- Email: [email protected]
InGaN quantum-well structure blue LEDs and LDs
JOURNAL OF
LUMINESCENC Journal of Luminescence 72-74 (1997) 55-58
InGaN quantum-well
structure blue LEDs and LDs
Shuji Nakamura* R&D Department, Nichia Chemical Industries Ltd., 491 Oka, Kaminaka, Anan, Tokushima 774, Japan
Abstract High-power InGaN single-quantum-well structure (SQW) blue/green light-emitting diodes (LEDs) and violet multiquantum-well (MQW) structure laser diodes (LDs) were fabricated. The LDs emitted coherent light at 390-440 nm from an InGaN-based MQW structure at room temperature. Lasing was observed up to a pulsed current duty ratio of 40%. The operating voltage of the LDs at the threshold was around 11 V. The emission of the SQW LEDs is due to a recombination of excitons localized at certain potential minima in InGaN quantum well. Keywords:
InGaN; Exciton; Blue LEDs; LDs
1. Introduction GaN and related materials such as AlGaInN are III-V nitride semiconductors with the wurtzite crystal structure and a direct energy band structure which is suitable for light-emitting devices. The band-gap energy of AlGaInN varies between 6.2 and 1.95 eV depending on its composition at room temperature. Therefore, these III-V nitride semiconductors are useful for light-emitting devices especially in the short wavelength regions. Recent research on III-V nitrides has paved the way for the realization of high-quality crystals of GaN, AlGaN and InGaN, and of p-type conduction in GaN and AlGaN [l-3]. High-brightness blue LEDs have been fabricated on the basis of these results, and luminous intensities over 1 cd have been achieved [4]. Also, high brightness singlequantum-well structure (SQW) blue, green and
*Tel. + 81-884-22-2311; fax: [email protected].
+ 81-884-23-1802; e-mail:
yellow InGaN light-emitting diodes (LEDs) with a luminous intensity of 10cd have been achieved and commercialized [S]. Then, in 1995, the first current-injection III-V nitride-based laser diodes (LDs) were fabricated using the InGaN multiquantum-well (MQW) structure as an active layer [6]. The laser emission wavelength of 390440nm was the shortest one ever generated by a semiconductor LD. Here, recent studies on an AlGaInNbased light-emitting device are described.
2. InGaN single-quantum-well (SQW) structure LEDs Thegreen SQW LED device structures consist of a 300A GaN buffer layer grown at a low temperature0(550”C), a 4 urn-thick layer of n-type GaN : Si, a 30A-thick active layer of undoped In0.45Ga,,55N, a lOOOA-thick layer of p-type Alo,zGao.aN: Mg, and a 0.5 urn-thick layer of p-type GaN: Mg [S]. The sctive region is a SQW structure consisting of a 30A-thick Ino,45Ga 0.55N well layer sandwiched
0022-2313/97/$17.00 c’ 1997 Elsevier Science B.V. All rights reserved PI1 SOO22-2313(96)00335-3
56
S. Nakamura 1 Journal of Luminescence
100
I-
400
(a) Blue
(b) Green
72- 74 (I 997) 55-58
3. InGaN multi-quantum-well
(c) Yellow
(MQW)
structure
LDs
450
500
550
600
650
700
Wavelength(nm) Fig. 1. EL of (a) blue, (b) green and (c) yellow SQW LEDs at a forward current of 20mA.
by 4 urn-thick n-type GaN and 1000 A-thick p-type A10.2Ga0,8N barrier layers. These structures were grown on a (0001) face sapphire substrate. Fig. 1 shows the typical electroluminescence (EL) of the blue, green and yellow SQW LEDs with different indium mole fractions of the InGaN well layer at a forward current of 20 mA. The peak wavelength and the full-width at half-maximum (FWHM) of the typical blue SQW LEDs are 450nm and 20nm, respectively, those of the green SQW LEDs are 520 and 30nm, respectively, and those of yellow are 600 and 50 nm, respectively. The EL peak energy of the green SQW LEDs showed blue shifts by about 100meV with increasing the driving current from i uA to 80mA. Similar blue shift was found in both of blue and yellow SQW LEDs. When the peak wavelength becomes longer, the FWHM of the EL spectra increases, probably due to the inhomogeneities of InGaN layer or the strain between well and barrier layers of the SQW, which is caused by mismatch of the lattice and the thermal expansion coefficients between well and barrier layers. At 20 mA, the output power and the external quantum efficiency of the blue SQW LEDs are 5 mW and 9.1%, respectively. Those of the green SQW LEDs are 3 mW and 6.3%, respectively. A typical on-axis luminous intensity of the green SQW LEDs with a lo” cone viewing angle is 10 cd at 20mA. These values of output power, external quantum efficiency and luminous intensity of blue/green SQW LEDs are the highest ever reported for blue/green LEDs.
