Article pubs.acs.org/crystal
Growth of Metamorphic InGaAs on GaAs (111)A: Counteracting Lattice Mismatch by Inserting a Thin InAs Interlayer Takaaki Mano,*,† Kazutaka Mitsuishi,† Neul Ha,†,‡ Akihiro Ohtake,† Andrea Castellano,†,§ Stefano Sanguinetti,§ Takeshi Noda,† Yoshiki Sakuma,† Takashi Kuroda,†,‡ and Kazuaki Sakoda† †
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Graduate School of Engineering, Kyushu University−NIMS, 1-1 Namiki, Tsukuba 305-0044, Japan § Dip. di Scienza dei Materiali, Università di Milano-Bicocca, Via Cozzi 55, I-20125 Milano, Italy ‡
S Supporting Information *
ABSTRACT: We have successfully grown high quality InxGa1−xAs metamorphic layer on GaAs (111)A using molecular beam epitaxy. Inserting a thin 3.0−7.1 monolayer (ML) InAs interlayer between the In0.25Ga0.75As and GaAs allowed the formation of a nearly lattice-relaxed In0.25Ga0.75As with a very flat upper surface. However, when the thickness of the inserted InAs is thinner or thicker than these values, we observed degradation of crystal quality and/or surface morphology. We also revealed this technique to be applicable to the formation of a high quality metamorphic InxGa1−xAs layer with a range of In compositions (0.25 ≤ x ≤ 0.78) on GaAs (111)A. Cross-sectional scanning transmission electron microscope studies revealed that misfit dislocations formed only at the interface of InAs and GaAs, not at the interface of In0.25Ga0.75As and InAs. From the dislocation density analysis, it is suggested that the dislocation density was decreased by growing In0.25Ga0.75As on InAs, which effectively contribute the strain relaxation of In0.25Ga0.75As. The InGaAs/InAlAs quantum wells that were formed on the metamorphic layers exhibit clear photoluminescence emissions up to room temperature.
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INTRODUCTION Metamorphic InxGa1−xAs layers grown on GaAs have been intensively investigated for their application to a range of optical and electronic devices, such as light-emitting diodes, lasers, solar cells, and high-mobility transistors.1−8 For the formation of metamorphic layers with high crystalline and optical qualities, the key issue is how to relax the strain while minimizing the threading dislocation density. Linear- or stepgraded buffer layers and strained superlattices are commonly used to form high quality metamorphic layers on GaAs (100) substrates.9−13 Although high quality layers can be achieved, thick and complicated layer growth sequences are required, which impedes their application in practical scenarios. It has also been reported recently that the insertion of a thin amorphous In0.6Ga0.4As buffer layer effectively enhances the crystal quality of In0.3Ga0.7As on GaAs (111) substrates.14 However, the formation of amorphous layers between the substrate and the epitaxial layer might become problematic if current flow along the growth direction is required in device applications. Hence, simple and adaptable methods for forming high quality metamorphic InGaAs on GaAs are still needed. In the case of InAs growth on GaAs (111)A, it has been reported that high quality lattice-relaxed (metamorphic) InAs can be formed despite the large lattice mismatching of 7%.15−17 During the initial stage of growth, a misfit dislocation network is formed at the interface and lattice-mismatching is relaxed15 in © 2016 American Chemical Society
contrast to the well-known InAs-on-GaAs (100), (311)A, and (311)B systems in which Stranski-Krastanow growth (threedimensional growth) occurs.18−21 As a result, two-dimensional growth of high quality InAs is continuous. A similar type of strain relaxation has also been observed when InAs is grown on Si (111) in which lattice mismatching is even as high as 11%.22 However, no successful growth of InxGa1−xAs (instead of InAs) on GaAs (111)A has been reported. Since the growth of InGaAs film degrades as the In composition is decreased,16 the growth of high quality metamorphic InGaAs is more challenging than that of InAs, especially when In composition (x) is low. In this study, we present the formation of high quality metamorphic InGaAs on GaAs (111)A, achieved by inserting a thin InAs interlayer: high-quality InxGa1−xAs with a wide range of In composition can be realized on top of the InAs interlayer. We first studied the thickness dependence of the InAs interlayer on the quality of InGaAs with low In composition. We found the optimum thickness of InAs and discuss a possible mechanism of strain relaxation. Received: June 13, 2016 Revised: August 1, 2016 Published: August 18, 2016 5412
