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Alternative plasmonic materials have attracted considerable attention due to their advantages compared to conventional metals, including compatibility with Si processing, tunability of optical properties, and reduced losses. In this work, we demonstrate that post-deposition annealing of materials fabricated by magnetron sputtering allows large tuning of the structural and the optical dispersion properties of Indium Tin Oxide (ITO), Al-doped ZnO (AZO) and Titanium Nitride (TiN) nano-layers. By measuring their optical bandgaps, we show that thermal annealing treatments can dramatically modulate the carrier concentration in these materials, thus providing tunability of the optical losses and enabling the engineering of Epsilon-Near-Zero (ENZ) regime. Besides, we perform X-ray diffraction (XRD) measurements to show that thermal annealing can also effectively tune the materials grain sizes. Eventually, the effect of different annealing gases on the free carrier concentration has also been investigated. The wide tunability and control of the optical and structural properties that we demonstrated in this work is important to engineer resonant optical responses across a wide frequency spectrum for device applications to plasmonics, metamaterials and transformation-optics.
© 2015 Optical Society of America
1. Introduction
The quest for the high-density integration of optical and electrical devices for compact photonics transducers and ultrasensitive chemical sensors has significantly driven the recent advancements of plasmonics and metamaterials technologies [1–3]. However, most of the current plasmonic and metamaterial devices utilize metallic components, which exhibit high extinction losses in the visible and near-infrared spectral ranges, limited tunability, and lack of compatibility with the widespread silicon technology, severely limiting practical device implementations. In recent years, Transparent conductive oxides (TCOs), such as Indium Tin Oxide (ITO) and Al-doped ZnO (AZO), and transition-metal nitrides, such as TiN, have emerged as alternative plasmonic materials featuring reduced optical losses compared with conventional noble metals such as Au and Ag in the visible and near-infrared spectral range [4–7]. These materials also feature outstanding thermal and chemical stability [8,9], and have found widespread applications in plasmonics, metamaterials and nonlinear optics, such as negative refraction [10], single photon source [11], perfect absorption of light [12], spontaneous emission control [13,14], and nonlinear generation enhancement [15,16]. In general, these materials are non-stoichiometric in nature and therefore their optical properties significantly depend on their deposition conditions [8,9]. For instance, ITO, AZO and TiN have demonstrated tunable optical dispersion by controlled deposition temperature [17,18].
In the present work, by using post-deposition annealing treatments in controlled gaseous atmospheres, we demonstrate wide tunability of the optical and structural properties of ITO, AZO and TiN thin films and a significant reduction of their optical losses. By measuring the optical bandgaps of the investigated materials, we show that the tunability of their optical properties originates from the modulation of the free carrier concentration upon annealing treatment. Then, we perform XRD characterization of the fabricated films that indicates that annealing can also effectively tune the grain size as a function of annealing temperature, which is consistent with the modification of the optical properties. At last, we investigate the role of annealing gases for ITO and AZO, demonstrating that the free-carrier modulation in ITO and AZO is due to the change in the density of oxygen vacancies induced by the post-deposition annealing.
2. Sample deposition and characterization
ITO and AZO have been used widely as transparent contacts in microelectronic industry [19,20], while TiN has been utilized as a coating material to improve surface properties [21], and as a conductive barrier in microelectronics [22]. The near-IR permittivity ε(ω) of ITO, AZO and TiN can be adequately described by Drude-Sommerfeld model [9]:
Here ε∞ is the background permittivity (high frequency limit of ε), ωp is the plasma angular frequency and Γ describes the charge carrier collision rate, which results in the optical losses inside the material. The plasma angular frequency can be expressed as:
where e is elementary charge, n is the carrier concentration (which can be experimentally inferred from ellipsometry), m* is the effective mass of electron (which in ITO, AZO and TiN is approximately 0.35 m0 [23], 0.38 m0 [24], 1.10 m0 [25], respectively. m0 is the free electron mass), and ε0 is the permittivity of free space.
