Low temperature deposition of VO2 thin filmsFrancine C. Case Citation: Journal of Vacuum Science & Technology A 8, 1395 (1990); doi: 10.1116/1.576888 View online: http://dx.doi.org/10.1116/1.576888 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/8/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Tuning the properties of VO2 thin films through growth temperature for infrared and terahertz modulationapplications J. Appl. Phys. 114, 113509 (2013); 10.1063/1.4821846 Pulsed laser-deposited VO2 thin films on Pt layers J. Appl. Phys. 113, 123503 (2013); 10.1063/1.4795813 Low temperature deposition of Ga2O3 thin films using trimethylgallium and oxygen plasma J. Vac. Sci. Technol. A 31, 01A110 (2013); 10.1116/1.4758782 Pulsed laser deposition of VO2 thin films Appl. Phys. Lett. 65, 3188 (1994); 10.1063/1.112476 Ionassisted laser deposition of CaF2 thin films at low temperatures J. Vac. Sci. Technol. A 7, 2983 (1989); 10.1116/1.576303

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low temperature deposition of V02 thin films Francine C. Case LTV Aerospace and Defense Company, Missiles and Electronics Division, Dallas, Texas 75265-0003

(Received 30 August 1989; accepted 6 November 1989)

V02 thin films have been deposited by a variety of techniques such as thermal oxidation, reactive evaporation, and magnetron or ion beam sputtering. These methods routinely require deposition temperatures in excess of 400"C in order to promote crystallization of the V02 phase. We report on a technique, employing low energy ion bombardment of reactively evaporated VOz, which promotes crystallization of the VOl phase at temperatures as low as 300°C. Properties of films deposited at these unusually low substrate temperatures will be discussed.

I. INTRODUCTION

Vanadium dioxide undergoes a semiconductor-to-mctal phase transition from a monoclinic to tetragonal structure at 67 DC, accompanied by large changes in electrical and in­frared optical properties. \-3 The successful exploitation of V02 properties in thin film devices is dependent, however, on its compatibility with other materials. With regard to deposition conditions, VOl thin films have been formed by a variety of techniques which require substrate temperatures in the 400-500·C range. For example, the thermal oxidation of a vanadium metal film into V02 occurs efficiently at tem­peratures above 480°C.' Deposition of VOl films at 400 DC has been reported for de planar magnetron reactive sputter­ing.s

A lower V02 deposition temperature is desirable for a number of reasons. First, diffusion and reactivity between thin film layers is significantly enhanced at high tempera­tures. We cite, for example, the considerable mixing that occurs across a vanadium-silicon interface even at 350°C." Some useful infrared materials, such as CdTe and ZnSe, have low softening temperatures, and V02 films deposited on these materials above the softening point deform with deleterious effects on film morphology. Finally, extrinsic stress in film layers (due to differential thermal expansion) is higher at elevated deposition temperatures, oftcn resulting in poor adhesion. For example, both reactively evaporated and thermally oxidized VOL thin films deposited near 500 ·C onto ZnS or ZnSe are known to delaminate.

Unfortunately, as the deposition temperature is lowered, crystallite size decreases and the vanadium-oxygen system becomes less favorably reactive to the formation ofV02, re­sulting in a degradation of phase transition properties. Whether or not a lack of substrate crystallinity will impede growth of switchab!e crystalline material at low temperature is also in question.

In ion assisted deposition energetic ions impart energy to the depositing thin film, thereby increasing atom mobility and reactivity and promoting crystallization.7

-9 Low energy

ions have been used for controlled growth of Ni-oxide-Ni and Cr-oxide-Ni tunneling junctions, \0 and stable thin films of Zr02 have been deposited at room temperature and 300 ·C by ion assisted electron beam deposition. Ion bom­bardment not only increases the packing density of these Zr02 films, but also promotes an amorphous to crystalline

transition of the film structure. l\ In light of the advantages outlined for low temperature VOz and the attributes of ion assistance, we report on the successful reduction ofV02 de­position temperature to 300°C by the incorporation of si­multaneous low energy ion bombardment into the V02 reac­tive evaporation process.

