- Email: [email protected]
Czochralski crystal growth of Zn2Te3O8
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 4217–4220
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Czochralski crystal growth of Zn2Te3O8 Jalal M. Nawash ,1, Kelvin G. Lynn Center for Materials Research, Materials Science Program, Washington State University, P.O. Box 642711, Pullman, WA 99164-2711, USA
a r t i c l e in fo
abstract
Article history: Received 2 January 2008 Received in revised form 24 March 2008 Accepted 12 June 2008 Communicated by A.G. Ostrogorsky Available online 26 June 2008
Crystal growth in the ZnO–TeO2 system was investigated using Czochralski technique in a 2.5 kHz induction heating system. Several runs and experiments helped optimize the Zn2Te3O8 growth process, which was limited by quite a few difficulties. These difficulties include the evaporation of TeO2 material above 700 1C, the formation of more than one phase during the growth, and the lack of a Zn2Te3O8 single crystal to initiate the growth. The main and most persisting problem is that there is no stable phase in the system that forms a line compound at which the crystal growth should be attempted. The resulting material was formed of many single crystals and a mixture of other phases. Single Zn2Te3O8 crystals of sizes ranging between 50 and 200 mm3 resulted when the pulling rate was 1.1 mm/h and the rotation speed was 12 rpm. These single crystals were extracted and their optical and electrical properties were studied for the first time. Using other pulling rates and rotation speeds returned smaller crystals with sizes ranging between 15 and 35 mm3. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.10. Fq 61.50.Ah Keywords: A1. Crystal structure A2. Czochralski method A2. Growth from melt A2. Single-crystal growth
1. Introduction Although the development of crystal growth started early in the 20th century [1], Czochralski (CZ) crystal growth was only well established by the mid-1950s. It had shown a great potential to pull oxide crystals [2–4], as well as semiconductor crystals such as silicon [5] and germanium [6]. Many other types of crystals were also grown by the CZ technique [7–9]. In CZ growths, the material is placed inside a suitable crucible and heated either by a radio frequency (RF) coil [10] or a regular ceramic heater. The mixture of the materials is preferred to be at the congruent melting point of the constituents to avoid complications of forming undesired phases while growing. Some researchers were able to pull single crystals at incongruent points [11]. Other workers reported the growth of single crystals from non-stoichiometric melts [12,13], while others grew multiphase semiconductor crystals at the peritectic phase transformation [14]. The II–VI oxide crystals have high refractive indices and are optically active [15]. They present non-linear optical properties [11,16], second harmonic generation effect, and birefringence. This
makes the crystals to be good materials in manufacturing fiber optics, polarizers, wave plates, depolarizers, and optical filters. The zinc oxide (ZnO) crystals have a wide band gap width of 3.3 eV at room temperature [17]. This makes the crystal a good candidate for applications in optoelectronic devices such as short-wavelength lasers and light-emitting diodes [18]. The average refractive index and the average static dielectric constant of ZnO crystals are 2.0 and 10.0, respectively [19]. Paratellurite (TeO2) crystals have useful applications in acousto-optic devices [20,21], many of which are used in data display devices [22]. TeO2 crystal has a band gap of 3.5 eV [23] and a refractive index of about 2.2 [19]. There are several phases that could form as ZnO–TeO2 melt is cooled down [24]. A quick investigation of the phase diagram and phase formation was done by several authors [25–28]. Most of their studies focused on the glass form of the material as it is quenched from melt. In this research paper, the procedure to grow Zn2Te3O8 single crystals was established and optimized for the first time. The sizes of these crystals permitted the performance of important electrical and optical measurements.
