Nakamura group flaunts non-polar GaN laser
University of California, Santa Barbara (UCSB) scientists claim to have made the first laser diode to be based on non-polar GaN.
The leading research team, which is headed up by GaN pioneers Shuji Nakamura, Steven DenBaars and James Speck, says that these lasers will be more efficient and have longer lifetimes than lasers based on conventional GaN material that is polar in nature (see our related magazine feature for an in-depth look at the advantages of non-polar GaN).
UCSB revealed few details about the device, other than that it operates at 405 nm "“ the wavelength used in high-definition optical data storage applications "“ and in pulsed mode. Details of the output power, which has been lower than predicted for the first non-polar GaN LEDs, the deposition method, or the substrate material, were not quoted.
According to UCSB, the latest findings have yet to be scrutinized by peer-review, but that has not stopped the researchers from planning a public demonstration of the device in early February, prior to publication of the work in a journal.
However, the non-polar laser still needs a good deal of further development before it could be considered for commercial use. For example, continuous-wave lasers are required for applications like high-definition DVD players and recorders.
The team also quotes a threshold current density for laser action of 7.5 kA/cm2. Although encouraging for a first device, Nakamura and colleagues will need to reduce this figure by around half to generate laser diodes with an output power and reliability good enough for commercial use.
As a comparison, conventional GaN-on-sapphire lasers sold by Nichia have threshold current densities in the region of 1.5 kA/cm2 (see related magazine article).
However, the UCSB team s threshold figure does compare well with that for the first GaN-based laser diode fabricated using MBE rather than the more conventional MOCVD process. In January 2004, Sharp Laboratories Europe quoted 30 kA/cm2 (see related story) for its first device made in this way, and has since worked hard to reduce it.
Another practical difficulty for volume manufacturing could arise from the need to use "r-plane" sapphire - upon which the non-polar, or "c-plane" GaN material can be deposited. This particular crystal orientation of sapphire tends to more difficult to cleave from bulk material than conventional substrates.