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Samsung And Toyoda Gosei Unveil Progress In Violet Lasers

As more companies develop commercially viable violet laser diodes, and both LEDs and lasers move into the UV, many improvements continue to be made at the device level, writes Max Shatalov.
The 5th International Symposium on Blue Laser and Light Emitting Diodes (ISBLLED-5) was held in Gyeongju, Korea, on March 15-19, 2004. A total of 242 papers, including 22 invited talks, were presented by participants from 19 different countries. The talks described ongoing research and development efforts in UV, blue and white LEDs, as well as in violet laser diodes. In the latter area, several companies demonstrated that their devices are almost ready for commercial production.

A number of major trends could be identified at the symposium: for high-power blue LEDs, participants spoke about the development of reflecting contacts, substrate removal by laser lift-off (LLO), and backside surface texturing. Other important topics included the development of GaN-free structures for high-power near-UV LEDs, and the key issue for deep-UV LEDs of reducing threading dislocation density (TDD) by lateral epitaxial overgrowth (LEO).

Developers of 405 nm laser diodes speaking at the conference were concerned with areas such as ridge geometry optimization and reliability. There is also a trend towards reducing threshold currents via lower TDD values, using the LEO technique or bulk GaN substrates.

Samsung targets Blu-rayAccording to O Nam of the Samsung Advanced Institute of Technology in Korea, Samsung plans to supply 405 nm laser diodes for Blu-ray optical data storage systems by the end of 2004. For such consumer products, reliability issues are naturally of great concern. Laser diodes grown on LEO GaN (with a minimum TDD of 1-2 x 106 cm-2) had a threshold current (Ith) of 33.4 mA, a threshold voltage (Vth) of 4.25 V, and a slope efficiency of 1.53 W/A. The relative intensity noise (RIN) was less than -130 dB/Hz at 2 mW. The projected operational lifetime was 10,000 h at 30 mW output (53 mA, 4.67 V) and 235 h at 60 mW (84 mA). The Samsung team has also demonstrated a maximum CW power of 300 mW at 280 mA.

Laser diodes grown on bulk GaN substrates (with TDD of 7 x 106 cm-2) showed similar threshold characteristics (Ith = 30.7 mA, Vth = 4.22 V) but a lower slope efficiency of 0.85 W/A. Only 1000 h lifetime was estimated for 30 mW operation (64 mA, 4.88 V). According to the Samsung work, reliability is determined by magnesium diffusion under bias from p-type layers into the active region through the threading dislocations. This was confirmed by the comparison of reliability testing of laser diodes with different TDD values, followed by SIMS analysis. Samples with a lower TDD showed less magnesium diffusion and longer operating lifetime. Also, it was shown that epi-down bonding needs to be used for better thermal dissipation.

Violet lasers at Toyoda GoseiAlready a leading supplier of GaN-based LEDs, Toyoda Gosei of Japan is also developing 405 nm laser diodes for next-generation optical data storage. H Watanabe presented recent results for devices grown on both LEO GaN templates and bulk GaN substrates. Single-mode operation with an output power of 30 mW at 65 mA was demonstrated for a device grown on a LEO GaN template with a TDD of about 2-20 x 106 cm-2. Ith was 36 mA and the slope efficiency was 1 W/A for ridge waveguide lasers with a width of 1.5-2 mm and length of 500-600 mm. RIN measured less than -130 dB/Hz at 2.5 mW.

Pulsed, single-mode operation at 150 mW was also demonstrated at 170 mA. Laser diodes grown on bulk GaN substrates, with a TDD of less than 5 x 106 cm-2, had an output power of 30 mW at 51 mA with a threshold current of 31 mA and 1.5 W/A slope efficiency. Under pulsed conditions, the devices had a projected operational lifetime of more than 7000 h at 60 mW.

Sharp uses MBE growthRecently, a team at the UK-based Sharp Laboratories of Europe (SLE) became the first group to successfully demonstrate GaN laser diodes grown by MBE. According to SLE s Jon Heffernan, the advantages of MBE growth for III-N lasers include the use of in situ monitoring techniques such as reflective high-energy electron diffraction and a significant reduction in ammonia flow. There is also no need for a post-growth anneal stage to activate the p-type dopant. Laser diodes with a 5 µm wide ridge and 500-1500 µm long cavities emitted at 395-405 nm with a threshold current of about 1.1 A (22 kA/cm2, 34 V) under pulsed operation.

ZnSe-based white LEDsWhile most of the industry is firmly focused on GaN-based devices, Japan-based Sumitomo Electric (now part of Eudyna Devices) has been at the forefront of the development of ZnSe-based white LEDs, which operate without the need for a phosphor for downconversion. T Nakamura presented recent results on reliability improvements of ZnSe-based LEDs. White emission with very stable color co-ordinates can be obtained by combining the blue emission at 485 nm due to band-to-band transitions and yellow emission due to deep level states. However, poor reliability due to electron overflow into p-regions is a known problem. By incorporation of an i-ZnMgBeSe/p-ZnMgSSe double cladding structure with a p-ZnMgSSe hole injector layer and suppression of the electron overflow, the operating lifetime (at 25 ºC) was increased from 3000 to 10,000 h (figure 2). For 5 mm LEDs, lamp power of 8.1 mW was demonstrated at 20 mA and 2.5 V, corresponding to an external quantum efficiency (EQE) of 16.9%.

UV LEDs and lasersYu Bilenko of the US-based company Sensor Electronic Technology presented details of the first commercially available UV LED with peak emission at 340 nm, which has been developed for biological and chemical sensor applications. The UVTOP-340 LED is hermetically flip-chip packaged into a metal-glass TO-39 header with a UV-transparent window (figure 1). It has an output power of up to 1 mW at 100 mA, with a projected operating lifetime of 2000 h. Bilenko also presented results of a joint project with the University of South Carolina, US, which has achieved sub-milliwatt and milliwatt operation of UV LEDs in the 250-340 nm range (Compound Semiconductor May 2004).

Meanwhile, Nichia s S Nagahama discussed advances in the development of UV LEDs with emission at 365 nm, which replicate the I-line of Hg-lamps. To achieve high LED output power several key issues were addressed: absorption in the LED structure; LLO of the sapphire substrate and backside surface texturing for enhanced light extraction; and reflecting contact schemes. After optimization, Nichia s UV LEDs exhibited an EQE of 21% under CW operation with an output power of 360 mW (500 mA, 5.0 V) and 24% EQE under pulsed operation at 410 mW (500 mA, 5.3 V).

Nagahama also described improvements in Nichia s UV laser diodes. The TDD in the laser structures has been reduced to 4 x 107 cm-2 using LEO. Devices operating at 365 nm had a lifetime of 2000 h at 3 mW, with a 25 ºC case temperature. The laser threshold current density at 50 mA was 3.6 kA/cm-2, with a threshold voltage of 4.8 V. Shorter-wavelength devices operating at 354.6 nm had a threshold current density of 14.3 kA/cm-2.


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