TiNxOy films with controllable optical properties have been fabricated by reactive mid-frequency magnetron sputtering from titanium nitride target. The optical and electrical properties were studied as a function of the reactive gas flow and were correlated with the film stoichiometry. The results showed that the behavior of TiNxOy films can be adjusted from metallic to dielectric by increasing oxygen content, which is of great significance to their extensive applications. Owing to the accurate control of optical properties, a TiNxOy based solar selective absorbing coating has been designed and prepared with the aid of TiO2/Si3N4/SiO2 antireflection layers. Its solar absorbance is as high as 97.5% and thermal emissivity is 4.3% with total thickness of 230 nm. The solar absorbance can maintain above 90% for a broad incident angle range from 0° to 65°.
© 2014 Optical Society of America
As representative transition metal oxynitrides, titanium oxynitrides (TiNxOy) have attracted durative research interests due to their remarkable optoelectronic properties, mechanical behavior, as well as chemical stability [1,2]. TiNxOy films can go from pure nitride compound to pure oxide compound with variable composition, covering almost all the possible stoichiometry (0≤x≤1.0 and 0≤y≤2.0) and resulting in a broad physicochemical properties [1,3,4]. One can tune the band gap and structure and hence the optical and electronic properties of TiNxOy between oxide and nitride by changing the nitride/oxide (N/O) ratio [1,4,5], which is the essence of their extensive applications. TiNxOy films with low oxygen content have been utilized as decorative coatings, wear-resistance coatings , transparent IR window electrodes, effective diffusion barriers [2,7], bipolar plate of polymer electrolyte membrane fuel cell  and energy efficient glazing . They are also exceptionally promising plasmonic materials with tunable optical properties [10,11]. TiNxOy films with high oxygen content have been used for insulating layers in metal-insulator-metal (MIM) capacitive structures, thin film resistors [7,12], high-k dielectric materials , visible-light photocatalyst [14,15] and improving light-to-electricity conversion of dye-sensitized solar cells . For some significant applications, such as solar selective absorbing coatings [17,18], the N/O ratio of TiNxOy films should be controlled within a specific range. The TiNxOy films will become transparent in the solar spectral region if the N/O ratio is too low, or will change to metallic behavior and become high reflection if the N/O ratio is too high, both of which will result in inefficient absorption and utilization of solar energy.
Consequently, it is critical to control and adjust the compositions and properties of TiNxOy films effectively. Various deposition techniques have been proposed to fabricate TiNxOy films, such as chemical vapour deposition (CVD) , sol-gel methods , activated reactive evaporation , pulsed laser deposition [19,20], implantation of O ions into TiN films  and magnetron sputtering [1–3,5–8,12,13,18,22]. Among these methods, reactive magnetron sputtering is an effective and convenient method for stoichiometry control and preparation of TiNxOy films with different N/O ratio . Two reactive gases mixture of O2 and N2 are usually used simultaneously when sputtering this type of metal oxynitrides. However, instability will emerge and it is very difficult to control the N/O ratio when mixed plasmas are used [21,23]. Moreover, unexpected residual oxygen or very low O2 partial pressures presenting in the chamber will lead to O rich TiNxOy films, since oxygen exhibits stronger reactivity than nitrogen with regards to metallic titanium. A very high nitrogen mass flow rate in comparison to that of the oxygen is necessary for preparing such films, which limits the range of oxygen and nitrogen composition [1,21]. Some modified techniques have been proposed to improve the process of reactive magnetron sputtering, such as reactive gas pulsing technique , inductively coupled plasma (ICP) assisted reactive sputtering  and reactive high-power pulsed magnetron sputtering . However, continuous control of N/O ratio and properties of TiNxOy films is still a challenge.
In this work, a new approach for depositing TiNxOy films with only one reactive gas of O2 has been demonstrated by reactive mid-frequency magnetron sputtering from a TiN target. The N/O ratio of TiNxOy films can be controlled continuously in a wide range, and the deposition process is much more reproducible and stable than using two reactive gases mixture in traditional methods. The optical and electrical properties of TiNxOy films changing with the flow of reactive gas O2 have been studied systematically. Based on the accurate control of optical properties, a TiNxOy based high performance solar selective absorbing coating has been demonstrated with the aid of TiO2/Si3N4/SiO2 antireflection layers.
