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GaAs surface quantum dots with high areal density
Physica E 42 (2010) 2455–2459
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Physica E journal homepage: www.elsevier.com/locate/physe
Abnormal temperature dependent photoluminescence of self-assembled InAs/GaAs surface quantum dots with high areal density X.L. Zhou, Y.H. Chen n, J.Q. Liu, B. Xu, X.L. Ye, Z.G. Wang Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China
a r t i c l e in fo
abstract
Article history: Received 26 April 2010 Received in revised form 1 June 2010 Accepted 4 June 2010 Available online 15 June 2010
We have investigated temperature dependent photoluminescence of both buried and surface selfassembled InAs/GaAs quantum dots with an areal density up to 1011/cm2. Different from the buried quantum dots, the peak energy of surface quantum dots shows a blueshift relative to the bulk material variation from 15 to 130 K. Besides the line width and the integrated intensity both first decrease and then increase in this temperature interval. The observed phenomena can be explained by carrier trapping effects by some shallow localized centers near the surface quantum dots. & 2010 Elsevier B.V. All rights reserved.
Keywords: Quantum dots Temperature dependent Photoluminescence Surface localized centers
1. Introduction In the last few years, semiconductor quantum dots (QDs) have attracted much attention due to their widely potential application in optoelectric devices such as near-infrared lasers and detectors [1–4], or in quantum communication areas such as solid single photon source [5,6]. The quantum dots are often formed via the strain induced Stranski–Krastanow (S–K) mode and capped with the barrier materials to confine the injected carriers into dots. To study the carrier dynamic of QDs system, different methods have been adopted, e.g., time resolved photoluminescence [7,8], deep levels transient spectrum [9]; however, the general photoluminescence is common due to its simplicity. For the variabletemperature photoluminescence of the buried QDs (BQDs), it has been frequently observed that a rapid peak redshift, as well as an S-shaped full width at half maximum (FWHM), occurs in the midtemperature interval of 100–200 K [10–12]. Such phenomena can be attributed to the thermal-assisted carrier redistribution between QDs with different energy states, which can be viewed as a signature of inhomogeneous size distribution of the QDs system. However, very few studies have concentrated on the temperature dependent PL of QDs without capping layers, i.e., surface quantum dots (SQDs), which may be due to their poor optical behaviors. For SQDs, it has been observed that the temperature dependence presents a peak redshift similar to the variation of bulk material and an almost invariant FWHM in
n
Corresponding author. E-mail address: [email protected] (Y.H. Chen).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.06.008
the whole temperature interval [13]. Additionally, the intensity quenching of SQDs is also demonstrated to be faster than in the BQDs, which is caused by its smaller activation energy [14]. However, the temperature dependence of SQDs may be different for specific growth structures, which is still not very clear. On the other hand, in spite of the poor optical performance, selfassembled SQDs can be a promising sensor candidate for sensing biological agents than surface bonded colloidal QDs due to their sensitive response to surface environments [15]. Recently, Liang et al. [15–18] have systematically studied the correlation between BQDs and SQDs. They found that optical performance of SQDs can be tuned by varying the growth structure of multilayer BQDs beneath. Besides, it is also observed that when the space layer thickness is decreased, the carrier transfer from BQDs to SQDs can be enhanced. Especially, the authors have pointed that such carrier transfer is sensitive to the surface environment and may be influenced by carrier transfer between some surface states and SQDs [15]. However, such carrier transfer mechanisms, which are important for the design of surface sensitive structures, are still not well understood. So, temperature dependent photoluminescence, which often supplies the information of carrier transfer between different QDs assemblies, is also expected to be a useful tool to clarify the mechanism of carrier transfer between SQDs and surface states. In this paper, we study an SQDs-contained sample that with a high sheet density up to 1.3 1011/cm2 is expected to enhance the optical response. The PL from SQDs can be maintained even at room temperature. An abnormal temperature dependence of PL is observed and the experimental results are discussed in terms of carrier processes between the SQDs and some localized surface states.
