Grossmann, M.

Liedtke, R.

Waser, R.

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APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 13 25 SEPTEMBER 2000Capacitance and admittance spectroscopy analysis of hydrogen-degradedPtÕBa,SrTiO3 ÕPt thin-film capacitorsR. Liedtke,a) M. Grossmann, and R. WaserInstitut fu¨r Werkstoffe der Elektrotechnik, RWTH Aachen, D-52056 Aachen, Germany~Received 8 May 2000; accepted for publication 25 July 2000!One of the problems occurring in conjunction with the integration of Ba0.7Sr0.3TiO3 ~BST! thin filmcapacitors into the Si technology is the large increase of leakage current after a forming gas heattreatment. In order to reveal the underlying mechanism, we studied the electric properties of Pt/BST/Pt ~metal–insulator–metal! ~MIM! structures after annealing in atmospheres containinghydrogen (H2) or carbon monoxide by means of admittance spectroscopy. Frequency-dependentcapacitance measurements on these MIM structures revealed a thermally activated relaxation step atlow frequencies with an activation energy of 0.62 eV. Admittance spectroscopy, in which theconductance is monitored as a function of temperature and frequency, verifies the Schottky barrierheights at the Pt/BST interface revealed by dc measurements. We found that the Schottky barrierheight decreased by 0.4 eV after annealing in a reducing atmosphere, independent of the presenceof protons. © 2000 American Institute of Physics. @S0003-6951~00!00139-X#Because of rapid evolution in the integration technologyof high-dielectric-constant ferroelectric materials likeBa0.7Sr0.3TiO3 ~BST!, the integration of ferroelectric capaci-tors in complementary metal–oxide–semiconductor~CMOS!-based devices is one of the most importantchallenges.1,2 For practical device applications of these newmaterials, their electrical properties must be understoodproperly with respect to dependence on the CMOS-integration processes. One of the most serious problems thatPt/BST/Pt ferroelectric capacitors suffer from during integra-tion is the large increase of the leakage current by more thanfour orders of magnitude after annealing in a strongly reduc-ing atmosphere,3,4 e.g., a typical forming gas anneal (N295% and H2 5%!. To avoid dramatical resistance degradationit is important to determine its origin.BST films, typically 130 nm thick, were deposited bychemical solution deposition ~CSD!3,5 on a platinized Si wa-fer. The Pt top electrodes ~100 nm! were deposited by rfmagnetron sputtering and patterned by a lift off process. Thesurface area of the top electrodes is 0.19 mm2. The resultingcapacitors were postannealed under an oxygen atmosphere atT5550– 600 °C for 30 min, and are referred to as the refer-ence capacitors. Additionally, some capacitors were annealedat T5450 °C under a forming gas atmosphere ~FGA! or amixture of CO 5% and argon 95% ~COA!. These atmo-spheres have comparable oxygen partial pressure pO2 , whichare lower than 10215 bar. The electrical data were acquiredusing a Keithley 617 electrometer as the voltage source andpicoamperemeter. A voltage-step technique is used to pre-cisely reveal the leakage currents and to eliminate dielectriccharging currents of the capacitor. For ac measurements afrequency response analyzer SI1260 from Schlumberger Ltd.was used in combination with a Solatron 1296 dielectric in-terface.Figure 1~a! shows comparison of the leakage current fora!Electronic mail: liedtke@iwe.rwth-aachen.de2040003-6951/2000/77(13)/2045/3/$17.00Downloaded 15 Dec 2006 to 134.94.122.39. Redistribution subject toa Pt/BST/Pt capacitor before and after a reducing atmosphereheat treatment at 450 °C. The leakage current increases bymore than four orders of magnitude, whereas the COA-treated samples still show a higher leakage current than theforming gas annealed samples. Some samples were recoveryannealed in air at very low temperatures for 30 min on a hotplate. Figure 1~b! shows the leakage current of a capacitor atan applied voltage of 2 V as a function of the recovery tem-perature. It is clearly observed that around 150 °C the leak-age current density at an applied voltage of 2 V decreasesrapidly. Both the FGA- and COA-treated samples show thisrecovery annealing behavior.The capacitance of the test structures was measured as afunction