Near‐Infrared Photoresponse of Waveguide‐Integrated Carbon Nanotube–Silicon Junctions

Single-wall carbon nanotubes (CNTs) have been recently integrated into optical waveguides and operated as on-chip electro-optical transducers and nonclassical light sources. Here, first steps are taken toward complementary, waveguide-integrated optoelectrical transducers based on CNTs. Few-chiral CNTs are deposited from solution onto silicon waveguide and silicon electrodes, to form waveguide-integrated CNT-silicon junctions. The junctions are characterized by means of spatially and spectrally resolved photocurrent measurements. Photocurrent spectra show wavelength-dependent photocurrent contribution from silicon and from CNTs, however with the opposite sign. This behavior is explained by the relative positions of the Si band edges and the molecular orbitals of the CNTs. In CNT/Si junctions, only small diameter (7, 5) and (7, 6) CNTs are photoactive and contribute to photocurrent. When these CNTs are excited, electrons are injected into the silicon giving rise to photocurrent, whereas photoexcited larger diameter CNTs such as (8, 6), (8, 7), and (9, 7) do not contribute because of the lowest unoccupied molecular orbital (LUMO) position being too low for exciton dissociation. On the other hand, excitation of silicon yields photocurrents flowing in the opposite direction due to contact to larger diameter CNTs having suitable highest occupied molecular orbital (HOMO)-LUMO positions. The findings are supported by photocurrent maps and reflectance measurements.