Advances in Cdzntese for Radiation Detector Applications

Article Advances in CdZnTeSe for Radiation Detector Applications

Utpal N. Roy 1,*, Giuseppe S. Camarda 2, Yonggang Cui 2 and Ralph B. James 1

1 Savannah River National Laboratory, Aiken, SC 29808, USA; [email protected] 2 Brookhaven National Laboratory, Upton, NY 11973, USA; [email protected] (G.S.C.); [email protected] (Y.C.) * Correspondence: [email protected]

Simple Summary: Cadmium-zinc-telluride is the most important semiconductor material today for room temperature gamma-ray detector applications. The radiation detectors are widely used for medical imaging, homeland security, and X- and gamma-ray astronomy. However, the material has longstanding technological problems. Despite significant improvements over the past 3 decades, the technology still suffers from major detrimental defects such as a high concentration of tellurium inclusions and sub-grain boundary network/dislocation walls, which adversely impact the yield of high-quality detectors fabricated from the crystals. To address these challenges, selenium was added to the cadmium zinc telluride matrix. The Se addition was found to have a profound effect to reduce the deleterious defects in the crystals. The resulting cadmium-zinc-telluride-selenide material was found to be free from a sub-grain boundary network with reduced tellurium inclusions. Thus, cadmium zinc telluride selenide material is a promising approach to increase the yield of high-quality radiation detectors as compared to cadmium zinc telluride. This paper reports a path to the advancement of the quaternary material to achieve the best detector performance. This advance may resolve the longstanding issues associated with cadmium zinc telluride-based X- and gamma-ray detectors. Abstract: Detection of X- and gamma-rays is essential to a wide range of applications from medical Citation: Roy, U.N.; Camarda, G.S.; imaging to high energy physics, astronomy, and homeland security. Cadmium zinc telluride (CZT) is Cui, Y.; James, R.B. Advances in CdZnTeSe for Radiation Detector the most widely used material for room-temperature detector applications and has been fulfilling Applications. Radiation 2021, 1, the requirements for growing detection demands over the last three decades. However, CZT still 123–130. https://doi.org/10.3390/ suffers from the presence of a high density of performance-limiting defects, such as sub-grain radiation1020011 boundary networks and Te inclusions. Cadmium zinc telluride selenide (CZTS) is an emerging material with compelling properties that mitigate some of the long-standing issues seen in CZT. This Academic Editor: Leonardo Abbene new quaternary is free from sub-grain boundary networks and possesses very few Te inclusions. In addition, the material offers a high degree of compositional homogeneity. The advancement Received: 16 March 2021 of CZTS has accelerated through investigations of the material properties and virtual Frisch-grid Accepted: 8 April 2021 (VFG) detector performance. The excellent material quality with highly reduced performance- Published: 25 April 2021 limiting defects elevates the importance of CZTS as a potential replacement to CZT at a substantially lower cost. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: radiation detector; CdZnTeSe; X-ray topography; defects; Te inclusions published maps and institutional affil- iations.

1. Introduction Radiation detectors, especially for the X- and gamma-ray range, are being developed Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. and utilize the advantages of semiconductor radiation detectors operating at room temper- This article is an open access article ature. These detectors have a large number of applications ranging from homeland security distributed under the terms and to medical imaging, astronomy, and high energy physics [1–4]. Intense global search has conditions of the Creative Commons been carried out to develop efﬁcient semiconductors with high detection efﬁciency at lower Attribution (CC BY) license (https:// cost [4]. CdZnTe remains the leading material of choice today and has dominated the global creativecommons.org/licenses/by/ market for more than three decades. However, the performance and yield of high-quality 4.0/). detector-grade materials are still limited by the presence of high concentrations of randomly

