# Aluminum-Based Plasmonic Photodetector for Sensing Applications
## 4 Results and Discussion  
Using the obtained results of theoretical modeling, experimental PPPD samples were fabricated with the above-mentioned optimal parameters of the Al grating. The electro-physical properties of the manufactured PPPDs were investigated by measuring their current-voltage (I-V) characteristics. The I-V characteristics were measured in the dark using a CHI660E (CH Instruments, Bee Cave, TX, USA) workstation in the two-wire connection mode. Figure 8 presents the I-V characteristics of the PPPDs with different frontal contacts (gratings), namely aluminum and gold, for comparison. The rest of the parameters of both PPPDs, including the characteristics of the gratings, were practically the same. It can be seen that the type of metal has little influence on the current-voltage characteristics of the investigated samples. This indicates that the p-n junction in silicon has a crucial effect on the current. The type of plasmon-active metal on the front surface of the PPPD does not affect the I-V characteristics. Both types of PPPDs, namely with the Au and Al frontal plasmon-active contacts, have almost the same reverse saturation currents and the same slope of the direct I-V characteristics. The differences may be related to the different contact areas and heterogeneity of the properties of the p-n junctions on silicon used for the production of PPPD. Both types of PPPDs have a non-ideality parameter of the I-V characteristics n-4 and a reverse saturation current \(I_{o}\)\(\sim\)\(10^{-5}\) A, which describe their diode properties in the range of 0.1-0.6 V by the general law without taking into account the series and shunt resistances:  
\[I(V)=I_{o}\cdot(\mathrm{e}^{\frac{\mathrm{a}V}{\mathrm{e}^{\mathrm{a}E_{b}T}}}- 1). \tag{5}\]  
Here, \(q\) is the electron charge, \(k_{b}\) is the Boltzmann constant, and \(T\) is the temperature. The spectral and angular dependences of the photocurrent for the fabricated PPPDs are presented in Figure 9a,b and Figure 10a,b, respectively. As can be seen in Figure 9a, the PPPD photocurrent exhibits a pronounced resonance for the case of p-polarized light. The resonance peak shifts toward longer wavelengths as the incidence angle increases, as predicted by the respective Expression (2) of the dispersion dependence for a surface plasmon-polariton wave corresponding to an m = +1 diffraction order. The polarization sensitivity of the PPPD was determined as the ratio of the photocurrent in resonance in p-polarization to the corresponding value when irradiated with s-polarized light, \(\mathrm{I_{p}/I_{s}}\). For the investigated PPPDs with the optimal parameters of the Al grating, we obtained a value of \(\mathrm{I_{p}/I_{s}}\) = 8, an FWHM of the photocurrent spectrum from 50 to 100 nm, and sensitivity at the resonance maximum of \(\mathrm{I_{ph}}\) = 0.04-0.06 A/W. The obtained PPPD characteristics are comparable to the best characteristics for PPPDs  
Figure 8: Forward (I, 2) and reverse (3, 4) current–voltage characteristics of the manufactured PPPD with Au (I, 3) and Al (2, 4) frontal plasmon-active contact. with gold gratings [20]. These practical results open up prospects for replacing gold in the production of PPPD with aluminum, which significantly lowers their cost. Figure 10: Angular dependences of photocurrent for Al-based PPPD in p-polarized light (1–3, 1’–3’) and in s-polarized light (4–6) for different wavelengths: 650 nm (1); 808 nm (2); 900 nm (3). (**a**) Experimentally measured data, and (**b**) numerically simulated light absorption by Si substrate. The photocurrent value is normalized to the power of the incident radiation. Figure 9: Spectral (**a**) dependences of photocurrent for Al-based PPPD in p-polarized light (1–5) and in s-polarized light (6, 7) at different incidence angles: \(\theta=5^{\circ}\) (1, 6), \(10^{\circ}\) (2), \(15^{\circ}\) (3), \(20^{\circ}\) (4), and \(25^{\circ}\) (5, 7). (**b**) Numerically simulated Si substrate absorption spectra at the same incidence angles.