  We use scanning photocurrent microscopy (SPCM) to investigate the properties
of internal p-n junctions as well as local defects in ambipolar carbon nanotube
(CNT) transistors. Our SPCM images show strong signals near metal contacts
whose polarity and positions change depending on the gate bias. SPCM images
analyzed in conjunction with the overall conductance also indicate the
existence and gate-dependent evolution of internal p-n junctions near contacts
in the n-type operation regime. To determine the p-n junction position and the
depletion width with a nanometer scale resolution, a Gaussian fit was used. We
also measure the electric potential profile of CNT devices at different gate
biases, which shows that both local defects and induced electric fields can be
imaged using the SPCM technique. Our experiment clearly demonstrates that SPCM
is a valuable tool for imaging and optimizing electrical and optoelectronic
properties of CNT based devices.
