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High-index low-loss gallium phosphide thin films fabricated by radio frequency magnetron sputtering
Thin Solid Films 519 (2011) 5424–5428
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
High-index low-loss gallium phosphide thin films fabricated by radio frequency magnetron sputtering Jian Gao ⁎, Qiwen Zhan, Andrew M. Sarangan Electro-Optics Graduate Program, University of Dayton, 300 College Park, Dayton OH 45469, USA
a r t i c l e
i n f o
Article history: Received 20 September 2010 Received in revised form 14 February 2011 Accepted 18 February 2011 Available online 2 March 2011 Keywords: Gallium Phosphide Thin films Refractive index Absorption loss Radio-frequency sputtering Ellipsometry
a b s t r a c t High-index low-loss Gallium Phosphide thin films for visible light have been produced by radio frequency magnetron sputtering in an argon environment. This broadens the high refractive index limit of transparent optical materials using a physical deposition process. Energy-dispersive x-ray analysis and spectroscopic ellipsometry were used to characterize the stoichiometry and optical properties. A post-deposition hightemperature anneal was found to be necessary to restore the proper stoichiometric ratio and to reduce the absorption. The annealing conditions were optimized by an in-situ fiber-optic transmission spectrum monitoring system. The films exhibit a high refractive index (N = 3.23) and a low extinction coefficient (K = 0.029) at 633 nm. Such high index GaP films have broad applications in nanophotonic device designs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Optical materials with very high and very low refractive indices are sought after for many photonics applications in the visible spectral range. For high refractive indices, the most common materials are TiO2, ZnS, ZnSe, CdS, ZnTe etc., which are widely used in anti-reflection (AR) coating applications [1], bio-sensing and imaging [2], photonic crystals [3] and high-Q micro-resonators [4]. However, these materials all have refractive indices that are lower than 3, which is significantly lower than semiconductors such as Si and Ge in the infra-red region. Gallium Phosphide (GaP) is a semiconductor material that is known to be transparent for the yellow and red spectral portions of the visible spectrum with a high real part (N~ 3.4) of the complex refractive index n = N-iK [5]. GaP is usually grown in crystalline form using epitaxial techniques primarily for the light emitting diode market [6]. In optical coating applications, amorphous or polycrystalline films from physical deposition techniques are generally used over crystal growth methods. However, the films produced by physical vapor deposition will exhibit different optical properties from their crystal counterpart depending on the techniques used: radio frequency (RF) sputtering, e-beam evaporating, chemical vapor deposition and pulse laser deposition (PLD) etc. It is known that optical properties are significantly influenced by the stoichiometric composition of the films [7]. In the case of GaP, the resulting films appear to be either Ga-rich, which will exhibit a metallic shiny appearance, or P-rich, which will be dark brown. Both cases will ⁎ Corresponding author at: 300 College Park, College Park Center, RM572, Dayton, OH 45469–2951, USA. Tel.: +1 937 654 0102; fax: +1 937 229 2097. E-mail address: [email protected] (J. Gao). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.02.068
result in high absorption of visible light. One way to achieve proper stoichiometric balance during deposition is to flow reactive background gasses [8]. However, the precursor to phosphorus is usually phosphine which is a toxic and flammable gas. The precursor to gallium is trimethygallium, which is pyrophoric. Both gasses are rarely utilized in coating applications outside of metalorganic chemical vapor deposition. In this report, we present the results of the optimization of RF magnetron sputtered GaP films in argon background through a high temperature annealing process to restore the proper stoichiometry. Compared with e-beam evaporation and PLD, RF sputtering does not involve high temperatures of the source material or the substrate. It is known that the atomic bond between Ga and P dissociates at high temperatures, resulting in volatile phosphorous and its oxidized compounds as byproducts [9]. In the case of electron-beam evaporation, when continuously heated with high-energy electrons, the GaP evaporating source will become extremely Ga-rich due to the net release of phosphorous. This was observed by the GaP pellets turning metallic after a brief evaporation. RF sputtering, on the other hand, occurs at a fairly low temperature (b45 °C) where the target is in direct contact with a water cooled backing plate. The temperatures of the target and substrate were verified by attaching temperature sensors on the surfaces. 2. Experimental details 2.1. Fabrication of GaP thin films The target was a 3 in. GaP crystalline wafer bonded to a 3 in. copper backing plate and mounted to the Onyx-3 Angstrom Science magnetron cathode on the Denton Vacuum Explorer 14 coating system with a Seren
