Effect of post-deposition annealing on phase formation and

properties of RF magnetron sputtered PLZT thin films

Ravindra Singh a, T.C. Goel b, Sudhir Chandra a,*a Centre for Applied Research in Electronics, IIT Delhi, New Delhi 110016, India

b BITS Pilani, Goa Campus, Zuari Nagar, Goa 403726, India

Received 19 October 2006; received in revised form 20 February 2007; accepted 28 February 2007

Available online 6 March 2007

Abstract

In this work, we report the preparation of lanthanum-modified lead zirconate titanate (PLZT) thin films by RF magnetron

sputtering on platinized silicon (Pt/Ti/SiO2/Si) substrate. Sputtering was done in pure argon at 100 W RF power without external

substrate heating. X-ray diffraction studies were performed on the films to study the effect of post-deposition furnace annealing

temperature and time on the perovskite phase formation of PLZT. Annealing at 650 8C for 2 h was found to be optimum for the

preparation of PLZT films in pure perovskite phase. The effect of different annealing conditions on surface morphology of the films

was examined using AFM. The dielectric, ferroelectric and electrical properties of these films were also investigated in detail as a

function of different annealing conditions. The pure perovskite film exhibits better properties than the other films which have some

fraction of unwanted pyrochlore phase. The remanent polarization for pure perovskite film was found to be �29 mC/cm2 which is

almost double compared to the films having mixed phases. The dc resistivity of the pure perovskite film was found to be

7.7 � 1010 V cm at the electric field of �80 kV/cm.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Thin films; B. Sputtering; C. X-ray diffraction; D. Dielectric properties; D. Ferroelectricity

1. Introduction

Ferroelectric lead zirconate titanate (PZT) and lanthanum-modified PZT (PLZT) thin films have greatly attracted

researchers during recent years because of their potential applications in non-volatile memories, optical waveguide

devices, IR detectors, piezoelectric devices, sensors and actuators, electro-optic devices, microelectromechanical

systems (MEMS), etc. [1–6]. More recently, these films have shown great potential for biosensor applications [7]. For

the best ferroelectric/piezoelectric response, it is a requirement that these films should be prepared in pure perovskite

phase. Amongst the various methods, sol–gel and sputtering are the most attractive techniques for preparing these

films. Sol–gel technique has the advantage that the composition of the film can be easily tailored for optimum

performance [8]. Also, the pure perovskite phase can be easily achieved using this technique. However, the sputtering

process is more suitable for integration of these films with IC and MEMS fabrication technology [9]. Furthermore,

sputtering process also yields more reproducible and stable film properties. Therefore, these two techniques are

complimentary in developing the films for a given application.

www.elsevier.com/locate/matresbu

Materials Research Bulletin 43 (2008) 384–393

* Corresponding author. Tel.: +91 11 26591105; fax: +91 11 26596219.

E-mail address: schandra@care.iitd.ernet.in (S. Chandra).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2007.02.044

Preparation of these films in pure perovskite phase by sputtering process is a challenging task. In the processing of

these films, a common problem comes from the lead loss due to high deposition temperature [10]. The resulting non-

stoichiometry of the films produces the formation of the non-ferroelectric pyrochlore phases which degrade the film

properties. In the sputtering process, excess lead (upto 30%) is added in the target to compensate for the lead

volatilization during high temperature deposition [11]. In another technique, the lead loss is minimized by depositing

the films at a relatively low temperature followed by post-deposition annealing at high temperature in which lead

diffusion rather than evaporation is dominant [12,13]. However, in this process, pure perovskite phase formation

strongly depends on post-deposition annealing conditions. Therefore, it is essential to study the effect of annealing

temperature and time on the phase formation and stabilization of these films.

In the present work, PLZT thin films were prepared on Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. For

this purpose, a PLZT (8/60/40) target of 75 mm diameter was prepared in-house by conventional solid state reaction

route. Only 5% excess lead was added into the target which compensates the lead volatilization during high

temperature sintering of the target. The detailed process of the target preparation has been reported earlier [14]. The 8/

60/40 composition of the target was selected on the basis of our previous study on sol–gel prepared PLZT films [15].

The post-deposition furnace annealing temperature and time were optimized to prepare PLZT thin films in pure

perovskite phase. The effect of different annealing conditions on structural, morphological, dielectric, ferroelectric

and electrical properties of these films have been investigated.

