Csaba Szeles, Stephen A. Soldner, Member, IEEE, Steve Vydrin, Jesse Graves, and Derek S. Bale
Abstract—Next generation high-flux x-ray imaging technology isexpected to advance towards multi-color or spectroscopic imagingand will significantly expand the capabilities of the techniquein a multitude of applications. Spectroscopic x-ray imaging willrequire energy-sensitive detector arrays. In this work we evalu-ated the applicability of pulse-mode CdZnTe detector arrays tohigh-flux spectroscopic imaging. To study the material and deviceperformance limitations of currently available CdZnTe detectorsunder high-flux x-ray irradiation we designed a 2D monolithicCdZnTe test array and associated test system. The detector arrayswere 16 16 pixel devices with 0.4 mm 0.4 mm area pixels on a0.5 mm pitch and were fabricated using 8.7 mm 8.7 mm 3.0mm CdZnTe single crystals. We measured the high-flux perfor-mance of over 1200 such arrays with various bulk CdZnTe crystalproperties using a 120 kVp x-ray source and our custom built testsystem.
We studied the various static and dynamic charge collectioneffects typically not observed in low-flux applications. These in-cluded dynamic polarization, static charge steering and dynamiclateral polarization and charge steering. In parallel with theexperimental effort we developed a dynamic charge transport andtrapping model to describe the experimentally observed static,dynamic and transient phenomena.
For the first time we demonstrated 15 106
counts s mm2 count-rate for several hundred such CdZnTedetector arrays. In addition we demonstrated good 1% shortterm count-rate stability of the detector arrays.
Index Terms—Cadmium compounds, semiconductor radiationdetectors, x-ray detectors, x-ray image sensors.
I. INTRODUCTION
IMPROVEMENTS in the crystal growth and device fab-rication technology of CdZnTe x-ray and gamma ray
detectors and detector arrays enabled the proliferation ofthis room-temperature semiconductor detector technologyinto many applications including industrial monitoring andgauging, medical and industrial imaging, nuclear safeguardsand non-proliferation, transportation security and safety, and arange of scientific applications [1], [2]. One large applicationarea where CdZnTe detector technology has not yet made manyinroads is high-flux x-ray imaging. These applications such asstate of the art Helical Multi-Detector Computed Tomography(MDCT) require very fast data acquisition and hence usefast-response current-mode detectors to operate in the tens andhundreds of millions of flux range. There is
Manuscript received January 15, 2007; revised November 20, 2007. Thiswork was supported in part by the U.S. Army Armament Research, Develop-ment, and Engineering Center (ARDEC) under contract no. DAAE 30-03-C-1171. Paper no. TNS-00034-2007.
The authors are with eV PRODUCTS, a division of II-VI Incorporated, Sax-onburg, PA 16056 USA (e-mail: [email protected]).
Digital Object Identifier 10.1109/TNS.2007.914034
however a rapidly growing interest in energy-selective pulsemode detectors to enable high-flux high-speed multi-color orspectroscopic x-ray imaging including MDCT. Semiconductordetectors offer excellent energy resolution, and the devicefabrication technology is mature enough to fabricate segmentedelectrode devices for high spatial resolution imaging arrays.Detector arrays for these applications require
a) room-temperature or near-room temperature operation toavoid bulky, power hungry and maintenance heavy refrig-eration systems,
b) short transit time of the x-ray induced charge carriersthrough the detector crystal to minimize pile-up and max-imize count-rate,
c) negligible charge trapping to minimize space-charge for-mation and polarization of the device, and
d) sufficiently fast and accurate readout electronics to enablethe processing of the large number of signals without sig-nificant count-rate degradation.
In this paper we discuss our efforts to evaluate the applica-bility of pulse-mode CdZnTe detector arrays to high-flux spec-troscopic imaging and to establish the material and device per-formance limitations of currently available CdZnTe crystals anddetectors for such applications.
In our experimental approach we developed a test platformto characterize the pulse-mode response of CdZnTe monolithicdetector arrays to high-flux x-ray irradiation. In addition we de-signed and built a custom x-ray characterization system to studythe flux dependence, linearity, response uniformity and stabilityof the detector arrays. We used the test system to survey thedetector response in a wide range of material and device param-eters and operation conditions.
We complemented our experimental work with the develop-ment of a dynamic charge transport and trapping model to de-scribe the experimentally observed static, dynamic and transientphenomena.
In this paper we discuss the performance of two-dimensional(2D) 16 16 pixel monolithic CdZnTe detector arrays operatedin pulse mode. We demonstrate for the first time larger than15 count-rate performance of the arraysin the x-ray energy range. In addition we demon-strate good short term (1 hour) count-rate stability of thedetector arrays. We describe the device fabrication technologythat enables the industrial-scale manufacturing of these devices.We also describe the device configuration and a custom probearray that allows testing of the devices before bonding them tothe readout chip. In addition we discuss our test system em-ploying 8-channel fast bipolar front-end application specific in-tegrated circuits (FE-ASIC), a count-rate correction techniqueand the data acquisition and x-ray source control modules.
