Correlation between Structural and Physical Properties in Ge–Sb–Se

Article pubs.acs.org/JPCC

Correlation between Structural and Physical Properties in Ge−Sb−Se Glasses Wen-Hou Wei,†,‡ Rong-Ping Wang,*,† Xiang Shen,§ Liang Fang,‡ and Barry Luther-Davies† †

Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), Laser Physics Centre, Research School of Physics and Engineering, Australian National University, Canberra ACT 0200, Australia ‡ Department of Applied Physics, Chongqing University, Chongqing 401331, P. R. China § College of Information Science and Engineering, Ningbo University, Ningbo 315211, P. R. China ABSTRACT: Three groups of Ge−Sb−Se glasses with compositions GexSb10Se90−x, GexSb15Se85−x, and GexSb20Se80−x have been systematically studied with the aim of understanding the role of chemical composition and mean coordination number (MCN) in determining their structural and physical properties. For each group of glasses, it was found that the optical bandgap increases and the refractive index decreases with increasing Ge concentration up to a transition point which corresponds to glasses that have chemically stoichiometric compositions. Raman spectra were measured and decomposed into different structural units. While the relative number of the heteropolar bonds changes in a reasonable manner with chemical composition, the evolution of the optical bandgap and refractive index correlated closely with the number of the homopolar bonds, suggesting that the band-tails formed by homopolar bonds could reduce the optical bandgap. On the other hand, the transitions at the chemically stoichiometric compositions could be attributed to “demixing” of networks above the chemical thresholds. These transition thresholds in the three groups of glasses demonstrated that the chemical composition has significant effects on the physical properties in the Ge−Sb−Se system. underconstrained “floppy” network to an overconstrained “rigid” phase. Tanaka7 envisaged that a rigidity transition existed at MCN = 2.67 due to a topological change from a 2-D to 3-D “stressed-rigid” phase. Another convincing version of the origin of the transition at stoichiometric composition is due to the existence of network demixing or nanoscale phase separation effects above the chemical threshold.8−12 The fact that the physical properties are predominately determined by MCN is easily understood for binary chalcogenide glasses, since MCN has a one to one correspondence with chemical composition. However, the situation is different for ternary chalcogenide glasses, for example, GexAs(Sb)ySe1−x−y ternary glasses where one can make two groups of glasses with the same MCN, one group being Se-rich and the other Se-poor. From the structural viewpoint, the glasses with the same MCN but different compositions will certainly exhibit significantly different local bonding coordination which should affect their physical, electronic, and optical properties. Therefore, the extent to which the concept of MCN can be used to explain the change of physical properties in ternary glass system is still debatable. To answer this question experimentally, it is essential to design the glass compositions to distinguish the different effects of MCN from chemical composition. In particular, it has been found that the effect of chemical composition on the structure

1. INTRODUCTION Chalcogenide glasses contain the chalcogen elements (S, Se, and Te) as a major constituent covalently bonded with other network forming elements such as Si, Ge, As, and Sb. They are promising for photonics because they have attractive optical properties which include high linear and nonlinear refractive indices, high photosensitivity, and exceptional transmission range spanning from the visible to the mid-infrared. Over the past decade, various applications have been developed including chalcogenide planar waveguides for high speed all-optical processing of telecommunications signals1−3 and chalcogenide optical fibers and waveguides for chemical sensors.4 Generally, chalcogenides have a large glass-forming region, and thus their physical properties can be easily tuned via chemical composition. At a fundamental level, it is the detailed chemical bonding combined with the topology of the glass network that determine the physical properties of the glasses. Therefore, it is essential to understand the structure of these glasses in order to seek the glass compositions with the best performance for the applications in photonics. For glasses consisting of a network of covalent bonds obeying the 8 − N rule, Phillips5 and Thorpe6 developed a constraint-counting theory based on the balance between the constraints on the bond length/angle and the total number of degrees of freedom available to a mole of threedimensionally connected atoms. They predicted the existence of a threshold transition at a mean coordination number of 2.4 (MCN, which is the sum of the products of the individual atomic abundance times the valency of the constituent atoms, irrespective of their actual chemical composition) from an © 2013 American Chemical Society

