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Raman scattering from GaAs-InxGa1−xAs strained-layer superllatices
Solid State Communications, Vol. 51, No. 5, pp. 343-345, 1984. Printed in Great Britain.
0038-1098/84 $3.00 + .00 Pergamon Press Ltd.
RAMAN SCATTERING FROM GaAs-InxGal_xAs STRAINED-LAYER SUPERLATTICES M. Nakayama, K. Kubota, H. Kato and N. Sano Faculty of Science, Kwansei Gakuin University, Uegahara, Nishinomiya, 662, Japan
(Received 4 April 1984 by J. Kanamori) Measurements of Raman scattering were performed on GaAs-InxGal_xAs strained.layer supedattices, grown b~, molecular beam epitaxy, with lattice periods ranging from 30 "" 250 g and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-AlxGal_xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk InxGal_xAs alloys as the ratio of the one-layer thickness of InxGal_xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer supedattice and the phonen frequency is determined by the averaged In concentration.
STRAINED-LAYER SUPERLATTICES (SIS's) made from lattice mismatched constituents have been recently of great interest for their unique electronic [ 1] and optical [2] properties. In this letter we report the first Raman scattering investigation of GaAs-InxGal_xAs SLS's with variations of In concentrations and lattice periodicities and discuss about the GaAs-like optical phonon mode which is very different from that of lattice matched supeflattices like GaAs-A1As and GaAsAlzGal_xAs. The samples used for the present measurements were grown on a semi-insulating (0 0 I) oriented GaAs substrate by molecular beam epitaxy and their total thickness was ~- 1/an. A 3000 A buffer layer of InyGal_~As in which the In concentration is half that of InxGat_xAs layers in the SIS was grown between the SIS and its GaAs substrate to ensure that the SIS, taken as a whole, was scarcely strained [3]. The lattice periods of the SLS's ranged from 30 to 250A and the In concentrations were 0.22 and 0.37. The lattice periods, onelayer thicknesses and In concentrations were determined by X-ray diffraction and photoluminescence measurements. All the Raman measurements were made at room temperature on (0 01) faces in backscattering geometry with a 5145 A line of an Ar+ laser. In this geometry, the scattering from zone center longitudinal optical (LO) phonons is allowed and that from transverse optical (TO) phonons is forbidden. Figure 1 shows Raman spectra from (a) a GaAsIn~zGao.~As SIS with a lattice period of 24 A GaAs25 A Ino.22Gao.~As and (b) a GaAs-Alo.3Gao.vAs superlattice with a lattice period of 20 A GaAs22A Alo.aGao.TAs. In Fig. l(a), the peak at 290cm -x 343
C~As-1~2GamAs
(a)
"E D L_ m v >.
I
!
c,=~s- AhC.~TAS
(b)
Z UJ Z
t
200
i
300
zOO
RAMAN SHIFT (¢m"a)
Fig. 1. Raman spectra from (a) a GaAs-Ino.t2Gao.7aAs SIS with a lattice period of 24 A GaAs-25 Ino.2zGao.vsAs, (b) a GaAs-AI o aGaonAs superlattice with a lattice period of 20A GaAs-22 A Alo.3Gao.TAs. corresponds to the GaAs-like LO phonon of the SLS. The sharpness of the LO phonon line of the SIS implies a high degree of crystal perfection and uniformity of the
344
Vol. 51, No. 5"
GaAs-InxGal_xAs STRAINED-LAYER SUPERLATTICES •
300 ~k zn~TSAs
300
bulk C~As
LO
290 ~ LO~..~
• ~, x = o.22 SLS • o x=o.37 SLS
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290
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100
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LATTICE PERIOD (~)
0
o
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|
|
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0.2
0.4
0.6
0.8
3.0
ONE-LAYER THICKNESS OFInGaAs/PERI00
Fig. 2. Dependence of the GaAs-like phonon frequencies of GaAs-I%~zGao.7sAs SLS's on the lattice period. The one-layer thickness of Ino.22Gao.TsAsis 1.3 times as large as that of GaAs.
