Giampiero de Cesare

The HASPIDE project of INFN exploits hydrogenated amorphous silicon (a-Si:H) as a sensitive material for innovative detectors, mainly focused on dosimetry both for standard and FLASH radiotherapy, but also fo other applications. The detector, deposited on a thin and flexible substrate of polyimide, has been developed with different contacts configurations during past years, giving rise to the necessity to compare them in order to determine which configuration is best, for a specific application. In particular, three types of contacts are under exam in this work: the first one is a classical n-i-p junction, where a thick layer of intrinsic a-Si:H material is sandwiched between n- and p-doped layers of hydrogenated amorphous silicon; the second configuration is a charge-selective contacts device (CSC), where the doped regions are replaced with metal oxides which selectively enhance the mobility of one charge-carrier, blocking the other and the last one is the hybrid type, in which the intrinsic region is coupled with a n-doped layer on the bottom and a hole selective metal oxide on the top. This work aims to give a comparison in the performance of three devices differing only in the contact configuration, in terms of dark current analysis and sensitivity to X-ray beams, so that the characteristics of each one can be analyzed and exploited.

Flexible hydrogenated amorphous silicon (a-Si:H) detectors are attractive candidates for clinical dosimetry, particularly for applications requiring lightweight devices and operation without external bias. In this work, three flexible a-Si:H detector architectures are experimentally compared under clinical photon and electron beams: a conventional n-i-p diode, a charge selective contact (CSC) detector, and a very thin p-i-n diode. All detectors show excellent dose linearity under both irradiation modalities. However, large differences in sensitivity are observed at zero bias, exceeding two orders of magnitude across architectures. The conventional n-i-p device shows a response that is limited by the internal electric field, while CSC and very thin p-i-n detectors achieve sensitivities in the pC/cGy range without applied bias. Electron beams yield higher charge per unit dose than photons, compatible with a larger local energy deposition in the active layer. The results demonstrate that sensitivity in flexible a-Si:H dosimeters is governed by contact design and intrinsic layer thickness rather than by active volume alone. Taken together, the present results suggest that combining charge selective contacts with thinner intrinsic layers may represent a promising strategy to further enhance zero-bias sensitivity. These findings provide clear design guidelines for the development of flexible dosimeters for surface and transmission measurements in advanced radiotherapy.

This paper presents a wearable potentiometric sensor system for sweat chloride measurement, targeting cystic fibrosis screening. The platform integrates Ag/AgCl electrodes, an instrumentation amplifier, and analog signal conditioning to enable low-noise acquisition of electrochemical signals. A low-power microcontroller performs on-device signal preprocessing and implements a TinyML-based regression model for sensor calibration and chloride concentration estimation. Environmental validation is achieved through an integrated humidity sensor to ensure reliable sweat detection. Experimental results demonstrate accurate chloride quantification within the clinically relevant diagnostic range, while maintaining low power consumption and autonomous operation, making the proposed system suitable for point-of-care and wearable sensing applications. This study validates the feasibility of deploying "smart," self-contained sensors that can autonomously map raw potentiometric transients to clinical values.

The INFN HASPIDE project investigates hydrogenated amorphous silicon (a-Si:H) as an innovative detector technology for dosimetry in FLASH radiotherapy. Conventional dosimeters, such as ionization chambers, show non-linear behavior when exposed to ultra-high dose-per-pulse beams used in FLASH radiotherapy, where several Grays are delivered in microsecond pulses. Thanks to its radiation hardness, mechanical flexibility and compatibility with large-area deposition, a-Si:H is a promising material for in-vivo and surface dosimetry. This work presents the response of a thin a-Si:H detector exposed to a high-dose-per-pulse 10 MeV electron beam, together with an analytical model used to describe the single-pulse dynamics and extract quantitative parameters. Measurements were performed at the University of Torino using a modified clinical LINAC operated in Ultra High Dose Rate regime, where the single-pulses satisfy the FLASH criteria. The a-Si:H detector, read out with a TetrAMM picoammeter (100 kHz sampling frequency), resolves individual radiation pulses with high repeatability. Both pulse charge and peak current scale linearly with the applied electric field up to 6 V (2.4 V/μm), beyond which the readout electronics saturates. The analytical model is validated through direct comparison with raw data, showing excellent agreement in extracted pulse parameters.

