Dielectric surface-controlled low-voltage organic transistors via n-alkyl phosphonic acid self-assembled monolayers on high-k metal oxide

In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO ₂) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO ₂ with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/ disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO ₂ hybrid dielectrics, pentacene-based OTFTs operate under -2.0 V with low hysteresis, on-off current ratios above 1 × 10 ⁶, threshold voltages below -0.6 V, subthreshold slopes as low as 100 mV dec-1, and field-effect mobilities as high as 1.8 cm2 V ^-1 s ^-1. © 2010 American Chemical Society.