Cao Z. (Ed.) Thin Film Growth: Physics, materials science and applications

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258 Thin ﬁ lm growth

moment in a slab of N unit cells can be approximated with the help of a

plate capacitor model:

tota

VN =VN

[11.2]

whereby e

and e

are the vacuum permittivity and the dielectric constant of the

material, respectively (Kittel, 1996). To get an idea of the energies involved,

the electrostatic potential is calculated for a free-standing MgO(111) bilayer

~ 10). The system consists of a positively-charged Mg

and a negative-

charged O

2–

plane (s = 2|e|/7.8 Å

) separated by the bulk lattice parameter

of 1.8 Å and already has an electrostatic energy of 8.3 eV. Apparently, the

values for thicker slabs easily reach the lattice energy in ionic crystals of

20–30 eV, rendering polar materials energetically unstable. Bulk-like structures

with uncompensated polarity should therefore not exist in nature.

The latter statement seems to be in con ict with the experimental

observation of systems with polar character. Moreover, polar terminations

are often characterized by a lower surface free energy than their non-polar

counterparts. For example, MgO cubes terminated by non-polar (100) surfaces

were found to develop polar (111) facets in a humid environment, although

this should be accompanied by a dramatic increase of the electrostatic energy

(Hacquart and Jupille, 2007). Similarly, a (111)-type NiO surface has been

identi ed as the one with the lowest free energy even with respect to the

non-polar (100) and (110) planes (Barbier et al., 2000). This discrepancy

can be solved by considering the various polarity-healing mechanisms that

are able to remove the electrostatic dipole and therewith the polarity of the

respective surface (Goniakowski et al., 2008). From a purely mathematical

viewpoint, polarity cancellation is achieved by adjusting a distinct charge

density at the surface that compensates the bulk dipole. For simple systems,

the required surface charge density is given by:

[11.3]

(a) (b)

0 sd 2sd

Thickness

11.2 (a) Vertical cut through a polar system. The layer distance d

and the size of the unit cell D are indicated. (b) Dependence of the

electrostatic energy on the thickness of the polar slab.

Energy

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259Electronic properties and adsorption behaviour of thin ﬁlms

with D being the height of the unit cell and d the interlayer distance in the

crystal. As an example, layers with 25% and 50% of the bulk charge density

need to be formed on the wurzite (0001) (d/D = 0.25) and the rocksalt (111)

surface (d/D = 0.5) in order to remove the polarity in the whole system. In

simple terms, the polarity annihilation might be understood in these cases

as the result of an additional dipole that is created between the upper and

lower surface of the slab and has exactly the same size but an opposite sign

as the bulk moment.

Several mechanisms have been identied that enable an adjustment of

the required charge density at the surface (Noguera, 1996; Goniakowski

et al., 2008). A rst possibility is the restructuring of the top-most surface

layers, creating planes with fractional atom lling and hence modied

charge distribution (Fig. 11.3a). The best studied example is the octopolar

reconstruction of the rocksalt (111) surface, which was predicted by

Wolf in 1992 and later veried experimentally through X-ray diffraction

measurements on NiO(111) and MgO(111) (Barbier et al., 2000; Finocchi

et al., 2004). The octopolar reconstruction comprises two surface layers

with 75% and 25% atom lling with respect to the ideal plane, which adds

up to the required 50% charge density at the surface according to Eq. 11.3.

The dipole compensation in this case is so efcient that the free energy of

the reconstructed surface drops below the value for comparable non-polar

oxide planes (Barbier et al., 2000). Alternative reconstructions have been

identied for other lattice geometries and different chemical environments.

For instance, a spinel-like termination was found to develop on NiO(111) at

low O

partial pressures (Barbier and Renaud, 1997; Barbier et al., 1999),

while several non-stoichiometric surface compositions were revealed for

srTiO

(111) in a Sr- or O-rich environment (Bottin et al., 2003). For the

Zn-terminated ZnO(0001) surface, polarity healing has been investigated

at the local scale with the help of scanning tunnelling microscopy (STM)

(Parker et al., 1998; Dulub et al., 2003; Ostendorf et al., 2008). The studies

revealed triangular-shaped islands and pits of 1 ML height that cover the

Reconstruction

(a)

Metallization

(b)

Adsorption

(c)

11.3 Sketch of the three main polarity-healing mechanisms in ionic

systems.

