chapter 7 - physical characterization on a nanometer scaleof the spm tip during pfm on electrostatic...

CHAPTER 7

Physical Characterizationon a Nanometer Scale7.1 Piezoresponse ForceMicroscopy (PFM)

Nina Balke*, Tony Schenk†, Igor Stolichnov‡, Alexei Gruverman§*Oak Ridge National Laboratory, Oak Ridge, TN, United States†NaMLab gGmbH, Dresden, Germany‡Nanoelectronic Devices Laboratory, Ecole Polytechnique F�ed�erale de Lausanne (EPFL), Lausanne,Switzerland§Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, NE, United States

7.1.1 Introduction

The rise of piezoresponse force microscopy (PFM) as a characterization

technique came about in the 1990s when multiple groups started to use this

technique to image and manipulate ferroelectric domains in ferroelectric

materials [1–8]. PFM is detecting the electromechanical material response

when locally biased with a conductive scanning probe microscopy (SPM)

tip. The response is based on the inverse piezoelectric effect, which connects

the electric field with a change in sample deformation through the piezo-

electric coefficient d. The PFM tip supplies the electric field and detects

the sample deformation locally through a change in cantilever deflection

(out-of-plane PFM) or cantilever torsion (in-plane PFM), which is mea-

sured by a reflected laser and a photo detector (Fig. 7.1.1A). PFM is a

dynamic characterization technique, which means oscillating AC voltages

are applied at a certain frequency and the resulting deformation change is

measured using lock in-based approaches. The result is information about

the local strength of the effective piezoelectric coefficient deff, the

so-called PFM amplitude, the phase shift between the applied field and sam-

ple deformation, and the PFM phase, indicating the domain orientation

(Fig. 7.1.1B). Additional DC voltages can be introduced to manipulate

the orientation of ferroelectric domains. In this case, the DC voltage can

be applied at the same time as the probing AC voltages or sequentially. These

cases are distinguished in the so-called on-field or off-field loops where the

291Ferroelectricity in Doped Hafnium Oxide © 2019 Elsevier Ltd.https://doi.org/10.1016/B978-0-08-102430-0.00014-0 All rights reserved.

https://doi.org/10.1016/B978-0-08-102430-0.00014-0

Fig. 7.1.1 (A) Basic principles of the PFM set-up probing local domains oriented up anddown. (B) PFM amplitude and phase along the dotted line.

292 Ferroelectricity in Doped Hafnium Oxide

first probes the bias-induced domain state and the latter probes the remanent

domain state. Because the tip radius is in the range of tens of nm, PFM allows

for imaging and switching of local ferroelectric domains, making it possible

to identify local variations originating from changes in structure, chemistry,

or functionality. This is not available with other techniques and is highly

important for device miniaturization and structure-function relationships.

In recent years, PFM has become a well-established and commercially

available characterization technique that is used for a variety of electrome-

chanically active materials such as thin films and ceramics [9–15], but alsopolymers [16–21], and biological materials [22–26]. At the same time,

PFM developed into a multidimensional characterization platform using

sophisticated excitation principles such as multifrequency approaches utiliz-

ing contact-resonance enhancement [27–29], ultrafast imaging [30], liquid

environments [31–33], or coupled interferometric approaches [34, 35].

These developments enabled PFM to be used to characterize samples push-

ing the boundaries of ferroelectricity, including ultrathin materials [36–42],heterostructures [43], and strain-induced ferroelectricity [44–46]. Here, the

presence of PFM hysteresis loops in which the electromechanical response is

measured as a function of applied DC bias, Vdc, is often interpreted as an

unambiguous indicator of ferroelectricity together with PFM images of fer-

roelectric domains before or after poling [47–52]. However, similar to clas-

sical P-E measurements [53, 54], PFM hysteresis loops can originate from a

number of alternative mechanisms [55]. For example, electrostatic interac-

tions between the tip and sample [56] and hysteretic surface charging [57, 58]

or ionic mechanism [43, 59–62] can lead to electromechanical hysteresis as

well. The same is true for domain images after poling. Charge writing and

strong electrostatic tip-sample interactions can appear as written domains in

293Physical Characterization on a Nanometer Scale

PFM [58]. Therefore, it was pointed out that it is crucial to understand dif-

ferent signal origins to correctly interpret PFMmeasurements. One approach

is to understand and explore the DC and AC voltage dependence of ferro-

electric behavior and perform additional measurements to explore the mea-

sured electromechanical response. This includes recent developments of

contact Kelvin probe force microscopy (KPFM), the comparison of on- and

off-field PFM hysteresis loops, loop shape as a function of the DC voltage

window, and AC probing voltage above coercive voltages [63].

7.1.2 PFM on Bare HfO2/ZrO2-Based Thin Films

Due to the high spatial resolution, PFM has been implemented recently to

explore local ferroelectric properties of mostly doped HfO2 thin films, which

cannot be done with macroscopic measurements. Therefore, the use of PFM

is very attractive to study the effect of doping, crystallographic structure, or

wake-up effects. The local electromechanical response is either probed

directly on the bareHfO2 surface [64–77] or on top capacitors [64, 65, 77–79].On bare oxide film surfaces, PFM image contrast and PFM hysteresis

loops are used as indicators of ferroelectric material properties. Some work

shows a strong PFM phase contrast, but most PFM amplitude hysteresis

loops do not show saturation for high DC voltages, as is the norm for fer-

roelectric materials when probed by PFM. The PFM amplitude loops typ-

ically present themselves as shifted V-shaped curves indicating a large linear

signal contribution, which cannot come from piezoelectricity that should be

voltage independent once the domains are oriented [80]. In this case, a

detailed study of the signal contributions is necessary. However, no work

so far has gone beyond simple PFM characterization to explore the origin

of the PFM signal on bare HfO2 thin films. Especially important here is

the tracking of bias-induced topographical changes, which can be observed

routinely on sub-10 nm thin ferroelectric films measured in air [81], electro-

static signal contributions as has been demonstrated for lithium niobate crys-

tals [80], or the amount of change in surface charges by performing KPFM

and PFM in the same location.

Another approach is to look at the expected quantitative numbers for

measured displacement. Typical values for the piezoelectric constant of

doped HfO2 thin films of �10 nm thickness are around 10 pm/V, as

obtained macroscopically from double-beam laser interferometry on capac-

itor structures of several 100 μm in diameter [82, 83]. This number is rela-

tively low in comparison to classical ferroelectrics [84, 85] and is the reason

why PFMmeasurements are often performed utilizing the contact resonance


enhancement. If the electrical contact between the SPM tip and sample is esti-

mated simply as a parallel-plate capacitor and an AC voltage of 1 V is applied,

the expected expansion is 10 pm. This has to be considered the upper limit of

the estimate due to asymmetric field distributions, mechanical clamping, and

already dead layer effects. In recent SPM studies focusing on the displacement

of the SPM tip during PFM on electrostatic forces alone, it was found that

typical displacements for standard PFM tips with a stiffness of �4 N/m and

a free resonance of 75 kHz are around 1–2 pm on amorphous, nonferroelec-

tric HfO2 thin films [86]. This value will depend strongly on the surface

potential present. This estimate is problematic, showing that electrostatic

forces can be as large as or even dominate the piezoelectric signal contribution

in doped HfO2 when probed with PFM.

In order to reduce the electrostatic tip-sample interactions, the use of stif-

fer cantilevers becomes logical. Cantilevers with a higher force constant

result in a higher tip-sample contact stiffness k*, which is inversely propor-

tional to the AC deflection changeDac through the electrostatic force acting

during dynamic PFM measurements, according to Eq. (7.1.1) [81].

Dac ¼ k∗�1C0VacVdc (7.1.1)

Here, C0 indicates the capacitance gradient, and Vac and Vdc are the applied

voltages during PFM voltage spectroscopy in analogy to traditional noncon-

tact KPFM signal descriptions [87]. While a stiff cantilever reduces the elec-

trostatic signal contribution, the piezoelectric signal contribution should

remain because it is a material property and should be independent of the

tip used if the applied fields are contact forces are comparable. While this

approach works for classical ferroelectrics such as PZT, it does not work

on bare HfO2 thin films, as shown in Fig. 7.1.2.

For a 50-nm-thick Pb(Zr,Ti)O3 (PZT) thin film, the PFM hysteresis

loop depends only a little on the stiffness of the cantilever. Both the soft can-

tilever (k�4 N/m) and the stiff cantilever (k�35 N/m) produce very com-

parable ferroelectric hysteresis loops when normalized by the sensitivity in

[nm/V] obtained through static force-distance curves (Fig. 7.1.2A). The

same is not true for doped HfO2 films where the signal is strongly reduced

with the stiff cantilever (Fig. 7.1.2B). This indicates a large electrostatic sig-

nal contribution in the case of HfO2, which is also obvious from looking at

the contact resonance that was used to perform these experiments. The tip-

sample contact resonance is described as a simple harmonic oscillator with a

180 degree (π) phase shift across the contact resonance frequency [88].

