Grüne oberflächenemittierende Halbleiterlaser (VCSEL) auf Basis von II-VI-Verbindungen

Abstract

Semiconductor-based laser diodes represent a key technology, which is used e.g. for optical data storage, data transmission and metrology purposes. However, the usual edge-emitting device design has some drawbacks concerning the properties of the emitted laser beam. This can be overcome by a more sophisticated approach called vertical-cavity surface emitting laser (VCSEL). The aim of the research within this thesis was the realization of a green fully-epitaxial VCSEL based on the II-VI material system deposited on a GaAs substrate. The structures are prepared by molecular beam epitaxy (MBE). The different building blocks for such a device like high- reflectivity distributed Bragg reflectors (DBRs) and monolithic microcavities with a sufficent optical quality (Q) factor were successfully developed. The main challenge was the achievement of a large difference delta n between high and low refractive index of the DBR layers in combination with a pseudomorphic epitaxial growth in zincblende crystal structure. A value of delta n = 0.6 has been reached for a DBR with the reflectivity maximum centered at a wavelength of 520 nm. This was possible by a special approach for the low index material, which consist of a short-period superlattice (SL) made from thin MgS and ZnCdSe layers. As a prerequisite, the growth of MgS in zincblende structure was investigated in detail in order to find suitable growth conditions. This was one crucial part of the work since the MgS compound naturally forms a rocksalt crystal structure. By choosing sulphur-rich conditions and taking advantage of a sulphur cracker evaporation cell, single zincblende MgS layers of up to 15 nm thickness were realized in good structural quality. However, the MgS thickness within the low-index SLs of the DBRs is typically much lower, i.e. in the range of 1 nm, which allows the effective stabilization in zincblende structure also for complex mulitilayer microcavity structures. Complete VCSEL structures with Q factors up to 3,000 were realized, showing lasing at a wavelength of 511 nm and a threshold excitation density of 21 kW/cm-2 under optical pumping. Starting from these planar cavity structures, micropillars have been prepared by focussed ion beam in order to create a three-dimensional confinement of the optical wave within the cavity. In addition, also first investigations with regard to an electrically-pumped VCSEL device had been performed. Since the conductivity of a p-type doped DBR is expected to be low due to the wide band-gap materials used, a reversed biased tunnel-junction light-emitting diode was realized. This injection scheme seems to be the most promising approach besides the preparation of intra-cavity contacts, which require complex etching and processing steps. Furthermore, the emission properties of CdSe quantum dots (QDs) were improved by embedding them between thin MgS barriers. These structures show a bright photo- luminescence of the QD ensemble up to room temperature, making them interesting for the realization of single photon sources as required for applications concerning quantum information science. In conclusion, within the scope of this thesis high-quality wide band-gap monolithic microcavities emitting in the green spectral range were developed, which might be used for practical applications as well as basic experimental investigations concerning quantum electrodynamics, e.g. the Purcell effect or cavity polaritons, in the future.