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Abstract
The objective of this work is to quantify the refractive index of germanium films relative to film thickness. The films are fabricated using radio-frequency sputter deposition with film thickness spanning 820 nm to 3950 nm. The dispersive refractive index of the films is measured by Fabry-Perot transmission measurements. Using this data, it is then verified by full-spectral matching of the resonant response of guided-mode resonance grating structures fabricated into the films. This is a new method to quantify the dispersion response of materials that can be deposited to form thin films. At a wavelength of 10 µm, the refractive index is found to vary from 3.84 for an 820 nm thick film to a value of 4.6 for a 3950 nm thick film. This, respectively, represents a ∼4% decrease and a ∼15% increase from the intrinsic crystalline refractive index. Chemical composition of the films is verified using energy dispersive spectroscopy with the film structure analyzed using X-ray diffraction.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Germanium is important in photonic technology due to numerous advantageous properties in the near-infrared (NIR) spectral region and beyond. It boasts a high index of refraction and excellent optical transparency starting at the communications wavelengths near 1500 nm and extending through the mid-wave infrared (MWIR) zone from 2.5-5 µm and past the long-wave infrared (LWIR) from 7-13 µm. Germanium has been alloyed with silicon to form infrared (IR) optical wires [1], applied to improve the efficiency of IR solar cells [2–4], and used to create photodiodes [5,6], photodetectors [7], and anti-reflective (AR) surfaces [8]. MWIR photonic devices taking advantage of the atmospheric transparency window from 3-5 µm apply germanium as a natural material candidate-yielding development of various waveguide, cavity, and resonator designs [9–12]. The LWIR region is of great interest as well, with work on various waveguide and resonant designs reported [13–17].
A key advantage of germanium is its high index of refraction which enables strong optical mode confinement. Previous studies have shown that the method of deposition can have a strong effect on the morphology and optical constants of the film [18,19]. In particular, the authors of [19] observed sputter deposited germanium films with refractive index values higher than that of intrinsic bulk crystalline germanium (c-Ge) for ultra-violet (UV) to NIR wavelengths. This increase in optical density is supported by material studies showing that sputtered germanium can form films with a 5% increase in material density relative to c-Ge [20]. To our knowledge, no analogous studies have been reported on the refractive-index properties of sputtered Ge films for the MWIR and LWIR ranges.
In this work, we measure the refractive index of germanium films deposited using standard 13.56 MHz radio-frequency (RF) magnetron sputtering over the 2.5-13 µm spectral range. Films with thicknesses ranging from 820 nm to 3950 nm are treated. Film thickness is measured using a combination of atomic force microscopy (AFM) and scanning electron microscopy (SEM). The refractive index of the films is measured in a two-fold manner using Fabry-Perot (FP) thin-film measurements and the guided-mode resonance (GMR) effect [21]. Fourier transform infrared (FTIR) spectroscopy is used to measure the transmission response in all cases. The measured FP response is fitted to numerical data using the measured film thickness by varying the refractive index (RI) of the simulated film. The measured and calculated GMR resonant peaks are fitted in a similar manner. Refractive index (RI) values calculated for the FP and GMR measurements are then compared for verification. All devices are fabricated on zinc sulfide multi-spectral (ZnS-MS) substrates. This is a high purity, near mono-crystalline material with high transparency and a minimally dispersive RI ≈ 2.2 throughout the MWIR and LWIR regions. Test samples used in cross-sectional measurements are fabricated on Si <100 > wafers to allow for wafer cleaving. More details of the fabrication process can be found in [17].
