- Email: [email protected]
Investigation on growth and defects of Ho3+:BaY2F8 crystals grown by Czochralski method
Journal of Alloys and Compounds 648 (2015) 803e808
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation on growth and defects of Ho3þ:BaY2F8 crystals grown by Czochralski method Hui Luo a, b, Zhouguo Guan b, Zhiyu He a, *, Wei Huang a, Wei Zhang b, Ruihua Niu b, Chao Yao b, Yongqiang Yang b, Huirong Zhang b, Zhibin Zhang b a b
Department of Materials Science, Sichuan University, 610064 Chengdu, China Southwest Institute of Technical Physics, Chengdu 610041, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2015 Received in revised form 1 July 2015 Accepted 7 July 2015 Available online 9 July 2015
Large and heavily Ho3þ-doped BaY2F8 single crystals were grown by the Czochralski method. X-ray powder diffraction was applied to analyze the phase of the crystal samples. Simultaneously, metallographic microscope, scanning electron microscopy and energy dispersive spectrometer were employed to observe and investigate defects in the as grown crystals. Two significant kinds of defects, namely cracking and impurities were discovered in the samples of Ho3þ:BaY2F8 single crystals. Theoretical analyses suggested that mechanisms concerning the formation of the impurities such as bubbles and inclusions were considered to be closely related to the growth temperature and atmosphere while the former defect was primarily brought by the lattice distortion relating to the thermal stress and the impurities. Based on the results of experiments and theoretical analyses, the parameters of growth process were optimized and a crack free 20 mol% Ho3þ:BaY2F8 single crystal has been successfully obtained. Furthermore, the UVeVis-IR (0.2e10 mm) absorption spectra of BaY2F8 single crystal and the crystal heavily doped with Ho3þ ions (20 mol%) have been investigated at room temperature. © 2015 Elsevier B.V. All rights reserved.
Keywords: Crystal defects Formation mechanism Inclusion Czochralski method Ho3þ:BaY2F8
1. Introduction The BaY2F8 (BYF) crystal is an attractive crystalline matrix for effective mid-infrared laser because of its low phonon energy (ħu ~ 360e380 cm1 [1]) and its laser output at room temperature has already been realized [2e7]. Therefore, a wide range of stimulated emission mechanisms in rare-earth-ions-doped BYF crystals have been investigated (e.g. Ho3þ [8], Tm3þ [9], Er3þ [10], Nd3þ [11]). In order to obtain BYF crystals, the Czochralski [12], Bridgman and temperature gradient technique method [13] were widely adopted. However, high quality crystals can be grown by Czochralski method because of avoiding the difficulties in growing fluoride crystals [14]. The BYF crystal belongs to monoclinic-symmetry class. After it is doped with Ho3þ, the Ho3þ ions are expected to occupy the Y3þ site [15], and the Y3þ sites exhibit 8-fold coordination to the 8 F ions, which manifest a C2 point symmetry [16]. Based on such a structure, the attention of the previous studies [17e19] was focused on the
* Corresponding author. Tel.: þ86 2885222120. E-mail address: [email protected] (Z. He). http://dx.doi.org/10.1016/j.jallcom.2015.07.062 0925-8388/© 2015 Elsevier B.V. All rights reserved.
characteristics of infrared emission bands of heavily Ho:BYF crystal. Efficient laser radiation was achieved at 3.9 mm transition with pulsed resonant laser pumping in the Ho:BYF single crystal [20]. Nonetheless, few studies have been carried out for the purpose of revealing the relationship between the optimization of the Ho:BYF crystal growth process and the reduction of the defects. The present work, therefore, is devoted to the defects of heavily Ho:BYF crystal grown by Czochralski method; two defects, namely cracking and impurities will be closely examined. In this paper, the reasons why the crystals crack along certain planes and how the cracking can be eliminated were investigated. Through our experiments and theoretical analyses, the optimal growth parameters were obtained by the Czochralski method under CF4 atmosphere. Moreover, the optical properties of 20 mol% Ho:BYF crystal were discussed. 2. Experiment Single crystals of heavily Ho:BYF were grown by the Czochralski method. The crystal growth was carried out with 10 kW graphite heating system. High purity powders (99.999%) of oxide reagents (such as Y2O3, BaCO3, Ho2O3) were utilized. According to the
804
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
stoichiometric amounts, Ho:BYF polycrystalline materials were gained by the hydrothermal method and the coprecipitation synthesis method. The concentration of Ho3þ ions in BYF was 20 mol% in the melt. The polycrystalline materials were carried out in a graphite crucible, heating at 800 C for a period of 10 h. To eliminate effectively the water and oxygen present in the chamber and raw materials, overall system was vacuumized by a rotary and a diffusion pump below 103 Pa, since even traces of those compounds are well known to be very detrimental for the optical quality of fluoride crystals [21]. In order to obtain high quality crystals with low defect densities like inclusions, growing parameters (growth speed, temperature gradient, atmosphere content and pressure) have to be chosen very carefully. Therefore, high purity CF4 gas (99.999%) was slowly flowed into the furnace according to a certain proportion while the polycrystalline powders were melted at about 1258 K. Using a seed rod with direction of [010], the crystal was grown with a rotation speed at 5e10 rpm and a pulling rate at 0.5e1.5 mm/h. The phase structures of the powders and as-grown crystals were done via X-ray diffraction (XRD) measurements which were performed in a Rigaku D/MAX 2000v/PC diffract meter in the 2q range from 10 to 80 in step scan mode, with steps of 0.06 , using CuK a radiation (40 kV/25 mA) at room temperature. The metallographic microscope and scanning electron microscopy (SEM) were employed to view the defects of as-grown Ho:BYF crystal. The defects were investigated by Energy Dispersive Spectrometer (EDS), and the samples were coated with a Gold conductive film to evacuate charged particles from the surface. In order to illustrate the source of impurities found in the above testing for defects, the inductively coupled plasma mass spectrometry (ICP-MS, XSenes7) and elemental analyzer (PerkineElmer PE2400) were utilized to determinate the relative content of impurity