News

4 June 2010

 

True green laser light from c-plane nitride semiconductors

Osram Opto-Semiconductors of Regensburg, Germany has reported ‘true green’ (520–570nm) laser diodes (LDs) on traditional c-plane free-standing gallium nitride (GaN) substrates [Adrian Avramescu et al, Appl. Phys. Express, vol3, p061003, 2010]. The researchers achieved continuous-wave (cw) 524nm laser emissions with output power of 50mW and wall-plug efficiencies as high as 2.3%. Pulsed-mode operation allowed even longer wavelengths of 531.7nm to be produced.

These operation parameters are seen as an “important step forward on the way to commercial applications” for direct green indium gallium nitride (InGaN) laser diodes. Green LDs are of great interest for producing compact, more efficient color projection displays. At present, the red and blue light components have well established LD parts, but the green component has to be produced using a bulky and inefficient frequency doubling of light from an infrared LD.

Osram Opto-Semiconductors’ achievements are credited to “improvements of epitaxial design and material quality on c-plane GaN substrates”.

Previously, longer-wavelength nitride semiconductor LDs used non-standard ‘semi-polar’ crystal orientation substrates to reduce internal polarization fields that can pull electrons and holes apart and hence reduce their ability to recombine as photons. On the other hand, the internal polarization fields can red-shift the emission frequency, making it easier to produce longer-wavelength devices using lower-indium-content InGaN layers.

It becomes more difficult to grown high-quality InGaN material as the indium content increases due to segregation effects where high-indium-content regions tend to clump together, creating non-uniformities. In fact, the lack of polarization of m-plane grown material means that much higher indium levels are needed, and has made producing ‘non-polar’ m-plane LDs a challenge beyond 500nm.

The Osram Opto-Semiconductors team believes that their results “suggest that the main limiting factor for achieving high-performance green InGaN lasers is not the piezoelectric field but it is related to the crystal quality of the In-rich InGaN QWs”.

The LDs used commercial 2-inch GaN substrates as the base for the InGaN multi-quantum wells (MQWs) that formed the active region of the LDs. Aluminum gallium nitride (AlGaN) and GaN layers were used for the cladding and waveguiding, respectively. The nitride semiconductor layers were grown using metal-organic chemical vapor deposition (MOCVD).

The resulting epitaxial material was formed into broad-area and ridge-waveguide (RWG) lasers. The ends of the 600μm cavities were coated with dielectric mirrors. The broad-area devices were used for testing the material under pulsed operation (0.4μsec, 6% duty cycle). The 1.8μm wide RWG LDs were packaged into transistor outline TO38 packages to allow “appropriate heat removal”.

The pulsed-operation testing was performed at 25°C. The aim of pulsing is to remove self-heating effects that can affect the performance of laser diodes. The 531.7nm emission had a threshold current density of 18kA/cm2. Below threshold the full-width half maximum (FWHM) of the peak was 30nm, which reduced to ~0.8nm above threshold.

Figure 1: Wall-plug efficiency of 524nm cw-emitting ridge laser device in TO-package at 25°C.

The 524nm cw emission of the packaged device had a threshold current of 97mA and a slope efficiency of ~330mW/A. The wall-plug efficiency peaked at an output power of 50mW (Figure 1). The rather broad spectrum (with a FWHM of ~1nm) is seen as a positive for reduced speckle for projection onto a screen (Figure 2).

Figure 2: High-resolution spectrum of packaged LD at 25°C in cw operation with 50mW optical output power.

The research was funded by the German Federal Ministry for Education and Research (BMBF).

See related items:

Green laser diode market $500m by 2016 as pico-projector market drives growth

Osram details record output & efficiency cw direct green InGaN laser diodes

Sumitomo boosts 520nm green InGaN laser to continuous-wave operation

Osram develops 50mW direct-emitting green InGaN laser

Search: Osram Green laser diodes Free-standing GaN substrates InGaN MQWs AlGaN

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The author Mike Cooke is a freelance technology journalist who has worked in the semiconductor and advanced technology sectors since 1997.