Ambiguous Role of Growth-Induced Defects on the Semiconductor-to

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The ambiguous role of growth-induced defects on the semiconductorto-metal characteristics in epitaxial VO2/TiO2 thin films. Cristian Nicolae Mihailescu, Elli Symeou, Efthymios Svoukis, Raluca Negrea, Corneliu Ghica, Valentin S Teodorescu, Liviu Tanase, Catalin Negrila, and John Giapintzakis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01436 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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ACS Applied Materials & Interfaces

The ambiguous role of growth-induced defects on the semiconductor-to-metal characteristics in epitaxial VO2/TiO2 thin films.

Cristian N. Mihailescu, †, ‡ Elli Symeou, † Efthymios Svoukis, † Raluca F. Negrea, § Corneliu Ghica, § Valentin Teodorescu, § Liviu C. Tanase, § Catalin Negrila, § and John Giapintzakis *† †

Department of Mechanical and Manufacturing Engineering, University of Cyprus, 75 Kallipoleos Avenue, PO Box 20537, 1678 Nicosia, Cyprus ‡

National Institute for Laser, Plasma and Radiation Physics, 409 Atomistilor Street, PO Box MG36, 077125 Magurele, Romania §

National Institute of Materials Physics, RO-077125 Magurele, Romania

ABSTRACT: Controlling the semiconductor-to-metal transition temperature in epitaxial VO2 thin films remains an unresolved question both at the fundamental as well as the application level. Within the scope of this work, the effects of growth temperature on the structure, chemical composition, interface coherency and electrical characteristics of rutile VO2 epitaxial thin films grown on TiO2 substrates are investigated. It is hereby deduced that the transition temperature is lower than the bulk value of 340 K. However, it is found to approach this value as a function of increased growth temperature even though it is accompanied by a contraction along the V4+V4+ bond direction, the crystallographic c-axis lattice parameter. Additionally, it is demonstrated that films grown at low substrate temperatures exhibit a relaxed state and a strongly reduced transition temperature. It is suggested that, besides thermal and epitaxial strain, growthinduced defects may strongly affect the electronic phase transition. The results of this work reveal the difficulty in extracting the intrinsic material response to strain, when the exact contribution of all strain sources cannot be effectively determined. The findings also bear implications on the limitations in obtaining the recently predicted novel semi-Dirac point phase in VO2/TiO2 multilayer structures. KEYWORDS: growth of VO2 films on TiO2, thin film epitaxy, semiconductor-metal phase transition, pulsed laser deposition, thermal and epitaxial strain, interdiffusion between film and substrate

* E-mail: [email protected]

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

seems to be a strong belief that it should be

1. INTRODUCTION Transition

a

considered as a Peierls-Mott insulator

plethora of interesting properties that are

where electron-electron correlations and

beyond the capabilities of conventional

dimerization of the V4+ ions contribute to

semiconductors,

in

the opening of an insulating gap. 7-12 Recent

electronics and optoelectronics. Therefore,

reports have revealed that, in the insulating

they provide the basis for novel concepts in

state, VO2 can form other structural phases,

oxide-based electronic devices.1-4 Oxides

i.e. M2-phase, in which only half of the

exhibiting electronic phase transitions are

vanadium atoms dimerize, while the other

at the focal point of materials physics, since

half remain equidistant along the resulting

the mechanisms governing such transitions

zigzag chain. Nevertheless, this polymorphic

are a topic of nearly half a century of

structure is a metastable one that can be

debate, yet still not fully understood.5

obtained only by doping

Among them, vanadium dioxide (VO2) - and

stress

its

temperature-driven

thermodynamically favored phase). Above

semiconductor-to-metal transition (SMT)

the TSM, the system undergoes a lattice

near room temperature - have recently

distortion to a higher-symmetry rutile-type

attracted a great deal of attention. This is

tetragonal structure (R-phase, space group

due, not only to its intriguing nature, but

P42 /mnm) that behaves as a half-filled

also to the increasing number of promising

metal (S=1/2, metallic paramagnet). 5,16-18, 19

first

applications.

metal

Page 2 of 33

oxides

commonly

order

Below

the

exhibit

used

7-10, 14, 15

11-13

or applied

(M1 is the stable and

The majority of experimental and

transition

temperature (TSM~340 K), VO2 adopts an

theoretical

insulating ground state and a monoclinic

electronic SMT and the structural phase

structure (space group P21 / c, M1-phase)

transition, (SPT), occur simultaneously.

