1.350272 LEER

Switchable vanadium oxide films by a solgel processD. P. Partlow, S. R. Gurkovich, K. C. Radford, and L. J. Denes

Citation: Journal of Applied Physics 70, 443 (1991); doi: 10.1063/1.350272 View online: http://dx.doi.org/10.1063/1.350272 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/70/1?ver=pdfcov Published by the AIP Publishing

Articles you may be interested in Sol-gel derived nanostructured cerium oxide film for glucose sensor Appl. Phys. Lett. 92, 263901 (2008); 10.1063/1.2953686

A modified sol-gel process for multiferroic nanocomposite films J. Appl. Phys. 102, 083911 (2007); 10.1063/1.2800804

RBS and NRA of cobalt oxide thin films prepared by the sol-gel process AIP Conf. Proc. 576, 440 (2001); 10.1063/1.1395343

Preparation and properties of transparent conductive aluminum-doped zinc oxide thin films by solgelprocess J. Vac. Sci. Technol. A 19, 1642 (2001); 10.1116/1.1340659

Magnetic thin films produced by solgel processes J. Appl. Phys. 57, 3812 (1985); 10.1063/1.334928

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:132.248.254.178 On: Wed, 18 Jun 2014 20:02:30

Switchable vanadium oxide films by a sol-gel process D. P. Partlow, S. R. Gurkovich, K. C. Radford, and L. J. Denes Westinghouse Science and Technology Center, 1310 Be&ah Road, Pittsburgh, Pennsylvania I5235

(Received 4 February 1991; accepted for publication 26 March 1991)

Thin polycrystalline films of VOZ and V303 were deposited on a variety of substrates using a sol-gel process. The orientation, microstructure, optical constants, and optical and electrical switching behavior are presented. These films exhibited sharp optical switching behavior even on an amorphous substrate such as fused silica. The method yields reproducible results and is amenable to the coating of large substrates and curved surfaces such as mirrors and lenses.

I. INTRODUCTION

The vanadium-oxygen system is an interesting and complex subject for study, involving as many as 13 distinct phases, each identifiable by its lattice structure and spacings. A further complication is the substantial variability in stoichiometry occurring in some structures, most notably the Magneli phases based on r-utile, with compositions of V,OZn _ i, where n is an integer. Semiconductor-to-metal transitions have been reported in at least eight vanadium- oxygen compounds at temperatures ranging from - 147 to 68 oC.2*3 These changes are associated with a first-order crystallographic transition affecting the electrical, magnetic, and optical properties of the materials, permitting them to be used as switches. VO3 and VZ03 exhibit the largest changes in properties and are therefore the most useful compositions, with VOZ receiving the most atten- tion. This is perhaps because the transition temperature of V02 is conveniently close to room temperature, at 68 C, making it useful for a variety of applications.2k7 Studies involving the transition of V303 require measurements in the range of - 121 C, and a few properties have been reported through its transition.-l3

was prepared and films deposited by spinning or dipping. Organic components were first removed during conversion of the film to V,O,, and reduction to V02 or V203 was done by heating the sample in a suitable reducing atmosphere. This method offers the advantages typical of the sol-gel method such as high purity, simplicity of equip- ment, and easy scaling for large substrates. In addition, a high degree of control was evident in the reduction process; as films could be cycled at will from single-phase V02 to V3O3 and back again. Optical properties were the primary interest in these experiments, and very good, reproducible optical switching behavior was obtained on several substrates including fused silica, showing that lattice matching and orientation were not preconditions for good switching.

