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Recrystallization mechanism of abnormal large grains during long growth of CVD-ZnS
Accepted Manuscript Recrystallization mechanism of abnormal large grains during long growth of CVD-ZnS Naiguang Wei, Hai Yang, Deyu Yang, Hongwei Li, Chengsong Huo, Jianming Li, Dongxu Li, Jianchun Yang, Jingjing Shi, Li Guo PII: DOI: Reference:
S0022-0248(19)30207-6 https://doi.org/10.1016/j.jcrysgro.2019.04.006 CRYS 25053
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
23 October 2018 18 March 2019 3 April 2019
Please cite this article as: N. Wei, H. Yang, D. Yang, H. Li, C. Huo, J. Li, D. Li, J. Yang, J. Shi, L. Guo, Recrystallization mechanism of abnormal large grains during long growth of CVD-ZnS, Journal of Crystal Growth (2019), doi: https://doi.org/10.1016/j.jcrysgro.2019.04.006
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.
Recrystallization mechanism of abnormal large grains during long growth of CVD-ZnS Naiguang Weia*, Hai Yanga, Deyu Yanga,b, Hongwei Lib, Chengsong Huoa, Jianming Lia , Dongxu Lia, Jianchun Yanga, Jingjing Shia, LiGuoa (a. GRIREM Guojing Advanbced Material Co., Ltd., Sanhe 065201, China; b. GRIREM Advanced Materials Co., Ltd., Beijing 100088, China) E-mail:[email protected] Abstract ： Abnormal large grains influencing highly the optical and electrical properties are commonly observed in the bottom part of CVD-ZnS layers. We have analyzed the films using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and infrared transmittance tests. Compared to normal grains, more homogeneous and simpler structures have been observed in abnormal large grains in CVD-ZnS. In addition to the obvious difference in sizes, the crystalline outline and interior structure of normal and abnormal large grains are found to be markedly different. Distorted structures and stacking faults, such as wurtzite phase and polytypes of ZnS, are not detected in the large grain areas. Preferred orientation of abnormal large grains is found to be sphalerite (111). These results could be explained as due to the recrystallization mechanism of abnormal large grains. Samples with large grains show lower infrared absorption band (at 6.2 µm wavelength) compared to samples with normal size grains. However, the infrared transmittance is found to be lower for the samples with large grains. Microstructure patterns show the presence of plenty of microdefects in boundaries between large grains, which indicates that the recrystallization in the volume with large grains is not as perfect as the recrystallization in hot isostatic pressure process (HIP). Keywords: A1.CVD-ZnS, B1.Abnormal grains, C1.Optical properties, D1. Microstructure. 1. Introduction In modern industry, chemical vapor deposition (CVD) has become the most widely-used method to produce large-scale bulk ZnS material. CVD-ZnS is a promising infrared-transmitting ceramic material for optical applications, in particular for the fabrication of infrared windows or domes. Since its first appearance in the 1970s[1] it has drawn great attention due to its superior infrared optical, and adequate mechanical properties. The industrial CVD process normally uses the reaction of H2S gas and zinc vapor to get bulk ZnS on graphite substrates[2]. However, the complexity and instability of CVD process cause various stacking faults and crystal defects to occur in the products. There are two main phases in ZnS: sphalerite (αZnS-cubic lattice) and wurtzite (βZnS-hexagonal lattice). Sphalerite is the main phase in CVD-ZnS with homogeneous optical properties. Wurtzite is an undesired phase which will cause scattering and reduce the infrared transmittance. In addition to these
two stable structures, about 194 polytypes of ZnS have been reported by researchers. These structures are stable interim phases which are formed from repeated stacking of 2D “modular layers”. They represent 1D disorder compared to the parent structures. Stacking faults, caused by various factors, can always lead to the appearance of wurtzite and polytypes[3]. In addition to the complicated phase structure, Zn-H bond is another intrinsic defect of CVD-ZnS, which causes an infrared absorption band to appear at around 6.2 μm wavelength. Formation of this defect was attributed to the reaction between the byproduct hydrogen and Zn[4]. Lewis[5] correlated the Zn-H defect to scattering, color, and the presence of sulfur vacancies. Hot isostatic pressure (HIP) treatment is normally used to eliminate these defects and get multispectral ZnS (m-ZnS)[3-6]. Recrystallization in HIP process amends the structural flaws responsible for light scattering while causing growth of some grains at the expense of others. The optical properties improve after recrystallization, while mechanical properties decrease [7,8]. In addition to the above-mentioned microdefects, abnormal large grain formation is a typical phenomenon occuring during CVD, which impacts on mechanical and optical properties of the material. The grains in the bottom part of the layer (nearer to the substrate) are larger than those in other areas. And the thickness of large grain layer varies with duration of deposition. Tilman Zscheckel[8] has described the variation trend of CVD-ZnS grain size in different layers. At a distance of 400 μm away from the substrate, crystal growth led to grains smaller than 50 μm in the plane parallel to the substrate. At a distance ≥ 1000 μm away from the substrate, the grain size decreased to around 20 μm. Some researchers[9,10] have reported that this trend occurred because of twinning, grain fragmentation, as well as the newly proposed nucleation on grain facets and change of the primary growth direction. However, these theories could not explain some phenomena perfectly, such as the thickness of bottom large grain layer increasing with the rise in deposition time. Thus, a new explanation need to be proposed for this abnormal phenomenon. In the present work, SEM (Scanning electron microscopy), TEM (Transmission electron microscopy) and XRD (X-ray diffraction) characterization methods have been used to explore the differences in microstructure, phase and elemental distribution between large grain and normal volumes. Infrared transmittance was also investigated. The mechanism of bottom large grain formation has been suggested through the above analyses. 2. Experimental 2.1. Materials and synthesis A CVD furnace built by GRINM Guojing Advanced Materials Co Ltd was used for the synthesis. Zinc vapor (99.999 % purity) and H2S gas (99.99 % purity) were inlet into the deposition chamber with argon gas to grow CVD-ZnS on graphite substrates (thickness: 32.5cm; surface area: 88cm×190cm). Table 1 shows the thermal expansion coefficients of CVD-ZnS and graphite. The deposition process lasted for about 450±5 h at a temperature of 923±10 K and pressure of 3000±100 Pa. Flow rates of Ar and H2S gases were 28±2 and 5±1 l/min respectively. The deposition rate and the