2. Experimental procedure Corresponding author. Tel.: +1 509 335 8145; fax: +1 509 335 4145.
E-mail address: [email protected] (J.M. Nawash). Currently at: Physics Department, Gonzaga University, 502 E Boone Avenue, AD51, Spokane, WA 99258, USA. Tel: +1 509 313 5979; fax: +1 509 313 5718. 1
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.06.058
ZnO and TeO2 powders (Alpha Aesar 99.999%) were mixed using a jar mill. The average mixing time was 15 h. Grinding zirconia beads were used to enhance mixing and milling. The mixture was melted in a platinum crucible heated by a RF coil in
ARTICLE IN PRESS 4218
J.M. Nawash, K.G. Lynn / Journal of Crystal Growth 310 (2008) 4217–4220
Table 1 A summary of 35.5:64.5 runs used to grow Zn2Te3O8 single crystals Run #
Crucible
Seed used
Pulling speed (mm/h)70.1 mm/h
Rotation speed (rpm)71 rpm
General color
Single crystal size (mm3)
Microprobe analysis of single crystals
1
60 ml Pt dish
0.9
10
Yellow
5–15
Zn2Te3O8 only
2
60 ml Pt dish
Zn2Te3O8 with minor TeO2 inclusions Zn2Te3O8 from ZTO8)1
0.9
15
10–35
3 4
15 ml Pt dish 125 ml 95%Pt/5%Au straight wall
Platinum wire Multicrystals from a previous 35.5 run
0.8 1.1
20 12
Light yellow. Single crystals are greenish Yellow and green Clear white and green
Zn2Te3O8 with minor TeO2 inclusions N/A Zn2Te3O8 only done by X-ray
5–20 50–200
Fig. 1. A bottom view of the 35.5:64.5 as grown crystals. Fig. 2. Zn2Te3O8 single crystal.
air. The crucible was placed inside a cylindrical build up constructed from insulating materials. A set of thermocouples were used to measure the temperature of the crucible. After the material melted, the temperature was raised between 20 and 40 1C and kept at that temperature to stabilize for 1 h. A seed attached to the seed holder was dipped onto the melt surface. The seed was made to rotate at a speed between 10 and 15 rpm while being pulled at a speed ranging from 0.8 to 3.0 mm/h. The most frequent mole percentages that were tested for growths are (in ZnO:TeO2 order): 33.3:66.7, 36.5:63.5, and 35.5:64.5. The choices were limited by several factors, such as the melting temperature and phase formation. Surplus phases always formed during each growth. Microprobe analysis of the grown single crystals of 33.3:66.7 showed an excess of TeO2. The second mole percentage (36.5:63.5) formed 5–15 mm3 crystals. Other growths showed that the 35.5:64.5 mol% formed single crystals of sizes that are bigger than 35 mm3 and less surplus phases. This mole percentage falls at the peritectic phase transformation and has a melting point of 650 1C. To optimize the growth of 35.5:64.5, several runs were conducted. A brief description of the procedures and results of these runs are discussed in the following table. For run number 4 in Table 1, a conglomeration of 40–50 single crystals of numerous sizes were grown; each one is separated from the other by the surplus phases. A further increase in the pulling rate resulted in the detachment of the crystal from the melt. Fig. 1 shows the bottom view of the grown material. It is also noticed that bigger single crystals were grown in a bigger crucible. Single crystals were isolated, cut, polished, and then sputtered
with gold contacts for analysis. Fig. 2 shows a polished sample crystal. The crystal structure, the lattice constants, and the direction of the samples were obtained using a single-crystal diffractometer equipped with a software package that includes a PDF and CIF libraries.
3. Results and discussion Crystals were tested for glow discharge mass spectroscopy; it shows that these single crystals included some impurities, such as (in ppm): 350 Al, 30 Zr, 72 Pt, and 8 Au. The unexpected large presence of aluminum in the crystal, perhaps, came from the aluminum foil where the powder was separated from the grinding media after mixing the two powders. Zirconium impurities came from the grinding zirconia beads, while platinum and gold came from the crucible. 3.1. Electrical and optical properties 3.1.1. Dielectric constant measurement The dielectric constant and the loss factor are measured using the Tenny Environmental Test Chamber, where the temperature was increased in 5 1C steps starting from 75 to 185 1C. The chamber is connected to the QuadTech 7600 Precision LCR meter, and interfaced with a computer. The (0 0 1) dielectric constant as a function of temperature is shown in Fig. 3. The dielectric constant
ARTICLE IN PRESS J.M. Nawash, K.G. Lynn / Journal of Crystal Growth 310 (2008) 4217–4220
4219
state means that the crystal stores charge when exposed to a voltage difference and keeps this charge when the voltage goes to zero. Such a performance is more likely to be a capacitor-like behavior.
