Structural investigation of chalcogenide and chalcohalide glasses using Raman spectroscopy

Journal of Non-Crystalline Solids 248 (1999) 103±114

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Structural investigation of chalcogenide and chalcohalide glasses using Raman spectroscopy V.Q. Nguyen

a,*

, J.S. Sanghera b, J.A. Freitas c, I.D. Aggarwal b, I.K. Lloyd

d

a Virginia Polytechnic Institute, Blacksburg VA 24061, USA Code 5606, Naval Research Laboratory, Washington DC 20375-5000, USA c Code 6874, Naval Research Laboratory, Washington DC 20375-5000, USA Materials and Nuclear Engineering Department, University of Maryland, College Park MD 20742, USA b

d

Received 6 November 1997; received in revised form 22 March 1999

Abstract HV- and HH-polarized Raman spectra of the chalcogenide glasses Ge30 As10 Se…60ÿx† Tex (25 6 x 6 35) and the chalcohalide glasses Ge30 As10 Se30 Te…30ÿy† Iy (0 6 y 6 30), Ge30 As10 Se35 Te…25ÿz† Iz (0 6 z 6 20) were investigated. For the chalcogenide glasses, the main structural units include [AsSe3ÿx Tex ] mixed pyramidal units, [GeSe4ÿx Tex ] mixed tetrahedral units, and Ge±Te±Ge chains. The substitution of iodine for tellurium in the chalcohalide glasses results in the formation of Ge±I, As±I, and Se±I bonds which break up the three-dimensional network. The main structural units for the chalcohalide glasses are [AsSex Tey Iz ] mixed pyramidal units where x ‡ y ‡ z ˆ 3, [GeSex Tey Iz ] mixed tetrahedral units where x ‡ y ‡ z ˆ 4, and GeSe3=2 I mixed tetrahedral units. The symmetry properties of these structural units has been determined through the dependence of the depolarization ratio on the frequency shift. Ó 1999 Elsevier Science B.V. All rights reserved.

1. Introduction Chalcogenide glasses are made from mixtures of the chalcogen elements S, Se, and Te and have received attention due to their ranges of infrared transmission, typically from 1 to 13 lm, depending upon composition [1]. The chalcogenide optical ®bers are currently being used in power delivery of CO (5.4 lm) or CO2 (10.6 lm) laser energy and ®ber optic chemical sensor systems using absorption, evanescent and di€usive re¯ectance spec-

* Corresponding author. Present address: Naval Research Laboratory, Code 5606.1, Washington, DC 20375-5000, USA. Tel.: +1 202 767 9324; fax: +1 202 404 3891; e-mail: [email protected]

troscopy for environmental and Department of Defense (DOD) facility clean up [2,3]. Also, these ®bers are used in infrared countermeasure (IRCM) and laser threat warning systems to increase aircraft survivability [4]. For transmission at longer wavelengths in the infrared, it is desirable to use tellurium containing glasses, due to the heavier atomic mass of tellurium and consequently longer wavelength transmission. The chalcogenide glasses in the Ge±As±Se±Te system have been reported by Inagawa et al. [5] to have a glass forming region in which 660 at.% Te can be introduced into the glasses before crystallization is discernible. These tellurium containing glasses have good chemical durability, stability against crystallization, low thermal expansion coecients and are relatively easy to ®berize. The lowest optical loss reported to

0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 0 3 - 8

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V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

