Surface effects in segmented silicon sensors

Abstract

PROF. DR. MICHAEL POTTHOFFii "Τὸν δὲ νοῦ καί ἐπιστήμης ἐραστὴν ἀνάγκη τὰς τῆς ἔμφρονος φύσεως αἰτίας πρώ-τας μεταδιώκειν." Πλάτωνος, Τίμαιος, Κεφ. 16 "The lover of reason and knowledge must first seek for the causes which belong to the rational order."Plato, Timaeus, Ch. 16 iii To my parents. . . iv AbstractSilicon detectors in Photon Science and Particle Physics require silicon sensors with very demanding specifications. New accelerators like the European X-ray Free Electron Laser (EuXFEL) and the High Luminosity upgrade of the Large Hadron Collider (HL-LHC), pose new challenges for silicon sensors, especially with respect to radiation hardness. High radiation doses and fluences damage the silicon crystal and the SiO 2 layers at the surface, thus changing the sensor properties and limiting their life time. Non-Ionizing Energy Loss (NIEL) of incident particles causes silicon crystal damage. Ionizing Energy Loss (IEL) of incident particles increases the densities of oxide charge and interface traps in the SiO 2 and at the Si-SiO 2 interface.In this thesis the surface radiation damage of the Si-SiO 2 system on high-ohmic Si has been investigated using circular MOSFETs biased in accumulation and inversion at an electric field in the SiO 2 of about 500 kV/cm. The MOSFETs have been irradiated by X-rays from an X-ray tube to a dose of about 17 kGy(SiO 2 ) in different irradiation steps. Before and after each irradiation step, the gate voltage has been cycled from inversion to accumulation conditions and back. From the dependence of the drain-source current on gate voltage the threshold voltage of the MOSFET and the hole and electron mobility at the Si-SiO 2 interface were determined. In addition, from the measured drain-source current the change of the oxide charge density during irradiation has been determined. The interface trap density and the oxide charge has been determined separately using the subthreshold current technique based on the Brews charge sheet model which has been applied for first time on MOSFETs built on high-ohmic Si. The results show a significant field-direction dependence of the surface radiation parameters. The extracted parameters and the acquired knowledge can be used to improve simulations of the surface radiation damage of silicon sensors.

On Monolithic Silicon Array Detectors for Small-Field Photon Beam Dosimetry

“…Following recommendations reported in the same references, a concentration of trapped charges at the Si-SiO interfaces and within the SiO layers of C 10 cm and C 10 cm for the pre-irradiated bulk Octa and for the epitaxial Octa respectively was considered. The concentration saturates between 1.5 10 cm and 3.5 10 cm [31].…”

Determination of the electric field in highly-irradiated silicon sensors using edge-TCT measurements

“…In Refs. [24,25,26] the electric field for a p + n strip detector in the region close to the plane of the readout strips has been studied as a function of ionising dose, biasing conditions and time after switching on the voltage. It has been found, that the boundary condition changes with time because of the finite surface resistivity of the SiO 2 , which is a strong function of humidity and temperature.…”

“…In Ref. [26] simulations of the changes of the electric field as a function of the time after biasing a segmented n + p-sensor are given. In the data analysis presented in this paper, the effect of the lack of knowledge of the field close to the strip plane is investigated by performing fits starting at different minimal y-values.…”

Signal coupling to embedded pitch adapters in silicon sensors

“…To emulate the effect of irradiation, the charge density is increased to its saturation value of 3 × 10 12 /cm 2 . These numbers are based on typical values for silicon-silicon dioxide boundaries [8,9], but have not been measured on the tested sensors. It is possible that the tested sensors start with a lower charge density than 1 × 10 11 /cm 2 when unirradiated, but changes to the qualitative behavior of the sensors are not observed below this value.…”