2. Experimental work

2.1. Preparation of PLZT thin films

PLZT thin films were prepared on Pt/Ti/SiO2/Si substrates of 2-in. diameter by RF magnetron sputtering using the

synthesized PLZT target. The thickness of the PLZT films was kept 0.6 mm. The Ti (20 nm) and Pt (150 nm) films

were prepared by planar RF diode sputtering at 10 mTorr pressure in the same sputtering system. The SiO2 (1 mm

thickness) was grown by wet oxidation process at 1150 8C. The chamber was evacuated to 1 � 10�5 Torr pressure

before introducing the sputtering gas in the chamber. Sputtering of PLZT was carried out in argon by supplying 100 W

RF power at a frequency of 13.56 MHz. The sputtering pressure was maintained at 5 mTorr for all the depositions. No

external heating was provided to the substrate during the sputtering process. The as-deposited films were formed in

amorphous phase, as confirmed by XRD. Therefore, the films were subsequently annealed in the furnace in air ambient

to obtain the perovskite (ferroelectric) phase. The sputtering and post-deposition annealing parameters used for

preparing PLZT thin films are summarized in Table 1.

2.2. Characterization of the films

The thickness of the films was determined using surface profiler (Tencor Instruments) and was kept 0.6 mm for all

the films reported in the present work. A step was formed by lithography and etching of the film in 5% HF solution.

The structural and phase analysis were carried out on PW 1830 Philips X-ray diffractometer using Cu Ka

(l = 1.5405 A) radiation. The surface morphology and the root-mean-square (rms) surface roughness of the films were

studied using atomic force microscope (AFM). Top electrodes of Cr–Au of area 0.442 mm2 were sputter deposited for

R. Singh et al. / Materials Research Bulletin 43 (2008) 384–393 385

Table 1

RF magnetron sputtering and post-deposition furnace annealing parameters used for the preparation of PLZT films

Parameter Conditions

RF power 100 W

Sputtering gas Pure argon

Sputtering pressure 5 mTorr

Target-to-substrate spacing 40, 50 and 60 mm

Deposition temperature No external substrate heating (self-heating temperature reaches up to 155 8C)

Post-deposition furnace annealing

temperatures and times

� 600 8C for 1 and 5 h

� 650 8C for 1, 2 and 3 h

� 700 8C for 1 h

dielectric, ferroelectric and leakage current measurements. Dielectric properties of the films were measured using an

Impedance Analyzer (HP 4192 A). Polarization versus electric field hysteresis loops (ferroelectric properties) were

obtained using a Sawyer-Tower circuit-RT66A (Radiant Technologies Inc.). The leakage current (I–V) measurements

were performed on a computer interfaced Source Meter (Keithley 2410).

3. Results and discussion

3.1. Target-to-substrate spacing and self-heating of the substrate

Optimization of target-to-substrate spacing was carried out to prepare a uniform PLZT film with smooth surface

texture. Fig. 1 shows the deposition rates of PLZT thin films on a 2-in. wafer at different target-to-substrate spacings. A

reasonable value of the deposition rate with good uniformity (92 � 6 A) is obtained at 50 mm target-to-substrate

spacing. Therefore, we selected 50 mm target-to-substrate spacing to prepare the PLZT films for the present study. It is

well known that during the sputtering process, the substrate temperature rises due to self-heating in plasma [16]. To

exploit this phenomenon for substrate heating during sputtering process, the substrate holder was designed to

minimize the heat loss to the surroundings. This enabled the substrate temperature to rise to somewhat higher value

during film deposition. Fig. 2 shows the substrate temperature with time during the sputtering when no external heating

was provided. The substrate temperature rises gradually during deposition and stabilizes at 155 8C after 50 min.

R. Singh et al. / Materials Research Bulletin 43 (2008) 384–393386

Fig. 1. Deposition rates of PLZT thin films on a 2-in. wafer at different target-to-substrate spacings.

Fig. 2. Change in substrate temperature with time during the sputtering when no external heating was provided.

