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A comparison of alane precursors for growth of AlAs and AlGaAs by metalorganic molecular beam epitaxy
Pergamon
0038-1101(94)00188-X
Solid-State Electronics Vol. 38. No. 3. DD. . . 737-138. 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-I lOI/ $9.50+0.00
NOTE A COMPARISON OF ALANE PRECURSORS FOR GROWTH OF AlAs AND AlGaAs BY METALORGANIC MOLECULAR BEAM EPITAXY (Received
18 August
1. INTRODUCTION
High purity AlGaAs alloys and AlAs are becoming increasingly important for use in GaAs-based optoelectronic devices. Because of the high reactivity of the Al-rich surface, these materials are highly susceptible to contamination during growth, especially by oxygen. This is particularly true in metalorganic molecular beam epitaxy (MOMBE) where the absence of gas phase interactions allows any impurities in the gaseous precursors to be incorporated in the grown layer. It was shown, for example, that AlGaAs grown by MOMBE using alkyl-Al precursors such as triethylaluminum (TEA) contained high levels of oxygen due to the presence of alkoxides in the Al source[l-31. To overcome this problem, trimethylamine alane (TMAA) was introduced as an altemative[l,2,4,5]. Unlike the alkyl Al sources, the amine saturated Al sources do not form volatile Al-0 species. As a result, Al,Ga, _,As layers with X,, up to 0.5 grown with this source showed a marked reduction in oxygen background to levels as low as 3 x 10” cm-3[6]. Dimethylethylamine alane (DMEAA) has been suggested as a liquid alternative to TMAA[7], which is a solid. In principle, this change should not alter the growth chemistry, and hence the impurity background, obtained in MOMBE grown material. Joyce et a1.[7], however, reported much higher oxygen levels when replacing TMAA with DMEAA, lW” cm-3 for DMEAA vs -7 x 10” ctn3 for TMAA, which they attribute to contamination of the DMEAA. Clearly, even though the alane itself is not a ready contributor of oxygen, there are other potential contaminants which can lead to high oxygen backgrounds. One source of this contamination is the solvent used to synthesize the alane, which is often ether for TMAA and various alkanes for DMEAA[8]. As this paper will demonstrate, this difference can have a striking impact on the impurity background in AlAs.
1994)
either TMAA or DMEAA, as one would expect (see Fig. I).
Previous work with alternative Ga precursors has shown that the oxygen background in AlGaAs grown at 500°C is limited by the presence of alkoxides in the Ga source[6]. Similarly, the carbon level was found to be controlled by the strength of the Ga-C bond and is not due to decomposition of the amide functional group[6]. Thus one would not expect the alane precursor to strongly effect the impurity background. From this work it was also assumed that the increase in oxygen usually observed with increasing temperature was due to enhanced decomposition of the Ga-alkoxides. However, replacing the TMAA with DMEAA completely eliminates this increase, suggesting either that the increase in oxygen obtained with TMAA is due to the presence of an oxygen impurity in the TMAA rather than in the TEG, and the incorporation of oxygen from alkoxides is essentially saturated over the entire temperature range, or that the use of DMEAA somehow suppresses the decomposition of Ga-alkoxides. This issue can be explored in more detail in the case of AlAs where the issue of contamination of the Ga source can be eliminated. In light of the low oxygen backgrounds obtained in AlGaAs grown from TMAA at the standard growth temperature, -500°C one would expect similar levels in AlAs grown under similar conditions. However, as shown in Fig. 2, such growth produces material with oxygen levels of _ lo*’ cm-‘. By contrast, the same conditions show levels of only u 5 x 10” cm-) when DMEAA is used at temperatures of 500°C or higher. Carbon levels are substantially lower as well. Given the similar behavior of these two compounds, one would not expect such variance in the incorporation of impurities, particularly oxygen. However, it is important to note that these compounds are synthesized using slightly different processes. The TMAA process utilizes ether while
2. EXPERIMENTAL Samples were grown in an INTEVAC Gas Source Gen II on 2 in. diameter semi-insulating GaAs substrates. Growth temperatures were monitored using an optical pyrometer. ASH, was decomposed in a low pressure cracker which was heated to 1100°C. Triethylgallium (TEG) was used as the gallium source. TMAA and DMEAA, both obtained from Air Products and Chemicals Inc., were used as aluminum sources for growth of undoped AlAs while triethylgallium (TEG) was used as the Ga source. Both of the Al source bubblers were kept at 9.2”C. All of the Group III sources were transported with a He carrier gas. SIMS analysis was obtained from Evans East using a Cs+ beam in a PHI 6300. Concentrations were determined by comparison with ion implanted standards.
CARBON: -0DMEAA, -O-- TMAA OXYGEN: --A-- DMEAA. --A-- TMAA
I
‘“‘?A0 400
I 450
500
550
600
GROWTH TEMPERATURE (“C)
3. RESULTS AND DISCUSSION At a growth
temperature
in impurity contamination SSE 1 3-N
of 500°C there is little difference
in A1022Ga,,78A~grown from
Fig. I. Dependence on growth temperature of carbon and oxygen backgrounds in Ab.22Gao78As grown from TEG, ASH, and either TMAA or DMEAA. 737
Note
738 10Z2 ~-,-A-OXYGEN
A
+,-O-CARBON -7
102'
\
1
\
‘::; ,‘X:
/
IOr1cmv3. Carbon backgrounds were substantially reduced as well. This is presumably due to the absence of oxygenated solvents in the synthesis of DMEAA as compared to TMAA which is synthesized with ether. For growth of Al&&,.,sAs, the differences in material grown at 500°C are not significant, suggesting that the impurity backgrounds are limited by the Ga-precursor. As the temperature is increased, however, the oxygen background in material grown from TMAA begins to rise, suggesting that the ether is beginning to play a role in the oxygen incorporation process. In spite of the higher purity material which can be obtained with DMEAA, it is not yet clear whether this source possesses sufficient stability to be=used as a replacement for TMAA. C. R. Abernathy
350
400
450
GROWTH
500 TEMPERATURE
550
600
(“C)
Fig. 2. Comparison of impurity backgrounds in AlAs grown from ASH, and either TMAA or DMEAA at various temperatures.
that for DMEAA uses only alkane solvents[S]. At low Al contents and low growth temperatures, this difference is unimportant as the growth surface is not sufficiently reactive to decompose the ether. As the surface becomes increasingly reactive, as in the growth of AlAs or growth at high temperatures, the ether is apparently decomposed resulting in the incorporation of high levels of oxygen. Given this difference, it would appear that DMEAA is the obvious choice for AlAs growth. However, there remains some question about the stability of this source which may ultimately limit its usefulness. We have not conducted a rigorous study of the stability of DMEAA. however we did experience some decomposition in the bubbler as evidenced by a large rise in bubbler pressure when stored overnight at 9.2”C. While some pressure build-up is also observed with TMAA, the pressure obtained in the DMEAA bubbler in the DMEAA bubbler was higher than that seen with TMAA and appeared to increase at a faster rate. Whether this decomposition is extensive enough to recommend against the use of this source requires further testing. 4.
CONCLUSIONS
DMEAA was shown to produce dramatically lower oxygen levels in AlAs than TMAA, 5 x IO” cmm3 vs
Department of Materials University of Florida Gainesoille, FL 32611 U.S.A.
Science and Engineering,
AT & T Bell Laboratories Murray Hill, NJ 07974 U.S.A.
P. W. Wisk
REFERENCES I. C. R. Abernathy, A. S. Jordan, S. J. Pearton, W. S. Hobson, D. A. Bohling and G. T. Muhr, Appl. Phys. Lett. 56, 2654 (1990). 2. C. R. Abernathy, S. J. Pearton,
F. A. Baiocchi, T. Ambrose, A. S. Jordan, D. A. Bohling and G. T. Muhr, J. Cryst. Growth 110, 457 (1991). 3. V. Frese, G. K. Regel, H. Hardtdegen, A. Brauners, P. Balk, M. Hostalek, M. Lokai, L. Pohl, A. Miklis and K. Werner, J. Electron Mater. 19, 305 (1990). 4. J. L. Benchimol, X. Q. Zhang, Y. Gao, G. Le Roux, H. Thibierge and F. Alexandre, J. Cryst. Growth 120, 189 (I 992). 5. C. R. Abernathy and D. A. Bohling, J. Crust. Growth
120, 195 (1992). 6. C. R. Abernathy, P. W. Wisk, A. C. Jones and S. A. Rushworth, Appl. Phys. Lett. 61, 180 (1992). 7. T. B. Joyce, T. J. Bullough, P. Knightley, C. J. Kiely, Y. R. Xing and P. J. Goodhew, J. Cryst. Growth 120,206 (1992). 8. D. A. Bohling and G. T. Muhr, private communication.
0038-1101(94)00188-X
Solid-State Electronics Vol. 38. No. 3. DD. . . 737-138. 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-I lOI/ $9.50+0.00
NOTE A COMPARISON OF ALANE PRECURSORS FOR GROWTH OF AlAs AND AlGaAs BY METALORGANIC MOLECULAR BEAM EPITAXY (Received
18 August
1. INTRODUCTION
High purity AlGaAs alloys and AlAs are becoming increasingly important for use in GaAs-based optoelectronic devices. Because of the high reactivity of the Al-rich surface, these materials are highly susceptible to contamination during growth, especially by oxygen. This is particularly true in metalorganic molecular beam epitaxy (MOMBE) where the absence of gas phase interactions allows any impurities in the gaseous precursors to be incorporated in the grown layer. It was shown, for example, that AlGaAs grown by MOMBE using alkyl-Al precursors such as triethylaluminum (TEA) contained high levels of oxygen due to the presence of alkoxides in the Al source[l-31. To overcome this problem, trimethylamine alane (TMAA) was introduced as an altemative[l,2,4,5]. Unlike the alkyl Al sources, the amine saturated Al sources do not form volatile Al-0 species. As a result, Al,Ga, _,As layers with X,, up to 0.5 grown with this source showed a marked reduction in oxygen background to levels as low as 3 x 10” cm-3[6]. Dimethylethylamine alane (DMEAA) has been suggested as a liquid alternative to TMAA[7], which is a solid. In principle, this change should not alter the growth chemistry, and hence the impurity background, obtained in MOMBE grown material. Joyce et a1.[7], however, reported much higher oxygen levels when replacing TMAA with DMEAA, lW” cm-3 for DMEAA vs -7 x 10” ctn3 for TMAA, which they attribute to contamination of the DMEAA. Clearly, even though the alane itself is not a ready contributor of oxygen, there are other potential contaminants which can lead to high oxygen backgrounds. One source of this contamination is the solvent used to synthesize the alane, which is often ether for TMAA and various alkanes for DMEAA[8]. As this paper will demonstrate, this difference can have a striking impact on the impurity background in AlAs.
1994)
either TMAA or DMEAA, as one would expect (see Fig. I).
Previous work with alternative Ga precursors has shown that the oxygen background in AlGaAs grown at 500°C is limited by the presence of alkoxides in the Ga source[6]. Similarly, the carbon level was found to be controlled by the strength of the Ga-C bond and is not due to decomposition of the amide functional group[6]. Thus one would not expect the alane precursor to strongly effect the impurity background. From this work it was also assumed that the increase in oxygen usually observed with increasing temperature was due to enhanced decomposition of the Ga-alkoxides. However, replacing the TMAA with DMEAA completely eliminates this increase, suggesting either that the increase in oxygen obtained with TMAA is due to the presence of an oxygen impurity in the TMAA rather than in the TEG, and the incorporation of oxygen from alkoxides is essentially saturated over the entire temperature range, or that the use of DMEAA somehow suppresses the decomposition of Ga-alkoxides. This issue can be explored in more detail in the case of AlAs where the issue of contamination of the Ga source can be eliminated. In light of the low oxygen backgrounds obtained in AlGaAs grown from TMAA at the standard growth temperature, -500°C one would expect similar levels in AlAs grown under similar conditions. However, as shown in Fig. 2, such growth produces material with oxygen levels of _ lo*’ cm-‘. By contrast, the same conditions show levels of only u 5 x 10” cm-) when DMEAA is used at temperatures of 500°C or higher. Carbon levels are substantially lower as well. Given the similar behavior of these two compounds, one would not expect such variance in the incorporation of impurities, particularly oxygen. However, it is important to note that these compounds are synthesized using slightly different processes. The TMAA process utilizes ether while
2. EXPERIMENTAL Samples were grown in an INTEVAC Gas Source Gen II on 2 in. diameter semi-insulating GaAs substrates. Growth temperatures were monitored using an optical pyrometer. ASH, was decomposed in a low pressure cracker which was heated to 1100°C. Triethylgallium (TEG) was used as the gallium source. TMAA and DMEAA, both obtained from Air Products and Chemicals Inc., were used as aluminum sources for growth of undoped AlAs while triethylgallium (TEG) was used as the Ga source. Both of the Al source bubblers were kept at 9.2”C. All of the Group III sources were transported with a He carrier gas. SIMS analysis was obtained from Evans East using a Cs+ beam in a PHI 6300. Concentrations were determined by comparison with ion implanted standards.
CARBON: -0DMEAA, -O-- TMAA OXYGEN: --A-- DMEAA. --A-- TMAA
I
‘“‘?A0 400
I 450
500
550
600
GROWTH TEMPERATURE (“C)
3. RESULTS AND DISCUSSION At a growth
temperature
in impurity contamination SSE 1 3-N
of 500°C there is little difference
in A1022Ga,,78A~grown from
Fig. I. Dependence on growth temperature of carbon and oxygen backgrounds in Ab.22Gao78As grown from TEG, ASH, and either TMAA or DMEAA. 737
Note
738 10Z2 ~-,-A-OXYGEN
A
+,-O-CARBON -7
102'
\
1
\
‘::; ,‘X:
/
IOr1cmv3. Carbon backgrounds were substantially reduced as well. This is presumably due to the absence of oxygenated solvents in the synthesis of DMEAA as compared to TMAA which is synthesized with ether. For growth of Al&&,.,sAs, the differences in material grown at 500°C are not significant, suggesting that the impurity backgrounds are limited by the Ga-precursor. As the temperature is increased, however, the oxygen background in material grown from TMAA begins to rise, suggesting that the ether is beginning to play a role in the oxygen incorporation process. In spite of the higher purity material which can be obtained with DMEAA, it is not yet clear whether this source possesses sufficient stability to be=used as a replacement for TMAA. C. R. Abernathy
350
400
450
GROWTH
500 TEMPERATURE
550
600
(“C)
Fig. 2. Comparison of impurity backgrounds in AlAs grown from ASH, and either TMAA or DMEAA at various temperatures.
that for DMEAA uses only alkane solvents[S]. At low Al contents and low growth temperatures, this difference is unimportant as the growth surface is not sufficiently reactive to decompose the ether. As the surface becomes increasingly reactive, as in the growth of AlAs or growth at high temperatures, the ether is apparently decomposed resulting in the incorporation of high levels of oxygen. Given this difference, it would appear that DMEAA is the obvious choice for AlAs growth. However, there remains some question about the stability of this source which may ultimately limit its usefulness. We have not conducted a rigorous study of the stability of DMEAA. however we did experience some decomposition in the bubbler as evidenced by a large rise in bubbler pressure when stored overnight at 9.2”C. While some pressure build-up is also observed with TMAA, the pressure obtained in the DMEAA bubbler in the DMEAA bubbler was higher than that seen with TMAA and appeared to increase at a faster rate. Whether this decomposition is extensive enough to recommend against the use of this source requires further testing. 4.
CONCLUSIONS
DMEAA was shown to produce dramatically lower oxygen levels in AlAs than TMAA, 5 x IO” cmm3 vs
Department of Materials University of Florida Gainesoille, FL 32611 U.S.A.
Science and Engineering,
AT & T Bell Laboratories Murray Hill, NJ 07974 U.S.A.
P. W. Wisk
REFERENCES I. C. R. Abernathy, A. S. Jordan, S. J. Pearton, W. S. Hobson, D. A. Bohling and G. T. Muhr, Appl. Phys. Lett. 56, 2654 (1990). 2. C. R. Abernathy, S. J. Pearton,
F. A. Baiocchi, T. Ambrose, A. S. Jordan, D. A. Bohling and G. T. Muhr, J. Cryst. Growth 110, 457 (1991). 3. V. Frese, G. K. Regel, H. Hardtdegen, A. Brauners, P. Balk, M. Hostalek, M. Lokai, L. Pohl, A. Miklis and K. Werner, J. Electron Mater. 19, 305 (1990). 4. J. L. Benchimol, X. Q. Zhang, Y. Gao, G. Le Roux, H. Thibierge and F. Alexandre, J. Cryst. Growth 120, 189 (I 992). 5. C. R. Abernathy and D. A. Bohling, J. Crust. Growth
120, 195 (1992). 6. C. R. Abernathy, P. W. Wisk, A. C. Jones and S. A. Rushworth, Appl. Phys. Lett. 61, 180 (1992). 7. T. B. Joyce, T. J. Bullough, P. Knightley, C. J. Kiely, Y. R. Xing and P. J. Goodhew, J. Cryst. Growth 120,206 (1992). 8. D. A. Bohling and G. T. Muhr, private communication.