PhD Alumni

Caironi Mario

Present position: Research Associate in Physics, Cavendish Laboratory, Cambridge University, UK,

Thesis title:  Organic Photodetectors and Conductance Switching Memories
Advisor:  Marco Sampietro
Research area:  Microelectronics and Emerging Technologies
Thesis abstract:  
The emerging field of organic electronics have raised considerable interest in the last decades, thanks to some peculiar characteristics of organic semicon-ductors, such as ease of processability, mechanical exibility, chemical tailoring of physical and electronic properties, and electroluminescence in the visible range. A research work on electronic devices based on this class of active materials is the subject of this thesis, which is concerned with the realization, the characterization and the development of organic conductance switching memories and photodetectors for visible and near infrared radiation.
Organic devices for data storage applications are under investigation with the aim to address some scalability shortcomings of nonvolatile memories based on complementary metal-oxide-semiconductor silicon technology. In this work conductance switching memory cells based on hindered phenol sub-stituted bithiophenes, showing long time retention, very high endurance to electrical stress and operation in ambient air condition are presented. The basic switching mechanism was widely investigated by comparing cells with aluminum and mercury top contacts, by means of transition time and capacitive measurements, FT-IR spectroscopy, cyclic voltammetry of the active materials and theoretical calculations. This allowed to exclude some material independent phenomena related to the particular adopted architecture as the origin of the observed conductance switching, and to demonstrate an active role of the organic layer in determining the memory behaviour of the device.
An electro-oxidative process, triggered by holes injection and leading to a modification of the injection barriers, is consistently proposed.
The field of organic photodetectors has been investigated to a lesser extent with respect to other applications where the optoelectronic properties of organic molecules are exploited. However it deserves in principle the same attention thanks to the ease of deposition of the active materials that offers a potentially low-cost solution-process technology, opening new optoelectronic applications fields and the possible integration in back-end-of-line processes of consolidated technologies. Concerning this field, photodetectors both for visible and near infrared radiation are reported. The adoption of a simple and versatile planar architecture fostered some interesting basic studies on
the influence of the charge transport on photodetectors performances: the importance of a balanced holes and electrons transport on the external quantum efficiency, and of the role of the slower carrier and of traps in determining the active area and the frequency response of the device were demonstrated.
Furthermore, the adoption of highly chemical and photochemical stable coordination complexes showing strong absorption in the near infrared was demonstrated to successfully address the issue of stability for low energy gap organic materials. By means of these complexes, detectors spectrally matched to all the three optical fiber windows, able to operate at repetition rates up to hundreds of kbit/s, were fabricated, thus demonstrating the feasibility of organic based detectors for signal applications in the near infrared range of the spectrum.