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Growth of GaN films by chloride vapour phase epitaxy
ARTICLE IN PRESS
Journal of Crystal Growth 268 (2004) 475–477
Growth of GaN films by chloride vapour phase epitaxy E. Varadarajan, J. Kumar, R. Dhanasekaran* Crystal Growth Centre, Anna University, Chennai 600 025, India
Abstract Chloride vapour phase epitaxial growth of GaN on (0 0 0 1) sapphire substrate has been investigated at various growth temperatures. The crystalline and optical qualities of the GaN have been improved significantly by increasing the growth temperature from 925 C to 990 C. For GaN films grown at 990 C, the FWHM for the (0 0 0 2) peak of the HRXRD curve have been determined. The GaN films show band-edge emission dominated PL at low temperature. From the f-scan studies, it is confirmed that the GaN films grown on (0 0 0 1) sapphire substrate are single crystalline. r 2004 Elsevier B.V. All rights reserved. PACS: 81.05 Ea; 81.10; 61.72Vv Keywords: A1. Photoluminescence; A1. X-ray diffraction; A3. Vapor phase epitaxy; B1. Gallium nitride
1. Introduction In the last few years, GaN has attracted a great interest owing to the possibility of producing optoelectronic devices in the blue/green spectral region as well as electronic devices working at very high temperature [1,2]. GaN films are mostly grown by MOCVD and MBE on sapphire substrate using buffer layers of AlN [3] and GaN [4]. The drawbacks lie in the expensive reactants, high gas consumption, ultra high vacuum conditions and large number of parameters that need to be adjusted to obtain epilayers. Another simple *Corresponding author. Tel.: +91-44-235-2774; fax: +9144-235-2774/235-2870. E-mail addresses: [email protected], [email protected] (R. Dhanasekaran).
approach consists of growing GaN is vapour phase epitaxy. VPE of GaN has been widely used because of the relatively high growth rate, near equilibrium process and cost effectiveness. In conventional VPE of GaN, the Ga source is gallium monochloride (GaCl) which is stable only at temperature above 600 C and is produced by the reaction of Ga and HCl gas. In this study, we have used gallium trichloride (GaCl3) as the Ga source, due to its high vapour pressure. Ammonia and nitrogen have been used as the nitrogen source and carrier gas, respectively. In this work, we have grown GaN films on sapphire substrate by reaction between GaCl3 and NH3 in a home-built chloride vapour phase (Cl-VPE) growth reactor. GaN films are grown at various growth temperatures and characterized by the determination of structural and optical properties.
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.075
ARTICLE IN PRESS 476
E. Varadarajan et al. / Journal of Crystal Growth 268 (2004) 475–477
2. Experiment 990°C
4k
The films were deposited on (0 0 0 1) sapphire substrate. GaN films have been grown as described in our previous report. The details of growth parameters are discussed elsewhere [5]. The growth of GaN films was carried out at various growth temperatures and at a fixed flow rates (2 slm) of the ammonia gas. High resolution X-ray diffraction (HRXRD) has been employed to investigate the structural properties of GaN films using Philips X’pert X-ray diffractometer. Photoluminescence (PL) spectra were recorded at low temperature (20 K) using He–Cd laser of 325 nm excitation wavelength with maximum input power of 15 mW to study the optical properties of GaN.
3. Results and discussions 3.1. HRXRD In order to minimize the number of experimental parameters, GaN films were grown at a fixed flow rate of ammonia (2 slm) and carrier gas (2 slm). X-ray diffraction was used to determine the crystallinity and orientation of the GaN films. Fig. 1 shows the X-ray diffraction pattern for GaN films on sapphire substrate. Only c- plane peaks are observed. This implies that GaN films and sapphire substrate basal planes are parallel to each other (GaN (0 0 0 1)/Al2O3 (0 0 0 1)). The peak intensities increase with increase of growth temperature. The FWHM of the GaN layers grown at 990 C is about 0.632 as shown in the insert of Fig. 1. The XRD peaks observed in this study are in good agreement with the values reported earlier [6]. Fig. 2 shows the f-scan of the 990 C grown GaN (2% 2 0 3) plane to determine the in-plane
Al2O3 (0006)
925°C 4k 990°C
Intensity (a. u.)
