Nanoscale imaging and control of resistance switching in VO2at room temperature

Jeehoon Kim,1 Changhyun Ko,2 Alex Frenzel,1 Shriram Ramanathan,2 andJennifer E. Hoffman1,a�

1Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA2School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

�Received 21 February 2010; accepted 4 May 2010; published online 25 May 2010�

We demonstrate controlled local phase switching of a VO2 film using a biased conducting atomicforce microscope tip. After application of an initial, higher “training” voltage, the resistancetransition is hysteretic with IV loops converging upon repeated voltage sweep. The threshold Vset toinitiate the insulator-to-metal transition is on order �5 V at room temperature, and increases at lowtemperature. We image large variations in Vset from grain to grain. Our imaging technique opens upthe possibility for an understanding of the microscopic mechanism of phase transition in VO2 aswell as its potential relevance to solid state devices. © 2010 American Institute of Physics.�doi:10.1063/1.3435466�

An insulator-to-metal transition may be triggered in VO2as a function of temperature,1 strain,2 electric field,3 or opti-cal excitation.4 This transition has useful properties, such asfast 80 fs switching time,5 high resistivity ratio �RR�, largechange in optical reflectance,6 and tunability near room tem-perature. Proposed applications include bolometers,7

memristors,8 tunable-frequency metamaterials,9 and datastorage.10

Sensor applications typically require negligible hyster-esis, while memory applications call for maximum hyster-esis, but all applications seek to maximize the RR. Singlecrystal VO2 exhibits RR up to 105, but bulk single crystalspose problems for real devices due to cracking on repeatedcycle through the transition.11 Epitaxial films on insulatingAl2O3 may have RR up to 104.12 As a route to interface withexisting electronics, recent effort has been devoted to filmgrowth on Si substrates,13 where voltage-controlled switch-ing of VO2 in capacitor geometry has been demonstrated atroom temperature.14 However, the lattice mismatch results inpolycrystalline VO2 films with RR so far limited to �103. Itis therefore important to understand the effects of grain sizeand the role of grain boundaries in determining hysteresisloop properties, RR, Vset, and Vreset. To date, the voltage trig-gered transition has been demonstrated down to 200 nm, butonly in a fixed area composed of multiple grains.15 Here wepresent nanoscale images of the voltage-triggered transition:we resolve single grains as small as tens of nanometers.

We study a 200 nm thick VO2 film grown by rf sputter-ing from a VO2 target onto a heavily As-doped Si substrate�n-type, with resistivity 0.002–0.005 � cm�.14 X-ray dif-fraction �XRD� data in Fig. 1�a� corresponds to a polycrys-talline, monoclinic VO2 phase. Figure 1�b� shows the ther-mal phase transition with RR�102. From atomic forcemicroscope �AFM� topography, the typical lateral diameterof a VO2 grain is �100 nm, and the rms surface roughnessis �6 nm.

To investigate this film, we use a home-built force mi-croscope with a conducting cantilever of spring constantkc=40 N /m.17 We touch down on the surface with feedback

to fix cantilever deflection at a typical setpoint of �4 nm,corresponding to a force of 160 nN, and a pressure of�800 bar �assuming a tip contact area diameter �50 nm�.The transition temperature has been shown to shift mostsignificantly with c-axis uniaxial stress, at a rate of �1.2K/kbar.18 The force applied by the tip therefore correspondsto change in local transition temperature �Tc of at most1 K.19

Upon first upward voltage sweep at a given location, wetypically observe a sudden transition from the insulating tothe metallic state at a “training” voltage VT�12 V. Thetransition is hysteretic, returning to the insulating state onlyat a much lower voltage Vreset. Subsequent sweeps show tran-sitions at lower Vset, but the hysteresis remains, and IV loopsroughly stabilize with typical �5% jitter around Vset�5 Vand Vreset�3 V. This is comparable to the 3% stability oftransition temperatures over 102 thermal cycles previouslyobserved in a macroscopic junction on a VO2 film on Al2O3substrate.20 Typical voltage sweeps are shown in Fig. 2. Thefollowing points are worth noting in this unique measure-ment geometry: �1� Unlike in prior work on 200 nm Audots,15 we rarely see multiple jumps in a single curve; we

a�Electronic mail: jhoffman@physics.harvard.edu.

