Synthesis and Investigation of the Structure and Optical Properties of Nano -Ni-Cr Films

The thermal evaporation technique was used to prepare the Ni-Cr films with a thickness of 200 nm and a rate of deposition of 0.22nm/Sec. The annealing was performed at 373 and 473 K. The structural and optical analyses of the grown layers were achieved and XRD patterns showed amorphous structure transferred to polycrystalline for film annealed at 373 and 473 K. AFM analysis showed that the surface of Ni-Cr films is homogenous and the average roughness, optical energy gap and absorption coefficient were increased with increasing annealing temperature (Ta).


Introduction
In the recent years, many research groups studied the physical properties of Ni-Cr films using different deposition techniques, because of their usages in various microelectronic applications such as strain gauge, fusible links, type-π attenuators, potentiometers, thin-film resistors, read-only memories, and humidity sensors [1][2][3][4][5]. NiCr alloy is one of the highly important materials because of its low temperature coefficient of resistance, high electrical resistivity, relatively large resistivity and outstanding thermal long-term stability [2,6]. NiCr thin films can be produced on a wide variety of substrates for the deposition of Ni-Cr alloys, such as stainless steel, silicon wafer, glass ceramic, and polymers like polyimide [7]. Ni-Cr alloys on the above mentioned substrates have been widely used as a sensor or chip resistor for common integrated circuits [7]. Physical and chemical properties of NiCr thin films depend on the formation conditions used in the deposition environment [5]. Ni-C films have been deposited by using sputtering, PLD and chemical method [8,9] ISSN: 0067-2904 Awadh Iraqi Journal of Science, 2020, Vol. 61, No. 9, pp: 2251-2256 2252 In this paper, the dependence of structure, morphology and optical characteristics of NiCr films on annealing temperature is investigated.

Experimental Part
NiCr films were deposited on the glass substrate by thermal evaporation technique in a unit vacuum (Edwards 306A). The evaporation was achieved under vacuum pressure of Torr at room temperature. The annealing was performed under vacuum at 10 -2 mbar with temperatures of 373K and 473K and annealing time of around 60 minutes. The structural properties were studied by X-ray diffraction (XRD) (PANalytical). Atomic Force Microscopy (AFM, AA2000 Nanoscope III Digital Instruments) was used to study the topography of the deposited films. The optical properties were studied by UV-spectrophotometer (UV-1601C).

Results and discussion X-Ray Diffraction
XRD was carried out to identify the Ni-Cr thin film phase present in the film. From fig.(1.a ), we can observe that the Ni-Cr thin film before annealing was of an amorphous structure with no apparent crystalline phases. Similar results were reported by a previous work [6,9]. After the annealing process, Ni-Cr was converted to a polycrystalline phase at (111) in 2θ (44.51 and 44.45) for films annealed at 373 and 473 K, respectively, as shown in Table-1. These results are in agreement with those of Bizhouet et al. [10]. The NiCr film growth was accompanied by an oxidation process. NiCr oxide was observed by comparing with ASTM data (PDF #210596) for NiCr-oxide. This behavior suggests that the annealing temperature has an influence on the improvement of Ni-Cr films

Atomic Force Microscopy (AFM) Analysis
The morphological characteristics of NiCr -oxide thin films were studied. AFM was used to observe the nanostructure of NiCr thin films. Fig. (2) (a-b-c) shows the topography of NiCr thin films grown Awadh Iraqi Journal of Science, 2020, Vol. 61, No. 9, pp: 2251-2256 2253 by thermal evaporation technique on the glass substrate with different annealing temperatures of 373 K and473 K. This figure shows that the NiCr films have a uniform surface. The average roughness was increased from 0.812 nm at RT to 2.28 nm at 473 K. The root mean square was increased from 1.16 nm at RT to 3.54 nm at 473K, as shown in Table-2. This indicates that the increase in annealing temperature leads to a reduction in the density of state and grain boundary [10]. This behavior leads to increasing the grain size, as shown in the x-ray diffraction where the grain size increased from 4.7 to 4.9 nm when the annealing temperature increased from 373 to 474 K. AFM analysis enhanced the results of x-ray diffraction measurement.

Optical properties
NiCr-oxide thin films deposited at room temperature has a metallic color change to gray through the annealing process. Figure-3 shows the transmission as a function of wavelength for different annealing temperatures. We observe from this figure that the transmission was decreasing with increasing the annealing temperature and wavelength. NiCr films prepared at room temperature had a high reflection compared with the one annealed at 473, as shown in Figure-  The absorption coefficient (α) properties as a function of wavelength are shown in Figure-5. It is clear from this figure that the absorption coefficient of NiCr-oxide films at RT and 373 was increased with increasing wave length, but decreased for films annealed at 473. This behavior is due to the fact that the annealing process leads to the oxidation of the films and the formation of clusters as NiO and Cr 2 O 3 at high Ta. A similar behavior of thin films was also reported earlier [11]. The optical energy gap of the as-deposited NiCr-oxide films was determined from the extrapolated energy intercept and calculated to be 0.47 eV, which is much higher than that of bulk NiCr. The band gap energy remained at 0.81 and 1 eV for the films annealed at 373 and 473 K, respectively, as shown in Figure-6 and Table-3). This result supports the XRD, AFM and absorption coefficient results and proves that we obtained NiCr-oxide films at 473K.

Conclusions
This study investigated the structure, morphology and optical properties of NiCr-oxid (50-50%) thin films on glass substrate at different annealing temperatures. X-ray diffraction of the deposited films showed an amorphous structure and polycrystalline nanostructure for films annealed at 373 and 473 K. This growth was accompanied by an oxidation process of NiCr-oxid. AFM measurements showed that the NiCr-oxid films have a uniform surface. The average roughness and root mean square were increased with increasing the annealing temperature, because of the grain size which affects the grain boundary. Heat treatments resulted in increasing the optical band gap, the absorption coefficient and reflection, whereas the transmission was decreased with increasing annealing temperature.