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1.3 μm Exciton resonances in InGaAs quantum wells grown by molecular beam epitaxy using a slowly graded buffer layer
Journal of Crystal Growth 127 (1993) 759—764 North-Holland
~
~
CRYSTAL GROWT H
1.3 p~mExciton resonances in InGaAs quantum wells grown by molecular beam epitaxy using a slowly graded buffer layer S.M. Lord, B. Pezeshki, S.D. Kim and J.S. Harris, Jr. Solid State Electronics Laboratory, Stanford University, Stanford, California 94305, USA
We achieve sharp excitonic resonances near 1.3 tzm in InGaAs quantum wells grown on a GaAs substrate by molecular beam epitaxy using a slowly graded InGaAs buffer layer. Our results indicate that linear grading is preferable to step grading for high In composition InGaAs on GaAs. SIMS results confirm the linearity of the grading. Cross-section TEM analysis of the graded sample reveals dislocations within the buffer but none which are threading up to reach the quantum wells. In addition to a well-defined exciton at 1.25 ~smunder zero bias, we observe pronounced quantum confined Stark effect in the absorption spectrum of a sample including a graded buffer layer while the spectra from samples with step-graded buffers exhibit no excitonic features. We demonstrate a transmission electro-absorption modulator with a relative transmission modulation (i~IT/T) of 12% at 1.3 sm.
1. Introduction The strained system of InGaAs grown on GaAs has many technological applications including the potential for operation within the 1.3—1.55 .tm regime, which is optimal for fiber optic communications. Increasing the indium mole fraction yields devices with higher operating wavelengths. However, growth of good quality quantum wells with high In content has been difficult. Attempts to achieve long wavelength operation in InGaAs/ GaAs have included reduced area growth [11and various buffers [2—4].However, the longest wavelength modulated was only 1.06 ~tm [2,4,51.Fewer results have been obtained using quantum wells near 1.3 ~m. Melman et al. [61 demonstrated a 1.3 ~m band edge using strained layer superlattices to separate spatially portions of the biaxial compressive mismatch stress from the well layer. Roan and Chang [71recently demonstrated 1.3 ~sm 300 K luminescence using an InAs/ GaAs short period superlattice. It has been suggested that the material quality of high In concentration layers was too poor, due to islanding and/or threading dislocations, for excitonic features to be observed. Recently, several groups reported success in growing another strained system, Si1~Ge~,using graded buffer 0022-0248/93/$06.OO © 1993
—
layers [8]. They claim that the graded buffers are particularly useful for preventing the propagation of threading dislocations and thus yield superior quality material to that grown on step-graded buffers. Using molecular beam epitaxy (MBE), we have grown high In content InGaAs quantum wells on GaAs substrates with both step-graded buffers and a slowly graded InGaAs buffer layer. In this report, we describe the first observation of the quantum confined Stark effect (QCSE) near 1.3 ~am in InGaAs/ GaAs. Secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and 300 K transmission measurements are used to characterize the material.
2. Experimental procedure For this study, samples were grown on semi-insulating GaAs substrates in a Varian Gen-Il MBE system using As2. Schematics of the targeted structures are shown in fig. 1. Samples I and II were grown at 480°Cwhile Sample III was grown at 450°C.An n-type buffer layer was grown atop a 500 A GaAs buffer layer. Sample I (one-step) had a buffer of 1 ~.tmof 1n034Ga066As. For Sample II (two-step), the buffer was 5500 A of
Elsevier Science Publishers B.V. All rights reserved
760
/ 1.3
SM. Lord et al. SAMPLE 1(1-STEP BUFFER) ~In
n-In0 ~Ga0~As
SAMPLE III (GRADED BUFFER)
A
~
g
J
____________________
SI GaAs
SAMPLE 11(2-STEP BUFFER)
~In0~Ga0~As 5000 A p-In0~Ga0 ~As 5000 A 75 A In~Ga~~As ~ 75 A ln~Ga~As •~ x 30 35 A AI~~Ga~As jx 30 35 A AI~Ga•67As j.X 30 n-In0~Ga0~As5500 A n-ln~5Ga065As 1 ~sm n- In0 17Ga0 83As 10 A ‘x 50 Graded Bufler 2.3 J.tm fl- Ifl04~5Ga~~,5A$ 20 A 15% In4tm
0~Ga0~As 5000 75 A In50Ga~As 35 A AI33Ga57As
n-GaAs
sm Exciton resonances in InGaAs QWs grown by MBE
500
A
n-In0 17Gao83As 5500 A
n-GaAs
500 A
n-GaAs
n-GaAs SI GaAs
500
A
SI GaAs
Fig. 1. Schematics of the targeted MBE-grown p—i—n structures with various n-type buffer layers.
