Technology Research Review
First in situ STM images in MOCVD kit
Researchers from the Technical University of Berlin, Germany and Tor Vergata University, Italy, have made the first ever in situ scanning tunneling microscopy (STM) images inside an MOCVD reactor.
These images offer direct monitoring of growth at an atomic level, which is not possible with today s commonly used techniques for process control, like reflectance anisotropy spectroscopy or ellipsometry.
STM has not been applied to in situ studies of an MOVCD growth up until now because the high-temperature, vibration-ridden environment is incompatible with the STM instrument s requirements.
The researchers have prevented the growth temperatures from damaging the microscope by adding an active cooling shield, which is cooled by a stream of gaseous nitrogen. With this method the instrument s coarse approach motor can be maintained below 80 °C for growth temperatures of up to 650 °C. The vibrations caused by the MOCVD reactor s rotary pump were reduced by attaching two coupled springs to the top of the instrument. With these modifications the team obtained in situ images of the steps of GaAs (001) substrate offcut at 2 ° along the [11–0] direction at a reactor temperature of 500 °C (see figure). This type of structure forms an ideal test sample because the topography – step bunches 2–3 nm high and separated by 50–100 mm – is known down to the nanoscale.
Research team member Markus Pristovsek explained to Compound Semiconductor that they are now investigating the evolution of In(Ga)As quantum dots at different growth temperatures. "With a large scan area the current density was small enough to not affect the surface," said Pristovsek.
• Journal reference B Rähmer et al. 2006 Appl. Phys. Lett. 89 063108.
Fuji develops longer lifetime for red lasers
Japan s Fuji Photo Film Company has developed 660 nm red-emitting lasers that have a sufficiently long lifetime to be used in plastic optical fiber (POF) communication networks in offices, hospitals and data centers.
The team s 5 mW lasers, which have an estimated lifetime of 1 × 106h at 60 °C, exceed the minimum lifetime requirement for deployment in networks operating at this temperature by a factor of 10.
In comparison, commercial red lasers used in DVD players – which also emit at a wavelength close to the plastic fiber s attenuation minimum at 650 nm – have an estimated median lifetime of only 5000 hours at 70 °C.
The improvements to the lasers were made by optimizing the MOCVD growth conditions of the AlGaInP cladding layers, the device s cavity length and doping concentration.
Fuji s laser was mounted in a junction-up configuration on a copper heatsink, produced modulation speeds of up to 1.25 Gbit/s and a threshold current of 65 mA at 25 °C. At a 5 mW continuous wave output the diodes have estimated lifetimes of 3 × 105 h and 1 × 106 h at 70 °C and 60 °C, respectively.
Tsoyoshi Ohgoh and co-workers have recently made further improvements to laser reliability that were announced at the Japan Society of Applied Physics Autumn Meeting. The latest diodes show no degradation after 3500 h at 90 °C, the operating temperature of sources used in POF-based car networks.
• Journal reference T Ohgoh et al. 2006 Elec. Lett. 42 1033.
Lumileds doubles brightness of devices
LED chip manufacturer Lumileds has more than doubled the brightness of its GaN-based flip-chip devices by separating the sapphire substrate from the epilayers.
Using this technique – which also involved roughening of the exposed GaN – Lumileds researchers fabricated 1 × 1 mm blue-emitting thin-film flip-chips (TFFCs), which produced a radiance of 168 mW/mm2 sr and an output power of 750 mW when operated at a 1 A drive current in pulsed mode. The radiance is 130% higher than that of conventional flip-chip LEDs produced from the same epiwafer and 20% greater than that of vertical thin-film chips with a high-reflectivity silver contact.
Lumileds Oleg Shchekin and colleagues say that their TFFC emitters produce a higher radiance and output power than vertical thin-film chips because they eliminate the occlusion by top-contact and wire bond pads.
According to the researchers, even though the patterning of a top metal contact in a vertical thin-film device can be further optimized, there is a trade-off between extraction efficiency and high current operation. The top contact has to be minimized to boost extraction, but brighter devices need a larger contact area to reduce current crowding effects.
The team coated blue-emitting TFFC LEDs with a phosphor to produce white lamps with peak luminous efficiencies of 96 lm/W at 20 mA, 60 lm/W at 350 mA and 41 lm/W at 1 mA.
• Journal reference O B Shchekin et al. 2006 Appl. Phys. Lett. 89 1109.
