- Email: [email protected]
ZnSe quantum dot grown by an alternate molecular beam supply
Journal of Crystal Growth 237–239 (2002) 1320–1325
Alloying of CdSe/ZnSe quantum dot grown by an alternate molecular beam supply Masakazu Ohishi*, Katsuya Tanaka, Tomohiro Fujimoto, Minoru Yoneta, Hiroshi Saito Department of Applied Physics, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan
Abstract Self-assembled CdSe quantum dots have been fabricated by means of an alternate molecular beam supply. For the CdSe dot embedded in ZnSe and annealed at the growth temperature (2501C), the PL peak showed a blueshift with increase of the annealing period. This denotes that the size of embedded CdSe dots becomes small by the alloy formation with the surrounding ZnSe. The bare CdSe dots exposed to vacuum (without any molecular beam irradiation) evaporate, and the CdSe dots gradually changed to ZnCdSe dots. It was confirmed that the ZnCdSe dots were stable compared with pure CdSe dots. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.66.Hf; 78.55.Et; 68.65.+g; 81.10.Bk Keywords: A1. Nanostructures; A3. Molecular beam epitaxy; B1. Alloys; B1. Cadmium compounds; B1. Zinc compounds; B2. Semiconducting II–VI materials
1. Introduction CdSe-based quantum structures, such as the quantum well (QW) and quantum dots (QDs), are attracting much attention for an opto-electronic device application. Usually, the formation of selfassembled dots during growth is confirmed either by the appearance of diffraction spots or the damping of a RHEED intensity oscillation [1–5]. Usually, CdSe dots are fabricated by means of a molecular beam epitaxy (MBE) and an atomic layer epitaxy (ALE), where the low growth rate is used. We have shown that CdSe dots are created immediately after 3 ML-CdSe formed on ZnSe and
the dot growth by ripening follows under the Cd beam irradiation by inspecting the specular spot intensity (Isp ). We have also demonstrated that the CdSe dots evaporate when all the beams are interrupted prior to the ZnSe cap layer deposition [6,7]. One of the problems in CdSe growth is to know whether CdSe dots remain pure CdSe or make alloy with ZnSe. For this purpose, we examined the alloying of CdSe dots embedded in ZnSe and the alloying of bare CdSe dots exposed to vacuum by means of photoluminescence (PL) measurement.
2. Experimental procedure *Corresponding author. Tel.: +81-86-256-9410; fax: +8186-255-7700. E-mail address: [email protected] (M. Ohishi).
Self-assembled CdSe dots were fabricated by supplying Cd- and Se-molecular beams alternately
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 5 9 - 0
M. Ohishi et al. / Journal of Crystal Growth 237–239 (2002) 1320–1325
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(ALS growth) on ZnSe buffer layer on GaAs (1 0 0). To grow ZnCdSe structure, Zn and Cd beams were supplied simultaneously, where Cd beam flux is fixed and Zn beam flux is varied to change the Zn composition. The molecular beam pressures near the substrate position were PCd ¼ 4:0 107 and PSe ¼ 1:5 106 : The PZn was varied between 0 and 3.5 107 Torr. The growth of CdSe and ZnCdSe were initiated on Znterminated ZnSe buffer layer grown on GaAs (1 0 0). The ZnSe buffer layer (B500 nm) was grown by MBE mode at 3501C. After several ALE cycles of ZnCdSe was deposited at 2501C, the ZnSe cap layer (B100 nm) was deposited by MBE mode. Throughout the CdSe growth, the intensity of specular spot (Isp ) in the RHHED pattern was monitored. PL spectra are measured by the excitation of 325 nm line from He–Cd laser at liquid helium temperature.
