Temperature induced change in the hysteretic behavior of the capacitance-voltage characteristics of Pt–ZnO–Pb„Zr0.2Ti0.8…O3–Pt heterostructures

L. Pintilie,a� C. Dragoi, R. Radu, A. Costinoaia, V. Stancu, and I. PintilieNational Institute of Materials Physics, P.O. Box MG-7, Bucharest-Magurele 077125, Romania

�Received 6 November 2009; accepted 14 December 2009; published online 4 January 2010�

Pt–ZnO–Pb�Zr0.2Ti0.8�O3–Pt �PZT-ZnO� heterostructures were fabricated by using a sol-gelprocess. Capacitance-voltage measurements performed on a wide temperature range �20–450 K�have revealed the presence of a hysteresis that undergo a change of direction from clockwiseat temperatures below 350 K to counter-clockwise at higher temperatures. In the first case,the hysteresis is produced by charge injection, similar to the case of classicalmetal-oxide-semiconductor capacitors. In the last case, the hysteresis is the fingerprint ofpolarization reversal, as reported for metal-ferroelectric-semiconductor �MFS� structures based onn-Si. The memory window at 450 K is about 6 V. This result suggests that PZT-ZnO MFSheterostructures can be used for memory devices working at elevated temperatures, in which theZnO plays the role of the semiconductor. © 2010 American Institute of Physics.�doi:10.1063/1.3284659�

The metal-ferroelectric-semiconductor �MFS� structuresbased on Si were investigated for applications in memorydevices due to the hysteresis observed in the capacitance-voltage �C-V� characteristics. The hysteresis due to the ferro-electric polarization effect should be counter clockwise incase of n-type Si, and clockwise in case of p-type Siwafers.1–5

The memory window of MFS structures is given by thevoltage shift of the C-V characteristics when the externalbias voltage is swept from depletion to accumulation andback. For an ideal MFS structure the memory windowshould be twice the value of the voltage corresponding to thecoercive field.6 The larger the memory window is the betterthe MFS structure is for applications in memory devices.Unfortunately, the MFS structures based on Si had shownserious drawbacks, especially when the ferroelectric layer isof lead zirconate-titanate �PZT� material. This is due to theimperfect ferroelectric-Si interface �interdiffusion, secondaryphases, etc.�, reflected in a poor retention of the writteninformation.7,8 Attempts were made to enhance the quality ofthe interface by using various buffer layers of dielectric na-ture �metal-ferroelectric-insulator-semiconductor structures�,but they are affecting the properties of the ferroelectric film,especially the magnitude of the spontaneous polarization,due to the imperfect screening of the polarization charges atthe ferroelectric-insulator interface.9,10

The recent surge of the ZnO as a wide gap semiconduc-tor, suitable for optoelectronic applications in the ultravioletrange and for electronic devices working at elevated tem-peratures and high electric fields, had opened new perspec-tives for MFS structures also. Combinations of PZT typematerials with ZnO offer even the possibility to grow epitax-ial structures, which is not possible in the case of Si withoutthe use of buffer layers.11,12 However, there are very fewreports analyzing the electric properties of MFS structuresbased on PZT-ZnO combination and reporting the presence

of hysteresis in the C-V characteristic. In addition, when thehysteresis was observed, this was clockwise although the as-grown ZnO is an n-type semiconductor �in which case thehysteresis should be counter clockwise�.13,14 Therefore, thepresence of hysteresis effects explained as caused by theferroelectric polarization becomes questionable.

In this letter, we report on the temperature change in thehysteresis orientation observed in the case of a MFS struc-ture based on PZT-ZnO layers. It was found that the hyster-esis is clockwise below a temperature around 350 K andbecomes counter clockwise at higher temperatures.

