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GaAs(311B) surface quantum dots clearly identified by the piezoreflectance technique
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Applied Surface Science 254 (2008) 4626–4631 www.elsevier.com/locate/apsusc
Optical transitions of InAs/In0.36Ga0.64As/GaAs(311B) surface quantum dots clearly identified by the piezoreflectance technique C. Wang a,b,*, Y. Yang a, X.M. Chen c, Z.L. Liu b, H.Y. Cui b, S. Zhang a, X.S. Chen b, W. Lu b a
Research Institute of Engineering and Technology, Yunnan University, Kunming 650091, China b National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China c Basic Research Center for Detectors, Kunming Institute of Physics, Kunming 650034, China
Received 3 October 2007; received in revised form 11 January 2008; accepted 15 January 2008 Available online 26 January 2008
Abstract The bilayer InAs/In0.36Ga0.64As/GaAs(311B) quantum dots (QDs), including one InAs buried quantum dot (BQD) layer and the other InAs surface quantum dot (SQD) layer, have been grown by molecular beam epitaxy (MBE). The optical properties of these three samples have been studied by the piezoreflectance (PzR) spectroscopy. The PzR spectra do not exhibit only the optical transitions originated from the InAs BQDs, but the features originated from the InAs SQDs. After the InAs SQDs have been removed chemically, those optical transitions from InAs SQDs have been demonstrated clearly by investigating the PzR spectra of the residual InAs BQDs in these samples. The great redshift of these interband transitions of InAs SQDs has been well discussed. Due to the suitable InAs SQD sizes and the thickness of In0.36Ga0.64As layer, the interband transition of InAs SQDs has been shifted to 1.55 mm at 77 K. # 2008 Elsevier B.V. All rights reserved. Keywords: Surface quantum dots; Molecular beam epitaxy; Photoluminescence; Piezoreflectance spectroscopy
1. Introduction Inspired by the great potential applications in the various nano-optoelectronic devices, the fabrication and the characterization of self-assembled InAs quantum dots (QDs) have become an extensively research area over the last decade [1]. Some properties, such as the high QD density, the less QD size fluctuation and the controllable QD distribution in two dimensional plane, are important to meet the high quality of QD-based optoelectronic devices, while these are also the major challenge for those QDs grown on the GaAs(0 0 1) due to the self-assembled QD formation with Stranski-Krastanov growth mode influenced by many factors [2]. Growing the InAs QDs on the GaAs(311B) has been demonstrated to be an effective method to form high-density and well-ordered QD array on surface with less coalescence of dots, the inherent growth mechanism could be attributed to a complex phase separation and strain-relief mechanism [3,4]. Recently, the
* Corresponding author. Tel.: +86 871 5031869; fax: +86 871 5037399. E-mail address: [email protected] (C. Wang). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.089
InAs nucleations for forming QDs, which is based on the strain driven growth instability, have been well controlled by growing a strained modulated InxGa1 xAs template on GaAs(311B) substrate. It is found that these InAs QDs on the InxGa1 xAs template exhibit superior photoluminescence (PL) properties over that of InAs QDs grown on GaAs(0 0 1) directly [5]. Although the growth characteristics of InxGa1 xAs QDs on GaAs(311B) have been studied by the atomic force microscope (AFM) [3], PL [5] and time-resolved PL technique [6], the optical and electronic properties of these QDs on GaAs(311B) are not understood clearly at present. On the other hand, recently, a weak emission peak, which originate, from the exciton-state recombination in the InAs/GaAs(0 0 1) surface quantum dots (SQDs) has been observed with a rather broadening linewidth at 1.6 mm [7]. However, such interband transition in the InAs SQDs has not been demonstrated by the absorbed-type spectroscopy. Piezoreflectance spectroscopy (PzR) is a powerful tool to investigate the optical properties of III–V compounds semiconductors [8]. Since the PzR is an absorbed-type technique, the acquired information reflects the optical transition behaviors, which are more directly related to the
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theoretical absorption model. In addition, due to the stressmodulated nature, the PzR is believed to be more useful than other modulation spectra to study the strained QD system [9]. In this article, the PzR spectroscopy is used to study the optical properties of three InAs/In0.36Ga0.64As/GaAs(311B) SQD samples. 2. Experimental details Three InAs/In0.36Ga0.64As QD samples were grown by solid source molecular-beam epitaxy (MBE) on GaAs(311B) substrates. After oxide desorption, the 300 nm thick GaAs buffer layer was deposited at 580 8C. The temperature was decreased to 500 8C to grow the first In0.36Ga0.64As template with the thickness of 2.0, 2.5 and 2.5 nm for samples A, B and C, respectively. Then, the 0.8 ML thick InAs was deposited to grow the InAs buried quantum dots (BQDs) at 500 8C in each sample. The InAs BQDs were covered by the 75, 100, and 120 nm thick GaAs capping layers in the three samples, respectively. Finally, another In0.36Ga0.64As template and InAs QDs, which are identical with the first In0.36Ga0.64As template and InAs BQDs, were grown repeatedly in each sample to form InAs SQDs without the GaAs capping layer. That is to say, all the three samples are double QD layers in structure, including a BQD layer and a SQD one. The growth rate for growing InAs BQDs is the same as that for InAs SQDs in each sample, but that is different among the three samples, as 0.08, 0.05, and 0.02 ML/s for the InAs BQDs (SQDs) of samples A, B, and C, respectively. For all the samples, the growth rate of the GaAs space is the same as 1.3 ML/s. PzR spectra were accomplished by mounting the InAs SQD samples on a lead-zirconate-titanate transducer. The transducer was cooled by continuous flow of liquid nitrogen and driven by a sinusoidal electric field [8]. The differential reflectivity signals were detected with a phase-sensitive lock-in amplifier. PL experiments of the three samples were conducted with a focused He-Ne laser beam (l = 632.8 nm) at the same excited power density. The PL emission was detected by using a liquidnitrogen-cooled InGaAs detector. The InAs SQD layer in each sample was removed by using the hydrochloric acid. After the InAs SQDs and BQDs had been removed ordinally, the PzR experiments were also performed for the residual InAs BQDs and the residual GaAs matrix, respectively. 3. Results and discussion The PL spectroscopy of initial sample A was measured at 77 K, as shown in Fig. 1(a). The PzR spectroscopy of initial sample A, and that of this sample with the InAs SQD and BQD removed ordinally are all shown in Fig. 1(b). Meanwhile, the PL curve was fitted with Gaussian profile, and the PzR curves were fitted very well by using the first derivative of Gaussian line shape. There are four emission peaks observed in PL spectroscopy of sample A. A rather narrow BQD0 peak with its Gaussian linewidth of 28 meV is located at 1.301 eV. It suggests that the InAs QDs of sample A grown on the In0.36Ga0.64As template is highly uniform in sizes. The BQD0
Fig. 1. (a) The PL spectroscopy of sample A measured at 77 K and (b) the PzR spectra of initial sample A (PzR1), PzR spectra of this sample with the InAs SQD (PzR2) and InAs BQD (PzR3) removed, respectively. The inset is the conduction band diagram of InAs/In0.36Ga0.64As/GaAs SQD structure; the bidirectional arrows in inset denote the coupling effects between the ground state of In0.36Ga0.64As/GaAs surface QW and the confined SQD states.