300 320 340 360 380

102

103

104

105HeatingCooling

25 30 35 40 45 500

10

20

30

T (Kelvin)2θ (Degrees)

CPS R(Ω)

(011)

(200)(020)

(012)

Monoclinic VO2(room temperature)

(a) (b)

VO2

n-type Si

Pd

FIG. 1. �Color online� �a� XRD profile, performed on a Scintag XDS2000diffractometer using Cu K� radiation ���1.5418 Å� at incidence angle of1°. The observed peaks correspond to the monoclinic VO2 phase and dem-onstrate polycrystallinity �Ref. 16�. �b� The RR is �102 as a function oftemperature, measured in two-point geometry as shown in the inset. For thisdata, voltage was applied and current was measured between two neighbor-ing 500�500 m2 Pd contact pads, centered 1 mm apart. Varying the dis-tance between Pd pads did not alter the resistance, demonstrating that theresistance is entirely vertical through the VO2 film, with negligible contri-bution from the doped Si substrate or the SiOx interface.

APPLIED PHYSICS LETTERS 96, 213106 �2010�

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0 2 4 6 8 100

100

200

300

400

500

600

Bias (V)

Current(μA)

0 2 4 6 8 10 12Bias (V)

Current(μA)

Cr/Au coated tip

n-type Si

V

VO film(~200 nm)

SiO nativeoxide

(3-4 nm)

2

x

(a)

(b)

(c)

(d) (e)

(f)

200

1

T = 293 K T = 293 K

T = 106.75 K

Position (μm)

Heigh

t(nm

)

1

14

0 50 100 150 2004

5

6

Sweep number

V(V)

set

50 100 150 200Sweep number

0 0.5 1 1.5 20

4020

(g)

50

100

150

200

250

300

FIG. 2. �Color online� �a� Schematic of the microscopetip and sample geometry. �b� Height trace from AFMtopography demonstrates the tip resolution and VO2

surface roughness. �c� 14 consecutive IV sweeps at asingle location at T=107 K. ��d� and �e�� 200 consecu-tive IV sweeps at two different representative locations,at room temperature. Iterations start from black and runthrough grey �light blue online�. Training voltage isVT�12 V in �d� and �e� and �40 V in �c�. ��f� and�g�� Vset as a function of iteration number for the datafrom �d� and �e�.

(g)(a)

(e)

(d) 4.44 V

4.93 V(b) 3.45 V

40 nm

0

3.95 V

4.44 V

4.93 V

5.43 V

(f) 5.43 V(c)

500 nm

3.95 V

0 50 100 150Current (A)

3.45 V

FIG. 3. �Color online� �a� AFM topography of the VO2

surface. ��b�–�f�� Current maps at increasing bias volt-age show the metallic puddle seeded at two grains �out-lined in black�, growing with increasing bias. Grainboundaries are drawn in white in the upper right cornersof �a� and �b�, which were acquired simultaneously, toemphasize the correlation between grain locations andregions of constant current. �g� Current distributions for�b�–�f� are bimodal, showing the jump between insulat-ing and metallic state conductivity. A second set of im-ages �not shown�, acquired in this same area, as theapplied voltage was subsequently decreased by thesame increments, show the shrinking and disappearanceof the metallic puddle.

213106-2 Kim et al. Appl. Phys. Lett. 96, 213106 �2010�

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really can access single grains! �2� As in this prior work, themeasured RR for the entire tip VO2–Si structure is limited to10, which we attribute to resistance Rs�15 k�, in serieswith the VO2 film. �3� Loops do not depend on sweep speedfrom 6 to 16 V/s. �4� Loop characteristics exhibit negligibledependence on force within the range 120 to 420 nN used inthis study.

