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Abstract
The stability of aluminum (Al) and silver (Ag) metallic thin films (MTFs) under helium ion bombardment has been investigated in the laboratory to replicate the effect of alpha particle bombardment on spacecrafts and satellites in a space environment. The implanted helium ions have varying fluence and energies ranging from 0.5 - 3 keV. The helium ion fluence in the present study has been chosen according to 4 and 6 years journey of a solar orbiter. The reflectivity of Al and Ag MTFs is investigated over a wide range of electromagnetic radiation covering ultraviolet to near infrared (200 - 2500 nm), prior and post helium ion implantation. It is observed that the degradation in the reflectivity of the above-mentioned MTFs is reasonably low for helium ion implantation and no significant impact is observed on reflectivity of both (Al and Ag) MTFs in the investigation. This opens a channel of utilization of these MTFs to provide better protection for the optical components used in spacecrafts. Surface characterization such as surface roughness is carried out to investigate the surface morphology of MTFs prior and post implantation using atomic force microscopy (AFM). It is observed that the effect of implantation on surface morphology is in accordance with the experimental results of reflectivity. SRIM/TRIM simulations help to obtain the distribution profile and penetration depth of helium ions inside the Al and Ag MTFs.
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1. Introduction
The investigation of space environment has long been a subject of interest because of its direct or indirect impact on our earth atmosphere or in missions related to earth exploration. The spacecrafts, satellites, and other space investigating tools such as solar orbiter have been deployed in space to gather information about the solar activity or to study the solar environment in order to achieve deeper understanding about its impact on Earth's atmosphere [1]. These space investigating tools encounter the solar wind particles (mainly high energy protons and alpha particles) and cosmic rays (protons, electrons and heavy ions) coming from the distant stars and galaxies [1,2]. The performance of optical components used in space vehicles is therefore subjected to degradation in harsh space environment [3,4]. Hence, prior knowledge about the irradiation effect of helium ions on Al and Ag MTFs could be useful for their possibility of being used in the space environment and for making robust metallic film mirrors that can sustain the exposure. The alpha particles and protons from the solar wind and cosmic rays bombard the metallic thin films (MTFs) used in spacecrafts and satellites and can hamper the proper functioning of various components connected to the MTFs [5]. In order to study the impact of protons and alpha particles bombardment on various MTFs, it is imperative to have a detailed knowledge of energy range of protons and alpha particles with which they hit the target and their doses (total number of particles during the journey). A significant amount of research regarding the implantation effect of high energy protons has already been reported [6,7], however there are very few studies on irradiation effect of low energy protons and alpha particles. The velocity range of alpha particles and protons coming from the solar winds lies typically in the range of 300 km/s to faster than 750 km/s, with average speed of $\sim$ 400 - 468 km/s [8], and their corresponding kinetic energy typically lies in the range of $\sim$ (0.4 - 7.5) keV for protons and $\sim$ (1.6 - 30) keV for alpha particles. In addition to the above mentioned energy range, there are solar wind particles which are produced from solar events such as Coronal Mass Ejection (CME) and solar flares, have energies in the order of hundreds of keV or in MeV [9].
To this regard, research has been planned according to the European space agency program, which had been set up to explore the solar events by sending a sun orbiting satellite i.e., solar orbiter (SOLO) to its closest distance (0.28 AU at perihelion) [8]. It has been reported that the optical components mainly degrade due to bombardment of low as well as high energy particles such as protons and alpha particles including electrons [1,8]. The degradation of optical materials by low energy (1 keV) proton bombardment has been investigated by M. G. Pelizzo [1] et al. In their investigation, Si/Mo and Ir/Si multi-layer coatings are exposed to low energy protons with the total dose of protons accumulated by a solar orbiter in 3 months and 1 year of space journey around the sun. Such type of coatings were specifically designed for multi element telescope and imaging spectroscopy for solar orbiter [10,11]. It has also been reported earlier that even multi layer coatings used in ultra-violet photo-lithographic apparatus degrade upon exposure to low energy protons [12]. M. Nardello et al. [8] have investigated the bombardment effect of low energy (4 keV) solar wind alpha particles on the nanostructured optical coatings. They have used the multi-layer coatings of Si/Mo and Ir/Si for irradiation and the fluence (total dose) was equivalent to one, two, and four years journey of a solar orbiter. There have been many other experiments to understand the surface properties of irradiated MTFs [13–18]. S. Zuccon et al. [19] have demonstrated the effect of bombardment of low energy (4 keV) helium ions on the optical properties (reflectivity) of gold and iridium thin films in the ultraviolet and visible regions of electromagnetic spectrum. The chosen flux of helium ions was equivalent to the solar winds alpha particle flux obtained during one, two, and four years. Hence, investigation of the optical properties of various MTFs in a space-like environment is essential to prevent optical components from both degradation and malfunctioning. It is with this aim that the present investigation is carried out.