J. Gao et al. / Thin Solid Films 519 (2011) 5424–5428
The optical properties of GaP films were characterized using an Angstrom Advanced PhE-102 spectroscopic ellipsometer. By choosing the extended Cauchy model [12] with six fitting parameters (An, Bn, Cn, α, β and γ), ψ = tan− 1(|rp| / |rs|) can be calculated through varying N&K and fitted them to the measured ψ. The expressions of N&K as a function of the wavelength are written as Bn C + n4 λ2 λ 1 1 K = α exp 1:24β − : λ γ N = An +
With proper fitting parameters, the measured and calculated ψ will overlap with each other indicating the closest N&K. The uniqueness of our fitting was verified by using multiple angle measurements. The fitted thickness was confirmed by physical measurements using the Ambios XP-1 Stylus Profiler. Although GaP was sputtered at fairly low RF powers, the Ar plasma still caused the dissociation of the Ga―P atomic bonds and their different volatilities that resulted in slight stoichiometric changes in the film. The stoichiometric composition analysis was done to study the chemical change on the Zeiss EVO-50XVP Environmental Scanning Electron Microscope with EDAX Genesis 2000 energy dispersive spectroscopy system. 3. Results and analysis 3.1. GaP thin films fabricated with different RF power Ellipsometry test showed that the GaP films sputtered at lower power had lower refractive indices and extinction coefficients than those made with higher power. At the wavelength of 532 nm, N ~ 3.58 and K ~ 0.47 indicate a relatively high absorption for the sample fabricated at 100 W. The measured thicknesses are 105.2 and 97.4 nm on ellipsometer and profiler respectively, which are very close to the reading of crystal monitor. Although these low power samples are much transparent, K remained at high levels (~0.29). A comparison
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λ (nm) Fig. 1. Optical constants of GaP thin films (100 nm) fabricated with 25 and 100 W RF power before anneal.
over the optical constants of the two GaP films with the same targeted thickness (~100 nm) is shown in Fig. 1. The band edge of the sample deposited at 25 W blue-shifts towards a shorter wavelength and the refractive indices drop to ~2.7 at 900 nm. Ten locations were scanned by energy dispersive X-ray spectroscopy (EDX) on each surface of the GaP films deposited on silicon substrates with different RF power. The integration time of each counting process is set to be 30 s. A high degree of non-uniformity was observed, as 5 of the total 50 locations gave Ga-rich results and the other 45 locations gave P-rich results as shown in Fig. 2(a). As the sputtered species will contain a mixture of atomic Ga and P, as well as molecular GaP at around room temperature during sputtering, the higher volatility of Ga due to lower melting point will cause the phosphorous atoms having a greater chance to accumulate on the substrate. Stoichiometric non-uniformity at the surface also indicates that potential atomic porosity between GaP particles exists in these films. At the same time, oxygen is absorbed into the films as shown in
a 3
Ga:P
2.2. Characterization of GaP thin films
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R601 RF supply and Pfeifer TMH 261 turbo molecular drag pump. Silicon and fused silica substrates were used to facilitate the chemical and optical properties tests. Both substrates were soaked in a piranha solution for 20 min to clean the organic contaminations and followed by deionized water rinse and N2 blown dry. The coating chamber was pumped down to 1.33× 10−4 Pa (1× 10−6 Torr) vacuum. A 15 min presputtering step was conducted with the shutter closed before each deposition to clean the surface oxides of the target. The GaP films were deposited using different RF plasma discharge power (100, 80, 60, 40 and 25 W) at 0.533 Pa (4 × 10−3 Torr) Ar pressure on a rotating unheated substrate stage. The deposition rate was 1.8 A/s at 100 W RF power and dropped down to 0.2 A/s at 25 W. An in-situ quartz crystal microbalance was used to monitor the thickness growth. The intention of trying different power levels was to find the optimal conditions conducive for low loss GaP film growth. In other words, more optically transparent films were expected by lowering the power, which was found to be the case. However, the optical property measurements were still far from the ideal in comparison to crystalline GaP values. It is known that high temperature annealing results in the re-crystallization and structural improvement of thin films fabricated with physical deposition. Annealing the substrate at a relatively low temperature (300–500 °C) has been reported for film deposition [7,10], ion implanting and doping [11]. The thermal energy helps to rearrange the material from amorphous to crystalline structure. A post-process atmospheric anneal was found to significantly improve the film quality, thus different anneal time was experimented for the same thickness at 700°C.
