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Growth of Si and in double-doped dislocation-free conductive GaAs crystal by the LEC technique
Vofume
4, number
MATERIALS
4
GROWTH OF Si AND In DOUBLE-DOPED CRYSTAL BY THE LEC TECHNIQUE Takashi FUJII, Masato NAKAJIMA
June 1986
LETTERS
DISLOCATION-FREE
CONDUCTIVE
GaAs
and Tsuguo FUKUDA
Optoelectronics Joint Research Loboratory I333 Kamikodanaku, Nakuhara-ku, Kawasaki 21 I, Jqan Received
3 January
1986
Dislocation-free and slip-free n-type conductive GaAs single crystals up to 2 inches in diameter were grown by the Si and In double-doped LEC technique. The crystals were characterized by KOH etching technique, XRT, ICP, SIMS and Hall effect measurement, and compared with undoped, Si-doped and in-doped crystal. With double doping a larger dislocation-free region (g = 0.7) is obtained, compared with In-doped crystal (g = 0.4). In the double-doped crystal, strong striations are observed, but carrier concentration is uniformly distributed in the wafer. It is possible for this crystal to be applied to optical devices.
1. Introduction Conductive GaAs single crystal has been widely used for fabricating light emitting diodes (LEDs), laser diodes (LDs), solar cells, gunn diodes and microwave diodes. Recently, with advances in molecular beam epitaxy (MBE), metal organic chemical vapor deposition ~OCVD) and ion implantation technique, devices of new structure have been reported and tested and the performance of devices of the usual structure is being improved. Nowadays, most conductive GaAs crystals which are used for these devices are produced by the boat growth technique. For the growth of large-diameter round substrate crystals at low cost, the liquid encapsulated Czochralski (LEC) technique has advantages over the boat growth technique. The LEC technique, however, had a limitation for application to optical devices due to the high dislocation density. Very recently, low-di~ocation or dislocation-free crystals were grown by reducing the temperature gradient [ 1,23 or by the impurity doping [3,4] technique. Fornari et al. [5] reported (111) oriented almost dislocation-free n-type conductive GaAs single crystal by high Si doping up to 5 X 10L8 atoms/cm3 _However, in (100) oriented Si doped crystal, which is required for the present device applications, dislocation density was higher than 5 X lo3 cme2 [6]. 0 167-577xi841~ 03.50 0 Elsevier Science Publishers B.V. ~No~h-Ho~~d Physics Publis~ng Division)
We have successfully grown 2-inch diameter dislocation-free and slip-free (100) n-type conductive GaAs singte crystals by the LEC technique using double doping with Si (donor) and In (isolator)$mpurities. The grown GaAs ingot was all single crystal from the seed end to the tail end. This paper reports the growth and characterization of double-doped n-type conductive GaAs single crystals, and crystal quality is discussed compared with that of undoped GaAs crystals, n-type GaAs crystals with only Si doping and semi-insulating GaAs crystals with only In doping.
2. Experimental We grew 2-inch diameter ( 100) oriented GaAs crystals under 20 atm of argon ambient by the direct synthesis LEC technique using Ga (6N) and As (7N) raw materials and SiOZ crucible. The initial charge weight was 1 kg and the As/(Ga + In t As) ratio in the initial melt was 0.506. Very low water content B,O, (under 100 ppm wt.) was used for an encapsulant and B,O, weight was = 300 g. The depth of the B203 layer was 28 mm in a 4-inch diameter crucible. The axial temperature gradient in this growth system was @ 80”C/cm in the upper B,O,-melt interface and m GO”C/cmin the lower, which was measured by 189
Volume 4, number 4
MATERIALS LETTERS
inserting thermocouples into the B203 and the melt. The crystal pulling speed was 5 mm/h, with counter rotation between crystal and crucible - 6 rpm and 20 rpm, respectively. Under the crystal growth conditions mentioned above, we grew undoped GaAs crystals, Indoped GaAs crystals, Si-doped GaAs crystals and In and Si double-doped GaAs crystals. In impurity-doped crystals, we added 250 mg (3 X 10lg atoms/cm3) of Si and/or 35 g (1 X 1021 atoms/cm3) of In into the melt. The crystal growing process was automatically controlled by computer [7]. Crystals were characterized by KOH etching technique, X-ray topography (XRT), inductively coupled radio frequency plasma atomic emission spectroscopy (IPC) analysis, secondary-ion mass spectroscopy (SIMS) analysis and Hall effect measurement.
3. Result and discussion Dislocation-free
and slip-free ( 100) oriented GaAs
Fig. 1. Typical example of KOH etching of a double-doped
190
June 1986
single crystals up to 2 inches in diameter were grown by the Si and In double-doped technique. Each crystal weighing w 700 g was single crystal from seed end to tail end, and the fraction of melt solidified (g) was about 0.7. Fig. 1 shows typical KOH etched figures of a double-doped GaAs wafer, of whichg is 0.57, as compared with that of the undoped crystal, of which g is 0.50. The double-doped wafer is entirely free of dislocation and slip, all over the wafer, whereas many etch pits are observed in the undoped GaAs wafer. Etch pit density (BPD) of crystals grown under the crystal growth conditions mentioned above is * 1 X lo4 cm-2 in the undoped crystal, = 1 X IO3 cmp2 in the crystal doped with Si only and nearly 0 cme2 in the crystal doped only with In. But in the In-doped wafer and Si-doped wafer many slip lines are observed near the edge of wafer. In In-doped crystal the EPD free region was obtained only when g was under 0.5, above which remarkable cellular substructure and polygonization was observed. We confirmed that doping the GaAs crystal with In alone or Si alone reduced the
2-inch GaAs (100) wafer and an undoped wafer.
MATERIALS LETTERS
Volume 4, number 4
June 1986
QP
Double
doping
In k
1Omm
(g=O.52)
doping
4
kpo.45
(a) Fig. 2. X-ray topographs of (a) doubledoped
1
(b) wafer of which g is 0.52 and (b) In-doped wafer of which g is 0.45.
number of dislocations. These results are in good agreement with the results reported by Fornari et al. [5,6] and Jacob et al. [4,8,9]. Further, we found that doping In and Si together had a double-doping effect and was useful in reducing the number of dislocations and slip-lines. To clarify this double-doping effect we compared the In and Si double-doped crystal with the crystal doped with In alone and examined the dislocations along the growth direction. Fig. 2a shows an X-ray topograph of a double-doped (100) wafer of which g is 0.52 and fig. 2b that of an In-doped wafer of which g is 0.45. X-ray transmission topographs were taken with the Lang method using MO Ka radiation. In the double-doped wafer dislocations and slip-lines are not observed while in the In-doped wafer cellular substructure is observed at the center and many slip-lines near the edge of the wafer. In In-doped crystal the cellular substructure shown in the center of fig. 2b appeared when g was = 0.4, while in the doubledoped crystal they were not observed untilg was= 0.7
in spite of the same growth conditions. These results suggest that double doping is effective to expand the cell-free region along the growth axial direction as compared with doping with In alone and we con firmed the reproducibility of this result in several experiments. Fig. 3 shows plots of - log(Cs/Co) against - log( 1 -g) for In in the double-doped crystal and I .2 l
Double-doped
&AS
0.41
0
0. I
0.2 -log
0.3
0.4
0.5
( 6
(I-g)
Fig. 3. Logarithmic plot of In segregation along doubledoped crystal (0) and In-doped crystal (0).
191
Volume 4, number 4
MATERIALS LETTERS
the In-doped crystal, where C’s and Cu are the concentrations in the crystal and the initial melt, respectively. Cs was measured by using the ICP technique and C, was estimated from initial charge weight. From analysis of the In content as a function of g the effective segregation coefficient (keg) of 0.1 for In in double-doped GaAs and In-doped GaAs are determined. It is recognized that there is not so much difference between k,, of an In-doped crystal and that of the double-doped crystal and that there is no relation between the generation of cellular substructure and keg of In. We are studying this result, but we note the following. As is well-known, the boundary condition for the generation of cellular substructure is CIR < (mC@)
(1-
kO)lkO , gradient in the melt. R
where G is the temperature
June 1986
the solidification velocity, m the liquidus gradient, D the diffusion coefficient and k, the equilibrium distribution coefficient. Here, it is seen that G, R, Co and k, are constants in this case because of the same growth conditions for the two crystals, and we suppose that the value of m becomes smaller or the value of D becomes bigger in proportion as the Si concentration increases. As is shown fig. 2a, dislocations and slips are not observed, but very strong striations are seen in the double-doped wafer, similar to the case of impurity doping, such as In, Si, etc. It is supposed that these striations result from condensation of the impurities. Fig. 4 shows the radial line scan data of Ci 10) and (010) direction of the impurity concentration analyzed by SIMS. The In concentration is 9 X 1019-1 X 1020 atoms/cm3 and uniformly distributed in the wafer and the Si is uniform, too. The B concentration is 7 X ld73 X 1018 atoms/cm3 and the distribution is slightly inhomogeneous. About 101’ atoms/cm3 of B atoms are normally contained in the LEC grown GaAs crystals and particularly = 101* atoms/cm3 of B atoms are contained in the Si-doped LEC grown GaAs crystals because Si reacts with B2O3 in the LEC
3000
_
I I
0
2 5
o-o\ 2000
“E 0
t
o-o1
--
distance
(mm)
o-o-o-o-o-o-o
.Z
a
2
I I
20
IO
0
Radial
distance
10
1000
20
(mm)
Fig. 4. Radial distributions of impurity concentration analyzed by SIMS along the (110) and (010) direction.
192
-0
*
A
c
o-o
Radial
Fig. 5. Radial distribution of carrier concentration mobility along the (110) direction.
and
Volume 4, number 4
MATERIALS LETTERS
system [6]. B atoms have less effect on reducing the dislocation density than In atoms or Si atoms [8], but the effect of B atoms on GaAs crystal cannot be separated from that of Si atoms in Si-doped LEC grown GaAs crystal. Fig. 5 shows the radial distribution of carrier concentration and mobility along the (110) direction of the wafer. Carrier concentration was = 9 X 1Ol7 atoms/cm3 and was uniformly distributed in the wafer. We fabricated LEDs and LDs on the double-doped wafer by the conventions commer~i~ method. And we confirmed that the characters of the LEDs and LDs are equal to those fabricated on boat grown wafers. These results indicate that the double-doped conductive GaAs crystal is suitable for LED and LD devices.
4. Summary We have successfully grown dislocation-free and slip-free 2-inch diameter <100) n-type conductive GaAs crystals by the double-dop~g technique. Double doping the GaAs crystal with In and Si is more effective in making the GaAs crystals dislocationfree and slip-free than doping with Si or doping with In, and shows a tendency of the dislocation-free region to expand along the growth direction. In the doubledoped wafer strong striations are observed, but the impurity concentration and the carrier concentration are uniformly distributed in the wafer. It is possible for this crystal to be applied to LEDs and LDs fabricated by the currently available process.
June 1986
Acknowledgement The authors are indebted to Drs. T. Iizuka, M. Hirano and I. Hayashi for helpful discussions. The present research effort is part of the large-scale project “Optical measurement and control systems” conducted under a program set up by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry.
References [ 1] T. Shimada, K. Terashima, H. Nakajima and T. Fukuda, Japan. J. Appl. Phys. 23 (1984) L24. ]2] T. Shiida, T. Obokata and T. Fukuda, Japan. J. Appl. Phys. 23 (1984) L44t f3] M.G. Mil’vidsky, V.B. Osvensky and S.S. Shifrin, J. Crystal Growth 52 (1981) 396. [ 41 G. Jacob, M. Duseaux, J.P. Farges, M.M.B. van den Boom and P.J. Roksnoer, J. Crystal Growth 61 (1983) 417. ]5] R. Fornari, C. Paorici, L. Zanotti and G. Zucaliy, J. Crystal Growth 63 (1983) 415. f6] R. Form&, C. Paorici and L. Zanotti, Crystal Res. Technol. 18 (1983) 157. [7] T. Fukuda, Japan. J. Appl. Phys. 22, Suppl. 1 (1983) 413. ]S] H.M. Hobgood, R.N. Thomas, D.L. Barrett, G.W. Eldridge, M.M. Sopira and M.C. Driver, in: Proceedings of the Conference on Semi-Ins~t~g III-V Materials, Kahneeta (1984) p. 149. (91 H. Nakanishi, H. Kohda, K. Yamada and K. Hoshikawa, in: Proceedings of the 16th Conference on Solid State Devicesand Materials, Kobe (1984) p. 63.
193
4, number
MATERIALS
4
GROWTH OF Si AND In DOUBLE-DOPED CRYSTAL BY THE LEC TECHNIQUE Takashi FUJII, Masato NAKAJIMA
June 1986
LETTERS
DISLOCATION-FREE
CONDUCTIVE
GaAs
and Tsuguo FUKUDA
Optoelectronics Joint Research Loboratory I333 Kamikodanaku, Nakuhara-ku, Kawasaki 21 I, Jqan Received
3 January
1986
Dislocation-free and slip-free n-type conductive GaAs single crystals up to 2 inches in diameter were grown by the Si and In double-doped LEC technique. The crystals were characterized by KOH etching technique, XRT, ICP, SIMS and Hall effect measurement, and compared with undoped, Si-doped and in-doped crystal. With double doping a larger dislocation-free region (g = 0.7) is obtained, compared with In-doped crystal (g = 0.4). In the double-doped crystal, strong striations are observed, but carrier concentration is uniformly distributed in the wafer. It is possible for this crystal to be applied to optical devices.
1. Introduction Conductive GaAs single crystal has been widely used for fabricating light emitting diodes (LEDs), laser diodes (LDs), solar cells, gunn diodes and microwave diodes. Recently, with advances in molecular beam epitaxy (MBE), metal organic chemical vapor deposition ~OCVD) and ion implantation technique, devices of new structure have been reported and tested and the performance of devices of the usual structure is being improved. Nowadays, most conductive GaAs crystals which are used for these devices are produced by the boat growth technique. For the growth of large-diameter round substrate crystals at low cost, the liquid encapsulated Czochralski (LEC) technique has advantages over the boat growth technique. The LEC technique, however, had a limitation for application to optical devices due to the high dislocation density. Very recently, low-di~ocation or dislocation-free crystals were grown by reducing the temperature gradient [ 1,23 or by the impurity doping [3,4] technique. Fornari et al. [5] reported (111) oriented almost dislocation-free n-type conductive GaAs single crystal by high Si doping up to 5 X 10L8 atoms/cm3 _However, in (100) oriented Si doped crystal, which is required for the present device applications, dislocation density was higher than 5 X lo3 cme2 [6]. 0 167-577xi841~ 03.50 0 Elsevier Science Publishers B.V. ~No~h-Ho~~d Physics Publis~ng Division)
We have successfully grown 2-inch diameter dislocation-free and slip-free (100) n-type conductive GaAs singte crystals by the LEC technique using double doping with Si (donor) and In (isolator)$mpurities. The grown GaAs ingot was all single crystal from the seed end to the tail end. This paper reports the growth and characterization of double-doped n-type conductive GaAs single crystals, and crystal quality is discussed compared with that of undoped GaAs crystals, n-type GaAs crystals with only Si doping and semi-insulating GaAs crystals with only In doping.
2. Experimental We grew 2-inch diameter ( 100) oriented GaAs crystals under 20 atm of argon ambient by the direct synthesis LEC technique using Ga (6N) and As (7N) raw materials and SiOZ crucible. The initial charge weight was 1 kg and the As/(Ga + In t As) ratio in the initial melt was 0.506. Very low water content B,O, (under 100 ppm wt.) was used for an encapsulant and B,O, weight was = 300 g. The depth of the B203 layer was 28 mm in a 4-inch diameter crucible. The axial temperature gradient in this growth system was @ 80”C/cm in the upper B,O,-melt interface and m GO”C/cmin the lower, which was measured by 189
Volume 4, number 4
MATERIALS LETTERS
inserting thermocouples into the B203 and the melt. The crystal pulling speed was 5 mm/h, with counter rotation between crystal and crucible - 6 rpm and 20 rpm, respectively. Under the crystal growth conditions mentioned above, we grew undoped GaAs crystals, Indoped GaAs crystals, Si-doped GaAs crystals and In and Si double-doped GaAs crystals. In impurity-doped crystals, we added 250 mg (3 X 10lg atoms/cm3) of Si and/or 35 g (1 X 1021 atoms/cm3) of In into the melt. The crystal growing process was automatically controlled by computer [7]. Crystals were characterized by KOH etching technique, X-ray topography (XRT), inductively coupled radio frequency plasma atomic emission spectroscopy (IPC) analysis, secondary-ion mass spectroscopy (SIMS) analysis and Hall effect measurement.