3. Results and discussion 3.1. Band-edge emission of CdSe dots embedded in ZnSe In order to examine the thermal stability of CdSe dots, the capped CdSe dots were annealed at 2501C (growth temperature: Tg ). The CdSe dots were fabricated by 5 ALS cycles, and the cap layer is about 500 nm thick. Fig. 1 shows the PL spectra of capped CdSe dots annealed for several periods. PL peak shifted monotonically from 524.7 to 516.5 nm as the increase of annealing period from 0 to 30 min. This denotes that the capped CdSe dots is not stable at Tg : The PL blueshift is explained by the size reduction of CdSe dots, which is due to the alloying of CdSe dots with surrounding ZnSe. It is natural to consider that the alloying of CdSe dot proceeds from the dot surface and the surrounding ZnSe also becomes alloy with CdSe. If the entire CdSe dots changed to ZnCdSe dots, the boundary between ZnCdSe dot and surrounding ZnSe becomes vague, the size of ZnCdSe dot becomes large and the exciton confinement is reduced. The blueshift described above implies that the core part of initial CdSe dots remains pure CdSe dots after 30 min. The
Fig. 1. PL spectra of capped CdSe QDs annealed at 2501C. Five cycles of Cd and Se beam supplied on ZnSe. Annealing was performed immediately after the cap layer deposition.
observed emission from the annealed CdSe dots originate from CdSe dots surrounded by ZnCdSe alloy. We do not know that the further long annealing really cause the redshift or not. 3.2. PL vs. beam interrupted period In order to study the thermal stability of uncapped CdSe dots, all the beam were interrupted prior to cap layer deposition at Tg ¼ 2501C, that is, as-grown bare dots were exposed to vacuum for prescribed time without any beam irradiation. The ALS cycles are 1, 2, 3 and 5 cycles, and the beam interruption is 0, 5, 10 and 30 min. In succession, the ZnSe cap layer was deposited. To avoid the influence of annealing, after the cap layer deposition the sample was cooled down to room temperature as soon as
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Fig. 3. PL peak wavelength vs. beam interrupted period.
Fig. 2. PL spectra of CdSe QDs on ZnSe. Cycle of alternately supplied beam is 1, 2, 3 and 5 cycles, respectively. Prior to the cap layer deposition, all the beams are interrupted for 0, 5, 10 and 30 min, respectively.
possible. Fig. 2 shows the PL spectra of the capped CdSe/ZnSe structures, and Fig. 3 represents the wavelength of PL peak as a function of the beam interrupted time. Even in the structures embedded 1 ALS cycle of CdSe (1 ML CdSe) and 2 ALS cycles of CdSe (2 ML CdSe), the intense emission is observed after the 30 min beam interruption. The PL spectra of 1 ML CdSe shows distinct double peaks, which do not coincide with any emission lines of ZnSe layer nor the reported emission peak from CdSe/ ZnSe QW [3,8]. In these samples, the nominal CdSe thickness is thinner than 2 ML wetting layer (WL) and any traces suggesting dot formation was not appreciated in the temporal Isp measurement [6,7], This results imply that there exists another mechanism of dot growth in addition to the Stranski–Krastanow (S–K) mode. We consider, at present stage, that the CdSe growth by an alternate beam supply is slightly away from an
ideal ALE mode. The Isp curve during ALS growth shows a square waveform, and high and low intensity levels correspond to Zn(Cd)- and Sesurface, respectively. Whereas, the Isp from the surface exposed to cation beam keeps constant intensity level, the Isp from the surface under Se beam irradiation does not show the constant intensity but gradually loose its intensity. This means that the Se molecules are adsorbed partially on the Se-terminated surface. The excessively adsorbed Se molecules and impinged Cd beam give rise to the CdSe microdots. Recently, it is reported that the formation of micro island starts already when the first monolayer is deposited [10]. This is consistent with our conclusion. It is noteworthy that the PL peak of each sample converges to different wavelength with the increase of the beam interrupted time. Remarkable PL blueshift occurs within 10 min after the beam interruption, and the PL blueshift becomes slow after 10 min. If the CdSe evaporates from the dot surface, the dot size decreases gradually as the interruption time proceeds. Then, it is expected that the PL peak related to dots shifts to higher energy and disappears finally. However, the intense PL peak is observed after long beam interruption and which shows the dot existence. To interpret the dot stability after long beam interruption, we considered the alloying of CdSe
