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MBE growth and optical properties of self-organized dots of CdTe and (Cd,Mn)Te
Journal of Alloys and Compounds 371 (2004) 31–36
MBE growth and optical properties of self-organized dots of CdTe and (Cd,Mn)Te Shinji Kuroda a,∗ , Naohiro Itoh a , Yoshikazu Terai a,1 , Kˆoki Takita a , Tsuyoshi Okuno b , Mitsuhiro Nomura b , Yasuaki Masumoto b a
Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan b Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received 10 December 2002; received in revised form 10 December 2002; accepted 18 May 2003
Abstract MBE growth of self-organized dots of CdTe and (Cd,Mn)Te on the lattice-mismatched ZnTe surface and their optical properties are reported. The formation of nano-scale dots was confirmed when CdTe or Cd1−x Mnx Te with the Mn composition x ≤ 0.1 was deposited on the ZnTe (0 0 1) surface beyond the critical thickness of 1.5–2 monolayers (MLs). In the PL measurement on CdTe dots, the zero-dimensional (0D) excitonic luminescence from the dots was observed. Though the emission from the wetting layer was not observed in the PL spectra, the existence of the wetting layer state was confirmed in the PLE measurement. The PL spectra from (Cd,Mn)Te dots were split into two lines in low temperatures less than 20 K. The lower-energy line showed anomalous behaviors in the dependences on temperature and magnetic field. This is considered due to the magnetic polaron effect enhanced by the 0D confinement. © 2003 Elsevier B.V. All rights reserved. PACS: 68.65.Hb; 75.50Pp; 78.55.Et; 78.67.Hc Keywords: Semiconductors; Nanostructures; Magnetic films; Multilayers; Luminescence
1. Introduction For these decades, self-organized island growth in the lattice-mismatched expitaxy has been gathering great interests both for the basic physics related to the zerodimensional excitons and for the application in the optoelectronics. The island formation in Stranski–Krastanow growth mode has been reported in the various combinations of materials including II–VI semiconductors. The advantageous features of II–VI semiconductors, such as large excitonic effect and continuous coverage of a wide spectral range, make quantum nanostructures promising for the optical
∗ Corresponding author. Current address: D´ epartement de Recherche Fondamentale sur la Mati`ere Condens´ee, SP2M/PSC, CEA/Grenoble, 17 avenue des Martyrs, 38054 Grenoble Cedex 9, France. Tel.: +33-438-784022; fax: +33-438-785197. E-mail address: [email protected] (S. Kuroda). 1 Present address: Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan.
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.05.004
applications. Furthermore, magnetic elements can be easily incorporated in II–VI semiconductors without deteriorating the optical quality. In nanostructures of semiconductors incorporating magnetic elements, i.e., diluted magnetic semiconductors (DMS), new phenomena can be expected arising from the combination of the quantum confinement and the exchange interaction, such as the enhancement of magnetic polaron effect or the modification of magnetic properties. In the present article, our recent studies on the self-organized dots of CdTe and (Cd,Mn)Te grown on the ZnTe surface are reported [1–5]. There have been many studies in selenide dots, but the studies on telluride dots have been relatively few [6,7]. In the combination of CdTe-on-ZnTe, the lattice mismatch ratio is a/a = 6.2%, which is close to the value of InAs-on-GaAs and CdSe-on-ZnSe. In the case of Cd1−x Mnx Te dots, the lattice mismatch ratio is decreased from this value depending on the Mn composition x, as expressed by (6.2− 2.4 × x)%.
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Fig. 1. The RHEED images at the respective stages in the ALE growth of CdTe. (a) Te flux supply at the first ALE cycle; (b) Cd flux supply at the first ALE cycle; and (c) Te flux supply at the seventh ALE cycle. The electron beam was directed parallel to the crystallographic axis of [1 1 0].