date for a tellurium containing glass ®ber is 0.11 dB/m at 6.6 lm in a Ge30 As10 Se30 Te30 glass [6]. Chalcohalide glasses have been studied for optoelectronic applications [7±10] because of their properties including electrical, memory switching and infrared transmission. The chalcohalide glasses are formed by adding halide elements or halide salts [11] to the chalcogenide glasses. The addition of halogens such as chlorine, bromine and iodine to the chalcogenide glasses results in a disruption of the three-dimensional network. In the TeX glasses with X ˆ Cl, Br and I, Zhang et al. [9] added selenium to stabilize the glasses. This addition leads to stable IR transmitting glasses with an optical window ranging from 1 to 20 lm. Unfortunately, the glass transition temperature (Tg ) of the TeX glasses typically range from about 80°C±120°C and are therefore not practically useful. Iodine has been added to the Ge±S glass system [12]. A decrease in the Tg was observed and attributed to the formation of Ge±I bonds which help to disrupt the three-dimensional connectivity resulting in the formation of a chain-like structure. We have developed and fabricated stable chalcogenide glasses which transmit to beyond 10 lm and possess Tg typically greater than 200°C [13]. Although, Schottmiller et al. [14] and Shirafuji et al. [15] investigated the structure of binary chalcogenide glasses As±Se, Se±S, Se±Te, and Ge±Se using Raman spectroscopy, the structure of multicomponent chalcogenide and chalcohalide glasses is not well understood. Therefore, the objective of this paper is to investigate the local structure of the stable chalcogenide (Ge30 As10 Se…60ÿx† Tex ) and chalcohalide (Ge30 As10 Se30 Te…30ÿy† Iy and Ge30 As10 Se35 Te…25ÿz† Iz ) glass systems using Raman spectroscopy. These glass systems were chosen since they are stable and ®berizable glasses. 2. Experimental procedure 2.1. Glass preparation The compositions for the chalcogenide glasses were based on Ge30 As10 Se…60ÿx† Tex with x ˆ 25, 28, 30, and 35 at.%. The chalcohalide glass compositions were based on Ge30 As10 Se30 Te…30ÿy† Iy ,

with y ˆ 5, 10, 20 and 30 at.% and Ge30 As10 Se35 Te…25ÿz† Iz , with z ˆ 2, 6, 10, and 20 at.%, respectively. A selenide glass, Ge33 As12 Se55 , was also prepared. The tellurium containing glasses and chalcohalide glasses were made from as-received chemicals with six 9s purity: As (All Chemie), Se (Sogem Afrimet) and Te (United Mineral & Chemical) were baked at 450°C, 300°C and 475°C, respectively, for 8 h to remove oxide impurities such as As2 O3 , As2 O5 , SeO2 , SeO3 , Se2 O3 , TeO, and TeO3 . The three times zone-re®ned Ge (United Mineral & Chemical, 6N purity) and iodine (Johnson Matthey, 5N purity) were used as-received. Quartz ampoules (<30 ppm OH, General Electric) (inner and outer diameter were 6 and 10 mm, respectively) were etched with 50 mol% HF for 2 min and then rinsed with deionized water several times. The ampoules were dried in a vacuum oven at 115°C for 4 h. The chemicals were batched in the quartz ampoules inside a glove box under a nitrogen atmosphere. The ampoules containing 16 g of the puri®ed chemicals were evacuated to 10ÿ5 Torr for 5 h, sealed with an oxygen±methane torch and melted in a rocking furnace between 850°C and 950°C for 10 h. Approximately 10 ppm of elemental Al was added to the batch with composition Ge30 As10 Se30 Te30 to getter the oxygen impurities and the melt was subsequently distilled to remove the oxides and impurities. The ampoules containing the chalcohalide glass batches were placed inside a mixture of liquid nitrogen and isopropanol to prevent the sublimation of iodine while under evacuation. At the end of the melting cycle these liquids in their ampoules were quenched in water for about 2 s and annealed at the appropriate Tg to obtain 6 mm diameter rods with lengths greater than 10 cm. The glass rods were cut into 6 mm thick plates with parallel surfaces. The end faces were polished to an optical ®nish on both end faces using a 0.1 lm Al2 O3 grit and with isopropanol as the polishing medium. 2.2. Raman scattering measurement HV- and HH-polarized Raman spectra of the chalcogenide and chalcohalide glasses were measured at room temperature. To minimize

V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

photo-induced structural and/or compositional changes during the experiments, the samples were excited with the 514.5 nm line of an Ar ion laser at a power density of 1.1 W/cm2 . The laser polarization was controlled with a Glan±Thomson prism and a polarizer rotator. The polarizations of the scattered light were selected with a polarizing plate. The scattered light was dispersed by a triplo spectrometer (Dilor) ®tted with 1800 gr/mm grating and a charge coupled device. To reduce the spectrometer polarization response a polarizer scrambler was placed between the focusing lens and the spectrometer entrance slit. The Raman shift was measured between 100 and 650 cmÿ1 , with a spectrometer bandpass 65 cmÿ1 . IHV is the scattered intensity when the incident light is polarized horizontal to the scattering plane and the scattered light is polarized perpendicular to the scattering plane. IHH is the scattered intensity when both incident and scattered light are polarized horizontal to the scattering plane.

