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Investigation of Te atmosphere annealing on the properties of detector-grade CdMnTe:In single crystals
Journal of Crystal Growth 430 (2015) 103–107
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Investigation of Te atmosphere annealing on the properties of detector-grade CdMnTe:In single crystals Pengfei Yu a,b,n, Lijun Luan b, Yuanyuan Du c, Jiahong Zheng b, Wanqi Jie a a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China School of Materials Science and Engineering, Chang'an University, Xi'an 710061, China c Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 March 2015 Received in revised form 30 June 2015 Accepted 20 August 2015 Communicated by: A. Burger Available online 29 August 2015
In this paper, detector-grade CdMnTe:In (CMT:In) single crystals were annealed under Te atmosphere with various annealing times. The results indicated that the density of Te inclusions had not changed as the annealing time increased, whereas the resistivity exhibited an initial increase followed by a decrease. The conduction type was changed from weak n-type conduction in as-grown crystal to p-type conduction in 60 h annealed crystal. The IR transmittance decreased obviously as the annealing time increased. In the PL spectra, the obvious reduction of the intensity of (D°,X) peak and the increase of the intensity of the Dcomplex peak in the annealed CMT:In crystals indicated a degradation of the crystal quality. The energy resolution of the detector fabricated with 15 h annealed crystal was improved, whereas the μτ values of the detectors fabricated with all annealed crystals were reduced. Specially, the characteristic peak of 241Am γ-ray could not be observed in the detectors fabricated by 60 h annealed crystals. Therefore, optimal annealing temperature and the duration are 773 K and 15 h, respectively. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A2. Bridgman Technique B1. Cadmium compounds B2. Semiconducting II–VI materials
1. Introduction Recently, research on materials of room radiation detectors has focused on CdTe-based compound semiconductors with wide band-gap and high average atomic number. Among these materials, Cd1 xMnxTe (CMT) has been demonstrated to be a promising candidate for radiation detector application due to its wide bandgap, high resistivity, and good electron-transport properties [1–5]. In comparison with CdZnTe, CMT has some advantages. CMT with a band-gap in the range 1.7–2.2 eV can be achieved by a relatively low Mn content because the energy band-gap of CMT increases about 13 meV per atomic percent of Mn compared with 6.7 meV of Zn in CdZnTe [6]. The near-unity segregation coefficient of Mn in CdTe crystal compared to 1.35 for Zn in CdTe results in a more homogeneous Mn distribution and therefore more uniform CdMnTe crystals [7]. Therefore, it is possible to grow large CMT crystals with homogeneous composition by vertical Bridgman method. And gamma-ray detectors with high quality can be fabricated by this material theoretically. As we know, room radiation detectors with well energy resolution and charge collection efficiency require materials to possess n Corresponding author at: State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China. Tel.: þ86 29 88486065; fax: þ 86 29 88495414. E-mail address: [email protected] (P. Yu).
http://dx.doi.org/10.1016/j.jcrysgro.2015.08.017 0022-0248/& 2015 Elsevier B.V. All rights reserved.
high purity and resistivity, good structure integrity and carrier mobility-lifetime product. However, as-grown CMT crystals usually contain lots of defects, such as Cd vacancies, Te inclusions, twins and impurities, which seriously deteriorate the performance of CMT crystals and detectors [8–10]. Post-growth annealing is a very effective way to reduce defects and improve crystal quality [11]. Few investigations are related to Cd atmosphere annealing. Zhang et al. [7] reported that Te inclusions were reduced and the resistivity was increased in Cd vapor atmosphere for CMT crystals. Kochanowska et al. [12] reported two different annealing methods of CMT crystals in Cd vapor. The results indicated that Cd vacancies were reduced by annealing at uniform temperature while Te precipitates were decreased and crystal quality was improved in the temperature gradient. In the investigation of Egarievwe et al. [13], Te inclusions were eliminated completely in Cd vapor, whilst electrical resistivity fell by an order of 102. However, study of Te atmosphere annealing is not involved. It is known that the Fermi level is pinned to the deepdonor level of the doubly ionized Te antisites, and then, the material becomes high resistivity. For CdZnTe, appropriate concentration of Te antisite will improve resistivity of crystals [14]. But, Te antisite above effective concentration will destroy the performance of detectors, and the conduction type of crystals was also changed [15]. For detector-grade CMT crystals, what is the influence of excess Te is not clear. Therefore, investigation of Te atmosphere annealing for CMT is necessary. In this paper, the
104
P. Yu et al. / Journal of Crystal Growth 430 (2015) 103–107
effects of annealing temperature and time on the properties of detector-grade CMT crystals were investigated. The performance of CMT detectors was also measured.
