Compositional dependence of the optical properties of new quaternary chalcogenide glasses of Ge–Sb–(S,Te) system
V. Pamukchieva
a, A. Szekeres
a,*, K. Todorova
a, E. Svab
b, M. Fabian
baInstitute of Solid State Physics, Bulgarian Academy of Sciences, Tzarigradsko Chaussee 72, 1784 Sofia, Bulgaria
bResearch Institute for Solid State Physics and Optics, Konkoly Thege str. 29-33, H-1525 Budapest, P.O.B. 49, Hungary
a r t i c l e i n f o
Article history:
Received 5 February 2009
Received in revised form 1 June 2009 Accepted 15 June 2009
Available online 17 July 2009
Keywords:
Multicomponent chalcogenide glasses Spectral ellipsometry
Neutron diffraction Optical constants
a b s t r a c t
New quaternary telluride glassy materials with composition of GexSb40xS50Te10and GexSb40xS55Te5
(x= 10, 20, and 27) have been synthesized and their optical properties have been studied by means of spectroscopic ellipsometry in the range of 400–820 nm. The optical constants, i.e. the refractive index, extinction coefficient, absorption coefficient, and the optical band gap energy are determined and their compositional dependence is considered. These parameters are characteristics for amorphous structure of the synthesized glasses, revealed from the neutron diffraction measurements.
Ó2009 Elsevier B.V. All rights reserved.
1. Introduction
Chalcogenide glasses have unique properties for potential appli- cation in infrared optics, fiber optics, memory devices, inorganics photoresists and antireflection coatings [1–6]. These glasses are low phonon energy materials, and are light transparent in the vis- ible and mid-infrared region. This underlines the importance of the characterization of these glassy materials through determination of their optical constants, such as refractive index and extinction coefficient, as well as the corresponding optical band gaps.
The ternary Ge–Sb–S glasses with nonstoichiometric composi- tions have been subject of intensive studies and their properties are well established[7–10]. Introduction of small amount of Te into these glasses alters the glassy structure, and leads to high refractive index values and photosensitivity. Therefore, telluride glasses from quaternary systems are potential candidates for inte- grated optics.
In recent years the number of papers dealing with telluride glasses from systems, such as Ge–Sb–Te[11–15], Te–As–Se[16–
19], has increased reflecting the growing interest in these materi- als. Nowadays the attention is extended over quaternary systems as possible candidates for optoelectronic applications.
Recently we have synthesized new quaternary telluride glasses based on Ge–S system with addition of Sb and partial substitution of Te for S, and have considered the basic physicochemical param- eters in dependence of glass composition[20]. To the best of our
knowledge there is no data in the literature on glasses from a qua- ternary Ge–Sb–S–Te system. In general, the optical properties are closely related to the material’s atomic structure, electronic band structure and electrical properties, which are strongly correlated with the glass composition. Thus the main goal of investigations of these materials is clarifying the influence of Te substitution for S on the optical properties of the glasses.
In this paper we present results on the ellipsometric study of the optical properties of glasses, synthesized by us, from the qua- ternary Ge–Sb–S–Te system, as the compositional dependence of the complex refractive index and optical band gap energy values are considered. These parameters have been obtained from the data analysis of ellipsometric measurements performed in the vis- ible spectral range of light.
2. Experimental
The bulk glasses with four compositions, namely GexSb40x- S50Te10 (x= 10 and 27 at.%) and GexSb40xS55Te5 (x= 20 and 27 at.%), were synthesized from 5 N purity elements by the con- ventional melt-quenching method. The synthesis was performed in a rotary furnace, as the glass components of a proper composi- tion were placed in quartz ampoules, which was evacuated (103Pa). The ampoules were heated up to 950°C, and were kept at this temperature for 24 h, rotating the furnace for homogeneous melting. Ending the process, the ampoules were pulled out, and were quenched in air. Part of the samples was cut into 4 mm thick slices with a diameter of 10 mm and the slices were 0925-3467/$ - see front matterÓ2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.optmat.2009.06.003
* Corresponding author.
E-mail addresses:amszekeres@yahoo.com,szekeres@issp.bas.bg(A. Szekeres).
Optical Materials 32 (2009) 45–48
Contents lists available atScienceDirect
Optical Materials
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / o p t m a t
polished on one side for optical measurements. The other part of the samples was powdered for neutron diffraction measurement.
The elemental composition of the glasses was cross-checked by means of prompt
c
-ray activation analyses (PGAA), the results of which are given in details elsewhere[21]. A good coincidence be- tween the measured compositions and those calculated for the material synthesis is established.The phase state of the glasses was examined by neutron diffrac- tion (ND) measurements, carried out on a ‘PSD’ neutron diffrac- tometer with a monochromatic wavelength ofk0= 1.068 Å. From the ND intensity data the total structure factor S(Q) was calculated after correction and normalization procedures, described in our previous work[22].
