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PAPER View Article OnlineView Journal | View Issue

a State Key Laboratory of Solidification Processing, School of Materials Science

and Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi 710072,China. E-mail: jwq@nwpu.edu.cn; Fax: +86 29 88495414; Tel: +86 29 88460496bNorthwest Institute of Nuclear Technology, Xi'an, Shaanxi 710024, China

10304 | CrystEngComm, 2013, 15, 10304–10310 This journal is © The Ro

Cite this: CrystEngComm, 2013, 15,10304

Received 29th August 2013,Accepted 25th September 2013

DOI: 10.1039/c3ce41734d

www.rsc.org/crystengcomm

Radiation damage on CdZnTe:In crystals under highdose 60Co γ-rays

Lingyan Xu,a Wanqi Jie,*a Gangqiang Zha,a Yadong Xu,a Xiaochuan Zhao,b

Tao Feng,a Lin Luo,a Wenlong Zhang,a Ruihua Nana and Tao Wanga

A radiation damage mechanism of CdZnTe:In crystals under 60Co γ-rays with a cumulated radiation dose of

~2.4 kGy was proposed and discussed. Thermally stimulated current (TSC) measurements were carried out to

characterize the γ-ray induced radiation damage. Four main trap levels (T1–T4) originating at four main micro-

scopic defective states were identified. We attributed traps T1 and T2 to shallow donor impurities and shallow

acceptor A-centers, respectively. Trap T3 was ascribed to dislocations and a great increase in defect concentra-

tion occurred after radiation. Trap T4 originated at deep acceptor Cd vacancies and vacancy related defect

complexes and a considerable increase in the trap density, after radiation, lead to the conversion of conduction

type from n to p in the Hall measurements and a Fermi level shift in the temperature-dependent resistivity analyses.

Radiation induced electrically active defects contributed to the variation of the electrical compensation conditions

and the worsening of the charge transport properties, which were consequently reflected by a significant deteriora-

tion in CZT detector performance.

1 Introduction

Cadmium zinc telluride (CdZnTe or CZT) is regarded as themost promising compound semiconductor material forroom-temperature X-ray and γ-ray radiation detectors.1 Theseapplications require CZT detectors to be operative in a hostileenvironment, for example, being exposed to high-dose radia-tion for a long time, which can markedly alter the detectorperformance, and cause so-called radiation damage. Thus, itis very important to have detailed knowledge of the radiationeffects on the crystal properties and detector performances.

High energy radiation can produce a number of primaryknock-on atoms (PKAs) which may lead to further displacedatoms, resulting in a collision cascade.2 Thus, the major form ofradiation damage is a high concentration of vacancy–interstitialpairs (Frenkel defects). Kinchin and Pease have developed asimple and widely used model to determine the total numberof atomic displacements in an irradiated solid.3 Lindhard et al.acquired the damage energy model based on a modifiedKinchin–Pease model by using the LSS stopping theory, whichpartitions the initial energy between the displacement damageof lattice atoms and electron excitations as heat dissipation.4

The concept of damage energy has proven useful in givinga schematic description of defect production in a collision cas-cade. However, there are still some difficulties: (1) displacement

occurs during collision when the energy transferred to thelattice atom exceeds the threshold value Ed. Ed is determinedby the interaction potential energy between lattice atoms andis affected by the crystallographic structure, the crystal orien-tation in which the displaced atom is knocked off, etc.5 Thus,this energy is not constant for all collisions and cannot beeasily obtained by calculation or by experiment. (2) Between60% and 80% of the Frenkel defects predicted by theKinchin–Pease model will be annihilated by recombination.6

A significant fraction of the point defects will aggregate intoclusters, which may also migrate and re-incorporate into defectsinks, such as voids, dislocations, dislocation loops, grainboundaries, precipitates, etc.7 And the remaining point defectsappear as single vacancies or isolated interstitials. The entirechain of events only takes about 10−11 s.8,9 Therefore, it isnearly impossible to follow or to predict the intensely complexradiation damage process. Considering the imperfections intransport theory, it's necessary to carry out measurements toacquire reliable defect information, thus getting a full under-standing of the radiation effects theoretically and experimentally.