2. Experiment and measurements
2.1 Refractive index measurements
We begin our analysis with two germanium films 850 nm and 820 nm in thickness. The first film (film 1) is deposited on cleaned and prepared substrates using a Kurt J. Lesker Lab 18 sputtering system. The system is programmed for an 8675 second deposition at 150 W, 3.3 mT process pressure, and 25 sccm Ar flow rate. The ZnS-MS and Si <100 > substrates are both run simultaneously to ensure that identical films are deposited. The substrates are held ∼25 cm from the sputter target on a rotating stage. During deposition, the temperature of the substrate holder is monitored via a proximity thermocouple and found to remain near room temperature (23° C) throughout the process. After the germanium deposition, a 10 nm Si layer is sputtered on the surface to passivate the film and promote photoresist adhesion. The films are then patterned with a photoresist (PR) grating using Shipley s1813 positive photoresist (PR). The photoresist grating height is measured using AFM and one of the Si samples is then cleaved to produce an SEM cross-sectional image. The AFM PR grating height measurements are compared to the SEM cross sections to determine the thickness of the sputtered Ge layer. AFM PR grating measurement results and an SEM cross section of the first sample are shown in Fig. 1. The PR grating height is measured to be 1230 nm, and the Ge layer thickness is determined to be 850 nm.
Fig. 1. Demonstration of the film-thickness measurement technique. (a) AFM scan of a developed photoresist (PR) grating. The left image is a topological view of the sample. The right image is a profile of the measured section highlighted in red. Red arrows on the profile measure the grating height, blue arrows mark the grating width, and green arrows are used to measure the device period. (b) SEM cross-section that is cross-referenced with the AFM measurement and used to determine the Ge layer thickness. Here, dPR = 1230 nm and dGe = 850 nm. The SEM is taken on a dummy sample fabricated on a Si substrate for easy cleaving. The striations visible in the image are due to the cleaving process.
Transmission measurements are then conducted on the 850 nm Ge-on-ZnS-MS film using a Nicolet iN10 FTIR with an iZ10 transmission measurement attachment as shown schematically in Fig. 2. The light is assumed to be normally incident and background calibration measurements are first taken by inserting a blank ZnS-MS wafer into the sample area.
The transmission measurements of the film result in a series of minima and maxima that stem from classically understood Fabry-Perot interference [22]. The RI of the film is determined by matching these measured peak locations to a calculated map of the peak locations as a function of the index of refraction as shown in Fig. 3. Figure 3(a) shows the mapping of the transmission peaks for an 850 nm film in the range of 2.5–12.5 µm and for RI values of 3.9-4.8. Figure 3(b) shows the transmittance of the 850 nm film where red lines connect the measured peak locations to their corresponding points on the RI map. Figure 3(c) plots the sputtered-film index of refraction using a logarithmic curve fit to the matched points. When compared to RI values for intrinsic Ge [23], we see that the sputtered-film index values diverge and are significantly lower across the MWIR and LWIR bands.
Fig. 3. Fabry-Perot measurements and RI curve fitting for film 1. (a) RI fit map showing the transmission peak locations as a function of index of refraction. Peak locations lie at the center of the bars. (b) FP transmission spectrum of the 850 nm film. The blue curve is measured FTIR transmission data. Redlined segments connect the FP peaks to their location on the fit map. The dashed line is the calculated FP response using the RI fit data. (c) RI data extrapolated from the FP peak locations. Solid line shows intrinsic Ge RI; the sputtered film RI shown with the dashed line is calculated by applying a logarithmic curve fit to the measurement points.
The RI measurements of film 1 are next verified by patterning a GMR element onto the surface using reactive-ion etching (RIE) utilizing SF6 and CHF3 gas chemistries. After patterning and O2 ashing to remove residual PR, unpolarized transmission measurements are taken of the Ge-on-ZnS-MS device and compared to simulations made using rigorous coupled-wave analysis (RCWA) [24]. Figure 4 shows the unpolarized FTIR transmittance with the diffractive GMR spectrum taken in grey compared to simulations shown in red using the logarithmically fit index of refraction from Fig. 3(c). The inset shows a cross-section of a simultaneously etched Ge-on-Si dummy device with the PR mask layer unremoved; here the PR that was used to verify the device dimensions also serves as the etch mask. Measured grating dimensions are dg = 500 nm, dh = 350 nm, Λ = 3.67 µm, and f = 0.61. There is excellent overlap between the simulations and measured results which serves to verify the dispersive Fabry-Perot derived RI values used in the simulation.
