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Growth and characterization of GaN films on Si(111) substrate using high-temperature AlN buffer layer
Surface & Coatings Technology 198 (2005) 350 – 353 www.elsevier.com/locate/surfcoat
Growth and characterization of GaN films on Si(111) substrate using high-temperature AlN buffer layer Xianfeng Ni, Liping Zhu*, Zhizhen Ye, Zhe Zhao, Haiping Tang, Wei Hong, Binghui Zhao State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China Available online 26 November 2004
Abstract Hexagonal GaN films exceeding 1 Am have been prepared on Si(111) substrates using high-temperature AlN buffer layers, and no cracks were observed. The FWHM of X-ray rocking curve for GaN (0002) was 560 arcsec. Strong band-edge photoluminescence (PL) was present in PL spectra. Micro-Raman spectra using shifts of E2 phonon showed that GaN films were in compressive stress, which agreed with the characterization result of X-ray lattice parameters method. D 2004 Elsevier B.V. All rights reserved. PACS: 68.55N9; 71.55E9; 78.55Cr; 81.15Gh Keywords: [X] Buffe layer; [B] Scanning electron microscopy; [B] X-ray diffraction; [B] Raman scattering spectroscopy; [C] Organometallic CVD; [D] Nitrides
1. Introduction Due to the lack of substrate for homo-epitaxy, metalorganic chemical vapor deposition (MOCVD) of GaN is usually performed on sapphire [1] and SiC [2]. These substrates have several severe disadvantages, such as either insulating behavior (sapphire) requiring more elaborate device processing for light emitting diodes or a very high price (SiC). In addition, both substrates have a high lattice and thermal mismatch to GaN [3]. An alternative substrate is silicon, which offers high electrical and thermal conductivity, as well as large diameters at low price (about 1/10 of sapphire, 1/100 of SiC). Additionally, it is enabling an integration of Si electronics with GaN-based electronics and opto-electronics on the same chip. However, it also shows a high lattice and thermal mismatch to GaN. In order to obtain high-quality GaN and related nitride epilayers on Si substrate, the two-step growth method has been proposed, and various types of buffer layers have been investigated: AlN buffer layer [4], AlAs [5], amorphous GaN [6], SiC [7]. * Corresponding author. Tel.: +86 571 879 53139; fax: +86 571 879 52625. E-mail address: [email protected] (L. Zhu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.073
Using AlN as buffer layer is a very promising way to grow high-quality GaN on Si. The lattice and thermal mismatch between AlN and GaN are much smaller than those between Si and GaN, and AlN has good wetting properties on Si. In 2000, Dadgar et al. [4] obtained crack-free GaN on Si exceeding 1 Am by introducing thing, low-temperature AlN buffer layer between GaN and Si substrate. Successful fabrication of InGaN/GaN light emitting diodes on Si(111) was subsequently achieved by this group in 2002 [8]. In this paper, we report the growth of crack-free hexagonal GaN films exceeding 1 Am on Si(111) substrate using high-temperature AlN (HT-AlN) buffer layer by MOCVD technique. X-ray rocking curve results indicated that the crystal quality of the GaN films is reasonably good. X-ray parameters method and micro-Raman spectra were used to measure the residual stress of GaN films.
2. Experimental procedure P-type Si(111) wafers off-cut ~48 with a resistivity of 8–12 V cm were used as substrates. The substrates were cleaned with RCA1 solution (NH4OH/H2O2/H2O, 1:1:6), followed by RCA2 solution (HCl/H2O2/H2O, 1:1:6), each for 15 min
X. Ni et al. / Surface & Coatings Technology 198 (2005) 350–353
at 85 8C. Then they were dipped into 10% HF solution for 30 s to remove native SiO2 on the Si surface [9]. The substrate was then immediately transferred into the chamber. The growth was carried out in a home-made two-flow MOCVD system. Trimethylaluminum (TMA), trimethylgallium (TMG) and ammonia (NH3) were used as precursors of Al, Ga, and N, respectively. Purified H2 was used as the carrier gas. The growth pressure for AlN buffer layer and GaN was 100 and 760 Torr, respectively. After annealing of the substrates in H2 ambient at 1060 8C for 15 min to clean the surface, a few monolayers of Al were pre-deposited respectively in the Si substrates to prevent the formation of Si3N4 before the growth of AlN buffer layers. The growth of AlN buffer was carried out at 1070 8C, with the flow rates of TMA and NH 3 being 12.5 Amol/min and 4 sl/min, respectively. The thickness of buffer layer was about 70 nm. Then, the substrate temperature was ramped to 1060 8C for the epitaxy of GaN thin films. The V/III ratio for the growth of GaN layers was about 4500. Scanning electron microscopy (SEM), atom force microscopy (AFM), X-ray diffraction (XRD) with Bede D1 system, photoluminescence (PL) with excitation wavelength of 300 nm at room temperature was used to characterize the surface morphology and crystalline quality of the samples. Besides, samples were characterized by micro-Raman spectroscopy using 514.5 nm line of the argon ion laser, with Z(XX)-Z configurations.
