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, its lack of absorption in the infrared precludes its use in infrared detectors and imaging sensors, severely constraining its implementation in telecommunications. Here we show that this limitation can be overcome by exploiting resonant absorption in ultrathin metal films (<20 nm). Through appropriate optical design, a zeroth-order Fabry–Perot resonance is achieved, enabling ∼80% light absorption below the bandgap of the semiconductor. Absorption within the metal film results in excitation and injection of hot carriers through a Schottky junction into the Si. We experimentally demonstrate this phenomenon with four ultrathin planar metal films (Pt, Fe, Cr, and Ti), chosen to satisfy the resonant condition over a wide range of wavelengths (1200–1600 nm), and realize a near-infrared imaging detector. Our approach paves the way to implement a scalable, lithography free, and low-cost route to obtain silicon-based optoelectronics beyond the material bandgap.

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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. From an optical point-of-view, pure thin metal films yield enhanced light absorption due to their cavity effects. However, an ideal metal–semiconductor photodetector requires not only high absorption, but also long hot carrier attenuation lengths in order to efficiently collect excited carriers. Here we demonstrate that Ag-Au alloys provide an ideal model system for controlling the optical and electrical responses in nanoscale thin 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. Instead, we find that alloying Ag and Au enhances the absorption by ∼50% while maintaining attenuation lengths >15 nm, currently limited by grain boundary scattering, although the electron attenuation length of pure Au outperforms pure Ag as well as all of the alloys investigated here. 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. Combined, these findings suggest a route to high efficiency hot carrier devices based on metallic alloying with potential applications ranging from photodetectors and sensors to improved catalytic materials.

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. Here we utilize a subwavelength Fabry-Pérot-like resonance in conjunction with an index-near-zero (INZ) substrate to achieve near-unity absorption and hot carrier photocurrent in nanoscale metal films. 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 and find that the photocurrent generated in Pt on Si 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.

In atomic force microscopy (AFM), the angle relative to the vertical ($\theta_{i}$) that the tip apex of a cantilever moves is determined by the tilt of the probe holder and the geometries of the cantilever and actuated eigenmode $i$. Even though the effects of $\theta_{i}$ on static and single-frequency AFM are known (increased effective spring constant, sensitivity to sample anisotropy, etc), the higher eigenmodes used in multifrequency force microscopy lead to additional effects that have not been fully explored. Here we use Kelvin probe force microscopy (KPFM) to investigate how $\theta_{i}$ affects not only the signal amplitude and phase, but can also lead to behaviors such as destabilization of the KPFM voltage feedback loop. We find that longer cantilevers and modified sample orientations improve voltage feedback loop stability, even though variations to scanning parameters such as cantilever shake amplitude and lift height do not.Comment: 5 pages of text, 5 figures. Revision replaces discussion of bimodal AFM with more discussion of experimental result