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Nuclear Instruments and Methods in Physics Research A 368 (1995) 199-204 NUCLEAR

INSTRUMENTS

Section A

Radiation damage to silicon detectors G. Hall *

Blacken Laboratory, Imperial College. London SW7 2AZ UK

Abstract A of recent in understanding fundamental causes bulk damage high resistivity

detectors is A model on deep states in material appears explain most the experimental Candidates for traps have tentatively identified vacancy-oxygen complexes the aid numerical

simulations. concentration of and carbon the silicon important in the concentration deep traps may allow possibility of the hardness detectors.

1. Introduction

In the last few years a large amount of experimental data has been gathered on the properties of silicon detectors after irradiation by different types of incident particle, at fluences in the range that are expected at the CERN LHC. The mea- surements have been carried out by several collaborations and many individual groups, using diodes, test structures and microstrip detectors [l-l I]. There is consensus on the main finding, which is that the principal obstacle to long term operation of silicon detectors at the LHC arises from damage to the bulk of the material where displacements of atoms from their lattice sites gives rise to production of com- plex defects, some of which appear to change the effective doping of the material by behaving as acceptors. Although many other changes take place during and after irradiation, for example at the surface of microstrip detectors, it seems that at least up to the particle fluences expected in inner tracking detectors at the LHC, ~10’~ cm-‘, these are not a fundamental obstacle to long term operation.

The origin of bulk damage in silicon at the microscopic level is poorly understood as, although some of the most prominent defect complexes have been identified in many studies, there are a large number of less well understood defects and some of these may be implicated in the obser- vations. Recently, however, a combination of experimental measurements of primary defect introduction rates and a nu- merical model of the evolution of complex defects, has shed more light on the phenomena which underlie the experi- mental data and may offer real possibilities of hardening the detector material if the concentration of various impurities, such as oxygen and carbon, can be altered. The model pro- vides a semi-quantitative explanation of the observed data

* E-mail g.hall@ic.ac.uk

0168-9002/95/$39.50 @ 1995 Elsevier Science B.V. All rights reserved

SSDlOl68-9002(95)00886-l

to date but needs further extensions and tests to be fully ver- ified.

2. Summary of experimental data

Most of the irradiations carried out to date have been with hadrons, particularly neutrons, protons and pions, since these will be abundant at LHC and are expected to produce the most damage to the silicon lattice. The dependence of silicon bulk damage on particle type has been calculated based on the hypothesis that damage scales in proportion to non-ionising energy deposited in the material. Although not tested over the full range of particle energies and types, the hypothesis does seem to be in reasonably good agree- ment with the data and allows approximate scaling of dam- age from one particle type and energy to another [ 121. For this reason many bulk damage studies are related to 1 MeV neutrons as a reference [ 13,141.

Recently, concern about the dominance of low energy pi- ons in the inner regions of tracking detectors placed close to the beam pipe led to new calculations of the effects of pions in the range around 300 MeV/c [ 151. At these energies, res- onance production in pion-nucleon scattering can enhance displacement damage and lead to higher effective damage constants than expected from minimum ionising particles. Measurements appear to confirm these calculations and the overall effect is to increase the rate of damage by 20-30% more than was previously expected [ 6,161.

A large number of surface related effects have been ob- served, many of which have been studied in some detail [ 6.91 although this is the area where most dependence is ob- served on the manufacturing process and details of the inter- nal design of the detectors. Some of the most important ef- fects are a significant decrease in interstrip resistance on the junction side of microstrip detectors and an increase in in-

V. DETECTOR AND TRIGGER PROJECTS

200 G. HalUNucl. Instr. and Meth. in Phys. Res. A 368 (1995) 199-204

terstrip capacitance, although at the high frequencies where LHC electronics will operate the magnitude of the change can be confined to IO-20%, with careful design [6,17]. In double sided microstrip detectors, there was concern that

the strip isolation on the ohmic surface would be affected by irradiation increasing oxide charge at the silicon inter- face. However it has been demonstrated that both field plate and p-stop isolation techniques work satisfactorily. Concerns

about the need to operate LHC detectors at higher voltages than normal has led to a preference for p-stop isolation since

micro-discharges at field plates have been observed to lead to premature breakdown under bias. Integrated resistors and capacitors appear to be quite feasible under LHC conditions, especially using polysilicon as the resistor material. The use

of silicon nitride in addition to oxide as the capacitor dielec- tric appears to lead to greater reliability and fewer break-

down problems with no major drawbacks. Clearly much fur- ther work is needed to fully define the LHC microstrip de- tectors and the interested reader is referred elsewhere for

further details. The other principal observations which have been made

are briefly summarised:

Diode leakage currents increase in linear proportion to fluence. After high energy hadron irradiations the current per unit volume (Jv) increases with fluence (Q) as Jv = cu@ with (Y z 5-10 x lo-l7 A cm-‘, depending on the particle

type and energy. Fewer results exist for photons but the damage constant is much smaller, (Y M 2 x 10ez3 Acm-’

[ 181. Annealing of bulk damage is important and must be

taken into account since most irradiations are carried out at much higher fluxes than will be encountered in experiments. Leakage currents are observed to decrease of by factors of 2-

3 compared to initial values after very short term irradiations. The current continues to be sensitive to temperature and can be reduced quite considerably by relatively modest cooling.

During irradiation there are changes in the effective dop- ing of the bulk material, either as a consequence of donor

removal or by acceptor introduction, or possibly both. After sufficient fluence of heavy particles, the bulk silicon, which is normally n-type, is observed to invert type and behave as p-type silicon. A typical fluence of 1 MeV neutrons to bring this about is @in” - 2 x lOI cm-*; more detailed studies as a function of starting material resistivity [ 191 have pa- rameterised the dependence on initial doping concentration

( NO) as @i,, = 19Na, with NO in cmW3. During annealing at ambient temperature there is an initial

recovery of donors (or acceptor reduction) but later a slow increase of acceptor density is observed (“anti-annealing”). This is especially marked after high fluence irradiations and,

since depletion voltage depends on doping concentration, leads to a need to operate the detectors at higher voltage to maintain full depletion and sufficiently rapid charge col- lection. Cooling to below about 5°C has been observed to control this effect but at the expense of the initial beneti- cial anneal, during which some of the damage is apparently repaired. Thus a compromise is required to optimise the op-

erational lifetime by minimising the leakage currents and anti-annealing but avoiding freezing in all the damage which

occurs during irradiation. The optimum temperature appears to be in the range -0’C.

3. Models of bulk damage in silicon

Unfortunately a complete explanation of all these obser-

vations has so far eluded those investigating these effects. A few models of the bulk damage mechanism have been proposed, some of which appear to ignore previous data reported in the literature (many experimental studies have been carried out over the last few decades). Few of the mod- els have sound foundations in solid state physics. In one simple model the doping changes are explained by donor re-

moval through the phosphorus dopant being rendered electri- cally inactive by combining with a lattice vacancy. The intro- duction of a shallow acceptor was then assumed to give rise to the type inversion, once the phosphorus was exhausted. The combination of donor removal and acceptor creation provides a satisfactory parametedsation of much of the data on doping changes during irradiation, but does not explain the anti-annealing effects without a better understanding of the nature of the defects present in the silicon.

Despite being initially attractive, some problems are posed by this simple picture. There is little or no direct evidence for sufficient phosphorus removal to explain inversion and

recent Deep Level Transient Spectroscopy (DLTS) studies show a relatively low removal rate. There is no clear can- didate for the shallow acceptor state and it is not obvious why apparently few donors are introduced. Past studies on high resistivity silicon have observed that unbiased material behaves as almost intrinsic and suggest that type inversion is observed only in diodes. Although there is not complete

consensus between experimenters on this point, it has be- come clear that the phenomenon of type inversion is more complicated than the simplest model would suggest.

