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Sensors and Materials, Vol. 30, No. 7 (2018) 1605–1610
MYU Tokyo
S & M 1617

Carrier Transport Properties of CdTe Detector
under Polarization Condition
Hisaya Nakagawa,1* Tsuyoshi Terao,1 Tomoaki Masuzawa,2 Tetsu Ito,2Akifumi Koike,2,3 Hisashi Morii,2,3 and Toru Aoki1,2,3
1Graduate Schoool of Science and Technology, Shizuoka University,
3-5-1 Johoku, Nakaku, Hamamatsu, Shizuoka 432-8011, Japan
2Reserch Institute of Electronics, Shizuoka University,
3-5-1 Johoku, Nakaku, Hamamatsu, Shizuoka 432-8011, Japan
3ANSeeN Inc., 3-5-1 Johoku, Nakaku, Hamamatsu, Shizuoka 432-8011, Japan(Received February 1, 2018; accepted April 25, 2018)

Keywords: radiation detector, semiconductor detector, CdTe, polarization, carrier transport properties

Cadmium telluride (CdTe) is a promising material for semiconductor-based roomtemperature radiation detectors because of its wide bandgap and relatively good carriertransport properties. One remaining issue of CdTe detectors is an instability arising in longterm operation. This instability, called polarization, is explained by charge accumulation
due to carrier trapping/detrapping at deep defect levels. However, previous explanations did
not include carrier transport in CdTe. In this study, we have investigated carrier transport
properties by measuring the carrier transit time at each stage of polarization. As a result, wehave developed a polarization model including carrier transport and calculated the electric field
distribution across the CdTe detector during polarization.

1.

Introduction

Cadmium telluride (CdTe) is considered to be a suitable material for semiconductor-based
X-ray and gamma-ray detectors.(1–3) CdTe-based detectors have high X-/gamma-ray detection
efficiencies because CdTe is composed of high-atomic-number elements and have high density.
Moreover, CdTe can realize low thermal noise operation at room temperature owing to its
relatively wide bandgap of ~1.4 eV. Potential applications of such detectors range from medicaland dental radiography, computed tomography, to nondestructive inspection.
Despite these advantages, the application of CdTe-based radiation detectors is limited torelatively short time operations; a severe degradation of detector performance is known in
long-term operation and is called polarization.(1) The degradation caused by polarization
includes a decrease in peak count and the shifting of the peak energy over operation time. The
polarization is often explained using the charge accumulation model.(4–6) In this model, thepolarization phenomenon is explained by carrier trapping/detrapping at deep acceptor levels.
Electrons trapped in the deep acceptor levels act as negative space charges and alter the potential*

Corresponding author: e-mail: nakagawa.hisaya.14@shizuoka.ac.jp
http://dx.doi.org/10.18494/SAM.2018.1929
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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distribution across the CdTe detector. As the charges build up, the depletion layer width in
CdTe decreases, which prevents the collection of signal charges. However, the ohmic CdTe
detector has not been reported.(7) Therefore, many mechanisms have been suggested to explain
polarization and the details have been discussed.(4–9) In a previous study, a modified model ofpolarization was suggested from current–voltage (I–V) characteristics by Toyama et al.(8) Othersmeasured the electric field distribution in the detector under polarization conditions, utilizing
the Pockels effect.(10–12) However, few studies have focused on carrier transport and collection
during polarization.
In this study, we measured carrier transport properties during polarization by measuring
the pulse height and carrier transit time of signal charges. For this purpose, we developed anovel measurement system that enables the simultaneous measurement of both pulse height andcarrier transit time.(13) The carrier transport properties obtained using this system were applied
to the previous charge accumulation model(8) to estimate the electric field distribution in theCdTe detector to investigate the validity of this model.

2.

