ELSEVIER Journal of Crystal Growth 171 (1997) 131-135

, . . . . . . . . C R Y S T A L

G R O W T H

Flux growth of single crystals of spinel ZnGa204 and CdGa204

Zheng Yan a,* ,1, Humih iko Takei b

Institute for Solid State Physics, University of Tokyo, Roppongi Minato-ku, Tokyo 106, Japan b Faculty of Science, Osaka University, Machikaneyama, Toyonaka-shi, Osaka 560, Japan

Received 25 March 1996

Abstract

Single crystals of spinel ZnGa204, which has been reported to be a promising material as a transparent electronic conductor, have been grown from PbO-B203 flux. Inclusion free and optical clear single crystals of maximum dimensions of 10 mm along the edge have been obtained. The lattice constant of the obtained single crystal is 8.332(3) A and the molar ratio of Ga/Zn is strictly stoichiometric and homogeneous along any directions, typically as Zno.96_l.07Gal.94_2.0204. High crystallinity has been proven and the space group of Fd3m has been confirmed from X-ray precession photographs. Single crystals of spinel CdxGa204_ x have also been obtained from the same flux system. Due to the volatility of CdO, the obtained crystals are Cd-poor in composition, ranging from 0.7 to 0.9 in x. The lattice constant of Cd09Ga203.9 is 8.615 ,~.

1. Introduct ion

A material with optical transparency and metallic conductivity has various applications, such as trans- parent electrodes in liquid crystal display (LCD) and solar cells. Until recently, the oxide phases reported to possess both properties are limited to indium tin oxide (ITO) (In203 doped with SnO2), SnO 2 doped with Sb205 and Cdln204 [1-3]. From recent re- search the cubic structured spinel group oxides are suggested to be promising materials. A lot of works have been done to study the properties of the ceram- ics ZnGa204 [4], CdGa204 [5,6] and MgIn204 [7,8],

* Corresponding author. E-mail: yan@ee.t.u-tokyo.ac.jp. i Present address: Department of Electrical Engineering, Fac-

ulty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo- ku. Tokyo 113, Japan.

where an n-type semiconducting behavior within the temperature range 77-293 K and a wide band gap of > 4.0 eV have been reported. However, the poly- crystal ZnGa204 thin film on glass substrate pre- pared by the sputtering method has been reported as insulating [9]. A fundamental study of the single crystals becomes necessary in understanding those phenomena.

In the present paper, we report the results of the crystal growth of single crystals of both spinel ZnGa204 and CdGa204 by the flux growth method. The growth of ZnGa204 single crystals has been reported by using a PbO-PbF2-B203 flux [10,11]. Van der Straten et al. [12] have prepared large single crystals of ZnGa204 from the above flux with SiO 2 additions at a very slow cooling rate of 0.5°C/h. However, based on the results of our research, a PbF2-rich flux induced a contamination of Pb into the ZnGa204 crystal lattice. Furthermore, the high

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132 Z. Yah, H. Takei / Journal of Crvstal Growth 171 (1997) 131-135

Table 1 The initial flux composition of the crystal growth of ZnGa204 in molar ratio

# PbO PbF 2 B203 ZnO Ga203

1 1 3 0.02 0.1 0.1 2 1 - 1 0.5 0.5 3 1 - 2 1 1

the obtained crystals have been reported and the compositional influence of the growth flux and the basic crystal nuclei formation mechanism have been suggested and discussed. The study of the physical properties of the ZnGa204 single crystals will be reported separately.

volatility of PbF 2 stirred a vehement flux concentra- tion instability and the static crystal growth became impossible. As a result, the use of PbF 2 leads to a poor quality of the single crystals. Solving this prob- lem, a low growth temperature is required to reduce the evaporation of the solvent, which must inevitably result in the incompleteness of the spinel phase reaction and a low solubility of the solute at the current system. To avoid this dilemma, B 2 0 3 has been chosen as a substitute of PbF 2 in the flux and single crystals of ZnGa204 have been successfully grown from the PbF2-free PbO-B203 system. No previous report of single crystal growth of CdGa204 has been made. The same growth flux of ZnGazO 4 has been tried to grow CdGazO 4 crystals. As CdO evaporates easily above 800°C, the composition of the obtained crystals deviated from the stoichiometry having a Cd-poor tendency. Usually in this case, a sealed container should be employed to prevent the evaporation at high temperature, which demerits in the limited volume, the mechanical difficulty and danger when the evaporation pressure inside the container increases at a temperature as high as 1250°C. Instead of the sealed container, we have chosen the method of adding an excess amount of CdO into the flux and lowering the soaking tempera- ture to improve the stoichiometry, which has still not given a satisfactory result. The characterizations of

2. Experimental procedure

Powders of ZnO(3N), CdO(3N), Ga203(3N), PbO(3N), PbF2(3N) and B203(3N) were used as raw materials directly without any precalcination. To de- termine the optimum growth conditions, heating- quenching experiments of ZnGa204 were carried out at the first stage. A 25 ml platinum crucible was used as a flux container. While the highest temperature was kept at 1250°C for each single run, the initial flux compositions were changed. Table 1 gives some examples of the flux compositions used. The quenched residues of the flux were cracked mechani- cally and scrutinized by mainly an optical micro- scope and a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA). The same experiments were also carried out in case of CdGaeO 4. In stead of the solvent composition of the flux, the soaking temperature and the initial CdO/Ga203 molar ratio were changed. Table 2 is a list of the examples used where the solvent composi- tion was kept unchanged as PbO/B203 = 0.5 in molar ratio.

