I. Pintilie

Second-order generation of point defects in gamma-irradiated float-zone silicon, an explanation for “type inversion”

Radiation-induced defects in silicon diodes were investigated after exposure to high doses of Co60-gamma irradiation using the thermally stimulated current method. We have found that, for high irradiation doses, a second-order defect can be detected. This defect is largely suppressed in oxygen-enriched material while it is the main cause for the space charge sign inversion effect observed in standard float-zone material.

Close to midgap trapping level in 60Co gamma irradiated silicon detectors

The deep level transient spectroscopy method was applied on standard and oxygenated float-zone silicon detectors exposed to high doses of 60Co–gamma irradiation. We have detected and characterized a close to midgap trapping level having an ionization energy of EC−(0.545±0.005) eV and electron/hole capture cross sections of σn=(1.7±0.2)×10−15 cm2/σp=(9±1)×10−14 cm2 respectively. This level has a strong impact on the detector performance being responsible for more than 90% of the change in the effective doping concentration. The defect is strongly oxygen related and a possible connection with the V2O complex is discussed.

Cluster related hole traps with enhanced-field-emission—the source for long term annealing in hadron irradiated Si diodes

Cluster related defects were investigated by the thermally stimulated current (TSC) method in neutron irradiated n-type Si diodes during 80°C annealing. Three hole traps labeled H (116K), H (140K), and H (152K) proved to have an electric-field-enhanced emission characteristic of Coulombic wells. Their zero field emission rates were deduced describing the TSC peaks with the three-dimensional Poole-Frenkel formalism when accounting for the electric field distribution. As acceptors in the lower half of the gap, these centers have a direct impact on the effective doping of the n-type diodes. They are revealed as causing the long term annealing effects.

Thermally stimulated current method applied on diodes with high concentration of deep trapping levels

We propose an improved method of thermally stimulated currents (TSC) spectra analysis in the case of diodes having a concentration of traps higher than that of doping impurities. Beside the calculation of trap concentrations from TSC peaks analysis, the method allows us to evaluate the density and the type of the very deep trapping level which, due to the contribution of leakage current, can not be detected in a real TSC experiment. The proposed method is applied to a p+-n Silicon diode irradiated with 1.82×1013neutrons/cm2.

Study of radiation damage induced by 12 keV X-rays in MOS structures built on high-resistivity n-type silicon.

Imaging experiments at the European X-ray Free Electron Laser (XFEL) require silicon pixel sensors with extraordinary performance specifications: doses of up to 1 GGy of 12 keV photons, up to 10(5) 12 keV photons per 200 µm × 200 µm pixel arriving within less than 100 fs, and a time interval between XFEL pulses of 220 ns. To address these challenges, in particular the question of radiation damage, the properties of the SiO(2) layer and of the Si-SiO(2) interface, using MOS (metal-oxide-semiconductor) capacitors manufactured on high-resistivity n-type silicon irradiated to X-ray doses between 10 kGy and 1 GGy, have been studied. Measurements of capacitance/conductance-voltage (C/G-V) at different frequencies, as well as of thermal dielectric relaxation current (TDRC), have been performed. The data can be described by a dose-dependent oxide charge density and three dominant radiation-induced interface states with Gaussian-like energy distributions in the silicon band gap. It is found that the densities of the fixed oxide charges and of the three interface states increase up to dose values of approximately 10 MGy and then saturate or even decrease. The shapes and the frequency dependences of the C/G-V measurements can be quantitatively described by a simple model using the parameters extracted from the TDRC measurements.

Radiation induced point- and cluster - related defects with strong impact to damage properties of silicon detectors

This work is focusing on the investigation of those radiation induced defects causing degradation effects of Silicon detector performance. Comparative studies of the defects induced by irradiation with Co60- γ rays, 23 GeV protons and 1 MeV equivalent reactor neutrons revealed the existence of some point defects and cluster related centers having a strong impact to damage properties of Si diodes. The detailed relation between the “microscopic” reasons as based on defect analysis and their “macroscopic” consequences for detector performance are presented. In particular, it is shown that the changes in the Si device properties (depletion voltage and leakage current) after exposing to high levels of Co60- γ irradiation can be completely understood by the microscopically investigated formation of two point defects: i) a defect formed via a second order process (I p ) that can be associated with the long searched for V 2 O complex or with a Carbon related center and is the cause for the observed type inversion effect in Oxygen lean material; ii) a bistable donor (BD) created during irradiation that is strongly generated in Oxygen rich material, associated with one of the earlier thermal donors in Si. It is the cause for the observed positive space charge induced by irradiation in oxygenated Si diodes. Specific for hadron irradiation are the annealing effects which decrease resp. increase the originally observed damage effects as seen by the changes of the depletion voltage (effects known as “beneficial” and “reverse” annealing, respectively). A group of four cluster related defects proved to be responsible for these annealing effects. Their formation is not affected by the Oxygen content or Si growth procedure suggesting that they are complexes of multi-vacancies located inside extended disordered regions.

Investigation of point and extended defects in electron irradiated silicon—Dependence on the particle energy

This work is focusing on generation, time evolution, and impact on the electrical performance of silicon diodes impaired by radiation induced active defects. n-type silicon diodes had been irradiated with electrons ranging from 1.5 MeV to 27 MeV. It is shown that the formation of small clusters starts already after irradiation with high fluence of 1.5 MeV electrons. An increase of the introduction rates of both point defects and small clusters with increasing energy is seen, showing saturation for electron energies above ∼15 MeV. The changes in the leakage current at low irradiation fluence-values proved to be determined by the change in the configuration of the tri-vacancy (V3). Similar to V3, other cluster related defects are showing bistability indicating that they might be associated with larger vacancy clusters. The change of the space charge density with irradiation and with annealing time after irradiation is fully described by accounting for the radiation induced trapping centers. High resolution ...

Field effect enhanced signal-to-noise ratio in chemically deposited PbS thin films on Si3N4/n-Si substrates

p-type lead sulphide (PbS) thin films were chemically deposited onto Si3N4/n-Si substrates. A three-contact pseudo-metal–oxide–semiconductor structure was designed in order to investigate the possibility of enhancing the signal-to-noise ratio in the PbS film by field effect. It was proved that on the entire sensitivity domain of PbS (1–3 μm at room temperature), the magnitude of the photoconductive signal generated by the PbS film depends on the polarity and value of the gate voltage. It is shown that the signal-to-noise ratio can be enhanced in this way with by at least 25% compared with the case when the gate electrode is in air (standard configuration for photoconductive measurements).

Theoretical background of the optical charging spectroscopy method used for investigation of trapping levels

An analytical formula for the optical charging spectroscopy (OCS) discharging current in the case of homogeneous semiconductors is deduced. The computation shows that the OCS current is proportional with the square of the initially trapped carrier concentrations. This result leads to a higher sensitivity in case of the OCS method compared with the thermally stimulated current method. The OCS current formula obtained for one trapping level is generalized in the case of many trapping levels. From the generalized formula it can be seen that the OCS current can change the sign if both electron and hole traps exist in the sample.

Study of the radiation hardness of silicon sensors for the XFEL

Imaging experiments at the XFEL pose unprecedented requirements to the detectors in terms of radiation tolerance: Fluxes of up to 1016(12 keVphotons/cm2) corresponding to approximately 109 Gy in silicon, are expected. An irradiation station has been set up in the DORIS beam line F4, MOS test structures have been irradiated, and first results on the dose dependence of the C/V-characteristics, surface current density, and interface trap density have been obtained.