- Email: [email protected]
InP quantum dot superlattice
PERGAMON
Solid State Communications 117 (2001) 465±469
www.elsevier.com/locate/ssc
Arsenic/phosphorus exchange and wavelength tuning of in situ annealed InAs/InP quantum dot superlattice Q.D. Zhuang*, S.F. Yoon, H.Q. Zheng School of Electrical and Electronic Engineering, Block S1, Nanyang Technological University, Nanyang Avenue, Singapore, Singapore 639798 Received 27 August 2000; received in revised form 13 November 2000; accepted 24 November 2000 by H. Akai
Abstract We report the solid-source molecular beam epitaxial (SSMBE) growth of InAs/InP quantum dots (QDs) superlattices and the effect of As/P exchange. The InAs QDs were found to have an average lateral diameter of ,40 nm and density of 3 to 4 £ 10 10 cm 22. The single-layer QDs have photoluminescence (PL) emission centred at 0.78 eV with a linewidth of 64 meV at low temperature (4 K). Double-crystal X-ray diffraction (DCXRD) spectra showed evidence of signi®cant As/P exchange during in situ annealing under P2 pressure before growing the spacer layer. An average P composition of ,30% in the resulting InAsP QDs in samples annealed for 50 s was deduced from dynamical simulations of the experimental DCXRD spectra. The QDs superlattice PL emission exhibits a blueshift with increase of annealing time, and emission at 1.55 mm at 300 K was achieved. This observation holds promise for possible telecommunication device applications at long wavelength. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: B. Epitaxy; D. Optical properties; E. Luminescence PACS: 61.10.-i; 68.65.-k; 68.35.-p; 78.67.-Hc
1. Introduction Presently, there is extensive research on the fabrication of self-organised quantum dots (QDs) via the Stranski±Krastanow growth mode, which is induced by the lattice mismatch between the epitaxial layer and substrate. These QDs were reported to have promising electrical and optical properties [1,2]. The incorporation of QDs was found to lead to large improvements in device performance, such as in lasers [3], detectors [4], and optical memories [5]. For example, lasers based on QDs have been shown to have lower threshold current density and higher characteristic temperature in comparison to quantum well (QW) lasers. Injection lasers containing InAs and InGaAs QDs embedded in a GaAs and AlGaAs matrix have been successfully demonstrated and showed improved characteristics as predicted [6,7]. However, the longest emission wavelength for In(Ga)As/GaAs QDs is 1.3 mm [8,9], and lasing occurred * Corresponding author. Tel.: 165-793-3318; fax: 165-7904528. E-mail address: [email protected] (Q.D. Zhuang).
at shorter wavelength owing to state ®lling in the QDs discrete quantum levels [10], which restricts applications in optical ®bre communication systems. From a practical point of view, it is desirable to realise QDs lasers whose lasing wavelength is in the range of 1.3±1.55 mm, which are suitable for optical ®bre communication systems. It has been reported that InAs QDs grown on an InP substrate showed photoluminescence (PL) emission at around 1.5 mm at 2 K [11]. The effective masses of the carriers in the strained InAs quantum structures grown on InP are smaller than those grown on GaAs, and their energy levels are further apart. This has a bene®cial effect for improving the thermal stability of the devices based on this material system [12]. Despite these advantages, there are limited reports on InAs/InP QDs [13±16], due in part to the small mismatch (3.2%) and the complexity of the QD formation mechanism associated with As/P exchange reactions at the growth surface. Although the effect of As/P exchange on the modi®cation of InAs QDs size, and on the tuning of the PL emission from QDs have been reported recently [14,15], the detailed mechanism of the As/P exchange is still relatively unclear.
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(00)00506-8
466
Q.D. Zhuang et al. / Solid State Communications 117 (2001) 465±469
2. Experimental procedure
ÊFig. 1. TEM plan-view of single layer InAs QDs capped by a 350-A thick InP layer.
In this paper, we report the preparation of an InAs/InP QDs superlattice grown by solid-source molecular beam epitaxy (SSMBE), and investigate the As/P exchange behaviour in QDs during the in situ annealing process under P2 pressure before the growth of the spacer layer. It is observed that large InAs QDs (average of ,40 nm in diameter) with a density of ,3 to 4 £ 10 10 cm 22 were successfully prepared. The results of in situ annealing demonstrated that signi®cant As/P exchange occurs in the QDs, as deduced from dynamical simulations of the double-crystal X-ray diffraction (DCXRD) spectra. Furthermore, the PL emission wavelength of the QDs can be tuned effectively. Through this method, a QDs superlattice, which emits at 1.55 mm, has been successfully achieved.
The samples were grown in an SSMBE system equipped Ê with valved As and P cracker sources. After growing 500 A of InP buffer layer on the InP substrate at 5008C at a growth Ê s 21, a ®ve-period InAs/InP QDs superlattice, rate of 1 A Ê -thick InAs layers separated by a which consists of 20-A Ê -thick InP spacer layer was prepared. To prevent 200-A group V intermixing, the P valve was closed for 3 s, followed by opening of the As valve for 3 s, before the Ê InAs InAs growth. The QDs were formed with a 20-A deposition at 5008C, and followed by a 60-s annealing under arsenic ¯ux. In order to investigate the As/P exchange effect on the QDs formation, four samples (samples A±D) of ®ve-period QDs superlattices were prepared with in situ annealing under P2 pressure for 0, 5, 20, and 50 s, respecÊtively, at 5008C. All the samples were ®nished with a 350-A thick InP cap layer. For the sample to be subjected to the transmission electron microscopy (TEM) measurements, a Ê -thick InP cap layer was grown after the single layer 350-A of InAs QDs deposition. The QDs formation was monitored in situ by re¯ection high-energy electron diffraction oscillation (RHEED) observation. The morphology of QDs is characterised by TEM. The low temperature of 4 K and room temperature PL were used to characterise the optical properties of the QDs, the PL was excited by an Ar 1 ion laser and detected by a cooled Ge detector, and DCXRD measurements were performed with a diffractometer.
3. Results and discussion Fig. 1 shows the plan-view of the TEM image of a typical InAs QDs structure grown on an InP (100) substrate. It is clearly found that the QDs exhibit an average lateral Ê with a density in the range of 3 to diameter of ,400 A 10 22 4 £ 10 cm . The QDs dimension is smaller than the
Fig. 2. Low-temperature (4 K) PL spectrum of single InAs QD layer with InP cap layer.
Q.D. Zhuang et al. / Solid State Communications 117 (2001) 465±469
467
Fig. 3. PL spectra at room temperature of InAs/InP QDs superlattice samples annealed under P2 pressure for 0 s (sample A), 5 s (sample B), 20 s (sample C) and 50 s (sample D).
islands grown in the InAlAs matrix on the (100) InP substrate by gas-source molecular beam epitaxy (GSMBE) [17], and comparable to the QDs grown on the higher-index (311)B InP substrate [15]. Low-temperature (4 K) PL measurement of a single layer QD sample is shown in Fig. 2. A strong emission peak at ,0.78 eV with a linewidth of 64 meV was observed. The narrower linewidth compared with that previously reported (,90 meV), indicates the improvement in the size distribution [18]. Fig. 3 shows the room-temperature PL spectra of the InAs/InP QDs superlattice samples, which were subjected to different annealing times of 0 s (sample A), 5 s (sample B), 20 s (sample C) and 50 s (sample D). It shows clearly that the QDs superlattice PL emission shifts to higher energy
Fig. 4. Experimental (004) DCXRD rocking curves of samples A±D (inset ®gure), and the corresponding simulated spectrum of sample A (curve SA) and D (curve SD).
