A tunable positive and negative photoconductive photodetector based on a gold/graphene/p-type silicon heterojunction

An interdigitated gold (Au) nanofilm was introduced on graphene, supported by a p-type silicon (p-Si) substrate, as an optoelectronic photodetector based on an Au/graphene/p-Si heterojunction by a low-cost method. Tunable photocurrent (photoconductivity) response to laser power density was realized under the modulation of the Au nanofilm and the Si layer on the carrier transport through graphene. Photoelectric measurements demonstrated that the device generated positive photocurrent when illuminated at low laser power density and negative photocurrent when illuminated at high laser power density. Positive photocurrents of 9 A and 5 A were generated at 1.13 W cm(-2) (power approximate to 20 nW) and 0.32 W cm(-2) (power approximate to 40 nW) under irradiation of 532 nm and 1064 nm lasers, respectively; responsivities of up to 450 A W-1 and 125 A W-1, were achieved, respectively, owing to the increase in carrier density caused by photo-induced carrier injection. At high power density, the mobility of graphene declined due to the scattering effect between carriers and phonons (bolometric effect), which was further strengthened by the local high-temperature field caused by the Au nanofilm. The negative photocurrent of up to several milliamperes was clearly observed under irradiation of 532 nm and 1064 nm lasers. The photocurrent response versus power density behavior of the photodetector could be modulated by the source-drain bias and laser wavelength. The device had a rapid response rate of the order of several hundred microseconds and good stability. The photodetector realized high-speed conversion of light density to photocurrent with different signs, demonstrating good potential for future applications in optoelectronic devices.