thickness of the CVD-ZnS crystal layer 68 μm/h and 30 mm respectively. All the samples discussed herein were taken from the same cycle. The expected chemical reaction is: H 2 S ( g ) Zn( g ) Z n S(s) H 2 ( g )
(1)
Table 1 The thermal expansion coefficients of CVD-ZnS and graphite
Temperature(K)
CVD-ZnS Thermal expansion coefficients(/K)
Graphite Thermal expansion coefficients(/K)
600
8.7×10-6
1.5×10-9
900
1.2×10-5
3.4×10-9
2.2 Characterization methods Powder and bulk samples were required for the characterization studies. The CVD-ZnS was powdered in a quartz crucible to get the powdered samples. The bulk CVD-ZnS samples were polished using various sizes of polishing powders to Ra = 0.08 µm level. XRD patterns were recorded on both bulk and powder samples, with 2θ angle 20-100 ° in 0.02 °/s step, using an X-ray powder diffractometer (Rigaku, Japan) with CoK α radiation (λ= 0.178752 nm) operated at a voltage of 40 kV and current 120 mA. SEM analysis was performed using a scanning electron microscope (Hitachi S-4800) operated at a voltage of 20 kV. Samples for SEM tests to observe the grain morphology were obtained by etching the polished bulk samples for 15 min at 90 ℃ with K3Fe(CN)6 and KOH (both 15 %) solutions mixed in 1:1 proportion. Surface spraying treatment with Pt layer at a pressure of 10-2 Pa was used to increase the electrical conductivity of SEM samples. Electron diffraction patterns and defects, in the interior of the grains, were observed using a transmission electron microscope (TEM, JEM-2010F). TEM samples were prepared in the following way. Firstly, the polished bulk sample was sliced and the edges were coated with acid-resistant paint to prevent the edges from dissolving too quickly. Secondly, it was washed with alcohol and then thinned using a chemical thinner (75 % HNO 3 with 25 % H 2O). Finally, this process was repeated until the desired thickness was reached. Infrared transmittance was measured at room temperature for polished samples (with a thickness of 2 mm) using a Fourier transform infrared (FTIR) spectrophotometer (Perkin Elmer LR-64912C) in the wavelength range of 2-15
μm. 3. Results and discussion 3.1. Analysis of crystal structure The direction of crystal growth in bulk CVD-ZnS materials is perpendicular to the substrate. We call, the S-orientation that along the growth direction and the P-orientation that parallel to the substrate surface. Figure 1(a) shows a polished slab cut along the S-orientation. Figure 1(b) shows the slabs cut that parallel to the P-orientation. Here, the Top, Middle and Bottom A/B are 25~29 mm, 12~16 mm and 1~5 mm respectively from graphite substrate along the growth direction.
Figure 1: (a) Slab of CVD-ZnS cut from the core perpendicular to substrate surface (Sorientation); (b) Slabs of CVD-ZnS cut from the core parallel to substrate surface (Porientation), the samples A and B come from different positions of Figure 1(a), Top A/B is 25-29 mm from the substrate surface, Middle A/B is 12-16 mm from the substrate surface, Bottom A/B is 1-5 mm from the substrate surface.
4mm thick slabs were cut in two adjacent positions (A and B, as shown in fig.1a) and at three layer heights. Slabs cut along P-orientation, presented in Figure 1(b), show similar color differences for samples A and B. The SEM images patterns, shown in Figures 2(a) to (e), present the microstructures of various areas in the bulk material. Figures 2(a) to (c) show the appearance of grains in S-orientation samples. From Figure 2(a) and 2(d), it can be seen that the grain size along the growth direction is longer than that along the direction parallel to the substrate surface in the normal areas. However, the microstructure seen in Figure 2(c) (Bottom layer) does not show this characteristic: the elongation of grain morphology is not present. And the grain size is enlarged to about 50-100 μm. The grain boundaries are also thicker. Figure 2(b) shows that the large and normal grains coexist in some part of the layer. Some large grains even appear above the small grains. The appearance of large and small grains cannot be explained by the available theories as the smaller grains formed after the grain size transition at 1000 μm are also twinned [8]. The formation of twins could be influenced by many factors such as the presence of zinc sulfide powder, zinc clusters and other impurities in the initial stage of deposition.
Figure 2: (a) SEM image of normal areas in S-orientation (9-30 mm from the substrate surface); (b) SEM image of transition layers in S-orientation (8.5-9 mm from the substrate surface); (c) SEM image of large grain areas in S-orientation (0-0.2 mm from the substrate surface); (d) SEM image of normal grain layer in P-orientation (~9 mm from the substrate surface); (e) SEM image of bottom layer in P-orientation (~0.3 mm from the substrate surface).
Figures 2(d) and (e) show the microstructure of P-orientation slabs. A decrease of grain size is found between the bottom and normal grain layers. The EDS results (insets of Figures 2(d) and (e)) indicate that there is no difference in element distribution between the two kinds of grain areas. Grains in the bottom layer do not show the typical characteristics of CVD-ZnS grains. Some of the enlarged grain boundaries can be regarded as microcracks. Meanwhile, twinning structure can easily be observed in the large grain areas. These microstructures degrade the mechanical properties. All of these changes are similar to those which took place in HIP-ZnS samples, as reported in several Refs. [7, 8, 11-12]. Recrystallization in HIP process leads to the same transformation of grain appearance that we observe within our sample. Thus, it is possible that a similar process happens in the large grain (bottom part) layer of CVD-ZnS. When the temperature reaches about 40 % of the melting point of crystal materials, recrystallization may happen in most materials[13]. The recrystallization due to thermal treatments is based on this theory. The melting point of ZnS is within
1973 - 2103 K according to different reports[14,15]. The deposition temperature of CVD-ZnS in the present study is within 900 - 1000 K. Thus, our conditions of CVD process are suitable for recrystallization. And the bottom parts of the layer may stay at high temperature for enough time for this structure change to happen.
Figure 3: (a) Microstructure image inside grain of normal area via TEM; (b) Electron diffraction pattern of normal area; (c) Microstructure image inside grain of bottom large grain area via TEM; (d, e) Electron diffraction pattern of bottom large grain area.
