Open Access
Add to BibSonomy
Abstract
Ge5AsxTe95-x amorphous thin films (x=20∼60) have been deposited by thermal evaporation and the change of their optical parameters—like refractive index and optical bandgap as a function of thermal annealing time—have been studied with an aim to screen the composition of the film with stable optical and thermal properties for applications in optical waveguide devices. The film with a composition around x=38.0 was found to be stable, while the optical band gap and refractive index decreases in the films with x<38.0, and opposite changes can be observed in the film with x>38.0. Further structural characterization showed no any observable changes of the Raman spectra in the as-prepared and annealed Ge5.2As38.0Te56.8 film, confirming the stability of the optical and thermal properties in this composition.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Chalcogenide glasses, consisting of chalcogen S, Se and Te elements that are covalently bonded with glass-forming elements (like Ge, As, Sb, etc.), have excellent transmission from visible to far infrared, high optical nonlinearity that is hundreds or thousands of times than silica, low phonon energy, and thus are finding increasing applications in many areas, such as phase change memory, optical waveguides and infrared optical systems. While S- and Se-based glasses have been widely used in various fields where some of them have been commercially available, it is well known that, the transmission of these glasses are limited to 10 and 15 µm for S- and Se-based glasses, respectively [1,2]. Emerging applications of the glasses require further extension of the transmission range of the glasses to the far-infrared. Generally, the replacement of S and Se by Te can extend the transmission range since Te is heavier than S and Se. However, previous investigations have indicated that, Ge-Te binary glasses have a narrow glass-forming region with Ge content from 10% to 25% [3], therefore the unique feature of the chalcogenide glasses with tunable properties via changeable compositions is greatly suppressed. One of the solutions is to add As into Ge-Te system and extend its glass-forming region into the compositions of Ge from 0 to 20% and As from 10% to 80% [4]. Our recent results have demonstrated that, while glass-forming region is greatly extended, the third order optical nonlinearity can also be tuned in a large range [5].
Our on-going effort is to prepare chalcogenide-based planar waveguide devices for various applications like sensing, supercontinuum generation or astrometry [6–8]. Obviously high quality of the materials with the formation of thin films are essential for such applications. For the films created under thermodynamically non-equilibrium conditions like vacuum evaporation, they generally exhibit more defective bonds compared with their bulk counterparts that are prepared under thermodynamically equilibrium conditions like melt-quenching [9,10]. Moreover, due to amorphous nature of the films, they usually exhibit strong structural relaxation upon external energy input like thermal, optical and ion irradiation [11,12]. Therefore, screening high quality Te-based film with stable structural and optical properties is perquisite to prepare high quality optical waveguide devices, but this has seldom been explored systematically before.
Previously, K. A. Aly et.al. reported to deposit a series of Ge-As-Te films with different compositions. It was found that as the Ge content increases, the optical band gap (Eg) increases and the refractive index (n) decreases [13]. P. Hawlova et.al. reported influence of laser irradiation on the optical properties of Ge-As-Te films, where a minimum change of band gap can be found in Ge20As20Te60 thin films, and thus considered photostable [14]. However, the influence of different annealing times on the optical properties of thin films has not been studied yet.
In this paper, Ge5AsxTe95-x thin films with different compositions were prepared by thermal evaporation. Ge content was fixed at around 5% in all cases and thus making it much easier to see the effect of the change of As content on physical properties of the films. We annealed the films below its respective glass transition temperature for different durations and then investigated their optical properties like linear refractive index and optical bandgap. The results showed that, while the films generally exhibit the change of the optical parameters against thermal annealing, relative stable composition exists in Ge5.2As38.0Te56.8. Such composition can be expected to be used in Te-based optical waveguides for the applications in the far-infrared.
2. Experiments
GeAsTe glasses with different compositions were prepared by the conventional melting-quench technique. High purity Ge, As and Te elements were used as starting materials. After they were weighted according to the required proportion, they were put into a pre-cleaned quartz tube that would be subsequently evacuated to 10−4 Pa and then sealed. Then they were introduced into a rocking furnace homogenizing for no less than 12 h at 900 °C, and eventually quenched by cold water. In order to remove the inner stress, the glass boules were subsequently annealed at 20 °C below the glass transition temperature Tg for 2 h and then slowly cooled at a rate of 10 °C /h to room temperature.