Fig. 3. Dependence of the OaAs.like phonon frequencies of GaAs-InxGal_xAs SLS's on the ratio of the one-layer thickness of InxGal_xAs to the lattice period. The onelayer thickness of GaAs is 24 A in each sample.
interracial stress between the GaAs and In0.22Gao.7sAs layers since a random stress would have broadened it [4]. The weak peak at 267.5 cm -1 corresponds to the GaAs-like TO phonon, resulting from the complexity of the interfaces between the GaAs and Ino.22Gao.78As layers. InAs-like phonon peaks are not observed. All the Raman spectra obtained from GaAs-InxGal_xAs SLS's have the same features. Comparing Fig. l(a) with Fig. l(b), it is clear that the GaAs-like LO phonon modes are different between two spectra. It is well-known that in GaAs-AlxGal_xAs superlattices, observed GaAs-like LO phonon modes have two peaks. In Fig. l(b), the two peaks at 280 and 290 cm -1 correspond to the GaAs-like LO phonon in the Alo.sGao.TAs layers and the LO phonon in the GaAs layers, respectively. The peak at 267 cm -~ corresponds to the GaAs-like TO phonons and cannot be decomposed into two peaks. Figure 2 shows the dependence of the GaAs-like phonon frequencies on the lattice periods of GaAsIno.22Gao.TaAs SIS's in the range of 30 ~ 250A. The one-layer thickness of Ino.22Gao.TsAsis 1.3 times as large as that of GaAs in each sample. The phonon frequencies of bulk GaAs and a bulk Ino.22Gao.TsAs alloy are indicated at the right hand side and left hand side in Fig. 2, respectively. It is clear that the GaAs-like phonon frequencies of the SLS's do not depend on the lattice periods and disagree with those of bulk GaAs and a bulk Ino.22Gao.TsAsalloy. Figure 3 shows the dependence of the GaAs-like phonon frequencies of GaAs-InxGal_xAs SLS's on the ratio of the one-layer thickness of InxGal_xAs to the lattice period of the SLS. The oneJayer thickness of GaAs is 24 A in each sample. The phonon frequencies at the ratio equal to zero and one correspond to those of bulk GaAs and
InxGal_xAs alloys, respectively. In each case of the In concentration 0.22 and 0.37, the GaAs-like phonon frequencies of the GaAs-InxGal_xAs SLS's shift from those of bulk GaAs to bulk InxGal_xAs alloys as the ratio increases from zero to one.. In the GaAs-A1As superlattices, the GaAsJike and AlAs-like phonon frequencies shift to the low frequency side as the one-layer thickness decreases [5]. When the one-layer thickness is above 50 A, the phonon frequencies are almost the same as the bulk ones. This result indicates that the phonon modes of GaAs-AlAs superlattices independently stand in each layer and show the bulk crystal features of the constituents. It is the same with GaAs-AlxGal_xAs superlattices. On the other hand, the phonon mode of GaAs-InxGal_xAs SLS's seems to be a uniform mode of the whole SLS and its frequency disagrees with those of the constituents. Using high resolution electron microscopy in conjunction with electron diffraction, a direct observation of stress.induced distortion in a GaAs-Ino.2Gao.sAs SLS with a period of 150A GaAs-150A Ino.2Gao.sAs has been reported [6]. The result indicates that the lattice mismatch in the SLS induces tetragonal distortion of each layer on either side of the interface. In lattice matched superlattices like GaAs-AlAs and GaAsAlxGal_xAs , this strain effect can be ignored and the lattice conditions of GaAs, AlAs and AlxGal_xAs layers in the superlattices are bulk crystal like. On the other hand, as lattice distortions occur on the interface regions in GaAs-InzGal_xAs SIS's, the lattice conditions of the constituents are not bulk crystal like. It is supposed that this strain effect is a cause for the uniform phonon mode. A feature of the uniform phonon mode, as shown in Fig. 3, is that the phonon
Vol. 51, No. 5
GaAs-InxGa,_xAs STRAINED-LAYER SUPERLATTICES
300 • '~
X=0.22
SLS
• o
x=0.37
SLS
._, 290
¥
*- 280
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260 250
I
I
I
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0.2
0.3
0-4
AVERAGED In CONCENTRATION
345
phonon frequencies of bulk InxGa,_xAs alloys. From Fig. 4, it is clear that the GaAs-like phonon frequencies of the GaAs-InxGal_xAs SLS agree with those of the bulk InyGal_yAs alloy which has the averaged In concentration of the SLS. In conclusion, the GaAs-like phonon mode of GaAs-InxGal_xAs SLS's is a uniform mode of the whole SLS, in contrast to the feature of lattice matched superlattices like GaAs-AIAs and GaAs-AlxGal_xAs. The GaAs-like phonon frequencies can be controlled in the range from those of bulk GaAs to those of bulk InxGa,_xAs alloys by the one.layer thicknesses of GaAs and InxGa,_xAs and be determined by the averaged In concentration of the SLS.
