This thesis is striving in the development and performance assessment
of GaAs/AlGaAs avalanche photodiodes (APDs) with separated
absorption and multiplication regions, which complement existing
silicon detectors providing higher efficiency for X-ray detection. During
the course of this thesis several APDs were fabricated utilizing
molecular beam epitaxy and lithography and subsequently have been
thoroughly characterized. This thesis is subdivided into six chapters.
It sets in with a general description of APD structures and their functionalities
prescinding the advantages of the developed APDs, which
are fabricated on mesas with a diameter of 200\u3bcm and consists of
an absorption and multiplication region separated by a thin p-doped
layer of carbon. In particular the benefits on impact ionization and
charge multiplication when using a superlattice of some (6, 12, 24)
nanometric layers of GaAs/AlGaAs hetero-junctions are described,
which enhances the charge amplification of electrons while reducing
the multiplication of holes thus lowering the overall detector noise.
The second chapter deals with device simulation and points out the
limitations of the established local model to describe impact ionization
in thick multiplication regions. In order to simulate APDs with narrow
intrinsic areas a new and improved nonlocal history-dependent model
for gain and noise based on the energy balance equation has been
developed and is thoroughly described at the end of this chapter. The
materials and method section provides in the third chapter a comprehensive
description of the techniques and machinery employed during
the device manufacturing, while in the fourth chapter the experimental
setups, which were involved to test the devices are outlined. Both, the
used readout and acquisition electronics and the light/particle sources
are thoroughly described. In chapter 5 the different measurements
and associated datamining are presented and discussed. In particular
the role of different doping levels in the p-doped layers has been
deeply investigated revealing that a planar doping with the maximum
effective acceptor density is favored as it maximized the potential drop
in the multiplication region thus enhancing the impact ionization. Furthermore,
measurements and associated results of the time resolution
of the APDs utilizing visible table-top lasers and X-rays are described
in this section, revealing a rise time of 80 ps for the 24-step device.
A study of the noise versus gain behavior is present as well and is
compared to the results of the simulation. Moreover, utilizing a charge
sensitive amplifier both the spectroscopic capabilities and the charge
collection efficiency of the APDs could be determined by means of a
pulsed table-top laser and an Americium source. The thesis finishes
with the conclusions in chapter 6