NTD Ge sensors for low-temperature thermometry

(for Neutrinoless Double Beta Decay)

S. Mathimalar DNAP-TIFR 11/09/2015

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Plan of the talk

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Temperature sensors

Neutrino less Double Beta Decay (NDBD)

Bolometer for NDBD

NTD Ge sensor development for bolometer

Hall Effect Measurement – carrier concentration

Low temperature resistance measurement of NTD Ge sensor

Summary

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Temperature sensors

Mercury thermometer Liquid Nitrogen -196oC (77K) Liquid Helium -269oC (~4K)

Need sensors working at ~mK temperature

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Neutrinoless Double Beta Decay (in 124Sn)

Significance of 0νββ: Presently the only experiment which can tell whether neutrino is its own antiparticle (Majorana particle) or not (Dirac particle)

Experiments will look at: Constancy of sum energy of two simultaneously emitted electrons, which requires high energy resolution and large scale detector

Coun

ts

Experimental signature

T1/2: ms to 109 yrs 1019 – 1021 yrs >1022 – 1025 yrs

Bolometer for NDBD

When DBD occur, energy of electrons heats the crystal Thermalization leads to rise in the crystal temperature T+δT The detector will be operated at an optimum temperature of ~10mK, in order to get a reasonable δT (~100μK/MeV). Need a high sensitivity, reproducibility sensor works at mK.

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~ms ~100 ms

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Temperature sensors at mK

Essential properties : Low specific heat Fast rise time Radio pure Bare sensor (no packing material) High temperature sensitivity Mass producible Reproducibility

Commercially available : Carbon speer (resistance) RuO2 (resistance) CMN (susceptibility) Melt growth doped semiconductor (resistance) Cerenox (resistance)

Preferable sensor: Most of the commercial sensors do not posses all the essential properties of a bolometric sensor. Hence sensors need to be developed indigenously for NDBD. Lightly doped semiconductor Ge posses most of the needed properties and it works on Variable Hopping Mechanism (VRH) at low temperature.

Neutron Transmutation Doped (NTD) Ge sensor is preferred due to its high temperature sensitivity, reproducibility, mass production, low specific heat, fast rise time and simple read out. Low neutron capture cross-section - homogenous doping. Melt-doped Ge with large concentration will be inhomogeneous and can have different characteristics. The resistivity of a NTD Ge is given by Mott’s law as,

In a Variable Hopping mechanism, the α is predicted to be 0.5. ρo and To can be tuned by neutron fluence.

Neutron Transmutation Doped Germanium (NTD Ge) Sensor

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ρ

Dopant details of irradiated Ge

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Isoto

pe

Isotopic Abundance (%) (SIMS)

Products formed (half-

life)

Decay into(decay mode)

Type

Set 1 Set 2 70Ge 21.5 21.9 71Ge*

(11.43 d)

71Ga (e- capture)

p

72Ge 26.8 27.0 73Ge (stable ) 73Ge - 73Ge 10.8 8.8 74Ge (stable ) 74Ge - 74Ge 34.1 35.1 75Ge*

(82.78min)

75As (β-decay)

n

76Ge 6.8 7.2 77Ge* (11.3, 38.8hrs)

77As*,77Se (β, β-decay)

n

The net carrier concentration is given by =

For a natural Ge, the neutron doping results in p-type due to its isotopic abundance and thermal neutron capture cross section.

Neutron Irradiation of Ge

Semiconductor grade Ge irradiated with thermal neutron from Dhruva Reactor (max. power 100MW), Bhabha Atomic Research Centre (BARC). Samples were cleaned, wrapped with (a) quartz tube or (b) High purity Aluminium foil, cold sealed in Al container, placed in the permissible region of reactor and then irradiated with reactor neutrons. Packing material has been carefully chosen to minimize the residual radioactive contamination.

