Modelling Semiconductor Pixel Detectors

Mathieson, Keith (2001) Modelling Semiconductor Pixel Detectors. PhD thesis, University of Glasgow.

Abstract

In this thesis the work will focus on the modelling of highly pixellated solid- state devices. Results are presented on the performance of a pixellated spectroscopic silicon detector - the Dash-E detector. The electronic noise is measured to be 228 eV with the system observed to be close to the Fano limit at room temperature. The characteristic X-rays of Mn-55, Cu, Rb, Mo, Ag and Ba have been used to examine the spectroscopic performance of the detectors. Good linearity over the required energy range 1 keV to 25 keV has been observed. A higher than expected background is evident in all of the spectra taken - approximately a 1:1 correspondence in the peak counts to background counts. The modelled performance of a 3-D GaAs detector is analysed. The effect of the metallic column-like electrode structure on the particle interactions is simulated via the Monte Carlo code MCNP. The effective dead area due to these electrodes is reduced from 8% to 4.5% due to the secondary particle interactions. The modelled structure becomes depleted at 50 V with a slight over-depletion of 75 V necessary to minimise the inter-electrode low field regions. The principle benefit of these depletion voltages is that they remain constant for whatever detection thickness that fabrication allows. The charge transport in the devices are also examined, with the damage effects of dry-etching included - a technique used to form the electrode holes in the GaAs material. Full charge collection can be expected after 200 ps in most cases. The effect of reducing the charge carrier lifetime and examining the charge collection efficiency has been utilised to explore how these detectors would respond in a harsh radiation environment. It is predicted that over critical carrier lifetimes (10 ps to 0.1 ns) an improvement of 40 % over conventional detectors can be expected. This also has positive implications for fabricating detectors, in this geometry, from materials which might otherwise be considered substandard. An analysis of charge transport in CdZnTe pixel detectors has been performed. The analysis starts with simulation studies into the formation of contacts and their influence on the internal electric field of planar detectors. The models include a number of well known defect states and these are balanced to give an agreement with a typical experimental I-V curve. The charge transport study extends to the development of a method for studying the effect of charge sharing in highly pixellated detectors. The case of X-ray, as well as higher energy ?-ray, interactions are considered. The charge lost is studied for these interactions over a range of pixel sizes (1mum to 10 mm). The aforementioned Dash-E detector has been used to obtain experimental data for comparison with the models developed, with close agreement being observed. The combination of MCNP and MEDICI is used to form a complete picture of photon interactions in semiconducting materials and also compares well with experiment. The models predict that the dominant term in the sharing of charge is due to diffusion and that the difference with photon energy is due to the energy given to the photoelectron.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Ken Smith
• Keywords: Condensed matter physics, Nuclear physics and radiation
• Date of Award: 2001
• Depositing User: Enlighten Team
• Unique ID: glathesis:2001-76094
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Dec 2019 09:15
• Last Modified: 19 Dec 2019 09:15
• URI: https://theses.gla.ac.uk/id/eprint/76094