Next generation calorimetric experiments for the search of rare events rely
on the detection of tiny amounts of light (of the order of 20 optical photons)
to discriminate and reduce background sources and improve sensitivity.
Calorimetric detectors are the simplest solution for photon detection at
cryogenic (mK) temperatures. The development of silicon based light detectors
with enhanced performance thanks to the use of the Neganov-Luke effect is
described. The aim of this research line is the production of high performance
detectors with industrial-grade reproducibility and reliability.Comment: 4 pages, 2 figure

Biassoni, M.

Brofferio, C.

Capelli, S.

Cassina, L.

Clemenza, M.

Cremonesi, O.

Faverzani, M.

Ferri, E.

Giachero, A.

Giordano, C.

Gironi, L.

Gotti, C.

Maino, M.

Margesin, B.

Nucciotti, A.

Pavan, M.

Pessina, G.

Previtali, E.

Puiu, A.

Sisti, M.

Terranova, F.

English

arXiv

M. Biassoni

C. Brofferio

S. Capelli

L. Cassina

M. Clemenza

O. Cremonesi

M. Faverzani

E. Ferri

A. Giachero

L. Gironi

C. Giordano

C. Gotti

M. Maino

B. Margesin

A. Nucciotti

M. Pavan

G. Pessina

E. Previtali

A. Puiu

M. Sisti

F. Terranova

Springer - Publisher Connector

Large area Si low-temperature light detectors with Neganov–Luke effect

Crossref

Eur. Phys. J. C (2015) 75:480DOI 10.1140/epjc/s10052-015-3712-2Special Article - Tools for Experiment and TheoryLarge area Si low-temperature light detectors with Neganov–LukeeffectM. Biassoni1,2, C. Brofferio1,2, S. Capelli1,2, L. Cassina1,2, M. Clemenza1,2, O. Cremonesi2, M. Faverzani1,2,E. Ferri1,2, A. Giachero2, L. Gironi1,2,a , C. Giordano1,3, C. Gotti1,2, M. Maino1,2, B. Margesin3, A. Nucciotti1,2,M. Pavan1,2, G. Pessina2, E. Previtali2, A. Puiu2, M. Sisti1,2, F. Terranova1,21 Dipartimento di Fisica, Università di Milano-Bicocca, Milano, Italy2 INFN, Sezione di Milano-Bicocca, Milano, Italy3 Fondazione Bruno Kessler, Trento, ItalyReceived: 3 August 2015 / Accepted: 29 September 2015 / Published online: 6 October 2015© The Author(s) 2015. This article is published with open access at Springerlink.comAbstract Next generation calorimetric experiments for thesearch of rare events rely on the detection of tiny amounts oflight (of the order of 20 optical photons) to discriminate andreduce background sources and improve sensitivity. Calori-metric detectors are the simplest solution for photon detectionat cryogenic (mK) temperatures. The development of siliconbased light detectors with enhanced performance thanks tothe use of the Neganov–Luke effect is described. The aimof this research line is the production of high performancedetectors with industrial-grade reproducibility and reliabil-ity.1 IntroductionIn the last decade low temperature thermal detectors haveproved to be powerful tools for high sensitivity spectroscopyin particle and nuclear physics. Calorimeters are used inmany of the most sensitive experiments for the search forrare events, like double beta decay, rare nuclear decays anddark matter direct detection. On the path to the improvementof their sensitivity, the reduction of the background is oneof the strategies to be pursued, and hybrid detectors for theactive rejection of spurious events are the most promisingoption. The simultaneous detection of the heat and light pro-duced by an interaction in the absorbing crystal makes parti-cle identification possible, both in scintillating calorimeters[1,2] (where the light yields of nuclear recoils, alpha andbeta/gamma particles are different) and in non-scintillatingcalorimeters by means of ancillary scintillating structures[3] or by the detection of Cherenkov photons [4]. In all thesecases, especially when the corresponding technology has toa e-mail: luca.gironi@mib.infn.itbe applied to future generation experiments, high detectionefficiency and low threshold are required as the amount oflight to be measured is as small as ∼50 eV. At the same timescalability and reproducibility are mandatory when dealingwith ton-scale experiments with hundreds to thousands ofchannels [5,6].2 Calorimetric light detectorsIn order to detect the photons emitted by large mass crys-tals operated as calorimeters at few mK, other calorimetersare usually the most effective tool. Presently running experi-ments or advanced R&Ds [2,7] use calorimetric light detec-tors of various materials (germanium, SOS - silicon on sap-phire) and temperature reading techniques (Ge NTD ther-mistors, TES, composite TES). These detectors proved goodto excellent performance in terms of energy resolution andthresholds