Nuclear radiation detectors with volumes of approximately 1 cu cm was fabricated from single crystals of germanium-silicon alloy containing as much as 20 weight percent germanium. The properties of these detectors were investigated and will be discussed. Tests reveal that the gamma ray photoelectric peak efficiency of an alloy detector with only 12 weight percent germanium is approximately 4 times that of a silicon detector of equal volume. The room temperature roomure appears to be a good possibility. Storage for long periods at room temperature does not seem to adversely affect these devices. The results of preliminary radiation damage experiments suggest that the alloy detectors possess a radiation damage resistance far greater than that of silicon

Alexander, P.

Shulman, H.

English

NASA Technical Reports Server

A HIGH RESOLUTION SEMICONDUCTOR DETECTOR FOR APPLICATIONS IN SPACE
Peter Alexander and Herman Shulman
#
Teledyne Isotopes
Nuclear radiation detectors with Volumes of _i cm 3 have been fabricated from single
crystals of Germanium-Silicon alloy containing as much as 20 weight percent germanium.
The properties of these detectors have been investigated and will be discussed. Tests
reveal that the gamma ray photoelectric peak efficiency of an alloy detector with only
12 weight percent germanium is _4 times that of a silicon detector of equal volume. The
alloy detector will operate with excellent energy resolution at -75°C. Operation at
room temperature appears to be a good possibility. Storage for long periods at room
temperature does not seem to adversely affect these devices. The results of preliminary
radiation damage experiments suggest that the alloy detectors possess a radiation damage
resistance far greater than that of silicon.
INTRODUCTION
The superior energy resolution obtainable with
semiconductor nuclear radiation detectors has made
their use widespread in all fields involving gamma
ray and charged-particle spectroscopy. For the
purpose of nuclear spectroscopy in spaee_ it is
desirable to have a detecting device which will
yield the best (lowest) energy resolution, the
highest detection efficiency, and the fewest oper-
ating difficulties. In addition this detector
should be light weight, rugged in construction, use
little power, be able to operate at temperatures a-
bove -195°Cj have room temperature storage cap-
ability, and the ability to withstand considerable
damage from the high radiation zones in space. Un-
fortunately at present these attributes are not all
combined in any one type of detector. Sodium Iodide
is relatively efficient as a gamma ray detector.
However, due to the relatively poor resolution of
sodium iodide it becomes extremely difficult to ex-
tract accurate energy and intensity information
from spectra containing more than a few lines. Con-
versely semiconductor detector spectra containing
hundreds of lines can be accurately analyzed for
energy and intensity information. The most common-
ly used semiconductor detector materials are ger-
manium and silicon.
The photoelectric interaction probability for
a gamma ray is roughly proportional to the fifth
power of the atomic number (Z) of the stopping
material. Thusj because of its low Z(14), sili-
con is very poor relative to germanium (Z=32), for
detection of gamma rays. Silicon detectors do
have an advantage over germanium dptectors in that
they can be operated and stored at room temperature.
This room temperature operating ability stems from
the larger band gap possessed by silicon (1.2 eV).
The narrower band gap of germanium (0.7 eV) re-
quires that the detector be cooled to the point at
which thermal generation of carriers across the gap
does not interfere with detection of the ionizing
radiation. The lithium, used to compensate the
impurities present in the detector, does not pre-
cipitate in silicon at room temperature as it does
_This work was supported in part by the Advanced Research Projects Agency under Contract DASA-01-70C-O040
with the Defense Atomic Support Agency and also by the National Aeronautics and Space Administration under
Contract NAS 1-6654.
$0 Van Buren Avenue, Westwood, New Jersey
723
https://ntrs.nasa.gov/search.jsp?R=19720010048 2020-03-17T03:46:57+00:00Z
in germanium. Such precipitation renders the de-
tector useless.