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Fig. 1. Typical monolithic CdZnTe detector array. This example shows a16� 16 pixel 2D array using a 34 mm� 34 mm� 5 mm CdZnTe crystal.
We also discuss experimental results on static and dynamiccharge transport phenomena such as detector polarization,static charge steering and dynamic lateral polarization andcharge steering.
II. DETECTOR ARRAY DESIGN AND FABRICATION CHALLENGES
Design and fabrication of CdZnTe monolithic detector arrayspose significant and unique challenges for the manufacturersof these devices. Unlike most semiconductor devices wheretypical device length-scales are in the micron range, x-rayand gamma-ray detectors require comparatively large devicevolumes to achieve satisfactory detector efficiency. The thick-ness of CdZnTe monolithic detector arrays is typically in the2–15 mm range with need for even larger thicknesses. Detectorcrystal surface areas are currently in the 10 mm 10 mm to40 mm 40 mm range with continuously increasing demandfor larger surface areas. A typical CdZnTe monolithic detectorarray is shown in Fig. 1. The device consists of a full-areaelectrode on one surface (typically the entrance window of thephotons) and a segmented electrode at the opposite surface. Thelarge surface area and the large individual detector elementsor pixels that are typically in the 100 100 to 2.5mm 2.5 mm range require very uniform surface processingand electrode deposition technology to ensure uniform deviceresponse. The large thickness of these devices requires thedevelopment of macroscopic scale advanced side surface pro-cessing and passivation techniques. The fabricated detectors arebonded directly to a Read-Out Integrated Circuit (ROIC) chipor to an interface board as illustrated in Fig. 2. Because CdZnTedetector arrays are sensitive to high temperature exposure typ-ically a low-temperature interconnect technology is employedthat maintains the detector temperature during hybridization inthe 60–150 range.
Another set of unique design and fabrication criteria are dic-tated by the charge transport properties of CdZnTe detectors. Inorder to ensure that the entire volume of the detectors are active,depletion of the crystals is required to mm and cm depths to en-sure that the radiation induced charge is collected from the en-tire volume of the crystal. Depletion to such macroscopic depthsrequires electrically compensated semi-insulating crystals withbulk electrical resistivity in the - range and theapplication of blocking electrodes such as Schottky barrier andp-n junction type contacts. In such high resistivity electricallycompensated CdZnTe crystals the Fermi level is in the middle
Fig. 2. Typical structure of CdZnTe detectors arrays.
of the band gap [3]. Contact induced band bending in such de-vices can be perilous to the operation of the device by activatingdeep-level traps and inducing catastrophic charge trapping andspace-charge formation in the proximity of the contact. Such ef-fects are particularly important in high-flux x-ray applicationswhere the rate of charge injection can approach the charge re-moval rate from the device. The contact barrier structure inCdZnTe detectors has to be carefully designed and tailored tothe electrical compensation condition of the bulk crystal to min-imize contact-induced trapping and defect charging effects. Inorder to achieve the deigned barrier properties of the electricalcontacts state-of-the-art semiconductor device processing tech-niques need to be implemented in the fabrication line. TodayCdZnTe device fabrication technology employs a combinationof industry standard wet and dry device processing techniquessuch as low-damage wire saw slicing and dicing, wet chem-ical etching, chemo-mechanical polishing, advanced ex-situ andin-situ surface cleaning, photolithography, physical and chem-ical vapor transport for electrode deposition and surface passi-vation [3].
In addition to the critical electrical properties of the con-tacts, the electrode films also have to have excellent adhesionproperties and appropriate mechanical properties to enable thehybridization of the detector arrays to the read-out integratedcircuits and packaging of the integrated assemblies. Manufac-turers of CdZnTe detector arrays face very similar challengesto those found during integration of other semiconductor de-vices. The chosen interconnect and packaging technology hasto ensure long-term stability and low failure rate of the devicesduring thermal cycling and other environmental conditions en-countered during usage of the device in the field.
High-flux high-speed x-ray imaging applications pose the fol-lowing requirements for monolithic CdZnTe detector array de-sign and fabrication:
A. Detector Response
• High response speed to minimize pile-up and dead time.• High count-rate limitation to enable high-flux operation.
Typical applications require count-rates in the 5 –100range.
• Good response uniformity including count-rate uniformity,gain uniformity and energy resolution uniformity.
• Good short- and long-term stability.
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Fig. 3. Electrode configuration of the 256-pixel 2D CdZnTe detector arraysused in this study.
B. Device Fabrication
• High quality crystals with uniform charge transport prop-erties that allow operation under high-flux charge injectionconditions without device polarization effects.
• Low noise devices. Most of the detector noise originatesfrom surface, interface and subsurface defects. Minimizingthe concentration of such electrically active defects is a keyrequirement for device fabrication.
• High breakdown voltage. High response speed of detectorarrays requires short transit time of the photon induced car-riers which in turn requires high applied bias. Increasingthe breakdown voltage of CdZnTe devices is a primary goalfor advanced device fabrication technology.
