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Abstract
Ge-Se thin film waveguide is used in optical devices because of its excellent optical properties. We investigated the structural and optical properties of as-deposited and thermally annealed Ge18Se82 films and the associated waveguides. The optimized annealing condition at 170 °C was determined for Ge18Se82 films. This study reveals that the annealing process can reduce the density of homopolar bonds and voids in the films. After the annealing process, Ge18Se82 waveguides with the dimensions of 1.0 µm×4.0 µm and 1.5 µm×4.0 µm present 0.22 dB/cm and 0.26 dB/cm propagation loss reduction, respectively. This finding suggests that thermal annealing is an appropriate method for improving the performance of chalcogenide glass devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Chalcogenide glasses (ChGs) are considered promising materials in planar photonics due to their remarkable optical properties, such as wide transparency window, high linear refractive index, tunable material properties through compositional tailoring, striking ultrafast response, and large third-order nonlinearity [1–3]. In recent years, chalcogenide material integrated with photonics has formed a unique area, namely, chalcogenide photonics [4]. The research has aimed to realize the manufacture, integration, and application of various optical devices on the chalcogenide material platform, such as waveguides [5,6], resonators [7,8], supercontinuum generation [9,10], and flexible integrated photonics [11,12].
Ge-Se glasses are nontoxic and transparent to IR radiation from 1 µm to 15 µm [13]which is interested for IR optical photonic devices. Ge-Se glasses are materials with a large number of cross-links within a covalent network, which can be increased by compositional modification. The structure of these systems can evolve steadily from floppy to rigid [14,15]. Within such structural transition, the connectivity is not only a good parameter that describes many thermodynamic properties of the glass transition but also minimizes residual stresses in its immediate vicinity [16,17]. The near absence of stress in the network has generated phenomena, such as large-scale photo reversible glass state in bulk chalcogenide samples (for example, GeSe4) [18]. Researchers have found that close to this composition in the same Ge-Se system, structural motifs corresponding to surface reliefs and trenches can be readily created when amorphous films are exposed to electron beams [19].
For the waveguide, the effective guide length Leff is determined as Leff= [1−exp (−aL)]/a, where a is the loss coefficient of the structure. A small effective mode area (Aeff) is desirable for boosting the light intensity in devices. However, it normally leads to an increased amount of light scattering from the core/cladding interfaces due to the strong interaction between the light and the sidewalls. Our previous works focused on optimizing the waveguide fabrication to reduce the scattering loss of a device [20–22]. Two principal scattering mechanisms are available, namely, surface scattering arising from the interface between waveguide core and cladding and Rayleigh scattering from microscopic refractive index fluctuations in the material [23]. The thin films produced in highly nonequilibrium conditions, which contain a large number of unsaturated chemical bonds and phase-separated clusters, will result in considerably high extrinsic absorption [24]. Rayleigh scattering caused by these unstable physical properties usually degrades devices and reliability [25]. Studies have reported the effect of thermal treatment of as-deposited films [26,27]. These studies have unequivocally concluded that annealing accelerates the relaxation of a film toward its equilibrium state and reduces phase separation and structural defects. In addition, annealing below the glass transition temperature (Tg) allows the film to settle to a thermodynamically favorable state [28–31]. Thermally annealed glass thin film exhibits the smallest index drift in the order of 10−4, which suggests that proper heat treatment is the preferred method for stabilizing the optical response of ChG devices.
Generally, the structure and properties of ChG films, which are different to those of bulk glass, are sensitive to the film deposition method. In thermal evaporation technique, this phenomenon is caused by the fact that films are produced under nonequilibrium conditions by rapidly condensing vapor onto a cold substrate. By contrast, bulk glasses are produced under near-equilibrium conditions by melt quenching [24]. Moreover, the differences in elemental vapor pressure, incongruent melting, and vapor dissociation will result in the compositional variation of the deposited films.
Here, we investigated the thermal annealing process of Ge-Se thin films and its influence on waveguides. The thin films were thermally evaporated from Ge20Se80 glasses, whereas the composition was measured as Ge18Se82. Se exhibits higher vapor pressure than Ge, which generates a higher deposition rate and increased concentration. The films used in this study were deposited on oxide-coated Si wafer substrate. These films were fabricated under the same conditions. Then, the thermally induced structural characteristics before and after thermal annealing were measured and analyzed using atomic force microscopy (AFM) and Raman spectroscopy. Finally, a simulation was performed for the optical mode profile of the waveguide. Waveguide losses with as-deposited and thermal annealing Ge18Se82 thin films were measured and analyzed using fiber coupling and cut-back method.