Radiation 2021, 1, 123–130. https://doi.org/10.3390/radiation1020011 https://www.mdpi.com/journal/radiation Radiation 2021, 1 124

distributed performance-limiting defects, such as Te inclusions and sub-grain boundary networks [5]. These defects act as trapping centers, severely hindering the localized charge transport and imposing severe spatial inhomogeneity in the charge transport characteristics of the detector, which adversely affects the device performance [5–8]. Cd1−xZnxTe1−ySey (CZTS), a new quaternary II–VI compound semiconductor material, has emerged as a next-generation detector material in the last few years and has the potential to surpass CZT at a lower cost due to its higher yield of high-quality detectors. The addition of selenium in the CZT matrix successfully circumvents many of the long-standing issues associated with the material [9,10], thus gaining attention for further studies by various groups [11–16]. The addition of selenium to CZT provides multipronged advantages over conventional CZT. The compositional homogeneity of Cd1−xZnxTe1−ySey has been found to increase signiﬁcantly for ingots grown by both the traveling heater method (THM) and Bridgman method [9–11] as compared with CZT [17,18]. The addition of selenium in the CZT matrix has also been found to increase hardness, with reduced cadmium vacancy and signiﬁcant reduction of Te inclusions with respect to CZT, plus the resulting CZTS materials have been found to be free from sub-grain boundary networks [9,10,19,20]. Various compositions with selenium concentrations of 1.5, 2, 4, and 7 atomic % have been studied while keeping Zn at 10 atomic % for all the compositions. The Cd1−xZnxTe1−ySey composition with x = 0.1 and y = 0.02 has been found to be optimal in terms of charge transport characteristics and device performance. In this article, we report the advancement of Cd1−xZnxTe1−ySey (x = 0.1, y = 0.02) by the THM technique and discuss the performance of Frisch-grid detectors fabricated from as-grown CZTS ingots.

2. Methods and Materials

Indium-doped Cd0.9Zn0.1Te0.98Se0.02 ingots with 52 mm diameter were grown by the THM technique. The starting materials were 6N purity Cd0.9Zn0.1Te from 5N Plus Inc. and 6N purity CdSe from Azelis Inc. Tellurium was used as the solvent for the THM growth. Both 6N purity tellurium and 6N purity indium were procured from Alfa Aesar. The compound Cd0.9Zn0.1Te0.98Se0.02 was first synthesized from a stoichiometric amount of Cd0.9Zn0.1Te and CdSe in a 52 mm inner diameter ampoule. The inner wall of the ampoule was precoated with carbon film by cracking spectroscopic-grade acetone at 900 ◦C, followed by annealing at ~1150 ◦C. The synthesized material was used for the THM growth, which was carried out in carbon-coated 52 mm internal and 58 mm outer diameter quartz ampoules. The growth conditions were reported earlier [10,21] for the CZTS ingots. The optimization of the growth parameters mainly involved the fine-tuning of the height for the solution zone, which depends on parameters such as the amount of tellurium, growth temperature, temperature gradient, and translation speed [22]. The grown ingots were cut into pieces of desired sizes using a programmable diamond impregnated wire saw for further material characterization and device fabrication. The samples were then lapped on silicon carbide (SiC) papers subsequently polished on a felt pad using alumina suspensions of successive grit sizes. The final polishing was performed with 0.05 µm alumina suspension to obtain mirror-finished surfaces. Infrared (IR) transmission microscopic and X-ray topographic studies were performed using a Nikon Eclipse LV 100 Microscope and LBNL Advanced Light Source (ALS) Synchrotron Beamline 3.3.2 with an X-ray beam energy ranging from 4 to 25 keV, respectively. Virtual Frisch-grid (VFG) detector geometries were used to evaluate the detector performances fabricated from the as-grown Cd0.9Zn0.1Te0.98Se0.02 ingots. Figure1 shows a schematic of the VFG detector configuration. The geometries of VFG detector samples in general have large aspect ratio parallelepiped bar shapes, as shown in Figure1. Details of VFG detector geometry have been discussed earlier [23]. Radiation 2021, 1, FOR PEER REVIEW 3 Radiation 2021, 1 125

FigureFigure 1. Schematic 1. Schematic diagram diagram of a virtual-grid of a virtual CZTS detector.-grid CZTS detector. The side walls of the detector sample were wrapped with an insulator, Kapton tape in the presentThe side case. walls A metal of strip the of detector copper was sample wrapped we onre the wrapped insulating layerwith known an insulator, Kapton tape asin the the Frisch present grid, ascase. illustrated A metal in Figure strip1. Theof copper end faces wa weres wrapped coated with on a metallic the insulating layer known film, gold in the present investigation. The gold films acted as the anode and cathode contacts.as the Frisch The gold grid contacts, as illustrated on the anode in and Figure cathode 1. facesThe end were faces deposited were by coated a gold with a metallic film, chloridegold in solution the present on as-polished investigation CZTS surfaces.. The A highergold leakagefilms act currented as was the observed anode for and cathode contacts. bromine–methanolThe gold contacts etched on surfaces the anode compared and with cathode as-polished faces surfaces. were Thus, deposited as-polished by a gold chloride so- surfaces were chosen to fabricate the detectors to achieve a better signal-to-noise ratio becauselution of on the as lower-polished leakage CZTS current surfaces. of the detector. A h Theigher cathode leakage and the current metallic was Frisch observed for bromine– gridmethanol were electrically etched connected, surfaces and compared the cathode with side was as- irradiatedpolished by surfaces. different gamma Thus, as-polished surfaces sourceswere tochosen evaluate to the fabricate detector response. the detectors The induced to achieve signal was a registeredbetter signal through-to an-noise ratio because of eV Products preamplifier (A 4039), an Ortec 672 shaping amplifier, and MCA-3 series/P 7882the cardlower from leakage FAST ComTec. current All theof the detector detector. measurements The cathode were performed and the at roommetallic Frisch grid were ◦ temperatureelectrically in theconnected range of 21–23, andC the on as-growncathode Cd side0.9Zn wa0.1Tes 0.98irradiatedSe0.02 samples. by differe All the nt gamma sources to resultsevaluat presentede the detector here are on response. as-grown material; The induced no postgrowth signal thermal was registered treatments were through an eV Products performed for the THM-grown CZTS samples. preamplifier (A 4039), an Ortec 672 shaping amplifier, and MCA-3 series/P 7882 card from 3.FAST Results ComTec. and Discussion All the detector measurements were performed at room temperature in We have been developing CdZnTeSe detectors over the last 4 years and steadily the range of 21–23 °C on as-grown Cd0.9Zn0.1Te0.98Se0.02 samples. All the results presented improved the material quality and VFG detector performances. In the early phase of the research,here are we on achieved as-grown an energy material resolution; no in thepostgrowth range of ~1.3% thermal to ~2% attreatments 662 keV for thewere performed for the VFGTHM detectors-grown with CZTS a length samples. of 9–10 mm fabricated from as-grown Cd0.9Zn0.1Te0.98Se0.02 ingots. Figure2 summarizes some of the VFG detector performances fabricated from as- grown ingot, depicting all the detectors as having very few Te inclusions. The dimensions of the3. detectorsResults are and noted Discussion in the figure. The measured dark current density at 500 V obtained were 91,We 49, have 53, 63 pA/mmbeen develop2 for the fouring devices,CdZnTeSe respectively detectors and the over resistivity the last values 4 years and steadily im- are in the range of ~1–3 × 1010 ohm-cm. The current densities for the three detectors out ofprov four,ed were the found material to be reasonably quality uniform.and VFG Although detector all the performances. detectors were fabricated In the early phase of the re- fromsearch, the same we ingot,achieved the observed an energy variation resolution of resistivity in was the reasonably range of large. ~1.3% However, to ~2% at 662 keV for the theVFG repeatability detectors of with the measured a length resistivity of 9–10 values mm variedfabricated for the from same sampleas-grown when Cd0.9Zn0.1Te0.98Se0.02 in- refabricated using a similar surface preparation process. Similar effects were also observed forgots CZT. Figure samples. 2 The summarizes operating bias some voltages of mentioned the VFG in detector Figure2 were performances the optimum fabricated from as- valuesgrown for ingot the corresponding, depicting detectors all the for detectors the best energy as having resolution. very The few relatively Te inclusions wide . The dimensions variationsof the detectors of the energy are resolutions, noted in and the optimum figure. operatingThe measured bias voltages dark suggest current the density at 500 V ob- presence of a fair amount of inhomogeneity in2 the grown ingot and surface properties of thetained detectors. were Such 91, wide 49, variations 53, 63 pA/mm of the electronic for propertiesthe four indevices, CZT are very respectively common and the resistivity duevalues to randomly are in distributedthe range high of concentrations~1–3 × 1010 ohm of sub-grain-cm. The boundary current networks densities and Te for the three detectors inclusions.out of four, However, were due found to the to absence be reasonably of sub-grain boundariesuniform. inA CZTSlthough material, all the the detectors were fabricated from the same ingot, the observed variation of resistivity was reasonably large. However, the repeatability of the measured resistivity values varied for the same sample when refabricated using a similar surface preparation process. Similar effects were also observed for CZT samples. The operating bias voltages mentioned in Figure 2 were the optimum values for the corresponding detectors for the best energy resolution. The relatively wide variations of the energy resolutions, and optimum operating bias voltages suggest the presence of a fair amount of inhomogeneity in the grown ingot and surface properties of the detectors. Such wide variations of the electronic properties in CZT are very common due to randomly distributed high concentrations of sub-grain boundary networks and Te inclusions. However, due to the absence of sub-grain boundaries in CZTS material, the material uniformity and yield of high-resolution detectors were higher than CZT. Figure 3a shows the optical photograph of a typical bar detector with gold contact corresponding to the detector displayed in the bottom left corner of Figure 2. The