wherein the 3d electrons are localized on V

Based on this premise, several scenarios

sites that form a singlet spin state

including, but not limited to, electron-

(insulating diamagnet).6,7 The nature of this

lattice coupling (Peierls transition),10,

low-temperature phase is the subject of a

spin–Peierls,19,

long-standing

electron interactions (Mott-Hubbard) are

debate;

however,

there


studies

20-22

suggest

that

the

12

and strong electron-

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nowadays considered as key theoretical

an intermediate monoclinic structure (M3).

components in explaining the mechanisms

Based on these observations the authors

triggering the SMT in VO2.10,

23

suggested that the SMT is not driven solely

However, the recent observation of distinct

by the dimerization of V4+ ions, which is

time scale for the SPT and resistance

considered to be the microscopic origin of

changes

phase

the transition, but also by the formation of

transition boundary has cast doubt once

hole carriers in the VO2 layer. In fact, even

again on the validity of the aforementioned

after more than half a century since the first

SMT

femtosecond

report by Morin, in 1959, the driving

pump-probe measurements on VO2/TiO2

mechanism still remains under debate,

thin films, Zhenshen et al.24 observed that,

swaying between two or more factors.7, 10,

after the occurrence of SMT, the structural

12, 20

across

the

mechanisms.

18,

electronic

Using

20,

M1-R SPT is delayed by the stabilization of

2. BACKGROUND

design new technologies, e.g. the MIT

Over the last decade the research

transistor concept, increasingly small -

paradigm of VO2 has evolved from pure

reaching a few nm thick -VO2 films are

academic curiosity to pursuit of real-world

required.18,31 Although high-quality and

functional devices such as next-generation

phase-pure VO2 thin films have so far been

transistors, sensors, memristive systems,

grown by a series of deposition techniques,

energy

brain-inspired

the constant and technologically relevant

neuromorphic or Non-Boolean computing,

need of adjusting the TSM to suit different

memory meta-materials and so on.18,25-30

needs has generated an impressive number

Considering that all envisaged applications

of experimental studies. In this direction,

rely on the SMT and that bulk VO2 single

many approaches have been considered to

crystals

modulate the TSM, such as the substrate

saving

cannot

heating/cooling

windows,

withstand cycles, the

repeated

symmetry

heightened

and

interest on the growth of VO2 in thin film

orientation,32,33

form is comprehensible.16 In addition, to

39

growth


layer

crystallographic thickness,31,33,34-

temperature,32,33,40

effect

of


doping,11-13,41,42

Page 4 of 33

hydrogenation,43,44

effecting a net decrease of the electron-

interface quality, usage of a buffer layer,

electron interactions and hence, causing a

etc.

3,16,22,34,36,45

published

phase transition at a lower temperature.9,36

As a general rule, in all

studies

ones

Following this line of thought, the

investigating the hydrogenation or the

commonly employed strategy towards a

doping

preferentially

controlled strain-induced modulation of the

stabilize the R structure or alter the phase

c-axis length requires lattice considerations

diagram to favor the formation of the

and predominantly involves variation of the

metastable M2 structure,19 the stability of

film’s thickness, i. e. the upper limit of

the metallic phase over temperature is

strain retained in the film structure is

associated with the film’s strained nature.

restricted by the film-substrate lattice

More specifically, it is associated with the

mismatch and can be preserved only for

lattice distortion along the V4+-V4+ bond

small layer thicknesses. Alternatively, by

(crystallographic c-axis) direction in the R-

increasing the layer thickness the structure

phase and its effect on the bandwidth of

gradually

the V 3d orbitals. In the conducting phase,

parameters, as well as the TSM, return to the

the low-energy t2g band originating from the

corresponding bulk values. We note that

splitting of the V 3d levels will further split

this scenario refers only to the isolated case

into a d// and a π* state as a result of the

of heteroepitaxial growth of R-VO2 films

anisotropic crystal field generated by the six

that invariably involves the presence of

O atoms in the rutile structure; the

strain, and can be obtained only on

overlapping of the d// states depends on the

substrates that exhibit a mismatch with the

relative ratio of cR/aR (in an ideal tetragonal

R-VO2 lattice smaller than 1%. If the lattice

structure cR/aR = 0.66). Since the d// orbitals

mismatch is too high then it becomes

spread along the V4+-V4+ chain, it is to be

energetically favorable to reduce the strain

expected that any contraction along this

energy through the formation of semi-

direction will induce a direct overlap of the

coherent interfaces, defects or the growth

d orbitals (upward shift of d// bands and a

of randomly oriented deposits.46 Even

downward shift of the π* bands) further

though

effect

that

besides

can

the


relaxes

and

polycrystalline

VO2

the

lattice

films

are

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accepted in a wide range of applications, in

this case it is almost impossible to speculate

Figure 1 Dependence of the TSM on the c-axis length (a) and different epilayer thicknesses in strained VO2 films (b).

or predict the origin of the strain and

from the contribution of defects; as the

further understand its contribution to the

accumulated

physical

this

linearly with the thickness of the epilayer,

thickness-related strategy cannot separate

above a certain thickness - specific for each

the material intrinsic response to strain

material

properties.