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260 Thin ﬁlm growth

complete oxide surface (Fig. 11.4a). According to density functional theory

(DFT) calculations, the edges of those triangular structures are saturated with

O atoms, rendering the surface oxygen-rich with respect to the bulk (Kresse

et al., 2003). As discussed above, the polarity of wurzite(0001) is cancelled

by creating a surface layer with 25% extra charges, which corresponds to

a local Zn:O stoichiometry of 3:4 with respect to the bulk value of 1:1.

Exactly this ratio is adjusted by introducing the triangular islands and pits

into the ZnO surface. Each equilateral triangle comprises ½ · n · (n + 1)

anions and ½ · n · (n – 1) cations (n being the side length) and has an excess

of n oxygen ions. The desired 3:4 surface stoichiometry is now realized by

forming islands with seven edge atoms (n = 7), a structure that is indeed

frequently found on the surface (Fig. 11.4b). For larger islands, additional

atoms have to be removed from the layer underneath to reach the ideal Zn:O

surface ratio (Fig. 11.4c,d). The patched Zn-terminated ZnO(0001) surface

is therefore an ideal example for polarity healing via surface reconstruction

(Dulub et al., 2003).

(a)

(b)

(c)

(d)

[1010]

[1100] [0110]

11.4 (a) Empty-states STM image of Zn-terminated ZnO(0001) (50 ¥

50 nm

). The terraces are covered with triangular islands and pits

that are involved in healing the surface polarity. (b–d) Structure

models of three triangular islands of different size. The island

borders are saturated with oxygen. The resulting non-stoichiometry

produces a charged surface layer that compensates the bulk

dipole moment. Reprinted with permission from Dulub et al., 2003,

ThinFilm-Zexian-11.indd 260 7/1/11 9:43:27 AM

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261Electronic properties and adsorption behaviour of thin ﬁlms

Alternatively, dipole compensation might be achieved via purely electronic

effects, leaving the atomic structure of the surface unchanged (Fig. 11.3b).

One proposed mechanism is the creation of surface states, the electron lling

of which is adjusted to reach the required charge density for polarity healing

(Goniakowski et al., 2008). The formation of a partly-lled electronic state is

always connected with the metallization of the oxide surface. This particular

mechanism has been predicted for bulk Al

(0001) (Wang et al., 2000)

and thin, unreconstructed MgO(111) lms (Goniakowski et al., 2007). The

formation of dipole-compensating surface states becomes particularly easy

when the polar oxide is capped by a metal lm, as demonstrated for Pd and

Cu over-layers on MgO(111) and ZnO(0001), respectively (Goniakowski

and Noguera, 2002; Meyer and Marx, 2004).

A third way to compensate the polarity of oxide materials is the binding of

ad-species that become charged upon adsorption (Fig. 11.3c). The prototype

adsorbate to heal surface polarity is hydrogen, which forms hydroxyl groups

consisting of a surface oxygen ion and a positively charged H

ion. The

hydroxylation of oxide surfaces is often triggered by the heterolytic splitting

of water, which renders this compensation mechanism especially efcient in

an ambient environment. Hydroxylation was predicted to occur spontaneously

on most rocksalt (111) surfaces (Pojani et al., 1997), on ZnO(0001) and

on Al

(0001) (Wang et al., 2000). It has been revealed experimentally

for instance on MgO(111) (Poon et al., 2006; Hacquart and Jupille, 2007),

NiO(111) (Rohr et al., 1994; Kitakatsu et al., 1998) and ZnO(0001) (Wang,

2008) by detecting the O–H vibrational bands. Also combined mechanisms

are reported, where molecular adsorption induces the formation of a partly-

lled surface state at the Fermi level, which in turn removes the surface

polarity (Wang et al., 2005). Adsorbate-mediated polarity healing, in general,

is responsible for the unique binding properties of polar systems and their

enhanced chemical reactivity with respect to non-polar materials (Sun et al.,

2009).

11.2 Polar oxide ﬁlms

Whereas for bulk materials the polarity needs to be healed in order to

avoid a divergence in the electrostatic energy, thin lms grown on metal

and semiconductor supports can be stabilized even in a polar state. This

difference to bulk materials relies on two effects. First, the electrostatic

energy might be kept below the lattice energy of the lm, as the number

of polar units is small (see Eq. 11.2). As a consequence, reconstruction of

the surface can be avoided and the lm keeps its polar nature. Second, the

substrate contributes to a reduction of the lm dipole, especially when using a

polarizable metal support. In this case, the required charge density that heals

the polarity according to Eq. 11.3 is provided by the substrate and localized

ThinFilm-Zexian-11.indd 261 7/1/11 9:43:27 AM

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262 Thin ﬁlm growth

at the metal–oxide interface (Goniakowski and Noguera, 2002, 2009). The

interplay between a polar lm and a metal support shall be demonstrated in

the following for a simple bilayer structure, consisting of a cationic and an

anionic plane (Fig. 11.5).

