CW GaN Laser Diodes Grown by Molecular Beam Epitaxy
Next-generation DVD technology -- whether it's Blu-ray, as supported by Sony, Sharp and about a dozen other companies, or HD-DVD, supported by Toshiba, NEC and several others -- will require 405-nm blue lasers. Thus, millions of dollars are being spent in laboratories around the world to develop reliable, inexpensive, high-quality blue semiconductor lasers.
Figure 1. Sharp Laboratories of Europe Ltd. has demonstrated CW 405-nm output from InGaN laser diodes grown using molecular beam epitaxy. Courtesy of Jonathan Heffernan.
There are two fundamental approaches to fabricating the GaN-based lasers that generate blue light: molecular beam epitaxy (MBE) and metallorganic chemical vapor deposition (MOCVD). Both growth techniques are widely used to fabricate GaAs- or GaInP-based red lasers, but until now, blue lasers have been fabricated entirely by MOCVD. Numerous attempts to apply MBE to blue lasers have been unsuccessful.
Figure 2. The InGaN laser reached threshold at a current of 125 mA, or a current density of 5.7 kA/cm2.
Scientists at Sharp Laboratories of Europe Ltd. in Oxford, UK, reported the first successful MBE fabrication of a blue-violet InGaN diode laser in January 2004 (see "InGaN Laser Diodes Are Grown by Molecular Beam Epitaxy," Photonics Spectra, April 2004, page 30). It was a pulsed laser, but continuous-wave operation is required for the DVD application. Recently, the scientists reported the first room-temperature, CW operation of an MBE-grown InGaN blue laser (Figure 1).
The laser is based on multiple quantum wells in InGaN/GaN. The scientists fabricated 2.2-µm ridge waveguides on their wafers, cleaved them into 1-mm-long bars and deposited high-reflective coatings on the ends.
Figure 3. The lifetime of the first-generation laser was only several minutes. The scientists hope to extend this in future lasers.
Laser threshold was reached at a pump current of 125 mA, corresponding to a current density of 5.7 kA/cm2 (Figure 2). The scientists also observed the characteristic sharp reduction in spectral bandwidth at laser threshold. The high-reflectivity coating on the facets (~90 percent at 405 nm) minimized the threshold current but limited the output power to 0.7 mW from each facet.
The lifetime of these first-generation lasers was limited to several minutes, even at a reduced output of 0.15 mW (Figure 3). Experiments are under way to reduce the operation voltage and threshold current to extend the lifetime.
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