Nevertheless,


strain

and

energy

known

increases

as

the


Page 6 of 33

pseudomorphism limit - a maximum storage

epilayer thickness and c-axis length on the

capacity of elastic energy is reached.

SMT, even when the growth approach and

Consequently, defects are created, i.e. low–

the applied conditions are similar (see

angle grain boundaries, twinning, misfit

Figure 1).

dislocations, edge dislocations at the

In the current literature, the majority of

interface, and/or, surface roughening and

experimental

changes of growth mode occur, all leading

decreasing the TSM in epitaxial VO2 thin

to the minimization of the epitaxial strain

films follows the thickness-dependence of

developed in the early stages of the growth

only one relaxation mechanism, occurring

process. Because such defects provide

at a certain level such as the surface, inner

relaxation paths that are typically difficult

volume or the interface, leaving the issue of

to monitor, and, more importantly, can also

other possible contribution - or their

serve as local sources of strain or generate

coexistence - unaddressed. Herein, owing

inhomogeneous strain, their actual effect is

to the natural isomorphism between the R-

always complex and difficult to interpret

VO2 lattice and the R-TiO2 substrate, but

within a simple framework. Moreover, each

also their different thermal expansion

growth method yields different nucleation

coefficients, we were able to control the

and growth modes, therefore, the critical

film’s strained nature without any (or with

thickness below which the epitaxial strain

minimal) change of their thickness. The VO2

can be accommodated coherently changes

thin films were grown at different substrate

not only from one material to another but

temperatures (Ts), and the structural

also with growing method and applied

evolution,

conditions,

whose

are

composition, interface quality and electrical

nowadays

essentially

In

characteristics have been systematically

combinations limitless.

investigated

contribution, based on studies following

diffraction

this

spectroscopy

has

proven

to

be

troublesome and the results are often

microscopy

controversial concerning the effect of the

transmission


dealing

morphology,

consequence, to quantify the exact strain

approach,

studies

by (XRD),

chemical

high-resolution X-ray

(XPS), (AFM), electron

with

X-ray

photoemission atomic

force

high-resolution microscopy

(HR-

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TEM), electron energy loss spectroscopy

detail in Ref.47, the oxidation state of

(EELS)

vanadium

and

temperature-dependent

can

be

altered

by

small

electrical resistance measurements. Based

modifications of the partial pressure of

on these characterization techniques it

oxygen P(O2) used during film growth. Here,

became feasible to evaluate the films’

prior to each deposition the chamber was

quality and discuss a rational interpretation

evacuated to a base pressure < 7 x 10–3

for the observed behavior of SMT. The

mTorr; while during the ablation process

effect of the deposition temperature on the

the P(O2) was kept constant at 12 mTorr.

growth mode, interface reactions, as well as

The films have been grown at substrates

their combined influence on the TSM are

temperatures, TS, in the range of 280°C to

also discussed. Additionally, we have

600°C using heating and cooling rates of 10

reviewed the literature in the context of

°C / min. All films were grown using the

defects and the related strain effects on the

same number of laser pulses (5000).

SMT nature in epitaxially grown TiO2/VO2

Sample Characterization. The films’ crystal

thin films.

structure, growth orientation, thicknesses and lattice parameters were determined by

3. MATERIALS AND METHODS Growth of the films. Epitaxial VO2 films

XRD

were grown on (0 0 1) oriented TiO2

measurements performed using a Rigaku

substrates by pulsed laser deposition (PLD).

SmartLab (9kW rotating anode, Cu Kα1

A KrF* laser (λ = 248 nm, τ = 25 ns)

radiation) diffractometer in parallel beam

operated at 5 Hz was used to ablate a high-

configuration,

purity (99.99 %) dense vanadium target

transmission

using an average energy fluence of ~ 1.3 J /

TEM). The TEM/HR-TEM studies were

cm2. The use of a dense target ensures a

performed using the analytical transmission

low deposition rate (0.12-0.2 Å / pulse),

electron

which is expected to promote slow

operated at 200 kV. The cross – section

oxidation kinetics and induce a larger

specimens for TEM observations (XTEM)

resistance

transition

were prepared by mechanical polishing

temperature. As has been discussed in

followed by ion milling at low angle. The

change

at

the

and


X-ray

and electron

microscope

reflectivity

(XRR)

high-resolution microscopy

JEOL

ARM

(HR-

200F


Page 8 of 33

films’ strained nature was further evaluated

The films’ surface morphology was

by Geometrical Phase Analysis (GPA) in

analyzed by AFM in non-contact mode using

conjunction with XRD and HR-TEM.

an NT-MDT INTEGRA Probe NanoLaboratory

One key limitation in the growth of VO2/TiO2

thin

films

is

the

system (NT-MDT NSG01 cantilever with tip

atomic

radius

of

10

nm).

