K. Kanxheri

Radiation hardness of three-dimensional polycrystalline diamond detectors

The three-dimensional concept in particle detection is based on the fabrication of columnar electrodes perpendicular to the surface of a solid state radiation sensor. It permits to improve the radiation resistance characteristics of a material by lowering the necessary bias voltage and shortening the charge carrier path inside the material. If applied to a long-recognized exceptionally radiation-hard material like diamond, this concept promises to pave the way to the realization of detectors of unprecedented performances. We fabricated conventional and three-dimensional polycrystalline diamond detectors, and tested them before and after neutron damage up to 1.2 ×1016 cm−2, 1 MeV-equivalent neutron fluence. We found that the signal collected by the three-dimensional detectors is up to three times higher than that of the conventional planar ones, at the highest neutron damage ever experimented.

Numerical simulation of ISFET structures for biosensing devices with TCAD tools

BackgroundIon Sensitive Field Effect Transistors (ISFETs) are one of the primitive structures for the fabrication of biosensors (BioFETs). Aiming at the optimization of the design and fabrication processes of BioFETs, the correlation between technological parameters and device electrical response can be obtained by means of an electrical device-level simulation. In this work we present a numerical simulation approach to the study of ISFET structures for bio-sensing devices (BioFET) using Synopsys Sentaurus Technology Computer-Aided Design (TCAD) tools.MethodsThe properties of a custom-defined material were modified in order to reproduce the electrolyte behavior. In particular, the parameters of an intrinsic semiconductor material have been set in order to reproduce an electrolyte solution.By replacing the electrolyte solution with an intrinsic semiconductor, the electrostatic solution of the electrolyte region can therefore be calculated by solving the semiconductor equation within this region.ResultsThe electrostatic behaviour (transfer characteristics) of a general BioFET structure has been simulated when the captured target number increases from 1 to 10. The ID current as a function of the VDS voltage for different positions of a single charged block and for different values of the reference electrode have been calculated.The electrical potential distribution along the electrolyte-insulator-semiconductor structure has been evaluated for different molar concentrations of the electrolyte solution.ConclusionsWe presented a numerical simulation approach to the study of Ion-Sensitive Field Effect Transistor (ISFET) structures for biosensing devices (BioFETs) using the Synopsys Sentaurus Technology Computer-Aided Design (TCAD) tools.A powerful framework for the design and optimization of biosensor has been devised, thus helping in reducing technology development time and cost. The main finding of the analysis of a general reference BioFET shows that there is no linear relationship between the number of charges and the current modulation. Actually, there is a strong position dependent effect: targets localized near the source region are most effective with respect to targets localized near the drain region. In general, even randomly distributed targets are more efficient with respect to locally grouped targets on the current modulation. Moreover, for the device at hand, a small positive biasing of the electrolyte solution, providing that the transistor goes on, will result in a greater enhancement of the current levels, still retaining a good sensitivity but greatly simplifying the operations of a real device.

Simulation and test of Silicon-on-Diamond sensors for particle detection

A laser bonding technique has been developed recently to create an innovative material based on a silicon/diamond interface. In this work, we propose the development and the application of a numerical model for TCAD simulations of poly-crystalline diamond conceived for Silicon-on-Diamond (SoD) sensors to be used, e.g., as particle detectors in High Energy Physics (HEP) experiments. The model is based on the introduction of an articulated, yet physically based, picture of deep-level defects acting as a recombination centers and/or trap states. The modelling scheme has been validated by comparing the simulation findings with experimental measurements carried out on real devices featuring a thinned CMOS Active Pixel Sensor chip bonded to a poly-crystalline diamond substrate. Eventually, this technique could foster the exploration of innovative semiconductor devices conjugating the properties of diamond substrates and the capabilities of CMOS electronics.

Evaluation of a 3D diamond detector for medical radiation dosimetry

Synthetic diamond has several properties that are particularly suited to applications in medical radiation dosimetry. It is tissue equivalent, not toxic and shows a high resistance to radiation damage, low leakage current and stability of response. It is an electrical insulator, robust and realizable in small size; due to these features there are several examples of diamond devices, mainly planar single-crystalline chemical vapor depositation (sCVD) diamond, used for relative dose measurement in photon beams. Thanks to a new emerging technology, diamond devices with 3-dimensional structures are produced by using laser pulses to create graphitic paths in the diamond bulk. The necessary bias voltage to operate such detector decreases considerably while the signal response and radiation resistance increase. In order to evaluate the suitability of this new technology for measuring the dose delivered by radiotherapy beams in oncology a 3D polycrystalline (pCVD) diamond detector designed for single charged particle detection has been tested and the photon beam profile has been studied. The good linearity and high sensitivity to the dose observed in the 3D diamond, opens the way to the possibility of realizing a finely segmented device with the potential for dose distribution measurement in a single exposure for small field dosimetry that nowadays is still extremely challenging.

Polycrystalline CVD diamond device level modeling for particle detection applications

Diamond is a promising material whose excellent physical properties foster its use for radiation detection applications, in particular in those hostile operating environments where the silicon-based detectors behavior is limited due to the high radiation fluence. Within this framework, the application of Technology Computer Aided Design (TCAD) simulation tools is highly envisaged for the study, the optimization and the predictive analysis of sensing devices. Since the novelty of using diamond in electronics, this material is not included in the library of commercial, state-of-the-art TCAD software tools. In this work, we propose the development, the application and the validation of numerical models to simulate the electrical behavior of polycrystalline (pc)CVD diamond conceived for diamond sensors for particle detection. The model focuses on the characterization of a physically-based pcCVD diamond bandgap taking into account deep-level defects acting as recombination centers and/or trap states. While a definite picture of the polycrystalline diamond band-gap is still debated, the effect of the main parameters (e.g. trap densities, capture cross-sections, etc.) can be deeply investigated thanks to the simulated approach. The charge collection efficiency due to β -particle irradiation of diamond materials provided by different vendors and with different electrode configurations has been selected as figure of merit for the model validation. The good agreement between measurements and simulation findings, keeping the traps density as the only one fitting parameter, assesses the suitability of the TCAD modeling approach as a predictive tool for the design and the optimization of diamond-based radiation detectors.