2. Sample growth and optical properties 2.1. CdTe dots The method of MBE growth of CdTe dots was described in detail in [1,2]; CdTe dots were grown by atomic layer epitaxy (ALE) at the substrate temperature of 300 ◦ C on the flat surface of a thick ZnTe buffer layer grown on a GaAs (1 0 0) substrate. In this condition, half monolayer (ML) of CdTe is grown at each cycle of the alternate supply of Cd and Te fluxes [8]. In Fig. 1, the RHEED images are shown at the supply of (a) Te flux and (b) Cd flux of the first ALE cycle, respectively. It was clearly observed that the reconstructed pattern switched between the Te-covered (2×1) pattern and the Cd-covered c-(2×2) pattern. However, with the repetition of the ALE cycle, it was changed into a combination of streak and spotty patterns, as shown in Fig. 1(c) at the seventh ALE cycle. It was observed that the change started at the fourth or fifth ALE cycle. The above transformation of the RHEED image, which is considered to correspond to the change from 2D to 3D growth mode, coincides with the result of the AFM measurements of the surface morphology in the air. The surface of 1.5 ML-thick CdTe layer deposited on the ZnTe surface was almost flat
only with some small swellings (height: h < 1 nm), while the surface of 3.5 ML-thick CdTe layer was covered with dots in a high density, as shown in Fig. 2(a). The average size of dots in this figure was given by the diameter D = 20 nm and the height h = 2.7 nm. It should be noted that no ripening process was observed in CdTe dots, differently from the CdSe-on-ZnSe case. The optical properties were investigated on CdTe dots overgrown by 30 nm-thick ZnTe capping layer [1]. Fig. 3(a) shows the time-integrated PL spectra from the samples of CdTe layers with thicknesses of 1, 2 and 3.5 ML, respectively. The samples of 1 ML- and 2 ML-thick CdTe layers are single quantum wells (SQW) with thin coherent 2D layers and the sample of 3.5 ML-thick CdTe layer is quantum dots (QDs) embedded in ZnTe. The PL peak energies from 1 ML- and 2 ML-SQWs agree relatively well with the calculated energies in SQWs with the respective well widths (indicated by arrows in figure), but that of 3.5 ML-QDs deviates to the higher energy. However, the observed PL energy of 3.5 ML-QDs neither coincides with the energy level in the dot with the same size as measured by AFM, which is roughly estimated at 1.9 eV. This deviation of the PL energy is considered due to the interdiffusion of Zn inside the dots during the overgrowth of ZnTe, rather than the lateral
Fig. 2. AFM top view of the surface of 3.5 ML-thick CdTe or Cd1−x Mnx Te layer deposited on the ZnTe (0 0 1) surface. (a) CdTe; (b) Cd1−x Mnx Te with the Mn composition x = 0.03; and (c) x = 0.051, respectively. The scanned size is 0.5 m×0.5 m.
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Fig. 3. (a) Time-integrated PL spectra from the samples with 1 ML-, 2 ML- and 3.5 ML-thick CdTe layers sandwiched by ZnTe. 1 ML- and 2 ML-CdTe samples are SQWs and 3.5 ML-CdTe sample is QDs. Arrows in figure show the calculated optical transition energies for SQWs with the respective well widths. (b) The temperature dependence of the PL decay time in the respective samples. The dashed lines are guides to the eye.
confinement in the dots. It is to be mentioned that the emission from the wetting layer (WL) was not observed even in a high excitation intensity. The results of time-resolved PL measurement are summarized in Fig. 3(b), in which the PL decay time is plotted against temperature. The temperature dependence of the PL decay time of 3.5 ML-QDs shows characteristic behaviors of 0D-confined excitons; that is, the constant decay time at low temperatures reflects the δ-function-like density of state in the 0D confinement. On the other hand, the increase of the decay time with temperature in 2 ML-SQW is typical of the 2D excitons because the thermal distribution of dark excitons away from the zone center at elevated temperatures contributes to prolonged PL decay times. In order to extract the detailed information from the optical point of view, the measurements of photoluminescence excitation (PLE) and PL under the quasi-resonant excitation were performed on 3.5 ML-QDs [5]. Fig. 4(a) shows the PLE spectra when the emission was detected at three different positions within the PL line (indicated by arrows I–III in the inset) and the excitation energy was swept continuously. In the respective PLE spectra, well-defined oscillatory structures were observed. They are attributed to the multiple emission of LO phonons. The respective PLE peak energies Eexc were separated from the detection energy Edet by the multiple of the LO phonon energy ELO ; i.e., E = Eexc − Edet = n · ELO . It is to be noted that the order of the multi-phonon emission reached as high as 19th at maximum when detected at the peak position of the PL line (Spectrum I). The observation to such a higher order, in particular, extending over the range of 0.5 eV across the ZnTe band-gap, should be attributed to strong electron–phonon coupling in II–VI compounds. However it suggests as well that the excitons are captured very efficiently by CdTe dots after the energy relaxation by emitting the LO phonons in the ZnTe matrix. Other than the oscillatory structure due to the multi-phonon emission, there existed two peaks at constant energies of 2.25 and 2.38 eV in the PLE spectra. This can be clearly seen in Fig. 4(b), in which the PLE peak ener-
