$(Ba,Sr)TiO_3$(BST) thin film is a very promising high dielectric constant material for use in Giga-bit scaled dynamic random access memories (DRAMs). It has been known that the leakage current of BST in the low field region consists of dielectric relaxation current and DC leakage current, which cause charge loss in DRAM. Especially, the dielectric relaxation current should be seriously considered since its magnitude is much larger than that of the DC leakage current in DRAM operation voltage. Therefore, it is important to investigate the origin of the dielectric relaxation and reduce it. On the other hand, it has been reported that the dielectric relaxation current is closely related with traps within BST film. However, it is necessary to analyze the traps in detail by using direct methods such at TSC(Thermally Stimulated Current), TSCAP(Themally Stimulated CAPacitor), DLTS(Deep Level Transient Spectroscopy), etc. In this study, thermally stimulated current(TSC) measurement is used to investigate the activation energy and density of the traps and identify the origin of the dielectric relaxation current. TSC result gives some advantages such as high sensitivity and resolving power for the traps.
First, the polarization condition such ad electric field, time, temperature and heating rate is investigated for reliable TSC measurement. The activation energy of trap is evaluated using the initial rise method and the peak cleaning technique from TSC raw data. From the TSC measurement, the energy level of traps for the BST thin film has been investigated and evaluated to be 0.2eV and 0.45eV. This result almost corresponds to other researcher`s results. Post-annealing process has been carried out to investigate the origin of the traps and verify the relationship between the measured traps and the dielectric relaxation current. We employed the Rapid Thermal Annealing (RTA) processes with different ambients. We have compared the electrical properties of the capacitors with post-annealing in vacuum ambient and in oxygen ambient after capacitor formation. Both the dielectric relaxation current and trap density are increased after vacuum annealing and decreased after oxygen annealing. Based on post annealing results. oxygen vacancy is concluded to be the origin of the traps.
In this study, in order to identify the origin of the dielectric relaxation current, we adapted TSC measurement and set up polarization condition for BST capacitor and investigated the effects of post-annealing in oxygen and vacuum ambient. From comparison of the TSC curves of as-deposited and post-annealed capacitors, it is concluded that the oxygen vacancy is the origin of the traps. TSC characteristics of the MIM BST capacitor have same trends with current-voltage (I-V) and current-time (I-t) characteristics. Thus this means that TSC measurement is compatible with I-V and I-t measurement to investigate BST thin film.
