficient peaks near the coercive field completely disappear under the increase of disordering caused by defects. This effect is similar to the well-known "square to slim transition" of the ferroelectric hysteresis loops in relaxor ferroelectrics. Also the increase of defect concentration leads to the drastic decrease of the coercive field typical for disordered ferroelectrics. Usually pyroelectric hysteresis loops of doped and inhomogeneous ferroelectrics have typical smooth shape without any pyroelectric coefficient peaks and coercive field values much lower than the thermodynamic one. Therefore our approach qualitatively explains available experimental results. Rather well quantitative agreement between our modelling and typical Pb(Zr,Ti)O3-film pyroelectric and ferroelectric loops has been obtained.Comment: 14 pages, 5 figure

Dielectric, ferroelectric and piezoelectric properties of sputtered PZT thin films on Si substrates: influence of film thickness and orientation

Lead titanate zirconate Pb(Zr,Ti)O₃ (PZT) thin films were deposited on platinized silicon substrates by r.f. magnetron sputtering and crystallized with preferred (110) or (111) orientation by conventional annealing treatment. The film structure evolution was observed as a function of the film thickness. Whatever the film thickness in the range 0.07 - 3 mm, the preferred orientation of the film is maintained. The film microstructure and, in particular, grain sizes varied with the film thickness; more precisely, grain sizes increases, both for (111) and (110) films with the film thickness. The electrical properties such as dielectric, ferroelectric and piezoelectric ones were systematically evaluated functions of the film thickness and their orientation. The relative dielectric constant increases with the film thickness; a saturation value of 920 is attained for film thicknesses higher than 0.6 µm independently of the film orientation. The ferroelectric properties seems to be independent of the film orientation; the coercive field decrease with increasing the film thickness to attain a minimum value of 30 kV/cm for films thicker than 1 mm. The remanent polarization increases with the film thickness and reaches the maximum value of 20 mC/cm². An increase in the piezoelectric constant e₃₁ with increasing the film thickness was observed for two types of films. For films thicker than 0.6 mm, the e₃₁ coefficient remains constant: e₃₁eff.rem. = -4.5 C/m² (which corresponds to d₃₁eff.rem. = -38 pm/V). Identical behavior is observed for the d₃₃eff. coefficient but no saturation effect with the film thickness is observed. The ferroelectric domain walls motion and the interfacial effects could explain partly the observed behavior

Modelling of pyroelectric response in inhomogeneous ferroelectric-semiconductor films

We have modified the Landau-Khalatnikov approach and shown that the pyroelectric response of inhomogeneous ferroelectric-semiconductor films can be described using six coupled equations for six order parameters: average displacement, its mean-square fluctuation and correlation with charge defect density fluctuations, average pyroelectric coefficient, its fluctuation and correlation with charge defect density fluctuations. Coupled equations demonstrate the inhomogeneous reversal of pyroelectric response in contrast to the equations of the Landau-Khalatnikov type, which describe the homogeneous reversal with the sharp pyroelectric coefficient peak near the thermodynamic coercive field value. Within the framework of our model, the pyroelectric hysteresis loop becomes much smoother, thinner and lower as well as the pyroelectric coefficient peaks near the coercive field completely disappear with increase in disordering caused by defects. This effect is similar to the well-known "square to slim transition" of the ferroelectric hysteresis loops in relaxor ferroelectrics. Also the increase of defect concentration leads to the drastic decrease of the coercive field typical for disordered ferroelectrics. Usually pyroelectric hysteresis loops of doped and inhomogeneous ferroelectrics have typical smooth shape without any pyroelectric coefficient peaks, and the coercive field values much lower than the thermodynamic one. Therefore, our approach qualitatively explains available experimental results. A rather well quantitative agreement between our modelling and typical Pb(Zr,Ti)O₃ film pyroelectric and ferroelectric loops has been obtained

Thin film PZT-Si structure with quadrant-diagonal electrode system as an element of position sensitive pyroelectric detector

