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Growth of α-HgI2 single crystals from physical vapor transport in an oil-bath furnace
Materials Research Bulletin 38 (2003) 1987–1992
Growth of a-HgI2 single crystals from physical vapor transport in an oil-bath furnace Han-Tang Zhoua, Cheng-Hsu Leea, Jia-Ming Chunga, Chen-Tsung Shina, Kuan-Cheng Chiua,*, San-Min Lanb a
Department of Physics, Chung-Yuan Christian University, Chung-Li 32023, Taiwan, ROC b Institute of Nuclear Energy, Lung-Tan 325, Taiwan, ROC
Received 19 May 2003; received in revised form 20 August 2003; accepted 5 September 2003
Abstract The growth of a-HgI2 crystals from physical vapor transport in a closed ampoule immerged in a simple and economical oil-bath furnace with an external heating coil imposed is discussed. The temperatures of the source side, Tsou, of the maximum value, Tmax, and of the crystal growth side, Tcry, together with dT/dx are changed for different growth conditions. The physical properties of the as-grown a-HgI2 crystals are characterized by scanning electron microscopy, X-ray diffraction, and I–V measurements with respect to different growth conditions. It is found that the a-HgI2 crystals grown from Tsou ¼ 125 8C, Tmax ¼ 135 8C, Tcry ¼ 114 8C, dT/dx ¼ 2:6 8C/mm, and with 5 Torr inert gas inside the growth ampoule exhibit the best qualities, such as, observable facets, transparent red color, and a low density of defects and grain boundaries. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductors; B. Crystal growth; D. Defects
1. Introduction Red mercuric iodide (a-HgI2) is considered to be a potential photoconductor for X-ray and low energy g-ray spectrometry at room temperature with low noise, compact size, and efficient detection because of the following useful properties, such as large band gap of 2.15 eV at 300 K, low electron– hole pair creation energy of 4.2 eV, and high atomic number of 80 for Hg and 53 for I [1–6]. However, the applications of a-HgI2 are limited by the difficulty in obtaining high-quality single crystals and the long-term reliability problems in devices made from these crystals.
*
Corresponding author. Tel.: þ886-32653213; fax: þ886-32653299. E-mail address: [email protected] (K.-C. Chiu). 0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2003.09.016
1988
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
Fig. 1. Schematic diagram of the oil-bath furnace used for a-HgI2 crystals growth from PVT: (1) lard oil; (2) heating plate; (3) heating coil; (4) temperature controllers and thermal couples; (5) growth ampoule; (6) source powder; (7) a-HgI2 crystal; (8) rotating and pulling motors; (9) temperature profile.
Mercuric iodide undergoes a solid–solid phase transition from an orthorhombic yellow phase (b-HgI2) to a tetragonal red phase (a-HgI2) at 127 8C [7,8]. Thus, crystal growth from melt (resulted in b-HgI2 first) is prohibited, although the melting temperature is only 259 8C. In general, the solution growth and/or vapor growth methods operating below 127 8C are adopted to grow a-HgI2 single crystals. It is known that crystal growth from solution has contamination problems from solvent molecules, and crystal growth from physical vapor (sublimation and re-condensation) usually results in a higher purity with a well-defined structure [9]. Furthermore, a-HgI2 has high enough vapor pressure (about 0.1 Torr) at 120 8C [10,11], it can be grown efficiently by physical vapor transport (PVT) in closed ampoules. Here, the growth of a-HgI2 crystals from PVT in a closed ampoule immersed in a simple and economical oil-bath furnace is discussed. This oil-bath furnace gives a stable temperature profile to operate at temperature around 120 8C. An external heating coil is imposed in this study to create a nonuniform temperature profile as shown in Fig. 1, where the temperature of source powder side, Tsou, the maximum temperature in vapor phase transport region, Tmax, the temperature of crystal growth side, Tcry, and the temperature gradient at crystal growth side dT/dx can be varied for a systematical study. This non-uniform temperature profile, which is a modification from our previous work [11], has two purposes. First, it provides an even larger value of dT/dx in front of the growing crystal interface (as comparing to the one without external heating coil) that effectively reduces the sidewall nucleation due to the constitutional supersaturation [12,13]. This is very useful for nucleation control. Second, it gives a larger value of Tcry that can enhance the surface kinetics of the adhered molecules. Finally, the physical properties of the as-grown a-HgI2 crystals are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and I–V measurements with respect to different growth conditions.
