High-quality GaN epitaxially grown on Si substrate with serpentine channels

Accepted Manuscript High-quality GaN epitaxially grown on Si substrate with serpentine channels Tiantian Wei, Hua Zong, Shengxiang Jiang, Yue Yang, Hui Liao, Yahong Xie, Wenjie Wang, Junze Li, Jun Tang, Xiaodong Hu PII:

S0749-6036(18)30358-6

DOI:

10.1016/j.spmi.2018.04.010

Reference:

YSPMI 5624

To appear in:

Superlattices and Microstructures

Please cite this article as: Tiantian Wei, Hua Zong, Shengxiang Jiang, Yue Yang, Hui Liao, Yahong Xie, Wenjie Wang, Junze Li, Jun Tang, Xiaodong Hu, High-quality GaN epitaxially grown on Si substrate with serpentine channels, Superlattices and Microstructures (2018), doi: 10.1016/j.spmi.2018.04.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Highlights 1. A novel serpentine-channeled mask was introduced to Si substrate and the fully coalesced GaN film on the masked Si substrate was achieved for the first time. 2. This new growth method can improve the quality of GaN significantly. high-dislocation region per mask opening.

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3. This growth method only requires one-step epitaxial growth of GaN which has only one 4. High-quality GaN with low dislocation density ~2.4×107 cm-2 was obtained, which

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accounted for about eighty percent of the GaN film in area.

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High-quality GaN epitaxially grown on Si substrate with serpentine channels Tiantian Weia, Hua Zonga, Shengxiang Jianga, Yue Yanga, Hui Liaoa, Yahong Xieb, Wenjie Wangc,

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Junze Lic, Jun Tangd, and Xiaodong Hua,∗ State Key Laboratory for Artificial Microstructure and Microscopic Physics, School of Physics, Peking University, Beijing

100871, China

Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, USA

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Microsystem and Terahertz Research Center, China Academy of Engineering Physics, Chengdu 610200, China

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Hefei IRICO Epilight Technology CO., Ltd., Hefei 230000, China

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ABSTRACT

A novel serpentine-channeled mask was introduced to Si substrate for low-dislocation GaN epitaxial growth and the fully coalesced GaN film on the masked Si substrate was achieved for the first time. Compared with the epitaxial lateral overgrowth (ELOG) growth method, this innovative mask only requires one-step epitaxial growth of GaN which has only one high-dislocation region per mask opening. This new growth method can effectively reduce

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dislocation density, thus improving the quality of GaN significantly. High-quality GaN with low dislocation density ~2.4×107 cm-2 was obtained, which accounted for about eighty percent of the GaN film in area. This innovative technique is promising for the growth of high-quality GaN

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templates and the subsequent fabrication of high-performance GaN-based devices like transistors, laser diodes (LDs), and light-emitting diodes (LEDs) on Si substrate.

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Keywords: GaN; Si; serpentine-channeled mask; high quality; low dislocation density

1. Introduction

Ⅲ-Nitrides compound semiconductors, especially GaN, have attracted much attention in recent years for their tremendous potential in electronic and optoelectronic devices, such as light-emitting diodes (LEDs), laser diodes (LDs) and high-power, high-frequency transistors[1-4]. GaN is usually heteroepitaxially grown on sapphire, SiC and Si substrates, accompanied with a ∗ Corresponding author. E-mail address: [email protected](X. Hu)

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high density of dislocations due to large in-plane lattice and thermal mismatch between GaN and substrates[5]. Dislocations in GaN degrade device performance, especially the electrical and optical qualities[6-8]. It is urgent to improve the crystal quality of heteroepitaxial grown-GaN. Many strategies have been proposed to achieve epitaxial GaN with lower dislocation density.

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For example, the most noted epitaxial lateral overgrowth (ELOG) approach and its different derivatives, which employ a masking layer SiO2 or Si3N4 to pattern a substrate and promote growth from lateral surfaces, have been proven to improve the GaN quality effectively[5,9-12]. However, these methods need two epitaxial growth steps and there are two high-dislocation

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regions per mask opening. To further reduce the dislocation density, we proposed an innovative serpentine channel patterned mask which requires only once growth and just have one

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high-dislocation region every mask opening[13]. We have previously applied the similar mask on sapphire substrates and obtained high-quality GaN films with reduced dislocation density [14-16]. Herein, we introduce this novel mask to Si substrate successfully. Compared with grown on sapphire or SiC, GaN epitaxially grown on Si substrate has various advantages, including its low cost, high thermal and electrical conductivity, and the most important one is the good compatibility with large-scale Si processing and the potential for

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integration with Si-based devices[17,18]. However, GaN directly grown on Si substrate always suffers a significant tensile stress because of large lattice mismatch and thermal mismatch, resulting in dislocations and cracks. The quality of GaN grown on Si still needs to be improved. In this paper, we introduce the serpentine-channeled mask to Si substrate and achieve fully

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coalesced GaN film for the first time. We confirm that the surface defect regions of GaN film are further reduced compared with the conventional ELOG method and the dislocation density in

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GaN is reduced effectively by using the serpentine masked structure. Detailed properties of the masked substrate and epitaxial GaN are characterized by optical microscopy, field emission scanning electron microscopy (FE-SEM) and cathodoluminescence (CL) spectroscopy.

2. Experimental

In this experiment, a masked Si substrate with the serpentine channels was fabricated before loaded into a metal organic chemical vapor deposition (MOCVD) reactor for the epitaxial growth of GaN. Figure 1 sketches the fabrication process of the serpentine-channeled mask on Si substrate. First, a 180-nm-thick AlN film was deposited on Si(111) substrate by a MOCVD

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system, followed by a thin Si3N4 layer deposited onto AlN by low pressure chemical vapor deposition (LPCVD) as a mask layer. AlN buffer layer was employed to avoid the meltback etching of Si by Ga happened when GaN was directly grown on Si[19]. Next, a set of parallel stripe-shape windows with a width of 2µm and periodicity of 12µm were defined with standard

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photolithography and reactive-ion etching (RIE) technology. Then, SiO2 was deposited by plasma enhanced chemical vapor deposition (PECVD), followed by the 2nd Si3N4 film which was thicker than the 1st Si3N4 layer and also deposited by LPCVD. The same treatment as above was carried out to obtain the 2µm top stripe-shape windows which had the same periodicity and

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orientation as bottom windows but a lateral phase shift to form the serpentine cross-sectional channels. Finally, SiO2 was partly removed by buffered oxide etchant (BOE) until AlN was

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exposed in the bottom windows. The Si substrate with serpentine channels before MOCVD GaN growth is shown in Figure 2. Fig. 2(a) and (b) show the plan-view and cross-sectional images of the three-layer stack structure with Si3N4 and SiO2 of the mask characterized by the optical microscopy and SEM, respectively. Different parts of serpentine-channeled mask are shown clearly in Fig. 2(a) with distinctive colors, and corresponding regions in cross-sectional image of Fig. 2(b) are well connected by red arrows. Fig. 2(c) is a photo of a prepared Si substrate with

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serpentine channels taken under fluorescent lamps.

Fig. 1. Fabrication process of the serpentine-channeled mask on Si substrate.

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Fig. 2. (a) Plan-view optical microscopy image and (b) cross-sectional SEM image of the Si substrate masked with serpentine channels before GaN growth. (c) A photo of a complete

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masked Si substrate taken under fluorescent lamps.

The growth of GaN was carried out in a multi-wafer Veeco TurboDisc K465i MOCVD system with trimethylgallium (TMGa) and ammonia (NH3) as precursors and high-purity H2 as

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carrier gas. The GaN growth process consists of two steps: first, growth of GaN was initiated at 530℃ to obtain a nucleation layer. Due to the lower nucleation barrier of GaN over crystalline

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AlN surface compared to that over the amorphous surfaces of SiO2 and Si3N4, the exclusively selective nucleation of GaN started from the exposed AlN surface at the bottom windows. Next, the substrate temperature was increased to 1060℃ for GaN growth on the existing nuclei with the time of 240min. The pressure was kept 300Torr throughout the whole growth procedure.

