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GaN multiple quantum wells grown by MOCVD
Journal of Crystal Growth 314 (2011) 202–206
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells grown by MOCVD Shanjin Huang, Bingfeng Fan, Yulun Xian, Zhiyuan Zheng, Zhisheng Wu, Hao Jiang n, Gang Wang State Key Laboratory of Optoelectronic Materials and Technology, Sun Yat-sen University, Guangzhou 510275, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2010 Received in revised form 7 November 2010 Accepted 9 November 2010 Communicated by K.W. Benz Available online 18 November 2010
We have investigated the influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs) with an indium composition around 20%. The samples with five-pairs InGaN/GaN MQWs were grown on sapphire substrates by metalorganic chemical vapor deposition. Photoluminescence spectra at 8 K showed that the MQWs grown with a low amount of TEGa flow gave a strong single peak and a higher emission energy. High-resolution X-ray diffraction measurements showed a deterioration of the InGaN/GaN interfaces in the sample grown with the large TEGa flow. The luminescence thermal quenching characteristics suggested that more structural defects acting as non-radiative recombination centers formed in the MQWs when the TEGa flow increased. The results indicate that decreasing the TEGa flow help to build up a new growth balance during the growth of InGaN wells, leading to less structural defects, more homogeneous indium distribution and the abrupt MQWs interfaces. & 2010 Elsevier B.V. All rights reserved.
Keywords: Al. High resolution X-ray diffraction A1. Photoluminescence A3. Metalorganic chemical vapor deposition B1. Nitrides B1. Triethylgallium
1. Introduction The InGaN/GaN multiple quantum wells (MQWs) structure is widely used as the active layer in the entire visible and near-UV nitride-based light emitters and therefore the optimization of the InGaN/GaN MQWs is significant [1,2]. However, the growth of high quality MQWs has been proved difficult due to the lattice mismatch between InGaN and GaN as well as the trade-off between the thermal instability of InN above 500 1C and the low cracking efficiency of ammonia below 1000 1C [2]. In order to get the high indium composition, low growth temperature (less than 800 1C) is used when growing InGaN quantum wells. Under such a low growth temperature, poor quality InGaN alloys with indium droplets, phase separation and compositional inhomogeneity were reported [3,4]. To address these problems, many works on optimizing the growth parameters of InGaN/GaN MQWs have been carried out [5–7]. On the other hand, though high efficiency GaN-based blue light emitting diodes (LEDs) have been successfully fabricated recently [1], the efficiencies of GaNbased green LEDs are still relatively low. The growth conditions of InGaN with an indium composition around 20% in MQWs, which is the key component of the green LEDs, need to be further optimized. As important growth parameters, the gases introduced into the reactor during growing InGaN/GaN MQWs by metalorganic chemical vapor deposition (MOCVD), namely the trimethylindium (TMIn) [8], ammonia [9] and hydrogen [10], were widely investigated. However, the
n
Corresponding author. Tel.: + 86 133 187 441 46; fax: +86 20 39943836. E-mail address: [email protected] (H. Jiang).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.065
effect of the triethylgallium (TEGa) flow, which is usually used as the Ga source when growing InGaN quantum wells, is seldom reported. Meanwhile, some works had focused on the effect of growth rate via simultaneously varying the flows of TMIn and TEGa [3,7]. But the different TMIn flow might introduce additional influences. Therefore, a further investigation on the influence of TEGa flow on the green light emitting InGaN/GaN MQWs, getting rid of the additional effects due to other gases, is necessary. In this paper, the effects of the TEGa flow on the optical and structural properties of InGaN/GaN MQWs were investigated. It was found that the red shift of photoluminescence (PL) emission peak and the degradation of optical and structural properties took place with increase in the TEGa flow. The temperature dependent PL measurements gave evidences that more structural defects acting as non-radiative recombination centers were formed in the MQWs grown with high TEGa flow. The experiment results showed that despite a slight drop in the indium composition, decreasing the TEGa flow during the growth of wells could improve the well/ barrier interface quality and suppress the indium fluctuation in InGaN wells. It also suggested that InGaN/GaN MQWs-based green light emitting devices with higher internal quantum efficiency (IQE) could be fabricated by adopting low TEGa flow.
2. Experimental procedure The InGaN/GaN MQWs samples were grown on (0 0 0 1) c-plane sapphire substrates in a MOCVD system with the Thomas Swan
S. Huang et al. / Journal of Crystal Growth 314 (2011) 202–206
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Table 1 Details of the wells’ growth parameters of samples A–D together with the calculated results of the well thicknesses and indium compositions via the HR-XRD. Samples
fTEGa (mmol/min)
Si doping in barriers
Well (nm)
Barrier (nm)
Indium composition (%)
Sample Sample Sample Sample
4.47 5.05 5.44 5.44
No No No Yes
2.62 2.80 2.89 2.80
17.05 17.85 19.46 18.60
19.2 20.2 22.8 20.5
A B C D
closely spaced showerhead reactor. TMIn and ammonia (NH3) were used as precursors for In and N, respectively. TEGa was used as the Ga source when growing InGaN wells while trimethylgallium (TMGa) was used instead during other layers. Hydrogen was used as the carrier gas throughout the growth run except the growth of the MQWs. During the growth of InGaN/GaN MQWs, the carrier gas was switched to nitrogen to avoid compromising the indium incorporation. A nucleation layer was first grown on the substrate at 530 1C, followed by a 4 mm undoped GaN layer deposited at 1050 1C. Then, five pairs of InGaN/GaN MQWs were grown. The wells and barriers were grown at 715 and 815 1C, respectively. In the MQWs structure, a 1 nm GaN cap layer was also deposited after each InGaN well at 715 1C before the temperature was ramped up for growing barriers. Finally, the structure was terminated with a 35 nm GaN layer. During the growth of quantum wells, the flows of NH3 and TMIn were held constant to eliminate the potential additional effects. Three samples (samples A, B and C) with various TEGa flow during the quantum-wells’ growth were investigated. In addition, many works had reported that the Si doping in the barriers resulted in better interface properties of the MQWs [11,12]. Therefore, another sample (sample D) using Si-doping barriers was also studied. The silane (SiH4) used in sample D is in the amount of 0.223 nmol/min. The different growth conditions of the four samples are listed in Table 1. The structural properties of the samples were investigated by the high resolution X-ray diffraction (HR-XRD) measurements using a Bruker-AXS D8 Discover instrument. The indium composition and the thicknesses of the wells and barriers were calculated by the LEPTOS software. The optical properties were analyzed by the temperature-dependent PL measurements in the 8–300 K range using a He–Cd laser (25 mW at 325 nm) as the excitation source. The excited luminescence was dispersed and detected by a monochromator equipped with a photomultiplier tube.