C. Device Testing
• Considering the finite yield of both detector and read-outchip fabrication it is important to develop and implementCdZnTe detector array testing prior to hybridization.CdZnTe detector crystals and Read-Out Integrated Circuit(ROIC) chips failing to meet specifications are screenedout in this process.
D. Hybridization
• CdZnTe detector arrays are sensitive to high temperatureexposure. Typically low-temperature interconnect tech-nology that maintains the detector temperature duringbonding in the 60–150 range is employed to hybridizethe detector array to the ROIC chip and/or interface board.
E. Read Out Integrated Circuit
• High-speed signal processing to enable high count-rate,high dynamic range good linearity operation without se-rious pile-up and electronics dead time.
• Multi-channel parallel readout for efficient processing sig-nals from multiple pixels in parallel.
• To obtain the energy information from the signal ampli-tudes multiple energy bin counting is required.
III. DETECTOR ARRAY CONFIGURATION AND FABRICATION
The 2D 256-pixel detector arrays used in this study were fab-ricated from 8.7 mm 8.7 mm 3.0 mm CdZnTe single crys-tals grown by the high-pressure electro-dynamic gradient freezetechnique [4]. The 0.4 mm 0.4 mm pixels form a 2D 16 16array on a 0.5 mm pitch. The pixel array is surrounded by a0.1-mm wide guard electrode to eliminate possible side-surfaceleakage current, breakdown and electric-field distortion effects(Fig. 3).
More than 1200 individual CdZnTe crystals were fabricatedinto pixilated detector arrays. After the material was cut to 3 mmthick slices using a multi-wire saw single crystal pieces wereselected for dicing some of which contained twin defects. Ei-ther fixed abrasive blade saw or multi-wire saw was used fordicing out the 8.7 mm 8.7 mm 3.0 mm CdZnTe single crys-tals for detector fabrication. The crystals were first chemicallyetched in a Br-methanol solution to remove damage from slicingand dicing. The 8.7 mm 8.7 mm area surfaces of the crystalswere polished using a proprietary chemo-mechanical polishingprocess. In the next step photo-resist mask was deposited on thepolished surface to form the pixel electrodes and guard electrodeusing, a standard photolithography technique. The Pt electrodeswere deposited using DC sputtering. In the next step the exposedCdZnTe surface between the pixel electrodes and on the sidesof the device between the opposing electrode surfaces was pas-sivated by depositing a high-resistivity film using a proprietaryprocess. Finally, the side surface of the device was encapsulatedwith an epoxy-based coating.
Probe testing CdZnTe detector arrays with small-area seg-mented electrodes is a challenging task. It requires careful de-sign of the probe tips, minimizing the applied force and mechan-ically hardened electrode structures. When small size spring-loaded probe tips are used very high local force is applied to thethin Pt and Au electrodes (typically 1000–5000 Å thick) at theprobe tip contact area. The probe can easily scratch or punchtrough the thin metal layer and can cause catastrophic damageto the Pt-CdZnTe interface or to the crystal itself as illustrated inFig. 4. This damage region becomes a significant noise sourcecausing performance degradation of the device.
In order to enable the reliable and reproducible testing ofthe CdZnTe detector arrays without the risk of damaging thedetector array before permanently bonding the device to thereadout chip, an interface substrate was designed as shownin Fig. 5. This substrate has matching pixel size and pitch tothe CdZnTe detector array on one surface. In this arrangementthe probe pins are contacting the thicker more robust substratepads rather than the delicate CdZnTe array pixels. The pads onthe opposing surfaces of the substrate are connected with filledvias. The matching array of smaller area pads is used for probetesting. The pad size and distribution on this side of the sub-strate matches the ROIC ASIC chip pad size and distribution.In this case we matched the CdZnTe detector array pad pitchand distribution to the NOVA R&D Inc. Hilda chip (16 16array on 0.5 mm pitch) [5]. The substrates were bonded to theCdZnTe monolithic detector array using proprietary Ag-epoxyflip-chip or Z-bond™ technology [6].
The design of the ceramic substrate was validated and thesubstrates were carefully inspected using white light interfer-ometric microscopy as illustrated in Fig. 6.
IV. PROBE TEST SYSTEM
Fig. 7 shows the top view of the pin probe array and socketdesigned to probe test the CdZnTe detector array and substrateassembly. The extra space at the left side is to accommodate thesection of the substrate where the pads for the guard electrodeare located. The pins are spring loaded gold studs with a round
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Fig. 4. Typical pin probe damage to the metal electrode and CdZnTe detector. The diameter of the probe damage is around 50 �m.
Fig. 5. CdZnTe detector array bonded to an interface ceramic substrate thatenables reliable probe testing of the device and facilitates bonding of the deviceto a ROIC chip at a later stage of manufacturing.
tip. For the current device the pins are arranged in 16 16 arrayon a 0.5-mm pitch. However, they can be customized for anyother geometrical arrangement as required by the CdZnTe de-tector array and interface substrate configuration. The pin probearray is mounted on an interface circuit board with connectorsto connect to the front-end readout electronics boards (FE-ASICboards).