with increase of annealing time from 0 to 50 s. A shift in energy of up to 53 meV was observed in the sample annealed for 50 s (sample D). Note that at annealing time of 5 s (sample B), the QDs superlattice PL emission is centred around 0.8 eV, which corresponds to an emission wavelength around 1.55 mm. It is worth pointing out that the room temperature PL emission peak energy of sample A is around 0.784 eV, which is nearly the same as the sample of single-layer QDs measured at low temperature. This coincidence indicates the shortening of PL emission wavelength of the multi-QDs structure compared with single QDs, and should not be attributed to the change of QDs size and shape caused by the multi-layer structure owing to the wide spacer layer. We have reported that the redshift of PL emission from InAs/InP QDs on the temperature is weak, and only a 10-meV redshift is observed from 4 K to room temperature [19]. Here, we propose that the coincidence is caused by the longer growth time for the multi-layer structure, which leads to a possible As/P exchange, hence weakening quantum con®nement, and shortening PL emission consequently. The modi®cation of the QDs PL emission caused by the annealing process could be attributed to the As/P exchange at the interface during the annealing process under P2 pressure. It has been reported that the As/P exchange can cause a signi®cant change in the QDs aspect ratio through enhancing exchange reactions at the periphery of the QDs base due to the concentration of localised strain at the periphery of the QDs base [14]. Signi®cant reduction of QDs height was observed during the InP cap layer growth process, which is attributed to the As/P exchange [15]. We propose that this reduction occurs during the annealing process, and increases the quantum con®nement, which shortens the QDs PL emission wavelength consequently. Furthermore, the As/ P exchange could cause the formation of InAs12xPx QDs, which directly reduces the QDs PL emission wavelength because of its larger bandgap. Certainly, the formation of InAs12xPx QDs could also reduce the compressive stress in
468
Q.D. Zhuang et al. / Solid State Communications 117 (2001) 465±469
the QDs, and hence increase the emission wavelength [20], but this effect is relatively weak. Therefore, the net effect of the formation of InAs12xPx QDs is to reduce the PL emission wavelength as observed. The DCXRD technique is a very sensitive technique for measuring the structural parameters and inter-diffusivity effects in composition-modulated thin ®lms. Hence, DCXRD measurements on the samples were taken, and the spectra simulated using the dynamical theory to evaluate the As/P exchange effects in the superlattice. Fig. 4 shows the measured (004) DCXRD rocking curves of samples A (0 s annealing), B (5 s annealing), C (20 s annealing), and D (50 s annealing) (shown as inset ®gure), as well as the corresponding simulated spectra of sample A (curve SA) and D (curve SD). From the inset ®gure, a signi®cant shift of the zero-order satellite peak to the InP substrate following an increase in annealing time was observed, indicating a relaxation of the strain. We propose that the strain relaxation could be attributed to the As/P exchange during the annealing process. Krost et al. [21] have demonstrated that rocking curve simulations based on the dynamical theory could be used to determine the indium composition in the InGaAs QDs. In our simulations, the QDs superlattice was treated as a quantum-well-like superlattice. It can be seen that curve ®tting of the simulated DCXRD spectrum to the measured DCXRD spectrum of sample A reveals a close match of the superlattice structural parameters to the designed values Ê InAs and 200 A Ê InP). The As/P exchange during the (20 A annealing process causes an incorporation of P atoms into the InAs layers. It is expected that the effective P composition in the InAs layers will increase with the increase in annealing time. Dynamical simulations of the measured DCXRD spectra revealed that the P composition in samples B±D, which best matches the experimental spectrum, are 0.05, 0.15, and 0.3, respectively. These results clearly show that the As/P exchange that occurs on the surface is anomalously large. The effect of this large As/P exchange on the surface growth kinetics enhances the complexity of the QDs formation process. According to the dynamical simulation results of sample D, where the P composition was determined to be 0.3, the annealing process could cause bandgap widening of 230 meV, and will lead to a large blue shift of the QDs PL emission. However, in practice, sample D only exhibited a much smaller blue shift of only 53 meV (Fig. 3). In fact, the InAs-grown surface is mainly covered by an InAs wetting layer, the P is mainly incorporated into the InAs layers; thus the P composition measured by DCXRD is mainly associated to the InAs wetting layers, then it is possible that the P composition in the QDs is less than the measured value. Furthermore, the discrepancy could be attributed partly to the modi®cation of the QDs size, and stress reduction caused by As/P exchange during the annealing process as previously discussed. Experiments are in progress to further understand the As/P exchange effect on the QDs size modi®cation.
Since the As/P exchange mechanism at the QDs surface is a complex phenomenon, the question remains whether the QDs are still present after annealing for 50 s. Observation of spotty RHEED patterns after P2 annealing for 50 s suggests that the QDs are still present. Furthermore, it was found that three-dimensional structures of the QDs are still observable from TEM examination of the samples. However, further annealing for a longer time of ,100 s results in the disappearance of the QDs. It is possible that under a longer annealing time, the QDs have expanded and merged to form quantum-well-like structures. 4. Conclusions In conclusion, we report the preparation of InAs/InP QDs superlattice structures using SSMBE. The effect of As/P exchange on the InAs QDs during in situ annealing under P2 pressure was investigated. Strong PL emission at 4 K was observed from single-layer InAs QDs. In the case of InAs/ InP QDs superlattice structures, annealing under P2 pressure has the effect of reducing the PL emission (room temperature) wavelength due to the formation of InAs12xPx QDs arising from As/P exchange. Dynamical simulations of the DCXRD spectra provided evidence of increase in the P composition with increase of annealing time. Hence, our study shows that annealing under P2 pressure is an effective means for tuning the PL emission wavelength. Room temperature emission at 1.55 mm has been observed in the QDs superlattice structure annealed for 5 s.