Fig. 4. Fabricated guided-mode resonant device with measured and simulated spectrum for the entire mid and long IR range. Inset shows fabricated grating with PR layer unremoved. Grating dimensions are dg = 500 nm, dh = 350 nm, Λ = 3.67 µm, and f = 0.61. Again, the SEM is for a dummy sample.
A second device, (film 2), is prepared separately, but in an identical manner to film 1. The thickness is determined to be 820 nm through SEM and AFM measurements as before, and Fabry-Perot transmission measurements are taken to determine the refractive index. A GMR grating is fabricated on the film with measured dimensions dg = 330 nm, dh = 490 nm, Λ = 3670 nm, and f = 0.66. The model of this device is shown in Fig. 5(a). This design produces a sharp resonance in the transverse electric (TE) polarization at 9.9 µm when using the intrinsic RI value for c-Ge as shown in Fig. 5(b). The position of this resonance point is most sensitive to the RI of the film and will shift with changing RI as shown in Fig. 5(c). The measured position of this resonance in the fabricated device will serve as a precise indicator of the RI of the sputtered Ge at the resonant point.
Fig. 5. GMR device with resonant refractive index measuring point. (a) Grating schematic and dimensions. (b) Calculated TE resonance using RI of c-Ge. (c) Transmission map showing the calculated resonant shift with a changing RI of the Ge layer. TE polarization corresponds to the electric-field vector pointing along the grating grooves.
Figure 6 shows the Fabry-Perot measurement results for film 2 obtained in the same manner as with film 1 along with the logarithmic fit curve to the FP data and spectral results for the TE resonant response of the fabricated grating. Polarized measurements are made by inserting a 10,000:1 wire grid polarizer into the transmission path. The FP fit curve again indicates a reduced index of refraction from intrinsic values as shown in Fig. 6(a) and matches very closely with the results for film 1 shown in Fig. 3. Figure 6(b) shows the measured TE resonant response of the fabricated device along with the simulated response which is calculated using the logarithmically fit RI values from Fig. 6(a). As with the previous device patterned onto film 1, we see again a very strong agreement between the measured and calculated data across the entire spectral range. Of particular note here is the matching of the designed resonant point in the LWIR from Fig. 5(b) which now falls at 9.7 µm due to the decrease in RI of the film. We note that whereas films 1 and 2 have comparable thicknesses, the resonance response differs significantly as seen by comparing Fig. 4 and Fig. 6(b). This difference occurs because the resonance signature of a given film is easily diversified by choice of different parameter sets for the periodic structure.
Fig. 6. Film 2 measurement results. (a) Fabry-Perot peak location map and logarithmic curve fit. (b) TE measured results (red) and simulated results (black) for the fabricated grating with grating dimensions dg = 330 nm, dh = 490 nm, Λ = 3670 nm, and f = 0.66 and with calculations made using the RI curve fit from FP measurements.
Next, two thicker films are deposited using a sputter time of 22,963 seconds with all other process parameters being the same as for films 1 & 2. This results in a 2200 nm film (film 3) and a 2250 nm film (film 4) measured through AFM and SEM methods as previously described. The FP transmission response of the films is measured and fitted to calculated data in the same manner as the previous devices with a summary of the curve fitting results shown in Fig. 7. In this case, we see that the RI curves for both films are significantly higher than the RI for intrinsic c-Ge. Even though these films have similar thickness, the refractive index differs as shown. We attribute these deviations to the ∼50 nm thickness difference as well as to statistical sputter chamber variations that we estimate to be on the order of +- 2%.
Fig. 7. FP curve fitting results for film 3 (2200 nm, square indicators) and film 4 (2250 nm, diamond indicators).