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material uniformity and yield of high-resolution detectors were higher than CZT. Figure3a shows the optical photograph of a typical bar detector with gold contact corresponding detector dimensions are 3.4 × 3.6 × 9.7 mm3. Figure 3b,c shows scanned microscopic im- to the detector displayed in the bottom left corner of Figure2. The detector dimensions ages of the detector in the reflection and IR transmission mode. As mentioned earlier, are 3.4 × 3.6 × 9.7 mm3. Figure3b,c shows scanned microscopic images of the detector inFigure the reflection 3c demo andnstrates IR transmission the presence mode. of very As few mentioned Te inclusions earlier,, which Figure can3c demonstrates be considered thea major presence achievement of very few in the Te inclusions,development which of the can quaternary be considered compound a major CZTS achievement. Modeling in 4 −3 theindicates development that the of presence the quaternary of Te inclusions compound with CZTS. concentrations Modeling indicates of less than that the~2 × presence 10 cm , 3 −3 2 −3 of~3Te × 10 inclusions cm , and with ~7–8 concentrations × 10 cm for 5, of 10 less, and than 15 µm ~2 ×sizes104, respectivelycm−3, ~3 ×, in10 3CZTcm −material3, and ~7–8can be× 10tolerated2 cm−3 tofor achieve 5, 10, and an energy 15 µm sizes,resolution respectively, in the range in CZT of ~0.5 material%–0.6% can at be 662 tolerated keV [5]. toIn achieve the present an energy case for resolution CZTS, the in the presence range ofof ~0.5%–0.6%very few Te at inclusions 662 keV [ 5suggests]. In the presentthat the casematerial for CZTS, is capable the presence of producing of very very few-high Te inclusions-energy-resolution suggests thatdetectors the material with higher is capable yield ofas producing compared very-high-energy-resolutionwith CZT. Another major hurdle detectors that withCZT higheris facing yield is the as presence compared of with high CZT.concentrations Another major of sub hurdle-grain that boundary CZT is facingnetworks is the distributed presence of randomly high concentrations in the CZT ofmatrix sub- grainthat evidently boundary impose networks a significant distributed impact randomly on the in detector the CZT performance matrix that evidently [5]. This imposemotivates a significantthe investigation impact of on the the sub detector-grain performanceboundaries and [5]. their This motivatesnetworks in the CZTS investigation. X-ray topogra- of the sub-grainphy is perhaps boundaries the most and powerful their networks tool to in explore CZTS. X-raythe presence topography of sub is perhaps-grain boundaries the most powerfuland their toolnetwork to explores. In this the present presence study, of sub-grain we conducted boundaries measurements and their networks. using a synchro- In this presenttron radiation study, we source, conducted especially measurements in the reflection using a synchrotronmode. Prior radiationto the measurements, source, especially mir- inror thelike reflection polished mode.samples Prior were to etched the measurements, in a 2% bromine mirrorlike–methanol polished solution samples for 2 min were to etchedremove in any a 2% possible bromine–methanol mechanically solution damaged for layer 2 mins that to remove might have any possiblebeen produced mechanically during damagedthe polishing layers process. that might Sub have-grain been boundaries produced are during essentially the polishing dislocations process. arranged Sub-grain in boundariesplanes, which are form essentially dislocation dislocations walls, and arranged their network in planes, is whichknown form as sub dislocation-grain boundary walls, andnetwork. theirnetwork Such an isisolated known dislocation as sub-grain wall boundary, namely, network. sub-grain Such boundary an isolated, is visible dislocation in Fig- wall,ure 4 namely, along the sub-grain arrow heads. boundary, Figure is visible4 depicts in Figurean X-ray4 along topographic the arrow image heads. of theFigure whole4 depictsdetector an shown X-ray topographicin Figure 3a.image As opposed of the wholeto CZT, detector the topographic shown in Figureimage3 ofa. the As opposedCZTS de- totector CZT, is the free topographic from the presence image of of a the sub CZTS-grain detectorboundary is network. free from The the CZTS presence samples of a sub-were grainconsistently boundary found network. to be free The from CZTS a sub samples-grain boundary were consistently network for found all the to becompositions free from awith sub-grain selenium boundary concentrati networkons between for all the1.5 compositionsand 7 at%. However, with selenium occasional concentrations presence of betweenisolated 1.5sub and-grain 7 at%. boundaries However, was occasional observed presence [10,23]. ofThese isolated sub- sub-graingrain boundary boundaries networks was observedin CZT are [10 known,23]. These to degrade sub-grain the boundarydetector response networks severely. in CZT Thus are known, the absence to degrade of a sub the- detectorgrain boundary response network severely. in Thus, CZTS the samples absence is ofhighly a sub-grain promising boundary and has network the potential in CZTS to samplesincrease is the highly yield promising of very high and- hasquality the potentialdetectors to from increase a given the yieldingot. of The very production high-quality of detectorssub-grain from boundary a given-free ingot. CZTS The material production is an of important sub-grain boundary-freeimprovement, CZTSwhich material has other- is anwise important been impossible improvement, to achieve which for has CZT otherwise due to beenits poor impossible thermophysical to achieve properties for CZT duenear toand its below poor thermophysical its melting point. properties The selenium near and plays below an its important melting point.role in The the selenium CZT matrix plays as anan importanteffective solid role insolution the CZT hardening matrix as agent. an effective solid solution hardening agent.

