Elektrisch betriebene oberflächen-emittierende Bauelemente auf Basis von II-VI Halbleiter

Abstract

The key technology towards electrically pumped vertical-cavity surface-emitting lasers (VCSELs) are high reflectivity Bragg-mirrors with high conductivity in vertical direction. A Bragg pair is made of a high and a low refractive index layer with an optical thickness of a quater wavelength each. The ZnSe-based material system is the only one to realize epitaxial high reflectivity distributed Bragg reflectors (DBRs) for monolithic devices lattice matched to GaAs in the blue-green spectral region. The use of II-VI materials enables an emission in the green spectral region which is of high interrest for many applications. With green being a primary color it can be used together with red andblue emitters for display technologies. Another application is the use in an optical network using plastic optical fibre (POF) with a absorption minima at 560 nm. A green-emitting VCSEL will enhance the performance of these networks.In literature approaches of II-VI-DBRs are reported using ZnSe/ZnS or ZnSe/ZnTe-DBR structures. DBRs wtih ZnSe/MgSSLsas low index material have been reported but with ZnSe ashigh index material and fewer SL-periods. These structures reach reflectivities around 93%. Also nitride-based structures with high reflectivities have been reported. But up to now these structures are not conductive.Previous investigations showed that the doping does not affect the optical properties within the accuracy of measurement and p-type doped DBRs have a much lower conductivity than n-type doped DBRs. Therefore only n-doped DBRs seem to be suitable for the use in electrically pumped devices. This work is concentrating on the optimaziation of the design of n-type DBRs, suitable metal contacts and the incorporation of such DBRs into devices. In a first step metal contacts for n- and p-type doped semiconductorswere inverstigated. For the n-type structures Aluminium is asiutable contact material. Current densities of over 160 A/cm2 at 3 volts have been achieved. Concerning the p-type contacts Pd/Au is the best material. The introduction of a epitactically grown ZnTe/ZnSe contact layer leeds to the best results but also contact without that layer are applicable having the benefit of a better crystal quality of the top mirror. The second step was then to optimize the conductivity of the Bragg-mirrors. To achieve this the MgS layer thickness in the low index super lattice was reduced. This leeds also to a reduction of reflectivity. But even after compensation the latter effct with more periodes a conductivity increase of almost a factor of two can beachieved.In the third and final step the above shown concepts were incorporated into electrically driven surface-emitting devices. Here a RCLED demonstrated the feasibillity of such devices. Different approaches for the contacting are displayed, i. e. intra-cavity contacts and dielectric mirrors.