elements of polycrystalline raw materials. Thermal expansion curves of the crystals were measured by NETZSCH DIL402C thermal dilatometer with a measured range of 40e900 C. The crystal was cut vertically along with the [001] orientation and polished for optical measurements. Absorption spectra of BYF and 20 mol% Ho:BYF single crystal in the wavelength range 0.2e2.5 mm was measured with a PerkinElmer lambda-1050 spectrophotometer whose spectral resolution is 1 nm at room temperature, and in the wavelength range 2.5e10 mm, it was measured with a PerkinElmer infrared spectrometer whose spectral resolution is about 3 nm at room temperature. The dimensions of the tested sample were 10 mm 3 mm 2.23 mm. 3. Results and discussion Fig. 1 shows the photographs of the Ho:BYF single crystals grown by Czochralski technique. It can be seen from Fig. 1(a) and (b) that one of two the Ho:BYF crystals was pale pink under the fluorescent lamp irradiation while the other one was pale yellow after being processed under the natural light. In the process of crystal growth, crystals sometimes crack along certain planes, which are regarded as the cleavage planes. This crack, one of the reasons of which is the high thermal stress, is manifestly presented in Fig. 1(b). Fig. 1(c) shows the XRD pattern of the cleavage surface. From the noteworthy fact that the Two-Theta angle of the diffraction peak is equal to 25.8 , we can draw the conclusion that the crystals have cleaved along the (100) plane. Since the solideliquid (SeL) interface could not be controlled as an ideal flat interface and was thus unstable during the growth of the crystal, the thermal stress could not be avoided. This kind of stress was in direct proportion to the temperature gradient, and in inverse proportion to the curvature radius of SeL interface. Fig. 2 shows the expansion coefficient curve
of vertical along (100) plane; it also indicates that the thermal expansion intensifies in the temperature range of 800e900 C. But, this kind of stress can be alleviated by annealing for, for example, more than 60 h [22]. The rest of the cracks, which are less significant and which are perpendicular to the (010) planes, can also be perceived in Fig. 1(b). This phenomenon may be attributed to the impurity induced stress. The experimental X-ray powder diffraction patterns of the pure BYF single crystal and the single crystal in the concentrations of 20 mol% of Ho3þ ions are presented in Fig. 3. All powder samples were extracted from the crystals with cracks shown in Fig. 1(b). The strong diffraction peaks are collected in a narrow angle range and are well matched with those in the BaY2F8 standard card (JCPDS450246); the identity of our BYF samples can be thus confirmed. Taking into account the peak positions of the diffraction angle, the XRD patterns of all the samples are consistent with the standard BYF powder diffraction pattern. In addition, it can be observed that undesirable phases such as Ba4Y3F17 and YOF do not exist in these samples. Yet small amounts of the phase of carbon can be proved to exist by comparing the hash-marked peaks in Fig. 3 with those in the standard card (JCPDS46-0943). Since the radius of an Ho3þ ion is slightly smaller than that of a Y3þ ion (r (Ho3þ) ¼ 1.015 Å, r (Y3þ) ¼ 1.019 Å [23]), when the Ho3þ ions enter into the crystal and occupy the Y3þ lattice sites, lattice distortion occurs; as a result, it is observed in Fig. 3(a) that the diffraction peak positions have shifted to the right. Furthermore, the higher content of Ho3þ ions is doped in the crystal, the more serious the lattice distortion becomes. Apart from that, impurities such as bubbles and inclusions may also give rise to lattice distortion. The crystal slice which was cut from the as-grown Ho:BYF crystal boule with [100] orientation and whose thickness is 1.0 mm after optically polishing is shown in Fig. 4. That several bubbles, most of which assume the form of a cylinder with a diameter of about 10 mm and a length of 50e200 mm, can be observed in the slice samples under the metallographic microscope, as shown in Fig. 5. In addition, the figure demonstrates that in the interior of these cylinders, there are some inclusions, the composition of which will be discussed at length in further part of this section. By observing boules paralleling to the (010) plane, it can be concluded that the bubbles and inclusions were crystallized in the vicinity of the growth front. The possible causes, which can both be attributed to the lower molten temperature and the shorter time for constant temperature, are larger viscosity and incomplete volatilization of the molten material in nearsolidification layers. The SEM measurement for sampling point in Fig. 5(a) illustrates the morphology and the structure of the inclusion inside the bubble (Fig. 6(a)). The size of the inclusion is about 10 mm 15 mm. Moreover, the comparison between the composition of the inclusion area and that of the single-crystal area for the same Ho:BYF sample can be conducted, based on the EDS recorded spectrums shown in Fig. 6(c) and (d). As impurities, the carbon (C), iron (Fe), hafnium (Hf) and selenium (Se) elements have been detected near the sampling point; In contrast, no C, Fe, Hf and Se have been detected in single-crystal area. The above finding indicates that impurities gathered within the inclusions, but the impurity content is few in single crystal area, thus failed to detect. The reason for above phenomenon could be that the defect concentration is high within the inclusion, and easily, impurities are accumulated around those defects. Table 1 presents the weight composition of ICP analysis made for impurities and Ho in polycrystalline materials. Because ICP is not suitable for accurate measurement for some lighter elements such as carbon, the element analyzer is used to measure the
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
805
Fig. 1. Photographs of as-grown Ho:BYF single crystal under the fluorescent lamp irradiation (a), as-grown Ho:BYF single crystal under natural light (b), and the XRD pattern of cleavage surface of cracks(c).
polycrystalline material. The carbon element was not detected in polycrystalline material. The result can be obtained by comparing the Fig. 6(c) and Table 1 that there are following impurities in polycrystalline materials and inclusions: Fe, Hf and Se element; but the carbon element was found only in inclusions.