Fig. 8 shows the schematic arrangement of the probe system.An alignment socket is used to accurately position the CdZnTedetector array over the pin array. A conductive rubber or foam isused to gently push the device against the pin array and ensurereliable electrical connection for all 256 channels. The same isused to connect the device to the high voltage (HV). The socketand devices are covered with a Tungsten electrical and radiationshield with a window for the x-ray radiation. The pin array andalignment socket is integrated into a printed circuit board andthe channels are fanned out to a series of connectors at the otherside of the circuit board. The 32-channel front end electronicsboards are connected to the interface board in a perpendiculargeometry.
V. MEASUREMENT AND DATA ACQUISITION SYSTEM
Fig. 9 shows the circuit diagram of the measurement anddata acquisition system. We used the eV-230B 8-channelbi-polar ASIC that contains a charge sensitive preamplifier andshaping amplifier (FE-ASIC). The peaking time of the five-poleshaping amplifier is 190 ns and the pulse width is 800 ns. FourFE-ASICs are integrated on a 32-channel readout board andthe selected signal is fed to a buffer amplifier through a 32:1multiplexer. Eight such 32-channel boards are used to readout the 256 detector pixels. After another 2:1 multiplexing theanalog signal arrives to a comparator and a buffer operationalamplifier. The analog signals from the operational amplifiersare fed to a 4-channel multi-channel analyzer (MCA) (Multi-port II by Canberra) for amplitude analysis.
The comparators in each group generate trigger pulses for theanalog signals above a threshold. The triggers from each pixelare counted by a field programmable gate array chip (FPGA)from all four channels simultaneously over a user defined col-lection period (frame, selectable from 1 ms to 1 min). Once all256 frames have been collected the FPGA sends them to the hostPC over a SPI/USB interface for data processing, storage and vi-sualization. The second output of the comparator is wired to aDAC that sets the global threshold voltage for all comparatorgroups above the noise level at 15 keV.
The measurement settings such as detector bias voltage,signal threshold and integration time are loaded from the dataacquisition PC trough the FPGA and via the USB port. Thehost PC controls the x-ray source as well via an RS232 serialinterface. The x-ray flux (tube current and voltage) and dataacquisition time and measurement sequence are programmedby the user using a graphical interface developed in LabView.
We used a 120 kVp, 400 x-ray generator mounted in alead-shielded enclosure to perform the throughput, count-rateuniformity and stability studies of the fabricated 256-channelmonolithic detector arrays. We designed a single-hole col-limator system to ensure that the entire array is exposed
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Fig. 6. White-light interferometric microscope image of the ceramic substrate. The left pane shows the entire substrate. The right pane shows the close-up of oneAu pad.
Fig. 7. 16� 16 pin probe array and socket employed to test the CdZnTe de-tector arrays through the interface ceramic substrate (left pane). The two pinsat the left hand corners are maintaining the connection to the guard electrode.Detector arrays mounted to the ceramic substrate (right pane).
uniformly to x-rays and no part of the alignment socket isirradiated.
A typical measurement sequence consists of an x-ray fluxsweep by ramping up the tube current from 0 to 400 ata selected fixed tube voltage in the 60–120 keV range. Whenmaximum flux (tube current) was achieved counts were col-lected at fixed current for preset period of time for stability test.The data acquisition system also allows the collection of gammaray spectra if the measurement is performed with a radioactivesource. The system can collect spectra from four pixels at thetime and the entire detector array is tested in a programmed au-tomatic sequence.
VI. RESULTS
The typical pixel counting response of the CdZnTe detectorarrays to increasing x-ray flux is shown in Fig. 10 for 3 differentbias voltages. At high bias (900 V) the response is linear in theentire dynamic range. At moderate bias (600 V) some nonlin-earity occurs at higher x-ray flux. At low bias (300 V) the de-vice demonstrates a space-charge formation caused polarizationat relatively low x-ray flux. In an extensive experimental study
Fig. 8. Probe test system schematics.
(over 1200 individual CdZnTe detector arrays were fabricatedand tested) we evaluated the 1) count-rate limitation (dynamicrange), 2) linearity, 3) response uniformity, and 4) stability of thedevices. For this aim we designed the testing protocol illustratedin Fig. 11. To test the dynamic range (count- rate limit) and lin-earity of the device the pixel counts were measured against agradually increased x-ray flux (#1 and #2). The pixel-to-pixelcounting uniformity is evaluated at the maximum flux (#3). Thisis visualized as a count-rate histogram where the width of thehistogram is a quantitative measure of response uniformity ofthe device. The count-rate stability of the devices was measuredby collecting counts for a preset period of time at the highest flux
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Fig. 9. Measurement and data acquisition system.
Fig. 10. Typical single-pixel counting behavior of the fabricated 256-channelCdZnTe detector arrays as a function of x-ray flux (tube current) for three dif-ferent bias voltages.
