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Magneto-photoluminescence from self-organized quantum dots of Cd1−xMnxTe grown by molecular beam epitaxy
Journal of Luminescence 87}89 (2000) 396}398
Magneto-photoluminescence from self-organized quantum dots of Cd\V MnV Te grown by molecular beam epitaxy Yoshikazu Terai , Shinji Kuroda *, Ko( ki Takita , Tadashi Takamasu, Giyuu Kido Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan National Research Institute for Metals, Tsukuba, Ibaraki 305-0047, Japan
Abstract Photoluminescence (PL) studies of self-organized quantum dots (QDs) of Cd\V MnV Te under magnetic "elds were reported. Cd\V MnV Te QDs were grown successfully in the composition range of x"0}10% on the ZnTe (1 0 0) surface by MBE. The PL spectra from capped Cd Mn Te QDs were composed of two lines with a separation of about 20 meV \V V at low temperatures. In the dependence on the temperature and magnetic "eld, the PL line in the lower-energy side exhibited peculiar behaviors, compared to that in the higher-energy side; with increasing temperature, the intensity of the lower-energy line was reduced rapidly and almost disappeared above 20 K, and Zeeman shift under magnetic "elds was smaller and the degree of the circular polarization was larger and saturated at low magnetic "elds. These features of the lower-energy line were discussed in relation to excitonic magnetic polaron in QDs. 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Self-organized dots; Cd Mn Te; Magneto-photoluminescence \V V
Diluted magnetic semiconductors (DMS) show marked magneto-optical e!ects due to the sp}d exchange interaction between band carriers and magnetic ions. In nanostructures of DMS (such as quantum dots } QDs), some new features are expected to arise in the magnetooptical properties caused by both of the exchange interaction and strong quantum con"nement. Experimentally, the magneto-optical studies have been reported in DMS nanocrystals ((Cd,Mn)Se in SiO [1], (Zn,Mn)S in glass [2]). However there seems to be some di$culties for quantitative investigations arising from the size distribution and the interface quality. On the other hand, self-organized QDs formed in the epitaxial growth onto a lattice-mismatched material are another object of the optical studies, which have the advantage of a relative uniform size and high emission e$ciency. Recent
* Corresponding author. Tel.: #81-298-53-5365/5020; fax: #81-298-55-7440. E-mail address: [email protected] (S. Kuroda)
successful fabrications of self-organized QDs in II}VI compounds such as CdSe on ZnSe [3] and CdTe on ZnTe [4] open the possibility of DMS QDs incorporating Mn. In the present paper, we report photoluminescence (PL) studies of self-organized QDs of Cd Mn Te, which were successfully grown on the \V V ZnTe (1 0 0) surface by MBE. The growth of Cd Mn Te QDs was performed in \V V an almost similar way to that of CdTe QDs in our previous study [4]; that is, ZnTe bu!er layer was "rst grown on a GaAs (1 0 0) substrate to a thickness of 0.6}1 lm, and Cd Mn Te layer was grown on that \V V surface by supplying a #ux of Cd#Mn and that of Te alternately at a substrate temperature of 3003C. In this condition, similarly to CdTe, half a mono-layer (ML) of Cd Mn Te was grown at one cycle of the alternate \V V supply of the anion and cation #uxes in an auto-regulated manner (atomic layer epitaxy } ALE [5]), because the composition of Mn is not high in the present case. The uncapped surface of the Cd Mn Te layer \V V was examined by atomic force microscope (AFM)
0022-2313/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 4 1 5 - 9
Y. Terai et al. / Journal of Luminescence 87}89 (2000) 396}398
measurement in the air. The formation of self-organized dots was observed on the surface of 3.5-ML thick Cd Mn Te when the Mn composition x was less \V V than 10%. Typically, the average size of Cd Mn Te \V V dots with x"3% was given by 23 nm in diameter and 2 nm in height and the density was 1;10 cm\. The dot density was almost constant in the range of x"0}10%, but it decreased abruptly when x was over 10%. On the other hand, when the deposition thickness was less than 2 ML, the surface was almost #at. This result indicates that the critical thickness for the self-organized island growth was a little more than 2 ML. For PL measurements, we prepared capped samples of QDs in which the 3.5-ML-thick Cd Mn Te layer was \V V overgrown by ZnTe capping layer. For comparison, we also prepared single-quantum well (SQW) samples, in which 2-ML thick Cd Mn Te wetting layer grown in \V V the same condition (i.e., the same Mn-cell temperature during the supply of cation #ux) was sandwiched by ZnTe layer. Fig. 