Improved VO2 thin films for infrared switching

Francine C. Case

Vanadium dioxide (VO,) undergoes a thermally induced phase transition from a semiconductor to a metalnear 680 C. The deposition of VO, thin films by using a process of activated-reactive evaporation provideshigh-quality VO,-film material; specifically, the semiconducting phase-extinction coefficient in theinfrared is reduced by an order of magnitude without detrimental effect on the corresponding metal phasecoefficient. The materials improvement significantly enhances accessible performance limits for opticalswitching devices, as compared with VO2 thin films deposited by both standard reactive and ion-assistedreactive evaporation.

Introduction

Vanadium dioxide (V 2 ) undergoes a semiconductor-to-metal phase transition at 680C, which is accompa-nied by large changes in electrical and optical proper-ties."4 As part of a properly designed film stack, VO2can provide fast and effective infrared optical switch-ing, the magnitude of which depends on the quality ofthe thin-film material. The extinction coefficient k is adirect indicator of the optical quality of dielectric(semiconductor) or metallic materials. In switchablethin films of VO2, the bistate optical contrast atinfrared wavelengths is dominated by the change in k.The most obvious optimization is for one to lower thevalue of the semiconducting-state extinction coeffi-cient k,, while raising its metallic-state counterpart,km.

At 3.4 rim, for example, 5 typical values for thesemiconductor- and metal-state extinction coeffi-cients for reactively evaporated films are k = 0.02-0.10 and km = 4.4-5.5.

I have found that the introduction of activationinto the electron-beam reactive evaporation processfor the deposition of VO2 thin films significantlylowers k, without sacrificing km. In turn, the reduc-tion of k, enhances the potential of high-contrastswitching.

Several films exhibiting low semiconducting-stateabsorption were obtained previously as part of a studyon the effects of ion bombardment on the propertiesof reactively evaporated VO2.5 Those films were depos-

The author is with LTV Missiles and Electronics Group, P. 0.Box 650003, MS WT-50, Dallas, Texas 75265-0003.

Received 2 November 1990.0003-6935/91/284119-05$05.00/0.3 1991 Optical Society of America.

ited as part of case 1 of Ref. 5 and were prepared byelectron-beam deposition of low-oxide material atroom temperature followed by a lengthy annealing inoxygen at 520'C. The metal-state absorption for thosefilms was slightly lower than that for the samples inthis paper. V02 thin films prepared by the activated-reactive-evaporation (ARE) technique discussed be-low require no such annealing procedure. They ex-hibit extremely high contrast in the as-depositedstate.

ARE is a technique whereby electrons from theelectron-beam melt are accelerated, by an appropri-ately placed anode, to energies favorable for ioniza-tion and excitation of background gas atoms, therebypromoting increased reactivity in the physical vapor-deposition process.7'8 Lower background-gas pressuremay thus be used, accompanied by improvement inprocess control.

Figure 1 compares the temperature dependence oftransmission at 3.4 m for two 100-nm VO2 filmsdeposited under identical temperature and pressureconditions, with and without anode activation. TheARE film is characterized by higher semiconducting-state transmission and a more abrupt transition. Inthis research, ARE conditions are presented for thedeposition of VO2 thin films that exhibit unusuallylow semiconducting-state extinction coefficients. Theproperties of these films are compared with thosedeposited under standard reactive and ion-beam-assisted (LAD) reactive evaporation.

Experimental

Films were deposited in an Edward's electron-beamcoating system equipped with a stainless-steel blockfor substrate heating. A Chromel-Alumel thermocou-ple was attached to each substrate by means ofthermally conductive epoxy. The substrate tempera-

1 October 1991 / Vol. 30, No. 28 / APPLIED OPTICS 4119

100

90

80

70

60

50

40

30

20

10

00 20 30 40 50 60 70

TEMPERATURE (C)

.10I

.08

k5.06

.04

.02 .

.00

2.0

80 90 100

Fig. 1. Temperature dependence of transmission for VO, thinfilms prepared by reactive evaporation and ARE.

7.0

kin 6.01

5.00.08O

0.07

0.06

0.05

k 0.04

0.03

0.02

0.01

0.00

0 5 10 15 20 25

Ia (mA)

Fig. 2. Semiconductor- and metal-state extinction coefficients at3.4 pm as functions of anode current at 2.60- and 2.88-mbarpressure during activated-reactive evaporation of VO, thin films.

ture was held near 5000C during the reactive deposi-tion of V metal at 0.1 nm/s in a controlled oxygenbackground. The oxygen was activated by use of acircular anode placed midway between the melt andthe substrate, which was biased at 90-200 V. Films,deposited upon oriented sapphire and silicon sub-

6.0

km 5.0

4.0

2.0

3.0

WAVELENGTH (m)

3.0

4.0

4.0WAVELENGTH (pm)

Fig. 3. Semiconductor- and metal-state extinction coefficients asfunctions of wavelength for VO, thin films deposited with andwithout activation.

strates were analyzed by scanning electron micros-copy, infrared spectrophotometry, x-ray diffraction,and four-point probe resistance measurements aboveand below the phase transition.