II. EXPERlMEN+

A. Film deposition and pyrolysis

The volume change associated with the crystallographic transition in VO3 and V2O3 is catastrophic in bulk material but is usually not a problem in thin films. Many thin-film formation methods have been used to prepare these materials, but results vary widely, particularly with respect to switching behavior.11.2*4-24 Some of these differences can be attributed to microstructure, since grain size and shape will clearly have a strong effect on electrical properties, the most common criterion used to evaluate switching in fllms.25 Another factor affecting film performance is the degree of crystallinity, which modifies both the optical and electrical switching, because amorphous material cannot participate in the crystallographic transition. Variability in stoichiometry and/or mixed phases also influences these properties, and post-deposition heat treatments have been used to adjust the stoichiometry of films deposited by vacuum techniques.2*23242627

A sol-gel method similar to that of Greenberg16 was used, in which the starting material was commercial vanadium oxide isopropoxide, ( VO ( OC3H7) 3 (Morton Thiokol, Alfa Products, Danvers, MA), a clear liquid sol- uble in a variety of organic solvents. Analysis by emission spectroscopy showed that the purity of this material was equal to or above that of the chemical standards used in the analysis, except for 40 ppm of Ti, which was judged acceptable. Figure 1 gives the range of sol-gel solution concentrations and the amount of hydrolysis water that could be tolerated at each concentration to yield a clear, stable solution suitable for making films. The dilution fluid in this case was dry ethanol, and exchange of the isopropyl and ethyl groups was evident, as the pale yellow isopropoxide became green. Similar results were obtained using the equivalent n-propoxide vanadium compound, and it is likely that a wide variety of organometallics and solvents can be used, since the final oxidation state of the vanadium ion is dependent on the pyrolysis conditions rather than the starting material. The clear solutions from Fig. 1 produced acceptable coatings if the deposition rate was controlled to yield appropriate film thicknesses.

In this work a relatively simple method was used to The deposition apparatus was a dipping machine ca- prepare V02 and V,03 films on a variety of substrates by pable of pull rates from 40-85 cm/min. This apparatus was combining previously reported techniques. 13-16,28 Begin- enclosed in a nitrogen-filled dry box to prevent uptake of ning with a vanadium alkoxide, a sol-gel precursor solution atmospheric moisture, which would induce precipitation in

443 J. Appl. Phys. 70 (I), 1 July 1991 0021-8979/91 /0104&3-l 0$03.00 @ 1991 American institute of Physics 443 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.248.254.178 On: Wed, 18 Jun 2014 20:02:30

54 000 oo 6.

j3 001

s 2 0

1

01 1 1 1 I I I I I I I I I 0 0.4 0.8 1.2 1.6 2 0

Hydrolysis ( mol H20/mol V)

FIG. 1. Combinations of solution concentration and hydrolysis water that permit the preparation of stable, clear solutions from VO(OGH,),.

. ,:

the precursor solution. Solution concentrations of 2-8 wt. % equivalent VO3 gave acceptable films using this apparatus, but the thinnest films were only 60 nm thick and were not opaque in the metallic state. Films that were too thickly cracked or spalled from the substrate during sub- sequent heating steps used to convert the fihri to the desired oxidation state. The maximum film thickness capable of being deposited in a single layer was substrate dependent, but intermediate film thicknesses of 200-400 nm were eas- ily obtained. Thicker films could be prepared by depositing multiple layers, with an intermediate heating step under a hot lamp at about 250 C! to prevent dissolution of prior layers by the solution. This process worked..yell for coating substrates of fused silica, sapphire, ZnS, ZnSe, silicon, and germanium.

The dry film was heated in flowing air at temperatures 2370 C!, since thermal analysis indicated that this temperature was required to remove -organic components from the film. For most of the work reported here, oxidation temperatures of either 450 or 550 C were used, avoiding the 670 C! melting point of V205. Polycrystalline V20, films with preferred orientation resulted from this process, and these were reduced either to V02 or to V2O3 by heating in an appropriate reducing gas atmosphere.3-5 It was possi-

WE. JCPDS 9-387

Degrees 29

FIG. 3. X-ray diffraction pattern for the V,O, film in Fig. 2 showing preferred (001) orientation.

ble to accomplish this by changing the llowing atmosphere without removing the sample from the furnace. It was also possible to reduce the dried film directly to V02 or V3O3, but films made by this direct reduction method28 did not display the sharp switching and narrow hysteresis charac- teristic of the other films, most likely because of their very fine grain size.