1 shows the time-integrated PL spectra from QDs and SQW samples with di!erent Mn compositions. These spectra were obtained at 5.5 K using 488 nm-line of an Ar> laser with an intensity as low as 5 mW/cm. The spectra in the "gure are arranged in the order of the Mn composition; A (QDs), A (SQW) } x"0% (CdTe), B, B } x"3% and C, C } x"3.6%. Here the Mn composition x of SQW samples B , C was determined from the PL peak energies and that of QDs samples B, C was assumed to be the same as SQW samples B , C , respectively, which were grown with the same Mn-cell temperature. The emission energies of QD samples were also shifted to a higher energy in a systematic way in the order of samples A}C. In our previous study [4], it was concluded that the luminescence from CdTe QDs (sample A) assumed the zero-dimensional excitonic character from the temperature dependence of the PL decay time. Therefore it could be deduced that the luminescence from samples B, C was also the emission of excitons con"ned in Cd Mn Te QDs. However the \V V PL spectra from these samples consisted of two distinct lines separated by about 20 meV. The dotted lines in Fig. 1 show the result of the "tting to two Gaussian functions. It should be noted that the appearance of these two separated lines was observed only in QDs containing Mn. Fig. 2 shows the PL spectra from sample C at temperatures of 2}20 K. At the lowest temperature of 2 K, the intensity of the lower-energy line (line-L) was higher than that of the higher-energy line (line-H), but with the increase of temperature the intensity of line-L decreased rapidly and disappeared at about 20 K. This suggests that line-L was a luminescence from a state which is unstable with increasing temperature. We performed magneto-PL measurement at 4.2 K with magnetic "elds applied parallel to the growth direc-
397
Fig. 1. Time-integrated PL spectra from QDs and SQW samples at 5.5 K. Samples A}C are Cd Mn Te QDs and A }C \V V are SQWs in which 2 ML wetting layer was sandwiched by ZnTe.
Fig. 2. PL spectra from sample C at various temperatures.
tion of the sample. In Fig. 3, the peak energies of the respective lines, which was obtained from the separation into two Gaussian functions, are plotted against magnetic "elds. The both lines were red-shifted due to the gigantic Zeeman splitting of the exciton states, but the red-shift was much smaller in line-L (10 meV at 10 T) than in line-H (23 meV at 10 T). Fig. 4 shows the results of the circularly polarized PL measurements under magnetic "elds at 2 K. In this "gure, the polarization degree o"(I !I )/(I #I ) of the two PL lines is plotted > \ > \ against magnetic "elds, where I and I are the lumines> \ cence intensities of the p> and p\ circular polarization, respectively. The "eld dependence of the polarization degree o of line-H was almost similar to that of luminescence from Cd Mn Te SQW samples (not shown in \V V the "gure). On the other hand, o of line-L was much larger and was saturated at a magnetic "eld as low as 0.2 T. Further it was found that this "eld dependence of o did not change when the temperature was increased up to 6 K. As described above, line-L exhibited anomalous behaviors, in contrast to those of line-H, which was similar to those of the PL line from SQW samples. Similar features in the behavior of line-L were also observed in
398
Y. Terai et al. / Journal of Luminescence 87}89 (2000) 396}398
Fig. 3. Peak energies of the two PL lines in sample C as a function of magnetic "eld. The magnetic "eld B was applied along the growth direction z of the sample. The measurements were performed at 4.2 K.
an e!ect of the magnetic polaron formation, which is enhanced in zero-dimensional dots. In addition, the quenching of line-L above 20 K could be interpreted as related to the instability of magnetic polarons at elevated temperatures. Therefore it is suggested that the anomalous behaviors of line-L re#ected the character of excitonic magnetic polarons in the zero-dimensional dots. In conclusion, we have studied the photoluminescence from Cd Mn Te QDs, which were successfully fab\V V ricated in the composition range of x"0}10% by the self-organized growth on the ZnTe (1 0 0) surface. The PL spectra from Cd Mn Te QDs consisted of two lines \V V separated by about 20 meV. In the dependence on the temperature and magnetic "eld, the lower-energy line exhibited peculiar behaviors } rapid quenching with increasing temperature and a smaller Zeeman shift and a larger degree of the polarization under magnetic "elds. These experimental results suggest that the excitonic state involved in the emission of the lower-energy line may be related to the magnetic polaron formation in QDs.
