Institute of Solid State Physics, University of Latvia

CONFOCAL SPECTROMICROSCOPY OF AMORPHOUS AND NANOCRYSTALLINE TUNGSTEN OXIDE FILMS

A. Kuzmin, R. Kalendarev, A. Kursitis, J. Purans

E-mail: [email protected]

NCM-10, Praha (Czech Republic), September 18-22, 2006 .

Institute of Solid State Physics, University of Latvia

State-of-the-Art

•

“Blu-ray” disks, having capacity of more than 25 GB per recording layer, use a 405 nm laser, focussed through a high numerical aperture (NA=0.8-1.0) objective lens to a spot size of about 300 nm.

•

The most frequently used rewritable phase change recording materials, belong to the group of semiconductor chalcogenides .

For example: ternary GeSbTe and quaternary AgInSbTe alloys.

•

Other materials tungsten oxides : - a reversible photoredox reaction under two-wavelength laser excitation of tungsten oxide in air J.M. Osman, R.J. Bussjager, F. Nash, J. Chaiken, R.M. Villarica, Appl. Phys. A 66 (1998) 223. - heat treatment of WO 3 /metal thin-film bilayered structures Y. Takeda, N. Kato, T. Fukano, A. Takeichi, T. Motohiro, S. Kawai, J. Appl. Phys. 96 (2004) 2417.

- rewritable electrically selective multi-layered optical recording disk, based on the electrochromic behaviour of WO 3 R. Sato, N. Ishii, N. Kawamura, H. Tokumaru, in: Proc. 3rd European Symp. on Phase Change and Ovonic Sci., Balzers, Liechtenstein, September 04-07, 2004. - write-once optical recording was demonstrated in WO 2 film T. Aoki, T. Matsushita, A. Suzuki, K. Tanabe, M. Okuda, Thin Solid Films 509 (2006) 107.

Institute of Solid State Physics, University of Latvia

Present work goal

•

To demonstrate the possible use of WO 3 & AWO 4 thin films for write-once phase change optical recording.

•

To propose the multilayer AWO 4 phase-change media structure based on Raman scattering detection of the highest frequency stretching W-O mode.

Institute of Solid State Physics, University of Latvia 3D scanning confocal microscope with spectrometer

"Nanofinder-S"

produced by SOLAR TII, Ltd.

Simultaneous / Multifunctional Analysis:

• Optical and Confocal Microscopy • Raman Spectroscopy • Luminescence Spectroscopy • 0D, 1D, 2D & 3D High-speed Imaging and Spectroscopy

Institute of Solid State Physics, University of Latvia

"Nanofinder-S" modular optical layout

He-Cd 441.6 nm 50 mW

Institute of Solid State Physics, University of Latvia

Commercial Compact Disk Imaging in Confocal Mode

CD-ROM CD-R CD-RW Track pitch = 1.6 µm Track pitch = 1.6 µm Images size: 20



24 µm Track pitch = 1.6 µm

Institute of Solid State Physics, University of Latvia

Thin Film Preparation by DC Magnetron co-Sputtering

SUBSTRATE N S S N Sputter Gas Metallic targets: W (99.95%) Ni (99.0%) Zn (99.9%) Substrates: Si, glass Plasma Glow Metallic Target / Cathode Magnets Sputter gas: Ar (80%) + O 2 (20%) Total gas pressure: 6.7 Pa Discharge power: 100 W

Institute of Solid State Physics, University of Latvia

Crystalline Structure of WO

3 W O WO 3 : [WO 6 ] Well known electrochromic material based on valence change of tungsten ions: W 6+ (transparent)



W 5+ (blue)

Institute of Solid State Physics, University of Latvia

Crystalline Structure of AWO

4 (A = Ni, Zn ) AWO 4 : [AO 6 ] & [WO 6 ] Tungstates are known as scintillators and Raman shifters.

A W O

Institute of Solid State Physics, University of Latvia a-W 6+ O 3

Optical Recording in t.f.-WO

3 W (6-y)+ 15 mW Si c-W 6+ 25 mW O-W-O stretching modes O 3 50 mW Si (c) (b) (a) O-W-O & W=O stretching modes 4 00 600 8 00 Raman shift (cm -1 ) 10 00 1 20 0 * A. Kuzmin, J. Purans, E. Cazzanelli, C. Vinegoni, G. Mariotto, J. Appl. Phys. 84 (1998) 5515.

Institute of Solid State Physics, University of Latvia

Optical Recording in t.f.-NiWO

4 15 mW 25 mW 50 mW Si O-W-O stretching modes 700 °C 40 0 Si 6 00 80 0 Raman shift (cm -1 ) 10 00 (d) (c) (b) (a) 1 200 * A. Kuzmin, J. Purans, R. Kalendarev, D. Pailharey, Y. Mathey, Electrochim. Acta 46 (2001) 2233.

Institute of Solid State Physics, University of Latvia

Optical Recording in t.f.-ZnWO

4

Si

O-W-O stretching modes

ZnWO

4 700 °C W-O-W bending modes 400

Si

50 mW 600 800

Raman shift (cm

-1

)

1000 1200

Institute of Solid State Physics, University of Latvia Possible Mechanisms of Optical Recording in Tungsten Oxides W 6+ O 3-x & AW 6+ O 4-x (A = Ni, Zn) Formation of metastable color centers W 6+



W (6-y)+

• short term life time in air

Change in reflectivity

(10-20%)

Crystallization to WO 3 / AWO 4

• long term life time • good thermal stability

Change in phase & reflectivity Multilayer phase-change media structure based on Raman scattering detection

Institute of Solid State Physics, University of Latvia Multilayer write-once phase-change media structure based on Raman scattering detection 1,..., n 1 Raman signal Dielectric layer Phase-change A 1 WO 4-x layer Dielectric layer ...

Phase-change A n WO 4-x layer Dielectric layer Metallic mirror layer Substrate Sequential Writing AWO 4 band gap ~ 3.0-3.8 eV Dielectric layer Phase-change A 1 WO 4-x layer Dielectric layer ...

Phase-change A n WO 4-x layer Dielectric layer Metallic mirror layer Substrate Parallel Reading

Institute of Solid State Physics, University of Latvia

Thank you !

For more information look at the Internet: http://www.cfi.lu.lv/exafs

This work was supported by the Latvian Government Research Grants and National Research Program in Materials Science.