- Email: [email protected]
patterned sapphire substrates
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 3063–3066
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Defect reduction of laterally regrown GaN on GaN/patterned sapphire substrates Dong-Sing Wuu a,, Hsueh-Wei Wu a, Shih-Ting Chen a, Tsung-Yen Tsai a, Xinhe Zheng a, Ray-Hua Horng b a b
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan, Republic of China Institute of Precision Engineering, National Chung Hsing University, Taichung 40227, Taiwan, Republic of China
a r t i c l e in fo
abstract
Available online 24 January 2009
Structural properties of GaN epilayers on wet-etched protruding and recess-patterned sapphire substrates (PSSs) have been investigated in detail using high-resolution double-crystal X-ray diffraction (DCXRD) and etch-pit density methods. The DCXRD results reveal various dislocation configurations on both types of PSSs. The etch pits of GaN on the recess PSS exhibit a regular distribution, i.e. less etch pits or threading dislocation density (TDD) onto the recess area than those onto the sapphire mesas. On the contrary, an irregular distribution is observed for the etch pits of GaN on the protruding PSS. A higher crystal quality of the GaN epilayer grown onto the recess PSS can be achieved as compared with that onto the protruding PSS. These data reflect that the GaN epilayer on the recess PSS could be a better template for the second epitaxial lateral overgrowth (ELOG) of GaN. As a result, the GaN epilayers after the ELOG process display the TDDs of around 106 cm2. & 2009 Elsevier B.V. All rights reserved.
PACS: 61.72.Dd 61.72.Ff 78.67.De 81.05.Ea 81.15.Gh 85.60.Jb Keywords: A1. Defects A1. Etching A3. Metalorganic chemical vapor deposition B1. Nitrides B1. Patterned sapphire substrate B3. Light-emitting diodes
1. Introduction Recently, tremendous progress has been achieved in nitridebased semiconductor materials. The high-emission InGaN lightemitting diodes (LEDs) have become commercialized products between the near ultraviolet (NUV) and the green spectra region, while attracting considerable application in the areas of photonic devices, such as traffic light, out-door full color displays, and solid-state lighting [1]. Even though the blue/green GaN LEDs are commercially available, it is still difficult to manufacture highpower NUV GaN LEDs. Recently, the NUV LEDs can be used as a pumping source for developing the white-light LEDs to solve the low color-rendering-index problem [2]. However, NUV InGaN-based LEDs are more sensitive to dislocation than blue InGaN-based LEDs, as indicated from the previous studies [3]. It is well known that a dislocation density in the order of 109–1011 cm2 is inherent in the epitaxial GaN films on sapphire substrates due to the large mismatch in lattice constant and thermal expansion coefficient. These defects will influence both the electrical and the optical properties of the device, such as
Corresponding author. Tel.: +886 4 22840500x714; fax: +886 4 22855046.
E-mail address: [email protected] (D.-S. Wuu). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.107
device lifetime, electron mobility, and the quantum efficiency of radiative recombination [4]. Therefore, how to further reduce the dislocation density of GaN, a template for the subsequent growth of InGaN-based active epilayers, is an important issue for fabricating high-performance NUV LEDs. Many different growth approaches, such as epitaxial lateral overgrowth (ELOG) [5,6] and its derivatives, pendeo-epitaxy [7] and facet-controlled epitaxial lateral overgrowth [8], have been proven to significantly reduce threading dislocation density (TDD) in GaN or AlGaN to a range 106–107 cm2. Recently, due to its single-growth process with no interruption, patterned sapphire substrate (PSS) is another alternative way to reduce the dislocation density and the percentage of total internal light reflection through its geometrical effect [9–12]. To further reap the benefits of ELOG and PSS, a combination of ELOG and PSS was demonstrated to reduce the defect density to a level of 105 cm2 [13], which significantly enhance the internal quantum efficiency and light output power. Additionally, the authors in Refs. [9–11,14–17] found that different shapes or sizes of the PSS will influence the growth behavior and dislocation distribution of the as-grown GaN epilayers. However, most studies were focused on the line or stripe-patterned sapphire substrates, where the low and high dislocation regions correlated well with the periodic stripe pattern as examined by cathodoluminescence [14].
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The dislocation distribution decorated by the etched pits for GaN on dot-patterned sapphire substrates has been less reported. In this study, a regular distribution of defect density in the GaN film grown on the recess dot-featured PSS is explored. Meantime, the protruding dot-featured PSS is also employed to form the GaN for an evident comparison. Finally, the lateral regrown GaN on these GaN/dot-featured PSSs is carried out. Details of the etch-pit or dislocation density and structural characteristics of the GaN epilayers will be described.
2. Experiments The protruding and recess PSSs were prepared on the (0 0 0 1) sapphire substrates using the same mask with a periodic pattern (diameter: 3 mm; spacing: 3 mm) as shown in Fig. 1(a). A standard photolithography process using respective positive and negative photoresist was carried out to form a wet-chemical-etching mask with different SiO2 patterns on the surface of sapphire substrates. The required SiO2 patterns were generated using CF4 in an inductively coupled-plasma etcher at a dc power of 800 W. Then, a mixture of H2SO4:H3PO4 (3:1) solution was used to etch the sapphire substrate with positive-photoresist and negative-
SiO2 Sapphire
3H2SO4 :1 H3PO4 at 280°C for 30 min
Sapphire
Sapphire
Sapphire
1 um
1 um
photoresist formed SiO2 patterns at 280 1C to achieve the protruding and recess PSS, respectively. Both the etched depth and height for the two PSS are approximately 1.5 mm. The corresponding scanning electron microscope (SEM) images of the protruding and recess PSSs are shown in Fig. 1(b) and (c). The GaN buffer layer with an initial thickness of 3.8 mm was then grown on the wet-etched PSS by MOCVD. Following that a dielectric SiO2 mask (100 nm thick) was deposited on the GaN/PSS buffer layers where a dot-array pattern offset over the underlying PSS was developed. After that, GaN epi-wafers were grown onto the conventional sapphire substrates and the PSSs. A prime concern about the ELOG/SiO2/GaN/PSS samples is their defect reduction revealed by the etch-pit-density (EPD) measurement. The etching process was carried out in a H2SO4 and H3PO4 mixture solution with a 1:3 ratio at 250 1C for 10 min. The surface morphology and microstructure were examined by a SEM (Hitachi, S-3000H) and a double-crystal X-ray diffractometer (DCXRD, Panalytical X’Pert PRO MPD) to characterize the GaNon-PSS samples.
3U. Results and discussion Fig. 2(a) and (b) shows the cross-section SEM images of the GaN epilayers grown onto the protruding and recess PSSs, respectively. It is shown that the coalesced lateral epitaxial overgrowth with a smooth surface of GaN layers is performed on both protruding and recess PSS LEDs. Additionally, the GaN film is found to grow not only on the mesas but also on the sidewalls and the bottoms of the recess PSS. Furthermore, due to a reasonable lateral growth and coalescence, small voids are observed underneath the completely coalesced GaN. This result implies that the coalesced GaN region could have lower TDDs as compared with GaN directly grown on the sapphire mesas. However, we observe that there are large-height voids towards the growth direction, indicating that the coalescence fronts are clearly shown over the protruding sapphire. This growth mode leads to a narrow-area GaN with a low dislocation density. The growth differences of GaN epilayers on the protruding and recess PSSs are reflected by SEM images of etched pits after the GaN surface was etched, as shown in Fig. 2(c) and (d). A closer visual inspection shows that the etched pits for the case of the recess PSS have a regular distribution (a complementary dot-array pattern of the underneath PSS) on the surface, marked by dashed circles in Fig. 2(d). The dashed-circle region corresponds to the coalesced GaN with a lower density of etched pits, while the GaN onto the mesas exhibits a little higher etch-pit density. For the case of the protruding PSS, an irregular distribution of the etched pits is observed. Additionally, the total etch-pit density is higher than the case of the recess PSS. The diagrams are schematically shown to compare the distribution of threading dislocations within the GaN onto the two patterned substrates, shown in Fig. 2(e) and (f). To further characterize the structural properties of GaN epilayers grown onto respective protruding and recess PSSs, DCXRD and etch-pit methods are used to measure and/or calculate TDDs. For DCXRD, the measurements of rocking curves through (00.2) and (30.2) reflections are carried out. The dislocation density r is calculated based on the formula [18]
r¼ 5 um
5 um
Fig. 1. (a) Schematic process flow of patterned sapphire substrate using a wet etching technique. SEM micrographs of various wet-etched PSS structures: (b) dotarray protruding PSS and (c) dot-array recess PSS.