Acknowledgements
Fig. 4. The degree of the circular polarization of the two PL lines in sample C as a function of magnetic "eld. The temperature was 2 K.
The authors are indebted to Prof. Kawabe for AFM measurements. This work is partially supported by Grant-in-Aids from the Ministry of Education, Science, Sports and Culture.
References sample B. As an origin of two PL lines in Cd Mn Te \V V QDs, a simple picture such as bound exciton [6] or bi-exciton seems to be inadequate to explain the observed features. It is expected that the formation of magnetic polarons a!ects signi"cantly on the relaxation process of excitons in DMS QDs. According to the theoretical calculation [7], the binding energy of magnetic polarons is increased with the reduction of the size of DMS nanocrystals. The observed features of line-L, in particular a small Zeeman shift and a large degree of the polarization at low magnetic "elds, could be explained as
[1] K. Yanata, K. Suzuki, Y. Oka, J. Appl. Phys. 73 (1993) 4595. [2] Y. Wang, N. Herron, K. Moller, T. Bein, Solid State Commun. 77 (1991) 33. [3] S.H. Xin, P.D. Wang, A. Yin, C. Kim, M. Dobrowolska, J.L. Merz, J.K. Furdyna, Appl. Phys. Lett. 69 (1996) 3884. [4] Y. Terai, S. Kuroda, K. Takita, T. Okuno, Y. Masumoto, Appl. Phys. Lett. 73 (1998) 3757. [5] J.M. Hartmann, G. Feuillet, M. Charleux, H. Mariette, J. Appl. Phys. 79 (1996) 3035. [6] A. Golnik, J. Ginter, J.A. Gaj, J. Phys. C 16 (1983) 6073. [7] A.K. Bhattacharjee, C. Benoit a` la Guillaume, Phys. Rev. B 55 (1997) 10 613.
Magneto-photoluminescence from self-organized quantum dots of Cd\V MnV Te grown by molecular beam epitaxy Yoshikazu Terai , Shinji Kuroda *, Ko( ki Takita , Tadashi Takamasu, Giyuu Kido Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan National Research Institute for Metals, Tsukuba, Ibaraki 305-0047, Japan
Abstract Photoluminescence (PL) studies of self-organized quantum dots (QDs) of Cd\V MnV Te under magnetic "elds were reported. Cd\V MnV Te QDs were grown successfully in the composition range of x"0}10% on the ZnTe (1 0 0) surface by MBE. The PL spectra from capped Cd Mn Te QDs were composed of two lines with a separation of about 20 meV \V V at low temperatures. In the dependence on the temperature and magnetic "eld, the PL line in the lower-energy side exhibited peculiar behaviors, compared to that in the higher-energy side; with increasing temperature, the intensity of the lower-energy line was reduced rapidly and almost disappeared above 20 K, and Zeeman shift under magnetic "elds was smaller and the degree of the circular polarization was larger and saturated at low magnetic "elds. These features of the lower-energy line were discussed in relation to excitonic magnetic polaron in QDs. 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Self-organized dots; Cd Mn Te; Magneto-photoluminescence \V V
Diluted magnetic semiconductors (DMS) show marked magneto-optical e!ects due to the sp}d exchange interaction between band carriers and magnetic ions. In nanostructures of DMS (such as quantum dots } QDs), some new features are expected to arise in the magnetooptical properties caused by both of the exchange interaction and strong quantum con"nement. Experimentally, the magneto-optical studies have been reported in DMS nanocrystals ((Cd,Mn)Se in SiO [1], (Zn,Mn)S in glass [2]). However there seems to be some di$culties for quantitative investigations arising from the size distribution and the interface quality. On the other hand, self-organized QDs formed in the epitaxial growth onto a lattice-mismatched material are another object of the optical studies, which have the advantage of a relative uniform size and high emission e$ciency. Recent
* Corresponding author. Tel.: #81-298-53-5365/5020; fax: #81-298-55-7440. E-mail address: [email protected] (S. Kuroda)
