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Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method
Journal Pre-proofs Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method H.P. Hsu, D.Y. Lin, C.W. Chen, Y.F. Wu, K. Strzałkowski, P. Sitarek PII: DOI: Reference:
S0022-0248(20)30014-2 https://doi.org/10.1016/j.jcrysgro.2020.125491 CRYS 125491
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
10 September 2019 7 January 2020 14 January 2020
Please cite this article as: H.P. Hsu, D.Y. Lin, C.W. Chen, Y.F. Wu, K. Strzałkowski, P. Sitarek, Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method, Journal of Crystal Growth (2020), doi: https://doi.org/10.1016/j.jcrysgro.2020.125491
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© 2020 Published by Elsevier B.V.
Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method
H. P. Hsu 1, D. Y. Lin 2,*, C. W. Chen2, Y. F. Wu1, K. Strzałkowski3, and P. Sitarek4
1 Department
of Electronic Engineering, Ming Chi University of Technology, Taishan, No. 84 Gungjuan Rd., New Taipei City 24301, Taiwan 2 Department of Electronic Engineering, National Changhua University of Education, No.2 Shi-Da Rd., Changhua 50074, Taiwan 3 Institute of Physics, Nicolas Copernicus University, Grudziądzka 5/7, 87-100 Toruń, Poland 4 Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
Abstract Cadmium zinc telluride is a tunable band gap II–VI compound semiconductor, which has received considerable attention due to its applications such as nuclear radiation detectors and industrial process monitoring. We have studied the optical properties of the ternary compound semiconductors Cd1-xZnxTe grown by vertical Bridgman-Stockbarger method in the whole range of zinc content 0
1
dependence of band gap energy were also performed. Based on these analyses, the relationship between the band gap energies and the zinc content are verified and discussed. Furthermore, we will study the defect recombination mechanism by using temperature-dependent PL experiments.
Keywrods: A1.Characterization, A2.Single crystal growth, B1.Cadmium compounds, B2.Semiconducting II-VI materials
2
1. Introduction Semiconductor nuclear radiation detectors for medical imaging have developed rapidly in recent years. The II-VI semiconductors are believed to be among the promising materials for the applications in laser diodes [1], nuclear radiation detectors [2], and spintronics [3]. The II-VI binary semiconductors such as ZnSe, ZnTe, CdS, and CdTe have been extensively investigated due to their potential applications in optoelectronics. [4-7] In nuclear medical imaging and industrial process monitoring, the ability to detect and perform energy-dispersive spectroscopy of high energy radiation such as x-ray and gamma-ray is an important issue. Among the various II-VI semiconductors, CdTe is proposed for the use in the gamma-ray and x-ray detectors. [8-10] However, the band gap of a binary alloy is fixed, which makes its applications limited. In II-VI semiconductors, ternary alloys allow a smooth change of the band gap and lattice constants, allowing for tunable band gaps. Cd1-xZnxTe crystals with low Zn content in the range x = 0.1 to 0.2 have become among the most studied ternary materials for gamma-ray and x-ray spectroscopic imaging in nuclear radiation detection [11, 12]. Compared to the traditional high performance spectrometers based on silicon and germanium, Cd1-xZnxTe detectors show good energy resolution, high detection efficiency and stable room temperature operation [13]. Due to their potential applications, work on the optical properties of Cd1-xZnxTe bulk materials in the whole range of zinc content 0
(PL)
measurements
to
3
study
Cd1-xZnxTe
(x=0–1)
bulk
semiconductors grown by vertical Bridgman-Stockbarger method. The identification of band gap energies of Cd1-xZnxTe mixed crystals at each temperature has been achieved by photo energy dependence of the absorption coefficient spectra. Temperature dependence of the band gap energies in the range from 20 to 300 K was investigated.
2. Experimental The Cd1-xZnxTe mixed crystals were grown by the high temperature and high pressure vertical Bridgman-Stockbarger method. The pure binary CdTe and ZnTe (6N, Koch-Light) powders were mixed in stoichiometric proportion and put into a graphite crucible. The crucible was kept at the temperature of 1650 K for 5–6 hours and then moved out from the heating zone with a speed of 2.4 mm/h. To prevent evaporation during the crystal growth process, an argon overpressure of about 150 atm was applied. An argon overpressure of about 100 atm was maintained during the growth process of Cd1-xZnxTe mixed crystals. Once the growth process was finished a cooling procedure was applied. The temperature was decreased linearly with the rate 4K/minute. Several Cd1-xZnxTe mixed crystals with zinc content from 0 to 1 were grown. The dimensions of the ingots were of 1 cm in diameter and up to 5 cm length. The obtained crystals were cut into plates of about 1 mm thickness and mechanically polished. Each ingot was cut into 1-1.5 mm thick slices. Depending on the thickness and the length of the growing rod, it can be cut into a maximum of 15 plates. The composition of grown crystals was determined every third plate for all Cd1-xZnxTe alloys. The same plate is used for absorption and PL measurement in this work. X-ray diffractometer (XRD) equipped with Cu K radiation source was used to examine the crystallographic characteristics. The
4
beam size of X-ray is ~ 10 mm x 0.6 mm. The content of Cd, Zn, and Te were determined by energy-dispersive X-ray spectroscopy (EDS). The uncertainty in Zn content (x) of Cd1-xZnxTe mixed crystals in this work is around ± 2%. The optical properties of the Cd1-xZnxTe bulk plates were studied by absorption and PL measurements in the temperature range of 20–300 K. A 150 W quartz-halogen lamp was passed through a PTI 0.25 m grating monochromator and focused on the sample and the spectra were collected using a silicon detector. The samples were mounted on a copper sample holder with near normal light incidence. In PL measurements, the materials were excited using a 405 nm laser diode with a power of ~20 mW with a spot size ~ 0.5 mm x 1.5 mm, then the luminescence signals were detected by using a spectrometer equipped with a silicon CCD detector. For temperature dependent measurements, a closed-cycle cryogenic refrigerator equipped with a digital thermometer controller with temperature stability better than 0.5 K was used.
