Ultraviolet detectors have a wide range of applications, including environmental monitoring, missile launch detectors, ozone detectors, remote control, space research, optical telecommunications, secure telecommunications, and biological imaging. These devices have been able to attract many research goals due to their great potential and wide applications. Fast time response, optimal signal-to-noise ratio and high responsiveness are the parameters of interest for such applications. Recently, materials with a large energy gap have been investigated in order to improve the response and stability of UV optical detectors due to their characteristics [1], [2]. Zinc oxide is considered the most desirable choice for making UV detectors with high sensitivity [3]. On the other hand, benefiting from the advantages of the nanoscale in materials and their electronic applications, especially zinc oxide nanomaterials and one-dimensional nanostructures, has made them multipurpose, stable and low-cost [4], [5], [6].

Zinc oxide is an n-type semiconductor material that has a large energy gap (3.37 eV) at room temperature, and its large exciton binding energy (60 meV) enables effective exciton emission at room temperature. Its low refractive index makes it easy for light to escape from an optical device. Its wurtzite crystal structure with lattice parameters a and c are equal to the values of 3.2495 Å and 5.2062 Å, respectively, and its large piezoelectric constants enable strong piezoelectric polarization in the material. Due to the high surface-to-volume ratio and surface reactivity of some ZnO morphologies, these structures are considered potential candidates for particle absorption applications, whether photons in dye-sensitized solar cells or molecules, be biochemical in biomedical sensors or gas molecules in gas sensors. In addition, zinc oxide nanostructures and nanoparticles, due to their low toxicity, biocompatibility and biodegradability, make it an attractive material in biomedical and environmental systems. [7] Also, one-dimensional zinc oxide nanostructures can be created with different shapes and sizes, different density and surfaces, and it is possible to achieve these morphologies with different methods, including physical, chemical, and electrochemical methods [4]. These nanostructures are good choices for making widely used devices such as solar cells, UV detectors and UV emitting diodes [8], [9], [10]. Hoa et al. [11] fabricated heterojunction p-Si/n-ZnO NRs by hydrothermal method, which improved the photocatalytic activity in this structure. Fabrication of an ultraviolet photodiode has been reported [12], which was created by growing zinc oxide nanorods on p-type silicon substrate. The I–V characteristic of the device shows that the link exhibits a good rectification characteristic and the reported rectification ratio is 210 at 5 V. The response of the fabricated photodiode at the wavelength of 370 nm was obtained as 0.6 A/W. Faisal et al. [13] showed that the synthesis of zinc oxide nanorods on silicon substrate by hot water method can be used in the manufacture of various optical devices, and the optical and structural characteristics of these nanorods were investigated and reported. Rajab et al. [14] produced zinc oxide rods for UV detection applications under laser irradiation and showed that the response of the device increased up to 2.441 mA W−1. Kathalingam et al. [15] were able to transform the heterogeneous structure of n-ZnO/p-Si into p-type under a thermal process. It has been reported [16] that self-biased UV–visible photodiodes based on heterogeneous bonding of ZnO and silicon nanorods with modified surface and improved structure have been fabricated. Mazandarani et al. [17] reported UVB photodiodes that were made using an array of zinc oxide nanorods on a silicon substrate by with a hydrothermal process and showed that the optical and electrical characteristics of this device were improved. Hassan et al. [18] investigated various characteristics, including the voltage current characteristic, whose dark current was reported to be approximately 60μA at a voltage of −1 V.

Methods have been proposed to reduce the dark current of photodiodes, especially ultraviolet photodiodes, and to improve their response, and the use of some of them requires special conditions that may not be easily accessible. In using a very easy and low-cost hydrothermal growth method using minimal precursors under the same growth conditions only by changing the concentration of precursors, ultraviolet photodiodes were made, which reduced the dark current and significantly improved the responses that were made only by the diameter of zinc oxide nanorods. This process was able to reduce the duration of construction and possible errors in time-consuming processes and facilitate the way of checking and making improved samples.

The remaining of the paper is organized as follows:

In Section 2, the Experimental details, which include two Section 2.1 and Section 2.2, Synthesis and Measurement, are described. In Section 3, the Results and discussion are mentioned, which includes six different Subsections, and the results of SEM (3.1), XRD (3.2), PL(3.3), UV–Vis (3.4), Raman (3.5) and I–V characteristics (3.6) analysis are included in it. Finally the paper is concluded in Section 4.