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

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.

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(Zr_xTi_1−x )O₃ film with x = 0.54 of 1-2 μm of thickness was deposited on the Pt/TiO_x/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 𝑉_𝑑 of 1 Hz of frequency in the amplitude range of 0≤𝑉_𝑑𝑎≤11 V.

To eliminate PR-current contribution, the examination of dynamic current-voltage characteristics (J-V-curves) was performed at unipolar saw-tooth drive voltage 𝑉_𝑑 with 0.5 s of durability and 1 Hz of repetitive frequency in the amplitude range of 0≤𝑉_𝑑𝑎≤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 of App-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 voltage 𝑉_𝑑 cycled in the range −10V≤𝑉_𝑑≤+10 V was applied to the bottom Pt electrode of the film. The amplitude of 𝑉_𝑑 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 of 𝑉_𝑑 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 Figures 1 and 2, respectively. Under increasing 𝑉_𝑑𝑎, 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 𝑉_𝑑𝑎=10 V that can be considered as forming.

(a)

(b)

Figure 1 

Bipolar charge-voltage loops of polarization reversal (a) and drive voltage dependences of charge asymmetry factors (b).

(a)

(b)

Figure 2 

Bipolar current-voltage loops of polarization reversal (a) and drive voltage dependence of capacitive asymmetry factor (b).

3.1.1. The Peculiarities of Bipolar Charge-Voltage Loops

Under increasing 𝑉_𝑑𝑎, 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 𝑉_𝑑𝑎 increasing. At 𝑉_𝑑𝑎=10 V, the coercive voltages 𝑉_𝑐 obtained from Q-V-loop are 𝑉⁻_𝑐=−3.4 V and 𝑉⁺_𝑐=+5.2 V. The vertical shift changes its sign (see Figure 1(a)) under decreasing 𝑉_𝑑𝑎 after forming at 𝑉_𝑑𝑎=10 V.

Figure 1(b) presents the 𝑉_𝑑𝑎dependences of charge asymmetry factors Δ𝑄_𝑖=(𝑄⁺_𝑖+𝑄⁻_𝑖)/2 considered as a measure of the vertical shift of Q-V-loop (𝑖=𝑚, 𝑟 correspond to maximal (𝑉_𝑑=𝑉_𝑑𝑎) and remanent (𝑉_𝑑=0) polarization, resp.). Under 𝑉_𝑑𝑎 increasing the positive Δ𝑄_𝑚 and Δ𝑄_𝑟 appear in 𝑉⁺_𝑐 vicinity. After forming under subsequent 𝑉_𝑑𝑎 decreasing the negative Δ𝑄_𝑚 and Δ𝑄_𝑟 disappear in 𝑉⁻_𝑐 vicinity. This behaviour of Δ𝑄_𝑚(𝑉_𝑑𝑎) and Δ𝑄_𝑟(𝑉_𝑑𝑎) shows that the vertical shift of Q-V-loops is connected with PR processes in the coercive 𝑉⁻_𝑐 and 𝑉⁺_𝑐 vicinities.

3.1.2. The Peculiarities of Bipolar Current-Voltage Loops

The observed I-V-loops are noticeably asymmetrical (Figure 2(a)). The absolute value of the voltage 𝑉⁺_𝑚 of positive current maximum 𝐼⁺_𝑚 is higher than that of the voltage 𝑉⁻_𝑚 of the negative current maximum 𝐼⁻_𝑚. At that, 𝐼⁺_𝑚 value is significantly higher than that of 𝐼⁻_𝑚. At 𝑉_𝑑𝑎=10 V, the voltages 𝑉_𝑚 obtained from I-V-loop are 𝑉⁻_𝑚=−3.7 V and 𝑉⁺_𝑚=+6.7 V.

The forming under 𝑉_𝑑𝑎=10 V results in the change of 𝑉⁺_𝑚 and 𝑉⁻_𝑚 values and increase of 𝐼⁺_𝑚 and 𝐼⁻_𝑚 values under subsequent 𝑉_𝑑𝑎 decrease (see Figure 2(a)).

Because the vertical (along I-axis) size of I-V-loop is proportional to the sample capacity 𝐶 [6], the value of Δ𝐼₀=(𝐼⁻₀+𝐼⁺₀)/2𝑉_𝑑𝑎 (where 𝐼⁻₀ and 𝐼⁺₀ are the currents at 𝑉_𝑑=0) can be considered as a measure of the capacitive asymmetry Δ𝐶₀=(𝐶⁻₀−𝐶⁺₀) of I-V-loop.