The

temperature

interdiffusion, 37 which was evaluated from

dependence of electrical resistance was

EELS

X-ray

measured by a conventional four probe

spectroscopy (EDS) maps at the interface

method using a Quantum Design Physical

level. The surface composition of the

Property Measurement System (QD-PPMS).

substrates was investigated by XPS. The XPS

To ensure a good ohmic contact, double-

measurements were performed in an AXIS

electrodes were used, 20nm Ti/100nm Au,

Ultra DLD (Kratos Surface Analysis) setup

which

equipped with an 180° hemispherical

temperature and further annealed at 300°C

analyzer, using Al Kα1 (1486.74 eV) radiation

for 1 h in an Ar flow (15 cc/min).

and

energy

dispersive

were

dc

sputtered

at

room

produced by a monochromatized X-Ray source at operating power of 300 W (15 kV

4. RESULTS AND DISCUSSIONS

× 20 mA). Partial charge compensation was

In most experimental studies, the (001)

reached by using a flood gun operating at

oriented R-TiO2 single-crystals are used as a

1.52 A filament current, 2.73 V charge

template to favor a highly ordered growth

balance, and 2.02 V filament bias. High

of

resolution core level spectra were recorded

parameters;48,49 Table 1 reports their

using Field of View 2 lens mode, 20 eV pass

crystallographic properties and lattice misfit

energy and a 110-µm aperture.

(γ).

R-VO2

due

to

the

close

lattice

Table 1 Crystal lattice parameters R-TiO2(Å)

R-VO2(Å)

γ (%)

a (nm)

4.593(3)

4.554(6)

0.84

c (nm)

2.958(7)

2.855(7)

3.48

The lattice misfit given in percent has been calculated using, γ = (asubstrateafilm)/asubstrate*100. 8 ACS Paragon Plus Environment

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Assuming that the only way strain, (ε), can

the [001] direction. The total room-

be induced in a hetero-epitaxial film is

temperature ε developed along the [100]

through lattice mismatching with the

direction has basically three contributions:

thermal contribution

lattice misfit

α α dT ∗ 100 and , ε

* + #$ %&' ()

"

*

*100,

along the [100] direction,

(1)

∗ 100, along the 20013 direction,

(2)

substrate crystal, it is to be expected that

(a) epitaxial strain due to lattice parameters

the thermal expansion of the substrate

misfit

lattice

the

temperature), (b) thermal strain developed

substrate-film lattice mismatching, (γ), and,

during the cooling down from the high

thus, in the film’s strained nature. From the

temperature of growth due to the different

values presented in Table 1, we expect that

thermal expansion coefficients of film and

the stabilized VO2 epilayers will probably

substrate, and (c) intrinsic strain due to

undergo tensile ε along the [100] direction

kinetics of film growth (coalescence of

because the a-axis lattice parameter of the

grains and islands), which depends on

TiO2 substrate is larger than that of R-VO2.

growth rate and temperature.45 We note

Compressive

be

that the calculated strain values do not take

correspondingly developed in the VO2

into account the intrinsic ε. To determine

epilayers along the [001] direction. Figure

the intrinsic ε will require a powerful in-situ

2a shows the calculated thermal expansion

characterization tool that can map the

of the R-TiO2 substrate and that of the R-

films’ structural evolution in real-time. The

VO2 along the [100] direction, while Figure

values presented in Figure 2b have been

2b illustrates the expected amount of

determined from:

will

induce

strain

variations

should

in

then

(calculated

room-temperature ε developed in the VO2 epilayers as a function of Ts along the [100] direction, as well as the residual one along


at

the

growth


where ΔT is the change in temperature, α is

room temperature (Figure 3a). Apart from

the thermal expansion coefficient along the

the substrate peak, only the (002) Bragg

[100] direction, and a VO2, cVO2, VVO2 and

diffraction peak of R-VO2 could be observed

aTiO2 represent the bulk-like values of VO2 and

in the entire scanned range up to 2θ=90°

TiO2 unit cells. We underline that the expected

(see also Supporting Information Figure

ε values are estimates, which have been

S1a).

determined based on the assumptions: (i) of a “defect-free growth process”, hence excluding the existence of any defects that may assist the ε development or relaxation (coherent epitaxy); (ii) that the close lattice parameters and similar symmetries (including the same space group, P42 /mnm) of the two materials will force the film’s lattice to adopt the in-plane lattice parameter of the substrate at all temperatures (fully strained); and (iii) of the preservation of the unit cell volume (Poisson effect).

