Te inclusion-induced electrical field perturbation in CdZnTe single crystals revealed by Kelvin probe force microscopy

Micron 88 (2016) 48–53

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Te inclusion-induced electrical field perturbation in CdZnTe single crystals revealed by Kelvin probe force microscopy Yaxu Gu a,b , Wanqi Jie a,b,∗ , Linglong Li c , Yadong Xu a,b,∗ , Yaodong Yang c , Jie Ren a , Gangqiang Zha a,b , Tao Wang a,b , Lingyan Xu a,b , Yihui He a,b , Shouzhi Xi a,b a State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China b Key Laboratory of Radiation Detection Materials and Devices of Ministry of Industry and Information Technology, Northwestern Polytechnical University, Xi’an 710072, PR China c Multidisciplinary Material Research Center, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710049, PR China

a r t i c l e

i n f o

Article history: Received 17 May 2016 Received in revised form 10 June 2016 Accepted 10 June 2016 Available online 16 June 2016 Keywords: CdZnTe Te inclusion Kelvin probe force microscopy Electrical property Bias dependent

a b s t r a c t To understand the effects of tellurium (Te) inclusions on the device performance of CdZnTe radiation detectors, the perturbation of the electrical field in and around Te inclusions was studied in CdZnTe single crystals via Kelvin probe force microscopy (KPFM). Te inclusions were proved to act as lower potential centers with respect to surrounding CdZnTe matrix. Based on the KPFM results, the energy band diagram at the Te/CdZnTe interface was established, and the bias-dependent effects of Te inclusion on carrier transportation is discussed. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction II–VI compound semiconductors, CdTe and CdZnTe (CZT), have been widely used for hard X- and gamma-ray detection (Franc et al., 1999). Melt growth under a slight excess of tellurium is often adopted to grow high resistivity CdTe and CdZnTe ingots by forming Te antisites (Lindström et al., 2016). However, one of the limitations of this method is the trapping of Te-rich droplets by the growth interface (Rudolph et al., 1995). Therefore, Te inclusions, usually in the size of 1–50 ␮m, exist prevalently in the as-grown crystals. Much attention has been devoted to the influence of Te inclusions on the local carrier transport properties both experimentally (Carini et al., 2007) and numerically (Bolotnikov et al., 2007a). Te inclusions were found to reduce the local charge collection efficiency (CCE) (Amman et al., 2002), and thus to degrade the energy resolution of CdZnTe radiation detectors (Bolotnikov et al., 2007b). For example, Carini et al. (Carini et al., 2006) and Hansson et al. (Hansson et al., 2012) observed a clear one to one

∗ Corresponding authors at: State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China. E-mail addresses: [email protected] (W. Jie), [email protected] (Y. Xu). http://dx.doi.org/10.1016/j.micron.2016.06.001 0968-4328/© 2016 Elsevier Ltd. All rights reserved.

correspondence between the low CCE regions and the location of Te inclusion using X-ray response mapping. Bolotnikov et al. (Bolotnikov et al., 2007c) proposed a geometry model to explain charge loss induced by Te inclusion, and simulated the effects of size and density of Te inclusions on gamma-ray spectra. Bale (Bale, 2010) further modeled the cumulative effect of Te inclusions in CdZnTe radiation defectors. A noticeable temperature-dependent influence of Te inclusions on charge collection was observed and attributed to polarization effect by Hossain et al. (Hossain et al., 2011). Even though progress has been made on understanding the role of Te inclusions, little is known on the electrical behaviors of Te inclusions in CdZnTe crystals. Since electrical field plays an important role in the charge collection property of radiation-generated carriers (Cola and Farella, 2013), revealing the effect of Te inclusions on the electrical field is therefore of importance to optimize the operation properties of CdZnTe detectors. Besides, it may also advance the knowledge of nanoscale Te precipitates (Rudolph et al., 1993) and other inclusions in corresponding devices, e.g., indium inclusions in InN (Liu et al., 2012) and carbon inclusions in GaN (Lefeld-Sosnowska and Frymark, 2001). In this work, Te inclusion-induced electrical property variation in CdZnTe single crystals is studied via KPFM. The measured potential distribution in and around Te inclusions is analyzed

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drift velocity, and further deteriorate the imaging performance in pixelated detectors (Kim et al., 2011). Fig. 2(c) and (d) shows the topography and potential profile along the dotted line in Fig. 2(b). The dotted line is chosen perpendicular to the Te/CdZnTe boundary. According to the tetrakaidecahedron model on the morphology evolution of Te inclusions (He et al., 2012), Te inclusions are embodied with oriented CdZnTe planes, preferentially with the orientations −

