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MOVPE growth of thick homogeneous InGaN directly on sapphire substrate using AlN buffer layer
Pergamon
Solid-State Electronics Vol. 41, No. 2, pp. 145-147, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved P I I : S0038-1101(96)00155-4 0038-1101/97 $17.00 + 0.00
MOVPE GROWTH OF THICK HOMOGENEOUS InGaN DIRECTLY ON SAPPHIRE SUBSTRATE USING AIN BUFFER LAYER MASAYA SHIMIZU, YASUTOSHI KAWAGUCHI, KAZUMASA HIRAMATSU and NOBUHIKO SAWAKI Department of Electronics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Abstract--InGaN films have usually been grown on GaN epitaxial layers but it is difficult to grow the thick InGaN films on GaN because of strong stress induced by large lattice mismatch between InGaN and GaN. The thick InGaN films grown on GaN have poor surface morphologies. In this paper it is described that the growth of thick InGaN films of a few microns thickness has successfullybeen achieved directly on sapphire substrates using AIN buffer layers. On AIN buffer layers homogeneous InGaN with 2.5 pm thickness and large indium mole fraction of 0.2 could be grown. The growth rate of 2.5 #m/h is much larger than has been reported previously. © 1997 Elsevier Science Ltd
l. INTRODUCTION
2. EXPERIMENTAL
The materials used for optical devices in blue or near UV regions are required to have direct band structures with more than 2.5eV energy gaps. A1GalnN, III-V nitride compound semiconductors, proposed by Matsuoka et al.[l], having direct energy gaps from 1.9 to 6.2eV with group III composition, are promising materials for short wavelength light-emitting devices. Especially, InGaN attracts the attention as the important material for the visible ray region. In recent years, great progress has been made in crystal growth of nitride semiconductors, especially rapid advances in InGaN growth. The equilibrium vapor pressure of N2 over InN is much higher than that over GaN[1,2], and thus InGaN should be grown at lower temperatures (~500°C) than GaN ( ~ 1000°C)[3]. Yoshimoto et al. reported the growth of InGaN at comparably high temperatures (800°C)[4], but the films grown did not have a high enough quality to be applied to devices. Nakamura et al. reported that high quality InGaN could be obtained on GaN films[5], and fabricated blue LEDs using InGaN as an active layer of a doublehetero (DH) structure[6]. It is difficult to grow high quality thick InGaN films. The InGaN films reported up to now have only been a few hundred nanometers in thickness and very slow growth rate should be required to improve film quality of thin InGaN[5,7,8]. It is necessary to obtain thick InGaN films for the study of InGaN properties. In this paper we describe the growth of thick InGaN films directly on sapphire substrate using AIN buffer layer.
lnGaN films were grown using sapphire ((0001) ct-AI203) substrates by MOVPE at atmospheric pressure in two kinds of methods: (1) growth on GaN films and (2) growth directly on sapphire substrates using AIN buffer layer. Trimethylgallium (TMG), trimethylindium (TMI), trimethylaluminium (TMA) and ammonia (NH3) were used as Ga, In, AI and N sources, respectively. In the case of (1) growth, prior to InGaN growth, the sapphire substrate was heated to 1150°C for 10 min thermal cleaning in a stream of H2. A thin AIN buffer layer of 50 nm thickness was deposited on the sapphire substrate at 600°C using TMA and NH3. The substrate temperature was raised to 1050°C and about 2 #m GaN film was grown. InGaN was grown on the GaN film at 800°C. In case of (2) growth, an AIN buffer layer was deposited on sapphire substrate at 600°C following 10 min thermal treatment in H2 flow at 1150°C. The substrate was heated for 5 s at 1050°C in a stream of NH3. InGaN growth was performed at 800°C. During the growth of InGaN on both GaN and A1N buffer layers, the flow rates of TMG, TMI and NH3 were 7.3 #mol/min, 18.7#mol/min and 4 1/min, respectively. In order to investigate the quality of grown InGaN, photoluminescence (PL) measurements were performed at room temperature (RT). 3. RESULTS AND DISCUSSION
Figure 1 shows cross-sectional SEM images of InGaN on (0001) GaN epitaxial films and A1N buffer layers. Growth time was changed from 5 to 60 min.
145
146
M. Shimizu et al.
N ,hire
IN
)hire Fig. 1. Cross-sectional SEM images of lnGaN on GaN films (a~t) and AIN buffer layers (e h). lnGaN layers were grown for (a) and (e) 5 rain, (b) and (f) 10 rain, (c) and (g) 15 min and (d) and (h) 60 min. Markers represent 1 ILm.
On G a N smooth thin l n G a N films were obtained in the initial growth stages (Figs l(a) and (b)). However the surfaces began to be in rough at about 15rain (Fig. l(c)). In Fig. l(d) two I n G a N layers could be observed in spite of continuous growth under the constant growth condition. The lower is homogeneous but the upper, including large grains,
•
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laGaN/C_mN/Al~qsapltldm
•
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RT
~-~'~_~~
(a)
0,)
rough. On the other hand I n G a N films on AIN buffer layers were slightly rough in the initial growth stages (Fig. l(e)). However films became smoother with increasing growth time (Figs l(f), (g)) and the smooth I n G a N film with about 2.5/~m thickness could be obtained after 6 0 m i n growth (Fig. l(h)). The growth rate of I n G a N was 2.5/~m/h, which is much larger than previously reported. Figure 2 shows PL spectra at RT. An He-Cd laser was used as an excitation source. The peak positions of PL spectra from I n G a N on GaN were at about 386 nm in the initial growth stages and shifted to longer wavelength with the growth time. The spectra were sharp in the initial stages and got broader with the growth time. In contrast with I n G a N on G a N the PL spectra from I n G a N on A1N buffer layers had constant peak positions at about 420 nm and became narrower with the growth time.
InC,~JdNt~tda~ a.
~
(e)
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i
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!
,
|
.
i
.
i
.
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.
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.
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,
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Wavelength (nm) Fig. 2. PL spectra at RT. Notations correspond to those in Fig. 1.
• 0
InOaNIAIN/sapphire I n G s N / G m N / A I N / s a p p
hirt
O,O
Growllh Time (mln) Fig. 3. Indium mole fractions in InGaN layers decided from peak energies of PL spectra in Fig. 2.
MOVPE growth of InGaN
InGaNIGaNIAIN/Sapphir¢ InGaN (x,,.o.2)
(x-,o.oT)
GIN AIN - Sapphin C-face
InGaNIAINISapphir¢ InC~IN (X-0.2)
AIN Sapphire C-face
I
-i~ig. 4. Schematic images of InGaN growth on GaN films and AIN buffer layers.
147
reported in LPE growth of InGaAs on InP and InGaP on GaAs[9]. Over the thickness of about 0.5/tm, rough surfaces due to large grains were generated in order to relax the large stress and InGaN with a large indium mole fraction could be grown. In the case of InGaN growth on A1N buffer layers, the stress between InGaN and sapphire might be relaxed near the interface between InGaN films and A1N buffer layers, hence smooth thick homogeneous InGaN films with a large indium mole fraction of 0.2 could be grown. The stress reduction mechanism on InGaN on A1N buffer layer is similar to that of GaN on A1N buffer layer[10]. 4. CONCLUSIONS
In the growth of InGaN on GaN, films were smooth only in the initial growth stage and film surfaces became rough because of large grains generated over 0.5 mm thickness. Smooth homogeneous InGaN film with 2.5/~m thickness could be obtained by the growth directly on sapphire substrate using AIN buffer layer. The growth rate of 2.5 ym/h was larger than reported previously. REFERENCES
Figure 3 shows indium mole fraction in InGaN films decided from peak wavelength of PL spectra in Fig. 2. Indium mole fraction in InGaN on GaN was about 0.07 in the initial growth stages and raised rapidly to 0.2. On the other hand indium mole fraction in InGaN on AIN buffer was out of relation of the growth time and was constant at about 0.2. This indicates that InGaN on AIN were homogeneous films. The schematic images of InGaN growth on GaN and AIN buffer layers are shown in Fig. 4. In the case of InGaN growth on GaN, smooth thin films, containing large stress due to the lattice mismatch between InGaN and GaN, should be grown in the initial growth stage and the stress might cause a small indium mole fraction of 0.07. The change in indium mole fraction caused a reduction in the large stress in the alloy films, which is known as the "substrate-pulling effect". This effect was
SSE 41/I--B
1. T. Matsuoka, H. Tanaka, T. Sasaki and A. Katsui, in GaAs and Related Compounds 1989, Inst Phys. Conf. Ser. 106, (Edited by T. Ikoma and H. Watanabe), p. 141. Institute of Physics, Bristol (1990). 2. T. Matsuoka, J. Cryst. Growth 124, 433 (1992). 3. T. Nagatomo, T. Kuboyama, H. Minamino and O. Omoto, Japan. J. Appl. Phys. 32, L1334 (1989). 4. N. Yoshimoto, T. Matsuoka, T. Sasaki and A. Katsui, Appl. Phys. Lett. 59, 2251 (1991). 5. S. Nakamura and T. Mukai, Japan. J. Appl. Phys. 31, L1457 (1992). 6. S. Nakamura, M. Senoh and Y. Mukai, Japan. J. Appl. Phys. 32, L8 (1993). 7. M. Asif Khan, Skrishnankutty, R. A. Skogman, J. N. Kuznia and D. T. Olson, Appl. Phys. Lett. 65, 520 (1994). 8. H. Amano, T. Tanaka, Y. Kunii, K. Kato, S. T. Kim and I. Akasaki, Appl. Phys. Lett. 64, 1377 (1994). 9. J. Ohta, M. Ishikawa, R. Ito and N. Ogasawara, Japan. J. Appl. Phys. 22, L136 (1983). 10. H. Amano, N. Sawaki, I. Akasaki and Y. Toyoda, Appl. Phys. Lett. 48, 353 (1986).
Solid-State Electronics Vol. 41, No. 2, pp. 145-147, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved P I I : S0038-1101(96)00155-4 0038-1101/97 $17.00 + 0.00
MOVPE GROWTH OF THICK HOMOGENEOUS InGaN DIRECTLY ON SAPPHIRE SUBSTRATE USING AIN BUFFER LAYER MASAYA SHIMIZU, YASUTOSHI KAWAGUCHI, KAZUMASA HIRAMATSU and NOBUHIKO SAWAKI Department of Electronics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan Abstract--InGaN films have usually been grown on GaN epitaxial layers but it is difficult to grow the thick InGaN films on GaN because of strong stress induced by large lattice mismatch between InGaN and GaN. The thick InGaN films grown on GaN have poor surface morphologies. In this paper it is described that the growth of thick InGaN films of a few microns thickness has successfullybeen achieved directly on sapphire substrates using AIN buffer layers. On AIN buffer layers homogeneous InGaN with 2.5 pm thickness and large indium mole fraction of 0.2 could be grown. The growth rate of 2.5 #m/h is much larger than has been reported previously. © 1997 Elsevier Science Ltd
l. INTRODUCTION
2. EXPERIMENTAL
The materials used for optical devices in blue or near UV regions are required to have direct band structures with more than 2.5eV energy gaps. A1GalnN, III-V nitride compound semiconductors, proposed by Matsuoka et al.[l], having direct energy gaps from 1.9 to 6.2eV with group III composition, are promising materials for short wavelength light-emitting devices. Especially, InGaN attracts the attention as the important material for the visible ray region. In recent years, great progress has been made in crystal growth of nitride semiconductors, especially rapid advances in InGaN growth. The equilibrium vapor pressure of N2 over InN is much higher than that over GaN[1,2], and thus InGaN should be grown at lower temperatures (~500°C) than GaN ( ~ 1000°C)[3]. Yoshimoto et al. reported the growth of InGaN at comparably high temperatures (800°C)[4], but the films grown did not have a high enough quality to be applied to devices. Nakamura et al. reported that high quality InGaN could be obtained on GaN films[5], and fabricated blue LEDs using InGaN as an active layer of a doublehetero (DH) structure[6]. It is difficult to grow high quality thick InGaN films. The InGaN films reported up to now have only been a few hundred nanometers in thickness and very slow growth rate should be required to improve film quality of thin InGaN[5,7,8]. It is necessary to obtain thick InGaN films for the study of InGaN properties. In this paper we describe the growth of thick InGaN films directly on sapphire substrate using AIN buffer layer.