peak is attributed to the ground state emission of the InAs QDs. The BQD1 peak at 1.362 eV should originate from the first exited-state emission of the InAs QDs. It is worthy to note that more optical features have been observed in the PzR spectra of initial sample A, as the PzR1 line shown in Fig. 1(b). The BQD0 and BQD1 features at 1.302 and 1.368 eV, which should share the same originations with the corresponding two features in PL spectroscopy. They are ascribed to the ground and first excited state transitions of the InAs BQDs, respectively. Well then, one probably would like to know the real identities of the other five features observed in the lower energy region of 0.8–1.25 eV. It is proposed that these peaks labeled as SQDn (n = 0, 1, 2, 3, and 4) originate from the interband transitions of the InAs SQDs. The feature ‘‘HH’’ at 1.410 eV is attributed to the heavy-hole transition in the twodimensional In0.36Ga0.64As/GaAs quantum well (QW) formed by the GaAs buffer, the first In0.36Ga0.64As template, and the GaAs capping layer. To identify the origination of the SQDn features exactly, the PzR spectroscopy has also been performed with the InAs SQDs removed by the hydrochloric acid solution, as the PzR2 curve shown in Fig. 1(b). Corresponding to those features in PzR1 curve, the BQD0, BQD1 and HH features in PzR2 line can still been observed clearly. It indicates that all these three optical transitions (BQD0, BQD1, and HH) originate from the buried InAs/In0.36Ga0.64As/GaAs structure. However, the SQDn peaks do not appear in the PzR2 curve again. It supports our identification that these optical features denoted by SQDn are the interband transitions between the corresponding confined
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Fig. 2. (a) The PL spectroscopy of initial sample B measured at 77 K and (b) the PzR spectra of initial sample B (PzR1), PzR spectra of this sample with the InAs SQD (PzR2) and InAs BQD (PzR3) removed, respectively.
states in conduction and valence band of the InAs SQDs. With the InAs BQD layer further removed, the PzR spectroscopy of the residual GaAs matrix of sample A only shows the absorbededge transition peak of GaAs, as the PzR3 curve shown in Fig. 1(b). The broad peak-like structure near 1.45 eV in PzR3 curve probably is related to the abnormal absorption of the incomplete etched In0.36Ga0.64As template on the GaAs matrix. Similar results have been observed in the samples B and C. Fig. 2(a) and (b) shows the PL and PzR spectra of sample B measured at 77 K, respectively. The PL spectrum does not present the peak from the emission of InAs SQDs yet. Only two emission peaks of the InAs BQDs are observed at 1.255 and 1.323 eV. Compared with that of sample A, the BQD0 peak of sample B shows an enlarged linewidth of 54 meV and a redshift of 47 meV. After the InAs SQDs were removed, the four interband transitions, which are labeled as SQDn, disappear in PzR2. As a result, the optical features labeled as SQDn are proved to originate from the confined state transition of the InAs SQDs once again. Photon emission in PL experiment is induced by the electron-hole recombination between the confined states of QDs. Due to the complicate surface conditions [10], the lifetime and effective number of carriers in InAs SQDs will be reduced greatly by many non-radiative recombination mechanisms, such as surface tunneling effects, carrier captured by the surface states, surface adsorption effects, and thermal activation, etc. Therefore, the weak emission from the exciton recombination of InAs SQDs is hard to be detected, even by further increasing the excited laser power. Nevertheless, PzR is an absorbed-type spectroscopy; the acquired information reflects the optical absorption and transition behaviors of InAs
QDs [9]. The complicated carrier-dynamics process does not affect the photon absorption and the interband transitions between the confined states of InAs SQDs. Furthermore, even the weak optical absorption of confined SQD states can induce the notable variation of differential reflectivity DR, which makes the optical transitions measurable at the corresponding wavelength. Probably this is the reason that the optical features from InAs SQDs can be observed clearly by the PzR, instead of the PL spectroscopy. In comparison with the BQD0 features of InAs BQDs, the SQD0 features in all the three samples exhibit a pronounced redshift, as the PzR1 curves of samples A and B shown in Figs. 1(b) and 2(b). For sample A, the redshift of SQD0 is up to 467 meV. This redshift is larger than that observed in the InAs/ GaAs(0 0 1) SQDs [7]. Due to the discrepancy of the strain and the potential barrier height (GaAs vs. surface potential), the confined energies of electron states in InAs SQDs and BQDs are definitely different, as well as that of confined hole states. The theoretical calculations indicate that the strain energies on the surface and top of InAs SQDs are much smaller than that of BQDs capped with the GaAs-based materials [11]. Our calculations based on three-dimensional effective mass approximation (EMA) [9] indicate that the strain relaxation of SQDs induces the decrease of the energy gap of 210 meV. Meanwhile, the strain effects have been taken into account in these self-consistently calculations. However, only the strain relaxation would not produce such enormous redshift of SQD0. In the case of removing the GaAs capping layer of SQDs, it has been proved that the height variation induced in the growth process leads to a 20–30 meV redshift of the SQDs [12–14]. The coupling between the surface states and the confined InAs SQD states [7] may contribute the transition-energy shrinkage of 90 meV. In addition, it can be expected that there is a strong coupling between the ground state confined in the In0.36Ga0.64As/GaAs surface QW with the confined SQD states. This coupling further interrupts the wave function and the confined energy of the SQD states. In comparison with that in the capped InxGa1 xAs/GaAs QW, the interband gap of ground state in the InxGa1 xAs/GaAs surface QW undergoes a shrinkage due to the interaction of the surface states [15]. Then, one can expect that the interaction between the ground state of InAs SQDs and the redshifted quantum state in the surface InxGa1 xAs/GaAs QW can induce a coupled state to the lower energy position, as the schematic coupling effects and the conduction band diagram shown in the inset of Fig. 1(a). These coupling effects are estimated to be responsive for the additional redshift of 50–60 meV of each SQDn feature. Therefore, in the case of substituting the GaAs barrier with a quasi-infinite surface potential (5.1 eV) [15–17], the strain relaxation, the height variation of SQDs, the coupling effects on the confined SQD states from the surface states and ground state in In0.36Ga0.64As/GaAs surface QW together lead to the enormous redshift of the SQD0 feature. On the other hand, the strong surface potential effects on the confined states in InAs SQDs and In0.36Ga0.64As/GaAs surface QW have also been unfolded clearly by the great redshift of SQD0 peaks.
