(based on Cree's Press Release dated July 29, 1997) CREE, the world's leading manufacturer and supplier of silicon carbide (SiC) wafers and SiC based semiconductor products, announced that it has achieved another milestone in its blue laser development project. The company has demonstrated continuous wave (CW) laser operation at room temperature. Improved high reflectivity facet coatings, which lowered the threshold current for lasing, were applied to new Cree laser devices by Dr. Arto Nurmikko's group at Brown University. The CW emission lasted for more than fifteen seconds, Cree officials noted. ``We are quite pleased with this latest laser development. It was reached far ahead of our expectations,'' says Cree's President and CEO Neal Hunter.
Just last month CREE announced pulsed laser operation from its gallium nitride based blue laser with lifetimes of over an hour. The company previously used a one microsecond long pulse to minimize device heating. Improvements in the laser structure obtained by facet coating reduces device heating and allows for continuous operation.
``This success really emphasizes the advantage of growing nitride laser structures on SiC. SiC is a vastly superior heat sink to sapphire or other substrates on which blue laser structures have been grown,'' Cree Senior Scientist, Dr. Kathy Doverspike, remarked.
CW operation is considered to be much more difficult than pulsed operation because of the heating that can occurr with the high currents needed to acheive lasing. GaN lasers have been reported to fail due to degradation of ohmic contacts at during laser operation. To avoid these problems, one must either lower the operating current of the laser, extract the heat better, or make more robust contacts. The CREE work indicates that the use of a substrate with high thermal conductivity is may be effective at lowering the operating temperature, but an efficient laser structure is also necesary. Ultimately, all three strategies may be necessary.
Just last week, Fujitsu Researchers announced that they had reproduced CREE's earlier result (see below) of pulsed operation of a GaN based laser structure grown on sapphire.
(based on Cree's Press Releases dated June 9, 1997 and June 19, 1997)
Cree Research Inc. the world's leading manufacturer and supplier of silicon carbide (SiC) wafers and SiC based semiconductor products, announced on Jun 9, 1997, that it had demonstrated an electrically pulsed gallium nitride based blue laser. The breakthrough was accomplished by scientists at Cree who designed and fabricated the device using Cree's silicon carbide wafers combined with a proprietary gallium nitride thin film process.
Cree co-founder and the senior scientist in charge of the development program, Dr. John Edmond remarked, "We are elated about demonstrating electrically pulsed laser operation at room temperature. Preliminary measurements indicate a device lifetime of greater than a few minutes. This is a milestone toward achieving a commercially viable laser, but we still have a lot of development work ahead of us. This is similar to the development cycle we went through for our blue LED. The next step is to improve device performance and process repeatability."
The introduction of a commercially viable blue laser could initially benefit optical storage systems such as high-end mass storage optical recording systems and eventually consumer products such as CDROM, CD audio and DVD by increasing storage capacity four or five times. The shorter wavelength of blue light as compared to longer wavelength red or infrared lasers used today is responsible for the increased storage capability.
Cree has been working for the past two years to develop its laser device with partial funding from the Defense Advanced Research Projects Agency (DARPA). That funding has been critical in achieving the blue laser result said Cree's president, Neal Hunter.
DARPA program director, Dr. Anis Husain stated, "We are excited about Cree's development. This demonstration is a critical step to realizing a key military component. Short wavelength blue lasers are critical for high data storage military systems and chemical biological reagent sensing, as well as covert communications. The base technology could also be extended for solar blind missile detection systems. We are extremely pleased with this accomplishment, although I concur with Dr. Edmond that more work remains to achieve a viable blue laser component for military systems."
Dr. Jan Schetzina, professor of physics at North Carolina State University and a partner in the DARPA effort discovered the initial laser demonstration. Dr. Schetzina stated, "This is the first known nitride laser using a silicon carbide substrate and the laser emission demonstrated was textbook. The heat going into a laser is a principal failure mechanism, and as an ideal heat sink, silicon carbide overcomes this far better than sapphire." The limitations of the device made on silicon carbide were not reported.
Dr. Arto Nurmikko, professor of physics at Brown University, also observed blue laser operation from sample devices received from Cree.
On June 19,1997, Cree announced that the devices have achieved lifetimes in excess of one hour. The company also noted that lasing had been demonstrated by devices that contained eight quantum wells. Quantum wells are used in the devices to confine the electrons moving through the material.
Dr. Gary Bulman, Lead Scientist on the laser development program remarked, ``Given the high power levels used to drive these devices, the resulting laser operation is amazing. However, much more work remains to achieve the 10,000 hours of continuous wave operation necessary for commercialization.''
``While a commercially viable blue laser must have a lifetime of several thousand hours, we believe Cree's demonstration of device operation of over an hour shows the promise of SiC as the preferred substrate for this technology,'' says Cree President and CEO, Neal Hunter.
Although Cree's press release did not make it clear, Wade Brooks of Cree has clarified that the lifetime is more than 1 hour in pulsed oeration.
Electrically pumped GaN based blue lasers were first acheived by researchers at Nichia Chemical Industries Ltd. of Japan. A technical paper describing their work is available on this site. Two other Japanese groups have reported GaN blue diode lasers.
SiC is the third substrate on which GaN diode lasers have been demonstrated. The Nichia Group has reported lasers on two orientations of sapphire (Al2O3) and on spinel (Al2MgO4). The high cost of SiC wafers has limited their use outside of the companies, such as Cree, who produce their own SiC.
Information about Cree can be found at their Web Site
Eric Hellman