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GaAs multiple quantum wells induced by rapid thermal annealing
Journal of Crystal Growth 483 (2018) 190–194
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/crys
Redshift and blueshift of GaNAs/GaAs multiple quantum wells induced by rapid thermal annealing Yijun Sun a,b, Zhiyuan Cheng a,b,⇑, Qiang Zhou b, Ying Sun b, Jiabao Sun a, Yanhua Liu a, Meifang Wang a, Zhen Cao b, Zhi Ye b, Mingsheng Xu b, Yong Ding b, Peng Chen c,d, Michael Heuken e,f, Takashi Egawa g a
ZJU Micro-Nano Fabrication Center, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China College of Information Science & Electronic Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronics Science and Engineering, Nanjing University, Nanjing 210093, China d Institute of Opto-Electronics, Nanjing University & Yangzhou, Yangzhou 225009, China e AIXTRON SE, Dornkaulstr. 2, Herzogenrath 52134, Germany f GaN Device Technology, RWTH Aachen University, Sommerfeldstr. 24, Aachen 52074, Germany g Research Center for Nano Devices and Advanced Materials, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b c
a r t i c l e
i n f o
Article history: Received 24 October 2017 Received in revised form 22 November 2017 Accepted 24 November 2017 Available online 24 November 2017 Communicated by H. Asahi Keywords: A3. Chemical beam epitaxy B2. Semiconducting III–V materials A3. Quantum wells A1. Optical properties A1. Rapid thermal annealing
a b s t r a c t The effects of rapid thermal annealing (RTA) on the optical properties of GaNAs/GaAs multiple quantum wells (MQWs) grown by chemical beam epitaxy (CBE) are studied by photoluminescence (PL) at 77 K. The results show that the optical quality of the MQWs improves significantly after RTA. With increasing RTA temperature, PL peak energy of the MQWs redshifts below 1023 K, while it blueshifts above 1023 K. Two competitive processes which occur simultaneously during RTA result in redshift at low temperature and blueshift at high temperature. It is also found that PL peak energy shift can be explained neither by nitrogen diffusion out of quantum wells nor by nitrogen reorganization inside quantum wells. PL peak energy shift can be quantitatively explained by a modified recombination coupling model in which redshift nonradiative recombination and blueshift nonradiative recombination coexist. The results obtained have significant implication on the growth and RTA of GaNAs material for high performance optoelectronic device application. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, dilute nitrides such as (In)GaNAs alloys have attracted much attention due to their applications in optoelectronic devices grown on GaAs substrates [1–3]. Unlike conventional III-V semiconductor alloys where the band gap energy changes almost linearly between the band gap energies of parental binary or ternary compounds with a small bowing parameter, the large differences in electronegativity and lattice constant between GaAs and GaN result in a very large optical bowing parameter (14–20 eV) and a significant band gap energy decrease even for small nitrogen content in GaNAs [4,5]. However, because the incorporation efficiency of nitrogen is hundreds of times lower than that of arsenic, the nitrogen content in (In)GaNAs is very low. In order to increase nitrogen content, (In)GaNAs alloys are generally grown at low temperatures compared to N-free samples, and then rapid
⇑ Corresponding author at: ZJU Micro-Nano Fabrication Center, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China. E-mail addresses: [email protected] (Y. Sun), [email protected] (Z. Cheng). https://doi.org/10.1016/j.jcrysgro.2017.11.028 0022-0248/Ó 2017 Elsevier B.V. All rights reserved.
thermal annealing (RTA) is used to improve optical quality [6]. It has been widely demonstrated that for most of (In)GaNAs alloys grown by different methods, including molecular beam epitaxy (MBE) [7–9], metal organic vapor phase epitaxy (MOVPE) [10,11], and chemical beam epitaxy (CBE) [12–14], with increasing RTA temperature, PL peak energy blueshifts. However, concerning RTA-induced redshift of PL peak energy of (In)GaNAs, very limited reports can be found. Wang et al. [15] grew GaNAs by solid source MBE, and only redshift of PL peak energy with annealing was observed in the investigated temperature range from 873 to 1073 K. Loke et al. [16] also grew GaNAs by solid source MBE and found that with increasing RTA temperature from 798 to 1123 K, redshift dominates at temperatures below 973 K, while blueshift dominates at temperatures above 973 K. Although both redshift and blueshift of PL peak energy were observed with annealing, the experimental results were not explained quantitatively. For (In)GaNAs alloys grown by CBE, only blueshift with annealing was observed [12–14], and there are no detailed reports on redshift of PL peak energy induced by RTA.
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In this paper, GaNAs/GaAs quadruple quantum wells (MQWs) are grown by CBE, and the effects of RTA on PL peak energy of the MQWs are investigated in detail by photoluminescence (PL) at 77 K. With increasing RTA temperature from 873 to 1223 K, both redshift and blueshift of PL peak energy of the MQWs are observed experimentally. It is also shown that both the redshift and the blueshift with annealing can be quantitatively explained by a modified recombination coupling model in which redshift nonradiative recombination and blueshift nonradiative recombination coexist. To the best of our knowledge, this is the first report to quantitatively explain both redshift and blueshift of PL peak energy of GaNAs induced by RTA.
2. Experimental The MQWs used for this study are grown on semi-insulating (0 0 1) GaAs substrates by CBE. Triethylgallium (TEG) and arsine (AsH3) are used as gallium and arsenic sources, respectively. Before introduction to the growth chamber, 100% AsH3 is cracked at 1000 °C and nitrogen source is obtained by cracking pure N2 in a radio frequency radical cell (200 W). After growing a 100-nm thick GaAs buffer layer at 580 °C, four periods of GaN0.007As0.993/GaAs MQWs are grown at 436 °C. The growth temperatures have been calibrated by a pyrometer. The thicknesses of the well layer and barrier layer are 7 and 10.5 nm, respectively. The nitrogen content is determined to be 0.7% by X-ray diffraction (XRD, Philips X’Pert MRD). The top layer of the MQWs is GaAs. Samples used for RTA are cleaved pieces of the same sample to avoid scatter due to possible material variations. RTA experiments are performed on a rapid thermal annealing system (VHC-610CP, ULVAC RIKO Inc.) that uses halogen lamps under N2 flow. During RTA, a GaAs wafer covers the sample face to face to prevent surface desorption. After growth, RTA experiments are performed at different temperatures from 873 to 1223 K for 30 s. The optical properties of the MQWs are examined by PL measurements at 77 K, in which an argon ion laser (514.5 nm) is used as the excitation source.