gies Eexc were plotted against the detection energy Edet . The peaks attributed to the multi-phonon emission, represented by black squares in figure, were distributed along a series of inclined parallel lines expressed by the equation Eexc = Edet + n · ELO . In addition, two sets of peaks appeared at constant energies of 2.38 and 2.25 eV, as represented by triangles and circles, respectively. The peaks at 2.38 eV come from the ZnTe matrix, while the ones at 2.25 eV are assigned to the WL state with a thickness of 2 ML, because it is close to the emission energy from the 2 ML-SQW sample. The WL state, the emission from which was not observed at all in PL even in a high excitation intensity, was detected definitely in the excitation spectra. A series of the equal-spaced peaks due to the multi-phonon emission was also seen in the PL spectra in a magnified scale. The separation between the adjacent peaks gives the energy of the LO phonon emitted by the excitons. The LO-phonon energy emitted during the exciton relaxation was investigated in a wide energy range by changing the excitation energy [5]. The result is summarized in Fig. 5, in which the energy separation between the adjacent phonon emission peaks was plotted against the peak position. As clearly seen in figure, the LO-phonon energy jumped by about 0.6 meV when the peak energy decreased across 2.25 eV. This energy of the abrupt change also corresponds to that of the WL state. The LO-phonon energy above the WL energy, 26.1 meV on average, almost coincides with that of bulk ZnTe (26.0 meV). On the other hand, the LO-phonon energy below the WL energy, 25.5 meV on average, is shifted by 0.6 meV from the former value and it corresponds to that in a mixed crystal Cd1−x Znx Te with the Zn composition x = 0.8. The abrupt change of the LO-phonon energy is considered to originate from a change of the exciton surroundings across the WL energy; the exciton above the WL energy, with its wavefunction extended into the ZnTe matrix, emits the LO phonon of ZnTe, while on the other hand, the exciton below the WL energy, localized in the CdTe dots, emits the LO phonon corresponding to Cd0.2 Zn0.8 Te. The latter suggests that the surroundings
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S. Kuroda et al. / Journal of Alloys and Compounds 371 (2004) 31–36
Fig. 4. (a) PLE spectra of 3.5 ML-thick CdTe QDs. The three spectra I–III correspond to the different detection positions within the PL line indicated by arrows in the inset. (b) The PLE peak positions as a function of the detection energy. The peaks due to the mutiple LO-phonon emission were represented by closed squares and the ones assigned to the ZnTe matrix and the wetting layer were represented by open triangles and circles, respectively.
of the exciton localized in the dots become a mixed crystal, which is considered to be resulting from the interdiffusion of Zn inside the dots during the overgrowth of ZnTe. The significant interdiffusion in the dots suggested above is in qualitative agreement with the observed PL energy, which was blue-shifted from the one expected from the AFM dot size. It is also consistent with the result obtained by lattice constant analysis of the cross-sectional TEM image [7]. 2.2. (Cd,Mn)Te dots Self-organized dots of (Cd,Mn)Te were grown by an almost similar procedure to that of CdTe QDs except for the addition of Mn flux [3]. The Mn composition was controlled by changing the Mn cell temperature in the range of 670 − 730 ◦ C. The AFM images of the surface
Fig. 5. The LO-phonon energies deduced from the energy separation between adjacent peaks of the multi-phonon emission are plotted against the emission peak position. A jump of the LO-phonon energies was clearly seen across the energy of the wetting layer.
of 3.5 ML-thick Cd1−x Mnx Te layer are shown in Fig. 2(b) and (c) for the Mn compositions x = 0.03 and 0.051, respectively. As shown in figure, an almost similar dot formation was confirmed even with the incorporation of a small amount of Mn. As a result of the investigation of the x dependence, it was found that the dot density showed a slightly decreasing tendency from 8 × 1010 to 4×1010 cm−2 when x was increased in the range of x ≤ 0.1, but it decreased abruptly by about one order of magnitude at x = 0.1. PL measurements were performed on 3.5 ML-thick Cd1−x Mnx Te QDs layer and 2 ML-thick SQW both overgrown by ZnTe. Fig. 6 shows the obtained PL spectra from
Fig. 6. Time-integrated PL spectra from 3.5 ML-QDs (A)–(D) and 2 ML-SQW (A’)–(D’) of Cd1−x Mnx Te with the respective Mn compositions. The double-peak PL spectra from Cd1−x Mnx Te QDs were decomposed into the two lines by Gaussian fitting and the results of the fitting were represented by dotted lines.
S. Kuroda et al. / Journal of Alloys and Compounds 371 (2004) 31–36
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Fig. 7. (a) The temperature dependence of the peak energies of line-H (triangles) and line-L (circles) in Cd1−x Mnx Te QDs with x = 0.036 (sample C in Fig. 6). (b) The magnetic-field dependence of the peak energies of line-H (triangles) and line-L (circles) in the same sample. Dashed line represents the parallel shift of the energies of line-H. The blue-shift of line-L in the temperature dependence (12 meV) is close to the difference of the Zeeman shift energy up to 10 T between the two lines (13 meV).