References [1] D. Leonard, K. Pond, P.M. Petroff, Phys. Rev. B 50 (1994) 11687. [2] P.M. Petroff, S.P. Denbaars, Superlattices Microstruct. 15 (1994) 15 (and references therein). [3] Y. Arakawa, H. Sasaki, Appl. Phys. Lett. 40 (1982) 939. [4] D. Pan, E. Towe, Electron. Lett. 34 (1998) 1883. [5] K. Imamura, Y. Sugiyama, Y. Nakata, S. Muto, N. Yokoyarna, Jpn J. Appl. Phys., Part 2 34 (1995) L1445. [6] N. Kirstaedter, N.N. Ledentsov, M. Grundmann, D. Bimberg, V.M. Ustinov, S.S. Ruvimov, M.V. Maximov, P.S. Kop'ev, Zh.I. Alferov, U. Richter, P. Werner, U. Gosele, J. Heydenreich, Electron. Lett. 30 (1994) 1416. [7] K. Kamath, P. Bhattacharya, T. Sosnowski, T. Norris, J. Phillips, Electron. Lett. 32 (1996) 1374. [8] V.M. Ustinov, N.A. Maleev, A.E. Zhukov, A.R. Kovsh, Egorov A.Yu, A.V. Lunev, B.V. Volovik, I.L. Krestnikov, Yu.G. Musikhin, N.A. Bert, P.S. Kop'ev, Zh.I. Alferov, N.N. Ledentsov, D. Bimberg, Appl. Phys. Lett. 74 (1999) 2815. [9] D.L. Huffaker, D.G. Deppe, Appl. Phys. Lett. 73 (1998) 520. [10] R. Mirin, A. Gossard, J. Bowers, Electron. Lett. 32 (1996) 1732. [11] A. Ponchet, A. Le Corre, H. L'haridon, B. Lambert, S. Salaun, J. Groenen, R. Carles, Solid-State Electron. 40 (1996) 615.
Q.D. Zhuang et al. / Solid State Communications 117 (2001) 465±469 [12] S. Fafard, Z. Wasilewski, J. McCaffrey, S. Raymond, S. Charbonneau, Appl. Phys. Lett. 68 (1996) 991. [13] A. Ponchet, A. Le Corre, H. L'Earidon, B. Lambert, S. Salau, Appl. Phys. Lett. 67 (1995) 1850. [14] S. Yoon, Y. Moon, Tae-Wan Lee, E. Yoon, Y.D. Kim, Appl. Phys. Lett. 74 (1999) 2029. [15] S. Frechengues, N. Bertru, V. Drouot, B. Lambert, S. Robinet, S. Loualiche, D. Lacombe, A. Ponchet, Appl. Phys. Lett. 74 (1999) 3356. [16] K. Nishi, M. Yamada, T. Anan, A. Gomyo, S. Sugou, Appl. Phys. Lett. 73 (1998) 526.
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[17] S. Fafard, Z. Wasilewski, J. McCaffrey, S. Raymond, S. Charbonneau, Appl. Phys. Lett. 68 (1996) 991. [18] H. Marchand, P. Desjardins, S. Guillon, J.-E. Paultre, Z. Bougrioua, R.Y.-F. Yip, R.A. Masut, Appl. Phys. Lett. 71 (1997) 527. [19] Q.D. Zhuang, S.F. Yoon, H.Q. Zheng, K.H. Yuan, J. Crystal Growth 216 (2000) 57. [20] M.-E. Pistol, N. Carlsson, C. Persson, W. Seifert, L. Samuelson, Appl. Phys. Lett. 67 (1995) 1438. [21] A. Krost, F. Heinrichsdorff, D. Bimberg, A. Darhuber, G. Bauer, Appl. Phys. Lett. 68 (1996) 785.