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Fig. 3. The PzR spectra of initial samples A (PzRA), B (PzRB) and C (PzRC) at 77 K, respectively. Every solid circle denotes four experimental points.
The ground state SQD0 and the first excited SQD1 of sample A are located at 0.835 eV (1.51 mm) and 0.933 eV in PzR1 curve, respectively. The two transition features show an energy s separation (DE0;1 ) of 98 meV in Fig. 1(b). So, another s interesting characteristic of the InAs SQDs is that the DE0;1 B is obviously larger than the energy separation DE0;1 between s BQD0 and BQD1 feature (63 meV). Moreover, this DE0;1 value is also larger than that of the conventional InAs BQDs (generally in the range of 50–90 meV) reported in literatures. s The larger DE0;1 observed in the InAs SQDs can be well resolved in terms of the absent GaAs capping layer. The strain relaxation effects will reduce the lateral confinement at the surface and top of InAs SQD. This supports the theoretical s results of Nabetani et al. [11]. Since the DE0;1 is inversely related to the lateral confinement [18], the decrease of lateral s confinement of InAs SQD implies the increase of DE0;1 . The PzR spectra of samples A, B and C has been shown in Fig. 3, respectively. Corresponding to spectral experiments of sample A, the BQD0 and BQD1 originated from InAs BQDs, and HH features from the embedded In0.36Ga0.64As/GaAs QW have also been observed in PzR and PL spectra of samples B s and C. However, the larger DE0;1 (110 meV) and less transition features of SQDs have been observed in the PzR of samples B and C. This probably can be attributed to the
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difference of structural properties of SQDs induced by the different SQD growth rates for each sample. The increasing growth rate leads to the decrease of both height and base diameter of SQD, but the decrease of base size is less than that of the former. This increase of the ratio of base diameter to height induces the further decrease of the lateral confinement of s of samples B and C. On the SQD [19], and the increase of DE0;1 other hand, the PzR linewidth (G) of SQDn features of all the three samples are generally wider (9–15 meV) than that of the BQDn peaks. This is another strong proof, which supports our identification that the physics origin of the SQDn peaks are different from that of BQDn ones, the SQDn transitions originate from the InAs SQDs while BQDn ones originate from the InAs BQDs. The SQD height variation due to the absence of GaAs capping layer and the strong coupled interactions from the surface states may broaden the energy distribution range of confined SQD states, and then enhance the scattering of joint density of states [20]. Then, the spread of absorption-edge of confined SQD levels result in the greater broadening of optical absorption gradually. Probably this is the reason for the observations of the broadening of SQDn peaks in PzR spectra. The SQD0 peak of samples B and C exhibit a small redshift of 15 and 28 meV in comparison with the SQD0 peak of sample A, respectively. Probably the redshift can be ascribed to the increasing In0.36Ga0.64As template thickness and SQD sizes induced by using the lower growth rate. In order to give a quantitative description, we have also studied the InAs SQDs by using atomic force microscopy. The well ordered, uniform, and relative high density of SQDs grown on GaAs(311B) have been shown in Fig. 4. The InAs SQD densities in samples A, B, and C are 9.5 1010, 9.1 1010, and 4.8 1010 cm 2, respectively. The average base-diameter (d) of SQDs in samples A, B and C are 42, 44 and 55 nm, and the average height (h) are 3.9, 4.2 and 6.8 nm, respectively. That is to say, the SQD sizes in our samples increase while the density reduces with decreasing growth rate. Generally, the low growth rate (0.08 ML/s) based on S–K model is a favorable method to form larger QDs on GaAs(0 0 1) [21,22] and to realize the large redshift of the emission peak. However, the super-low growth rate is not easy to achieve such high QD density as the observation in our InAs/In0.36Ga0.64As/GaAs(311B) samples, and the QDs grown directly on GaAs(1 0 0) following this
Fig. 4. The 0.5 0.5 mm2 AFM pictures of InAs surface quantum dots in samples A (a), B (b) and C (c), respectively.
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method is hard to be applied to fabricating device with high optical efficiency. The relative high SQD density realized in our samples can be ascribed to the strain modulation created by the In0.36Ga0.64As template and the well controllable ability of GaAs(311B) for growing ordered nucleation [21]. The strain modulation based on the strain-driven growth instability reduces the bottom and surface strain of InAs nucleation. Therefore, more new nucleation is expected to form among the sites of the old nucleation due to the decreasing repulsion interactions between InAs SQDs. This leads to the increase of the InAs SQD density on GaAs(311B) even by using the low rate growth technique. Further compared with the BQD0 peak in the PzR of sample A, the BQD0 peaks show large redshift of 47 meV in sample B and 72 meV in sample C, respectively. Although the growth parameters for growing the InAs BQDs and SQDs are identical in each sample, it is interesting to find that the redshift of BQD0 peaks are 3 times larger than that of SQD0 in sample B (47 meV vs. 15 meV) and sample C (72 meV vs. 28 meV). This behavior is more clearly shown in Fig. 3. According to our calculations based on the three-dimensional EMA, actually the increase of the In0.36Ga0.64As template thickness from 2.0 to 2.5 nm only contributes to the 12 meV redshift of BQD0 peaks in samples B and C. That is to say, besides the two factors, the increase of In0.36Ga0.64As thickness and the increase of BQD size induced by the decreasing growth rate, there is another mechanism to shift the BQD0 peaks of samples B and C to the longer wavelength. In this case, the overgrowth process of the first In0.36Ga0.64As template has been proposed to be the third factor. Compared with sample A, the more indium adatoms can be expected to segregate from the first In0.36Ga0.64As template and