P. Kiesel

40% efficient thin-film surface-textured light-emitting diodes by optimization of natural lithography

In conventional light-emitting diodes (LEDs), the external efficiency is limited by total internal reflection at the semiconductor-air interface. The problem can be overcome by the concept of the nonresonant cavity LED, which is an LED with a textured top surface and a rear reflector. The surface is textured using natural lithography. A monolayer of randomly positioned polystyrene spheres acts as a mask for dry etching. We present details about the optimization of the parameters of the texturing process for GaAs/AlGaAs LEDs. The studied parameters are the size of the spheres, the distribution of the spheres on the surface and the etching depth. Using optimized texturing conditions, we have realized un-encapsulated top-emitting oxide-confined GaAs/AlGaAs nonresonant cavity LEDs with an external quantum efficiency of 40%.

Polarization threshold switches based on ordered GaInP

We report on optoelectronic switching devices with high photoconductive gain which are sensitive to the polarization direction of the optical input. Due to the large polarization anisotropy of the absorption coefficient of ordered GaInP, an electrical output can be switched “on” and “off” by rotating the linear polarization of the input light. Depending on the external adjustment of the working point, the structure can operate as a polarization threshold switch or a polarization detector, respectively. First results exhibit a switching contrast of 25 dB and a contrast of the detector signal of 7 dB between two perpendicularly polarized incident light beams, respectively.

Polarization effect in light emitting diodes with ordered GaInP active layers

We report on polarization effects in surface emitting light emitting diodes based on p‐i‐n AlGaInP/GaInP double heterostructures grown by metal organic vapor phase epitaxy. Devices with an ordered GaInP active layer show polarized light output with a ratio of 4:3 for polarization along the [011] and [011] crystal direction, respectively. This polarization is nearly independent of diode current and mesa geometry, but is not observed if disordered GaInP active layers are used. The effect is considered to be an important means for polarization control in vertical cavity surface emitting lasers.

Polarization anisotropy in the electroabsorption of ordered GaInP

The polarization dependence of the Franz–Keldysh effect (FKE) in metalorganic vapor phase epitaxially grown AlGaInP/GaInP/AlGaInP p-i-n double heterostructures was investigated for samples with different ordering parameters. We determine an ordering induced shift of the FK spectra of up to 13 meV between light polarized along the [011] and [01-1] crystal directions. Due to the FKE the transmitted light intensity ratio between the respective polarizations was changed from 0.2 to 2.8 dB by applying an electric field of 335 kV/cm.

Novel shadow mask molecular beam epitaxial regrowth technique for selective doping

We present a novel molecular beam epitaxial regrowth technique which provides a simple and convenient way for the in situ lateral structuring of the doping profiles and growth rates on a μm scale. We achieve excellent selective contacts to the respective doping layers for device dimensions varying from several 100 μm down to several μm. Keldysh based n‐i‐p‐i modulator structures, fabricated with our new method, exhibit an on/off ratio of 6:1 for a voltage swing of 7 V without enhancement by additional Bragg mirrors.

Femtosecond differential transmission measurements on low temperature GaAs metal–semiconductor–metal structures

We report on differential transmission measurements on low temperature grown (LT)-GaAs with and without applied electrical fields at different wavelengths. Electrical fields up to 100 kV/cm can be applied via an interdigitated contact structure to our LT GaAs samples which have been removed from the substrate by epitaxial lift off. In the presence of an electric field, both, the absorption bleaching due to phase space filling and field induced absorption changes due to the Franz–Keldysh effect contribute to the transmission changes. We observe an extended carrier lifetime with applied field. The response time of a biased metal–semiconductor–metal detector, therefore, exceeds the carrier life time of the substrate material.

Annealing induced refractive index and absorption changes of low‐temperature grown GaAs

Large changes of the refractive index (Δn≊0.25) and absorption coefficient (Δα≊16 000 cm −1) of GaAs grown by molecular beam epitaxy (MBE) at low substrate temperature (LT‐GaAs) induced by annealing are reported. The refractive index difference between the LT‐GaAs layer and the GaAs substrate are determined from both, the amplitude of the Fabry–Perot oscillations and the shift of their extrema towards shorter wavelengths yielding nearly the same results. With increasing anneal temperature the excess refractive index as well as the strong absorption at photon energies below the GaAs band gap (determined by transmission measurements) disappear around 700  °C.

Low power (bistable) opto‐electrical threshold switches with high gain based on n‐i‐p‐i doping superlattices

We report on the realization of an electro‐optical switch on n‐i‐p‐i doping superlattices for both bistable and step‐like threshold operation at very low input power levels (≂200 pW) and with high opto‐electrical gain (6.8×106). The configuration presented in this letter consists of a (two terminal) photodiode with negative current‐voltage characteristic and a novel three‐terminal photoconductive detector structure. The minimum optical switching power is determined by the reverse dark currents of the devices that are below pA. The switches are suitable for monolithical integration into arrays.

Quantum dot micro-LEDs for the study of few-dot electroluminescence, fabricated by focussed ion beam

Abstract We present first results from a micrometer sized micro-LED fabricated for the electroluminescence (EL) investigation of very few to one self-assembled quantum dot. Utilizing focussed ion beam (FIB) implantation we were able to fabricate a LED consisting of crossed p- and n-doped stripes of a few micrometer width. Confined between both stripes is a layer of quantum dots symmetrically embedded in a thin i-layer. We show that good quality quantum dots can be grown on FIB implemented regions and present first EL spectra which agree very well with PL measurements.

CLASS IDENTIFICATION OF BIO-MOLECULES BASED ON MULTI-COLOR NATIVE FLUORESCENCE SPECTROSCOPY

Laser-induced native fluorescence is measured on a set of bio-molecules from different classes (bacteria, proteins, fungi) for excitation at 266nm and 355nm. A method of preprocessing the spectra to obtain an inherently normalized set of data is described. Class identification on the normalized data set is demonstrated.