In general, there are several possibilities for the source of the impurity in the crystal. Firstly, impurity elements exist in raw materials inevitably; secondly, impurity elements may be add in the polycrystalline artificially in the process of the synthesis of raw materials; thirdly, the reason for impurity elements is that there is the volatility of heater and crucible at high temperature in the process of crystal growth.
Fig. 2. Expansion coefficient curve of vertical along (100) plane.
Fig. 3. XRD powder diffraction pattern of BYF and Ho:BYF crystals.
806
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
Fig. 4. The inset is the photograph of tested samples, which are two slices cut from the Ho:BYF crystal. The thickness of the two slices is about 1.0 mm after optical polishing.
The following reasons could explain the source of impurities in the crystal and the formation mechanism of the inclusions. The EDS measurement implying that the inclusion is composed of some impurity elements such as C, Fe, Hf and Se and their compounds. The testing results for ICP (Table 1) shows that the impurity elements (Fe, Hf and Se) are derived from the polycrystalline materials. But, there are only graphite crucible and heater composed of carbon element inside the chamber of the growth furnace, the main source of C in the crystal is, probably, the injection of particles from the walls of the graphite crucible and from the source according to the volatility mechanism of graphite heater at higher temperature in the process of crystal growth. Such a hypothesis [24] is supported by the fact that the C inclusions are typically observed in the case of high temperature gradients or at pronounced exhaustion of the source (a large number of weakly bonded carbon atoms) or under unstable conditions in the system. Agglutinations of C gave rise to dislocation nucleation and agglutinations of other impurity elements formed inclusions eventually. In other words, if the graphite crucible is made of high-purity material, which becomes so nonvolatile under high temperature that it is difficult to be dissolved into the molten material; and ultimately, the carbon element will hardly any be smuggled into the crystal along the formation of the bubbles during the crystal growth. With the above analysis, some impurities like bubbles and inclusions were introduced into crystal during crystal growth. The
impurities led to the crystal lattice distortion. So, the stress around the impurities was generated. All the results and above analysis illustrate that cracks were also caused by the lattice distortion related to the impurities. Such a conclusion can be deduced from the calculated results of the Ho3þ doping concentration, which is about 18 at.% (Fig. 6(d)). The deviation of the Ho3þ doping concentration can be explained by that the EDS method allows only qualitative and semiquantitative measurement of atomic component relative concentration. In order to obtain high quality and large Ho:BYF single crystal which has no cracks, the following measures were adopted: (1) the graphite crucible and heater with higher purity have been chosen; (2) the raw materials with higher purity have been chosen, and the risk of pollution polycrystalline material has been reduced in the synthesis process by process optimization; (3) the temperature gradient near-solidification layers was optimized; (4) the time for constant temperature at the stage of melting materials was prolonged. Fig. 7 shows the large Ho:BYF single crystal whose diameter is greater than 36.4 mm and 40 mm length at least grown by the above optimization of process. Fig. 8 shows the absorption spectra of 20 mol% Ho:BYF and pure BYF crystals at room temperature. The electronic configuration is 4f10 for a free Ho3þ ion [25,26]. We only consider the spineorbit coupling of the trivalent ions and do not take account of the Stark splitting, which is caused by the crystal field. In the Fig. 8, the absorption peak position is 532 nm, 639 nm, 889 nm, 1150 nm, 1192 nm, 1935 nm and 2055 nm, corresponding to the transitions from the 5I8 ground state to the 5S2, 5F5, 5I5, 5I6 (1135e1205 nm) and 5 I7 (1850e2095 nm) excited states. By comparing the spectra of doped sample with that of a pure one, together with observing only lines due to Ho3þ in the doped samples, it may conclude that the crystal samples doped with 20% Ho3þ ions do not manifest any host absorption in the 200e8000 nm spectral range. According to the relationship between the absorption coefficient and the integrated absorption cross section which can be expressed as:
sabs ¼ X abs
kðlÞ DðlÞ ¼ Nc dNc lge
Z ¼
DðlÞ 1 dl ¼ dNc lge 0:4343dNc
(1) Z DðlÞdl
(2)
where, k(l) is the absorption coefficient, sabs is the absorption cross section, D(l) is the optical density, d is the thickness of the sample and Nc is the Ho3þ ion density per cubic centimeter in the BYF
Fig. 5. Bubbles in the slice sample.
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
807
Fig. 6. The analyzed results of the composition for sampling point in Fig. 4 by SEM and EDS (a) inclusion photograph by SEM; (b) single-crystal photograph by SEM; (c) The chemical composition of inclusion sampling point by EDS; (d) The chemical composition of single-crystal by EDS.
