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Vertical Bridgman growth and characterization of large ZnGeP2 single crystals
Journal of Crystal Growth 314 (2011) 306–309
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Vertical Bridgman growth and characterization of large ZnGeP2 single crystals Shixing Xia a, Meng Wang a, Chunhui Yang a,n, Zuotao Lei a, Guoli Zhu b, Baoquan Yao b a b
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, P.R. China National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 June 2010 Received in revised form 25 October 2010 Accepted 8 November 2010 Communicated by R.S. Feigelson Available online 18 November 2010
The growth of ZnGeP2 crystals by seeded Vertical Bridgman method was studied. High-quality nearstoichiometric ZnGeP2 single crystals obtained were of 20–30 mm in diameter and 90–120 mm in length. By selection of the seed crystallographic orientation the single crystal ingots without cracks and twins were grown, as shown by X-ray diffraction. The infrared transmission property of the ZnGeP2 crystals was studied by the calculated optical absorption coefficient spectra. The results showed that after thermal annealing of the crystals the optical absorption coefficient was 0.10 cm 1 at 2.05 mm, and 0.01 cm 1 at 3–8 mm. The rocking curves patterns of the (4 0 0) reflection demonstrated that the as-grown single crystals possessed a good structural quality. The composition of the crystals was close to the ideal stoichiometry ratio of 1:1:2. The low-loss typical ZnGeP2 samples of 6 mm 6 mm 15 mm in sizes were cut from the annealed ingots for optical parametric oscillation experiments. The output power of 3.2 W was obtained at 3–5 mm when the incident pumping power of 2.05 mm laser was 9.4 W, and the corresponding slope efficiency and the conversion efficiency were 44% and 34%, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A2. Vertical Bridgman method B1. Zinc germanium diphosphide B2. Nonlinear optical crystals
1. Introduction Zinc Germanium Diphosphide (ZnGeP2, ZGP) is a semiconducting compound of II–IV–V2 group. ZGP has an anisotropic chalcopyrite lattice in the tetragonal crystallographic system, space group I42 d. ZGP single crystals are used as efficient laser sources in the 3–5 mm and atmospheric transmission windows in 8–12 mm for various applications, including high resolution spectroscopy, remote sensing of atmosphere, and also defense systems [1,2]. The compound is characterized by a potentially wide transparency range (0.65–12 mm), high non-linear coefficient (d36 ¼75 pm/V), and high thermal conductivity (0.35 W/cm K) [1,3,4]. ZGP crystals can significantly increase the possibilities of the laser devices by using the Second Harmonic Generation (SHG) and especially by Optical Parametric Oscillation (OPO) to shift the wavelengths of various laser sources into the mid-infrared region (approximately 3–8 mm) [5–7]. The possibility of wider application of ZGP in nonlinear optics depends to a large degree on the growth of ZGP large single crystals with perfect structural and optical qualities. However, the synthesis and growth of ZGP were proved to be difficult because of various problems, such as the high melting point of ZGP (1027 1C), the high vapor pressure produced mainly by phosphorus, and considerable transport of zinc vapor towards the temperature gradient region, when the material is synthesized the by two-
n
Corresponding author. Tel.: + 86 451 86413707; fax: + 86 451 86418270. E-mail address: [email protected] (C. Yang).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.062
temperature technique. Many attempts were performed to solve these problems, several of them had a success, presented in papers [8–11]. In order to overcome the above problems, our present work was focused on developing the growth procedure of ZGP single crystals.
2. Experimental procedure 2.1. Single crystals growth ZGP compound was synthesized from elementary components: high purity zinc, germanium, and phosphorus. The original twotemperature synthesis technique was used as described previously [12], in which more than 300 g of ZGP material could be obtained in a synthesis process. The furnace, composed of nine-sections, was used to grow ZGP single crystals by the Vertical Bridgman technique. Schematic representation of the furnace together with growth ampule inside and the temperature profile are shown in Fig. 1. The upper hot zone temperature was 1050–1070 1C and lower cold zone temperature was 980–1010 1C during experiments. The temperature gradient between the zones was about 6–15 1C/cm near crystallization front depending on temperatures of the hot and cold zones. The crystals were grown from ZGP polycrystalline single phase material, located in Pyrolytic Boron Nitride (PBN) crucibles, which was then placed in high purity quartz ampoules. At the bottom of PBN crucible a single crystal seed was placed, having been machined fitted to the
Heating Element Quartz Ampoule PBN Crucible ZGP Melt Growing Crystal
Fumace Positon, X
S. Xia et al. / Journal of Crystal Growth 314 (2011) 306–309
Growth Direction
Melting Point: ZGP:1027°C
Seed
Temperature, T Fig. 1. Schematic representation of a furnace used for seeded Vertical Bridgman growth of ZGP crystals. As a scale reference, the PBN crucible is 20 cm in length.
