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Composition and structure of Ge islands grown on Si(001) and of SiGe grown on Si mesa
Thin Solid Films 336 (1998) 73±75
Composition and structure of Ge islands grown on Si(001) and of SiGe grown on Si mesa D.V. Regelman a,*, V. Magidson a, R. Beserman a, K. Dettmer b a
Solid State Institute, Technion, Haifa, Israel Institute of Semiconductor Physics and Optics, T.U. Braunschweig, Germany
b
Abstract The thickness, the composition and the quality, of Si12xGex epilayers grown by molecular beam epitaxy on Si mesa structures, have been studied by Raman scattering. It has been shown that the layer deposited on the (001) plane of the mesa is thicker and of better quality that the one deposited on the (111) plane. Big Ge islands were grown on Si substrates, the composition of the islands vary with their size and with their thickness. In 2 £ 2 mm and 150-nm thick islands, the Si composition varies from ù10% at the edge of the island to ù30% at the maximum height. In 200 £ 200 nm and ù30 nm height islands the Si content is ù10%. This results points to a high surface mobility of the Si atoms on the substrate surface. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Materials; Silicon; Germanium; Islands; Mesa structures
1. Introduction Raman scattering has long been used to characterize the composition, the stress and the quality of various materials. The Raman frequency depends on the distance between nearest neighbors but the next nearest ones and even further atoms in¯uence the vibration frequency also. In a perfect material, interaction between the incoming light and the optical branches takes place close to the center of the Brilloin zone, but in less perfect materials, the phonon momentum ceases to be a good quantum number and the interaction between light and vibration will take place in an extended range of the Brilloin zone, which results in a broadening and in an asymmetry of the Raman peak [1]. When the periodicity is completely lost, as in an amorphous structure, the Raman spectrum is that of the phonon density weighted by a frequency dependent factor [2]. Calculations of the Si Raman spectra as a function of the size of spherical nano-crystals show a continuous evolution from a single Si peak at 520 cm 21 toward a broad band which frequency decreases down to 510 cm 21 for a 3-nm spherical crystal [3]. Long ago this relationship has been established experimentally [4] and the link between the line width and the frequency shift has been established. Raman scattering has been widely used to characterize and to measure the crystal quality of materials [5], to determine the stress inside a
* Corresponding author.
crystal [6], in fact this tool is a standard and ef®cient way of characterization. Here we want to use Raman scattering to study two types of structures. First, Si12xGex epilayers grown on Si mesa structures, and second nominally pure Ge islands deposited on Si substrates. We shall see that the quality and the thickness of the epilayers depend on the plane of growth, and that the nominally pure Ge islands contain a fair amount of Si, which varies with the thickness of the islands.
2. Experimental The samples studied here, were grown by molecular beam epitaxy (MBE) technique using a Balzers ultrahigh vacuum unit (UHV) UMS500. The Si(001) wafers were chemically precleaned in HF, and heated in UHV at 9708C. The Si12xGex epilayers were grown on Si substrates patterned using conventional photolithography to give stripes with lateral width ranging from 2 to 200 mm, and with a 1 mm depth, the etching gave a (111) oriented side wall. Two epilayers were grown, a Si0.6Ge0.4 60 nm thick, and a Si0.75Ge0.25 70 nm thick one. Both have nominal thickness above the theoretical critical one [7] but close to the empirical values determined experimentally. A 5.33 Ge monolayer was deposited on a Si substrate heated at 6408C. Islands which vary in size from ù200 nm to ù2 mm are formed on preexisting nucleation centers,
0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0 040-6090(98)012 72-3
74
D.V. Regelman et al. / Thin Solid Films 336 (1998) 73±75
Fig. 1. Schematic representation of a SiGe epilayer deposited on Si mesa patterned substrate and backscattering Raman measurement of the (111) plane of the mesa structure.
due to the 4.2% lattice mismatch between the incoming Ge atoms and the Si substrate. A Dilor triple spectrometer micro-Raman system with a confocal optical entrance, equipped with a OMA4000 CCD camera was used to investigate the SiGe epilayers and the Ge islands. The penetration depth of the exciting laser (wavelength 514.5 nm) depends on the Ge concentration, and is more than the thickness of the SiGe epilayers and less that of the large island. The optical resolution was proven to be 0.4 mm, the sample was mounted on a XY scanner with 0.1 mm steps. The islands shape and size distribution was measured by atomic force microscopy (AFM) from Digital Instruments in the tapping mode. A typical Raman spectrum of a mixed SiGe is composed of four main peaks [8]: The Ge±Ge vibration at ù300 cm 21, the Ge±Si one at ù400 cm 21, clusters of Si vibrate in the
Fig. 2. Raman spectra of the Si0.6Ge0.4 epilayer deposited on the (001) (broken line) and on the (111) (doted line) planes and of the Si substrate (solid line). The inset gives the normalized spectra of the Si±Si layer peaks for shape comparison.
Fig. 3. AFM image of the Ge islands grown on a Si substrate: (a) 3 mm base and 0.2 mm height, (b) 0.6 mm base and 0.1 mm height, (c) 0.2 mm base and 0.03 mm height.
frequency range 430±470 cm 21 depending on their size, and the Si±Si vibration is at ù500 cm 21, all these frequencies depend on the Ge concentration and on the stress, their shape depend on the material quality. In addition the peak at 520 cm 21 is the phonon frequency of the Si substrate.
3. SiGe Epilayers on mesa patterned Si substrates Our aim is to study the homogeneity of the SiGe epilayers, and their crystalline quality. The inset of Fig. 1 gives a schematic representation of the structure, the epilayers are deposited on the (001) and on the (111) planes, in order to compare the crystalline quality of the epilayers grown on these two planes, we have to be in the back scattering geometry in both cases. Fig. 1 shows the experimental setting of the sample, which was glued on a glass substrate and tilted by 54.78 precisely, so that the exciting laser arrives perpendicular to the (111) plane. Otherwise, under standard conditions, with the laser beam perpendicular to the (001) plane, we tend to enhance the Raman intensity scattered from the edge of the mesa. In Fig. 2 we compare the Raman spectra of the Si0.6Ge0.4 epilayer scattered from the (001) and from the (111) planes. The spectrum of pure Si is represented by the solid line, the layer on the (001) plane is represented by the broken line, and the spectrum of the layer on the (111) plane by the doted line. All spectra are normalized to the intensity of the Si±Si substrate peak to correct for selection rules. The intensity of the (111) Si±Si peak is strongly reduced compared to that of the (001) peak, the Ge±Si vibration is also reduced, in addition, as seen in the inset, the width of the Si±Si peak is
D.V. Regelman et al. / Thin Solid Films 336 (1998) 73±75
Fig. 4. Raman spectra as a function of position on a `big' island, the points on the AFM image correspond to the points A±E on the Raman spectra. The darkness corresponds to the intensity of Raman signal on the logarithmic scale. The Si content is a minimum at points A and E and maximum at points B and D, with an intermediate value at C.
higher on the (111) plane than in the (001), which indicates a poorer crystalline quality. These results are even more pronounced for the Si0.75Ge0.25 epilayer. The poorer quality and the thinner epilayer grown on the (111) plane, could be due to the different nature of dangling bonds on the different planes, dimers on the (001) plane, and the single bonds on the (001) one.
