**CERN/LHCC 2000-009**

**LEB Status Report/RD48**

**31 December 1999**

**3^RD^ RD48 STATUS REPORT**

**The ROSE Collaboration**

**(R&d On Silicon for future Experiments)**

**Co-Spokespersons:** **Francois Lemeilleur, Gunnar Lindström, Steve
Watts**

**[Member Groups:]{.underline}**

Bari University, Italy

V. Augelli, M. Angarano, D. Creanza, M. De Palma,

L. Schiavulli

Berkeley, University of California, Dept. of Materials Science, USA

E. Weber, H. Feick

Brookhaven National Laboratory, USA

Z. Li, B. Dezillie

Bucharest, Institute of Nuclear Physics and Engineering, Romania

A. Vasilescu

Bucharest, Institute of Physics and Technology of Materials, Romania

T. Botila, D. Petre, I. Pintilie, L. Pintilie, C. Tivarus

Catania University, Italy

S. Albergo, D. Boemi, R. Potenza, A. Tricomi

Centro Nacional de Microelectronica (CNM-CSIC),\
Barcelona, Spain and\
Instituto de Fisica Corpuscular, (IFIC-CISIC), Valencia

F. Campabadal, M.J. Costa, L. Fonseca, J. Fuster,\
M. Lozano, C. Martinez, J.M. Rafi

CERN, Switzerland

F. Lemeilleur, M. Letheren, M. Glaser, M. Moll,

P. Riedler, S. Roe, P. Weilhammer, K. Zankel

Demokritos, Institute of Nuclear Physics, Greece

G. Fanourakis, D. Loukas, A. Markou, I. Siotis,

S. Tzamarias, A. Vayaki

Dortmund University, Germany

R. Wunstorf, J. Wüstenfeld

Fermilab, Batavia (IL), USA

S. Kwan, D. Anderson

Firenze University, Italy\
M.Bruzzi, U. Biggeri, E.Borchi, E.Catacchini, E. Focardi, G. Parrini

Gent University, Belgium

P. Clauws

Glasgow University, U.K.

K. Smith, R. Bates, S. Manolopoulos, V. Oshea,

A. Pickford, C. Raine

Hamburg University, Germany

G. Lindström, E. Fretwurst, M. Kuhnke, M. Moll

Karlsruhe University

W. de Boer, G. Grigoriev, F.Hauler, S. Heising,

L. Jungermann

Kiev, Institute for Nuclear Research, Academy

of Sciences, Ukraine

P. Litovchenko

Lancaster University, UK

T. Sloan, T. Brodbeck, A. Chilingarov,

G. Hughes, P. Ratoff, B.K.Jones

Liverpool University, U.K.

P. Allport, G.Casse, M. Hanlon

London, Brunel University, UK

S. Watts, A. Holmes-Siedle, M. Ahmed, M. Solanky,

C. Da Via

London, Imperial College, UK

G. Hall, B. MacEvoy, A. Santocchia

London, Kings College, UK

G. Davies

Ljubljana, J. Stefan Institute, Slovenia

V. Cindro, G. Kramberger, M. Mikuz, D. Zontar

Modena University, Italy

G. Ottaviani

Montreal University, Canada\
C. Leroy, P. Roy

Munich, Max Planck Institute, Germany\
G. Lutz, R.H. Richter

Padova University, Italy\
D. Bisello, N. Bacchetta, J. Wyss

Perugia University, Italy

G.M. Bilei, P. Bartalini, P. Ciampolini, D. Passeri

Pisa, INFN, Italy.

G. Tonelli, R. Dell\'Orso, A. Messineo, P. Verdini,

R. Wheadon

Prague, Nuclear Center of Charles University

I. Wilhelm

Prague, Czech Technical University, Czech Republic

B. Sopko, S. Pospisil, V.Linhart

Prague, Institute of Physics, Academy of Sciences, Czech Republic\
V. Vrba, P. Sicho

Stockholm, Royal Institute of Technology, Sweden

B. Svensson

St Petersburg, Ioffe Physico-Technical Institute, Russia

E. Verbitskaya, V. Eremin, A. Ivanov, I. Ilyashenko

Tel Aviv University, Department of Engineering, Israel

A. Ruzin

Villigen, PSI, Switzerland

K. Gabathuler, R. Horisberger

Warsaw, Institute of Electronic Materials Technology (ITME), Poland

Z. Luczynski, E. Nossarzewska, P. Zabierowski

Warsaw, Institute of Electron Technology (ITE), Poland

M. Wegrzecki, I. Wegrzecki, W. Slysz

**[Associated Companies:]{.underline}**

Canberra Semiconductor, Belgium

P. Burger

Centro Nacional de Microelectronica (CNM-CSIC), Spain M. Lozano

CiS, Erfurt, Germany

K. Stolze

Institute of Electron Technology (ITE), Warsaw, Poland\
M. Wegrzecki, I. Wegrzecki, W. Slysz

Institute of Electronic Materials Technology

(ITME), Warsaw, Poland

Z. Luczynski, E. Nossarzewska, P. Zabierowski

Micron Semiconductor, U.K.

C. Wilburn

SINTEF, Norway

B. Sundby Avset

**[Observers:]{.underline}**

European Space Agency, ESTEC, Solar System Division, Holland

B. Johlander

IMEC, Belgium

C. Claeys, E. Simoen

Max Planck Institute, Munich, Germany

J. Kemmer, N. Meidinger

Executive Summary

This is the third report of the ROSE Collaboration. The Collaboration
has existed since October 1995 and was formally approved as RD48 by the
LHCC in June 1996. Its objectives are:

a\) To develop radiation hard detectors that can operate beyond the
limits of present devices and that ensure guaranteed operation for the
whole lifetime of the LHC experimental programme.

b\) To make recommendations to experiments on the optimum silicon for
detectors and quality control procedures required to ensure optimal
radiation tolerance.

**\
**The Collaboration has performed a systematic evaluation of the effect
of oxygen and carbon impurities on the radiation tolerance of silicon
detectors.

The key scientific results of the project are:

-   The leakage current damage parameter is material independent (no
    impurity, resistivity or conduction type dependence). It has been
    linked to defect clusters which are not affected by the material.
    Annealing of the leakage current is also material independent. The
    damage parameter and its annealing has been shown to scale ideally
    with NIEL (non ionising energy loss), i.e. without any remaining
    particle or energy dependence.

-   Effective doping changes can be improved by oxygenation of the
    material (factor 3) Such improvement is only observed when the
    radiation environment contains a significant charged particle
    component. This has been understood in terms of the production of
    larger numbers of isolated vacancy/interstitial pairs during charged
    particle irradiation.

-   Lower resistivity oxygenated material is beneficial for detectors
    that operate in a radiation environment dominated by reactor energy
    neutrons.

-   Reverse annealing has been linked to defect clusters. After proton
    irradiation, this process is found to saturate at high fluence ( 2
    10^14^p.cm^-2^ ) for oxygenated silicon. This provides a significant
    safety margin. In addition, the time constant for the process is
    found to be a factor 4 larger. This would allow detectors to remain
    at room temperature for longer periods during maintenance periods
    and thus offers a substantial safety margin.

-   Detailed correlations have been found between microscopic defect
    formation and macroscopic damage parameters. Defect kinetics models
    and device models can predict macroscopic behaviour well, even for
    hadron irradiation.

-   A macroscopic damage parameter model has been developed which can be
    used to predict detector parameters in a given radiation
    environment. This model has been used already in operational
    projections for major LHC experiments.

The key technological results of the project are:

-   Two methods were found to highly oxygenate silicon. Firstly, at the
    ingot growing stage. Secondly by diffusion of oxygen into ANY wafer
    using a high temperature drive-in (a minimum of 16 hours at 1150
    ^O^C seems to be sufficient).

-   This technology has been successfully transferred to several silicon
    detector manufacturers (SINTEF, Micron, ST, CIS) and full-scale
    microstrip detectors produced.

Results from full scale oxygenated microstrip and pixel detectors are
now becoming available. The key results so far are:

-   DOFZ (Diffusion Oxygenated Float Zone) wafers produce detectors
    which prior to irradiation are no different to those produced on
    standard material.

-   Depletion voltage curves derived from signal measurements with LHC
    speed electronics for irradiated oxygenated strip detectors look to
    be significantly lower than those on corresponding detectors
    processed on standard material. However, more data on detectors from
    a variety of suppliers is urgently required. The degree of
    improvement looks consistent with results obtained from signal
    studies on simple diode test structures. Consistency with CV derived
    depletion voltages requires further investigation.

The following work needs to be performed in the next few months:

-   The minimum diffusion time required to give radiation hardening
    needs further study. The beneficial effect that oxygen has on the
    reverse annealing process needs more work. As this effect is crucial
    to the maximum maintenance period that can be used by the
    experiments, it needs further investigation. This work is extremely
    time consuming.

-   The physics of bulk damage should be the same in full-scale
    detectors as in simple diodes. Nevertheless, bulk damage parameters
    should be extracted from irradiated strip detectors and compared to
    the well-measured parameters obtained with diodes.

-   The violation of NIEL by charged hadrons in oxygenated material
    needs further study. Testing with radiation sources that better
    represent the environment in the LHC experiments needs to be
    performed. The neutron spectrum in the LHC experiments extends to
    much higher energy than for reactor sources. There are good reasons
    to believe that oxygenated silicon will perform better than standard
    material in such a neutron environment.

In conclusion, the Collaboration has achieved its main objectives.
Diffused oxygen technology has been successfully transferred to the LHC
experiment detector groups and industry. Some further work is required
to answer the remaining questions.

1\. Introduction

This is the third status report of the RD48 (ROSE) Collaboration. The
two earlier ones of 1997 and 1998 are listed as ref. \[[^1]\] and
\[[^2]\]. Silicon pixel and microstrip detectors have been selected as
best choice for most tracking applications in the forthcoming LHC
experiments. However the required ten years of safe operability poses an
extreme challenge to their radiation hardness, predominantly due to the
hadron induced damage in the silicon bulk. The collaboration is
concentrating on these problems, performing systematic investigations of
all detector relevant effects, including the search for an improved
radiation tolerance by special defect engineering methods. The main
scientific results are outlined, of which the following two points are
of major importance. Oxygen enrichment of the silicon bulk, introduced
by RD48 and achieved within the manufacturing process, has resulted in
substantial radiation hardening of the devices. Secondly, a model
description has been developed allowing reliable predictions of the
detector performance for the LHC operational scenario. Based on these
results recommendations are outlined concerning the choice of material
and detector processing for silicon trackers. This technique has been
successfully transferred to several manufacturers and test devices for
the large LHC experiments are presently under study in close cooperation
with RD48. In addition, considerable progress has been achieved in the
understanding of the microscopic processes responsible for the
macroscopically observed effects. Finally we will survey the main open
questions which still have to be answered regarding several aspects of
the macroscopic features pertaining to detector performance.

It should be noted that the results presented in this report reflect the
combined continuous effort of all collaborating groups, working together
in the preparation and characterisation of test devices, the irradiation
experiments, and other related investigations and analysis. The RD48
groups have carried out numerous studies and a proper presentation of
all individual results is not possible within the limited scope of this
report. Therefore, the illustrations included in this report are
necessarily selected in order to show the different effects in the
clearest way. In each case references to the relevant papers, technical
notes or workshop talks are given. A complete publication list of the
collaboration (numbering about 100 papers) is in preparation and will be
available shortly on the ROSE web page \[8\].

The next RD48 workshop will be held at CERN in the beginning of March
2000, when the results given in this report will be discussed in more
detail, together with many other findings that have not been mentioned
or only been referenced briefly here. This Workshop will also be a
platform for a broader discussion with both the detector manufacturers
and the tracker groups of the large LHC experiments.

2\. The ROSE Collaboration -- Defect EnginEering.

The ROSE Collaboration currently consists of 37 international groups
working on detectors for particle physics experiments at the LHC. The
Collaboration benefits from the very valuable input of solid state
physicists and the expertise of silicon manufacturers, who are also
members of RD48. In addition the close involvement of Canberra, CNM,
Micron and SINTEF is shown through their \"Associated Company\" status.
Inputs are also acknowledged from the European Space Agency, IMEC,
Belgium and the MPI Semiconductor Laboratory in Munich all of which are
cooperating via an \"Observer\" status. Work at ITE and ITME (both RD48
members) has been vital for the rapid development, production and
processing of various materials. More recently collaboration with ITME
has resulted in high quality material characterisation. Finally the RD48
technique for Oxygen enrichment had been successfully transferred to CiS
(Germany), Micron (Great Britain), SINTEF (Norway) and
ST-Microelectronics (Italy). These manufacturers have produced dedicated
ROSE test detectors and full scale detector prototypes for LHC
experimental groups on oxygenated silicon.