307
also dried in vacuum. The vacuumed and sealed ampoule was placed into the furnace, where the melting of ZGP charge and the partial part of the seed was occurred. After soaking into the hot zone for 24–48 h, the ampule was pulled slowly down with a speed of 1–3 mm/h to crystallize the ZGP material. When the melt completely solidified, the grown crystal was cooled down with a rate of 5–10 1C/h to 800 1C and then 30–50 1C/h to room temperature. Subsequently, the quartz ampoules were opened, and crystal ingots were removed from crucibles, and the crystals were checked for grains, twins, and optical transparency. The single crystal ingots were then annealed in vacuumed and sealed quartz ampoules at 600 1C for 300–400 h in order to reduce the optical absorption [1]. Three ZGP crystals with different sizes were grown by the developed Vertical Bridgman method: two ingots with 20 mm in diameter and the other one with 30 mm in diameter. Fig. 2,a and b demonstrate the ZGP crystals with 20 and 30 mm in diameter as well as non-linear optical elements cut from our crystals. 2.2. Crystal measurements The powder X-Ray Diffraction (XRD) analysis of the samples that were taken from different parts of one of the as-grown single crystal was conducted on a Shimazu X-ray diffractometer (D/max2000, CuKa radiation). The diffraction patterns were recorded over the 2y range 101–1201, with scan step of 0.021/sec at room temperature. In order to determine a composition of constituents at different places of the as-grown crystals, X-Ray Fluorescence (XRF) measurements were carried out with error within 2%. Rocking curve analysis of the studied ZGP samples were carried out by reflection in double-crystal variant. For monochromatization of NiKa1 radiation the Ge plate of high quality was used. Optical transparency measurements were performed on Perkin Elmer UV–vis–NIR spectrometer (Lambda900) for the near-IR spectrum and a BRUKER IR spectrometer (EQUINOX55) for the middle-IR spectrum. Optical absorption coefficient over transparency ZGP range was calculated using the Sellmeier coefficients provided by Zelmon, et. al. [13] in order to take into account the reflectivity losses of ZGP.
3. Results and discussion 3.1. Crystal growth
Fig. 2. ZGP single crystal ingots and nonlinear optical elements: (a) ZGP single crystal ingots with 20 mm diameter and non-linear optical elements and (b) ZGP single crystal ingot with 30 mm diameter.
crucible walls. In addition, phosphorus, whose amount was calculated for vapor phase, was added into the crucible. After the charging the quartz ampule with charged crucible was pumped down and sealed off. The quartz ampoules and crucible were etched in aqua regia, washed in deionized water, and dried in vacuum before pumping them. Polycrystalline ZGP and seed were etched in HCl/HNO3 mixed solution (HCl:HNO3 ¼1:1), washed by deionized water, and
Several experiments were initially carried out with spontaneous growth, i.e. without seed and ZGP ingots were obtained. These crystal ingots had one twin, as a rule, i.e. they were almost monocrystalline, the crystallographic orientation of the growth axis was /1 1 6S, as a rule. From the grown ingots the seeds were cut with different orientations and growth experiments have been carried out with them. It turned out that only seeds, oriented along a or c axis of ZGP tetragonal lattice generated single crystals ingots without cracks and twins. Usually, these crystal ingots had one twin, i.e. they were almost monocrystalline, the crystallographic orientation of the growth axis was /1 1 6S, as a rule. This fact is in agreement with early data of Feigelson and Route [14], and results, given recently [1]. Thus, in the following growth experiments the standard seed crystallographic orientations are /0 0 1S or /1 0 0S were used. Using developed Vertical Bridgman method, several single crystals were grown of 20–30 mm in diameter and 90–130 mm in length. The weight loss during growth was about 1.8% in the final crystal from the starting ZGP charge. Most probably, the weight losses may be attributed to the formation of binary volatile compound ZnP2 outside the crucible. But there is also a possibility, as shown by Fiechter, et al. [15], that the GeO gas can be formed at
308
S. Xia et al. / Journal of Crystal Growth 314 (2011) 306–309
melting temperature or higher as a by-product of interaction of the Ge vapor species with the walls of a quartz ampule, having OH ions as free radicals. So, it cannot be excluded that some vapor species containing Ge in their composition formed, which then precipitated outside the crucible. The vapor phase deposits outside the crucible after the growth experiments was not yet been studied. Fig. 2, a presents the two ZGP single crystal ingots with 20 mm in diameter and non-linear optical elements, cut from earlier grown annealed single crystals. Fig. 2, b demonstrates the ZGP single crystal ingot with 30 mm in diameter.