ð1Þ
950°C
3k
Intensity (arb. units)
GaCl3 þ NH3 ¼ GaN þ 3 HCl
GaN (0002)
2k
3k 950°C
2k 1k 925°C
0
1k
16.0
16.5
17.0
17.5
18.0
angle (θ)
0
16
17
18
19
20
21
22
Angle (θ)
Fig. 1. High resolution X-ray diffraction (HRXRD) pattern for GaN films on sapphire substrate grown at different temperature. 250
GaN ( 2 203) 2θ=91.1, ω=45.6183, ϕ=51.510 200
Intensity (arb. units)
We have employed a horizontal chloride vapour phase reactor. GaCl3 and NH3 have been used as Ga and N precursors, respectively. GaCl3 is transported by N2 as a carrier gas into the reactor. The overall reaction of the growth process is given by the following equation:
150
100
50
0 50
100
150
200
250
300
350
φ angle Fig. 2. High resolution X-ray diffraction (HRXRD) pattern of the f-scan of the GaN (2% 2 0 3) plane grown at 990 C.
orientation. Only one set of reflection peaks from (2% 2 0 3) plane has been observed with 60 spacing. This indicates that the GaN films grown on sapphire are single crystal.
ARTICLE IN PRESS E. Varadarajan et al. / Journal of Crystal Growth 268 (2004) 475–477
tures in the range 925–990 C. HRXRD spectra for GaN grown at 990 C exhibit a sharp peak of wurzite GaN (0 0 0 2) and the sapphire substrate. Low temperature PL spectrum for GaN grown at various temperature exhibit YL, BB and band edge emissions of GaN. It is concluded that higher growth temperature (>970 C) and longer growth time are very important to obtain good quality crystalline GaN films.
Band-edge
4.0x10-5
990°C 975°C
Intensity (arb. units)
477
925°C
2.0x10-5
YL BB
Acknowledgements
0.0 2.2
2.4
2.6
2.8
3.0
3.2
3.4
Energy (eV)
Fig. 3. Low temperature (20 K) photoluminescence spectrum of GaN/Al2O3 grown at different temperature.
3.2. Photoluminescence (PL) PL spectra for GaN grown at 925 C and 950 C exhibit deep level yellow line (YL) blue band (BB) and band edge (BE) emissions at peak energies of 2.42, 3.17 and 3.45 eV, respectively. For the films grown at 990 C, however, band edge emission is sharp, whereas the deep level yellow emission was greatly suppressed as shown in the insert of Fig. 3. This means that the optical properties of the GaN films greatly depend on the growth temperature. The FWHM of the GaN layers grown at 990 C is about 60 meV. The values observed in this study are in good agreement with the values reported earlier [7,8]
4. Conclusion Wurtzite GaN films have been grown on sapphire substrate at different growth tempera-
Department of Science and Technology, Government of India is acknowledged for financial support to carryout this work. One of the author (E.Varadarajan) is grateful to CSIR, Government of India for the award of Senior Research Fellowship. The authors thank Prof. B. M. Arora, Tata Institute of Fundamental Research, Mumbai for helping to obtain the HRXRD spectra and PL measurements.
References [1] R. Fornari, M. Bosi, N. Armani, G. Attolini, C. Ferrari, C. Pelosi, G. Salviati, Mater. Sci. Eng. B 79 (2001) 159. [2] S.C. Jain, M. Wilander, J. Narayan, R. Van Overstraeten, J. Appl. Phys. 87 (2000) 965. [3] H. Morkoc, A. Salvator, B. Sverdlov, J. Crystal Growth 150 (1995) 887. [4] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [5] E. Varadarajan, P. Puviarasu, J. Kumar, R. Dhanasekaran, J. Crystal Growth 260 (2004) 43. [6] H. Lee, J.S. Harris, J. Crystal Growth 169 (1996) 689. [7] M. Shibata, T. Furuya, H. Sakaguchi, S. Kuma, J. Crystal Growth 196 (1999) 47. [8] H. Morkoc, Mater. Sci. Eng. R 33 (2001) 135.