The TEM results, shown in Figures 3(a) to (e), indicate the differences inside grains. Figure 3(a) shows the grain appearance of normal grain parts in CVD-ZnS. The surface alternates with bright and dark bands. Some researchers[16-18], concerned with nanobelts of ZnS, also reported similar band structure in monocrystalline ZnS. The simultaneous presence of twinned cubic and hexagonal ZnS phases was described in these papers. Scocioreanu[19] has reported that, in addition to the cubic twinning, cubic and hexagonal layers with some disordered structures coexist in this region. All the patterns show only cubic twinning structure as can be seen in Figure 3(b). According to Ref.[20], the phase transformation temperature of ZnS between cubic and hexagonal phases is 1293 K. The cubic phase is the stable one under our conditions of chemical vapor deposition (deposition temperature is within 900 - 1000 K). Owing to some transient changes, such as the non-uniform distribution of reactant concentration, distorted layer stacking composed of stacking faults, twin boundaries, and dislocation pileups can lead to growth faults[21,22]. Hexagonal phase and some polytypes [23] of ZnS could
form in some regions or layers of the products. However, if the regions without stacking faults are irradiated by the TEM beam, the diffraction spots of hexagonal phase can not be found in the indexed pattern. Although hexagonal structure is not present, features of stacking faults could be seen in Figure 3(b). Huge amount of distorted structure makes the diffraction spots almost connected with each other through the white lines. Figures 3(c) to (e) are TEM patterns of inside one of the bottom large grains. There is no alternating bright/dark stripes in this region, as seen in Figure 3(c). Compared to Figure 3(a), the appearance grains in this region is more homogeneous. The haphazard distribution of white and black stripes or dots, appearing in some grains, could be caused by nitric acid etching. Diffraction patterns, shown in Figures 3(d) and 3(e), prove that both the twinning structure and fine cubic structure exist. However, when compared to Figure 3(b), these twinning diffractiion spots are not from the same structure. The twinning structure seen in Figure 3(b) could be caused by the alternating bright/dark bands inside grains. The twinning spots seen in Figure 3(e) are more independent of each other. The clearer diffraction background proves that the distorted grains become more ordered and the crystal structure becomes more perfect in the large grain region. Some researchers[22,24] have reported that when CVD-ZnS was reheated above 773 K, distorted structure becomes mobile and tends to realign toward the more-stable cubic phase. Thus, recrystallization of grains in the bottom volume of the layer is highly possible. This transition process makes the inside structure of grains more ordered. And the distorted structure can be amended effectively. 3.2. Phase and texture
Figure 4: (a) XRD patterns of powder CVD-ZnS from various layers; (b) XRD patterns of bulk CVD-ZnS from various layers (size: 10×10×4 mm3).
In order to understand more about the possibility of crystal recrystallization, XRD was used to analyze the phase and texture from various regions in CVD-ZnS. The results obtained are shown in Figures 4(a) and (b). It can be seen that the hexagonal
(100) diffraction peaks with similar intensity appear at 31.25°. A more complex diffraction background can be seen in the range of 35-37°. The “w” represents wurtzite (hexagonal) phase while “s” is sphalerite (cubic) phase. Samples, designed as top and middle layers, were cut from normal areas with small size grains. The transition layers contain both large and small grains. The samples of large grain layers were taken from the bottom of the layer. The powder XRD patterns, shown in Figure 4(a), show hexagonal structure peaks for the samples cut from normal areas and from the transition layer. Peaks of w (100) and w (101) are clearly seen in these patterns. And broad background around peaks of s (111), w (100) and w (101) are observable for these samples. For the bottom large grain layer sample, however, only cubic peaks appear. These results prove that stacking faults (hexagonal phase and polytypes) are absent or in low density in the large grain areas. Compared with Figure 4(a), hexagonal structure peaks are of lower intensity for bulk samples, as seen in Figure 4(b). The peak of w (100) cannot be seen for some samples from normal areas, while w (101) peak always appears for these samples. The patterns for bulk samples with large grains still do not contain any hexagonal structure peaks, just as in the powder XRD results. This is coincident with TEM results that hexagonal phase is only present in local parts of CVD-ZnS. Because of the limited penetration depth of X-rays, the hexagonal structure peak cannot be present if the diffracting sample does not contain wurtzite phase. Besides, according to the model of unit cells [19] , the 3C (111) and 2H (001) directions are the close-packed ones. Both powder and bulk XRD results indicate that the difference of peak strength between s (111) and other peaks is larger for the samples from close to large grain areas. The s (111) is the preferred orientation in the large grain area, which agrees with the close-packed theory. And the diffraction backgrounds of bottom layers are of lower intensity than those for other regions. These results indicate that the crystals in the large grain volume have more homogeneous structure, with more uniform phase and texture distribution. Hexagonal phase and polytypes of ZnS, caused by stacking faults in CVD process, can be removed effectively during the recrystallization process. These results show high comparability with HIP experiments previously reported [3, 6-8, 19]. 3.3. Infrared transmittance test
Figure 5: Infrared transmittance result of different layers: (a) Group A; (b) Group B.
Figure 6: Microstructure patterns of CVD-ZnS : (a) normal area in S-orientation; (b) large grain area in S-orientation.