This article reports modifications in the reflectivity of Ag and Al MTFs upon irradiation of low energy helium ion beams with varying energy (0.5, 1, 2, and 3 keV) and varying fluences ranging from 1.10 $\times$ 1016 cm−2 to 1.56 $\times$ 1016 cm−2. The fluence of helium ions is chosen according to the four (1.1 $\times$ 1016 cm−2) and six (1.56 $\times$ 1016 cm−2) years doses of alpha particles encountered by a solar orbiter [8]. It has been observed from the obtained results that helium implanted MTFs have lower reflectivity in case of aluminum and higher in case of silver than the pristine MTF with some anomalies that will be discussed in the manuscript. It has also been seen in the results of both MTFs that for fixed doses of helium ions be it four year dose or six year dose, as the energy of helium ions increases, the penetration depth increases and in turn reflectivity decreases for aluminium in whole range of wavelength, and increases for silver in the UV and visible region as the refractive index gets modified with energy and fluence of helium ions. The variation of both real and imaginary parts of Al MTFs is presented in the discussion part. Interband transition in the reflectivity of each MTF i.e., $\sim$ 814 nm for Al and $\sim$ 320 nm for Ag, is clearly visible in their spectra [20,21]. Hence, the penetration depth and distribution profile of foreign ions, and surface modifications due to ion implantation are the main factors behind the observed results of reflectivity. SRIM/TRIM simulations have been carried out for both Al and Ag MTFs to find out the penetration depth and the ion distribution profile of helium ions inside the MTFs, and it is found that helium ions penetrate more inside the Al than Ag MTFs. It has also been found that as the ion energy increases the peak of ion distribution shifts inwards in the MTF in both cases. This simulation has been carried out using 104 atoms as the input parameter in the SRIM/TRIM simulation. The surface morphology of Al and Ag MTFs before and after implantation has been studied using atomic force microscopy (AFM).
This article is divided into the following sections. Section 2 presents the experimental setup and methodology. Section 3 is on the experimental results followed by discussion. Finally, section 4 summarizes the results and concludes the paper.
2. Experimental setup and methodology
The schematic diagram of the experimental setup is shown in Fig. 1. There are two main chambers in the experimental setup, i.e., the plasma generation chamber or vacuum chamber (VC) and the ion extraction chamber. In the plasma chamber, a plasma is created using microwaves of 2.45 GHz generated by a microwave generator (magnetron: Alter - TMA20). Isolation of magnetron from reflected microwaves is achieved using a water-cooled isolator (ISO). Forward and reflected powers are measured simultaneously from a directional coupler (DC), and a triple stub tuner (TST) is used to match the source (magnetron) and load (plasma) impedance for maximum power transfer. In the experiment, the working gas is helium which is fed into the vacuum chamber (VC) through a gas inlet (GI), and the gas flow is controlled by a mass flow controller (MFC, MKS 1179A). A helium (purity is $\sim$ 99.999 $\%$) plasma is created inside a magnetic multicusp, which is kept inside the plasma chamber, having a length of 50 cm and diameter of 20 cm. The multicusp confines the plasma radially as well as axially due to end-plugging, where the magnets polarities are reversed at both ends [22,23]. The base pressure of the plasma chamber has been kept around $\sim$ 1 $\times$ 10−6 Torr using a Varian 301 Navigator Turbo-molecular pump (TMP) backed by a rotary pump. The working gas pressure and microwave power are kept constant at $\sim$ 4 $\times$ 10−3 Torr and 180 W, respectively during the irradiation process. The ion extraction part of the experimental system is used to extract the helium ions from the plasma chamber and make them impinge on the metallic thin films of aluminum (Al) and silver (Ag) mounted on the collector plate, which is floated with varying negative high voltages (0.5 – 3 kV). Al and Ag samples are mounted $\sim$ 8 - 10 mm away from the multicusp boundary. A high vacuum typically higher than that in the plasma chamber, is maintained in extraction chamber using another pumping set consisting of a TMP backed by a rotary pump. The differential pumping ensures that ions do not suffer from collisions with neutral atoms in the extraction chamber.
