With the maturity of germanium (Ge) growth on Si, Ge photodetectors have drawn great interests worldwide, which are potentially used in NIR/MIR light detecting, optical telecommunications, single photon detecting, biosensor applications. Lateral and vertical structured Ge-on-Si PIN photodetectors were fabricated and investigated. A dark current density of 20.4 mA/cm<sup>2</sup> was obtained, and small size devices resulted in low dark current values. The responsivity as a function of the wavelength was tested, and the highest responsivity of 0.8 A/W at the wavelength of 1310nm was obtained in vertical structured photodetectors, while the lateral structured photodetectors had the best 3dB bandwidth of 0.5 GHz, which was evaluated from the response time of 0.7 ns. The quantum efficiency was ~76%, and the reason of low 3dB bandwidth was discussed.
We report a high external quantum efficiency (EQE) photodiode detector with PtSi/Si-nanostructures. Black silicon nanostructures were fabricated by metal-assist chemical etching (MCE), a 2 nm Pt layer was subsequently deposited on black silicon surface by DC magnetron sputtering system, and PtSi/Si-nanostructures were formed in vacuum annealing at 450 <sup>o</sup>C for 5 min. As the PtSi/Si-nanostructures presented a spiky shape, the absorption of incident light was remarkably enhanced for the repeat reflection and absorption. The breakdown voltage, dark current, threshold voltage and responsivity of the device were investigated to evaluate the performance of the PtSi/Si-nanostructures detector. The threshold voltage and dark currents of the PtSi/Si-nanostructure photodiode tends to be slightly higher than those of the standard diodes. The breakdown voltage remarkably was reduced because of existing avalanche breakdown in PtSi/Si-nanostructures. However, the photodiodes had high response at room temperature in near infrared region. At -5 V reverse bias voltage, the responsivity was 0.72 A/W in 1064 nm wavelength, and the EQE was 83.9%. By increasing the reverse bias voltage, the responsivity increased. At -60 V reverse bias voltage, the responsivity was 3.5 A/W, and the EQE was 407.5%, which means the quantum efficiency of PtSi/Si-nanostructure photodiodes was about 10 times higher than that of a standard diode. Future research includes how to apply this technology to enhance the NIR sensitivity of image sensors, such as Charge Coupled Devices (CCD).