The typical pyroelectric detector is structured with LiTaO₃ material in a central layer between two metal electrodes. The pyroelectric detectors were fabricated from a polarized LiTaO₃ crystal of 1.2 mm×1.2 mm and a thickness of about . Chemical–mechanical polishing method is used to thin down the LiTaO₃ to a thickness of about . The absorber structure is deposited onto the sensitive element area of LiTaO₃ crystal. The sputtering equipment (Varian 3280) was used to prepare NiCr thin films by magnetron sputtering of a high purity NiCr target containing 80% nickel and 20% chromium in an Ar atmosphere. To obtain NiCr films with different structures, layers were deposited with different deposition rates, which can be determined by the parameters of different pressure atmospheres and sputtering currents. The pressure was chosen as 4 Pa–12 Pa for best refractive index adjustment. The sputtering current was adjusted from 0.2 A to 0.4 A with a deposition rate of up to 7.5 A/s. The 3-layer NiCr absorber was carried out at a base pressure of 2 × 10⁻⁴ Pa and with a chamber temperature of 80 °C. The total thickness of the nano-structured NiCr absorption was 470 nm–500 nm. Meanwhile, the single layer NiCr absorbers with the same film thickness were prepared by different pressure atmospheres (4 Pa, 8 Pa, and 12 Pa) and sputtering currents (0.2 A, 0.3 A, and 0.4 A) for comparison. Ag film with a thickness of 30 nm is deposited on the backside of LiTaO₃ crystal.

To characterize the IR absorption properties of the NiCr absorption, normal incidence reflection measurements are acquired on LiTaO₃ crystal coated with NiCr films using a Fourier transform infrared spectroscopy (FTIR) system (PerkinElmer Spectrum 400). As the metal Ag film as a perfect reflector is deposited on the backside of LiTaO₃ crystal, the transmission is equal to 0. The absorption for the film is calculated as .

Figure 1 shows the scanning electron microscopy (SEM) top view of NiCr absorption deposited at different pressure atmospheres 4 Pa, 8 Pa, and 12 Pa in Figs. 1(a), 1(b), and 1(c), and different sputter currents 0.4 A, 0.3 A, and 0.2 A in Figs. 1(d), 1(e), and 1(f), respectively. Owing to extreme atomic shadowing effects and limited adatom diffusion, deposition at very high pressure atmosphere leads to a nanostructured and porous film; as shown in Fig. 1(c). The film deposited at low pressure atmosphere (4 Pa) has a smoother surface which produces grains between 20 nm–50 nm in diameter. If the pressure is increased to 8 Pa, then some grains with a diameter of 50 nm–100 nm are created. Using a 12-Pa pressure atmosphere produces grains with a diameter of 100 nm–200 nm all over the surface. Clearly, the general size of the grains is dependent on the pressure atmosphere. Similar evolution of grains is not found on the films deposited with different sputter currents. As shown in Figs. 1(d)–1(f), it seems that the films deposited by different sputter currents have similar grain size with a diameter of 20 nm–50 nm. It is worth noting that the deposition rate of films is decreased from 11 nm/s to 5 nm/s and from 7 nm/s to 3.5 nm/s as the pressure increases and sputter current decreases, respectively, as shown in Fig. 2. This indicates that the deposition rate is not the main reason for the formation of grains with different sizes.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. SEM top view of NiCr absorption deposited at different pressure atmospheres (a) 4 Pa, (b) 8 Pa, and (c) 12 Pa and sputter currents (d) 0.4 A, (e) 0.3 A, and (f) 0.2 A.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. Deposition rate of NiCr films with (a) different gas pressures, and (b) sputter current.

