- Email: [email protected]
blue GaN light emitting devices
Physica B 185 (1993) North-Holland
PHYSICA 1
428-432
Conductivity control of GaN and fabrication light emitting devices
of UV/blue
GaN
I. Akasaki and H. Amano Meijo University, Department of Electrical and Electronic Engineering,
Nagoya 468, Japan
N. Koide, M. Kotaki and K. Manabe Toyoda Gosei Co., Ltd, Department of Research and Development,
Haruhi-mura,
Aichi-prefecture.
Japan
The development of the technology for heteroepitaxial growth of high-crystalline-quality GaN films on sapphire substrates using AlN buffer layers, the establishment of a technique for control of conductivity for n-type GaN by Si-doping, and the realization of p-type Mg-doped GaN using low-energy electron-beam irradiation treatment have enabled us to fabricate a high performance, short wavelength LED based on GaN. In this paper, we review the recent developments of AlGaNiGaN multi-layered structures showing quantum-size effects, and of LEDs with AlGaNlGaN double heterostructures which emit light in the blue to UV region with output power of more than a few milliwatts at room temperature
1. Introduction
Bright light emitting diodes (LED) in the red to green region and a few milliwatt laser diodes (LD) in the red region have been commercially available for some time. To realize a new fullcolor display system, a pure blue LED has been eagerly demanded. The practical shorter wavelength LD also enables us to develop compact and high density optical storage systems and small medical apparatus. For the realization of these devices, the research and development of semiconductors with wide band gaps, so-called wide-band-gap semiconductors, is necessary. A great advance in this field has recently been given by 3M [l]. Blue-green LDs with emission wavelength of 512 nm at room temperature have been recently developed using a ZnCdSe-based quantum well structure. However, the wavelengths of their LDs or LEDs are still in the blue /green to green/blue region. Therefore, development of a pure blue light emitter and much Correspondence to: I. Akasaki, Meijo University, Department of Electrical and Electronic Engineering, l-501 Shiogamaguchi, Tempaku-ku, Nagoya-city 468, Japan. 0921-4526/93/$06.00
0
1993 - Elsevier
Science
Publishers
shorter wavelength (blue to UV) LDs is strongly desired. Gallium nitride (GaN) is a promising candidate as the material for fabrication of such short wavelength light emitters, because it has a direct transition-type band structure with a band gap energy of 3.39 eV at room temperature, which corresponds to a wavelength of 375 nm. The emission wavelength of a GaN-based LD will be less than three-fourths of that of a ZnCdSebased LD. Accordingly, the realization of much higher density optical storage systems can be anticipated by using a GaN-based LD. Concerning the blue and violet LED, bright LEDs with high external quantum efficiency can be obtained by using GaN-based materials because internal absorption loss in GaN and column III-nitride alloys is negligible in the blue and violet region. In contrast with other III-V compounds such as GaAs and InP, however, it had been fairly difficult to grow high quality epitaxial films with a flat surface free from cracks, because of the large lattice mismatch and the large difference in thermal expansion coefficient between the epitaxial film and the sapphire substrate.
B.V. All rights
reserved
I. Akasaki et al. I Conductivity control of GaN and fabrication of GaN LEDs
In 1986, we succeeded in overcoming these problems and growing high quality GaN films with a specular surface free from cracks by the prior deposition of a thin AlN buffer layer in MOVPE growth of GaN [2]. The electrical and optical properties as well as the crystalline quality can be remarkably improved at the same time [3]. By using such a high quality GaN film, UV stimulated emission at room temperature by optical pumping was achieved for the first time [4,5]. Practical bright MIS-type blue-LEDs have been developed by using the same GaN film. The brightness is typically about 100 mcd, and 200mcd maximum at a forward current of 10 mA, which is the highest in the blue region and comparable with that of commercially available GaAsP red LEDs and GaP green LEDs. Silicon was found to act as a donor impurity in GaN [6,7] as well as in AlGaN [8], and the free electron concentration has been controlled from undoped level up to near 10” cm-‘. It has been well known that undoped GaN shows n-type conduction and p-type GaN had never been realized. We succeeded in producing p-type GaN in 1989 [9] and p-type AlGaN in 1991 by low-energy electron-beam irradiation (LEEBI) treatment of Mg-doped films. On the basis of these result, we developed the first p-n junction GaN UV/blue LED. In this paper, (1) the conductivity control of GaN for both n-type and p-type and (2) the performance of the UV/blue LED are described.