The ideal detector for space applications would
combine the high resolution, good gamma ray effi-
ciency and large detector volume available in a ger-
manium detector with the desirable properties of a
silicon detector. These properties include room
temperature operating capability, and prolonged
room temperature storage capability. Such an ideal
detector can_ in principle, be obtained by judi-
ciously alloying germanium and silicon in a large
single crystal. Figure i, taken from the work of
Braunstein et al 1, gives the dependence of the ger-
manium-silicon alloy band gap on the percentage of
the constituents present. From this figure it be-
comes apparent that a silicon content of at. least
15 atomic percent should be present in the alloy.
At this silicon concentration the crystal has a
band gap almost midway between that of germanium
and silicon but has retained a large germanium con-
centration which will yield a high gamma ray effi-
ciency. It may be necessary to push the silicon
concentration still higher in order to produce a
wider band gap and thereby permit the detector fab-
ricated from this material to operate at more ele-
vated temperatures.
In pure silicon as in pure germanium both inter-
stitials and vacancies are sufficently mobile to in-
teract with dopants and other defects so as to pro-
duce stable secondary complexes. Removal of these
radiation induced complexes by annealing requires a
temperature so high as to destroy the properties of
the germanium and silicon which enable it to func-
tion as a detector. In the germanium-silicon alloy,
the formation of complexes will be hindered not only
by the low vacancy-interstitial mobility but also
by the thermal annealing which may be expected to
proceed at reduced temperature. If the detector
bias is maintained during the annealing cycle_ the
resulting lithium ion drift will tend to compensate
both primary and secondary damage effects. Such an
"in situ" cycling procedure should enable greatly
extended detector operating lifetimes.
1.2
I.!
I.O
o.
o
c_ 0.9
z
rn
O.e
0.70 I I I i I20 40 60 80 I00
SILICON CONCENTRATION (otomic %)
l'igu_'e I. Variation o[ _iv*.,_iHium :_i]J_;_1,alloy ],and ga;, _,_th si]itoll cOJJCeH~
trat lollI .
GERMANIUM-SILICON ALLOY SINGLE CRYSTALS
It has been found that extremely high purity
levels are requisite in silicon and germanium for
fabrication of good detectors. The presence of such
contaminants as copper, carbon or dissolved gases,
particularly oxygen, in quantities as small as 1
contaminant atom in 10 9 silicon or germanium atoms
is sufficient to degrade detector performance. This
effect is rendered through trapping of the charge
carriers and/or breakdown of the large electric
field which must be applied across the detector in
order to collect these carriers. The specifications
724
for germanium-silicon alloy crystals used for nu-
clear radiation detector fabrication are consistent
with the crystal requirements necessary to produce
good germanium or silicon detectors.
In this experiment measurements were made of
those crystal parameters which most strongly in-
fluence radiation detector performance. These para-
meters include: minority carrier lifetime, trace
oxygen content, homogeneity and degree of crys-
taline perfection, lithium drift mobility, and
lithium diffusion constant. The measurements which
define the last two quantities are detailed below.
The rate at which lithium can be drifted into
germanium-silicon alloy under the influence of an
applied electric field was studied, in a sample con-
taining 10% Ge - 90_ St, as a function of tempera-
ture in the range iO0°C to 160°C. The lithium
drift rate is an important quantity in that it re-
veals how deeply a germanium-silicon alloy slice may
be lithium compensated in a given time period. The
drift depth measurements were performed by vacuum
evaporating lithium onto the face of a germanium-
silicon alloy slice, then heating at 400°C for I0
minutes in an argon atmosphere furnace, and then
drifting in a silicone fluid medium maintained at
the desired temperature. The progress of the n-i
and i-p juntions were monitored by the copper
stain technique. The change in depth of the in-
trinsic region was followed at a given temperature
for about 12 days with a fixed applied reverse bias
of 75v. Intrinsic depth versus drift time curves
as measured for this alloy sample,are plotted in
Figure 2 for temperatures of 120°C, 140°C and 150°C.
Comparison drift curves for germanium and silicon
are also presented. It may be observed that the
germanium-silicon alloy drifts considerably more
slowly than either germanium or silicon.
!
