Transport modeling and measurements in solid state photo-detectors

The research is part of a project to study the 𝐼𝐼−𝑉𝐼 semi-insulating materials and especially the binary compound CdTe and the ternary compound Cd1-xZnxTe(CZT). The great interest about these materials lies in their ability to solve high-energy photons from the spectroscopic point of view. With a very remarkable stopping power, due to the high atomic number of its components and a high signal to noise ratio, due to a resistivity of the order of 1011 Ω⋅ cm, the CZT ranks as one of the best radiation detectors at room temperature. Furthermore, the charge collection efficiency is closely related to the product between mobility μ and the lifetime τ of the carriers (electrons and holes) and it depends strongly on the spatial profile of the electric field. The non-uniformity of the electric field within the CZT is confirmed both by numerical simulations and by experimental evidence (Pockels effect). The spatial profile of the electric field, that governs the transport of the charge carriers, should be investigated. The technique used to obtain information about the transport and the electric field inside the material is a technique TOF (time of flight) called Laser Excited-Transient Current Technique: (LE-TCT). The transient measurements performed on full-area detectors have involved the study and the comparison of different samples. The analysis of current transients was made using a new model (2τ model) that allows to obtain both the transport parameters and the electric field profile within the material. Recently, complex geometries such as pixelated detectors and strip detectors have been developed with the intention to couple a good spectroscopic performance to a good spatial resolution. In this way, a single device can identify the type of X and γ rays source, either to its location. A complex geometry, however, entails complications from the point of view of the calculation of the current signal. A new model (1τ model), developed and proposed in this thesis, allows both to decouple the contribution of the weighting (geometrical field) from that due to the electric field (physical field), and to obtain the mobility and lifetime of the carriers. Finally, a new diffusion model suitable for the study of the thermal spread of the carriers, a phenomenon assumed to be negligible in the previous models, is presented. The model allows to obtain the diffusion coefficient, directly proportional to the mobility of carriers which then can be calculated in two independent ways starting from the current transients.