Figure 2 (a) Thermal expansion of R-TiO2 and R-VO2 along the [100] direction. (b) The expected ε evolution in the VO2 epilayers lattice as a function of Ts; αTiO2= 7.249 x 10– 6 + 2.198 x 10–9 (T-273) + 1.298 x 10–12 (T273)2, αVO2= 5.828 x 10–6 – 7.091 x 10–9 (T273) + 6.946 x ·10–12 (T-273)2. 48,49

Structural characterization. To evaluate the films’ structural quality, detailed 2θ/ω XRD scans of VO2 thin films were acquired at 10

ACS Paragon Plus Environment

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The absence of other diffraction peaks

and a similar structural quality between the

implies that the films are single phase and

VO2 films and the TiO2 substrates, see

highly textured along the [001] direction.

Supplementary Information Figure S1b. It is

Also, the presence of well-defined layer

noted that the FWHM increases when Ts is

fringes suggests atomic-scale smoothness,

increased above 400°C. This variation could

high crystallinity and low defect density for

be attributed either to the change in the ε

the films. Both, the AFM images and the

or to the increased number of defects

existence of thickness-interference fringes

(diffusion-related as will be discussed later).

in the XRR profiles confirmed the uniformity

The epitaxy of the films was confirmed by

of VO2 films and the smooth surface (root

symmetric XRD φ-scans of the (2 2 0) peaks

mean square: ~0.4 nm). Furthermore, the

of the TiO2 substrates and VO2 films (shown

full width at the half maximum (FWHM) of

in the Supporting Information Figure S2),

the (0 0 2) rocking curve varies between

where four peaks separated by 90°

0.069° and 0.0955°. These values are

appeared over a full circle due to the four-

comparable with those of the underlying

fold symmetry of the crystal structure. The

TiO2 substrates demonstrating a high

fact that the peaks for the films and the

degree of alignment of the (00Ɩ) R-VO2

substrates

planes parallel to the substrate’s surface

azimuthal


are

located

at

the

same


Figure 3 (a) Detailed 2θ/ω XRD scans. (b) Symmetric 2θχ/φ XRD scans around the (2 2 0) diffraction position of the R-TiO2 and R-VO2. The in-plane scans were carried out with 2θχ = 56.4244° for TiO2 and 57.1592° for VO2, ω = 0.33 - 0.62°, φ = 160 - 270° and χ = 0.1 - 0.3°. The scans have been recorded exsitu at room temperature. (c) The VO2 lattice evolution as a function of Ts.

angles

confirms

the

single-crystal-like

temperature, Ts = 280°C, the diffraction

character of the films and indicates that the

position of the (0 0 2) VO2 peak was the

epitaxial relationships are: VO2 [0 0 2] ∥ TiO2

same as that of the bulk R-VO2 (ICDD 79-

[0 0 2] and VO2 [2 2 0] ∥ TiO2 [2 2 0]. The

1655), suggesting, in this case, a relaxed

evolution of the a- and c-axis lattice

state of the lattice. In fact, the presence of

parameter lengths as a function of Ts were

lattice relaxation was also confirmed by

calculated based on a Gaussian fit of the (0

GPA measurements, as discussed later.

0 2) and (2 2 0) peaks, see Figures 3a,b. We

Based on the experimentally obtained

underline that for the lowest deposition

values, Figure 3c illustrates that for Ts ≤ 12


Page 12 of 33

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axis lattice parameter continues to follow the thermal expansion of the TiO2 substrate lattice. This observation can be explained either by changes of the growth mode as a function of Ts, which, however, do not justify the abrupt increase of the epilayer’s thickness, or by the Ti ions diffusion from the substrate into the VO2 lattice. As Figure 4b shows, for Ts > 400°C the epilayer’s thickness exhibits an abrupt increase from 27 nm to 38 nm at Ts = 420°C; meanwhile, the insert of Figure 4a shows the existence of double critical angles in the XRR profiles of films grown at high Ts – indicative of secondary phases in the films’ structure.