1223K
1123K 1073K
Fig. 1 shows PL spectra at 77 K for the MQWs after RTA at different temperatures from 873 to 1223 K for 30 s. It can be seen from Fig. 1 that with increasing RTA temperature, both redshfit and blueshift of PL peak energy are clearly observed as indicated by arrows with a solid line. It should be noted that before RTA, PL intensity is too low to be detected for as-grown sample. After RTA, the PL peak from GaNAs/GaAs MQWs can be clearly observed. This indicates that the optical quality of the MQWs improves significantly after RTA. The remarkable increase in PL intensity after RTA is attributed to the removal of defects and impurities in the wells, barriers, and heterointerface regions. Fig. 2 summarizes the effect of RTA temperature on PL peak energy. It can be seen from Fig. 2 that with increasing RTA temperature, the PL peak energy redshifts at temperatures below 1023 K, while it blueshifts at temperatures above 1023 K. That is to say, redshfit dominates at low temperatures while blueshift dominates at high temperatures. Both redshift and blueshift of PL peak energy can be clearly observed at the same time, and the transition temperature is determined to be around 1023 K. This indicates that two competitive processes occur simultaneously, low temperature process and high temperature process, respectively. Although RTA-induced blueshift has been widely observed in (In)GaNAs alloys grown by different methods, concerning redshift induced by RTA, very limited data can be found in literature. In order to understand the mechanism for the PL peak energy shift, the effect of RTA temperature on PL intensity is shown in Fig. 3. Generally speaking, PL intensity (IPL ) increases with increasing RTA temperature. Further investigations show that with increasing RTA temperature, two different processes, low temperature process and high temperature process, can be clearly observed. It can be seen from Fig. 3 that below 1023 K, the dependence of PL intensity follows Arrhenius equation. That is to say, the dependence of logðIPL Þ on reciprocal temperature can be well fitted by a straight line below 1023 K, as can be seen from Fig. 3. However, at temperatures above 1023 K, the dependence of logðIPL Þ on reciprocal temperature cannot be well fitted by a straight line, indicating that the dependence of PL intensity above 1023 K does not follow Arrhenius equation any more. Although at present time it is very difficult for us to use a suitable function to simulate the high temperature process above 1023 K, the different dependences of PL intensity on RTA temperature in different temperature regions strongly suggest that with increasing RTA temperature, two processes with different mechanisms occur simultaneously during RTA. Based upon the results of Figs. 2 and 3, it is concluded that the low temperature process which follows Arrhenius equation is responsible for the redshift while the high temperature process which does not follow Arrhenius equation is responsible for the blueshift and that the compe-
1023K
1.42
PL Peak Energy (eV)
Normalized PL Intenstiy
1173K
3. Results and discussion
973K 923K 873K
1.35
1.37
1.39
1.41
1.43
1.45
Energy (eV)
Experimental Model
1.41
1.40
1.39
900
1000
1100
1200
RTA Temperature (K) Fig. 1. 77K PL spectra for the MQWs after RTA at different temperatures for 30 s. Peak positions are indicated by arrows with a solid line.
Fig. 2. Effect of RTA temperature on PL peak energy.
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PL Intensity Arrhenius Plot
3
10
2
10
1
XRD Intensity (a.u.)
PL Intensity (a.u.)
10
(a)
(b) (c)
-0.6
10
0.8
0.9
1.0
1.1
-0.2
0.0
0.2
0.4
0.6
Relative Omega (degrees)
1.2
1000/T (1/K)
-0.4
Fig. 4. XRD spectra for the MQWs (a) before and (b) after RTA, and (c) corresponding dynamical simulation results.
Fig. 3. Effect of RTA temperature on PL intensity.
PL Intensity (a.u.)
tition between these two processes results in the observed redshift and blueshift in Fig. 2. Due to the fact that redshift occurs at low temperatures while blueshift occurs at high temperatures, with increasing RTA temperature redshift can be more easily annealed out compared with blueshift. This is the reason why blueshift is more commonly observed whereas there are only limited reports on redshift in literature. It has been widely reported that PL peak energy shifts with RTA temperature, especially blueshift. Up to now, three different models are mainly used to quantitatively account for the observed blueshift with RTA temperature. Li et al. [7] investigated the effects of RTA on the optical properties of GaNAs/GaAs single quantum well grown by MBE using low temperature PL, and found that PL peak energy blueshifts with RTA temperature. They explained the blueshift with RTA temperature by nitrogen diffusion out of quantum well, and suggested that nitrogen diffusion from well layer to barrier layer is responsible for the observed blueshift. They modeled the blueshift by assuming error function diffusion and solving the Schrodinger equation for an arbitrary potential well using a particle transmission calculation. After assuming an isotropic diffusion of N and As, the dependence of the observed blueshift on RTA temperature was calculated. Although this model can quantitatively explain RTA-induced blueshift by nitrogen diffusion from well layer to barrier layer, the model is obviously contrary to the observations that even if there is no nitrogen diffusion from well layer to barrier layer, there is still a blueshift with increasing RTA temperature. For both GaNAs QWs and thick epilayers, RTAinduced blueshift was clearly observed in case that there is no macroscopic nitrogen diffusion from well layer to barrier layer [8,12,16]. In order to further check the validity of this model to our case, XRD is used to characterize nitrogen contents of the MQWs before and after RTA, and the results are shown in Fig. 4. It can be seen from Fig. 4 that all the satellite peak positions before RTA are in good agreement with those after RTA. Both XRD spectra before and after RTA can be well fitted by dynamic simulation, and the nitrogen contents are the same before and after RTA, 0.7% according to the simulation results. Therefore, based upon above analysis and the results of Figs. 2 and 4, it is concluded that the nitrogen diffusion model is not suitable to account for the observed redshift and blueshift in this work. In order to quantitatively explain RTA-induced blueshift in case of no nitrogen diffusion out of GaNAs, nitrogen reorganization model was proposed. Grenouillet et al. [8] reported the effect of RTA temperature on PL peak energy, and found that with increasing RTA temperature PL peak energy blueshifts for both GaNAs epilayers and GaNAs/GaAs QWs grown by gas source MBE. They explained the observed blueshift with increasing RTA temperature
in terms of RTA-induced nitrogen reorganization by assuming nitrogen diffusion inside GaNAs. After assuming that PL peak energy increases with the amplitude of nitrogen composition fluctuations, the dependences of PL peak energy on RTA temperature and time were proposed. This model can quantitatively explain RTA-induced blueshift by nitrogen composition fluctuations for the case that nitrogen composition is kept constant before and after RTA. However, in the case that there are no nitrogen composition fluctuations, the model is not suitable any more. In order to check the validity of this model to the observed redshift and blueshift in this work, 77 K PL spectrum for the MQWs annealed at 1123 K for 30 s is fitted by Gauss function, and the results are shown in Fig. 5. It can be seen from Fig. 5 that the PL spectrum for the MQWs is symmetric and can be well fitted by Gauss function. It should be noted that the low energy tail which is characteristic of nitrogen composition fluctuations is not observed from Fig. 5. After line shape analysis, no low energy tail can be found for all the PL spectra of the MQWs annealed at different RTA temperatures, although there is a high energy shoulder for the MQWs annealed at low temperatures as can be seen from Fig. 1. Therefore, it is concluded that the nitrogen reorganization model is not suitable to account for the observed redshift and blueshift in this work either. The PL peak energy shift induced by RTA can also be quantitatively explained by a recombination coupling model. Sun et al. [12] studied the effect of RTA temperature on PL peak energy for the triple QWs grown by CBE and found that with increasing RTA temperature from 873 to 1273 K, PL peak energy blueshifts. In the whole temperature range investigated, no redshift was observed. Due to the fact that neither nitrogen diffusion model nor nitrogen reorganization model could explain the observed blueshfit with increasing RTA temperature, the recombination coupling model was proposed in which it was assumed that there is a
1.34
Experimental Gauss fit
1.36
1.38
1.40
1.42
1.44
1.46
Energy (eV) Fig. 5. 77K PL spectrum for the MQWs annealed at 1123 K for 30 s.