2. Experimental A Cd0.9Mn0.1Te ingot with the In-doping concentration of 5 1017 atoms/cm3 was grown under Te-rich conditions by the modified vertical Bridgman method (MVB) in our laboratory. The accelerated crucible rotation technique (ACTR) was introduced to homogenize the crystals in growing process. First, CMT polycrystalline materials were synthesized before crystal growth. The raw materials of Cd (7 N), Mn (5 N), Te (7 N) and In (7 N) were sealed in vacuum and synthesized in carbon-coated quartz crucibles using a rocking furnace. Next, using these synthesized polycrystals, crystal growth was undertaken at the withdrawal rate of 1 mm h 1 and the temperature gradient of 12 K cm 1 in a twozone furnace. By this modified Bridgman method, a CMT:In ingot was obtained. CMT wafers were cut from the ingot along (111) face and then diced into small single crystal slices with the size of 5 5 2 mm3. The resistivity of the CMT:In slices was in the order of 109 Ω cm. Before annealing, all the slices were polished mechanically with MgO suspension and then etched with 5% bromine in methanol (Br2–MeOH) for 2 min to remove the damaged surface layer. For the annealing treatment, high-purity Te (7 N) was sealed with CMT:In slices in the same quartz crucible under a vacuum of 10 5 Pa. Annealing experiments were carried out in a two-zone furnace. In order to avoid the formation of Te precipitates at low temperature (about 723 K) and reduce the evaporation of Cd during the annealing process, the annealing temperature of both slices and the source was chosen as 773 K. When the annealing temperature is 773 K, the Cd vapor pressure in CMT is only about 0.043 atm. If the temperature is up to 1073 K, the Cd vapor pressure is close to 0.68 atm. This will result in deviation of stoichiometric ratio of CMT crystals and change of
200μm
crystal structure. The samples were heated to 773 K with the heating-up rate 50 K/h. The annealing time was chosen as 15 h, 30 h and 60 h, respectively. After annealing, the crucible was cooled down to the room temperature in air. IR transmission microscopy (Micronviewer 7290A, American Electrophysics Company) was used to detect precipitates/inclusions. IR transmittances were tested by a Nicolet Nexus 670 spectrometer in the wave number range of 4000–500 cm 1. For PL measurements, the sample was attached on a cold copper finger in a closed-cycle cryostat with grease, and the sample temperature was 10 K. An argon ion laser operating at a wavelength of 488 nm was used to excite PL spectra. A Triax 550 tri-grating monochrometer with a photo-multiplier tube (PMT) possessing a resolution better than 0.3 nm was employed to collect and to analyze the emissions of the sample. Meanwhile, the electrical properties of the samples were measured at room temperature using an Agilent 4155C instrument. The energy resolution of the detector was measured with an ORTEC measurement system using 241Am γ-ray source at room temperature.
3. Results and discussion The typical IR microscope images (IRM) of as-grown and 60 h annealed CMT:In slices are given in Fig. 1. The observation is in situ. From Fig. 1A and B, it can be seen that the density of Te inclusions is almost the same before and after annealing. For further observation (Fig. 1a and b), Te inclusions have not been changed in the same region before and after annealing. The size of Te inclusions is about 4–10 μm and the distribution of Te inclusions is not homogenous. For as-grown and annealed CMT:In slices, the density of Te inclusions is about (6–7) 104 cm 2. It is found that the density of Te inclusions does not change as the annealing time increases. The reason is that the annealing temperature is slightly higher than the melting point of Te, Te inclusions are small and the annealing time is shorter ( r60 h).
200μm
Fig. 1. IR images of as-grown (A and a) and 60 h annealed (B and b) CMT:In crystals. The size of images A and B is 4.5 mm 4.5 mm.
P. Yu et al. / Journal of Crystal Growth 430 (2015) 103–107
80 60 40
I/nA
20 0
as-grown 15h 30h 60h
-20 -40 -60 -80
-100
-50
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50
100
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80
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40 30h annealing
30 20
60h annealing
10 0 4000
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Fig. 3. IR transmittance spectra of CMT:In crystals before and after annealing.
1.2
( D0,X)
Dcomplex
0 DAP ( D ,h)
1.0
PL intensity /a.u.
Therefore, the change of Te inclusions is not observed and the density of Te inclusions does not reduce. The results of current–voltage measurement of as-grown and annealed CMT:In slices are shown in Fig. 2. The resistivity of asgrown CMT:In crystals is 2.4 109 Ω cm. The leakage current is within 6.5 nA when the bias voltage is 100 V. After annealing, the variation of the resistivity is interesting. The resistivity of the annealed CMT:In slices can be calculated to 2.2 1010 Ω cm (15 h), 1.5 109 Ω cm (30 h) and 2.6 108 Ω cm (60 h). As annealing time increases, the resistivity increases first and then decreases. The leakage current of 15 h annealed CMT:In slice is within 1 nA, whereas that of 60 h annealed slice is even close to 61 nA. Hall effect measurements indicated that as-grown CMT:In slices had weak n-type conduction. After 15 h and 30 h annealing, the slices were weak p-type conduction. However, it changed to p-type conduction after 60 h annealing. In addition, as annealing time increases, the concentration of carriers decreases first and then increases. Annealing under Te atmosphere creates saturated vapor pressures of Te, and evaporation of lattice Cd is possible. Cd evaporation from the lattice increases the concentration of Cd vacancy. Such an increase, in turn, results in the increase of hole concentration and p-type conductivity. IR transmittance is an important property for CMT crystals. Researchers used IR transmittance at room temperature to evaluate the quality of CMT crystals [5,7]. Typical IR transmittance spectra of as-grown and annealed CMT:In slices are shown in Fig. 3. Obviously, the transmittance curves of as-grown and annealed CMT:In slices had straight lines in the wave number
105
as-grown
0.8 15h 0.6 30h 0.4 60h 0.2 1.4
1.5
1.6
1.7
1.8
1.9
Energy/eV Fig. 4. PL spectra (10 K) of CMT:In crystals before and annealing.
