The leakage current response of high-permittivity columnar-grown ͑Ba, Sr͒TiO 3 thin films has been studied at elevated temperatures under dc load. We observe a thermally activated current prior to the onset of the resistance degradation with an activation energy of E A = 1.1 eV. A point defect model is applied to calculate the migration of electronic and ionic defects under the dc field as well as the current response of the system. We find that the peak in current is not caused by a space-charge-limited transient of oxygen vacancies, but related to a modulation of the electronic conductivity upon oxygen vacancy redistribution. Furthermore, we show that after the redistribution of electronic and ionic defects, no further increase in conductivity takes place in the simulation.

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One of the problems occurring in conjunction with the integration of Ba 0.7 Sr 0.3 TiO 3 ͑BST͒ thin film capacitors into the Si technology is the large increase of leakage current after a forming gas heat treatment. In order to reveal the underlying mechanism, we studied the electric properties of Pt/ BST/Pt ͑metal-insulator-metal͒ ͑MIM͒ structures after annealing in atmospheres containing hydrogen (H 2 ) or carbon monoxide by means of admittance spectroscopy. Frequency-dependent capacitance measurements on these MIM structures revealed a thermally activated relaxation step at low frequencies with an activation energy of 0.62 eV. Admittance spectroscopy, in which the conductance is monitored as a function of temperature and frequency, verifies the Schottky barrier heights at the Pt/BST interface revealed by dc measurements. We found that the Schottky barrier height decreased by 0.4 eV after annealing in a reducing atmosphere, independent of the presence of protons. © 2000 American Institute of Physics. ͓S0003-6951͑00͒00139-X͔ Because of rapid evolution in the integration technology of high-dielectric-constant ferroelectric materials like Ba 0.7 Sr 0.3 TiO 3 ͑BST͒, the integration of ferroelectric capacitors in complementary metal-oxide-semiconductor ͑CMOS͒-based devices is one of the most important challenges. 1,2 For practical device applications of these new materials, their electrical properties must be understood properly with respect to dependence on the CMOSintegration processes. One of the most serious problems that Pt/BST/Pt ferroelectric capacitors suffer from during integration is the large increase of the leakage current by more than four orders of magnitude after annealing in a strongly reducing atmosphere, 3,4 e.g., a typical forming gas anneal (N 2 95% and H 2 5%͒. To avoid dramatical resistance degradation it is important to determine its origin.BST films, typically 130 nm thick, were deposited by chemical solution deposition ͑CSD͒ 3,5 on a platinized Si wafer. The Pt top electrodes ͑100 nm͒ were deposited by rf magnetron sputtering and patterned by a lift off process. The surface area of the top electrodes is 0.19 mm 2 . The resulting capacitors were postannealed under an oxygen atmosphere at Tϭ550-600°C for 30 min, and are referred to as the reference capacitors. Additionally, some capacitors were annealed at Tϭ450°C under a forming gas atmosphere ͑FGA͒ or a mixture of CO 5% and argon 95% ͑COA͒. These atmospheres have comparable oxygen partial pressure p O 2 , which are lower than 10 Ϫ15 bar. The electrical data were acquired using a Keithley 617 electrometer as the voltage source and picoamperemeter. A voltage-step technique is used to precisely reveal the leakage currents and to eliminate dielectric charging currents of the capacitor. For ac measurements a frequency response analyzer SI1260 from Schlumberger Ltd. was used in combination with a Solatron 1296 dielectric interface. Figure 1͑a͒ shows comparison of the leakage current for a Pt/BST/Pt capacitor before and after a reducing atmosphere...

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