down to hundreds of eV. Even though germaniumslabs are commercial products, the semiconductor industryis almost completely focused on silicon substrates. Siliconon sapphire detectors are also not very common outside thescientific environment. As a consequence, silicon is muchcheaper and the available technological expertise of the semi-conductor industry is wider. Silicon detectors with Neganov–Luke enhanced thermal signal can therefore be produced withreproducible processes and their properties tuned with highprecision. Silicon has also a wider range of processing tech-nologies, including micromachining processes, that poten-tially allow the integration of thermal sensors and mechani-cal suspension structures. A further advantage of silicon overgermanium is the fact that specific heat of silicon is a factor∼4.5 smaller than germanium [8], opening the possibility ofbuilding substantially larger detectors without compromis-123480 Page 2 of 4 Eur. Phys. J. C (2015) 75 :480ing the signal amplitude which is inversely proportional tothe heat capacity of the device.2.1 Neganov–Luke amplificationThe Neganov–Luke effect [9] is a mechanism that leadsto thermal signal amplification in semiconductor calorime-ters when a static electric field is applied. In standard oper-ating conditions, the signal is the temperature rise gen-erated by the conversion of the energy deposited by aninteracting particle into heat. Under the assumption thatall the energy deposited by the particle converts into ther-mal phonons in a time lapse much smaller than the typ-ical time response of the temperature sensors, the ampli-tude of the temperature variation is directly proportionalto the originally deposited energy. Given the large numberof information carriers (thermal phonons) in the calorime-ter, the intrinsic energy resolution is beyond reach ofany other detector. The energy resolution that can realis-tically be achieved is limited by imperfect thermalisationmechanisms and temperature sensor performance, but itis still of the order of 100 eV for small size light detec-tors.When a static electric field is applied to the semiconductorabsorber of the calorimeter, the electron-hole pairs producedby the primary particle interactions drift and acquire kineticenergy. This energy is converted into heat while the pairsscatter the absorber lattice during their motion, producing anadditional temperature increase, or thermal signal gain. Aslong as the electron-hole pairs drift and are collected withoutsignificant recombination, the additional heat produced isproportional to the number of pairs and to the voltage appliedto generate the electric field. The total energy resulting in athermal signal is therefore:Etot = E − Eproduction + Efield= E − ne−H δ + ne−H qV= E + E(−δ + qV )= E(1 − δ+ qV). (1)where E is the energy deposited by the particle interac-tion, Eproduction is the fraction of energy used to produceelectron-hole pairs, while Efield is the energy of the elec-tric field converted into heat to enhance the thermal signal.The former equals the number of electrons and holes (ne−H)times the band gap δ (1.17 eV for silicon [10]), while thelatter is ne−H times the charge of the electron (q) multi-plied by the potential V . The number of electron-hole pairsequals the energy originally deposited divided by the meanenergy  needed to produce a pair in the considered semi-conductor. In the case of silicon this energy is ∼3.8 eV at77 K.It is interesting to note that, as soon as V is largerthan a few volts, the last term of Eq. 1 quickly becomesdominant. In this regime the thermal detector is work-ing like a ionization detector because the thermal signalis almost entirely generated by the drifting charges, andits energy resolution is therefore expected to be deter-mined by the fluctuations in the number of electron-hole pairs generated in the primary interaction. As faras the noise is not affected and the pulse amplitude isincreased, the detection threshold is reduced by the samefactor.Some literature exists where attempts to exploit this effectto amplify the signal produced by light absorbed in calori-metric detectors are described [11,12]. In this work we reporton similar measurements performed with different electrodelayouts, in a wider voltage range and with carefully selectedhigh resistivity silicon substrates.3 Experimental setupThe detectors we discuss are Si crystals held by means ofPTFE pieces fixed to a copper frame. Each of them con-sists of a 20×20 mm2, 625µm thick, crystalline Si absorber.The room-temperature resistivity