Again, the RI values are verified by patterning the films with GMR gratings designed this time with two resonant markers present in both the transverse magnetic (TM) and TE polarizations for the LWIR band. The resonant points will shift with refractive index changes in the same manner as the single resonance design shown in Fig. 5(c) and will enable mapping of the RI across the LWIR. In Fig. 8 we show the fabricated devices, measured and calculated spectra, and comparison graphs of the logarithmically fit RI curve to the intrinsic values. For film 3, it is found that using the RI fit data from the FP measurements produces a very close match between the measured resonance spectra and simulations as seen in Figs. 8(c) and (d). For film 4, the measured resonant points are used to create a new RI curve shown in Fig. 8(h) that agrees reasonably well with the results from the FP measurements shown in Fig. 7 for film 4. The GMR responses measured for both devices reinforce the FP measurements showing that for these thicker films, the RI is significantly increased relative to the intrinsic c-Ge values. Films 3 and 4 show a ∼1-2% statistical refractive index variation across the range.
Fig. 8. Resonant device results for films 3 and film 4. (a) SEM cross section of the fabricated film 3 device with dimensions dg = 500 nm, dh = 1700 nm, Λ = 3670 nm, f = 0.63. (b) SEM cross section of fabricated film 4 device with remaining PR grating and dimensions dg = 720 nm, dh = 1530 nm, Λ = 3670 nm, f = 0.63. (c) Film 3 TE response. (d) Film 3 TM response. (e) RI values used in film 3 simulations. (f) Film 4 TE response. (g) Film 4 TM response. (h) RI values used in film 4 simulations. Measured data are shown in red, calculated spectra using c-Ge RI are shown in blue, and calculated spectra using logarithmically fit RI data are shown in black.
Lastly, film 5 is grown using a deposition time of 40,312 seconds and otherwise identical parameters as the previous devices which results in a film thickness of 3950 nm as shown in Fig. 9(a). FP transmission measurements are taken for the film as shown in Fig. 9(b), and the FP peak fitting results are shown in Fig. 9(c) with the results indicating elevated RI values beyond those seen in films 3 & 4. The FP curve fit data indicates a 16% increase in the RI at 2.5 µm and a 13% increase at 12.5 µm. Figure 10 summarizes the results for all five films by displaying the RI values derived from FP measurements along with the RI for intrinsic c-Ge.
Fig. 9. Film 5 FP measurements and RI curve fitting. (a) SEM cross section of the 3950 nm sputtered film along with a PR grating used to calibrate thickness measurements. (b) FP transmission measurement results. (c) RI curve resulting from matching FP transmission peak locations.
2.2 Material analysis
A chemical analysis is performed on films 1, 3, and 5 in order to rule out any material contamination that may have occurred during the sputtering process. Energy dispersive spectroscopy (EDS) utilizing a Bruker SEM with a 20 keV beam is used to analyze the chemical makeup of the films. Table 1 shows the normalized mass percentage and atomic percentage for the three films. Germanium comprises the bulk of the material. Zinc and sulfur are present originating in the substrate; in the thicker films they present smaller returns as the beam has to tunnel through more Ge to reach the substrate layer. Silicon and oxygen are present deriving from the passivation layer and surface oxides, trace amounts of argon remain trapped from the sputtering process, and there is a consistent amount of carbon in all three samples resulting from surface contamination. Verification of this analysis is provided next through X-ray diffraction measurements.
Table 1. EDS measurement results
X-ray diffraction (XRD) analysis is performed on the films to gain insight on their structural morphology. Figure 11 shows the spectral results for films 1, 3, and 5 zoomed in on the 2θ region from 20-30°. This range is selected to look for signatures from the Ge <111 > diffraction peak which exists near 27.4° [25]. The gap in the spectrum between 28° and 29° is due to the ZnS <111 > signal [26] which would dominate the spectrum if shown at full scale due to the strong crystallinity of the substrate. The figure highlights an area in red in which the detection intensity reaches an arbitrary value of 60 where there is a general broadening of the spectral return in the Ge <111 > region as the film thickness increases. A broadened XRD return indicates a more amorphous material in contrast to a sharp peak that would be seen in the case of high crystallinity; in this case the results imply that as the sputtered film thickness is increased, it is accompanied by an increase in amorphousness. An analysis of the results showed no other significant peaks from other alloys that may have formed during the deposition process which reinforces the assertion that the measured RI results are for pure germanium.