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the lower energy peak is only one-fifth of the high energy one. So it is almost impossible for the high energy peak to originate from the excited states, which are often observed to be weaker due to the fast carrier relaxation between confined sublevels. Besides, the energy difference between the two peaks is about 190 meV, which is also much higher than the general energy difference between excited states and ground states. Additionally, the peaks that come from the multimodal distributed QDs can also be excluded according to the Gaussian shaped height distribution histogram. So here it can be sure that the high energy peak comes from the ground states of buried quantum dots, and the low energy peaks can be attributed to the emission of surface ones. The large peak redshift of SQDs can be first attributed to reduced compressive strain inside the dots without the GaAs capping layer. An estimation based on a simple hemisphere mode has shown that the strain induced redshift is about 200 meV [19], which is close to our results. Besides, the peak redshift and the broadening of PL line width of SQDs can also be caused by the lack of In/Ga atom interdiffusion at the interfaces between InAs QDs and the surrounding GaAs barrier [20]. The In/Ga interdiffusion at a high growth temperature is expected to increase the bandgap and to decrease the inhomogeneous distribution of QDs materials, which is similar to that observed after the postgrowth rapid thermal annealing process [21,22]. Additionally, the weaker emission intensity of SQDs than BQDs may be ascribed to two reasons: the first is the large amount of surface states arising from the dangling bonds or defects [13,14], which means most of the carrier cannot be injected into the quantum dots and may be lost non-radiatively. Another possible explanation can be the small average dot height of only 1.4 nm. Wang et al. [13] have observed that there is no or few confined levels for the SQDs, especially for those with small sizes. The small size also means weak
2. Experiments High density surface QDs sample was grown on a GaAs (0 0 1) substrate in our Riber 32p molecular beam epitaxy (MBE) system. The growth structure is shown in Fig. 1(a). After oxide desorption at 580 1C, a 200 nm GaAs buffer layer was grown at 610 1C. Subsequently, a 10 GaAs/Al0.25Ga0.75As (10 nm/20 nm) superlattice was grown at the rate of 0.6 mm/h. Later, the temperature was cooled down to 510 1C for 2 monolayer (ML) InAs deposition for QDs formation. Finally, the temperature was raised to 610 1C again and a 10 GaAs/Al0.25Ga0.75As layer (10 nm/20 nm) was grown as the capping layer. The surface QDs were then grown at the same condition as those of the buried quantum dots except the capping layer. The surface morphologies of QDs were characterized by our Solver P47 AFM at the contact mode. Continuous wave PL measurements were performed at a Fourier transform infrared spectrometer setup equipped with an In(Ga)As detector. The samples were mounted in a cryostatat temperature from 15 to 300 K and excited by a 532 nm solid state laser with an utmost excitation power of 100 mW.
3. Results and discussion The height statistic histogram and 1 mm 1 mm AFM image are shown in Fig. 1(b). The size distribution holds a single Gaussian shape. The areal density of QDs is 1.3 1011/cm2 and the corresponding average height and diameter are 34.3 nm and 1.4 nm, respectively. The PL spectra measured at room temperature are shown at Fig. 1(c). It is clear that there are two peaks: one is centered at 1.12 eV with an FWHM of 74 meV and the other is centered at 0.93 eV with an FWHM of 112 meV. The intensity of
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Fig. 1. Growth structure (a) and AFM height statistic histogram (b) of the SQD sample. Inset of (b): AFM image (1 mm 1 mm). (c) Photoluminescence spectra measured at room temperature of both BQDs and SQDs. Dashed lines are Gaussian shape fitting.
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overlapping of electron and hole wavefunctions, which gives rise to poor radiative recombination strength [23]. However, in our case, the strong PL from SQDs should benefit from the high sheet density, which supplies enough energy states for carrier population. Besides, the direct carrier capture into SQDs is expected to be the dominant injection mechanism for the high density sample [24], which may reduce the non-radiative loss via wetting layer or interface defects. The variable-temperature photoluminescence measured from 15 to 280 K is shown in Fig. 2. To observe the PL spectra from SQDs more clearly, the whole spectra are normalized according to the BQDs because they become much stronger than SQDs at lower temperature. The absolute spectra of SQDs are also shown in the
Fig. 2. Normalized photoluminescence measured from 15 to 280 K. Inset shows the enlarged figure of PL from SQDs.
