2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Chan, HLW

Chen, WP

Choy, CL

Dai, J

Lee, PF

Lu, SG

Wang, XX

Wang, Y

Applied Physics Letters

2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

Spontaneous recovery of hydrogen-degraded TiO2 ceramic capacitorsW. P. Chen,a) Y. Wang, J. Y. Dai, S. G. Lu, X. X. Wang, P. F. Lee, H. L. W. Chan,and C. L. ChoyDepartment of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University,Hong Kong, People’s Republic of China~Received 2 September 2003; accepted 12 November 2003!Degradation of TiO2-based ceramic capacitors was observed after hydrogen incorporation on thetermination electrodes of the capacitors via electrolysis of water. Fourier-transform infrared ~FTIR!absorption spectra analysis of polished rutile single crystals clearly showed that hydrogen wasincorporated into the TiO2 lattice through the treatment. Hydrogen reduces Ti41 to Ti31 andincreases the concentration of charge carriers. The degradation was found to exhibit a strongdependence on time at room temperature. The degraded properties were spontaneously recoveredthrough an aging process, showing a spontaneous recovery unique to TiO2-based ceramiccapacitors. It is proposed that hydrogen is metastable in TiO2 and that hydrogen-induceddegradation has different stabilities among various oxide-based components and devices. © 2004American Institute of Physics. @DOI: 10.1063/1.1637942#The effects of hydrogen incorporation in materials havebeen extensively studied, particularly on metals ~hydrogenembrittlement! and silicon-based devices ~passivation ofdangling bonds!. In recent years, however, more and moreattention has been paid to the reactions between hydrogenand metal oxide materials.1–3 In our previous studies, wefound that hydrogen reacts with many metal oxides at anambient temperature in some electrochemical processes, in-cluding electroplating4 and the electrolysis of water.5,6 Theseambient temperature reactions usually induce obvious degra-dations in the properties of these oxide-based componentsand devices. A systematic understanding of the reactions isof great importance for improving the reliability of metaloxide-based electronic components and devices.7For many oxide-based components and devices, the deg-radation induced by the reaction of hydrogen is permanentand stable at room temperature. Sometimes, an annealing inan oxidizing atmosphere can recover the degradedproperties.4,8 Obviously, the hydrogen-induced defects inthose oxides responsible for the degradation can only beeliminated by high-temperature oxidizing reactions. Whenwe studied hydrogen-induced degradation in TiO2-based ce-ramic capacitors, however, we obtained a quite unexpectedresult. Although hydrogen induces an obvious degradation inthe capacitors, the degraded properties exhibit a strong de-pendence on time, and the degradation is gradually recoveredthrough an aging process without any thermal treatment.This indicates that hydrogen-induced defects are only meta-stable in TiO2-based ceramic capacitors at room temperature,in contrast to hydrogen-induced defects in many other ox-ides.The composition and sintering of the TiO2-basedtemperature-compensating ceramic capacitors used in thepresent investigation were reported in another paper.9 Thecapacitors were 0.70 mm thick and 7.5 mm in diameter, witha 4.5 mm diameter silver electrode on one surface and a 5.5mm diameter silver electrode on the other surface. Somecapacitors were placed in a 0.01 M NaOH solution and dcvoltages were imposed between the silver electrode of thecapacitors and an anode in the solution. The electrolysis ofwater occurred because the applied dc voltages and hydrogenevolved on the silver electrode of the capacitors. This treat-ment is hereafter referred to as hydrogen charging. The dcvoltages were disconnected after designated periods of timeand the capacitors were taken out, cleaned with de-ionizedwater and dried. An Agilent 4294A impedance analyzer wasused to measure the frequency spectra of capacitance anddielectric loss of the capacitors. The current density–voltage(J – V) characteristics of the capacitors were measuredthrough a Keithley 6517 electrometer/high resistance meter.Polished rutile ~001! single crystals ~1031030.5 mm3 insize! have been used for studying the reaction mechanismbetween hydrogen and TiO2 . A 3 mm diameter silver elec-trode was fired onto a corner of a rutile single crystal forhydrogen charging. Fourier-transform infrared ~FTIR! ab-sorption spectra were obtained with a Nicolet’s Magna-TRTM 750 FTIR spectrometer by measuring the transmit-tance spectroscopy. Raman spectra were recorded through aJobin-Yvon T6400 instrument, with an Ar1 laser source of514 nm wavelength and an incident power of 100 mW.SEM analysis revealed that the capacitors were verydense in microstructure. Some samples had been immersedin the NaOH solution for 100 h. After washing and drying, itwas found that the immersion had no noticeable influence onthe