Fig. 1. Schematic diagram of experimental setup. MWG, microwave generator; ISO, isolator; DC, directional coupler; TST, triple stub tuner; SSC, straight section.
A high flux of helium ions is obtained, and this is possible due to confinement of the plasma in a small multicusp geometry in our system. The obtained flux in the system ranges from 0.63 $\times$ 1016 cm−2 sec−1 to 1.39 $\times$ 1016 cm−2 sec−1 for energies ranging from 0.5 to 3 keV, respectively, which allows obtaining the helium ions dose corresponding to 4 and 6 years within a few seconds ($\lneq$ 3 seconds). Hence, the irradiation time in our system has been reduced drastically compared to the other ion beam systems used in such work [1,8]. The required fluence is decided by time of irradiation (treatment time). The maximum uncertainty in measured values of fluence is $\sim$ 10 $\%$ as reported in our previous articles [17,18]. Al and Ag thin films are deposited on glass substrates using thermal evaporation technique and the glass substrates are cleaned thoroughly by employing ultrasonication with acetone, isopropyl alcohol and deionized water. The deposition rate is kept $\sim$ 10 angstrom/sec and the base pressure of the chamber is kept $\sim$ 4 $\times$ 10−6 Torr while it increases up to 4 - 6 $\times$ 10−5 Torr (one order higher than the base pressure) during the deposition of Al and Ag MTFs. The thickness of both Ag and Al samples has been kept $\sim$ 330 nm to block the transmission completely. Hence, the absorptivity (A = 1 - R) can be easily calculated using the reflectivity data. The thickness of samples is measured using the thickness monitor installed in the thermal evaporation system and is verified additionally by a profilometer. Al and Ag MTFs of size $\sim$ 1.25 $\times$ 1.25 cm2 are irradiated uniformly with a beam of diameter $\sim$ 2 cm. The samples are prevented from oxidation and atmospheric contamination by keeping them in a vacuum desiccator. Samples have only been taken out from the desiccator at the time of irradiation and spectrophotometric measurement. The reflectivity of Ag and Al MTFs prior (pristine) to and post implantation (irradiated) has been experimentally measured at 8° of normal incidence using a L950 UV-VIS-NIR spectrophotometer in the wavelength ranging from 200 to 2500 nm with the resolution of 5 nm, covering UV to NIR regions of the electromagnetic spectrum. Incoming light of spectrophotometer has 0.5 $\times$ 1 cm2 slit size and its wavelength range is 175 to 3300 nm with 0.05–5.00 nm and 0.20–20.00 nm resolution, in the UV/VIS and NIR regions, respectively. The uncertainty in the measurement of reflectivity R is around (0.2 - 0.3) $\%$ as per the specification sheet of spectrophotometer.
3. Experimental results
3.1 Reflectivity of aluminum (Al) MTFs irradiated with helium ions
Reflectivity of aluminum has been investigated for both pristine and irradiated MTFs from UV (200 nm) to NIR (2500 nm) regions. The reflectivity of irradiated samples is compared to the pristine MTF and is presented in Figs. 2 and 3. The change in Al reflectivity after irradiation with helium ions at fixed fluence (4 years dose) and with varying energy (0.5, 1, 2, and 3 keV) is presented in Fig. 2. While Fig. 3 shows the reflectivity of MTFs irradiated with 6 years dose of helium ions. The reflectivity is shown in three separate regions (UV, VIS, and NIR) of the electromagnetic spectrum in Figs. 2 and 3. It is observed from Fig. 2 that the reflectivity after irradiation with the fluence corresponding to 4 years dose of helium ions shows significant change in the ultraviolet (200 - 400 nm) and visible (400 - 800 nm) region while it shows almost negligible change of reflectivity in the NIR (800 - 2500 nm) region, and this change in reflectivity increases in case of fluence corresponding to 6 years dose, as shown in Fig. 3.