The ridge-geometry InGaN MQW LDs were fabricated [6]. The active layer is an InO.zGaO,eN/ In o,,osGao.95N MQW structure consisting of three 30 A-thick undoped In,,,GaO.aN well layer! forming the gain medium, separated by 60A-thick undoped Ino.osGao.gs N barrier layers. The 0.1 urnthick n-type and p-type GaN layers were lightguiding layers. The 0.5 urn-thick n-type and p-type Alo. 1ZGa0.88N layers were cladding layers for confinement of the carriers and the light emitted from the active region of the InGaN MQW structure. p-type GaN layer was used as a contact layer of p-electrode. The area of the ridge-geometry LD was 2 urn x 700 urn. High-reflection facet coatings (30%) consisting of two pairs of quarter-wave Ti02/SiOz dielectric multilayers were used to reduce the threshold current. The electrical characteristics of LDs were measured under pulsed current-biased conditions at RT. No stimulated emission was observed up to a threshold current of 90 mA, which corresponded to a threshold current density of 6.4 kA/cm2. A differential quantum efficiency of 13% per facet and a pulsed output power of 3mW per facet were obtained at a current of lOOmA. The operating voltage at the threshold current was 11 V. When the duty ratio of the pulsed current was increased from 1% to 30 %, the threshold current increased from 90mA to 120mA due to heat generation. At a duty ratio of 40%, the output power of the stimulated emission saturated at a current of 160mA. Fig. 2 shows typical optical spectra of the InGaN MQW LDs under pulsed current injection at room temperature. At injection currents around the threshold currents, many sharp peaks appeared with a peak separation of 0.04 nm, as shown in Fig. 2(a), where J is a current density and Jth is a threshold current density. If these peaks arise from the longitudinal modes, the mode separation AE,is given by Ai, = Ao2/2/L/n, where n is the refractive index and &, is the emission wavelength (404.2nm). L was 0.07 cm. A value of 2.54 was used for the refractive index. Thus, Ai, is calculated as 0.04 nm. Therefore, the observed peak separation is the longitudinal mode separation. When the forward current was
57
S. Nakamura /Journal of Luminescence 72-74 (1997) 55-58
observed, as shown in Fig. 2(c). The origin of these subband emissions is not clear at present.
(c) J = 1.164Jtb
4. Emission mechanism
403
404
405
406
Wavelength (nm) (b) J = 1.073Jth
Static (DC) electroluminesence (EL), photovoltage (PV) and modulated-electroabsorption (EA) spectra were measured on the above-mentioned SQW LEDs and MQW LDs [7]. Fig. 3 summarizes room-temperature EL, PV and EA spectra of green, blue SQW LED and MQW LD structure. The EL spectrum of the MQW LD structure was measured below the threshold current density. The
-!
InGaN SQW-LED,
MQw-LD
Photovoltage, Electroabsorption, and Electroluminescence Q 300K
r
Green LED x=0.45
402
403
404
405
I
406
Wavelength (urn)
I
402
403
(a) J = 1.018Jth
404
405
406
Wavelength (urn) Fig. 2. Optical spectra for the InGaN MQW LD. (a) at J = 1.0185& (b) at J = 1.073J,h; (c) at J = 1.1645,,. Intensity scales for these three spectra are in arbitrary units, and are different. 2.0
increased, the main peak become dominant at the wavelength of 404.2nm, as shown in Fig. 2(b). At J = 1.1645,,, , several peaks which had a different peak separation from the longitudinal mode were
2.5
PHOTON
3.0
ENERGY
3.5
4.0
(eV)
Fig. 3. EL, PV and EA spectra for InGaN green and blue SQW LED structure and MQW laser structure. The EL spectrum of the MQW structure was measured below the threshold current.
58
S. Nakamura / Journal
qf Luminescence 72- 74 (1997) 55-58
lasing emission of this MQW LD appeared at 3.052eV (406nm) at the threshold current density of 11.3 kA/cm2. In general, the low-field EA monitors exciton resonance rather than band-to-band transition even at RT in wide gap semiconductor such as GaN under certain condition that the modulation field is smaller than that to dissociate excitons [8]. The PV spectra were taken using a monochromated light, and the open-circuit voltage of the device was measured spectroscopically. The PV peak at 3.21,2.91 and 2.93eV for MQW, blue and green SQW structures correspond to exciton absorption in the quantum well, because the energies agree with those in the EA spectra. It is recognized that FWHM of the PV peak increases with increasing x. The PV peak energy decreases from 3.21 to 2.91 eV with increasing x from 0.2 to 0.45. However, the peak energy is almost unchanged for x = 0.3 and x = 0.45. This implies that InGaN does not form perfect alloys, but form compositional tailing especially for larger x. Such a compositional tailing in the quantum-well plane can produce two-dimentional potential minima in the InGaN well layer. The EL peak energy is smaller by 100, 215 and 570 meV than the free exciton resonance absorption energy from MQW LD, blue and green SQW LED, respectively. All EL peaks are located at
the low-energy tail of the free exciton resonance. Such low-energy tails of the exciton structure reflect a presence of certain potential minima in the quantum-well plane. These EL emissions are considered as recombination of excitons localized at certain potential minima in the InGaN quantum well.
References 111H. Morkoc.
S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov and M. Burns, J. Appl. Phys. 76 (1994) 1363. c21 H. Amano, M. Kito, K. Hiramatsu and I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112. c31S. Nakamura and T. Mukai, Jpn. J. Appl. Phys. 31 (1992) L1457. r41 S. Nakamura, T. Mukai and M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. M. Senoh, N. Iwasa, S. Nagahama, T. c51S. Nakamura, Yamada and T. Mukai, Jpn. J. Appl. Phys. 34(1995) L1332. M. Senoh, S. Nagahama, N. Iwasa, T. 161S. Nakamura, Yamada, T. Matsushita, H. Kiyoku and Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74; Jpn. J. Appl. Phys. 35 (1996) L217; Appl. Phys. Lett. 68 (1996) 2105; Appl. Phys. Lett. 68 (1996) 3269. 38th [71 S. Chichibu, T. Azuhata, T. Sota and S. Nakamura, Electronic Material Conf. WlO (Santa Barbara, USA, June 28, 1996) Appl. Phys. Lett., submitted (unpublished). PI S. Chichibu, T. Azuhata, T. Sota and S. Nakamura, J. Appl. Phys. 79 (1996) 2784.
LUMINESCENC Journal of Luminescence 72-74 (1997) 55-58
InGaN quantum-well
structure blue LEDs and LDs
Shuji Nakamura* R&D Department, Nichia Chemical Industries Ltd., 491 Oka, Kaminaka, Anan, Tokushima 774, Japan
Abstract High-power InGaN single-quantum-well structure (SQW) blue/green light-emitting diodes (LEDs) and violet multiquantum-well (MQW) structure laser diodes (LDs) were fabricated. The LDs emitted coherent light at 390-440 nm from an InGaN-based MQW structure at room temperature. Lasing was observed up to a pulsed current duty ratio of 40%. The operating voltage of the LDs at the threshold was around 11 V. The emission of the SQW LEDs is due to a recombination of excitons localized at certain potential minima in InGaN quantum well. Keywords:
InGaN; Exciton; Blue LEDs; LDs
1. Introduction GaN and related materials such as AlGaInN are III-V nitride semiconductors with the wurtzite crystal structure and a direct energy band structure which is suitable for light-emitting devices. The band-gap energy of AlGaInN varies between 6.2 and 1.95 eV depending on its composition at room temperature. Therefore, these III-V nitride semiconductors are useful for light-emitting devices especially in the short wavelength regions. Recent research on III-V nitrides has paved the way for the realization of high-quality crystals of GaN, AlGaN and InGaN, and of p-type conduction in GaN and AlGaN [l-3]. High-brightness blue LEDs have been fabricated on the basis of these results, and luminous intensities over 1 cd have been achieved [4]. Also, high brightness singlequantum-well structure (SQW) blue, green and
*Tel. + 81-884-22-2311; fax: [email protected].
+ 81-884-23-1802; e-mail:
yellow InGaN light-emitting diodes (LEDs) with a luminous intensity of 10cd have been achieved and commercialized [S]. Then, in 1995, the first current-injection III-V nitride-based laser diodes (LDs) were fabricated using the InGaN multiquantum-well (MQW) structure as an active layer [6]. The laser emission wavelength of 390440nm was the shortest one ever generated by a semiconductor LD. Here, recent studies on an AlGaInNbased light-emitting device are described.
2. InGaN single-quantum-well (SQW) structure LEDs Thegreen SQW LED device structures consist of a 300A GaN buffer layer grown at a low temperature0(550”C), a 4 urn-thick layer of n-type GaN : Si, a 30A-thick active layer of undoped In0.45Ga,,55N, a lOOOA-thick layer of p-type Alo,zGao.aN: Mg, and a 0.5 urn-thick layer of p-type GaN: Mg [S]. The sctive region is a SQW structure consisting of a 30A-thick Ino,45Ga 0.55N well layer sandwiched
0022-2313/97/$17.00 c’ 1997 Elsevier Science B.V. All rights reserved PI1 SOO22-2313(96)00335-3
56
S. Nakamura 1 Journal of Luminescence
100
I-
400
(a) Blue
(b) Green
72- 74 (I 997) 55-58
3. InGaN multi-quantum-well
(c) Yellow
(MQW)
structure
LDs
450
500
550
600
650
700
Wavelength(nm) Fig. 1. EL of (a) blue, (b) green and (c) yellow SQW LEDs at a forward current of 20mA.
by 4 urn-thick n-type GaN and 1000 A-thick p-type A10.2Ga0,8N barrier layers. These structures were grown on a (0001) face sapphire substrate. Fig. 1 shows the typical electroluminescence (EL) of the blue, green and yellow SQW LEDs with different indium mole fractions of the InGaN well layer at a forward current of 20 mA. The peak wavelength and the full-width at half-maximum (FWHM) of the typical blue SQW LEDs are 450nm and 20nm, respectively, those of the green SQW LEDs are 520 and 30nm, respectively, and those of yellow are 600 and 50 nm, respectively. The EL peak energy of the green SQW LEDs showed blue shifts by about 100meV with increasing the driving current from i uA to 80mA. Similar blue shift was found in both of blue and yellow SQW LEDs. When the peak wavelength becomes longer, the FWHM of the EL spectra increases, probably due to the inhomogeneities of InGaN layer or the strain between well and barrier layers of the SQW, which is caused by mismatch of the lattice and the thermal expansion coefficients between well and barrier layers. At 20 mA, the output power and the external quantum efficiency of the blue SQW LEDs are 5 mW and 9.1%, respectively. Those of the green SQW LEDs are 3 mW and 6.3%, respectively. A typical on-axis luminous intensity of the green SQW LEDs with a lo” cone viewing angle is 10 cd at 20mA. These values of output power, external quantum efficiency and luminous intensity of blue/green SQW LEDs are the highest ever reported for blue/green LEDs.
The ridge-geometry InGaN MQW LDs were fabricated [6]. The active layer is an InO.zGaO,eN/ In o,,osGao.95N MQW structure consisting of three 30 A-thick undoped In,,,GaO.aN well layer! forming the gain medium, separated by 60A-thick undoped Ino.osGao.gs N barrier layers. The 0.1 urnthick n-type and p-type GaN layers were lightguiding layers. The 0.5 urn-thick n-type and p-type Alo. 1ZGa0.88N layers were cladding layers for confinement of the carriers and the light emitted from the active region of the InGaN MQW structure. p-type GaN layer was used as a contact layer of p-electrode. The area of the ridge-geometry LD was 2 urn x 700 urn. High-reflection facet coatings (30%) consisting of two pairs of quarter-wave Ti02/SiOz dielectric multilayers were used to reduce the threshold current. The electrical characteristics of LDs were measured under pulsed current-biased conditions at RT. No stimulated emission was observed up to a threshold current of 90 mA, which corresponded to a threshold current density of 6.4 kA/cm2. A differential quantum efficiency of 13% per facet and a pulsed output power of 3mW per facet were obtained at a current of lOOmA. The operating voltage at the threshold current was 11 V. When the duty ratio of the pulsed current was increased from 1% to 30 %, the threshold current increased from 90mA to 120mA due to heat generation. At a duty ratio of 40%, the output power of the stimulated emission saturated at a current of 160mA. Fig. 2 shows typical optical spectra of the InGaN MQW LDs under pulsed current injection at room temperature. At injection currents around the threshold currents, many sharp peaks appeared with a peak separation of 0.04 nm, as shown in Fig. 2(a), where J is a current density and Jth is a threshold current density. If these peaks arise from the longitudinal modes, the mode separation AE,is given by Ai, = Ao2/2/L/n, where n is the refractive index and &, is the emission wavelength (404.2nm). L was 0.07 cm. A value of 2.54 was used for the refractive index. Thus, Ai, is calculated as 0.04 nm. Therefore, the observed peak separation is the longitudinal mode separation. When the forward current was
57
S. Nakamura /Journal of Luminescence 72-74 (1997) 55-58
observed, as shown in Fig. 2(c). The origin of these subband emissions is not clear at present.
(c) J = 1.164Jtb
4. Emission mechanism
403
404
405
406
Wavelength (nm) (b) J = 1.073Jth
Static (DC) electroluminesence (EL), photovoltage (PV) and modulated-electroabsorption (EA) spectra were measured on the above-mentioned SQW LEDs and MQW LDs [7]. Fig. 3 summarizes room-temperature EL, PV and EA spectra of green, blue SQW LED and MQW LD structure. The EL spectrum of the MQW LD structure was measured below the threshold current density. The
-!
InGaN SQW-LED,
MQw-LD
Photovoltage, Electroabsorption, and Electroluminescence Q 300K
r
Green LED x=0.45
402
403
404
405
I
406
Wavelength (urn)
I
402
403
(a) J = 1.018Jth
404
405
406
Wavelength (urn) Fig. 2. Optical spectra for the InGaN MQW LD. (a) at J = 1.0185& (b) at J = 1.073J,h; (c) at J = 1.1645,,. Intensity scales for these three spectra are in arbitrary units, and are different. 2.0
increased, the main peak become dominant at the wavelength of 404.2nm, as shown in Fig. 2(b). At J = 1.1645,,, , several peaks which had a different peak separation from the longitudinal mode were
2.5
PHOTON
3.0
ENERGY
3.5
4.0
(eV)
Fig. 3. EL, PV and EA spectra for InGaN green and blue SQW LED structure and MQW laser structure. The EL spectrum of the MQW structure was measured below the threshold current.
58
S. Nakamura / Journal
qf Luminescence 72- 74 (1997) 55-58
lasing emission of this MQW LD appeared at 3.052eV (406nm) at the threshold current density of 11.3 kA/cm2. In general, the low-field EA monitors exciton resonance rather than band-to-band transition even at RT in wide gap semiconductor such as GaN under certain condition that the modulation field is smaller than that to dissociate excitons [8]. The PV spectra were taken using a monochromated light, and the open-circuit voltage of the device was measured spectroscopically. The PV peak at 3.21,2.91 and 2.93eV for MQW, blue and green SQW structures correspond to exciton absorption in the quantum well, because the energies agree with those in the EA spectra. It is recognized that FWHM of the PV peak increases with increasing x. The PV peak energy decreases from 3.21 to 2.91 eV with increasing x from 0.2 to 0.45. However, the peak energy is almost unchanged for x = 0.3 and x = 0.45. This implies that InGaN does not form perfect alloys, but form compositional tailing especially for larger x. Such a compositional tailing in the quantum-well plane can produce two-dimentional potential minima in the InGaN well layer. The EL peak energy is smaller by 100, 215 and 570 meV than the free exciton resonance absorption energy from MQW LD, blue and green SQW LED, respectively. All EL peaks are located at
the low-energy tail of the free exciton resonance. Such low-energy tails of the exciton structure reflect a presence of certain potential minima in the quantum-well plane. These EL emissions are considered as recombination of excitons localized at certain potential minima in the InGaN quantum well.
References 111H. Morkoc.
S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov and M. Burns, J. Appl. Phys. 76 (1994) 1363. c21 H. Amano, M. Kito, K. Hiramatsu and I. Akasaki, Jpn. J. Appl. Phys. 28 (1989) L2112. c31S. Nakamura and T. Mukai, Jpn. J. Appl. Phys. 31 (1992) L1457. r41 S. Nakamura, T. Mukai and M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. M. Senoh, N. Iwasa, S. Nagahama, T. c51S. Nakamura, Yamada and T. Mukai, Jpn. J. Appl. Phys. 34(1995) L1332. M. Senoh, S. Nagahama, N. Iwasa, T. 161S. Nakamura, Yamada, T. Matsushita, H. Kiyoku and Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74; Jpn. J. Appl. Phys. 35 (1996) L217; Appl. Phys. Lett. 68 (1996) 2105; Appl. Phys. Lett. 68 (1996) 3269. 38th [71 S. Chichibu, T. Azuhata, T. Sota and S. Nakamura, Electronic Material Conf. WlO (Santa Barbara, USA, June 28, 1996) Appl. Phys. Lett., submitted (unpublished). PI S. Chichibu, T. Azuhata, T. Sota and S. Nakamura, J. Appl. Phys. 79 (1996) 2784.