properties of the capacitors. By contrast, changes wereobserved in both the appearance and properties of thosesamples treated by hydrogen charging. The as-sinteredsamples were yellowish in color. The blank ceramic surfacegradually changed to bluish–gray and finally to blue–blackas hydrogen charging proceeded. Associated with the colorchange were dramatic increases in dielectric loss and in leak-age current of the capacitors. Similar to the blank ceramicsurface of TiO2 capacitors, the blank part of the rutile singlecrystal with a 3 mm silver electrode on a corner also changeda!Electronic mail: apchenwp@polyu.edu.hkAPPLIED PHYSICS LETTERS VOLUME 84, NUMBER 1 5 JANUARY 20041030003-6951/2004/84(1)/103/3/$22.00 © 2004 American Institute of Physicsto blue–black after hydrogen charging and the transparencywas greatly decreased. The FTIR absorption spectra for anas-received rutile single crystal and the hydrogen-chargedrutile single crystal are shown in Fig. 1. Hydrogen charginghad been conducted for 80 h with the application of a 6 V dcvoltage and a cathodic current density of 0.5 mA/cm2. TheFTIR absorption spectra were taken at 60 h after hydrogencharging. One can see that for the hydrogen-charged sample,the transmittance had been greatly decreased above 1300cm21 and there is a significant absorption peak around thewavenumber of 3280 cm21. This is a characteristic absorp-tion for interstitial hydrogen identification.10 So it is directevidence that hydrogen has been incorporated into the TiO2lattice at an ambient temperature by means of electrochemi-cal hydrogen charging. It is well known that Ti41 can bereduced to Ti31. Therefore, the reaction between hydrogengenerated by the electrolysis of water and TiO2 capacitorscan be expressed asH2O1e2→OH21Hads, ~1!Hads1Hads→H2 , ~2!Hads1TiTi→Hi•1TiTi8 , ~3!where Hads represents an adsorbed hydrogen atom, Hi• repre-sents an ionized hydrogen in an interstitial site and TiTi8 rep-resents a trivalent titanium in the lattice site of a tetravalenttitanium. The electron in TiTi8 is weakly bonded and is easilyactivated to the conduction band, which results in great in-creases in dielectric loss and in leakage current.The spontaneous recovery of hydrogen-induced degrada-tion in TiO2 capacitors is shown by a representative samplein the following. The capacitor had been treated by hydrogencharging for 12 h, with the application of a 4.5 V dc voltageand a cathodic current density of around 0.3 mA/cm2. Tomeasure the time dependence, the moment when the samplewas disconnected from the dc voltage was taken as the start-ing point of aging. After cleaning and drying, the sample waskept at 20 °C for both aging and measuring. Figure 2 showsthe frequency spectra of capacitance and dielectric loss of thecapacitor measured at different aging times. It is clear thatthe spectra depend greatly on aging time. For the as-sinteredcapacitor, the capacitance is about 24.9 pF from 1 kHz to 1MHz and the dielectric loss is very low over this frequencyrange. For the degraded capacitor, at the early stages of ag-ing, the capacitance has obviously increased, and it decreaseswith increasing testing frequency; the dielectric loss has alsogreatly increased and a peak appears in its frequency spec-trum. The increase in capacitance must be due to the incor-porated hydrogen atoms. They ionize and form dipoles thatmake a noticeable contribution to the polarization, especiallyin the low frequency region. It is interesting to note that thespectra of capacitance and dielectric loss both change sys-tematically with aging time. As aging proceeds, the dielectricloss peak moves to the low frequency side and the peakheight decreases; finally, the dielectric loss becomes verysmall over the testing frequency range. The change in thecapacitance-frequency curve with aging time is also veryclear. Roughly speaking, the slope of the curve decreases asthe aging time increases, until the capacitance becomes in-dependent of frequency. After 240 h of aging, the capacitorexhibits the same capacitance and dielectric loss versus fre-quency characteristics as that of the capacitor before electro-chemical hydrogen charging; i.e., the dielectric properties re-covered after 240 h of aging.The J – V characteristics of the capacitor also exhibited astrong dependence on time, as illustrated in Fig. 3. Accord-ing to the J – V curve measured after 1 h of aging, the leak-age current density of the degraded capacitor is more thanthree orders of magnitude larger than that of the as-sinteredFIG. 