- Email: [email protected]
AlGaN distributed Bragg reflectors
Journal of Crystal Growth 237–239 (2002) 961–967
MOCVD growth of high reflective GaN/AlGaN distributed Bragg reflectors Naoyuki Nakadaa,*, Hiroyasu Ishikawab, Takashi Egawab, Takashi Jimboc, Masayoshi Umenoa,b a
Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Research Center for Micro-Structure Devices, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan c Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Abstract GaN/AlGaN distributed Bragg reflectors have been grown by low-pressure metal organic chemical vapor deposition. Those structures were fabricated on the atmospheric pressure grown GaN layer on sapphire substrate. The aluminum content in low-pressure grown AlGaN layers was estimated to be 0.60 by X-ray diffraction. The GaN layers grown under the low-pressure condition in GaN/Al0.60Ga0.40N multilayer were compressively strained. The flat surfaces without cracks were successfully obtained for the growth of GaN/Al0.60Ga0.40N distributed Bragg reflector. For the 45.5 pairs, a peak reflectivity of over 98% was obtained at a wavelength of 421 nm. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Cr; 42.55.Sa; 42.55.Px Keywords: A3. Low pressure metal organic chemical vapor deposition; A3. Metal organic chemical vapor deposition; B3. Vertical cavity surface emitting lasers
1. Introduction GaN, AlN, InN and their alloys have been highly popular as optical devices in the blueultraviolet region and have created much interest for high-temperature, high-power electric devices at microwave frequencies. GaN-based high brightness light emitting diodes (LEDs) and edge *Corresponding author. Tel.: +81-52-735-5540; fax: +8152-735-5546. E-mail address: [email protected] (N. Nakada).
emitting laser diodes (LDs) have demonstrated continuous-wave operation at room temperature [1]. Recently, GaN-based vertical cavity surface emitting lasers (VCSELs) with a distributed Bragg reflector (DBR) have been studied in many research groups because of their many advantages for various applications such as full color display, photolithography, super high-density optical memory and bright white light source [2]. While the fabrication of smooth edge mirror is difficult for edge emitting lasers, VCSELs can get the smooth mirror without a cleavage technique. However, it is necessary to fabricate high reflective
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 2 2 - X
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mirrors on both sides of the active region to form their short cavity length. In this material system, GaN/AlGaN multilayers are usually used as high reflective mirrors on the bottom side [3]. To realize a high reflective DBR, the contrast of the refractive index between two materials of the multilayers and the flat surface of each layer are important [4]. Because the difference of refractive index between GaAs and AlAs, so many periods of GaN/AlGaN layered structures are required to obtain a high reflectivity. When making such numbers of periods, many cracks are grown and the surface flatness is spoiled. More recently, optical pumping VCSEL, which performed the cavity by GaN/AlGaN DBR for the bottom side mirror and ZrO2/SiO2 dielectric mirror for the top, was reported at room temperature [5], and many authors reported the GaN/AlGaN based reflectors [4,6–11]. We have obtained improved characteristics of InGaN MQW LED on sapphire by use of the 15 pairs of GaN/AlGaN DBR [12]. In this letter, we reported the high reflective GaN/Al0.60Ga0.40N DBRs with crack-free surface fabricated under the low-pressure condition.