While this is measured for the soft cantilever on both PZT andHfO2, it does

–3 –2 –1 0 1 2 3

–10

–5

0

5

10

15

Dac

]mp[

Vdc

[V]

stiff soft

on-field, Sr:HfO2

using S

–6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6

–40

–20

0

20

40D

ac]

mp[

Vdc

[V]

stiff soft

on-field, PZT

720 740 760 780 800–4

–3

–2

–1

0

1

2

3

4

Dac

]dar[ esahp

f [kHz]

Vdc,max

Vdc,min

PZT, stiff

p

660 670 680 690 700 710–4

–3

–2

–1

0

1

2

3

4

Dac

]dar[ esahp

f [kHz]

Vdc,max

Vdc,min

Sr:HfO2, stiff

p

(A) (B)

(C) (D)Fig. 7.1.2 Comparison of PFM on-field hysteresis loops measured using contactresonance enhancement on (A) 50 nm PZT and (B) 10 nm Sr-doped HfO2 with a softtip and a stiff tip. Phase jumps obtained with the stiff tip on (C) 50 nm PZT and(D) 10 nm Sr-doped HfO2 around the resonance frequency at maximum andminimum applied VDC.


not hold true for the stiff cantilever. While the cantilever is driven purely by

the changes in sample volume in the case of PZT through the piezoelectric

effect showing the expected phase profiles (Fig. 7.1.2C), the phase across the

contact resonance for HfO2 delivers values below the expected 180 degree

phase shift (Fig. 7.1.2D). This can be explained by electrostatic driving forces

that are not strong enough to excite the complete contact resonance in the

case of the stiff cantilever.

Another approach to minimize the electrostatic signal contribution is the

measurements through top electrodes in the capacitor structures instead of

the bare oxide film surface. In general, the difficulty of studying bare oxide

films is the unknown surface condition that affects PFM as a surface-sensitive

technique. The existence of dead layers [89–92] or changes in surface-near

chemistry [93] can alter the measured PFM response. In addition, the role of


the measurement environment, for example, humidity and its effects on fer-

roelectric switching [94], is rarely considered and might prevent standard

PFM measurements on HfO2 thin films. Fig. 7.1.3 demonstrates the differ-

ence in ferroelectric hysteresis loops when measured on and off a capacitor

structure for 10-nm-thick Sr-doped HfO2.While the response is mostly lin-

ear in the on-field loop (Fig. 7.1.3A) with little hysteresis in the off-field loop

(Fig. 7.1.3B), the identical measurements on the capacitor show saturated

ferroelectric hysteresis loops and reduced electrostatics, as evident from

the differential loops [95] (Fig. 7.1.3C). By plotting the hysteresis loops

on the same scale, it can be seen that the measurement on the oxide only

changes around one of the polarization states (Fig. 7.1.3A). Ferroelectric

switching is not induced by the biased SPM tip. The reasons for this behavior

are unknown and require more detailed studies in the future.

Therefore, capacitor structures are utilized to minimize the impact of

artifacts in the next section, which is devoted to exploring the evolution

of domain patterns during polarization switching in hafnia-based thin films.

When attempting such studies, it should be kept in mind that the thicker the

electrode, the more it impairs the lateral resolution. The thinnest possible

electrodes are favorable as long as they can provide sufficient displacement

currents to ensure that the whole capacitor area follows the potential curve

given by the external excitation signal.

7.1.3 PFM on HfO2/ZrO2-Based Thin Film Capacitors

On a microscopic level, polarization reversal occurs via the nucleation and

growth of a large number of domains. The dynamic characteristics of

domain growth as well as the static properties of domain structure to a large

extent determine the ferroelectric device performance. Application of PFM

provided a breakthrough in understanding the static and dynamic character-

istics of the ferroelectric films via nanoscale visualization of the electrically

induced evolution of domain structures [96, 97]. One of the key features of

PFM is its capability to visualize the domain structure not just on a free fer-

roelectric surface, but also through the top electrode [98–100], which pro-

vides a unique possibility for investigation of the domain structure dynamics

in the ferroelectric capacitors emulating the real device conditions.

In PFM imaging of the ferroelectric capacitors, the probing tip is in con-

tact with the deposited top electrode. Although in this case the whole vol-

ume underneath the electrode is electrically excited, the electromechanical

response is still probed locally. Scanning the electrode surface while

–3 –2 –1 0 1 2 3

–4

–2

0

2

4

6

8

Dac

[pm

]

Vdc [V]

Cap Oxide

On-field

–3 –2 –1 0 1 2 3

–2.0–1.5–1.0–0.50.00.51.01.52.0

Dac

]mp[

Vdc [V]

Cap Oxide

Off-field

–3 –2 –1 0 1 2 3

–6

–4

–2

0

2

4

6

Dac

[pm

]

Vdc [V]

Cap Oxide

Differential

(A) (B) (C)Fig. 7.1.3 Comparison of PFM hysteresis loop measurements on bare oxide surface versus top electrodes (25 nm Pt, 10 nm Ti, 10 nm TiN) of10 nm-thick Sr-doped HfO2. Shown are (A) on-field loops, (B) off-field loops, and (C) differential loops.

297PhysicalC

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measuring the local strain provides spatially resolved information on the

domain structure underneath the electrode. The external bias applied to

the top electrode generates a uniform electrical field within the capacitor

so that the PFM tip senses the response from the whole thickness of the fer-

roelectric layer. Thus, this approach allows not only alleviating a problem of

electrostatic contribution to the PFM signal, but also getting around a prob-

lem of an inhomogeneous field distribution generated by the probing tip in a

film without a top electrode. As such, this approach has a significant advan-

tage over conventional electrical testing of the switching behavior as it

allows direct assessment of the relative contribution of nucleation and

domain wall motion into the polarization reversal process, the field-

dependent motion of domain walls, and the capacitor-size effect on its

switching behavior [97, 100–111].However, the application of PFM to capacitor structures is not without

certain challenges. For example, the lateral resolution w, determined as the

domain wall image profile, linearly scales with the thicknesses of the ferro-

electric layer tFE and the top electrode L. For typical ferroelectric film

parameters and a relatively thin top electrode (L≪ tFE), the resolution is

expected to be w�0.2tFE, which presents the ultimate limit on PFM reso-

lution in capacitors [112]. Nevertheless, it has been shown that domain

imaging can even be performed through top electrodes as thick as

250 nm [113]. Furthermore, this approach cannot be used for the investiga-

tion of domain wall interaction with microstructural features, such as defects

and grain boundaries, which calls for complementary PFM studies on the

free surfaces or cross-sectional samples.

Additional limitations have to be considered when using the tip to drive

an AC excitation voltage between the top and bottom electrode of a capac-

itor. Often, top electrodes are prepared by, for example, deposition through

a shadow mask, and therefore they exhibit dimensions of 50�50 μm2 and

more. This represents an upper limit for the tolerable RC delay resulting

from the resistance R and capacitance C of the circuit. The resistance R

of the tip and the tip-surface contact is typically on the order of 10 MΩ,while C is given by C ¼ ε0εrA/tf, with the vacuum permittivity ε0, the rel-ative permittivity εr, the capacitor areaA, and the electrode distance tf. Rely-

ing on the resonance enhancement techniques mentioned earlier usually

requires driving frequencies of >100 kHz, which equals a maximum time

constant of τmax ¼ RC ¼ 10 μs. This limits the maximum allowed capaci-

tance to a value of 1 pF, or better 0.1 pF to be on the safe side. Otherwise,

the voltage on the capacitor plates cannot follow the intended signal as


supplied by the voltage source. Taking the example of a relative permittivity

of 300 for PZT and 30 for HfO2 and a film thickness of 10 nm, the resulting

maximum top-electrode dimensions are 600�600 nm2 and 2�2 μm2,

respectively. It becomes clear that even for larger thicknesses of 100 nm

or even 1 μm, the simple capacitors from a shadow mask process cannot

be properly driven to guarantee the required excitation for resonance-

enhanced PFM. Even off-resonance measurements relax the RC-time issue

only by about one order of magnitude (resonance frequencies of several tens

of kHz), which is still insufficient.

This RC-time issue can be circumvented by using an external probe

[114] to establish a low-resistance electrical contact with the top electrode

the electric field application while using the PFM tip only as a sensor to

detect the local electromechanical displacement (Fig. 7.1.4).

Fig. 7.1.5 illustrates a PFM investigation of the switchability of La:HfO2-

based capacitors as a function of the wake-up cycling via domain visualiza-

tion through the top electrode. A 300�300 μm2 capacitor has been precon-

ditioned by the application of 103 and then 104 alternating voltage cycles

with the amplitude of 3 V and frequency of 100 Hz. The result of the

600

550

500

450

400

350

300

mm

250

200

150

100

50

00 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850

mm

Fig. 7.1.4 An optical image showing an external macroscopic probe and a cantileverused for PFM testing of large ferroelectric capacitors.