1. Room temperature Fourier-transform infrared ~FTIR! absorptionspectra of an as-received rutile single crystal and a hydrogen-charged rutilesingle crystal.FIG. 2. Frequency spectra of capacitance and dielectric loss for a hydrogencharging degraded TiO2-based ceramic capacitor measured at various agingtimes.FIG. 3. J – V characteristics for a hydrogen charging degraded TiO2-basedceramic capacitor measured at various aging times.104 Appl. Phys. Lett., Vol. 84, No. 1, 5 January 2004 Chen et al.capacitor. But as aging time increases, the difference gradu-ally narrows. The relationship between resistance ~calculatedfrom the J – V curves at the voltage of 250 V! and agingtime is shown in Fig. 4. It can be seen that the decreasedinsulation resistance increases monotonously to approachthat of the as-sintered capacitor. The recovery is fast at theearly stage of aging, but becomes slower as aging time in-creases. After 577 h of aging, the resistance reaches about50% of that before hydrogen charging.It should be pointed out that the darkened color inducedby hydrogen charging fades gradually in the course of aging.Together with the recovery observed in various properties, itis reasonable to assume that the concentration of the defects~interstitial hydrogen in this case! responsible for the degra-dation decreases as aging time increases. A possible mecha-nism is expressed as2Hi•12TiTi8 →2TiTi1H2 . ~4!It is assumed that interstitial hydrogen ions are meta-stable in TiO2-based ceramics. As a matter of fact, the char-acteristic FTIR absorption peak for the O–H stretch mode,which is around 3500 cm21,10,11 cannot be seen in Fig. 1 forthe hydrogen-charged rutile single crystal. Figure 5 showsthe Raman spectra for an as-received rutile single crystal andfor the hydrogen-charged rutile single crystal of Fig. 1. Thevibration modes of three peaks are 240 cm21 ~disorder andsecond order scattering!; 451 cm21 (Eg); 610 cm21 (A1g).12There is no noticeable change for the three modes betweenthe two samples; no special Raman scattering peak for theO–H stretch mode, which was located around 3700cm21,1,13 was observed for the hydrogen-charged crystal, ei-ther. These results suggest that incorporated hydrogen doesnot form an O–H bond with oxygen ion in rutile, which mayexplain why hydrogen is only metastable in the rutile lattice.A similar result had been reported for LiNbO3 and LiTaO3 ,10in which hydrogen corresponding to the FTIR absorptionpeak of 3280 cm21 is also only metastable. On the otherhand, when incorporated hydrogen forms an O–H bond withan oxygen ion, such as in Pb~Nb,Zr,Ti!O3 ,14 it shows ahigher stability and it is driven out only at some elevatedtemperatures.To summarize, this study clearly shows that hydrogengenerated by the electrolysis of water is incorporated in theTiO2 lattice and induces an obvious degradation inTiO2-based ceramic capacitors. Through an aging process,however, the degradation vanishes gradually as the variousdegraded properties systematically return to their originalvalues. It is proposed that the degradation results from thereduction of titanium and the formation of interstitial hydro-gen. These hydrogen-induced defects are only metastable inthe TiO2 lattice, and their disappearance results in the recov-ery. Hydrogen-induced defects exhibit different stabilitiesbetween TiO2 and many other oxides. It is imperative tobuild up a systematic understanding on the reactions betweenhydrogen and metal oxides.1 S. Aggarwal, S. R. Perusse, C. W. Tipton, R. Ramesh, H. D. Drew, T.Venkatesan, D. B. Romero, V. B. Podobedov, and A. Weber, Appl. Phys.Lett. 73, 1973 ~1998!.2 N. Poonawala, V. P. Dravid, O. Auciello, J. Im, and A. R. Krauss, J. Appl.Phys. 87, 2227 ~2000!.3 J. H. Ahn, P. C. McIntyre, L. W. Mirkarimi, S. R. Gilbert, J. Amano, andM. Schulberg, Appl. Phys. Lett. 77, 1378 ~2000!.4 W. P. Chen, L. T. Li, Y. Wang, and Z. L. Gui, J. Mater. Res. 13, 1110~1998!.5 W. P. Chen, H. L. W. Chan, F. C. H. Yiu, K. M. W. Ng, and P. C. K. Liu,Appl. Phys. Lett. 80, 3587 ~2002!.6 W. P. Chen, X. P. Jiang, Y. Wang, Z. Peng, and H. L. W. Chan, J. Am.Ceram. Soc. 86, 735 ~2003!.7 J. D. Baniecki, J. S. Cross, M. Tsukada, and J. Watanabe, Appl. Phys. Lett.81, 3837 ~2002!.8 J. Im, O. Auciello, A. R. Krauss, D. M. Gruen, R. P. H. Chang, S. H. Kim,and A. I. Kingon, Appl. Phys. Lett. 78, 1162 ~1999!.9 W. P. Chen, Y. Wang, X. X. Wang, J. Wang, and H. L. W. Chan, Mater.Chem. Phys. 82, 520 ~2003!.10 P. Baldi, M. P. De Micheli, K. El Hadi, S. Nouh, A. C. Cino, P. Aschieri,and D. B. Ostrowsky, Opt. Eng. 37, 1193 ~1998!.11 X. L. Tong, M. Zhang, A. Yariv, A. Agranat, R. Hofmeister, and V. Leyva,Appl. Phys. Lett. 69, 479 ~1996!.12 J. Arbiol, J. Cerda, G. Dezanneau, A. Cirera, F. Peiro, A. Cornet, and J. R.Morante, J. Appl. Phys. 92, 853 ~2002!.13 A. R. Krauss, A. Dhote, O. Auciello, J. Im, R. Ramash, and S. Aggarwal,Integr. Ferroelectr. 27, 1191 ~1999!.14 S. Aggarwal, S. R. Perusse, C. J. Kerr, R. Ramesh, J. T. Evans, Jr., L.Boyer, and G. Velasquez, Appl. Phys. Lett. 76, 918 ~2000!.FIG. 5. Raman spectra for an as-received rutile single crystal and for thehydrogen-charged rutile single crystal of Fig. 1.FIG. 4. The relationship between the resistance and aging time for a hydro-gen charging degraded TiO2-based ceramic capacitor.105Appl. Phys. Lett., Vol. 84, No. 1, 5 January 2004 Chen et al.

Spontaneous recovery of hydrogen-degraded TiO₂ ceramic capacitors

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