IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 4, AUGUST 2013 2853

Cd Zn Te Crystal Growth and Fabrication ofLarge Volume Single-Polarity Charge Sensing

Gamma DetectorsSandeep K. Chaudhuri, Ramesh M. Krishna, Kelvin J. Zavalla, Liviu Matei, Vladimir Buliga, Michael Groza,

Arnold Burger, Senior Member, IEEE, and Krishna C. Mandal, Member, IEEE

Abstract—Detector grade Cd Zn Te single crystals weregrown using a tellurium solvent method. Single crystal blocksof volume cm were prepared for detector fabrication andcharacterization. The grown crystals were characterized usinginfra-red transmission imaging and Pockel’s effect measurements.Two detectors in single-polarity charge sensing configurations viz.,small pixel, and virtual Frisch grid were fabricated on two crystalsobtained from the same section of the ingot. Current-voltagemeasurements performed in planar configuration exhibited a verylow leakage current of nA at 1000 V and resistivities of theorder of cm. Electron drift mobilities of the order of840 cm /V.s and electron mobility-lifetime products of the orderof cm /V were calculated from alpha spectroscopyusing detectors in planar configuration. The small pixel and thevirtual Frisch grid detector showed similar energy resolutionof 3.7% for 662 keV gamma rays however, the virtual Frischgrid configuration revealed a better overall performance with apeak-to-Compton ratio of 2.8. A digital spectrometer and relatedsoftware has been developed using a digitizer card and used toemploy offline correction schemes to compensate for the chargeloss effects, resulting in significant improvement of the 662 keVpeak resolution (1.8% as compared to 3.7% without correction)obtained in the case of small pixel detector.

Index Terms—Biparametric correlation, Cd Zn Te (CZT),crystal growth, digital data analyses, radiation detector, small pixeleffect, Te solvent method, virtual Frisch grid detector.

I. INTRODUCTION

A MONG all other direct conversion high-Z radiation de-tectors, Cd Zn Te (CZT) based devices have turned

out to be the most popular as compact and high energy gammadetectors since the availability of high-quality detector gradematerial. CZT has high gamma ray absorption coefficient andalso have properties like low leakage currents, wide band gap atroom temperature, and high density which are essential prereq-uisites for nuclear radiation detectors used in the field of home-

Manuscript received December 01, 2012; revised March 02, 2013, May 10,2013, and June 13, 2013; accepted June 16, 2013. Date of publication July 18,2013; date of current version August 14, 2013. This work was supported in partby the DOE Office of Nuclear Energy’s Nuclear Energy University Programs,Grant DE-AC07-05ID14517 and in part by ASPIRE—I (Advanced Supportfor Innovative Research Excellence—I), University of South Carolina, Grant15530-A401.S. K. Chaudhuri, R. M. Krishna, K. J. Zavalla, and K. C. Mandal are with the

Electrical Engineering Department, University of South Carolina, Columbia,SC 29208 USA (e-mail: mandalk@cec.sc.edu).L. Matei, V. Buliga, M. Groza, and A. Burger are with the Physics Depart-

ment, Fisk University, Nashville, TN 37208 USA (e-mail: aburger@fisk.edu).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TNS.2013.2270289

land security, medical imaging, infrared focal plane arrays, en-vironmental monitoring etc. [1]–[7].Performance of large volume CZT detectors is limited by the

poor charge transport properties which in turn are due to thepresence of various macroscopic and microstructural defects[1]. Vast amount of research is being performed in order to growhigh quality detector grade crystals with better charge transportproperties.Most CZTmaterials used for x-/gamma ray detectionare grown by travelling heater method (THM) and high pressureBridgman technique. Modified Bridgman technique with lowtemperature gradient is also used to grow better quality crystals.In the present work we have used a tellurium solvent method togrow high quality detector grade Cd Zn Te single crystals[8], [9].Apart from the attempt of growing high-quality defect free

crystals [6], [10], detectors can be fabricated with single-po-larity charge sensing geometry e.g., virtual Frisch grid config-uration [11]–[13], coplanar grid structure [14] and small pixelgeometry [15]. These kind of detector geometries eliminate theeffect of poor hole transport properties to a great extent.The effect of charge trapping can also be compensated for by