Materials and Methods

The principle of radiation detection in our system is described as follows. When radiation
is absorbed in the CdTe detector, multiple electron–hole pairs are generated. The generated
carriers are driven by the electric field in the CdTe detector and collected at the electrodesof the detector. The collected charges are converted to a signal pulse by a charge-sensitiveamplifier (CSA), and the pulse signals are analyzed with a signal processing system. Figure 1
shows the output pulse signal from the CSA. The height of the pulse signal is proportional tothe incident radiation energy, and the pulse rise time indicates the time needed to collect signalcarriers. The signal processing system is designed to measure both the height and rise time ofthe signal pulses simultaneously, so that carrier transit time can be measured at each degree ofpolarization.
The CdTe detectors used in this study were Schottky diodes based on a p-type crystal
fabricated by the travelling heater method (THM) at Acrorad Co., Ltd. Indium (In) wasdeposited on one side of the CdTe detector to form a Schottky electrode, and platinum (Pt)

Fig. 1. Schematic diagram of signal pulse from CSA. The novel signal processing system can yield the pulse
height and rise time as energy and carrier transit time, respectively.

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was deposited on the other side of the detector to serve as an ohmic electrode. The size of theCdTe detector was 4 × 4 mm2 and the thickness of CdTe was 0.5 mm. The CdTe detector wasconnected to the CSA (CLEAR-PULSE Co., Ltd., Model 5102) and the output signals from theCSA were introduced to a specially designed signal processing system (ANSeeN Inc. ANSHSMCA1-2D).
The measurement is conducted as follows. A reverse bias of 100 V is applied to the detector
to obtain the energy spectrum of a gamma-ray from 241Am radioisotopes. The detector isoperated for over 6 h to observe the progress of polarization over time. At the beginning, thedetector is assumed to be fully depleted by the applied bias. With time, polarization occurs andthe gamma-ray spectra measured by the detector show peak shifts and a reduction in energyresolution. The energy spectra of gamma-rays are obtained every 60 min to evaluate the effectof polarization over time. It should be noted that each gamma-ray spectrum is observed in 5
min so that the polarization condition does not change during each measurement.
In this study, gamma-rays from the 241Am radioisotope (59.5 keV) were measured atroom temperature. The gamma-rays from 241Am have relatively low energies and are mostly
absorbed near the surface of the CdTe detector. By irradiating either side of the CdTe detector,
we can select which carriers (electrons or holes) are transported longer in the CdTe detector.
When the In-deposited side of the detector is irradiated with gamma-rays, electrons are read out
immediately, and holes drift across the detector before being collected at the other side of thedetector. On the other hand, when the Pt-deposited side is irradiated, electrons drift across thedetector before being collected as the signal. Therefore, we can evaluate the carrier transport
properties of both electrons and holes by changing the irradiation conditions.

3.

Results and Discussion

Figure 2 shows energy spectra of gamma-rays from 241Am, obtained every 60 min. The
pulse height of radiation signals is converted to channel number, which corresponds to theenergy of the incident gamma-ray. Figure 2(a) shows the spectra obtained by irradiating the(a)(b)

Fig. 2. (color online) Pulse height spectrum for 241Am irradiation from (a) In-deposited side (anode side) and(b) Pt-deposited side (cathode side) with time. The channel number at the start of bias voltage application (0 min)corresponds to the 241Am photoelectric peak energy of 59.5 keV.

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gamma-rays from the In-deposited side of the CdTe detector, and Fig. 2(b) shows the spectrawith gamma-rays irradiated from the Pt-deposited side. It was confirmed that the peaks of241
Am gamma-rays shifted to a lower channel number (i.e., energy) and the counts of 241Am
peaks decreased with time. These features are characteristic of a detector suffering frompolarization. It was also confirmed that the effect of polarization became more significant as
the device was operated longer. Figure 3 shows the carrier transit time of the signal pulses forthe main peak (peak due to photoelectric absorption) after each measurement interval. Here,the transit time represented the rise time of the output signal pulse from the CSA. In the caseof In-deposted side irradiation, the carrier transit time did not change up to 120 min. However,the carrier transit time increased after 120 min. With an increase in operation time, the carriertransit time began to decrease. A similar result was obtained for Pt-deposited side irradiation.
No decrease in transit time beyond 180 min was expected. A possible explanation is as follows:at the early stage of polarization, electric field accumulates at the surface owing to the spacecharge, and the electric field across the CdTe detector weakens. This results in a longer carriertransit time. As the polarization advances, more space charges accumulate, and at some point,
the width of the depletion layer begins to decrease. Since the electric field exists only in thedepletion layer, carriers generated outside the depletion layer cannot be read out as a signal, andthose generated in the depletion layer will be read out. This results in a smaller carrier transport
length, and thus carrier transit time is shortened. The change in carrier transit time suggeststhat the polarization not only changes the intensity of electric field but also reduces the width ofthe depletion layer.
Since the reduction in depletion layer width was expected, we evaluated the depletion
layer width by integrating the peak count. The number of detected gamma-ray photons wasproportional to the depletion layer width. Figure 4 shows the change in the integrated count ofphotoelectric peaks obtained from the energy spectra of irradiation on each side. When 241Am
gamma-rays were irradiated on the In-deposited side of the CdTe detector, the integrated peak
number did not change with time. This result means that 241Am gamma-rays are absorbed near
the In-deposited side and cannot reach the Pt-deposited side of the CdTe detector. On the otherhand, in the case of Pt-deposited side irradiation, the integrated count started to decrease afterFig. 3. Transit time upon 241Am irradiation from each side with time. Circles correspond to In-deposited sideirradiation and crosses to Pt-deposited side irradiation.