The MFS structures were fabricated by sol-gel methodon platinized silicon wafers �Pt /TiO2 /SiO2 /Si�. First, thePbZr0.2Ti0.8O3 thin films were deposited. The procedure isdescribed in detail in a previous publication.15 The crystalli-zation was realized by conventional thermal annealing at650 °C for 30 min. The ZnO layer was deposited on top ofthe PZT film using the method proposed by Farley.16 Thecrystallization annealing for the ZnO was performed at640 °C for 3 h, in air. Witness samples of only PZT depos-ited on Pt /TiO2 /SiO2 /Si substrate, were grown in order totest the ferroelectric properties of the PZT films.

Two sets of samples with area of 2�2 cm2 were pre-pared for investigations. Half of the samples’ area was usedfor analyzing the structure by x-ray diffraction �XRD�. Onthe other half of the samples’ area Pt electrodes of 0.2 mm2

were deposited on top of PZT film and PZT-ZnO heterostruc-tures resulting in more than 50 contacts per sample for elec-trical measurements. The capacitance was measured with anAgilent LCR meter, while the hysteresis was measured witha TF2000 ferroelectric tester. For measuring at different tem-peratures the samples were mounted in a closed cycle Hecryostat able to achieve temperatures between 20 and 450 K.

The results of the XRD structural investigations of thePZT-ZnO heterostructures are shown in Fig. 1. The charac-teristic peaks of tetragonal PZT are observed. The dominantorientation is �100�. A prominent �111� peak is also observed,most probably due to the Pt bottom electrode which isknown to promote the �111� growth of the PZT films depos-ited by sol-gel.17 Beside the characteristic peaks of PZT,

a�Author to whom correspondence should be addressed. Electronic mail:pintilie@infim.ro.

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some small ones are observed. They fit to the �002� and �101�peaks of ZnO having the wurtzite structure. The small am-plitude of these peaks suggests that the ZnO crystallites arevery small. Within the XRD resolution one can conclude thatthe heterostructure is composed only of PZT and ZnO, with-out parasitic phases.

The ferroelectric properties of the PZT films alone aswell as of the PZT-ZnO bilayer with top and bottom Pt elec-trodes were investigated by performing hysteresis measure-ments at room temperature. The obtained results are shownin Fig. 2. It is interesting to observe that a clear ferroelectrichysteresis is obtained in both cases. The average value ofremnant polarization is about the same, around 35 �C /cm2.The hysteresis loops are shifted along the voltage axis, sug-gesting the presence of an internal electric field. Beingpresent in both cases, this field cannot be associated to thePZT-ZnO interface.

The results of the hysteresis measurements indicate thatthe ZnO layer plays the role of an electrode for the PZT film,of finite resistance and capacitance. The applied voltage onthe PZT-ZnO structure will divide between the two layers,leading to a higher coercive field than in the case of singlePZT film. This is confirmed by the experimental results pre-sented in Fig. 2. The finite capacitance of the ZnO film will

appear in series with that of the PZT layer, lowering thecapacitance of the structure. Therefore, the equivalent dielec-tric constant will be lower for the PZT-ZnO structure than forthe single PZT film. This fact is also confirmed by experi-ment, if one considers that the slope of the hysteresis loop inthe voltage range, where the spontaneous polarization issaturated, is proportional to the static dielectric constant ac-cording to the relation D=�E+PS.18 Here D is the electricdisplacement �the quantity that is, in fact, recorded duringthe hysteresis measurement�, E is the applied electric field,PS is the spontaneous polarization, and � is the permittivityof the ferroelectric material ��=�st�0, with �st being the staticdielectric constant and �0 the permittivity of free space�. Ascan be observed in Fig. 2, the slope in the saturation range issmaller for PZT-ZnO structure compared to single PZT film.