3. Result and discussion Dislocation-free
and slip-free ( 100) oriented GaAs
Fig. 1. Typical example of KOH etching of a double-doped
190
June 1986
single crystals up to 2 inches in diameter were grown by the Si and In double-doped technique. Each crystal weighing w 700 g was single crystal from seed end to tail end, and the fraction of melt solidified (g) was about 0.7. Fig. 1 shows typical KOH etched figures of a double-doped GaAs wafer, of whichg is 0.57, as compared with that of the undoped crystal, of which g is 0.50. The double-doped wafer is entirely free of dislocation and slip, all over the wafer, whereas many etch pits are observed in the undoped GaAs wafer. Etch pit density (BPD) of crystals grown under the crystal growth conditions mentioned above is * 1 X lo4 cm-2 in the undoped crystal, = 1 X IO3 cmp2 in the crystal doped with Si only and nearly 0 cme2 in the crystal doped only with In. But in the In-doped wafer and Si-doped wafer many slip lines are observed near the edge of wafer. In In-doped crystal the EPD free region was obtained only when g was under 0.5, above which remarkable cellular substructure and polygonization was observed. We confirmed that doping the GaAs crystal with In alone or Si alone reduced the
2-inch GaAs (100) wafer and an undoped wafer.
MATERIALS LETTERS
Volume 4, number 4
June 1986
QP
Double
doping
In k
1Omm
(g=O.52)
doping
4
kpo.45
(a) Fig. 2. X-ray topographs of (a) doubledoped
1
(b) wafer of which g is 0.52 and (b) In-doped wafer of which g is 0.45.
number of dislocations. These results are in good agreement with the results reported by Fornari et al. [5,6] and Jacob et al. [4,8,9]. Further, we found that doping In and Si together had a double-doping effect and was useful in reducing the number of dislocations and slip-lines. To clarify this double-doping effect we compared the In and Si double-doped crystal with the crystal doped with In alone and examined the dislocations along the growth direction. Fig. 2a shows an X-ray topograph of a double-doped (100) wafer of which g is 0.52 and fig. 2b that of an In-doped wafer of which g is 0.45. X-ray transmission topographs were taken with the Lang method using MO Ka radiation. In the double-doped wafer dislocations and slip-lines are not observed while in the In-doped wafer cellular substructure is observed at the center and many slip-lines near the edge of the wafer. In In-doped crystal the cellular substructure shown in the center of fig. 2b appeared when g was = 0.4, while in the doubledoped crystal they were not observed untilg was= 0.7
in spite of the same growth conditions. These results suggest that double doping is effective to expand the cell-free region along the growth axial direction as compared with doping with In alone and we con firmed the reproducibility of this result in several experiments. Fig. 3 shows plots of - log(Cs/Co) against - log( 1 -g) for In in the double-doped crystal and I .2 l
Double-doped
&AS
0.41
0
0. I
0.2 -log
0.3
0.4
0.5
( 6
(I-g)
Fig. 3. Logarithmic plot of In segregation along doubledoped crystal (0) and In-doped crystal (0).
191
Volume 4, number 4
MATERIALS LETTERS
the In-doped crystal, where C’s and Cu are the concentrations in the crystal and the initial melt, respectively. Cs was measured by using the ICP technique and C, was estimated from initial charge weight. From analysis of the In content as a function of g the effective segregation coefficient (keg) of 0.1 for In in double-doped GaAs and In-doped GaAs are determined. It is recognized that there is not so much difference between k,, of an In-doped crystal and that of the double-doped crystal and that there is no relation between the generation of cellular substructure and keg of In. We are studying this result, but we note the following. As is well-known, the boundary condition for the generation of cellular substructure is CIR < (mC@)
(1-
kO)lkO , gradient in the melt. R
where G is the temperature
June 1986
the solidification velocity, m the liquidus gradient, D the diffusion coefficient and k, the equilibrium distribution coefficient. Here, it is seen that G, R, Co and k, are constants in this case because of the same growth conditions for the two crystals, and we suppose that the value of m becomes smaller or the value of D becomes bigger in proportion as the Si concentration increases. As is shown fig. 2a, dislocations and slips are not observed, but very strong striations are seen in the double-doped wafer, similar to the case of impurity doping, such as In, Si, etc. It is supposed that these striations result from condensation of the impurities. Fig. 4 shows the radial line scan data of Ci 10) and (010) direction of the impurity concentration analyzed by SIMS. The In concentration is 9 X 1019-1 X 1020 atoms/cm3 and uniformly distributed in the wafer and the Si is uniform, too. The B concentration is 7 X ld73 X 1018 atoms/cm3 and the distribution is slightly inhomogeneous. About 101’ atoms/cm3 of B atoms are normally contained in the LEC grown GaAs crystals and particularly = 101* atoms/cm3 of B atoms are contained in the Si-doped LEC grown GaAs crystals because Si reacts with B2O3 in the LEC
3000
_
I I
0
2 5
o-o\ 2000
“E 0
t
o-o1
--
distance
(mm)
o-o-o-o-o-o-o
.Z
a
2
I I
20
IO
0
distance
10
1000
20
Fig. 4. Radial distributions of impurity concentration analyzed by SIMS along the (110) and (010) direction.
192
-0
*
A
c
o-o
Radial
Fig. 5. Radial distribution of carrier concentration mobility along the (110) direction.
and
Volume 4, number 4
MATERIALS LETTERS
system [6]. B atoms have less effect on reducing the dislocation density than In atoms or Si atoms [8], but the effect of B atoms on GaAs crystal cannot be separated from that of Si atoms in Si-doped LEC grown GaAs crystal. Fig. 5 shows the radial distribution of carrier concentration and mobility along the (110) direction of the wafer. Carrier concentration was = 9 X 1Ol7 atoms/cm3 and was uniformly distributed in the wafer. We fabricated LEDs and LDs on the double-doped wafer by the conventions commer~i~ method. And we confirmed that the characters of the LEDs and LDs are equal to those fabricated on boat grown wafers. These results indicate that the double-doped conductive GaAs crystal is suitable for LED and LD devices.
4. Summary We have successfully grown dislocation-free and slip-free 2-inch diameter <100) n-type conductive GaAs crystals by the double-dop~g technique. Double doping the GaAs crystal with In and Si is more effective in making the GaAs crystals dislocationfree and slip-free than doping with Si or doping with In, and shows a tendency of the dislocation-free region to expand along the growth direction. In the doubledoped wafer strong striations are observed, but the impurity concentration and the carrier concentration are uniformly distributed in the wafer. It is possible for this crystal to be applied to LEDs and LDs fabricated by the currently available process.
June 1986
Acknowledgement The authors are indebted to Drs. T. Iizuka, M. Hirano and I. Hayashi for helpful discussions. The present research effort is part of the large-scale project “Optical measurement and control systems” conducted under a program set up by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry.
References [ 1] T. Shimada, K. Terashima, H. Nakajima and T. Fukuda, Japan. J. Appl. Phys. 23 (1984) L24. ]2] T. Shiida, T. Obokata and T. Fukuda, Japan. J. Appl. Phys. 23 (1984) L44t f3] M.G. Mil’vidsky, V.B. Osvensky and S.S. Shifrin, J. Crystal Growth 52 (1981) 396. [ 41 G. Jacob, M. Duseaux, J.P. Farges, M.M.B. van den Boom and P.J. Roksnoer, J. Crystal Growth 61 (1983) 417. ]5] R. Fornari, C. Paorici, L. Zanotti and G. Zucaliy, J. Crystal Growth 63 (1983) 415. f6] R. Form&, C. Paorici and L. Zanotti, Crystal Res. Technol. 18 (1983) 157. [7] T. Fukuda, Japan. J. Appl. Phys. 22, Suppl. 1 (1983) 413. ]S] H.M. Hobgood, R.N. Thomas, D.L. Barrett, G.W. Eldridge, M.M. Sopira and M.C. Driver, in: Proceedings of the Conference on Semi-Ins~t~g III-V Materials, Kahneeta (1984) p. 149. (91 H. Nakanishi, H. Kohda, K. Yamada and K. Hoshikawa, in: Proceedings of the 16th Conference on Solid State Devicesand Materials, Kobe (1984) p. 63.
193