FigureFigure 2. 2.Performance Performance of of Frisch-grid Frisch-grid detectors detectors fabricated fabricated from from the the same same THM-grown THM-grown ingot. ingot.

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Figure 3. (a) Optical photograph of the as-grown CZTS bar detector with gold contact, (b) microscopic scanned image of the detector in reflection mode, and (c) microscopic scanned image of the detector in IR transmissionFigure mode.3. (a) Optical Detector photograph dimensions: of the3.4 ×as 3.6-grown × 9.7 CZTSmm3. bar detector with gold contact, (b) micro- FigureFigure 3. (a )3. Optical (a) Optical photograph photograph of the as-grown of CZTS the bar as detector-grown with CZTS gold contact, bar (bdetector) microscopic with gold contact, (b) micro- scannedscopic imagescanned of the image detector of in reﬂectionthe detector mode, in and reflection (c) microscopic mode scanned, and image (c) microscopic of the detector scanned image of the scopic scanned image of the detector in reflection3 mode, and (c) microscopic scanned image of the indetector IR transmission in IR mode.transmission Detector dimensions: mode. Detector 3.4 × 3.6 ×dimensions:9.7 mm . 3.4 × 3.6 × 9.7 mm3. detector in IR transmission mode. Detector dimensions: 3.4 × 3.6 × 9.7 mm3.

