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Hollow single crystals of ZnS
Journal of Crystal Growth 7 (1970) 61-64
8 North-Holland
HOLLOW
Publishing Co.
SINGLE
CRYSTALS
E. LENDVAY
and
OF ZnS
P. KOVACS
Research Institute for Technical Physics of the Hungarian Academy of Sciences, Budapest, Hungary
Received
10 July
1969
The vapor growth and morphology of hollow ZnS single crystals have been investigated. Two types of hollow crystals were observed, large crystals with dendritic morphology and needle-like crystals probably grown by a simple Frank mechanism. The
morphology was determined by the temperature distribution of the growth system; for the same growth conditions, it was the same for doped and undoped crystals.
1. Introduction
The temperatures of the source and the zone of crystallization were measured by Pt-PtRh thermocouples. Pure and doped ZnS powder produced in our Institute was used as source material. For doping we used CuCI, and MnCl, in the concentration range of 10-5-10-2 g Me/g ZnS. The gas used for transport was a mixture of N,, H,S and HCl transport agent (1: 9: 6) and the flow-rate varied between 2-20 cm3 set- I. In some cases we first passed the H,S-N, gas mixture through a furnace heated to 800 “C, containing ZnS powder. All the gases were carefully dried. The growth time was 30 hr, and the cooling rate was about 0.5 “C/min. For both undoped and doped source materials large, hollow crystals lo-15 mm long and 4-5 mm in diameter (A-type) were formed at the end of the boat containing the ZnS powder, on the quartz tube containing the gas inlet and the first thermocouple, and on the
The growth of hollow ZnS crystals was described by Soxman’) in 1963. The growth of hollow CdS single crystals of different forms has been described by Mash and Firth2), Paorici3), Dreeben4), Maeda et a1.5) and Fujisaki et a1.6). In these crystals, which usually have hexagonal structure and morphology, the cavity is parallel to the c-axis. Sometimes this cavity runs through the entire crystal, but often it is observable only on the basal plane of the hexagonal prisms. The size of the crystals varies considerably. The length of the ZnS crystals described in ref. 1 did not exceed 1 mm, whereas some of the CdS crystals were up to 15 mm long. Along the cavity the wall thicknesses varied from 20 /lrn to 1 mm. According to some authors’-3) the appearance of the special morphology is caused by oxygen and alkali contamination. Dreeben4), however, observed the formation of hollow CdS crystals even under very pure conditions. We have produced hollow ZnS single crystals about 15 times longer than those mentioned in ref. 1. In the following the formation and some features of these large, hollow ZnS crystals will be discussed.
In.3
SWRCE
MoCOUPLEZ ET .c , ACWSSB
TVPE CRVSTALS
2. Experimental The growth of ZnS singIe crystals was performed in an open-tube transport system, using a 5-zone resistance furnace (fig. 1). The second zone was wound with Pt wire, the rest with Kanthal A-l. The source was in the Pt unit, in the temperature range of 1300-1400 “C. 61
1.. 0 10 10
Fig.
1.
Schematic
.-cm
a
drawing
10
50 60
10
m
*I
of Z&zone crystal-growing
furnace.
E.
62
LENDVAY
AND
P.
KOVP;CS
diameters, even when they are the same length (B-type). Shallow holes were often found on the basal plane of the crystals. Microscopic examination of the bottoms of the holes was carried out by using mirror objectives of large focal length, or by cleaving the crystal and using a standard
metallographic
microscope.
3. The morphology of hollow ZnS crystals The large hollow crystals usually have irregular cross sections. Fig. 3 shows a top’view of a characteristic A-type prism. At the points Larked with arrows the crystal contractions probably correspond to twin or grain boundaries. Inside the hole there is a multitude
Fig.
Large,
2.
hollow
A-type
ZnS crystal.
quartz tube directly after the source. Fig. 2 shows such a hollow ZnS prism. Hollow prisms are also obtained at lower temperature (T N 1280 “C), but these have considerably smaller
Fig.
4.
Top view of A-type
ZnS crystal.
Fig.
At the marked
3.
points
Top view of A-type ZnS crystal, traces of twin or grain boundaries
hook
connections
are shown
with arrows marking (mag. 27 x).
(mag.