Fig. 2. Reflectivity of aluminum metallic thin films irradiated with helium ions with varying energy and at 4 years dose (1.1 $\times$ 10 16 cm−2) in (A) ultraviolet (UV) region, (B) visible region, (C) near infrared (NIR) region, and (D) aluminum reflectivity in the whole range of wavelength.
Fig. 3. Reflectivity of aluminum metallic thin films irradiated with helium ions with varying energy and at 6 years dose (1.56 $\times$ 10 16 cm−2) in (A) ultraviolet (UV) region, (B) visible region, (C) near infrared (NIR) region, and (D) aluminum reflectivity in the whole range of wavelength.
The change in reflectivity is highest for 2 keV and 6 years dose among the other irradiation combinations of energy and fluence. At fixed fluences (4 or 6 years dose), the reflectivity is found to decrease with increase in helium ion energy with some anomalies for 0.5 and 3 keV in case of 4 years dose, and 1 and 3 keV for 6 years dose. The observed dip at $\sim$ 814 nm (cf. Figures 2(D) and 3(D)) in the reflectivity of both pristine and irradiated samples, corresponds to the interband transition [21]. The position of interband transition does not change with the energy and fluence (either 4 or 6 years doses) of implanted helium ions. Reflectivity of aluminum at $\sim$ 814 nm reduces for 1 and 2 keV and it increases for 0.5 and 3 keV for 4 years dose. In case of 6 years dose, reflectivity increases at $\sim$ 814 nm for each energy except 2 keV. The change in reflectivity is higher for 2 keV compared to the other energies at interband transition. The reflectivity of Al MTFs changes more for 6 years dose than 4 years dose and this modification in reflectivity is more dominant in the ultraviolet (200 - 400 nm) and visible (400 - 800 nm) regions, while the reflectivity changes maximum $\sim$ 2 - 5 $\%$ in infrared (800 - 2500 nm) region of the electromagnetic spectrum after irradiation. The magnified view of reflectivity in the NIR region (1200 - 2500 nm) is shown in the inset of Figs. 2(C) and 3(C). Hence, the Al MTFs may be the suitable candidate to reflect most of the heat radiation (mainly the infrared radiation) coming from the sun. Al MTFs may work without any significant degradation in their reflectivity or optical performance in the infrared region for 4 and 6 years of a space journey. The possible reason behind this anomaly in reflectivity is due to the random value of surface roughness, generated after irradiation. It is observed from the AFM results that the RMS surface roughness of the samples irradiated with ions of energy 0.5 and 3 keV in case of 4 years dose, and 1 and 3 keV in case of 6 years dose, is lower than that of the pristine (unirradiated) as shown in Fig. 4. Lowering of the surface roughness enhances the reflectivity. The effect of roughness is linked to the abnormal behaviour in reflectivity, has been reported earlier [17] and also discussed by S. Zuccon et al. [19]. The RMS surface roughness of Al samples has been shown in Figs. 4(A) and (B) for 4 and 6 years dose. The RMS surface roughness of pristine and irradiated samples has been investigated using tapered AFM (Asylum Research MFP-3D) mode with non coated Si probe tip of radii $\sim$ 8 - 9 nm. Surface morphology of two different areas: 20 $\times$ 20 µm2 and 5 $\times$ 5 µm2 has been investigated and the AFM images of pristine and irradiated (4 years dose) samples having 5 $\times$ 5 µm2 area are presented in Fig. 5. It is found that no specific structures are observed in case of both pristine as well as samples irradiated with 0.5 and 2 keV energy and 4 years dose of helium ions.
Fig. 4. RMS surface roughness of aluminum irradiated MTFs with varying helium ion energy at (A) 4 years dose, and (B) 6 years dose fluence.
Fig. 5. Surface morphology of Al samples (A) pristine, (B) irradiated with 0.5 keV and 4 years dose, and (C) irradiated with 2 keV and 4 years dose.
