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Study of stress in ammonothermal non-polar and semi-polar GaN crystal grown on HVPE GaN seeds
Journal Pre-proofs Study of stress in ammonothermal non-polar and semi-polar GaN crystal grown on HVPE GaN seeds Tengkun Li, Guoqiang Ren, Jingjing Yao, Xujun Su, Shunan Zheng, Xiaodong Gao, Lei Xu, Ke Xu PII: DOI: Reference:
S0022-0248(19)30638-4 https://doi.org/10.1016/j.jcrysgro.2019.125423 CRYS 125423
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
31 August 2019 7 December 2019 10 December 2019
Please cite this article as: T. Li, G. Ren, J. Yao, X. Su, S. Zheng, X. Gao, L. Xu, K. Xu, Study of stress in ammonothermal non-polar and semi-polar GaN crystal grown on HVPE GaN seeds, Journal of Crystal Growth (2019), doi: https://doi.org/10.1016/j.jcrysgro.2019.125423
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© 2019 Published by Elsevier B.V.
Study of stress in ammonothermal non-polar and semi-polar GaN crystal grown on HVPE GaN seeds Tengkun Li1, 2, Guoqiang Ren1,2*, Jingjing Yao2, Xujun Su2, Shunan Zheng2, Xiaodong Gao2, Lei Xu2 and Ke Xu1,2,3* 1
School of Nano-Tech and Nano-Bionics, University of Science and Technology of
China, Hefei, 230026, Anhui, China 2Suzhou
Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences,
Suzhou, 215123, Jiangsu, China 3Suzhou
Nanowin Science and Technology Co, Ltd., Suzhou, 215123, Jiangsu, China
Abstract GaN crystals were grown on non-polar and semi-polar HVPE GaN seeds by basic ammonothermal method. Stress distributions were investigated in cross-section of (1120) plane, (10-10) plane, (20-21) plane and (10-11) plane GaN crystal. The cathodoluminescence (CL) images show cross-section information clearly and each examined object consisted of hydride vapor phase epitaxy (HVPE) seed and ammonothermal GaN (Am-GaN). The impurity concentration and free carrier concentration were estimated by secondary-ion mass spectroscopy (SIMS) and Hall. Moreover, Raman spectroscopy was used for studying stress distribution in the cross section. Shifts of E2(high) phonon lines were analyzed to determine stress. Our results indicate that the stress is about 35MPa in bulk GaN and the stress is less than 60MPa in the interface of Am-GaN. Keywords: A1. Characterization, A2. Ammonothermal crystal growth, B1. Gallium compounds, B1. Nitrides
Introduction Recently, high quality bulk Gallium nitride (GaN) substrates are strongly demanded for both optoelectronic and electronic devices. [1, 2] The nonpolar and semipolar planes of GaN crystals also have gained considerable attention due to the potential for highefficiency, low-droop light emitting diodes (LEDs) as a result of reduced or eliminated
detrimental polarization-related effects. Several groups reported remarkable results for LEDs on specific semi-polar or non-polar GaN substrate materials.[3-6] Such substrate materials have been obtained by the hydride vapor phase epitaxy (HVPE) method by slicing along the proper direction of the boule crystal grown along c-direction. However, considerably large threading dislocation density is caused by the growth on heterogeneous substrate. In order to solve this problem, ammonothermal method was developed. Ammonothermal is grown on native GaN seed and is analogous to the hydrothermal growth of quartz. It is considered appropriate method to fulfill the need for large area, bulk GaN. [7-11] Both polar and non-polar bulk GaN crystals were grown in supercritical ammonia with basic mineralizer [7, 12, 13] and acidic mineralizer[14]. The surface morphology of non-polar and semi-polar ammonothermal GaN (Am-GaN) have been studied [15]. In our previous work, the growth behaviors of GaN crystals grown on non-polar and semi-polar HVPE GaN seeds by ammonothermal method were investigated. [16] In particularly, ammonothermal growth typically incorporates high concentrations of impurities in the crystal. The impurities will induce lattice strain and enough strain to cause cracking of the substrate[17]. The stress distribution between the Am-GaN and HVPE seed is not yet well known and it is crucial for developing GaN bulk growth technology in ammonothermal method. In this paper, Ammonothermal growth of GaN crystals on HVPE seeds of different orientations were realized. The growth crystal qualities were characterized and the cross section of the crystals were investigated by cathodoluminescence (CL). The impurity concentration and free carrier concentration were estimated by SIMS and Hall. Moreover, micro-Raman spectroscopy is used for studying stress in GaN crystallized by ammonothermal on HVPE seed and E2(high) phonon lines were analyzed to determine stress. Experiment The Am-GaN was grown on HVPE-GaN seed, and the growth run was carried out in home-made autoclave with 30mm inner diameter. The inner room of the autoclave is divided into two regions by the baffle, the upper nutrient region and the lower growth
