Finally, the optical properties of Cu3N thin films deposited on glass substrates were analysed by UV-VIS-NIR spectroscopy in the range of 300-2000nm. Figure 5 pictures the transmittance spectra of the deposited thin films at 3.5 Pa and 5.0 Pa, where a greater number of fringes appear as RF power increases, attributed to the thicker films obtained by increasing the deposition rate with the RF power. These spectra clearly show a low transparency of the films in the visible region, and a maximum optical transmission close to 80%, showed for the samples deposited at low values of RF power, in the near infrared region.
Figure 5. Transmittance spectra of the Cu3N in study. In the inset, representative (hνα)2 and (hνα)1/2 vs hν plots of the sample deposited at 150 W, for the determination of the direct and indirect band gap.
From the transmittance spectra and the Tauc equation [23], the band gap of the Cu3N films were calculated, with the aim of determining their potential as solar absorbers.
\(\left(\text{hυα}\right)^{\frac{1}{p}}=A\left(h\nu-Eg\right)\)Eq.1
where A is the band tailing parameter and p is the parameter associated with type of transition, i.e. p=2 for an indirect allowed transition and p=1/2 is direct allowed transition, α is the absorption coefficient, and hν is photon energy. In the inset of Figure 5, the (hνα)2 and (hνα)1/2 vs hν plots for the sample deposited at 150 W are pictured as example. Table 1 summarised the values of the direct and indirect band energies obtained varying the RF power. They are very close to those found in the literature [9, 23]. In the case of the samples deposited at 3.5 Pa, we observe a slightly blue-shift in the band gap energy as the RF power increases. This may be attributed to the increasing presence of unbonded Cu atoms, which would form a defect level, leading to a reduction in the band gap. We would be faced with the effect of Cu supersaturation. However, in the case of the samples deposited at 5.0 Pa, that tendency is not so well appreciated. This leads us to believe that the Cu oversaturation effect would not be occurring due to the increase of the gas pressure [24].
Table 1. Direct and indirect band gap calculated by using Tauc plot for the samples deposited at different RF powers and a total pressure of 3.5 Pa and 5.0 Pa, respectively.