2. Experimental The commercially available a-HgI2 powder of 99.9% purity contains a certain amount of various impurities. To obtain a higher purity, a 5X purification process is applied for a-HgI2 powder in a multichamber, where X stands for a continuous sublimation in dynamic evacuation [14]. Details of the cleaning processes for the multi-chamber and the purification steps for the source powder have been published elsewhere [11].
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
1989
Table 1 Growth conditions (GCs) used in this work
GC-1 GC-2 GC-3 GC-4
Tsou (8C)
Tmax (8C)
Tcry (8C)
dT/dx (8C/mm)
Inert gas inside (Torr)
Growth rate (mm per day)
120 120 125 130
125 125 135 140
106 106 114 120
1.5 1.5 2.6 2.5
No 5 5 5
1.2 0.2 0.7 0.1 0.4 0.1 –
Fig. 1 schematically depicts the oil-bath furnace used in this work. The system consists of a bath of lard oil (with a smoking temperature of about 380 8C) on top of a controllable heating plate which controls Tsou via a thermal couple; an external heating wire embedded into a glass coil which controls Tmax via another thermal couple; a growth ampoule made of pyrex glass; and a rotation motor and a pulling motor to rotate and pull the growth ampoule, respectively. Usually, at the initial nucleation stage it needs a larger supersaturation. The supersaturation is related to Tcry, Tsou (DT ¼ Tsou Tcry ), dT/dx, and the crystal growth rate. Once initial nucleation is obtained and acts as the seed for the subsequent growth, DT is reduced by increasing Tcry (i.e. moving the growth ampoule downward; see Fig. 1), and then the crystal growth process starts. Table 1 lists the growth conditions (GCs) used in this work.
3. Results and discussion Fig. 2 shows the as-grown a-HgI2 crystals corresponding to different growth conditions. To reveal the quality of the crystals, the cleaved samples from the crystals along the easy-cut plane are prepared. After etching in KI solution for 5 min and drying by clean air, the SEM pictures of the cleavage surface (see Fig. 3) show the interface morphology. To further identify the Miller index of the cleavage surface as well as the facets on the as-grown crystals, XRD is applied and the results are depicted as Fig. 4. The Miller index of the easy-cut plane is confirmed to be (0 0 2). For GC-1, there is no inert gas inside the growth ampoule, thus, the crystal growth rate about 1:2 0:2 mm per day is relatively high. The growth direction as shown in Fig. 2 is along c-axis of the as-grown crystal and no clear facet is observed. The color of the crystal is dark red and not very transparent. The SEM picture on the cleavage surface reveals a high density of defects and grain boundaries. Furthermore, while in preparing the cleaved samples, one can feel that the crystals from GC-1 are relatively soft, i.e. low mechanical strength, as compared to those from GC-2 and GC-3.
Fig. 2. The as-grown a-HgI2 crystals correspond to different growth conditions.
1990
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
Fig. 3. SEM pictures correspond to the cleaved interface for a-HgI2 from (a) GC-1, (b) GC-2, and (c) GC-3.
Above results indicate that the quality of the as-grown crystals from GC-1 is rather poor. We believe that the poor quality is resulted from the large mass transport rate inside the growth ampoule. To control the growth mechanism in a mass transport limited regime [9], Ar at a pressure of about 5 Torr is put into the closed ampoule to reduce the mass transport rate for GC-2 (with growth rate about 0:7 0:1 mm per day) and GC-3 (0:4 0:1 mm per day). The as-grown a-HgI2 crystals for GC-2 and GC-3 then show observable facets with growth direction about 408 deviating from (0 0 2) direction. The color of these essentially transparent crystals is red. Furthermore, the SEM pictures on the cleavage surface indicate that the density of defects and grain boundaries is markedly reduced. For GC-3, the much higher Tcry, results in shifting the whole temperature profile up about 5–10 8C, which should increase the surface kinetics of the depositing HgI2 molecules. Thus, the relatively better surface morphology of the cleaved interface for GC-3 as compared to that for GC-2 is obtained as shown in Fig. 3.
Fig. 4. XRD patterns for (a) the cleaved interface, (b) the (0 0 2) facet on as-grown crystals from GC-2 or GC-3, and (c) the (2 2 0) facet on as-grown crystals from GC-2 or GC-3.
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
1991
Table 2 The dark resistivity for electric field parallel or perpendicular to the c-axis corresponding to the as-grown a-HgI2 crystals from different GCs GC-1
GC-2 E ? c-axis
E || c-axis r (O cm)
(8.4 0.3) 10
11
(2.7 0.1) 10
GC-3 E ? c-axis
E || c-axis 10
11
(5.1 0.2) 10
E ? c-axis
E || c-axis 11
(3.5 0.2) 10
12
(4.0 0.2) 10
(7.8 0.2) 1011
For GC-3, though some temperatures exceed the solid–solid phase transition temperature of 127 8C inside the vapor transport region, the solid phase formed by nucleation or subsequent growth are found only in the a form. No b-HgI2 phase is observed. However, for GC-4 with Tsou > 127 8C, metastable bHgI2 with a yellow color appears in the crystal growth side though Tcry is only 120 8C. In the temperature range studied here, it was reported [15] that solid-HgI2 sublimates into vapor-HgI2 congruently without decomposition. Hence, our experiment suggests that the vaporized HgI2 molecules (which maybe in the form of small clusters, {HgI2}n, within the transport region) tend to preserve their favorite structure from the source region and this memory effect dominates the formation of metastable b-HgI2 phase in the crystal growth side once Tsou > 127 8C. As mentioned above, Fig. 4a shows the typical XRD patterns of the cleavage interface. Moreover, a shift in 2y is observed for the corresponding peaks from different samples. Therefore, the values of the lattice constant c along c-axis for samples from GC-1, GC-2, and GC-3 are evaluated (especially from ˚ , 12:43 0:01 A ˚ , and 12:45 0:01 A ˚ , respectively. The standard (0 0 8) peak) to be 12:40 0:01 A ˚ . For poor quality samples from GC-1, the shrinkage on the c value may due reference value is 12.45 A to the embedded defects that distort the tetragonal structure of a-HgI2 phase. For the I–V measurements, two different samples are cut from each bulk crystal with electrodes (i.e. applied electric field, E) either parallel or perpendicular to the c-axis. The conductive carbon cement is used as electrode. The sample is first put in a dark box for 30 min. The external voltage is applied from 0 up to 50 V for the positive cycle and then from 0 to 50 V for the negative cycle both with increment of 5 V every 1 min. The measured I–V curves from these samples are quite linear and the calculated results of dark resistivity, r, of the as-grown a-HgI2 crystals corresponding to different GCs are listed in Table 2. Due to the inherent layer structure of a-HgI2, the values of r with E parallel to the c-axis are higher than those with E perpendicular to the c-axis. However, for GC-1, because of the poor quality, i.e. a high density of defects embedded into the layers, this difference is exaggerated. Furthermore, the larger values of r for crystals from GC-3 as comparing to those from GC-2 is attributed to the higher quality.
4. Summary We have presented the growth of a-HgI2 crystals from physical vapor transport in a closed ampoule immersed in a simple and economical oil-bath furnace with an external heating coil. By varying the growth conditions, it is found that the physical properties of the as-grown a-HgI2 crystals from GC-3 exhibit the best qualities, such as, observable facets, very clear red color, low density of defects and grain boundaries, and high dark resistivity. Though for GC-3, inside the vapor transport region some temperatures exceed the solid–solid phase transition temperature of 127 8C, the solid phases from
1992
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
nucleation or subsequent growth are still in a-HgI2 phase, no metastable b-HgI2 phase is observed. However, for GC-4 with Tsou > 127 8C, metastable b-HgI2 phase appears in the crystal growth side (though Tcry ¼ 120 8C). The above results suggest that in the transport region the vaporized HgI2 molecules in the form of small clusters tend to preserve their favorite structure from the source region.
Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially supporting this research.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
W.R. Willig, Nucl. Instrum. Methods 96 (1971) 615. M. Schieber, M. Roth, W.F. Schnepple, J. Cryst. Growth 65 (1983) 353. Y.J. Wang, B.E. Patt, J.S. Iwanczyk, Nucl. Instrum. Methods A 353 (1994) 50. T. Pal, S.L. Sharma, H.N. Acharya, J. Phys. D Appl. Phys. 28 (1995) 1439. J.P. Ponpon, R. Stuck, M. Amann, Solid-State Electronics 44 (2000) 29. M. Zahangir, S.O. Kasap, Appl. Phys. Lett. 80 (2002) 1664. J.B. Newkirk, Acta Metall. 4 (1956) 316. A. Burger, S. Morgan, C. He, E. Silberman, L. van den Berg, C. Ortale, L. Franks, J. Cryst. Growth 99 (1990) 988. F. Rosenberger, Phys. Chem. Hydrodyn. 1 (1980) 3. R. Cadoret, P. Brisson, A. Magnan, Nucl. Instrum. Methods A 283 (1989) 339. K.C. Chiu, P. Luo, H.A. Tu, B.M. Chuang, J.Y. Chen, Chin. J. Mater. Sci. 30 (1998) 191. T.B. Reed, W.J. LaFleur, Appl. Phys. Lett. 5 (1964) 191. F. Rosenberger, M.C. DeLong, D.W. Greenwell, J.M. Olson, G.H. Westphal, J. Cryst. Growth 29 (1975) 49. H.A. Lamonds, Nucl. Instrum. Methods 213 (1983) 5. M. Piechotka, E. Kaldis, J. Less-Common Metals 115 (1986) 315.
Growth of a-HgI2 single crystals from physical vapor transport in an oil-bath furnace Han-Tang Zhoua, Cheng-Hsu Leea, Jia-Ming Chunga, Chen-Tsung Shina, Kuan-Cheng Chiua,*, San-Min Lanb a
Department of Physics, Chung-Yuan Christian University, Chung-Li 32023, Taiwan, ROC b Institute of Nuclear Energy, Lung-Tan 325, Taiwan, ROC
Received 19 May 2003; received in revised form 20 August 2003; accepted 5 September 2003
Abstract The growth of a-HgI2 crystals from physical vapor transport in a closed ampoule immerged in a simple and economical oil-bath furnace with an external heating coil imposed is discussed. The temperatures of the source side, Tsou, of the maximum value, Tmax, and of the crystal growth side, Tcry, together with dT/dx are changed for different growth conditions. The physical properties of the as-grown a-HgI2 crystals are characterized by scanning electron microscopy, X-ray diffraction, and I–V measurements with respect to different growth conditions. It is found that the a-HgI2 crystals grown from Tsou ¼ 125 8C, Tmax ¼ 135 8C, Tcry ¼ 114 8C, dT/dx ¼ 2:6 8C/mm, and with 5 Torr inert gas inside the growth ampoule exhibit the best qualities, such as, observable facets, transparent red color, and a low density of defects and grain boundaries. # 2003 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductors; B. Crystal growth; D. Defects
1. Introduction Red mercuric iodide (a-HgI2) is considered to be a potential photoconductor for X-ray and low energy g-ray spectrometry at room temperature with low noise, compact size, and efficient detection because of the following useful properties, such as large band gap of 2.15 eV at 300 K, low electron– hole pair creation energy of 4.2 eV, and high atomic number of 80 for Hg and 53 for I [1–6]. However, the applications of a-HgI2 are limited by the difficulty in obtaining high-quality single crystals and the long-term reliability problems in devices made from these crystals.
*
Corresponding author. Tel.: þ886-32653213; fax: þ886-32653299. E-mail address: [email protected] (K.-C. Chiu). 0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2003.09.016
1988
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
Fig. 1. Schematic diagram of the oil-bath furnace used for a-HgI2 crystals growth from PVT: (1) lard oil; (2) heating plate; (3) heating coil; (4) temperature controllers and thermal couples; (5) growth ampoule; (6) source powder; (7) a-HgI2 crystal; (8) rotating and pulling motors; (9) temperature profile.
Mercuric iodide undergoes a solid–solid phase transition from an orthorhombic yellow phase (b-HgI2) to a tetragonal red phase (a-HgI2) at 127 8C [7,8]. Thus, crystal growth from melt (resulted in b-HgI2 first) is prohibited, although the melting temperature is only 259 8C. In general, the solution growth and/or vapor growth methods operating below 127 8C are adopted to grow a-HgI2 single crystals. It is known that crystal growth from solution has contamination problems from solvent molecules, and crystal growth from physical vapor (sublimation and re-condensation) usually results in a higher purity with a well-defined structure [9]. Furthermore, a-HgI2 has high enough vapor pressure (about 0.1 Torr) at 120 8C [10,11], it can be grown efficiently by physical vapor transport (PVT) in closed ampoules. Here, the growth of a-HgI2 crystals from PVT in a closed ampoule immersed in a simple and economical oil-bath furnace is discussed. This oil-bath furnace gives a stable temperature profile to operate at temperature around 120 8C. An external heating coil is imposed in this study to create a nonuniform temperature profile as shown in Fig. 1, where the temperature of source powder side, Tsou, the maximum temperature in vapor phase transport region, Tmax, the temperature of crystal growth side, Tcry, and the temperature gradient at crystal growth side dT/dx can be varied for a systematical study. This non-uniform temperature profile, which is a modification from our previous work [11], has two purposes. First, it provides an even larger value of dT/dx in front of the growing crystal interface (as comparing to the one without external heating coil) that effectively reduces the sidewall nucleation due to the constitutional supersaturation [12,13]. This is very useful for nucleation control. Second, it gives a larger value of Tcry that can enhance the surface kinetics of the adhered molecules. Finally, the physical properties of the as-grown a-HgI2 crystals are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and I–V measurements with respect to different growth conditions.
2. Experimental The commercially available a-HgI2 powder of 99.9% purity contains a certain amount of various impurities. To obtain a higher purity, a 5X purification process is applied for a-HgI2 powder in a multichamber, where X stands for a continuous sublimation in dynamic evacuation [14]. Details of the cleaning processes for the multi-chamber and the purification steps for the source powder have been published elsewhere [11].
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
1989
Table 1 Growth conditions (GCs) used in this work
GC-1 GC-2 GC-3 GC-4
Tsou (8C)
Tmax (8C)
Tcry (8C)
dT/dx (8C/mm)
Inert gas inside (Torr)
Growth rate (mm per day)
120 120 125 130
125 125 135 140
106 106 114 120
1.5 1.5 2.6 2.5
No 5 5 5
1.2 0.2 0.7 0.1 0.4 0.1 –
Fig. 1 schematically depicts the oil-bath furnace used in this work. The system consists of a bath of lard oil (with a smoking temperature of about 380 8C) on top of a controllable heating plate which controls Tsou via a thermal couple; an external heating wire embedded into a glass coil which controls Tmax via another thermal couple; a growth ampoule made of pyrex glass; and a rotation motor and a pulling motor to rotate and pull the growth ampoule, respectively. Usually, at the initial nucleation stage it needs a larger supersaturation. The supersaturation is related to Tcry, Tsou (DT ¼ Tsou Tcry ), dT/dx, and the crystal growth rate. Once initial nucleation is obtained and acts as the seed for the subsequent growth, DT is reduced by increasing Tcry (i.e. moving the growth ampoule downward; see Fig. 1), and then the crystal growth process starts. Table 1 lists the growth conditions (GCs) used in this work.
3. Results and discussion Fig. 2 shows the as-grown a-HgI2 crystals corresponding to different growth conditions. To reveal the quality of the crystals, the cleaved samples from the crystals along the easy-cut plane are prepared. After etching in KI solution for 5 min and drying by clean air, the SEM pictures of the cleavage surface (see Fig. 3) show the interface morphology. To further identify the Miller index of the cleavage surface as well as the facets on the as-grown crystals, XRD is applied and the results are depicted as Fig. 4. The Miller index of the easy-cut plane is confirmed to be (0 0 2). For GC-1, there is no inert gas inside the growth ampoule, thus, the crystal growth rate about 1:2 0:2 mm per day is relatively high. The growth direction as shown in Fig. 2 is along c-axis of the as-grown crystal and no clear facet is observed. The color of the crystal is dark red and not very transparent. The SEM picture on the cleavage surface reveals a high density of defects and grain boundaries. Furthermore, while in preparing the cleaved samples, one can feel that the crystals from GC-1 are relatively soft, i.e. low mechanical strength, as compared to those from GC-2 and GC-3.
Fig. 2. The as-grown a-HgI2 crystals correspond to different growth conditions.
1990
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
Fig. 3. SEM pictures correspond to the cleaved interface for a-HgI2 from (a) GC-1, (b) GC-2, and (c) GC-3.
Above results indicate that the quality of the as-grown crystals from GC-1 is rather poor. We believe that the poor quality is resulted from the large mass transport rate inside the growth ampoule. To control the growth mechanism in a mass transport limited regime [9], Ar at a pressure of about 5 Torr is put into the closed ampoule to reduce the mass transport rate for GC-2 (with growth rate about 0:7 0:1 mm per day) and GC-3 (0:4 0:1 mm per day). The as-grown a-HgI2 crystals for GC-2 and GC-3 then show observable facets with growth direction about 408 deviating from (0 0 2) direction. The color of these essentially transparent crystals is red. Furthermore, the SEM pictures on the cleavage surface indicate that the density of defects and grain boundaries is markedly reduced. For GC-3, the much higher Tcry, results in shifting the whole temperature profile up about 5–10 8C, which should increase the surface kinetics of the depositing HgI2 molecules. Thus, the relatively better surface morphology of the cleaved interface for GC-3 as compared to that for GC-2 is obtained as shown in Fig. 3.
Fig. 4. XRD patterns for (a) the cleaved interface, (b) the (0 0 2) facet on as-grown crystals from GC-2 or GC-3, and (c) the (2 2 0) facet on as-grown crystals from GC-2 or GC-3.
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
1991
Table 2 The dark resistivity for electric field parallel or perpendicular to the c-axis corresponding to the as-grown a-HgI2 crystals from different GCs GC-1
GC-2 E ? c-axis
E || c-axis r (O cm)
(8.4 0.3) 10
11
(2.7 0.1) 10
GC-3 E ? c-axis
E || c-axis 10
11
(5.1 0.2) 10
E ? c-axis
E || c-axis 11
(3.5 0.2) 10
12
(4.0 0.2) 10
(7.8 0.2) 1011
For GC-3, though some temperatures exceed the solid–solid phase transition temperature of 127 8C inside the vapor transport region, the solid phase formed by nucleation or subsequent growth are found only in the a form. No b-HgI2 phase is observed. However, for GC-4 with Tsou > 127 8C, metastable bHgI2 with a yellow color appears in the crystal growth side though Tcry is only 120 8C. In the temperature range studied here, it was reported [15] that solid-HgI2 sublimates into vapor-HgI2 congruently without decomposition. Hence, our experiment suggests that the vaporized HgI2 molecules (which maybe in the form of small clusters, {HgI2}n, within the transport region) tend to preserve their favorite structure from the source region and this memory effect dominates the formation of metastable b-HgI2 phase in the