b2 2
4:35 b
(1)
where b is the absolute value of the Burgers vector and b stands for the FWHM of the rocking curves. For screw-type dislocation and edge-type dislocation, their Burgers vectors are /0 0 0 2S and 1/3/11 2¯ 0S, respectively. By doing this, the calculated results of TDDs are listed in Table 1. In the meantime, the variation
ARTICLE IN PRESS D.-S. Wuu et al. / Journal of Crystal Growth 311 (2009) 3063–3066
GaN
3065
GaN
Sapphire
2 um
5 um
2 um
Sapphire
5 um
1 um
Dislocation
Etch-pit
Void
Dislocation
Sapphire
Etch-pit
Void
Sapphire
Fig. 2. Typical cross-section SEM micrographs of GaN epilayers grown onto (a) protruding PSS and (b) recess PSS. (c) and (d) demonstrate the corresponding etch-pit distributions of (a) and (b) samples, where the inset shows an enlarged etch pit. The threading dislocation distributions within the GaN epilayers onto the protruding and recess PSSs are schematically shown in (e) and (f), respectively.
Table 1 Structural properties of GaN epilayers on conventional sapphire (flat surface) and patterned sapphire (protruding and recess type) substrates. Sample
Conventional sapphire Protruding PSS Recess PSS
Dislocation density (cm2)
FWHM (arcsec)
Etch pit density (cm2)
(0 0 2)
(3 0 2)
Screw-type
Edge-type
224 245 225
486 422 388
1.0 108 1.2 108 1.0 108
1.6 109 1.2 109 1.0 109
of crystal quality using different growth buffer layers or substrates is summarized. Note that the GaN samples grown on the conventional sapphire substrates and recess PSSs have almost the same FWHM value at the (00.2) reflections, while the GaN on the protruding PSS shows a larger FWHM as compared with the two former ones. An inspection reveals that in comparing results of the GaN on the protruding PSS, the GaN on the recess PSS displays a significant decrease in FWHM value for the (30.2) reflections. The reduction in FWHM values exhibits the lower density of edge/mixed dislocations. Actually, the photoluminesence (PL) measurements also indicate an increase in PL intensity tendency (not shown here), which reflects that the edge/mixed dislocations act as the non-radiative recombination center. These results imply that probably they have different growth modes for different substrate patterns. The exact reason is not clear yet. The related
1.4 109 8.2 108 6.8 108
research to understand the exact reason is still in progress. Additionally, it is worth noting that the TDDs using DCXRD are a little larger than the etch-pit density using chemical-etching decoration. This could relate to their respective metrology principle. In the case of DCXRD, a large X-ray beam and penetration depth could make the radiated area extend to some regions with a higher dislocation density. While for the etch-pit method, actually, a critical remark has to be made concerning the accuracy for an exact determination of the defect density. The number of etch pits varies more or less depending on the place from where the measurement is done on the sample and depending on the etching behavior of the surface. Fig. 3(a) and (b) shows the cross-section transmissionelectron-microscopy (TEM) images of GaN on recess PSS and ELOG GaN onto the GaN/recess–PSS buffer layer, respectively. It is found that the dislocations in the mask area are terminated by
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0.5µm
0.5µm Dislocation
SiO2
GaN
5 um
Sapphire
Void
Fig. 3. Cross-section TEM micrographs of (a) GaN onto recess PSS and (b) ELOG GaN onto GaN/PSS template. Top-view SEM image of the etched surface of ELOG GaN is shown in (c). The threading dislocation distribution terminated by oxide mask insertion within the GaN epilayer onto recess PSS can be further interpreted by (d).
