- Email: [email protected]
The evolution of group III nitride semiconductors
Materials Science and Engineering B74 (2000) 101 – 106 www.elsevier.com/locate/mseb
The evolution of group III nitride semiconductors Seeking blue light emission Isamu Akasaki * Department of Electrical and Electronic Engineering, High-Tech Research Center, Meijo Uni6ersty 1 -501 Shiogamaguchi, Tempaku-ku, Nagoya 468 -8502, Japan
Abstract Renaissance and progress in crystal growth and achievement of conductivity control of group III nitride semiconductors in the past quarter century are reviewed as the groundwork for recently developed high-performance blue and green light emitting diodes, laser diodes and transistors based on nitrides. Quite recent advance in crystal growth is also reported. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Group III nitrides; Organometallic vapor phase epitaxy; Low-temperature buffer; Conductivity control; Light emitting diode; Laser
1. Introduction Due to their superior intrinsic properties such as wide range of direct transition type band structure, chemical and physical stability, high thermal conductivity and high electron saturation velocity, group III nitrides are regarded as one of the most promising materials for applications to short wavelength light emitting diodes (LEDs), laser diodes (LDs) and photodiodes (PDs) as well as high-temperature electronic devices. Moreover, nitrides are the most ‘environmentally friendly’ material available. To realize such novel devices, it is essential to grow high-quality nitride single crystal and to control their electrical conductivity. During the past two decades before 1985, pioneering work on GaN has been done by many researchers. However, it had been quite difficult to grow high-quality epitaxial GaN film with a flat surface free from cracks. This is mainly caused by the lack of thermally stable substrate materials with lattice constants and thermal expansion coefficients close to those of GaN. Moreover, it has generally been found that undoped GaN was strongly n-type with high residual electron concentrations, indeed p-type GaN and GaN p–n junctions are a relatively recent discovery. And many researchers decided to retire from the * Tel.: +81-52-8321151; fax: + 81-52-8321244. E-mail address: [email protected] (I. Akasaki)
nitride field. Thus, GaN activities slowly declined and this started a big gap in the history of the group III nitride research, – – the wide gap as seen in Fig. 1 [1].
2. Renaissance and progress in crystal growth of nitride semiconductors In order to overcome difficulties due to the large lattice mismatch between sapphire substrate and GaN, ‘ the method of the low temperature deposited buffer layer (LT-buffer)’, was proposed by the present author. By using this method in organometallic vapor phase epitaxy (OMVPE), an extremely high-quality GaN single crystal with specular surface free from cracks has been successfully grown, as shown in Fig. 2a [2]. The essence of this method is to insert a slightly softer material in order to reduce the interfacial free energy between the epitaxial layer and the highly mismatched substrate. Fig. 3 illustrates this concept. By insertion of the LT-AlN buffer layer deposited under the optimum condition in OMVPE, not only the crystalline quality, but also the electrical and optical properties of GaN can be dramatically improved at the same time [3]. Immediately, a bright metal-insulatorsemiconductor (MIS)-type blue LED has been fabricated by using such a high-quality GaN. UV stimulated emission from the same GaN film was also achieved by optical pumping for the first time at room temperature
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 5 4 3 - 7
102
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
Fig. 3. Illustration of the concept of the low-temperature deposited buffer layer inserted between epitaxial layer and highly-mismatched substrate.
3. Achievement of conductivity control of nitride semiconductors
3.1. p-type nitrides
Fig. 1. Number of publications (INSPEC) and activities in GaN over the years on a logarithmic scale. All events are marked in the year when they were first achieved.
[4]. This development (LT-Buffer) was in 1986 [2]. In 1991, this LT-buffer layer method was implemented by Nakamura in OMVPE, who used an LT-GaN buffer layer [5]. Today, these LT-AlN or LT-GaN buffer layer methods are indispensable and standard in the growth of high quality GaN and nitride alloys on sapphire substrate by OMVPE.