The ellipsometric measurements were done on a manual Rudolf Research ellipsometer with PCSA configuration in the spectral re- gion of 400–820 nm (1.5–3.1 eV) and at an angle of light incidence of 50°. The accuracy of the polarizer, analyzer and incidence angles was within ±0.01°. The systematic errors in the experimental elli- psometric angles were eliminated through averaging the four zones measurements. From the SE data analysis, the complex refractive index (ñ=n–jk), where nis the refractive index andk is the extinction coefficient, absorption coefficient (
a
= 4p
k/k) and optical bandgap energy (Eg) of the glasses were evaluated.3. Results and discussion
InFig. 1, the dispersion curves of the refractive index (1a) and extinction coefficient (1b) for the studied compositions are pre- sented. The refractive index dependences (Fig. 1a) follow the or- dinary dispersion behavior, namely in the weak absorption region thenvalue increases towards shorter wavelengths and be- lowk480 nm it starts to decrease due to the stronger absorption of light (Fig. 1b).
The shape of the dispersion curves of both quantities is similar but their values are compositionally dependent. Increasing the Ge content up to 27, correspondingly decreasing the Sb content, at constant amount of Te leads to reduction of the refractive index values throughout in the studied spectral region (Fig. 1a). Decrease of the Te content from 10 to 5 results in a further reduction in then values. Similar tendency in the dispersion curves ofkis observed (Fig. 1b).
In the dependence of the extinction coefficient on photon en- ergy (Fig. 1b), a shift of the whole curve toward lower energy re- gion (red shift) is observed when the Ge content decreases or Te content increases, as the latter has weaker influence on the ob- served displacement.
The observed compositional dependences of the optical con- stantsnandk can be explained by the different polarizability of the glass constituent elements [23]. Since the Ge cations have smaller polarizability than the Sb cations, increase of their amount
in the glass most probably causes the observed decrease of the refractive index. On the other hand, the Te ions, being larger than S ions, have higher polarizability and, hence, for 10 at.% Te the refractive index values are larger.
The absorption coefficient was determined from thekdata by using the relationship
a
= 4p
k/k. In the high absorption region (a
> 104cm1), the absorption coefficient can be described by the relation (a
hm
)1/m=B(hm
Eg) [24,25], where the parameter B is dependent on the electron transition probability, while the power m is an integer number characterizing the transition process. Plot- ting the (a
hm
)1/mversus photon energy (hm
) and assuming an indi- rect type electron transition (m= 2), as is the case for chalcogenide materials, the energy band gap (Eg) was determined by extrapolat- ing the linear part of the curves toward zero absorption; the inter- ception with the photon energy axis providing theEgvalue. The results are summarized inFig. 2, where the spectral dependence ofa
and the corresponding inserted plot of (a
hm
)1/mversus (hm
) are given for all compositions studied. By extrapolation, theEgval- ues were obtained with an accuracy of ±0.05 eV and are presented inTable 1. The tendency of the compositional dependence ofEg values is that the optical band gap energy increases with increasing Ge and it becomes smaller with increasing the amount of Te atoms.400 500 600 700 800 900 1.5
2.0 2.5 3.0 3.5
Refractive index
Wavelength (nm)
Ge10Sb30S50Te10 Ge27Sb13S50Te10 Ge20Sb20S55Te5 Ge27Sb13S55Te5
400 500 600 700 800 900
0.0 0.4 0.8 1.2
Extinction coefficient
Wavelength (nm)
Ge10Sb30S50Te10 Ge27Sb13S50Te10 Ge20Sb20S55Te5 Ge27Sb13S55Te5
Fig. 1.Spectral dependences of the refractive index (a) and extinction coefficient (b) of chalcogenide glasses with compositions, inserted.
Table 1
Optical band gap energyEg, coordination numberZ, heat of atomizationHsand the ratioHs/Zof the synthesized chalcogenide glasses.
Composition Eg(eV) Z Hs(kcal/g) Hs/Z(kcal/g)
Ge10Sb30S50Te10 1.30 2.50 64.01 25.60
Ge27Sb13S50Te10 1.42 2.67 66.54 24.92
Ge20Sb20S55Te5 1.47 2.60 66.57 25.6
Ge27Sb13S55Te5 1.49 2.67 67.50 25.28
1.5 2.0 2.5 3.0
104 105
1.2 1.6 2.0 2.4 2.8 3.2 0
200 400 600 800 1000
hν. (eV) (αhν)1/2 (cm-1eV)1/2
α (cm-1 )
Photon energy. hν. (eV) Ge10Sb30S50Te10
Ge27Sb13S50Te10 Ge20Sb
20S
55Te
5
Ge27Sb13S55Te5
Fig. 2.Absorption coefficientain dependence on photon energy of chalcogenide glasses with compositions, inserted. The corresponding plots of (ahm)1/2versushm are also inserted.