Fig. 7.1.5 (A–C) PFM amplitude and phase images of the domain structures appearingafter application of a single +4 V, 100 ms voltage pulse in the pristine La:HfO2 capacitor(A) and the same capacitor subjected to AC training by 104 (B) and 105 (C) cycles.(D) Corresponding P-V loops acquired in the pristine and AC trained capacitor.


wake-up procedure is tested by applying a single +4 V, 100 ms voltage pulse

followed by the PFM imaging through the top electrode and performing a

comparative analysis of the observed domain patterns in the pristine and

trained capacitors (Note that the top electrode morphological features do

not affect the PFM domainmaps.) It can be seen that the single domain state1

cannot be achieved in either case. However, the volume of the residual

domains appearing with the dark contrast in the PFM phase images is

decreasing with an increase in the number of the AC cycles. The same ten-

dency is observed in the case that poling is done by the negative pulses, sug-

gesting that the AC training leads to the relaxation of the trapped charges at

both interfaces that pin the polarization, which is in accordance with what

has been deduced from other electrical and structural techniques (see also

Chapter 9.2) [115–117]. The corresponding P-V loops illustrate the effect

of the wake-up cycling from the macroscopic point of view. By bridging

the data sets obtained at different length scales, it is possible to prove

unequivocally that the transition from the pinched to the open-shape

P-V loop is caused by the decrease in the volume of the pinned domains.

Generally, the application of PFM to the investigation of domain switch-

ing dynamics is limited by its low time resolution determined by the acqui-

sition time for a single frame (of the order of several minutes). A high-speed

1The term “single domain state” is used for the sake of brevity. In the narrowest sense of the

word “domain,” a polycrystalline film can never exhibit a single domain state. Here it is used

in a wider sense referring to all grains being switched to the same of the two possible sat-

urated phase angles as a reflection of the effective polarization direction when projecting the

polarization onto the direction of the DC electric field.


version of PFM (HSPFM) has been developed by Huey’s group to allow

complete image acquisition in several seconds, thus increasing time resolu-

tion by two orders of magnitude over the conventional PFM imaging [118].

This approach, which involves high-speed scanning of a bare ferroelectric

surface with a tip under a superposition of a switching and imaging bias,

allows effective studies of the dynamics of domain nucleation and growth

but requires relatively smooth surfaces.

The development of a stroboscopic PFM (S-PFM) method extended the

time resolution of PFM imaging into the sub-100 ns range [109]. This

method is based on visualization of domain configurations developing in fer-

roelectric capacitors during step-by-step polarization reversal. Switching

characteristics such as nucleation rate and domain wall velocity can be cal-

culated from a set of PFM snapshots taken at different time intervals by mea-

suring the time dependence of the number and size of growing domains.

The time resolution of the S-PFM method is determined by the rise time

and duration of the switching pulses and, depending on the capacitor size

and time constant of the external circuitry, can be on the order of 10 ns.

Two main modifications of the stroboscopic PFM method have been

suggested.

In one approach, the domain switching behavior is visualized by apply-

ing a series of short input pulses with fixed amplitude and incrementally

increasing duration (τ1 < τ2 <… < τn < tsτ < ts, where ts is a switching

time for a given voltage) [104]. PFM imaging of the resulting domain pattern

is performed after each pulse. At the beginning of each switching cycle, the

capacitor is reset into the initial polarization state. The applicability of this

approach depends upon the reproducibility of domain switching kinetics

from cycle to cycle. Indeed, the deterministic nature of domain nucleation

has been shown earlier in bulk crystals of lead germanate by means of optical

stroboscopy [119]. The same behavior has been observed at the nanoscale

level in ferroelectric thin film capacitors via S-PFM studies: in each switch-

ing cycle, domain nucleation occurs in the predetermined sites most likely

corresponding to the local defects at the film-electrode interface [111]. In

epitaxial PZT capacitors, nucleation probability is above 90% while in poly-

crystalline capacitors it is close to 100%.

Another approach was proposed to reduce the detrimental effect of sto-

chastic nucleation events in epitaxial structures after a capacitor is being set.

Here, the switching pulses of the same duration are applied to the capacitor

(τ1 ¼ τ2 ¼… ¼ τn < ts) with PFM imaging between the pulses [120]. In

this case, it is assumed that the PFM image obtained after the n-th pulse


is the same as that after a single pulse with duration of t ¼ τ1 + τ2 +… + τn.Then, all the PFM images taken before the (n+1)th pulse reveal the succes-

sive domain wall evolution during the time period of t.

Both variations of the S-PFM method rely on the stability of instanta-

neous domain patterns between pulse applications. Whether polarization

relaxation takes place or not can be checked by comparing the PFM switch-

ing with the transient current measurements. Little or no discrepancy

between the two sets of data obtained in most reports is a solid proof of

the reliability of the S-PFM approach [121].

An example of S-PFM imaging is illustrated in Fig. 7.1.6. It shows snap-

shots of domain structure evolution during switching in La:HfO2-based

capacitors. Switching as a whole occurs via lateral growth of the residual

domains as well as nucleation of new domains. It is interesting to mention

that the size of the nuclei is usually about 100 nm, which is larger than the

average grain size (�30 nm). The time-dependent evolution of the domain

structure reveals that the wall velocity varies depending on the azimuthal

direction, possibly reflecting the effect of the grain boundaries. In addition,

some domains exhibit an increase in their PFM amplitude response without

a change in their lateral dimensions, which might be an indication of either

polarization vector rotation or local phase transition induced by the

electric field.

The use of capacitor structures as well as the correlation of integrated

transient current to the phase image obtained from PFM is already very con-

vincing [114]. However, to overcome any potential remaining doubts about

ferroelectricity as the dominating signal origin, the next section uses nonre-

sonant PFM. This helps to rule out the artificial enhancement of a wealth of

signal origins that results from the resonant excitation of the cantilever and

the related outstanding sensitivity. Considering the historical development

of PFM, this appears as a step back while it represents an essential step for-

ward in terms of clarity of result interpretation, as will be shown.

7.1.4 Frequency-Independent (Nonresonant) PFM: A NewPotential of the Classic Approach

The recent PFM results summarized above are generally consistent with fer-

roelectricity on HfO2-based thin films. However, the detailed picture

remains rather contradictory. A number of issues, including the discrepancy

between the piezoresponse loops measured with field-on and field-off,

unclear PFM results measured on capacitors through the top electrode,

Fig. 7.1.6 PFM phase images of the instantaneous domain configurations developing at different stages of polarization reversal in the La:HfO2 capacitor under the electric field of 2.5 MV/cm.

303PhysicalC

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and difficulties with observing naturally formed ferroelectric domain pat-

terns, have to be addressed. In order to properly analyze the local piezoelec-

tric response of the material and separate it from the experimental artifacts, a

combination of different approaches is required. This subsection presents a

strategy that relies on maximum simplification of the tip-sample interaction

by using the off-resonance measurements for more clear PFM data with

most straightforward interpretation.

Themainstream PFM technique that has been adopted as a standard since

the early 2000s is resonance PFM, where the electromechanical coupling is

detected near the resonance frequency of the cantilever in contact with the

sample surface. Different approaches such as dual AC resonance tracking

(DART) [28] or band excitation [27, 29] are used in order to closely track

the resonance frequency while doing the PFM scan or PFM loop measure-

ments. All the PFM data discussed in the previous subsection have been

obtained using this approach. The reasons why resonant PFM has almost

completely replaced the traditional off-resonance methods are quite obvi-

ous: The detected signal gains one to two orders of magnitude compared

to the off-resonance measurements. This enhancement allows for the detec-

tion of weak electromechanical responses that would be more difficult to

measure otherwise. For conventional ferroelectrics, the measurements at

resonance frequency allow for spectacularly fast PFM mapping and efficient

PFM spectroscopy with the acquisition of large arrays of PFM loops. On the

other hand, the tip-sample interaction close to the resonance frequency can

be more difficult for analysis. In particular, the results are strongly dependent

on the accuracy of the resonance tracking and parasitic nonlocal probe-

sample interactions. Furthermore, the quantitative analysis of the results

can be difficult because the shape and quality of the resonance curve may

change depending on the DC voltage applied and the topography, and even

the position of the laser spot on the cantilever plays a significant role [35]. In

this context, the use of “frequency-independent” off-resonance PFM rep-

resents a significant simplification and allows for a more straightforward anal-

ysis of the piezoresponse signal [122].

The off-resonance PFM, which was widely used in the early PFM era

(the late 1990s), is a relatively simple technique where the tip is driven with

AC or AC+DC voltage with the frequency that can be arbitrarily chosen

without approaching the system’s resonance frequency. The choice of fre-

quency is only limited by the electrical circuitry andRC constant, so that the

PFM results (both qualitative and quantitative) have to be basically

frequency-independent, which is an important criterion of credibility of


the measurements. The interpretation of the results, in this case, is generally

more straightforward than in resonance PFM. In particular, the amplitude of

the tip deflection can be calibrated more easily to quantify the effective trans-

verse piezoelectric response directly. The obvious disadvantage of the tech-

nique is a relatively low amplitude of the detected piezoelectric signal, which

dictates using very long times of data acquisition and consequently very slow

measurements (typically one to two orders of magnitude longer compared to

the standard DART measurements).

