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Fabrication and optical study of quantum dots, quantum wires and quantum wells of II–VI diluted magnetic semiconductors
Journal of Crystal Growth 214/215 (2000) 183}186
Fabrication and optical study of quantum dots, quantum wires and quantum wells of II}VI diluted magnetic semiconductors N. Takahashi , K. Takabayashi , E. Shirado , I. Souma , J.X. Shen , Y. Oka * Research Institute for Scientixc Measurements, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
Abstract Nanometer-scale structures of II}VI diluted magnetic semiconductors were synthesized by the microfabrication techniques. Quantum dots of Cd Mn Se (x"0.03) with diameters of 9.0}9.4 nm were grown by the self-organized \V V mode in the molecular beam epitaxy. Quantum wires of Cd Mn Se (x"0.08) were synthesized with the width of \V V 60}125 nm from the two-dimensional quantum wells by the electron beam lithography and chemical etching. The optical properties and the exciton dynamics of these quantum structures were studied by the transient photoluminescence spectroscopy. The magneto-optical properties of the con"ned excitons in the quantum dots, the wires and the wells are discussed. 2000 Elsevier Science B.V. All rights reserved. PACS: 71.10.Pm; 71.20.Nr; 78.47.#p Keywords: Magnetic semiconductors; Quantum wells; Excitons
1. Introduction Physics and applications of diluted magnetic semiconductors (DMSs) are of current interests because of the appearance of the large magnetooptical e!ect and the dynamics of the Faraday rotation [1,2]. The exchange interaction of band electrons with magnetic ions is the physical origin of the enhanced magneto-optical e!ect. Nanometer-scale structures of DMSs are expected to show varieties of the optical phenomena due to the con"nement e!ect and the exchange e!ect. The fabrication of new-types of nanostructure * Corresponding author. Research Institute for Scienti"c Measurements, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Tel./fax.: #81-22-217-5360. E-mail address: [email protected] (Y. Oka).
DMS has been made by the molecular beam epitaxy (MBE) and electron-beam lithography techniques [3]. Quantum dots (QDs) and quantum wires (QWRs) of Cd Mn Se as well as the quan\V V tum wells (QWs) of Cd Mn Te are successfully \V V fabricated by these nanofabrication techniques. We study the magneto-optical properties of these DMS nanostructures by ultrafast time-resolved photoluminescence spectroscopy. Magneto-optical properties and the exciton dynamics in these quantum nanostructures are discussed.
2. Fabrication of DMS nanostructures Low-dimensional DMSs are fabricated by the epitaxy and lithography techniques. QDs of
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 0 6 9 - 5
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N. Takahashi et al. / Journal of Crystal Growth 214/215 (2000) 183}186
Cd Mn Se were grown on a GaAs substrate by \V V the atomic-layer-epitaxy (ALE) and subsequent self-organization in a MBE-growth chamber. A ZnSe bu!er layer was "rst grown on the GaAs substrate. The subsequent growth of CdSe with 2 ML by the ALE mode makes a smooth wetting layer of CdSe on the ZnSe bu!er. Then the Cd Mn Se layer was grown with 5 ML thick\V V ness at a substrate temperature of 2303C by using the vapor-#uxes of Cd, Se and Mn. By increasing the substrate temperature to 3703C, the Cd Mn Se layer changed to QDs due to the \V V self-organization growth. The formation of Cd Mn Se QDs from the 5 ML was monitored \V V by the re#ected high-energy electron di!raction (RHEED). The self-organization of Cd Mn Se \V V QDs was con"rmed by the change of the RHEED signal from a streaky pattern to the spotty one. The #ux rate of the Mn vapor beam determines the mole fraction of Mn ions in QDs as x"0.03. Fig. 1 shows the self-organized Cd Mn Se QDs \V V observed by the scanning electron microscopy (SEM) in the sample without any overlayer on the dots. The observed dot size is distributed to 30}50 nm in diameter. In this uncovered sample the dot size is a!ected by the ripening of the dots during the extraction from the high vacuum MBE chamber and the observation by SEM. For the optical measurements, the Cd Mn Se QDs were \V V covered by a ZnSe cap layer to prevent ripening of the dots from the initially grown sizes.
Fig. 1. SEM image of the uncovered Cd Mn Se QDs. The dot size is 30}50 nm.
Cd Mn Se QWRs were fabricated by using \V V the two-dimensional quantum wells of Cd Mn Se/Zn Cd Se grown on the GaAs \V V \W W substrates. A pattern of 40}50 wires was drawn by the electron-beam-drawing equipment (ElionixERA8000FE) in an area of 0.5;0.5 mm. Several patterns with the wire width of 30}200 nm were drawn on one substrate and etched with a solution of K Cr O , HBr and H O. The obtained quan tum wires of Cd Mn Se (x"0.08) are \V V 60}125 nm in the width. Two-dimensional QWs of Cd Mn Te/Cd Mg Te were also grown by \V V \W W MBE. Multi-quantum wells and asymmetric double quantum wells were fabricated to realize the exciton dynamics in the two-dimensional wells. Time resolved photoluminescence (PL) from these DMS nanostructures was measured by using optical excitation with femtosecond pulses from a mode-locked Ti : sapphire laser or by excitations with the tunable femtosecond pulses from the optical paramemetric ampli"er. Detection of the transient PL was made by a streak camera combined with a spectrometer.
3. Magneto-optical results and discussion PL spectra of the exciton in Cd Mn Se QDs are shown in Fig. 2 for the magnetic "eld of 0}7 T. At 0 T the PL peak is located at 2.445 eV and the spectral width is 30 meV in the full-width at the half-maximum. The peak energy and the spectral width indicate that the average diameter of QDs is 9.2 nm and the size-distribution is 9.0}9.4 nm. The exciton PL spectrum shows low-energy Zeeman shifts with increasing the magnetic "eld. The shift reaches 19 meV at 7 T, which gives the e!ective g-value of the QD exciton as 91. The result shows the strong exchange interaction between excitons and Mn ions in the QDs. Furthermore, we see a marked increase of the luminescence intensity with increasing the magnetic "eld. Therefore, the lifetime of the QD exciton increases by the magnetic "eld. The time-resolved PL measurement shows the variation of the exciton lifetime depending on the magnetic "eld strength. The lifetime of the excitons in the QDs is 15 ps at 0 T, which is markedly short, while the lifetime
N. Takahashi et al. / Journal of Crystal Growth 214/215 (2000) 183}186
185
Fig. 3. Exciton photoluminescence of the Cd Mn Se QWRs for the wire width of 60}126 nm.
Fig. 2. Exciton photoluminescence of the Cd Mn Se QDs for the magnetic "eld of 0}7 T.
increases to 40 ps at 7 T. In nonmagnetic CdSe QDs the exciton lifetime was about 200 ps. Thus, the exciton lifetime in the present DMS QDs is much shorter than that in the nonmagnetic QDs. The short exciton lifetime is possibly due to dominance of the nonradiative recombination process caused by the Mn ions in the DMS QDs. On the other hand, the lifetime increase in the DMS QDs by the magnetic "eld indicates the decrease of non-radiative recombination channels by the "eld, which is possibly due to the e!ect of the shrinkage of the exciton wavefunction by the "elds. The nonradiative recombination centers are possibly located at the surface of the QDs and the shrinked excitons encounter less non-radiative recombination centers. PL spectra from these QWRs are shown in Fig. 3. The bottom spectrum is the case of a single quantum well of Cd Mn Se. These QWRs show the exciton luminescence peak at 2.090}2.094 eV region with the spectral width of 39 meV. The peak energy of the exciton PL does not depend strongly on the wire widths of 60}126 nm range. The almost constant exciton energy in these QWRs is interpreted as follows: in the wire structure, the Cd Mn Se well layers are \V V
Fig. 4. The exciton energy in the Cd Mn Se QWRs as a function of the wire width. Quantum con"nement and lattice strain e!ects are calculated.