The as-prepared glasses were broken and put into an evaporation boat for thermal evaporation. The chamber was evacuated to 6.6×10−4 Pa, and the evaporation temperature was raised to 280 °C, and then the films with different compositions were prepared on thermal oxidation silicon and quartz wafers. Before the experiments, the substrates were cleaned by deionized water and ethanol solution using ultrasonic cleaner. The space between the source and the wafers is 40 cm. The evaporation rate was fixed at 2 Å/s, and the thickness of the film around 500 nm was in-situ monitored by the quartz oscillator and ex-situ measured by a probe-type surface profiler. Energy dispersive x-ray spectrometer (EDX) equipped in a scanning electron microscope was used to measure the composition of the film over several different positions on the surface.
The as-deposited films were annealed at vacuum around 10−3 Pa. Generally, the temperature was slowly raised to a point that is 20 °C below its respective Tg for different durations. The films were annealed with different durations, and the possible crystallization was examined by X-ray diffraction (XRD) with a scanning angle 2θ range from 10° to 60°. The transmission and absorption spectra of the films were measured using a 950UV-type spectrophotometer at a range from 400 to 2500 nm. The films thickness and refractive index were measured by infrared variable angle spectroscopic ellipsometry (IR-VASE).
3. Results and discussion
It is challenging to copy the target composition into the final film using thermal evaporation, especially for ternary or quaternary chalcogenide glasses [9]. In our experiments, we found that the film composition can be tuned via the change of the evaporation rates. A general tendency is that, high evaporation rate causes the loss of Te and the increase of As. However, it is impossible to copy Ge content from the target to the films exactly. Therefore, in our experiments, we carefully control the evaporation rate and finally get a group of materials with almost identical composition of Ge at around 5%, and varied As content from 20% to 60%. For the bulk Ge-As-Te glasses, previous investigation has shown that the glasses can be formed in a compositional range with Ge content up to 20% and As content up to 80% [4]. Therefore, the film composition used in the paper is well located within such a glass-forming region. On the other hand, it is well known that, both the change of the four-coordinated Ge and three-coordinated As can alter the glass structure and thus the physical properties of glasses [1,2]. Since variation range of Ge content is relatively narrow in the glass-forming region, the change of As content enables the physical properties to be tuned in a large range. Moreover, the monotonous change of the As content can distinguish the different role of the topological and chemical order in determining physical properties of the glasses.
To determine thermal annealing temperature of the film, we measured glass transition temperature Tg of both the film and bulk glass with the same compositions using a flash differential scanning calorimetry (Mettler-Toledo Flash DSC 1) where only tens of nanograms of materials are needed. Therefore, it is possible to just scratch pieces of the films for Tg measurement. We found the difference of Tg between the bulk and film is less than 5 µC. Therefore, we measured Tg values of the bulk glasses with the same compositions of the films using a conventional DSC (DSC 2010 TA). Table 1 lists the target and film composition, and Tg of the bulk glasses corresponding to the Ge-As-Te films investigated in the paper.as listed in Table 1. We annealed the films at a temperature that is 20 µC below their respective Tg [15].
Table 1. The target and film composition, and the Tg of the bulk glasses corresponding to the Ge-As-Te films.
Figure 1 shows the typical XRD profiles of Ge5AsxTe95-x films annealed at 20 °C below Tg with an annealing time of 35 h. It can be seen that, the films are always amorphous without any crystallization features.
The optical band gap of the film can be derived from the Tauc plots,
where K is a constant related to the transition probability, Eg is the optical band gap, and hν is the photon energy [16].Figure 2(a) is Eg of the as-prepared and annealed films with different compositions. It can be found that, Eg increases with increasing annealing time for the films with As content more than 38.0%, while it decreases for the films with As content less than 38.0% until it reaches a stable value with prolonged annealing time. However, it exhibits almost constant for the films with As content around 38.0%.
Fig. 2. (a) Annealing time dependent optical band gap of Ge5AsxTe95-x films. (b) The relationship between As content and band gap of Ge5AsxTe95-x films.