Fig. 11. XRD analysis of sputtered films. (a) Film 1. (b) Film 3. (c) Film 5. Red bars indicate regions where the peak return signal is greater than 60 in detector intensity. The highlighted areas show a general broadening of the signal as the film thickness increases. To the right is a zoomed-out view of each spectrum where the broadening is more visually apparent.
3. Discussion
The refractive index of sputtered germanium films with varying thickness is measured and confirmed through two-part independent measurements including a novel guided-mode resonance based approach. This method is shown to be an effective means of refractive-index verification as applied here. The method is not limited to the particular case treated but is universally applicable for materials systems that can be fashioned into thin films on which periodic patterns can be inscribed. The results indicate a varying refractive index that can range in value from 10% lower than intrinsic c-Ge at 10 µm wavelength for the thinnest depositions to up to 13% higher for the thickest film at that wavelength. EDS measurements show that the films do not contain any unexpected elements, and XRD results indicate a generally amorphous material composition with no unexpected additional alloys or compounds. The XRD results also suggest an increase in amorphousness as the deposition time and corresponding film thickness is increased.
It is likely that these films contain a graded refractive index (GRIN) resulting from a gradual change in the film morphology during deposition as evidenced by differing refractive index results between the FP and resonant device measurements recorded for film 5 and for the slight differences seen in the results for films 3 and 4; the GMR fit data for film 5 (not presented) did not allow a convincing match for this reason.
The explanation for the RI shift between FP and GMR measurements for film 5 may lie in slight lattice constant mismatches between ZnS (lattice constant 5.41 Å) and Ge (lattice constant 5.66 Å) or in induced surface roughness due to ion bombardment during substrate preparation that may lead to the formation of small voids or island growth during the initial deposition, creating a micro-porous structure and leading to the decreased effective refractive index measured here. As the deposition continues, the film structure homogenizes into a denser structure as reported in [20] for films less than 12 µm in thickness consistent with the observed increased RI in our samples. Etching into the material while patterning the device would remove material from the denser upper layers with lower index material being left behind to form the homogenous layer in the base of the device which would cause a shift in the resonant response. Further studies would be necessary to determine the precise nature of the effect and particularly the extent of GRIN formation.
The effective indices seen by the FP measurements and GMR devices would also vary due to the nature of the effects. FP measurement peaks result from phase delays in normally incident light passing through the film and will record only the aggregate response of the film. GMR, in contrast, relies on laterally traveling modes within the waveguide grating and the resonant response will be strongly affected by any gradient in the RI profile of the structure. The minimally differing RI results observed between the FP and GMR in films 3 and 4 and the more pronounced results from the thicker film 5 therefore support the idea of a gradient in the RI of the films.
4. Conclusion
In this work, we have shown that it is possible to considerably increase in the index of refraction of sputtered germanium films in the mid and long wave IR bands. The source of this increase likely lies in the increased density of the sputtered film during the deposition. This discovery can have a significant impact on the development on any number of photonic devices operating in the MWIR and LWIR regions as many applications rely on the ability to create strong optical mode confinement enabled by high refractive index materials. Additional research is needed to standardize the process to achieve high index films regardless of the deposition thickness, possibly through the use of techniques such as time-gated, ion-assisted deposition to create uniform high-density films in the early stages of the deposition.
Funding
Texas Emerging Technology Fund; Texas Instruments (Distinguished University Chair in Nanoelectronics).
Acknowledgements
Photomasks were fabricated by Gordon Pollack in the UT Dallas Cleanroom Research Laboratory. Parts of this research were conducted in the UT Arlington Shimadzu Institute Nanotechnology Research Center. The research was supported, in part, by the UT System Texas Nanoelectronics Research Superiority Award funded by the State of Texas Emerging Technology Fund as well as by the Texas Instruments Distinguished University Chair in Nanoelectronics endowment.
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