- Email: [email protected]
Reducing dislocations of thick AlGaN epilayer by combining low-temperature AlN nucleation layer on c-plane sapphire substrates
Journal of Alloys and Compounds 555 (2013) 311–314
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Reducing dislocations of thick AlGaN epilayer by combining low-temperature AlN nucleation layer on c-plane sapphire substrates Wang Dang-Hui a,⇑, Hao Yue b, Xu Sheng-Rui b, Xu Tian-Han a, Wang Dang-Chao c, Yao Ting-Zhen a, Zhang Ya-Ni a a b c
School of Materials Science and Engineering of Xi’an Shiyou University, Xi’an 710065, China State Key Lab. of Fundamental Science on Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, China Department of Physics and Electron Engineering of Xianyang Normal College, Xianyang 712000, China
a r t i c l e
i n f o
Article history: Received 15 November 2012 Received in revised form 7 December 2012 Accepted 8 December 2012 Available online 20 December 2012 Keywords: Metal–organic chemical vapor deposition Crystal quality Surface morphology AlN nucleation layer
a b s t r a c t In this study, we have reported on growth of thick AlGaN layer on the c-plane sapphire substrate with low-temperature AlN (LT-AlN) nucleation layer by low-pressure metal–organic chemical vapor deposition (LPMOCVD). High resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements have been employed to study the crystal quality, threading dislocation density, surface morphology, and optical properties of thick AlGaN layer. Results indicate that the insertion of LT-AlN nucleation layer between sapphire substrate and high-temperature AlN nucleation layer effectively improves the thick AlGaN crystal quality, reduces the surface roughness and eliminates the threading dislocation density. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, III-nitride (GaN, AlN and InN) and its compounds are being established as materials of extreme significance for next generation high-density power devices. They are used in microwave telecommunications in radar and mobile operations [1–3]. AlGaN alloys are important for opto-electronic devices such as light-emitting diodes (LEDs) and photo-detectors in the UV spectral region between the range of 200 nm–365 nm, which can be tuned according to the band gap of AlGaN [4]. Despite the rapid commercialization of III-nitride semiconductor devices for applications in visible and ultraviolet opto-electronics and in high-power and high-frequency electronics, their full potential is limited by a primary obstacle: a high defect density and biaxial strain due to the heteroepitaxial growth on foreign substrates, which result in lower performance and shortened device lifetime. In order to obtain high-quality epitaxial layers of III-nitrides, many researcher groups have adopted to different growth methods (such as hydride vapor phase epitaxy (HVPE), epitaxial lateral overgrowth (ELO) and molecular beam epitaxy (MBE) or other technologies (incorporating a low temperature GaN or AlN nucleation layer could improve crystal quality of the AlGaN epitaxial layer [5–7]) to reduce the threading dislocation density and strain/stress due to the large differences in lattice constant and thermal expansion coefficient between the epitaxial layer and the underneath sapphire substrate. ⇑ Corresponding author. Tel.: +86 02988382539. E-mail address: [email protected] (W. Dang-Hui). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.018
Although many research studies have been carried out on this topic and several papers on this aspect have been published [8–10], however, there exists more complex growth conditions and more serious crystal quality in AlGaN epilayer. The typical growth temperature for AlGaN alloys lies between 700 °C and 800 °C [11], researchers hoped to obtain the higher crystal quality AlGaN through the simpler growth processing. Up to now, there have been few general studies on the crystal quality and surface morphology for thick AlGaN layer by inserting low-temperature (LT) AlN nucleation layer between sapphire substrate and high-temperature (HT) AlN nucleation layer. In this paper, we indicated that the insertion of LT-AlN nucleation layer effectively improves the thick AlGaN crystal quality, reduces surface roughness and eliminates the threading dislocation density.