a!ected by the compressional strain e!ect from the barrier layers due to the mismatch of the lattice constant between the well and the barrier. The narrower QWRs are less a!ected by the strain because the mismatched lattice sites in the wellbarrier interface are small. The exciton in the QWRs is in#uenced by the one-dimensional quantum con"nement e!ect and also by the strain e!ect. Fig. 4 depicts the observed exciton energy as a function of the wire width. A calculation is also shown of the exciton energy by considering both the con"nement and strain e!ects. These two e!ects cause the shift of the exciton energy to the opposite energy directions. Therefore, the energy shift of the
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exciton *E remains within 5 meV of the initial value for the decrease of the wire width to 50 nm. The exciton luminescence of the Cd Mn Se \V V QWRs is polarized parallel to the wire. At 7 T the exciton peak shifts by 21.2 meV due to the Zeeman e!ect, which gives the e!ective g-value of 108. Further the polarization degree is decreased from 0.65 (0 T) to 0.55 (7 T). This result indicates the dominance of the exciton dipole-oscillation along the wire-direction at zero-"eld and the depolarization of the oscillation due to the cyclotron motion of the electrons and holes by the magnetic "eld applied perpendicular to the wire. The exciton lifetime of the QWRs is mostly 2.2 ns at 0 T, while it decreases to 700 ps at 7 T. These lifetimes and the magnetic-"eld variation do not strongly depend on the wire width for the wires wider than 50 nm. The observed decrease of the lifetime by the magnetic "eld is the typical e!ect occuring in the singlet and triplet exciton states (or the bright and dark excitons). The exchange interaction of the exciton with the Mn ions induces the ferromagnetic polarization of Mn spins. This state is a magnetic polaron state of the exciton. The formation process of the magnetic polarons was con"rmed in the QWRs by the transient low energy shift of the time-resolved exciton PL peak. In the two-dimensional QWs of Cd Mn Te/ \V V Cd Mg Te, the detailed exction states and the \W W energy relaxation process were studied [4]. The existence of the dark exciton in the DMS QWs was clari"ed by the analysis of the double-exponentialdecay characteristics of the luminescence and also by the magnetic-"eld suppression of the double-exponential-decay to the single decay [5]. The asymmetric double QWs of Cd Mn Te/ \V V
Cd Mg Te show the tunneling of carriers from \W W the narrow QW to the wide QW depending on the width of the barrier between the QWs. The analysis of the results was made by using the tunneling model of excitons and also by that of electrons. These two models give similar transient luminescence characteristics of the excitons in the present asymmetric double QW structures. In summary, we have fabricated nanostructure DMSs by the epitaxy and microlithograpy techniques. The obtained DMSs showed the exchangeinduced large magneto-optical e!ects in the quantum con"ned electronic states.
Acknowledgements This work was supported by the research project of CREST by the Japan Science and Technology Corporation and also by the project on the spin related phenomena by the Ministry of Education, Science and Culture, Japan.
References [1] S.A. Crooker, J.J. Baumberg, F. Flack, N. Samarth, D.D. Awschalom, Phys. Rev. Lett. 77 (1996) 2814. [2] A. Haury, A. Wasiela, A. Arnoult, J. Ciebert, S. Tatarenko, T. Dietl, Y. Merle d'Aubigne, Phys. Rev. Lett. 79 (1997) 511. [3] Y. Oka, J.X. Shen, K. Takabayashi, N. Takahashi, H. Mitsu, I. Souma, R. Pittini, J. Lumin. 83/84 (1999) 83. [4] H. Mitsu, K. Egawa, M.C. Debnath, J.X. Shen, Y. Oka, Proceedings of the third International Conference on Excitonic Processes in Condensed Matter, Boston, The Electrochemical Society, Inc., 1998, p. 204. [5] M.C. Debnath, I. Souma, E. Shirado, H. Mitsu, T. Sato, J.X. Shen, Y. Oka, J. Appl. Phys. 85 (1999) 5941.
Fabrication and optical study of quantum dots, quantum wires and quantum wells of II}VI diluted magnetic semiconductors N. Takahashi , K. Takabayashi , E. Shirado , I. Souma , J.X. Shen , Y. Oka * Research Institute for Scientixc Measurements, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan CREST, Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
Abstract Nanometer-scale structures of II}VI diluted magnetic semiconductors were synthesized by the microfabrication techniques. Quantum dots of Cd Mn Se (x"0.03) with diameters of 9.0}9.4 nm were grown by the self-organized \V V mode in the molecular beam epitaxy. Quantum wires of Cd Mn Se (x"0.08) were synthesized with the width of \V V 60}125 nm from the two-dimensional quantum wells by the electron beam lithography and chemical etching. The optical properties and the exciton dynamics of these quantum structures were studied by the transient photoluminescence spectroscopy. The magneto-optical properties of the con"ned excitons in the quantum dots, the wires and the wells are discussed. 2000 Elsevier Science B.V. All rights reserved. PACS: 71.10.Pm; 71.20.Nr; 78.47.#p Keywords: Magnetic semiconductors; Quantum wells; Excitons
1. Introduction Physics and applications of diluted magnetic semiconductors (DMSs) are of current interests because of the appearance of the large magnetooptical e!ect and the dynamics of the Faraday rotation [1,2]. The exchange interaction of band electrons with magnetic ions is the physical origin of the enhanced magneto-optical e!ect. Nanometer-scale structures of DMSs are expected to show varieties of the optical phenomena due to the con"nement e!ect and the exchange e!ect. The fabrication of new-types of nanostructure * Corresponding author. Research Institute for Scienti"c Measurements, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Tel./fax.: #81-22-217-5360. E-mail address: [email protected] (Y. Oka).
DMS has been made by the molecular beam epitaxy (MBE) and electron-beam lithography techniques [3]. Quantum dots (QDs) and quantum wires (QWRs) of Cd Mn Se as well as the quan\V V tum wells (QWs) of Cd Mn Te are successfully \V V fabricated by these nanofabrication techniques. We study the magneto-optical properties of these DMS nanostructures by ultrafast time-resolved photoluminescence spectroscopy. Magneto-optical properties and the exciton dynamics in these quantum nanostructures are discussed.
2. Fabrication of DMS nanostructures Low-dimensional DMSs are fabricated by the epitaxy and lithography techniques. QDs of
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 0 6 9 - 5
184
N. Takahashi et al. / Journal of Crystal Growth 214/215 (2000) 183}186
Cd Mn Se were grown on a GaAs substrate by \V V the atomic-layer-epitaxy (ALE) and subsequent self-organization in a MBE-growth chamber. A ZnSe bu!er layer was "rst grown on the GaAs substrate. The subsequent growth of CdSe with 2 ML by the ALE mode makes a smooth wetting layer of CdSe on the ZnSe bu!er. Then the Cd Mn Se layer was grown with 5 ML thick\V V ness at a substrate temperature of 2303C by using the vapor-#uxes of Cd, Se and Mn. By increasing the substrate temperature to 3703C, the Cd Mn Se layer changed to QDs due to the \V V self-organization growth. The formation of Cd Mn Se QDs from the 5 ML was monitored \V V by the re#ected high-energy electron di!raction (RHEED). The self-organization of Cd Mn Se \V V QDs was con"rmed by the change of the RHEED signal from a streaky pattern to the spotty one. The #ux rate of the Mn vapor beam determines the mole fraction of Mn ions in QDs as x"0.03. Fig. 1 shows the self-organized Cd Mn Se QDs \V V observed by the scanning electron microscopy (SEM) in the sample without any overlayer on the dots. The observed dot size is distributed to 30}50 nm in diameter. In this uncovered sample the dot size is a!ected by the ripening of the dots during the extraction from the high vacuum MBE chamber and the observation by SEM. For the optical measurements, the Cd Mn Se QDs were \V V covered by a ZnSe cap layer to prevent ripening of the dots from the initially grown sizes.
Fig. 1. SEM image of the uncovered Cd Mn Se QDs. The dot size is 30}50 nm.
Cd Mn Se QWRs were fabricated by using \V V the two-dimensional quantum wells of Cd Mn Se/Zn Cd Se grown on the GaAs \V V \W W substrates. A pattern of 40}50 wires was drawn by the electron-beam-drawing equipment (ElionixERA8000FE) in an area of 0.5;0.5 mm. Several patterns with the wire width of 30}200 nm were drawn on one substrate and etched with a solution of K Cr O , HBr and H O. The obtained quan tum wires of Cd Mn Se (x"0.08) are \V V 60}125 nm in the width. Two-dimensional QWs of Cd Mn Te/Cd Mg Te were also grown by \V V \W W MBE. Multi-quantum wells and asymmetric double quantum wells were fabricated to realize the exciton dynamics in the two-dimensional wells. Time resolved photoluminescence (PL) from these DMS nanostructures was measured by using optical excitation with femtosecond pulses from a mode-locked Ti : sapphire laser or by excitations with the tunable femtosecond pulses from the optical paramemetric ampli"er. Detection of the transient PL was made by a streak camera combined with a spectrometer.
3. Magneto-optical results and discussion PL spectra of the exciton in Cd Mn Se QDs are shown in Fig. 2 for the magnetic "eld of 0}7 T. At 0 T the PL peak is located at 2.445 eV and the spectral width is 30 meV in the full-width at the half-maximum. The peak energy and the spectral width indicate that the average diameter of QDs is 9.2 nm and the size-distribution is 9.0}9.4 nm. The exciton PL spectrum shows low-energy Zeeman shifts with increasing the magnetic "eld. The shift reaches 19 meV at 7 T, which gives the e!ective g-value of the QD exciton as 91. The result shows the strong exchange interaction between excitons and Mn ions in the QDs. Furthermore, we see a marked increase of the luminescence intensity with increasing the magnetic "eld. Therefore, the lifetime of the QD exciton increases by the magnetic "eld. The time-resolved PL measurement shows the variation of the exciton lifetime depending on the magnetic "eld strength. The lifetime of the excitons in the QDs is 15 ps at 0 T, which is markedly short, while the lifetime
N. Takahashi et al. / Journal of Crystal Growth 214/215 (2000) 183}186
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Fig. 3. Exciton photoluminescence of the Cd Mn Se QWRs for the wire width of 60}126 nm.
Fig. 2. Exciton photoluminescence of the Cd Mn Se QDs for the magnetic "eld of 0}7 T.
increases to 40 ps at 7 T. In nonmagnetic CdSe QDs the exciton lifetime was about 200 ps. Thus, the exciton lifetime in the present DMS QDs is much shorter than that in the nonmagnetic QDs. The short exciton lifetime is possibly due to dominance of the nonradiative recombination process caused by the Mn ions in the DMS QDs. On the other hand, the lifetime increase in the DMS QDs by the magnetic "eld indicates the decrease of non-radiative recombination channels by the "eld, which is possibly due to the e!ect of the shrinkage of the exciton wavefunction by the "elds. The nonradiative recombination centers are possibly located at the surface of the QDs and the shrinked excitons encounter less non-radiative recombination centers. PL spectra from these QWRs are shown in Fig. 3. The bottom spectrum is the case of a single quantum well of Cd Mn Se. These QWRs show the exciton luminescence peak at 2.090}2.094 eV region with the spectral width of 39 meV. The peak energy of the exciton PL does not depend strongly on the wire widths of 60}126 nm range. The almost constant exciton energy in these QWRs is interpreted as follows: in the wire structure, the Cd Mn Se well layers are \V V
Fig. 4. The exciton energy in the Cd Mn Se QWRs as a function of the wire width. Quantum con"nement and lattice strain e!ects are calculated.
a!ected by the compressional strain e!ect from the barrier layers due to the mismatch of the lattice constant between the well and the barrier. The narrower QWRs are less a!ected by the strain because the mismatched lattice sites in the wellbarrier interface are small. The exciton in the QWRs is in#uenced by the one-dimensional quantum con"nement e!ect and also by the strain e!ect. Fig. 4 depicts the observed exciton energy as a function of the wire width. A calculation is also shown of the exciton energy by considering both the con"nement and strain e!ects. These two e!ects cause the shift of the exciton energy to the opposite energy directions. Therefore, the energy shift of the
186
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exciton *E remains within 5 meV of the initial value for the decrease of the wire width to 50 nm. The exciton luminescence of the Cd Mn Se \V V QWRs is polarized parallel to the wire. At 7 T the exciton peak shifts by 21.2 meV due to the Zeeman e!ect, which gives the e!ective g-value of 108. Further the polarization degree is decreased from 0.65 (0 T) to 0.55 (7 T). This result indicates the dominance of the exciton dipole-oscillation along the wire-direction at zero-"eld and the depolarization of the oscillation due to the cyclotron motion of the electrons and holes by the magnetic "eld applied perpendicular to the wire. The exciton lifetime of the QWRs is mostly 2.2 ns at 0 T, while it decreases to 700 ps at 7 T. These lifetimes and the magnetic-"eld variation do not strongly depend on the wire width for the wires wider than 50 nm. The observed decrease of the lifetime by the magnetic "eld is the typical e!ect occuring in the singlet and triplet exciton states (or the bright and dark excitons). The exchange interaction of the exciton with the Mn ions induces the ferromagnetic polarization of Mn spins. This state is a magnetic polaron state of the exciton. The formation process of the magnetic polarons was con"rmed in the QWRs by the transient low energy shift of the time-resolved exciton PL peak. In the two-dimensional QWs of Cd Mn Te/ \V V Cd Mg Te, the detailed exction states and the \W W energy relaxation process were studied [4]. The existence of the dark exciton in the DMS QWs was clari"ed by the analysis of the double-exponentialdecay characteristics of the luminescence and also by the magnetic-"eld suppression of the double-exponential-decay to the single decay [5]. The asymmetric double QWs of Cd Mn Te/ \V V
Cd Mg Te show the tunneling of carriers from \W W the narrow QW to the wide QW depending on the width of the barrier between the QWs. The analysis of the results was made by using the tunneling model of excitons and also by that of electrons. These two models give similar transient luminescence characteristics of the excitons in the present asymmetric double QW structures. In summary, we have fabricated nanostructure DMSs by the epitaxy and microlithograpy techniques. The obtained DMSs showed the exchangeinduced large magneto-optical e!ects in the quantum con"ned electronic states.
Acknowledgements This work was supported by the research project of CREST by the Japan Science and Technology Corporation and also by the project on the spin related phenomena by the Ministry of Education, Science and Culture, Japan.
References [1] S.A. Crooker, J.J. Baumberg, F. Flack, N. Samarth, D.D. Awschalom, Phys. Rev. Lett. 77 (1996) 2814. [2] A. Haury, A. Wasiela, A. Arnoult, J. Ciebert, S. Tatarenko, T. Dietl, Y. Merle d'Aubigne, Phys. Rev. Lett. 79 (1997) 511. [3] Y. Oka, J.X. Shen, K. Takabayashi, N. Takahashi, H. Mitsu, I. Souma, R. Pittini, J. Lumin. 83/84 (1999) 83. [4] H. Mitsu, K. Egawa, M.C. Debnath, J.X. Shen, Y. Oka, Proceedings of the third International Conference on Excitonic Processes in Condensed Matter, Boston, The Electrochemical Society, Inc., 1998, p. 204. [5] M.C. Debnath, I. Souma, E. Shirado, H. Mitsu, T. Sato, J.X. Shen, Y. Oka, J. Appl. Phys. 85 (1999) 5941.