Ramanathan Sh. (Ed.) Thin Film Metal-Oxides: Fundamentals and Applications in Electronics and Energy

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58 D. Ruzmetov and S. Ramanathan

Fig. 2.6 Monoclinic M

lattice of the low temperature (below T

SPT

) semiconducting phase of

. Two types of oxygen atoms [29, 34] are differentiated. (From Eyert [29] with permission.

The structural phase transition involving VO

lattice transformation from

tetragonal to monoclinic is generally believed to happen in concomitance with

the metal insulator transition .T

SPT

D T

MIT

/ [6, 35, 36]. However, it has been also

suggested recently that the formation of the tetragonal lattice occurs at a different

temperature than MIT does .T

SPT

¤ T

MIT

/ with the difference being up to 9

C[37].

Thin ﬁlm VO

is often of polycrystalline structure with the grain size in the

range of 30–120nm depending strongly on the ﬁlm growth conditions [8, 15, 38–

40]. Single crystal thin ﬁlm VO

can be epitaxially grown on sapphire substrates

by means of MOCVD [41]. Figure 2.7 shows the XRD spectrum taken using Cu

K’ radiation in ™–2™ geometry from thin ﬁlm VO

on a Si (001) substrate at

room temperature. The thin (100-nm thick) ﬁlm was reactively DC sputtered in

Ar.91:2%/ CO

.8:8%/ environment at 10 mTorr from a V target. The details of the

measurement are given elsewhere [16]. The d-spacings are inscribed in the ﬁgure

and the line assignment is done for VO

peaks. The XRD spectrum corresponds to

stoichiometric polycrystalline VO

in insulating phase.

2 Metal-Insulator Transition in Thin Film Vanadium Dioxide 59

Fig. 2.7 XRD spectrum from a VO

thin ﬁlm on Si.001/=SiO

(native oxide) substrate. d values

(in

A) of the peaks are inscribed and for VO

lines corresponding Miller indices of the Bragg

planes are given in brackets (From Ruzmetov et al. [16] with permission. Copyright (2008) by the

IOP Publishing Ltd)

2.4 The Relationship between Electron Transport

and Material Morphology

The electrical parameters of the metal insulator transition (MIT) in VO

vary

signiﬁcantly for thin ﬁlms and bulk crystals, and for thin ﬁlms prepared at different

conditions [9]. The magnitude of the electrical resistance change at the MIT in thin

ﬁlms and the temperature width of the transition are generally not as sharp as it is

found in single crystal VO

. Although there has been considerable research done on

thin ﬁlms, the mechanisms responsible for the deterioration of the MIT param-

eters – the decrease in R and spread of T

MIT

– in polycrystalline thin ﬁlms are

not yet well-understood. In this section, we analyze how material synthesis condi-

tions and material morphology affect the electrical parameters of the phase transition

in an attempt to uncover the microscopic mechanisms underlying the macroscopic

observables of the MIT.

Figure 2.8a shows the temperature dependence of the resistance of 50-nm

thick VO

ﬁlms RF-sputtered from a VO

target under identical conditions on

Si=SiO

.400 nm/ and r-plane sapphire .Al

/ substrates [24]. These electron

transport measurements demonstrate that the sputtering method described in [24]

allows synthesizing VO

ﬁlms on technologically important Si-based substrates

with the MIT parameters comparable to some of the best reported VO

thin ﬁlms

[9]. In order to precisely determine the transition temperature .T

MIT

/ and its width

we show in Fig. 2.8b the derivative of log

R.T / for the data in Fig. 2.8awhich

60 D. Ruzmetov and S. Ramanathan

Fig. 2.8 (a) Resistance versus temperature of 50-nm thick VO

ﬁlms on Si=SiO

(400 nm) and

r-plane Al

(sapphire) substrates showing a sharp metal–insulator transition. (b) The derivatives

of Log

R.T / for the curves in (a) are shown. Symbols are data points, the lines are Gaussian ﬁts

whose minima and widths determine the T

MIT

and MIT width. (From Ruzmetov et al. [24] with

permission. Copyright (2007) by American Institute of Physics)

are ﬁtted with Gaussians. The centers and widths of the Gaussian peaks are taken

as T

MIT

and MIT widths. Different substrates exert tensile or compressive strain

on a VO

lattice causing the shift of T

MIT

[14]. The VO

ﬁlm grown on Si has

2 Metal-Insulator Transition in Thin Film Vanadium Dioxide 61

Fig. 2.9 (a) The normalized resistance of VO

ﬁlms of different thickness: 50 nm and approxi-

mately 7 nm. Sputtering conditions are identical for the two data sets. The substrate is Al

.The

thinner ﬁlm has considerably sharper MIT. (b) The normalized resistance of two 2-terminal devices

and an unpatterned ﬁlm. The devices are VO

stripes (size d  d ,whered D 20 m; 500 m)

contacted with Pt leads. No size dependence is observed for devices scaled down to 20 m. The

increased R value of the 20-m stripe in the metallic state is due to the higher contact resistance.

