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Influence of the pulling rate on the properties of ZnGeP2 crystal grown by vertical Bridgman method
Journal of Crystal Growth 445 (2016) 37–41
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Influence of the pulling rate on the properties of ZnGeP2 crystal grown by vertical Bridgman method Liang Shen, Dong Wu n State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 December 2015 Received in revised form 4 April 2016 Accepted 12 April 2016 Available online 13 April 2016
Zinc–germanium diphosphide (ZGP) crystals (15 mm in diameter and 65 mm in length) were successfully grown by the modified vertical Bridgman method on seeds at different pulling rates (0.5 mm/h and 0.75 mm/h) in order to study the defect generation during crystal growth. At the different positions (the onset, middle and end) in single crystals, their properties of ZGP crystals were investigated by X-ray diffraction, etching technique and optical transmission spectra. The results indicate that the increase in the pulling rate deteriorates the crystal quality at the onset part of the single crystals. The etch pit density (EPD) and the full width at half maximum (FWHM) of the X-ray rocking curves increase, while the optical transmittance decreases with increasing pulling rate. However, the increase in the pulling rate hardly influences the crystal quality at the middle and end part of the single crystals. & 2016 Elsevier B.V. All rights reserved.
Keywords: Pulling rate Defects Bridgman technique Single crystal growth Nonlinear optic materials
1. Introduction Because of its multiple excellent properties including optical, thermal and mechanical properties, Zinc–germanium diphosphide (ZGP) crystal can be applied in optical parametric oscillators (OPOs) based on type I phase-matching to produce laser sources at 3–12 mm [1], which are widely used in infrared countermeasures, molecular spectroscopy, invasive surgery and detection of gas [2,3]. Since the 1960s, ZGP crystals have attracted much interest [4]. But at first the purity and quantity of ZGP polycrystalline charges were low, and the size of ZGP single crystals was small [5–7]. Until 1997, Schunemann et al. successfully used the horizontal gradient freeze (HGF) method to grow large single crystals and the optical absorption coefficient could reach 0.09 cm 1 at 2.05 mm, which is the basic requirement for the OPO applications [8]. Later, the ZGP single crystal up to 27 39 140 mm3 in size was grown by the HGF method [9]. In 1997, G.A. Verozubova et al. made a considerable improvement in ZGP polycrystal synthesis and could synthesize 500 g charges in one synthesis run [10]. They grew ZGP single crystals by the vertical Bridgman (VB) method and the vertical gradient freeze (VGF) method [10]. At present using the thermal annealing and electron irradiation, the optical absorption coefficient can be decreased down to 0.02–0.04 cm 1 at 2.06 mm by G.A. Verozubova group [11]. Since entering the 21st century, n
Corresponding author. E-mail address: [email protected] (D. Wu).
http://dx.doi.org/10.1016/j.jcrysgro.2016.04.025 0022-0248/& 2016 Elsevier B.V. All rights reserved.
ZGP crystals have attracted many related research topics, including growth, polishing, annealing, laser-induced damage and theoretical studies on native defects [9,11–16]. However, research on the single crystal growth technique is still one of the most important topics. Among many ZGP crystal growth methods such as the VB method [10], the VGF method [10], the HGF method [8], the highpressure vapor transport (HPVT) method [17] and the liquid encapsulated Czochralski (LEC) method [18], the VB method and the HGF method are more widely used. In this paper we also used the VB method to grow ZGP single crystals. In any ZGP single crystal growth process, the quality of the ZGP crystal is affected by a number of factors such as pulling rate, rotation rate, temperature gradient, growth ampoule shape, seed orientation, etc. In the VB method, pulling rate is an important parameter to control the thermal balance at the solid/melt interface. A suitable pulling rate can make the melt–solid phase transition continuous and stable. A high pulling rate usually induces a large thermal stress, which will result in more risks of crystal cracking. Conversely, a low pulling rate takes more time and reduces the growth efficiency. So choosing a suitable pulling rate is favorable for both crystal quality and growth efficiency. Furthermore, different crystals usually need the different pulling rates, for example, for III–V ternary compounds with a temperature gradient of about 10 °C/cm, the pulling rate lies within the range of 0.02 to 0.5 mm/h [19]. For ZGP single crystal growth by the VB method, the pulling rate is usually in the range of 0.5 to 0.75 mm/h when the temperature gradient is about 3–10 °C/cm [10,13,20–23]. As far as we know, there is no study of the influence of the pulling rate
38
L. Shen, D. Wu / Journal of Crystal Growth 445 (2016) 37–41
on the properties of ZGP crystal. In this paper, after ZGP crystals were grown at 0.5 mm/h and 0.75 mm/h by the VB method, three slices were cut from the onset, middle and end of each crystal in order to compare crystal quality. The X-ray rocking curves, etch pit densities and optical spectra were adapted to characterize the crystals quality. By comparing the crystal quality at the different positions, we can infer what will happen to the crystal quality with an increase of pulling rate.