SiO2 masks and most of them bend toward the mask area, which leads to a significant reduction in TDDs. Meanwhile, we observe that part of the dislocations, with a lower density due to the coalescence of GaN over the protruding sapphire substrate, propagates to the surface. In some regions, they are almost free of TDs. The behavior is very similar to the previous ELOG works [4,19]. Obviously, the TDDs within the entire ELOG GaN film are significantly decreased. To further confirm the reduction of dislocations within the ELOG GaN, its surface is etched using a mixture of acid solution, shown in Fig. 3(c). The figure clearly reveals that the total etch-pit density is 106 cm2, implying a very low TDD after the employment of a combination of ELOG and PSS techniques [13]. Judging from the measured results above, it could be inferred that the protruding PSS is reasonably a better buffer layer for the second ELOG of GaN.
4. Summary We compared the structural properties of GaN epilayers on the recess and protruding PSSs. The results confirm that the GaN crystal quality of the former is better than that of the latter. Using GaN onto the recess PSS could prove to be a better template for the lateral regrown GaN as compared with that on the protruding PSS.
Acknowledgements This work was financially supported by the National Science Council (Taiwan) and Wafer Works Corporation (Taoyuan, Taiwan) under Contract no. NSC 94-2622-E-005-001.
References [1] S. Nakamura, G. Fasol, S.J. Pearton, The Blue Laser Diode: The Complete Story, Springer, Berlin, 2000. [2] Y. Narukawa, I. Niki, K. Izuno, M. Yamada, Y. Murazki, T. Mukai, Jpn. J. Appl. Phys. Lett. 41 (2002) L371. [3] J. Han, M.H. Crawford, R.J. Shul, J.J. Figiel, L. Zhang, Y.K. Song, H. Zhou, A.V. Nurmikko, Appl. Phys. Lett. 73 (1998) 1688. [4] B. Garni, J. Ma, N. Perkins, J. Liu, T.F. Kuech, M.G. Lagally, Appl. Phys. Lett. 68 (1996) 1380. [5] Y. Kato, S. Kitamura, K. Hiramatsu, N. Sawaki, J. Cryst. Growth 144 (1994) 133. [6] K. Iida, T. Kawashima, A. Miyazaki, H. Kasugai, S. Mishima, A. Honshio, Y. Miyake, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki, J. Cryst. Growth 272 (2004) 270. [7] T.S. Zheleva, D. Thomson, S. Smith, P. Pajagopal, K. Linthicum, T. Gehrke, R.F. Davis, Ext. Abstract MRS Fall Meeting, Boston, 1998, p. 338. [8] Y. Kawaguchi, M. Shimizu, M. Yamaguchi, K. Hiramatsu, N. Sawaki, W. Taki, H. Tsuda, N. Kuwano, K. Oki, T. Zheleva, R.F. Davis, J. Cryst. Growth 189/190 (1998) 24. [9] K. Tadatomo, H. Okagawa, Y. Ohuchi, T. Tsunekawa, Y. Imada, M. Kato, T. Taguchi, Jpn. J. Appl. Phys. 40 (2001) L583. [10] K.M. Yamada, T. Mitani, Y. Narukawa, S. Shioji, I. Niki, S. Sonobe, K. Deguchi, M. Sano, T. Mukai, Jpn. J. Appl. Phys. 41 (2002) 1431. [11] W.K. Wang, D.S. Wuu, S.H. Lin, P. Han, R.H. Horng, T.C. Hsu, D.T.C. Huo, M.J. Jou, Y.H. Yu, A. Lin, IEEE J. Quantum Electron. 