3.2. Characterization of the ZGP crystals 3.2.1. XRD analysis The powder XRD pattern of the ZGP single crystal is shown in Fig. 3 with scanning time of 40 min. The unit cell parameters of tetragonal phase ZGP were calculated as follows: a ¼b¼5.465 A˚ ˚ which are consistent with the standard data and c¼10.710 A, (JCPDS card #33-1471). A wafer of 20 mm in diameter and 4.0 mm in thickness was cut from the bottom of one of the grown ZGP single crystal ingots, its plane is (0 0 1). The photograph of this wafer is presented in Fig. 4. The XRD pattern of the wafer is shown in Fig. 5, from which it can be seen that multiple diffraction peaks of the {0 0 4} face is presented
Fig. 5. The XRD pattern of ZGP wafer ((0 0 1) plane), shown in Fig. 4.
Fig. 6. X-ray rocking curve for as-grown ZGP single crystal.
Fig. 3. The powder XRD pattern of the ZGP single crystal.
Fig. 4. A photograph of 4.0 mm thickness ZGP wafer with (0 0 1) plane.
and the intensity of the diffraction peaks is high and the shape of the peaks has a good symmetry. This result demonstrates that a single crystallinity of the as-grown ZGP wafer is very good. The structural quality of ZGP single crystals was also estimated from the rocking curves. A typical X-ray rocking curve for as-grown ZGP single crystal of the (4 0 0) reflection is shown in Fig. 6. The Half-Width of the Peak on the Half-Height (HWHH) is 13.5 angular seconds. Dispersion of the measurement related to difference of the ZGP and Ge monochromator that shows a value of about 5 angular seconds. Therefore, it suggests that the own HWHH of the rocking curve is about 8 sec. The shape of the peak had a good symmetry and its intensity was very high. Thus, the studied ZGP sample demonstrates a good structural quality. A modest broadening of the peak can be related to native point defects, dislocations, and other structural defects, present in the studied as-grown sample.
3.2.2. Composition analysis The composition of the crystals was determined by XRF spectra. The results show that the crystals have a stoichiometry ratio of Zn:Ge:P¼1.08:1.03:2.00, which is quite close to the ideal stoichiometry ratio of 1:1:2. This result demonstrates that the as-grown crystals are quite homogeneous.
S. Xia et al. / Journal of Crystal Growth 314 (2011) 306–309
3.2.3. Optical absorption The ZGP wafer, studied by XRD and presented in Fig. 4 was annealed for 300 h at 600 1C in mixed P and ZGP powder. After grinding and polishing, it was used to measure the optical transparency spectra. The calculated optical absorption coefficient spectra are shown in Fig. 7. It was found that the optical absorption coefficient for both ordinary (o) and extraordinary (e) lights were typical, as shown earlier by other researchers [4,7]. In case of o-polarized light, the absorption coefficient of the crystal is less than 0.01 cm 1 over the range of 3–8 mm, while it is 0.10 cm 1 at 2.05 mm. In case of e-polarized light, the absorption coefficient is less than 0.03 cm 1 over the range of 3–8 mm, while it is 0.13 cm 1 at wavelength 2.05 mm. In addition, measurements of the absorption coefficient of the 2.05 mm Tm, Ho:GdVO4 pump laser were also carried out by using of 6 mm 6 mm 15 mm ZGP crystal. The end-face of the ZGP crystal were Anti-Reflectivity (AR) coated at 2.0 mm (reflectivityo0.1% at 2.05 mm). The experimentally measured o-polarized absorption coefficient was 0.10 cm 1, and the e-polarized absorption coefficient was 0.13 cm 1, which had a good agreement with the spectra measurement as shown in Fig. 7.
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3.2.4. ZGP OPO The ZGP crystals were cut to 6 mm 6 mm 15 mm in sizes after thermal annealing. Both surfaces of the crystal were carefully polished and antireflection coated for pump, signal, and idler lights. The crystal was cut at y ¼551 and f ¼901 for type I phase-match. High pulse repetition frequency of 10 kHz cryogenic cooled Tm, Ho:GdVO4 laser emitting at 2.05 mm. Which was used as the pumping source for ZGP OPO. The ZGP OPO output wavelengths were in the spectral range of 3–5 mm. Fig. 8 shows the output power of the ZGP OPO as a function of 2.05 mm Tm, Ho:GdVO4 pumping power. It is shown in Fig. 8 that up to 3.2 W output power was obtained under an incident pump power of 9.4 W. The corresponding slope efficiency was 44% and the conversion efficiency was 34%. Thus, from above results, it can be suggested that the ZGP is an excellent OPO material and is acceptable for the fabrication of the infrared nonlinear optical devices. To depress the absorption at 2.05 mm this is an effective way to improve the ZGP OPO property. Further, improvement of the polishing technology and the AR film technology is also useful to increase ZGP OPO output power.
4. Conclusion Using the polycrystalline ZGP single phase material, synthesized in our laboratory, the seeded Vertical Bridgman method was developed to grow ZGP single crystals. The crack-free ZGP single crystals of 20–30 mm in diameter and 90–120 mm in length have been grown. The quality of the grown crystals was studied by XRD, rocking curves analysis, XRF spectra, IR spectrophotometry, and OPO. All results demonstrate that the developed growth method is very promising to produce ZGP single crystals and the quality of the grown crystals is good. As it has been shown by XRF spectra, the composition of the crystals is close to the ideal stoichiometry ratio of 1:1:2, which demonstrates that the as-grown crystal is quite homogeneous. The optical absorption coefficient for annealed ZGP crystals is 0.01 cm 1 at 3–8 mm and 0.10 cm 1 at 2.05 mm for o-light. The output power of 3.2 W was obtained when the incident pumping power of 2.05 mm laser was 9.4 W, the corresponding slope efficiency was 44% and the conversion efficiency was 34%. In conclusion, the crystals are acceptable for the fabrication of the infrared nonlinear optical devices, in particular, OPO devices.
Fig. 7. Optical absorption coefficient spectra, typical for our annealed ZGP crystals, using o- and e-polarized lights at the wavelength range over 1–13 mm.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. E50872023) and the Key Science and Technology Program of Heilongjiang Province (No. GC05A205). References
Fig. 8. OPO output power as a function of pump power.
[1] G.A. Verozubova, A.I. Gribenyukov, Crystallogr. Rep. 53 (2008) 158. [2] E. Lippert, G. Rustad, G. Arisholm, K. Stenersen, Opt. Express 16 (18)(2008) 13878. [3] J.L. Shay, J.H. Wernick., Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties, and Applications., Pergamon Press, New York, 1975 174. [4] Kevin T. Zawilski, Peter G. Schunemann, Scott D. Setzler, et al., J. Cryst. Growth 310 (2008) 1891. [5] Y. Jiang, Y. Ding, Opt. Express 15 (2007) 12699. [6] M. Henriksson, M. Tiihonen, V. Pasiskevicius, F. Laurell, Appl. Phys. B 88 (2007) 37. [7] K.T. Zawilski, S.D. Setzler, P.G. Schunemann, Proc. SPIE 5991 (2005) 599104-1. [8] D.F. Bliss, M. Harris, J. Horrigan, W.M. Higgins, A.F. Armington, J. Cryst. Growth 137 (1994) 145. [9] G.A. Verozubova, A.I. Gribenyukov, Yu.P. Mironov, Inorg Mater 43 (10)(2007) 1040. [10] X. Zhao, S. Zhu, B. Zhao, B. Chen, Z. He, R. Wang, J. Cryst. Growth 311 (2008) 190. [11] T.Y. Wang, R. Sivakumar, D.K. Rai, W.T. Hsu, J. Chin, Inst. Chem. Eng. 39 (2008) 385. [12] M. Wang, C. Yang, Z. Lei, S. Xia, C. Zhu, Cryst. Res. Technol 45 (1)(2010) 25. [13] D.E. Zelmon, E.A. Hanning, P.G. Schunemann, J. Opt. Soc. Am. B 18 (2001)1307 18 (2001). [14] R.S. Feigelson, R.K. Route, J. Cryst. Growth 49 (1980) 61. [15] S. Fiechter, R.H. Castleberry, M. Angelov, K.J. Bachmann, Mater. Res. Soc. Symp. Proc 450 (1997) 315.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Vertical Bridgman growth and characterization of large ZnGeP2 single crystals Shixing Xia a, Meng Wang a, Chunhui Yang a,n, Zuotao Lei a, Guoli Zhu b, Baoquan Yao b a b
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, P.R. China National Key Laboratory of Tunable Laser Technology, Harbin Institute of Technology, Harbin 150001, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 June 2010 Received in revised form 25 October 2010 Accepted 8 November 2010 Communicated by R.S. Feigelson Available online 18 November 2010
The growth of ZnGeP2 crystals by seeded Vertical Bridgman method was studied. High-quality nearstoichiometric ZnGeP2 single crystals obtained were of 20–30 mm in diameter and 90–120 mm in length. By selection of the seed crystallographic orientation the single crystal ingots without cracks and twins were grown, as shown by X-ray diffraction. The infrared transmission property of the ZnGeP2 crystals was studied by the calculated optical absorption coefficient spectra. The results showed that after thermal annealing of the crystals the optical absorption coefficient was 0.10 cm 1 at 2.05 mm, and 0.01 cm 1 at 3–8 mm. The rocking curves patterns of the (4 0 0) reflection demonstrated that the as-grown single crystals possessed a good structural quality. The composition of the crystals was close to the ideal stoichiometry ratio of 1:1:2. The low-loss typical ZnGeP2 samples of 6 mm 6 mm 15 mm in sizes were cut from the annealed ingots for optical parametric oscillation experiments. The output power of 3.2 W was obtained at 3–5 mm when the incident pumping power of 2.05 mm laser was 9.4 W, and the corresponding slope efficiency and the conversion efficiency were 44% and 34%, respectively. & 2010 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A2. Vertical Bridgman method B1. Zinc germanium diphosphide B2. Nonlinear optical crystals
1. Introduction Zinc Germanium Diphosphide (ZnGeP2, ZGP) is a semiconducting compound of II–IV–V2 group. ZGP has an anisotropic chalcopyrite lattice in the tetragonal crystallographic system, space group I42 d. ZGP single crystals are used as efficient laser sources in the 3–5 mm and atmospheric transmission windows in 8–12 mm for various applications, including high resolution spectroscopy, remote sensing of atmosphere, and also defense systems [1,2]. The compound is characterized by a potentially wide transparency range (0.65–12 mm), high non-linear coefficient (d36 ¼75 pm/V), and high thermal conductivity (0.35 W/cm K) [1,3,4]. ZGP crystals can significantly increase the possibilities of the laser devices by using the Second Harmonic Generation (SHG) and especially by Optical Parametric Oscillation (OPO) to shift the wavelengths of various laser sources into the mid-infrared region (approximately 3–8 mm) [5–7]. The possibility of wider application of ZGP in nonlinear optics depends to a large degree on the growth of ZGP large single crystals with perfect structural and optical qualities. However, the synthesis and growth of ZGP were proved to be difficult because of various problems, such as the high melting point of ZGP (1027 1C), the high vapor pressure produced mainly by phosphorus, and considerable transport of zinc vapor towards the temperature gradient region, when the material is synthesized the by two-
n
Corresponding author. Tel.: + 86 451 86413707; fax: + 86 451 86418270. E-mail address: [email protected] (C. Yang).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.062
temperature technique. Many attempts were performed to solve these problems, several of them had a success, presented in papers [8–11]. In order to overcome the above problems, our present work was focused on developing the growth procedure of ZGP single crystals.
2. Experimental procedure 2.1. Single crystals growth ZGP compound was synthesized from elementary components: high purity zinc, germanium, and phosphorus. The original twotemperature synthesis technique was used as described previously [12], in which more than 300 g of ZGP material could be obtained in a synthesis process. The furnace, composed of nine-sections, was used to grow ZGP single crystals by the Vertical Bridgman technique. Schematic representation of the furnace together with growth ampule inside and the temperature profile are shown in Fig. 1. The upper hot zone temperature was 1050–1070 1C and lower cold zone temperature was 980–1010 1C during experiments. The temperature gradient between the zones was about 6–15 1C/cm near crystallization front depending on temperatures of the hot and cold zones. The crystals were grown from ZGP polycrystalline single phase material, located in Pyrolytic Boron Nitride (PBN) crucibles, which was then placed in high purity quartz ampoules. At the bottom of PBN crucible a single crystal seed was placed, having been machined fitted to the
Heating Element Quartz Ampoule PBN Crucible ZGP Melt Growing Crystal
Fumace Positon, X
S. Xia et al. / Journal of Crystal Growth 314 (2011) 306–309
Growth Direction
Melting Point: ZGP:1027°C
Seed
Temperature, T Fig. 1. Schematic representation of a furnace used for seeded Vertical Bridgman growth of ZGP crystals. As a scale reference, the PBN crucible is 20 cm in length.
307
also dried in vacuum. The vacuumed and sealed ampoule was placed into the furnace, where the melting of ZGP charge and the partial part of the seed was occurred. After soaking into the hot zone for 24–48 h, the ampule was pulled slowly down with a speed of 1–3 mm/h to crystallize the ZGP material. When the melt completely solidified, the grown crystal was cooled down with a rate of 5–10 1C/h to 800 1C and then 30–50 1C/h to room temperature. Subsequently, the quartz ampoules were opened, and crystal ingots were removed from crucibles, and the crystals were checked for grains, twins, and optical transparency. The single crystal ingots were then annealed in vacuumed and sealed quartz ampoules at 600 1C for 300–400 h in order to reduce the optical absorption [1]. Three ZGP crystals with different sizes were grown by the developed Vertical Bridgman method: two ingots with 20 mm in diameter and the other one with 30 mm in diameter. Fig. 2,a and b demonstrate the ZGP crystals with 20 and 30 mm in diameter as well as non-linear optical elements cut from our crystals. 2.2. Crystal measurements The powder X-Ray Diffraction (XRD) analysis of the samples that were taken from different parts of one of the as-grown single crystal was conducted on a Shimazu X-ray diffractometer (D/max2000, CuKa radiation). The diffraction patterns were recorded over the 2y range 101–1201, with scan step of 0.021/sec at room temperature. In order to determine a composition of constituents at different places of the as-grown crystals, X-Ray Fluorescence (XRF) measurements were carried out with error within 2%. Rocking curve analysis of the studied ZGP samples were carried out by reflection in double-crystal variant. For monochromatization of NiKa1 radiation the Ge plate of high quality was used. Optical transparency measurements were performed on Perkin Elmer UV–vis–NIR spectrometer (Lambda900) for the near-IR spectrum and a BRUKER IR spectrometer (EQUINOX55) for the middle-IR spectrum. Optical absorption coefficient over transparency ZGP range was calculated using the Sellmeier coefficients provided by Zelmon, et. al. [13] in order to take into account the reflectivity losses of ZGP.
3. Results and discussion 3.1. Crystal growth
Fig. 2. ZGP single crystal ingots and nonlinear optical elements: (a) ZGP single crystal ingots with 20 mm diameter and non-linear optical elements and (b) ZGP single crystal ingot with 30 mm diameter.
crucible walls. In addition, phosphorus, whose amount was calculated for vapor phase, was added into the crucible. After the charging the quartz ampule with charged crucible was pumped down and sealed off. The quartz ampoules and crucible were etched in aqua regia, washed in deionized water, and dried in vacuum before pumping them. Polycrystalline ZGP and seed were etched in HCl/HNO3 mixed solution (HCl:HNO3 ¼1:1), washed by deionized water, and
Several experiments were initially carried out with spontaneous growth, i.e. without seed and ZGP ingots were obtained. These crystal ingots had one twin, as a rule, i.e. they were almost monocrystalline, the crystallographic orientation of the growth axis was /1 1 6S, as a rule. From the grown ingots the seeds were cut with different orientations and growth experiments have been carried out with them. It turned out that only seeds, oriented along a or c axis of ZGP tetragonal lattice generated single crystals ingots without cracks and twins. Usually, these crystal ingots had one twin, i.e. they were almost monocrystalline, the crystallographic orientation of the growth axis was /1 1 6S, as a rule. This fact is in agreement with early data of Feigelson and Route [14], and results, given recently [1]. Thus, in the following growth experiments the standard seed crystallographic orientations are /0 0 1S or /1 0 0S were used. Using developed Vertical Bridgman method, several single crystals were grown of 20–30 mm in diameter and 90–130 mm in length. The weight loss during growth was about 1.8% in the final crystal from the starting ZGP charge. Most probably, the weight losses may be attributed to the formation of binary volatile compound ZnP2 outside the crucible. But there is also a possibility, as shown by Fiechter, et al. [15], that the GeO gas can be formed at
308
S. Xia et al. / Journal of Crystal Growth 314 (2011) 306–309
melting temperature or higher as a by-product of interaction of the Ge vapor species with the walls of a quartz ampule, having OH ions as free radicals. So, it cannot be excluded that some vapor species containing Ge in their composition formed, which then precipitated outside the crucible. The vapor phase deposits outside the crucible after the growth experiments was not yet been studied. Fig. 2, a presents the two ZGP single crystal ingots with 20 mm in diameter and non-linear optical elements, cut from earlier grown annealed single crystals. Fig. 2, b demonstrates the ZGP single crystal ingot with 30 mm in diameter.
3.2. Characterization of the ZGP crystals 3.2.1. XRD analysis The powder XRD pattern of the ZGP single crystal is shown in Fig. 3 with scanning time of 40 min. The unit cell parameters of tetragonal phase ZGP were calculated as follows: a ¼b¼5.465 A˚ ˚ which are consistent with the standard data and c¼10.710 A, (JCPDS card #33-1471). A wafer of 20 mm in diameter and 4.0 mm in thickness was cut from the bottom of one of the grown ZGP single crystal ingots, its plane is (0 0 1). The photograph of this wafer is presented in Fig. 4. The XRD pattern of the wafer is shown in Fig. 5, from which it can be seen that multiple diffraction peaks of the {0 0 4} face is presented
Fig. 5. The XRD pattern of ZGP wafer ((0 0 1) plane), shown in Fig. 4.
Fig. 6. X-ray rocking curve for as-grown ZGP single crystal.
Fig. 3. The powder XRD pattern of the ZGP single crystal.
Fig. 4. A photograph of 4.0 mm thickness ZGP wafer with (0 0 1) plane.
and the intensity of the diffraction peaks is high and the shape of the peaks has a good symmetry. This result demonstrates that a single crystallinity of the as-grown ZGP wafer is very good. The structural quality of ZGP single crystals was also estimated from the rocking curves. A typical X-ray rocking curve for as-grown ZGP single crystal of the (4 0 0) reflection is shown in Fig. 6. The Half-Width of the Peak on the Half-Height (HWHH) is 13.5 angular seconds. Dispersion of the measurement related to difference of the ZGP and Ge monochromator that shows a value of about 5 angular seconds. Therefore, it suggests that the own HWHH of the rocking curve is about 8 sec. The shape of the peak had a good symmetry and its intensity was very high. Thus, the studied ZGP sample demonstrates a good structural quality. A modest broadening of the peak can be related to native point defects, dislocations, and other structural defects, present in the studied as-grown sample.
3.2.2. Composition analysis The composition of the crystals was determined by XRF spectra. The results show that the crystals have a stoichiometry ratio of Zn:Ge:P¼1.08:1.03:2.00, which is quite close to the ideal stoichiometry ratio of 1:1:2. This result demonstrates that the as-grown crystals are quite homogeneous.
S. Xia et al. / Journal of Crystal Growth 314 (2011) 306–309
3.2.3. Optical absorption The ZGP wafer, studied by XRD and presented in Fig. 4 was annealed for 300 h at 600 1C in mixed P and ZGP powder. After grinding and polishing, it was used to measure the optical transparency spectra. The calculated optical absorption coefficient spectra are shown in Fig. 7. It was found that the optical absorption coefficient for both ordinary (o) and extraordinary (e) lights were typical, as shown earlier by other researchers [4,7]. In case of o-polarized light, the absorption coefficient of the crystal is less than 0.01 cm 1 over the range of 3–8 mm, while it is 0.10 cm 1 at 2.05 mm. In case of e-polarized light, the absorption coefficient is less than 0.03 cm 1 over the range of 3–8 mm, while it is 0.13 cm 1 at wavelength 2.05 mm. In addition, measurements of the absorption coefficient of the 2.05 mm Tm, Ho:GdVO4 pump laser were also carried out by using of 6 mm 6 mm 15 mm ZGP crystal. The end-face of the ZGP crystal were Anti-Reflectivity (AR) coated at 2.0 mm (reflectivityo0.1% at 2.05 mm). The experimentally measured o-polarized absorption coefficient was 0.10 cm 1, and the e-polarized absorption coefficient was 0.13 cm 1, which had a good agreement with the spectra measurement as shown in Fig. 7.
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3.2.4. ZGP OPO The ZGP crystals were cut to 6 mm 6 mm 15 mm in sizes after thermal annealing. Both surfaces of the crystal were carefully polished and antireflection coated for pump, signal, and idler lights. The crystal was cut at y ¼551 and f ¼901 for type I phase-match. High pulse repetition frequency of 10 kHz cryogenic cooled Tm, Ho:GdVO4 laser emitting at 2.05 mm. Which was used as the pumping source for ZGP OPO. The ZGP OPO output wavelengths were in the spectral range of 3–5 mm. Fig. 8 shows the output power of the ZGP OPO as a function of 2.05 mm Tm, Ho:GdVO4 pumping power. It is shown in Fig. 8 that up to 3.2 W output power was obtained under an incident pump power of 9.4 W. The corresponding slope efficiency was 44% and the conversion efficiency was 34%. Thus, from above results, it can be suggested that the ZGP is an excellent OPO material and is acceptable for the fabrication of the infrared nonlinear optical devices. To depress the absorption at 2.05 mm this is an effective way to improve the ZGP OPO property. Further, improvement of the polishing technology and the AR film technology is also useful to increase ZGP OPO output power.
4. Conclusion Using the polycrystalline ZGP single phase material, synthesized in our laboratory, the seeded Vertical Bridgman method was developed to grow ZGP single crystals. The crack-free ZGP single crystals of 20–30 mm in diameter and 90–120 mm in length have been grown. The quality of the grown crystals was studied by XRD, rocking curves analysis, XRF spectra, IR spectrophotometry, and OPO. All results demonstrate that the developed growth method is very promising to produce ZGP single crystals and the quality of the grown crystals is good. As it has been shown by XRF spectra, the composition of the crystals is close to the ideal stoichiometry ratio of 1:1:2, which demonstrates that the as-grown crystal is quite homogeneous. The optical absorption coefficient for annealed ZGP crystals is 0.01 cm 1 at 3–8 mm and 0.10 cm 1 at 2.05 mm for o-light. The output power of 3.2 W was obtained when the incident pumping power of 2.05 mm laser was 9.4 W, the corresponding slope efficiency was 44% and the conversion efficiency was 34%. In conclusion, the crystals are acceptable for the fabrication of the infrared nonlinear optical devices, in particular, OPO devices.
Fig. 7. Optical absorption coefficient spectra, typical for our annealed ZGP crystals, using o- and e-polarized lights at the wavelength range over 1–13 mm.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. E50872023) and the Key Science and Technology Program of Heilongjiang Province (No. GC05A205). References
Fig. 8. OPO output power as a function of pump power.
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