105

3. Results 3.1. Raman spectra Ge30 As10 Se…60ÿx† Tex samples

of

chalcogenide

Fig. 1(a) and (b) show the room temperature HV- and HH-polarized Raman spectra of glasses in the Ge30 As10 Se…60ÿx† Tex system, respectively. For comparison, Fig. 1(b) shows the spectrum for a Ge33 As12 Se55 sample. The substitution of tellurium for selenium produces a band between 140 and 190 cmÿ1 , a shoulder between 190 and 230 cmÿ1 , and a smaller band between 230 and 280 cmÿ1 . The amplitude of the band between 140 and 190 cmÿ1 increases as Te is substituted for Se. In addition, the band position shifts to smaller wave number. The amplitudes of the shoulder between 190 and 230 cmÿ1 shifts to smaller wave number as the Te content in the glass increases. Furthermore, the intensity of the smaller band between 230 and 280 cmÿ1 does not change nor shift to smaller wave

Fig. 1. Room temperature (a) HV- and (b) HH-polarized Raman spectra of chalcogenide Ge30 As10 Se…60ÿx† Tex glasses as a function of Te content.

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V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

Table 1 Wave number and relative intensity of the peaks in the Raman spectra of glasses in the chalcogenide Ge30 As10 Se…60ÿx† Tex system x ˆ 25

x ˆ 28

x ˆ 30

x ˆ 35

HV

HH

HV

HH

HV

HH

HV

HH

167 205 ÿ

168 205 246

166 204 ÿ

167 204 245

165 203 ÿ

166 203 243

162 202 ÿ

158 202 240

number within errors of measurement. Table 1 lists the wave number and the relative maximum amplitudes of the bands in the chalcogenide Ge30 As10 Se…60ÿx† Tex glass system. 3.2. Raman spectra of chalcohalide Ge30 As10 Se30 Te…30ÿy† Iy and Ge30 As10 Se35 Te…25ÿz† Iz samples Fig. 2(a) and (b) show the room temperature HV- and HH-polarized Raman spectra of the

chalcohalide Ge30 As10 Se30 Te…30ÿy† Iy samples, respectively. With no iodine substitution (y ˆ 0 at.% I) there is a band between 140 and 190 cmÿ1 , a shoulder between 190 and 220 cmÿ1 , and a smaller amplitude band between 230 and 280 cmÿ1 . As more iodine is added to the glass (y ˆ 5, 10, 20, and 30 at.% I), the maximum amplitude of the band between 140 and 190 cmÿ1 increases and shifts to larger wave number. At 30 at.% I, a band is observed centered at about 190 cmÿ1 . However, the shoulder between 190 and 220 cmÿ1 decreases in

Fig. 2. Room temperature (a) HV- and (b) HH-polarized Raman spectra of chalcohalide Ge30 As10 Se30 Te…30ÿy† Iy glasses as a function of I content.

V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

amplitude and eventually disappears with increasing I. The band between 230 and 280 cmÿ1 increases in amplitude as more iodine is added and the position shifts to larger wave number. There is a band located at about 253 cmÿ1 in the spectrum of sample with 30 at.% I substitution. Table 2 lists the wave numbers and relative amplitudes of the bands for the Ge30 As10 Se30 Te…30ÿy† Iy glass system. Fig. 3(a) and (b) also show the HV- and HH-polarized Raman spectra of the

107

Ge30 As10 Se35 Te…25ÿz† Iz samples measured at room temperature, respectively. As the content of iodine is increased, the band positions and the maximum amplitudes have a similar dependence on I content as do the Ge30 As10 Se30 Te…30ÿy† Iy samples. Table 3 lists the wave number and the relative amplitude of the bands in the Ge30 As10 Se35 Te…25ÿz† Iz glass system. Figs. 4±6 compare the amplitudes of the room temperature HH- and HV-polarized Raman spectra

Table 2 Wave number and relative intensity of the peaks in the Raman spectra of glasses in the chalcohalide Ge30 As10 Se30 Te…30ÿy† Iy system yˆ0

yˆ5

y ˆ 10

y ˆ 20

y ˆ 30

HV

HH

HV

HH

HV

HH

HV

HH

HV

HH

165 203 ÿ

166 203 243

166 204 ÿ

167 204 246

167 206 ÿ

169 207 247

169 208 248

173 209 249

182 ÿ 252

184 ÿ 253

Fig. 3. Room temperature (a) HV- and (b) HH-polarized Raman spectra of chalcohalide Ge30 As10 Se35 Te…25ÿz† Iz glasses as a function of I content.