3. Results and discussion 3.1. Additional effects of TMIn TMIn and TEGa are the two main source gases used in growing InGaN alloys by MOCVD technology. The amount of the TEGa flow is considered to play a key role on the growth rate of InGaN alloys, while the influence of the TMIn flow is thought to be so weak that its effect is always neglected. In order to characterize the effect of the TMIn on the InGaN growth rate, a series of InGaN films with different TMIn flows but constant TEGa flow at 4.47 mmol/min were grown directly on the 4 mm-thick undoped GaN layer. Fig. 1 shows the InGaN growth rate as a function of the TMIn flow under our experiment conditions. As the TMIn flow became larger, an increase in trend of the growth rate was presented. The growth rate increased 52% when the TMIn flow was changed from 2.33 to 10.04 mmol/min. For comparison, the growth rate as a function of the TEGa flow for another series of InGaN films grown with a constant TMIn flow at 19.84 mmol/min is also shown in the inset of Fig. 1. It is necessary to mention that, the growth rates deduced
Fig. 1. InGaN growth rate as a function of the TMIn flow with a constant TEGa flow at 4.47 mmol/min. The growth rate as a function of the TEGa flow with a constant TMIn flow at 19.84 mmol/min is also shown in the inset for comparison.
from the scanning electron microscopy images of these InGaN films are consistent with that of the InGaN wells in the MQWs structure under our experimental conditions, as confirmed by the transmission electron microscopy (TEM) and XRD results of our MQWs samples. The obvious increase of the growth rate under larger TMIn flow indicates that the influence of the TMIn flow on the InGaN growth rate cannot be neglected, which is in contradiction with what we previously expected. Keller et al. [3] and Lee et al. [7] had studied the effect of growth rate on the InGaN/GaN MQWs growth by simultaneously varying the TMIn and TEGa. However, we do not know what exact role the TEGa played in their works since both the TMIn and the TEGa contributed to the change of the growth rate, just as shown in our experiment. Based on these findings, a series of MQWs samples were deposited for a deeper investigation on the TEGa flow during the MQWs growth, getting rid of the effects introduced by TMIn.
3.2. Influence of the TEGa flow Four MQWs samples with different TEGa flows but constant TMIn flow were grown by MOCVD. HR-XRD studies were performed to investigate their structural properties. The wells/barriers thicknesses and the In compositions in MQWs were determined by fitting the position and the relative intensity of the high-order satellite peaks [13]. In the calculation, we also assumed that the InGaN wells were under fully strained condition, which is often valid for such thin wells. The calculated information of the InGaN/GaN MQWs is also summarized in Table 1. As can be seen, the indium composition increased with increase in the TEGa flow. This increase may be due to an enhancing
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(FWHM) of satellites peaks compared with the flat ones. Assuming a Gaussian distribution function is valid; the interface roughness (IRN) can be described as [14]
indium incorporation effect caused by faster-growing layers under the large TEGa flow [5]. Fig. 2 shows the HR-XRD (0 0 0 4) o/2y scan patterns of the InGaN/ GaN MQWs with different TEGa flows during the wells’ growth. Highorder satellite peaks of these four MQWs samples are well resolved. Especially, satellite peak up to 8th can be clearly distinguished for sample A. Compared with sample A, a distinguishable deterioration of the satellites peaks is clearly seen in samples C and D. It is known that the rough interfaces will broaden the full width at half maximum
Wn ¼ W0 þ ðln 2Þ1=2 DyM
qz / (1/Å)
3.52 3.54 3.56 3.58 3.6 3.62 3.64 3.66 3.68 3.7 3.72
Sample A
9.65
GaN
ð1Þ
where n is the satellite peaks’ order, Wn and W0 are the FWHM of zeroth- and nth-order peaks, respectively, DyM is the angle periods for satellite peaks, and s/L represents the IRN. According to Eq. (1), the IRN are 0.3%, 1.4%, 2.4% and 1.6% for samples A–D, respectively. The inset in Fig. 2 shows the IRNs of the four samples as a function of the TEGa flow. Obviously, the IRN increases with the flow value of the TEGa during the wells’ growth. As the growth parameters are kept constant except the TEGa flow amount, the degradation of the XRD results in sample C implied the low crystal quality with poor well/ barrier interfaces resulting from the large TEGa flow. In the meanwhile, Si doping in the barriers can improve the interface quality of InGaN/GaN MQWs. Adopting Si-doping barriers in sample D, the IRN is reduced from 2.4% to 1.6%. For deeper understanding the influence of the TEGa flow and to investigate the overall strain states of the MQWs samples, the reciprocal space mapping (RSM) scans of the (1 0 5) diffraction for samples A and C were also measured, as presented in Fig. 3. The asymmetric RSMs reveal that the InGaN/GaN MQWs in samples A and C are both coherently strained to the 4 mm undoped GaN layers, evidenced by the vertical alignment of the satellite peaks. This result suggests that the variation of the TEGa flow does not change the overall strain state or cause strain relaxation in the MQWs. However, we also observed that the satellite peaks in sample C were dispersive and could be hardly distinguished, while the 4 satellite peaks in sample A were clearly detected. The detectable high order satellite peaks evidence the superior crystal quality in sample A, which is in good agreement with the results shown in Fig. 2. The RSM analyses suggest that by changing the TEGa flow, we
Fig. 2. HR-XRD (0 0 0 4) o/2y scan patterns of the InGaN/GaN MQWs grown with different TEGa flows. The inset shows the IRN of four samples as a function of the TEGa flow.
