- Email: [email protected]
Si strained-layer superlattices grown by phase-locked epitaxy
548
Applied
Surface
Science 41/42
(1989) 5488552 North-Holland
RAMAN SCATTERING AND PHOTOLUMINESCENCE CHARACTERIZATION OF Ge / Si STRAINED-LAYER SUPERLA’ITICES GROWN BY PHASE-LOCKED EPITAXY H. OKUMURA, Elecirotechnrcal Received
K. MIKI,
Luhorutory,
8 November
K. SAKAMOTO,
l-l -4, Umezono,
1988; accepted
Tsukuba,
for publication
T. SAKAMOTO, Ibaraki
K. END0
and S. YOSHIDA
305, Japan
5 April 1989
Raman and photoluminescence spectra of Ge,Si, (m = 4n, 3n) strained-layer superlattices with periods of several tens of angstroms grown on Si (001) substrates by phase-locked epitaxy method were measured. The superlattice periodicity was observed as zone-folded acoustic phonon peaks for the samples with n > 2. It was found that the structures of the zone-folded acoustic phonons are described by an elastic mode1 considering the lattice strain in the Ge layers. The phonon structures above 200 cm-’ for the samples with n 2 4 are described by a linear-chain model and the concept of superlattice zone-folding. However, those for the samples with n s 2 are close to those of GeSi alloys and cannot be explained as superlattices. Ge,Si,, superlattices showed emission in the spectral region from 0.7 to 0.9 eV at 4.2 K. Quite strong emission was observed for a Ge,Si,z superlattice.
1. Introduction Ge/Si strained-layer superlattices are of much interest from the viewpoint of band engineering through lattice strain and band alignment. In addition, it is noted that a direct type band structure was predicted for certain Ge/Si superlattice structures [l]. The recent development of molecular beam epitaxy technique enables us to obtain Ge/Si strained-layer superlattices with very short periods [2]. For short-period superlattices, the formation of interfaces is very important, because interfaces have much influence on the properties of the superlattices. In this paper, we report the results of Raman scattering and photoluminescence measurements of Ge/Si strained-layer superlattices with periods of several tens of %ngstriims, and discuss the superlattice structures and their electronic structures.
2. Experiments Ge/Si superlattices were prepared on Si (001) substrates by molecular beam epitaxy observing the intensity oscillation of reflection high energy electron diffraction (RHEED) patterns. The de0169-4332/89/$03.50 (North-Holland)
6 Elsevier Science Publishers
B.V.
tails of the growth were described elsewhere [2]. Structures of (Ge,Si,,),W (n = 1, 2, 4, 6) and (Ge,Si,,), (n = 4, 6) covered with Si cap layers were prepared. From the X-ray diffraction measurements, it was found that lattice relaxation due to the introduction of misfit dislocations did not occur. Raman and photoluminescence measurements were carried on using the 488 nm line of an Ar+ laser. Raman spectra were measured in the backscattering configuration with a photon counting system at room temperature. Photoluminescence spectra were measured with a Ge detector at 4.2 K. GeSi alloys with the equivalent compositions, the thickness of which was below the critical thickness for lattice relaxation, were also measured.
3. Results and discussion If the designed superlattice structures were properly prepared, the periodicity of the superlattice structures is expected to be observed as zone-folded acoustic phonon peaks in Raman spectra. Fig. 1 shows the Raman spectra below 200 cm-’ for (Ge,,Si,,) (n = 2, 4 and 6) superlattices. The spectra exhibit the peaks assigned to
H. Okumura et al. / Characterization of Ge/Si
549
strained-layer superlattices
such zone-folded modes. In the case of GaAs/AlAs superlattices, it was reported that folded acoustic phonon peaks can be described in terms of the elastic model [3]. The lattice mismatch between Ge and Si is 4%, which is quite large compared with that between AlAs and GaAs. We tried to calculate folded acoustic phonon frequencies according to the elastic model, assuming the lattice constant of Ge layers perpendicular to the surfaces is 8% larger than that of the bulk. The frequencies calculated and actually measured are shown in fig. 2 as a function of n. Although the observed peaks are quite broad and some peaks n
Fig. 2. Observed and calculated Raman frequencies due to zone-folded acoustic phonons for (Ge,Si,,) superlattices as a function of n. The calculated curves are shown by solid lines.
x
.z : 2 0
40
80
120
160
200
0
40
80
120
160
200
‘2 3 d s
I
40.3
‘;;.
