Crystallographic transition in conventional bronze andanomalous blue V0 2 thin films

Francine C. Case

X-ray diffraction spectra both above and below the 67CC phase transition were obtained for conventionalbronze and anomalous blue V0 2 thin films. Linewidth and d spacing changes which occur in the dominantx-ray line, when the material is switched from the high temperature tetragonal to low temperature monoclinicphase, are discussed. A comparison is made between XRD spectra for anomalous material prepared by twodifferent methods.

1. Introduction

Thin films of vanadium dioxide exhibit a thermallyinduced semiconductor-metal phase transition near670 C.1-3 Below 671C the material is a semiconductorwith a distorted monoclinic structure. Above 670 C itbecomes metallic, transforming to a higher symmetrytetragonal structure.2 4 These two crystalline phasesare represented in Fig. 1. Equations (1) and (2) speci-fy the d spacing between planes corresponding toMiller indices hkl for both phases of the material.5These spacings are, of course, also functions of latticeparameters a, b, c, and /3, the values of which arespecified in Fig. 1 for the bulk material:

h 2/a

2+ 1

2/c

2- (2hl/ac) cos3 k 2

-1/2monoclinic dhkl Iin_ , (1)

L sin2fl b

2j

tetragonal dhkl = [h2/a2 + k2/a2 + 12/c2]-1 /2 (2)

Conventional films deposited near 5000 by eitherthermal oxidation of vanadium metal or by reactiveelectron-beam evaporation display large changes inoptical properties in the IR spectral region; however,not much contrast is observed at visible wavelengths.6They are bronze in transmission and do not changecolor appreciably when heated. The microstructureconsists of tightly packed coherent grains ranging from0.1 to 0.5 m in diameter, depending on depositionconditions.

Anomalous films of V0 2 have been deposited forwhich a difference in behavior in the visible wave-

The author is with LTV Missiles & Electronics Group, P.O. Box650003, Dallas, Texas 75265-0003.

Received 22 June 1988.0003-6935/89/142731-05$02.00/0.© 1989 Optical Society of America.

length region is observed. 7 8 These anomalous V0 2films exhibit a color change in transmittance from blueto red in room light on being heated through the phasetransition. In contrast to the bronze material, theseanomalous films consist of large grains, typically 0.7-1gm in diameter, separated by wide grain boundaries.Figure 2 illustrates the transmission at 24 and 1000Cfor bronze and anomalous blue V0 2 thin films from 870to 390 nm. Also included are scanning electron micro-graphs of each type of material.

X-ray diffraction data were obtained using a Sie-mens x-ray diffractometer equipped with a Hubergrazing-angle thin film arrangement. This configura-tion samples a greater effective film thickness andprovides signal enhancement over that of a standarddiffractometer. 910 Figure 3 indicates some importantparameters for the Seeman-Bohlin geometry in x-raydiffraction. An x-ray wavelength of 1.5406 A is used,and y is the mean angle between the incident x-raybeam and the plane of the film, 0 is the angle betweenthe diffracting plane PP' and the plane of the film, and20 is twice the Bragg angle in the expression

mX = 2d sinO. (3)

For the data presented, -y is 5, and the intensity isplotted as a function of 20. A resistive heater assemblywas installed so that the V0 2 samples could be exam-ined both above and below the transition temperature.

II. Experimental

Samples were deposited by three methods onto sap-phire substrates, which were oriented with the c axisnormal to the substrate plane to minimize interferencewith V0 2 x-ray diffraction lines. A standard bronzeV0 2 film was prepared by the high temperature (480-510°C) oxidation of a thin film of evaporated vanadi-um metal. The temperature was monitored by a ther-mocouple attached directly to the substrate by means

15 July 1989 / Vol. 28, No. 14 / APPLIED OPTICS 2731

OPEN CIRCLES 2

CLOSED CIRCLES V4

TETRAGONAL

Ct = 2.87A at = 4.53A

d- 2ct

1

= 122.6°

PERCENTTRANSMISSION

BLUE BRONZE40 - t t TOTAL TRANSMISSION (100'C)

TOTAL TRANSMISSION ( 25C)

20 -/

0o390 450 510 570 630 690 750 810 870

WAVELENGTH (nm)

Fig. 2. Total transmission for standard bronze and two-step anom-alous blue V0 2 thin films.

Fig. 1. Vanadium dioxide in the tetragonal(metallic) and monoclinic (semiconducting) phases

(after Ref. 2).