I0
0,1
I I
1.0 tO
INTRINSIC DWPTH (mat)
P_&_re 2. IAthium drlf% depth _n germnni_tm-_llico, alloy_ containSng io_ Oe -
90;; Si, as a fu.ction nf t_me at various t_q_ratu_'es. Tile b_as applied
across tl,t,(let,±etorwas 75 volts: Also plotted are comparable curves
for ger.wmim_ a[id silicon•
In order to make good semiconductor diodes it
is necessary to diffuse lithium into the material
to just the proper depth. If lithium is diffused
too deeply, a thick "dead layer" results on the face
of the detector. If lithium is not diffused deeply
enough, oxidation and surface preparation will re-
move the lithium before it can be drifted into the
material. Although the germanium-silicon alloy un-
der study is silicon-rich, it is not at all obvious
that the diffusion rate of the alloy will be ex-
pressible as some linear combination of the dif-
fusion constants for germanium and silicon. There-
fore, the diffusion of lithium in the germanium-
silicon alloy is being studied as a function of
temperature and Ge/Si ratio. In this study lithi-
ated slices of both silicon and germaniUm-silicon
alloy were heated to a preselected temperature. The
slices were removed from the oven and the diffusion
death was determined by the copper stain technique.
725
From this data plus a knowledge of the surface lith-
ium concentration and the accepter dopant level, the
lithium diffusion coefficient could be calculated
for a particular diffusion temperature and Ge/Si
ratio.
The results of the measurements described a-
bove are shown in Figure 3 where D is plotted ver-
sus the reciprocal of the absolute temperatures for
pure silicon specimens, for samples from an ingot
containing i0_ germanium by weight, and for samples
from an ingot with 16_ germanium by weight. The
solid line shown is based on Pell's empirical e-
quation 2 for the diffusion constant of lithium in
silicon. The data from the _easurements on the al-
loy samples and the silicon samples agree with Pell':
estimates above 500°K. Below this temperature there
is departure of the data for the alloy samples from
both Pell's curve and from the data for silicon.
This deviation is consistent with an oxygen concen-
tration of 1017 atoms/cm 3 in the base alloy. The
effects of this oxygen on the diffusion constant be-
come more pronounced as the diffusion temperature
is lowered.
i0 "l
o
iO-m
I I I I I I I I I I ]
+
+ -$i
iO-al
1.0 1,25 I I._ Z 0 2._ ?-5
103/T (°K)
BEHAVIOR OF Ge-Si ALLOY DETECTORS
A number of gamma ray and charged particle de-
tectors have been fabricated from germanium-silicon
alloy. Due to the very low drift rate of lithium
in the germanium-silicon alloy ingots_ most detec-
tors had active depths ranging from 200 to 800 p.
This depth corresponds to about 1 month of drift
time and yields maximum active volumes of _i cm 3,
The alloy detectors deliver good gamma ray res-
olution at temperatures up to -7S°C. At room tem-
perature the resolution is greatly degraded due to
surface leakage. A grooved or guard ring detector
configuration would substantially improve this re-
sponse. Typical alloy detector gamma ray resolu-
tion values range from 2 to 5 keV for 57Co gamma
rays. Comparable resolution is obtained using ger-
manium detectors of the same size. At 356 keV the
photopeak gamma ray efficiency of an alloy detector
containing 12_ Ge - 88_ Si is 4 times that of a sil-
icon detector of the same size.
Several alloy detectors have been stored for
periods of up to 3 months at room temperature while
exposed to the room atmosphere. In all cases it
was possible to operate these devices_ when they
were eoooled back down to -175°C_ with no loss in
resolution or degradation of reverse bias charac-
teristics.
As described in reference 3_ germanium-silicon
alloy detectors have also been used for detection
of alpha particles yielding extremely good spectral
energy resolution. Application of a thin gold en-
trance window on the p-face of the diode yields an
241Am spectrum such as that presented in Figure 4.
726
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E
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/_136or_ _'" 13'so ' 14'00 1420
CHANNEL NUMB£R
Fi_re 4. 241_ alpha s_33ectnlm as observed wlth a gez=anlum-s£_tcon a1.1oy de-
t_o_. T _ _ _ r_v s_e_trua observed mt the _ t_me t|
u_d for e.erg)' ca1_brat_on p_r_ses.
