- Email: [email protected]
InAlGaAs quantum dots
Thin Solid Films 517 (2009) 3979–3982
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Effects of group-III elements on the growth kinetics of shape-engineered InAs/InAlGaAs quantum dots Youngsin Yang a, Byounggu Jo a, Jaesu Kim a, Kwang Jae Lee a, Myoungkuk Ko a, Cheul-Ro Lee a, Jin Soo Kim a,⁎, Dae Kon Oh b, Jong Su Kim c, Jae-Young Leem d a
Division of Advanced Materials Engineering, Research Center of Advanced Materials Development (RCAMD), Chonbuk National University, Jeonju 561-756, Chonbuk, Republic of Korea Electronics and Telecommunication Research Institute (ETRI), Daejeon 305-350, Republic of Korea Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea d School of Nano Engineering, Inje University, Gimhae 621-749, Republic of Korea b c
a r t i c l e
i n f o
Available online 4 February 2009 Keywords: Quantum dot Shape-engineered Optical properties
a b s t r a c t We studied the influences of the group-III elements on the shape-engineered InAs/InAlGaAs quantum dots (SEQDs) by photoluminescence (PL) spectroscopy. By alternately depositing a thin InAs layer and a thin InAlGaAs layer on an InAlGaAs buffer layer (so called alternate growth method, AGM), the shape of QDs, especially height, was significantly manipulated. To optically investigate the effect of the introduction of Al and Ga atoms to InAs/InAlGaAs SEQDs (SEQD1), InAs/GaAs (SEQD2) and InAs/AlAs (SEQD3) SEQDs were respectively grown by using the same AGM. The emission peak of the InAs/InAlGaAs SEQDs was 1427 nm with a linewidth broadening of 36 meV at 15 K. The emission peak of the InAs/GaAs SEQDs was red-shifted by 215 nm from the SEQD1 sample. On the other hand, the emission peak for the InAs/AlAs SEQDs was blueshifted by 111 nm from the SEQD1 sample. From the temperature-dependent PL measurements, the emission peak for the SEQD1, SEQD2, and SEQD3 samples were respectively red-shifted by 18, 5, and 40 nm with increasing temperature. The different behavior in the PL results for the SEQD1, SEQD2, and SEQD3 samples can be attributed to the different atomic distribution of the group-III elements inside the SEQDs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the past several years, considerable research has been carried out to control the formation characteristics of self-assembled semiconductor quantum dots (QDs) grown by the Stranski–Kratanov (S–K) mode [1]. Self-assembled QDs have been attracted lots of attentions in terms of the fundamental physics and potential device application [2]. For example, the laser diode with self-assembled QDs as an active medium has been extensively studied due to the expectation on the high temperature stability, high differential gain, and low threshold current, because of their delta function-like density of states [3,4]. However, the excellent device performances reflecting the zero-dimensional properties has not been sufficiently demonstrated to date mainly because of the difficulties in forming highquality QDs by the S–K growth method. Therefore, much research effort has been made to control QD size, shape, and size distribution, thus changing the optical properties. For example, changing the growth parameters, varying the lattice-mismatch between a QD and a buffer layer, and using a stressor may effectively modify the strain energy for the formation of a QD [1,5]. In particular, for In(Ga)As QDs ⁎ Corresponding author. Tel.: +82 63 270 2291; fax: +82 63 270 2305. E-mail address: [email protected] (J.S. Kim). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.01.109
on InP used for the emission wavelength of 1.55 µm, the growth of high-quality QDs was relatively difficult compared to that of an In(Ga) As/GaAs QD system due mainly to the complexity of the QD formation associated with lower strain of 3.2% and possible chemical reaction at the surface [2,5,6]. As an example, the formation of In(Ga)As QDs on InAlGaAs lattice-matched to an InP substrate was influenced by the intrinsic phase separation of InAlGaAs at the interface leading to the growth of quantum dashes, wires, and asymmetric QDs. Usually, the lateral size of self-assembled In(Ga)As QDs on InP is relatively larger than the height. Because of the small height of the InP-based In(Ga)As QDs, the confinement of carrier wave-function in the vertical direction may be small resulting that the performances of the QD devices may not be satisfactory as theoretically expected [7,8]. Several approaches to enhance the confinement of the carriers in the QDs were studied by using an additional barrier with high potential or changing the shape of QDs. Paranthoen et al. reported a way to improve the uniformity of InAs QDs embedded in an InP matrix by so called double-cap procedure, where the height of QDs was controlled resulting in the narrower PL linewidth. However, the height of InP-based QDs was not sufficient to totally confine the carrier confinements in the vertical direction [9]. So, to improve the zero-dimensionality of self-assembled In(Ga)As QDs on InP, J. S. Kim et al. reported a plausible way to fabricate the shape-engineered InAs/InAlGaAs QDs (SEQDs) by
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alternately depositing a thin InAs layer and a thin InAlGaAs layer on an InAlGaAs buffer layer (so called alternated growth method, AGM), where the shape of the QDs was significantly changed, especially height, from the conventionally-grown S–K QDs [10]. In Ref. [10], the shape of InAs/InAlGaAs QDs was controlled by simultaneously using the strain relaxation and phase separation mechanism between InAs and InAlGaAs. However, there was not systematic evaluation on the introduction of the Al and Ga atoms in the InAs/InAlGaAs SEQDs. In this paper, we optically analyzed the effects of the introduction of the Al and Ga atoms for the InAs/InAlGaAs SEQDs grown by the AGM. To investigate the introduction of the group III-elements to the InAs/InAlGaAs SEQD region, the InAs/GaAs and InAs/AlAs SEQDs were respectively grown by the same AGM. 2. Experimental details The QD samples used in this study were grown by a V80 molecularbeam-epitaxy system with solid sources on n-type InP (001) substrates. The InAs/InAlGaAs SEQDs (SEQD1) were fabricated by using alternately depositing an InAs layer with a thickness of 1 monolayer (ML) and an InAlGaAs layer with a thickness of 1 ML on an InAlGaAs buffer layer. The total period of alternate deposition was 18. The InAs/InAlGaAs SEQDs were formed mainly due to both strain relaxation and phase separation properties between InAs and InAlGaAs. The schematic illustration of the SEQD samples was described in Fig. 1. More details on the growth method and the structural and optical properties of InAs/InAlGaAs SEQDs were described in Ref. [10]. In the InAs/InAlGaAs SEQDs, there is more possibility for the existence of the Al and Ga atoms in QD regions than conventionally-grown InAs QDs on InAlGaAs by the S–K growth mode. So, to investigate the effect of the Al and Ga atoms on the InAs/ InAlGaAs SEQDs, the InAs/GaAs SEQDs (SEQD2), and the InAs/AlAs SEQDs (SEQD3) were respectively grown by the same growth method, AGM. The growth conditions such as growth temperature and deposition periods for the SEQD2 and SEQD3 are exactly same for those of the SEQD1 sample. For PL measurements, an argon ion laser with a wavelength of 514.5 nm was used as an excitation source. The luminescence light from the QD samples was focused with collection lenses, dispersed by a 1.2 m SPEX single grating monochromater, and detected by an InGaAs array detector with the detection range from 850 to 1700 nm. The temperature-dependent PL spectra were obtained at the temperature range from 15 to 240 K. 3. Results and discussions Fig. 2(a) shows the normalized PL spectra from the SEQD samples at 15 K. The PL spectra of the conventional InAs QDs grown by S–K mode, where InAs was contributory supplied above the critical thickness to form QDs, was also shown as a reference. The emission peak of the ground states for the SEQD1, SEQD2, and SEQD3 samples were 1427, 1642, and 1316 nm, respectively. The emission peak of the SEQD2 sample was red-shifted by 215 nm from the SEQD1 sample. On the other hand, the emission peak for the SEQD3 sample was blue-
Fig. 1. Schematic diagram for the SEQD samples.
Fig. 2. From the SEQD sample, (a) normalized PL spectra measured at 15 K (b) PL spectrum (solid line) with Gaussian fitting curves (dotted lines).
shifted by 111 nm from the SEQD1 sample. The different behavior in the PL emission between the SEQD2 and SEQD3 samples with respect to the SEQD1 sample can be attributed to the individual introduction of the Al and Ga atoms to the SEQDs during the formation of the QD layer. The InAs/AlAs SEQDs for the SEQD3 samples has more Al atoms inside the QDs than conventional InAs QDs on InAlGaAs because of the thin AlAs layer. Since the energy band-gap for AlAs is larger than that of InAs and GaAs, the InAs/AlAs SEQD3 sample with more Al atoms should have a larger bandgap than that of the pure InAs QDs and InAs/ GaAs SEQDs. Since GaAs has smaller energy bandgap than AlAs, the InAs/GaAs SEQD2 sample with more Ga atoms inside the QDs have longer emission wavelength than the SEQD3 sample. As a result, the InAs/InAlGaAs SEQDs with Al and Ga atoms in QD region have the emission wavelength between the SEQD2 and SEQD3 samples [4,11,12]. That is, the band structure of the InAs QDs was tailored by the introduction of the Al and Ga atoms. Fig. 2(b) shows the PL spectra (solid line) with fitting curves (dot lines) for the SEQD1, SEQD2, and SEQD3 samples, respectively. The PL spectrum for the SEQD1 sample was separated to three peaks corresponding to the ground, 1st, and 2nd excited-state transitions [13]. The linewidth broadening for the ground states of the SEQD1 sample was 36 meV, which is significantly narrower than that of the conventional InAs QDs. Even though the linewidth broadening of the SEQD3 sample is smaller than that of the conventional InAs QDs, the excited-state transitions were not observed, where the ground states and excited states are likely to overlap each other [10]. No observation on the separation between the ground-state and excited-state transitions for the InAs/AlAs SEQDs can be explained by the unsatisfactory diffusion mechanism of the Al atoms due to relatively low growth temperature. These results indicated that the size uniformity of the InAs/InAlGaAs SEQD1 was better compared to that of the InAs/AlAs SEQDs [1]. The PL spectrum for the InAs/GaAs SEQDs (SEQD2) shows the double-peak feature
Y. Yang et al. / Thin Solid Films 517 (2009) 3979–3982
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with the 1st excited-state transitions. The linewidth broadening for the InAs/GaAs SEQDs was 32 meV, which is slightly narrower than that of that InAs/InAlGaAs SEQDs. The temperature-dependent PL measurements at the temperature range from 15 to 240 K were carried out to investigate the thermal stability of carriers in the SEQD samples. The PL spectra with temperature are shown in Fig. 3(a), (b), and (c) for the SEQD1, SEQD2, and SEQD3 samples, respectively. In all the SEQD samples, the PL intensity was continuously decreased with increasing temperature. Usually, when the temperature is increased, the PL intensity is decreased due to the thermal escape of carriers in the localized states. Also, the red-shift in PL emission peak was observed when increasing temperature, which can be explained by the effects of the dilation of lattice and the electron–lattice interaction. Fig. 4(a) shows the summary on the emission wavelengths of the SEQD samples with temperature, where the dashed lines are only guides for the eyes. The emission wavelengths of the SEQD1, SEQD2,
Fig. 4. (a) Summary on the emission wavelength as function of temperature. (b) The integrated PL intensity of the SEQD samples. (The solid lines are only guides).
Fig. 3. Temperature-dependent PL spectra for the (a) SEQD1, (b) SEQD2, and (c) SEQD3 samples at the temperature range from 15 to 240 K.
and SEQD3 samples at 240 K were respectively red-shifted by 18, 5, and 40 nm from those at 15 K. The amount of the red-shift for the InAs/AlAs SEQDs is 40 nm, which is relatively larger than that of the InAs/GaAs SEQDs (SEQD2 sample). If we only consider the thermal expansion coefficients for the alloy materials composed of InAs, GaAs, and AlAs, the change in the emission wavelength for the SEQD2 sample should be larger [14–16]. However, the change in the emission wavelength for the SEQD2 sample is smaller than those of the SEQD1 and SEQD3 samples. The relatively large variation in the emission wavelength for the SEQD3 sample can be attributed to the large size inhomogeneity of QDs. According to the PL spectra in Fig. 2, the size uniformity for the SEQD3 sample was improved compared to the conventional InAs QDs. However, the QD uniformity of InAs/AlAs SEQDs was still poor resulting that the separation between the ground states and the excited states was not observed in the PL spectrum. For the SEQD3 sample, the carriers in the localized states of the relatively small QDs can be thermally excited with increasing temperature, and then, the thermally-excited carriers may be in part recaptured in larger QDs. As a result, the emission wavelength seemed to be effectively red-shifted with increasing temperature. On the other hand, because the uniformity of the InAs/GaAs SEQDs was better than the InAs/AlAs SEQDs resulting in the concentrated state-distribution and narrower PL linewidth, the probability for the repopulation process of the carriers may be significantly reduced. This result led to the relatively small variation in the emission wavelength in the InAs/ GaAs SEQDs. The change in the emission wavelength for the InAs/ InAlGaAs SEQDs can be explained by the same way. The behavior of the integrated PL intensity with temperature shown in Fig. 4(b) supported the variation in the emission wavelength in Fig. 4(a). The reduction in the integrated PL intensity for the InAs/AlAs SEQDs was more severe than those of the InAs/GaAs and InAs/InAlGaAs SEQD samples. The meaning of the reduction in the PL intensity may be
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related to the thermal escape from the localized states in the QDs with increasing temperature [15]. 4. Conclusion The influences of the Ga and Al atoms in the InAs/InAlGaAs SEQDs were investigated by optically analyzing the InAs/GaAs SEQDs and the InAs/AlAs SEQDs. The emission peak for the InAs/InAlGaAs SEQDs was strongly related to the existence of the Al and Ga atoms inside the QD region by considering the PL spectra of InAs/AlAs and InAs/GaAs SEQDs. From the temperature-dependent PL results, the variation in the emission wavelength and the integrated PL intensity for the SEQD samples were attributed to the size uniformity of the QDs. Acknowledgments This work was supported in part by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Funds, KRF-2007-331-D00246 and KRF-2008-314D00249), in part by the Basic Research of the Korea Science and Engineering Foundation (Grant No. R0A-2008-000-20031-0), and in part by the Ministry of Knowledge and Economy of Korea through the Ultrashort Quantum Beam Facility Program. References [1] Y.F. Li, X.L. Ye, F.Q. Liu, D. Ding, W.H. Jiang, Z.Z. Sun, Y.C. Zhang, H.Y. Liu, Z.G. Wang, J. Cryst. Growth 218 (2000) 451. [2] J.S. Kim, Y. Yang, C.-R. Lee, I.H. Lee, Y.T. Yu, H.K. Ahn, K.W. Seol, J.S. Kim, J.-Y. Leem, M.-Y. Ryu, J. Appl. Phys. 102 (2007) 113526. [3] M. Borgstrom, M.P. Pires, T. Bryllert, S. Landi, W. Seifert, P.L. Souza, J. Cryst. Growth 252 (2003) 481. [4] Z. Zhen, D.A. Bedarev, B.V. Volovik, N.N. Ledentsov, A.V. Lunev, M.V. maksimou, A.F. Tsatsul, ' nikv, A.Yu. Egorov, A.E. Zhukov, A.R. Kovsh, V.M. Ustinov, P.S. Kop, ' ev, Semiconductors 33 (1999) 80.
[5] H. Li, T. Daniels-Race, M.-A. Hasan, Appl. Phys. Lett. 80 (2002) 1367. [6] X.-Q. Huang, F.-Q. Liu, X.-L. Che, J.-Q. Liu, W. Lei, Z.-G. Wang, J. Cryst. Growth 270 (2004) 364. [7] J.S. Kim, C.-R. Lee, I.H. Lee, J.-Y. Leem, J.S. Kim, M.-Y. Ryu, J. Appl. Phys. 102 (2007) 073501. [8] X.H. Zhang, J.R. Dong, S.J. Chua, J. Zhang, A. Yong, J. Cryst. Growth (2004) 420. [9] C. Paranthoen, N. Bertru, B. Lambert, O. Dehaese, A.L. Corre, J. Even, S. Loualiche, F. Lissillour, G. Moreau, J.C. Simon, Semicond. Sci. Technol. 17 (2002) L5. [10] J.S. Kim, J.H. Lee, S.U. Hong, H.-S. Kwack, B.S. Choi, D.K. Oh, Appl. Phys. Lett. 87 (2005) 053102. [11] C.H. Roh, H.J. Song, D.H. Kim, J.S. Park, Y.-S. Choi, H. Kim, C.-K. Hahn, J. Appl. Phys. 101 (2007) 064320. [12] N.N. Ledentsov, V.A. Shchukin, M. Grundmann, N. Kirstaedter, J. Bohrer, O. Schmidt, D. Bimberg, V.M. Ustinov, A.Y. Egorov, A.E. Zhukov, P.S. Kop, ' eV, S.V. Zaitsev, N.Yu. Gordeev, Z.I. Alferv, A.I. Borovkov, A.O. Kosogov, S.S. Ruvimov, P. Werner, U. Gosele, J. Heydenreich, Phys. Rev. B 54 (1996) 8743. [13] T.V. Torchynska, M. Dybiec, S. Ostpenko, Phys. Rev. B 72 (2005) 195341. [14] K. Mukai, M. Sugawara, Appl. Phys. Lett. 74 (1999) 3963. [15] J.S. Kim, D.K. Oh, P.W. Yu, J.-Y. Leem, J.I. Lee, C.-R. Lee, J. Cryst. Growth 261 (2004) 38. [16] W. Lei, Y.H. Chen, Y.L. Wang, X.Q. Huang, Ch. Zhao, J.Q. Liu, B. Xu, P. Jin, Y.P. Zeng, Z.G. Wang, J. Cryst. Growth 286 (2006) 23.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t s f
Effects of group-III elements on the growth kinetics of shape-engineered InAs/InAlGaAs quantum dots Youngsin Yang a, Byounggu Jo a, Jaesu Kim a, Kwang Jae Lee a, Myoungkuk Ko a, Cheul-Ro Lee a, Jin Soo Kim a,⁎, Dae Kon Oh b, Jong Su Kim c, Jae-Young Leem d a
Division of Advanced Materials Engineering, Research Center of Advanced Materials Development (RCAMD), Chonbuk National University, Jeonju 561-756, Chonbuk, Republic of Korea Electronics and Telecommunication Research Institute (ETRI), Daejeon 305-350, Republic of Korea Advanced Photonics Research Institute, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea d School of Nano Engineering, Inje University, Gimhae 621-749, Republic of Korea b c
a r t i c l e
i n f o
Available online 4 February 2009 Keywords: Quantum dot Shape-engineered Optical properties
a b s t r a c t We studied the influences of the group-III elements on the shape-engineered InAs/InAlGaAs quantum dots (SEQDs) by photoluminescence (PL) spectroscopy. By alternately depositing a thin InAs layer and a thin InAlGaAs layer on an InAlGaAs buffer layer (so called alternate growth method, AGM), the shape of QDs, especially height, was significantly manipulated. To optically investigate the effect of the introduction of Al and Ga atoms to InAs/InAlGaAs SEQDs (SEQD1), InAs/GaAs (SEQD2) and InAs/AlAs (SEQD3) SEQDs were respectively grown by using the same AGM. The emission peak of the InAs/InAlGaAs SEQDs was 1427 nm with a linewidth broadening of 36 meV at 15 K. The emission peak of the InAs/GaAs SEQDs was red-shifted by 215 nm from the SEQD1 sample. On the other hand, the emission peak for the InAs/AlAs SEQDs was blueshifted by 111 nm from the SEQD1 sample. From the temperature-dependent PL measurements, the emission peak for the SEQD1, SEQD2, and SEQD3 samples were respectively red-shifted by 18, 5, and 40 nm with increasing temperature. The different behavior in the PL results for the SEQD1, SEQD2, and SEQD3 samples can be attributed to the different atomic distribution of the group-III elements inside the SEQDs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the past several years, considerable research has been carried out to control the formation characteristics of self-assembled semiconductor quantum dots (QDs) grown by the Stranski–Kratanov (S–K) mode [1]. Self-assembled QDs have been attracted lots of attentions in terms of the fundamental physics and potential device application [2]. For example, the laser diode with self-assembled QDs as an active medium has been extensively studied due to the expectation on the high temperature stability, high differential gain, and low threshold current, because of their delta function-like density of states [3,4]. However, the excellent device performances reflecting the zero-dimensional properties has not been sufficiently demonstrated to date mainly because of the difficulties in forming highquality QDs by the S–K growth method. Therefore, much research effort has been made to control QD size, shape, and size distribution, thus changing the optical properties. For example, changing the growth parameters, varying the lattice-mismatch between a QD and a buffer layer, and using a stressor may effectively modify the strain energy for the formation of a QD [1,5]. In particular, for In(Ga)As QDs ⁎ Corresponding author. Tel.: +82 63 270 2291; fax: +82 63 270 2305. E-mail address: [email protected] (J.S. Kim). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.01.109
on InP used for the emission wavelength of 1.55 µm, the growth of high-quality QDs was relatively difficult compared to that of an In(Ga) As/GaAs QD system due mainly to the complexity of the QD formation associated with lower strain of 3.2% and possible chemical reaction at the surface [2,5,6]. As an example, the formation of In(Ga)As QDs on InAlGaAs lattice-matched to an InP substrate was influenced by the intrinsic phase separation of InAlGaAs at the interface leading to the growth of quantum dashes, wires, and asymmetric QDs. Usually, the lateral size of self-assembled In(Ga)As QDs on InP is relatively larger than the height. Because of the small height of the InP-based In(Ga)As QDs, the confinement of carrier wave-function in the vertical direction may be small resulting that the performances of the QD devices may not be satisfactory as theoretically expected [7,8]. Several approaches to enhance the confinement of the carriers in the QDs were studied by using an additional barrier with high potential or changing the shape of QDs. Paranthoen et al. reported a way to improve the uniformity of InAs QDs embedded in an InP matrix by so called double-cap procedure, where the height of QDs was controlled resulting in the narrower PL linewidth. However, the height of InP-based QDs was not sufficient to totally confine the carrier confinements in the vertical direction [9]. So, to improve the zero-dimensionality of self-assembled In(Ga)As QDs on InP, J. S. Kim et al. reported a plausible way to fabricate the shape-engineered InAs/InAlGaAs QDs (SEQDs) by
3980
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alternately depositing a thin InAs layer and a thin InAlGaAs layer on an InAlGaAs buffer layer (so called alternated growth method, AGM), where the shape of the QDs was significantly changed, especially height, from the conventionally-grown S–K QDs [10]. In Ref. [10], the shape of InAs/InAlGaAs QDs was controlled by simultaneously using the strain relaxation and phase separation mechanism between InAs and InAlGaAs. However, there was not systematic evaluation on the introduction of the Al and Ga atoms in the InAs/InAlGaAs SEQDs. In this paper, we optically analyzed the effects of the introduction of the Al and Ga atoms for the InAs/InAlGaAs SEQDs grown by the AGM. To investigate the introduction of the group III-elements to the InAs/InAlGaAs SEQD region, the InAs/GaAs and InAs/AlAs SEQDs were respectively grown by the same AGM. 2. Experimental details The QD samples used in this study were grown by a V80 molecularbeam-epitaxy system with solid sources on n-type InP (001) substrates. The InAs/InAlGaAs SEQDs (SEQD1) were fabricated by using alternately depositing an InAs layer with a thickness of 1 monolayer (ML) and an InAlGaAs layer with a thickness of 1 ML on an InAlGaAs buffer layer. The total period of alternate deposition was 18. The InAs/InAlGaAs SEQDs were formed mainly due to both strain relaxation and phase separation properties between InAs and InAlGaAs. The schematic illustration of the SEQD samples was described in Fig. 1. More details on the growth method and the structural and optical properties of InAs/InAlGaAs SEQDs were described in Ref. [10]. In the InAs/InAlGaAs SEQDs, there is more possibility for the existence of the Al and Ga atoms in QD regions than conventionally-grown InAs QDs on InAlGaAs by the S–K growth mode. So, to investigate the effect of the Al and Ga atoms on the InAs/ InAlGaAs SEQDs, the InAs/GaAs SEQDs (SEQD2), and the InAs/AlAs SEQDs (SEQD3) were respectively grown by the same growth method, AGM. The growth conditions such as growth temperature and deposition periods for the SEQD2 and SEQD3 are exactly same for those of the SEQD1 sample. For PL measurements, an argon ion laser with a wavelength of 514.5 nm was used as an excitation source. The luminescence light from the QD samples was focused with collection lenses, dispersed by a 1.2 m SPEX single grating monochromater, and detected by an InGaAs array detector with the detection range from 850 to 1700 nm. The temperature-dependent PL spectra were obtained at the temperature range from 15 to 240 K. 3. Results and discussions Fig. 2(a) shows the normalized PL spectra from the SEQD samples at 15 K. The PL spectra of the conventional InAs QDs grown by S–K mode, where InAs was contributory supplied above the critical thickness to form QDs, was also shown as a reference. The emission peak of the ground states for the SEQD1, SEQD2, and SEQD3 samples were 1427, 1642, and 1316 nm, respectively. The emission peak of the SEQD2 sample was red-shifted by 215 nm from the SEQD1 sample. On the other hand, the emission peak for the SEQD3 sample was blue-
Fig. 1. Schematic diagram for the SEQD samples.
Fig. 2. From the SEQD sample, (a) normalized PL spectra measured at 15 K (b) PL spectrum (solid line) with Gaussian fitting curves (dotted lines).
shifted by 111 nm from the SEQD1 sample. The different behavior in the PL emission between the SEQD2 and SEQD3 samples with respect to the SEQD1 sample can be attributed to the individual introduction of the Al and Ga atoms to the SEQDs during the formation of the QD layer. The InAs/AlAs SEQDs for the SEQD3 samples has more Al atoms inside the QDs than conventional InAs QDs on InAlGaAs because of the thin AlAs layer. Since the energy band-gap for AlAs is larger than that of InAs and GaAs, the InAs/AlAs SEQD3 sample with more Al atoms should have a larger bandgap than that of the pure InAs QDs and InAs/ GaAs SEQDs. Since GaAs has smaller energy bandgap than AlAs, the InAs/GaAs SEQD2 sample with more Ga atoms inside the QDs have longer emission wavelength than the SEQD3 sample. As a result, the InAs/InAlGaAs SEQDs with Al and Ga atoms in QD region have the emission wavelength between the SEQD2 and SEQD3 samples [4,11,12]. That is, the band structure of the InAs QDs was tailored by the introduction of the Al and Ga atoms. Fig. 2(b) shows the PL spectra (solid line) with fitting curves (dot lines) for the SEQD1, SEQD2, and SEQD3 samples, respectively. The PL spectrum for the SEQD1 sample was separated to three peaks corresponding to the ground, 1st, and 2nd excited-state transitions [13]. The linewidth broadening for the ground states of the SEQD1 sample was 36 meV, which is significantly narrower than that of the conventional InAs QDs. Even though the linewidth broadening of the SEQD3 sample is smaller than that of the conventional InAs QDs, the excited-state transitions were not observed, where the ground states and excited states are likely to overlap each other [10]. No observation on the separation between the ground-state and excited-state transitions for the InAs/AlAs SEQDs can be explained by the unsatisfactory diffusion mechanism of the Al atoms due to relatively low growth temperature. These results indicated that the size uniformity of the InAs/InAlGaAs SEQD1 was better compared to that of the InAs/AlAs SEQDs [1]. The PL spectrum for the InAs/GaAs SEQDs (SEQD2) shows the double-peak feature
Y. Yang et al. / Thin Solid Films 517 (2009) 3979–3982
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with the 1st excited-state transitions. The linewidth broadening for the InAs/GaAs SEQDs was 32 meV, which is slightly narrower than that of that InAs/InAlGaAs SEQDs. The temperature-dependent PL measurements at the temperature range from 15 to 240 K were carried out to investigate the thermal stability of carriers in the SEQD samples. The PL spectra with temperature are shown in Fig. 3(a), (b), and (c) for the SEQD1, SEQD2, and SEQD3 samples, respectively. In all the SEQD samples, the PL intensity was continuously decreased with increasing temperature. Usually, when the temperature is increased, the PL intensity is decreased due to the thermal escape of carriers in the localized states. Also, the red-shift in PL emission peak was observed when increasing temperature, which can be explained by the effects of the dilation of lattice and the electron–lattice interaction. Fig. 4(a) shows the summary on the emission wavelengths of the SEQD samples with temperature, where the dashed lines are only guides for the eyes. The emission wavelengths of the SEQD1, SEQD2,
Fig. 4. (a) Summary on the emission wavelength as function of temperature. (b) The integrated PL intensity of the SEQD samples. (The solid lines are only guides).
Fig. 3. Temperature-dependent PL spectra for the (a) SEQD1, (b) SEQD2, and (c) SEQD3 samples at the temperature range from 15 to 240 K.
and SEQD3 samples at 240 K were respectively red-shifted by 18, 5, and 40 nm from those at 15 K. The amount of the red-shift for the InAs/AlAs SEQDs is 40 nm, which is relatively larger than that of the InAs/GaAs SEQDs (SEQD2 sample). If we only consider the thermal expansion coefficients for the alloy materials composed of InAs, GaAs, and AlAs, the change in the emission wavelength for the SEQD2 sample should be larger [14–16]. However, the change in the emission wavelength for the SEQD2 sample is smaller than those of the SEQD1 and SEQD3 samples. The relatively large variation in the emission wavelength for the SEQD3 sample can be attributed to the large size inhomogeneity of QDs. According to the PL spectra in Fig. 2, the size uniformity for the SEQD3 sample was improved compared to the conventional InAs QDs. However, the QD uniformity of InAs/AlAs SEQDs was still poor resulting that the separation between the ground states and the excited states was not observed in the PL spectrum. For the SEQD3 sample, the carriers in the localized states of the relatively small QDs can be thermally excited with increasing temperature, and then, the thermally-excited carriers may be in part recaptured in larger QDs. As a result, the emission wavelength seemed to be effectively red-shifted with increasing temperature. On the other hand, because the uniformity of the InAs/GaAs SEQDs was better than the InAs/AlAs SEQDs resulting in the concentrated state-distribution and narrower PL linewidth, the probability for the repopulation process of the carriers may be significantly reduced. This result led to the relatively small variation in the emission wavelength in the InAs/ GaAs SEQDs. The change in the emission wavelength for the InAs/ InAlGaAs SEQDs can be explained by the same way. The behavior of the integrated PL intensity with temperature shown in Fig. 4(b) supported the variation in the emission wavelength in Fig. 4(a). The reduction in the integrated PL intensity for the InAs/AlAs SEQDs was more severe than those of the InAs/GaAs and InAs/InAlGaAs SEQD samples. The meaning of the reduction in the PL intensity may be
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related to the thermal escape from the localized states in the QDs with increasing temperature [15]. 4. Conclusion The influences of the Ga and Al atoms in the InAs/InAlGaAs SEQDs were investigated by optically analyzing the InAs/GaAs SEQDs and the InAs/AlAs SEQDs. The emission peak for the InAs/InAlGaAs SEQDs was strongly related to the existence of the Al and Ga atoms inside the QD region by considering the PL spectra of InAs/AlAs and InAs/GaAs SEQDs. From the temperature-dependent PL results, the variation in the emission wavelength and the integrated PL intensity for the SEQD samples were attributed to the size uniformity of the QDs. Acknowledgments This work was supported in part by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Funds, KRF-2007-331-D00246 and KRF-2008-314D00249), in part by the Basic Research of the Korea Science and Engineering Foundation (Grant No. R0A-2008-000-20031-0), and in part by the Ministry of Knowledge and Economy of Korea through the Ultrashort Quantum Beam Facility Program. References [1] Y.F. Li, X.L. Ye, F.Q. Liu, D. Ding, W.H. Jiang, Z.Z. Sun, Y.C. Zhang, H.Y. Liu, Z.G. Wang, J. Cryst. Growth 218 (2000) 451. [2] J.S. Kim, Y. Yang, C.-R. Lee, I.H. Lee, Y.T. Yu, H.K. Ahn, K.W. Seol, J.S. Kim, J.-Y. Leem, M.-Y. Ryu, J. Appl. Phys. 102 (2007) 113526. [3] M. Borgstrom, M.P. Pires, T. Bryllert, S. Landi, W. Seifert, P.L. Souza, J. Cryst. Growth 252 (2003) 481. [4] Z. Zhen, D.A. Bedarev, B.V. Volovik, N.N. Ledentsov, A.V. Lunev, M.V. maksimou, A.F. Tsatsul, ' nikv, A.Yu. Egorov, A.E. Zhukov, A.R. Kovsh, V.M. Ustinov, P.S. Kop, ' ev, Semiconductors 33 (1999) 80.
[5] H. Li, T. Daniels-Race, M.-A. Hasan, Appl. Phys. Lett. 80 (2002) 1367. [6] X.-Q. Huang, F.-Q. Liu, X.-L. Che, J.-Q. Liu, W. Lei, Z.-G. Wang, J. Cryst. Growth 270 (2004) 364. [7] J.S. Kim, C.-R. Lee, I.H. Lee, J.-Y. Leem, J.S. Kim, M.-Y. Ryu, J. Appl. Phys. 102 (2007) 073501. [8] X.H. Zhang, J.R. Dong, S.J. Chua, J. Zhang, A. Yong, J. Cryst. Growth (2004) 420. [9] C. Paranthoen, N. Bertru, B. Lambert, O. Dehaese, A.L. Corre, J. Even, S. Loualiche, F. Lissillour, G. Moreau, J.C. Simon, Semicond. Sci. Technol. 17 (2002) L5. [10] J.S. Kim, J.H. Lee, S.U. Hong, H.-S. Kwack, B.S. Choi, D.K. Oh, Appl. Phys. Lett. 87 (2005) 053102. [11] C.H. Roh, H.J. Song, D.H. Kim, J.S. Park, Y.-S. Choi, H. Kim, C.-K. Hahn, J. Appl. Phys. 101 (2007) 064320. [12] N.N. Ledentsov, V.A. Shchukin, M. Grundmann, N. Kirstaedter, J. Bohrer, O. Schmidt, D. Bimberg, V.M. Ustinov, A.Y. Egorov, A.E. Zhukov, P.S. Kop, ' eV, S.V. Zaitsev, N.Yu. Gordeev, Z.I. Alferv, A.I. Borovkov, A.O. Kosogov, S.S. Ruvimov, P. Werner, U. Gosele, J. Heydenreich, Phys. Rev. B 54 (1996) 8743. [13] T.V. Torchynska, M. Dybiec, S. Ostpenko, Phys. Rev. B 72 (2005) 195341. [14] K. Mukai, M. Sugawara, Appl. Phys. Lett. 74 (1999) 3963. [15] J.S. Kim, D.K. Oh, P.W. Yu, J.-Y. Leem, J.I. Lee, C.-R. Lee, J. Cryst. Growth 261 (2004) 38. [16] W. Lei, Y.H. Chen, Y.L. Wang, X.Q. Huang, Ch. Zhao, J.Q. Liu, B. Xu, P. Jin, Y.P. Zeng, Z.G. Wang, J. Cryst. Growth 286 (2006) 23.