region from 500 cm 1 to 4000 cm 1. The average transmittance of as-grown CMT:In slices is 53%. After 15 h, 30 h and 60 h annealing, the average transmittances were decreased to 43%, 32% and 21%, respectively. The above results can be explained by the absorption properties of CMT crystals. Du et al. [5] and Zhang et at [7] explained IR absorption behavior in CMT. In this work, IR transmittance is mainly influenced by the lattice absorption and the free-carrier absorption [16]. Te inclusions and dislocation in CMT crystals can destroy the uniformity of the lattice which will enlarge lattice absorption. Therefore, for as-grown CMT:In crystals, Te inclusions and dislocation with high density result in the increase of lattice absorption. Although the concentration of carrier is relatively low (the resistivity is 109 Ω cm). Therefore, the actual IR transmittance is lower than theoretical value (65%). After annealing, the density of Te inclusions is not changed. However, Te atmosphere annealing results in the high density of Cd vacancies due to the volatilization of Cd in CMT. The long annealing time will increase the density of Cd vacancies and the density of dislocation. Therefore, as the annealing time increases, IR transmittance decreases. Although the concentration of carrier has changed after annealing due to the variation of the resistivity, the influence of the variation on IR transmittance is not obvious. The typical low-temperature PL spectra of as-grown and annealed CMT:In slices at 10 K are shown in Fig. 4. Some changes can be found in the PL spectra. Referring to our previous investigation about the PL spectra of CdZnTe crystals [17]. Firstly, the intensity of the neutral donor bound exciton (D°,X) peak centered at 1.7440 eV reduces obviously in near-band-edge region. It indicates that the crystal quality degrades after annealing. As the annealing time increases, the intensity of (D°,X) peak decreases. Moreover, (D°,h) peak centered at 1.7288 eV corresponding to the jump of the neutral donor to valence band disappears in annealed CMT:In slices. Usually, the neutral donor is due to the dopant In. Secondly, in donor–acceptor region, a donor–acceptor pairs (DAP) peak centered at 1.6685 eV has relatively low emission intensity before annealing. However, their intensities become very high after annealing due to the gathering of impurities in Te inclusions or the increase of donor–acceptor pairs because of Cd vacancies and Te antisites. Thirdly, in the defect band, the intensity of Dcomplex peak centered at 1.5305 eV increases after annealing. According to Du et al. [5], Dcomplex peak is not only ascribed to Cd vacancy-related defects, but also ascribed to dislocations in CdMnTe. The concentration of Cd vacancy and the dislocation density increase because of Cd evaporation, which results in the increase of the intensity of Dcomplex peak.
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P. Yu et al. / Journal of Crystal Growth 430 (2015) 103–107
1400
1800
[email protected] Bias=300V
[email protected] Bias=300V
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as-grwon(16.87%)
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[email protected] Bias=300V
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1000 as-grwon(17.06%)
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after 60h annealing(no signal )
400 200 0
0
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Channel/Number Fig. 5.
241
Am γ-ray spectra of CMT:In detectors before and after annealing for (a) 15 h; (b) 30 h; (c) 60 h.
1.0
Charge collection efficiency/ η
Charge collection efficiency/ η
1.0 0.9 0.8 experimental data theoretical fitting
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0.9 0.8 experimental data theoretical fitting
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Fig. 6. Collection efficiency of CZT detectors: (a) as-grown; (b) after 15 h annealing.