then accumulate at the surface of InAs BQDs during the overgrowth process. No matter as a template or a capping layer in the QD structures, the InxGa1 xAs layer is used to improve the growth and optical characteristics of InAs QDs. However, the exact x value in the InxGa1 xAs layer usually disaccords with nominal indium concentration [23,24]. One can obtain the real indium concentration x in the first In0.36Ga0.64As template by studying the transition energy of HH peak in the strained In0.36Ga0.64As/ GaAs QW. Therefore, the calculations based on one-dimensional EMA have also been performed. The band offset between GaAs and In0.36Ga0.64As are taken to be 0.60 in these calculations. However, our calculations based on nominal indium concentration x = 0.36 do not match with the spectral results. Further calculations emphasize the actual indium concentration in the first In0.36Ga0.64As template of samples A, B and C are x = 0.33, 0.31 and 0.295, respectively, instead of the nominal value (0.36). The missing indium in the first In0.36Ga0.64As template probably is driven by the strong strain field [25], and then undergoes a transfer and accumulation at the surface of InAs BQDs. It supports the strain-driven alloy phase segregation (SDAPS) model in which the indium adatoms migrate to the InAs BQD stressors on expense of the indium concentration in the first In0.36Ga0.64As template during the overgrowth [26]. Due to the longer time used in growing the thicker GaAs capping layer of samples B and C, the stronger
SDAPS effects are expected to occur in the first In0.36Ga0.64As template of the two samples during the overgrowth. Then, the more segregated indium adatoms from In0.36Ga0.64As template further increase the initial InAs BQD sizes. At the same time, the more effective strain relaxation of InAs BQDs can be expected in samples B and C. As a result, the stronger SDAPS effects in the first In0.36Ga0.64As template of these two samples are responsive for the larger redshift of BQD0 feature in comparing with that of SQD0 peak. Additionally, the transfer and incorporation of indium adatoms during overgrowth are also a self-assembled process. So, the stronger SDAPS effects weaken the uniformity of InAs BQD sizes unavoidably, and result in the broadening of the BQD0 peak in the PL spectra of samples B and C. As shown in the Fig. 2(a), the PL linewidth of BQD0 peak of sample B is about two times larger than that of sample A. 4. Conclusion In summary, the optical properties of three InAs/ In0.36Ga0.64As/GaAs(311B) surface quantum dot samples have been studied by PL and PzR spectra. Since the photon absorption involved in the interband transition process is not affected by the carrier dynamics, several interband transitions related to InAs surface quantum dots have been observed by the PzR instead of the PL spectra. By lowering the growth rate of InAs SQDs and increasing the thickness of the In0.36Ga0.64As temperate, the redshift of SQD0 has been observed. The optical transitions of InAs surface quantum dots show different optical properties from that of InAs buried quantum dots. On the other hand, the optical discrepancy between surface and buried quantum dots can be used to identify the origination of the optical features. Acknowledgements The authors thank Prof. B. Zhang and Q. Gong for their fruitful discussion. This work was financially supported by Chinese National Science Foundation (No. 60567001), Chinese National Key Basic Research Special Fund (No. 2006CB921507), and the Cultivated Foundation for the Academic cadreman of Yunnan University. Reference [1] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203. [2] Zh.M. Wang, Sh. Seydmohamadi, J.H. Lee, G.J. Salamo, Appl. Phys. Lett. 85 (2004) 3411. [3] K. Akahane, T. Kawamura, K. Okino, H. Koyama, S. Lan, Y. Okada, M. Kawabe, Appl. Phys. Lett. 73 (1998) 3411. [4] Y. Okada, M. Miyagi, K. Akahane, Y. Iuchi, M. Kawabe, J. Appl. Phys. 90 (2001) 192. [5] Q. Gong, R. No¨tzel, G.J. Hamhuis, T.J. Eijkemans, J.H. Wolter, Appl. Phys. Lett. 81 (2002) 3254. [6] S. Lan, K. Akahane, H.Z. Song, Y. Okada, M. Kawabe, T. Nishimura, O. Wada, Phys. Rev. B61 (2000) 16847. [7] Z.L. Miao, Y.W. Zhang, S.J. Chua, Y.H. Chye, P. Chen, S. Tripathy, Appl. Phys. Lett. 86 (2005) 031914.
C. Wang et al. / Applied Surface Science 254 (2008) 4626–4631 [8] C. Wang, P.P. Chen, N.Y. Tang, C.S. Xia, X.S. Chen, W. Lu, Phys. Lett. A 350 (2006) 269. [9] C. Wang, P.P. Chen, N.Y. Tang, T.X. Li, W. Lu, F.Z. Wang, Z.H. Chen, J. Cryst. Growth 289 (2006) 547. [10] B.J. Skromme, C.J. Sandroff, E. Yablonovitch, T. Gmitter, Appl. Phys. Lett. 51 (1987) 2022. [11] Y. Nabetani, T. Matsumoto, G. Sasikala, I. Suemune, J. Appl. Phys. 98 (2005) 063502. [12] J. Dreybrodt, A. Forchel, J.P. Reithmaier, Phys. Rev. B48 (1993) 14741. [13] L. Bhusal, A. Alemu, A. Freundlich, Nanotechnology 15 (2004) S245. [14] R. Songmuang, S. Kiravittaya, O.G. Schmidt, G. Le, J. Cryst. Growth 249 (2003) 416. [15] Z.L. Miao, P.P. Chen, X.S. Chen, W. Lu, S.C. Shen, Phys. Lett. A 225 (2000) 175. [16] H. Shen, M. Dutta, L. Fotiadis, P.G. Newman, W.H. Chang, Appl. Phys. Lett. 57 (1990) 2118. [17] P.P. Chen, Z.L. Miao, W. Lu, J. Cryst. Growth 227–228 (2001) 108.
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[18] F. Guffarth, R. Heitz, A. Schliwa, O. Stier, N.N. Ledentsov, V.M. Ustinov, D. Bimberg, Phys. Rev. B64 (2001) 085305. [19] N.Y. Tang, Doctoral dissertation, Shanghai Institute of Technical Physics, Chinese Academy of Science, Shanghai, 2005, p. 20. [20] S.C. Shen, Spectra and Optical Properties of Semiconductors, Science Press, Beijing, 2002, p. 168 (in Chinese). [21] Y. Qiu, P. Gogna, S. Forouhar, A. Stinz, L.F. Lester, Appl. Phys. Lett. 79 (2001) 3570. [22] H.Y. Liu, M. Hopkinson, C.N. Harrison, M.J. Steer, R. Frith, D.J. Mowbray, M.S. Skolnick, J. Appl. Phys. 93 (2003) 2931. [23] J.E. Guyer, P.W. Voorhee, Phys. Rev. Lett. 74 (1995) 4031. [24] C. Wang, Z.L. Liu, X.M. Chen, C.S. Xia, S. Zhang, Y. Yang, W. Lu, Chin. Phys. Lett. 24 (2007) 3260. [25] M.V. Maximov, A.F. Tsatsul’nikov, R. Heitz, D. Bimberg, Yu.G. Musikhin, Phys. Rev. B62 (2000) 16671. [26] W.H. Chang, H.Y. Chen, H.S. Chang, W.Y. Chen, T.M. Hsu, N.T. Yeh, Appl. Phys. Lett. 86 (2005) 131917.