41 (2005) 1403. [12] Y.P. Hsu, S.J. Chang, Y.K. Su, J.K. Sheu, C.T. Lee, T.C. Wen, L.W. Wu, C.H. Kuo, C.S. Chang, S.C. Shei, J. Cryst. Growth 261 (2004) 466. [13] D.S. Wuu, W.K. Wang, K.S. Wen, S.C. Huang, S.H. Lin, S.Y. Huang, C.F. Lin, R.H. Horng, Appl. Phys. Lett. 89 (2006) 1611051. [14] C.C. Pan, C.H. Hsieh, C.W. Lin, J.I. Chyi, J. Appl. Phys. 102 (2007) 084503. [15] H. Gao, F. Yan, Y. Zhang, J. Li, Y. Zeng, G. Wang, J. Phys. D: Appl. Phys. 41 (2008) 115106. [16] J. Wang, L.W. Guo, H.Q. Jia, Z.G. Xing, Y. Wang, J.F. Yan, N.S. Yu, H. Chen, J.M. Zhou, J. Cryst. Growth 290 (2006) 398. [17] D.H. Kang, J.C. Song, B.Y. Shim, E.A. Ko, D.W. Kim, S. Kannappan, C.R. Lee, Jpn. J. Appl. Phys. 46 (2007) 2563. [18] R. Chierchia, T. Bottcher, H. Heinke, S. Einfeldt, S. Figge, D. Hommel, J. Appl. Phys. 93 (2003) 8918. [19] M. Kappelt, D. Bimberg, J. Electrochem. Soc. 143 (1996) 3271.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Defect reduction of laterally regrown GaN on GaN/patterned sapphire substrates Dong-Sing Wuu a,, Hsueh-Wei Wu a, Shih-Ting Chen a, Tsung-Yen Tsai a, Xinhe Zheng a, Ray-Hua Horng b a b
Department of Materials Science and Engineering, National Chung Hsing University, Taichung 40227, Taiwan, Republic of China Institute of Precision Engineering, National Chung Hsing University, Taichung 40227, Taiwan, Republic of China
a r t i c l e in fo
abstract
Available online 24 January 2009
Structural properties of GaN epilayers on wet-etched protruding and recess-patterned sapphire substrates (PSSs) have been investigated in detail using high-resolution double-crystal X-ray diffraction (DCXRD) and etch-pit density methods. The DCXRD results reveal various dislocation configurations on both types of PSSs. The etch pits of GaN on the recess PSS exhibit a regular distribution, i.e. less etch pits or threading dislocation density (TDD) onto the recess area than those onto the sapphire mesas. On the contrary, an irregular distribution is observed for the etch pits of GaN on the protruding PSS. A higher crystal quality of the GaN epilayer grown onto the recess PSS can be achieved as compared with that onto the protruding PSS. These data reflect that the GaN epilayer on the recess PSS could be a better template for the second epitaxial lateral overgrowth (ELOG) of GaN. As a result, the GaN epilayers after the ELOG process display the TDDs of around 106 cm2. & 2009 Elsevier B.V. All rights reserved.