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inset figure for clarity. Both the BQDs and SQDs hold a Gaussian shape PL and keep almost separate from each other in the entire temperature interval, which is suitable for further shape fitting. The temperature dependent peak energies of BQDs and SQDs are shown in Fig. 3(a). In the temperature interval 80–160 K, the peak energy of BQDs decreases faster than that of the InAs bulk material, i.e., the Varshni law [25]. Accordingly, as shown in Fig. 3(b), the FWHM variation of BQDs first decreases and then increases with increasing temperature. This can be explained by the thermally activated carrier transfer from small size quantum dots to large size ones. Differently, the peak variation of SQDs shows a strange shape. There is a slower redshift below 130 K compared to the Varshni law. The maximum relative blueshift is about 13 meV at 130 K, which cannot be treated trivially. On the other hand, the FWHM first decreases slightly from 15 to 75 K and then increases strongly up to 130 K with an utmost increment of 60 meV. Meanwhile, as shown in Fig. 3(c), the intensity of SQDs also first decreases and then increases, similar to the FWHM. The intensity almost reduces by 40% from 15 to 75 K and then recovers again from 75 to 130 K. Now we discuss the abnormal temperature dependence in the interval 15–130 K. Generally, the PL peak energy of QDs ensemble reflects the center of carrier population in the inhomogeneously distributed energy states. The slower redshift of SQDs means that the center of carrier population moves to the high energy side with increase in temperature, which implies a preferential population of small size quantum dots with higher energy levels. On the other hand, the FWHM can reflect the energy range of populated states. The FWHM variation of SQDs implies some quantum dots are not occupied below 75 K but more are occupied above 75 K. Similarly, the PL intensity also varies according to the number of captured carriers by SQDs. So it can be imagined that in the low temperature interval, the carrier capture into surface quantum dots must be first impeded and then recovered again as temperature increases. It can be first excluded that the abnormal
Fig. 3. Temperature dependence of peak energy (a), FWHM (b) and normalized intensity (c) of BQDs (circle) and SQDs (square). The red dashed and solid lines in (c) represent fitting of intensity quenching for BQDs and SQDs according to the Arrhenius formula.
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temperature dependence of SQDs originates from the correlation between SQDs and BQDs due to the large space layer thickness of several hundred nanometers in our sample, which is much different from that of Ref. [15]. On the other hand, the temperature is also very different from that reported in Refs. [16,17], where a rapid thermal quenching of SQDs emission was observed. However, it is noted that such abnormal temperature dependence is very similar to that of disordered group-III nitride structures, which show an S-shaped peak energy dependence and W-shaped line width dependence caused by thermally assisted exciton motion via hopping through localized states at the interface of heterostructures [26,27]. In our case, such localized states can also exist because the surface is exposed without a capping layer. Surface fluctuation, dangling bond and surface defects (e.g., dislocations) can be viewed as the localized centers for carrier capture. The growth rate of our sample is so high that quite a number of quantum dots are still waiting to increase, i.e., immature QDs-like precursors [28]. From the height statistic histogram above, there are many quantum dots with a height below 1 nm, which can be viewed as QDs precursors. Such small precursors almost cannot capture the carrier for radiative recombination due to their less-confined levels. However, they can be viewed as shallow localized centers for carrier trapping. As illustrated by the inset figure of Fig. 3(c), there are some localized centers in the neighbor of QDs with a higher energy level than the QDs ground states. As the temperature is very low, the injected carriers can be mostly trapped by these localized centers but not the QDs. The initial temperature increase from 15 to 75 K can enhance carrier thermal diffusion and hopping via these localized centers, so as to enhance the carrier trapping probability. However, as temperature increases more, the carrier can be excited out of such localized centers due to their weaker quantum confinement, and then captured by the neighboring quantum dots. Such processes can explain first decrease and then the increase of both FWHM and intensity from 15 to 130 K. However, the peak blueshift seems to indicate that such a process is more obvious for those size quantum dots with high energy levels. There may be two explanations: first, as mentioned above, the immature precursors (i.e., localized centers) are more extensively distributed around the small size QDs than the large size ones. As observed by several reports, the large size QDs are formed by consuming the material of their neighboring immature ones [28]. So the number of immature precursors around the large size QDs is thought to be less than those around the small size ones. More localized centers around small size QDs means enhanced carrier exchange between the two, which gives rise to the more obvious carrier population of small QDs as temperature increases. Another explanation is based on the different capture efficiencies for QDs of different sizes. It is often accepted that the QDs with large size often have larger capture cross section than the smaller ones [24]. So the large size quantum dots are more easily populated than the smaller ones at the lowest temperature. The population increment is then expected to be more prominent for small QDs due to the saturation of larger ones as temperature increases. To further demonstrate the second explanation, the power variable experiments at 15 K are performed for the buried quantum dots. As shown in Fig. 4, the normalized PL from 1 to 100 mW shows a peak blueshift of about 5 meV, as well as an FWHM broadening of 6 meV. This means, as the power is low, carrier capture efficiency is much higher for large quantum dots than for small ones. So the power induced population is more obvious for small QDs due to their small population initially. However, it has to be stressed that such size dependent capture differences are due to high density, as well as the large inhomogeneous broadening of our sample, which both supply enough energy difference to be reflected from the photoluminescence spectra.