Fig. 5 shows 241Am γ-ray spectra of the detectors fabricated with as-grown and annealed CMT:In slices after the preparation of planar-electrode on both sides of the detectors. The bias voltage is 300 V, the shaping time is 2 μs. The energy resolution of the detectors fabricated by as-grown CMT:In slices can be calculated to be 17.69%, 16.87% and 17.06%. After annealing, the energy resolution of the detectors fabricated by annealed CMT:In slices can be calculated to be 12.32% (15 h), 19.14% (30 h) and no signal (60 h). Obviously, the energy resolution is improved after 15 h annealing. However, when the annealing time is over 30 h, the energy resolution becomes bad even no characteristic peak. For the detector fabricated by as-grown slice, the characteristic peak of 241Am γ-ray cannot be observed when bias voltage is higher than 350 V. However, for the detector fabricated by 15 h annealed slice, the
characteristic peak of 241Am γ-ray can be observed even when bias voltage is 800 V. This indicates that the resistivity is enhanced after annealing. The improvement of resistivity ameliorates the energy resolution. Notably, for the detectors fabricated by 60 h annealed slices, the characteristic peak of 241Am γ-ray cannot be observed, as shown in Fig. 5c. Only a peak caused by noise with high intensity is seen. I–V measurements indicate that the resistivity of CMT crystal decreases when the annealing time is more than 30 h. The leakage current increases as the resistivity decreases, thus the noise of detectors increases. Moreover, after annealing, PL measurements show that dislocation density increases because of Cd evaporation. Electrons and holes are easily scattered by dislocations. Dislocations hinder the migration of electrons and holes, which will reduce μτ value. Therefore, both
P. Yu et al. / Journal of Crystal Growth 430 (2015) 103–107
the decrease of the resistivity and the increase of dislocation density may lead to the disappearance of the characteristic peak of 241 Am γ-ray. As is well-known, the energy resolution and the position are changed as the variation of voltage. Fig. 6 shows the collection efficiency of the detectors fabricated by as-grown and 15 h annealed slices. According to the collection efficiency of the detectors at different voltages, the mobility-lifetime products for electrons (μτ)e can be calculated by Hecht equation [18]. Therefore, μτ values of the detectors fabricated by as-grown CMT:In slices can be calculated to be 4.01 10 4 cm2/V, 3.96 10 4 cm2/V and 4.05 10 4 cm2/V. And the corresponding μτ values of the detectors fabricated by annealed CMT:In slices are 3.92 10 4 cm2/V (15 h) and 2.46 10 4 cm2/V (30 h). μτ value of the detectors fabricated by 60 h annealed CMT:In slices cannot be calculated because no signal is obtained. Obviously, as the annealing time increases, μτ value reduces. The reason is that the annealing treatment deteriorates the crystal quality.
4. Conclusions Detector-grade CMT:In crystals were annealed under Te atmosphere with different annealing times of 15 h, 30 h and 60 h at 773 K. After annealing, the density of Te inclusions was not changed. However, the resistivity increased first and then decreased as the annealing time increased. The conduction type was changed from weak n-type in as-grown crystal to p-type in 60 h annealed crystal. IR transmittance decreased obviously as the annealing time increased. In PL spectra, the obvious reduction of the intensity of (D°,X) peak and the increase of the intensity of Dcomplex peak in the annealed CMT:In crystals indicated that the crystal quality was degraded. The energy resolution of the detector fabricated by 15 h annealed crystal was improved, whereas μτ values of the detectors fabricated with all annealed crystals were low. The characteristic peak of 241Am γ-ray could not be observed in the detectors fabricated by 60 h annealed crystal. This may be explained by the decrease of the resistivity and the increase of dislocation density. The optimal annealing conditions are a temperature of 773 K and duration of 15 h.
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Acknowledgments This work was supported by the China Postdoctoral Science Foundation (Grant no. 2014M550509), the National Natural Science Foundations of China (Grant nos. 51201297 and 51402022), the fund of the State Key Laboratory of Solidification Processing in NWPU (No. SKLSP201514), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2015JQ5144) and also supported by Special Fund for Basic Science Research of Central Colleges of Chang'an University (Nos. 310831151084, 0009-2014G1311083 and 0009-2014G1221017).
