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Impurity phases analysis of ZnGeP2 single crystal grown by Bridgman method
Accepted Manuscript Impurity phases analysis of ZnGeP2 single crystal grown by Bridgman method Denghui Yang, Beijun Zhao, Baojun Chen, Shifu Zhu, Zhiyu He, Wei Huang, Zhangrui Zhao, Mengdi Liu PII:
S0925-8388(16)33481-8
DOI:
10.1016/j.jallcom.2016.11.023
Reference:
JALCOM 39518
To appear in:
Journal of Alloys and Compounds
Received Date: 17 August 2016 Revised Date:
29 October 2016
Accepted Date: 2 November 2016
Please cite this article as: D. Yang, B. Zhao, B. Chen, S. Zhu, Z. He, W. Huang, Z. Zhao, M. Liu, Impurity phases analysis of ZnGeP2 single crystal grown by Bridgman method, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Impurity phases analysis of ZnGeP2 single crystal grown by Bridgman method
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Denghui Yang, Beijun Zhao*, Baojun Chen, Shifu Zhu, Zhiyu He, Wei Huang, Zhangrui Zhao, Mengdi Liu Department of Materials Science, Sichuan University, Chengdu 610064, China
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Abstract: X-ray photoelectron spectroscopy (XPS) was proposed as suitable method to study the low content of impurity phases on ZnGeP2 (ZGP) single crystals which are excellent materials in non-linear infrared fields. Besides it, X-ray diffraction (XRD) and Energy Dispersive Spectrometer (EDS) tests were carried out to characterize the components homogeneity of the crystal. It is found that impurity phases Zn3P2 and ZnP2, which are undetectable by X-ray diffraction (XRD) method due to low content, results in the heterogeneity of components and phases of ZGP single crystal. Meanwhile, Oxides of P2O5 and GeO2 were found to exist on the surface of the crystal. Key words: ZnGeP2; impurity phases; components homogeneity; X-ray photoelectron spectroscopy; X-ray diffraction;
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1. Introduction ZnGeP2 (ZGP), as one of the most promising high infrared nonlinear optical materials in IR
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region, possesses a unique set of advantages including wide transmission range (0.7~12 µm), large nonlinear coefficient (d36=75 pm/V), high thermal conductivity (360 mW/(cm·K)), high laser induced damage threshold (30 GW/cm2), and suitable birefringence (0.040~0.042) [1-2]. These properties make it widely used as medium materials of optical parametric amplification, second-harmonic generation, and optical parametric oscillators, etc [3-12]. However, it is difficult to gain ZGP single crystals with large size and low absorption coefficient. One of the main factors is the non-stoichiometry of ZGP single crystals resulting from P and Zn which are easy to evaporate from molten ZGP. In addition, according to quasi-binary ZnP2-Ge section of ternary Zn-Ge-P diagram, impurity phases such as ZnP2 and Ge are most likely present in as-grown ZGP [13]. Non-stoichiometry and homogeneity of ZGP have been studied by G.A. Verozubova et al. [14] and they have reported that the probable composition of the precipitates is a mixture of Ge, Zn3P2 and ZnGeP2. I.G. Vasilyeva et al. [15] use chemical analysis combined with the inductively coupled plasma atomic emission spectroscopy to detect impurity phases ZnP2 and Ge which are in a low content and distribute irregularly in the bulk ZGP. These studies manifest that some impurity phases such as ZnP2 and Ge exist in grown ZGP. Meanwhile, these impurity phases are difficult to be detected by conventional component test methods because of their low content. In this paper, XPS was employed to analyze the impurity phases and their space distribution *
Corresponding author. e-mail address: [email protected]
ACCEPTED MANUSCRIPT in as-grown ZGP single crystal while low content impurity phases are undetectable by conventional XRD method. In addition, heterogeneity along the growth axis was studied as well.
