GaN-based high-power laser diodes

Materials Science and Engineering B82 (2001) 248– 252 www.elsevier.com/locate/mseb

GaN-based high-power laser diodes Takao Miyajima a,*, Hiroshi Yoshida a, Katsunori Yanashima a, Takashi Yamaguchi a, Tsunenori Asatsuma a, Kenji Funato a, Shigeki Hashimoto a, Hiroshi Nakajima a, Masafumi Ozawa a, Toshimasa Kobayashi a, Shigetaka Tomiya b, Takeharu Asano c, Shiro Uchida c, Satoru Kijima c, Tsuyoshi Tojyo c, Tomonori Hino c, Masao Ikeda c a

Sony Corporation Core Technology and Network Co., 2 -1 -1 Shinsakuragaoka, Hodogaya, Yokohama 240 -0036, Japan b Technology Support Center, Sony Corporation, Hodogaya, Yokohama 240 -0036, Japan c Sony Shiroishi Semiconductor Incorporation, Shiroishi, Miyagi 989 -0734, Japan

Abstract We report our recent progress on GaN-based laser diodes (LDs) which will be applied as a light source in high-density optical storage systems. Recently we achieved a lifetime of more than 500 hours under continuous-wave operation with a constant power 20 mW at 25°C using GaN-based LDs with a standard ridge structure. We also report the potential of GaN-based LDs with another structure of a buried-ridge. The far-field pattern of the LDs with a buried-ridge structure strongly depended on the Al content of the Alx Ga1 − x N burying layer. This dependency showed that the device characteristics change from gain-guiding to refractive index-guiding. The critical point was around x= 0.30 of an Al content which corresponds to Dn= 0.007 of a lateral index step. It was, therefore, found that the optical transverse mode can be controlled by adjusting the Al content of the Alx Ga1 − x N burying layer. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ridge structure; Buried-ridge structure; Refractive index-guiding; Gain-guiding; Index step; Alx Ga1 − xN burying layer

1. Introduction GaN-based semiconductors have been studied as a promising material for the light source of high-density optical storage systems. In 1994, candela-class highbrightness GaN-based light-emitting diodes (LEDs) were reported [1], although huge threading dislocations of 108 cm − 2 existed in the device. At the time, it was believed that the dislocations do not act as efficient non-radiative recombination centers [2,3]. On the other hand, Nakamura et al. used epitaxial lateral overgrowth (ELO) techniques [4 – 6] for GaN-based LDs as a novel method of reducing dislocation density, and achieved a lifetime of more than 10 000 h under an output power of 2 mW [7,8]. This shows us that highdensity dislocations limit the lifetime of GaN-based LDs and their reduction is necessary in a practical device. * Corresponding author. Tel.: + 81-45-3536867; fax: +81-453536905. E-mail address: [email protected] (T. Miyajima).

We reduced dislocation density in GaN layer to the order of 108 cm − 2 using raised-pressure metal-organic chemical vapor deposition (RP-MOCVD) [9]. This growth method improved the crystalline quality of the GaN layer [10] and allowed room-temperature continuous-wave (cw) operation [9]. As the next step, we also achieved the lower dislocation density of several times 108 cm − 2 using the ELO technique [8], which allowed us to attain a lifetime of 500 h under an output power of 20 mW at 25°C [11,12]. Comparing the characteristics of the LDs grown on ELO GaN and on sapphire, we confirmed that lifetime is strongly dependent not only dislocation density but also initial input power [11]. It is now necessary to study the non-radiative nature of dislocations in LDs again. This problem has been discussed elsewhere [13,14]. Many research groups have achieved room-temperature cw operation of a GaN-based LD which was always based a ridge structure. This simple structure is suitable for mass-production, but has a disadvantage in that the distribution of current and light parallel (lateral) to the junction plane cannot be controlled sepa-

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T. Miyajima et al. / Materials Science and Engineering B82 (2001) 248–252

rately. To address this problem, we have suggested a GaN-based LD with a buried-ridge structure as shown in Fig. 1 and demonstrated its room-temperature cw operation [15]. With this structure, it will be easy to vary the light distribution in the lateral direction by changing the Al contents of Alx Ga1 − x N burying layer while maintaining the current distribution. In this paper, we report the device characteristics of GaN-based LDs with a buried-ridge structure as the Al content of the Alx Ga1 − x N burying layer is changed, and the device characteristics changing from that of an index-guiding type to a gain-guiding type. We then report the improved device characteristics of GaNbased LDs with a ridge structure.

2. Experimental All samples were grown on (0001) sapphire substrates using MOCVD. The epitaxial growth was carried out in a horizontal reactor. The source materials were trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), solution-trimethylindium (TMI) and ammonia. Monosilane and bis-methylcyclopentadienylmagnesium were used as dopants.

3. The new buried-ridge structure and the LD with the structure

3.1. Regrowth of AlGaN burying layer For the material of the burying layer, we chose the Alx Ga1 − x N alloy whose refractive index can be varied by changing the Al content of the Alx Ga1 − x N alloy. This was reasonable based on an analogy with GaAsbased LDs. Many cracks, however, were observed in the Alx Ga1 − x N layer with an Al content higher than x = 0.1, because there are large differences in lattice

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constants and thermal expansion coefficients between GaN and AlN. The existence of cracks is considered to have a fatal influence on the laser performance. To prevent the cracks, we investigated the growth of the Alx Ga1 − x N layer at lower growth temperatures and found that Alx Ga1 − x N with a high Al content (x= 0.4–1.0) could be grown without cracks at low growth temperatures from 520 to 730°C. The details have been reported previously [15,16].

3.2. De6ice fabrication Fig. 1 shows the structure of a GaN-based LD with a buried-ridge structure. A 1.2 mm-thick n-Al0.08Ga0.92N cladding layer was grown on a n-GaN contact layer, followed by a 0.1 mm-thick n-GaN guiding layer, GaInN multiple quantum well (MQW) active layers consisting of four pairs of 3.5 nm-thick Ga0.92In0.08N well layers and a 7.0 nm-thick Ga0.97In0.03N barrier layer, a 20 nm-thick p-Al0.16Ga0.84N capping layer, a 0.1 mm-thick p-GaN guiding layer, a 0.6 mm-thick p-Al0.06Ga0.94N cladding layer, and a 0.1 mm-thick pGaN contact layer. After patterning of SiO2 masks, ridge stripes were formed on these layers using reactive ion etching. The stripe direction was the Ž1 –100 direction of GaN. On the ridge structure, an undoped Alx Ga1 − x N layer (x= 0.15, 0.30 and 0.40) was grown as the burying layer at the growth temperature of 530°C. The Al contents of Alx Ga1 − x N burying layer were measured using X-ray diffraction spectroscopy. Because the Alx Ga1 − x N burying layer was deposited even on the SiO2 mask, we developed a self-aligned process to remove the Alx Ga1 − x N overgrown layer and to make a p-type ohmic contact on the top layer. Two parameters of W (stripe width) and d (the distance between the active layer and the ridge bottom) are defined as shown in Fig. 1. W was varied between 2.5 and 3.0 mm. d was fixed at 0.15 mm. These values, which were measured using SEM after device fabrication, have a significant effect on current and light distribution in the lateral direction of the LD and the device characteristics. The cavity length of the LD was 700 mm.

3.3. De6ice characteristics

Fig. 1. Schematic diagrm of LD with a buried-ridge.

Fig. 2 and Fig. 3 show the typical L–I characteristics and far-field patterns of a LD with a buried-ridge structure. The Al content of the Alx Ga1 − x N burying layer was x =0.40. W was 2.5 mm. No kinks were observed with an output power up to 35 mW. The threshold current was 47 mA. The emission wavelength was 406.5 nm. The angles parallel (u//) and perpendicular (uÞ) to the junction plane were 8 and 24°, respectively.

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Fig. 2. L – I curve at the LD with a buried-ridge structure. Al content of the burying layer was x=0.40.

Fig. 3. Far-field patterns of the LD with a buried-ridge structure in which Al content was x= 0.40.

and equivalent index method. Dn signifies the difference of the effective refractive index between the inside and outside of the lasing area in the direction lateral to the active layer We calculated the stripe-width dependence of q// for various values of Dn, as shown in Fig. 4 [17]. Experimental data of q// for x= 0.40 and 0.30 were plotted on the calculated lines for Dn = 0.010 and 0.007, respectively. We conclude, therefore, that index guiding dominates in the LD with the Alx Ga1 − x N burying layer (x= 0.40 and 0.30). On the other hand, q// for x= 0.15 was much larger than that of x=0.40 and 0.30, and corresponded to the large index step of Dn \ 0.012 although the smaller index step (Dn B0.007) was predicted. This shows that the transverse mode of the LD with the Alx Ga1 − x N burying layer (x=0.15) can not be explained by index-guiding, but can be by gain-guiding. In order to verify this, we measured the astigmatism of the LD, because the gain-guiding LD has a large astigmatism ascribed to the bent wavefront [18]. Actually, the astigmatism of LDs with an Alx Ga1 − x N burying layer (x= 0.15) was estimated to be 2.5 mm, which was larger than 1.6 mm of that for both x=0.30 and 40. From these considerations, it can be said that the optical transverse mode can be controlled by adjusting the Al content of Alx Ga1 − x N burying layer, and that the critical point between gain-guiding and index-guiding is around x= 0.30, which corresponds to Dn= 0.007 of lateral index step.

4. Improving the device characteristics of standard ridge-type LDs We improved the device characteristics of the ridgetype LDs which are widely used as GaN-based LDs.

4.1. De6ice fabrication

Fig. 4. Calculated dependence of FFP angles on stripe width. Experimental values are also plotted.

We studied which type of guide is responsible for the device characteristics of a LD with a buried-ridge structure. The transverse mode parallel to the active layer is theoretically controlled by either gain-guiding or refractive index-guiding. If index-guiding dominates, the farfield patterns should be calculated using a stripe width (W), a lateral index step (hereafter referred to as Dn),

We applied the ELO technique [6] to our LDs to reduce the dislocation density. First, a seed layer of 2 mm GaN film was grown on a (0001) sapphire substrate, and silicon dioxide (SiO2) was deposited and patterned to form 2-mm-wide stripe masks with a periodicity of 12 mm. The seed GaN layer was then etched out using Reactive Ion Etching (RIE) and the SiO2 mask was removed using wet etching. Laser epitaxy was performed on these rectangular GaN films. From planview TEM, both the seed GaN and the coalescence regions have a relatively high defect density over 1× 108 cm − 2. Indeed, dislocation-free regions are limited to a width within (12− 2)/2 = 5 mm, but it is not difficult to align a 2–3 mm stripe on this narrow region. A 1.0 mm-thick n-Al0.08Ga0.92N cladding layer was grown on the ELO GaN film, followed by a 0.1 mmthick n-GaN optical guiding layer, GaInN multiple

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quantum well (MQW) active layers consisting of 3 pairs of 3.5 nm-thick Ga0.9In0.1N well layers and a 7.0 nmthick Ga0.98In0.02N barrier layer, a 20 nm-thick pAl0.16Ga0.84N layer to minimize the electron overflow, a 0.1 mm-thick p-GaN optical guiding layer, a 0.5 mmthick modulation doped AlGaN/GaN superlattice cladding layer consisting of 100 pairs of 2.5 nm-thick undoped Al0.15Ga0.85N and 2.5 nm-thick Mg-doped GaN, and a 0.1 mm-thick p-GaN contact layer. A 2.0 –3.5 mm ridge stripe was formed using RIE, and a p-type electrode consisting of Pd/Pt/Au was evaporated on the p-GaN contact layer. An n-type electrode of Ti/Pt/Au was evaporated on n-GaN, which was also exposed by RIE. Wafer thickness was reduced to approx. 100 mm, for easy cleaving. The 500– 750 mm long cavity was formed parallel to the Ž1 – 100 direction of GaN using the conventional cleavage technique. The rear facet was coated with TiO2/SiO2 quarter – wave high-reflection films of 96% reflectivity and the front facet was coated with anti-reflection films of 10% reflectivity.

4.2. De6ice characteristics Fig. 5 shows typical light output power versus current (L–I) and voltage versus current (V – I) characteristics for 600 mm-cavity lasers under CW condition at

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20°C. The threshold current was 42.7 mA, corresponding to a threshold current density of 3.6 kA/cm2, and the voltage at the threshold was 4.9 V. The output power was as high as 35 mW without a kink, and it has stable linearity up to 35 mW. The slope efficiency was as high as 0.85 W/A. The emission wavelength was 406.7 nm for the output power of 30 mW. The operating current and voltage at 30 mW output power was 74.4 mA and 5.7 V, respectively, corresponding input power of 0.42 W. This low value is a consequence of optimized laser factors such as p-type ohmic metal, p-type superlattice cladding layer, stripe width (W) and the distance between the active layer and the ridge bottom (d), as defined in Fig. 1. The effect of introducing Pd/Pt/Au as a p-type ohmic metal and a modulation-doped super lattice cladding layer yielded a 2 V reduction of operating voltage, compared to our conventional lasers. The input power was also strongly dependent on W and d [11]. We chose W= 2.3 mm and d= 0.15 mm to minimize the input power. A lifetime test of the 500 mm-long cavity laser grown on ELO GaN was performed under 25°C CW conditions at a constant output power of 20 mW. It has more than 500 h lifetime. Lifetime is closely related to not only dislocation density in the laser stripe but also the initial input power Iop × Vop as shown in the Fig. 6. It seems that the reliability of LDs on ELO GaN is superior to that of LDs on sapphire due to the reduction of the defect density. In order to obtain a longer lifetime, it is effective to reduce the operating current and the operating voltage simultaneously.

5. Conclusion

Fig. 5. Typical L – I and V–I characteristics in cw operation at 20°C.

Fig. 6. Consumption power (Iop × Vop) dependence of lifetime.

We confirmed the potential of GaN-based LDs with a buried-ridge structure. The far-field pattern depends on the Al content of the Alx Ga1 − x N burying layer. This dependency shows that the dominant nature is changed from gain-guiding to index-guiding. The critical point is about Dn = 0.007. It was found that the optical transverse mode in GaN-based LDs can be controlled by adjusting the Al content of the Alx Ga1 − xN burying layer. The device characteristics of GaN-based LDs with ridge-type structure were improved by introducing ELO GaN, Pd/Pt/Au of p-type ohmic metal, p-type GaN/AlGaN superlattice of cladding layer and suitable devicestructure parameters of stripe width and remaining etching depth. These LDs showed an output power as high as 35 mW in room temperature cw conditions. The lifetime was more than 500 h in cw operation with a constant power of 20 mW at 25°C. From the device characteristics, we found that the lifetime of GaN-based

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LDs was strongly dependent on both the initial input power and the level of dislocation density.

[8]

[9]

Acknowledgements The authors would like to thank S. Ansai, M. Takeya, K. Shibuya, S. Ikeda, Y. Yabuki, T. Aoki, K. Nagamuma, S. Tomioka, E. Morita and R. Minatoya for their technical support and helpful discussions. They also thank K. Honda, K. Tamamura, and Dr O. Kumagai for their encouragement during this work.

[10] [11]

[12]

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