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RF Power (watt) Fig. 2. (a) Stoichiometry analysis of Ga:P ratio in GaP thin films (100 nm) fabricated using different RF power; (b) Atom percentage of absorbed oxygen in the films.
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maintain a high refractive index for the material, high power is an important condition.
8 Ga
7
3.2. Results of annealing at 700 °C
P
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O
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Time (hrs) Fig. 3. Stoichiometry changes of GaP thin films fabricate with 80 W RF power for different anneal time.
Fig. 2(b). Although the native oxide of the silicon substrate is measured to be around 0.8%, the trend in oxygen content for GaP films with the same thickness but different deposition power is consistent with the fact that films made at lower RF powers tend to have lower packing density, greater porosity and lower refractive indices. Thus, in order to
With the RF power maintained at a high level, a set of samples (100 nm film thickness) on fused silica were annealed in air at the atmospheric pressure with the temperature of 700°C. The annealing time was gradually reduced from 4 h to 2 h, 1 h and 30 min. For excessive anneal like 4 h, a transparent light yellow film was obtained with N dropping down to 2.8 and K ~ 0.01 at 532 nm. This is nearly lossless for green light. It was also found that a significant amount of oxygen was absorbed into the film (approximately 8% after annealing compared with just 2% oxygen atoms before annealing), and the films turned into slightly Ga-rich. The increased oxidation of the films continuously reduced both the index of refraction and the extinction coefficient. As the oxygen percentage increases, a more complex oxide compound is formed that the six parameter Cauchy model is no longer able to get a good fit for ψ on the ellipsometer. With less annealing time, it limits the oxidation process to a certain degree that a compromised transparency can be achieved while maintaining a high refractive index. EDX tests of the samples with different annealing time show that the oxidation rate grows rapidly within 1 h and eventually the oxygen atomic percentage gets stabilized around 8% (Fig. 3). The sample that annealed for 30 min has a high index of refraction and transparency (shown in Fig. 4). The refractive index slowly drops from 3.5 at 400 nm to 3.0 at 900 nm and extinction coefficient decreases to nearly zero (within the measurement limits of the ellipsometer). At the wavelength of 633 nm, the index of refraction N = 3.19 and extinction coefficient K = 0.05 are obtained. The fitted thickness from ellipsometry is 87.5 nm which is in good agreement with the measured thickness of 94.5 nm from stylus profiler. To confirm these optical properties obtained from the ellipsometer, a HeNe laser was used to test the transmission and reflection of the annealed sample. Except for very large incident angle, where the light is partially coupled into the substrate and exits through the sidewalls, the calculated transmittance (T) and reflectance (R) based on ellipsometer result agree very well with the experiment data (shown in Fig. 5). The Filmetrics F10-VC spectrometer was also used to verify the dispersion relation of refractive indices for the spectrum ranging from 400 to 900 nm (shown in Fig. 6)
Transmission, Reflection
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0 Fig. 4. Ellipsometer test of GaP (87.5 nm) on fused silica annealed for 30 min (a) Ellipsometer fitting of calculated Psi to the test data at the incident angles of 40 and 50°. (b) Dispersion spectrum of Cauchy optical constants.
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Fig. 5. Transmission and reflection test of the GaP thin film on fused silica substrate deposited at 80 W RF power and annealed for 30 min.
J. Gao et al. / Thin Solid Films 519 (2011) 5424–5428
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Wavelength (nm) Fig. 6. Transmission test for 400–900 nm spectral range using Filmetrics F10-VC spectrometer at normal incidence.
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at normal incidence. The red line is the measured transmission spectrum which is very close to the theoretical calculation (blue line) using the refractive indices from ellipsometer test.
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time (min) Fig. 7. Photon counts of the transmitted light monitored by spectrometer for different wavelengths (510, 532, 633 and 690 nm), the GaP film is fabricated with 100 W RF power.