The PFM experiments presented in this subsection demonstrate the

potential of classic off-resonance PFM for analysis of “nontraditional” fer-

roelectrics [122], where the data acquired using the conventional resonance

PFM are sometimes confusing. The parallel-plate capacitor geometry has

been chosen in order to ensure the uniform electric field independent of

the tip radius, eliminate the risk of electrochemical reactions on the surface,

and reduce the nonlocal interaction between the tip and sample. The mate-

rial used for the experiment was Hf0.5Zr0.5O2 (HZO), which is an archetyp-

ical representative of the family with a well-established and robust hysteretic

dielectric response. The layered capacitor structure of Si/SiO2/TiN/HZO/

TiN/Pt was prepared according to the well-established procedure described

elsewhere [123]. The HZO film had a thickness of 12 nm, and the 40 nm

top electrode consisted of 15 nm of TiN covered with 25 nm of Pt. Small

5�5 μm2 capacitors were fabricated by dry etching in order to avoid the

frequency limitations imposed by the RC constant for the capacitor driven

through the conductive AFMprobe. The electromechanical coupling in this

geometry is detected through the top electrode.

Fig. 7.1.7 shows a series of local piezoelectric loops measured on the

same spot of the capacitor at different frequencies. The effective piezoelec-

tric coefficient d33 is proportional to the tip deflection under AC voltage,

which was fixed for all three loops at 0.8 V. In these off-resonance measure-

ments, the electromechanical coupling sensed on 12 nm film through the

40 nm electrode produces a relatively weak signal of tens of μVwhile in con-

ventional resonance techniques, typically some mV-range signal is detected.

Therefore, slow measurements with a large lock-in time constant within the

range of 50–500 ms are necessary for measurements with acceptable signal/

noise ratio. The results in Fig. 7.1.7 have been produced at the data acqui-

sition speed of 1 s/point, thus the whole time of a single loop consisting of

200 measurement points exceeds 3 min. This time is much longer compared

to the resonance measurements, where a single loop is typically measured

within 1–10 s.

6×10–5 12 kHz

12 kHz

4×10–5

0

Am

plitu

de (

µV)

Pha

se (

deg)

2×10–5

–2×10–5

–4×10–5

–6×10–5

–4

100

0

50

–50

–100

–4 –3 –2 –1 0 1 2 3 4

–3 –2 –1 0 1 2 3

Voltage (V)

DC onDC off

Voltage (V)

DC onDC off

Pha

se (

deg)

100

0

50

–50

–100

–4 –3 –2 –1 0 1 2 3 4

Voltage (V)

DC onDC off

Pha

se (

deg)

100

0

50

–50

–100

–4 –3 –2 –1 0 1 2 3 4

Voltage (V)

DC onDC off

4

6×10–5 92 kHz

92 kHz

4×10–5

0

Am

plitu

de (

µV)

2×10–5

–2×10–5

–4×10–5

–6×10–5

–4 –3 –2 –1 0 1 2 3

Voltage (V)

DC onDC off

4

6×10–5 230 kHz

230 kHz

4×10–5

0

Am

plitu

de (

µV)

2×10–5

–2×10–5

–4×10–5

–6×10–5

–4 –3 –2 –1 0 1 2 3

Voltage (V)

DC onDC off

4

(A) (B) (C)

(D) (E) (F)Fig. 7.1.7 Off-resonance piezoelectric hysteresis loops measured at three different frequencies: amplitude (A, B, C) and phase (D, E, F) of localpiezoresponse measured at 12, 92, and 230 kHz, respectively.

306Ferroelectricity

inDoped

Hafnium

Oxide


For PFMmeasurements performed off resonance, one of the key criteria

of reliability is the frequency independence of results within a wide fre-

quency range. The loops presented in Fig. 7.1.7 have beenmeasured at three

different frequencies (lower than the contact resonance frequency): 12, 92,

and 230 kHz. All loops in Fig. 7.1.7 show nearly the same amplitude, sug-

gesting that the detected amplitude represents the true piezoelectric defor-

mation. The phase difference of 180�5 degrees between the two opposite

polarization states indicates that the polarization response is not affected by

leakage or any other mechanism resulting in a phase shift. Another essential

characteristic of proper piezoelectric response measurements in capacitor

geometry is the consistency between the on-field and off-field loops. The

data in Fig. 7.1.7 show that these loops closely follow each other and have

very similar amplitudes and virtually identical coercive fields. Furthermore,

the difference between on-field and off-field amplitudes represents another

important argument in favor of the true piezoelectric nature of measured

loops. Indeed, all loops measured at the three different frequencies show

the same trend: The amplitude measured on-field decreases with a DC volt-

age increase while the off-field amplitude does not change after reaching sat-

uration. This behavior is very similar to conventional ferroelectrics such as

PZT and agrees well with the theory of piezoelectric effect [124]. In partic-

ular the observation of d33 (on-field)<d33 (off-field) is consistent with the

lattice dielectric constant decrease under DC voltage, observed in ferroelec-

trics. Note that for competing scenarios of hysteretic behavior originating

from mobile charged defect redistribution, the opposite trend is expected:

The DC-on amplitude has to be higher or equal to the DC-off response.

Hence, the shape of on-field/off-field loops can be considered an argument

in favor of true ferroelectric switching in the HZO films.

In addition to single-spot local piezoelectric hysteresis loops, the off-

resonance PFM provides high-quality domain maps, as illustrated in

Fig. 7.1.8. Comparison of topography in Fig. 7.1.8A (very small grains with

RMS roughness <1 nm), piezoelectric response amplitude (Fig. 7.1.8B),

and phase (Fig. 7.1.8C) strongly suggest that the observed pattern represents

the polarization domains, similar to that typically observed in PZT film

capacitors. The lateral resolution of the measurements is limited by the

40-nm-thick top electrode, which covers the 12-nm HZO ferroelectric

layer. In spite of this limitation, the amplitude and phase maps resolve clearly

the domains with size down to 50 nm. The scan had been taken at 0.03 Hz/

line in order to reach acceptable signal/noise ratio. The mixed polarization

state with coexisting domains of polarization oriented toward the top and

4×10–5

Amplitude Phase 100

50

0

Phase (deg)

–50

–100

2×10–5

–2×10–5

–4×10–5

Am

plitu

de (

µv)

0

Voltage (V)

–4 –3 –2 –1 0 1 2 3 4

4×10–5

Amplitude Phase 100

50

0

Phase (deg)

–50

–100

2×10–5

–2×10–5

–4×10–5

Am

plitu

de (

µv)

0

Voltage (V)

–4 –3 –2 –1 0 1 2 3 4

0.0 0.5 1.0µm

1.5 2.0

–1.0

–0.5

0.0

0.50.5

1.0

nm

0.0 0.5 1.0µm

1.5 2.0

0.00.51.01.52.02.53.0

v

0.0 0.5 1.0µm

1.5 2.0

v

10

–1–2–3–4–5

(A)

(B)

(D)

(E)(C)

Fig. 7.1.8 Maps of topography (A), amplitude (B), and phase (C) of local piezoelectricresponse measured on 12 nm HZO capacitor with an AC signal of 0.8 V/92 kHz. (D),(E): Local piezoelectric response loops measured with AC voltage of 0.5 V ondomains with opposite polarization gives same-state loops (D) or opposite stateloops (E), depending on the polarization orientation.


bottom electrodes has been created by a special poling procedure. First, the

capacitor was cycled with 1000 pulses of �3 V, 1000 Hz in order to max-

imize switching polarization. The “trained” capacitor was poled with the tip

voltage of +3 V/1 s and then partially switched with�1.5 V/1 s in order to

reverse a part of the polarization domains (seen as bright areas in the phase

map, Fig. 7.1.8C). Local piezoresponse measured on the spots with polar-

ization oriented “upward” (dark) and “downward” (bright) yield same-state

and opposite-state loops, respectively (Fig. 7.1.8D and E). The loops have

beenmeasured with the identical setting, at AC voltage amplitude 0.5 V, the

negative DC voltage being applied first. This behavior is also very similar to

the conventional ferroelectrics such as PZT, and the absence of gaps in both

same-state and opposite-state loops attests to good polarization stability.

The data presented in this subsection directly demonstrate switchable

ferroelectric domains in 12 nmHZO layers in the most application-relevant


capacitor geometry. Another important proof of true ferroelectricity in this

material comes from a comparison of the on-field and off-field piezoelectric

loops. In particular, the decrease of d33 with a voltage increase in on-field

loops and the characteristic loop shape with the tips pointing downward

are consistent with the hysteretic piezoelectric response commonly observed

in conventional ferroelectrics. Remarkably, the piezoelectric properties

measured off-resonance in HZO are quite similar to the properties typical

for high-quality PZT ferroelectric films. These results illustrate the hidden

potential of the classic PFM approach based on the off-resonance technique.

