Project development
The OPTICS project foresees that the device and the final prototype will be designed and fabricated entirely in the team’s laboratories. To this aim, the project team will work closely together, even if, of course, each unit is more specialized in one of the aspects necessary for the development of the prototype.
In the first phase the study and optimization of different types of OPTs, in order to select the most suitable device for the purpose. The study of the device will deal with the analysis of thin film growths, of the interfaces between semiconductor and dielectric, of their electrical and optical response. The final objective of the second phase is the realization of the prototype and its test.
In this phase, it is planned to design, build and optimize the transmitter and receiver control electronics and to carry out a full characterization of the prototype.
Metodology
- Organic Photo-Transistors
The research team proposes highly innovative developing new devices in broadband visible light communication [12]. The innovative nature of the project goes far beyond the mere development of devices for VLC, proposing optical detectors based on organic semiconductors, on flexible substrates, thus impacting also on wearable electronics. Different organic materials are available, sensitive to different wavelengths. For example, in the case of dinaphtho [2,3-b: 2 ‘, 3’-f] thieno [3,2-b] thiophene (DNTT), blue-sensitive phototransistors are obtained, while in the case of pentacene (C22H14 ), red sensitive devices are realized.
Among the different device structures possible for OPTs, Bottom Gate/Bottom Contact (BGBC) and Bottom Gate/Top Contact (BGTC) structures are considered. Both the selected OPT structures, developed following multilayer technology, shown in Fig. 2, allow the organic semiconductor scintillator to be in the upper part of the device, useful for receiving the light. However, in the case of BGBC, 100% of the material is exposed to light, improving, in principle, the response of the device, while for BGTC structure, the sensitive surface is only 33% of the semiconductor, even if the coupling of the semiconductor to the dielectric is maximized, leading to a better response to the field effect. So, both proposed structures will be explored to fine-tune the electrical properties of the two configurations and, above all, the optical device characteristics.
Single layer OPTs are developed by using high mobility organic semiconductor, like DNTT, that allows good response for λ 450 nm (blue color). The absorbing layer is selected between several organic semiconductors optimizing the transmission of visible light. Furthermore, for improving the OPT characteristics, reducing the response time, the fabrication of heterojunction [15] between absorbing and active layer will be investigated.
Both polymeric, as PMMA and Cytop [16], and inorganic, as AlOx [17] gate dielectrics are evaluated for the OPTs. The optimization of the gate dielectric aims at improving the responsivity of the OPT. A first way is to use materials with high dielectric constant and/or by reducing the film thickness, as in the case of Al2O3 thin films. The second one is related to the organic semiconductor quality deposited on top of the gate dielectric in both BGBC and BGTC configurations. Indeed, the final morphology of the semiconductor film is strongly influenced by the characteristics of the dielectric surface. Since this is a key point to obtain high field effect mobility, the film structure and morphology is investigated by X-ray based techniques and Atomic Force Microscope (AFM). The results provide an important feedback to fine selection of materials as well as growth and process parameters. Finally, electrical stability of the OPT is influenced by the gate dielectric, since charge trapping at the interface causes hysteresis and variation of the electrical characteristics under gate bias, that must be minimized to improve the OPT response. In addition to the sensitivity to blue light, DNTT, not particularly suffering the presence of humidity and oxygen, is the main candidate for the development of organic devices for VLC. Nonetheless, it is considered appropriate to encapsulate the device, preserving it from the environment. It can be encapsulated by using a highly resistant and transparent material, such as Cytop deposited by plasma or spin-coating and whose geometry can be defined by standard photolithographic processes and reactive ion etching. The absorption spectrum of Cytop shows a high degree of transparency to visible light. Moreover, Cytop has very good properties as barrier layer [18-19], making this material the ideal candidate for the protection of the OPT to use in VLC.
- Electrical and optical characterization
Electrical and optical measurements are at the basis of the development and optimization of OPTs. Standard electrical characterization is performed on test facilities including OPT with different layouts. It includes three-terminal current-voltage characteristics, transfer and output curves at different temperatures, and low-frequency impedance spectroscopy measurements, for the evaluation of the dielectric parameters like dielectric constant and breakdown voltage. In addition, the contact resistance analysis will be performed on several OPTs through the Transmission Line Method. The OPT static photo-response will be investigated in the range from 350 to 1100 nm with calibrated light sources. Dynamic photo-response will be investigated with LED pulser developed ad-hoc to match the OPT layout, designed to generate light pulses with programmable irradiance (from 5nW/cm^2 to 200 uW/cm^2) and pulse width (from 200ns upward), at specific wavelength (namely 460nm, 520nm, 580nm and 620nm). The analysis of the current variation of the device under illumination and during the recovery to dark condition, gives information about the charge trapping and de-trapping dynamics, which mainly determine the response time of the OPTs. These measurements are correlated with measurements of electrical noise, as is described below. The electrical characterization includes also the evaluation of the electrical stability of the device, important for obtaining a reliable optical response of the OPTs, which is evaluated through bias stress measurements, environmental stability, aging, and bending experiments.
Electrical noise is at the basis of the conduction and photoconduction mechanisms, as well as the measure of main optical parameters, like the specific detectivity D*, that quantifies the capability to detect light of the OPTs in a practical environment. D* (Eq. 7) is related to the Noise Equivalent Power (NEP), defined as the impinging signal power which results in a unity signal to noise ratio in the measurement bandwidth. Here f is the frequency and SI is the current noise spectral density. However, NEP is seldom measured because of the specific equipment needed to probe SI is dominated by low-frequency 1/f noise. Moreover, noise measurements allow to correlate the performance of OPTs with the origin of the noise, typically produced by the presence of hole/electron traps and defects inside both the dielectric and organic layers, an essential aspect for the optimization of a device which must respond to a light signal by increasing its transport current. The noise comes from the receiving device itself and from the preamplifier stage used to locally increase the signal-to-noise ratio. In dark conditions, there are fluctuations generated inside the device, which are the result of the charge transport and the interactions of the mobile charges with atoms and defects of the materials of which the device is composed. The power associated to these background fluctuations can completely mask the impinging signal power. To this regard, a correct characterization of the NEP is fundamental to understand the limit of sensitivity of any photo-receiver. Noise measurements at very high levels of sensitivity are very difficult. Indeed, beside the required shielding from external electromagnetic, mechanical, and temperature fluctuations, the measuring system needs to be properly optimized, in term of
sensitivity, bandwidth, gain, stability and bias. A major problem regards the stability of the measurement system in case of large devices under test like organic device. The large device capacitance can result in system instability if the bandwidth is not properly reduced. However, this can be in contrast with the measurement of the device noise in a required larger bandwidth. Differently from the charge conduction in inorganic crystalline semiconductors, specific noise models for organic materials are not present, so that they are borrowed from the former. In this context, higher levels of effective trap density and Coloumb scattering parameter have been found in devices based on organic materials [21-22], possibly indicating the higher state of disorder in such systems. 1/f and low frequency noise measurements, with such standard models, will be used to investigate the quality of the materials and interfaces. The relevance of such characterization resides in the fact that defects can increase recombination reducing the conversion efficiency and also mobility and speed.