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coupling between radiative recombination of PL emission and nonradiative recombination of nonradiative centers, affecting PL peak energy. After assuming that the magnitude of the blueshift is proportional to the relative change in the concentration of nonradiative centers, PL peak energy EðT; tÞ after RTA at temperature T for time t can be calculated by
EB EðT; tÞ ¼ E0 þ gB 1 exp k2B DB t exp ; kT
ð1Þ
where E0 is initial PL peak energy before annealing, gB is the coupling strength between the radiative recombination of PL emission and the nonradiative recombination of nonradiative centers. EB is activak2B
is a constant, and the subtion energy, DB is diffusion constant, script B represents blueshift. It has been demonstrated that this quantitative model well explains the blueshift induced by RTA in GaNAs grown by different methods and under different experimental conditions [12], and that the simulation parameters of E0 are in good agreement with experimental data of E0 [12]. However, although this model can explain the observed blueshift quantitatively, this model cannot explain the case where there is only redshift and the case where redshift and blueshift coexist. Actually, in addition to RTA-induced blueshift, RTA-induced redshift was observed already [15]. Especially, RTA-induced redshift and blueshift of PL peak energy have also been observed simultaneously for GaNAs films grown by gas source MBE [16] and for the MQWs grown by CBE in this work. Therefore, in order to quantitatively explain all the three cases, the case where there is only redshift, the case where there is only blueshift, and the case where redshift and blueshift coexist, it is necessary to modify the model. Based upon the analysis for the recombination coupling model, it is concluded that the reason why the model is not able to explain the case where there is only redshift and the case where blueshift and redshift coexist is that, only one type of nonradiative recombination was proposed in the model. That is to say, although the blueshift nonradiative recombination which results in blueshift with annealing by coupling with radiative recombination was proposed, the redshift nonradiative recombination which results in redshift with annealing by coupling with radiative recombination was not proposed. In this case, based upon the observed redshift and blueshift of Fig. 2 and the redshifts observed in literature [15,16], it is reasonable to propose that there are two types of nonradiative recombination, redshfit nonradiative recombination and blueshift nonradiative recombination, respectively. Redshift nonradiative recombination results in redshift with annealing while blueshift nonradiative recombination results in blueshift with annealing. Nonshift nonradiative recombination which has no contribution to the perturbation of PL peak energy is not considered here. The competition between redshift and blueshift results in all the three cases, the case of only redshift, the case of only blueshift, and the case where redshift and blueshift coexist. Therefore, PL peak energy EðT; tÞ after RTA at temperature T for time t can be described theoretically by
ERi kT i¼1 n X E gBj 1 exp k2Bj DBj t exp Bj þ : kT j¼1
EðT; tÞ ¼ E0
m X
gRi 1 exp k2Ri DRi t exp
ð2Þ
where the subscripts R and B represent redshift and blueshift, respectively, m and n are respectively the total numbers of redshift and blueshift nonradiative recombination coupled with radiative recombination. Practically, and for simplicity, all the redshifts induced by RTA can be regarded as one effective redshift, and all the blueshifts induced by RTA can be regarded as one effective blueshift. Then, Eq. (2) changes to
ER EðT; tÞ ¼ E0 gR 1 exp k2R DR t exp kT EB 2 : þ gB 1 exp kB DB t exp kT
ð3Þ
The experimental results in Fig. 2 are fitted with Eq. (3), and the fitting results are also shown in the figure for comparison. It can be seen from Fig. 2 that the experimental results can be well fitted by Eq. (3) using fitting parameters as shown in Table 1. It can be seen from Table 1 that both the coupling strengths for redshift and blueshfit are on the same order (102 eV), consistent with that obtained for the triple QWs grown by CBE [12]. The coupling strength of redshift is comparable with that of blueshift, resulting that redshift and blueshift are observed with annealing simultaneously. If one of the coupling strengths is obviously higher than the other, redshift and blueshift will not be observed simultaneously with increasing RTA temperature. In this case only redshift or blueshift can be observed. It can also be seen from Table 1 that, the activation energy for the redshift is 1.23 eV, while the activation energy for the blueshift is 2.24 eV. The higher activation energy for the blueshift compared with that for the redshift clearly indicates that redshift process is easier to be annealed out with annealing compared with blueshift process. This is in good agreement with the fact that the redshift occurs at low temperatures while the blueshift occurs at high temperatures and that blueshift is more commonly observed in literature. It can also be seen from Eq. (3) that the change in PL peak energy induced by RTA is controlled by the competition between redshift and blueshift. If PL peak energy is dominated by RTA-induced blueshift, the second term of Eq. (3) can be ignored (gR ¼ 0), then Eq. (3) changes to Eq. (1). If PL peak energy is dominated by RTA-induced redshift, the third term of Eq. (3) can be ignored. Then Eq. (3) changes to
ER : EðT; tÞ ¼ E0 gR 1 exp k2R DR t exp kT
ð4Þ
In order to further check the validity of the modified recombination coupling model, the experimental results in Ref. [15] are fitted with Eq. (4), and the fitting results are also shown in Fig. 6 for comparison. Ref. [15] is the first report for GaNAs in which only redshift was observed after RTA. It can be seen from Fig. 6 that the experimental results can be well fitted by Eq. (4) and the fitting parameters are shown in Table 1. It can be seen from Table 1 that the coupling strength 0.048 eV obtained from Ref. [15] is very close to the coupling strength 0.50 eV obtained in Ref. [12] in which only blueshift was observed. Further investigations show that the coupling strengths from Refs. [15,12] are about three times higher than the coupling strength for redshift obtained in this work. This is the reason why only redshift and only blueshift were observed from Refs. [15,12], respectively, while redshift and blueshift are observed simultaneously in this work.