The Collaboration formed following the First Workshop on Radiation
Hardening of Silicon Detectors at CERN in October 1995. The proposal
\[[^3]\] was approved by the LHCC in June 1996. The Second Workshop was
held at CERN in February 1997 \[[^4]\], the third one at DESY in
February 1998 \[[^5]\] and the fourth at CERN in December 1998 \[[^6]\]
followed by a Meeting in June 1999 \[[^7]\]. As noted above, the Fifth
Workshop will be held shortly in March 2000. A World-Wide-Web page also
provides useful information \[[^8]\].

The objectives of the collaboration are:

-   The development of radiation hard silicon detectors that can operate
    beyond the limits of present devices and that ensure guaranteed
    operation for the whole lifetime of the LHC experimental programme.

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-   The outline of recommendations to experiments on the optimum silicon
    for detectors and quality control procedures required to ensure
    optimal radiation tolerance.

The key idea is that one can improve the radiation tolerance of silicon
by defect engineering. Defect engineering involves the deliberate
addition of impurities to silicon in order to affect the formation of
electrically active defect centres and thus control the macroscopic
parameters of devices. According to current models RD48 had relied on
oxygen and carbon as the key ingredients for a possible change in the
radiation hardness. Oxygen and carbon capture silicon vacancies and
interstitials respectively. The carbon is converted from a
substitutional to an interstitial position which is mobile at room
temperature. It eventually forms stable defects with oxygen and
substitutional carbon. Migrating silicon interstitials and vacancies
escape from a region of silicon where an intense concentration of
Frenkel pairs are produced by a Primary Knock-On Atom (PKA). The PKA is
produced by the incident radiation. Vacancies can react with one another
to form multivacancy defects. This leads to clustering of intrinsic
defects especially at the end of the PKA range. This so-called
\"cluster\" region controls many of the electrical parameters of
irradiated silicon.

![](media/image1.wmf){width="5.55in" height="3.8270833333333334in"}

Fig.1: Oxygen and carbon concentrations in substrates investigated by
the ROSE Collaboration.

Various types of silicon have been investigated in the past, covering
most of the accessible phase space in Carbon and Oxygen concentration,
see Fig.1. The important role of Oxygen was first demonstrated by the
use of Czochralski (Cz) silicon, where concentrations of up to 10^18^
cm^-3^ are normal due to the growing process in a Quartz crucible.
However this material is not available in detector grade quality and the
results were hence of limited value. The use of epitaxial silicon was
also investigated. In this case O-diffusion from the Oxygen rich Cz
substrate leads to an appreciable enhancement of the O-concentration
with respect to standard FZ (Float Zone) silicon. The results obtained
with such test devices, with a thickness of up to 200 m, were
encouraging but were not pursued further because the cost of production
was not suitable for mass application. Gas doping during the FZ growing
of the silicon ingot was first applied by Polovodice in Prague but they
could not get concentrations higher than about 10^16^ cm^-3^. ITME in
Warsaw then grew highly oxygenated silicon using a gas jet technique
during the FZ process, achieving an O-concentration of up to 3 10^17^
cm^-3^. A method which results in the same enrichment but has a
substantially higher cost effectiveness was finally initiated by RD48
and carried out by ITE. The technique consists of diffusion of oxygen
from a thick oxide layer grown via a prolonged oxidation step. It should
be mentioned that this had been invented originally several years ago by
BNL but did not lead to any sizeable enhancement of the radiation
tolerance \[[^9]\].

The initial RD48 tests on neutron damage with this material were at
first also discouraging. It was only in the second half of 1998 that the
CERN group found an appreciable hardening effect both by using jet
oxygenation and O-diffusion processed detectors following proton induced
damage. Possible reasons for the difference between charged hadron and
neutron damage will be discussed later. The success demonstrated in the
1998 workshop \[Error: Reference source not found\] then led to the fast
transfer of the O-diffusion technique to various manufacturing
companies, of which SINTEF was the first one, followed later on in 1999
and likewise successful by CiS and ST-Microelectronics. Micron also used
a modified technique, using an O-implanted layer as the supply for the
succeeding diffusion throughout the bulk. Because of the success
obtained with the O-enriched detectors, other possibilities for
radiation hardening were not pursued anymore in the past year. Among
those alternatives which would be very interesting is the Sn-enrichment
of silicon. Ref. \[[^10]\] shows that tin highly suppresses divacancy
production. Both Topsil and ITME have grown Sn-doped FZ silicon
achieving a Sn-concentration of 2 to 3 10^17^ cm^-3^. A future
investigation of such material could also help in clarifying the role of
Oxygen in the hardening process.

The proper characterisation of the material before exposing the
detectors to radiation damage was vital for our investigations.
Secondary Ion Mass Spectroscopy (SIMS) has proven to be the most
reliable technique for evaluating the O-concentration profiles. In Fig.
2 an example is shown comparing the depth profile of Oxygen after
standard oxidation with the result obtained in a high temperature long
term diffusion process, performed at BNL \[[^11]\]. The homogeneity of
the \[O\]-profile is extremely flat and the concentration of 4 10^17^
cm^-3^ is by far the highest yet achieved with this technique. In this
case a dedicated Quartz tube was used, allowing for continuous tempering
at 1200C but in routine production lines the temperature is limited to
1150C. This limitation can be overcome by using a SiC tube instead and
the results obtained this way have shown to be very comparable to those
reached with a standard Quartz
oven.![](media/image2.wmf){width="3.4069444444444446in"
height="2.3534722222222224in"}

  --------------------------------------------------- -----------------------------------------
  ![](media/image3.png){width="3.314583333333333in"   ![](media/image4.png){width="3.31875in"
  height="2.216666666666667in"}                       height="2.1930555555555555in"}

  Fig.3: Oxygen concentration depth profiles.         Fig.4: Carbon concentration depth
                                                      profiles.
  --------------------------------------------------- -----------------------------------------

The results shown in Fig.3 have been obtained by a cooperation between
SINTEF and CNM. One of the key questions is how much O-enrichment is
really necessary and how can that be obtained. A short diffusion of not
more than 16 hours at 1150C (\[O\] = 1.5 10^17^ cm^-3^) may already be
enough for the required radiation hardening effect. This point of
optimisation is under further study. In contrast to the improvement
gained by O-enrichment, an increased Carbon-concentration does have an
adverse effect. Hence it is reassuring that diffusion in a SiC tube does
not enhance the C-contribution in the silicon bulk. While TCA (a Carbon
containing compound) is used in helping the normal oxidation process it
must be strictly avoided during the O-diffusion process - see Fig.4
(same source as for Fig.3). The best process now in wide use is a
standard oxidation followed by diffusion in a Nitrogen atmosphere.
Another point of worry is whether the RD48 oxygenation process changes
the resistivity of the bulk material or has any other adverse effect. A
resistivity profile before and after oxygenation is shown in Fig.5,
contributed by ITE. No appreciable change of the resistivity or its
homogeneity throughout the depth of the wafer is detected. Finally
detectors produced by this method show a similarly low bulk current as
those following standard processes. This has been convincingly
demonstrated e.g. by SINTEF where the I/V characteristics have shown
suitable high voltage stability and a current at room temperature of
about 5 nAcm^-3^ at 4 times the depletion voltage!
![](media/image5.png){width="3.7708333333333335in"
height="5.145833333333333in"}

All materials used for the experiments covered in this report were
standard FZ silicon of both \<111\> as well as \<100\> orientation and
resistivity between 1 and 15 kcm, supplied by Polovodice, Topsil and
Wacker. Different enrichment processes have been used, either during the
FZ growing or the detector processing stage. Prior to irradiation the
detectors have fully been characterised as to their resistivity and as
to the O- and C-concentration.

**3. MACROSCOPIC EFFECTS --BASIC FEATURES**

This section provides a brief overview of radiation effects in silicon
detectors to make the document self contained and provide a proper
reference for the results given in the later sections.

Three main macroscopic effects are seen in high-resistivity diodes
following energetic hadron irradiation, these are:

-   Change of the doping concentration with severe consequences for the
    operating voltage needed for total depletion.

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-   Fluence proportional increase in the leakage current, caused by
    creation of recombination/generation centres

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-   Deterioration of charge collection efficiency due to charge carrier
    trapping leading eventually to a reduction in the signal height
    produced by mip's.

The second and third effect have consequences to the S/N ratio for
mip\'s. However trapping has been found to be tolerable and the reverse
current can be largely reduced by operating the detectors at a
moderately reduced temperature of about -10C. The first effect is the
most severe, as the operating voltage cannot be increased to very high
values. This limitation is predominantly set by the diode processing,
but note that a large operating voltage together with the increased
reverse current leads also to an increase of dissipation power which may
affect the cooling system, and if distributed nonhomogenously could also
lead to thermal run away effects. Details of these basic damage produced
deterioration effects have been described in \[Error: Reference source
not found\]. However the following important points should be repeated.

[NIEL-scaling and irradiation sources]{.underline}

Detectors to be used in the LHC trackers will have a thickness of
between 250 and 300 m and they are processed on 1 to 5 kcm mostly n-type
silicon wafers. The radiation effects to be encountered depend in
principal on the particle type and energy. However, macroscopic changes
normally scale with the Non Ionising Energy Loss (NIEL). For the leakage
current increase this scaling has recently been verified using different
kinds of silicon material as will be shown later. Exceptions from this
rule have been found when comparing the effect that neutron and charged
hadron damage have on the effective doping concentration. However, in
order to get a good perception on what could be expected from different
particles and energies the folding of a given energy distribution with
the relevant NIEL function gives quite reliable results. The differences
observed from charged or neutral hadrons are then projected onto the
parameters describing the radiation effects, see later. A compilation of
recommended NIEL values for different particles in the whole relevant
energy range can be found in \[[^12]\]. It is common practice to
represent the intensity of any hadron irradiation by its equivalent
fluence of 1 MeV neutrons. All results in this report are given as
function of these *~eq~*-values. The different sources are then
characterised by a so called hardness factor , describing the damage
efficiency of a particular source. The hardness factor is defined by
* = ~eq~/~tot~* with *~tot~* being the total measured particle fluence.
For the inner detector tracking regions pions and neutrons are the most
relevant hadrons. Consequently RD48 test-experiments have been performed
with monoenergetic (PTB Braunschweig) and reactor neutrons (energy
ranging to about 10 MeV, TRIGA type reactor at Josef Stefan Institute
Ljubljana), 200 MeV pions (PSI Villigen) and 24GeV/c protons (CERN PS).
The proton irradiation facility at CERN \[[^13]\] has been widely used
to represent the effects of charged hadrons in comparison to neutrons.

[Effective Doping Concentration and Depletion Voltage]{.underline}

Under reverse bias the electric field in a silicon diode extends from
the p^+^n junction through the n-type silicon bulk and the total field
depth (depletion depth) increases with increasing voltage. The bias
value causing the electric field to finally reach through the total
depth of the diode is called the depletion voltage *V~dep~*. This value
depends on the diode thickness *d* and the effective doping
concentration *N~eff~* in the following way:

\(1\)
$V_{\text{dep}} = \frac{q_{0}}{2\text{εε}_{0}}\left| N_{\text{eff}} \right|d^{2}$

This equation holds not only for the original n-type silicon with
*N~eff~* governed by an abundance of donors but also after severe
irradiation when the effective doping concentration changes its sign,
due both to a removal of donors and the increasing generation of
acceptor like defects. In any case \|*N~eff~*\| = \|*N~d~-N~a~*\|, with
*N~d~* being the donor-concentration and *N~a~* that of the acceptors.
Especially for the original material it is useful to remember the
relation between the specific resistivity , the doping concentration
*N~eff~* and the depletion voltage *V~dep~*.:

\(2\) $N_{\text{eff}} = \left( q_{0}\mu_{n,p}\rho_{n,p} \right)^{- 1}$;
$V_{\text{dep}} = \frac{d^{2}}{2\text{εε}_{0}\mu_{n,p}\rho_{n,p}}$

Here *~n,p~* denotes the mobility of electrons and holes in *n* or
*p-type* material.