2. Experimental procedure Polycrystalline ZGP charges were synthesized from stoichiometric amounts of high-purity (99.9999%) phosphorus (P), zinc (Zn) and germanium (Ge) elements by the two-temperature-zone method described in Ref. [24]. Because of the high dissociation pressure (3.5 atm) at the melting point of ZGP [25], some excess P (E 2 g) was added into the fused silica synthesis ampoule. By this modified method a polycrystalline ZGP ingot weighted 250 g could be obtained in one synthesis run. ZGP single crystals were grown by the modified VB method described in Ref. [24]. The hot zone temperature was 1050 °C and the cold zone temperature was 984 °C. The temperature gradient at the melting point (1027 °C) was about 4–8 °C/cm. The crystals were pulled down at different rates R, and labeled R0.5 (0.5 mm/ h), R0.75 (0.75 mm/h). The cylindrical o112 4-oriented ZGP seed crystals were used for the single crystal growth. Each seed (20– 30 mm in length) was fixed in the seed well at the bottom of the pyrolytic boron nitride (PBN) crucible. In order to make sure the seed was melted until its size was reduced from 20–30 mm to 10– 15 mm, the first step was to align the top of seed to the melting point (1027 °C). The next step was to slowly raise the seed until the temperature at its middle reaches 1027 °C. Finally, the growth crucible was rotated and pulled down to start the single crystal growth. After the ZGP melt was completely solidified, the grown crystal was cooled down to 920 °C at the rate of 5 °C/h, then at 20 °C/h to 620 °C and at 40–50 °C/h to room temperature. For each pulling rate, three cross slices were cut from different parts of each single crystal ingot and the cutting surface was perpendicular to the growth axis, as shown in Fig. 1. The powder X-ray diffraction (XRD) analysis of the synthesized polycrystalline charges was performed by a Rigaku D/max-IIIA diffractometer using Cu Kα (λ ¼0.15406 nm) radiation. The pattern was recorded from 15° to 80° (2θ) at a rate of 0.02 °/s at room temperature. The X-ray rocking curves of the cross slices were recorded on Bruker D8 Discover diffractometer with Cu Kα (λ ¼0.15406 nm) radiation and a Ge (222) monochromator. The etch pits of the slices were observed by a FEI Quanta 400FEG scanning electron microscope (SEM) instrument. Finally optical transparency was measured using a Perkin-Elmer UV–Vis–NIR spectrophotometer (lambda 950) with unpolarized incident light.
3. Results and discussion 3.1. X-ray diffraction analysis The powder XRD pattern of the polycrystalline ZGP charges, as shown in Fig. 2, is in good agreement with the standard diffraction of ZnGeP2 (PDF Card no.73–0398). The calculated lattice parameters are a¼ b¼5.4675 Å and c¼ 10.7151 Å, which are also consistent with the standard data of the PDF card (no.73–0398). These results indicate that this modified two-temperature-zone method can successfully synthesize polycrystalline ZGP charges as the starting material for single crystal growth. Before the properties measurements and chemical etching of
Fig. 1. The photographs of as-grown ZnGeP2 crystal and the cross slice.
Fig. 2. The powder X-ray diffraction patterns of the polycrystalline ZnGeP2 charges.