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2. Experimental ZGP single crystals were grown using modified Bridgman method [16] from polycrystalline materials that were synthesized by two-temperature mechanical oscillation method [17]. Growth was carried out in a crucible, enclosed in evacuated fused quartz ampoules under 5×10-4 Pa, located in a six-zone furnace which can produce proper and precise temperature gradient. The six-zone furnace can mainly produce three steady temperature regions. Temperature of the upper region of the furnace was 1070 oC and the lower region temperature was 900 oC, while the temperature gradients of the growth region were 10 oC/cm near solid-liquid interface. The crystal growth started by spontaneous nucleation and the maximum descending rate of the ampoule was 0.6 cm/day to prevent cracking of the crystal during solidification and the phase transformation at about 950 oC. After the melt was completely solidified, the grown crystals were cooled at 10 oC/h through 900 oC to room temperature. After growth, a ZGP crystal was obtained.
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Figure 1 is as-grown ZGP single crystal with the size of φ24 mm×70mm. Figure 2 shows three 2 mm thick wafers a, b and c which were cut from the neck, main body and the near the end of the grown crystal respectively. The three wafers were polished by diamond polishing liquid for test use. Elements, phases and chemical state analysis of the three wafers were carried out by EDS, XRD and XPS. In addition, elements and phases homogeneity were studied by the three test methods as well.
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3. Results and discussion 3.1 XRD Small pieces were cut off from a, b, c wafers and were ground into powders respectively. Then the three samples were characterized by XRD (λ (Cu Kα) =0.154184 nm). X-ray generator operating at 40 kV, 25 mA was used to record the X-ray diffraction patterns, the 2θ range was from 10o to 90o, and the scanning speed was 0.01 o/s. The obtained pattern is shown in Fig. 3, which are all in agreement with the standard XRD pattern (PDF 73–0398). Figure 3 indicate that grown crystal with tetragonal structure, single-phase was successfully obtained. In addition, phases of the neck, main body and the end are almost the same, and these indicate that the whole ingot we obtained has good phase homogeneity. 3.2 Components homogeneity Elemental compositions (P, Ge and Zn) of a, b and c wafers of the as-grown crystal were
measured by EDS. The results are shown in Table 1. According to the results, although the average ratio of the Ge, Zn and P is close to the ZGP stoichiometric ratio 1:1:2, contents of P reduce along the growth direction, and this results from vaporization of P which is the most volatile component. And the P gathered at the end of the quartz tube. Furthermore, it can be found that element ratios of Zn, Ge and P are nearly uniform along the growth axis of the crystal. The results indicate that the grown crystal has little composition segregation and the whole ingot we obtained has good elements homogeneity.
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Zn (%)
Ge (%)
P (%)
Zn:Ge:P
a b c
15.48 15.54 17.07
15.28 15.30 16.85
29.07 28.49 30.92
1:0.987:1.878 1:0.985:1.833 1:0.987:1.811
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3.3 XPS analysis of as-grown ZGP single crystal X-ray photoelectron spectroscopy analyses of the samples were performed with an Axis Ultra DLD spectrometer using monochromatic Al-Kα source. The instrument work function was calibrated to give a C 1s binding energy (BE) of 284.6 eV. Figure 4, Figure 5 and Figure 6 display the Zn 2p, P 2p and Ge 3d XPS comparison spectra acquired on the outer surfaces of the wafers of a, b and c respectively. All the spectra are nearly similar for the same kind of element respectively. The results indicate that the main components of a, b and c specimens are consistent. In addition, the neck and main body of the crystal have good components homogeneity which is in agreement with XRD and EDS results. However, spectra of wafer c are slightly different from that of wafer a and b. The binding energy value corresponding to the peak of wafer c is 0.2 eV larger than that of wafer a and b. For the same kind and valence of one elements analyzed by XPS, binding energy value is related to electronegativity of atoms around, the larger the electronegativity, the