In0 17Ga0 83As followed by a superlattice of 50 x 10 A In0 17Ga083As/20 A In0 425Ga0575As and then another 5500 A of In035Ga065As. Each step was chosen to be 17% In, based on the findings of Ribas et al. [3] that dislocations in InGaAs/ GaAs will thread down rather than up for steps of 18% In. For Sample III (graded), the In GaAs buffer’s In composition was graded from 0% to 35% at a rate of 15% In per p.m. On top of the various buffers, superlattices consisting of thirty 75 A In~5Ga05As quantum wells with thirty-one 35 A Al033Ga067As barriers were grown. To complete the p—i—n structures, 5000 A of p-type 1n035Ga065As or 1n034Ga066As was grown on top. Samples were etched down to the n-layer to form 500 x 500 p.m square mesas. Electrical connections were made via an alloyed Au/ Ge/ Ni/ Au contact to the n-layer and an annular Ti—Au contact to the top of the mesas. The devices were packaged and wire bonded into a drilled mount and the optical transmission measured with a half-meter spectrometer and a white light source. Cross-sectional TEM analysis was conducted using a Philips EM400 at 120 kV. SIMS was performed at Charles Evans and Associates using Cst
3. Results and discussion Several features of the growth merit discussion. As2 was chosen because improved surface
morphology has been demonstrated for MBE growth of InGaAs on InP with As2 instead of As4 [9]. As2 may also be beneficial at the low substrate temperatures employed during growth, since As2 incorporates more efficiently than As4. Studies have shown that the critical thickness is highly dependent on growth temperature and increases as the temperature decreases [10]. Thus, low substrate temperatures are particularly important for the successful growth of high In cornpositions. TEM analysis showed that 30 A In05Ga05As quantum wells grown on GaAs at 490°C had considerably better morphology than identical wells grown at 535°C [11]. Also, the entire buffer region in Sample III is considerably thicker than in Samples I and II which may affect its role in relaxation. Considerable variation was apparent in the surface morphologies of these samples as observed in optical and scanning electron microscopy. Sample I’s surface was extremely rough with no distinct features. The surface of Sample II was also quite rough, but it showed crosshatching characteristic of dislocation formation. The surface of Sample III showed considerable cross-hatching, however, the area between the dislocation lines appeared fairly smooth in comparison with that of Sample II. The samples with step-graded buffers exhibited significantly higher leakage currents than did the graded buffer sample. For 500 x 500 p.m devices under 9 V reverse bias, the leakage currents roughly for 30 2, 400 mA/cm2, and were 80 p.A/cm2 mA/cm
S.M. Lord et al.
E
/
1.3 ~m Exciton resonances in InGaAs QWs grown by MBE
761
14000
The absorption coefficient (a) was calculated
12000
from transmission measurements including the dependence of the white light source on wavelength, considering the reflection at the surface, and subtracting off the value of absorption for a
10000 8000
4V
12500 13000 13500 wavelength (A) Fig. 2. 300 K absorption spectra for Sample III at various reverse bias voltages,
blank GaAs wafer. The nonzero a after the peak may be due to variation in the thicknesses of GaAs wafers and uncertainities in the surface reflection. An exciton is clearly visible at 1.25 p.m zero bias. The absorption per well is estimated to be 1.1%, which is comparable to that of the best GaAs/AIGaA5 quantum wells. Applica2 causes tion an electric to 300 this of peak to red field shift up while theky/cm half-width at
Samples I, II, and III, respectively. Thus, Sample III is the best candidate for a low leakage photodetector. Both of the samples with step-graded buffers exhibited extremely broad absorption edges with no excitonic behavior during 300 K photocurrent measurements. This is in marked contrast to the excellent excitonic features and accompanying QCSE observed in photocurrent measurements of Sample III. Room temperature absorption spectra at various applied biases for the sample with the graded buffer layer are shown in fig. 2.
half-maximum (HWHM) of the exciton resonance remains less than 25 rneV. The total shift obtamed was about 600 A yielding an exciton at 1.31 p.m. SIMS analysis confirms that the grading in Sample III is basically linear as seen in fig. 3. This sample was etched to remove 4000 A of the top p-layer, since SIMS resolution degrades as the sputtered depth into the sample increases. The remainder of this top layer was assumed to be the targeted composition of 35% In and used as an internal calibration for the subsequent analysis. Although it is possible to see the oscillations
~
6000
8V
.