Solid State Communications 117 (2001) 465±469
www.elsevier.com/locate/ssc
Arsenic/phosphorus exchange and wavelength tuning of in situ annealed InAs/InP quantum dot superlattice Q.D. Zhuang*, S.F. Yoon, H.Q. Zheng School of Electrical and Electronic Engineering, Block S1, Nanyang Technological University, Nanyang Avenue, Singapore, Singapore 639798 Received 27 August 2000; received in revised form 13 November 2000; accepted 24 November 2000 by H. Akai
Abstract We report the solid-source molecular beam epitaxial (SSMBE) growth of InAs/InP quantum dots (QDs) superlattices and the effect of As/P exchange. The InAs QDs were found to have an average lateral diameter of ,40 nm and density of 3 to 4 £ 10 10 cm 22. The single-layer QDs have photoluminescence (PL) emission centred at 0.78 eV with a linewidth of 64 meV at low temperature (4 K). Double-crystal X-ray diffraction (DCXRD) spectra showed evidence of signi®cant As/P exchange during in situ annealing under P2 pressure before growing the spacer layer. An average P composition of ,30% in the resulting InAsP QDs in samples annealed for 50 s was deduced from dynamical simulations of the experimental DCXRD spectra. The QDs superlattice PL emission exhibits a blueshift with increase of annealing time, and emission at 1.55 mm at 300 K was achieved. This observation holds promise for possible telecommunication device applications at long wavelength. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: B. Epitaxy; D. Optical properties; E. Luminescence PACS: 61.10.-i; 68.65.-k; 68.35.-p; 78.67.-Hc
1. Introduction Presently, there is extensive research on the fabrication of self-organised quantum dots (QDs) via the Stranski±Krastanow growth mode, which is induced by the lattice mismatch between the epitaxial layer and substrate. These QDs were reported to have promising electrical and optical properties [1,2]. The incorporation of QDs was found to lead to large improvements in device performance, such as in lasers [3], detectors [4], and optical memories [5]. For example, lasers based on QDs have been shown to have lower threshold current density and higher characteristic temperature in comparison to quantum well (QW) lasers. Injection lasers containing InAs and InGaAs QDs embedded in a GaAs and AlGaAs matrix have been successfully demonstrated and showed improved characteristics as predicted [6,7]. However, the longest emission wavelength for In(Ga)As/GaAs QDs is 1.3 mm [8,9], and lasing occurred * Corresponding author. Tel.: 165-793-3318; fax: 165-7904528. E-mail address: [email protected] (Q.D. Zhuang).
at shorter wavelength owing to state ®lling in the QDs discrete quantum levels [10], which restricts applications in optical ®bre communication systems. From a practical point of view, it is desirable to realise QDs lasers whose lasing wavelength is in the range of 1.3±1.55 mm, which are suitable for optical ®bre communication systems. It has been reported that InAs QDs grown on an InP substrate showed photoluminescence (PL) emission at around 1.5 mm at 2 K [11]. The effective masses of the carriers in the strained InAs quantum structures grown on InP are smaller than those grown on GaAs, and their energy levels are further apart. This has a bene®cial effect for improving the thermal stability of the devices based on this material system [12]. Despite these advantages, there are limited reports on InAs/InP QDs [13±16], due in part to the small mismatch (3.2%) and the complexity of the QD formation mechanism associated with As/P exchange reactions at the growth surface. Although the effect of As/P exchange on the modi®cation of InAs QDs size, and on the tuning of the PL emission from QDs have been reported recently [14,15], the detailed mechanism of the As/P exchange is still relatively unclear.
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(00)00506-8
466
Q.D. Zhuang et al. / Solid State Communications 117 (2001) 465±469
2. Experimental procedure
ÊFig. 1. TEM plan-view of single layer InAs QDs capped by a 350-A thick InP layer.
In this paper, we report the preparation of an InAs/InP QDs superlattice grown by solid-source molecular beam epitaxy (SSMBE), and investigate the As/P exchange behaviour in QDs during the in situ annealing process under P2 pressure before the growth of the spacer layer. It is observed that large InAs QDs (average of ,40 nm in diameter) with a density of ,3 to 4 £ 10 10 cm 22 were successfully prepared. The results of in situ annealing demonstrated that signi®cant As/P exchange occurs in the QDs, as deduced from dynamical simulations of the double-crystal X-ray diffraction (DCXRD) spectra. Furthermore, the PL emission wavelength of the QDs can be tuned effectively. Through this method, a QDs superlattice, which emits at 1.55 mm, has been successfully achieved.