We next investigate the spatial dependence of the transi-tion. After training an area by scanning with a bias voltage of11.1 V, we rescan with increasing bias to watch the details ofmetallic puddle growth. As shown in Fig. 3, the insulatingstate displays variations in conductivity up to 100% of themode value. Conductivity appears constant within eachgrain, but slowly varies from one grain to the next. Conduc-tivity appears lower in the grain boundaries, which suggestsa different stoichiometric phase in the grain boundaries. �Wecannot rule out a topographic artifact from variations in thecontact area of the tip between grains and grain boundariesbut such an effect would likely result in increased contactarea in the grain boundaries, thus apparent higher conductiv-ity�. On increasing voltage, the metallic state nucleates at thegrains with largest insulating-state conductivity, grows into alarger metallic puddle, and shrinks again as the voltage isdecreased. This type of switching is typically referred to as“threshold switching” �as opposed to “memory switching” inwhich the resistance state remains changed after removal ofthe applied voltage�. Granularity and lower conductivitygrain boundaries remain apparent in the metallic as well asthe insulating phase.

Although previous researchers have suggested that fieldor carrier injection alone may be sufficient to induce thetransition,21,22 evidence suggests that in our experimentalgeometry the transition results most directly from Jouleheating. At room temperature, the local power injectionimmediately prior to the insulator-to-metal transition shownin Fig. 2�d�, is P�100 W. Given the specific heat C=690 J / �K kg� and the mass density �=4340 kg /m3 ofVO2, the time to heat a single grain of volume V��100 nm�3 from room temperature �TRT�293 K� to thetransition temperature �Tc�340 K� would be only 1.4 ns.The empirical fact that the grain does not transition soonerimplies significant thermal conduction away from each grain.The hysteresis may be explained as follows: on upwardsvoltage sweep, the switching occurs just as the Joule heatingexceeds the heat flow out by enough to raise the grain tem-perature above its Tc. Upon transition, the current flow andresultant Joule heating suddenly increase, so the grain tem-perature does not immediately drop below Tc as the voltageis swept back down. But the increased Joule heating at thehigher current state causes the surrounding grains and Si sub-strate to also increase in temperature, which increases theirthermal conductivity, so heat flows away more quickly. Thegrain may then return to the insulating state on downwardsvoltage sweep even though the power input is still higherthan the power input on insulator-to-metal transition.

The hypothesis of Joule heating is further suggested bythe following observations: �1� transition occurs at the sameabsolute value of voltage, to within �10%, for positive andnegative applied bias. �2� The transition is seeded at thegrains with the largest insulating state conductance �i.e., thelargest Joule heating for a given applied voltage�. �3� Thetransition can also be triggered by voltage at the much re-

duced global temperature of 107 K as shown in Fig. 2�c� butthe transition is shifted to higher voltage and power, aswould be required to achieve the much larger �T=234 K.

The origin of the training voltage remains unknown. Werule out surface contamination, because use of the tip toscrape the VO2 surface, in situ in vacuum, with enough forceto physically remove up to 10% of the VO2 material, doesnot lower the initial transition voltage. Possibly, the trainingalters the native SiOx layer. More likely, the training mayoccur within the VO2 film itself, possibly due to O migrationwhich improves grain stoichiometry, or due to formation ofmore conductive phases such as Magneli �VnO2n−1� phases23

in the grain boundaries.In conclusion, single grain switching with reproducible

hysteresis demonstrates the scaling of the insulator-to-metaltransition in VO2 down to tens of nanometers. Nanoscalevoltage-triggered switching at room temperature may lead toadditional VO2 applications. High resolution conductanceimaging of VO2 has the potential to elucidate the micro-scopic mechanism of phase transition, and to inform the op-timization of grain size and grain boundary composition inpractical VO2 film devices.

This work is supported by the Harvard NSEC under NSFGrant No. PHY-0117795 and by AFOSR Grant No. FA9550-08-1-0203.

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213106-3 Kim et al. Appl. Phys. Lett. 96, 213106 �2010�

Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp