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The liquid surface tension as a factor influencing the VLS growth of silicon crystals
Materials Science and Engineering, 20 (1975) 171--177
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
The Liquid Surface Tension as a Factor Influencing the VLS Growth of Silicon Crystals
J. WEYHER Military Academy of Technology, Warsaw-Bemowo (Poland)
(Received February 12, 1975)
SUMMARY By means of measurements of the VL to LS interface areas ratio made after VLS silicon crystals growth the effect of the liquid phase composition and of its initial a m o u n t upon the liquid surface tension has been qualitatively determined. The differences in silicon crystal morphology, structure and growth kinetics observed after VLS processes depending on liquid surface tension values have been described and explained. Gold, platinum and a gold - platinum alloy were used as liquid forming agents. Silicon crystals with diameter ranging from about 1 to 140/am have been examined.
1. INTRODUCTION The liquid surface tension plays an important role in vapour - liquid - solid (VLS) growth. Depending on the proportions and equilibrium configurations of the surface tension forces 7VL, 7LS and 7vs (corresponding to specific free energies aVL, (~LS and a v s ) the differences in VLS crystal morphologies, structures and growth kinetics can be achieved. Wagner and Ellis [1] predicted and James and Lewis [2], as well as Filby and Nielsen [3], confirmed the dependence of the VLS growth of silicon layers or whiskers upon the wetting characteristic of liquid phase - substrate. Givargizov and Sheftal' [4] demonstrated the possibility of controlling the silicon crystal diameters by changing the contents of the vapor phase. Wagner and Doherty [ 5 ] and Wagner [6] have found that the cross-section shapes and, consequently, silicon crystal
morphologies depend on the growth temperature. Bootsma e t al. [7] have presented detailed considerations concerning the dependence of a- and/3-SIC crystal morphologies upon the interfacial equilibrium state of the vapor liquid - solid phases. After an analysis of the VLS growth rates of silicon and germanium, Bootsma and Gassen [8] have introduced the corrected accommodation coefficient a' which depends on the ratio of the V - L to L - S surfaces. Thornton et al. [9], Givargizov and Sheftal' [10] and Givargizov [11] have found that the periodic instability during Ge and Si whiskers growth is related to such experimental conditions as the gas stream velocity [9], temperature, supersaturation and surface active impurities [10,11]. These conditions influence the absolute value of the surface tension components 7LS and 7VL. According to Wagner [12] and Wagner and Doherty [13] the instability at the liquid solid interface results in branching, kinking and liquid phase entrapment during VLS silicon growth. Wagner and Ellis [1] determined the limitation of VLS whisker lower diameters as a consequence of the dependence of liquid surface energy upon the change of liquid droplet radius (the Gibbs - Thomson effect). Givargizov and Czernov [14] have given detailed data concerning the above mentioned effect in the case of VLS silicon growth with gold as liquid forming agent. From the above, as well as from the author's own investigations, it follows that the liquid surface tension can be controlled, during VLS growth, by: vapor phase composition; substrate type and its crystallographic orientation; growth temperature; liquid phase composition; and liquid phase initial amount.
172
The aim of this paper is to describe some observations concerning the effect o f the last three factors listed above upon the VLS silicon crystallization.
2. EXPERIMENTAL
The VLS crystallization was effected by means of radio-frequency heating inside a horizontal reaction tube having a silicon coated graphite susceptor. The hydrogen reduction of SiCla in an open system was applied, the hydrogen being purified in a palladium diffuser. Details concerning the geometry of the reaction zone have been published in Ref. 15. The experimental procedure preceding VLS growth was similar to those described in Refs. 1 and 5. Platinum and gold (i.e. the liquid-forming impurities) were prepared by slicing a wire 50 pm in diameter. The wire slices were then placed on conventionally cleaned and etched (111} single crystal silicon wafers (Czochralski grown, dislocation-free, p-type, ca. 20 o h m / c m ones). However, some of the silicon wafers were covered in a vacuum with 1000 A gold layers and then the platinum wire slices were placed on top so t h a t Au - Pt liquid agent could be formed. The growth processes took place at temperatures of 950 ° and 1050°C, the SIC14 to hydrogen mole ratio being 0.009 to 0.01. The total hydrogen flow rate was 800 cc/min and the growth time 1 - 2 hours. After the VLS growth the samples were examined by means of SEM, using the surface topography method. Measurements of the ratio of V - L to L - S interface areas were made on the enlarged SEM photographs of the alloy silicon crystal terminations. This part of the regrowth zone, which is formed on cooling down from the growth temperature to the room temperature, was taken into account, because the boundary between VLS grown silicon and silicon solidified on cooling corresponds to the L - S interface at the equilibrium state, that is when the liquid composition is CE [5].
3. RESULTS AND DISCUSSION
The liquid phase composition The typical pictures of the alloy terminations of VLS silicon crystals grown with gold, platinum
and gold - platinum alloys respectively are shown in Fig. 1. Crystal diameters d exceed 100 pm and differ slightly but it has been shown [15] that for d = 20 - 130 pm the ratio (FvL/FLS)Au-Si is constant and the ratio (FvL/ F L S ) P t - Si changes slightly (see Fig. 3), therefore the quantitative comparisons of this parameter for the respective alloy can be made. The comparisons show that in the diameter range in question, and for the growth temperature equal to 1050°C, the ratio (FvL/FLs)P t. Si is 1.3 to 1.5 times greater than (FvL/FLS)Au-Si. The examples of FvL/FLs values for the terminations of different compositions are tabulated in Table 1, Section A. From the above the following dependence is evident:
FLs] Au-
Si •
\ F L s / A u - Pt- Si < \ FLS ] Pt- Si "
(1) It has been concluded in the earlier paper [16] that for the three-component (Au - Pt - Si) alloy used for VLS silicon growth, continuous change in the alloy composition within the range determined by the phase diagram is possible. Therefore, a continuous change of the liquid surface tension within the range described by the inequality (1) may be obtained.
Growth temperature Growth temperature is an effective factor influencing the liquid surface tension and it is easy to control. Figure 2 shows the two alloy (Au - Si) and (Au - Pt - Si) terminations which " s e e d e d " VLS silicon growth in 950°C. The ratios F v L / F L s in those two cases are given in Table 1, Section B. From Table 1 it could be calculated that lowering the growth temperature by 100 deg C results in 22% growth of FvL/FLs for gold - silicon alloy, and for gold platinum - silicon alloy, in a growth not smaller than 40%.
Liquid phase amount From detailed investigations made on the alloy Si crystal terminations of different diameters it has been found that higher FvL/FLs values correspond to lower crystal diameters (especially for d smaller than 10 pm). The results of measurements of platinum - silicon terminations are summed up in the graph presented in Fig. 3, while some of the terminations are shown in Fig. 4. For crystals with diameters
173
(c)
(a)
Fig. 1. The alloy t e r m i n a t i o n s o f d i f f e r e n t c o m p o s i t i o n a f t e r VLS silicon g r o w t h at 1 0 5 0 ° C : (a) Au - Si alloy, m a g n i f i c a t i o n 640x, SEM; (b) Pt - Si alloy, m a g n i f i c a t i o n 8 0 0 x , SEM; (c) Au - Pt - Si alloy, m a g n i f i c a t i o n 8 0 0 x , SEM.
smaller than 0.7 pm the Gibbs - Thomson effect was detected. These observations concerning the critical droplet diameter are in good agreement with analytical [1] and experimental [14] values. On the other hand the assumption of constant parameter FvL/FLs a s a function of crystal diameters made in Refs. 14 and 17 does not seem to be correct, which is evident from Fig. 3. Undoubtedly the measurements of FvL/FLs (for d not exceeding 5 pm) on SEM photographs may be made with an accuracy of about 50% of the calculated value but even such a greater scatter makes no change in the nature of the curve shown in Fig. 3. (b)
TABLE 1 Characteristics o f VLS g r o w n silicon crystal alloy t e r m i n a t i o n s
Growth temperature
°C
Liquid c o m p o s i t i o n
FvL/FLs Crystal d i a m e t e r d Figure
pm
Section A
Section B
1050
950
Au - Si
Pt - Si
Au - Pt - Si
Au - Si
Au - Pt - Si
2.0
2.8
2.5
2.4
3.8
120
105
103
100
97
la
lb
lc
2a
2b
174
FvL
rLs
Crystal diameter d [pm']
Fig. 3. R e l a t i o n b e t w e e n t e r m i n a t i o n g e o m e t r y ( F v L / FLS ) and crystal d i a m e t e r for V L S silicon g r o w t h w i t h platinum.
Ca)
(a)
(b) Fig. 2. The alloy terminations of different composition after V L S silicon growth at 950°C: (a) A u - Si alloy, magnification 960x, S E M ; (b) A u - Pt - Si alloy, magnification 800x, S E M .
The role o f the liquid surface tension in VLS grow th The different values of the liquid surface tension play a significant role for VLS silicon growth since they cause the variations in: crystal morphology, structure (or structure perfection) and growth kinetics. The liquid surface tension values become higher when the growth temperature is being gradually reduced; the slhcon crystal cross-section changes from a 6-sided to a 12-sided one, when durinK VLS
iiiiiii:!~: ilii!ii!iil
Fig. 4(a,b)
(b)
175
When silicon crystals grow with gold and platinum under the same experimental conditions their morphologies are always slightly different. It follows from the inequality (1): ( T L S ) P t - Si > (TLS)Au - Si
(2)
and this fact accounts for formation of the (311) faces at the base of platinum grown crystals (see Fig. 6a), while by using gold these faces do not appear (see Fig. 6b). In the writer's opinion the above relation gives rise to the tendency of Pt - Si entrapment formation, especially at the early stage of VLS
(c) Fig. 4. Silicon crystal t e r m i n a t i o n s o f d i f f e r e n t diameters a f t e r VLS g r o w t h w i t h p l a t i n u m : (a) d = 19.7 p m FvL/FLs = 3.1, SEM; (b) d = 5 p m , F v L / F L s = 3.4, SEM; (c) d = 2.2 p m , F v L / F L s = 4.2, SEM.
crystallization with gold - platinum alloy the temperature is being reduced from 1050 ° to 950°C (see Fig. 5). The same type of changes has been described for silicon growth with gold [6].
(a)
Fig. 5. C h a n g e o f crystal m o r p h o l o g y d u r i n g c o n t i n u o u s decrease o f t h e V L S g r o w t h t e m p e r a t u r e f r o m 1 0 5 0 ° to 950°C. Gold - p l a t i n u m liquid f o r m i n g agent. Magn i f i c a t i o n 1 8 0 x , SEM.
(b) Fig. 6. Silicon crystal m o r p h o l o g i e s a f t e r VLS g r o w t h at 1 0 5 0 ° C w i t h : (a) p l a t i n u m ; (b) gold; SEM, ~ = 30 40 ° .
176
growth. Wagner [6,12,13] discussed the reasons for entrapment formation and pointed out that most of the silicon crystals which had been grown with gold were perfect [12]. On the other hand all sectioned silicon crystals with diameters of 80 - 150 pm grown with platinum had entrapments of different sizes, although they were morphologically perfect and had grown with a relatively low SIC14 to hydrogen mole ratio. One of such crystals which contains an entrapment at the base is shown in Fig. 7. The analysis of the scheme shown in Fig. 8 enables one to explain the entrapment formation. During VLS growth there occurs a change from the A growth stage to the D one in the configuration of the surface tension forces as well as in their absolute values. In addition, in the (B - C) stage, with the decrease of the crosssection of the growing crystal the linear VLS velocity rises. Simultaneously there is an increase of the FvL/Fvs parameter which may cause a local liquid supersaturation in the vicinity of the VL interface. As a result the
Fig. 7. Etqlhed {110} section of a VLS crystal grown with platinum at 1050°C.
/
Z~L
~'vs
A
B-C
D
Fig. 8. Configuration of the surface tension forces for Pt - Si alloy termination in the successive VLS growth stages.
external part of the crystal grows faster, the solid - liquid interface becomes nonplanar and an entrapment is formed. Therefore the (B - C) stage is the critical m o m e n t for entrapment formation. For the gold - silicon alloy, for the growth temperature of 1050°C and d = 10 - 140 pm the FvL/FLs parameter does not exceed 2.2 (see Table 1). This factor, in addition to the small VLS growth rate, prevents the entrapment formation, this small growth rate being characteristic of the VLS silicon growth with gold [15]. The liquid surface tension also seems to be the main factor which differentiates VLS growth rate, depending on crystal diameters; " t h i n " crystals, with higher FvL/Fbs parameter, grow faster than the " t h i c k " ones (see Fig. 9). The enlarged pictures of the terminations
Fig. 9. The VLS growth rates of silicon crystals of different diameters, SEM.
]77
marked in Fig. 9 with cipher 1 to 4 are shown in Figs. l b and 4a-c respectively; in the case of crystal number 5 (Fig. 9) the Gibbs - Thomson effect stopped the VLS process. Obviously crystals of the same diameters but of different liquid phase compositions grow with different VLS rates [15]; however, the quantitative comparisons are difficult because of the different platinum and gold catalytic effect upon the reaction in the vapor phase and therefore upon the silicon deposition rates [ 1 ].
4. CONCLUSIONS
The differences in VLS silicon crystal structures, morphologies and growth kinetics, depending on applied liquid forming agent, have been revealed. These differences result from the different values of the liquid surface tension, the values being expressed by the FvL/ FLS parameter. This parameter can be changed either continuously or by steps, by the liquid phase composition and by its amount. Experiments were carried out using gold, platinum and a gold - platinum alloy as liquid forming impurities. The measurements of FvL/FLs (for the constant liquid phase composition) have shown the dependence of the liquid surface tension upon the crystal diameter. For the VLS silicon growth with gold and platinum as impurities the liquid surface tension increases rapidly for the crystals with diameters not exceeding 10 pm.
ACKNOWLEDGEMENTS
The author is indebted to Professor S. Wojciechowski for making valuable sug-
gestions which have been incorporated into the text; the author is also grateful to Professor B. Ciszewski for his advice and to Mrs. M. Pawlowska for her kind assistance in SEM investigation.
REFERENCES 1 R.S. Wagner and W.C. Ellis, Trans. AIME, 233 (1965) 1053. 2 D.W.F. James and C. Lewis, Brit. J. Appl. Phys., 16 (1965) 1089. 3 J.D. Filby and S. Nielsen, Brit. J. Appl. Phys., 17 (1966) 81. 4 E.I. Givargizov and N.N. Sheftal', J. Crystal Growth, 9 (1971) 326. 5 R.S. Wagner and C.J. Doherty, J. Electrochem. Soc., 113 (1966) 1300. 6 R.S. Wagner, J. Appl. Phys., 38 (1967) 1554. 7 G.A. Bootsma, W.F. Knippenberg and G. Verspui, J. Crystal Growth, 11 (1971) 297. 8 G.A. Bootsma and H.J. Gassen, J. Crystal Growth, 10 (1971) 223. 9 P.R. Thornton, D.W.F. James, C. Lewis and A. Bradford, Phil. Mag., 14 (1966) 165. 10 E.I. Givargizov and N.N. Sheftal', Krist. Tech., 7 (1972) 37. 11 E.I. Givargizov, J. Crystal Growth, 20 (1973) 217. 12 R.S. Wagner, Crystal Growth, Pergamon Press, Oxford and New York, 1967, 347. 13 R.S. Wagner and C.J. Doherty, J. Electrochem. Soc., 115 (1968) 93. 14 E.I. Givargizov and A.A. Chernov, Krystallografiya,
18 (1973) 147. 15 J. Weyher, Doctor Thesis, Military Academy of Technology, Warsaw-Bemowo, 1974. 16 L. Kaczyfiski and J. Weyher, Paper presented at VII Intern. Conf. on X-Ray Optics and Microanalysis, Moscow-Kiev, July 1974, to be published. 17 E.I. Givargizov and Yu.G. Kostjuk, Rost Kristallov (Crystal Growth), Nauka, Moscow, 9 (1972) 242.
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
The Liquid Surface Tension as a Factor Influencing the VLS Growth of Silicon Crystals
J. WEYHER Military Academy of Technology, Warsaw-Bemowo (Poland)
(Received February 12, 1975)
SUMMARY By means of measurements of the VL to LS interface areas ratio made after VLS silicon crystals growth the effect of the liquid phase composition and of its initial a m o u n t upon the liquid surface tension has been qualitatively determined. The differences in silicon crystal morphology, structure and growth kinetics observed after VLS processes depending on liquid surface tension values have been described and explained. Gold, platinum and a gold - platinum alloy were used as liquid forming agents. Silicon crystals with diameter ranging from about 1 to 140/am have been examined.
1. INTRODUCTION The liquid surface tension plays an important role in vapour - liquid - solid (VLS) growth. Depending on the proportions and equilibrium configurations of the surface tension forces 7VL, 7LS and 7vs (corresponding to specific free energies aVL, (~LS and a v s ) the differences in VLS crystal morphologies, structures and growth kinetics can be achieved. Wagner and Ellis [1] predicted and James and Lewis [2], as well as Filby and Nielsen [3], confirmed the dependence of the VLS growth of silicon layers or whiskers upon the wetting characteristic of liquid phase - substrate. Givargizov and Sheftal' [4] demonstrated the possibility of controlling the silicon crystal diameters by changing the contents of the vapor phase. Wagner and Doherty [ 5 ] and Wagner [6] have found that the cross-section shapes and, consequently, silicon crystal
morphologies depend on the growth temperature. Bootsma e t al. [7] have presented detailed considerations concerning the dependence of a- and/3-SIC crystal morphologies upon the interfacial equilibrium state of the vapor liquid - solid phases. After an analysis of the VLS growth rates of silicon and germanium, Bootsma and Gassen [8] have introduced the corrected accommodation coefficient a' which depends on the ratio of the V - L to L - S surfaces. Thornton et al. [9], Givargizov and Sheftal' [10] and Givargizov [11] have found that the periodic instability during Ge and Si whiskers growth is related to such experimental conditions as the gas stream velocity [9], temperature, supersaturation and surface active impurities [10,11]. These conditions influence the absolute value of the surface tension components 7LS and 7VL. According to Wagner [12] and Wagner and Doherty [13] the instability at the liquid solid interface results in branching, kinking and liquid phase entrapment during VLS silicon growth. Wagner and Ellis [1] determined the limitation of VLS whisker lower diameters as a consequence of the dependence of liquid surface energy upon the change of liquid droplet radius (the Gibbs - Thomson effect). Givargizov and Czernov [14] have given detailed data concerning the above mentioned effect in the case of VLS silicon growth with gold as liquid forming agent. From the above, as well as from the author's own investigations, it follows that the liquid surface tension can be controlled, during VLS growth, by: vapor phase composition; substrate type and its crystallographic orientation; growth temperature; liquid phase composition; and liquid phase initial amount.
172
The aim of this paper is to describe some observations concerning the effect o f the last three factors listed above upon the VLS silicon crystallization.
2. EXPERIMENTAL
The VLS crystallization was effected by means of radio-frequency heating inside a horizontal reaction tube having a silicon coated graphite susceptor. The hydrogen reduction of SiCla in an open system was applied, the hydrogen being purified in a palladium diffuser. Details concerning the geometry of the reaction zone have been published in Ref. 15. The experimental procedure preceding VLS growth was similar to those described in Refs. 1 and 5. Platinum and gold (i.e. the liquid-forming impurities) were prepared by slicing a wire 50 pm in diameter. The wire slices were then placed on conventionally cleaned and etched (111} single crystal silicon wafers (Czochralski grown, dislocation-free, p-type, ca. 20 o h m / c m ones). However, some of the silicon wafers were covered in a vacuum with 1000 A gold layers and then the platinum wire slices were placed on top so t h a t Au - Pt liquid agent could be formed. The growth processes took place at temperatures of 950 ° and 1050°C, the SIC14 to hydrogen mole ratio being 0.009 to 0.01. The total hydrogen flow rate was 800 cc/min and the growth time 1 - 2 hours. After the VLS growth the samples were examined by means of SEM, using the surface topography method. Measurements of the ratio of V - L to L - S interface areas were made on the enlarged SEM photographs of the alloy silicon crystal terminations. This part of the regrowth zone, which is formed on cooling down from the growth temperature to the room temperature, was taken into account, because the boundary between VLS grown silicon and silicon solidified on cooling corresponds to the L - S interface at the equilibrium state, that is when the liquid composition is CE [5].
3. RESULTS AND DISCUSSION
The liquid phase composition The typical pictures of the alloy terminations of VLS silicon crystals grown with gold, platinum
and gold - platinum alloys respectively are shown in Fig. 1. Crystal diameters d exceed 100 pm and differ slightly but it has been shown [15] that for d = 20 - 130 pm the ratio (FvL/FLS)Au-Si is constant and the ratio (FvL/ F L S ) P t - Si changes slightly (see Fig. 3), therefore the quantitative comparisons of this parameter for the respective alloy can be made. The comparisons show that in the diameter range in question, and for the growth temperature equal to 1050°C, the ratio (FvL/FLs)P t. Si is 1.3 to 1.5 times greater than (FvL/FLS)Au-Si. The examples of FvL/FLs values for the terminations of different compositions are tabulated in Table 1, Section A. From the above the following dependence is evident:
FLs] Au-
Si •
\ F L s / A u - Pt- Si < \ FLS ] Pt- Si "
(1) It has been concluded in the earlier paper [16] that for the three-component (Au - Pt - Si) alloy used for VLS silicon growth, continuous change in the alloy composition within the range determined by the phase diagram is possible. Therefore, a continuous change of the liquid surface tension within the range described by the inequality (1) may be obtained.
Growth temperature Growth temperature is an effective factor influencing the liquid surface tension and it is easy to control. Figure 2 shows the two alloy (Au - Si) and (Au - Pt - Si) terminations which " s e e d e d " VLS silicon growth in 950°C. The ratios F v L / F L s in those two cases are given in Table 1, Section B. From Table 1 it could be calculated that lowering the growth temperature by 100 deg C results in 22% growth of FvL/FLs for gold - silicon alloy, and for gold platinum - silicon alloy, in a growth not smaller than 40%.
Liquid phase amount From detailed investigations made on the alloy Si crystal terminations of different diameters it has been found that higher FvL/FLs values correspond to lower crystal diameters (especially for d smaller than 10 pm). The results of measurements of platinum - silicon terminations are summed up in the graph presented in Fig. 3, while some of the terminations are shown in Fig. 4. For crystals with diameters
173
(c)
(a)
Fig. 1. The alloy t e r m i n a t i o n s o f d i f f e r e n t c o m p o s i t i o n a f t e r VLS silicon g r o w t h at 1 0 5 0 ° C : (a) Au - Si alloy, m a g n i f i c a t i o n 640x, SEM; (b) Pt - Si alloy, m a g n i f i c a t i o n 8 0 0 x , SEM; (c) Au - Pt - Si alloy, m a g n i f i c a t i o n 8 0 0 x , SEM.
smaller than 0.7 pm the Gibbs - Thomson effect was detected. These observations concerning the critical droplet diameter are in good agreement with analytical [1] and experimental [14] values. On the other hand the assumption of constant parameter FvL/FLs a s a function of crystal diameters made in Refs. 14 and 17 does not seem to be correct, which is evident from Fig. 3. Undoubtedly the measurements of FvL/FLs (for d not exceeding 5 pm) on SEM photographs may be made with an accuracy of about 50% of the calculated value but even such a greater scatter makes no change in the nature of the curve shown in Fig. 3. (b)
TABLE 1 Characteristics o f VLS g r o w n silicon crystal alloy t e r m i n a t i o n s
Growth temperature
°C
Liquid c o m p o s i t i o n
FvL/FLs Crystal d i a m e t e r d Figure
pm
Section A
Section B
1050
950
Au - Si
Pt - Si
Au - Pt - Si
Au - Si
Au - Pt - Si
2.0
2.8
2.5
2.4
3.8
120
105
103
100
97
la
lb
lc
2a
2b
174
FvL
rLs
Crystal diameter d [pm']
Fig. 3. R e l a t i o n b e t w e e n t e r m i n a t i o n g e o m e t r y ( F v L / FLS ) and crystal d i a m e t e r for V L S silicon g r o w t h w i t h platinum.
Ca)
(a)
(b) Fig. 2. The alloy terminations of different composition after V L S silicon growth at 950°C: (a) A u - Si alloy, magnification 960x, S E M ; (b) A u - Pt - Si alloy, magnification 800x, S E M .
The role o f the liquid surface tension in VLS grow th The different values of the liquid surface tension play a significant role for VLS silicon growth since they cause the variations in: crystal morphology, structure (or structure perfection) and growth kinetics. The liquid surface tension values become higher when the growth temperature is being gradually reduced; the slhcon crystal cross-section changes from a 6-sided to a 12-sided one, when durinK VLS
iiiiiii:!~: ilii!ii!iil
Fig. 4(a,b)
(b)
175
When silicon crystals grow with gold and platinum under the same experimental conditions their morphologies are always slightly different. It follows from the inequality (1): ( T L S ) P t - Si > (TLS)Au - Si
(2)
and this fact accounts for formation of the (311) faces at the base of platinum grown crystals (see Fig. 6a), while by using gold these faces do not appear (see Fig. 6b). In the writer's opinion the above relation gives rise to the tendency of Pt - Si entrapment formation, especially at the early stage of VLS
(c) Fig. 4. Silicon crystal t e r m i n a t i o n s o f d i f f e r e n t diameters a f t e r VLS g r o w t h w i t h p l a t i n u m : (a) d = 19.7 p m FvL/FLs = 3.1, SEM; (b) d = 5 p m , F v L / F L s = 3.4, SEM; (c) d = 2.2 p m , F v L / F L s = 4.2, SEM.
crystallization with gold - platinum alloy the temperature is being reduced from 1050 ° to 950°C (see Fig. 5). The same type of changes has been described for silicon growth with gold [6].
(a)
Fig. 5. C h a n g e o f crystal m o r p h o l o g y d u r i n g c o n t i n u o u s decrease o f t h e V L S g r o w t h t e m p e r a t u r e f r o m 1 0 5 0 ° to 950°C. Gold - p l a t i n u m liquid f o r m i n g agent. Magn i f i c a t i o n 1 8 0 x , SEM.
(b) Fig. 6. Silicon crystal m o r p h o l o g i e s a f t e r VLS g r o w t h at 1 0 5 0 ° C w i t h : (a) p l a t i n u m ; (b) gold; SEM, ~ = 30 40 ° .
176
growth. Wagner [6,12,13] discussed the reasons for entrapment formation and pointed out that most of the silicon crystals which had been grown with gold were perfect [12]. On the other hand all sectioned silicon crystals with diameters of 80 - 150 pm grown with platinum had entrapments of different sizes, although they were morphologically perfect and had grown with a relatively low SIC14 to hydrogen mole ratio. One of such crystals which contains an entrapment at the base is shown in Fig. 7. The analysis of the scheme shown in Fig. 8 enables one to explain the entrapment formation. During VLS growth there occurs a change from the A growth stage to the D one in the configuration of the surface tension forces as well as in their absolute values. In addition, in the (B - C) stage, with the decrease of the crosssection of the growing crystal the linear VLS velocity rises. Simultaneously there is an increase of the FvL/Fvs parameter which may cause a local liquid supersaturation in the vicinity of the VL interface. As a result the
Fig. 7. Etqlhed {110} section of a VLS crystal grown with platinum at 1050°C.
/
Z~L
~'vs
A
B-C
D
Fig. 8. Configuration of the surface tension forces for Pt - Si alloy termination in the successive VLS growth stages.
external part of the crystal grows faster, the solid - liquid interface becomes nonplanar and an entrapment is formed. Therefore the (B - C) stage is the critical m o m e n t for entrapment formation. For the gold - silicon alloy, for the growth temperature of 1050°C and d = 10 - 140 pm the FvL/FLs parameter does not exceed 2.2 (see Table 1). This factor, in addition to the small VLS growth rate, prevents the entrapment formation, this small growth rate being characteristic of the VLS silicon growth with gold [15]. The liquid surface tension also seems to be the main factor which differentiates VLS growth rate, depending on crystal diameters; " t h i n " crystals, with higher FvL/Fbs parameter, grow faster than the " t h i c k " ones (see Fig. 9). The enlarged pictures of the terminations
Fig. 9. The VLS growth rates of silicon crystals of different diameters, SEM.
]77
marked in Fig. 9 with cipher 1 to 4 are shown in Figs. l b and 4a-c respectively; in the case of crystal number 5 (Fig. 9) the Gibbs - Thomson effect stopped the VLS process. Obviously crystals of the same diameters but of different liquid phase compositions grow with different VLS rates [15]; however, the quantitative comparisons are difficult because of the different platinum and gold catalytic effect upon the reaction in the vapor phase and therefore upon the silicon deposition rates [ 1 ].
4. CONCLUSIONS
The differences in VLS silicon crystal structures, morphologies and growth kinetics, depending on applied liquid forming agent, have been revealed. These differences result from the different values of the liquid surface tension, the values being expressed by the FvL/ FLS parameter. This parameter can be changed either continuously or by steps, by the liquid phase composition and by its amount. Experiments were carried out using gold, platinum and a gold - platinum alloy as liquid forming impurities. The measurements of FvL/FLs (for the constant liquid phase composition) have shown the dependence of the liquid surface tension upon the crystal diameter. For the VLS silicon growth with gold and platinum as impurities the liquid surface tension increases rapidly for the crystals with diameters not exceeding 10 pm.
ACKNOWLEDGEMENTS
The author is indebted to Professor S. Wojciechowski for making valuable sug-
gestions which have been incorporated into the text; the author is also grateful to Professor B. Ciszewski for his advice and to Mrs. M. Pawlowska for her kind assistance in SEM investigation.
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