Received: April 22, 2013 Revised: July 18, 2013 Published: July 22, 2013 16571

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the areas do not indicate the absolute concentrations, but only indicate the relative change in amount of each structural unit as the glass composition is changed. Furthermore, we emphasize that although we decomposed the broad bands at high wavenumber in Se-poor samples into several Gaussian components, it is quite conceivable that a larger number of vibrational modes in fact contribute to the overall spectrum. However, due to the broad nature of these amorphous vibrational peaks, only a negligible change in the position of the vibrational peaks occurs as one element is replaced by another, and in most cases such substitution only broadens the bands in the amorphous phase. Therefore, it is almost impossible to decompose the spectra with any meaning into a larger number of Gaussian components.

and physical properties of glasses can be obscured if the elements comprising the glasses have similar atomic radius and mass as is the case for Ge−As−Se. However, chemical effects can be enhanced if As is replaced by Sb due to the larger mass difference between Sb and (Ge, Se). Therefore, in the paper we investigated the structural and physical properties in Ge−Sb−Se glasses. To distinguish the different role of chemical composition and MCN, we prepared three groups of the Ge−Sb−Se glasses with a fixed Sb concentration of 10, 15, and 20 at. %, respectively, and measured their optical bandgap, refractive index, and Raman spectra. We found a correlation between the bandgap, refractive index, and the number of the homopolar bonds in the glasses.

2. EXPERIMENTS 2.1. Sample Preparation and Characterization. Ge−Sb− Se bulk glasses were prepared from high-purity elements using the conventional melt quenching technique. High-purity (5N) germanium, antimony, and selenium metals used as starting materials were weighed inside a dry nitrogen glovebox and loaded into a precleaned quartz ampule. The loaded ampule was dried under vacuum (10−6 Torr) at 110 °C for 4 h to remove surface moisture from the raw materials. The ampule was then sealed under vacuum using an oxygen−hydrogen torch and introduced into a rocking furnace to melt the contents at 900 °C for a period not less than 30 h. Before being air quenched, the ampule was removed from the rocking furnace at a typical temperature around 600−700 °C depending on the actual glass composition. The resulting glass boule was subsequently annealed at a temperature 30 °C below the glass transition temperature Tg and then slowly cooled to room temperature. Following the annealing process, the glass boules were sectioned to form disks of 10 mm diameter and approximately 1 mm thick. The disks had their opposite surfaces ground plane parallel and then polished to optical quality. The quality of the glasses was examined using X-ray diffraction and infrared microscopy. XRD patterns indicated the glass structure was amorphous, and IR microscopy revealed the absence of any observable bubbles for all of our glasses. The refractive indices of bulk Ge−Sb−Se glasses were measured using a Metricon Model 2010 prism coupler. The absorption spectra were recorded using a Varian Cary 5000 UV−Vis−NIR spectrophotometer in the wavelength range from 400 to 1500 nm. The optical gap was estimated at α ≈ 103 cm−1. Raman spectra were recorded using a T64000 HORIBA Jobin Yvon micro-Raman spectrometer excited by an 830 nm laser line. The excitation power was kept as small as possible to avoid any photoinduced effects. The resolution of the spectrometer was about 0.5 cm−1. 2.2. Raman Peak Fitting and Analysis. To understand the detailed structural evolution, it is prerequisite that each Raman spectrum should be decomposed into the different structural units. We used PeakFit software and an iterative method simulating the peak center, width, and height of each Raman vibrational peak. The criterions for the best fit were (1) the fit to the whole experimental spectra should lead to the maximum value of r2, representing the goodness of the fit, and (2) the evolution of each structural unit should be physically reasonable with the varying chemical compositions. For example, the Ge−Se content should increase while Se−Se content should decrease with increasing Ge concentration until all the Se atoms in the glasses are consumed. These constraints allowed the software to extract the evolution of the integrated area of particular Raman vibrations in a reasonable manner. It should be pointed out that

3. RESULTS AND DISCUSSION Figure 1 shows the plot of the optical bandgap as a function of chemical composition. For the glasses with the same Ge