Figure 4. X-ray topographic image of an as-grown detector. Detector dimensions: 3.4 × 3.6 × 9.7 mm3.

The pulse heightFigureFigure spectr 4. X-ray4. X-umray topographic topographicof the image VFG of image andetector as-grown of an detector.for as- growna 137 DetectorCs detector. sealed dimensions: sourceDetector 3.4 × 3.6registered dimensions× 9.7 mm3. at: 3.4 × 3.6 × 9.7 mm3. room temperature under an applied bias of −1800 V and shaping time of 6 micro-second. FigureThe pulse4. X- heightray topographic spectrum of theimage VFG detectorof an as for-grown a 137Cs detector. sealed source Detector registered dimensions at : 3.4 × 3.6 × 9.7 mm3. 137 is shown in Figureroom 5, reveal temperatureThe ingpulse a underwell height- anresolved applied spectr biaslowum of -ofenergy− 1800the VFG V barium and detector shaping X-ray time for photopeak of a 6 micro-second.Cs sealed at ~32 source registered at isroom shown temperature in Figure5, revealing under a well-resolvedan applied low-energybias of −1800 barium V X-rayand photopeakshaping time at of 6 micro-second. keV and a characteristicThe 662 keVpulse photo heightpeak. spectr Figureum 5b of is the a magnified VFG detector version for of a the137Cs spec- sealed source registered at trum to enhance the~32is shown keV662 andkeV in a characteristic photoFigurepeak. 5, reveal 662 The keVing energy photopeak. a well at-resolved Figure662 keV5b is lowwas a magniﬁed- energy~1.3% version withbarium ofa thepeak X-ray- photopeak at ~32 spectrumroom temperature to enhance the 662 under keV photopeak. an applied The energybias of at 662−1800 keV wasV and ~1.3% shaping with a time of 6 micro-second. to-valley ratio of ~20.keV and a characteristic 662 keV photopeak. Figure 5b is a magnified version of the spec- peak-to-valleyis shown in ratio Figure of ~20. 5, revealing a well-resolved low-energy barium X-ray photopeak at ~32 trum to enhance the 662 keV photopeak. The energy at 662 keV was ~1.3% with a peak- tokeV-valley and ratioa characteristic of ~20. 662 keV photopeak. Figure 5b is a magnified version of the spectrum to enhance the 662 keV photopeak. The energy at 662 keV was ~1.3% with a peak- to-valley ratio of ~20.