Hydrogenated amorphous silicon (a-Si:H) is a mature thin-film technology for large-area devices and thin-film sensors, and its low-temperature growth via Plasma-Enhanced Chemical Vapor Deposition (PECVD) makes it particularly suitable for biomedical flexible and wearable platforms. However, the reliable integration of a-Si:H sensors on polymer substrates requires a quantitative assessment of their electrical stability under mechanical stress, since bending-induced variations may affect sensor accuracy. In this work, we provide a quantitative, direction-dependent evaluation of the static-bending robustness of both single-doped a-Si:H layers and complete p-i-n junction stacks on polyimide (Kapton^®), thereby linking material-level strain sensitivity to device-level functionality. First, n- and p-doped a-Si:H layers were deposited on 50 µm thick Kapton^® and then structured as two-terminal thin-film resistors to enable resistivity extraction under bending conditions. Electrical measurements were performed on multiple samples, with the current path oriented either parallel (longitudinal) or perpendicular (transverse) to the bending axis, and resistance profiles were determined as a function of bending radius. While n-type layers exhibited limited and mostly gradual variations, p-type layers showed a stronger sensitivity to mechanical stress, with a critical-radius behavior under transverse bending and a more progressive evolution in the longitudinal one. This directional response identifies a practical bending condition under which doped layers, particularly p-type films, are more susceptible to strain-induced degradation. Subsequently, a linear array of a-Si:H p-i-n sensors was fabricated on Kapton^® substrates with two different thicknesses (25 and 50 µm thick) and characterized under identical bending conditions. Despite the increased strain sensitivity observed in the single-layers, the p-i-n diodes preserved their rectifying behavior down to the smallest radius tested. Indeed, across the investigated radii, the reverse current at −0.5 V remained consistent, confirming stable junction operation under bending. Only minor differences, related to substrate thickness, were observed in the reverse current and in the high-injection regime. Overall, these results demonstrate the mechanical robustness of stacked a-Si:H junctions on polyimide and support their use as sensors for wearable biosensing architectures. By establishing a quantitative, orientation-aware stability benchmark under static bending, this study supports the design of reliable a-Si:H flexible sensor platforms for curved and wearable surfaces.

This work presents an advanced electro-physical model for hydrogenated amorphous silicon (a-Si:H) Junction Field Effect Transistors (JFETs) to enable the design of devices with energy-efficient analog interface building blocks for Lab-on-Chip (LoC) systems. The presence of this device can support monolithic integration with thin-film sensors and circuit-level design through a validated compact formulation. The model accurately describes the behavior of a-Si:H JFETs addressing key physical phenomena, such as the channel thickness dependence on the gate-source voltage when the channel approaches full depletion. A comprehensive framework was developed, integrating experimental data and mathematical refinements to ensure robust predictions of JFET performance across operating regimes, including the transition toward full depletion and the associated current-limiting behavior. The model was validated through a broad set of fabricated devices, demonstrating excellent agreement with experimental data in both the linear and saturation regions. Specifically, the validation was carried out at 25 °C on 15 fabricated JFET configurations (12 nominally identical devices per configuration), using the mean characteristics of 9 devices with standard-deviation error bars. In the investigated bias range, the devices operate in a sub-µA regime (up to several hundred nA), which naturally supports µW-level dissipation for low-power interfaces. This work provides a compact, experimentally validated modeling basis for the design and optimization of a-Si:H JFET-based LoC front-end/readout circuits within technology-constrained and energy-efficient operating conditions.

Radiation damage tests in hydrogenated amorphous silicon (a-Si:H) flexible flux and dose-measuring devices have been performed with a 3 MeV proton beam, to evaluate combined displacement and total ionizing dose damage. The tested devices had two different configurations and thicknesses. The first device was a 2 μm thick n-i-p diode having a 5 mm x 5 mm area. The second device was a 5 μm thick charge selective contact detector having the same area. Both the devices were deposited on a flexible polyimide substrate and were irradiated up to the fluence of 10¹⁶ n_eq/cm². The response to different proton fluxes has been measured before irradiation and after irradiation at 10¹⁶ n_eq/cm² for charge-selective contacts and n-i-p devices. The effect of annealing for partial performance recovery at 100°C for 12 hours was also studied and a final characterization on annealed devices was performed. This test is the first combined displacement and total ionizing dose test on flexible a-Si:H devices.

Hydrogenated amorphous silicon (a-Si:H) devices on flexible substrates are currently being studied for application in dosimetry and beam flux measurements. The necessity of in vivo dosimetry requires thin devices with maximal transparency and flexibility. For this reason, a thin (7 to 2.49 × 10¹⁰ p/(cm² s).