The motivation for investigating the use of GaAs as a material for detecting
particles in experiments for High Energy Physics (HEP) arose from its perceived
resistance to radiation damage. This is a vital requirement for detector
materials that are to be used in experiments at future accelerators where the
radiation environments would exclude all but the most radiation resistant of
detector types.Comment: 5 pages. PS file only - original in WORD Also available at
  http://ppewww.ph.gla.ac.uk/preprints/97/06

Bates, R. L.

Da'Via, C.

O'Shea, V.

Pickford, A.

Raine, C.

Smith, K. M.

English

arXiv

The motivation for investigating the use of GaAs as a material for detecting particles in experiments for High Energy Physics (HEP) arose from its perceived resistance to radiation damage. This is a vital requirement for detector materials that are to be used in experiments at future accelerators where the radiation environments would exclude all but the most radiation resistant of detector types.The motivation for investigating the use of GaAs as a material for detecting particles in experiments for high-energy physics (HEP) arose from its perceived resistance to radiation damage. This is a vital requirement for detector materials that are to be used in experiments at future accelerators where the radiation environments would exclude all but the most radiation resistant of detector type

Bates, R.

Da Via, C.

Smith, K.

CERN Document Server

physics/9709034   24 Sep 1997GLAS-PPE/97-0628 August 1997Department of Physics & AstronomyExperimental Particle Physics GroupKelvin Building, University of  Glasgow,Glasgow, G12 8QQ, Scotland.Telephone: +44 - (0)141 – 3398855 Fax: +44 – (0)141 - 3305881Radiation Induced Damage in GaAs Particle Detectors.R. Bates, C. Da Via, V. O'Shea, A. Pickford, C. Raine and K.Smith.Department of Physics & Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland.AbstractThe motivation for investigating the use of GaAs as a material for detecting particles in experimentsfor High Energy Physics  (HEP) arose from its perceived  resistance to radiation damage. This is avital requirement for detector materials that are to be used in experiments at future accelerators wherethe radiation environments would exclude all but the most radiation resistant of detector types.Radiation Induced Damage in GaAs Particle Detectors.R. Bates, C. Da Via, V. O'Shea, A. Pickford, C. Raine and K.Smith.Department of Physics & Astronomy, University of Glasgow, Glasgow G12 8QQ, Scotland.AbstractThe motivation for investigating the use of GaAs as a material for detecting particles in experiments for High EnergyPhysics  (HEP) arose from its perceived  resistance to radiation damage. This is a vital requirement for detector materialsthat are to be used in experiments at future accelerators where the radiation environments would exclude all but the mostradiation resistant of detector types.INTRODUCTIONGaAs particle detectors were developed for use in theexperiments currently being designed as part of the newaccelerator (the LHC) to be built at CERN, Geneva. Theestimated radiation fluence foreseen for the innermostforward tracking layer of the experiment is of the order ofseveral 1014 cm-2 of particles, consisting of roughly equalfluxes of charged hadrons and neutrons with a typicalenergy of 1 MeV. This will be accumulated over the life ofthe experiment, which is projected to be 10 years.GaAs had been proposed as a radiation hard detectormaterial for tracking particles in this very forward region,where the radiation environment will be harshest andprecludes the use of standard types of semiconductorradiation detectors. Extensive studies[1] have already beencarried out on the resistance to radiation damange of thismaterial using  Gamma radiation. The purpose of this studywas to evaluate the damage from neutrons and chargedparticles with particular emphasis on pions, as these areforeseen to be one of the dominant sources of damage tosemiconductor particle detectors at the LHC.MEASUREMENTS AND RESULTSAll the detectors used in this study were 3 mm diametercircular Schottky diodes with a 500 µm wide guard ringseparated by 10 µm on 200 µm thick semi-insulating GaAssubstrates. The detector operates as a reverse biased diode,the energy deposited by a particle creating electron-holepairs which are swept out of the depleted region by theapplied field, which create a signal in the front-end chargeamplifier. The main detector characteristics of interest forHEP are the leakage current, the full depletion voltage (Vfd)and the charge collection efficiency (cce). The leakagecurrent is measured as an I-V characteristic up to thebreakdown voltage or until the diode reaches a compliancecurrent. Vfd is rather more difficult to measure as there isincomplete charge collection in the first place. This value ismeasured by looking for alpha particle detection on theback contact as the range of this type of particle