The preferred adsorption geometry of the lm is given by the electron

afnity of the substrate. Whereas the interface is formed by positively charged

ions on electronegative metals (e.g. Au and Pt), a negatively charged oxygen

layer sits on top of electropositive materials (e.g. Mg and Al) (Goniakowski

and Noguera, 2009). In the rst case, electrons accumulate in the topmost

substrate plane, leading to the following sequence of charged layers: metal

surface(–)/cations(+) / anions(–). In the second scenario, the charge distribution

changes to metal(+)/anions(–)/cations(+), as the electron density in the metal

surface is depleted by the polar lm. In both cases, the tri-layer structure has

a reduced vertical dipole moment and hence low polarity. This substrate-

mediated effect on the polarity might be enhanced by a real charge transfer,

whereby electrons ow out of the polar lm on electronegative metals but

into the ad-layer on electropositive supports. The resulting charge-driven

dipole always aligns in opposite direction to the lm moment and further

quenches the polarity of the metal–oxide system. The various electrostatic

interactions that occur between a metal surface and a polar lm, as well as

the dipole moments involved are summarized in Fig. 11.5.

Thanks to efcient polarity stabilization schemes, thin oxide lms with

residual dipole moment can be prepared on metal surfaces, even if the respective

bulk oxides are thermodynamically unstable. Well-studied examples for polar

oxide lms are MgO(111) on Ag(111) and Au(111) (Kiguchi et al., 2003;

Arita et al., 2004; Mantilla et al., 2008; Myrach et al., 2011), ZnO(0001)

on Ag(111) (Tusche et al., 2007), CoO(111) on Ir(100) (Giovanardi et al.,

2006) as well as FeO(111) on Pt(111) (Vurens et al., 1988; Ritter et al., 1998;

Rienks et al., 2005). STM topographic images of the respective lms are

displayed in Fig. 11.6. The residual polarity of the various systems depends

ﬁlm

supp

Electronegative support Electropositive support

(a) (b)

11.5 Sequence of oxide layers at the interface to an electronegative

(a) and an electropositive (b) support. In both cases, the intrinsic

dipole of the ﬁlm is oriented in opposite direction to the interface

dipole that results from a polarization/charge transfer between oxide

and metal support. This compensation effect partly quenches the

polarity of the combined system.

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263Electronic properties and adsorption behaviour of thin ﬁlms

largely on the lm structure and the nature of the metal–oxide interactions

and is rather different in all cases. Rocksalt MgO(111) is only polar in the

limit of ultrathin lms, but becomes non-polar with increasing thickness due

to the formation of 3D oxide islands (Myrach et al., 2011). In ZnO(0001)/

Ag(111), on the other hand, the polarity is suppressed already in the rst

atomic planes, which adopt a hexagonal boron nitride structure with Zn and

O ions lying in the same layer (Tusche et al., 2007). The wurzite structure

of bulk ZnO is only restored in thicker lms, which are, however, subject

to considerable surface roughening that quenches the reappearing surface

dipole. In rocksalt CoO(111) and spinel Co

(111), polarity healing is

achieved by a substantial decrease in the interlayer distance between the

top-most O and Co planes and a concomitant reduction of the ionicity of

the surface species (Giovanardi et al., 2006). A similar means to reduce the

surface polarity has been identied for FeO(111) on Pt(111). Using X-ray

diffraction techniques, the Fe-O layer separation in the lm was determined

(a)

(c)

(b)

(d)

22.5 Å

11.6 STM topographic images of polar oxide ﬁlms prepared on

metal supports. (a) ZnO/Ag(111) (9 ¥ 9 nm

, 1.2 V). Reprinted with

permission from Tusche et al. (2007), Copyright (2010) by the

American Physical Society. (b) FeO/Pt(111) (9 ¥ 9 nm

, 0.65 V). (c)

CoO/Ir(100) (4.5 ¥ 4.5 nm

, 0.15 V). Reprinted with permission from

Giovanardi et al. (2006), Copyright (2010) by the American Physical

Society. (d) VO/Rh(111) (6 ¥ 6 nm

, 2.0 V). Reprinted with permission

from Parteder et al. (2008), Copyright (2010) by Elsevier.