In addition to an instantaneous change in the effective doping
concentration (both donor removal and acceptor generation) the silicon
bulk exhibits long term annealing effects after irradiation which are
beneficial during a short time period but are adverse for very long
times. This latter so called reverse annealing has become a point of
much concern as it finally limits the operability of the detectors. The
long term behaviour depends however, due to the underlying defect
kinetics, very strongly on the detector temperature. In fact, the
reverse annealing has shown to be efficiently frozen at temperatures
below *-10C*. Hence the LHC experiments will keep their detectors cold
during the beam periods for the sake of a reduced reverse current but
also for most of the remaining time without beam, in order to reduce the
overall reverse annealing effect. Only very short warm up periods of
around 2 weeks are foreseen for maintenance each year. A summary of the
complex fluence and time dependence of the effective doping
concentration is shown schematically in Fig.6.

[Reverse current]{.underline}

The reverse current increase exhibits far less complexity than that
observed for the depletion voltage. It is entirely due to the generation
of electron/hole-pairs in the silicon bulk. All detectors have guard
ring structures in order to guarantee a defined value of the sensitive
volume *V* (junction area times detector thickness)*.* Provided a guard
ring is used, the measured current *I* is strictly proportional to the
volume of the bulk and it is also proportional to the equivalent
irradiation fluence that the detector has received. One therefore
defines a *current related damage rate* by:

\(3\) $I = \alpha \cdot \Phi_{\text{eq}} \cdot V$

The damage induced bulk current decreases strongly with lower operating
temperature according to:

\(4\)
$I(T) \propto T^{2}\text{exp}\left( - \frac{E_{g}}{2k_{B}T} \right)$

where *T* is the operating temperature, *E~g~* the band gap energy of
*1.12 eV* and *k~B~* is the Boltzman constant. Operating the detectors
at e.g. *-10C* reduces the current with respect to its room temperature
value by a factor of *20*. Normally the values for the current related
damage rate are given for *20C* using equation (4). These values can
then easily be transferred to any given operating temperature. The
damage induced bulk current undergoes also a temperature depending
beneficial annealing effect, its universal functional dependence will be
shown in section 4.

[Charge collection]{.underline}

Bulk damage induces trapping of charge carriers which leads to a
deterioration of the charge collection efficiency and thus a reduction
in the signal height for mip's. The prime condition for achieving a
reasonable charge collection is to establish a sizeable electric field
strength throughout the detector. To only apply a bias voltage necessary
for total depletion is certainly not enough. In this situation the
electric field decreases from its maximum value at the junction side to
practically zero level at the opposite electrode. Therefore charge
carriers reach zero velocity at that point and consequently the charge
collection does not have a finite duration. The trapping probability of
any charge carrier during its drift through the detector is proportional
to the ratio of the charge collection time *t~c~* and the trapping time
constant . Hence a large value of *t~c~/* must be avoided. For severe
damage in the order of 10^14^ -10^15^ cm^-2^ an appreciable overbias may
be needed to ensure optimum conditions. Illustrations will be shown in
section 4.

**4. MACROSCOPIC EFFECTS -- LATEST RESULTS**

[Current related damage rate]{.underline}

As stated in the preceding section, the damage induced increase of the
reverse current appears to be of less complexity than those effects
responsible for the change in the depletion voltage. In fact, a careful
measurement of the current related damage rate has shown that its value
does not depend on any material property so far studied, no matter
whether it is *n-* or *p-type* silicon. Likewise it is independent of
the resistivity of the silicon and most important of all it is identical
for *neutron,* *proton* and *pion* induced damage and shows no variation
with fluence in a range between *10^11^ - 10^15^* *cm^-2^*. This is
nicely documented in Fig.7, where the exact fluence proportionality of
the volume related current increase at a specific annealing stage is
displayed. In fact this behaviour has then led to use of the measured
increase of the reverse current for an exact determination of the *1 MeV
neutron equivalent fluence* to which the detectors have been irradiated
\[[^14]\]. Equally impressive is the existence of a universal annealing
function, again not dependent on material properties, particle type and
energy as well as the fluence used for irradiation. The curve
representing this function is given in Fig.8 in an example for an
annealing temperature of *60C*. Using the measured parameters governing
the temperature dependence, given in \[Error: Reference source not
found\], one can easily derive the annealing behaviour at any given
temperature. It should be emphasised that in contrast to other damage
effects, the functional dependence of on the annealing duration is
exactly the same in Oxygen-enriched and standard material. The method of
deriving the equivalent fluence from the measured current increase is
therefore considered to be very reliable. Measurements were normally
done after 80 minutes annealing at 60 C resulting in a value of

\(5\)
$\alpha_{\frac{\text{80}}{\text{60}}} = \left( 3\text{.}\text{99} \pm 0\text{.}\text{03} \right) \cdot \text{10}^{- \text{17}}\text{Acm}^{- 1}$

Using this method for defining the equivalent fluence for a given
particle source and comparing it with the otherwise measured total
fluence (e.g. by activation analysis of irradiated Al as in case of a
high energetic proton beam or Au as for energetic neutrons) hardness
factors have been established, see Table 1.

  ---------------------------------------------------- ---------------------------------------
  ![](media/image6.wmf){width="3.3069444444444445in"   ![](media/image7.wmf){width="3.325in"
  height="2.3979166666666667in"}                       height="2.325in"}

  Fig. 7.: Fluence dependence of leakage current for   Fig.8: Current related damage rate as
  detectors produced by various process technologies   function of cumulated annealing time at
  from different silicon materials. The current was    60C. Comparison between data obtained
  measured after a heat treatment for 80 min at 60C    for oxygen diffused silicon and
  \[Error: Reference source not found\].               parameterisation given in Ref. \[Error:
                                                       Reference source not found\].
  ---------------------------------------------------- ---------------------------------------

Table 1.: Hardness factors for various sources as determined by the
value and expected from NIEL calculations \[Error: Reference source not
found,Error: Reference source not found, [^15]\].

  ----------------------- ----------------------- -----------------------
  Irradiation source      Hardness factor\        Hardness factor\
                          from value              theoretical expectation
                                                  (NIEL)

  PTB Be(d,n) \<En\> =    (used as reference)     1.45
  5.2 MeV                                         

  CERN PS 24 GeV/c        0.51 0.01               0.5
  protons                                         

  Ljubljana reactor       0.90 0.05               0.91 0.05
  neutrons                                        

  UKE ^3^H(d,n) 14.1 MeV  1.7                     1.80
  n                                               

  ^60^Co-gamma            1.910^-6^               6.410^-3^
  ----------------------- ----------------------- -----------------------

[CERN-scenario measurements for rapid comparisons]{.underline}

As outlined in section 3, the damage induces an instantaneous change of
the effective doping concentration as determined from the depletion
voltage. The effective doping concentration undergoes a short term
beneficial annealing and subsequently a reverse annealing effect on a
much longer time scale. Measurements taken immediately after irradiation
could be misleading because longer irradiation times would also include
some self-annealing and thus the results would depend both on fluence
and irradiation time. A much better method consists of consecutive
irradiation steps with intermittent short term annealing of *4 minutes*
*at 80C* and measurement thereafter. This scenario, introduced by the
CERN group of RD48 \[[^16]\] has been found to be extremely useful,
since at this stage the change in the effective doping concentration
reaches a flat minimum and thus the long term reverse annealing affects
the results only slightly. In addition such an annealing is
approximately equivalent to 2 weeks at room temperature, a storage time
which is foreseen as the yearly maintenance period e.g. in the
ATLAS-SCT. The method allows measurements of important damage features
as function of fluence almost simultaneously with the ongoing
irradiation steps and a comparison between many different diodes can
thus be obtained rapidly. At low fluences the change in the effective
doping concentration is dominated by donor removal thus leading to a
decrease in the full depletion voltage. Once the donor concentration is
exhausted or compensated by acceptors, *N~eff~* reaches a minimum, at
which the conduction type of the material changes from n- to p-type. The
introduction rate of negative space charge beyond the inversion point,
resulting from the generation of deep acceptors, is called the -value,
defined as the slope of a linear line fit to the data. Fig.9 shows the
results of such measurements for standard and Oxygen enriched diodes
after irradiation with *neutrons, pions* and *protons*. It can be
clearly seen that the oxygen rich material reveals a much lower -value
for *pion* and *proton* irradiation than observed for standard silicon.
On the other hand for *neutron* induced damage such an improvement is
not visible. Reasons for this different behaviour will be given later.
Fig.10 displays data obtained by the same method but compares
*standard*, *Carbon-* and *Oxygen-rich* samples, exposed to *proton*
irradiation. The improvement of the *O-rich* diodes with respect to
standard ones is *~st~/~\[O\]~ = 3.5*, while *Carbon-rich* samples show
an appreciable worsening by a factor of *3*! It can also be seen from
Fig.10, that different diffusion times used in this case for oxygenation
do not affect the -improvement. Two other important features have also
been revealed by the CERN-scenario measurements.

Fig.11 and 12 show the results with *O-rich* diodes using silicon of
different resistivity. In Fig.11, displaying the proton damage, one
deduces that the effective doping concentration reached at high fluences
well above inversion point, is independent of the initial resistivity.
In terms of our present understanding this means that the removal effect
has exhausted the initial donor concentration completely and hence the
space charge at larger fluences is only governed by the generation of
acceptors. However for neutron irradiation, as shown in Fig.12, the
situation is different. At least part of the difference in the initial
doping concentration present before irradiation remains available even
at a fluence of *210^14^ cm^-2^*. Thus the increase of the acceptor
concentration is partly compensated due to this incomplete donor
removal. The consequence for the choice of material is then: while using
silicon with different resistivity for detectors does not have any
effect on the needed depletion voltage at larger fluences following
proton irradiation one gets an appreciable beneficial effect for neutron
induced damage by choosing O-rich low resistivity material. As an
example, *1 kcm n-type* silicon would result in an initial depletion
voltage of *290 V* for a *300 m* thick diode, whereas for *15 kcm* one
would need only *20 V*. For a donor removal of only *30%* as to be
deduced from Fig.12, *70%* of the initial difference in depletion
voltage, i.e. *190 V* less voltage would be needed for the *1 kcm*
Oxygen-enriched diode after high fluence.

![](media/image8.wmf){width="6.378472222222222in"
height="3.9270833333333335in"}

Figure 9: Dependence of N~eff~ on the accumulated 1 MeV neutron
equivalent fluence for standard and oxygen enriched FZ silicon
irradiated with reactor neutrons (Ljubljana), 23 GeV protons (CERN PS)
and 192 MeV pions (PSI).

![](media/image9.wmf){width="6.311805555555556in"
height="4.403472222222222in"}

Figure 10: Effective space charge density and full depletion voltage
versus proton fluence for standard, carbon-enriched and three types of
oxygen diffused samples: 24, 48 and 72 hour diffusion at 1150C
\[[^17]\].

  ----------------------------------------------------- ----------------------------------------------------
  ![](media/image10.wmf){width="3.2944444444444443in"   ![](media/image11.wmf){width="3.234027777777778in"
  height="2.1743055555555557in"}                        height="2.126388888888889in"}

  Fig. 11.: 24 GeV/c proton irradiation of O-rich       Fig.12: Reactor neutron irradiation of O-rich diodes
  diodes with different resistivity.                    with different resistivity.
  ----------------------------------------------------- ----------------------------------------------------

[Full annealing cycles and complete modelling]{.underline}

As stated above, the CERN-scenario measurements have proven to be
extremely useful for a fast damage evaluation of different material.
Only this way was it possible to select the most promising defect
engineering process resulting in an appreciable improvement of radiation
hardness. However these measurements allow one to measure only one of
the relevant parameters, namely the effective doping concentration at or
around the minimum of the annealing function. A different approach,
introduced by the Hamburg group \[[^18]\], uses a set of diodes
processed from the same material and individually irradiated by
different fluences. Each diode then undergoes a full annealing cycle.
This is a time consuming effort but allows the study of all components
in the change of the effective doping concentration in the most
systematic way. An example of the whole complex behaviour is given in
Fig.13. *N~eff~* is the damage induced change in the effective doping
concentration with respect to its initial value before irradiation.
![](media/image12.wmf){width="3.917361111111111in"
height="2.4590277777777776in"}

\(6\)
${\Delta N}_{\text{eff}}\left( \Phi_{\text{eq}},t\left( T_{a} \right) \right) = N_{\text{eff},0} - N_{\text{eff}}\left( \Phi_{\text{eq}},t\left( T_{a} \right) \right)$

As function of time and fluence *N~eff~* can be described as:

\(7\)
${\Delta N}_{\text{eff}}\left( \Phi_{\text{eq}},t\left( T_{a} \right) \right) = N_{A}\left( \Phi_{\text{eq}},t\left( T_{a} \right) \right) + N_{C}\left( \Phi_{\text{eq}} \right) + N_{Y}\left( \Phi_{\text{eq}},t\left( T_{a} \right) \right)$

In this equation it has been explicitly denoted that the time dependence
is in itself subject to the annealing temperature *T~a~*. As indicated
in Fig.13 and obvious from eq.(7) *N~eff~* consists of three components,
a short term beneficial annealing *N~A~*, a stable damage part *N~C~*
and the reverse annealing component *N~Y~*. *N~C~* can be described by
an incomplete donor removal, depending exponentially on the fluence with
a final value *N~C0~* and a fluence proportional introduction of stable
acceptors with an introduction rate *g~C~*:

(8) $$N_{C}\left( \Phi_{\text{eq}} \right) = N_{C0}\left( 1 - \text{exp}\left( - {c\Phi}_{\text{eq}} \right) \right) + g_{C}\Phi_{\text{eq}}$$

$$$$As can be seen from Fig.13, disregarding the still present component
from the beneficial annealing and a starting reverse annealing part,
*N~C~* should obey a very similar dependence on *~eq~* as exhibited in
the CERN scenario experiments and hence there is a close relation
between of those measurements and the introduction rate *g~C~* given
here. Finally the reverse annealing component appears to be best
described by a 2^nd^ order process with amplitude *N~Y~* = *g~Y~* *~eq~*
and a dependence on the annealing time according to *\[1- 1/(1 +
t/~Y~)\]*. It should be noted that this function is quite identical to a
1^st^ order approach according to *\[1 -- exp(-t/~Y~)\]* for moderately
short annealing times but differs appreciably in approaching the
saturation value. It has also been shown that the temperature dependence
of the time constants involved in the annealing processes are strictly
governed by activation energies and having measured these values one can
easily transfer the annealing function obtained at high temperature (for
the sake of acceleration of the measurement) to any given value. With
respect to a room temperature measurement this acceleration factor for
the reverse annealing is e.g. 550 for a 60C and 7400 for an annealing at
80 C.