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(a) (b)

Samples details with neutron fluence

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Samples (Plane,

Thickness, Resistivity)

Tag (size in mm2)

Neutron fluence (n/cm2) Thermal

(<0.625eV) Epithermal

(.625eV - 0.821 MeV)

Fast (.821MeV -

10MeV)

Set 1 (<111>, 0.4 mm,

>30 Ωcm)

B (10 x 10) 1.40 x 1019 2.56 x 1018 1.72 x 1017

C (10 x 10) 9.13 x 1018 1.72 x 1018 1.12 x 1017

Set 2 (<100>, 1.0 mm,

>35 Ωcm)

D (30 x 10) 4.57 x 1018 8.02 x 1017 5.32 x 1016

E (30 x 10) 2.11 x 1018 3.52 x 1017 2.46 x 1016

F (30 x 10) 3.52 x 1018 5.87 x 1017 4.11 x 1016 G (30 x 10) 3.57 x 1018 5.95 x 1017 4.16 x 1016

H (10 x 10) 1.8 x 1018 3.01 x 1017 2.1 x 1016

I (30 x 10) 1.8 x 1018

3.01 x 1017

2.1 x 1016

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Surface studies and annealing details

(Before HF cleaning) (After HF cleaning)

Oxides

Samples were cleaned with HF for 90sec and rinsed with distilled water and then blow dried with dry N2 at every stage of measurement. SEM images before and after etching shows removal of oxide layer from Ge surface with HF treatment.

The samples were annealed at 600oC for 2 hours in a sealed quartz tube to cure defects and were cut into the required size for various studies.

Fabrication of NTD Ge

For contact making In, Al, Au were tried and finally optimized with Au-Ge pads. For Hall Effect measurement six contacts pads (with Au (88 %) – Ge (12 %)) were made using standard Vander Pauw method. Hall Effect measurement used as tool to estimate thermal neutron dose by estimating the carrier concentration at 77K. The samples were rapid thermally annealed at 400°C for 2 min. Electrical connections were made by wedge bonding 25 μm Al wires to the contact pad.

Schematic view

NTD Ge with Al wedge bonding

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The Hall voltage is measured with 100 μA current in the varying magnetic field. The Hall voltages were measured at both 77 K and at 300 K. The carrier concentration were estimated by assuming single carrier approximation using the formula:

Hall Effect Measurement

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= Hall voltage = Applied Mag field

= Supplied current = thickness = carrier concentration = charge

The contribution of magneto-resistance was taken into account during carrier concentration estimation. At 300K the intrinsic carriers due to thermal agitation can participate in conductivity, hence the values are higher than the actual dopant concentration.

Hall Effect Result

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S. Mathimalar et al., NIM B, 345 (2015) 33

Sample Carrier concentration (/cm3) error < 2% 77K 300K From n-fluence

B 3.42 x 1017 3.89 x 1017 3.34 x 1017 C 2.11 x 1017 2.60 x 1017 2.18 x 1017 D 1.11 x 1017 2.13 x 1017 1.11 x 1017 E 4.45 x 1016 7.25 x 1016 5.12 x 1016 I 4.17 x 1016 4.87 x 1016 4.37 x 1016

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Hall Effect Results cont..

The carrier concentration estimated using Hall Effect at 77K is compared with the carrier concentration estimated from the reactor power data.

Low temperature resistance measurement in dilution refrigerator

Probe setup

Gas Handling system with the dilution unit

V. Singh, S. Mathimalar, et al., Pramana 81 (2013) 719.

NDBD lab @ TIFR

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Base temperature stability ± 10mK at 8 mK (measured with CMN)

Minimum temperature without probe ~ 8 mK, with probe ~ 12 mK.

Cooling power 1.4 mW @120 mK with about 50 l of 3He.

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Development of DAQ system

Lakeshore, AVS 47 - the two commercial resistance bridge and NI – PXIe – 6229 were used for resistance measurement.

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Labview program – Block diagram

PID Control block

Feedback

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Labview program - FP

Measurement possible for a given range of temperature with given stability. Eg: 5K to 15K, in steps of 0.5K, stability check for 16sec within 0.1K

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Readout system

AVS Readouts

NI Readouts

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Issues with resistance measurement: Thermal anchoring of the sensors Vibrational issues Grounding issues RF pickups Base temperature stabilization.

Reducing noise: Faraday cage Ground Isolator Vibration damper Suitable filter Shielded twisted pair cables EMI proof cabinet

Faraday cage made with Cu and GI sheets

Spectra with noise

Spectra with reduced noise

Noise issues

NTD Ge sample mounts

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Cu mount

Cu mount with read out connectors

Masks

Low temperature resistance measurement (AVS)

The resistance values are compared with that of commercial RuO2 sensors. Clearly the NTD Ge sensor showed higher dR/dT compared to RuO2. The dR/dT is about ~2.3kΩ/mK at 100mK.

Logarithmic sensitivity α =

For a semiconductor thermistor will range between 1 to 10. Clearly the figure of merit is higher for NTD Ge as compared to that of RuO2.