M. Ohishi et al. / Journal of Crystal Growth 237–239 (2002) 1320–1325
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dots with ZnSe buffer layer. By the inter-diffusion between CdSe WL and underlying ZnSe buffer layer, the WL varies to the ZnCdSe. At the same time, the inter-diffusion between CdSe dots and WL takes place. After long beam interruption, the CdSe dot gradually changes to Zn-rich ZnCdSe dot. The smaller the dot size is, the higher the alloying rate is. As described later, we confirmed that the ZnCdSe alloy dot is stable and the dot evaporation hardly occurs. Then, we concluded that the PL emission originates from ZnCdSe QDs after long beam interruption. 3.3. Zn beam dependence on the Isp variation during ZnCdSe growth We have reported that the temporal Isp during CdSe growth by alternate beam supply gives the information on the dot generation and the ripening [6,7]. In the ZnCdSe growth on ZnSe buffer layer, the lattice misfit between ZnCdSe layer and ZnSe buffer layer decreases with the increase of Zn composition. Since the self-assembling of dots is induced by the surface strain, it is expected that the increase of Zn content in Zn+Cd beam thickens the WL thickness and suppress the S–K dots generation. Fig. 4 shows the temporal Isp recorded during the growth of Cd(Zn)Se. Firstly, ZnSe buffer layer were grown on GaAs(1 0 0) by MBE mode. Then, ZnCdSe were grown on Zn-terminated ZnSe buffer by supplying Se and Cd+Zn beams alternately (Se beam is supplied first). Each beam irradiation period is 10 s, and no beam interruption time was inserted between beam irradiation. Finally, the ZnSe cap layer was grown by MBE mode. The upper trace in Fig. 4 is the Isp observed during CdSe growth (without Zn flux). As reported before, the steep Isp drop corresponding to dot generation is observed during the third Cd beam irradiation [6,7]. As the Zn beam is added in the Cd beam (PCd ¼ 4:7 108 Torr, PZn ¼ 4:7 108 Torr), the slight Isp drop is observed at fourth Cd beam irradiation. This denotes that the slight reduction of the lattice misfit delays the S–K dot generation. When the PZn is more than 1.3 107 Torr, and no Isp drop during Cd beam irradiation is observed.
Fig. 4. Specular spot intensity during the growth of ZnCdSe on ZnSe. Cd and Zn beams are supplied simultaneously. The beam shutter program is shown in the figure. PCd ¼ 4:2 107 Torr, PSe ¼ 1:0 106 Torr and PZn is varied from 0 to 3.1 107 Torr.
In this case, the RHEED intensity oscillation indicating the layer-by-layer growth (2D growth) was clearly observed at the onset of cap layer deposition. This means that the surface of ZnCdSe is smooth enough and no large S–K dots are formed on the surface. (Here we comment that the microdot creation does not affect on the Isp :) 3.4. PL spectra of ZnCdSe/ZnSe Fig. 5 shows the PL spectra of ZnCdSe capped with ZnSe (same samples shown in Fig. 3). As the small amount of Zn flux (PZn ¼ 1:7 108 Torr) was added in the Cd beam, PL peak showed large blueshift (from 523.3 to 491.0 nm). And the full-width of half-maximum (FWHM) of PL peak is decreased from 39.8 to 31.6 meV. For PZn > 4:7 108 Torr, the blueshift became small and the FWHM of PL peak became narrower
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described previously, the lattice misfit is dissolved and no ripening takes place. Thus, we concluded that the dot stability depends on whether the dots and WL are pure CdSe or ZnCdSe.
4. Conclusion
Fig. 5. PL spectra of ZnCdSe grown on ZnSe. The sample surface is capped with ZnSe.
(below 25 meV). In the very initial stage of the growth, the CdSe growth proceeds by both coherent growth mode and micro island growth mode. When the lattice misfit is large, both the layer-by-layer growth and the micro island growth occur in the initial stage. When the deposited thickness exceeds the WL, the S–K dots appear. For the smaller lattice misfit, however, only the layer-by-layer growth and the micro island growth occur. Furthermore, we notice that PL emission of 1 ML CdSe is composed of two peaks after 30 min growth interruption, which is attributed to the bimodal dot size distribution [9,11,12]. We also studied the effect of beam interruption prior to cap layer deposition on the PL of ZnCdSe (Tg =2501C). However, no appreciable blueshift of PL peak was observed. This result means that both the sublimation and the ripening hardly occur. Thus, we concluded that ZnCdSe dots become stable as the increase of Zn composition in ZnCdSe dots. Recently, it is reported that CdSe dots do not ripen on laboratory time scale [13]. This result has brought up the question whether the CdSe dots ripen or not. Since both the CdSe dots and the WL becomes ZnCdSe alloy as
Annealing of CdSe QDs embedded in ZnSe at growth temperature of 2501C caused the blueshift of PL peak, which is due to the alloying of CdSe dots. From the PL of CdSe dots fabricated with growth interruption prior to ZnSe cap layer deposition, the alloying of CdSe dots is also confirmed. Once the ZnCdSe alloy is formed on the CdSe dot surface, the alloying rate of CdSe decreases, that is, the CdSe dots covered with ZnCdSe become stable. Furthermore, it becomes clear that micro CdSe islands (dots) exist on the surface deposited 1 and 2 ML CdSe. Therefore, both S–K growth and micro island growth modes give two kinds of CdSe dots. During the dot growth, both the WL and the CdSe dots make alloy with ZnSe. We also confirmed that the PL peak position of ZnCdSe dots does not change by beam interruption prior to cap layer deposition, suggesting the ZnCdSe dots are stable.
Acknowledgements Part of this work has been supported by a special grant for cooperative research administered by Japan private school promotion foundation. We are also grateful for the Grant-In Aid for Scientific Research from the Ministry of Education and Culture.
References [1] S.H. Xin, P.D. Wang, A. Yin, C. Ki, M. Dobrowolska, J.L. Merz, J.K.M. Furdyna, Appl. Phys. Lett. 68 (1996) 3834. [2] H.-C. Ko, Y. Kawakami, Sz. Fujita, Sg. Fujita, J. Crystal Growth 184/185 (1998) 283. [3] M. Rabe, M. Lowisch, F. Hennenberger, J. Crystal Growth 184/185 (1998) 248. [4] J.L. Merz, S. Lee, J.K.M. Furdyna, J. Crystal Growth 184/ 185 (1998) 228.
M. Ohishi et al. / Journal of Crystal Growth 237–239 (2002) 1320–1325 [5] Shizo Fujita, Yi-hong Wu, Yoichi Kawakami, Shigeo Fujita, J. Appl. Phys. 72 (1992) 5233. [6] Kenta Arai, Takahashi Hanada, Takfumi Yao, J. Crystal Growth 214/215 (2000) 703. [7] M. Ohishi, M. Yoneta, H. Saito, T. Ichikawa, T. Fujimoto, J. Crystal Growth 214/215 (2000) 690. [8] M. Ohishi, H. Saito, M. Yoneta, T. Ichikawa, T. Fujimoto, Jpn. J. Appl. Phys. 39 (2000) 4584. [9] H. Zajicek, P. Juza, E. Abramof, O. Pankratov, H. Zitter, H. Helm, G. Brunthler, W. Faschinger, Appl. Phys. Lett. 62 (1993) 717. [10] D. Schikora, S. Schwedhelm, I. Kudryashov, K. Lischka, D. Litvinov, A. Rosenauer, D. Gerthsen, M. Strassburg,
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A. Hoffmann, D. Bimberg, J. Crystal Growth 214/215 (2000) 698. [11] M. Strassburg, Th. Deniozou, A. Hoffmann, S. Rodt, V. Turck, R. Heitz, U.W. Pohl, D. Bimberg, D. Litvinov, A. Rosenauer, D. Gerthsen, S. Schwedhelm, I. Kudryashov, K. Lischka, D. Schikora, J. Crystal Growth 214/215 (2000) 756. [12] C.S. Kim, M. Kim, S. Lee, J.K. Furdyna, M. Dobrowolska, H. Rho, L.M. Smith, H.E. Jackson, J. Crystal Growth 214/215 (2000) 761. [13] P.R. Kratzert, M. Rabe, F. Hennenberger, Phys. Rev. Lett. 83 (1999) 239.
