STUDY OF NiO/Si BASED HETEROSTRUCTURE FOR BROADBAND SELF-POWERED PHOTODETECTION

Abstract

Researchers are currently focusing their efforts on developing smart, environmentally friendly technologies that apply to the Internet of Things (IoT). According to reports, the number of sensors involved in IoT technology has surpassed 50 billion, with this figure expected to touch 200 billion by 2025. As a result, demand for highly efficient sensors in the IoT market is increasing at an unexpected rate. Photodetectors (PDs), one of these sensors, have a lot of promise for application in the next nanodevices. One of the primary issues with today's PDs is that they must be driven by external sources (batteries), which limits their use in ecologically friendly and sustainable technologies. Thus, significant research has been dedicated to the fabrication of self-powered PDs in the current scientific industries. Self-powered PDs have additional advantages over conventional PDs, such as lower power usage, minimization of unnecessary energy-related difficulties, and reduction in overall size. Therefore, to cater to the need, an effort has been made in the present thesis to develop a self-powered broadband photodetector based on NiO/Si heterostructure. The knowledge of band alignment properties is helpful to understand the contribution of charge carriers in the NiO/Si photodiode. For the band alignment study, the p-type NiO thin film was deposited on a Si substrate at room temperature, under 30 mTorr ambient oxygen pressure conditions by using the pulsed laser deposition technique (PLD). By using the X-ray photoelectron spectroscopy (XPS) characterization technique, the values of valence and conduction band offset are calculated to be 0.34 and 1.68 eV, respectively, and accordingly, the energy band diagram for the NiO/Si photodiode is constructed. In addition, the optical constant of NiO thin film is obtained by modeling the data measured by Spectroscopic Ellipsometer. The extinction coefficient plot of the NiO thin film grown on Si substrate indicates absorption below its optical bandgap, which may be due to the presence of defects. X-ray diffraction (XRD) and XPS results confirm the Ni vacancy in the NiO thin film, which indicates the p-type conducting nature of the NiO thin film. The Hall measurement also confirmed the p-type conducting nature of the NiO thin film having the hole mobility of 4.67 cm2/V.s. Further, the self-powered broadband photodetection properties of the photodiode have been explored under the illumination of 365 nm, 405 nm, 455 nm, 485 nm, 530 nm, 655 nm, 735 nm, and 850 nm wavelength LED sources. When the device is operated in selfbiased mode, the photodetector shows the responsivity of 12.5 mA/W, 24.6 mA/W, and 30.8 mA/W under the exposures of 365 nm, 485 nm, and 850 nm wavelength LED sources, respectively. In addition, the photodiode exhibits rapid response in UV to NIR region. As the plasmonic metal nanoparticles have huge potential in improving the optoelectronic device performance, an effort has been made to enhance the photodetection properties of NiO/Si photodiode by incorporating the silver nanoparticles (Ag-NPs) of different sizes and distribution, sandwich between NiO and Si. Optimized Ag-NPs incorporated photodiode (NiO/Ag-NPs/Si) exhibits better photo-absorption under AM1.5G sunlight illumination and enhanced self-powered photoresponse (20 times enhancement under sunlight illumination and 8 times enhancement under UV illumination) as compared to the NiO/Si heterostructure. The improvement of the self-powered photoresponse performance of the device is explained by localized surface plasmon-induced hotelectron generation in Ag-NPs. Defects engineering plays an important role in modifying the physical properties of metal oxides to enhance the optoelectronic properties of a system. Aiming to that, the defects in the NiO thin film were modified by changing the ambient oxygen pressure (AOP) during growth by using the PLD technique. Spectroscopic ellipsometry and first-principles DFT + U simulations are compared to better understand the influence of varying oxygen pressure on the physical characteristics of NiO film. As a result, the structural, optical, and electrical characteristics of NiO films may be modified by adjusting the oxygen working pressure, making it a critical feature in optimizing device performance.