bigger the binding energy is. Therefore, components of wafer c is not the same as wafer a and b. Figure 4 displays the comparison of high-resolution Zn 2p XPS spectra acquired on the surface of a, b and c specimens. The intensity of spectrum of c sample is lower than that of a and b which may attribute to the contents of the elements. It is interesting to note that while the a and b specimens show only one single component on the high resolution spectrum of ZGP, a second component appears at higher binding energies of 133.2 eV and 32.4 eV respectively from Fig. 5 and Fig. 6 which is associated to the presence of oxides that may be considered to be passivation layer of the samples surface. In addition, on the surface sample c, the content of oxides are much higher than sample a and b which may result from non-stoichiometry of the tail of the crystal. Figure 7 displays the high-resolution P 2p XPS spectrum acquired on the outer surface of the b wafer. The spectrum may be fitted to four components with different intensities: the first and most intense situated at 128.3 eV and 129.0 eV that correspond to the P 2p3/2 and P 2p1/2 are attributed to the presence of the main phase of ZnGeP2. At 128.8 eV, a second component appears, which may be associated to the presence of Zn3P2 that is similar to wafers a and b. At 129.6 eV, a third component appears, which is associated to the presence of ZnP2. The existence of a small shoulder intensity at the 132.6 eV of the high-resolution P 2p peak spectrum may indicate the existence of P2O5 which is not in agreement with NIST XPS database (P2O5:135.1~135.6 eV) because of the existence of atoms (Ge, Zn) around P of different electronegativity (O P Ge Zn). Therefore, a chemical shift of binding energy of P2O5 to 132.6 eV occurred. It may indicate that impurity phases of ZGP single crystal are derived from its raw materials because XPS spectra P 2p of ZGP single crystal are similar to those of ZGP polycrystals. In addition, low content impurity phase Zn3P2 and ZnP2 cannot be eliminated during ZGP polycrystals synthesis progress[18]. It is known that a proper temperature gradient and growth rate facilitate impurity rejection[19]. This study, therefore, provide important information for our
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further ZGP single crystal growth progress improvement. Figure 8 displays the high-resolution Ge 3d XPS spectrum acquired on the outer surface of the c wafer. The spectrum may be fitted to two components with different intensities: the first and most intense situated at 30.1 eV is attributed to the presence of the main phase of ZnGeP2. At 32.2 eV, a second low content component appears, which may be attributed to the presence of GeO2 that is difficult to detect in wafers a and b. I.G. Vasilyeva et al. [15] have reported that low content Ge exist in bulk ZGP crystal, since the peak of Ge 3d of ZGP obtained here is of symmetry and overlaps with Ge element completely. It has not been possible to determine from this peak the presence of Ge in metallic state in the as-grown ZGP single crystal. As a result, at the end of the crystal, low content impurity phase exists on the surface of the samples in the form of GeO2 that cannot be detected by XRD measurement.
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4. Conclusions XRD analyses reveal that the as-grown crystal is in good structural integrity and crystallization. EDS analysis indicates that composition distribution along the growth axis is uniform and the ratios of the Zn, Ge and P elements are found to be close to the ZGP stoichiometric ratio 1:1:2. However, XPS analyses manifest that a fairly low content impurity phase exists in the form of Zn3P2 and ZnP2 in the whole as-grown crystal which cannot be detected by XRD measurement. Oxides in the form of P2O5 and GeO2 exist on the surface of the sample as passivation layer. In addition, the impurity phases in ZGP single crystal may be derived from polycrystals. Fairly low content impurity phases and their distribution of the as grown crystal provide important information to optimize our ZGP single crystal’s growth progress for its further application.
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Acknowledgement: This study was financially supported by the National Natural Science Foundation General and Key Programs of China (Nos.50672061 and 50732005) and the 863 High-Tech program of China (No.2007AA03Z443).