3. Results and discussion In Figure 1(a), the surface treatment process of the investigated crystals made clearly visible the boundaries between the few different seeds. This is a common disadvantage of the Bridgman growing technique. For all content, we expect a low level of inclusions. It is noticed here one can see the crystals are not monocrystals, they consist of two-three grains merged together. The plates are therefore more fragile and sometimes can be broken quite easily along these boundaries which cause the slices are partial. Figure 1(b) depicts the X-ray diffraction (XRD) patterns of Cd1-xZnxTe mixed crystals for various Zn contents. The XRD patterns of Cd1-xZnxTe mixed crystals show the (111) orientation at
5
2θ = 23.81 degree for CdTe and the peak gradually shifts to 25.24 degree for ZnTe. The results show the crystal growth of Cd1-xZnxTe mixed crystals is with the expected lattice composition. Under ideal conditions, all the diffraction waves taken place at the Bragg angles of reflection from parallel planes of atoms are coherent to construct narrow and intense peaks. In unperfected crystals X-rays also be reflected incoherently from planes without long-range atomic order or scattered by defect atoms, which will reduce the coherent intensity. The XRD peak intensities is rather weak for the samples with Zn content = 0.89 and 0.94. This result implies coherent scattering domains are very small due to many crystal defects. Another reason for the decreased peak intensities might be attributed to the smaller crystallite size of crystals [14]. Figure 1(c) shows the Raman spectrum obtained for Cd1-xZnxTe mixed crystals. The peaks in the range from 164 cm-1 (CdTe) to 206 cm-1 (ZnTe) are the longitudinal optic (LO) modes of Cd1-xZnxTe mixed crystals. As noticed here, there are two peaks appearing at 124 and 141 cm-1 were raised from elemental of Te-rich phases [15]. This is the common occurrence for Te-containing semiconductors [16,17]. The XRD measurements and Raman spectra provide a signature for identifying the crystal structure and material phase of Cd1-xZnxTe mixed crystals. Figures 2(a) and 2(b) show the room temperature absorption and PL spectra of Cd1-xZnxTe mixed crystals with various zinc contents of x = 0, 0.10, 0.49, 0.89, 0.94, and 1, respectively. As shown in figure 2(a), the absorption spectra gradually shift to the high energy side with increasing zinc content. The band gap energies (Eg) for Cd1-xZnxTe mixed crystals with different zinc content were determined from the extrapolation of the slope of the absorption coefficient squared versus the photon energy to the base line. The obtained band gap energy values of Cd1-xZnxTe mixed crystals at room temperature are
6
1.457, 1.465, 1.732, 2.035, 2.121, and 2.174 eV for zinc contents of x = 0, 0.10, 0.49, 0.89, 0.94, and 1, respectively. The results show the band gap energy was blue-shifted by the incorporation of zinc. In figure 2(b), the PL spectra of Cd1-xZnxTe mixed crystals gradually shift to higher energies with increasing Zn content and show a broad lineshape. The broad PL lineshape in low energy side of the low Zn content samples were attributed to the D-A pair recombination which is common seen in II-VI compounds [18]. In the case of Zn-rich crystal one can observe a wide PL band emission in the red region, which is due to O in Te site. That O(Te)-related band is quite wide because of the high level of coupling with phonons [19]. A plot of energy band gap versus zinc content of Cd1-xZnxTe mixed crystals, obtained from room temperature absorption measurement, is shown in Figure 3. It is well known that the variation of the band gap of ternary alloys as a function of the composition can be described by a quadratic expression. In the present work, the composition dependence behavior of band gap of Cd1-xZnxTe mixed crystals is described by the quadratic equation ECd1-xZnxTe(x) = xEZnTe + (1-x)ECdTe - bx(1-x), where EZnTe and ECdTe are the band gap energies of the binary components, and b is the bowing parameter. The solid curve is the least-squares fit of a quadratic equation with bowing parameter b = 0.376. The obtained value of bowing parameter in this study is in reasonable agreement with those reported in previous works for CdZnTe (b = 0.458-0.463) [20-22], BeZnTe (b = 0.5) [23], and MgZnTe (b = 0.45) [24] alloys. The small difference in bowing parameter in this study might be attributed to the uncertainty in the band gap energies extracted from absorption measurements. For application to optoelectronics, it is important to study the temperature dependence optical properties. Displayed by solid curves in Figures 4(a) and 4(b) are,
7
respectively, the experimental absorption spectra of samples with zinc contents x = 0.49 and 0.89 at several temperatures between 20 and 300 K with temperature step 20 K. Figures 4(c) and 4(d) show the temperature dependent PL spectra of Cd1-xZnxTe mixed crystals with Zn = 0.49 and 0.89, respectively. It is shown here the Zn-rich crystal, the wide PL band emission in the red region were attribute to the O in Te site. The broadening effect can be attributed to the O(Te)-related band involved in the emission. As is the general property of most semiconductors, when the measuring temperature is increased, the band gap energies in the absorption and PL spectra red-shift. The temperature variations of the band gap energies extracted from absorption spectra for Cd1-xZnxTe mixed crystals are depicted in Figure 5. The solid curves are the temperature dependent band gap energies of Cd1-xZnxTe mixed crystals fitted by Varshni semi-empirical relationship [25]:
ECd1-xZnxTe(x) = ECd1-xZnxTe(0) – T2/(+T)
(1)
where ECd1-xZnxTe (0) is the band gap energy at 0 K. The constant α is related to the electron (exciton)-average phonon interaction strength and β is closely related to the Debye temperature. The solid curves are least-squares fits to the Varshni semi-empirical relationship. The obtained parameters that describe the temperature dependence behavior of band gap energies of Cd1-xZnxTe mixed crystals are listed in Table I. For comparison, the parameters for band gap energies of Be1-xZnxTe [26], ZnTe [27], and CdTe [5, 28] II-VI materials are also listed in Table I.