Fig. 4. Normalized PL of BQDs at the excitation power 1, 10 and 100 mW measured at 15 K.
In the higher temperature interval above 130 K, the peak energy of SQDs varies according to the bulk material gradually and the FWHM begins to drop again, accompanied by quenching of PL intensity. The quenching can be fitted using the Arrhenius relation and the activation energies are 133 and 60 meV for BQDs and SQDs, respectively. The smaller activation energy of SQDs can be attributed to the nonradiative role of surface defects.
4. Conclusion In summary, we have grown high density surface quantum dots without capping layers. The surface QDs emission can be maintained even at room temperature and is comparable with the PL from buried QDs. The temperature dependent PL of surface QDs presents an extraordinary variation compared with the buried ones, as well as other reported results in the literature. In the interval 15–130 K, the slower peak redshift, first decrease then increase variation of line width and intensity, can be explained by the thermal transfer processes between some surface localized states and the surface quantum dots, which is especially obvious for the small size QDs due to their smaler carrier population initially. It is demonstrated that the carrier processes between surface QDs and the localized surface states can be studied via temperature dependent photoluminescence and further studies are needed to elucidate the detailed influence of surface states on the carrier transfer between buried and surface quantum dots in the vertically coupled system.
Acknowledgments This work was supported by the National Natural Science Foundation of China (nos. 60625402 and 60990313), and the 973 Program (2006CB604908 and 2006CB921607). References [1] A.E. Zhukov, A.R. Kovsh, V.M. Ustinov, Y.M. Shernyakov, S.S. Ruvimov, N.A. Maleev, G. Musikhin, N.N. Ledentsov, P.S. Kop’ev, D. Bimberg, IEEE Photon. Technol. Lett. 11 (1999) 1345. [2] G. Park, O.B. Shchekin, D.L. Huffaker, D.G. Deppe, IEEE Photon. Technol. Lett. 13 (2000) 230. [3] E.T. Kim, A. Madhukar, Z. Ye, J.C. Campbell, Appl. Phys. Lett. 84 (2004) 3277.
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[4] S.D. Gunapala, S.V. Bandara, C.J. Hill, D.Z. Ting, J.K. Liu, S.B. Rafol, E.R. Blazejewski, J.M. Mumolo, S.A. Keo, S. Krishna, Y.C. Chang, C.A. Shott, Proc. SPIE 6206 (2006) 62060J. [5] C. Santori, M. Pelton, G.S. Solomon, Y. Dale, Y. Yamamoto, Phys. Rev. Lett. 86 (2001) 1502. [6] B. Lounis, M. Orrit, Rep. Prog. Phys. 68 (2005) 1129. [7] A. Fiore, P. Borri, W. Langbein, J.M. Hvam, U. Oesterle, R. Houdre, R.P. Stanley, M. Ilegems, Appl. Phys. Lett. 76 (2000) 3430. [8] F. Pulizzi, A.J. Kent, A. Patane, L. Eaves, M. Henini, Appl. Phys. Lett. 84 (2004) 3046. [9] A. Schramm, S. Schulz, Ch. Heyn, W. Hansen, Phys. Rev. B 77 (2008) 153308. [10] Z.Y. Xu, Z.D. Lu, X.P. Yang, Z.L. Yuan, B.Z. Zheng, J.Z. Xu, W.K. Ge, Y. Wang, L.L. Chang, Phys. Rev. B 54 (1996) 11528. [11] Y.T. Dai, J.C. Fan, Y.F. Chen, J. Appl. Phys. 82 (1997) 4489. [12] S. Sanguinetti, M. Henini, M. Grassi Alessi, M. Capizzi, Pl Frigeri, S. Franchi, Phys. Rev. B 60 (1999) 8276. [13] J.Z. Wang, Z. Yang, C.L. Yang, Appl. Phys. Lett. 77 (2000) 2837. [14] Z.F. Wei, S.J. Xu, R.F. Duan, Q. Li, J. Wang, J. Appl. Phys. 98 (2005) 084305. [15] B.L. Liang, Zh.M. Wang, Yu.I. Mazur, G.J. Salamo, Appl. Phys. Lett. 89 (2006) 043125.
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[16] B.L. Liang, Zh.M. Wang, Yu.I. Mazur, G.J. Salamo, Appl. Phys. Lett. 89 (2006)243124 89 (2006). [17] B.L. Liang, Zh.M. Wang, Yu.I. Mazur, Sh. Seydmohamadi, M.E. Ware, G.J. Salamo, Opt. Express 15 (2007) 8157. [18] B.L. Liang, Yu.I. Mazur, Vas.P. Kunets, Zh.M. Wang, G.J. Salamo, E.A. DeCuir Jr., B. Passmore, M.O. Manasreh, Nanotechnology 19 (2008) 065705. [19] H. Saito, K. Nishi, S. Sugou, Appl. Phys. Lett. 73 (1998) 2742. [20] R. Leon, Y. Kim, C. Jagadish, M. Gal, J. Zou, D.J.H. Cockayne, Appl. Phys. Lett. 69 (1888) 1996. [21] S. Malik, C. Roberts, R. Murray, Appl. Phys. Lett. 71 (1987) 1997. [22] Z.L. Miao, Y.W. Zhang, S.J. Chua, Y.H. Chye, P. Chen, S. Tripathy, Appl. Phys. Lett. 86 (2005) 031914. [23] J. Johansen, S. Stobbe, I.S. Nikolaev, T.L. Hansen, P.T. Kristensen, J.M. Hvam, W.L. Vos, P. Lodahl, Phys. Rev. B 77 (2008) 073303. [24] F.V. de Sates, J.M.R. Cruz, S.W. de Silva, M.A.G. Soler, P.C. Morais, M.J. da Silva, A. Quivy, J.R. Leite, J. Appl. Phys. 94 (2003) 1787. [25] Y.P. Varshni, Physica 34 (1967) 149. [26] Y.-H. Cho, T.J. Schmidt, S. Bidnyk, G.H. Gainer, J.J. Song, S. Keller, U.K. Mishra, S.P. DenBaars, Phys. Rev. B 61 (2000) 7571. [27] P.G. Eliseev, P. Perlin, J. Lee, M. Osinski, Appl. Phys. Lett. 71 (1997) 569. [28] T.J. Krzyzewski, P.B. Joyce, G.R. Bell, T.S. Jones, Phys. Rev. B 66 (2000) 121307.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Abnormal temperature dependent photoluminescence of self-assembled InAs/GaAs surface quantum dots with high areal density X.L. Zhou, Y.H. Chen n, J.Q. Liu, B. Xu, X.L. Ye, Z.G. Wang Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China
a r t i c l e in fo
abstract
Article history: Received 26 April 2010 Received in revised form 1 June 2010 Accepted 4 June 2010 Available online 15 June 2010
We have investigated temperature dependent photoluminescence of both buried and surface selfassembled InAs/GaAs quantum dots with an areal density up to 1011/cm2. Different from the buried quantum dots, the peak energy of surface quantum dots shows a blueshift relative to the bulk material variation from 15 to 130 K. Besides the line width and the integrated intensity both first decrease and then increase in this temperature interval. The observed phenomena can be explained by carrier trapping effects by some shallow localized centers near the surface quantum dots. & 2010 Elsevier B.V. All rights reserved.
Keywords: Quantum dots Temperature dependent Photoluminescence Surface localized centers
1. Introduction In the last few years, semiconductor quantum dots (QDs) have attracted much attention due to their widely potential application in optoelectric devices such as near-infrared lasers and detectors [1–4], or in quantum communication areas such as solid single photon source [5,6]. The quantum dots are often formed via the strain induced Stranski–Krastanow (S–K) mode and capped with the barrier materials to confine the injected carriers into dots. To study the carrier dynamic of QDs system, different methods have been adopted, e.g., time resolved photoluminescence [7,8], deep levels transient spectrum [9]; however, the general photoluminescence is common due to its simplicity. For the variabletemperature photoluminescence of the buried QDs (BQDs), it has been frequently observed that a rapid peak redshift, as well as an S-shaped full width at half maximum (FWHM), occurs in the midtemperature interval of 100–200 K [10–12]. Such phenomena can be attributed to the thermal-assisted carrier redistribution between QDs with different energy states, which can be viewed as a signature of inhomogeneous size distribution of the QDs system. However, very few studies have concentrated on the temperature dependent PL of QDs without capping layers, i.e., surface quantum dots (SQDs), which may be due to their poor optical behaviors. For SQDs, it has been observed that the temperature dependence presents a peak redshift similar to the variation of bulk material and an almost invariant FWHM in
n
Corresponding author. E-mail address: [email protected] (Y.H. Chen).
1386-9477/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2010.06.008
the whole temperature interval [13]. Additionally, the intensity quenching of SQDs is also demonstrated to be faster than in the BQDs, which is caused by its smaller activation energy [14]. However, the temperature dependence of SQDs may be different for specific growth structures, which is still not very clear. On the other hand, in spite of the poor optical performance, selfassembled SQDs can be a promising sensor candidate for sensing biological agents than surface bonded colloidal QDs due to their sensitive response to surface environments [15]. Recently, Liang et al. [15–18] have systematically studied the correlation between BQDs and SQDs. They found that optical performance of SQDs can be tuned by varying the growth structure of multilayer BQDs beneath. Besides, it is also observed that when the space layer thickness is decreased, the carrier transfer from BQDs to SQDs can be enhanced. Especially, the authors have pointed that such carrier transfer is sensitive to the surface environment and may be influenced by carrier transfer between some surface states and SQDs [15]. However, such carrier transfer mechanisms, which are important for the design of surface sensitive structures, are still not well understood. So, temperature dependent photoluminescence, which often supplies the information of carrier transfer between different QDs assemblies, is also expected to be a useful tool to clarify the mechanism of carrier transfer between SQDs and surface states. In this paper, we study an SQDs-contained sample that with a high sheet density up to 1.3 1011/cm2 is expected to enhance the optical response. The PL from SQDs can be maintained even at room temperature. An abnormal temperature dependence of PL is observed and the experimental results are discussed in terms of carrier processes between the SQDs and some localized surface states.
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the lower energy peak is only one-fifth of the high energy one. So it is almost impossible for the high energy peak to originate from the excited states, which are often observed to be weaker due to the fast carrier relaxation between confined sublevels. Besides, the energy difference between the two peaks is about 190 meV, which is also much higher than the general energy difference between excited states and ground states. Additionally, the peaks that come from the multimodal distributed QDs can also be excluded according to the Gaussian shaped height distribution histogram. So here it can be sure that the high energy peak comes from the ground states of buried quantum dots, and the low energy peaks can be attributed to the emission of surface ones. The large peak redshift of SQDs can be first attributed to reduced compressive strain inside the dots without the GaAs capping layer. An estimation based on a simple hemisphere mode has shown that the strain induced redshift is about 200 meV [19], which is close to our results. Besides, the peak redshift and the broadening of PL line width of SQDs can also be caused by the lack of In/Ga atom interdiffusion at the interfaces between InAs QDs and the surrounding GaAs barrier [20]. The In/Ga interdiffusion at a high growth temperature is expected to increase the bandgap and to decrease the inhomogeneous distribution of QDs materials, which is similar to that observed after the postgrowth rapid thermal annealing process [21,22]. Additionally, the weaker emission intensity of SQDs than BQDs may be ascribed to two reasons: the first is the large amount of surface states arising from the dangling bonds or defects [13,14], which means most of the carrier cannot be injected into the quantum dots and may be lost non-radiatively. Another possible explanation can be the small average dot height of only 1.4 nm. Wang et al. [13] have observed that there is no or few confined levels for the SQDs, especially for those with small sizes. The small size also means weak
2. Experiments High density surface QDs sample was grown on a GaAs (0 0 1) substrate in our Riber 32p molecular beam epitaxy (MBE) system. The growth structure is shown in Fig. 1(a). After oxide desorption at 580 1C, a 200 nm GaAs buffer layer was grown at 610 1C. Subsequently, a 10 GaAs/Al0.25Ga0.75As (10 nm/20 nm) superlattice was grown at the rate of 0.6 mm/h. Later, the temperature was cooled down to 510 1C for 2 monolayer (ML) InAs deposition for QDs formation. Finally, the temperature was raised to 610 1C again and a 10 GaAs/Al0.25Ga0.75As layer (10 nm/20 nm) was grown as the capping layer. The surface QDs were then grown at the same condition as those of the buried quantum dots except the capping layer. The surface morphologies of QDs were characterized by our Solver P47 AFM at the contact mode. Continuous wave PL measurements were performed at a Fourier transform infrared spectrometer setup equipped with an In(Ga)As detector. The samples were mounted in a cryostatat temperature from 15 to 300 K and excited by a 532 nm solid state laser with an utmost excitation power of 100 mW.
3. Results and discussion The height statistic histogram and 1 mm 1 mm AFM image are shown in Fig. 1(b). The size distribution holds a single Gaussian shape. The areal density of QDs is 1.3 1011/cm2 and the corresponding average height and diameter are 34.3 nm and 1.4 nm, respectively. The PL spectra measured at room temperature are shown at Fig. 1(c). It is clear that there are two peaks: one is centered at 1.12 eV with an FWHM of 74 meV and the other is centered at 0.93 eV with an FWHM of 112 meV. The intensity of
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Fig. 1. Growth structure (a) and AFM height statistic histogram (b) of the SQD sample. Inset of (b): AFM image (1 mm 1 mm). (c) Photoluminescence spectra measured at room temperature of both BQDs and SQDs. Dashed lines are Gaussian shape fitting.
X.L. Zhou et al. / Physica E 42 (2010) 2455–2459
overlapping of electron and hole wavefunctions, which gives rise to poor radiative recombination strength [23]. However, in our case, the strong PL from SQDs should benefit from the high sheet density, which supplies enough energy states for carrier population. Besides, the direct carrier capture into SQDs is expected to be the dominant injection mechanism for the high density sample [24], which may reduce the non-radiative loss via wetting layer or interface defects. The variable-temperature photoluminescence measured from 15 to 280 K is shown in Fig. 2. To observe the PL spectra from SQDs more clearly, the whole spectra are normalized according to the BQDs because they become much stronger than SQDs at lower temperature. The absolute spectra of SQDs are also shown in the
Fig. 2. Normalized photoluminescence measured from 15 to 280 K. Inset shows the enlarged figure of PL from SQDs.
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inset figure for clarity. Both the BQDs and SQDs hold a Gaussian shape PL and keep almost separate from each other in the entire temperature interval, which is suitable for further shape fitting. The temperature dependent peak energies of BQDs and SQDs are shown in Fig. 3(a). In the temperature interval 80–160 K, the peak energy of BQDs decreases faster than that of the InAs bulk material, i.e., the Varshni law [25]. Accordingly, as shown in Fig. 3(b), the FWHM variation of BQDs first decreases and then increases with increasing temperature. This can be explained by the thermally activated carrier transfer from small size quantum dots to large size ones. Differently, the peak variation of SQDs shows a strange shape. There is a slower redshift below 130 K compared to the Varshni law. The maximum relative blueshift is about 13 meV at 130 K, which cannot be treated trivially. On the other hand, the FWHM first decreases slightly from 15 to 75 K and then increases strongly up to 130 K with an utmost increment of 60 meV. Meanwhile, as shown in Fig. 3(c), the intensity of SQDs also first decreases and then increases, similar to the FWHM. The intensity almost reduces by 40% from 15 to 75 K and then recovers again from 75 to 130 K. Now we discuss the abnormal temperature dependence in the interval 15–130 K. Generally, the PL peak energy of QDs ensemble reflects the center of carrier population in the inhomogeneously distributed energy states. The slower redshift of SQDs means that the center of carrier population moves to the high energy side with increase in temperature, which implies a preferential population of small size quantum dots with higher energy levels. On the other hand, the FWHM can reflect the energy range of populated states. The FWHM variation of SQDs implies some quantum dots are not occupied below 75 K but more are occupied above 75 K. Similarly, the PL intensity also varies according to the number of captured carriers by SQDs. So it can be imagined that in the low temperature interval, the carrier capture into surface quantum dots must be first impeded and then recovered again as temperature increases. It can be first excluded that the abnormal
Fig. 3. Temperature dependence of peak energy (a), FWHM (b) and normalized intensity (c) of BQDs (circle) and SQDs (square). The red dashed and solid lines in (c) represent fitting of intensity quenching for BQDs and SQDs according to the Arrhenius formula.