2. Preparation and characterization
2.1 Sample preparation
As mentioned in section 1, the physicochemical properties of TiNxOy thin films are very sensitive to their chemical composition. It is difficult to control the composition precisely from metallic Ti target with two reactive gases mixture of O2 and N2 simultaneously as reported in most papers. We provide a new approach for depositing TiNxOy films with TiN target by reactive mid-frequency magnetron sputtering. The deposition process can be greatly simplified by using only one reactive gas of O2 and becomes more stable and reproducible for controlling the physicochemical properties of TiNxOy films.
All the TiNxOy thin films and antireflection films were deposited by using a home-made multi target magnetron sputtering system. Titanium nitride (TiN) target (99.95% in purity) and TiO2 ceramic target (99.99% in purity) and Si target (99.99% in purity) were used with size of 322 mm × 140 mm. The purity of working gas argon and reactive gas oxygen are both 99.99%. The substrates are 2 inch K9 glass, double-polished silicon wafer and polished copper foil. They are cleaned with alcohol and acetone in an ultrasonic bath and rinsed with deionized water, then dried by nitrogen gas. All the depositions were carried out under a sputtering power of 1 kW with frequency of 30 kHz at room temperature with base pressure of 3.0 × 10−4 Pa. The thickness of the films was controlled by the sputtering time according to the deposition rate.
The TiNxOy thin films were deposited by the TiN target in Ar/O2 mixture gas. The flow rate of working gas argon was kept constant at 30.0 sccm (mL/min). The flow rate of reactive gas oxygen was adjusted from 0 sccm to 8.0 sccm to tune the optical and electrical properties of TiNxOy thin films. The sputtering time was kept the same in all of the deposition processes. The detailed process parameters for the deposition of TiNxOy thin films are listed in Table 1.
2.2 Characterization and measurements
The solar energy photo-thermal conversion performance of absorbing coatings is usually characterized by two basic parameters: solar absorbance (α) and thermal emissivity (ε). Solar absorbance was calculated in the range of 0.3-2.5 μm, covering almost all of the solar radiation energy at AM1.5. Thermal emissivity was calculated in the range of 2.5-25 μm, covering most of the thermal radiation at the temperature of 373 K (100 °C). So the solar absorbance and thermal emissivity can be defined as the following formulas:
λ is the wavelength in units of micrometer, is the absorbance at wavelength λ, is the solar spectral radiation at AM1.5, defined by the ISO standard 9845-1 (1992). is the blackbody spectral radiation at temperature T.
The transmission and reflectance spectra of all samples in the range of 0.3-2.5 μm were measured by PerkinElmer Lamda 950 UV/VIS/NIR spectrometer equipped with an integration sphere. The transmission and reflectance spectra in the range of 2.5-25 μm were obtained with Bruker IFS 125HR Fourier transform infrared (FTIR) spectrometer. The film thicknesses provided in Table 1 were measured by Dektak 8 Stylus Profiler. The sheet resistance and resistivity of the films was measured by a four-probe meter (Model RTS-8). The chemical composition of the thin films deposited on Si substrates has been determined from Rutherford backscattering spectrometry (RBS) using a 3.5 MeV He+ beam by a tandem accelerator. The microstructure of TiNxOy based solar selective absorbing coatings was obtained by Philips CM200/FEG Transmission Electron Microscope (TEM).
3. Results and discussion
3.1 Hysteresis effect of the reactive sputtering process and the film stoichiometry
To begin with, the hysteresis behavior was investigated as an important feature of reactive magnetron sputtering. Sputtering voltage of TiN target was studied against O2 mass flow rates. The flow of O2 was increased from 0 sccm to 8.0 sccm with interval of 0.5 sccm and kept 5 min each time for stability, and then decreased backwards step by step.