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Fig. 4. Changes in counts of 241Am photoelectric peaks upon In-and Pt-deposited side irradiations. Circles
correspond to In-deposited side irradiation and crosses to Pt-deposited side irradiation.

120 min and was reduced by about 20% after 360 min. From the change in integrated peakcount, the depletion layer thickness was confirmed to decrease with time and the dead layer forinteraction was formed from the Pt-deposited side of the CdTe detector.
From these results, we estimated the polarization mechanism. The initial state is defined by
the depletion layer being fully formed in the CdTe detector, and the generated carriers can fully
be collected. When polarization begins, the potential of the CdTe detector is contorted by theaccumulated space charges. The electric field is changed by the accumulated charges, and thedrift velocity decreases. As the polarization proceeds further, the dead layer is formed and thedepletion layer width decreases. For these reasons, the carrier transit time becomes shorter than
the initial state and carriers are not collected sufficiently. To compare our estimation modelwith the existing model, the electric field distributions of the modified charge accumulation
model suggested by Toyama et al.(8) were calculated as
 qNTqNT  2t  VR
20≤ x≤λ− ε x + 2εD  D − ( D − λ ) exp(− τ )  + D ,ε ( x, t ) = 
− qNT 1 − exp(− t )  x + qNT  D 2 − D 2 + λ 2 exp(− t )  + VR , λ ≤ x ≤ Dτ τ  D2εD  ε ((1))

where q is the magnitude of electric charge, NT is the total concentration of deep acceptors, ϵis the dielectric constant of the semiconductor (ϵ = 10.2ϵ0 for CdTe, where ϵ0 is the dielectric
constant in vacuum), x is the position from the interface of the Schottky contact, D is thethickness of the CdTe detector, τ is the detrapping time, t is the operation time, VR is the reverse
bias voltage, and λ is the point of intersection between the deep acceptor level and the Fermilevel. Moreover, the carrier transit time was calculated from this distribution. Figure 5 shows
the carrier transit time derived using the modified charge accumulation model. The calculated
carrier transit time also gradually increases with time. In addition, the carrier transit timedecreases with increasing operation time. This result indicates almost the same tendency asthat of the carrier transit time obtained by our measurement.1610

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Fig. 5.

4.

Transit time calculated using charge accumulation model.

Conclusions

In this study, we measured changes in carrier transport properties during polarization, by
measuring both the pulse height and rise time of a signal pulse using a newly developed signal
processing system. The effect of polarization was confirmed by the shifted peak energy, lossof energy resolution, and the decrease in peak intensity in the energy spectra of gamma-rays
from 241Am. During polarization, the carrier transit time first became longer, but then became
shorter with time. This unexpected tendency was explained by the combination of a decreasein electric field across the CdTe detector and a decrease in depletion layer width. From theseresults, we estimated the mechanisms of polarization from carrier transport properties. The
electric field distribution was calculated using the model, which showed good agreement withthe measured carrier transit time and previous charge accumulation model.(8)Acknowledgments
This research is based on the Cooperative Research Project of Research Center for
Biomedical Engineering/Research Institute of Electronics, Shizuoka University.

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