of frequency at different temperatures at zero volt-age dc bias. In the case of the FGA- and COA-treatedsamples the capacitance shows a Debye-like relaxation be-FIG. 1. ~a! Variation of the leakage current density of the Pt/BST/Pt capaci-tors annealed under the forming gas and CO containing Ar in comparison tothe reference capacitor and ~b! leakage current at an applied voltage of 2 Vas a function of the recovery annealing temperature.5 © 2000 American Institute of Physics AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp2046 Appl. Phys. Lett., Vol. 77, No. 13, 25 September 2000 Liedtke, Grossmann, and Waserhavior with a relaxation step height of DC5Cl f2Ch f’3 nF @Fig. 2~a!#. The measured capacitance can be ex-pressed byCp~v ,T !5Ch f1Cl f2Ch f11~v/vp!2, ~1!where Cl f is the capacitance measured at low frequency (v!vp), and Ch f is the capacitance measured at high fre-quency (v@vp). The temperature dependence of the relax-ation frequency vp is given byvp5v0 exp~2Ea /kbT !, ~2!where v0 is the characteristic relaxation frequency at infinitetemperature, Ea is the activation energy for relaxation, kbBoltzmann’s constant, and T the absolute temperature. Themeasured data @see insert of Fig. 2~a!# reveal an activationenergy of 0.62 eV. After low temperature annealing at T5200 °C in air, the relaxation step disappears @Fig. 2~b!# andthe capacitance shows only a slight dependence of the fre-quency.The current conduction through Pt/BST/Pt thin films ca-pacitors is usually controlled by the interfacial potential bar-rier, of which the magnitude is affected by the work functionof the contacting metal and interface trap states.6–9 The leak-age current in such a MIM structure is assumed to be deter-minded by the reversed biased Schottky contacts with theSchottky barrier height Fb . Although additional conductionFIG. 2. Temperature and frequency dependence of the capacitance for ~a!the FGA-annealed capacitors and ~b! the capacitors recovered in air at T5200 °C.Downloaded 15 Dec 2006 to 134.94.122.39. Redistribution subject tomechanisms like tunneling at the Pt–BST interface arediscussed10 the leakage current data in this article were ana-lyzed with respect to Schottky conduction.In order to determine the zero-voltage Schottky barrierheight Fb0 the admittance was measured as a function offrequency at different temperatures with an ac signal of 30mV. The temperature dependence of ln(s/T2) of the MIMstructure after FGA and after the subsequent recovery an-nealing at T5200 °C is shown in Figs. 3~a! and 3~b!, respec-tively. It is obvious that in the low-temperature regime, theac conductivities depend significantly on the frequency. Ifthe temperature is increased a linear relation betweenln(s/T2) and 1/T becomes apparent. The activation energieswere determined from the slope in this region for the FGA-treated samples as EaFGA50.56 eV and for the recoveredsamples as EaREC50.96 eV, respectively. These activationenergies should now be compared to the zero-voltageSchottky barrier height determined by dc measurements.The experimental procedure for determining theSchottky barrier heights from dc measurements is describedelsewhere.7,8 For the same set of samples the zero-voltageSchottky barrier height was measured for the top and bottomcontacts and for the FGA and recovered samples ~Fig. 4!.The activation energies which are shown in Table I are invery good agreement with the zero-voltage Schottky barrierheights determined by dc measurements, both techniques re-vealed a lowering of the Schottky barrier height after a re-ducing heat treatment of about DFb0;0.4 eV.The physical origin of the large increase of the leakagecurrent after the forming gas anneal has been discussed in theliterature from different point of views. It is known that hy-FIG. 3. Relation between ln(s/T2) and 1000/T as a function of frequency for~a! the FGA-annealed capacitors and ~b! the capacitors recovered in air atT5200 °C. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp2047Appl. Phys. Lett., Vol. 77, No. 13, 25 September 2000 Liedtke, Grossmann, and Waserdrogen dissolves within the BST lattice during FGA.11 Thedissociation process of molecular hydrogen should be sup-ported by the catalytic nature of the Pt surface.12 The H1ions then diffuse through the Pt layer and accumulate in theBST thin film. The increase of leakage current was then at-tributed to the lowering of the Schottky barrier by an inter-face hydrogen layer.3,13 Due to the comparable degradationof BST thin film capacitors in forming gas and a CO-containing Ar atmosphere we can clearly exclude the pres-ence of dissolved hydrogen in the BST lattice as a reason forthe increase in leakage current.After oxidizing treatment ~i.e., initially or after the re-covery anneal! the film shows a high resistivity throughoutits entire thickness. We suggest that due to annealing in areducing atmosphere oxygen vacancies are produced (O05V0••12e211/2O2) and a layer of lower resistivity (%L) isgenerated within the film. Based on the impedance spectraFIG. 4. Variations of the Schottky barrier heights as a function of AV forPt/BST/Pt capacitor after FG anneal and after a subsequent low temperaturerecovery anneal in air at T5200 °C ~top electrode positively biased!.TABLE I. Zero-voltage Schottky barrier heights Fb0 for FGA-annealed andrecovered Pt/BST contacts in comparison to the activation energies deter-mined by ac measurements.FGA recovered at T5200 °Cdc~1!a 0.6 eV 1.05 eVdc~2!a 0.55 eV 0.95 eVac 0.56 eV 0.96 eVaBias of the top electrode.Downloaded 15 Dec 2006 to 134.94.122.39. Redistribution subject toand the assumption that the real permittivity of the BST doesnot change drastically during FG anneal, we can state fromthe ratio of Cl f and Ch f that the layer of low resisitivity, %L ,consumes approximately 50% of the film thickness. Accord-ing to the Maxwell Wagner14 relaxation model of serial RCcombinations of two layer within the film the ratio of theresistivities of the high resistive (%H) and the low resistive(%L) layer can be estimated to be %L /%H;0.1. The physicalorigin and the location of the highly resistive layer whichremains after FG anneal are still under investigation. Possi-bly it is located at the interfaces and is generated by carrierdepletion. Additionally, we suppose that interface trap statesare generated. These interface trap states, which create aninterface dipole,15 reduce the zero voltage Schottky barrierheight of about 0.4 eV. In conclusion, protons are not theorigin of the increase of leakage current in Pt/BST/Pt MIMstructures after a forming gas anneal. It was found that thezero voltage Schottky barrier height was decreased by ap-proximately 0.4 eV after a reducing heat treatment, whichwas measured by ac and dc experiments. The strong Debye-like relaxation steps in the capacitance at low frequencieswas discussed by means of Maxwell Wagner relaxation.1 T. Ueda, A. Noma, and D. Ueda, Integr. Ferroelectr. 7, 45 ~1995!.2 P.-Y. Lesaicherre, H. Yamaguchi, Y. Miyasaka, H. Watanabe, H. Ono,and M. Yoshida, Integr. Ferroelectr. 8, 201 ~1995!.3 R. Liedtke, S. Hoffmann, M. Grossmann, and R. Waser, in Proceedings ofSemiconductor Advances for Future Electronics ~SAFE99!, Mierlo, 1999.4 D. Hadad, T.-S. Chen, V. Balu, B. Jiang, S. H. Kuah, P. McIntyre, S.Summerfelt, J. M. Anthony, and J. C. Lee, Integr. Ferroelectr. 17, 461~1997!.5 U. Hasenkox, S. Hoffmann, and R. Waser, J. Sol-Gel Sci. Technol. 12, 67~1998!.6 J. Robertson and C. W. Chen, Appl. Phys. Lett. 74, 1168 ~1999!.7 C. S. Hwang, B. T. Lee, C. S. Kang, K. H. Lee, H. J. Cho, H. Hideki, W.D. Kim, S. I. Lee, and M. Y. Lee, J. Appl. Phys. 85, 287 ~1999!.8 G. W. Dietz, M. Schumacher, R. Waser, S. K. Streiffer, C. Basceri, and A.Kingon, J. Appl. Phys. 82, 2359 ~1997!.9 J. F. Scott, Ferroelectr. Rev. 1, 1 ~1999!.10 J. C. Shin, J. Park, C. S. Hwang, and H. J. Kim, J. Appl. Phys. 86, 506~1999!.11 R. Waser, J. Am. Ceram. Soc. 71, 58 ~1988!.12 J. P. Han and T. P. Ma, Appl. Phys. Lett. 71, 1267 ~1997!.13 W. Mo¨nch, J. Vac. Sci. Technol. B 14, 2985 ~1996!.14 H. Neumann and G. Arlt, Ferroelectrics 69, 179 ~1986!.15 J. Winterhalter, D. G. Ebling, D. Maier, and J. Honerkamp, J. Appl. Phys.82, 5488 ~1997!. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Capacitance and admittance spectroscopy analysis of hydrogen-degraded Pt/(Ba,Sr)TiO/Pt thin-film capacitors

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Capacitance and admittance spectroscopy analysis of hydrogen-degraded Pt/(Ba,Sr)TiO/Pt thin-film capacitors

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