ù150 nm. high, and the small ones which are about one order of magnitude smaller whose height is ù50 nm. The insert in Fig.3 shows the AFM pro®le of a big island. We take a Raman spectrum every 0.1 mm along the A-B-CD-E line. As seen in Fig. 4, where we plot the Raman spectrum as a function of the position every 0.1 mm, the frequency of the Ge±Ge vibration varies along the line AB-C-D-E, in addition we see the Ge±Si phonon of a mixed crystal as well as the weak Si±Si vibrations of the Si clusters in the Ge island. The Si content in the island which is deduced from the Raman experiment, shows that at the A point the Si concentration is ù10% it reaches a maximum value of ù30% at the summit of the island, the Si concentration follows the thickness of the islands. The Si content in the smaller Ge islands is of the order of 10%. These results are not yet understood, they seem to imply that at a growth temperature much smaller than the melting temperature of Si, the Si atoms are extremely mobile and can diffuse easily into the Ge. Why is the Si concentration proportional to the thickness of the island? The reason could be that Si atoms in the Ge island reduce the stress between the substrate and the island, but the thickness involved here is much higher than the critical thickness of Ge on Si. Further experiments are needed to understand the mechanism which leads to the intermixing between the two types of atoms.
References [1] [2] [3] [4] [5]
4. Ge islands grown on (001) Si substrates Fig. 3 shows the AFM image of the islands grown on Si, two main types of structures can be seen, the big islands which have dimensions of the order of 2 £ 2 mm and are
75
[6] [7] [8]
P. Paraynthal, F.H. Pollak, Phys. Rev. Lett. 52 (1984) 1822. R. Shuker, R. Gammon, Phys. Rev. Lett. 25 (1970) 222. P.M. Faucher, Semiconductors Semimetals 49 (1998) 205. Z. Iqbal, S. Veprek, A.P. Webb, P. Capezzuto, Solid State Comm. 37 (1981) 993. K.K. Tiang, P. Amirtharaj, F.H. Pollack, Appl. Phys. Lett. 44 (1984) 122. F. Cerdeira, C.J. Buchenauer, F.H. Pollack, M. Cardona, Phys. Rev. B 5 (1972) 580. S.J. Jain, H.E. Maes, K. Pinardi, I.D. Wolf, J. Appl. Phys. 79 (1996) 8145. W.J. Brya, Solid State Commun. 12 (1973) 253.
Composition and structure of Ge islands grown on Si(001) and of SiGe grown on Si mesa D.V. Regelman a,*, V. Magidson a, R. Beserman a, K. Dettmer b a
Solid State Institute, Technion, Haifa, Israel Institute of Semiconductor Physics and Optics, T.U. Braunschweig, Germany
b
Abstract The thickness, the composition and the quality, of Si12xGex epilayers grown by molecular beam epitaxy on Si mesa structures, have been studied by Raman scattering. It has been shown that the layer deposited on the (001) plane of the mesa is thicker and of better quality that the one deposited on the (111) plane. Big Ge islands were grown on Si substrates, the composition of the islands vary with their size and with their thickness. In 2 £ 2 mm and 150-nm thick islands, the Si composition varies from ù10% at the edge of the island to ù30% at the maximum height. In 200 £ 200 nm and ù30 nm height islands the Si content is ù10%. This results points to a high surface mobility of the Si atoms on the substrate surface. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Materials; Silicon; Germanium; Islands; Mesa structures
1. Introduction Raman scattering has long been used to characterize the composition, the stress and the quality of various materials. The Raman frequency depends on the distance between nearest neighbors but the next nearest ones and even further atoms in¯uence the vibration frequency also. In a perfect material, interaction between the incoming light and the optical branches takes place close to the center of the Brilloin zone, but in less perfect materials, the phonon momentum ceases to be a good quantum number and the interaction between light and vibration will take place in an extended range of the Brilloin zone, which results in a broadening and in an asymmetry of the Raman peak [1]. When the periodicity is completely lost, as in an amorphous structure, the Raman spectrum is that of the phonon density weighted by a frequency dependent factor [2]. Calculations of the Si Raman spectra as a function of the size of spherical nano-crystals show a continuous evolution from a single Si peak at 520 cm 21 toward a broad band which frequency decreases down to 510 cm 21 for a 3-nm spherical crystal [3]. Long ago this relationship has been established experimentally [4] and the link between the line width and the frequency shift has been established. Raman scattering has been widely used to characterize and to measure the crystal quality of materials [5], to determine the stress inside a
* Corresponding author.
crystal [6], in fact this tool is a standard and ef®cient way of characterization. Here we want to use Raman scattering to study two types of structures. First, Si12xGex epilayers grown on Si mesa structures, and second nominally pure Ge islands deposited on Si substrates. We shall see that the quality and the thickness of the epilayers depend on the plane of growth, and that the nominally pure Ge islands contain a fair amount of Si, which varies with the thickness of the islands.
2. Experimental The samples studied here, were grown by molecular beam epitaxy (MBE) technique using a Balzers ultrahigh vacuum unit (UHV) UMS500. The Si(001) wafers were chemically precleaned in HF, and heated in UHV at 9708C. The Si12xGex epilayers were grown on Si substrates patterned using conventional photolithography to give stripes with lateral width ranging from 2 to 200 mm, and with a 1 mm depth, the etching gave a (111) oriented side wall. Two epilayers were grown, a Si0.6Ge0.4 60 nm thick, and a Si0.75Ge0.25 70 nm thick one. Both have nominal thickness above the theoretical critical one [7] but close to the empirical values determined experimentally. A 5.33 Ge monolayer was deposited on a Si substrate heated at 6408C. Islands which vary in size from ù200 nm to ù2 mm are formed on preexisting nucleation centers,
0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0 040-6090(98)012 72-3
74
D.V. Regelman et al. / Thin Solid Films 336 (1998) 73±75
Fig. 1. Schematic representation of a SiGe epilayer deposited on Si mesa patterned substrate and backscattering Raman measurement of the (111) plane of the mesa structure.
due to the 4.2% lattice mismatch between the incoming Ge atoms and the Si substrate. A Dilor triple spectrometer micro-Raman system with a confocal optical entrance, equipped with a OMA4000 CCD camera was used to investigate the SiGe epilayers and the Ge islands. The penetration depth of the exciting laser (wavelength 514.5 nm) depends on the Ge concentration, and is more than the thickness of the SiGe epilayers and less that of the large island. The optical resolution was proven to be 0.4 mm, the sample was mounted on a XY scanner with 0.1 mm steps. The islands shape and size distribution was measured by atomic force microscopy (AFM) from Digital Instruments in the tapping mode. A typical Raman spectrum of a mixed SiGe is composed of four main peaks [8]: The Ge±Ge vibration at ù300 cm 21, the Ge±Si one at ù400 cm 21, clusters of Si vibrate in the
Fig. 2. Raman spectra of the Si0.6Ge0.4 epilayer deposited on the (001) (broken line) and on the (111) (doted line) planes and of the Si substrate (solid line). The inset gives the normalized spectra of the Si±Si layer peaks for shape comparison.
Fig. 3. AFM image of the Ge islands grown on a Si substrate: (a) 3 mm base and 0.2 mm height, (b) 0.6 mm base and 0.1 mm height, (c) 0.2 mm base and 0.03 mm height.
frequency range 430±470 cm 21 depending on their size, and the Si±Si vibration is at ù500 cm 21, all these frequencies depend on the Ge concentration and on the stress, their shape depend on the material quality. In addition the peak at 520 cm 21 is the phonon frequency of the Si substrate.
3. SiGe Epilayers on mesa patterned Si substrates Our aim is to study the homogeneity of the SiGe epilayers, and their crystalline quality. The inset of Fig. 1 gives a schematic representation of the structure, the epilayers are deposited on the (001) and on the (111) planes, in order to compare the crystalline quality of the epilayers grown on these two planes, we have to be in the back scattering geometry in both cases. Fig. 1 shows the experimental setting of the sample, which was glued on a glass substrate and tilted by 54.78 precisely, so that the exciting laser arrives perpendicular to the (111) plane. Otherwise, under standard conditions, with the laser beam perpendicular to the (001) plane, we tend to enhance the Raman intensity scattered from the edge of the mesa. In Fig. 2 we compare the Raman spectra of the Si0.6Ge0.4 epilayer scattered from the (001) and from the (111) planes. The spectrum of pure Si is represented by the solid line, the layer on the (001) plane is represented by the broken line, and the spectrum of the layer on the (111) plane by the doted line. All spectra are normalized to the intensity of the Si±Si substrate peak to correct for selection rules. The intensity of the (111) Si±Si peak is strongly reduced compared to that of the (001) peak, the Ge±Si vibration is also reduced, in addition, as seen in the inset, the width of the Si±Si peak is
D.V. Regelman et al. / Thin Solid Films 336 (1998) 73±75
Fig. 4. Raman spectra as a function of position on a `big' island, the points on the AFM image correspond to the points A±E on the Raman spectra. The darkness corresponds to the intensity of Raman signal on the logarithmic scale. The Si content is a minimum at points A and E and maximum at points B and D, with an intermediate value at C.
higher on the (111) plane than in the (001), which indicates a poorer crystalline quality. These results are even more pronounced for the Si0.75Ge0.25 epilayer. The poorer quality and the thinner epilayer grown on the (111) plane, could be due to the different nature of dangling bonds on the different planes, dimers on the (001) plane, and the single bonds on the (001) one.
ù150 nm. high, and the small ones which are about one order of magnitude smaller whose height is ù50 nm. The insert in Fig.3 shows the AFM pro®le of a big island. We take a Raman spectrum every 0.1 mm along the A-B-CD-E line. As seen in Fig. 4, where we plot the Raman spectrum as a function of the position every 0.1 mm, the frequency of the Ge±Ge vibration varies along the line AB-C-D-E, in addition we see the Ge±Si phonon of a mixed crystal as well as the weak Si±Si vibrations of the Si clusters in the Ge island. The Si content in the island which is deduced from the Raman experiment, shows that at the A point the Si concentration is ù10% it reaches a maximum value of ù30% at the summit of the island, the Si concentration follows the thickness of the islands. The Si content in the smaller Ge islands is of the order of 10%. These results are not yet understood, they seem to imply that at a growth temperature much smaller than the melting temperature of Si, the Si atoms are extremely mobile and can diffuse easily into the Ge. Why is the Si concentration proportional to the thickness of the island? The reason could be that Si atoms in the Ge island reduce the stress between the substrate and the island, but the thickness involved here is much higher than the critical thickness of Ge on Si. Further experiments are needed to understand the mechanism which leads to the intermixing between the two types of atoms.
References [1] [2] [3] [4] [5]
4. Ge islands grown on (001) Si substrates Fig. 3 shows the AFM image of the islands grown on Si, two main types of structures can be seen, the big islands which have dimensions of the order of 2 £ 2 mm and are
75
[6] [7] [8]
P. Paraynthal, F.H. Pollak, Phys. Rev. Lett. 52 (1984) 1822. R. Shuker, R. Gammon, Phys. Rev. Lett. 25 (1970) 222. P.M. Faucher, Semiconductors Semimetals 49 (1998) 205. Z. Iqbal, S. Veprek, A.P. Webb, P. Capezzuto, Solid State Comm. 37 (1981) 993. K.K. Tiang, P. Amirtharaj, F.H. Pollack, Appl. Phys. Lett. 44 (1984) 122. F. Cerdeira, C.J. Buchenauer, F.H. Pollack, M. Cardona, Phys. Rev. B 5 (1972) 580. S.J. Jain, H.E. Maes, K. Pinardi, I.D. Wolf, J. Appl. Phys. 79 (1996) 8145. W.J. Brya, Solid State Commun. 12 (1973) 253.