Three-layer NiCr absorbers deposited by gradient pressure and gradient current are obtained. The SEM cross-sectional views of the three-layer NiCr absorber structure deposited at an increasing pressure of 4 Pa, 8 Pa, and 12 Pa and a decreasing pressure of 12 Pa, 8 Pa, and 4 Pa are shown in Figs. 3(a) and 3(b), respectively. The thickness of these absorber films has a similar value of 500 nm. It is seen that the resulting film structure shows nano-columns with growing size. In Fig. 3(a), the nano-columns grow larger in size and the first layer on the substrate under 4 Pa shows dense column, which makes excellent adhesion properties. In contrast, the first layer in Fig. 3(b) shows oblique orientation under 12 Pa due to extreme atomic shadowing effects and limited adatom diffusion. Three-layer NiCr absorber structures deposited at an increasing current of 0.2 A, 0.3 A, and 0.4 A and a decreasing current of 0.4 A, 0.3 A, and 0.2 A are shown in Figs. 3(c) and 3(d), respectively. Similar to the trend in Figs. 1(d)–1(f), no apparent gradient layers are shown by the different current parameters.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. Cross-sectional SEM images of the nanostructured NiCr film fabricated using sputtering deposition with (a) increasing gas pressure of 4 Pa, 8 Pa, and 12 Pa, (b) decreasing gas pressure of 12 Pa, 8 Pa, and 4 Pa, (c) increasing sputter current of 0.2 A, 0.3 A, and 0.4 A, and (d) decreasing sputter current of 0.4 A, 0.3 A, and 0.2 A.

The measured reflection curves of NiCr films produced by different argon pressures and gradient pressures with similar thickness are measured. The absorption curves are then calculated based on the relationship between reflection R and absorption A: R+A=1, because the transmission is equal to 0. It is clear that the absorption of NiCr films is increased at an optimal working pressure of 12 Pa (as shown in Fig. 4), while the gradient layer system shows higher absorption than single layers. The absorber features high absorption efficiency ( ) peaks in the wavelength of , , and . The performance of the NiCr absorption layer is comparable to those metal absorption films. Schossig reported a 600-nm thick NiCr film fabricated by glancing angle deposition (GLAD) technique, which features a high absorbance of 70% in mid IR range but low absorbance of less than 50% in the wavelength of .^[8] Shang et al. prepared a 1.5- thick nanostructured Cr thin-film by sputtering at a high incident angle of 110°, revealing an absorbance of 55% at which linearly decreases to 35% at .^[11] The thin metal NiCr film with the thickness of less than 10 nm represents broadband absorbance radiation, while its maximum absorptivity is less than 50%.^[12] Compared with these metal absorbers, the NiCr absorption layer that we proposed is more advantageous for mid/long wave infrared and very long wavelength applications such as military spacecraft. Apparently, the nano-column structure shown in SEM images (Fig. 1) on the top layer by higher argon atmosphere has a rough surface, which increases the absorption. It is known that the absorption of a metal film consists of two components: the intrinsic absorption of an ideally smooth surface and the physical structures that contributes to the enhancement of infrared absorption. In principle, the optical functionality of gradient layers can be imagined as a combination of nano-structures on the top layer and anti-reflection property induced by graded-refractive-index layers.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. Absorption of single NiCr films with different argon pressures and 3-layer gradient pressure films.

The optical properties of absorbing layers can be analytically modeled based on the optical modeling proposed by Parsons et al.^[13] Since the index of refraction n and extinction coefficient k are essential parameters in the simulation modeling, spectroscopic ellipsometry (SE, V-VASE ellipsometer) was used to determine these two optical constants. An ultraviolet (UV)-visible (Vis)-near-IR spectrophotometer (Cary500, Agilent) with a wavelength range of was used to measure the reflectance and transmittance of the samples. A simple three-phase model consisting of Ag/LiTaO₃/NiCr thin films has been used to represent the sample. The variables in the fitting procedure include the layer thickness and all Tauc–Lorentz (TL) parameters. The fitting quality is poor when simple TL dispersion function is used, indicating that more than one single type of electronic transition dominates the optical absorption for NiCr films. Double TL dispersion function is then applied to fit the measured ellipsometric data in the present work. The index of refraction and the extinction coefficient as a function of wavelength can then be obtained, as shown in Fig. 5. It can be seen that the index of refraction increases as the argon atmosphere increases. NiCr with gradient layer has an average equivalent refractive index compared to that of single NiCr film with the same thickness in the wavelength range of . For the extinction coefficient, gradient NiCr layer has higher value than single films, which mainly contributes to the enhancement of infrared absorption.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. Extracted effective (a) n and (b) k for the single NiCr films and 3-layer gradient film.