IOV
~..
2000
under 12000
:
10.3
~
0.00
0.30 0.40 0.50 0.60 0.70 0.80 depth (microns) Fig. 3. SIMS spectra for Sample III. Inset shows higher resolution analysis. 0.10
0.20
762
SM. Lord et a!.
/ 1.3
p.m Exciton resonances in InGaAs QWs grown by MBE
in In and Al concentrations which correspond to the quantum wells and barriers within the superlattice, the resolution was insufficient to provide a direct estimate of the In composition in the
wells. However, assuming that the ratio of the thickness of the barriers to the thickness of the wells was as targeted, one may estimate the In composition in the quantum wells from the aver-
[L~
Fig. 4. Cross-section TEM micrographs ot Sample 111 showing: (a) close-up of superlattice region and (b) top, superlattice, and about 1 p.m of the graded buffer.
SM. Lord et a!.
/
1.3 p.m Exciton resonances in InGa~4sQWs grown by MBE
age composition in the superlattice. X-ray diffraction analysis showed that the superlattice period was indeed within a few percent of the targeted value. The average value of 33% In measured in the superlattice yields 48.5% In in the quantum wells. We expect the slowly graded buffer to relax fully at the top, i.e. with a lattice constant corresponding to the targeted 35% In. The superlattice should be symmetrically strained with the InGaAs quantum wells under compressive strain and the AlGaAs barriers under tensile strain. However, the wavelength at which we observe the zero bias exciton in 300 K absorption measurements does not agree with this hypothesis. Electron microprobe data shows the top p-type layer of Sample III to be 29% In rather than the targeted 35%. Attributing this difference to uncertainties in the In growth rate throughout the structure yields reasonable agreement with the absorption spectra. From the SIMS data, if the top layer is taken to be 29%, the In composition in the quantum wells~is about 40% which is consistent with the absorption data. Cross-section TEM shows smooth interfaces within Sample III’s superlattice as shown in fig. 4a. For quantum wells with such high In content, these interfaces look particularly good. No evidence of islanding, 3D growth, or clustering is visible. The 75 A thick quantum wells exceed the Matthews—Blakeslee critical thickness [121 and would result in a rough superlattice if grown directly on GaAs. Thus, the graded buffer has enabled more In to be added without degrading material quality. Fig. 4b shows a lower magnification image including some of the buffer. Note that no dislocations are visible within about 3000 A beneath the superlattice. Deeper into the graded buffer, although many dislocations are visible, none within the observable area are threading dislocations which would destroy material quality and device performance. We believe that the mechanism involved here may be related to that described by LeGoues et al. [13]. The changes in absorption and thus transmission as a function of voltage, as seen in fig. 2, makes the quantum wells in Sample III suitable for use in an electro-absorption modulator. The
763
device functions as a transmission modulator with an insertion loss of about 3 dB. Operation on the exciton peak at 1.25 p.m results in a normally off modulator with a relative transmission modulation (4T/T) of 17%. Operation at 1.3 p.m, where the initial absorption is zero, yields a normally on modulator with a 4T/T of 12%. Of course, better performance can be obtained by either antireflection coating both sides of the wafer or incorporating a Fabry—Perot cavity.
4. Conclusions In conclusion, we have demonstrated exciton resonances near 1.3 p.m for InGaAs grown on GaAs. The key to our achievement is the incorporation of a slowly graded InGaAs buffer layer beneath the quantum wells. Absorption spectra of a sample including such a buffer show well-defined QCSE, while those from samples with step-graded buffers exhibit no excitonic features. We also demonstrated an electro-absorption modulator with zlT/T of 12% at 1.3 p.m.
Acknowledgments We would like to thank C. Kirshbaum for SIMS analysis. SML acknowledges fellowship support from AT&T. This work was partially supported by DARPA and ONR through contract N00014-90-J-4056.
References [1] E.A. Beam III and Y.C. Kao, J. AppI. Phys. 69 (1991) 4253. [2] S. Niki, W.S.C. Chang, H.H. Wieder and T.E. Van Eck, J. Crystal Growth 111 (1991) 419. [3] P. Ribas, V. Krishnamoorthy and R.M. Park, AppI. Phys. Letters 57 (1990) 1040. [4] T.K. Woodward, T. Sizer, DL Sivco and A.Y. Cho, AppI. Phys. Letters 57 (1990) 548. [51I.J. Fritz, D.R. Myers, G.A. Vawter, TM. Brennan and B.E. Hammons, AppI. Phys. Letters 58 (1991) 1608. [6] P. Melman, B. Elman, C. Jagannath, E.S. Koteles, A. Silletti and D. Dugger, Appi. Phys. Letters 55 (i989) 1436.
764
S.M Lord et a!.
/
1.3 p.m Exciton resonances in InGaAs QWs grown by MBE
[7] E.J. Roan and KY. Cheng, AppI. Phys. Letters 59 (1991) 2688. [8] E.A. Fitzgerald, Y.-H. Xie, M.L. Green, D. Brasen, AR. Kortan, J. Michel, Y.-J. Mu and BE. Weir, AppI. Phys. Letters 59 (1991) 881; F.K. LeGoues, B.S. Meyerson and J.F. Morar, Phys. Rev. Letters 66 (i991) 2903. [9] Y.C. Pao, PhD Thesis, Stanford University (1990).
[10] M.J. Ekenstedt, SM. Wang and T.G. Andersson, AppI. Phys. Letters 58 (1991) 854. [11] SM. Lord, B. Pezeshki, S.D. Kim and J.S. Harris, Jr., unpublished. [12] J.W. Matthews and A.E. Blakeslee, J. Crystal Growth 27 (1974) 118. [13] F.K. LeGoues, B.S. Myerson, J.F. Morar and PD. Kirchner, J. AppI. Phys. 71(1992) 4230.
~
~
CRYSTAL GROWT H
1.3 p~mExciton resonances in InGaAs quantum wells grown by molecular beam epitaxy using a slowly graded buffer layer S.M. Lord, B. Pezeshki, S.D. Kim and J.S. Harris, Jr. Solid State Electronics Laboratory, Stanford University, Stanford, California 94305, USA
We achieve sharp excitonic resonances near 1.3 tzm in InGaAs quantum wells grown on a GaAs substrate by molecular beam epitaxy using a slowly graded InGaAs buffer layer. Our results indicate that linear grading is preferable to step grading for high In composition InGaAs on GaAs. SIMS results confirm the linearity of the grading. Cross-section TEM analysis of the graded sample reveals dislocations within the buffer but none which are threading up to reach the quantum wells. In addition to a well-defined exciton at 1.25 ~smunder zero bias, we observe pronounced quantum confined Stark effect in the absorption spectrum of a sample including a graded buffer layer while the spectra from samples with step-graded buffers exhibit no excitonic features. We demonstrate a transmission electro-absorption modulator with a relative transmission modulation (i~IT/T) of 12% at 1.3 sm.
1. Introduction The strained system of InGaAs grown on GaAs has many technological applications including the potential for operation within the 1.3—1.55 .tm regime, which is optimal for fiber optic communications. Increasing the indium mole fraction yields devices with higher operating wavelengths. However, growth of good quality quantum wells with high In content has been difficult. Attempts to achieve long wavelength operation in InGaAs/ GaAs have included reduced area growth [11and various buffers [2—4].However, the longest wavelength modulated was only 1.06 ~tm [2,4,51.Fewer results have been obtained using quantum wells near 1.3 ~m. Melman et al. [61 demonstrated a 1.3 ~m band edge using strained layer superlattices to separate spatially portions of the biaxial compressive mismatch stress from the well layer. Roan and Chang [71recently demonstrated 1.3 ~sm 300 K luminescence using an InAs/ GaAs short period superlattice. It has been suggested that the material quality of high In concentration layers was too poor, due to islanding and/or threading dislocations, for excitonic features to be observed. Recently, several groups reported success in growing another strained system, Si1~Ge~,using graded buffer 0022-0248/93/$06.OO © 1993
—
layers [8]. They claim that the graded buffers are particularly useful for preventing the propagation of threading dislocations and thus yield superior quality material to that grown on step-graded buffers. Using molecular beam epitaxy (MBE), we have grown high In content InGaAs quantum wells on GaAs substrates with both step-graded buffers and a slowly graded InGaAs buffer layer. In this report, we describe the first observation of the quantum confined Stark effect (QCSE) near 1.3 ~am in InGaAs/ GaAs. Secondary ion mass spectroscopy (SIMS), transmission electron microscopy (TEM), and 300 K transmission measurements are used to characterize the material.