Table 1 Weight composition of impurities and Ho in polycrystalline materials obtained by ICP. Element
Mass fraction/%
Fe Se Hf Ho
2.4 3.8 2.8 9.94
104 106 106 102
Relative ratio for molar fraction of the Ho ions/% 0.7 102 0.8 104 0.26 104
crystal. The absorption cross section can be estimated from Eq. (1) (sabs ¼ 1.29 1021 cm3 @ l ¼ 889 nm, sabs ¼ 3.64 1021 cm3 @ l ¼ 532 nm). So, the conclusion can be confirmed that broader absorption band and larger absorption cross section serve to absorb the pump light and improve the conversion efficiency of pump light [17]. Further study will be focused on the properties of Ho:BYF single crystal for its application in laser output. 4. Conclusion The growth of large Ho:BYF single crystal was performed by the Czochralski method under CF4 atmosphere. Various measurements, including XRD, SEM, EDS, ICP-MS and etc., were adopted to characterize the properties of the Ho:BYF single crystal. It is found that the existence of thermal stress during the growth process and the lattice distortion caused by the impurities, such as bubbles and inclusions, give rise to the cracks of the as grown Ho:BYF crystal. Based on the results of EDS, ICP-MS and elements analyzer, the impurity elements gathered in the inclusions are Fe, Hf, Se and C. Among them, Fe, Hf and Se are derived from the polycrystalline raw materials, whereas C is derived from the graphite crucible and heater. Hence, higher purity raw materials, higher purity graphite
Fig. 7. Photograph of the as-grown 20 mol% Ho:BYF crystal (pale yellow) under the natural light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
crucibles and heaters and various optimized growth conditions for reducing the thermal stress were used. As a result, an integrated and large Ho:BYF single crystal (Ø36.4 40 mm3) was successfully obtained. Additionally, the absorption spectra show that the crystal
808
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
Fig. 8. Wavelength range 0.2e2.5 mm (a) and 2.5e10 mm (b) absorption spectra of 20 mol% Ho:BYF (1) and pure BYF (2) crystals at room temperature.
samples doped with 20% Ho3þ ions do not manifest any host absorption in the 200e8000 nm spectral range, and the estimated absorption cross sections are up to 1.29 1021 cm3 at 889 nm and 3.64 1021 cm3 at 532 nm which facilitates the absorption of the pump light. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
E.B. Svesnikova, A.A. Stroganov, N.T. Timofeev, Opt. Spectrosk. 64 (1993) 43. L.F. Johnson, H.J. Guggenheim, Appl. Phys. Lett. 20 (1972) 474. L.F. Johnson, H.J. Guggenheim, J. IEEE, Quantum Electron. QE-10 (1974) 442. A.A. Kaminskii, S.E. Sarkisov, F. Below, Opt. Quantum Electron. 22 (1990) S95. M.F. Joubert, B. Jacquier, C. Linares, J. Lumin. 47 (1991) 269. X.X. Zhang, P. Hong, M. Bass, Appl. Phys. Lett. 63 (1993) 2606. M. Pollnau, W. Lüthy, H.P. Weber, Opt. Lett. 21 (1996) 48. E. Osiac, I. Sokolska, S. Kück, J. Alloys Compd. 323 (2001) 283. M.A. Noginov, M. Curley, P. Venkateswarlu, A. Williams, H.-P. Jenssen, J. Opt. Soc. Am. B 14 (1997) 2126. [10] M. Pollnau, W. Luethy, H.-P. Weber, K. Kraemer, H.U. Guedel, R.A. McFarlane, Appl. Phys. B 62 (1996) 339. [11] S. Guy, M.-F. Joubert, B. Jacquier, M. Bouazaoui, Phys.Rev. B 47 (1993) 11001. [12] F. Cornacchia, A. Toncelli, M. Tonelli, Prog. Quantum Electron. 33 (2009) 61.
[13] Shuai Wang, Yongfeng Ruan, Taiju Tsuboi, Shouchao Zhang, Youfa Wang, Zhouli Wu, Hongshuang Tong, Cryst. Res. Technol. 47 (2012) 928. [14] F. Cornacchia, A. Toncelli, M. Tonelli, Prog. Quantum Electron. 33 (2009) 75. [15] J.B. Amaral, M.A.C. Santos, M.E.G. Valerio, R.A. Jackson, Appl. Phys. 81 (2005) 841. [16] J.C. van’t Spijker, et al., J. Lumin. 85 (1999) 11. [17] Taiju Tsuboi, KyoungHyuk Jang, Silviu Polosan, Kiyoshi Shimamura, Marco Bettinelli, Hyo Jin Seo, J. Lumin. 131 (2011) 695e700. [18] M. Mazzera, A. Baraldi, R. Capelletti, M. Tonelli, Opt. Mater. 31 (2009) 1366e1369. [19] E. Osiac, I. Sokolska, J. Alloys Compd. 323e324 (2001) 283e287. [20] A.M. Tabirian, D.P. Stanley, R.R. Selleck, L.H. Morton, S. Guch, Lasers, ElectroOptics Society, Publication Date, in: LEOS 2007. The 20th Annual Meeting of the IEEE, 2007, pp. 21e25. Oct. 2007, On page(s): 507-508, ISSN: 1092-8081, ISBN: 978-1-4244-0925-9, INSPEC Accession Number: 9858491. [21] S.L. Baldochi, I.M. Ranieri, Acta Phys. Pol. A 124 (2013) 286e294. [22] Zhang Shouchao, Ruan Yongfeng, Wei Xiaoping, et al., J. Synth. Cryst. 39 (2010) 625e626. [23] R.D. Shannon, Acta Cryst. A 32 (1976) 751. [24] D.D. Avrov, S.I. Dorozhkin, et al., Mater. Electron. Eng. 42 (2008) 1474. [25] G.H. Dieke, in: H.M. Crosswhite, H. Crosswhite (Eds.), Wiley Interscience Publishers, New York, 1968. [26] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, Report Argonne National Laboratory, Chemistry Division, Argonne, Illinois, 1977.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Investigation on growth and defects of Ho3þ:BaY2F8 crystals grown by Czochralski method Hui Luo a, b, Zhouguo Guan b, Zhiyu He a, *, Wei Huang a, Wei Zhang b, Ruihua Niu b, Chao Yao b, Yongqiang Yang b, Huirong Zhang b, Zhibin Zhang b a b
Department of Materials Science, Sichuan University, 610064 Chengdu, China Southwest Institute of Technical Physics, Chengdu 610041, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2015 Received in revised form 1 July 2015 Accepted 7 July 2015 Available online 9 July 2015
Large and heavily Ho3þ-doped BaY2F8 single crystals were grown by the Czochralski method. X-ray powder diffraction was applied to analyze the phase of the crystal samples. Simultaneously, metallographic microscope, scanning electron microscopy and energy dispersive spectrometer were employed to observe and investigate defects in the as grown crystals. Two significant kinds of defects, namely cracking and impurities were discovered in the samples of Ho3þ:BaY2F8 single crystals. Theoretical analyses suggested that mechanisms concerning the formation of the impurities such as bubbles and inclusions were considered to be closely related to the growth temperature and atmosphere while the former defect was primarily brought by the lattice distortion relating to the thermal stress and the impurities. Based on the results of experiments and theoretical analyses, the parameters of growth process were optimized and a crack free 20 mol% Ho3þ:BaY2F8 single crystal has been successfully obtained. Furthermore, the UVeVis-IR (0.2e10 mm) absorption spectra of BaY2F8 single crystal and the crystal heavily doped with Ho3þ ions (20 mol%) have been investigated at room temperature. © 2015 Elsevier B.V. All rights reserved.
