Preparation & Characterization of n-Type Amorphous Selenium Films as Blocking Layers in a-Se X-ray Detectors
Date
2009
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Degree Level
Masters
Abstract
The "n-like layer" is important in multilayer layer amorphous selenium (a-Se) based Xray
detector structures because it blocks the injection of holes from the positive electrode. The dark current in these devices is controlled primarily by hole injection, and the introduction of the n-like layer to block hole injection was a key development in the commercialization of a-Se X-ray detectors. An n-like a-Se layer is defined as a layer in which the electron range is much greater than the hole range, μₑτₑ >> μₕτₕ, where τ and μ are the lifetime and drift mobility of the charge carriers and the subscript e and h represent electrons and holes.
This thesis examines the effect of doping a-Se with Group II elements (in particular Mg) towards finding a better n-like layer – that with relatively long electron range (drift mobility × lifetime) , trap limited hole transport and which is stable against crystallization. Conventional Time of Flight (TOF) and Interrupted Field Time of Flight (IFTOF) transient photoconductivity measurements were used to characterize the
electron and hole transport in various Group II doped a-Se layers. The dependence of
the electron and hole lifetimes and drift mobilities on the composition of the n-like layer
was examined. The addition of Group II materials converts the a-Se starting material
from p-like into n-like. It was found that increasing the concentration of Mg increases
the electron range while limiting the hole range by modifying the population of deep
traps. The addition of As further limits the hole transport but does not alter the electron
range. The clear reproducibility of the thermal properties obtained from the Differential
Scanning Calorimetry (DSC) implies that small amounts of Mg can be used as a suitable n-type dopant.
Description
Keywords
Amorphous Selenium, n-layers, X-ray Deetectors
Citation
Degree
Master of Science (M.Sc.)
Department
Electrical Engineering
Program
Electrical Engineering