Figure 2(b) shows the relations between As content and optical band gap of the as-deposited films. As we see that, Eg increases with increasing As content for the as-deposited films. This is due to the decrease of the number of the lone electron pairs with the increase of As content in the film, resulting in a decrease in the molecular orbital at the top of the valence band and a decrease in the band gap [17].
The refractive indices of all the films were determined by variable angle spectroscopic ellipsometry (VASE) [18]. The Cody-Lorentz model was used for the VASE data analysis where the refractive index is achieved with the lowest fitting values of mean square error (MSE) typically less than 2.
The evolution of the refractive index for the films (at 3.0 µm) with different annealing time is shown in Fig. 3(a). It is found that, the refractive increases for the films with As content more than 38.0 %, while it decreases for the films with As content less than 38.0 % until it reaches a stable value with increasing annealing time. However, it is almost constant for the films with As content around 38.0 %. Such behaviors of the changes are same for the refractive index at any wavelength, and this can be clearly seen in Fig. 3(b), where the dispersion curves of the refractive index in the films with three typical compositions are plotted, and the refractive index remains almost unchanged for Ge5.2As38.0Te56.8 film before and after thermal annealing at any wavelength.
Fig. 3. (a) Annealing time dependence of refractive index at 3.0 µm for all Ge5AsxTe95-x films. (b) Refractive index dispersion of the films with three typical compositions before and after thermal annealing. (c) The relationship between As content and refractive index of Ge5AsxTe95-x films.
Figure 3(c) is the change of the as-deposited film with As content. It can be seen from the figure that as the As content in the film increases, the refractive index decreases. According to the Lorentz–Lorenz relationship [19],
The thickness of the film is also affected by thermal annealing. The film thickness ratio is defined as $\varDelta d/d = ({d_i} - {d_a})/{d_i}$, where di and da is the thickness of initial film and annealed films, respectively. As shown in Fig. 4, the film thickness increases for the films with As content less than 38.0 % while it has a significant decrease for the films with As content more than 38.0 %. However, it has a slight fluctuation for the films with As content around 38.0 %. It is well known that, thermal annealing can induce the change of the material density [20]. The decrease in density is reflected by the increase in film thickness, leading to a decrease of the refractive index.
We further measured Raman spectra of the as-deposited and annealed films (35 h) in order to understand the structural origin of the change of the optical properties. Raman spectra of three typical films before and after thermal annealing were shown in Fig. 5. The Raman peak at 122 cm-1 is attributed to the symmetric stretching of the GeTe4 tetrahedron, symmetric bending of AsTe3 pyramid, or antisymmetric stretching of Te3 pyramid. The peak at 150-155 cm-1 is attributed to symmetric stretching of Te3 pyramid, and the peak around 190 cm-1 is attributed to the symmetrically extended As3 pyramid structure [21,22].
In Fig. 5(a), a Raman peak at 122 cm-1 is considerable weak in the as-deposited film but becomes strong in the annealing film, representing the generation of more GeTe4 or AsTe3 upon thermal annealing. Moreover, a weak Raman feature at 141 cm-1 corresponds to the vibration of Ge-Te in the annealed film [22], which is reasonable since the as-deposited films contain a lot of defective bonds, and thermal annealing can break the homopolar Ge-Ge and Te-Te bonds and induce the rebonding of Ge-Te bonds.
In Fig. 5(c), we observed similar behaviors, e.g., the appearance of Raman features at 122 cm-1 and 141 cm-1 upon thermal annealing. However, the physical origin is different since Ge5.2As60.1Te34.7 is Te-poor, and thus phase separation could be easily induced upon thermal annealing, and similar phenomenon has been observed in Se-poor Ge33As12Se55 film where the evolution of the chemical bonds clearly demonstrated the existence of the phase separation [23].
However, the Raman features remains almost unchanged before and after thermal annealing of Ge5.2As38.0Te56.8 film as shown in Fig. 5(b). It proves that the film structure is stable in Ge5.2As38.0Te56.8, which is consistent with the results in Figs. 3 and 4.