2e

of a high temperature thermally conducting epoxy.The anomalous samples were prepared by two differ-ent techniques. The first is a two-step process fromRef. 8 which relies on annealing an ambiently deposit-ed reactively evaporated VO (x > 2) thin film. Thesecond technique is a one-step process similar to thatdiscussed in Ref. 7, in which the vanadium metal isreactively evaporated in an oxygen background onto asubstrate held at temperatures in excess of 5600C.

X-ray diffraction data were obtained for each sam-ple at both room temperature and 830C. It should benoted that the number and relative intensity of thepeaks in the x-ray spectrum need not necessarily, andmost often do not, reflect those peaks documented inthe JCPDS V0 2 powder diffraction files,"1 since onlypolycrystalline films with a random distribution ofcrystallites would adequately represent a true powdersample. Deposition conditions greatly influence thedegree of crystallinity and the orientation of thesecrystallites in the thin film.

Fig. 3. Schematic of Seeman-Bohlin x-ray diffraction arrange-ment: y = angle of incidence of x-ray beam to sample; 0 = anglebetween diffracting plane (PP') and substrate plane; = Bragg

angle.

Ill. Results and Discussion

Figure 4 shows both the room temperature and 830CXRD spectrum for the anomalous blue V02 thin filmprepared by the two-step process. The plot is of inten-sity as a function of 20. The spectrum is characterizedby multiple peaks, most of which can be assigned toV0 2 when compared to the powder diffraction files. Itmust be emphasized, however, that x-ray data cannotbe the sole determiner of the phase or phases present inthe sample. Because of overlap in the many peaks ofthe possible oxidation states of vanadium, x-ray spec-tra alone are generally insufficient to determine abso-lutely the presence of vanadium dioxide; however,these data, in conjunction with electrical and/or opti-

2732 APPLIED OPTICS / Vol. 28, No. 14 / 15 July 1989

iw - nILETA (DEGREES)

Fig. 4. X-ray diffraction patterns of a two-step anomalous blueV02 thin film in both semiconducting and metallic states.

cal measurements, can serve to clarify whether V0 2dominates the film composition. The semiconductor-to-metal resistivity ratios for the bronze and one-stepanomalous blue films are 2.3 X 103 and 2.4 X 104,respectively. The presence of spatially isolated grainsprevented a meaningful resistivity measurement onthe two-step blue film.

Changes are observed in the XRD pattern of Fig. 4when the anomalous blue (two-step) sample is heatedthrough its phase transition. A shift is observed in anumber of peaks. Figure 5 shows a portion of thespectrum which encompasses the strongest peak at 20equal -27.6°. The set of lattice planes correspondingto this dominant peak (011) is very nearly parallel tothe substrate surface, that is, at an angle 0 9°. Theseries of nearly coincident curves on the right of Fig. 5was scanned for the sample in its semiconducting stateheld at temperatures of 24, 33, 43, and 530C. Thecurve which is shifted to lower 20 in the figure wasobtained for the sample in its metallic state at 830C.Table I lists the relevant XRD data for this sample atintervals over the temperature range mentioned. In-cluded in the table are data for the sample held at630C. At this temperature, the film is partiallyswitched.

As seen in Table I, the line is observed to shifttoward lower 20 as the sample is heated above roomtemperature while in its monoclinic phase. Thischange in 20 becomes more pronounced when the sam-ple switches to the tetragonal state. The latter ispartially explained by the change in d spacing due tothe change in crystal structure at 670 C. As shown inTable I, a shift toward lower 20 represents an increasein the d spacing for this set of planes. This may bequalitatively explained as follows.

The samples are prepared and, therefore, are in astate of minimal extrinsic stress at high substrate tem-perature. Specific d spacings for various diffractingplanes would be associated with this state of minimal

330

I- ~ ~ ~ Z

C/) ~~~~~~~430 C

21 23 25 27. 29 31 33TWO-THETA (DEGREES)

Fig. 5. Shifts in the dominant x-ray diffraction peak for a two-stepanomalous blue V0 2 thin film held at various temperatures.

stress. As the temperature is reduced, the differencesin thermal expansion coefficients a between film andsubstrate induce a macrostress that is tensile in na-ture.12 One would expect this increase in tensile stressto manifest itself as a decrease (increase) in d spacingfor those planes nearly parallel (perpendicular) to thesubstrate face as the temperature is reduced belowthat of deposition. Over the temperature range dis-cussed, neglecting changes in a at the transition, thesample should experience maximum tensile stress atroom temperature. This would correspond to a mini-mal d spacing (maximum in the associated 20 value) fordiffracting planes which lie nearly parallel to the sub-strate. As the sample is heated above room tempera-ture, these planes would then be expected to relax,with d increasing, as is observed for the dominantpeak.

