UC San Diego

Nanowire photodetectors have been attracting increased attention due to their potential for very high sensitivity detection stemming from the unique properties of these quasi-one dimensional structures. Silicon photodetectors are of particular interest due to their low cost, ease of processing and ability for integration with conventional fabrication techniques. This work focuses on utilizing silicon nanowires towards creating a very high responsivity detector sensitive to a wide range of wavelengths from the ultraviolet to the near infrared spectrum. A physical understanding of the silicon nanowire phototransistors studied in this work is crucial for applying them towards high sensitivity imaging. The novel device concept is first presented qualitatively, illustrating how surface states in conjunction with geometrical effects of the nanowire create a large phototransistive gain. The device theory is then formalized mathematically, revealing the important physical quantities responsible for gain and the theoretical sensitivity achievable in such devices. Subsequently, simulations are performed to validate the concept and determine the parameters which govern device behavior. A top-down fabrication approach is utilized in creating these devices, allowing for precise control over geometry, traditional doping techniques, large area device formation, and compatibility with industrial fabrication lines. The devices are patterned through either traditional technology with e-beam lithography or maturing technology with nanoimprint lithography, and the nanowires formed through highly anisotropic dry etching. The finished devices are embedded in dielectric to support a top transparent contact. Characterization of these devices exhibits very high responsivity and phototransistive gain. Static measurements at room temperature show the initial demonstration of the device. Spectral measurements are then performed, showing absorption enhancement effects in vertical nanowire structures. Temperature dependent measurements demonstrate the capabilities of the device to detect illumination levels down to the sub-femtowatt at visible wavelengths, and picowatt at infrared wavelengths, unseen in bulk or thin-film devices. Finally, dynamic measurements determine the bandwidth of the device and a critical time constant in the kHz range. The characterization of these devices reveals both their potential and limitations. Future work on this device is proposed to engineer more reliable control over gain, lowered dark current for room temperature operation, and increased sensitivity