Fig. 4. Dependence of the GaAs-like phonon frequencies of GaAs-InxGa~_xAs SLS's on the averaged In concentration. All the plotted data are the same as in Fig. 3. Solid lines show the dependence of the phonon frequencies of bulk InxGal_xAs alloys.
Acknowledgement - This work and the MBE facilities were supported by the Yamada Science Foundation. The authors would like to acknowledge the assistance of Mr. S. Matsudaira in making measurements.
frequencies shift from those bulk GaAs to those of bulk InxGa,_xAs alloys as the ratio of the one-layer thickness of InxGa,_xAs to the lattice period increases from zero to one. Under the condition that the ratio is constant, the GaAs-like phonon frequencies, as shown in Fig. 2, are constant over the range of the lattice period. Figure 4 shows the dependence of the GaAs-like phonon frequencies of GaAs-InxGaa_xAs SLS's on the averaged In concentration which is obtained from multiplying x by the ratio of the one-layer thickness of InxGal_xAs to the lattice period. All the plotted data are the same as in Fig. 3. Solid lines show the dependence of the GaAs-like
REFERENCES 1. 2. 3. 4. 5. 6.
G.C. Osbourn, Phys. R ev. B27, 5126 (1983). G.C. Osbourn, R.M. Biefeld & P.L. Goudey, Appl. Phys. Lett. 41,172 (1982). I.J. Fritz, L.R. Dawson & T.E. Zipperian, Appl. Phys. Lett. 43,846 (1983). DJ. Evans & S. Ushioda, Phys. Rev. B9, 1638 (1974). K. Kubota, M. Nakayama, H. Katoh & N. Sane, Solid State Commun. 49, 157 (1984). J.M. Brown, N. Holonyak, Jr., MJ. Ludowise, W.T. Dietze & C.R. Lewis, Appl. Phys. Lett. 43, 863 (1983).
0038-1098/84 $3.00 + .00 Pergamon Press Ltd.
RAMAN SCATTERING FROM GaAs-InxGal_xAs STRAINED-LAYER SUPERLATTICES M. Nakayama, K. Kubota, H. Kato and N. Sano Faculty of Science, Kwansei Gakuin University, Uegahara, Nishinomiya, 662, Japan
(Received 4 April 1984 by J. Kanamori) Measurements of Raman scattering were performed on GaAs-InxGal_xAs strained.layer supedattices, grown b~, molecular beam epitaxy, with lattice periods ranging from 30 "" 250 g and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-AlxGal_xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk InxGal_xAs alloys as the ratio of the one-layer thickness of InxGal_xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer supedattice and the phonen frequency is determined by the averaged In concentration.
STRAINED-LAYER SUPERLATTICES (SIS's) made from lattice mismatched constituents have been recently of great interest for their unique electronic [ 1] and optical [2] properties. In this letter we report the first Raman scattering investigation of GaAs-InxGal_xAs SLS's with variations of In concentrations and lattice periodicities and discuss about the GaAs-like optical phonon mode which is very different from that of lattice matched supeflattices like GaAs-A1As and GaAsAlzGal_xAs. The samples used for the present measurements were grown on a semi-insulating (0 0 I) oriented GaAs substrate by molecular beam epitaxy and their total thickness was ~- 1/an. A 3000 A buffer layer of InyGal_~As in which the In concentration is half that of InxGat_xAs layers in the SIS was grown between the SIS and its GaAs substrate to ensure that the SIS, taken as a whole, was scarcely strained [3]. The lattice periods of the SLS's ranged from 30 to 250A and the In concentrations were 0.22 and 0.37. The lattice periods, onelayer thicknesses and In concentrations were determined by X-ray diffraction and photoluminescence measurements. All the Raman measurements were made at room temperature on (0 01) faces in backscattering geometry with a 5145 A line of an Ar+ laser. In this geometry, the scattering from zone center longitudinal optical (LO) phonons is allowed and that from transverse optical (TO) phonons is forbidden. Figure 1 shows Raman spectra from (a) a GaAsIn~zGao.~As SIS with a lattice period of 24 A GaAs25 A Ino.22Gao.~As and (b) a GaAs-Alo.3Gao.vAs superlattice with a lattice period of 20 A GaAs22A Alo.aGao.TAs. In Fig. l(a), the peak at 290cm -x 343
C~As-1~2GamAs
(a)
"E D L_ m v >.