2. Growth of GaN/AlGaN DBRs A metal organic chemical vapor deposition (MOCVD) equipment with horizontal quartz reactor (Nippon Sanso, SR-2000) was employed for the growth of GaN/AlGaN DBRs. (0 0 0 1)oriented, 2-in diameter sapphire substrates were used for the growth of samples. Trimethylgallium (TMGa) and trimethylaluminum (TMAl) were used as group III and ammonia (NH3) as group V source materials, respectively. The surface morphology of the DBRs was studied with an optical microscope and the atomic force microscopy (AFM) operated in contacting mode. The reflectivity of the DBRs was measured by an ultraviolet–visible spectrometer as a function of wavelength. The aluminum content in the AlGaN layer was estimated by X-ray diffraction (XRD) measurement using a Philips X’Pert MRD system and electron probe micro analysis (EPMA). Reciprocal space maps (RSMs) of XRD intensity
were performed on a four-circle goniometer around GaN ð1 0 1% 4Þ Bragg peak as the asymmetrical diffraction spot. A thermal cleaning process was carried out at 11801C for 10 min in a stream of hydrogen ambient before the growth of nitride layers. After depositing a 30-nm-thick GaN nucleation layer at 5001C, the substrate was heated up to 11301C, then a GaN layer was grown. The flow rates of TMGa were 36 and 72 mmol/min for the growth of GaN nucleation layer and GaN layer, respectively. Then a 20-nm-thick Al0.15Ga0.85N layer was grown. The flow rates of TMGa and TMAl were 24 and 6.0 mmol/min for the growth of AlGaN layer, respectively. During the growth, the flow rate of NH3 was maintained at 5 l/min. After growth of an AlGaN layer, we changed the growth condition by changing the pressure inside the reactor. It was vacuumed down to a pressure of 100 Torr by dry pump, keeping the temperature of the substrate at 11301C. After stability of the pressure at 100 Torr, GaN/AlGaN multilayers were grown at 11301C. The flow rate of TMGa was 72 mmol/min for the growth of GaN layer which constructed the multilayer structure and those of TMGa and TMAl were 18 and 18 mmol/min for the AlGaN layer, respectively. During the growth of the multilayer structure, the flow rate of NH3 was maintained at 2.5 l/min. The growth rates of GaN were 3.1 mm/h for atmospheric-pressure growth and 3.3 mm/h for low–pressure, respectively, and that of AlGaN was 1.27 mm/h for low–pressure growth. In this structure, we introduced the 20nm-thick Al0.15Ga0.85N as the cap layer to protect the growth layers, because it needed approximately 2.5 min of waiting time to stabilize the pressure at 100 Torr. If this cap layer was not introduced, the surface flatness would be inferior due to the thermal decomposition during the waiting time and we could not have obtained a high reflectivity.
3. Results and discussion We prepared four samples and changed the thickness of GaN layer grown under the atmo-
N. Nakada et al. / Journal of Crystal Growth 237–239 (2002) 961–967
70 10 10
50
Counts [cps]
Reflectivity [%]
60
40 30 20 10 0 0
2000
4000
6000
Thickness of GaN [nm] Fig. 1. Maximum reflectivity of GaN/AlGaN multilayers as a function of the thickness of GaN, which was grown at atmospheric–pressure before the growth of GaN/AlGaN multi-layers at low-pressure condition. Triangle markers indicate the same position on the 2-in sapphire substrate. Longitudinal bars show the distributions of the reflectivity, which was measured at nine areas taken across the 2-in sapphire substrate.
spheric pressure condition (AP-GaN) to 50, 500, 1500 and 6000 nm, respectively. Fig. 1 shows the maximum reflectivity of multilayers as a function of thickness of the AP-GaN layer. The measurement was carried out on the nine areas with a diameter of 7 mm for each sample, taken across the 2-in substrate to study the distribution. In the case of 50 nm, the reflectivity was low and had a large distribution. It was caused by cracks, but the peak wavelength also had a large distribution. Moreover, as the thickness of the AP-GaN layer became thicker, it also became lower in reflectivity. In the case of 500 nm, the reflectivity is the highest and the distribution is the smallest. From this result, we used the 500-nm-thick APGaN layer for the fabrication of DBRs. Then 30.5 pairs of GaN/AlGaN DBR which were designed at a peak wavelength of 420 nm were grown. Fig. 2(a) shows the XRD spectra from symmetrical (0 0 0 4) Bragg peak of GaN/ AlGaN DBR. For comparison, it was also measured on a 700-nm-thick AlGaN grown on
10 10
4
963
(a)
3 2
GaN
Al0.60Ga0.40 N
1
Al0.60Ga0.40 N
GaN 5
104 103 102 101 10
(b)
-2000
0
2000
4000
Diffraction angle [arcsec] Fig. 2. XRD curve from symmetrical (0 0 0 4) Bragg peak of AlGaN/GaN grown on under low-pressure condition on sapphire substrate: (a) 30.5 pairs of GaN/AlGaN DBR and (b) 700-nm thick AlGaN grown on GaN. The arrows indicate the position of AP-GaN and Al0.60Ga0.40N, respectively.