3.2. Structural properties and phase stabilization

In order to investigate the effect of post-deposition annealing conditions on phase formation of PLZT, the films

were annealed at different temperatures for varying durations in the furnace in air ambient. Fig. 3 shows XRD patterns

of as-deposited film and the films annealed at different temperatures for 1 h. It can be seen that as-deposited film

exhibits no crystallinity and was found to be completely amorphous in nature. After annealing, these films exhibited

crystalline structure with a mix of non-ferroelectric pyrochlore and ferroelectric perovskite phases. The film annealed

at 600 8C shows the low intensity peaks at 21.98, 31.18, 38.48, 44.78, 49.98, 55.38, 64.88 and 73.68 corresponding to

perovskite phase with some pyrochlore peaks at 29.88, 34.88 and 58.88 apart from the platinum peaks at 408, 46.58 and

67.88. In this case, the perovskite to pyrochlore ratio was estimated to be about 2:1 by comparing the intensities of their

peaks. As the annealing temperature was increased to 650 8C, the intensity of the peaks corresponding to perovskite

phase increased drastically while that of pyrochlore phase decreased. The perovskite to pyrochlore ratio increased to

about 10:1. Further increment in the annealing temperature to 700 8C resulted a drastic reduction in the intensity of the

perovskite peaks while the intensity of pyrochlore peaks again increased. In this case, pyrochlore phase is dominant

and the perovskite to pyrochlore ratio was calculated to be about 1:6. It can be concluded from these results that the

appearance of low intensity perovskite peaks along with pyrochlore phase at 600 8C annealing temperature may be due

to either insufficient energy required for perovskite phase formation or the deficiency of oxygen which arises at low

temperatures [13]. However, the formation of pyrochlore phase at 700 8C annealing temperature is governed by lead

loss which is observed during high temperature treatments of PLZT due to high volatility of lead.

Although the perovskite phase formation is enhanced by increasing the annealing temperature from 600 to 650 8C,

pure perovskite phase could not be obtained in 1 h annealing. Therefore, annealing for longer time was investigated to

obtain pure perovskite phase. Fig. 4 shows the XRD patterns of the films annealed at 600 and 650 8C for various time

durations. It is evident that the annealing at 600 8C even for 5 h is not sufficient for complete perovskite phase

formation. Although the perovskite to pyrochlore ratio increased to about 8:1 after 5 h annealing, the pyrochlore phase

was not completely removed. In the case of annealing at 650 8C, as the annealing time was increased from 1 to 2 h, the

pyrochlore peaks disappeared completely and the film was obtained in pure perovskite phase. Further increase in the

annealing time resulted in reappearance of pyrochlore peaks in which perovskite to pyrochlore ratio was found to be

about 7:1. This may be attributed to the lead loss during longer time annealing at higher temperature. Based on the

above results, the annealing at 650 8C for 2 h in air ambient was considered optimum for the preparation of PLZT films

in pure perovskite phase.

The pure perovskite film shows tetragonal structure which is consistent with the phase diagram of PLZT system for

the same composition [17]. The calculated lattice parameters of this film are as: a = 4.045 A and c = 4.076 A. The

R. Singh et al. / Materials Research Bulletin 43 (2008) 384–393 387

Fig. 3. XRD of PLZT films: (a) as-deposited, (b–d) annealed for 1 h at 600, 650 and 700 8C, respectively.

splitting of X-ray diffraction peaks such as (1 0 0)/(0 0 1), (1 1 0)/(1 0 1) doublets, which is the characteristic of

tetragonal structure, is not visible here. This may be due to limitation of the resolution of X-ray diffractometer which is

not able to resolve the fine structure of the film having low tetragonality (low c/a ratio of 1.008). For the precise

identification of film structure, the XRD analysis was carried out using ‘‘POWDMULT (Version 2.5)’’ software. All

the dobs., 2u and relative intensities were fed in the program and the different symmetries of structures were evaluated.

The assignment for a particular structure was selected and refined using the least-square refinement method, for which

standard deviation, Dd (=dobs. � dcal.) was found to be minimum. A good agreement between the observed and

calculated d-values suggests the correctness of the selected system and cell parameters.