References [1] A. Mycielski, A. Burger, M. Sowinska, M. Groza, A. Szadkowski, P. Wojnar, B. Witkowska, W. Kaliszek, P. Siffert, Phys. Status Solidi C 2 (2005) 1578. [2] K.H. Kim, A.E. Bolotnikov, G.S. Camarda, G. Yang, A. Hossain, Y. Cui, R.B. James, J. Hong, S.U. Kim, J. Appl. Phys. 106 (2009) 023706-1. [3] O.S. Babalola, A.E. Bolotnikov, M. Groza, A. Hossain, S. Egarievwe, R.B. James, A. Burger, J. Cryst. Growth 311 (2009) 3702. [4] K.H. Kim, G.S. Camarda, A.E. Bolotnikov, R.B. James, Jinki Hong, SunUng Kim, J. Appl. Phys. 105 (2009) 093705-1. [5] Y.Y. Du, W.Q. Jie, T. Wang, X. Zheng, Y.D. Xu, L.J. Luan, J. Cryst. Growth 355 (2012) 33. [6] J. Parkin, P.J. Sellin, A.W. Davies, A. Lohstroh, M.E. Özsan, P. Seller, Nucl. Instrum. Meth. A 573 (2007) 220. [7] J.J. Zhang, L.J. Wang, J.H. Min, K.F. Qin, J. Huang, K. Tang, X.Y. Liang, P. Shen, J. Cryst. Growth 358 (2012) 12. [8] Y.Y. Du, W.Q. Jie, X. Zheng, T. Wang, X.X. Bai, Hui Yu, Trans. Nonferr. Met. Soc. China 22 (2012) S143. [9] A. Hossain, Y. Cui, A.E. Bolltnikov, G.S. Camarda, G. Yang, D. Kochanowska, M. Witkowska-baran, A. Mycielski, R.B. James, J. Electron. Mater. 4 (2009) 1593. [10] K.H. Kim, G. Jeng, P. Kim, J. Choi, A.E. Bolotnikov, G.S. Camarda, R.B. James, J. Appl. Phys. 114 (2013) 063706-1. [11] P.F. Yu, W.Q. Jie, T. Wang, CrystEngComm 13 (2011) 3521. [12] D. Kochanowska, A. Mycielski, M. Witkowska-Baran, A. Szadkowski, B. Witkowska, W. Kaliszek, Y. Cui, R.B. James, IEEE Nucl. Sci. Symp. Conf. Rec. R12-1 (2008) 202. [13] S.U. Egarievwe, G. Yang, A.A. Egarievwe, I.O. Okwechime, J. Gray, Z.M. Hales, A. Hossain, G.S. Camarda, A.E. Bolotnikov, R.B. James, Nucl. Instrum. Meth. A 784 (2015) 51. [14] P.F. Yu, W.Q. Jie, T. Wang, J. Mater. Sci. 46 (2011) 3749. [15] P.F. Yu, W.Q. Jie, J. Cryst. Growth 383 (2013) 126. [16] G.Q. Li, W.Q. Jie, Z. Gu, H. Hua, Chin. Phys. Lett. 20 (2003) 1600. [17] P.F. Yu, W.Q. Jie, J. Lumin. 146 (2014) 382. [18] K.A. Jones, G. Ciampi, C.E. Skrip, K.G. Lynn, Proc. SPIE 6706 (2007) 670607-4.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Investigation of Te atmosphere annealing on the properties of detector-grade CdMnTe:In single crystals Pengfei Yu a,b,n, Lijun Luan b, Yuanyuan Du c, Jiahong Zheng b, Wanqi Jie a a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China School of Materials Science and Engineering, Chang'an University, Xi'an 710061, China c Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 20 March 2015 Received in revised form 30 June 2015 Accepted 20 August 2015 Communicated by: A. Burger Available online 29 August 2015
In this paper, detector-grade CdMnTe:In (CMT:In) single crystals were annealed under Te atmosphere with various annealing times. The results indicated that the density of Te inclusions had not changed as the annealing time increased, whereas the resistivity exhibited an initial increase followed by a decrease. The conduction type was changed from weak n-type conduction in as-grown crystal to p-type conduction in 60 h annealed crystal. The IR transmittance decreased obviously as the annealing time increased. In the PL spectra, the obvious reduction of the intensity of (D°,X) peak and the increase of the intensity of the Dcomplex peak in the annealed CMT:In crystals indicated a degradation of the crystal quality. The energy resolution of the detector fabricated with 15 h annealed crystal was improved, whereas the μτ values of the detectors fabricated with all annealed crystals were reduced. Specially, the characteristic peak of 241Am γ-ray could not be observed in the detectors fabricated by 60 h annealed crystals. Therefore, optimal annealing temperature and the duration are 773 K and 15 h, respectively. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A2. Bridgman Technique B1. Cadmium compounds B2. Semiconducting II–VI materials
1. Introduction Recently, research on materials of room radiation detectors has focused on CdTe-based compound semiconductors with wide band-gap and high average atomic number. Among these materials, Cd1 xMnxTe (CMT) has been demonstrated to be a promising candidate for radiation detector application due to its wide bandgap, high resistivity, and good electron-transport properties [1–5]. In comparison with CdZnTe, CMT has some advantages. CMT with a band-gap in the range 1.7–2.2 eV can be achieved by a relatively low Mn content because the energy band-gap of CMT increases about 13 meV per atomic percent of Mn compared with 6.7 meV of Zn in CdZnTe [6]. The near-unity segregation coefficient of Mn in CdTe crystal compared to 1.35 for Zn in CdTe results in a more homogeneous Mn distribution and therefore more uniform CdMnTe crystals [7]. Therefore, it is possible to grow large CMT crystals with homogeneous composition by vertical Bridgman method. And gamma-ray detectors with high quality can be fabricated by this material theoretically. As we know, room radiation detectors with well energy resolution and charge collection efficiency require materials to possess n Corresponding author at: State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China. Tel.: þ86 29 88486065; fax: þ 86 29 88495414. E-mail address: [email protected] (P. Yu).
http://dx.doi.org/10.1016/j.jcrysgro.2015.08.017 0022-0248/& 2015 Elsevier B.V. All rights reserved.
