SEMICONDUCTOR LASERS: BeZnCdSe laser diode reaches true green
While much progress has been made toward creating green laser diodes that have practical qualities such as a reasonably long lifetime, there are areas where it appears to be difficult to push indium gallium nitride/gallium nitride (InGaN/GaN) laser diodes-for example, the true-green portion of the visible spectrum between 530 and 550 nm.
While much progress has been made toward creating green laser diodes that have practical qualities such as a reasonably long lifetime (see "The quest for practical green laser diodes," page 65, this issue), there are areas where it appears to be difficult to push indium gallium nitride/gallium nitride (InGaN/GaN) laser diodes—for example, the true-green portion of the visible spectrum between 530 and 550 nm (see figure).
One possible way around this difficulty is to use a quaternary semiconductor based on beryllium and zinc. A year and a half ago, a group at Sophia University (Tokyo, Japan) created an optically pumped, pulsed green laser diode with beryllium chalcogenide (BeZnSeTe) active layers that emitted at 548 nm. Now, researchers at the National Institute of Advanced Industrial Science and Technology (AIST; Ibaraki, Japan), Hitachi (Tokyo, Japan), and Sony Corp. (Kanagawa, Japan) have gone a couple of steps further and created a beryllium zinc cadmium selenide (BeZnCdSe) room-temperature quantum-well laser diode that emits at a similar 545 nm wavelength, but is electrically pumped and emits continuous-wave light—two qualities necessary for practical use in picoprojectors, for example.1 (In addition to the various combinations of Be, Zn, Cd, and Se in the diode's many layers, parts of the laser's structure also contain tellurium.)
|The pure-green portion of the visible spectrum between 530 and 550 nm (a, CIE x-y chromaticity diagram) is a region that is very hard for conventional InGaN/GaN laser-diode technology to break into (b, red data points). In contrast, a laser diode based on the BeZnCdSe semiconductor readily achieves output in the pure green (b, green data point). The information in the graph was compiled by the AIST, Hitachi, and Sony researchers from earlier published data.|
Reasonable threshold-current density
Grown on a gallium arsenide substrate, the laser has a 5 μm wide gain region, a threshold current of 68 mA, a threshold-current density of 1.7 kA/cm2, and a threshold voltage of 10.4 V. The laser light output jumps at threshold, indicating some saturable absorption; in addition, the light-output power curve shows some other kinks, which originate from a gain-guided structure that consists of a stripe that is a bit too wide. Both these flaws can be corrected in future versions. The threshold voltage is quite high, which the researchers say is the result of a nonoptimized p-contact structure. This too can be corrected in future versions, they note.
One big problem with InGaN/GaN laser diodes is that their threshold current rises drastically as their operating wavelength approaches the pure-green region. In contrast, the BeZnCdSe laser diode has a threshold current that remains reasonably low in the pure green. The wavelength of this new laser can be easily shortened if desired by decreasing the Cd content in the active layer, say the researchers.
No lifetime data is yet available—and even if it were, prolonging the lifetime of a new type of laser diode usually involves many iterations of diode structure and material purity, so end results should not be expected soon. In addition, green-emitting laser diodes of more conventional InGaN/GaN materials are increasing in lifetime due to great effort by companies such as Osram, Soraa, Nichia, and others (see "Scientists and engineers: A vital alliance," page 48, this issue). However, the ZnSe-based diode structure does appear to be a promising new type of compact pure-green laser, at least so far. —John Wallace
1. J. Kasai et al., Appl. Phys. Exp., 3, 091201 (2010).