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High-Power and High-Efficiency True Green Laser Diodes Pdf 285 KB ELECTRONICS High-Power and High-Efficiency True Green Laser Diodes Shimpei TAKAGI*, Masaki UENO, Koji KATAYAMA, Takatoshi IKEGAMI, Takao NAKAMURA and Katsunori YANASHIMA The authors demonstrated InGaN green laser diodes (LDs) that were grown on semipolar {2021} GaN substrate and achieved output power of over 100 mW in the spectral region beyond 530 nm. In the range of 525-532 nm, these LDs realized wall plug efficiencies as high as 7.0-8.9%, which exceed those reported for c-plane LDs. Moreover, the InGaN green LDs are expected to have a long lifetime of over 5,000 hours under auto power control of 50 mW at a case temperature of 55˚C. These results suggest that the InGaN green LDs grown on the {2021} plane are ideal as light sources for applications that require wavelengths of over 525 nm. Keywords: semipolar, GaN, green, semiconductor, laser 1. Introduction Although lasing wavelengths of 499 nm on a nonpolar m- plane(7) and 426 nm on a semipolar (1122) plane have been Recently, the development of InGaN-based green laser reported(8), both planes are subject to the problem of the diodes (LDs) has been the subject of extensive studies since crystalline quality of InGaN QWs. Under these circum- these lasers would find immediate application in red-green- stances, we have been investigating other novel planes, put- blue (RGB) laser projectors, enabling high color repro- ting priority on the crystalline quality rather than reducing ducibility, focus-free operation, and power saving at the the piezoelectric fields to zero, and have found out the su- same time. Although green lasers based on SHG*1 tech- periority of a {2021} plane. By fabricating LDs on this plane, nologies are already available, semiconductor LDs have ad- we have successfully demonstrated the world’s first true vantages in size, stability, and efficiency for commercial green pulsed operation at 531 nm in 2009(9), followed by application. CW operation at 520 nm(10) and lower threshold CW oper- 2 (11) It is known that two problems exist in achieving longer ation at 525 nm with Jth = 4.3 kA/cm . wavelength on InGaN-based green LDs. One is a decrease This time, in a joint development with Sony Corpora- in luminescent efficiency due to piezoelectric fields. Blue- tion, we have succeeded in the CW operation of InGaN violet LDs, which were the first InGaN-based LDs used in green LDs fabricated on semipolar {2021} GaN substrates Blue-ray players, employ (0001) plane (c -plane) GaN sub- with output power of over 100 mW in the spectral region strates. On the c -plane, large piezoelectric fields are cre- beyond 530 nm. We have also demonstrated the long-term ated in the direction of the c -axis due to a spontaneous reliability of these LDs at 55˚C. In this paper, we report on polarization caused by the asymmetry of the crystal as well the recent progress on development of the semipolar as the strain resulting from a lattice mismatch between the {2021} green LDs. InGaN active layer and the GaN substrate. These piezoelec- tric fields spatially separate an electron wave function and a hole wave function in the quantum wells (QWs) and re- duce luminescent efficiency. The second problem is In 2. Experiment compositional fluctuations with increasing In contents in the InGaN active layer caused by the immiscibility of InN First, we focus on the semipolar {2021} plane GaN sub- with GaN. Although these fluctuations improve lumines- strates we utilized. As shown in Fig. 1, this is a typical low- cent efficiency on blue LEDs*2 due to the localized car- index plane tilted at 75.1˚ to the c -plane towards the rier(1), they generate non-radiative defects on green LDs. m-axis, and both Ga and N atoms exist at the surface of this Moreover, these fluctuations increase the luminescence plane. While the piezoelectric fields on a nonpolar m-plane spectrum width and reduce the differential gain, leading and a semipolar (1122) plane reduce to zero, those on the to the increase of operating currents(2),(3). {2021} plane are one-third of those on the c -plane and ori- In spite of these problems, many groups have suc- ented in opposite direction. ceeded in lasing in the spectral range from the blue-green The semipolar {2021} plane GaN substrates were pro- to the green regions since 2009. Although the wavelength duced by HVPE*4. The substrates exhibit n-type conductivity over 500 nm was believed to be difficult to achieve on the and threading dislocation densities of less than 1 × 106 cm-2 (12). c -plane, CW*3 operation at 515 nm has been demonstrated The LD structures were grown by OMVPE*5. An n-type GaN at