To date, most efforts to produce green lasers for display applications have focused on frequency doubling IR lasers. IR lasers are common and relatively cheap, as is the most common non-linear conversion material, periodically polled Lithium Niobate (PPLN). Therefore, it should be easy to produce cheap frequency-doubled green lasers, right? Not necessarily, according to the experience of numerous companies, including Novalux, QPC and Collinear. Other companies, including Corning, Osram and EpiCrystals continue to work on the problem and are expected to have commercial quantities of frequency-doubled green lasers real soon now.

Matt Brennesholtz
Insight Media Analyst

If frequency doubled lasers are so difficult and expensive to make, why not just make direct-emission green, the way they make direct emission red and blue lasers? Material science is the short answer, although material science is not a simple answer in most people’s book. To make a direct emission green laser you need a material with a band gap equal to the energy of the green photons you want to produce. Materials like this exist, in fact they are used all the time to make green LEDs. The III-V compound semiconductor material gallium nitride (GaN) is doped with indium to produce InGaN and used in green LEDs. Unfortunately, the very high defect density in conventional InGaN prevents its use in lasers. II-VI compound semiconductor materials have also been used to produce demonstration direct-emission green lasers in the lab, but these materials are far from ready for a mass market like displays.

Over a year ago, the Defense Advanced Research Projects Agency (DARPA) started a project called VIGIL, which stands for Visible InGaN Injection Lasers, to develop a new version of InGaN that would have a lower defect density and be suited for direct-emission green lasers. This project, including 9 research teams and reported in the February 2008 issue of our newsletter, Large Display Report, had a goal of producing a working green laser with non-polar InGaN by June 2009.

What is non-polar InGaN, you may ask? Conventional InGaN is produced in the c-plane orientation on non-InGaN substrates such as sapphire or silicon carbide. The lattice constant mismatch between the InGaN and the substrate is the main reason for the high defect density. The c-plane orientation of conventional InGaN produces polar material. Non-polar material is based on producing the GaN oriented along the m- or a- planes.

While Insight Media has not followed the progress inside the research labs, the first commercial products are beginning to emerge. Inlustra Technologies, a start-up company that was founded by Drs. Ben Haskell and Paul Fini in 2005 as a spin-out of University of California Santa Barbara (UCSB), has begun to deliver non-polar GaN substrates to customers. The UCSB GaN effort is led by Professors Shuji Nakamura, Jim Speck, and Steve DenBaars and is a member of the VIGIL project. Nakamura, as you may remember, developed the first high brightness blue GaN LED, which was introduced commercially by Nichia in 1993.

The Inlustra product is not a green laser: it is non-polar GaN substrates. Inlustra customers will then grow InGaN green lasers or other devices on these substrates. Since the InGaN is grown on a GaN substrate instead of sapphire or silicon carbide, there will be little or no lattice constant mismatch and therefore much lower defect densities.

While this does not mean cheap direct emission green lasers are imminent, it is one key step along the supply chain to produce them. Today is tax day and I am glad to see our tax dollars bearing fruit in a way that will help the display industry in the long term.