PACS: 61.72.Dd 61.72.Ff 78.67.De 81.05.Ea 81.15.Gh 85.60.Jb Keywords: A1. Defects A1. Etching A3. Metalorganic chemical vapor deposition B1. Nitrides B1. Patterned sapphire substrate B3. Light-emitting diodes
1. Introduction Recently, tremendous progress has been achieved in nitridebased semiconductor materials. The high-emission InGaN lightemitting diodes (LEDs) have become commercialized products between the near ultraviolet (NUV) and the green spectra region, while attracting considerable application in the areas of photonic devices, such as traffic light, out-door full color displays, and solid-state lighting [1]. Even though the blue/green GaN LEDs are commercially available, it is still difficult to manufacture highpower NUV GaN LEDs. Recently, the NUV LEDs can be used as a pumping source for developing the white-light LEDs to solve the low color-rendering-index problem [2]. However, NUV InGaN-based LEDs are more sensitive to dislocation than blue InGaN-based LEDs, as indicated from the previous studies [3]. It is well known that a dislocation density in the order of 109–1011 cm2 is inherent in the epitaxial GaN films on sapphire substrates due to the large mismatch in lattice constant and thermal expansion coefficient. These defects will influence both the electrical and the optical properties of the device, such as
Corresponding author. Tel.: +886 4 22840500x714; fax: +886 4 22855046.
E-mail address: [email protected] (D.-S. Wuu). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.107
device lifetime, electron mobility, and the quantum efficiency of radiative recombination [4]. Therefore, how to further reduce the dislocation density of GaN, a template for the subsequent growth of InGaN-based active epilayers, is an important issue for fabricating high-performance NUV LEDs. Many different growth approaches, such as epitaxial lateral overgrowth (ELOG) [5,6] and its derivatives, pendeo-epitaxy [7] and facet-controlled epitaxial lateral overgrowth [8], have been proven to significantly reduce threading dislocation density (TDD) in GaN or AlGaN to a range 106–107 cm2. Recently, due to its single-growth process with no interruption, patterned sapphire substrate (PSS) is another alternative way to reduce the dislocation density and the percentage of total internal light reflection through its geometrical effect [9–12]. To further reap the benefits of ELOG and PSS, a combination of ELOG and PSS was demonstrated to reduce the defect density to a level of 105 cm2 [13], which significantly enhance the internal quantum efficiency and light output power. Additionally, the authors in Refs. [9–11,14–17] found that different shapes or sizes of the PSS will influence the growth behavior and dislocation distribution of the as-grown GaN epilayers. However, most studies were focused on the line or stripe-patterned sapphire substrates, where the low and high dislocation regions correlated well with the periodic stripe pattern as examined by cathodoluminescence [14].
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D.-S. Wuu et al. / Journal of Crystal Growth 311 (2009) 3063–3066
The dislocation distribution decorated by the etched pits for GaN on dot-patterned sapphire substrates has been less reported. In this study, a regular distribution of defect density in the GaN film grown on the recess dot-featured PSS is explored. Meantime, the protruding dot-featured PSS is also employed to form the GaN for an evident comparison. Finally, the lateral regrown GaN on these GaN/dot-featured PSSs is carried out. Details of the etch-pit or dislocation density and structural characteristics of the GaN epilayers will be described.
2. Experiments The protruding and recess PSSs were prepared on the (0 0 0 1) sapphire substrates using the same mask with a periodic pattern (diameter: 3 mm; spacing: 3 mm) as shown in Fig. 1(a). A standard photolithography process using respective positive and negative photoresist was carried out to form a wet-chemical-etching mask with different SiO2 patterns on the surface of sapphire substrates. The required SiO2 patterns were generated using CF4 in an inductively coupled-plasma etcher at a dc power of 800 W. Then, a mixture of H2SO4:H3PO4 (3:1) solution was used to etch the sapphire substrate with positive-photoresist and negative-
SiO2 Sapphire
3H2SO4 :1 H3PO4 at 280°C for 30 min
Sapphire
Sapphire
Sapphire
1 um
1 um
photoresist formed SiO2 patterns at 280 1C to achieve the protruding and recess PSS, respectively. Both the etched depth and height for the two PSS are approximately 1.5 mm. The corresponding scanning electron microscope (SEM) images of the protruding and recess PSSs are shown in Fig. 1(b) and (c). The GaN buffer layer with an initial thickness of 3.8 mm was then grown on the wet-etched PSS by MOCVD. Following that a dielectric SiO2 mask (100 nm thick) was deposited on the GaN/PSS buffer layers where a dot-array pattern offset over the underlying PSS was developed. After that, GaN epi-wafers were grown onto the conventional sapphire substrates and the PSSs. A prime concern about the ELOG/SiO2/GaN/PSS samples is their defect reduction revealed by the etch-pit-density (EPD) measurement. The etching process was carried out in a H2SO4 and H3PO4 mixture solution with a 1:3 ratio at 250 1C for 10 min. The surface morphology and microstructure were examined by a SEM (Hitachi, S-3000H) and a double-crystal X-ray diffractometer (DCXRD, Panalytical X’Pert PRO MPD) to characterize the GaNon-PSS samples.