How well this model describes the real data obtained for a large set of
different fluences and annealing times spanning 3 orders of magnitude,
may be demonstrated in Fig.14. Each full line is the result of a fit
employing the functional dependence as described above. Using such
evaluations the different damage parameters, i.e. *N~A~, ~A~, N~C~,*
*N~Y~* and *~Y~* can be derived.

![](media/image13.wmf){width="4.460416666666666in" height="2.875in"}

Fig. 14: Annealing of oxygen enriched diodes at 60C after irradiation
with 24 GeV/c protons

The results of such analysis for neutron irradiated Oxygen enriched
diodes are given in Fig.15 for the beneficial annealing, Fig.16 for the
stable damage component and Fig.17 for the reverse annealing. As stated
above the introduction rate for stable acceptors *g~C~*, analysed this
way and found to be *2 10^‑2^ cm^-2^* (Fig.16) is indeed very close to
the *-value* measured in the CERN scenario experiment of Fig.13. The
nice fluence proportionality of the amplitudes for the beneficial and
reverse annealing and their independence of the annealing temperature as
seen in Fig.15 and Fig.17 plus the good fit in Fig.16 according to
eq.(8) are certainly a convincing demonstration of the quality of the
model description.

  ----------------------------------------------------- -----------------------------------------------------
  ![](media/image14.wmf){width="3.2666666666666666in"   ![](media/image15.wmf){width="3.2666666666666666in"
  height="3.248611111111111in"}                         height="3.248611111111111in"}

  Fig.15: Beneficial annealing *N~A\ ~*for neutron      Fig.16: Stable Damage component *N~C~* for neutron
  irradiated oxygen enriched silicon.                   irradiated oxygen enriched silicon.
  ----------------------------------------------------- -----------------------------------------------------

![](media/image16.wmf){width="3.3986111111111112in"
height="3.283333333333333in"}

In Figs.18 to 20 the proton damage results for Oxygen enriched diodes
are given and compared to those obtained with standard material. Again,
as was clear already from the *-values* extracted from the CERN
scenario measurements (Fig.10), we see a very pronounced improvement in
the introduction rate of stable acceptors. With respect to standard
material the oxygen enriched silicon is radiation harder by a factor of
*g~C~(standard)/g~C~(oxygen.) = 3.5* (Fig.18). In addition we have
observed the really surprising effect, that in contrast to standard
silicon, the reverse annealing amplitude is not proportional to the
fluence anymore in oxygenated silicon but shows a significant saturation
effect, as shown in Fig.19. In fact due to the steep increase of that
amplitude for standard material the largest fluence for which the full
annealing cycle could be measured was *~eq~=3.5 10^14^ cm^-2^* whereas
the Oxygenated silicon measurements could be extended to almost double
that value. The observed reduction in *N~Y~* is at least a factor of 2,
and would even increase at larger fluence. It was likewise surprising
that in addition to this effect, the time constant for the reverse
annealing process is increasing, and depends on the Oxygen
concentration. It shows an improvement by a factor of 4 with respect to
standard diodes (Fig.20). Both these effects, the reduction of the
reverse annealing amplitude and the much longer time period before the
final value will be reached are regarded to be extremely important for
the LHC applications. As the reverse annealing is mainly encountered
during the warm up periods necessary every year, this behaviour offers a
much welcome safety margin for operation.

  ----------------------------------------------------- -----------------------------------------------------
  ![](media/image17.wmf){width="3.1395833333333334in"   ![](media/image18.wmf){width="3.0131944444444443in"
  height="3.3875in"}                                    height="3.372916666666667in"}

  Fig.18: Damage parameters *N~C~* for oxygenated and   Fig.19: Damage parameters *N~Y~* for oxygenated and
  standard material as obtained from annealing          standard material as obtained from annealing
  experiments at 60C after irradiation with 24 GeV/c    experiments at 60C after irradiation with 24 GeV/c
  protons.                                              protons.
  ----------------------------------------------------- -----------------------------------------------------

![](media/image19.wmf){width="5.129861111111111in"
height="3.535416666666667in"}

Fig.20: Reverse annealing for standard and oxygen enriched diodes after
irradiation with 24 GeV/c protons (~eq~ = 2 10^14^cm^-2^). The time
constant is increased for oxygen enriched diodes.

[Charge collection measurements]{.underline}

The LHC experiments are naturally less concerned with the depletion
voltage measured by normal C/V techniques which gives the effective
doping concentration but concentrate on the operating voltage needed to
guarantee a good S/N ratio for the detection of mip's. Hence the prime
objective is to assure a given charge collection efficiency. Due to
trapping and other effects, the bias voltage needed for this might in
fact be appreciably larger than the depletion voltage deduced from C/V
measurements or from the model. A proper comparison between results
obtained from the C/V method and those extracted directly from charge
collection measurements is therefore required. Such experiments can be
performed with a beta-source which provides real minimum ionising
particles or using a long wavelength IR-laser light with an ultrashort
pulse duration. The laser source simulates a mip like ionisation in the
diode. Both techniques are extremely time consuming in contrast to the
C/V-method. Hence they have so far only been performed on selected
samples as a consistency check. Fig. 21 gives an example for a real
ATLAS prototype strip detectors processed by MICRON irradiated with 3
10^14^ p/cm^2^ (the standard ATLAS test fluence) and annealed at 80C for
4 minutes \[[^19]\].

The following two results are evident: The oxygenated detector shows an
improved charge collection as function of bias. 85% of the full charge
collection, as measured with the Ru-beta-source is already reached at
150 V whereas 225 V are needed for the non oxygenated one. This is in
quite good agreement with expectations from C/V measurements. In
addition, the measurements performed with the Ru-source agree very well
with those using the pulsed laser, which are much easier to perform.
Fig.22a and b show examples for two different test diodes (oxygenated
and standard) irradiated with 4 and 8 10^14^ p/cm^2^ respectively and
annealed at 80C for 4 minutes. The agreement between the charge
collection results and those obtained by C/V measurements are quite
satisfactory. As a matter of fact the bias voltage needed for 85% charge
collection efficiency is in both examples very close to the depletion
voltage, the remaining 15% of the signal height is attributed to
trapping. A complementary presentation of these results is shown in
Fig.23, where the charge collection inefficiency is plotted versus the
inverse bias voltage for values above the depletion voltage. It nicely
demonstrates that test diodes and strip detectors behave almost
identically while there is a substantial improvement to be seen for
oxygenated silicon against the use of standard material. Another example
of comparison between the results obtained from C/V measurements and
those from charge collection is shown in Fig.24. Here the test diodes
have been completely annealed after irradiation. In this case the
illumination is made from the p^+^-side with a short wavelength laser of
only 670 nm. The carriers are generated in a very shallow layer of only
a few microns due to the very short absorption length of this light. As
the material is type inverted at these fluences, the electric field
starts to grow from the rear side and reaches the p^+^-electrode only at
full depletion. Hence measuring the charge collection as function of
bias provides a very sensitive tool for the measurement of the depletion
voltage. The almost complete agreement between the two measurements
supports the results for the saturation of the reverse annealing
displayed in Fig.19 very nicely. The *N~eff~-values* extracted from the
data of Fig.24 suggests an even larger saturation effect than that shown
in Fig.19.

Finally measurements on charge collection using an oxygenated detector
have been performed in a strong magnetic field \[[^20]\]. Examples of
these results are presented in Fig. 25. Also in this case a short
wavelength IR-laser ( = 850nm) ensuring a shallow absorption was used
and the diode was illuminated from the p^+^ side for measurement of the
electron contribution and from the rear one for the holes. The full
depletion voltage after 7 10^13^ p/cm^2^ was of the order of 100 V and
the material was type inverted. As the holes are thus generated in an
area of high electric field and the electrons at low field strength, the
difference in their behaviour as function of bias voltage is
understandable. It is reassuring that the reduction in signal height
observed due to a magnetic field of 4T is not appreciable. This effect
is a consequence of the Lorentz angle resulting in larger drift lengths
and hence increased trapping.

![](media/image20.jpeg){width="3.3097222222222222in"
height="3.182638888888889in"}

Fig. 21 CCE of ATLAS prototype strip detectors processed by MICRON and
irradiated with 3 10^14^ p/cm^2^ \[19\].

![](media/image21.jpeg){width="3.296527777777778in"
height="3.196527777777778in"}

Fig.22a: CCE (1060 nm laser) of standard and oxygenated diodes
irradiated with 4 10^14^ p/cm^2^ \[19\].

![](media/image22.jpeg){width="3.3097222222222222in"
height="3.011111111111111in"}

Fig.22b: CCE (1060 nm laser) of standard and oxygenated diodes
irradiated with 8 10^14^ p/cm^2^ \[19\].

![](media/image23.wmf){width="3.5694444444444446in"
height="2.759027777777778in"}

Fig.23: Charge collection inefficiency as function of bias voltage above
full depletion. Data from Figs. 21 and 22a.

  ----------------------------------------- -----------------------------------
  ![](media/image24.wmf){width="3.2875in"   ![](media/image25.wmf)
  height="2.379861111111111in"}             

  Fig.24: 24 GeV/c proton irradiated        Fig.25: CCE (850 nm laser, 25ns
  oxygenated silicon detectors (2, 4 and    integration) of proton irradiated
  610^14^ p/cm^2^). Comparison of depletion oxygenated diodes with and without
  voltages determined by CV (10 kHz) and    magnetic field \[Error: Reference
  TCT (front illumination, 670nm, 40ns      source not found\].
  integration).                             
  ----------------------------------------- -----------------------------------

Table 2.: Damage parameters for oxygen enriched and standard silicon.

  --------------- -------------- -------------- ----------------- --------------
                  Standard                      Oxygen-enriched   
                  Silicon                       Silicon           

                  Neutrons       Protons        Neutrons          Protons

  g~a~            1.810^-2^      \-             1.410^-2^ cm^-1^  \-
                  cm^-1^                                          

  ~a~(20C)        55 h           \-             70 h              \-

  g~C~            1.510^-2^      1.910^-2^      2.010^-2^ cm^-1^  5.310^-3^
                  cm^-1^         cm^-1^                           cm^-1^

  N~C0~/N~eff0~   0.70           \-             0.45              1.0

  g~Y~            5.210^-2^      6.610^-2^      4.810^-2^ cm^-1^  2.310^-2^
                  cm^-1^         cm^-1^                           cm^-1\ ()^

  ~Y~(20C)        480 d          \-             800 d             950 d
  --------------- -------------- -------------- ----------------- --------------

^()^ saturation value measured for ~eq~ = 6 10^14^cm^-2^

[Parameters of the Hamburg model and application to the LHC]{.underline}

Once a set of reliable parameters has been extracted from the various
damage test experiments it is then possible to apply the model
description outlined above to the operational scenario of the LHC
experiments. It should be emphasised here that the conservative values
listed in Table 2 are still subject to some uncertainties. In contrast
to the behaviour of the current related damage parameter , which has
been shown to be completely independent of material properties and
different processing techniques, variations for the parameters
describing the change of the effective impurity concentration are in the
order of 50%. They are certainly much smaller than the really
outstanding improvements observed with Oxygenated silicon in comparison
to standard material. The main results using O-rich material in
comparison to standard silicon are:

### Proton irradiation

-   The introduction rate for stable acceptors *g~C~* is a factor of 3.5
    lower.

-   The reverse annealing effect is considerably reduced both due to a
    saturation at high fluences (resulting in at least a factor of 2)
    and the much longer time constant (by a factor of 4).

-   There is no effect from the initial resistivity of the material (due
    to complete donor removal)

-   The results obtained with C/V measurements have been nicely
    reproduced with those obtained from charge collection.

### Neutron irradiation

-   The introduction rate for stable acceptors appears to be the same as
    for standard silicon.

-   The reverse annealing is not affected by Oxygenation.

-   There is an appreciable beneficial effect using O-rich low
    resistivity silicon

It should be noted that operational projections for the behaviour of the
silicon detectors to be incorporated in both the ATLAS, CMS and LHCb
have already been carried out on the basis of the Hamburg model. An
update using the new parameters obtained for the Oxygenated silicon
detectors is shown in Fig. 26a and b. As an example, the two pixel
layers of the ATLAS detector are chosen, where the pion irradiation is
most severe. The ATLAS B-layer is subject to a total fluence of *~eq~
=3 10^15^* *cm^-2^* accumulated within 10 years of LHC operation.
Simulation results are shown in Fig.26a. Here the pixel-detectors have a
thickness of *200m* and will be operated at a maximum bias voltage of
*600 V*. The depletion voltage will reach this limit already after 4
years of operation, using standard silicon detectors, whilst the
application of Oxygen enriched silicon would extend that time to 6
years. However, the ATLAS pixel group envisages to operate the detectors
in partially depleted mode, requiring only a signal height of *6000
e/mip*. Using this reduced requirement the operational lifetime for
standard silicon would be extended to *6 years* while Oxygen enriched
detectors could be almost operated for the full *10 year* period!

In Fig.26b we display the results for the 1^st^ layer at *10 cm* again
in the ATLAS pixel subdetector. Here the calculations have included the
mixed field (approximately 70% pions, 30% neutrons), which account for a
total fluence of *eq = 7 10^14^ cm^-2^*. In this case the detectors will
have a thickness of *250m* and total depletion is required at a maximum
voltage of *600V*. This limit is reached after *8 years* for standard
silicon while Oxygen rich silicon is easily operable during the full LHC
period. From both examples it is obvious that the use of the radiation
hardening process offered by the RD48 results, provides an appreciable
improvement for safe operation of silicon detectors in the inner
detectors of the large LHC experiments. It should also be emphasised at
this point that the operational scenario for e.g. the ATLAS detector
requires a warm up period of only 2 weeks every year. The results of
Fig.26a and b were based on the assumption that this requirement can
really be fulfilled during 10 years! As longer warm up periods or
accidents would result in a much larger contribution from the reverse
annealing effect, the appreciable improvements obtained by Oxygenated
silicon in this respect (see above) provide an additional invaluable
safety margin.

  ----------------------------------------------------- ----------------------------------------------------
  ![](media/image26.wmf){width="3.3097222222222222in"   ![](media/image27.wmf){width="3.222916666666667in"
  height="2.0805555555555557in"}                        height="2.1145833333333335in"}

  Fig.26a.: Damage projection for the ATLAS B-layer     Fig.26b.: Damage projection for the ATLAS
  demonstrating the difference between standard and     1^st^-layer demonstrating the difference between
  oxygenated silicon.                                   standard and oxygenated silicon.
  ----------------------------------------------------- ----------------------------------------------------

**5. EFFECTS AT THE Si/SiO~2~ INTERFACE**

In addition to standard test diodes intended for investigations of bulk
damage effects, a set of gate-controlled diodes were included in the
ROSE mask layout \[[^21]\]. These structures are perfectly suited to the
analysis of the oxide quality, as both oxide charge and the e-h pair
generation process at the interface can be determined independently.
Ionising radiation is know to lead to an increasing oxide charge and
interface state density, which results in corresponding changes in the
depletion depth and the leakage current of strip detectors. The
as-processed quality of the interface as well as its sensitivity to
radiation are strongly dependent on the details of the device
processing. It is therefore very important to doublecheck, how the use
of oxygen-enriched and lower resistivity material affect the interface
properties.

Fig. 27 displays two measurements of the interface generation current on
oxygen-enriched samples processed with different processing parameters.
The interface generation current is equal to the height of the current
step at the flat-band voltage of about 2.5 V. Comparing the values with
non-oxygenated silicon, a similar interface quality is found for the
as-processed devices.

![](media/image28.png){width="4.299305555555556in"
height="2.7881944444444446in"}

Fig. 27. Measurements of the interface generation current for two
differently\
processed gate-controlled diodes manufactured on oxygen-enriched
material.

The relatively low flat-band voltage indicates a high-quality oxide.
After exposing the oxides to electrons from a Scanning Electron
Microscope, the density of positive trapped oxide charges increased,
which is manifested by the shift of the flat-band voltage, see Fig. 28.
The curves differ according to the different oxide thickness, but the
oxide charge density is similar for both devices, and as expected its
value is about 310^12^ cm^-2^. The results of the study are summarised
in Table 3.

Table 3. Results of the study on the radiation sensitivity\
of the oxides grown on oxygen-enriched silicon.

  ---------------- -------------------- ---------------------- --------------
  **Device**       **N~ox~              **D~it~                **Dose
                   \[10^11^/cm^2^\]**   \[10^10^/cm^2^eV\]**   \[kGy\]**

  **W19-S-TF-4**   0.6                  0.7                    0

  **T31-S-TF-4**   0.6                  0.7                    0

  **W19-S-TF-4**   21                   30                     200

  **T31-S-TF-4**   35                   30                     200
  ---------------- -------------------- ---------------------- --------------

![](media/image29.png){width="5.010416666666667in" height="3.1625in"}

Fig. 28. Capacitance-voltage characteristics on the gate electrode of
the\
gate-controlled diodes before and after exposure to ionising radiation.

**6. DEFECT SPECTROSCOPY**

Along with the phenomenological studies on the damage-induced
deterioration of the detectors, defect spectroscopy has always been
carried out within the collaboration. A detailed knowledge about the
microscopic origin of the changes in leakage current, effective doping
concentration, and carrier trapping is inevitably needed as the
groundwork for the defect-engineering approach: Detrimental defects must
be structurally and chemically identified before wise choices can be
made regarding the optimisation of the impurity content of the silicon
in order to suppress the defect's production following irradiation. In
fact, the decision in favour of oxygen-enriched, carbon-lean, lower
resistivity material is to a large extent motivated by the known
interactions between the primary displacement damage defects
(interstitials and vacancies) with the most prominent impurities
(Oxygen, Carbon, Phosphorus, Boron), well reported in literature for
more than a decade. Using this heuristic approach, more radiation
tolerant silicon has been successfully engineered by the collaboration
even though there is much still to learn about the microscopic processes
leading to the deterioration of silicon detectors. Much time consuming
work has been carried out in order to discover previously unreported
defects, to account for the peculiarities of heavy-particle damage, and
to handle the experimentally difficult high-resistivity material. As
will be shown below, many significant new results have been obtained in
this field.

The following section is organised into three parts. In the first part
the main spectroscopic tools available within the collaboration are
briefly introduced, highlighting the kind of information that can be
gained with them and indicating experimental requirements for their
application. The second part discusses results obtained after
irradiation with various particle types and using different materials,
and the third part reviews examples where spectroscopic signals could
indeed be unambiguously related to a certain macroscopic quantity.

[Damage-Induced Defects as Observed with Various Spectroscopic
Characterisation Tools]{.underline}

*Capacitance-Deep Level Transient Spectroscopy (C-DLTS)*

In C-DLTS the capacitance of a reverse-biased diode is used as a probe
of the charges in the space charge region. The charges are the doping
atoms and defects that were allowed to trap free carriers during a
preceding filling pulse. Following the pulse, the capacitance approaches
its steady state value exponentially with a time constant characteristic
for the emission of carriers from the traps into the band. The DLTS
signal is derived from this transient, e.g., using the difference of the
capacitance at two different times (box-car). Plotting the DLTS signal
versus temperature gives a spectrum with a peak for each level in the
band gap of the semiconductor, see Fig. 29.

Good C-DLTS set-ups provide the emission-parameters (activation energy E
and cross section ) and the concentrations N~t~ of all traps within one
temperature scan. Traps throughout the entire band gap, except for very
shallow levels, can be detected, and majority and minority carrier traps
can be distinguished. With some extra effort, the free carrier trapping
kinetics (capture coefficients) can be studied. While the trap
concentrations derived from DLTS measurements are the most accurate
values available compared with all other tools discussed below, C-DLTS
is strictly limited to trap concentrations well below the doping
concentration. For the high-resistivity material studied here this means
that the fluence must be very low, orders of magnitude lower than is
anticipated during the operation of the detectors.

In Fig. 29 we observe all well-known signals from damage-induced defects
in silicon and the corresponding isochronal annealing behaviour.
Evidently Oxygen and Carbon play an important role as sinks for the
primary damage defects (interstitials and vacancies). In fact, the ratio
between the C~i~C~s~ and C~i~O~i~ signals in conjunction with their
annealing rate can be exploited to determine both the O and C
concentration. The more relevant observations for today's discussion
concern the less abundant and unidentified peaks (e.g. E~V~ + 0.312 eV),
the left and right-hand side broadening of the VV^‑/0^ transition,
and/or the apparent concentration difference between the doubly charged
and singly charged divacancy peak observed before the 280°C annealing
step. The two latter observations are currently discussed within the
framework of the defect cluster strain model and the inter-centre charge
transfer model described in refs. \[[^22]\] and \[[^23]\], respectively.

*Thermally Stimulated Current (TSC)*

In order to obtain a TSC spectrum, the defect filling pulse is applied
only once at low temperature, and the TSC signal is simply given by the
leakage current through the reverse-biased detector during the heating
period, see Fig. 30. While TSC was originally used analogous to DLTS, it
was realised in this collaboration that a TSC signal can still be
measured even if the trap concentrations are higher than the doping
concentration, if forward current injection or optical injection of free
carriers is used for the trap filling. However, in this case it is not
straight-forward to distinguish between majority and minority carrier
traps or to determine trap concentrations. Also, measuring E and
requires much more experimental effort as compared to DLTS. Traps
throughout the entire band gap are observed, including very shallow
traps (e.g. the Phosphorus donor level). However, signals from deep
traps are often hidden below the steady state leakage current of the
device, which constitutes the baseline signal of the method.

![](media/image30.wmf){width="6.2868055555555555in"
height="4.4222222222222225in"}

Fig. 29. Isochronal annealing (T = 20°C, t = 20 min) of DLTS spectra
after radiation damage with 10^11^ \<5.3 MeV\> neutrons per cm^2^ (3 kcm
n-type epitaxial material) \[[^24]\].

![](media/image31.wmf){width="4.057638888888889in"
height="2.8368055555555554in"}

Fig. 30. Reverse-bias dependence of TSC spectra \[[^25]\].

### Current-DLTS (I-DLTS) and Photo-Induced Current Transient Spectroscopy (PICTS)

In order to combine the higher sensitivity and faster access to E and
provided by C-DLTS with the ability to measure trap-compensated
high-resistivity material, the detector leakage current can be used as
the signal for the DLTS method. This approach is known as I-DLTS and/or
PICTS, where the latter represents an effort to determine trap
concentrations from photo-induced current transients exploiting
additional experimental information. Also I-DLTS is commonly used in
combination with optical excitation. Rather detailed spectra can be
obtained, see Fig. 31, but the status of trap assignment is not as
advanced as compared to C-DLTS.

![](media/image32.wmf){width="3.9006944444444445in"
height="3.001388888888889in"}

Fig. 31. Experimental I-DLTS spectra of silicon detectors irradiated to
a high fluence, and the corresponding simulated spectra. Dashed/dotted
lines show the components of the simulated spectra \[[^26]\].

### Optical Charging Spectroscopy (OCS)

OCS is a method related to TSC. Traps are filled at low temperature
using illumination with light of various wavelengths to modulate the
depth profile. The short-circuit current is measured during warm-up. See
Fig. 32 for a comparison between TSC and OCS. This method is sensitive
to traps close to the junction of the diode.

![](media/image33.wmf){width="3.8722222222222222in"
height="2.7868055555555555in"}

Fig. 32. TSC (V~up~ = 150 V) and OCS (V~up~ = 0 V) currents (arb.
units), ~eq~ = 1.810^13^ n/cm^2^ \[[^27]\].

### Transient Current Technique (TCT)

Here free carriers are generated with a short (1 ns) laser pulse below
either front or backside of the detector. The signal is given by the
current induced by the drifting carriers. At low temperatures the free
carriers can also be trapped at defects, which modulates the electric
field in the detector and thus affects the pulse shape, compare Fig. 33.
Studying the signal as function of temperature gives spectroscopic
information about the traps. TCT is applicable for medium radiation
fluence (about 10^13^ cm^‑2^), where the electric field in the fully
depleted detector is so low that the drift time of the carriers is well
above the resolution of about 1 ns. TCT is a complementary method as it
is very sensitive to traps close to the middle of the band gap. For
example, a deep hole trap at E~V~ + 0.5 eV has commonly been observed,
which might be very important for the generation of leakage current
\[[^28],[^29]\].

![](media/image34.wmf){width="4.60625in" height="3.61875in"}

Fig. 33. Measured steady-state electron current pulse shapes at various
temperatures for an irradiated detector R1: 170K; R2: 165K; R3: 160K;
R4: 155K (4 kcm, d = 630 m, V = 100V, ~n~ = 2.310^12^ cm^‑2^, 31 months
room temperature) \[Error: Reference source not found\].

### Photo Luminescence (PL)

PL is an optical method, and does not require electrical contacts to the
sample. Electron-hole pairs are excited in the material with above
band-gap light. At low temperature the electron and hole form a weakly
bonded pair, which can diffuse through the crystal as an entity. This
so-called exciton can get trapped at a defect, where it eventually will
recombine and thus cause the emission of a photon characteristic of the
defect. Fig. 34 shows a typical PL spectrum as observe on standard
float-zone material after proton irradiation. Note that besides the
known C~i~C~s~ and C~i~O~i~ signals the mysterious W-line is observed.
Recent quantum mechanical modelling indicates that the W-line may be the
tri-interstitial. Optical selection rules and oscillator strengths
determine whether a deep level defect gives rise to a PL signal and how
strong it is. It is therefore not in general possible to obtain
concentrations from PL measurements; many defects cannot be detected at
all, e.g. VO~i~. PL is very sensitive (10^10^ cm^‑3^) given the number
of nonradiative recombination centres is sufficiently small.

![](media/image35.wmf){width="4.814583333333333in" height="3.41875in"}

Fig. 34. Annealing of Photoluminescence spectra measured on
heavy-particle damaged silicon \[[^30]\].

[Results for Different Particle Types and Materials]{.underline}

In the following we will present systematic studies on the influence of
the particle type used for irradiation and of the impurity content of
the material. In part, these experiments were designed to elucidate the
circumstances leading to the improved radiation tolerance of
oxygen-enriched silicon only observed for charged-particle irradiation.

Gammas Versus Neutrons

A direct comparison of the radiation defects produced by ^60^Co- and
14 MeV neutrons has been carried out on 3 kcm standard Wacker FZ
material, see Fig. 35 \[Error: Reference source not found\]. With
gammas, only randomly distributed point defects are produced, whereas
neutrons give rise to damage cascades and end-of-range damage. The
samples were maintained at room temperature for a few months before the
measurements. While for the gamma-irradiated sample the ratio
\[VV(‑/0)\]/\[VV(=/‑)\] between the apparent concentration of the two
divacancy-related peaks is found to be 1.0, the neutron-irradiated
sample displays a 3.3 times weaker signal from the doubly-to-singly
charged transition. This discrepancy was discovered earlier by Svensson
et al. and was explained by lattice strain effects, which then also
would account for the peak broadening \[Error: Reference source not
found\]. The strain arises from divacancies being predominantly produced
within each others lattice strain field at the end of range of the
recoil cascades. Another explanation was put forward by S. Watts et al.,
realising that such closely spaced defects might as well be able to
exchange electrical charges \[Error: Reference source not found\]. This
is the so-called inter-centre charge transfer model also explains the
suppression of the VV(=/-) signal.

Divacancies are produced in much larger amounts in neutron-damaged
samples as compared to gammas. The corresponding ratios between the
introduction rates g = N~t~/ found in this study are:

gammas: g(VV^-/0^)/g(VO~i~) = 0.029, and neutrons: g(E(205))/g(VO~i~) =
1.49/0.69 = 2.15.

![](media/image36.wmf){width="4.561111111111111in"
height="3.042361111111111in"}

Fig. 35. Comparison of DLTS spectra after ^60^Co- and neutron
irradiation on identical samples (3 kcm) \[Error: Reference source not
found\].

In addition to the qualitative differences between the DLTS spectra
observed after ^60^Co- and neutron-damage, heavy-particle damaged
samples also display a unique annealing behaviour, compare Fig. 27. Two
annealing steps at 70°C (E~C~ -- 0.46 eV) and 170°C (E~C~ --0.40 eV) are
now well established. The same annealing steps are observed in the
leakage current annealing, proving that the cluster peak at about 205 K
is a main contributor to the leakage current generation in
heavy-particle damaged silicon.

Oxygen-Content Dependence of DLTS Spectra

Using the materials listed in Table 4, the effect that the Oxygen
content has on the formation and annealing of radiation-induced defects
has been studied. For this example, the samples were exposed to
\<5.3 MeV\> neutrons at a Be(d,n) neutron generator. They were
subsequently annealed at 60°C for 80 min. Fig. 36 shows the
corresponding DLTS spectra. The introduction rate and the shape of the
cluster peak at 205 K is found to be independent of the Oxygen
concentration. This is expected, as it is known that the leakage current
scales very nicely with the concentration of the cluster peak, and that
the oxygen content does not affect the leakage current-related damage
constant. To a first approximation the generation rate of VO~i~
(A-centre) defects does not depend on the oxygen concentration, since
under the given irradiation conditions interstitial oxygen is the
dominant sink for freely migrating vacancies. That means \[VO~i~\] is
simply a measure of the total dose. Freely migrating silicon
interstitials, on the other hand, are all converted into interstitial
Carbon, which then has two reaction partners available to form stable
defects: substitutional Carbon and interstitial Oxygen. Therefore the
ratio between the two resulting reaction products \[C~i~C~s~\] and
\[C~i~O~i~\] is proportional to the ratio between \[C~s~\] and \[O~i~\].
This explains why for the oxygen-lean material a larger C~i~C~s~
concentration is observed superimposing the VO~i~ peak. Finally we note
the existence of Thermal Donors (TD), presumably a conglomerate of four
oxygen atoms, in the CZ material. In principal these donors can also be
created in oxygen-enriched float-zone material, and thus pose the danger
of deteriorating the resistivity of the material. However, recipes to
dissolve these donors and to circumvent their detrimental effects are
well adopted within the silicon device processing industries.

Table 4. Materials used for the comparative DLTS study.

  ----------------------------------- -------------- --------------------
  Material                            Resistivity    Oxygen Content

  ITME, FZ, n-type, \<111\>           120 cm         \<510^16^ cm^-3^

  ITME, FZ, n-type, \<111\>           800 cm         1.710^17^ cm^-3^

  Polovodice, Cz, n-type, \<111\>     100 cm         910^17^ cm^-3^
  ----------------------------------- -------------- --------------------

![](media/image37.wmf){width="3.90625in" height="2.7847222222222223in"}

Fig. 36. DLTS spectra (normalised to introduction rate at 200K) obtained
after irradiation with neutrons and a subsequent 80 min heat treatment
at 60°C for different materials (see legend) \[[^31]\].

Comparison of DLTS Spectra after Irradiation with Neutrons, Protons,
Pions

A gradual transition from spectra characteristic of neutron damage to
those characteristic of ^60^Co-gammas is expected when one considers
charged-particle damage with varying energies. At low charged-particle
energy the relative importance of Coulomb scattering, imparting only
small energies to the PKA, is increasing. The result of such a study is
presented in Fig. 37, where \<5.3 MeV\> neutrons from a Be(d,n)
generator, 192 MeV pions, and 27 MeV and 23 GeV protons have been used.
The material (ITME FZ n-type, \<111\>, 800 cm, jet-oxygenated, \[O\] =
1.710^17^ cm^‑3^) was the same in every case. Moreover, all samples were
stored at 60°C for 80 min, in order to exclude any artefacts from
differences in the annealing history.

Evidently, the cluster-peak at 205 K is introduced at the same rate in
every case, reconfirming the intrinsic nature of the comprising defects
and their location within the end-of-range region of the damage
cascades. The introduction rate of the isolated point defects,
represented by VO­~i~ and C~i~C~s,~ varies for the different particle
types, in agreement with the expected enhancement due to Coulomb
scattering. Here it is interesting to note that in this respect the very
high-energetic (23 GeV) protons behave similarily to the 192 MeV pions.

Another striking observation concerns the peak normally assigned to the
doubly-charged divacancy. Its variation with particle type resembles the
one observed for the point defects. Also, quite obviously, the shape of
the cluster peak at 205 K does not depend on the particle type. The
broadening of the VV^‑/0^ transition and the apparent reduction of the
apparent VV^\--/-^ concentration with respect to VV^-/0^ can therefore
not have the same origin. These observations need to be understood in
terms of the lattice strain and the inter-centre charge transfer models.

![](media/image38.wmf){width="4.709722222222222in"
height="3.151388888888889in"}

Fig. 37. DLTS spectra obtained on samples from the same wafer obtained
after irradiation with different particles (see legend) and a 80 min
lasting heat treatment at 60°C. The spectra are normalised to the 1 MeV
neutron equivalent introduction rate at 200 K \[Error: Reference source
not found\].

[Correlation with Macroscopic Effects.]{.underline}

Short Term Annealing

It had been noted earlier that the activation energy for the short term
annealing of the leakage current and the effective doping concentration
are similar. Also it was known that the annealing at 70°C of the
right-hand side broadening of the VV^-/0^ transition, see Fig. 29,
correlates with the leakage current annealing. It is therefore tempting
to assume that the macroscopic effects have a common cause, namely, the
deep level at E~C~ - 0.46 eV, and that this can be reasonably discussed
on the grounds of simple Shockley-Read/Hall statistics. In order to
substantiate this hypothesis, the deep level in question, labeled E4b in
the following, was monitored during an isothermal annealing study at
60°C using DLTS, see Fig. 38. To obtain reliable defect parameters and
concentrations for the annealing traps, the difference of two subsequent
DLTS spectra was subjected to the standard DLTS evaluation procedure.
The results are given in numerical form in Table 5. E4b was noted to be
accompanied by a more shallow level at E~C~ -- 0.36 eV (E4a), which, in
analogy to the divacancy, tentatively can be considered a second
ionization state of E~C~ -- 0.46 eV (E4b). A very good correlation is
observed between the change in leakage current between two annealing
steps and the corresponding change in the concentration of E4b, see Fig.
39.

From the DLTS measurement, only the capture cross section for electrons
~n~ is obtained. In order to calculate the leakage current from E4b
using the Shockley/Read-Hall formulae, also the capture cross section
for holes ~p~ must be known. Vice versa, the measured leakage current
can be exploited to obtain ~p~ from such a calculation. Here we obtain a
value of about 10^‑13^ cm^2^, which is quite conceivable. From forward
current injection DLTS spectra it is in fact known that ~p~ \> ~n~.
Using these parameters we can also calculate the contribution to the
space charge from this trap. The value is however a factor five smaller
than what is observed experimentally. Still, adjusting both cross
sections ~p~ and ~n~ in order to account for the macroscopically
observed effects gives ~n~ = 8.610^‑15^ cm^2^ and ~p~ =
3.110^‑13^ cm^2^, demonstrating that a comparatively small uncertainty
of the cross section can have a big effect here. It should be noted that
the macroscopic measurements were carried out at room temperature
whereas the DLTS data was gathered at about 200 K. Systematic studies on
the temperature dependence of the cross sections and the trap ionization
energy are needed to draw final conclusions.

Table 5. Defect parameters. The given introduction rates refer to
certain annealing states at 60°C: (a) after 5 min and after 82000 min,
(b) annealed in the period 5min to 10000 min and (c) after anneal to
82000 min.