ZGP slices with (112) crystallographic plane, they were all finely polished. The X-ray rocking curves of the slices grown at different rates are shown in Fig. 3. It can be seen that the full width at half maximum (FWHM) of the peaks of the six samples is about 19– 22″. Besides, the diffraction peaks of the curves are all perfectly symmetrical and their intensities are very high. These results demonstrate that the modified VB method is suitable to grow good crystallinity and high structural quality crystals, even if the pulling rate increases from 0.5 mm/h to 0.75 mm/h. However, the FWHM increases with increasing pulling rate, especially at the onset of the crystals. The FWHM of the onset pulled at 0.75 mm/h is 22.3″, which is higher than that of the onset pulled at 0.5 mm/h. The FWHM of the middle and end of the crystal pulled at 0.75 mm/h are close to those of the crystal pulled at 0.5 mm/h, the difference
L. Shen, D. Wu / Journal of Crystal Growth 445 (2016) 37–41
Fig. 3. X-ray rocking curves of the different positions of the crystals pulled at 0.5 mm/h and 0.75 mm/h.
between the middle positions is 0.18″ and that between the end positions is 0.035″. An important implication of these results is that increasing pulling rate significantly influences the quality of the onset of the crystals.
Fig. 5. The SEM micrograph illustrating the typical morphology of the etch pit on ZnGeP2 slice. Table 1 EPDs of the six slices of R ¼0.5 mm/h and 0.75 mm/h. Pulling rate (mm/h)
3.2. Etch pit density (EPD) 0.50
The defect-selective etching method is an important way to observe the dislocations in the crystals [26,27]. The six ZGP slices were etched by a solution of HF, HNO3, H2O, CH3COOH and I2 (6 ml:6 ml:3 ml:3 ml:12 mg), which was proved to be very effective to produce etch pits on ZGP crystals by Refs. [24,28,29]. The etching process lasted for about ten minute at room temperature with ultrasonic vibration. Fig. 4 shows the SEM micrographs of the etch pits of the six slices and Fig. 5 shows the SEM micrograph of one etch pit. The calculated EPD for eaching slice is listed in Table 1. It can be seen that the EPDs of no samples exceed
39
0.75
Position of crystals
EPD (cm 2)
onset middle end onset middle end
388 402 320 796 411 298
796 cm 2. This is the evidence of the superiority of the modified VB method. Note that the EPD of the onset of the crystal pulled at 0.75 mm/h is about twice that of crystal pulled at 0.5 mm/h. However, the EPDs of the crystals pulled at the different rates are almost equal at the middle and end of the single crystals. These
Fig. 4. The SEM micrographs of etch pits on cross section ZnGeP2 slices pulled at 0.5 mm/h and 0.75 mm/h.
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L. Shen, D. Wu / Journal of Crystal Growth 445 (2016) 37–41
results are consistent with the values of the FWHM of X-ray rocking curves. As mentioned in Section 3.1, when the pulling rate is increased, the quality is worse for the onset of the crystals and almost the same for the middle and end of the crystals. The increase of dislocations should be due to the more latent heat induced by the higher pulling rate. Normally, there are some relative large thermal stresses at the transition region (from the seed to the full-diameter body) [30,31]. However, when the pulling rate is small (0.5 mm/h), the thermal stress at the transition region does not exceed the critical resolved shear stress (CRSS). As the pulling rate is increased (0.75 mm/h), more latent heat is released so that the solid/liquid interface becomes increasingly concave. It is well known that an increase in the curvature of the growing interface usually corresponds to an increase in thermal stress. When the thermal stress is bigger than the CRSS, this means that more dislocations generate at the onset of grown crystal. Thus, a nearly fat solid/liquid interface is always preferred for the single crystal growth [32–34].
