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Surface tension controlled growth of uniform GaAs crystals
Volume 7, number
11
MATERIALS
SURFACE TENSION
CONTROLLED
February
LETTERS
GROWTH OF UNIFORM
1989
GaAs CRYSTALS
Ju WU and Peigen MO Shanghai Institute ofMetallurgy, Academia Sinrca, Chang Ning Road 865. Shanghai, China Received
19 October
1988
It IS demonstrated that surface tension may be used to maintain a constant diameter A homogeneous and low dislocation crystal has been grown with a constant diameter
ofa GaAs crystal during the growth process. controlled by surface tension.
1. Introduction Controlling the diameter during the growth of LEC GaAs is critical to the yield of crystal. Moreover, any sharp variation in the diameter during growth may result in a high defect density in the crystal. Therefore, expensive and complex automatic systems have been designed to control the diameter of the crystal during growth. In this study, a more convenient way of controlling the diameter using the surface tension is proposed and a low dislocation crystal with a constant diameter maintained by surface tension has been grown.
2. Simple considerations in maintaining diameter by surface tension
Fig. I. The meniscus
noted that near the inner wall of the crucible, the liquid-B,O, interface is round due to surface tension. This can be observed macroscopically by cooling down the melt in the crucible to solidify it. It can be imagined that, when the diameter of the growing crystal is increased to a given anomalously large value, it cannot be further increased without changing the angle 8, due to the curvature of the round liquid-B203 interface near the inner wall of the crucible. In other words, when the diameter of the growing crystal is beyond a certain value, the force equilibrium condition at the highest point of the meniscus would be destroyed and a thermodynamic force must be overcome for the diameter to increase further. If the barrier formed by the thermodynamic force is appreciably larger to accommodate some variation in the growth rate, the diameter controlling by surface tension can be realized in a limited range.
the
In the LEC system, a meniscus surrounds the solidliquid interface, as shown in fig. 1. Three phases meet at the highest point of the meniscus and, when the surface of the melt is flat, equilibrium must be established among the forces resulting from the surface tension and acting at that point. In fig. 1, 8, is defined as the angle between the tangents of the solidliquid interface and of the solid-B,O1 interface, and 6, as the angle between the tangents of the liquidBz03 interface. Both angles 8, and 8, should be constant and are determined by the surface tension and independent of the growth process parameters [ 11. It should be 0167-577x/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
in the LEC system
B.V.
395
Volume 7, number
I1
MATERIALS
LETTERS
February
1989
3. Crystal growth Although the effect of the surface tension described above cannot be estimated exactly at present, a ( 100 ) crystal of 2-inch constant diameter surface-tension controlled with precision of almost 100% has been grown routinely in the PBN crucible with a low pressure puller in our laboratory. Most of the crystals weigh about 500 g and the length of the constant-diameter portion is generally about 30-40 mm. The shape of the bottom of some crystals resembles that of the crucible, indicating its role in controlling the diameter. Furthermore, in the conventional LEC method, the growing crystal has to be subjected to severe thermal conditions which result in a high dislocation density with characteristic W-shaped distribution pattern as calculated by Jordan et al. [ 21. Jacob [ 31 has proposed the liquid encapsulated Kyropoulos (LEK) method to improve the thermal environment to grow low dislocation crystals. The main characteristics of the LEK method, with full encapsulation, is that the lateral surface of the growing crystal is in contact with the inner wall of the crucible. In this method, the thermal stress is independent of the diameter of the crystal and is reduced to a lower value [ 31, and a low dislocation crystal can be grown. Unfortunately an electrically homogeneous crystal cannot be obtained, because the growing crystal and the crucible, being in contact with each other, are prohibited from rotating. The existing LEC crystals with their surface tension controlled diameters have been grown in similar thermal conditions to that of the LEK method. Low dislocation crystals with surface tension con-
396
600
400
”
200
‘,E g w
104
103
1
0.5
I 0
I
0.5
r/r0 Fig. 2. EPD distribution
along the ( 110) radial direction.
trolled diameter have been grown. The radial distribution of the etch pit density (EPD) of the crystals is shown in fig. 2. It can be seen from the figure that the In-doped crystal is almost dislocation-free and in the undoped crystal, the W-shaped pattern is obviously suppressed.
References [ 1] N.B. Min, Preliminary physics of crystal growth (Shanghai Science &Technology Press, Shanghai, 1982). [ 2) A.S. Jordan, R. Caruse and A.R. von Neida, Bell System Tech. J. 59 (1980) 593. [ 31 G. Jacob, J. Crystal Growth 58 (1982) 455. [4] M. Duseaux, J. Crystal Growth 61 (1983) 576.
