Sol-gel BST thin films for FeFET applications : fabrication and characterization

Abstract

The effect of the chemical composition and film thickness of the ferroelectric barium strontium titanate (BST) at the memory window behavior of Al/BST/SiO2/Si-Gatefield effect transistor structure has been investigated. BaxSr1-xTiO3 thin films with different x values and film thickness have been fabricated as metal-ferroelectricinsulator- semiconductor (MFIS) and metal-ferroelectric-metal (MFM) configurations using a sol-gel technique. The surface morphology and grain size have been studied in detail using an atomic force microscope (AFM). The AFM micrographs show that BST films have good surface properties with increasing in the grain size and surface roughness as both Ba content and film thickness increase. The grain size of BST for different Ba:Sr ratio and film thickness varies between 48 ± 3.56 nm and 120 ± 3.56 nm. In order to confirm the ferroelectric behavior of BST, the ferroelectric hysteresis characteristics of the films have been studied with MFM structure using both Sawyer- Tower circuit and capacitance-voltage (C-V) characteristic tests. The results show that the ferroelectric hysteresis strengthens as both Ba content and film thickness increase. The conduction mechanism and dielectric relaxation mechanism for BST film within MFM structure have been investigated using an impedance spectroscopy in the frequency range of 1 Hz to 1 MHz. The results show that the conductivity of the films decreases as both the Ba content and film thickness increase which are attributed to the grain size effect. The complex impedance (Z*) plane for all BST thin films possesses two regions, which attributed to the film’s regions conduction mechanism i.e. grains, and grain boundaries and electrodes. The variation of the dielectric constant (ε) and tan δ with frequency have been studied to ensure the dielectric quality of the material. The results show that the dielectric constant for BST films decreases as both of the Ba content and film thickness increase, which also attributed to the grain size effect. The values of the dielectric permittivity constant of the tested samples vary between 151 and 335 at 105 Hz frequency for different Ba:Sr ratio and film thickness. The electric modulus results show that the relaxation mechanism within BST films is non-Debye type. The frequency dependent conductivity plots show three regions of conduction processes, i.e. lowfrequency region due to DC conduction, mid-frequency region due to translational hopping motion and high-frequency region due to localized hopping and/or reorientational motion. In addition, depending on the complex impedance and complex modulus plots, an equivalent circuit has been suggested for each film in the current study to obtain further understanding for the conduction mechanism of BST film. The ferroelectric memory behavior of the MFIS samples has been investigated using capacitance-voltage (C-V) characteristics. The C-V characteristics show that the memory window width increases with the increase both of Ba content and film thickness. This is attributed to the grain size and dipole dynamics effect. The values of the memory window for the studied films vary between 0.3 V and 3.3 V for different Ba:Sr ratios and film thickness. Furthermore, the width of the memory window increases as the applied voltage increases; however, it is not affected by the voltage sweeping rate. In addition, the operating mode of the gate structure, i.e. accumulation, depletion and inversion conditions, has been also discussed in detail.