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The growth mode and Raman scattering characterization of m-AlN crystals grown by PVT method
Journal Pre-proof The growth mode and Raman scattering characterization of m-AlN crystals grown by PVT method L. Jin, H.L. Wu, Y. Zhang, Z.Y. Qin, Y.Z. Shi, H.J. Cheng, R.S. Zheng, W.H. Chen PII:
S0925-8388(20)30298-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.153935
Reference:
JALCOM 153935
To appear in:
Journal of Alloys and Compounds
Received Date: 26 November 2019 Revised Date:
5 January 2020
Accepted Date: 19 January 2020
Please cite this article as: L. Jin, H.L. Wu, Y. Zhang, Z.Y. Qin, Y.Z. Shi, H.J. Cheng, R.S. Zheng, W.H. Chen, The growth mode and Raman scattering characterization of m-AlN crystals grown by PVT method, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153935. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
CRediT author statement L. Jin: Conceptualizationm, Methodology, Writing - Original Draft H. L. Wu: Conceptualization, Methodology, Writing - Review & Editing Y. Zhang: Formal analysis, Investigation Z. Y. Qin: Validation, Investigation Y. Z. Shi: Investigation, Validation H. J. Cheng: Investigation R. S. Zheng: Supervision W. H. Chen: Resources
The growth Mode and Raman Scattering Characterization of m-AlN Crystals grown by PVT method
L. Jin1*, H. L. Wu1*, Y. Zhang2, Z. Y. Qin1, Y. Z. Shi2, H. J. Cheng2, R. S. Zheng1, W. H. Chen1 1. College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518000, China 2. The 46th Research Institute, CETC, Tianjin 300220, China
Corresponding author: E-mail address: [email protected] (L. Jin); [email protected] (H. L. Wu)
Abstract: The growth mode of m-plane AlN crystal grown by physical vapor transport method is studied. The m-plane AlN crystal fabricated by spontaneous nucleation was used as a seed to grow bulk crystal, and 3-mm thick AlN crystals with a maximum lateral dimension of 20 mm were obtained. The Raman tensor elements of A1(TO), E2(high) and E1(TO) Raman models from the m-plane AlN crystal are investigated by angle-dependent polarized Raman scattering. The Raman tensor comparing with our previous report indicates the possible enhancement of the Raman tensor in the directions vertical to [0002] direction at higher growth temperature. Key words: Physical vapor transport; Raman spectroscopy; semiconductor; nucleation of recrystallization
1. Introduction: AlxGa1-xN materials with low dislocation densities are rather demanding for ultraviolet (UVC) light emitting diodes (LEDs) which will enhance the UVC LEDs quantum efficiency [1-3]. As the widely used substrate for UVC LEDs, sapphire and silicon limit the quality of AlxGa1-xN resulted high dislocation densities of 108~1010cm-2 [4-6]. As an important III-V semiconductor, aluminum nitride (AlN) has many desirable physical properties suitable as a substrate of AlxGa1-xN, such as lowest lattice and thermal mismatch, wide band-gap (~6.20 eV), high thermal conductivity, and high chemical and thermal stability. It has been reported that the dislocation density of AlxGa1-xN epitaxial heterostructures on AlN substrate was 104~106 cm-2[7, 8] and 229nm UVC LEDs were fabricated on [0001] plane AlN substrate with AlxGa1-xN as the multi-quantum wells[9]. However, it was well observed that piezoelectric polarization field of substrate will lead the sharp band bending of multi-quantum wells and result the quantum efficiency degrading[10,11]. Moreover, the luminescence anisotropic property of AlN and AlGaN lead the UV light should emit from the side of the LED which grown on [0001] orientation[12]. By employing nonpolar orientations AlN substrate those problems can be circumvented [13]. Thus, to obtain m-plane AlN crystal and to investigate its polarization tensor is of great importance. Physical vapor transport (PVT) is effective method to prepare bulk AlN single crystal. Currently, for the growth of (0001) face AlN crystal, 2 inch diameter AlN wafer had been reported by R. T. Bondokov’ group and T. Y. Chemekova’ group, however, the substrates have some polycrystalline regions of the total area [14,15]. High quality N-polar bulk AlN crystal exhibit a zonar structure was grown by HexaTech, Inc [16] and the largest known single crystal AlN substrate (2 inch) that is completely free of macroscopic defects had been reported recently [17]. To data, the growth of m-orientation bulk AlN crystal has been reported by R. Collazo’s group [18] and our group [19], however, few details were illustrated about the growth mode and the effect of grown temperature on crystal structure. In this work, the growth mode of m-plane AlN crystal fabricated by spontaneous nucleation strategy was investigated. And we use the m-plane AlN as the seed to prepare centimeter-sized bulk crystal. The angle-dependent Raman spectroscopy was presented to investigate the polarization anisotropic of m-plane AlN bulk crystal, and the angular dependencies of Al(TO), E2(high) and El(TO) mode are discussed in terms of the Raman selection rules to investigate AlN crystal structure.