These results validate the results of X-ray diffraction and defectselective etching. The three measurements mentioned above indicate that the higher pulling rate degrades the crystal quality at the onset, but the quality of the middle and end of the crystals is not significantly impacted by the increase in the pulling rate. Thus when using the vertical Bridgman method for ZGP growth, the pulling rate can be adjusted as follows: firstly, at the beginning of the growth process (about from the seed/melt interface to the full diameter) the pulling rates should be kept at 0.4–0.5 mm/h; secondly, in the next 2 cm, the pulling rate could be slowly increased to 0.75 mm/h; finally, the pulling rate is kept at 0.75 mm/h until all of the melt is solidified. These adjustments of pulling rate can efficiently save the growth time and avoid dislocations formation.
4. Conclusion 3.3. Optical spectra Structural imperfections identified by the above-mentioned X-ray diffraction and defect-selective etching can affect the optical transmittance. Because the optical transmittance of ZGP crystals generally exceeds 56% in the wavelength range 3–8 μm [22,24], Fig. 6 just shows the transmission spectra of the six slices (1.9 mm in thickness) pulled at different rates in the wavelength range 0.5– 3 μm. As shown in Fig. 6(a), the pulling rate of 0.5 mm/h can yield a good uniformity of optical transmittance from the onset to the end of the crystal. From Figs. 6(b) to (d), one can see that: (i) the optical transmittance of the onset becomes lower with an increasing of pulling rate; (ii) there is just tiny difference between 0.5 mm/h and 0.75 mm/h at the middle and end of the crystals.
In summary, the influence of pulling rate on the properties of ZnGeP2 crystals during vertical Bridgman growth has been investigated. It is found that the crystal quality at the onset of single crystal growth is negatively impacted by the increasing of pulling rate. Meanwhile, the crystal quality of other positions (the middle and end) is essentially stable in spite of a significant increase in the pulling rate. Along with crystal growth process, when the pulling rate is large enough, the thermal stress at the onset of the crystals will be first to exceed the CRSS so that more dislocations generate. This also means that the middle and end of the crystal allow higher pulling rate. These results are helpful in optimizing the ZGP crystal pulling rate during the vertical Bridgman growth process.
Fig. 6. The optical transmission spectra of the six slices pulled at different rates.
L. Shen, D. Wu / Journal of Crystal Growth 445 (2016) 37–41
Acknowledgments This work was partially supported by the National Natural Science Foundation of China (No. 11204384). The authors thanks Shengfei Su for valuable discussions.
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Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Influence of the pulling rate on the properties of ZnGeP2 crystal grown by vertical Bridgman method Liang Shen, Dong Wu n State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, China
art ic l e i nf o
a b s t r a c t
Article history: Received 28 December 2015 Received in revised form 4 April 2016 Accepted 12 April 2016 Available online 13 April 2016
Zinc–germanium diphosphide (ZGP) crystals (15 mm in diameter and 65 mm in length) were successfully grown by the modified vertical Bridgman method on seeds at different pulling rates (0.5 mm/h and 0.75 mm/h) in order to study the defect generation during crystal growth. At the different positions (the onset, middle and end) in single crystals, their properties of ZGP crystals were investigated by X-ray diffraction, etching technique and optical transmission spectra. The results indicate that the increase in the pulling rate deteriorates the crystal quality at the onset part of the single crystals. The etch pit density (EPD) and the full width at half maximum (FWHM) of the X-ray rocking curves increase, while the optical transmittance decreases with increasing pulling rate. However, the increase in the pulling rate hardly influences the crystal quality at the middle and end part of the single crystals. & 2016 Elsevier B.V. All rights reserved.
Keywords: Pulling rate Defects Bridgman technique Single crystal growth Nonlinear optic materials
1. Introduction Because of its multiple excellent properties including optical, thermal and mechanical properties, Zinc–germanium diphosphide (ZGP) crystal can be applied in optical parametric oscillators (OPOs) based on type I phase-matching to produce laser sources at 3–12 mm [1], which are widely used in infrared countermeasures, molecular spectroscopy, invasive surgery and detection of gas [2,3]. Since the 1960s, ZGP crystals have attracted much interest [4]. But at first the purity and quantity of ZGP polycrystalline charges were low, and the size of ZGP single crystals was small [5–7]. Until 1997, Schunemann et al. successfully used the horizontal gradient freeze (HGF) method to grow large single crystals and the optical absorption coefficient could reach 0.09 cm 1 at 2.05 mm, which is the basic requirement for the OPO applications [8]. Later, the ZGP single crystal up to 27 39 140 mm3 in size was grown by the HGF method [9]. In 1997, G.A. Verozubova et al. made a considerable improvement in ZGP polycrystal synthesis and could synthesize 500 g charges in one synthesis run [10]. They grew ZGP single crystals by the vertical Bridgman (VB) method and the vertical gradient freeze (VGF) method [10]. At present using the thermal annealing and electron irradiation, the optical absorption coefficient can be decreased down to 0.02–0.04 cm 1 at 2.06 mm by G.A. Verozubova group [11]. Since entering the 21st century, n
Corresponding author. E-mail address: [email protected] (D. Wu).