The results of C-V measurements on PZT-ZnO structureat different temperatures �150, 350, and 450 K� are shown inFig. 3. A hysteresis is obtained when the voltage is swept upand down. The direction of the hysteresis is changing fromclockwise at low temperatures �Fig. 3�a�� to counter clock-wise at high temperatures �Fig. 3�c��. An intermediate situa-tion can be recorded for temperatures around 350 K�Fig. 3�b��. Thus, at low temperatures the measured hyster-esis is similar to that measured on a standard metal-oxide-semiconductor �MOS based on SiO2� structures in case ofcharge injection, and only for temperatures higher than 350K the hysteresis becomes similar to what was reported in thecase of MFS structures on n-type Si.1 In our opinion, thisvery interesting phenomenon of changing the hysteresis di-rection with increasing the temperature, is entirely due to thePZT-ZnO interface. A qualitative explanation can be given ifwe consider trapping centers close to the interface region.Thus, when the voltage is swept up, starting with minus po-larity on the PZT, the ZnO is in depletion and the capacitanceof the PZT-ZnO structure is low. When the voltage on PZTbecomes positive the ZnO switch to accumulation and aninjection of charge will take place in the interface region. Apart of this charge will be trapped in the interface region. Atlow temperatures the trapped charge will not be able to fol-

FIG. 1. XRD spectra in case of a PZT-ZnO heterostructure with a totalthickness of 270 nm ��65 nm ZnO and 205 nm of PZT�.

FIG. 2. The hysteresis loops obtained in case of PZT single films and incase of PZT-ZnO structures. Measurement performed by using a triangularwave, with a frequency of 10 kHz. The lines are suggesting the linear de-pendence after saturation of spontaneous polarization PS, with slope propor-tional with the static dielectric constant.

FIG. 3. C-V characteristics at 150 K �a�, 350 K �b�, and 450 K �c�, obtainedin the case of a MFS structure based on PZT-ZnO bilayer. Measurementswere performed at 100 kHz, with an amplitude of the test signal of 100 mV.

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low the voltage variations of the small ac signal. Therefore, aclockwise hysteresis will occur, as was reported in case ofcharge injection for MOS structures.19,20 Thus, the presenceof interface states can surpass the effect of polarization re-versal in the PZT layer and determine a clockwise orienta-tion of the hysteresis at low temperatures. At elevated tem-perature, the interface traps are able to respond to the acsmall signal in the capacitance measurements and the hyster-etic behavior associated to interface states is significantlyreduced, leaving the polarization switching in the PZT layerthe dominant mechanism for the observed hysteresis. Thisbecomes counter clockwise as it should be in the case of aMFS structure with n-type semiconductor.

Another aspect of interest in the C-V characteristics isthe presence of two capacitance peaks observed both at lowand high temperatures. They can be associated with themaxima present in the well known butterfly shaped C-Vcharacteristic obtained in the case of single PZT layer.13 Weremind that the capacitance peaks observed in the C-V char-acteristic of a ferroelectric film are associated to polarizationswitching.21 If the hysteresis is close to rectangular, as it is inthe case of PZT-ZnO �see Fig. 2�, the peaks are sharp andcan remain visible in the equivalent capacitance of the struc-ture.

In conclusion, MFS type structures based on PZT-ZnObilayers were realized by sol-gel method on Pt/Si substrates.The electrical measurements performed on the structure haverevealed very good ferroelectric properties for the PZT layerand the presence of a hysteretic behavior in the case of C-Vcharacteristics. The hysteresis orientation is dependent ontemperature, being clockwise below about 350 K and counterclockwise above 350 K. This change can be explained by thepresence of trapped charges at the PZT-ZnO interface, domi-nating the hysteretic behavior at low temperatures. At el-evated temperatures the PZT-ZnO bilayer behaves as a nor-mal MFS structure. At 450 K, the memory window is ofabout 6 V, significantly larger than was reported in literatureso far for other MFS structures. This opens the possibility to

use PZT-ZnO heterostructures in memory devices operatingat temperatures above 100 °C.

The authors acknowledge the financial support from theRomanian Ministry of Education, Research, and Innovation-National Authority for Scientific Research through the Con-tract No. 72 149—HETOX.

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