2. Experimental procedure Growth of thick AlGaN layer was achieved using a cold-wall showerhead lowpressure metal organic chemical vapor deposition (LPMOCVD) system. As shown in Fig. 1, a 210-nm-thick HT-AlN nucleation layer was first deposited on (0 0 0 1) sapphire at 1100 °C. Then a 1200-nm-thickness AlGaN epifilm layer was deposited on HT-AlN nucleation layer at 1030 °C denoted as sample A. The sample B has the same structure besides that the LT-AlN nucleation layer was grown at 660 °C between sapphire substrate and HT-AlN nucleation layer. The as-grown samples were characterized by high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and photoluminescence (PL). For the purpose to reveal the complex of the defects, and to obtain a comprehensive knowledge of the microstructure of AlGaN and its correlation with the growth dynamics, we measured X-ray rocking curves (XRCs) for both symmetry and asymmetry diffraction planes by HRXRD.
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Normalization intensity
(a) 1.0
Fig. 1. Cross-section structures of sample A (a) and sample B (b).
The HRXRD was performed using Bruker D8-discover system equipped with Ge (2 2 0) monochromator and channel-cut analyzer, delivering a pure Cu Ka line of wavelength k = 0.15406 nm.
XRC-(002)
0.8
Sample A
0.4 0.2 0.0 -0.2
-0.1
AlGaN-(002)-Sample B
300k
AlGaN-(002)-Sample A
(b) Normalization intensity
The 2h x scan XRD profiles for the two samples are shown in Fig. 2. The peak located at 34.67° is diffraction from the AlGaN (0 0 2) planes of the two samples, obviously. As is shown in Fig. 2, 35.92° and 36.00° are diffraction from the AlN (0 0 2) planes for sample A and sample B, respectively. Using Vegard’s law, the Al mole fraction x in thick AlxGa1xN layer was calculated from the observed peaks shown in Fig. 2 to be about 7.50% for the two samples. The intensity of AlGaN (0 0 2) for sample B is much stronger than that of sample A. Fig. 3 shows measured symmetrical (0 0 2) and asymmetrical (1 0 2) x-scan HRXRD full-width at half-maximum (FWHM) of the two AlGaN epitaxial layers. A comparison of the FWHM of Xray rocking curve (XRC) of sample A and sample B is made. It can be seen that (0 0 2) HRXRD FWHM of the AlGaN epitaxial layers for sample A and sample B were 227 arcsec and 134 arcsec, respectively. It can also be seen that (1 0 2) HRXRD FWHM of the AlGaN epitaxial layers were 1800 arcsec and 1447 arcsec, respectively. In other words, we can significantly reduce (0 0 2) and (1 0 2) HRXRD FWHMs of the AlGaN epitaxial layers by insertion LT-AlN nucleation layer. Previously, it has been demonstrated that the X-ray rocking curve for the symmetric (0 0 2)-reflecting plane is related to screw and mixed dislocations, whereas the X-ray rocking curve for the asymmetric (1 0 2)-reflecting plane is directly influenced by all threading dislocations, including edge dislocations [12–14]. We believe that the improved crystal quality is related to the alleviation of lattice mismatch between AlGaN and the underneath sapphire substrate. Therefore, the smaller FWHMs suggest that we can indeed reduce the dislocation density through the use of LT-AlN nucleation layer. Furthermore, it was well known that the FWHMs of the x-scan rocking curve for the symmetric and skew symmetric planes
Intensity/a.u.
0.0
0.1
0.2
w-scan/Deg.
3. Results and discussions
400k
Sample B
0.6
1.0 RC-(102) 0.8
Sample B
Sample A
0.6 0.4 0.2 0.0 -0.6
-0.3
0.0
0.3
0.6
w-sacn/Deg. Fig. 3. X-ray rocking curve for (0 0 2) reflection for sample A and sample B. (a) is the (0 0 2) planes and (b) is the (1 0 2) planes.
indirectly represented the density of screw and edge dislocations and were correlated with the growth processing. Eq. (1) is used to be described as the relationship about threading dislocation density as following [15,16].
