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.