27 Y).
HOLLOW
SINGLE
OF ZnS
CRYSTALS
63
of curved step formations. In the other hollow crystals the prism is either composed of two crystals connected by hooks (fig. 4) or has the form described by Fujisaki et al.6). A gradually growing lamellar structure can often be seen on the inner wall of the hole, as shown in fig. 4. The formations on the bottoms of the holes can also be divided into 2 groups. Curved steps of dendritic formations like those in fig. 3 are found on the inner surface of almost every crystal. The other growth picture, which occurs on rare occasions, is not dendritic. Inside the cavity the crystal is separated by an always well observable grain boundary, on both sides of which one can observe a curved growth, as shown in fig. 5.
Fig. 6. Top view of B-type ZnS crystal. At the points marked with arrows one can observe the growth of thin, high walls. On the basal plane spiral growth steps can be seen relating to screw dislocations (mag. about 200 x ; Reichert-Nomarski interferencecontrast equipment).
any oriented kinks on the growth magnification.
steps even at higher
4. Discussion We succeeded in producing reproducible large hollow ZnS crystals with variable Cu and Mn concentrations in an oxygen-free medium of a low alkali contamination (see table 1). This proves that in contradiction
Fig.
5. Growth pattern of an A-type crystal containing boundary on the bottom of the cavity (msg. 120 x).
grain
The B-type crystals generally have shallow holes on their basal plane. On the bottom of these holes neither dendritic formations nor that of fig. 5 can be found. In some of these crystals the inner and outer walls of the hole show a very strong polygonization as shown in fig. 6, similar to that of the CdS crystals described by Maeda et al.5). Other crystals do not show such polygonization. Thus the crystal in fig. 7 has a hole with strongly curved inner wall, where one cannot observe
Fig.
7.
Basal plane
of B-type
ZnS crystal
(msg.
180 x).
64
Ii.
LENDVAY
AND
I’.
KOVACS
TABLE 1
Analytical
data
for ZnS source
Fe Source material __~ Hollow crystal from the source material + 5 % MnClp Hollow crystal from the source material +O. 1 % cuc12
__
..__
material
Ni
and
hollow
co
Mn
1 x 10m6 1 x 1O-b
<1O-5
<1o-6
3x10-6
1-8x10-3
2x10-6
1 x10-6
<10-e
crystals determined by spectrographic Impurities (g Me/g ZnS) cu 1.5 Y 10-e
3x10-h
1-16x
peratures
where the ZnS + 2 HCl e ZnCl,
+ H,S
Ag
Pb
1.5x10-6
<10-h
1 x10-5
1O-3 -cIO-~
refs. 1-3, this extraordinary growth probably is not caused by the described cationic or oxygen impurities. Morphologic examination shows that the growth of most A-type crystals is probably a special case of dendritic growth. This conclusion is supported by the formation of twins like those shown in fig. 3. The high supersaturation necessary for fast, dendritic growth can occur in the region where hollow crystals are formed. The high supersaturation is proved by the observed two-dimensional nucleation on the (OOOi) plane of Atype crystals. In our system it is even possible that the ZnCl, (whose concentration reaches its maximum in the vicinity of the source) developed by the presence of HCl at high temperature is adsorbed on the growing crystal plane and gives rise to dendritic growth. At lower temwith
and activation
Al
Mg 8rlo-’
52 10-e
analytical
Ca 1.5x10-5
lo/ 10-s
methods
Na -.10-3
10-x
.:t0-3
K ‘-lo-”
10-Z
10-3
equilibrium transport reaction is strongly shifted to the ZnS side, the possibility of dendritic growth decreases. The change in the gas flow rate did not appreciably change the yield of hollow crystals either in the A cases or in the B ones. Spiral growth relating to screw dislocations can be seen on the basal planes of most B-type crystals (fig. 6). Similar observations on CdS were made by Maeda et a1.5). It is most probable that these crystals grow first of all by a dislocation mechanism, so they differ considerably from the A-type ones not only morphologically but in their growth mechanism as well. References 1) E. J. Soxman,
J. Appl. Phys. 34 (1963) 948. D. H. Mash and F. Firth, J. Appl. Phys. 34 (1963) 3636. C. Paorici, J. Crystal Growth 2 (1968) 324. A. Dreeben, J. Appl. Phys. 35 (1964) 2549. M. Maeda, F. Goto and K. Miyata, Japan. J. Appl. Phys. 3 (1964) 426. 6) H. Fujisaki, M. Takahashi, H. Shoji and Y. Tanabe, Japan. J. Appl. Phys. 2 (1963) 665.