Keywords: Crystal defects Formation mechanism Inclusion Czochralski method Ho3þ:BaY2F8
1. Introduction The BaY2F8 (BYF) crystal is an attractive crystalline matrix for effective mid-infrared laser because of its low phonon energy (ħu ~ 360e380 cm1 [1]) and its laser output at room temperature has already been realized [2e7]. Therefore, a wide range of stimulated emission mechanisms in rare-earth-ions-doped BYF crystals have been investigated (e.g. Ho3þ [8], Tm3þ [9], Er3þ [10], Nd3þ [11]). In order to obtain BYF crystals, the Czochralski [12], Bridgman and temperature gradient technique method [13] were widely adopted. However, high quality crystals can be grown by Czochralski method because of avoiding the difficulties in growing fluoride crystals [14]. The BYF crystal belongs to monoclinic-symmetry class. After it is doped with Ho3þ, the Ho3þ ions are expected to occupy the Y3þ site [15], and the Y3þ sites exhibit 8-fold coordination to the 8 F ions, which manifest a C2 point symmetry [16]. Based on such a structure, the attention of the previous studies [17e19] was focused on the
* Corresponding author. Tel.: þ86 2885222120. E-mail address: [email protected] (Z. He). http://dx.doi.org/10.1016/j.jallcom.2015.07.062 0925-8388/© 2015 Elsevier B.V. All rights reserved.
characteristics of infrared emission bands of heavily Ho:BYF crystal. Efficient laser radiation was achieved at 3.9 mm transition with pulsed resonant laser pumping in the Ho:BYF single crystal [20]. Nonetheless, few studies have been carried out for the purpose of revealing the relationship between the optimization of the Ho:BYF crystal growth process and the reduction of the defects. The present work, therefore, is devoted to the defects of heavily Ho:BYF crystal grown by Czochralski method; two defects, namely cracking and impurities will be closely examined. In this paper, the reasons why the crystals crack along certain planes and how the cracking can be eliminated were investigated. Through our experiments and theoretical analyses, the optimal growth parameters were obtained by the Czochralski method under CF4 atmosphere. Moreover, the optical properties of 20 mol% Ho:BYF crystal were discussed. 2. Experiment Single crystals of heavily Ho:BYF were grown by the Czochralski method. The crystal growth was carried out with 10 kW graphite heating system. High purity powders (99.999%) of oxide reagents (such as Y2O3, BaCO3, Ho2O3) were utilized. According to the
804
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
stoichiometric amounts, Ho:BYF polycrystalline materials were gained by the hydrothermal method and the coprecipitation synthesis method. The concentration of Ho3þ ions in BYF was 20 mol% in the melt. The polycrystalline materials were carried out in a graphite crucible, heating at 800 C for a period of 10 h. To eliminate effectively the water and oxygen present in the chamber and raw materials, overall system was vacuumized by a rotary and a diffusion pump below 103 Pa, since even traces of those compounds are well known to be very detrimental for the optical quality of fluoride crystals [21]. In order to obtain high quality crystals with low defect densities like inclusions, growing parameters (growth speed, temperature gradient, atmosphere content and pressure) have to be chosen very carefully. Therefore, high purity CF4 gas (99.999%) was slowly flowed into the furnace according to a certain proportion while the polycrystalline powders were melted at about 1258 K. Using a seed rod with direction of [010], the crystal was grown with a rotation speed at 5e10 rpm and a pulling rate at 0.5e1.5 mm/h. The phase structures of the powders and as-grown crystals were done via X-ray diffraction (XRD) measurements which were performed in a Rigaku D/MAX 2000v/PC diffract meter in the 2q range from 10 to 80 in step scan mode, with steps of 0.06 , using CuK a radiation (40 kV/25 mA) at room temperature. The metallographic microscope and scanning electron microscopy (SEM) were employed to view the defects of as-grown Ho:BYF crystal. The defects were investigated by Energy Dispersive Spectrometer (EDS), and the samples were coated with a Gold conductive film to evacuate charged particles from the surface. In order to illustrate the source of impurities found in the above testing for defects, the inductively coupled plasma mass spectrometry (ICP-MS, XSenes7) and elemental analyzer (PerkineElmer PE2400) were utilized to determinate the relative content of impurity elements of polycrystalline raw materials. Thermal expansion curves of the crystals were measured by NETZSCH DIL402C thermal dilatometer with a measured range of 40e900 C. The crystal was cut vertically along with the [001] orientation and polished for optical measurements. Absorption spectra of BYF and 20 mol% Ho:BYF single crystal in the wavelength range 0.2e2.5 mm was measured with a PerkinElmer lambda-1050 spectrophotometer whose spectral resolution is 1 nm at room temperature, and in the wavelength range 2.5e10 mm, it was measured with a PerkinElmer infrared spectrometer whose spectral resolution is about 3 nm at room temperature. The dimensions of the tested sample were 10 mm 3 mm 2.23 mm. 3. Results and discussion Fig. 1 shows the photographs of the Ho:BYF single crystals grown by Czochralski technique. It can be seen from Fig. 1(a) and (b) that one of two the Ho:BYF crystals was pale pink under the fluorescent lamp irradiation while the other one was