(From Ruzmetov et al. [24] with permission. Copyright (2007) by American Institute of Physics)

MIT

D 71

C, whereas VO

on sapphire has T

MIT

D 64

C, which are on different

sides from the published single crystal value T

MIT

D 66

C. Therefore, the substrate

can be used to tune the MIT parameters.

In order to use vanadium dioxide in scaled phase transition devices it is important

to investigate how the MIT parameters scale with the device size. Figure 2.9ashows

62 D. Ruzmetov and S. Ramanathan

the normalized resistance of VO

on sapphire measured at two spots on a sample

with varying ﬁlm thickness. We see considerable improvement of the MIT parame-

ters when the ﬁlm thickness is reduced from 50 nm to approximately 7 nm. We can

conclude that under these synthesis conditions the thinner ﬁlm shows larger resis-

tance drop at MIT and narrower transition width. The reason is discussed further in

the text.

On the other hand the change of the lateral size of a two-terminal VO

device

does not produce signiﬁcant changes in MIT parameters as shown in Fig. 2.9b. In

Fig. 2.9b, three data sets are shown for lithographically patterned VO

stripes with

Pt/Ti (adhesion layer of 4 nm) contacts and unpatterned ﬁlm directly contacted by

electrical probes separated by 3 mm. The stripe sizes between the Pt contacts are

20 m  20 m  .50 nm thick/ and 500 m  500 m  50 nm. The increased

value of the resistance in the metallic phase for 20 m stripe is due to higher contact

Pt=Ti=VO

resistance of the smaller device (inversely proportional to the lateral

size). The data demonstrate no changes of the MIT upon lateral scaling down to

20-m devices.

It was previously demonstrated that VO

ﬁlms sputtered at higher substrate

temperatures have larger grain sizes and higher crystalline order [39]. Here we in-

vestigate how VO

morphology affects MIT parameters. Figure 2.10ashowsMIT

in three samples sputtered at different substrate temperatures. We see that the mag-

nitude of the resistivity drop increases monotonously with increasing sputtering

temperature and consequently the increase in VO

grain size and crystalline order.

However, we also observe the non-monotonous behavior in transition width which

may be a result of two competing phenomena affecting the MIT sharpness in oppos-

ing manner with respect to the sputtering temperature: the stabilization of the pure

phase and improving of the crystalline order. Higher substrate temperature

during the sputtering was shown by means of electron diffraction measurements to

improve crystalline order of VO

[26], which is expected to narrow the MIT width

and enhance the transition since the strongest MIT values were found in VO

sin-

gle crystals. On the other hand, higher substrate temperature promotes oxygen loss

which deteriorates the overall VO

stoichiometry and results in the larger proportion

of the additional substoichiometric phases of the vanadium oxide, such as V

for

instance. The appearance of such VO

phases .x ¤ 2/ makes the MIT due to VO

component less pronounced. If this scenario is true then we expect that an addition of

small amount of oxygen in the sputtering gas should compensate for the oxygen loss

and improve (make more pronounced) the transition in samples sputtered at high

temperatures. Figure 2.10b shows the MIT of three VO

ﬁlms sputtered in varying

partial oxygen pressures. Adding 2% of air to the Ar-sputtering gas increases the

resistance drop by an order of magnitude with respect to the ﬁlms sputtered in pure

Ar. Further increasing of the oxygen partial pressure results in ﬁlms of higher oxi-

dation states of vanadium so that the MIT almost disappears: see resistance curve in

Fig. 2.10b for the ﬁlm sputtered in 2% oxygen in addition to Ar.

Figure 2.11 shows thermal hysteresis loops of VO

ﬁlms sputtered on a sapphire

substrate at different conditions. For comparison, representative data from a ﬁlm

sputtered from a VO

target is given along with the data from a VO

ﬁlm reactively

2 Metal-Insulator Transition in Thin Film Vanadium Dioxide 63

Fig. 2.10 (a) The MIT in three VO

ﬁlms sputtered in Ar at different substrate temperatures.