The reduction of V205 films to the V02 state utilized the method of MacChesney et a1.4T1 who heated the samples in. a mixture of 50% CO/50% CO2 at temperatures between 480 and 530 C. However, it was possible to carry out this- reduction at virtually any temperature above 400 C. At 525 C the reduction time was on the order of 30 min and increased to 2 h at 450 C. Reduction of VsO, films to the V2O3 state was done by heating for 2 h. at 350-850 C in dry H& as described by Kosuge.13 To obtain good crystallinity in V3O3 Elms temperatures above 700 C! were preferred.

It was determined that a v&3 metastable-phase could be formed by quenching before reduction was complete, but no evidence of this or any phase intermediate between V02 and V203.was found by x-ray diffraction analyses of the finished tilms or of bulk powders prepared similarly. Furthermore any V,O, material could be converted to V02 or V2O3 using these heat treatments, as shown, for example, by the full conversion of sputtered films contain- ing mixed phases.

B. Measurement of film properties

Film microstructure and crystallinity were determined by scanning electron microscopy, transmission electron microscopy, electron diffraction, and x-ray diffraction. Opti- cal and electrical switching characteristics were evaluated using a commercial variable temperature dewar assembly that was installed in the sample compartment of an IR spectrophotometer. Since V02 and V203 films are transparent throughout the mid IR spectral region, property

FIG. 2 Scanning electron micrograph of a V,O, film pyrolyzed at 400 C on a fused SO, substrate (tilt angle = 70 C).

444 J. Appl. Phys., Vol. 70, No. I, 1 July 1991 Partlow et al. 444


a

b

d

100 80 60 40 20 0

JCPDSQ-142

Deprees 20

FIG. 4. X-ray diffraction patterns for VO, film; showing preferred orien- tations of (011) and/or (020) on (a) and (c) fused SiOr substrates; (b) and (d) (001) sapphire substrates. -- -*_

__.- a/

measurements were made at a wavelength of 2.5 pm; which could be attained using several available spectropho- tometers. The optical fixture was modified to permit simul- taneous measurement of the sheet resistanceand optical transmission. of the film. The temperature was -measured

FIG. 5. Scanning electron micrograph of a typical VO, film.

. .

Direct Reduced Alkoxide Film on

?O ; . 30 40 50 60 70 80 90 Temperature, C

FIG. 6. Comparison of the optical switching behavior of a VOz film prepared in this work (shaded) with that of films reported in references as indicated.

using a copper-constantan thermocouple placed firmly in contact with the film under test. Because the thermocouple contact was displaced from the area of impingement of the optical probe beam, it was imperative that the temperature cycling rate be sloti and constant to obtain accurate hysteresis curves. Uncontrolled temperature cycling. can pro- duce misleading results in terms of the sharpness of switching and width of the hysteresis loop. For this work, heating

iid cooling rates of 1 Wmin were found to yield accurate results.

III. RESULTS AND DISCUSSION ~. A. V206 films .

The microstructure of a 300~nm V,O, film pyrolyzed at 400 C on -a fused silica substrate is shown in Fig. 2, which indicates a fine granular structure. An x;ray diffraction -pattern for this film in Fig. 3 suggests a preferred orientation, with the (001) planesof the crystallitea in the iilm lying parallel to the plane of the substrate. As the pyrolysis temperature apgroached the mehing point of V&I5 the film grain size increasedto500-1000 nm. Higher magnification transmission electron microscopy revealed that the film grains were tabular in shape. All of the Vz05 films in this work showed preferred orientation with either (001) or (hO0) planes parallel to the substrate, depending on whether the tabular grains were primarily ver- tical or horizontal. The factors governing this orientation were not explored, because these effects were overridden upon reduction by lattice matching considerations with the substrate when applicable.