I
I
,
,
I
I
-
,x _ ._ 2 6) _ z-, -0
, 40
,
, 80
,
120
,
,
160
200
Raman shift (cm -‘) Fig. 1. Raman
spectra below 200 cm-’ for (Ge,Sis), and (GGSi,,) superlattices.
(Ge,Si,,)
do not split, the measured peak frequencies agree with the calculated ones quite well. This result indicates that the long range periodicity due to the (Ge,Si,,) (n = 2, 4 and 6) superlattice structures is properly formed as designed, and that the folded acoustic phonon structure can be explained with the elastic model even for highly-mismatched superlattices, assuming lattice strain. We also investigated (Ge,Si,) superlattices. However, peaks assigned to folded acoustic phonon modes were not observed. In X-ray diffraction measurements, peaks due to the superlattice periodicity were also observed for the samples with n 2 2, which is consistent with the Raman results. For superlattices, the peaks in the high frequency region are generally interpreted as folded optic phonon peaks. Regarding the optic phonon structure of Ge/Si superlattices, it must be noted that the selection and polarization rules are different from those of compound semiconductor superlattices like AlAs/GaAs, because of the following reasons. (1) In the case of compound semiconductor superlattices, one kind of atoms is always located at only one sublattice in zincblende structure. In Ge/Si superlattices, however, one kind of atoms can be located at two kinds of sublattices. (2) Under the treatment as a one dimensional system, the inversion center is always at the atomic plane sites for compound semiconductor superlattices. However, for Ge/Si superlattices, the in-
550
H. Okumura
L
I
,
1
1
I
200
2x0
360
200
280
360
II
200
11
280
Ranam
et (11. / Characterrzution
'
1
360
shift
440
520
600
440
520
600
"ill'
440
1
520
600
(cm -’ )
Fig. 3. Raman spectra in the region from 200 to 600 cm-’ for (Ge4Si,,), (Ge,Si,) superlattices and Ge0.2Si,I x allQy. The calculated peak positions for allowed phonon branches of the superlattices are shown in the figures by arrows.
version center can also be at intermediate points between atomic planes. In fig. 3, the Raman spectra in the frequency region from 200 to 600 cm-’ are shown for the (Ge,Si,) and (Ge,Si,,) superlattices and for a Ge,,,Si,,, alloy. The optic phonon frequencies of Ge/Si superlattices were calculated using the linear-chain model. Considering the calculated frequencies and the selection rules, the positions of folded optic phonon peaks allowed for Ge/Si superlattices are shown by arrows in fig. 3. In the case of GeSi alloys, it was reported that three kinds of peaks are observed in the spectral regions around 520, 420 and 300 to Si-Si, Si-Ge and cm-‘, which are attributed
of Gr/Sr
strained-lu_ver
superlattices
Ge-Ge. bonds, respectively [4]. In our experiments, a Ge,,Si,, alloy epilayer sample was lattice-matched to the Si substrate without dislocations and different from a bulk sample in ref. [4]. However, it shows essentially similar spectral features to those in ref. [4]. The doublet peaks at around 280 and 420 cm- ’ are considered to be due to LO-TO splitting, and the strongest peak at around 520 cm-’ comes from Si substrates. For the superlattice samples, peaks were observed in similar regions as for alloys. However, there are large differences between the spectra of the (Ge,Si,) and (Ge,Si,,) superlattices regarding peak intensity and line shape. Besides. the frequencies of the peaks observed for the (Ge,Si 16) superlattice agree with the calculated frequencies as seen in fig. 3. In the calculation of allowed LO phonon frequencies, peak intensities were not taken into account. The allowed peaks at about 250, 360 and 470 cm-’ are considered to be too weak to be observed. The intensities of the observed peaks at around 300 and 420 cm ’ are quite high. The modes of these peaks are found to be confined mainly at the interfaces between Ge and Si layers, and in Ge layers, respectively. Therefore, it is reasonable that the energies of these vibrational modes are close to those of alloys and their intensities are quite strong. The Raman peaks of (Ge,Si,,) superlattices can be explained by the concept of zone-folding of the optic phonon mode. On the contrary, for the (Ge,Si,) superlattice, the peak intensities in the regions around 420 and 300 cm- ’ are weak compared with the (Ge,Si,,) superlattice, and the observed to any peaks below 480 cm- ’ do not correspond calculated frequencies of an allowed phonon mode. In addition, the shapes of the peaks observed for the (Ge,Si,) superlattice are similar to those for the Ge,,Si o x alloy. The optic phonon modes in the region from 200 to 480 cm-’ have their vibrational amplitude mainly in Ge layers. The differences of the spectral features between (GelSi,) and (Ge,Si,,) superlattices can be attributed to Ge layers. For alloys, three kinds of Raman peaks are interpreted as vibrations of Si-Si, Si-Ge and Ge-Ge bonds. respectively. The experimental result means that the optic phonon structure of (Ge,Si x) superlattices has the same local character