46 V. Pamukchieva et al. / Optical Materials 32 (2009) 45–48
The increase in theEgvalues can be explained on the basis of the
‘‘density of states” model proposed by Davis and Mott[26]. Accord- ing to this model, the width of the localized states near the mobil- ity edges depends on the degree of disorder and defects present in the amorphous state. High concentration of localized states in the band structure narrows the energy gap and, therefore, broadening of energy gap, observed in our case, can be an indication for lesser degree of disorder and less defects in the studied glasses. In chal- cogenide glasses containing a high concentration of group VI ele- ments (Te and S in our case) the lone-pair electrons form the top of the valence band (bonding band) and the antibonding band form the conduction band[27]. The number of states in the conduction band depends only on the number of different bonds in the given composition, while the number of states in the valence band is determined by the number of lone-pair states related to the chal- cogenide atoms. Since in the compositions considered by us the amount of chalcogenides is constant (S50Te10or S55Te5), the va- lence band remains the same. Therefore, the increase of the optical band gap with the Ge content, keeping the S and Te contents con- stant, can be attributed to the narrowing of the conduction band tail caused by increasing the amount of stronger (53.5 kcal/mol) Ge–S bonds at the expense of the amount of weaker (47.6 kcal/
mol) Sb–S bonds[28]. When the Te content decreases from 10 to 5 at.%, the increase of the number of the much stronger Ge–S and Sb–S bonds at the expense of weaker (35.5 kcal/mol) Ge–Te and (31.6 kcal/mol) Sb–Te bonds[28]leads to an increase of the aver- age stabilization energy of chemical bonds and, therefore, to an in- crease of the gap energy, as was observed (Table 1).
The decrease ofEgvalues with increasing the Te content can be also explained on the bases of the suggestion that the change of the band gap is caused by alloying effect, namely a compositional change in the host material itself[29]due to the change in bond angles and/or bond lengths modifying the glassy structure and dis- turbing the ordering. The lower value of the optical band gap for compositions with higher Te content can be related to the ten- dency of Te atoms to form chemical disordering and to create local- ized states in the band gap.
The energy band gap value is correlated with the physical parameters of the glasses, such as average coordination number (Z) and average heat of atomization (Hs), the latter being a measure of cohesive energy and representing the relative bond strength.
The average coordination numberZis defined by the expression Z= 4XGe+ 3XSb+ 2XS+ 2XTe, whereXis the molar fraction of con- stituent elements and the numbers 4, 3, 2 and 2 are the coordina- tion number of Ge, Sb, S, Te, respectively. The coordination number Zcharacterizes the electronic properties of semiconducting materi- als, and shows the bonding character in the nearest-neighbour re- gion[30]. The heat of atomization,Hs, for our quaternary glassy system GeaSbbScTed can be calculated as HS¼ ð
a
HGes þbHSbs þc
HSsþdHTes Þ=ða
þbþc
þdÞ[31], whereHsis the heat of atomiza- tion of constituent atoms, and corresponds to the average nonpolar bond energy of the Ge–Ge, Sb–Sb, S–S and Te–Te chemical bonds [28];a
,b,c
anddare the atomic percent of the corresponding ele- ments. The calculatedZandHsvalues, as well as their ratio are summarized inTable 1.According to the constrain theory[32], the chalcogenide glasses can be organized into three different categories: (i) floppy, or un- der-coordinated glasses with Z< 2.4; (ii) optimally-coordinated or ideal glasses withZ= 2.4; (iii) stressed-rigid and over-coordi- nated glasses withZ> 2.4. In accordance with that, the glassy com- positions studied by us are over-coordinated, stressed-rigid and with lower connectivity, as the values ofZare larger than 2.4 (Ta- ble 1). The ratio of the heat of atomization and coordination num- ber, parameter Hs/Z, given also in Table 1, is almost constant independently on composition and, therefore, one may conclude
that the average heat of atomization has no essential influence on the band gap energy values.
The results from the neutron diffraction measurements of these Ge–Sb–S–Te glasses are summarized inFig. 3. As is seen, there are no sharp peaks in the measured spectra of the structure factor S(Q), which could be related to crystalline phase and, therefore, the syn- thesized glasses are fully amorphous. Based on these results we can conclude that all optical parameters considered here are char- acteristics for the amorphous glassy material with the given composition.
4. Conclusion
By spectroscopic ellipsometry, the optical constants of new amorphous quaternary telluride glassy materials with composition of GexSb40xS50Te10and GexSb40xS55Te5(x= 10, 20, and 27) have been defined in the spectral range of 400-820 nm. The observed in- crease of the optical band gap energy value with increase of Ge content or decrease of Te content is explained in terms of chemical bonds formation and density of states, both affected by composi- tional variation.
Acknowledgements
The authors gratefully acknowledge the financial support from the EU 6FP:MNI3 Access Program under contract RII-CT-2003- 505925.
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