In the situation where the results coming from the standard resonance tech-

nique are not always conclusive, these simple measurements with relatively

simple interpretation add credibility to the analysis. In spite of unavoidable

limitations of the off-resonance PFM, in particular, weak signals and conse-

quently a very slow data acquisition process, the method offers a valuable

addition to the PFM toolbox and brings important proofs of true ferroelec-

tricity in HfO2-based materials [122].

7.1.5 Outlook/Ways Around the Problem

Going forward, local characterization using PFM on bare surfaces needs to

be addressed and the underlying problems of why it does not seem feasible

need to be resolved. While measurements on capacitors are promising, they

come with their own challenges and cannot provide as good a lateral reso-

lution as measurements on bare surfaces. Some of the big unknowns are the

chemistry and structure of the surface itself and their changes under applied

bias. Here, chemical information should be combined with PFM measure-

ments similar to what has been done on other ferroelectric films [93].

Another alternative is to combine PFM with other measurements, which

are less sensitive to measurement artifacts such as current-based techniques,

static strain loops, or microwavemicroscopy [125] or, as it has been shown in

the last section, to go back to the roots and use nonresonant PFM.

AcknowledgmentsThe resonance enhanced PFM measurements were performed at the Center for Nanophase

Materials Sciences, which is a DOE Office of Science User Facility (N.B.). T.S. gratefully

acknowledges the German Research Foundation (Deutsche Forschungsgemeinschaft) for

funding part of this research at NaMLab in the frame of the “Inferox” project (MI

1247/11-2). I.S. acknowledges the Swiss National Science Foundation for support through

grant 200021_169339.


References[1] P. Guthner, K. Dransfeld, Local poling of ferroelectric polymers by scanning force

microscopy, Appl. Phys. Lett. 61 (1992) 1137–1139.[2] K. Franke, J. Besold, W. Haessler, C. Seegebarth, Modification and detection of

domains on ferroelectric PZT films by scanning force microscopy, Surf. Sci.302 (1994) L283–L288.

[3] K. Franke, M. Weihnacht, Evaluation of electrically polar substances by electric scan-ning force microscopy. Part I: measurement signals due to Maxwell stress, Ferroelectr.Lett. Sect. 19 (1995) 25–33.

[4] A. Gruverman, O. Auciello, H. Tokumoto, Scanning Force microscopy for the studyof domain structure in ferroelectric thin films, J. Vac. Sci. Technol. B14 (1996)602–605.

[5] L. Eng, M. Friedrich, J. Fousek, P. G€unter, Scanning force microscopy of ferroelectriccrystals, Ferroelectrics 186 (1996) 49–52.

[6] J.W. Hong, S.I. Park, Z.G. Khim, Measurement of hardness, surface potential, andcharge distribution with dynamic contact mode electrostatic force microscope,Rev. Sci. Instrum. 70 (1999) 1735–1739.

[7] A. Roelofs, U. Bottger, R. Waser, F. Schlaphof, S. Trogisch, L.M. Eng, Differenti-ating 180 degrees and 90 degrees switching of ferroelectric domains with three-dimensional piezoresponse force microscopy, Appl. Phys. Lett. 77 (2000) 3444–3446.

[8] V. Shvartsman, A. Tyunina, J. Levoska, A. Kholkin, Local electromechanical proper-ties of PbMg1/3Nb2/3O3 thin films studied by piezoelectric force microscopy,Ferroelectrics 302 (2004) 569–572.

[9] A. Gruverman, O. Auciello, H. Tokumoto, Imaging and control of domain structuresin ferroelectric thin films via scanning force microscopy, Ann. Rev. Mater. Sci.28 (1998) 101–124.

[10] J. Munoz-Saldana, G.A. Schneider, L.M. Eng, Stress induced movement of ferroelas-tic domain walls in BaTiO3 single crystals evaluated by scanning force microscopy,Surf. Sci. 480 (2001) L402–L410.

[11] M.T. Buscaglia, V. Buscaglia, M. Viviani, J. Petzelt, M. Savinov, L. Mitoseriu,A. Testino, P. Nanni, C. Harnagea, Z. Zhao, M. Nygren, Ferroelectric propertiesof dense nanocrystalline BaTiO3 ceramics, Nanotechnology 15 (2004) 1113–1117.

[12] L. Mitoseriu, C. Harnagea, P. Nanni, A. Testino, M.T. Buscaglia, V. Buscaglia,M. Viviani, Z. Zhao, M. Nygren, Local switching properties of dense nanocrystallineBaTiO3 ceramics, Appl. Phys. Lett. 84 (2004) 2418–2420.

[13] A.N. Salak, V.V. Shvartsman, M.P. Seabra, A.L. Kholkin, M. Ferreira, Ferroelectric-to-relaxor transition behaviour of BaTiO3 ceramics doped with La(Mg1/2Ti1/2)O3,J. Phys. Condens. Matter 16 (2004) 2785–2794.

[14] V.V. Shvartsman, A.L. Kholkin, A. Orlova, D. Kiselev, A.A. Bogomolov,A. Sternberg, Polar nanodomains and local ferroelectric phenomena in relaxor leadlanthanum zirconate titanate ceramics, Appl. Phys. Lett. 86 (2005).

[15] R. Dittmer, W. Jo, J. Rodel, S. Kalinin, N. Balke, Nanoscale insight into lead-freeBNT-BT-xKNN, Adv. Funct. Mater. 22 (2012) 4208–4215.

[16] V.S. Bystrov, I.K. Bdikin, D.A. Kiselev, S. Yudin, V.M. Fridkin, A.L. Kholkin,Nanoscale polarization patterning of ferroelectric Langmuir-Blodgett P(VDF-TrFE)films, J. Phys. D Appl. Phys. 40 (2007) 4571–4577.

[17] B.J. Rodriguez, S. Jesse, S.V. Kalinin, J. Kim, S. Ducharme, V.M. Fridkin, Nanoscalepolarization manipulation and imaging of ferroelectric Langmuir-Blodgett polymerfilms, Appl. Phys. Lett. 90 (2007).

[18] B.J. Rodriguez, S. Jesse, J. Kim, S. Ducharme, S.V. Kalinin, Local probing of relax-ation time distributions in ferroelectric polymer nanomesas: time-resolved piezore-sponse force spectroscopy and spectroscopic imaging, Appl. Phys. Lett. 92 (2008).

http://refhub.elsevier.com/B978-0-08-102430-0.00014-0/rf0010































































[19] M. Nakayama, Y. Uenaka, S. Kataoka, Y. Oda, K. Yamamoto, Y. Tajitsu, Piezoelec-tricity of ferroelectret porous polyethylene thin film, Jpn. J. Appl. Phys. 48 (2009).

[20] P. Sharma, T. Reece, D. Wu, V.M. Fridkin, S. Ducharme, A. Gruverman, Nanoscaledomain patterns in ultrathin polymer ferroelectric films, J. Phys. Condens. Matter21 (2009).

[21] Y.Y. Choi, J. Hong, S. Hong, H. Song, D.S. Cheong, K. No, Nanoscale piezore-sponse of 70 nm poly(vinylidene fluoride-trifluoro-ethylene) films annealed at differ-ent temperatures, Phys. Stat. Sol. 4 (2010) 94–96.

[22] S.V. Kalinin, B.J. Rodriguez, S. Jesse, T. Thundat, A. Gruverman, Electromechanicalimaging of biological systems with sub-10 nm resolution, Appl. Phys. Lett. 87 (2005).

[23] S.V. Kalinin, B.J. Rodriguez, J. Shin, S. Jesse, V. Grichko, T. Thundat, A.P. Baddorf,A. Gruverman, Bioelectromechanical imaging by scanning probe microscopy: Galva-ni’s experiment at the nanoscale, Ultramicroscopy 106 (2006) 334–340.

[24] A. Gruverman, D. Wu, B.J. Rodriguez, S.V. Kalinin, S. Habelitz, High-resolutionimaging of proteins in human teeth by scanning probe microscopy, Biochem. Bio-phys. Res. Commun. 352 (2007) 142–146.

[25] M. Minary-Jolandan, M.F. Yu, Nanoscale characterization of isolated individual typeI collagen fibrils: polarization and piezoelectricity, Nanotechnology 20 (2009).

[26] T. Li, K. Zeng, Piezoelectric properties and surface potential of green abalone shellstudied by scanning probe microscopy techniques, Acta Mater. 59 (2011) 3667–3679.

[27] S. Jesse, S.V. Kalinin, R. Proksch, A.P. Baddorf, B.J. Rodriguez, The band excitationmethod in scanning probe microscopy for rapid mapping of energy dissipation on thenanoscale, Nanotechnology 18 (2007) 435503.

[28] B.J. Rodriguez, C. Callahan, S.V. Kalinin, R. Proksch, Dual-frequency resonance-tracking atomic force microscopy, Nanotechnology 18 (2007).

