Broad area, high power CW operated InGaN laser diodes

Broad area, high power CW operated InGaN laser diodes P. Wiśniewski 1, R. Czernecki 2, P. Prystawko 1, M. Maszkowicz 3, M. Leszczyński 1,2, T. Suski 1, I. Grzegory 1,2, S. Porowski 1, M. Marona 1, T. Świetlik 1 and P. Perlin 1,2 1.Institute of High Pressure Physics, Unipress Sokolowska 29/37, 1-142 Warsaw, Poland 2. TopGaN, Sokolowska 29/37, 1-142 Warsaw, Poland 3. Warsaw University of Technology, Pl. Politechniki 1, -661 Warsaw, Poland ABSTRACT We demonstrate the operation of wide-stripe InGaN laser diodes grown on bulk gallium nitride substrates obtained by high-pressure synthesis. The use of almost dislocation-free substrates resulted in very low defect densities of obtained laser structures - typically in the range of 1 5 cm -2. We tested 3 types of devices of the dimensions: 2µmx5µm, 2µmx1µm and 5µmx5µm. All three types of lasers showed good properties during pulse current experiments, exhibiting threshold currents of 4, 85 and 95 ma, respectively. The lasing wavelength varied between 45 and 42 nm, depending on the particular device. After p-down mounting on diamond heatspreaders, the first two types of lasers showed CW operation with a total output power reaching 2 mw. These devices, after optimization, offer good prognostics for reaching an optical power in the 1 W range needed for the applications in large area displays. Keywords: GaN, InGaN, laser diode, wide stripe, high power Novel In-Plane Semiconductor Lasers V, edited by Carmen Mermelstein, David P. Bour, Proc. of SPIE Vol. 6133, 6133Q, (26) 277-786X/6/$15 doi: 1.1117/12.64549 Proc. of SPIE Vol. 6133 6133Q-1

1. Introduction. The realization of the high-power blue/uv light-emitting nitride laser diodes remains a challenging task for the optoelectronic industry. These devices are needed for many applications, including the fabrication of large area color displays (e.g. laser projection TV), high-speed printing, photochemical processing and photolithography 1. For these applications the required level of optical power is between 2 mw and 1 W. High power laser diodes, in case of conventional red and infrared devices, are manufactured as wide-stripe devices, which means that their stripe width varies typically between 5 and 1 µm (Ref. 2). The employment of wide stripe laser geometry limits the optical field density and current density of the laser diode operating at high power. In Fig. 1, we show the limits in achievable optical output power imposed by the Catastrophic Optical Damage (COD) threshold ( 5 MW/cm 2 ) 3. One can see that for narrow stripe devices (2 µm) the maximum output power is around 3 mw. However, because of the specific properties of substrate materials used for the laser structure epitaxy, it is not easy to use the wide stripe geometry for the construction of GaN LDs,. These substrates are typically fabricated by means of the so-called ELO (Epitaxial Lateral Overgrowth) method 4. This technique was invented to reduce the number of dislocations in the material and leads to local dilution of the dislocation density. These improved areas usually have the shape of stripes with widths not exceeding a few microns and are too small to accommodate a large area laser structure. Fortunately, the laser diodes designed for application in optical storage (DVD) can and should be constructed as narrow stripe devices. Wide-stripe devices, urgently needed for the realization of large area displays, were demonstrated only once, by the Sony team 5, which used special GaN substrates fabricated by Sumitomo. So far, however, all the commercial devices are manufactured as narrow stripe devices with stripe widths typically smaller than 2 µm (Ref. 5). In the present paper we demonstrate the construction of wide-area devices on top of low-dislocation highpressure grown GaN substrate crystals. These crystals allows for the fabrication of lasers of almost arbitrary dimensions. 2. Device fabrication We grow our GaN/AlGaN/InGaN laser structures in a home-made vertical flow MOVPE reactor using TMGa, TEGa, TMAl, TMIn, CP 2 Mg, SiH 4 and ammonia as precursors. The substrates used for epitaxy are GaN bulk crystals obtained by high-pressure synthesis 7. The initial density of dislocations in the substrate is at the level of 1 2 cm -2. The Proc. of SPIE Vol. 6133 6133Q-2

