Our fourth publication

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 3, JUNE 2008 1567
Spectral Response of THM Grown CdZnTe Crystals
Henry Chen, Salah A. Awadalla, Fraser Harris, Pinghe Lu, Robert Redden, Glenn Bindley, Antonio Copete,
Jaesub Hong, Jonathan Grindlay, Mark Amman, Julie S. Lee, Paul Luke, Irfan Kuvvetli, and Carl Budtz-Jorgensen
Abstract—The spectral response of several crystals grown by
the Traveling Heater Method (THM) were investigated. An energy
resolution of 0.98% for a Pseudo Frisch-Grid of 4 4 9 mm3
and 2.1% FWHM for a coplanar-grid of size 11 11 5 mm3
were measured using 137Cs-662 keV. In addition a 4% FWHM
at 122 keV has also been measured on 20 20 5 mm3
monolithic pixellated devices. The material shows great potential
toward producing large-volume detectors with spectral perfor-
mance that meets the requirement for high-resolution gamma-ray
spectroscopy.
Index Terms—Coplanar-grid, CZT, depth correction, gamma-
ray, THM.
I. INTRODUCTION
ONE of the most noticeable facts in the field of room
temperature semiconductor radiation detectors is the
increasing interest of major medical equipment, governments,
and academic research institutions, to develop CdZnTe (CZT)-
based spectrometer and imaging systems in an effort to replace
scintillator and cryogenic Ge based technologies. The qualities
of CZT as a room temperature radiation detector have been
reported for more than a decade [1]–[16]. Significant improve-
ments have been made in device design, detector array readout
electronics and crystal growth. However, the majority of the
progress has been primarily demonstrated on small quantities
of moderate volume single channel devices. Large volume
detectors exist but their usage has always been very limited due
to crystal yield, cost and availability issues.
In our previous publication [17]–[19] we have shown that
THM growth method is applicable for producing spectroscopic
CZT crystal with both good material uniformity and electron
transport properties. In this paper, we present the gamma re-
sponse of both single channel and monolithic segmented detec-
tors produced by the THM method. In addition, other charac-
terization results demonstrating the quality of the THM crystals
will also be discussed.
Manuscript received November 26, 2006; revised December 27, 2007.
H. Chen, S. A. Awadalla, F. Harris, P. Lu, R. Redden, and G. Bindley
are with Redlen Technologies, Sidney, BC V8L 5Y8 Canada (e-mail: henry.
chen@redlen.com; salah.awadalla@redlen.com; bob.redden@redlen.com;
glenn.bindley@redlen.com).
A. Copete, J. Hong, and J. Grindlay are with Harvard University, Cambridge,
MA 02138 USA (e-mail: acopete@cfa.harvard.edu; jhong@cfa.harvard.edu;
jgrindlay@cfa.harvard.edu).
M. Amman, J. S. Lee, and P. Luke are with Lawrence Berkeley National Lab-
oratory, Berkeley, CA 94720 USA (e-mail: mark_amman@lbl.gov; julie_lee@
lbl.gov; pnluke@lbl.gov).
I. Kuvvetli and C. Budtz-Jorgensen are with the Danish National Space
Center, Copenhagen, Denmark.
Digital Object Identifier 10.1109/TNS.2008.924089
Fig. 1. Schematic diagram of multi-channel analyzer.
II. DEVICE FABRICATION AND CHARACTERIZATION
Several tiles with different area and thickness ( mm ,
mm , mm , mm , mm ,
and mm ) were cut from the same wafer. The leakage
current along with the resistivity were measured for all tiles. The
mm , mm , and mm tiles were
fabricated as pseudo Frisch Grid form [20]–[31] where by the
cathode area is extended up to 2/3 of the device volume. The
mm and mm tiles were fabricated in
pixilated monolithic detectors of 4 4 and 8 8 pixels patterns.
The 8 8 pixel pattern is shown in Fig. 6. The mm
tile was fabricated as coplanar grid detectors as shown in Fig. 9.
The devices are characterized by different systems as discussed
below.
A. Single Channel
The detector performance of the pseudo Frisch Grid tiles was
tested using Cs and Co gamma sources through a multi-
channel analyzer (MCA) system. The MCA system combines
several electrical components to record the spectral response of
samples to radiation exposure. The block diagram of this system
is shown below in Fig. 1. The benefit of this type of measure-
ment in the industry is that the performance of different crystals
can be compared quantifiably and radioisotopes are identified
by their characteristic gamma energies.
Fig. 2 shows the Cs 662 keV response, 1.1% FWHM, at
780 V, 120s live time and 0.5 s shaping time, of pseudo Frisch
Grid mm [20]–[31]. In addition, an improved energy res-
olution was achieved by optimizing the aspect ratio of the de-
vice. Figs. 3 and 4 show the spectra of the mm and
mm of pseudo Frisch Grid devices. An energy resolu-
tion of 0.78% and 0.98% FWHM were obtained, using 1190 V,
120s live time and 0.5 s shaping time, at room temperature
without any additional signal processing correction that will be
0018-9499/$25.00 © 2008 IEEE

1568 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 3, JUNE 2008
Fig. 2. Cs 662 keV showing 1.1% FWHM spectrum of a pseudo Frisch Grid
3 2 3 2 5 mm CZT detector, at 780 V.
Fig. 3. Cs-662 keV showing a 0.78% FWHM of response of a Pseudo Frisch
Grid 3 2 3 2 9 mm CZT detector.
discussed at the end of the paper. This is a good indication of the
good charge transport properties of the material, specifically, for
electrons due to the single charge carrier nature of the device.
Notice that values of 0.78% FWHM and 3.37% FWTM have
rarely been reported in the literature for CZT detector (without
additional electronic signal correction). This evidence not only
demonstrates the good transport properties of THM CZT but
also demonstrates that it is possible to achieve sub-1% 662 keV
resolution at room temperature with CZT, a very important
requirement in advanced radiation detector applications. It
should be mentioned that while the photo-peak’s FWHM is
very narrow, a significant portion of the photo-peak events lie
in the tail and background region on the low energy side of the
peak. This is a common and unavoidable property of pseudo
Frisch Grid type of detectors as well as any detectors utilizing
the small-pixel effect when detecting high energy gamma rays
Fig. 4. Cs-662 keV showing a 0.98% FWHM of response of a Pseudo Frisch
Grid 4 2 4 2 9 mm CZT detector.
Fig. 5. Co 122 keV of the same detector as above, at 700 V, 130 s collection
and 0.5 s shaping time.
[32]. However, this drawback can in many cases be corrected
electronically via the use of bi-parametric (pulse height vs rise
time) correction, which lead to improved detector’s properties.
The Co response of the same device is shown in Fig. 5
where the 136 keV and the 14.4 keV peaks are well resolved
while the 122 keV main photo-peak exhibits a 2.8% FWHM
energy resolution with 7.8% FWTM. This is another indication
of the quality of THM grown CZT.
B. Pixellated Detector
To demonstrate more on the viability of THM grown CZT
crystals, a mm was fabricated with extended
shielding cathode as electrical field shaping to improve the edge
pixel performance. The detector was fabricated as 8 8 pixels
with pixel size of mm and 2.46 mm pitch as shown in
Fig. 6.

CHEN et al.: SPECTRAL RESPONSE OF THM GROWN CDZNTE CRYSTALS 1569
Fig. 6. The 20 22025 mm pixellated detector with additional electric field
shaping design on the edges.
Both the I-V and the energy resolution of these detectors
were carried out by the Harvard-Smithsonian Center for As-
trophysics group. The I-V measurement uses a computer-con-
trolled Keithley 237 source unit that measures the leakage cur-
rent on individual pixels, which are read sequentially through
a custom-made set of logic gates while the radiation setup has
the pixellated anode side mounted on a pogo pin fixture, with
the unsoldered electrical contact provided by a fuji-poly con-
nector. The pogo pins rest on a chip-on-board module whose
contacts are routed to a pair of ASICs, where charge signals are
collected, pre-amplified and read out by an IDEAS XaIm 3.2
system [33]. Fig. 7(a)–(c) show the I-V, done at 900 V, from
a mm pixellated detector shown above. As is
seen, the leakage current level of all the pixels are very low, in
the sub-nA level [Fig. 7(a)] which transfers to a very high de-
vice (apparent) resistivity, ohm-cm. In addition, our de-
vices have s semi-blocking contact at low bias that extends to be
Ohmic at higher; therefore, the I-V curve is nonlinear. The resis-
tivity and leakage current distribution of the pixel groups, inner,
second edge and edge, are show in Fig. 7(b) and (c), for com-
pleteness, respectively. Edge pixels have higher than average
leakage possibly due to surface contribution.
The spectra response of the detector is shown in Fig. 8 which
acquired using both Am and Co with 500 V applied bias
and 2 s shaping time, at room temperature, without any ad-
ditional signal processing correction scheme. The responses of
this pixellated monolithic detector can be clearly seen via the en-
ergy resolution of the Co 122 keV photo-peak and the Am
59.6 keV photo-peak as mapped out in Fig. 8(a) and (b) respec-
tively. Notice that the FWHM at 122 keV is as low as 3.4% min
to 4.7% max whereas that of 59.6 keV is 6.4% min to 8.6%
max. The distribution of the FWHM of all the pixels is shown
in Fig. 8(c) with the average FWHM of the detector at 122 keV
being 4% ( 4.8 keV). As is seen in the figure, the average
FWHM of the edge pixels is greater than middle and inner pixels
which could be an indication of an unfocused electrical field.
Again, this quality ( 4% FWHM @ 122 keV) for monolithic
pixellated CZT grown by THM is phenomenal, which demon-
strates not only high spectrometer grade performance but also
good material uniformity.
C. Coplanar Grid Detector
To further test the spectrometer capability of THM CZT, a
different yet very effective device for higher energy gamma
Fig. 7. I-V measurements of a typical 20 2 20 2 5 pixellated detector:
(a) leakage current; (b) resistivity; (c) leakage current distribution.
rays, the coplanar grid detector, has been fabricated. This effort
has been carried out at Lawrence Berkeley National Laboratory
(LBNL). The coplanar grid is a single carrier, electron, sensing
technique that can achieve a high uniform response. The details
of this technique have been reported in the literature [34]–[36].
Fig. 9 shows the mm coplanar grid detector
fabricated from THM CZT crystal. The room temperature Cs
662 keV spectrum, using detector bias of 700 V, grid bias of 34
V and shaping time of 2 s, is shown in Fig. 10, showing about
2% energy resolution. The coplanar-grid electrode pattern we

Fig. 8. Am and Co energy resolution of all pixels on a 2022025 mm
pixellated detector. (a) 59.6 keV min and max energy resolution of all the pixels;
(b) 122 keV. Min and Max energy resolution of all the pixels; (c) Distribution
of 122 keV and 59.6 keV energy resolution of all pixels.
Fig. 9. 11 211 25 mm coplanar grid detector fabricated from THM CZT.
used for this test was produced with the standard LBNL mask
set designed for a detector area of 10 mm 10 mm. This is not
optimal for this particular detector, and is likely to have caused
some degradation in the spectral response and contributing to
the pronounced “wing” near the base of the spectral line. The
result could have been significantly better if the grid pattern and
the size of the crystal were properly matched [34]. Nevertheless,
the result indicates that our THM CZT materials are promising
for the use as large-volume coplanar-grid detectors.
Through out the paper, it has been mentioned that the results
of the energy resolution were obtained without any additional
Fig. 10. Cs spectral response of the 11:5211:324:8 mm Coplanar-grid
detector fabricated from THM CZT.
electronic signal correction. To show the effect of the electronic
signal correction a mm tile from the same series
of growth conditions of the coplanar grid detector was fabri-
cated into 4 4 pixels with pixel size of mm , 2.5 mm
pitch and 100 m gap. The tile was sent overseas and the testing
was done by the Danish National Space Center. Testing was
carried out using Cs source by applying an in-house devel-
oped Depth of interaction using Drift Strip Method (DSM). The
total number of the strip, in DSM, are configured as the drift
detector cells with a group of 9 strips; 8 out of the 9 strips acts
as drift strip electrode and one as anode readout strip. The drift
strip electrodes are biased by a voltage divider that supplies each
drift strip with different bias whereas the anode strips are held
at ground potential. The drift electrodes and the planar elec-
trode are biased such that electrons are drifted toward the anode
readout strip [37]. Out of the 16 total pixels detector 12 pixels
(inner and edge) have FWHM 1% while the corner pixels have
FWHM of 1.3%–1.5%. This variation is possibly due to the
nonlinear electrical field distribution around the corner pixels.
Fig. 11 is an aliquot of the 4 4 pixels detector showing the
Cs spectra of the inner pixels obtained with 800 V applied
bias on the cathode and 1.2 s shaping time.
III. CONCLUSION
In conclusion, electron mobility-lifetime values and unifor-
mity have been measured in CZT crystals grown from THM
method. The material also exhibits a high value of resistivity
and structural qualities such as single crystal, macro-defect
free, good optical quality and relatively smaller Te precipitates
size and density. Large area and thick detectors have been
produced from which excellent spectrometer performance has
been achieved in both single channel and monolithic array
devices. The material shows great potential for producing
large-volume detectors with appropriate spectral performance
for high-resolution gamma-ray spectroscopy, for use in fields
and applications.

CHEN et al.: SPECTRAL RESPONSE OF THM GROWN CDZNTE CRYSTALS 1571
Fig. 11. Cs response at 0800 V bias of the inner pixels for monolithic 424 pixel using Depth of interaction method.
REFERENCES
[1] T. E. Schlesinger and R. B. James, Eds., Semiconductors for Room
Temperature Nuclear Detector Applications. New York: Academic,
1995, vol. 43, Semiconductors and Semimetals.
[2] R. B. James, Ed., Semiconductors for Room Temperature Radiation De-
tector Applications II. Pittsburg, PA: Materials Research Soc., 1998,
vol. 487.
[3] P. N. Luke, “Coplanar-grid CdZnTe detector with three-dimensional
position sensitivity,” Appl. Phys. Lett., vol. 65, p. 2884, 1994.
[4] H. Chen, S. U. Egarriewe, Z. Hu, J. Tong, D. T. Shi, G. H. Wu, K.
T. Chen, M. A. George, W. E. Collins, A. Burger, R. B. James, C. M.
Stahle, and L. M. Bartlett, “Study of gamma-ray detector performance
of Cd Zn Te crystal treated by different etchants,” Proc. SPIE, vol.
2859, p. 254, 1996.
[5] H. Chen, J. Tong, Z. Hu, D. T. Shi, K.-T. Chen, M. A. George, W.
E. Collins, A. Burger, R. B. James, C. M. Stahle, and L. M. Bartlett,
“Low temperature photoluminescence of detector grade Cd Zn Te
crystal treated by different chemical etchants,” J. Appl. Phys., vol. 80,
no. 6, p. 3509, Sep. 15, 1996.
[6] A. Burger, H. Chen, J. Tong, D. Shi, M. A. George, K.-T. Chen, W. E.
Collins, R. B. James, C. M. Stahle, and L. M. Bartlett, “Investigation of
electrical contacts for Cd Zn Te nuclear radiation detectors,” IEEE
Trans. Nucl. Sci., vol. 44, no. 3, pp. 934–938, Jun. 1997.
[7] Z. He, G. F. Knoll, D. K. Wehe, and J. Miyamoto, “Position-sensitive
single carrier CdZnTe detectors,” Nucl. Instrum. Meth. Phys. Res. A,
vol. 388, p. 180, 1997.
[8] D. S. McGregor, Z. He, H. A. Seifert, D. K. Wehe, and R. A. Rojeski,
“Single charge carrier type sensing with parallel strip pseudo-Frisch-
grid CdZnTe semiconductor radiation detector,” Appl. Phys. Lett., vol.
72, p. 792, 1998.
[9] H. Chen, K. Chattopadhyay, A. Burger, M. A. George, J. J. Weimer, J.
C. Gregory, P. Nag, and R. B. James, “Passivation of CdZnTe surfaces
by oxidation in low energy atomic oxygen,” J. Vac. Sci.Technol. A, vol.
17, no. 1, p. 97, Jan./Feb. 1999.
[10] A. Burger, H. Chen, K. Chattopadhyay, D. Shi, S. H. Morgan, W. E.
Collins, and R. B. James, “Characterization of metal contacts and sur-
face of Cadmium Zinc Telluride,” Nucl. Instrum. Meth. Phys. Res. A,
vol. 428, p. 8, 1999.