applying offline correction schemes to digitally obtained pulse-height spectra [16]. Biparametric correlation based correction isone such scheme in which the pulse-heights from same energyevents are correlated to their corresponding depth of interactionand any discrepancies are taken care of by applying suitablecorrection factor.In the present work we have used a tellurium solvent

method to grow large volume and high quality detector gradeCd Zn Te single crystals and fabricated detectors withsingle-polarity charge sensing configurations viz., small pixeland virtual Frisch grid. The CZT crystals were characterizedoptically and electrically prior to detector fabrication. Electrondrift-mobility and mobility-lifetime products havebeen measured using a time-of-flight technique and alpharay spectroscopy respectively. The detectors were tested fortheir performance as high-energy gamma ray detector using aCs radiation source. Biparametric corrections were applied

to compensate for the charge loss effects in the small-pixeldetector.

II. EXPERIMENTAL PROCEDURES

A. Crystal Growth and Detector Fabrication

For refinement, 5N purity Cd, Zn, and Te precursors werepassed through a zone refiner at a rate of mm per hour

0018-9499 © 2013 IEEE

2854 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 4, AUGUST 2013

Fig. 1. Photograph of a small pixel (a), and a virtual Frisch grid (b) detectorfabricated on cm CZT crystals (dimensions given in Table I).

TABLE IPHYSICAL PROPERTIES OF THE CRYSTALS USED FOR DETECTOR FABRICATION

in an average of 35 cycles to achieve purity. Zone re-fined (ZR) precursors were weighed in stoichiometric amountsfor Cd Zn Te crystal growth with excess of 50% ZR Teand were vacuum ( Torr) sealed in a carbon-coated quartzampoule ( mm wall thickness, ampoule ID 25 mm). Thesealed ampoule was then loaded into a two-zone horizontal fur-nace for synthesis. The CZT polycrystalline charge was pre-pared by slowly heating the ampoule to a maximum tempera-ture of about 1085 C. Continuous rotation during the synthesiswas used to ensure homogeneity. The polycrystalline ingot wasthen placed in a conically tipped thick-walled ( mm) carboncoated quartz ampoule, which holds the CZT seed crystal, andsealed under a vacuum of Torr. The sealed ampoule wasloaded in the Bridgman crystal growth furnace and connectedto a slow-speed (20 rph) motor for axial rotation. The growncrystal directionally solidified at a constant velocity ofcm/day. Several CZT crystals of various dimensions have beencut out from the ingot and, ground, lapped (down to 1 m SiCpaper) and polished (down to 0.05 m alumina powder). Moredetails on the growth method can be found elsewhere [8], [9].Two detectors, one in virtual Frisch grid and the other in small

pixel configuration, were fabricated in two different crystals ob-tained from the same section of the ingot. Table I provides thedetails of the crystal properties and the detectors used in thisstudy. Crystal A was used for fabricating a small pixel detectorand crystal B was used for fabricating a virtual Frisch grid de-tector. Fig. 1(a) shows a detector in small pixel configuration ac-complished by depositing 3.2 mm diameter circular gold contacton the Te rich face (parallel to (111) crystallographic planes) anda full planar contact on the opposite face. The detector in smallpixel geometry was also equipped with a guard ring structure.The virtual Frisch grid configuration (Fig. 1(b)) was achievedwith a copper sheath tightly wrapped around the crystal. Theheight of the copper sheath was mm and was flush with thecathode. The sheathwas electrically insulated from the CZTma-terial by lining the crystal side surfaces with insulating Kapton

tape. The copper sheath contained a projected tab which wasused to connect it to the cathode.