Figure 1. Optical bandgap as a function of chemical composition in Ge− Sb−Se glasses. The dotted lines through the data points are guides to the eye.

concentrations, the glasses with lower Sb concentration have higher optical bandgap compared with those with higher Sb concentration. It is evident that a “threshold” behavior can only be found when the glasses with the same Sb concentrations are compared. The trend in three groups of the glasses with the Sb contents of 10, 15, and 20 at. % is the same: the optical bandgap increases with increasing Ge content before it reaches a maximum and then decreases with further increasing Ge content. The maximum bandgap in each group of the glasses occurs at different Ge concentrations but each corresponds to the chemically stoichiometric composition. Figure 2 shows the refractive index of the glasses at 1.55 μm as a function of Ge concentration. It was found that in glasses with

Figure 2. Refractive index at 1.55 μm as a function of Ge concentration in Ge−Sb−Se glasses. 16572

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the same Ge content, those with lower Sb concentrations had lower refractive index compared with those with higher Sb concentration but the same Ge content. When the refractive index of glasses with the same Sb concentration was compared, the refractive index decreased before it reached a minimum and then increased with further increase in Ge concentration. Again, the transition from decreasing to increasing index with Ge content occurs at the chemically stoichiometric compositions. Comparing our data on the optical bandgap with those in refs 13 and 14, the value in our paper is ∼0.1 eV smaller because we estimated the bandgap from the 103 cm−1 absorption level of 1 mm thick samples rather than from a Tauc plot of a thin sample. Nevertheless, this has little effect on the way the bandgap evolves as a function of the Ge content shown in Figure 1 since we measured the bandgap of glasses with the same thickness. Moreover, the correlation of the bandgap and refractive index is in excellent agreement with the Moss rule15 where the value of Egn4 should be a constant. In our case the value of Egn4 is 73 ± 0.5, 81 ± 0.5, and 89 ± 0.5 eV for the three groups of the glasses with Sb concentration of 10, 15, and 20%, respectively. We measured Raman spectra of the glasses in order to understand their structure. The reduced Raman spectra and some typical peak-fitting results of GexSb10Se90−x, GexSb15Se85−x, and GexSb20Se80−x are shown in Figures 3 and 4, respectively. The general features can be summarized as followings: (1) The peak at around 203 cm−1 and its shoulder at 218 cm−1 can be attributed to the heteropolar Ge−Se bonds in the corner-sharing and the edge-sharing GeSe4/2 tetrahedra,16 respectively. (2) The Raman peak at 256 cm−1 in Se-rich glasses is due to the vibration of Se chains or Se rings,17 and this gradually shifts toward higher wavenumber with increasing Ge concentration, furthermore, above the chemically stoichiometric composition, which evolves into a broad band extending from 235 to 330 cm−1, which contains contributions from the stretching vibration of Ge−Ge bonds in modified Se3Ge−(GeSe2)0.1−GeSe3 and in Ge− GemSe4−m (m = 1, 2, 3, 4) units18 due to shortage of Se in the glasses. (3) The vibrations related to dominant homopolar bonds in the band between 150 and 175 cm−1 are absent in glasses with low Ge concentration but obviously appear in the glasses with Ge concentration more than 27.5, 22.5, and 20% respectively in Figures 3a, 3b, and 3c. This is probably due to the presence of Ge−Ge and Sb−Sb vibrations at 17016,19,20 and 160 cm−1, respectively. (4) The decomposed peak at 195 cm−1 is ascribed to the vibration of Sb−Se bonds in SbSe3/2 pyramids,21 although the peak is not clearly shown in the Raman spectra due to its overlapping with the Ge−Se vibrational bands. The decomposed relative contribution of each structural unit is plotted in Figure 5, where the transition-like features again can be found. For example, both corner-sharing and edge-sharing GeSe4/2 vibrations exhibit a maximum relative concentration around the chemically stoichiometric compositions. In contrast, the contribution of the SbSe3/2 pyramids remains constant with increasing Ge content and then begins to decrease at the chemically stoichiometric compositions. The number of homopolar bonds like Ge−Ge and Sb−Sb is zero in the glasses with low Ge concentration and then begins to appear near the chemically stoichiometric compositions and to increase with increasing Ge concentration, while the number of Se−Se bonds decrease and becomes close to zero beyond the chemically stoichiometric compositions. These evolutions are generally in agreement with the expectations for ternary glasses. Starting from the glasses with low Ge concentrations, Se-rich glasses possess ideal GeSe4/2 and

