E. Verbitskaya

Effect of radiation induced deep level traps on Si detector performance

The main factor, which leads to semiconductor detector degradation in high-energy physics experiments, is the introduction of lattice defects in the detector material produced by radiation. Based on the spectrum of radiation induced defects in the silicon bulk, the overview of effects and mechanisms responsible for the changes in the main detector parameters such as effective concentration of the space charge in the depleted region, space charge sign inversion, charge collection efficiency, and detector breakdown voltage are considered. Special attention is paid to the electric field distortion related with high concentration of radiation induced deep traps, which is the key question for the design of detectors operating at cryogenic temperature. In particular, the charge collection recovery at low temperature, often refereed as the Lazarus effect, and the limitation for the detection rate related to the polarization effect are considered.

ANALYSIS OF DIVACANCY RELATED TRAPS INDUCED BY PROTON, NEUTRON AND GAMMA RADIATION IN HIGH RESISTIVITY SILICON DETECTORS

Abstract Defects with deep levels induced in high-resistivity silicon detectors by low and high radiation fluence of protons and neutrons are studied using capacitance and current DLTS. Numerical simulation of I-DLTS and C-DLTS spectra based on the model of charge carrier emission and redistribution of electric field in the detector enabled one to perform the detailed investigation of DLTS spectra. It has been shown that the main DLTS peak in the range of 200 to 260 K may be considered as a result of the interference of deep levels near the midgap – negatively charged divacancy VV− and the Ci–Oi complex. The model describing the broadening of the VV− component of the spectrum, which arises from the divacancy localization inside a cluster, is discussed. The results are compared with those obtained for gamma irradiation, for which the dominant contribution in DLTS spectra arises just from the Ci–Oi complex.

Direct observation and measurements of neutron-induced deep levels responsible for Neff changes in high-resistivity silicon detectors using TCT

Neutron induced deep levels responsible for changes of space charge concentration {ital N{sub eff}} in high resistivity silicon detectors have been observed directly using the transient current technique (TCT). It has been observed by TCT that the absolute value and sign of {ital N{sub eff}} experience changes due to the trapping of non- equilibrium free carriers generated near the surface (about 5 micrometers depth into the silicon) by short wavelength laser pulses in fully depleted detectors. Electron trapping causes {ital N{sub eff}} to change toward negative direction (or more acceptor-like space charges) and hole trapping causes {ital N{sub eff}} to change toward positive direction (or more donor-like space charges). The specific temperature associated with these {ital N{sub eff}} changes are those of the frozen-up temperatures for carrier emission of the corresponding deep levels. The carrier capture cross sections of various deep levels have been measured directly using different free carrier injection schemes. 10 refs., 12 figs., 3 tabs.

Relaxation of radiation damage in silicon planar detectors

The behavior of radiation‐induced carbon‐related defects in high‐resistivity silicon detectors has been investigated. The defects were introduced by α‐particle irradiation and investigated by deep‐level transient spectroscopy. An unusual defect behavior consists in low‐temperature annealing, including self‐annealing at room temperature, of the interstitial carbon Ci with a simultaneous increase of the Ci‐Oi‐complex concentration. The kinetic parameters of the process have been determined from the increase of the Ci‐center concentration versus time. Two annealing velocities have been observed, which arise from different heat treatments during the detector fabrication process.

Study of GaAs as a material for solar neutrino detectors

Abstract Semi-insulating GaAs crystals grown by liquid encapsulated Czochralski technique from Ga-rich melts were evaluated as a possible material for radiation detectors with a high active layer thickness. The density of deep traps, particularly the midgap EL2 donors pinning the Fermi level, was measured by various techniques in conducting and semi-insulating samples. For EL2 traps, a direct evidence of their partial neutralization in the space charge region of reverse biased Schottky diodes due to nonequilibrium capture of electrons is presented for the first time. It is shown that the density of EL2 centers decreases with decreased As composition of the melt very gradually, especially for post-growth annealed samples. Subsequently, if one aims to decrease the EL2 density to such an extent that it would make a serious impact on the depletion layer width in GaAs-based detectors one has to grow semi-insulating GaAs crystals from melts with As composition below about 43% which poses a problem for the preservation of high resistivity of the material due to the relatively high concentration of compensating acceptors.

