50
Nuclear Instruments and Methods in Physics Research 226 (1984) 50-55
North-Holland, Amsterdam
R E S P O N S E S T U D I E S O N SILICON SURFACE BARRIER D E T E C T O R S FOR LOW ENERGY
ION SPECTROMETRY
I N SPACE
J,J. V A N R O O I J E N ,
P. L O W E S a n d W , A . M E L S
Laboratory for Space Research Utrecht, Beneluxlaan 21, 3527 HS Utrecht, The Netherlands
J. H E N R I O N
Space Science Department of ESA, ESTEC, 2200 AG Noordwijk, The Netherlands
Accurate measurements of the three-dimensional directional distribution of low energy ions in space require a careful calibration
of the silicon surface barrier detectors mostly employed. This paper describes a systematic search for the pulse-height defects in the
detector response to protons in the 25-150 keV energy range. The results ensured a proper pre-fiight calibration of the ISEE-3 proton
spectrometer and also allow some conclusions on the ratio of the average energies % and %_ expended for electron-hole pair
generation in silicon.
I. Introduction
In the last decade, several instruments for measuring
low energy ions in interplanetary space have been flown.
In particular, there is a growing interest in detailed
studies of three-dimensional directional distributions in
the energy range ~ 10-1000 keV, in combination with
good spectral mapping, The particle environment, as
encountered in interplanetary space, is briefly as follows. There is a continuous plasma flow (solar wind) at
a speed in the range 300-900 k m / s . The ion component
consists mainly of protons, having a nominal flux density of the order of 10S/cm 2. s. The proton energies,
corresponding to the solar wind velocity, are in the
range 0.5-4.0 keV. Frequently also proton (ion) populations with energies up to ~ 1 MeV are observed. These
intense flux enhancements of energetic particles are
mostly associated with interplanetary shocks and the
investigations concern the origin, acceleration and propagation of the particles.
The requirements to be met in order to achieve
reliable directional distribution measurements in this
energy range have been discussed elsewhere [1], In case
a multiple telescope system is employed, one should aim
at telescopes with identical performance, in order to
eliminate any conceivable "instrumental count rate
modulation". Usually the required instrumentation is
equipped with silicon surface barrier detectors (SSBD).
The reliability of the detectors to be flown must be
thoroughly tested; particularly because of the long lifetime requirement on several missions and the detectors
must be properly calibrated.
This paper deals with some of the results obtained
0168-9002/84/$03.00 © Elsevier Science Publishers B.V.
(North-Holland Physics Publishing Division)
during calibration of the detectors for the ISEE-3 proton spectrometer [1]. For quick reference, the data flow
from this instrument and a schematic representation of
the telescopes are depicted in figs. 1 and 2, respectively.
The telescopes have a certain geometric factor, which
has to be calibrated, and their construction ensures an
effective elimination of instrument response to sunlight,
electrons and several other unwanted events. The front
detector in the "2-element" telescope, operated at
- 2 5 ° C , is the actual spectrometer.
The response of SSBDs to low energy ions involves
many competing interactions, normally not relevant in
the MeV-range, as well as phenomena in the silicon
wafer, which may critically depend on the choice of
base material and on the details of detector processing
[2]. We have carried out systematic studies on the SSBD
response to protons in the energy range 25-150 keV.
We shall present and discuss in the subsequent sections
the observed pulse-height defects and their dependence
on some operational characteristics of the detector. We
will also evaluate the data in terms of the ratio of the
average energies ~p and % expended for electron-hole
pair generation in silicon.
2. Elements of the SSBD calibration
The program of SSBD calibrations we have carried
out involves: 1) a systematic determination of pulseheight defects (PHD) in the response of n-type detectors
to protons in the energy range 25-150 keV; 2) a verification of calculated backscatter fractions (Au contact)
at the lowest energies in this range; 3) a determination
J.J. van Rooijen et aL / Silicon surface barrier detectors
51
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The data flow consists of 192 channels of information on the proton distribution function, transmitted every 16 s.
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II. APPLICATIONS
52
J.J. van Rooijen et al. / Silicon surface barrier detectors
of the effective threshold of SSBD-response (Au contact) to proton energies in the keV region.