Reactively evaporated films, deposited at 300 °C with and without ion bombardment, are compared, and effects of low temperature on the adhesion ofV02 to some generally trou­blesome substrates are discussed. In order to determine how critical substrate crystallinity is to the formation of switch­able VOz at these low temperatures, data for a deposition on fused quartz is also included.

It EXPERIMENTAL

Vanadium dioxide films were deposited by reactively evaporating vanadium metal in an oxygen background in an Edwards electron-beam coater equipped with an MKS pres­sure/flow controller, a Commonwealth Kaufman-type 3-cm ion gun with dual Vacuum General mass flow controllers, and a movable Faraday cup. The ion gun axis was 19 a rela­tive to the substrate normal. Ion flux was recorded routinely both before and after vanadium oxide deposition. In order to minimize potential effects on V02 phase transition proper­ties by tungsten poisoning of the films 12 a plasma bridge neutralizer and tharia-coated iridium were substituted for tungsten neutralizer and cathode filaments, respectively. The sample was mounted opposite the electron beam source and in contact with a stainless-steel heating block.

Substrate cleaning consists of a Buehler Micropolish scrub, followed by a rinse and 60-s sonnication in deionized water. The substrate is then rinsed in isopropyl alcohol, fol­lowed by a IS-min soak at 82.4 ac in an isopropyl alcohol degreaser.

Films were analyzed by electrical, optical, scanning elec­tron microscopy (SEM), and x-ray diffraction (XRD) techniques. Infrared spectral measurements were made on a Perkin-Elmer model 580 infrared spectrophotometer, and the temperature dependence of optical transmission at 3.4 j-l was obtained by temperature cycling the sample. The sample was heated radially from the edge using a ring afthermoelec­tric heaters coated with thermal grease and connected to an annular aluminum backing plate against which the sample was held. Temperature cycling was done at 2.0 ·C/min.

1395 J. Vac. Sci. Technol. A 8 (3), May/Jun 1990 0734-2101190/031395-04$01.00 @ 1990 American Vacuum Society 1395

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1396 Francine C. Case: low temperature deposition of V02 thin films 1396

Sample structure and composition were examined using a lEOL scanning electron microscope and Siemens-Allis x­ray diffractometer equipped with a Huber thin-film attach­ment. Film thickness was measured with a Tencor Alpha Step profilometer.

Three 1200 A films were reactively evaporated onto single crystal sapphire substrates at 300°C. One of the films was simultaneously bombarded with 200 eV oxygen ions during deposition. In this case, the flux to the Faraday cup was preset at 0.1 mA/cm2 with the only oxygen source being 5 sccm flowing through the ion gun itself. Background oxygen was then admitted into the system and controlled at 2.2 mT for the deposition, dropping the observed ion flux to 0.06 mA/cm2

• The second and third films were deposited at 2.2 and 3.0 mT, respectively, but with no ion assistance. A series of films were then deposited onto ZnSe, CdTe, and fused quartz under nominally the same conditions as the ion-as­sisted film.

III. RESULTS AND DISCUSSION

All films were deposited at a rate of 1 Als as measured by a quartz crystal thickness monitor located out of the ion beam path. Deposition time for bombarded films was in­creased to offset ion beam etching of the depositing film and to provide film thicknesses comparable with nonbombarded samples. The two films deposited without ion assistance at 300°C onto single crystal sapphire differ optically and elec­trically when temperature cycled. Figures 1 (a) and (b) show transmission vs temperature at 3.4 !t for these films which were reactively evaporated at oxygen pressures of 2.2 and 3.0 mT, respectively. No phase transition is observed for either film, and the resistivity changes from 3.34 X 1O~ 1

(ambient) to 7.4 X 10 3 n cm (80°C) for the first sample and from 2.3 X 10 C (ambient) to 6.39 X 10- 1 n cm (80 DC)

i

~ i

40 60 80

TEMPERATURE ("C)

FIG. 1. Temperaturedcpendence of transmission for YO, films deposited at 300"C: (a) and (b) without ion assistance, (c) with ion assistance.