ThinFilm-Zexian-11.indd 263 7/1/11 9:43:27 AM

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264 Thin ﬁlm growth

as 0.68 Å, which is 50% smaller than the bulk value and substantially

decreases the surface dipole (Kim et al., 1997). As a result of this vertical

contraction, the FeO layer is expanded within the surface plane and the Fe

nearest-neighbour distance increases from its bulk value of 3.0 Å to 3.1 Å

in the lm. A similar tetragonal distortion has been revealed for other polar

lms, e.g. for VO/Rh(111) (Schoiswohl et al., 2005; Parteder et al., 2008)

and TiO

/Pt(111) (Sedona et al., 2005), indicating the universal nature of

this polarity healing mechanism.

In all these experiments, changes in the oxide stoichiometry and parasitic

adsorption processes have been excluded as means to heal the oxide polarity.

Thin oxide lms that can be prepared on various metal supports are therefore

well suited to study the adsorption behaviour of polar materials. However,

before this issue is addressed in Section 11.4, experimental techniques

to quantify the amount of oxide polarity will be introduced in the next

Section.

11.3 Measuring polarity of thin oxide ﬁlms

In most experiments, the polar nature of thin lms is concluded from indirect

evidence, for instance from a reconstruction of the surface (Dulub et al.,

2003; Ostendorf et al., 2008), unusual electronic properties (Kiguchi et al.,

2003; Arita et al., 2004) or an adsorption behaviour that strongly deviates

from the non-polar case (Rohr et al., 1994; Wang et al., 2005; Poon et al.,

2006). Such indirect indications give no information on the nature of the

actual polarity healing mechanism and on the size of the remaining surface

dipole. However, this kind of data can be obtained with STM, a technique

that is usually employed to probe the surface topography. In the following,

two spectroscopic applications of STM are discussed that enable detection of

the local surface potential as a measure for the polarity. The techniques are

based on the evaluation of (i) energy positions of eld emission resonances

(FER) and (ii) effective barrier heights for electron-tunnelling into the

polar lm. They are demonstrated using the example of the FeO(111) lm

mentioned above (Rienks et al., 2005), but can be applied to any other polar

system with sufcient conductivity to perform STM.

The FeO lm has a bilayer structure, consisting of a hexagonal O layer

at the surface and a hexagonal Fe plane at the interface to the Pt(111) (Fig.

11.7). It has a polar character due to the ionic nature of the Fe

and O

d–

species and belongs to type III materials in the Tasker scheme (Tasker,

1979). The FeO lm has an in-plane lattice parameter of 3.1 Å, being 11%

larger than that of Pt(111) (2.76 Å). This mismatch leads to the formation of

a coincidence lattice with 25 Å edge length and a crystallographic relation

of (√91 ¥ √91)R ± 5.2° with respect to the Pt support (Ritter et al., 1998).

Within the coincidence cell, three stacking domains are distinguishable that

ThinFilm-Zexian-11.indd 264 7/1/11 9:43:27 AM

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265Electronic properties and adsorption behaviour of thin ﬁlms

differ in their Fe binding geometry on the Pt(111) surface (Fig. 11.7a). In the

top domain, the interfacial Fe atoms bind on top of Pt, while the O atoms in

the layer above occupy fcc hollow sites. In the fcc (hcp) domains, Fe binds

to fcc (hcp) hollow sites while O sits in hcp (top) positions of the Pt(111)

lattice. The different binding congurations give rise to a distinct contrast of

the FeO lm in low-bias STM images (Fig. 11.7b). The discernible regions

have been assigned to the underlying stacking domains with the help of

model calculations based on DFT (Giordano et al., 2007b) and a quantum-

chemical scattering approach (Galloway et al., 1996).

The binding of Fe and O atoms either to Pt hollow or top sites modulates

the interlayer distance and hence the surface dipole in the three oxide domains.

The resulting change in polarity is an observable quantity and produces a

strong bias-dependent contrast of the FeO lm in STM images taken at

high sample bias (Fig. 11.8b). At 4.25 V, one region of the coincidence

cell (marked with a circle) is imaged with a particularly bright contrast.

