Development of NTD Ge sensors for low
temperature thermometry
∗ India
S. Mathimalar∗,† , V. Singh∗,† , N. Dokania∗,† , V. Nanal‡ , R.G. Pillay‡ , S. Pal‡ ,
S. Ramakrishnan†† , A. Shrivastava§ , Priya Maheshwari∗∗ , P.K. Pujari∗∗ ,
S. Ojha , D. Kanjilal , K.C. Jagadeesan¶ , and S.V. Thakare¶ .
based Neutrino Observatory, Tata Institute of Fundamental Research, Mumbai 400 005, India.
† Homi Bhabha National Institute, Anushaktinagar, Mumbai 400 094, India.
‡ Department of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Mumbai 400 005, India.
Email: nanal@tifr.res.in (V. Nanal)
†† Department of Condensed Matter Physics, Tata Institute of Fundamental Research, Mumbai 400 005, India.
§ Nuclear Physics Divison, Bhabha Atomic Research Centre, Mumbai 400 085, India.
∗∗ Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.
Inter University Accelerator Centre, New Delhi 110 067, India.
¶ Isotope Applications & Radiopharmaceuticals Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.
Abstract—The development of NTD Ge sensors for use in
cryogenic bolometric detector to search for neutrinoless double
beta decay (0νββ) in 124 Sn is reported. The samples made
from device grade Ge wafers are irradiated with thermal
neutrons at Dhruva reactor, Bhabha Atomic Research Centre
(BARC), Mumbai. The carrier concentration in irradiated
Ge samples is estimated by Hall effect measurement at 77K.
The fast neutron induced defects are studied using Positron
Annihilation Lifetime Spectroscopy and Channeling. It is
found that vacuum annealing of the samples at 600o C for
2 hours is necessary to cure the defects. Sensors are made
from annealed NTD samples using Au-Ge Ohmic contact.
Preliminary measurements have shown a significantly large
dR/dT ∼ 2.3 kΩ/mK at 100 mK. Details of these measurements
are presented.
Index Terms—NTD Ge sensors, PALS, Channeling
I. I NTRODUCTION
The Neutron Transmutation Doped (NTD) Ge thermistors
are used as mK sensors in cryogenic bolometric detectors [1, 2]. The NTD technique is preferred over other
conventional methods due to high precision and homogeneity of dopants [3, 4]. We have initiated the development
of NTD Ge sensor [5] for use in the feasibility study
of 0νββ in 124 Sn cryogenic bolometer at the upcoming
underground facility of INO [6]. Thermal neutron irradiation is carried out at Dhruva reactor, BARC, Mumbai.
During the irradiation Ge crystals are also exposed to fast
neutrons which can cause significant damage to the crystal.
The performance of NTD Ge sensors critically depends
on defects formed during neutron irradiation, electrical
contacts and carrier concentration. The defect density depends on fast neutron flux encountered during irradiation
and annealing conditions need to be optimized for the
samples. The carrier concentrations are estimated from Hall
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effect measurements. The fast neutron induced damage is
studied using different techniques like Positron Annihilation
Lifetime Spectroscopy (PALS) and Channeling. Both these
methods are expected to give complementary information.
Sensors for low temperature (mK) measurement are fabricated using these NTD Ge samples and have shown a
large dR/dT compared to a commercial Ruthenium oxide
(RuO2 ). Details of these measurements are described in the
following sections.
II. E XPERIMENTAL DETAILS
Semiconductor grade Ge crystals of resistivity ≥ 35 Ω
cm, having < 100 > cleavage plane and 1 mm thickness
were used for NTD sensor production. A sample of 10
mm x 30 mm size was irradiated with thermal neutrons
at Dhruva reactor, BARC (India) for approximately 4 days.
The thermal and the fast neutron fluence (En ∼ 0.625 eV
to 0.821 MeV) during the exposure are estimated to be
∼ 7x1018 /cm2 and ∼ 1.2x1018 /cm2 , respectively. Due to
the natural isotopical composition of Ge and the neutron
capture cross sections, the resultant doping is of p-type.
After a cool-down period of 45 days since irradiation, the
sample was taken out and cut into several smaller pieces
for further studies. Some of these samples were vacuum
annealed at 600o C for 2 hours. The annealed samples were
rinsed in electronic grade isopropyl alcohol, followed by
etching in HF to remove oxide layers and blow-dried with
dry N2 . Figure 1A SEM picture of a typical NTD Ge sample
prior to annealing. figure.caption.1 and 2A SEM picture of
annealed NTD Ge sample after HF etching. figure.caption.2
show SEM (Scanning Electron Microscope) pictures for the
irradiated sample prior to annealing and after annealing,
respectively. A few micron size patches are clearly visible
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in the irradiated sample prior to annealing. The EDAX
(Energy Dispersive X-ray Analysis) showed these patches
to be rich in Oxygen content. The annealed and HF etched
sample exhibits a reasonably clean surface and also a
significant reduction in Oxygen content is observed in
EDAX.
diagram of the V-I measurement. figure.caption.3 shows a
picture of the contact pattern. Electrical connections to the
sample were made by wedge bonding Aluminum wires (φ =
25 μm) to the Au-Ge contact pad and to the mounting chip.
