3.1. X-Ray Diffraction analysis
For investigation the crystalline structures of the deposited thin films, XRD measurements were used at room temperature. Figure 1 shows the X-ray diffraction pattern of HgS/glass thin films produced at different deposition times of 50, 100 and 150 minutes. The XRD pattern shows single crystal of α phase of HgS with only (220) reflections characteristic at 2ϴ=32.44o which are in good agreement with the JCPDS-pattern (JCPDS file No.: 73-1593). The films tend to crystalized in a certain direction.
As shown, increasing deposition time gradually leads to higher lattice volume as specified by peak shift towards lower diffraction angle (higher d value) due to increasing the compressive stress in the films.
3.1.2. Energy dispersive X-ray spectroscopy
EDAX results are shown in figure 2. In agreement with our purpose, the peaks of Hg and S were present in the spectrum with the large peak of Si. The Hg and S peak intensity increased by increasing deposition time. We can consider that by increasing deposition time, oxygen can also come from the air during the deposition process and resulting in lower O amount. The layers deposited at 50 minutes have a more oxygen-rich composition. The increased in deposition time lead to decrease in oxygen content in the layers.
3.3. Optical properties with Kramers-kronig relations
The relation between deposition time and optical properties were investigated. In this work Kramers- Kronig relations were used to calculate the phase angle θ (E) [13-14]:
Where E denotes the photon energy, E2 the asymptotic limitation of the free-electron energy, and R(E) the reflectance. Hence, if E2 is known, the θ (E) can be calculated. Then the real and imaginary parts of the refractive index were calculated, from which other parameters were obtained.
Figure 3 show transmittance and reflectance curves for mercury sulfide layers produced by chemical bath deposition method in different deposition times of 50, 100 and 150 minutes, for this work.
The spectra’s are obtained in visible light energy range (1.5-3 eV). As it can be seen from figure 1, before 2 eV, curves have been separated. This region belongs to hot colors as red and infrared. By increasing deposition time, transmittances decreases that are because of configuration of complete layers by increasing deposition time. Also in this range of energy (1.5-2 eV) transmittances are high and are about 80% up to 90%, for the layers produced in this work. Oblique and almost same transmittance and reflectance observe for 2 eV to 2.3 eV energy ranges that belongs to orange color. For 2.3 eV up to 3 eV energies that belongs to violet, blue, green and yellow colors, curves are almost the same and are in straight lines also transmittance of this region is very low and reflectance (figure 3b) is very high, that means mercury sulfide is offended layer in this range, HgS layers are almost transparent in hot color wavelengths and offended in cold color wavelengths. Figure 3 shows the reflectance of mercury sulfide produced
in this work. On the contrary of transmittance curves, there are high reflections for high energies and low reflections for lower energies. In lower energy range by increasing deposition time reflection increases that is because of formation complete layers.
Figure 4 shows the real part of reflective index for mercury sulfide layers produced in this work. Real part of refractive index begins from a minimum and reaches to a maximum and continues in a straight line for all layers. As it can be seen, by increasing deposition time, n has an increasing trend that is because of formation complete and dense layers by increasing deposition time. The results of real part of refractive indices are in agreement with reflection curves.
Figure 5 shows the imaginary part of refractive index (k). These curves are in exact agreement with transmittance curves. In infrared, red and orange energy regions, there is a low extinction coefficient for mercury sulfide layers and the k amounts are almost the same. By increasing deposition time in the range of 2.3 eV- 3 eV energies, transmittance of layers decreases therefor absorbance increases. In the violet, blue, green and yellow energy region, extinction coefficients are high.
Figure 6 shows the real part of dielectric function. All curves begin from a minimum and increase to a maximum. By increasing deposition time real parts of dielectric functions (ε1) increase, that is because of formation more HgS molecules on layers by increasing deposition time. ε1 for low energies is almost the same. Figure 7 shows the imaginary parts of dielectric functions (ε2). For low energies range, ε2 curves are almost the same. By increasing deposition time, ε2 curves increases in high energy ranges. ε2 results are in exact agreement with extinction coefficient curves. Figure 8 and 9 show real parts (σ1) and imaginary parts (σ2) of optical
conductivities for layers produced in this work, respectively. By increasing the deposition time, real part of conductivities decrease and imaginary parts of conductivities increase. Results are in the exact agreement with dielectric function curves. There are a high conductivities for violet, blue, green and yellow colors and low conductivity for orange, red and infrared colors. Hg atoms for layer produced at 50 minutes have high densities and S atoms in same layer have low densities. For layer produced at 100 minutes, number of Hg atoms decrease and number of HgS molecules increases and also for layer produced at 150 minutes, number of Hg atoms has low densities and HgS molecules configure more on layer.
Figure 10 shows the absorption coefficient for mercury sulfide layers produced in this work. In high energy ranges, by increasing deposition time absorption coefficient decreases. There are high absorbance for this region, Therefor mercury sulfide layers for violet, blue, green, yellow colors are offended and for orange, red and infrared colors are transparent. As it can be seen from all optical results, orange is a frontier color for HgS layers and mercury sulfide layers have photolminisance property.
Figure 11 shows the values of band gap energy for the layers produced in this work. By increasing deposition time, present of Hg atoms decrease and formation of HgS molecules increase, therefor dielectric property increases and value of band gap also increases. Table 1 shows the values of bang gap energy for mercury sulfide layers produced in this work.
Table I: Band gap energy values of β- HgS nano layers
Deposition time (minutes)
|
band gap energy (eV)
|
50
|
1.96
|
100
|
2.06
|
150
|
2.08
|