The main characteristics of position sensitive systems of pyroelectric detectors of radiation (PDR) with sensitive elements based on PZT film on Si-substrate were investigated by photopyromodulation method. Pt-PZT-Pt/Ti-SiO₂/Si structures with 1.9 mm (111)-oriented PZT (54/46) layer were manufactured by R.F. magnetron sputtering. Top Pt electrodes in a shape of isosceles rectangular triangle together make the quadrant-diagonal system. The results of pyroelectric undersurface probing and “along surface” pyroelectric scanning show that the investigated quadrant-diagonal system of SE makes it possible to examine all main types of differential position-sensitive PDR. The experimental results for investigated 2-element single-coordinate PDR variants are in a good agreement with calculations for the corresponding systems

Peculiarities and asymmetry of polarization reversal in Pt/PZT-film/Pt:Ti/SiO₂/Si-substrate structures in pyroelectric response investigations

By RF magnetron sputtering method the Pt/PZT-film/Pt:Ti-sublayer/SiO₂/Si-substrate structures were prepared and pyroelectric response amplitude and phase behaviour under external voltage application was investigated by photopyroelectric modulation method. The results of investigation of pyroelectric response – external voltage loops of polarization reversal, pyroelectric response – voltage poling curves and pyroelectric response – time repolarization curves and also dynamic current-voltage characteristics of Pt/PZT/Pt:Ti/SiO₂/Si-substrate structures are presented. From variation of pyroelectric response in the current and voltage modes the capacity-voltage loops of polarization reversal and poling curves were derived. From asymmetric pyroelectric response – time repolarization curves the voltage behaviour of characteristic times of zero response and saturation was analyzed. Observed transformations of current-voltage characteristics display the considerable voltage and time dependent variation of charge transfer conditions. The performed investigation has shown the strong correlation between the poling pyroelectric and so ferroelectric and electrical asymmetries. Presented data on the polar and time asymmetry of the conditions of polarization reversal are discussed in the terms of influence of dynamics of space charge asymmetry on pinning conditions under the different polarity of applied voltage in the course of polarization reversal

Partial polarization switching in ferroelectrics-semiconductors with charged defects

We propose the phenomenological description of ferroelectric disordering caused by charged defects in ferroelectric-semiconductors. The good agreement between the obtained experimental results for PZT films and theoretical calculations has been shown. We suppose that proportional to the averaged charge density of defects improper conductivity is sufficiently high to provide the screening of charge density random fluctuations drs in the absence of external field. When external electric field is applied, inner field fluctuations and induction fluctuations dD appear in the inhomogeneously polarized system “charged fluctuation + screening cloud”. We show that the macroscopic state of ferroelectric-semiconductor with random charged defects and sufficiently high improper conductivity can be described by three coupled equations for three order parameters. Averaged over sample volume induction determines the ferroelectric ordering in the system, its square fluctuation determines disordering caused by electric field fluctuations appeared around charged fluctuations drs, and reflects the correlations between the free carriers screening cloud and charged defects drs. For the first time, we derive the following system of three coupled equations: Also the obtained system of coupled equations qualitatively describes the peculiarities of polarization switching (footprint and minor hysteresis loops) in such ferroelectric materials with charged defects as PZT films with growth imperfections, PLZT ceramics and SBN single crystals doped with cerium

Growth of piezoelectric thin films by sputtering

Lead titanate thin films have been deposited on different substrates by radio frequency magnetron sputtering from pressed powders targets. The films have been deposited without substrate heating. The amorphous films were then annealed in order to obtain the perovskite structure. Two types of post-annealing were study : conventional annealing and rapid thermal annealing (R.T.A.). The main advantage of the RTA process is to avoid (or to limit) the problem of interdiffusion or of interfacial disturbance between the substrate (in particular Silicium substrate) and the lead titanate thin film. The composition, structure and microstructure of PT films were evaluated as a function of the annealing parameters for conventional and rapid annealing

Modulateur electro-optique à onde guidée sur GaAs

We present a phase-shifter and a Mach-Zehnder type modulator that we have realized on n-/n+ GaAs homojonction using classical photolithography techniques. The characterization of the optical components has been performed and shows a 50 V Vπ voltage, a 10 dB extinction ratio and 5 dB insertion loss for the modulator.Nous présentons un déphaseur ainsi qu'un modulateur de type interféromètre de Mach-Zehnder que nous avons réalisés sur homojonction n- /n + GaAs en utilisant des techniques classiques de photolithographie. La caractérisation de ces composants a montré, pour le modulateur, une tension d'extinction de 50 V, un taux d'extinction de 10 dB et des pertes d'insertion de 5 dB