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Crack-free AlGaN growth aids UV lasing

A group that includes a GaN growth pioneer has developed a new method for UV laser diode fabrication.

A Japanese collaboration has successfully made UV laser diodes using a low-defect growth method.

The hook-up between Hamamatsu Photonics and Meijo University grew GaN/AlGaN UV laser diodes on a 2-inch sapphire substrate without producing any cracks in the material.

AlGaN layers with a high proportion of AlN are used as cladding and guiding layers in UV laser diodes, but typically suffer from poor crystalline quality that affects performance.

In this case, crystal quality was improved by using a method called hetero-facet-controlled epitaxial lateral overgrowth (hetero-FACELO), in which the first facet-controlled epitaxy step produces triangular GaN seed crystals.

“After growth of the triangular GaN seeds, AlGaN is laterally grown from their inclined facets,” commented Harumasa Yoshida, corresponding author of the September 7 Japanese Journal of Applied Physics paper.

“This growth of AlGaN can be carried out under stress-relaxed conditions, even on lattice-mismatched GaN, and then dislocations propagate mainly in the horizontal direction during lateral growth of the AlGaN,” he added.

“These mechanisms result in crack-free, low-dislocation material.”

In the paper, Yoshida s group reported growing GaN/AlGaN multiple quantum wells over the base hetero-FACELO layer to assess dislocation density, which was recorded as 3.9 x 108 cm-2 by cathodoluminescence.

Existing UV sources find uses in a number of material processing and characterization applications, including fluorescence spectroscopy. Hamamatsu Photonics is hoping to produce UV laser diodes that offer advantages over existing sources in terms of energy efficiency, lifetime and size, while avoiding the use of highly-toxic materials.

The UV diodes resulting from hetero-FACELO lased between 355.4 and 361.6 nm at room temperature, under pulsed operation, but with current threshold densities above desirable commercial targets. However, Hamamatsu is currently improving device performance, according to Yoshida.

“In order to lower the threshold current density of these UV-LDs, it is necessary to reduce overflow current due to low conductivity of p-type AlGaN, as well as to decrease defects that act as non-radiative centers,” he commented.

Leading the collaboration at Meijo University is Hiroshi Amano, who has won several awards for the development of GaN growth methods that led to blue LEDs.

If his reputation is anything to go by, then perhaps UV laser diodes will be the next semiconductor devices that Amano helps towards commercial success.

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