InGaN Laser Diodes Are Grown by Molecular Beam Epitaxy
Daniel S. Burgess
Researchers at Sharp Laboratories of Europe Ltd. in Oxford, UK, have reported the successful fabrication of blue-violet InGaN laser diodes using molecular beam epitaxy. Currently, these emitters are grown using metallorganic chemical vapor deposition, which consumes more source materials and which requires an additional postgrowth processing step to activate the P-type dopant.
Sharp Laboratories of Europe Ltd. has demonstrated 400-nm InGaN laser diodes grown using molecular beam epitaxy. Courtesy of Jonathan Heffernan, Sharp Laboratories of Europe.
According to the company, compound semiconductor laser diodes are produced using molecular beam epitaxy or metallorganic chemical vapor deposition roughly equally. Both techniques offer the necessary control over deposition with atomic-scale accuracy, and the decision to employ one rather than the other is largely a matter of preference on the part of the manufacturer. Until now, however, molecular beam epitaxy had not proved suitable for the construction of InGaN-based devices, yielding only low-power blue LEDs and no laser diodes.
The team employed a gas-source molecular beam epitaxy system in its work. A commercially available wafer of silicon-doped GaN on sapphire served as the template substrate. Ammonia was the nitrogen source, and elemental gallium, indium and aluminum were the group-III sources. Elemental silicon and bis(cyclopentadienyl)magnesium were used as the sources of the N- and P-type dopants, respectively.
The laser diodes feature a separate confinement heterostructure design with a multiple-quantum-well active region consisting of 2-nm-thick InGaN separated by 12-nm-thick undoped GaN barriers. After the growth process, the researchers used electron cyclotron resonance plasma etching to form 5 × 1000-µm ridge waveguide structures and the laser facets. No coating was applied to the facets to minimize loss.
Tests at room temperature under pulsed current injection confirmed that the devices emit at a wavelength of ~400 nm, with a linewidth of less than 0.2 nm, and display a threshold current density of ~30 kA/cm2, which the company notes is similar to that of the first violet laser diodes grown by metallorganic chemical vapor deposition. The group is investigating facet coatings and ways to optimize the P-type AlGaN cladding layer, with hopes of reducing the operating voltage and threshold current to achieve CW lasing at room temperature.
A target application of such a CW emitter is in the writing and reading of data in the Blu-ray next-generation optical disk format. Under development by 12 companies in the consumer electronics and computer industries, including Sharp, Blu-ray systems will employ 405-nm laser diodes to boost the storage capacity of optical disks.
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