Contributions to the PHD stem from energy losses,
in the metallic contact as well as in a possible insensitive layer underneath the contact (the "dead-layer"),
but also from "end of range effects": the ionization
process caused by the ions has a threshold, for protons
of the order of hundreds of eV [3]. Also, the unproductive nuclear collisions, at low energies enhanced with
respect to the electronic stopping power, add to the
PHD. There is obviously the need for a comparison
standard of detector response. Assuming that the average energy c expended for electron-hole pair generation
is independent of particle species and energy, one may
either choose MeV a particles of well-defined energy or
lower energy (conversion) electrons. In both cases, the
above defects are relatively small. The inclusion of a
charge collection deficit throughout the whole silicon
wafer in the PHD of low energy ions is less meaningful,
since to a first approximation it affects the absolute
response to all particles to the same extent.
In view of some aspects related to operation and
handling of space instrumentation and considering the
well-known inconsistencies in dead-layer results as obtained by several groups (e.g. ref. [2], and the references
therein), the PHD was measured as a function of proton
energy, detector bias, temperature, time (years) and
radiation damage. We have measured the PHD of SSBDs
from several manufacturers, both for Au and A1 contact
irradiations.
3. Experimental procedure
Elad et al. [2] have discussed several methods for
measuring the dead-layer of a SSBD. For our purpose,
we used a different procedure, particularly useful at the
low energies to be studied and frequently applied by
space instrumentation groups, (e.g. ref. [4]). The method
involves the use of the small potential drop accelerator
at Goddard Space Flight Center. This facility is capable
of providing low intensity beams in the 1-150 keV
energy range. At a preset accelerator terminal voltage,
e - and H - beams of equal energy E 0 are produced.
These beams are alternatively directed towards the detector, with the help of analyzing magnets. The major
fraction of the detector area is irradiated. The telescope
with the detector is placed in a vacuum chamber and
can be cooled as desired. Pulse-height distributions are
recorded with the help of a multichannel analyzer. The
spectra are real-time analyzed using a peak evaluation
routine and are stored on magnetic tape for later
processing if desired. The energy calibration of the
detector response is obtained with the help of the 127
keV conversion electrons from the decay of C e 139. In
first approximation, the PHD of the protons is defined
as AE(keV) = E o - g p = g e - E p , where E e_ and g p
are the detected electron and proton energy, respectively. The consistency of the results was verified by
varying the beam intensity, the amplifier time constant
and the energy resolution; the latter in view of the small
total absorption peaks of > 100 keV electrons in 30 t~m
thick detectors.
By changing the polarity of the terminal voltage, the
facility provides beams of H + and H~- ions, which are
very useful for the backscattering and keV-response
experiments.
4. Experimental results
In fig. 3, representative pulse-height distributions are
shown. These distributions indicate that the dead-layer
is fairly homogeneous across the Si wafer. Fig. 4 shows
representative PHD results for a 30 ttm thick Philips
detector. The PHD is strongly dependent on the applied
detector bias. Within the experimental accuracy, no
temperature dependence is found. Ortec detectors of the
same dimensions, but made from 300-600 ~2cm material,
on the other hand, have no clear bias dependence. The
relation between bias dependence of the PHD and the
resistivity of the base material has been found earlier.
Caywood et al. [5] provided a model to account for
these findings, but there are still conflicting experimental results in this area [2]. The major point of interest is
the observed shape of the PHD(Ep) function, which
applies equally well to Philips and Ortec detectors; it
deviates strongly from electronic stopping power predictions, for both the Au and A1 contacts. We shall comment on this point in sect. 5. In fig. 5, we have displayed
the relevant portions of the stopping power functions
for gold and aluminium, which have been used in our
analysis of the observations.
Fig. 6 shows some results on 250 ~m thick detectors.
The detectors are operated at a 50% overbias (Vdepl = 25
V). Irradiation through the Au contact does not reveal
any temperature dependence of the PHD. On the other
hand, such an effect is clearly present in the case of
irradiation through the A1 contact. It is remarkable that
the PHD increases with decreasing temperature. To the
authors' best knowledge, no such effect has been reported so far. It is also not accounted for in the model
of Caywood et al. [5], which predicts a dead-layer
proportional to k T due to carrier diffusion. Again, the
PHD(Ep) function deviates from the predicted shape,
for both Au and AI contact irradiations.
