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Compound III-V semiconductor avalanche photodiodes for X-ray spectroscopy

Gomes, Rajiv (2012) Compound III-V semiconductor avalanche photodiodes for X-ray spectroscopy. PhD thesis, University of Sheffield.

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A theoretical investigation into the avalanche statistics limited energy resolution of avalanche photodiodes (APDs) was carried out using a random path length (RPL) model. The probability density function (PDF) of the avalanche gain which directly affects the energy resolution was computed taking into account the effects of the incident photon energy, the pair creation energy of the interacting material, the electron and hole ionisation coefficients and the mean avalanche gain. The results show that the use of conventional excess noise factors obtained from light measurements to calculate the spread in energy resolution due to avalanche statistics is incorrect as the gain PDF generated by X-ray absorption is considerably different from that obtained from light detection. The effect of dead space on the PDF was also investigated. Significant dead space leads to a more deterministic multiplication process leading to a narrow gain PDF and hence an improvement in the spectral resolution. Simulations were performed to study the experimental pulse height spectra obtained from a GaAs/Al0.8Ga0.2As separate absorption and multiplication (SAM) APD. The simulated spectra are found to be in good agreement with experimental results when the noise from APD and read out electronics along with the statistics of photon energy loss were accounted for. The spectral performance of the SAM APD at room temperature is also presented. In addition to the SAM APD, narrow bandgap InAs has also been investigated. InAs has the potential to achieve spectral resolution beyond that of elemental Si and Ge, closer to that obtained by superconductors. Its larger atomic number and crystal density also ensure that its linear stopping power is higher than elemental semiconductors. In addition, InAs APDs exhibit the highly desirable single carrier ionisation characteristic that leads to low avalanche excess noise. Electrical and X-ray characterisation of InAs n+ip+ APDs was carried out at liquid nitrogen temperature. The 5.9 keV X-ray energy peak from a 55Fe radioisotope source was detected at zero bias with a full width half maximum (FWHM) of 2.8 keV. At increasing gain the 5.9 keV peak shifts away from the electronic noise floor improving the signal to noise performance of the detector. This is also complemented by a rapid improvement in the energy resolution with an FWHM of 950 eV obtained at a gain of 5.3. Modelling of the X-ray pulse height spectra was carried out using the RPL technique. The intrinsic resolution limited by avalanche statistic was found to be independent of avalanche gain.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Engineering (Sheffield) > Electronic and Electrical Engineering (Sheffield)
Depositing User: Mr Rajiv Gomes
Date Deposited: 07 Aug 2012 14:57
Last Modified: 08 Aug 2013 08:49
URI: http://etheses.whiterose.ac.uk/id/eprint/2560

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