Figure 6 shows the relations between As content and absolute change ratio in optical band gap |ΔEg|, refractive index (at 3.0 µm) |Δn|, thickness |Δd|. The change of all three parameters is less than 0.05. It is interesting to compare such changes with those in S- and Se-based glasses, but unfortunately it is hard to find the S- or Se-based films with the same compositions. Especially extremely small glass forming region in Ge-As-Te glasses limits Ge content at a range less than 20%. Therefore, it is practical to compare the change of these parameters in the film with the similar chalcogen content. It was reported that, Ge7.0As30.3Se62.7 film has a |ΔEg| of 0.012 eV and |Δn| of 0.0597 [9], both of which are less than the data derived from Fig. 6 for Ge5As30Te65 film. On the other hand, Ge30.8As5.7S63.4 has a |ΔEg| of 0.41 eV and |Δn| of 0.2 [24], both of which are far larger than GeAsTe or GeAsSe film with chalcogen content around 60%. Moreover, it can be seen that the thermal-induced change of the optical parameters is the largest in S-based film and the smallest in Te-based film. The same conclusion has been drawn in photo-induced effect in S-, Se- and Te-based glasses [1]. Furthermore, we see from Fig. 6 that, the film with As content of 38.0% (Ge5.2As38.0Te56.8) has the smallest change ratios in optical band gap, refractive index, as well as the thickness, and thus are potential for the applications in mid-and far-infrared photonics.
4. Conclusion
In this work, Ge5AsxTe95-x films were prepared using thermal evaporation, and the effect of thermal annealing on the optical properties of the films was studied. With prolonged annealing time, the band gap decreases for the films with As content less than 38.0%, while it increases for the films with As content more than 38.0% and exhibits almost constant for the films with As content around 38.0%. In addition, the refractive index also shows the same trend as the annealing time increases. The results show that, the optical band gap, refractive index, and thickness in Ge5.2As38.0Te56.8 exhibit the lowest changes with the increase of the annealing time. Further comparison of Raman spectra of the-as-prepared and annealed Ge5.2As38.0Te56.8 film shows no changes of any structural features, indicating that there is no any structural transformation upon thermal annealing. Therefore, Ge5.2As38.0Te56.8 with the best thermal stability could be useful in the mid- and far-infrared optical devices.
Funding
National Natural Science Foundation of China (61775109, 61775111, 61904091); Ningbo University (K. C. Wong Magna Fund); 3315 Innovation Team in Ningbo City; Natural Science Foundation of Ningbo (2019A610065); Natural Science Foundation of Zhejiang Province (LR18E010002).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. K. Tanaka and K. Shimakawa, Amorphous Chalcogenide Semiconductors and Related Materials (Springer, 2011).
2. R. P. Wang, Amorphous Chalcogenides: Advances and Applications (Pan Stanford Publishing, 2014).
3. Z. U. Borisova, Glassy Semiconductor (Plenum Press, 1981).
4. H. Pan, Z. Yang, Y. Chen, R. Wang, and X. Shen, “X-ray photoelectron spectra of Ge-As-Te glasses,” AIP Adv. 8(7), 075208 (2018). [CrossRef]
5. Q. Li, R. P. Wang, F. Xu, X. S. Wang, Z. Yang, and X. Gai, “Third-order nonlinear optical properties of Ge-As-Te chalcogenide glasses in mid-infrared,” Opt. Mater. Express 10(6), 1413–1420 (2020). [CrossRef]
6. Y. Yu, X. Gai, P. Ma, K. Vu, Z. Y. Yang, R. P. Wang, D.-Y. Choi, S. Madden, and B. Luther-Davies, “Experimental Demonstration of Linearly Polarized 2-10 µm Supercontinuum Generation in a Chalcogenide Rib Waveguide,” Opt. Lett. 41(5), 958–961 (2016). [CrossRef]
