This work reports the processing and properties of a new chalcogenide glass film that can be photo-patterned by multiphoton lithography (MPL) with enhanced post-fabrication stability. Thermally evaporated germanium-doped arsenic selenide [Ge5(As2Se3)95] thin films were photo-patterned using the output of a mode-locked titanium:sapphire laser. The morphology, chemical structure, and optical properties of the material were studied before and after photo-patterning and compared for their long-term aging behavior and stability to previously investigated arsenic trisulfide (As2S3) films fabricated using similar MPL conditions. Relative to As2S3, thermally deposited Ge5(As2Se3)95 is found to offer higher photo-sensitivity and greater chemical stability after photo-patterning, as evidenced by lack of age-induced crystallization and reduced feature degradation over a four year aging period. These findings demonstrate the suitability of a new photo-patternable material for the creation of robust, long-lived functional infrared anti-reflective coatings and meta-optics.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Infrared (IR) optical systems offer capabilities in spectroscopy, imaging, astrophysics, diagnostic medicine, sensing, safety, defense and environmental monitoring that are not achievable using visible (Vis) and near-infrared (NIR) wavelengths [1–4]. Legacy IR optical materials hinder innovation in these areas because they afford no means for controlling optical, thermal, and mechanical properties. Conventional IR optics created from crystalline Ge, Si, and ZnSe are difficult to process, costly, and have fixed optical, mechanical and thermo-optic properties, which are far from optimum for most applications. Typical processing techniques, such as single-point diamond turning, are expensive and produce brittle optics. Less costly alternatives like molding produce bulky/heavy optics with limited optical functionality [5–13].
Semiconducting photo-patternable multi-component glasses known as chalcogenide glasses (ChGs) possess tunable properties that include excellent IR transparency, large index of refraction, photo-sensitivity, modest coefficients of thermal expansion (CTE), thermo-optic coefficient (dn/dT), and unique optical functionality over a range of form factors and environmental conditions . ChGs have applications in photonic waveguides, sensors, acousto-optics, and optics [14–18].
Micro-scale 3D devices have function in optics, imaging, sensing, consumer electronics, micro-electromechanical systems, and medicine [16–18]. Multiphoton lithography (MPL) has been used to create functional micro-scale three-dimensional (3D) devices in photopolymers, glasses, and composites . However, there is a lack of viable semiconductor material systems that can be directly photo-patterned. New semiconductor materials systems are needed to create structures with improved electronic, magnetic, thermal, optical, or mechanical function, or the ability to respond to a chemically active environment, as needed for sensing.
The ChG that has been most used for MPL is thermally deposited arsenic trisulfide (As2S3) [15–18]. As2S3 offers potential for IR applications due to its useful optical bandgap of Eg = 2.35 eV (λ ≈ 517 nm), high transparency from 620 nm to 11 µm, and large refractive index of n = 2.45 − 2.53 [15–18]. As2S3 can be thermally deposited onto substrates as thin photo-sensitive glassy films of molecular clusters consisting of homopolar bonds. Photo-excitation of the thermally deposited film cross-links the material back into a glassy network solid by converting the homopolar bonds to heteropolar bonds. Photo-patterned As2S3 films can then be immersed in a polar solvent to selectively dissolve away unexposed material, revealing the targeted structure. MPL in As2S3 films has been used to create 3D structures such as woodpile photonic crystals, waveguide, and nanowires . However, As2S3 is unstable toward crystallization, and it forms oxides under ambient conditions, which significantly diminishes its usefulness for practical applications . Additionally, As2S3 has fixed optical, mechanical, and thermo-optical properties that are poorly suited for IR applications. No other ChG composition has been comprehensively studied for photo-processing by MPL. New photo-processible ChGs for MPL are needed, including better fundamental understanding of the relationship between their composition and processing and the resulting properties and performance, to enable fabrication of high-performance IR optical and photonic devices.