B. Crystal and Detector Characterization

The polished crystals were characterized for integrity (singlecrystallinity), internal stress and internal electric field throughinfra-red (IR) transmission imaging by using a 75 W high sta-bility Xenon arc lamp. For the electrical and elctro-optical mea-surements, planar gold contacts were deposited on two oppo-site crystal faces parallel to the (111) crystallographic planes.The I-V measurements for leakage current and resistivity deter-mination were carried out using a Keithley 237 source-meter.For Pockels effect imaging we used an Electrophysics GaAscamera Model 8320 TE cooled and sensitive in the 900–1700nm range, a Newport light source Model 66475 equipped witha 75 W high-stability Xenon arc lamp, an Andover IR narrowband filter P/N 115FS10-50 (pass band nm), and twoEdmund Optics polarizers P/N NT48–889 covering a range of1000 to 2000 nm with an extinction factor greater than 1000:1.The high voltage power supply used was a Tennelec model TC952. Neutral density filters were used to reduce the incident lightintensity as needed to limit the photo carrier generation.The electron drift-mobility and mobility-lifetime product

measurements were carried out on the same planar geometry.The electron drift-mobility was measured using a time-of-flightmethod [17] where the waveform of the response to alphaparticles impinging on the cathode are recorded and averagedon a storage oscilloscope. The average signal is analyzed todetermine the transit time for electrons, /( E), whered is the thickness of the detector, E is the electric field andis the electron drift-mobility. The value of the electric field isexperimentally optimized and chosen to prevent obtaining atransit time that is too short and comparable with the responsetime of the electronics, yet has a sufficiently high value tominimize trapping effects.The mobility-lifetime products were measured using Hecht

equation fit to the bias dependence of charge-collection effi-ciency calculated using alpha spectroscopy. A Am alphasource (primarily 5.48 MeV) was used for both the studies. Thegamma spectroscopic measurements were carried out usingan analog spectrometer comprising of an Amptek A250CFpre-amplifier, an Ortec 671 shaping amplifier and a CanberraMultiport II multichannel analyzer. The energy resolutionof the detectors was measured in terms of the full width athalf maxima (FWHM) of the full-energy peak in the gammapulse-height spectra. The peak-to-valley (P/V) ratio was cal-culated as the ratio of full-energy peak height to the averageheight at a distance from the peak centroid ( being thestandard deviation of the Gaussian fit of the peak) and thepeak-to-Compton (P/C) ratio was calculated as the ratio offull-energy peak height to the Compton height measured ap-proximately 100 keV below the Compton edge.

C. Digital Pulse-Height Correction

A Labview based data-acquisition software was developed toacquire and store the digitized pulses. PCI-5122 enables to ac-quire pulses with a sampling rate of 100MS/s and 14 bit verticalresolution. A separate program was developed to process andanalyze the digitized data using Labview and MATLAB codes.

CHAUDHURI et al.: CD ZN TE CRYSTAL GROWTH AND FABRICATION 2855

Fig. 2. IR transmission image of the CZT crystals showing Te inclusions/pre-cipitations.

Fig. 3. I-V curve obtained for CZT crystal B with planar gold contacts. Insetshows the linear region of the I-V curve for resistivity measurements.

The data analyses involved digital semi-Gaussian (CR-RC )[18], [19] shaping of the pulses followed by pulse-height de-termination. Biparametric correlation was carried out by map-ping the pulse-height of the signal generated from a particularevent and the corresponding depth of interaction of that par-ticular event in a 2D graph. The depth of interaction was as-sumed to be proportional to the rise-time of the pulse. Deviationof the correlation plots from the ideal behavior can be noticedwhen there are substantial charge loss effects associated with thegamma interaction events. Such deviations can be corrected forby incorporating proper correction factors which restores the bi-parametric plots to a near-ideal situation. Corrected pulse-heightspectra were regenerated from the corrected biparametric plots.

III. RESULTS AND DISCUSSIONS

A. Spectroscopic Characterization

Fig. 2 shows an IR transmission image of one of the CZTcrystals (B) used in this study. Te inclusions with diametersgreater than 10 m can act as potential charge trapping cen-ters and significantly degrade the detector’s performance [20],[21]. The images revealed an average tellurium inclusion/pre-cipitate size of m. Fig. 3 shows the I-V curve obtained fora CZT detector (crystal B) in planar configuration by applying

Fig. 4. Pockels images of the CZT crystal B at zero bias, parallel polarizers inimage (a), zero bias, cross polarizers high sensitivity (integration time 0.5 s) inimage (b), no bias, cross polarizers, lower sensitivity (integration time 0.1 s) inimage (c) V at bottom contact, cross polarizers same integration time(0.1 s) in image (d) and electric field distribution as a function of depth (e).Electric field is relatively uniform throughout the crystal thickness with slightenhancement at cathode side.

