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Zn1−yMny Se quantum dots and quantum wells
Physica E 10 (2001) 358–361
www.elsevier.nl/locate/physe
Magneto-luminescence in Cd 1−x Mnx Se=ZnSe and CdSe=Zn1−y Mny Se quantum dots and quantum wells Katsuhiro Shibataa; b; ∗ , Kazumasa Takabayashia; b , Izuru Soumaa; b , Jinxi Shena; b , Kouhei Yanataa; b , Yasuo Okaa; b a RISM,
b CREST,
Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
Abstract Radiative and non-radiative recombination processes of excitons in quantum dots of diluted magnetic semiconductors were studied in magnetic 5elds. The Cd 1−x Mnx Se quantum dots fabricated by the self-organized mode on ZnSe showed marked increases of the photoluminescence intensity and the decay time in the magnetic 5eld. These phenomena are related to the c 2001 Elsevier Science B.V. All rights reserved. magnetic 5eld induced shrinkage of the exciton Bohr radius. PACS: 75.50.P; 73.20.D; 75.70.C; 78.66; 32.50 Keywords: Magnetic semiconductors; Quantum dots; Quantum wells; Luminescence
1. Introduction Diluted magnetic semiconductors (DMSs) show remarkable magneto-optical properties such as a giant Zeeman e>ect, which is due to the strong exchange interaction between the carrier spin and the spin of the magnetic ions. Semiconductor quantum dots (QDs) also have particular properties, such as the discrete energy levels due to the three-dimensional quantum con5nement e>ects. Recently, the application of the ∗ Corresponding author. RISM, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81-22-217-5362; fax: +81-22-217-5363. E-mail address: [email protected] (K. Shibata).
QDs for optical devices is also investigated extensively. However, up to now, the reports on fabrication of DMS QDs are few [1– 4] and their properties are not well clari5ed. Theoretical investigation of DMS QDs has been made recently [5]. In this study, we fabricate the Cd 1−x Mnx Se QDs and investigate their magneto-optical properties, especially the inIuence of quantum con5nement on the magneto-optical e>ect.
2. Growth of the Cd1−x Mnx Se QDs The self-organized growth has been proven to be a promising method to fabricate semiconductor QDs
c 2001 Elsevier Science B.V. All rights reserved. 1386-9477/01/$ - see front matter PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 1 1 6 - 3
K. Shibata et al. / Physica E 10 (2001) 358–361
[6,7]. This method can provide QDs of high crystal quality and good optical properties. In the present work, the Cd 1−x Mnx Se QDs are fabricated by using the atomic layer epitaxy (ALE), which is followed by annealing to grow the QDs by the self-organized mode. The ZnSe bu>er layer was 5rst grown on the ◦ (1 0 0) GaAs substrate at 300 C. The substrate tem◦ perature was then decreased down to 230 C. At this temperature, 2 monolayers (ML) of CdSe (wetting layer) and 2–7 ML of Cd 1−x Mnx Se were deposited onto the ZnSe bu>er by ALE. The sample was an◦ nealed at 370 C for 1 min and then cooled down to ◦ 300 C. During these processes, the Cd 1−x Mnx Se QDs were formed as well as the CdSe wetting layer. The ZnSe cap layer was then grown on the QDs for optical measurements. The formation of the QD structure was monitored by the reIection high energy electron di>raction (RHEED). Fig. 1(a) shows the atomic force microscopy (AFM) image of the self-organized Cd 1−x Mnx Se QDs without the ZnSe cap layer. From this 5gure, one can see that the typical diameter of the dots is about 100 nm, the height is 10 nm and the density of dots is about 109 cm−2 . However, since the dots in Fig. 1(a) were exposed to open air, they were growing to larger dots with time due to the Ostwald ripening [8]. Therefore, the size of the QDs covered by the ZnSe cap is expected to have much smaller diameter and the density is expected to be higher. The Mn concentration in the self-organized QDs is 3%, determined by the X-ray di>raction. 3. Magneto-optical properties of Cd0:97 Mn0:03 Se QDs The time resolved photoluminescence (PL) was measured by exciting the samples with femtosecond light pulses of a frequency-doubled mode-locked Ti : sapphire laser. The wavelength of excitation light is 390 nm. Detection of the transient PL of the sample was made by a streak camera combined with a spectrometer. The PL spectrum of the Cd 0:97 Mn0:03 Se QDs is shown in Fig. 1(b). The spectrum displays the exciton PL peaks from the ZnSe, the CdSe wetting layer, the Cd 0:97 Mn0:03 Se QDs and the Mn2+ d–d transition at 2.8, 2.6, 2.45 and 2:25 eV, respectively. The PL
359
Fig. 1. (a) The AFM image of the Cd 0:97 Mn0:03 Se QDs (1 m × 1 m). (b) A PL spectrum of the Cd 0:97 Mn0:03 Se QDs sample (B = 0 T).