9.7
s L
3.52 3.54 3.56 3.58 3.6 3.62 3.64 3.66 3.68 3.7 3.72
9.7
9.7
9.7
Sample C
9.65 9.65
9.65 GaN
9.6
0
9.6
9.6
0
9.6
9.55
-1
9.55 9.55
-1
9.55
9.5
-2
9.5
9.5
-2
9.5
9.45
-3
9.45 9.45
-3
9.45
9.4
-4
9.4
9.4
9.4
9.35 9.35
9.35
9.3
9.3
9.35
9.3
9.3
3.52 3.54 3.56 3.58 3.6 3.62 3.64 3.66 3.68 3.7 3.72
3.52 3.54 3.56 3.58 3.6 3.62 3.64 3.66 3.68 3.7 3.72
qx / (1/Å)
qx / (1/Å)
Fig. 3. Reciprocal space maps of the (1 0 5) diffraction for samples A and C. The dot lines indicate the vertical direction of the GaN peaks.
S. Huang et al. / Journal of Crystal Growth 314 (2011) 202–206
can improve the structural and interfaces’ qualities of InGaN MQWs without introducing strain relaxation. In general, the main influence of the increase in TEGa flow during wells growth is the higher grow rate and lower V/III ratio. The wells growth rate is 1.57 and 2.31 nm/min for samples A and C, respectively. Lee et al. [7] has reported that the lower growth rate allows adatoms on the surface to have enough time to move to the two-dimensional step edges of the growth front and thereby effectively enhances the crystal quality. Also, the lower growth rates are found to be able to minimize the formation of structural defects such as threading dislocations during the crystal growth [6]. The poor crystal quality in sample C may then be ascribed to the higher growth rate. On the other hand, there is strong indium surface segregation on the growth front when growing InGaN alloy due to the different formation enthalpies for the InN and GaN. Some investigations had been reported that high V/III ratios were able to suppress this indium segregation during the growth of InGaN [15]. Therefore, the lower V/III ratios in well growth for sample C may cause a strong indium surface segregation on the quantum wells’ interfaces, deteriorating the interfaces’ quality. As a result of these two effects, sample C presents the inferior crystal quality in the MQWs. The PL spectra of the four InGaN/GaN MQWs samples measured at 8 K are shown in Fig. 4. A single sharp and strong PL peak located at 2.51 eV was detected from sample A. The PL peak was red-shifted from 2.51 to 2.23 eV when the TEGa flow increased from 4.47 to 5.44 mmol/min. This red-shift is related to the stronger indium incorporation in the quantum wells and the quantum-confined Stack effect (QCSE) caused by the huge piezoelectric field in the MQWs increasing with the wells thickness [16]. The 8 K PL intensity and the emission energy as a function of the TEGa flow are also shown in the inset of Fig. 4. It is clear that the PL intensity increases with reducing the amount of TEGa flows. Moreover, we also observed in sample C that there was another emission peak at 2.18 eV. It has been reported that indium-rich clusters caused by inhomogeneous indium distribution in InGaN MQWs would result in the appearance of the lower energy peak [17,18]. Therefore, we ascribe the single and strong PL peak of sample A to the more homogeneous In distribution in the MQWs resulting from the low TEGa flow. One possible mechanism for these behaviors is that the absorption and desorption of the indium in InGaN are existing simultaneously during the wells’ growth, reaching a growth
205
balance. Decrease in the TEGa flow would lead to an enhancement of the random indium desorption and help to build up a new growth balance, resulting in a uniform indium distribution in InGaN wells. This assumption is supported by the lower indium composition in sample A, which is due to the enhancement of indium desorption. The luminescence thermal-quenching characteristics deduced from the temperature-dependent PL measurements are significant factors and have been widely used to estimate the qualities of InGaN MQWs recently [19,20]. Usually, the radiative recombination mechanism dominates at low temperature. When the temperature increases, the non-radiative recombination will become stronger and finally dominates the emission process at room temperature. Therefore, the luminescence thermal quenching processes of MQWs can be used to estimate the density of non-radiative recombination centers (NRCs) in them [21]. A good thermal quenching characteristic also indicates a high IQE of MQWs. Fig. 5 shows the Arrhenius plot of the integrated PL intensity for the four samples. We can see that the luminescence of sample C quenched much faster than that of the other samples. Bimberg et al. [22] had suggested that the normalized integrated PL intensity of the MQWs could be calculated by the following equation: X E IðTÞ=Ið0Þ ¼ 1= 1þ ð2Þ ai exp ai kB T where Eai is the activation energy of the corresponding NRCs, ai is a rate parameter related to the density of there centers and kB is the Boltzmann’s constant. Two types of NRCs are expected in our samples. Using Eq. (2), the temperature-dependent integrated PL intensities were well fitted as shown in Fig. 5. The fitting parameters are summarized in Table 2. It is shown that the activation energies of
Fig. 5. Arrhenius plot of the temperature dependent integrated PL intensity of all samples. The solid lines are the corresponding fitting curves according to Eq. (2).
Table 2 The fitting parameters of the normalized temperature-dependent PL integrated intensities for samples A–D.
Fig. 4. 8 K PL spectra of InGaN/GaN MQWs grown with different TEGa flows. The inset shows the corresponding 8 K PL intensity and the emission energy as a function of the TEGa flow.
Samples
a1
E1 (meV)
a2
E2 (meV)
I(0)
Sample Sample Sample Sample
1.04 0.77 1.75 1.92
14.7 10.7 9.04 18.3
40.01 46.10 203.3 79.75
70.21 74.05 69.66 88.32
0.00030 0.00022 0.00013 0.00010
A B C D
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S. Huang et al. / Journal of Crystal Growth 314 (2011) 202–206
the dominant a2-type NRCs are around Ei ¼70 meV for all samples, but their rate constants (a2) are different. Due to the large TEGa flow, more structural defects (likely to be threading dislocations) are expected in the sample C resulting in the highest rate constant a2 ¼203.3. Meanwhile, the lowest rate constant a2 ¼40.02 is presented in sample A, which is with the best crystal quality confirmed by the HR-XRD and PL results. It is also found that the Si-doping in barriers is an effective method to improve the thermal quenching characteristics of the MQWs’ luminescence. Using Si-doping barriers in sample D, its rate constant is reduced to a2 ¼79.75. However, it still cannot compensate the deterioration resulting from the larger TEGa flow. It thus can be concluded that the formation of structural defects in the MQWs is suppressed in sample A due to the adopting of the small TEGa flow. Therefore, a guide line is provided that one could reduce the structural defects and achieve higher IQE for green light emitting MQWs reducing the TEGa flow when growing InGaN wells.