region. And the baffle has an open to closed area ratio of ~ 15%. Polycrystalline GaN, which was by-product HVPE GaN on the edge of the susceptor used in the growth of thick GaN boules, was used as nutrient. In addition, the KNH2 was used as mineralizer. Finally, the autoclave was sealed and filled with pure ammonia. The growth zone temperature was in the range 500-580℃ and the system pressure between 270 MPa and 250 MPa. The locations of temperature measurement were in the middle of the growth zone and the nutrient zone. The growth run was performed ten days. GaN seed crystals were prepared by cutting thick c-axis bulk GaN which were grown at NANOWIN by HVPE. The bulk GaN was sliced at (11-20), (10-10), (20-21), (1011) and (0001) plane. And the seed crystals were used after mechano-chemical polishing. Crystal quality of Am-GaN was characterized by determining the full width at half maximum (FWHM) of ω-rocking curve using high-resolution X-ray diffraction for a spot size of 1mm × 10mm on a Bruker D8 Discover and double-crystal X-ray diffraction was used for determining FWHMs. The concentrations of impurities were examined by SIMS. Hall measurements were carried by the van der Pauw method to estimate the free carrier concentration. The cross section structure was characterized with scanning electron microscopy (SEM) (Quanta400FEG) and its attached cathodoluminescence (CL) (Mono-CL3+) at room temperature with an accelerating voltage of 20 kV. Raman-scattering measurements were performed with a He–Ne laser (532 nm excitation wavelength) and a Horiba-Jobin-Yvon HR800 spectrometer. In the Raman measurement, the excitation density is about 6.5×105 W/cm2, the spectral resolution of the system is 0.2cm-1, the measurement uncertainty is about 0.2cm-1.
Results and discussion
Fig. 1 Schematic illustration and photography of ammonothermal crystals grown on HVPE seeds of varying crystallographic plane. (a) Schematic hexagonal lattice showing (11-20) plane, (10-10) plane, (20-21) plane, (10-11) plane. (b)-(e) Photography of as-grown GaN on the seed corresponding to those shown in (a). The growth run with multiple seeds was performed. Fig.1 (a)-(e) are the schematic illustration and photography of basic ammonothermal crystals grown on HVPE seeds of varying crystallographic plane. Fig. 1(a) Schematic hexagonal lattice showing (1120) plane, (10-10) plane, (20-21) plane, (10-11) plane. The growth rates of a-plane, mplane, (20-21) and (10-11) is about 125 μm/d, 4 μm/d, 53μm/d and 41μm/d, respectively. In the same growth conditions, the growth rate of (11-20) plane is the highest. Owing to the impurity incorporation play important roles in the color of the Am-GaN, the color of the crystals is nearly brown. Because the ammonothermal process is closed system growth process leading to strictly requirements on purity of sources materials. Any soluble impurity in the supercritical ammonia solution may be doped into the GaN crystal in the growth system. [18] And unintentional impurities concentrations of AmGaN crystals were measured by SIMS. Concentrations of O is about 5×1019 cm−3. H concentrations in excess of 1020 cm−3. Concentrations of Si is about mid-1017 cm−3 and concentrations of C is 3×1017 cm−3. The impurity concentrations is list in the table1.
Table 1. Impurity concentrations measured by SIMS in GaN crystals Impurity concentration by element (atoms/cm3) Stress in the bulk Am-GaN (MPa)
H
O
Si
C
~1020
5×1019
5×1017
3×1017
The crystal quality of Am-GaN was characterized by determining the FWHM of the ω-rocking (shown in Fig. 2). The FWHMs of the ω-XRC of as grown GaN are about 76-140 arcsec. It is broader than the underlying HVPE seed crystal (The FWHMs of the ω-XRC of the seed is about 40 arcsec).
Fig.2 X-ray rocking curves for ammonothermal-GaN grown on different seeds, (a), (b), (c) (d) are for (11-20), (10-10), (20-21), and (10-11), respectively.
Fig.3 Cross section Panchromatic CL micrograph of the Am-GaN grown on (a) (1120), (b) (10-10), (c) (20-21) and (d) (10-11) HVPE seed, respectively. Areas measured by Raman spectroscopy are marked by white dash lines. Four samples were prepared and their cross-sections were shown in Fig3 (a)-(d), respectively. Fig. 3 shows cross section Panchromatic CL micrograph of the Am-GaN grown on HVPE seeds. Cathodoluminescence constitutes a powerful technique for distinguish the interface between Am-GaN and HVPE-seed crystals, since it gives information about the incorporation of impurities under the spectrally resolved mode. Luminescence efficiency depends on the presence of the concentration of impurities. [19] CL images show the interface and cross-section information clearly. The areas measured by Raman spectroscopy are marked by white lines in these figures. Each examined object consisted of HVPE seed and Am-GaN.
Fig.4 The Raman shift E2 (high) phonon mode line mapping of cross section (a) (1120), (b) (10-10), (c) (20-21) and (d) (10-11) Am-GaN grown on HVPE seed. The HVPE seeds were marked between the dash lines.
Table 2. The stress distribution in the Am-GaN relative to HVPE seed Crystallographic plane
(11-20)
(10-10)
(20-21)
(10-11)
Stress in the interface (MPa)
<35
<35
<60
<48
Stress in the bulk Am-GaN (MPa)
~35
—
~35
~35
The cross sections of Am-GaN grown on HVPE were examined along the white lines in the CL images. It should be noted that the measuring points were taken every 5μm. Fig. 4 (a)-(d) represents Raman shift scan of lines corresponding to the positions of E2(high) peaks. It is known that the variation of Raman shift is an indication of the stress variation. The E2(high) phonon mode is sensitive to biaxial stress. [20, 21] A shift of the E2(high) phonon line is a linear function of the biaxial stress and can be expressed as: ∆ω = Kσ𝑥𝑥 = 𝑦𝑦
where Δω represents the shift of the phonon line, σ𝑥𝑥 = 𝑦𝑦 is the element of stress tensor, and K is the pressure coefficient. For our calculations K = 4.2 [cm−1/GPa] was chosen[21, 22]. As shown in Fig. 4, the dashed line indicates the interface (the interface is about 30μm width) between Am-GaN and HVPE seed. The frequency of E2(high) peaks exhibiting a blue shift in the Am-GaN. So compressive stress is observed in AmGaN and the stress is gradually increasing in the interface region. In fig. 4 (c) (d), the frequencies difference of E2(high) peaks in the interface is increasing to about 0.25cm−1 and 0.2cm−1, so the stress is about 60MPa and 48Mpa. On the other crystalline growth direction, the stress in (20-2-1) plane and (10-1-1) plane is lower. Although the frequencies of E2(high) peaks in the seed crystals have some deviations, the frequencies relative difference of E2(high) peaks between HVPE seed and Am-GaN of E2(high) peaks is approximate to 0.15 cm−1 and in the bulk material the stress value decrease to 35MPa. More important, in the bulk material the stress is stable or decreased trend. Specifically, in the fig. 4(b), because the growth rate is low in m-plane, in the same period Am-GaN of m-plane is in the interface region and hasn't reached a steady trend yet. The stress distribution in the Am-GaN relative to HVPE seed was list in the table 2. As was already mentioned, any soluble impurity in the supercritical ammonia solution may be doped into the GaN crystal. In our work, Concentrations of O is about 5×1019 cm−3. H concentrations in excess of 1020 cm−3. Concentrations of Si is about mid-1017 cm−3 and concentrations of C is 3×1017 cm−3. The free carrier concentrations was estimated by the hall method. The free carrier concentrations of a-plane Am-GaN is about 4.3×1019 cm-3 and (20-21) plane Am-GaN is about 2.9×1019 cm-3. (10-11) plane and m-plane Am-GaN were cleaved in other testing process. And these crystals were grown in the same condition, so the free carrier concentrations will be on the same order of magnitude. According to M. Amilusik et al. [21] when HVPE-GaN is grown on an Am-GaN seed, the oxygen concentration in GaN grown in the lateral directions is always the order of 1019 cm−3. Higher oxygen concentration in the GaN leads to higher free carrier concentration in this material. [20]This, in turn, results in an increase of the lattice constants. And for GaN, the increase of lattice parameters by free electrons was
measured by doping the crystal with silicon.[23] Therefore, due to high free carrier concentration leading lattice mismatch between Am-GaN and HVPE seed, the AmGaN is under compressive stress. As for the origin of the higher stress in the, there is no clear conclusion has been presented. Looking closer the interface region, during first stages of ammonothermal process, there was back etching of the seed. The cross section panchromatic CL micrograph of the Am-GaN grown on HVPE seed was shown in the fig. 5(a). In the first stage of the ammonothermal growth, there may be more incorporation aggregation, like the “black particles” in the interface (the interface is about 20μm width) in the fig.5(a). In this case, the introduction of “black particles” with in the interface will result in performed degradation of the crystalline quality. This may be corelated with the higher stress in the interface region. The schematic diagram of etch back and growth of GaN crystals was shown in the fig.5 (b).