2. Experimental method
2.1. Bulk glass preparation
The Ge-Se glasses were obtained by conventional melt-quenching technique using high-purity Ge and Se (all of 5N) as raw materials. All the elements were appropriately weighted, placed in quartz ampoule, evacuated to 10−3 Pa, and finally sealed. The ampoule was placed in a rotating and rocking furnace at 950 °C for 10 h for thorough mixing and homogenization. The long periods of synthesis and frequently rocking of the melt were necessary for homogeneity of the material composition. The molten materials were then rapidly quenched in ice water mixture to obtain a sample in a glassy state.
2.2. Film deposition
Ge18Se82 thin films were prepared on oxidized silicon wafers by thermal evaporation techniques in a chamber evacuated to 2 × 10−4 torr. The evaporation rates and the thickness of the films were controlled by quartz crystal monitoring. The thin films were thermally annealed at different temperatures up to below glass transition temperature Tg (≈182 °C) for 12 h in a vacuum annealing furnace to suppress the simultaneous surface oxidation. The transmission spectra of the films were measured using a spectrophotometer (Perkin Elmer Lambda 950) The refractive indices were measured by ellipsometry (J.A. Woollam IR-Vase II). The surface morphologies of the as-deposited and annealing films were investigated by AFM over 3×3 µm2 scanning area.
2.3. Waveguide fabrication
The waveguides were fabricated by ultraviolet lithography and plasma etching on the prepared thin films at CF4/CHF3 atmosphere. The details of the fabrication process were shown in document [21]. Waveguide end facets were obtained by hand cleaving the wafer along the {101} silicon cleavage planes. The propagation losses of the waveguides were determined via the standard cutback method on Newport auto align measurement system.
3. Result and discussion
3.1. Thermally induced structural characterization
The as-deposited Ge18Se82 films contain a large number of Ge2(Se1/2) and Sen clusters and other structural disorder. Although their bond structure corresponds to a higher potential energy than that of the ideal glass, the physical properties of Ge18Se82 films are unstable. Hence, chemical bonds have a tendency to be released when the film is exposed to energy in the form of heat. This process results in a change in microscopic properties, such as the optical band gap and refraction index, with time compromising the long-term stability of fabricated devices.
When thermal annealing is applied to as-deposited films, the heat temperature is practically limited to ensure that the film is free from any crack or bubble. In the experiment, we track the annealing of several groups of films. Each group is treated with identical fabrication protocols but with different temperatures below Tg, from 110 °C to 170 °C with every 20 °C (Table 1). The variation of the root mean square (RMS) roughness (RRMS) of the film surfaces is measured by AFM. RRMS is ∼1.4 nm for as-deposited film and decreases with the increase in temperature, although the change is not evident. The mean square error (MSE) evidently decreases with the increase in temperature. By increasing the temperature further to 180 °C near Tg, some cracks and bubbles in the film appear under a microscope. This phenomenon is possibly caused by the considerable thermal mismatch between Ge-Se film and silicon substrate, which results in tensile stress in the film. As a result of heating and cooling cycle, we conclude that the best annealing temperature in our experiment is 170 °C. In Fig. 1, the topographical AFM images for the as-deposited and annealing Ge18Se82 films show the fine granular structure of the Ge-Se glasses. The films are produced in nonequilibrium conditions under the rapid condensation of a vapor onto a cold substrate, which will result in disordered surfaces. Accordingly, the surfaces of the thin films are covered with nanometric grains. In comparison with the as-deposited film, the grains in the film annealing at 170 °C are smaller, and some valleys (visible as irregular black dots or black islands), which have increased the surface porosity, also decrease.
Fig. 1. AFM topographical image of Ge18Se82 film surfaces taken at 3×3 µm scanning area: (a) as-deposited film and (b) film annealing at 170 °C.
Table 1. Surface amplitude parameter for Ge18Se82 films
3.2. Optical property and structural changes
As-deposited Ge18Se82 films produced by thermal evaporation are known to contain large numbers of homopolar, dangling bonds, voids, and subphases, compared with bulk glass produced by melt quenching [23]. Such inhomogeneities will result in enhanced levels of Rayleigh (volume) scattering and increased absorption associated with the existence of tail states within the bandgap. Some studies have demonstrated that thermal annealing of as-deposited films can make the chemical bonds relax toward those of the bulk glass, reduce the number of defective bonds in the glass network, and decrease several sources of optical attenuation [32,33].