Samples Logarithmic sensitivity at 100mK

NTD Ge 1.7 RuO2 0.7

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S. Mathimalar et al., IEEE-WOLTE, doi:10.1109/WOLTE.2014.6881014 .

Low temperature resistance measurement cont..

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Resistance measured using AVS resistance bridge as a function of temperature for various neutron fluence.

NA – ND from Hall Effect:

NTD Ge Carriers/cm3

B 3.42 x 1017

D 1.11 x 1017

E 4.45 x 1016 *F 8.55 x 1016

*G 8.68 x 1016

*Estimated from reactor power data

VI measurement using NI-PXIe

25 The logarithmic resistance of NTD Ge goes linear with T-0.5.

R vs T

ln R vs T-0.5

Summary

NTD Ge sensor is chosen for the upcoming TIN.TIN (NDBD experiment at INO). Ge wafers are doped by neutron transmutation doping technique. Sensors are fabricated with irradiated and annealed samples (600°C for 2hrs in a sealed quartz tube) by sputtering Au-Ge contact pads. Hall probe measurement at 77K is used as a tool to verify the neutron fluence. NTD Ge (φth = 4.6 x 1018/cm2) has higher dR/dT compared to that of the commercial RuO2. For our semiconductor grade natural Ge, φth > 1019/cm2

(NA-ND: 3.42 x 1017) shows metallic behaviour. Preferable neutron fluence for natural Ge samples: 2 x 1018/cm2 < φth < 5 x 1018/cm2.

(corresponding carrier concentration: 4 x 1016/cm3 < NA-ND < 11 x 1016/cm3 ).

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Collaborators

Dr. V Singh Dr. Neha Dokania Mr. Abhijit Garai Prof. V. Nanal Prof. R.G. Pillay Dr. Sanjoy Pal Prof. S. Ramakrishnan Dr. A. Shrivastava Dr. Priya Maheswari Dr. P.K. Pujari Mr. Sunil Ojha Dr. D. Kanjilal Mr. K.C. Jagadeesan Dr. S.V. Thakare

Dr. G. Ravikumar Prof. V.M. Datar Prof. Mandar Deshmukh Prof. Shriganesh S. Prabhu Ms. Sadhana Mishra Mr. J.N. Karande Mr. Mahesh Pose Mr. Chandan Ghosh Mr. Kiran Divekar Mr. Mallikarjunachary Ms. Sudipta Dubey Mr. John Mathew Mr. Abhishek Singh Mr. Harshad S Surdi Mr. Rudheer D Bapat Ms. Bagyshri A Chalke

Acknowledgement

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Thank you

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Back ups

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Sensors used at mK

Semiconductor thermistor (NTD Ge)

(semiconductor resistance dependence of the temperature)

Transition edge sensors (TES)

(Superconducting transition resistance dependence of the temperature)

Magnetic micro calorimeters (MMC) paramagnetic

(Magnetization dependence of the temperature)

Multiplexed Kinetic Inductance Detectors (MKID)

(Temperature dependence of AC Impedance of a superconductor)

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Sample Thermal n-fluence (n/cm2)

T0 (K) R0 ( )

D 4.57 x 1018 4.9(0.2) 4.3(0.3) G 3.57 x 1018 9.8(0.2) 7.9(0.5) E 2.11 x 1018 41(1) 4.5(0.9)

Extracted parameters of NTD Ge

For NTD Ge sensor (E. Pasca et al., Proc. 8th Int. Conf. Adv. Tech. Part. Phys. 2 (2004) 93) depends on intrinsic properties of Ge, T0 depends on the doping level and constant α ~ 0.5 (A.L. Woodcraft et al., J. Low Temp. Phys. 134 (2004) 925)

Variation in R0 can be due to residual stress, unannealed radiation damage etc. (also reported by N. Wang et al., Phys. Rev. B 41 (1990) 3761).

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Readout system

Voltage noise f = 1kHz ~ 28 nv/ √(Hz) fB = 0.1Hz to 10Hz ~ 2μV p-p

Current noise f = 1kHz ~ 0.1 fA/√(Hz)

Current source is a Voltage to Current converter with an output adjustment range of ± 1000 pA. The voltage to the unit is supplied by an analog output channel of NI-PXI 6229 (16 bit DAC)

INA116 – 1st stage amplifier

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Prototype Bolometer

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Noise level at lower frequency

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Example for Homogenous doping (NTD Ge D)