crystal growth side once Tsou > 127 8C. As mentioned above, Fig. 4a shows the typical XRD patterns of the cleavage interface. Moreover, a shift in 2y is observed for the corresponding peaks from different samples. Therefore, the values of the lattice constant c along c-axis for samples from GC-1, GC-2, and GC-3 are evaluated (especially from ˚ , 12:43 0:01 A ˚ , and 12:45 0:01 A ˚ , respectively. The standard (0 0 8) peak) to be 12:40 0:01 A ˚ . For poor quality samples from GC-1, the shrinkage on the c value may due reference value is 12.45 A to the embedded defects that distort the tetragonal structure of a-HgI2 phase. For the I–V measurements, two different samples are cut from each bulk crystal with electrodes (i.e. applied electric field, E) either parallel or perpendicular to the c-axis. The conductive carbon cement is used as electrode. The sample is first put in a dark box for 30 min. The external voltage is applied from 0 up to 50 V for the positive cycle and then from 0 to 50 V for the negative cycle both with increment of 5 V every 1 min. The measured I–V curves from these samples are quite linear and the calculated results of dark resistivity, r, of the as-grown a-HgI2 crystals corresponding to different GCs are listed in Table 2. Due to the inherent layer structure of a-HgI2, the values of r with E parallel to the c-axis are higher than those with E perpendicular to the c-axis. However, for GC-1, because of the poor quality, i.e. a high density of defects embedded into the layers, this difference is exaggerated. Furthermore, the larger values of r for crystals from GC-3 as comparing to those from GC-2 is attributed to the higher quality.
4. Summary We have presented the growth of a-HgI2 crystals from physical vapor transport in a closed ampoule immersed in a simple and economical oil-bath furnace with an external heating coil. By varying the growth conditions, it is found that the physical properties of the as-grown a-HgI2 crystals from GC-3 exhibit the best qualities, such as, observable facets, very clear red color, low density of defects and grain boundaries, and high dark resistivity. Though for GC-3, inside the vapor transport region some temperatures exceed the solid–solid phase transition temperature of 127 8C, the solid phases from
1992
H.-T. Zhou et al. / Materials Research Bulletin 38 (2003) 1987–1992
nucleation or subsequent growth are still in a-HgI2 phase, no metastable b-HgI2 phase is observed. However, for GC-4 with Tsou > 127 8C, metastable b-HgI2 phase appears in the crystal growth side (though Tcry ¼ 120 8C). The above results suggest that in the transport region the vaporized HgI2 molecules in the form of small clusters tend to preserve their favorite structure from the source region.
Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially supporting this research.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
W.R. Willig, Nucl. Instrum. Methods 96 (1971) 615. M. Schieber, M. Roth, W.F. Schnepple, J. Cryst. Growth 65 (1983) 353. Y.J. Wang, B.E. Patt, J.S. Iwanczyk, Nucl. Instrum. Methods A 353 (1994) 50. T. Pal, S.L. Sharma, H.N. Acharya, J. Phys. D Appl. Phys. 28 (1995) 1439. J.P. Ponpon, R. Stuck, M. Amann, Solid-State Electronics 44 (2000) 29. M. Zahangir, S.O. Kasap, Appl. Phys. Lett. 80 (2002) 1664. J.B. Newkirk, Acta Metall. 4 (1956) 316. A. Burger, S. Morgan, C. He, E. Silberman, L. van den Berg, C. Ortale, L. Franks, J. Cryst. Growth 99 (1990) 988. F. Rosenberger, Phys. Chem. Hydrodyn. 1 (1980) 3. R. Cadoret, P. Brisson, A. Magnan, Nucl. Instrum. Methods A 283 (1989) 339. K.C. Chiu, P. Luo, H.A. Tu, B.M. Chuang, J.Y. Chen, Chin. J. Mater. Sci. 30 (1998) 191. T.B. Reed, W.J. LaFleur, Appl. Phys. Lett. 5 (1964) 191. F. Rosenberger, M.C. DeLong, D.W. Greenwell, J.M. Olson, G.H. Westphal, J. Cryst. Growth 29 (1975) 49. H.A. Lamonds, Nucl. Instrum. Methods 213 (1983) 5. M. Piechotka, E. Kaldis, J. Less-Common Metals 115 (1986) 315.