B; VOp films

-Uniform V02 films were prepared with excellent adhe- sion to different substrates including SiOz, A120s, Si, Ge, ZnS, and ZnSe as confirmed by a pull test using transparent tape. The VOz films prepared were predominantly the commonly reported bronze type, appearing golden brown in transmission. Films of the blue type as de-

445 J. Appl. Phys., Vol. 70, No. 1, 1 July 1991 Partlow et al. 445 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.248.254.178 On: Wed, 18 Jun 2014 20:02:30

Fused Si02 (100) Sapphire

c 103

ii 102 .P 8 c 10

100

lo 1

,03; 4o j T$pera;e) ria 1;

60-

zf? 50 .i *- 40- g 3c- I= 20

lc- LL 0, I I 8

30 40 50 60 70 80 90 100 Temperature C

10-l ! I I 30 40 60 60 70 60 90 1

Temperature C

FIG. 7. Comparison of optical and electrical switching curves for VOz films on fused SiOz and sapphire substrates (A = 2;5 pm). Differing switched transmission values are attributable to film thickness differences.

scribed by Case and by Nyberg and Buhrman23v30 were also produced. Experimental conditions leading to the formation of blue V02 were consistent with the observations of Nyberg that biue V02 formed at higher partial pressures of oxygen. Microstructural characteristics of these films were similar to those reported by Case, who observed coarsening of the microstructure concomitant with the formation of blue VOz. These films, however, did not exhibit significant differences in x-ray diffraction patterns or optical switching performance compared to bronze films. Further discussion will be limited to bronze-type films.

The x-ray diffraction patterns for V02 films on two different substrates are shown in Fig. 4. These films were polycrystalline showing preferred orientation with the (011) plane lying parallel to the plane of the substrate even on amorphous silica substrates. This orientation is likely a remnant of the orientation of the V205 platelets as described above. Figures 4(a) and 4(b), involving fused silica and single-crystal sapphire substrates, illustrate typical x-ray diffractio.n patterns for these V02 films.

Another orientation commonly observed in V02 films prepared from sol-gel solutions is (020). The (020) reflection is not shown in the JCPDS powder diffraction pattern for V02 (y-142), but is possible in the monoclinic crystal system and has been calculated to have an intensity which is 3% of that of the (011) reflection.24 The occurrence of this orientation has been reported by others and has been attributed to lattice matching with sapphire.24 However,

446 J. Appl. Phys., Vol. 70, No. 1, 1 July 1991

Figs. 4(c) and 4(d) show that this orientation was observed on amorphous silica as well as sapphire substrates, indicating that lattice matching is not a prerequisite for the (020) orientation in V02 films. The (020) orientation was observed primarily after prolonged annealing of films and was accompanied by a mottled ti appearance, suggesting the possibility of recrystallization. However, this was not found to affect optical switching performance. X-ray diffraction analyses of V02 films on ( 111) silicon substrates produced results similar to those described for silica and sapphire substrates, although confirmation of the (011) orientation was not possible because of the interfering reflection from the substrate.,

A typical microstructure for a VOZ film on silicon is shown in Fig. 5. This scanning electron micrograph shows primarily needlelike grains with diameters of 200-500 nm and lengths of 1000-3000 nm.

Optical switching measurements of V02 films on fused silica, sapphire, and silicon showed hysteresis widths of 2-6 C (.measured halfway between maximum and mini- mum transmission), and temperature changes of 2.5-6 C! (measured between 10% and 90% of the transmission range) were required to switch the films. An example of switching performance is shown by the shaded curve in Fig. 6 for a 233-nm film on a (100) sapphire substrate. This film exhibited a 2 C hysteresis width and required a temperature change of 2.5 C to switch from a transmission of 60 to < 0.5%. This behavior is similar to that of the best

Partlow et a/. 446


g? 60 r 50

$ 40

-2 30 tn g 20

F 10 - Calculated d OF.*,...........