active layer of the device is composed of one to five In.1 Ga.9 N quantum wells of a width of 4 Å. The quantum barriers were In.2 Ga.98 N:Si layers of a thickness of 7 Å. The quantum well system is followed by a 2Å-thick Al.2 Ga.8 N:Mg electron blocking layer..7 µm-thick undoped upper and.1 µm-thick n-type lower GaN layer form the device waveguide. A.5 µm-thick n-type Al.16 Ga.84 N/GaN superlattice (11x24Å/24Å) was used as bottom cladding and.37µm- thick p-type Al.15 Ga.85 N/GaN superlattice (8x23Å/23Å) as upper cladding to confine the light emitted from the active region of the InGaN MQW structure. A 4nm-thick p-type GaN layer was used as a contact layer of the p-electrode. The devices were processed as ridge-waveguide, oxide-isolated lasers. The mesa structure was etched out in the wafer down to a depth of.3 µm (roughly to the middle of the upper cladding layer). The laser structure was then isolated by the e-beam deposition of a.1 µm layer of ZrO 2. Ni/Au contacts, of typical contact resistance between 1-3 - 1-4 Ω cm 2, were deposited on the top surface of the device, while Ti/Au contacts were deposited on the backside of the highly conducting n-gan substrate crystal. The stripe width was set to 2 or 5 µm and the resonator length was 5 or 1 µm. The mirrors were coated with quarter-wavelength layers of SiO 2 /TiO 2 to increase the mirror reflectivity. The structures were characterized by a density of dislocations at the level of around 1 5 cm -2. Most of these dislocations were initiated in the upper part of the structure, especially in electron blocking layer. III. Performance and properties of laser diodes grown on bulk GaN substrates. One of the most important issues related to the operation of nitride laser diodes is the successful removal of the large amount of heat dissipated during diode operation. Nitride laser diodes are characterized by a 1-fold larger threshold current density and two times larger threshold voltage as compared with their GaAs counterparts. This results in a more than 2 times higher electrical power consumption at threshold comparing to GaAs based devices. In case of our wide-stripe laser we have the electrical power density at threshold in the range of 24-3 kw/cm 2. We constructed three types of wide-stripe lasers: Type A: laser with a 2µmx5µm stripe Type B: laser with a 2µmx1µm stripe Type C: laser with a 5 µmx5µm stripe Proc. of SPIE Vol. 6133 6133Q-3

Fig. 2 shows a Type C laser just prior to final chip separation. All these diodes were mounted p-side down on metalized diamond heat-spreaders. The electrical parameters of these devices are listed in Table I. Laser type I thr (ma) J thr (ka/cm 2 ) V thr (V) Maximum current at CW operation (ma) Emitting wavelength (nm) 2µmx5µm 395-55 3.95-5.5 6.2-6.7 13 412-415 2µmx1µm 1-13 5-6.5 6.8-7.1 19 45-42 5 µmx5µm 1296-1722 5.2-6.9 7-8 Not tested 42-415 As is shown in the Table I and in Fig. 3, these devices are characterized by similar parameters in terms of threshold current density and threshold voltage, which implies that the device current remains proportional to the stripe area. The dissipated power roughly varies between 2.5 W and 8 W, depending on the stripe geometry. This large amount of heat forces us to use an active thermoelectric cooling system. Our measurements indicate that the temperature roll-off of L-I characteristics occurs for an electrical power density of 8 kw/cm 2. In Figs. 4 and 5 we can see the optical and electrical characteristics under CW operation of lasers A and B. Their performance seems to be similar. So far slightly higher optical output powers were achieved by using a 2 µm device. The maximum ratings of our lasers are so far limited by the slope efficiency and by the thermal resistance of the package, which should both be improved. So far, laser C was not successfully tested under CW operation because of thermal management problems. However, we believe that, with an improved mounting scheme, this type of laser may provide a good solution for the 1 W emission range. Fig. 6 shows the pulsed current test performance of a class A device. We could demonstrate over 2.5 W of optical power on the output mirror. The failure of this device for larger power coincides with the COD level (5 MW/cm 2 ) 3. We expect that type C devices would be able to emit over 5 W of radiation from a single stripe (at least under pulse operation). Proc. of SPIE Vol. 6133 6133Q-4