2. Experimental procedure For this study, samples were grown on semi-insulating GaAs substrates in a Varian Gen-Il MBE system using As2. Schematics of the targeted structures are shown in fig. 1. Samples I and II were grown at 480°Cwhile Sample III was grown at 450°C.An n-type buffer layer was grown atop a 500 A GaAs buffer layer. Sample I (one-step) had a buffer of 1 ~.tmof 1n034Ga066As. For Sample II (two-step), the buffer was 5500 A of
Elsevier Science Publishers B.V. All rights reserved
760
/ 1.3
SM. Lord et al. SAMPLE 1(1-STEP BUFFER) ~In
n-In0 ~Ga0~As
SAMPLE III (GRADED BUFFER)
A
~
g
J
____________________
SI GaAs
SAMPLE 11(2-STEP BUFFER)
~In0~Ga0~As 5000 A p-In0~Ga0 ~As 5000 A 75 A In~Ga~~As ~ 75 A ln~Ga~As •~ x 30 35 A AI~~Ga~As jx 30 35 A AI~Ga•67As j.X 30 n-In0~Ga0~As5500 A n-ln~5Ga065As 1 ~sm n- In0 17Ga0 83As 10 A ‘x 50 Graded Bufler 2.3 J.tm fl- Ifl04~5Ga~~,5A$ 20 A 15% In4tm
0~Ga0~As 5000 75 A In50Ga~As 35 A AI33Ga57As
n-GaAs
sm Exciton resonances in InGaAs QWs grown by MBE
500
A
n-In0 17Gao83As 5500 A
n-GaAs
500 A
n-GaAs
n-GaAs SI GaAs
500
A
SI GaAs
Fig. 1. Schematics of the targeted MBE-grown p—i—n structures with various n-type buffer layers.
In0 17Ga0 83As followed by a superlattice of 50 x 10 A In0 17Ga083As/20 A In0 425Ga0575As and then another 5500 A of In035Ga065As. Each step was chosen to be 17% In, based on the findings of Ribas et al. [3] that dislocations in InGaAs/ GaAs will thread down rather than up for steps of 18% In. For Sample III (graded), the In GaAs buffer’s In composition was graded from 0% to 35% at a rate of 15% In per p.m. On top of the various buffers, superlattices consisting of thirty 75 A In~5Ga05As quantum wells with thirty-one 35 A Al033Ga067As barriers were grown. To complete the p—i—n structures, 5000 A of p-type 1n035Ga065As or 1n034Ga066As was grown on top. Samples were etched down to the n-layer to form 500 x 500 p.m square mesas. Electrical connections were made via an alloyed Au/ Ge/ Ni/ Au contact to the n-layer and an annular Ti—Au contact to the top of the mesas. The devices were packaged and wire bonded into a drilled mount and the optical transmission measured with a half-meter spectrometer and a white light source. Cross-sectional TEM analysis was conducted using a Philips EM400 at 120 kV. SIMS was performed at Charles Evans and Associates using Cst
3. Results and discussion Several features of the growth merit discussion. As2 was chosen because improved surface
morphology has been demonstrated for MBE growth of InGaAs on InP with As2 instead of As4 [9]. As2 may also be beneficial at the low substrate temperatures employed during growth, since As2 incorporates more efficiently than As4. Studies have shown that the critical thickness is highly dependent on growth temperature and increases as the temperature decreases [10]. Thus, low substrate temperatures are particularly important for the successful growth of high In cornpositions. TEM analysis showed that 30 A In05Ga05As quantum wells grown on GaAs at 490°C had considerably better morphology than identical wells grown at 535°C [11]. Also, the entire buffer region in Sample III is considerably thicker than in Samples I and II which may affect its role in relaxation. Considerable variation was apparent in the surface morphologies of these samples as observed in optical and scanning electron microscopy. Sample I’s surface was extremely rough with no distinct features. The surface of Sample II was also quite rough, but it showed crosshatching characteristic of dislocation formation. The surface of Sample III showed considerable cross-hatching, however, the area between the dislocation lines appeared fairly smooth in comparison with that of Sample II. The samples with step-graded buffers exhibited significantly higher leakage currents than did the graded buffer sample. For 500 x 500 p.m devices under 9 V reverse bias, the leakage currents roughly for 30 2, 400 mA/cm2, and were 80 p.A/cm2 mA/cm
S.M. Lord et al.
E
/
1.3 ~m Exciton resonances in InGaAs QWs grown by MBE
761
14000
The absorption coefficient (a) was calculated
12000
from transmission measurements including the dependence of the white light source on wavelength, considering the reflection at the surface, and subtracting off the value of absorption for a
10000 8000
4V
12500 13000 13500 wavelength (A) Fig. 2. 300 K absorption spectra for Sample III at various reverse bias voltages,
blank GaAs wafer. The nonzero a after the peak may be due to variation in the thicknesses of GaAs wafers and uncertainities in the surface reflection. An exciton is clearly visible at 1.25 p.m zero bias. The absorption per well is estimated to be 1.1%, which is comparable to that of the best GaAs/AIGaA5 quantum wells. Applica2 causes tion an electric to 300 this of peak to red field shift up while theky/cm half-width at
Samples I, II, and III, respectively. Thus, Sample III is the best candidate for a low leakage photodetector. Both of the samples with step-graded buffers exhibited extremely broad absorption edges with no excitonic behavior during 300 K photocurrent measurements. This is in marked contrast to the excellent excitonic features and accompanying QCSE observed in photocurrent measurements of Sample III. Room temperature absorption spectra at various applied biases for the sample with the graded buffer layer are shown in fig. 2.
half-maximum (HWHM) of the exciton resonance remains less than 25 rneV. The total shift obtamed was about 600 A yielding an exciton at 1.31 p.m. SIMS analysis confirms that the grading in Sample III is basically linear as seen in fig. 3. This sample was etched to remove 4000 A of the top p-layer, since SIMS resolution degrades as the sputtered depth into the sample increases. The remainder of this top layer was assumed to be the targeted composition of 35% In and used as an internal calibration for the subsequent analysis. Although it is possible to see the oscillations
~
6000
8V
.
IOV
~..
2000
under 12000
:
10.3
~
0.00
0.30 0.40 0.50 0.60 0.70 0.80 depth (microns) Fig. 3. SIMS spectra for Sample III. Inset shows higher resolution analysis. 0.10
0.20
762
SM. Lord et a!.
/ 1.3
p.m Exciton resonances in InGaAs QWs grown by MBE
in In and Al concentrations which correspond to the quantum wells and barriers within the superlattice, the resolution was insufficient to provide a direct estimate of the In composition in the
wells. However, assuming that the ratio of the thickness of the barriers to the thickness of the wells was as targeted, one may estimate the In composition in the quantum wells from the aver-
[L~
Fig. 4. Cross-section TEM micrographs ot Sample 111 showing: (a) close-up of superlattice region and (b) top, superlattice, and about 1 p.m of the graded buffer.
SM. Lord et a!.