I
!
c,=~s- AhC.~TAS
(b)
Z UJ Z
t
200
i
300
zOO
RAMAN SHIFT (¢m"a)
Fig. 1. Raman spectra from (a) a GaAs-Ino.t2Gao.7aAs SIS with a lattice period of 24 A GaAs-25 Ino.2zGao.vsAs, (b) a GaAs-AI o aGaonAs superlattice with a lattice period of 20A GaAs-22 A Alo.3Gao.TAs. corresponds to the GaAs-like LO phonon of the SLS. The sharpness of the LO phonon line of the SIS implies a high degree of crystal perfection and uniformity of the
344
Vol. 51, No. 5"
GaAs-InxGal_xAs STRAINED-LAYER SUPERLATTICES •
300 ~k zn~TSAs
300
bulk C~As
LO
290 ~ LO~..~
• ~, x = o.22 SLS • o x=o.37 SLS
LO --~
290
u I-I,I.
280
~_ 280 LI.
U~ Z <{
270
TO
TO
--
2?0
O
~E <{
-TO
26O
o: 260
250
I
0
100
,
250
I
200
°~
300
LATTICE PERIOD (~)
0
o
I
|
|
I
I
0.2
0.4
0.6
0.8
3.0
ONE-LAYER THICKNESS OFInGaAs/PERI00
Fig. 2. Dependence of the GaAs-like phonon frequencies of GaAs-I%~zGao.7sAs SLS's on the lattice period. The one-layer thickness of Ino.22Gao.TsAsis 1.3 times as large as that of GaAs.
Fig. 3. Dependence of the OaAs.like phonon frequencies of GaAs-InxGal_xAs SLS's on the ratio of the one-layer thickness of InxGal_xAs to the lattice period. The onelayer thickness of GaAs is 24 A in each sample.
interracial stress between the GaAs and In0.22Gao.7sAs layers since a random stress would have broadened it [4]. The weak peak at 267.5 cm -1 corresponds to the GaAs-like TO phonon, resulting from the complexity of the interfaces between the GaAs and Ino.22Gao.78As layers. InAs-like phonon peaks are not observed. All the Raman spectra obtained from GaAs-InxGal_xAs SLS's have the same features. Comparing Fig. l(a) with Fig. l(b), it is clear that the GaAs-like LO phonon modes are different between two spectra. It is well-known that in GaAs-AlxGal_xAs superlattices, observed GaAs-like LO phonon modes have two peaks. In Fig. l(b), the two peaks at 280 and 290 cm -1 correspond to the GaAs-like LO phonon in the Alo.sGao.TAs layers and the LO phonon in the GaAs layers, respectively. The peak at 267 cm -~ corresponds to the GaAs-like TO phonons and cannot be decomposed into two peaks. Figure 2 shows the dependence of the GaAs-like phonon frequencies on the lattice periods of GaAsIno.22Gao.TaAs SIS's in the range of 30 ~ 250A. The one-layer thickness of Ino.22Gao.TsAsis 1.3 times as large as that of GaAs in each sample. The phonon frequencies of bulk GaAs and a bulk Ino.22Gao.TsAs alloy are indicated at the right hand side and left hand side in Fig. 2, respectively. It is clear that the GaAs-like phonon frequencies of the SLS's do not depend on the lattice periods and disagree with those of bulk GaAs and a bulk Ino.22Gao.TsAsalloy. Figure 3 shows the dependence of the GaAs-like phonon frequencies of GaAs-InxGal_xAs SLS's on the ratio of the one-layer thickness of InxGal_xAs to the lattice period of the SLS. The oneJayer thickness of GaAs is 24 A in each sample. The phonon frequencies at the ratio equal to zero and one correspond to those of bulk GaAs and
InxGal_xAs alloys, respectively. In each case of the In concentration 0.22 and 0.37, the GaAs-like phonon frequencies of the GaAs-InxGal_xAs SLS's shift from those of bulk GaAs to bulk InxGal_xAs alloys as the ratio increases from zero to one.. In the GaAs-A1As superlattices, the GaAsJike and AlAs-like phonon frequencies shift to the low frequency side as the one-layer thickness decreases [5]. When the one-layer thickness is above 50 A, the phonon frequencies are almost the same as the bulk ones. This result indicates that the phonon modes of GaAs-AlAs superlattices independently stand in each layer and show the bulk crystal