400°C the lattice evolution of the VO2 epilayers is in a good agreement with the

This is postulated to be due to the diffusion

expected one. That is by increasing the

of Ti ions into the VO2 at Ts > 400°C, leading

substrate temperature, the VO2 a-axis

to the formation of multiple phases (VO2

lattice parameter exhibits a linear increase;

and V1-xTixO2). Similar results have been

this is due to the tensile ε imposed by the

reported by Muraoka et al. where the

substrate’s thermal expansion leading to

expansion along the c-axis direction was

the development of a residual ε along the c-

attributed to the Ti ions diffusion from the

axis direction into VO2 epilayers, see also Figure 2a. On the other hand, in the case of the samples grown using Ts > 400°C, the caxis lattice parameter has shown a linear increase

suggesting

that

above

this

temperature the epilayer’s unit cell cannot preserve the bulk material volume, as the a13 ACS Paragon Plus Environment


Page 14 of 33

substrate into the epilayer’s lattice and the

images)

formation of secondary (V, Ti)O2 phases.32,33

micrographs displayed in Figures 5a, g, m

It is interesting to note that this correlation

reveal a columnar type of growth for the

was based only on the observation of

samples deposited at 400°C and 500°C,

multistep SM transitions, without any

while the sample deposited at 300°C

structural

exhibits

or

compositional

supporting

measurements.

a

pyramidal

The

TEM

(Volmer-Weber)

information. In a recent experiment from

growth mode. The epitaxial nature of the

the same group using in situ synchrotron

films was confirmed by selected area

radiation photoemission spectroscopy, it

electron diffraction (SAED) patterns from

was shown that even for Ts = 300°C an

the interface region, which indicated the

insulating (V, Ti)O2 solid solution is formed

presence

near

in

structures: the R-TiO2 and the R-VO2. The

contradiction with the previous work, in the

presence of these two structures is marked

latter experiment the SMT did not occur

by the 002 spot splitting in the SAED

Figure 4 (a) XRR profiles of the VO2 thin films grown at different Ts. (b) Layer thickness vs. Ts. The film thickness was extracted from the period of the layer fringes using the following equation: t=λ/2(sinθ2 - sin θ1) where t is the film thickness, λ the wavelength of Cu Kα1 radiation and θ1 and θ2 the diffraction angles of two consecutive fringes. The obtained values were consistent with the ones determined from the XRR profiles and TEM analysis.

patterns (shown by arrows and zoomed into

the

interface.

However,

of

two

tetragonal

crystal

the inset in Figures 5b, h, n). The orientation

relationship

between

the

crystallographic axes of the R-VO2 thin films and TiO2 substrates is: VO2 [002] ∥ TiO2 [002] (in the out-of-plane direction) and RVO2 [101] ∥ TiO2 [101] (along the in-plane

stepwise, although the concentration of Ti

direction). The chemical composition at the

was found to be high near the interface and

interface between TiO2 substrates and VO2

decreasing toward the surface of the film.37

epilayers was investigated by EELS - SI

This indicates that the role of Ti doping on

elemental analysis. Figures 5f, l, s show the

SMT is not clearly understood.

elemental maps corresponding to the V-L2, 3

In

order

to

illustrate

the

edges at 513 and 521 eV, Ti-L2,

film’s

3

edges at

microstructure evolution as a function of Ts,

456 and 462 eV, and the O-K edge at 532

we performed TEM and EELS – SI (spectrum

eV, respectively, extracted from EELS-SI


Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


data cube. Interestingly, despite the high

interdiffusion

growth temperature, even in the case of

interdiffusion was, however, detected for

the sample grown at Ts = 500 °C, the

Ts=600 °C, see Figure 6.

interface was coherent and no atomic


was

observed.

Ti


Page 16 of 33

Figure 5 (a), (g), (m) TEM images at low - magnification showing the TiO2 substrate and VO2 thin films grown at Ts = 300°C, 400°C and 500°C. (b), (n), (h) The SAED patterns corresponding to TEM images. (c), (d), (i), (j), (o), (p) HR-TEM images of the VO2/TiO2 structures along the [010] orientation. (e), (k), (r) Highangle annular dark-field (HAADF – STEM) images. (f), (l), (s) EELS – SI maps revealing the elemental distribution of V, Ti and O elements in the samples.