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Table 3 Wave number and relative intensity of the peaks in the Raman spectra of glasses in the chalcohalide Ge30 As10 Se35 Te…25ÿz† Iz system zˆ0

zˆ2

zˆ6

z ˆ 10

z ˆ 20

HV

HH

HV

HH

HV

HH

HV

HH

HV

HH

167 205 246

168 205 246

168 205 246

170 205 246

171 206 247

173 207 248

172 207 248

174 208 249

173 209 249

183 210 251

Fig. 4. Room temperature HH- and HV-polarized Raman spectra for Ge30 As10 Se25 Te35 glass.

Fig. 5. Room temperature HH- and HV-polarized Raman spectra for Ge30 As10 Se30 Te10 I20 glass.

for the Ge30 As10 Se25 Te35 , Ge30 As10 Se30 Te10 I20 , and Ge30 As10 Se35 Te5 I20 samples, respectively. In each ®gure, there are two spectra, one representing the HV scattered intensity (IHV ) and the other representing the HH scattered intensity (IHH ). For amorphous materials the depolarization ratio, q(x), is de®ned as the ratio of IHV (x,T)/IHH (x,T) [16] and ranges from 0 to 0.75. Raman bands with q(x) approaching 0.75 are depolarized and those for which q(x) < 0.70 are polarized. Information regarding the symmetries of the local active vibration modes can be obtained through the dependence of the depolarization ratio on the Raman shift. Although these samples are amor-

phous, Figs. 4±6 show that the amplitudes of Raman bands di€er for the two polarizations, indicating that the vibrations are larger in the HHpolarized spectrum. Figs. 7±9 show the depolarization ratio spectrum as a function of Raman shift at room temperature for the Ge30 As10 Se25 Te35 , Ge30 As10 Se30 Te10 I20 , and Ge30 As10 Se35 Te5 I20 samples, respectively. For the chalcogenide Ge30 As10 Se25 Te35 sample the depolarization ratio is between 0.55 and 0.7. For the chaland cohalide samples Ge30 As10 Se30 Te10 I20 Ge30 As10 Se35 Te5 I20 , the depolarization ratio is between 0.55 and 0.7 and 0.45 and 0.6, respectively.

V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

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Fig. 8. Depolarization spectrum q(x) of Ge30 As10 Se30 Te10 I20 glass.

Fig. 6. Room temperature HH- and HV-polarized Raman spectra for Ge30 As10 Se35 Te5 I20 glass.

Fig. 9. Depolarization spectrum q(x) of Ge30 As10 Se35 Te5 I20 glass.

4. Discussion 4.1. Chalcogenide Ge30 As10 Se…60ÿx† Tex glasses Fig. 7. Depolarization spectrum q(x) of Ge30 As10 Se25 Te35 glass.

Chalcogenide glasses are made from mixtures of the chalcogen elements such as sulfur, selenium,

110

V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

and tellurium which usually have two-fold coordination. The addition of the network formers such as germanium or silicon (four-fold coordinated), and arsenic or antimony (three-fold coordinated) establishes cross-linking between chains and facilitates stable glass formation. Since the electronegativities of the chalcogen elements are similar to those of the network formers, the chemical bonding of the chalcogenide glasses is predominantly covalent with well-de®ned directional bonds. The amorphous chalcogenide glass structure can be best described by using the `random covalent model' (RCM) [17] in which all local valence requirements are satis®ed. In this model, the germanium atom is coordinated by four atoms and each chalcogen element is coordinated by 2 atoms, in accordance with Mott's 8 ÿ N bonding rule where N is the number of valence electrons, and the factor …8 ÿ N † gives the number of nearest neighbors in the structure [18]. This 8 ÿ N bonding rule states that all electrons are taken up in bonding so that changes in conductivity are small (1%) changes of composition. Thus the longrange structure of chalcogenide glass is completely random and disordered. In accordance with these valence requirements, the four-fold coordinated germanium and three-fold coordinated arsenic form tetrahedral units [GeX4 ] and pyramidal units [AsX3 ], respectively, where X are the two-fold coordinated chalcogen elements, S, Se, and Te. For the Ge30 As10 Se…60ÿx† Tex samples shown in Fig. 1(a) and (b), the band between 140 and 190 cmÿ1 is associated with Ge±chalcogen tetrahedral units. This association is because the spectra of germanium selenide glasses have a band centered at around 190 cmÿ1 which is due to the m1 symmetric stretching vibration of GeSe4=2 units as shown in Fig. 1(b) for Ge33 As12 Se55 glass [19]. The presence of Te in the current glasses will shift the band to lower frequencies due to the increased reduced mass due to Te. The atomic mass of Te is 127.6 compared with 78.96 for Se. Consequently, this band is expected to shift to smaller wave numbers with increasing Te content. Therefore, this band can be attributed to [GeSe…4ÿx† Tex ] tetrahedral units. In an analogous argument, the presence of AsSe…3ÿx† Tex structural units cannot be ignored,