351
Fig. 2. X-ray diffraction pattern of AlN/Si(111).
Fig. 1 shows the SEM image of surface morphology of AlN buffer layer annealed at 1100 8C in NH3 atmosphere for 10 min. The surface is relatively smooth and uniform. It is clear that the silicon substrate is covered completely by AlN buffer layer, which can greatly suppress the nitridation of Si substrate when exposed in NH3 for GaN growth. The result of AFM measurement shows a surface root-mean
square (RMS) roughness of 5.69 nm. Cross-sectional SEM image of the AlN buffer layer was also measured (not shown here), showing a thickness of about 70 nm. The Xray diffraction pattern indicates that the obtained AlN buffer layer on Si(111) is highly c-axis oriented, as shown in Fig. 2. This suggests that the HT-AlN buffer layer could serve as a good template for the subsequent growth of high-quality GaN layer. Based on the above AlN buffer layer, GaN epitaxy was carried out in our MOCVD system. Fig. 3 shows a typical X-ray diffraction pattern of GaN films on AlN/Si(111) substrate. Only peaks of GaN (0002), (0004) and Si(111) are observed in this pattern, indicating hexagonal structure of GaN epilayer with its [0001] direction parallel to the [111] of Si. The c constant calculated from GaN (0002) peak is 5.20932, which is a little larger than that of bulk GaN (5.18552 [10]). This indicates that GaN films with HT-AlN buffer layer are in compressive stress. Fig. 4 shows the cross-sectional SEM image of GaN/HT-AlN/Si(111) structure. The GaN film has crystallized well and its thickness is about 3 Am with no cracks being observed.
Fig. 1. SEM image of the surface of high-temperature AlN buffer layer on Si(111) substrate.
Fig. 3. X-ray diffraction pattern of GaN films on AlN/Si(111) substrate.
3. Results and discussions
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X. Ni et al. / Surface & Coatings Technology 198 (2005) 350–353
Fig. 4. Cross-sectional SEM image of GaN/HT-AlN/Si(111) structure. Fig. 6. Room-temperature photoluminescence spectra of GaN/Si(111) samples using HT-AlN buffer layer.
In order to determine the crystalline quality of our GaN films, high-resolution X-ray diffraction has been performed on our samples using omega scan mode. Fig. 5 shows a typical X-ray rocking curve of GaN films using HT-AlN buffer layer, with a FWHM of 560 arcsec, indicating high crystalline quality of GaN on Si(111). Fig. 6 shows roomtemperature PL spectra of GaN/Si(111) samples with HT-AlN buffer layers. Strong band edge luminescence can be observed, with FWHM in the range of 15–17 nm, and no yellow luminescence (YL) is present. Meanwhile, in order to estimate residual stress in GaN films, micro-Raman measurements were carried out on GaN samples, using shifts of the E2 phonon. The E2 phonon line in GaN is sensitive to strain in the layer, and its frequency can be accurately measured by Raman spectroscopy. Fig. 7 shows a typical Raman spectrum for GaN film grown in our MOCVD system using AlN buffer layer. In Raman spectrum, the intense peak at 520 cm 1 is the O(G) phonon from the Si(111) substrate, no matter what kind of buffer layer has been used. Another peak can also be found, which is associated with the E2(TO) peak of GaN films.