Fig.5 (a) Cross section panchromatic CL micrograph of the Am-GaN grown on HVPE seed. (b) The schematic diagram of etch back and growth of GaN crystals
Conclusion The study of ammonothermal growth of non-polar and semi-polar GaN crystal on HVPE seeds was presented in this paper. The CL images show the interface and crosssection information clearly. The impurity concentration and free carrier concentration were estimated by SIMS and Hall. Raman spectroscopy was used for determining the
stress, and then calculating stress in Am-GaN grown on HVPE GaN seed. It was shown that due to high free carrier concentration leading lattice mismatch, the stress in the interface is higher than the stress in the bulk Am-GaN. In the bulk material the stress is stable or in decreased trend. It will be a solution to reduce the stress in the bulk material that the crystals were grown in ultrahigh purity growth environment to improve purity.
Corresponding Author Email: [email protected]; Email: [email protected] Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 61574162, 61604169), National Key Research and Development Program of China (Grant No. 2017YFB0404100). We would like to express our thanks to Suzhou Nanowin Science and Technology Co, Ltd for supporting HVPE GaN seeds.
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2499-2502. [8] T. Hashimoto, E.R. Letts, D. Key, B. Jordan, Two inch GaN substrates fabricated by the near equilibrium ammonothermal (NEAT) method, Jpn J Appl Phys, 58 (2019) SC1005. [9] R. Kucharski, M. Zajac, R. Doradzinski, M. Rudzinski, R. Kudrawiec, R. Dwilinski, Non-polar and semipolar ammonothermal GaN substrates, Semicond Sci Tech, 27 (2012) 024007. [10] P. Kruszewski, P. Prystawko, I. Kasalynas, A. Nowakowska-Siwinska, M. Krysko, J. Plesiewicz, J. Smalc-Koziorowska, R. Dwilinski, M. Zajac, R. Kucharski, M. Leszczynski, AlGaN/GaN HEMT structures on ammono bulk GaN substrate, Semicond Sci Tech, 29 (2014) 075004. [11] N.A. Mahadik, S.B. Qadri, J.A. Freitas, Structural Inhomogeneities and Impurity Incorporation in Growth of High-Quality Ammonothermal GaN Substrates, Crystal Growth & Design, 15 (2015) 291-294. [12] S. Pimputkar, S. Kawabata, J.S. Speck, S. Nakamura, Improved growth rates and purity of basic ammonothermal GaN, Journal of Crystal Growth, 403 (2014) 7-17. [13] R. Kucharski, M. Rudzinski, M. Zajac, R. Doradzinski, J. Garczynski, L. Sierzputowski, R. Kudrawiec, J. Serafinczuk, W. Strupinski, R. Dwilinski, Nonpolar GaN substrates grown by ammonothermal method, Applied Physics Letters, 95 (2009) 131119. [14] Y. Mikawa, T. Ishinabe, S. Kawabata, T. Mochizuki, A. Kojima, Y. Kagamitani, H. Fujisawa, Ammonothermal Growth of Polar and Non-polar Bulk GaN Crystal, in: J.I. Chyi, H. Fujioka, H. Morkoc (Eds.) Gallium Nitride Materials and Devices X, 2015, pp. 936302. [15] S. Pimputkar, S. Kawabata, J.S. Speck, S. Nakamura, Surface morphology study of basic ammonothermal