the bias on the Cd rich face. Both the crystals showed a distinctasymmetry in the current behavior in the negative and positivebias regime indicating the presence of active deep centers at thesurface which can render the Au/CZT interface as non-Ohmic[22]. The typical leakage currents at V was found to be

nA and nA at V. Inset in Fig. 3 shows a lowrange I-V measurements for resistivity determination. The bulkresistivity was found to be and -cm incrystal A and B respectively which are desirable values for gooddetection performance.Compared with other published studies on the Pockels char-

acterization technique [23]–[25], images in Fig. 4 indicate arelatively low concentration of mechanical stresses inside the

2856 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 4, AUGUST 2013

Fig. 5. (a) Variation of electron transit time as a function of inverse of biasvoltage and (b) Hecht plot for determining the mobility-lifetime product. isthe coefficient of determination which shows the goodness of the fit. The closer

is to 1, the better is the fit.

crystal (b) and a good uniformity of internal electric field (e),both being prerequisites for a high performance CZT detector.Fig. 5(a) shows a plot of transit time of electrons as a func-

tion of inverse of applied bias voltage obtained for crystal B.The drift-mobility was calculated from the slope of the linear fitto the plot. The drift-mobility values obtained for the CZT crys-tals A and B were and cm /V.s respectively.The decent electron drift-mobility values further confirms thedetector grade quality of the crystal. Fig. 5(b) shows a plot ofthe alpha peak channel number as a function of the bias voltagein the case of crystal B. The plot was fitted with the single car-rier Hecht equation [26] given by

(1)

where CCE is the charge collection efficiency, V is the appliedbias and d is the detector thickness. The product was cal-culated from the fitted parameters. A value of coefficient of de-termination so close to 1 implies a very goodfitting. The product obtained for crystals A and B were

% and % cm /V respectively.Fig. 6 shows a Cs spectrum obtained using crystal A with

the small guarded pixel on Te rich face and solid cathode on Cdrich face biased at V. The guard ring was connected tothe ground. The 662 keV gamma peak was seen to be distinctlyresolved along with the Compton edge and the backscatteredpeak. The energy resolution for the 662 keV gamma ray peak

Fig. 6. Cs spectrum obtained using the CZT detector in small-pixelconfiguration.

Fig. 7. Cs spectrum obtained using the CZT detector in virtual Frisch gridconfiguration.

was calculated to be % with a P/V ratio of and a P/Cratio of . The width of the pulser peak, acquired simultane-ously, was seen to be much narrower than the full-energy peakimplying that the detector performance was not limited by theelectronic noise of the detection circuit. Although the full-en-ergy peak was well resolved, a tailing at the low energy sidecould be observed which is normally ascribed to the charge lossin various kinds of defects leading to partial charge induction[27]. An attempt to compensate for the effect of charge loss wasmade and has been presented in Section B.The crystal B when configured in a virtual Frisch grid ge-

ometry provided a similar energy resolution of % for the662 keV gamma rays at a comparatively lower bias voltage of1300 V. Fig. 7 shows a Cs spectrum obtained using the de-tector in virtual Frisch grid configuration. The narrower widthof the pulser peak implies that the electronic noise did not limitthe detector performance in this case either. The P/C ratio wasfound to be better compared to that of the small pixel detector.However the P/V ratio was found to be more for the small pixel

CHAUDHURI et al.: CD ZN TE CRYSTAL GROWTH AND FABRICATION 2857

Fig. 8. Biparametric plots obtained for the small pixel CZT detector using aCs source before correction (a) and after correction (b).

detector as the involved active volume is less compared to thatof the full area virtual Frisch collar detector.

