Neutron Irradiated Silicon Research Articles

Electron paramagnetic resonance studies of multi-defect clusters in neutron irradiated silicon carbide

High-temperature stable defect complexes in 6H-SiC crystals created by heavy neutron irradiation and following high-temperature annealing have been discovered by electron paramagnetic resonance. After thermal annealing at 1500 °C at least five new axially symmetric centres with electron spin S = 1/2 and S = 1 were shown to arise in 6H-SiC crystals. The striking feature of all discovered centres is a strong hyperfine interaction with a great number (up to 12) of equivalent host Si (C) atoms. Two models, a four-vacancy complex VSi–3VC, and a split-interstitial antisite (C2)Si or a pair of two antisites (C2)Si–SiC are discussed. As a result of multi-defect cluster decay new triplet centres labelled as N-V have been observed in 1900 °C annealed 6H-SiC crystals which were concluded to consist of silicon vacancy and carbon substitutional nitrogen in the adjacent lattice cites oriented along the c-axis. The parameters of these centres are similar to those for the well-known N-V centre in diamond.

Annealing studies of effective trapping times in silicon detectors

Annealing of effective trapping times of electrons and holes in neutron irradiated silicon detectors was measured at different temperatures 40, 60, 80 ° C . The evolution of effective trapping times seems to be governed by the first-order process. The effective trapping probability of holes was found to increase by 40% and of electrons to decrease by 20% during annealing. The time constants are of order 600 min at 60 ° C . The scaling to different temperatures can be obtained from Arrhenius relation with activation energies of around 1 eV.

Signal fluctuation in neutron-irradiated silicon Avalanche Photodiodes

Light from PbWO 4 crystals in the barrel part of the CMS electromagnetic calorimeter will be detected by Avalanche Photodiodes (APDs). The crystal-APD system has to operate reliably in a high radiation environment. In the present paper, APD signal variation with wavelength and gain fluctuations for PbWO 4 scintillation light was investigated as a function of the neutron fluence. Calculations were made with a single-particle Monte Carlo simulation technique. The results show that the neutron fluence has an influence on the APD signal during 10 years CMS operation.

Identification of the Triplet State N-V Defect in Neutron Irradiated Silicon Carbide by Electron Paramagnetic Resonance

Two types of a new triplet centers labeled as N-V have been observed in heavily neutron irradiated (dose of 1021 cm-2) and high-temperature annealed (2000°C) 6H-SiC crystals. The centers have an axial symmetry along c-axis. Anisotropic hyperfine splitting due to the one 14N nucleus has been observed. The EPR spectra of N-V defects in the triplet state in 6H-SiC reveal strong temperature dependence. The parameters of these centers are similar to that for well-known N-V center in diamond. It seems to consist of silicon vacancy and carbon substitutional nitrogen in the adjacent lattice cites oriented along c-axis. Similar to the diamond N-V centers in SiC have been produced by neutron irradiation and high-temperature annealing of the crystals containing nitrogen. For the first shell the structure of the N-V defect in 6H-SiC is practically identical with that in diamond. The charge state of this defect seems to be +1 compare with neutral state for nitrogensilicon vacancy defect in 6H-SiC with S=1/2.

Annealing behaviors of vacancy in varied neutron irradiated Czochralski silicon

The difference of annealing behaviors of vacancy-oxygen complex (VO) in varied dose neutron irradiated Czochralski silicon: (S1 5×10 17 n/cm 3 and S2 1.07×10 19 n/cm 3) were studied. The results show that the VO is one of the main defects formed in neutron irradiated Czochralski silicon (CZ-Si). In this defect, oxygen atom shares a vacancy, it is bonded to two silicon neighbors. Annealed at 200 °C, divacancies are trapped by interstitial oxygen(O i) to form V 2O (840 cm −1). With the decrease of the 829 cm −1 (VO) three infrared absorption bands at 825 cm −1 (V 2O 2), 834 cm −1 (V 2O 3) and 840 cm −1 (V 2O) will rise after annealed at temperature range of 200–500 °C. After annealed at 450–500 °C the main absorption bands in S1 sample are 834 cm −1, 825 cm −1 and 889 cm −1 (VO 2), in S2 is 825 cm −1. Annealing of A-center in varied neutron irradiated CZ-Si is suggested to consist of two processes. The first is due to trapping of VO by O i in low dose neutron irradiated CZ-Si (S1) and the second is due to capture the wandering vacancy by VO, etc, in high dose neutron irradiated CZ-Si (S2), the VO 2 plays an important role in the annealing of A-center. With the increase of the irradiation dose, the annealing behavior of A-center is changed.