11
MATERIALS
SURFACE TENSION
CONTROLLED
February
LETTERS
GROWTH OF UNIFORM
1989
GaAs CRYSTALS
Ju WU and Peigen MO Shanghai Institute ofMetallurgy, Academia Sinrca, Chang Ning Road 865. Shanghai, China Received
19 October
1988
It IS demonstrated that surface tension may be used to maintain a constant diameter A homogeneous and low dislocation crystal has been grown with a constant diameter
ofa GaAs crystal during the growth process. controlled by surface tension.
1. Introduction Controlling the diameter during the growth of LEC GaAs is critical to the yield of crystal. Moreover, any sharp variation in the diameter during growth may result in a high defect density in the crystal. Therefore, expensive and complex automatic systems have been designed to control the diameter of the crystal during growth. In this study, a more convenient way of controlling the diameter using the surface tension is proposed and a low dislocation crystal with a constant diameter maintained by surface tension has been grown.
2. Simple considerations in maintaining diameter by surface tension
Fig. I. The meniscus
noted that near the inner wall of the crucible, the liquid-B,O, interface is round due to surface tension. This can be observed macroscopically by cooling down the melt in the crucible to solidify it. It can be imagined that, when the diameter of the growing crystal is increased to a given anomalously large value, it cannot be further increased without changing the angle 8, due to the curvature of the round liquid-B203 interface near the inner wall of the crucible. In other words, when the diameter of the growing crystal is beyond a certain value, the force equilibrium condition at the highest point of the meniscus would be destroyed and a thermodynamic force must be overcome for the diameter to increase further. If the barrier formed by the thermodynamic force is appreciably larger to accommodate some variation in the growth rate, the diameter controlling by surface tension can be realized in a limited range.
the
In the LEC system, a meniscus surrounds the solidliquid interface, as shown in fig. 1. Three phases meet at the highest point of the meniscus and, when the surface of the melt is flat, equilibrium must be established among the forces resulting from the surface tension and acting at that point. In fig. 1, 8, is defined as the angle between the tangents of the solidliquid interface and of the solid-B,O1 interface, and 6, as the angle between the tangents of the liquidBz03 interface. Both angles 8, and 8, should be constant and are determined by the surface tension and independent of the growth process parameters [ 11. It should be 0167-577x/89/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
in the LEC system
B.V.
395
Volume 7, number
I1
MATERIALS
LETTERS
February
1989
3. Crystal growth Although the effect of the surface tension described above cannot be estimated exactly at present, a ( 100 ) crystal of 2-inch constant diameter surface-tension controlled with precision of almost 100% has been grown routinely in the PBN crucible with a low pressure puller in our laboratory. Most of the crystals weigh about 500 g and the length of the constant-diameter portion is generally about 30-40 mm. The shape of the bottom of some crystals resembles that of the crucible, indicating its role in controlling the diameter. Furthermore, in the conventional LEC method, the growing crystal has to be subjected to severe thermal conditions which result in a high dislocation density with characteristic W-shaped distribution pattern as calculated by Jordan et al. [ 21. Jacob [ 31 has proposed the liquid encapsulated Kyropoulos (LEK) method to improve the thermal environment to grow low dislocation crystals. The main characteristics of the LEK method, with full encapsulation, is that the lateral surface of the growing crystal is in contact with the inner wall of the crucible. In this method, the thermal stress is independent of the diameter of the crystal and is reduced to a lower value [ 31, and a low dislocation crystal can be grown. Unfortunately an electrically homogeneous crystal cannot be obtained, because the growing crystal and the crucible, being in contact with each other, are prohibited from rotating. The existing LEC crystals with their surface tension controlled diameters have been grown in similar thermal conditions to that of the LEK method. Low dislocation crystals with surface tension con-
396
600
400
”
200
‘,E g w
104
103
1
0.5
I 0
I
0.5
r/r0 Fig. 2. EPD distribution
along the ( 110) radial direction.
trolled diameter have been grown. The radial distribution of the etch pit density (EPD) of the crystals is shown in fig. 2. It can be seen from the figure that the In-doped crystal is almost dislocation-free and in the undoped crystal, the W-shaped pattern is obviously suppressed.
References [ 1] N.B. Min, Preliminary physics of crystal growth (Shanghai Science &Technology Press, Shanghai, 1982). [ 2) A.S. Jordan, R. Caruse and A.R. von Neida, Bell System Tech. J. 59 (1980) 593. [ 31 G. Jacob, J. Crystal Growth 58 (1982) 455. [4] M. Duseaux, J. Crystal Growth 61 (1983) 576.