energy of the QDs is much higher than that of the Cd 0:97 Mn0:03 Se thick epilayer (1:88 eV). This large blue shift is resulted from the three-dimensional quantum con5nement e>ect for the exciton state. Fig. 2 shows the PL energy of the Cd 0:97 Mn0:03 Se QDs as a function of the diameter of the QDs, which are self-organized from the epitaxial layers of 2–7 ML. The solid line displays the calculated value by assuming the QDs to be a disk shape with diameter d and thickness d=2. Open circles correspond to the PL peak energies of the fabricated QDs samples. By comparing the theoretical curve and the experimental data, the diameter of the QDs under the ZnSe cap is determined as 5.9 –8:4 nm.
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K. Shibata et al. / Physica E 10 (2001) 358–361
Fig. 2. The PL peak energy as a function of the diameter of the Cd 0:97 Mn0:03 Se QDs. The solid line is the calculated value and the open circles are experimental values.
Fig. 3. Streak camera image of the exciton PL of the Cd 0:97 Mn0:03 Se QDs (d = 7:5 nm).
Fig. 3 shows the transient PL images of the Cd 0:97 Mn0:03 Se QDs at 0 and 7 T detected by the streak camera. The magnetic 5eld was applied parallel to the growth direction. The decay time at 0 T is 20 ps. We have also fabricated the CdSe QDs in the Zn1−y Mny Se matrices, where the optically created excitons in the QDs have the exchange interaction with the Mn ions outside the QDs. In this case, the decay time of the exciton is also 20 ps, while the CdSe QDs with the ZnSe matrices show the decay time of 200 ps. Therefore, the short decay time is related with the non-radiative recombination center located at the QD surface, where the Mn ions exist. The decay time of the QD excitons increases to 104 ps at 7 T in Fig. 3.
Fig. 4. (a) Magnetic 5eld dependence of the exciton PL intensity of the Cd 0:97 Mn0:03 Se QDs and the CdSe QDs, where the diameter of the QDs is 6:0 nm. (b) Magnetic 5eld dependence of the exciton PL intensity of the Cd 0:97 Mn0:03 Se QDs (5.9 –8:4 nm) and the Cd 0:97 Mn0:03 Se SQW.
Fig. 4(a) shows the magnetic 5eld dependence of the exciton PL intensities in the Cd 0:97 Mn0:03 Se QDs and the CdSe QDs with the diameter of 6:0 nm. For the magnetic of 6 T, the exciton PL intensity of the Cd 0:97 Mn0:03 Se QDs increases 7 times of that at 0 T, while that of the CdSe QDs remains unchanged. In Fig. 4(b), the PL intensity of the Cd 0:97 Mn0:03 Se QDs and SQW are shown as a function of the magnetic 5eld. The QDs of 5.9 –6:0 nm diameter show the marked increase of the PL intensity, while in the QDs of 8:4 nm diameter and the SQW the PL intensities are not a>ected signi5cantly by the magnetic 5eld.
K. Shibata et al. / Physica E 10 (2001) 358–361
Fig. 5. Size dependence of the exciton PL decay time of the Cd 0:97 Mn0:03 Se QDs.
361
enhancement of the PL intensity and longer decay time can be expected by the shrinkage of the exciton wave function especially for the QDs of 5.9 –6:0 nm diameter. On the other hand, no signi5cant increases of PL intensity and decay time were observed in the perpendicular con5guration of the magnetic 5eld [10]. Since the excitons in the QDs are more tightly con5ned in the growth direction, owing to the disk shape of dots, much higher magnetic 5eld is required to induce a noticeable exciton shrinkage. Therefore, in the perpendicular con5guration of the magnetic 5eld, the increase of PL intensity and decay time were not obviously observed. 4. Summary
Fig. 5 shows the decay time of the exciton in the QDs as a function of the QD diameter. The decay time is 110 ps for the 8:4 nm QDs and it decreases to 50 ps for the 5:9 nm QDs at 0 T. Since the decay time of the exciton PL in the QDs increases with increasing the magnetic 5eld as shown in Fig. 3, the strong enhancement of the exciton PL intensity by the 5eld is caused by the increase in the decay time. The observed magnetic 5eld dependence of the PL intensity and the decay time were not seen in epilayers and SQW of the Cd 0:97 Mn0:03 Se and also CdSe QDs in ZnSe matrix. (Fig. 4) Therefore, the magnetic 5eld dependent PL have inherent properties in the Cd 1−x Mnx Se QDs. The short decay time is the dominance of the non-radiative centers at the surface of the Mn doped QDs. When the magnetic 5eld is applied parallel to the growth direction of the sample, the shrinkage of the exciton Bohr radius occurs in the plane perpendicular to the growth direction. If the exciton Bohr radius is shrunk into the QDs the non-radiative exciton trapping rate decreases. Thus, the decay time and the PL intensities increase by increasing the magnetic 5eld. The shrinkage of the exciton Bohr radius in the Cd 0:97 Mn0:03 Se QDs is estimated to be 10% at 7 T from the Yafet, Keyes and Adams theory [9]. However, the diameter of the present QDs is 5.9 –8:4 nm, which is close to the size of the exciton with the Bohr radius of 3:6 nm at 0 T. Therefore, the signi5cant
The Cd 1−x Mnx Se=ZnSe; CdSe=ZnSe QDs were fabricated by using the self-organized mode. Their magneto-optical properties were studied by time resolved PL under magnetic 5eld. Marked increase of the exciton PL intensity and the decay time of the QDs were induced by the magnetic 5eld. The results were interpreted by the shrinkage of the exciton Bohr radius in the DMS QDs by the magnetic 5eld. References [1] Ying Wang, H. Norman, K. Moller, T. Bein, Solid State Commun. 77 (1990) 33. [2] Y. Oka, K. Yanata, J. Lumin. 70 (1996) 35. [3] Y. Oka, J.X. Shen, K. Takabayashi, N. Takahashi, H. Mitsu, I. Souma, R. Pittini, J. Lumin. 83=84 (1999) 83. [4] T. Kuroda, N. Hasegawa, F. Minami, Y. Terai, S. Kuroda, K. Takita, J. Lumin. 83=84 (1999) 321. [5] A.K. Bhattacharjee, Phys. Rev. 51 (1995) 9912. [6] B.D. Min, Y. Kim, E.K. Kim, S.K. Min, M.J. Park, Phys. Rev. 57 (1998) 11 879. [7] E. Kurtz, T. Sekiguchi, Z. Zhu, T. Yao, J.X. Shen, Y. Oka, M.Y. Shen, T. Goto, Superlattices Microstruct. 25 (1999) 119. [8] S. Lee, I. Daruka, C.S. Kim, A.-L. Barabasi, J.L. Merz, J.K. Furdyna, Phys. Rev. Lett. 81 (1998) 3479. [9] Y. Yafet, R.W. Keyes, E.N. Adams, J. Phys. Chem. Solids 1 (1956) 137. [10] K. Takabayashi, K. Shibata, I. Souma, J.X. Shen, Y. Oka, Extended Abstracts of The Fifth Symposium on the PASPS, 1999, p. 76.
