Growth and Patterning of Organic Single-Crystal by Controlled Microfabrication Processes
authors Domingos, Ismael D.; Oliveira, Ana P. P.; Serra, João M. P. S.; Sequeira, Sara I. H.; Maçôas, Ermelinda M. S.; Cardoso, Susana; Alves, Helena C. R. J. D. e Leitão, Diana C. P.
nationality International
abstract Organic semiconductors have been used in optoelectronic and photonic applications leading to successful developments such as light emitting diodes (OLEDs), photovoltaic cells or photosensors. These can be produced in their amorphous state with the objective of large area and printing production, but also as single-crystals with long range structural order, which makes possible to achieve superior device efficiency due to much higher charge carrier mobility and current density. In field-effect-transistors (FETs), these structures exhibit the highest mobility of organic semiconductors materials [1]. In phototransistors and photosensors they present high mobility, low-contact resistance, high photoresponse in the visible range, with low-operating voltage, revealing excellent optical sensing capabilities [2,3]. Such properties, makes organic single-crystals very attractive for optoelectronic applications, including lasers or sensor arrays [4,5]. Even though these materials present very promising performances comparable with state-of-the-art organic semiconductors, the fabrication process is difficult, non-scalable and rely in manual skills. Device fabrication by handpicking, layering and contact deposition leads to shape, size and quality control problems and therefore a problem in reproducibility and scalability of the devices. To improve device reliability, industrial potential and expand device applications it is essential to have the ability to tune shape and size down to the microscale. This work presents a microfabrication process used to produce and test devices with organic single-crystals, with rubrene as organic semiconductor. Crystals were grown by physical vapor transport, handpicked and laminated directly onto glass substrates containing several previously patterned gold contacts levelled with Al2O3 and SiO2. A clean room microfabrication process adequate for organic single-crystals was developed and tested on controlled different geometries, rectangular and circular. During the process, the crystals were protected to reduce the damage inflicted by distinct thermal expansion coefficients. Samples were coated with a photoresist layer and the pattern was defined by optical lithography and reactive ion etching (RIE). The etching conditions were optimized and the etch rates of the distinct layers used were calibrated for different reactive agents, CF4, Ar, O2, CHF3 and He. The samples where then passivated and diced, separating the sampled into independent dies containing several devices each. These were attached to chip carriers and tested for photo response taking in account different lengths, shape and diverse contact geometry. The electrical properties and photoresponse of these devices were tested for different wavelengths, associating the device structure with performance. [1] A. S. Molinari, H. Alves, Z. Chen, A. Facchetti, A. Morpurgo, J. Am. Chem. Soc., Vol. 131, (2009), pp 2462 [2] R. M. Pinto, W. Gouveia, A. I. S. Neves and H. Alves, Appl. Phys. Lett., Vol. 107 (2015), pp 223301 [3] H. Alves, R. M. Pinto, and E. M. S. Maçoas, Nature Commun., Vol. 4 (2013), pp 1842 [4] Fang, H. H., et al. (2013) Laser & Photonics Reviews, 7(2), 281-288 [5] Briseno, et al. (2006). Nature, 444(7121), 913-917
year published 2018

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