features of the constituents. It is the same with GaAs-AlxGal_xAs superlattices. On the other hand, the phonon mode of GaAs-InxGal_xAs SLS's seems to be a uniform mode of the whole SLS and its frequency disagrees with those of the constituents. Using high resolution electron microscopy in conjunction with electron diffraction, a direct observation of stress.induced distortion in a GaAs-Ino.2Gao.sAs SLS with a period of 150A GaAs-150A Ino.2Gao.sAs has been reported [6]. The result indicates that the lattice mismatch in the SLS induces tetragonal distortion of each layer on either side of the interface. In lattice matched superlattices like GaAs-AlAs and GaAsAlxGal_xAs , this strain effect can be ignored and the lattice conditions of GaAs, AlAs and AlxGal_xAs layers in the superlattices are bulk crystal like. On the other hand, as lattice distortions occur on the interface regions in GaAs-InzGal_xAs SIS's, the lattice conditions of the constituents are not bulk crystal like. It is supposed that this strain effect is a cause for the uniform phonon mode. A feature of the uniform phonon mode, as shown in Fig. 3, is that the phonon
Vol. 51, No. 5
GaAs-InxGa,_xAs STRAINED-LAYER SUPERLATTICES
300 • '~
X=0.22
SLS
• o
x=0.37
SLS
._, 290
¥
*- 280
_
LL
"1" U3
z 270 ~'.,=,.-a.._
TO
260 250
I
I
I
I
0.l
0.2
0.3
0-4
AVERAGED In CONCENTRATION
345
phonon frequencies of bulk InxGa,_xAs alloys. From Fig. 4, it is clear that the GaAs-like phonon frequencies of the GaAs-InxGal_xAs SLS agree with those of the bulk InyGal_yAs alloy which has the averaged In concentration of the SLS. In conclusion, the GaAs-like phonon mode of GaAs-InxGal_xAs SLS's is a uniform mode of the whole SLS, in contrast to the feature of lattice matched superlattices like GaAs-AIAs and GaAs-AlxGal_xAs. The GaAs-like phonon frequencies can be controlled in the range from those of bulk GaAs to those of bulk InxGa,_xAs alloys by the one.layer thicknesses of GaAs and InxGa,_xAs and be determined by the averaged In concentration of the SLS.
Fig. 4. Dependence of the GaAs-like phonon frequencies of GaAs-InxGa~_xAs SLS's on the averaged In concentration. All the plotted data are the same as in Fig. 3. Solid lines show the dependence of the phonon frequencies of bulk InxGal_xAs alloys.
Acknowledgement - This work and the MBE facilities were supported by the Yamada Science Foundation. The authors would like to acknowledge the assistance of Mr. S. Matsudaira in making measurements.
frequencies shift from those bulk GaAs to those of bulk InxGa,_xAs alloys as the ratio of the one-layer thickness of InxGa,_xAs to the lattice period increases from zero to one. Under the condition that the ratio is constant, the GaAs-like phonon frequencies, as shown in Fig. 2, are constant over the range of the lattice period. Figure 4 shows the dependence of the GaAs-like phonon frequencies of GaAs-InxGaa_xAs SLS's on the averaged In concentration which is obtained from multiplying x by the ratio of the one-layer thickness of InxGal_xAs to the lattice period. All the plotted data are the same as in Fig. 3. Solid lines show the dependence of the GaAs-like
REFERENCES 1. 2. 3. 4. 5. 6.
G.C. Osbourn, Phys. R ev. B27, 5126 (1983). G.C. Osbourn, R.M. Biefeld & P.L. Goudey, Appl. Phys. Lett. 41,172 (1982). I.J. Fritz, L.R. Dawson & T.E. Zipperian, Appl. Phys. Lett. 43,846 (1983). DJ. Evans & S. Ushioda, Phys. Rev. B9, 1638 (1974). K. Kubota, M. Nakayama, H. Katoh & N. Sane, Solid State Commun. 49, 157 (1984). J.M. Brown, N. Holonyak, Jr., MJ. Ludowise, W.T. Dietze & C.R. Lewis, Appl. Phys. Lett. 43, 863 (1983).