Fabrication and Characterization of Diamond RadiationDetector as an Alternative to Silicon Detectors

Manoj Jadhav∗ and Shyam Kumar, K. Das Gupta, D. S. Misra, P. Sarin, R. VarmaDepartment of Physics, Indian Institute of Technology Bombay, Mumbai - 400076, INDIA

Introduction

Diamond with its unique properties is ex-pected to replace the place that silicon occu-pies in radiation detection today. This is pri-marily driven by the need to prevent frequentreplacement of the inner tracking system ofalmost all major collaborations at LHC afterits upgrade. For after LS-1 (Long Shutdown)LHC will deliver an instantaneous luminosityof 1034cm−2s−1 while in LS-2, the luminos-ity will be nearly doubled. Moreover, any fu-ture competitive international accelerator fa-cility will have to match LHC in terms of lu-minosity, which renders diamond as future de-tector material for sensors placed close to theinteraction point. It is with this aim that wehave taken up the development of diamonddetectors at IIT Bombay. This developmentis also in tune with our future contributionto the PANDA collaboration where we expectthe sub nano second timing of diamond sen-sors to improve the Particle Identification ca-pabilities of the PANDA detector.

Diamond Detector

With a high band gap of Eg = 5.5 eV, di-amond gives negligible intrinsic carrier den-sities even at room temperature, allowing tooperate diamond as a detector. As there is nopn-junction, the polarity of the electric field isirrelevant. The dark current of the diamondsamples, including both bulk and surface cur-rents, is less than 1 nA.cm−2 at an electricfield of 1 V.µm−1[2]. Due to the high carriermobilities in diamond, the charge collection isvery fast, taking about few picosec in detec-tors of approximately 300 µm thickness.

∗Electronic address: manoj.jadhav10@iitb.ac.in

Fabrication of DetectorDetector is prepared from 300 µm thick sin-

gle crystal CVD diamond samples with anarea of 3.5 mm × 3.5 mm.

A. Diamond Preparation

Before contacts were deposited, the dia-mond surface was cleaned. Firstly to re-move graphite and grease residue from thin-ning. The diamond was cleaned using a sat-urated solution of chromic acid, rinsed withdeionized water, cleaned in dilute solutions ofammonium hydroxide and hydrochloric acidand finally rinsed in deionized water. Thento remove any traces of chemicals, finger-prints, etc., diamond was cleaned in ammo-nium hydroxide; rinsed with deionized water,acetone, methanol, and deionized water; andthen placed in an oxygen plasma etcher forfinal surface preparation[3].

B. Metallization

A metallic thermal evaporation techniquewas used to coat both sides of the diamondwith successive metals: Cr (300 A) and Au(2000 A) [4]. Chromium was used since iteasily forms carbide structure which providesohmic contacts, Au was used to prevent ox-idation of the Cr layer and for ease of wirebonding. The evaporations were performedsuccessively, first Cr coated tungsten wire andthen the gold from an alumina coated tung-sten evaporating source boat. Then samplewas annealed at 580◦C in an N2 environmentto allow the chromium to form a carbide withthe diamond.For preliminary tests, the sensor was con-nected to outside electronics using copper wirebonded by silver epoxy on gold. The chip wasmounted on a modified IC-base. Fig.1 showsSensor with Cr/Au contacts and copper bond-ing using epoxy mounted on IC mounter.

Proceedings of the DAE Symp. on Nucl. Phys. 58 (2013) 872

Available online at www.sympnp.org/proceedings

FIG. 1: Diamond Sensor with Cr-Au coating.

1. CharacterizationGenerally, the metal in contact with the di-

amond determines the electrical properties ofthe contact. To supply voltage, Keithley 2410source meter was used which also measuredthe current through device. I-V characteristicsvoltage range selected from -1000 V to +1000V. A current-voltage curve from contacts isshown in Fig.2(a) indicating high resistivityand low leakage current which is comparablewith a data of a commercial detector samplefrom element6 UK shown in Fig.2(b).

(a)3.5 mm x 3.5 mm = 12.25 mm2, 300µmIIT Bombay

(b)4 x 4 mm2, 470µm Element6, UK

FIG. 2: I-V characteristic indicates dark current.

For capacitance study Keithley 4200 SMUsetup was used. Capacitance of the device ob-tained with the above measurement is 2.2 pF

at 0 V (shown in Fig.3) which is also in goodagreement with Element6 data which is hav-ing capacitance of 3 pF for 470 µm device[5].Considering a 70-80 Volts of operating volt-age, the detector should show timing of fewpico sec.

FIG. 3: C-V characteristics

Future Plans1. Characterize the detector developed by

studying the charge collection distanceand charge collection efficiency with aradiation source as well as with cosmics.

2. To test this detector with minimum ion-izing particles to test the improvementin particle identification capabilities byTOF if silicon is replaced with dia-mond. Simulations have already beenperformed and will be presented.

3. Develop a strip detector with silicon &diamond to be installed in PANDA col-laboration after suitable test beam runs.

AcknowledgmentsWe are thankful to CEN lab facility at IIT

Bombay. Also we are thankful to the PANDAcollaboration.

References[1] Technical design Report PANDA, 2009.[2] W. Adam et al. (RD42 Collaboration),

CERN/LHCC 98-20, 1998.[3] NIMPR A 354 (1995) 318-327[4] S. Zhao, PhD. Thesis, The Ohio State

University (1994).[5] Pernegger et al, J. Appl. Phys. 97, 073704

(2005)

Proceedings of the DAE Symp. on Nucl. Phys. 58 (2013) 873

Available online at www.sympnp.org/proceedings