The samples were grown in an SSMBE system equipped Ê with valved As and P cracker sources. After growing 500 A of InP buffer layer on the InP substrate at 5008C at a growth Ê s 21, a ®ve-period InAs/InP QDs superlattice, rate of 1 A Ê -thick InAs layers separated by a which consists of 20-A Ê -thick InP spacer layer was prepared. To prevent 200-A group V intermixing, the P valve was closed for 3 s, followed by opening of the As valve for 3 s, before the Ê InAs InAs growth. The QDs were formed with a 20-A deposition at 5008C, and followed by a 60-s annealing under arsenic ¯ux. In order to investigate the As/P exchange effect on the QDs formation, four samples (samples A±D) of ®ve-period QDs superlattices were prepared with in situ annealing under P2 pressure for 0, 5, 20, and 50 s, respecÊtively, at 5008C. All the samples were ®nished with a 350-A thick InP cap layer. For the sample to be subjected to the transmission electron microscopy (TEM) measurements, a Ê -thick InP cap layer was grown after the single layer 350-A of InAs QDs deposition. The QDs formation was monitored in situ by re¯ection high-energy electron diffraction oscillation (RHEED) observation. The morphology of QDs is characterised by TEM. The low temperature of 4 K and room temperature PL were used to characterise the optical properties of the QDs, the PL was excited by an Ar 1 ion laser and detected by a cooled Ge detector, and DCXRD measurements were performed with a diffractometer.
3. Results and discussion Fig. 1 shows the plan-view of the TEM image of a typical InAs QDs structure grown on an InP (100) substrate. It is clearly found that the QDs exhibit an average lateral Ê with a density in the range of 3 to diameter of ,400 A 10 22 4 £ 10 cm . The QDs dimension is smaller than the
Fig. 2. Low-temperature (4 K) PL spectrum of single InAs QD layer with InP cap layer.
Q.D. Zhuang et al. / Solid State Communications 117 (2001) 465±469
467
Fig. 3. PL spectra at room temperature of InAs/InP QDs superlattice samples annealed under P2 pressure for 0 s (sample A), 5 s (sample B), 20 s (sample C) and 50 s (sample D).
islands grown in the InAlAs matrix on the (100) InP substrate by gas-source molecular beam epitaxy (GSMBE) [17], and comparable to the QDs grown on the higher-index (311)B InP substrate [15]. Low-temperature (4 K) PL measurement of a single layer QD sample is shown in Fig. 2. A strong emission peak at ,0.78 eV with a linewidth of 64 meV was observed. The narrower linewidth compared with that previously reported (,90 meV), indicates the improvement in the size distribution [18]. Fig. 3 shows the room-temperature PL spectra of the InAs/InP QDs superlattice samples, which were subjected to different annealing times of 0 s (sample A), 5 s (sample B), 20 s (sample C) and 50 s (sample D). It shows clearly that the QDs superlattice PL emission shifts to higher energy
Fig. 4. Experimental (004) DCXRD rocking curves of samples A±D (inset ®gure), and the corresponding simulated spectrum of sample A (curve SA) and D (curve SD).
with increase of annealing time from 0 to 50 s. A shift in energy of up to 53 meV was observed in the sample annealed for 50 s (sample D). Note that at annealing time of 5 s (sample B), the QDs superlattice PL emission is centred around 0.8 eV, which corresponds to an emission wavelength around 1.55 mm. It is worth pointing out that the room temperature PL emission peak energy of sample A is around 0.784 eV, which is nearly the same as the sample of single-layer QDs measured at low temperature. This coincidence indicates the shortening of PL emission wavelength of the multi-QDs structure compared with single QDs, and should not be attributed to the change of QDs size and shape caused by the multi-layer structure owing to the wide spacer layer. We have reported that the redshift of PL emission from InAs/InP QDs on the temperature is weak, and only a 10-meV redshift is observed from 4 K to room temperature [19]. Here, we propose that the coincidence is caused by the longer growth time for the multi-layer structure, which leads to a possible As/P exchange, hence weakening quantum con®nement, and shortening PL emission consequently. The modi®cation of the QDs PL emission caused by the annealing process could be attributed to the As/P exchange at the interface during the annealing process under P2 pressure. It has been reported that the As/P exchange can cause a signi®cant change in the QDs aspect ratio through enhancing exchange reactions at the periphery of the QDs base due to the concentration of localised strain at the periphery of the QDs base [14]. Signi®cant reduction of QDs height was observed during the InP cap layer growth process, which is attributed to the As/P exchange [15]. We propose that this reduction occurs during the annealing process, and increases the quantum con®nement, which shortens the QDs PL emission wavelength consequently. Furthermore, the As/ P exchange could cause the formation of InAs12xPx QDs, which directly reduces the QDs PL emission wavelength because of its larger bandgap. Certainly, the formation of InAs12xPx QDs could also reduce the compressive stress in