/
1.3 p.m Exciton resonances in InGa~4sQWs grown by MBE
age composition in the superlattice. X-ray diffraction analysis showed that the superlattice period was indeed within a few percent of the targeted value. The average value of 33% In measured in the superlattice yields 48.5% In in the quantum wells. We expect the slowly graded buffer to relax fully at the top, i.e. with a lattice constant corresponding to the targeted 35% In. The superlattice should be symmetrically strained with the InGaAs quantum wells under compressive strain and the AlGaAs barriers under tensile strain. However, the wavelength at which we observe the zero bias exciton in 300 K absorption measurements does not agree with this hypothesis. Electron microprobe data shows the top p-type layer of Sample III to be 29% In rather than the targeted 35%. Attributing this difference to uncertainties in the In growth rate throughout the structure yields reasonable agreement with the absorption spectra. From the SIMS data, if the top layer is taken to be 29%, the In composition in the quantum wells~is about 40% which is consistent with the absorption data. Cross-section TEM shows smooth interfaces within Sample III’s superlattice as shown in fig. 4a. For quantum wells with such high In content, these interfaces look particularly good. No evidence of islanding, 3D growth, or clustering is visible. The 75 A thick quantum wells exceed the Matthews—Blakeslee critical thickness [121 and would result in a rough superlattice if grown directly on GaAs. Thus, the graded buffer has enabled more In to be added without degrading material quality. Fig. 4b shows a lower magnification image including some of the buffer. Note that no dislocations are visible within about 3000 A beneath the superlattice. Deeper into the graded buffer, although many dislocations are visible, none within the observable area are threading dislocations which would destroy material quality and device performance. We believe that the mechanism involved here may be related to that described by LeGoues et al. [13]. The changes in absorption and thus transmission as a function of voltage, as seen in fig. 2, makes the quantum wells in Sample III suitable for use in an electro-absorption modulator. The
763
device functions as a transmission modulator with an insertion loss of about 3 dB. Operation on the exciton peak at 1.25 p.m results in a normally off modulator with a relative transmission modulation (4T/T) of 17%. Operation at 1.3 p.m, where the initial absorption is zero, yields a normally on modulator with a 4T/T of 12%. Of course, better performance can be obtained by either antireflection coating both sides of the wafer or incorporating a Fabry—Perot cavity.
4. Conclusions In conclusion, we have demonstrated exciton resonances near 1.3 p.m for InGaAs grown on GaAs. The key to our achievement is the incorporation of a slowly graded InGaAs buffer layer beneath the quantum wells. Absorption spectra of a sample including such a buffer show well-defined QCSE, while those from samples with step-graded buffers exhibit no excitonic features. We also demonstrated an electro-absorption modulator with zlT/T of 12% at 1.3 p.m.
Acknowledgments We would like to thank C. Kirshbaum for SIMS analysis. SML acknowledges fellowship support from AT&T. This work was partially supported by DARPA and ONR through contract N00014-90-J-4056.
References [1] E.A. Beam III and Y.C. Kao, J. AppI. Phys. 69 (1991) 4253. [2] S. Niki, W.S.C. Chang, H.H. Wieder and T.E. Van Eck, J. Crystal Growth 111 (1991) 419. [3] P. Ribas, V. Krishnamoorthy and R.M. Park, AppI. Phys. Letters 57 (1990) 1040. [4] T.K. Woodward, T. Sizer, DL Sivco and A.Y. Cho, AppI. Phys. Letters 57 (1990) 548. [51I.J. Fritz, D.R. Myers, G.A. Vawter, TM. Brennan and B.E. Hammons, AppI. Phys. Letters 58 (1991) 1608. [6] P. Melman, B. Elman, C. Jagannath, E.S. Koteles, A. Silletti and D. Dugger, Appi. Phys. Letters 55 (i989) 1436.
764
S.M Lord et a!.
/
1.3 p.m Exciton resonances in InGaAs QWs grown by MBE
[7] E.J. Roan and KY. Cheng, AppI. Phys. Letters 59 (1991) 2688. [8] E.A. Fitzgerald, Y.-H. Xie, M.L. Green, D. Brasen, AR. Kortan, J. Michel, Y.-J. Mu and BE. Weir, AppI. Phys. Letters 59 (1991) 881; F.K. LeGoues, B.S. Meyerson and J.F. Morar, Phys. Rev. Letters 66 (i991) 2903. [9] Y.C. Pao, PhD Thesis, Stanford University (1990).
[10] M.J. Ekenstedt, SM. Wang and T.G. Andersson, AppI. Phys. Letters 58 (1991) 854. [11] SM. Lord, B. Pezeshki, S.D. Kim and J.S. Harris, Jr., unpublished. [12] J.W. Matthews and A.E. Blakeslee, J. Crystal Growth 27 (1974) 118. [13] F.K. LeGoues, B.S. Myerson, J.F. Morar and PD. Kirchner, J. AppI. Phys. 71(1992) 4230.