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

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

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

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

Exciton-polaritons gas as a nonequilibrium coolant

Using angle-resolved Raman spectroscopy, we show that a resonantly excited ground-state exciton-polariton fluid behaves like a nonequilibrium coolant for its host solid-state semiconductor microcavity. With this optical technique, we obtain a detailed measurement of the thermal fluxes generated by the pumped polaritons. We thus find a maximum cooling power for a cryostat temperature of $50$K and below where optical cooling is usually suppressed, and we identify the participation of an ultrafast cooling mechanism. We also show that the nonequilibrium character of polaritons constitutes an unexpected resource: each scattering event can remove more heat from the solid than would be normally allowed using a thermal fluid with normal internal equilibration.Comment: 5 pages, 3 figures + supplemental materia

Band gap bowing of binary alloys: Experimental results compared to theoretical tight-binding supercell calculations for CdZnSe

Compound semiconductor alloys of the type ABC find widespread applications as their electronic bulk band gap varies continuously with x, and therefore a tayloring of the energy gap is possible by variation of the concentration. We model the electronic properties of such semiconductor alloys by a multiband tight-binding model on a finite ensemble of supercells and determine the band gap of the alloy. This treatment allows for an intrinsic reproduction of band bowing effects as a function of the concentration x and is exact in the alloy-induced disorder. In the present paper, we concentrate on bulk CdZnSe as a well-defined model system and give a careful analysis on the proper choice of the basis set and supercell size, as well as on the necessary number of realizations. The results are compared to experimental results obtained from ellipsometric measurements of CdZnSe layers prepared by molecular beam epitaxy (MBE) and photoluminescence (PL) measurements on catalytically grown CdZnSe nanowires reported in the literature.Comment: 7 pages, 6 figure

Impact of contacting geometries on measured fill factors

The fill factor determined from a measured current-voltage characteristic of a bare solar cell depends on the number and positions of the electrical contacting probes. Nine different geometries for contacting the front side busbars are used to measure the current-voltage (I-V) characteristics of a 5 busbar industrial-type passivated emitter and rear totally diffused (PERT) solar cell under standard testing conditions. The fill factors of the measured I-V characteristics vary from 78.5 %abs to 80.6 %abs. We further measure the contacting resistance of 3 different contacting probes to estimate the sensitivity of measurements with different contacting geometries on random resistance variations. The contacting resistance is 60 mΩ for nine-point probes and 80 mΩ for four- and single-point probes. We determine the magnitude of contacting resistance variations from measurements at different probe positions to be ±30 mΩ. Using this variation, we perform numerical simulations and find a larger sensitivity on random resistance variations for tandem- (pairs of current- and sense probes) compared to triplet (one sense- between two current probes) configurations. The corresponding fill factor deviation is approximately 0.1%abs for tandem configurations when the contacting resistances of up to two current probes are altered. The sensitivity for triplet configurations is negligible