2) 3) 4) 5)
8 North-Holland
HOLLOW
Publishing Co.
SINGLE
CRYSTALS
E. LENDVAY
and
OF ZnS
P. KOVACS
Research Institute for Technical Physics of the Hungarian Academy of Sciences, Budapest, Hungary
Received
10 July
1969
The vapor growth and morphology of hollow ZnS single crystals have been investigated. Two types of hollow crystals were observed, large crystals with dendritic morphology and needle-like crystals probably grown by a simple Frank mechanism. The
morphology was determined by the temperature distribution of the growth system; for the same growth conditions, it was the same for doped and undoped crystals.
1. Introduction
The temperatures of the source and the zone of crystallization were measured by Pt-PtRh thermocouples. Pure and doped ZnS powder produced in our Institute was used as source material. For doping we used CuCI, and MnCl, in the concentration range of 10-5-10-2 g Me/g ZnS. The gas used for transport was a mixture of N,, H,S and HCl transport agent (1: 9: 6) and the flow-rate varied between 2-20 cm3 set- I. In some cases we first passed the H,S-N, gas mixture through a furnace heated to 800 “C, containing ZnS powder. All the gases were carefully dried. The growth time was 30 hr, and the cooling rate was about 0.5 “C/min. For both undoped and doped source materials large, hollow crystals lo-15 mm long and 4-5 mm in diameter (A-type) were formed at the end of the boat containing the ZnS powder, on the quartz tube containing the gas inlet and the first thermocouple, and on the
The growth of hollow ZnS crystals was described by Soxman’) in 1963. The growth of hollow CdS single crystals of different forms has been described by Mash and Firth2), Paorici3), Dreeben4), Maeda et a1.5) and Fujisaki et a1.6). In these crystals, which usually have hexagonal structure and morphology, the cavity is parallel to the c-axis. Sometimes this cavity runs through the entire crystal, but often it is observable only on the basal plane of the hexagonal prisms. The size of the crystals varies considerably. The length of the ZnS crystals described in ref. 1 did not exceed 1 mm, whereas some of the CdS crystals were up to 15 mm long. Along the cavity the wall thicknesses varied from 20 /lrn to 1 mm. According to some authors’-3) the appearance of the special morphology is caused by oxygen and alkali contamination. Dreeben4), however, observed the formation of hollow CdS crystals even under very pure conditions. We have produced hollow ZnS single crystals about 15 times longer than those mentioned in ref. 1. In the following the formation and some features of these large, hollow ZnS crystals will be discussed.
In.3
SWRCE
MoCOUPLEZ ET .c , ACWSSB
TVPE CRVSTALS
2. Experimental The growth of ZnS singIe crystals was performed in an open-tube transport system, using a 5-zone resistance furnace (fig. 1). The second zone was wound with Pt wire, the rest with Kanthal A-l. The source was in the Pt unit, in the temperature range of 1300-1400 “C. 61
1.. 0 10 10
Fig.
1.
Schematic
.-cm
a
drawing
10
50 60
10
m
*I
of Z&zone crystal-growing
furnace.
E.
62
LENDVAY
AND
P.
KOVP;CS
diameters, even when they are the same length (B-type). Shallow holes were often found on the basal plane of the crystals. Microscopic examination of the bottoms of the holes was carried out by using mirror objectives of large focal length, or by cleaving the crystal and using a standard
metallographic
microscope.
3. The morphology of hollow ZnS crystals The large hollow crystals usually have irregular cross sections. Fig. 3 shows a top’view of a characteristic A-type prism. At the points Larked with arrows the crystal contractions probably correspond to twin or grain boundaries. Inside the hole there is a multitude
Fig.
Large,
2.
hollow
A-type
ZnS crystal.
quartz tube directly after the source. Fig. 2 shows such a hollow ZnS prism. Hollow prisms are also obtained at lower temperature (T N 1280 “C), but these have considerably smaller
Fig.
4.
Top view of A-type
ZnS crystal.
Fig.
At the marked
3.
points
Top view of A-type ZnS crystal, traces of twin or grain boundaries
hook
connections
are shown
with arrows marking (mag. 27 x).
(mag.