of the Si is >5 kcm. Forthe application of the Neganov–Luke voltage the absorber isequipped with two Al electrodes directly evaporated onto itssurface. Below the electrodes the silicon substrate is heavilydoped in order to build a ohmic contact. We tested differ-ent geometries and the best results were obtained with thegeometry shown in Fig. 1.The detectors were installed in an Oxford InstrumentTL200 dilution refrigerator located in the Cryogenic Lab-oratory of Milano-Bicocca University.The temperature sensor is a 3×1×0.5 mm3 NeutronTransmutation Doped (NTD) germanium thermistor. It isthermally coupled to the Si crystal via glue spots ofAraldite. When the NTDs are biased with a polarizingcurrent, the strong dependence of their resistance on thetemperature translates in a variation of the voltage acrossthe thermistor each time a temperature pulse is recorded.This electrical signal (voltage) is amplified, filtered toremove high frequency noise, digitized and continuouslyrecorded. The off-line analysis allows to determine thepulse amplitude, as well as many pulse shape parame-ters.Two types of light sources have been faced to the detec-tors: BGO (Bismuth Germanate) crystals illuminated by a232Th source located outside the cryostat and YAP (YttriumAluminum Perovskite) scintillators with a monochromatic αsource deposited on a surface.123Eur. Phys. J. C (2015) 75 :480 Page 3 of 4 480Fig. 1 Neganov–Luke detectorgeometry. Left technicaldrawing of the electrode layout.Right picture of the actualdetector with aluminumelectrodes and bonding pads.The central dot is the bondingpad and the electrode itself. Theannular electrode has a pad forwire bonding at the midpoint ofone side. The square in theupper left corner is a gold patchforeseen for the placement ofthe NTD temperature sensorFig. 2 Comparison of pulseswith and withoutNeganov–Luke amplification:black line represents a pulsewithout any amplification whilethe red line (and red scale on theright) is a pulse correspondingto the same energy depositionamplified by a 240 V bias. Thesignal-to-noise ratio is improvedby a factor ∼60Samples0 200 400 600 800 1000 1200 1400Amplitude [mV]−6−4−2024681012140V240VAmplitude [mV]200−100−01002003004005006004 Detector performanceThe performance of the detectors have been studied by apply-ing different polarizing voltages at the Neganov–Luke effectelectrodes. The polarity of the bias has also been invertedto characterize any asymmetric behaviour of the device anddetermine the best operating conditions. Voltages up to 240V have been applied on 12 detectors without encounteringany failure and observing a consistent behaviour.4.1 Detectors noise and energy resolutionThe noise level of the detectors has been evaluated during thedata taking at different values of Neganov–Luke voltage byrandomly sampling the baseline after the application of anoptimal filtering algorithm. No dependence of the noise levelon the applied voltage has been observed up to the maximumapplied voltage of 240 V. The energy resolution has also beenestimated by measuring the width of the peak correspondingto the light pulses produced by the YAP source. As expected,already at the lowest value of applied voltage (40 V) the rel-ative energy resolution is compatible with the width of thelight source emission of about 7 % (dominated by the alphasource smearing), and remains constant when the voltage isincreased.4.2 Pulse amplitude and gainWhen a voltage is applied to the electrodes the amplitude ofthe pulses generated by light interacting with the absorber islargely increased. The corresponding gain G(VLuke), definedas the ratio between the amplitude of a pulse with a givenbias voltage VLuke and the amplitude of a pulse producedby a particle of the same energy with no voltage applied,shows a smooth and nearly linear dependence on the volt-age, with absolute value depending on the electrodes design.The gain stability over time has been characterized and a gen-eral behaviour has been identified on all the different elec-trode designs: after an initial transient during which the gaindecreases exponentially, the pulse amplitude stabilizes. Thetime constant of the transient strongly depends on the rateof energy deposition on the absorber, and hence on the rateof charge produced and drifted by the electric field: the lightdetectors facing the BGO source with low rate of interac-123480 Page 4 of 4 Eur. Phys. J. C (2015) 75 :480tion are stable over long time scales, while the gain of thedetectors facing the YAP sources quickly decays by a factor20–40 %. This behaviour, already observed in similar appli-cations [11–13], is ascribed to charge trapping phenomena inthe detector and the consequent generation of a shielding fieldwhose features can be observed and studied when the polar-izing voltage is removed. This effect can be reduced with acareful selection of high purity silicon absorbers and by fol-lowing a proper procedure when the voltage is applied; theresidual charge can be easily removed by flashing the detec-tor with light. Given the rate of interaction that the detectorsmust sustain in rare events experiments (typically few mHz),the gain reduction can be considered negligible.By applying a Neganov–Luke voltage of 240 V to thedetectors described and shown in Fig. 1, a gain of ∼60 hasbeen obtained on the pulse amplitude, with no visible effectson the noise level. An equal improvement of the signal-to-noise ratio has therefore been achieved, as shown for examplein Fig. 2. Since no misbehaviour has been observed in anyof the detectors up to 240 V, higher voltages will be tested ina future measurement campaign with a dedicated cabling toovercome the limits of a standard cryostat wiring system.5 ConclusionsA technique that exploits the Neganov–Luke effect toenhance the thermal signal in cryogenic detectors has beensuccessfully applied to silicon light detectors with a noveldesign. A large gain in the signal-to-noise ratio has beendemonstrated and the good behaviour of the detectors testedin this work proves that further improvements are realisticallyachievable. Thanks to the very low energy threshold, largedetection surface, low cost and performance reproducibil-ity, these detectors are suitable for many applications in thephysics of rare events with low temperature detectors, wheresmall amounts of light (scintillation or Cherenkov photons)must be detected in next generation experiments in order toreject the spurious background.OpenAccess This article is distributed under the terms of the CreativeCommons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution,and reproduction in any medium, provided you give appropriate creditto the original author(s) and the source, provide a link to the CreativeCommons license, and indicate if changes were made.Funded by SCOAP3.References1. A. Alessandrello et al., Phys. Lett. B 420, 109 (1998)2. G. Angloher et al., Astropart. Phys. 18, 1 (2002)3. L. Canonica et al., Nucl. Instr. Methods Phys. Res. A 732, 286(2013)4. T. Tabarelli de Fatis, arXiv:0908.0164 [nucl-ex]5. The CUPID Interest Group, arXiv:1504.03599 [physics.ins-det]6. The CUPID Interest Group, arXiv:1504.03612 [physics.ins-det]7. J.W. Beeman et al., Adv. High Energy Phys. 2013, 237973 (2013)8. Noel Coron et al., Opt. Eng. 43, 1568 (2004)9. P.N. Luke, J. Appl. Phys. 64, 6858 (1988)10. G.G. Macfarlane et al., Phys. Rev. 111, 1245 (1958)11. M. Stark et al., Nucl. Instr. Methods Phys. Res. A 545, 738 (2005)12. C. Isaila et al., Phys. Lett. B 716, 160 (2012)13. R. Agnese et al., Phys. Rev. D 91, 052021 (2015)123

The European Physical Journal C

Next generation bolometric experiments for the search for rare events relay on the detection of tiny amount of light (of the order of 20 optical photons) to discriminate and reduce background sources and improve their sensitivity. Bolometric detectors are the simplest solution for photon detection at cryogenic temperatures. The development of silicon based light
detectors with enhanced performance thanks to the use of the Neganov-Luke eect is described. The aim of this research line is the production of high performance detectors with industrial-grade reproducibility and reliability

Giordano, Claudia

Margesin, Benno

Archivio della ricerca - Fondazione Bruno Kessler

Next generation calorimetric experiments for the search of rare events rely on the detection of tiny amounts of light (of the order of 20 optical photons) to discriminate and reduce background sources and improve sensitivity. Calorimetric detectors are the simplest solution for photon detection at cryogenic (mK) temperatures. The development of silicon based light detectors with enhanced performance thanks to the use of the Neganov–Luke effect is described. The aim of this research line is the production of high performance detectors with industrial-grade reproducibility and reliability

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Large area Si low-temperature light detectors with Neganov-Luke effect

Large area Si low-temperature light detectors with Neganov-Luke effect

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