Results

Anode current varies with anode voltage, backgroundpressure, and metal evaporation rate. Figure 2 showsmetal- and semiconducting-state extinction coeffi-cients, km and k, respectively, at 3.4 pum as functionsof anode current for VO2 thin films deposited at 50000by ARE at a rate of 0.1 nm/s at two pressures, P1 =2.60 andp 2 = 2.88 mbars. It is seen from Fig. 2 that,although metal-state extinction coefficients km are not

4120 APPLIED OPTICS / Vol. 30, No. 28 / 1 October 1991

* CONTROL

o ARE

I I I I I

I I I I I

I

.12

Fig. 4. X-ray diffraction data for VO2 thinfilms deposited with and without activation.

.06

.04

.02

.00

8.0

km

6.0 -

5.0

4.0_2.0 3.0 4.0 5.0 6.0

X (pm)

Fig. 5. Semiconductor- and metal-state extinction coefficients asfunctions of wavelength for activated films deposited upon sap-phire and silicon substrates.

100

o-

l 1-2

I I 1,_ I I I I i40 80 120 160

TEMPERATURE (C)

Fig. 6. Temperature dependence of resistivity for a VO2 thin filmdeposited by ARE.

seriously affected by variations in these variables,those of the semiconducting state, k, are highlysensitive to both 02 pressure and anode current. Forfilms deposited at a background pressure of 2.88mbars, k decreases with increasing anode current. At15-20 mA, corresponding to a voltage near 180 V, anorder-of-magnitude reduction in k is observed. Forthe film deposited at 2.60 mbars, the activationprocess is not so efficient because a higher voltage isrequired at this reduced pressure to generate thesame anode current; however, the ionization proba-

1 October 1991 / Vol. 30, No. 28 / APPLIED OPTICS 4121

I I I I I I I I

v02 (311)

(320)

_ An__ A=~~~ ATIVATE

CONTROLI I I II I I I

21.0 41.0 61.0 81.0

20 (degrees)

101.0 121

I I I I I I I

I I I

V02 /Si

V 2/Al 2 3

4--N --

l l

/o~~~

// / 1< VO /AlV02

.

_ I I I,

k5

7. 0 _-

Table 1. Comparison of Properties for VO, Thin Films Deposited byStandard, Activated, and on-Assisted Reactive Evaporation

ReactiveProcess (Pa/Pm)8

nb kb ne ke,

Standard 2.93 x 103 3.13 0.073 3.23 5.90IAD(200V) 1.91 x 104 3.19 0.036 3.17 5.81IAD(350 V) 6.60 x 103 3.28 0.075 3.28 6.43ARE 5.40 x 104 2.88 0.002 3.56 5.71

apX semiconductor-state resistivity; pm, metal-state resistivity.bn - ik,, semiconductor-state complex refractive index.Cfnm - ikm, metal-state complex refractive index.

Table 1. Comparison of Optical Performance for Two MultilayersContaining VO2 With k, = 0.02 and 0.002a

Design 1 Design 2

Layer nb k," dd nb kc dd

1 3.00 0.0 0.283 3.00 0.0 0.2832 1.63 0.0 0.521 1.63 0.0 0.5213 3.00 0.0 0.283 3.00 0.0 0.2834 1.63 0.0 0.521 1.63 0.0 0.5215 3.00 0.2 x 10-2 0.283 3.00 0.2 x 10-l 0.2836 7.84 0.0 0.108 7.19 0.0 0.119

Semiconductor Metal Semiconductor MetalTransmit-

tance 97.5 0.002 81.8 0.003Reflec-

tance 0.00 82.3 0.00 79.6

aMetal-state results are for n = 3.0, k 5.5; kin, metal-stateextinction coefficient.

bn, real part of the index of refraction.0k8 , semiconducting-state extinction coefficient.dd, thickness (p1m).

bility is diminished for electrons accelerated to thesehigher energies. Figure 3 compares k3 and km from 2.5to 3.5 pm for films upon sapphire substrates, depos-ited under identical conditions of temperature andpressure, with and without voltage on the anode.Filled circles represent the nonactivated control film,whereas open circles refer to the activated film depos-ited at 180-V and 17-mA anode current. Large differ-ences are observed in k but not in km between the twofilms. For example, at 3.4 m the relative percentchange in km with or without activation is <4%compared with > 98% for k8 . X-ray diffraction dataindicate that all the films deposited on sapphire at thelow rate of 0.1 nm/s exhibit preferential orientation,regardless of whether activation was used. Figure 4shows these data for both activated and nonactivatedfilms on sapphire. Two x-ray lines are present for theactivated film and represent the (320) and (311) VO2orientations, the latter appearing for the nonacti-vated film as well, although, a higher degree ofcrystallization is apparent for the ARE film. Theremaining line is due to the substrate.