8
The temperature dependence of band gap energies can also be described by the Bose–Einstein expression (dotted curves) [29, 30]:
ECd1-xZnxTe(x) = ECd1-xZnxTe(0) – 2aB/[exp(B/T)-1]
(2)
where ECd1-xZnxTe(0) is the band gap energy at 0 K, aB represents the strength of the electron (exciton)-average phonon interaction, and B corresponds to the average phonon temperature. The values obtained for the various parameters are also presented in Table I, together with the parameters for band gap energies of Be1-xZnxTe [26], ZnTe [27], and CdTe [5,28] II-VI materials for comparison. The parameter of Eq. (1) can be related to aB and B in Eq. (2) by taking the high-temperature limit of both expressions. This yields
= 2aB/B. Comparison of the numbers presented in Table I show that this relation is fairly satisfied. From Eq. (2), it is straightforward to show that the high temperature limit of the slope of E(T) vs T curve approaches a value of -2aB/B. The calculated value of -2aB/B for conduction to heavy-hole (light-hole) near band edge transition energies equals -0.29, -0.30, -0.38, -0.40, -0.45, and -0.47 meV/K for x = 0, 0.10, 0.49, 0.89, 0.94, and 1, respectively, which agrees well with the value of [dE/dT] = -0.29, -0.29, -0.37, -0.41, -0.43, and -0.46 meV/K as obtained from the linear extrapolation of the high temperature (140–300 K) absorption experimental data.
4. Conclusion In conclusion, the XRD measurement and Raman spectra provide the crystal structure and material phase for the identification of Cd1-xZnxTe mixed crystals. We have 9
characterized the temperature dependent band gap energies of Cd1-xZnxTe mixed crystals by absorption and PL measurements in the range from 20 – 300 K. The band gap energies of a series of Cd1-xZnxTe mixed crystals were determined by extrapolating the slope of the absorption coefficient squared versus the photon energy to the base line. The band gap energies show a blue-shift with higher zinc content, the bandgap bowing effect was also studied. The broad PL lineshape in low energy side results from the D-A pair recombination of low Zn content samples were observed. In the case of Zn-rich crystal, a PL band emission in the red region due to O(Te)-related band is very wide because of the high level of coupling with phonons. Varshni and Bose–Einstein’s models were used to fit the temperature dependence of the band gap energies. The parameters that describe the temperature variation of the band gap energies have been evaluated and found to gradually increase with adding more zinc. 5. Acknowledgement This work was financial supported by Ministry of Science and Technology of Taiwan under grand nos. MOST 107-2112-M-018-002 and 108-2221-E-018-010.
10
References: [1] T. Nakamura, K. Katayama, H. Mori, S. Fujiwara, Phys. Status Solidi B 241 (2004) 2659. [2] Y. Nemirovsky, J. of Appl. Phys. 85 (1999) 8. [3] H.Katayama-Yoshida and K.Satob, J. Phys. Chem. Solids 64 (2003) 1447. Y. Niiyama, M. Watanabe, Semicond. Sci. Technol. 20 (2005) 1187. [4] N. C. Giles‐Taylor, R. N. Bicknell, D. K. Blanks, T. H. Myers, J. F. Schetzina, J. Vac. Sci. Technol. A 3 (1985) 76. [5] K. Strzałkowski, J. Zakrzewski, M. Maliński, Int. J. Thermophys. 34 (2013) 691. [6] Y. L. Sun, D. Xie, M. X. Sun, C. J. Teng, L. Qian, R. S. Chen, L. Xiang, T. L. Ren, Sci. Rep. 8 (2018) 5107. [7] K. Sato and S. Adachi, J. Appl. Phys. 73 (1993) 926. [8] H. Choi, J, Korean Phys. Soc. 66 (2015) 27. [9] C. Szeles, S. E. Cameron, J. O. Ndap, W. C. Chalmers, IEEE Trans. Nucl. Sci. 49 (2002) 2535. [10] S. Del Sordo, L. Abbene, E. Caroli, A. M. Mancini, A. Zappettini, P. Ubertini, Sensors 9 (2009) 3491. [11] T. E. Schlesinger, J. E. Toney, H. Toon, E. Y. Lee, B. A. Brunett, L. Franks, R. B. James, Mater. Sci. Eng. R 32 (2001) 103. [12] C. Szeles, S. A. Soldner, S. Vydrin, J. Graves, D. S. Bale, IEEE Trans. Nucl. Sci. 55 (2008) 572. [13] S. Del Sordo, L. Abbene, E. Caroli, A. M. Mancini, A. Zappettini, P. Ubertini, Sensor, 9 (2009) 3491. [14] T. Rattana, S. Suwanboon, P. Amornpitoksuk, A. Haidoux, P. Limsuwan, J. Alloys Compd. 480 (2009) 603. [15] K. Ersching, C. E. M. Campos, J. C. de Lima, T. A. Grandi, S. M. Souza, D. L. da Silva, P. S. Pizani, J. Appl. Phys. 105 (2009) 123532. [16] P. M. Amirtharaj and F. H. Pollak, Appl. Phys. Lett. 45 (1984) 789. [17] S. H. Shin, J. Bajaj, L. A. Moudy, D. T. Cheung, Appl. Phys. Lett. 43 (1983) 68. [18] J. Z. Wang, P. J. Huang, H. P. Hsu, Y. S. Huang, F. Firszt, S. Łȩgowski, H. Mȩczyńska, A. Marasek, K. K. Tiong, J. Appl. Phys. 101 (2007) 103539. [19] H. Tews, M. Schneider, C. An, Appl. Phys. Lett. 40 (1982) 41. [20] K. Strzałkowski, J. Phys. D: Appl. Phys. 46 (2016) 435106. [21] B. Samanta, S. L. Sharma, A. K. Chauduri, Vacuum 46 (1995) 739. [22] J. L. Reno and E. D. Jones, Phys. Rev. B 45 (1992) 1440. [23] O. Maksimov and M. C. Tamargo, Appl. Phys. Lett. 79 (2001) 782. [24] D. Barbier, B. Montegu, and A. Lauguer, Solid State Commun. 28 (1978) 525. [25] Y. P. Varshni, Physica 34 (1967) 149. [26] Y. C. Shih, Y. S. Huang, F. Firszt, S. Łęgowski, H. Męczyńska, K. K. Tiong, J. Phys.: Condens. Matter 20 (2008) 255227. [27] Y. M. Yu, S. Nam, K. S. Lee, Y. D. Choi, B. O, J. Appl. Phys. 90 (2001) 807. [28] U. Pal, J. L. Herrera Pérez, J. Piqueras, E. Dieguéz, Mater. Sci. Eng. B 42 (1996) 297. [29] P. Lautenschlager, M. Garriga, M. Cardona, Phys. Rev. B 36 (1987) 4813. [30] P. Lautenschlager, M. Garriga, S. Logothetidis, M. Cardona, Phys. Rev. B 35 (1987) 9174. 11
Table I. Values of the Varshni– and Bose–Einstein type fitting parameters, which describe the temperature dependence of band gap energies for Cd1-xZnxTe mixed crystals. The parameters of CdTe, ZnTe, and Be1-xZnxTe materials are also included for comparison.