The Hall voltages were measured at 77 K and at 300 K with
100 μA current and the magnetic field was varied between
–1 to 1 Tesla. The voltages V12 (hall voltage) and V23
(longitudinal) were measured simultaneously with two lockin amplifiers. This enabled the correction for the magnetoresistance effect as well as effects due to lack of parallelism
in connections, thereby allowing unambiguous estimation of
the Hall coefficient and the carrier concentration (Nc ).
Figure. 1 – A SEM picture of a typical NTD Ge sample
prior to annealing.
Figure. 2 – A SEM picture of annealed NTD Ge sample
after HF etching.
Hall effect measurements were carried out using Van
der Pauw method on a 7 x 4 mm2 symmetrical, annealed
sample. Six Au(88%)–Ge(12%) contact pads of 0.8 mm
diameter and about 50 nm thickness were made and rapid
thermally annealed at 400o C for 2 min. Figure 3(a) A
picture of the NTD Ge sample with contacts pads for Hall
effect measurement by Van der Pauw method (b) Schematic
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Figure. 3 – (a) A picture of the NTD Ge sample with
contacts pads for Hall effect measurement by Van der Pauw
method (b) Schematic diagram of the V-I measurement.
Fast neutron induced defect studies were carried out
using PALS and Channeling on 3 sets of samples, namely,
virgin sample (unirradiated), irradiated sample before annealing and irradiated sample after annealing. For PALS
measurement a 22 Na (∼ 8 μCi) source, covered with Kapton
foil, was sandwiched within two identical (virgin/irradiated)
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Ge samples and mounted between two plastic scintillators
kept at 180◦ w.r.t. each other. The β + lifetime was obtained
from Time to Amplitude Converter (TAC) spectrum with a
1.27 MeV γ-ray as a start signal and the 511 keV γ-ray
as a stop signal. The time resolution measured with 60 Co
source was 262 ps and the ADC calibration constant was
12.5 ps/channel. The lifetime spectrum was analysed using
PATFIT program [7].
The crystal channeling is a sensitive tool for probing
interstitial and strain related defects [8]. The channeling
experiment with 2 MeV alpha particles has been carried out
using Pelletron Accelerator RBS-AMS Systems (PARAS)
facility at Inter University Accelerator Centre (IUAC), New
Delhi [9]. Rutherford backscattering was measured at a
θlab = 166◦ with a silicon surface barrier detector. The
beam current on the sample was maintained below 5 nA
throughout the measurement to minimize the damage to
sample.
The sensor performance in T = 75 to 250 mK was tested
with the sample used in Hall measurement. The resistance
measurement was carried out in a high cooling power
(1.4 mW at 120 mK) cryogen free dilution refrigerator
(CFDR) setup at TIFR, Mumbai [10] using a four probe
technique. All connecting wires below 4K station were
superconducting (NbTi/Al) to minimize the heat load. Data
were acquired using a commercial resistance bridge AVS47B with a dedicated Labview interface, specially developed for this setup. To avoid the self heating of the sensor
during measurement, the net power dissipated in the sensor
was maintained below 10 fW.
III. R ESULTS
The carrier concentration (Nc ) is estimated from the data
corresponding to |B| > 0.5 Tesla. To eliminate the magnetoresistance contribution, the average Vav
Hall corresponding
to ± B is used. The estimated carrier concentrations at
77 K and 300 K are found to be 1.11 x 1017 /cm3 and
2.13 x 1017 /cm3 , respectively. The errors are within 0.1 %.
Morin [11] has explained that due to lattice scattering, the
Hall factor (ratio of Hall mobility to conductivity mobility)
for p-type Ge at 300 K is ∼ 1.8 while it nearly saturates
to ∼ 1.1 below 100 K. In the present case, the Hall factor
estimated from enhanced Nc value at 300 K w.r.t. 77 K is
somewhat higher (∼ 1.9). Since the Hall factor at 77 K
is close to unity, the Nc at lower temperature is used
to obtain thermal neutron fluence as ∼ 4.6 x 1018 /cm2 .
The corresponding fast neutron fluence is estimated from
the ratio of thermal to fast neutrons in Dhruva reactor as
∼ 8 x 1017 /cm2 .