The contrast is not related to a geometric effect, as the low-bias corrugation

of the lm that reects its true morphology is very small (0.3 Å). It is, in

fact, of electronic origin and caused by a strong increase in the conductance

through the STM junction at this particular bias. To verify this statement, the

differential conductance (dI/dV) through the FeO/Pt(111) system has been

measured directly as a function of bias voltage, as shown in Fig. 11.8a. In

correspondence to the topographic image, high dI/dV intensity at 4.25 V

is observed in the domain marked by the circle. However, with increasing

bias the dI/dV maximum moves to an adjacent domain, being agged by

the square, and nally appears in the triangle region at 4.9 V sample bias.

These bias-dependent conductance changes are caused by electron transport

through conned states that develop in the classical part of an STM junction

and become accessible for electrons at elevated bias (Binnig et al., 1982;

fcc

hcp

top

25 Å

–Fe

–Pt

(a) (b)

11.7 (a) Structure model of the coincidence cell formed between FeO

and Pt(111). The O-top layer is omitted for the sake of clarity. (b)

STM topographic image taken at 65 mV (7 ¥ 7 nm

). The different

stacking domains are assigned in accordance to the DFT calculations

presented in Giordano et al. (2007b).

ThinFilm-Zexian-11.indd 265 7/1/11 9:43:27 AM

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266 Thin ﬁlm growth

Becker et al., 1985). Those states may be considered as eigenstates in a

triangular potential, being conned by the sample surface on one side and the

down-sloping vacuum barrier on the other (Fig. 11.9b). Electrons penetrating

this classical region are able to form standing waves, if multiples of half

their electron wavelength match the distance between the two boundaries.

In this case, propagating and reected waves interfere constructively and

quasi-bound electronic states with a high transmission probability develop in

the STM junction. These states are termed eld emission resonances (FER)

and dominate the STM image contrast at elevated bias. Interestingly, they

also contain information on the local surface potential F and might therefore

be used to probe oxide polarity (Fig. 11.9b). The interplay between FER

and F can be explained with a simple picture. The electrostatic energy due

to uncompensated polarity (Eq. 11.2) produces an offset on the surface

potential that shifts the FER to higher energy and modulates their availability

for electron transport through the oxide lm. Up-shifted FER are therefore

indicative for polar regions, while down-shift states occur in areas with

compensated polarity.

Already this crude model is sufcient to connect the contrast changes

observed in the dI/dV maps of FeO/Pt(111) to spatial modulations of the

surface polarity (Rienks et al., 2005). The domain that turns bright at 4.25 V

is the one with lowest surface potential, as the rst FER becomes available

at relatively low bias (Fig. 11.8a). Based on a simple hard-sphere model of

the lm, this region is assigned to the FeO top domain, in which Fe and

O atoms occupy top and hollow sites of the Pt surface, respectively, and

the Fe-O interlayer distance and hence the surface dipole are small. At 4.5

V, the FER become available in the square regions, which have a slightly

higher F as indicated by the up-shifted resonances. The region marked by the

(a)

(b)

4.3 V 4.5 V 4.7 V 4.8 V 4.9 V

11.8 (a) Conductance and (b) topographic images of FeO/Pt(111)

taken as a function of the bias voltage (9 ¥ 9 nm

). The contrast

change reﬂects the varying contributions of ﬁeld emission

resonances to the electron transport through the different stacking

domains. Reprinted with permission from Rienks et al. (2005),

ThinFilm-Zexian-11.indd 266 7/1/11 9:43:28 AM

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267Electronic properties and adsorption behaviour of thin ﬁlms

triangle turns bright only at 4.9 V and consequently has the highest F and

hence the highest degree of polarity. The hard-sphere model connects this

region to the hcp domains, as the vertical distance between the hollow-bound

Fe atoms and the top-bound O atoms is expected to be largest there. The

sequential availability of FER in the different domains is not so evident in the

topographic images, which contain information on the integral conductance

probed over a large bias window (Fig. 11.8b).

top

fcc

hcp

Conductance

Energy

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Sample bias (V)

(a)

vac

Tip

Sample

-eV

Tip

FeO

Barrier

Pt(111)

–

FER

levels

n = 5

n = 4

n = 3

n = 2

n = 1

(b)

11.9 (a) Conductance spectra obtained with closed feedback loop

on the three stacking domains of the FeO ﬁlm, as indicated in the

STM image shown in the inset. Each maximum marks the position

of a ﬁeld emission resonance (FER) in the tip-sample junction. (b)

Potential diagram visualizing the formation of FER at high bias

voltage and their dependence on the local work function. Reprinted

American Physical Society.

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