Applied Surface Science 254 (2008) 4626–4631 www.elsevier.com/locate/apsusc
Optical transitions of InAs/In0.36Ga0.64As/GaAs(311B) surface quantum dots clearly identified by the piezoreflectance technique C. Wang a,b,*, Y. Yang a, X.M. Chen c, Z.L. Liu b, H.Y. Cui b, S. Zhang a, X.S. Chen b, W. Lu b a
Research Institute of Engineering and Technology, Yunnan University, Kunming 650091, China b National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China c Basic Research Center for Detectors, Kunming Institute of Physics, Kunming 650034, China
Received 3 October 2007; received in revised form 11 January 2008; accepted 15 January 2008 Available online 26 January 2008
Abstract The bilayer InAs/In0.36Ga0.64As/GaAs(311B) quantum dots (QDs), including one InAs buried quantum dot (BQD) layer and the other InAs surface quantum dot (SQD) layer, have been grown by molecular beam epitaxy (MBE). The optical properties of these three samples have been studied by the piezoreflectance (PzR) spectroscopy. The PzR spectra do not exhibit only the optical transitions originated from the InAs BQDs, but the features originated from the InAs SQDs. After the InAs SQDs have been removed chemically, those optical transitions from InAs SQDs have been demonstrated clearly by investigating the PzR spectra of the residual InAs BQDs in these samples. The great redshift of these interband transitions of InAs SQDs has been well discussed. Due to the suitable InAs SQD sizes and the thickness of In0.36Ga0.64As layer, the interband transition of InAs SQDs has been shifted to 1.55 mm at 77 K. # 2008 Elsevier B.V. All rights reserved. Keywords: Surface quantum dots; Molecular beam epitaxy; Photoluminescence; Piezoreflectance spectroscopy
1. Introduction Inspired by the great potential applications in the various nano-optoelectronic devices, the fabrication and the characterization of self-assembled InAs quantum dots (QDs) have become an extensively research area over the last decade [1]. Some properties, such as the high QD density, the less QD size fluctuation and the controllable QD distribution in two dimensional plane, are important to meet the high quality of QD-based optoelectronic devices, while these are also the major challenge for those QDs grown on the GaAs(0 0 1) due to the self-assembled QD formation with Stranski-Krastanov growth mode influenced by many factors [2]. Growing the InAs QDs on the GaAs(311B) has been demonstrated to be an effective method to form high-density and well-ordered QD array on surface with less coalescence of dots, the inherent growth mechanism could be attributed to a complex phase separation and strain-relief mechanism [3,4]. Recently, the
* Corresponding author. Tel.: +86 871 5031869; fax: +86 871 5037399. E-mail address: [email protected] (C. Wang). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.01.089
InAs nucleations for forming QDs, which is based on the strain driven growth instability, have been well controlled by growing a strained modulated InxGa1 xAs template on GaAs(311B) substrate. It is found that these InAs QDs on the InxGa1 xAs template exhibit superior photoluminescence (PL) properties over that of InAs QDs grown on GaAs(0 0 1) directly [5]. Although the growth characteristics of InxGa1 xAs QDs on GaAs(311B) have been studied by the atomic force microscope (AFM) [3], PL [5] and time-resolved PL technique [6], the optical and electronic properties of these QDs on GaAs(311B) are not understood clearly at present. On the other hand, recently, a weak emission peak, which originate, from the exciton-state recombination in the InAs/GaAs(0 0 1) surface quantum dots (SQDs) has been observed with a rather broadening linewidth at 1.6 mm [7]. However, such interband transition in the InAs SQDs has not been demonstrated by the absorbed-type spectroscopy. Piezoreflectance spectroscopy (PzR) is a powerful tool to investigate the optical properties of III–V compounds semiconductors [8]. Since the PzR is an absorbed-type technique, the acquired information reflects the optical transition behaviors, which are more directly related to the
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theoretical absorption model. In addition, due to the stressmodulated nature, the PzR is believed to be more useful than other modulation spectra to study the strained QD system [9]. In this article, the PzR spectroscopy is used to study the optical properties of three InAs/In0.36Ga0.64As/GaAs(311B) SQD samples. 2. Experimental details Three InAs/In0.36Ga0.64As QD samples were grown by solid source molecular-beam epitaxy (MBE) on GaAs(311B) substrates. After oxide desorption, the 300 nm thick GaAs buffer layer was deposited at 580 8C. The temperature was decreased to 500 8C to grow the first In0.36Ga0.64As template with the thickness of 2.0, 2.5 and 2.5 nm for samples A, B and C, respectively. Then, the 0.8 ML thick InAs was deposited to grow the InAs buried quantum dots (BQDs) at 500 8C in each sample. The InAs BQDs were covered by the 75, 100, and 120 nm thick GaAs capping layers in the three samples, respectively. Finally, another In0.36Ga0.64As template and InAs QDs, which are identical with the first In0.36Ga0.64As template and InAs BQDs, were grown repeatedly in each sample to form InAs SQDs without the GaAs capping layer. That is to say, all the three samples are double QD layers in structure, including a BQD layer and a SQD one. The growth rate for growing InAs BQDs is the same as that for InAs SQDs in each sample, but that is different among the three samples, as 0.08, 0.05, and 0.02 ML/s for the InAs BQDs (SQDs) of samples A, B, and C, respectively. For all the samples, the growth rate of the GaAs space is the same as 1.3 ML/s. PzR spectra were accomplished by mounting the InAs SQD samples on a lead-zirconate-titanate transducer. The transducer was cooled by continuous flow of liquid nitrogen and driven by a sinusoidal electric field [8]. The differential reflectivity signals were detected with a phase-sensitive lock-in amplifier. PL experiments of the three samples were conducted with a focused He-Ne laser beam (l = 632.8 nm) at the same excited power density. The PL emission was detected by using a liquidnitrogen-cooled InGaAs detector. The InAs SQD layer in each sample was removed by using the hydrochloric acid. After the InAs SQDs and BQDs had been removed ordinally, the PzR experiments were also performed for the residual InAs BQDs and the residual GaAs matrix, respectively. 3. Results and discussion The PL spectroscopy of initial sample A was measured at 77 K, as shown in Fig. 1(a). The PzR spectroscopy of initial sample A, and that of this sample with the InAs SQD and BQD removed ordinally are all shown in Fig. 1(b). Meanwhile, the PL curve was fitted with Gaussian profile, and the PzR curves were fitted very well by using the first derivative of Gaussian line shape. There are four emission peaks observed in PL spectroscopy of sample A. A rather narrow BQD0 peak with its Gaussian linewidth of 28 meV is located at 1.301 eV. It suggests that the InAs QDs of sample A grown on the In0.36Ga0.64As template is highly uniform in sizes. The BQD0
Fig. 1. (a) The PL spectroscopy of sample A measured at 77 K and (b) the PzR spectra of initial sample A (PzR1), PzR spectra of this sample with the InAs SQD (PzR2) and InAs BQD (PzR3) removed, respectively. The inset is the conduction band diagram of InAs/In0.36Ga0.64As/GaAs SQD structure; the bidirectional arrows in inset denote the coupling effects between the ground state of In0.36Ga0.64As/GaAs surface QW and the confined SQD states.