Journal of Crystal Growth 268 (2004) 475–477
Growth of GaN films by chloride vapour phase epitaxy E. Varadarajan, J. Kumar, R. Dhanasekaran* Crystal Growth Centre, Anna University, Chennai 600 025, India
Abstract Chloride vapour phase epitaxial growth of GaN on (0 0 0 1) sapphire substrate has been investigated at various growth temperatures. The crystalline and optical qualities of the GaN have been improved significantly by increasing the growth temperature from 925 C to 990 C. For GaN films grown at 990 C, the FWHM for the (0 0 0 2) peak of the HRXRD curve have been determined. The GaN films show band-edge emission dominated PL at low temperature. From the f-scan studies, it is confirmed that the GaN films grown on (0 0 0 1) sapphire substrate are single crystalline. r 2004 Elsevier B.V. All rights reserved. PACS: 81.05 Ea; 81.10; 61.72Vv Keywords: A1. Photoluminescence; A1. X-ray diffraction; A3. Vapor phase epitaxy; B1. Gallium nitride
1. Introduction In the last few years, GaN has attracted a great interest owing to the possibility of producing optoelectronic devices in the blue/green spectral region as well as electronic devices working at very high temperature [1,2]. GaN films are mostly grown by MOCVD and MBE on sapphire substrate using buffer layers of AlN [3] and GaN [4]. The drawbacks lie in the expensive reactants, high gas consumption, ultra high vacuum conditions and large number of parameters that need to be adjusted to obtain epilayers. Another simple *Corresponding author. Tel.: +91-44-235-2774; fax: +9144-235-2774/235-2870. E-mail addresses: [email protected], [email protected] (R. Dhanasekaran).
approach consists of growing GaN is vapour phase epitaxy. VPE of GaN has been widely used because of the relatively high growth rate, near equilibrium process and cost effectiveness. In conventional VPE of GaN, the Ga source is gallium monochloride (GaCl) which is stable only at temperature above 600 C and is produced by the reaction of Ga and HCl gas. In this study, we have used gallium trichloride (GaCl3) as the Ga source, due to its high vapour pressure. Ammonia and nitrogen have been used as the nitrogen source and carrier gas, respectively. In this work, we have grown GaN films on sapphire substrate by reaction between GaCl3 and NH3 in a home-built chloride vapour phase (Cl-VPE) growth reactor. GaN films are grown at various growth temperatures and characterized by the determination of structural and optical properties.
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.04.075
ARTICLE IN PRESS 476
E. Varadarajan et al. / Journal of Crystal Growth 268 (2004) 475–477
2. Experiment 990°C
4k
The films were deposited on (0 0 0 1) sapphire substrate. GaN films have been grown as described in our previous report. The details of growth parameters are discussed elsewhere [5]. The growth of GaN films was carried out at various growth temperatures and at a fixed flow rates (2 slm) of the ammonia gas. High resolution X-ray diffraction (HRXRD) has been employed to investigate the structural properties of GaN films using Philips X’pert X-ray diffractometer. Photoluminescence (PL) spectra were recorded at low temperature (20 K) using He–Cd laser of 325 nm excitation wavelength with maximum input power of 15 mW to study the optical properties of GaN.
3. Results and discussions 3.1. HRXRD In order to minimize the number of experimental parameters, GaN films were grown at a fixed flow rate of ammonia (2 slm) and carrier gas (2 slm). X-ray diffraction was used to determine the crystallinity and orientation of the GaN films. Fig. 1 shows the X-ray diffraction pattern for GaN films on sapphire substrate. Only c- plane peaks are observed. This implies that GaN films and sapphire substrate basal planes are parallel to each other (GaN (0 0 0 1)/Al2O3 (0 0 0 1)). The peak intensities increase with increase of growth temperature. The FWHM of the GaN layers grown at 990 C is about 0.632 as shown in the insert of Fig. 1. The XRD peaks observed in this study are in good agreement with the values reported earlier [6]. Fig. 2 shows the f-scan of the 990 C grown GaN (2% 2 0 3) plane to determine the in-plane
Al2O3 (0006)
925°C 4k 990°C
Intensity (a. u.)
