Thin Solid Films, 204 (1991) L1-L4 LI

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Optical properties and electron energy-loss diagnostics of vanadium dioxide thin films

MERCY THOMAS* AND ELIZABETH E. CHAIN The University of Texas at Dallas, Erik Jonsson School of Engineering and Computer Science and the Center for Applied Optics, P.O. Box 830688, BE 28, Richardson, TX 75083-0688 (U.S.A.)

(Received November 22 1990; accepted May 22, 1991)

Measurement of the energy loss in a fast electron beam via characteristic plasmon excitation in electron energy loss spectroscopy can be used to probe free carrier density as vanadium dioxide (VO2) is thermally cycled through its phase transition. The energy loss is related to material optical properties via the dielectric function e. Expected energy losses in VO2 have been correlated with the observed optical properties, with a prediction of electron energy loss in transmission through a 1000 A, thin film.

The optical constants of thin film materials can be determined from energy loss measurements of electrons over a wide range of energies via electron energy loss spectroscopy (EELS) 1-3, since this energy loss spectrum is quantitatively described by the dielectric theory. The intensity due to electrons which have suffered an energy loss E is given by the energy loss function, which is proportional to the probability that an electron undergoes an energy loss event:

loss function = I m ( - ! ) e2 energy el 2 ..1_ e22 (1)

where e = el + ie2 is the complex dielectric function. Once I r a ( - l/e) is known from an energy loss measurements, one can obtain Re(I/e) by a Kramers-Kronig transformation 2'3, so that both real and imaginary parts of the dielectric function can be known. In this manner the optical properties of the material can be obtained from an electron energy loss experiment.

In the current work published data on the optical properties of VO2 have been utilized in an analysis of material electronic behavior both above and below the critical temperature Tc = 68°C T M at which the material experiences a semi- conductor-to-metal transition. At temperatures below Tc VO2 displays semi- conductor behavior, and the Lorentz model for bound oscillators is suitable for use in the explanation of electronic behavior. For T > T~, where VO 2 displays metallic behavior, the Drude model for free electron behavior works well to explain the

* Present address: ITT Technical Institute, 15621 Blue Ash Drive, Suite 160, Houston TX 77090-5818, U.S.A.

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material electronic behavior. The electron energy loss function for a metal or insulator is written in terms of the plasma energy Ep and relaxation time v:

{ 1 t Ep2Eh/-c Im - ~ :~ = (E 2 _ Ep2) 2 q - ( E h / r ) 2 (21

The energy loss function, and thus the optical constants, can be determined from the energy loss spectrum.

The dielectric function e has been determined for VO 2 between 0.1 and 12 eV v's both below and above the semiconductor metal transition temperature by reflectance and transmit tance studies. On the basis of the Drude free electron theory, used to describe VO2 behavior for T > T~, the real and imaginary parts ofe can be utilized to obtain the relaxation time r :

(1 -- ~:l)h r - {3)

Ee2

The best fit of this data provides r as 0.72 × 10 15 s, small compared with that for other metals (for which r ~ 10 1,, s typically). The Drude best fit to the optical data g i v e s Ep = 1.6 eV. For T > T c the Lorentz theory was used to determine the relaxation time r as 1.1 × 10 15 s from the optical properties. The maximum value of e2 occurs at E = E o, with

ez{max ) - EP~r (4) hEo

Since Eo corresponds approximately to the band gap energy, 0.6eV 4, the corresponding semiconduct ing state plasma energy is 1.2 eV, The values determined here for Ep a r e in agreement with results obtained by other researchers ;'~°.

The free carrier density N is related to the plasma frequency (% = Ep/h:

~)p = - - (5)

F r o m optical and electronic measurements, the electron effective mass m in both the VO 2 semiconduct ing and metallic states has been determined to be nearly equal to the free electron mass mog; therefore m ~ m 0 was used in the determination of the effective free electron density N for VO2. The free carrier density, calculated on this basis for the Drude model, is Nm~,ajlicph,s e = 5.5 × 102~ electrons cm 3 which is in agreement with other results 9'1° when the VO2 molecular density (about 3.4 x 1022 molecules cm - 3) is taken into account. VO2 in the metallic phase makes a relatively poor metal since it has an effective free carrier density of only 0.3 electrons per vanadium atom, low compared with a good metal, for which about 1 electron per a tom is expected. The electron density determined from the Lorentz model is Nsemiconduc t ing phase = 9.5 x 1020 electrons cm 3.

The electron energy loss diagnostics determined for VO 2 is summarized in Table I. A compar ison of the Drude and Lorentz predictions of the carrier density N for VO2 in its metallic and semiconduct ing states shows that Nmetamc ph,~ > Nsemiconducting ph . . . . as expected. Below the transition temperature, the effective electron density is small because the number of electrons that participate in

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TABLE I ELECTRON ENERGY LOSS DIAGNOSTICS FOR V O 2 , ABOVE AND BELOW THE METALS-SEMICONDUCTOR

TRANSITION TEMPERATURE T¢

Plasma energy Ep Free carrier density N (eV) (x 102~ electrons cm -3)

T > T~ 1.6 5.5 T < T, 1.2 0.95

conduction is low in this semiconducting phase. Above the transition temperature, where VO2 displays metalic behavior, the IR spectra show an increase in reflectance because of the increase in the carrier density. It is reasonable that z is smaller in the metallic phase than in the semiconducting phase of VO2, because a higher free carrier density should result in less time, on average, between collisions.

The energy loss function I m ( - l/e), determined from the optical properties for electron transmission through a 1000/~ film of VO2 in its semiconducting and metallic states, is plotted in Fig. 1, and shows a discrete peak at the plasma

0.45

k~

0.4

0.35

0.3

0.25

0.2

0.15

(1.1

0,05

, /

,, . - ,

, ' ,,,'-

• , .,"

., . , .- . - .,..

0 I S - / 4 ~ i'0 12

E n e r g y , in e V

Fig. 1. Energy loss vs. energy loss function fora 1000~thickcoatingofVO2aboveandbelowthemetal - semiconductor transition temperature T~: - --, metallic state; - - - , semiconducting state.

frequency. The energy loss spectrum for the semiconducting phase has a peak near 1.2 eV, in agreement with the Lorentz model prediction, while the loss spectrum for the metallic phase has a strong peak near 1.6 eV, in agreement with the Drude model prediction. Comparison of these spectra demonstrates that metallic VO2 has a fairly sharp plasmon line compared with semiconducting VO 2.

In this study electronic properties were deduced from known optical properties. The reverse analysis can also be accomplished; that is, the VO2 optical properties can be determined from measured electron energy loss. In future, experiments will use the direct measurement of the VO2 energy loss parameters for T < X c and T > Tc to compare results with measured optical properties 12. Analysis of the

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energy loss measurements additionally provides the relative electron density as a function of material temperature.

In conclusion, it has been shown that electron energy loss diagnostics can successfully describe observed differences in the electronic behavior of VO 2 above and below the transition temperature. The Lorentz model was applied to the semiconducting phase of VO2 and provided a successful description of the frequency-dependent dielectric properties as they are observed in EELS. Similarly, the Drude free electron model was used to describe the electronic properties of the metallic phase of VO2. In this study VO2 optical properties were used to predict an electron response, which has been shown to be appreciable and measurable, and to be significantly different for the two VO2 phases. In a more general sense it can be concluded that the electron beam energy loss measurement technique can form the basis for the analysis of the optical properties of VO 2 and other phase transition materials.

The authors wish to thank J. S. Chivian for suggesting electron energy loss analysis for VO2. There have also been profitable discussions about this work with D. G. Crowe, P. E. Batson, D. M. Byrne and D. L MacFarlane. This work was supported by the Erik Jonsson School of Engineering and Computer Science, under the direction of B. E. Cherrington.

l F. Bassani, lnterband transitions and optical properties. In E. D. Haidemenakis, (ed.}, Optical Properties of Solids', Gordon and Breach, London, 1970, p. I.

2 D . W . Johnson, Optical properties determined from electron energy loss distributions. In J. V. Sanders and D. J. Goodchild (eds.), Electron Microscopy, 8th Int. Congr., Australian Academy of Science, Canberra, 1974, p. 388.

3 J. Daniels, C. ,. Festenberg, H. Raether and K. Zcppenfeld, Optical constants of solids by electron spectroscopy. In Springer Tracts in Modern Physics, Vol. 54, Springer, Berlin, 1970, p. 78.

4 E.E. Chain, The optical properties of vanadium dioxide and vanadium pentoxide thin films, Appl. Opt., ( 1991 ), to be published.

5 K. Chhotani , E. E. Chain and C. L. Hawkins, Properties of vanadium dioxide films prepared by d.c. magnetron sputtering, in preparation.

6 J .S . Chivian, M. W. Scott, W. E Case and N. J. Krasutsky, An improved scan laser with VO 2 programmable mirror, IEEE J. Quantum Eh, ctron., 21 (1985) 383.

7 B.S. Borisov, S. T. Koretskaya, V. G. Mokerov. A. V. Rakov and S. G. Solovev, Electrical and optical properties of VO2 near the semiconductor semimetal transition point, Sot. Phys. Solid State, 12(1971) 1763.

8 G . F . Derbenwick, Tech. Rep,, 5220-2, Stanford Electronics Laboratories, 1969. 9 C . N . Berglund and H. J. Guggenheim, Electronic poroperties of VO2 near the semiconductor

metal transition, Phys. Rev., 185 (1969) 1022. 10 A. Bianconi, S. Stizza and R. Bernardini, Critical behavior of the plasmon resonance at the metal

insulator transition in VO 2, Phys. Rev. B, 24 (1981) 4406. 11 M. Thomas and E. E. Chain, Electron energy-loss diagnostics of phase transition thin films, in

preparation. 12 U. Mandava, E. E. Chain and J. S. Chivian, Electron energy loss spectroscopy analysis of optical

and electronic material, in preparation.