[29] S. Jesse, S.V. Kalinin, Band excitation in scanning probe microscopy: sines of change,J. Phys. D Appl. Phys. 44 (2011).

[30] R. Nath, Y.H. Chu, N.A. Polomoff, R. Ramesh, B.D. Huey, High speed piezore-sponse force microscopy: < 1 frame per second nanoscale imaging, Appl. Phys. Lett.93 (2008) 072905.

[31] B.J. Rodriguez, S. Jesse, A.P. Baddorf, S.V. Kalinin, High resolution electromechan-ical imaging of ferroelectric materials in a liquid environment by piezoresponse forcemicroscopy, Phys. Rev. Lett. 96 (2006).

[32] B.J. Rodriguez, S. Jesse, S. Habelitz, R. Proksch, S.V. Kalinin, Intermittent contactmode piezoresponse force microscopy in a liquid environment, Nanotechnology20 (2009).

[33] N. Balke, S. Jesse, Y.-H. Chu, S.V. Kalinin, High-frequency electromechanical imag-ing of ferroelectrics in a liquid environment, ACS Nano 6 (2012) 5559–5565.

[34] S. Lepadatu, M. Stewart, M.G. Cain, Quantification of electromechanical couplingmeasured with piezoresponse force microscopy, J. Appl. Phys. 116 (2014) 066806.

[35] A. Labuda, R. Proksch, Quantitative measurements of electromechanical responsewith a combined optical beam and interferometric atomic force microscope, Appl.Phys. Lett. 106 (2015) 253103.

[36] V.Nagarajan, S. Prasertchoung, T. Zhao, H. Zheng, J. Ouyang, R.Ramesh,W. Tian,X.Q. Pan, D.M. Kim, C.B. Eom, H. Kohlstedt, R. Waser, Size effects in ultrathinepitaxial ferroelectric heterostructures, Appl. Phys. Lett. 84 (2004) 5225–5227.

[37] V. Nagarajan, J. Junquera, J.Q. He, C.L. Jia, R. Waser, K. Lee, Y.K. Kim, S. Baik,T. Zhao, R. Ramesh, P. Ghosez, K.M. Rabe, Scaling of structure and electrical prop-erties in ultrathin epitaxial ferroelectric heterostructures, J. Appl. Phys. 100 (2006).

[38] A. Gruverman, D. Wu, H. Lu, Y. Wang, H.W. Jang, C.M. Folkman, M.Y. Zhuravlev, D. Felker, M. Rzchowski, C.-B. Eom, E.Y. Tsymbal, Tunneling elec-troresistance effect in ferroelectric tunnel junctions at the nanoscale, Nano Lett.9 (2009) 3539–3543.
























































[39] A.R. Chaudhuri, M. Arredondo, A. Hahnel, A. Morelli, M. Becker, M. Alexe,I. Vrejoiu, Epitaxial strain stabilization of a ferroelectric phase in PbZrO3 thin films,Phys. Rev. B 84 (2011).

[40] H. Lu, X. Liu, J.D. Burton, Y.Wang, Y. Zhang, D.J. Kim, A. Stamm, P. Lukashev, C.W. Bark, D.A. Felker, C.M. Folkman, P. Gao, X.Q. Pan, M.S. Rzchowski, C.-B. Eom, E.Y. Tsymbal, A. Gruverman, Enhancement of ferroelectric polarization sta-bility by interface engineering, Adv. Mater. 24 (2012) 1209–1216.

[41] P.Maksymovych, M. Huijben,M. Pan, S. Jesse, N. Balke, Y.-H. Chu, H.J. Chang, A.Y. Borisevich, A.P. Baddorf, G. Rijnders, D.H.A. Blank, R. Ramesh, S.V. Kalinin,Ultrathin limit and dead-layer effects in local polarization switching of BiFeO3, Phys.Rev. B 85 (2012).

[42] A. Zenkevich, M. Minnekaev, Y. Lebedinskii, K. Bulakh, A. Chouprik, A. Baturin,R. Mantovan, M. Fanciulli, O. Uvarov, Pulsed laser deposition of ultrathin BaTiO3/Fe bi-layers: structural characterization and piezoelectric response, Thin Solid Films520 (2012) 4586–4589.

[43] C.W. Bark, P. Sharma, Y. Wang, S.H. Baek, S. Lee, S. Ryu, C.M. Folkman, T.R. Paudel, A. Kumar, S.V. Kalinin, A. Sokolov, E.Y. Tsymbal, M.S. Rzchowski,A. Gruverman, C.B. Eom, Switchable induced polarization in LaAlO3/SrTiO3 hetero-structures, Nano Lett. 12 (2012) 1765–1771.

[44] J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S. Choudhury,W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefer, J. Levy, D.G. Schlom, Room-temperature ferroelectricityin strained SrTiO3, Nature 430 (2004) 758–761.

[45] A. Kholkin, I. Bdikin, T. Ostapchuk, J. Petzelt, Room temperature surface piezoelec-tricity in SrTiO3 ceramics via piezoresponse force microscopy, Appl. Phys. Lett.93 (2008).

[46] D. Lee, H. Lu, Y. Gu, S.-Y. Choi, S.-D. Li, S. Ryu, T.R. Paudel, K. Song,E. Mikheev, S. Lee, S. Stemmer, D.A. Tenne, S.H. Oh, E.Y. Tsymbal, X. Wu,L.-Q. Chen, A. Gruverman, C.B. Eom, Emergence of room-temperature ferroelec-tricity at reduced dimensions, Science 349 (2015) 1314–1317.

[47] I.K. Bdikin, J. Gracio, R. Ayouchi, R. Schwarz, A.L. Kholkin, Local piezoelectricproperties of ZnO thin films prepared by RF-plasma-assisted pulsed-laser depositionmethod, Nanotechnology 21 (2010).

[48] T.S. Herng, A. Kumar, C.S. Ong, Y.P. Feng, Y.H. Lu, K.Y. Zeng, J. Ding, Inves-tigation of the non-volatile resistance change in noncentrosymmetric compounds,Sci. Rep. 2 (2012).

[49] Y. Liu, Y. Zhang, M.-J. Chow, Q.N. Chen, J. Li, Biological ferroelectricity uncov-ered in aortic walls by piezoresponse force microscopy, Phys. Rev. Lett. 108 (2012)078103.

[50] J. Varghese, S. Barth, L. Keeney, R.W.Whatmore, J.D. Holmes, Nanoscale ferroelec-tric and piezoelectric properties of Sb2S3 nanowire arrays, Nano Lett. 12 (2012)868–872.

[51] A.V. Kolobov, D.J. Kim, A. Giussani, P. Fons, J. Tominaga, R. Calarco,A. Gruverman, Ferroelectric switching in epitaxial GeTe films, APL Mater. 2 (2014).

[52] N. Deepak, M.A. Caro, L. Keeney, M.E. Pemble, R.W. Whatmore, Interesting evi-dence for template-induced ferroelectric behavior in ultra-thin titanium dioxide filmsgrown on (110) neodymium gallium oxide substrates, Adv. Funct. Mater. 24 (2014)2844–2851.

[53] J.F. Scott, Ferroelectrics go bananas, J. Phys. Condens. Matter 20 (2008) 021001.[54] L. Jin, F. Li, S. Zhang, Decoding the fingerprint of ferroelectric loops: comprehension

of the material properties and structures, J. Am. Ceram. Soc. 97 (2014) 1–27.




























































[55] R.K. Vasudevan, N. Balke, P. Maksymovych, S. Jesse, S.V. Kalinin, Ferroelectric ornon-ferroelectric: why so many materials exhibit “ferroelectricity” on the nanoscale,Appl. Phys. Rev. 4 (2017) 021302.

[56] S.V. Kalinin, D.A. Bonnell, Contrast mechanism maps for piezoresponse forcemicroscopy, J. Mater. Res. 17 (2002) 936–939.

[57] Q. Li, Y. Liu, D.Y. Wang, R.L. Withers, Z.R. Li, H.S. Luo, Z. Xu, Switching spec-troscopic measurement of surface potentials on ferroelectric surfaces via an open-loopKelvin probe force microscopy method, Appl. Phys. Lett. 101 (2012) 242906.

[58] N. Balke, P. Maksymovych, S. Jesse, I. Kravchenko, Q. Li, S.V. Kalinin, Exploringlocal electrostatic effects with scanning probe microscopy: implications for piezore-sponse force microscopy and triboelectricity, ACS Nano 8 (2014).

[59] N. Balke, S. Jesse, Y. Kim, L. Adamczyk, A. Tselev, I.N. Ivanov, N.J. Dudney, S.V. Kalinin, Real space mapping of Li-ion transport in amorphous Si anodes with nano-meter resolution, Nano Lett. 10 (2010) 3420–3425.