It may also be seen from Table I that the linewidthincreases as the sample is heated while still in its mono-clinic phase. Once the sample begins to switch to thetetragonal phase at 630 C, there is a reversal in thistrend, and a dramatic narrowing of the line is observedwhen the phase change is complete.

Figure 6 shows the XRD spectra for the convention-al thermally oxidized bronze V0 2 thin film. The dot-ted upwardly displaced curve was taken at room tem-perature, and the solid curve is for the sample held at

Table 1. Relevant Data as a Function of Temperature for Dominant XRDPeak for a Two-Step Anomalous Blue V02 Thin Film

Temperature 20 d FWHM(0C) (deg) (A) (deg)

24 27.46343 3.1998 0.6311333 27.46047 3.2002 0.6774743 27.45581 3.2007 0.6861953 27.40648 3.2063 0.7461463 27.25284 3.2237 0.6116383 27.23816 3.2254 0.49808

15 July 1989 / Vol. 28, No. 14 / APPLIED OPTICS 2733

*'C| r24CLU R

LU ~~~~~~~~~83C'C

21 35 49 63 77 91TWO-THETA (DEGREES)

Fig. 6. X-ray diffraction spectra for a standard bronze V0 2 thinfilm at 24 and 830 C.

830 C. Characteristics for the dominant peak areshown in Table II.

A 21% reduction in linewidth occurs between theroom temperature monoclinic and tetragonal phasesfor the two-step anomalous blue film, whereas only a2.8% linewidth reduction is observed for the bronzematerial. The corresponding percent increases in dspacing, on the other hand, are comparable at 0.80 and0.88%, respectively.

The width of a diffraction line is largely determinedby crystallite size (instrumental broadening is also afactor). In cases where crystallite size is not expectedto change, line broadening can be attributed to disloca-tions and microstrains. The larger reduction inFWHM in the two-step anomalous film on switchingmay indicate this strain relief to be more pronouncedin the anomalous blue than in the bronze film.

The d spacing appropriate to each hkl set can becalculated from' Eqs. (1) and (2) for the low tempera-ture monoclinic and high temperature tetragonalphases, respectively. The use of bulk lattice constantsin these expressions is an approximation, since theunstressed lattice constants peculiar to the film may bedifferent from those of the bulk.

In the case of the dominant line for the two-stepanomalous film of Table I, the d spacing increased onswitching from the monoclinic to tetragonal phasewith a difference of 1.91 X 10-2 A. The expecteddifference as determined from Eqs. (1) and (2), usingbulk lattice constants, is 5.42 X 10-3. The change incrystal structure does not in itself, therefore, appear to

Table 11. Relevant Data for Dominant XRD Peak for Standard Bronze V02Thin Film

COLD 1/COLD 2

65 69 73 77 81 85TWO-THETA (DEGREES)

Fig. 7. XRD spectra for highly oriented one-step V0 2 thin filmcycled from 241C (cold 1) to 84 0C (hot) and back to 24 0C (cold 2).

explain the observed shift in this line. The remainderof the shift may be attributed to a change in expansioncoefficient along the 011 plane as the film passesthrough the phase transition.

Referring to Fig. 1, the 011 plane in the monoclinicphase is comparable with the 110 plane in the tetrago-nal phase. Should these planes be aligned nearly par-allel to the substrate face, expansion coefficients alongthe a (monoclinic) and c (tetragonal) axes would bestenter into a description of macrostress introduced intothe film due to the film-substrate interface.

For the monoclinic phase, the only expansion coeffi-cient available is a = 24.0 X 10- 6/K (). 13 For thetetragonal phase, the a of interest is 4.9 X 10-6 /K(la),' 2 that is, along the 110 plane. Although a directcomparison cannot be made due to lack of informationof a (IIa) for the monoclinic phase, it may be expectedthat some value other than 4.9 X 10-6/K describes a forthat case and that there is a sufficient change in a atthe phase transition to account for the measured shiftin d spacing.