Figure 3. Reduced Raman spectra of the glasses with different chemical compositions: (a) Ge x Sb 10Se 90−x, (b) Ge xSb 15Se 85−x, and (c) GexSb20Se80−x.

Figure 4. Typical decomposed Raman spectra of GexSb10Se90−x glasses.

SbSe3/2 structural units, and the edge-sharing and corner-sharing units increase with increasing Ge concentration while SbSe3/2 units remain constant until the glasses become stoichiometric 16573

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Figure 6. Raman spectra of two groups of samples with the same Ge concentration at 22.5 and 25%.

bonds in heavily Se-poor GeSbSe glasses. It should be noted that Ge−Sb vibrations were excluded in the simulations since they are located between the Ge−Ge (170 cm−1) and Sb−Sb (160 cm−1) vibrations, and it would not be meaningful to attempt to fit three peaks in such a narrow wavenumber range. A remarkable feature extracted from the curve fitting is the evolution of the ratio of the integrated areas representing the total number of defective bonds to that of the whole Raman spectra as shown in Figure 7. The three groups of data have the

Figure 5. Relative number of the bonds derived from the simulation of each Raman spectrum.

where all the excess Se has been consumed. With further increase of Ge content, homopolar bonds such as Ge−Ge and Sb−Sb can be formed. Consequently, the intensity of 170 cm−1 band and the broad band between 235 and 330 cm−1 increase as shown in Figure 3 since the Se becomes deficient. By comparing their respective evolutions in Figure 5, it is also evident that with increasing Ge concentration, Ge−Ge bonds are formed before Sb−Sb bonds. In addition, a distinct difference can be found in the concentrations of Ge−Ge and Sb−Sb homopolar bonds in extremely Se-poor glasses. The number of Ge−Ge bonds appears to saturate at a certain value while that of Sb−Sb increases with increasing Sb concentration. To clearly confirm this, we plotted in Figure 6 the Raman spectra of two groups of samples with the same Ge concentration at 22.5 and 25%, respectively. From the bottom, Ge22.5Sb10Se67.5 glass is Se-rich, and no Ge−Ge and Sb−Sb vibrations can be found. However, on increasing the Sb concentration to 15%, a weak shoulder appears at 170 cm−1 due to Ge−Ge vibrations. A further increase of Sb concentration to 20% induces vibrations at 160 cm−1 that are due to Sb−Sb bonds. The featured position at the maximum intensity of each band indicates that, as more Sb is added into the glasses while Ge is kept constant, Sb is preferentially bonded with Se since the Ge−Ge vibrations are the first to appear. A similar behavior can be found for another group of the glasses with fixed Ge concentration of 25%, and this suggests that such a bondforming trend could be general in Ge−Sb−Se ternary glasses. Previous X-ray photoelectron spectra investigations on Ge− As−Se ternary glasses indicated that Se−Se bonds still exist in heavily Se-poor glasses where Ge−As bonds can be formed.22 However, we find only a negligible contribution from Se−Se

Figure 7. Relative number of the homopolar bonds derived from the simulation of each Raman spectrum.