So far, many groups attempted to produce p-type GaN. No groups, however, succeeded. It was found in 1989 that good controllability of Mg concentration in OMVPE growth of GaN using the LT-buffer layer by employing biscyclopentadienyl Mg (CP2Mg) or methylCP2Mg as the Mg precursor [6]. However, the added Mg was mostly inactive as-grown. In 1989, we realize, for the first time in the world, distinct p-type GaN with low-resistivity by a low-energy electron beam irradiation (LEEBI) treatment of such a high-quality GaN doped with Mg grown with the LT-buffer layer [7]. These breakthroughs (the LT-buffer layer method, p-type doping) were in 1986 and in 1989, respectively, and as seen in Fig. 1, a tremendous growth of the field started. In 1991, p-type AlGaN was achieved in the same manner [8]. And p-type GaN was also obtained
Fig. 2. SEM image of GaN epitaxial film grown on sapphire (a) with and (b) without the LT-AlN buffer layer.
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
103
by thermal annealing in N2 atmosphere of Mg-doped GaN using CP2Mg grown with the LT-buffer layer [5]. It was shown that Mg is passivated by H2 in asOMVPE grown state. It was also reported that p-type GaN can be grown with H2-free atmosphere (e.g. by molecular beam epitaxy (MBE)) without any postgrowth treatment. Indeed, we had a lot of H2 in our OMVPE growth process. Fig. 4 shows temperature dependence of the hole concentration of Mg-doped GaN, AlGaN and GaInN. At present, in OMVPE all the p-type GaN and nitride alloys are prepared by Mg doping using CP2Mg or methyl-CP2Mg followed by the LEEBI or thermal annealing in an H2-free atmosphere.
3.2. n-type nitrides Regarding the n-type doping, a group had tried doping SiH4 before, but it had been difficult to control the conductivity, probably due to high density of residual donors [9]. In 1989, we found that Si behaves as a shallow donor in nitrides, and SiH4 is suitable for Si doping. The electron concentration in GaN and AlGaN and later in GaInN can be controlled from undoped levels (1016 1017 cm − 3) to levels \ 1020 cm − 3 by changing SiH4 flow rate [10]. Fig. 5. Change of the external quantum efficiency (hext) of GaN-based blue LEDs.
Today, this Si doping in OMVPE growth in combination with the LT-buffer layer method is widely adopted for the conductivity control of n-type nitrides, GaN, AlGaN and GaInN. This conductivity control of n-type nitrides is practically important as well as that of p-type conductivity. Then everythings went very quickly and work in field started growing tremendously (Fig. 1). These achievements in crystal growth and conductivity control have led to the development of high-performance devices such as LEDs, LDs, FETs and UV detectors.
4. Brief history of light emitting devices
4.1. LEDs
Fig. 4. Temperature dependence of hole concentration of Mg-doped GaN, AlGaN and GaInN.
The GaN MIS-type blue LED was firstly reported by Pankove et al. [11]. The first p–n junction blue LED was developed in 1989 [7]. In 1992, the LED with an efficiency (hext) of 1.5% was achieved by us [12], and in 1993 a GaN blue LED with efficiency of about 2.7% was commercialized by Nichia Chemical Co. [13]. As seen in Fig. 5, the efficiency of GaN-based blue LED began to increase steeply soon after the success in
104
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
producing the high-quality nitrides using the LT-buffer method, which resulted in p – n junction LEDs. External quantum efficiencies of blue and green LEDs are about 12 13%, typically at present. The luminous efficiencies of these LEDs are comparable to or exceed those of the AlGaAs- and AlGaInP-based LEDs.
4.2. Laser diodes Similar to the case of the LED, threshold power (Pth) for stimulated emission by optical pumping began to decrease steeply soon after the successes in producing the high-quality nitrides, quantum wells and p –n junctions as shown in Fig. 6. Thus, in late 1995 the onset of stimulated emission by current injection was observed [14]. Today, more than ten groups develop nitride-based LDs. The longest life time of more than 104 h has been achieved [15].
5. Recent advance in crystal growth Until recently, the density of dislocations in the active layers of LEDs and LDs, which are grown on
Fig. 7. Timing chart for growth of GaN: (a) conventional single LT-buffer layer and (b) two-step LT-layers.