QDs (A)–(D) and SQW (A’)–(D’) samples at 2.2 K. In figure, the QDs and SQW samples denoted by the same alphabet were grown with the same Mn flux and accordingly assumed to contain the identical Mn compositions. As shown in figure, the PL lines are shifted to the higher energy with the increase of the Mn composition x. One of the noticeable features of the PL spectra from Cd1−x Mnx Te QDs was the splitting into two lines. This splitting was observed in all Cd1−x Mnx Te QDs samples in the range of x = 0.6–10.2%. The energy separation of the two lines was 20–26 meV and had an increasing tendency with x. The behaviors of two PL lines of Cd1−x Mnx Te QDs were investigated in detail by changing temperature, excitation intensity, and by applying magnetic fields. As a result, it was found that the lower-energy line (line-L) showed anomalous behaviors in the dependences on these parameters, while on the other hand, the behaviors of the higher energy line (line-H) were quite similar to the PL line in Cd1−x Mnx Te SQWs [4]. As most representative examples among them, the dependences on temperature and magnetic field in sample C with x = 0.036 are shown in Fig. 7(a) and (b), respectively. In Fig. 7(a), the peak energies of line-H and -L were plotted as a function of temperature. When the temperature is increased from 6 to 12 K, line-L was blue-shifted by about 12 meV, together with a rapid decrease in the intensity. In Fig. 7(b), the peak energies of the both lines were plotted as a function of magnetic field. Here, the measurement was performed at 4.2 K with magnetic fields applied perpendicular to the layer. Though the both PL lines were red-shifted with magnetic fields due to the gigantic Zeeman splitting, the amount of the Zeeman shift of line-L, 10 meV at 10 T, was much smaller than that of line-H, 23 meV at 10 T. The anomalous behaviors of line-L as described above, an unusually large blue-shift in a narrow range of the temperature and a much smaller Zeeman shift, could be explained by the enhancement of the magnetic polaron (MP) formation of the excitons confined in the (Cd,Mn)Te dots. The stabilization of the excitons due to the MP formation should be enhanced in the 0D confinement because of the large
sp–d coupling between the exciton wavefunction shrinking in QDs and the localized magnetic moments [9]. A large Stokes shift at zero fields as a result of the increased MP formation energy, and its suppression by applying magnetic fields, give rise to a much smaller Zeeman shift in the luminescence energy. In addition, the destruction of MP at elevated temperatures results in a blue-shift with the increase of temperature. It is worthy noting that the difference in the Zeeman shift energy up to 10 T between line-H and -L, 13 meV, is very close to the temperature-dependent blue-shift of line-L, 12 meV. The reason why the MP formation energy was enhanced only in the exciton states involved the line-L emission has not been clarified at present. However, it might be possible that a small valence band offset in CdTe/ZnTe and the inhomogeneous strain distribution produce the both the localized and extended hole states, and only the localized state be affected by the enhancement of the MP formation.
3. Summary The MBE growth and optical properties of self-organized dots of CdTe and (Cd,Mn)Te grown on the lattice-mismatched ZnTe surface were reported. The 3D island growth was confirmed through the transformation of the RHEED image and the AFM examination. In the PL measurement on the CdTe dots overgrown by ZnTe, the intense emission from the 0D excitons was observed. Though the emission from the wetting layer was not observed in PL, it was detected as a peak in the PLE spectra. The shift of the LO-phonon energy in the multi-phonon emission peaks clarified the intermixing surrounding the dots. In Cd1−x Mnx Te dots, the high-density dot formation was confirmed in the Mn composition x ≤ 0.1. The PL spectra from Cd1−x Mnx Te QDs exhibited the splitting into two lines. The anomalous behaviors of the lower-energy line suggested the enhancement of the magnetic polaron formation due to the 0D confinement.
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Acknowledgements One of the authors (S. Kuroda) would like to thank Prof. H. Mariette for valuable discussions. This work is partially supported by Grant-in-Aids from the Ministry of Education, Science, Sports and Culture of Japanese Government (Monbukagakushˆo). References [1] Y. Terai, S. Kuroda, K. Takita, T. Okuno, Y. Masumoto, Appl. Phys. Lett. 73 (1998) 3757.