Multiple peak XRD spectra are observed for boththe two-step anomalous blue and for the standardbronze V0 2 thin films; however, the one-step anoma-lous film exhibits strong preferential alignment char-acterized by few diffraction peaks. Figure 7 shows theonly peaks observed for a one-step V0 2thin film cycledfrom room temperature to 83 0C and back to roomtemperature. Scans were run at these end points.The two ambient scans coincide, whereas the 830 Cscan shows a shift to lower 20 and change in the relativeintensities of the x-ray peaks. The d spacing corre-sponding to these peaks is not found for any oxide ofvanadium in the JCPDS files; however, these linescould correspond to the set of planes for V02 withMiller indices 230. The large semiconductor-to-metalresistivity ratio of 2.4 X 104, as well as the abrupttemperature-induced changes in these diffractionpeaks, argues for their association with V0 2. The

2734 APPLIED OPTICS / Vol. 28, No. 14 / 15 July 1989

Temperature 20 d FWHM(0C) (deg) (A) (deg)

24 27.56165 3.1888 0.7192283 27.31225 3.2169 0.69878

absence of these peaks from the JCPDS files maysimply indicate incompleteness of powder XRD mea-surements at high 20.

In an attempt to locate other diffracting planeswhich might be sensitive to sample orientation about ahorizontal axis through its center, a spinner for thefilms was installed. The sample was rapidly rotated ateach detector position. No other lines were found forthe one-step anomalous film, and it appears that it isindeed preferentially oriented.

IV. Summary

Multiple-peak x-ray diffraction spectra were ob-tained for both bronze and two-step anomalous blueV02 thin films, whereas the one-step anomalous mate-rial exhibits preferential orientation and a disappear-ance of the dominant x-ray peak near 20 27°. Thedecrease in d spacing for the 011 planes (approximate-ly parallel to the substrate face) as the samples arecooled is attributed to increasing tensile stress lockedin by cooling the film-substrate combination fromhigh deposition temperature. The large change in d atthe phase transition may be explained by both struc-tural and thermal expansion coefficient differencesbetween the two states. The increase in linewidthfrom the tetragonal (film-deposited state) to themonoclinic is attributed to increased microstrainbrought about by the loss of symmetry incurred by theswitch to the distorted structure.

References

1. F. J. Morin, "Oxides which Show Metal-to-Insulator Transi-tions at the Neel Temperature," Phys. Rev. Lett. 3, 34-36(1959).

2. J. B. Goodenough, "The Two Components of the Crystallo-graphic Transition in V0 2," J. Solid State Chem. 3, 490-500(1971).

3. F. C. Case, "Modifications in the Phase Transition Properties ofPre-Deposited V02 Films," J. Vac. Sci. Technol. A 2, 1509-1512(1984).

4. C. H. Griffiths and H. K. Eastwood, "Influence of Stoichiometryon the Metal-Semiconductor Transition in Vanadium Di-oxide," J. Appl. Phys. 45, 2201-2206 (1974).

5. H. P. Klug and L. E. Alexander, X-Ray Diffraction Proceduresfor Polycrystalline and Amorphous Materials (Wiley, NewYork, 1974).

6. H. W. Verleur, A. S. Barker, Jr., and C. N. Berglund, "OpticalProperties of V0 2 Between 0.25 and 5 eV," Phys. Rev. 172,788-798 (1968).

7. G. A. Nyberg and R. A. Buhrman, "Preparation and OpticalProperties of Reactivity Evaporated V02 Thin Films," J. Vac.Sci. Technol. A 2, 301-302 (1984).

8. F. C. Case, "Reactive Evaporation of Anomalous Blue V0 2 ,"Appl. Opt. 26, 1550-1553 (1987).

9. R. Feder and B. S. Berry, "Seeman-Bohlin X-Ray Diffracto-meter for Thin Films," J. Appl. Cryst. 3, 372-379 (1970).

10. A. Segmuller and M. Murakami, Thin Films from Free Atomsand Particles (Academic, New York, 1985), Chap. 8.

11. JCPDS International Centre for Diffraction Data, 1601 ParkLane, Swarthmore, PA 19081.

12. M. Guntersdorfer, "Conductivity Anomaly in Vanadium Di-oxide," Solid State Electron. 13, 355-367 (1970).

13. Y. Hazony and H. K. Perkins, "Electronic Structure and Anom-alous Thermal Expansion in FeF2 and V0 2 ," J. Appl. Phys. 41,5130-5131 (1970).

The author would like to acknowledge H. E. Ratzlafffor his ingenious technical assistance, W. A. Patey forassistance in film deposition, C. J. Day for opticalcharacterization, and L. C. Coleman for obtainingXRD data of the thin films. Also appreciated are themany helpful discussions and insights offered by mycolleagues F. E. Moore and W. E. Case.

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