2, Institute of Materials Science, University of Stuttgart, Stuttgart, , Germany
3, Fachbereich Physik, Philipps-Universität Marburg, Marburg, , Germany
Organic thin-film transistors (TFTs) are devices in which the active layer is an organic semiconductor with a thickness of several molecular monolayers that form a homogenous, polycrystalline film . The growth, morphology and stability of the semiconductor films on the surface of the gate dielectric are of particular interest, since the gate-field-induced charge-carrier channel is located in close vicinity to the semiconductor-dielectric interface. Using atomic force microscopy (AFM) and scanning electron microscopy (SEM), we have monitored the growth of thin films of the small-molecule semiconductor dinaphthothienothiophene (DNTT) [2,3] deposited by sublimation in vacuum onto a hybrid aluminum oxide/self-assembled monolayer (SAM) gate dielectric as a function of the film thickness. This was accomplished by depositing a DNTT film with a linear thickness gradient from 0 to 5 nm. At each point along the thickness gradient, i.e. for nominal semiconductor thicknesses between 0 and 5 nm, a significant temporal change in the stability and morphology of the semiconductor film was observed. Breuer et al.  have previously observed the phenomenon of post-deposition dewetting in DNTT films with a thickness of 2 nm, both on silicon dioxide and on silicon dioxide modified with an alkyl-silane SAM. They observed a transformation of the initially formed, nearly closed monolayer of DNTT molecules into separated islands of different height. This dewetting process was significantly less pronounced in thicker DNTT films (40 nm), but its potential influence on device stability needs to be nonetheless examined in more detail. We have therefore also fabricated bottom-gate, bottom-contact TFTs based on 2-nm- and 25-nm-thick DNTT films. We have observed a sharp decrease in the charge-carrier mobility in the TFTs with the 2-nm-thick DNTT films, from 0.2 cm2/Vs (15 mins after device fabrication) to 0.011 cm2/Vs (72 hours after device fabrication) and correlate this to post-depositional dewetting occurring in various stages. For comparison, the TFTs with the 25-nm-thick DNTT film are stable, showing a virtually constant carrier mobility of 1.1 cm2/Vs. Not only does this TFT architecture allow us to study the effect of dewetting on the characteristics and the stability of the semiconductor films and the TFTs, but it also opens up the possibility of new device designs if post-deposition semiconductor dewetting is successfully suppressed. This work also explores different strategies to that effect, such as post-deposition cooling. References:  R. Yi, et al., Sci. China Technol. Sci., 57, 1142, 2014.  K. Takimiya, et al., J. Am. Chem. Soc., 128, 12604, 2006.  U. Zschieschang, et al., Organic Eletronics, 12, 1370, 2011.  T. Breuer, et al., ACS Appl. Mater. Interfaces, 9, 8384, 2017.