The practical details of depositing zinc oxide nanorods and coating them with a thin layer of a material are mentioned in detail in our previously published papers [19]. Figure 1 display the steps of double layers thin films deposition.
The prepared ZnO/CdZnS thin films were thermally treated by annealing them at 200 with different annealing time and the details revealed in Table 1. UV detectors were fabricated and tested as shown in Fig. 2.
Table 1
the information about samples annealing process.
Sample | Structure | Annealing temp. (CO) | Annealing time (min.) |
S0 | ZnO | - | - |
S1 | ZnO/CdZnS | - | - |
S2 | ZnO/CdZnS | 200 | 30 |
S3 | ZnO/CdZnS | 200 | 60 |
S4 | ZnO/CdZnS | 200 | 120 |
Figure 3 demonstrations the UV-visible transmission spectra ZnO/CdZnS annealed at different time periods. All samples show no transmittance at UV region from 300–400 nm and the films show an average transmittance of 20–50% within the visible light region. which comes from the strong light scattering by the nanorods. Moreover, the transmittance intensity in the visible range increase when the annealing time is 60 min and drop again when the annealing time is 120 min. This can be attributed to the increased scattering effect induced by the rough CdZnS shell [1].
Figure 4 displays the absorption spectra of all samples, UV light is substantially absorbed by ZnO NRs (S0), while visible light is only weakly absorbed. In the UV and visible bands, the absorption spectrum of ZnO NRs/CdZnS before annealing (S1) is much more intense than that of ZnO NRs. And there is a shift toword the short wavelengths [20]. According to research the absorption edge of CdZnS around 450 nm [21, 22]. After annealing were attended (S2,S3 and S4), the absorption was faced a dropping in the intensity.
To investigate the photoelectric properties of the photodetectors, their current-voltage (I-V) curve were studied in the absence of light, in the presence of UV light, and in the presence of visible light. The results of these investigations are depicted in Fig. 5, which can be seen here. Current–voltage characteristics were obtained in under UV radiation at 390 nm, − 5–5 V to examine the UV photodetectors based on ZnO/CdZnS thin films at different annealing time. The dark current of ZnO/CdZnS is larger than that of ZnO NRs film s due to the enhanced conductivity [23].
It can be seen that all the prepared samples are sensitive to ultraviolet light strongly and their sensitivity is zero or little to visible light, whether they are annealed or not. It has been discovered that, when exposed to UV wavelength, all of the constructed detectors show an increase in their photocurrent density. Under the influence of UV light, electronhole pairs are produced in ZnO/CdZnS, and some of these holes are snatched up and adsorbed by oxygen on the surface of the material. This phenomenon can be linked to this phenomenon. As a result of the photoexcited electrons and holes moving down the NRs in opposite directions, which is caused by the external electric field, the photoconductance of the photodetector increases [24].
I-V characteristics for S3 and S2 samples show rectifying behavior which confirms formation of junction between ZnO and CdZnS .
As can be seen in Fig. 6, all of the materials' responsivities were determined using ultraviolet light with a wavelength of 385 nanometers and room temperature. It has been noticed that the responsivity increases with an increase in the bias voltage that is applied, and the responsivity of sample S3 when illuminated with UV light has been shown to be the greatest of all.
The photo switching measurements were performed by periodically exposing the photodetectors to UV light at (385 nm), with a power density of 2000 μW/cm2. The switching On/Off condition of the photodetectors within the time of 10 sec/10 sec at a 5V applied voltage bias; all of the samples were measured at room temperature. Figures 7 show the current characteristics with time (I-T) of the S0, S1, S2, S3, and S4 samples under UV illumination, it can be seen that the photocurrent is quick, consistent, and repeatable. The response (Tr) and recovery (Td) times of prepared samples are displayed in Figure 8. The (Tr) and (Td) times of all prepared devices were calculated at 385 nm wavelengths of UV light.
Figure 9. represents the rise and fall times of the prepared samples, as it shows from the figure that the fourth sample (S4) has the lowest rise and fall time than the rest of the samples, which means that the increasing of annealing time leads to decreasing the rise and fall time of the detectors.