qs ¼
Dx2s 4:35c2
qe ¼
and
Dx2e 4:35a2 ð1Þ
In which qs and qe are the screw and edge threading dislocation densities, respectively; the quantities Dxs and Dxe refer to the FWHM of (0 0 2) and (1 0 2), respectively; c and a are the relevant Burgers vectors of the AlGaN epifilms. The calculated results of threading dislocation densities are shown as Table 1. We listed the measured values about the two samples as shown in Table 1. As can be seen, whatever for (0 0 2) planes and (1 0 2) planes, the FWHM of the rocking curve for sample B is smaller than sample A, which indicates that the sample B has a higher crystal quality than sample A has. In additional, the edge dislocation density is a majority factor in the two AlGaN epifilms, and the total
200k 100k
AlN-(002)
Table 1 Experimental results of the u and x scan FWHM values and the calculated threading dislocation densities of the sample A and sample B. Sample
0 34.5
35.0
35.5
36.0
2Theta/deg. Fig. 2. 2h x scan XRD profiles for sample A and sample B for AlGaN (0 0 2) planes.
A B
HRXRD FWHM of rocking curve/arcsec (002)
(102)
227 134
1800 1447
qs /108cm2
qe /1010cm2
1.042 0.363
1.754 1.134
313
W. Dang-Hui et al. / Journal of Alloys and Compounds 555 (2013) 311–314
Fig. 4. AFM images of 5 5lm2 Surface morphology for sample A and sample B.
0.5
PL Voltage/mV
threading dislocation density for sample B is much smaller than that of sample A. These results suggested that both screw dislocation density and edge dislocation density can be reduced. Therefore, we can draw a conclusion that thick AlGaN layer with higher crystal quality and lower threading dislocation density can be obtained using insertion the LT-AlN nucleation layer between sapphire and HT-AlN nucleation layer. Fig. 4 shows the 5 5lm2 AFM images of the two thick AlGaN samples. We can see that the difference in surface morphology between sample A and sample B. It was found that dark spots, originated from the threading dislocations in the AlGaN epitaxial layers, could be clearly observed in both AlGaN samples. The sample B has a uniform and regular surface morphology and little dark spots comparison with sample A has [17]. One can see from these images that the thick AlGaN surface morphology is becoming smoother when the LT-AlN nucleation layer grown on the sapphire substrates. Fig. 4(c) and (d) show the 3D surface morphology of the two thick AlGaN layer samples. The sample B clearly shows a lower fluctuation on surface than sample A does. In addition, the root mean square (RMS) value is 0.253 nm for sample A and 0.185 nm for sample B, which indicates that the sample B has a smoother surface morphology than sample A does. We thought it is the larger compressive strain induced these morphology differences between sample A and sample B. So the higher crystal quality thick AlGaN epifilms can be obtained by insertion the LT-AlN nucleation layer between sapphire substrate and HT-AlN nucleation layer. We carried out the PL measurement to examine the optical properties of the two thick AlGaN layer samples at room temperature. The PL spectral of sample A and sample B are shown in Fig. 5 for a comparison. As can be shown, a spectral peak is at 338.1 nm (3.67 eV) for sample A and sample B, which are the corresponding band emission peak of the two thick AlGaN samples. It has been noted that a weak subsidiary peak is at 357.4 nm (3.47 eV) below the main peak for the sample A, which is closer to bandgap of GaN (3.42 eV). The origin of this peak is probably ascribed to the inhomogeneity AlGaN epifilms during the growth processing at
Sample A
0.4 0.3
Sample B
0.2 0.1 0.0
320
340
360
380
400
Photon wavelength/nm
420
Fig. 5. PL spectra recorded at room temperatures for sample A and sample B.