Fig. 1. Schematic diagram of KPFM scanning over a Te inclusion embedded on cleaved (110) CdZnTe surface.

by employing the Poisson’s equation, and possible carrier transportation across Te inclusion is discussed. Based on KPFM results, electrical potential distribution around Te inclusions under different biases are numerically simulated using the software COMSOL Multiphysics 4.3a. 2. Experimental Cd0.9 Zn0.1 Te: In single crystals grown by modified vertical Bridgman method in Imdetek were used in the experiments. The effective doping level of In in the as-grown ingot is about 30 ppm. To avoid the influence of surface contamination on KPFM results, CdZnTe samples were kept in nitrogen atmosphere immediately after cleavage in clear and dry air. KPFM measurements were carried out on a commercial AFM platform (Asylum, Cypher S) as illustrated in Fig. 1. Te inclusion with a flat cross-section was observed on the cleaved (110) CdZnTe surface, which meets the requirement of surface evenness of KPFM test and subsequent interfacial analysis. Since (110) CdZnTe surface is electrically neutral and is free of charged states, the influence of surface dipole as well as charged adsorbate on the measured potential distribution can therefore be minimized. Frequency modulation (FM) mode was used in KPFM measurements to achieve high spatial resolution. The lift height of Ti/Ir-coated Si probe was set to be about 20 nm, and the applied ac voltage was 800 mV in all experiments. Potential fluctuation caused by systematic noise is below 0.50 mV, and the vertical noise floor in Z direction is less than 15 pm. Fig. 2(a) and (b) shows the topography and surface potential (or Kelvin potential) in and around a Te inclusion on the cleaved (110) CdZnTe surface, respectively. The root-mean-square roughness of CdZnTe surface in Fig. 2(a) is about 0.14 nm. 3. Results and discussion 3.1. Electrical potential distribution at Te/CdZnTe interface Corresponding to the topography of Te inclusion shown in Fig. 2(a), surface potential distribution is obtained with KPFM (Fig. 2(b)), which demonstrates that Te inclusions act as lower potential centers in CdZnTe crystal. According to the results of Gaussian fitting to the histogram of pixel potentials in Fig. 2(b), the potential of Te inclusion is about 0.32 V lower than that of CdZnTe matrix. Moreover, the low potential region around the Te inclusion extends into CdZnTe matrix for several ␮m. This electrical field inhomogeneity will expectedly cause a fluctuation of electron

{0111}Te {111}CdZnTe and {0001}Te {100}CdZnTe . Since CdZnTe lattice is body-centred cubic, the cleaved (110) CdZnTe surface or Te cross-section is thus perpendicular to corresponding intersecting Te/CdZnTe interfaces. Therefore, the dotted line is perpendicular to the intersecting Te/CdZnTe interface, which is beneficial to employ one-dimensional approximation in the analysis of Te/CdZnTe interface. To fully understand Te/CdZnTe interfacial electrical properties, such as electrical field and space charge density distribution, the potential profile in Fig. 2(d) are further analyzed by solving the one-dimensional Poisson’s equation. Due to the influence of surface undulation (Li and Li, 2005), however, potential profile within Te inclusion fluctuates substantially, which introduces significant noise to the calculated electrical properties. In order to suppress the influence of this unreasonable potential fluctuation on the calculated electrical field and the space-charge distribution, the Savitzky-Golay (SG) smoothing (Luo et al., 2005) with the even number m = 7, polynomial of degree p = 2 and differential order d = 0 was performed on the measured potential profile in Fig. 2(d). The smoothed potential profile is shown by the dotted line in Fig. 2(d). Given the dielectric constants εCZT = 10.4 (Li et al., 2012), εTe = 30.0 (Madelung, 1996) and ε0 = 8.9 × 10−14 F cm−1 , the calculated electrical field and space-charge density profile are obtained (see Fig. 2(e)–(f)). Fig. 2(e) shows that the electrical field reaches up to its maximum of about 2000 V cm−1 at Te/CdZnTe interface. The calculated space-charge distribution suggests that high density of positive space charge exists in CdZnTe crystals, while negative space charge accumulates at Te inclusion side (see Fig. 2(f)). The space-charge density in CdZnTe matrix reaches up to 6 × 1012 q0 ·cm−3 at 5 ␮m and 13.5 ␮m in X axis, with the elementary charge q0 . This value is about 2–3 orders of magnitude higher than the fully-depleted CdZnTe bulk crystals measured using transient current technique by Uxa et al. (Uxa et al., 2012) or the results of Pockels electrooptical technique by Sellin and Franc et al. (Franc et al., 2011; Sellin et al., 2010), which indicates a higher density of defects in the region surrounding Te inclusions. 3.2. Energy band diagram and proposed carrier transport process across Te/CdZnTe interface The surface potential obtained from KPFM presents the variation of local vacuum level (Cahen and Kahn, 2003). Since the electron affinity stays constant for a given surface, the measured potential distribution thus further predicts the energy band alignment of the regions investigated. The change of local vacuum level Evac , usually expressed as qVbi , corresponds to the variation of potential profile in Fig. 2(d). Furthermore, the potential difference is mainly distributed near Te/CdZnTe boundary, and thus the band bending across the interface can be determined accordingly, which provides guidance for understanding the negative effects of Te inclusions. A qualitative energy band diagram across Te/CdZnTe interface (Fig. 3(a)) is proposed based on the potential profile shown in Fig. 2(d). As seen in Fig. 3(a), a downward band bending from the interface to CdZnTe crystals acts as a potential well for holes, which demonstrates that severe hole trapping can be caused by Te inclusions (Bolotnikov et al., 2009). Besides, X-ray response

