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InP MQW p-i-n modulator structures
Microelectronic Engineering 19 (1992) 895-898 Elsevier
895
E l e c t r o - O p t i c Characterization of I n G a A s / I n P M Q W p - i - n M o d u l a t o r Structures R. Schwedler, H. Mikkelsen, R. Kersting, D. Laschet, A. Kohl, K. Wolter, K. Leo, H. Kurz Institut ffir Halbleitertechnik II, RWTH Aachen, Sommerfeldstrasse 24, 5100 Aachen, Germany Abstract For self electro-optic effect device applications, multiple quantum well modulator devices in the material system InGaAs/InP are studied. The applied experimental techniques are differential electrotransmission and photoluminescence with and without electric field. The optoelectronic properties of the modulators, including transport and recombination processes, axe studied in the experiments. The theory describes the electric field dependence of respectively the confined state energies, overlap of electron-hole wavefunctions, the dielectric constants el and e2 of the multiple quantum well material and the differential electrotransmission spectra.
1. T H E O R Y The operation of a M Q W diode as a light modulator is based on the Quantum Confined Stark Effect. In order to model the optical response of the p - i - n diode modulator the field dependent dielectric functions el and e2 of the M Q W material are calculated. At first, the SchrSdinger equation is solved with an electronic transfer-matrix method to determine the confined state energies and wavefunctions of electrons and holes. The polarisation dependent dielectric function e2 of the well material is calculated [1-3] from the optical transition energies and the electronhole wavefunction overlap matrixes. We use an oscillator model for the excitonic transitions and a continuum above the excitons, el of the well material is calculated from a Kramers Kronig integration over e2. The contribution to el of the well material from transitions with energies far above the bandgap energy is also included. These cause the largest absolute contribution to el. The field dependent dielectric functions of the M Q W axe calculated from those of the well and barrier material [4] in an effective medium model, as shown in Fig. 1. The differential electrotransmission (DET) is finally calculated with an optical transfer-matrix method, using the field dependent dielectric functions of the MQW.
2. E X P E R I M E N T A L The samples are grown by L P - M O C V D on a n+-InP substrate followed by a 300 nm n - I n P buffer layer. The M Q W section is embedded between two 100 nm i-InP 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishel:s B.V. All rights reserved.
896
R. Schwedler et al. / InGaAs/lnP M Q W p-i-n modulator structures
0.2
12 ©I
!!~
F=OkV/cm F=70kV/cln
0.1
11
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0
0.7
I
0.8
,
'
,
I
0.9 1 Energy [eV]
,
I
1.1
,
1.2
10
Figure 1: The dielectric functions q and e2 of the InGaAs/InP M Q W material at 0 k V / c m (whole lines) and 70 k V / c m (dotted lines). layers and consists of 100 periods of 7 nm InGaAs wells and 20 nm InP barriers. On top of this, 700 nm p + - I n P and finally a contact layer of 100 nm p+-InGaAs is grown. Metal contacts are deposited on both the p+-InGaAs and the substrate and alloyed. The p+-InGaAs is removed selectively with a wet chemical etchant for optical windows, and the M Q W p-i-n diode devices are separated by a mesa etch process. The breakdown voltage of the p-i-n diodes is -60 V.
3. R E S U L T S
AND
DISCUSSION
The presented DET data of the M Q W p - i - n diode are compared with the results of the model calculations. In Fig. 2 the calculated spectrum is shown together with results of the room temperature DET experiment. In our experiments the electric field is modulated from 0 to 70 kV/cm. A good qualitative agreement between experiment and theory is demonstrated. The several allowed and forbidden transitions are indicated in Fig. 1. The influence of the parity forbidden E1H2, E2H1 and E2H3 transitions is small compared to the other transitions. While the nominal well thickness is 7 nm, the experimental transition energies rather correspond to the 8.3 nm used in the calculation. This difference can be explained by interface layers at the heterointerfaces due to growth conditions [5]. The M Q W has a background donor doping of about 101Scm -3 leading to an inhomogeneous electric field which is not yet included in the model. This effect leads to a broadening of the optical transitions observed. The PL spectrum under electric field depends (i) on the field dependent electronhole matrix, (ii) transport processes and (iii) radiative recombination processes. In the case of 633 nm excitation, all carriers are generated within the p - I n P layer. If no external field is applied, the carriers diffuse to the Q W region and recombine.
897
R. Schwedler et al. / InGaAs/InP M Q W p-i-n modulator structures
0.3 r~ ,, ~
0.2
-0.1 -
calc" exp.
;/
,
0.8
0.9 1 Energy [eV]
\'
-0.2 0.7
1.1
1.2
Figure 2: Differential Transmission spectra for the InGaAs/InP p-i-n Diode. The field is modulated from 0 to 70 kV/cm. The spectra of the emitted PL are depicted in Fig. 3a. As soon as a low electric field of i k V / c m is applied, hole transport to the QWs is suppressed. The PL is reduced by about one order of magnitude and cannot be observed at higher fields. In order to avoid these transport processes, an optical excitation below the InP bandgap is needed. Fig. 3b depicts the PL spectra at 1150 n m excitation wavelength (1.07 eV). In this case all carriers are generated in the MQW region. At all biases, the traces show EH1 as well as EL1 PL. However, the EH1 PL intensity decreases significantly with increasing bias while the intensity of the EL1 signal is approximately constant. This leads to an appearant blue shift of the PL maximum by 25 meV with increasing field overcompensating red shifts due to the QCSE. This feature results from the radiative recombination matrix element which is proportional to the overlap of the electron and hole wave functions. The electron-heavy hole overlap is strongly reduced by the electric field, while the electron-light hole overlap is almost constant. In consequence, the overall radiative recombination rate decreases. Nevertheless, as depicted in Fig. 3b, the integrated PL intensity decreases because the dominant recombination channel is non-radiative at all biases. 4.
CONCLUSIONS
Through the experimental and theoretical analysis of the modulator structures some fundamentals neccessary for design optimization of optoelectronic modulators like self electro optic effect devices have been studied. Theoretical model calculations predict energy shifts in the optical transitions when an electric field is applied on the MQW structure. This is confirmed by DET experiments. From the calculations, a reduced electron-hole overlap matrix is predicted too. This is confirmed both in the DET as well as in the PL experiments. The strong PL signal quenching
898
R. Schwedler et al. / lnGaAs/lnP M Q W p-i-n modulator structures
f-1
1.8
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I
I
n
i
3
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0 kVlcm
.........
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.62
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0.5
~, n
4 kV/cm
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~--' 0.8
0.85
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"
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kV/cm
30 k V / c m
C I
/ n
0.9
Energy E eV ]
0.8
0.85
0.9
Energy E eV I
Figure 3: a) PL spectra obtained with 1.96 eV illumination. Carrier generation takes place within the top layer of InP. b) PL spectra obtained by direct excitation of the QWs by 1.07 eV radiation. for the EIH1 transition for high electric fields is accompanied by an approximately constant ElL1 transition intensity, which is due to the almost constant overlap of these electron-light hole wavefunctions.
ACKNOWLEDGEMENTS This work has been supported by the Deutsche Forschungsgemeinschaft (Ku 540/11). HM has been entirely supported through the Royal Norwegian Council for Scientific and Industrial Research (NTNF) by a DEMINEX PhD grant.
References 1 Y. Kan, H. Nagai, M. Yamanishi, I. Suemune. IEEE J. QE 23, 2167 (1987). 2 M. Yamanishi, I. Suemone. Jap. J. Appl. Phys. 23, L35 (1984). 3 J.-Th. Zettler, H. Mikkelsen, K. Leo, H. Kurz, R. Carius, A. FSrster. Submitted to Phys. Rev. B. 4 S. Adachi. Phys. Rev. B 39, 12612 (1989). 5 S. Juillaguet, J. P. Laurenti, R. Schwedler, K. Wolter, J. Camassel, H. Kurz. In: Non-Stoichiometry in Semiconductors, Edited by K. J. Bachmann, H.-L. Hwang, C. Schwab, p. 155, Elsevier, 1992.
895
E l e c t r o - O p t i c Characterization of I n G a A s / I n P M Q W p - i - n M o d u l a t o r Structures R. Schwedler, H. Mikkelsen, R. Kersting, D. Laschet, A. Kohl, K. Wolter, K. Leo, H. Kurz Institut ffir Halbleitertechnik II, RWTH Aachen, Sommerfeldstrasse 24, 5100 Aachen, Germany Abstract For self electro-optic effect device applications, multiple quantum well modulator devices in the material system InGaAs/InP are studied. The applied experimental techniques are differential electrotransmission and photoluminescence with and without electric field. The optoelectronic properties of the modulators, including transport and recombination processes, axe studied in the experiments. The theory describes the electric field dependence of respectively the confined state energies, overlap of electron-hole wavefunctions, the dielectric constants el and e2 of the multiple quantum well material and the differential electrotransmission spectra.
1. T H E O R Y The operation of a M Q W diode as a light modulator is based on the Quantum Confined Stark Effect. In order to model the optical response of the p - i - n diode modulator the field dependent dielectric functions el and e2 of the M Q W material are calculated. At first, the SchrSdinger equation is solved with an electronic transfer-matrix method to determine the confined state energies and wavefunctions of electrons and holes. The polarisation dependent dielectric function e2 of the well material is calculated [1-3] from the optical transition energies and the electronhole wavefunction overlap matrixes. We use an oscillator model for the excitonic transitions and a continuum above the excitons, el of the well material is calculated from a Kramers Kronig integration over e2. The contribution to el of the well material from transitions with energies far above the bandgap energy is also included. These cause the largest absolute contribution to el. The field dependent dielectric functions of the M Q W axe calculated from those of the well and barrier material [4] in an effective medium model, as shown in Fig. 1. The differential electrotransmission (DET) is finally calculated with an optical transfer-matrix method, using the field dependent dielectric functions of the MQW.
2. E X P E R I M E N T A L The samples are grown by L P - M O C V D on a n+-InP substrate followed by a 300 nm n - I n P buffer layer. The M Q W section is embedded between two 100 nm i-InP 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishel:s B.V. All rights reserved.
896
R. Schwedler et al. / InGaAs/lnP M Q W p-i-n modulator structures
0.2
12 ©I
!!~
F=OkV/cm F=70kV/cln
0.1
11
J I iI
0
0.7
I
0.8
,
'
,
I
0.9 1 Energy [eV]
,
I
1.1
,
1.2
10
Figure 1: The dielectric functions q and e2 of the InGaAs/InP M Q W material at 0 k V / c m (whole lines) and 70 k V / c m (dotted lines). layers and consists of 100 periods of 7 nm InGaAs wells and 20 nm InP barriers. On top of this, 700 nm p + - I n P and finally a contact layer of 100 nm p+-InGaAs is grown. Metal contacts are deposited on both the p+-InGaAs and the substrate and alloyed. The p+-InGaAs is removed selectively with a wet chemical etchant for optical windows, and the M Q W p-i-n diode devices are separated by a mesa etch process. The breakdown voltage of the p-i-n diodes is -60 V.
3. R E S U L T S
AND
DISCUSSION
The presented DET data of the M Q W p - i - n diode are compared with the results of the model calculations. In Fig. 2 the calculated spectrum is shown together with results of the room temperature DET experiment. In our experiments the electric field is modulated from 0 to 70 kV/cm. A good qualitative agreement between experiment and theory is demonstrated. The several allowed and forbidden transitions are indicated in Fig. 1. The influence of the parity forbidden E1H2, E2H1 and E2H3 transitions is small compared to the other transitions. While the nominal well thickness is 7 nm, the experimental transition energies rather correspond to the 8.3 nm used in the calculation. This difference can be explained by interface layers at the heterointerfaces due to growth conditions [5]. The M Q W has a background donor doping of about 101Scm -3 leading to an inhomogeneous electric field which is not yet included in the model. This effect leads to a broadening of the optical transitions observed. The PL spectrum under electric field depends (i) on the field dependent electronhole matrix, (ii) transport processes and (iii) radiative recombination processes. In the case of 633 nm excitation, all carriers are generated within the p - I n P layer. If no external field is applied, the carriers diffuse to the Q W region and recombine.
897
R. Schwedler et al. / InGaAs/InP M Q W p-i-n modulator structures
0.3 r~ ,, ~
0.2
-0.1 -
calc" exp.
;/
,
0.8
0.9 1 Energy [eV]
\'
-0.2 0.7
1.1
1.2
Figure 2: Differential Transmission spectra for the InGaAs/InP p-i-n Diode. The field is modulated from 0 to 70 kV/cm. The spectra of the emitted PL are depicted in Fig. 3a. As soon as a low electric field of i k V / c m is applied, hole transport to the QWs is suppressed. The PL is reduced by about one order of magnitude and cannot be observed at higher fields. In order to avoid these transport processes, an optical excitation below the InP bandgap is needed. Fig. 3b depicts the PL spectra at 1150 n m excitation wavelength (1.07 eV). In this case all carriers are generated in the MQW region. At all biases, the traces show EH1 as well as EL1 PL. However, the EH1 PL intensity decreases significantly with increasing bias while the intensity of the EL1 signal is approximately constant. This leads to an appearant blue shift of the PL maximum by 25 meV with increasing field overcompensating red shifts due to the QCSE. This feature results from the radiative recombination matrix element which is proportional to the overlap of the electron and hole wave functions. The electron-heavy hole overlap is strongly reduced by the electric field, while the electron-light hole overlap is almost constant. In consequence, the overall radiative recombination rate decreases. Nevertheless, as depicted in Fig. 3b, the integrated PL intensity decreases because the dominant recombination channel is non-radiative at all biases. 4.
CONCLUSIONS
Through the experimental and theoretical analysis of the modulator structures some fundamentals neccessary for design optimization of optoelectronic modulators like self electro optic effect devices have been studied. Theoretical model calculations predict energy shifts in the optical transitions when an electric field is applied on the MQW structure. This is confirmed by DET experiments. From the calculations, a reduced electron-hole overlap matrix is predicted too. This is confirmed both in the DET as well as in the PL experiments. The strong PL signal quenching
898
R. Schwedler et al. / lnGaAs/lnP M Q W p-i-n modulator structures
f-1
1.8
'
I
I
n
i
3
'"'"'"1
.........
I"'"''"l
IEHI
0 kVlcm
.........
0 kVlcm
D
.62
EL l
t.
>. q) E
0.5
~, n
4 kV/cm
Ill
tJ
Icm 0
~--' 0.8
0.85
'~'~'
"
fll
kV/cm
30 k V / c m
C I
/ n
0.9
Energy E eV ]
0.8
0.85
0.9
Energy E eV I
Figure 3: a) PL spectra obtained with 1.96 eV illumination. Carrier generation takes place within the top layer of InP. b) PL spectra obtained by direct excitation of the QWs by 1.07 eV radiation. for the EIH1 transition for high electric fields is accompanied by an approximately constant ElL1 transition intensity, which is due to the almost constant overlap of these electron-light hole wavefunctions.
ACKNOWLEDGEMENTS This work has been supported by the Deutsche Forschungsgemeinschaft (Ku 540/11). HM has been entirely supported through the Royal Norwegian Council for Scientific and Industrial Research (NTNF) by a DEMINEX PhD grant.
References 1 Y. Kan, H. Nagai, M. Yamanishi, I. Suemune. IEEE J. QE 23, 2167 (1987). 2 M. Yamanishi, I. Suemone. Jap. J. Appl. Phys. 23, L35 (1984). 3 J.-Th. Zettler, H. Mikkelsen, K. Leo, H. Kurz, R. Carius, A. FSrster. Submitted to Phys. Rev. B. 4 S. Adachi. Phys. Rev. B 39, 12612 (1989). 5 S. Juillaguet, J. P. Laurenti, R. Schwedler, K. Wolter, J. Camassel, H. Kurz. In: Non-Stoichiometry in Semiconductors, Edited by K. J. Bachmann, H.-L. Hwang, C. Schwab, p. 155, Elsevier, 1992.