FigureFigure 5. (a 5.) Pulse (a) Pulse height spectrum height ofspectrum the VFG detector of the for VFG a 137 Csdetector source and for (b )a a 137 magniﬁedCs source version and of the(b) same a magnified spectrum ver- 3 tosion enhance of the 662 same keV peak.spectrum Detector to dimensions: enhance 3.4the× 6623.6 × keV9.7 mmpeak. . Detector dimensions: 3.4 × 3.6 × 9.7 mm3. FigureThe overall5. (a) Pulse performance height of spectrum the as-grown of CZTSthe VFG detectors detector was observedfor a 137Cs to besource excellent and (b) a magnified ver- The overall performanceevension atof the the early same of stage thespectrum of as our-grown research. to enhance CZTS The underlying the detectors 662 keV reason was peak is observed the. Detector drastic reduction dimensionsto be excel- of : 3.4 × 3.6 × 9.7 mm3. lent even at the earlyperformance-limiting stage of our research. defects as a The result underlying of the addition reason of selenium is the in the drastic CZT matrix. reduction The of performance-limitingFigureThe defects 5. overall(a) Pulse as aperformance resultheight of spectrum the ofaddition the of asthe- grownof VFG selenium detector CZTS in detectors forthe a CZT 137Cs wasmatrix. source observed and (b to) a bemagnified excel- ver- The presence of highlentsion concentrations evenof the at same the early spectrum of sub stage-grain to of enhance our boundary research. the 662network The keV underlying peaks and. DetectorTe inclusions reason dimensions is thein drastic: 3.4 ×reduction 3.6 × 9.7 mm 3. CZT has been an unresolvedof performance major-limiting hurdle thatdefects restrict as as result the yield of the of highaddition-quality of selenium detector -in the CZT matrix. grade material fromThe grown presenceThe ingotoverall ofs ,high resulting performance concentrations in a high of thecostof sub as of- growngrainend product boundary CZTSs. detectorsT networkhe addition swas and observedTe inclusions to bein excel- of selenium has beenCZTlent found evenhas been toat successfullythe an unresolvedearly stage alleviate majorof our many hurdle research. of that the The restrictissues underlying sand the enhanceyield reason of highthe is-quality the drastic detector reduction- potential for low-productiongradeof performance material cost. from A-limiting continuous grown defects ingot efforts, asresulting is a being result inmade of a thehigh to addition improvecost of end ofthe seleniumproduct ma- s. Tinhe the addition CZT matrix. terial quality of theofThe new selenium presence quaternary has of been material high found concentrations by tooptimizing successfully of the sub growthalleviate-grain parameters. boundarymany of the Withinnetwork issues ands and enhance Te inclusions the in a short period ofpotential CZT time, has we forbeen have low a n-successfullyproduction unresolved cost. improve major A c ontinuoushurdled the materialthat effort restrict is properties beings the made yield and toof improvehigh-quality the ma- detector- achieved as-measuredterialgrade energy quality material resolutions of the from new grown quaternaryin the rangeingot material sof, resulting 1 ± 0.1% by optimizing atin 662 a high keV the costfor growth ~ of10 endmm parameters. products. WithinThe addition a short period of time, we have successfully improved the material properties and long VFG detectorsof [10,2 selenium4]. The besthas energybeen found resolution to successfully achieved with alleviate VFG CZTS many detectors of the issues and enhance the at room temperatureachieved thus far as was-measured ~0.77% energyat 662 keVresolutions without in any the charge range- lossof 1 corrections.± 0.1% at 662 keV for ~10 mm potential for low-production cost. A continuous effort is being made to improve the ma- long VFG detectors [10,24]. The best energy resolution achieved with VFG CZTS detectors atterial room quality temperature of the newthus quaternaryfar was ~0.77% material at 662 bykeV optimizing without any the charge growth-loss parameters. corrections. Within a short period of time, we have successfully improved the material properties and

achieved as-measured energy resolutions in the range of 1 ± 0.1% at 662 keV for ~10 mm long VFG detectors [10,24]. The best energy resolution achieved with VFG CZTS detectors

at room temperature thus far was ~0.77% at 662 keV without any charge-loss corrections.

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presence of high concentrations of sub-grain boundary networks and Te inclusions in CZT has been an unresolved major hurdle that restricts the yield of high-quality detector-grade material from grown ingots, resulting in a high cost of end products. The addition of selenium has been found to successfully alleviate many of the issues and enhance the potential for low-production cost. A continuous effort is being made to improve the material quality of the new quaternary material by optimizing the growth parameters. Within a short period of time, we have successfully improved the material properties and achieved as-measured energy resolutions in the range of 1 ± 0.1% at 662 keV for ~10 mm long VFG detectors [10,24]. The best energy resolution achieved with VFG CZTS detectors at room temperature thus far was ~0.77% at 662 keV without any charge-loss corrections. The profound improvements of the material quality of CZTS and detector performances within a span of about 4 years is very compelling. However, the new CZTS material still suffers from issues pertaining to impurity content. The impurities responsible for deep levels have been found to be very high in tens of ppb (parts per billion) as conﬁrmed by glow discharge mass spectrometry (GDMS). These impurities in general have not been detectable in commercial CZT, except for Fe at ~22 ppba [25]. Table1 lists the concentrations of some of the selected impurities present in two separate CZTS ingots. The presence of such high impurities in the CZTS matrix is believed to originate from the CdSe starting material. Thus, CZTS has room for further improvement of the material properties and the detector performance by purifying the starting materials.

