Structural and electrical properties of self-assembled monolayers on Ge as passivating and insulating layers

International audienceDue to its high intrinsic mobility, germanium is emerging as a likely alternative material to replace silicon in the next generation of high-mobility and high-frequency field effect transistors. However, the preparation of an interfacial layer enabling to passivate and insulate Ge surface is still problematic. A promising approach consists in designing new self-assembled molecular monolayers (SAMs) [1] grafted on Ge exhibiting highly insulating and passivating properties as new high-K self-assembled nanodielectrics [2]. We have studied SAMs of model molecules such as alkylthiols and fluoro-alkylthiols, and of specially synthesized non-charged novel push-pull chromophores bearing electron donor and acceptor groups, separated by a pi-conjugated bithiophene bridge which promotes electron transfer and a subsequent dipole formation [3]. Indeed, due to the alignment of the oriented dipoles promoted by the SAM deposition strategy, such push-pull chromophores have been shown to form highly polarizable insulating films in the literature [2]. We have adapted and developed the original Ge deoxidation/grafting technique in hydro-alcoholic solution [4] and shown that, compared to the usual deoxidizing acid treatment, it gives smoother surfaces and well-organized SAMs, which is proven by ellipsometry, wettability, and scanning probe microscopy analyses. The grafting of alkylthiols and fluoro-alkylthiols on Ge has been performed directly in a single step, whereas for the push-pull chromophores designed with a carboxylic anchoring group, we have achieved a two-step grafting with amide bonding on pre-assembled amine-terminated sticking layers. Among the latter, we have demonstrated aminothiophenol SAMs exhibit a better arrangement than cysteamine, with a smooth monolayer film suitable for grafting ordered push-pull SAMs on top. UV-Visible absorption spectroscopy of push-pull chromophores in solution was used to determine the concentration limit to avoid aggregation. X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIR) analyses demonstrate the oxide removal from the Ge surface after the SAM formation. Statistical electrical analyses revealed that with such push-pull SAMs, we have been able to decrease the current by a factor of 105 compared to Ge, and 104 compared to dodecane SAMs of similar thickness. Results have been analyzed by transition voltage spectroscopy [5], and successfully correlated with spectroscopic analyses of molecular levels, using inverse photoemission spectroscopy and XPS valence band determination for probing the unoccupied and occupied molecular orbitals respectively, as well as with DFT calculations, thus allowing to identify the highest occupied molecular orbital as the level involved in the electronic transport through the push-pull SAM. Dipole formation has also been evidenced in the SAM. References:[1]. A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press (Ed.), Boston (1991)[2]. A. Facchetti et al., Adv. Mater. 17 (2005) 1705; Y.G. Ha et al., Chem. Mater. 21 (2009) 1173[3]. V. Malytskyi et al., Tetrahedron 73 (2017) 5738 [4]. J.N. Hohman et al., Chem. Sci. 2 (2011) 1334[5]. X. Lefevre et al., J. Phys. Chem. C 119 (2015) 570