RADIATION DAMAGE MEASUREMENTS
The germanium-silicon alloy detectors describ-
ed in the preceding section have been employed to
study the effects of gamma ray damage on detector
performance. Exposure to the gamma rays was accom-
plished in the 3000 Curie 60Co hot cell facility of
the Rutgers University Reactor. Four lithium-drift-
ed diodes were exposed to gamma radiation. Two of
the diodes had been fabricated from i000 ohm-cm
boron-doped silicon, while the remaining two diodes
were made from 300 ohm-cm gallium-and-boron-doped
germanium-silicon alloy (88% St, 12% Ge by weight).
The four diodes were mounted on a copper plate in
the vacuum space of a liquid nitrogen cooled cryo-
star. The diodes were maintained under vacuum and
at liquid nitrogSn temperature throughout the irra-
diation and during the post-radiation measuring
period. The gamma flux to which the diodes were ex-
posed was l. Sx10 I$ photons/cm 2 per hour, which is
equivalent to a dose rate of 217 rads (Si)/sec.
They were exposed for an accumulated period of
1,006 minutes, to a total fluence of 2.14xi016
photons/cm 2 .
During the course of the above experiment mea-
surements were periodically made of the detector:
reverse-bias leakage current, reverse-bias photo-
current induced by the 60Co gamma ray fluence, and
resolution. The resolution of the silicon detec-
tors was observed to degrade many times faster than
the alloy detector resolution. The $7Co gamma ray
spectra observed with one of the germanium-silicon
detectors both before and after exposure to 2xlO 16
photons/em 2 is presented in Figure 5. A similar
set of spectra for one of the silicon detectors is
presented in Figure 6. The resolution of both of
the silicon detectors was greatly degraded by the
radiation damage while the alloy detectors were
hardly affected. Post-irradiation annealing at sev-
eral temperatures up to 278°K, completely restored
the properties of the two alloy detectors while sim-
ilar annealing caused a monotonic deterioration in
the resolution of the silicon detectors. The res-
olution of the silicon detectors did not fully re-
cover e=en after 24-hour lithium-ion drift cycle at
727
360_. The reverse bias current measurements are
also consistent with the above data. Additional d_-
tails of these experiments will be presented in an-
other publication.
CONCLUSION
A number afpoints concerning the properties of
germanium-silicon alloy remain to be clarified by
future experiments. Some conclusions concerning
this material, based on the experiments described
in this paper, are listed below.
i. Germanium-silicoh alloy single crystals of de-
tector grade can be produced with as much as
20 weight percent germanium.
2. Lithium can be drifted into these crystals to
compensate them to reasonable depths.
3. Detectors of volumes _i cm 3 have been achieved
and these yielded gamma ray resolution com-
parable to that of a silicon detector of simi-
lar size.
4. Alloy detectors have been found to possess a
gamma ray efficiency significantly better than
that of silicon detectors of comparable volume.
5. The alloy detectors work quite well at -78°C
and with geometric alterations will probably
function at room temperature.
6. Long term room temperature storage does not ap-
pear to harm the alloy detector.
_-_ _uy U_L_CLOr appears to evidence a gamma
ray radiation damage resistance greatly exceed-
ing that of silicon (and implicitly germaniumS.
8. Annealing the alloy detector at temperatures be-
low 300°K appears to remove the effects of the
radiation damage. In silicon similar thermal
treatment of gamma irradiated detectors is found
to cause additional degradation.
_1,97 KIv x IEFORE ._A_ATK_
i
• _'TER m_TtO_
xl
/.
.." ...
GAMMA RAY ENERSY
REFERENCES
i. Braunstein, R; Moore, A.R.; and Herman, F: Instrinsic Optical Absorption in Ge-Si alloys, Physical
'Review, 109, 1958j p. 695.
2. Pell, E.M., Diffusion of Li in Si at High Temperature and the Isotope Effects, Physical Review, 119,
1960, p. 1014.
3. Alexander_ P. and Shulman, H.; A Semiconductor Material Research Program., NASA-CRIII793.
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A high resolution semiconductor detector for applications in space

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