http://dx.doi.org/10.1016/j.jcrysgro.2016.04.025 0022-0248/& 2016 Elsevier B.V. All rights reserved.
ZGP crystals have attracted many related research topics, including growth, polishing, annealing, laser-induced damage and theoretical studies on native defects [9,11–16]. However, research on the single crystal growth technique is still one of the most important topics. Among many ZGP crystal growth methods such as the VB method [10], the VGF method [10], the HGF method [8], the highpressure vapor transport (HPVT) method [17] and the liquid encapsulated Czochralski (LEC) method [18], the VB method and the HGF method are more widely used. In this paper we also used the VB method to grow ZGP single crystals. In any ZGP single crystal growth process, the quality of the ZGP crystal is affected by a number of factors such as pulling rate, rotation rate, temperature gradient, growth ampoule shape, seed orientation, etc. In the VB method, pulling rate is an important parameter to control the thermal balance at the solid/melt interface. A suitable pulling rate can make the melt–solid phase transition continuous and stable. A high pulling rate usually induces a large thermal stress, which will result in more risks of crystal cracking. Conversely, a low pulling rate takes more time and reduces the growth efficiency. So choosing a suitable pulling rate is favorable for both crystal quality and growth efficiency. Furthermore, different crystals usually need the different pulling rates, for example, for III–V ternary compounds with a temperature gradient of about 10 °C/cm, the pulling rate lies within the range of 0.02 to 0.5 mm/h [19]. For ZGP single crystal growth by the VB method, the pulling rate is usually in the range of 0.5 to 0.75 mm/h when the temperature gradient is about 3–10 °C/cm [10,13,20–23]. As far as we know, there is no study of the influence of the pulling rate
38
L. Shen, D. Wu / Journal of Crystal Growth 445 (2016) 37–41
on the properties of ZGP crystal. In this paper, after ZGP crystals were grown at 0.5 mm/h and 0.75 mm/h by the VB method, three slices were cut from the onset, middle and end of each crystal in order to compare crystal quality. The X-ray rocking curves, etch pit densities and optical spectra were adapted to characterize the crystals quality. By comparing the crystal quality at the different positions, we can infer what will happen to the crystal quality with an increase of pulling rate.