  ----------- ------------- -------------- -------------------------------
  Defect      E~t~ \[eV\]   ~n~ \[cm^2^\]  Introduction rate \[cm^-1^\]

  E3          0.24          10^-14^        0.240.41 (a)

  E4a         0.36          10^-14^        0.26 (b)

  E4b         0.46          110^-14^       0.62 (b)

  E4          0.41          1.510^-15^     1.04 (c)
  ----------- ------------- -------------- -------------------------------

  ---------------------------------------------------- -----------------------------------------------------
  ![](media/image39.wmf){width="4.094444444444444in"   ![](media/image40.wmf){width="2.4458333333333333in"
  height="2.963888888888889in"}                        height="2.58125in"}

  Fig. 38. Evolution of the DLTS spectrum at 60°C for  Fig. 39. Correlation between trap E4b and leakage
  a neutron irradiated sample produced from Cz silicon current \[Error: Reference source not found\].
  \[Error: Reference source not found\].               
  ---------------------------------------------------- -----------------------------------------------------

Reverse Annealing of the Effective Doping Concentration

So far, only a weak correlation between the W-line observed with PL,
compare Fig. 34, and the change in doping concentration during the
reverse annealing phase have been reported. A systematic isothermal
annealing study at 80°C using TSC on a sample irradiated with
10^13^ cm^‑2^ \<5.3 MeV\> neutrons has provided additional insight, see
Fig. 40. The defect concentrations at the given fluence are too high to
allow a quantitative analysis of the TSC spectra. Most importantly the
spectra will exhibit a memory effect: a change in the peak signal at low
temperature implies a change at all higher temperatures because of the
variation of the space charge layer width with the concentration of
ionised traps. It can still be argued that the signal H(116K) is indeed
growing during the annealing sequence.

Fig. 41 demonstrates a good correlation between the integrated peak
charge versus the change in space charge during the reverse annealing
period. H(116K) had already previously been unambiguously connected with
negative space charge by exploiting the bistability of this level. The
simplest approach would mean that H(116K) is an acceptor level. This
assignment would roughly be in agreement with the lower limit of the
concentration derived for this peak from the TSC spectrum. Moreover, for
an acceptor level one would expect a fairly large capture cross section,
because of the Coulombic attraction between the ionised acceptor and the
emitted hole. Previously reported level parameters (E~t~ = 0.32 eV and
~p~ = 1.710^‑13^ cm^2^) would also match this criteria and could be
reproduced here using C-DLTS, see Fig. 27. However, another set of
parameters has been reported in ref. \[Error: Reference source not
found\], which is surprisingly similar to the C~i~^+/0^ donor
transition. Further studies are needed to ascertain the chemical nature
of this defect.

  ----------------------------------------- ----------------------------------------------------
  ![](media/image41.wmf){width="3.8625in"   ![](media/image42.wmf){width="2.640277777777778in"
  height="3.129166666666667in"}             height="2.4875in"}

  Fig. 40. Annealing of TSC spectra during\ Fig. 41. Correlation between amount\
  the reverse annealing period \[Error:     of reverse annealing and concentration\
  Reference source not found\].             of the trap H(116K).
  ----------------------------------------- ----------------------------------------------------

**7. MODELLING OF DEFECT PRODUCTION AND RADIATION DAMAGE**

A key strategy of the ROSE Collaboration has been to understand
macroscopic effects by using semiconductor device physics and known
microscopic defect concentrations. A further ambitious aim has been to
predict defect concentrations using defect kinetics models. This has
been discussed in detail in previous status reports \[Error: Reference
source not found,Error: Reference source not found\]. Consequently, only
a brief summary will be given here together with recent modelling that
explains why oxygen is beneficial to the radiation hardness of silicon
when irradiated with protons. The defect kinetics model of Davies
\[[^32]\] has been adapted for detector material to predict defect
concentrations after irradiation with ^60^Co photons and fast hadrons.
The reactions used in the model \[[^33]\] are listed in Table 6.
Reaction rates are controlled by the concentrations of impurities and
their relative capture radii. Full details may be found in Ref. \[Error:
Reference source not found\]. The input parameters required by the model
are the oxygen and carbon impurity concentrations (determined by SIMS,
IR absorption etc.) and the introduction rates of the primary defects.
The rates used in the calculation are given in Table 7.

Table 6. Defect kinetics model reaction scheme.

+-----------------------+-----------------------+-----------------------+
| Table 6A. Cluster     |                       |                       |
| reactions             |                       |                       |
+-----------------------+-----------------------+-----------------------+
| I reactions           | V reactions           | C~i~ reactions        |
+-----------------------+-----------------------+-----------------------+
| I+VSi                 | V+VV~2~               | \.....                |
+-----------------------+-----------------------+-----------------------+
| Table 6B. Diffusion   |                       |                       |
| reactions             |                       |                       |
+-----------------------+-----------------------+-----------------------+
| I+C~s~C~i~            | V+VV~2~               | C~i~+C~s~CC           |
|                       |                       |                       |
| I+CCCCI               | V+V~2~V~3~            | C~i~+OCO              |
|                       |                       |                       |
| I+CCICCII             | V+OVO                 |                       |
|                       |                       |                       |
| I+COCOI               | V+VOV~2~O             |                       |
|                       |                       |                       |
| I+COICOII             | V+PVP                 |                       |
|                       |                       |                       |
| I+VOO                 |                       |                       |
|                       |                       |                       |
| I+V~2~OVO             |                       |                       |
|                       |                       |                       |
| I+V~2~V               |                       |                       |
|                       |                       |                       |
| I+VPP                 |                       |                       |
+-----------------------+-----------------------+-----------------------+

Table 7. Introduction rates used in the modelling (refs. \[[^34]\] and
\[[^35]\])

  ------------------ ------------------------- --------------------------
  Radiation Source   V Introduction rate       V~2~ Introduction Rate
                     (cm^-1^)                  (cm^-1^)

  ^60^Co gammas      2.710^-4^                 7.110^-6^

  1 MeV neutrons     0.58                      0.96

  24 GeV/c protons   0.61                      0.46
  ------------------ ------------------------- --------------------------

In order to calculate device characteristics, the theory of Shockley,
Read and Hall has been combined with the predicted defect
concentrations. The states included in the SRH calculation of the
effective doping, *N*~eff~, and leakage current density, *J*~v~, are
listed in Table 8. It should be noted that there is some uncertainty in
the energy level assignment of the divacancy-oxygen (V~2~O) defect
\[[^36]\].

Table 8. Defect states considered in the SRH calculation.

  ------------------- ------------------ ---------------- ----------------
  Identity            Energy (eV)        Type             Charge

  VO                  E~c~-0.17          Acceptor         (0/-)

  V~2~O               E~c~-0.50±.05      Acceptor         (0/-)

  V~2~                E~v~+0.20          Donor            (+/0)

                      E~c~-0.41          Acceptor         (0/-)

                      E~c~-0.23          Acceptor         (-/=)

  VP                  E~c~-0.45          Acceptor         (0/-)

  CO                  E~v~+0.36          Donor            (+/0)

  CC                  E~c~-0.17          Acceptor         (0/-)
  ------------------- ------------------ ---------------- ----------------

[Gamma Irradiation]{.underline}

The predictions of the model for the evolution of *N*~eff~ during gamma
irradiation are compared with experimental data from detectors with
various oxygen concentrations in Fig. 42. The agreement between the
model and data is impressive. The dominant contribution to *N*~eff~
arises from the V~2~O state; an energy level of *E*~V2O~ = *E*~c~-0.54
eV is required to obtain agreement with the data, a value which lies
within 1 of the experimental energy measurement. Oxygenated material is
more radiation hard to gammas because V~2~O production is suppressed by
the competing reaction V + O VO. The leakage current in these detectors
has also been modelled. The V~2~O defect dominates on account of its
proximity to mid-gap. The model therefore suggests that is reduced as
the oxygen content rises, a prediction which has been borne out by
experiment, ref. \[[^37]\].

![](media/image43.wmf){width="4.614583333333333in"
height="3.4854166666666666in"}

Fig. 42. Model predictions (lines) and experimental data (markers) for
^60^Co gamma irradiation of samples with various oxygen concentrations
and a carbon concentration of \~10^16^ cm-3.

[Hadron irradiation]{.underline}

Using the value of *E*~V2O~ fixed by the gamma data, the same modelling
procedure was applied to the case of hadron irradiation. For 1 MeV
neutrons, it was found that the predicted type inversion fluence was an
order of magnitude higher than that observed experimentally. The
dominant contribution to the doping changes predicted by the model again
arose from the V~2~O defect. The leakage current was underestimated by
some 2 orders of magnitude. The potential errors in the model were
examined carefully but could not possibly explain these discrepancies.

The presence of defect clusters in hadron-irradiated material, and their
absence in gamma-irradiated material, suggests a possible cause. Several
defects are strongly produced within the terminal clusters, including
the divacancy (V~2~) and two defects known as E70 and E170. The E70
state is multivalent, giving rise to acceptor levels at *E*~c~-0.45 eV
(0/-) and *E*~c~-0.35 eV (-/=), whereas E170 gives rise to a single
acceptor at *E*~c~-0.37 eV (0/-) \[[^38]\]. E70 and E170 are strongly
correlated with annealing of the leakage current after irradiation, but
neither state is sufficiently close to mid-gap to explain the magnitude
of the observed current decrease in the conventional SRH picture
\[Error: Reference source not found\].

Simple calculations indicate that the local density of V~2~, E70 and
E170 within the terminal clusters is of the order of 10^19^ cm^-3^. It
is evident, therefore, that some of these defects are close enough to
exchange charge directly. There is compelling experimental evidence in
the literature for such inter-centre charge transfer (see, for example,
Ref. \[[^39]\]). By writing general expressions for the rates of
emission and capture between all levels of the V~2~, E70 and E170
defects, and assuming equal carrier capture cross-sections throughout,
it is possible to calculate the steady-state occupancy of each defect
and the carrier generation rate numerically. The predicted value for the
damage constant, , which is the sum of the three components, is
\~1010^-17^ A.cm^-1^ in close agreement with what is actually observed.

The introduction rate of negative space charge is now \~510^-3^ cm^-1^.
To explain the experimental data, however, an introduction rate of
\~2510^-3^ cm^-1^ is required. Given that this preliminary calculation
assumes equal carrier capture cross-sections, which is known not to be
the case, it is not surprising that there is not a perfect agreement
with the data. A more complete calculation is now in progress; from work
to date it is evident that even small deviations from unity in the
capture cross-section ratios can cause further enhancements in the
introduction rate of negative space-charge. In order to make a step
forward we have assumed that the total contribution of the clustered
defects to the space charge is indeed of the order of \~2510^-3^ cm^-1^
for 1 MeV neutrons and scales with the cluster defect introduction
rates.

The results of combining the standard SRH calculation for point defects
and the non-SRH calculation for clustered defects with this assumption
are shown in Fig. 43 and Fig. 44. The plots show the predicted evolution
of *N*~eff~ for various detector impurity concentrations for 1 MeV
neutrons and 24 GeV protons respectively. The experimental data are
described well. There are several points to note. In the case of neutron
irradiation, the various detectors display similar radiation tolerance;
the only differences observed are due to differences in the initial
resistivity of the devices. In the case of proton irradiation, however,
the oxygen-enriched detector is more rad-hard and the carbon-enriched
detector less rad-hard than the standard device. The reason for this is
that, in the case of proton irradiation, the primary introduction rate
of the vacancy is proportionally larger than in the case of neutron
irradiation. Hence, the role played by V~2~O in determining *N*~eff~ is
greater for protons. As in the case of gamma irradiation, a high oxygen
concentration suppresses V~2~O production and thus the changes in
*N*~eff~. Conversely, a high carbon concentration encourages V~2~O
production because substitutional carbon acts as a sink for
interstitials and suppresses capture of interstitials at V~2~O itself
and its pre-cursor states (Table 6).

  ----------------------------------------------------- ----------------------------------------------------
  ![](media/image44.wmf){width="3.2604166666666665in"   ![](media/image45.wmf){width="3.265972222222222in"
  height="2.3833333333333333in"}                        height="2.3868055555555556in"}

  Fig. 43. Model predictions (lines) and experimental   Fig. 44. Model predictions (lines) and experimental
  data (markers) for 1 MeV neutron irradiation.         data (markers) for 24 GeV proton irradiation.
  Circles - *\[O\]* = 310^15^ cm^-3^, *\[C\]* = 510^15^ Circles - *\[O\]* = 310^15^ cm^-3^, *\[C\]* =
  cm^-3^; triangles - *\[O\]* =\                        510^15^ cm^-3^; triangles - *\[O\]* =\
  1.710^16^ cm^-3^, *\[C\]* \< 210^16^ cm^-3^;          1.710^16^ cm^-3^, *\[C\]* \< 210^16^ cm^-3^;
  squares - *\[O\]* \< 510^16^ cm^-3^, *\[C\]* =        squares - *\[O\]* \< 510^16^ cm^-3^, *\[C\]* =
  1.810^16^ cm^-3^.                                     1.810^16^ cm^-3^.
  ----------------------------------------------------- ----------------------------------------------------

It follows that it is misleading to compare the results of radiation
damage studies by simply scaling *N~eff~* data with the hardness factor
of the source. The figures which determine radiation hardness are the
actual defect introduction rates themselves, which do not necessarily
scale with NIEL.