pale yellow after being processed under the natural light. In the process of crystal growth, crystals sometimes crack along certain planes, which are regarded as the cleavage planes. This crack, one of the reasons of which is the high thermal stress, is manifestly presented in Fig. 1(b). Fig. 1(c) shows the XRD pattern of the cleavage surface. From the noteworthy fact that the Two-Theta angle of the diffraction peak is equal to 25.8 , we can draw the conclusion that the crystals have cleaved along the (100) plane. Since the solideliquid (SeL) interface could not be controlled as an ideal flat interface and was thus unstable during the growth of the crystal, the thermal stress could not be avoided. This kind of stress was in direct proportion to the temperature gradient, and in inverse proportion to the curvature radius of SeL interface. Fig. 2 shows the expansion coefficient curve
of vertical along (100) plane; it also indicates that the thermal expansion intensifies in the temperature range of 800e900 C. But, this kind of stress can be alleviated by annealing for, for example, more than 60 h [22]. The rest of the cracks, which are less significant and which are perpendicular to the (010) planes, can also be perceived in Fig. 1(b). This phenomenon may be attributed to the impurity induced stress. The experimental X-ray powder diffraction patterns of the pure BYF single crystal and the single crystal in the concentrations of 20 mol% of Ho3þ ions are presented in Fig. 3. All powder samples were extracted from the crystals with cracks shown in Fig. 1(b). The strong diffraction peaks are collected in a narrow angle range and are well matched with those in the BaY2F8 standard card (JCPDS450246); the identity of our BYF samples can be thus confirmed. Taking into account the peak positions of the diffraction angle, the XRD patterns of all the samples are consistent with the standard BYF powder diffraction pattern. In addition, it can be observed that undesirable phases such as Ba4Y3F17 and YOF do not exist in these samples. Yet small amounts of the phase of carbon can be proved to exist by comparing the hash-marked peaks in Fig. 3 with those in the standard card (JCPDS46-0943). Since the radius of an Ho3þ ion is slightly smaller than that of a Y3þ ion (r (Ho3þ) ¼ 1.015 Å, r (Y3þ) ¼ 1.019 Å [23]), when the Ho3þ ions enter into the crystal and occupy the Y3þ lattice sites, lattice distortion occurs; as a result, it is observed in Fig. 3(a) that the diffraction peak positions have shifted to the right. Furthermore, the higher content of Ho3þ ions is doped in the crystal, the more serious the lattice distortion becomes. Apart from that, impurities such as bubbles and inclusions may also give rise to lattice distortion. The crystal slice which was cut from the as-grown Ho:BYF crystal boule with [100] orientation and whose thickness is 1.0 mm after optically polishing is shown in Fig. 4. That several bubbles, most of which assume the form of a cylinder with a diameter of about 10 mm and a length of 50e200 mm, can be observed in the slice samples under the metallographic microscope, as shown in Fig. 5. In addition, the figure demonstrates that in the interior of these cylinders, there are some inclusions, the composition of which will be discussed at length in further part of this section. By observing boules paralleling to the (010) plane, it can be concluded that the bubbles and inclusions were crystallized in the vicinity of the growth front. The possible causes, which can both be attributed to the lower molten temperature and the shorter time for constant temperature, are larger viscosity and incomplete volatilization of the molten material in nearsolidification layers. The SEM measurement for sampling point in Fig. 5(a) illustrates the morphology and the structure of the inclusion inside the bubble (Fig. 6(a)). The size of the inclusion is about 10 mm 15 mm. Moreover, the comparison between the composition of the inclusion area and that of the single-crystal area for the same Ho:BYF sample can be conducted, based on the EDS recorded spectrums shown in Fig. 6(c) and (d). As impurities, the carbon (C), iron (Fe), hafnium (Hf) and selenium (Se) elements have been detected near the sampling point; In contrast, no C, Fe, Hf and Se have been detected in single-crystal area. The above finding indicates that impurities gathered within the inclusions, but the impurity content is few in single crystal area, thus failed to detect. The reason for above phenomenon could be that the defect concentration is high within the inclusion, and easily, impurities are accumulated around those defects. Table 1 presents the weight composition of ICP analysis made for impurities and Ho in polycrystalline materials. Because ICP is not suitable for accurate measurement for some lighter elements such as carbon, the element analyzer is used to measure the
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
805
Fig. 1. Photographs of as-grown Ho:BYF single crystal under the fluorescent lamp irradiation (a), as-grown Ho:BYF single crystal under natural light (b), and the XRD pattern of cleavage surface of cracks(c).
polycrystalline material. The carbon element was not detected in polycrystalline material. The result can be obtained by comparing the Fig. 6(c) and Table 1 that there are following impurities in polycrystalline materials and inclusions: Fe, Hf and Se element; but the carbon element was found only in inclusions.
