# Asymmetry of Polarization Reversal and Current-Voltage Characteristics of Pt/PZT-Film/Pt:Ti/SiO2/Si-Substrate Structures

Authors: S. L. Bravina; N. V. Morozovsky; J. Costecalde; C. Soyer; D. Remiens; D. Deresmes
Journal: Smart Materials Research
 (2011)

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## Abstract

The characterization of the asymmetries of bipolar charge-voltage and current-voltage loops of polarization reversal and unipolar current-voltage curves for Pt/PZT-film/Pt:Ti/SiO2/Si-substrate systems was performed in the dynamic mode. The asymmetry of local deformation-voltage loops was observed by piezoresponse force microscopy. The comparison of the dependences of introduced asymmetry factors for the bipolar charge-voltage and current-voltage loops and unipolar current-voltage curves on drive voltage indicates the interconnection of ferroelectric and electrical space charge transfer asymmetries.


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## Body

## 1. Introduction

At present, lead zirconate titanate (PZT) film on silicon structures are considered among the best ones for creating elements of Si-integrated converters of various types. However, the practically important electrical characteristics of “metal-PZT film-metal-Si-substrate” systems manifest the well-known set of natural and technological asymmetries.Earlier [1–3], we reported the results of investigations of polar and poling asymmetries of the set of pyroelectric and related characteristics of Pt/PZT-film/Pt:Ti/SiO2/Si-substrate sandwich type structures.In this paper, we concentrate our attention on the characterization of the complex of the asymmetries of dynamic bipolar charge-voltage (Q-V-) and current-voltage (I-V-) loops and unipolar current-voltage (J-V-) curves as well as bipolar deformation-voltage (X-V-) loop asymmetry. We present the results of investigation of interrelated polarization reversal and current-voltage dynamic asymmetries of Pt/PZT-film/Pt:Ti/SiO2/Si-substrate systems.


## 2. Experiment

### 2.1. Samples

The samples under investigation were PZT films prepared by the radiofrequency magnetron sputtering method. The bottom Pt electrode of 150 nm of thickness with Ti adhesive layer of 10 nm of thickness was deposited on the 350 nm SiO2 layer on a 350 μm (100) n-type Si wafer. Pb(ZrxTi1−x)O3 film with x = 0.54 of 1-2 μm of thickness was deposited on the Pt/TiOx/SiO2/Si-substrate structure just after its stabilization by annealing treatment at 650–700°C.The perovskite ferroelectric phase of PZT was obtained by annealing at 625°C for 30 min. Then, a top Pt (150 nm) electrode of≈1 mm2 of area was deposited by sputtering.The sputtering conditions and the results of X-ray and microstructure characterization were described in details in [4].


### 2.2. Measurements

For the investigations of polarization reversal (PR) and current-voltage characteristics, the measuring set for complete ferroelectric characterization [2] was used. The examination of PR characteristics was performed in two actual modes, namely, ferroelectric hysteresis loops of charge (Q-V-loops) and current (I-V-loops) were registered.The measurements were performed by Sawyer-Tower like circuit [5] in the multicycle mode under applied a.c. triangular drive voltage Vd of 1 Hz of frequency in the amplitude range of 0≤Vda≤11 V.To eliminate PR-current contribution, the examination of dynamic current-voltage characteristics (J-V-curves) was performed at unipolar saw-tooth drive voltageVd with 0.5 s of durability and 1 Hz of repetitive frequency in the amplitude range of 0≤Vda≤10 V.Drive voltage was applied to the circuits of series connection of tested sample with reference resistor for I-V-loops and J-V-curves and with reference capacitor for Q-V-loops. The corresponding characteristics were observed by means of two-beam digital storage oscilloscope YB 54060 operating in I-V-mode. The voltage on the reference elements was monitored and plotted versus drive voltage.Piezoelectric response force microscopy (PFM) observations of deformation-voltage (X-V-) loops were carried out with a commercial AFM Veeco “Dimension 3100” using the probe ofApp-Nano ANSCM-PA type with cantilever spring constant of ≈40 N/m (resonance frequency ≈300 kHz). The probe was equipped by Si tip with double layer of chromium and platinum/iridium5 (Pt/Ir5) coating of 23 nm thick. After localization of the top Pt electrode by AFM imaging, the PFM measurements were performed in the contact mode with PFM tip located over the PZT grain (1–3 μm) visible through the top Pt electrode.A modulation ac voltage of 2 kHz of frequency and 1.5 V of amplitude was applied to the conductive PFM tip, and dc driving voltageVd cycled in the range -10V≤Vd≤+10 V was applied to the bottom Pt electrode of the film. The amplitude of Vd was chosen to be higher than the coercive voltage for polarization reversal in the examined PZT film.PFM response amplitude hysteresis loops were recorded as a function ofVd by demodulating photoelectrically transformed cantilever deflection signal with a lock-in amplifier Signal Recovery 7270.