Alloying of CdSe/ZnSe quantum dot grown by an alternate molecular beam supply Masakazu Ohishi*, Katsuya Tanaka, Tomohiro Fujimoto, Minoru Yoneta, Hiroshi Saito Department of Applied Physics, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan
Abstract Self-assembled CdSe quantum dots have been fabricated by means of an alternate molecular beam supply. For the CdSe dot embedded in ZnSe and annealed at the growth temperature (2501C), the PL peak showed a blueshift with increase of the annealing period. This denotes that the size of embedded CdSe dots becomes small by the alloy formation with the surrounding ZnSe. The bare CdSe dots exposed to vacuum (without any molecular beam irradiation) evaporate, and the CdSe dots gradually changed to ZnCdSe dots. It was confirmed that the ZnCdSe dots were stable compared with pure CdSe dots. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.66.Hf; 78.55.Et; 68.65.+g; 81.10.Bk Keywords: A1. Nanostructures; A3. Molecular beam epitaxy; B1. Alloys; B1. Cadmium compounds; B1. Zinc compounds; B2. Semiconducting II–VI materials
1. Introduction CdSe-based quantum structures, such as the quantum well (QW) and quantum dots (QDs), are attracting much attention for an opto-electronic device application. Usually, the formation of selfassembled dots during growth is confirmed either by the appearance of diffraction spots or the damping of a RHEED intensity oscillation [1–5]. Usually, CdSe dots are fabricated by means of a molecular beam epitaxy (MBE) and an atomic layer epitaxy (ALE), where the low growth rate is used. We have shown that CdSe dots are created immediately after 3 ML-CdSe formed on ZnSe and
the dot growth by ripening follows under the Cd beam irradiation by inspecting the specular spot intensity (Isp ). We have also demonstrated that the CdSe dots evaporate when all the beams are interrupted prior to the ZnSe cap layer deposition [6,7]. One of the problems in CdSe growth is to know whether CdSe dots remain pure CdSe or make alloy with ZnSe. For this purpose, we examined the alloying of CdSe dots embedded in ZnSe and the alloying of bare CdSe dots exposed to vacuum by means of photoluminescence (PL) measurement.
2. Experimental procedure *Corresponding author. Tel.: +81-86-256-9410; fax: +8186-255-7700. E-mail address: [email protected] (M. Ohishi).
Self-assembled CdSe dots were fabricated by supplying Cd- and Se-molecular beams alternately
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 5 9 - 0
M. Ohishi et al. / Journal of Crystal Growth 237–239 (2002) 1320–1325
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(ALS growth) on ZnSe buffer layer on GaAs (1 0 0). To grow ZnCdSe structure, Zn and Cd beams were supplied simultaneously, where Cd beam flux is fixed and Zn beam flux is varied to change the Zn composition. The molecular beam pressures near the substrate position were PCd ¼ 4:0 107 and PSe ¼ 1:5 106 : The PZn was varied between 0 and 3.5 107 Torr. The growth of CdSe and ZnCdSe were initiated on Znterminated ZnSe buffer layer grown on GaAs (1 0 0). The ZnSe buffer layer (B500 nm) was grown by MBE mode at 3501C. After several ALE cycles of ZnCdSe was deposited at 2501C, the ZnSe cap layer (B100 nm) was deposited by MBE mode. Throughout the CdSe growth, the intensity of specular spot (Isp ) in the RHHED pattern was monitored. PL spectra are measured by the excitation of 325 nm line from He–Cd laser at liquid helium temperature.
3. Results and discussion 3.1. Band-edge emission of CdSe dots embedded in ZnSe In order to examine the thermal stability of CdSe dots, the capped CdSe dots were annealed at 2501C (growth temperature: Tg ). The CdSe dots were fabricated by 5 ALS cycles, and the cap layer is about 500 nm thick. Fig. 1 shows the PL spectra of capped CdSe dots annealed for several periods. PL peak shifted monotonically from 524.7 to 516.5 nm as the increase of annealing period from 0 to 30 min. This denotes that the capped CdSe dots is not stable at Tg : The PL blueshift is explained by the size reduction of CdSe dots, which is due to the alloying of CdSe dots with surrounding ZnSe. It is natural to consider that the alloying of CdSe dot proceeds from the dot surface and the surrounding ZnSe also becomes alloy with CdSe. If the entire CdSe dots changed to ZnCdSe dots, the boundary between ZnCdSe dot and surrounding ZnSe becomes vague, the size of ZnCdSe dot becomes large and the exciton confinement is reduced. The blueshift described above implies that the core part of initial CdSe dots remains pure CdSe dots after 30 min. The
Fig. 1. PL spectra of capped CdSe QDs annealed at 2501C. Five cycles of Cd and Se beam supplied on ZnSe. Annealing was performed immediately after the cap layer deposition.
observed emission from the annealed CdSe dots originate from CdSe dots surrounded by ZnCdSe alloy. We do not know that the further long annealing really cause the redshift or not. 3.2. PL vs. beam interrupted period In order to study the thermal stability of uncapped CdSe dots, all the beam were interrupted prior to cap layer deposition at Tg ¼ 2501C, that is, as-grown bare dots were exposed to vacuum for prescribed time without any beam irradiation. The ALS cycles are 1, 2, 3 and 5 cycles, and the beam interruption is 0, 5, 10 and 30 min. In succession, the ZnSe cap layer was deposited. To avoid the influence of annealing, after the cap layer deposition the sample was cooled down to room temperature as soon as
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Fig. 3. PL peak wavelength vs. beam interrupted period.
Fig. 2. PL spectra of CdSe QDs on ZnSe. Cycle of alternately supplied beam is 1, 2, 3 and 5 cycles, respectively. Prior to the cap layer deposition, all the beams are interrupted for 0, 5, 10 and 30 min, respectively.
possible. Fig. 2 shows the PL spectra of the capped CdSe/ZnSe structures, and Fig. 3 represents the wavelength of PL peak as a function of the beam interrupted time. Even in the structures embedded 1 ALS cycle of CdSe (1 ML CdSe) and 2 ALS cycles of CdSe (2 ML CdSe), the intense emission is observed after the 30 min beam interruption. The PL spectra of 1 ML CdSe shows distinct double peaks, which do not coincide with any emission lines of ZnSe layer nor the reported emission peak from CdSe/ ZnSe QW [3,8]. In these samples, the nominal CdSe thickness is thinner than 2 ML wetting layer (WL) and any traces suggesting dot formation was not appreciated in the temporal Isp measurement [6,7], This results imply that there exists another mechanism of dot growth in addition to the Stranski–Krastanow (S–K) mode. We consider, at present stage, that the CdSe growth by an alternate beam supply is slightly away from an
ideal ALE mode. The Isp curve during ALS growth shows a square waveform, and high and low intensity levels correspond to Zn(Cd)- and Sesurface, respectively. Whereas, the Isp from the surface exposed to cation beam keeps constant intensity level, the Isp from the surface under Se beam irradiation does not show the constant intensity but gradually loose its intensity. This means that the Se molecules are adsorbed partially on the Se-terminated surface. The excessively adsorbed Se molecules and impinged Cd beam give rise to the CdSe microdots. Recently, it is reported that the formation of micro island starts already when the first monolayer is deposited [10]. This is consistent with our conclusion. It is noteworthy that the PL peak of each sample converges to different wavelength with the increase of the beam interrupted time. Remarkable PL blueshift occurs within 10 min after the beam interruption, and the PL blueshift becomes slow after 10 min. If the CdSe evaporates from the dot surface, the dot size decreases gradually as the interruption time proceeds. Then, it is expected that the PL peak related to dots shifts to higher energy and disappears finally. However, the intense PL peak is observed after long beam interruption and which shows the dot existence. To interpret the dot stability after long beam interruption, we considered the alloying of CdSe
M. Ohishi et al. / Journal of Crystal Growth 237–239 (2002) 1320–1325
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dots with ZnSe buffer layer. By the inter-diffusion between CdSe WL and underlying ZnSe buffer layer, the WL varies to the ZnCdSe. At the same time, the inter-diffusion between CdSe dots and WL takes place. After long beam interruption, the CdSe dot gradually changes to Zn-rich ZnCdSe dot. The smaller the dot size is, the higher the alloying rate is. As described later, we confirmed that the ZnCdSe alloy dot is stable and the dot evaporation hardly occurs. Then, we concluded that the PL emission originates from ZnCdSe QDs after long beam interruption. 3.3. Zn beam dependence on the Isp variation during ZnCdSe growth We have reported that the temporal Isp during CdSe growth by alternate beam supply gives the information on the dot generation and the ripening [6,7]. In the ZnCdSe growth on ZnSe buffer layer, the lattice misfit between ZnCdSe layer and ZnSe buffer layer decreases with the increase of Zn composition. Since the self-assembling of dots is induced by the surface strain, it is expected that the increase of Zn content in Zn+Cd beam thickens the WL thickness and suppress the S–K dots generation. Fig. 4 shows the temporal Isp recorded during the growth of Cd(Zn)Se. Firstly, ZnSe buffer layer were grown on GaAs(1 0 0) by MBE mode. Then, ZnCdSe were grown on Zn-terminated ZnSe buffer by supplying Se and Cd+Zn beams alternately (Se beam is supplied first). Each beam irradiation period is 10 s, and no beam interruption time was inserted between beam irradiation. Finally, the ZnSe cap layer was grown by MBE mode. The upper trace in Fig. 4 is the Isp observed during CdSe growth (without Zn flux). As reported before, the steep Isp drop corresponding to dot generation is observed during the third Cd beam irradiation [6,7]. As the Zn beam is added in the Cd beam (PCd ¼ 4:7 108 Torr, PZn ¼ 4:7 108 Torr), the slight Isp drop is observed at fourth Cd beam irradiation. This denotes that the slight reduction of the lattice misfit delays the S–K dot generation. When the PZn is more than 1.3 107 Torr, and no Isp drop during Cd beam irradiation is observed.
Fig. 4. Specular spot intensity during the growth of ZnCdSe on ZnSe. Cd and Zn beams are supplied simultaneously. The beam shutter program is shown in the figure. PCd ¼ 4:2 107 Torr, PSe ¼ 1:0 106 Torr and PZn is varied from 0 to 3.1 107 Torr.
In this case, the RHEED intensity oscillation indicating the layer-by-layer growth (2D growth) was clearly observed at the onset of cap layer deposition. This means that the surface of ZnCdSe is smooth enough and no large S–K dots are formed on the surface. (Here we comment that the microdot creation does not affect on the Isp :) 3.4. PL spectra of ZnCdSe/ZnSe Fig. 5 shows the PL spectra of ZnCdSe capped with ZnSe (same samples shown in Fig. 3). As the small amount of Zn flux (PZn ¼ 1:7 108 Torr) was added in the Cd beam, PL peak showed large blueshift (from 523.3 to 491.0 nm). And the full-width of half-maximum (FWHM) of PL peak is decreased from 39.8 to 31.6 meV. For PZn > 4:7 108 Torr, the blueshift became small and the FWHM of PL peak became narrower
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described previously, the lattice misfit is dissolved and no ripening takes place. Thus, we concluded that the dot stability depends on whether the dots and WL are pure CdSe or ZnCdSe.
4. Conclusion
Fig. 5. PL spectra of ZnCdSe grown on ZnSe. The sample surface is capped with ZnSe.
(below 25 meV). In the very initial stage of the growth, the CdSe growth proceeds by both coherent growth mode and micro island growth mode. When the lattice misfit is large, both the layer-by-layer growth and the micro island growth occur in the initial stage. When the deposited thickness exceeds the WL, the S–K dots appear. For the smaller lattice misfit, however, only the layer-by-layer growth and the micro island growth occur. Furthermore, we notice that PL emission of 1 ML CdSe is composed of two peaks after 30 min growth interruption, which is attributed to the bimodal dot size distribution [9,11,12]. We also studied the effect of beam interruption prior to cap layer deposition on the PL of ZnCdSe (Tg =2501C). However, no appreciable blueshift of PL peak was observed. This result means that both the sublimation and the ripening hardly occur. Thus, we concluded that ZnCdSe dots become stable as the increase of Zn composition in ZnCdSe dots. Recently, it is reported that CdSe dots do not ripen on laboratory time scale [13]. This result has brought up the question whether the CdSe dots ripen or not. Since both the CdSe dots and the WL becomes ZnCdSe alloy as
Annealing of CdSe QDs embedded in ZnSe at growth temperature of 2501C caused the blueshift of PL peak, which is due to the alloying of CdSe dots. From the PL of CdSe dots fabricated with growth interruption prior to ZnSe cap layer deposition, the alloying of CdSe dots is also confirmed. Once the ZnCdSe alloy is formed on the CdSe dot surface, the alloying rate of CdSe decreases, that is, the CdSe dots covered with ZnCdSe become stable. Furthermore, it becomes clear that micro CdSe islands (dots) exist on the surface deposited 1 and 2 ML CdSe. Therefore, both S–K growth and micro island growth modes give two kinds of CdSe dots. During the dot growth, both the WL and the CdSe dots make alloy with ZnSe. We also confirmed that the PL peak position of ZnCdSe dots does not change by beam interruption prior to cap layer deposition, suggesting the ZnCdSe dots are stable.
Acknowledgements Part of this work has been supported by a special grant for cooperative research administered by Japan private school promotion foundation. We are also grateful for the Grant-In Aid for Scientific Research from the Ministry of Education and Culture.
References [1] S.H. Xin, P.D. Wang, A. Yin, C. Ki, M. Dobrowolska, J.L. Merz, J.K.M. Furdyna, Appl. Phys. Lett. 68 (1996) 3834. [2] H.-C. Ko, Y. Kawakami, Sz. Fujita, Sg. Fujita, J. Crystal Growth 184/185 (1998) 283. [3] M. Rabe, M. Lowisch, F. Hennenberger, J. Crystal Growth 184/185 (1998) 248. [4] J.L. Merz, S. Lee, J.K.M. Furdyna, J. Crystal Growth 184/ 185 (1998) 228.
M. Ohishi et al. / Journal of Crystal Growth 237–239 (2002) 1320–1325 [5] Shizo Fujita, Yi-hong Wu, Yoichi Kawakami, Shigeo Fujita, J. Appl. Phys. 72 (1992) 5233. [6] Kenta Arai, Takahashi Hanada, Takfumi Yao, J. Crystal Growth 214/215 (2000) 703. [7] M. Ohishi, M. Yoneta, H. Saito, T. Ichikawa, T. Fujimoto, J. Crystal Growth 214/215 (2000) 690. [8] M. Ohishi, H. Saito, M. Yoneta, T. Ichikawa, T. Fujimoto, Jpn. J. Appl. Phys. 39 (2000) 4584. [9] H. Zajicek, P. Juza, E. Abramof, O. Pankratov, H. Zitter, H. Helm, G. Brunthler, W. Faschinger, Appl. Phys. Lett. 62 (1993) 717. [10] D. Schikora, S. Schwedhelm, I. Kudryashov, K. Lischka, D. Litvinov, A. Rosenauer, D. Gerthsen, M. Strassburg,
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