ð1Þ
950°C
3k
Intensity (arb. units)
GaCl3 þ NH3 ¼ GaN þ 3 HCl
GaN (0002)
2k
3k 950°C
2k 1k 925°C
0
1k
16.0
16.5
17.0
17.5
18.0
angle (θ)
0
16
17
18
19
20
21
22
Angle (θ)
Fig. 1. High resolution X-ray diffraction (HRXRD) pattern for GaN films on sapphire substrate grown at different temperature. 250
GaN ( 2 203) 2θ=91.1, ω=45.6183, ϕ=51.510 200
Intensity (arb. units)
We have employed a horizontal chloride vapour phase reactor. GaCl3 and NH3 have been used as Ga and N precursors, respectively. GaCl3 is transported by N2 as a carrier gas into the reactor. The overall reaction of the growth process is given by the following equation:
150
100
50
0 50
100
150
200
250
300
350
φ angle Fig. 2. High resolution X-ray diffraction (HRXRD) pattern of the f-scan of the GaN (2% 2 0 3) plane grown at 990 C.
orientation. Only one set of reflection peaks from (2% 2 0 3) plane has been observed with 60 spacing. This indicates that the GaN films grown on sapphire are single crystal.
ARTICLE IN PRESS E. Varadarajan et al. / Journal of Crystal Growth 268 (2004) 475–477
tures in the range 925–990 C. HRXRD spectra for GaN grown at 990 C exhibit a sharp peak of wurzite GaN (0 0 0 2) and the sapphire substrate. Low temperature PL spectrum for GaN grown at various temperature exhibit YL, BB and band edge emissions of GaN. It is concluded that higher growth temperature (>970 C) and longer growth time are very important to obtain good quality crystalline GaN films.
Band-edge
4.0x10-5
990°C 975°C
Intensity (arb. units)
477
925°C
2.0x10-5
YL BB
Acknowledgements
0.0 2.2
2.4
2.6
2.8
3.0
3.2
3.4
Energy (eV)
Fig. 3. Low temperature (20 K) photoluminescence spectrum of GaN/Al2O3 grown at different temperature.
3.2. Photoluminescence (PL) PL spectra for GaN grown at 925 C and 950 C exhibit deep level yellow line (YL) blue band (BB) and band edge (BE) emissions at peak energies of 2.42, 3.17 and 3.45 eV, respectively. For the films grown at 990 C, however, band edge emission is sharp, whereas the deep level yellow emission was greatly suppressed as shown in the insert of Fig. 3. This means that the optical properties of the GaN films greatly depend on the growth temperature. The FWHM of the GaN layers grown at 990 C is about 60 meV. The values observed in this study are in good agreement with the values reported earlier [7,8]
4. Conclusion Wurtzite GaN films have been grown on sapphire substrate at different growth tempera-
Department of Science and Technology, Government of India is acknowledged for financial support to carryout this work. One of the author (E.Varadarajan) is grateful to CSIR, Government of India for the award of Senior Research Fellowship. The authors thank Prof. B. M. Arora, Tata Institute of Fundamental Research, Mumbai for helping to obtain the HRXRD spectra and PL measurements.
References [1] R. Fornari, M. Bosi, N. Armani, G. Attolini, C. Ferrari, C. Pelosi, G. Salviati, Mater. Sci. Eng. B 79 (2001) 159. [2] S.C. Jain, M. Wilander, J. Narayan, R. Van Overstraeten, J. Appl. Phys. 87 (2000) 965. [3] H. Morkoc, A. Salvator, B. Sverdlov, J. Crystal Growth 150 (1995) 887. [4] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [5] E. Varadarajan, P. Puviarasu, J. Kumar, R. Dhanasekaran, J. Crystal Growth 260 (2004) 43. [6] H. Lee, J.S. Harris, J. Crystal Growth 169 (1996) 689. [7] M. Shibata, T. Furuya, H. Sakaguchi, S. Kuma, J. Crystal Growth 196 (1999) 47. [8] H. Morkoc, Mater. Sci. Eng. R 33 (2001) 135.