arXiv subjects

Thomas Mathis

Publications and source records attributed to Thomas Mathis.

3 recordsLinked to original sources

arXiv2016

Unusual anisotropic response of the charge carrier mobility to uniaxial mechanical strain in Rubrene crystals

Charge transport in Rubrene single crystals under uniaxial mechanical strain is systematically investigated in the crystal's two in-plane transport directions both under tensile and compressive strain applied parallel or perpendicular to the current direction. The density of trap states remains unchanged. The field-effect mobility as a benchmark figures for intermolecular transport is found to increase with compressive strain and vice versa with a magnitude of -1.5 cm2/Vs per percent of strain independently of tranport direction. A very remarkable result is the mobility change when the crystal is strained perpendicular to the transport direction. While this enhancement could be quantitatively explained from an improved wave-function overlap, mobility in the perpendicular direction improves even more, contrary to simple geometric considerations based od later expansion and usual Poisson ratios. This result emphasises the central role of the stress induced variations of the dynamics wave function overlap in organic molecular crystals.

cond-mat.mtrl-sci↗

arXiv2010

The trap DOS in small molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

We show that it is possible to reach one of the ultimate goals of organic electronics: producing organic field-effect transistors with trap densities as low as in the bulk of single crystals. We studied the spectral density of localized states in the band gap (trap DOS) of small molecule organic semiconductors as derived from electrical characteristics of organic field-effect transistors or from space-charge-limited-current measurements. This was done by comparing data from a large number of samples including thin-film transistors (TFT's), single crystal field-effect transistors (SC-FET's) and bulk samples. The compilation of all data strongly suggests that structural defects associated with grain boundaries are the main cause of "fast" hole traps in TFT's made with vacuum-evaporated pentacene. For high-performance transistors made with small molecule semiconductors such as rubrene it is essential to reduce the dipolar disorder caused by water adsorbed on the gate dielectric surface. In samples with very low trap densities, we sometimes observe a steep increase of the trap DOS very close (<0.15 eV) to the mobility edge with a characteristic slope of 10-20 meV. It is discussed to what degree band broadening due to the thermal fluctuation of the intermolecular transfer integral is reflected in this steep increase of the trap DOS. Moreover, we show that the trap DOS in TFT's with small molecule semiconductors is very similar to the trap DOS in hydrogenated amorphous silicon even though polycrystalline films of small molecules with van der Waals-type interaction on the one hand are compared with covalently bound amorphous silicon on the other hand.

cond-mat.mtrl-sci↗

arXiv2007

Organic small molecule field-effect transistors with Cytop(TM) gate dielectric: eliminating gate bias stress effects

We report on organic field-effect transistors with unprecedented resistance against gate bias stress. The single crystal and thin-film transistors employ the organic gate dielectric Cytop(TM). This fluoropolymer is highly water repellent and shows a remarkable electrical breakdown strength. The single crystal transistors are consistently of very high electrical quality: near zero onset, very steep subthreshold swing (average: 1.3 nF V/(dec cm2)) and negligible current hysteresis. Furthermore, extended gate bias stress only leads to marginal changes in the transfer characteristics. It appears that there is no conceptual limitation for the stability of organic semiconductors in contrast to hydrogenated amorphous silicon.

cond-mat.mtrl-sci↗