In general, a large free carrier concentration of the order of 1020-1022 cm−3 is needed to achieve negative permittivity, i.e. a metallic behavior in the optical spectrum. In TCOs, such high doping levels are very challenging due to the fundamental solid-solubility limit [4,9]. Therefore, TCOs typically display metal behavior in the NIR range. On the contrary, it has been shown that TiN supports a carrier concentration of approximately ~1022 cm−3 and it shows metal-like properties even in the visible spectrum. However, this functionality comes at the cost of a significantly increased value of optical losses, since Eq. (1) implies that , i.e. ε2 grows linearly with the free carrier concentration.
The frequency at which the real part of the permittivity goes to zero is known as screened plasma frequency, and it can be expressed as. For TiN, the imaginary permittivity at the screened plasma wavelength λPS is larger than 1. On the contrary, it has been demonstrated that ITO and AZO can show an epsilon-near-zero (ENZ) condition, denoted by λENZ, at the screened plasma frequency [4,9,26]. For these materials, not only the real permittivityis zero, but also the imaginary part of the permittivity is significantly smaller than one. In particular, the condition guarantees enhanced internal fields in nanolayers composed of materials that supports the ENZ condition. Achieving the ENZ condition in a widely tunable spectral range is important for applications to optical cloaking [27–29], molecular emission control [30,31] and nonlinear optics [16,32].
In the following, we systematically investigate a set of ITO, AZO and TiN thin films deposited by magnetron sputtering. We show that post-deposition annealing allows to effectively modulate the free carrier concentration in these materials, and to achieve full tunability of their optical and structural properties. In this article, for simplicity, in all the tables that summarize our annealing parameters we use the symbol λPS to represent both the ENZ condition and screened plasma wavelength.
2.1 Optical properties of ITO and AZO
We deposited ITO and AZO thin films using RF magnetron sputtering at room temperature in a Denton Discovery 18 confocal-target system, and we tailored their optical dispersion properties by a post-deposition annealing. ITO and AZO thin films were grown on Si substrates in Ar atmosphere, the chosen sputtering targets for ITO, AZO thin films were ITO (99.99% purity), Al (99.99% purity)/ZnO (99.99% purity), respectively. The sputtering power for ITO thin films was held constant at 200 W, as for AZO thin films, the Al/ZnO targets power were 30 W/123 W, respectively. The base pressure was 1.0 × 10−7 Torr and the Ar gas flow was kept at 12 sccm. For all the fabricated samples, two different thicknesses have been grown at 37 nm ± 5 nm (thin) and 300 nm ± 5 nm (thick), irrespective of the deposition conditions.
Post-deposition annealing treatments have been performed using a rapid thermal annealing (RTA) furnace, which resulted in a large tunability of the measured optical dispersion for ITO and AZO in the near-IR spectrum. The film thickness remains almost constant after annealing (thickness error: ± 5 nm), as independently verified by surface profilometry. For ITO thin films, annealing processes were performed in Ar gas atmosphere at temperatures between 350 and 750 °C for 30 min, while for AZO thin films, they were performed in N2 gas atmosphere at temperatures between 100 and 400 °C for 60 min. The annealing conditions of ITO and AZO samples are listed in Table 1 and Table 2, respectively. (We did not list annealing parameters of AZO annealed at 400 °C, 300 °C, 100 °C and as deposited, because their λENZ exceeds the wavelength range 300-2000 nm of our ellipsometer, as shown in Fig. 2(a).)
Table 1. ITO thin films annealing parameter on Si substrate
Table 2. AZO thin films annealing parameter on Si substrate
The experimentally measured optical permittivity data for ITO and AZO samples are shown in Fig. 1 and in Fig. 2, which display the effect of annealing on the real (ε1) and imaginary (ε2) parts of the complex permittivity. Two values of sample thickness have been investigated, 37 nm (continuous lines) and 300 nm (dashed lines) in order to demonstrate consistency. Our data demonstrate that the λENZ of all the ITO and AZO samples can be largely tuned in the range of NIR (1200-2000 nm for ITO, 1400-2000 nm for AZO), depending on the post deposition annealing conditions.
