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Influence of high hydrostatic pressure–high temperature treatment on defect structure of AlGaAs layers
Journal of Alloys and Compounds 286 (1999) 279–283
L
Influence of high hydrostatic pressure–high temperature treatment on defect structure of AlGaAs layers a, a a a a b J. Ba¸k-Misiuk *, J. Adamczewska , J. Domagal«a , Z.R. Zytkiewicz , J. Trela , A. Misiuk , c c d d ´ M. Leszczynski , J. Jun , H.B. Surma , A. Wnuk a
´ 32 /46, 02 -668 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, Al.Lotnikow b ´ 32 /46, 02 -668 Warsaw, Poland Institute of Electron Technology, Al.Lotnikow c High Pressure Research Centre, Polish Academy of Sciences, Sokol«owska 29 /37, 01 -142 Warsaw, Poland d ´ ´ 133, 01 -919 Warsaw, Poland Institute of Electronic Materials Technology, Wolczynska
Abstract Influence of high pressure–high temperature treatment on structural properties of AlGaAs / GaAs structures was studied by high resolution x-ray diffractometry and photoluminescence techniques. The treatment-induced changes in lattice parameter are explained by relaxation of the misfit strain via creation of misfit dislocations and other extended defects, as well as by diffusion of Al to dislocations. 1999 Elsevier Science S.A. All rights reserved. Keywords: Semiconductors; X-ray diffraction; Defect structure; High-pressure treatment
1. Introduction Material response to stress at high temperature is of a fundamental interest. Annealing at enhanced (hydrostatic) pressure (HP treatment) of ambient gas can influence the defect structure of homo or heterogeneous systems in many ways. Firstly, if there are any imperfections of different compressibility than that of the bulk crystal, the creation of additional defects at the boundary between these imperfections and the matrix can be expected. Thus, HP treatment can enhance the development of the primary existing structure irregularities. On the other hand, an improvement of the structure perfection has been reported for some systems [1]. In both cases, the studies on HP treatment effects can help to understand the initial sample defect structure, as well as physical phenomena related to strain at the film / substrate interface. Recently, the influence of HP at high temperature (HT) on the properties of semiconductors was very intensively investigated. However, mostly the silicon crystals were studied (e.g. [2–5]) and only a few reports concerned other semiconductors [1,6–8]. It has been established that Si, GaP and GaAs *Corresponding author.
single crystals containing precipitates or inclusions of different compressibility and thermal expansion with respect to that of the matrix, can have their defect structure changed at HP–HT [1–8]. This effect was explained as being due to the higher strain at the precipitate / matrix boundary, as well as to enhanced diffusivity of components at HP conditions. Additional defects can be created if the stress exceeds a critical value. For epitaxial layers, the change of defect structure can be also expected, when the critical strain between the layer and substrate is exceeded. The influence of HP on lattice misfit relaxation has been reported for the Si 12x Ge x / Si [5], Al 0.27 Ga 0.73 As / GaAs [7,8] and GaAs / Si systems [1]. Moreover, it was found that HP–HT treatment induced changes of defect structure of semiconductor layers as well as strongly influencing their optical and electronic properties [8]. The aim of our paper is to report on the influence of the HP–HT treatment on crystallographic properties of Al x Ga 12x As heteroepitaxial layers on GaAs substrates, as a function of their initial strain state, with respect to the GaAs substrate. The influence of HP–HT treatment on the properties of layers was studied using x-ray diffraction and photoluminescence (PL) for the thin layers of Al x Ga 12x As
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )01021-4
J. Ba¸k-Misiuk et al. / Journal of Alloys and Compounds 286 (1999) 279 – 283
280
having various values of lattice mismatch in relation to GaAs.
2. Experimental Al x Ga 12x As layers of thickness in the range between 2–13 mm were grown on the (001) oriented GaAs substrates by molecular beam epitaxy (MBE) and by liquid phase epitaxy (LPE) techniques at a temperature of about 1000 K. The following treatment procedures were applied to the samples: 1. (HT): Annealing at 770 K during 1 h at atmospheric pressure. 2. (HP–HT): Annealing at 770 K under a 1.2 GPa pressure of argon during 1 h. 3. (HP): High pressure treatment under a 1.2 GPa pressure of argon at room temperature during 1 h. X-ray measurements were carried out for each sample before and after treatments using x-ray high resolution diffractometry and reflection topography. In order to determine the strain status of layers, the in-plane (a i ) and out-of-plane (a ' ) lattice parameters were measured and the symmetrical and asymmetrical reciprocal space maps were recorded. The lattice parameters were determined by the Bond method or by the method of the direct measurements of the Bragg angle [9] using the symmetrical 004 and asymmetrical 117, 115 and 335 reflections. Cu Ka 1 radiation was used in the experiment. The measured lattice parameters a i and a ' served for evaluation of the relaxed lattice parameters: a relax 5 (a ' 1 2a i C) /(1 1 2C)
(1)
where: C5(12 n ) /(11 n ), n 5Poisson ratio (we assumed
the linear relation between nAlAs 50.275 and nGaAs 50.311 [10]). It has been shown (e.g. in [11]), that contour maps of diffuse scattering measured near the reciprocal lattice point are related to the defect structure of the sample. Therefore, the reciprocal space maps were also recorded to study the changes of the defect structure induced by the HP–HT treatment. Moreover, the Al x Ga 12x As layers with the direct energy gap (x,0.4) were examined by photoluminescence spectroscopy at a temperature of 6 K. PL was excited by a 488-nm line of argon laser.
3. Results All investigated samples did not exhibit any measurable structural changes after atmospheric pressure annealing at 770 K (HT) or after HP treatment at room temperature. Such changes occurred only after simultaneous application of the HP and HT. For all Al x Ga 12x As samples, an increase of the full width at half maximum (FWHM) of the 004 rocking curve, and a decrease of PL intensity, were observed after the HP–HT treatment. Also the changes of the lattice parameters were detected (see Table 1). We were able to find the following correlation of these changes with the initial state of the strain in the as-grown samples. For initially strained thin layers (samples A, B, C), the increase of the out-of-plane lattice parameter, as well as of the relaxed one, a relax , took place. No change of the in-plane lattice parameter was detected. It means that layers after annealing at HP remain fully strained (see Table 1). The partially relaxed Al x Ga 12x As layers (samples D, E, F, G) exhibit different changes of the lattice parameters after the HP–HT treatment. For such samples, the out-ofplane lattice parameters and the relaxed ones decreased (Table 1). It is necessary to admit that an additional
Table 1 Features of AlGaAs samples before and after the HP–HT treatment a Samples
t (mm)
gb
Growth method
x
a' before treatment 62310 25 ˚ (A)
ai before treatment 63310 25 ˚ (A)
Da ' 62.5310 25 ˚ (A)
Da relax. 62.5310 25 ˚ (A)
FWHM as grown (arcsec)
FWHM after HP–HT (arcsec)
A B C D E F G
3 3 3 12 13 3 2
1 1 1 0.91 0.95 0.99 0.97
MBE MBE MBE LPE LPE LPE MBE
0.22 0.27 0.37 0.21 0.32 0.75 0.8
5.65692 5.65752 5.65903 5.65663 5.65831 5.66483 5.66538
5.65353 5.65355 5.65352 5.65363 5.65362 5.65358 5.65365
1.2310 24 5.4310 24 1.5310 24 21310 24 21310 24 22310 24 22310 24
6310 25 2.7310 24 7310 25 25310 25 25310 25 21310 24 21310 25
22 19 22 36 34 22 43
26 21 33 46. 48 26 67
b a
The change of the g parameter after the HP–HT treatment for partially relaxed samples was about 0.0260.01. t5layer thickness; g 5strain parameter; Da ' and Da relax are HP–HT induced change of out-of-plane and of relaxed lattice parameters, respectively.
J. Ba¸k-Misiuk et al. / Journal of Alloys and Compounds 286 (1999) 279 – 283
281
Fig. 1. Photoluminescence for strained (A) and partially relaxed (B) AlGaAs / GaAs samples.
relaxation process took place during the HP–HT treatment, because the in-plane lattice parameters increased by about ˚ 5310 25 A. PL spectra of the strained and partially relaxed Al x Ga 12x As layers before and after the HP–HT treatment are presented in Fig. 1. The decrease of the PL intensity due to the creation of nonradiative defects and the shift of exciton lines after the HP–HT treatment were observed for both kinds of samples. The change of Al x Ga 12x As defect structure after HP– HT treatment is seen on reciprocal space maps. Reciprocal space maps for a partially strained layer (x50.75) before and after the HP–HT treatment are presented in Fig 2. A widespread diffuse scattering (Fig. 2B) around the reciprocal lattice point of layer was detected for the sample subjected to the HP–HT treatment. The lattice parameters changes and that of FWHM for
the as-grown and HP–HT treated samples with different initial strain state are shown in Table 1. The strain state of the samples is described by the strain parameter g, where g is defined by 1-g 5 (a i 2 a s ) /(a relax 2 a s )
(2)
The symbols a i and a relax denote the in-plane lattice parameter and the relaxed one of the layer material, respectively; a s is the lattice parameter of GaAs substrate. . Therefore, g 51 corresponds to a fully strained structure, whereas g 50 corresponds to a full relaxation. The in-plane lattice parameter for strained layers was the ˚ at 258C). It same as that for the GaAs substrate (5.65351 A is visible (Table 1), that the initial strain state of the samples depends on Al content and layer thickness. Reflection topographs of an initially partially relaxed
Fig. 2. Reciprocal space maps around Cu Ka 1 335 reciprocal lattice point for as-grown (A) and HP–HT treated sample (B). Axes are marked in l / 2d units ( l-wavelength, d-interplanar distance). The neighbouring contours represent the intensity ratio, 10.
J. Ba¸k-Misiuk et al. / Journal of Alloys and Compounds 286 (1999) 279 – 283
282
Fig. 3. 044 reflection topographs of initially partially relaxed (x50.32) samples. (A) As-grown sample, (B) sample after HP–HT treatment.
sample before and after the HP–HT treatment are shown in Fig. 3. The topograph of the as-grown sample (Fig. 3A) does not shows dislocations, whereas they are visible for the HP–HT treated sample (Fig. 3B). A similar effect was found for some of the fully strained samples in this and previous works [7].
4. Discussion There are four main observations of our work: 1. Separate HP or HT treatments do not change the microstructure of Al x Ga 12x As layers. 2. HP–HT treatment induces new defects in the Al x Ga 12x As / GaAs system (observed in reciprocal space maps). The most probable newly created defects are the dislocation loops pushed out from the precipitates of both different compressibility and thermal expansion with respect to the matrix material. It is known, that at sufficiently severe HP–HT conditions, the stresses at the precipitate / matrix boundary can reach the critical value for emitting dislocation loops and other defects [2]. 3. The HP–HT treatment causes a creation of misfit dislocations. 4. After the HP–HT treatment, the relaxed lattice parameters increase for the initially fully strained layers and decrease for the initially partially relaxed layers. These changes in the lattice parameters were in agreement with PL results. Due to the negative sign of the deformation potential of Al x Ga 12x As, the HP–HT induced increase or decrease of lattice parameter lead to decrease or increase of the energy gap, respectively, in agreement with our observations. The two last (3, 4) statements need an additional explanation. The HP–HT conditions are not favourable for lattice relaxation. The lattice mismatch at the layer / sub-
strate interface induced at HP–HT conditions, (´HP – HT ), can be estimated [2] from:
´HP – HT 5 DT(a1 2 a2 ) 1 ( p / 3)(1 /B2 2 1 /B1 )
(3)
where: p5applied pressure, DT 5 T treatment 2 T room , a1 , a2 and B1 , B2 are the linear thermal expansion coefficients and bulk moduli for the substrate and layer materials, respectively. These values for GaAs and layer materials are as follows [12]: aGaAs 56.7310 26 K 21 , BGaAs 5 75 GPa, aAlAs 55310 26 K 21 , BAlAs 578 GPa. According to the formula (3), the mismatch at the HP–HT conditions at the layer / GaAs boundary should be lower than that for the as-grown layer. For example, for the Al 0.8 Ga 0.2 As layer, the ´HP – HT value equals to 6310 24 , whereas ´as-grown 5 1.3310 23 . Therefore, no relaxation resulting in the creation of misfit dislocation would occur at HP–HT. We cannot attribute the observed creation of misfit dislocations to the greater lattice mismatch at high pressure treatment. Instead, we propose an explanation, in which newly created defects (see point 2 above) are the source of misfit dislocations. Concerning point 4, we should be aware that the presence of misfit dislocations can enhance the diffusion of Al to them. Such a phenomenon was observed for SiGe / Si system [5]. For both kinds of AlGaAs layers (fully strained and partially strained) there are two competitive processes leading to a change of lattice parameters: 1. Creation of defects leading to an increase of lattice parameters, 2. Diffusion of Al towards dislocations leading to a decrease of lattice parameters. For initially fully strained layers with small dislocation density, the first process would be a dominant one. The opposite situation occurs for the initially partially relaxed
J. Ba¸k-Misiuk et al. / Journal of Alloys and Compounds 286 (1999) 279 – 283
layers with a higher dislocation density, for which diffusion of Al towards defects is expected to prevail.
Acknowledgements This work was supported in part by the grants of Polish Committee for Scientific Research (KBN) No. 8 T11B 03010, 8 T11B 00913 and 2 P03B02109.
References [1] T. Jimbo, H. Ishiwara, Jpn. J. Phys. 36 (1997) 327. [2] J. Jung, Philos. Mag. A 50 (1984) 257. [3] A. Misiuk, J. Wolf, L. Datsenko, J. Adamczewska, J. Ba¸k-Misiuk, Nukleonika 39 (1994) 281.
283
¨ [4] L. Datsenko, V. Khrupa, S. Krasulya, A. Misiuk, J. Hartwig, B. Surma, Acta Phys. Polon. A 91 (1997) 929. [5] P. Zaumseil, G. Fischer, Ch. Quick, A. Misiuk, Solid State Phenomena 47–48 (1995) 517. [6] J. Ba¸k-Misiuk, J. Domagal«a, B. Kozankiewicz, A. Misiuk, J. Adamczewska, M. Skibska, Acta Phys. Polon. A 83 (1993) 87. ´ [7] J. Ba¸k-Misiuk, M. Leszczynski, J. Domagacl«a, J. Trela, A. Misiuk, ¨ J. Hartwig, E. Prieur, Acta Phys. Polon. A 89 (1996) 405. [8] W. Szuszkiewicz, W. Ge¸bicki, J. Ba¸k-Misiuk, J. Domagal«a, M. ´ ¨ Leszczynski, J. Hartwig, Acta Phys. Polon. A 91 (1997) 1003. [9] P.F. Fewster, N.L. Andrew, J. Appl. Cryst. 28 (1995) 451. [10] B.K. Tanner, A.G. Turnbull, C.R. Stanley, A.H. Kean, M. McElhinney, Appl. Phys. Lett. 56 (1991) 2272. [11] P.F. Fewster, Appl. Surf. Sci. 50 (1991) 9. [12] S. Adachi, in: S. Adachi (Ed.), Properties of Aluminium Gallium Arsenide, INSPEC, the Institution of Electrical Engineers, London, 1993, p. 17.
L
Influence of high hydrostatic pressure–high temperature treatment on defect structure of AlGaAs layers a, a a a a b J. Ba¸k-Misiuk *, J. Adamczewska , J. Domagal«a , Z.R. Zytkiewicz , J. Trela , A. Misiuk , c c d d ´ M. Leszczynski , J. Jun , H.B. Surma , A. Wnuk a
´ 32 /46, 02 -668 Warsaw, Poland Institute of Physics, Polish Academy of Sciences, Al.Lotnikow b ´ 32 /46, 02 -668 Warsaw, Poland Institute of Electron Technology, Al.Lotnikow c High Pressure Research Centre, Polish Academy of Sciences, Sokol«owska 29 /37, 01 -142 Warsaw, Poland d ´ ´ 133, 01 -919 Warsaw, Poland Institute of Electronic Materials Technology, Wolczynska
Abstract Influence of high pressure–high temperature treatment on structural properties of AlGaAs / GaAs structures was studied by high resolution x-ray diffractometry and photoluminescence techniques. The treatment-induced changes in lattice parameter are explained by relaxation of the misfit strain via creation of misfit dislocations and other extended defects, as well as by diffusion of Al to dislocations. 1999 Elsevier Science S.A. All rights reserved. Keywords: Semiconductors; X-ray diffraction; Defect structure; High-pressure treatment
1. Introduction Material response to stress at high temperature is of a fundamental interest. Annealing at enhanced (hydrostatic) pressure (HP treatment) of ambient gas can influence the defect structure of homo or heterogeneous systems in many ways. Firstly, if there are any imperfections of different compressibility than that of the bulk crystal, the creation of additional defects at the boundary between these imperfections and the matrix can be expected. Thus, HP treatment can enhance the development of the primary existing structure irregularities. On the other hand, an improvement of the structure perfection has been reported for some systems [1]. In both cases, the studies on HP treatment effects can help to understand the initial sample defect structure, as well as physical phenomena related to strain at the film / substrate interface. Recently, the influence of HP at high temperature (HT) on the properties of semiconductors was very intensively investigated. However, mostly the silicon crystals were studied (e.g. [2–5]) and only a few reports concerned other semiconductors [1,6–8]. It has been established that Si, GaP and GaAs *Corresponding author.
single crystals containing precipitates or inclusions of different compressibility and thermal expansion with respect to that of the matrix, can have their defect structure changed at HP–HT [1–8]. This effect was explained as being due to the higher strain at the precipitate / matrix boundary, as well as to enhanced diffusivity of components at HP conditions. Additional defects can be created if the stress exceeds a critical value. For epitaxial layers, the change of defect structure can be also expected, when the critical strain between the layer and substrate is exceeded. The influence of HP on lattice misfit relaxation has been reported for the Si 12x Ge x / Si [5], Al 0.27 Ga 0.73 As / GaAs [7,8] and GaAs / Si systems [1]. Moreover, it was found that HP–HT treatment induced changes of defect structure of semiconductor layers as well as strongly influencing their optical and electronic properties [8]. The aim of our paper is to report on the influence of the HP–HT treatment on crystallographic properties of Al x Ga 12x As heteroepitaxial layers on GaAs substrates, as a function of their initial strain state, with respect to the GaAs substrate. The influence of HP–HT treatment on the properties of layers was studied using x-ray diffraction and photoluminescence (PL) for the thin layers of Al x Ga 12x As
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )01021-4
J. Ba¸k-Misiuk et al. / Journal of Alloys and Compounds 286 (1999) 279 – 283
280
having various values of lattice mismatch in relation to GaAs.
2. Experimental Al x Ga 12x As layers of thickness in the range between 2–13 mm were grown on the (001) oriented GaAs substrates by molecular beam epitaxy (MBE) and by liquid phase epitaxy (LPE) techniques at a temperature of about 1000 K. The following treatment procedures were applied to the samples: 1. (HT): Annealing at 770 K during 1 h at atmospheric pressure. 2. (HP–HT): Annealing at 770 K under a 1.2 GPa pressure of argon during 1 h. 3. (HP): High pressure treatment under a 1.2 GPa pressure of argon at room temperature during 1 h. X-ray measurements were carried out for each sample before and after treatments using x-ray high resolution diffractometry and reflection topography. In order to determine the strain status of layers, the in-plane (a i ) and out-of-plane (a ' ) lattice parameters were measured and the symmetrical and asymmetrical reciprocal space maps were recorded. The lattice parameters were determined by the Bond method or by the method of the direct measurements of the Bragg angle [9] using the symmetrical 004 and asymmetrical 117, 115 and 335 reflections. Cu Ka 1 radiation was used in the experiment. The measured lattice parameters a i and a ' served for evaluation of the relaxed lattice parameters: a relax 5 (a ' 1 2a i C) /(1 1 2C)
(1)
where: C5(12 n ) /(11 n ), n 5Poisson ratio (we assumed
the linear relation between nAlAs 50.275 and nGaAs 50.311 [10]). It has been shown (e.g. in [11]), that contour maps of diffuse scattering measured near the reciprocal lattice point are related to the defect structure of the sample. Therefore, the reciprocal space maps were also recorded to study the changes of the defect structure induced by the HP–HT treatment. Moreover, the Al x Ga 12x As layers with the direct energy gap (x,0.4) were examined by photoluminescence spectroscopy at a temperature of 6 K. PL was excited by a 488-nm line of argon laser.
3. Results All investigated samples did not exhibit any measurable structural changes after atmospheric pressure annealing at 770 K (HT) or after HP treatment at room temperature. Such changes occurred only after simultaneous application of the HP and HT. For all Al x Ga 12x As samples, an increase of the full width at half maximum (FWHM) of the 004 rocking curve, and a decrease of PL intensity, were observed after the HP–HT treatment. Also the changes of the lattice parameters were detected (see Table 1). We were able to find the following correlation of these changes with the initial state of the strain in the as-grown samples. For initially strained thin layers (samples A, B, C), the increase of the out-of-plane lattice parameter, as well as of the relaxed one, a relax , took place. No change of the in-plane lattice parameter was detected. It means that layers after annealing at HP remain fully strained (see Table 1). The partially relaxed Al x Ga 12x As layers (samples D, E, F, G) exhibit different changes of the lattice parameters after the HP–HT treatment. For such samples, the out-ofplane lattice parameters and the relaxed ones decreased (Table 1). It is necessary to admit that an additional
Table 1 Features of AlGaAs samples before and after the HP–HT treatment a Samples
t (mm)
gb
Growth method
x
a' before treatment 62310 25 ˚ (A)
ai before treatment 63310 25 ˚ (A)
Da ' 62.5310 25 ˚ (A)
Da relax. 62.5310 25 ˚ (A)
FWHM as grown (arcsec)
FWHM after HP–HT (arcsec)
A B C D E F G
3 3 3 12 13 3 2
1 1 1 0.91 0.95 0.99 0.97
MBE MBE MBE LPE LPE LPE MBE
0.22 0.27 0.37 0.21 0.32 0.75 0.8
5.65692 5.65752 5.65903 5.65663 5.65831 5.66483 5.66538
5.65353 5.65355 5.65352 5.65363 5.65362 5.65358 5.65365
1.2310 24 5.4310 24 1.5310 24 21310 24 21310 24 22310 24 22310 24
6310 25 2.7310 24 7310 25 25310 25 25310 25 21310 24 21310 25
22 19 22 36 34 22 43
26 21 33 46. 48 26 67
b a
The change of the g parameter after the HP–HT treatment for partially relaxed samples was about 0.0260.01. t5layer thickness; g 5strain parameter; Da ' and Da relax are HP–HT induced change of out-of-plane and of relaxed lattice parameters, respectively.