Ge-doped As/Se ChG glasses offer advantages over As2S3 glasses. It has been reported that films are more photo-sensitive when they contain As-Se and Ge-Se molecular units . The polarizability and therefore the index of refraction of the thermally deposited films and the laser patterned structures should increase with Ge content . The inclusion of Ge into the material system is expected to improve the mechanical toughness of the thermally deposited films and the laser patterned structures . Replacing S by Se and adding Ge has been shown to increase the stability of As-based bulk ChGs [23–26]. Comparisons of As/S, As/Se, and Ge/Se ChGs show that Se-containing glasses are more stable toward light-induced ageing [27–29]. Consistent with this, the commercial series of AMTIR glasses based on Ge/As/Se are environmentally stable in bulk form . Additionally, studies of thermally deposited GeSe2 show that Ge scavenges oxygen and remains amorphous, which prevents further crystallization . Therefore, incorporating Ge and Se into a thermally deposited ChG should help stabilize the laser-cross-linked material. To date, MPL in Ge/Se-based thermally deposited films has not been reported, and there are only a few reports of photo-exposure and etch processing of bulk Ge/Se-based ChGs .
This work reports the first detailed study of MPL in thermally deposited Ge/Se-based films. Fundamental understanding has been obtained for how chemical composition, laser patterning conditions, and etch processing affect the nano-structure morphology, chemical networking, and appearance of the features generated by MPL in Ge/Se-based films. This knowledge should help advance the use of ChGs for manufacturing IR optics with tunable properties and expanded applications.
2.1 Thermal deposition of films
Bulk Ge5(As2Se3)95 and Ge5(As3Se7)95 material was thermally deposited onto polished silicon wafers. The bulk ChGs were prepared using traditional melt-quench processes described elsewhere . All films were deposited using a Kurt Lesker thermal evaporation chamber. The substrate temperature was not controlled during deposition. The Ge5(As2Se3)95 film was deposited at a rate of 55 Å s−1 – 75 Å s−1, had a final thickness of 511 nm ± 6.8 nm, and was silver in color. The Ge5(As3Se7)95 film was deposited at a rate of 55 Å s−1 – 65 Å s−1, had a final thickness of 526 nm ± 4.9 nm, and was also silver in color. The film colors were determined by visual inspection. All films were placed in light-tight containers and stored in a desiccator for protection from ambient light and moisture. The films were prepared and handled as reported earlier [16–18, 34].
The methods of laser exposure and film processing have been described previously but are summarized here for completeness [16–18, 34]. A mode-locked femtosecond laser (Coherent-Mira, 800-nm center wavelength, pulse width τp = 120 fs, repetition rate f = 76 MHz) was used to photo-pattern the films. An acousto-optic modulator (AOM) was used to electronically control beam power. The laser beam was first guided through a beam expander then into a 100× /1.4 NA oil-immersion objective (Nikon) which focused the beam in to the film. The transverse spatial profile of the beam is approximated as a radially symmetric Gaussian having a 1/e2 radius of ω0 ≈ 175 nm, and the minor effects of input beam polarization are neglected . The laser beam was focused along the z-axis of an arbitrary coordinate system defined so that the substrate lies in the xy-plane. The average power <P> used to photo-pattern the films was measured at the exit aperture of the objective using an integrating sphere. The corresponding peak irradiance of the focused beam Ip can be estimated using Eq. (1) .
The film was affixed to a sample stage that was translated at a speed of 50 μm s−1 in the x-, y-, and z-axes relative to the laser beam to define patterns. To produce vertical nano-scale pillars, the film was moved to a given (x, y) coordinate, the AOM was activated, and the film was translated in the z-direction to expose over a vertical length of 1 µm − 2 µm. This exposure distance was necessary to bury the beam up to 1 µm into the substrate and to travel completely through the film thickness.
Sets of nano-pillars referred to as “dose arrays” were photo-patterned over a range of travel speeds and average (avg.) focused laser power that varied from 0.02 mW (1.15 GW cm−2) to 3.0 mW (173 GW cm−2). Individual arrays consisted of a 9 × 9 grid of nano-pillars separated by a pitch of 500 nm. Each array had a starting area of approximately 4 μm × 4 μm which increased based on dose and was separated from the next array by 5 μm. The term “dose array” will hereafter refer to the entire set of 9 × 9 grids of nano-pillars separated by a pitch of 500 nm and fabricated using laser powers that varied from 0.02 mW (1.15 GW cm−2) to 3.0 mW (173 GW cm−2) .
2.3 Etch processing
Etch processing conditions were identified that could be used to dissolve unexposed regions of a ChG film selectively, leaving behind material that had been exposed and structurally modified. The optimal etchant would yield structures with excellent contrast (ratio of rates at which the exposed and unexposed material from the same film type dissolve in the etchant) and feature fidelity limited only by the dimensions of the focused laser spot.