(#4). In the current study only short-term stability was evaluatedand the preset time was 1 hour.
Fig. 12 shows the typical spectral performance of an indi-vidual pixel in a detector array to radiation. The full-widthhalf maximum (FWHM) energy resolution is 4.7% at 122 keV.The peaking time was 190 ns and the threshold was set at 15keV for this measurement. The single pixel response of the de-vice clearly shows the typical small pixel effect, although thegeometry was not optimized for achieving the highest possibleenergy resolution. Because the pixel linear dimensions (0.4 mm)are too small for the chosen device thickness (3.0 mm); as ex-pected a high-energy shoulder and broadening deteriorates theenergy resolution of the 122 keV photopeak.
The throughput, response uniformity and stability of the16 16 pixel CdZnTe monolithic detector arrays were tested
Fig. 11. Schematic illustration of the testing protocol of the 16� 16 pixelCdZnTe monolithic detector arrays.
Fig. 12. Typical single-pixel pulse-height spectrum of the 16� 16 pixelCdZnTe monolithic detector arrays measured with a Co radioactive sourceat a low 37 counts/s rate.
578 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 1, FEBRUARY 2008
Fig. 13. Test result for a typical 16� 16 pixel CdZnTe monolithic detector array. The left pane shows the count-rate as a function of flux (tube current). Thisis followed by stability and uniformity tests at constant maximum tube current. The right pane shows the counts distribution at maximum current indicating theresponse uniformity of the device.
according the above protocol. Fig. 13 shows a typical test re-sult. The left pane shows the count data collected from the 256pixels. In the first segment the x-ray flux was increased linearlyto the maximum achievable at the highest current of our x-raytube (400 ). In the next segment the maximum photon fluxwas maintained for a period of time (1 min in this example).In the first segment we see the typical nonlinear increase of thecount-rate. In the second segment, where the photon flux wasconstant the count-rates of the individual pixels did not changeappreciably indicating good short-term device stability. In thisexample all 256 pixels show closely the same flux-dependence.The maximum count-rate of individual pixels varies between600,000 counts/s/pixel and 1.1 . The rightpane of Fig. 13 shows the count-rate distribution histogramof the device. This distribution is close to Gaussian and ischaracterized by a standard deviation. The averagecount-rate is about 8.5 . Consideringthe 0.5 0.5 effective cross sections of the individualdetector elements (pixels) this count-rate corresponds to about3.4 . The count-rate is limited by theshaping time of the amplifier and the electronics dead-time andis significantly less than the throughput of the CdZnTe detectordevice.
The first segment of the test where the counting rate of theindividual pixels was measured as a function of the x-ray flux(tube current) provides a nonlinear response curve (regions #1and #2 in Fig. 11). At higher flux, where the photon arrival fre-quency approaches the shaping time of the bi-polar shaping am-plifier, the pulse pile-up and electronics dead time depresses themeasured counting rate from the true event rate. Although, ourcounting system has a variable dead time (i.e., the width of thecomparator output pulse depends on the signal amplitude of thefront-end amplifier and hence on pulse pile-up), the entire signalchain behaves as a fixed dead time non-paralyzable system. Therelevant effective dead-time of the system is somewhat shorterthan the width of the bi-polar pulse . We found thatthe classical model for non-paralyzable counting rates does notdescribe satisfactorily the observed dependence of the countingrate on the photon flux [7]. The dependence in our countingsystem can be better described by the bi-parameter nonlinearfunction
Fig. 14. Count rate dependence on x-ray flux (tube current) of a single pixel.The experimental data (squares) were fitted with the classical non-paralyzable(dashed line, R = 0:99497) and paralyzable (dotted line, R = 0:99599)and the modified non-paralyzable model (full line, R = 0:99974).
where is the effective electronics dead time proportional tothe pulse width of the bipolar signal, is the x-ray tube currentand is a proportionality constant. Note that is the eventrate in the detector and that this formula approaches the asymp-totic value of the classical non-paralyzable model at high photonflux . Fig. 14 shows the counting rate dependenceof one pixel on the x-ray flux (tube current). We fitted the clas-sical paralyzable and non-paralyzable models [7] as well as ourmodel described by the above equation to the experimental data.Fig. 14 clearly shows that our non-paralyzable model (#2) de-scribes the dependence the most accurately.
The raw count-rate non-uniformity of the device shown inFig. 13 is worse than what is typically acceptable in x-rayimaging applications. Nevertheless, because all pixels of thedevice show the same count-rate dependence on the photon-fluxit is relatively straightforward to develop an effective calibra-tion and non-uniformity correction in the downstream dataacquisition system or image reconstruction software. In ad-dition, the simple functional form of the flux dependenceallows the measurement of the true throughput of the CdZnTedetector array independent of the electronics dead-time causedcount-rate deficit at higher photon flux.