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Fig. 2. KPFM measurement of a Te inclusion on the cleaved (110) CdZnTe surface (a) topography, (b) surface potential, (c) topography profile, (d) surface potential profile along the dotted line in (b), (e) corresponding calculated electrical field profile, (f) space-charge density profile along the dotted line in (b). The overall scan area is 20 × 20 ␮m2 . The vertical dot-dash lines in (c)–(f) suggest the boundary between Te inclusion and CdZnTe crystal.

Fig. 3. Schematic diagram of energy band alignments at Te/CdZnTe interface (a) without applied bias, (b) with applied bias of V.

map for holes reveals that the sizes of degraded CCE regions are much larger than the physical sizes of Te inclusions (Bolotnikov et al., 2009). Lower potential regions around Te inclusions may steer holes into Te inclusions, and should be responsible for the

enlarged CCE degradation area. Furthermore, the electron transport process across Te/CdZnTe interface was analyzed. In principle, three processes, namely diffusion, thermionic emission and tunneling, are considered to determine the carrier transport across

Table 1 Parameters used in the electrostatic simulation using COMSOL Multiphysics 4.3a. Relative permittivity ␧Te

␧CdZnTe

30

10.2

Surface charge density in Te (C cm−2 )

Space charge density around Te (C cm−3 )

Thickness of Au electrode (␮m)

Size of mesh element (␮m)

1.0E-5

0.85

0.1

0.0075 ≤ L ≤ 0.5

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Fig. 4. Geometry model and calculated electrical potential distribution in and around a Te inclusion in CdZnTe single crystals. (a) Schematic diagram of geometry model used in COMSOL Multiphysics, (b) electrical potential distribution under the applied bias of 1 V, electrical potential profile along the dashed line in (a) under the electrical field of (c) 70 V cm−1 and (d) 700 V cm−1 .

a barrier. Given the temperature T = 300 K, the effective mass of electrons me * /m0 = 0.14 (Schlesinger et al., 2001), electron mobility e = 1000 cm2 V−1 s−1 (Szeles, 2004), electron lifetime  e = 1 ␮s (Szeles, 2004) and k0 = 8.617 × 10−5 eV K−1 , the diffusion length Ln and the mean free path of electron n are calculated to be 51.0 ␮m and 25.9 nm based on the Einstein relation (Sze and Ng, 2006), respectively. The barrier width xd in CdZnTe crystals is observed to be about 2 ␮m, as illustrated in Fig. 2(d). Therefore, a qualitative relation can be obtained as follows, Ln  xd  n , suggesting that electron trapping due to the process of thermionic emission

and tunneling may be negligible compared to the diffusion. Consequently, part of drifting electrons is expected to diffuse into Te inclusions under low applied bias, leading to the degradation of local charge collection efficiency. This is supported by Carini et al. using high-resolution X-ray mapping that charge loss induced by Te inclusions is possible even at applied bias as low as −25 V for planar CdZnTe detectors (Carini et al., 2007). The fact that Te inclusions trap both electrons and holes suggests its role as recombination centers, as also proven by cathodoluminescence (Fernandez, 2003; He et al., 2014). In addition to the

Fig. 5. (a) Variation of potential barrier qV’CZT under different electrical field simulated using COMSOL Multiphysics 4.3a; (b) change of charge loss induced by Te inclusion with increasing applied biases measured by ion beam induced charge technique. Fig. 5(b) reprinted with permission from Ref. (Gu et al., 2015). Copyright 2015, Elsevier.

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band bending, the significant difference of bandgaps between Te inclusion and CdZnTe crystal may also play a crucial role in determining the feature of recombination centers for Te inclusions. As shown in Fig. 3(a), the band gap of tellurium (Eg(Te) = 0.33 eV) (Anzin et al., 1977) is much smaller than that of CdZnTe single crystals (Eg(CZT) = 1.56 eV) (Xu et al., 2013), suggesting that Te inclusion itself acts as potential well both for electrons and holes. Also, the conduction type of tellurium is p-type, whereas CdZnTe crystal is often of weak n-type with its Fermi level pinned at Ec -0.74 eV close to the middle of bandgap at room temperature. By shifting the Fermi level in bandgap of Te inclusions, the conduction-band offset Ec and valence-band offset Ev for Te/CdZnTe contact can be roughly estimated to be at least 0.92 eV and 0.30 eV, respectively. This means the band offsets of Te/CdZnTe are much larger than the carrier’s thermal activation energy 0.026 eV at room temperature, and carriers can hardly be detrapped from Te inclusions. The combined effects of remarkable bandgap difference and distinct band offset between Te and CdZnTe crystal determine Te inclusions as recombination centers. 3.3. Numberical simulation of potential barriers qVCZT under different applied biases The aforementioned analysis of energy band diagram at Te/CdZnTe interface is based on KPFM results where no electrical field is applied to CdZnTe crystals. However, CdZnTe radiation detectors are usually operated at applied bias of several hundreds of Volts to realize high detection efficiency and energy resolution. In this case, potential difference at the cathode side VCZT will be reduced by the external electrical field, and potential barrier qVCZT decreases to qV’CZT accordingly, as shown in Fig. 3(b). When the potential barrier qV’CZT is offset by the external electrical field, trapping probability of drifting electrons will supposedly increase. In order to determine the critical field strength that nullifies the potential barrier qV’CZT , electric potential distribution around a Te inclusion embedded in CdZnTe planar detector is simulated under different applied biases by using the commercial software COMSOL Multiphysics 4.3a. The size of Te inclusion is set to be 8 ␮m, and the thickness of CdZnTe detector and Au electrode is 100 ␮m and 0.1 ␮m, respectively. Corresponding geometry of the model is illustrated in Fig. 4(a). In the simulation, the space charge density in CdZnTe crystal is employed with reference to the values calculated from KPFM results. Besides, since the space charge density in Te inclusions contributes little to the potential difference or potential barrier qV’CZT in CdZnTe crystal, we substitute it with a surface charge density to reduce the computation burden. The parameters used for the simulation are listed in Table 1. Fig. 4(b) shows the simulated potential distribution under the electrical field of 100 V cm−1 . Fig. 4(c) and (d) shows the potential profiles along the dashed line in Fig. 4(a) under the electrical field of 70 V cm−1 and 700 V cm−1 , respectively. As expected in Fig. 3(b), V’CZT is determined to be the difference between the highest potential in CdZnTe crystal and that at the interface both on the cathode side (Fig. 4(c)). Note that the potential difference V’CZT is totally offset under electrical field of 700 V cm−1 . Fig. 5(a) shows the variation of simulated potential barrier qV’CZT under different external electrical field. It is shown that qV’CZT decreases from about 0.28 eV to 0 as the electrical field increases from 0 to 700 V cm−1 . In particular, corresponding potential barrier qV’CZT is predicted to be 0.03 eV at an electrical field of 500 V cm−1 , approximately equal to the thermal energy of the charge carriers (0.026 eV) at room temperature. Therefore, Te inclusion-induced charge loss is expected to increase significantly under electrical fields from 500 V cm−1 to 700 V cm−1 . Measured charge loss induced by Te inclusions using ion beam induced charge (IBIC) demonstrate that there is a sharp increase of charge loss at applied

biases between 80 V and 150 V, equivalent to the electrical field of 400 V cm−1 to 750 V cm−1 (Gu et al., 2015), as shown in Fig. 5(b). Though the effect of Te inclusions on carrier transport is quite a complex problem, which may be complicated by its shape, size, location and its interaction with other defects, etc, the agreement between the ranges of applied electrical field in Fig. 5(a) and (b) suggests that the potential barrier qV’CZT around Te inclusion may also play an important role on carrier transportation in CdZnTe radiation detectors.

4. Conclusions We demonstrate that Te inclusions induce severe electrical field non-uniformity and act as lower potential centers in CdZnTe single crystals. Charge carrier transportation near Te inclusions is analyzed on basis of the energy band diagram at Te/CdZnTe interface. By calculating the variation of potential distribution around Te inclusion under different bias voltages, it was suggested that the potential barrier at Te/CdZnTe interface may play an important role on carrier transportation in CdZnTe radiation detectors.

Acknowledgements This work was supported by the National Natural Science Foundations of China (Nos. 51202197 and 51372205). Work was also supported by the 111 Project of China (No. B08040) and by the Natural Science Basic Research Plan in Shaanxi Province of China (2016KJXX-09).

References Amman, M., Lee, J.S., Luke, P.N., 2002. Electron trapping nonuniformity in high-pressure-Bridgman-grown CdZnTe. J. Appl. Phys. 92, 3198–3206. Anzin, V.B., Eremets, M.I., Kosichkin, Y.V., Nadezhdinskii, A.I., Shirokov, A.M., 1977. Measurement of energy-gap in tellurium under pressure. Phys. Status Solidi A: Appl. Res. 42, 385–390. Bale, D.S., 2010. Fluctuations in induced charge introduced by Te inclusions within CdZnTe radiation detectors. J. Appl. Phys. 108, 024504–024508. Bolotnikov, A.E., Camarda, G.S., Carini, G.A., Cui, Y., Li, L., James, R.B., 2007a. Modelling the effects of Te precipitates on the electron transport in CdZnTe radiation detectors. Nucl. Instrum. Methods Phys. Res. Sect. A 579, 125–129. Bolotnikov, A., Abdul-Jabber, N., Babalola, S., Camarda, G., Cui, Y., Hossain, A., Jackson, E., James, J., Konman, K., Luryi, A., 2007b. Effects of Te inclusions on the performance of CdZnTe radiation detectors. Nuclear Science Symposium Conference Record NSS’07. IEEE vol. 3, 1788–1797. Bolotnikov, A.E., Camarda, G.S., Carini, G.A., Cui, Y., Li, L., James, R.B., 2007c. Cumulative effects of Te precipitates in CdZnTe radiation detectors. Nucl. Instrum. Methods Phys. Res. Sect. A 571, 687–698. Bolotnikov, A.E., et al., 2009. Extended defects in CdZnTe radiation detectors. IEEE Trans. Nucl. Sci. 56, 1775–1783. Cahen, D., Kahn, A., 2003. Electron energetics at surfaces and interfaces: concepts and experiments. Adv. Mater. 15, 271–277. Carini, G.A., Bolotnikov, A.E., Camarda, G.S., Wright, G.W., James, R.B., Li, L., 2006. Effect of Te precipitates on the performance of CdZnTe detectors. Appl. Phys. Lett. 88, 143515–1435153. Carini, G.A., Bolotnikov, A.E., Camarda, G.S., James, R.B., 2007. High-resolution X-ray mapping of CdZnTe detectors. Nucl. Instrum. Methods Phys. Res. Sect. A 579, 120–124. Cola, A., Farella, I., 2013. Electrical fields and dominant carrier transport mechanisms in CdTe Schottky detectors. Appl. Phys. Lett. 102, 113502. Fernandez, P., 2003. Defect structure and luminescence properties of CdTe based compounds. J. Optoelectron. Adv. Mater. 5, 369–388. Franc, J., Höschl, P., Belas, E., Grill, R., Hlı´ıdek, P., Moravec, P., Bok, J., 1999. CdTe and CdZnTe crystals for room temperature gamma-ray detectors. Nucl. Instrum. Methods Phys. Res. Sect. A 434, 146–151. Franc, J., Dˇediˇc, V., Sellin, P.J., Grill, R., Veeramani, P., 2011. Radiation induced control of electrical field in Au/CdTe/In structures. Appl. Phys. Lett. 98, 232115. Gu, Y., Rong, C., Xu, Y., Shen, H., Zha, G., Wang, N., Lv, H., Li, X., Wei, D., Jie, W., 2015. Effects of Te inclusions on charge-carrier transport properties in CdZnTe radiation detectors. Nucl Instrum. Methods Phys. Res. Sect. B 343, 89–93. Hansson, C.C.T., Owens, A., Quarati, F., Kozorezov, A., Gostilo, V., Lumb, D., 2012. Synchrotron radiation studies of spectral response features caused by Te inclusions in a large volume coplanar grid CdZnTe detector. Nucl. Instrum. Methods Phys. Res. Sect. A 695, 283–287.

Y. Gu et al. / Micron 88 (2016) 48–53 He, Y.H., Jie, W.Q., Xu, Y.D., Wang, T., Zha, G.Q., Yu, P.F., Zheng, X., Zhou, Y., Liu, H., 2012. Matrix-controlled morphology evolution of Te inclusions in CdZnTe single crystal. Scr. Mater. 67, 5–8. He, Y., Jie, W., Xu, Y., Wang, Y., Zhou, Y., Liu, H., Wang, T., Zha, G., 2014. Dislocation-mediated coupling mechanism between the microstructural defects and Te inclusions in CdZnTe single crystals. Scr. Mater. 82, 4. Hossain, A., Bolotnikov, A.E., Camarda, G.S., Gul, R., Kim, K.-H., Cui, Y., Yang, G., Xu, L., James, R.B., 2011. Effect of Te inclusions in CdZnTe crystals at different temperatures. J. Appl. Phys. 109, 04450–04454. Kim, J.C., Kaye, W.R., Wang, W., Zhang, F., He, Z., 2011. Impact of electrical field non-uniformity on large CdZnTe crystals. 23–29 Oct., 2011, 4494–5000. Lefeld-Sosnowska, M., Frymark, I., 2001. Extended defects in GaN single crystals. J. Phys. D: Appl. Phys. 34, A148. Li, W., Li, D.Y., 2005. On the correlation between surface roughness and work function in copper. J. Chem. Phys. 122, 1–3. Li, Q., Fan, H., Jie, W., Peng, B., Long, C., 2012. Dielectric properties and relaxation behavior of the indium doped cadmium zinc telluride single crystal. J. Appl. Phys. 111, 084111. Lindström, A., Mirbt, S., Sanyal, B., Klintenberg, M., 2016. High resistivity in undoped CdTe: carrier compensation of Te antisites and Cd vacancies. J. Phys. D: Appl. Phys. 49, 035101. Liu, H.F., Liu, W., Dolmanan, S.B., Tripathy, S., Chua, S.J., Chi, D.Z., 2012. Effects of doping and indium inclusions on the structural and optical properties of InN thin films grown by MOCVD. J. Phys. D: Appl. Phys. 45, 195102. Luo, J., Ying, K., Bai, J., 2005. Savitzky–Golay smoothing and differentiation filter for even number data. Signal Process. 85, 1429–1434.

53

Madelung, O., 1996. Semiconductors—Basic Data. Springer Verlag, Berlin Heidelberg. Rudolph, P., Neubert, M., M:uhlberg, M., 1993. Defects in CdTe bridgman monocrystals caused by nonstoichiometric growth conditions. J. Cryst. Growth 128, 582–587. Rudolph, P., Engel, A., Schentke, I., Grochocki, A., 1995. Distribution and genesis of inclusions in CdTe and (Cd Zn)Te single crystals grown by the Bridgman method and by the travelling heater method. J. Cryst. Growth 147, 297–304. Schlesinger, T., Toney, J., Yoon, H., Lee, E., Brunett, B., Franks, L., James, R., 2001. Cadmium zinc telluride and its use as a nuclear radiation detector material. Mater. Sci. Eng. R 32, 103–189. Sellin, P., Prekas, G., Franc, J., Grill, R., 2010. Electrical field distributions in CdZnTe due to reduced temperature and x-ray irradiation. Appl. Phys. Lett 96, 133509. Sze, S.M., Ng, K.K., 2006. Physics of Semiconductor Devices. Wiley-Interscience, New York. Szeles, C., 2004. CdZnTe and CdTe materials for X-ray and gamma ray radiation detector applications. Phys. Status Solidi B: Basic Res. 241, 783–790. ˇ Belas, E., Grill, R., Praus, P., James, R.B., 2012. Determination of electric-field Uxa, S., profile in CdTe and CdZnTe detectors using transient-current technique. IEEE Trans. Nucl. Sci. 59, 2402–2408. Xu, L.Y., Jie, W.Q., Zha, G.Q., Xu, Y.D., Zhao, X.C., Feng, T., Luo, L., Zhang, W.L., Nan, R.H., Wang, T., 2013. Radiation damage on CdZnTe:In crystals under high dose Co-60 gamma-rays. CrystEngComm 15, 10304–10310.