Researchers at Sharp Laboratories of Europe (SLE; Oxford, England), a subsidiary of Sharp (Osaka, Japan), have demonstrated what the company claims to be the world's first blue-violet laser diodes made by molecular-beam epitaxy (MBE). The devices, based on nitride semiconductors, are being developed to comply with the Blu-ray standard for next-generation DVD storage and video applications.
The indium gallium nitride (InGaN) laser diodes were fabricated in a newly developed gas-source MBE system that uses ammonia as a source of nitrogen and elemental sources for gallium, indium, and aluminum. The demonstrated lasers operate at 405 nm. Grown on a sapphire substrate, the quantum-well ridge-waveguide lasers operate in a pulsed mode at room temperature with a threshold current density of approximately 30 kA/cm2. This is claimed to be comparable to the first metal-organic chemical vapor deposition (MOCVD)-grown blue-violet lasers.
A 405-nm-emitting InGaN laser diode is fabricated by molecular-bream epitaxy, a process that reduces consumption of source materials and increases control of growth (one of the laser's facets is seen in this scanning-electron micrograph).
Blue-violet InGaN lasers produced by MOCVD have been around for some time, while MBE has been used to produce low-power blue-violet light-emitting diodes. In other compound semiconductor materials, such as gallium arsenide (used in lasers for CD optical pickups), devices are produced by both MOCVD and MBE, with little difference in material quality and device characteristics. Sharp currently produces a large share of the world's red DVD lasers by MBE at its new laser compound-semiconductor factory in Hiroshima.
"Blue laser diodes developed by MBE offer a number of advantages," says Jon Heffernan, manager of the advanced optoelectronic devices group at SLE. "There is a significant reduction in the consumption of source materials, which can in turn have implications for laser production costs and in reducing the environmental impact of the production method."
He notes that MBE also has advantages in offering more accurate control of growth than the traditional MOCVD method. "Such an advantage may allow the development of new device structures with significantly improved features," he says. "The properties of InGaN are very unusual and the crystalline structure responsible for its high optical efficiency is still a matter of debate. MBE has the advantage of in-situ monitoring techniques such as RHEED (reflection high-energy electron diffraction), which allow a direct monitoring of the growth surface. We hope to exploit this advantage to optimize our devices for better performance."
The project is now concentrating on improving characteristics of the devices with the aim of demonstrating long-lived continuous-wave operation and evaluating the devices for commercial applications. Blue-violet lasers form the basis for the next generation of optical-storage devices. The current generation of DVD systems uses a red laser diode to read and write information and has a storage capacity of approximately 4.7 GBytes, or about two hours of standard video. Shorter-wavelength lasers will increase storage capacity. Blu-ray, the next generation of DVD systems, will incorporate blue-violet lasers, leading to a storage capacity of 27 GBytes, or approximately 13 hours of video. Thirteen companies, including Sharp, Sony, Matsushita, Hitachi, Pioneer, Samsung, Philips and Thomson, are backing the new Blu-ray standard.