# Aluminum-Based Plasmonic Photodetector for Sensing Applications
## 2 Materials and Methods  
### Fabrication of PPPDs on Si Substrates  
For fabricating PPPDs, a pre-made p-n junction on n-Si (111) with a burial depth of 100 nm and 25/250 nm Ti/Ni ohmic rear contacts were used. The necessary periodic relief structures (gratings) on the silicon surface were created using interference lithography with thermally deposited chalcogenide photoresists in a vacuum [20]. A particular feature of this technology, compared to the standard one where the grating is etched into the semiconductor, is the profiling of the metallic plasmon-carrying Al film only. This maintains a flat metal/semiconductor interface with low recombination of the photo-generated carriers. Such a PPPD has a simpler construction and good resonant properties [20].  
A schematic representation of the step-by-step (1-7) process of fabricating Al gratings on a Si surface is shown in Figure 1. Before installation into the vacuum chamber, the working side of the Si wafer was cleaned in ethyl alcohol and a 5% aqueous solution of HF, followed by rinsing with distilled water and drying with compressed air (Figure 1, 1). The subsequent steps involved the sequential vacuum thermal deposition of an adhesive Cr layer with a mass thickness of ~3 nm and an Al layer of optimal thickness onto the Si surface (Figure 1, 2). This thickness determined the depth of modulation of the surface relief, \(D\). After cooling the wafer to room temperature, a layer of chalcogenide photoresist As\({}_{40}\)S\({}_{40}\)Se\({}_{20}\) with a thickness of ~100 nm was deposited on the Al surface (Figure 1, 3). The photoresist was exposed in the interference zone of two coherent laser beams emitted by a helium-cadmium laser (wavelength \(\lambda\) = 441.6 nm). The spatial period of the light intensity distribution in the interference zone depended on the convergence angle of the beams and determined the grating period, a. After exposure, the photoresist was selectively etched to form a mask, which allowed access of the etchant to the open areas of the Al layer (Figure 1, 4).  
The selective etchant used for the photoresist As\({}_{40}\)S\({}_{40}\)Se\({}_{20}\) contained ethylenediamine (C\({}_{2}\)H\({}_{8}\)N\({}_{2}\)) as an active substance and dimethylsulfoxide (C\({}_{2}\)H\({}_{6}\)OS) and acetone (C\({}_{3}\)H\({}_{6}\)O) as solvents at a ratio of 2:55:43. At the optimal etchant temperature (20-25 \({}^{\circ}\)C), the etching process took about 3 min. After creating the relief lithographic mask, the Al layer in the mask openings was removed using an etchant based on orthophosph  
Figure 1: Key stages of manufacturing a PPPD using aluminum as the plasmon-carrying metal.  
and acetic (H\({}_{3}\)COOH) acids, and distilled water at a ratio of 6:2:2 (Figure 1, 5). The final steps of forming the relief structure of the PPPD involved removing the lithographic mask (Figure 1, 6) and the vacuum thermal deposition of an additional Al layer of thickness \(d\) onto the obtained relief (Figure 1, 7). The etchant used for removing the photoresist mask contained ethylenediamine (C\({}_{2}\)H\({}_{8}\)N\({}_{2}\)) and dimethylsulfoxide (C\({}_{2}\)H\({}_{6}\)OS) at a ratio of \(\sim\)1:9. The aluminum grating on the silicon wafer surface provided the excitation of SPR and also served as the upper electrical contact.