3. Results and discussion The plan-view SEM image of the coalesced GaN film is shown in Figure 3(a). The smooth surface of the film confirms that the fully coalescent and flat GaN film was obtained over the masked Si substrate. Figure 3(b) displays the cross-sectional SEM image of GaN film with a 4

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thickness of ~5µm over the top Si3N4 layer. Window, wing, coalescence regions are marked distinctly by white arrows. During the growth process, GaN firstly grew from the bottom windows to the channels and grew out of the top windows finally, which exhibited a serpentine growth behavior. After GaN grew out of the top windows, a lateral GaN growth above the

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serpentine-channeled mask took place and the neighboring GaN merged with each other

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eventually.

Fig. 3. (a) Plan-view and (b) cross-sectional SEM images of completely coalesced GaN film on

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the masked Si substrate.

In the CL image at 365nm of Fig. 4(a), meeting fronts marked with red rectangles are shown clearly by the black lines constituted of dark dots. The meeting front areas, where GaN

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grew out of the adjacent top windows and coalesced above the upper Si3N4 layer, are highly defective because of the newly generated dislocations which are shown in the form of intensive

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dark dots[20,21]. GaN out of the top windows and at the wing areas only have several little dark dots, corresponding to higher crystal quality. By counting the numbers of dislocations in Fig. 4(a), we can estimate the dislocation density in the contiguous window and wing regions between two adjacent coalescence areas is ~2.4×107 cm-2, while the dark dots in coalescence regions cannot be identified clearly. As shown in the CL line scan intensity map of the obtained GaN sample in Fig. 4(b), intensity of coalescence region is considerably weaker than that of wing region and window region. In terms of window area, a small center part exhibits a slightly lower intensity, and for wing area, the CL intensity decreases gradually when approaching to the wing/coalescence interface. The optical properties of different regions are quite sensitive to the 5

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spatial variations of crystal quality, which is well consistent with the defect distribution in GaN as shown in Figure 4(a). Fig. 4(c) displays the CL spectra of three typical regions on GaN film, which further confirms the different GaN quality in different areas. Wing region has the highest optical intensity and the narrowest full width at half maximum (FWHM), then the window

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region, and coalescence region has the weakest optical intensity and the largest FWHM. By this, we can say that our growth method could effectively reduce the dislocations of GaN film, compared with the conventional ELOG method where the window regions were always highly

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defective[5].

Fig. 4. (a) CL image at 365nm of the coalesced GaN film. (b) Intensity map of GaN measured by CL linescan under 15 kV at room-temperature within a mask period. (c) CL spectra recorded from three typical regions on GaN film. (d) Schematic diagram of dislocation evolution for GaN grown on the masked Si substrate. Black lines represent dislocations.

Based on the CL results, the evolution of dislocations during GaN growth process can be deduced as drawn in Fig. 4(d), in which dislocations are shown by black lines. At the beginning of GaN growth, a large amount of dislocations was generated at the interface between AlN and 6

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GaN, stemmed from the large lattice and thermal mismatch between the two materials. In addition, some of the dislocations originated from the underneath AlN layer penetrated into GaN layer grown in the bottom windows. Along with the GaN growth, some dislocations bent to AlN layer after a short distance going up, some grew vertically and then bent to the side wall of SiO2

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supporting layer or bent to the opposite direction followed by annihilation. Nevertheless, most of them propagated vertically to the top without bending and blocked by the upper Si3N4 layer finally. Blocking and bending are the most effective procedures of the innovative serpentine-channeled mask to reduce dislocation density compared with conventional ELOG

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method. An obvious dislocation reduction in the subsequent GaN growth was achieved and thus the quality of GaN was significantly improved, as observed in this experiment. This dislocation

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bending phenomenon can be attributed to the image force of the structure as the dislocations tend to extend along the line[22]. There were a few dislocations survived which reached to the surface of GaN eventually by two 90° bending when GaN grew out of the top windows. Before coalescence, GaN grown laterally above the upper Si3N4 layer was almost free of dislocations, indicating the high quality of GaN layer over the mask. At coalescence areas, a large number of dislocations generated when the neighboring GaN merged. Up to about 80% of the total area is

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occupied by wing and window regions, where GaN is of high material quality. As shown in the CL image of Fig. 4(a), there is only one highly defective region every period which is called coalescence area, while wing and window areas are of low dislocations. The specific mechanism

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for dislocation reduction of this novel growth method needs further investigation.