albeit their concentration is expected to be less based on the smaller As concentration. The concentration of Se±Te bonds is likely to be much smaller since these glasses are considered to be anion de®cient. Table 1 shows that at x ˆ 35 at.% Te, the position of the band between 140 and 190 cmÿ1 is 158 cmÿ1 for the HH-polarized Raman spectra compared with 190 cmÿ1 for the selenide glass, Ge33 As12 Se55 , which has zero Te. Thus the band located between 140 and 190 cmÿ1 is assumed to be due to the stretching vibration of [GeSe4ÿx Tex ] mixed tetrahedral units where 1 6 x 6 4. The band located between 190 and 230 cmÿ1 lies close to the vibrational band (240 cmÿ1 ) found in As2 Se3 glass which is attributed to AsSe3=2 pyramidal units [20]. However, since our samples contain Te, we propose modi®ed structural units which are represented by [AsSe…3ÿx† Tex ] structural units. Hence, we expected the vibrational band to be shifted to smaller wave number, as observed, due to the presence of the heavier Te in the structural units. The band position shifts to smaller wave number as Te content increases from 25 to 35 at.%. This shift is attributed to the larger atomic mass of Te compared with that of Se. Thus the shoulder located between 190 and 230 cmÿ1 is assumed to be due to the vibration of [AsSe3ÿx Tex ] mixed pyramidal units where 1 6 x 6 3. In addition, there is a smaller band located between 230 and 280 cmÿ1 . The m3 stretching vibration associated with GeSe4=2 structural units is centered around 262 cmÿ1 [19]. Therefore we tentatively ascribe the band in the 230 to 280 cmÿ1 region to stretching vibrations associated with [GeSe…4ÿx† Tex ] tetrahedral units. Furthermore, the vibrational band associated with the m1 symmetric stretching mode of Ge±Se±Ge occurs at 288 cmÿ1 [19]. It is expected that Te replaces the Se and forms the Ge±Te±Ge chains. However, it is anticipated that the concentration of the m3 stretching vibration of [GeSe…4ÿx† Tex ] tetrahedral units and the Ge±Te±Ge chains will be much smaller due to the glasses being anion de®cient. This de®ciency is correlate with the features observed in the 230±280 cmÿ1 region. When Te is substituted for Se in the Ge30 As10 Se…60ÿx† Tex glass system, the structure can

V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

Fig. 10. Schematic representation Ge30 As10 Se…60ÿx† Tex system.

of

glasses

in

the

be schematically represented as shown in Fig. 10. The Se and Te atoms are bonded to As and Ge atoms so that the main structural units present are the [AsSe3ÿx Tex ] mixed pyramids, [GeSe4ÿx Tex ] mixed tetrahedra, and Ge±Te±Ge chains with Ge±Ge, As±As, Ge±As, and Se±Te bonds connected together to form an amorphous structure. The stoichiometric glasses would have formula Ge30 As10 (Se/Te)75 whereas we have Ge30 As10 (Se/ Te)60 . Based on these numbers there will not be a prevalence of Ge±Ge, As±As, and Ge±As semihomopolar bonds. 4.2. Chalcohalide Ge30 As10 Se30 Te…30ÿy† Iy Ge30 As10 Se35 Te…25ÿz† Iz samples

and

As discussed above for the Ge30 As10 Se…60ÿx† Tex samples, the band between 140 and 190 cmÿ1 is assumed to be due to the stretching vibration of the [GeSe4ÿx Tex ] mixed tetrahedral units. As iodine is substituted for tellurium (y ˆ 5, 10 and 20 at.% I), the peak position for the band between 140 and 190 cmÿ1 shifts to larger wave number. This shift is due to a decrease in e€ective mass because an iodine atom (126.90) has a smaller atomic mass than does a tellurium atom (127.60) and the di€erence depends on the force constants of the Ge±I and Ge±Te bonds [21]. The band between 140 and 190 cmÿ1 is assumed to be due to the presence of [GeSex Tey Iz ] mixed tetrahedal units (x ‡ y ‡ z ˆ 4). In the extreme cases where z ˆ 0 or y ˆ 0, the samples contain the [GeSe4ÿy Tey ] and [GeSe4ÿz Iz ] mixed tetrahedral