E(0)
β
aB
B
dE/dT
(eV)
( m eV/ K)
(K)
(m eV)
(K)
( m eV/ K)
1.533
0.364
111
20
137
-0.29
Cd0.89Zn0.10Tea
1.541
0.374
112
21
141
-0.29
Cd0.51Zn0.49Tea
1.824
0.445
123
28
148
-0.37
Cd0.11Zn0.89Tea
2.134
0.488
150
34
168
-0.41
Cd0.06Zn0.94Tea
2.224
0.540
171
39
174
-0.43
ZnTea
2.281
0.570
185
44
185
-0.46
ZnTeb
2.381
0.56
150
-0.47
Be0.106Zn0.894Teb
2.578
0.57
140
-0.51
ZnTec
2.374
0.54
150
CdTed
1.575
0.43
183
CdTee
1.588
0.46
111
Materials CdTea
Feature Eg
aPresent
work (bulk, absorption). 25 (bulk, contactless electroreflectance/photoreflectance). cReference 26 (film on GaAs, photoluminescence). dReference 27 (bulk, photoreflectance). eReference 5 (bulk, photoluminescence). bReference
12
Figure Captions Fig. 1. The (a) crystal photo, (b) XRD patterns, and (c) Raman spectra of Cd1-xZnxTe mixed crystals as a function of Zn content. Fig. 2. Room temperature (a) absorption and (b) PL spectra for Cd1-xZnxTe mixed crystals with various Zn contents (x = 0 – 1). Fig. 3. Band gap energy of Cd1-xZnxTe mixed crystals as a function of zinc content x. The solid line is the result fitted by the quadratic equation ECd1-xZnxTe(x) = xEZnTe + (1-x)ECdTe - bx(1-x). Fig. 4. Temperature dependent experimental (a, b) absorption and (c, d) PL spectra of Cd1-xZnxTe mixed crystals for Zn content x = 0.49 and x = 0.89. The band gap energies in the absorption and PL emission spectra exhibit a red-shift characteristic with increasing temperature. Fig. 5. Temperature variations of the experimental absorption values of band gap energies for Cd1-xZnxTe mixed crystals for Zn content x = 0 - 1. The temperature dependence of the band gap energies of Cd1-xZnxTe mixed crystals. The solid curves are least-squares fits to Varshni semi-empirical relationship and the dashed curves are least-squares fits to Bose–Einstein expression.
13
Fig. 1(a)
Fig. 1(b) (111)
(a) XRD
Intensity (a. u.)
Cd 1-xZn xTe
x=1 x=0.94 x=0.89 x=0.49 x=0.1 x=0
20
22
24
26
28
2 (degree)
30
Fig. 1(c) (b)
LO
Cd 1-xZn xTe Intensity (a. u.)
x=1 x=0.94 x=0.89
x=0.49
Te
100
Te
x=0.1 x=0
150
200
250
-1
Raman shift (cm )
14
300
350
Fig. 2(a) (a)
CdTe
ZnTe
2
(h ) (a. u.)
Zn=0 Zn=0.1 Zn=0.49 Zn=0.89 Zn=0.94 Zn=1
300 K Cd 1-xZn xTe 1.2
1.4
1.6
1.8
2.0
2.2
2.4
Energy (eV)
PL Intensity (a. u.)
Fig. 2(b) (b)
Cd 1-xZn xTe Cd 0.90Zn 0.10Te x15 Cd 0.51Zn 0.49Te x2 Cd 0.11Zn 0.89Te Cd 0.006Zn 0.94Te
x=1
ZnTe
x=0.94 x=0.1
1.2
1.4
x=0.49 x=0.89
1.6
1.8
2.0
Energy (eV)
15
300 K 2.2
2.4
Fig. 3 2.4
Cd1-xZnxTe this work Ref. [20] Ref. [21] Ref. [22]
Energy (eV)
2.2 2.0 1.8
b=0.376
1.6 1.4
0.0
0.2
0.4
0.6
Zn content (x)
16
0.8
1.0
Fig. 4(a) (a)
Absorption (a. u.)
Cd 0.51Zn 0.49Te
300 K
1.4
1.5
1.6
20 K
1.7
1.8
1.9
Energy (eV)
Fig. 4(b)
Absorption (a. u.)
Cd 0.11Zn 0.89Te
(b)
300 K
1.7
1.8
1.9
20 K
2.0
2.1
Energy (eV)
17
2.2
Fig. 4(c) Cd 0.51Zn 0.49Te PL Intensity (a. u.)
(c)
300 K 1.3
1.4
1.5
1.6
20 K 1.7
Energy (eV)
1.8
1.9
2.0
Fig. 4(d) Cd 0.11Zn 0.89Te PL Intensity (a. u.)
(d)
1.4
20 K
300 K
1.5
1.6
1.7
1.8
1.9
Energy (eV)
18
2.0
2.1
2.2
Fig. 5 Cd 1-xZn xTe
Energy (eV)
2.4
Varshni fit Bose-Einstein fit
2.2
ZnTe
2.0 1.8 1.6 1.4
CdTe 0
50
100
150
200
250
Temperature (K)
19
300
350
Conflict of interest: Our manuscript “Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method” for submission as an article to JOURNAL OF CRYSTAL GROWTH. This manuscript does not have any interests to conflict. So we will state 'Conflict of interest: none'. Der-Yuh Lin Professor Department of Electronic Engineering National Changhua University of Education E-mail: [email protected]
20
Declaration of interest: Our manuscript “Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method” for submission as an article to JOURNAL OF CRYSTAL GROWTH. This manuscript does not have any interests to declare. So we will state 'declare of interest: none'. Der-Yuh Lin Professor Department of Electronic Engineering National Changhua University of Education E-mail: [email protected]
21
Highlights ￭
Cd1-xZnxTe grown by grown by vertical Bridgman-Stockbarger method.