lnGaN films were grown using sapphire ((0001) ct-AI203) substrates by MOVPE at atmospheric pressure in two kinds of methods: (1) growth on GaN films and (2) growth directly on sapphire substrates using AIN buffer layer. Trimethylgallium (TMG), trimethylindium (TMI), trimethylaluminium (TMA) and ammonia (NH3) were used as Ga, In, AI and N sources, respectively. In the case of (1) growth, prior to InGaN growth, the sapphire substrate was heated to 1150°C for 10 min thermal cleaning in a stream of H2. A thin AIN buffer layer of 50 nm thickness was deposited on the sapphire substrate at 600°C using TMA and NH3. The substrate temperature was raised to 1050°C and about 2 #m GaN film was grown. InGaN was grown on the GaN film at 800°C. In case of (2) growth, an AIN buffer layer was deposited on sapphire substrate at 600°C following 10 min thermal treatment in H2 flow at 1150°C. The substrate was heated for 5 s at 1050°C in a stream of NH3. InGaN growth was performed at 800°C. During the growth of InGaN on both GaN and A1N buffer layers, the flow rates of TMG, TMI and NH3 were 7.3 #mol/min, 18.7#mol/min and 4 1/min, respectively. In order to investigate the quality of grown InGaN, photoluminescence (PL) measurements were performed at room temperature (RT). 3. RESULTS AND DISCUSSION
Figure 1 shows cross-sectional SEM images of InGaN on (0001) GaN epitaxial films and A1N buffer layers. Growth time was changed from 5 to 60 min.
145
146
M. Shimizu et al.
N ,hire
IN
)hire Fig. 1. Cross-sectional SEM images of lnGaN on GaN films (a~t) and AIN buffer layers (e h). lnGaN layers were grown for (a) and (e) 5 rain, (b) and (f) 10 rain, (c) and (g) 15 min and (d) and (h) 60 min. Markers represent 1 ILm.
On G a N smooth thin l n G a N films were obtained in the initial growth stages (Figs l(a) and (b)). However the surfaces began to be in rough at about 15rain (Fig. l(c)). In Fig. l(d) two I n G a N layers could be observed in spite of continuous growth under the constant growth condition. The lower is homogeneous but the upper, including large grains,
•
r
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•
,
-
i
-
i
-
,
•
i
•
|
•
!
laGaN/C_mN/Al~qsapltldm
•
!
RT
~-~'~_~~
(a)
0,)
rough. On the other hand I n G a N films on AIN buffer layers were slightly rough in the initial growth stages (Fig. l(e)). However films became smoother with increasing growth time (Figs l(f), (g)) and the smooth I n G a N film with about 2.5/~m thickness could be obtained after 6 0 m i n growth (Fig. l(h)). The growth rate of I n G a N was 2.5/~m/h, which is much larger than previously reported. Figure 2 shows PL spectra at RT. An He-Cd laser was used as an excitation source. The peak positions of PL spectra from I n G a N on GaN were at about 386 nm in the initial growth stages and shifted to longer wavelength with the growth time. The spectra were sharp in the initial stages and got broader with the growth time. In contrast with I n G a N on G a N the PL spectra from I n G a N on A1N buffer layers had constant peak positions at about 420 nm and became narrower with the growth time.
InC,~JdNt~tda~ a.
~
(e)
i •
!
,
i
,
!
,
|
.
i
.
i
.
i
.
i
.
!
,
i
Wavelength (nm) Fig. 2. PL spectra at RT. Notations correspond to those in Fig. 1.