It has been observed that for each dose (4 and 6 years) of helium ions the roughness variation profile of samples is almost same for both the areas investigated. The RMS roughness of samples (both pristine and irradiated) lies in the range of $\sim$ 19 - 23 nm and $\sim$ 19 - 26 nm for 4 and 6 years dose, respectively. Error bars in roughness data represent the deviation from the average of 2 - 3 scans. The RMS roughness of the irradiated samples at 2 keV and 6 years dose is higher than any other combination of energy and fluence, and this could be the reason behind the maximum change in reflectivity.
An estimate of penetration depth and distribution profile of helium ions inside the Al metallic thin films has been obtained using SRIM/TRIM simulations [24]. The projected range (the perpendicular distance from top surface of the target material to the position where projectile atom/ion stops) and ion distribution profile of helium ions are presented in Fig. 6. Higher energy ions penetrate more than the lower energy ions as shown in the simulation results of Fig. 6(B). The range of ions is $\sim$ 5 - 25 nm for energies of 0.5 - 3 keV. However, it has already been reported that the SRIM/TRIM simulation does not include various effects involved during the irradiation process such as inelastic collisions, many body interaction and thermal diffusion of implanted ions due to heat generated at the time of irradiation, in its Monte Carlo simulation [16]. Hence, it usually underestimates the penetration depth and deviates from the actual value [15], and the ion range has been found much higher $\sim$ 20 nm in EDX spectroscopy than the SRIM/TRIM values ($\sim$ 0.8 nm) as reported by S. Chatterjee et al. [15] where gold metallic thin films were irradiated with 0.5 keV argon ions at 3.47 $\times$ 1016 ions/cm−2. Hence, the range of helium ions inside the Al metallic thin films is expected to be a little higher than the SRIM ($\sim$ 5 - 25 nm) data [15,16].
Fig. 6. (A) He ion distribution profile in aluminum at different energies, and (B) projected range of helium ions of varying energy inside the Al MTF obtained from SRIM/TRIM simulations.
In addition to the formation of heterogeneous medium, the projectile ions lose most of their energy by colliding elastically to the target atoms and the target atoms displace from their mean position and produces the defects in crystal lattice structure. The refractive index or dielectric constants of the heterogeneous medium differs from the pristine metallic films [17,18,25,26] because of voids, lattice defects, and surface roughness, which causes the reflectivity to change in the irradiated samples as compared to the pristine MTF, in addition to the inhomogeneity effect.
3.2 Reflectivity of silver (Ag) MTFs irradiated with helium ions
Reflectivity of silver (Ag) metallic thin films irradiated with helium ions of varying energy and fluence (equivalent to 4 and 6 years dose of alpha particles) is shown in Figs. 7 and 8. Reflectivity is shown separately in three regions (UV, VIS, and NIR) of the electromagnetic spectrum in Figs. 7 and 8(A), (B) and (C), and the combined reflectivity is shown in Figs. 7(D) and 8(D).
Fig. 7. Reflectivity of silver metallic thin films irradiated with helium ions with varying energy and at 4 years dose (1.1 $\times$ 10 16 cm−2) in (A) ultraviolet (UV) region, (B) visible region, (C) near infrared (NIR) region, and (D) silver reflectivity in the whole range of wavelength.
Fig. 8. Reflectivity of silver metallic thin films irradiated with helium ions with varying energy and at 6 years dose (1.56 $\times$ 10 16 cm−2) in (A) ultraviolet (UV) region, (B) visible region, (C) near infrared (NIR) region, and (D) silver reflectivity in the whole range of wavelength.
The reflectivity tends to increase with energy of helium ions at fixed ion dose in ultraviolet region unlike in aluminum, while it shows decrease in reflectivity for 1 keV energy (as shown in the inset of Fig. 7(B)) in visible and near infrared region at 4 years dose. Except at energy of 1 keV, it is found that reflectivity increases with energy of implanted ions. The increase and decrease in reflectivity of irradiated samples are compared to the reflectivity of pristine sample and it is found that the reflectivity is changed by maximum of $\sim$ $20\%$ in UV region while it is $\sim$ $(3-4)\%$ and less than $\sim$ $1\%$ in visible and infrared regions, respectively. The reflectivity of Ag samples has also been investigated for 6 years dose of helium ions and it is observed that the reflectivity for each helium ion energy is higher than the pristine sample in both UV and visible regions (as shown in the inset of Fig. 8(B)). The reflectivity is found to decrease with energy of helium ions in the higher wavelength region ($\sim$ 1500 - 2500 nm). The change in reflectivity is higher towards the shorter wavelength side and this difference reduces as we move toward the higher wavelength side. Reflectivity of Ag MTFs (pristine as well as irradiated) shows a dip in its spectra at $\sim$ 320 nm ($\sim$ 3.8 eV) which corresponds to the interband transition of silver [20], and the position of the dip does not change with energy and fluence of helium ions, similar to aluminum. Enlarged view of reflectivity at $\sim$ 320 nm (interband transition) is shown in the inset of Figs. 7(D) and 8(D). It is observed that reflectivity at interband transition increases with increase in the energy of helium ions for both 4 and 6 years doses.
