Structural and electrical characteristics of SrTiO₃ thin films for dynamic random access memory applications

Abstract

Polycrystalline SrTiO₃ thin films having perovskite structure were prepared by the metallo-organic solution deposition technique on platinum coated silicon and bare silicon substrates. Crack free and crystalline films with uniform composition and thickness were fabricated by spinning and post deposition rapid thermal annealing at a low temperature of 550° C for 60 s. The films exhibited good structural, dielectric, and insulating properties. The dielectric constant was found to depend on film thickness and annealing temperature. The small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 225 and 0.008, respectively, for a 0.8-μm-thick film annealed at 700° C for 60 s. The frequency dependence of the dielectric constant and the dielectric loss was also studied. The measurement of the current-voltage (I-V) characteristics on films in metal-insulator-metal configuration indicated the conduction process to be bulk limited. The I-V characteristics were ohmic at low fields and space-charge limited at high fields. Room temperature resistivity of 10¹³ Ωcm and leakage current density of less than 10^-8 A/cm² were obtained for a 0.8-µm-thick film at an applied electric field of 200 kV/cm, establishing good insulating behavior. The interfacial properties of gold-strontium titanate-silicon structures were studied experimentally by measuring the capacitance-voltage (C-V) characteristics. The C-V curves exhibited anomalous frequency dispersion behavior and a hysteresis phenomenon. The hysteresis in the C-V curve was found to be about 0.8 V and of a charge injection type. The density of interface states recharged during the bias cycle in hysteresis measurement was estimated to be of the order of 8.3× 10¹¹ cm^-2. The density of interface states at the flatband voltage was found to have a value of 1.38× 10¹² eV^-1cm^-2. For a 0.5-µ m-thick SrTiO3 film, a unit area capacitance of 3.5 fF/µ m² and a charge storage density of 36.7 fC/µ m² were obtained at an applied electric field of 200 kV/cm. These measured parameters place SrTiO₃ as one of the suitable dielectric material for dynamic random access memory applications.