AP-GaN under the same growth condition (Fig. 2(b)). As shown in Fig. 2(b), the separation of the peak angles between GaN and AlGaN was 3750 arcsec, and the aluminum content x in the AlxGa1xN layer was estimated at 0.60. In Fig. 2(a), there are some satellite peaks and the highest diffraction intensity was shown at 300 arcsec around GaN peak; however, we estimated the GaN peak which, when relaxed, was about 0 arcsec in curve (a). Because the satellite peaks were broadened compared with the simulation curve and AlGaN peaks of curve (a) and that of curve (b) were in good agreement at 3700 arcsec, we consider that GaN layers grown under the lowpressure condition (LP-GaN), which perform the GaN/AlGaN multilayers, were strained. Furthermore, the results of EPMA measurement were very close to this estimation on curve (b). To determine the strain in the DBR, we proceeded with the XRD measurement on RSM around GaN ð1 0 1% 4Þ Bragg peak. Fig. 3 shows the RSMs of 5.5, 10.5, 20.5 and 40.5 pairs of GaN/AlGaN DBR. The lattice parameter c along the growth direction was represented by the perpendicular axis and the
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Fig. 3. X-ray RSMs around ð1 0 1% 4Þ Bragg peak of GaN: (a) 5.5 pairs of GaN/Al0.60Ga0.40N DBR, (b) 10.5 pairs, (c) 20.5 pairs and (d) 40.5 pairs.
in-plane lattice parameter a by the parallel axis. Both axes were inversely proportional to each lattice constant. In Fig. 3, as the number of pairs increases with 5.5, 10.5, 20.5 and 40.5, the diffraction spot from the GaN layer seems to be close to that from AlGaN in the parallel axis. As
shown in Fig. 3(d), in the case of 40.5 pairs, the diffraction spot from the GaN layer is almost perfectly aligned to that of AlGaN. This indicates that the LP-GaN layers, which formed GaN/ AlGaN DBR, were under the compressive stress. When the number of pairs is small, the total
N. Nakada et al. / Journal of Crystal Growth 237–239 (2002) 961–967
thickness of LP-GaN layer in the GaN/AlGaN DBR was thinner than 500-nm-thick AP-GaN and the highest diffraction intensity was suggested to be the AP-GaN layer. When the number of pairs was large, as shown in Fig. 3(d), the total thickness of LP-GaN layer in the GaN/AlGaN DBR was thicker than 500-nm-thick AP-GaN, and the highest diffraction intensity was suggested to be the LP-GaN layers. This indicates that the lowpressure grown AlGaN layers (LP-AlGaN) are not coherently grown on 500-nm-thick AP-GaN, although the LP-GaN layers are coherently grown on LP-AlGaN layers. Reflectance spectra of 30.5 pairs of GaN/ Al0.60Ga0.40N DBR were shown in Fig. 4. To study the distribution of reflectivity, we measured on nine areas with 7 mm diameters taken across the 2-in wafer in the direction of a cross. Fig. 4(a) shows the reflectance spectra of three areas A, B and C, which are located in order from the center
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of a wafer to the radius direction. At area A, the peak reflectivity of 93% was obtained at wavelength of 417 nm, and at area B, 93% at 421 nm. This peak wavelength was in good agreement with the required value. At area C, the peak wavelength was 448 nm. This was caused by the difference of growth rate due to the temperature distribution of heater, not by cracks. We would like to emphasize that a flat surface morphology without cracks was obtained by optical microscope. Fig. 4(b) shows the four spectra of the areas B, D, F and H which were at the same distance away from the center of a wafer. These spectra were in good agreement with each other and this suggested that this DBR had good uniformity. The number of pair dependence of reflectivity was shown in Fig. 5. In this measurement, we also measured on nine areas with 7 mm diameters taken across the 2-in wafer in the direction of a cross for each sample. Longitudinal bars show the distributions of the reflectivity. This
100 (a)
80
C B A
A B C
60
Reflectivity [%]
40 20 0 100
(b)
80 60
B D F H
B F
H D
40 20 0 350
400
450
500
550
Wavelength [nm] Fig. 4. Reflectance spectra of the 30.5 pairs of GaN/Al0.60Ga0.40N DBR measured at room temperature as a function of wavelength taken across the 2-in wafer: (a) three areas which are located in order from the center of the wafer to the edge along the radius and (b) four areas, which are at same distance away from the center of wafer.