111

units, respectively. The band between 160 and 190 cmÿ1 is due to the vibrational mode of the [GeSe4ÿy Iy ] mixed tetrahedra. This assignment is con®rmed with Usuki et al.'s work for the Ge±Se±I glass system [22]. Thus for y ˆ 5, 10 and 20 at.% I, the band between 140 and 190 cmÿ1 consists of vibrational modes associated with the stretching of [GeSe4ÿy Tey ], [GeSe4ÿz Iz ], and [GeSex Tey Iz ] …x ‡ y ‡ z ˆ 4† mixed tetrahedral units. At 20 at.% I substitution, the contribution from the stretching vibration of [GeSe4ÿz Iz ] mixed tetrahedra is larger than that of the stretching vibration of [GeSe4ÿy Tey ] mixed tetrahedra since the majority of the tellurium atoms are replaced by iodine atoms. For y ˆ 30 at.% I, the band originally between 140 and 190 cmÿ1 shifts to between 150 and 220 cmÿ1 with the band position moving to a larger wave number (190 cmÿ1 ) since there is no tellurium in the glass. Therefore, we attribute the band between 150 and 220 cmÿ1 to [GeSe4ÿy Iy ] tetrahedra and [AsSe…3ÿy† Iy ] pyramidal structural units. Also from our previous discussion of the chalcogenide glass, the shoulder between 190 and 230 cmÿ1 is due to the stretching of the [AsSe3ÿy Tey ] mixed pyramids. As the iodine content is increased from 0 to 20 at.%, the amplitude of the shoulder between 190 and 230 cmÿ1 decreases. This shoulder disappears for 30 at.% I substitution since there is no tellurium in the glass network to make up the [AsSe3ÿy Tey ] mixed pyramids. Thus 6 20 at.% I, the shoulder between 190 and 230 cmÿ1 is due to the vibration of [AsSe3ÿy Tey ] mixed pyramids. With no iodine added to the glass, there is a smaller band between 230 and 280 cmÿ1 . Although the origin of this was discussed in the previous section, to recap, the band between 230 and 280 cmÿ1 has been assigned to the t3 stretching vibrations associated with the [GeSe4ÿx Tex ] mixed tetrahedra units. As the tellurium is being replaced by iodine, the amplitude of the band between 230 and 280 cmÿ1 also increases and shifts to larger wave number (253 cmÿ1 ). At 30 at.% I substitution, there is a band located at about 253 cmÿ1 . In Gey Se1ÿy …0 < y < 0:5† glasses, the t3 stretching vibration associated with GeSe4=2 structural units is centered around 262 cmÿ1 [19]. Therefore, we assign the band in the 230 and 280 cmÿ1 region to the t3

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V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

Table 4 Wave number and assignment of the Raman bands in chalcogenide and chalcohalide glasses Glass Chalcogenide Ge30 As10 Se…60ÿx† Tex Chalcohalide Ge30 As10 Se30 Te…30ÿy† Iy Ge30 As10 Se35 Te…25ÿz† Iz

Wave number (cmÿ1 )

Assignments

140±190 190±230 230±280

[GeSe4ÿx Tex ] mixed tetrahedra [AsSe3ÿx Tex ] mixed pyramids Combination of m3 [GeSe4ÿx Tex ] mixed tetrahedra and Ge±Te±Ge chains

140±190

Combination of [GeSe4ÿy Iy ], [GeSe4ÿy Tey ], and [GeSex Tey Iz ] (x + y + z ˆ 4) mixed tetrahedra for 0±20 at% I Combination of [AsSe3ÿy Iy ] mixed pyramids and [GeSe4ÿy Iy ] mixed tetrahedra for 30 at.% I [GeSe4ÿy Iy ] mixed tetrahedra [AsSe3ÿy Tey ] mixed pyramids for 0±20 at.% I t3 of GeSe3=2 I mixed tetrahedra

