Nuclear Instruments and Methods in Physics Research A 729 (2013) 265–272

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Nuclear Instruments and Methods inPhysics Research A

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Synchrotron characterisation of non-uniformities in a small pixelcadmium zinc telluride imaging detector

M.C. Veale a,n, S.J. Bell a,b, D.D. Duarte a,b, A. Schneider a, P. Seller a, M.D. Wilson a,V. Kachkanov c, K.J.S. Sawhney c

a Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, UKb University of Surrey, Surrey GU2 7UU, UKc Diamond Light Source Synchrotron, Oxfordshire OX11 0QX, UK

a r t i c l e i n f o

Article history:Received 17 April 2013Received in revised form11 July 2013Accepted 15 July 2013Available online 20 July 2013

Keywords:CdZnTeSynchrotronX-ray detectorSpectroscopicElectric field

02/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.nima.2013.07.054

esponding author. Tel.: +44 123 544 5030.ail addresses: matthew.veale@stfc.ac.uk,ewVeale@Outlook.com (M.C. Veale).

a b s t r a c t

A small pixel cadmium zinc telluride detector has been fabricated, assembled and tested at theRutherford Appleton Laboratory. The detector consists of 74�74 pixels on a 250 μm pitch with a50 μm spacing. Flat field irradiations with an 241Am γ-ray source have demonstrated that there aresignificant variations in the number of counts detected by each pixel as well as large differences in theFWHM of the X-ray photo-peak. A 10�10 μm microbeam of 20 keV X-rays has been used to characterisethese non-uniformities. These measurements have shown that variations in the counting and spectro-scopic performance of individual pixels are due to the presence of a non-uniform electric field within thedetector.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Over the last decade, a hard X-ray imaging detector system hasbeen developed at the Rutherford Appleton Laboratory [1,2]. Eachsystem consists of either a small pixel cadmium telluride (CdTe) orcadmium zinc telluride (CdZnTe) detector with up to a maximumof 80�80 pixels on a 250 μm pitch, with an inter-pixel spacing of50 μm, flip-chip-bonded to the HEXITEC ASIC [3]. These imagingsystems are currently in use in a number of application areas suchas synchrotron science, homeland security, medical imaging andastrophysics [1–6].

To date the best system performance has been achieved using1 mm thick CdTe detectors fabricated with Schottky contacts[7–9]. These detectors have achieved sub 1 keV energy resolutionin the energy range 5–160 keV but, due to decreasing materialstopping power with photon energy, the collection efficiency isreduced to less than 20% at the highest energies. Many of thetarget application areas require efficient detection of X-rays andγ-rays in the range 100–300 keV. To improve the detector perfor-mance at higher energies, thicker detectors are required. Therelatively low resistivity of CdTe material, and the presence of

ll rights reserved.

defects such as inclusions, currently limits the detector thicknessto 1 mm [10,11].

In CdZnTe, typically, 10% of the cadmium in the crystal isreplaced with zinc. The presence of zinc acts to reduce the averageatomic spacing in the crystal increasing the band-gap and resis-tivity of the material. Resistivities of CdZnTe are in the range of1010–1011 Ω cm compared with that of CdTe which is ∼109 Ω cm[12]. To achieve high spectroscopic performance the detectorleakage (dark) current must be kept to a minimum and in CdTethis requires the use of Schottky contacts on thin crystals whichare prone to polarisation [13,14]. The increased resistivity ofCdZnTe allows thicker detectors with Ohmic contacts to beproduced which have improved performance at higher energies.

Prior to 2006, the majority of available CdZnTe material wasgrown from the melt using variations of the Bridgman techniquessuch as the high pressure Bridgman and modified vertical Bridgmanmethods [15,16]. Although these growth techniques were capable ofproducing high quality detectors the overall yield was low and largevariations in crystal quality were common. The development of thetravelling heater method for crystal growth and related post-growthtreatments has led to large improvements in both crystal yield anduniformity [17]. CdZnTe material is now being routinely producedthat has high resistivities 41011 Ω cm and excellent charge trans-port properties, μeτe41�10�2 cm2 V�1 s�1.