3.3. Optimization of annealing conditions in Fig. 10). Regarding the high atom percentage in Ga2O3, P2O5 and GaxPyOz compound, oxidation was eliminated within a reasonable range which maintained the high index and reduced the extinction coefficient. High temperature anneal in an Ar environment was also tried to study the impact of oxidation. At vacuum and low Ar pressures 1066 Pa (8 Torr), the GaP films (100 nm) were all evaporated and disappeared within 15 min. However, at atmospheric Ar environment, the film survived after a 46 min anneal at 700°C with an ellipsometer measured thickness of 87 nm. The index of refraction and extinction coefficient gave N = 3.32 and K = 0.023 at 633 nm, which were on the same level as the optimal atmospheric annealing. Higher Ar pressure slowed down the film evaporation rate and no major increase of the
7 6 5
Atom %
Since the atmospheric anneal process is a tradeoff between oxidation and absorption loss, a systematic optimization will require real-time monitoring of these parameters during the anneal process. Hence an annealing monitoring system was designed with a 200 μm diameter high-temperature fiber connected to a Tungsten Halogen Light source and another 1 mm diameter fiber connected to an OceanOptics HR2000 Spectrometer. The GaP sample is placed between these two fibers on a stainless steel fixture in the high-temperature furnace. The transmission spectrum is recorded so that the optimum anneal time can be found to achieve reduced extinction coefficient while still maintain high index of refraction. Transmitted intensities at four wavelength channels (510, 532, 633 and 690 nm) were recorded during the annealing (Fig. 7). For longer wavelengths, the transmission increases rapidly in 20 min and then oscillates periodically while for short wavelengths the increase of transmission slows down after 20 min. Structural refinement of the thin film dominates the beginning of the annealing process, with oxidation taking over slowly and decreases the index of refraction and extinction coefficient. The level of oxidation as a function of time was measured by EDX on a set of samples with different annealing times (shown in Fig. 8) close to the optimal annealing time (20 min) which confirms the expected trends in atomic compositions. A comparison of GaP films fabricated with high and low RF power before and after annealing is shown in Fig. 9. Both samples are annealed for their optimized times (19 min and 7 min) to achieve the large transmission for 633 nm. The color of the 100 W sample (100 nm) changes from dark brown to transparent medium yellow. Since it has a higher film density (due to the high sputter energy), the refractive index N is maintained at a high level (N= 3.35 at 532 nm, 3.23 at 633 nm). The extinction coefficient decreases to 0.039 and 0.029, which causes 25% and 7% energy loss for normal incident light of 532 nm and 633 nm respectively. For the low power sample (25 W, 100 nm), refractive index drops to 2.90 and extinction coefficient is 0.05 at 633 nm (shown
4 3 2 O
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Time (mins) Fig. 8. Stoichiometric change of GaP film (fabricated with 100 W RF power) annealed for different time around optimal annealing time.
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Fig. 9. Comparison of GaP films fabricated with 100 and 25 W RF power before and after optimal anneal.
4. Conclusions
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References
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In summary, high quality GaP thin films with high refractive index and low absorption loss have been made using RF magnetron sputtering. It is found that high power RF sputtering is preferred for a higher film density, and an additional annealing process is applied to restore the proper stoichiometry and achieve re-crystallization. An insitu fiber-optic transmission monitoring system was constructed for finding the optimal anneal time. GaP thin films with optical properties close to the crystalline GaP have been obtained.
900
λ (nm) Fig. 10. Optical constants of GaP thin films (100 nm) fabricated with 25 and 100 W RF power after optimal anneal.
oxygen percentage was observed, but after continually annealing the sample for more than 2 h, the film eventually disappeared. Compared with annealing in Argon, the partial oxidation that occurs during atmospheric anneal helped to prevent the films from evaporating.
[1] J.-Q. Xi, M.F. Schubert, J.K. Kim, E.F. Schubert, M. Chen, S.-Y. Lin, W. Liu, J.A. Smart, Nature Photon. 1 (2007) 176. [2] J. Gao, A. Widjaja, M. Pan, A. Sarangan, Q. Zhan, in: M. Razeghi, H. Mohseni (Eds.), SPIE Optics and Photonics, San Diego, U.S.A., August 28, 2008, Proceedings of SPIE 7035, , 2008, p. 70350W. [3] F.J. Schuurmans, D. Vanmaekelbergh, J. van de Lagemaat, A. Lagendijk, Science 284 (1999) 141. [4] J. Heebner, R. Grover, T.A. Ibrahim, Optical Microresonators: Theory, Fabrication, and Applications, Springer-Verlag, London, 2007. [5] D.E. Aspnes, A.A. Studna, Phys. Rev. B 27 (1983) 985. [6] M.M. Sobolev, V.G. Nikitin, Tech. Phys. Lett. 24 (1998) 329. [7] Y. Li, Z. Liu, Acta Phys. Sin-ch Ed. 58 (2009) 5022. [8] F.M. Davidson, R. Wiacek, B.A. Korgel, Chem. Mater. 17 (2005) 230. [9] D.M. Rowe, G. Min, Y.A. Chen, Appl. Phys. 27 (1994) 182. [10] N.G. Tsvelykh, V.A. Tsendrovskii, Sov. Phys. J. 14 (1971) 1233. [11] D.L. Cui, J.Q. Pan, Z.C. Zhang, B.B. Huang, X.Y. Qin, M.H. Jiang, S.M. Gao, Prog. Cryst. Growth Charact. Mater. 40 (2000) 145. [12] H.G. Tompkins, W.A. McGahan, Spectroscopic Ellipsometry and Reflectometry, Wiley, New York, 1999.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