DOI: 10.1021/acs.cgd.6b00899 Cryst. Growth Des. 2016, 16, 5412−5417
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Article
EXPERIMENTAL PROCEDURES
surface in Sample C, the streak pattern also maintains up to ∼80 nm growth. After the further growth of In0.25Ga0.75As, however, the RHEED pattern becomes diffuse and spotty, as seen in Figure 1c, indicating drastic degradation of crystal quality and surface morphology. Figure 2 shows AFM images of the surfaces of samples A, B, and C. As observed in the RHEED patterns, Sample A exhibits
The samples were grown on semi-insulating GaAs (111)A substrates using a standard solid-source molecular beam epitaxy system. After the growth of 50 nm GaAs layers at 500 °C, 0−85 monolayer (ML) InAs layers were grown at 450 °C. On top of these InAs layers, InxGa1−xAs (x = 0.25, 0.55, and 0.78) were grown at the same temperature. The growth rates of GaAs, InAs, and InxGa1−xAs were 0.3, 0.085, and 0.3 ML/s, respectively. These rates were determined by reflection highenergy electron diffraction (RHEED) oscillation and cross-sectional scanning transmission electron microscope (STEM) imaging. Since crystal growth on (111)A substrates requires a high V/III ratio, high As4 pressure was used: the As4 flux intensity was (6−7) × 10−5 Torr beam equivalent pressure during growth. With growth complete, their structural properties were characterized by atomic force microscope (AFM), cross-sectional STEM, and X-ray diffraction (XRD) measurements. The photoluminescence (PL) spectra were measured at 10 and 300 K in a closed-cycle refrigerator using the 532 nm line of a Nd:YAG laser (35 mW). The PL signal was dispersed with a spectrometer and detected using a cooled InGaAs photodiode array.
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RESULTS AND DISCUSSION We first fix the In composition of InGaAs layer at 0.25 (as a typical example of difficult-to-grow InGaAs with a low In composition) and investigate the InAs interlayer thickness dependence (thickness of InAs: 0, 1.7, 3.0, 4.3, 7.1, 14.2, and 85 ML). Figure 1 shows the RHEED patterns after the growth of
Figure 2. 5 × 5 μm2 AFM images of 150 nm In0.25Ga0.75As on (a) 4.3 ML InAs, (b) 85 ML InAs, and (c) GaAs (111)A. (d) Plot of log RMS roughness as a function of thickness of InAs interlayers.
a very flat surface (Figure 2a). The root-mean-square (RMS) roughness is only 0.57 nm. A step-and-terrace structure is visible. Most of the surface of Sample B is also flat, but large islands are formed, as seen in Figure 2b, which might be the origin of the less streaky RHEED patterns. Due to the formation of these islands, the RMS roughness (1.27 nm) is not as good as that of Sample A. In Sample C, a very rough surface is formed, as expected from the RHEED patterns. Large islands and dislocations (hatch patterns) are visible in Figure 2c. The RMS roughness is 14.4 nm. Next, we carried out XRD measurements to investigate strain relaxation in the In0.25Ga0.75As. Figure 3 shows a twodimensional reciprocal space map (RSM) around asymmetric 115 reflections of GaAs and In0.25Ga0.75As (and InAs) for Samples (a) A, (b) B, and (c) C. A one-dimensional detector was used for these measurements. The vertical and horizontal axes represent the indices along the [111] and [1̅1̅2] directions, respectively. In all the samples, diffraction peaks originated from GaAs and InGaAs were clearly observed as shown in Figure 3. In sample A, the diffraction peak from 4.3 ML InAs was not detectable in our setup because of the thinness of the InAs interlayer. In Sample A, the 115 peak of In0.25Ga0.75As is located close to the position of unstrained In0.25Ga0.75As (red dotted lines). Peak position analysis suggests the lattice spacings along the vertical (d111) and in-plane (d1̅1̅2) direction to be 0.3321 and 0.2359 nm, respectively. If In0.25Ga0.75As is fully relaxed, the in-plane lattice spacing (d1̅1̅2) should be 0.2349 nm. The observed value therefore indicates that a small tensile strain is accumulated in the In0.25Ga0.75As layer along the
Figure 1. RHEED patterns taken along the [110̅ ] and [11̅ 2̅ ] directions of 150 nm In0.25Ga0.75As on (a) 4.3 ML InAs, (b) 85 ML InAs, and (c) GaAs (111)A.