3U. Results and discussion Fig. 2(a) and (b) shows the cross-section SEM images of the GaN epilayers grown onto the protruding and recess PSSs, respectively. It is shown that the coalesced lateral epitaxial overgrowth with a smooth surface of GaN layers is performed on both protruding and recess PSS LEDs. Additionally, the GaN film is found to grow not only on the mesas but also on the sidewalls and the bottoms of the recess PSS. Furthermore, due to a reasonable lateral growth and coalescence, small voids are observed underneath the completely coalesced GaN. This result implies that the coalesced GaN region could have lower TDDs as compared with GaN directly grown on the sapphire mesas. However, we observe that there are large-height voids towards the growth direction, indicating that the coalescence fronts are clearly shown over the protruding sapphire. This growth mode leads to a narrow-area GaN with a low dislocation density. The growth differences of GaN epilayers on the protruding and recess PSSs are reflected by SEM images of etched pits after the GaN surface was etched, as shown in Fig. 2(c) and (d). A closer visual inspection shows that the etched pits for the case of the recess PSS have a regular distribution (a complementary dot-array pattern of the underneath PSS) on the surface, marked by dashed circles in Fig. 2(d). The dashed-circle region corresponds to the coalesced GaN with a lower density of etched pits, while the GaN onto the mesas exhibits a little higher etch-pit density. For the case of the protruding PSS, an irregular distribution of the etched pits is observed. Additionally, the total etch-pit density is higher than the case of the recess PSS. The diagrams are schematically shown to compare the distribution of threading dislocations within the GaN onto the two patterned substrates, shown in Fig. 2(e) and (f). To further characterize the structural properties of GaN epilayers grown onto respective protruding and recess PSSs, DCXRD and etch-pit methods are used to measure and/or calculate TDDs. For DCXRD, the measurements of rocking curves through (00.2) and (30.2) reflections are carried out. The dislocation density r is calculated based on the formula [18]
r¼ 5 um
5 um
Fig. 1. (a) Schematic process flow of patterned sapphire substrate using a wet etching technique. SEM micrographs of various wet-etched PSS structures: (b) dotarray protruding PSS and (c) dot-array recess PSS.
b2 2
4:35 b
(1)
where b is the absolute value of the Burgers vector and b stands for the FWHM of the rocking curves. For screw-type dislocation and edge-type dislocation, their Burgers vectors are /0 0 0 2S and 1/3/11 2¯ 0S, respectively. By doing this, the calculated results of TDDs are listed in Table 1. In the meantime, the variation
ARTICLE IN PRESS D.-S. Wuu et al. / Journal of Crystal Growth 311 (2009) 3063–3066
GaN
3065
GaN
Sapphire
2 um
5 um
2 um
Sapphire
5 um
1 um
Dislocation
Etch-pit
Void
Dislocation
Sapphire
Etch-pit
Void
Sapphire
Fig. 2. Typical cross-section SEM micrographs of GaN epilayers grown onto (a) protruding PSS and (b) recess PSS. (c) and (d) demonstrate the corresponding etch-pit distributions of (a) and (b) samples, where the inset shows an enlarged etch pit. The threading dislocation distributions within the GaN epilayers onto the protruding and recess PSSs are schematically shown in (e) and (f), respectively.