4. Conclusions In conclusion, the influence of the different TEGa flows during the wells’ growth on the structural and optical properties of the InGaN/GaN MQWs has been investigated. A blue shift of the 8 K PL emission energy was observed with the decrease of the TEGa flow, accompanied with the higher emission intensity. HR-XRD results showed that the well/barrier interfaces deteriorated with the increase in TEGa flow, though the indium composition in quantum wells was a little higher. The luminescence thermal quenching data provided evidences that more structural defects were formed in the MQWs with the increase of TEGa flow during the InGaN well growth. The results presented here were explained by the enhancement of the indium desorption and the new growth balance built up with the decrease of the TEGa flow, which results in the suppression of the indium fluctuation in the InGaN layers and the higher well/barrier interface quality. The investigation also confirms the positive effect of Si-doping barriers on the structural and optical properties of MQWs, which however cannot compensate the deterioration caused by the increased TEGa flow. It is suggested that high quality green light emitting InGaN/GaN MQWs with the homogeneous indium distribution, fewer structural defects and abrupt MQWs interfaces can be grown by means of optimizing the TEGa flow during the wells’ growth.
Acknowledgments This work was supported by the Foundation of the Key Technologies R&D Program of Guangdong Province under Grant 2009498A2202 and the Science and Technology Projects in Guangdong Province under Grant 2008A080800014. References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [2] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Appl. Phys. Lett. 67 (1995) 1868. [3] S. Keller, S.F. Chichibu, M.S. Minsky, E. Hu, U.K. Mishra, S.P. DenBaars, J. Cryst. Growth 195 (1998) 285. [4] M.D. McCluskey, L.T. Romano, B.S. Krusor, D.P. Bour, N.M. Johnson, S. Brennan, Appl. Phys. Lett. 72 (1998) 1730. [5] S. Keller, B.P. Keller, D. Kapolnek, A.C. Abare, H. Masui, L.A. Coldren, U.K. Mishra, S.P.Den Baars, Appl. Phys. Lett. 68 (1996) 3147. [6] S. Keller, S.F. Chichibu1, M.S. Minsky, E. Hu, U.K. Mishra, S.P. DenBaars, J. Cryst. Growth 195 (1998) 258. [7] S. Lee, T. Sakong, W. Lee, H. Paek, M. Seon, I. Lee, O. Nam, Y. Park, J. Cryst. Growth 250 (2003) 256. [8] T.K. Kim, S.K. Shim, S.S. Yang, J.K. Son, Y.K. Hong, G.M. Yang, Curr. Appl. Phys. 7 (2007) 469. [9] A. Oliver, M.J. Kappers, C.J. Humphreys, G. Andrew, D. Briggs, J. Cryst. Growth 272 (2004) 393. [10] E.L. Piner, M.K. Behbehani, N.A. El-Masry, F.G. McIntosh, J.C. Roberts, K.S. Boutros, S.M. Bedair, Appl. Phys. Lett. 70 (1997) 461. [11] Y.-H. Cho, J.J. Song, S. Keller, M.S. Minsky, E. Hu, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 73 (1998) 1128. [12] L.W. Wu, S.J. Chang, T.C. Wen, Y.K. Su, J.F. Chen, W.C. Lai, C.H. Kuo, C.H. Chen, J.K. Sheu, IEEE J. Quant. Electron. 38 (2002) 446. [13] M.E. Vickers, M.J. Kappers, T.M. Smeeton, E.J. Thrush, J.S. Barnard, C.J. Humphreys, J. Appl. Phys. 94 (2003) 1565. [14] Z. Pan, Y.T. Wang, Y. Zhuang, Y.W. Lin, Z.Q. Zhou, L.H. Li, R.H. Wu, Q.M. Wang, Appl. Phys. Lett. 75 (1999) 223. [15] W. Van der Stricht, I. Moerman, P. Demeester, L. Considine, E.J. Thrush, J.A. Crawley, MRS Internet J. Nitride Semicond. Res. 2 (1997) 16. [16] S.F. Chichibu, A.C. Abare, M.P. Mack, M.S. Minsky, T. Deguchi, D. Cohen, P. Kozodoy, S.B. Fleischer, S. Keller, J.S. Speck, J.E. Bowers, E. Hu, U.K. Mishra, L.A. Coldren, S.P. DenBaars, K. Wada, T. Sota, S. Nakamura, Mater. Sci. Eng. B 59 (1999) 298. [17] Y.-S. Lin, K.-J. Ma, C. Hsu, S.-W. Feng, Y.-C. Cheng, C.-C. Liao, C.C. Yang, C.-C. Chou, C.-M. Lee, J.-I. Chyi, Appl. Phys. Lett. 77 (2000) 2988. [18] Y.-L. Lai, C.-P. Liu, Z.-Q. Chen, Appl. Phys. Lett. 86 (2005) 121915. [19] N. Watanabe, M. Yamada, Y. Nagashima, C. Ueki, Y. Sasaki, T. Yamada, K. Taguchi, H. Tadatomo, Okagawa, H. Kudo, Appl. Phys. Lett. 83 (2003) 4906. [20] Q. Dai, M.F. Schubert, M.H. Kim, J.K. Kim, E.F. Schubert, D.D. Koleske, M.H. Crawford, S.R. Lee, A.J. Fischer, G. Thaler, M.A. Banas, Appl. Phys. Lett. 94 (2009) 111109. [21] Y. Wang, X.J. Pei, Z.G. Xing, L.W. Guo, H.Q. Jia, H. Chen, J.M. Zhou, J. Appl. Phys. 101 (2007) 033509. [22] D. Bimberg, M. Sondergeld, E. Grobe, Phys. Rev. B 4 (1971) 3451.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells grown by MOCVD Shanjin Huang, Bingfeng Fan, Yulun Xian, Zhiyuan Zheng, Zhisheng Wu, Hao Jiang n, Gang Wang State Key Laboratory of Optoelectronic Materials and Technology, Sun Yat-sen University, Guangzhou 510275, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2010 Received in revised form 7 November 2010 Accepted 9 November 2010 Communicated by K.W. Benz Available online 18 November 2010
We have investigated the influence of the TEGa flow on the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs) with an indium composition around 20%. The samples with five-pairs InGaN/GaN MQWs were grown on sapphire substrates by metalorganic chemical vapor deposition. Photoluminescence spectra at 8 K showed that the MQWs grown with a low amount of TEGa flow gave a strong single peak and a higher emission energy. High-resolution X-ray diffraction measurements showed a deterioration of the InGaN/GaN interfaces in the sample grown with the large TEGa flow. The luminescence thermal quenching characteristics suggested that more structural defects acting as non-radiative recombination centers formed in the MQWs when the TEGa flow increased. The results indicate that decreasing the TEGa flow help to build up a new growth balance during the growth of InGaN wells, leading to less structural defects, more homogeneous indium distribution and the abrupt MQWs interfaces. & 2010 Elsevier B.V. All rights reserved.