Optimization of electric field distribution by free carrier injection in silicon detectors operated at low temperatures

We present a study of the modeling of the electric field distribution, which is controlled by injection and trapping of nonequilibrium carriers, in Si detectors irradiated by high neutron fluences. An analytical calculation of the electric field distribution in detectors irradiated by neutrons up to fluences of 1 /spl middot/ 10/sup 14/ to 5 /spl middot/ 10/sup 15/ cm/sup -2/ shows the possibility of reducing the full depletion voltage at low temperatures via hole injection. For this calculation, we use the detector operating parameters and equivalent neutron fluences expected for Large Hadron Collider experiments. The results of the calculation are in good qualitative agreement with published experimental data, lending strong support for the model and for an earlier proposal of electric field manipulation by free carrier injection.

Elevated temperature annealing of the neutron induced reverse current and corresponding defect levels in low and high resistivity silicon detectors

A new aspect of degradation phenomena of neutron irradiated silicon detectors has been revealed which consists in the significant influence of carbon related defect transformation on the detector reverse current (I/sub rev/). The annealing of the reverse current at elevated temperatures and the corresponding changes of the deep level transient spectroscopy (DLTS) spectra of defects for fast neutron irradiated silicon detectors, fabricated on high (4-6) k/spl Omega/-cm, moderate (0.5-1.0 k/spl Omega/-cm) and low ( >

Spin-dependent recombination electron paramagnetic resonance spectroscopy of defects in irradiated silicon detectors

Spin-dependent recombination (SDR) electron paramagnetic resonance (EPR) spectroscopy is applied for investigation of paramagnetic recombination centers in irradiated silicon p−n junction detectors (diodes) formed on float-zone (FZ) silicon wafers. The main radiation defects, associated with SRD-EPR spectra arising from excited triplet states, are assigned to complexes of two substitutional carbon atoms and one interstitial silicon atom (CS+SiI+CS) and to oxygen + vacancy (O+V) complexes (A-centers). In spite of the low concentration of oxygen in FZ silicon the A-centers are found to play an important role in the recombination process in the diodes. At temperatures T<100 K the SDR-EPR spectra are well observable by measurements of the microwave conductivity or by detecting a dc forward current IF below a forward-blocking voltage UFBL. At UFBL the IF has a steep jump followed by a decrease of the voltage over the diode and a negative resistance region with oscillations of the current. The SDR-EPR spectrum ...

TEMPERATURE-STIMULATED ABNORMAL ANNEALING OF NEUTRON-INDUCED DAMAGE IN HIGH-RESISTIVITY SILICON DETECTORS

Abstract Neutron-irradiated high-resistivity silicon detectors have been subjected to elevated temperature annealing (ETA). It has been found that both detector full depletion voltage and leakage current exhibit abnormal annealing (or “reverse annealing”) behaviour for highly irradiated detectors: increase with ETA. Laser-induced current measurements indicate a net increase of acceptor type space charges associated with the full depletion voltage increase after ETA. Current deep level transient spectroscopy ( I -DLTS) and thermally stimulated current (TSC) data show that the dominant effect is the increase of a level at 0.39 eV below the conduction band ( E c − 0.39 eV) or a level above the valence band ( E v + 0.39 eV). Candidates tentatively identified for this level are the singly charged double vacancy (V-V − ) level at E c − 0.39 eV, the carbon interstitial-oxygen interstitial (C i O i ) level at E v +0.36 eV, and/or the tri-vacancy-oxygen center (V 3 O) at E v +0.40 eV.

Scanning transient current study of the I-V stabilization phenomena in silicon detectors irradiated by fast neutrons

Abstract Investigations of the I - V stabilization phenomenon in neutron-irradiated silicon detectors have been carried out using scanning transient current technique (STCT) on non-irradiated p + -p-n + detectors. The p + -p-n + structure was used to simulate the p + -n-n + detectors irradiated beyond the space charge sign inversion (SCSI). Two mechanisms partially responsible for the I - V stabilization have been identified.