Despite the improvement in CdZnTe crystal quality the produc-tion of small pixel detectors (o500 μm pitch) is still technically

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challenging [18,19]. The role of the metal–semiconductor interfaceis crucial to the performance of the final detector and thefabrication process for these detectors is currently an area ofintense interest [20,21]. Characterising the performance of thesedetectors post-fabrication also represents a challenge.

In this paper the small pixel, spectroscopic, read-out HEXITECASIC [3] and an X-ray microbeam have been combined to study thesource of non-uniformities observed in the response of a pixelatedCdZnTe detector.

2. Experimental arrangement

A small pixel detector was fabricated from a single crystal ofRedlen Technologies CdZnTe. Prior to the deposition of contacts,the faces of the crystal were lapped and mechanically polished to amirror finish using 0.3 and 0.05 μm alumina slurries. After proces-sing the crystal had dimensions of 19.3�19.3�2.0 mm; as thesize of the CdZnTe is too small to accommodate the full array of80�80 pixels on a HEXITEC ASIC, a modified mask was used toproduce a detector with 74�74 pixels.

Fig. 1. (left) A simple schematic of the experimental arrangement. The detector is mou(right) The positions of the microbeam scan sites A–C on the 74�74 pixel detector high

Fig. 2. A typical single pixel 241Am spectrum taken with the CdZnTe detector (a) w

Gold contacts were deposited using a gold(III) chloride solutionin the ratio 1:25 HAuCl4:H2O [20]. The small anode pixels weredeposited on the (1 1 1)A cadmium face of the crystal while thelarge planar cathode was deposited on the (1 1 1)B tellurium face.The electroless deposition process was chosen due to theimproved adhesion and leakage current performance relative toother processes such as sputtering and thermal evaporation.Following fabrication the detector was flip-chip-bonded to theHEXITEC readout ASIC using gold studs and silver epoxy.

The performance of the detector was assessed using an 241Amγ-ray source. The detector was mounted in a custom data acquisi-tion system, held at a constant temperature of 8 oC, a humidity ofo10% and biased at �500 V. The 241Am source was positioned30 cm from the detector to ensure a flat field irradiation; the fluxat the detector was 60 photons s�1 cm�2. Data was collected for45 h to provide adequate statistics for an energy calibration ofeach pixel of the detector using multiple peaks in the 241Amspectrum.

Micro-beam characterisation of the detector was carried out onthe beam line B16 at the Diamond Light Source Synchrotron [22].A simple schematic of the experimental arrangement is shown in

nted on an optics table, translation stages allow the detector to be moved in X–Z.lighted in black. The position of the detector guard band is also indicated in black.

ithout any correction and (b) after charge sharing discrimination correction.

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Fig. 1. The storage ring was operated at 3 GeV with a current of250 mA. The X-ray energy was tuned to 20 keV by a Si(1 1 1)double crystal monochromator; a small amount of flux is alsoproduced at the 3rd and 4th harmonics which have energies of 60and 80 keV, respectively; the 2nd harmonic is forbidden.A 10�10 μm collimated beam of monochromatic photons and aflux of ∼1�109 photons s�1 cm�2 was scanned across areas of thedetector in 25 μm steps. At each step a 20 s exposure was taken.The position of the scan sites is shown in Fig. 1. During themovement of the translation stage the detector bias voltage wasset to 0 V for 2 s before returning the bias to �500 V and allowingthe detector to settle for 5 s; this was consistent with previousmicrobeam measurements [23].