J. Vac. Sci. Techno!. A, Vol. 8, No.3, May/Jun 1990

for the second. The ion-assisted film, on the other hand, as seen from Fig. 1 ( c) does go through a phase tranl'>ition at a temperature of 57 DC, ten degrees below that typically ob­served for VOl' Its resistivity change is more than three or­ders of magnitude from 2.26 X 10 0 (ambient) to 4.7 X 10-- 4

n cm (80T). The low transition temperature may be partially attribut­

ed to a reduction in extrinsic stress, which is proportional to flail Twhere Ila is the difference in thermal expansion coef­ficients between the film and substrate and I:lT is the now reduced difference between room and deposition tempera­ture. A correlation between extrinsic stress and VOz transi­tion temperature has been reported. \3.14 Ion bombardment can affect transition temperature by several means. Tung­sten doping of V02 thin films has been observed to reduce the phase transition temperature, and doping can occur to some degree by incorporation of tungsten into the film via bombardment of the cathode or neutralizer filaments by en­ergetic ions. ! 2, i 5- 17 In this work, however, tungsten has been eliminated from the ion gun, and the reduction in transition temperature, therefore, is likely due to a second important factor, i.e., changes in intrinsic stress brought about by ions impinging on the depositing film surface.

Evidence of the uniform crystallization promoted by ion bombardment can be seen in Fig. 2, which is an x-ray diffrac­tion comparison of the bombarded and nonassisted films. It must be emphasized that x-ray data cannot be the sole deter­miner of the phase or phases present in the sample. Because of overlap in the many peaks of the possible oxidation states of vanadium, x-ray spectra alone are generally insufficient to determine absolutely the presence of vanadium dioxide; however, these data, in conjunction with electrical and/or optical measurements can serve to clarify whether V02

dominates the film composition. The film deposited without ion assistance at 2.2 mToH oxygen pressure, exhibits the x­ray diffraction pattern in Fig. 2(a) consisting of one peak which may be attributable to the presence of partially crys­tallized VOl' When deposition background pressure is in­creased to 3.0 mTorr, all crystallinity disappears, and the resulting amorphous film shows no diffraction signature as

(il)

K£LATl'IE INTEN51 rv (b)

Ie)

21 41 61 81 101 121

rwo-TH[ r A (OeGREeS)

FIG. 2. X·ray diffraction comparison of YO, thin films reactively evaporat­ed at 300 'Co (a) and (h) 2.2 and 3.0 mT oxygen pressure without ion assistance. (c) 2.2 mT oxygen pressure with ion assistance.

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1397 Francine C. Case: Low temperature deposition of V02 thin films 1397

in Fig. 2(b), The incorporation of ion assistance in conjunc­tion with 2.2 mTorr oxygen pressure results in a film of pre­dominantly VOl phase as in Fig. 2(c) with a dominant peak near 2(J~ 70.8. An SEM comparison indicates the presence of structure, in the form of small grains on thc order of 0.1 fl in diameter on the ion bombarded sample, but somewhat smaner structures on the other two films. Figure 3(a) is a plot of resistivity as a function of temperature for the ion­bombarded film. The increase of the resistivity following switching at 57 PC indicates true metallic behavior as the sample is heated above the phase transition. Figure 4 is a spectral plot of transmission at 24 and 80°C for the ion­assisted film deposited at 300 "c and for one deposited at 500°C on Al20 3 without ion assistance. Spectral data for these two samples compare favorably. Note that the drop in room temperature transmission above 4.5 f1 is not a property of the V02 film, but is due rather, to the onset of absorption in the Al]Ol substrate.