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cracks grew from the edge of wafer. However, we would like to emphasize that a flat surface morphology without cracks was obtained until the 50.5 pair of DBR. Fig. 6 shows the root mean square (rms) roughness of DBRs as a function of the number of pairs that was obtained by AFM measurement scanning four different areas of 3 3 mm for each sample. The value of rms was the worst at 10.5 pairs. When the number of pairs was larger, the value of rms became smaller. To obtain a high reflective DBR, we considered the 40 or 50 pairs of DBR the best. From this result, we fabricate the 45.5 pairs of GaN/Al0.60Ga0.40N DBR. The reflectivity of over 98% at a peak wavelength of 421 nm was obtained. This DBR had a flat surface free of cracks.
100
Reflectivity [%]
80 60 40 20 0 0
20
40
60
80
Number of pairs Fig. 5. Maximum reflectivity of GaN/Al0.60Ga0.40N DBR as a function of the number of pairs. Triangle markers indicate the same position on the 2-in wafer. Longitudinal bars show the distributions of the reflectivity, which were measured at nine
We fabricated the low-pressure grown GaN/ Al0.60Ga0.40N DBR on AP-GaN by MOCVD. Optimizing the thickness of AP-GaN, the surface morphology has been improved. We also fabricated 45.5 pairs of GaN/Al0.60Ga0.40N DBR with a reflectivity of over 98% at 421 nm. This DBR had a flat surface free of cracks, even though the total thickness of DBR was about 4.5 mm. We believe that this DBR should play an important role in the realization of GaN-based VCSELs.
1.6 1.4 1.2
Rms [nm]
4. Conclusions
1.0 0.8 0.6 0.4
References
0.2 0.0 0
20
40
60
80
Number of pairs Fig. 6. Root mean square roughness of the surface of GaN/ Al0.60Ga0.40N DBR as a function of the number of pairs. Root mean square is measured by AFM measurement, which was scanned on four different areas of 3 3 mm2 for each sample.
result shows that as the number of pairs was increasing, the reflectivity became higher. When the number of pairs were larger than 60, the DBR had a large distribution of reflectivity because the
[1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, H. Kiyoku, Appl. Phys. Lett. 69 (1996) 1477. [2] K. Iga, Proceedings of the First International Symposium on Blue Laser and Light Emitting Diodes, 1996, p. Th-11. [3] T. Honda, A. Katsube, T. Sakaguchi, F. Koyama, K. Iga, Jpn. J. Appl. Phys. 34 (1995) 3527. [4] T. Shirasawa, N. Mochida, A. Inoue, T. Honda, T. Sakaguchi, F. Koyama, K. Iga, J. Crystal Growth 189/190 (1998) 124. [5] T. Someya, R. Werner, A. Forchel, M. Catalano, R. Cingolani, Y. Arakawa, Science 285 (1999) 1905. [6] M.A. Khan, J.N. Kuznia, J.M. Van Hove, D.T. Olson, Appl. Phys. Lett. 59 (1991) 1449.
N. Nakada et al. / Journal of Crystal Growth 237–239 (2002) 961–967 [7] I.J. Fritz, T.J. Drummond, Electron. Lett. 31 (1995) 68. [8] H. Ishikawa, N. Nakada, G.Y. Zhao, T. Egawa, T. Jimbo, M. Umeno, Proceedings of the Second International Symposium on Blue Laser and Light Emitting Diodes, 1998, p. Fr-15. [9] T. Someya, Y. Arakawa, Appl. Phys. Lett. 73 (1998) 3653.
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[10] H.M. Ng, T.D. Moustakas, S.N.G. Chu, Appl. Phys. Lett. 76 (2000) 2818. [11] R. Langer, A. Barski, J. Simon, N.T. Pelekanos, O. Konovalov, R. Andr!e, L.S. Dang, Appl. Phys. Lett. 74 (1999) 3610. [12] N. Nakada, M. Nakaji, H. Ishikawa, T. Egawa, M. Umeno, T. Jimbo, Appl. Phys. Lett. 76 (2000) 1804.
MOCVD growth of high reflective GaN/AlGaN distributed Bragg reflectors Naoyuki Nakadaa,*, Hiroyasu Ishikawab, Takashi Egawab, Takashi Jimboc, Masayoshi Umenoa,b a
Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Research Center for Micro-Structure Devices, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan c Department of Environmental Technology and Urban Planning, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Abstract GaN/AlGaN distributed Bragg reflectors have been grown by low-pressure metal organic chemical vapor deposition. Those structures were fabricated on the atmospheric pressure grown GaN layer on sapphire substrate. The aluminum content in low-pressure grown AlGaN layers was estimated to be 0.60 by X-ray diffraction. The GaN layers grown under the low-pressure condition in GaN/Al0.60Ga0.40N multilayer were compressively strained. The flat surfaces without cracks were successfully obtained for the growth of GaN/Al0.60Ga0.40N distributed Bragg reflector. For the 45.5 pairs, a peak reflectivity of over 98% was obtained at a wavelength of 421 nm. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Cr; 42.55.Sa; 42.55.Px Keywords: A3. Low pressure metal organic chemical vapor deposition; A3. Metal organic chemical vapor deposition; B3. Vertical cavity surface emitting lasers
1. Introduction GaN, AlN, InN and their alloys have been highly popular as optical devices in the blueultraviolet region and have created much interest for high-temperature, high-power electric devices at microwave frequencies. GaN-based high brightness light emitting diodes (LEDs) and edge *Corresponding author. Tel.: +81-52-735-5540; fax: +8152-735-5546. E-mail address: [email protected] (N. Nakada).
emitting laser diodes (LDs) have demonstrated continuous-wave operation at room temperature [1]. Recently, GaN-based vertical cavity surface emitting lasers (VCSELs) with a distributed Bragg reflector (DBR) have been studied in many research groups because of their many advantages for various applications such as full color display, photolithography, super high-density optical memory and bright white light source [2]. While the fabrication of smooth edge mirror is difficult for edge emitting lasers, VCSELs can get the smooth mirror without a cleavage technique. However, it is necessary to fabricate high reflective
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 2 2 - X
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mirrors on both sides of the active region to form their short cavity length. In this material system, GaN/AlGaN multilayers are usually used as high reflective mirrors on the bottom side [3]. To realize a high reflective DBR, the contrast of the refractive index between two materials of the multilayers and the flat surface of each layer are important [4]. Because the difference of refractive index between GaAs and AlAs, so many periods of GaN/AlGaN layered structures are required to obtain a high reflectivity. When making such numbers of periods, many cracks are grown and the surface flatness is spoiled. More recently, optical pumping VCSEL, which performed the cavity by GaN/AlGaN DBR for the bottom side mirror and ZrO2/SiO2 dielectric mirror for the top, was reported at room temperature [5], and many authors reported the GaN/AlGaN based reflectors [4,6–11]. We have obtained improved characteristics of InGaN MQW LED on sapphire by use of the 15 pairs of GaN/AlGaN DBR [12]. In this letter, we reported the high reflective GaN/Al0.60Ga0.40N DBRs with crack-free surface fabricated under the low-pressure condition.