## 2.1. Samples

The samples under investigation were PZT films prepared by the radiofrequency magnetron sputtering method. The bottom Pt electrode of 150 nm of thickness with Ti adhesive layer of 10 nm of thickness was deposited on the 350 nm SiO2 layer on a 350 μm (100) n-type Si wafer. Pb(ZrxTi1−x)O3 film with x = 0.54 of 1-2 μm of thickness was deposited on the Pt/TiOx/SiO2/Si-substrate structure just after its stabilization by annealing treatment at 650–700°C.The perovskite ferroelectric phase of PZT was obtained by annealing at 625°C for 30 min. Then, a top Pt (150 nm) electrode of≈1 mm2 of area was deposited by sputtering.The sputtering conditions and the results of X-ray and microstructure characterization were described in details in [4].


## 2.2. Measurements

For the investigations of polarization reversal (PR) and current-voltage characteristics, the measuring set for complete ferroelectric characterization [2] was used. The examination of PR characteristics was performed in two actual modes, namely, ferroelectric hysteresis loops of charge (Q-V-loops) and current (I-V-loops) were registered.The measurements were performed by Sawyer-Tower like circuit [5] in the multicycle mode under applied a.c. triangular drive voltage Vd of 1 Hz of frequency in the amplitude range of 0≤Vda≤11 V.To eliminate PR-current contribution, the examination of dynamic current-voltage characteristics (J-V-curves) was performed at unipolar saw-tooth drive voltageVd with 0.5 s of durability and 1 Hz of repetitive frequency in the amplitude range of 0≤Vda≤10 V.Drive voltage was applied to the circuits of series connection of tested sample with reference resistor for I-V-loops and J-V-curves and with reference capacitor for Q-V-loops. The corresponding characteristics were observed by means of two-beam digital storage oscilloscope YB 54060 operating in I-V-mode. The voltage on the reference elements was monitored and plotted versus drive voltage.Piezoelectric response force microscopy (PFM) observations of deformation-voltage (X-V-) loops were carried out with a commercial AFM Veeco “Dimension 3100” using the probe ofApp-Nano ANSCM-PA type with cantilever spring constant of ≈40 N/m (resonance frequency ≈300 kHz). The probe was equipped by Si tip with double layer of chromium and platinum/iridium5 (Pt/Ir5) coating of 23 nm thick. After localization of the top Pt electrode by AFM imaging, the PFM measurements were performed in the contact mode with PFM tip located over the PZT grain (1–3 μm) visible through the top Pt electrode.A modulation ac voltage of 2 kHz of frequency and 1.5 V of amplitude was applied to the conductive PFM tip, and dc driving voltageVd cycled in the range -10V≤Vd≤+10 V was applied to the bottom Pt electrode of the film. The amplitude of Vd was chosen to be higher than the coercive voltage for polarization reversal in the examined PZT film.PFM response amplitude hysteresis loops were recorded as a function ofVd by demodulating photoelectrically transformed cantilever deflection signal with a lock-in amplifier Signal Recovery 7270.