High-index low-loss gallium phosphide thin films fabricated by radio frequency magnetron sputtering Jian Gao ⁎, Qiwen Zhan, Andrew M. Sarangan Electro-Optics Graduate Program, University of Dayton, 300 College Park, Dayton OH 45469, USA
a r t i c l e
i n f o
Article history: Received 20 September 2010 Received in revised form 14 February 2011 Accepted 18 February 2011 Available online 2 March 2011 Keywords: Gallium Phosphide Thin films Refractive index Absorption loss Radio-frequency sputtering Ellipsometry
a b s t r a c t High-index low-loss Gallium Phosphide thin films for visible light have been produced by radio frequency magnetron sputtering in an argon environment. This broadens the high refractive index limit of transparent optical materials using a physical deposition process. Energy-dispersive x-ray analysis and spectroscopic ellipsometry were used to characterize the stoichiometry and optical properties. A post-deposition hightemperature anneal was found to be necessary to restore the proper stoichiometric ratio and to reduce the absorption. The annealing conditions were optimized by an in-situ fiber-optic transmission spectrum monitoring system. The films exhibit a high refractive index (N = 3.23) and a low extinction coefficient (K = 0.029) at 633 nm. Such high index GaP films have broad applications in nanophotonic device designs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Optical materials with very high and very low refractive indices are sought after for many photonics applications in the visible spectral range. For high refractive indices, the most common materials are TiO2, ZnS, ZnSe, CdS, ZnTe etc., which are widely used in anti-reflection (AR) coating applications [1], bio-sensing and imaging [2], photonic crystals [3] and high-Q micro-resonators [4]. However, these materials all have refractive indices that are lower than 3, which is significantly lower than semiconductors such as Si and Ge in the infra-red region. Gallium Phosphide (GaP) is a semiconductor material that is known to be transparent for the yellow and red spectral portions of the visible spectrum with a high real part (N~ 3.4) of the complex refractive index n = N-iK [5]. GaP is usually grown in crystalline form using epitaxial techniques primarily for the light emitting diode market [6]. In optical coating applications, amorphous or polycrystalline films from physical deposition techniques are generally used over crystal growth methods. However, the films produced by physical vapor deposition will exhibit different optical properties from their crystal counterpart depending on the techniques used: radio frequency (RF) sputtering, e-beam evaporating, chemical vapor deposition and pulse laser deposition (PLD) etc. It is known that optical properties are significantly influenced by the stoichiometric composition of the films [7]. In the case of GaP, the resulting films appear to be either Ga-rich, which will exhibit a metallic shiny appearance, or P-rich, which will be dark brown. Both cases will ⁎ Corresponding author at: 300 College Park, College Park Center, RM572, Dayton, OH 45469–2951, USA. Tel.: +1 937 654 0102; fax: +1 937 229 2097. E-mail address: [email protected] (J. Gao). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.02.068
result in high absorption of visible light. One way to achieve proper stoichiometric balance during deposition is to flow reactive background gasses [8]. However, the precursor to phosphorus is usually phosphine which is a toxic and flammable gas. The precursor to gallium is trimethygallium, which is pyrophoric. Both gasses are rarely utilized in coating applications outside of metalorganic chemical vapor deposition. In this report, we present the results of the optimization of RF magnetron sputtered GaP films in argon background through a high temperature annealing process to restore the proper stoichiometry. Compared with e-beam evaporation and PLD, RF sputtering does not involve high temperatures of the source material or the substrate. It is known that the atomic bond between Ga and P dissociates at high temperatures, resulting in volatile phosphorous and its oxidized compounds as byproducts [9]. In the case of electron-beam evaporation, when continuously heated with high-energy electrons, the GaP evaporating source will become extremely Ga-rich due to the net release of phosphorous. This was observed by the GaP pellets turning metallic after a brief evaporation. RF sputtering, on the other hand, occurs at a fairly low temperature (b45 °C) where the target is in direct contact with a water cooled backing plate. The temperatures of the target and substrate were verified by attaching temperature sensors on the surfaces. 2. Experimental details 2.1. Fabrication of GaP thin films The target was a 3 in. GaP crystalline wafer bonded to a 3 in. copper backing plate and mounted to the Onyx-3 Angstrom Science magnetron cathode on the Denton Vacuum Explorer 14 coating system with a Seren
