Sol-Gel derived Ferroelectric Thin Films for Voltage Tunable Applications

Abstract

Ferroelectric perovskite thin films for voltage tunable applications, namely (Ba,Sr)TiO3 (Barium Strontium Titanate or BST) and (Pb,Sr)TiO3 (Lead Strontium Titanate or PST), are synthesized via the so-called sol-gel route. While BST shows the tendency to severe film cracking, PST can be grown crack free onto platinised Si standard substrates and even directly onto SiO2, SiNx or bare Si. The growth kinetics of PST on platinised SiO2/Si and directly on SiO2/Si are studied in detail using X-ray diffractometry (XRD), scanning electron and atomic force microscopy, SEM and AFM respectively. It is shown that PST begins to crystallise at 500°C on Ti/Pt and 550°C directly on SiO2. After a thermal treatment of 650°C for 15 min both films are fully crystallised with random (100) and (110) orientation and a smooth surface. The dielectric properties, e.g. dielectric constant, loss and tunability, of PST 50/50 are measured using a standard Ti/Pt bottom electrode with Cr/Au top electrodes and a TiW/Cu bottom electrode on which the PST thin film was bonded with TiW/Cu top electrodes. The Cu/PST/Cu system shows an enhanced performance in terms of loss resulting in a larger device quality factor and a figure of merit (FOM) of 18.25 compared to 16.6 for the configuration using a Pt bottom electrode. The maximum tunability is 73% with an applied voltage of 35V and the dielectric constant at zero bias is ~ 420 with a loss < 4 %. (Pb0.4Sr0.6)(MnxTi1-x)O3 (Mn doped PST 40/60) thin films with x = 0, 0.01, 0.03, and 0.05 are grown on Ti/Pt coated SiO2/Si substrates. The surface morphologies, dielectric and tunable properties of these films are investigated as a function of Mn content (x). It is found that the grain size/roughness, dielectric constant, loss, tunability and figure of merit are affected by the Mn doping level. Further on it is found that the ferroelectricity and the transition temperature between the cubic paraelectric and tetragonal ferroelectric state increase with Mn content. The dielectric constant at zero bias reaches a maximum of 1100 and the maximum FOM is 23.96 with 3 mol% Mn; whereas the maximum value of the tunability is 76.72% at 10 V with 1 mol% Mn. A detailed understanding of the effect of Mn doping is developed and presented. It is found and explained that a doping level of 2 mol% Mn results in optimal properties in terms of tunability and loss. Auger spectroscopy is used to study the compositional change in the interfacial region between PST and PZT thin films and SiO2/Si substrates to understand the growth kinetics of PST directly onto SiO2 in more detail. The thin films from both materials are annealed under the same conditions (temperature and time). It is found that strontium stops the lead diffusion into SiO2 by forming SrSiO3/Sr2SiO4 and/or SrO, maintaining a well defined SiO2 region, while PbSiO3 is formed in the PZT/SiO2 system. It is shown that SrO covalently saturates all Si dangling bonds by forming SrSiO3 and/or Sr2SiO4. This provides the necessary ionic template towards the perovskite SrO-terminated SrTiO3, on which PST can grow further on. A single layer of PST is finally used as a buffer layer for the growth of piezoelectric PZT directly onto SiO2 to replace the common Ti/Pt bottom electrode. The initial characterisation of PZT device structures shows that PZT films with PST as a diffusion buffer had fully crystallized in the perovskite phase exhibiting good dielectric and ferroelectric behaviour. Although the piezoelectric coefficients of the PZT films were not measured directly in this study, it is envisaged from the experimental data of the dielectric constant and hysteresis loop that the PZT/PST composite has the potential to provide good and comparable piezoelectric performances as typically observed in PZT device structures grown on commonly used Ti/Pt.