150 nm In0.25Ga0.75As (a) on 4.3 ML InAs/GaAs (111)A (sample A), (b) on 85 ML InAs/GaAs(111)A (sample B), and (c) directly (0 ML InAs) on GaAs (111)A (sample C), respectively. In samples A and B, streak (2 × 2) RHEED patterns are maintained during the entire growth of In0.25Ga0.75As (Figure 1a,b).23 The patterns in Sample A are streakier than those in Sample B where nodes are visible in the streaks. When In0.25Ga0.75As is directly grown on GaAs (111)A 5413
DOI: 10.1021/acs.cgd.6b00899 Cryst. Growth Des. 2016, 16, 5412−5417
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Figure 4. Cross-sectional STEM images of 150 nm In0.25Ga0.75As on 4.3 ML InAs with (a) low and (b) high magnification. The inset of (b) shows the highly magnified lattice image of the area where dislocation is formed at the interface of InAs and GaAs. (c) shows cross-sectional bright-field STEM images of 150 nm In0.25Ga0.75As on 85 ML InAs.
Figure 3. XRD-RSM around asymmetric 115 reflections of 150 nm In0.25Ga0.75As on (a) 4.3 ML InAs, (b) 85 ML InAs, and (c) GaAs (111)A. The red and blue dotted lines indicate the position of the 115 reflections of bulk In0.25Ga0.75As and InAs, respectively. (d) Plot of d1̅1̅2 of In0.25Ga0.75As estimated from the XRD-RSM as a function of thickness of InAs. The green dotted line indicates the strain-free state.
in-plane direction (expanded by 0.51% from the unstrained bulk). In Sample B, In0.25Ga0.75As is more relaxed than in Sample A. The d1̅1̅2 value is 0.2357 nm, and is, therefore, expanded by 0.33%. In Figure 3b, the peak from 85 ML InAs is clearly visible, in which the d111 and d1̅1̅2 values are estimated to be 0.3515 and 0.2443 nm, respectively. These values indicate that a compressive strain is accumulated in the InAs interlayer along the in-plane direction in Sample B (compressed by 1.2% from the unstrained bulk). It should also be noted that the d1̅1̅2 values of InAs and In0.25Ga0.75As are not the same, which means that strain relaxation also occurs between 85 ML InAs and In0.25Ga0.75As. In contrast to Samples A and B, two peaks originated from the fully strained In0.25Ga0.75As (marked by red arrowhead in Figure 3c) and from the relaxed In0.25Ga0.75As are visible in Sample C. We have confirmed that only a peak related to the fully strained In0.25Ga0.75As is visible in XRD-RSM measurement when 80 nm-In0.25Ga0.75As was grown on GaAs (111)A (Figure S1). These results, together with the RHEED observations, suggest that strained In0.25Ga0.75As was grown at the initial stage of the growth up to ∼80 nm. The thickness, then, reaches a critical value and strain relaxation occurs by forming misfit dislocations in the In0.25Ga0.75As. Most likely, these dislocations in the In0.25Ga0.75As layer drastically degrade the surface morphology and crystal quality. Figure 4a,b shows cross-sectional STEM images of Sample A. In Figure 4a, contrast related to the dislocations at the interface is clearly visible. On the other hand, only a small number of defects are formed in the In0.25Ga0.75As layer, suggesting the high quality of In0.25Ga0.75As. To check the dislocation density in the In0.25Ga0.75As layer, we have carried out wet chemical etching of the surface of sample A and counted the etch pit density (EPD). For the formation of etch pits we dipped the sample in HNO3:H2O (1:3) solution for 1 min at room temperature.24 Figure 5 shows an optical microscope image of the etched surface of sample A. The average EPD was calculated to be 1.1 × 106/cm2, which is a low value comparable
Figure 5. Optical microscope image of the surface of sample A after etching.
