- Email: [email protected]
LPE growth of silicon by yo-yo solute feeding method
598
Journal of Crystal Growth 108 (1991) 598-602 North-Holland
LPE growth of silicon by yo-yo solute feeding method Tokuzo Sukegawa, Masakazu Kimura and Akira Tanaka Research Institute of Electronics, Shizuoka University, Johoku 3-5-I, Hatnamatsu 432, Japan
Received 20 June 1990; manuscript received in final form 13 August 1990
Stripe lines were observed on stained cross-sections of epitaxial layers of silicon grown by yo-yo solute feeding method using indium solvent. These stripe lines were found to be flat and parallel to the interface between the epitaxial layer and the substrate. The origin of these lines is discussed and it is made clear that the growth occurs in each cycle of yo-yo temperature repetitions.
1. Infroduction The chemical vapor deposition of epitaxial layers of silicon has been extensively studied. In contrast, little work has been reported on the epitaxial growth of silicon from the liquid phase [1—4].Growth of epitaxial layers from the liquid phase has the advantage of a lower growth ternperature than vapor phase epitaxy. This is important in the fabrication of silicon devices because this reduces junction deformation during growth, and also allows growth of layers with an abrupt interface between the epitaxial layer and the substrate. In addition, this technology has the unique attribute of complete suppression of autodoping [21. We have investigated the gravity effect on solute transport during crystal growth and dissolution using a horizontal substrate—solution—substrate sandwich system. With indium solvent, it was found that dissolution of silicon occurred mainly on the lower substrate, while growth on the upper substrate was larger than that on the lower substrate under near isothermal conditions [5]. Based on these phenomena, we have developed a new growth technique, called yo-yo solute feeding method [6]. This method has many applications for the fabrication of semiconductor devices [7]. In the yo-yo method, solute transport is not caused by the spatial temperature gradient, but by the 0022-0248/91/$03.50 © 1991
—
temperature modulation. Corresponding to the repetitions of the temperature cycle, the epitaxial layer with a desirable thickness can be grown. Recently, we have found that the stripe lines were revealed on the cross-sections of epitaxial layers after stain etching. These stripe lines were clearly observed, especially in the case of nondoped growth using indium solvent. In this paper, the origin of the observed lines is discussed. Further, it will be shown that the growth occurs in each cycle of the yo-yo temperature repetitions.
2. Experimental procedure All epitaxial growth was performed using indium as solvent in the Pd-diffused H2. The specific gravity of indium at the growth temperature (974— 3 994°C) estimated at approximately g/cm [8]. Thisisvalue is about 2.5 times larger6.45 than that of silicon (2.51 g/cm3). Details of the growth system and the growth procedure are described in ref. [6]. The upper substrate and lower source crystal are horizontally set face to face with a gap of 4 mm in a graphite boat. Prior to each experiment, the solution was saturated with silicon at 994°C. In this study, (111) or (100) oriented 2 x 2 cm2 silicon substrates were used. The growth experiments were conducted as follows. The temperature of substrate, source and the
Elsevier Science Publishers B.V. (North-Holland)
T. Sukegawa et al.
/ LPE
I-
____________________________________ __________
W —
—~
-—--
— -
—
~
599
growth of Si by vo-yo solute feeding method
solution was uniformly increased to the initial temperature (994°C). The indium solution saturated with silicon was then inserted between the substrate and source crystal and the temperature was modulated up and down periodically with an appropriate amplitude under isothermal conditions. In this experiment, the high and low temperature levels were maintained at 994 and 974°C, respectively. The cooling rate of each cycle was 0.33°C/mm and the period of one cycle was 170 mm. As reported before, the layer thickness per cycle is approximately 15 p.m/cycle using this condition [6]. The cross-section of the grown layers was prepared for microscopic observation by lapping, polishing and stain etching.
~
Substrate
3. Results and discussion
-~
~ _______
____________________________________________ _____
Fig. 1. Cross-sectional views of epitaxial layers grown on (111) substrate (a) and (100) substrate (b). Stripe lines were revealed by stain etching. In both samples, the epitaxial growths, were conducted by the yo-yo solute feeding method using indium solvent with 10 repetition times of the temperature cycle (yo-yo
times). Marker represents 50 ~sm.
When the grown layers were stain etched, the stripe lines were observed. Fig. 1 shows the typical cross-sectional views of epitaxial layers on the (111) and (100) substrates. In both cases, the stripe lines are very flat and parallel to the interface between the epitaxial layer and the substrate. Further, the number of stripe lines are in exact accordance with the repetition times of the temperature cycle (yo-yo times). Fig. 2 shows the relation between the spreading resistance and the stripe lines. The sample was grown on p~(111) substrate with 20 yo-yo times. Corresponding to the stripe lines, the spreading resistance is slightly changing periodically. Namely, it is thought that these stripe lines caused by the change of indium concentration in the grown layer. According to the yo-yo temperature cycle, solid solubility of indium in silicon is changing, as shown in fig. 3a. Therefore the concentration of indium is changing along the growth direction (fig. 3b). The distances between the lines are almost equal (about 15 j.tm). This implies that the growth occurs in each temperature cycle. We have conducted the same measurement for the heavily phosphorus-doped sample. However, a change of spreading resistance was not observed in this case, stnce the difference of solid solubility of phos.
.
.
.
phorus in silicon between 994—974°C is very small [9]. For the device fabrication, this result is useful.
600
T. Sukegawa et al.
/ LPE growth of Si by yo-yo solute feeding method
Fig. 4 shows the cross-sectional views of the epitaxial layer with 1, 2 and 3 yo-yo repetitions. Obviously, the layers corresponding to the yo-yo repetitions can be observed in each photograph. In our growth system, it is impossible to remove the substrates from the solution. Therefore, the layers
grown during the cooling process to room temperature are also observed. The layer thickness for one yo-yo process is nearly equal to the averaged layer thickness per cycle described before. This value is in fairly good agreement with the value of 17 p.m calculated from the solubility difference of
0~
I 102
.
-
101
Epitaxal Layer
Substrate
non—dopedn—type
0
I 50
I 100
I 150
I 200
I 250
DEPTH (pm)
Fig. 2. Relation between the stripe lines and spreading resistance.
300
T Sukegawa et al.
/ LPE growth of Si by yo-yo solute feeding method
silicon in indium corresponding to 994—974°C. This means that solute transport to the growing interface of the upper substrate is sufficiently fast.
.
601
.
~
Epi Layer
4. Conclusion Stripe lines were observed in the grown layer by stain etching. From the measurement of the spreading resistance, it is concluded that these stripe lines show the grown layers corresponding to the yo-yo repetitions of the temperature cycle. Namely, the growth occurs in each cycle of yo-yo repetitions. Further, there was not much difference of growth thickness between (111) and (100) substrates. Under these growth conditions, the growth seems to be limited by supply rate of silicon from the solution. Using this method, an
a
Substrate
—
2nd Layer * —
Substrate
5Opm Growth
(a
_______________
V
~
~ T~ Temperature
>
—
•~ *~‘
_____
~_~~ri-~
In Conc
b
Ci
Ch~
~ —
____2nc
_____
~
Stripe Line
Layer Layer
Substrate
/
...
/
7—~
Fig. 4. Cros~-~ectionaI~ie~ s of the gro~n L~erwiih I (a), 2 (b) and 3 (c) yo-yo repetitions of the temperature cycle.
1 yo-yo Layer
//
Growth Direction Fig. 3. The change of solid solubility of indium in silicon during growth (a) and schematic diagram for the explanation of the relation between the stripe lines and the concentration of indium in grown layer (b). In this figure, T 5 and T1 indicate the high and low temperature levels of the yo-yo process, respectively. Ch and C1 show the solid solubility of indium in silicon corresponding to T5 and T,.
epitaxial layer with a desirable thickness can be grown by optimizing the temperature program. It should be noted that these experiments were performed without externally applied temperature difference. Acknowledgments .
The authors wish to thank Dr. Chikao Kimura of New Japan Radio Co., Ltd., for valuable dis-
602
T. Sukegawa et al.
/ LPE growth of Si by yo-yo solute feeding method
cussions. This work was supported in part by a Grant-in-Aid scientific Research from the Ministry of Education, Science and Culture, Japan.
References [1] B.J. Baliga, J. Crystal Crowth 41(1977)199. [2] B.J. Baliga, J. Electrochem. Soc. 133 (1986) SC. [3] Y. Suzuki and T. Nishinaga, Japan. J. Appl. Phys. 28 (1989) ~iz~o.
[41Y.
Suzuki, T. Nishinaga and T. Sanada, J. Crystal Growth 99 (1990) 229. [5] M. Kimura, A. Tanaka and T. Sukegawa, J. Crystal Growth 99 (1990) 1295. [6] T. Sukegawa, M. Kimura and A. Tanaka. J. Crystal Growth 92 (1988) 46. [7] T. Sukegawa, M. Kimura and A. Tanaka, IEEE Electron Device Letters EDL-l0 (1989) 20. [8] C.J. Smithells, Metals Reference Book, 5th ed. (Butterworths, London, 1976) p. 945. [9] T. Sukegawa, M. Kimura and A. Tanaka, J. Crystal Growth 96 (1989) 584.
Journal of Crystal Growth 108 (1991) 598-602 North-Holland
LPE growth of silicon by yo-yo solute feeding method Tokuzo Sukegawa, Masakazu Kimura and Akira Tanaka Research Institute of Electronics, Shizuoka University, Johoku 3-5-I, Hatnamatsu 432, Japan
Received 20 June 1990; manuscript received in final form 13 August 1990
Stripe lines were observed on stained cross-sections of epitaxial layers of silicon grown by yo-yo solute feeding method using indium solvent. These stripe lines were found to be flat and parallel to the interface between the epitaxial layer and the substrate. The origin of these lines is discussed and it is made clear that the growth occurs in each cycle of yo-yo temperature repetitions.
1. Infroduction The chemical vapor deposition of epitaxial layers of silicon has been extensively studied. In contrast, little work has been reported on the epitaxial growth of silicon from the liquid phase [1—4].Growth of epitaxial layers from the liquid phase has the advantage of a lower growth ternperature than vapor phase epitaxy. This is important in the fabrication of silicon devices because this reduces junction deformation during growth, and also allows growth of layers with an abrupt interface between the epitaxial layer and the substrate. In addition, this technology has the unique attribute of complete suppression of autodoping [21. We have investigated the gravity effect on solute transport during crystal growth and dissolution using a horizontal substrate—solution—substrate sandwich system. With indium solvent, it was found that dissolution of silicon occurred mainly on the lower substrate, while growth on the upper substrate was larger than that on the lower substrate under near isothermal conditions [5]. Based on these phenomena, we have developed a new growth technique, called yo-yo solute feeding method [6]. This method has many applications for the fabrication of semiconductor devices [7]. In the yo-yo method, solute transport is not caused by the spatial temperature gradient, but by the 0022-0248/91/$03.50 © 1991
—
temperature modulation. Corresponding to the repetitions of the temperature cycle, the epitaxial layer with a desirable thickness can be grown. Recently, we have found that the stripe lines were revealed on the cross-sections of epitaxial layers after stain etching. These stripe lines were clearly observed, especially in the case of nondoped growth using indium solvent. In this paper, the origin of the observed lines is discussed. Further, it will be shown that the growth occurs in each cycle of the yo-yo temperature repetitions.
2. Experimental procedure All epitaxial growth was performed using indium as solvent in the Pd-diffused H2. The specific gravity of indium at the growth temperature (974— 3 994°C) estimated at approximately g/cm [8]. Thisisvalue is about 2.5 times larger6.45 than that of silicon (2.51 g/cm3). Details of the growth system and the growth procedure are described in ref. [6]. The upper substrate and lower source crystal are horizontally set face to face with a gap of 4 mm in a graphite boat. Prior to each experiment, the solution was saturated with silicon at 994°C. In this study, (111) or (100) oriented 2 x 2 cm2 silicon substrates were used. The growth experiments were conducted as follows. The temperature of substrate, source and the
Elsevier Science Publishers B.V. (North-Holland)
T. Sukegawa et al.
/ LPE
I-
____________________________________ __________
W —
—~
-—--
— -
—
~
599
growth of Si by vo-yo solute feeding method
solution was uniformly increased to the initial temperature (994°C). The indium solution saturated with silicon was then inserted between the substrate and source crystal and the temperature was modulated up and down periodically with an appropriate amplitude under isothermal conditions. In this experiment, the high and low temperature levels were maintained at 994 and 974°C, respectively. The cooling rate of each cycle was 0.33°C/mm and the period of one cycle was 170 mm. As reported before, the layer thickness per cycle is approximately 15 p.m/cycle using this condition [6]. The cross-section of the grown layers was prepared for microscopic observation by lapping, polishing and stain etching.
~
Substrate
3. Results and discussion
-~
~ _______
____________________________________________ _____
Fig. 1. Cross-sectional views of epitaxial layers grown on (111) substrate (a) and (100) substrate (b). Stripe lines were revealed by stain etching. In both samples, the epitaxial growths, were conducted by the yo-yo solute feeding method using indium solvent with 10 repetition times of the temperature cycle (yo-yo
times). Marker represents 50 ~sm.
When the grown layers were stain etched, the stripe lines were observed. Fig. 1 shows the typical cross-sectional views of epitaxial layers on the (111) and (100) substrates. In both cases, the stripe lines are very flat and parallel to the interface between the epitaxial layer and the substrate. Further, the number of stripe lines are in exact accordance with the repetition times of the temperature cycle (yo-yo times). Fig. 2 shows the relation between the spreading resistance and the stripe lines. The sample was grown on p~(111) substrate with 20 yo-yo times. Corresponding to the stripe lines, the spreading resistance is slightly changing periodically. Namely, it is thought that these stripe lines caused by the change of indium concentration in the grown layer. According to the yo-yo temperature cycle, solid solubility of indium in silicon is changing, as shown in fig. 3a. Therefore the concentration of indium is changing along the growth direction (fig. 3b). The distances between the lines are almost equal (about 15 j.tm). This implies that the growth occurs in each temperature cycle. We have conducted the same measurement for the heavily phosphorus-doped sample. However, a change of spreading resistance was not observed in this case, stnce the difference of solid solubility of phos.
.
.
.
phorus in silicon between 994—974°C is very small [9]. For the device fabrication, this result is useful.
600
T. Sukegawa et al.
/ LPE growth of Si by yo-yo solute feeding method
Fig. 4 shows the cross-sectional views of the epitaxial layer with 1, 2 and 3 yo-yo repetitions. Obviously, the layers corresponding to the yo-yo repetitions can be observed in each photograph. In our growth system, it is impossible to remove the substrates from the solution. Therefore, the layers
grown during the cooling process to room temperature are also observed. The layer thickness for one yo-yo process is nearly equal to the averaged layer thickness per cycle described before. This value is in fairly good agreement with the value of 17 p.m calculated from the solubility difference of
0~
I 102
.
-
101
Epitaxal Layer
Substrate
non—dopedn—type
0
I 50
I 100
I 150
I 200
I 250
DEPTH (pm)
Fig. 2. Relation between the stripe lines and spreading resistance.
300
T Sukegawa et al.
/ LPE growth of Si by yo-yo solute feeding method
silicon in indium corresponding to 994—974°C. This means that solute transport to the growing interface of the upper substrate is sufficiently fast.
.
601
.
~
Epi Layer
4. Conclusion Stripe lines were observed in the grown layer by stain etching. From the measurement of the spreading resistance, it is concluded that these stripe lines show the grown layers corresponding to the yo-yo repetitions of the temperature cycle. Namely, the growth occurs in each cycle of yo-yo repetitions. Further, there was not much difference of growth thickness between (111) and (100) substrates. Under these growth conditions, the growth seems to be limited by supply rate of silicon from the solution. Using this method, an
a
Substrate
—
2nd Layer * —
Substrate
5Opm Growth
(a
_______________
V
~
~ T~ Temperature
>
—
•~ *~‘
_____
~_~~ri-~
In Conc
b
Ci
Ch~
~ —
____2nc
_____
~
Stripe Line
Layer Layer
Substrate
/
...
/
7—~
Fig. 4. Cros~-~ectionaI~ie~ s of the gro~n L~erwiih I (a), 2 (b) and 3 (c) yo-yo repetitions of the temperature cycle.
1 yo-yo Layer
//
Growth Direction Fig. 3. The change of solid solubility of indium in silicon during growth (a) and schematic diagram for the explanation of the relation between the stripe lines and the concentration of indium in grown layer (b). In this figure, T 5 and T1 indicate the high and low temperature levels of the yo-yo process, respectively. Ch and C1 show the solid solubility of indium in silicon corresponding to T5 and T,.
epitaxial layer with a desirable thickness can be grown by optimizing the temperature program. It should be noted that these experiments were performed without externally applied temperature difference. Acknowledgments .
The authors wish to thank Dr. Chikao Kimura of New Japan Radio Co., Ltd., for valuable dis-
602
T. Sukegawa et al.
/ LPE growth of Si by yo-yo solute feeding method
cussions. This work was supported in part by a Grant-in-Aid scientific Research from the Ministry of Education, Science and Culture, Japan.
References [1] B.J. Baliga, J. Crystal Crowth 41(1977)199. [2] B.J. Baliga, J. Electrochem. Soc. 133 (1986) SC. [3] Y. Suzuki and T. Nishinaga, Japan. J. Appl. Phys. 28 (1989) ~iz~o.
[41Y.
Suzuki, T. Nishinaga and T. Sanada, J. Crystal Growth 99 (1990) 229. [5] M. Kimura, A. Tanaka and T. Sukegawa, J. Crystal Growth 99 (1990) 1295. [6] T. Sukegawa, M. Kimura and A. Tanaka. J. Crystal Growth 92 (1988) 46. [7] T. Sukegawa, M. Kimura and A. Tanaka, IEEE Electron Device Letters EDL-l0 (1989) 20. [8] C.J. Smithells, Metals Reference Book, 5th ed. (Butterworths, London, 1976) p. 945. [9] T. Sukegawa, M. Kimura and A. Tanaka, J. Crystal Growth 96 (1989) 584.