same trend that the relative ratio of the homopolar bonds first decreases and then increases with increasing Ge content, and the minima appear at stoichiometric composition, i.e., Ge25Sb10Se65, Ge20.83Sb15Se64.17, and Ge16.67Sb20Se63.33. This interprets the variation of the optical bandgap shown in Figure 1. In amorphous material, the existence of defective units forms band-tails below the conduction band or above the valence band;23,24 the presence of more defects will broaden the energy level of the band-tails and narrow the optical bandgap. Regarding the microscopic nature of the transitions at a molecular level, the stochastic agglomeration theory (SAT) suggested that the glass transition temperature (Tg) is a good measure of global connectivity in a glass network.25 Mahadevan et al. reported that Tg increases with increasing Ge content in the Se-rich range until it reaches a maximum at the stoichiometric composition and then sharply decreases with further increase of Ge concentration in Ge−Sb−Se glasses.26 Boolchand et al. investigated the threshold behavior in Tg and suggested that, above the threshold, phase separation can occur in the nano- or micrometer scale; i.e., the Ge−Sb−Se glass network is made up of demixed structural units of GeSe4/2 and SbSe3/2.8,9,27 The 16574

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heteropolar bonds segregated from the main backbone, destroying the connectivity of the glass network and reducing Tg. The maximum Tg values observed in the chemical stoichiometric compositions8,9,26 are strikingly similar to the threshold behaviors that have been observed in the evolution of the optical bandgap in Figure 1 and the refractive index in Figure 2 as well as the concentration of homopolar bonds in Figure 7. This suggests that these threshold behaviors of physical properties in the Ge− Sb−Se system can be traced to “demixing” of networks above the chemical thresholds. The transition thresholds observed at the three chemically stoichiometric compositions of Ge25Sb10Se65, Ge20.83Sb15Se64.17, and Ge16.67Sb20Se63.33 correspond to a MCN of 2.60, 2.57, and 2.54, respectively. In the “demixing” model, the decrease of the MCN threshold with increasing Sb content from 10 to 20% is a direct consequence of the chemical threshold shifting down as 3-fold Sb replaces 4-fold Ge in the backbone. To sum up, we concluded that chemical composition has significant effects on the physical properties in the Ge−Sb−Se system.

REFERENCES

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4. CONCLUSIONS We have prepared several series of GexSb10Se90−x, GexSb15Se85−x, and GexSb20Se80−x glasses and investigated their structure and physical properties systematically. It was found that the evolution of optical bandgap and the refractive index exhibit threshold behavior at the chemically stoichiometric compositions. On the other hand, we measured Raman spectra of the glasses and decomposed them into different structural units. A striking correlation between the evolution of the bandgap, refractive index, and relative number of the homopolar bonds in the glasses was found. The glass with a stoichiometric composition generally has fewer defective bonds, higher optical bandgap, and smaller refractive index. The threshold behaviors of physical properties in the Ge−Sb−Se system could be traced to “demixing” of networks above the chemical thresholds. Therefore, we concluded that chemical composition has significant effects on the physical properties of Ge−Sb−Se glasses.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +61 2 6125 1591; Fax +61 2 6125 0029 (R.-P.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Australian Research Council (ARC) Centre of Excellence for Ultrahigh bandwidth Devices for Optical System (project CE110001018), the ARC Discovery programs (project DP110102753), the National Program on Key Basic Research Project (973 Program) (Grant No. 2012CB722703), the International Science & Technology Cooperation Program of China (Grant No. 2011DFA12040), and the Natural Science Foundation of China (Grant No. 61008041). The financial support from the China Scholarship Council (CSC) is also acknowledged.

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ABBREVIATIONS MCN, mean coordination number; Ge, germanium; Sb, antimony; Se, selenium; 3-D, three-dimensional. 16575

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(24) Vanderbilt, D.; Joannopoulos, J. D. Theory of Defect States in Glassy As2Se3. Phys. Rev. B 1981, 23, 2596−2606. (25) Micoulaut, M.; Naumis, G. G. Glass Transition Temperature Variation, Cross-Linking and Structure in Network Glasses: A Stochastic Approach. Europhys. Lett. 1999, 47, 568−574. (26) Mahadevan, S.; Giridhar, A. Floppy to Rigid Transition and Chemical Ordering in Ge-Sb(as)-Se Glasses. J. Non-Cryst. Solids 1992, 143, 52−58. (27) Boolchand, P.; Chen, P.; Vempati, U. Intermediate Phases, Structural Variance and Network Demixing in Chalcogenides: The Unusual Case of Group V Sulfides. J. Non-Cryst. Solids 2009, 355, 1773− 1785.

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