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GaAs multiple quantum wells on (111)A GaAs
Materials Science and Engineering B66 (1999) 123 – 125 www.elsevier.com/locate/mseb
Photoreflectance evaluation of MOVPE AlGaAs/GaAs multiple quantum wells on (111)A GaAs Soohaeng Cho a,*, A. Sanz-Herva´s a, O.V. Kovalenkov a, A. Majerfeld a, C. Villar b, B.W. Kim c b
a Department of Electrical and Computer Engineering, CB425, Uni6ersity of Colorado, Boulder, CO 80309, USA Departamento de Tecuologı´a Electronica, E.T.S.I. Telecomunicacio´n, UPM, Ciudad Uni6ersitaria, 28040 Madrid, Spain c Electronics and Telecommunications Research Institute, P.O. Box 106, Yusong, Taejon 305 -600, South Korea
Abstract The Photoreflectance (PR) technique was applied to evaluate a 25-period Al0.27Ga0.73As/GaAs multiquantum-well (MQW) structure with a well length of 55 A, grown by atmospheric pressure metalorganic vapor phase expitaxy (MOVPE) on a (111)A GaAs substrate. Structural parameters such as well and barrier lengths, and the Al fraction in the barriers were accurately determined by high resolution X-ray diffractometry (HRXRD). The PR spectrum exhibits all the possible confined QW transitions between electron and hole sub-bands. The theoretically calculated transition energies are in very close agreement ( 91 meV) with those experimentally determined from the PR spectrum even up to the highest possible transition for these wells. From a detailed monolayer (ML) analysis of the various transition energies it is concluded that the QW interfaces have much less than a 91 ML fluctuation over the 25 periods and that the interfaces are smooth, abrupt and uniform. In addition, photoluminescence (PL) measurements were also used to further assess the optical quality. The PL full width at half maximum (FWHM) is 12.5 meV, which corresponds to less than a 91 ML fluctuation throughout the 25-period MQW in agreement with the PR analysis. This FWHM is the best value reported to date for AlGaAs/GaAs MQW structures grown on {111} GaAs substrates either by MBE or MOVPE. © 1999 Elsevier Science S.A. All rights reserved.
1. Introduction The growth of compound semiconductors on nonconventional 111 crystallographic directions has recently attracted significant attention due to their special properties and their possible use for lasers and novel optoelectronic devices. It was recently reported that the first successful fabrication of high quality AlGaAs/ GaAs multiquantum-well (MQW) structures on (111)A GaAs substrates by the metalorganic vapor phase epitaxy (MOVPE) process [1,2]. Several techniques such as photoluminescence (PL), high-resolution X-ray diffractometry (HRXRD) and modulation spectroscopy are commonly employed to characterize QW structures. In particular, photoreflectance (PR), which is a modula-
* Corresponding author. Tel.: +1-303-492-7164; fax: + 1-303-4922758.
tion spectroscopic technique, has been shown to be a very powerful tool to study all the confined states in quantum well structures due to its non-destructive, contactless and highly sensitive nature. Previous investigations have employed PR to study photovoltage effects [3], to measure directly the piezoelectric field in strained MQW structures [4] and to analyze higher interband optical transitions at room-temperature [5]. In this paper we present the application of low temperature PR spectroscopy to assess the abruptness and roughness of the interfaces, as well as to determine the structural parameters of (111)A MOVPE grown AlGaAs/GaAs MQW structures. Low temperature PR measurements permit a direct comparison with low temperature PL peak energies and linewidth broadening by obtaining its full width at half maximum (FWHM). HRXRD analyses have also been performed to independently obtain the structural parameters of the MQWs.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 1 3 1 - 2
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S. Cho et al. / Materials Science and Engineering B66 (1999) 123–125
2. Experimental The sample studied in this work was grown on a (111)A exactly-oriented GaAs substrate in a horizontal quartz MOVPE reactor operated at atmospheric pressure. The AlGaAs/GaAs quantum well structure reported in this paper has the following layer sequence in order of growth: a 0.36 mm buffer, a 25-period AlGaAs/GaAs MQW with an Al fraction of 27% in the barriers and well and barrier thicknesses of Lw = 55 A, and Lb =235 A, , respectively, and a 46 A, GaAs cap on top. The MQW parameters listed were determined from HRXRD measurements carried out with a Bede D3 diffractometer (l=Cu Ka1). Photoluminescence measurements at 11 K were performed using an Ar+ laser with an excitation intensity of 0.25 W cm − 2 and a double pass monochromator. Photoreflectance measurements at 11 K were made by using the beam from a tungsten light source passed through a double pass monochromator as the probe beam, with the chopped beam of the Ar+ laser tuned to 5145 A, with an excitation intensity of 0.025 W cm − 2 as the pump beam. A Si diode with a longpass filter (Corning 3 – 68) was used to detect the reflectance signal.
3. Results and discussion The low temperature PR spectrum versus energy, which shows a number of optical transitions, was obtained and analyzed. The modulated signal can be expressed as [6]: DR/R= %Re[Aeif(E − Ei +iG) − n] where A is the amplitude, f is a phase angle, E is the photon energy, Ei is the transition energy, and G is the broadening parameter. Here, the values of n are 3 and 2.5 for two-dimensional critical points and for the bulk GaAs feature, respectively. The 11 K PR spectrum and the lineshape fit are shown in Fig. 1. The energies obtained are indicated by arrows, and the notation Em-H (L)n denotes a transition between the mth conduction state and nth valence state of heavy (H) or light (L) hole character, respectively. The experimental energies are clearly identified with all the allowed transitions (E1-H1, E1-L1 and E2-H2) and one weakly allowed transition (E1-H3) together with the AlGaAs related peak at the 1.862 eV. The peak related to the AlGaAs barriers was used to obtain the band gap and Al fraction in the AlGaAs barriers. The peak at 1.862 eV yields 27% [7], which is identical to the value deduced from HRXRD. An exciton binding energy of 6 meV was added to the peak at 1.862 eV for theoretical calculations. In the
computations, the following effective masses were used: me = 0.067m0, mhh = 0.952m0 and mlh =0.079m0 for GaAs wells, and me = 0.084m0, mhh =0.999m0, and mlh = 0.092m0 for AlGaAs barriers [8]. A 67% conduction-band offset of 234 and 115 meV for the valence-band offset were used. After deducting an exciton binding energy of 10 meV from all the theoretical eigenvalues [9], the comparison between the experimental transition energies and the theoretical eigenvalues obtained for Lw = 55 A, and Lw 91 monolayer (ML) up to E2-H2 is summarized in Table 1. The excellent agreement, to within 1 meV, between the experimental and calculated energies reveals a high degree of structural quality and uniformity of the interfaces, and also indicates an overall well length fluctuation of much less than 9 1 ML. The PL spectrum at 11 K is also shown in Fig. 1. The sample exhibits a single E1-H1 peak. The FWHM of 12.5 meV also demonstrates a well length variation throughout the MQW of B 9 1 ML in good agreement with the PR measurements.
4. Conclusions In summary, we have presented a comprehensive photoreflectance evaluation of a 25-period Al0.27 Ga0.73As/GaAs MQW structure with a well length of 55 A, grown by atmospheric pressure MOVPE on a (111)A GaAs substrate. The structural parameters were accurately determined from an HRXRD study which shows an excellent period repeatability. The theoretically calculated transition energies are in very close agreement (9 1 meV) with those experimentally obtained from the PR spectrum up to the highest possible transitions for these wells. From a detailed
Fig. 1. PR experimental curve (dotted line), theoretical lineshape fit (solid line) and PL spectrum (dashed line) for the 25-period AlGaAs/ GaAs MQW grown on a (111)A GaAs substrate.
S. Cho et al. / Materials Science and Engineering B66 (1999) 123–125
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Table 1 Summary of the experimental PR transition energies, broadening parameters, and relative amplitudes deduced from the best fit to the 11 K PR spectrum for the 25-period AlGaAs/GaAs MQW grown on a (111)A GaAs substrate and the theoretical eigenvalues for the structure Spectral feature
Energy (eV) PR energy
E1-H1 E1-L1 E1-H3 E2-H2 Al0.27Ga0.73As
1.586 1.622 1.653 1.771 1.862
Broadening parameter (meV)
Relative amplitude
26.7 31.9 65.3 19.2 681
1.0 1.2 7.0 0.15 8962
Calculation* Lw-l ML
Lw
Lw+l ML
1.593 1.633 1.666 1.781
1.586 1.623 1.652 1.770
1.579 1.614 1.639 1.756
* After deducting an exciton binding energy of 10 meV [9].
ML analysis it is concluded that the quantum well interfaces have less than a 91 ML fluctuation in well length over the 25 periods and that the interfaces are smooth, abrupt and uniform. The sample exhibits a single PL peak with a FWHM of 12.5 meV, which corresponds to less than a 91 ML well length fluctuation throughout the 25-period MQW in agreement with the PR analysis. This FWHM is the best value reported to date for an AlGaAs/GaAs MQW structures grown on {111} GaAs either by MBE or MOVPE.
Acknowledgements The work at the University of Colorado was supported by the Electronics and Telecomunications Research Institute, Taejon, South Korea.
.
References [1] E. Mao, S.A. Dickey, A. Majerfeld, A. Sanz-Hervas, B.W. Kim, Microelectron J. 28 (1997) 727. [2] A. Sanz-Herva´s, Soohaeng Cho, O.V. Kovalenkov, S.A. Dickey, A. Majerfeld, C. Villar, B.W. Kim, Proceedings of the IEEE International Symposium on Compound Semiconductors, 8 – 11 September, 1997, San Diego, USA, (1998), p. 291. [3] X. Yin, H.-M. Chen, F.H. Pollak, Y. Chan, P.A. Montano, P.D. Kirchner, G.D. Pettit, J.M. Woodall, Appl. Phys. Lett. 58 (3) (1991) 260. [4] S.A. Dickey, A. Majerfeld, J.L. Sanchez-Rojas, A. Sacedon, E. Mun˜oz, A. Sanz-Herva´s, M. Aguilar, B.W. Kim, Microelectron. Eng. J. 43 – 44 (1998) 171. [5] G. Wang, P. Tronc, R. Melliti, E. Mao, A. Majerfeld, B.W. Kim, Phys. Stat. Sol. (a) 164 (1997) 117. [6] D.E. Aspnes, in: T.S. Moss (Ed.), Handbook on Semiconductors, vol. 2, North-Holland, New York, 1980, p. 109. [7] H.C. Casey, M.B. Panish, Heterostructure Lasers; part A, Academic Press, New York, 1978, p. 187. [8] J. Los, A. Fasolino, A. Catellani, Phys. Rev., B 53 (1996) 4630. [9] L. Vin˜a, L. Mun˜oz, F. Calle, N. Mestres, J.M. Calleja, W.I. Wang, Phys. Rev., B 46 (1992) 13234.
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Photoreflectance evaluation of MOVPE AlGaAs/GaAs multiple quantum wells on (111)A GaAs Soohaeng Cho a,*, A. Sanz-Herva´s a, O.V. Kovalenkov a, A. Majerfeld a, C. Villar b, B.W. Kim c b
a Department of Electrical and Computer Engineering, CB425, Uni6ersity of Colorado, Boulder, CO 80309, USA Departamento de Tecuologı´a Electronica, E.T.S.I. Telecomunicacio´n, UPM, Ciudad Uni6ersitaria, 28040 Madrid, Spain c Electronics and Telecommunications Research Institute, P.O. Box 106, Yusong, Taejon 305 -600, South Korea
Abstract The Photoreflectance (PR) technique was applied to evaluate a 25-period Al0.27Ga0.73As/GaAs multiquantum-well (MQW) structure with a well length of 55 A, grown by atmospheric pressure metalorganic vapor phase expitaxy (MOVPE) on a (111)A GaAs substrate. Structural parameters such as well and barrier lengths, and the Al fraction in the barriers were accurately determined by high resolution X-ray diffractometry (HRXRD). The PR spectrum exhibits all the possible confined QW transitions between electron and hole sub-bands. The theoretically calculated transition energies are in very close agreement ( 91 meV) with those experimentally determined from the PR spectrum even up to the highest possible transition for these wells. From a detailed monolayer (ML) analysis of the various transition energies it is concluded that the QW interfaces have much less than a 91 ML fluctuation over the 25 periods and that the interfaces are smooth, abrupt and uniform. In addition, photoluminescence (PL) measurements were also used to further assess the optical quality. The PL full width at half maximum (FWHM) is 12.5 meV, which corresponds to less than a 91 ML fluctuation throughout the 25-period MQW in agreement with the PR analysis. This FWHM is the best value reported to date for AlGaAs/GaAs MQW structures grown on {111} GaAs substrates either by MBE or MOVPE. © 1999 Elsevier Science S.A. All rights reserved.
1. Introduction The growth of compound semiconductors on nonconventional 111 crystallographic directions has recently attracted significant attention due to their special properties and their possible use for lasers and novel optoelectronic devices. It was recently reported that the first successful fabrication of high quality AlGaAs/ GaAs multiquantum-well (MQW) structures on (111)A GaAs substrates by the metalorganic vapor phase epitaxy (MOVPE) process [1,2]. Several techniques such as photoluminescence (PL), high-resolution X-ray diffractometry (HRXRD) and modulation spectroscopy are commonly employed to characterize QW structures. In particular, photoreflectance (PR), which is a modula-
* Corresponding author. Tel.: +1-303-492-7164; fax: + 1-303-4922758.
tion spectroscopic technique, has been shown to be a very powerful tool to study all the confined states in quantum well structures due to its non-destructive, contactless and highly sensitive nature. Previous investigations have employed PR to study photovoltage effects [3], to measure directly the piezoelectric field in strained MQW structures [4] and to analyze higher interband optical transitions at room-temperature [5]. In this paper we present the application of low temperature PR spectroscopy to assess the abruptness and roughness of the interfaces, as well as to determine the structural parameters of (111)A MOVPE grown AlGaAs/GaAs MQW structures. Low temperature PR measurements permit a direct comparison with low temperature PL peak energies and linewidth broadening by obtaining its full width at half maximum (FWHM). HRXRD analyses have also been performed to independently obtain the structural parameters of the MQWs.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 1 3 1 - 2
124
S. Cho et al. / Materials Science and Engineering B66 (1999) 123–125
2. Experimental The sample studied in this work was grown on a (111)A exactly-oriented GaAs substrate in a horizontal quartz MOVPE reactor operated at atmospheric pressure. The AlGaAs/GaAs quantum well structure reported in this paper has the following layer sequence in order of growth: a 0.36 mm buffer, a 25-period AlGaAs/GaAs MQW with an Al fraction of 27% in the barriers and well and barrier thicknesses of Lw = 55 A, and Lb =235 A, , respectively, and a 46 A, GaAs cap on top. The MQW parameters listed were determined from HRXRD measurements carried out with a Bede D3 diffractometer (l=Cu Ka1). Photoluminescence measurements at 11 K were performed using an Ar+ laser with an excitation intensity of 0.25 W cm − 2 and a double pass monochromator. Photoreflectance measurements at 11 K were made by using the beam from a tungsten light source passed through a double pass monochromator as the probe beam, with the chopped beam of the Ar+ laser tuned to 5145 A, with an excitation intensity of 0.025 W cm − 2 as the pump beam. A Si diode with a longpass filter (Corning 3 – 68) was used to detect the reflectance signal.
3. Results and discussion The low temperature PR spectrum versus energy, which shows a number of optical transitions, was obtained and analyzed. The modulated signal can be expressed as [6]: DR/R= %Re[Aeif(E − Ei +iG) − n] where A is the amplitude, f is a phase angle, E is the photon energy, Ei is the transition energy, and G is the broadening parameter. Here, the values of n are 3 and 2.5 for two-dimensional critical points and for the bulk GaAs feature, respectively. The 11 K PR spectrum and the lineshape fit are shown in Fig. 1. The energies obtained are indicated by arrows, and the notation Em-H (L)n denotes a transition between the mth conduction state and nth valence state of heavy (H) or light (L) hole character, respectively. The experimental energies are clearly identified with all the allowed transitions (E1-H1, E1-L1 and E2-H2) and one weakly allowed transition (E1-H3) together with the AlGaAs related peak at the 1.862 eV. The peak related to the AlGaAs barriers was used to obtain the band gap and Al fraction in the AlGaAs barriers. The peak at 1.862 eV yields 27% [7], which is identical to the value deduced from HRXRD. An exciton binding energy of 6 meV was added to the peak at 1.862 eV for theoretical calculations. In the
computations, the following effective masses were used: me = 0.067m0, mhh = 0.952m0 and mlh =0.079m0 for GaAs wells, and me = 0.084m0, mhh =0.999m0, and mlh = 0.092m0 for AlGaAs barriers [8]. A 67% conduction-band offset of 234 and 115 meV for the valence-band offset were used. After deducting an exciton binding energy of 10 meV from all the theoretical eigenvalues [9], the comparison between the experimental transition energies and the theoretical eigenvalues obtained for Lw = 55 A, and Lw 91 monolayer (ML) up to E2-H2 is summarized in Table 1. The excellent agreement, to within 1 meV, between the experimental and calculated energies reveals a high degree of structural quality and uniformity of the interfaces, and also indicates an overall well length fluctuation of much less than 9 1 ML. The PL spectrum at 11 K is also shown in Fig. 1. The sample exhibits a single E1-H1 peak. The FWHM of 12.5 meV also demonstrates a well length variation throughout the MQW of B 9 1 ML in good agreement with the PR measurements.
4. Conclusions In summary, we have presented a comprehensive photoreflectance evaluation of a 25-period Al0.27 Ga0.73As/GaAs MQW structure with a well length of 55 A, grown by atmospheric pressure MOVPE on a (111)A GaAs substrate. The structural parameters were accurately determined from an HRXRD study which shows an excellent period repeatability. The theoretically calculated transition energies are in very close agreement (9 1 meV) with those experimentally obtained from the PR spectrum up to the highest possible transitions for these wells. From a detailed
Fig. 1. PR experimental curve (dotted line), theoretical lineshape fit (solid line) and PL spectrum (dashed line) for the 25-period AlGaAs/ GaAs MQW grown on a (111)A GaAs substrate.
S. Cho et al. / Materials Science and Engineering B66 (1999) 123–125
125
Table 1 Summary of the experimental PR transition energies, broadening parameters, and relative amplitudes deduced from the best fit to the 11 K PR spectrum for the 25-period AlGaAs/GaAs MQW grown on a (111)A GaAs substrate and the theoretical eigenvalues for the structure Spectral feature
Energy (eV) PR energy
E1-H1 E1-L1 E1-H3 E2-H2 Al0.27Ga0.73As
1.586 1.622 1.653 1.771 1.862
Broadening parameter (meV)
Relative amplitude
26.7 31.9 65.3 19.2 681
1.0 1.2 7.0 0.15 8962
Calculation* Lw-l ML
Lw
Lw+l ML
1.593 1.633 1.666 1.781
1.586 1.623 1.652 1.770
1.579 1.614 1.639 1.756
* After deducting an exciton binding energy of 10 meV [9].
ML analysis it is concluded that the quantum well interfaces have less than a 91 ML fluctuation in well length over the 25 periods and that the interfaces are smooth, abrupt and uniform. The sample exhibits a single PL peak with a FWHM of 12.5 meV, which corresponds to less than a 91 ML well length fluctuation throughout the 25-period MQW in agreement with the PR analysis. This FWHM is the best value reported to date for an AlGaAs/GaAs MQW structures grown on {111} GaAs either by MBE or MOVPE.
Acknowledgements The work at the University of Colorado was supported by the Electronics and Telecomunications Research Institute, Taejon, South Korea.
.
References [1] E. Mao, S.A. Dickey, A. Majerfeld, A. Sanz-Hervas, B.W. Kim, Microelectron J. 28 (1997) 727. [2] A. Sanz-Herva´s, Soohaeng Cho, O.V. Kovalenkov, S.A. Dickey, A. Majerfeld, C. Villar, B.W. Kim, Proceedings of the IEEE International Symposium on Compound Semiconductors, 8 – 11 September, 1997, San Diego, USA, (1998), p. 291. [3] X. Yin, H.-M. Chen, F.H. Pollak, Y. Chan, P.A. Montano, P.D. Kirchner, G.D. Pettit, J.M. Woodall, Appl. Phys. Lett. 58 (3) (1991) 260. [4] S.A. Dickey, A. Majerfeld, J.L. Sanchez-Rojas, A. Sacedon, E. Mun˜oz, A. Sanz-Herva´s, M. Aguilar, B.W. Kim, Microelectron. Eng. J. 43 – 44 (1998) 171. [5] G. Wang, P. Tronc, R. Melliti, E. Mao, A. Majerfeld, B.W. Kim, Phys. Stat. Sol. (a) 164 (1997) 117. [6] D.E. Aspnes, in: T.S. Moss (Ed.), Handbook on Semiconductors, vol. 2, North-Holland, New York, 1980, p. 109. [7] H.C. Casey, M.B. Panish, Heterostructure Lasers; part A, Academic Press, New York, 1978, p. 187. [8] J. Los, A. Fasolino, A. Catellani, Phys. Rev., B 53 (1996) 4630. [9] L. Vin˜a, L. Mun˜oz, F. Calle, N. Mestres, J.M. Calleja, W.I. Wang, Phys. Rev., B 46 (1992) 13234.
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