GaN grown on non-polar GaN seed crystals of varying surface orientations from mplane to a-plane, Journal of Crystal Growth, 368 (2013) 67-71. [16] T. Li, G. Ren, X. Su, J. Yao, Z. Yan, X. Gao, K. Xu, Growth behavior of ammonothermal GaN crystals grown on non-polar and semi-polar HVPE GaN seeds, CrystEngComm, 21 (2019) 4874-4879. [17] S. Sintonen, P. Kivisaari, S. Pimputkar, S. Suihkonen, T. Schulz, J.S. Speck, S. Nakamura, Incorporation and effects of impurities in different growth zones within basic ammonothermal GaN, Journal of Crystal Growth, 456 (2016) 43-50. [18] S. Suihkonen, S. Pimputkar, S. Sintonen, F. Tuomisto, Defects in Single Crystalline Ammonothermal Gallium Nitride, Advanced Electronic Materials, 3 (2017) 1600496. [19] V. Hortelano, O. Martinez, J. Jimenez, B. Wang, S. Swider, M. Suscavage, D. Bliss, Cathodoluminescence Study of Ammonothermal GaN Crystals, in: H. YamadaKaneta, A. Sakai (Eds.) Defects-Recognition, Imaging and Physics in Semiconductors Xiv, 2012, pp. 63-66. [20] P. Schustek, M. Hocker, M. Klein, U. Simon, F. Scholz, K. Thonke, Spectroscopic study of semipolar (112¯2)-HVPE GaN exhibiting high oxygen incorporation, Journal of Applied Physics, 116 (2014) 163515. [21] M. Amilusik, D. Wlodarczyk, A. Suchocki, M. Bockowski, Micro-Raman studies of strain in bulk GaN crystals grown by hydride vapor phase epitaxy on ammonothermal GaN seeds, Jpn J Appl Phys, 58 (2019) SCCB32. [22] C. Kisielowski, J. Kruger, S. Ruvimov, T. Suski, J.W. Ager, E. Jones, Z. LilientalWeber, M. Rubin, E.R. Weber, M.D. Bremser, R.F. Davis, Strain-related phenomena in GaN thin films, Physical Review B, 54 (1996) 17745-17753. [23] M. Leszczynski, P. Prystawko, T. Suski, B. Lucznik, J. Domagala, J. Bak-Misiuk, A. Stonert, A. Turos, R. Langer, A. Barski, Lattice parameters of GaN single crystals, homoepitaxial layers and heteroepitaxial layers on sapphire, Journal of Alloys and Compounds, 286 (1999) 271-275.
Highlights 1. Ammonothermal non-polar and semi-polar GaN crystal were grown on HVPE GaN seeds. 2. CL images show cross-section information clearly. 3. The stress is about 35Mpa in bulk GaN and less than 70Mpa in the interface.
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Conflict of Interest Forms: The authors declare no conflict of interest.
The manuscript was written through contributions of all authors.
S0022-0248(19)30638-4 https://doi.org/10.1016/j.jcrysgro.2019.125423 CRYS 125423
To appear in:
Journal of Crystal Growth
Received Date: Revised Date: Accepted Date:
31 August 2019 7 December 2019 10 December 2019
Please cite this article as: T. Li, G. Ren, J. Yao, X. Su, S. Zheng, X. Gao, L. Xu, K. Xu, Study of stress in ammonothermal non-polar and semi-polar GaN crystal grown on HVPE GaN seeds, Journal of Crystal Growth (2019), doi: https://doi.org/10.1016/j.jcrysgro.2019.125423
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.
© 2019 Published by Elsevier B.V.