Some time ago the suggestion was made that more rigor- ous semiconductor physics could explain many of the obser- vations by means of the creation of “deep” acceptor states. Deep levels are those states which are found closer to the centre of the band gap, while shallow levels are found closer to the band edges. In silicon shallow acceptors and donors, such as boron and phosphorus, are used to dope the mate- rial since they generate states which are a few meV from either the conduction band (phosphorus) or valence band (boron) and are therefore normally ionised at room temper- ature, providing either an electron or hole to the appropriate band. In contrast deep levels can exist in different charge states depending on other conditions in the material.

Most deep levels have two charge states, so an acceptor is negatively charged when occupied by an electron and neutral when empty, while a donor is positively charged when un- occupied but neutral otherwise. In unbiased material, where thermal equilibrium holds, the occupation of a state depends

G. HaWNucl. Instr. and Meth. in Phys. Res. A 368 (1995) 199-204

on Fermi-Dirac statistics and is calculable if the concentra-

tion of states and their energy levels are known. In biased

material the situation is more complex because the charge state depends on the energy level but also on the density of

free carriers. Thus it is necessary to solve Poisson’s equa- tion for the electric field, satisfy the current continuity con- ditions and include the correct occupancy statistics. These

are coupled together in a subtle way which makes a numer- ical solution essential in most cases. However some insight can be gained by simplified analytical calculations.

Such calculations were carried out by Watts and collabo-

rators [ 181. They hypothesised that after high particle flu- ences the bulk material was close to intrinsic silicon and,

to simplify the calculations, assumed a single deep acceptor level at the middle of the forbidden energy gap. Using the measured value of current density as a function of fluence

and the state occupancy from Shockley-Read-Hall statis- tics, they carried out a numerical semiconductor simulation in which they calculated the depletion voltage as a function

of fluence, and thus the effective doping concentration.

In this calculation there is only one free parameter, which is the introduction rate of the acceptor state. This was ad- justed to ensure that effective doping at high fluence con-

formed with data. Changing the assumption that the accep- tor is not at mid-gap leads to a simple scaling of the result so

. Acceptors -high-temperature irradiation Acceptors - low-temperahue irradiation

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4

Defect energy level relative to centre of band-gap (ev)

Fig. 1. The lines ascending to the right show the introduction rate for an

acceptor state at a given energy level required to give p = 0.016 cm-’ [ 181 for different assumptions of electron and hole cross-sections. Circles

show known or possible acceptor state introduction rates from the model [ 231, squares show known donors. The tines ascending to the left show the

introduction rate of donor states required to be consistent with the presence

of CO and CC states. They correspond to &onor = 0.0007 cm-‘.

3 l”‘*

_ 2101*

?lj 1.5 10’2

% 1 1o12 z

5 10”

0

E ,nll

CI Neff p-type 0 Neff n-type t

-J I”

0 2 10” 4 lolJ 6 10” 8 lOI 1 10” Neutron Fluence (n axi*)

Fig. 2. Effective doping concentration (Ned as a function of 1 MeV neutron

flux for n- and p-type silicon. Lines represent fits to data from the RD2

collaboration [ 3 I and symbols are from model calculations [ 18 I.

that the required introduction rate can be plotted as a func- tion of the position of the state (Fig. 1) . With these simple assumptions it is possible to explain the observed behaviour

of silicon diodes after neutron irradiation: detectors show effective type-inversion when under bias and the trend of effective doping concentration with fluence is explained sur-

prisingly well (Fig. 2). In this calculation, no phosphorus removal was invoked.

Of course the model does not exclude phosphorus re-

moval, nor does it require that only a single deep acceptor state be present. However it does suggest that an alternative explanation based on deep levels generated during irradia- tion is likely. The essence of the hypothesis has been ver-

ified by observing the depletion behaviour of diodes under illumination; it is clearly observed that irradiated detectors behave differently to non-irradiated ones in a manner consis-

tent with deep traps being filled by photo-generated carriers

[W.