Figure 1 shows that there exists an obvious hysteresis loop which can be divided into three modes: (i) metal, (ii) transition and (iii) oxide. They are directly correlated with the stoichiometry and the N/O radio of the films . When without O2, the sputtering voltage was as low as 334 V. The TiNxOy films can be controlled in metal mode when the O2 flow is no more than 2.0 sccm. The sputtering voltage changed slowly and the sputtering pressure kept almost unchanged at 0.117 Pa as shown in Table 1. The sputtering voltage increased rapidly when the O2 flow increased from 2.5 sccm to 5.0 sccm, as well as the sputtering pressure increasing from 0.118 Pa to 0.125 Pa. This region is transition mode, corresponding to a consequence of the increasing oxygen content of the films. The N/O ratio of TiNxOy film can be widely tuned in this transition mode . When the O2 flow is larger than 5.0 sccm, the sputtering voltage goes to about 408 V and tends to saturate. This sputtering mode is oxide mode, corresponding to oxygen-rich films. At the process of O2 flow rate decrement, the sputtering voltage remained almost unchanged and kept in the oxide mode until the O2 flow rate decreased to 4.5 sccm. The turning point was 0.5 sccm less than process of increment, showing a hysteresis effect. Then the sputtering mode returned to transition mode and metal mode.
Chemical composition of TiNxOy films deposited on Si substrates at different oxygen flow rates was obtained from RBS analysis. Figure 2 presents RBS spectra of TiNxOy films with oxygen flow rate from 0 sccm to 3.5 sccm. The corresponding atomic concentration and stoichiometry of TiNxOy thin films are listed in Table 2.The trace amounts of oxygen content in TiNxOy films with oxygen flow rate of 0 sccm is from the surface oxidation. Figure 3 shows that the N/O ratio in TiNxOy films was tuned from 16.3 to 0.7 when the oxygen flow rate increased from 0 sccm to 3.5 sccm. When the oxygen flow rate increases further, the N/O ratio will trend closer to zero.
Therefore, the N/O ratio and properties of TiNxOy films can be continuously adjusted by precisely controlling the O2 flow rate. The optical and electrical properties of TiNxOy films changing with the flow of O2 will be investigated systematically next to proof this point of view.
3.2 Control of the optical properties of TiNxOy films
As mentioned in section 1, it is particularly essential to control the optical properties of TiNxOy films in a wide range for many important applications. Figures 4(a) and 4(b) show that the transmittance of TiNxOy films gradually increases with the increasing oxygen flow while the reflectance gradually decreases with the increasing oxygen flow in the near infrared region. The optical properties of TiNxOy films are dominated by their optical constants which can be deduced from the measured transmittance and reflectance spectra based on appropriate dielectric function models. Considering contributions of intraband and interband transitions, the dielectric function of TiNxOy was described as the sum of the following models:
is the contribution of interband transitions at much higher energies than the spectral region of interest and can be considered as a constant. The OJL model proposed by Stephen K. O’Leary, S. R. Johnson and P. K. Lim was an interband transition model for amorphous materials , which describes the contribution of optical transition from the valence band to the conduction band. The Drude model represents unbound electron oscillators, describing the intraband transitions of the electrons in the conduction band. The Lorenz function model represents the bound harmonic oscillator model, used to describe the interband transitions into the upper half of the conduction band . The fitting results agreed with the measured data very well as shown in Fig. 4.
The corresponding complex refractive indices deduced from the transmittance and reflectance spectra are shown in Fig. 5.It can be seen that the optical constants are significantly affected by the O2 flow rate, corresponding to N/O ratio. There exists a significantly different variation rules for the refractive index n between the visible region and the near infrared region. The refractive index n decreases continuously with the O2 flow rate in the near infrared region (when wavelength >1 μm) but firstly increases and then decreases in the visible region. This phenomenon can be linked with the hysteresis loops in Fig. 1, indicating that before the O2 flow rate reaches the turning point of the hysteresis loops, that is, before oxide mode, the refractive index in the visible region will increase with the O2 flow rate but decrease after entering the oxide mode. As shown in Fig. 6, the refractive index increases from 1.5 to 2.6 and then decreases to 2.2 at 0.5 μm but monotonically decreases from 4.9 to 2 at 2.5 μm with the increase of O2 flow rate. However, the extinction coefficient decreases with the O2 flow rate in the whole range of 0.3~2.5 μm and decreases from 1.37 to 0.0023 at 0.5 μm.