H. Okumura et al. / Characterization
as seen for GeSi alloys. In this sense, the optic phonon structure of (Ge,Si,) superlattices is considered to be different from that of (Ge,Si,,) superlattices. For (Ge,Si,,) superlattices with M= 1 and 2, it is adequate to treat the optic phonon structure as alloy-like local vibrations. From the Raman spectra of folded acoustic phonon branches described above, the superlattice periodicity was confirmed for (Ge,Si,) samples with n 2 2. However, the optic phonon behavior of (Ge,Si,) samples could not be explained as superlattices. Folded optic phonon branches of superlattices are usually confined to some part of the superlattice unit. The results for (Ge,Si,) superlattices suggest that the formation of Ge, layers is not ideal for the phonon structure of the superlattice. Two monatomic layers of Ge may be too thin to confine the optic phonon. Another possible explanation is that mixing of Ge and Si atoms occurs at the interfaces. The fact that Ge showed an agglomerating tendency more than Si during the growth supports the latter explanation. From the viewpoint of the formation of interfaces and superlattice phonon structures, the observed Raman results indicate the minimum limit for the formation of Ge, thin layers is n = 2. Next, we discuss the results of photoluminescence measurements, which were performed for the investigation of the fundamental band gaps of Ge/Si superlattices. In fig. 4, photoluminescence spectra of the near-infrared region for the (Ge,Si,,) and (Ge,Si,,) superlattices are shown. For these samples, the designed superlattice structures are considered to be formed properly, judging from the Raman results. For the (Ge,Si,,) sample, a weak and broad emission peak was observed in the spectral region from 0.7 to 0.9 eV. The band gaps of GeSi alloys lattice-matched to Si are between 1.17 and 0.55 eV [5]. The observed emission peak is located in this region. However, Si substrates and cap layers as well as GeSi alloys with equivalent compositions showed no emission in this region. In a short period superlattice, socalled mini-band formation is expected. The lowest transition energy of such superlattice should be between the band gap energies of two component materials. Therefore, the observed emission peaks in our experiments are considered to come from
of Ge/Si
stramed-layer
I
700
551
superlattices
1
780
Emission
I
860
I
I
i
940
energy
1020
1100
(meV)
Fig. 4. Photoluminescence spectra in the spectral region from 700 to 1100 meV for (Ge,Si,,) and (Ge,Si,,) superlattices.
the Ge/Si superlattice portion and result from the mini-band formation. Similar new transitions of Ge-Si system were also observed by electroreflectance [6] and photoluminescence [7] measurements. Compared with the (Ge,Si,,) superlattice, the spectrum of the (Ge,Si,,) superlattice exhibits quite remarkable features. The emission intensity is more than one order of magnitude enhanced. The main peak is at 0.80 eV with a width of 20 meV. In the Brillouin zone of Si, the lowest conduction band minima are located at the intermediate points between the I (000) and X (001) points. On account of the formation of the superlattice structure along the [OOl] direction, the conversion of the conduction band minima to the IY point by zone-folding is predicted under certain conditions, which results in direct band gap transition. The difference between these two Ge/Si superlattices is that of the period along the [OOl] direction. The conversion to direct band gap may be one explanation for the striking enhancement
552
H. Okumura
et al. / Characterrration
of emission intensity observed for the (Ge,Si,,) superlattice. Further experiments are required to obtain a final conclusion.
4. Conclusion Raman and photoluminescence spectra were measured for short period Ge,,Si, (m = 4n, 3n) strained-layer superlattices. Superlattice periods were confirmed for the samples with n 2 2. It was found that the structure of folded acoustic phonon branches is described with the elastic model including lattice strain. The structure of the optic phonon is similar to that of alloys rather than superlattices, when the thickness of Ge layers is 2 monatomic layers or less. The minimum limit for
of Ge/ Si strained-layer
superlattices
the formation of thin Ge layer is 2 monatomic layers in terms of superlattice phonon structure. New optical emission peaks were observed in 0.7-0.9 eV for Ge,,Si, superlattices. In particular, superlattice shows a quite strong and a Ge,Si,, sharp emission peak at 0.80 eV.
References [l] U. Gnutzmann and K. Clausecker, Appl. Phys. 3 (1974) 9. [2] K. Miki et al.. in\‘Proc. 5th Intern. Conf. on Molecular Beam Epitaxy (Sapporo, 1988) p. 90. [3] M. Nakayama et al., Japan. J. Appl. Phys. 24 (7985) 1331. [4] W.J. Brya, Solid State Commun. 12 (1973) 253. [S] D.V. Lang et al.. Appl. Phys. Letters 47 (1985) 1333. (61 T.P. Pearsall et al., Phys. Rev. Letters 58 (1987) 729. [7] K. Eberl et al.. J. Phys. (Paris) Colloq. 11 (1987) C5-329.
Applied
Surface
Science 41/42
(1989) 5488552 North-Holland
RAMAN SCATTERING AND PHOTOLUMINESCENCE CHARACTERIZATION OF Ge / Si STRAINED-LAYER SUPERLA’ITICES GROWN BY PHASE-LOCKED EPITAXY H. OKUMURA, Elecirotechnrcal Received
K. MIKI,
Luhorutory,
8 November
K. SAKAMOTO,
l-l -4, Umezono,
1988; accepted
Tsukuba,
for publication
T. SAKAMOTO, Ibaraki
K. END0
and S. YOSHIDA
305, Japan
5 April 1989
Raman and photoluminescence spectra of Ge,Si, (m = 4n, 3n) strained-layer superlattices with periods of several tens of angstroms grown on Si (001) substrates by phase-locked epitaxy method were measured. The superlattice periodicity was observed as zone-folded acoustic phonon peaks for the samples with n > 2. It was found that the structures of the zone-folded acoustic phonons are described by an elastic mode1 considering the lattice strain in the Ge layers. The phonon structures above 200 cm-’ for the samples with n 2 4 are described by a linear-chain model and the concept of superlattice zone-folding. However, those for the samples with n s 2 are close to those of GeSi alloys and cannot be explained as superlattices. Ge,Si,, superlattices showed emission in the spectral region from 0.7 to 0.9 eV at 4.2 K. Quite strong emission was observed for a Ge,Si,z superlattice.
1. Introduction Ge/Si strained-layer superlattices are of much interest from the viewpoint of band engineering through lattice strain and band alignment. In addition, it is noted that a direct type band structure was predicted for certain Ge/Si superlattice structures [l]. The recent development of molecular beam epitaxy technique enables us to obtain Ge/Si strained-layer superlattices with very short periods [2]. For short-period superlattices, the formation of interfaces is very important, because interfaces have much influence on the properties of the superlattices. In this paper, we report the results of Raman scattering and photoluminescence measurements of Ge/Si strained-layer superlattices with periods of several tens of %ngstriims, and discuss the superlattice structures and their electronic structures.
2. Experiments Ge/Si superlattices were prepared on Si (001) substrates by molecular beam epitaxy observing the intensity oscillation of reflection high energy electron diffraction (RHEED) patterns. The de0169-4332/89/$03.50 (North-Holland)
6 Elsevier Science Publishers
B.V.
tails of the growth were described elsewhere [2]. Structures of (Ge,Si,,),W (n = 1, 2, 4, 6) and (Ge,Si,,), (n = 4, 6) covered with Si cap layers were prepared. From the X-ray diffraction measurements, it was found that lattice relaxation due to the introduction of misfit dislocations did not occur. Raman and photoluminescence measurements were carried on using the 488 nm line of an Ar+ laser. Raman spectra were measured in the backscattering configuration with a photon counting system at room temperature. Photoluminescence spectra were measured with a Ge detector at 4.2 K. GeSi alloys with the equivalent compositions, the thickness of which was below the critical thickness for lattice relaxation, were also measured.
3. Results and discussion If the designed superlattice structures were properly prepared, the periodicity of the superlattice structures is expected to be observed as zone-folded acoustic phonon peaks in Raman spectra. Fig. 1 shows the Raman spectra below 200 cm-’ for (Ge,,Si,,) (n = 2, 4 and 6) superlattices. The spectra exhibit the peaks assigned to
H. Okumura et al. / Characterization of Ge/Si
549
strained-layer superlattices
such zone-folded modes. In the case of GaAs/AlAs superlattices, it was reported that folded acoustic phonon peaks can be described in terms of the elastic model [3]. The lattice mismatch between Ge and Si is 4%, which is quite large compared with that between AlAs and GaAs. We tried to calculate folded acoustic phonon frequencies according to the elastic model, assuming the lattice constant of Ge layers perpendicular to the surfaces is 8% larger than that of the bulk. The frequencies calculated and actually measured are shown in fig. 2 as a function of n. Although the observed peaks are quite broad and some peaks n
Fig. 2. Observed and calculated Raman frequencies due to zone-folded acoustic phonons for (Ge,Si,,) superlattices as a function of n. The calculated curves are shown by solid lines.
x
.z : 2 0
40
80
120
160
200
0
40
80
120
160
200
‘2 3 d s
I
40.3
‘;;.
I
I
,
,
I
I
-
,x _ ._ 2 6) _ z-, -0
, 40
,
, 80
,
120
,
,
160
200
Raman shift (cm -‘) Fig. 1. Raman
spectra below 200 cm-’ for (Ge,Sis), and (GGSi,,) superlattices.
(Ge,Si,,)
do not split, the measured peak frequencies agree with the calculated ones quite well. This result indicates that the long range periodicity due to the (Ge,Si,,) (n = 2, 4 and 6) superlattice structures is properly formed as designed, and that the folded acoustic phonon structure can be explained with the elastic model even for highly-mismatched superlattices, assuming lattice strain. We also investigated (Ge,Si,) superlattices. However, peaks assigned to folded acoustic phonon modes were not observed. In X-ray diffraction measurements, peaks due to the superlattice periodicity were also observed for the samples with n 2 2, which is consistent with the Raman results. For superlattices, the peaks in the high frequency region are generally interpreted as folded optic phonon peaks. Regarding the optic phonon structure of Ge/Si superlattices, it must be noted that the selection and polarization rules are different from those of compound semiconductor superlattices like AlAs/GaAs, because of the following reasons. (1) In the case of compound semiconductor superlattices, one kind of atoms is always located at only one sublattice in zincblende structure. In Ge/Si superlattices, however, one kind of atoms can be located at two kinds of sublattices. (2) Under the treatment as a one dimensional system, the inversion center is always at the atomic plane sites for compound semiconductor superlattices. However, for Ge/Si superlattices, the in-
550
H. Okumura
L
I
,
1
1
I
200
2x0
360
200
280
360
II
200
11
280
Ranam
et (11. / Characterrzution
'
1
360
shift
440
520
600
440
520
600
"ill'
440
1
520
600
(cm -’ )
Fig. 3. Raman spectra in the region from 200 to 600 cm-’ for (Ge4Si,,), (Ge,Si,) superlattices and Ge0.2Si,I x allQy. The calculated peak positions for allowed phonon branches of the superlattices are shown in the figures by arrows.
version center can also be at intermediate points between atomic planes. In fig. 3, the Raman spectra in the frequency region from 200 to 600 cm-’ are shown for the (Ge,Si,) and (Ge,Si,,) superlattices and for a Ge,,,Si,,, alloy. The optic phonon frequencies of Ge/Si superlattices were calculated using the linear-chain model. Considering the calculated frequencies and the selection rules, the positions of folded optic phonon peaks allowed for Ge/Si superlattices are shown by arrows in fig. 3. In the case of GeSi alloys, it was reported that three kinds of peaks are observed in the spectral regions around 520, 420 and 300 to Si-Si, Si-Ge and cm-‘, which are attributed
of Gr/Sr
strained-lu_ver
superlattices
Ge-Ge. bonds, respectively [4]. In our experiments, a Ge,,Si,, alloy epilayer sample was lattice-matched to the Si substrate without dislocations and different from a bulk sample in ref. [4]. However, it shows essentially similar spectral features to those in ref. [4]. The doublet peaks at around 280 and 420 cm- ’ are considered to be due to LO-TO splitting, and the strongest peak at around 520 cm-’ comes from Si substrates. For the superlattice samples, peaks were observed in similar regions as for alloys. However, there are large differences between the spectra of the (Ge,Si,) and (Ge,Si,,) superlattices regarding peak intensity and line shape. Besides. the frequencies of the peaks observed for the (Ge,Si 16) superlattice agree with the calculated frequencies as seen in fig. 3. In the calculation of allowed LO phonon frequencies, peak intensities were not taken into account. The allowed peaks at about 250, 360 and 470 cm-’ are considered to be too weak to be observed. The intensities of the observed peaks at around 300 and 420 cm ’ are quite high. The modes of these peaks are found to be confined mainly at the interfaces between Ge and Si layers, and in Ge layers, respectively. Therefore, it is reasonable that the energies of these vibrational modes are close to those of alloys and their intensities are quite strong. The Raman peaks of (Ge,Si,,) superlattices can be explained by the concept of zone-folding of the optic phonon mode. On the contrary, for the (Ge,Si,) superlattice, the peak intensities in the regions around 420 and 300 cm- ’ are weak compared with the (Ge,Si,,) superlattice, and the observed to any peaks below 480 cm- ’ do not correspond calculated frequencies of an allowed phonon mode. In addition, the shapes of the peaks observed for the (Ge,Si,) superlattice are similar to those for the Ge,,Si o x alloy. The optic phonon modes in the region from 200 to 480 cm-’ have their vibrational amplitude mainly in Ge layers. The differences of the spectral features between (GelSi,) and (Ge,Si,,) superlattices can be attributed to Ge layers. For alloys, three kinds of Raman peaks are interpreted as vibrations of Si-Si, Si-Ge and Ge-Ge bonds. respectively. The experimental result means that the optic phonon structure of (Ge,Si x) superlattices has the same local character
H. Okumura et al. / Characterization
as seen for GeSi alloys. In this sense, the optic phonon structure of (Ge,Si,) superlattices is considered to be different from that of (Ge,Si,,) superlattices. For (Ge,Si,,) superlattices with M= 1 and 2, it is adequate to treat the optic phonon structure as alloy-like local vibrations. From the Raman spectra of folded acoustic phonon branches described above, the superlattice periodicity was confirmed for (Ge,Si,) samples with n 2 2. However, the optic phonon behavior of (Ge,Si,) samples could not be explained as superlattices. Folded optic phonon branches of superlattices are usually confined to some part of the superlattice unit. The results for (Ge,Si,) superlattices suggest that the formation of Ge, layers is not ideal for the phonon structure of the superlattice. Two monatomic layers of Ge may be too thin to confine the optic phonon. Another possible explanation is that mixing of Ge and Si atoms occurs at the interfaces. The fact that