In general, there are several possibilities for the source of the impurity in the crystal. Firstly, impurity elements exist in raw materials inevitably; secondly, impurity elements may be add in the polycrystalline artificially in the process of the synthesis of raw materials; thirdly, the reason for impurity elements is that there is the volatility of heater and crucible at high temperature in the process of crystal growth.
Fig. 2. Expansion coefficient curve of vertical along (100) plane.
Fig. 3. XRD powder diffraction pattern of BYF and Ho:BYF crystals.
806
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
Fig. 4. The inset is the photograph of tested samples, which are two slices cut from the Ho:BYF crystal. The thickness of the two slices is about 1.0 mm after optical polishing.
The following reasons could explain the source of impurities in the crystal and the formation mechanism of the inclusions. The EDS measurement implying that the inclusion is composed of some impurity elements such as C, Fe, Hf and Se and their compounds. The testing results for ICP (Table 1) shows that the impurity elements (Fe, Hf and Se) are derived from the polycrystalline materials. But, there are only graphite crucible and heater composed of carbon element inside the chamber of the growth furnace, the main source of C in the crystal is, probably, the injection of particles from the walls of the graphite crucible and from the source according to the volatility mechanism of graphite heater at higher temperature in the process of crystal growth. Such a hypothesis [24] is supported by the fact that the C inclusions are typically observed in the case of high temperature gradients or at pronounced exhaustion of the source (a large number of weakly bonded carbon atoms) or under unstable conditions in the system. Agglutinations of C gave rise to dislocation nucleation and agglutinations of other impurity elements formed inclusions eventually. In other words, if the graphite crucible is made of high-purity material, which becomes so nonvolatile under high temperature that it is difficult to be dissolved into the molten material; and ultimately, the carbon element will hardly any be smuggled into the crystal along the formation of the bubbles during the crystal growth. With the above analysis, some impurities like bubbles and inclusions were introduced into crystal during crystal growth. The
impurities led to the crystal lattice distortion. So, the stress around the impurities was generated. All the results and above analysis illustrate that cracks were also caused by the lattice distortion related to the impurities. Such a conclusion can be deduced from the calculated results of the Ho3þ doping concentration, which is about 18 at.% (Fig. 6(d)). The deviation of the Ho3þ doping concentration can be explained by that the EDS method allows only qualitative and semiquantitative measurement of atomic component relative concentration. In order to obtain high quality and large Ho:BYF single crystal which has no cracks, the following measures were adopted: (1) the graphite crucible and heater with higher purity have been chosen; (2) the raw materials with higher purity have been chosen, and the risk of pollution polycrystalline material has been reduced in the synthesis process by process optimization; (3) the temperature gradient near-solidification layers was optimized; (4) the time for constant temperature at the stage of melting materials was prolonged. Fig. 7 shows the large Ho:BYF single crystal whose diameter is greater than 36.4 mm and 40 mm length at least grown by the above optimization of process. Fig. 8 shows the absorption spectra of 20 mol% Ho:BYF and pure BYF crystals at room temperature. The electronic configuration is 4f10 for a free Ho3þ ion [25,26]. We only consider the spineorbit coupling of the trivalent ions and do not take account of the Stark splitting, which is caused by the crystal field. In the Fig. 8, the absorption peak position is 532 nm, 639 nm, 889 nm, 1150 nm, 1192 nm, 1935 nm and 2055 nm, corresponding to the transitions from the 5I8 ground state to the 5S2, 5F5, 5I5, 5I6 (1135e1205 nm) and 5 I7 (1850e2095 nm) excited states. By comparing the spectra of doped sample with that of a pure one, together with observing only lines due to Ho3þ in the doped samples, it may conclude that the crystal samples doped with 20% Ho3þ ions do not manifest any host absorption in the 200e8000 nm spectral range. According to the relationship between the absorption coefficient and the integrated absorption cross section which can be expressed as:
sabs ¼ X abs
kðlÞ DðlÞ ¼ Nc dNc lge
Z ¼
DðlÞ 1 dl ¼ dNc lge 0:4343dNc
(1) Z DðlÞdl
(2)
where, k(l) is the absorption coefficient, sabs is the absorption cross section, D(l) is the optical density, d is the thickness of the sample and Nc is the Ho3þ ion density per cubic centimeter in the BYF
Fig. 5. Bubbles in the slice sample.
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
807
Fig. 6. The analyzed results of the composition for sampling point in Fig. 4 by SEM and EDS (a) inclusion photograph by SEM; (b) single-crystal photograph by SEM; (c) The chemical composition of inclusion sampling point by EDS; (d) The chemical composition of single-crystal by EDS.
Table 1 Weight composition of impurities and Ho in polycrystalline materials obtained by ICP. Element
Mass fraction/%
Fe Se Hf Ho
2.4 3.8 2.8 9.94
104 106 106 102
Relative ratio for molar fraction of the Ho ions/% 0.7 102 0.8 104 0.26 104
crystal. The absorption cross section can be estimated from Eq. (1) (sabs ¼ 1.29 1021 cm3 @ l ¼ 889 nm, sabs ¼ 3.64 1021 cm3 @ l ¼ 532 nm). So, the conclusion can be confirmed that broader absorption band and larger absorption cross section serve to absorb the pump light and improve the conversion efficiency of pump light [17]. Further study will be focused on the properties of Ho:BYF single crystal for its application in laser output. 4. Conclusion The growth of large Ho:BYF single crystal was performed by the Czochralski method under CF4 atmosphere. Various measurements, including XRD, SEM, EDS, ICP-MS and etc., were adopted to characterize the properties of the Ho:BYF single crystal. It is found that the existence of thermal stress during the growth process and the lattice distortion caused by the impurities, such as bubbles and inclusions, give rise to the cracks of the as grown Ho:BYF crystal. Based on the results of EDS, ICP-MS and elements analyzer, the impurity elements gathered in the inclusions are Fe, Hf, Se and C. Among them, Fe, Hf and Se are derived from the polycrystalline raw materials, whereas C is derived from the graphite crucible and heater. Hence, higher purity raw materials, higher purity graphite
Fig. 7. Photograph of the as-grown 20 mol% Ho:BYF crystal (pale yellow) under the natural light. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
crucibles and heaters and various optimized growth conditions for reducing the thermal stress were used. As a result, an integrated and large Ho:BYF single crystal (Ø36.4 40 mm3) was successfully obtained. Additionally, the absorption spectra show that the crystal
808
H. Luo et al. / Journal of Alloys and Compounds 648 (2015) 803e808
Fig. 8. Wavelength range 0.2e2.5 mm (a) and 2.5e10 mm (b) absorption spectra of 20 mol% Ho:BYF (1) and pure BYF (2) crystals at room temperature.
samples doped with 20% Ho3þ ions do not manifest any host absorption in the 200e8000 nm spectral range, and the estimated absorption cross sections are up to 1.29 1021 cm3 at 889 nm and 3.64 1021 cm3 at 532 nm which facilitates the absorption of the pump light. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
E.B. Svesnikova, A.A. Stroganov, N.T. Timofeev, Opt. Spectrosk. 64 (1993) 43. L.F. Johnson, H.J. Guggenheim, Appl. Phys. Lett. 20 (1972) 474. L.F. Johnson, H.J. Guggenheim, J. IEEE, Quantum Electron. QE-10 (1974) 442. A.A. Kaminskii, S.E. Sarkisov, F. Below, Opt. Quantum Electron. 22 (1990) S95. M.F. Joubert, B. Jacquier, C. Linares, J. Lumin. 47 (1991) 269. X.X. Zhang, P. Hong, M. Bass, Appl. Phys. Lett. 63 (1993) 2606. M. Pollnau, W. Lüthy, H.P. Weber, Opt. Lett. 21 (1996) 48. E. Osiac, I. Sokolska, S. Kück, J. Alloys Compd. 323 (2001) 283. M.A. Noginov, M. Curley, P. Venkateswarlu, A. Williams, H.-P. Jenssen, J. Opt. Soc. Am. B 14 (1997) 2126. [10] M. Pollnau, W. Luethy, H.-P. Weber, K. Kraemer, H.U. Guedel, R.A. McFarlane, Appl. Phys. B 62 (1996) 339. [11] S. Guy, M.-F. Joubert, B. Jacquier, M. Bouazaoui, Phys.Rev. B 47 (1993) 11001. [12] F. Cornacchia, A. Toncelli, M. Tonelli, Prog. Quantum Electron. 33 (2009) 61.
[13] Shuai Wang, Yongfeng Ruan, Taiju Tsuboi, Shouchao Zhang, Youfa Wang, Zhouli Wu, Hongshuang Tong, Cryst. Res. Technol. 47 (2012) 928. [14] F. Cornacchia, A. Toncelli, M. Tonelli, Prog. Quantum Electron. 33 (2009) 75. [15] J.B. Amaral, M.A.C. Santos, M.E.G. Valerio, R.A. Jackson, Appl. Phys. 81 (2005) 841. [16] J.C. van’t Spijker, et al., J. Lumin. 85 (1999) 11. [17] Taiju Tsuboi, KyoungHyuk Jang, Silviu Polosan, Kiyoshi Shimamura, Marco Bettinelli, Hyo Jin Seo, J. Lumin. 131 (2011) 695e700. [18] M. Mazzera, A. Baraldi, R. Capelletti, M. Tonelli, Opt. Mater. 31 (2009) 1366e1369. [19] E. Osiac, I. Sokolska, J. Alloys Compd. 323e324 (2001) 283e287. [20] A.M. Tabirian, D.P. Stanley, R.R. Selleck, L.H. Morton, S. Guch, Lasers, ElectroOptics Society, Publication Date, in: LEOS 2007. The 20th Annual Meeting of the IEEE, 2007, pp. 21e25. Oct. 2007, On page(s): 507-508, ISSN: 1092-8081, ISBN: 978-1-4244-0925-9, INSPEC Accession Number: 9858491. [21] S.L. Baldochi, I.M. Ranieri, Acta Phys. Pol. A 124 (2013) 286e294. [22] Zhang Shouchao, Ruan Yongfeng, Wei Xiaoping, et al., J. Synth. Cryst. 39 (2010) 625e626. [23] R.D. Shannon, Acta Cryst. A 32 (1976) 751. [24] D.D. Avrov, S.I. Dorozhkin, et al., Mater. Electron. Eng. 42 (2008) 1474. [25] G.H. Dieke, in: H.M. Crosswhite, H. Crosswhite (Eds.), Wiley Interscience Publishers, New York, 1968. [26] W.T. Carnall, H. Crosswhite, H.M. Crosswhite, Report Argonne National Laboratory, Chemistry Division, Argonne, Illinois, 1977.