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References [1] D.N. Nikogosyan. Springer Science & Business Media, 2006. [2] V.G. Dmitriev, G.G. Gurzadyan, D.N. Nikogosyan. Springer, 2013. [3] K.T. Zawilski, P. G. Schunemann, S. D. Setzler, T.M. Pollak, J. Cryst. Growth. 2008, 310(7-9): 1891-1896. [4] D. Creeden, P.A. Ketteridge, P.A. Budni, S.D. Setzler, Y.E. Young, J. C. McCarthy, K. Zawilski, P.G. Schunemann, T. M. Pollak, E.P. Chicklis, M. Jiang, Opt. Lett. 2008, 33, 315-317. [5] P.B. Phua, K.S. Lai, R.F. Wu, T.C. Chong, Appl. Opt. 1999, 38, 563-565. [6] M. Henriksson, M. Tiihonen, V. Pasiskevicius, F. Laurell, Opt. Lett. 2006, 31, 1878-1880. [7] M. Gebhardt, C. Gaida, P. Kadwani, A. Sincore, N. Gehlich, C. Jeon, L. Shah, M. Richardson, Opt. Lett. 2014, 39, 1212-1215. [8] M.W. Haakestad, H. Fonnum, E. Lippert, Opt. Express. 2014, 22, 8556-8564. [9] M. Eichhorn, M. Schellhorn, M. W. Haakestad, H. L. E. Fonnum, Opt. Lett. 2016, 41, 2596-2599.
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[10] S.V. Chuchupal, G.A. Komandin, E.S. Zhukova, O.E. Porodinkov, I.E. Spektor, A.I. Gribenyukov, Phys. Solid State. 2015, 57, 1607-1612. [11] C. Kieleck, A. Berrou, B. Donelan, B. Cadier, T. Robin, M. Eichhorn. Opt. Lett. 2015, 40, 1101-1104. [12] A.A. Ionin, I.O. Kinyaevskii, Y.M. Klimachev. Bull. Lebedev Phys. Instittute. 2014, 41, 222-225. [13] I.G. Vasilyeva, R.E. Nikolaev, G.A. Verozubova. J. Solid State Chem. 2010, 183, 2242-2247. [14] G.A. Verozubova, A.I. Gribenyukov, V.V. Korotkova, A.W. Vere, C.J. Flynn, J. Cryst. Growth. 2002, 2000-2004. [15] I.G. Vasilyeva, M.G. Demidova, Talanta. 2012, 101, 187-191. [16] J. Cheng, S.F. Zhu, B.J. Zhao, B.J. Chen, Z.Y. He, Q. Fan, T. Xu. J. Cryst. Growth. 2013, 362, 125-129. [17] X. Zhao, S.F. Zhu, B.J. Zhao, B.J. Chen, Z.Y. He, R.L. Wang, H.G. Yang, Y.Q. Sun, J. Cheng, J. Cryst. Growth. 2008, 311, 190-193. [18] G.A. Verozubova, A.I. Gribenyukov, Y.P. Mironov, Inorg. Mater. 2007, 43, 1040-1045. [19] G.A. Verozubova, A Yu. Trofimov, E. M. Trukhanov, A. V. Kolesnikov, A. O. Okunev, Yu. F. Ivanov, P. R. J. Galtier, S A Said. Hassani, Crystallogr. Rep. 2010, 55, 65-70.
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Fig. 1. Photograph of as-grown ZGP single crystal Fig. 2. Mechanically polished three wafers cutting from corresponding position Fig. 3. XRD patterns of a, b, c specimens Fig. 4. Comparison of Zn 2p spectra of the samples Fig. 5. Comparison of P 2p spectra of the samples Fig. 6. Comparison of Ge 3d spectra of the samples Fig. 7. XPS spectra P 2p (black line) of sample b, peaks from deconvolution analysis (colorized lines) Fig. 8. XPS spectra Ge 3d (black line) of sample c, peaks from deconvolution analysis (colorized lines)
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1. XPS was proposed to study the low content impurity phases in ZnGeP2 crystal. 2. XPS results indicate the impurity phases are ZnP2 and Zn3P2. 3. The impurity phases may be considered to be derived from polycrystals. 4. The obtained crystal is of good structural integrity and has high crystallinity.