2. Growth of GaN/AlGaN DBRs A metal organic chemical vapor deposition (MOCVD) equipment with horizontal quartz reactor (Nippon Sanso, SR-2000) was employed for the growth of GaN/AlGaN DBRs. (0 0 0 1)oriented, 2-in diameter sapphire substrates were used for the growth of samples. Trimethylgallium (TMGa) and trimethylaluminum (TMAl) were used as group III and ammonia (NH3) as group V source materials, respectively. The surface morphology of the DBRs was studied with an optical microscope and the atomic force microscopy (AFM) operated in contacting mode. The reflectivity of the DBRs was measured by an ultraviolet–visible spectrometer as a function of wavelength. The aluminum content in the AlGaN layer was estimated by X-ray diffraction (XRD) measurement using a Philips X’Pert MRD system and electron probe micro analysis (EPMA). Reciprocal space maps (RSMs) of XRD intensity
were performed on a four-circle goniometer around GaN ð1 0 1% 4Þ Bragg peak as the asymmetrical diffraction spot. A thermal cleaning process was carried out at 11801C for 10 min in a stream of hydrogen ambient before the growth of nitride layers. After depositing a 30-nm-thick GaN nucleation layer at 5001C, the substrate was heated up to 11301C, then a GaN layer was grown. The flow rates of TMGa were 36 and 72 mmol/min for the growth of GaN nucleation layer and GaN layer, respectively. Then a 20-nm-thick Al0.15Ga0.85N layer was grown. The flow rates of TMGa and TMAl were 24 and 6.0 mmol/min for the growth of AlGaN layer, respectively. During the growth, the flow rate of NH3 was maintained at 5 l/min. After growth of an AlGaN layer, we changed the growth condition by changing the pressure inside the reactor. It was vacuumed down to a pressure of 100 Torr by dry pump, keeping the temperature of the substrate at 11301C. After stability of the pressure at 100 Torr, GaN/AlGaN multilayers were grown at 11301C. The flow rate of TMGa was 72 mmol/min for the growth of GaN layer which constructed the multilayer structure and those of TMGa and TMAl were 18 and 18 mmol/min for the AlGaN layer, respectively. During the growth of the multilayer structure, the flow rate of NH3 was maintained at 2.5 l/min. The growth rates of GaN were 3.1 mm/h for atmospheric-pressure growth and 3.3 mm/h for low–pressure, respectively, and that of AlGaN was 1.27 mm/h for low–pressure growth. In this structure, we introduced the 20nm-thick Al0.15Ga0.85N as the cap layer to protect the growth layers, because it needed approximately 2.5 min of waiting time to stabilize the pressure at 100 Torr. If this cap layer was not introduced, the surface flatness would be inferior due to the thermal decomposition during the waiting time and we could not have obtained a high reflectivity.
3. Results and discussion We prepared four samples and changed the thickness of GaN layer grown under the atmo-
N. Nakada et al. / Journal of Crystal Growth 237–239 (2002) 961–967
70 10 10
50
Counts [cps]
Reflectivity [%]
60
40 30 20 10 0 0
2000
4000
6000
Thickness of GaN [nm] Fig. 1. Maximum reflectivity of GaN/AlGaN multilayers as a function of the thickness of GaN, which was grown at atmospheric–pressure before the growth of GaN/AlGaN multi-layers at low-pressure condition. Triangle markers indicate the same position on the 2-in sapphire substrate. Longitudinal bars show the distributions of the reflectivity, which was measured at nine areas taken across the 2-in sapphire substrate.