The recorded data was processed using either a charge sharingdiscrimination (CSD) or charge sharing addition (CSA) correctionalgorithm. These algorithms were implemented in MathWorksMATLAB [23]. A charge sharing event is defined here as aninteraction where more than 1 pixel registers an event above thedetector low energy threshold. Each frame of data, consisting of74�74 pixels, was inspected in turn for charge sharing events.In the CSD algorithm shared events are simply removed from thedata set and are not included in the later data analysis. In the CSAalgorithm the total energy deposited during an interaction iscalculated from the magnitude of each of the pixel outputs. Theenergy measured in the individual pixels are summed together torecreate the original interaction energy. The full energy event isassigned to the pixel which had the largest share of the interactionenergy; neighbouring pixels are set to zero. A prior energycalibration, produced using the 241Am measurements, ensuresthe addition is accurate.

3. Results

3.1. 241Am γ-ray source measurement

The entire detector cathode was exposed to X-rays and γ-raysfrom an 241Am radioactive source. The source was placed ∼30 cmfrom the face of the detector to produce a flat field irradiation. Theexposure lasted 45 h over which time the average current drawnby the detector was 15.071.8 nA. The data was processed usingthe CSD algorithm removing any shared events to produce the bestspectroscopic performance. Fig. 2 shows an example of a singlepixel spectrum (a) before and (b) after CSD; this clearly removeslower energy counts. The number of events detected in the photo-

Fig. 3. (a) The distribution of the number of events in the 60 keV photo-peak and (b) theFWHM of calibration peaks produced using a test input to each pixel is also shown in (

peak, defined here as between 45 and 65 keV, was also calculatedfor each pixel. Fig. 3(a) shows the variation in the total number ofphoto-peak counts per pixel. The distribution in the number ofcounts has a width of 66%, much larger than in previous measure-ments with Acrorad CdTe detectors where the width is of the orderof 20% [23].

The distribution of FWHM for all the pixels of the detector isalso shown in Fig. 3(b). The average FWHM of the 60 keV photo-peaks for the entire detector was calculated to be 1.7470.39 keV.The peak counts and FWHM were stable over the course of theexposure. The energy resolution of the detector is consistent withpreviously published results on small pixel CdZnTe detectorsfabricated by Redlen Technologies bonded to the HEXITEC ASIC[24,25] but is poorer than that observed in Acrorad CdTe detectorswhere, typically, FWHM of 0.8 keV are observed [23]. The ASICconsists of a test input circuit that allows a voltage pulse to beapplied to the input of each pixel to measure the performance ofthe detector electronics. While under identical bias and tempera-ture conditions, a test pulse of 50 mV was applied to each pixeland the FWHM of the resulting peak measured. The variation inthe number of test pulse counts per pixel was negligible.A histogram of the test pulse peak FWHM is shown in Fig. 3(b).The average FWHM of the peak was measured to be1.0870.24 keV. Both the average value and deviation in theFWHM are lower than the data taken with the 241Am sourcedemonstrating that the additional variation, and lower resolution,is a property of the CdZnTe detector rather than the detectorelectronics.

The 241Am γ-ray source measurements have shown that, whilethe detector is capable of good energy resolution, the largevariation in the total number of counts and FWHM measured ineach pixel could lead to a degradation of the detectors imagingperformance. To investigate these effects in greater detail, mea-surements were made using a 20 keV monochromatic microbeam.

3.2. Microbeam characterisation

Using the 241Am source measurements three regions of inter-est, each covering a 2�2 pixel area, were identified (see Fig. 1).The variation in the number of counts detected and the energyresolution of individual pixels were used to define these areas; seeTable 1. In Area A the number of counts detected, and the peakFWHM, were representative of the averages of the distributionsshown in Fig. 3(a) and (b). In Areas B and C, large variations in thenumber of events detected and the peak FWHM suggest the

distribution of measured FWHM of the 60 keV photo-peak. The distribution of theb).

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presence of non-uniformities. In all three areas the variationsobserved in the number of counts are larger than previouslyreported in CdTe detectors [23].

Table 1The average values of photo-peak counts and FWHM for 241Am measurements inregions of interest A–C. The standard error in the average values was used to definethe different regions.

Region of interest Counts (N) rN (%) FWHM (keV) rFWHM (%)

A 2730 1.9 1.5 3.0B 2767 9.0 1.6 8.5C 3361 9.9 1.6 9.2

Fig. 4. Examples of the spectra from Area A for (a) a position in the centre of a pixel andtest capacitance is shown in (a). In (b) the individual response of neighbouring pixels i

Fig. 5. Maps of the total numbers of counts detected per scan position in Area A, (a) Rawaddition correction.

Fig. 6. Maps of the (a) Raw data, measured photo-peak energy in keV and (b) CSA data, tcorrection.

Each of the areas selected were 6 mm or greater from thephysical edge of the detector; this was to avoid the contribution ofthe crystal edges which previous authors have shown to effect thedetector response [28]. Areas were mapped using a 20 keVmonochromatic X-ray microbeam with dimensions 10�10 μm.The small beam size and discrete energy allows the variationsobserved in the pixels to be investigated on a sub-pixel scale.

3.2.1. Area AFig. 4(a) and (b) shows examples of the microbeam spectrum

measured at the centre of a pixel and in an inter-pixel region of AreaA; the raw data and the data after charge sharing addition are shown.When the beam is positioned at the centre of a pixel a sharp photo-

(b) a position between the two pixels. A test pulse peak produced using a pulser ands shown as well as the CSA corrected spectrum.

data, without correction for charge sharing and (b) CSA data, after charge sharing

he peak FWHM in keV of Area A. Both maps show data after charge sharing addition

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peak of energy 20 keV is observed with negligible differencesobserved in the raw and CSA corrected spectra; see Fig. 4(a). Whenthe beam is incident on the inter-pixel region the energy deposited isshared between the neighbouring pixels. The energy deposited in eachindividual pixel is dependent on the beam position relative to thepixel. If the beam is positioned at the centre of the inter-pixel regionthen the energy is shared equally between the two. Fig. 4(b) shows anexample of the spectra produced by a beam position between 2 pixels.The deposited energy is shared between the pixels appearing as an8 keV peak in 1 pixel and 12 keV in the other. The larger proportion ofenergy deposited in Pixel 2 is consistent with the beam position beingcloser to this pixel than the neighbour. A small number of counts atthe full energy of 20 keV are still observed between pixels but theseare due to scattering of photons between the slits (used to define thebeam size) and the detector surface by the air and the detectorwindows. These scattered events account for ∼6% of the total countsdetected at all positions.

At each of the scan positions the total number of countsdetected in the scan area was calculated. Maps of the total numberof counts, before and after charge sharing addition, are shown inFig. 5. The raw data, Fig. 5(a), shows an increase in the totalnumber of counts when the beam passes through an inter-pixelregion. This increase is expected as in these regions the energy ofthe X-ray is shared between 2 or more pixels. Some minor non-uniformities in the pixel shapes are also visible. The use of the CSAcorrection algorithm successfully corrects for the charge sharingevents as shown in Fig. 5(b) where a uniform number of counts isobserved at each scan position.

Fig. 7. Maps of the total numbers of counts detected per scan position in Area B, (a) Rawaddition correction.

Fig. 8. Maps of (a) Raw data, the measured photo-pea

At each position of the scan the energy of the main photo-peak,the peak centroid, was also determined. A map of the photo-peakenergy after charge sharing addition correction is shown in Fig. 6(a). The use of the CSA correction recovers the majority of thisenergy although losses of up to 5 keV are observed at the pixeledge; this is due to the non-zero low energy threshold of eachpixel which has a value of 3–5 keV. For events where a portion ofcharge less than, or equal to, 5 keV is shared, the detector is unableto identify the event as shared and it is registered as a single eventwith an energy of between 15 and 19.75 keV. In interactionsinvolving more than 2 pixels the energy lost may be greater.