S0925-8388(16)33481-8
DOI:
10.1016/j.jallcom.2016.11.023
Reference:
JALCOM 39518
To appear in:
Journal of Alloys and Compounds
Received Date: 17 August 2016 Revised Date:
29 October 2016
Accepted Date: 2 November 2016
Please cite this article as: D. Yang, B. Zhao, B. Chen, S. Zhu, Z. He, W. Huang, Z. Zhao, M. Liu, Impurity phases analysis of ZnGeP2 single crystal grown by Bridgman method, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT
Impurity phases analysis of ZnGeP2 single crystal grown by Bridgman method
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Denghui Yang, Beijun Zhao*, Baojun Chen, Shifu Zhu, Zhiyu He, Wei Huang, Zhangrui Zhao, Mengdi Liu Department of Materials Science, Sichuan University, Chengdu 610064, China
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Abstract: X-ray photoelectron spectroscopy (XPS) was proposed as suitable method to study the low content of impurity phases on ZnGeP2 (ZGP) single crystals which are excellent materials in non-linear infrared fields. Besides it, X-ray diffraction (XRD) and Energy Dispersive Spectrometer (EDS) tests were carried out to characterize the components homogeneity of the crystal. It is found that impurity phases Zn3P2 and ZnP2, which are undetectable by X-ray diffraction (XRD) method due to low content, results in the heterogeneity of components and phases of ZGP single crystal. Meanwhile, Oxides of P2O5 and GeO2 were found to exist on the surface of the crystal. Key words: ZnGeP2; impurity phases; components homogeneity; X-ray photoelectron spectroscopy; X-ray diffraction;
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1. Introduction ZnGeP2 (ZGP), as one of the most promising high infrared nonlinear optical materials in IR
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region, possesses a unique set of advantages including wide transmission range (0.7~12 µm), large nonlinear coefficient (d36=75 pm/V), high thermal conductivity (360 mW/(cm·K)), high laser induced damage threshold (30 GW/cm2), and suitable birefringence (0.040~0.042) [1-2]. These properties make it widely used as medium materials of optical parametric amplification, second-harmonic generation, and optical parametric oscillators, etc [3-12]. However, it is difficult to gain ZGP single crystals with large size and low absorption coefficient. One of the main factors is the non-stoichiometry of ZGP single crystals resulting from P and Zn which are easy to evaporate from molten ZGP. In addition, according to quasi-binary ZnP2-Ge section of ternary Zn-Ge-P diagram, impurity phases such as ZnP2 and Ge are most likely present in as-grown ZGP [13]. Non-stoichiometry and homogeneity of ZGP have been studied by G.A. Verozubova et al. [14] and they have reported that the probable composition of the precipitates is a mixture of Ge, Zn3P2 and ZnGeP2. I.G. Vasilyeva et al. [15] use chemical analysis combined with the inductively coupled plasma atomic emission spectroscopy to detect impurity phases ZnP2 and Ge which are in a low content and distribute irregularly in the bulk ZGP. These studies manifest that some impurity phases such as ZnP2 and Ge exist in grown ZGP. Meanwhile, these impurity phases are difficult to be detected by conventional component test methods because of their low content. In this paper, XPS was employed to analyze the impurity phases and their space distribution *
Corresponding author. e-mail address: [email protected]
ACCEPTED MANUSCRIPT in as-grown ZGP single crystal while low content impurity phases are undetectable by conventional XRD method. In addition, heterogeneity along the growth axis was studied as well.
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2. Experimental ZGP single crystals were grown using modified Bridgman method [16] from polycrystalline materials that were synthesized by two-temperature mechanical oscillation method [17]. Growth was carried out in a crucible, enclosed in evacuated fused quartz ampoules under 5×10-4 Pa, located in a six-zone furnace which can produce proper and precise temperature gradient. The six-zone furnace can mainly produce three steady temperature regions. Temperature of the upper region of the furnace was 1070 oC and the lower region temperature was 900 oC, while the temperature gradients of the growth region were 10 oC/cm near solid-liquid interface. The crystal growth started by spontaneous nucleation and the maximum descending rate of the ampoule was 0.6 cm/day to prevent cracking of the crystal during solidification and the phase transformation at about 950 oC. After the melt was completely solidified, the grown crystals were cooled at 10 oC/h through 900 oC to room temperature. After growth, a ZGP crystal was obtained.