Fig. 5. (a) Cross-sectional SEM image of coalesced GaN layer. White arrow shows the direction of CL linescan performed vertical to the substrate surface. (b) CL spectra of positions 1−10

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collected along the arrow direction under 15kV at room-temperature.

To further verify the crystal quality of GaN, cross-sectional CL linescan from channel upward to surface was carried out to study the optical property. Figure 5(a) shows the spatial

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distribution of ten points measured by CL line scan and the result is displayed in Figure 5(b). The emission centered at about 367nm (3.38eV) is ascribed to near band edge (NBE) emission of GaN. As clearly shown in the CL image, the intensity keeps increasing and the FWHM of NBE peak becomes narrower as GaN grows perpendicularly to the masked substrate. This obvious

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phenomenon in CL spectra powerfully proves the improved crystal quality of GaN grown over the mask. In conclusion, improved crystal quality is observed during the GaN epitaxial growth

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over the serpentine channels masked Si substrate.

4. Conclusions

In summary, we have successfully achieved the fully coalesced GaN film with low dislocation density on an serpentine-channel masked Si substrate for the first time. Compared with the conventional ELOG growth, the novel mask needs only once epitaxial growth step.

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More importantly, the range of high-quality GaN is much larger than conventional ELOG method, including both wing and window regions, while the coalescence region is the only area with large-density dislocations generated at the meeting fronts where GaN grew out of the adjacent top windows and coalesced above the upper Si3N4 layer. From the measurements of

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coalesced GaN surface, periodical spatial variation of the crystal quality is observed. In addition, according to the cross-section tests, improved crystal quality is achieved with GaN growing upwards. The contiguous area of wing and window regions occupies about 80% of the total area

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with the dislocation density about 2.4×107 cm-2, which will make a great contribution to the device performance. The significant dislocation reduction from the serpentine-channeled mask make the technique promising for growing high-quality GaN templates and GaN-based devices like transistors, LDs, and LEDs on Si substrate. If we stop the GaN growth just before coalescence, wing and window regions can be used as a whole for micro-devices fabrication such as micro-LEDs. Besides, if we design the windows further away from each other, we can expect to obtain a larger GaN area with low dislocation density, which will further broaden its application for device fabrication.

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Acknowledgements This work was supported by the Key National Research and Development Program (No.2016YFB0401801), Science Challenge Project (No. TZ2016003-2), the National Natural

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Science Foundation of China (No. 61334005), and Beijing Municipal Science and Technology Project, China (No. Z61100002116037).

References

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[1]S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, T. Yamada, T.Mukai, Superbright Green InGaN Single-Quantum-Well-Structure Light-Emitting Diodes, Jpn. J.Appl. Phys. 34 (1995)

M AN U

L1332-L1335.

[2]E. F. Schubert, J. K. Kim, Solid-State Light Sources Getting Smart, Science 308 (2005) 1274-1278.

[3]T.-C. Lu, S.-W. Chen, T.-T. Wu, P.-M. Tu, C.-K. Chen, C.-H. Chen, Z.-Y. Li, H.-C. Kuo, S.-C. Wang, Continuous wave operation of current injected GaN vertical cavity surface emitting lasers at room temperature, Appl. Phys. Lett. 97 (2010) 071114.

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[4]U. K. Mishra, P. Parikh, Y. F. Wu, AlGaN/GaN HEMTs—An Overview of DeviceOperation and Applications, Proc. IEEE 90 (2002) 1022-1031. [5]P. Gibart, Metal organic vapour phase epitaxy of GaN and lateral overgrowth, Rep. Prog. Phys. 67 (2004) 667-715.

EP

[6]S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, InGaN/GaN/AlGaN-based laser diodes

AC C

with modulation-doped strained-layer superlattices grown on an epitaxially laterally overgrown GaN substrate, Appl. Phys. Lett. 72 (1998) 211. [7]D. B. Li, X. J. Sun, H. Song, Z. M. Li, Y. R. Chen, G. Q. Miao, H. Jiang, Influence of threading dislocations on GaN-based metal-semiconductor-metalultraviolet photodetectors, Appl. Phys. Lett. 98 (2011) 011108. [8]A. Khan, K. Balakrishnan, T. Katona, Ultraviolet light-emitting diodes based on group three nitrides, Nat. Photonics 2 (2008) 77-84. [9]S. Nakamura, The Roles of Structural Imperfections in InGaN-Based Blue Light-Emitting Diodes and Laser Diodes, Science 281 (1998) 956-961.