7. A. Arriola, S. Gross, M. Ams, T. Gretzinger, D. Le Coq, R. P. Wang, H. Ebendorff-Heidepriem, J. Sanghera, S. Bayya, L. B. Shaw, M. Ireland, P. Tuthill, and M. Withford, “Mid-infrared astrophotonics: study of ultrafast laser induced index change in compatible materials,” Opt. Mater. Express 7(3), 698–711 (2017). [CrossRef]
8. P. Ma, D.-Y. Choi, Y. Yu, Z. Y. Yang, K. Vu, T. Nguyen, A. Mitchell, B. Luther-Davies, and S. Madden, “High Q factor chalcogenide ring resonators for cavity-enhanced MIR spectroscopic sensing,” Opt. Express 23(15), 19969–19979 (2015). [CrossRef]
9. D. A. P. Bulla, R. P. Wang, A. Prasad, A. V. Rode, S. J. Madden, and B. Luther-Davies, “On the properties and stability of thermally evaporated Ge-As-Se thin films,” Appl. Phys. A 96(3), 615–625 (2009). [CrossRef]
10. X. Q. Su, R. P. Wang, and B. Luther-Davies, “The dependence of photosensitivity on composition for thin films of GexAsySe1-x-y chalcogenide glasse,” Appl. Phys. A 113(3), 575–581 (2013). [CrossRef]
11. R. P. Wang, A. V. Rode, S. J. Madden, C. J. Zha, R. A. Javis, and B. Luther-Davies, “Structural relaxation and optical properties in amorphous Ge33As12Se55 films,” J. Non-Cryst. Solids 353(8-10), 950–952 (2007). [CrossRef]
12. R. P. Wang, A. V. Rode, C. J. Zha, S. J. Madden, and B. Luther-Davies, “Annealing induced phase transformation in amorphous As2S3 films,” J. Appl. Phys. 100(6), 063524 (2006). [CrossRef]
13. K. A. Aly, A. M. Abd Elnaeim, and M. A. M. Uosif, “Optical properties of Ge–As–Te thin films,” Phys. B 406(22), 4227–4232 (2011). [CrossRef]
14. P. Hawlova, F. Verger, V. Nazabal, R. Boidin, and P. Nemec, “Test-photostability of pulsed laser deposited amorphous thin films from Ge-As-Te system,” Sci. Rep. 5(1), 9310 (2015). [CrossRef]
15. Z. Zhang, S. Xu, Y. M. Chen, X. Shen, and R. P. Wang, “Photo-induced effects in Ge-As-Se films in various states,” Opt. Mater. Express 10(2), 540–548 (2020). [CrossRef]
16. P. K. Singh and D. K. Dwivedi, “Impact of Heat Treatment on The Structural and Optical Properties of Ge4Se60Te30In6 Phase Change Thin Film,” Mater. Today: Proc. 17(Part 1), 118–123 (2019). [CrossRef]
17. M. Kastner, “Bonding Bands, Lone-Pair Bands, and Impurity States in Chalcogenide Semiconductors,” Phys. Rev. Lett. 28(6), 355–357 (1972). [CrossRef]
18. P. Němec, J. Prikryl, V. Nazabal, and M. Frumar, “Optical characteristics of pulsed laser deposited Ge–Sb–Te thin films studied by spectroscopic ellipsometry,” J. Appl. Phys. 109(7), 073520 (2011). [CrossRef]
19. P. Sharma and S. C. Katyal, “Effect of substrate temperature on the optical parameters of thermally evaporated Ge–Se–Te thin films,” Thin Solid Films 517(13), 3813–3816 (2009). [CrossRef]
20. J. I. Thornton, C. Langhauser, and D. Kahane, “Correlation of Glass Density and Refractive Index—Implications to Density Gradient Construction,” J. Forensic Sci. 29(3), 711–713 (1984).
21. S. Sen, E. L. Gjersing, and B. G. Aitken, “Physical properties of GexAs2xTe100−3x glasses and Raman spectroscopic analysis of their short-range structure,” J. Non-Cryst. Solids 356(41-42), 2083–2088 (2010). [CrossRef]
22. K. Šútorová, P. Hawlova, L. Prokes, P. Nemec, R. Boidin, and J. Havel, “Laser desorption ionization time-of-flight mass spectrometry of Ge-As-Te chalcogenides,” Rapid Commun. Mass Spectrom. 29(5), 408–414 (2015). [CrossRef]
23. R. P. Wang, D. Y. Choi, A. V. Rode, S. J. Madden, and B. Luther-Davies, “Rebonding of Se to As and Ge in Ge33As12Se55 films upon thermal annealing: Evidence from x-ray photoelectron spectra investigations,” J. Appl. Phys. 101(11), 113517 (2007). [CrossRef]
24. V. Pamukchiev, A. Szekeres, and D. Arsova, “Spectroscopic ellipsometry study of the effect of illumination and thermal annealing on the optical constants of thin Ge–As–S films,” Phys. Scr. 83(2), 025405 (2011). [CrossRef]
