- Email: [email protected]
Growth of large ZnSe single crystal by CVT method
Journal of Crystal Growth 186 (1998) 60—66
Growth of large ZnSe single crystal by CVT method S. Fujiwara*, H. Morishita, T. Kotani, K. Matsumoto, T. Shirakawa Basic High-Technology Laboratories, Sumitomo Electric Industries Ltd., 1-1-3, Shimaya, Konohana-ku, Osaka 554, Japan Received 28 March 1997; accepted 28 May 1997
Abstract The growth of ZnSe single crystals by chemical vapor transport (CVT) method using iodine as transport agent was investigated for the purpose of growing a large ZnSe crystal with a diameter of 1 in. The optimum growth temperature, seed orientation associated with the angle at the conical tip and iodine concentration were determined from the viewpoint of high growth rate and morphological stability. A 1A£ ZnSe single crystal was successfully grown under determined growth conditions. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction ZnSe, which is a wide-band-gap semiconductor, is a promising material for the blue-emitting laser diodes. Therefore, considerable effort has been devoted to the growth of epitaxial ZnSe layers on GaAs substrates. In recent years, homoepitaxial growth of ZnSe has been tried in an attempt to avoid various difficulties of heteroepitaxial growth on GaAs substrates [1, 2]. However, the attempts have been obstructed by the difficulty of growing bulk ZnSe single crystals for substrates. Furthermore, the growth of ZnSe single crystals with large size and high quality, such as low dislocation density and low resistance, is desired.
* Corresponding author. Fax: #81 6 464 3564; e-mail: [email protected].
In general, the characteristic feature of ZnSe crystal growth by CVT using iodine as transport agent is that a twin-free single crystal with low dislocation density less than 1]104 cm~2 and ntype conductivity less than 0.1 ) cm can be grown [3—5]. These qualities are desirable for the substrate of homoepitaxial growth. However, the maximum size of ZnSe single crystal grown by CVT, which is about 0.5 in at most, is insufficient for the substrate. ZnSe single crystals with a diameter of more than 1 in have neither been grown by CVT, nor has adequate investigation been conducted on the possibility. In this paper, we first describe the optimum growth conditions about growth temperature, seed orientation, ampoule geometry and iodine concentration for the purpose of growing large ZnSe single crystals with high growth rate. Next, the results of the growth of a ZnSe single crystal with a diameter of 1A under the determined optimum growth conditions are presented.
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 4 4 1 - 7
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
2. Optimum growth conditions 2.1. Experimental procedure Crystals were grown in closed quartz ampoules as indicated in Fig. 1. A ZnSe polycrystal measuring 5]5]10 mm3 with a purity of 6N, synthesized by the CVD method, was etched by a boiling 30% NaOH solution and then washed with deionized water. A ZnSe single crystal with (1 1 1)A or (1 1 1)B face with dimensions of 5]5]1 mm3, 10]10]1 mm3 or 15]15]1 mm3 was prepared as a seed crystal from ZnSe single crystals grown by the recrystallization method developed by Terashima et al. [6]. The ZnSe polycrystal and seed crystal were baked in a quartz tube at 150°C for 2 h in a vacuum of less than 3]10~7 Torr. After baking, iodine was charged into the quartz tube under N flow with a humidity of less than 3%. The 2 quartz tube was then evacuated to less than 3]10~7 Torr and sealed off. During these processes after baking, the inner surface of the quartz tube, the polycrystal and seed crystal were not exposed to the atmosphere. Horizontal furnaces A and B, which were equipped with three and four heating elements, respectively, and a vertical furnace C with four heating elements were employed. In the case of furnace A or
61
B, ampoules were set in the furnace in such a way that a seed crystal was located at the upper part of the ampoule. The ampoule was placed in the furnace, where the seed temperature was about 10°C higher than the source temperature so that the seed surface was etched. After 3 h, the seed temperature was lowered to initiate the growth on the seed crystal. At the end of growth, ampoules were pulled out of the furnace after measuring the temperature profile along the ampoule periphery. 2.1.1. Growth temperature The dependence of the growth rate and the crystalline quality on the growth temperature was investigated in growth runs Nos. 1-A—D (see Table 1) in order to determine the optimum growth temperature. In a series of these growth experiments, an identical type-1 ampoule was repeatedly used for all the runs without breaking the ampoule after growth. In other words, the grown crystal was used as a seed crystal in the following growth experiment. The growth rate was monitored in-situ by measuring the shift of the center-of-gravity in the ampoule. The details of this growth rate monitor are described in our previous work [7]. For that purpose, furnace C was employed because a horizontal configuration was required for the in-situ growth rate monitor. The crystalline quality was evaluated by observing the surface morphology of the grown crystal from the outside of the quartz tube. 2.1.2. Seed orientation and ampoule structure The effect of the seed orientation associated with the angle h at the conical tip of the ampoule (see Fig. 1) on the morphological stability was examined. The combinations of seeds with (1 1 1)A face or (1 1 1)B face and the 30 or 60° angle at the conical tip were tested in growth runs Nos. 2—4 (see Table 1).