B. Digital Corrections

Fig. 8(a) shows a biparametric plot generated for the smallpixel detector irradiated with a Cs source. The x-axis rep-resents the pulse-height, the y-axis represents the rise-time andthe z-axis represents the number of correlated counts given bythe shaded color (dark being low and bright being high). The662 keV photoelectric events could be seen as the inclined bandof events shown within the white dotted lines. These eventswould have appeared as a vertically straight band (parallel to therise-time axis) of correlated events had there been no trappingcenters. The inclination shows the variation of signal amplitudeas a function of depth of interaction for the 662 keV photo-elec-tric events because of charge-loss in defects. The tailing at thelower energy side of the 662 keV peak e.g., in Fig. 6 can beeasily realized from the biparametric plots by considering thefact that the pulse-height spectrum could be obtained by pro-jecting the events in the biparametric plot onto the pulse-heightaxis. The affected pulse-height spectrum can be recovered byapplying suitable correction factors to the affected events iden-tified from the biparametric plots. The correction scheme canbe summarized as follows. First of all the position of the eventswithout any charge loss is located on the energy (pulse-height)

Fig. 9. Cs pulse-height spectra regenerated from the corrected biparametricplot (Fig. 8(b)) for the small-pixel detector.

axis for the concerned set of events (662 keV interactions inthis case). In a defect free detector, where there is no chargetrapping, all the events are expected to be found in the sameenergy position but with different rise-times depending on thedepth of interactions essentially forming a vertically straightband of events parallel to the rise-time axis. In the presence ofdefects, charge carriers can be lost during the transit and theywill induce partial charge on the collecting electrode. Due tothis reason a deviation from the straight band towards the lowerenergy side is noticed. All these events are then shifted to thevertically straight band position by applying suitable correc-tion. The details about the biparametric correction scheme canbe found in [27]. Fig. 8(b) shows such a corrected biparametricplot corresponding to the one shown in Fig. 8(a). The corre-sponding corrected pulse-height spectrum is shown in Fig. 9.The pulse-height spectra indeed improved after applying thecorrection scheme. An improved FWHM of 1.8% along witha P/V ratio of and P/C ratio of 2.1 was obtained from thecorrected spectrum.

IV. CONCLUSION

Large volume detector grade CZT single crystals were grownusing a Te solvent method and radiation detectors were fabri-cated in small pixel and virtual Frisch grid geometry to elimi-nate the poor hole-movement related effects in the case of highenergy gamma ray detection. The crystals were characterizedwith resistivities of the order of -cm and elec-tron mobility-lifetime product and drift mobilities of the orderof cm /V and 840 cm /V.s respectively. The ef-fects of the high resistivity and high electron product anddrift-mobility values were reflected in the high resolution re-sponse of the detectors in single polarity charge sensing geome-tries for high-energy gamma rays. Detectors fabricated in smallpixel (Crystal A) and virtual Frisch grid (Crystal B) configura-tions were found to exhibit similar energy resolution of 3.7%for 662 keV gamma rays. Although the quality of the crystal Bwas better compared to that of the crystals A, the ultimate en-ergy resolution of the small pixel detector was found to be sim-ilar to that of the virtual Frisch grid detector due the smaller

2858 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 4, AUGUST 2013

active volume of the small pixel detector. It can also be no-ticed that the optimum operating bias for the Frisch detector( V) was much lower compared to the small pixel one( V). In order to investigate the effect of charge loss onthe energy resolution for these crystals, biparametric correla-tion studies were carried out for the small pixel detector. It wasobserved that charge trapping and charge loss has considerableeffect on the energy resolution of this detector. Offline digitalcorrection schemes were applied to the case of the small pixeldetector and considerable improvement was achieved in termsof energy resolution (1.8% at 662 keV).

REFERENCES

[1] T. E. Schlesinger, J. E. Toney, H. Yoon, E. Y. Lee, B. A. Brunett, L.Franks, and R. B. James, “Cadmium zinc telluride and its use as anuclear radiation detector material,” Mater. Sci. Eng. R, vol. 32, pp.103–189, 2001.

[2] A. E. Bolotnikov, J. Butcher, G. S. Camarda, Y. Cui, G. De Geronimo,J. Fried, R. Gul, P. M. Fochuk, M. Hamade, A. Hossain, K. H. Kim, O.V. Kopach, M. Petryk, E. Vernon, G. Yang, and R. B. James, “Arrayof virtual Frisch-grid CZT detectors with common cathode readout forcorrecting charge signals and rejection of incomplete charge-collectionevents,” IEEE Trans. Nucl. Sci., vol. 59, no. 6, pp. 1544–1551, Dec.2012.