4. Microscopic explanation of bulk damage

The deep acceptor model was in fact motivated by exper- imental measurements carried out by the same group, using DLTS as a tool to measure the introduction rates of certain defects [ 181. These showed that in detector grade material

the rate of phosphorus removal was significantly less than required to agree with the donor removal-shallow accep- tor model. However, one drawback of the DLTS method is that it can only be used in conditions where the trap con- centrations are less than the doping concentrations, so it is restricted to low fluence irradiations. In contrast other tech- niques, such as Thermally Stimulated Current (TSC) , can be used after high fluences but also have difficulty in identi- fying states close to the centre of the band gap. In addition DLTS provides additional information on the type (acceptor or donor) of the trap, whereas TSC is blind in this respect. Thus a combination of experimental techniques is required to characterise fully irradiated detector material. Even then interpretation of the results may not be simple and the ab- sence of a complete explanation of these phenomena after many years of work by solid state scientists is evidence of

V. DETECI-OR AND TRIGGER PROJECTS

202 G. Hall/Nucl. Insrr. and Meth in Phys. Res. A 368 (19951 199-204

this; a guide to the evolution of defects in the material is also required.

This may be provided by considering from first princi- ples the effects which occur during irradiation. A frequent effect of a high energy heavy particle is to displace an atom

from its lattice site, often with a high recoil energy. This re- coil atom produces a cascade with many interactions, some of which also produce energetic secondary recoils. Each of

these terminates in a primary damage cluster which contains a high density of self-interstitial (1) and vacancy (V) pairs as well as significant amounts of disordered regions which

probably amalgamate into multi-vacancy complexes. In con-

trast photons and electrons rarely generate multiple recoils and give rise to a more uniform distribution of isolated point

defects than that produced by heavy particles.

irradiation to levels expected at the CERN LHC. The results

were compared in several cases with experimental data in the literature and found to be in good agreement.

5. Candidates for a deep-level acceptor

Vacancies and interstitials are highly mobile at room tem- perature and much below. At the centre of the cluster, where the initial concentrations of these defects are very high, di- rect I-V recombination is the dominant process and most

(>90%) of the primary defects are annihilated in a very short time. Interactions between the primary defects also take place in the cluster volume. The interstitial atoms dif-

fuse more rapidly than the vacancies after the initial recom- bination so some multiple vacancy complexes, especially di- vacancies (Vz), are formed. Because photons and electrons do not produce clusters, direct divacancy production is neg-

ligible in comparison to heavy particle irradiation. Interactions of defects with impurities are initially rare

because of the size of the cluster (linear dimensions -100 A). Later quasichemical reactions with impurities become more important, leading to the formation of stable radia- tion defect clusters. Most impurities (carbon is the notable exception) and defect species are immobile, for example the divacancy is immobile until -560 K. Reaction rates

are controlled by the concentration of impurities and de- fects and their capture radii. Some typical reactions are:

I+Cs +Ci, v+o+vo, Ci +C, +CC,

I+v? -+v. v +P-+vP, c,,+ 0 --+ co,

I+vP+P, V+V0+V20,

I + VT0 + vzo. V+V20-+V30

v+v+vz,

An attempt was made to identify candidates for a deep-

level acceptor, by deriving the introduction rates of all the defect species from the numerical model over the range 0 < @ < lOI ncm-‘. Some of the states, such as CO and CC,

have been identified as hole traps in DLTS spectra so are not

candidates for an acceptor state. Two strong candidates were identified - VZO and VsO (Fig. 1). If it is indeed the case that V20 and VsO may act as acceptors, one or the other could account for the changes in Nee with neutron fluence. However, only V20 is a candidate for the acceptor causing type inversion since at fluences N 2 x lOi3 n cme2 there are too few VsO defects to compensate the remaining phospho- rus. The typical V20 concentration at the inversion fluence is 3 x lOI cmp3. VsO could however be more important in

subsequent annealing. Other experimental data also support the explanation using deep levels. This can be seen by con- sidering the situation following type inversion supposing the presence of a single deep level with concentration NA and occupation probability f~. The effective doping concentra-

tion Ncs is

NOR = LANA .

0.025

I+V-+Si, v fV2 +v3.