2. Experimental details The growth of m-plane AlN crystals were synthesized by PVT method in a resistance-heated furnace. AlN powders were sintered twice at 2000oC~2200oC for 8 h to reduce the impurity content in the samples. The chamber was vacuumed to 2×10-4 Pa, and then filled to (6~9)×104 Pa with high purity N2. M-plane AlN crystals on tungsten substrate were obtained with the source temperature kept at about 2260~2290 oC for 6~10 h and the gradient between source and crucible lid set as 0.2~1 o
C /mm. Then m-plane AlN crystals obtained were used as native seed to get bulk AlN crystal. AlN crystal microscope images were acquired by laser confocal microscopy. Atomic force
microscopy (AFM) was performed with an intelligent mode using a Bruker Dimension Icon to characterize the nuclei morphologies. X-ray diffraction (XRD) measurements were performed using a high resolution diffractometer (Rigaku, TTRAX III, Cu Ka, 18W) to define the AlN crystal orientation and quality. Polarization Raman spectra of m-plane AlN were collected by a HR800 Raman spectrometer system from Horiba Jobin Yvon with a 532 nm solid-state laser as the exciting source. In the optical path of the scattered signal, a rotatable polarizer was inserted, enabling the measurement component of the scattered light with polarization parallel (perpendicular) to that of incoming light. Raman spectra were acquired in the range of 550-700 cm-1 wave-number by a lens-based spectrometer with a cooled charge-coupled device (CCD) detector.
3. Results and discussion 3.1. The growth model of m-plane AlN crystal By spontaneous nucleation method, the multiple and single growth centers of quadrilateral AlN crystals are found. It is shown that the top surface of multiple growth centers are flat with stripes surround and the detail of this kind center exhibits irregular morphology (Fig. 1(a)~(c)). The nonlinear growth stripes indicate the AlN crystal grown in large supersaturation at the growing surface. The single growth center is shown in Fig. 1 (d)~(f), and the particulars of the growth center exhibit a rectangular shape and the stripes surround are also regular in morphology. In fact, the single growth center AlN crystal is fabricated in low supersaturation at the growing surface. The growth direction of the tetragonal AlN crystal is determined along [10¯10] axis compared to the schematic of crystal structure as shown in Fig. 1 (g). X-ray Omega-scans curves of tetragonal AlN crystal are carried out in Fig. 1 (h) and the rocking curves is inserted in Fig. 1 (h). The diffraction peak located at 16.04o angle further reveals that the tetragonal AlN crystal is grown along [10¯10] axis, and about
0.54o angle is detected between the growth plane and m-plane which may be caused by the temperature distribution of the substrate. The ω-scan rocking curve full width at half maximum (FWHM) values is 100.2 arcsec reflects the high crystal quality of the as-grown AlN crystal. The growth of m-plane AlN crystals is only possible in a small parameter range with an adequate supersaturation of gaseous aluminum Al(g) and N2(g), especially the growth of m-plane AlN crystal with single growth center. In order to prepare bulk m-plan AlN crystals, AFM image of three different regions are taken from the growth center to investigate growth mode of m-plane AlN crystal. Selected area as marked by red box in Fig. 2 (a) is shown in Fig. 2 (b), and it is found that the width of terraces is about 5~10µm and the height is about 30~80nm. The long-side of the step bunches is parallel to the c-axis. The step bunches as marked by black box in Fig. 2 (a) is shown in Fig. 2 (c). Compare to the area in Fig. 2 (b), the terraces are narrow with 1~3µm in width. The terrace as marked in white box is shown in Fig. 2 (d) and (e). The smooth surface represents a 2D layer growth mechanism without any island formation. In fact, each step bunch consists of many parallel steps being bunched together. The unique nucleation point is fund as shown in Fig. 2 (f). The morphology of the nucleus is triangle in the first layer, and then evolves into irregular hexagon as the second and third layers. Crystal grown by the single growth center mode seems to show better structural quality [20]. Summarily, the growth surface is divided into nucleation area, step flow area and step bunching area. Firstly, the nucleus occurs on nucleation area and the shape develops into triangle. Generally, crystal grown with a single nucleation center follows the screw dislocation growth model, such as c plane SiC, c plane GaN and c plane AlN. Whereas the growth center of m-plane AlN crystal exhibit the concentric, closed ring structures which may correspond to edge dislocations and need further investigation[21, 22]. Secondly, the step-flow begins at this point smoothly flows to all around on step flow area. The observed steps are very straight and they flow in line with significantly varying spacing along the steps and no inclusions are noticed microscopically. The m-plane AlN crystal grows up at this stage. Finally, steps origin from nucleation center bunch together on the step bunching area. The steps flow cease for many reasons, such as the supersaturation fluctuation on growth surface, or the foreign particles and impurities deposited on the surface[23]. 3.2 The structure anisotropy The m-plane AlN crystal fabricated by spontaneous nucleation is used as a seed to grow bulk crystal
