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Growth and Patterning of Organic Single-Crystal for Whispering-Gallery-Mode devices
Keywords: Organic semiconductors, Reactive ion etching, Whispering gallery modes
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 are usually explored in two trends. The first is based in amorphous thin-films with the objective of large area and printing production. In a second approach, these materials can be obtain 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]. Parallel to materials development there is progress towards the miniaturization of devices, with controlled optical path and the ability to tune shape and size down to the microscale. One way to control the optical path length is by coupling light into a guided mode. One particular interesting approach is the whispering-gallery-modes (WGM), where the light is forced to couple within closed circular structures by total internal reflexion in order to increase the optical path length and improve light absorption [6]. These have demonstrated the capacity to store and manipulate light in small volumes with unique sensitivity. By controlling the material and geometry dispersion of the device, these structures can promote applications such as multicolour lasers, broadband memories, and multiwavelength optical networks. It has also been suggested that such devices, offering tens or even hundreds of well-defined narrowband optical carriers in massively parallel structures, could revolutionize high-speed optical communications. For such achievements, suitable materials and innovation in fabrication and sensing are essential in a mutually reinforcing cycle. This work presents a fabrication process used to produce and test WGM 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 different geometries, rectangular and circular, in order to analyse the effect of a circular structure on the photoresponse of rubrene single- crystals. During the process, the crystals were protected by PVA and PMMA, in a formulation optimized to reduce the damage inflicted to the crystals due to 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. Different rates and selectivity of the organic materials were observed. The samples where then passivated with an Al2O3 layer 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. Photoresponse in different wavelengths was characterized, was well associated with the device structure and performance.
Ismael Domingos,
Universidade de Aveiro
Aveiro, Aveiro
Portugal


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