￭
Absorption and photoluminescence spectroscopy. Temperature dependent optical spectroscopy of Cd1-xZnxTe mixed crystals.
￭
22
S0022-0248(20)30014-2 https://doi.org/10.1016/j.jcrysgro.2020.125491 CRYS 125491
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
10 September 2019 7 January 2020 14 January 2020
Please cite this article as: H.P. Hsu, D.Y. Lin, C.W. Chen, Y.F. Wu, K. Strzałkowski, P. Sitarek, Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method, Journal of Crystal Growth (2020), doi: https://doi.org/10.1016/j.jcrysgro.2020.125491
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method
H. P. Hsu 1, D. Y. Lin 2,*, C. W. Chen2, Y. F. Wu1, K. Strzałkowski3, and P. Sitarek4
1 Department
of Electronic Engineering, Ming Chi University of Technology, Taishan, No. 84 Gungjuan Rd., New Taipei City 24301, Taiwan 2 Department of Electronic Engineering, National Changhua University of Education, No.2 Shi-Da Rd., Changhua 50074, Taiwan 3 Institute of Physics, Nicolas Copernicus University, Grudziądzka 5/7, 87-100 Toruń, Poland 4 Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
Abstract Cadmium zinc telluride is a tunable band gap II–VI compound semiconductor, which has received considerable attention due to its applications such as nuclear radiation detectors and industrial process monitoring. We have studied the optical properties of the ternary compound semiconductors Cd1-xZnxTe grown by vertical Bridgman-Stockbarger method in the whole range of zinc content 0
1
dependence of band gap energy were also performed. Based on these analyses, the relationship between the band gap energies and the zinc content are verified and discussed. Furthermore, we will study the defect recombination mechanism by using temperature-dependent PL experiments.
Keywrods: A1.Characterization, A2.Single crystal growth, B1.Cadmium compounds, B2.Semiconducting II-VI materials
2
1. Introduction Semiconductor nuclear radiation detectors for medical imaging have developed rapidly in recent years. The II-VI semiconductors are believed to be among the promising materials for the applications in laser diodes [1], nuclear radiation detectors [2], and spintronics [3]. The II-VI binary semiconductors such as ZnSe, ZnTe, CdS, and CdTe have been extensively investigated due to their potential applications in optoelectronics. [4-7] In nuclear medical imaging and industrial process monitoring, the ability to detect and perform energy-dispersive spectroscopy of high energy radiation such as x-ray and gamma-ray is an important issue. Among the various II-VI semiconductors, CdTe is proposed for the use in the gamma-ray and x-ray detectors. [8-10] However, the band gap of a binary alloy is fixed, which makes its applications limited. In II-VI semiconductors, ternary alloys allow a smooth change of the band gap and lattice constants, allowing for tunable band gaps. Cd1-xZnxTe crystals with low Zn content in the range x = 0.1 to 0.2 have become among the most studied ternary materials for gamma-ray and x-ray spectroscopic imaging in nuclear radiation detection [11, 12]. Compared to the traditional high performance spectrometers based on silicon and germanium, Cd1-xZnxTe detectors show good energy resolution, high detection efficiency and stable room temperature operation [13]. Due to their potential applications, work on the optical properties of Cd1-xZnxTe bulk materials in the whole range of zinc content 0
(PL)
measurements
to
3
study
Cd1-xZnxTe
(x=0–1)
bulk
semiconductors grown by vertical Bridgman-Stockbarger method. The identification of band gap energies of Cd1-xZnxTe mixed crystals at each temperature has been achieved by photo energy dependence of the absorption coefficient spectra. Temperature dependence of the band gap energies in the range from 20 to 300 K was investigated.
2. Experimental The Cd1-xZnxTe mixed crystals were grown by the high temperature and high pressure vertical Bridgman-Stockbarger method. The pure binary CdTe and ZnTe (6N, Koch-Light) powders were mixed in stoichiometric proportion and put into a graphite crucible. The crucible was kept at the temperature of 1650 K for 5–6 hours and then moved out from the heating zone with a speed of 2.4 mm/h. To prevent evaporation during the crystal growth process, an argon overpressure of about 150 atm was applied. An argon overpressure of about 100 atm was maintained during the growth process of Cd1-xZnxTe mixed crystals. Once the growth process was finished a cooling procedure was applied. The temperature was decreased linearly with the rate 4K/minute. Several Cd1-xZnxTe mixed crystals with zinc content from 0 to 1 were grown. The dimensions of the ingots were of 1 cm in diameter and up to 5 cm length. The obtained crystals were cut into plates of about 1 mm thickness and mechanically polished. Each ingot was cut into 1-1.5 mm thick slices. Depending on the thickness and the length of the growing rod, it can be cut into a maximum of 15 plates. The composition of grown crystals was determined every third plate for all Cd1-xZnxTe alloys. The same plate is used for absorption and PL measurement in this work. X-ray diffractometer (XRD) equipped with Cu K radiation source was used to examine the crystallographic characteristics. The
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beam size of X-ray is ~ 10 mm x 0.6 mm. The content of Cd, Zn, and Te were determined by energy-dispersive X-ray spectroscopy (EDS). The uncertainty in Zn content (x) of Cd1-xZnxTe mixed crystals in this work is around ± 2%. The optical properties of the Cd1-xZnxTe bulk plates were studied by absorption and PL measurements in the temperature range of 20–300 K. A 150 W quartz-halogen lamp was passed through a PTI 0.25 m grating monochromator and focused on the sample and the spectra were collected using a silicon detector. The samples were mounted on a copper sample holder with near normal light incidence. In PL measurements, the materials were excited using a 405 nm laser diode with a power of ~20 mW with a spot size ~ 0.5 mm x 1.5 mm, then the luminescence signals were detected by using a spectrometer equipped with a silicon CCD detector. For temperature dependent measurements, a closed-cycle cryogenic refrigerator equipped with a digital thermometer controller with temperature stability better than 0.5 K was used.