The resistance drop is increasing with increasing sputtering temperature. The MIT width changes

nonmonotonously with sputtering temperature. (b) The MIT of three ﬁlms sputtered on sapphire

substrate in different gas environments. The substrate temperature during sputtering is 450

(From Ruzmetov et al. [24] with permission. Copyright (2007) by American Institute of Physics)

sputtered from a V target in 84% Ar C 16% air environment. We see qualitatively

different hysteresis shapes for the ﬁlms sputtered by different methods. The hys-

teresis loop in Fig. 2.11a extends for 34

C with its width not exceeding 4:3

C. The

hysteresis of the reactively sputtered ﬁlm is squarer and extends for 24

C with width

64 D. Ruzmetov and S. Ramanathan

Fig. 2.11 Thermal hysteresis curves of VO

thin ﬁlms on Al

substrate. (a)VO

ﬁlm is

sputtered in pure Ar from a VO

target at substrate temperature T

subs

D 550

C. The width of

the hysteresis loop is T D 4:3

C. (b)VO

ﬁlm is reactively sputtered from a V target at

subs

D 450

C. The width of the hysteresis loop is T D 10

C. (From Ruzmetov et al. [24]

with permission. Copyright (2007) by American Institute of Physics)

up to 10

C. These differences may reﬂect different morphologies of the ﬁlms. As

argued by Lopez et al. [42], the phase transition in VO

is nucleated on defects since

the homogeneous nucleation is inhibited due to a high potential barrier originating

from surface free energy increase on a nucleating site. According to the formalism

suggested by Lopez et al., the probability of ﬁnding a potent defect in a particle of

2 Metal-Insulator Transition in Thin Film Vanadium Dioxide 65

volume V increases with either V or jT  T

MIT

j. Then in smaller particles larger

thermal hysteresis is expected as was conﬁrmed experimentally [42]. We can con-

clude from the data in Fig. 2.11 that the reactively sputtered ﬁlm in Fig. 2.11bhas

smaller average grain size than the ﬁlm in Fig. 2.11a, which is reﬂected in the larger

hysteresis width. These arguments do not explain the difference in the sharpness, or

transition width T

MIT

, in the two graphs. The MIT widths, measured as FWHM of

d.Log

R.T //=dT ,ofthinVO

ﬁlms synthesized in our experiments range from 6

to 20

C, which is considerably larger than a few degrees widths of the transition in

epitaxial VO

ﬁlms [8, 9]. A possible explanation may be that our polycrystalline

ﬁlms contain crystallites with different MIT temperatures. Then the transition in the

whole ﬁlm is spread over the distribution of T

MIT

of the constituent crystallites. The

cause of the distribution of T

MIT

in individual crystallites may be a strain induced by

the substrate and propagating to the top of the ﬁlm. It is likely that the variation in

the crystallites (or the strain) goes in the direction perpendicular to the ﬁlm surface

due to in-plane symmetry. Then a thinner ﬁlm would have a narrower distribution of

crystallites and correspondingly narrower MIT width. This argument is supported

by measurements shown in Fig. 2.9a, where we studied the VO

ﬁlm of variable

thickness. The thinner ﬁlm has sharper MIT even though it was deposited under

identical conditions as the thicker ﬁlm.

The comparison of the resistance curves above the MIT temperature in Fig. 2.11

allows better understanding of the reason for the deterioration of the resistance drop

magnitude in polycrystalline ﬁlms whose synthesis conditions are not successfully

optimized. The ﬁlm in Fig. 2.11a exhibits MIT with approximately 3 orders of mag-

nitude resistance drop, whereas the drop is 2 orders for the reactively sputtered ﬁlm

in Fig. 2.11b. We see that the resistance in Fig. 2.11b continues to decrease above

MIT

, which is characteristic to a semiconductor. The resistance decrease above T

MIT

is much less pronounced in Fig. 2.11a. In single crystal VO

the resistivity goes up

with increasing temperature immediately after the transition as is expected for the

metallic behavior [43]. The thin ﬁlms may be a composite of VO

crystallites and

grains of another substoichiometric VO

phase which do not experience MIT in the

measured temperature region. Then the total resistance reﬂects the temperature de-

pendence of all the composite phases and is apparently dominated by the non-VO

semiconducting phase above T

MIT

in Fig. 2.11. Less amount of the VO

.x ¤ 2/

phase is manifested by a larger resistance drop at MIT and weaker semiconducting

behavior above T

MIT

The hysteresis loops in the resistance (see Fig. 2.11) are reproducible upon

thermal cycling. We found that the resistance of the RF-sputtered ﬁlms depends

primarily on the temperature and thermal history of the material. For example, if

the sample temperature is ramped up to the middle of the MIT and is left at the

temperature up to 10 hours, the resistance will not drift within the precision of the

measurement. We also did not ﬁnd any sample deterioration with time in VO

ﬁlms

sputtered under optimal parameters (all samples presented in [24]), so that resistance

curves are reproducible within at least 6 months of the sample synthesis. RF sputter-

ing in pure Ar from a VO

target at low substrate temperatures .<150

C/ yields thin

ﬁlms with weak MIT signature superimposed on an overall semiconducting slope

66 D. Ruzmetov and S. Ramanathan

in resistivity versus temperature curve. Such ﬁlms are thought to be composed of a

mixture of VO

and other VO phases and when sputtered on Si substrates may expe-

rience degradation with time. Results on change in hysteresis upon thermal cycling

in VO

ﬁlms deposited at low temperatures by electron-beam evaporation have been

reported [44].