Study of stress in ammonothermal non-polar and semi-polar GaN crystal grown on HVPE GaN seeds Tengkun Li1, 2, Guoqiang Ren1,2*, Jingjing Yao2, Xujun Su2, Shunan Zheng2, Xiaodong Gao2, Lei Xu2 and Ke Xu1,2,3* 1
School of Nano-Tech and Nano-Bionics, University of Science and Technology of
China, Hefei, 230026, Anhui, China 2Suzhou
Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences,
Suzhou, 215123, Jiangsu, China 3Suzhou
Nanowin Science and Technology Co, Ltd., Suzhou, 215123, Jiangsu, China
Abstract GaN crystals were grown on non-polar and semi-polar HVPE GaN seeds by basic ammonothermal method. Stress distributions were investigated in cross-section of (1120) plane, (10-10) plane, (20-21) plane and (10-11) plane GaN crystal. The cathodoluminescence (CL) images show cross-section information clearly and each examined object consisted of hydride vapor phase epitaxy (HVPE) seed and ammonothermal GaN (Am-GaN). The impurity concentration and free carrier concentration were estimated by secondary-ion mass spectroscopy (SIMS) and Hall. Moreover, Raman spectroscopy was used for studying stress distribution in the cross section. Shifts of E2(high) phonon lines were analyzed to determine stress. Our results indicate that the stress is about 35MPa in bulk GaN and the stress is less than 60MPa in the interface of Am-GaN. Keywords: A1. Characterization, A2. Ammonothermal crystal growth, B1. Gallium compounds, B1. Nitrides
Introduction Recently, high quality bulk Gallium nitride (GaN) substrates are strongly demanded for both optoelectronic and electronic devices. [1, 2] The nonpolar and semipolar planes of GaN crystals also have gained considerable attention due to the potential for highefficiency, low-droop light emitting diodes (LEDs) as a result of reduced or eliminated
detrimental polarization-related effects. Several groups reported remarkable results for LEDs on specific semi-polar or non-polar GaN substrate materials.[3-6] Such substrate materials have been obtained by the hydride vapor phase epitaxy (HVPE) method by slicing along the proper direction of the boule crystal grown along c-direction. However, considerably large threading dislocation density is caused by the growth on heterogeneous substrate. In order to solve this problem, ammonothermal method was developed. Ammonothermal is grown on native GaN seed and is analogous to the hydrothermal growth of quartz. It is considered appropriate method to fulfill the need for large area, bulk GaN. [7-11] Both polar and non-polar bulk GaN crystals were grown in supercritical ammonia with basic mineralizer [7, 12, 13] and acidic mineralizer[14]. The surface morphology of non-polar and semi-polar ammonothermal GaN (Am-GaN) have been studied [15]. In our previous work, the growth behaviors of GaN crystals grown on non-polar and semi-polar HVPE GaN seeds by ammonothermal method were investigated. [16] In particularly, ammonothermal growth typically incorporates high concentrations of impurities in the crystal. The impurities will induce lattice strain and enough strain to cause cracking of the substrate[17]. The stress distribution between the Am-GaN and HVPE seed is not yet well known and it is crucial for developing GaN bulk growth technology in ammonothermal method. In this paper, Ammonothermal growth of GaN crystals on HVPE seeds of different orientations were realized. The growth crystal qualities were characterized and the cross section of the crystals were investigated by cathodoluminescence (CL). The impurity concentration and free carrier concentration were estimated by SIMS and Hall. Moreover, micro-Raman spectroscopy is used for studying stress in GaN crystallized by ammonothermal on HVPE seed and E2(high) phonon lines were analyzed to determine stress. Experiment The Am-GaN was grown on HVPE-GaN seed, and the growth run was carried out in home-made autoclave with 30mm inner diameter. The inner room of the autoclave is divided into two regions by the baffle, the upper nutrient region and the lower growth
region. And the baffle has an open to closed area ratio of ~ 15%. Polycrystalline GaN, which was by-product HVPE GaN on the edge of the susceptor used in the growth of thick GaN boules, was used as nutrient. In addition, the KNH2 was used as mineralizer. Finally, the autoclave was sealed and filled with pure ammonia. The growth zone temperature was in the range 500-580℃ and the system pressure between 270 MPa and 250 MPa. The locations of temperature measurement were in the middle of the growth zone and the nutrient zone. The growth run was performed ten days. GaN seed crystals were prepared by cutting thick c-axis bulk GaN which were grown at NANOWIN by HVPE. The bulk GaN was sliced at (11-20), (10-10), (20-21), (1011) and (0001) plane. And the seed crystals were used after mechano-chemical polishing. Crystal quality of Am-GaN was characterized by determining the full width at half maximum (FWHM) of ω-rocking curve using high-resolution X-ray diffraction for a spot size of 1mm × 10mm on a Bruker D8 Discover and double-crystal X-ray diffraction was used for determining FWHMs. The concentrations of impurities were examined by SIMS. Hall measurements were carried by the van der Pauw method to estimate the free carrier concentration. The cross section structure was characterized with scanning electron microscopy (SEM) (Quanta400FEG) and its attached cathodoluminescence (CL) (Mono-CL3+) at room temperature with an accelerating voltage of 20 kV. Raman-scattering measurements were performed with a He–Ne laser (532 nm excitation wavelength) and a Horiba-Jobin-Yvon HR800 spectrometer. In the Raman measurement, the excitation density is about 6.5×105 W/cm2, the spectral resolution of the system is 0.2cm-1, the measurement uncertainty is about 0.2cm-1.
Results and discussion
Fig. 1 Schematic illustration and photography of ammonothermal crystals grown on HVPE seeds of varying crystallographic plane. (a) Schematic hexagonal lattice showing (11-20) plane, (10-10) plane, (20-21) plane, (10-11) plane. (b)-(e) Photography of as-grown GaN on the seed corresponding to those shown in (a). The growth run with multiple seeds was performed. Fig.1 (a)-(e) are the schematic illustration and photography of basic ammonothermal crystals grown on HVPE seeds of varying crystallographic plane. Fig. 1(a) Schematic hexagonal lattice showing (1120) plane, (10-10) plane, (20-21) plane, (10-11) plane. The growth rates of a-plane, mplane, (20-21) and (10-11) is about 125 μm/d, 4 μm/d, 53μm/d and 41μm/d, respectively. In the same growth conditions, the growth rate of (11-20) plane is the highest. Owing to the impurity incorporation play important roles in the color of the Am-GaN, the color of the crystals is nearly brown. Because the ammonothermal process is closed system growth process leading to strictly requirements on purity of sources materials. Any soluble impurity in the supercritical ammonia solution may be doped into the GaN crystal in the growth system. [18] And unintentional impurities concentrations of AmGaN crystals were measured by SIMS. Concentrations of O is about 5×1019 cm−3. H concentrations in excess of 1020 cm−3. Concentrations of Si is about mid-1017 cm−3 and concentrations of C is 3×1017 cm−3. The impurity concentrations is list in the table1.
Table 1. Impurity concentrations measured by SIMS in GaN crystals Impurity concentration by element (atoms/cm3) Stress in the bulk Am-GaN (MPa)
H
O
Si
C
~1020
5×1019
5×1017
3×1017
The crystal quality of Am-GaN was characterized by determining the FWHM of the ω-rocking (shown in Fig. 2). The FWHMs of the ω-XRC of as grown GaN are about 76-140 arcsec. It is broader than the underlying HVPE seed crystal (The FWHMs of the ω-XRC of the seed is about 40 arcsec).
Fig.2 X-ray rocking curves for ammonothermal-GaN grown on different seeds, (a), (b), (c) (d) are for (11-20), (10-10), (20-21), and (10-11), respectively.
Fig.3 Cross section Panchromatic CL micrograph of the Am-GaN grown on (a) (1120), (b) (10-10), (c) (20-21) and (d) (10-11) HVPE seed, respectively. Areas measured by Raman spectroscopy are marked by white dash lines. Four samples were prepared and their cross-sections were shown in Fig3 (a)-(d), respectively. Fig. 3 shows cross section Panchromatic CL micrograph of the Am-GaN grown on HVPE seeds. Cathodoluminescence constitutes a powerful technique for distinguish the interface between Am-GaN and HVPE-seed crystals, since it gives information about the incorporation of impurities under the spectrally resolved mode. Luminescence efficiency depends on the presence of the concentration of impurities. [19] CL images show the interface and cross-section information clearly. The areas measured by Raman spectroscopy are marked by white lines in these figures. Each examined object consisted of HVPE seed and Am-GaN.
Fig.4 The Raman shift E2 (high) phonon mode line mapping of cross section (a) (1120), (b) (10-10), (c) (20-21) and (d) (10-11) Am-GaN grown on HVPE seed. The HVPE seeds were marked between the dash lines.