Fig. 15 illustrates the uniformity and non-linearity (flux de-pendence) correction applied to another device. The individualpixel count-rate vs. photon-flux (tube current) curve was fittedby the bi-parametric nonlinear response curve discussed above.The curves were then normalized to the average as shown in the
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Fig. 15. Non-linearity (dead-time) and uniformity correction of the count-rate flux dependence for a typical 256-pixel CdZnTe monolithic detector array.
bottom left pane of Fig. 15. Note that this correction is flux inde-pendent and provides calibration in the entire photon-flux rangeover 6 orders of magnitude. The top right pane of Fig. 15 showsthe count-rate distribution after application of the flux-indepen-dent normalization. The width of this distribution is now only
. Such response uniformity is well within the realm ofhigh-flux x-ray imaging applications.
Finally the bottom right pane in Fig. 15 shows the flux depen-dence of the count-rates of all 256 pixels after the nonlinearityand uniformity corrections. This correction allows the measure-ment of the true count-rate capability of the CdZnTe detectorarray independent of the dead-time of the readout electronics.The average of the maximum count-rates (at the highest photonflux) is 4.2 corresponding to a true av-erage pixel count-rate of 16.8 for this de-tector.
We have successfully fabricated several hundred arrays withsuch performance as part of our development program. Thelarger than 15 count-rate for 120 kVpx-ray irradiation demonstrates an unprecedented performanceof CdZnTe monolithic detector arrays that exceeds the perfor-mance we demonstrated earlier for CdZnTe devices by a factorof 3 [8]. While the uncorrected response non-uniformity is
, after correction we regularly achieve better thancount-rate uniformity. The short-term stability (1 hour) of thedevices was better than .
VII. DISCUSSION
A. Polarization
Depending on the type and concentration of electrically ac-tive lattice defects, some devices show a rapid decline of the
Fig. 16. Count-rate performance of a CdZnTe monolithic array showing pre-mature polarization.
count-rate at a critical photon flux. This device polarization phe-nomenon is caused by the formation of space charge in thevolume of the CdZnTe crystal due to the trapping of the chargecarriers generated by the x-ray photons. The polarization occursat a photon flux and bias voltage where the rate of electron-holegeneration by the x-ray photons exceeds the removal rate of thischarge by drift and recombination.
To achieve the exceptional count-rate performance of these devices, several material selection anddevice design criteria have to be considered. In order to avoidspace-charge build up in the CdZnTe crystal that would lead tothe collapse of the electric field and catastrophic device failure(polarization) it must be ensured that the charge generated bythe x-ray radiation is removed from the device at a sufficientlyhigh rate through drift and recombination.
Fig. 16 shows the performance of a CdZnTe detector arrayfor which device polarization takes place once the x-ray fluxsurpasses a critical value. Above the count-rate de-creases as the x-ray flux increases. This is a typical polarization
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phenomenon: the space-charge built-up in the device collapsesthe electric field and drastically reduces the signal amplitudefrom the detector. In order to gain understanding of the polar-ization phenomenon we studied the critical flux for polarizationonset ( is the maximum of the counts-flux curve) as a func-tion of x-ray energy ( ), carrier mobility-lifetime products( , ), bias voltage ( ), and temperature ( ) both exper-imentally and theoretically. The critical flux can be described atlowest order by the
equation where is the device thickness, is the detector area,the elementary charge, is the dielectric constant, is the
bias voltage and is the residence time of holes in holetraps [9]. This equation was developed by recognizing that thespace charge build-up by the slow carriers i.e., holes in CdZnTeis responsible for the collapse of the electric field and the onsetof polarization. Our experiments confirmed that the transportproperties of the holes are indeed the most critical parametercontrolling the critical x-ray flux of the CdZnTe devices. Theabove equation is the outcome of the theory where we first cal-culate the critical amount of charge from holes that required tocollapse the internal electric field underneath the cathode in adepth characterized by the absorption of x-rays of certain en-ergy. Next we calculate the rate of charge injection to the ab-sorption depth for a given x-ray energy and for a given photonflux. This injected charge is removed by drift to the electrodes.As holes are the slow moving particles the hole transit time con-trols the removal rate of the injected charge from the device. Ifthe injected charge within one hole transit time exceeds the crit-ical amount of charge required to collapse the electric field po-larization occurs. The solution of this charge balance equationyields the above equation for the critical flux needed for polar-ization. The complete theory of polarization in CdZnTe detec-tors is given in [10]. The central role of hole transport is cap-tured in the above equation by the expression in the parenthesesdescribing the stop-and-go nature of hole transport and the re-sulting reduction of the effective hole drift velocity. Because theresidence time of holes in traps ( ) is typically much longerthan their lifetime ( ) the holes spend much less time in thefree state than in the trapped state causing strong reduction oftheir effective drift speed.
Fig. 17 shows the experimental validation of the above equa-tion. The critical flux was measured as a function of the appliedbias. was determined from the maximum of the counts vs.flux curve averaged over all 256 pixels in the device. A fit ofthe experimental vs. the bias voltage results in a quadraticdependence as predicted by the equation above [9].