As to aging effects, none of the detectors studied has
provided evidence for a change of the dead-layer in the
course of time, neither at the Au constant nor at the A!
contact.
For one of the detectors in the badge from which the
flight detectors were eventually selected, the effect of
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53
/ Silicon surface barrier detectors
J.J. van Rooijen et aL
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Fig. 3. Pulse-height spectra for several proton energies, as recorded during the detector calibrations.
radiation damage on the P H D was studied. An intense
beam of 10 keV H ÷ ions was directed to the Au contact
of the detector. The detector was operated at - 2 5 o C.
At regular intervals, detector current and noise and the
P H D at 90 keV proton energy were recorded. Electrical
detector breakdown occurred at a dose level of --- 10 ]3
p r o t o n s / c m 2. Just prior to breakdown, the P H D at 90
keV incident protons was found to have grown by 1.3
and 2.3 keV at detector bias voltages of 15 and 5 V,
respectively. The conclusion, relevant to instrument performance in space, is that the noise increase (measured
in-flight) will be the limiting factor, long before the
increase in dead-layer is significant.
Backscattering from the Au contact was studied with
the help of a beam of H~- ions. Upon entering the
detector contact, the ion is split up into 2 protons,
having a small chance of being backscattered [8]. The
backscattered fraction of protons is thus found by comparing the relative intensities in the full and half energy
peaks. The results agree well with calculated values [9],
based on a quasi-Thomas-Fermi potential for the gold
atom scattering centres. At 30 keV proton energy, the
backscattered fraction amounts to 1.5%. Thus, the
counting efficiency at the lower energies is hardly reduced.
Finally, irradiations with proton energies below 10
keV were carried out. The detector response was studied
in terms of the increase of the noise-width. It turned out
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size of the dots and crosses indicates the experimental error.)
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Fig. 5. Electronic stopping power data used in the analysis of
the detector response. The data are taken from refs. [6,7] and
the references therein.
II. APPLICATIONS
54
J.J. van Rooijen et aL / Silicon surface barrier detectors
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250 /Lm detector: (a) AI contact irradiation; (b) Au contact
irradiation. (Errors are indicated by the single error bar at the
side of the plot.)
that the practical cut-off in the detector response occurs
at about 1.5 keV.
5. Discussion and conclusions
The reported measurements were made to achieve a
proper calibration of a low energy proton spectrometer
for interplanetary space. Our measurements illustrate
the need for careful response studies on individual
detectors. Some of the P H D variations can be eliminated
by choosing appropriate detector operating conditions,
e.g. strong overbias. Obviously, reliable operation of the
detectors should not be affected. On the whole, the ion
response at the A1 contact is not very satisfactory. The
dead-layer is several keV larger than at the Au contact.
Also, even for detectors of the same badge, the deadlayers may differ by several keV. Therefore, we have
flown the detectors with the Au contact looking outward from the spacecraft, despite the greater sensitivity
to scattered sunlight and the enhanced backscattering
from the contact.
For the conventional n-type SSBDs, the P H D at
= 50 keV proton energy can be as low as 3 keV (with 20
f f g / c m 2 gold contact). The recent detectors fabricated
using the planar process [10] are less satisfactory in this
respect. Their dead-layer is = 1000 A thick, corresponding to an energy loss of --. 13 keV.
Let us now consider the deviation of the measured
P H D ( E p ) curves (fig. 7) from the predictions. It can be
shown [9] that the enhancement of the energy loss at
lower energies due to multiple scattering in the metallic
contacts is too small to account for the discrepancy.
Explanations based on enhanced trapping in some layers
of the silicon wafer can also be excluded, since the
discrepancies persist when different semiconductor base
materials and different operational parameters are chosen.
We will show that energy independent ~e /~p ratios
> 1 can account for our results. We start with a "trial
window thickness", which is composed of the thickness
of the contact as specified in the detector data sheets
and some additional insensitive layer in the silicon.
