Spectroscopic ellipsometric determination of the optical constants of chalcogenide films of the Ge-Sb-S-Te system

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Spectroscopic ellipsometric determination of the optical constants of chalcogenide films of the Ge-Sb-S-Te system

V Pamukchieva ¹, A Szekeres ^1,4, E Svab ², M Fabian ², Z Revay ³ and L Szentmiklosi ³

1 Georgi Nadjakov Institute of Solid State Physics,

Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria

2 Research Institute for Solid State Physics and Optics, 29-33 Konkoly Thege Str., H-1525 Budapest, Hungary

3 Institute of Isotopes, 29-33 Konkoly Thege Str., H-1525 Budapest, Hungary

E-mail: szekeres@issp.bas.bg

Abstract. Spectroscopic ellipsometric studies of GexSb40-xS50Te10 (x=10, 20, 27) and Ge27Sb13S55Te5 films evaporated on glass substrates demonstrated the compositional dependence of the optical constants, namely, as the Ge fraction in the films is increased from 10 to 27, the values of the refractive index and extinction coefficient gradually decrease, while the optical band gap increases from 1.04 eV to 1.33 eV.

1. Introduction

Chalcogenide glasses have received a great deal of attention due to their transparency in the infrared region, high refractive index and photosensitivity [1-4]. These materials are heavy anion glasses since S, Se and Te are the main constituents of their compositions. The addition of Ge or As facilitates the formation of covalent bonds and reduces the atomic diffusivity providing sufficient amorphous stability. The heavy elements Te and Sb have substantial influence on the physical properties of glasses, making changeable their coordination number. An important advantage of tellurium- containing glasses is that due to the heavier mass of Te the transmission in the infrared region is shifted toward the longer wavelengths. While the physical properties of chalcogenide films from ternary stoichiometric Ge-Sb-Te compositions have been intensively studied [5, 6], quaternary systems based on Ge-S are still to be thoroughly explored. Recently, the number of papers dealing with ternary or quaternary telluride systems has been growing [7-12]. Some of these studies demonstrated that such glasses become increasingly non-linear as S is replaced by the heavier and more polarizable Se or Te.

Our recent research interest has turned to investigating quaternary telluride systems based on Ge-S glasses with addition of Sb and partial substitution of Te for S. The composition of these glasses influences their optical properties and underlines the importance of characterizing these glassy materials through determination of the optical constants, (refractive index and extinction coefficient), as well as the corresponding optical band gaps. In this work we present results on the spectroscopic ellipsometric studies of amorphous Ge-Sb-S-Te films, as the effect of Ge and Te contents on the optical properties is considered.

4 To whom any correspondence should be addressed.

1 Fifteenth International Summer School on Vacuum, Electron and Ion Technologies IOP Publishing Journal of Physics: Conference Series 113 (2008) 012054 doi:10.1088/1742-6596/113/1/012054

c 2008 IOP Publishing Ltd 1

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2. Experimental details

The bulk glasses with composition of Ge_xSb_40-xS₅₀Te₁₀ (x=10, 20, 27 atomic percent) and Ge₂₇Sb₁₃S₅₅Te₅ were prepared by the conventional melt-quenching method from 5N purity elements.

The syntheses were performed in evacuated quartz ampoules (10^-3 Pa) using a rotary furnace. The ampoules were heated up to 950°C and were kept at this temperature for 24 h while rotating the furnace so that the melt be homogenized. After 24 hours the ampoules were pulled out from the furnace and were quenched in air. The materials thus obtained were powdered.

In order to check the glass composition, prompt gamma-ray activation analyses (PGAA) were carried out at 10 MW in Budapest Neutron Center (BNC). During the analysis we focused on the determination of the major components (Ge, Sb, Te, As, S) and on some characteristic trace elements.

The major components of each sample were determined with a statistical error of 2-3 %. The results obtained correspond completely to the expected compositions with x=10, 20, 27 within the measurements error. The results are summarized in table 1.

Table 1. The elemental composition in atomic percent of the synthesized chalcogenide glasses determined by PGAA.

Expected composition Measured composition Ge₁₀Sb₃₀S₅₀Te₁₀ Ge_9.4Sb_29.7S_50.2Te_10.7 Ge₂₀Sb20S₅₀Te₁₀ Ge_19.4Sb_19.5S_50.6Te_10.4 Ge₂₇Sb₁₃S₅₀Te₁₀ Ge_25.4Sb_13.3S_51.1Te_10.2

Ge₂₇Sb₁₃S₅₅Te₅ Ge_25.2Sb_13.1S_56.4Te_5.3

The neutron diffraction measurements performed using a PSD diffractometer (λ=1.069Å) in BNC revealed that the glasses synthesized of 4-component Ge_xSb_40-xS_yTe_60-y (x=10, 20, 27 and y=50 and 55 at x=27) were fully amorphous, as evidenced the lack of characteristic Bragg peaks of a crystalline phase and the wavy curves of the structural factor S(Q). A detailed interpretation of the PSD data is in progress.

The films were prepared by vacuum evaporation of the powdered parent glassy material from molybdenum boats on the glass substrates at room temperature. The deposition rate was about 7 nm/s, measured using a quartz microbalance. The thickness of the films was around 1 μm.