According to this equation the critical flux for polarizationonset is a function of the ratio of the hole lifetime ( ) andthe residence time of holes in the traps ( ). As the holelifetime decreases (increasing hole trap concentration) andthe residence time of holes increases (deeper hole traps) thecritical flux decreases. In order to evaluate this prediction
Fig. 17. Critical flux of polarization onset as a function of bias voltage.
experimentally we performed a survey of device response asa function of electron mobility-lifetime product ( ) in the(1.0 – 9.0) range and hole in the (1.5 –7.0) range. The experimental results confirmedthat hole lifetime is the dominant material parameter control-ling the onset of polarization in CdZnTe detectors. We alsohave to point out that the critical flux has a strong temperaturedependence due to the strong temperature dependence of thehole residence time in the traps ( ) [3].
B. Static Charge Steering
In addition to the polarization phenomenon we studied thecount-rate response uniformity of the 256-channel CdZnTe de-tector arrays. We can categorize the spatial count-rate non-uni-formity to long-range and short-range non-uniformity. Long-range spatial response non-uniformity is typically associatedwith structural defects that cause charge trapping and recom-bination and lead to charge transport non-uniformity in CdZnTedevices. The transport non-uniformity is caused either by non-uniform trapping and recombination or by non-uniform elec-tric field distribution due to the presence of structural defectssuch as grain boundaries, twin planes, and sub-grain boundariesdecorated with Te inclusions. Fig. 18 illustrates the count-ratenon-uniformity caused by a series of twin planes that intersectthe volume of the CdZnTe crystal. The twin planes run fromthe cathode to the anode surface. These planes are decoratedwith Te inclusions of diameter in the 15 – 35 range. Theeffect of the decorated twin planes is to increase count-rates inthe volume of the crystal containing the planes and reducing thecount-rate in the immediate vicinity of the twin planes. It is tobe noted that the count-rate non-uniformity shown in Fig. 18
SZELES et al.: CDZNTE SEMICONDUCTOR DETECTORS FOR SPECTROSCOPIC X-RAY IMAGING 581
Fig. 18. 2D contour map of the count-rate uniformity of 16� 16 CdZnTe de-tector array. The high count-rate region at the x = 3�4mm position coincideswith a location of a twin defect in the crystal.
Fig. 19. 3D count-rate distribution map of a typical 256-channel CdZnTemonolithic detector array.
often can be corrected and does not necessarily lead to unusablemonolithic arrays.
Short-range response non-uniformity occurs on length-scalescommensurate with the size of pixels in twin-free single crystalCdZnTe detectors as show in Figs. 13, 14 and 18 and canamount to as much as pixel-to-pixel count non-unifor-mity. Short-range non-uniformity is characterized by a broadcount-rate distribution and a large variation of this distributionfrom one detector to another. In contrast we found that the av-erage count-rate of all pixels in a single detector varies less than3% from one device to another. This important characteristicof the short-range response non-uniformity phenomenon showsthat the charge clouds induced by the x-ray photons are notlost through trapping and recombination but rather collectedby neighboring pixels. As a result typically low counting-effi-ciency pixels have high counting-efficiency neighbors as seenin the 3D plot in Fig. 19. Further we found that the phenomenonis nearly completely independent of photon flux and occurseven at very low x-ray fluxes. These findings suggest thatthis phenomenon is a result of static charge steering in thedevice. (We use the word static to distinguish it from dynamic
Fig. 20. Dynamic lateral steering and polarization phenomenon in CdZnTemonolithic detector arrays. The circles indicate the irradiated area.
flux-dependent phenomena.) It can be understood in terms ofthe charge up of small structural defects. The charged defectscause lateral drift (steering) of the passing charge clouds awayform one pixel to the neighboring pixels. As a result somepixels gain counting efficiency at the expense of neighboringpixels.
C. Dynamic Lateral Steering and Polarization
Fig. 20 shows a very interesting flux-dependent dynamicphenomenon that occurs in CdZnTe crystals having low holemobility-lifetime product. Here the 8.7 mm 8.7 mm area256-channel monolithic CdZnTe detector array was irradiatedthrough a 5-mm diameter hole. Fig. 20 shows the grey-scalecounting-rate map of the irradiated pixels. At low photon fluxall the illuminated pixels are responding to the irradiation asshow in the left pane of Fig. 20. As the photon flux increased thecounting rate of the inner pixels increases while the countingrate closer to the edge of the illuminated area dramaticallyreduces as shown in the right pane of Fig. 20. As the photonflux is increased the number of counting pixels is graduallyshrunk to a handful of pixels in the center of the irradiated areawhile the counting rate of these pixels increases. Once a criticalflux is surpassed the counting rate of the center pixels alsodecreases until the entire irradiated area undergoes polarization.This phenomenon is reversible. If the x-ray irradiation flux isreduced the outer pixels recover and start to count again. Atlow fluxes the uniform counting efficiency is achieved again.