2. MOVPE growth of high-quality films on sapphire buffer layers
substrates
GaN thin using AIN
A horizontal type MOVPE reactor operated at atmospheric pressure was used for the growth of the GaN film. Trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia (NH,) were used as source gases and hydrogen (H,) as a carrier gas. Polished sapphire (0 0 0 1) crystals were used as the substrate. Misorientation was less than 1”. In our process, before GaN growth, a thin AlN
429
layer about 50 nm thick was deposited at 600°C by feeding TMAl and NH, diluted with H,. Then the temperature was raised to 105O”C, and a single crystal GaN film several micrometers thick was grown. The surface morphology of GaN films can be remarkably improved by the preceding deposition of the AlN as a buffer layer. GaN films with optically flat surfaces free from cracks can be grown on the sapphire substrate covered with an AlN buffer layer. On the contrary, island growth occurred in the growth of GaN on the bare sapphire substrate surface. X-Ray rocking curve (XRC) measurements also revealed that GaN grown using the AlN buffer layer has high quality. The full width at half maximum of the GaN film grown with the AlN buffer layer is about 110 arcsec, which is the narrowest to date in this material; that of the GaN film grown directly on the sapphire substrate was more than 1000 arcsec. Photoluminescence (PL) measurements also showed that the luminescence properties of the GaN film can be improved by using the AlN buffer layer. In the PL spectrum at 4.2 K of GaN grown with the buffer layer, the free exciton line (E,) and the donor-bound exciton line (I,) clearly appear, while emission bands in the long wavelength region, which may be due to deeplevel defects, are scarcely observed. On the other hand, emission bands at long wavelengths dominated in the spectrum of the GaN film grown directly on the sapphire substrate. This indicates that the generation of deep-level defects can be suppressed by using the buffer layer. GaN films grown by the above-mentioned process show very high resistivity. This fact indicates that our undoped film is very pure and nearly intrinsic. Slightly Si-doped GaN films grown using the AlN buffer layer have n-type conductivity with an electron mobility higher than 500 cm2/V s at room temperature, which is one order of magnitude higher than that of directly grown films. All these results (surface morphology, XRC, PL and electrical properties) clearly show that by the preceding deposition of the AlN buffer layer, the electrical and optical properties as well as the
430
I. Akasaki et al. I Conductivity control of GaN and fabrication of GaN LEDs
crystalline quality of GaN films can be remarkably improved. We have also succeeded in observing the first room temperature stimulated emission from a GaN film grown using the AlN buffer layer, which was cleaved in a l-2 mm stripe and excited with a 337.1 nm line of a pulsed nitrogen laser [4,5]. The peak photon energy of the stimulated emission was found to be 3.32 eV, which is about 70 meV lower than the band gap energy of GaN at room temperature.
3. Conductivity
control for n-type GaN films
The electron concentrations and resistivities of GaN can be easily controlled by changing the silane flow rate from the undoped level of less than 10” cme3 up to near lOi cmp3. The intensity of cathodoluminescence of the near band-edge emission increases with the increase in the doping level of Si in the GaN film.