room temperature (RT) by improving the crystalline layer was grown directly on the GaN substrate, followed by quality of QWs(4),(5). More recently, a lasing wavelength of an n-type InAlGaN cladding layer, an n-type InGaN wave- 531 nm has also been reported(6). In addition, an approach guiding layer, an InGaN multiple QW active region, a p-type using a nonpolar or semipolar plane has been carried out AlGaN electron-blocking layer, a p-type InGaN waveguiding with the expectation of lowering the piezoelectric fields. layer, a p-type InAlGaN cladding layer, and a p-type GaN 102 · High-Power and High-Efficiency True Green Laser Diodes (a) (b) spectral region beyond 530 nm. The output power was 168 mW at 522.0 nm, 167 mW at 525.1 nm, 105 mW at 531.3 nm, Ga 107 mW at 532.1 nm, and 75 mW at 535.7 nm under CW op- N [0001] eration at RT. In our previous work, we reported on the ex- ceptionally low threshold current densities of these LDs in comparison with those of the c-plane LDs in the green spec- [1010] tral region of 520-530 nm. For practical application, how- (11) [1120] ever, the slope efficiencies needed to be improved . This time, we optimized the cavity length and mirror reflectivity, Fig. 1. (a) c-plane GaN substrate, (b) {2021} plane GaN substrate and succeeded in improving the slope efficiency fourfold while suppressing the increase of the threshold current. This improvement of the slope efficiency has led to the remark- able progress in the maximum output power. contact layer. The quaternary InAlGaN cladding layer lat- We now discuss the WPE*6 of semipolar {2021} and c- tice-matched to the GaN substrate is a key factor to high plane green LDs. As shown in Fig. 2 (b), in the green spectral crystalline quality. A ridge waveguide of 2-µm width was fab- region, our {2021} plane LDs clearly exhibit higher WPEs. ricated by conventional dry etching and photolithography The difference in the WPE between two crystal orientations techniques. The 500-µm-long cavities and mirror facets increases with increasing wavelength. This difference may were formed by cleaving. The front facets were coated with be attributed to the higher compositional homogeneity of dielectric mirrors of approximately 50% in reflectivity. In-rich InGaN QWs on the {2021} plane and the weaker piezoelectric fields. As discussed in the previous works, the narrow electroluminescence line width, uniform micro- scopic PL image, and abrupt interfaces of QWs indicated 3. Result the homogeneity of InGaN QWs grown on the {2021} plane(16). This is supported by other experimental results Figure 2 (a) compares the output power of different obtained by time-resolved photoluminescence measure- types of green LDs(13)-(15). The green LDs fabricated on the ments(17). The experimental results obtained in this work {2021} plane exhibit output power of over 100 mW in the indicate that the deterioration in the crystalline quality of the InGaN QWs with increasing In composition is inhibited by fabricating the InGaN QWs on the {2021} plane. In ad- dition, the improvement of the WPE has enabled CW las- (a) ing at 536.6 nm, as shown in Fig. 3, which is the longest 200 180 lasing wavelength by an InGaN LD, to the best of our 160 knowledge. 140 Next, we focus on the device characteristics of our re- 120 cent semipolar {2021} green LDs. The temperature depen- 100 dences of light output power vs. current (L-I ) and voltage This work 80 vs. current (V-I ) for a typical green LD under CW opera- 60 c-plane tion are shown in Figs. 4 (a) and (b), respectively. The Output power (mW) power Output 40 Ref. 13 threshold current, threshold current density, and thresh- 20 semipolar old voltage at a case temperature (Tc) of 25˚C were 59 mA, Refs. 14 and 15 0 510 515 520 525 530 535 540 Wavelength (nm) (b) 10 8 6 This work 4 c-plane Intensity (arb. (arb. unit) Intensity 2 Ref. 13 Wall plug efficiency (%) plug Wall semipolar Refs. 14 and 15 0 510 515 520 525 530 535 540 525 530 535 540 545 550 Wavelength (nm) Wavelength (nm) Fig. 2. Dependence of (a) output power and (b) wall plug efficiency on lasing wavelength for three different types of InGaN green Fig. 3. Lasing spectrum of the {2021} plane LD under CW LDs operation at RT SEI TECHNICAL REVIEW · NUMBER 77 · OCTOBER 2013 · 103 (a) (a) 100 1 50 mW Tc: 25˚C 80 25˚C Tc: 40˚C 40°C Tc: 55˚C 60 60°C 70°C 40 80°C Output power (mW) Output 20 0 0 50 100 150 200 250 300 Normalized Intensity (arb. units) Current (mA) 0 -45 -30 -15 0 15 30 45 (b) 10 Horizontal Angle (deg.) (b) 8 1 2525˚C˚C 50 mW Tc: 25˚C 6 4040˚C˚C Tc: 40˚C 6060˚C˚C Tc: 55˚C Voltage (V) Voltage 4 70˚C 8080˚C˚C 2 0 050100150200250300 Current (mA) Normalized Intensity (arb.
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