Patterning organic single-crystal transistor arrays (original) (raw)

Field-effect transistors made of organic single crystals are ideal for studying the charge transport characteristics of organic semiconductor materials 1 . Their outstanding device performance 2-8 , relative to that of transistors made of organic thin films, makes them also attractive candidates for electronic applications such as active matrix displays and sensor arrays. These applications require minimal cross-talk between neighbouring devices. In the case of thin film systems, simple patterning of the active semiconductor layer 9,10 minimizes cross-talk. But when using organic single crystals, the only approach currently available for creating arrays of separate devices is manual selection and placing of individual crystals-a process prohibitive for producing devices at high density and with reasonable throughput. In contrast, inorganic crystals have been grown in extended arrays , and efficient and large-area fabrication of silicon crystalline islands with high mobilities for electronic applications has been reported 14,15 . Here we describe a method for effectively fabricating large arrays of single crystals of a wide range of organic semiconductor materials directly onto transistor source-drain electrodes. We find that film domains of octadecyltriethoxysilane microcontact-printed onto either clean Si/SiO 2 surfaces or flexible plastic provide control over the nucleation of vapour-grown organic single crystals. This allows us to fabricate large arrays of high-performance organic single-crystal field-effect transistors with mobilities as high as 2.4 cm 2 V 21 s 21 and on/off ratios greater than 10 7 , and devices on flexible substrates that retain their performance after significant bending. These results suggest that our fabrication approach constitutes a promising step that might ultimately allow us to utilize high-performance organic single-crystal field-effect transistors for large-area electronics applications. illustrates the single-crystal patterning process (see Methods for a detailed description). First, thick octadecyltriethoxysilane (OTS) films (average thickness ,13 6 2 nm, measured by ellipsometry) are printed by microcontact printing onto a clean SiO 2 /Si substrate, using a polydimethylsiloxane (PDMS) stamp with a relief structure in the desired pattern. We note that although the results discussed here mainly relate to structures grown on arrays of micrometre-sized OTS squares, it is straightforward to create more-complex OTS film patterns for single-crystal nucleation ( . After film deposition, crystals are grown using a vapour transport method 16 applicable to a broad range of materials, including high-mobility p-type materials such as rubrene, pentacene and tetracene, and n-type materials such as C 60 , fluorinated copper phthalocyanine (F 16 CuPc) and tetracyanoquinodimethane (TCNQ) . We find that sublimation of the organic material and crystal growth are accomplished in as little as five minutes for pentacene and as much as two hours for C 60 , and that crystal nucleation is restricted to OTS-stamped regions and not observed on the SiO 2 background. The crystals show strong birefringence in optical micrographs recorded under cross-polarized light ( ), confirming their crystalline nature. We also observe intense and narrow diffraction peaks in the X-ray diffraction patterns of pentacene, rubrene and C 60 single crystals, which are indicative of a high degree of crystallinity .