The E2(TO) peak has a value of 569 cm 1, showing a little blue shift, relative to the bulk value of GaN film, 567.5 cm 1 of a 400 Am free-standing GaN layer [11]. The blue shift suggests that compressive stress exists in GaN using HT-AlN buffer [11], which is usually beneficial for the growth of device structures, usually having thickness of several microns. The conclusion drawn from Raman spectra is consistent with the above-mentioned results of Xray parameters method. In summary, GaN films have been successfully grown on Si(111) substrate using HT-AlN buffer layer. X-ray rocking curve result shows good crystalline quality. The GaN film is about 3 Am thick and no cracks can be found. In addition, characterization results of X-ray parameters method and micro-Raman spectra show that GaN films using HT-AlN buffer layer have compressive residual stress, which is desirable for the epitaxy of devicestructured nitride layers.
Fig. 5. X-ray rocking curve of GaN films on Si(111) substrate using HT-AlN buffer.
Fig. 7. Raman spectrum for GaN film on Si(111) substrate.
X. Ni et al. / Surface & Coatings Technology 198 (2005) 350–353
Acknowledgments This research work is supported by the Special Foundation of the Education Ministry of China for Overseas Returnees 2003 [406] and the state major Fundamental Research Project (973) [No. G20000683].
References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [2] S.N. Mohammed, H. Morkoc, Proc. IEEE 83 (1995) 1306.
353
[3] L. Liu, J.H. Edgar, Mater. Sci. Eng., R Rep. 37 (2002) 61. [4] A. Dadgar, J. Blasing, A. Diez, et al., Jpn. J. Appl. Phys. 39 (2000) L1183. [5] A. Strittmatter, A. Krost, V. Tqrck, et al., Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 59 (1999) 29. [6] P. Chen, S.Y. Xie, Z.Z. Chen, et al., J. Cryst. Growth 213 (2000) 27. [7] H.M. Liaw, R. Venugopal, J. Wan, et al., Solid-State Electron. 44 (2000) 685. [8] A. Dadgar, M. Poschenrieder, J. Blasing, et al., Appl. Phys. Lett. 80 (2002) 3670. [9] Z.Z. Ye, X.B. Jiang, J. Yuan, et al., Chin. J. Semicond. 17 (1996) 380. [10] S. Porowski, J. Cryst. Growth 166 (1996) 583. [11] S. Tripathy, S.J. Chua, P. Chen, et al., J. Appl. Phys. 92 (2002) 3503.
Growth and characterization of GaN films on Si(111) substrate using high-temperature AlN buffer layer Xianfeng Ni, Liping Zhu*, Zhizhen Ye, Zhe Zhao, Haiping Tang, Wei Hong, Binghui Zhao State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China Available online 26 November 2004
Abstract Hexagonal GaN films exceeding 1 Am have been prepared on Si(111) substrates using high-temperature AlN buffer layers, and no cracks were observed. The FWHM of X-ray rocking curve for GaN (0002) was 560 arcsec. Strong band-edge photoluminescence (PL) was present in PL spectra. Micro-Raman spectra using shifts of E2 phonon showed that GaN films were in compressive stress, which agreed with the characterization result of X-ray lattice parameters method. D 2004 Elsevier B.V. All rights reserved. PACS: 68.55N9; 71.55E9; 78.55Cr; 81.15Gh Keywords: [X] Buffe layer; [B] Scanning electron microscopy; [B] X-ray diffraction; [B] Raman scattering spectroscopy; [C] Organometallic CVD; [D] Nitrides
1. Introduction Due to the lack of substrate for homo-epitaxy, metalorganic chemical vapor deposition (MOCVD) of GaN is usually performed on sapphire [1] and SiC [2]. These substrates have several severe disadvantages, such as either insulating behavior (sapphire) requiring more elaborate device processing for light emitting diodes or a very high price (SiC). In addition, both substrates have a high lattice and thermal mismatch to GaN [3]. An alternative substrate is silicon, which offers high electrical and thermal conductivity, as well as large diameters at low price (about 1/10 of sapphire, 1/100 of SiC). Additionally, it is enabling an integration of Si electronics with GaN-based electronics and opto-electronics on the same chip. However, it also shows a high lattice and thermal mismatch to GaN. In order to obtain high-quality GaN and related nitride epilayers on Si substrate, the two-step growth method has been proposed, and various types of buffer layers have been investigated: AlN buffer layer [4], AlAs [5], amorphous GaN [6], SiC [7]. * Corresponding author. Tel.: +86 571 879 53139; fax: +86 571 879 52625. E-mail address: [email protected] (L. Zhu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.10.073
Using AlN as buffer layer is a very promising way to grow high-quality GaN on Si. The lattice and thermal mismatch between AlN and GaN are much smaller than those between Si and GaN, and AlN has good wetting properties on Si. In 2000, Dadgar et al. [4] obtained crack-free GaN on Si exceeding 1 Am by introducing thing, low-temperature AlN buffer layer between GaN and Si substrate. Successful fabrication of InGaN/GaN light emitting diodes on Si(111) was subsequently achieved by this group in 2002 [8]. In this paper, we report the growth of crack-free hexagonal GaN films exceeding 1 Am on Si(111) substrate using high-temperature AlN (HT-AlN) buffer layer by MOCVD technique. X-ray rocking curve results indicated that the crystal quality of the GaN films is reasonably good. X-ray parameters method and micro-Raman spectra were used to measure the residual stress of GaN films.
2. Experimental procedure P-type Si(111) wafers off-cut ~48 with a resistivity of 8–12 V cm were used as substrates. The substrates were cleaned with RCA1 solution (NH4OH/H2O2/H2O, 1:1:6), followed by RCA2 solution (HCl/H2O2/H2O, 1:1:6), each for 15 min
X. Ni et al. / Surface & Coatings Technology 198 (2005) 350–353
at 85 8C. Then they were dipped into 10% HF solution for 30 s to remove native SiO2 on the Si surface [9]. The substrate was then immediately transferred into the chamber. The growth was carried out in a home-made two-flow MOCVD system. Trimethylaluminum (TMA), trimethylgallium (TMG) and ammonia (NH3) were used as precursors of Al, Ga, and N, respectively. Purified H2 was used as the carrier gas. The growth pressure for AlN buffer layer and GaN was 100 and 760 Torr, respectively. After annealing of the substrates in H2 ambient at 1060 8C for 15 min to clean the surface, a few monolayers of Al were pre-deposited respectively in the Si substrates to prevent the formation of Si3N4 before the growth of AlN buffer layers. The growth of AlN buffer was carried out at 1070 8C, with the flow rates of TMA and NH 3 being 12.5 Amol/min and 4 sl/min, respectively. The thickness of buffer layer was about 70 nm. Then, the substrate temperature was ramped to 1060 8C for the epitaxy of GaN thin films. The V/III ratio for the growth of GaN layers was about 4500. Scanning electron microscopy (SEM), atom force microscopy (AFM), X-ray diffraction (XRD) with Bede D1 system, photoluminescence (PL) with excitation wavelength of 300 nm at room temperature was used to characterize the surface morphology and crystalline quality of the samples. Besides, samples were characterized by micro-Raman spectroscopy using 514.5 nm line of the argon ion laser, with Z(XX)-Z configurations.
351
Fig. 2. X-ray diffraction pattern of AlN/Si(111).
Fig. 1 shows the SEM image of surface morphology of AlN buffer layer annealed at 1100 8C in NH3 atmosphere for 10 min. The surface is relatively smooth and uniform. It is clear that the silicon substrate is covered completely by AlN buffer layer, which can greatly suppress the nitridation of Si substrate when exposed in NH3 for GaN growth. The result of AFM measurement shows a surface root-mean
square (RMS) roughness of 5.69 nm. Cross-sectional SEM image of the AlN buffer layer was also measured (not shown here), showing a thickness of about 70 nm. The Xray diffraction pattern indicates that the obtained AlN buffer layer on Si(111) is highly c-axis oriented, as shown in Fig. 2. This suggests that the HT-AlN buffer layer could serve as a good template for the subsequent growth of high-quality GaN layer. Based on the above AlN buffer layer, GaN epitaxy was carried out in our MOCVD system. Fig. 3 shows a typical X-ray diffraction pattern of GaN films on AlN/Si(111) substrate. Only peaks of GaN (0002), (0004) and Si(111) are observed in this pattern, indicating hexagonal structure of GaN epilayer with its [0001] direction parallel to the [111] of Si. The c constant calculated from GaN (0002) peak is 5.20932, which is a little larger than that of bulk GaN (5.18552 [10]). This indicates that GaN films with HT-AlN buffer layer are in compressive stress. Fig. 4 shows the cross-sectional SEM image of GaN/HT-AlN/Si(111) structure. The GaN film has crystallized well and its thickness is about 3 Am with no cracks being observed.