• 0
InOaNIAIN/sapphire I n G s N / G m N / A I N / s a p p
hirt
O,O
Growllh Time (mln) Fig. 3. Indium mole fractions in InGaN layers decided from peak energies of PL spectra in Fig. 2.
MOVPE growth of InGaN
InGaNIGaNIAIN/Sapphir¢ InGaN (x,,.o.2)
(x-,o.oT)
GIN AIN - Sapphin C-face
InGaNIAINISapphir¢ InC~IN (X-0.2)
AIN Sapphire C-face
I
-i~ig. 4. Schematic images of InGaN growth on GaN films and AIN buffer layers.
147
reported in LPE growth of InGaAs on InP and InGaP on GaAs[9]. Over the thickness of about 0.5/tm, rough surfaces due to large grains were generated in order to relax the large stress and InGaN with a large indium mole fraction could be grown. In the case of InGaN growth on A1N buffer layers, the stress between InGaN and sapphire might be relaxed near the interface between InGaN films and A1N buffer layers, hence smooth thick homogeneous InGaN films with a large indium mole fraction of 0.2 could be grown. The stress reduction mechanism on InGaN on A1N buffer layer is similar to that of GaN on A1N buffer layer[10]. 4. CONCLUSIONS
In the growth of InGaN on GaN, films were smooth only in the initial growth stage and film surfaces became rough because of large grains generated over 0.5 mm thickness. Smooth homogeneous InGaN film with 2.5/~m thickness could be obtained by the growth directly on sapphire substrate using AIN buffer layer. The growth rate of 2.5 ym/h was larger than reported previously. REFERENCES
Figure 3 shows indium mole fraction in InGaN films decided from peak wavelength of PL spectra in Fig. 2. Indium mole fraction in InGaN on GaN was about 0.07 in the initial growth stages and raised rapidly to 0.2. On the other hand indium mole fraction in InGaN on AIN buffer was out of relation of the growth time and was constant at about 0.2. This indicates that InGaN on AIN were homogeneous films. The schematic images of InGaN growth on GaN and AIN buffer layers are shown in Fig. 4. In the case of InGaN growth on GaN, smooth thin films, containing large stress due to the lattice mismatch between InGaN and GaN, should be grown in the initial growth stage and the stress might cause a small indium mole fraction of 0.07. The change in indium mole fraction caused a reduction in the large stress in the alloy films, which is known as the "substrate-pulling effect". This effect was
SSE 41/I--B
1. T. Matsuoka, H. Tanaka, T. Sasaki and A. Katsui, in GaAs and Related Compounds 1989, Inst Phys. Conf. Ser. 106, (Edited by T. Ikoma and H. Watanabe), p. 141. Institute of Physics, Bristol (1990). 2. T. Matsuoka, J. Cryst. Growth 124, 433 (1992). 3. T. Nagatomo, T. Kuboyama, H. Minamino and O. Omoto, Japan. J. Appl. Phys. 32, L1334 (1989). 4. N. Yoshimoto, T. Matsuoka, T. Sasaki and A. Katsui, Appl. Phys. Lett. 59, 2251 (1991). 5. S. Nakamura and T. Mukai, Japan. J. Appl. Phys. 31, L1457 (1992). 6. S. Nakamura, M. Senoh and Y. Mukai, Japan. J. Appl. Phys. 32, L8 (1993). 7. M. Asif Khan, Skrishnankutty, R. A. Skogman, J. N. Kuznia and D. T. Olson, Appl. Phys. Lett. 65, 520 (1994). 8. H. Amano, T. Tanaka, Y. Kunii, K. Kato, S. T. Kim and I. Akasaki, Appl. Phys. Lett. 64, 1377 (1994). 9. J. Ohta, M. Ishikawa, R. Ito and N. Ogasawara, Japan. J. Appl. Phys. 22, L136 (1983). 10. H. Amano, N. Sawaki, I. Akasaki and Y. Toyoda, Appl. Phys. Lett. 48, 353 (1986).