When the O2 flow rate is no more than 2.0 sccm, the N/O radio of TiNxOy films is high, in this case, both the refractive index and extinction coefficient increase with the wavelength in the near infrared region and the real part of permittivity shows negative values, which is a characteristic behavior of metallic films, confirming the metal mode of the sputtering process. The N/O radio of TiNxOy films decreases continuously with the increase of O2 flow rate when the O2 flow rate is between 2.0 sccm and 4.5 sccm in the transition mode. In this region, the extinction coefficient reaches a maximum around 1.0 μm and then decreases with the wavelength, which is a characteristic behavior for a semiconductor material . As expected, when in the oxide mode region (O2 flow rate larger than 4.5 sccm), TiNxOy films change to be oxygen-rich. The refractive index does not show appreciable variation with wavelength and the extinction coefficient is near zero, which is consistent with a dielectric behavior. All these results demonstrate that the optical behavior of TiNxOy films can be continuously adjusted from metallic to semiconducting and then to dielectric by varying the O2 flow rate.
The change of optical constants of TiNxOy films with the O2 flow rate is intrinsically due to the change of electronic structure dominated by the N/O ratio. Intraband and interband electronic transitions occurring during the interaction of material with incident light determines the optical properties of a material . TiN exhibits free-electron-like behavior from the electrons in the Ti d band, which means that it contains conduction electrons resulting in metal-like electrical conductivity. The optical properties can be engineered if the density of these free electrons can be systematically varied via changes in the stoichiometry parameter . With the incorporation of oxygen, the electronic structure can be systematically varied, resulting in the adjustable optical and electrical properties of TiNxOy films. The optical properties of TiNxOy films with high N/O ratio are dominated by the intraband transition from free electrons and behave like metallic films. With increasing oxygen content the contribution of the interband transitions, due to bound electrons, became more pronounced, while the contribution of the intraband transitions is less important . When TiNxOy films change to be oxygen-rich, the optical properties will be governed by the interband transitions, while the band gap will increase with the O concentration , which are characteristic of semiconducting and insulating materials.
3.3 Control of the electrical properties of TiNxOy films
The resistivity of TiNxOy films is also studied as a function of the O2 flow rate (taken at room temperature) and plotted in log scale and shown in Fig. 7.It is obvious that the resistivity of TiNxOy films depends dramatically on the O2 flow rate. When the O2 flow rate is less than 2.0 sccm, the resistivity is lower than 1 × 10−3 Ω·cm, showing high electrical conductivity like metal. When the O2 flow rate is larger than 2.0 sccm, the resistivity increases rapidly and reaches more than 1 Ω·cm at 4.0 sccm. When the O2 flow rate increases further and reaches oxide mode, the resistivity will increased sharply and reaches about 106 Ω·cm at 5.0 sccm. The resistivity will increase further and close to that of TiO2 film (more than 109 Ω·cm) and behaves as an insulator. It is consistent to the very low extinction coefficient of optical property. Therefore, the trend of electrical properties changing with O2 flow rate is consistent with the optical properties of TiNxOy films, showing that TiNxOy films can be continuously tuned from conductor to insulator by the N/O radio. The broadly adjustable electrical properties can be utilized as thin film resistors, insulating layers in metal-insulator-metal (MIM) capacitive structures and bipolar plate of polymer electrolyte membrane fuel cell.
3.4 Reproducibility of the optical properties of TiNxOy films
To effectively control the optical properties of TiNxOy films, the reproducibility and stability of deposition process should be considered as well. The same processing parameters with O2 flow rate of 3.3 sccm were repeated for 3 times, the measured transmittance and reflectance spectra of the three samples (S10, S11 and S12) are plotted together for comparison, as shown in Fig. 8.The consistency of the spectra demonstrates that the optical properties of TiNxOy films are reproducible and controllable by our deposition method.
3.5 Solar absorbance of TiNxOy single layer on Cu substrate
For solar energy application, such as solar collector and concentrating solar power systems, solar absorbers should have high solar absorbance (α) and low thermal emissivity (ε) at the same time for high solar energy utilizing efficiency. TiNxOy films will exhibit excellent spectrally selective properties and can be used as solar selective absorbing coatings when deposited on highly infrared reflective metal substrates such as Cu or Al [17,18]. As demonstrated in section 3.2, the refractive index n and extinction coefficient k of TiNxOy films can be continuously adjusted by the O2 flow rate, which is a significant behavior conducive to adapted for optimal selectivity.
