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HUPD-9013, August1990.
Surface Damage Effects of PIN Photodiodes
by Gamma Rays
T. Ohsugi, H. Kojima and H. Himeno
Department of Physics, Hiroshima University
Hiroshima ?30, Japan
T. Kondo and M. Noguchi
National Laboratory for High Energy Physics (KEK)
Ibaraki 305, Japan
M. Hasegawa and S. Mori
ofrsukuba
Institure
y;;r"*
"o"lTlililxi:
Abstract
We have investigated the radiation damage effects of PIN photodiodesby
gamma rays from a 60CO source. An interesting breakdown-like behavior of the
I-V curve has been observedbetween 10 3 and 10 5 Gy (Si). At irradiation dose of
8.7 x 10 5 Gy (Si), this behavior disappeared.This damage effects can be
interpreted as a positive charge buildup in the silicon dioxide, which gives rise
to a formation of inversion layer at the edge of p+ channel.
1 Introduction
The radiation damage of the detector is still one of the major concernsfor
practical applications to high energy and high intensity hadron collider
experiments. Although a silicon strip detector has many outstanding features
as a high energy particle detector, the radiation damage may be the most urgent
problem for the silicon detector.
Intense efforts have been focused on the radiation damage study of the
microelectronicsmade with silicon semiconductorby the space-physicspeople.
A considerable amount of data for the damage of MOS structures have been
accumulated.and their damage mechanism is interpreted by a surface damage
model [1]. However, experimental data of the silicon parficle detector which size
is signiAcantlylarge compared with the micrOelectrotticS are not
evaluate perfo4nance of silicOn detector for large scall applicatiOn tO the
experilnents inl五 gh radiation en宙 ronmentso We repttt here the results on the
1
ganllna―ray damlge test of PIN photodiodes.
2 Expe五mentatt PrOcedures
■
￨
PIN photodiOdes which are the same as thoseedtぉ
in the previous
neutron irradiatiorL teSt[2]were exposedito gamma rays froII1 60cO source at
the gamma‐
raプ
apanOf」
lrradiatton facility ofthe TOKAI Establishment
Atomic Energy慶
にsearch lnstituteo The.exposure watt perfOrmed two times and
the resultS frOtt these twO exposures were cOnsistent"ith each other.
The structure of this PIN photodiOde is depicted schematically in Fig.1.
T h e r e s i st ty iば宙
t h e w a f e r u s e d f o r t h i s p h o5 t ok dΩfi o rd e b Oi ts h 3 p∼ a n d n
type detectors.The
e,high resistive substrate
(n―
)p―
type detector has a p_(n_)tyわ
which is fully depleted and becomes sensitive to a penetrating particl
readout electro4e is surrounding a light receiving宙
Ⅲdow a10ng the detector
edge.
￨
The totalirradiatton dose was varied from 80 to l.7x105 Gy(Si),which
w a s e v a l u a t e d t t t暉hcee ■
to tm s 協
the 60cO gammalay source and the
irradiation tima The samples
re wё
contained in a plastic
thiれ case and
irradiated.During the irradiation no bias voltage we」
じsupplied.The irradiation
t i m e s w e r e f r oF■
ew hours t0 0ne day.:
￨
The radiation damage was measured by meanslofthe leakage curreit
and pulse helght distributions for lniniコ爆
um ionizing particles.Since the
temperature coettcients Of a senliconductor is considerably large,an
C in the
parameters were measured at room temperature kept,within 24」 °
electromagnetiOand light shielding boxl
.
Elect五ca:meaSurements were performed,aftelthe shOrt― teFm reCOvery
or annealing"4s cOmpleted.Du五 ng the measuremel乱 s,slow but tertain
「
amount of anne・
ahng was still observed。
￨
3 Results and DliScussiOn
a)Leakage Current and Surface Damage
i
The leaktte Currents as a Functiorllof bias evoltaな
for Various irradiation
dose are shownin Fig。
2.The sharp breakdown―like increase ofleakage current
i s s e e n a r o u n d 8 1 0 V f o r t h e i F r a d i a t i o n d )o 。
sTeh eo fb r8e0a kGdyo(w鎌
n voltage
gets lower and the current increase becomesless steep with increase of
irradiation dose. It reachesto 50 V with the doseof 2.4 x 10 4 Gy (Si). Beyond a
few times 10 4 Gy (Si), the breakdown voltage turns into a recovery phase. At 8 x
10 4 Gy (Si) the breakdown-like voltage comesback to 70 V and finally the
breakd.own-likeincrease of the leakage current is disappeared below 100 V at 8.7
x 10 5 Gv (Si).
Another interesting feature of this breakdown-like behavior of leakage
current is a current saturation with increase of bias voltage, which is clearly
different from usual breakdown current behavior.
How do we interpret this curious behavior of leakage current versus
ir-radiation dose? An interesting hint was given by the investigation for
microelectronics [1]. An ionized charge is trapped and accumulated in the
insulator, SiO2 and interface between insulator and semiconductor. Normally
positive charge is accumulated in the SiOZ insulator and negative charge in the
interface, which makes inversion layer in the p channel facing to SiO2
insulator. The edge of the p+ channel in which a dopant concentration is
usually low due to diffusion processeasily inverted into n- by the electron
attracted with positive charge accumulated on the silicon dioxide facing to the pn junction, as shown in Fig. 3. In the thin, inverted region between the edge of
the p+ channel and the silicon dioxide insulator, the field strength becomes
strong, as increasing of charge accumulation and as increasing reversed bias
voltage, enough to generate an electron-hole pair across the energy gap. This
kind of generation current should behave like a breakdown cunent as a function
of bias voltage. This is a possible scenario why the leakage current increase
steeply with increase of dose. To confirm this scenario, pinpoint irradiation test
on the region in which the p-n junction facing to SiOZ insulator is proposed and
its result must be compared with the data irradiated on the region in which
there is only p channel under the SiOZ insulator.
There is one possiblescenarioto explain the phenomenonof the
breakdown voltage recovery at higher dose as follows: The leakage current due
to the bulk damage exceedsa certain level and then the current flow eats the
electric field at the sharp edge and prevent the breakdown-like current increase
or the free charge supplied by enough current compensatesthe field generated
by the charge accumulation in the insulator. This scenario might be supported
by the fact that for the non-irradiated samples the breakd.ownvoltage shifted
somewhat higher voltage with the secondtrial of the breakdown voltage
measurement iS gurrent flows more than a certain leraelbeyond breakdown
voltage at the first trial. This self-improvement effct decayedwith one or two
days time cons麟
t。
b)DifFerenoe tttweenp‐
type and‐
typedetectOr
We have exanlined radiation hardness of p―
shom in ng.3,the.p―
l
typeiand n‐type detectors.As
type detector seeml to be harder'than n,type detector sO
long as gamma■
ay irradiation is concerntd:Between 10 3 Gy and 10 5 Gy(Si
type detector s趨
餘)rs from cOnsiderable breakdown―
li卜
l phenomenon,while the
type detector ttas little change Of I_V curve.This catt be interpreted,as
p‐
discussed abovtt by the forma饉 on ofthe inversion lay■ at the interface between
the silicon dio対
囃e and p―type silicon,not at the interfaCe between silicon dioxide
and n―type silic轟。Then the questiOn is that the p‐typeldetector is surely better
than n‐type detOこtoro we do五 ot have a conclusive ans,er because it depends on
a detail structutt of the detector.For the Case ofthe p=otodiOde used in this
expettlnent,p‐
ousl,better than nltype one.On the other
句わe subStrate is ob宙
hand,ifwe conttder abOut st五
n‐ st五
p detector,lthetype
p― substrate andtype
p
structure may l饉
ve seriOus problem of channel isolatiわ
n between n+strips.Two
attaCё
n t n+strips may be comected thrlugh the invelsion layer at the inter
between sittcon颯
10対de and p―
type substFate.
￨
￨
At 8.7x10も
Gy(Si)there is no qualita髄
ve difFer4nce betweenp and n_typ
detector. The童擬gnitude ofleakage cur】哺nt is still bellw ILA which is not yet
se五ois compared with the proton and neutron damatt efFects。
c) Bulk Damagi
The leakale current below bias voltage of the brbakdown-like increase
indicates usual plateau and increases gradually with increase of irradiation
dose. This part can be explained with the bulk damagd by gamma rays.
Assuming linear increase of leakage current at bias vdtage of 20 V, the leakage
t
current constant , CI1is defined [2] as
;
(A . cm -3)
Alleak = q.y. D
where Alleak is the leakage current increase and D istthe irradiation dosein
- 11 .
e
unit of Gy. The hakage current constant, srywas evaludted to be 1.7 x tO
- 1^.e
cm-'. This value should be comparedwith the 12 GeV proton value [3], 1.0
Gy
- 1.
.--3. The bulk damage by gamma rays is almost three order
x 10 -8 A . Gy
of magnitude less than that of protons.
d) Long Term Annealing
Fig. 3 shows the leakage current measured at one year after the
irradiation. Still the breakdown-like behavior persists, but a signifrcant
annealing (reeovery)at room temperature is seen both for the breakdown voltage
and.the magnitude of leakage current. There seems to be a considerably long
component of the decay of the charge accumulated by the ionization dose.
e) Particle Signal
Pulse height spectra for penetrating B rays from 90Y have been measured
with the bias voltage of 40 V by a coineidencemethod. The pule height
distributions for the non-irradiated sample, the irradiated with 2.4 x 10 a Gy (Si)
and with 8.4 x t0 4 Gy (Si) are shown in Fig. 4. The separation between signal
and noise degraded gradually with increase of irradiation dose. The peak pulse
height of the distribution is depicted as a function of irradiation dose in Fig. 5.
The pulse height degrades gradually with increase of irradiation dose and
seemsto reach lower limit of |Vo less level at a few times 10 4 Gy (Si).
4 Conclusion
We have investigated the radiation damage effects on the PIN photodiodes
5
by gamma rays from a 60CO source with irradiation dose up to 8 x 10 Gy (Si).
The leakage current as a function of irradiation dose were measured as a
indication of radiation damage effects.
The breakdown-like behavior of the leakage current increase (a steep
increase of leakage current) was observed and the breakdown voltage (starting
voltage of the steepincrease)moved lower with increase of doseup to a few times
t0 4 Gy (Si). Beyond this irradiation dose,the breakdown voltage moved back and
at 8.? x 10 5 Gy (Si) it exceeded100V which is more than nominal breakdown
voltage. This phenomenon can be interpreted by the surface damage effects
which is due to charge accumulation in the silicon dioxide layer. This damage
effects seemsnot to be serious for particle detection by looking at the signals of
penetrating B-ray.
The buik damageby gamma rays was more than two order of magnitude
less than that of protons so long as leakage current measurement.
The radtttiOn hardness cOmpans6n between
n‐ type
type and
p― detector
was discussed。
I The surface damage efFects are surel,difFerent but the
hardness is strongly depend On the det〔
五l structure of the detector.
1
Acknowledgmclnt
・
1
‐
We wishtO thank Mr.Yamamoto,Hamamats,photOnics,fOr provi
us thё
test samples an(l for valuable discussiOn in th('cou
We wOuld like“
thank the member ofthe
on Radia伍
Slfty Center at xEK.
T h i s w o r k w a s 3 u p p O r t e d i n p a r t b y t h e liGnr―
aaindt ‐
fOi Scientiflc Research
under the Miniぶtry of EducatiOn,Science and Culturα 5」apan and in part by the
」apan_us c。 01籐
ratiVe Research Program for High Elergy PhySics.
References
l
[1]See,fOr example,G.Ct Messenger and MoS.Ash,'Thw EfFects 6f
RadiatiOn On Elここtronic systems",Van Nostrand Reitthold Company,New
York,1986,p2161
1
[2]M.Hasegaマ嘩こt al.,Nud.Instr.and MethOds,A277(1989)395,
H o W o K r a n e f e t a l . , N u c l . I n s t r . a n d M e t h O d s , A12978或
9)266,
T.A五
ma et域
。
apaneseJourna1 0fApplied Physits 28(1989)1957
,」
E.Fretwurst et al.,Nucl.Instr.and Methods,A288(1990)1.
[3]T.Ohsugi etal。,Nucl.Instr.and Methods,A265(19188)105。
1
MoNakamura et al。 ,Nucl.Instr.and MethOds,A2菊 (1988)42.
Figure CaptiOn轟
￨
‐
Fig。1.The schematic diagram for the sttucture of thO PIN photodibde.
Fig。2.The lёa聴 ge currents as a function of bias voltage for variOus
lrradiation dos16fgamma rays.
1
Fig. 3. The mechanism of making breakdown by the idnization irradiation.
Fig.4. The compirrison of the leahage cunent between the n-type and the p-
l
.W,J€
Fig. 5 The leak{ge currents measured one/ ear after.
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2.
Comparisons with 1993 milestones-
One year ago we foresaw to "test the 32 channel version of our front end system
in coirventi6nal and radiation hard form as a Precursor to constructing a full size
readout chip. It will be evaluated in a beam with full size microgtrip detectors.
We aim to produce a 128 channel version of the front e1{ c\ig with a
multiplexing Llu^et t incorporated. A radiation hard version of the chip will be
submitted flr manufacturi. We will construct a double sided module, to be
partially insfiumented untit the 128channel chiPs are available."
The DRDC targetswere to
comptetEthe design and fabricationof LHC prototype single- and doublesided detectors,
d.emonstratea radiation hard (l0Mrad) 128 channel readout chip,
demonstratean LHC detectorand electronicsreadout system,
which we did not ocpectto completefully in one year only'
The first milestone has been largely completed with both full size single-and
double sided prototypes produc-ed.a furtner production run is currently in
which have emerged
ft"gr"s whicir e*bodi"s'all the miigr_ desigrr features
iioti ont srudies in a set of double sided detectorsand this should be completeby
the end of the year.
︱︱︱︱︱︱︱︱︱︱︱ｌｌｌｌｌｌ︰︰︱︰日日︱︱︱︱︱︱︱︱︱︱︱︱︱︱︱ ■ ■■ ■■ ■■ ■■ ■■ ■
The 32 channel chip was in fabrication one year ago. Oyq highest pti.odly was to
r,rU*it the compl"i" fZA c-hannelchip as soon as possible after we had verified
that the 32 chainel version functionld correctly. Late deliverie from both the
manufacturers of the conventional chip (FElix) and hardened chip (APV3),
aeiayea this evaluation but both chips were-found to work extremely well. The
submission of the hardened 128 channel chip with multiplexer was made in
Sepember and.we exPectdelivery in ]anuary 1{5: Our measurementsshow that
thi Harris process,tsed for our ti8 charmelchip is hard to well beyond l0Mrad.
The systemsissues alluded to by the DRDC are being addressedby-members of
RD20; mainly in the context of their contributions to ATLAS and CMS
i"p*it"iy. Thu d"*o*rration of an LHC detector fully equipped with front end
electronicswill become a high priority of RD20 in the coming year. The full
electronicsreadout systeminJuding daia transfer from the detector is considered
to be experiment specific. A beani test is underway at the time of writing in
which a detector, ApVg chip, off chip multiplexers and analogue optical link
have been connectedtogethir for the-first time to read out minimum ionising
particle pulses. This is ihe baseline sys_teTfor CMS, where a fully -analogue
;y;t"* ii at a fairly advancedstage-oi-design,and is also bging consideredby
anteS, which miftrt implement i fully analogue or digital s-ystemusing the
uirderstanding of the implications of constructing a
RD20 iront end. tt
"
completesystem are important in eviluating the-merits of one particular front
end'readout chip in comparisonwith another. Flowever, we foresaw that this
milestone wouldbecome a higher priority during the coming year.
Many technicalnotes have been produced in the course of this work; they-are
nste6 below. Severalpapers have been published or submitted for publication,
also listed below. The papers and technicalnotes are all avaiiable on request.
s uOrl1994
RD20 S鯰 `“
Far 2

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