The photo-peak FWHM was also calculated at each positionand can be seen in Fig. 6(b). Variation in the FWHM is observedfrom pixel to pixel but little variation is observed at a sub-pixellevel. For comparison, a test pulse was applied to each of the pixelsto measure the electronic performance. An example of a test pulsepeak can be seen in Fig. 4(a). The average value of the FWHM forthe calibration peaks for the four pixels was 0.9070.03 keV whilethe average FWHM of the corrected 20 keV photo-peaks was1.4670.16 keV. These measurements demonstrate that the varia-tion in the pixel resolution observed is a property of the CdZnTedetector not the ASIC.

If the charge sharing addition correction applied was 100%efficient the expected FWHM in the inter-pixel regions would beequal to the FWHM of the individual pixels added in quadrature.In reality, a larger FWHM than expected is measured due to thenon-zero low energy threshold of each pixel which limits theefficiency of the correction.

data, without correction for charge sharing and (b) CSA data, after charge sharing

k energy and (b) CSA data, the FWHM in Area B.

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3.2.2. Area BMaps of the total number of counts in Area B, before and after

charge sharing addition, are shown in Fig. 7(a) and (b), respec-tively. Unlike the mapping data shown in Fig. 5(a), the shape of theindividual pixels and their effective area show clear non-uniformities in the raw data. This difference in effective area isconsistent with the variation in the number of counts detected inthe earlier 241Am source measurements (see Table 1) whichsuggests that, despite a difference of eight orders of magnitude,the effect is independent of beam flux over the range studied.Following charge sharing addition correction the number of

Fig. 9. A comparison of the spectroscopic performance at positions X14 Y24 andX24 Y24. The calibration peak shown was taken for the poorly performing pixel.

Fig. 10. Maps of the total numbers of counts detected per scan position in Area C, (a) Raaddition correction.

Fig. 11. Maps of (a) Raw data, the measured photo-pe

counts detected, Fig. 7(b), produces a flat field response asobserved in Fig. 5(b). As the number of counts after correction isuniform across the scan area, despite the observed non-uniformi-ties, no counts have been lost.

Fig. 8(a) and (b) shows maps of the measured photo-peakenergy and FWHM after charge sharing addition correction.As shown in Fig. 8(a), in the majority of positions the full energypeak is observed at 20 keV. As in Area A, small losses are observedin the inter-pixel regions due to the non-zero low energy thresh-olds. In the case of the FWHM map, Fig. 8(b), distinct differences inthe peak FWHM are detected for each pixel. Fig. 9 compares thespectra taken from two central positions within the lower pixels ofArea B. Large differences in the peak FWHM were measured withvalues of 1.0 and 2.5 keV, respectively, poorly performing pixelsshowed events shifted to lower energies. The measured FWHM didnot show any correlation with the width of calibration peaksdemonstrating the differences in peak FWHM are a detectorproperty.

3.2.3. Area CFig. 10(a) and (b) shows the number of counts detected before

and after charge sharing addition correction for Area C.As observed in Area B, large non-uniformities are observed inthe effective size of the pixels within the scan area but this doesnot result in the loss of any events as after correction a flat-fieldresponse is observed in the corrected count map.

The map of peak energy shown in Fig. 11(a) is consistent withthe behaviour observed in both Area A and B with the full peakenergy measured in the majority of positions with only small

w data, without correction for charge sharing and (b) CSA data, after charge sharing

ak energy and (b) CSA data, the FWHM in Area C.

Fig. 12. A comparison of the spectroscopic performance at positions X13 Y21 andX23 Y21. A test pulse peak recorded for the poorly performing pixel is also show.

Fig. 13. The correlation between the relative pixel areas measured from mappingdata and the measured FWHM of the 20 keV photo-peak. Data is included from allthree areas mapped. The dotted line is a linear fit to the data points.

