Nano-materials and nano-technologies for novel photon detection systems

The transition from macro to micro-sensor systems in the last decades has been made possible by the use of silicon and by development of micro-technology. Semiconductors opened the door to the realization of sensors based on Very Large Scale Integration Systems in which the external information is transduced into an electrical signal managed directly inside the micro-devices by the use of microelectronics. Up to now, the most powerful integrated systems may be considered the Medipix chip for medical imaging and avalanche photodiodes operated in Geiger mode (SiPMs) for photon radiation detection. A similar transition from micro to nano is now knocking at the door. New nanotechnologies allow the realization of finely pixelled surfaces and manipulation of nano- structured materials at a few nanometer scales. Sensors can be created by flipping from a top-down process to a bottom-up approach. To do this, one must develop new materials having similar electrical properties of silicon but chemical properties appropriates for a bottom-up building process. Among the wide variety of materials which can be managed with nano-technological processes, Carbon Nanotubes (CNT) dominate for their unique mechanical and electrical characteristics. They can be grown chemically in a very easy and cheap way, assembled in the desired geometry by means of nano-lithography and directly connected to readout electronics devices. In addition, they can be coupled to silicon substrates to obtain mixed micro-nano structures with intermediate electronic properties. First radiation detectors prototypes have been realized using CNTs grown through a CVD (Chemical Vapour Deposition) process. These devices, sensitive to the radiation in the range from 335 to 850 nm, exibit a relevant increase in the photocurrent toward UV wavelengths both with continuous light and with pulsed radiation. The surface pixellization can be easily achieved using nano-lithography techniques at a very low cost. This approach looks very promising also for the realization of nano-opto-electronic devices directly coupled with readout electronics as in the Medipix chip. In this talk, we propose a review of results obtained by the authors with detector prototypes made of multiwalled carbon nanotubes grown on sapphire and silicon substrates. Electrical and optical properties of detectors have been intensively studied as well as the coupling between nanotubes and silicon substrates. The strong matching between silicon and CNTs found suggests the possibility to realize a finely pixelled large area detector, UV sensitive, with submicron position accuracy.