[2] S. Kuroda, Y. Terai, K. Takita, T. Okuno, Y. Masumoto, Jpn. J. Appl. Phys. 38 (1999) 2524. [3] Y. Terai, S. Kuroda, K. Takita, Appl. Phys. Lett. 76 (2000) 2400. [4] S. Kuroda, Y. Terai, K. Takita, T. Takamasu, G. Kido, N. Hasegawa, T. Kuroda, F. Minami, J. Cryst. Growth 214/215 (2000) 140. [5] T. Okuno, M. Nomura, Y. Masumoto, Y. Terai, S. Kuroda, K. Takita, J. Phys. Soc. Jpn. 71 (2002) 3052. [6] G. Karczewski, S. Ma´ckowski, M. Kutrowski, T. Wojtowicz, J. Kossut, Appl. Phys. Lett. 74 (1999) 3011. [7] L. Marsal, L. Besombes, F. Tinjod, K. Kheng, A. Wasiela, B. Gilles, J.-L. Rouvière, H. Mariette, J. Appl. Phys. 91 (2002) 4936. [8] J.M. Hartmann, G. Feuillet, M. Charleux, H. Mariette, J. Appl. Phys. 79 (1996) 3035. [9] A.K. Bhattacharjee, C. Benoit à la Guillaume, Phys. Rev. B 55 (1997) 10613.
MBE growth and optical properties of self-organized dots of CdTe and (Cd,Mn)Te Shinji Kuroda a,∗ , Naohiro Itoh a , Yoshikazu Terai a,1 , Kˆoki Takita a , Tsuyoshi Okuno b , Mitsuhiro Nomura b , Yasuaki Masumoto b a
Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan b Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received 10 December 2002; received in revised form 10 December 2002; accepted 18 May 2003
Abstract MBE growth of self-organized dots of CdTe and (Cd,Mn)Te on the lattice-mismatched ZnTe surface and their optical properties are reported. The formation of nano-scale dots was confirmed when CdTe or Cd1−x Mnx Te with the Mn composition x ≤ 0.1 was deposited on the ZnTe (0 0 1) surface beyond the critical thickness of 1.5–2 monolayers (MLs). In the PL measurement on CdTe dots, the zero-dimensional (0D) excitonic luminescence from the dots was observed. Though the emission from the wetting layer was not observed in the PL spectra, the existence of the wetting layer state was confirmed in the PLE measurement. The PL spectra from (Cd,Mn)Te dots were split into two lines in low temperatures less than 20 K. The lower-energy line showed anomalous behaviors in the dependences on temperature and magnetic field. This is considered due to the magnetic polaron effect enhanced by the 0D confinement. © 2003 Elsevier B.V. All rights reserved. PACS: 68.65.Hb; 75.50Pp; 78.55.Et; 78.67.Hc Keywords: Semiconductors; Nanostructures; Magnetic films; Multilayers; Luminescence
1. Introduction For these decades, self-organized island growth in the lattice-mismatched expitaxy has been gathering great interests both for the basic physics related to the zerodimensional excitons and for the application in the optoelectronics. The island formation in Stranski–Krastanow growth mode has been reported in the various combinations of materials including II–VI semiconductors. The advantageous features of II–VI semiconductors, such as large excitonic effect and continuous coverage of a wide spectral range, make quantum nanostructures promising for the optical
∗ Corresponding author. Current address: D´ epartement de Recherche Fondamentale sur la Mati`ere Condens´ee, SP2M/PSC, CEA/Grenoble, 17 avenue des Martyrs, 38054 Grenoble Cedex 9, France. Tel.: +33-438-784022; fax: +33-438-785197. E-mail address: [email protected] (S. Kuroda). 1 Present address: Department of Materials Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan.
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.05.004
applications. Furthermore, magnetic elements can be easily incorporated in II–VI semiconductors without deteriorating the optical quality. In nanostructures of semiconductors incorporating magnetic elements, i.e., diluted magnetic semiconductors (DMS), new phenomena can be expected arising from the combination of the quantum confinement and the exchange interaction, such as the enhancement of magnetic polaron effect or the modification of magnetic properties. In the present article, our recent studies on the self-organized dots of CdTe and (Cd,Mn)Te grown on the ZnTe surface are reported [1–5]. There have been many studies in selenide dots, but the studies on telluride dots have been relatively few [6,7]. In the combination of CdTe-on-ZnTe, the lattice mismatch ratio is a/a = 6.2%, which is close to the value of InAs-on-GaAs and CdSe-on-ZnSe. In the case of Cd1−x Mnx Te dots, the lattice mismatch ratio is decreased from this value depending on the Mn composition x, as expressed by (6.2− 2.4 × x)%.
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Fig. 1. The RHEED images at the respective stages in the ALE growth of CdTe. (a) Te flux supply at the first ALE cycle; (b) Cd flux supply at the first ALE cycle; and (c) Te flux supply at the seventh ALE cycle. The electron beam was directed parallel to the crystallographic axis of [1 1 0].