Fig. 1. Schematic illustration of the ampoules. Two types of ampoules (type 1 and type 2) are prepared. h indicates the angle at the conical tip.
2.1.3. Iodine concentration The effect of the total pressure, the sum of the partial pressures of ZnI , Se , I and I, in the 2 2 2 ampoule, on the growth rate and morphological stability was investigated, using type-1 and type-2 ampoules (see Table 2). In the case of type-1
62
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
Table 1 Growth conditions. Run Nos. 1-A—D are the experiments to determine the optimum growth temperature. Run Nos. 2—4 are the experiments to determine the optimum seed orientation associated with the angle h at the conical tip of the ampoule Run No.
Ampoule
Furnace
Seed orientation
Seed size (mm3)
Iodine Seed concentration temperature (mg/cm3) (°C)
Source temperature (°C)
Angle at conical tip (deg)
1-A 1-B 1-C 1-D 2 3 4
Type Type Type Type Type Type Type
C C C C A A A
(1 1 1)A (1 1 1)A (1 1 1)A (1 1 1)A (1 1 1)A (1 1 1)B (1 1 1)B
5]5]1 5]5]1 5]5]1 5]5]1 10]10]1 5]5]1 10]10]1
0.55 0.55 0.55 0.55 1.1 3.4 1.1
856 904 952 805 875 871 868
60 60 60 60 60 60 30
1 1 1 1 1 1 1
846 893 939 796 855 851 861
Table 2 Growth conditions of the experiments to determine the optimum iodine concentration Run No.
Ampoule
Furnace
Iodine concentration (mg/cm3)
5-A—D 6-A—H 7-A—D
Type 1 Type 1 Type 2
A B C
1.1—4.4 0.55—4.4
ampoules, the total pressure was estimated by the equilibrium condition, which satisfies the given iodine concentration, of the following two reactions: ZnSe(s)#I (g)"ZnI (g)#1Se (g), (1) 2 2 2 2 I (g)"2I(g). (2) 2 The equilibrium constants for the above reactions were estimated from thermodynamical data [8, 9] using the average temperature of seed temperature and source temperature. In the case of the type-2 ampoule, the total pressure was assumed to equal the iodine pressure at the reservoir temperature. The angle at the conical tip of the ampoules was 30° with an aperture diameter of 12 mm, and seeds with (1 1 1)B face measuring 15]15]1 mm3 were used. Furnace C for type-2 ampoules, was used with a subheater which controlled the temperature of iodine in the reservoir. The growth conditions such as ampoule structure, seed temperature and source temperature, except for iodine concentration, were the same in the series of growth experiments using
Reservoir temperature (°C)
Total pressure (atm)
Seed temperature (°C)
Source temperature (°C)
131—220
0.58—2.1 0.29—2.1 0.22—2.15
860 860 860
865 865 865
an identical furnace. Note that measuring of the temperature contains large systematic error, which was presumed of the order of 10°C, due to the indirect temperature measurement of seed and source using the thermocouple placed outside the ampoules. Therefore, it is senseless to compare the growth rates between the experiments using a different furnace. However, it is possible to compare the growth rates from the identical furnace, because the position of the ampoule and heater temperature were the same in the series of growth experiments using the same furnace. 2.2. Results and discussion 2.2.1. Growth temperature The growth rate dependence on the growth temperature are shown in Fig. 2. Starting from 850°C, the growth rate decreased with the increase of the growth temperature maintaining the growth of single crystals which were bounded by smooth symmetric facets, whereas polygonization occurred
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
63
Fig. 2. Dependence of growth rate and crystallinity on the growth temperature. These results are obtained using an identical ampoule. The arrows represent the order of the experiments. Solid circles (v) represent the growth of single crystal. Open circle (L) represents the growth of polycrystal.
with loosing smooth facets when the seed temperature was lowered to about 800°C. Therefore, the optimum growth temperature (seed temperature), which enables the growth of single crystal and high growth rate, is around 850°C. These results agree with Parker’s result that the growth temperature less than 830°C gave polycrystalline growth while temperature greater than 890°C gave no crystal growth [10]. 2.2.2. Seed orientation and ampoule structure Photograph of the crystals grown in growth experiments run Nos. 2—4 are shown in Fig. 3. The periphery of the crystal grown in run No. 2 is bounded by threefold facets with M1 0 0N and M1 1 1NB faces, however, the front of the crystal is not bounded by stable facets, where the formation of a prevailing (1 1 1)A facet had been expected, and substitutively a deep basin and some projections are observed there. The front of the crystal grown in Run No. 3 is bounded by a stable facet with (1 1 1)B face. The periphery of the front part neighboring the (1 1 1)B facet is also bounded by stable threefold M1 0 0N facets. However, the periphery of the tail part neighboring the seed crystal is bounded by out-developing thin plates with a M1 1 1NB facet and basins. As a result, the tail part
Fig. 3. Photograph of the grown ZnSe crystal grown in run Nos. 2—4 (upper row) and the schematic illustration of the facets and basins in each crystal (lower row). Smallest divisions of the scale represent 1 mm. The hatching in the illustration indicate basins.
of the crystal contains numerous voids. To the contrary, the tail part of the crystal grown in run No. 4 is bounded by a rough surface bordering the ampoule wall without forming voids. Since the front part of the crystal as well as the crystal grown in run No. 3 are bounded by stable facets, the whole crystal grown in run No. 4 is dense without voids. From these results, it is concluded that a seed with M1 1 1NB face should be used for the purpose of forming stable facets at the front of the crystal and a small angle at the conical tip should be adopted for the purpose of forming a dense tail part without voids. The same results were obtained in our many growth experiments, though only three typical examples are presented here. It is presumed that the morphological stability at the tail part is determined by the comparison between the angle at the conical tip and the angle of the out-developing thin plate. That is, if the angle at the conical tip is smaller than the angle of the out-developing plate (38.9°), out-developing thin plates do not form,
64
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
resulting in the stable growth with rough surface in the tail part. Koyama’s results [4], that polycrystals were obtained at an angle of 120°and single crystals were obtained at the angle of 60° in case of using a (1 1 1)A seed, can be similarly explained considering out-developing thin plates with a M1 0 0N facet. Therefore, the discrepancy in the optimum seed orientation among the previous works [4, 10, 11] can be completely accounted for by the growth rate and ampoule geometry, if very low growth rate enables the growth of single crystal on a (1 1 1)A seed. 2.2.3. Iodine concentration The obtained dependence of growth rate and morphological stability on the total pressure, which is equivalent to the iodine concentration, is shown in Fig. 4. The increase of the growth rate in region C can be accounted for by taking into account of the convective mass transport, assuming that mass transport limits the growth rate, since the thermal convection prevails at high pressure. It is generally noted that the convective mass transport causes the supersaturation in front of the crystal surface giving rise to random nucleation there, resulting in polycrystalline growth [12—15]. In our growth experiments, polycrystalline growth due to the random nucleation was prevented by using seed crystals even in region C. However, maintaining morphological stability in region C was unsuccessful, resulting in the formation of many voids. Furthermore, twin boundaries are frequently observed in the crystals grown in this region. Therefore, it is concluded that the growth conditions where the contribution of the convective mass transport is significant should be avoided in the growth of ZnSe by CVT even if seed crystal is used. The increase of growth rate in region A does not originate from the change of the convective mass transport, since the influence of the thermal convection is negligible even in region B where the thermal convection is stronger than in region A. That the influence of the deviation from stoichiometry of the gas in the ampoule or the influence of the oxygen from the source polycrystal [16], should both be more prominent under the condition of less mass of charged iodine, does not account for these results, because this increase was also observed in the series
Fig. 4. The dependence of growth rate and morphological stability on the total pressure in the ampoule. Symbols (v L) represent the results from run No. 5-A—D using furnace A. Symbols (r e) represent the results from run No. 6-A—H using furnace B. Symbols (j h) represent the results from run No. 7-A—D using furnace C. Solid symbols (v j r) indicate the growth of single crystal with stable facets. Open symbols (L h e) indicate unstable growth. Solid lines show the calculated curves taking account of only diffusive mass transport.
of growth experiments using the type-2 ampoules equipped with an iodine reservoir. The increase must, therefore, result from the increase of the diffusive mass transport. In order to confirm this assumption, the change of the diffusive mass transport against the change of the total pressure was estimated. The diffusive transport rate is approximately given by ¼AD I" (p (S)!p (C)), Z/IÈ ¸R¹ Z/IÈ
(3)
where ¼, A, D, ¸, R and ¹ represent the weight of ZnSe synthesized 1 mol ZnI , the cross section of 2 the ampoule, diffusion coefficient, the length between source and seed and temperature, respectively. Suffix S and C indicate source and seed, respectively. The diffusion coefficient D is expressed approximately by (see Ref. [17]) D"D (¹/¹ )1.8(p /p ), (4) 0 0 0 T where ¹ "273 K, p "1 atm and p is the total 0 0 T pressure in the ampoule. The partial pressures of ZnI at source and seed, p (S) and p (C), 2 Z/IÈ Z/IÈ
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
65
can be evaluated from Eqs. (1) and (2) within the constraint imposed by the given total pressure and temperature. The calculated results for three series of growth experiments are shown by solid lines in Fig. 4. Here, the diffusion coefficient D and the 0 temperature at seed were chosen in such a way that the calculated growth rates agreed with the experimental growth rates in region B. Since the calculated results also agree well with the experimental results in region A, the increase of the growth rate in region A is infallibly due to the increase of the diffusive mass transport. Therefore, the optimum iodine concentration should be situated in region B from the viewpoint of morphological stability and high growth rate.