peak is attributed to the ground state emission of the InAs QDs. The BQD1 peak at 1.362 eV should originate from the first exited-state emission of the InAs QDs. It is worthy to note that more optical features have been observed in the PzR spectra of initial sample A, as the PzR1 line shown in Fig. 1(b). The BQD0 and BQD1 features at 1.302 and 1.368 eV, which should share the same originations with the corresponding two features in PL spectroscopy. They are ascribed to the ground and first excited state transitions of the InAs BQDs, respectively. Well then, one probably would like to know the real identities of the other five features observed in the lower energy region of 0.8–1.25 eV. It is proposed that these peaks labeled as SQDn (n = 0, 1, 2, 3, and 4) originate from the interband transitions of the InAs SQDs. The feature ‘‘HH’’ at 1.410 eV is attributed to the heavy-hole transition in the twodimensional In0.36Ga0.64As/GaAs quantum well (QW) formed by the GaAs buffer, the first In0.36Ga0.64As template, and the GaAs capping layer. To identify the origination of the SQDn features exactly, the PzR spectroscopy has also been performed with the InAs SQDs removed by the hydrochloric acid solution, as the PzR2 curve shown in Fig. 1(b). Corresponding to those features in PzR1 curve, the BQD0, BQD1 and HH features in PzR2 line can still been observed clearly. It indicates that all these three optical transitions (BQD0, BQD1, and HH) originate from the buried InAs/In0.36Ga0.64As/GaAs structure. However, the SQDn peaks do not appear in the PzR2 curve again. It supports our identification that these optical features denoted by SQDn are the interband transitions between the corresponding confined
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Fig. 2. (a) The PL spectroscopy of initial sample B measured at 77 K and (b) the PzR spectra of initial sample B (PzR1), PzR spectra of this sample with the InAs SQD (PzR2) and InAs BQD (PzR3) removed, respectively.
states in conduction and valence band of the InAs SQDs. With the InAs BQD layer further removed, the PzR spectroscopy of the residual GaAs matrix of sample A only shows the absorbededge transition peak of GaAs, as the PzR3 curve shown in Fig. 1(b). The broad peak-like structure near 1.45 eV in PzR3 curve probably is related to the abnormal absorption of the incomplete etched In0.36Ga0.64As template on the GaAs matrix. Similar results have been observed in the samples B and C. Fig. 2(a) and (b) shows the PL and PzR spectra of sample B measured at 77 K, respectively. The PL spectrum does not present the peak from the emission of InAs SQDs yet. Only two emission peaks of the InAs BQDs are observed at 1.255 and 1.323 eV. Compared with that of sample A, the BQD0 peak of sample B shows an enlarged linewidth of 54 meV and a redshift of 47 meV. After the InAs SQDs were removed, the four interband transitions, which are labeled as SQDn, disappear in PzR2. As a result, the optical features labeled as SQDn are proved to originate from the confined state transition of the InAs SQDs once again. Photon emission in PL experiment is induced by the electron-hole recombination between the confined states of QDs. Due to the complicate surface conditions [10], the lifetime and effective number of carriers in InAs SQDs will be reduced greatly by many non-radiative recombination mechanisms, such as surface tunneling effects, carrier captured by the surface states, surface adsorption effects, and thermal activation, etc. Therefore, the weak emission from the exciton recombination of InAs SQDs is hard to be detected, even by further increasing the excited laser power. Nevertheless, PzR is an absorbed-type spectroscopy; the acquired information reflects the optical absorption and transition behaviors of InAs
QDs [9]. The complicated carrier-dynamics process does not affect the photon absorption and the interband transitions between the confined states of InAs SQDs. Furthermore, even the weak optical absorption of confined SQD states can induce the notable variation of differential reflectivity DR, which makes the optical transitions measurable at the corresponding wavelength. Probably this is the reason that the optical features from InAs SQDs can be observed clearly by the PzR, instead of the PL spectroscopy. In comparison with the BQD0 features of InAs BQDs, the SQD0 features in all the three samples exhibit a pronounced redshift, as the PzR1 curves of samples A and B shown in Figs. 1(b) and 2(b). For sample A, the redshift of SQD0 is up to 467 meV. This redshift is larger than that observed in the InAs/ GaAs(0 0 1) SQDs [7]. Due to the discrepancy of the strain and the potential barrier height (GaAs vs. surface potential), the confined energies of electron states in InAs SQDs and BQDs are definitely different, as well as that of confined hole states. The theoretical calculations indicate that the strain energies on the surface and top of InAs SQDs are much smaller than that of BQDs capped with the GaAs-based materials [11]. Our calculations based on three-dimensional effective mass approximation (EMA) [9] indicate that the strain relaxation of SQDs induces the decrease of the energy gap of 210 meV. Meanwhile, the strain effects have been taken into account in these self-consistently calculations. However, only the strain relaxation would not produce such enormous redshift of SQD0. In the case of removing the GaAs capping layer of SQDs, it has been proved that the height variation induced in the growth process leads to a 20–30 meV redshift of the SQDs [12–14]. The coupling between the surface states and the confined InAs SQD states [7] may contribute the transition-energy shrinkage of 90 meV. In addition, it can be expected that there is a strong coupling between the ground state confined in the In0.36Ga0.64As/GaAs surface QW with the confined SQD states. This coupling further interrupts the wave function and the confined energy of the SQD states. In comparison with that in the capped InxGa1 xAs/GaAs QW, the interband gap of ground state in the InxGa1 xAs/GaAs surface QW undergoes a shrinkage due to the interaction of the surface states [15]. Then, one can expect that the interaction between the ground state of InAs SQDs and the redshifted quantum state in the surface InxGa1 xAs/GaAs QW can induce a coupled state to the lower energy position, as the schematic coupling effects and the conduction band diagram shown in the inset of Fig. 1(a). These coupling effects are estimated to be responsive for the additional redshift of 50–60 meV of each SQDn feature. Therefore, in the case of substituting the GaAs barrier with a quasi-infinite surface potential (5.1 eV) [15–17], the strain relaxation, the height variation of SQDs, the coupling effects on the confined SQD states from the surface states and ground state in In0.36Ga0.64As/GaAs surface QW together lead to the enormous redshift of the SQD0 feature. On the other hand, the strong surface potential effects on the confined states in InAs SQDs and In0.36Ga0.64As/GaAs surface QW have also been unfolded clearly by the great redshift of SQD0 peaks.