high purity and resistivity, good structure integrity and carrier mobility-lifetime product. However, as-grown CMT crystals usually contain lots of defects, such as Cd vacancies, Te inclusions, twins and impurities, which seriously deteriorate the performance of CMT crystals and detectors [8–10]. Post-growth annealing is a very effective way to reduce defects and improve crystal quality [11]. Few investigations are related to Cd atmosphere annealing. Zhang et al. [7] reported that Te inclusions were reduced and the resistivity was increased in Cd vapor atmosphere for CMT crystals. Kochanowska et al. [12] reported two different annealing methods of CMT crystals in Cd vapor. The results indicated that Cd vacancies were reduced by annealing at uniform temperature while Te precipitates were decreased and crystal quality was improved in the temperature gradient. In the investigation of Egarievwe et al. [13], Te inclusions were eliminated completely in Cd vapor, whilst electrical resistivity fell by an order of 102. However, study of Te atmosphere annealing is not involved. It is known that the Fermi level is pinned to the deepdonor level of the doubly ionized Te antisites, and then, the material becomes high resistivity. For CdZnTe, appropriate concentration of Te antisite will improve resistivity of crystals [14]. But, Te antisite above effective concentration will destroy the performance of detectors, and the conduction type of crystals was also changed [15]. For detector-grade CMT crystals, what is the influence of excess Te is not clear. Therefore, investigation of Te atmosphere annealing for CMT is necessary. In this paper, the
104
P. Yu et al. / Journal of Crystal Growth 430 (2015) 103–107
effects of annealing temperature and time on the properties of detector-grade CMT crystals were investigated. The performance of CMT detectors was also measured.
2. Experimental A Cd0.9Mn0.1Te ingot with the In-doping concentration of 5 1017 atoms/cm3 was grown under Te-rich conditions by the modified vertical Bridgman method (MVB) in our laboratory. The accelerated crucible rotation technique (ACTR) was introduced to homogenize the crystals in growing process. First, CMT polycrystalline materials were synthesized before crystal growth. The raw materials of Cd (7 N), Mn (5 N), Te (7 N) and In (7 N) were sealed in vacuum and synthesized in carbon-coated quartz crucibles using a rocking furnace. Next, using these synthesized polycrystals, crystal growth was undertaken at the withdrawal rate of 1 mm h 1 and the temperature gradient of 12 K cm 1 in a twozone furnace. By this modified Bridgman method, a CMT:In ingot was obtained. CMT wafers were cut from the ingot along (111) face and then diced into small single crystal slices with the size of 5 5 2 mm3. The resistivity of the CMT:In slices was in the order of 109 Ω cm. Before annealing, all the slices were polished mechanically with MgO suspension and then etched with 5% bromine in methanol (Br2–MeOH) for 2 min to remove the damaged surface layer. For the annealing treatment, high-purity Te (7 N) was sealed with CMT:In slices in the same quartz crucible under a vacuum of 10 5 Pa. Annealing experiments were carried out in a two-zone furnace. In order to avoid the formation of Te precipitates at low temperature (about 723 K) and reduce the evaporation of Cd during the annealing process, the annealing temperature of both slices and the source was chosen as 773 K. When the annealing temperature is 773 K, the Cd vapor pressure in CMT is only about 0.043 atm. If the temperature is up to 1073 K, the Cd vapor pressure is close to 0.68 atm. This will result in deviation of stoichiometric ratio of CMT crystals and change of
200μm
crystal structure. The samples were heated to 773 K with the heating-up rate 50 K/h. The annealing time was chosen as 15 h, 30 h and 60 h, respectively. After annealing, the crucible was cooled down to the room temperature in air. IR transmission microscopy (Micronviewer 7290A, American Electrophysics Company) was used to detect precipitates/inclusions. IR transmittances were tested by a Nicolet Nexus 670 spectrometer in the wave number range of 4000–500 cm 1. For PL measurements, the sample was attached on a cold copper finger in a closed-cycle cryostat with grease, and the sample temperature was 10 K. An argon ion laser operating at a wavelength of 488 nm was used to excite PL spectra. A Triax 550 tri-grating monochrometer with a photo-multiplier tube (PMT) possessing a resolution better than 0.3 nm was employed to collect and to analyze the emissions of the sample. Meanwhile, the electrical properties of the samples were measured at room temperature using an Agilent 4155C instrument. The energy resolution of the detector was measured with an ORTEC measurement system using 241Am γ-ray source at room temperature.
3. Results and discussion The typical IR microscope images (IRM) of as-grown and 60 h annealed CMT:In slices are given in Fig. 1. The observation is in situ. From Fig. 1A and B, it can be seen that the density of Te inclusions is almost the same before and after annealing. For further observation (Fig. 1a and b), Te inclusions have not been changed in the same region before and after annealing. The size of Te inclusions is about 4–10 μm and the distribution of Te inclusions is not homogenous. For as-grown and annealed CMT:In slices, the density of Te inclusions is about (6–7) 104 cm 2. It is found that the density of Te inclusions does not change as the annealing time increases. The reason is that the annealing temperature is slightly higher than the melting point of Te, Te inclusions are small and the annealing time is shorter ( r60 h).