2. Experimental procedure Polycrystalline ZGP charges were synthesized from stoichiometric amounts of high-purity (99.9999%) phosphorus (P), zinc (Zn) and germanium (Ge) elements by the two-temperature-zone method described in Ref. [24]. Because of the high dissociation pressure (3.5 atm) at the melting point of ZGP [25], some excess P (E 2 g) was added into the fused silica synthesis ampoule. By this modified method a polycrystalline ZGP ingot weighted 250 g could be obtained in one synthesis run. ZGP single crystals were grown by the modified VB method described in Ref. [24]. The hot zone temperature was 1050 °C and the cold zone temperature was 984 °C. The temperature gradient at the melting point (1027 °C) was about 4–8 °C/cm. The crystals were pulled down at different rates R, and labeled R0.5 (0.5 mm/ h), R0.75 (0.75 mm/h). The cylindrical o112 4-oriented ZGP seed crystals were used for the single crystal growth. Each seed (20– 30 mm in length) was fixed in the seed well at the bottom of the pyrolytic boron nitride (PBN) crucible. In order to make sure the seed was melted until its size was reduced from 20–30 mm to 10– 15 mm, the first step was to align the top of seed to the melting point (1027 °C). The next step was to slowly raise the seed until the temperature at its middle reaches 1027 °C. Finally, the growth crucible was rotated and pulled down to start the single crystal growth. After the ZGP melt was completely solidified, the grown crystal was cooled down to 920 °C at the rate of 5 °C/h, then at 20 °C/h to 620 °C and at 40–50 °C/h to room temperature. For each pulling rate, three cross slices were cut from different parts of each single crystal ingot and the cutting surface was perpendicular to the growth axis, as shown in Fig. 1. The powder X-ray diffraction (XRD) analysis of the synthesized polycrystalline charges was performed by a Rigaku D/max-IIIA diffractometer using Cu Kα (λ ¼0.15406 nm) radiation. The pattern was recorded from 15° to 80° (2θ) at a rate of 0.02 °/s at room temperature. The X-ray rocking curves of the cross slices were recorded on Bruker D8 Discover diffractometer with Cu Kα (λ ¼0.15406 nm) radiation and a Ge (222) monochromator. The etch pits of the slices were observed by a FEI Quanta 400FEG scanning electron microscope (SEM) instrument. Finally optical transparency was measured using a Perkin-Elmer UV–Vis–NIR spectrophotometer (lambda 950) with unpolarized incident light.
3. Results and discussion 3.1. X-ray diffraction analysis The powder XRD pattern of the polycrystalline ZGP charges, as shown in Fig. 2, is in good agreement with the standard diffraction of ZnGeP2 (PDF Card no.73–0398). The calculated lattice parameters are a¼ b¼5.4675 Å and c¼ 10.7151 Å, which are also consistent with the standard data of the PDF card (no.73–0398). These results indicate that this modified two-temperature-zone method can successfully synthesize polycrystalline ZGP charges as the starting material for single crystal growth. Before the properties measurements and chemical etching of
Fig. 1. The photographs of as-grown ZnGeP2 crystal and the cross slice.
Fig. 2. The powder X-ray diffraction patterns of the polycrystalline ZnGeP2 charges.
ZGP slices with (112) crystallographic plane, they were all finely polished. The X-ray rocking curves of the slices grown at different rates are shown in Fig. 3. It can be seen that the full width at half maximum (FWHM) of the peaks of the six samples is about 19– 22″. Besides, the diffraction peaks of the curves are all perfectly symmetrical and their intensities are very high. These results demonstrate that the modified VB method is suitable to grow good crystallinity and high structural quality crystals, even if the pulling rate increases from 0.5 mm/h to 0.75 mm/h. However, the FWHM increases with increasing pulling rate, especially at the onset of the crystals. The FWHM of the onset pulled at 0.75 mm/h is 22.3″, which is higher than that of the onset pulled at 0.5 mm/h. The FWHM of the middle and end of the crystal pulled at 0.75 mm/h are close to those of the crystal pulled at 0.5 mm/h, the difference
L. Shen, D. Wu / Journal of Crystal Growth 445 (2016) 37–41
Fig. 3. X-ray rocking curves of the different positions of the crystals pulled at 0.5 mm/h and 0.75 mm/h.
between the middle positions is 0.18″ and that between the end positions is 0.035″. An important implication of these results is that increasing pulling rate significantly influences the quality of the onset of the crystals.