The phenomenon can be understood in terms of a dynamiclateral steering and polarization phenomenon [11]. Underhigh-flux x-ray irradiation conditions as the photon flux isincreased a localized positive charge builds up under the irradi-ated area due to strong hole trapping in the crystal. This positivespace charge creates a lateral electric field that is perpendicularto the transport direction of the charge clouds generated by thex-ray photons. Because the space charge is positive it steersthe electron clouds from the periphery of the irradiated area to-wards the center. As a result, the counting rate of the peripheralpixels decreases and the center pixels increases. As the x-rayflux increased further the device undergoes polarization and thecounting rate of the innermost pixels show rapid decrease withincreasing photon flux. As the x-ray flux decreased the spatiallylocalized holes are removed through drift and recombinationand the device returns to normal operation.
VIII. SUMMARY
The performance of 2D CdZnTe monolithic detector arraysdesigned for high flux x-ray imaging applications was studied.
582 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 1, FEBRUARY 2008
The 2D CdZnTe monolithic arrays were 16 16 pixel deviceshaving 0.4 mm 0.4 mm area pixels on a 0.5 mm pitch and werefabricated using 8.7 mm 8.7 mm 3.0 mm CdZnTe singlecrystals grown by the high-pressure electro-dynamic gradientfreeze technique. The CdZnTe detector arrays were bonded toa ceramic substrate to allow performance testing of the indi-vidual arrays before bonding to the read-out chip. The deviceswere probe tested in a system consisting of a custom 16 16pin probe array, and a 256 channel read-out electronics utilizing8-channel fast bipolar ASIC chips and computer controlled 120kVp x-ray source.
In order to assess the true throughput of the CdZnTe detec-tors independent of electronics dead-time a nonlinear correctionmethod was implemented. We demonstrated for the first timelarger than 15 average pixel count-ratefor CdZnTe monolithic detector arrays. This ultra high flux per-formance was achieved for several hundred individual mono-lithic arrays. In addition we demonstrated good short term(1 hour) count-rate stability of the detector arrays.
We performed an extensive survey of the device response ina wide range of material and device parameters and operationconditions and established that the hole mobility-lifetime is thekey material charge transport property that governs the criticalx-ray flux for the onset of device polarization.
We also developed a dynamic charge transport and trappingmodel to describe the experimentally observed static, dynamicand transient phenomena.
We studied the various static and dynamic charge collectioneffects typically not observed in low-flux applications. These in-cluded dynamic polarization, static charge steering and dynamiclateral polarization and charge steering.
ACKNOWLEDGMENT
The authors are grateful for the numerous discussions with J.B. Glick and M. Prokesch and to A. Narvette for developing theLabView control and data acquisition program.
REFERENCES
[1] K. Zanio, Cadmium Telluride, Semiconductors and Semimetals. NewYork: Academic, 1978, vol. 13.
[2] R. Triboulet, Y. Marfaing, A. Cornet, and P. Siffert, “Undoped high-resistivity cadmium telluride for nuclear radiation detectors,” J. Appl.Phys., vol. 45, pp. 2759–2765, 1974.
[3] C. Szeles, “Advances in the crystal growth and device fabrication tech-nology of CdZnTe room-temperature radiation detectors,” IEEE Trans.Nucl. Sci., vol. 51, no. 3, pt. 3, pp. 1242–1249, Jun. 2004.
[4] C. Szeles, S. E. Cameron, S. A. Soldner, J.-O. Ndap, and M. D. Reed,“Development of the high-pressure electro-dynamic gradient crystalgrowth technology for semi-insulating CdZnTe growth for radiationdetector applications,” J. Electron. Mater., vol. 33, pp. 742–751, 2004.
[5] M. Clajus, V. Cajipe, S. Hayakawa, T. O. Tumer, and P. D. Willson,Multi-Energy, Fast Counting Hybrid CZT Pixel Detector With Dedi-cated Read-Out Integrated Circuit, IEEE RTSD 2006 (R3-3).
[6] [Online]. Available: http://www.evproducts.com.[7] G. F. Knoll, Radiation Detection and Measurement. New York:
Wiley, 2000, pp. 119–122.[8] C. Szeles, S. A. Soldner, S. Vydrin, J. Graves, and D. S. Bale, “Ultra
high flux 2D CdZnTe monolithic detector arrays for X-ray imaging ap-plications,” IEEE Trans. Nucl. Sci., vol. 54, no. 4, pt. 3, pp. 1350–1358,Aug. 2007.
[9] D. Bale and C. Szeles, The Nature of Polarization in Wide Band-GapSemiconductor Detectors Under High-Flux Irradiation: Application toCdZnTe IEEE RTSD 2006 (R2-5).
[10] D. Bale and C. Szeles, “The nature of polarization in wide band-gapsemiconductor detectors under high-flux irradiation: Application toCdZnTe,” Phys. Rev. B, B77, 035205–035220, 2008.
[11] S. A. Soldner, D. Bale, and C. Szeles, “Dynamic lateral polarizationin CdZnTe under high flux X-ray irradiation,” IEEE Trans. Nucl. Sci.,vol. 54, no. 5, pt. 2, pp. 1723–1727, Oct. 2007.