The probability for a specific reaction is determined by the concentrations of reactants weighted by their capture ra- dius. Much oxygen is present ( ~10L6 cmp3), so phospho- rus removal is limited. Similarly, high carbon concentrations (also - 1 016 cm-‘) provide the major sink for interstitials.

z 0.01

0.005

A computer model of defect evolution which was devel- oped to explain optical absorption data [ 2 I] from electron irradiated silicon has been applied to attempt to understand better the concentrations of important defect species dur- ing irradiation [ 22,231. Measured introduction rates of in- terstitials, vacancies and primary divacancies from electron and neutron irradiation [ 18,241 were used as input and the model was applied to investigate the evolution of defects in high resistivity detector material during neutron and gamma

Fig. 3. The relationship between the leakage current damage constant (a)

and the effective doping density ( N,R) for diodes irradiated at different temperatures to similar Ruences [ 221. The data plotted are points generated

using tbe parameterisation by the authors of Ref. ( 11 I.

G. HaWNucl. Instr. and Meth. in Phys. Rex A 368 (1995) 199-204 203

Under bias fA, Nes and the current density, (Y@, are related

by Poisson and continuity equations. This leads to the rela- tion

where k is a constant and ni is the intrinsic dopingconcentra- tion. Fig. 3 shows the linear relationship predicted during the post-irradiation annealing of the detectors. In other words,

the trap occupancy, and consequently Ned, is limited by the free electron density in the material, which is determined by the leakage current. The same correlation between leakage

current and effective doping has been observed by measur- ing irradiated detectors at different temperatures [ 251.

If a second level contributes with occupancy f2 and con- centration N2, then. provided it is present at much lower concentration,

Ncn = fANA + f?Nz.

In this case the relationship becomes a little more complex

which seems to explain the non-zero intercept in the data.

6. Detector hardening

If the model is generally correct, then key ingredients in possible radiation harder detectors are the concentrations of

oxygen and carbon since these are the dominant capture sites for vacancies and interstitials. If the carbon concentration is reduced, the accumulation of multi-vacancy complexes should be suppressed because more interstitials will be cap- tured at multi-vacancy sites in the early stages of irradiation, and the carbon will be depleted, later releasing more inter- stitials for multi-vacancy defect reactions.

Conversely, a high oxygen concentration suppresses phos- phorus removal and encourages A-centre formation at the expense of VZO and V10. The A-centre appears to be rela- tively harmless because of its distance in energy from mid- gap, so it seems that an increase in the initial oxygen con- centration would be beneficial.

The modelling supports these conclusions [23] but clearly experimental tests are now required to verify if in- deed the multi-vacancy oxygen complexes are at the root of the bulk damage observed. This will require a close interaction with silicon manufacturers. However this will probably be essential for material quality control since radi- ation hardness could be severely compromised by a choice of non-standard material which would not be apparent until detectors had been operated for some time at the LHC.

7. Hypothesis for anti-annealing

To be fully satisfactory a model of bulk damage ought, if possible, to provide an explanation of anti-annealing. The most obvious mechanism in the framework of this model is by release of vacancies during annealing of clusters. This im- mediately suggests that anti-annealing should not be present

after photon irradiation, which is supported by experiment [ 201, although type inversion did occur after gamma doses in the range 100-200 Mrad, also as expected in the model.

There is some DLTS evidence for an unidentified defect 0.4 eV below the conduction band which anneals at -350 K leading to increased A-centre concentration. A possible

hypothesis is that this is a @i-vacancy (V3) produced during

the early stages of the evolution of the damage clusters. The vacancies released by annealing of the &i-vacancy are

subsequently captured and thus increase the concentration of

the deep-level acceptor candidates V20 and V30. The defect kinetics model has been used to compare the predictions of

this mechanism with the observed doping changes in n-type detectors during reverse-annealing assuming V? + V2 + V or V3 --t 3V. At present it is only possible to say that the data are not in disagreement with this hypothesis and a stronger

conclusion awaits a full calculation on Nerr in detectors under bias.

8. Conclusions

The deep acceptor model appears to provide a key to

understanding more deeply the origin of radiation damage in silicon. If the model of defect evolution influenced by impurities in the bulk is correct, then there is real hope to improve the hardness of irradiated detectors by modifying

the oxygen or carbon content.

Acknowledgements

I warmly thank Steve Watts, Barry MacEvoy and Karl Gill for valuable input.

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