## 3. Results and Discussion

### 3.1. Polarization Reversal Characteristics

The obtained Q-V-loops and I-V-loops of polarization reversal are presented in Figures1 and 2, respectively. Under increasing Vda, the start of saturation of Q-V-loops (Figure 1(a)) is accompanied by appearance of maxima on I-V-loops (Figure 2(a)). The main characteristics of Q-V- and I-V-loops are changed after several minutes of cycling under Vda=10 V that can be considered as forming.Bipolar charge-voltage loops of polarization reversal (a) and drive voltage dependences of charge asymmetry factors (b).
(a)(b)Bipolar current-voltage loops of polarization reversal (a) and drive voltage dependence of capacitive asymmetry factor (b).
(a)(b)


#### 3.1.1. The Peculiarities of Bipolar Charge-Voltage Loops

Under increasingVda, the tendency of Q-V-loops for saturation is accompanied by vertical (along Q-axis) shift (Figure 1(a)), which reaches the maxima and starts to decrease under subsequent Vda increasing. At Vda=10 V, the coercive voltages Vc obtained from Q-V-loop are Vc-=-3.4 V and Vc+=+5.2 V. The vertical shift changes its sign (see Figure 1(a)) under decreasing Vda after forming at Vda=10 V.Figure1(b) presents the Vdadependences of charge asymmetry factors ΔQi=(Qi++Qi-)/2 considered as a measure of the vertical shift of Q-V-loop (i=m,r correspond to maximal (Vd=Vda) and remanent (Vd=0) polarization, resp.). Under Vda increasing the positive ΔQm and ΔQr appear in Vc+ vicinity. After forming under subsequent Vda decreasing the negative ΔQm and ΔQr disappear in Vc- vicinity. This behaviour of ΔQm(Vda) and ΔQr(Vda) shows that the vertical shift of Q-V-loops is connected with PR processes in the coercive Vc- and Vc+ vicinities.


#### 3.1.2. The Peculiarities of Bipolar Current-Voltage Loops

The observed I-V-loops are noticeably asymmetrical (Figure2(a)). The absolute value of the voltage Vm+ of positive current maximum Im+ is higher than that of the voltage Vm- of the negative current maximum Im-. At that, Im+ value is significantly higher than that of Im-. At Vda=10 V, the voltages Vm obtained from I-V-loop are Vm-=-3.7 V and Vm+=+6.7 V.The forming underVda=10 V results in the change of Vm+ and Vm- values and increase of Im+ and Im- values under subsequent Vda decrease (see Figure 2(a)).Because the vertical (along I-axis) size of I-V-loop is proportional to the sample capacityC [6], the value of ΔI0=(I0-+I0+)/2Vda (where I0- and I0+ are the currents at Vd=0) can be considered as a measure of the capacitive asymmetry ΔC0=(C0--C0+) of I-V-loop.Figure2(b) presents Vdadependences of capacitive asymmetry factor ΔC0∝ΔI0. Under Vda increasing and its subsequent decreasing after forming the values of ΔC0 are near the same at Vda<Vm- and at Vda>Vm+. However, in the range Vm-<Vda<Vm+ not only the behaviour of ΔC0(Vda) but also ΔC0 signs are different before and after forming. At that, the maximal ΔC0 difference before and after forming is observed at Vda≈|Vc+|=5.2 V. This behaviour of ΔC0(Vda) shows that the current asymmetry of I-V-loops is connected with PR processes in Vm- and Vm+ vicinities.Direct modelling by means of the electrical circuit that includes inversely connected Zener diodes divided by parallel RC circuit [7] demonstrates the possibility of controlling the value of vertical shifts of Q-V-loops and peculiarities of I-V-loops by adjusting Zener voltages.


