Characterization of afterpulse like noise in silicon single photon avalanche diodes

During the last decades, electronics plays a important role in medicine, more specifically in the bioimages field. Diverse techniques and physical principles that are used in this field of studies and extend from the use of ionizing radiation until pressure waves; all of these methods give invaluable information of the metabolism and the anatomy of the patient. One of these techniques is being used in the field of Near-Infrared Spectroscopy (NIRS) where recent studies demonstrate the possibility to give a new optic to photon migration by measuring the reflectance of a biological tissue with null interfiber distance between the light source and the detector. In fact, this particular research shows how by reducing the distance between laser and detector there is an enhancement in the quality of the obtained bioimages due to the fact that its effect reduces the sensitivity profile at all the instants of time, which translates into a better localization of the scattered photons, spatial resolution (specially at shallowness tissues) and contrast of the resulting image. Nevertheless, a price must be paid in order to get the advantages mentioned before. First of all, the measurement of weak optical signals that have led to the continuous development of single-photon detectors with diverse structures and fabrication processes (Silicon, InGaAs/InP). These kinds of detectors are able to acquire fast waveforms by means of a Time-Correlated Single-Photon Counting (TCSPC) technique, while slow waveforms just by counting the number of the impinging photons on the detector (photon counting technique). Another consequence of the reduce distance between the light source and the detector is defined by the so called “early photons”, which are the ones reflected in the superficial tissues of the matter under examination. These photons can eventually saturate the detector when increasing the optical power injected; this represents a limitation in terms of detection of “late photons”, which carry information about deep tissues of the matter under examination. In order to reduce the contribution of early photons some electronic systems are been developed like Ultra-Fast Gated SPAD which allows to reject the photons by enabling the detector with high precision and with ultra-fast rising and falling times (in the range of hundreds of picoseconds). The necessity of separate in time slots the scattered photons, allows increasing the optical power injected in the tissue. In this way obtaining information about the deeper layers of the object. Notwithstanding, a strange effect had been discovered which represents the main limitation on the dynamic range of the system when using Ultra-Fast Gated Single-Photon Avalanche Diodes (SPADs) technique; this phenomenon has been called “Memory Effect” due to direct relation between its amplitude and the impinging optical power on the detector during the OFF state. On Chapter 1 named “Single-Photon Avalanche Diode (SPAD)” are presented diverse fabrication and structure devices like Silicon, standard CMOS and InGaAs/InP SPADs. A brief presentation of the main parameters of these kinds of devices, the quenching circuits for biasing the circuit under breakdown voltage after a detected photon and the Time-Correlated Single Photon Counting technique for waveform reconstruction are introduced. On Chapter 2 named “Silicon SPADs in Gated-Mode Operation” is described the Near-Infrared Spectroscopy technique for brain imaging and some waveform reconstruction methods like Continuous wave, time-resolved and frequency-domain techniques. Moreover, the Fast-Gated SPAD technique for rejecting photons with very precise and fast rising and falling times of hundreds of picoseconds is presented. On Chapter 3 named “Characterization of Memory Effect in custom technology and thin junction Silicon SPADs” are shown the experimental results for thin Silicon SPADs and its dependence on temperature, wavelength and excess bias with a brief introduction about some possible origins of the internal noise of the devices and the nature of the known noise sources in the different kind of SPADs like diffusion of photogenerated carriers and deep-level trapping.