Photodetection using ultrathin metal films
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Silicon is the most widely used material for visible photodetection, with extensive applications in both consumer and industrial products. Further, its excellent optoelectronic properties and natural abundance have made it nearly ideal for microelectronic devices and solar cells. However, silicon's lack of absorption in the infrared limits its use in infrared detectors and imaging sensors, severely constraining its implementation in telecommunications for low-cost integrated optical circuitry. In this thesis, we show that this limitation can be overcome by exploiting resonant absorption in ultrathin metal films (< 20 nm). Our approach paves the way to implement scalable, lithography-free, and low-cost silicon-based optoelectronics beyond the material bandgap.
Light absorption in metal films can excite hot carriers, which are useful for photodetection, solar energy conversion, and many other applications. However, metals are highly reflective, and therefore, careful optical design is required to achieve high absorption in these films. Through appropriate optical design, we achieved a Fabry-Perot-like resonance in ultrathin metal films deposited on a semiconductor enabling> 70% light absorption below the bandgap of the semiconductor. We experimentally demonstrate this phenomenon with four ultrathin planar metal films: Pt, Fe, Cr, and Ti. These metals were chosen to satisfy the resonant condition for high absorption over a wide range of wavelengths, and with these designs we realize a near-infrared imaging detector.
In addition, we utilize an index-near-zero (INZ) substrate to further improve the absorption to near-unity. By employing aluminum-doped zinc oxide (AZO) as the INZ medium in the near-infrared range, we enhance the metal film absorption by nearly a factor of 2. To exploit this absorption enhancement in an optoelectronic device, we fabricate a Schottky photodiode with a Pt film on Si and and that the photocurrent generated in the photodiode is enhanced by > 80% with the INZ substrate. The enhancement arises from a combination of improved carrier generation and carrier transport resulting from the addition of the AZO film.
Finally, we explore the tunability of material properties through alloying metals. Alloying of metals provides a vast parameter space for tuning of material, chemical, and mechanical properties, impacting disciplines ranging from photonics and catalysis to aerospace. We demonstrate that AgAu alloys provide an ideal model system for controlling the optical and electrical responses in ultrathin metal films for hot carrier photodetectors with improved performance. While pure Ag and Au have long hot carrier attenuation lengths > 20 nm, their optical absorption is insufficient for high efficiency devices. We and that alloying Ag and Au enhances the absorption by ~50% while maintaining attenuation lengths > 15 nm, currently limited by grain boundary scattering. Further, our density functional theory analysis shows that the addition of small amounts of Au to the Ag lattice significantly enhances the hot hole generation rate.