As mentioned above, the synthesis of good quality VO

ﬁlms involves high

(above 300

C) fabrication temperatures. This complicates lithographic patterning

of VO

into devices using common photo- and e-beam sensitive resists and deterio-

rates interfaces due to enhanced diffusion. Low temperature synthesis techniques are

preferred for the purpose of incorporating the material in nanoscale devices and also

for heterogeneous integration. Therefore, novel methods of oxidation of vanadium

and its oxide phases need to be explored. Ultraviolet (UV) radiation during oxida-

tion of thin metal ﬁlms has been shown to enhance the oxidation process resulting in

high-quality oxide layers at room temperature [45]. Ruzmetov et al. [24] have stud-

ied the effect of UV radiation on the oxidation of vanadium and vanadium oxide thin

ﬁlms using Hg vapor lamp with a primary wavelength of 254 nm and other ancillary

major wavelength at 185 nm. These wavelengths are close to the bond energies of

molecules so that the radiation creates oxygen radicals and ozone.

In one approach, we started with an oxygen-deﬁcient VO compound sputtered

reactively from a V target in a gas mixture of 86% Ar C14% air, while 16% air was

considered to be optimal to stabilize VO

phase (as for the sample in Fig. 2.11b)

[24]. The resistance curve for this sample, a thin (60-nm) ﬁlm on sapphire, shows

a weak MIT transition on an overall semiconducting background (decreasing with

increasing temperature). Then the ﬁlm was exposed to UV radiation for 100-min

at 45

C at atmospheric pressure. The resistance change after the exposure with re-

spect to the original R versus T curve is shown in Fig. 2.12. We see a clear change of

the resistance which implies an oxidation enhancement caused by the UV exposure

even near room temperature, whereas as was stated above, without UV, the resis-

tance curves were stable with time in ambient environment. The observed change

in the resistance may be explained by the addition near the surface of the ﬁlm of an

oxidized layer, which does not exhibit MIT and, therefore, ﬂattens the overall R ver-

sus T curve being negative below the transition and positive above. This argument

agrees well with the reasoning above ascribing the deterioration of the MIT sharp-

ness in thin ﬁlms to the presence of different stoichiometric VO

phases. Given

that the thickness of the additionally oxidized layer is expected to be only a few

nanometers [46], it is interesting to note that it produces such a noticeable change in

the resistance of a 60-nm ﬁlm. Using the resistance change as a feedback one may

attempt to optimize the UV-enhanced oxidation procedure in order to obtain phase

pure VO

in a similar manner as reactive oxidation parameters during sputtering

were optimized to obtain VO

ﬁlms (e.g., see Fig. 2.11b).

Further studies on UV illumination-driven resistance changes were performed by

Ko et al. [47]. Four vanadium oxide samples were studied: optimized stoichiometric

thin ﬁlm (ST), lightly overoxidized VO

thin ﬁlm (LO), lightly vanadium rich

(LV), and heavily vanadium rich VO

ﬁlm (HV) that still exibits MIT [47].

Figure 2.13a shows that electrical resistance at 25

C can be altered by up to 30%

2 Metal-Insulator Transition in Thin Film Vanadium Dioxide 67

Fig. 2.12 The resistance change of a vanadium oxide thin (60 nm) ﬁlm after the ﬁlm was exposed

to UV radiation. The UV light affects only a few nm layer near the surface of the ﬁlm [46], i.e., a

small fraction of the whole ﬁlm. The observed clear resistance difference outlined by a straight line

ﬁt evidences a signiﬁcant change in the electrical resistance of the UV-affected material demon-

strating the possibility to control the oxidation process with UV radiation. (From Ruzmetov et al.

Fig. 2.13 The effect of UV irradiation on electrical parameters of vanadium oxide ﬁlms exhibit-

ing MIT: ST stoichiometric VO

, LO low oxygen excess, LV and HV light and heavy vanadium

excess. (a) Relative change of resistance at 25

C.R

r25

/;(b) Ratio of relative resistances at 25 and

100

C;R

r25

r100

of Physics)

upon UV irradiation. The relative resistance ratio below and above MIT can be

noticeably affected by UV as well (Fig. 2.13b). The relative resistance ratio was

deﬁned in [47]as