Fig. 1 Real (a) and imaginary (b) part of permittivity of ITO samples on Si substrate annealed at different temperatures: As Deposited (black), 350 ◦C (red), 550 ◦C (green), 750 ◦C (blue). The annealing is performed in Ar atmosphere for 30 min for all samples. Continuous lines are for samples with thickness 37 nm, dashed lines are for samples with thickness 300 nm.
Fig. 2 Real (a) and imaginary (b) part of permittivity of AZO samples on Si substrate annealed at different temperatures: As Deposited (black), 100 ◦C (red), 200 ◦C (green), 225 ◦C (blue), 250 ◦C (cyan), 275 ◦C (magenta), 300 ◦C (dark yellow), 400 ◦C (orange). The annealing is performed in N2 atmosphere for 60 min for all samples. Continuous lines are for samples with thickness 37 nm, dashed lines are for samples with thickness 300 nm.
It is well known that the optical properties of TCOs have thickness dependence, due to reasons including the trapping states at the interface of TCOs and substrates [33,34], and different microstructures of thin films [35]. The developed fabrication process has largely solved this problem, as evidenced in Fig. 3 that shows the λENZ and ε2 as a function of annealing temperature for both thick and thin samples. The data indicate that as the annealing temperature is increased, both the λENZ and ε2 are decreased. Most importantly, ITO thin films show very small thickness dependence of their optical properties upon post-deposition annealing, as can be seen clearly from Fig. 1. We also note that the imaginary parts of the permittivity ε2 assumes at λENZ values in the range 0.541 – 0.951 for thin ITO, 0.347 – 0.845 for thick ITO (see Table 1), which are significantly smaller than what previously reported in plasmonic metals in the targeted telecommunication window [36].
Fig. 3 λENZ (black curves on left axis) and ε2 (blue curves on right axis) as a function of the annealing temperature. Solid triangular lines are for ITO samples with thickness 37 nm, and solid circle lines are for ITO samples with thickness 300 nm.
On the other hand, we found that AZO materials have a larger thickness dependence of their optical properties compared with ITO, as evidenced in Fig. 4. Differently from ITO, there is an optimal annealing temperature (250 °C) where we can achieve the smallest λENZ. Moreover, the imaginary parts of the permittivity ε2 at λENZ are 1.47 – 2.19 for thin samples, and 0.66 – 0.78 for thick AZO (see Table 2), which are improved compared to typical plasmonic metals in the same wavelength range [36]. However, it should be noticed that thin-film AZO does not support the ENZ condition (), while thick AZO samples display the ENZ condition λENZ (). We ascribe this difference to the improved crystallinity degree of the thick AZO samples.
Fig. 4 λENZ (black curves on left axis) and ε2 (blue curves on right axis) as a function of the annealing temperature. Solid triangular lines are for AZO samples with thickness 37 nm, and solid circle lines are for AZO samples with thickness 300 nm.
2.2 Optical properties of TiN
TiN thin films on fused silica and Si substrates were deposited using DC reactive sputtering with Ti target (99.99% purity) in N2 atmosphere at room temperature. The base pressure was 2.5 × 10−7 Torr and the N2 gas flow was kept at 10sccm. Similar to ITO and AZO, two different thicknesses have been prepared at 40 nm ± 5 nm (thin) and 300 nm ± 5 nm (thick), irrespective of the deposition conditions. Post-deposition annealing processes have been performed using a Mellen thermal furnace, which again resulted in a large tunability of the screened plasma wavelengths λPS in the 500– 720 nm spectral range. Annealing processes were performed in vacuum with pressure 2.3 × 10−3 Torr at temperatures between room temperature and 900 °C for 60 min. Again, the film thickness stays almost constant after annealing (thickness error: ± 5 nm). The annealing conditions are the same for samples on both Si and fused silica substrates, and its corresponding annealing parameters are included in Table 3 and Table 6, respectively.
