Investigation of Green Emitting Monolithic II-VI Vertical Cavity Surface Emitting Laser
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Investigation of Green Emitting Monolithic II-VI Vertical Cavity Surface Emitting Laser C. Kruse, G. Alexe, R. Kröger, M. Klude, H. Heinke, D. Hommel, S. Ulrich, P. Michler, J. Gutowski Institute of Solid State Physics University of Bremen, Kusteiner Strasse NW1 28359 Bremen, Germany
ABSTRACT In this paper, we are investigating the growth of a ZnSe based vertical cavity surface emitting laser (VCSEL). Undoped and p-type doped distributed Bragg reflectors (DBRs) with reflectivities exceeding 99% have been grown by molecular beam epitaxy (MBE) using Zn(S)Se layers for the high refractive index material and ZnSe/MgS superlattices (SLs) for the low index material. An undoped monolithic VCSEL structure containing a ZnCdSSe quantum well (QW) emitting in the blue-green reaches a quality factor (Q-factor) of 100, which is the best value reported so far. Temperature dependent photoluminescence (PL) measurements show that the emission of the QW is effectively guided by the cavity resonance. INTRODUCTION
Up to now, there is no alternative to the II-VI material system for the realisation of laser diodes emitting in the blue-green spectral region. However, the cw-lifetime of such devices is still limited to approximately 400 h, which is not sufficient for commercial applications. A further reduction of the threshold current density is expected to increase the device lifetime significantly. Semiconductor microcavities are of high interest for both fundamental investigations concerning exciton-photon interaction as well as devices like VCSELs and resonant cavity light emitting diodes (RCLEDs). The II-VI material system provides emission in the blue-green spectral region using ZnCdSe QWs [1] or CdSe QDs [2]. Especially the combination of QDs inserted into a vertical resonator is expected to reduce the threshold current density and increase the lifetime of II-VI laser diodes. An important application of such VCSELs would be cost-effective short-range data communication using plastic optical fibers (POFs) which have the absorption minimium in the green spectral region. Up to now, only optically pumped VCSEL using ZnCdSe QWs with dielectric mirrors have been realised [3]. These structures are suitable for studies of threshold behaviour and emission characteristics, but the lack of dopability does not allow effective current injection. Therefore no electrically pumped ZnSe-based has been realized yet. Undoped monolithic ZnSe-based microcavities with a pronounced cavity resonance in the blue-green spectral region were achieved using short-period ZnSe/MgS SLs [4]. These SLs were used for the low index material which allow stabilization of the high-bandgap MgS (Egap = 4.5 eV) in zincblende crystal structure. This is important in order to reach a refractive index as low as possible for the low index material (higher bandgap means lower refractive index in general). However, the reflectivities of the DBRs used in these structures were limited to 92% and the MOVPE growth method does not allow p-type doping up to now which is needed for ele
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