Semiconductor-dielectric interfacial study using spectral-spatial photocurrent probes and 1/f noise probe in organic field effect transistors

摘要

Organic Field Effect Transistors (OFETs) are sensitive to the chemistry of the gate dielectric/semiconductor boundary both during and after fabrication. In particular, surface states can be introduced on polymer gate dielectrics that shift the threshold voltage if the dielectric layer is exposed to oxidizing agents prior to semiconductor deposition. To understand the interfacial properties can not only benefit practical applications but also further the understanding of transport mechanism in OFETs. Photocurrent and noise probes are well suited for investigating the properties of such process-induced interface trap states thanks to their high sensitivity to the electronic transport processes in the material. We have measured and analyzed the gate bias dependence of the photocurrent in pentacene organic field effect transistors which have been doped using a UV-ozone treatment and compared these to the response of identical devices produced in an oxygen and ozone free environment. The wavelength dependent photocurrent spectrum shows new photocurrent peaks in oxygen doped samples in the range of 350 nm to 450 nm, which corresponds to energy transitions (2.66 eV, 2.76 eV, 2.95 eV, 3.15 eV) larger than the pentacene HOMO-LUMO gap. The acceptor-like states generated by UV treatment of the dielectric layer, although outside the HOMO-LUMO gap, can free some holes into the accumulated charge pool and thus change the threshold voltage. We have also characterized the dependence of $1/f$ noise on the drain current in control OFETs fabricated with no air exposure and those whose dielectric has been exposed to UV-ozone. The noise in control devices is proportional to $I_{D}⁁{2}$, while noise in UV-exposed devices is proportional to $I_{D}$. This difference indicates that in the UV-exposed devices, noise is generated by acceptor-like states introduced into the HOMO, while in the control devices, interface states within the HO-nMO-LUMO gap are the dominant noise source. Under 405nm illumination, the change in 1/f noise is proportional to the change in I_{D} for both devices implying that no additional noise-generation sites are created in the empty states which have donated charge to the channel. Spatially localized photocurrent is futher utilized to study mobility variance along the channel and preliminary results are presented.