as shown in Fig. 3 (a). Very careful process parameters are applied to avoid polycrystalline generation. 3-mm thick bulk crystals with 12mm×20mm in diameter are obtained as shown in Fig. 3 (b) and the m-plane was marked by red arrow which is used to detect by angle-dependent Raman scattering. The configuration of the angle-dependent Raman scattering experiment is shown in Fig. 3 (c). The XYZ is the measurement coordinate system. The wave-vector of incident laser is parallel to X-axis, more specifically,
was known as the unit vector in the direction of the incident light,
was considered as the unit vector in the direction of the scattered light. The angle between the unit and Y-axis was severally presented as θ in the m-plane. The AlN sample can be rotated
vector of
360o around the X-axis from 0o to 360o with step of 10o. Raman spectra measured without polarizer inserted in the optical path was shown in Fig. 3 (d), and five peaks were detected at 248.1 cm-1, 609.7 cm-1, 656.2 cm-1, 669.8 cm-1, and 912.5 cm-1 which were E2(low), A1(TO), E2(high), E1(TO) and E1(LO) modes, respectively. The Raman spectra of X(YZ)¯X (perpendicular) and X(YY)¯X (parallel) configurations are shown in Fig. 3 (e) and (f), and the A1(TO), E2(high) and E1(TO) models were observed at 609.7 cm-1, 655.8 cm-1 and 669.3 cm-1, respectively. The peak intensities of A1(TO), E2(high) and E1(TO) models change periodically with the polarization angle, indicating the anisotropic Raman signals of the m-plane AlN crystal. Theoretically, the scattering intensities in a Raman experiment can be calculated by[24~26] I∞|
where
i
and
s
∙
∙
|2
(1)
are the polarization vectors of the incident and scattered radiation, respectively, and
R is the complex second-rank Raman tensor which ascribe to the polarization behavior as shown in equation (2). R=
(2)
where Rij represents the Raman tensor in j direction excited by the electric field in the i direction. The Raman tensor of A1, E1, and E2 mode in a hexagonal material is given by R[A1]= 0 0
R[E2]= d 0
0
0
0 0
d 0 −d 0 0 0
(3)
(4)
0 R[E1]= 0 −
0 − 0 c c 0
(5)
For Raman backscattering measurements on the m-plane surface, the polarization vectors of incident and scattering light referred to the hexagonal axes can be written as 0 sin ( ) = i cos( ) 0 ‖ s= sin( ) cos ( )
0 s= cos( ) −sin ( )
(6)
(7)
(8)
For these configurations, according to Eq. (1)~(8), the Raman intensity A1(TO), E2(high) and E1(TO) modes can be expressed as shown in table 1. We fitted the experiment data with the equations, and the results are plotted in Fig. 4. In perpendicular-polarization configuration, the A1(TO) mode shows obvious 90° periodic variations with the sample rotation angle for perpendicular-polarization, and the four maximum intensity angles occur at 45°, 135°, 225° and 315° for A1(TO) mode. While in parallel-polarization configuration, the A1(TO) shows obvious 180° periodic variations with firstly maximum intensities at 0° and 180°, and secondary maximum intensities at 90° and 270°. The ratio of firstly maximum intensities to secondary maximum is about 6.7 which originate from the large phase difference between the complex Raman tensor elements a and b. As shown in Fig. 4 (c) and (d), the E2(high) mode shows obvious 90° periodic variations, yielding a 4-lobed shape with four maximum intensity angles at about 45°, 135°, 225° and 315° in perpendicular-polarization configuration, and in parallel-polarization configuration shows 180° periodic variations, yielding a 2-lobed shape with two maximum intensity angles at about 90°, and 270°. The E1(TO) mode, as shown in Fig. 4 (e) and (f), both in perpendicular-polarization and parallel-polarization configuration are 90° periodic variations, while the difference is that maximum intensity angles located at 0°, 90°, 180° , 270° and 45°, 135°, 225° and 315°, respectively. From the fitting parameters, the relative values of the Raman tensor elements for AlN are calculated, which are summarized in Table 2. And the relative values of the Raman tensor elements of AlN crystal that we have reported before[25, 26] is listed for a comparison to give a further understanding about the structure anisotropy under different growth temperature. The crystal
fabrication temperature in this work and previous work are 2260oC and 2150oC, respectively. The phase difference φa-b between elements | | and | | in this work is 91.69° and 91.41° for parallel
and cross-polarized scattering geometries, which have similar values to our previous work. From the calculation data, the ratio of | / |, | / | and | / | of AlN crystal in this work become smaller,
whereas, the ratio of | / | | / | and | / | become larger than our previous work indicated that | | become bigger and | | become smaller. From the complex second-rank Raman tensor R and
fitted equation in Table 1, we can see that the Raman tensor | | is along ZZ polarization represented that | | is in [0002] direction, and the Raman tensor | | is along YY polarization represented that
| | is vertical to [0002] direction. Above all, comparing to AlN grown at 2150oC, the Raman tensor
in [0002] direction of AlN grown at 2260oC is becoming smaller. Theoretically, growth morphologies
of AlN crystal in PVT method are temperature dependent which may originate from the effect of the polar-surface[27]. Higher growth temperature will result a larger growth rates vertical to [0002] direction of AlN crystal which origins from the enhancement of the polarization of chemical bonds in the m and a direction, thus enhancing the Raman tensor in those directions. The detail influence of growth temperature on the anisotropy of Raman tensor in AlN crystals is not reported, therefore, further investigations are needed which advantageous to improve the performance of AlN based devices.