Table 1. Selected impurity concentrations present in two different THM-grown Cd0.9Zn0.1Te0.98Se0.02 ingots as measured by GDMS.

CZTS Ingot #1 CZTS Ingot #2 Elements Concentration (ppb at.) Concentration (ppb at.) Fe 42 42 Cu <4 22 Ti <5 <5 Cr 36 <20 Pb 11 10 Ni 16 <4 Sn <100 <100

4. Summary The long-standing issues suffered by CZT pertaining to the presence of high concentrations of performance-limiting defects (sub-grain boundary network and Te inclusions) due to poor thermophysical properties motivated the researchers to ﬁnd a suitable alternative material with better detector performance at a lower cost of production. We pursued an approach to improve CZT material properties by adding selenium to the matrix and discussed the advancement of CdZnTeSe. The effectiveness of adding selenium to the CdTe matrix was observed three decades ago by the infrared substrate community for night vision applications. The resulting ternary compound CdTeSe with as low as 0.4% Se was reported to be free from a sub-grain boundary network [26], while codoping with Zn and Se in the CdTe matrix was also observed to produce sub-grain boundary-free quaternary CdZnTeSe material [27]. The reported advantages of selenium addition to the CdTe/CZT matrix have motivated the development of CdZnTeSe for radiation detector applications. The optimum composition of the ternary compound (Cd0.9Zn0.1Te0.98Se0.02) has been determined for realizing the best detector performance. As discussed, in the early phase of the research, we attained an energy resolution in the range of ~1.3% to ~2% at 662 keV for the VFG detectors with a length of 9–10 mm fabricated from as-grown CZTS ingots. Within a short period of time, we have successfully improved the material properties, and very-high-energy-resolution VFG detectors have been achieved with Radiation 2021, 1 129

average FWHM values of 1 ± 0.1% at 662 keV for ~10 mm long detectors [10,24]. The new compound CZTS has been found to be free from sub-grain boundary networks with very few Te inclusions embedded in the matrix, which improves the yield of high-quality detectors from the as-grown ingot with the potential of a significant reduction of detector cost. Achieving high-resolution detectors from as-grown CZTS in the early stage of our research reflects the efficacy of adding selenium to the CZT matrix due to the reduction of many performance-limiting defects. The quaternary CZTS has also been reported to be superior to CZT for some medical imaging applications, particularly those requiring a high flux of X-rays [14]. The observed high concentrations of external impurities suggest that further improvement of detector performances is possible through the growth of the quaternary material using purified starting materials. The material properties and the achieved detector performance supersede the present-day CZT material. Thus, the new material Cd0.9Zn0.1Te0.98Se0.02 with dramatically reduced defects offers tremendous potential to successfully replace CdZnTe material at a reduced cost.

Author Contributions: Conceptualization, U.N.R., R.B.J.; methodology, G.S.C., Y.C., U.N.R.; valida- tion, G.S.C., Y.C., R.B.J.; investigation, writing—original draft preparation, U.N.R.; writing—review and editing, U.N.R., R.B.J., Y.C.; visualization, U.N.R., Y.C., G.S.C.; supervision, U.N.R., R.B.J. All the authors participated in scientific discussions and critically reviewed the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the U.S. Department of Energy Office of Defense Nuclear Nonproliferation Research and Development (DNN R&D.) The authors from SRNL acknowledges the partial support from the Laboratory Directed Research and Development (LDRD) program within the Savannah River National Laboratory (SRNL). This document was prepared in conjunction with work accomplished under Contract No. DE-AC09-08SR22470 with the U.S. Department of Energy (DOE) Office of Environmental Management (EM). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: This study did not report any data. Conflicts of Interest: The authors declare no conflict of interest.

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