J. Ba¸k-Misiuk et al. / Journal of Alloys and Compounds 286 (1999) 279 – 283
281
Fig. 1. Photoluminescence for strained (A) and partially relaxed (B) AlGaAs / GaAs samples.
relaxation process took place during the HP–HT treatment, because the in-plane lattice parameters increased by about ˚ 5310 25 A. PL spectra of the strained and partially relaxed Al x Ga 12x As layers before and after the HP–HT treatment are presented in Fig. 1. The decrease of the PL intensity due to the creation of nonradiative defects and the shift of exciton lines after the HP–HT treatment were observed for both kinds of samples. The change of Al x Ga 12x As defect structure after HP– HT treatment is seen on reciprocal space maps. Reciprocal space maps for a partially strained layer (x50.75) before and after the HP–HT treatment are presented in Fig 2. A widespread diffuse scattering (Fig. 2B) around the reciprocal lattice point of layer was detected for the sample subjected to the HP–HT treatment. The lattice parameters changes and that of FWHM for
the as-grown and HP–HT treated samples with different initial strain state are shown in Table 1. The strain state of the samples is described by the strain parameter g, where g is defined by 1-g 5 (a i 2 a s ) /(a relax 2 a s )
(2)
The symbols a i and a relax denote the in-plane lattice parameter and the relaxed one of the layer material, respectively; a s is the lattice parameter of GaAs substrate. . Therefore, g 51 corresponds to a fully strained structure, whereas g 50 corresponds to a full relaxation. The in-plane lattice parameter for strained layers was the ˚ at 258C). It same as that for the GaAs substrate (5.65351 A is visible (Table 1), that the initial strain state of the samples depends on Al content and layer thickness. Reflection topographs of an initially partially relaxed
Fig. 2. Reciprocal space maps around Cu Ka 1 335 reciprocal lattice point for as-grown (A) and HP–HT treated sample (B). Axes are marked in l / 2d units ( l-wavelength, d-interplanar distance). The neighbouring contours represent the intensity ratio, 10.
J. Ba¸k-Misiuk et al. / Journal of Alloys and Compounds 286 (1999) 279 – 283
282
Fig. 3. 044 reflection topographs of initially partially relaxed (x50.32) samples. (A) As-grown sample, (B) sample after HP–HT treatment.
sample before and after the HP–HT treatment are shown in Fig. 3. The topograph of the as-grown sample (Fig. 3A) does not shows dislocations, whereas they are visible for the HP–HT treated sample (Fig. 3B). A similar effect was found for some of the fully strained samples in this and previous works [7].
4. Discussion There are four main observations of our work: 1. Separate HP or HT treatments do not change the microstructure of Al x Ga 12x As layers. 2. HP–HT treatment induces new defects in the Al x Ga 12x As / GaAs system (observed in reciprocal space maps). The most probable newly created defects are the dislocation loops pushed out from the precipitates of both different compressibility and thermal expansion with respect to the matrix material. It is known, that at sufficiently severe HP–HT conditions, the stresses at the precipitate / matrix boundary can reach the critical value for emitting dislocation loops and other defects [2]. 3. The HP–HT treatment causes a creation of misfit dislocations. 4. After the HP–HT treatment, the relaxed lattice parameters increase for the initially fully strained layers and decrease for the initially partially relaxed layers. These changes in the lattice parameters were in agreement with PL results. Due to the negative sign of the deformation potential of Al x Ga 12x As, the HP–HT induced increase or decrease of lattice parameter lead to decrease or increase of the energy gap, respectively, in agreement with our observations. The two last (3, 4) statements need an additional explanation. The HP–HT conditions are not favourable for lattice relaxation. The lattice mismatch at the layer / sub-