Fig. 6. Change of the threshold power (Pth) for stimulated emission from group III nitrides by optical pumping.
sapphire substrate covered with the LT-buffer layer, is the order of 109 1010 cm − 2. These dislocations do not affect severely the LED lifetime, but affect LD lifetime and FET performances. Recently, reduction of dislocation density was achieved by using epitaxial lateral overgrowth (ELO) proposed by Nishinaga in the case of GaAs on Si as the micro-channel epitaxy (MCE) [16] and developed by Usui et al. in GaN system [17], and the effectiveness on the LD lifetime was proven by Nakamura [15]. We also succeeded in reduction of both etch pit density as well as dislocation density to a great extent by insertion of the second LT-buffer layer between a high temperature (HT) grown GaN [18]. Fig. 7 shows the time chart for the growth of GaN on sapphire for the conventional (one-step) LT-buffer layer sequence mentioned in 2 (Fig. 7a) and the newly developed multi-step LT-buffer layer sequence (Fig. 7b). In the latter case (Fig. 7b), the sapphire substrate was first covered with either an AlN or a GaN buffer layer. Then at a high temperature (HT) of above 1050°C, GaN with a thickness of about 1 mm was grown. Then the supply of group III alkyls was stopped and the substrate temperature was lowered to 400°C. Next, either the LT-AlN layer or the LT-GaN layer (we call
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
105
Fig. 8. (a) Cross-sectional TEM of GaN film grown on a sapphire (0001) substrate using a three-step LT-AlN buffer layer sequence. Triangles show threading dislocation. (b) Dislocation density in HT-GaN film as a function of number of LT-interlayers. Fig. 9. (a) Lattice constant c of HT-grown GaN as a function of number of LT-interlayers. (b) Surface morphology of uppermost HT-grown GaN film.
106
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
hereafter the LT-interlayer) about 20 nm thick was deposited. After deposition, supply of the group III alkyls was stopped again and substrate was heated to 1050°C. Then GaN was grown. Cross-sectional transmission electron micrograph (TEM) (Fig. 8a) shows that by using the multistep LT-interlayers, dislocation density in GaN decreased to mid 107 cm − 2, which is two to three orders of magnitude lower than those of conventional LTbuffer layer method. The dislocation density decreases with the number of the LT-interlayer as shown in Fig. 8b, while simultaneously increasing the tensile stress during growth (Fig. 9a), ultimately resulting in cracking of the HT-GaN film grown on top of it (Fig. 9b (center)) in the case of LT-GaN interlayers [19,20]. On the other hand, LT-AlN inerlayers were found to be effective in suppressing cracking by reducing tensile stress (Fig. 9a) resulting in crack-free surface as seen in Fig. 9b (top), as well as in reducing the dislocation density (Fig. 8). Also, using LT-AlN interlayer, crack-free and high quality AlGaN has been firstly achieved. This interlayer approach permits tailoring of the film stress to optimize film structure and properties of nitrides.
6. Summary Renaissance and progress in crystal growth and conductivity control of group III nitride semiconductors in the past quarter century, are reviewed as the groundwork for recently developed high-performance blue and green LEDs, and UV and violet LDs based on nitrides. Today, the performance of these devices are still progressing, owing to the steady progress in the areas of crystal growth, process technologies and optimization of device structure. Much further improvements of crystalline quality of quantum wells and p-type conductivity as well as the understanding of intrinsic properties of nitrides will lead to the development of much higher-performance devices which are most environmentally friendly ones available and able to operate in harsh environments.
.
Acknowledgements The author is grateful to the following collaborators who have made major contributions to this work; H. Amano, M. Iwaya and T. Takeuchi. This work was partly supported by JSPS Research for the Future Program in the Area of Atomic-Scale Surface and Interface Dynamics under the project of ‘Dynamic Process and Control of the Buffer Layer at the Interface in a Highly-Mismatched Systems’, and the Ministry of Education, Science, Sports and Culture of Japan (contract nos. 09450133 and 09875083).
References [1] I. Akasaki, Mat. Res. Soc. Symp. Proc. 482 (1998) 3. [2] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [3] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J. Crystal Growth 98 (1989) 209. [4] H. Amano, T. Asahi, I. Akasaki, Jpn. J. Appl. Phys. 29 (1990) L205. [5] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. [6] H. Amano, M. Kitoh, K. Hiramatsu, I. Akasaki, J. Electrochem. Soc. 137 (1990) 1639. [7] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112. [8] I. Akasaki, H. Amano, Mat. Res. Soc. Symp. Proc. 242 (1992) 383. [9] K. Sayyah, B.C. Chung, M. Gershenzon, J. Cryst. Growth 77 (1986) 424. [10] H. Amano, I. Akasaki, Mat. Res. Soc. Ext. Abs. EA21 (1990) 165. [11] J.I. Pankove, E.A. Miller, D. Richman, J.E. Berkeyheiser, J. Lumin. 4 (1971) 63. [12] I. Akasaki, H. Amano, K. Itoh, N. Koide, K. Manabe, Inst. Phys. Conf. Ser. 129 (1992) 851. [13] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [14] I. Akasaki, H. Amano, S. Sota, H. Sakai, T. Tanaka, M. Koike, Jpn. J. Appl. Phys. 34 (1995) L1517. [15] S. Nakamura, Mat. Res. Soc. Symp. 537 G1,1, Boston (1998). [16] T. Nishinaga, T. Nakano, S. Zhang, Jpn. J. Appl. Phys. 27 (1988) 1964. [17] A. Usui, H. Sunagawa, A. Sakai, A. Yamaguchi, Jpn. J. Appl. Phys. 2 (36) (1997) L899. [18] M. Iwaya, T. Takeuchi, S. Yamaguchi, C. Wetzel, H. Amano, I. Akasaki, Jpn. J. Appl. Phys 37 (1998) L316. [19] H. Amano, M. Iwaya, N. Hayashi, I. Akasaki, et al., Proceedings of the 3rd Symposium on Atomic-Scale Surface and Interface Dynamics 337, March 4 – 5, 1999, Fukuoka, Japan. [20] H. Amano, M. Iwaya, I. Akasaki, et al., Jpn. J. Appl. Phys. 37 (1998) L1540.
The evolution of group III nitride semiconductors Seeking blue light emission Isamu Akasaki * Department of Electrical and Electronic Engineering, High-Tech Research Center, Meijo Uni6ersty 1 -501 Shiogamaguchi, Tempaku-ku, Nagoya 468 -8502, Japan
Abstract Renaissance and progress in crystal growth and achievement of conductivity control of group III nitride semiconductors in the past quarter century are reviewed as the groundwork for recently developed high-performance blue and green light emitting diodes, laser diodes and transistors based on nitrides. Quite recent advance in crystal growth is also reported. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Group III nitrides; Organometallic vapor phase epitaxy; Low-temperature buffer; Conductivity control; Light emitting diode; Laser
1. Introduction Due to their superior intrinsic properties such as wide range of direct transition type band structure, chemical and physical stability, high thermal conductivity and high electron saturation velocity, group III nitrides are regarded as one of the most promising materials for applications to short wavelength light emitting diodes (LEDs), laser diodes (LDs) and photodiodes (PDs) as well as high-temperature electronic devices. Moreover, nitrides are the most ‘environmentally friendly’ material available. To realize such novel devices, it is essential to grow high-quality nitride single crystal and to control their electrical conductivity. During the past two decades before 1985, pioneering work on GaN has been done by many researchers. However, it had been quite difficult to grow high-quality epitaxial GaN film with a flat surface free from cracks. This is mainly caused by the lack of thermally stable substrate materials with lattice constants and thermal expansion coefficients close to those of GaN. Moreover, it has generally been found that undoped GaN was strongly n-type with high residual electron concentrations, indeed p-type GaN and GaN p–n junctions are a relatively recent discovery. And many researchers decided to retire from the * Tel.: +81-52-8321151; fax: + 81-52-8321244. E-mail address: [email protected] (I. Akasaki)
nitride field. Thus, GaN activities slowly declined and this started a big gap in the history of the group III nitride research, – – the wide gap as seen in Fig. 1 [1].