3.3. Surface morphology

The effect of annealing conditions on surface morphology (grains distribution, their sizes and roughness) of the

films was investigated using AFM as shown in Fig. 5. It appears from the micrograph of as-deposited film that the

grains growth is in the early stage and therefore, the grains are not distinctly visible. This film shows average grain size

and rms roughness values of around 70 and 1.8 nm, respectively. As the films undergo post-deposition annealing

treatment, the grains growth accelerates and individual grains are more clearly visible. Also, the merging of grains to

form clusters starts appearing in the annealed films. The average grain size for the films annealed at 650 8C for 1, 2 and

3 h are 130, 162 and 168 nm, respectively, while that the rms roughness are 2.23, 2.46 and 2.58 nm, respectively. A

slight increment in both the grain size and roughness was observed with increasing annealing time. In the case of the

film annealed at 650 8C for 3 h, very small gaps between the clusters of grains are also visible. For the film annealed at

600 8C for 5 h, the average grain size and rms roughness were found to be 158 and 2.35 nm, respectively.

3.4. Dielectric, ferroelectric and electrical properties

The dielectric constant (er) and loss tangent (tan d) were measured as a function of frequency and temperature for

the films annealed under different conditions. The frequency variation of er and tan d at room temperature for these

films are shown in Fig. 6. In all the cases, dielectric constant decreases with frequency whereas loss tangent shows a

different behavior. Loss is found to be high at low frequencies, which decreases as the frequency increases upto

100 kHz. However, it again starts increasing at higher frequencies (100 kHz to 1 MHz). The decrease in loss tangent

with frequency is due to dielectric relaxation, which is common in ferroelectric materials. The observed high values of

loss tangent in the frequency range 100 kHz to 1 MHz may be attributed to the resonance effect. The stray induction of

the contact and leads induce L-C resonance {f r=1/H(LC)} [18]. Accordingly, for the films having capacitance of the

order of nanofarad (as in the present case), a small contact inductance of the order of few microhenry can induce

R. Singh et al. / Materials Research Bulletin 43 (2008) 384–393388

Fig. 4. XRD of PLZT films annealed for (a) 1 h, (b) 5 h at 600 8C, and (c) 1 h, (d) 2 h and (e) 3 h at 650 8C.

resonance near the MHz region [19]. For all films, the values of dielectric constant are lower than those reported for

bulk PLZT [20]. The low value of dielectric constant for thin films as compared to that for bulk samples is attributed to

the sub-micron grain size and stresses from the substrate. It can be seen from Fig. 6 that the dielectric constant of the

film increases significantly with an increment in annealing time from 1 to 2 h at 650 8C while the loss tangent

decreases drastically. This is consistent with the XRD results as the crystallization of the film improves after increasing

R. Singh et al. / Materials Research Bulletin 43 (2008) 384–393 389

Fig. 5. 3-D AFM of PLZT films: (a) as-deposited, (b–d) annealed at 650 8C for 1, 2 and 3 h, respectively, and (e) annealed at 600 8C for 5 h.

the annealing time from 1 to 2 h, showing the pure perovskite phase formation of PLZT. Further increase in the annealing

time upto 3 h results in a drastic reduction in the dielectric constant (almost half the value of the film annealed for 2 h)

while the loss tangent again increases. We attribute this to the reappearance of pyrochlore phase. As discussed in the

subsequent section, this film exhibits higher leakage current density compared to the films annealed for 1 and 2 h, which

may also be responsible for low dielectric constant and high loss tangent values. The film annealed at 600 8C for 5 h

exhibits lower dielectric constant compared to the films annealed at 650 8C which we attribute to weak crystallization

(low intensities XRD peaks) and incomplete perovskite phase formation. The er and tan d values of the pure perovskite

film (annealed at 650 8C for 2 h) closely matches with the values reported by Thomas et al. for PLZT (8/65/35) films

deposited by sol–gel technique [21]. Although they observed a sudden decrease in er after 100 kHz, the present film shows

the constant dispersion in er over the whole frequency range (100 Hz to 1 MHz). However, tan d shows similar behavior in

both the cases and increases sharply after 100 kHz. Also, the present film has shown better dielectric properties than the

film of same composition prepared by sol–gel technique in our previous work [15].

The temperature dependence of er and tan d at 10 kHz are shown in Fig. 7. The measured room temperature values

of er and tan d at 10 kHz for the films annealed in different conditions are given in Table 2. The dielectric constant

R. Singh et al. / Materials Research Bulletin 43 (2008) 384–393390

Fig. 6. Frequency dependence of dielectric constant and dielectric loss of PLZT films annealed under different conditions.

