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Structure of GaAs heteroepitaxial layer grown on GaP(001) by molecular beam epitaxy
Surface
166
Structure of GaAs heteroepitaxial by molecular beam epitaxy
Science 242 (1991) 166&170 North-Holland
layer grown on GaP(OO1)
Takashi Nomura ‘, Kenji Murakami ‘, Kenji Ishikawa ‘. Masahiro Tsuyoshi Yamaguchi b, Akira Sasaki b and Minoru Hagino a
Miyao ‘,I.
” Re.wurch lnstrtute o/ Elec~romcs, Shizuoko Umvrrsity, 3-5-l Johoku, Hamamutsu 432, Jupun h Fuc~ul!,~of Engrneenng, Shizuoka lJmner.wy, 3-5-l Johoku, Hamamcrrru 432, Japan Kcce~vd
15 May 1990; accepted
for publication
4 July 1990
This paper describes a growth mechanism in the early stage of GaAs heteroepitaxy on GaP (001). The GaAs film is grown on ;L pre-grown GaP layer by molecular beam epitaxy (MBE). The GaAs layer grows two-dimensionally on a GaP substrate up to 2 ML. The two-dimensional growth mode changes to the three-dimensional one over 2 ML in the layer thickness. The transformation from the two-dimensional layer into the islands caused by the migration of the GaAs is investigated by using the intensity profile of the RHEED pattern and the angle-resolved XPS. The islands formed in the initial growth stage are also observed by scanning tunneling microscopy (STM) in real space. The STM images revealed the island formation with an anisotropic structure.
1. Introduction
The lattice mismatch between a growth layer and a substrate affects the quality of the layer and the growth mechanism by molecular beam epitaxy (MBE). Schaffer et al. [l] reported the island formation and the coalescence in the initial growth stage of InAs on GaAs. Munekata et al. [2] reported the influences of the As/In beam flux ratio on the growth mode of InAs on GaAs. Lewis et al. [3] observed a change from a streaky pattern to a spotty one by reflection high-energy diffraction (RHEED) after the growth of a few monolayers of InGaAs on GaAs. which has a lattice mismatch of 3.5%. The three-dimensional (3D) growth mode in the initial stage is reported for GaAs on Si [4,5]. The GaAs heteroepitaxial layer grown on the Si substrate is known to have many defects. The influence of the lattice mismatch on the GaAs layer is not clear because of complex effects of the lattice mismatch, the polar-nonpolar interface and
’ Present address: Sendai National College Kami-Ayashl. Sendai. 989-31, Japan. 0034-602X/91/$03.50
of Technology.
I” 1991 - Elsevier Science Publishers
the difference in thermal expansion coefficients. The effects of the lattice mismatch on the GaAs heteroepitaxy can be derived from the GaAs growth on a GaP substrate. The lattice mismatch between GaAs and GaP is 3.7%, which is close to that between GaAs and Si, 4.1%. We have investigated the growth mechanism 01 GaAs on GaP(lO0) formed by MBE [6,7]. In this paper the more detailed observations of the early growth stage of GaAs heteroepitaxy are presented. The growth mechanism is analyzed by the intensity profile of the RHEED pattern and the results of an angle-resolved X-ray photoelectron spectroscopy (XPS). The structure of the GaAs film is observed by scanning tunneling microscopy (STM). The STM observation reveals an island formation and the structure of the islands in real space.
2. Experiments Prior to the heteroepitaxial growth of GaAs. 300 nm of the GaP homoepitaxial layer was grown
B.V. (North-Holland)
and Yamada
Science Foundation
T. Nomura et al. / Structure of GaAs heteroepitaxial layer on GaP(O01)
on a GaP (001) substrate under the phosphorus stable growth condition to produce a well-defined substrate surface. The GaAs layer was grown on the GaP pre-growth layer at the substrate temperature of 520’ C. The growth rate of GaAs was 0.4 pm/h with the As/Ga beam flux ratio of 3. The growth rate was derived from the RHEED intensity oscillation. The structure of the grown GaAs film was determined by the intensities of the XPS signal of phosphorus from the substrate with various exit angles and film thicknesses. The film thickness was calculated from the growth rate and time. The intensity profile of the RHEED pattern along the reciprocal lattice rod was also monitored during the growth to determine the structure of the film. The STM observations were performed using a Pt--1r probe tip in the vacuum of the order of 1o-6 Pa. The film grown by MBE was transferred into the STM chamber via air. However, measurements by RHEED and XPS, suggest that the effects of the air exposure have little influence on the film structure.