Keywords: Al. High resolution X-ray diffraction A1. Photoluminescence A3. Metalorganic chemical vapor deposition B1. Nitrides B1. Triethylgallium
1. Introduction The InGaN/GaN multiple quantum wells (MQWs) structure is widely used as the active layer in the entire visible and near-UV nitride-based light emitters and therefore the optimization of the InGaN/GaN MQWs is significant [1,2]. However, the growth of high quality MQWs has been proved difficult due to the lattice mismatch between InGaN and GaN as well as the trade-off between the thermal instability of InN above 500 1C and the low cracking efficiency of ammonia below 1000 1C [2]. In order to get the high indium composition, low growth temperature (less than 800 1C) is used when growing InGaN quantum wells. Under such a low growth temperature, poor quality InGaN alloys with indium droplets, phase separation and compositional inhomogeneity were reported [3,4]. To address these problems, many works on optimizing the growth parameters of InGaN/GaN MQWs have been carried out [5–7]. On the other hand, though high efficiency GaN-based blue light emitting diodes (LEDs) have been successfully fabricated recently [1], the efficiencies of GaNbased green LEDs are still relatively low. The growth conditions of InGaN with an indium composition around 20% in MQWs, which is the key component of the green LEDs, need to be further optimized. As important growth parameters, the gases introduced into the reactor during growing InGaN/GaN MQWs by metalorganic chemical vapor deposition (MOCVD), namely the trimethylindium (TMIn) [8], ammonia [9] and hydrogen [10], were widely investigated. However, the
n
Corresponding author. Tel.: + 86 133 187 441 46; fax: +86 20 39943836. E-mail address: [email protected] (H. Jiang).
0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.065
effect of the triethylgallium (TEGa) flow, which is usually used as the Ga source when growing InGaN quantum wells, is seldom reported. Meanwhile, some works had focused on the effect of growth rate via simultaneously varying the flows of TMIn and TEGa [3,7]. But the different TMIn flow might introduce additional influences. Therefore, a further investigation on the influence of TEGa flow on the green light emitting InGaN/GaN MQWs, getting rid of the additional effects due to other gases, is necessary. In this paper, the effects of the TEGa flow on the optical and structural properties of InGaN/GaN MQWs were investigated. It was found that the red shift of photoluminescence (PL) emission peak and the degradation of optical and structural properties took place with increase in the TEGa flow. The temperature dependent PL measurements gave evidences that more structural defects acting as non-radiative recombination centers were formed in the MQWs grown with high TEGa flow. The experiment results showed that despite a slight drop in the indium composition, decreasing the TEGa flow during the growth of wells could improve the well/ barrier interface quality and suppress the indium fluctuation in InGaN wells. It also suggested that InGaN/GaN MQWs-based green light emitting devices with higher internal quantum efficiency (IQE) could be fabricated by adopting low TEGa flow.
2. Experimental procedure The InGaN/GaN MQWs samples were grown on (0 0 0 1) c-plane sapphire substrates in a MOCVD system with the Thomas Swan
S. Huang et al. / Journal of Crystal Growth 314 (2011) 202–206
203
Table 1 Details of the wells’ growth parameters of samples A–D together with the calculated results of the well thicknesses and indium compositions via the HR-XRD. Samples
fTEGa (mmol/min)
Si doping in barriers
Well (nm)
Barrier (nm)
Indium composition (%)
Sample Sample Sample Sample
4.47 5.05 5.44 5.44
No No No Yes
2.62 2.80 2.89 2.80
17.05 17.85 19.46 18.60
19.2 20.2 22.8 20.5
A B C D
closely spaced showerhead reactor. TMIn and ammonia (NH3) were used as precursors for In and N, respectively. TEGa was used as the Ga source when growing InGaN wells while trimethylgallium (TMGa) was used instead during other layers. Hydrogen was used as the carrier gas throughout the growth run except the growth of the MQWs. During the growth of InGaN/GaN MQWs, the carrier gas was switched to nitrogen to avoid compromising the indium incorporation. A nucleation layer was first grown on the substrate at 530 1C, followed by a 4 mm undoped GaN layer deposited at 1050 1C. Then, five pairs of InGaN/GaN MQWs were grown. The wells and barriers were grown at 715 and 815 1C, respectively. In the MQWs structure, a 1 nm GaN cap layer was also deposited after each InGaN well at 715 1C before the temperature was ramped up for growing barriers. Finally, the structure was terminated with a 35 nm GaN layer. During the growth of quantum wells, the flows of NH3 and TMIn were held constant to eliminate the potential additional effects. Three samples (samples A, B and C) with various TEGa flow during the quantum-wells’ growth were investigated. In addition, many works had reported that the Si doping in the barriers resulted in better interface properties of the MQWs [11,12]. Therefore, another sample (sample D) using Si-doping barriers was also studied. The silane (SiH4) used in sample D is in the amount of 0.223 nmol/min. The different growth conditions of the four samples are listed in Table 1. The structural properties of the samples were investigated by the high resolution X-ray diffraction (HR-XRD) measurements using a Bruker-AXS D8 Discover instrument. The indium composition and the thicknesses of the wells and barriers were calculated by the LEPTOS software. The optical properties were analyzed by the temperature-dependent PL measurements in the 8–300 K range using a He–Cd laser (25 mW at 325 nm) as the excitation source. The excited luminescence was dispersed and detected by a monochromator equipped with a photomultiplier tube.