Etch studies were performed by immersing as-deposited films in either butylamine, propylamine, neat ethanolamine or solutions of ethanolamine in DMSO. The etching was monitored and recorded visually using a video camera. The recordings were reviewed afterward to obtain measures of the time required to etch the material fully. Propylamine, butylamine and ethanolamine were chosen because they have been reported to be good negative etchants for thermally deposited As2Se3 films [21, 37, 38]. Due to the reflectivity of the film layer and the silicon substrate are significantly different, the time at which the etchant reached the film/Si interface was easily identifiable from changes in the image contrast. The etch rate is defined per Eq. (2) as the ratio of its thickness to the time required to etch it.
Photo-patterned films were etched using ethanolamine to reveal the targeted nano-structures. The photo-patterned film was immersed in the etchant for up to 120 minutes and then rinsed in a fresh solution for 15 seconds then immersed in IPA for 2 minutes. Scanning electron microscope (SEM) images of the resulting nano-structures were recorded using a Zeiss ULTRA-55 FEG SEM in top-down and profile views. These images were used to measure the dimensions (x- and y-widths) and shapes of the features and correlate them with film composition, processing conditions, and fabrication laser power. The x- and y-widths are not expected to be exactly the same because a linearly polarized beam focused under a high numerical aperture does not produce a rotationally symmetric point spread function [17, 18, 39]. This effect is expected to cause the widths to differ by no more than 10%. Rekstyte et al. reported that the observed elongation of polymerized features due to the polarization of the incident laser beam was attributed to heat diffusion and local overheating and was found to be more pronounced for higher pulse energies, longer wavelengths, and longer pulses .
2.4 Spectroscopic and optical characterization
Using micro-Raman spectroscopy the film structure and chemical bonding were explored in as-deposited films or in UV-exposed films. A Bruker Senterra micro-Raman system with an average power of 10 mW and an excitation wavelength of 785 nm was used. The beam was focused using a 10× microscope objective (NA = 0.3, WD = 15 mm). The focused beam had a diameter of 50 µm – 200 µm. Peaks in the Raman spectra of the films were assigned to the molecular fragment as described in references [14, 33, 41]. As-deposited pristine films were exposed by irradiating samples for two minutes in a Zeta Loctite UV flood chamber equipped with high-pressure mercury lamps that emit broadband UV radiation (65 mW cm−2). An energy-dispersive X-ray spectrometer (EDS, Noran System 7) installed on the SEM was used for elemental analysis of the as-deposited films.
The dispersion of the refractive index of pristine and UV-exposed films was determined using a variable-angle spectroscopic ellipsometer (Woollam M2000). The probe wavelengths ranged from 600 nm to 1600 nm. Focusing optics were used to collect data from regions as small as 100 µm × 100 µm with a spectral resolution of 3 nm at visible wavelengths and 6 nm in the near infrared. A Tauc-Lorentz oscillator method was used to extract the n and k values . The surface roughness and optical bandgap were allowed to vary as free parameters. Film surface roughness was obtained using a Zygo New View 6300 white light interferometer with a 10× objective. The reported surface roughness is the average of the root mean square (RMS) roughness measured from five 0.70 mm × 0.53 mm sample regions.
3. Results and analysis
3.1 Composition and etch processing
Thermal deposition produced Ge-doped AsSe films that are stoichiometrically similar to their bulk starting materials, compositionally homogeneous, and structurally uniform. SEM images and interferometry measurements showed that the Ge-doped AsSe film surface is smooth and homogenous with an average RMS roughness value of 0.060 µm ± 0.004 µm (Fig. 1). The RMS roughness values for the Ge-doped AsSe film similar to reported RMS roughness values for Ge and Se films in which waveguides have been fabricated [43, 44].
Measurements from energy dispersive X-ray spectroscopy (EDX) of bulk glass and thermally deposited films are shown in Table 1. For the bulk glass, the Ge content was kept low (~5%) because high valence (Ge = 4) tends to favor extended 3D network structures, whereas lower valence (As = 3 and S/Se = 2) tends to favor sheet-like and chain structures that promote glass formation . The As-to-Se atomic ratios indicate that thermal deposition produced samples similar to the expected stoichiometric elemental composition; however, the Ge content was lower than expected for both films.