Physical properties. To elucidate the effect

strained nature each of the samples has

of Ts on the TSM, the temperature

revealed a SMT change of 2 – 3 orders of

dependence of the electrical resistivity of

magnitude in the resistivity; see Figure7a-c.

the VO2 epilayers was measured across the

For all investigated Ts, the TSM, taken here

SM phase transition. Irrespectively of their

as the midpoint of the jump in the 16


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solutions, see Figure 6, and the TSM increases due to the increasing content of Ti. It has been reported in the literature that the choice of Ts is the most critical among growth parameters, in particular for the films’ phase stability and the interfaces’ coherency;33, 37, 40,

41

thus, one may expect

that the Ts will affect the VO2 lattice, and, consequently,

the

structural-electrical

relationships, in a different way for each Ts used. First, for Ts ≤ 400°C the modulation of Figure 6(a) Low-magnification XTEM image of the VO2/TiO2 film grown at Ts=600°C (in the TSM is partially accounted by the orientation), (b) high magnification detail of the asperity formed on the Figure 7 Electrical resistivity vs. temperature for VO2 films grown at (a) Ts ≤ 400°C, (b) 400 < Ts ≤ surface of the film. The surface planes are 500°C, (c) Ts ≥ 500°C, and the corresponding derivatives of log10ρ(T) for the heating curves in of the (011) type, (c) SAED pattern, and the (d), (e) and (f), respectively. TEM-EDS analysis in (d) and (e). The VO2 film extends in the white and gray region modification of the c-axis length. In this of the Figure 6d. temperature range, we propose that the resistance curve measured on the heating SMT is also affected by the growth-induced cycle, was found to be lower than that of defects suppressing the TSM, as discussed the bulk counterpart.6 In Figure 7d-f, the later in detail. Second, for 400°C < Ts ≤ 500 Gaussian fittings on the differential curves °C the Ti ions start to diffuse into the VO2 of the resistivity in heating cycle are lattice leading to the formation of presented. As a function of Ts three distinct secondary V1-xTixO2 phases responsible for types of behavior can be observed: (i) for Ts the multiple transitions. Ref. 32,33 have ≤ 400 °C, the TSM increases linearly with the investigated the transport properties and growth temperature, (ii) for 400 < Ts ≤ electronic states of epitaxial VO2 thin films 500°C, multiple transitions can be observed grown on (0 0 1) TiO2 substrates and found suggesting that Ti ions diffused in the R-VO2 that high Ts can induce Ti doping into the epilayers, and (iii) above 500 °C, the films VO2 films leading to the formation of are transformed into V1-xTixO2 solid 17 ACS Paragon Plus Environment


Page 18 of 33

secondary phases, which alter the SMT.

grown at Ts = 600 °C. In good agreement

Here, the extent of Ti diffusion has a local

with the SAED pattern and the EDS line

character as it has not been observed in the

profiles,

TEM or EELS investigations but is consistent

magnification XTEM images show that the

with the multistep transitions in R(T) and

film stoichiometry has been completely

XRR, as well as the complete formation of

transformed into V1-xTixO2 solid solution by

V1-xTixO2 solid solution observed at Ts = 600

the massive diffusion of the Ti ions into the

°C. Such local information cannot be

VO2 lattice - the SAED pattern indicates the

detected by the aforementioned techniques

presence of only one rutile V1-xTixO2

because the diffusion area is too small. Jing

structure. We note that although much

et al.41 and Takahashi et al.42 have reported

research has been focused on the effect of

an increase of 20 K in the TSM for 2% of Ti

ion doping on the SMT of VO2, the

doping. This increase is very large compared

microscopic mechanism remains a matter

with the TSM variation presented here.

of great debate swinging between the role

Third, for Ts ≥ 550 °C the films are entirely

of charge doping and the effect of lattice

transformed into V1-xTixO2 solid solutions.

distortion. Moreover, recent reports have

Figure 6a shows the XTEM images of a film

indicated 18


Figures

6c,d,e,

contradictory

the

results.

low-

For

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example, Ref. 4, 38, 42, 50 - 56 reported

on the characterization of the films grown

that the TSM is dramatically elevated by

in the temperature range of 280°C to 400°C

increasing the doping concentrations of Ti,

since above this temperature another

while Ref.56, 57 found that doping of Ti

extrinsic factor affects the films’ strain

could lower the TSM of a VO2 film. More

nature and the SMT, i.e. Ti diffusion.

recently, Yanfei et al. have investigated the

Table 2 summarizes the films’ SMT

effect of Ti4+-doping in VO2 nanopowders

parameters, namely, the TSM, the sharpness

and

slightly

(ΔT) and the hysteresis (ΔH) of the

decreased initially, and then increased with

transition as a function of Ts, see also Table

increasing Ti concentration. 58 In this case it

S1 in Supplementary Information. It can be

was suggested that the distortion of VO6

observed that by increasing the Ts the

octahedra induced by Ti4+-doping can

transition

change the hybridization between V 3d and

gets

O 2p orbitals, resulting in the shift of π and

sharper,

π* bands near the Fermi level in the band

from ΔT =

observed

that

the

TSM

Figure 8 Dependence of the c-axis length and TSM on the Ts. The dash lines represent the corresponding bulk values.