Fig. 7. Temperature dependence of dielectric constant and dielectric loss of PLZT films annealed under different conditions.

increases with increasing temperature and shows a maximum value around 100, 80, 120 and 100 8C for the films

annealed for 1, 2 and 3 h at 650 8C, and 5 h at 600 8C, respectively. These maxima may be attributed to ferroelectric to

paraelectric transition at Curie temperature (Tc) and are fairly broad which is common in ferroelectric ceramics in thin

film form [22,23]. The possible reason for the absence of a well-defined sharp Curie transition peak in these materials

could be grain size variations and diffuse phase transition. The variation of grain size may lead to variation in stress

produced in the material and may effect the ferroelectric to paraelectric transition. The diffuse phase transition in the

materials may be due to the marginal structural disorder and compositional fluctuations. In the case of the films

annealed at 650 8C for 1 and 2 h, the behavior of loss tangent as a function of temperature also seems to be similar as

that of dielectric constant. The temperature difference between the peak dielectric constant and the peak dielectric loss

is the consequence of the Kramers-Kronig relations [17]. However, the loss tangent increases drastically at higher

temperatures in the case of the films annealed at 650 and 600 8C for 3 and 5 h, respectively.

Fig. 8 shows the polarization versus electric field hysteresis loops of the PLZT films annealed under different

conditions. The loops clearly show the ferroelectric nature of PLZT films. The saturation polarization (Ps), remanent

polarization (Pr) and coercive field (Ec) values of the films are summarized in Table 2. It is found that as the film attains

pure perovskite phase after increasing the annealing time from 1 to 2 h at 650 8C, the Pr value of the film increases

dramatically from 14.2 to 28.8 mC/cm2. However, further increment in the annealing time upto 3 h results in a sharp

reduction in Pr value (upto almost the same value as of the film annealed for 1 h). This we attribute to reappearance of

pyrochlore phase as seen in Fig. 4(e). This film also shows somewhat lossy type (‘football’ shaped) hysteresis loop,

which may be due to the higher leakage current density in this film (as shown latter) compared to the films annealed for

1 and 2 h. The Pr value for the film annealed at 600 8C for 5 h is found to be 10 mC/cm2, which is lowest amongst all

R. Singh et al. / Materials Research Bulletin 43 (2008) 384–393 391

Table 2

Values of various parameters of PLZT thin films annealed under different conditions

Parameters PLZT films annealed under different conditions

650 8C for 1 h 650 8C for 2 h (pure perovskite film) 650 8C for 3 h 600 8C for 5 h

Room temperature er at 10 kHz 336 421 275 253

Room temperature tan d at 10 kHz 0.078 0.031 0.034 0.029

Tc (8C) 100 80 120 100

Ps (mC/cm2) 24.3 49.4 – 21.8

Pr (mC/cm2) 14.2 28.8 13.8 10.0

Ec (kV/cm) 105 70 98 82

J (10�7 A/cm2) at �80 kV/cm 4.6 10.5 15.0 1.9

Fig. 8. Polarization vs. electric field hysteresis loops of PLZT films annealed under different conditions.

the films investigating in the present work. The low Pr value of this film is consistent with its XRD and dielectric

properties. The Ec value is found to be minimum (�70 kV/cm) for the pure perovskite film (annealed at 650 8C for

2 h). The Pr value of pure perovskite film is very close to the reported value for bulk PLZT (7/60/40) ceramic [24].

However, the Ec value is higher in the film compared to that of bulk ceramic. The increase in Ec value is attributed to

the small grain size in the films and the space-charge layers at the electrodes of the thin films. The Pr value (28.8 mC/

cm2) of pure perovskite film prepared in the present work is very much higher compared to the value reported by Cao

et al. (�14 mC/cm2) for PLZT (5/60/40) and Thomas et al. (�6 mC/cm2) for PLZT (8/65/35) films deposited by sol–

gel technique [25,21]. This is also higher than the reported Pr value (�22 mC/cm2) by Lu et al. for PLZT (7.5/65/35)

film deposited by RF magnetron sputtering [26]. Our Pr value is very close to that reported by Park et al. (30 mC/cm2)

for PLZT (3/52/48) film prepared by photochemical metal-organic deposition [27]. The Pr value of the present film is

also very much higher compared to the film of same composition prepared by sol–gel technique in our previous work

[15].