[60] N. Balke, S. Jesse, A.N.Morozovska, E. Eliseev, D.W. Chung, Y. Kim, L. Adamczyk,R.E. Garcia, N. Dudney, S.V. Kalinin, Nanoscale mapping of ion diffusion in alithium-ion battery cathode, Nat. Nanotechnol. 5 (2010) 749–754.

[61] N. Balke, S. Kalnaus, N.J. Dudney, C. Daniel, S. Jesse, S.V. Kalinin, Local detection ofactivation energy for ionic transport in lithium cobalt oxide, Nano Lett. 12 (2012)3399–3403.

[62] P. Sharma, S. Ryu, Z. Viskadourakis, T.R. Paudel, H. Lee, C. Panagopoulos, E.Y. Tsymbal, C.B. Eom, A. Gruverman, Electro-mechanics of ferroelectric-likebehavior of LaAlO3 thin films, Adv. Funct. Mater. 25 (2015) 6538–6544.

[63] N. Balke, P. Maksymovych, S. Jesse, A. Herklotz, A. Tselev, C.-B. Eom, I.I. Kravchenko, P. Yu, S.V. Kalinin, Differentiating ferroelectric and nonferroelectricelectromechanical effects with scanning probe microscopy, ACS Nano 9 (2015)6484–6492.

[64] D. Martin, J. Muller, T. Schenk, T.M. Arruda, A. Kumar, E. Strelcov, E. Yurchuk,S. Muller, D. Pohl, U. Schroder, S.V. Kalinin, T. Mikolajick, Ferroelectricity inSi-doped HfO2 revealed: a binary lead-free ferroelectric, Adv. Mater. 26 (2014)8198–8202.

[65] A. Chernikova, M. Kozodaev, A. Markeev, D. Negrov, M. Spiridonov, S. Zarubin,O. Bak, P. Buraohain, H.D. Lu, E. Suvorova, A. Gruverman, A. Zenkevich, UltrathinHf0.5Zr0.5O2 ferroelectric films on Si, ACS Appl. Mater. Interfaces 8 (2016)7232–7237.

[66] Z. Fan, J.X. Xiao, J.X. Wang, L. Zhang, J.Y. Deng, Z.Y. Liu, Z.L. Dong, J. Wang, J.S. Chen, Ferroelectricity and ferroelectric resistive switching in sputteredHf0.5Zr0.5O2 thin films, Appl. Phys. Lett. 108 (2016).

[67] S. Shibayama, L. Xu, S. Migita, A. Toriumi, Study of wake-up and fatigue propertiesin doped and undoped ferroelectric HfO2 in conjunction with piezo-response forcemicroscopy analysis, IEEE Symposium on VLSI Technology, 2016.

[68] F. Ambriz-Vargas, G. Kolhatkar, R. Thomas, R. Nouar, A. Sarkissian, C. Gomez-Yanez, M.A. Gauthier, A. Ruediger, Tunneling electroresistance effect in a Pt/Hf0.5Zr0.5O2/Pt structure, Appl. Phys. Lett. 110 (2017).

[69] R. Eskandari, X.D. Zhang, L.M. Malkinski, Polarization-dependent photovoltaiceffect in ferroelectric-semiconductor system, Appl. Phys. Lett. 110 (2017).

[70] S. Kim, J. Hong, Ferroelectricity in ultrathin yttrium-doped hafnium oxide films pre-pared by chemical solution deposition based on metal chlorides and alcohol, Ceram.Int. 43 (2017) S158–S161.

[71] S. Starschich, T. Schenk, U. Schroeder, U. Boettger, Ferroelectric and piezoelectricproperties of Hf1-xZrxO2 and pure ZrO2 films, Appl. Phys. Lett. 110 (2017) 182905.





































































[72] X. Tian, A. Toriumi, New Opportunity of Ferroelectric Tunnel Junction Memorywith Ultrathin HfO2-based Oxides, in: IEEE Electron Devices Technology andManufacturing Conference (EDTM), 2017.

[73] Y. Li, R. Liang, J. Wang, Y. Zhang, H. Tian, H. Liu, S. Li, W. Mao, Y. Pang, Y. Li,Y. Yang, T.-L. Ren, A ferroelectric thin film transistor based on annealing-freeHfZrO film, IEEE J. Electron Devices Soc. 5 (5) (2017) 378–383.

[74] M. Pesi�c, M. Hoffmann, C. Richter, S. Slesazeck, T. K€ampfe, L.M. Eng,T. Mikolajick, U. Schroeder, Anti-ferroelectric ZrO2, an enabler for low powernon-volatile 1T-1C and 1T random access memories, in: IEEE European Solid-StateDevice Research Conference (ESSDERC), 2017, , pp. 160–163.

[75] K. Florent, S. Lavizzari, M. Popovici, L. Di Piazza, U. Celano, G. Groeseneken, J. VanHoudt, Understanding ferroelectric Al:HfO2 thin films with Si-based electrodes for3D applications, J. Appl. Phys. 121 (2017) 204103.

[76] A. Wei, C. Chen, L. Tang, K. Zhou, D. Zhang, Chemical solution deposition of fer-roelectric Sr:HfO2 film from inorganic salt precursors, J. Alloys Compd. 731 (2018)546–553.

[77] S. Zarubin, E. Suvorova, M. Spiridonov, D. Negrov, A. Chernikova, A. Markeev,A. Zenkevich, Fully ALD-grown TiN/Hf0.5Zr0.5O2/TiN stacks: ferroelectric andstructural properties, Appl. Phys. Lett. 109 (2016) 192903.

[78] Y. Sharma, D. Barrionuevo, R. Agarwal, S.P. Pavunny, R.S. Katiyar, Ferroelectricityin rare-earth modified hafnia thin films deposited by sequential pulsed laser deposition,ECS Solid State Lett. 4 (2015) N13–N16.

[79] A. Chernikova, M. Kozodaev, A. Markeev, Y. Matveev, D. Negrov, O. Orlov, Con-finement-free annealing induced ferroelectricity in Hf0.5Zr0.5O2 thin films, Micro-electron. Eng. 147 (2015) 15–18.

[80] N. Balke, S. Jesse, P. Yu, B. Carmichael, S.V. Kalinin, A. Tselev, Quantification ofsurface displacements and electromechanical phenomena via dynamic atomic forcemicroscopy, Nanotechnology 27 (2016).

[81] S.V. Kalinin, S. Jesse, A. Tselev, A.P. Baddorf, N. Balke, The role of electrochemicalphenomena in scanning probe microscopy of ferroelectric thin films, ACS Nano5 (2011) 5683–5691.

[82] T.S. B€oscke, J. M€uller, D. Br€auhaus, U. Schr€oder, U. B€ottger, Ferroelectricity in haf-nium oxide thin films, Appl. Phys. Lett. 99 (2011) 102903.

[83] S. Starschich, D. Griesche, T. Schneller, R. Waser, U. B€ottger, Chemical solutiondeposition of ferroelectric yttrium-doped hafnium oxide films on platinum electrodes,Appl. Phys. Lett. 104 (2014) 202903.

[84] R.E. Newnham, Properties of Materials: Anisotropy, Symmetry, Structure, OUPOxford, 2004.

[85] G.W. Taylor, Piezoelectricity, Gordon and Breach Science Publishers, 1985.[86] N. Balke, S. Jesse, B. Carmichael, M.B. Okatan, I.I. Kravchenko, S.V. Kalinin,

A. Tselev, Quantification of in-contact probe-sample electrostatic forces withdynamic atomic force microscopy, Nanotechnology 28 (2017) 065704.

[87] M.Nonnenmacher, M.P. Oboyle, H.K.Wickramasinghe, Kelvin probe force micros-copy, Appl. Phys. Lett. 58 (1991) 2921–2923.

[88] U. Rabe, M. Kopycinska, S. Hirsekorn, W. Arnold, Evaluation of the contact reso-nance frequencies in atomic force microscopy as a method for surface characterisation,invited, Ultrasonics 40 (2002) 49–54.

[89] C. Zhou, D.M. Newns, Intrinsic dead layer effect and the performance of ferroelectricthin film capacitors, J. Appl. Phys. 82 (1997) 3081–3088.

[90] C. Zhou, D.M. Newns, Is there an intrinsic dead layer effect at ferroelectric surfaces?Superlattice. Microst. 23 (1998) 539–545.

[91] A.M. Bratkovsky, A.P. Levanyuk, Very large dielectric response of thin ferroelectricfilms with the dead layers, Phys. Rev. B 63 (2001).









































































[92] L.J. Sinnamon, R.M. Bowman, J.M. Gregg, Investigation of dead-layer thickness inSrRuO3/Ba0.5Sr0.5TiO3/Au thin-film capacitors, Appl. Phys. Lett. 78 (2001)1724–1726.

[93] A.V. Ievlev, P. Maksymovych, M. Trassin, J. Seidel, R. Ramesh, S.V. Kalinin,O.S. Ovchinnikova, Chemical state evolution in ferroelectric films during tip-inducedpolarization and electroresistive switching, ACS Appl. Mater. Interfaces 8 (2016)29588–29593.

[94] A.V. Ievlev, A.N. Morozovska, V.Y. Shur, S.V. Kalinin, Humidity effects on tip-induced polarization switching in lithium niobate, Appl. Phys. Lett. 104 (2014)092908.

[95] N. Balke, S. Jesse, Q. Li, P. Maksymovych, M.B. Okatan, E. Strelcov, A. Tselev,S.V. Kalinin, Current and surface charge modified hysteresis loops in ferroelectric thinfilms, J. Appl. Phys. 118 (2015).

[96] M. Alexe, A. Gruverman (Eds.), Nanoscale Characterization of Ferroelectric Mate-rials: Scanning Probe Microscopy Approach, Springer-Verlag, 2004.

[97] S. Jesse, A.P. Baddorf, S.V. Kalinin, Switching spectroscopy piezoresponse forcemicroscopy of ferroelectric materials, Appl. Phys. Lett. 88 (2006) 062908.

[98] A. Gruverman, B.J. Rodriguez, A.I. Kingon, R.J. Nemanich, J.S. Cross, M. Tsukada,Spatial inhomogeneity of imprint and switching behavior in ferroelectric capacitors,Appl. Phys. Lett. 82 (2003) 3071–3073.

[99] P. Bintachitt, S. Trolier-McKinstry, K. Seal, S. Jesse, S.V. Kalinin, Switching spectros-copy piezoresponse force microscopy of polycrystalline capacitor structures, Appl.Phys. Lett. 94 (2009) 042906.

[100] S. Hong, E.L. Colla, E. Kim, D.V. Taylor, A.K. Tagantsev, P. Muralt, K. No,N. Setter, High resolution study of domain nucleation and growth during polarizationswitching in Pb(Zr,Ti)O3 ferroelectric thin film capacitors, J. Appl. Phys. 86 (1999)607–613.

[101] A. Gruverman, Scaling effect on statistical behavior of switching parameters of ferro-electric capacitors, Appl. Phys. Lett. 75 (1999) 1452–1454.

[102] H. Fujisawa, T. Yagi, M. Shimizu, H. Niu, Investigation of polarization switchingprocesses in Pb(Zr,Ti)O3 capacitors using piezoresponse imaging, Ferroelectrics269 (2002) 21–26.

[103] A.Gruverman,B.J.Rodriguez,A.I.Kingon,R.J.Nemanich,A.K.Tagantsev, J.S.Cross,M. Tsukada, Mechanical stress effect on imprint behavior of integrated ferroelectriccapacitors, Appl. Phys. Lett. 83 (2003) 728–730.

[104] A. Gruverman, D. Wu, J.F. Scott, Piezoresponse force microscopy studies of switch-ing behavior of ferroelectric capacitors on a 100-ns timescale, Phys. Rev. Lett.100 (2008) 097601–097604.

[105] H. Fujisawa, M. Shimizu, H. Niu, Piezoresponse force microscopy observations ofswitching behavior in Pb(Zr,Ti)O3 capacitors, Jpn. J. Appl. Phys. 43 (2004)6571–6575.

[106] W.H. Ma, D. Hesse, Polarization imprint in ordered arrays of epitaxial ferroelectricnanostructures, Appl. Phys. Lett. 84 (2004) 2871–2873.

[107] J.M. Marshall, S. Dunn, R.W. Whatmore, FIB milled PZT nanocapacitors testedusing PFM, Integr. Ferroelectr. 61 (2004) 223–230.

[108] B.J. Rodriguez, A. Gruverman, A.I. Kingon, R.J. Nemanich, J.S. Cross, Three-dimensional high-resolution reconstruction of polarization in ferroelectric capacitorsby piezoresponse force microscopy, J. Appl. Phys. 95 (2004) 1958–1962.

[109] A. Gruverman, B.J. Rodriguez, C. Dehoff, J.D. Waldrep, A.I. Kingon, R.J. Nemanich, J.S. Cross, Direct studies of domain switching dynamics in thin film fer-roelectric capacitors, Appl. Phys. Lett. 87 (2005).

[110] B.J. Rodriguez, A. Gruverman, A.I. Kingon, R.J. Nemanich, J.S. Cross, Investigationof the mechanism of polarization switching in ferroelectric capacitors by three-






























































dimensional piezoresponse force microscopy, Appl. Phys. A Mater. Sci. Process.80 (2005) 99–103.

[111] D.J. Kim, J.Y. Jo, T.H. Kim, S.M. Yang, B. Chen, Y.S. Kim, T.W.Noh, Observationof inhomogeneous domain nucleation in epitaxial Pb(Zr,Ti)O3 capacitors, Appl. Phys.Lett. 91 (2007).

[112] S.V. Kalinin, B.J. Rodriguez, S.-H. Kim, S.-K. Hong, A. Gruverman, E.A. Eliseev,Imaging mechanism of piezoresponse force microscopy in capacitor structures, Appl.Phys. Lett. 92 (2008) 152906.

[113] A. Gruverman, J.S. Cross, W.S. Oates, Peculiar effect of mechanical stress on polar-ization stability in micrometer-scale ferroelectric capacitors, Appl. Phys. Lett.93 (2008) 242902–242904.

[114] P. Buragohain, C. Richter, T. Schenk, H. Lu, T. Mikolajick, U. Schroeder,A. Gruverman, Nanoscopic studies of domain structure dynamics in ferroelectricLa:HfO2 capacitors, Appl. Phys. Lett. 112 (2018) 222901.

[115] M. Pesic’, F.P.G. Fengler, L. Larcher, A. Padovani, T. Schenk, E.D. Grimley, X. Sang,J.M. LeBeau, S. Slesazeck, U. Schroeder, T. Mikolajick, Physical mechanisms behindthe field-cycling behavior of HfO2-based ferroelectric capacitors, Adv. Funct. Mater.26 (2016) 4601–4612.

[116] E.D. Grimley, T. Schenk, X. Sang, M. Pesic, U. Schroeder, T. Mikolajick, J.M. LeBeau, Structural changes underlying field-cycling phenomena in ferroelectricHfO2 thin films, Adv. Electron. Mater. 2 (2016) 1600173.

[117] T. Schenk, M. Hoffmann, J. Ocker, M. Pesic, T. Mikolajick, U. Schroeder, Complexinternal bias fields in ferroelectric hafnium oxide, ACSAppl.Mater. Interfaces 7 (2015)20224–20233.

[118] N.A. Polomoff, R. Nath, J.L. Bosse, B.D. Huey, Single ferroelectric domain nucle-ation and growth monitored by high speed piezoforce microscopy, J. Vac. Sci. Tech-nol. B 27 (2009) 1011.

[119] A. Gruverman, N. Ponomarev, K. Takahashi, Domain nucleation during polarizationreversal in lead germanate, Jpn. J. Appl. Phys. 33 (Part 1) (1994) 5536.

[120] S.M. Yang, J.Y. Jo, D.J. Kim, H. Sung, T.W.Noh, H.N. Lee, J.-G. Yoon, T.K. Song,Domain wall motion in epitaxial Pb(Zr,Ti)O3 capacitors investigated by modifiedpiezoresponse force microscopy, Appl. Phys. Lett. 92 (2008) 252901.

[121] D.A. Bonnell, S.V. Kalinin, A. Kholkin, A. Gruverman, Piezoresponse force micros-copy: a window into electromechanical behavior at the nanoscale, MRS Bull.34 (2009) 648.

[122] I. Stolichnov, M. Cavalieri, E. Colla, T. Schenk, T. Mittmann, T. Mikolajick,U. Schroeder, A.M. Ionescu, Genuinely ferroelectric sub-1-volt-switchable nanodo-mains in HfxZr(1-x)O2 ultrathin capacitors, ACS Appl. Mater. Interfaces (2018),https://doi.org/10.1021/acsami.8b07988. accepted for publication.

[123] M.H. Park, Y.H. Lee, H.J. Kim, Y.J. Kim, T. Moon, K.D. Kim, J. Muller, A. Kersch,U. Schroeder, T. Mikolajick, C.S. Hwang, Ferroelectricity and antiferroelectricity ofdoped thin HfO2-based films, Adv. Mater. 27 (2015) 1811–1831.

[124] N. Setter, D. Damjanovic, L. Eng, G. Fox, S. Gevorgian, S. Hong, A. Kingon,H. Kohlstedt, N.Y. Park, G.B. Stephenson, I. Stolitchnov, A.K. Taganstev, D.V. Taylor, T. Yamada, S. Streiffer, Ferroelectric thin films: review of materials, prop-erties, and applications, J. Appl. Phys. 100 (2006) 051606.

[125] A. Tselev, P. Yu, Y. Cao, L.R. Dedon, L.W. Martin, S.V. Kalinin, P. Maksymovych,Microwave a.c. conductivity of domain walls in ferroelectric thin films, Nat. Com-mun. 7 (2016).









































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chapter 7 - physical characterization on a nanometer scaleof the spm tip during pfm on electrostatic...

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