Nowadays, infrared (IR) photodetectors are mainly made from compound semiconductors due to the bandgap flexibility. However, compound semiconductors are mostly synthesized by expensive and energy-intensive epitaxy processes. Moreover, compound semiconductors are difficult to integrate with Si-based IC industry. Therefore, we used n-type Si (n-Si) wafers and thin NiSi to combine with localized surface plasmon resonance (LSPR) to form a Schottky IR detector. The incident IR light can induce thermionic effect to generate photocurrent, and the LSPR can enhance the light absorption and improve the photoresponse. The LSPR was created by NiSi covered inverted-pyramid array structures (IPAS) formed on n-Si substrates through photolithography and etching processes. After IPAS were prepared, 10-nm-thick Ni was thermally deposited on the IPAS and then the entire samples were annealed under 500 °C in 5 s to form NiSi/n-Si Schottky junctions. Finally, Ti and Au were thermally deposited successively on the NiSi and the back of n-Si wafers to be electrodes. A planar device was also prepared to be a control part. The photodetection ability of the device was examined by a 4.8-μm IR source with 1.8-mW optical power, which is in the absorption range of carbon monoxide. The IR source was turned on/off for each 15 s. Consequently, the planar NiSi/n-Si Schottky photodetector shows average 9.37-μA current change under 4.8-μm IR source illumination in 15 s. However, if 8-μm-period IPAS was used, the average current change improved to 30.9 μA. The response enhancement is 3.30 times of the planar device.
Infrared (IR) photodetector is widespread applied to spectroscopy, biosensing, and image detection. Nowadays, most of IR photodetectors are prepared from compound semiconductors, for example, SiGe, HgCdTe, and InSb. However, most of them are formed through high energy consumption and high expense processes, such as chemical vapor deposition. Also, compound devices are not compatible with Si-based IC manufacturing and very expensive. Therefore, here, we used n-type Si (n-Si) wafers and Ag thin films to form a Schottky IR detector. The detection principle is using IR source to induce thermionic effect on Schottky diode and then the scattering of photoelectrons excited by IR and visible light, respectively, induces current difference. Regarding device preparation, at first, the native oxide on n-Si wafers was removed by buffer HF; then the 10-nm-thick Ag films and 100-nm-thick Ag grid anode were thermally deposited on the n-Si successively. Afterward, Al was thermally deposited on the opposite side of n-Si wafers to be a cathode. The electric property of devices was determined through current-to-time (I-T) measurement with an 80-mW green laser illuminating on the Ag side constantly. A 3.22-μm IR source was illuminated through Ag side but turned on/off for each 5 s. The electric bias is 0 V. Consequently, if no green laser exposing, current increased after IR turned on due to the pure thermionic effect and the responsivity is 1.8 mA/W. While the green laser illuminating constantly, current decreased after IR turned on, and the responsivity increased to -15.7 mA/W.