Table 1 Fitting parameters for modified recombination coupling model.
Fig. 1 Fig. 6
E0 (eV)
gR (eV)
k2R DR t
ER (eV)
gB (eV)
k2B DB t
EB (eV)
1.408 1.263
0.014 0.048
5.0E6 1.2E4
1.23 0.71
0.023
6.1E9
2.24
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Y. Sun et al. / Journal of Crystal Growth 483 (2018) 190–194
4. Conclusions
PL Peak Energy (eV)
1.24 Experimental Model
1.23
1.22
1.21 850
900
950
1000
1050
1100
RTA Temperature (K) Fig. 6. Effect of RTA temperature on PL peak energy. The experimental data are from Ref. [15].
Based upon the above analysis, it is concluded that the modified recombination coupling model can well explain all the three cases for PL peak energy shift induced by RTA, the case where redshift is dominated, the case where blueshift is dominated, and the case where both redshift and blueshift are observed simultaneously with annealing. It is worth noting that the dependence of PL peak energy on RTA temperature in this work is very similar to that observed in Ref. [16] although the samples used are grown by different methods. That is to say, with increasing RTA temperature, redshift is dominated at low temperatures while blueshift is dominated at high temperatures. The transition temperature around 1023 K obtained in this work is also close to 973 K obtained for GaNAs with nitrogen content of 1.3% in Ref. [16]. It is also worth mentioning that for the quadruple QWs grown by CBE in this work both blueshift and redshift are observed simultaneously with annealing while for the triple QWs grown by CBE only blueshift was found [12]. This indicates that even if the growth method is the same, the dependence of PL peak energy on annealing is different due to different material structures. The thicknesses of well and barrier layers for the quadruple QWs in this work are 7 and 10.5 nm, respectively, whereas the thicknesses of well and barrier layers for the triple QWs are 8.5 and 27.8 nm, respectively. Similar results are also found for GaNAs grown by MBE. Wang et al. [15] reported that for GaN0.0145As0.9855 grown by MBE PL peak exhibited a redshift of 18 meV from 1.234 to 1.216 eV with annealing from 873 to 1073 K, while for GaN0.019As0.981 grown by the same MBE PL peak exhibited a blueshift of 75 meV from 0.948 to 1.023 eV. They attributed the difference in PL peak energy with annealing to different growth conditions. These results clearly show that the dependence of PL peak energy on annealing is very complex, depending on growth method, material structure, as well as growth conditions. Further investigation is necessary.
In conclusion, the effects of RTA on the optical properties of GaNAs/GaAs MQWs grown by CBE are studied by PL in detail with special emphasis on RTA-induced PL peak energy shift. The results show that the optical quality of the MQWs improves significantly after RTA. With increasing RTA temperature, both redshift and blueshift are observed simultaneously, and the transition temperature is determined to be around 1023 K. Low temperature process results in redshift while high temperature process results in blueshift. The competition between these two processes is responsible for the observed redshift and blueshift. Based upon the observed redshfit and blueshift with annealing in this work and the observations in literature, the recombination coupling model is modified in which it is proposed that there are two types of nonradiative recombination, redshift nonradiative recombination and blueshift nonradiative recombination, respectively. It is found that the modified recombination coupling model can quantitatively explain all the three cases for PL peak energy shift induced by RTA, the case where redshift is dominated, the case where blueshift is dominated, and the case where both redshift and blueshift are observed simultaneously. References [1] J.E. Stehr, S.L. Chen, M. Jansson, F. Ishikawa, W.M. Chen, I.A. Buyanova, Appl. Phys. Lett. 109 (2016) 203103. [2] D. Tang, G.K. Vijaya, A. Mehrotra, A. Freundlich, D.J. Smith, J. Vac. Sci. & Tech. B 34 (2016) 011210. [3] M. Baranowski, R. Kudrawiec, J. Misiewicz, Appl. Phys. A 118 (2015) 479. [4] J.Y. Duboz, J.A. Gupta, Z.R. Wasilewski, J. Ramsey, R.L. Williams, G.C. Aers, B.J. Riel, G.I. Sproule, Phys. Rev. B 66 (2002) 085313. [5] L. Auvray, H. Dumnot, J. Dazord, Y. Monteil, J. Bouix, C. Bru-Chevallier, L. Grenouillet, Mater. Sci. Semcon. Process. 3 (2000) 505. [6] Ł. Gelczuk, H. Stokowski, M. Da˛browska-Szata, R. Kudrawiec, J. Appl. Phys. 119 (2016) 185706. [7] L.H. Li, Z. Pan, Y.Q. Xu, Y. Du, Y.W. Lin, R.H. Wu, Appl. Phys. Lett. 78 (2001) 2488. [8] L. Grenouillet, C. Bru-Chevallier, G. Guillot, P. Gilet, P. Ballet, P. Duvaut, G. Rolland, A. Million, J. Appl. Phys. 91 (2002) 5902. [9] Z.L. Liu, P.P. Chen, C. Wang, T.X. Li, H.Y. Cui, Y.J. Li, X.S. Chen, W. Lu, J. Appl. Phys. 101 (2007) 113514. [10] P. Klangtakai, S. Sanorpim, K. Yoodee, W. Ono, F. Nakajima, R. Katayama, K. Onabe, J. Cryst. Growth 298 (2007) 140. [11] D. Sentosa, X. Tang, Z. Yin, S.J. Chua, J. Cryst. Growth 307 (2007) 229. [12] Y. Sun, T. Egawa, H. Ishikawa, J. Appl. Phys. 96 (2004) 2586. [13] A. Fotkatzikis, M.-A. Pinault, J.A.H. Coaquira, A. Freundlich, J. Vac. Sci. Tech. B 23 (2005) 1333. [14] T. Kageyama, T. Miyamoto, S. Makino, F. Koyama, K. Iga, Jpn. J. Appl. Phys. 38 (1999) L298. [15] S.Z. Wang, S.F. Yoon, W.K. Loke, C.Y. Liu, S. Yuan, J. Crystal Growth 255 (2003) 258. [16] W.K. Loke, S.F. Yoon, S.Z. Wang, T.K. Ng, W.J. Fan, J. Appl. Phys. 91 (2002) 4900.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/crys
Redshift and blueshift of GaNAs/GaAs multiple quantum wells induced by rapid thermal annealing Yijun Sun a,b, Zhiyuan Cheng a,b,⇑, Qiang Zhou b, Ying Sun b, Jiabao Sun a, Yanhua Liu a, Meifang Wang a, Zhen Cao b, Zhi Ye b, Mingsheng Xu b, Yong Ding b, Peng Chen c,d, Michael Heuken e,f, Takashi Egawa g a
ZJU Micro-Nano Fabrication Center, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China College of Information Science & Electronic Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronics Science and Engineering, Nanjing University, Nanjing 210093, China d Institute of Opto-Electronics, Nanjing University & Yangzhou, Yangzhou 225009, China e AIXTRON SE, Dornkaulstr. 2, Herzogenrath 52134, Germany f GaN Device Technology, RWTH Aachen University, Sommerfeldstr. 24, Aachen 52074, Germany g Research Center for Nano Devices and Advanced Materials, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b c