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temperature dependence of SQDs originates from the correlation between SQDs and BQDs due to the large space layer thickness of several hundred nanometers in our sample, which is much different from that of Ref. [15]. On the other hand, the temperature is also very different from that reported in Refs. [16,17], where a rapid thermal quenching of SQDs emission was observed. However, it is noted that such abnormal temperature dependence is very similar to that of disordered group-III nitride structures, which show an S-shaped peak energy dependence and W-shaped line width dependence caused by thermally assisted exciton motion via hopping through localized states at the interface of heterostructures [26,27]. In our case, such localized states can also exist because the surface is exposed without a capping layer. Surface fluctuation, dangling bond and surface defects (e.g., dislocations) can be viewed as the localized centers for carrier capture. The growth rate of our sample is so high that quite a number of quantum dots are still waiting to increase, i.e., immature QDs-like precursors [28]. From the height statistic histogram above, there are many quantum dots with a height below 1 nm, which can be viewed as QDs precursors. Such small precursors almost cannot capture the carrier for radiative recombination due to their less-confined levels. However, they can be viewed as shallow localized centers for carrier trapping. As illustrated by the inset figure of Fig. 3(c), there are some localized centers in the neighbor of QDs with a higher energy level than the QDs ground states. As the temperature is very low, the injected carriers can be mostly trapped by these localized centers but not the QDs. The initial temperature increase from 15 to 75 K can enhance carrier thermal diffusion and hopping via these localized centers, so as to enhance the carrier trapping probability. However, as temperature increases more, the carrier can be excited out of such localized centers due to their weaker quantum confinement, and then captured by the neighboring quantum dots. Such processes can explain first decrease and then the increase of both FWHM and intensity from 15 to 130 K. However, the peak blueshift seems to indicate that such a process is more obvious for those size quantum dots with high energy levels. There may be two explanations: first, as mentioned above, the immature precursors (i.e., localized centers) are more extensively distributed around the small size QDs than the large size ones. As observed by several reports, the large size QDs are formed by consuming the material of their neighboring immature ones [28]. So the number of immature precursors around the large size QDs is thought to be less than those around the small size ones. More localized centers around small size QDs means enhanced carrier exchange between the two, which gives rise to the more obvious carrier population of small QDs as temperature increases. Another explanation is based on the different capture efficiencies for QDs of different sizes. It is often accepted that the QDs with large size often have larger capture cross section than the smaller ones [24]. So the large size quantum dots are more easily populated than the smaller ones at the lowest temperature. The population increment is then expected to be more prominent for small QDs due to the saturation of larger ones as temperature increases. To further demonstrate the second explanation, the power variable experiments at 15 K are performed for the buried quantum dots. As shown in Fig. 4, the normalized PL from 1 to 100 mW shows a peak blueshift of about 5 meV, as well as an FWHM broadening of 6 meV. This means, as the power is low, carrier capture efficiency is much higher for large quantum dots than for small ones. So the power induced population is more obvious for small QDs due to their small population initially. However, it has to be stressed that such size dependent capture differences are due to high density, as well as the large inhomogeneous broadening of our sample, which both supply enough energy difference to be reflected from the photoluminescence spectra.
Fig. 4. Normalized PL of BQDs at the excitation power 1, 10 and 100 mW measured at 15 K.
In the higher temperature interval above 130 K, the peak energy of SQDs varies according to the bulk material gradually and the FWHM begins to drop again, accompanied by quenching of PL intensity. The quenching can be fitted using the Arrhenius relation and the activation energies are 133 and 60 meV for BQDs and SQDs, respectively. The smaller activation energy of SQDs can be attributed to the nonradiative role of surface defects.
4. Conclusion In summary, we have grown high density surface quantum dots without capping layers. The surface QDs emission can be maintained even at room temperature and is comparable with the PL from buried QDs. The temperature dependent PL of surface QDs presents an extraordinary variation compared with the buried ones, as well as other reported results in the literature. In the interval 15–130 K, the slower peak redshift, first decrease then increase variation of line width and intensity, can be explained by the thermal transfer processes between some surface localized states and the surface quantum dots, which is especially obvious for the small size QDs due to their smaler carrier population initially. It is demonstrated that the carrier processes between surface QDs and the localized surface states can be studied via temperature dependent photoluminescence and further studies are needed to elucidate the detailed influence of surface states on the carrier transfer between buried and surface quantum dots in the vertically coupled system.
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