4. Conclusion In summary, m-plane AlN crystal is fabricated by spontaneous nucleation strategy in PVT method. The single nucleation center with closed ring structures illustrates the growth mode of the m-plane AlN crystal generally follows edge dislocations driving growth model. 3-mm thick bulk AlN crystal with 12mm×20mm in diameter were obtained and used to study the structure anisotropic by angle-dependent Raman scattering. The ratio of Raman tensor elements of A1, E1, and E2 phonon modes is calculated and compared with our previous reports, indicative of the possible enhancement of the Raman tensor in the directions vertical to [0002] direction at a higher crystal growth temperature.
Acknowledgments This work is supported financially by the Natural Science Foundation of China (Grant numbers 51702297).
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Fig. 1 (a) Optical photograph of the quadrilateral AlN crystal with multiple growth centers; (b) One of the growth centers from (a); (c) The enlarge image of growth centers in (b); (d) Optical photograph of the quadrilateral AlN crystal with smooth surface; (e) The growth centers from (d); (f) The enlarge image of growth centers in (f); (g) The schematic of AlN crystal structure; (h) XRD Φ scans of tetragonal AlN crystal, and inset shows X-ray diffraction rocking curves of the m-plane AlN single crystals
Fig. 2 (a) AFM image of the growth center of m-plane AlN crystal; (b)~(d) Enlarged AFM images marked by red, blak and white box in Fig. (a); (e) Enlarged AFM images of Fig. (e); (f) AFM image of nucleation point; (g) Schematic diagram of growth mode
Fig. 3 (a) Photograph of the AlN seed from spontaneous nucleation; (b) Photograph of the m-plane AlN crysatal grown for five growth runs; (c) Schematic illustration of the Raman experimental coordinate system;(d) Raman spectrum of the AlN crystal without polarizer inserted in the optical path; (e) The Raman spectra of the AlN crystal for perpendicular configuration; (f) The Raman spectra of the AlN crystal for parallel configuration
Fig. 4 Polar plots of angle-resolved polarized Raman scattering intensities. (a), (b) show the intensity of A1(TO) signal under perpendicular and parallel polarization vectors, respectively, (c), (d) show the intensity of E2(High) signal under perpendicular and parallel polarization vectors, respectively, (e), (f) show the intensity of E1(TO) signal under perpendicular and parallel polarization vectors, respectively
Table 1 Selection rules for the allowed Raman modes of m-plane faced crystals for different polarizations of the incident e#$ and scattered (e#% ) polarization vectors: perpendicular (e#$ ⊥ ) and parallel (e#$ ‖e#% ), where θ is the angle between e#$ and the Y axis and '()* is the phase difference between the complex Raman tensor elements a and b. Raman #, ⊥#. # , ‖+ #/ + + mode 1 2 1 A1 | |2 <= > ( )＋| |2 =?@> ( )＋ | || |=?@2 (2 )cos ('()* ) sin (2θ)5|a|2 ＋|b|2 − 2|a||b|cos (φ9): ); 4 2 1 2 2 | |2 =?@> ( ) E2 | | =?@ (2 ) 4 E1
| |2 <= 2 (2 )
| |2 =?@2 (2 )
Table 2 Ratio of the Raman tensor elements of A1, E1, and E2 phonon modes of AlN estimated from the parameters of fits to angle-dependent Raman spectra for two different alignments of the incident (ei) and scattered (es) polarization vectors. The comparison of Ratio of the Raman tensor elements of AlN crystal that we have reported before is listed.
Tensor element （m-plane） | | | | | | | | |
/ / / / / / / / /
| | | | | | | | |
'()*
AlN 1.582 0.609 0.638 2.598 1.048 1.642 2.480 0.954 1.567 91.69o
?∥ =
AlN (ref.)