Several related experimental studies have been reportedon the radiation effects of various types of radiation sources(γ-rays, X-rays, electrons, protons, neutrons and ions) onCdTe and CdZnTe detectors.10–14 Some of the existing researchhas focused on radiation hardness under different conditions.Cavallini et al. demonstrated that an increase in the concen-tration of defect states becomes significant after ~10 kGy ofγ-ray radiation, and a total dose of 30–35 kGy will cause acomplete degradation of the spectroscopic capabilities of

yal Society of Chemistry 2013

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CdTe:Cl detectors.15,16 Fraboni et al. revealed that CdZnTedetector performance shows no severe degradation until ithas been subjected to a cumulated dose of 260 kGy, whileCdTe:Cl starts degrading after 130 kGy of X-ray radiation.17

It was found that CdZnTe has better radiation hardness thanCdTe.18,19 Furthermore, a CdTe detector with a Schottky barrierwould possibly undergo a deterioration in performance at asmaller radiation dose than one without a barrier.20,21 Someof the existing research has focused on material compensa-tion properties. Cavallini et al. monitored the evolution ofradiation induced trap defects with increasing radiation dose,and tried to correlate the balance between deep and shallowdonors and acceptors and the electrical compensation pro-cess, which determines the resulting material resistivity. Theyhave also compared radiation results obtained for CdTe andCdZnTe detectors in order to clarify the origins of identifieddefect levels.15,22 Very little attention has been paid to themicroscopic mechanism, however the first step in investigatingradiation effects is to understand the nature of radiationdamage on the atomic level. In this study, the authors areconcerned with the radiation damage mechanism for a seriesof energy transfer processes, including Compton scattering,Rutherford scattering and collision cascade, focusing on thehigh dose 60Co γ-ray irradiated CZT crystals. We are able toobtain general estimations of the type and amount of defectsthrough transport theory, which provide comparisons withexperimental results. We have also investigated the effects ofradiation induced defective states on the material propertiesand detector performances.

2 Experimental

A set of Cd0.9Zn0.1Te:In single crystals were cut from a CZTingot grown by a modified vertical Bridgman (MVB) method,with dimensions of 5 × 5 × 2 mm3. After chemical polishingwith a bromine–methanol solution, electrodes were depositedby electroless gold deposition. The crystals were radiated withhigh dose γ-rays from the 60Co radiation source, with twomain γ-ray energies at 1.17 MeV and 1.33 MeV. The cumu-lated radiation dose was about 2.4 kGy after 8 h of irradiationunder a high dose rate of ~0.3 kGy h−1, corresponding to2.6 × 1012 impinging photons. With an absorption coefficient ofaround 6.07%, approximately 1.6 × 1011 photons were absorbedby the CZT crystals.

We utilized thermally stimulated current (TSC) technologyto investigate the variation of energy distribution of traplevels in the bandgap, which are associated with defectivestates induced in the crystal lattice. The excitations of freecarriers by achromatic light from a halogen lamp were cap-tured by defects at low temperature. As the temperatureincreased in darkness from 80 to 320 K at a constant heatingrate of ~0.2 K s−1, trapped electrons and holes would bereleased by thermal emission, and the TSC current could bescanned and recorded under an applied bias of 10 V. Eachpeak in the TSC spectrum corresponds to a certain trap

This journal is © The Royal Society of Chemistry 2013

defect in the material, with which the trap parameters, likeactivation energy and capture cross section, could be acquiredby applying the Arrhenius plot.23–25 We also compared electri-cal properties, optical properties, carrier transport propertiesand spectroscopic characterizations before and after γ-rayradiation. An Agilent 4155C setup was employed for current–voltage analysis to assess ohmic contacts and to determinethe material resistivity. Photoluminescence (PL) technologywas carried out to obtain information on γ-ray inducedchanges in the structure. A 20 mW argon ion laser with awavelength of 488 nm was used for the excitation at 10 K.A Triax 550 tri-grating monochromator with a photo-multipliertube (PMT) possessing a spectral resolution of better than0.3 nm was employed to collect and to analyze the emissions,thus forming the PL spectra. Charge carrier mobility wasobserved by means of the Van der Pauw method based on theHall effect. The energy resolution and collection efficiency ofthe detector performance measurements were investigated byusing CZT planar detectors irradiated with γ-ray emissionsfrom an uncollimated 241Am source with an energy of 59.54 keV.The spectroscopy testing system consists of an ORTEC 710 biassupply, an eV-550 preamplifier, an ORTEC 570 shaping ampli-fier and the multi-channel analyzer Canberra Multiport II.The mobility–lifetime product (μτ) could be obtained by fittingthe Hecht equation.26,27

3 Results and discussion3.1 Effects of high dose 60Co γ-ray radiation on CZT crystals

An incoming high energy photon (γ-ray or X-ray) can interactwith matter in three different ways; the photoelectric effect,Compton scattering and electron–positron pair production,with the probability of each event occurring as a function ofphoton energy and the atomic number of the target mate-rial.28,29 Accordingly, Compton scattering is the dominatinginteraction in our case. This effect can be considered as anelastic collision between an incident γ-ray photon and anelectron, through which the radiation transfers part of itsenergy to the scattered Compton electron. The remainingenergy is taken by the degraded photon which remains avail-able to create additional free carriers until all of its initialenergy is lost.2,30,31 As shown in Fig. 1(a), the new photonwith a wavelength of λ1 is emitted at a scattering angle of θ1from the initial photon of wavelength λ. The wavelength shiftduring the primary Compton scattering is expressed as

1e

11 hm c

( cos ) ð1Þ

where h is the Planck constant, me is the electron rest mass,and c is the velocity of light.

Fig. 1(b) shows the energies of a new photon and theCompton electron after primary Compton scattering from theinitial γ-ray photon with an average energy of ~1.25 MeV in aCZT crystal as a function of the scattering angle. It is clear to

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Fig. 1 Radiation damage mechanism in a CZT crystal under high dose60Co γ-ray radiation: (a) a schematic diagram for a series of energy transfer processes; (b) energies of

a new photon and the Compton electron after primary Compton scattering from the initial γ-ray photon with an average energy of ~1.25 MeV as a function of the scattering

angle; (c) energies obtained by lattice atoms Te, Zn and Cd, after Rutherford scattering from Compton electrons with high energies (170 keV–1.04 MeV).

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see that the Compton electron with an energy in the rangeof 170 keV–1.04 MeV could be obtained during primaryCompton scattering with a pretty high probability of up to 89%.These energies are high enough to cause atomic displacements,since a Compton electron with an energy greater than 170 keVwould interact with a lattice atom through Rutherford scatter-ing.31 Relativistic quantum mechanics can be used to describethe collision process between Compton electrons and latticeatoms.2 As shown in Fig. 1(a), the maximum energy transferred

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to an atom of mass M from a high energy electron of energy Ee1could be expressed as

Max( ae e eE E E m cMc1

1 12

2

2 2) ( )

ð2Þ

where Ea1 is the energy of the lattice atom.Fig. 1(c) shows that energies obtained by the lattice atoms

Te, Zn and Cd, reach up to 36.1, 70.3 and 40.9 eV,

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respectively. As described earlier, the displacement thresholdenergy Ed is not constant for all collisions and cannot beaccurately determined by calculation or experiment. The typi-cal value has been taken to be around 20–30 eV.3,5,32,33 Weapproximately consider Ed = Eb, since the minimum energyfor displacing an atom from its lattice site Ed is closelyrelated to the energy binding an atom to its lattice site Eb(~19.01 eV for Te, ~19.39 eV for Zn, and ~18.99 eV for Cd).7,34,35