[3] K. C. Mandal, S. H. Kang, M. Choi, J. Bello, L. Zheng, H. Zhang, M.Groza, U. N. Roy, A. Burger, D. E. Holcomb, G. W. Wright, and J. A.Williams, “Simulation, modeling, and crystal growth of Cd Zn Tefor nuclear spectrometers,” J. Electron. Mater., vol. 35, no. 6, pp.1251–1256, 2006.

[4] K. C. Mandal, S. H. Kang, M. Choi, A. Kargar, M. J. Harrison, D. S.McGregor, A. E. Bolotnikov, G. A. Carini, G. C. Camarda, and R. B.James, “Characterization of low-defect Cd Zn Te and CdTe crys-tals for high-performance Frisch collar detectors,” IEEE Trans. Nucl.Sci., vol. 54, no. 4, pp. 802–806, Aug. 2007.

[5] K. C. Mandal, R. M. Krishna, P. G. Muzykov, and T. C. Hayes, “Fabri-cation and characterization of Cd Zn Te Schottky diodes for highresolution nuclear radiation detectors,” IEEE Trans. Nucl. Sci., vol. 59,no. 4, pp. 1504–1509, Aug. 2012.

[6] H. Chen, S. A. Awadalla, F. Harris, L. Pinghe, R. Redden, G. Bindley,A. Copete, J. Hong, J. Grindlay, M. Amman, J. S. Lee, P. Luke,I. Kuvvetli, and C. Budtz-Jorgensen, “Spectral response of THMgrown CdZnTe crystals,” IEEE Trans. Nucl. Sci., vol. 55, no. 3, pp.1567–1572, Jun. 2008.

[7] P. J. Sellin, “Recent advances in compound semiconductor radiationdetectors,” Nucl. Instrum. Methods Phys. Res. A, vol. 513, no. 1-2, pp.332–339, 2003.

[8] R. M. Krishna, T. C. Hayes, P. G. Muzykov, and K. C. Mandal, “Lowtemperature crystal growth and characterization of Cd Zn Te forradiation detection applications,” in Proc. Mater. Res. Soc. Symp.,2011, vol. 1341, pp. 39–44.

[9] R. M. Krishna, P. G. Muzykov, and K. C. Mandal, “Electron beaminduced current imaging of dislocations in Cd Zn Te crystal,” J.Phys. Chem. Solids, vol. 74, no. 1, pp. 170–173, 2013.

[10] U. N. Roy, A. Gueorguiev, S. Weiler, and J. Stein, “Growth of spectro-scopic grade Cd Zn Te:In by THM technique,” J. Cryst. Growth,vol. 312, no. 1, pp. 33–36, 2009.

[11] D. S. McGregor, Z. He, H. A. Seifert, D. K. Wehe, and R. A. Rojeski,“Single charge carrier type sensing with a parallel strip pseudo-Frisch-grid CdZnTe semiconductor radiation detector,” Appl. Phys. Lett., vol.72, no. 7, pp. 792–794, 1998.

[12] W. J. McNiel, D. S. McGregor, A. E. Bolotnikov, G. W.Wright, and R.B. James, “Single-charge-carrier-type sensing with an insulated Frischring CdZnTe semiconductor radiation detector,” Appl. Phys. Lett., vol.84, no. 11, pp. 1988–1990, 2004.

[13] A. Kargar, A. M. Jones, W. J. McNeil, M. J. Harrison, and D. S. Mc-Gregor, “CdZnTe Frisch collar detectors for -ray spectroscopy,”Nucl.Instrum. Methods Phys. Res. A, vol. 558, no. 2, pp. 497–503, 2006.

[14] P. N. Luke, “Single-polarity charge sensing in ionization detectorsusing coplanar electrodes,” Appl. Phys. Lett., vol. 65, no. 22, pp.2884–2886, 1994.

[15] H. H. Barrett, J. D. Eskin, and H. B. Barber, “Charge transport in arraysof semiconductor gamma-ray detectors,” Phys. Rev. Lett., vol. 75, no.1, pp. 156–159, 1995.