successful fabrications of self-organized QDs in II}VI compounds such as CdSe on ZnSe [3] and CdTe on ZnTe [4] open the possibility of DMS QDs incorporating Mn. In the present paper, we report photoluminescence (PL) studies of self-organized QDs of Cd Mn Te, which were successfully grown on the \V V ZnTe (1 0 0) surface by MBE. The growth of Cd Mn Te QDs was performed in \V V an almost similar way to that of CdTe QDs in our previous study [4]; that is, ZnTe bu!er layer was "rst grown on a GaAs (1 0 0) substrate to a thickness of 0.6}1 lm, and Cd Mn Te layer was grown on that \V V surface by supplying a #ux of Cd#Mn and that of Te alternately at a substrate temperature of 3003C. In this condition, similarly to CdTe, half a mono-layer (ML) of Cd Mn Te was grown at one cycle of the alternate \V V supply of the anion and cation #uxes in an auto-regulated manner (atomic layer epitaxy } ALE [5]), because the composition of Mn is not high in the present case. The uncapped surface of the Cd Mn Te layer \V V was examined by atomic force microscope (AFM)
0022-2313/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 4 1 5 - 9
Y. Terai et al. / Journal of Luminescence 87}89 (2000) 396}398
measurement in the air. The formation of self-organized dots was observed on the surface of 3.5-ML thick Cd Mn Te when the Mn composition x was less \V V than 10%. Typically, the average size of Cd Mn Te \V V dots with x"3% was given by 23 nm in diameter and 2 nm in height and the density was 1;10 cm\. The dot density was almost constant in the range of x"0}10%, but it decreased abruptly when x was over 10%. On the other hand, when the deposition thickness was less than 2 ML, the surface was almost #at. This result indicates that the critical thickness for the self-organized island growth was a little more than 2 ML. For PL measurements, we prepared capped samples of QDs in which the 3.5-ML-thick Cd Mn Te layer was \V V overgrown by ZnTe capping layer. For comparison, we also prepared single-quantum well (SQW) samples, in which 2-ML thick Cd Mn Te wetting layer grown in \V V the same condition (i.e., the same Mn-cell temperature during the supply of cation #ux) was sandwiched by ZnTe layer. Fig. 1 shows the time-integrated PL spectra from QDs and SQW samples with di!erent Mn compositions. These spectra were obtained at 5.5 K using 488 nm-line of an Ar> laser with an intensity as low as 5 mW/cm. The spectra in the "gure are arranged in the order of the Mn composition; A (QDs), A (SQW) } x"0% (CdTe), B, B } x"3% and C, C } x"3.6%. Here the Mn composition x of SQW samples B , C was determined from the PL peak energies and that of QDs samples B, C was assumed to be the same as SQW samples B , C , respectively, which were grown with the same Mn-cell temperature. The emission energies of QD samples were also shifted to a higher energy in a systematic way in the order of samples A}C. In our previous study [4], it was concluded that the luminescence from CdTe QDs (sample A) assumed the zero-dimensional excitonic character from the temperature dependence of the PL decay time. Therefore it could be deduced that the luminescence from samples B, C was also the emission of excitons con"ned in Cd Mn Te QDs. However the \V V PL spectra from these samples consisted of two distinct lines separated by about 20 meV. The dotted lines in Fig. 1 show the result of the "tting to two Gaussian functions. It should be noted that the appearance of these two separated lines was observed only in QDs containing Mn. Fig. 2 shows the PL spectra from sample C at temperatures of 2}20 K. At the lowest temperature of 2 K, the intensity of the lower-energy line (line-L) was higher than that of the higher-energy line (line-H), but with the increase of temperature the intensity of line-L decreased rapidly and disappeared at about 20 K. This suggests that line-L was a luminescence from a state which is unstable with increasing temperature. We performed magneto-PL measurement at 4.2 K with magnetic "elds applied parallel to the growth direc-
397
Fig. 1. Time-integrated PL spectra from QDs and SQW samples at 5.5 K. Samples A}C are Cd Mn Te QDs and A }C \V V are SQWs in which 2 ML wetting layer was sandwiched by ZnTe.