Therefore, without considering Zn for its small amount, Te mayhave insufficient energy to be displaced from its lattice site.The most probable cause of primarily induced defects is theproduction of a high concentration of Cd vacancy–interstitialpairs. A Cd vacancy with a relatively low formation energy(~0.6 eV) is the simplest point defect in the CZT crystal latticein Te-rich conditions.36–38 When the electrons can impartmore than 2Ed to the primary knock-ons (PKAs), at least onedisplaced lattice atom is created per PKA throughout the colli-sion cascade process, according to the simple Kinchin–Peasemodel.3 Thus, a total number of ~1011 Cd vacancies could beinitially generated from ~1.6 × 1011 γ-ray photons. Thesepoint defects would further interact with as-grown intrinsicdefects or impurities under appropriate conditions, forming atotal of ~1010 defect complexes in the irradiated CZT crystaland deep traps in the bandgap.7 All the electrically activedefects could affect the electrical compensation conditions andcharge carrier transport properties, which eventually causes asignificant deterioration of the CZT detector performance, aswill be discussed in detail later.

3.2 Radiation induced defect states

Thermally stimulated current (TSC) technology was used asthe main tool for the characterization of 60Co γ-ray inducedradiation damage, since TSC results could reveal trap levelslocated at microscopic defective states already present in theas-grown material and newly induced in the irradiated CZTlattice structure. As shown in Fig. 2, four main peaks T1–T4,corresponding to four main defect levels in the bandgap,

Fig. 2 Typical TSC curves before and after high dose γ-ray radiation, the enlarged

area involving traps T1 and T2 is inserted in the upper-left corner.

This journal is © The Royal Society of Chemistry 2013

are presented in the TSC curves for the CZT crystals beforeand after high energy γ-ray radiation. The enlarged areainvolving traps T1 and T2 is inserted in the upper-left corner.Clearly, traps T1 and T2 are not strongly affected by radiation,whereas the defect concentrations for the deep traps T3 andT4 greatly increase after radiation, which will subsequently beproved to fundamentally determine the worsening of spec-troscopic responses.

The level labeled T1 observed in the CZT crystals has anactivation energy of 0.02–0.04 eV below the conduction bandminimum, with the trap type (electron or hole) revealed bythermoelectric effect spectroscopy (TEES).39–41 This energylevel was assigned to a shallow donor originated at an In dopantor dopant related point defect (Incd, for example). The effectof the element In (EC − 0.014 eV) was revealed by photo-luminescence (PL) spectra and the defect Incd was theoreti-cally calculated to have an activation energy of about 0.04 eVin the In-doped CdTe material.42 We identified trap T2 as ashallow acceptor defect located at 0.09–0.13 eV above thevalence band maximum. This trap has been attributed to thesingly ionized Cd vacancy or vacancy related A-center. The Cdvacancy has a first ionization energy in the range of 0.13–0.21 eVaccording to theoretical calculations.38 The acceptor complexA-center including vacancies and donor impurities, [VCd + In]or [VCd + InCd], has an activation energy in the range of0.12–0.15 eV.43,44 The generation of Cd vacancies followed bythe creation of complexes with donors leads to a slight decreaseof trap density for donor T1 (In or InCd) and a slight increasefor acceptor T2 (Cd vacancy or A-center) in the TSC results.

The electron trap level T3, with an energy located at 0.18–0.23 eVbelow the conduction band minimum (close to the reporteddefect level of dislocations EC − 0.17 eV), is considered to con-sist of dislocation related intrinsic defects.45 This assumptionagrees well with potential defective states existing in the irra-diated CZT crystal. Damaged areas full of stresses caused byatomic displacements and lattice absorptions generated byhigh dose γ-ray radiation result in a high density of disloca-tions.46 Therefore, we could see a great increase in the con-centration of the dislocation related trap T3 after radiation.