Table 3. TiN thin films annealing parameter on Si substrate
The measured permittivity are shown in Fig. 5 and Fig. 6 for TiN samples with thickness 40 nm and 300 nm on Si substrate, respectively. It is important to notice that for all the TiN samples the imaginary permittivity ε2 has values larger than 1, and therefore the ENZ condition was not met. In fact, TiN thin films do not show any ENZ condition due to their higher optical losses () compared to ITO/AZO thin films.
Fig. 5 Real (a) and imaginary (b) part of permittivity of TiN samples with thickness 40 nm on Si substrate annealed at different temperatures: As Deposited (black), 200 ◦C (red), 300 ◦C (green), 400 ◦C (blue), 500 ◦C (cyan), 600 ◦C (magenta), 700 ◦C (dark yellow), 800 ◦C (navy), 900 ◦C (purple). The annealing is performed in vacuum for 60 min for all samples.
Fig. 6 Real (a) and imaginary (b) part of permittivity of TiN samples with thickness 300 nm on Si substrate annealed at different temperatures: As Deposited (black), 200 ◦C (red), 300 ◦C (green), 400 ◦C (blue), 500 ◦C (cyan), 600 ◦C (magenta), 700 ◦C (dark yellow), 800 ◦C (navy), 900 ◦C (purple). The annealing is performed in vacuum for 60 min for all samples.
Figure 7 clearly visualizes the effect of the post-deposition annealing on the thin film optical properties. For TiN samples with thickness 40 nm and 300 nm, as annealing temperature increases the corresponding λPS decrease, as well as the imaginary permittivity ε2. We note that the differences of λPS are small between the 40 nm and 300nm TiN samples, showing that TiN has small thickness-dependence for the screened plasma wavelength after the annealing treatment. In particular, for TiN films (both thin and thick samples) the optical losses can be reduced by almost a factor of 2 compared to the as-deposited samples. However, it should be noticed that the losses of the 300 nm-thick TiN sample is about twice that of the 40 nm-thin TiN sample, which indicates that TiN exhibits larger thickness-dependence for optical losses upon annealing. However, the data in Fig. 7 demonstrate that post-deposition annealing is an effective way to reduce the optical losses for both thicknesses and tune the optical properties of plasmonic TiN materials.
Fig. 7 λPS (black curves on left axis) and ε2 (blue curves on right axis) as a function of the annealing temperature. Solid triangular lines are for TiN samples with thickness 40 nm, and solid circle lines are for TiN samples with thickness 300 nm.
The modulation of optical losses is vividly demonstrated by the optical photographs in Fig. 8, which correspond to 40nm-thick TiN samples on Si annealed at different temperatures. Samples annealed above 700 ◦C exhibit Au-like metallic luster, which demonstrates the fundamental role of the annealing process in tuning the material properties of TiN, which can result in plasmonic behavior in the visible spectral range. A similar trend has also been observed for TiN samples with 300 nm thickness.
Fig. 8 Optical image of TiN samples with thickness 40 nm on Si substrate annealed at different temperatures from room temperature to 900 ◦C. For samples annealed above 700 ◦C, Au-like metallic luster starts to present.
By using post-deposition annealing, our findings demonstrate large tunability of the optical properties of these materials in terms of the ENZ conditions (for ITO/AZO), screened plasma wavelength (for TiN), and optical losses. In the following sections, we will address the origin of these effects and clarify the role of annealing treatments on the optical bandgaps and structural properties of the different plasmonic materials.
3. Optical bandgap and structural properties
3.1 Optical bandgap
In order to understand the connection between the annealing and the optical properties of our materials we experimentally measured the optical bandgaps of the fabricated ITO, AZO and TiN thin films on fused silica substrates using the Tauc’s method [37,38]. Due to the wide bandgap nature of the materials, we see that the optical gaps can be accurately estimated by considering a linear fit of the Tauc plots in the region Eg>3eV extrapolated to zero absorption (See Fig. 9(a), 10(a), 11(a)).Table 4, 5 and 6 summarize the annealing parameters for ITO, AZO and TiN thin films, respectively.