0 4 8 12 16 20 24 Wavelength, urn

70 ..I.............. vo2

L

60 : B i c 50 - 2 - ; 40 -

r

T=3OOK - Calculated

J

Si (0.3 mm)

140 nm Fi lm on St 1

301 0 4 8 12 16 20 24

Wavelength, pm

50 T3300K

& 0 40 - Calculated

g30 z E20 140 nm Fi lm on 0.3

8 9 10

n 0 4 8 12 16 20 24

Wavelength, pm

0 4 8 12 16 20 24 Wavelength, pm

t 22 1 60 . n E - 5 50 -

K : r - Calculated

,ot...'.'."*.'..."."...' 0 4 8 12 16 20 24

Wavelength, pm

0 4 8 12 16 20 Wavelength, pm

140 nm Fi lm on 0.3 m m Si 0

24

FIG. 8. Spectrophotometer data and model comparison for a VOa film on a Si substrate and for uncoated Si in the unswitched (300 K) and switched (360 K) states.

films reported and is compared in Fig. 6 with data from a film prepared by e-beam evaporation.24 Reproducibility in the switching behavior for the sol-gel films was demon- strated by three MO-nm and three 700-nm films on silicon substrates, all of which displayed hysteresis widths of 5& 1 C and required temperature changes of 3 f 1 C to switch.

Also shown in Fig. 6 is a film ( -300 nm) prepared using a sol-gel method involving direct conversion of the dried gel to V02,* without the intermediate oxidation to V,O, which removes organics. This film exhibits a broad hysteresis and requires a greater temperature change to switch as compared to the films described above. Such be-


havior was also observed in films prepared by direct conversion in this work and is associated with very fine grain size. For example, a 223-nm-thick film converted directly to VOz on a fused silica substrate displayed a hysteresis width of 16 C and required 10. C to switch from 65 to ~0.5% transmission. Note the low unswitched transmission ( - 33%) for the directly converted film * represented in Fig. 6. This is likely due to the presence of an absorbing second phase or to nonstoichiometry in that film and was not seen in the present .work.

A number of potential substrate materials were inher- ently unstable or reacted with vanadium oxides under conditions optimized for the production of VOi. This necessi-

Partlow et al. 447 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.248.254.178 On: Wed, 18 Jun 2014 20:02:30

COLD STATE REFRACTIVE INDICES HOT STATE REFRACTIVE INDICES

n 3.0

0 140nm 2.5 * 27Onm

+ 410nm

2.0 '..I.. .'. .-I.. ."--I'. . 0 4 8 12 16 -2.0' 24

Wavelength, $I

T t 300 K

k 0.1 r 0 140nm

6 27Onm + 410nm 1

o.o,......*......... 0 4 8 12 16 20 24

Wavelength, pm

n

7

6

5

4

3 2

1

0 0 4 8 12 16 20 24

Wavelength, pm

3 8

6

k 4 0 140nm *

2 l 270nm 1 + 410nm *

Oi................ 0 4 8 12 10 20 24

Wavelength, pm

FIG. 9. Optical constants (n, k) determined for three VC& samples at 300 and 360 K.

tated processing under less ideal conditions which led to finer grain size that produced effects similar to those observed in direct conversion, i.e., increases in hysteresis widths and in the temperature change required to switch. For example, a V02 film on ZnS showed a hysteresis width of 16 C! and required a temperature change of 10 C to switch from 71 to < 0.5% transmission.

Electrical measurements were also made using a simple .- series resistance measurement and computer-controlled system for heating and data acquisition. It was found that electrical switching characteristics were more sensitive to processing conditions than were optical switching characteristics. For the most part good electrical switching con- noted good optical switching, although the presence of a metallic phase such as V,Ois during the early stages of film optimization permitted good electrical switching along with poor unswitched optical transmission. Films on fused silica substrates showed a consistently smaller change in resistance upon switching than those on single-crystal sapphire. Optical and electrical switching curves for films on silica and sapphire are shown in Fig. 7. Despite comparable optical switching performance, the film on fused silica exhibited a resistance change of only three orders of mag- nitude compared to. four for the- film on sapphire. ,The comparable optical switching performance, which is dic- tated by the bulk properties of the films, indicates that the differences in electrical switching are strongly influenced by grain boundary. phenomena. =

