Optical Characterization of VO2 Smart Materialsusing Spectroscopic Ellipsometry

Céline Eypert, Mélanie Gaillet, Application Scientists HORIBA Scientific

The development of new types of ultra-fast switches operating in the RF-microwave and optical domainsis based on the use of a class of materials undergoing fast, reversible phase transitions from a semicon-ducting state to a metallic one (Semiconductor Metal Transition / SMT). An example of such a materialis vanadium dioxide (VO2).

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VO2 exhibits very fast SMT transition that can be triggeredby different external excitations such as temperaturechange, optical excitation or charge injection. During thephase transition the electrical resistivity of the VO2 thinfilm can decrease by several orders of magnitude. Opti-cally the material is transparent in the semiconductorstate and highly reflective in the metallic state for a largespectral range (from 1mm up to THz frequencies) (fig1).These remarkable properties can be exploited for opticalmicro mirrors, modulators, switches and microwavewaveguides (see Ref. 1) to overcome the current problemsencountered in communication systems (consumption,power handling, slow switching and integration).This note describes how to characterize the optical prop-erties of VO2 as it changes phase, with the transition in-duced by a temperature change.

Experimental

A HORIBA Scientific UVISEL Spectroscopic Phase Modu-lated Ellipsometer with temperature cell has been used tocharacterize the optical properties VO2 deposited on asapphire substrate (fig.2&3). The change in the opticalconstants (n,k) at various temperatures is characteristic ofthe SMT exhibited by VO2.

Ellipsometric measurements were performed at a range oftemperatures from ambient to 90°C using an angle of in-cidence of 70° and spectral range 190-2100 nm.

Ellipsometric Modelling and Results

The model used to describe the sample is represented be-low.It shows a thin roughness layer (58 Å) at the surface of theVO2 film that which improves the goodness of fit 2 pa-

Fig.1: VO2 SMT transition: electrical (left) and optical (right) properties

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50 nm thick VO2 film on Al2O3 (R)

Wavelength (nm)Temperature (°C)

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Fig.2: UVISEL with temperature cell

Fig.3: Schematic - positioning the temperature cell

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rameter (from 2=1.3 to 2=0.2). As the sapphire is atransparent substrate the model includes automatic back-side correction. It also takes into account the anisotropyof the sapphire.

The optical constants of the VO2 were determined usingthe Tauc Lorentz dispersion formula with multiple oscilla-tors.

The two graphs below show how the optical constantschange as a function of temperature from ambient to90°C. One can see that the refractive index decreases and theextinction coefficient strongly increases in the near infrared region. This behaviour is typical for metals.

Conclusion

A HORIBA Scientific spectroscopic ellipsometer equippedwith a heating cell allows exploration of material proper-ties as a function of temperature. It is well suited for thecharacterization of material transitions including thermaltransition of polymers, semiconductor metal transition,band structure for compound alloys, thermal hysteresis ofmagnetic materials, etc…

Acknowledgements

HORIBA Scientific, Thin Film Division thanks CorinneChampeaux and co-workers from SPCTS and XLIM re-search institutes at the University of Limoges for allowingus to publish this application note on VO2 optical charac-terization by spectroscopic ellipsometry.

Ref.1: F. DUMAS-BOUCHIAT, C. CHAMPEAUX, A. CATHERI-NOT, J. GIVERNAUD, A. CRUNTEANU, P. BLONDY, RF Micro-wave Switches Based On Reversible Metal-Semiconductor Transition Properties Of VO2 Thin Films:An Attractive Way To Realise Simple RF MicroelectronicDevices, Materials and Devices for Smart Systems III, Mat. Res.Soc. Symp. Proceedings Volume 1129, P. 275-286, 2009.

(n,k) = f(Ê) - VO2_50

Wavelength (nm)2 0001 8001 6001 4001 2001 000800600400200

n

3.400

3.300

3.200

3.100

3.000

2.900

2.800

2.700

2.600

2.500

2.400

2.300

2.200

2.100

2.000

k

1.600

1.400

1.200

1.000

0.800

0.600

0.400

0.200

0.000