Figure 2(a) shows the bandgaps for the films annealed at different temperatures, and the inset in Fig. 2(a) shows the transmittance spectra. Optical annealing with band edge light results in a red shift in the optical gap in Ge18Se82 films. We can explain this phenomenon through the localized density of states model proposed by Mott and Davis [34]. On the basis of this model, the localized width of the mobility edge depends on the degree of defects and disorder in the amorphous structure. Coordination defects will be formed due to the shortage of atom number (Ge) in the deposited film. These coordination defects will form localized states. When annealed below Tg, these coordination defects gradually decrease, which results in a continuous decrease of localized state density and eventually the continuous increase of optical band gap of the film. The evolution of the refractive index for as-deposited and annealing films at 1550 nm is plotted in Fig. 2(b). During the annealing process, the refractive index of the as-deposited film reduces from 2.443 to 2.368, closing to the value of bulk Ge18Se82 glass ≈2.350 at 1550 nm. The glassy structure changing from original to annealed state can be described as a transition from a random bond arrangement to a chemically ordered one, as indicated by far IR transmission spectra [35]. S. H. Wemple, et al demonstrated that the decrease in refractive index can be explained by the increase of bond ionicity with the number of heteropolar bonds [36]. Therefore, for the as-deposited films in the present study, the combination of chemical bonds is close to a random arrangement. However, many wrong bonds exist in the films, which make the refractive index of the thin film greater than that of the glass. The disorder of the internal structure of the thin film leads to more chemical bond defects. After the heating treatment, the internal chemical bonds of the thin film release and recombine, and the wrong bonds (homopolar bonds) of the thin film continue to decrease, thus, the refractive index of the thin film reduce and the structure gradually approaches the bulk glass.
Fig. 2. (a) Bandgaps of films at different annealing temperatures (the inset is the transmittance of the film annealed at different temperatures) and (b) variation in refractive index at 1550 nm in films annealed at different temperatures.
Raman spectroscopy is a useful tool for exploring the microstructure of thin films. Measurements are performed in a Renishaw in via Raman microscope using a 0.25 mW with 785 nm excitation source. To avoid any damage induced by laser irradiation, the measurements are performed within 10 s at 10 accumulations. For Se-rich Ge-Se system, a well-accepted structural model involves tetrahedral GeSe4 units cross-linked by Se-chain fragments in a stochastic fashion [25]. The Raman spectra of the Ge18Se82 films expose five different regions, which preserve the mode assignments of the structural units that build the films. The five regions are as follows: (1) Ge-Se corner-sharing clusters of GeSe4 tetrahedra centered at 195 cm−1; (2) edge-sharing (ES) clusters of GeSe4 tetrahedra centered at 214 cm−1; (3) a wider frequency band centered at ∼260 cm−1, which corresponds to the bond stretching in Se-Se chains and rings; (4) the band at 302 cm−1 correlated to the asymmetrical stretching of ES bonds [37–40]; and (5) Ge-Ge homopolar ethane-like (ETH) structure units at 175cm−1. We only consider the Ge-Ge and Se-Se homopolar bonds that contain phases. The Raman spectra of films at different states and the intensity change of Ge-Ge and Se-Se bonds generated by thermal treatment are shown in Fig. 3. The thermal treatment generates the drop of fraction of homopolar bonds in the film, which indicates that part of the Se-Se and Ge-Ge bonds are broken upon thermal treatment. Consequently, homopolar bonds, such as the Se-Se and Ge-Ge bonds in the as-deposited film, are unstable upon heating. The conversion of these homopolar bonds into heteropolar Ge-Se bonds occurs via the following process:
Fig. 3. (a) Raman spectra of as-deposited film and the films with different annealing temperatures. Decomposition of Raman-integrated area of (b) Ge-Ge bond and (c) Se-Se bond.
3.3. Optical propagation losses
Strip Ge18Se82 waveguides of 1.0 µm×4.0 µm and 1.5 µm×4.0 µm have been fabricated successfully. Figures 4(a)–4(d) show the SEM images of waveguide with as-deposited Ge18Se82 films and films annealed at 170 °C. The SEM images show that the annealing process under Tg of the waveguide materials does not change the structure and morphology of the waveguides. Measurements are performed using a tunable laser source, a polarization controller, and a power meter. Tapered lens-tip fibers are used to couple laser light in and out of the waveguides. Measurement data are obtained using a cut-back method at 1550 nm with different waveguide lengths. Several tests are performed, and the results are averaged to avoid statistical error in the loss measurement.