Optical parameters for VOz films were determined


from spectrophotometer measurements of transmission, T(A), and reflectance, R(A), of the substrate and of the film/substrate pair in addition to the flhn thickness, which was measured by profilometry. Optical constants were then calculated using commercial software, TF Calc (Software Spectra, Inc., Culver City, CA). Figure 8 presents the measured transmission and reflectance data and inferred absorption for a silicon substrate and a V02 film/Si substrate sample. The solid curves show the model fit to the data from which the optical parameter dependence was obtained. Note that the transmission, reflectance, and absorption nearly mimic those of the Si substrate in the cold (300 K or 27 C) state of V02. However, in the hot (360 K or 87 C) state VO;! exhibits lower transmission, higher reflectance, and appreciable absorption throughout the 2.5-22 pm range covered.

Figure 9 shows the n and k values as determined for three VOz samples of differing thicknesses. In all three samples a smoothly varying n and k dependence is found that fits the T(L) and R(A) data within their experimental uncertainties. There is some departure in detail among the three samples, most noticeably at shorter wavelengths. This is most likely due to the fact that these films were dipcoated on small substrates, and the large beam aperture used in this task extended close to the edge of the sample, where a meniscus effect produced a thickness nonunifor- mity. Our deduced n and k dependances for VOa films are within the range of the values observed by other research- ers 17,2611,32

Paitlow et a/. 448


1 a

7--. . , . . x .Z 2

49 r al

.z * cd

z a

C

JCPDS 34-187

10 24 38 52 80 Degrees 28

FIG. 10. X-ray diffraction patterns for V,O, films showing preferred orientation on (a) fused SiO*, (b) (100) sapphire/and (c) (001) sapphire.

C. V203 films .

X-ray diffraction patterns of sol-gel derived Vz03 films on several types of substrates are shown in Fig. 10, which also includes a standard random powder pattern for comparison. The film on fused silica appears to be randomly oriented, with some suppression of the (104) reflection. However, the films on sapphire show strong enhancement of reflections from V,Os planes which have lattice spacings matching those of the substrate. The alignment of parallel sets of planes such as (300) in V203 and ( 100) in sapphire is not surprising, since the crystal structures are both hex- agonal with similar lattice constants. The same can be said for the (006) V,Os enhancement on (001) sapphire. Al- though the relationship of the other two enhanced V,Os reflections, ( 1 lo), and (220)) with the (00 1) plane of sapphire is less obvious, analysis of both structures.shows that the ionic positions in these V,O, planes match certain positions in the basal plane of sapphire within 4%. V203 films on (111) silicon and germanium also showed preferred orientation, as indicated by the enhancement. of either ( 110) or ( 116), depending on the reduction temperature. Structural analysis again showed that this was due to lattice matching considerations, although the match was not as good as with sapphire.

- 130 140 150 160 170 180 180 - 190 190 : :

Temperature, K

40

Sample B146.3R Substrate: Si 2585 A ?. = 2.5 pm

0 130 ,140 150 160 170 180 190 2

-. Temperature, K

A scanning electron micrograph of a sol-gel VsOs film on silicon is presented in Fig. 11, which is representative of

FIG. 12. Similarity of optical switching for V20, films prepared on (a) fused SiOl and (b) (111) Si (A=2.5 pm).

FIG. 11. Scanning electron micrograph of V,O, film on a Si substrate.

films prepared similarly on other substrates. This figure shows that the conversion from V,O, to V,Os was accompanied by a coarsening of the microstructure. An--optical switching curve for this film is shown in Fig. 12 along with a switching curve for another film on fused silica. Note that the switching behavior of these films is very similar, even though one is random and the other of preferred orientation. Different transmissions in the cold state are attributable to optical interference effects. A hysteresis width of 7-10 C was typical, and the temperature change required

449 J. Appl. Phys., Vol. 70, No. 1, 1 July 1991 Partlow et al. 449 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.248.254.178 On: Wed, 18 Jun 2014 20:02:30

g? 60

$50 3 40 E 30 f 20 $10

210 nm Film on Si 4

- Calculated

ot......~ 0 4 8 12 16 20 24

Wavelength, pm

70

60 ai- E 0 50 2 E: a 40 K 1 f +210nmFilmonSi + 3ot......*....1

0 4 8 12 16 20 24 Wavelength, pm

50 ..1..1...I,..I.,.I.., v20.3

$40 E T.:13OK

- Calculated - I. 4 _.

g 30 Z Q $20

210 nm Film on Si

2 10

n "0 4 8 12 16 20 24

Wavelength, pm .-

70~*..1...I...I...I'..I.,.~ v203

#t 60 g" 50

*; 40

-; 30

g" 20 E

210 nm Film on SI

I- 10

0 0 4 8 12 16 20 24

Wavelength, pm

60 . $ g 50 5 J= Q) 40 K

v203 T+300K

- calaJlated 1 3or""""""."'""'"'

0 4 8 12 16 20 24 Wavelength, pm

$2 40 - Calculated

g30 3 e 2lOnmFilmo j 20 2 a IO

0 0 4 8 12 16 20 24

Wavelength, pm

FIG. 13. Spectrophotometer data and model comparison for a V,O, lilm on a Si substrate and for uncoated Si in the unswitched (300 R) and switched (360 K) states.

to switch the films from 10% to 90% of full range was 8 to 11 c!.

This switching response is comparable to the results obtained by others on single crystals of V20310p29 and is significantly better than the reported optical switching behavior of epitaxial and polycrystalline films. Good switching properties were obtained in V20s films on fused silica, silicon, germanium, and ( 100) sapphire substrates, but films on (001) sapphire exhibited rather sluggish switching.

The approach used in determining the optical parameters for V20s films was identical to that used for the

V02 films discussed above. Fig. 13 presents the measured transmission and reflectance data and inferred absorption for a silicon substrate and a V,03 film Si substrate sample. The solid curves again represent our model fit to the data from which the optical parameter dependence was obtained. As with V02, the transmission, reflectance, and absorption nearly mimic those of the silicon substrate in the cold ( 130 K or - 143 C!) state of V,Os, but there is a departure at wavelengths longer than about 16 ,um. In the hot (300 K or 27 C!) state V20s exhibits lower transmission, higher reflectance, and appreciable absorption throughout the 2.5-22 pm range, similar to VO,.

450 J. Appl. Phys., Vol. 70, No. 1, 1 July 1991 Partlow et al. 450


COLD STATE REFRACTIVE INDICES HOT STATE REFRACTIVE INDICES

n

k

..*I*..I...I...I,..,..

v203 Tr130K

+ (210 nm) Sol Gel Film on Si

0 ..~.......~... 0 4 8 12 i6 26 24

Wavelength, pm

0 Bulk, Ref. 31 + (210 nm) Sol-Gel Film on Si

0.01 ............-..... 0 4 8 12 16 20 24

Wavelength, pm

8 ..-I.-.I...I..I..*I... v203

A. T P 300 K

6

n 4

2

n

0 Sulk. Ref. 31 * Bulk, Ref. 33 + (210 nm) Sol-Gel Film on Si

-0 4 8 12 16 20 24 Wavelength, pm

6 k

4

0 4 8 12 16 20 24 Wavelength, pm

FIG. 14. Optical constants (n, k) determined for a 210~nm V,03 film on silicon at 130 and 300 K. Included for comparison are data reported by other workers.

Figure 14 shows the n and k values as determined for a V,Os film. As with the V02, a smoothly varying n and k dependence fits the r(n) and R(a) data that were illus- trated in Fig. 13. Also shown for comparison are the optical parameters for bulk V,O, reported by Fan31 and Zhuze et a1.33

IV. CONCLUSIONS

High-quality thin-film switches of VOz and V203 were produced using a sol-gel method in which the initial deposition was done from the liquid state. Film chemistry was then altered as desired by heating the coated substrate in appropriate atmospheres. The final films were polycrystalline and showed preferred orientation in cases where lattice matching with the substrate was possible. These films exhibited excellent optical switching behavior even on an amorphous substrate such as fused silica. The advantages of this method are simplicity of process, high purity of starting materials, compatibility with a wide range of substrates, ability to scale the process to handle large substrates, and reproducibility in switching properties.