Influence of rapid thermal process on intrinsic gettering in fast neutron irradiated Czochralski silicon

A rapid thermal process (RTP) was first introduced into the intrinsic gettering (IG) processes of fast neutron irradiated Czochralski (CZ) silicon. The effect of RTP conditions on bulk microdefects (BMDs) and denuded zone (DZ) was investigated. Fourier transform infrared absorption spectrometer (FTIR) was used to measure the concentration of interstitial oxygen ([O i]). Bulk microdefects were observed by optical microscope. The results show that, according to the variation of [O i], it is found that RTP doesn't change the processes of oxygen precipitation in fast neutron irradiated Czochralski silicon. Perfect denuded zone, dense oxygen precipitates and defects form in the bulk of irradiated samples. With increasing temperature of RTP, the width of denuded zone decreases. Increasing RTP cooling rate, the density of Bulk microdefects increases. DZ forms in the sample that annealed in nitrogen atmosphere.

Accelerated oxygen precipitation in fast neutron irradiated Czochralski silicon

Annealing effect of the oxygen precipitation and the induced defects have been investigated on the fast neutron irradiated Czochralski silicon (CZ-Si) by infrared absorption spectrum and the optical microscopy. It is found that the fast neutron irradiation greatly accelerates the oxygen precipitation that leads to a sharp decrease of the interstitial oxygen with the annealing time. At room temperature (RT), the 1107cm−1 infrared absorption band of interstitial oxygen becomes weak and broadens to low energy side. At low temperature, the infrared absorption peaks appear at 1078cm−1, 1096cm−1 and 1182cm−1, related to different shapes of the oxygen precipitates. The bulk microdefects, including stacking faults, dislocations and dislocation loops, were observed by the optical microscopy. New or large stacking faults grow up when the silicon self-interstitial atoms are created and aggregate with oxygen precipitation.

Vacancy-related complexes in neutron-irradiated silicon

Electrically active defects induced by neutron irradiation in n-type Czochralski-grown (Cz)Si crystals have been studied by means of capacitance transient techniques. Theseneutron-induced defects are compared with those created by electron irradiation andself-ion implantation. Four electron traps with the activation energies for electronemission of 0.12, 0.16, 0.24 and 0.42 eV were observed after neutron irradiation inphosphorous-doped Cz Si crystals. It is inferred that the E(0.12) and E(0.16) traps arerelated to the single-acceptor states of the silicon self-interstitial–oxygen dimer complex(IO2i) and the vacancy–oxygen pair (VO), respectively. The E(0.24) trap is associatedwith the electron emission from the double-acceptor state of the divacancy(V2). However, an asymmetric peak with its maximum at around 220 K and an activationenergy for electron emission of 0.42 eV dominated the spectra. We used high resolutionLaplace DLTS to investigate the structure of E(0.42) and found that this signal iscomplex, consisting of contributions from several defects. From the annealingbehaviour, it was revealed that as some of these defects anneal out they aresources of vacancies evidenced by an increase in the concentration of VO andV2. It is suggested that some of the defects contributing to the E(0.42) peak are related tosmall vacancy clusters.

Optical studies of defects generated in neutron‐irradiated Cz‐Si during HP‐HT treatment

AbstractNeutron‐irradiated Czochralski grown silicon subjected to heat treatment (HT) at 350 °C and 1000 °C under enhanced hydrostatic pressure (HP) was studied in this work. It has been shown that external hydrostatic pressure enhances the creation of VO2 defects in neutron irradiated silicon subjected to the HP ‐ HT treatment at 350 °C. Enhanced formation of platelet‐like oxygen precipitates was found in the samples treated at 1000 °C under 1.1 GPa. This effect was more pronounced in the samples with VO2 defects. Presented results seem to suggest that probably HP helps to transform VO2 to some kind of defects or change alone VO2 defects in the form that can act as an additional nucleus for an additional oxygen precipitation at 1000°C. No correlation between the plate‐like oxygen precipitates related absorption at 1225 cm‐1 and dislocation‐related emission has been confirmed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron spin resonance study of paramagnetic centers in neutron-irradiated heat-treated silicon