structure of VO2 that changes the energy

6.1 K, at TS = 280°C, to ΔT = 2.6 K, at Ts =

gap. Based on the obtained results and by

400°C. This result is attributable to an

comparing with other ion-doped systems,59

improvement of the films quality by

the authors concluded that the charge

increasing the Ts. The model proposed by

doping is more effective than the lattice

Narayan and Bhosle60 suggests that ΔT and

distortion in modulating the SMT behavior

ΔH of the transition are related to the

of VO2. In our case, a clear distinction

overall defect density per unit volume in

between these two effects cannot be easily

VO2. Here the sharpness is defined as

made since the VO2 host lattice is affected

ΔT ;< ∗ ρ> , where ;< is a constant, and

by thermal as well as epitaxial ε.

ρ> is the defect density, i. e. point defects,

From the observations discussed above we

dislocations,

conclude that phase-pure VO2 films can be

impurities. For high-quality films the model

obtained only for Ts ≤ 400°C. Therefore,

predicts a sharp transition (small ΔT) and a

grain

boundaries,

and

very small ΔH. The improvement of the

hereafter, the discussions will be focused



Page 20 of 33

films’ quality as a function of Ts is confirmed by the larger change in the resistivity, sharper transition and narrower hysteresis.

Ts (°C) 280 320 360 380 400

Table 2 SMT parameters TSM(K) ΔT(K) ΔA ΔH(K) 291 6.1 2.8 3.3 293 5.7 3.5 3.5 300 4.2 3.3 4.3 303 3.8 3.7 3.9 310 2.6 3.8 3

atoms along the c-axis direction in the R structure. Ref. 32 - 34, 36, 55 have investigated the effect of uniaxial stress along the c-axis on the SMT of epitaxial VO2 thin films and suggested that there is a

Furthermore, our results show that the TSM

correlation between the c-axis length and

increases as the c-axis length decreases, as

TSM, i.e. the shorter the c-axis length the

shown in Figure 8. It is worth mentioning

lower the TSM. The underlying mechanism

that the results presented in this study are

was attributed to a direct overlapping of

in good agreement with the current

the d orbitals, which increases the width of

literature only in what concerns the effect

the d band and stabilizes the metallic phase

of the Ts on the VO2 lattice and not on the

of the rutile structure. Following this line of

relationship between the c-axis length and

thought, one would expect a decreasing TSM

the TSM – i. e. the stabilization of the

as a function of Ts, since the epilayer’s

metallic

lattice has shown a contraction along the c-

ground

state

at

lower

temperatures by increasing the overlap

axis

between

However, from our measurements it is

the

vanadium

and

oxygen

direction

evident

by

that

monotonically

increasing

the as

the

TSM c-axis

the

Ts.

increases length

decreases. Moreover, the relaxed film grown at Ts = 280 °C with bulk-like lattice parameters had a TSM of 291 K whereas the


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strained film deposited at Ts = 400 °C with a

transition in epitaxial VO2 films to occur at

shorter c-axis length had a TSM of 310 K. For

lower

the sample, grown at Ts = 280°C, the

However, depending on the strain source

relaxed state has been confirmed also by

and

GPA (see Supplementary Information Figure

unusual cases have already been reported.

S5), which showed that the value of the

Nagashima et al.34 have studied the

strain along the in-plane direction is 1.2% ±

interface effects on the SMT of strained VO2

0.4% and in the out-of-plane direction is -

ultrathin films grown epitaxially on TiO2

2.7% ±0.5%. The obtained values are close

(001) single crystal substrates. They found

(considering the standard deviation) to the

that surface reconstructions, produced by

misfit between these two structures in bulk

treating the surface of the TiO2 substrates,

state implying that the VO2 thin film is

are detrimental in effectively applying strain

relaxed in both directions. From strain maps

effects to reduce the TSM, due to the

the existence of some dislocations can be

subsequent strain relaxation in the c-axis of

observed at the VO2-TiO2 interface. In

the films. Contrary to other works, they

addition, the HRTEM image showed that

reported a decrease of the TSM as the

the interface is not sharp, being quite

epilayer thickness is increased. It is noted

difficult to identify; thus, for this substrate

that all films characterized in our study have

temperature the low TSM could be also

been grown on identical substrates, with

partly

very flat surfaces (miscut angle

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