AlN
1.818 0.730 0.671 2.490 0.919 1.370 2.709 1.088 1.490 91.24o
1.571 0.605 0.688 2.597 137.1 1.653 2.283 0.879 1.453 91.41o
?⊥ =
AlN (ref.) 1.846 0.746 0.721 2.475 0.966 1.341 2.560 1.035 1.387 91.18o
1. First experiment report on the growth mode of m Plane AlN single crystals. 2. 3-mm thick m plane AlN crystals with a maximum lateral dimension of 20 mm were obtained using AlN seed fabricated by spontaneous nucleation. 3. The ratio of Raman tensor elements of A1, E1, and E2 phonon modes is calculated and compared with our previous reports, indicative of the enhancement of the Raman tensor in the directions vertical to [0002] direction at a higher crystal growth temperature.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30298-X
DOI:
https://doi.org/10.1016/j.jallcom.2020.153935
Reference:
JALCOM 153935
To appear in:
Journal of Alloys and Compounds
Received Date: 26 November 2019 Revised Date:
5 January 2020
Accepted Date: 19 January 2020
Please cite this article as: L. Jin, H.L. Wu, Y. Zhang, Z.Y. Qin, Y.Z. Shi, H.J. Cheng, R.S. Zheng, W.H. Chen, The growth mode and Raman scattering characterization of m-AlN crystals grown by PVT method, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153935. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
CRediT author statement L. Jin: Conceptualizationm, Methodology, Writing - Original Draft H. L. Wu: Conceptualization, Methodology, Writing - Review & Editing Y. Zhang: Formal analysis, Investigation Z. Y. Qin: Validation, Investigation Y. Z. Shi: Investigation, Validation H. J. Cheng: Investigation R. S. Zheng: Supervision W. H. Chen: Resources
The growth Mode and Raman Scattering Characterization of m-AlN Crystals grown by PVT method
L. Jin1*, H. L. Wu1*, Y. Zhang2, Z. Y. Qin1, Y. Z. Shi2, H. J. Cheng2, R. S. Zheng1, W. H. Chen1 1. College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518000, China 2. The 46th Research Institute, CETC, Tianjin 300220, China
Corresponding author: E-mail address: [email protected] (L. Jin); [email protected] (H. L. Wu)
Abstract: The growth mode of m-plane AlN crystal grown by physical vapor transport method is studied. The m-plane AlN crystal fabricated by spontaneous nucleation was used as a seed to grow bulk crystal, and 3-mm thick AlN crystals with a maximum lateral dimension of 20 mm were obtained. The Raman tensor elements of A1(TO), E2(high) and E1(TO) Raman models from the m-plane AlN crystal are investigated by angle-dependent polarized Raman scattering. The Raman tensor comparing with our previous report indicates the possible enhancement of the Raman tensor in the directions vertical to [0002] direction at higher growth temperature. Key words: Physical vapor transport; Raman spectroscopy; semiconductor; nucleation of recrystallization
1. Introduction: AlxGa1-xN materials with low dislocation densities are rather demanding for ultraviolet (UVC) light emitting diodes (LEDs) which will enhance the UVC LEDs quantum efficiency [1-3]. As the widely used substrate for UVC LEDs, sapphire and silicon limit the quality of AlxGa1-xN resulted high dislocation densities of 108~1010cm-2 [4-6]. As an important III-V semiconductor, aluminum nitride (AlN) has many desirable physical properties suitable as a substrate of AlxGa1-xN, such as lowest lattice and thermal mismatch, wide band-gap (~6.20 eV), high thermal conductivity, and high chemical and thermal stability. It has been reported that the dislocation density of AlxGa1-xN epitaxial heterostructures on AlN substrate was 104~106 cm-2[7, 8] and 229nm UVC LEDs were fabricated on [0001] plane AlN substrate with AlxGa1-xN as the multi-quantum wells[9]. However, it was well observed that piezoelectric polarization field of substrate will lead the sharp band bending of multi-quantum wells and result the quantum efficiency degrading[10,11]. Moreover, the luminescence anisotropic property of AlN and AlGaN lead the UV light should emit from the side of the LED which grown on [0001] orientation[12]. By employing nonpolar orientations AlN substrate those problems can be circumvented [13]. Thus, to obtain m-plane AlN crystal and to investigate its polarization tensor is of great importance. Physical vapor transport (PVT) is effective method to prepare bulk AlN single crystal. Currently, for the growth of (0001) face AlN crystal, 2 inch diameter AlN wafer had been reported by R. T. Bondokov’ group and T. Y. Chemekova’ group, however, the substrates have some polycrystalline regions of the total area [14,15]. High quality N-polar bulk AlN crystal exhibit a zonar structure was grown by HexaTech, Inc [16] and the largest known single crystal AlN substrate (2 inch) that is completely free of macroscopic defects had been reported recently [17]. To data, the growth of m-orientation bulk AlN crystal has been reported by R. Collazo’s group [18] and our group [19], however, few details were illustrated about the growth mode and the effect of grown temperature on crystal structure. In this work, the growth mode of m-plane AlN crystal fabricated by spontaneous nucleation strategy was investigated. And we use the m-plane AlN as the seed to prepare centimeter-sized bulk crystal. The angle-dependent Raman spectroscopy was presented to investigate the polarization anisotropic of m-plane AlN bulk crystal, and the angular dependencies of Al(TO), E2(high) and El(TO) mode are discussed in terms of the Raman selection rules to investigate AlN crystal structure.
2. Experimental details The growth of m-plane AlN crystals were synthesized by PVT method in a resistance-heated furnace. AlN powders were sintered twice at 2000oC~2200oC for 8 h to reduce the impurity content in the samples. The chamber was vacuumed to 2×10-4 Pa, and then filled to (6~9)×104 Pa with high purity N2. M-plane AlN crystals on tungsten substrate were obtained with the source temperature kept at about 2260~2290 oC for 6~10 h and the gradient between source and crucible lid set as 0.2~1 o
C /mm. Then m-plane AlN crystals obtained were used as native seed to get bulk AlN crystal. AlN crystal microscope images were acquired by laser confocal microscopy. Atomic force
microscopy (AFM) was performed with an intelligent mode using a Bruker Dimension Icon to characterize the nuclei morphologies. X-ray diffraction (XRD) measurements were performed using a high resolution diffractometer (Rigaku, TTRAX III, Cu Ka, 18W) to define the AlN crystal orientation and quality. Polarization Raman spectra of m-plane AlN were collected by a HR800 Raman spectrometer system from Horiba Jobin Yvon with a 532 nm solid-state laser as the exciting source. In the optical path of the scattered signal, a rotatable polarizer was inserted, enabling the measurement component of the scattered light with polarization parallel (perpendicular) to that of incoming light. Raman spectra were acquired in the range of 550-700 cm-1 wave-number by a lens-based spectrometer with a cooled charge-coupled device (CCD) detector.
3. Results and discussion 3.1. The growth model of m-plane AlN crystal By spontaneous nucleation method, the multiple and single growth centers of quadrilateral AlN crystals are found. It is shown that the top surface of multiple growth centers are flat with stripes surround and the detail of this kind center exhibits irregular morphology (Fig. 1(a)~(c)). The nonlinear growth stripes indicate the AlN crystal grown in large supersaturation at the growing surface. The single growth center is shown in Fig. 1 (d)~(f), and the particulars of the growth center exhibit a rectangular shape and the stripes surround are also regular in morphology. In fact, the single growth center AlN crystal is fabricated in low supersaturation at the growing surface. The growth direction of the tetragonal AlN crystal is determined along [10¯10] axis compared to the schematic of crystal structure as shown in Fig. 1 (g). X-ray Omega-scans curves of tetragonal AlN crystal are carried out in Fig. 1 (h) and the rocking curves is inserted in Fig. 1 (h). The diffraction peak located at 16.04o angle further reveals that the tetragonal AlN crystal is grown along [10¯10] axis, and about
0.54o angle is detected between the growth plane and m-plane which may be caused by the temperature distribution of the substrate. The ω-scan rocking curve full width at half maximum (FWHM) values is 100.2 arcsec reflects the high crystal quality of the as-grown AlN crystal. The growth of m-plane AlN crystals is only possible in a small parameter range with an adequate supersaturation of gaseous aluminum Al(g) and N2(g), especially the growth of m-plane AlN crystal with single growth center. In order to prepare bulk m-plan AlN crystals, AFM image of three different regions are taken from the growth center to investigate growth mode of m-plane AlN crystal. Selected area as marked by red box in Fig. 2 (a) is shown in Fig. 2 (b), and it is found that the width of terraces is about 5~10µm and the height is about 30~80nm. The long-side of the step bunches is parallel to the c-axis. The step bunches as marked by black box in Fig. 2 (a) is shown in Fig. 2 (c). Compare to the area in Fig. 2 (b), the terraces are narrow with 1~3µm in width. The terrace as marked in white box is shown in Fig. 2 (d) and (e). The smooth surface represents a 2D layer growth mechanism without any island formation. In fact, each step bunch consists of many parallel steps being bunched together. The unique nucleation point is fund as shown in Fig. 2 (f). The morphology of the nucleus is triangle in the first layer, and then evolves into irregular hexagon as the second and third layers. Crystal grown by the single growth center mode seems to show better structural quality [20]. Summarily, the growth surface is divided into nucleation area, step flow area and step bunching area. Firstly, the nucleus occurs on nucleation area and the shape develops into triangle. Generally, crystal grown with a single nucleation center follows the screw dislocation growth model, such as c plane SiC, c plane GaN and c plane AlN. Whereas the growth center of m-plane AlN crystal exhibit the concentric, closed ring structures which may correspond to edge dislocations and need further investigation[21, 22]. Secondly, the step-flow begins at this point smoothly flows to all around on step flow area. The observed steps are very straight and they flow in line with significantly varying spacing along the steps and no inclusions are noticed microscopically. The m-plane AlN crystal grows up at this stage. Finally, steps origin from nucleation center bunch together on the step bunching area. The steps flow cease for many reasons, such as the supersaturation fluctuation on growth surface, or the foreign particles and impurities deposited on the surface[23]. 3.2 The structure anisotropy The m-plane AlN crystal fabricated by spontaneous nucleation is used as a seed to grow bulk crystal
as shown in Fig. 3 (a). Very careful process parameters are applied to avoid polycrystalline generation. 3-mm thick bulk crystals with 12mm×20mm in diameter are obtained as shown in Fig. 3 (b) and the m-plane was marked by red arrow which is used to detect by angle-dependent Raman scattering. The configuration of the angle-dependent Raman scattering experiment is shown in Fig. 3 (c). The XYZ is the measurement coordinate system. The wave-vector of incident laser is parallel to X-axis, more specifically,
was known as the unit vector in the direction of the incident light,
was considered as the unit vector in the direction of the scattered light. The angle between the unit and Y-axis was severally presented as θ in the m-plane. The AlN sample can be rotated
vector of
360o around the X-axis from 0o to 360o with step of 10o. Raman spectra measured without polarizer inserted in the optical path was shown in Fig. 3 (d), and five peaks were detected at 248.1 cm-1, 609.7 cm-1, 656.2 cm-1, 669.8 cm-1, and 912.5 cm-1 which were E2(low), A1(TO), E2(high), E1(TO) and E1(LO) modes, respectively. The Raman spectra of X(YZ)¯X (perpendicular) and X(YY)¯X (parallel) configurations are shown in Fig. 3 (e) and (f), and the A1(TO), E2(high) and E1(TO) models were observed at 609.7 cm-1, 655.8 cm-1 and 669.3 cm-1, respectively. The peak intensities of A1(TO), E2(high) and E1(TO) models change periodically with the polarization angle, indicating the anisotropic Raman signals of the m-plane AlN crystal. Theoretically, the scattering intensities in a Raman experiment can be calculated by[24~26] I∞|
where
i
and
s
∙
∙
|2
(1)
are the polarization vectors of the incident and scattered radiation, respectively, and
R is the complex second-rank Raman tensor which ascribe to the polarization behavior as shown in equation (2). R=
(2)
where Rij represents the Raman tensor in j direction excited by the electric field in the i direction. The Raman tensor of A1, E1, and E2 mode in a hexagonal material is given by R[A1]= 0 0
R[E2]= d 0
0
0
0 0
d 0 −d 0 0 0
(3)
(4)
0 R[E1]= 0 −
0 − 0 c c 0
(5)
For Raman backscattering measurements on the m-plane surface, the polarization vectors of incident and scattering light referred to the hexagonal axes can be written as 0 sin ( ) = i cos( ) 0 ‖ s= sin( ) cos ( )
0 s= cos( ) −sin ( )
(6)
(7)
(8)
For these configurations, according to Eq. (1)~(8), the Raman intensity A1(TO), E2(high) and E1(TO) modes can be expressed as shown in table 1. We fitted the experiment data with the equations, and the results are plotted in Fig. 4. In perpendicular-polarization configuration, the A1(TO) mode shows obvious 90° periodic variations with the sample rotation angle for perpendicular-polarization, and the four maximum intensity angles occur at 45°, 135°, 225° and 315° for A1(TO) mode. While in parallel-polarization configuration, the A1(TO) shows obvious 180° periodic variations with firstly maximum intensities at 0° and 180°, and secondary maximum intensities at 90° and 270°. The ratio of firstly maximum intensities to secondary maximum is about 6.7 which originate from the large phase difference between the complex Raman tensor elements a and b. As shown in Fig. 4 (c) and (d), the E2(high) mode shows obvious 90° periodic variations, yielding a 4-lobed shape with four maximum intensity angles at about 45°, 135°, 225° and 315° in perpendicular-polarization configuration, and in parallel-polarization configuration shows 180° periodic variations, yielding a 2-lobed shape with two maximum intensity angles at about 90°, and 270°. The E1(TO) mode, as shown in Fig. 4 (e) and (f), both in perpendicular-polarization and parallel-polarization configuration are 90° periodic variations, while the difference is that maximum intensity angles located at 0°, 90°, 180° , 270° and 45°, 135°, 225° and 315°, respectively. From the fitting parameters, the relative values of the Raman tensor elements for AlN are calculated, which are summarized in Table 2. And the relative values of the Raman tensor elements of AlN crystal that we have reported before[25, 26] is listed for a comparison to give a further understanding about the structure anisotropy under different growth temperature. The crystal
fabrication temperature in this work and previous work are 2260oC and 2150oC, respectively. The phase difference φa-b between elements | | and | | in this work is 91.69° and 91.41° for parallel
and cross-polarized scattering geometries, which have similar values to our previous work. From the calculation data, the ratio of | / |, | / | and | / | of AlN crystal in this work become smaller,
whereas, the ratio of | / | | / | and | / | become larger than our previous work indicated that | | become bigger and | | become smaller. From the complex second-rank Raman tensor R and
fitted equation in Table 1, we can see that the Raman tensor | | is along ZZ polarization represented that | | is in [0002] direction, and the Raman tensor | | is along YY polarization represented that
| | is vertical to [0002] direction. Above all, comparing to AlN grown at 2150oC, the Raman tensor
in [0002] direction of AlN grown at 2260oC is becoming smaller. Theoretically, growth morphologies
of AlN crystal in PVT method are temperature dependent which may originate from the effect of the polar-surface[27]. Higher growth temperature will result a larger growth rates vertical to [0002] direction of AlN crystal which origins from the enhancement of the polarization of chemical bonds in the m and a direction, thus enhancing the Raman tensor in those directions. The detail influence of growth temperature on the anisotropy of Raman tensor in AlN crystals is not reported, therefore, further investigations are needed which advantageous to improve the performance of AlN based devices.
