- Email: [email protected]
Structural investigation of MOVPE grown InGaAs buffer layers
........ CRYSTAL GROWTH
ELSEVIER
Journal of Crystal Growth 170 (1997) 743-747
Structural investigation of MOVPE grown InGaAs buffer layers P. Maign6 *, D. Coulas Communications Research Centre, P.O. Box 11490, Station H, Ottawa, Ontario, Canada, K2H 8S2
Abstract We have used X-ray diffraction to study the residual elastic strain in InxGa ~_~As (0.07 < x < 0.14) layers grown by MOVPE on GaAs substrate, as a function of growth conditions, growth procedures and post-growth treatment. The samples have been grown at temperatures ranging from 625°C to 680°C with a growth rate between 0.45 and 2 Ixm/h. For thick layers, the residual elastic strain represents about 15% of the initial lattice mismatch and is independent of the Indium composition. The extent of strain relief is not significantly changed by an increase in growth temperature from 625°C to 680°C. On the other hand, we found that the extent of strain relief is dependant upon the growth rate. The residual strain can be reduced to 5% of the lattice mismatch by lowering the growth rate from 1 to 0.45 Ixm/h. This variation is related to the increased time at which the structure is at the growth temperature, since a post-growth annealing for one hour at growth temperature of a sample grown at 1 Ixm/h leads also to the same reduction of the residual strain. Due to dislocations interactions, the effective stress responsible for dislocations motion and consequently the dislocation gliding velocity are significantly reduced. In these conditions, the time required by a dislocation to glide to the interface is comparable to the growth time.
1. Introduction
In order to optimise the quality of epitaxial layers grown on mismatched substrates over their elastic limits, different procedures have been proposed [16]. Despite some success in device performance [7,8], it is usually found that, even for very thick layers, the lattice mismatch is not entirely accommodated by the formation of misfit dislocations and that some residual elastic strain is always present when the growth of the layer proceeds in a two dimensional fashion. The value of this residual strain is difficult to predict at the present time and is an obstacle for the routine growth of well controlled quasi-substrates capable of hosting device quality materials. There-
Corresponding author. Fax: +1 613 990 8382; E-mail: pascal.maigne @crc.doc.ca.
fore, this work is aimed at understanding the structural properties of partially relaxed InGaAs layers grown on (100) GaAs substrate. We have deliberately restricted our study to single layers of one composition with a particular interest in the growth procedures and the post-growth treatments that would reduce or eliminate this residual strain.
2. E x p e r i m e n t a l procedure
The samples have been grown by MOVPE in a commercial horizontal reactor, using trimethylgallium, trimethylindium and 100% arsine. The carrier gas was hydrogen purified through a palladium cell. The growth temperature was varied between 625°C and 680°C and the growth rate was varied between 0.45 and 2 I x m / h with a typical growth rate of about
0022-0248/97/$17.00 Copyright © 1997 P. Maign~. Published by Elsevier Science B.V. PI! S0022-0248(96)00562-3
P. Maign~, D. Coulas /Journal of Cm,stal Growth 170 (1997) 743-747
744
1 Ixm/h. The growth was carried out on epi-ready (100) GaAs substrates cut nominally flat with substrate misorientation less than 0.1 °. Following growth, one sample has been annealed for an hour at a temperature of 625°C under arsine flow. Thickness calibration curves have been produced from scanning electron microscopy measurements performed on test samples in which AlAs marker layers have been included. The structural properties of the samples have been investigated using a Rigaku Double Crystal X-ray diffractometer. Symmetrical 400 and asymmetrical 422 rocking curves have been recorded as a function of the azimuthal angle in order to measure the lattice parameters c and a respectively perpendicular and parallel to the interface. The residual strain, e, is then given by e = ( a - a r ) / a r, where a r is the epilayer lattice parameter in its completely relaxed state. Since the layers have different composition, we have derived, for each sample, a relaxation coefficient R, given by R = (1 - e l f ) , where f represents the lattice mismatch and is given by f = ( a r - a,~)/a~ with a s being the substrate lattice parameter. Experimental data can be represented by the variation of R as a function of the ratio of the layer thickness, t, to the critical layer thickness, t c [9]. This ratio represents the normalized thickness.
comparison, is data obtained from a previous study [10] on MOCVD grown InGaAs samples and data from Dunstan et al. [11] on MBE grown InGaAs samples. It must be pointed out that the two sets of MOCVD samples have been grown in different reactors. We note an excellent agreement between the three studies. The trends given by the three sets of data are also in good agreement with the predictions of the strain relaxation model via plastic flow of Dodson and Tsao [12]. In particular, the steep increase observed at t / t c = 10, corresponds to a region, where dislocation multiplication mechanisms are dominant. For larger values of the normalized thickness ( t / t c > 40), our data indicate that the relaxation coefficient reaches a plateau at 85% of the lattice mismatch, independent of the In composition. In most layered systems, it is found that differences in thermal expansion coefficients between materials leads to a measurable residual strain. Since the GaAs substrate has a larger thermal expansion coefficient than the InGaAs epilayer, it contracts more rapidly during the post-growth cooling period, therefore it imposes a compressive stress on the film, and consequently a decrease of the relaxation coefficient. However, using data from Berstein and Beals [13], it can be easily shown that the residual strain due to
3. Results and discussion
Table 1 Growth conditions (temperature, T and growth rate, G), structural parameters (thickness, t, In composition, x, and normalized thickness, t / t c) and results from X-ray diffraction experiments (relaxation coefficient, R, and strain anisotropy, A)
Results from X-ray diffraction experiments as well as the growth parameters are listed in Table 1 for the samples under investigation. We have included a parameter A, which represents the asymmetry in strain relaxation and is given by R[001] - R[011] a = 2 g[011] + R[0~I 1] '
(1)
where R[011] and R[011] represents the relaxation coefficient along the [011] and [011] direction respectively. In Fig. 1, we have reported the relaxation coefficient as a function of normalized thickness for the first 8 samples (sample A to H) which have been grown at the same growth temperature and with the same growth rate. Also represented in Fig. 1, for
Sample
T (°C)
G (l~m/h)
t x (p~m)
t / tc
R (%)
A (%)
A B C D E F G H I J K L M *
625 625 625 625 625 625 625 625 670 680 625 625 625
1.05 1.05 1.05 1.05 1 1.1 1.15 1.2 1.1 1.05 2.1 0.45 1
0.05 0.1 0.15 1.8 1.75 1.9 2 2.1 1.9 1.85 1.8 1.6 1.75
3 6 9 64 96 123 142 156 99 96 79 112 102
4 11 33 84 84 85.5 86.5 86.5 82 83 67 95 93.5
0 0 67 9 1 2 5 1 25 0 62 13 5
a
0.11 0.115 0.12 0.075 0.11 0.125 0.135 0.14 0.105 0.105 0.09 0.135 0.11
Post-growth annealing for 1 h at 625°C.
745
P. Maign~, D. Coulas / Journal of Co'stal Growth 170 (1997) 743- 747
in surface morphology has been observed in sample J, which is probably a sign changing from a 2D to a 3D growth mode. This shows that the dislocations are not as mobile as expected (in standard growth conditions, the velocity of a dislocations in InGaAs is about 1 c m / s [16,17]). Again, as the layer grows thicker, the interaction between dislocations leads to a reduction of the effective stress which is responsible for dislocation motion. Since the dislocation glide velocity is proportional to the effective stress acting on a dislocation, the gliding velocity is significantly reduced. It is not clear, however, whether dislocation motion is stopped or whether the gliding velocity is reduced so that the time necessary for a dislocation to reach the interface is much longer that the growth time. Drigo et al. [18] have shown that for layer thicknesses up to 1 Ixm, the residual strain measured in InGaAs layers is independent of the growth rate. For a given thickness, a variation of growth rate is directly translated into a variation of growth time. At the beginning of the relaxation process, the interaction between dislocations is negligible and the dislocation velocity is expected to be of the order of 1 c m / s . Consequently, the time of growth, even though reduced, is still very large with respect to the time required for a dislocation to glide to the interface.
the difference in thermal expansion coefficients between the epilayer and the substrate is negligible in the I n G a A s / G a A s system over the entire composition range. The fact that complete strain relaxation cannot be reached for thick buffer layers of single composition has been observed previously [14]. The concept of work hardening has been invoked [15] whereby, for partially relaxed layers in which the density of misfit dislocations is large, the formation of a new misfit dislocation is more difficult. This is due to the repulsive interaction between the threading dislocation and the array of existing misfit dislocations. In addition, repulsion between segments of same-sign Burgers vectors threading dislocations can occur. This results in a more difficult motion of the threading dislocations towards the edge of the substrate and consequently, an inefficiency in relieving the elastic strain. This description agrees qualitatively with our results. The growth temperature is expected to influence the relaxation process. An increase in growth temperature should favour the relaxation of the strain since the dislocations are expected to be more mobile. However, as shown in Table 1, by increasing the growth temperature from 625°C to 670°C (sample I) and to 680°C (sample J), the residual strain is not changed significantly. Moreover, a degradation
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AppI.Phys.Lett. 59 (1991)3390.
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N o r m a l i z e d Thickness Fig. 1. Variation of the relaxation coefficient R as a function of the layer thickness, t, normalized to the critical layer thickness, tc.
746
P. Maign~, D. Coulas / Journal of Crystal Growth 170 (1997) 743-747
This can explain the insensitivity of the residual strain upon the growth rate for thin layers. On the other hand, for thicker layers when dislocation interaction is no longer negligible, the growth time may influence the extent of strain relief. As indicated in Table 1, most of our samples have been grown with a growth rate between I and 1.2 txm/h. The increase of the growth rate to 2 ~ m / h (sample K) leads to an increase of the residual strain, where only 67% of the misfit is relieved. It is interesting to note that a strong asymmetry in strain relaxation along ~011 ) in-plane directions has been measured in sample K. The relaxation coefficient is still equal to 85% in the [011] direction but is equal to 43% in the [071] direction. This is in contrast with all the other thick layers, where the asymmetry in strain relief is small or negligible. In addition, when the anisotropy is measurable, the relaxation coefficient is always larger in the [011] direction showing a density of dislocations higher along the [011] direction. This reverse anisotropy observed in sample K is not understood at the present time. We note that the growth has been carried out on (100)-oriented substrates with misorientation angle less than 0,1 °. On the other hand, lowering the growth rate to 0.5 I~m/h (sample L) leads to a relaxation coefficient of 95% and a reduced anisotropy, with complete relaxation in the [011] direction and a relaxation of 90% in the [011] direction. These results indicate that, although the gliding velocity is severely reduced, the dislocations are still mobile. Lowering the growth rate further is probably not a practical and cost-effective solution since it significantly increases the growth time. In addition, it is doubtful that complete relaxation could be achieved. In order to confirm the relationship between residual strain reduction and growth time rather the growth rate, we have annealed for an one hour at 625°C a layer grown with a growth rate of 1 p~m/h (sample M). Consequently samples M and L have been at a temperature of 625°C for the same amount of time. We found a relaxation coefficient of 93.5%, in excellent agreement with results obtained on sample L. This result is interesting because it shows that, for thick layers, the relaxation process is not completed when the growth is stopped. Whaley and Cohen [19] have shown that the situation is different for thin layers. Using RHEED, they followed the variation of
the in-plane lattice parameter in the first stages of the relaxation process and showed that the relief of the elastic strain is stopped at each growth interruption. This again is related to the high dislocation velocity, when dislocation interaction is negligible. On the other hand for thick layers, our results confirm that for partially relaxed structures, the motion of dislocations is severely impeded by a high density of dislocations, but the dislocations are still mobile. In fact, this shows that the time required by a dislocation to reach the interface is comparable to the growth time. However, this reduction of the residual strain leads to a deterioration of the structural quality of sample M, as evidenced by a large increase of the full width at half maximum of the X-ray epilayer peak, when compared to samples E and L. This severe degradation of the structural properties, related to an increase of the density of threading dislocations, is not expected, since only an additional 10% of the lattice mismatch is accommodated by misfit dislocations. Nonetheless, this shows that, at the present time, a simple post-growth annealing procedure as the one used in this study, makes the top surface of the structure unsuitable for the subsequent growth of device quality materials.
4. S u m m a r y In conclusion, we have studied the extent of strain relief in InGaAs thick layers grown by MOCVD on GaAs substrates. We found that only 85% of the lattice mismatch is accommodated by the formation of misfit dislocations, which can be explained by a reduction of the effective stress acting on a dislocation due to interaction between dislocations. The amount of strain relief is independent of the growth temperature in the range under investigation. The residual strain is decreased by a decrease of the growth rate, the main effect of which is to double the growth time. This shows that the dislocation velocity has been significantly reduced but that the dislocations are still mobile. This can be reproduced by a post-growth annealing at growth temperature and normal growth rate. However despite the reduction of residual strain, this procedure leads to a severe degradation of the quality of the top surface, making it unsuitable for the realisation of quasi-substrate.
P. Maign~, D. Coulas / Journal of Crystal Growth 170 (1997) 743-747
References [1] A. Sacedon, F. Gonzalez-Sanz, E. Calleja, E. Munoz, S.I. Molina, F.J. Pacheco, D. Araujo, R. Garcia, M. Louren~o, Z. Yang, P. Kidd and D. Dunstan, Appl. Phys. Lett. 66 (1995) 3334. [2] D. Dunstan, Semicond. Sci. Technol. 6 (1991) A76. [3] P.M. Mooney, J.L. Jordan-Sweet, K. Ismail, J.O. Chu, R.M. Feenstra and F.K. LeGoues, Appl. Phys. Lett. 67 (1995) 2373. [4] R. Azoulay, A. Clei, L. Dugrand, N. Draidia, G. Leroux and S. Biblemont, J. Crystal Growth 107 (1991) 926. [5] S.P. Watkins, C.A. Tran, R. Ares and G. Soerensen, Appl, Phys. Lett. 66 (1995) 882. [6] E.A. Fitzgerald, J. Vac. Sci. Technol. B 7 (1989) 782. [7] R.J. Aggarwal and C.G. Fonstad, Jr., Electron. Lett. 31 (1995) 75. [8] S.F. Nelson, K. Ismail, J.O. Chu and B.S. Meyerson, Appl. Phys. Lett. 63 (1993) 367. [9] J.W. Matthews, A.E. Blakeslee, J. Crystal Growth 27 (1974) 118.
747
[10] P. Maign6 and J,-M. Baribeau, J. Appl. Phys. 76 (1994) 1962. [11] D.J. Dunstan, P. Kidd, L.K. Howard and R.H. Dixon, Appl. Phys. Lett, 59 (1991) 3390. [12] B.W. Dodson and J.Y. Tsao, Appl. Phys. Lett. 51 (1987) 1710. [13] L. Berstein and R.J. Beals, J. Appl. Phys. 32 (1961) 122. [14] R.M. Biefeld, C.R. Hills and S.R. Lee, J. Crystal Growth 91 (1988) 515. [15] B.W. Dodson, Appl. Phys. Lett. 53 (1988) 37. [16] P.J. Mar~e, J.C. Barbour, J.F. van der Veen, K.L. Kavanagh, C.W.T. Bulle-Lieuwa and M.P.A. Viegers, J. Appl. Phys. 62 (1987) 4413. [17] K. Sumino and I. Yonenaga, in: Proc. 7th Semi-insulating III-V Materials Conf., Eds. C.J. Miner, W. Ford and E.R. Weber (Institute of Physics Publishing, Bristol, 1992) p. 29. [18] A.V. Drigo, Y. Aydinli, A. Camera, F. Genova, C. Rigo, C. Ferrari, P. Franzosi and G. Salviati, J. Appl. Phys. 66 (1989) 1975. [19] G.J. Whaley and P.I. Cohen, Appl. Phys. Lett. 57 (1990) 144.
ELSEVIER
Journal of Crystal Growth 170 (1997) 743-747
Structural investigation of MOVPE grown InGaAs buffer layers P. Maign6 *, D. Coulas Communications Research Centre, P.O. Box 11490, Station H, Ottawa, Ontario, Canada, K2H 8S2
Abstract We have used X-ray diffraction to study the residual elastic strain in InxGa ~_~As (0.07 < x < 0.14) layers grown by MOVPE on GaAs substrate, as a function of growth conditions, growth procedures and post-growth treatment. The samples have been grown at temperatures ranging from 625°C to 680°C with a growth rate between 0.45 and 2 Ixm/h. For thick layers, the residual elastic strain represents about 15% of the initial lattice mismatch and is independent of the Indium composition. The extent of strain relief is not significantly changed by an increase in growth temperature from 625°C to 680°C. On the other hand, we found that the extent of strain relief is dependant upon the growth rate. The residual strain can be reduced to 5% of the lattice mismatch by lowering the growth rate from 1 to 0.45 Ixm/h. This variation is related to the increased time at which the structure is at the growth temperature, since a post-growth annealing for one hour at growth temperature of a sample grown at 1 Ixm/h leads also to the same reduction of the residual strain. Due to dislocations interactions, the effective stress responsible for dislocations motion and consequently the dislocation gliding velocity are significantly reduced. In these conditions, the time required by a dislocation to glide to the interface is comparable to the growth time.
1. Introduction
In order to optimise the quality of epitaxial layers grown on mismatched substrates over their elastic limits, different procedures have been proposed [16]. Despite some success in device performance [7,8], it is usually found that, even for very thick layers, the lattice mismatch is not entirely accommodated by the formation of misfit dislocations and that some residual elastic strain is always present when the growth of the layer proceeds in a two dimensional fashion. The value of this residual strain is difficult to predict at the present time and is an obstacle for the routine growth of well controlled quasi-substrates capable of hosting device quality materials. There-
Corresponding author. Fax: +1 613 990 8382; E-mail: pascal.maigne @crc.doc.ca.
fore, this work is aimed at understanding the structural properties of partially relaxed InGaAs layers grown on (100) GaAs substrate. We have deliberately restricted our study to single layers of one composition with a particular interest in the growth procedures and the post-growth treatments that would reduce or eliminate this residual strain.
2. E x p e r i m e n t a l procedure
The samples have been grown by MOVPE in a commercial horizontal reactor, using trimethylgallium, trimethylindium and 100% arsine. The carrier gas was hydrogen purified through a palladium cell. The growth temperature was varied between 625°C and 680°C and the growth rate was varied between 0.45 and 2 I x m / h with a typical growth rate of about
0022-0248/97/$17.00 Copyright © 1997 P. Maign~. Published by Elsevier Science B.V. PI! S0022-0248(96)00562-3
P. Maign~, D. Coulas /Journal of Cm,stal Growth 170 (1997) 743-747
744
1 Ixm/h. The growth was carried out on epi-ready (100) GaAs substrates cut nominally flat with substrate misorientation less than 0.1 °. Following growth, one sample has been annealed for an hour at a temperature of 625°C under arsine flow. Thickness calibration curves have been produced from scanning electron microscopy measurements performed on test samples in which AlAs marker layers have been included. The structural properties of the samples have been investigated using a Rigaku Double Crystal X-ray diffractometer. Symmetrical 400 and asymmetrical 422 rocking curves have been recorded as a function of the azimuthal angle in order to measure the lattice parameters c and a respectively perpendicular and parallel to the interface. The residual strain, e, is then given by e = ( a - a r ) / a r, where a r is the epilayer lattice parameter in its completely relaxed state. Since the layers have different composition, we have derived, for each sample, a relaxation coefficient R, given by R = (1 - e l f ) , where f represents the lattice mismatch and is given by f = ( a r - a,~)/a~ with a s being the substrate lattice parameter. Experimental data can be represented by the variation of R as a function of the ratio of the layer thickness, t, to the critical layer thickness, t c [9]. This ratio represents the normalized thickness.
comparison, is data obtained from a previous study [10] on MOCVD grown InGaAs samples and data from Dunstan et al. [11] on MBE grown InGaAs samples. It must be pointed out that the two sets of MOCVD samples have been grown in different reactors. We note an excellent agreement between the three studies. The trends given by the three sets of data are also in good agreement with the predictions of the strain relaxation model via plastic flow of Dodson and Tsao [12]. In particular, the steep increase observed at t / t c = 10, corresponds to a region, where dislocation multiplication mechanisms are dominant. For larger values of the normalized thickness ( t / t c > 40), our data indicate that the relaxation coefficient reaches a plateau at 85% of the lattice mismatch, independent of the In composition. In most layered systems, it is found that differences in thermal expansion coefficients between materials leads to a measurable residual strain. Since the GaAs substrate has a larger thermal expansion coefficient than the InGaAs epilayer, it contracts more rapidly during the post-growth cooling period, therefore it imposes a compressive stress on the film, and consequently a decrease of the relaxation coefficient. However, using data from Berstein and Beals [13], it can be easily shown that the residual strain due to
3. Results and discussion
Table 1 Growth conditions (temperature, T and growth rate, G), structural parameters (thickness, t, In composition, x, and normalized thickness, t / t c) and results from X-ray diffraction experiments (relaxation coefficient, R, and strain anisotropy, A)
Results from X-ray diffraction experiments as well as the growth parameters are listed in Table 1 for the samples under investigation. We have included a parameter A, which represents the asymmetry in strain relaxation and is given by R[001] - R[011] a = 2 g[011] + R[0~I 1] '
(1)
where R[011] and R[011] represents the relaxation coefficient along the [011] and [011] direction respectively. In Fig. 1, we have reported the relaxation coefficient as a function of normalized thickness for the first 8 samples (sample A to H) which have been grown at the same growth temperature and with the same growth rate. Also represented in Fig. 1, for
Sample
T (°C)
G (l~m/h)
t x (p~m)
t / tc
R (%)
A (%)
A B C D E F G H I J K L M *
625 625 625 625 625 625 625 625 670 680 625 625 625
1.05 1.05 1.05 1.05 1 1.1 1.15 1.2 1.1 1.05 2.1 0.45 1
0.05 0.1 0.15 1.8 1.75 1.9 2 2.1 1.9 1.85 1.8 1.6 1.75
3 6 9 64 96 123 142 156 99 96 79 112 102
4 11 33 84 84 85.5 86.5 86.5 82 83 67 95 93.5
0 0 67 9 1 2 5 1 25 0 62 13 5
a
0.11 0.115 0.12 0.075 0.11 0.125 0.135 0.14 0.105 0.105 0.09 0.135 0.11
Post-growth annealing for 1 h at 625°C.
745
P. Maign~, D. Coulas / Journal of Co'stal Growth 170 (1997) 743- 747
in surface morphology has been observed in sample J, which is probably a sign changing from a 2D to a 3D growth mode. This shows that the dislocations are not as mobile as expected (in standard growth conditions, the velocity of a dislocations in InGaAs is about 1 c m / s [16,17]). Again, as the layer grows thicker, the interaction between dislocations leads to a reduction of the effective stress which is responsible for dislocation motion. Since the dislocation glide velocity is proportional to the effective stress acting on a dislocation, the gliding velocity is significantly reduced. It is not clear, however, whether dislocation motion is stopped or whether the gliding velocity is reduced so that the time necessary for a dislocation to reach the interface is much longer that the growth time. Drigo et al. [18] have shown that for layer thicknesses up to 1 Ixm, the residual strain measured in InGaAs layers is independent of the growth rate. For a given thickness, a variation of growth rate is directly translated into a variation of growth time. At the beginning of the relaxation process, the interaction between dislocations is negligible and the dislocation velocity is expected to be of the order of 1 c m / s . Consequently, the time of growth, even though reduced, is still very large with respect to the time required for a dislocation to glide to the interface.
the difference in thermal expansion coefficients between the epilayer and the substrate is negligible in the I n G a A s / G a A s system over the entire composition range. The fact that complete strain relaxation cannot be reached for thick buffer layers of single composition has been observed previously [14]. The concept of work hardening has been invoked [15] whereby, for partially relaxed layers in which the density of misfit dislocations is large, the formation of a new misfit dislocation is more difficult. This is due to the repulsive interaction between the threading dislocation and the array of existing misfit dislocations. In addition, repulsion between segments of same-sign Burgers vectors threading dislocations can occur. This results in a more difficult motion of the threading dislocations towards the edge of the substrate and consequently, an inefficiency in relieving the elastic strain. This description agrees qualitatively with our results. The growth temperature is expected to influence the relaxation process. An increase in growth temperature should favour the relaxation of the strain since the dislocations are expected to be more mobile. However, as shown in Table 1, by increasing the growth temperature from 625°C to 670°C (sample I) and to 680°C (sample J), the residual strain is not changed significantly. Moreover, a degradation
100 90 4,,,a
80
°r,ii
70
o v...l
©
•
This Study
[]
AppI.Phys.Lett. 59 (1991)3390.