Historically, delamination has been a problem for V02

thin films deposited directly onto zinc selenide substrates at typical processing temperatures. We report that successful crystallization and adhesion of switchable VOl to zinc se1en­ide is possible when films are deposited under ion bombard­ment at 300 "Co Figure 5 shows the temperature dependence of transmission for such a film at 3.4/1. Also included are

1 j

1 ( ,j

Temperature ('C)

FIG. 3. Temperature dependence of resistivity for ion-assisted V02 films deposited at 300 'C on (a) AI20,. (b) ZnSe, and (c) fused quartz,

J. Vac. Sci. Technol. A, Vol. 8, No.3, May/Jun 1990

..•.• -••. ' •.••• '," ... ; •.• .-;-.••••••• -.; •••• -. ••• ' ....... :0;.; ••••••••••••.•.•.••••• , •.

eo

>-

~ ~ t;S

"- 40 \ T3CO (80"C)

'0 I "r-I \ r T500 (eo°c)

r I \ o r"l"·"'·'" ::,·;,,~J:t'i·"''''''·'''·,:·'''·-:'':~·,,:,·r·:-:·":':::-·,:,>'''''''' .""1' ... ., "''''1'''',"'' I

2.5 3.5 4.5 5.5 6.5

WAVELENGTH (MICRONS)

FIG. 4. A comparison of spectral transmission at 24 and 80"C for V02 thin films on A120, deposited at 300·C with ion assistance and 500 ·C witlJOut ion assistance.

data for material deposited onto CdTe and fused quartz at this same low temperature. Note the reduced transition tem­perature near 53 ·C for V02 on these materials. Figures 3 (b) and 3 (c) show the temperature dependence of resistivity for the films on ZnSe and fused quartz, respectively. Neither of these films behave as good metals at high temperatures. This could be due to the presence of some material which is either amorphous or "off stoichiometry." The presence of multiple peaks in the x .. ray diffraction data indicates that films depos­ited at 300 °C onto ZnSe, fused quartz, and CdTe are less preferentially oriented than those on sapphire.

100,..----,-__ -y_-,-_-,-_,

80

50

20 40 TEI~PERATURE (C,)C

60

FiG. 5. Temperarure dependence of transmission (..i = 3.4fi) forVO, films deposited at 3OO'C by ion assistance on ZnSe ( _.0 ). fused quartz ( -) and CdTc substrates ( , .. ).

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1398 Francine C. Case: Low temperature deposition of V02 thin films 1398

IV. SUMMARY

Switchable V02 thin films have been successfully deposit­ed at 300 "C, the lowest temperature reported thus far, onto sapphire, cadmium telluride, zinc selenide, and fused quartz substrates by means of ion-assisted reactive evaporation, thereby enhancing this material's potential use with other optical components in multilayer thin film structures. The propensity for delamination of V02 films on zinc selenide has been eliminated at this deposition temperature.

ACKNOWLEDGMENTS

The author would like to acknowledge H. E. Ratzlaff for his many technical contributions to this work, W. A. Patey for film deposition, and T. A. Shafer for x-ray diffraction data.

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J. Vac. Sci. Technol. A, Vol. 8, No.3, May/Jun 1990

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12F. C. Case. J. Vac. Sci. Techno!. A 6, 2010 (1988). "K. D. Ufert, Phys. Status Solidi A 34, K83 (1976). 14F. C. Case, J. Vac. Sci. Techno!. A 2,1509 (1984). [sT. Harlin, T. Niklcwski, and M. Nygren, Mater. Res. Bull. 7, 1515

(1972). 16M. Nygren and M. Israe1sson, Mater. Res. Bull. 4, 881 (1961). 17 J. M. Reyes, G. F. Lynch, M. Sayer, S. L. McBride, and T. S. Hutchison,

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