Development of avalanche photodiodes with engineered band gap based upon III-V semiconductors

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 200μm 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.