Fig. 3. Dielectric constant in the (0 0 1) direction as a function of temperature for Zn2Te3O8 single crystal.
3.1.3. Transmission and absorption measurement These measurements were performed using a DT 1000 CE UV/VIS light source and OOIBase 32 Spectrometer Operating Software. The sample was placed perpendicular between the source and the detector. Calibrating the setup included storing a dark and a reference spectrum before conducting the measurement. A single crystal was tested for transmission and absorption. Fig. 5 shows that the crystal absorbs the UV radiation up to a cutoff wavelength of 295 nm. An approximate maximum value for the optical band gap was calculated to be 4.2 eV. This value was calculated using E ¼ hc/lc, where h and c are Planck’s constant and the speed of light, respectively. lc is the 295 nm cut-off wavelength. The other band edge was not observed up to 5000 nm. The crystal was solarized for 1 h by a Xenon vapor lamp to create internal defects. The solarized crystal spectrum shows a minor difference from the unsolarized spectrum. This might be attributed to some type of defects in the crystal caused by the change in oxidation number of Te from +4 to +2. This change is due to a missing oxygen atom that was knocked away by UV radiation during solarization. 3.1.4. Birefringence The refractive index for this crystal was verified to be higher than 1.8 by matching oil of the appropriate refractive index method. Quantitative value of the birefringence effect was not obtainable. However, Fig. 6 demonstrates birefringence for a 0.8 mm thick crystal as it is rotated 901, where the vertical shift of light (left) turned to horizontal (right). 3.2. Crystal constants Using XRD results and JADE 6.5, the crystal structure of Zn2Te3O8 was confirmed to be monoclinic, with a, b, and c being (in A˚) 12.676, 5.1980, and 11.7810, respectively. The angle b was calculated to be 99.601. This structure belongs to the space group known as C2/c. Using CrystalDiffract 1.3 and other software packages, the estimated density is 5567.5 kg/m3 and the crystal’s volume is 765.37 A˚3. A 3D diagram of the crystal structure was
Fig. 4. Current–voltage relation for Zn2Te3O8 single crystal.
for 1 kHz at 18 1C was measured to be 23.570.6, with 0.0064 loss factor. The Curie temperature was not noticed in the temperature range between 75 and 180 1C as no sudden change in dielectric constant value took place. Using another chamber, the Curie temperature was not detected up to T ¼ 400 1C. 3.1.2. I–V tests This is a procedure where samples are exposed to a voltage difference and a leakage current is measured at each voltage step. The sample with known dimensions is placed between two poles in a dark box. The result is a relation between the voltage and the current by which the resistivity of the crystal was determined. Fig. 4 shows the resultant relation between the current and the voltage. Resistivity r is calculated to be 1.16 1015 O cm, and the leakage current at V ¼ +80 V is found to be 1.4 1013 A. At a voltage difference of 40 V, the current goes to zero, and at zero voltage, the current measures a non-zero value. This transient
Fig. 5. Absorption spectrum for both Zn2Te3O8 single crystal before and after solarization.
ARTICLE IN PRESS 4220
J.M. Nawash, K.G. Lynn / Journal of Crystal Growth 310 (2008) 4217–4220
Fig. 6. Birefringence of Zn2Te3O8 crystal. Photograph on the left-hand side shows birefringence in the vertical direction, but when the crystal was rotated 901, birefringence took place in the horizontal direction.