spheric pressure condition (AP-GaN) to 50, 500, 1500 and 6000 nm, respectively. Fig. 1 shows the maximum reflectivity of multilayers as a function of thickness of the AP-GaN layer. The measurement was carried out on the nine areas with a diameter of 7 mm for each sample, taken across the 2-in substrate to study the distribution. In the case of 50 nm, the reflectivity was low and had a large distribution. It was caused by cracks, but the peak wavelength also had a large distribution. Moreover, as the thickness of the AP-GaN layer became thicker, it also became lower in reflectivity. In the case of 500 nm, the reflectivity is the highest and the distribution is the smallest. From this result, we used the 500-nm-thick APGaN layer for the fabrication of DBRs. Then 30.5 pairs of GaN/AlGaN DBR which were designed at a peak wavelength of 420 nm were grown. Fig. 2(a) shows the XRD spectra from symmetrical (0 0 0 4) Bragg peak of GaN/ AlGaN DBR. For comparison, it was also measured on a 700-nm-thick AlGaN grown on
10 10
4
963
(a)
3 2
GaN
Al0.60Ga0.40 N
1
Al0.60Ga0.40 N
GaN 5
104 103 102 101 10
(b)
-2000
0
2000
4000
Diffraction angle [arcsec] Fig. 2. XRD curve from symmetrical (0 0 0 4) Bragg peak of AlGaN/GaN grown on under low-pressure condition on sapphire substrate: (a) 30.5 pairs of GaN/AlGaN DBR and (b) 700-nm thick AlGaN grown on GaN. The arrows indicate the position of AP-GaN and Al0.60Ga0.40N, respectively.
AP-GaN under the same growth condition (Fig. 2(b)). As shown in Fig. 2(b), the separation of the peak angles between GaN and AlGaN was 3750 arcsec, and the aluminum content x in the AlxGa1xN layer was estimated at 0.60. In Fig. 2(a), there are some satellite peaks and the highest diffraction intensity was shown at 300 arcsec around GaN peak; however, we estimated the GaN peak which, when relaxed, was about 0 arcsec in curve (a). Because the satellite peaks were broadened compared with the simulation curve and AlGaN peaks of curve (a) and that of curve (b) were in good agreement at 3700 arcsec, we consider that GaN layers grown under the lowpressure condition (LP-GaN), which perform the GaN/AlGaN multilayers, were strained. Furthermore, the results of EPMA measurement were very close to this estimation on curve (b). To determine the strain in the DBR, we proceeded with the XRD measurement on RSM around GaN ð1 0 1% 4Þ Bragg peak. Fig. 3 shows the RSMs of 5.5, 10.5, 20.5 and 40.5 pairs of GaN/AlGaN DBR. The lattice parameter c along the growth direction was represented by the perpendicular axis and the
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Fig. 3. X-ray RSMs around ð1 0 1% 4Þ Bragg peak of GaN: (a) 5.5 pairs of GaN/Al0.60Ga0.40N DBR, (b) 10.5 pairs, (c) 20.5 pairs and (d) 40.5 pairs.