Table 4. ITO thin films annealing parameter on fused silica substrate
Table 5. AZO thin films annealing parameter on fused silica substrate
Table 6. TiN thin films annealing parameter on fused silica substrate
The optical transmittance T of thin films fabricated on fused silica substrates has been measured at normal incidence. The absorption coefficient α has been calculated using the formula:
where R is the overall reflectance coefficient, and t is the film thickness. The optical bandgap Eg for direct transitions can be expressed as function of frequency by the formula:
Figures 9(a) and 10(a) show the bandgaps of ITO and AZO can be largely tuned by post-deposition annealing (3.63-3.94 eV for ITO, 3.68-3.93 eV for AZO). In the inset of Fig. 9(a) and 10(a), we report the bandgaps of ITO and AZO as a function of the optical free-carrier concentration N2/3, which describes the bandgap shift due to the increased free carrier density in the material, known as Burstein-Moss shift [39,40]:
Fig. 9 (a) Tauc plot for a set of ITO samples, where the linear fit (dashed lines) extrapolates the optical bandgap Eg = 3.94 eV (red, λENZ = 1160 nm), Eg = 3.90 eV (blue, λENZ = 1250 nm), Eg = 3.81 eV (green, λENZ = 1360 nm), Eg = 3.68 eV (cyan, λENZ = 1810 nm), Eg = 3.63 eV (magenta, λENZ = 2000 nm). Inset: Burstein-Moss shift of the bandgap Eg as a function of the carrier concentration N2/3. (b) λENZ (circles on left axis) and plasma frequency (triangles on right axis in energy units) as a function of the bandgap Eg.
Fig. 10 (a) Tauc plot for a set of AZO samples, where the linear fit (dashed lines) extrapolates the optical bandgap Eg = 3.93 eV (red, λENZ = 1441 nm), Eg = 3.82 eV (blue, λENZ = 1548 nm), Eg = 3.7 eV (green, λENZ = 1698 nm), Eg = 3.68 eV (cyan, λENZ = 1749 nm). Inset: Burstein-Moss shift of the bandgap Eg as a function of the carrier concentration N2/3. (b) λENZ (circles on left axis) and plasma frequency (triangles on right axis in energy units) as a function of the bandgap Eg.
In Fig. 9(b) and 10(b), we additionally plot the optical bandgaps versus the plasma frequency (expressed in energy units) for the measured ITO and AZO thin films. Finally, we correlate the values of the measured λENZ for all the samples with the measured optical bandgaps, thus directly demonstrating the critical role played by the annealing conditions in determining the ENZ transition wavelengths of ITO and AZO thin films. The increase of optical bandgaps is in accordance with the increase of their plasma frequency, which reduces the λENZ accordingly.
Figure 11(a) shows that the optical bandgap of TiN can also be tuned in a wide range (3.53-3.83 eV) by post-deposition annealing in vacuum. As the annealing temperature is increased from room temperature to 900 °C, not only the screened plasma wavelength λPS decreases, but also the optical bandgap Eg increases.
Fig. 11 (a) Tauc plot for a set of TiN samples, where the linear fit (dashed lines) extrapolates the optical bandgap Eg = 3.83 eV (black, λPS = 509 nm), Eg = 3.78 eV (red, λPS = 546 nm), Eg = 3.82 eV (green, λPS = 576 nm), Eg = 3.72 eV (blue, λPS = 602 nm), Eg = 3.65 eV (cyan, λPS = 612 nm), Eg = 3.64 eV (magenta, λPS = 624 nm), Eg = 3.62 eV (dark yellow, λPS = 638 nm), Eg = 3.56 eV (navy, λPS = 637 nm), Eg = 3.53 eV (purple, λPS = 647 nm). (b) λPS (circles on left axis) and optical bandgap (triangles on right axis in energy units) as a function of the annealing temperature.