## [Summary]{.underline} 

The changes in *N*~eff~ and dark current during gamma irradiation have
been modelled satisfactorily in terms of production of the V~2~O defect.
In the case of hadron irradiation, electrical characteristics are
controlled by cluster defects, with a contribution to *N*~eff~ from the
V~2~O state. The V~2~O defect is more copiously produced during proton
irradiation on account of the proportionally higher vacancy introduction
rate, hence the radiation hardness of materials to protons is more
sensitive to impurity concentrations than in the case of neutrons. This
modelling can be considered to be indirect evidence for the presence of
V~2~O. Direct experimental evidence for V~2~O has yet to be obtained.
There is evidence for a deep level close to mid-gap from TCT data but
its precise nature has not been determined. Quantum mechanical modelling
of this defect predicts that it is an acceptor close to mid-gap
\[[^40]\].

**8. OUTLOOK AND OPEN PROBLEMS**

The RD48-Collaboration has achieved significant results and reached its
original goal to develop radiation hard silicon detectors and provide
guidelines to the LHC experiments. However, there are still some
remaining issues both at a practical and fundamental level that need to
be resolved. These are addressed in this section.

[Final optimisation of the radiation hardening effect of oxygen
enrichment]{.underline}

To date, experiments have been performed with a variety of different
oxygenation processes from a 16h diffusion at 1150C up to 9 days at
1200C (see Table 9). The Quartz ovens used at the detector manufacturers
do not allow a higher temperature than 1150C. Even at this temperature
the lifetime of the quartz tubes is limited and hence replacement adds
to the price of detector production. Furthermore, safety requirements
may prohibit continuous operation of the ovens for a 24hr/day basis. For
all these reasons, the diffusion process should be minimised without
losing the advantages that oxygenation has on the radiation hardness. In
the last year, much has been done to systematically study this process.
However, there are still some open questions regarding the minimum
diffusion process required not only to maintain the hardening for the
stable damage effect but to also to control reverse annealing. The fact
that reverse annealing can be affected by high oxygen levels was
unexpected but very welcome. As this effect has important consequences
for the maintenance periods used by the experiments, progress in this
area needs urgent further investigation. These measurements take a long
time and results will only become available over the next few months.
Another open question which can only be tackled together with the
detector manufacturers will be the not yet understood variations seen in
standard control diodes and their possible relation to different
manufacturing techniques. Material characterisation and the measurement
of the complete set of damage parameters needs to be performed for each
manufacturer.

[Consistency of the RD48 results on diodes with pixel- and strip
detectors]{.underline}

Most of the RD48-results presented in this report have been obtained
using special ROSE test detectors, i.e. small pad diodes with usually
just one guard ring. The geometry of the real strip and pixel-detectors
differs appreciably from this simple structure. They have a much more
complex surface geometry with SiO~2~ between the individual strips or
pixels and employ complex guard-ring structures. The few measurements
that have been made on full-scale detectors show an acceptable agreement
with simple test diodes. We see no reason for considerable deviation
between the strip and diode results, nevertheless, the consistency of
the RD48 results on diodes should be carefully tested against full-scale
detectors. This task has already started and a close direct
collaboration between the CERN-MIC-SD group of RD48 and the large LHC
experiments has been initiated. Two joint meetings have already been
held and RD48 diodes have been used in parallel with LHC detectors
during irradiation experiments. For instance, prototype ATLAS strip
detectors have been irradiated together with our own diodes. These
consistency checks will be pursued further both with ATLAS and CMS.

Table 9.: Material produced under the auspices of the ROSE-collaboration
in 1999. Diode Producers: BNL: Brookhaven National Lab, Upton NY, USA;
Sintef: Sintef, Oslo, Norway; CIS: CIS, Erfurt, Germany; ST:
STMicroelectronics, Catania, Italy.

+-------+--------+--------+--------+-----------------------+---------+
| **Pro | **Pro  | **O    | **     | **Treatment**         | **SIMS  |
| ducer | ducer\ | rienta | Resist |                       | (       |
| di    | sil    | tion** | ivity\ | **(Oxygenation)**     | 150 m)\ |
| ode** | icon** |        | \[k    |                       | \[O\]   |
|       |        |        | cm\]** |                       | \[cm^   |
|       |        |        |        |                       | -3^\]** |
+-------+--------+--------+--------+-----------------------+---------+
| BNL   | Topsil | \      | 1.0    | 9d in N~2~ 1200C      | 4e17    |
|       |        | <100\> |        |                       |         |
+-------+--------+--------+--------+-----------------------+---------+
| CIS   | Topsil | \      | 1.0    | 9d in N~2~ 1200C\     | 3.4e17  |
|       |        | <100\> |        | (performed at BNL)    |         |
+-------+--------+--------+--------+-----------------------+---------+
| CIS   | Topsil | \      | 1.3    | no diffusion step     | 2.2e16  |
|       |        | <100\> |        |                       |         |
+-------+--------+--------+--------+-----------------------+---------+
| CIS   | Wacker | \      | 3.9    | no diffusion step     | \< 1e16 |
|       |        | <111\> |        |                       |         |
+-------+--------+--------+--------+-----------------------+---------+
| CIS   | Wacker | \      | 4.4    | 16h in N~2~ at 1150C  | 1.4e17  |
|       |        | <111\> |        |                       |         |
+-------+--------+--------+--------+-----------------------+---------+
| CIS   | Wacker | \      | 4.0    | 24h in N~2~ at 1150C  | 3.0e17  |
|       |        | <111\> |        |                       |         |
+-------+--------+--------+--------+-----------------------+---------+
| S     | Wacker | \      | 2      | 72h in N~2~ at 1150C  | 1       |
| intef |        | <111\> |        |                       | .5-2e17 |
+-------+--------+--------+--------+-----------------------+---------+
| S     | Wacker | \      | 2      | 72h in O~2~ at        | 1       |
| intef |        | <111\> |        | 1150C;\               | .5-2e17 |
|       |        |        |        | oxide thinning        |         |
+-------+--------+--------+--------+-----------------------+---------+
| S     | Wacker | \      | 2      | 72h in O~2~ at        | 1       |
| intef |        | <111\> |        | 1150C;\               | .5-2e17 |
|       |        |        |        | oxide removal +       |         |
|       |        |        |        | reoxidation           |         |
+-------+--------+--------+--------+-----------------------+---------+
| S     | Topsil | \      | 6      | 72h in N~2~ at 1150C  | 1       |
| intef |        | <111\> |        |                       | .5-2e17 |
+-------+--------+--------+--------+-----------------------+---------+
| S     | Topsil | \      | 6      | 72h in O~2~ at        | 1       |
| intef |        | <111\> |        | 1150C;\               | .5-2e17 |
|       |        |        |        | oxide thinning        |         |
+-------+--------+--------+--------+-----------------------+---------+
| S     | Topsil | \      | 2      | 72h in O~2~ at        | 1       |
| intef |        | <111\> |        | 1150C;\               | .5-2e17 |
|       |        |        |        | oxide removal +       |         |
|       |        |        |        | reoxidation           |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 1.0    | 30h in N~2~ at 1200C; | \-      |
|       |        | <111\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 1.1    | 30h in N~2~ at 1200C; | \-      |
|       |        | <100\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 1.0    | 60h in N~2~ at 1200C; | \-      |
|       |        | <111\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 1.0    | 60h in N~2~ at 1200C; | \-      |
|       |        | <100\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 2.2    | 30h in N~2~ at 1200C; | \-      |
|       |        | <111\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 2.4    | 30h in N~2~ at 1200C; | \-      |
|       |        | <100\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 2.0    | 60h in N~2~ at 1200C; | \-      |
|       |        | <111\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 2.4    | 60h in N~2~ at 1200C; | \-      |
|       |        | <100\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 16     | 30h in N~2~ at 1200C; | \-      |
|       |        | <111\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+
| ST    | Wacker | \      | 15     | 30h in N~2~ at 1200C; | \-      |
|       |        | <100\> |        | (SiC-oven)            |         |
+-------+--------+--------+--------+-----------------------+---------+

[The charged hadron-neutron puzzle and its consequences for
LHC]{.underline}

Many experiments in recent years have shown that to compare bulk damage
parameters for radiation sources using different particle types and
energy one should scale results using the Non-Ionising Energy Loss
(NIEL). This important result enables one to extrapolate results
obtained with particular sources to the complex particle and energy
spectra that will be encountered in the LHC experiments by appropriate
scaling using the NIEL data. It was completely unforeseen that this
assumption, which had been proven to be valid for standard material, was
incorrect for high energy proton or pion irradiation of oxygen enriched
silicon. It has been proposed that the origin of this difference can be
understood by a more detailed study of NIEL. Neutrons in the energy
range up to several MeV only interact with the silicon by elastic
scattering, resulting in high energy recoil Si-atoms. However, charged
hadron interactions incorporate a considerable contribution due to
Coulomb scattering resulting in low energy recoils. Assuming that recoil
atoms below a certain energy (5 keV) cannot produce defect clusters but
predominantly point defects, one would expect that for charged hadrons
much of the total NIEL contributes to impurity related defects which
leads to the hardening effect of oxygen. This is consistent with results
from microscopic studies (see Fig. 37 and comments in Section 6), which
show that there are more diffusing vacancies and interstitials for
protons and pions. Section 7 shows that device modelling is also
consistent with this hypothesis.

Fig. 45 shows results from a detailed study of NIEL versus the silicon
atom recoil energy for different particles \[[^41]\]. Only elastic
scattering (both nuclear and Coulomb) has been included. The functional
dependence shown in Fig. 45 also supports the above stated assumption.
The relative NIEL contribution for recoil energies lower than 5 keV is
negligible for 1 MeV neutrons. However, a considerable portion of the
NIEL for charged hadrons exists in that energy regime. One can argue
that elastic scattering of high energy pions or protons contributes only
30 (200 MeV pions) to 37% (9 GeV protons) of the total NIEL and that
therefore according to Fig. 45 the net contribution from recoils below
5 keV is only 6 to 10%. Hence it is hard to believe that this small
value could lead to the observed effect. However, for high energy
particles there are many higher order nuclear reactions which lead to a
variety of secondary reaction products which again interact with the
Si-atoms partly by the Coulomb interaction and hence give rise to a
significant number of low energy recoils. More detailed investigations
using Monte Carlo simulations are necessary. However one result is
already apparent: at high energies, nuclear reactions induced by charged
hadrons and neutrons are practically the same. Thus, the secondary
Coulomb contribution to low energy recoils in the silicon lattice can
also be expected following high energy neutron damage.

Fig. 46 shows the main contributions to the energy spectra present in
the inner detector of the ATLAS experiment folded with the appropriate
damage efficiency for the neutron and pion contribution \[[^42]\]. While
the pion spectrum is centred around 200 MeV, neutrons show an almost
homogeneous energy distribution between 0.1 and 100 MeV. From the
arguments outlined above, at least the high energy part of the neutron
spectrum could therefore also lead to low energy recoils and thus the
damage produced by them would partly result in the same behaviour as
observed for protons. However, all test experiments with neutrons
performed so far would have not shown this effect because the energy
spectra extended only to about 10 MeV. In most cases reactor neutrons
were used.

![](media/image46.wmf){width="4.550694444444445in"
height="3.345138888888889in"}

Fig. 45. Relative damage efficiency for different particles as function
of recoil energy.

![](media/image47.wmf){width="6.017361111111111in"
height="3.8569444444444443in"}

Fig. 46. Neutron and pion energy spectrum folded with NIEL as present in
ATLAS-SCT.

An important consequence of these considerations is the necessity for
dedicated test experiments employing radiation fields which more closely
resemble the LHC environment. The particle sources best suited for this
project are the PSI facility for 200 MeV pion irradiation and the
neutron shuttle, which was recently installed at the CERN-PS \[Error:
Reference source not found\]. In fact the spectral distribution,
displayed in Fig. 47, reproduces the LHC neutron spectrum in the inner
detector very well - compare with Fig. 46.

![](media/image48.wmf)

Fig. 47. Energy spectra of neutrons, pions, protons and gammas in the
beam axis of the IRRAD2 cavity in the CERN PS East Hall \[Error:
Reference source not found\].

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