Due to this optimization, single crystals of sizes between 50 and 200 mm3 were obtained. The size of the crucible might have an effect on the size of the Zn2Te3O8-grown crystals. The crystals have a refractive index that is higher than 1.8. They were found to be birefringent non-isotropic crystals. The crystals exhibit a very high resistivity that is of the order of 1015 O cm. They display a capacitance-like behavior but no piezoelectric effects were detected. The Curie temperature value does not fall in the temperature range between 75 and 400 1C. The dielectric constant at room temperature, 1 kHz, and in the (0 0 1) direction was found to be 23.5 with 0.0064 dielectric loss factor. The band gap was calculated to be 4.20 eV.
Acknowledgments We wish to thank Professor Roger D. Willett for his help using the single-crystal diffractometer at the Chemistry department at Washington State University. The authors are also grateful for Scott B. Cornelius for his technical assistance in X-ray and microprobe measurements. This research was sponsored by Space & Missile Defense Command (SMDC). Contract no. DASG60-02-C-0084.
References
Fig. 7. Zn2Te3O8 crystal structure built using CrystalMaker 1.3 for Windows.
constructed using the CrystalMaker 1.3 for Windows. The diagram is shown in Fig. 7. 3.3. Piezoelectric measurements A 0.37 mm thick Zn2Te3O8 single-crystal sample was tested to see if it possesses any piezoelectric properties. The sample was poled up to 300 V but there was no poling noticed. Thickness coupling coefficient factor calculations using data obtained from the Precision Impedance Analyzer 4294A show no sign of Piezoelectricity. This result is in accordance with the space group that the crystal belongs to. Poling did not seem to have any effect on the dielectric constant value.
4. Conclusions CZ crystal growth was used in an attempt to pull Zn2Te3O8 single crystals. The most persisting problem during growth is the formation of surplus phases. The process was optimized to obtain bigger crystals by choosing the appropriate ZnO:TeO2 mole percentage and then by optimizing the pulling and rotating rates.
[1] H.J. Scheel, The Development of Crystal Growth Technology: Crystal Growth Technology, Wiley, New York, 2003, 3. [2] C.D. Brandle, J. Crystal Growth 264 (2004) 593. [3] S. Kumaragurubaran, D. Krishnamurthy, C. Subramanian, P. Ramasamy, J. Cryst. Growth 211 (2000) 276. [4] L. Fornasiero, E. Mix, V. Peters, G. Petermann, G. Huber, Cryst. Res. Technol. 34 (2) (1999) 255. [5] N.V. Abrosimov, H. Riemann, W. Schro¨der, H.-J. Pohl, A.K. Kaliteevski, O.N. Godisov, V.A. Korolyov, A.J. Zhilnikov, Cryst. Res. Technol. 38 (7–8) (2003) 654. [6] P. Dold, Cryst. Res. Technol. 38 (7–8) (2003) 659. [7] K. Sankaranarayanan, P. Ramasamy, J. Cryst. Growth 193 (1998) 252. [8] N.V. Abrosimov, A. Lu¨dge, H. Riemann, N.V. Kurlov, D. Borissova, V. Klemm, H. Halloin, P.V. Ballmoos, P. Bastie, B. Hamelin, R.K. Smither, J. Cryst. Growth 278 (2005) e495. [9] J.L. Chen, G.H. Wu, D.Q. Zhao, S.X. Gao, W.S. Zhan, Y.X. Li, J.P. Qu, G.Z. Xu, J. Cryst. Growth 222 (2001) 779. [10] L.C.F. Blackman, P.H. Dundas, A.W. Moore, A.R. Ubbelohde, Br. J. Appl. Sci. 12 (1960) 377. [11] D.S. Robertson, N. Shaw, I.M. Young, J. Mater. Sci. 13 (1978) 1986. [12] J.R. Carruthers, G.E. Peterson, M. Grasso, P.M. Bridenbaugh, J. Appl. Phys. 42 (5) (1970) 1846. [13] T. Takahashi, O. Yamada, K. Ametani, J. Cryst. Growth 20 (1973) 89. [14] M. Akasak, T. Iida, G. Sakuragi, S. Furuyama, M. Noda, M. Ota, H. Suzuki, H. Sato, Y. Takanshi, S. Sakuragi, J. Alloys Compd 386 (2005) 228. [15] V. Kra¨mer, G. Brandt, Acta Crystallogr. C 41 (1985) 1152. [16] J.G. Bergman, G.D. Boyd, A. Ashken, J. Appl. Phys. 40 (7) (1969) 2860. [17] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (10) (1998) 5447. [18] X. Liu, X. Wu, H. Cao, R.P.H. Chang, J. Appl. Phys. 95 (6) (2004) 3141. [19] D.R. Lide, Handbook of Chemistry and Physics, 86th ed, Taylor & Francis Group, Boca Raton, FL, 2005. [20] S. Kumaragurubaran, D. Krishnamurthy, C. Subramanian, P. Ramasamy, J. Cryst. Growth 197 (1999) 210. [21] T. Lukasiewicz, A. Majchrowski, J. Cryst. Growth 116 (1992) 346. [22] J.G. Grabmaier, R.D. Plattner, M. Schieber, J. Cryst. Growth 20 (1973) 82. [23] R. Nayak, A. Nayak, V. Gupta, K. Sreenivas, IEEE Ultrason. Symp. (2003) 1129. [24] J. Nawash, B. Twamley, K. Lynn, Acta Crystallogr. C 63 (2007) i66. [25] H. Bu¨rger, K. Kneipp, H. Hobert, W. Vogel, J. Non-Cryst. Solids 151 (1992) 134. [26] A. Nukui, T. Taniguchi, M. Miyata, J. Non-Cryst. Solids 293–295 (2001) 255. [27] M.R. Marinov, V.S. Kozhouharov, C. R. Acad. Bulg. Sci. 25 (3) (1972) 329. ¨ vec- og˘lu, M.R. O ¨ zlap, G. O ¨ zen, F. Altin, V. Kalem, Key Eng. Mater. [28] M.L. O 264–268 (2004) 1891.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Czochralski crystal growth of Zn2Te3O8 Jalal M. Nawash ,1, Kelvin G. Lynn Center for Materials Research, Materials Science Program, Washington State University, P.O. Box 642711, Pullman, WA 99164-2711, USA
a r t i c l e in fo
abstract
Article history: Received 2 January 2008 Received in revised form 24 March 2008 Accepted 12 June 2008 Communicated by A.G. Ostrogorsky Available online 26 June 2008
Crystal growth in the ZnO–TeO2 system was investigated using Czochralski technique in a 2.5 kHz induction heating system. Several runs and experiments helped optimize the Zn2Te3O8 growth process, which was limited by quite a few difficulties. These difficulties include the evaporation of TeO2 material above 700 1C, the formation of more than one phase during the growth, and the lack of a Zn2Te3O8 single crystal to initiate the growth. The main and most persisting problem is that there is no stable phase in the system that forms a line compound at which the crystal growth should be attempted. The resulting material was formed of many single crystals and a mixture of other phases. Single Zn2Te3O8 crystals of sizes ranging between 50 and 200 mm3 resulted when the pulling rate was 1.1 mm/h and the rotation speed was 12 rpm. These single crystals were extracted and their optical and electrical properties were studied for the first time. Using other pulling rates and rotation speeds returned smaller crystals with sizes ranging between 15 and 35 mm3. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.10. Fq 61.50.Ah Keywords: A1. Crystal structure A2. Czochralski method A2. Growth from melt A2. Single-crystal growth
1. Introduction Although the development of crystal growth started early in the 20th century [1], Czochralski (CZ) crystal growth was only well established by the mid-1950s. It had shown a great potential to pull oxide crystals [2–4], as well as semiconductor crystals such as silicon [5] and germanium [6]. Many other types of crystals were also grown by the CZ technique [7–9]. In CZ growths, the material is placed inside a suitable crucible and heated either by a radio frequency (RF) coil [10] or a regular ceramic heater. The mixture of the materials is preferred to be at the congruent melting point of the constituents to avoid complications of forming undesired phases while growing. Some researchers were able to pull single crystals at incongruent points [11]. Other workers reported the growth of single crystals from non-stoichiometric melts [12,13], while others grew multiphase semiconductor crystals at the peritectic phase transformation [14]. The II–VI oxide crystals have high refractive indices and are optically active [15]. They present non-linear optical properties [11,16], second harmonic generation effect, and birefringence. This
makes the crystals to be good materials in manufacturing fiber optics, polarizers, wave plates, depolarizers, and optical filters. The zinc oxide (ZnO) crystals have a wide band gap width of 3.3 eV at room temperature [17]. This makes the crystal a good candidate for applications in optoelectronic devices such as short-wavelength lasers and light-emitting diodes [18]. The average refractive index and the average static dielectric constant of ZnO crystals are 2.0 and 10.0, respectively [19]. Paratellurite (TeO2) crystals have useful applications in acousto-optic devices [20,21], many of which are used in data display devices [22]. TeO2 crystal has a band gap of 3.5 eV [23] and a refractive index of about 2.2 [19]. There are several phases that could form as ZnO–TeO2 melt is cooled down [24]. A quick investigation of the phase diagram and phase formation was done by several authors [25–28]. Most of their studies focused on the glass form of the material as it is quenched from melt. In this research paper, the procedure to grow Zn2Te3O8 single crystals was established and optimized for the first time. The sizes of these crystals permitted the performance of important electrical and optical measurements.
2. Experimental procedure Corresponding author. Tel.: +1 509 335 8145; fax: +1 509 335 4145.
E-mail address: [email protected] (J.M. Nawash). Currently at: Physics Department, Gonzaga University, 502 E Boone Avenue, AD51, Spokane, WA 99258, USA. Tel: +1 509 313 5979; fax: +1 509 313 5718. 1
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.06.058
ZnO and TeO2 powders (Alpha Aesar 99.999%) were mixed using a jar mill. The average mixing time was 15 h. Grinding zirconia beads were used to enhance mixing and milling. The mixture was melted in a platinum crucible heated by a RF coil in
ARTICLE IN PRESS 4218
J.M. Nawash, K.G. Lynn / Journal of Crystal Growth 310 (2008) 4217–4220
Table 1 A summary of 35.5:64.5 runs used to grow Zn2Te3O8 single crystals Run #
Crucible
Seed used
Pulling speed (mm/h)70.1 mm/h
Rotation speed (rpm)71 rpm
General color
Single crystal size (mm3)
Microprobe analysis of single crystals
1
60 ml Pt dish
0.9
10
Yellow
5–15
Zn2Te3O8 only
2
60 ml Pt dish
Zn2Te3O8 with minor TeO2 inclusions Zn2Te3O8 from ZTO8)1
0.9
15
10–35
3 4
15 ml Pt dish 125 ml 95%Pt/5%Au straight wall
Platinum wire Multicrystals from a previous 35.5 run
0.8 1.1
20 12
Light yellow. Single crystals are greenish Yellow and green Clear white and green
Zn2Te3O8 with minor TeO2 inclusions N/A Zn2Te3O8 only done by X-ray
5–20 50–200
Fig. 1. A bottom view of the 35.5:64.5 as grown crystals. Fig. 2. Zn2Te3O8 single crystal.