The crystal growth experiments of ZnGa204 and CdGa204 were carried out with the starting flux composition of #3 as in Table 1 and that in Table 2, respectively. After being mixed mechanically at the room temperature, powders of the flux were moved

Table 2 The growth conditions and the results of the characterizations of the obtained crystals of CdGa204 (the solvent composition of the flux was kept as PbO/B203 = 0.5; the composition results and the lattice constants of the obtained crystals were based on the EPMA results and the X-ray diffraction profiles, respectively)

# Growth conditions Characterizations of the obtained crystals

Soaking temperature (°C) CdO Ga203 x in Cd,Ga204_~ Lattice constant a (4)

1 1250 1.0 2.0 0.71 8.627 2 1000 1.1 2.0 0.79 8.622 3 1100 1.1 2.0 0.92 8.615

Z Yan, H. Takei/Journal of Crystal Growth 171 (1997) 131-135 133

into a 50 ml platinum crucible container and then set to a perpendicular SiC furnace. After being soaked at 1250°C for ZnGazO 4 or 1100°C for CdGa204 for 8 h, the melts were slowly cooled to 850°C at a rate of l°C/h, and finally quenched. The flux residues were leached by heating with a dilute nitric acid solution. The as-grown single crystals were evaluated by us- ing the optical microscope, SEM-EPMA, XPD and X-ray precession photographs analyses.

3. Results and discussion

The heating-quenching experiments of ZnGa204 revealed that the PbF2-rich flux sample as #1 in Table 1 had a mass loss of over 30% even with a tightly covered crucible and the flux did not com- pletely melt at 1250°C. The obtained crystals from this flux were slightly yellow in color which could not be leached with the treatment of nitric acid. However, no significant mass loss was observed in cases of #2 and #3, and they were completely melted at 1250°C. The crystals obtained from those fluxes of #2 and #3 were optically clear. From the above observation, it is concluded that the flux com- position of #1 which includes excess amount of PbF 2 is not suitable for the crystal growth by the following reasons: PbF 2 has a high volatility at a temperature over 1000°C, resulting in an increase of the crystal growth rate. Furthermore, PbF 2 has a relatively high reactivity with ZnO and Ga203 which usually causes a compositional deviation and gives a non-stoichiometric result. The reactivity of PbF 2 and ZnGa204 is very certain from our previous study of the preparation of a ZnGa204 spinel single crystal thin film on MgO by the solvent evaporation epitaxy method [13]. B203 was chosen as a substitute of PbF 2 as those of #2 and #3 in Table 1. Although it has been known that, in the PbO-B203 system, the B203-rich compositions result in a porous and glassy melt which is believed not to be beneficial for single crystal synthesis [ 14]; in the present study, it is found that the B203-poor flux is powdery and has a high melting point with a low solubility value for ZnGa204, while the B203-rich melts are optically clear and have a high solubility value. The high solubility of the flux is crucial in the flux growth method. PbB204 is believed to be the main solvent

in the PbO-B203 system. A simple phase relation- ship of ZnGa204 has been suggested and only the stoichiometric composition of ZnO to GazO 3 has been used. A conclusion that the flux of #3 in Table 1 is the most suitable growth composition of ZnGa204 is drawn from the heating-quenching ex- periments.

A close study of the crystal formation has been carried out. Fig. 1 is a schematic image of the cross-sectional view of the flux in the crucible after the crystal growth. The crystals appear near the wall of the crucible and near the central surface of the flux. The temperature distribution of the employed perpendicular SiC furnace has been measured, which indicates that there is a maximum temperature differ- ence of 10°C in decreasing order toward the center along the radius direction of the furnace, and no significant temperature deviation around the setting position of the crucible in a length of 20-30 cm along the furnace axis is observed. By analyzing the phenomenon in Fig. 1, it is suggested that there are two kinds of crystal nuclei formation mechanism prevailing in this system. One is the wall effect, as of those of (a) in Fig. 1; and the other is the tempera- ture slope as those of (b). It is also assumed that ZnGa204 has a lower density than that of the flux in the liquid state, as the crystals tend to appear at the flux surface.

Crystal growth of ZnGa204 has been carried out for several independent runs, giving very similar and repeatable results. Large centimeter-sized and opti- cally clear ZnGa204 single crystals were obtained. The average size was about 6 × 5 × 5 mm 3 and the largest one was 10 × 8 × 7 mm 3 along the edge. A typical example of the as-grown single crystal is

b

flux

.crystals

/

, . . -a

crucible Fig. 1. A schematic cross-sectional image of the flux remaining in the crucible after the crystal growth of ZnGa204.