Ge showed an agglomerating tendency more than Si during the growth supports the latter explanation. From the viewpoint of the formation of interfaces and superlattice phonon structures, the observed Raman results indicate the minimum limit for the formation of Ge, thin layers is n = 2. Next, we discuss the results of photoluminescence measurements, which were performed for the investigation of the fundamental band gaps of Ge/Si superlattices. In fig. 4, photoluminescence spectra of the near-infrared region for the (Ge,Si,,) and (Ge,Si,,) superlattices are shown. For these samples, the designed superlattice structures are considered to be formed properly, judging from the Raman results. For the (Ge,Si,,) sample, a weak and broad emission peak was observed in the spectral region from 0.7 to 0.9 eV. The band gaps of GeSi alloys lattice-matched to Si are between 1.17 and 0.55 eV [5]. The observed emission peak is located in this region. However, Si substrates and cap layers as well as GeSi alloys with equivalent compositions showed no emission in this region. In a short period superlattice, socalled mini-band formation is expected. The lowest transition energy of such superlattice should be between the band gap energies of two component materials. Therefore, the observed emission peaks in our experiments are considered to come from
of Ge/Si
stramed-layer
I
700
551
superlattices
1
780
Emission
I
860
I
I
i
940
energy
1020
1100
(meV)
Fig. 4. Photoluminescence spectra in the spectral region from 700 to 1100 meV for (Ge,Si,,) and (Ge,Si,,) superlattices.
the Ge/Si superlattice portion and result from the mini-band formation. Similar new transitions of Ge-Si system were also observed by electroreflectance [6] and photoluminescence [7] measurements. Compared with the (Ge,Si,,) superlattice, the spectrum of the (Ge,Si,,) superlattice exhibits quite remarkable features. The emission intensity is more than one order of magnitude enhanced. The main peak is at 0.80 eV with a width of 20 meV. In the Brillouin zone of Si, the lowest conduction band minima are located at the intermediate points between the I (000) and X (001) points. On account of the formation of the superlattice structure along the [OOl] direction, the conversion of the conduction band minima to the IY point by zone-folding is predicted under certain conditions, which results in direct band gap transition. The difference between these two Ge/Si superlattices is that of the period along the [OOl] direction. The conversion to direct band gap may be one explanation for the striking enhancement
552
H. Okumura
et al. / Characterrration
of emission intensity observed for the (Ge,Si,,) superlattice. Further experiments are required to obtain a final conclusion.
4. Conclusion Raman and photoluminescence spectra were measured for short period Ge,,Si, (m = 4n, 3n) strained-layer superlattices. Superlattice periods were confirmed for the samples with n 2 2. It was found that the structure of folded acoustic phonon branches is described with the elastic model including lattice strain. The structure of the optic phonon is similar to that of alloys rather than superlattices, when the thickness of Ge layers is 2 monatomic layers or less. The minimum limit for
of Ge/ Si strained-layer
superlattices
the formation of thin Ge layer is 2 monatomic layers in terms of superlattice phonon structure. New optical emission peaks were observed in 0.7-0.9 eV for Ge,,Si, superlattices. In particular, superlattice shows a quite strong and a Ge,Si,, sharp emission peak at 0.80 eV.
References [l] U. Gnutzmann and K. Clausecker, Appl. Phys. 3 (1974) 9. [2] K. Miki et al.. in\‘Proc. 5th Intern. Conf. on Molecular Beam Epitaxy (Sapporo, 1988) p. 90. [3] M. Nakayama et al., Japan. J. Appl. Phys. 24 (7985) 1331. [4] W.J. Brya, Solid State Commun. 12 (1973) 253. [S] D.V. Lang et al.. Appl. Phys. Letters 47 (1985) 1333. (61 T.P. Pearsall et al., Phys. Rev. Letters 58 (1987) 729. [7] K. Eberl et al.. J. Phys. (Paris) Colloq. 11 (1987) C5-329.