The beam quality of conventional high-power laser diodes is usually hampered by the multi-mode character of their emission leading to deteriorated M 2 values of these devices. In order to combine highpowers and good singlemode operation we used an external cavity scheme (Littrow configuration). Using an external cavity we narrow down the laser spectrum and we also gain additional possibility of wavelength tuning using the external grating. Figure 7 shows the results of laser tuning. We demonstrate over 4 nm of tunability of such a laser without using AR coatings, which compares well with the previous literature values obtained for narrow stripe diodes 8. This latter experiment was performed on a pulse operated laser and experiments with CW operated devices are in progress. Conclusions. CW operation of lasers with a 2 µm wide stripe of cavity lengths 5 and 1 µm was demonstrated. To achieve this in the case of a 5 µm laser diode, the mounting is still to be improved. The maximum output power achieved for CW LDs was close to 2 mw, but a 1W range is envisioned for these devices. The use of low dislocation density GaN single-crystal substrates was crucial for construction of these wide-stripe devices. The combination of external cavity and wide stripe lasers is promising for applications requiring good beam quality and high power. We would like also to mention that we have constructed also type A laser diodes basing on MBE technology. This lasers achieved maximum power of more than 3 mw under room temperature CW operation 9. Acknowledgement We would like to acknowledge the financial support from EU within GaNano project: STREP 55641-1 and PRENABIO Project Proc. of SPIE Vol. 6133 6133Q-5

1.. H. Morkoc Nitride Semiconductors and devices, Springer-Verlag Berlin Heidelberg New York, 1999 2. R. Diehl, High-power diode lasers, Springer-Verlag Berlin Heidelberg New York, 2 3. M Takeya, T. Hashizu, M. Ikeda, in Novel in plane Semiconductor laser edited by C. Mermelstein, D.P. Bour, Proceedings of SPIE 5738, p. 63 4. P. Gibart, Rep. Prog. Phys. 67, 667 (24) 5. S. Goto, M. Ohta, Y. Yabuki, Y. Hoshina, K. Naganuma, K. Tamamura, T. Hashizu, M. Ikeda, phys. stat. sol. (a) 2, 122 (23) 6. T. Asano, M. Takeya, T. Tojyo, T. Mizuno, S. Ikeda, K. Shibuya, T. Hino, S. Uchida, M. Ikeda, Applied Phys. Lett. 8, 3497, (22) 7. I. Grzegory, J. Phys.: Condensed Matter 13, 6875 (22 8. L. Hildebrandt, R. Knispel, S. Stry, J. R. Sacher, F. Schael, 211 Appl. Optics 42, 211 (23) 9. C. Skierbiszewski et al., unpublished Proc. of SPIE Vol. 6133 6133Q-6

1 8 2 µm 2 µm 5 µm Power density (MW/cm 2 ) 6 4 2 24 mw 2.5 W 6 W 12 W Catastrophic Optical Damage Limit 1 µm 2 4 6 8 1 12 14 Optical output power (W) Figure 1. Calculated optical threshold for laser degradation. The value of COD (4 MW/cm 2 ) as quoted in Ref. 3.. Figure 2. Laser C. Laser bar just after cleavage Proc. of SPIE Vol. 6133 6133Q-7

5 45 4 laser A laser B laser C Optical power (mw) 35 3 25 2 15 1 5 1 2 3 4 5 6 7 8 9 1 11 12 13 current (ma) Figure 3 Light-current characteristics of the lasers of different stripe geometries. Lasers were measured at room temperature under pulsed current conditions (1 khz, 5 ns). 8 LD198 D9 T = 2 o C 15 Voltage (V) 6 4 2 1 5 Optical power (mw) 1 2 3 4 5 6 7 Current density ( ka/cm 2 ) Figure 4. Light-current and current-voltage characteristics of a laser diode of type A measured under CW conditions at room temperature. Proc. of SPIE Vol. 6133 6133Q-8

LD1981 D2 T = 2 o C λ = 417 nm 2 15 Voltage ( V ) 5 1 5 Optical power ( mw )..5 1. Current ( A ) Figure 5. Light-current and current-voltage characteristics of a laser diode of type B measured under CW conditions at room temperature. 3. 2.5 Optical power (W) 2. 1.5 1..5. 1 2 3 4 5 6 7 8 9 current (A) Figure 6 Light-current characteristic measured under pulsed current conditions (5 khz, 2 ns). Highly reflective coating causes almost all emission to exit through the output mirror. Proc. of SPIE Vol. 6133 6133Q-9

Blue laser diode in external cavity (Littrow configuration) Intensity (arb. units) 4 2 416 418 42 422 Wavelength (nm) Figure 7 The spectra of laser emission obtained in the Littrow external cavity configuration, for different grating angles. Proc. of SPIE Vol. 6133 6133Q-1