strate interface induced at HP–HT conditions, (´HP – HT ), can be estimated [2] from:
´HP – HT 5 DT(a1 2 a2 ) 1 ( p / 3)(1 /B2 2 1 /B1 )
(3)
where: p5applied pressure, DT 5 T treatment 2 T room , a1 , a2 and B1 , B2 are the linear thermal expansion coefficients and bulk moduli for the substrate and layer materials, respectively. These values for GaAs and layer materials are as follows [12]: aGaAs 56.7310 26 K 21 , BGaAs 5 75 GPa, aAlAs 55310 26 K 21 , BAlAs 578 GPa. According to the formula (3), the mismatch at the HP–HT conditions at the layer / GaAs boundary should be lower than that for the as-grown layer. For example, for the Al 0.8 Ga 0.2 As layer, the ´HP – HT value equals to 6310 24 , whereas ´as-grown 5 1.3310 23 . Therefore, no relaxation resulting in the creation of misfit dislocation would occur at HP–HT. We cannot attribute the observed creation of misfit dislocations to the greater lattice mismatch at high pressure treatment. Instead, we propose an explanation, in which newly created defects (see point 2 above) are the source of misfit dislocations. Concerning point 4, we should be aware that the presence of misfit dislocations can enhance the diffusion of Al to them. Such a phenomenon was observed for SiGe / Si system [5]. For both kinds of AlGaAs layers (fully strained and partially strained) there are two competitive processes leading to a change of lattice parameters: 1. Creation of defects leading to an increase of lattice parameters, 2. Diffusion of Al towards dislocations leading to a decrease of lattice parameters. For initially fully strained layers with small dislocation density, the first process would be a dominant one. The opposite situation occurs for the initially partially relaxed
J. Ba¸k-Misiuk et al. / Journal of Alloys and Compounds 286 (1999) 279 – 283
layers with a higher dislocation density, for which diffusion of Al towards defects is expected to prevail.
Acknowledgements This work was supported in part by the grants of Polish Committee for Scientific Research (KBN) No. 8 T11B 03010, 8 T11B 00913 and 2 P03B02109.
References [1] T. Jimbo, H. Ishiwara, Jpn. J. Phys. 36 (1997) 327. [2] J. Jung, Philos. Mag. A 50 (1984) 257. [3] A. Misiuk, J. Wolf, L. Datsenko, J. Adamczewska, J. Ba¸k-Misiuk, Nukleonika 39 (1994) 281.
283
¨ [4] L. Datsenko, V. Khrupa, S. Krasulya, A. Misiuk, J. Hartwig, B. Surma, Acta Phys. Polon. A 91 (1997) 929. [5] P. Zaumseil, G. Fischer, Ch. Quick, A. Misiuk, Solid State Phenomena 47–48 (1995) 517. [6] J. Ba¸k-Misiuk, J. Domagal«a, B. Kozankiewicz, A. Misiuk, J. Adamczewska, M. Skibska, Acta Phys. Polon. A 83 (1993) 87. ´ [7] J. Ba¸k-Misiuk, M. Leszczynski, J. Domagacl«a, J. Trela, A. Misiuk, ¨ J. Hartwig, E. Prieur, Acta Phys. Polon. A 89 (1996) 405. [8] W. Szuszkiewicz, W. Ge¸bicki, J. Ba¸k-Misiuk, J. Domagal«a, M. ´ ¨ Leszczynski, J. Hartwig, Acta Phys. Polon. A 91 (1997) 1003. [9] P.F. Fewster, N.L. Andrew, J. Appl. Cryst. 28 (1995) 451. [10] B.K. Tanner, A.G. Turnbull, C.R. Stanley, A.H. Kean, M. McElhinney, Appl. Phys. Lett. 56 (1991) 2272. [11] P.F. Fewster, Appl. Surf. Sci. 50 (1991) 9. [12] S. Adachi, in: S. Adachi (Ed.), Properties of Aluminium Gallium Arsenide, INSPEC, the Institution of Electrical Engineers, London, 1993, p. 17.