a r t i c l e
i n f o
Article history: Received 24 October 2017 Received in revised form 22 November 2017 Accepted 24 November 2017 Available online 24 November 2017 Communicated by H. Asahi Keywords: A3. Chemical beam epitaxy B2. Semiconducting III–V materials A3. Quantum wells A1. Optical properties A1. Rapid thermal annealing
a b s t r a c t The effects of rapid thermal annealing (RTA) on the optical properties of GaNAs/GaAs multiple quantum wells (MQWs) grown by chemical beam epitaxy (CBE) are studied by photoluminescence (PL) at 77 K. The results show that the optical quality of the MQWs improves significantly after RTA. With increasing RTA temperature, PL peak energy of the MQWs redshifts below 1023 K, while it blueshifts above 1023 K. Two competitive processes which occur simultaneously during RTA result in redshift at low temperature and blueshift at high temperature. It is also found that PL peak energy shift can be explained neither by nitrogen diffusion out of quantum wells nor by nitrogen reorganization inside quantum wells. PL peak energy shift can be quantitatively explained by a modified recombination coupling model in which redshift nonradiative recombination and blueshift nonradiative recombination coexist. The results obtained have significant implication on the growth and RTA of GaNAs material for high performance optoelectronic device application. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, dilute nitrides such as (In)GaNAs alloys have attracted much attention due to their applications in optoelectronic devices grown on GaAs substrates [1–3]. Unlike conventional III-V semiconductor alloys where the band gap energy changes almost linearly between the band gap energies of parental binary or ternary compounds with a small bowing parameter, the large differences in electronegativity and lattice constant between GaAs and GaN result in a very large optical bowing parameter (14–20 eV) and a significant band gap energy decrease even for small nitrogen content in GaNAs [4,5]. However, because the incorporation efficiency of nitrogen is hundreds of times lower than that of arsenic, the nitrogen content in (In)GaNAs is very low. In order to increase nitrogen content, (In)GaNAs alloys are generally grown at low temperatures compared to N-free samples, and then rapid
⇑ Corresponding author at: ZJU Micro-Nano Fabrication Center, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China. E-mail addresses: [email protected] (Y. Sun), [email protected] (Z. Cheng). https://doi.org/10.1016/j.jcrysgro.2017.11.028 0022-0248/Ó 2017 Elsevier B.V. All rights reserved.
thermal annealing (RTA) is used to improve optical quality [6]. It has been widely demonstrated that for most of (In)GaNAs alloys grown by different methods, including molecular beam epitaxy (MBE) [7–9], metal organic vapor phase epitaxy (MOVPE) [10,11], and chemical beam epitaxy (CBE) [12–14], with increasing RTA temperature, PL peak energy blueshifts. However, concerning RTA-induced redshift of PL peak energy of (In)GaNAs, very limited reports can be found. Wang et al. [15] grew GaNAs by solid source MBE, and only redshift of PL peak energy with annealing was observed in the investigated temperature range from 873 to 1073 K. Loke et al. [16] also grew GaNAs by solid source MBE and found that with increasing RTA temperature from 798 to 1123 K, redshift dominates at temperatures below 973 K, while blueshift dominates at temperatures above 973 K. Although both redshift and blueshift of PL peak energy were observed with annealing, the experimental results were not explained quantitatively. For (In)GaNAs alloys grown by CBE, only blueshift with annealing was observed [12–14], and there are no detailed reports on redshift of PL peak energy induced by RTA.
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In this paper, GaNAs/GaAs quadruple quantum wells (MQWs) are grown by CBE, and the effects of RTA on PL peak energy of the MQWs are investigated in detail by photoluminescence (PL) at 77 K. With increasing RTA temperature from 873 to 1223 K, both redshift and blueshift of PL peak energy of the MQWs are observed experimentally. It is also shown that both the redshift and the blueshift with annealing can be quantitatively explained by a modified recombination coupling model in which redshift nonradiative recombination and blueshift nonradiative recombination coexist. To the best of our knowledge, this is the first report to quantitatively explain both redshift and blueshift of PL peak energy of GaNAs induced by RTA.
2. Experimental The MQWs used for this study are grown on semi-insulating (0 0 1) GaAs substrates by CBE. Triethylgallium (TEG) and arsine (AsH3) are used as gallium and arsenic sources, respectively. Before introduction to the growth chamber, 100% AsH3 is cracked at 1000 °C and nitrogen source is obtained by cracking pure N2 in a radio frequency radical cell (200 W). After growing a 100-nm thick GaAs buffer layer at 580 °C, four periods of GaN0.007As0.993/GaAs MQWs are grown at 436 °C. The growth temperatures have been calibrated by a pyrometer. The thicknesses of the well layer and barrier layer are 7 and 10.5 nm, respectively. The nitrogen content is determined to be 0.7% by X-ray diffraction (XRD, Philips X’Pert MRD). The top layer of the MQWs is GaAs. Samples used for RTA are cleaved pieces of the same sample to avoid scatter due to possible material variations. RTA experiments are performed on a rapid thermal annealing system (VHC-610CP, ULVAC RIKO Inc.) that uses halogen lamps under N2 flow. During RTA, a GaAs wafer covers the sample face to face to prevent surface desorption. After growth, RTA experiments are performed at different temperatures from 873 to 1223 K for 30 s. The optical properties of the MQWs are examined by PL measurements at 77 K, in which an argon ion laser (514.5 nm) is used as the excitation source.
