# Aluminum-Based Plasmonic Photodetector for Sensing Applications
## 1 Introduction  
Surface plasmon-polaritons (SPPs) are electromagnetic surface waves that propagate along the boundary between a metal and a dielectric or semiconductor, with the maximum field located at this interface and decaying exponentially in both media [1]. Being a non-radiative mode, such a surface wave requires matching its wave vector with the wave vector of the incident light, which, in most cases, is accomplished using a coupling prism [2; 3; 4] or diffraction grating [5; 6; 7]. Under optimal conditions, the complete transformation of light energy into the energy of surface plasmon-polaritons is achieved, accompanied by a local increase in the intensity of the electromagnetic field at the interface with the metal plasmon-carrying film. Such spatial localization of the electromagnetic field at the incident photon energy greater than the band gap of the semiconductor contributes to an increase in the generation of electron-hole pairs in it near the contact with the metal and a corresponding increase in the photocurrent in the presence of a built-in electric field separating the charge carriers. At energies lower than the band gap, the excitation ofhot electrons in the metal film with subsequent ballistic transport to the semiconductor region with the built-in electric field is possible [8; 9; 10; 11]. Thus, the spectral sensitivity range of such photodetectors can extend beyond the band-to-band generation region by expanding into the red spectral region. Based on this phenomenon, the design of a so-called plasmon-polariton photodetector (PPPD) was proposed [8; 12; 13]. To excite SPPs in such detectors, a periodically textured semiconductor surface (diffraction grating) with a thin layer of plasmonic metal deposited on it is used. GaAs, Si, and InP were proposed as semiconductors, and films of the noble metals Au and Ag, as well as Al films, were used as plasmon-carrying metals [14; 15]. The application area of such photodetectors is quite wide: they are used in sensing, photovoltaics, telecommunications, etc. [14; 15; 16; 17; 18]. The basic principle underlying the devices' operation is the resonant enhancement of the short-circuit photocurrent generated under surface plasmon resonance (SPR) conditions in a Schottky junction at the metal-semiconductor contact [8; 19]. However, the presence of a corrugated interface at the Schottky junction contributes to enhanced carrier recombination and scattering, leading to the attenuation of the useful signal. In this regard, PPPD designs with a flat metal-semiconductor interface have been proposed, with the grating relief formed on the outer side of the plasmon-carrying metallic film [20]. For PPPDs based on n-type doped silicon, a scheme with a shallow p-n junction and a distance of about 100 nm from the plasmon-carrying metal layer to the p-n junction has been proposed [21]. This approach eliminates the need to form a high Schottky barrier at the metal-semiconductor interface, which is not always possible for Au/Si or Al/Si pairs. In this case, the separation of photo-carriers occurs in the p-n junction region, i.e., remote from the metal/Si interface, which is characterized by a high surface recombination rate. The influence of deep levels formed by Au in Si on the increase in recombination is also reduced. Moreover, the metal/Si interface can remain flat, simplifying the manufacturing process. Most studies on PPPDs have been carried out on structures comprising noble metals, mainly gold. Gold has slightly worse plasmonic characteristics (in particular, the ratio of the real and imaginary parts of the dielectric constant) than silver, but it is much more resistant to degradation during operation. It was shown in [18; 22] that cheaper aluminum has plasmonic characteristics even better than gold in a significant spectral range, especially in the blue and ultraviolet parts of the spectrum.