Photoconductivity of SrBi2Ta2O9 thin films

https://doi.org/10.1016/S0955-2219(98)00460-9Get rights and content

Abstract

Photoconductive properties of SrBi2Ta2O9 thin films in the 250–400 nm wavelength range were investigated. The films were deposited on Pt/SiO2/Si substrates using the Chemical Solution Deposition and were crystallized by conventional thermal annealing at 850°C. One sensitivity maximum was observed in the spectral distribution of the photoconductive signal and was attributed to band-to-band generation in the film. The wavelength corresponding to this maximum was found to be dependent on the applied voltage, on the delay time, defined as the time between the application of light on the film and the reading of the generated photocurrent, and on the wavelengths sweeping direction (up or down). The gap value was estimated to be around 3·94–4 eV.

Introduction

Ferroelectric thin films were subjected to an intensive research in the last decade due to their potential use in memory devices.1, 2The effort was focused mainly on the preparation method, structural quality and ferroelectric properties of the films. The major problems regarding the ferroelectric memories are the polarization fatigue in case of memory cells based on ferroelectric capacitors[3]and the low retention time in case of memory cells based on ferroelectric field effect transistors.[4]In both cases the read–write procedure used until now was purely electric and is based on ferroelectric polarization reversal. To overcome the above mentioned problems new optical read–write procedures were proposed.[5]Such procedures can be applied only if the ferroelectric films have photoelectric properties. Besides this aspect, the photoelectric properties investigation of ferroelectric thin films can give information about the semiconductor properties like energy gap, energetic trap distribution in the gap and presence of internal electric fields.6, 7, 8Even the switching behavior can be affected by the presence of the photogenerated carriers.[9]New applications for the ferroelectric films, such as UV detectors, can also be found.[10]Until now only a few studies were devoted to the investigation of photoelectric properties in ferroelectric thin films with perovskite or layered perovskite structure.11, 12, 13The present study investigates the photoconductive properties in SrBi2Ta2O9 (SBT) thin films, which is known as a representative layered-perovskite material widely used in ferroelectric memories' devices.

Section snippets

Experimental

SBT thin films were deposited by Chemical Solution Deposition (CSD) method on Pt/SiO2/Si substrates. The metalorganic solution was prepared using Bi-diethylhexanoate, Sr-diethylhexanoate and Ti-isopropoxide as metal precursors and xylene as solvent. The films were obtained by solution spinning at 2500 rpm for 35 s followed by a pyrolize at 300°C for 5 min. Deposition–pyrolize process was repeated three times to obtain 550 nm thick SBT films. The films were crystallized by conventional thermal

Discussions

There are several aspects which have to be considered when we start to judge the experimental results. First, depending on the polarity of the applied voltage, the photogenerated electrons or holes are extracted from the sample.[14]Second, the light penetration depth, defined as the reverse of the absorption coefficient α, is dependent on the incident light wavelength. For short wavelengths compared with the peak wavelength in the spectral distribution, the value of α can reach values as high

Conclusions

Photoelectric properties of ferroelectric SBT thin films deposited on Pt/SiO2/Si substrates by Chemical Solution Deposition method were investigated in the 250–400 nm wavelength range. A 3·94–4 eV gap value was determined. The wavelength corresponding to the maximum in spectral distribution is dependent on the delay time and on the wavelength sweeping direction (up or down). The presence of an internal electric field in the film and the non-equivalence of the two metal–ferroelectric interfaces

Acknowledgements

The authors would like to thank to Professor U. Gösele for continuing support and to Dr. F. Müller from the Max Planck Institute for Microstructure Physics, Halle-Saale, Germany, for technical support.

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