The increase in reflectivity of Ag MTFs after irradiation with helium ions is attributed to the lowering of RMS surface roughness as shown in Fig. 9, and also mentioned above in case of aluminum, and this effect of roughness has also been reported in previous articles [17,19]. It is found that RMS surface roughness of each irradiated samples is lower than the RMS roughness value of the pristine sample. It is considered that the increase in grain size after irradiation of helium ions could be the possible reason of lowering the roughness [19]. The RMS roughness of silver MTFs is found to lie in the range of $\sim$ 1 - 5 nm and $\sim$ 3 - 5 nm for 4 years and 6 years dose, respectively. The penetration depth and ion distribution profile of helium ions inside the silver MTF are shown in Fig. 10. It is found that the penetration depth of helium ions increases with increase in ion energy and lies in the range of $\sim$ 2 - 12 nm for energy ranging from 0.5 - 3 keV as shown in Fig. 10(B). The penetration depth of helium ions is found to be lower in case of silver than aluminum because of higher atomic density of silver.
Fig. 9. RMS surface roughness of silver irradiated MTFs with varying helium ion energy at (A) 4 years dose, and (B) 6 years dose fluence.
Fig. 10. (A) He ion distribution profile in silver at different energies, and (B) projected range of helium ions of varying energy inside the Ag MTF obtained from SRIM/TRIM simulations.
4. Discussion of results
The optical properties of the medium strongly depend upon the refractive index and its surface morphology. The refractive index as well as the surface morphology both can be changed by employing ion implantation technique and that in turn changes the optical properties [17,18]. The effect of implantation of foreign atoms (He ions in our case) on refractive index or dielectric constants of the medium can be understood using the effective medium approximations such as Maxwell-Garnett approximation [27] and Bruggeman approximation [26] as discussed in our previous article [17]. Both the models consider a different micro-structure of a random unit cell in heterogeneous medium. Formula (Eqs. (1)) for the effective dielectric constant in the MG approximation model is described below,
The reflectivity of metallic thin films at normal incidence can be calculated using the following Eq. (3) to investigate the effect of He ions fluence on reflectivity of pristine MTFs. Thickness of the films is maintained at $\sim$ 330 nm which is much higher than the skin depth to make the films completely opaque. The reflectivity is given by
where, $n_{0}$ is the refractive index of incident media (air, $n_{0}$ $\sim$ 1). $n$ and $k$ are the real and imaginary parts of the refractive index of He ion irradiated samples as mentioned in Eq. (2). Both $n$ and $k$ obtained from MG approximation decrease with He ion fluence and causes the reflectivity of metallic medium to decrease which can be verified from Eq. (3), and the experimental results of reflectivity presented for aluminum in section 3.1 in NIR ($\sim$ 1200 - 2500 nm) region can be understood by incorporating both the change in refractive index and RMS surface roughness of irradiated MTFs. This agreement becomes better as presented in Table 1 after including the RMS surface roughness in Eq. (3) by employing the following Debye Waller equation [28], where, $R_{0}$ is the reflectivity obtained from Eq. (3), $\sigma$ is the RMS surface roughness in $nm$, $\theta$ is the incidence angle from normal, and $\lambda$ is the wavelength of incident light in $nm$. Reflectivity measurements correlates with RMS surface roughness well in UV region, because at shorter wavelengths, light scatters more from the rough surface than at larger wavelengths since the scattering of light becomes prominent when the light wavelength reaches close to the object size. Reflectivity measurements are done at a fixed angle and hence the specular reflectivity changes upon exposure of ions. The reflectivity of aluminum at 670 nm obtained from both experiment and theory is presented in Table 1, and it is observed that reflectivity gets closer to the experimental values after including the RMS surface roughness by employing Debye Waller equation. By increasing the fluence, the total number of helium ions per unit area bombarded on to the sample increases and which in turn increases the density and hence fractional volume f. The f, calculated for 0.5 and 2 keV energies and 4 and 6 years dose of helium ions, is presented in Table 1 which in turn is employed to calculate the refractive index of aluminum using MG approximation. The refractive index of aluminum has also been calculated by employing pseudo- Brewster (PB) angle technique and is given in the following table. Reflectivity is calculated from Eq. (3) by using the refractive index obtained from MG and PB angle technique and the results are presented in Table 1.