The results of infrared transmittance test are shown in Figures 5(a) and 5(b). The samples were cut from bulk CVD-ZnS. Top and middle layers were cut from the normal volume with small grains, while the bottom layer was cut from the large grain area. There is always an absorption peak at around 6.2 μm wavelength in normal CVD-ZnS. H2, one of the by-products in CVD process, is always remaining in the bulk materials and occupying vacancies of sulfur to form Zn-H complex. This defective structure can lead to the resonance absorption at around 6.2 μm wavelength. This absorption peak is universally seen in CVD-ZnS. And HIP method is always used to remove it, via recrystallization . The redundant H can be expelled from the materials during recrystallization[4]. Two sets of results of infrared transmittance show similar optical features. The absorption peaks at around 6.2 μm are weaker for the bottom layer samples, their transmittance at this wavelength about 5 % higher than that of CVD-ZnS taken in the other locations. This indicates that the Zn-H bond in the large grain region is eliminated effectively during the recrystallization process. This trend is similar to what was observed with HIP samples, which means the recrystallization in bottom area can promote the expelling of H. However, the large grains in the bottom layer lead to the decrease in short to middle wavelength infrared transmittance (around 2 to 7 μm except for the characteristic absorption peak at around 6.2 μm). This trend may be caused by the enlarged grain boundaries in the large grain area. From Figure 6, it can be understood that the grains in normal areas are arranged closely. Almost no holes or micro-cracks can be found between them. However, in the large grains area, holes or micro-cracks whose sizes are within 1 to 5 μm can always be observed between grains. It indicates that the recrystallization of grains is not as perfect as the recrystallization process in HIP. Vast defective structures are left in the large grain area. And their dimension is similar to short to middle infrared wavelength. In addition to causing adverse impacts on mechanical properties, these defects can be considered as scattering centers which decrease the transmission of short to middle wavelength infrared [15]. Conclusions The analyses of crystal structure, phase and textures proved that the grains in the bottom area (up to 5mm thickness) of a 30mm thick CVD-ZnS layer had
gone through a recrystallization process before the end of the 450 hrs-long chemical vapor deposition at 900 K. According to crystal microstructure analysis, this transition prompted the enlargement of grain size and their boundaries. TEM patterns indicated that the alternating dark/bright band structure in CVD-ZnS grains could not be observed in the large grain area. Electron diffraction and XRD patterns showed coincident results. Clearer diffraction backgrounds were obtained from the large grain area. And the preferred orientation in this region was s (111). The occurence of twinned cubic and hexagonal ZnS phases was confirmed in different regions. These results proved that some unexpected and distorted structures, such as wurtzite phase and polytypes of ZnS, were removed during the recrystallization growth of grains. The results of XRD also showed that there are similar phases and textures inside and outside the large grain area, the cubic phase is dominant, and there are a few hexagonal phases inside the material, possible a polyhedral ZnS structure formed by the wrong stacking of hexagonal and cubic phases. Infrared transmittance tests indicated that areas with large grains had a higher transmittance at wavelength of around 6.2 μm. The 6.2 μm absorption band caused by Zn-H was weakened in the large grain area. The recrystallization growth process could perfect the homogeneity inside grains. However, the transmittance of large grain layers in short to middle wave infrared region was lower than in the normal grains area in CVD-ZnS. The microstructure of large grain area was not as perfect as that in an HIP recrystallized crystal. A mass of holes and microcracks between grains could lead to scattering and degrade infrared transmittance. Acknowledgements This work was financially supported by the General Armament Department, People's Liberation Army. References [1] D. C. Harris, Development of hot-pressed and chemical-vapor-deposited zinc sulfide and zinc selenide in the United States for optical windows, Proc. SPIE. 6545(2007) 654502. [2] J. McCloy, International development of chemical vapor deposited zinc sulfide, Proc. SPIE. 6545 (2007) 654503. [3] Myers S M, Baskes M I, Haller E E, Hydrogen interactions with defects in crystalline solids, Review of Modern Physics. 64(2008) 559–617. [4] Lewis K L, Arthur G S, Banyard S A, Hydrogen-related defects in vapour-deposited zinc sulphide, Journal of Crystal Growth. 66(1984) 125-136. [5]A. F. Shchurov, E. M. Gavrishchuk, V. B. Ikonnikov, Effect of hot isostatic pressing on the elastic and optical properties of polycrystalline CVD ZnS, Inorg. Mater. 40(2004)336–339. [6]E. Karaksina, V. Ikonnikov, E. Gavrishchuk, Recrystallization behavior of ZnS
during hot isostatic pressing, Inorg. Mater. 43(2007)452–454. [7]T Zscheckel, W Wisniewski, A Gebhardt, Mechanisms counteracting the growth of large grains in industrial ZnS grown by Chemical Vapor Deposition, ACS Applied materials & interfaces. 6(2014)394-400. [8]T. Zscheckel, W. Wisniewski, C. Rüssel, Microstructure and texture of polycrystalline CVD-ZnS analyzed via EBSD, Advanced Functional Materials. 22(2012)4969–4974. [9]John. S. McCloy, Ralph Korenstein, Brian Zelinski, Effects of temperature, pressure, and metal promoter on the recrystallized structure and optical transmission of Chemical Vapor Deposited Zinc Sulfide, Journal of the American Ceramic Society. 92(2009)7. [10]X. Fang, Y. Bando, U. K. Gautam, ZnO and ZnS nanostructures: ultraviolet-light emitters, lasers, and sensors, CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 34(2009)190-223. [11]N. K. Morozova, I. A. Karetnikov, V. G. Plotnichenko, E. M. Gavrishchuk, E. V. Yashina, Transformation of luminescence centers in CVD ZnS films subject to a high hydrostatic pressure, Semiconductors. 38(2004)36–41. [12]K. L. Lewis, G. S. Arthur, S. A. Banyard, Hydrogen-related defects in vapour-deposited zinc sulphide, J. Cryst. Growth. 66(1984)125–136. [13]H. Z. Yu, Infrared optical materials, National Defense Industry Press, Beijing, 2015. [14]John Mccloy, Randal Tustison, Chemical vapor deposited zinc sulfide. Washington: SPIE. (2013)74-75. [15] Yang De Yu, Yang Hai, Review of defects in bulk CVD ZnS, Bulletin of The Chinese Ceramic Society. 36(2017). [16]L. Henneman, L. LaCroix, C. Wilson, Thermal, structural, and optical properties of cleartran multispectral zinc sulfide, Opt. Eng. 47(2009)099801-099801-1. [17]Z. L. Wang, Piezoelectric nanogenerators-their principle and potential applications, Physics. 35(2006) 897-903. [18]J.S. McCloy, Properties and processing of Chemical Vapor Deposited Zinc Sulfide, Dissertations & Theses-Gradworks, 2008. [19]M. Scocioreanu, M. Baibarac, Photoluminescence and Raman evidence for mechanico-chemical interaction of polyaniline-emeraldine base with ZnS in cubic and hexagonal phase, J. Solid State Chem. 186(2012)217–223. [20]John Mccloy, Randal Tustison, Chemical vapor deposited zinc sulfide. Washington: SPIE. (2013)9-11. [21]Cheng C, Fei J D, Numerical simulation on the optical transmission distortions throughout the laminar flow field, Infrared & Laser Engineering. 34(2005)548-552. [22]Y. Li, Transparent and luminescent ZnS ceramics consolidated by vacuum hot pressing method, J. Am. Ceram. Soc. 98(2015)2972–2975. [23]G. E. Engel, R. J. Needs, Total energy calculations on zinc sulphide polytypes, J. Phys. Condens. Matter. 2(1990)367-376. [24]C. A. Klein, R. N. Donadio, Infrared-active phonons in cubic zinc sulfide, J. Appl. Phys. 51(1980)797–800.
Highlights： Normal and abnormal grain areas for CVD- ZnS using TEM were performed. Preferred orientation of abnormal large grains was sphalerite (111). Microstructure patterns show the presence of plenty of microdefects in boundaries between large grains.
S0022-0248(19)30207-6 https://doi.org/10.1016/j.jcrysgro.2019.04.006 CRYS 25053
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
23 October 2018 18 March 2019 3 April 2019
Please cite this article as: N. Wei, H. Yang, D. Yang, H. Li, C. Huo, J. Li, D. Li, J. Yang, J. Shi, L. Guo, Recrystallization mechanism of abnormal large grains during long growth of CVD-ZnS, Journal of Crystal Growth (2019), doi: https://doi.org/10.1016/j.jcrysgro.2019.04.006
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Recrystallization mechanism of abnormal large grains during long growth of CVD-ZnS Naiguang Weia*, Hai Yanga, Deyu Yanga,b, Hongwei Lib, Chengsong Huoa, Jianming Lia , Dongxu Lia, Jianchun Yanga, Jingjing Shia, LiGuoa (a. GRIREM Guojing Advanbced Material Co., Ltd., Sanhe 065201, China; b. GRIREM Advanced Materials Co., Ltd., Beijing 100088, China) E-mail:[email protected] Abstract ： Abnormal large grains influencing highly the optical and electrical properties are commonly observed in the bottom part of CVD-ZnS layers. We have analyzed the films using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and infrared transmittance tests. Compared to normal grains, more homogeneous and simpler structures have been observed in abnormal large grains in CVD-ZnS. In addition to the obvious difference in sizes, the crystalline outline and interior structure of normal and abnormal large grains are found to be markedly different. Distorted structures and stacking faults, such as wurtzite phase and polytypes of ZnS, are not detected in the large grain areas. Preferred orientation of abnormal large grains is found to be sphalerite (111). These results could be explained as due to the recrystallization mechanism of abnormal large grains. Samples with large grains show lower infrared absorption band (at 6.2 µm wavelength) compared to samples with normal size grains. However, the infrared transmittance is found to be lower for the samples with large grains. Microstructure patterns show the presence of plenty of microdefects in boundaries between large grains, which indicates that the recrystallization in the volume with large grains is not as perfect as the recrystallization in hot isostatic pressure process (HIP). Keywords: A1.CVD-ZnS, B1.Abnormal grains, C1.Optical properties, D1. Microstructure. 1. Introduction In modern industry, chemical vapor deposition (CVD) has become the most widely-used method to produce large-scale bulk ZnS material. CVD-ZnS is a promising infrared-transmitting ceramic material for optical applications, in particular for the fabrication of infrared windows or domes. Since its first appearance in the 1970s[1] it has drawn great attention due to its superior infrared optical, and adequate mechanical properties. The industrial CVD process normally uses the reaction of H2S gas and zinc vapor to get bulk ZnS on graphite substrates[2]. However, the complexity and instability of CVD process cause various stacking faults and crystal defects to occur in the products. There are two main phases in ZnS: sphalerite (αZnS-cubic lattice) and wurtzite (βZnS-hexagonal lattice). Sphalerite is the main phase in CVD-ZnS with homogeneous optical properties. Wurtzite is an undesired phase which will cause scattering and reduce the infrared transmittance. In addition to these
two stable structures, about 194 polytypes of ZnS have been reported by researchers. These structures are stable interim phases which are formed from repeated stacking of 2D “modular layers”. They represent 1D disorder compared to the parent structures. Stacking faults, caused by various factors, can always lead to the appearance of wurtzite and polytypes[3]. In addition to the complicated phase structure, Zn-H bond is another intrinsic defect of CVD-ZnS, which causes an infrared absorption band to appear at around 6.2 μm wavelength. Formation of this defect was attributed to the reaction between the byproduct hydrogen and Zn[4]. Lewis[5] correlated the Zn-H defect to scattering, color, and the presence of sulfur vacancies. Hot isostatic pressure (HIP) treatment is normally used to eliminate these defects and get multispectral ZnS (m-ZnS)[3-6]. Recrystallization in HIP process amends the structural flaws responsible for light scattering while causing growth of some grains at the expense of others. The optical properties improve after recrystallization, while mechanical properties decrease [7,8]. In addition to the above-mentioned microdefects, abnormal large grain formation is a typical phenomenon occuring during CVD, which impacts on mechanical and optical properties of the material. The grains in the bottom part of the layer (nearer to the substrate) are larger than those in other areas. And the thickness of large grain layer varies with duration of deposition. Tilman Zscheckel[8] has described the variation trend of CVD-ZnS grain size in different layers. At a distance of 400 μm away from the substrate, crystal growth led to grains smaller than 50 μm in the plane parallel to the substrate. At a distance ≥ 1000 μm away from the substrate, the grain size decreased to around 20 μm. Some researchers[9,10] have reported that this trend occurred because of twinning, grain fragmentation, as well as the newly proposed nucleation on grain facets and change of the primary growth direction. However, these theories could not explain some phenomena perfectly, such as the thickness of bottom large grain layer increasing with the rise in deposition time. Thus, a new explanation need to be proposed for this abnormal phenomenon. In the present work, SEM (Scanning electron microscopy), TEM (Transmission electron microscopy) and XRD (X-ray diffraction) characterization methods have been used to explore the differences in microstructure, phase and elemental distribution between large grain and normal volumes. Infrared transmittance was also investigated. The mechanism of bottom large grain formation has been suggested through the above analyses. 2. Experimental 2.1. Materials and synthesis A CVD furnace built by GRINM Guojing Advanced Materials Co Ltd was used for the synthesis. Zinc vapor (99.999 % purity) and H2S gas (99.99 % purity) were inlet into the deposition chamber with argon gas to grow CVD-ZnS on graphite substrates (thickness: 32.5cm; surface area: 88cm×190cm). Table 1 shows the thermal expansion coefficients of CVD-ZnS and graphite. The deposition process lasted for about 450±5 h at a temperature of 923±10 K and pressure of 3000±100 Pa. Flow rates of Ar and H2S gases were 28±2 and 5±1 l/min respectively. The deposition rate and the
thickness of the CVD-ZnS crystal layer 68 μm/h and 30 mm respectively. All the samples discussed herein were taken from the same cycle. The expected chemical reaction is: H 2 S ( g ) Zn( g ) Z n S(s) H 2 ( g )
(1)
Table 1 The thermal expansion coefficients of CVD-ZnS and graphite
Temperature(K)
CVD-ZnS Thermal expansion coefficients(/K)
Graphite Thermal expansion coefficients(/K)
600
8.7×10-6
1.5×10-9
900
1.2×10-5
3.4×10-9
2.2 Characterization methods Powder and bulk samples were required for the characterization studies. The CVD-ZnS was powdered in a quartz crucible to get the powdered samples. The bulk CVD-ZnS samples were polished using various sizes of polishing powders to Ra = 0.08 µm level. XRD patterns were recorded on both bulk and powder samples, with 2θ angle 20-100 ° in 0.02 °/s step, using an X-ray powder diffractometer (Rigaku, Japan) with CoK α radiation (λ= 0.178752 nm) operated at a voltage of 40 kV and current 120 mA. SEM analysis was performed using a scanning electron microscope (Hitachi S-4800) operated at a voltage of 20 kV. Samples for SEM tests to observe the grain morphology were obtained by etching the polished bulk samples for 15 min at 90 ℃ with K3Fe(CN)6 and KOH (both 15 %) solutions mixed in 1:1 proportion. Surface spraying treatment with Pt layer at a pressure of 10-2 Pa was used to increase the electrical conductivity of SEM samples. Electron diffraction patterns and defects, in the interior of the grains, were observed using a transmission electron microscope (TEM, JEM-2010F). TEM samples were prepared in the following way. Firstly, the polished bulk sample was sliced and the edges were coated with acid-resistant paint to prevent the edges from dissolving too quickly. Secondly, it was washed with alcohol and then thinned using a chemical thinner (75 % HNO 3 with 25 % H 2O). Finally, this process was repeated until the desired thickness was reached. Infrared transmittance was measured at room temperature for polished samples (with a thickness of 2 mm) using a Fourier transform infrared (FTIR) spectrophotometer (Perkin Elmer LR-64912C) in the wavelength range of 2-15
μm. 3. Results and discussion 3.1. Analysis of crystal structure The direction of crystal growth in bulk CVD-ZnS materials is perpendicular to the substrate. We call, the S-orientation that along the growth direction and the P-orientation that parallel to the substrate surface. Figure 1(a) shows a polished slab cut along the S-orientation. Figure 1(b) shows the slabs cut that parallel to the P-orientation. Here, the Top, Middle and Bottom A/B are 25~29 mm, 12~16 mm and 1~5 mm respectively from graphite substrate along the growth direction.
Figure 1: (a) Slab of CVD-ZnS cut from the core perpendicular to substrate surface (Sorientation); (b) Slabs of CVD-ZnS cut from the core parallel to substrate surface (Porientation), the samples A and B come from different positions of Figure 1(a), Top A/B is 25-29 mm from the substrate surface, Middle A/B is 12-16 mm from the substrate surface, Bottom A/B is 1-5 mm from the substrate surface.
4mm thick slabs were cut in two adjacent positions (A and B, as shown in fig.1a) and at three layer heights. Slabs cut along P-orientation, presented in Figure 1(b), show similar color differences for samples A and B. The SEM images patterns, shown in Figures 2(a) to (e), present the microstructures of various areas in the bulk material. Figures 2(a) to (c) show the appearance of grains in S-orientation samples. From Figure 2(a) and 2(d), it can be seen that the grain size along the growth direction is longer than that along the direction parallel to the substrate surface in the normal areas. However, the microstructure seen in Figure 2(c) (Bottom layer) does not show this characteristic: the elongation of grain morphology is not present. And the grain size is enlarged to about 50-100 μm. The grain boundaries are also thicker. Figure 2(b) shows that the large and normal grains coexist in some part of the layer. Some large grains even appear above the small grains. The appearance of large and small grains cannot be explained by the available theories as the smaller grains formed after the grain size transition at 1000 μm are also twinned [8]. The formation of twins could be influenced by many factors such as the presence of zinc sulfide powder, zinc clusters and other impurities in the initial stage of deposition.
Figure 2: (a) SEM image of normal areas in S-orientation (9-30 mm from the substrate surface); (b) SEM image of transition layers in S-orientation (8.5-9 mm from the substrate surface); (c) SEM image of large grain areas in S-orientation (0-0.2 mm from the substrate surface); (d) SEM image of normal grain layer in P-orientation (~9 mm from the substrate surface); (e) SEM image of bottom layer in P-orientation (~0.3 mm from the substrate surface).
Figures 2(d) and (e) show the microstructure of P-orientation slabs. A decrease of grain size is found between the bottom and normal grain layers. The EDS results (insets of Figures 2(d) and (e)) indicate that there is no difference in element distribution between the two kinds of grain areas. Grains in the bottom layer do not show the typical characteristics of CVD-ZnS grains. Some of the enlarged grain boundaries can be regarded as microcracks. Meanwhile, twinning structure can easily be observed in the large grain areas. These microstructures degrade the mechanical properties. All of these changes are similar to those which took place in HIP-ZnS samples, as reported in several Refs. [7, 8, 11-12]. Recrystallization in HIP process leads to the same transformation of grain appearance that we observe within our sample. Thus, it is possible that a similar process happens in the large grain (bottom part) layer of CVD-ZnS. When the temperature reaches about 40 % of the melting point of crystal materials, recrystallization may happen in most materials[13]. The recrystallization due to thermal treatments is based on this theory. The melting point of ZnS is within
1973 - 2103 K according to different reports[14,15]. The deposition temperature of CVD-ZnS in the present study is within 900 - 1000 K. Thus, our conditions of CVD process are suitable for recrystallization. And the bottom parts of the layer may stay at high temperature for enough time for this structure change to happen.
Figure 3: (a) Microstructure image inside grain of normal area via TEM; (b) Electron diffraction pattern of normal area; (c) Microstructure image inside grain of bottom large grain area via TEM; (d, e) Electron diffraction pattern of bottom large grain area.
The TEM results, shown in Figures 3(a) to (e), indicate the differences inside grains. Figure 3(a) shows the grain appearance of normal grain parts in CVD-ZnS. The surface alternates with bright and dark bands. Some researchers[16-18], concerned with nanobelts of ZnS, also reported similar band structure in monocrystalline ZnS. The simultaneous presence of twinned cubic and hexagonal ZnS phases was described in these papers. Scocioreanu[19] has reported that, in addition to the cubic twinning, cubic and hexagonal layers with some disordered structures coexist in this region. All the patterns show only cubic twinning structure as can be seen in Figure 3(b). According to Ref.[20], the phase transformation temperature of ZnS between cubic and hexagonal phases is 1293 K. The cubic phase is the stable one under our conditions of chemical vapor deposition (deposition temperature is within 900 - 1000 K). Owing to some transient changes, such as the non-uniform distribution of reactant concentration, distorted layer stacking composed of stacking faults, twin boundaries, and dislocation pileups can lead to growth faults[21,22]. Hexagonal phase and some polytypes [23] of ZnS could
form in some regions or layers of the products. However, if the regions without stacking faults are irradiated by the TEM beam, the diffraction spots of hexagonal phase can not be found in the indexed pattern. Although hexagonal structure is not present, features of stacking faults could be seen in Figure 3(b). Huge amount of distorted structure makes the diffraction spots almost connected with each other through the white lines. Figures 3(c) to (e) are TEM patterns of inside one of the bottom large grains. There is no alternating bright/dark stripes in this region, as seen in Figure 3(c). Compared to Figure 3(a), the appearance grains in this region is more homogeneous. The haphazard distribution of white and black stripes or dots, appearing in some grains, could be caused by nitric acid etching. Diffraction patterns, shown in Figures 3(d) and 3(e), prove that both the twinning structure and fine cubic structure exist. However, when compared to Figure 3(b), these twinning diffractiion spots are not from the same structure. The twinning structure seen in Figure 3(b) could be caused by the alternating bright/dark bands inside grains. The twinning spots seen in Figure 3(e) are more independent of each other. The clearer diffraction background proves that the distorted grains become more ordered and the crystal structure becomes more perfect in the large grain region. Some researchers[22,24] have reported that when CVD-ZnS was reheated above 773 K, distorted structure becomes mobile and tends to realign toward the more-stable cubic phase. Thus, recrystallization of grains in the bottom volume of the layer is highly possible. This transition process makes the inside structure of grains more ordered. And the distorted structure can be amended effectively. 3.2. Phase and texture
Figure 4: (a) XRD patterns of powder CVD-ZnS from various layers; (b) XRD patterns of bulk CVD-ZnS from various layers (size: 10×10×4 mm3).
In order to understand more about the possibility of crystal recrystallization, XRD was used to analyze the phase and texture from various regions in CVD-ZnS. The results obtained are shown in Figures 4(a) and (b). It can be seen that the hexagonal
(100) diffraction peaks with similar intensity appear at 31.25°. A more complex diffraction background can be seen in the range of 35-37°. The “w” represents wurtzite (hexagonal) phase while “s” is sphalerite (cubic) phase. Samples, designed as top and middle layers, were cut from normal areas with small size grains. The transition layers contain both large and small grains. The samples of large grain layers were taken from the bottom of the layer. The powder XRD patterns, shown in Figure 4(a), show hexagonal structure peaks for the samples cut from normal areas and from the transition layer. Peaks of w (100) and w (101) are clearly seen in these patterns. And broad background around peaks of s (111), w (100) and w (101) are observable for these samples. For the bottom large grain layer sample, however, only cubic peaks appear. These results prove that stacking faults (hexagonal phase and polytypes) are absent or in low density in the large grain areas. Compared with Figure 4(a), hexagonal structure peaks are of lower intensity for bulk samples, as seen in Figure 4(b). The peak of w (100) cannot be seen for some samples from normal areas, while w (101) peak always appears for these samples. The patterns for bulk samples with large grains still do not contain any hexagonal structure peaks, just as in the powder XRD results. This is coincident with TEM results that hexagonal phase is only present in local parts of CVD-ZnS. Because of the limited penetration depth of X-rays, the hexagonal structure peak cannot be present if the diffracting sample does not contain wurtzite phase. Besides, according to the model of unit cells [19] , the 3C (111) and 2H (001) directions are the close-packed ones. Both powder and bulk XRD results indicate that the difference of peak strength between s (111) and other peaks is larger for the samples from close to large grain areas. The s (111) is the preferred orientation in the large grain area, which agrees with the close-packed theory. And the diffraction backgrounds of bottom layers are of lower intensity than those for other regions. These results indicate that the crystals in the large grain volume have more homogeneous structure, with more uniform phase and texture distribution. Hexagonal phase and polytypes of ZnS, caused by stacking faults in CVD process, can be removed effectively during the recrystallization process. These results show high comparability with HIP experiments previously reported [3, 6-8, 19]. 3.3. Infrared transmittance test
Figure 5: Infrared transmittance result of different layers: (a) Group A; (b) Group B.
Figure 6: Microstructure patterns of CVD-ZnS : (a) normal area in S-orientation; (b) large grain area in S-orientation.
The results of infrared transmittance test are shown in Figures 5(a) and 5(b). The samples were cut from bulk CVD-ZnS. Top and middle layers were cut from the normal volume with small grains, while the bottom layer was cut from the large grain area. There is always an absorption peak at around 6.2 μm wavelength in normal CVD-ZnS. H2, one of the by-products in CVD process, is always remaining in the bulk materials and occupying vacancies of sulfur to form Zn-H complex. This defective structure can lead to the resonance absorption at around 6.2 μm wavelength. This absorption peak is universally seen in CVD-ZnS. And HIP method is always used to remove it, via recrystallization . The redundant H can be expelled from the materials during recrystallization[4]. Two sets of results of infrared transmittance show similar optical features. The absorption peaks at around 6.2 μm are weaker for the bottom layer samples, their transmittance at this wavelength about 5 % higher than that of CVD-ZnS taken in the other locations. This indicates that the Zn-H bond in the large grain region is eliminated effectively during the recrystallization process. This trend is similar to what was observed with HIP samples, which means the recrystallization in bottom area can promote the expelling of H. However, the large grains in the bottom layer lead to the decrease in short to middle wavelength infrared transmittance (around 2 to 7 μm except for the characteristic absorption peak at around 6.2 μm). This trend may be caused by the enlarged grain boundaries in the large grain area. From Figure 6, it can be understood that the grains in normal areas are arranged closely. Almost no holes or micro-cracks can be found between them. However, in the large grains area, holes or micro-cracks whose sizes are within 1 to 5 μm can always be observed between grains. It indicates that the recrystallization of grains is not as perfect as the recrystallization process in HIP. Vast defective structures are left in the large grain area. And their dimension is similar to short to middle infrared wavelength. In addition to causing adverse impacts on mechanical properties, these defects can be considered as scattering centers which decrease the transmission of short to middle wavelength infrared [15]. Conclusions The analyses of crystal structure, phase and textures proved that the grains in the bottom area (up to 5mm thickness) of a 30mm thick CVD-ZnS layer had
gone through a recrystallization process before the end of the 450 hrs-long chemical vapor deposition at 900 K. According to crystal microstructure analysis, this transition prompted the enlargement of grain size and their boundaries. TEM patterns indicated that the alternating dark/bright band structure in CVD-ZnS grains could not be observed in the large grain area. Electron diffraction and XRD patterns showed coincident results. Clearer diffraction backgrounds were obtained from the large grain area. And the preferred orientation in this region was s (111). The occurence of twinned cubic and hexagonal ZnS phases was confirmed in different regions. These results proved that some unexpected and distorted structures, such as wurtzite phase and polytypes of ZnS, were removed during the recrystallization growth of grains. The results of XRD also showed that there are similar phases and textures inside and outside the large grain area, the cubic phase is dominant, and there are a few hexagonal phases inside the material, possible a polyhedral ZnS structure formed by the wrong stacking of hexagonal and cubic phases. Infrared transmittance tests indicated that areas with large grains had a higher transmittance at wavelength of around 6.2 μm. The 6.2 μm absorption band caused by Zn-H was weakened in the large grain area. The recrystallization growth process could perfect the homogeneity inside grains. However, the transmittance of large grain layers in short to middle wave infrared region was lower than in the normal grains area in CVD-ZnS. The microstructure of large grain area was not as perfect as that in an HIP recrystallized crystal. A mass of holes and microcracks between grains could lead to scattering and degrade infrared transmittance. Acknowledgements This work was financially supported by the General Armament Department, People's Liberation Army. References [1] D. C. Harris, Development of hot-pressed and chemical-vapor-deposited zinc sulfide and zinc selenide in the United States for optical windows, Proc. SPIE. 6545(2007) 654502. [2] J. McCloy, International development of chemical vapor deposited zinc sulfide, Proc. SPIE. 6545 (2007) 654503. [3] Myers S M, Baskes M I, Haller E E, Hydrogen interactions with defects in crystalline solids, Review of Modern Physics. 64(2008) 559–617. [4] Lewis K L, Arthur G S, Banyard S A, Hydrogen-related defects in vapour-deposited zinc sulphide, Journal of Crystal Growth. 66(1984) 125-136. [5]A. F. Shchurov, E. M. Gavrishchuk, V. B. Ikonnikov, Effect of hot isostatic pressing on the elastic and optical properties of polycrystalline CVD ZnS, Inorg. Mater. 40(2004)336–339. [6]E. Karaksina, V. Ikonnikov, E. Gavrishchuk, Recrystallization behavior of ZnS
during hot isostatic pressing, Inorg. Mater. 43(2007)452–454. [7]T Zscheckel, W Wisniewski, A Gebhardt, Mechanisms counteracting the growth of large grains in industrial ZnS grown by Chemical Vapor Deposition, ACS Applied materials & interfaces. 6(2014)394-400. [8]T. Zscheckel, W. Wisniewski, C. Rüssel, Microstructure and texture of polycrystalline CVD-ZnS analyzed via EBSD, Advanced Functional Materials. 22(2012)4969–4974. [9]John. S. McCloy, Ralph Korenstein, Brian Zelinski, Effects of temperature, pressure, and metal promoter on the recrystallized structure and optical transmission of Chemical Vapor Deposited Zinc Sulfide, Journal of the American Ceramic Society. 92(2009)7. [10]X. Fang, Y. Bando, U. K. Gautam, ZnO and ZnS nanostructures: ultraviolet-light emitters, lasers, and sensors, CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 34(2009)190-223. [11]N. K. Morozova, I. A. Karetnikov, V. G. Plotnichenko, E. M. Gavrishchuk, E. V. Yashina, Transformation of luminescence centers in CVD ZnS films subject to a high hydrostatic pressure, Semiconductors. 38(2004)36–41. [12]K. L. Lewis, G. S. Arthur, S. A. Banyard, Hydrogen-related defects in vapour-deposited zinc sulphide, J. Cryst. Growth. 66(1984)125–136. [13]H. Z. Yu, Infrared optical materials, National Defense Industry Press, Beijing, 2015. [14]John Mccloy, Randal Tustison, Chemical vapor deposited zinc sulfide. Washington: SPIE. (2013)74-75. [15] Yang De Yu, Yang Hai, Review of defects in bulk CVD ZnS, Bulletin of The Chinese Ceramic Society. 36(2017). [16]L. Henneman, L. LaCroix, C. Wilson, Thermal, structural, and optical properties of cleartran multispectral zinc sulfide, Opt. Eng. 47(2009)099801-099801-1. [17]Z. L. Wang, Piezoelectric nanogenerators-their principle and potential applications, Physics. 35(2006) 897-903. [18]J.S. McCloy, Properties and processing of Chemical Vapor Deposited Zinc Sulfide, Dissertations & Theses-Gradworks, 2008. [19]M. Scocioreanu, M. Baibarac, Photoluminescence and Raman evidence for mechanico-chemical interaction of polyaniline-emeraldine base with ZnS in cubic and hexagonal phase, J. Solid State Chem. 186(2012)217–223. [20]John Mccloy, Randal Tustison, Chemical vapor deposited zinc sulfide. Washington: SPIE. (2013)9-11. [21]Cheng C, Fei J D, Numerical simulation on the optical transmission distortions throughout the laminar flow field, Infrared & Laser Engineering. 34(2005)548-552. [22]Y. Li, Transparent and luminescent ZnS ceramics consolidated by vacuum hot pressing method, J. Am. Ceram. Soc. 98(2015)2972–2975. [23]G. E. Engel, R. J. Needs, Total energy calculations on zinc sulphide polytypes, J. Phys. Condens. Matter. 2(1990)367-376. [24]C. A. Klein, R. N. Donadio, Infrared-active phonons in cubic zinc sulfide, J. Appl. Phys. 51(1980)797–800.
Highlights： Normal and abnormal grain areas for CVD- ZnS using TEM were performed. Preferred orientation of abnormal large grains was sphalerite (111). Microstructure patterns show the presence of plenty of microdefects in boundaries between large grains.