Fig. 2. PL spectra from sample C at various temperatures.
tion of the sample. In Fig. 3, the peak energies of the respective lines, which was obtained from the separation into two Gaussian functions, are plotted against magnetic "elds. The both lines were red-shifted due to the gigantic Zeeman splitting of the exciton states, but the red-shift was much smaller in line-L (10 meV at 10 T) than in line-H (23 meV at 10 T). Fig. 4 shows the results of the circularly polarized PL measurements under magnetic "elds at 2 K. In this "gure, the polarization degree o"(I !I )/(I #I ) of the two PL lines is plotted > \ > \ against magnetic "elds, where I and I are the lumines> \ cence intensities of the p> and p\ circular polarization, respectively. The "eld dependence of the polarization degree o of line-H was almost similar to that of luminescence from Cd Mn Te SQW samples (not shown in \V V the "gure). On the other hand, o of line-L was much larger and was saturated at a magnetic "eld as low as 0.2 T. Further it was found that this "eld dependence of o did not change when the temperature was increased up to 6 K. As described above, line-L exhibited anomalous behaviors, in contrast to those of line-H, which was similar to those of the PL line from SQW samples. Similar features in the behavior of line-L were also observed in
398
Y. Terai et al. / Journal of Luminescence 87}89 (2000) 396}398
Fig. 3. Peak energies of the two PL lines in sample C as a function of magnetic "eld. The magnetic "eld B was applied along the growth direction z of the sample. The measurements were performed at 4.2 K.
an e!ect of the magnetic polaron formation, which is enhanced in zero-dimensional dots. In addition, the quenching of line-L above 20 K could be interpreted as related to the instability of magnetic polarons at elevated temperatures. Therefore it is suggested that the anomalous behaviors of line-L re#ected the character of excitonic magnetic polarons in the zero-dimensional dots. In conclusion, we have studied the photoluminescence from Cd Mn Te QDs, which were successfully fab\V V ricated in the composition range of x"0}10% by the self-organized growth on the ZnTe (1 0 0) surface. The PL spectra from Cd Mn Te QDs consisted of two lines \V V separated by about 20 meV. In the dependence on the temperature and magnetic "eld, the lower-energy line exhibited peculiar behaviors } rapid quenching with increasing temperature and a smaller Zeeman shift and a larger degree of the polarization under magnetic "elds. These experimental results suggest that the excitonic state involved in the emission of the lower-energy line may be related to the magnetic polaron formation in QDs.
Acknowledgements
Fig. 4. The degree of the circular polarization of the two PL lines in sample C as a function of magnetic "eld. The temperature was 2 K.
The authors are indebted to Prof. Kawabe for AFM measurements. This work is partially supported by Grant-in-Aids from the Ministry of Education, Science, Sports and Culture.
References sample B. As an origin of two PL lines in Cd Mn Te \V V QDs, a simple picture such as bound exciton [6] or bi-exciton seems to be inadequate to explain the observed features. It is expected that the formation of magnetic polarons a!ects signi"cantly on the relaxation process of excitons in DMS QDs. According to the theoretical calculation [7], the binding energy of magnetic polarons is increased with the reduction of the size of DMS nanocrystals. The observed features of line-L, in particular a small Zeeman shift and a large degree of the polarization at low magnetic "elds, could be explained as
[1] K. Yanata, K. Suzuki, Y. Oka, J. Appl. Phys. 73 (1993) 4595. [2] Y. Wang, N. Herron, K. Moller, T. Bein, Solid State Commun. 77 (1991) 33. [3] S.H. Xin, P.D. Wang, A. Yin, C. Kim, M. Dobrowolska, J.L. Merz, J.K. Furdyna, Appl. Phys. Lett. 69 (1996) 3884. [4] Y. Terai, S. Kuroda, K. Takita, T. Okuno, Y. Masumoto, Appl. Phys. Lett. 73 (1998) 3757. [5] J.M. Hartmann, G. Feuillet, M. Charleux, H. Mariette, J. Appl. Phys. 79 (1996) 3035. [6] A. Golnik, J. Ginter, J.A. Gaj, J. Phys. C 16 (1983) 6073. [7] A.K. Bhattacharjee, C. Benoit a` la Guillaume, Phys. Rev. B 55 (1997) 10 613.