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Fig. 3. The PzR spectra of initial samples A (PzRA), B (PzRB) and C (PzRC) at 77 K, respectively. Every solid circle denotes four experimental points.
The ground state SQD0 and the first excited SQD1 of sample A are located at 0.835 eV (1.51 mm) and 0.933 eV in PzR1 curve, respectively. The two transition features show an energy s separation (DE0;1 ) of 98 meV in Fig. 1(b). So, another s interesting characteristic of the InAs SQDs is that the DE0;1 B is obviously larger than the energy separation DE0;1 between s BQD0 and BQD1 feature (63 meV). Moreover, this DE0;1 value is also larger than that of the conventional InAs BQDs (generally in the range of 50–90 meV) reported in literatures. s The larger DE0;1 observed in the InAs SQDs can be well resolved in terms of the absent GaAs capping layer. The strain relaxation effects will reduce the lateral confinement at the surface and top of InAs SQD. This supports the theoretical s results of Nabetani et al. [11]. Since the DE0;1 is inversely related to the lateral confinement [18], the decrease of lateral s confinement of InAs SQD implies the increase of DE0;1 . The PzR spectra of samples A, B and C has been shown in Fig. 3, respectively. Corresponding to spectral experiments of sample A, the BQD0 and BQD1 originated from InAs BQDs, and HH features from the embedded In0.36Ga0.64As/GaAs QW have also been observed in PzR and PL spectra of samples B s and C. However, the larger DE0;1 (110 meV) and less transition features of SQDs have been observed in the PzR of samples B and C. This probably can be attributed to the
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difference of structural properties of SQDs induced by the different SQD growth rates for each sample. The increasing growth rate leads to the decrease of both height and base diameter of SQD, but the decrease of base size is less than that of the former. This increase of the ratio of base diameter to height induces the further decrease of the lateral confinement of s of samples B and C. On the SQD [19], and the increase of DE0;1 other hand, the PzR linewidth (G) of SQDn features of all the three samples are generally wider (9–15 meV) than that of the BQDn peaks. This is another strong proof, which supports our identification that the physics origin of the SQDn peaks are different from that of BQDn ones, the SQDn transitions originate from the InAs SQDs while BQDn ones originate from the InAs BQDs. The SQD height variation due to the absence of GaAs capping layer and the strong coupled interactions from the surface states may broaden the energy distribution range of confined SQD states, and then enhance the scattering of joint density of states [20]. Then, the spread of absorption-edge of confined SQD levels result in the greater broadening of optical absorption gradually. Probably this is the reason for the observations of the broadening of SQDn peaks in PzR spectra. The SQD0 peak of samples B and C exhibit a small redshift of 15 and 28 meV in comparison with the SQD0 peak of sample A, respectively. Probably the redshift can be ascribed to the increasing In0.36Ga0.64As template thickness and SQD sizes induced by using the lower growth rate. In order to give a quantitative description, we have also studied the InAs SQDs by using atomic force microscopy. The well ordered, uniform, and relative high density of SQDs grown on GaAs(311B) have been shown in Fig. 4. The InAs SQD densities in samples A, B, and C are 9.5 1010, 9.1 1010, and 4.8 1010 cm 2, respectively. The average base-diameter (d) of SQDs in samples A, B and C are 42, 44 and 55 nm, and the average height (h) are 3.9, 4.2 and 6.8 nm, respectively. That is to say, the SQD sizes in our samples increase while the density reduces with decreasing growth rate. Generally, the low growth rate (0.08 ML/s) based on S–K model is a favorable method to form larger QDs on GaAs(0 0 1) [21,22] and to realize the large redshift of the emission peak. However, the super-low growth rate is not easy to achieve such high QD density as the observation in our InAs/In0.36Ga0.64As/GaAs(311B) samples, and the QDs grown directly on GaAs(1 0 0) following this
Fig. 4. The 0.5 0.5 mm2 AFM pictures of InAs surface quantum dots in samples A (a), B (b) and C (c), respectively.
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method is hard to be applied to fabricating device with high optical efficiency. The relative high SQD density realized in our samples can be ascribed to the strain modulation created by the In0.36Ga0.64As template and the well controllable ability of GaAs(311B) for growing ordered nucleation [21]. The strain modulation based on the strain-driven growth instability reduces the bottom and surface strain of InAs nucleation. Therefore, more new nucleation is expected to form among the sites of the old nucleation due to the decreasing repulsion interactions between InAs SQDs. This leads to the increase of the InAs SQD density on GaAs(311B) even by using the low rate growth technique. Further compared with the BQD0 peak in the PzR of sample A, the BQD0 peaks show large redshift of 47 meV in sample B and 72 meV in sample C, respectively. Although the growth parameters for growing the InAs BQDs and SQDs are identical in each sample, it is interesting to find that the redshift of BQD0 peaks are 3 times larger than that of SQD0 in sample B (47 meV vs. 15 meV) and sample C (72 meV vs. 28 meV). This behavior is more clearly shown in Fig. 3. According to our calculations based on the three-dimensional EMA, actually the increase of the In0.36Ga0.64As template thickness from 2.0 to 2.5 nm only contributes to the 12 meV redshift of BQD0 peaks in samples B and C. That is to say, besides the two factors, the increase of In0.36Ga0.64As thickness and the increase of BQD size induced by the decreasing growth rate, there is another mechanism to shift the BQD0 peaks of samples B and C to the longer wavelength. In this case, the overgrowth process of the first In0.36Ga0.64As template has been proposed to be the third factor. Compared with sample A, the more indium adatoms can be expected to segregate from the first In0.36Ga0.64As template and then accumulate