Ethanolamine dissolved the as-deposited Ge5(As2Se3)95 in 60 minutes, whereas etching in propylamine and butylamine required over 120 minutes and left behind material on the surface. The variation in dissolution kinetics among the amines is likely due to the difference in each solvents polarity . Ethanolamine has the highest polarity out of the three and dissolved the as-deposited film faster and more completely than propylamine or butylamine . Ethanolamine, even with its relatively low rate compared to As2S3 (0.139 nm s−1 ± 0.02 nm s−1 compared to an average value of 18.8 nm s−1 ± 6 nm s−1 for As2S3 films) was the fastest etchant amongst the amines and therefore used as the developer for all further experiments with Ge5(As2Se3)95 film. The as-deposited Ge5(As3Se7)95 film remained unchanged after immersing in either ethanolamine, propylamine, or butylamine for over 120 minutes.
UV exposed Ge5(As2Se3)95 films did not show any signs of dissolution after being immersed in ethanolamine for over 120 minutes. UV exposed Ge5(As2Se3)95 films also have a slightly higher content of heteropolar bonds. Kumaresan et al. reported that UV exposed As2Se3 films have an increased fraction of As-Se heterpolar bonds over as-deposited films and that these heterpolar bonds are responsible for slower etching . This is consistent with the as-deposited Ge5(As3Se7)95 film, which has a higher Se content, resisting etching completely.
3.2 Photo-patterning in Ge5(As2Se3)95
Dose arrays like those shown in Fig. 2 were used to study the exposure conditions needed to photo-pattern the films. Ge5(As2Se3)95 and Ge5(As3Se7)95 films were exposed over a range of average focused laser powers (corresponding peak irradiance can be found in Fig. 3) at a fixed scan speed to identify the minimum power needed to create robust features which could survive photo-exposure and subsequent etching. Also measured was the maximum power above which damage or distortion occurs, and how the feature size scales with power between the lower and upper limits. After exposure, the unexposed films were removed by etching with ethanolamine.
The Ge5(As3Se7)95 films could not be photo-patterned, only ablated (not shown). The apparent ablation threshold is at 1.1 mW. The difference in photo-sensitivity of the two films is due to differences in Se content and resulting As-Se bonds. Additionally, the complete resistance to amine-based etchants would make it impossible to etch away unexposed material and reveal the photo-patterned structures. Given that no viable nano-structures were formed as a result of laser exposure in Ge5(As3Se7)95 films, and the film could not be etched, the rest of the photo-patterning and characterization studies were conducted only using Ge5(As2Se3)95 films.
Vertically scanning the focal spot through the films during MPL was expected to produce cylindrical pillars, or possibly bead shaped pillars, based on other studies using As2S3 films . However, micro-structures patterned in the Ge5(As2Se3)95 film are pyramid shaped and smaller (~250 nm) than the film height (~500 nm). The form of the micro-structure suggests a differential etching rate which may be due to cross-linking at the substrate during thermal deposition on an uncooled sample stage. A stronger amine based etchant must be used for Ge5(As2Se3)95 films since it is more resistant to amine-based etchants than As2S3 films . The increased etchant strength along with the low etch rate of the film suggest that some of the photo-patterned material is also unintentionally etched. This decrease in etch selectivity may cause structure deformation and height decrease. Even though the fabrication did not result in the production of cylindrical or bead shaped nano-structures, the pyramid shaped nano-structures may be useful for creating graded index AR coatings .
Exposing the films with higher average laser power produced larger features in the Ge5(As2Se3)95 film, consistent with that typically found for a threshold-dependent photochemical process [34, 46]. SEM top-view images were used to obtain the widths of the photo-patterned micro-structures over a range of powers. The measurements were used to create a plot of pillar widths versus power (Fig. 3). For a threshold-dependent MPL process, the relationship between laser-patterned structure-width, spot size of the focused laser beam r0, average laser power <P>, and the threshold laser power for producing a feature <P>Th can be described by Eq. (3) .
In Fig. 3, the smooth curve is a global fit of the pillar widths to Eq. (3) with r0 and <P>Th set as free parameters. The fit is consistent with photo-patterning in Ge5(As2Se3)95 resulting from a threshold-dependent process. However, deviations from the theoretical curve may be due to variations in the widths of features measured over a given array. Due to the pyramid shape nature of the micro-structures, accurate measurements of the structure width are difficult to obtain. Furthermore, the structure widths may vary across an array as the high viscosity of the etchant increases the potential for uneven etching of the material.
The Ge5(As2Se3)95 films were found to be more photo-sensitive than thermally deposited As2S3 [16–18]. Structures with widths of 200 nm are formed at an average power of 0.050 mW. In comparison, an average power of 0.18 mW is needed to produce this sized feature in As2S3 films. The apparent increased photo-sensitivity of thermally deposited Ge5(As2Se3)95 results from the addition of Se and Ge into the system. The electronic band gap for bulk As2S3 and As2Se3 is 1.93 eV and 1.1 eV, respectively [47, 48]. The electronic band-edge of bulk As2S1-xSex red-shifts with increasing Se content. If the same can be expected for thermally deposited films, then increasing the Se-content should produce a more photo-sensitive ChG film at a given wavelength. Thermal deposition of As2S3 is known to produce films containing molecular clusters with homopolar As-As and S-S bonds that allow facile bond cleavage and network cross-linking with photo-excitation [16–18, 49]. Ge atoms are more polarizable than S atoms , and the electronic band-edge of bulk ChGs also red-shifts with increasing Ge content . Consequently, increasing the Ge content of thermally deposited films should also increase their linear and nonlinear absorptivity [23,51] making them more photo-sensitive. The photo-sensitivity of a material also depends on the strength of bonds being activated with light. Materials having weaker bonds will be more photo-sensitive at a given wavelength. The photo-sensitivity of thermally deposited chalcogenide glasses originates with the homopolar bonds. The bond dissociation energies for S-S and Se-Se bonds are 226 kJ/mol and 172 kJ/mol, respectively . Given that Se-Se bonds are weaker than S-S bonds, and As/Se glasses are more absorptive than As/S glasses at the same wavelength, AsSe glasses can be expected to be more photo-sensitive.
3.3 Raman spectroscopy of Ge5(As2Se3)95 films
Raman spectroscopy was used to study the chemical structure of bulk-, as-deposited-, and UV-exposed films (circa 1 μm Raman sampling depth). In Fig. 4, Raman spectra of the film have peaks indicative of molecular clusters typically found in thermally deposited films containing Ge, As, and Se . The peak appearing at 300 cm−1 is due to the silicon substrate. The film displays low-energy peaks at 203 cm−1, 215 cm−1, 223 cm−1, 238 cm−1, 240 cm−1, 248 cm−1 indicating the presence of Ge2Se4, GeSe4, AsSe3, As4Se3, As2Se3 and As4Se4 molecular clusters, respectively (Table 2) [14, 33, 41, 53]. In Fig. 4, the peak at 256 cm−1 is assigned to AsSe3 and the broad shoulder at 265 cm−1 is assigned to Se8 rings. The film lacks the sharp Ge2Se4 peak which is found for the bulk glass (Fig. 4). This indicates that thermal deposition rearranges the Ge/Se bonds to form the molecular clusters that transport through the vapor phase to the substrate, just as is observed for thermal deposition of As2S3 .
Figure 4 also shows how the Ge5(As2Se3)95 film behaves under UV-exposure. The bands at 203 cm−1, 215 cm−1, 223 cm−1, 238 cm−1, 240 cm−1, and 248 cm−1 decrease slightly following UV exposure, indicating that irradiation breaks or rearranges the homopolar bonds of Ge2Se4, GeSe4, AsSe3, As4Se3, As2Se3 and As4Se4 units in the glassy film. These findings are consistent with previous studies of Ge- and Se-based films [21, 38]. Khan et al. reported that the local glass energy increases with the ratio of edge-sharing to corner-sharing Ge2Se4 bonds, so laser irradiation of GexAs35-xSe65 thin films relaxes the glass-energy by removing the edge-sharing bonds . Similarly, Olivier and associates reported that there was a decrease in homopolar Ge-Ge bonds after irradiation of thermally deposited (GeSe2)70(Sb2Se3)30 films .
3.4 Index of refraction for as-deposited films
The index of refraction n and extinction coefficient k were obtained from spectroscopic ellipsometry measurements performed on as-deposited Ge5(As2Se3)95 film (Fig. 5). The extinction coefficient k (not shown) was effectively zero for the film over the 500 nm to 1600 nm wavelength range. The index of refraction n of the Ge5(As2Se3)95 film is approximately 0.446 units greater than that of thermally deposited As2S3 . The high refractive index and photo-sensitivity of the Ge5(As2Se3)95 film suggests that thermally deposited Ge/Se-based films could be useful for fabricating functional infrared photonic devices.
