Preparation, Microstructure and Physical Characteristics of Ferroelectric Pb 5 Ge 3 O 11 Thin Films for Memory Applicati
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Preparation, Microstructure and Physical Characteristics of Ferroelectric Pb5Ge3O11 Thin Films for Memory Application Y. X. Liu, C. Caragianis-Broadbridge*, A.H. Lehman**, J. McGuinness*, and T. P. Ma Department of Electrical Engineering, Yale Univ., New Haven, CT 06520 *Department of Physics, Southern Connecticut State Univ., New Haven, CT 06515 **Facility for Electron Microscopy; Trinity College, Hartford, CT 06106 ABSTRACT We report sol-gel process of Pb5Ge3O11 (PGO) as well as the microstructure and physical properties of ferroelectric PGO films for memory applications. The PGO sol was prepared from lead acetate hydrate, germanium isopropoxide, and di(ethylene glycol) ethyl ether. The reactions taking place during the sol-gel process were examined in detail. Diethanolamine (DEA) was added to help maintain the desired species ratio and prevent germanium oxide precipitation. The preferred orientation of the PGO thin films was well controlled by the heating and reflux procedures in the sol-gel preparation process. Additionally, to examine the impact of postdeposition processing, selected samples were oxygen annealed at temperatures ranging from 450-650°C. The samples were characterized with X-ray diffraction (XRD), non-contact (planview) atomic force microscopy (NC-AFM). The resulting data indicate that the microstructure and physical properties of PGO films depend strongly on the precursor preparation as well as the post deposition annealing temperature. INTRODUCTION Ferroelectric thin films have great potential for memory applications. The transistor with metal-ferroelectrics-insulator-semiconductor (MFIS) gate structure has many desirable properties, such as small cell size, low-voltage operation, fast writing time, and non-destructive reading. Thus, the MFIS transistor is one of the promising candidates for future high-density, high-speed, and low-power memory technology. However, for the long retention time and lowvoltage operation desired, there is a demand for ferroelectric films that have as small a dielectric constant as possible [1, 2]. Therefore, the PGO film with a relatively low dielectric constant (εr=50~100) and preferred c-axis orientation [1, 3] has potential as a ferroelectric material that offers the possibility of increased retention time and reduced memory writing voltage. EXPERIMENT PGO precursor solution was synthesized through a three-step process with lead acetate trihydrate, germanium isopropoxide, and di(ethylene glycol) ethyl ether as starting materials [4]. In the first step, lead acetate trihydrate was dissolved in di (ethylene glycol) ethyl ether and heated at 180oC for 1 hour, and vacuum-distilled to dehydrate. During the second step the dehydrated solution was moved to a glove box with N2 atmosphere. Germanium isopropoxide and DEA were then added and stirred for one hour. In the third step, the solution was moved out of the glove box and heated in an oil bath at 180oC in air for half an hour. After cooling down, the deep red-brown solution was filtered through a 0.2um filte
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