4. Conclusion In summary, m-plane AlN crystal is fabricated by spontaneous nucleation strategy in PVT method. The single nucleation center with closed ring structures illustrates the growth mode of the m-plane AlN crystal generally follows edge dislocations driving growth model. 3-mm thick bulk AlN crystal with 12mm×20mm in diameter were obtained and used to study the structure anisotropic by angle-dependent Raman scattering. The ratio of Raman tensor elements of A1, E1, and E2 phonon modes is calculated and compared with our previous reports, indicative of the possible enhancement of the Raman tensor in the directions vertical to [0002] direction at a higher crystal growth temperature.
Acknowledgments This work is supported financially by the Natural Science Foundation of China (Grant numbers 51702297).
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Fig. 1 (a) Optical photograph of the quadrilateral AlN crystal with multiple growth centers; (b) One of the growth centers from (a); (c) The enlarge image of growth centers in (b); (d) Optical photograph of the quadrilateral AlN crystal with smooth surface; (e) The growth centers from (d); (f) The enlarge image of growth centers in (f); (g) The schematic of AlN crystal structure; (h) XRD Φ scans of tetragonal AlN crystal, and inset shows X-ray diffraction rocking curves of the m-plane AlN single crystals
Fig. 2 (a) AFM image of the growth center of m-plane AlN crystal; (b)~(d) Enlarged AFM images marked by red, blak and white box in Fig. (a); (e) Enlarged AFM images of Fig. (e); (f) AFM image of nucleation point; (g) Schematic diagram of growth mode
Fig. 3 (a) Photograph of the AlN seed from spontaneous nucleation; (b) Photograph of the m-plane AlN crysatal grown for five growth runs; (c) Schematic illustration of the Raman experimental coordinate system;(d) Raman spectrum of the AlN crystal without polarizer inserted in the optical path; (e) The Raman spectra of the AlN crystal for perpendicular configuration; (f) The Raman spectra of the AlN crystal for parallel configuration
Fig. 4 Polar plots of angle-resolved polarized Raman scattering intensities. (a), (b) show the intensity of A1(TO) signal under perpendicular and parallel polarization vectors, respectively, (c), (d) show the intensity of E2(High) signal under perpendicular and parallel polarization vectors, respectively, (e), (f) show the intensity of E1(TO) signal under perpendicular and parallel polarization vectors, respectively
Table 1 Selection rules for the allowed Raman modes of m-plane faced crystals for different polarizations of the incident e#$ and scattered (e#% ) polarization vectors: perpendicular (e#$ ⊥ ) and parallel (e#$ ‖e#% ), where θ is the angle between e#$ and the Y axis and '()* is the phase difference between the complex Raman tensor elements a and b. Raman #, ⊥#. # , ‖+ #/ + + mode 1 2 1 A1 | |2 <= > ( )＋| |2 =?@> ( )＋ | || |=?@2 (2 )cos ('()* ) sin (2θ)5|a|2 ＋|b|2 − 2|a||b|cos (φ9): ); 4 2 1 2 2 | |2 =?@> ( ) E2 | | =?@ (2 ) 4 E1
| |2 <= 2 (2 )
| |2 =?@2 (2 )
Table 2 Ratio of the Raman tensor elements of A1, E1, and E2 phonon modes of AlN estimated from the parameters of fits to angle-dependent Raman spectra for two different alignments of the incident (ei) and scattered (es) polarization vectors. The comparison of Ratio of the Raman tensor elements of AlN crystal that we have reported before is listed.
Tensor element （m-plane） | | | | | | | | |
/ / / / / / / / /
| | | | | | | | |
'()*
AlN 1.582 0.609 0.638 2.598 1.048 1.642 2.480 0.954 1.567 91.69o
?∥ =
AlN (ref.)
AlN
1.818 0.730 0.671 2.490 0.919 1.370 2.709 1.088 1.490 91.24o
1.571 0.605 0.688 2.597 137.1 1.653 2.283 0.879 1.453 91.41o
?⊥ =
AlN (ref.) 1.846 0.746 0.721 2.475 0.966 1.341 2.560 1.035 1.387 91.18o
1. First experiment report on the growth mode of m Plane AlN single crystals. 2. 3-mm thick m plane AlN crystals with a maximum lateral dimension of 20 mm were obtained using AlN seed fabricated by spontaneous nucleation. 3. The ratio of Raman tensor elements of A1, E1, and E2 phonon modes is calculated and compared with our previous reports, indicative of the enhancement of the Raman tensor in the directions vertical to [0002] direction at a higher crystal growth temperature.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: