Displacement Damage in Silicon
last update: 15-Aug-00
General Remarks
Numerous observations have led to the result that damage effects by energetic particles in the bulk of any material can be described as being proportional to the so called displacement damage cross section D. This quantity is equivalent to the Non Ionizing Energy Loss (NIEL) and hence the proportionality between the NIEL-value and the resulting damage effects is referred to as the NIEL-scaling hypothesis (for deviations to this rule see below). D is normally quantified in [MeVmb], whereas the NIEL-value is given in [keVcm²/g]. For silicon with A=28.086 g/mol the relation between D and NIEL is: 100MeVmb=2.144 keVcm²/g. The D or NIEL value is depending on the particle type and energy. According to an ASTM standard, the displacement damage cross section for 1 MeV neutrons is set as a normalizing value: Dn(1MeV) = 95 MeVmb. On the basis of the NIEL scaling the damage efficiency of any particle with a given kinetic energy E can then be described by the hardness factor k, defined as kparticle (E)=Dparticle/Dn(1MeVn). Therefore, instead of D(E) the normalized values D(E)/95MeVmb are listed in this compilation.
The NIEL-value can also be referred to as the displacement-KERMA, i.e. the Kinetic Energy Released to MAtter (in this case silicon). The quantity, no matter whether envisoned as D, NIEL or KERMA, is responsible for displacements of atoms in the crystal lattice. These displacements are actually causing the damage effects to be seen in the behaviour of the silicon bulk, leading to the deterioration effects of the detector properties. In contrast to the displacement kerma one has to view the pure ionization losses, monitored by theradiation dose as causing the damage effects related to the surface and interface layers of the detectors and electronics. It should be noted that these two quantities have strictly to be distinguished. They are in no way proportional to each other. Here we deal only with the bulk damage effects and hence document only the knowledge on the displacement damage cross section resp. its relative value D/95MeVmb.
From the available literature only those tabulations are selected which proved (as to the physics approach behind and also in comparison to existing experimental data) to be the most reliable ones. E.g. the old and widely used Fermi-Lab compilation provided by Van Ginneken (FN-522, 1989) had to be rejected as its results are too coarse, did not include important physical features and proved to be largely wrong at high energies. From later compilations finally the following sources had been exclusively selected as best:
neutrons:
P.J. Griffin et al., SAND92-0094 (Sandia Natl. Lab.93), priv. comm. 1996: E = 1.025E-10 - 1.995E+01 MeV
A. Konobeyev, J. Nucl. Mater. 186 (1992) 117: E = 2.000E+01 - 8.000E+02 MeV
M. Huhtinen and P.A. Aarnio, NIM A 335 (1993) 580 and priv. comm.: E = 8.050E+02 - 8.995E+03 MeV
protons:
G.P. Summers et al., IEEE NS40 (1993) 1372: E = 1.000E-03 - 2.000E+02 MeV
M. Huhtinen and P.A. Aarnio, NIM A 335 (1993) 580 and priv. comm.: E = 1.500E+01 - 9.005E+03 MeV
pions:
M. Huhtinen and P.A. Aarnio, NIM A 335 (1993) 580 and priv. comm.: E = 1.500E+01 - 9.005E+03 MeV
electrons:
G.P. Summers et al., IEEE NS40 (1993) 1372: E = 3.000E-01 - 2.000E+02 MeV
A warning about the applicability of the NIEL scaling hypothesis:
The following two examples may be taken as evidence that the NIEL scaling hypothesis should not be regarded as a universally and ideally valid rule. In fact it can be argued that the real damage depends not only on the integral NIEL value, which summarizes all existing reaction channels with their respective recoil energy distributions folded with the efficiency for producing displacements, but that it might also depend on the specifics of these energy transfers, which are not entirely described by the NIEL value. Hence one has to be cautious in applying the NIEL scaling as a strict rule. Its application is however always useful, in order to cancel out most of the particle and energy dependences of the observed damage in silicon detectors.
1. Difference between proton and neutron induced damage:
Recently it was found by the CERN-RD48 (ROSE) collaboration, that the NIEL scaling does not hold ideally for charged hadrons. In fact detecors fabricated using the ROSE developed radiation hardening by Oxygenation (DOFZ), show appreciably less damage caused by high energy protons or pions than that due to neutrons if normalized to the NIEL equivalent fluence. Although this effect is restricted to the change in the effective doping concentrational it is of course largely favorable for the applications in the inner region of the LHC tracking detectors as in that region the overall radiation field is governed by pions. The obtained improvements of the radiation tolerance are described in the latest ROSE status report CERN-LHCC-2000-009 (see ROSEstatus reports).
2. Reduced damage by electrons:
Comparing damage effects of hadrons with that produced by electrons one observes an even worse breaking down of the NIEL hypothesis. As an example, the relative hardness of 1.8 MeV electrons is according to Summers (see above) D/95MeVmb = 2.3E-02. The current related damage rate for this energy had been measured to be 4.5E-20 A/cm*) and hence the NIEL normalized value would be 2E-18 A/cm. However for neutrons, protons and pions a universal value of 8E-17 A/cm had been found under the same experimental conditions (prompt measurement at 20C). Thus the electrons of that energy appear to be less damaging than hadrons by a factor of 40. The reason for this is most likely twofold: For hadron irradiation an abundance of displacement clusters is formed via nuclear interaction leading to a high generation current whereas for electrons this part is almost negligible. In addition the Coulomb interaction, responsible solely for the energy transfer by electrons, leads to much lower Si recoil energies and such a higher ratio of close pairs is formed among the primary Frenkel defects. Recombining to a large degree, these close pairs will consequently not produce any permanent damage.
*) R. Wunstorf, PhD thesis 1992, DESY-FH1K-92-01
Forthcoming publication:
A much more detailed discussion of the whole NIEL-issue, commenting also the following tabulations and figures will be given in a forthcoming paper:
A. Vasilescu (INPE Bucharest) and G. Lindström (Univ. of Hamburg):
Fluence Normalization of Radiation Damage in Silicon Detectors.
The reader is also referred to:
A. Vasilescu (INPE Bucharest) and G. Lindström (Univ. of Hamburg):
Notes on the fluence normalisation based on the NIEL scaling hypothesis, ROSE/TN/2000-02,
to be accessed under ROSE-Technical Notes 2000 in the ROSE homepage
Links to the tabulations of the relative damage functions:
note: the .ps- resp. .pdf-files aretransscripted versions of the excel files.
neutrons.ps
neutrons.pdf / protons.xls
protons.ps
protons.pdf / pions.xls
pions.ps
pions.pdf / electrons.xls
electrons.ps
electrons.pdf
Links to ps- resp. pdf-files in picture book:
NIEL-n.pdf
(neutrons E: 10e-10 - 10e+04 MeV) / NIEL-nr.ps
NIEL-nr.pdf
(neutrons E: 10e-04 - 10e+04 MeV)
NIEL-p.ps
NIEL-p.pdf
(protons E:10e-03 -10e+04 MeV) / NIEL-pr.ps
NIEL-pr.pdf
(protons E: 10e+00 - 10e+04 MeV)
NIEL-pi.ps
NIEL-pi.pdf
(pions E: 10e+01 - 10e+04 MeV) / NIEL-e.ps
NIEL-e.pdf
(electrons E: 10e-01 - 10e+03 MeV)
NIEL-all.ps
NIEL-all.pdf
(n, p, pi, e E: 10e-10 - 10e+04 MeV) / NIEL-allr.ps
NIEL-allr.pdf
(n, p, pi, e E: 10e-04 - 10e+04 MeV)
Use of this compilation:
You are welcome to use these freely, however in any publication or report a reference should be given as follows:
A. Vasilescu (INPE Bucharest) and G. Lindstroem (University of Hamburg),
Displacement damage in silicon, on-line compilation
For any questions and/or comments please contact us via email:
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