[11] P. N. Luke, M. Amman, J. S. Lee, and H. Yaver, “Coplanar-grid
CdZnTe detector with three-dimensional position sensitivity,” Nucl.
Instrum. Meth. Phys. Res. A, vol. 439, p. 611, 2000.
[12] 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 a nu-
clear radiation detector maerial,” Mater. Sci. Eng., vol. 32, p. 103, 2001.
[13] W. Li, Z. He, G. F. Knoll, D. K. Wehe, and J. E. Berry, “Experimental
results from an Imarad 8 28 pixellated CZT detector,” Nucl. Instrum.
Meth. Phys. Res. A, vol. 458, p. 518, 2001.
[14] J. L. Matteson, M. R. Pelling, and R. T. Skelton, “CZT detectors with
3-D readout for gamma-ray spectroscopy and imaging,” Proc. SPIE,
vol. 4784, p. 1, 2002.
[15] L. Verger, M. C. Gentet, L. Gerfault, R. Guillemaud, C. Mestais, O.
Monnet, G. Montemont, G. Petroz, J. P. Rostaing, and J. Rystiquue,
“Performance and perspective of a CdZnTe-based gamma camera for
medical imaging,” presented at the IEEE NSS/MIC Conf. Rec., 2003.
[16] F. Zhang, Z. He, and D. Xu, “Analysis of detector response using 3-D
position-sensitive CZT gamma-ray spectrometers,” IEEE Trans. Nucl.
Sci., vol. 51, no. 6, pp. 3098–3104, Dec. 2004.
[17] H. Chen, S. A. Awadalla, J. Mackenzie, R. Redden, G. Bindley, A. E.
Bolotnikov, G. S. Camarda, G. Carini, and R. B. James, “Characteri-
zation of Traveling Heater Method (THM) grown Cd Zn Te crys-
tals,” IEEE Trans. Nucl. Sci., vol. 54, no. 4, pp. 811–816, Aug. 2007.
[18] H. Chen, S. A. Awadalla, K. Iniewski, P. H. Lu, F. Harris, J. Mackenzie,
T. Hasanan, W. Chen, R. Redden, G. Bindley, I. Kuvveti, C. B. Jor-
gensen, P. Luke, M. Amman, J. S. Lee, A. E. Bolotnikov, G. S. Ca-
marda, G. Carini, and R. B. James, “Characterization of large CdZnTe
crystals grown by Traveling Heater Method (THM),” Proc, SPIE, vol.
6706, p. 1, 2007.
[19] P. J. Sellin, A. Davies, A. Lohstroh, M. E. Ozsan, J. Parkin, V. Perumal,
M. Veale, H. Chen, S. A. Awadalla, J. Mackenzie, R. Redden, and G.
Bindley, “Photoluminescence and charge transport studies of CdZnTe
grown by the travelling heater method,” presented at the IEEE Conf.
Rec., 2007.
[20] P. Chenvast, U. El-Hanany, D. Schneider, and R. Triboulet, “CdTe and
CdZnTe crystal growth by horizontal bridgman technique,” J. Cryst.
Growth, vol. 101, p. 270, 1990.
[21] U. El-Hanany, A. Shahar, and A. Tsigelman, “CZT detectors equipped
with effective Ohmic contact; their spectroscopic performance and the
enigma of why theythus behave,” Proc. SPIE, vol. 3768, p. 2, 1999.