isapproximately 20 µm in GaAs. This effect is also seen onthe I-V curve as a knee in the leakage current correspondingto an increase in the injection current from the back contact.05101520253035Leakage Current  (nA/mm2 )0 5 10 15 20 25 30Fluence (1013 p/cm2)Fig[1]  Detector leakage current increase vs. proton irradiationfluence. The current increases initially and then remains flat at afactor of 2 higher value in the range of interest.The cce of the device is defined as the ratio of themeasured charge to the total charge deposited in thedetector by the ionising particle. The deposition of energyin the bulk of the detector depends on the type of radiationused to illuminate the detector. Beta particles with anenergy of over 2 MeV deposit energy uniformly along theirpath through the detector material with a Landau energydistribution and are known as minimum ionising particles(MIPs). The most probable value of this deposited energy is56 keV per 100 µm of GaAs traversed. Alpha particles loseall their energy close to the surface of the material becauseof their short range (approximately 20 µm for 5.5 MeValphas from Am241). This property is very useful as itpermits the contribution to the total charge collected by theholes and the electrons to be separated by illuminating thedetector from either side. This technique has been used toestimate the mean free path for each carrier in the material.Proton Fluence (1013/cm2)-100-75-50-250Current (nA/mm2 )-250 -200 -150 -100 -50 0Bias (V)8.93.91.6PreVfdVfd Fig[2] Leakage current for different fluences of protons. Thechange in Vfd is indicated.In previous studies devices had been irradiated up to 100Mrad with Co60 gammas and showed negligibledegradation.Neutrons from a spallation source at ISIS with a peakenergy of 1 MeV, 24 GeV/c protons from the PS (ProtonSynchrotron) at CERN and 300 MeV/c pions from the PSIfacility at Villigen, Switzerland were used to irradiate thesamples. The energy of the neutrons is the typical energyexpected for the background neutron flux at the LHC.0510152025Charge collection eficiency (%)0 200 400 600 800 1000Reverse Bias (V)Hole Charge CollectionElectron Charge CollectionFig[3] Electron and hole contributions to the charge signal after2x1014 p cm-2 measured using alphas on the front and back of thedevice.The pion energy is at the delta resonance and results inmaximum damage. The proton energy is that used toirradiate silicon detectors for similar applications to thoseproposed for their GaAs  counterparts.The change in leakage current as a function ofirradiation is of the order of a factor of two over the rangeof interest for all types of irradiation (Fig.[1]). This changeis orders of magnitude lower than that for silicon detectors.This is not a cause for concern for the use of GaAs devicesin the LHC environment, as the leakage current increasewould contribute, at most, a 30% decrease in signal to noiseratio - an effect which is even further reduced by the fastshaping times proposed for the LHC experiments.   The voltage at which full depletion occurs (Vfd) decreaseswith increasing fluence for all types of charged particleirradiation (as illustrated in Fig.2 for protons) and isconsistent with a decrease in free carrier concentrationsimilar to the donor removal effect seen in silicon beforetype inversion. From an initial fluence of around 2 x 1013 pcm-2, Vfd falls by about 35% from its pre-irradiation valuewhich is always around 1 V µm-1 of substrate thickness forthe type of detectors used in this study. At higher fluencesVfd remains almost unchanged.The cce for MIPs is the parameter most affected byparticle irradiation. The loss of charge is due to the creationof radiation induced trapping centres in the bulk. Theseeither trap the charge created by the traversing particle ormodify the electric field within the bulk of the detector so asto enhance the trapping efficiencies of the traps present.Protons020406080100Charge Collection Efficiency (%)0 2 4 6 8 10Integrated non-ionising Energy (1014 keV per gm)PionsNeutronsProtonsFig[4] Charge collection efficiency as a function of integratedNIEL for protons, pions and neutrons. The energy deposited bydifferent particles has been normalised according to Table 1.The cce of a 200 µm thick detector, reverse biased at200 V, before irradiation is typically >75% whichcorresponds to a signal greater than 20,000 electrons.The reduction of cce with fluence depends on the typeand energy of the irradiating particle. At a bias of 200 V a10,000 electron signal is seen for a MIP (~40% cce) after1.4 x 1014 n cm-2, 6.0 x 1013 p cm-2 and only  3.0 x 1013 picm-2, for the energies used. Initially, the cce falls rapidly asa function of fluence and then decreases at a lower rate. Thetwo stage cce dependence on fluence is similar to thatobserved in the dependence of leakage current as a functionof fluence.0102030Charge collection efficiency (%)0 100 200 300 400 500Reverse Bias (V)Hole Charge Collection Post