150±220 160±190 190±230 230±280

stretching vibration associated with GeSe3=2 I mixed tetrahedral units since the peak position shifts to smaller wave number (253 cmÿ1 ) according to the increase in reduced mass attributed to I. In addition, we also see a shift in the peak position to shorter wavelength with the GeSe3=2 I mixed tetrahedral units in our infrared studies of Ge±As± Se±Te±I glasses [21]. Table 4 shows the assignment of Raman bands at the corresponding wave number for the chalcogenide and chalcohalide glasses. By adding iodine into the chalcogenide glass (Ge±As±Se±Te), the structure for the Ge±As±Se± Te±I glass system could be schematically represented as shown in Fig. 11. As the iodine content is increased, the formation of Ge±I, As±I, and Se±I bonds partially replace the Ge±Te, As±Te, and Se±Te bonds. Since iodine is a network terminator, it disrupts the chalcogenide structural units which include the [AsSe3ÿy Tey ] mixed pyramids and [GeSe4ÿy Tey ] mixed tetrahedra. The resulting structure then becomes [AsSex Tey Iz ] …x ‡ y ‡ z ˆ 3† mixed pyramids, [GeSex Tey Iz ] …x ‡ y ‡ z ˆ 4† mixed tetrahedra, and GeSe3=2 I mixed tetrahedra units, as well as including Ge±Ge, As±As, and Ge±As, together with Se±Te bonds connected to form the chalcohalide glass structure. Fig. 4 shows the room temperature HV- and HH-Raman spectra for the chalcogenide sample, Ge30 As10 Se25 Te35 . The maximum amplitude at about 158 cmÿ1 is relatively larger in both the HV- and HH-Raman spectra. The amplitude at 202 cmÿ1 is larger in the HH scattering con®gu-

Fig. 11. Schematic representation of glasses in the Ge±As±Se± Te±I system.

ration than in the HV scattering con®guration. Similar dependence is also found for the amplitude at about 240 cmÿ1 . Since the ®rst order Raman spectra in glasses is continuous, the depolarization ratio, q(x), is also a continuous function of the Raman shift. The di€erence in symmetry properties of the active vibrational modes can be determined through the depolarization spectrum. Fig. 7 shows that at 158 cmÿ1 , the depolarization ratio is 0.61. Thus the 158 cmÿ1 feature is polarized and the mode giving rise to this feature is the stretching vibration of [GeSe4ÿx Tex ] mixed tetrahedral units as discussed in the previous paragraphs. At 202 cmÿ1 the depolarization ratio is 0.60. The 202 cmÿ1 feature is also polarized and ascribed to the

V.Q. Nguyen et al. / Journal of Non-Crystalline Solids 248 (1999) 103±114

stretching vibration of [AsSe3ÿx Tex ] mixed pyramidal units. At 240 cmÿ1 the depolarization ratio is 0.70. The 240 cmÿ1 feature is less polarized and is attributed to the vibration of [GeSe…4ÿx† Tex ] tetrahedral units and Ge±Te±Ge chains. Fig. 5 shows the room temperature HV- and HH-Raman spectra for the chalcohalide sample, Ge30 As10 Se30 Te10 I20 . At 173 cmÿ1 and 249 cmÿ1 the amplitudes are larger in the HH scattering con®guration. For the 173 cmÿ1 feature, the depolarization ratio is 0.54 as shown in Fig. 8. The 173 cmÿ1 feature is more polarized. This vibrational mode is associated with the stretching modes of the [GeSe4ÿy Tey ], [GeSe4ÿy Iy ], and [GeSex Tey Iz ] …x ‡ y ‡ z ˆ 4† mixed tetrahedra. The small shoulder at 209 cmÿ1 in the HV con®guration in Fig. 5 has a depolarization ration of 0.63. The 209 cmÿ1 feature is less polarized and is attributed to the stretching vibrational modes of [AsSe3ÿy Tey ] and [AsSe3ÿy Iy ] mixed pyramids. At 249 cmÿ1 in the HH con®guration, the depolarization ratio is 0.58. The 249 cmÿ1 feature is polarized and due to the t3 stretching vibration associated with GeSe3=2 I mixed tetrahedral units. Similarly, Fig. 6 shows the room temperature HV- and HH-Raman spectra for the chalcohalide sample, Ge30 As10 Se35 Te5 I20 . The HH-amplitudes at 177 cmÿ1 and 251 cmÿ1 are much larger than that of the HV scattering con®guration. At 177 cmÿ1 the depolarization ratio is 0.47 (Fig. 9). The 177 cmÿ1 feature is more polarized and due to the stretching vibrations of the [GeSe4ÿy Iy ], [GeSe4ÿy Tey ], and [GeSex Tey Iz ] …x ‡ y ‡ z ˆ 4† mixed tetrahedra. At 251 cmÿ1 the depolarization ratio is 0.50. The 251 cmÿ1 band is polarized and due to the t3 stretching vibration associated with GeSe3=2 I mixed tetrahedral units. Note that the Raman peaks in the chalcohalide sample Ge30 As10 Se35 Te5 I20 are more polarized than the chalcohalide sample Ge30 As10 Se30 Te10 I20 . Also from Tables 1±3, the band positions have a smaller Raman shift in the HV scattering con®guration when compared to their shift in the HH scattering con®guration. This di€erence is due to the band dependence of the coupling coecients which can be explained through the Shuker±Gammon theory [23]. This property is also seen in Kobliska's work [16] for the amorphous As2 S3 system.