J. Gao et al. / Thin Solid Films 519 (2011) 5424–5428
The optical properties of GaP films were characterized using an Angstrom Advanced PhE-102 spectroscopic ellipsometer. By choosing the extended Cauchy model [12] with six fitting parameters (An, Bn, Cn, α, β and γ), ψ = tan− 1(|rp| / |rs|) can be calculated through varying N&K and fitted them to the measured ψ. The expressions of N&K as a function of the wavelength are written as Bn C + n4 λ2 λ 1 1 K = α exp 1:24β − : λ γ N = An +
With proper fitting parameters, the measured and calculated ψ will overlap with each other indicating the closest N&K. The uniqueness of our fitting was verified by using multiple angle measurements. The fitted thickness was confirmed by physical measurements using the Ambios XP-1 Stylus Profiler. Although GaP was sputtered at fairly low RF powers, the Ar plasma still caused the dissociation of the Ga―P atomic bonds and their different volatilities that resulted in slight stoichiometric changes in the film. The stoichiometric composition analysis was done to study the chemical change on the Zeiss EVO-50XVP Environmental Scanning Electron Microscope with EDAX Genesis 2000 energy dispersive spectroscopy system. 3. Results and analysis 3.1. GaP thin films fabricated with different RF power Ellipsometry test showed that the GaP films sputtered at lower power had lower refractive indices and extinction coefficients than those made with higher power. At the wavelength of 532 nm, N ~ 3.58 and K ~ 0.47 indicate a relatively high absorption for the sample fabricated at 100 W. The measured thicknesses are 105.2 and 97.4 nm on ellipsometer and profiler respectively, which are very close to the reading of crystal monitor. Although these low power samples are much transparent, K remained at high levels (~0.29). A comparison
3.4
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500
600
700
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500
600
700
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λ (nm) Fig. 1. Optical constants of GaP thin films (100 nm) fabricated with 25 and 100 W RF power before anneal.
over the optical constants of the two GaP films with the same targeted thickness (~100 nm) is shown in Fig. 1. The band edge of the sample deposited at 25 W blue-shifts towards a shorter wavelength and the refractive indices drop to ~2.7 at 900 nm. Ten locations were scanned by energy dispersive X-ray spectroscopy (EDX) on each surface of the GaP films deposited on silicon substrates with different RF power. The integration time of each counting process is set to be 30 s. A high degree of non-uniformity was observed, as 5 of the total 50 locations gave Ga-rich results and the other 45 locations gave P-rich results as shown in Fig. 2(a). As the sputtered species will contain a mixture of atomic Ga and P, as well as molecular GaP at around room temperature during sputtering, the higher volatility of Ga due to lower melting point will cause the phosphorous atoms having a greater chance to accumulate on the substrate. Stoichiometric non-uniformity at the surface also indicates that potential atomic porosity between GaP particles exists in these films. At the same time, oxygen is absorbed into the films as shown in
a 3
Ga:P
2.2. Characterization of GaP thin films
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R601 RF supply and Pfeifer TMH 261 turbo molecular drag pump. Silicon and fused silica substrates were used to facilitate the chemical and optical properties tests. Both substrates were soaked in a piranha solution for 20 min to clean the organic contaminations and followed by deionized water rinse and N2 blown dry. The coating chamber was pumped down to 1.33× 10−4 Pa (1× 10−6 Torr) vacuum. A 15 min presputtering step was conducted with the shutter closed before each deposition to clean the surface oxides of the target. The GaP films were deposited using different RF plasma discharge power (100, 80, 60, 40 and 25 W) at 0.533 Pa (4 × 10−3 Torr) Ar pressure on a rotating unheated substrate stage. The deposition rate was 1.8 A/s at 100 W RF power and dropped down to 0.2 A/s at 25 W. An in-situ quartz crystal microbalance was used to monitor the thickness growth. The intention of trying different power levels was to find the optimal conditions conducive for low loss GaP film growth. In other words, more optically transparent films were expected by lowering the power, which was found to be the case. However, the optical property measurements were still far from the ideal in comparison to crystalline GaP values. It is known that high temperature annealing results in the re-crystallization and structural improvement of thin films fabricated with physical deposition. Annealing the substrate at a relatively low temperature (300–500 °C) has been reported for film deposition [7,10], ion implanting and doping [11]. The thermal energy helps to rearrange the material from amorphous to crystalline structure. A post-process atmospheric anneal was found to significantly improve the film quality, thus different anneal time was experimented for the same thickness at 700°C.
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RF Power (watt) Fig. 2. (a) Stoichiometry analysis of Ga:P ratio in GaP thin films (100 nm) fabricated using different RF power; (b) Atom percentage of absorbed oxygen in the films.
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maintain a high refractive index for the material, high power is an important condition.
8 Ga
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3.2. Results of annealing at 700 °C
P
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Time (hrs) Fig. 3. Stoichiometry changes of GaP thin films fabricate with 80 W RF power for different anneal time.
Fig. 2(b). Although the native oxide of the silicon substrate is measured to be around 0.8%, the trend in oxygen content for GaP films with the same thickness but different deposition power is consistent with the fact that films made at lower RF powers tend to have lower packing density, greater porosity and lower refractive indices. Thus, in order to
With the RF power maintained at a high level, a set of samples (100 nm film thickness) on fused silica were annealed in air at the atmospheric pressure with the temperature of 700°C. The annealing time was gradually reduced from 4 h to 2 h, 1 h and 30 min. For excessive anneal like 4 h, a transparent light yellow film was obtained with N dropping down to 2.8 and K ~ 0.01 at 532 nm. This is nearly lossless for green light. It was also found that a significant amount of oxygen was absorbed into the film (approximately 8% after annealing compared with just 2% oxygen atoms before annealing), and the films turned into slightly Ga-rich. The increased oxidation of the films continuously reduced both the index of refraction and the extinction coefficient. As the oxygen percentage increases, a more complex oxide compound is formed that the six parameter Cauchy model is no longer able to get a good fit for ψ on the ellipsometer. With less annealing time, it limits the oxidation process to a certain degree that a compromised transparency can be achieved while maintaining a high refractive index. EDX tests of the samples with different annealing time show that the oxidation rate grows rapidly within 1 h and eventually the oxygen atomic percentage gets stabilized around 8% (Fig. 3). The sample that annealed for 30 min has a high index of refraction and transparency (shown in Fig. 4). The refractive index slowly drops from 3.5 at 400 nm to 3.0 at 900 nm and extinction coefficient decreases to nearly zero (within the measurement limits of the ellipsometer). At the wavelength of 633 nm, the index of refraction N = 3.19 and extinction coefficient K = 0.05 are obtained. The fitted thickness from ellipsometry is 87.5 nm which is in good agreement with the measured thickness of 94.5 nm from stylus profiler. To confirm these optical properties obtained from the ellipsometer, a HeNe laser was used to test the transmission and reflection of the annealed sample. Except for very large incident angle, where the light is partially coupled into the substrate and exits through the sidewalls, the calculated transmittance (T) and reflectance (R) based on ellipsometer result agree very well with the experiment data (shown in Fig. 5). The Filmetrics F10-VC spectrometer was also used to verify the dispersion relation of refractive indices for the spectrum ranging from 400 to 900 nm (shown in Fig. 6)
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Fig. 5. Transmission and reflection test of the GaP thin film on fused silica substrate deposited at 80 W RF power and annealed for 30 min.
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time (min) Fig. 7. Photon counts of the transmitted light monitored by spectrometer for different wavelengths (510, 532, 633 and 690 nm), the GaP film is fabricated with 100 W RF power.