The calculated optical absorption spectra of the TiNxOy single layer on Cu substrate at the optimized thicknesses for different O2 flow rates are shown in Fig. 9(a) and the corresponding solar absorbance is shown in Fig. 9(b). These results indicate that there exists a maximum solar absorbance of 81.5% when the O2 flow rate is 3.3 sccm. The solar absorbance decreases rapidly when the O2 flow rate is larger than 4.0 sccm. The measured reflection and absorption spectra (from 0.3 to 25 μm) of deposited TiNxOy single layer at 3.3 sccm on Cu substrates is shown in Fig. 10.The normalized solar radiation (AM1.5) and thermal radiation spectra (T = 100 °C) are also plotted for reference. The solar absorbance in solar radiation region (0.25-2.5 μm) is 81.7% and the emissivity (at 100 °C) in thermal infrared region (≥2.5 μm) is as low as 2.5%, showing a very good property for solar selective absorption.
3.6 Design and preparation of TiNxOy based high performance solar selective absorbing coating
In general, the solar absorbance of homogeneous single absorption layer is limited since the reflection losses from refractive index mismatch with air. Ungraded single cermet layers with isotropic metal volume fractions, deposited on metal reflectors show normal solar absorbance of about 80% , the emissivity will increase dramatically to achieve higher absorbance since the increase of thickness. It should make a balance between absorbance and emissivity. In order to achieve absorbance higher than 90% and keep lower emissivity at the same time, graded and double layer cermet concepts should be used [29,30].
Different from the traditional graded or double layer cermet structure, homogeneous TiNxOy single absorption layer with the aid of TiO2/Si3N4/SiO2 antireflection layers was designed to obtain a high performance solar selective absorbing coating. In this structure, TiO2/Si3N4/SiO2 antireflection layers were designed with gradient refractive index and proper thickness to match the phase and amplitude of solar light propagated in the multilayer structure, resulting a greatly decrease of reflection loss in solar radiation region. With this structure, the thickness of absorption layer is relatively thin so that the emissivity can be kept low. Meanwhile, the stability and controllability of the fabrication can be improved greatly than traditional graded or double layer cermet structure.
For the TiNxOy/TiO2/Si3N4/SiO2 multilayer absorber on Cu substrate, the TiNxOy thin film at O2 flow rate of 3.3 sccm was chosen to design and deposit the multilayer absorber based on the results of section 3.5. The thickness of each layer was adjusted to reduce the reflection loss in the solar radiation wavelength range while maintain the reflectance larger than 50% for wavelength longer than 2.5 μm to ensure low thermal emissivity. The multilayer structure was obtained when the thicknesses of TiNxOy, TiO2, Si3N4 and SiO2 were 83 nm, 20 nm, 42 nm and 86 nm, respectively. The thickness of absorption layer is only 83 nm and the total thickness of the coating is about 230 nm. The multilayer absorber was deposited on Cu substrate and Si substrate at the same time. The latter is used for the study of bright field cross-sectional transmission electron microscopy (TEM) micrograph as shown in Fig. 11(a).The four-layer structure is clearly shown in the TEM micrograph and the platinum segment (Pt) in the top of the multilayer is post-deposited to increase the conductivity of the sample and improve the imaging quality. The corresponding electron diffraction pattern of TiNxOy, TiO2, Si3N4 and SiO2 are shown in Figs. 11(b)-11(e), respectively. From the TEM micrograph and corresponding selected area diffraction pattern, it can be found that the TiNxOy film is polycrystalline structure while TiO2, Si3N4 and SiO2 are amorphous.