3. Results and discussions The measurement of the intensity profile of the RHEED pattern during the growth shows the change of the growth mode. The ratio between the full width at half maximum (FWHM) of the specular spot along and pe~endicular to the reciprocal lattice rod is plotted in fig. 1 as a function of the layer thickness. The RHEED pattern was observed with a 20 kV electron beam incident along the (110) direction. The initial increase of the FWHM ratio in the figure is due to the surface roughness induced by the initial growth. The FWHM ratio decreases drastically when the layer thickness increases from 1 to 2 ML. The decrease corresponds to the transition from a streaky pattern to a spotty pattern. The result shows the transition of the growth mode from 2D to 3D around 2 ML. The FWHM ratio is constant between 2 ML and 10 ML. The rapid decrease of the FWHM ratio is observed again around 70 ML. These decreases show the change of the film structure both at 2 ML and at 10 ML. The analysis of the intensity profile along the
167
”
g
2.0
0
0 00 0030
ic
ooco
00
0 0 0
000
1.0
0
0 000
kc+, ’ Yto Layer’ ’ Thickness ’ ’ ““’
10.0
i
’ ’ ’ ““”100.0 ’ t ML)
’
Fig. 1. The ratio between the FWHM of the specular spot along and perpendicular to the reciprocal lattice rod as a function of the layer thickness. The layer thickness is calculated from the growth rate and time.
reciprocal lattice rod of the RHEED pattern indicates the transformation from the GaAs layer into the islands on the GaP substrate. Fig. 2 shows the intensity profile of the RHEED pattern after the
I
I
A
B
1 1
I
Distance (arb. unit ) Fig. 2. The intensity profile around the specular spot in the RHEED pattern after 6 ML growth of C&As. Arrows A and $3 show the specular spot and the diffraction spot, respectively
16X
T. Nomuru
et al. / Structure
of GaAs heteroeprtrrxiul
GaAs growth stops at 6 ML. The diffraction condition was slightly off-Bragg. Arrows and A and B show the specular spot and one of the reciprocal lattice spots of bulk, respectively. The intensity of the specular spot decreases and that of reciprocal lattice spot increases with time even after the Ga beam is shut off. The change of the intensity profile shows the structural transformation of the grown film without further growth. The decrease in the intensity of the specular spot and the increase in that of the diffraction spots indicate the transformation from the two-dimensional layer into the islands as the result of the migration of GaAs. The result shows that the layer structure of GaAs is not stable on GaP at a substrate temperature of 520” C. Fig. 3 shows the intensity ratio of P 2p to Ga 3d from XPS as a function of the layer thickness. The XPS signal of phosphorus from the substrate is affected by the overlayer structure. The solid lines in the figure show the intensity ratio calculated based on the exponential decrease of the substrate intensity assuming a two-dimensional layer growth. The measured points are in good agreement with the calculated lines up to 2 ML for various exit angles. On the other hand. the mea-
Fig. 3. The Intensity ratio of P2p to Ga3d from XPS as a function of GaAs layer thickness. The squares. open circles and filled circles indicate the intensity ratios measured at the electron extt angles of So, 45 o and 90 O. respectively. The solid line5 in the figure show the Intensity ratios calculated based on the exponential decrease of the substrate intensity assuming a two-dtmensional layer growth.
Iu_yrr on GuP(OO1)
Fig. 4. The RHEED pattern observed from GaAs films with 4 ML in the layer thickness. The electron beam incidents along (I IO) and (Ii(I) for (a) and (h), respectively.
sured points exceed the calculated lines over 4 ML. The result suggests island formation over 4 ML, which is the cause of the change in the growth mode of GaAs heteroepitaxy from 2D to 3D around 2 ML. The structural change from the two-dimensional layer into the islands by the migration of GaAs is also observed in XPS results. The intensity of the phosphorus signal increases with the layer thickness from 2 ML to 10 ML. The intensity from the substrate should decrease monotonically with an increase in the thickness if the film grows in the Stranski-Krastanov mode. The increase in the intensity from the substrate asserts the decrease of the surface coverage of the GaAa. The reduction of surface coverage also supports the transformation of the GaAs two-dimensional layer into the islands.
T Nomura et al. / Structure of GuAs heteroepitaxiul
The RHEED pattern shows the anisotropic structure of the GaAs islands formed in the early stages of heteroepitaxial growth. The RHEED patterns for the GaAs film with the layer thickness of 4 ML are shown in fig. 4. A spotty pattern is observed by the electron beam incident along the (110) direction due to the island formation of GaAs. The v-shaped spotty pattern is observed for (110) electron beam incidence as shown in fig.
layer on GaP(OO1)
169
4(b). The results suggest that the formed islands have an anisotropic structure and elongate along the (110) direction. The angle of the v-shape, 24”. shows the existence of (114) facets in the elongated side of the islands. Figs. 5(a), (b) and (c) show the STM images for the surfaces of the GaAs films with 4 ML, 10 ML and 18 ML in the layer thickness, respectively. The bias voltage, -5 V, is applied to the Pt-lr
Fig. 5. The STM images for the surfaces of the G&s films with the layer thicknesses of 4 ML, 10 ML and 18 ML. The bias voltage. - 5 V is applied to the Pt-Ir probe tip and the tunneling current is 50 pA.
170
T. Nomum
et al. / Structure of GuAs heteroepitcrxd
probe tip and the tunneling current is 50 pA. The size of the scanning area and the crystallographic direction are illustrated in the figure. The vertical size is about 20 A from dark to bright. The images show that the islands elongate along the (110) direction. The observed structure is in good agreement with the suggestion from the RHEED pattern (fig. 4). The population of the islands increases rapidly with the thickness of the GaAs film. The size of each island, however, does not have a significant change. The images obtained by STM also support the growth model predicted from RHEED and XPS measurements. The STM images reveal the formation of islands and its structural anisotropy in real space.
4. Conclusions The growth mode and of GaAs heteroepitaxy on The growth mode changes layer thickness of about 2
the initial growth stage GaP were investigated. from 2D to 3D over a ML. The analysis of the
layer on GnP(OO1)
intensity profile of the RHEED pattern and the measurement of XPS show the island formation caused from the migration of GaAs. The RHEED pattern suggests the anisotropic structure of the islands. Finally, the STM observation reveals in real space the formation of the islands and its anisotropic structures.
References [I] W.J. Schaffer, M.D. Lind, S.P. Kowalczyk and R.W. Grant, J. Vat. Sci. Technol. B 1 (1983) 688. [2] H. Munekata, L.L. Chang. S.C. Woronick and Y.H. Kao. J. Cryst. Growth 81 (1987) 237. [3] B.F. Lewis, T.C. Lee, F.J. Grunthaner, A. Madhukar, R. Fernandez and J. Maserjian. J. Vat. Sci. Technol. B 2 (1984) 419. [4] H. Takasugi, M. Kawabe and Y. Bando, Jpn. J. Appl. Phys. 26 (1987) L584. [5] S.M. Koch, S.J. Ronsner, R. Hull, G.W. Yofee and J.S. Harris. J. Cryst. Growth 81 (1987) 205. [6] T. Nomura, Y. Maeda, M. Miyao, M. Hagino and K. Ishikawa, Jpn. J. Appl. Phys. 6 (1987) 908. [7] T. Nomura, M. Miyao. K. Ishikawa. Y. Suzuki and M. Hagino, Appl. Surf. Sci. 33/34 (1988) 1176.
166
Structure of GaAs heteroepitaxial by molecular beam epitaxy
Science 242 (1991) 166&170 North-Holland
layer grown on GaP(OO1)
Takashi Nomura ‘, Kenji Murakami ‘, Kenji Ishikawa ‘. Masahiro Tsuyoshi Yamaguchi b, Akira Sasaki b and Minoru Hagino a
Miyao ‘,I.
” Re.wurch lnstrtute o/ Elec~romcs, Shizuoko Umvrrsity, 3-5-l Johoku, Hamamutsu 432, Jupun h Fuc~ul!,~of Engrneenng, Shizuoka lJmner.wy, 3-5-l Johoku, Hamamcrrru 432, Japan Kcce~vd
15 May 1990; accepted
for publication
4 July 1990
This paper describes a growth mechanism in the early stage of GaAs heteroepitaxy on GaP (001). The GaAs film is grown on ;L pre-grown GaP layer by molecular beam epitaxy (MBE). The GaAs layer grows two-dimensionally on a GaP substrate up to 2 ML. The two-dimensional growth mode changes to the three-dimensional one over 2 ML in the layer thickness. The transformation from the two-dimensional layer into the islands caused by the migration of the GaAs is investigated by using the intensity profile of the RHEED pattern and the angle-resolved XPS. The islands formed in the initial growth stage are also observed by scanning tunneling microscopy (STM) in real space. The STM images revealed the island formation with an anisotropic structure.
1. Introduction
The lattice mismatch between a growth layer and a substrate affects the quality of the layer and the growth mechanism by molecular beam epitaxy (MBE). Schaffer et al. [l] reported the island formation and the coalescence in the initial growth stage of InAs on GaAs. Munekata et al. [2] reported the influences of the As/In beam flux ratio on the growth mode of InAs on GaAs. Lewis et al. [3] observed a change from a streaky pattern to a spotty one by reflection high-energy diffraction (RHEED) after the growth of a few monolayers of InGaAs on GaAs. which has a lattice mismatch of 3.5%. The three-dimensional (3D) growth mode in the initial stage is reported for GaAs on Si [4,5]. The GaAs heteroepitaxial layer grown on the Si substrate is known to have many defects. The influence of the lattice mismatch on the GaAs layer is not clear because of complex effects of the lattice mismatch, the polar-nonpolar interface and
’ Present address: Sendai National College Kami-Ayashl. Sendai. 989-31, Japan. 0034-602X/91/$03.50
of Technology.
I” 1991 - Elsevier Science Publishers
the difference in thermal expansion coefficients. The effects of the lattice mismatch on the GaAs heteroepitaxy can be derived from the GaAs growth on a GaP substrate. The lattice mismatch between GaAs and GaP is 3.7%, which is close to that between GaAs and Si, 4.1%. We have investigated the growth mechanism 01 GaAs on GaP(lO0) formed by MBE [6,7]. In this paper the more detailed observations of the early growth stage of GaAs heteroepitaxy are presented. The growth mechanism is analyzed by the intensity profile of the RHEED pattern and the results of an angle-resolved X-ray photoelectron spectroscopy (XPS). The structure of the GaAs film is observed by scanning tunneling microscopy (STM). The STM observation reveals an island formation and the structure of the islands in real space.
2. Experiments Prior to the heteroepitaxial growth of GaAs. 300 nm of the GaP homoepitaxial layer was grown
B.V. (North-Holland)
and Yamada
Science Foundation
T. Nomura et al. / Structure of GaAs heteroepitaxial layer on GaP(O01)
on a GaP (001) substrate under the phosphorus stable growth condition to produce a well-defined substrate surface. The GaAs layer was grown on the GaP pre-growth layer at the substrate temperature of 520’ C. The growth rate of GaAs was 0.4 pm/h with the As/Ga beam flux ratio of 3. The growth rate was derived from the RHEED intensity oscillation. The structure of the grown GaAs film was determined by the intensities of the XPS signal of phosphorus from the substrate with various exit angles and film thicknesses. The film thickness was calculated from the growth rate and time. The intensity profile of the RHEED pattern along the reciprocal lattice rod was also monitored during the growth to determine the structure of the film. The STM observations were performed using a Pt--1r probe tip in the vacuum of the order of 1o-6 Pa. The film grown by MBE was transferred into the STM chamber via air. However, measurements by RHEED and XPS, suggest that the effects of the air exposure have little influence on the film structure.
3. Results and discussions The measurement of the intensity profile of the RHEED pattern during the growth shows the change of the growth mode. The ratio between the full width at half maximum (FWHM) of the specular spot along and pe~endicular to the reciprocal lattice rod is plotted in fig. 1 as a function of the layer thickness. The RHEED pattern was observed with a 20 kV electron beam incident along the (110) direction. The initial increase of the FWHM ratio in the figure is due to the surface roughness induced by the initial growth. The FWHM ratio decreases drastically when the layer thickness increases from 1 to 2 ML. The decrease corresponds to the transition from a streaky pattern to a spotty pattern. The result shows the transition of the growth mode from 2D to 3D around 2 ML. The FWHM ratio is constant between 2 ML and 10 ML. The rapid decrease of the FWHM ratio is observed again around 70 ML. These decreases show the change of the film structure both at 2 ML and at 10 ML. The analysis of the intensity profile along the
167
”
g
2.0
0
0 00 0030
ic
ooco
00
0 0 0
000
1.0
0
0 000
kc+, ’ Yto Layer’ ’ Thickness ’ ’ ““’
10.0
i
’ ’ ’ ““”100.0 ’ t ML)
’
Fig. 1. The ratio between the FWHM of the specular spot along and perpendicular to the reciprocal lattice rod as a function of the layer thickness. The layer thickness is calculated from the growth rate and time.
reciprocal lattice rod of the RHEED pattern indicates the transformation from the GaAs layer into the islands on the GaP substrate. Fig. 2 shows the intensity profile of the RHEED pattern after the
I
I
A
B
1 1
I
Distance (arb. unit ) Fig. 2. The intensity profile around the specular spot in the RHEED pattern after 6 ML growth of C&As. Arrows A and $3 show the specular spot and the diffraction spot, respectively
16X
T. Nomuru
et al. / Structure
of GaAs heteroeprtrrxiul
GaAs growth stops at 6 ML. The diffraction condition was slightly off-Bragg. Arrows and A and B show the specular spot and one of the reciprocal lattice spots of bulk, respectively. The intensity of the specular spot decreases and that of reciprocal lattice spot increases with time even after the Ga beam is shut off. The change of the intensity profile shows the structural transformation of the grown film without further growth. The decrease in the intensity of the specular spot and the increase in that of the diffraction spots indicate the transformation from the two-dimensional layer into the islands as the result of the migration of GaAs. The result shows that the layer structure of GaAs is not stable on GaP at a substrate temperature of 520” C. Fig. 3 shows the intensity ratio of P 2p to Ga 3d from XPS as a function of the layer thickness. The XPS signal of phosphorus from the substrate is affected by the overlayer structure. The solid lines in the figure show the intensity ratio calculated based on the exponential decrease of the substrate intensity assuming a two-dimensional layer growth. The measured points are in good agreement with the calculated lines up to 2 ML for various exit angles. On the other hand. the mea-
Fig. 3. The Intensity ratio of P2p to Ga3d from XPS as a function of GaAs layer thickness. The squares. open circles and filled circles indicate the intensity ratios measured at the electron extt angles of So, 45 o and 90 O. respectively. The solid line5 in the figure show the Intensity ratios calculated based on the exponential decrease of the substrate intensity assuming a two-dtmensional layer growth.