higher temperature. The mole fraction of Al was calculated by compositional dependence of the optical band gap of ternary alloys:
Eg;AlGaN ðxÞ ¼ Eg;AlN x þ Eg;GaN ð1 xÞ bxð1 xÞ
ð2Þ
Here, Eg, x and b are the optical band gap, AlN mole fraction and bowing parameter, respectively. Eg,GaN, Eg,AlN and b is 3.4 eV, 6.2 eV and 0.9 eV, respectively [18]. Using Eq. (2), we calculated the Al mole fraction is 13.25% for the two samples. Ref. [19] studied this difference between XRD and PL in details, and thought it is the variable bowing parameter for the whole composition range and larger value for smaller Al mole fraction that made this difference. It should be noted that there does not exist a common deep level emission center, it is so-called yellow luminescence (YL) [20], which indicated that the crystal quality of the two thick AlGaN layer is better. 4. Conclusions In this paper, we have reported on growth of thick AlGaN layer on c-plane sapphire substrate with different temperature AlN
314
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nucleation layers by low-pressure metal-organic chemical vapor deposition. High resolution X-ray diffraction, atomic force microscopy, and photoluminescence (PL) measurements have been employed to study the crystal quality, surface morphology and optical properties of thick AlGaN layer. The results indicated that AlGaN epifilms crystal quality can be improved greatly by insertion the LT-AlN nucleation layer. The threading dislocation density in obtained thick AlGaN epifilm has a lower threading dislocation density, and which is consistent with the surface morphology of the AlGaN alloys probed by AFM. Acknowledgements This study was supported by state science fund for youths (Grant No. 61204006), youth science and technology innovation fund of Xi’an Shiyou University (No. Z12180), and science research program of department of education of Shaanxi province (Grant No. 12JK0440). The authors would like to thank Xidian university and Xi’an Shiyou University for the financial and technical support as well as for their kind permission to publish this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
S. Nuttinck, E. Gebara, J. Laskar, et al., IEEE Microw. Mag. 3 (2002) 80. M. Kuzuhara, H. Miyamoto, Y. Ando, et al., Phys. Stat. Sol. A 200 (2003) 161. M.X. Xiao, M. Zhao, Q. Jiang, Chem. Phys. Lett. 512 (2011) 251–254. M. Iwaya, S. Terao, T. Sano, et al., J. Cryst. Growth 237 (2002) 951. H. Amano, N. Sawaki, I. Akasaki, et al., Appl. Phys. Lett. 48 (1986) 353. S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. V. Kueller et al., J. Cryst. Growth 315 (2011) 200. A. Redondo-cubero, R. Gago, González-posada, et al., Thin Solid Films 516 (2008) 8447. S.V. Novikov, C.R. Staddon, R.E.L. Powell, et al., J. Cryst. Growth 322 (2011) 23. H.S. Ahn, K.H. Kim, M. Yang, et al., J. Cryst. Growth 243 (2005) 178. O. Ambacher, J. Phys. D: Appl. Phys. 31 (1998) 2653–2710. M.Z. Peng, L.W. Guo, J. Zhang, et al., Superlattices 310 (2008) 1088. B. Heying, X.H. Wu, S. Keller, et al., Appl. Phys. Lett. 68 (1996) 643. Y. Fu, Y.T. Moon, F. Yun, et al., Appl. Phys. Lett. 86 (2005) 043108. C.S. Gallinat, G. Koblmuller, F. Wu, et al., J. Appl. Phys. 107 (2010) 053517. Zhang. YuChao, Xing. ZhiGang, Ma. ZiGuang, et al., Sci. Chin. Phys. Mech. Astron. 53 (2010) 465–468. K.T. Lam, C.L. Yu, P.C. Chang, et al., Superlattices Microstruct. 43 (2008) 147. L.F. Semond, F. Natali, D. Byrne, et al., Superlattices Microstruct. 36 (2004) 659. P.G. Moses, C.G. Van de Walle, Appl. Phys. Lett. 96 (2010) 021908. Y. Guo et al., Appl. Surf. Sci. 256 (2010) 3352.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Reducing dislocations of thick AlGaN epilayer by combining low-temperature AlN nucleation layer on c-plane sapphire substrates Wang Dang-Hui a,⇑, Hao Yue b, Xu Sheng-Rui b, Xu Tian-Han a, Wang Dang-Chao c, Yao Ting-Zhen a, Zhang Ya-Ni a a b c
School of Materials Science and Engineering of Xi’an Shiyou University, Xi’an 710065, China State Key Lab. of Fundamental Science on Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, China Department of Physics and Electron Engineering of Xianyang Normal College, Xianyang 712000, China
a r t i c l e
i n f o
Article history: Received 15 November 2012 Received in revised form 7 December 2012 Accepted 8 December 2012 Available online 20 December 2012 Keywords: Metal–organic chemical vapor deposition Crystal quality Surface morphology AlN nucleation layer