134 Z Yan, H. Takei/Journal of Crystal Growth 171 (1997) 131-135

shown in Fig. 2, where the octahedral habits of ZnGa204 spinel can clearly be seen. The out-shaped surfaces were determined to be of the (111) surface from the X-ray precession photographs. Besides a small quantity of needles and plates, the majority of the outcome were very thick, octahedron-like and optically clear crystals with (111) well-developed surfaces. In the X-ray diffraction profile of the single crystal ZnGa204, a single phase of spinel ZnGa204 was clearly illustrated, and the lattice constant of a = 8.332(3) ,~ was derived. This value has a very high comparability with that of 8.335 A in JCPDS Cards [15]. Fig. 3 is the X-ray precession pho- tographs of the (111) plane of a ZnGa204 single crystal. The reflections of 220, 420, etc. were de- tected. All reflection spots are undistorted, which identifies that the as-grown ZnGa204 is a single crystal with high crystallinity, and the precession pattern is of the (111) plane of a cubic symmetry. Along with the precession photographs of the higher levels and the other directions, the space group of Fd3m has been confirmed. The EPMA revealed that the crystals were almost homogeneous in composi- tion both of the as-grown surfaces and the bulk, typically being Zn0.96_l.07Gal.94_2.020 4. It is con- cluded that the stoichiometric and homogeneous large single crystals of ZnGa204 with ideal spinel struc- ture are obtained. The present growth technique of ZnGa204 crystals from the chosen flux is very stable and produces a high reproducible results.

In the process of CdGa204 single crystal growth, it is found that CdO evaporates vehemently over

Fig. 2. The SEM photograph of the as-grown crystal of ZnGa204.

Fig. 3. The (111) plane X-ray precession photograph of the as-grown single crystal of the ZnGa204 spinel.

800°C. We have lowered the soaking temperature in purpose of decreasing this evaporation and added an excess amount of CdO as a compensation of its loss in heating process. All obtained crystals were still Cd-poor from the EPMA results and of single spinel phase based on the X-ray diffraction profiles. In Table 2, the EPMA results and the lattice constant are listed. Comparing with the lattice constant of 8.602 A of JCPDS Card [16], the obtained crystals give larger results, which is obviously due to the Cd loss. A clear relation between the lattice constant and the crystal composition was drawn from the data in Table 2. Fig. 4 is the SEM photograph of a CdGa204 single crystal grown from lot #3, where the (111) habits are illustrated. The surface defects of irregular high steps shown in Fig. 4 might be considered as a reflection of the instability of the growth process. From the crystal analysis results, lot #3 in Table 2 had the best crystallinity, where the maximum ob- tained crystal was 1.2 × 1.0 × 0.8 mm 3. CdO was found remained in the fluxes of #2 and #3 after the growth, indicating a Cd-rich growth environment, and the over-lowered soaking temperature, which caused an incomplete reaction of forming CdGa204 as that of #2, is not beneficial.

In comparison of the grow process of ZnGa204 with that of CdGa204 in the PbO-B203 flux, the

z. Yan, H. Takei/Journal of Crystal Growth 171 (1997) 131-135 135

soaking temperature was lowered and the CdO/Ga203 molar ration was changed to improve the stoichiometry, which gave a best result as

Cd0 .9Ga203 .9 .

Fig. 4. The SEM photograph of the as-grown crystal of CdGa204 obtained from lot #3 in Table 2.

size of the related ions should be considered. The radii of Pb 2÷, Zn 2+ and Cd 2÷ cations are 1.18, 0.60 and 0.95 ,~ respectively. As the large radial differ- ence between Pb 2÷ and Zn 2÷, the substitution of Pb to Zn site is hardly to occur. However, a high chance of the substitution of Pb to the Cd site is assumed because of the apparent smaller size difference, espe- cially when the Cd site is empty (Cd-poor). Though Pb was not detected at the EPMA level, it is still suggested that the slight yellow color of all obtained crystals of CdGa204 is due to the Pb impurity. This substitution is certainly not beneficial for the crystal growth. Moreover the volatility of CdO is critically disturbing the growth of CdGa204 crystals. As a result, the obtained crystals of CdGa204 were smaller and lower in crystallinity than those of ZnGa204.

4. Conclusions

Large single crystals of spinel ZnGa204 with high crystallinity were synthesized from the developed PbO-B203 flux. From the study of the flux compo- sition, a PbF2-free, B203-rich flux was believed to be suitable for the growth of ZnGa204. The volatil- ity of CdO leaded to Cd-poor CdGa204 crystals. The

Acknowledgements

The authors would like to show their gratitude to Dr. M. Hasegawa, Mr. M. Koike, Ms. F. Sakai and Dr. T.R. Zhao for their valuable discussions as well as the experimental advice.

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