Figure 2(b) presents 𝑉_𝑑𝑎dependences of capacitive asymmetry factor Δ𝐶₀∝Δ𝐼₀. Under 𝑉_𝑑𝑎 increasing and its subsequent decreasing after forming the values of Δ𝐶₀ are near the same at 𝑉_𝑑𝑎<𝑉⁻_𝑚 and at 𝑉_𝑑𝑎>𝑉⁺_𝑚. However, in the range 𝑉⁻_𝑚<𝑉_𝑑𝑎<𝑉⁺_𝑚 not only the behaviour of Δ𝐶₀(𝑉_𝑑𝑎) but also Δ𝐶₀ signs are different before and after forming. At that, the maximal Δ𝐶₀ difference before and after forming is observed at 𝑉_𝑑𝑎≈|𝑉⁺_𝑐|=5.2 V. This behaviour of Δ𝐶₀(𝑉_𝑑𝑎) shows that the current asymmetry of I-V-loops is connected with PR processes in 𝑉⁻_𝑚 and 𝑉⁺_𝑚 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 (Figure 3(a)) under 𝑉_𝑑𝑎 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 𝑉⁺_𝑡 and 𝑉⁻_𝑡 of transition between sublinear and superlinear J-V-regions.

(a)

(b)

Figure 3 

Unipolar current-voltage curves (a) and drive voltage dependences of transition voltages and asymmetry ratio (b).

As a measure of J-V-curve asymmetry the ratio 𝐴_𝑅=𝐽⁺_𝑚/𝐽⁻_𝑚 of the maximal (index “𝑚”) current values of positive 𝐽⁺_𝑚(𝑉_𝑑𝑎) and negative 𝐽⁻_𝑚(𝑉_𝑑𝑎) branches was taken.

The 𝑉_𝑑𝑎 dependences of 𝐴_𝑅 and also 𝑉⁻_𝑡/𝑉_𝑑𝑎 and 𝑉⁺_𝑡/𝑉_𝑑𝑎 for initial and formed J-V-curves are presented in Figure 3(b). The values of 𝐴_𝑅 for initial and formed J-V-curves are different at 𝑉_𝑑𝑎<𝑉⁺_𝑚 and became near equal at 𝑉_𝑑𝑎≥𝑉⁺_𝑚. Under increasing 𝑉_𝑑𝑎 the dependence 𝐴_𝑅(𝑉_𝑑𝑎) reaches the maximum at 𝑉_𝑑𝑎≈|𝑉⁻_𝑐|. After forming 𝐴_𝑅(𝑉_𝑑𝑎) dependence has the smooth step with the start at 𝑉⁻_𝑚, the centre near 𝑉_𝑑𝑎≈𝑉⁺_𝑐, and the finish at 𝑉⁺_𝑚.

The difference in 𝑉⁺_𝑡(𝑉_𝑑𝑎) and 𝑉⁻_𝑡(𝑉_𝑑𝑎) behaviour is displayed by 𝑉_𝑑𝑎 dependences of 𝑉⁺_𝑡/𝑉_𝑑𝑎, and 𝑉⁻_𝑡/𝑉_𝑑𝑎 ratios (see Figure 3(b)). The equality of 𝑉⁺_𝑡/𝑉_𝑑𝑎 and 𝑉⁻_𝑡/𝑉_𝑑𝑎 is observed up to 𝑉_𝑑𝑚≈|𝑉⁻_𝑐| and the value of (𝑉⁺_𝑡−𝑉⁻_𝑡)/𝑉_𝑑𝑎 reaches maximum at 𝑉_𝑑𝑎≈𝑉⁺_𝑚.

This behaviour of asymmetry ratio 𝐴_𝑅(𝑉_𝑑𝑎) and also transition voltages 𝑉⁺_𝑡(𝑉_𝑑𝑎) and 𝑉⁻_𝑡(𝑉_𝑑𝑎) of unipolar J-V-curves manifests the peculiarities in the coercive 𝑉⁻_𝑐 and 𝑉⁺_𝑐 vicinities as bipolar Q-V-loops and in 𝑉⁺_𝑚 and 𝑉⁻_𝑚 vicinities as bipolar I-V-loops.

3.3. Deformation-Voltage Loops

Figure 4 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.

(a)

(b)

Figure 4 

PFM amplitude-voltage loops initial (a) and after forming by drive voltage sweeping with decreasing amplitude (b).

As it is seen from Figure 4(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 𝑉_𝑑 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.