www.elsevier.nl/locate/physe
Magneto-luminescence in Cd 1−x Mnx Se=ZnSe and CdSe=Zn1−y Mny Se quantum dots and quantum wells Katsuhiro Shibataa; b; ∗ , Kazumasa Takabayashia; b , Izuru Soumaa; b , Jinxi Shena; b , Kouhei Yanataa; b , Yasuo Okaa; b a RISM,
b CREST,
Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Japan Science and Technology Corporation, Kawaguchi 332-0012, Japan
Abstract Radiative and non-radiative recombination processes of excitons in quantum dots of diluted magnetic semiconductors were studied in magnetic 5elds. The Cd 1−x Mnx Se quantum dots fabricated by the self-organized mode on ZnSe showed marked increases of the photoluminescence intensity and the decay time in the magnetic 5eld. These phenomena are related to the c 2001 Elsevier Science B.V. All rights reserved. magnetic 5eld induced shrinkage of the exciton Bohr radius. PACS: 75.50.P; 73.20.D; 75.70.C; 78.66; 32.50 Keywords: Magnetic semiconductors; Quantum dots; Quantum wells; Luminescence
1. Introduction Diluted magnetic semiconductors (DMSs) show remarkable magneto-optical properties such as a giant Zeeman e>ect, which is due to the strong exchange interaction between the carrier spin and the spin of the magnetic ions. Semiconductor quantum dots (QDs) also have particular properties, such as the discrete energy levels due to the three-dimensional quantum con5nement e>ects. Recently, the application of the ∗ Corresponding author. RISM, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81-22-217-5362; fax: +81-22-217-5363. E-mail address: [email protected] (K. Shibata).
QDs for optical devices is also investigated extensively. However, up to now, the reports on fabrication of DMS QDs are few [1– 4] and their properties are not well clari5ed. Theoretical investigation of DMS QDs has been made recently [5]. In this study, we fabricate the Cd 1−x Mnx Se QDs and investigate their magneto-optical properties, especially the inIuence of quantum con5nement on the magneto-optical e>ect.
2. Growth of the Cd1−x Mnx Se QDs The self-organized growth has been proven to be a promising method to fabricate semiconductor QDs
c 2001 Elsevier Science B.V. All rights reserved. 1386-9477/01/$ - see front matter PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 1 1 6 - 3
K. Shibata et al. / Physica E 10 (2001) 358–361
[6,7]. This method can provide QDs of high crystal quality and good optical properties. In the present work, the Cd 1−x Mnx Se QDs are fabricated by using the atomic layer epitaxy (ALE), which is followed by annealing to grow the QDs by the self-organized mode. The ZnSe bu>er layer was 5rst grown on the ◦ (1 0 0) GaAs substrate at 300 C. The substrate tem◦ perature was then decreased down to 230 C. At this temperature, 2 monolayers (ML) of CdSe (wetting layer) and 2–7 ML of Cd 1−x Mnx Se were deposited onto the ZnSe bu>er by ALE. The sample was an◦ nealed at 370 C for 1 min and then cooled down to ◦ 300 C. During these processes, the Cd 1−x Mnx Se QDs were formed as well as the CdSe wetting layer. The ZnSe cap layer was then grown on the QDs for optical measurements. The formation of the QD structure was monitored by the reIection high energy electron di>raction (RHEED). Fig. 1(a) shows the atomic force microscopy (AFM) image of the self-organized Cd 1−x Mnx Se QDs without the ZnSe cap layer. From this 5gure, one can see that the typical diameter of the dots is about 100 nm, the height is 10 nm and the density of dots is about 109 cm−2 . However, since the dots in Fig. 1(a) were exposed to open air, they were growing to larger dots with time due to the Ostwald ripening [8]. Therefore, the size of the QDs covered by the ZnSe cap is expected to have much smaller diameter and the density is expected to be higher. The Mn concentration in the self-organized QDs is 3%, determined by the X-ray di>raction. 3. Magneto-optical properties of Cd0:97 Mn0:03 Se QDs The time resolved photoluminescence (PL) was measured by exciting the samples with femtosecond light pulses of a frequency-doubled mode-locked Ti : sapphire laser. The wavelength of excitation light is 390 nm. Detection of the transient PL of the sample was made by a streak camera combined with a spectrometer. The PL spectrum of the Cd 0:97 Mn0:03 Se QDs is shown in Fig. 1(b). The spectrum displays the exciton PL peaks from the ZnSe, the CdSe wetting layer, the Cd 0:97 Mn0:03 Se QDs and the Mn2+ d–d transition at 2.8, 2.6, 2.45 and 2:25 eV, respectively. The PL
359
Fig. 1. (a) The AFM image of the Cd 0:97 Mn0:03 Se QDs (1 m × 1 m). (b) A PL spectrum of the Cd 0:97 Mn0:03 Se QDs sample (B = 0 T).
energy of the QDs is much higher than that of the Cd 0:97 Mn0:03 Se thick epilayer (1:88 eV). This large blue shift is resulted from the three-dimensional quantum con5nement e>ect for the exciton state. Fig. 2 shows the PL energy of the Cd 0:97 Mn0:03 Se QDs as a function of the diameter of the QDs, which are self-organized from the epitaxial layers of 2–7 ML. The solid line displays the calculated value by assuming the QDs to be a disk shape with diameter d and thickness d=2. Open circles correspond to the PL peak energies of the fabricated QDs samples. By comparing the theoretical curve and the experimental data, the diameter of the QDs under the ZnSe cap is determined as 5.9 –8:4 nm.
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K. Shibata et al. / Physica E 10 (2001) 358–361
Fig. 2. The PL peak energy as a function of the diameter of the Cd 0:97 Mn0:03 Se QDs. The solid line is the calculated value and the open circles are experimental values.
Fig. 3. Streak camera image of the exciton PL of the Cd 0:97 Mn0:03 Se QDs (d = 7:5 nm).
Fig. 3 shows the transient PL images of the Cd 0:97 Mn0:03 Se QDs at 0 and 7 T detected by the streak camera. The magnetic 5eld was applied parallel to the growth direction. The decay time at 0 T is 20 ps. We have also fabricated the CdSe QDs in the Zn1−y Mny Se matrices, where the optically created excitons in the QDs have the exchange interaction with the Mn ions outside the QDs. In this case, the decay time of the exciton is also 20 ps, while the CdSe QDs with the ZnSe matrices show the decay time of 200 ps. Therefore, the short decay time is related with the non-radiative recombination center located at the QD surface, where the Mn ions exist. The decay time of the QD excitons increases to 104 ps at 7 T in Fig. 3.
Fig. 4. (a) Magnetic 5eld dependence of the exciton PL intensity of the Cd 0:97 Mn0:03 Se QDs and the CdSe QDs, where the diameter of the QDs is 6:0 nm. (b) Magnetic 5eld dependence of the exciton PL intensity of the Cd 0:97 Mn0:03 Se QDs (5.9 –8:4 nm) and the Cd 0:97 Mn0:03 Se SQW.
Fig. 4(a) shows the magnetic 5eld dependence of the exciton PL intensities in the Cd 0:97 Mn0:03 Se QDs and the CdSe QDs with the diameter of 6:0 nm. For the magnetic of 6 T, the exciton PL intensity of the Cd 0:97 Mn0:03 Se QDs increases 7 times of that at 0 T, while that of the CdSe QDs remains unchanged. In Fig. 4(b), the PL intensity of the Cd 0:97 Mn0:03 Se QDs and SQW are shown as a function of the magnetic 5eld. The QDs of 5.9 –6:0 nm diameter show the marked increase of the PL intensity, while in the QDs of 8:4 nm diameter and the SQW the PL intensities are not a>ected signi5cantly by the magnetic 5eld.
K. Shibata et al. / Physica E 10 (2001) 358–361
Fig. 5. Size dependence of the exciton PL decay time of the Cd 0:97 Mn0:03 Se QDs.
361
enhancement of the PL intensity and longer decay time can be expected by the shrinkage of the exciton wave function especially for the QDs of 5.9 –6:0 nm diameter. On the other hand, no signi5cant increases of PL intensity and decay time were observed in the perpendicular con5guration of the magnetic 5eld [10]. Since the excitons in the QDs are more tightly con5ned in the growth direction, owing to the disk shape of dots, much higher magnetic 5eld is required to induce a noticeable exciton shrinkage. Therefore, in the perpendicular con5guration of the magnetic 5eld, the increase of PL intensity and decay time were not obviously observed. 4. Summary
Fig. 5 shows the decay time of the exciton in the QDs as a function of the QD diameter. The decay time is 110 ps for the 8:4 nm QDs and it decreases to 50 ps for the 5:9 nm QDs at 0 T. Since the decay time of the exciton PL in the QDs increases with increasing the magnetic 5eld as shown in Fig. 3, the strong enhancement of the exciton PL intensity by the 5eld is caused by the increase in the decay time. The observed magnetic 5eld dependence of the PL intensity and the decay time were not seen in epilayers and SQW of the Cd 0:97 Mn0:03 Se and also CdSe QDs in ZnSe matrix. (Fig. 4) Therefore, the magnetic 5eld dependent PL have inherent properties in the Cd 1−x Mnx Se QDs. The short decay time is the dominance of the non-radiative centers at the surface of the Mn doped QDs. When the magnetic 5eld is applied parallel to the growth direction of the sample, the shrinkage of the exciton Bohr radius occurs in the plane perpendicular to the growth direction. If the exciton Bohr radius is shrunk into the QDs the non-radiative exciton trapping rate decreases. Thus, the decay time and the PL intensities increase by increasing the magnetic 5eld. The shrinkage of the exciton Bohr radius in the Cd 0:97 Mn0:03 Se QDs is estimated to be 10% at 7 T from the Yafet, Keyes and Adams theory [9]. However, the diameter of the present QDs is 5.9 –8:4 nm, which is close to the size of the exciton with the Bohr radius of 3:6 nm at 0 T. Therefore, the signi5cant
The Cd 1−x Mnx Se=ZnSe; CdSe=ZnSe QDs were fabricated by using the self-organized mode. Their magneto-optical properties were studied by time resolved PL under magnetic 5eld. Marked increase of the exciton PL intensity and the decay time of the QDs were induced by the magnetic 5eld. The results were interpreted by the shrinkage of the exciton Bohr radius in the DMS QDs by the magnetic 5eld. References [1] Ying Wang, H. Norman, K. Moller, T. Bein, Solid State Commun. 77 (1990) 33. [2] Y. Oka, K. Yanata, J. Lumin. 70 (1996) 35. [3] Y. Oka, J.X. Shen, K. Takabayashi, N. Takahashi, H. Mitsu, I. Souma, R. Pittini, J. Lumin. 83=84 (1999) 83. [4] T. Kuroda, N. Hasegawa, F. Minami, Y. Terai, S. Kuroda, K. Takita, J. Lumin. 83=84 (1999) 321. [5] A.K. Bhattacharjee, Phys. Rev. 51 (1995) 9912. [6] B.D. Min, Y. Kim, E.K. Kim, S.K. Min, M.J. Park, Phys. Rev. 57 (1998) 11 879. [7] E. Kurtz, T. Sekiguchi, Z. Zhu, T. Yao, J.X. Shen, Y. Oka, M.Y. Shen, T. Goto, Superlattices Microstruct. 25 (1999) 119. [8] S. Lee, I. Daruka, C.S. Kim, A.-L. Barabasi, J.L. Merz, J.K. Furdyna, Phys. Rev. Lett. 81 (1998) 3479. [9] Y. Yafet, R.W. Keyes, E.N. Adams, J. Phys. Chem. Solids 1 (1956) 137. [10] K. Takabayashi, K. Shibata, I. Souma, J.X. Shen, Y. Oka, Extended Abstracts of The Fifth Symposium on the PASPS, 1999, p. 76.