468
Q.D. Zhuang et al. / Solid State Communications 117 (2001) 465±469
the QDs, and hence increase the emission wavelength [20], but this effect is relatively weak. Therefore, the net effect of the formation of InAs12xPx QDs is to reduce the PL emission wavelength as observed. The DCXRD technique is a very sensitive technique for measuring the structural parameters and inter-diffusivity effects in composition-modulated thin ®lms. Hence, DCXRD measurements on the samples were taken, and the spectra simulated using the dynamical theory to evaluate the As/P exchange effects in the superlattice. Fig. 4 shows the measured (004) DCXRD rocking curves of samples A (0 s annealing), B (5 s annealing), C (20 s annealing), and D (50 s annealing) (shown as inset ®gure), as well as the corresponding simulated spectra of sample A (curve SA) and D (curve SD). From the inset ®gure, a signi®cant shift of the zero-order satellite peak to the InP substrate following an increase in annealing time was observed, indicating a relaxation of the strain. We propose that the strain relaxation could be attributed to the As/P exchange during the annealing process. Krost et al. [21] have demonstrated that rocking curve simulations based on the dynamical theory could be used to determine the indium composition in the InGaAs QDs. In our simulations, the QDs superlattice was treated as a quantum-well-like superlattice. It can be seen that curve ®tting of the simulated DCXRD spectrum to the measured DCXRD spectrum of sample A reveals a close match of the superlattice structural parameters to the designed values Ê InAs and 200 A Ê InP). The As/P exchange during the (20 A annealing process causes an incorporation of P atoms into the InAs layers. It is expected that the effective P composition in the InAs layers will increase with the increase in annealing time. Dynamical simulations of the measured DCXRD spectra revealed that the P composition in samples B±D, which best matches the experimental spectrum, are 0.05, 0.15, and 0.3, respectively. These results clearly show that the As/P exchange that occurs on the surface is anomalously large. The effect of this large As/P exchange on the surface growth kinetics enhances the complexity of the QDs formation process. According to the dynamical simulation results of sample D, where the P composition was determined to be 0.3, the annealing process could cause bandgap widening of 230 meV, and will lead to a large blue shift of the QDs PL emission. However, in practice, sample D only exhibited a much smaller blue shift of only 53 meV (Fig. 3). In fact, the InAs-grown surface is mainly covered by an InAs wetting layer, the P is mainly incorporated into the InAs layers; thus the P composition measured by DCXRD is mainly associated to the InAs wetting layers, then it is possible that the P composition in the QDs is less than the measured value. Furthermore, the discrepancy could be attributed partly to the modi®cation of the QDs size, and stress reduction caused by As/P exchange during the annealing process as previously discussed. Experiments are in progress to further understand the As/P exchange effect on the QDs size modi®cation.
Since the As/P exchange mechanism at the QDs surface is a complex phenomenon, the question remains whether the QDs are still present after annealing for 50 s. Observation of spotty RHEED patterns after P2 annealing for 50 s suggests that the QDs are still present. Furthermore, it was found that three-dimensional structures of the QDs are still observable from TEM examination of the samples. However, further annealing for a longer time of ,100 s results in the disappearance of the QDs. It is possible that under a longer annealing time, the QDs have expanded and merged to form quantum-well-like structures. 4. Conclusions In conclusion, we report the preparation of InAs/InP QDs superlattice structures using SSMBE. The effect of As/P exchange on the InAs QDs during in situ annealing under P2 pressure was investigated. Strong PL emission at 4 K was observed from single-layer InAs QDs. In the case of InAs/ InP QDs superlattice structures, annealing under P2 pressure has the effect of reducing the PL emission (room temperature) wavelength due to the formation of InAs12xPx QDs arising from As/P exchange. Dynamical simulations of the DCXRD spectra provided evidence of increase in the P composition with increase of annealing time. Hence, our study shows that annealing under P2 pressure is an effective means for tuning the PL emission wavelength. Room temperature emission at 1.55 mm has been observed in the QDs superlattice structure annealed for 5 s.
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