Fig. 5. The SEM micrograph illustrating the typical morphology of the etch pit on ZnGeP2 slice. Table 1 EPDs of the six slices of R ¼0.5 mm/h and 0.75 mm/h. Pulling rate (mm/h)
3.2. Etch pit density (EPD) 0.50
The defect-selective etching method is an important way to observe the dislocations in the crystals [26,27]. The six ZGP slices were etched by a solution of HF, HNO3, H2O, CH3COOH and I2 (6 ml:6 ml:3 ml:3 ml:12 mg), which was proved to be very effective to produce etch pits on ZGP crystals by Refs. [24,28,29]. The etching process lasted for about ten minute at room temperature with ultrasonic vibration. Fig. 4 shows the SEM micrographs of the etch pits of the six slices and Fig. 5 shows the SEM micrograph of one etch pit. The calculated EPD for eaching slice is listed in Table 1. It can be seen that the EPDs of no samples exceed
39
0.75
Position of crystals
EPD (cm 2)
onset middle end onset middle end
388 402 320 796 411 298
796 cm 2. This is the evidence of the superiority of the modified VB method. Note that the EPD of the onset of the crystal pulled at 0.75 mm/h is about twice that of crystal pulled at 0.5 mm/h. However, the EPDs of the crystals pulled at the different rates are almost equal at the middle and end of the single crystals. These
Fig. 4. The SEM micrographs of etch pits on cross section ZnGeP2 slices pulled at 0.5 mm/h and 0.75 mm/h.
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results are consistent with the values of the FWHM of X-ray rocking curves. As mentioned in Section 3.1, when the pulling rate is increased, the quality is worse for the onset of the crystals and almost the same for the middle and end of the crystals. The increase of dislocations should be due to the more latent heat induced by the higher pulling rate. Normally, there are some relative large thermal stresses at the transition region (from the seed to the full-diameter body) [30,31]. However, when the pulling rate is small (0.5 mm/h), the thermal stress at the transition region does not exceed the critical resolved shear stress (CRSS). As the pulling rate is increased (0.75 mm/h), more latent heat is released so that the solid/liquid interface becomes increasingly concave. It is well known that an increase in the curvature of the growing interface usually corresponds to an increase in thermal stress. When the thermal stress is bigger than the CRSS, this means that more dislocations generate at the onset of grown crystal. Thus, a nearly fat solid/liquid interface is always preferred for the single crystal growth [32–34].
These results validate the results of X-ray diffraction and defectselective etching. The three measurements mentioned above indicate that the higher pulling rate degrades the crystal quality at the onset, but the quality of the middle and end of the crystals is not significantly impacted by the increase in the pulling rate. Thus when using the vertical Bridgman method for ZGP growth, the pulling rate can be adjusted as follows: firstly, at the beginning of the growth process (about from the seed/melt interface to the full diameter) the pulling rates should be kept at 0.4–0.5 mm/h; secondly, in the next 2 cm, the pulling rate could be slowly increased to 0.75 mm/h; finally, the pulling rate is kept at 0.75 mm/h until all of the melt is solidified. These adjustments of pulling rate can efficiently save the growth time and avoid dislocations formation.
4. Conclusion 3.3. Optical spectra Structural imperfections identified by the above-mentioned X-ray diffraction and defect-selective etching can affect the optical transmittance. Because the optical transmittance of ZGP crystals generally exceeds 56% in the wavelength range 3–8 μm [22,24], Fig. 6 just shows the transmission spectra of the six slices (1.9 mm in thickness) pulled at different rates in the wavelength range 0.5– 3 μm. As shown in Fig. 6(a), the pulling rate of 0.5 mm/h can yield a good uniformity of optical transmittance from the onset to the end of the crystal. From Figs. 6(b) to (d), one can see that: (i) the optical transmittance of the onset becomes lower with an increasing of pulling rate; (ii) there is just tiny difference between 0.5 mm/h and 0.75 mm/h at the middle and end of the crystals.
In summary, the influence of pulling rate on the properties of ZnGeP2 crystals during vertical Bridgman growth has been investigated. It is found that the crystal quality at the onset of single crystal growth is negatively impacted by the increasing of pulling rate. Meanwhile, the crystal quality of other positions (the middle and end) is essentially stable in spite of a significant increase in the pulling rate. Along with crystal growth process, when the pulling rate is large enough, the thermal stress at the onset of the crystals will be first to exceed the CRSS so that more dislocations generate. This also means that the middle and end of the crystal allow higher pulling rate. These results are helpful in optimizing the ZGP crystal pulling rate during the vertical Bridgman growth process.
Fig. 6. The optical transmission spectra of the six slices pulled at different rates.
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Acknowledgments This work was partially supported by the National Natural Science Foundation of China (No. 11204384). The authors thanks Shengfei Su for valuable discussions.
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