2. Sample growth and optical properties 2.1. CdTe dots The method of MBE growth of CdTe dots was described in detail in [1,2]; CdTe dots were grown by atomic layer epitaxy (ALE) at the substrate temperature of 300 ◦ C on the flat surface of a thick ZnTe buffer layer grown on a GaAs (1 0 0) substrate. In this condition, half monolayer (ML) of CdTe is grown at each cycle of the alternate supply of Cd and Te fluxes [8]. In Fig. 1, the RHEED images are shown at the supply of (a) Te flux and (b) Cd flux of the first ALE cycle, respectively. It was clearly observed that the reconstructed pattern switched between the Te-covered (2×1) pattern and the Cd-covered c-(2×2) pattern. However, with the repetition of the ALE cycle, it was changed into a combination of streak and spotty patterns, as shown in Fig. 1(c) at the seventh ALE cycle. It was observed that the change started at the fourth or fifth ALE cycle. The above transformation of the RHEED image, which is considered to correspond to the change from 2D to 3D growth mode, coincides with the result of the AFM measurements of the surface morphology in the air. The surface of 1.5 ML-thick CdTe layer deposited on the ZnTe surface was almost flat
only with some small swellings (height: h < 1 nm), while the surface of 3.5 ML-thick CdTe layer was covered with dots in a high density, as shown in Fig. 2(a). The average size of dots in this figure was given by the diameter D = 20 nm and the height h = 2.7 nm. It should be noted that no ripening process was observed in CdTe dots, differently from the CdSe-on-ZnSe case. The optical properties were investigated on CdTe dots overgrown by 30 nm-thick ZnTe capping layer [1]. Fig. 3(a) shows the time-integrated PL spectra from the samples of CdTe layers with thicknesses of 1, 2 and 3.5 ML, respectively. The samples of 1 ML- and 2 ML-thick CdTe layers are single quantum wells (SQW) with thin coherent 2D layers and the sample of 3.5 ML-thick CdTe layer is quantum dots (QDs) embedded in ZnTe. The PL peak energies from 1 ML- and 2 ML-SQWs agree relatively well with the calculated energies in SQWs with the respective well widths (indicated by arrows in figure), but that of 3.5 ML-QDs deviates to the higher energy. However, the observed PL energy of 3.5 ML-QDs neither coincides with the energy level in the dot with the same size as measured by AFM, which is roughly estimated at 1.9 eV. This deviation of the PL energy is considered due to the interdiffusion of Zn inside the dots during the overgrowth of ZnTe, rather than the lateral
Fig. 2. AFM top view of the surface of 3.5 ML-thick CdTe or Cd1−x Mnx Te layer deposited on the ZnTe (0 0 1) surface. (a) CdTe; (b) Cd1−x Mnx Te with the Mn composition x = 0.03; and (c) x = 0.051, respectively. The scanned size is 0.5 m×0.5 m.
S. Kuroda et al. / Journal of Alloys and Compounds 371 (2004) 31–36
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Fig. 3. (a) Time-integrated PL spectra from the samples with 1 ML-, 2 ML- and 3.5 ML-thick CdTe layers sandwiched by ZnTe. 1 ML- and 2 ML-CdTe samples are SQWs and 3.5 ML-CdTe sample is QDs. Arrows in figure show the calculated optical transition energies for SQWs with the respective well widths. (b) The temperature dependence of the PL decay time in the respective samples. The dashed lines are guides to the eye.
confinement in the dots. It is to be mentioned that the emission from the wetting layer (WL) was not observed even in a high excitation intensity. The results of time-resolved PL measurement are summarized in Fig. 3(b), in which the PL decay time is plotted against temperature. The temperature dependence of the PL decay time of 3.5 ML-QDs shows characteristic behaviors of 0D-confined excitons; that is, the constant decay time at low temperatures reflects the δ-function-like density of state in the 0D confinement. On the other hand, the increase of the decay time with temperature in 2 ML-SQW is typical of the 2D excitons because the thermal distribution of dark excitons away from the zone center at elevated temperatures contributes to prolonged PL decay times. In order to extract the detailed information from the optical point of view, the measurements of photoluminescence excitation (PLE) and PL under the quasi-resonant excitation were performed on 3.5 ML-QDs [5]. Fig. 4(a) shows the PLE spectra when the emission was detected at three different positions within the PL line (indicated by arrows I–III in the inset) and the excitation energy was swept continuously. In the respective PLE spectra, well-defined oscillatory structures were observed. They are attributed to the multiple emission of LO phonons. The respective PLE peak energies Eexc were separated from the detection energy Edet by the multiple of the LO phonon energy ELO ; i.e., E = Eexc − Edet = n · ELO . It is to be noted that the order of the multi-phonon emission reached as high as 19th at maximum when detected at the peak position of the PL line (Spectrum I). The observation to such a higher order, in particular, extending over the range of 0.5 eV across the ZnTe band-gap, should be attributed to strong electron–phonon coupling in II–VI compounds. However it suggests as well that the excitons are captured very efficiently by CdTe dots after the energy relaxation by emitting the LO phonons in the ZnTe matrix. Other than the oscillatory structure due to the multi-phonon emission, there existed two peaks at constant energies of 2.25 and 2.38 eV in the PLE spectra. This can be clearly seen in Fig. 4(b), in which the PLE peak ener-
gies Eexc were plotted against the detection energy Edet . The peaks attributed to the multi-phonon emission, represented by black squares in figure, were distributed along a series of inclined parallel lines expressed by the equation Eexc = Edet + n · ELO . In addition, two sets of peaks appeared at constant energies of 2.38 and 2.25 eV, as represented by triangles and circles, respectively. The peaks at 2.38 eV come from the ZnTe matrix, while the ones at 2.25 eV are assigned to the WL state with a thickness of 2 ML, because it is close to the emission energy from the 2 ML-SQW sample. The WL state, the emission from which was not observed at all in PL even in a high excitation intensity, was detected definitely in the excitation spectra. A series of the equal-spaced peaks due to the multi-phonon emission was also seen in the PL spectra in a magnified scale. The separation between the adjacent peaks gives the energy of the LO phonon emitted by the excitons. The LO-phonon energy emitted during the exciton relaxation was investigated in a wide energy range by changing the excitation energy [5]. The result is summarized in Fig. 5, in which the energy separation between the adjacent phonon emission peaks was plotted against the peak position. As clearly seen in figure, the LO-phonon energy jumped by about 0.6 meV when the peak energy decreased across 2.25 eV. This energy of the abrupt change also corresponds to that of the WL state. The LO-phonon energy above the WL energy, 26.1 meV on average, almost coincides with that of bulk ZnTe (26.0 meV). On the other hand, the LO-phonon energy below the WL energy, 25.5 meV on average, is shifted by 0.6 meV from the former value and it corresponds to that in a mixed crystal Cd1−x Znx Te with the Zn composition x = 0.8. The abrupt change of the LO-phonon energy is considered to originate from a change of the exciton surroundings across the WL energy; the exciton above the WL energy, with its wavefunction extended into the ZnTe matrix, emits the LO phonon of ZnTe, while on the other hand, the exciton below the WL energy, localized in the CdTe dots, emits the LO phonon corresponding to Cd0.2 Zn0.8 Te. The latter suggests that the surroundings
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Fig. 4. (a) PLE spectra of 3.5 ML-thick CdTe QDs. The three spectra I–III correspond to the different detection positions within the PL line indicated by arrows in the inset. (b) The PLE peak positions as a function of the detection energy. The peaks due to the mutiple LO-phonon emission were represented by closed squares and the ones assigned to the ZnTe matrix and the wetting layer were represented by open triangles and circles, respectively.
of the exciton localized in the dots become a mixed crystal, which is considered to be resulting from the interdiffusion of Zn inside the dots during the overgrowth of ZnTe. The significant interdiffusion in the dots suggested above is in qualitative agreement with the observed PL energy, which was blue-shifted from the one expected from the AFM dot size. It is also consistent with the result obtained by lattice constant analysis of the cross-sectional TEM image [7]. 2.2. (Cd,Mn)Te dots Self-organized dots of (Cd,Mn)Te were grown by an almost similar procedure to that of CdTe QDs except for the addition of Mn flux [3]. The Mn composition was controlled by changing the Mn cell temperature in the range of 670 − 730 ◦ C. The AFM images of the surface
Fig. 5. The LO-phonon energies deduced from the energy separation between adjacent peaks of the multi-phonon emission are plotted against the emission peak position. A jump of the LO-phonon energies was clearly seen across the energy of the wetting layer.
of 3.5 ML-thick Cd1−x Mnx Te layer are shown in Fig. 2(b) and (c) for the Mn compositions x = 0.03 and 0.051, respectively. As shown in figure, an almost similar dot formation was confirmed even with the incorporation of a small amount of Mn. As a result of the investigation of the x dependence, it was found that the dot density showed a slightly decreasing tendency from 8 × 1010 to 4×1010 cm−2 when x was increased in the range of x ≤ 0.1, but it decreased abruptly by about one order of magnitude at x = 0.1. PL measurements were performed on 3.5 ML-thick Cd1−x Mnx Te QDs layer and 2 ML-thick SQW both overgrown by ZnTe. Fig. 6 shows the obtained PL spectra from
Fig. 6. Time-integrated PL spectra from 3.5 ML-QDs (A)–(D) and 2 ML-SQW (A’)–(D’) of Cd1−x Mnx Te with the respective Mn compositions. The double-peak PL spectra from Cd1−x Mnx Te QDs were decomposed into the two lines by Gaussian fitting and the results of the fitting were represented by dotted lines.
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Fig. 7. (a) The temperature dependence of the peak energies of line-H (triangles) and line-L (circles) in Cd1−x Mnx Te QDs with x = 0.036 (sample C in Fig. 6). (b) The magnetic-field dependence of the peak energies of line-H (triangles) and line-L (circles) in the same sample. Dashed line represents the parallel shift of the energies of line-H. The blue-shift of line-L in the temperature dependence (12 meV) is close to the difference of the Zeeman shift energy up to 10 T between the two lines (13 meV).
QDs (A)–(D) and SQW (A’)–(D’) samples at 2.2 K. In figure, the QDs and SQW samples denoted by the same alphabet were grown with the same Mn flux and accordingly assumed to contain the identical Mn compositions. As shown in figure, the PL lines are shifted to the higher energy with the increase of the Mn composition x. One of the noticeable features of the PL spectra from Cd1−x Mnx Te QDs was the splitting into two lines. This splitting was observed in all Cd1−x Mnx Te QDs samples in the range of x = 0.6–10.2%. The energy separation of the two lines was 20–26 meV and had an increasing tendency with x. The behaviors of two PL lines of Cd1−x Mnx Te QDs were investigated in detail by changing temperature, excitation intensity, and by applying magnetic fields. As a result, it was found that the lower-energy line (line-L) showed anomalous behaviors in the dependences on these parameters, while on the other hand, the behaviors of the higher energy line (line-H) were quite similar to the PL line in Cd1−x Mnx Te SQWs [4]. As most representative examples among them, the dependences on temperature and magnetic field in sample C with x = 0.036 are shown in Fig. 7(a) and (b), respectively. In Fig. 7(a), the peak energies of line-H and -L were plotted as a function of temperature. When the temperature is increased from 6 to 12 K, line-L was blue-shifted by about 12 meV, together with a rapid decrease in the intensity. In Fig. 7(b), the peak energies of the both lines were plotted as a function of magnetic field. Here, the measurement was performed at 4.2 K with magnetic fields applied perpendicular to the layer. Though the both PL lines were red-shifted with magnetic fields due to the gigantic Zeeman splitting, the amount of the Zeeman shift of line-L, 10 meV at 10 T, was much smaller than that of line-H, 23 meV at 10 T. The anomalous behaviors of line-L as described above, an unusually large blue-shift in a narrow range of the temperature and a much smaller Zeeman shift, could be explained by the enhancement of the magnetic polaron (MP) formation of the excitons confined in the (Cd,Mn)Te dots. The stabilization of the excitons due to the MP formation should be enhanced in the 0D confinement because of the large
sp–d coupling between the exciton wavefunction shrinking in QDs and the localized magnetic moments [9]. A large Stokes shift at zero fields as a result of the increased MP formation energy, and its suppression by applying magnetic fields, give rise to a much smaller Zeeman shift in the luminescence energy. In addition, the destruction of MP at elevated temperatures results in a blue-shift with the increase of temperature. It is worthy noting that the difference in the Zeeman shift energy up to 10 T between line-H and -L, 13 meV, is very close to the temperature-dependent blue-shift of line-L, 12 meV. The reason why the MP formation energy was enhanced only in the exciton states involved the line-L emission has not been clarified at present. However, it might be possible that a small valence band offset in CdTe/ZnTe and the inhomogeneous strain distribution produce the both the localized and extended hole states, and only the localized state be affected by the enhancement of the MP formation.
3. Summary The MBE growth and optical properties of self-organized dots of CdTe and (Cd,Mn)Te grown on the lattice-mismatched ZnTe surface were reported. The 3D island growth was confirmed through the transformation of the RHEED image and the AFM examination. In the PL measurement on the CdTe dots overgrown by ZnTe, the intense emission from the 0D excitons was observed. Though the emission from the wetting layer was not observed in PL, it was detected as a peak in the PLE spectra. The shift of the LO-phonon energy in the multi-phonon emission peaks clarified the intermixing surrounding the dots. In Cd1−x Mnx Te dots, the high-density dot formation was confirmed in the Mn composition x ≤ 0.1. The PL spectra from Cd1−x Mnx Te QDs exhibited the splitting into two lines. The anomalous behaviors of the lower-energy line suggested the enhancement of the magnetic polaron formation due to the 0D confinement.
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Acknowledgements One of the authors (S. Kuroda) would like to thank Prof. H. Mariette for valuable discussions. This work is partially supported by Grant-in-Aids from the Ministry of Education, Science, Sports and Culture of Japanese Government (Monbukagakushˆo). References [1] Y. Terai, S. Kuroda, K. Takita, T. Okuno, Y. Masumoto, Appl. Phys. Lett. 73 (1998) 3757.
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