1223K
1123K 1073K
Fig. 1 shows PL spectra at 77 K for the MQWs after RTA at different temperatures from 873 to 1223 K for 30 s. It can be seen from Fig. 1 that with increasing RTA temperature, both redshfit and blueshift of PL peak energy are clearly observed as indicated by arrows with a solid line. It should be noted that before RTA, PL intensity is too low to be detected for as-grown sample. After RTA, the PL peak from GaNAs/GaAs MQWs can be clearly observed. This indicates that the optical quality of the MQWs improves significantly after RTA. The remarkable increase in PL intensity after RTA is attributed to the removal of defects and impurities in the wells, barriers, and heterointerface regions. Fig. 2 summarizes the effect of RTA temperature on PL peak energy. It can be seen from Fig. 2 that with increasing RTA temperature, the PL peak energy redshifts at temperatures below 1023 K, while it blueshifts at temperatures above 1023 K. That is to say, redshfit dominates at low temperatures while blueshift dominates at high temperatures. Both redshift and blueshift of PL peak energy can be clearly observed at the same time, and the transition temperature is determined to be around 1023 K. This indicates that two competitive processes occur simultaneously, low temperature process and high temperature process, respectively. Although RTA-induced blueshift has been widely observed in (In)GaNAs alloys grown by different methods, concerning redshift induced by RTA, very limited data can be found in literature. In order to understand the mechanism for the PL peak energy shift, the effect of RTA temperature on PL intensity is shown in Fig. 3. Generally speaking, PL intensity (IPL ) increases with increasing RTA temperature. Further investigations show that with increasing RTA temperature, two different processes, low temperature process and high temperature process, can be clearly observed. It can be seen from Fig. 3 that below 1023 K, the dependence of PL intensity follows Arrhenius equation. That is to say, the dependence of logðIPL Þ on reciprocal temperature can be well fitted by a straight line below 1023 K, as can be seen from Fig. 3. However, at temperatures above 1023 K, the dependence of logðIPL Þ on reciprocal temperature cannot be well fitted by a straight line, indicating that the dependence of PL intensity above 1023 K does not follow Arrhenius equation any more. Although at present time it is very difficult for us to use a suitable function to simulate the high temperature process above 1023 K, the different dependences of PL intensity on RTA temperature in different temperature regions strongly suggest that with increasing RTA temperature, two processes with different mechanisms occur simultaneously during RTA. Based upon the results of Figs. 2 and 3, it is concluded that the low temperature process which follows Arrhenius equation is responsible for the redshift while the high temperature process which does not follow Arrhenius equation is responsible for the blueshift and that the compe-
1023K
1.42
PL Peak Energy (eV)
Normalized PL Intenstiy
1173K
3. Results and discussion
973K 923K 873K
1.35
1.37
1.39
1.41
1.43
1.45
Energy (eV)
Experimental Model
1.41
1.40
1.39
900
1000
1100
1200
RTA Temperature (K) Fig. 1. 77K PL spectra for the MQWs after RTA at different temperatures for 30 s. Peak positions are indicated by arrows with a solid line.
Fig. 2. Effect of RTA temperature on PL peak energy.
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PL Intensity Arrhenius Plot
3
10
2
10
1
XRD Intensity (a.u.)
PL Intensity (a.u.)
10
(a)
(b) (c)
-0.6
10
0.8
0.9
1.0
1.1
-0.2
0.0
0.2
0.4
0.6
Relative Omega (degrees)
1.2
1000/T (1/K)
-0.4
Fig. 4. XRD spectra for the MQWs (a) before and (b) after RTA, and (c) corresponding dynamical simulation results.
Fig. 3. Effect of RTA temperature on PL intensity.
PL Intensity (a.u.)
tition between these two processes results in the observed redshift and blueshift in Fig. 2. Due to the fact that redshift occurs at low temperatures while blueshift occurs at high temperatures, with increasing RTA temperature redshift can be more easily annealed out compared with blueshift. This is the reason why blueshift is more commonly observed whereas there are only limited reports on redshift in literature. It has been widely reported that PL peak energy shifts with RTA temperature, especially blueshift. Up to now, three different models are mainly used to quantitatively account for the observed blueshift with RTA temperature. Li et al. [7] investigated the effects of RTA on the optical properties of GaNAs/GaAs single quantum well grown by MBE using low temperature PL, and found that PL peak energy blueshifts with RTA temperature. They explained the blueshift with RTA temperature by nitrogen diffusion out of quantum well, and suggested that nitrogen diffusion from well layer to barrier layer is responsible for the observed blueshift. They modeled the blueshift by assuming error function diffusion and solving the Schrodinger equation for an arbitrary potential well using a particle transmission calculation. After assuming an isotropic diffusion of N and As, the dependence of the observed blueshift on RTA temperature was calculated. Although this model can quantitatively explain RTA-induced blueshift by nitrogen diffusion from well layer to barrier layer, the model is obviously contrary to the observations that even if there is no nitrogen diffusion from well layer to barrier layer, there is still a blueshift with increasing RTA temperature. For both GaNAs QWs and thick epilayers, RTAinduced blueshift was clearly observed in case that there is no macroscopic nitrogen diffusion from well layer to barrier layer [8,12,16]. In order to further check the validity of this model to our case, XRD is used to characterize nitrogen contents of the MQWs before and after RTA, and the results are shown in Fig. 4. It can be seen from Fig. 4 that all the satellite peak positions before RTA are in good agreement with those after RTA. Both XRD spectra before and after RTA can be well fitted by dynamic simulation, and the nitrogen contents are the same before and after RTA, 0.7% according to the simulation results. Therefore, based upon above analysis and the results of Figs. 2 and 4, it is concluded that the nitrogen diffusion model is not suitable to account for the observed redshift and blueshift in this work. In order to quantitatively explain RTA-induced blueshift in case of no nitrogen diffusion out of GaNAs, nitrogen reorganization model was proposed. Grenouillet et al. [8] reported the effect of RTA temperature on PL peak energy, and found that with increasing RTA temperature PL peak energy blueshifts for both GaNAs epilayers and GaNAs/GaAs QWs grown by gas source MBE. They explained the observed blueshift with increasing RTA temperature
in terms of RTA-induced nitrogen reorganization by assuming nitrogen diffusion inside GaNAs. After assuming that PL peak energy increases with the amplitude of nitrogen composition fluctuations, the dependences of PL peak energy on RTA temperature and time were proposed. This model can quantitatively explain RTA-induced blueshift by nitrogen composition fluctuations for the case that nitrogen composition is kept constant before and after RTA. However, in the case that there are no nitrogen composition fluctuations, the model is not suitable any more. In order to check the validity of this model to the observed redshift and blueshift in this work, 77 K PL spectrum for the MQWs annealed at 1123 K for 30 s is fitted by Gauss function, and the results are shown in Fig. 5. It can be seen from Fig. 5 that the PL spectrum for the MQWs is symmetric and can be well fitted by Gauss function. It should be noted that the low energy tail which is characteristic of nitrogen composition fluctuations is not observed from Fig. 5. After line shape analysis, no low energy tail can be found for all the PL spectra of the MQWs annealed at different RTA temperatures, although there is a high energy shoulder for the MQWs annealed at low temperatures as can be seen from Fig. 1. Therefore, it is concluded that the nitrogen reorganization model is not suitable to account for the observed redshift and blueshift in this work either. The PL peak energy shift induced by RTA can also be quantitatively explained by a recombination coupling model. Sun et al. [12] studied the effect of RTA temperature on PL peak energy for the triple QWs grown by CBE and found that with increasing RTA temperature from 873 to 1273 K, PL peak energy blueshifts. In the whole temperature range investigated, no redshift was observed. Due to the fact that neither nitrogen diffusion model nor nitrogen reorganization model could explain the observed blueshfit with increasing RTA temperature, the recombination coupling model was proposed in which it was assumed that there is a
1.34
Experimental Gauss fit
1.36
1.38
1.40
1.42
1.44
1.46
Energy (eV) Fig. 5. 77K PL spectrum for the MQWs annealed at 1123 K for 30 s.
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coupling between radiative recombination of PL emission and nonradiative recombination of nonradiative centers, affecting PL peak energy. After assuming that the magnitude of the blueshift is proportional to the relative change in the concentration of nonradiative centers, PL peak energy EðT; tÞ after RTA at temperature T for time t can be calculated by
EB EðT; tÞ ¼ E0 þ gB 1 exp k2B DB t exp ; kT
ð1Þ
where E0 is initial PL peak energy before annealing, gB is the coupling strength between the radiative recombination of PL emission and the nonradiative recombination of nonradiative centers. EB is activak2B
is a constant, and the subtion energy, DB is diffusion constant, script B represents blueshift. It has been demonstrated that this quantitative model well explains the blueshift induced by RTA in GaNAs grown by different methods and under different experimental conditions [12], and that the simulation parameters of E0 are in good agreement with experimental data of E0 [12]. However, although this model can explain the observed blueshift quantitatively, this model cannot explain the case where there is only redshift and the case where redshift and blueshift coexist. Actually, in addition to RTA-induced blueshift, RTA-induced redshift was observed already [15]. Especially, RTA-induced redshift and blueshift of PL peak energy have also been observed simultaneously for GaNAs films grown by gas source MBE [16] and for the MQWs grown by CBE in this work. Therefore, in order to quantitatively explain all the three cases, the case where there is only redshift, the case where there is only blueshift, and the case where redshift and blueshift coexist, it is necessary to modify the model. Based upon the analysis for the recombination coupling model, it is concluded that the reason why the model is not able to explain the case where there is only redshift and the case where blueshift and redshift coexist is that, only one type of nonradiative recombination was proposed in the model. That is to say, although the blueshift nonradiative recombination which results in blueshift with annealing by coupling with radiative recombination was proposed, the redshift nonradiative recombination which results in redshift with annealing by coupling with radiative recombination was not proposed. In this case, based upon the observed redshift and blueshift of Fig. 2 and the redshifts observed in literature [15,16], it is reasonable to propose that there are two types of nonradiative recombination, redshfit nonradiative recombination and blueshift nonradiative recombination, respectively. Redshift nonradiative recombination results in redshift with annealing while blueshift nonradiative recombination results in blueshift with annealing. Nonshift nonradiative recombination which has no contribution to the perturbation of PL peak energy is not considered here. The competition between redshift and blueshift results in all the three cases, the case of only redshift, the case of only blueshift, and the case where redshift and blueshift coexist. Therefore, PL peak energy EðT; tÞ after RTA at temperature T for time t can be described theoretically by
ERi kT i¼1 n X E gBj 1 exp k2Bj DBj t exp Bj þ : kT j¼1
EðT; tÞ ¼ E0
m X
gRi 1 exp k2Ri DRi t exp
ð2Þ
where the subscripts R and B represent redshift and blueshift, respectively, m and n are respectively the total numbers of redshift and blueshift nonradiative recombination coupled with radiative recombination. Practically, and for simplicity, all the redshifts induced by RTA can be regarded as one effective redshift, and all the blueshifts induced by RTA can be regarded as one effective blueshift. Then, Eq. (2) changes to
ER EðT; tÞ ¼ E0 gR 1 exp k2R DR t exp kT EB 2 : þ gB 1 exp kB DB t exp kT
ð3Þ
The experimental results in Fig. 2 are fitted with Eq. (3), and the fitting results are also shown in the figure for comparison. It can be seen from Fig. 2 that the experimental results can be well fitted by Eq. (3) using fitting parameters as shown in Table 1. It can be seen from Table 1 that both the coupling strengths for redshift and blueshfit are on the same order (102 eV), consistent with that obtained for the triple QWs grown by CBE [12]. The coupling strength of redshift is comparable with that of blueshift, resulting that redshift and blueshift are observed with annealing simultaneously. If one of the coupling strengths is obviously higher than the other, redshift and blueshift will not be observed simultaneously with increasing RTA temperature. In this case only redshift or blueshift can be observed. It can also be seen from Table 1 that, the activation energy for the redshift is 1.23 eV, while the activation energy for the blueshift is 2.24 eV. The higher activation energy for the blueshift compared with that for the redshift clearly indicates that redshift process is easier to be annealed out with annealing compared with blueshift process. This is in good agreement with the fact that the redshift occurs at low temperatures while the blueshift occurs at high temperatures and that blueshift is more commonly observed in literature. It can also be seen from Eq. (3) that the change in PL peak energy induced by RTA is controlled by the competition between redshift and blueshift. If PL peak energy is dominated by RTA-induced blueshift, the second term of Eq. (3) can be ignored (gR ¼ 0), then Eq. (3) changes to Eq. (1). If PL peak energy is dominated by RTA-induced redshift, the third term of Eq. (3) can be ignored. Then Eq. (3) changes to
ER : EðT; tÞ ¼ E0 gR 1 exp k2R DR t exp kT
ð4Þ
In order to further check the validity of the modified recombination coupling model, the experimental results in Ref. [15] are fitted with Eq. (4), and the fitting results are also shown in Fig. 6 for comparison. Ref. [15] is the first report for GaNAs in which only redshift was observed after RTA. It can be seen from Fig. 6 that the experimental results can be well fitted by Eq. (4) and the fitting parameters are shown in Table 1. It can be seen from Table 1 that the coupling strength 0.048 eV obtained from Ref. [15] is very close to the coupling strength 0.50 eV obtained in Ref. [12] in which only blueshift was observed. Further investigations show that the coupling strengths from Refs. [15,12] are about three times higher than the coupling strength for redshift obtained in this work. This is the reason why only redshift and only blueshift were observed from Refs. [15,12], respectively, while redshift and blueshift are observed simultaneously in this work.