3. Results and discussion 3.1. Additional effects of TMIn TMIn and TEGa are the two main source gases used in growing InGaN alloys by MOCVD technology. The amount of the TEGa flow is considered to play a key role on the growth rate of InGaN alloys, while the influence of the TMIn flow is thought to be so weak that its effect is always neglected. In order to characterize the effect of the TMIn on the InGaN growth rate, a series of InGaN films with different TMIn flows but constant TEGa flow at 4.47 mmol/min were grown directly on the 4 mm-thick undoped GaN layer. Fig. 1 shows the InGaN growth rate as a function of the TMIn flow under our experiment conditions. As the TMIn flow became larger, an increase in trend of the growth rate was presented. The growth rate increased 52% when the TMIn flow was changed from 2.33 to 10.04 mmol/min. For comparison, the growth rate as a function of the TEGa flow for another series of InGaN films grown with a constant TMIn flow at 19.84 mmol/min is also shown in the inset of Fig. 1. It is necessary to mention that, the growth rates deduced
Fig. 1. InGaN growth rate as a function of the TMIn flow with a constant TEGa flow at 4.47 mmol/min. The growth rate as a function of the TEGa flow with a constant TMIn flow at 19.84 mmol/min is also shown in the inset for comparison.
from the scanning electron microscopy images of these InGaN films are consistent with that of the InGaN wells in the MQWs structure under our experimental conditions, as confirmed by the transmission electron microscopy (TEM) and XRD results of our MQWs samples. The obvious increase of the growth rate under larger TMIn flow indicates that the influence of the TMIn flow on the InGaN growth rate cannot be neglected, which is in contradiction with what we previously expected. Keller et al. [3] and Lee et al. [7] had studied the effect of growth rate on the InGaN/GaN MQWs growth by simultaneously varying the TMIn and TEGa. However, we do not know what exact role the TEGa played in their works since both the TMIn and the TEGa contributed to the change of the growth rate, just as shown in our experiment. Based on these findings, a series of MQWs samples were deposited for a deeper investigation on the TEGa flow during the MQWs growth, getting rid of the effects introduced by TMIn.
3.2. Influence of the TEGa flow Four MQWs samples with different TEGa flows but constant TMIn flow were grown by MOCVD. HR-XRD studies were performed to investigate their structural properties. The wells/barriers thicknesses and the In compositions in MQWs were determined by fitting the position and the relative intensity of the high-order satellite peaks [13]. In the calculation, we also assumed that the InGaN wells were under fully strained condition, which is often valid for such thin wells. The calculated information of the InGaN/GaN MQWs is also summarized in Table 1. As can be seen, the indium composition increased with increase in the TEGa flow. This increase may be due to an enhancing
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(FWHM) of satellites peaks compared with the flat ones. Assuming a Gaussian distribution function is valid; the interface roughness (IRN) can be described as [14]
indium incorporation effect caused by faster-growing layers under the large TEGa flow [5]. Fig. 2 shows the HR-XRD (0 0 0 4) o/2y scan patterns of the InGaN/ GaN MQWs with different TEGa flows during the wells’ growth. Highorder satellite peaks of these four MQWs samples are well resolved. Especially, satellite peak up to 8th can be clearly distinguished for sample A. Compared with sample A, a distinguishable deterioration of the satellites peaks is clearly seen in samples C and D. It is known that the rough interfaces will broaden the full width at half maximum
Wn ¼ W0 þ ðln 2Þ1=2 DyM
qz / (1/Å)
3.52 3.54 3.56 3.58 3.6 3.62 3.64 3.66 3.68 3.7 3.72
Sample A
9.65
GaN
ð1Þ
where n is the satellite peaks’ order, Wn and W0 are the FWHM of zeroth- and nth-order peaks, respectively, DyM is the angle periods for satellite peaks, and s/L represents the IRN. According to Eq. (1), the IRN are 0.3%, 1.4%, 2.4% and 1.6% for samples A–D, respectively. The inset in Fig. 2 shows the IRNs of the four samples as a function of the TEGa flow. Obviously, the IRN increases with the flow value of the TEGa during the wells’ growth. As the growth parameters are kept constant except the TEGa flow amount, the degradation of the XRD results in sample C implied the low crystal quality with poor well/ barrier interfaces resulting from the large TEGa flow. In the meanwhile, Si doping in the barriers can improve the interface quality of InGaN/GaN MQWs. Adopting Si-doping barriers in sample D, the IRN is reduced from 2.4% to 1.6%. For deeper understanding the influence of the TEGa flow and to investigate the overall strain states of the MQWs samples, the reciprocal space mapping (RSM) scans of the (1 0 5) diffraction for samples A and C were also measured, as presented in Fig. 3. The asymmetric RSMs reveal that the InGaN/GaN MQWs in samples A and C are both coherently strained to the 4 mm undoped GaN layers, evidenced by the vertical alignment of the satellite peaks. This result suggests that the variation of the TEGa flow does not change the overall strain state or cause strain relaxation in the MQWs. However, we also observed that the satellite peaks in sample C were dispersive and could be hardly distinguished, while the 4 satellite peaks in sample A were clearly detected. The detectable high order satellite peaks evidence the superior crystal quality in sample A, which is in good agreement with the results shown in Fig. 2. The RSM analyses suggest that by changing the TEGa flow, we
Fig. 2. HR-XRD (0 0 0 4) o/2y scan patterns of the InGaN/GaN MQWs grown with different TEGa flows. The inset shows the IRN of four samples as a function of the TEGa flow.
9.7
s L
3.52 3.54 3.56 3.58 3.6 3.62 3.64 3.66 3.68 3.7 3.72
9.7
9.7
9.7
Sample C
9.65 9.65
9.65 GaN
9.6
0
9.6
9.6
0
9.6
9.55
-1
9.55 9.55
-1
9.55
9.5
-2
9.5
9.5
-2
9.5
9.45
-3
9.45 9.45
-3
9.45
9.4
-4
9.4
9.4
9.4
9.35 9.35
9.35
9.3
9.3
9.35
9.3
9.3
3.52 3.54 3.56 3.58 3.6 3.62 3.64 3.66 3.68 3.7 3.72
3.52 3.54 3.56 3.58 3.6 3.62 3.64 3.66 3.68 3.7 3.72
qx / (1/Å)
qx / (1/Å)
Fig. 3. Reciprocal space maps of the (1 0 5) diffraction for samples A and C. The dot lines indicate the vertical direction of the GaN peaks.
S. Huang et al. / Journal of Crystal Growth 314 (2011) 202–206
can improve the structural and interfaces’ qualities of InGaN MQWs without introducing strain relaxation. In general, the main influence of the increase in TEGa flow during wells growth is the higher grow rate and lower V/III ratio. The wells growth rate is 1.57 and 2.31 nm/min for samples A and C, respectively. Lee et al. [7] has reported that the lower growth rate allows adatoms on the surface to have enough time to move to the two-dimensional step edges of the growth front and thereby effectively enhances the crystal quality. Also, the lower growth rates are found to be able to minimize the formation of structural defects such as threading dislocations during the crystal growth [6]. The poor crystal quality in sample C may then be ascribed to the higher growth rate. On the other hand, there is strong indium surface segregation on the growth front when growing InGaN alloy due to the different formation enthalpies for the InN and GaN. Some investigations had been reported that high V/III ratios were able to suppress this indium segregation during the growth of InGaN [15]. Therefore, the lower V/III ratios in well growth for sample C may cause a strong indium surface segregation on the quantum wells’ interfaces, deteriorating the interfaces’ quality. As a result of these two effects, sample C presents the inferior crystal quality in the MQWs. The PL spectra of the four InGaN/GaN MQWs samples measured at 8 K are shown in Fig. 4. A single sharp and strong PL peak located at 2.51 eV was detected from sample A. The PL peak was red-shifted from 2.51 to 2.23 eV when the TEGa flow increased from 4.47 to 5.44 mmol/min. This red-shift is related to the stronger indium incorporation in the quantum wells and the quantum-confined Stack effect (QCSE) caused by the huge piezoelectric field in the MQWs increasing with the wells thickness [16]. The 8 K PL intensity and the emission energy as a function of the TEGa flow are also shown in the inset of Fig. 4. It is clear that the PL intensity increases with reducing the amount of TEGa flows. Moreover, we also observed in sample C that there was another emission peak at 2.18 eV. It has been reported that indium-rich clusters caused by inhomogeneous indium distribution in InGaN MQWs would result in the appearance of the lower energy peak [17,18]. Therefore, we ascribe the single and strong PL peak of sample A to the more homogeneous In distribution in the MQWs resulting from the low TEGa flow. One possible mechanism for these behaviors is that the absorption and desorption of the indium in InGaN are existing simultaneously during the wells’ growth, reaching a growth
205
balance. Decrease in the TEGa flow would lead to an enhancement of the random indium desorption and help to build up a new growth balance, resulting in a uniform indium distribution in InGaN wells. This assumption is supported by the lower indium composition in sample A, which is due to the enhancement of indium desorption. The luminescence thermal-quenching characteristics deduced from the temperature-dependent PL measurements are significant factors and have been widely used to estimate the qualities of InGaN MQWs recently [19,20]. Usually, the radiative recombination mechanism dominates at low temperature. When the temperature increases, the non-radiative recombination will become stronger and finally dominates the emission process at room temperature. Therefore, the luminescence thermal quenching processes of MQWs can be used to estimate the density of non-radiative recombination centers (NRCs) in them [21]. A good thermal quenching characteristic also indicates a high IQE of MQWs. Fig. 5 shows the Arrhenius plot of the integrated PL intensity for the four samples. We can see that the luminescence of sample C quenched much faster than that of the other samples. Bimberg et al. [22] had suggested that the normalized integrated PL intensity of the MQWs could be calculated by the following equation: X E IðTÞ=Ið0Þ ¼ 1= 1þ ð2Þ ai exp ai kB T where Eai is the activation energy of the corresponding NRCs, ai is a rate parameter related to the density of there centers and kB is the Boltzmann’s constant. Two types of NRCs are expected in our samples. Using Eq. (2), the temperature-dependent integrated PL intensities were well fitted as shown in Fig. 5. The fitting parameters are summarized in Table 2. It is shown that the activation energies of
Fig. 5. Arrhenius plot of the temperature dependent integrated PL intensity of all samples. The solid lines are the corresponding fitting curves according to Eq. (2).
Table 2 The fitting parameters of the normalized temperature-dependent PL integrated intensities for samples A–D.
Fig. 4. 8 K PL spectra of InGaN/GaN MQWs grown with different TEGa flows. The inset shows the corresponding 8 K PL intensity and the emission energy as a function of the TEGa flow.
Samples
a1
E1 (meV)
a2
E2 (meV)
I(0)
Sample Sample Sample Sample
1.04 0.77 1.75 1.92
14.7 10.7 9.04 18.3
40.01 46.10 203.3 79.75
70.21 74.05 69.66 88.32
0.00030 0.00022 0.00013 0.00010
A B C D
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the dominant a2-type NRCs are around Ei ¼70 meV for all samples, but their rate constants (a2) are different. Due to the large TEGa flow, more structural defects (likely to be threading dislocations) are expected in the sample C resulting in the highest rate constant a2 ¼203.3. Meanwhile, the lowest rate constant a2 ¼40.02 is presented in sample A, which is with the best crystal quality confirmed by the HR-XRD and PL results. It is also found that the Si-doping in barriers is an effective method to improve the thermal quenching characteristics of the MQWs’ luminescence. Using Si-doping barriers in sample D, its rate constant is reduced to a2 ¼79.75. However, it still cannot compensate the deterioration resulting from the larger TEGa flow. It thus can be concluded that the formation of structural defects in the MQWs is suppressed in sample A due to the adopting of the small TEGa flow. Therefore, a guide line is provided that one could reduce the structural defects and achieve higher IQE for green light emitting MQWs reducing the TEGa flow when growing InGaN wells.