2. Renaissance and progress in crystal growth of nitride semiconductors In order to overcome difficulties due to the large lattice mismatch between sapphire substrate and GaN, ‘ the method of the low temperature deposited buffer layer (LT-buffer)’, was proposed by the present author. By using this method in organometallic vapor phase epitaxy (OMVPE), an extremely high-quality GaN single crystal with specular surface free from cracks has been successfully grown, as shown in Fig. 2a [2]. The essence of this method is to insert a slightly softer material in order to reduce the interfacial free energy between the epitaxial layer and the highly mismatched substrate. Fig. 3 illustrates this concept. By insertion of the LT-AlN buffer layer deposited under the optimum condition in OMVPE, not only the crystalline quality, but also the electrical and optical properties of GaN can be dramatically improved at the same time [3]. Immediately, a bright metal-insulatorsemiconductor (MIS)-type blue LED has been fabricated by using such a high-quality GaN. UV stimulated emission from the same GaN film was also achieved by optical pumping for the first time at room temperature
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 5 4 3 - 7
102
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
Fig. 3. Illustration of the concept of the low-temperature deposited buffer layer inserted between epitaxial layer and highly-mismatched substrate.
3. Achievement of conductivity control of nitride semiconductors
3.1. p-type nitrides
Fig. 1. Number of publications (INSPEC) and activities in GaN over the years on a logarithmic scale. All events are marked in the year when they were first achieved.
[4]. This development (LT-Buffer) was in 1986 [2]. In 1991, this LT-buffer layer method was implemented by Nakamura in OMVPE, who used an LT-GaN buffer layer [5]. Today, these LT-AlN or LT-GaN buffer layer methods are indispensable and standard in the growth of high quality GaN and nitride alloys on sapphire substrate by OMVPE.
So far, many groups attempted to produce p-type GaN. No groups, however, succeeded. It was found in 1989 that good controllability of Mg concentration in OMVPE growth of GaN using the LT-buffer layer by employing biscyclopentadienyl Mg (CP2Mg) or methylCP2Mg as the Mg precursor [6]. However, the added Mg was mostly inactive as-grown. In 1989, we realize, for the first time in the world, distinct p-type GaN with low-resistivity by a low-energy electron beam irradiation (LEEBI) treatment of such a high-quality GaN doped with Mg grown with the LT-buffer layer [7]. These breakthroughs (the LT-buffer layer method, p-type doping) were in 1986 and in 1989, respectively, and as seen in Fig. 1, a tremendous growth of the field started. In 1991, p-type AlGaN was achieved in the same manner [8]. And p-type GaN was also obtained
Fig. 2. SEM image of GaN epitaxial film grown on sapphire (a) with and (b) without the LT-AlN buffer layer.
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
103
by thermal annealing in N2 atmosphere of Mg-doped GaN using CP2Mg grown with the LT-buffer layer [5]. It was shown that Mg is passivated by H2 in asOMVPE grown state. It was also reported that p-type GaN can be grown with H2-free atmosphere (e.g. by molecular beam epitaxy (MBE)) without any postgrowth treatment. Indeed, we had a lot of H2 in our OMVPE growth process. Fig. 4 shows temperature dependence of the hole concentration of Mg-doped GaN, AlGaN and GaInN. At present, in OMVPE all the p-type GaN and nitride alloys are prepared by Mg doping using CP2Mg or methyl-CP2Mg followed by the LEEBI or thermal annealing in an H2-free atmosphere.
3.2. n-type nitrides Regarding the n-type doping, a group had tried doping SiH4 before, but it had been difficult to control the conductivity, probably due to high density of residual donors [9]. In 1989, we found that Si behaves as a shallow donor in nitrides, and SiH4 is suitable for Si doping. The electron concentration in GaN and AlGaN and later in GaInN can be controlled from undoped levels (1016 1017 cm − 3) to levels \ 1020 cm − 3 by changing SiH4 flow rate [10]. Fig. 5. Change of the external quantum efficiency (hext) of GaN-based blue LEDs.
