Substrate temperature effects on infrared emissivity of TiN x films

Deposition of TiN _x films

The deposition of TiN _x films was carried out on glass substrates with a Ti target (99.995% in purity) using a direct current reactive magnetron sputtering system (JPG450). The base pressure of vacuum chamber was pumped to 5.0 × 10 ⁻⁴Pa. Before deposition, the target was pre-sputtered by Ar gas (99.999% in purity) for 15 min to remove the surface oxide layer. After pre-sputtering, the mixture of Ar and N₂ gas (99.999% in purity), fixed at 100 and 20 standard cubic centimetres, respectively, was introduced into the chamber using mass flow controllers to regulate the flow of gas. A total gas pressure of 0.6 Pa and a sputtering power of 120 W were used to deposit 400 nm films. During the film deposition, the substrate temperatures for different samples were held at 25, 150, 250 and 350°C, respectively.

Characterisation techniques

The crystal structure and grain size of TiN _x films were obtained by an X-ray diffraction analysis (Philips X'Pert Diffractometer). Cu Kα (λ = 0.1514 nm) radiation was used as the source for diffraction. The bonding states of elements and N/Ti ratios of the films were identified by X-ray photoelectron spectrometry (XPS, ESCALAB 250Xi). Composition analysis was performed after ion etching 360 s using 3KeV Ar⁺. The thickness was observed by a field-emission-gun scanning electron microscopy (ZEISS SUPRA 55). The resistivity of the films was measured with a standard four-point probe measurement method. The carrier concentration and Hall mobility were measured by a Hall measurement system. The infrared emissivity of TiN _x films in the wavelength of 3–5 and 8–14 μm was measured using an ISTP IR-2 dual-band emissivity tester at room temperature.

Phase constituent of TiN _x films

The XRD patterns of TiN _x films deposited at different substrate temperatures are shown in Figure 1. It confirms that all the films are composed of TiN phase with a faced-centered cubic crystal structure based on JCPDS No.87-0628. An obvious (200) peak is observed for all the films, indicating that TiN _x films with polycrystalline structure are formed. It can be observed that the (111) and (220) crystalline plane is negligible for the films except for the film deposited at 250°C, which exhibits a mixture of the structure in a more pronounced (200) orientation with less (111) and (220), showing changes in the crystalline orientations. As substrate temperature increases, the intensity of XRD peaks also increases and the full width at half maxima of the peaks becomes narrower, indicating a rise in grain size. The grain sizes of TiN _x films deposited at different substrate temperatures are shown in Figure 2. The grain size (D) of TiN _x films can be calculated using the Scherrer equation [17]: (1) where λ is the wavelength of the incident Cu Kα X-ray (0.1514 nm), B is the FWHM of the diffraction peak and θ is the diffraction angle. The result shows that the grain size grows up from 19 to 27 nm when the substrate temperature goes up from 25 to 350°C, which is attributed to the high temperature of the substrate to promote the growth of the grains.

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Figure 1

XRD patterns of TiN _x films deposited at various substrate temperatures.

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Figure 2

Grain sizes of TiN _x films deposited at various substrate temperatures.

Binding states of the elements in TiN _x films

The X-ray photoelectron spectra of Ti 2p and N 1s for TiN _x film deposited at 250°C are shown in Figure 3(a,b). The binding energies of the peaks are calibrated respect to C 1s peak at 284.8 eV. In Figure 3(a), Ti 2p states exhibit 5 binding peaks, which are 454.9, 456.2, 457.8 eV of Ti 2p_3/2 and 460.9, 463.0 eV of Ti 2p_1/2. The XPS peak of Ti2 p_3/2 at 454.9 eV can be ascribed to the formation of TiN phase [18,19]. The peaks at 456.2 eV and 457.8 eV suggest the formation of the titanium oxy-nitride and TiO₂ phase. The peaks of Ti 2p_1/2 are composed of two peaks at 460.9 eV (TiN) and 463.0 eV (TiO₂). In Figure 3(b), the peaks of N 1s spectra at 397.3 eV and 399.5 eV can be assigned to the presence of TiN and titanium oxy-nitride phase, respectively, [20,21].

