ZnSe‐based laser diodes: New approaches

Abstract

In this paper we report on recent results concerning conventional edge emitting laser structures containing either quantum wells or quantum dots as region and vertical-cavity surface-emitting lasers. In the first part a series of four similar laser structures containing quantum wells with an emission wavelength of 520 nm was grown by molecular beam epitaxy in order to perform a systematic study of lifetime improvement. They differed in the alternating implementation of an additional 5 nm thick ZnSSe layer with a high sulfur composition of 25% neighboring the quantum well. A high stability of the CdZnSSe active layer was observed by introducing such a kind of strain compensating layers. Lifetime measurements showed a significant improvement up to one order of magnitude using p- and n- & p-side layers. In the second part electro-optical characteristics of ridge and planar CdSe quantum dot laser diodes were compared. A reduction of the threshold current density by a factor of 4.7 for the ridge structure was obtained. This has to be associated to the reduction of current spreading inside the laser diodes. Furthermore, a significant slower degradation of CdSe quantum dot structures compared to common ZnSe-based QW structures was observed. An operating time over 2700 h in pulsed mode experiments at 50 A/cm2 in LED mode was achieved. In the third part we report on the realization of an optically pumped monolithic vertical-cavity surface-emitting laser operating at a wavelength of 511 nm. The microresonator has a quality factor of 3200 while the threshold excitation power density for the onset of lasing is 22 kW/cm2 at room temperature. Micropillars of different diameter fabricated out of this structure show discrete optical modes due to the three dimensional optical confinement of the optical wave. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)