9

ACCEPTED MANUSCRIPT

[10]K. Hiramatsu, K. Nishiyama, M. Onishi, H. Mizutani, M. Narukawa, A. Motogaito, H. Miyake, Y. Iyechika, T. Maeda, Fabrication and characterization of low defect density GaN using facet-controlled epitaxial lateral overgrowth (FACELO), J. Cryst. Growth 221 (2000) 316-326. [11] T. S. Zheleva, S. A. Smith, D. B. Thomson, K. J. Linthicum, P. Rajagopal, R. F. Davis,

RI PT

Pendeo-Epitaxy: A New Approach for Lateral Growthof Gallium Nitride Films, J. Electron. Mater. 28 (1999) L5-L8.

[12]O.-H. Nam, T. S. Zheleva, M. D. Bremser, R. F. Davis, Lateral Epitaxial Overgrowth of GaN

233-237. [13]Y. H. Xie, U.S. patent 6,633,056 (14 October 2003).

SC

Films on SiO2 Areasvia Metalorganic Vapor Phase Epitaxy, J. Electron. Mater. 27 (1998)

M AN U

[14]L. Li, JP. C. Liu, L Liu, D Li, L Wang, C. H. Wan, W. H. Chen, Z. J. Yang, Y. H. Xie, X. D. Hu, G. Y. Zhang, Defect Reduction via Selective Lateral Epitaxy of GaN on an Innovative Masked Structure with Serpentine Channels, Appl. Phys. Express 5 (2012) 051001. [15]W. Zhang, P. Liu, B. Jackson, T. S. Sun, S. J. Huang, H. C. Hsu, Y. K. Su, S. J. Chang, L. Li, D. Li, L. Wang, X. D. Hu, Y. H. Xie, Dislocation reduction through nucleation and growth selectivity of metal-organic chemical vapor deposition GaN, J. Appl. Phys.113 (2013) 144908.

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[16]Q. B. Ji, L. Li, W. Zhang, J. Wang, P. Liu, Y. H. Xie, T. X. Yan, W. Yang, W. H. Chen, X. D. Hu, Dislocation Reduction and Stress Relaxation of GaN and InGaN Multiple Quantum Wells with Improved Performance via Serpentine Channel Patterned Mask, ACS Appl. Mater. Interfaces 8 (2016) 21480-21489.

EP

[17]A. Strittmater, A. Krost, J. Blasing, D. Bimberg, High Quality GaN Layers Grown by Metalorganic Chemical Vapor Depositionon Si(111) Substrates, Phys. Status Solidi A 176

AC C

(1999) 611-614.

[18]A. Dadgar, P. Veit, F. Schulze, J. Blasing, A. Krtschil, H. Witte, A. Diez, T. Hempel, J. Christen, R. Clos, A. Krost, MOVPE growth of GaN on Si-Substrates and strain, Thin Solid Films 515 (2007) 4356-4361. [19]H. Ishikawa, K. Yamamoto, T. Egawa, T. Soga, T. Jimbo, M. Umeno, Thermal stability of GaN on (111) Si substrate, J. Cryst. Growth 189/190 (1998) 178-182. [20]Z. R. Zytkiewicz, J. Z. Domagala, D. Dobosz, L. Dobaczewski, A. Rocher, C. Clement, J. Crestou, Tilt and dislocations in epitaxial laterally overgrown GaAs layers, J. Appl. Phys. 101 (2007) 013508.

10

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[21]J.-S. Song, H. Rho, M. S. Jeong, J.-W. Ju, I.-H. Lee, Spatially resolved photoluminescence and Raman mapping of epitaxial GaN laterally overgrown on sapphire, Phys. Rev. B 81 (2010) 233304. [22]T. S. Zheleva, O.-H. Nam, W. M. Ashmawi, J. D. Griffin, R. F. Davis, Lateral epitaxy and

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dislocation density reduction in selectively grown GaN structures, J. Cryst. Growth 222 (2001)

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M AN U

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706-718.

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