200μm
Fig. 1. IR images of as-grown (A and a) and 60 h annealed (B and b) CMT:In crystals. The size of images A and B is 4.5 mm 4.5 mm.
P. Yu et al. / Journal of Crystal Growth 430 (2015) 103–107
80 60 40
I/nA
20 0
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Fig. 3. IR transmittance spectra of CMT:In crystals before and after annealing.
1.2
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Dcomplex
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PL intensity /a.u.
Therefore, the change of Te inclusions is not observed and the density of Te inclusions does not reduce. The results of current–voltage measurement of as-grown and annealed CMT:In slices are shown in Fig. 2. The resistivity of asgrown CMT:In crystals is 2.4 109 Ω cm. The leakage current is within 6.5 nA when the bias voltage is 100 V. After annealing, the variation of the resistivity is interesting. The resistivity of the annealed CMT:In slices can be calculated to 2.2 1010 Ω cm (15 h), 1.5 109 Ω cm (30 h) and 2.6 108 Ω cm (60 h). As annealing time increases, the resistivity increases first and then decreases. The leakage current of 15 h annealed CMT:In slice is within 1 nA, whereas that of 60 h annealed slice is even close to 61 nA. Hall effect measurements indicated that as-grown CMT:In slices had weak n-type conduction. After 15 h and 30 h annealing, the slices were weak p-type conduction. However, it changed to p-type conduction after 60 h annealing. In addition, as annealing time increases, the concentration of carriers decreases first and then increases. Annealing under Te atmosphere creates saturated vapor pressures of Te, and evaporation of lattice Cd is possible. Cd evaporation from the lattice increases the concentration of Cd vacancy. Such an increase, in turn, results in the increase of hole concentration and p-type conductivity. IR transmittance is an important property for CMT crystals. Researchers used IR transmittance at room temperature to evaluate the quality of CMT crystals [5,7]. Typical IR transmittance spectra of as-grown and annealed CMT:In slices are shown in Fig. 3. Obviously, the transmittance curves of as-grown and annealed CMT:In slices had straight lines in the wave number
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as-grown
0.8 15h 0.6 30h 0.4 60h 0.2 1.4
1.5
1.6
1.7
1.8
1.9
Energy/eV Fig. 4. PL spectra (10 K) of CMT:In crystals before and annealing.
region from 500 cm 1 to 4000 cm 1. The average transmittance of as-grown CMT:In slices is 53%. After 15 h, 30 h and 60 h annealing, the average transmittances were decreased to 43%, 32% and 21%, respectively. The above results can be explained by the absorption properties of CMT crystals. Du et al. [5] and Zhang et at [7] explained IR absorption behavior in CMT. In this work, IR transmittance is mainly influenced by the lattice absorption and the free-carrier absorption [16]. Te inclusions and dislocation in CMT crystals can destroy the uniformity of the lattice which will enlarge lattice absorption. Therefore, for as-grown CMT:In crystals, Te inclusions and dislocation with high density result in the increase of lattice absorption. Although the concentration of carrier is relatively low (the resistivity is 109 Ω cm). Therefore, the actual IR transmittance is lower than theoretical value (65%). After annealing, the density of Te inclusions is not changed. However, Te atmosphere annealing results in the high density of Cd vacancies due to the volatilization of Cd in CMT. The long annealing time will increase the density of Cd vacancies and the density of dislocation. Therefore, as the annealing time increases, IR transmittance decreases. Although the concentration of carrier has changed after annealing due to the variation of the resistivity, the influence of the variation on IR transmittance is not obvious. The typical low-temperature PL spectra of as-grown and annealed CMT:In slices at 10 K are shown in Fig. 4. Some changes can be found in the PL spectra. Referring to our previous investigation about the PL spectra of CdZnTe crystals [17]. Firstly, the intensity of the neutral donor bound exciton (D°,X) peak centered at 1.7440 eV reduces obviously in near-band-edge region. It indicates that the crystal quality degrades after annealing. As the annealing time increases, the intensity of (D°,X) peak decreases. Moreover, (D°,h) peak centered at 1.7288 eV corresponding to the jump of the neutral donor to valence band disappears in annealed CMT:In slices. Usually, the neutral donor is due to the dopant In. Secondly, in donor–acceptor region, a donor–acceptor pairs (DAP) peak centered at 1.6685 eV has relatively low emission intensity before annealing. However, their intensities become very high after annealing due to the gathering of impurities in Te inclusions or the increase of donor–acceptor pairs because of Cd vacancies and Te antisites. Thirdly, in the defect band, the intensity of Dcomplex peak centered at 1.5305 eV increases after annealing. According to Du et al. [5], Dcomplex peak is not only ascribed to Cd vacancy-related defects, but also ascribed to dislocations in CdMnTe. The concentration of Cd vacancy and the dislocation density increase because of Cd evaporation, which results in the increase of the intensity of Dcomplex peak.