Iu_yrr on GuP(OO1)
Fig. 4. The RHEED pattern observed from GaAs films with 4 ML in the layer thickness. The electron beam incidents along (I IO) and (Ii(I) for (a) and (h), respectively.
sured points exceed the calculated lines over 4 ML. The result suggests island formation over 4 ML, which is the cause of the change in the growth mode of GaAs heteroepitaxy from 2D to 3D around 2 ML. The structural change from the two-dimensional layer into the islands by the migration of GaAs is also observed in XPS results. The intensity of the phosphorus signal increases with the layer thickness from 2 ML to 10 ML. The intensity from the substrate should decrease monotonically with an increase in the thickness if the film grows in the Stranski-Krastanov mode. The increase in the intensity from the substrate asserts the decrease of the surface coverage of the GaAa. The reduction of surface coverage also supports the transformation of the GaAs two-dimensional layer into the islands.
T Nomura et al. / Structure of GuAs heteroepitaxiul
The RHEED pattern shows the anisotropic structure of the GaAs islands formed in the early stages of heteroepitaxial growth. The RHEED patterns for the GaAs film with the layer thickness of 4 ML are shown in fig. 4. A spotty pattern is observed by the electron beam incident along the (110) direction due to the island formation of GaAs. The v-shaped spotty pattern is observed for (110) electron beam incidence as shown in fig.
layer on GaP(OO1)
169
4(b). The results suggest that the formed islands have an anisotropic structure and elongate along the (110) direction. The angle of the v-shape, 24”. shows the existence of (114) facets in the elongated side of the islands. Figs. 5(a), (b) and (c) show the STM images for the surfaces of the GaAs films with 4 ML, 10 ML and 18 ML in the layer thickness, respectively. The bias voltage, -5 V, is applied to the Pt-lr
Fig. 5. The STM images for the surfaces of the G&s films with the layer thicknesses of 4 ML, 10 ML and 18 ML. The bias voltage. - 5 V is applied to the Pt-Ir probe tip and the tunneling current is 50 pA.
170
T. Nomum
et al. / Structure of GuAs heteroepitcrxd
probe tip and the tunneling current is 50 pA. The size of the scanning area and the crystallographic direction are illustrated in the figure. The vertical size is about 20 A from dark to bright. The images show that the islands elongate along the (110) direction. The observed structure is in good agreement with the suggestion from the RHEED pattern (fig. 4). The population of the islands increases rapidly with the thickness of the GaAs film. The size of each island, however, does not have a significant change. The images obtained by STM also support the growth model predicted from RHEED and XPS measurements. The STM images reveal the formation of islands and its structural anisotropy in real space.
4. Conclusions The growth mode and of GaAs heteroepitaxy on The growth mode changes layer thickness of about 2
the initial growth stage GaP were investigated. from 2D to 3D over a ML. The analysis of the
layer on GnP(OO1)
intensity profile of the RHEED pattern and the measurement of XPS show the island formation caused from the migration of GaAs. The RHEED pattern suggests the anisotropic structure of the islands. Finally, the STM observation reveals in real space the formation of the islands and its anisotropic structures.
References [I] W.J. Schaffer, M.D. Lind, S.P. Kowalczyk and R.W. Grant, J. Vat. Sci. Technol. B 1 (1983) 688. [2] H. Munekata, L.L. Chang. S.C. Woronick and Y.H. Kao. J. Cryst. Growth 81 (1987) 237. [3] B.F. Lewis, T.C. Lee, F.J. Grunthaner, A. Madhukar, R. Fernandez and J. Maserjian. J. Vat. Sci. Technol. B 2 (1984) 419. [4] H. Takasugi, M. Kawabe and Y. Bando, Jpn. J. Appl. Phys. 26 (1987) L584. [5] S.M. Koch, S.J. Ronsner, R. Hull, G.W. Yofee and J.S. Harris. J. Cryst. Growth 81 (1987) 205. [6] T. Nomura, Y. Maeda, M. Miyao, M. Hagino and K. Ishikawa, Jpn. J. Appl. Phys. 6 (1987) 908. [7] T. Nomura, M. Miyao. K. Ishikawa. Y. Suzuki and M. Hagino, Appl. Surf. Sci. 33/34 (1988) 1176.