Trap T4 is recognized as an acceptor defect level, locatedat ~0.35 eV above the valence band maximum. The micro-scopic origin of this trap has been attributed to the secondlyionized Cd vacancy or [TeCd + 2VCd] complex. The Cd vacancyhas a second ionization energy of EV + 0.36 eV from theoreti-cal calculations.47 An energy level at around EV + 0.40 eV wasfound in CdTe and attributed to a Cd vacancy.48 Further-more, the activation energy of [TeCd + 2VCd] was recently cal-culated as 0.32–0.40 eV using the first-principle method.38

The amplitude of a certain peak in the TSC spectrum is pro-portional to the trap density of the associated defect, and theconcentration of trap T4 was calculated to show an increasefrom 1.7 × 1011 to 3.0 × 1012 cm−3 in the γ-ray irradiated CZTcrystal. We can conclude from TSC measurements that theformation of a high concentration of Cd vacancies andvacancy related complexes is likely to be the major outcomeof high dose γ-ray radiation. This result is consistent with the

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radiation damage mechanism discussed above, in which atotal number of 1010–1011 (corresponding to a defect concen-tration of 1011–1012 cm−3) Cd vacancies and defect complexescould be newly induced after radiation, according to theKinchin–Pease model.

3.3 Electrical and optical properties and detectorperformance

Table 1 presents the charge carrier transport properties inves-tigated through Hall measurements before and after high doseγ-ray radiation. The Hall results clearly indicate the increase ofnet free carrier density and the conversion of conduction typefrom n to p in the irradiated CZT crystal, which is easy tounderstand based on the changes in the defect concentrationrevealed by the TSC results. The measured net carrier densityis mainly determined by the compensation between Teantisites (donor defects) and Cd vacancies (acceptor defects)in the CZT crystal.47,49 The existence of slightly more Teantisites than Cd vacancies results in a low net free electronconcentration of ~1.9 × 106 cm−3 before radiation. This com-pensation effect is responsible for the pinning of the Fermilevel near the mid-gap, which was confirmed by fitting thetemperature dependence of resistivity, as shown in Fig. 3(a).The Fermi level is positioned at 0.85 eV below the conductionband with respect to the band gap of ~1.6 eV for a CZT crystalat room temperature, and hence leads to high resistivity withn type conduction in the material. A considerable increase in

Table 1 Charge carrier transport properties investigated through Hall measureme

Net carrier density (cm−3)

Before radiation 1.9 × 106

After radiation 1.7 × 1012

Fig. 3 Electrical properties before and after high dose60Co γ-ray radiation: (a) the Fe

voltage analyses.

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the defect concentration of the Cd vacancy related deep acceptorT4 would strongly affect the electrical compensation ratio afterradiation, resulting in a high concentration of ~1.7 × 1012 cm−3

of net free holes. The Fermi level shift (0.52 eV above thevalence band) eventually contributes to low resistivity with p typeconduction in the material, which could explain the increaseof the leakage current in the irradiated CZT crystal, as shownin Fig. 3(b).

Typical photoluminescence (PL) spectra of the CZT crys-tals excited by an argon ion laser (488 nm) at 10 K before andafter high dose γ-ray radiation are shown in Fig. 4(a). Theincrease of the emission intensity ratio I (DComplex)/I (D0, X)from 3.26 to 10.38 suggests a higher dislocation density inthe irradiated CZT crystal, since the DComplex luminescencecenter is often attributed to extended dislocations.50 Thisphenomenon agrees well with the TSC results, in which theconsiderable increase in defect concentration of the disloca-tion related trap T3 was induced by 60Co γ-ray radiation. Fur-thermore, the full width at half maximum (FWHM) for theshallow donor bound exciton (D0, X) emission line broadensfrom 6.25 meV before radiation to 12.47 meV after radiation,and the intensity of the free exciton (FE) transition peakweakens significantly or even disappears after radiation.The broadening of the (D0, X) peak and the disappearing ofthe FE peak in the PL spectra clearly indicate the worseningof the crystal quality due to the disordering of the crystal-line lattice caused by a large amount of defects inducedby radiation.46

nts before and after high dose γ-ray radiation

Hall mobility (cm2 V−1 s−1) Conduction type (n or p)

315 n14.5 p

rmi level acquired by fitting the temperature dependence of resistivity; (b) current–

This journal is © The Royal Society of Chemistry 2013

Fig. 4 Optical properties and detector performances before and after high dose γ-ray radiation: (a) typical PL spectra of CZT crystals excited by an argon ion laser (488 nm)

at 10 K; (b) typical energy spectra of CZT planar detectors excited by an241

Am source (59.54 keV) at room temperature.