### 3.2. Unipolar Current-Voltage Characteristics

For the observed unipolar J-V-curves (Figure3(a)) under Vda increasing and its subsequent decreasing after forming is characteristic of the change of asymmetry degree and the shape of the positive and negative branches and also the change of the voltages Vt+ and Vt- of transition between sublinear and superlinear J-V-regions.Unipolar current-voltage curves (a) and drive voltage dependences of transition voltages and asymmetry ratio (b).
(a)(b)As a measure of J-V-curve asymmetry the ratioAR=Jm+/Jm- of the maximal (index “m”) current values of positive Jm+(Vda) and negative Jm-(Vda) branches was taken.TheVda dependences of AR and also Vt-/Vda and Vt+/Vda for initial and formed J-V-curves are presented in Figure 3(b). The values of AR for initial and formed J-V-curves are different at Vda<Vm+ and became near equal at Vda≥Vm+. Under increasing Vda the dependence AR(Vda) reaches the maximum at Vda≈|Vc-|. After forming AR(Vda) dependence has the smooth step with the start at Vm-, the centre near Vda≈Vc+, and the finish at Vm+.The difference inVt+(Vda) and Vt-(Vda) behaviour is displayed by Vda dependences of Vt+/Vda, and Vt-/Vda ratios (see Figure 3(b)). The equality of Vt+/Vda and Vt-/Vda is observed up to Vdm≈|Vc-| and the value of (Vt+-Vt-)/Vda reaches maximum at Vda≈Vm+.This behaviour of asymmetry ratioAR(Vda) and also transition voltages Vt+(Vda) and Vt-(Vda) of unipolar J-V-curves manifests the peculiarities in the coercive Vc- and Vc+ vicinities as bipolar Q-V-loops and in Vm+ and Vm- vicinities as bipolar I-V-loops.


### 3.3. Deformation-Voltage Loops

Figure4 presents the local deformation-voltage (X-V-) loops (PFM-amplitude) studied on the same Pt/PZT/Pt:Ti/SiO2/Si-substrate structure as were examined integral charge Q-V- and current I-V-loops of polarization reversal.PFM amplitude-voltage loops initial (a) and after forming by drive voltage sweeping with decreasing amplitude (b).
(a)(b)As it is seen from Figure4(a), the initial X-V-loop is remarkably asymmetrical. An apparent approach of the loop shape to a near symmetrical butterfly-like one (Figure 4(b)) was achieved by cycling under Vd sweeping with a decreasing from 10 V to zero amplitude (local forming procedure).So, the forming procedure results in similar symmetrization of corresponding characteristics related to integral and local PR processes.


## 3.1. Polarization Reversal Characteristics

The obtained Q-V-loops and I-V-loops of polarization reversal are presented in Figures1 and 2, respectively. Under increasing Vda, the start of saturation of Q-V-loops (Figure 1(a)) is accompanied by appearance of maxima on I-V-loops (Figure 2(a)). The main characteristics of Q-V- and I-V-loops are changed after several minutes of cycling under Vda=10 V that can be considered as forming.Bipolar charge-voltage loops of polarization reversal (a) and drive voltage dependences of charge asymmetry factors (b).
(a)(b)Bipolar current-voltage loops of polarization reversal (a) and drive voltage dependence of capacitive asymmetry factor (b).
(a)(b)


### 3.1.1. The Peculiarities of Bipolar Charge-Voltage Loops

Under increasingVda, the tendency of Q-V-loops for saturation is accompanied by vertical (along Q-axis) shift (Figure 1(a)), which reaches the maxima and starts to decrease under subsequent Vda increasing. At Vda=10 V, the coercive voltages Vc obtained from Q-V-loop are Vc-=-3.4 V and Vc+=+5.2 V. The vertical shift changes its sign (see Figure 1(a)) under decreasing Vda after forming at Vda=10 V.Figure1(b) presents the Vdadependences of charge asymmetry factors ΔQi=(Qi++Qi-)/2 considered as a measure of the vertical shift of Q-V-loop (i=m,r correspond to maximal (Vd=Vda) and remanent (Vd=0) polarization, resp.). Under Vda increasing the positive ΔQm and ΔQr appear in Vc+ vicinity. After forming under subsequent Vda decreasing the negative ΔQm and ΔQr disappear in Vc- vicinity. This behaviour of ΔQm(Vda) and ΔQr(Vda) shows that the vertical shift of Q-V-loops is connected with PR processes in the coercive Vc- and Vc+ vicinities.