a b s t r a c t In this study, we have reported on growth of thick AlGaN layer on the c-plane sapphire substrate with low-temperature AlN (LT-AlN) nucleation layer by low-pressure metal–organic chemical vapor deposition (LPMOCVD). High resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements have been employed to study the crystal quality, threading dislocation density, surface morphology, and optical properties of thick AlGaN layer. Results indicate that the insertion of LT-AlN nucleation layer between sapphire substrate and high-temperature AlN nucleation layer effectively improves the thick AlGaN crystal quality, reduces the surface roughness and eliminates the threading dislocation density. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction In the past decade, III-nitride (GaN, AlN and InN) and its compounds are being established as materials of extreme significance for next generation high-density power devices. They are used in microwave telecommunications in radar and mobile operations [1–3]. AlGaN alloys are important for opto-electronic devices such as light-emitting diodes (LEDs) and photo-detectors in the UV spectral region between the range of 200 nm–365 nm, which can be tuned according to the band gap of AlGaN [4]. Despite the rapid commercialization of III-nitride semiconductor devices for applications in visible and ultraviolet opto-electronics and in high-power and high-frequency electronics, their full potential is limited by a primary obstacle: a high defect density and biaxial strain due to the heteroepitaxial growth on foreign substrates, which result in lower performance and shortened device lifetime. In order to obtain high-quality epitaxial layers of III-nitrides, many researcher groups have adopted to different growth methods (such as hydride vapor phase epitaxy (HVPE), epitaxial lateral overgrowth (ELO) and molecular beam epitaxy (MBE) or other technologies (incorporating a low temperature GaN or AlN nucleation layer could improve crystal quality of the AlGaN epitaxial layer [5–7]) to reduce the threading dislocation density and strain/stress due to the large differences in lattice constant and thermal expansion coefficient between the epitaxial layer and the underneath sapphire substrate. ⇑ Corresponding author. Tel.: +86 02988382539. E-mail address: [email protected] (W. Dang-Hui). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.12.018
Although many research studies have been carried out on this topic and several papers on this aspect have been published [8–10], however, there exists more complex growth conditions and more serious crystal quality in AlGaN epilayer. The typical growth temperature for AlGaN alloys lies between 700 °C and 800 °C [11], researchers hoped to obtain the higher crystal quality AlGaN through the simpler growth processing. Up to now, there have been few general studies on the crystal quality and surface morphology for thick AlGaN layer by inserting low-temperature (LT) AlN nucleation layer between sapphire substrate and high-temperature (HT) AlN nucleation layer. In this paper, we indicated that the insertion of LT-AlN nucleation layer effectively improves the thick AlGaN crystal quality, reduces surface roughness and eliminates the threading dislocation density.
2. Experimental procedure Growth of thick AlGaN layer was achieved using a cold-wall showerhead lowpressure metal organic chemical vapor deposition (LPMOCVD) system. As shown in Fig. 1, a 210-nm-thick HT-AlN nucleation layer was first deposited on (0 0 0 1) sapphire at 1100 °C. Then a 1200-nm-thickness AlGaN epifilm layer was deposited on HT-AlN nucleation layer at 1030 °C denoted as sample A. The sample B has the same structure besides that the LT-AlN nucleation layer was grown at 660 °C between sapphire substrate and HT-AlN nucleation layer. The as-grown samples were characterized by high resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and photoluminescence (PL). For the purpose to reveal the complex of the defects, and to obtain a comprehensive knowledge of the microstructure of AlGaN and its correlation with the growth dynamics, we measured X-ray rocking curves (XRCs) for both symmetry and asymmetry diffraction planes by HRXRD.
312
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Normalization intensity
(a) 1.0
Fig. 1. Cross-section structures of sample A (a) and sample B (b).
The HRXRD was performed using Bruker D8-discover system equipped with Ge (2 2 0) monochromator and channel-cut analyzer, delivering a pure Cu Ka line of wavelength k = 0.15406 nm.