27 Y).
HOLLOW
SINGLE
OF ZnS
CRYSTALS
63
of curved step formations. In the other hollow crystals the prism is either composed of two crystals connected by hooks (fig. 4) or has the form described by Fujisaki et al.6). A gradually growing lamellar structure can often be seen on the inner wall of the hole, as shown in fig. 4. The formations on the bottoms of the holes can also be divided into 2 groups. Curved steps of dendritic formations like those in fig. 3 are found on the inner surface of almost every crystal. The other growth picture, which occurs on rare occasions, is not dendritic. Inside the cavity the crystal is separated by an always well observable grain boundary, on both sides of which one can observe a curved growth, as shown in fig. 5.
Fig. 6. Top view of B-type ZnS crystal. At the points marked with arrows one can observe the growth of thin, high walls. On the basal plane spiral growth steps can be seen relating to screw dislocations (mag. about 200 x ; Reichert-Nomarski interferencecontrast equipment).
any oriented kinks on the growth magnification.
steps even at higher
4. Discussion We succeeded in producing reproducible large hollow ZnS crystals with variable Cu and Mn concentrations in an oxygen-free medium of a low alkali contamination (see table 1). This proves that in contradiction
Fig.
5. Growth pattern of an A-type crystal containing boundary on the bottom of the cavity (msg. 120 x).
grain
The B-type crystals generally have shallow holes on their basal plane. On the bottom of these holes neither dendritic formations nor that of fig. 5 can be found. In some of these crystals the inner and outer walls of the hole show a very strong polygonization as shown in fig. 6, similar to that of the CdS crystals described by Maeda et al.5). Other crystals do not show such polygonization. Thus the crystal in fig. 7 has a hole with strongly curved inner wall, where one cannot observe
Fig.
7.
Basal plane
of B-type
ZnS crystal
(msg.
180 x).
64
Ii.
LENDVAY
AND
I’.
KOVACS
TABLE 1
Analytical
data
for ZnS source
Fe Source material __~ Hollow crystal from the source material + 5 % MnClp Hollow crystal from the source material +O. 1 % cuc12
__
..__
material
Ni
and
hollow
co
Mn
1 x 10m6 1 x 1O-b
<1O-5
<1o-6
3x10-6
1-8x10-3
2x10-6
1 x10-6
<10-e
crystals determined by spectrographic Impurities (g Me/g ZnS) cu 1.5 Y 10-e
3x10-h
1-16x
peratures
where the ZnS + 2 HCl e ZnCl,
+ H,S
Ag
Pb
1.5x10-6
<10-h
1 x10-5
1O-3 -cIO-~
refs. 1-3, this extraordinary growth probably is not caused by the described cationic or oxygen impurities. Morphologic examination shows that the growth of most A-type crystals is probably a special case of dendritic growth. This conclusion is supported by the formation of twins like those shown in fig. 3. The high supersaturation necessary for fast, dendritic growth can occur in the region where hollow crystals are formed. The high supersaturation is proved by the observed two-dimensional nucleation on the (OOOi) plane of Atype crystals. In our system it is even possible that the ZnCl, (whose concentration reaches its maximum in the vicinity of the source) developed by the presence of HCl at high temperature is adsorbed on the growing crystal plane and gives rise to dendritic growth. At lower temwith
and activation
Al
Mg 8rlo-’
52 10-e
analytical
Ca 1.5x10-5
lo/ 10-s
methods
Na -.10-3
10-x
.:t0-3
K ‘-lo-”
10-Z
10-3
equilibrium transport reaction is strongly shifted to the ZnS side, the possibility of dendritic growth decreases. The change in the gas flow rate did not appreciably change the yield of hollow crystals either in the A cases or in the B ones. Spiral growth relating to screw dislocations can be seen on the basal planes of most B-type crystals (fig. 6). Similar observations on CdS were made by Maeda et a1.5). It is most probable that these crystals grow first of all by a dislocation mechanism, so they differ considerably from the A-type ones not only morphologically but in their growth mechanism as well. References 1) E. J. Soxman,
J. Appl. Phys. 34 (1963) 948. D. H. Mash and F. Firth, J. Appl. Phys. 34 (1963) 3636. C. Paorici, J. Crystal Growth 2 (1968) 324. A. Dreeben, J. Appl. Phys. 35 (1964) 2549. M. Maeda, F. Goto and K. Miyata, Japan. J. Appl. Phys. 3 (1964) 426. 6) H. Fujisaki, M. Takahashi, H. Shoji and Y. Tanabe, Japan. J. Appl. Phys. 2 (1963) 665.
2) 3) 4) 5)