3.2 Structural properties
The structural properties of ITO, AZO and TiN have been investigated using X-ray diffraction with Cu-Kα (Bruker D8 Discover) in 2θ mode. Figure 12(a), 13(a) and 14(a) show the X-ray spectra of ITO, AZO and TiN samples with thickness 300 nm on Si substrate under different annealing conditions, respectively. For ITO, several peaks appears including (211) at 2θ = 21.45◦, (222) at 2θ = 30.63◦, (400) at 2θ = 35.46◦, (332) at 2θ = 41.88◦, (431) at 2θ = 45.63◦, (440) at 2θ = 51◦, (622) at 2θ = 60.6◦. These spectra clearly reveal the polycrystalline nature of the sputtered ITO samples. From Scherrer equation [41], we extract the estimated grain sizes from the full-width half-maximum (FWHM) at (211) peak. Figure 12(b) correlates the grain sizes for all the samples with λENZ as a function of annealing temperature. Grain size of ITO is increased from 21 to 29.6 nm as the annealing temperature increases from room temperature to 750 ◦C. The trend and grain size of ITO are in agreement with other findings as well [38,42,43].
Fig. 12 (a) X-ray diffraction pattern for a set of ITO samples with thickness 300 nm on Si substrate, with λENZ = 1826 nm (black, As Deposited), 1609 nm (red, 350 ◦C 30 min in Ar), 1476 nm (green, 550 ◦C 30 min in Ar), 1283 nm (blue, 750 ◦C 30 min in Ar). (b) ITO (211) crystallite size and λENZ as a function of annealing temperature.
As for AZO, the main peak is at (002) at 2θ = 34.48◦−34.70◦ depending on the annealing temperature, the small shift can be attributed to slightly different Al doping concentration for samples annealed at different temperature [44,45]. Figure 13(b) shows (002) grain size is increased from 17.7 to 26.2 nm from room temperature to 250 ◦C, and then decreased to 22 nm from 250 to 300 ◦C, which is consistent with the change of λENZ.
Fig. 13 (a) X-ray diffraction pattern for a set of AZO samples with thickness 300 nm on Si substrate, with λENZ = 1620 nm (black, As Deposited), 1525 nm (red, 100 ◦C 60 min in N2), 1405 nm (green, 200 ◦C 60 min in N2), 1382 nm (blue, 225 ◦C 60 min in N2), 1375 nm (cyan, 250 ◦C 60 min in N2), 1403 nm (magenta, 275 ◦C 60 min sin N2), 1423 nm (dark yellow, 300 ◦C 60 min in N2), N.A. (orange, 400 ◦C 60 min in N2). (b) AZO (002) crystallite size and λENZ as a function of annealing temperature.
For TiN, The main peak is at (200) at 2θ = 42.32◦−42.72◦, where the small shift is due to the decreased lattice constant after annealing [46–49]. Similarly, Fig. 14(b) depicts (200) grain size and λPS as a function of annealing temperature. The increase of annealing temperature can not only decrease the screened plasma wavelength λPS, but also significantly increase the crystallite size, which is also consistent with the trend of optical loss ε2 in Fig. 7.
Fig. 14 (a) X-ray diffraction pattern for a set of TiN samples with thickness 300 nm on Si substrate, with λPS = 717 nm (black, As Deposited), 691 nm (red, 200 ◦C 60 min in vacuum), 672 nm (green, 300 ◦C 60 min in vacuum), 659 nm (blue, 400 ◦C 60 min in vacuum), 633 nm (cyan, 500 ◦C 60 min in vacuum), 591 nm (magenta, 600 ◦C 60 min in vacuum), 553 nm (dark yellow, 700 ◦C 60 min in vacuum), 541 nm (navy, 800 ◦C 60 min in vacuum), 560 nm (purple, 900 ◦C 60 min in vacuum). (b) TiN (200) crystallite size and λPS as a function of annealing temperature.
Our findings demonstrate that post-deposition annealing treatments can effectively modulate the free carrier density in the material, thus enabling control over the plasma frequency, the optical bandgap, the ENZ condition (for ITO and AZO), and the screened plasma wavelength (for TiN), as well as the materials grain sizes.