Fig. 1. SEM image of the surface of high-temperature AlN buffer layer on Si(111) substrate.
Fig. 3. X-ray diffraction pattern of GaN films on AlN/Si(111) substrate.
3. Results and discussions
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X. Ni et al. / Surface & Coatings Technology 198 (2005) 350–353
Fig. 4. Cross-sectional SEM image of GaN/HT-AlN/Si(111) structure. Fig. 6. Room-temperature photoluminescence spectra of GaN/Si(111) samples using HT-AlN buffer layer.
In order to determine the crystalline quality of our GaN films, high-resolution X-ray diffraction has been performed on our samples using omega scan mode. Fig. 5 shows a typical X-ray rocking curve of GaN films using HT-AlN buffer layer, with a FWHM of 560 arcsec, indicating high crystalline quality of GaN on Si(111). Fig. 6 shows roomtemperature PL spectra of GaN/Si(111) samples with HT-AlN buffer layers. Strong band edge luminescence can be observed, with FWHM in the range of 15–17 nm, and no yellow luminescence (YL) is present. Meanwhile, in order to estimate residual stress in GaN films, micro-Raman measurements were carried out on GaN samples, using shifts of the E2 phonon. The E2 phonon line in GaN is sensitive to strain in the layer, and its frequency can be accurately measured by Raman spectroscopy. Fig. 7 shows a typical Raman spectrum for GaN film grown in our MOCVD system using AlN buffer layer. In Raman spectrum, the intense peak at 520 cm 1 is the O(G) phonon from the Si(111) substrate, no matter what kind of buffer layer has been used. Another peak can also be found, which is associated with the E2(TO) peak of GaN films.
The E2(TO) peak has a value of 569 cm 1, showing a little blue shift, relative to the bulk value of GaN film, 567.5 cm 1 of a 400 Am free-standing GaN layer [11]. The blue shift suggests that compressive stress exists in GaN using HT-AlN buffer [11], which is usually beneficial for the growth of device structures, usually having thickness of several microns. The conclusion drawn from Raman spectra is consistent with the above-mentioned results of Xray parameters method. In summary, GaN films have been successfully grown on Si(111) substrate using HT-AlN buffer layer. X-ray rocking curve result shows good crystalline quality. The GaN film is about 3 Am thick and no cracks can be found. In addition, characterization results of X-ray parameters method and micro-Raman spectra show that GaN films using HT-AlN buffer layer have compressive residual stress, which is desirable for the epitaxy of devicestructured nitride layers.
Fig. 5. X-ray rocking curve of GaN films on Si(111) substrate using HT-AlN buffer.
Fig. 7. Raman spectrum for GaN film on Si(111) substrate.
X. Ni et al. / Surface & Coatings Technology 198 (2005) 350–353
Acknowledgments This research work is supported by the Special Foundation of the Education Ministry of China for Overseas Returnees 2003 [406] and the state major Fundamental Research Project (973) [No. G20000683].
References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [2] S.N. Mohammed, H. Morkoc, Proc. IEEE 83 (1995) 1306.
353
[3] L. Liu, J.H. Edgar, Mater. Sci. Eng., R Rep. 37 (2002) 61. [4] A. Dadgar, J. Blasing, A. Diez, et al., Jpn. J. Appl. Phys. 39 (2000) L1183. [5] A. Strittmatter, A. Krost, V. Tqrck, et al., Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 59 (1999) 29. [6] P. Chen, S.Y. Xie, Z.Z. Chen, et al., J. Cryst. Growth 213 (2000) 27. [7] H.M. Liaw, R. Venugopal, J. Wan, et al., Solid-State Electron. 44 (2000) 685. [8] A. Dadgar, M. Poschenrieder, J. Blasing, et al., Appl. Phys. Lett. 80 (2002) 3670. [9] Z.Z. Ye, X.B. Jiang, J. Yuan, et al., Chin. J. Semicond. 17 (1996) 380. [10] S. Porowski, J. Cryst. Growth 166 (1996) 583. [11] S. Tripathy, S.J. Chua, P. Chen, et al., J. Appl. Phys. 92 (2002) 3503.