Radiation defects in neutron irradiated silicon

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Radiation defects in neutron irradiated silicon
with high oxygen concentration P.G.Litovchenkoa,
A.A.Grozaa, V.I.Varninaa, M.I.Starchika,
V.I.Khivricha, G.G.Shmatkoa, L.A. Polivzeva, M.B.
Pinkovskaa, D.Bisellob, A. Candelorib,
A.P.Litovchenkob, A. Kaminskyb, J. Wyssc, W.
Wahld.   aInstitute for Nuclear Research of NASU,
Prospect Nauki 47, 03028,Kiev, Ukraine bIstituto
Nazionale di Fisica Nucleare and Dipartimento di
Fisica,Università di Padova, via Marzolo 8,
I-35131, Padova, Italy cUniversity of Cassino,
Department DIMSAT, via DiBiasio 43, 03043,
Cassino(FR), Italy dGSF, Institute of Radiation
Protection, Neuherberg, Germany

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Contents 1. Introduction 2. Dose dependence of A
centers concentration (12 mkm band ) 3.
Spectra of IR absorption in irradiated silicon
with high oxygen concentration (4-9?1017cm-3) by
fast neutrons 4. Influence of pre-irradiation on
oxygen complex formation in silicon with high
oxygen concentration 5. Influence of neutron
irradiation on oxygen precipitation in silicon
during annealing 6. Conclusion
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I. Introduction Oxygen impurity is always
present in silicon. It strongly influences the
properties of silicon and its radiation hardness
1. Recently the interest of many research teams
is directed towards the investigation of silicon
with high oxygen concentrations for the purpose
of producing radiation hard silicon for
semiconductor detectors, for example, for the
experiments foreseen at the LHC at CERN. It is
therefore necessary to study comprehensively the
behaviour of the oxygen impurity in irradiated
silicon. The free oxygen concentration can be
obtained by measuring the intensity of the 9 mkm
band in infrared (IR) absorption spectra. It was
shown that the band is connected to the
interstitial oxygen and belongs to the IR active
antisymmetric valence-like vibration of the
SiOSi quasi-molecule . The study of simple
defects, such as A-centers (or SiB1 centers),
created by the vacancy capture by interstitial
oxygen, can give some information about radiation
defects. Silicon-oxygen bond vibrations in
A-center leads to the appearance of the band with
the maximum at 12 mkm in the IR absorption
spectrum. The changes of the oxygen precipitation
in silicon under irradiation can be used as a
method for studying the radiation behaviour of
Si-Cz .
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The saturation is not caused by the exhaustion
of interstitial oxygen, as there is still a lot
of free oxygen in the sample (more than a half of
initial oxygen concentration). The saturation can
be caused by the decrease of the fraction of free
mono-vacancies at high fluence values and the
formation of multi-vacancy complexes. A-center
annealing mechanism is not a simple defect
dissociation of the vacancy and oxygen
interstitial atom, as most authors propose. The
phenomena are more complicated. At higher
temperatures A-centers are transformed to more
complicated complexes that include some vacancies
and oxygen atoms.
Fig1. Dependence of the differential absorption
coefficient ?? (irradiated in comparison with
unirradiated)on neutron fluence F in the band
maximum 12 mkm.
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Figure 2. Dependence of the differential
absorption coefficient ?? on wave number for
n-Si, irradiated with neutrons to fluences 1019
n0/cm2 (solid curve) and 4,5?1016 n0/cm2 (dotted
curve) after annealing at 6500C. The temperature
for the measurements was equal to 80K.
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Figure 3 The differential absorption spectrum of
n-Si re-irradiated with neutrons, to the same
fluence of F1019 n0/cm2 , comparing an annealed
sample at 6000C to an unannealed sample.
The preliminary irradiation and annealing in the
mentioned temperature interval does not lead to
any significant creation of oxygen-containing
defect centers the creation of oxygenradiation
defect complexes during additional irradiation is
decreased. This effect can lead to an improvement
of the radiation hardness of silicon. Similar
effects of preliminary irradiation on the
near-edge absorption value observed.
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Figure 4. Dependence of the time for 50 oxygen
precipitation on neutron fluence for Si annealed
at 10000C. Differently marked points respond to
different ingots.
The reduction in time is due to the increase of
the number of nuclear centers and, possibly,
because of radiation induced diffusion processes.
The decrease in time for oxygen precipitation in
irradiated silicon is connected mainly with the
participation of vacancies and its complexes in
the creation and stabilization of precipitate
nuclei.
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One of the possible ways to increase the
radiation hardness of silicon devices, for
example semiconductor detectors, is creating
sinks for mobile radiation defects by introducing
inner getters, which can form local mechanical
stress that effectively absorb defects without
device parameter degradation. The
radiation-thermal treatment makes the silicon
specific resistivity and oxygen precipitation
more homogeneous leading to an increased device
stability. This method can be promising for
producing radiation hard silicon for
semiconductor detectors by using
radiation-thermal treating. The material appears
to be more stable to neutron irradiation, which
is difficult to achieve by other methods. An
explanation of this fact is the formation of
sinks for mobile radiation defects as inner
getters (oxygen-silicon precipitates). This
approach in semiconductor device technology,
named defect engineering, has developed
intensely over recent years and has lead to the
increase in radiation hardness of semiconductors
materials and devices.
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Conclusion. In neutron irradiated silicon with
high oxygen concentration, A-center (VO)
concentration depends linearly on neutron fluence
up to F1018 n0 /cm2 after which the formation is
reduced due to the decrease of the free vacancy
part with additional fluence. During annealing
some of the A-centers will lead to the creation
of new oxygen-defect centers, composed of oxygen
atoms and vacancies. These centers are stable up
to 7000C. Neutron irradiation accelerates oxygen
precipitation in silicon. At low radiation
fluences growth micro-precipitates dominate as
precipitate nucleation centers, while defects
with the participation of radiation centers
dominate at high fluences. Precipitation centers
consist of vacancies and oxygen atoms. Such
precipitates can serve as sinks for mobile
primary radiation defects or as getters for
impurities. Only irradiation and thermal
treatment can introduce homogeneously distributed
sinks to the required concentration. The creation
of the sinks can lead to an increased radiation
hardness of silicon and detectors made from
it. Oxygenation and thermal treatment of silicon
can create oxygenstructure defect complexes that
can act as sinks for primarily radiation defects.
This new approach may help understanding the
observed different radiation hardness of
oxygenated silicon and detectors fabricated with
it.