0
J.AppI.Phys. 76 (1994) 1962
[]
• ~
o~
[]
Go
~O []
60 O
c.) 50 o 40 ~t 30
g) 0
20
[]
10
°00
~C]~l
2
i
i
i
3 4 5 6
i i i i
101
I
I
I
I
2
3
4
5 6
I
~
I
I
I
1(~ 2
2
I
I
3
4
N o r m a l i z e d Thickness Fig. 1. Variation of the relaxation coefficient R as a function of the layer thickness, t, normalized to the critical layer thickness, tc.
746
P. Maign~, D. Coulas / Journal of Crystal Growth 170 (1997) 743-747
This can explain the insensitivity of the residual strain upon the growth rate for thin layers. On the other hand, for thicker layers when dislocation interaction is no longer negligible, the growth time may influence the extent of strain relief. As indicated in Table 1, most of our samples have been grown with a growth rate between I and 1.2 txm/h. The increase of the growth rate to 2 ~ m / h (sample K) leads to an increase of the residual strain, where only 67% of the misfit is relieved. It is interesting to note that a strong asymmetry in strain relaxation along ~011 ) in-plane directions has been measured in sample K. The relaxation coefficient is still equal to 85% in the [011] direction but is equal to 43% in the [071] direction. This is in contrast with all the other thick layers, where the asymmetry in strain relief is small or negligible. In addition, when the anisotropy is measurable, the relaxation coefficient is always larger in the [011] direction showing a density of dislocations higher along the [011] direction. This reverse anisotropy observed in sample K is not understood at the present time. We note that the growth has been carried out on (100)-oriented substrates with misorientation angle less than 0,1 °. On the other hand, lowering the growth rate to 0.5 I~m/h (sample L) leads to a relaxation coefficient of 95% and a reduced anisotropy, with complete relaxation in the [011] direction and a relaxation of 90% in the [011] direction. These results indicate that, although the gliding velocity is severely reduced, the dislocations are still mobile. Lowering the growth rate further is probably not a practical and cost-effective solution since it significantly increases the growth time. In addition, it is doubtful that complete relaxation could be achieved. In order to confirm the relationship between residual strain reduction and growth time rather the growth rate, we have annealed for an one hour at 625°C a layer grown with a growth rate of 1 p~m/h (sample M). Consequently samples M and L have been at a temperature of 625°C for the same amount of time. We found a relaxation coefficient of 93.5%, in excellent agreement with results obtained on sample L. This result is interesting because it shows that, for thick layers, the relaxation process is not completed when the growth is stopped. Whaley and Cohen [19] have shown that the situation is different for thin layers. Using RHEED, they followed the variation of
the in-plane lattice parameter in the first stages of the relaxation process and showed that the relief of the elastic strain is stopped at each growth interruption. This again is related to the high dislocation velocity, when dislocation interaction is negligible. On the other hand for thick layers, our results confirm that for partially relaxed structures, the motion of dislocations is severely impeded by a high density of dislocations, but the dislocations are still mobile. In fact, this shows that the time required by a dislocation to reach the interface is comparable to the growth time. However, this reduction of the residual strain leads to a deterioration of the structural quality of sample M, as evidenced by a large increase of the full width at half maximum of the X-ray epilayer peak, when compared to samples E and L. This severe degradation of the structural properties, related to an increase of the density of threading dislocations, is not expected, since only an additional 10% of the lattice mismatch is accommodated by misfit dislocations. Nonetheless, this shows that, at the present time, a simple post-growth annealing procedure as the one used in this study, makes the top surface of the structure unsuitable for the subsequent growth of device quality materials.
4. S u m m a r y In conclusion, we have studied the extent of strain relief in InGaAs thick layers grown by MOCVD on GaAs substrates. We found that only 85% of the lattice mismatch is accommodated by the formation of misfit dislocations, which can be explained by a reduction of the effective stress acting on a dislocation due to interaction between dislocations. The amount of strain relief is independent of the growth temperature in the range under investigation. The residual strain is decreased by a decrease of the growth rate, the main effect of which is to double the growth time. This shows that the dislocation velocity has been significantly reduced but that the dislocations are still mobile. This can be reproduced by a post-growth annealing at growth temperature and normal growth rate. However despite the reduction of residual strain, this procedure leads to a severe degradation of the quality of the top surface, making it unsuitable for the realisation of quasi-substrate.
P. Maign~, D. Coulas / Journal of Crystal Growth 170 (1997) 743-747
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