4. Realization
of p-type GaN films
Doping of magnesium (Mg) was carried out during the growth of the GaN film by supplying biscycropentadienylmagnesium (Cp,Mg) as a Mg source gas. Compared to Zn, the vapor pressure of Mg is rather low, and/or the sticking coefficient of Mg at GaN surface is rather high. Therefore, the Mg concentration in GaN changed linearly with the supply flow rate of Cp,Mg. This relationship was almost independent of the substrate temperature. Thus we can easily obtain the desired Mg concentration and its profile in GaN by controlling the supply flow rate of Cp,Mg. It is difficult to determine the type of conductivity of as-grown Mg-doped GaN, because the resistivity is too high. The Mg-doped GaN tends to become low resistivity by low-energy electronbeam irradiation treatment (LEEBI-treatment). It was found that the film tends to show p-type conduction after the LEEBI treatment. An ohmic contact to the LEEBI-treated Mgdoped GaN layer was achieved by depositing
Au. Hole concentrations at room temperature up to about 1.4 X 10” cme3 can be achieved. Therefore, it can be said that Mg behaves as an acceptor impurity in GaN. The detailed mechanism of the LEEBI treatment is being studied at present. In the PL spectrum at 4.2 K of the Mg-doped GaN with Mg concentration less than 2 x 10” cm-3, D-A pair emission and its LOphonon replica can be clearly observed. On the contrary, in the spectrum of undoped GaN, DA pair emission did not appear, and the E,-line and I,-line appeared. Therefore, the origin of the D-A pair emission is thought to be residual donor (D) and doped Mg acceptors (A). This also shows that Mg acts as an acceptor impurity in GaN. The activation energy of the Mg acceptor was found to be about 155-165 meV, which is somewhat shallower than that of Zn (210 meV). As seen in the spectra of Mg-doped GaN with Mg concentration higher than 5 x 10’” cm-“, a strong blue emission appears even at room temperature. Therefore, it should be emphasized that Mg forms the blue luminescence centers as well as acting as an acceptor in GaN. By the LEEBI treatment, the intensity of the blue emission is remarkably enhanced keeping the shape of the spectrum. The enhancement of the blue luminescence intensity suggests the increase of Mg-related blue luminescence centers, which may be due to the redistribution of Mg atoms by the LEEBI treatment.
5. Characteristics
of p-n junction
UV/blue
LEDs
A typical DC-EL spectrum at room temperature observed from the newly developed diode shows the broad blue emission peaking at 423 nm, which is due to the Mg-associated transition in the p-GaN layer. Strong and sharp UV emission peaking at 372 nm is also observed, which is thought to originate from band-to-band transition in the n-type GaN layer. With an increase in the injection current, the intensity of the latter emission overcomes that of the blue
I. Akasaki et al. I Conductivity control of GaN and fabrication of GaN LEDs
one. An output power of the UV emission at room temperature of more than 1.5 mW at a forward current of 90 mA and bias voltage of 5.0 V have been achieved, which is the highest power ever reported in the LED mode operation of UV LEDs.
431
indicate that the DH structure is effective for optical confinement. The P-Al,,,,Ga,,,,r./GaN/N-Al,,,,Ga,,,N DH diode showed good I-V characteristics, and the same EL spectrum as that of homojunction.
7. Summary 6. Development of UV/blue emitting devices with AIGaN/GaN double heterostructures The effectiveness of the AIN buffer layer on the improvement of the crystalline quality of AlGaN films has been recently proved [lo]. The LEEBI treatment has been found to have the same effect on AlGaN film as in the case of GaN film. The PL measurements of the AlGaN/GaN multi-layered structure clearly showed quantum size effects [ll]. By using an AlGaN/GaN double heterostructure (DH), the threshold power for UV stimulated emission by optical pumping can be decreased to about 110 kW/cm’, which is about one-sixth of that of a homostructure. The threshold power of the DH structure varied with the thickness of the GaN active layer. The experimental results agree quite well with the calculated results as shown in fig. 1. These facts
GaN/Gat_,Al,N
DH x=0.10
Room Temp.
1
By MOVPE using the AlN buffer layer, the crystalline quality as well as the electrical and optical properties of GaN film can be remarkably improved. Conductivity control for both n-type GaN and AlGaN films has been achieved. GaN films having distinct p-type conduction have been realized for the first time by Mg doping followed by LEEBI treatment. High performance p-n junction UVIblue light emitting devices have been achieved. P-n junction AlGaN/GaN double heterostructure diodes with much lower threshold power have been developed. AlGaN/ GaN multi-layered structures showed quantum size effects.
Acknowledgements The authors are indebted to Professor K. Hiramatsu and Professor N. Sawaki of Nagoya University for valuable discussions. This work was partly supported by The Mitsubishi Foundation and a Grant-in-Aide from the Ministry of Education, Science and Culture of Japan for Scientific Research on Priority Areas “Crystal Growth Mechanism in Atomic Scale”.
References
ACTIVE LAYER THICKNESS, d( ,um) Fig. 1. Dependence of the threshold current density on the thickness of the GaN active layer in an AlGaNiGaNiAlGaN double heterostructure. 0, experimental data measured from the optical pumping; -, calculated data.
(11 M.A. Haase, J. Qiu, J.M. Depuydt and H. Cheng, Appl. Phys. Lett. 59 (1991) 1272. [2] H. Amano, N. Sawaki. I. Akasaki and Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [3] H. Amano, I. Akasaki, K. Hiramatsu, N. Koide and N. Sawaki, Thin Solid Films 163 (1988) 41.5. [4] H. Amano, T. Asahi and I. Akasaki, Jpn. J. Appl. Phys. 29 (1990) L205. [S] H. Amano, T. Asahi, M. Kito and 1. Akasaki, J. Lumin. 48 & 49 (1991) 889.
432
1. Akasaki et al. 1 Conductivity control of GaN and fabrication of GaN LEDs
[6] H. Amano and I. Akasaki, Extended Abstract of MRS’90 Fall Meeting EA-21 (1990) p. 165. [7] N. Koide, H. Kato, M. Sassa, S. Yamasaki, K. Manabe, H. Amano, K. Hiramatsu and I. Akasaki, J. Cryst. Growth 115 (1991) 639. [8] H. Murakami, T. Asahi, H. Amano, K. Hiramatsu, N. Sawaki and I. Akasaki, J. Cryst. Growth 115 (1991) 648.
[9] H. Amano, M. Kito, K. Hiramatsu and I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112. [lo] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu and N. Sawaki, J. Cryst. Growth 98 (1989) 209. [ll] K. Itoh, T. Kawamoto, H. Amano, K. Hiramatsu and I. Akasaki, Jpn. Appl. Phys. 30 (1991) 1924.
PHYSICA 1
428-432
Conductivity control of GaN and fabrication light emitting devices
of UV/blue
GaN
I. Akasaki and H. Amano Meijo University, Department of Electrical and Electronic Engineering,
Nagoya 468, Japan
N. Koide, M. Kotaki and K. Manabe Toyoda Gosei Co., Ltd, Department of Research and Development,
Haruhi-mura,
Aichi-prefecture.
Japan
The development of the technology for heteroepitaxial growth of high-crystalline-quality GaN films on sapphire substrates using AlN buffer layers, the establishment of a technique for control of conductivity for n-type GaN by Si-doping, and the realization of p-type Mg-doped GaN using low-energy electron-beam irradiation treatment have enabled us to fabricate a high performance, short wavelength LED based on GaN. In this paper, we review the recent developments of AlGaNiGaN multi-layered structures showing quantum-size effects, and of LEDs with AlGaNlGaN double heterostructures which emit light in the blue to UV region with output power of more than a few milliwatts at room temperature
1. Introduction
Bright light emitting diodes (LED) in the red to green region and a few milliwatt laser diodes (LD) in the red region have been commercially available for some time. To realize a new fullcolor display system, a pure blue LED has been eagerly demanded. The practical shorter wavelength LD also enables us to develop compact and high density optical storage systems and small medical apparatus. For the realization of these devices, the research and development of semiconductors with wide band gaps, so-called wide-band-gap semiconductors, is necessary. A great advance in this field has recently been given by 3M [l]. Blue-green LDs with emission wavelength of 512 nm at room temperature have been recently developed using a ZnCdSe-based quantum well structure. However, the wavelengths of their LDs or LEDs are still in the blue /green to green/blue region. Therefore, development of a pure blue light emitter and much Correspondence to: I. Akasaki, Meijo University, Department of Electrical and Electronic Engineering, l-501 Shiogamaguchi, Tempaku-ku, Nagoya-city 468, Japan. 0921-4526/93/$06.00
0
1993 - Elsevier
Science
Publishers
shorter wavelength (blue to UV) LDs is strongly desired. Gallium nitride (GaN) is a promising candidate as the material for fabrication of such short wavelength light emitters, because it has a direct transition-type band structure with a band gap energy of 3.39 eV at room temperature, which corresponds to a wavelength of 375 nm. The emission wavelength of a GaN-based LD will be less than three-fourths of that of a ZnCdSebased LD. Accordingly, the realization of much higher density optical storage systems can be anticipated by using a GaN-based LD. Concerning the blue and violet LED, bright LEDs with high external quantum efficiency can be obtained by using GaN-based materials because internal absorption loss in GaN and column III-nitride alloys is negligible in the blue and violet region. In contrast with other III-V compounds such as GaAs and InP, however, it had been fairly difficult to grow high quality epitaxial films with a flat surface free from cracks, because of the large lattice mismatch and the large difference in thermal expansion coefficient between the epitaxial film and the sapphire substrate.
B.V. All rights
reserved
I. Akasaki et al. I Conductivity control of GaN and fabrication of GaN LEDs
In 1986, we succeeded in overcoming these problems and growing high quality GaN films with a specular surface free from cracks by the prior deposition of a thin AlN buffer layer in MOVPE growth of GaN [2]. The electrical and optical properties as well as the crystalline quality can be remarkably improved at the same time [3]. By using such a high quality GaN film, UV stimulated emission at room temperature by optical pumping was achieved for the first time [4,5]. Practical bright MIS-type blue-LEDs have been developed by using the same GaN film. The brightness is typically about 100 mcd, and 200mcd maximum at a forward current of 10 mA, which is the highest in the blue region and comparable with that of commercially available GaAsP red LEDs and GaP green LEDs. Silicon was found to act as a donor impurity in GaN [6,7] as well as in AlGaN [8], and the free electron concentration has been controlled from undoped level up to near 10” cm-‘. It has been well known that undoped GaN shows n-type conduction and p-type GaN had never been realized. We succeeded in producing p-type GaN in 1989 [9] and p-type AlGaN in 1991 by low-energy electron-beam irradiation (LEEBI) treatment of Mg-doped films. On the basis of these result, we developed the first p-n junction GaN UV/blue LED. In this paper, (1) the conductivity control of GaN for both n-type and p-type and (2) the performance of the UV/blue LED are described.
2. MOVPE growth of high-quality films on sapphire buffer layers
substrates
GaN thin using AIN
A horizontal type MOVPE reactor operated at atmospheric pressure was used for the growth of the GaN film. Trimethylgallium (TMGa), trimethylaluminum (TMAl) and ammonia (NH,) were used as source gases and hydrogen (H,) as a carrier gas. Polished sapphire (0 0 0 1) crystals were used as the substrate. Misorientation was less than 1”. In our process, before GaN growth, a thin AlN
429
layer about 50 nm thick was deposited at 600°C by feeding TMAl and NH, diluted with H,. Then the temperature was raised to 105O”C, and a single crystal GaN film several micrometers thick was grown. The surface morphology of GaN films can be remarkably improved by the preceding deposition of the AlN as a buffer layer. GaN films with optically flat surfaces free from cracks can be grown on the sapphire substrate covered with an AlN buffer layer. On the contrary, island growth occurred in the growth of GaN on the bare sapphire substrate surface. X-Ray rocking curve (XRC) measurements also revealed that GaN grown using the AlN buffer layer has high quality. The full width at half maximum of the GaN film grown with the AlN buffer layer is about 110 arcsec, which is the narrowest to date in this material; that of the GaN film grown directly on the sapphire substrate was more than 1000 arcsec. Photoluminescence (PL) measurements also showed that the luminescence properties of the GaN film can be improved by using the AlN buffer layer. In the PL spectrum at 4.2 K of GaN grown with the buffer layer, the free exciton line (E,) and the donor-bound exciton line (I,) clearly appear, while emission bands in the long wavelength region, which may be due to deeplevel defects, are scarcely observed. On the other hand, emission bands at long wavelengths dominated in the spectrum of the GaN film grown directly on the sapphire substrate. This indicates that the generation of deep-level defects can be suppressed by using the buffer layer. GaN films grown by the above-mentioned process show very high resistivity. This fact indicates that our undoped film is very pure and nearly intrinsic. Slightly Si-doped GaN films grown using the AlN buffer layer have n-type conductivity with an electron mobility higher than 500 cm2/V s at room temperature, which is one order of magnitude higher than that of directly grown films. All these results (surface morphology, XRC, PL and electrical properties) clearly show that by the preceding deposition of the AlN buffer layer, the electrical and optical properties as well as the
430
I. Akasaki et al. I Conductivity control of GaN and fabrication of GaN LEDs
crystalline quality of GaN films can be remarkably improved. We have also succeeded in observing the first room temperature stimulated emission from a GaN film grown using the AlN buffer layer, which was cleaved in a l-2 mm stripe and excited with a 337.1 nm line of a pulsed nitrogen laser [4,5]. The peak photon energy of the stimulated emission was found to be 3.32 eV, which is about 70 meV lower than the band gap energy of GaN at room temperature.
3. Conductivity
control for n-type GaN films
The electron concentrations and resistivities of GaN can be easily controlled by changing the silane flow rate from the undoped level of less than 10” cme3 up to near lOi cmp3. The intensity of cathodoluminescence of the near band-edge emission increases with the increase in the doping level of Si in the GaN film.
4. Realization
of p-type GaN films
Doping of magnesium (Mg) was carried out during the growth of the GaN film by supplying biscycropentadienylmagnesium (Cp,Mg) as a Mg source gas. Compared to Zn, the vapor pressure of Mg is rather low, and/or the sticking coefficient of Mg at GaN surface is rather high. Therefore, the Mg concentration in GaN changed linearly with the supply flow rate of Cp,Mg. This relationship was almost independent of the substrate temperature. Thus we can easily obtain the desired Mg concentration and its profile in GaN by controlling the supply flow rate of Cp,Mg. It is difficult to determine the type of conductivity of as-grown Mg-doped GaN, because the resistivity is too high. The Mg-doped GaN tends to become low resistivity by low-energy electronbeam irradiation treatment (LEEBI-treatment). It was found that the film tends to show p-type conduction after the LEEBI treatment. An ohmic contact to the LEEBI-treated Mgdoped GaN layer was achieved by depositing
Au. Hole concentrations at room temperature up to about 1.4 X 10” cme3 can be achieved. Therefore, it can be said that Mg behaves as an acceptor impurity in GaN. The detailed mechanism of the LEEBI treatment is being studied at present. In the PL spectrum at 4.2 K of the Mg-doped GaN with Mg concentration less than 2 x 10” cm-3, D-A pair emission and its LOphonon replica can be clearly observed. On the contrary, in the spectrum of undoped GaN, DA pair emission did not appear, and the E,-line and I,-line appeared. Therefore, the origin of the D-A pair emission is thought to be residual donor (D) and doped Mg acceptors (A). This also shows that Mg acts as an acceptor impurity in GaN. The activation energy of the Mg acceptor was found to be about 155-165 meV, which is somewhat shallower than that of Zn (210 meV). As seen in the spectra of Mg-doped GaN with Mg concentration higher than 5 x 10’” cm-“, a strong blue emission appears even at room temperature. Therefore, it should be emphasized that Mg forms the blue luminescence centers as well as acting as an acceptor in GaN. By the LEEBI treatment, the intensity of the blue emission is remarkably enhanced keeping the shape of the spectrum. The enhancement of the blue luminescence intensity suggests the increase of Mg-related blue luminescence centers, which may be due to the redistribution of Mg atoms by the LEEBI treatment.
5. Characteristics
of p-n junction
UV/blue
LEDs
A typical DC-EL spectrum at room temperature observed from the newly developed diode shows the broad blue emission peaking at 423 nm, which is due to the Mg-associated transition in the p-GaN layer. Strong and sharp UV emission peaking at 372 nm is also observed, which is thought to originate from band-to-band transition in the n-type GaN layer. With an increase in the injection current, the intensity of the latter emission overcomes that of the blue
I. Akasaki et al. I Conductivity control of GaN and fabrication of GaN LEDs
one. An output power of the UV emission at room temperature of more than 1.5 mW at a forward current of 90 mA and bias voltage of 5.0 V have been achieved, which is the highest power ever reported in the LED mode operation of UV LEDs.
431
indicate that the DH structure is effective for optical confinement. The P-Al,,,,Ga,,,,r./GaN/N-Al,,,,Ga,,,N DH diode showed good I-V characteristics, and the same EL spectrum as that of homojunction.
7. Summary 6. Development of UV/blue emitting devices with AIGaN/GaN double heterostructures The effectiveness of the AIN buffer layer on the improvement of the crystalline quality of AlGaN films has been recently proved [lo]. The LEEBI treatment has been found to have the same effect on AlGaN film as in the case of GaN film. The PL measurements of the AlGaN/GaN multi-layered structure clearly showed quantum size effects [ll]. By using an AlGaN/GaN double heterostructure (DH), the threshold power for UV stimulated emission by optical pumping can be decreased to about 110 kW/cm’, which is about one-sixth of that of a homostructure. The threshold power of the DH structure varied with the thickness of the GaN active layer. The experimental results agree quite well with the calculated results as shown in fig. 1. These facts
GaN/Gat_,Al,N
DH x=0.10
Room Temp.
1
By MOVPE using the AlN buffer layer, the crystalline quality as well as the electrical and optical properties of GaN film can be remarkably improved. Conductivity control for both n-type GaN and AlGaN films has been achieved. GaN films having distinct p-type conduction have been realized for the first time by Mg doping followed by LEEBI treatment. High performance p-n junction UVIblue light emitting devices have been achieved. P-n junction AlGaN/GaN double heterostructure diodes with much lower threshold power have been developed. AlGaN/ GaN multi-layered structures showed quantum size effects.
Acknowledgements The authors are indebted to Professor K. Hiramatsu and Professor N. Sawaki of Nagoya University for valuable discussions. This work was partly supported by The Mitsubishi Foundation and a Grant-in-Aide from the Ministry of Education, Science and Culture of Japan for Scientific Research on Priority Areas “Crystal Growth Mechanism in Atomic Scale”.
References
ACTIVE LAYER THICKNESS, d( ,um) Fig. 1. Dependence of the threshold current density on the thickness of the GaN active layer in an AlGaNiGaNiAlGaN double heterostructure. 0, experimental data measured from the optical pumping; -, calculated data.
(11 M.A. Haase, J. Qiu, J.M. Depuydt and H. Cheng, Appl. Phys. Lett. 59 (1991) 1272. [2] H. Amano, N. Sawaki. I. Akasaki and Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353. [3] H. Amano, I. Akasaki, K. Hiramatsu, N. Koide and N. Sawaki, Thin Solid Films 163 (1988) 41.5. [4] H. Amano, T. Asahi and I. Akasaki, Jpn. J. Appl. Phys. 29 (1990) L205. [S] H. Amano, T. Asahi, M. Kito and 1. Akasaki, J. Lumin. 48 & 49 (1991) 889.
432
1. Akasaki et al. 1 Conductivity control of GaN and fabrication of GaN LEDs
[6] H. Amano and I. Akasaki, Extended Abstract of MRS’90 Fall Meeting EA-21 (1990) p. 165. [7] N. Koide, H. Kato, M. Sassa, S. Yamasaki, K. Manabe, H. Amano, K. Hiramatsu and I. Akasaki, J. Cryst. Growth 115 (1991) 639. [8] H. Murakami, T. Asahi, H. Amano, K. Hiramatsu, N. Sawaki and I. Akasaki, J. Cryst. Growth 115 (1991) 648.
[9] H. Amano, M. Kito, K. Hiramatsu and I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112. [lo] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu and N. Sawaki, J. Cryst. Growth 98 (1989) 209. [ll] K. Itoh, T. Kawamoto, H. Amano, K. Hiramatsu and I. Akasaki, Jpn. Appl. Phys. 30 (1991) 1924.