Table Iτβ + using PALS in NTD Getable.caption.4 gives
the results of PALS for all 3 samples. Only single lifetime
component of 232 ps was obtained for virgin sample similar
to that of bulk Ge crystal (228 ps). In the irradiated sample
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TABLE I – τβ + using PALS in NTD Ge
Samples
Virgin Ge
Irradiated
Annealed
τβ +
(ps)
232 ± 0.3
294 ± 0.3
401 ± 30
233 ± 0.4
Intensity
%
100
90.0 ± 2.5
9.44 ± 2.44
100
τβ + [12]
(ps)
228 (bulk)
293 (Monovacant)
401 (vacancy clusters)
228 (bulk)
two lifetime components were observed. One with 294 ps
corresponding to monovacany in Ge and the other with 401
ps indicates the formation of vacancy clusters. It can be seen
that the τ ∼ 233 ps observed for the annealed sample is
identical to that of the virgin sample. This clearly indicates
that the present annealing procedure sufficiently cured the
defects created by fast neutrons during irradiation.
Figure. 4 – The RBS spectra of NTD Ge sample along
< 100 > axis before (red curve) and after annealing (blue
dash-dot line) together with that for the virgin sample (black
dash). The random orientation spectrum (green dotted line)
is also shown for comparison.
Figure 4The RBS spectra of NTD Ge sample along
< 100 > axis before (red curve) and after annealing (blue
dash-dot line) together with that for the virgin sample (black
dash). The random orientation spectrum (green dotted line)
is also shown for comparison. figure.caption.5 shows the
channeled spectra of the irradiated sample before and after
annealing together with that of the virgin sample. Increased
RBS yield resulting from dechanneling due to defects can
be clearly seen in the unannealed sample. Improvement
due to annealing is evident. It may be noted that in the
near surface region, the channeled spectrum of the annealed
sample is somewhat better than the virgin sample.
Figure 5Measured resistance for NTD Ge sensor as
a function of temperature. Data for a commercial RuO2
sensor is also shown for comparison. figure.caption.6 shows
the resistance of NTD Ge as a function of temperature. The
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R EFERENCES
Figure. 5 – Measured resistance for NTD Ge sensor as
a function of temperature. Data for a commercial RuO2
sensor is also shown for comparison.
data for a commercial RuO2 (room temperature resistance
= 1.5 kΩ) is also shown for comparison. It can be seen
that the NTD Ge has a larger dR/dT below 225 mK
indicating suitability for low temperature thermometry. It
may be mentioned that neutron dose > 1019 /cm2 yields a
dR/dT∼ 0.02 Ω/mK at 100 mK, implying that the NTD Ge
becomes nearly metallic.
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[4] K.M. ltoh, Appl. Phys. Lett. 64 (1994) 2121-2123.
[5] S. Mathimalar et al., DAE Symp. on Nucl. Phys. 58
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[6] V. Nanal, EPJ Web of Conferences 66 (2014) 08005
1-8.
[7] P. Kirkegaard, N.J. Pedersen, M. Eldrup, PATFIT-88,
Risφ-M-2740 (Roskilde, Denmark, 1989).
[8] D. S. Gemmell, Rev. Mod. Phys. 46 (1974) 129-236.
[9] http://www.iuac.res.in/accel/paras.
[10] V. Singh, S. Mathimalar, N. Dokania, V. Nanal,
R.G. Pillay, S. Ramakrishnan, Pramana 81 (2013) 719725.
[11] F. J. Morin, Physical Review, 93 (1954) 62-63.
[12] R.K. Rehberg, H.S. Leipner, Positron Annihilation in
Semiconductors - Defect Studies (Springer Pub.) (1999)
186.
IV. C ONCLUSION
The NTD Ge samples have been prepared by thermal
neutron irradiation at Dhurva reactor, BARC, Mumbai. The
carrier concentration is estimated to be 1.11 x 1017 /cm3
from the Hall voltage measurement at 77 K. The fast
neutron induced defects in NTD Ge samples have been
studied using PALS and Channeling. The PALS results
indicate that NTD Ge have predominantly ‘monovacancy’
defects. Both PALS and channeling have shown that
vacuum annealing at 600◦ C for 2 hours adequately cures
the defects. The sensor prepared from the NTD Ge sample
has shown very promising results in the temperature range
75 to 250 mK, with large dR/dT ∼ 2.3 kΩ/mK at 100 mK.
ACKNOWLEDGEMENT
We thank Ms. S. Mishra for help with sample preparation; Mr. J. Mathew, Mr. A. Singh, Dr. S.S. Prabhu, Prof.
M. Deshmukh for assistance with electrical measurements
and Ms. Bagyshri. A. Chalke for SEM measurements. We
are grateful to Dr. V.M. Datar, Dr. G. Ravikumar and Dr.
K. Dasgupta for useful discussions.
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