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Typical energy spectra of the CZT planar detectors excitedby an 241Am source (59.54 keV) at room temperature beforeand after high dose γ-ray radiation are shown in Fig. 4(b),which directly reflect the radiation effects in the CZT mate-rial. Radiation induced electrically active defects acting astrapping centers will reduce carrier lifetime, and scatteringfrom defects will lead to a decrease in carrier mobility. There-fore, the decrease of the (μτ)e product from 1.08 × 10−3 to7.12 × 10−4 cm2 V−1 after radiation was obtained by fitting theHecht equation.26,27 The worsening of the charge transportproperties will consequently deteriorate the energy resolutionand the charge collection efficiency (CCE) of the detectorcharacteristics. The FWHM of the photopeak broadens from38.2 to 67.4 eV, showing the loss of the energy resolutioncaused by charge carriers trapping in defects in the irradiatedCZT crystal. Another effect is the dramatic shift of thephotopeak position towards a lower channel, demonstratingincomplete collection efficiency, since the observed channelnumber is proportional to the incident photon energy. Inaddition, an increase in leakage current after radiation, asshown in Fig. 3(b), acts as the main source of noise in thedetector and becomes another reason for the deterioration ofdetector performance. The CZT crystal undergoes a trap den-sity increase and detector performance degradation at a totaldose of ~2.4 kGy. If the radiation dose goes even higher, thetrap density will increase in several orders of magnitude, thuscausing severe deterioration of the spectroscopic behavior oreven complete degradation of the CZT detector performance.15

4 Conclusions

The radiation damage mechanism for high dose (~2.4 kGy)60Co γ-rays on CdZnTe:In crystals involves a series of energytransfer processes, i.e., Compton scattering between incidentγ-ray photons and electrons, Rutherford scattering between

This journal is © The Royal Society of Chemistry 2013

Compton electrons and lattice atoms, and collision cascade,in which a total number of 1010–1011 Cd vacancies and defectcomplexes are induced according to the Kinchin–Peasemodel. TSC measurements were carried out to characterizethe radiation damage and we identified four main trap levels(T1, T2, T3 and T4) which were attributed to shallow donorimpurities, shallow acceptor A-centers, dislocations and deepacceptor Cd vacancies or vacancy related defect complexes,respectively. The considerable increase in the trap density(from 1.7 × 1011 to 3.0 × 1012 cm−3) of the Cd vacancy relatedtrap T4 contributed to the variation of the electrical compen-sation ratio from ~1.9 × 106 cm−3 net free electrons beforeradiation to ~1.7 × 1012 cm−3 net free holes after radiationand the conversion of the conduction type from n to p. Theincrease of the emission intensity ratio I (DComplex)/I (D0, X)in the PL spectra agrees well with the increase in the trapdensity of the dislocation related trap T3, and the decrease ofthe (μτ)e product in the energy spectra from 1.08 × 10−3 to7.12 × 10−4 cm2 V−1 clearly indicated the worsening of thecharge transport properties caused by radiation induced elec-trically active defects. Finally, the radiation effects weredirectly reflected by a significant deterioration of CZT detec-tor performance, corresponding to a loss of energy resolutionand incomplete charge collection efficiency.

Acknowledgements

This work was supported by the Special Fund of National KeyScientific Instrument and Equipment Development (2011YQ040082),the National 973 Project of China (2011CB610400), the 111 Pro-ject of China (B08040), the National Natural Science Founda-tion of China (NNSFC-50902114), the Doctorate Foundationof Northwestern Polytechnical University (CX201102), andMinistry of Education Fund for Doctoral Students NewcomerAwards of China.

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