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P. Yu et al. / Journal of Crystal Growth 430 (2015) 103–107
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[email protected] Bias=300V
[email protected] Bias=300V
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Counts
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[email protected] Bias=300V
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400 200 0
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Channel/Number Fig. 5.
241
Am γ-ray spectra of CMT:In detectors before and after annealing for (a) 15 h; (b) 30 h; (c) 60 h.
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Charge collection efficiency/ η
Charge collection efficiency/ η
1.0 0.9 0.8 experimental data theoretical fitting
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Bias voltage/V
0.9 0.8 experimental data theoretical fitting
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Fig. 6. Collection efficiency of CZT detectors: (a) as-grown; (b) after 15 h annealing.
Fig. 5 shows 241Am γ-ray spectra of the detectors fabricated with as-grown and annealed CMT:In slices after the preparation of planar-electrode on both sides of the detectors. The bias voltage is 300 V, the shaping time is 2 μs. The energy resolution of the detectors fabricated by as-grown CMT:In slices can be calculated to be 17.69%, 16.87% and 17.06%. After annealing, the energy resolution of the detectors fabricated by annealed CMT:In slices can be calculated to be 12.32% (15 h), 19.14% (30 h) and no signal (60 h). Obviously, the energy resolution is improved after 15 h annealing. However, when the annealing time is over 30 h, the energy resolution becomes bad even no characteristic peak. For the detector fabricated by as-grown slice, the characteristic peak of 241Am γ-ray cannot be observed when bias voltage is higher than 350 V. However, for the detector fabricated by 15 h annealed slice, the
characteristic peak of 241Am γ-ray can be observed even when bias voltage is 800 V. This indicates that the resistivity is enhanced after annealing. The improvement of resistivity ameliorates the energy resolution. Notably, for the detectors fabricated by 60 h annealed slices, the characteristic peak of 241Am γ-ray cannot be observed, as shown in Fig. 5c. Only a peak caused by noise with high intensity is seen. I–V measurements indicate that the resistivity of CMT crystal decreases when the annealing time is more than 30 h. The leakage current increases as the resistivity decreases, thus the noise of detectors increases. Moreover, after annealing, PL measurements show that dislocation density increases because of Cd evaporation. Electrons and holes are easily scattered by dislocations. Dislocations hinder the migration of electrons and holes, which will reduce μτ value. Therefore, both
P. Yu et al. / Journal of Crystal Growth 430 (2015) 103–107
the decrease of the resistivity and the increase of dislocation density may lead to the disappearance of the characteristic peak of 241 Am γ-ray. As is well-known, the energy resolution and the position are changed as the variation of voltage. Fig. 6 shows the collection efficiency of the detectors fabricated by as-grown and 15 h annealed slices. According to the collection efficiency of the detectors at different voltages, the mobility-lifetime products for electrons (μτ)e can be calculated by Hecht equation [18]. Therefore, μτ values of the detectors fabricated by as-grown CMT:In slices can be calculated to be 4.01 10 4 cm2/V, 3.96 10 4 cm2/V and 4.05 10 4 cm2/V. And the corresponding μτ values of the detectors fabricated by annealed CMT:In slices are 3.92 10 4 cm2/V (15 h) and 2.46 10 4 cm2/V (30 h). μτ value of the detectors fabricated by 60 h annealed CMT:In slices cannot be calculated because no signal is obtained. Obviously, as the annealing time increases, μτ value reduces. The reason is that the annealing treatment deteriorates the crystal quality.
4. Conclusions Detector-grade CMT:In crystals were annealed under Te atmosphere with different annealing times of 15 h, 30 h and 60 h at 773 K. After annealing, the density of Te inclusions was not changed. However, the resistivity increased first and then decreased as the annealing time increased. The conduction type was changed from weak n-type in as-grown crystal to p-type in 60 h annealed crystal. IR transmittance decreased obviously as the annealing time increased. In PL spectra, the obvious reduction of the intensity of (D°,X) peak and the increase of the intensity of Dcomplex peak in the annealed CMT:In crystals indicated that the crystal quality was degraded. The energy resolution of the detector fabricated by 15 h annealed crystal was improved, whereas μτ values of the detectors fabricated with all annealed crystals were low. The characteristic peak of 241Am γ-ray could not be observed in the detectors fabricated by 60 h annealed crystal. This may be explained by the decrease of the resistivity and the increase of dislocation density. The optimal annealing conditions are a temperature of 773 K and duration of 15 h.
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Acknowledgments This work was supported by the China Postdoctoral Science Foundation (Grant no. 2014M550509), the National Natural Science Foundations of China (Grant nos. 51201297 and 51402022), the fund of the State Key Laboratory of Solidification Processing in NWPU (No. SKLSP201514), the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2015JQ5144) and also supported by Special Fund for Basic Science Research of Central Colleges of Chang'an University (Nos. 310831151084, 0009-2014G1311083 and 0009-2014G1221017).
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