[22] L. Li, F. Lu, C. Lel, G. Wright, D. R. Rhiger, S. Sen, K. Shah, M.
R. Squillante, L. Cirignano, R. B. James, A. Burger, P. Luke, and R.
Olson, “Development of large single crystal (3-inch ingots) CdZnTe
for large volume nuclear radiation detectors,” Proc. SPIE, vol. 4784, p.
76, 2002.
[23] C. M. Greaves, B. A. Brunett, J. M. Van Scyoc, T. E. Schlesinger, and
R. B. James, “Material uniformity of CdZnTe grown by low-pressure
Bridgman,” Nucl. Instrum. Meth. Phys. Res. A, vol. 458, p. 96, 2001.
[24] E. Raiskin and J. Butler, “CdTe low level gamma detectors based on a
new crystal growth method,” IEEE Trans. Nucl. Sci., vol. 35, no. 1, pp.
81–84, Feb. 1988.
[25] F. P. Doty, J. F. Butler, J. F. Schetzina, and K. A. Bowers, “Properties
of CdZnTe crystals grown by a high pressure Bridgman method,” J.
Vac. Sci. Technol. B, vol. 10, no. 4, p. 1418, 1992.
[26] C. J. Johnson, E. E. Eissler, S. F. Cameron, Y. Kong, S. Fan, S. Ja-
vanovic, and K. G. Lynn, Semiconductor for Room Temperature Radi-
ation Detector Applications, R. B. James, T. E. Schlesinger, P. Siffert,
and L. Frank, Eds. Pittsburgh, PA: Materials Research Soc., 1993,
vol. 302, p. 463.
[27] C. Sezeles and E. E. Eissler, “Current issues of high pressure Bridgman
growth of semi-insulating CdZnTe,” in Proc. MRS Symp., 1998, vol.
487, p. 3.
[28] M. Ohmari, Y. Iwase, and R. Ohno, “High quality CdTe and its appli-
cation to radiation detectors,” Mater. Sci. Eng., vol. B16, p. 283, 1993.
[29] R. Triboulet, “The Travelling Heater Method (THM) for Hg Cd Te
and related materials,” Prog. Cryst. Growth Char. Matter, vol. 128, p.
85, 1994.
[30] S. McGregor and R. A. Rojeski, “High-resolution ionization detector
and array of such detectors,” U.S. Patent 6 175 120, Jan. 6, 2001.
[31] G. Montémont, M. Arques, L. Verger, and J. Rustique, “A capacitive
Frisch grid structure for CdZnTe detectors,” IEEE Trans. Nucl. Sci.,
vol. 48, no. 3, pp. 278–281, Jun. 2001.
[32] P. N. Luke, “Electrode conﬁguration and energy resolution in
gamma-ray detectors,” Nucl. Instrum. Meth. A, vol. 380, pp. 232–237,
1996.
[33] S. V. Vadawale, J. Hong, J. Grindlay, P. Williams, M. Zhang, E. Bellm,
T. Narita, W. Craig, B. Parker, C. Stahle, and F. Yan, “Multipixel char-
acterization of imaging CZT detectors for hard X-ray imaging and
spectroscopy,” Proc. SPIE, vol. 5540, p. 22, 2004.
[34] P. N. Luke, “Unipolar charge sensing with coplanar electrodes appli-
cation to semiconductor detectors,” IEEE Trans. Nucl. Sci., vol. 42, no.
4, pp. 207–213, Aug. 1995.
[35] P. N. Luke, M. Amman, T. H. Prettyman, P. A. Russo, and D. A. Close,
“Electrode design for coplanar-grid detectors,” IEEE Trans. Nucl. Sci.,
vol. 44, no. 3, pp. 713–720, Jun. 1997.
[36] P. N. Luke, M. Amman, J. S. Lee, B. A. Ludewigt, and H. Yaver, “A
CdZnTe coplanar-grid detector array for environmental remediation,”
Nucl. Instrum. Meth. Phys. Res. A, vol. 458, p. 319, 2001.
[37] M. A. J. van Pamelen and C. Budtz-Joergensen, “CdZnTe drift detector
with correction for hole trapping,” Nucl. Instrum. Meth. A, vol. 411, pp.
197–200, 1998.

Our fourth publication

More Related Content

What's hot

Viewers also liked

Similar to Our fourth publication

Our fourth publication