Using the stopping power data of fig. 5, we calculate the
energy loss A E involved. At all energies, we add an
estimated end-of-range defect ~ of 0.5 keV. The result is
a "trial P H D " curve. Then this trial curve is lowered by
an amount proportional to the incident electron energy.
Iteratively (varying window thickness and % / c p ) , a
satisfactory fit to the measurements is obtained (solid
lines in fig. 7). Conversely, we interpret the dashed lines
as the true P H D (Ep):
PHDt~ e = A E + 8 = P H D . . . . + ( ( e - / ( p
-- 1)E0.
The result suggests that the electrons are less "effective" than the protons, in particular for the thinner
detectors. The difference, of the order of 1-2%, is larger
than that reported for electrons vs a particles [11]. A
fundamental difference in % , ~p and c,, is unlikely.
Preferential hole trapping ([12] and the references
therein) has been suggested to account for the spread in
reported ~e J ( a values.
Considering that the difference appears to depend on
the detector thickness, and since electrons have a relatively large X-ray yield [13,14], we suggest that the
escape of Si K X-rays and (possibly) of continuum
radiation accounts for the observations. Note that Si K
X-rays can effectively penetrate through several micrometres of silicon.
Our results cannot reveal to what extent the l e e - / / ( p
ratio depends on the particle energy. At energies below
--- 50 keV, uncertainties in the stopping power limit the
significance of further evaluation. Complete X-ray production and escape calculations, integrating over the
full electron track through the silicon, would be required to support the analysis. But unfortunately, data
J.J. uan Rooijen et aL / Sificon surface barrier detectors
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Fig. 7. Measured PHD (crosses) for 30 and 250 #m thick detectors obtained by averaging the results for a number of similar detectors.
The cross size is a measure of the experimental uncertainty in the PHD values. Dashed and solid lines are discussed in the text.
o n X-ray production cross sections for Z < 2 0 a r e s c a r c e .
Alternatively, one might devise a suitable method for
measuring the X-ray escape fractions as a function o f
electron energy.
With a view to spectrometric applications, w e c o n c l u d e that electron response of SSBDs, particularly in
the c a s e o f t h i n detectors, does not provide a reliable
calibration standard.
This work would not have been possible without the
help of Steve Brown in running the low energy facility
at Goddard Space Flight Center.
References
[1] J.J. van Rooijen, G.J. van Dijen, H.Th. Lafleur and P.
Lowes, Space Sci. Instr. 4 (1979) 373.
[2] E. Elad, C.N. Inskeep, R.A. Sareen and P. Nestor, IEEE
Trans. Nucl. Sci. NS-20 (1) (1973) 534.
[3] H.C. Schweinler, in: Semiconductor nuclear particle detec-
tors, eds., J.W.T. Dabbs and FJ. Walter, NAS-NRC Publication 871 (1961) p. 91.
[4] F.M. Ipavich, R.A. Lundgren, B.A. Lambird and G.
Gloeckler, Nucl. Instr. and Meth. 154 (1978) 291.
[5] J.M. Caywood, C.A. Mead and J.W. Mayer, Nucl. Instr.
and Meth. 79 (1970) 329.
[6] P. Mertens and T. Krist, Nucl. Instr. and Meth. 194 (1982)
57.
[7] A. Valenzuela, W. Meckbach, A.J. Kestelman and J.C.
Eckardt, Phys. Rev. B6 (1972) 95.
[8] A.H. Morton, D.A. Aldcroft and M.F. Payne, Phys. Rev.
165 (1968) 415.
[9] E.W. van Ravenswaaij, private communication.
[10] J. Kenmaer, these Proceedings (Semiconductor Detectors
'83), p. 89.
[11] R.D. Ryan, IEEE Trans. Nucl. Sci. NS-20 (1) (1973) 473.
[12] M. Martini, T.W. Raudorf, W.R. Stott and J.C. Waddington, IEEE Trans. Nucl. Sci. NS-22 (1975) 145.
[13] L.S. Birks, R.E. Seebold, A.P, Batt and J.S. Grosso, J.
Appl. Phys. 35 (1964) 2578.
[14] H.D. Fetzer, C. Kuebker, M.A. Heath and J.T. Dodd,
IEEE Trans. Nucl. Sci. NS-26 (1979) 1171.
II. APPLICATIONS