Today, this Si doping in OMVPE growth in combination with the LT-buffer layer method is widely adopted for the conductivity control of n-type nitrides, GaN, AlGaN and GaInN. This conductivity control of n-type nitrides is practically important as well as that of p-type conductivity. Then everythings went very quickly and work in field started growing tremendously (Fig. 1). These achievements in crystal growth and conductivity control have led to the development of high-performance devices such as LEDs, LDs, FETs and UV detectors.
4. Brief history of light emitting devices
4.1. LEDs
Fig. 4. Temperature dependence of hole concentration of Mg-doped GaN, AlGaN and GaInN.
The GaN MIS-type blue LED was firstly reported by Pankove et al. [11]. The first p–n junction blue LED was developed in 1989 [7]. In 1992, the LED with an efficiency (hext) of 1.5% was achieved by us [12], and in 1993 a GaN blue LED with efficiency of about 2.7% was commercialized by Nichia Chemical Co. [13]. As seen in Fig. 5, the efficiency of GaN-based blue LED began to increase steeply soon after the success in
104
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
producing the high-quality nitrides using the LT-buffer method, which resulted in p – n junction LEDs. External quantum efficiencies of blue and green LEDs are about 12 13%, typically at present. The luminous efficiencies of these LEDs are comparable to or exceed those of the AlGaAs- and AlGaInP-based LEDs.
4.2. Laser diodes Similar to the case of the LED, threshold power (Pth) for stimulated emission by optical pumping began to decrease steeply soon after the successes in producing the high-quality nitrides, quantum wells and p –n junctions as shown in Fig. 6. Thus, in late 1995 the onset of stimulated emission by current injection was observed [14]. Today, more than ten groups develop nitride-based LDs. The longest life time of more than 104 h has been achieved [15].
5. Recent advance in crystal growth Until recently, the density of dislocations in the active layers of LEDs and LDs, which are grown on
Fig. 7. Timing chart for growth of GaN: (a) conventional single LT-buffer layer and (b) two-step LT-layers.
Fig. 6. Change of the threshold power (Pth) for stimulated emission from group III nitrides by optical pumping.
sapphire substrate covered with the LT-buffer layer, is the order of 109 1010 cm − 2. These dislocations do not affect severely the LED lifetime, but affect LD lifetime and FET performances. Recently, reduction of dislocation density was achieved by using epitaxial lateral overgrowth (ELO) proposed by Nishinaga in the case of GaAs on Si as the micro-channel epitaxy (MCE) [16] and developed by Usui et al. in GaN system [17], and the effectiveness on the LD lifetime was proven by Nakamura [15]. We also succeeded in reduction of both etch pit density as well as dislocation density to a great extent by insertion of the second LT-buffer layer between a high temperature (HT) grown GaN [18]. Fig. 7 shows the time chart for the growth of GaN on sapphire for the conventional (one-step) LT-buffer layer sequence mentioned in 2 (Fig. 7a) and the newly developed multi-step LT-buffer layer sequence (Fig. 7b). In the latter case (Fig. 7b), the sapphire substrate was first covered with either an AlN or a GaN buffer layer. Then at a high temperature (HT) of above 1050°C, GaN with a thickness of about 1 mm was grown. Then the supply of group III alkyls was stopped and the substrate temperature was lowered to 400°C. Next, either the LT-AlN layer or the LT-GaN layer (we call
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
105
Fig. 8. (a) Cross-sectional TEM of GaN film grown on a sapphire (0001) substrate using a three-step LT-AlN buffer layer sequence. Triangles show threading dislocation. (b) Dislocation density in HT-GaN film as a function of number of LT-interlayers. Fig. 9. (a) Lattice constant c of HT-grown GaN as a function of number of LT-interlayers. (b) Surface morphology of uppermost HT-grown GaN film.