Table 1 Fitting parameters for modified recombination coupling model.
Fig. 1 Fig. 6
E0 (eV)
gR (eV)
k2R DR t
ER (eV)
gB (eV)
k2B DB t
EB (eV)
1.408 1.263
0.014 0.048
5.0E6 1.2E4
1.23 0.71
0.023
6.1E9
2.24
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4. Conclusions
PL Peak Energy (eV)
1.24 Experimental Model
1.23
1.22
1.21 850
900
950
1000
1050
1100
RTA Temperature (K) Fig. 6. Effect of RTA temperature on PL peak energy. The experimental data are from Ref. [15].
Based upon the above analysis, it is concluded that the modified recombination coupling model can well explain all the three cases for PL peak energy shift induced by RTA, the case where redshift is dominated, the case where blueshift is dominated, and the case where both redshift and blueshift are observed simultaneously with annealing. It is worth noting that the dependence of PL peak energy on RTA temperature in this work is very similar to that observed in Ref. [16] although the samples used are grown by different methods. That is to say, with increasing RTA temperature, redshift is dominated at low temperatures while blueshift is dominated at high temperatures. The transition temperature around 1023 K obtained in this work is also close to 973 K obtained for GaNAs with nitrogen content of 1.3% in Ref. [16]. It is also worth mentioning that for the quadruple QWs grown by CBE in this work both blueshift and redshift are observed simultaneously with annealing while for the triple QWs grown by CBE only blueshift was found [12]. This indicates that even if the growth method is the same, the dependence of PL peak energy on annealing is different due to different material structures. The thicknesses of well and barrier layers for the quadruple QWs in this work are 7 and 10.5 nm, respectively, whereas the thicknesses of well and barrier layers for the triple QWs are 8.5 and 27.8 nm, respectively. Similar results are also found for GaNAs grown by MBE. Wang et al. [15] reported that for GaN0.0145As0.9855 grown by MBE PL peak exhibited a redshift of 18 meV from 1.234 to 1.216 eV with annealing from 873 to 1073 K, while for GaN0.019As0.981 grown by the same MBE PL peak exhibited a blueshift of 75 meV from 0.948 to 1.023 eV. They attributed the difference in PL peak energy with annealing to different growth conditions. These results clearly show that the dependence of PL peak energy on annealing is very complex, depending on growth method, material structure, as well as growth conditions. Further investigation is necessary.
In conclusion, the effects of RTA on the optical properties of GaNAs/GaAs MQWs grown by CBE are studied by PL in detail with special emphasis on RTA-induced PL peak energy shift. The results show that the optical quality of the MQWs improves significantly after RTA. With increasing RTA temperature, both redshift and blueshift are observed simultaneously, and the transition temperature is determined to be around 1023 K. Low temperature process results in redshift while high temperature process results in blueshift. The competition between these two processes is responsible for the observed redshift and blueshift. Based upon the observed redshfit and blueshift with annealing in this work and the observations in literature, the recombination coupling model is modified in which it is proposed that there are two types of nonradiative recombination, redshift nonradiative recombination and blueshift nonradiative recombination, respectively. It is found that the modified recombination coupling model can quantitatively explain all the three cases for PL peak energy shift induced by RTA, the case where redshift is dominated, the case where blueshift is dominated, and the case where both redshift and blueshift are observed simultaneously. References [1] J.E. Stehr, S.L. Chen, M. Jansson, F. Ishikawa, W.M. Chen, I.A. Buyanova, Appl. Phys. Lett. 109 (2016) 203103. [2] D. Tang, G.K. Vijaya, A. Mehrotra, A. Freundlich, D.J. Smith, J. Vac. Sci. & Tech. B 34 (2016) 011210. [3] M. Baranowski, R. Kudrawiec, J. Misiewicz, Appl. Phys. A 118 (2015) 479. [4] J.Y. Duboz, J.A. Gupta, Z.R. Wasilewski, J. Ramsey, R.L. Williams, G.C. Aers, B.J. Riel, G.I. Sproule, Phys. Rev. B 66 (2002) 085313. [5] L. Auvray, H. Dumnot, J. Dazord, Y. Monteil, J. Bouix, C. Bru-Chevallier, L. Grenouillet, Mater. Sci. Semcon. Process. 3 (2000) 505. [6] Ł. Gelczuk, H. Stokowski, M. Da˛browska-Szata, R. Kudrawiec, J. Appl. Phys. 119 (2016) 185706. [7] L.H. Li, Z. Pan, Y.Q. Xu, Y. Du, Y.W. Lin, R.H. Wu, Appl. Phys. Lett. 78 (2001) 2488. [8] L. Grenouillet, C. Bru-Chevallier, G. Guillot, P. Gilet, P. Ballet, P. Duvaut, G. Rolland, A. Million, J. Appl. Phys. 91 (2002) 5902. [9] Z.L. Liu, P.P. Chen, C. Wang, T.X. Li, H.Y. Cui, Y.J. Li, X.S. Chen, W. Lu, J. Appl. Phys. 101 (2007) 113514. [10] P. Klangtakai, S. Sanorpim, K. Yoodee, W. Ono, F. Nakajima, R. Katayama, K. Onabe, J. Cryst. Growth 298 (2007) 140. [11] D. Sentosa, X. Tang, Z. Yin, S.J. Chua, J. Cryst. Growth 307 (2007) 229. [12] Y. Sun, T. Egawa, H. Ishikawa, J. Appl. Phys. 96 (2004) 2586. [13] A. Fotkatzikis, M.-A. Pinault, J.A.H. Coaquira, A. Freundlich, J. Vac. Sci. Tech. B 23 (2005) 1333. [14] T. Kageyama, T. Miyamoto, S. Makino, F. Koyama, K. Iga, Jpn. J. Appl. Phys. 38 (1999) L298. [15] S.Z. Wang, S.F. Yoon, W.K. Loke, C.Y. Liu, S. Yuan, J. Crystal Growth 255 (2003) 258. [16] W.K. Loke, S.F. Yoon, S.Z. Wang, T.K. Ng, W.J. Fan, J. Appl. Phys. 91 (2002) 4900.