3.5 Stability of structures photo-patterned in Ge5(As2Se3)95
The stability of structures photo-patterned in Ge5(As2Se3)95 was compared to those created using As2S3 and correlated with the environmental conditions, age, and composition of the parent-films. The surfaces of thermally deposited As2S3 films are known to become chemically altered and degraded due to exposure to ambient light and water vapor . Key attributes of the aged As2S3 materials which make them unsuitable for use (without a protective layer that mitigates environmental degradation) is that both homogeneous and patterned films exhibit surface crystallization and thus, increased optical loss with age. Before photo-patterning, as-deposited films were kept in closed petri dishes wrapped with aluminum foil and stored in an amber desiccator in a cupboard to minimize exposure to ambient light and humidity. After a film was photo-patterned and etched, the resulting micro-structures were stored in closed petri dishes in a cupboard, but permitted to have contact with ambient air and humidity. To compare the stability of films and structures produced with Ge5(As2Se3)95 and As2S3, SEM imaging and EDX spectroscopy were used to monitor the samples over time and detect changes in morphology, film topography, and chemical structure.
SEM images revealed that after three months, micro-structures created in As2S3 were significantly degraded (Fig. 6), whereas micro-structures created in Ge5(As2Se3)95 were unchanged. Features patterned in As2S3 had deformed and excess material formed on the surface of the micro-structures. EDX showed that this degradation included both crystallization and formation of As2O3. Un-patterned, as-deposited films of As2S3 degraded similarly within three months (Figs. 6g and 6h). It was also found that as As2S3 films age, their photo-sensitivity decreases. In contrast, films and micro-structures created in Ge5(As2Se3)95 are much more robust. After four years, structures photo-patterned in Ge5(As2Se3)95 at low power had shrunk slightly, but as seen in Fig. 7, features patterned at intermediate powers were largely unchanged. Additionally, neither as-deposited films of Ge5(As2Se3)95 nor micro-structures created in Ge5(As2Se3)95 exhibited any signs of surface crystallization. Raman and EDX spectroscopy of a square pad of laser-patterned and etched Ge5(As2Se3)95 that was aged for four years exhibited no change in atomic composition or chemical bonding within experimental error (Table 3 and Fig. 8). Thus, adding Ge and Se elements into the composition increases the stability of the films and decreases degradation with time relative to As2S3 films. These observations show that ChGs for MPL based on Ge and Se would be more stable than As2S3. This conclusion is important, because although more aggressive etchants are needed to process Ge5(As2Se3)95, the material is clearly better suited than As2S3 for creating nano- and micro-scale devices.
It is important to note that the slow etching rate and low contrast is a disadvantage with Ge/Se containing films as compared to As2S3 films. However, some organic resists etch slowly and have low etch selectivity have been used for anti-reflective coatings . Therefore, it is still reasonable that this material can be useful for manufacturing. In future work, thicker films as well as different deposition rates which are known to influence the etch rate of the film will be explored.
Compositions based on Ge-doped arsenic selenide were investigated to find new materials for MPL that offer higher stability as thermally deposited films and patterned structures than As2S3. Thermal deposition of Ge5(As2Se3)95 formed glassy films that were more photo-sensitive than As2S3, enabling fabrication at lower average power. Nano-structures created in Ge5(As2Se3)95 had a pyramid shape, which may be useful for a creating graded index anti-reflective surfaces. The addition of Ge and Se stabilizes the chalcogenide glass and decreases the degradation effects typical of As2S3. The increased photo-sensitivity, higher index, and improved stability of Ge5(As2Se3)95 suggests that Ge/Se-based chalcogenides may better suited for manufacturing via MPL of functional optical and photonic devices with improved optical, mechanical, and thermal properties.
National Science Foundation (NSF CAREER award DMR/CHE-0748712 and DMR-1337758); Lockheed Martin; the Florida High-Tech Corridor Council; the Space Research Initiative Program, through the Florida Space Institute, hosted at the University of Central Florida; and the National Aeronautics and Space Administration, through the University of Central Florida's NASA-Florida Space Grant Consortium.
The authors thank Dr. Pieter Kik and Dr. Chatdanai Lumdee for assistance with the ellipsometry measurements and Dr. Claudia Goncalves for additional Raman measurements. GDR and SL were supported by UCF SURF scholarships.
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