3.3. Optimization of annealing conditions in Fig. 10). Regarding the high atom percentage in Ga2O3, P2O5 and GaxPyOz compound, oxidation was eliminated within a reasonable range which maintained the high index and reduced the extinction coefficient. High temperature anneal in an Ar environment was also tried to study the impact of oxidation. At vacuum and low Ar pressures 1066 Pa (8 Torr), the GaP films (100 nm) were all evaporated and disappeared within 15 min. However, at atmospheric Ar environment, the film survived after a 46 min anneal at 700°C with an ellipsometer measured thickness of 87 nm. The index of refraction and extinction coefficient gave N = 3.32 and K = 0.023 at 633 nm, which were on the same level as the optimal atmospheric annealing. Higher Ar pressure slowed down the film evaporation rate and no major increase of the
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Since the atmospheric anneal process is a tradeoff between oxidation and absorption loss, a systematic optimization will require real-time monitoring of these parameters during the anneal process. Hence an annealing monitoring system was designed with a 200 μm diameter high-temperature fiber connected to a Tungsten Halogen Light source and another 1 mm diameter fiber connected to an OceanOptics HR2000 Spectrometer. The GaP sample is placed between these two fibers on a stainless steel fixture in the high-temperature furnace. The transmission spectrum is recorded so that the optimum anneal time can be found to achieve reduced extinction coefficient while still maintain high index of refraction. Transmitted intensities at four wavelength channels (510, 532, 633 and 690 nm) were recorded during the annealing (Fig. 7). For longer wavelengths, the transmission increases rapidly in 20 min and then oscillates periodically while for short wavelengths the increase of transmission slows down after 20 min. Structural refinement of the thin film dominates the beginning of the annealing process, with oxidation taking over slowly and decreases the index of refraction and extinction coefficient. The level of oxidation as a function of time was measured by EDX on a set of samples with different annealing times (shown in Fig. 8) close to the optimal annealing time (20 min) which confirms the expected trends in atomic compositions. A comparison of GaP films fabricated with high and low RF power before and after annealing is shown in Fig. 9. Both samples are annealed for their optimized times (19 min and 7 min) to achieve the large transmission for 633 nm. The color of the 100 W sample (100 nm) changes from dark brown to transparent medium yellow. Since it has a higher film density (due to the high sputter energy), the refractive index N is maintained at a high level (N= 3.35 at 532 nm, 3.23 at 633 nm). The extinction coefficient decreases to 0.039 and 0.029, which causes 25% and 7% energy loss for normal incident light of 532 nm and 633 nm respectively. For the low power sample (25 W, 100 nm), refractive index drops to 2.90 and extinction coefficient is 0.05 at 633 nm (shown
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Time (mins) Fig. 8. Stoichiometric change of GaP film (fabricated with 100 W RF power) annealed for different time around optimal annealing time.
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Fig. 9. Comparison of GaP films fabricated with 100 and 25 W RF power before and after optimal anneal.
4. Conclusions
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References
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In summary, high quality GaP thin films with high refractive index and low absorption loss have been made using RF magnetron sputtering. It is found that high power RF sputtering is preferred for a higher film density, and an additional annealing process is applied to restore the proper stoichiometry and achieve re-crystallization. An insitu fiber-optic transmission monitoring system was constructed for finding the optimal anneal time. GaP thin films with optical properties close to the crystalline GaP have been obtained.
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λ (nm) Fig. 10. Optical constants of GaP thin films (100 nm) fabricated with 25 and 100 W RF power after optimal anneal.
oxygen percentage was observed, but after continually annealing the sample for more than 2 h, the film eventually disappeared. Compared with annealing in Argon, the partial oxidation that occurs during atmospheric anneal helped to prevent the films from evaporating.
[1] J.-Q. Xi, M.F. Schubert, J.K. Kim, E.F. Schubert, M. Chen, S.-Y. Lin, W. Liu, J.A. Smart, Nature Photon. 1 (2007) 176. [2] J. Gao, A. Widjaja, M. Pan, A. Sarangan, Q. Zhan, in: M. Razeghi, H. Mohseni (Eds.), SPIE Optics and Photonics, San Diego, U.S.A., August 28, 2008, Proceedings of SPIE 7035, , 2008, p. 70350W. [3] F.J. Schuurmans, D. Vanmaekelbergh, J. van de Lagemaat, A. Lagendijk, Science 284 (1999) 141. [4] J. Heebner, R. Grover, T.A. Ibrahim, Optical Microresonators: Theory, Fabrication, and Applications, Springer-Verlag, London, 2007. [5] D.E. Aspnes, A.A. Studna, Phys. Rev. B 27 (1983) 985. [6] M.M. Sobolev, V.G. Nikitin, Tech. Phys. Lett. 24 (1998) 329. [7] Y. Li, Z. Liu, Acta Phys. Sin-ch Ed. 58 (2009) 5022. [8] F.M. Davidson, R. Wiacek, B.A. Korgel, Chem. Mater. 17 (2005) 230. [9] D.M. Rowe, G. Min, Y.A. Chen, Appl. Phys. 27 (1994) 182. [10] N.G. Tsvelykh, V.A. Tsendrovskii, Sov. Phys. J. 14 (1971) 1233. [11] D.L. Cui, J.Q. Pan, Z.C. Zhang, B.B. Huang, X.Y. Qin, M.H. Jiang, S.M. Gao, Prog. Cryst. Growth Charact. Mater. 40 (2000) 145. [12] H.G. Tompkins, W.A. McGahan, Spectroscopic Ellipsometry and Reflectometry, Wiley, New York, 1999.