AnnealElectron Charge Collection Post AnnealHole Charge Collection Pre AnnealElectron Charge Collection Pre AnnealFig[5] The effect of a thermal anneal at 450 °C for 90 seconds on adiode that has been irradiated with 1x1014 pi cm-2. From alpha cce data it has been shown that the signalfrom holes falls faster with increasing fluence than thatfrom electrons and so the MIP signal after irradiation ismainly due to the electron signal. If the bias is increased thesignal begins to increase again and this is due to a furtherincrease in the electron signal as shown in Fig[3].The differing effects from each type of irradiation on thecce can be attributed to the differing amounts of non-ionising energy loss (NIEL) as the particles pass through thecrystal lattice. Fig[4] shows the correlation between cce for200 µm thick detectors measured at 200 V and thecalculated NIEL[2,3] for the various particles used in thisstudy. The damage attributed to each particle type has beenweighted by its relative NIEL and the calculated energy lossis well correlated with the observed damage. The radiationinduced defects in the crystal may be reduced by thermalannealing. Arsenic anti-site defects introduced byirradiation have been shown to anneal at temperatures above450°C [3]. Detectors were thermally annealed at 450°C in arapid thermal annealer after exposure to 1 x 1014 p cm-2.The electron signal increased as shown in Fig[5] but thatdue to the holes hardly changed. The response to MIPs, asillustrated in Fig.[6], gave an increase in cce to 50% at abias of 300 V.DISCUSSIONThe radiation damage measured in GaAs particledetectors correlates very well with calculated values ofNIEL for different particle types. The exponential decreasein charge collection efficiency with increasing integratedNIEL cannot easily be explained, as it depends on too manyparameters which cannot be measured in isolation. In Sidetectors the displacement cross section for protons,neutrons and pions has been found to be  almost the samevalue for each particle at the energy of interest[5]. In thecase of GaAs detectors this is not so and a factor of almost 4has been measured in the damage caused by equivalentfluxes of neutrons and pions, with protons causing roughlythree times more damage than neutrons.020406080100Charge collection efficiency (%)0 100 200 300 400Reverse Bias (V)Post-IrradiationPost-Irradiation and AnnealingPre-IrradiationFig[6] Improvement in cce after annealing at 450 °C.Irradiation type NIEL Damage factor1 MeV neutrons 0.924 GeV/c protons 2.9300 MeV/c pions 3.6Table 1. Calculated damage factors for different types of radiationby Chilingarov et al[2].Calculations of the NIEL for each of these particlesindicate that the particle equivalence which applies toSilicon detectors is not valid for GaAs based detectors. TheGaAs detectors have a greater radiation hardness to neutronirradiation than Silicon but are more susceptible to damagefrom charged particle irradiation. Indeed, recent studieswith Silicon detectors show increased hardness by usingharder design rules and subtle processing improvements [6]which should enable them to be used in the harshest areas ofthe LHC experiments for the duration of the project.SUMMARY AND CONCLUSIONSThe irradiation studies show that a large signal loss ispresent in the detectors at high levels of incident chargedhadron fluence. This damage is consistent with increasednon-ionising energy loss from the charged particleirradiation. Post irradiation annealing helps to recover someof the lost charge by annealing out an electron trap. Furtherstudies are taking place in an attempt to improve theunderstanding and performance of these devices afterirradiation as their present radiation tolerance is notsufficiently hard enough for the forward regions in an LHCexperiment.REFERENCES[1] S. P. Beaumont et al., GaAs solid state detectors forparticle physics.Nucl. Instr. and Meth. A322 (1992) 472-482.[2] A. Chilingarov et al., Radiation damage due to NIEL inGaAs particle detectorsFourth International Workshop on Gallium Arsenideand Related Compounds, Aberfoyle, Scotland, UK,4-7 June 1996. To be published in Nucl. Instr. andMeth. A.[3] M. Rogalla et al., Radiation studies for GaAs in theATLAS inner detector.Fourth International Workshop on Gallium Arsenideand Related Compounds, Aberfoyle, Scotland, UK,4-7 June 1996. To be published in Nucl. Instr. andMeth. A.[4] R. Worner et al., Electron spin resonance of AsGaantisite defects in fast neutron irradiated GaAs.Appl. Phys. Lett. 40(2) (1982) 141-143.[5] G.N. Taylor et al., Radiation induced bulk damage inSilicon detectors.Nucl. Instr and Meth. A383 (1996) 144 - 154.[6] A. Chilingarov et al., Radiation studies and operationalprojections for silicon in the ATLAS inner detector.Nucl. Instr and Meth. A360 (1995) 432- 437.

Radiation Induced Damage in GaAs Particle Detectors

Radiation Induced Damage in GaAs Particle Detectors

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