Fig. 4. SEM images of strip waveguides: (a) 1-µm-thick and (c) 1.5-µm-thick waveguides on as-deposited Ge18Se82 films; (b) 1-µm-thick and (d) 1.5-µm-thick waveguides on Ge18Se82 films annealing at 170 °C.
Figure 5 plots the insertion losses as a function of waveguide length. From the fitting results, we estimate the coupling loss and calculate the propagation loss of the waveguides. The coupling losses between the waveguides and the lens–tip fibers can be estimated as approximately 6 dB/facet using the intercepts of fitting lines. The propagation losses of waveguides with as-deposited films are calculated to be approximately 1.54 dB/cm and 1.44 dB/cm for 1- and 1.5-µm-thick waveguides, respectively; for the annealed films, the propagation losses are 1.32 dB/cm and 1.18 dB/cm, respectively. The measurement error is calculated from the experimental data to be approximately ± 0.08 dB/cm. This approximate measurement error shows that waveguides after thermal annealing exhibit more than 0.2 dB/cm lower propagation losses compared with as-deposited waveguides.
We simulate and compare the fundamental quasi-TE mode fields supported in typical 1.5- and 1-µm-thick strip waveguides for as-deposited and annealing films at 170 °C, as shown in Fig. 6. In this manner, we determine the influence of the thermal annealing process to the optical transmission property of Ge18Se82 waveguide. From the simulated mode profile of as-deposited waveguides in Figs. 6(a) and 6(c), for the both types of structure, the mode field is tightly confined in the core of waveguide (99.25% for 1.5-µm-thick waveguide and 97.834% for 1-µm-thick waveguide) with an extremely small overlap with the waveguide sidewall (only 0.036%). These results suggest that these strip waveguides are less sensitive to the roughness of etched sidewalls. Moreover, more amount of light is confined in the core for 1.5-µm-thick waveguide, with less scattering loss from top and bottom interfaces. These results agree well with the measurement results (1.54 dB/cm for 1-µm-thick waveguide and 1.44 dB/cm for 1.5-µm-thick waveguide).
Fig. 6. TE mode profiles of as-deposited (a) 1-µm-thick and (c) 1.5-µm-thick waveguides with 4 µm width; annealing (b) 1-µm-thick and (d) 1.5-µm-thick waveguides with 4 µm width.
For annealed thin film waveguides in Figs. 6(b) and 6(d), the amount of light intercepting the sidewalls remain nearly unchanged (0.037% versus 0.036%). The amount of light outside the core of waveguide increases in both types of structure after the annealing process. This finding may be caused by the decrease of the refractive index of the core layer of waveguide by the annealing process. Thus, the annealed waveguide should show more propagation loss than the as-deposited one; however, the experimental data exhibit the opposite result in both types of structure. We verify the roughness of the plasma-etched top surfaces obtained using as-deposited and annealing films. During lithography and etching processes, the same protocol for as-deposited and annealing films is used. Thus, we assume that these processes will result in the same roughness. The etched top surfaces of as-deposited and annealing films are found almost on the same level and have caused a slight change in propagation loss. Moreover, the oxide layer is smooth, and the contribution to scattering from the lower interface between the film layer and the thermal oxide is negligible. Hence, we estimate that the more than 0.2 dB/cm decrease in propagation losses of the waveguides after annealing is mainly attributed to the decrease in the scattering of the core layer, which is caused by the reduction in the density of homopolar bonds, defects, and voids by the annealing of Raman scatter.
4. Conclusion
In this work, we studied the structural and optical properties of as-deposited and thermally annealed Ge18Se82 films. The bandgap, refractive index, surface roughness, and homopolar bond content of the films are investigated as functions of different annealing temperatures. Thermally induced structural characterization and optical property changes of the annealed films are investigated. Moreover, propagation losses of waveguides produced from as-deposited and annealed films are demonstrated. On the basis of the measurement and calculation results, we identify that thermal annealing decreases the propagation loss by 0.22 dB/cm and 0.26 dB/cm for 1.0 µm×4.0 µm and 1.5 µm×4.0 µm Ge18Se82 waveguides, respectively. These results are mainly attributed to the reduction in the density of homopolar bonds, defects, and voids in the thin films. This finding suggests that proper annealing treatment is the preferred method for improving the optical performance of ChG devices.
Funding
Natural Science Foundation of Zhejiang Province (LD19F050001, LGJ18F050001, LY18F050003); National Natural Science Foundation of China 61675105International Science and Technology Cooperation Project of Ningbo City (2017D10008); K. C. Wong Magna Fund at Ningbo University.
Disclosures
The authors declare no conflicts of interest.
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