[16] L. Verger, J. P. Bonnefoy, F. Glasser, and P. Ouvrier-Buffet, “New de-velopments in CdTe and CdZnTe detectors for X and -ray applica-tions,” J. Electron. Materials, vol. 26, no. 6, pp. 738–744, 1997.

[17] Z. Burshtein, H. N. Jayatirtha, A. Burger, J. F. Butler, B. Apotovsky,and F. P. Doty, “Charge carrier mobilities in Cd Zn Te single crys-tals used as nuclear radiation detectors,” Appl. Phys. Lett., vol. 63, no.1, pp. 102–104, 1993.

[18] S. D. Stearns, Digital Signal Analysis. New York, NY, USA: Hayden,1975.

[19] M. Nakhostin, “Recursive algorithms for real-time CR-(RC) digitalpulse shaping,” IEEE Trans. Nucl. Sci., vol. 58, no. 5, pp. 2378–2381,Oct. 2011.

[20] G. A. Carini, A. E. Bolotnikov, G. S. Camarda, G. W. Wright,R. B. James, and L. Li, “Effect of Te precipitates on the perfor-mance of CdZnTe detectors,” Appl. Phys. Lett., vol. 88, no. 14, pp.143515–143517, 2006.

[21] A. E. Bolotnikov, G. S. Camarda, G. A. Carini, Y. Cui, L. Li, and R.B. James, “Cumulative effects of Te precipitates in CdZnTe radiationdetectors,” Nucl. Instrum. Methods Phys. Res. A, vol. 571, no. 3, pp.687–698, 2007.

[22] J. Crocco, H. Bensalah, Q. Zheng, V. Corregidor, E. Avles, A. Castal-dini, B. Fraboni, D. Cavalcoli, A. Cavallini, O. Vela, and E. Dieguez,“Study of asymmetries of Cd(Zn)Te devices investigated using photo-induced current transient spectroscopy, Rutherford backscattering, sur-face photo-voltage spectroscopy, and gamma ray spectroscopies,” J.Appl. Phys., vol. 112, no. 7, pp. 074503-1–074503-9, 2012.

[23] A. Burger, M. Groza, Y. Cui, D. Hillman, E. Brewer, A. Billikiss, G.M.Wright, L. Li, F. Lu, and R. B. James, “Characterization of large singlecrystal gamma-ray detectors of cadmium zinc telluride,” J. Electron.Mater., vol. 32, no. 7, pp. 756–760, 2003.

[24] M. Groza, H. Krawczynski, A. Garson, J. W. Martin, K. Lee, Q. Li, M.Beilicke, Y. Cui, V. Buliga, M. Guo, C. Coca, and A. Burger, “Investi-gation of the internal electric field in cadmium zinc telluride detectorsusing Pockels effect and the analyses of charge transients,” J. Appl.Phys., vol. 107, no. 2, pp. 023704-1–023704-5, 2010.

[25] C. J. Sullivan, A. Burger, M. Groza, and T. H. Prettyman, “Bulk uni-formity of cadmium zinc telluride (CZT) crystals for large volumeco-planar gamma spectrometers,” presented at the IEEE Nuclear Sci-ence Symp. and Medical Imaging Conf., Honolulu, HI, USA, Oct.27–Nov. 3 2007, Symp. Conf. Rec., vol. 3, pp. 1805–1808.

[26] G. F. Knoll, Radiation Detection and Measurement, 3rd ed. NewYork, NY, USA: Wiley, 2000, p. 480.

[27] S. K. Chaudhuri, A. Lohstroh, M. Nakhostin, and P. J. Sellin, “Digitalpulse height correction in HgI -ray detectors,” J. Instrum., vol. 7, pp.T04002-1–T04002-13, 2012.

[28] U. N. Roy, S. Weiler, J. Stein, M. Groza, V. Buliga, and A. Burger,“Internal electric field estimation, charge transport and detector per-formance of as-grown Cd Zn Te:In by THM,” IEEE Trans. Nucl.Sci., vol. 58, no. 4, pp. 1949–1952, Aug. 2011.