to the previous reports.7,14 In Figure 4a, we found that some of the threading dislocations nucleate in the middle of InGaAs. The origin of these dislocations are possibly attributed to the growth instability of InGaAs on (111)A surfaces. Therefore, the dislocation density could be further reduced by optimizing the growth condition of InGaAs, such as growth temperatures and rates.25 In the magnified lattice image (Figure 4b and the inset), dislocations formed at the interface between InAs and GaAs are marked with red circles. These dislocations do not thread along the growth direction but remain at the interface.15,22,26 In addition, no dislocation is visible at the interface between In0.25Ga0.75As and InAs in Figure 4b. Figure 4c shows the cross-sectional bright-field STEM images of Sample B. As observed in the XRD-RSM, periodic contrasts related to the misfit dislocations are visible at both the GaAs/InAs and InAs/In0.25Ga0.75As interfaces. From the present experimental results, the strain relaxation of In0.25Ga0.75As on InAs interlayer with various thickness can be roughly classified into three stages, as shown in Figures 2d and 3d. When the thickness of InAs is 0−1.7 ML (stage I), the surface of In0.25Ga0.75As is very rough (RMS roughness is greater than 8 nm). In the In0.25Ga0.75As, compressive strain is accumulated. The insertion of 3.0−7.1 ML InAs (stage II) results in the formation of a flat surface (with RMS roughness 5414
DOI: 10.1021/acs.cgd.6b00899 Cryst. Growth Des. 2016, 16, 5412−5417
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Figure 6. 5 × 5 μm2 AFM images (left-hand panel) and XRD-RSM around asymmetric 115 reflection (right-hand panel) of (a) 150 nm In0.55Ga0.45As and (b) 150 nm In0.78Ga0.22As on 4.3 ML InAs/GaAs (111)A. The red dotted lines indicate the position of the 115 reflections of bulk In0.55Ga0.45As and In0.78Ga0.22As.
When the thickness of the InAs buffer layer on GaAs (111)A exceeds 1.5 ML, the in-plane lattice spacing (d1̅1̅2) of InAs starts to change (stage II).15,16 According to refs 15 and 16, the d1̅1̅2 value of InAs increases from 0.2318 to 0.2425 nm by increasing the thickness of InAs from 1.5 to 5 ML. Thus, the residual strain in the growing In0.25Ga0.75As on top can be minimized when the in-plane lattice spacing of the InAs layer is closely adjusted to that of the In0.25Ga0.75As by tuning the thickness of InAs. As seen in Figure 3d, insertion of 3.0 ML InAs results in the best results of highly relaxed In0.25Ga0.75As (d1̅1̅2 = 0.2352 nm) in our experiment. However, as we will show below, such a precise control of the in-plane lattice spacing is not necessarily required for the growth of high quality InGaAs. As described earlier in this paper, high quality InGaAs layer is grown using the 4.3 ML InAs interlayer. On the other hand, the d1̅1̅2 value of 4.3 ML InAs on GaAs (111)A (before the growth of InGaAs) is estimated to be 0.2419 nm, obtained from refs 15 and 16, which is much larger than the value for the growing In0.25Ga0.75As layer (0.2361 nm). Since no dislocation is observed at the interface between 4.3 ML InAs and In0.25Ga0.75As in Figure 4a, the d1̅1̅2 value of 4.3 ML InAs and In0.25Ga0.75As should closely match. This means that the d1̅1̅2 value of InAs was significantly changed from 0.2419 to 0.2361 nm (−2.4% change) by growing In0.25Ga0.75As on top, while the d1̅12̅ of growing In0.25Ga0.75As exhibits only a 0.51% difference from the unstrained bulk value. To understand this phenomenon, we focus on the dislocation density at the interface of InAs and GaAs. As shown in Figure 4b, the averaged spacing of the dislocations at the interface of InAs and GaAs is 11 nm. In contrast, after the growth of 4.3 ML InAs on GaAs (111)A, the d1̅1̅2 value of InAs should be 0.2419 nm, as