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5. Conclusion The main structural units for the chalcogenide Ge±As±Se±Te glass system are [AsSe3ÿx Tex ] mixed pyramidal units, [GeSe4ÿx Tex ] mixed tetrahedra units, and Ge±Te±Ge chains. The addition of the monovalent iodine to the chalcogenide glass system disrupts the three-dimensional network connectivity. For chalcohalide glasses, the main structural units include [AsSex Tey Iz ] …x ‡ y ‡ z ˆ 3† mixed pyramidal units, and [GeSex Tey Iz ] …x ‡ y ‡ z ˆ 4† mixed tetrahedral units, and GeSe3=2 I mixed tetrahedra units. Acknowledgements This paper is submitted in partial ful®llment of the requirements for V.Q.N.'s PhD degree at the University of Maryland at College Park. References [1] J.R. Gannon, in: L.G. DeShazer, C. Kao (Eds.), Infrared Fibers (0.8±12 mm) I, Proc. Soc. Photo-Opt. Instrum. Eng., 1981, vol. 266, pp. 62±68. [2] J.S. Sanghera, F.H. Kung, L.E. Busse, P.C. Pureza, I.D. Aggarwal, J. Am. Ceram. Soc. 78 (8) (1995) 2198. [3] G. Nau, F. Bucholtz, K.J. Ewing, S.T. Vohra, J.S. Sanghera, I.D. Aggarwal, Proc. Soc. Photo-Opt. Instrum. Eng. 2504 (1995) 291. [4] L.E. Busse, J. Moon, J.S. Sanghera, I.D. Aggarwal, Laser World Focus 32 (9) (1996) 143. [5] I. Inagawa, R. Iizuka, T. Yamagishi, R. Yokota, J. NonCryst. Solids 95&96 (1987) 810. [6] J.S. Sanghera, V.Q. Nguyen, P.C. Pureza, F.H. Kung, R.E. Miklos, I.D. Aggarwal, J. Lightwave Technol. 12 (1994) 737. [7] J. Porter, J. Non-Cryst. Solids 112 (1989) 15. [8] J. Heo, J.D. Mackenzie, J. Non-Cryst. Solids 113 (1989) 246. [9] X.H. Zhang, H.L. Ma, G. Fonteneau, J. Lucas, J. NonCryst. Solids 140 (1992) 47. [10] J.S. Sanghera, J. Heo, J.D. Mackenzie, J. Non-Cryst. Solids 103 (1988) 155. [11] A.B. Seddon, J. Non-Cryst. Solids 213&214 (1997) 22. [12] J. Heo, J.D. Mackenzie, J. Non-Cryst. Solids 111 (1989) 29. [13] J.S. Sanghera, V.Q. Nguyen, P.C. Pureza, F.H. Kung, F. Miklos, L. Busse, I.D. Aggarwal, Proc. Soc. Photo-Opt. Instrum. Eng. 2290 (1994) 89. [14] J. Schottmiller, M. Tabak, G. Lucovsky, A. Ward, J. NonCryst. Solids 4 (1970) 80.

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