The ellipsometric measurements were performed in the spectral range 400-820 nm on a Rudolf manual ellipsometer with polarizer-compensator-sample-analyzer configuration. The accuracy of the polarizer, analyzer and incidence angles was within ±0.01°. A mini-monochromator with a holographic grating was utilized. Using ~2 mm thick glass substrates and an angle of light incidence of 50^o the reflection from the substrate back surface was successfully eliminated. The optical constants, refractive index, n, extinction coefficient, k, absorption coefficient, α=4πk/λ) and optical band gap energy, Eog), of the films were evaluated from the SE data analysis. The considerably large thickness and strong absorption of the films allowed us to calculate the optical constants considering the films as a bulk material. However, in the longer wavelength region (above 600-700 nm depending on the film composition) interference fringes were observed which suggested that within this spectral range the substrate has contribution to the optical response and, therefore, the optical constants calculated above

~600 nm should be considered with caution.

3. Results and discussion

The dispersion curves of the refractive index and extinction coefficient for the compositions studied are presented in figure 1. For both the n and k values, a tendency to decrease as the Ge content increases and, correspondingly, the Sb content decreases is observed. A further decrease of the n and k values occurs when the Te content decreases from 10 to 5 percent. The variation of the refractive index with the film composition can be explained by the different polarizability of the elements. The

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Sb cations have larger polarizability than the Ge cations, while the S atoms, being smaller than the Te atoms, have lower polarizability [13]. All this contributes to the slight drop observed of the n values with the increase of the Ge content and the decrease of the Te content.

400 500 600 700 800

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5

n

n, k

Wavelength (nm)

k

Figure 1. Spectral dependences of the refractive index (n) and extinction coefficient (k) of chalcogenide films with given composition, as follows:

□ - Ge10Sb30S50Te10

∇ - Ge20Sb20S50Te10

○ - Ge27Sb13S50Te10

☆ - Ge₂₇Sb₁₃S₅₅Te_5.

The absorption coefficient α = 4πk/λ was calculated from the k values, presented in figure 2. Using the Tauc’s expression [14] and assuming an indirect type electron transition, the energy band gap (Eog) was determined by building the plots (αhν)^1/2 versus hν and extrapolating the linear part of the curves toward zero absorption; the interception with the photon energy (hν) axis provides the E_og value. The procedure is presented in figure 2 and the E_og values obtained are summarized in table 2.

As it is seen from figures 1 and 2, the absorption decreases and the optical absorption edge moves toward the higher energies (blue shift) as the Ge increases and the Te content decreases at constant values of the S/Te and Ge/Sb ratio, respectively. The absorption tails may arise due to structural defects, such as sulphur vacancies and atom displacements that create trap states within the Ge-Sb-S-Te band gap.

The optical band gap increases with the Ge content, while a decrease of the Te fraction from 10 to 5 results only in a small change within the extrapolation error (table 2). Since the optical absorption depends on the short-range order and the defects associated with it, the increase in the Eog values can

0,8 1,2 1,6 2,0 2,4 2,8 3,2 0

500 1000

1500 Ge10Sb30S50Te10 Ge20Sb20S50Te10 Ge27Sb13S50Te10

Ge27Sb13S55Te5

(α h ν )

1/2

(cm

-1

eV )

1/2

Photon energy (eV)

Figure 2. Plots (αhν)^1/2 vs. photon energy (hν) for Ge-Sb-S-Te films with different compositions.

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Table 2. Optical band gap energy (E_og) of Ge-Sb-S-Te films with different compositions, determined with an accuracy of ±0.05 eV.

Film composition Optical band gap energy (eV) Ge10Sb30S50Te10 1.04

Ge20Sb20S50Te10 1.14 Ge27Sb13S50Te10 1.33 Ge27Sb13S55Te5 1.38

be explained on the basis of the “density of states” model proposed by Davis and Mott [15]. According to this model, the width of the localized states near the mobility edges depends on the degree of disorder and defects present in the amorphous state. In chalcogenide glasses containing a high concentration of group VI elements (Te and S in our case) the lone-pair electrons form the top of the valence band (bonding band) and the antibonding band forms the conduction band [16].

In general, a high concentration of localized states in the band structure leads to low band gap energy. There exist in the films a large number of unsaturated bonds due to the insufficient number of atoms that produce localized states. Therefore, the increase of the optical band gap with the Ge content up to x=27, keeping the S and Te contents constant, can be attributed to the decrease of the width of localized states near the mobility edges due to the lesser degree of disorder and less defects in films. In the case of Ge_xSb_40-xS₅₀Te₁₀ compositions, the decreasing amount of weaker Sb-S bonds at the expense of the increasing amount of stronger Ge-S bonds leads to a narrowing of the conduction band tail, whereas the valence band tail remains the same, since it depends mainly on the amount of chalcogenide atoms. As a result the optical band gap energy increases with increasing the amount of Ge-S bonds.

As the Te content is decreased, the reduction of the number of the much weaker Ge-Te and Sb-Te bonds [17] leads to negligible changes of the average stabilization energy of chemical bonds and, therefore, the E_og value remains almost constant, as observed.

4. Conclusion

It is shown that the optical constants of Ge-Sb-S-Te films depend on the Ge content rather than Te content for the studied compositions. The changes observed are explained on the basis of chemical bond approach and the “density of states” model in amorphous chalcogenide solids.

Acknowledgments

The authors acknowledge the financial support from EU 6FP:MNI3 under contracts BRR-113 and BRR-91.

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