of around 0.6 nm). At this stage, small tensile strain is accumulated in the In0.25Ga0.75As. The tensile strain increases with increasing InAs thickness. By further increasing InAs thickness to greater than 7.1 ML (stage III), tensile strain slightly reduces. However, RMS roughness increases again due to island formation, as shown in Figure 2b. Therefore, the optimum thickness of InAs for growing the high quality In0.25Ga0.75As (i.e., lattice-relaxed and with a flat surface) is 3.0− 7.1 ML (stage II). To study the specified role of the InAs interlayer for the growth of high quality metamorphic In0.25Ga0.75As, we now analyze the changes in lattice spacings in more detail and discuss the growth mechanism. When InAs is grown on GaAs (111)A, considerable strain relaxation occurs by introducing a two-dimensional misfit dislocation network during the initial stage of growth (starting at around 1.5 ML).15−17 For the InAs thickness below this value (stage I), the in-plane lattice spacing is nearly the same as that of GaAs. On top of the GaAs substrate or these strained thin InAs interlayers (0 and 1.7 ML, respectively, in our experiment), In0.25Ga0.75As grows coherently (with its in-plane lattice spacing matched to that of GaAs) at the beginning of growth, followed by drastic degradation of surface morphology when the layer thickness of In0.25Ga0.75As exceeds a critical value (around 80 nm). Most probably, the misfit dislocations formed in the In0.25Ga0.75As have significant negative impact on the crystal quality, as usually observed in lattice mismatched heteroepitaxy, which is different from the two-dimensional misfit dislocation network formed at the interface of the InAs and GaAs. As a result, compressively strained In0.25Ga0.75As with very rough surface is formed. 5415
DOI: 10.1021/acs.cgd.6b00899 Cryst. Growth Des. 2016, 16, 5412−5417
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confirm the high quality of the metamorphic InGaAs. The MQW structures formed on the InxGa1−xAs (x = (a) 0.25 and (b) 0.55) are shown in the right-hand panels of Figure 7. In
discussed above, which requires the dislocations with the averaged spacing of 7 nm at the InAs/GaAs interface. Such a difference in the dislocation density could be explained by assuming that the thin InAs interlayer is elastically deformed by accumulating compressive strain applied by growing In0.25Ga0.75As. The change of dislocation structure associated with the amount of strain has actually been observed in ref 15. Due to this mechanism, nearly unstrained In0.25Ga0.75As with a very flat surface can be formed on the InAs even when the inplane spacing (i.e., the thickness) of InAs is not perfectly optimized. Here, it should be noted that the averaged dislocation spacing for the perfect relaxation of In0.25Ga0.75As on GaAs is calculated to be around 19 nm, which is larger than the observed value (11 nm). This difference might be the origin of the small tensile strain remaining in the In0.25Ga0.75As. For more quantitative study of this system, however, further studies are necessary. In stage III, the strain in InAs on GaAs (111)A is almost relaxed. The d1̅1̅2 value of 85 ML InAs on GaAs (111)A is 0.2455 nm,15,16 and from the XRD-RSM shown in Figure 3b the d1̅1̅2 value of InAs after the growth of In0.25Ga0.75As is 0.2443 nm. On the other hand, the growing In0.25Ga0.75As has a much smaller value of 0.2357 nm. This means that the in-plane lattice spacing of InAs is changed only slightly (0.49%) by growing In0.25Ga0.75As on top. Thus, to relax the lattice mismatch, misfit dislocations are formed at the interface of In0.25Ga0.75As and 85 ML InAs in addition to those at the interface of InAs and GaAs, as observed in Figure 4c. This result suggests that a thick InAs interlayer (stage III) cannot be elastically deformed, in marked contrast with a thin InAs interlayer (stage II). In the viewpoint of strain relaxation, the In0.25Ga0.75As in stage III is less strained than In0.25Ga0.75As on 4.3 and 7.1 ML InAs. However, the RMS is not as good as these In0.25Ga0.75As due to the island formation. The growth of the flat layer is possibly difficult in this kind of tensile epitaxy system.27,28 From these results we conclude that the use of a partially relaxed thin InAs interlayer is crucially important to promote the growth of high quality metamorphic In0.25Ga0.75As on top. To show the practical applicability of our method and validity of the above discussion, InxGa1−xAs with high In compositions (x = 0.55 and 0.78) were grown on 4.3 ML InAs under otherwise identical growth conditions. As shown in the AFM images (left-hand panels in Figure 6), both samples exhibit flat surfaces. The RMS values are (a) 0.489 and (b) 0.566 nm, respectively. From the XRD-RSM shown in the right-hand panels of Figure 6, it is clear that both InGaAs layers are nearly lattice-relaxed, similar