Table 1 Structural properties of GaN epilayers on conventional sapphire (flat surface) and patterned sapphire (protruding and recess type) substrates. Sample
Conventional sapphire Protruding PSS Recess PSS
Dislocation density (cm2)
FWHM (arcsec)
Etch pit density (cm2)
(0 0 2)
(3 0 2)
Screw-type
Edge-type
224 245 225
486 422 388
1.0 108 1.2 108 1.0 108
1.6 109 1.2 109 1.0 109
of crystal quality using different growth buffer layers or substrates is summarized. Note that the GaN samples grown on the conventional sapphire substrates and recess PSSs have almost the same FWHM value at the (00.2) reflections, while the GaN on the protruding PSS shows a larger FWHM as compared with the two former ones. An inspection reveals that in comparing results of the GaN on the protruding PSS, the GaN on the recess PSS displays a significant decrease in FWHM value for the (30.2) reflections. The reduction in FWHM values exhibits the lower density of edge/mixed dislocations. Actually, the photoluminesence (PL) measurements also indicate an increase in PL intensity tendency (not shown here), which reflects that the edge/mixed dislocations act as the non-radiative recombination center. These results imply that probably they have different growth modes for different substrate patterns. The exact reason is not clear yet. The related
1.4 109 8.2 108 6.8 108
research to understand the exact reason is still in progress. Additionally, it is worth noting that the TDDs using DCXRD are a little larger than the etch-pit density using chemical-etching decoration. This could relate to their respective metrology principle. In the case of DCXRD, a large X-ray beam and penetration depth could make the radiated area extend to some regions with a higher dislocation density. While for the etch-pit method, actually, a critical remark has to be made concerning the accuracy for an exact determination of the defect density. The number of etch pits varies more or less depending on the place from where the measurement is done on the sample and depending on the etching behavior of the surface. Fig. 3(a) and (b) shows the cross-section transmissionelectron-microscopy (TEM) images of GaN on recess PSS and ELOG GaN onto the GaN/recess–PSS buffer layer, respectively. It is found that the dislocations in the mask area are terminated by
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0.5µm
0.5µm Dislocation
SiO2
GaN
5 um
Sapphire
Void
Fig. 3. Cross-section TEM micrographs of (a) GaN onto recess PSS and (b) ELOG GaN onto GaN/PSS template. Top-view SEM image of the etched surface of ELOG GaN is shown in (c). The threading dislocation distribution terminated by oxide mask insertion within the GaN epilayer onto recess PSS can be further interpreted by (d).
SiO2 masks and most of them bend toward the mask area, which leads to a significant reduction in TDDs. Meanwhile, we observe that part of the dislocations, with a lower density due to the coalescence of GaN over the protruding sapphire substrate, propagates to the surface. In some regions, they are almost free of TDs. The behavior is very similar to the previous ELOG works [4,19]. Obviously, the TDDs within the entire ELOG GaN film are significantly decreased. To further confirm the reduction of dislocations within the ELOG GaN, its surface is etched using a mixture of acid solution, shown in Fig. 3(c). The figure clearly reveals that the total etch-pit density is 106 cm2, implying a very low TDD after the employment of a combination of ELOG and PSS techniques [13]. Judging from the measured results above, it could be inferred that the protruding PSS is reasonably a better buffer layer for the second ELOG of GaN.
4. Summary We compared the structural properties of GaN epilayers on the recess and protruding PSSs. The results confirm that the GaN crystal quality of the former is better than that of the latter. Using GaN onto the recess PSS could prove to be a better template for the lateral regrown GaN as compared with that on the protruding PSS.
Acknowledgements This work was financially supported by the National Science Council (Taiwan) and Wafer Works Corporation (Taoyuan, Taiwan) under Contract no. NSC 94-2622-E-005-001.
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