1352 OPTICS LETTERS / Vol. 24, No. 19 / October 1, 1999

High photosensitivity and nanometer-scale phase separationin GeO2–SiO2 glass thin films

Hideo Hosono

Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

Junji Nishii

Osaka National Research Laboratory, Midorigaoka, Ikeda, Osaka Prefecture 563, Japan

Received May 29, 1999

Amorphous xGeO2 �1 2 x�SiO2 thin films exhibit large negative index changes (4–8%) in the high GeO2 region�x . �0.25� on irradiation with ArF laser pulses. The sign of the index change is opposite the low GeO2 regionX , 0.25, and the magnitude of the index change is larger by an order of magnitude than that reported sofar. Cross-sectional transmission electron microscope observation has revealed that nanometer-scale phaseseparation is induced in these highly photosensitive glasses by irradiation with ArF excimer laser light pulsesor electron beams. This is a first f inding of microphase separation in SiO2 GeO2 glasses by irradiation andprovides an essential constraint on the modeling of photonic effects induced by irradiation in these glasses. 1999 Optical Society of America

OCIS codes: 160.2750, 160.5320, 310.3840, 350.3390.

Photosensitivity observed in GeO2 SiO2 glasses hasbeen of interest in recent years because an ultraviolet-induced index change is applicable for optical encodingof gratings in optical fibers.1 – 3 Although there is stilldebate concerning the source of photosensitive effectsin germanosilica glasses, the correlation between ahigh degree of photosensitivity and Ge-associatedoxygen-deficient defects4 is certain.5 Fabricationof highly photosensitive glass thin films is of im-portance for applications to planar gratings6 andwaveguides.7 It was reported by several groups of re-searchers8,9 that the photosensitivity of the thin filmswas much greater than the sensitivity of the bulk.There is a demand for more-photosensitive thin filmsin real-time image processing systems that use liquidcrystals. Here we report that a large negative indexchange, as much as 4–8%, is induced in the thin filmsof high-GeO2-containing glasses by ArF excimer laserirradiation and that nanometer-scale phase separationoccurs in these glass thin films by irradiation withan ArF excimer laser or electron beam. Althoughextensive studies of modification of local structuressuch as oxygen-deficient defects by excimer laser irra-diation have been accumulated, no information aboutthe nanometer-scale texture change in this system hasbeen reported to our best knowledge.

Thin films of xGeO2 �1 2 x�SiO2 glasses �0.05 # x #0.6� were deposited upon SiO2 glass substrates by con-ventional rf sputtering with targets that were obtainedby sintering of mixtures of SiO2 and GeO2 powdersat 1450 ±C in an O2 atmosphere. Sputtering was car-ried out in an Ar O2 mixture (99%:1% by volume; totalpressure, 1.33 Pa), and no intentional heating occurredduring deposition. The thicknesses of the depositedfilms were 200–300 nm. The chemical compositionsof the resultant thin films were determined by x-rayphotoelectron spectroscopy. The amorphous nature of

0146-9592/99/191352-03$15.00/0

the resultant thin films was confirmed by glancing-angle x-ray diffraction. Irradiation with excimer laserlight pulses was performed at ambient atmosphere.The repetition rate and the pulse duration were 10 Hzand �20 ns, respectively. The changes in the refrac-tive index and thickness of the films were measuredwith an ellipsometer at a wavelength of 633 nm with astylus, respectively.

Thin cross sections for cross-sectional TEM observa-tion were prepared by ultramicrotomy10 instead of byconventional ion beam thinning to avoid any artificialmodification by ion beams during the thinning process.An aluminum plate with anodic-oxidized amorphousAl2O3 was used in place of SiO2 glass as the sub-strate.11 Ultramicrotomy can be applied to this sub-strate because Al is soft enough to be cut with adiamond knife. TEM observation was carried out atan acceleration voltage of 200 kV.

Figure 1 shows changes in index and film thicknesson laser irradiation as a function of the GeO2 con-tent �x� in the films. The index change can be clas-sified into two concentration regions, x , �0.25 andx . �0.25. In the low-x region the sign of the indexchange Dn is positive and the magnitude is 0.2–0.5%;in the high-x region the sign of Dn is negative and themagnitude increases to 4–8%. Although the concen-trations of Ge E 0 centers induced by laser irradiationin the high-x and low-x specimens were comparable, aconspicuous expansion of the thickness was observedin the high-x samples. The index decrease estimatedfrom volume expansion by use of the Lorentz–Lorenzequation is comparable with the observed changes.These results reveal that the index changes observedin the specimens with high GeO2 content are controllednot by the color-center formation but by a volume ex-pansion. Figure 2 is a scanning-electron microscopicphoto of 40GeO2:60SiO2 glass thin films made by

1999 Optical Society of America

October 1, 1999 / Vol. 24, No. 19 / OPTICS LETTERS 1353

Fig. 1. Changes in refractive index (filled circles) andthickness (open circles) of the thin films on ArF laserirradiation as a function of GeO2 content. The filmthickness before irradiation was 1–200 nm. Irradiationconditions, �30 �mJ�cm2��pulse 3 1200 pulses at 10 Hz.

Fig. 2. Scanning-electron microscope photo of 0.38GeO20.62SiO2 glass thin films (�200 nm thick) after irradiationwith ArF excimer laser pulses ��30�mJ�cm2��pulse 31200 pulses� through a silica phase mask (period, 1.06 mm).

irradiation with ArF laser pulses through a phasemask. A periodic thickness modulation that ref lectsthe grating period of the mask can be seen.

Figure 3 shows cross-sectional TEM photographsof the high-GeO2 sample before and after ArF laserirradiation. It is evident that the specimen is ho-mogeneous in the as-deposited state. All specimensexamined here were homogeneous. This sampledif-fers from the as-deposited glasses prepared byvapor-phase axial deposition.12 On UV irradiation,nanometer-sized particles that give dark contrastdevelop [Fig. 3(b)]. Irradiation-induced changes ofthis kind were seen only for the high-GeO2 specimen.These features became more distinct for the speci-men irradiated with high-power-density pulses. Thecontrast seen in Fig. 3(b) is due to the heterogeneityof the chemical composition. Although the chemi-cal composition could not be determined by energydispersive x-ray analysis (the particles are too smallto be distinguished from the surrounding matrix),

the dark-contrast region evidently has a considerablyGe-richer composition than the nominal composition.

A more distinct evolution of nanometer-scale hetero-geneity in the specimen with the high GeO2 content isobserved by electron- �e-� beam irradiation. Figure 4shows a series of cross-sectional TEM photos of the as-deposited specimen during continuous irradiation with200-keV e beams under TEM observation. No hetero-geneity in the specimen was perceived until �3 minafter focusing under the present TEM observation con-ditions. However, after that, evolution of phase sepa-ration became distinct for xGeO2 �1 2 x�SiO2 glassthin films �x � 0.3, 0.38, 0.48�. The morphologiesof the separated phases appear to be interconnected.Such morphology is characteristic of the initial stage ofspinodal-type phase separation. When the specimenstarts to become deformed by action of the e beam, thisphase-separated morphology begins to appear. Theseresults indicate that nanometer-scale phase separationis induced in high GeO2-containing thin films by irra-diation of ArF excimer laser pulses or by e beams.

There may be two distinct mechanisms that producethe effects of excimer laser light pulses and focusedelectron beams on these specimens. One is heatingand the other is electronic excitation. An observationthat phase separation occurs simultaneously withsample deformation strongly suggests that a consid-erable rise in temperature is a requirement for phaseseparation. However, no immiscible region has beenreported to exist in the GeO2 SiO2 system as far aswe know. Thus it is implied that a combined effect of

Fig. 3. Cross-sectional TEM photos of the specimen�0.38GeO2 0.62SiO2� (a) before and (b) after ArF laserirradiation �50�mJ�cm2��pulse 3 1200 pulses�.

1354 OPTICS LETTERS / Vol. 24, No. 19 / October 1, 1999

Fig. 4. Cross-sectional TEM photos showing evolutionof nanometer-scale microstructure during electron-beam irradiation under TEM observation. Specimen,0.38GeO2 0.62SiO2. (a) Just after focusing, (b) �3 minafter photo (a) was taken, (c) �10 min after photo (a) wastaken.

heating and electronic excitation plays an importantrole in phase separation.

A plausible model for the large volume expansion inthe high GeO2-containing specimens is the explosiveevaporation of gaseous species such as GeO becauseof reduction of Ge concentration (2–3%) in the speci-mens after laser irradiation. The formation of Ge-richphases such as GeO2 by means of phase separation mayaccelerate the evaporation of GeO gaseous molecules bystrong absorption of ArF laser light. In fact, a similarvolume expansion was observed in the present experi-ment for pure GeO2 glass thin films.

According to de Neufville and Turnbull,13 there is asubliquidus immiscibility in the range GeO GeO1.85.If GeO is an end member of the phase separationinduced by the irradiation, the above model can be

reasonably understood. Further effort is in progressto elucidate in detail the mechanism for the large indexchange and the phase separation induced by ArF laseror e-beam irradiation.

The present finding of nanometer-scale phase sepa-ration in GeO2 SiO2 glasses by irradiation with ArFlaser pulses may inf luence the modeling of photonicproperties in these glasses by excimer laser irradia-tion. Irradiation with high-density laser pulses is re-quired for the formation of type II gratings (radiationdamage)14 and the emergence of a large second-ordernonlinearity by UV poling.15 We believe that laser-induced nanometer-scale phase separation is closely as-sociated with the emergence of these photonic effects.

The authors thank L. Skuja for reading the manu-script and K. Shimidza for his help with the TEMobservation. H. Hosono’s e-mail address is hosonol@rlem.titech.ac.jp.

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