3. Results and discussion In Figure 1(a), the surface treatment process of the investigated crystals made clearly visible the boundaries between the few different seeds. This is a common disadvantage of the Bridgman growing technique. For all content, we expect a low level of inclusions. It is noticed here one can see the crystals are not monocrystals, they consist of two-three grains merged together. The plates are therefore more fragile and sometimes can be broken quite easily along these boundaries which cause the slices are partial. Figure 1(b) depicts the X-ray diffraction (XRD) patterns of Cd1-xZnxTe mixed crystals for various Zn contents. The XRD patterns of Cd1-xZnxTe mixed crystals show the (111) orientation at
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2θ = 23.81 degree for CdTe and the peak gradually shifts to 25.24 degree for ZnTe. The results show the crystal growth of Cd1-xZnxTe mixed crystals is with the expected lattice composition. Under ideal conditions, all the diffraction waves taken place at the Bragg angles of reflection from parallel planes of atoms are coherent to construct narrow and intense peaks. In unperfected crystals X-rays also be reflected incoherently from planes without long-range atomic order or scattered by defect atoms, which will reduce the coherent intensity. The XRD peak intensities is rather weak for the samples with Zn content = 0.89 and 0.94. This result implies coherent scattering domains are very small due to many crystal defects. Another reason for the decreased peak intensities might be attributed to the smaller crystallite size of crystals [14]. Figure 1(c) shows the Raman spectrum obtained for Cd1-xZnxTe mixed crystals. The peaks in the range from 164 cm-1 (CdTe) to 206 cm-1 (ZnTe) are the longitudinal optic (LO) modes of Cd1-xZnxTe mixed crystals. As noticed here, there are two peaks appearing at 124 and 141 cm-1 were raised from elemental of Te-rich phases [15]. This is the common occurrence for Te-containing semiconductors [16,17]. The XRD measurements and Raman spectra provide a signature for identifying the crystal structure and material phase of Cd1-xZnxTe mixed crystals. Figures 2(a) and 2(b) show the room temperature absorption and PL spectra of Cd1-xZnxTe mixed crystals with various zinc contents of x = 0, 0.10, 0.49, 0.89, 0.94, and 1, respectively. As shown in figure 2(a), the absorption spectra gradually shift to the high energy side with increasing zinc content. The band gap energies (Eg) for Cd1-xZnxTe mixed crystals with different zinc content were determined from the extrapolation of the slope of the absorption coefficient squared versus the photon energy to the base line. The obtained band gap energy values of Cd1-xZnxTe mixed crystals at room temperature are
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1.457, 1.465, 1.732, 2.035, 2.121, and 2.174 eV for zinc contents of x = 0, 0.10, 0.49, 0.89, 0.94, and 1, respectively. The results show the band gap energy was blue-shifted by the incorporation of zinc. In figure 2(b), the PL spectra of Cd1-xZnxTe mixed crystals gradually shift to higher energies with increasing Zn content and show a broad lineshape. The broad PL lineshape in low energy side of the low Zn content samples were attributed to the D-A pair recombination which is common seen in II-VI compounds [18]. In the case of Zn-rich crystal one can observe a wide PL band emission in the red region, which is due to O in Te site. That O(Te)-related band is quite wide because of the high level of coupling with phonons [19]. A plot of energy band gap versus zinc content of Cd1-xZnxTe mixed crystals, obtained from room temperature absorption measurement, is shown in Figure 3. It is well known that the variation of the band gap of ternary alloys as a function of the composition can be described by a quadratic expression. In the present work, the composition dependence behavior of band gap of Cd1-xZnxTe mixed crystals is described by the quadratic equation ECd1-xZnxTe(x) = xEZnTe + (1-x)ECdTe - bx(1-x), where EZnTe and ECdTe are the band gap energies of the binary components, and b is the bowing parameter. The solid curve is the least-squares fit of a quadratic equation with bowing parameter b = 0.376. The obtained value of bowing parameter in this study is in reasonable agreement with those reported in previous works for CdZnTe (b = 0.458-0.463) [20-22], BeZnTe (b = 0.5) [23], and MgZnTe (b = 0.45) [24] alloys. The small difference in bowing parameter in this study might be attributed to the uncertainty in the band gap energies extracted from absorption measurements. For application to optoelectronics, it is important to study the temperature dependence optical properties. Displayed by solid curves in Figures 4(a) and 4(b) are,
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respectively, the experimental absorption spectra of samples with zinc contents x = 0.49 and 0.89 at several temperatures between 20 and 300 K with temperature step 20 K. Figures 4(c) and 4(d) show the temperature dependent PL spectra of Cd1-xZnxTe mixed crystals with Zn = 0.49 and 0.89, respectively. It is shown here the Zn-rich crystal, the wide PL band emission in the red region were attribute to the O in Te site. The broadening effect can be attributed to the O(Te)-related band involved in the emission. As is the general property of most semiconductors, when the measuring temperature is increased, the band gap energies in the absorption and PL spectra red-shift. The temperature variations of the band gap energies extracted from absorption spectra for Cd1-xZnxTe mixed crystals are depicted in Figure 5. The solid curves are the temperature dependent band gap energies of Cd1-xZnxTe mixed crystals fitted by Varshni semi-empirical relationship [25]:
ECd1-xZnxTe(x) = ECd1-xZnxTe(0) – T2/(+T)
(1)
where ECd1-xZnxTe (0) is the band gap energy at 0 K. The constant α is related to the electron (exciton)-average phonon interaction strength and β is closely related to the Debye temperature. The solid curves are least-squares fits to the Varshni semi-empirical relationship. The obtained parameters that describe the temperature dependence behavior of band gap energies of Cd1-xZnxTe mixed crystals are listed in Table I. For comparison, the parameters for band gap energies of Be1-xZnxTe [26], ZnTe [27], and CdTe [5, 28] II-VI materials are also listed in Table I.
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The temperature dependence of band gap energies can also be described by the Bose–Einstein expression (dotted curves) [29, 30]:
ECd1-xZnxTe(x) = ECd1-xZnxTe(0) – 2aB/[exp(B/T)-1]
(2)
where ECd1-xZnxTe(0) is the band gap energy at 0 K, aB represents the strength of the electron (exciton)-average phonon interaction, and B corresponds to the average phonon temperature. The values obtained for the various parameters are also presented in Table I, together with the parameters for band gap energies of Be1-xZnxTe [26], ZnTe [27], and CdTe [5,28] II-VI materials for comparison. The parameter of Eq. (1) can be related to aB and B in Eq. (2) by taking the high-temperature limit of both expressions. This yields
= 2aB/B. Comparison of the numbers presented in Table I show that this relation is fairly satisfied. From Eq. (2), it is straightforward to show that the high temperature limit of the slope of E(T) vs T curve approaches a value of -2aB/B. The calculated value of -2aB/B for conduction to heavy-hole (light-hole) near band edge transition energies equals -0.29, -0.30, -0.38, -0.40, -0.45, and -0.47 meV/K for x = 0, 0.10, 0.49, 0.89, 0.94, and 1, respectively, which agrees well with the value of [dE/dT] = -0.29, -0.29, -0.37, -0.41, -0.43, and -0.46 meV/K as obtained from the linear extrapolation of the high temperature (140–300 K) absorption experimental data.
4. Conclusion In conclusion, the XRD measurement and Raman spectra provide the crystal structure and material phase for the identification of Cd1-xZnxTe mixed crystals. We have 9
characterized the temperature dependent band gap energies of Cd1-xZnxTe mixed crystals by absorption and PL measurements in the range from 20 – 300 K. The band gap energies of a series of Cd1-xZnxTe mixed crystals were determined by extrapolating the slope of the absorption coefficient squared versus the photon energy to the base line. The band gap energies show a blue-shift with higher zinc content, the bandgap bowing effect was also studied. The broad PL lineshape in low energy side results from the D-A pair recombination of low Zn content samples were observed. In the case of Zn-rich crystal, a PL band emission in the red region due to O(Te)-related band is very wide because of the high level of coupling with phonons. Varshni and Bose–Einstein’s models were used to fit the temperature dependence of the band gap energies. The parameters that describe the temperature variation of the band gap energies have been evaluated and found to gradually increase with adding more zinc. 5. Acknowledgement This work was financial supported by Ministry of Science and Technology of Taiwan under grand nos. MOST 107-2112-M-018-002 and 108-2221-E-018-010.
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References: [1] T. Nakamura, K. Katayama, H. Mori, S. Fujiwara, Phys. Status Solidi B 241 (2004) 2659. [2] Y. Nemirovsky, J. of Appl. Phys. 85 (1999) 8. [3] H.Katayama-Yoshida and K.Satob, J. Phys. Chem. Solids 64 (2003) 1447. Y. Niiyama, M. Watanabe, Semicond. Sci. Technol. 20 (2005) 1187. [4] N. C. Giles‐Taylor, R. N. Bicknell, D. K. Blanks, T. H. Myers, J. F. Schetzina, J. Vac. Sci. Technol. A 3 (1985) 76. [5] K. Strzałkowski, J. Zakrzewski, M. Maliński, Int. J. Thermophys. 34 (2013) 691. [6] Y. L. Sun, D. Xie, M. X. Sun, C. J. Teng, L. Qian, R. S. Chen, L. Xiang, T. L. Ren, Sci. Rep. 8 (2018) 5107. [7] K. Sato and S. Adachi, J. Appl. Phys. 73 (1993) 926. [8] H. Choi, J, Korean Phys. Soc. 66 (2015) 27. [9] C. Szeles, S. E. Cameron, J. O. Ndap, W. C. Chalmers, IEEE Trans. Nucl. Sci. 49 (2002) 2535. [10] S. Del Sordo, L. Abbene, E. Caroli, A. M. Mancini, A. Zappettini, P. Ubertini, Sensors 9 (2009) 3491. [11] T. E. Schlesinger, J. E. Toney, H. Toon, E. Y. Lee, B. A. Brunett, L. Franks, R. B. James, Mater. Sci. Eng. R 32 (2001) 103. [12] C. Szeles, S. A. Soldner, S. Vydrin, J. Graves, D. S. Bale, IEEE Trans. Nucl. Sci. 55 (2008) 572. [13] S. Del Sordo, L. Abbene, E. Caroli, A. M. Mancini, A. Zappettini, P. Ubertini, Sensor, 9 (2009) 3491. [14] T. Rattana, S. Suwanboon, P. Amornpitoksuk, A. Haidoux, P. Limsuwan, J. Alloys Compd. 480 (2009) 603. [15] K. Ersching, C. E. M. Campos, J. C. de Lima, T. A. Grandi, S. M. Souza, D. L. da Silva, P. S. Pizani, J. Appl. Phys. 105 (2009) 123532. [16] P. M. Amirtharaj and F. H. Pollak, Appl. Phys. Lett. 45 (1984) 789. [17] S. H. Shin, J. Bajaj, L. A. Moudy, D. T. Cheung, Appl. Phys. Lett. 43 (1983) 68. [18] J. Z. Wang, P. J. Huang, H. P. Hsu, Y. S. Huang, F. Firszt, S. Łȩgowski, H. Mȩczyńska, A. Marasek, K. K. Tiong, J. Appl. Phys. 101 (2007) 103539. [19] H. Tews, M. Schneider, C. An, Appl. Phys. Lett. 40 (1982) 41. [20] K. Strzałkowski, J. Phys. D: Appl. Phys. 46 (2016) 435106. [21] B. Samanta, S. L. Sharma, A. K. Chauduri, Vacuum 46 (1995) 739. [22] J. L. Reno and E. D. Jones, Phys. Rev. B 45 (1992) 1440. [23] O. Maksimov and M. C. Tamargo, Appl. Phys. Lett. 79 (2001) 782. [24] D. Barbier, B. Montegu, and A. Lauguer, Solid State Commun. 28 (1978) 525. [25] Y. P. Varshni, Physica 34 (1967) 149. [26] Y. C. Shih, Y. S. Huang, F. Firszt, S. Łęgowski, H. Męczyńska, K. K. Tiong, J. Phys.: Condens. Matter 20 (2008) 255227. [27] Y. M. Yu, S. Nam, K. S. Lee, Y. D. Choi, B. O, J. Appl. Phys. 90 (2001) 807. [28] U. Pal, J. L. Herrera Pérez, J. Piqueras, E. Dieguéz, Mater. Sci. Eng. B 42 (1996) 297. [29] P. Lautenschlager, M. Garriga, M. Cardona, Phys. Rev. B 36 (1987) 4813. [30] P. Lautenschlager, M. Garriga, S. Logothetidis, M. Cardona, Phys. Rev. B 35 (1987) 9174. 11
Table I. Values of the Varshni– and Bose–Einstein type fitting parameters, which describe the temperature dependence of band gap energies for Cd1-xZnxTe mixed crystals. The parameters of CdTe, ZnTe, and Be1-xZnxTe materials are also included for comparison.
E(0)
β
aB
B
dE/dT
(eV)
( m eV/ K)
(K)
(m eV)
(K)
( m eV/ K)
1.533
0.364
111
20
137
-0.29
Cd0.89Zn0.10Tea
1.541
0.374
112
21
141
-0.29
Cd0.51Zn0.49Tea
1.824
0.445
123
28
148
-0.37
Cd0.11Zn0.89Tea
2.134
0.488
150
34
168
-0.41
Cd0.06Zn0.94Tea
2.224
0.540
171
39
174
-0.43
ZnTea
2.281
0.570
185
44
185
-0.46
ZnTeb
2.381
0.56
150
-0.47
Be0.106Zn0.894Teb
2.578
0.57
140
-0.51
ZnTec
2.374
0.54
150
CdTed
1.575
0.43
183
CdTee
1.588
0.46
111
Materials CdTea
Feature Eg
aPresent
work (bulk, absorption). 25 (bulk, contactless electroreflectance/photoreflectance). cReference 26 (film on GaAs, photoluminescence). dReference 27 (bulk, photoreflectance). eReference 5 (bulk, photoluminescence). bReference
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Figure Captions Fig. 1. The (a) crystal photo, (b) XRD patterns, and (c) Raman spectra of Cd1-xZnxTe mixed crystals as a function of Zn content. Fig. 2. Room temperature (a) absorption and (b) PL spectra for Cd1-xZnxTe mixed crystals with various Zn contents (x = 0 – 1). Fig. 3. Band gap energy of Cd1-xZnxTe mixed crystals as a function of zinc content x. The solid line is the result fitted by the quadratic equation ECd1-xZnxTe(x) = xEZnTe + (1-x)ECdTe - bx(1-x). Fig. 4. Temperature dependent experimental (a, b) absorption and (c, d) PL spectra of Cd1-xZnxTe mixed crystals for Zn content x = 0.49 and x = 0.89. The band gap energies in the absorption and PL emission spectra exhibit a red-shift characteristic with increasing temperature. Fig. 5. Temperature variations of the experimental absorption values of band gap energies for Cd1-xZnxTe mixed crystals for Zn content x = 0 - 1. The temperature dependence of the band gap energies of Cd1-xZnxTe mixed crystals. The solid curves are least-squares fits to Varshni semi-empirical relationship and the dashed curves are least-squares fits to Bose–Einstein expression.
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Fig. 1(a)
Fig. 1(b) (111)
(a) XRD
Intensity (a. u.)
Cd 1-xZn xTe
x=1 x=0.94 x=0.89 x=0.49 x=0.1 x=0
20
22
24
26
28
2 (degree)
30
Fig. 1(c) (b)
LO
Cd 1-xZn xTe Intensity (a. u.)
x=1 x=0.94 x=0.89
x=0.49
Te
100
Te
x=0.1 x=0
150
200
250
-1
Raman shift (cm )
14
300
350
Fig. 2(a) (a)
CdTe
ZnTe
2
(h ) (a. u.)
Zn=0 Zn=0.1 Zn=0.49 Zn=0.89 Zn=0.94 Zn=1
300 K Cd 1-xZn xTe 1.2
1.4
1.6
1.8
2.0
2.2
2.4
Energy (eV)
PL Intensity (a. u.)
Fig. 2(b) (b)
Cd 1-xZn xTe Cd 0.90Zn 0.10Te x15 Cd 0.51Zn 0.49Te x2 Cd 0.11Zn 0.89Te Cd 0.006Zn 0.94Te
x=1
ZnTe
x=0.94 x=0.1
1.2
1.4
x=0.49 x=0.89
1.6
1.8
2.0
Energy (eV)
15
300 K 2.2
2.4
Fig. 3 2.4
Cd1-xZnxTe this work Ref. [20] Ref. [21] Ref. [22]
Energy (eV)
2.2 2.0 1.8
b=0.376
1.6 1.4
0.0
0.2
0.4
0.6
Zn content (x)
16
0.8
1.0
Fig. 4(a) (a)
Absorption (a. u.)
Cd 0.51Zn 0.49Te
300 K
1.4
1.5
1.6
20 K
1.7
1.8
1.9
Energy (eV)
Fig. 4(b)
Absorption (a. u.)
Cd 0.11Zn 0.89Te
(b)
300 K
1.7
1.8
1.9
20 K
2.0
2.1
Energy (eV)
17
2.2
Fig. 4(c) Cd 0.51Zn 0.49Te PL Intensity (a. u.)
(c)
300 K 1.3
1.4
1.5
1.6
20 K 1.7
Energy (eV)
1.8
1.9
2.0
Fig. 4(d) Cd 0.11Zn 0.89Te PL Intensity (a. u.)
(d)
1.4
20 K
300 K
1.5
1.6
1.7
1.8
1.9
Energy (eV)
18
2.0
2.1
2.2
Fig. 5 Cd 1-xZn xTe
Energy (eV)
2.4
Varshni fit Bose-Einstein fit
2.2
ZnTe
2.0 1.8 1.6 1.4
CdTe 0
50
100
150
200
250
Temperature (K)
19
300
350
Conflict of interest: Our manuscript “Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method” for submission as an article to JOURNAL OF CRYSTAL GROWTH. This manuscript does not have any interests to conflict. So we will state 'Conflict of interest: none'. Der-Yuh Lin Professor Department of Electronic Engineering National Changhua University of Education E-mail: [email protected]
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Declaration of interest: Our manuscript “Optical characterizations of Cd1-XZnXTe mixed crystals grown by vertical Bridgman-Stockbarger method” for submission as an article to JOURNAL OF CRYSTAL GROWTH. This manuscript does not have any interests to declare. So we will state 'declare of interest: none'. Der-Yuh Lin Professor Department of Electronic Engineering National Changhua University of Education E-mail: [email protected]
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Highlights ￭
Cd1-xZnxTe grown by grown by vertical Bridgman-Stockbarger method.
￭
Absorption and photoluminescence spectroscopy. Temperature dependent optical spectroscopy of Cd1-xZnxTe mixed crystals.
￭
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