4. Effect of annealing gas
Several mechanisms have been proposed to account for carrier concentration and mobility in ITO including oxygen vacancies, structural deformations and tin dopants [40,42,50–52]. However, it has recently been demonstrated that the morphology of sputtered ITO thin films, which can be directly modified with the annealing temperature, plays a key role [40,42]. This is because free electrons in as-deposited ITO are trapped at the grain boundaries. With increasing annealing temperature the grain size increases, the density of grain boundaries decreases, and fewer carriers remain trapped resulting in a higher free carrier density. Moreover, grain boundaries behave as traps for free carriers and barriers for carrier transport, which explains the observed increase of carrier mobility with annealing temperature [40,42,50–52]. We notice to this regard that the measured optical band-gaps and free carrier concentrations for our samples are in good agreement with reported values for sputtered films [40,42].
For AZO, several other mechanisms have been proposed to explain the contribution of carrier concentration, such as Al3+ substitution sites, Al interstitial atoms and oxygen vacancies [53,54]. In order to further understand the mechanism which allows for tailoring the free-carrier concentration in our samples, we performed post-deposition annealing of thin ITO and AZO films on Si substrate in different gas atmospheres, at fixed temperature (750 °C for ITO, 250 °C for ITO) and time (60 min). The reason we choose 750 °C(for ITO)/250 °C(for AZO) is because it is the temperature we used to achieve the smallest λENZ (see Table 1 and Table 2). For ITO, the permittivity of the annealed samples measured by ellipsometry is shown in Fig. 15. The two samples annealed in Ar and N2 show a similar permittivity, with a clear ENZ condition around 1160 nm. On the contrary, the sample annealed in O2 displays a purely dielectric behavior (i.e. positive ε1). Similar trend has also been observed for the AZO thin films, as shown in Fig. 16. This is a strong indication that the conduction mechanism in the fabricated ITO and AZO thin films is certainly due to O2 vacancies, and that the free-carriers are strongly reduced by O2 annealing, consistently with the published literature [4,55].
Fig. 15 Real (a) and imaginary (b) part of permittivity of ITO thin (37 nm) film on Si substrate at a fixed temperature 750 °C for 60 min under different annealing gas ambient O2 (red), Ar (black), N2 (green).
Fig. 16 Real (a) and imaginary (b) part of permittivity of AZO thin (37 nm) film on Si substrate at a fixed temperature 250 °C for 60 min under different annealing gas ambient O2 (red), Ar (black), N2 (green).
On the other hand, for TiN, we kept annealing time 60 min fixed and tried different annealing atmosphere including vacuum, Ar, N2 and O2. Our results showed that only vacuum annealing treatments resulted in good quality TiN films with good film uniformity, while other annealing gases gave rise to surface non-uniformities.
5. Conclusion
We have demonstrated that by using post-deposition annealing treatments, the optical properties and the structural properties of ITO, AZO and TiN thin films can be effectively tuned in a wide range, due to the modulation of free-carrier concentration. In particular, ITO and AZO can serve as good candidates of plasmonic materials in NIR range, while TiN can be used in visible spectrum. Besides, we show that upon annealing, the measured optical properties ITO feature small thickness-dependence, while the ones of AZO possess larger thickness-dependence. Furthermore, we discovered that TiN exhibits small thickness-dependence for λENZ, but larger thickness-dependence for optical loss ε2. At last, by investigating the optical dispersion in different annealing gases, we show that oxygen vacancies are responsible for the free-carrier modulation in both ITO and AZO thin films. Our findings demonstrate the critical importance of annealing treatments to provide wide tunability of both optical and structural properties of ITO, AZO and TiN, thus providing a simple and promising method for the control and engineering of Si-based tunable plasmonic and metamaterial devices.
Acknowledgments
This work is supported by the National Science Foundation (NSF) EAGER program “Engineering light-matter interaction via topological phase transitions in photonic heterostructures with aperiodic order” under Award No. ECCS 1541678.
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