The measured reflection and absorption spectra (from 0.3 to 25 μm) of the fabricated sample on Cu substrate together with the designed results (from 0.3 to 2.5 μm) are shown in Fig. 12.The normalized solar radiation (AM1.5) and normalized thermal radiation spectra (at temperature of 373 K) are also plotted in Fig. 12 for reference. The solar absorbance is 97.5% and agrees with the designed result of 98% very well. The emissivity (at 100 °C) of the deposited sample deduced from the measured infrared reflection spectrum (from 2.5 to 25 μm) is 4.3%. The results show that the deposited mutilayer absorber sample exhibits a high solar selectivity (α/ε) of 22.7, showing a more excellent energy performance than most of the reported solar selective absorbing coatings, such as TiAlSiN/TiAlSiON/SiO2 optical stack showed solar absorbance of 96% and emissivity of 5% reported by L. Rebouta et. al.  corresponding to solar selectivity of 19.2, TiAlN/TiAlON/Si3N4 tandem absorber showed solar absorbance of 95% and emissivity of 7% reported by Harish C. Barshilia et. al. corresponding to solar selectivity of 13.6 .
Compared with the TiNxOy single layer, the reflectance in solar radiation region was minimized to improve the absorbance from 81% to 98% with the aid of TiO2/Si3N4/SiO2 antireflection layers. In addition, the reflection edge at NIR (from 2 to 2.5 μm) became more steep and hence further improve the solar selectivity. The position of reflection edge for this structure can also be shifted to accommodate different application temperature range by adjusting the thickness of each layer. To investigate the influence of antireflection layers, the calculated distributions of the absorbed energy and electric field along the profile of single absorbing layer and multilayers at the maximum absorption wavelength (0.58 μm) and minimum absorption wavelength (2.5 μm) in solar radiation region are compared as shown in Fig. 13 and Fig. 14, respectively.Figure 13 shows that the most important layer for energy absorption is TiNxOy layer. The Cu substrate also contributes a small part of energy absorption, while the energy absorption of the dielectric antireflection layers is very low. The energy absorption of multilayer absorber is stronger than TiNxOy single layer in any wavelengths. The energy absorption at 0.58 μm is much stronger than 2.5 μm for both single layer and multilayer absorber. Figure 14 shows that with the help of the antireflection layers, the electric field intensity in TiNxOy absorbing layer is largely enhanced. The electric field at 0.58 μm is much stronger than 2.5 μm for both single layer and multilayer absorber.
For solar energy applications, the influence of incident angle on absorbance is also an important factor, especially when the solar collectors are integrated in the building facade. Figure 15 shows the solar absorbance of our absorber as a function of incident angle. The results show that the absorbance can maintain higher than 90% in a broad range of angles (0°-65°). The measured results agree with the calculated ones very well. It means that the absorber we fabricated is angle-insensitive.
This study demonstrates a new approach to deposit TiNxOy films from TiN target by reactive mid-frequency magnetron sputtering using only one reactive gas O2, providing a reproducible process to continuously control the properties of TiNxOy films in a wide range. The N/O ratio in TiNxOy films was tuned from 16.3 to 0.7 when the oxygen flow rate increased from 0 sccm to 3.5 sccm. The refractive index of TiNxOy films decreases continuously with the O2 flow rate in the near infrared region (>1 μm), but firstly increases and then decreases in the visible region. The extinction coefficient is ever decreasing with the O2 flow rate. The refractive index can be tuned from 1.5 to 2.6 and the extinction coefficient from 1.37 to near 0 at the wavelength of 0.5 μm. The corresponding resistivity changed in a huge range from 10−4 Ω·cm to 106 Ω·cm. The results show that the optical and electrical properties of the TiNxOy films can be continuously adjusted from metallic to semiconducting and then to dielectric by varying the O2 flow rate. When used as solar selective absorbing coating, there exists a maximum solar absorbance for the TiNxOy single layer on Cu substrates when the O2 flow rate is 3.3 sccm and the maximum solar absorbance is 81.7%. A TiNxOy based high performance solar selective absorbing coating has been designed and prepared with the aid of TiO2/Si3N4/SiO2 antireflection layers. With total thickness of only 230 nm, its solar absorbance is as high as 97.5% with very low thermal emissivity of 4.3% (at 100 °C). The solar absorbance is angle-insensitive and can maintain above 90% for a broad incident angle range from 0° to 65°.
This work was partially supported by the Shanghai Science and Technology Foundations (12nm0502900, 12dz2293600) and National Natural Science Foundation of China (61223006). The authors thank Prof. Zhifeng Li and Xin Chen in our lab for the help of measuring parts of the reflectance and transmittance spectra.
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