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 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

Investigation of the behaviour of GaAs/AlGaAs SAM-APDs for synchrotron radiation

11noThis work reports on the fabrication and characterization of a novel high-speed, low-noise X-ray Avalanche Photodiode based on III-V compound semiconductors operating over an extended photon energy range. These materials were suggested as their higher atomic numbers allow for the absorption of higher photon energies; hence, shorter response times can be achieved by growing APDs with thinner active regions. In addition, the use of staircase hetero-junctions enhances electron multiplication and results in lower noise if compared with conventional p-i-n diodes. In this work, molecular beam epitaxy was used to produce GaAs/AlGaAs APDs with separated absorption and multiplication regions. The multiplication region, separated from the absorption region by a δ p-doped layer of carbon, contains a staircase structure composed of nanometric layers of AlGaAs and GaAs, which alternate periodically. The periodic modulation of the band gap enables a well-defined charge multiplication and results in low multiplication noise. Several devices were characterized in terms of dark current, photocurrents generated utilizing visible and hard X-ray sources as well as noise generated under laser light.openopenNichetti, Camilla; Steinhartova, Tereza; Antonelli, Matias; Cautero, Giuseppe; Menk, Ralf Hendrik; Pilotto, Alessandro; Driussi, Francesco; Palestri, Pierpaolo; Selmi, Luca; Arfelli, Fulvia; Biasiol, GiorgioNichetti, Camilla; Steinhartova, Tereza; Antonelli, Matias; Cautero, Giuseppe; Menk, Ralf Hendrik; Pilotto, Alessandro; Driussi, Francesco; Palestri, Pierpaolo; Selmi, Luca; Arfelli, Fulvia; Biasiol, Giorgi

Gain and noise in GaAs/AlGaAs avalanche photodiodes with thin multiplication regions

11noAvalanche photodiodes based on GaAs/AlGaAs with separated absorption and multi- plication regions (SAM-APDs) will be discussed in terms of capacitance, response to light (gain and noise) and time response. The structures have been fabricated by molecular beam epitaxy introducing a δ p layer doped with carbon to separate the multiplication and the absorption re- gions. The thickness of the latter layer defines the detection efficiency and the time resolution of the structure, which in turn allows tailoring the device for specific scientific applications. Within the multiplication region a periodic modulation of the bandgap is obtained by growing alternating nanometric layers of AlGaAs and GaAs with increasing Al content; this staircase structure enables the tuning of the bandgap and subsequently provides a well-defined charge multiplication. The use of such staircase hetero-junctions enhances electron multiplication and conversely reduces — at least in principle — the impact of the noise associated to hole multiplication, which should result in a decreased overall noise, when compared to p-i-n diodes composed by a single material. The first part of this paper focuses on the electrical characteristics of the grown structure and on the comparison with the simulated behaviour of such devices. In addition, gain and noise measure- ments, which have been carried out on these devices by utilizing photons from visible light to hard X-rays, will be discussed and will be compared to the results of a nonlocal history-dependent model specifically developed for staircase APDs.reservedmixedNichetti, C.; Steinhartova, T.; Antonelli, M.; Cautero, G.; Menk, R.H.; Pilotto, A.; Driussi, F.; Palestri, P.; Selmi, L.; Arfelli, F.; Biasiol, G.Nichetti, C.; Steinhartova, T.; Antonelli, M.; Cautero, G.; Menk, R. H.; Pilotto, A.; Driussi, F.; Palestri, P.; Selmi, L.; Arfelli, F.; Biasiol, G

Synchrotron Radiation Study of Gain, Noise, and Collection Efficiency of GaAs SAM-APDs with Staircase Structure

In hard X-ray applications that require high detection efficiency and short response times, such as synchrotron radiation-based Mössbauer absorption spectroscopy and time-resolved fluorescence or photon beam position monitoring, III–V-compound semiconductors, and dedicated alloys offer some advantages over the Si-based technologies traditionally used in solid-state photodetectors. Amongst them, gallium arsenide (GaAs) is one of the most valuable materials thanks to its unique characteristics. At the same time, implementing charge-multiplication mechanisms within the sensor may become of critical importance in cases where the photogenerated signal needs an intrinsic amplification before being acquired by the front-end electronics, such as in the case of a very weak photon flux or when single-photon detection is required. Some GaAs-based avalanche photodiodes (APDs) were grown by a molecular beam epitaxy to fulfill these needs; by means of band gap engineering, we realised devices with separate absorption and multiplication region(s) (SAM), the latter featuring a so-called staircase structure to reduce the multiplication noise. This work reports on the experimental characterisations of gain, noise, and charge collection efficiencies of three series of GaAs APDs featuring different thicknesses of the absorption regions. These devices have been developed to investigate the role of such thicknesses and the presence of traps or defects at the metal–semiconductor interfaces responsible for charge loss, in order to lay the groundwork for the future development of very thick GaAs devices (thicker than 100 [Formula: see text] m) for hard X-rays. Several measurements were carried out on such devices with both lasers and synchrotron light sources, inducing photon absorption with X-ray microbeams at variable and controlled depths. In this way, we verified both the role of the thickness of the absorption region in the collection efficiency and the possibility of using the APDs without reaching the punch-through voltage, thus preventing the noise induced by charge multiplication in the absorption region. These devices, with thicknesses suitable for soft X-ray detection, have also shown good characteristics in terms of internal amplification and reduction of multiplication noise, in line with numerical simulations