Table 1. Variation of refractive index of aluminum at 670 nm with varying fluence and energy of helium ions where PB: pseudo- Brewster angle technique, MG: Maxwell-Garnett approximation, DW: Debye Waller equation, R: reflectivity, $f$: fractional volume, E: energy of helium ions, Exp: experiment
The dielectric constants of the mixed medium depend upon the dielectric constants of both host (pristine metallic thin films) and inclusion (foreign helium atoms), and the fractional volume of inclusion. The fractional volume of helium ions inside the metallic thin films increases with increase in fluence (total dose in unit area). As the fractional volume increases, both real and imaginary parts of the refractive index tend to decrease with helium ion dose and hence, the optical properties (reflectivity, transmissivity and absorptivity) are expected to be modified upon irradiation. The effect of argon ion implantation on optical properties of aluminum, silver, copper, and gold has been discussed in a previous article by employing the approximation methods [17]. It is observed that the impact of helium ions on reflectivity of aluminum and silver is smaller than in case of argon ion implantation. The reason behind the change in optical properties of pristine metallic thin films is attributed to the polarizability change of host after implantation of impurity atoms [25]. Argon has higher dielectric constants ($\sim$ 1.000516) compared to that of helium ($\sim$ 1.0000684) and the atomic or electronic polarizability depends upon the size of atoms or magnitude of charge displaced in external field. Helium is smaller in size than argon and the magnitude of charge inside the nuclei is also lower than argon. Hence, it is expected that helium ion implantation will have less effect on optical properties (reflectivity, transmissivity, and absorptivity) of metallic thin films than argon implantation.
5. Summary and conclusion
The study has demonstrated the effect of helium ions on the optical properties (mainly reflectivity) of aluminum and silver metallic thin films. Both types of MTFs are bombarded with helium ions of varying energy and fluence. The fluences have been chosen in the present study corresponding to 4 and 6 years journey of a solar orbiter in its orbit of perihelion 0.28 AU. The reflectivity has been measured using UV-VIS-NIR spectrophotometer over the wide wavelength range of 200 - 2500 nm with the resolution of 5 nm. It has been observed that the reflectivity of Al decreases with increase in fluence as well as the energy of bombarding ions with some anomalies. Reflectivity reduces significantly in the ultraviolet and visible region, while it remains almost same in the near infrared region. Reflectivity of silver MTFs increases with ion energy except at 1 keV in the visible and infrared region at 4 years dose of helium ions, while, it shows increment in reflectivity for each energy in case of 6 years dose of helium ions except in higher wavelength region. The penetration depth of helium ions inside the aluminum and silver has been estimated using SRIM/TRIM. The surface morphology has been investigated using atomic force microscopy (AFM) and it is found that the RMS surface roughness of both pristine and irradiated samples is closely related with the experimental results of reflectivity obtained from UV-VIS-NIR spectrophotometer. We propose that these MTFs may be suitable candidate for satellites and spacecrafts as metallic mirrors to reflect the heat radiation without significant degradation in their performance in 4 and 6 years of time span space journey.
Funding
Indian Space Research Organisation (STC/PHY/2019433).
Acknowledgements
KPS sincerely acknowledges CSIR, India, for award of a Senior Research Fellowship (09/092(0954)/2016-EMR-I). KPS also thanks Dr. Dipak Bhowmik for his help in SRIM/TRIM simulations and to Mr. Sushanta Barman for his input toward enriching the manuscript figures. Finally, we thank the anonymous referees for their valuable suggestions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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