4. Conclusions In conclusion, the influence of the different TEGa flows during the wells’ growth on the structural and optical properties of the InGaN/GaN MQWs has been investigated. A blue shift of the 8 K PL emission energy was observed with the decrease of the TEGa flow, accompanied with the higher emission intensity. HR-XRD results showed that the well/barrier interfaces deteriorated with the increase in TEGa flow, though the indium composition in quantum wells was a little higher. The luminescence thermal quenching data provided evidences that more structural defects were formed in the MQWs with the increase of TEGa flow during the InGaN well growth. The results presented here were explained by the enhancement of the indium desorption and the new growth balance built up with the decrease of the TEGa flow, which results in the suppression of the indium fluctuation in the InGaN layers and the higher well/barrier interface quality. The investigation also confirms the positive effect of Si-doping barriers on the structural and optical properties of MQWs, which however cannot compensate the deterioration caused by the increased TEGa flow. It is suggested that high quality green light emitting InGaN/GaN MQWs with the homogeneous indium distribution, fewer structural defects and abrupt MQWs interfaces can be grown by means of optimizing the TEGa flow during the wells’ growth.
Acknowledgments This work was supported by the Foundation of the Key Technologies R&D Program of Guangdong Province under Grant 2009498A2202 and the Science and Technology Projects in Guangdong Province under Grant 2008A080800014. References [1] S. Nakamura, G. Fasol, The Blue Laser Diode: GaN Based Light Emitters and Lasers, Springer, Berlin, 1997. [2] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Appl. Phys. Lett. 67 (1995) 1868. [3] S. Keller, S.F. Chichibu, M.S. Minsky, E. Hu, U.K. Mishra, S.P. DenBaars, J. Cryst. Growth 195 (1998) 285. [4] M.D. McCluskey, L.T. Romano, B.S. Krusor, D.P. Bour, N.M. Johnson, S. Brennan, Appl. Phys. Lett. 72 (1998) 1730. [5] S. Keller, B.P. Keller, D. Kapolnek, A.C. Abare, H. Masui, L.A. Coldren, U.K. Mishra, S.P.Den Baars, Appl. Phys. Lett. 68 (1996) 3147. [6] S. Keller, S.F. Chichibu1, M.S. Minsky, E. Hu, U.K. Mishra, S.P. DenBaars, J. Cryst. Growth 195 (1998) 258. [7] S. Lee, T. Sakong, W. Lee, H. Paek, M. Seon, I. Lee, O. Nam, Y. Park, J. Cryst. Growth 250 (2003) 256. [8] T.K. Kim, S.K. Shim, S.S. Yang, J.K. Son, Y.K. Hong, G.M. Yang, Curr. Appl. Phys. 7 (2007) 469. [9] A. Oliver, M.J. Kappers, C.J. Humphreys, G. Andrew, D. Briggs, J. Cryst. Growth 272 (2004) 393. [10] E.L. Piner, M.K. Behbehani, N.A. El-Masry, F.G. McIntosh, J.C. Roberts, K.S. Boutros, S.M. Bedair, Appl. Phys. Lett. 70 (1997) 461. [11] Y.-H. Cho, J.J. Song, S. Keller, M.S. Minsky, E. Hu, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 73 (1998) 1128. [12] L.W. Wu, S.J. Chang, T.C. Wen, Y.K. Su, J.F. Chen, W.C. Lai, C.H. Kuo, C.H. Chen, J.K. Sheu, IEEE J. Quant. Electron. 38 (2002) 446. [13] M.E. Vickers, M.J. Kappers, T.M. Smeeton, E.J. Thrush, J.S. Barnard, C.J. Humphreys, J. Appl. Phys. 94 (2003) 1565. [14] Z. Pan, Y.T. Wang, Y. Zhuang, Y.W. Lin, Z.Q. Zhou, L.H. Li, R.H. Wu, Q.M. Wang, Appl. Phys. Lett. 75 (1999) 223. [15] W. Van der Stricht, I. Moerman, P. Demeester, L. Considine, E.J. Thrush, J.A. Crawley, MRS Internet J. Nitride Semicond. Res. 2 (1997) 16. [16] S.F. Chichibu, A.C. Abare, M.P. Mack, M.S. Minsky, T. Deguchi, D. Cohen, P. Kozodoy, S.B. Fleischer, S. Keller, J.S. Speck, J.E. Bowers, E. Hu, U.K. Mishra, L.A. Coldren, S.P. DenBaars, K. Wada, T. Sota, S. Nakamura, Mater. Sci. Eng. B 59 (1999) 298. [17] Y.-S. Lin, K.-J. Ma, C. Hsu, S.-W. Feng, Y.-C. Cheng, C.-C. Liao, C.C. Yang, C.-C. Chou, C.-M. Lee, J.-I. Chyi, Appl. Phys. Lett. 77 (2000) 2988. [18] Y.-L. Lai, C.-P. Liu, Z.-Q. Chen, Appl. Phys. Lett. 86 (2005) 121915. [19] N. Watanabe, M. Yamada, Y. Nagashima, C. Ueki, Y. Sasaki, T. Yamada, K. Taguchi, H. Tadatomo, Okagawa, H. Kudo, Appl. Phys. Lett. 83 (2003) 4906. [20] Q. Dai, M.F. Schubert, M.H. Kim, J.K. Kim, E.F. Schubert, D.D. Koleske, M.H. Crawford, S.R. Lee, A.J. Fischer, G. Thaler, M.A. Banas, Appl. Phys. Lett. 94 (2009) 111109. [21] Y. Wang, X.J. Pei, Z.G. Xing, L.W. Guo, H.Q. Jia, H. Chen, J.M. Zhou, J. Appl. Phys. 101 (2007) 033509. [22] D. Bimberg, M. Sondergeld, E. Grobe, Phys. Rev. B 4 (1971) 3451.