Table 2. The stress distribution in the Am-GaN relative to HVPE seed Crystallographic plane
(11-20)
(10-10)
(20-21)
(10-11)
Stress in the interface (MPa)
<35
<35
<60
<48
Stress in the bulk Am-GaN (MPa)
~35
—
~35
~35
The cross sections of Am-GaN grown on HVPE were examined along the white lines in the CL images. It should be noted that the measuring points were taken every 5μm. Fig. 4 (a)-(d) represents Raman shift scan of lines corresponding to the positions of E2(high) peaks. It is known that the variation of Raman shift is an indication of the stress variation. The E2(high) phonon mode is sensitive to biaxial stress. [20, 21] A shift of the E2(high) phonon line is a linear function of the biaxial stress and can be expressed as: ∆ω = Kσ𝑥𝑥 = 𝑦𝑦
where Δω represents the shift of the phonon line, σ𝑥𝑥 = 𝑦𝑦 is the element of stress tensor, and K is the pressure coefficient. For our calculations K = 4.2 [cm−1/GPa] was chosen[21, 22]. As shown in Fig. 4, the dashed line indicates the interface (the interface is about 30μm width) between Am-GaN and HVPE seed. The frequency of E2(high) peaks exhibiting a blue shift in the Am-GaN. So compressive stress is observed in AmGaN and the stress is gradually increasing in the interface region. In fig. 4 (c) (d), the frequencies difference of E2(high) peaks in the interface is increasing to about 0.25cm−1 and 0.2cm−1, so the stress is about 60MPa and 48Mpa. On the other crystalline growth direction, the stress in (20-2-1) plane and (10-1-1) plane is lower. Although the frequencies of E2(high) peaks in the seed crystals have some deviations, the frequencies relative difference of E2(high) peaks between HVPE seed and Am-GaN of E2(high) peaks is approximate to 0.15 cm−1 and in the bulk material the stress value decrease to 35MPa. More important, in the bulk material the stress is stable or decreased trend. Specifically, in the fig. 4(b), because the growth rate is low in m-plane, in the same period Am-GaN of m-plane is in the interface region and hasn't reached a steady trend yet. The stress distribution in the Am-GaN relative to HVPE seed was list in the table 2. As was already mentioned, any soluble impurity in the supercritical ammonia solution may be doped into the GaN crystal. In our work, Concentrations of O is about 5×1019 cm−3. H concentrations in excess of 1020 cm−3. Concentrations of Si is about mid-1017 cm−3 and concentrations of C is 3×1017 cm−3. The free carrier concentrations was estimated by the hall method. The free carrier concentrations of a-plane Am-GaN is about 4.3×1019 cm-3 and (20-21) plane Am-GaN is about 2.9×1019 cm-3. (10-11) plane and m-plane Am-GaN were cleaved in other testing process. And these crystals were grown in the same condition, so the free carrier concentrations will be on the same order of magnitude. According to M. Amilusik et al. [21] when HVPE-GaN is grown on an Am-GaN seed, the oxygen concentration in GaN grown in the lateral directions is always the order of 1019 cm−3. Higher oxygen concentration in the GaN leads to higher free carrier concentration in this material. [20]This, in turn, results in an increase of the lattice constants. And for GaN, the increase of lattice parameters by free electrons was
measured by doping the crystal with silicon.[23] Therefore, due to high free carrier concentration leading lattice mismatch between Am-GaN and HVPE seed, the AmGaN is under compressive stress. As for the origin of the higher stress in the, there is no clear conclusion has been presented. Looking closer the interface region, during first stages of ammonothermal process, there was back etching of the seed. The cross section panchromatic CL micrograph of the Am-GaN grown on HVPE seed was shown in the fig. 5(a). In the first stage of the ammonothermal growth, there may be more incorporation aggregation, like the “black particles” in the interface (the interface is about 20μm width) in the fig.5(a). In this case, the introduction of “black particles” with in the interface will result in performed degradation of the crystalline quality. This may be corelated with the higher stress in the interface region. The schematic diagram of etch back and growth of GaN crystals was shown in the fig.5 (b).
Fig.5 (a) Cross section panchromatic CL micrograph of the Am-GaN grown on HVPE seed. (b) The schematic diagram of etch back and growth of GaN crystals
Conclusion The study of ammonothermal growth of non-polar and semi-polar GaN crystal on HVPE seeds was presented in this paper. The CL images show the interface and crosssection information clearly. The impurity concentration and free carrier concentration were estimated by SIMS and Hall. Raman spectroscopy was used for determining the
stress, and then calculating stress in Am-GaN grown on HVPE GaN seed. It was shown that due to high free carrier concentration leading lattice mismatch, the stress in the interface is higher than the stress in the bulk Am-GaN. In the bulk material the stress is stable or in decreased trend. It will be a solution to reduce the stress in the bulk material that the crystals were grown in ultrahigh purity growth environment to improve purity.
Corresponding Author Email: [email protected]; Email: [email protected] Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant No. 61574162, 61604169), National Key Research and Development Program of China (Grant No. 2017YFB0404100). We would like to express our thanks to Suzhou Nanowin Science and Technology Co, Ltd for supporting HVPE GaN seeds.
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Highlights 1. Ammonothermal non-polar and semi-polar GaN crystal were grown on HVPE GaN seeds. 2. CL images show cross-section information clearly. 3. The stress is about 35Mpa in bulk GaN and less than 70Mpa in the interface.
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Conflict of Interest Forms: The authors declare no conflict of interest.
The manuscript was written through contributions of all authors.