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Figure 1 is as-grown ZGP single crystal with the size of φ24 mm×70mm. Figure 2 shows three 2 mm thick wafers a, b and c which were cut from the neck, main body and the near the end of the grown crystal respectively. The three wafers were polished by diamond polishing liquid for test use. Elements, phases and chemical state analysis of the three wafers were carried out by EDS, XRD and XPS. In addition, elements and phases homogeneity were studied by the three test methods as well.
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3. Results and discussion 3.1 XRD Small pieces were cut off from a, b, c wafers and were ground into powders respectively. Then the three samples were characterized by XRD (λ (Cu Kα) =0.154184 nm). X-ray generator operating at 40 kV, 25 mA was used to record the X-ray diffraction patterns, the 2θ range was from 10o to 90o, and the scanning speed was 0.01 o/s. The obtained pattern is shown in Fig. 3, which are all in agreement with the standard XRD pattern (PDF 73–0398). Figure 3 indicate that grown crystal with tetragonal structure, single-phase was successfully obtained. In addition, phases of the neck, main body and the end are almost the same, and these indicate that the whole ingot we obtained has good phase homogeneity. 3.2 Components homogeneity Elemental compositions (P, Ge and Zn) of a, b and c wafers of the as-grown crystal were
measured by EDS. The results are shown in Table 1. According to the results, although the average ratio of the Ge, Zn and P is close to the ZGP stoichiometric ratio 1:1:2, contents of P reduce along the growth direction, and this results from vaporization of P which is the most volatile component. And the P gathered at the end of the quartz tube. Furthermore, it can be found that element ratios of Zn, Ge and P are nearly uniform along the growth axis of the crystal. The results indicate that the grown crystal has little composition segregation and the whole ingot we obtained has good elements homogeneity.
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Zn (%)
Ge (%)
P (%)
Zn:Ge:P
a b c
15.48 15.54 17.07
15.28 15.30 16.85
29.07 28.49 30.92
1:0.987:1.878 1:0.985:1.833 1:0.987:1.811
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3.3 XPS analysis of as-grown ZGP single crystal X-ray photoelectron spectroscopy analyses of the samples were performed with an Axis Ultra DLD spectrometer using monochromatic Al-Kα source. The instrument work function was calibrated to give a C 1s binding energy (BE) of 284.6 eV. Figure 4, Figure 5 and Figure 6 display the Zn 2p, P 2p and Ge 3d XPS comparison spectra acquired on the outer surfaces of the wafers of a, b and c respectively. All the spectra are nearly similar for the same kind of element respectively. The results indicate that the main components of a, b and c specimens are consistent. In addition, the neck and main body of the crystal have good components homogeneity which is in agreement with XRD and EDS results. However, spectra of wafer c are slightly different from that of wafer a and b. The binding energy value corresponding to the peak of wafer c is 0.2 eV larger than that of wafer a and b. For the same kind and valence of one elements analyzed by XPS, binding energy value is related to electronegativity of atoms around, the larger the electronegativity, the bigger the binding energy is. Therefore, components of wafer c is not the same as wafer a and b. Figure 4 displays the comparison of high-resolution Zn 2p XPS spectra acquired on the surface of a, b and c specimens. The intensity of spectrum of c sample is lower than that of a and b which may attribute to the contents of the elements. It is interesting to note that while the a and b specimens show only one single component on the high resolution spectrum of ZGP, a second component appears at higher binding energies of 133.2 eV and 32.4 eV respectively from Fig. 5 and Fig. 6 which is associated to the presence of oxides that may be considered to be passivation layer of the samples surface. In addition, on the surface sample c, the content of oxides are much higher than sample a and b which may result from non-stoichiometry of the tail of the crystal. Figure 7 displays the high-resolution P 2p XPS spectrum acquired on the outer surface of the b wafer. The spectrum may be fitted to four components with different intensities: the first and most intense situated at 128.3 eV and 129.0 eV that correspond to the P 2p3/2 and P 2p1/2 are attributed to the presence of the main phase of ZnGeP2. At 128.8 eV, a second component appears, which may be associated to the presence of Zn3P2 that is similar to wafers a and b. At 129.6 eV, a third component appears, which is associated to the presence of ZnP2. The existence of a small shoulder intensity at the 132.6 eV of the high-resolution P 2p peak spectrum may indicate the existence of P2O5 which is not in agreement with NIST XPS database (P2O5:135.1~135.6 eV) because of the existence of atoms (Ge, Zn) around P of different electronegativity (O P Ge Zn). Therefore, a chemical shift of binding energy of P2O5 to 132.6 eV occurred. It may indicate that impurity phases of ZGP single crystal are derived from its raw materials because XPS spectra P 2p of ZGP single crystal are similar to those of ZGP polycrystals. In addition, low content impurity phase Zn3P2 and ZnP2 cannot be eliminated during ZGP polycrystals synthesis progress[18]. It is known that a proper temperature gradient and growth rate facilitate impurity rejection[19]. This study, therefore, provide important information for our
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further ZGP single crystal growth progress improvement. Figure 8 displays the high-resolution Ge 3d XPS spectrum acquired on the outer surface of the c wafer. The spectrum may be fitted to two components with different intensities: the first and most intense situated at 30.1 eV is attributed to the presence of the main phase of ZnGeP2. At 32.2 eV, a second low content component appears, which may be attributed to the presence of GeO2 that is difficult to detect in wafers a and b. I.G. Vasilyeva et al. [15] have reported that low content Ge exist in bulk ZGP crystal, since the peak of Ge 3d of ZGP obtained here is of symmetry and overlaps with Ge element completely. It has not been possible to determine from this peak the presence of Ge in metallic state in the as-grown ZGP single crystal. As a result, at the end of the crystal, low content impurity phase exists on the surface of the samples in the form of GeO2 that cannot be detected by XRD measurement.
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4. Conclusions XRD analyses reveal that the as-grown crystal is in good structural integrity and crystallization. EDS analysis indicates that composition distribution along the growth axis is uniform and the ratios of the Zn, Ge and P elements are found to be close to the ZGP stoichiometric ratio 1:1:2. However, XPS analyses manifest that a fairly low content impurity phase exists in the form of Zn3P2 and ZnP2 in the whole as-grown crystal which cannot be detected by XRD measurement. Oxides in the form of P2O5 and GeO2 exist on the surface of the sample as passivation layer. In addition, the impurity phases in ZGP single crystal may be derived from polycrystals. Fairly low content impurity phases and their distribution of the as grown crystal provide important information to optimize our ZGP single crystal’s growth progress for its further application.
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Acknowledgement: This study was financially supported by the National Natural Science Foundation General and Key Programs of China (Nos.50672061 and 50732005) and the 863 High-Tech program of China (No.2007AA03Z443).
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Fig. 1. Photograph of as-grown ZGP single crystal Fig. 2. Mechanically polished three wafers cutting from corresponding position Fig. 3. XRD patterns of a, b, c specimens Fig. 4. Comparison of Zn 2p spectra of the samples Fig. 5. Comparison of P 2p spectra of the samples Fig. 6. Comparison of Ge 3d spectra of the samples Fig. 7. XPS spectra P 2p (black line) of sample b, peaks from deconvolution analysis (colorized lines) Fig. 8. XPS spectra Ge 3d (black line) of sample c, peaks from deconvolution analysis (colorized lines)
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1. XPS was proposed to study the low content impurity phases in ZnGeP2 crystal. 2. XPS results indicate the impurity phases are ZnP2 and Zn3P2. 3. The impurity phases may be considered to be derived from polycrystals. 4. The obtained crystal is of good structural integrity and has high crystallinity.