*J. Stringer, J. Less-Common Met. 8, 1 (1965). F. A. Chudnovskii, Sov. Phys. Tech. Phys. 20, 999 (1975). D. Adler, Rev. Mod. Phys. 40, 714 (1968). 4A. R. Begishev, A. S. Ignatev, V. V. Kapaev, and V. G. Mokerov, Sov.

Phys. Tech. Phys. 24, 1263 (1979).


R. G. Cope and A. W. Penn, Brit. J. Appl. Phys. J. Phys. D 1, 161 (1968).

G. A. Rozgonyi and D. H. Hensler, J. Vat. Sci. Technol. 5, 194 ( 1968). I. Balberg and S. Trokman, J. Appl. Phys. 46, 2111 (1975). s A. C. Gossard, A. Menth, W. W. Warren, Jr., and J. P. Remeika, Phys.

Rev. B, 3, 3993 (1971). K. Kosuge, T. Takada, and S. Kachi, J. Phys. Sot. Jpn. 18,318 (1963).

J. Feinleib and W. Paul, Phys. Rev. 155, 841 ( 1967). V. G. Mokerov, I. V. Ryabinin, and G. B. Galiev, Sov. Phys. Solid State

22, 712-713 (1980). G. A. Rozgonyi and W. J. Polite, J. Electrochem. Sot. 115, 56 (1968). t3K. Kosuge, J. Phys. Chem. Solids 28, 1613 ( 1967). 14J. B. MacChesney, J. F. Potter, and H. J. Guggenheim, J. Am. Ceram.

Sot. 51, 176 (1968). Idem, J. Electrochem. Sot. 115, 52- ( 1968). r6C B. Greenberg, Thin Solid Films 110, 73 (1983). H. W. Verleur, A. S. Barker, Jr., and N. Berglund, Phys. Rev. 172,788

(1968). I* W. R. Roach and I. Balberg, Solid State Commun. 9, 551 (1971). H. Bruchlos and K.-D. Ufert, K&t. Tech. 12, 153 (1977). V. G. Mokerov and A. R. Begishev, Sov. Phys. Solid State 22, 629

(1980). E. V. Babkin, A. A. Charyev, A. P. Dolgarev, and H. 0. Urinov, Thin

Solid Films 150, 11 ( 1987). K Okabe, T. Mitsuishi, and Y. Sasaki, Jpn. J. Appl. Phys. 26, 1802

(1987). F. Case, Appl. Opt. 26, 1550 (1987). s4J. F. De Natale, P. J. Hood, and A. B. Harker, J. Appl. Phys. 66, 5844

(1989). 25 E. E. Chain, J. Vat. Sci. Technol. A, 4, 432 (1986). 26F. Case, Appl. Opt. 27, 1803 (1988). Y. Takahashi, M. Kanamori, H. Hashimoto, Y. Moritani, and Y. Ma-

suda, J. Mat. Sci. 24, 192 (1989).

Partlow et a/. 451 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

132.248.254.178 On: Wed, 18 Jun 2014 20:02:30

28K. R. Speck, S. S.-W. Hu, M. E. Shewin, and R. S. Pot%nber, Thin. J. C. C. Fan, Technical Report No. ARPA-43 and HP 28, 05ce of Solid Films 165, 317 (1988).

F. J. Morin, Phys. Rev. Lett. 3, 34 (1959,. Naval Research, May 1972.

32G. A. Nyberg and R. A. Buhrman, Thin Solid Films 147, 111 (1987). G. A. Nyberg and R. A. Buhrman, J. Vat. Sci. Technol. A 2, 301 33V. P. Zhuze, D. P. Lukirskii, and G. P. Startsev, Sov. Phys. Solid State

(1984). 13,260 (1971).

452 J. Appi. Phys.. Vol. 70, No. 1, 1 July 1991 Partlow et al. 452


1.350272 LEER

Documents

Transcript of 1.350272 LEER