In order to study the conduction mechanism and dc resistivity of these films, I–V measurements were performed on

metal insulator metal (MIM) structure. Fig. 9 shows the leakage current density (J) as a function of applied voltage for

the films annealed under different conditions. After applying a voltage, the leakage current is found to decay with time

before it stabilizes in a few seconds. Therefore, each measurement was taken after a delay of 10 s that was initially fed

in the computer interfaced with the setup. All the films exhibit a low leakage current density of the order of 10�6 A/cm2

upto the field strength of around�80 kV/cm. As the annealing time increases from 1 to 3 h at 650 8C, an increase in the

leakage current density is observed at higher voltages. As seen by AFM, the formation of clusters by merging of grains

increases with increasing annealing time. This result in the generation of small gaps between the individual clusters,

which are clearly visible in the AFM of the film annealed at 650 8C for 3 h. The gaps between clusters may act as

defect centers and trap charge carriers which lead to higher leakage current in the film. The gaps between the

individual clusters in the film annealed at 650 8C for 3 h are more compared to those of other films, which we believe is

responsible for higher leakage current in this film. In the case of the film annealed at 600 8C for 5 h, the gaps between

the clusters are less than those of the film annealed at 650 8C for 3 h. This could be a reason that the leakage current in

the film annealed at 600 8C for 5 h is lower compared to that of the film annealed at 650 8C for 3 h. Also, the perovskite

to pyrochlore ratio is 8:1 in the film annealed at 600 8C for 5 h which is slightly higher than that of the film annealed at

650 8C for 3 h. This may also be responsible for low leakage current in the film annealed at 600 8C for 5 h.

In all the cases, the current follows ohmic behavior (J / V) at low fields and square-law (J / V2) dependence in the

higher field region. On further increasing the voltage, a region of J / Vn, with n upto 6 was also observed. This nature

of log (J) versus log (V) plots is the characteristic of materials having single-carrier space charge limited (SCL)

injection current flow mechanism controlled by shallow trap levels which are distributed in energy, as described by

Kao and Hwang [28]. The slight changes in the shape of the I–V curves for the films annealed under different

conditions can be understood on the basis of different trap charge carrier densities in these films. The dc resistivity of

R. Singh et al. / Materials Research Bulletin 43 (2008) 384–393392

Fig. 9. Leakage current density as a function of dc voltage of PLZT films annealed under different conditions. The thickness of the films is 0.6 mm.

the pure perovskite film, calculated from I–V measurements, was found to be 7.7 � 1010 V cm at electric field of

80 kV/cm.

4. Conclusions

Ferroelectric perovskite PLZT thin films were prepared on Pt/Ti/SiO2/Si substrate by RF magnetron sputtering. The

phenomenon of self-heating in plasma has been exploited for substrate heating during the sputtering process in which

substrate temperature reached upto 155 8C without external heating. The effect of post-deposition furnace annealing

temperature and time on perovskite phase formation of PLZT films was investigated. The annealing at 650 8C for 2 h

in air ambient was found to be optimum for the preparation of the films in pure perovskite phase. The pure perovskite

film exhibited better dielectric and ferroelectric properties than the other films, which have small fractions of

pyrochlore phase along with perovskite phase. The room temperature dielectric constant and loss tangent of pure

perovskite film at 10 kHz were found to be 421 and 0.031, respectively. The pure perovskite film exhibited excellent

remanent polarization value of around 29 mC/cm2 which is almost double in comparison to other films and amongst

the few best reported values for PLZT films. The dc resistivity of the pure perovskite film was found to be

7.7 � 1010 V cm at the electric field of 80 kV/cm which is reasonably good for device applications. Hence, all the

properties of pure perovskite PLZT film prepared in the present work meet the requirements for its applications in

many devices such as non-volatile memories, MEMS, biosensors, etc.

Acknowledgements

This work was carried under a Sponsored Research Project of DRDO, Government of India. The authors would like

to thank Electroceramics group and Dr. Balakrishnan of Solid State Physics Laboratory, New Delhi for providing the

facility of hysteresis loop and IV measurements, respectively.

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