APPROACHING CRYOGENIC GE PERFORMANCE WITH PELTIER COOLED CDTE Page: 3 of 9
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2. CdTe P-I-N STRUCTURES
Recent remarkable progress has been achieved with CdZnTe (conduction counter) and CdTe (p-i-n) structures in the x-ray
and gamma ray spectroscopy area. CdZnTe can provide resistivity as high as 1011 Q-cm. Despite the use of a conduction
counter detector configuration, at -40 C with 5x103 V/cm electric field, the leakage current can be suppressed to the degree
that 240 eV FWHM at 5.9 keV is possible [5]. CdTe compensated with Cl can achieve a resistivity of about 109 Q-cm. The
use of p-i-n junction structures helps to control the leakage current and allows the use of higher electric fields. Optimally,
detectors should operate at electric field values assuring the saturation of the drift velocity of charge carriers. For CdTe
crystals this value is around 15 - 20 kV/cm, thus for a p-i-n structure with a 3-mm thick i-region, the p-i-n diode should not
exhibit a breakdown under a reverse bias condition at voltages below 3kV (for planar geometry). We have developed
fabrication technology for p-i-n detectors that satisfy these requirements. Also, we have incorporated detector surface
passivation providing long-term stability of the leakage current.
The CdTe p-i-n structures presented in this paper can tolerate bias voltages of 2.5 - 3.0 kV with leakage currents much lower
than 107"A at - 30 0 to - 40 C. This feature, in combination with the good charge collection for both electrons and holes
leads to construction of high-energy resolution spectrometers. In addition to good energy resolution there are a number of
other detector characteristics that are of particular high importance for commercial devices, including: a) stability of detector
characteristics over a long operating time b) good peak-to-background ratio and c) symmetry of peaks in the collected
spectra.
3. MEASUREMENTS OF MOBILITY-TRAPPING TIME PRODUCTS
The selection of a starting material for fabrication detectors includes measurements of pgr products and their non-
uniformities. The measurements are performed with alpha particles separately for electron and holes. The results of
measurements are fitted to the Hecht relation [6] from which values of (pt)e and (pT)0h are derived. The non-uniformity of
the charge collection parameters are measured by scanning of the detector with a collimated alpha source. Since the current
method for measurement of mobility-lifetime products for electrons and holes in CdTe and their distribution in the crystals
give reliable results, it is possible to predict the spectral characteristics of a detector produced from each crystal with a
sufficient accuracy.
Calculated spectrum
-- rna spectrum
FW HM818 keV
-_Iherebcal spectrum
3(00- FVMM:.22 keV
1000-
0
1)0 1)0 170 1800 19)0 2000 2100 22)0
Chanel number
Figure 1: Calculated vs. experimental spectrum obtained from CdTe detector as a response to
662 keV y-rays.
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Khusainov, A. K. (A. Kh.); Iwanczyk, J. S. (Jan S.); Patt, B. E. (Bradley E.); Prirogov, A. M. (Alexandre M.) & Vo, Duc T. APPROACHING CRYOGENIC GE PERFORMANCE WITH PELTIER COOLED CDTE, article, January 1, 2001; United States. (https://digital.library.unt.edu/ark:/67531/metadc927458/m1/3/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.