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amounts of charge lost in the inter-pixel regions due to the lowenergy threshold. In Fig. 11(b) large differences are seen in thespectroscopic performance of the individual pixels. Fig. 12 com-pares the spectra measured for the two lower pixels in the imageat scan positions X13 Y21and X23 Y21. The measured FWHM atthese positions was 4.2 and 0.8 keV, respectively; in the latter casethis matches the spectroscopic performance measured previouslyin CdTe detectors [7,24]. In the poorly performing pixel the largeFWHM is due to the shift of ∼50% of the events to a lower energyof 18 keV forming a double peak in the spectrum. The test pulsepeak of the poorly performing pixel, also shown in Fig. 12, showsno such broadening and demonstrates that the formation of adouble peak is a property of the CdZnTe detector.

4. Discussion and conclusions

The mapping measurements have demonstrated the presenceof non-uniformities within the CdZnTe detector. These non-uniformities manifest themselves as variations in the effectivearea of the pixels and the spectroscopic performance of individualpixels. Minor non-uniformities were observed even in the betterperforming regions of the detector relative to previously reportedmeasurements on CdTe detectors [23]. Fig. 13 demonstrates thatthere is a correlation between the effective pixel area and themeasured FWHM. For this purpose, the effective pixel size wasdefined as the region over which single pixel counts dominatedivided by the expected pixel area of 0.16 mm2 due to the physicalsize of the electrode. This suggests that the observed variations incounting efficiency and the spectroscopic performance have thesame source. The presence of the non-uniformities have beenmeasured to be independent of rate and energy in the ranges1�102–1�109 photons cm�2 s�1 and 5–80 keV, respectively.A lack of variation in the spectroscopic performance of thedetector on a sub-pixel scale also rules out secondary phases,such as inclusions, as the source of the non-uniformities[23,26,27].

The geometry of individual pixels in the detector is fixed at200�200 μm which, in an ideal detector, should produce a uni-form electric field. The variations observed in the distribution ofcounts, Figs. 7(a) and 10(a), are evidence for the presence of localvariations of the internal electric field in the CdZnTe detector;similar results have previously been reported on high pressureBridgman grown CdZnTe material but on larger length scales[28,29].

The CdZnTe detector used in the study was fabricated usingchemical deposition to form the electrodes; this was due to theirsuperior adhesion and leakage current performance comparedwith other deposition processes. The electroless process is knownto produce back to back Schottky contacts on the detector [20,21].The presence of potential barriers at the metal–semiconductorinterface can lead to a build-up of space charge within thedetector. The presence of a space charge may distort the fieldwithin the detector due to local variations in the effective fieldstrength [13,30,31]. Spatial variations in the concentration of spacecharge can lead to focusing, and defocusing, of the electric fieldmodifying the volume over which the pixel collects charge, asobserved in these measurements.

Recent work on the characterisation of chemically depositedcontacts has shown that the metal–semiconductor interface has acomplex structure containing crystalline defects, oxides and otherspecies [20,21,32]. Any variation in the properties of the interfaciallayer could produce local variations in space charge, leading tonon-uniformities in the electric field. Other authors have sug-gested that large stresses can develop at the metal–semiconductorinterface during electrode deposition. Induced stresses can lead toa modification of the internal electric field and degradation of thedetector response [33,34] and may be another explanation of thenon-uniformities presented in this paper. Bonding-induced stressin the detector is ruled out based on the excellent performance ofCdTe detectors [23] bonded using the same techniques.

241Am γ-ray source measurements have demonstrated thatsmall pixel CdZnTe detectors have good energy resolution butcan show spatial variation in detector response. X-ray microbeammeasurements at the Diamond Light Source showed that varia-tions in the electric field strength led to pixel-to-pixel variations inthe number of counts detected and energy resolution. Furtherwork characterising the metal–semiconductor interface andimproving the deposition process is required to improve thedetector uniformity.

Acknowledgments

The authors would like to thank Paul Adkin and Paul Booker forthe bonding of CdTe detectors at the Rutherford Appleton Labora-tory. Thanks are also given to the beam line staff of B16 and theDiamond Synchrotron Light Source for help arranging and runningthe beam time. This work has been supported by the EPSRCHEXITEC Translation Grant EP/H046577/1 (2011–2014).

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