XRC-(002)
0.8
Sample A
0.4 0.2 0.0 -0.2
-0.1
AlGaN-(002)-Sample B
300k
AlGaN-(002)-Sample A
(b) Normalization intensity
The 2h x scan XRD profiles for the two samples are shown in Fig. 2. The peak located at 34.67° is diffraction from the AlGaN (0 0 2) planes of the two samples, obviously. As is shown in Fig. 2, 35.92° and 36.00° are diffraction from the AlN (0 0 2) planes for sample A and sample B, respectively. Using Vegard’s law, the Al mole fraction x in thick AlxGa1xN layer was calculated from the observed peaks shown in Fig. 2 to be about 7.50% for the two samples. The intensity of AlGaN (0 0 2) for sample B is much stronger than that of sample A. Fig. 3 shows measured symmetrical (0 0 2) and asymmetrical (1 0 2) x-scan HRXRD full-width at half-maximum (FWHM) of the two AlGaN epitaxial layers. A comparison of the FWHM of Xray rocking curve (XRC) of sample A and sample B is made. It can be seen that (0 0 2) HRXRD FWHM of the AlGaN epitaxial layers for sample A and sample B were 227 arcsec and 134 arcsec, respectively. It can also be seen that (1 0 2) HRXRD FWHM of the AlGaN epitaxial layers were 1800 arcsec and 1447 arcsec, respectively. In other words, we can significantly reduce (0 0 2) and (1 0 2) HRXRD FWHMs of the AlGaN epitaxial layers by insertion LT-AlN nucleation layer. Previously, it has been demonstrated that the X-ray rocking curve for the symmetric (0 0 2)-reflecting plane is related to screw and mixed dislocations, whereas the X-ray rocking curve for the asymmetric (1 0 2)-reflecting plane is directly influenced by all threading dislocations, including edge dislocations [12–14]. We believe that the improved crystal quality is related to the alleviation of lattice mismatch between AlGaN and the underneath sapphire substrate. Therefore, the smaller FWHMs suggest that we can indeed reduce the dislocation density through the use of LT-AlN nucleation layer. Furthermore, it was well known that the FWHMs of the x-scan rocking curve for the symmetric and skew symmetric planes
Intensity/a.u.
0.0
0.1
0.2
w-scan/Deg.
3. Results and discussions
400k
Sample B
0.6
1.0 RC-(102) 0.8
Sample B
Sample A
0.6 0.4 0.2 0.0 -0.6
-0.3
0.0
0.3
0.6
w-sacn/Deg. Fig. 3. X-ray rocking curve for (0 0 2) reflection for sample A and sample B. (a) is the (0 0 2) planes and (b) is the (1 0 2) planes.
indirectly represented the density of screw and edge dislocations and were correlated with the growth processing. Eq. (1) is used to be described as the relationship about threading dislocation density as following [15,16].
qs ¼
Dx2s 4:35c2
qe ¼
and
Dx2e 4:35a2 ð1Þ
In which qs and qe are the screw and edge threading dislocation densities, respectively; the quantities Dxs and Dxe refer to the FWHM of (0 0 2) and (1 0 2), respectively; c and a are the relevant Burgers vectors of the AlGaN epifilms. The calculated results of threading dislocation densities are shown as Table 1. We listed the measured values about the two samples as shown in Table 1. As can be seen, whatever for (0 0 2) planes and (1 0 2) planes, the FWHM of the rocking curve for sample B is smaller than sample A, which indicates that the sample B has a higher crystal quality than sample A has. In additional, the edge dislocation density is a majority factor in the two AlGaN epifilms, and the total
200k 100k
AlN-(002)
Table 1 Experimental results of the u and x scan FWHM values and the calculated threading dislocation densities of the sample A and sample B. Sample
0 34.5
35.0
35.5
36.0
2Theta/deg. Fig. 2. 2h x scan XRD profiles for sample A and sample B for AlGaN (0 0 2) planes.
A B
HRXRD FWHM of rocking curve/arcsec (002)
(102)
227 134
1800 1447
qs /108cm2
qe /1010cm2
1.042 0.363
1.754 1.134
313
W. Dang-Hui et al. / Journal of Alloys and Compounds 555 (2013) 311–314
Fig. 4. AFM images of 5 5lm2 Surface morphology for sample A and sample B.
0.5
PL Voltage/mV
threading dislocation density for sample B is much smaller than that of sample A. These results suggested that both screw dislocation density and edge dislocation density can be reduced. Therefore, we can draw a conclusion that thick AlGaN layer with higher crystal quality and lower threading dislocation density can be obtained using insertion the LT-AlN nucleation layer between sapphire and HT-AlN nucleation layer. Fig. 4 shows the 5 5lm2 AFM images of the two thick AlGaN samples. We can see that the difference in surface morphology between sample A and sample B. It was found that dark spots, originated from the threading dislocations in the AlGaN epitaxial layers, could be clearly observed in both AlGaN samples. The sample B has a uniform and regular surface morphology and little dark spots comparison with sample A has [17]. One can see from these images that the thick AlGaN surface morphology is becoming smoother when the LT-AlN nucleation layer grown on the sapphire substrates. Fig. 4(c) and (d) show the 3D surface morphology of the two thick AlGaN layer samples. The sample B clearly shows a lower fluctuation on surface than sample A does. In addition, the root mean square (RMS) value is 0.253 nm for sample A and 0.185 nm for sample B, which indicates that the sample B has a smoother surface morphology than sample A does. We thought it is the larger compressive strain induced these morphology differences between sample A and sample B. So the higher crystal quality thick AlGaN epifilms can be obtained by insertion the LT-AlN nucleation layer between sapphire substrate and HT-AlN nucleation layer. We carried out the PL measurement to examine the optical properties of the two thick AlGaN layer samples at room temperature. The PL spectral of sample A and sample B are shown in Fig. 5 for a comparison. As can be shown, a spectral peak is at 338.1 nm (3.67 eV) for sample A and sample B, which are the corresponding band emission peak of the two thick AlGaN samples. It has been noted that a weak subsidiary peak is at 357.4 nm (3.47 eV) below the main peak for the sample A, which is closer to bandgap of GaN (3.42 eV). The origin of this peak is probably ascribed to the inhomogeneity AlGaN epifilms during the growth processing at
Sample A
0.4 0.3
Sample B
0.2 0.1 0.0
320
340
360
380
400
Photon wavelength/nm
420
Fig. 5. PL spectra recorded at room temperatures for sample A and sample B.
higher temperature. The mole fraction of Al was calculated by compositional dependence of the optical band gap of ternary alloys:
Eg;AlGaN ðxÞ ¼ Eg;AlN x þ Eg;GaN ð1 xÞ bxð1 xÞ
ð2Þ
Here, Eg, x and b are the optical band gap, AlN mole fraction and bowing parameter, respectively. Eg,GaN, Eg,AlN and b is 3.4 eV, 6.2 eV and 0.9 eV, respectively [18]. Using Eq. (2), we calculated the Al mole fraction is 13.25% for the two samples. Ref. [19] studied this difference between XRD and PL in details, and thought it is the variable bowing parameter for the whole composition range and larger value for smaller Al mole fraction that made this difference. It should be noted that there does not exist a common deep level emission center, it is so-called yellow luminescence (YL) [20], which indicated that the crystal quality of the two thick AlGaN layer is better. 4. Conclusions In this paper, we have reported on growth of thick AlGaN layer on c-plane sapphire substrate with different temperature AlN
314
W. Dang-Hui et al. / Journal of Alloys and Compounds 555 (2013) 311–314
nucleation layers by low-pressure metal-organic chemical vapor deposition. High resolution X-ray diffraction, atomic force microscopy, and photoluminescence (PL) measurements have been employed to study the crystal quality, surface morphology and optical properties of thick AlGaN layer. The results indicated that AlGaN epifilms crystal quality can be improved greatly by insertion the LT-AlN nucleation layer. The threading dislocation density in obtained thick AlGaN epifilm has a lower threading dislocation density, and which is consistent with the surface morphology of the AlGaN alloys probed by AFM. Acknowledgements This study was supported by state science fund for youths (Grant No. 61204006), youth science and technology innovation fund of Xi’an Shiyou University (No. Z12180), and science research program of department of education of Shaanxi province (Grant No. 12JK0440). The authors would like to thank Xidian university and Xi’an Shiyou University for the financial and technical support as well as for their kind permission to publish this work.
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