Design and fabrication of photodetectors on porous silicon substrates based on ZnO and carbon quantum dots
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Date
2026-07-18
Authors
Γανωτής, Νικόλαος
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Abstract
The subject of this thesis is the design, fabrication and characterization of photodetectors based on Zinc Oxide (ZnO) nanostructures. The ZnO nanostructures were synthesized using the hydrothermal method on a silicon substrate as well as on Porous Silicon (Porous Si), while Carbon Dots (CDs) were deposited using the spin-coating technique. These three materials were utilized due to their absorption in different spectral regions. To this end, the materials were selectively synthesized under different conditions and characterized optically using photoluminescence (PL) measurements and structurally by performing SEM measurements. Through the appropriate design of schematic layouts using microelectronic techniques, the photodetector device was integrated by using Photolithography. The device was characterized by performing Current-Voltage (I-V) and Photocurrent measurements and the time and electrical response were studied upon irradiation with different light sources. Photoluminescence measurements confirmed the characteristic dual emission of the ZnO films, comprising a near-band-edge ultraviolet peak and a broad deep-level emission centered near 550 nm, the latter attributed to oxygen vacancy defects that were deliberately exploited as the basis for visible-light sensitivity. Electrical characterization showed that the pristine ZnO device exhibited ohmic, ultraviolet-only behavior, whereas all carbon-dot-modified devices displayed rectifying current-voltage characteristics, demonstrating that the active junction is formed at the carbon-dot/ZnO interface rather than at the metal contacts. The hybrid devices achieved a filterless broadband response extending continuously from the ultraviolet through the visible and into the near-infrared. Among the three fabricated devices, the one employing spin-coated nitrogen- and phosphorus-co-doped carbon dots gave the best performance, reaching a peak responsivity of approximately 3.8 A/W together with the lowest dark current of the series (on the order of 10⁻¹⁰ A near zero bias) and a shot-noise-limited specific detectivity of about 1.75 × 10¹⁴ Jones at zero bias. Several devices produced a measurable photocurrent under zero applied bias, confirming self-powered operation driven by the built-in field at the carbon-dot/ZnO interface. Overall, the results establish that a carbon-dot-sensitized ZnO film integrated into an interdigitated metal-semiconductor-metal geometry yields a low-cost, eco-friendly, filterless broadband visible photodetector with self-powered capability.
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Keywords
Zinc oxide (ZnO), Metal-semiconductor-metal photodetectors, Photolithography, Photoluminescense (PL), Carbon dots (CDs), Porous silicon (PSi)