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Figure 3

XPS spectra of TiN _x films deposited at 250°C (a) Ti 2p; (b) N 1s.

The N/Ti ratios calculated from the XPS spectra of the films are shown in Figure 4. The N/Ti ratio rises when the substrate temperature increases, whereas the ratio of N/Ti at 250°C is exception. The N/Ti ratio is in range of 0.81–0.97, indicating that all the films are off stoichiometry and deficit in nitrogen. When the substrate temperature increases, the mobility and diffusion of reactive atoms are enhanced to reduce the detrimental effects and increase the N/Ti ratio. At 250°C, the N/Ti ratio shows a sudden decrease, which could be explained by the orientation of TiN _x films. Combadiere et al. [22] reported that the (111) planes consisted only of Ti atoms. As for the (220) and (200) planes, they contained both N and Ti atoms. The film deposited at 250°C exhibits an obvious (111) plane, so there are more Ti atoms in the film deposited at 250°C than others, which leads to the sudden decrease of the N/Ti ratio. This result is in good agreement with those obtained by XRD.

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Figure 4

N/Ti ratios of TiN_x films deposited at various substrate temperatures.

Electrical properties of TiN _x films

Figure 5(a,b) shows the resistivity, carrier concentration and Hall mobility of TiN _x films deposited at different substrate temperatures. It is observed that the substrate temperature has a significant effect on the electrical properties of TiN _x films. The resistivity of TiN _x films decreases obviously from 15.7 × 10³μΩ cm at 25°C to 0.9 × 10³ μΩ cm at 350°C. It is known that the non-stoichiometric TiN _x films contain more lattice defects, which increase the resistivity of the films. Therefore, the resistivity of TiN _x films is higher than the single-crystal TiN film, 18 µΩ cm [23].

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Figure 5

Electrical properties of TiN _x films deposited at various substrate temperatures (a) Resistivity; (b) Carrier concentration and Hall mobility.

The evolution of resistivity can be explained by the changes of carrier concentration and Hall mobility using Equation (2) [24]: (2) where ρ is the resistivity, n is the carrier concentration, e is the electron charge and μ is the mobility. The resistivity is inversely proportional to carrier concentration and mobility.

When the substrate temperature goes up, the N/Ti ratio of TiN _x film is closer to the stoichiometric ratio, as shown in Figure 5(b), the carrier concentration n increases. Meanwhile, with substrate temperature increase, the grain size of TiN _x films rises and the crystal structure of films grows more perfect, causing that the number of grain boundary and crystalline defects decreases. The structure of TiN _x films becomes denser, leading to an increase of mobility μ due to the decrement of grain boundary scattering and crystalline defects scattering. Consequently, the resistivity of TiN _x film goes down.

Infrared emissivity of TiN _x films

The infrared emissivity curves in the wavelength of 3–5 and 8–14 μm of TiN _x films deposited at various substrate temperatures are shown in Figure 6. The increase in substrate temperature leads to the decrease of infrared emissivity of the films. With the substrate temperature rising from 25 to 350°C, the infrared emissivity of TiN _x films in the wavelength of 3–5 μm decreases from 0.55 to 0.29, and the value in the wavelength of 8–14 μm drops from 0.54 to 0.27. The change in infrared emissivity of the films is consistent with their resistivity, which can be explained using the Hagen-Rubens equation [25]. According to Hagen-Rubens relation, the infrared emissivity ε can be expressed as Equation (3). (3) where ε₀ is the permittivity of the electron in vacuum, c₀ is the velocity of light in vacuum, ρ is the resistivity and λ is the wavelength of the infrared radiation. When the substrate temperature rises, as described in the resistivity results, the resistivity ρ of TiN _x films decreases. According to Equation (3), the infrared emissivity ε of TiN _x films drops with the decline of resistivity ρ. At 350°C, due to the minimum resistivity, the TiN _x films exhibit the lowest infrared emissivity.

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Figure 6

Infrared emissivity of TiN _x films deposited at various substrate temperatures.