to the case of x = 0.25. In more detail, since the d1̅1̅2 value of unstrained In0.55Ga0.45As (0.2399 nm) is smaller than that of 4.3 ML InAs (around 0.2419 nm), small tensile strain is accumulated in the In0.55Ga0.45As layer (the measured d1̅1̅2 value is 0.2399 nm). On the contrary, the d1̅1̅2 value of unstrained In0.78Ga0.22As (0.2437 nm) is larger than that of 4.3 ML InAs. As a result, compressive strain is accumulated in the In0.78Ga0.22As (d1̅1̅2 value is 0.2428 nm). These results reveal that a 4.3 ML InAs interlayer can assist the growth of high quality metamorphic InxGa1−xAs with a wide range of x (0.25 ≤ x ≤ 0.78), in which both compressive and tensile strain can be accommodated. Moreover, we have also confirmed that this technique can be applicable to InxAl1−xAs growth with the similar In compositions.29 Finally, we show the PL properties of InGaAs/InAlAs multiple quantum wells formed on the metamorphic InGaAs to
Figure 7. PL spectra of (1) In0.25Ga0.75As/In0.25Al0.75As and (b) In0.55Ga0.45As/In0.55Al0.45As multiple quantum wells grown on 4.3 ML InAs at 10 and 300 K. The details of the sample structures are shown in the right-hand panels.
both cases, clear PL emissions are visible around (a) 992 and (b) 1286 nm from the MQWs at 10 K. The peak widths are (a) 56 and (b) 33 meV, respectively. In Figure 7a, other PL peaks are also observed. The line at 850 nm stems from the GaAs substrates. The broad PL emission observed at around 1100 nm ∼1500 nm at 10 K have most probably originated from the 50 nm InGaAs and InAs interlayer. Since the In0.25Al0.75As is an indirect bandgap semiconductor, the PL emission from the In0.25Ga0.75As MQWs shown in Figure 7a is weaker than that shown in Figure 7b. As a result, these additional PL peaks become visible. Both of the MQWs exhibit clear PL emissions up to room temperature. The intensity reduction (∼1/500) might be mainly due to the threading dislocations formed in the MQWs. The clear and narrow PL emissions from the MQWs up to room temperature provide further evidence of the high quality of the metamorphic InGaAs layers.
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CONCLUSION We studied the growth of high quality InxGa1−xAs metamorphic layers on GaAs (111)A using thin epitaxial InAs interlayers with variable thickness by molecular beam epitaxy. High quality metamorphic In0.25Ga0.75As can be grown by inserting 3.0−7.1ML-thick InAs interlayers in between In0.25Ga0.75As and GaAs. In contrast, when the InAs interlayer is thinner or thicker than these values, the crystal quality and/or surface morphology of the In0.25Ga0.75As layers suffers. The cross-sectional STEM images reveal that misfit dislocations are formed only at the interface between InAs and GaAs, and not at the interface between In0.25Ga0.75As and InAs. InxGa1−xAs with high In composition (x = 0.55 and 0.78) can also be realized on top of the 4.3 ML InAs/GaAs (111)A. Detailed strain analysis suggests that the residual strain in the InxGa1−xAs can be minimized by adjusting the in-plane lattice spacing of the thin InAs interlayer to that of the growing InxGa1−xAs on top by 5416
DOI: 10.1021/acs.cgd.6b00899 Cryst. Growth Des. 2016, 16, 5412−5417
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tuning the thickness of InAs. However, even when the InAs thickness is not optimized, the thin InAs interlayer is elastically deformed by growing InxGa1−xAs on top together with the dislocation density change at the InAs/GaAs interface. As a result, nearly strain-free metamorphic InxGa1−xAs can be formed. The InGaAs/InAlAs MQWs formed on the metamorphic InGaAs exhibit clear PL emissions up to room temperature. This makes the use of thin InAs interlayers a highly promising technique for forming InGaAs containing different levels of indium.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00899.
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Figure S1. XRD-RSM (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written based on the contributions of all the authors. All the authors approved the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by Grant-in-Aid for Scientific Research (C) (No. 2539011) and (A) (No. 16H02203).
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DOI: 10.1021/acs.cgd.6b00899 Cryst. Growth Des. 2016, 16, 5412−5417