air. The crucible was placed inside a cylindrical build up constructed from insulating materials. A set of thermocouples were used to measure the temperature of the crucible. After the material melted, the temperature was raised between 20 and 40 1C and kept at that temperature to stabilize for 1 h. A seed attached to the seed holder was dipped onto the melt surface. The seed was made to rotate at a speed between 10 and 15 rpm while being pulled at a speed ranging from 0.8 to 3.0 mm/h. The most frequent mole percentages that were tested for growths are (in ZnO:TeO2 order): 33.3:66.7, 36.5:63.5, and 35.5:64.5. The choices were limited by several factors, such as the melting temperature and phase formation. Surplus phases always formed during each growth. Microprobe analysis of the grown single crystals of 33.3:66.7 showed an excess of TeO2. The second mole percentage (36.5:63.5) formed 5–15 mm3 crystals. Other growths showed that the 35.5:64.5 mol% formed single crystals of sizes that are bigger than 35 mm3 and less surplus phases. This mole percentage falls at the peritectic phase transformation and has a melting point of 650 1C. To optimize the growth of 35.5:64.5, several runs were conducted. A brief description of the procedures and results of these runs are discussed in the following table. For run number 4 in Table 1, a conglomeration of 40–50 single crystals of numerous sizes were grown; each one is separated from the other by the surplus phases. A further increase in the pulling rate resulted in the detachment of the crystal from the melt. Fig. 1 shows the bottom view of the grown material. It is also noticed that bigger single crystals were grown in a bigger crucible. Single crystals were isolated, cut, polished, and then sputtered
with gold contacts for analysis. Fig. 2 shows a polished sample crystal. The crystal structure, the lattice constants, and the direction of the samples were obtained using a single-crystal diffractometer equipped with a software package that includes a PDF and CIF libraries.
3. Results and discussion Crystals were tested for glow discharge mass spectroscopy; it shows that these single crystals included some impurities, such as (in ppm): 350 Al, 30 Zr, 72 Pt, and 8 Au. The unexpected large presence of aluminum in the crystal, perhaps, came from the aluminum foil where the powder was separated from the grinding media after mixing the two powders. Zirconium impurities came from the grinding zirconia beads, while platinum and gold came from the crucible. 3.1. Electrical and optical properties 3.1.1. Dielectric constant measurement The dielectric constant and the loss factor are measured using the Tenny Environmental Test Chamber, where the temperature was increased in 5 1C steps starting from 75 to 185 1C. The chamber is connected to the QuadTech 7600 Precision LCR meter, and interfaced with a computer. The (0 0 1) dielectric constant as a function of temperature is shown in Fig. 3. The dielectric constant
ARTICLE IN PRESS J.M. Nawash, K.G. Lynn / Journal of Crystal Growth 310 (2008) 4217–4220
4219
state means that the crystal stores charge when exposed to a voltage difference and keeps this charge when the voltage goes to zero. Such a performance is more likely to be a capacitor-like behavior.
Fig. 3. Dielectric constant in the (0 0 1) direction as a function of temperature for Zn2Te3O8 single crystal.
3.1.3. Transmission and absorption measurement These measurements were performed using a DT 1000 CE UV/VIS light source and OOIBase 32 Spectrometer Operating Software. The sample was placed perpendicular between the source and the detector. Calibrating the setup included storing a dark and a reference spectrum before conducting the measurement. A single crystal was tested for transmission and absorption. Fig. 5 shows that the crystal absorbs the UV radiation up to a cutoff wavelength of 295 nm. An approximate maximum value for the optical band gap was calculated to be 4.2 eV. This value was calculated using E ¼ hc/lc, where h and c are Planck’s constant and the speed of light, respectively. lc is the 295 nm cut-off wavelength. The other band edge was not observed up to 5000 nm. The crystal was solarized for 1 h by a Xenon vapor lamp to create internal defects. The solarized crystal spectrum shows a minor difference from the unsolarized spectrum. This might be attributed to some type of defects in the crystal caused by the change in oxidation number of Te from +4 to +2. This change is due to a missing oxygen atom that was knocked away by UV radiation during solarization. 3.1.4. Birefringence The refractive index for this crystal was verified to be higher than 1.8 by matching oil of the appropriate refractive index method. Quantitative value of the birefringence effect was not obtainable. However, Fig. 6 demonstrates birefringence for a 0.8 mm thick crystal as it is rotated 901, where the vertical shift of light (left) turned to horizontal (right). 3.2. Crystal constants Using XRD results and JADE 6.5, the crystal structure of Zn2Te3O8 was confirmed to be monoclinic, with a, b, and c being (in A˚) 12.676, 5.1980, and 11.7810, respectively. The angle b was calculated to be 99.601. This structure belongs to the space group known as C2/c. Using CrystalDiffract 1.3 and other software packages, the estimated density is 5567.5 kg/m3 and the crystal’s volume is 765.37 A˚3. A 3D diagram of the crystal structure was
Fig. 4. Current–voltage relation for Zn2Te3O8 single crystal.
for 1 kHz at 18 1C was measured to be 23.570.6, with 0.0064 loss factor. The Curie temperature was not noticed in the temperature range between 75 and 180 1C as no sudden change in dielectric constant value took place. Using another chamber, the Curie temperature was not detected up to T ¼ 400 1C. 3.1.2. I–V tests This is a procedure where samples are exposed to a voltage difference and a leakage current is measured at each voltage step. The sample with known dimensions is placed between two poles in a dark box. The result is a relation between the voltage and the current by which the resistivity of the crystal was determined. Fig. 4 shows the resultant relation between the current and the voltage. Resistivity r is calculated to be 1.16 1015 O cm, and the leakage current at V ¼ +80 V is found to be 1.4 1013 A. At a voltage difference of 40 V, the current goes to zero, and at zero voltage, the current measures a non-zero value. This transient
Fig. 5. Absorption spectrum for both Zn2Te3O8 single crystal before and after solarization.
ARTICLE IN PRESS 4220
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Fig. 6. Birefringence of Zn2Te3O8 crystal. Photograph on the left-hand side shows birefringence in the vertical direction, but when the crystal was rotated 901, birefringence took place in the horizontal direction.
Due to this optimization, single crystals of sizes between 50 and 200 mm3 were obtained. The size of the crucible might have an effect on the size of the Zn2Te3O8-grown crystals. The crystals have a refractive index that is higher than 1.8. They were found to be birefringent non-isotropic crystals. The crystals exhibit a very high resistivity that is of the order of 1015 O cm. They display a capacitance-like behavior but no piezoelectric effects were detected. The Curie temperature value does not fall in the temperature range between 75 and 400 1C. The dielectric constant at room temperature, 1 kHz, and in the (0 0 1) direction was found to be 23.5 with 0.0064 dielectric loss factor. The band gap was calculated to be 4.20 eV.
Acknowledgments We wish to thank Professor Roger D. Willett for his help using the single-crystal diffractometer at the Chemistry department at Washington State University. The authors are also grateful for Scott B. Cornelius for his technical assistance in X-ray and microprobe measurements. This research was sponsored by Space & Missile Defense Command (SMDC). Contract no. DASG60-02-C-0084.
References
Fig. 7. Zn2Te3O8 crystal structure built using CrystalMaker 1.3 for Windows.
constructed using the CrystalMaker 1.3 for Windows. The diagram is shown in Fig. 7. 3.3. Piezoelectric measurements A 0.37 mm thick Zn2Te3O8 single-crystal sample was tested to see if it possesses any piezoelectric properties. The sample was poled up to 300 V but there was no poling noticed. Thickness coupling coefficient factor calculations using data obtained from the Precision Impedance Analyzer 4294A show no sign of Piezoelectricity. This result is in accordance with the space group that the crystal belongs to. Poling did not seem to have any effect on the dielectric constant value.
4. Conclusions CZ crystal growth was used in an attempt to pull Zn2Te3O8 single crystals. The most persisting problem during growth is the formation of surplus phases. The process was optimized to obtain bigger crystals by choosing the appropriate ZnO:TeO2 mole percentage and then by optimizing the pulling and rotating rates.
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