Figure 5 is a comparison of k and km versuswavelength for VO2 deposited onto silicon and ori-ented-sapphire substrates. Values for k, for the film

on silicon are near 0.004 at both 3.4 and 5.0 [um, withthose on oriented sapphire being slightly lower. Onsetof a reaction at the vanadium-silicon interface attemperatures as low as 350'C has been reported,9 andthis could account for the slight difference in k,. Anincrease in km is observed for the film on silicon overthe range of 2.5-5.0 pm.

Figure 6 is a plot of resistance versus temperaturefor an activated film deposited upon a sapphiresubstrate. A resistivity change of more than 4 ordersof magnitude is observed. There is a slight increase inresistivity with temperature following the phase tran-sition from the semiconductor to the metal state.

Table I lists the complex indices of refraction andthe semiconductor-to-metal resistivity ratios for V02films deposited at 500'C by standard oxygen-ion-assisted evaporation and ARE. All films were depos-ited at 0.1 nm/s with a background oxygen pressure of2.88 mbars. The ion-assisted films were depositedunder both 200- and 350-V oxygen-ion bombard-ment.4 Table I shows that low-energy oxygen-ionbombardment increases the resistivity ratio and low-ers the semiconducting-state extinction coefficientover that of the standard reactively evaporated film.At higher ion bombardment energies, the reverse istrue. ARE, however, produces films with a resistivityratio of 5.4 x 104, significantly higher than for theother representative films and of the order of thatobserved for high-quality single crystals.'"'" Althoughits metal-state extinction coefficient is lower thanthose of the other films, the switching gain effected bythe much-reduced k3 value far exceeds any detrimentcaused by the modified km value. The indices listed inTable I are computed from transmittance, film-sidereflectance, and substrate-side reflectance averagedover three separate spectral runs. The upper limit onk8 for the ARE film on sapphire is 0.002.

Discussion

Consider two V02 thin films both of index n = 3 andextinction coefficients k = 0.02 and 0.002, respec-tively, the former representing good quality standardmaterial and the latter the improved ARE materialreported here. When deposited as single layers ofequal thickness, these two films will exhibit small,albeit measurable, differences in reflection and trans-mission (- 1%). However, the incorporation of V0 2into a properly designed multilayer stack can enhancethe optical switching performance of the stack.'2 Inthis case, the benefits of an order-of-magnitude reduc-tion in the semiconducting-state extinction coefficientbecome apparent. A relatively simple film designcould begin with a sapphire substrate and the addi-tion of four dielectric films with high-low-high-lowindices n = 3.0 and n = 1.63, respectively, followed bya V02 film and an antireflection coating. For simplic-ity the index and the thickness of a hypotheticalsingle layer (layer 6) have been computed to provide aperfect antireflection coating at 3.4 [um when V0 2 isin its semiconducting state. I compared responses for

4122 APPLIED OPTICS / Vol. 30, No. 28 / 1 October 1991

two cases in which the semiconductor-state materialexhibits k = 0.02 and k = 0.002, representingstandard and improved ARE material, respectively.The metal-state parameters for the VO, are fixed forboth films at n = 3.0 and km = 5.5. Table II lists thedesign parameters as well as the expected reflectionand transmission of each multilayer stack. Theseparameters indicate that, when the VO2 film is in itsmetal state, the transmission of either multilayerdesign is near zero, and the reflectances are verysimilar (82.3% and 79.6%). The small variation is dueto the slightly different antireflection coatings thatserve to maximize the semiconducting-state transmis-sion of each design. When the VO2 is in its semicon-ducting state, the reflectances of both stacks are zero;however, the transmission for the design that con-tains the ARE VO2 material is enhanced (97.5%) overthat containing the standard material (81.8%). Thistransmission increase of 15.7%, which improves theswitching characteristics of the design, was achievedby the order-of-magnitude reduction in k3 from 0.02to 0.002 for the phase-transition material.

Summary

VO2 films of improved quality have been deposited at500'C by using activated oxygen background gas in areactive evaporation process. These films have agreatly reduced semiconducting-state extinction coef-ficient with only minor degradation of the metallic-state properties. Incorporation of this material canenhance the performance of switchable multilayerthin-film devices.

The author thanks H. E. Ratzlaff for his consider-able contributions to this research and W. A. Pateyfor careful film depositions. Also appreciated are thecomments of W. E. Case.

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