106
I. Akasaki / Materials Science and Engineering B74 (2000) 101–106
hereafter the LT-interlayer) about 20 nm thick was deposited. After deposition, supply of the group III alkyls was stopped again and substrate was heated to 1050°C. Then GaN was grown. Cross-sectional transmission electron micrograph (TEM) (Fig. 8a) shows that by using the multistep LT-interlayers, dislocation density in GaN decreased to mid 107 cm − 2, which is two to three orders of magnitude lower than those of conventional LTbuffer layer method. The dislocation density decreases with the number of the LT-interlayer as shown in Fig. 8b, while simultaneously increasing the tensile stress during growth (Fig. 9a), ultimately resulting in cracking of the HT-GaN film grown on top of it (Fig. 9b (center)) in the case of LT-GaN interlayers [19,20]. On the other hand, LT-AlN inerlayers were found to be effective in suppressing cracking by reducing tensile stress (Fig. 9a) resulting in crack-free surface as seen in Fig. 9b (top), as well as in reducing the dislocation density (Fig. 8). Also, using LT-AlN interlayer, crack-free and high quality AlGaN has been firstly achieved. This interlayer approach permits tailoring of the film stress to optimize film structure and properties of nitrides.
6. Summary Renaissance and progress in crystal growth and conductivity control of group III nitride semiconductors in the past quarter century, are reviewed as the groundwork for recently developed high-performance blue and green LEDs, and UV and violet LDs based on nitrides. Today, the performance of these devices are still progressing, owing to the steady progress in the areas of crystal growth, process technologies and optimization of device structure. Much further improvements of crystalline quality of quantum wells and p-type conductivity as well as the understanding of intrinsic properties of nitrides will lead to the development of much higher-performance devices which are most environmentally friendly ones available and able to operate in harsh environments.
.
Acknowledgements The author is grateful to the following collaborators who have made major contributions to this work; H. Amano, M. Iwaya and T. Takeuchi. This work was partly supported by JSPS Research for the Future Program in the Area of Atomic-Scale Surface and Interface Dynamics under the project of ‘Dynamic Process and Control of the Buffer Layer at the Interface in a Highly-Mismatched Systems’, and the Ministry of Education, Science, Sports and Culture of Japan (contract nos. 09450133 and 09875083).
References [1] I. Akasaki, Mat. Res. Soc. Symp. Proc. 482 (1998) 3. [2] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [3] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J. Crystal Growth 98 (1989) 209. [4] H. Amano, T. Asahi, I. Akasaki, Jpn. J. Appl. Phys. 29 (1990) L205. [5] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. [6] H. Amano, M. Kitoh, K. Hiramatsu, I. Akasaki, J. Electrochem. Soc. 137 (1990) 1639. [7] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112. [8] I. Akasaki, H. Amano, Mat. Res. Soc. Symp. Proc. 242 (1992) 383. [9] K. Sayyah, B.C. Chung, M. Gershenzon, J. Cryst. Growth 77 (1986) 424. [10] H. Amano, I. Akasaki, Mat. Res. Soc. Ext. Abs. EA21 (1990) 165. [11] J.I. Pankove, E.A. Miller, D. Richman, J.E. Berkeyheiser, J. Lumin. 4 (1971) 63. [12] I. Akasaki, H. Amano, K. Itoh, N. Koide, K. Manabe, Inst. Phys. Conf. Ser. 129 (1992) 851. [13] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [14] I. Akasaki, H. Amano, S. Sota, H. Sakai, T. Tanaka, M. Koike, Jpn. J. Appl. Phys. 34 (1995) L1517. [15] S. Nakamura, Mat. Res. Soc. Symp. 537 G1,1, Boston (1998). [16] T. Nishinaga, T. Nakano, S. Zhang, Jpn. J. Appl. Phys. 27 (1988) 1964. [17] A. Usui, H. Sunagawa, A. Sakai, A. Yamaguchi, Jpn. J. Appl. Phys. 2 (36) (1997) L899. [18] M. Iwaya, T. Takeuchi, S. Yamaguchi, C. Wetzel, H. Amano, I. Akasaki, Jpn. J. Appl. Phys 37 (1998) L316. [19] H. Amano, M. Iwaya, N. Hayashi, I. Akasaki, et al., Proceedings of the 3rd Symposium on Atomic-Scale Surface and Interface Dynamics 337, March 4 – 5, 1999, Fukuoka, Japan. [20] H. Amano, M. Iwaya, I. Akasaki, et al., Jpn. J. Appl. Phys. 37 (1998) L1540.