3. Growth of large ZnSe single crystal The main issue in growing large ZnSe crystal by CVT is the suppression of the thermal convection, since enlargement of the diameter of the ampoule or the aperture at the seed crystal enhances the thermal convection as does the increase of the total pressure. In order to grow a ZnSe single crystal with a diameter of 1A, we used furnace A and type-1 ampoule with the same thermal conditions as used in run No. 5-A—D. The diameters of the seed crystal and the aperture at the seed were 27 mm and 23 m, respectively. The angle at the conical tip was 30° and the orientation of the seed crystal was (1 1 1)B. The determination of the iodine concentration was not so simple, since region B shrinks to a lower total pressure region by the extension of region C. Note that the boundary between region A and B is independent of the diameter of the ampoule or the aperture, since it is independent of the convective mass transport. Therefore, the iodine concentration of 1.1 mg/cm3, which corresponds to the lowest total pressure in region B, was chosen. In Fig. 5, the photographs of the grown crystal and the (1 0 0) polished wafer sawed from this crystal are presented. The crystal is a single crystal bounded by stable threefold facets. No voids or twin boundaries are observed and the whole crystal is transparent. The size of the grown crystal is 25 mm in diameter and 7 mm in length. This is the
Fig. 5. Photographs of a ZnSe single crystal with a diameter of 1A and (1 0 0) polished wafer.
largest ZnSe single crystal ever grown by CVT using iodine as transport agent.
4. Conclusions The optimum growth conditions of large ZnSe single crystals by CVT using iodine as transport agent were investigated from the viewpoint of high growth rate and morphological stability. The determined optimum growth conditions were as follows: f Growth temperature: near 850°C. f Seed orientation: (1 1 1)B with an angle of less than 30° at the conical tip. f Iodine concentration: near 1.1 mg/cm3. Although the previous work was confirmed only in terms of the growth temperature, we first determined the optimum seed orientation associated
66
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
with the angle at the conical tip of the ampoule and discussed the optimum iodine concentration aiming to grow a larger ZnSe single crystal with morphological stability and high growth rate. ZnSe single crystal with a diameter of 1 in was successfully grown under the above growth conditions. The crystal is transparent, containing no voids or twin boundaries. References [1] R.M. Park, C.M. Rouleau, M.B. Toffer, T. Koyama, T. Yodo, J. Mater. Res. 5 (1990) 475. [2] D.B. Eason, Z. Yu, W.C. Hughes, W.H. Roland, C. Boney, J.W. Cook Jr., J.F. Schetzina, G. Cantwell, W.C. Harsch, Appl. Phys. Lett. 66 (1995) 115. [3] S. Fujita, H. Mimoto, H. Takebe, T. Noguchi, J. Crystal Growth 47 (1979) 326. [4] T. Koyama, T. Yodo, H. Oka, K. Yamashita, T. Yamasaki, J. Crystal Growth 91 (1988) 639. [5] T. Koyama, K. Yamashita, K. Kumata, J. Crystal Growth 96 (1989) 217.
[6] K. Terashima, M. Kawachi, M. Takena, J. Crystal Growth 102 (1990) 387. [7] S. Fujiwara, K. Matsumoto, T. Shirakawa, J. Crystal Growth 165 (1996) 169. [8] H. Hartmann, R. Mach, B. Selle, in: E. Kaldis (Ed.), Current Topics in Materials Science, vol. 9, North-Holland, Amsterdam, 1982, p. 373. [9] O. Kubaschewski, C.B. Alcock, in: G.V. Raynor (Ed.), Metallurgical Thermochemistry, vol. 24, Pergamon, Oxford, 1979, p. 364. [10] S.G. Parker, J. Crystal Growth 9 (1971) 177. [11] U. Geissler, H. Hartmann, E. Krause, D. Siche, J. Crystal Growth 159 (1996) 175. [12] K. Bo¨ttcher, H. Hartmann, J. Crystal Growth 146 (1995) 53. [13] Y. Noda, H. Yonekura, Y. Okunuki, S. Nakazawa, Mater. Trans. JIM 36 (1995) 1067. [14] J. Mercier, J. Crystal Growth 56 (1982) 235. [15] H. Wiedemeier, D. Chandra, F.C. Klaessig, J. Crystal Growth 51 (1980) 345. [16] W. Palosz, J. Crystal Growth 61 (1983) 412. [17] E. Kaldis, M. Piechotka, in: D.T.J. Hurle (Ed.), Handbook of Crystal Growth, vol. 2, North-Holland, Amsterdam, 1994, p. 618.
Growth of large ZnSe single crystal by CVT method S. Fujiwara*, H. Morishita, T. Kotani, K. Matsumoto, T. Shirakawa Basic High-Technology Laboratories, Sumitomo Electric Industries Ltd., 1-1-3, Shimaya, Konohana-ku, Osaka 554, Japan Received 28 March 1997; accepted 28 May 1997
Abstract The growth of ZnSe single crystals by chemical vapor transport (CVT) method using iodine as transport agent was investigated for the purpose of growing a large ZnSe crystal with a diameter of 1 in. The optimum growth temperature, seed orientation associated with the angle at the conical tip and iodine concentration were determined from the viewpoint of high growth rate and morphological stability. A 1A£ ZnSe single crystal was successfully grown under determined growth conditions. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction ZnSe, which is a wide-band-gap semiconductor, is a promising material for the blue-emitting laser diodes. Therefore, considerable effort has been devoted to the growth of epitaxial ZnSe layers on GaAs substrates. In recent years, homoepitaxial growth of ZnSe has been tried in an attempt to avoid various difficulties of heteroepitaxial growth on GaAs substrates [1, 2]. However, the attempts have been obstructed by the difficulty of growing bulk ZnSe single crystals for substrates. Furthermore, the growth of ZnSe single crystals with large size and high quality, such as low dislocation density and low resistance, is desired.
* Corresponding author. Fax: #81 6 464 3564; e-mail: [email protected].
In general, the characteristic feature of ZnSe crystal growth by CVT using iodine as transport agent is that a twin-free single crystal with low dislocation density less than 1]104 cm~2 and ntype conductivity less than 0.1 ) cm can be grown [3—5]. These qualities are desirable for the substrate of homoepitaxial growth. However, the maximum size of ZnSe single crystal grown by CVT, which is about 0.5 in at most, is insufficient for the substrate. ZnSe single crystals with a diameter of more than 1 in have neither been grown by CVT, nor has adequate investigation been conducted on the possibility. In this paper, we first describe the optimum growth conditions about growth temperature, seed orientation, ampoule geometry and iodine concentration for the purpose of growing large ZnSe single crystals with high growth rate. Next, the results of the growth of a ZnSe single crystal with a diameter of 1A under the determined optimum growth conditions are presented.
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 4 4 1 - 7
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
2. Optimum growth conditions 2.1. Experimental procedure Crystals were grown in closed quartz ampoules as indicated in Fig. 1. A ZnSe polycrystal measuring 5]5]10 mm3 with a purity of 6N, synthesized by the CVD method, was etched by a boiling 30% NaOH solution and then washed with deionized water. A ZnSe single crystal with (1 1 1)A or (1 1 1)B face with dimensions of 5]5]1 mm3, 10]10]1 mm3 or 15]15]1 mm3 was prepared as a seed crystal from ZnSe single crystals grown by the recrystallization method developed by Terashima et al. [6]. The ZnSe polycrystal and seed crystal were baked in a quartz tube at 150°C for 2 h in a vacuum of less than 3]10~7 Torr. After baking, iodine was charged into the quartz tube under N flow with a humidity of less than 3%. The 2 quartz tube was then evacuated to less than 3]10~7 Torr and sealed off. During these processes after baking, the inner surface of the quartz tube, the polycrystal and seed crystal were not exposed to the atmosphere. Horizontal furnaces A and B, which were equipped with three and four heating elements, respectively, and a vertical furnace C with four heating elements were employed. In the case of furnace A or
61
B, ampoules were set in the furnace in such a way that a seed crystal was located at the upper part of the ampoule. The ampoule was placed in the furnace, where the seed temperature was about 10°C higher than the source temperature so that the seed surface was etched. After 3 h, the seed temperature was lowered to initiate the growth on the seed crystal. At the end of growth, ampoules were pulled out of the furnace after measuring the temperature profile along the ampoule periphery. 2.1.1. Growth temperature The dependence of the growth rate and the crystalline quality on the growth temperature was investigated in growth runs Nos. 1-A—D (see Table 1) in order to determine the optimum growth temperature. In a series of these growth experiments, an identical type-1 ampoule was repeatedly used for all the runs without breaking the ampoule after growth. In other words, the grown crystal was used as a seed crystal in the following growth experiment. The growth rate was monitored in-situ by measuring the shift of the center-of-gravity in the ampoule. The details of this growth rate monitor are described in our previous work [7]. For that purpose, furnace C was employed because a horizontal configuration was required for the in-situ growth rate monitor. The crystalline quality was evaluated by observing the surface morphology of the grown crystal from the outside of the quartz tube. 2.1.2. Seed orientation and ampoule structure The effect of the seed orientation associated with the angle h at the conical tip of the ampoule (see Fig. 1) on the morphological stability was examined. The combinations of seeds with (1 1 1)A face or (1 1 1)B face and the 30 or 60° angle at the conical tip were tested in growth runs Nos. 2—4 (see Table 1).
Fig. 1. Schematic illustration of the ampoules. Two types of ampoules (type 1 and type 2) are prepared. h indicates the angle at the conical tip.
2.1.3. Iodine concentration The effect of the total pressure, the sum of the partial pressures of ZnI , Se , I and I, in the 2 2 2 ampoule, on the growth rate and morphological stability was investigated, using type-1 and type-2 ampoules (see Table 2). In the case of type-1
62
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
Table 1 Growth conditions. Run Nos. 1-A—D are the experiments to determine the optimum growth temperature. Run Nos. 2—4 are the experiments to determine the optimum seed orientation associated with the angle h at the conical tip of the ampoule Run No.
Ampoule
Furnace
Seed orientation
Seed size (mm3)
Iodine Seed concentration temperature (mg/cm3) (°C)
Source temperature (°C)
Angle at conical tip (deg)
1-A 1-B 1-C 1-D 2 3 4
Type Type Type Type Type Type Type
C C C C A A A
(1 1 1)A (1 1 1)A (1 1 1)A (1 1 1)A (1 1 1)A (1 1 1)B (1 1 1)B
5]5]1 5]5]1 5]5]1 5]5]1 10]10]1 5]5]1 10]10]1
0.55 0.55 0.55 0.55 1.1 3.4 1.1
856 904 952 805 875 871 868
60 60 60 60 60 60 30
1 1 1 1 1 1 1
846 893 939 796 855 851 861
Table 2 Growth conditions of the experiments to determine the optimum iodine concentration Run No.
Ampoule
Furnace
Iodine concentration (mg/cm3)
5-A—D 6-A—H 7-A—D
Type 1 Type 1 Type 2
A B C
1.1—4.4 0.55—4.4
ampoules, the total pressure was estimated by the equilibrium condition, which satisfies the given iodine concentration, of the following two reactions: ZnSe(s)#I (g)"ZnI (g)#1Se (g), (1) 2 2 2 2 I (g)"2I(g). (2) 2 The equilibrium constants for the above reactions were estimated from thermodynamical data [8, 9] using the average temperature of seed temperature and source temperature. In the case of the type-2 ampoule, the total pressure was assumed to equal the iodine pressure at the reservoir temperature. The angle at the conical tip of the ampoules was 30° with an aperture diameter of 12 mm, and seeds with (1 1 1)B face measuring 15]15]1 mm3 were used. Furnace C for type-2 ampoules, was used with a subheater which controlled the temperature of iodine in the reservoir. The growth conditions such as ampoule structure, seed temperature and source temperature, except for iodine concentration, were the same in the series of growth experiments using
Reservoir temperature (°C)
Total pressure (atm)
Seed temperature (°C)
Source temperature (°C)
131—220
0.58—2.1 0.29—2.1 0.22—2.15
860 860 860
865 865 865
an identical furnace. Note that measuring of the temperature contains large systematic error, which was presumed of the order of 10°C, due to the indirect temperature measurement of seed and source using the thermocouple placed outside the ampoules. Therefore, it is senseless to compare the growth rates between the experiments using a different furnace. However, it is possible to compare the growth rates from the identical furnace, because the position of the ampoule and heater temperature were the same in the series of growth experiments using the same furnace. 2.2. Results and discussion 2.2.1. Growth temperature The growth rate dependence on the growth temperature are shown in Fig. 2. Starting from 850°C, the growth rate decreased with the increase of the growth temperature maintaining the growth of single crystals which were bounded by smooth symmetric facets, whereas polygonization occurred
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
63
Fig. 2. Dependence of growth rate and crystallinity on the growth temperature. These results are obtained using an identical ampoule. The arrows represent the order of the experiments. Solid circles (v) represent the growth of single crystal. Open circle (L) represents the growth of polycrystal.
with loosing smooth facets when the seed temperature was lowered to about 800°C. Therefore, the optimum growth temperature (seed temperature), which enables the growth of single crystal and high growth rate, is around 850°C. These results agree with Parker’s result that the growth temperature less than 830°C gave polycrystalline growth while temperature greater than 890°C gave no crystal growth [10]. 2.2.2. Seed orientation and ampoule structure Photograph of the crystals grown in growth experiments run Nos. 2—4 are shown in Fig. 3. The periphery of the crystal grown in run No. 2 is bounded by threefold facets with M1 0 0N and M1 1 1NB faces, however, the front of the crystal is not bounded by stable facets, where the formation of a prevailing (1 1 1)A facet had been expected, and substitutively a deep basin and some projections are observed there. The front of the crystal grown in Run No. 3 is bounded by a stable facet with (1 1 1)B face. The periphery of the front part neighboring the (1 1 1)B facet is also bounded by stable threefold M1 0 0N facets. However, the periphery of the tail part neighboring the seed crystal is bounded by out-developing thin plates with a M1 1 1NB facet and basins. As a result, the tail part
Fig. 3. Photograph of the grown ZnSe crystal grown in run Nos. 2—4 (upper row) and the schematic illustration of the facets and basins in each crystal (lower row). Smallest divisions of the scale represent 1 mm. The hatching in the illustration indicate basins.
of the crystal contains numerous voids. To the contrary, the tail part of the crystal grown in run No. 4 is bounded by a rough surface bordering the ampoule wall without forming voids. Since the front part of the crystal as well as the crystal grown in run No. 3 are bounded by stable facets, the whole crystal grown in run No. 4 is dense without voids. From these results, it is concluded that a seed with M1 1 1NB face should be used for the purpose of forming stable facets at the front of the crystal and a small angle at the conical tip should be adopted for the purpose of forming a dense tail part without voids. The same results were obtained in our many growth experiments, though only three typical examples are presented here. It is presumed that the morphological stability at the tail part is determined by the comparison between the angle at the conical tip and the angle of the out-developing thin plate. That is, if the angle at the conical tip is smaller than the angle of the out-developing plate (38.9°), out-developing thin plates do not form,
64
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
resulting in the stable growth with rough surface in the tail part. Koyama’s results [4], that polycrystals were obtained at an angle of 120°and single crystals were obtained at the angle of 60° in case of using a (1 1 1)A seed, can be similarly explained considering out-developing thin plates with a M1 0 0N facet. Therefore, the discrepancy in the optimum seed orientation among the previous works [4, 10, 11] can be completely accounted for by the growth rate and ampoule geometry, if very low growth rate enables the growth of single crystal on a (1 1 1)A seed. 2.2.3. Iodine concentration The obtained dependence of growth rate and morphological stability on the total pressure, which is equivalent to the iodine concentration, is shown in Fig. 4. The increase of the growth rate in region C can be accounted for by taking into account of the convective mass transport, assuming that mass transport limits the growth rate, since the thermal convection prevails at high pressure. It is generally noted that the convective mass transport causes the supersaturation in front of the crystal surface giving rise to random nucleation there, resulting in polycrystalline growth [12—15]. In our growth experiments, polycrystalline growth due to the random nucleation was prevented by using seed crystals even in region C. However, maintaining morphological stability in region C was unsuccessful, resulting in the formation of many voids. Furthermore, twin boundaries are frequently observed in the crystals grown in this region. Therefore, it is concluded that the growth conditions where the contribution of the convective mass transport is significant should be avoided in the growth of ZnSe by CVT even if seed crystal is used. The increase of growth rate in region A does not originate from the change of the convective mass transport, since the influence of the thermal convection is negligible even in region B where the thermal convection is stronger than in region A. That the influence of the deviation from stoichiometry of the gas in the ampoule or the influence of the oxygen from the source polycrystal [16], should both be more prominent under the condition of less mass of charged iodine, does not account for these results, because this increase was also observed in the series
Fig. 4. The dependence of growth rate and morphological stability on the total pressure in the ampoule. Symbols (v L) represent the results from run No. 5-A—D using furnace A. Symbols (r e) represent the results from run No. 6-A—H using furnace B. Symbols (j h) represent the results from run No. 7-A—D using furnace C. Solid symbols (v j r) indicate the growth of single crystal with stable facets. Open symbols (L h e) indicate unstable growth. Solid lines show the calculated curves taking account of only diffusive mass transport.
of growth experiments using the type-2 ampoules equipped with an iodine reservoir. The increase must, therefore, result from the increase of the diffusive mass transport. In order to confirm this assumption, the change of the diffusive mass transport against the change of the total pressure was estimated. The diffusive transport rate is approximately given by ¼AD I" (p (S)!p (C)), Z/IÈ ¸R¹ Z/IÈ
(3)
where ¼, A, D, ¸, R and ¹ represent the weight of ZnSe synthesized 1 mol ZnI , the cross section of 2 the ampoule, diffusion coefficient, the length between source and seed and temperature, respectively. Suffix S and C indicate source and seed, respectively. The diffusion coefficient D is expressed approximately by (see Ref. [17]) D"D (¹/¹ )1.8(p /p ), (4) 0 0 0 T where ¹ "273 K, p "1 atm and p is the total 0 0 T pressure in the ampoule. The partial pressures of ZnI at source and seed, p (S) and p (C), 2 Z/IÈ Z/IÈ
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
65
can be evaluated from Eqs. (1) and (2) within the constraint imposed by the given total pressure and temperature. The calculated results for three series of growth experiments are shown by solid lines in Fig. 4. Here, the diffusion coefficient D and the 0 temperature at seed were chosen in such a way that the calculated growth rates agreed with the experimental growth rates in region B. Since the calculated results also agree well with the experimental results in region A, the increase of the growth rate in region A is infallibly due to the increase of the diffusive mass transport. Therefore, the optimum iodine concentration should be situated in region B from the viewpoint of morphological stability and high growth rate.
3. Growth of large ZnSe single crystal The main issue in growing large ZnSe crystal by CVT is the suppression of the thermal convection, since enlargement of the diameter of the ampoule or the aperture at the seed crystal enhances the thermal convection as does the increase of the total pressure. In order to grow a ZnSe single crystal with a diameter of 1A, we used furnace A and type-1 ampoule with the same thermal conditions as used in run No. 5-A—D. The diameters of the seed crystal and the aperture at the seed were 27 mm and 23 m, respectively. The angle at the conical tip was 30° and the orientation of the seed crystal was (1 1 1)B. The determination of the iodine concentration was not so simple, since region B shrinks to a lower total pressure region by the extension of region C. Note that the boundary between region A and B is independent of the diameter of the ampoule or the aperture, since it is independent of the convective mass transport. Therefore, the iodine concentration of 1.1 mg/cm3, which corresponds to the lowest total pressure in region B, was chosen. In Fig. 5, the photographs of the grown crystal and the (1 0 0) polished wafer sawed from this crystal are presented. The crystal is a single crystal bounded by stable threefold facets. No voids or twin boundaries are observed and the whole crystal is transparent. The size of the grown crystal is 25 mm in diameter and 7 mm in length. This is the
Fig. 5. Photographs of a ZnSe single crystal with a diameter of 1A and (1 0 0) polished wafer.
largest ZnSe single crystal ever grown by CVT using iodine as transport agent.
4. Conclusions The optimum growth conditions of large ZnSe single crystals by CVT using iodine as transport agent were investigated from the viewpoint of high growth rate and morphological stability. The determined optimum growth conditions were as follows: f Growth temperature: near 850°C. f Seed orientation: (1 1 1)B with an angle of less than 30° at the conical tip. f Iodine concentration: near 1.1 mg/cm3. Although the previous work was confirmed only in terms of the growth temperature, we first determined the optimum seed orientation associated
66
S. Fujiwara et al. / Journal of Crystal Growth 186 (1998) 60—66
with the angle at the conical tip of the ampoule and discussed the optimum iodine concentration aiming to grow a larger ZnSe single crystal with morphological stability and high growth rate. ZnSe single crystal with a diameter of 1 in was successfully grown under the above growth conditions. The crystal is transparent, containing no voids or twin boundaries. References [1] R.M. Park, C.M. Rouleau, M.B. Toffer, T. Koyama, T. Yodo, J. Mater. Res. 5 (1990) 475. [2] D.B. Eason, Z. Yu, W.C. Hughes, W.H. Roland, C. Boney, J.W. Cook Jr., J.F. Schetzina, G. Cantwell, W.C. Harsch, Appl. Phys. Lett. 66 (1995) 115. [3] S. Fujita, H. Mimoto, H. Takebe, T. Noguchi, J. Crystal Growth 47 (1979) 326. [4] T. Koyama, T. Yodo, H. Oka, K. Yamashita, T. Yamasaki, J. Crystal Growth 91 (1988) 639. [5] T. Koyama, K. Yamashita, K. Kumata, J. Crystal Growth 96 (1989) 217.
[6] K. Terashima, M. Kawachi, M. Takena, J. Crystal Growth 102 (1990) 387. [7] S. Fujiwara, K. Matsumoto, T. Shirakawa, J. Crystal Growth 165 (1996) 169. [8] H. Hartmann, R. Mach, B. Selle, in: E. Kaldis (Ed.), Current Topics in Materials Science, vol. 9, North-Holland, Amsterdam, 1982, p. 373. [9] O. Kubaschewski, C.B. Alcock, in: G.V. Raynor (Ed.), Metallurgical Thermochemistry, vol. 24, Pergamon, Oxford, 1979, p. 364. [10] S.G. Parker, J. Crystal Growth 9 (1971) 177. [11] U. Geissler, H. Hartmann, E. Krause, D. Siche, J. Crystal Growth 159 (1996) 175. [12] K. Bo¨ttcher, H. Hartmann, J. Crystal Growth 146 (1995) 53. [13] Y. Noda, H. Yonekura, Y. Okunuki, S. Nakazawa, Mater. Trans. JIM 36 (1995) 1067. [14] J. Mercier, J. Crystal Growth 56 (1982) 235. [15] H. Wiedemeier, D. Chandra, F.C. Klaessig, J. Crystal Growth 51 (1980) 345. [16] W. Palosz, J. Crystal Growth 61 (1983) 412. [17] E. Kaldis, M. Piechotka, in: D.T.J. Hurle (Ed.), Handbook of Crystal Growth, vol. 2, North-Holland, Amsterdam, 1994, p. 618.