in-plane lattice parameter a by the parallel axis. Both axes were inversely proportional to each lattice constant. In Fig. 3, as the number of pairs increases with 5.5, 10.5, 20.5 and 40.5, the diffraction spot from the GaN layer seems to be close to that from AlGaN in the parallel axis. As
shown in Fig. 3(d), in the case of 40.5 pairs, the diffraction spot from the GaN layer is almost perfectly aligned to that of AlGaN. This indicates that the LP-GaN layers, which formed GaN/ AlGaN DBR, were under the compressive stress. When the number of pairs is small, the total
N. Nakada et al. / Journal of Crystal Growth 237–239 (2002) 961–967
thickness of LP-GaN layer in the GaN/AlGaN DBR was thinner than 500-nm-thick AP-GaN and the highest diffraction intensity was suggested to be the AP-GaN layer. When the number of pairs was large, as shown in Fig. 3(d), the total thickness of LP-GaN layer in the GaN/AlGaN DBR was thicker than 500-nm-thick AP-GaN, and the highest diffraction intensity was suggested to be the LP-GaN layers. This indicates that the lowpressure grown AlGaN layers (LP-AlGaN) are not coherently grown on 500-nm-thick AP-GaN, although the LP-GaN layers are coherently grown on LP-AlGaN layers. Reflectance spectra of 30.5 pairs of GaN/ Al0.60Ga0.40N DBR were shown in Fig. 4. To study the distribution of reflectivity, we measured on nine areas with 7 mm diameters taken across the 2-in wafer in the direction of a cross. Fig. 4(a) shows the reflectance spectra of three areas A, B and C, which are located in order from the center
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of a wafer to the radius direction. At area A, the peak reflectivity of 93% was obtained at wavelength of 417 nm, and at area B, 93% at 421 nm. This peak wavelength was in good agreement with the required value. At area C, the peak wavelength was 448 nm. This was caused by the difference of growth rate due to the temperature distribution of heater, not by cracks. We would like to emphasize that a flat surface morphology without cracks was obtained by optical microscope. Fig. 4(b) shows the four spectra of the areas B, D, F and H which were at the same distance away from the center of a wafer. These spectra were in good agreement with each other and this suggested that this DBR had good uniformity. The number of pair dependence of reflectivity was shown in Fig. 5. In this measurement, we also measured on nine areas with 7 mm diameters taken across the 2-in wafer in the direction of a cross for each sample. Longitudinal bars show the distributions of the reflectivity. This
100 (a)
80
C B A
A B C
60
Reflectivity [%]
40 20 0 100
(b)
80 60
B D F H
B F
H D
40 20 0 350
400
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500
550
Wavelength [nm] Fig. 4. Reflectance spectra of the 30.5 pairs of GaN/Al0.60Ga0.40N DBR measured at room temperature as a function of wavelength taken across the 2-in wafer: (a) three areas which are located in order from the center of the wafer to the edge along the radius and (b) four areas, which are at same distance away from the center of wafer.
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cracks grew from the edge of wafer. However, we would like to emphasize that a flat surface morphology without cracks was obtained until the 50.5 pair of DBR. Fig. 6 shows the root mean square (rms) roughness of DBRs as a function of the number of pairs that was obtained by AFM measurement scanning four different areas of 3 3 mm for each sample. The value of rms was the worst at 10.5 pairs. When the number of pairs was larger, the value of rms became smaller. To obtain a high reflective DBR, we considered the 40 or 50 pairs of DBR the best. From this result, we fabricate the 45.5 pairs of GaN/Al0.60Ga0.40N DBR. The reflectivity of over 98% at a peak wavelength of 421 nm was obtained. This DBR had a flat surface free of cracks.
100
Reflectivity [%]
80 60 40 20 0 0
20
40
60
80
Number of pairs Fig. 5. Maximum reflectivity of GaN/Al0.60Ga0.40N DBR as a function of the number of pairs. Triangle markers indicate the same position on the 2-in wafer. Longitudinal bars show the distributions of the reflectivity, which were measured at nine
We fabricated the low-pressure grown GaN/ Al0.60Ga0.40N DBR on AP-GaN by MOCVD. Optimizing the thickness of AP-GaN, the surface morphology has been improved. We also fabricated 45.5 pairs of GaN/Al0.60Ga0.40N DBR with a reflectivity of over 98% at 421 nm. This DBR had a flat surface free of cracks, even though the total thickness of DBR was about 4.5 mm. We believe that this DBR should play an important role in the realization of GaN-based VCSELs.
1.6 1.4 1.2
Rms [nm]
4. Conclusions
1.0 0.8 0.6 0.4
References
0.2 0.0 0
20
40
60
80
Number of pairs Fig. 6. Root mean square roughness of the surface of GaN/ Al0.60Ga0.40N DBR as a function of the number of pairs. Root mean square is measured by AFM measurement, which was scanned on four different areas of 3 3 mm2 for each sample.
result shows that as the number of pairs was increasing, the reflectivity became higher. When the number of pairs were larger than 60, the DBR had a large distribution of reflectivity because the
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