Electron spin resonance (ESR) was used to study neutron-induced defects in silicon as functions of anneal temperature T{sub an}. For T{sub an} below 200 deg. C, the ESR response is dominated by the Si-P3 and Si-P6 spectra, as observed before. At T{sub an}=200 deg. C, a low symmetry center Si-H8, only once reported before, appears. Here, a hyperfine doublet, corresponding to interaction with one {sup 29}Si nucleus, is resolved. In the range T{sub an}{>=}250 deg. C, two other paramagnetic defects dominate. The first is the tetragonal center Si-B3: Mapping of the full angular dependence of known and newly resolved {sup 29}Si hyperfine structure enables conclusive identification as the tetra-interstitial (I{sub 4}) in the positive charged state, based on compliance of the data with former theoretical calculations. Second, we report the observation of an unidentified trigonal center, Si-B5, exhibiting two hyperfine (hf) doublets corresponding with hf interaction with four and six equivalent Si sites. The tri-interstitial I{sub 3} is proposed as a provisional defect model. The annealing behavior and symmetry of these defects provides evidence for linking these paramagnetic B3 and B5 centers to the X and W optical centers, respectively.

Pressure stimulated creation of oxygen-related defects in oxygen-implanted and neutron-irradiated silicon

The present work reports some experimental results based on capacitance measurements on float-zone silicon (Fz-Si) and Czochralski-grown silicon (Cz-Si) subjected to oxygen implantation, subsequent neutron irradiation and finally high-pressure thermal anneals. The purpose of this work was the study of the effect of irradiation on the formation of thermal acceptors and donors in silicon. We found that oxygen ion implantation followed by neutron irradiation results in shallow and deep level acceptor-like defects formation. Prolonged heat treatment leads to thermal donor generation as usual in Cz-Si annealed at 720 K. The most striking result of the study is finding that high-pressure thermal anneals result in extra donor formation even for low oxygen concentration. The effects mentioned above lead to changes in the type of conductivity (p-n junction formation) depending on oxygen content in the material, hydrostatic pressure and an extent of damage caused by the irradiation.

FTIR study an VO2 defect in fast neutron irradiated Czochralski silicon

The vacancy_dioxygen complex(VO2) is one of the main defects formed i n fast neutron irradiated CZ_Si during annealing in the temperature range 400—500℃. In this defect,two oxygen atoms share a vacancy,each of which is bonded to two s ilicon neighbors.With the increase of the 889cm-1(VO2),two infrared absorption bands at 919.6 and 1006cm-1 will arise in neutron irradia ted CZ _Si after annealed in the temperature range 300—500℃.IR vibrational bands at 9 19.6 and 1006cm-1 can be assigned to the metastable defect (O-V-O)th at is composed of a VO(A center) and a neighboring interstitial oxygen(Oi)atom.By prolonging the annealing time from 2h up to 10h or increasing the annealing temperature,the metastable defect(O-V-O)will be converted into V O2.During annealing in the temperature range 400—500℃,the main defe cts for med in the high dose(1019) neutron irradiated CZ_Si is the multi_vac ancy type of defects and the formation of the VO2 will be depressed.

X-ray Diffraction Study of the Effect of Neutron Irradiation on the Defect Formation in Silicon Crystals Grown by the Czochralski Method and Annealed at High Temperatures

The effect of x-ray scattering by neutron-irradiated and reference (unirradiated) silicon crystals grown by the Czochralski method and annealed at temperatures of 850–1050°C on the defect formation is comparatively investigated using triple-crystal x-ray diffractometry. The sizes and concentrations of clusters composed of point defects and dislocation loops formed during decomposition of an oxygen-containing solid solution and subsequent clusterization of the point defects are calculated.

Determination of the effective dominant electron and hole trap in neutron-irradiated silicon detectors

An attempt to determine the effective dominant electron and hole traps in standard, neutron-irradiated FZ silicon was made. The method was based on measurements of effective dopant concentration and effective carrier trapping times in the presence of an enhanced carrier concentration. Measurements can be well described with a deep donor level in the bottom part of the band gap and a deep acceptor level in the upper part of the band gap.

Defect-related diffusion of hydrogen in silicon

Hydrogen-related effects in silicon with radiation defects are discussed in the present report. The first effect is the defect-related out-diffusion of hydrogen from an implanted layer. The second one is the saturation with hydrogen of some layers in neutron-irradiated silicon, during its boiling in pure water.

Atomic structure of theB3defect in neutron-irradiated silicon

The tetragonal ${(D}_{2d}$ symmetry) $\mathrm{Si}\ensuremath{-}B3$ defect is intensively observed by electron spin resonance (ESR) in neutron-irradiated Cz-grown c-Si after annealing in the range $250--500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ In addition to the limited known one, a rich ${}^{29}\mathrm{Si}$ hyperfine structure is revealed and fully angularly mapped. Spectra simulations show that the defect unpaired electron has its strongest hf interaction with two equivalent Si sites in the first shell, a next interaction with four equivalent Si sites in the second shell, and a last interaction with eight equivalent Si sites in the third shell. From comparison of the data with extensive theoretical calculations on the tetrainterstitial ${(I}_{4}),$ the $\mathrm{Si}\ensuremath{-}B3$ is identified as the ${I}_{4}$ defect.

Carbon-related complexes in neutron-irradiated silicon

In this work, we have studied point defects in carbon-doped Si material, irradiated by fast neutrons, via the observation of the Infrared absorption spectra. We mainly focus on carbon-related defects and their complexing with primary defects. We discuss the localized vibrational mode bands related to these defects, their annealing behavior and their interactions. Infrared spectra recorded at room temperature reveal the presence of a band at 544cm−1, appearing only in C-rich Si, and showing similar thermal stability to that of the di-carbon (CiCs) defect. In addition, its amplitude scales with the carbon content of the material. We attributed this band to the (CiCs) center. Two bands, at 987 and 993cm−1 were attributed to the CiCs(SiI) center. Furthermore, the origin of a C-related band at 943cm−1 is discussed.

Breakdown voltage analysis of neutron irradiated silicon detectors

The very intense radiation environment of high luminosity future colliding beam experiments (LHC, etc.) makes radiation hardness the most important issue for Si detectors. The crucial question is whether the Si strip detectors can withstand the harsh radiation environment for sufficiently long time (full LHC lifetime) and hence the central issue concerning all LHC experiments is the breakdown performance of these detectors. In this paper the simulations have been performed to analyze electrical parameters for the most deleterious long-term effect of radiation: the change in effective charge carrier concentration and resulting increase in full depletion bias. Detailed calculations using Hamburg Model have allowed the parameterization of these effects, and helped to simulate the operation scenario of Si detectors over 10 years of LHC operation. The systematic studies of the electric field to get an insight into the device behavior provide, for the first time, a possible explanation for the improvement in breakdown performance with radiation. Simulation study is also carried on optimized guard ring structures to study the change in guard voltage distribution after 10 years of neutron radiation damage.

Radiation defects in neutron irradiated silicon with high oxygen concentration

In silicon material with oxygen concentration in the range (4–9)×1017cm−3, grown by the CZ method and irradiated by reactor neutrons, the interstitial oxygen mainly participates in A-center (V+O) creation. A-center concentration is linearly dependent on neutron fluence up to Φ=1018n/cm2, value at which saturation occurs caused not by the exhaustion of dissolved oxygen but rather by vacancy-interstitial atom annihilation competitive mechanisms. A-center annealing (T>300°C) does not lead to the full restoring of interstitial oxygen concentration. Repeating neutron irradiation and annealing causes a decrease of oxygen complex activity and the silicon behaves as a low-oxygen material.

Recent progress in low-temperature silicon detectors

The CERN RD39 Collaboration studies the possibility to extend the detector lifetime in a hostile radiation environment by operating them at low temperatures. The outstanding illustration is the Lazarus effect, which showed a broad operational temperature range around 130 K for neutron irradiated silicon detectors.