at the surface of InAs BQDs during the overgrowth process. No matter as a template or a capping layer in the QD structures, the InxGa1 xAs layer is used to improve the growth and optical characteristics of InAs QDs. However, the exact x value in the InxGa1 xAs layer usually disaccords with nominal indium concentration [23,24]. One can obtain the real indium concentration x in the first In0.36Ga0.64As template by studying the transition energy of HH peak in the strained In0.36Ga0.64As/ GaAs QW. Therefore, the calculations based on one-dimensional EMA have also been performed. The band offset between GaAs and In0.36Ga0.64As are taken to be 0.60 in these calculations. However, our calculations based on nominal indium concentration x = 0.36 do not match with the spectral results. Further calculations emphasize the actual indium concentration in the first In0.36Ga0.64As template of samples A, B and C are x = 0.33, 0.31 and 0.295, respectively, instead of the nominal value (0.36). The missing indium in the first In0.36Ga0.64As template probably is driven by the strong strain field [25], and then undergoes a transfer and accumulation at the surface of InAs BQDs. It supports the strain-driven alloy phase segregation (SDAPS) model in which the indium adatoms migrate to the InAs BQD stressors on expense of the indium concentration in the first In0.36Ga0.64As template during the overgrowth [26]. Due to the longer time used in growing the thicker GaAs capping layer of samples B and C, the stronger
SDAPS effects are expected to occur in the first In0.36Ga0.64As template of the two samples during the overgrowth. Then, the more segregated indium adatoms from In0.36Ga0.64As template further increase the initial InAs BQD sizes. At the same time, the more effective strain relaxation of InAs BQDs can be expected in samples B and C. As a result, the stronger SDAPS effects in the first In0.36Ga0.64As template of these two samples are responsive for the larger redshift of BQD0 feature in comparing with that of SQD0 peak. Additionally, the transfer and incorporation of indium adatoms during overgrowth are also a self-assembled process. So, the stronger SDAPS effects weaken the uniformity of InAs BQD sizes unavoidably, and result in the broadening of the BQD0 peak in the PL spectra of samples B and C. As shown in the Fig. 2(a), the PL linewidth of BQD0 peak of sample B is about two times larger than that of sample A. 4. Conclusion In summary, the optical properties of three InAs/ In0.36Ga0.64As/GaAs(311B) surface quantum dot samples have been studied by PL and PzR spectra. Since the photon absorption involved in the interband transition process is not affected by the carrier dynamics, several interband transitions related to InAs surface quantum dots have been observed by the PzR instead of the PL spectra. By lowering the growth rate of InAs SQDs and increasing the thickness of the In0.36Ga0.64As temperate, the redshift of SQD0 has been observed. The optical transitions of InAs surface quantum dots show different optical properties from that of InAs buried quantum dots. On the other hand, the optical discrepancy between surface and buried quantum dots can be used to identify the origination of the optical features. Acknowledgements The authors thank Prof. B. Zhang and Q. Gong for their fruitful discussion. This work was financially supported by Chinese National Science Foundation (No. 60567001), Chinese National Key Basic Research Special Fund (No. 2006CB921507), and the Cultivated Foundation for the Academic cadreman of Yunnan University. Reference [1] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Appl. Phys. Lett. 63 (1993) 3203. [2] Zh.M. Wang, Sh. Seydmohamadi, J.H. Lee, G.J. Salamo, Appl. Phys. Lett. 85 (2004) 3411. [3] K. Akahane, T. Kawamura, K. Okino, H. Koyama, S. Lan, Y. Okada, M. Kawabe, Appl. Phys. Lett. 73 (1998) 3411. [4] Y. Okada, M. Miyagi, K. Akahane, Y. Iuchi, M. Kawabe, J. Appl. Phys. 90 (2001) 192. [5] Q. Gong, R. No¨tzel, G.J. Hamhuis, T.J. Eijkemans, J.H. Wolter, Appl. Phys. Lett. 81 (2002) 3254. [6] S. Lan, K. Akahane, H.Z. Song, Y. Okada, M. Kawabe, T. Nishimura, O. Wada, Phys. Rev. B61 (2000) 16847. [7] Z.L. Miao, Y.W. Zhang, S.J. Chua, Y.H. Chye, P. Chen, S. Tripathy, Appl. Phys. Lett. 86 (2005) 031914.
C. Wang et al. / Applied Surface Science 254 (2008) 4626–4631 [8] C. Wang, P.P. Chen, N.Y. Tang, C.S. Xia, X.S. Chen, W. Lu, Phys. Lett. A 350 (2006) 269. [9] C. Wang, P.P. Chen, N.Y. Tang, T.X. Li, W. Lu, F.Z. Wang, Z.H. Chen, J. Cryst. Growth 289 (2006) 547. [10] B.J. Skromme, C.J. Sandroff, E. Yablonovitch, T. Gmitter, Appl. Phys. Lett. 51 (1987) 2022. [11] Y. Nabetani, T. Matsumoto, G. Sasikala, I. Suemune, J. Appl. Phys. 98 (2005) 063502. [12] J. Dreybrodt, A. Forchel, J.P. Reithmaier, Phys. Rev. B48 (1993) 14741. [13] L. Bhusal, A. Alemu, A. Freundlich, Nanotechnology 15 (2004) S245. [14] R. Songmuang, S. Kiravittaya, O.G. Schmidt, G. Le, J. Cryst. Growth 249 (2003) 416. [15] Z.L. Miao, P.P. Chen, X.S. Chen, W. Lu, S.C. Shen, Phys. Lett. A 225 (2000) 175. [16] H. Shen, M. Dutta, L. Fotiadis, P.G. Newman, W.H. Chang, Appl. Phys. Lett. 57 (1990) 2118. [17] P.P. Chen, Z.L. Miao, W. Lu, J. Cryst. Growth 227–228 (2001) 108.
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[18] F. Guffarth, R. Heitz, A. Schliwa, O. Stier, N.N. Ledentsov, V.M. Ustinov, D. Bimberg, Phys. Rev. B64 (2001) 085305. [19] N.Y. Tang, Doctoral dissertation, Shanghai Institute of Technical Physics, Chinese Academy of Science, Shanghai, 2005, p. 20. [20] S.C. Shen, Spectra and Optical Properties of Semiconductors, Science Press, Beijing, 2002, p. 168 (in Chinese). [21] Y. Qiu, P. Gogna, S. Forouhar, A. Stinz, L.F. Lester, Appl. Phys. Lett. 79 (2001) 3570. [22] H.Y. Liu, M. Hopkinson, C.N. Harrison, M.J. Steer, R. Frith, D.J. Mowbray, M.S. Skolnick, J. Appl. Phys. 93 (2003) 2931. [23] J.E. Guyer, P.W. Voorhee, Phys. Rev. Lett. 74 (1995) 4031. [24] C. Wang, Z.L. Liu, X.M. Chen, C.S. Xia, S. Zhang, Y. Yang, W. Lu, Chin. Phys. Lett. 24 (2007) 3260. [25] M.V. Maximov, A.F. Tsatsul’nikov, R. Heitz, D. Bimberg, Yu.G. Musikhin, Phys. Rev. B62 (2000) 16671. [26] W.H. Chang, H.Y. Chen, H.S. Chang, W.Y. Chen, T.M. Hsu, N.T. Yeh, Appl. Phys. Lett. 86 (2005) 131917.