### 3.1.2. The Peculiarities of Bipolar Current-Voltage Loops

The observed I-V-loops are noticeably asymmetrical (Figure2(a)). The absolute value of the voltage Vm+ of positive current maximum Im+ is higher than that of the voltage Vm- of the negative current maximum Im-. At that, Im+ value is significantly higher than that of Im-. At Vda=10 V, the voltages Vm obtained from I-V-loop are Vm-=-3.7 V and Vm+=+6.7 V.The forming underVda=10 V results in the change of Vm+ and Vm- values and increase of Im+ and Im- values under subsequent Vda decrease (see Figure 2(a)).Because the vertical (along I-axis) size of I-V-loop is proportional to the sample capacityC [6], the value of ΔI0=(I0-+I0+)/2Vda (where I0- and I0+ are the currents at Vd=0) can be considered as a measure of the capacitive asymmetry ΔC0=(C0--C0+) of I-V-loop.Figure2(b) presents Vdadependences of capacitive asymmetry factor ΔC0∝ΔI0. Under Vda increasing and its subsequent decreasing after forming the values of ΔC0 are near the same at Vda<Vm- and at Vda>Vm+. However, in the range Vm-<Vda<Vm+ not only the behaviour of ΔC0(Vda) but also ΔC0 signs are different before and after forming. At that, the maximal ΔC0 difference before and after forming is observed at Vda≈|Vc+|=5.2 V. This behaviour of ΔC0(Vda) shows that the current asymmetry of I-V-loops is connected with PR processes in Vm- and Vm+ vicinities.Direct modelling by means of the electrical circuit that includes inversely connected Zener diodes divided by parallel RC circuit [7] demonstrates the possibility of controlling the value of vertical shifts of Q-V-loops and peculiarities of I-V-loops by adjusting Zener voltages.


## 3.1.1. The Peculiarities of Bipolar Charge-Voltage Loops

Under increasingVda, the tendency of Q-V-loops for saturation is accompanied by vertical (along Q-axis) shift (Figure 1(a)), which reaches the maxima and starts to decrease under subsequent Vda increasing. At Vda=10 V, the coercive voltages Vc obtained from Q-V-loop are Vc-=-3.4 V and Vc+=+5.2 V. The vertical shift changes its sign (see Figure 1(a)) under decreasing Vda after forming at Vda=10 V.Figure1(b) presents the Vdadependences of charge asymmetry factors ΔQi=(Qi++Qi-)/2 considered as a measure of the vertical shift of Q-V-loop (i=m,r correspond to maximal (Vd=Vda) and remanent (Vd=0) polarization, resp.). Under Vda increasing the positive ΔQm and ΔQr appear in Vc+ vicinity. After forming under subsequent Vda decreasing the negative ΔQm and ΔQr disappear in Vc- vicinity. This behaviour of ΔQm(Vda) and ΔQr(Vda) shows that the vertical shift of Q-V-loops is connected with PR processes in the coercive Vc- and Vc+ vicinities.


## 3.1.2. The Peculiarities of Bipolar Current-Voltage Loops

The observed I-V-loops are noticeably asymmetrical (Figure2(a)). The absolute value of the voltage Vm+ of positive current maximum Im+ is higher than that of the voltage Vm- of the negative current maximum Im-. At that, Im+ value is significantly higher than that of Im-. At Vda=10 V, the voltages Vm obtained from I-V-loop are Vm-=-3.7 V and Vm+=+6.7 V.The forming underVda=10 V results in the change of Vm+ and Vm- values and increase of Im+ and Im- values under subsequent Vda decrease (see Figure 2(a)).Because the vertical (along I-axis) size of I-V-loop is proportional to the sample capacityC [6], the value of ΔI0=(I0-+I0+)/2Vda (where I0- and I0+ are the currents at Vd=0) can be considered as a measure of the capacitive asymmetry ΔC0=(C0--C0+) of I-V-loop.Figure2(b) presents Vdadependences of capacitive asymmetry factor ΔC0∝ΔI0. Under Vda increasing and its subsequent decreasing after forming the values of ΔC0 are near the same at Vda<Vm- and at Vda>Vm+. However, in the range Vm-<Vda<Vm+ not only the behaviour of ΔC0(Vda) but also ΔC0 signs are different before and after forming. At that, the maximal ΔC0 difference before and after forming is observed at Vda≈|Vc+|=5.2 V. This behaviour of ΔC0(Vda) shows that the current asymmetry of I-V-loops is connected with PR processes in Vm- and Vm+ vicinities.Direct modelling by means of the electrical circuit that includes inversely connected Zener diodes divided by parallel RC circuit [7] demonstrates the possibility of controlling the value of vertical shifts of Q-V-loops and peculiarities of I-V-loops by adjusting Zener voltages.


## 3.2. Unipolar Current-Voltage Characteristics

For the observed unipolar J-V-curves (Figure3(a)) under Vda increasing and its subsequent decreasing after forming is characteristic of the change of asymmetry degree and the shape of the positive and negative branches and also the change of the voltages Vt+ and Vt- of transition between sublinear and superlinear J-V-regions.Unipolar current-voltage curves (a) and drive voltage dependences of transition voltages and asymmetry ratio (b).
(a)(b)As a measure of J-V-curve asymmetry the ratioAR=Jm+/Jm- of the maximal (index “m”) current values of positive Jm+(Vda) and negative Jm-(Vda) branches was taken.TheVda dependences of AR and also Vt-/Vda and Vt+/Vda for initial and formed J-V-curves are presented in Figure 3(b). The values of AR for initial and formed J-V-curves are different at Vda<Vm+ and became near equal at Vda≥Vm+. Under increasing Vda the dependence AR(Vda) reaches the maximum at Vda≈|Vc-|. After forming AR(Vda) dependence has the smooth step with the start at Vm-, the centre near Vda≈Vc+, and the finish at Vm+.The difference inVt+(Vda) and Vt-(Vda) behaviour is displayed by Vda dependences of Vt+/Vda, and Vt-/Vda ratios (see Figure 3(b)). The equality of Vt+/Vda and Vt-/Vda is observed up to Vdm≈|Vc-| and the value of (Vt+-Vt-)/Vda reaches maximum at Vda≈Vm+.This behaviour of asymmetry ratioAR(Vda) and also transition voltages Vt+(Vda) and Vt-(Vda) of unipolar J-V-curves manifests the peculiarities in the coercive Vc- and Vc+ vicinities as bipolar Q-V-loops and in Vm+ and Vm- vicinities as bipolar I-V-loops.


## 3.3. Deformation-Voltage Loops

Figure4 presents the local deformation-voltage (X-V-) loops (PFM-amplitude) studied on the same Pt/PZT/Pt:Ti/SiO2/Si-substrate structure as were examined integral charge Q-V- and current I-V-loops of polarization reversal.PFM amplitude-voltage loops initial (a) and after forming by drive voltage sweeping with decreasing amplitude (b).
(a)(b)As it is seen from Figure4(a), the initial X-V-loop is remarkably asymmetrical. An apparent approach of the loop shape to a near symmetrical butterfly-like one (Figure 4(b)) was achieved by cycling under Vd sweeping with a decreasing from 10 V to zero amplitude (local forming procedure).So, the forming procedure results in similar symmetrization of corresponding characteristics related to integral and local PR processes.


## 4. Conclusion

The developed conception of asymmetry factors clearly shows the interconnection of polarization reversal asymmetry and asymmetry of electrical space charge transfer in ferroelectric Pt/PZT-film/Pt:Ti/SiO2/Si-substrate structures.The obtained results evidence the interrelation of charge transfer and polarization reversal processes in PZT-film on Si structures. It is assumed that charge transfer-polarization reversal interplay takes place at least in the case of polarization reversal assisted by charged domain wall displacement proper to PZT films [8].The physical reason of the observed consequences of the electrical treatment is the space charge transfer symmetrization through the film thickness under alternative voltage forming.The observed asymmetry of the investigated polarization reversal and current-voltage characteristics is developed in the conditions of the space charge transfer in time.



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