The success of advanced quantum communication relies crucially on non-classical light sources emitting single indistinguishable photons at high flux rates and purity. We report on deterministically fabricated microlenses with single quantum dots inside which fulfil these requirements in a flexible and robust quantum device approach. In our concept we combine cathodoluminescence spectroscopy with advanced in situ three-dimensional electron-beam lithography at cryogenic temperatures to pattern monolithic microlenses precisely aligned to pre-selected single quantum dots above a distributed Bragg reflector. We demonstrate that the resulting deterministic quantum-dot microlenses enhance the photon-extraction efficiency to (23±3)%. Furthermore we prove that such microlenses assure close to pure emission of triggered single photons with a high degree of photon indistinguishability up to (80±7)% at saturation. As a unique feature, both single-photon purity and photon indistinguishability are preserved at high excitation power and pulsed excitation, even above saturation of the quantum emitter.

We probe the indistinguishability of photons emitted by a semiconductor quantum dot (QD) via time- and temperature-dependent two-photon interference (TPI) experiments. An increase in temporal separation between consecutive photon emission events reveals a decrease in TPI visibility on a nanosecond time scale, theoretically described by a non-Markovian noise process in agreement with fluctuating charge traps in the QD's vicinity. Phonon-induced pure dephasing results in a decrease in TPI visibility from (96±4)% at 10 K to a vanishing visibility at 40 K. In contrast to Michelson-type measurements, our experiments provide direct access to the time-dependent coherence of a quantum emitter on a nanosecond time scale.

We have found a new photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the BL C band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free electrons concentration of about 10 18 cm-3). The electron-and holecapture coefficients for this defect-related band are estimated as 10-9 and 10-10 cm 3 /s, respectively. The BL C band is observed only in GaN samples with relatively high concentration of carbon impurity, where the yellow luminescence (the YL1 band) with a maximum at 2.2 eV is the dominant defect-related PL. Both the YL1 and BL C bands likely originate from the C N defect, namely from electron transitions via the −/0 and 0/+ thermodynamic transition levels of the C N. BL C band appears only at high excitation intensities in n-type GaN samples co-doped with Si and C, and it can be found in wide range of excitation intensities in semi-insulating (presumably ptype) GaN samples doped with C. The properties and behavior of the YL1 and BL C bands can be explained using phenomenological models and first-principles calculations.

Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metal organic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered.Comment: 3 pages, 4 figure

This paper reviews semi-polar GaN surfaces, of interest for light emitting devices, from both theoretical and experimental perspectives. Theoretical results on polarization charges at InGaN/GaN heterointerfaces and In incorporation into InGaN films are presented for polar (0001), semi-polar (1122) and nonpolar (1100) surfaces. Specific features of semi-polar InGaN/ GaN structures are emphasized which can be beneficial for improving optical and transport properties of quantum-wellbased light emitting devices. The analysis favours semi-polar surfaces such as the (1122) surface as growth plane for longwavelength light emitters. Therefore, the experimental sections emphasize progress towards long-wavelength LEDs and lasers by growth of InGaN/AlGaN/GaN(1122) heterostructures on large-area GaN(1122)/m-sapphire templates. The current status of such templates as grown by hydride vapour phase epitaxy is presented. The implementation of an epitaxial lateral overgrowth method on such templates to improve device performances is demonstrated.

GaN growth on Si is very attractive for low-cost optoelectronics and high-frequency, high-power electronics. It also opens a route towards an integration with Si electronics. Early attempts to grow GaN on Si suffered from large lattice and thermal mismatch and the strong chemical reactivity of Ga and Si at elevated temperatures. The latter problem can be easily solved using gallium-free seed layers as nitrided AlAs and AlN. The key problem for device structure growth on Si is the thermal mismatch leading to cracks for layer thicknesses above 1 µm. Meanwhile, several concepts for strain engineering exist as patterning, Al(Ga)N/GaN superlattices, and low-temperature (LT) AlN interlayers which enable the growth of device-relevant GaN thicknesses. The high dislocation density in the heteropitaxial films can be reduced by several methods which are based on lateral epitaxial overgrowth using ex-situ masking or patterning and by in-situ methods as masking with monolayer thick SiN. With the latter method in combination with strain engineering by LT-AlN interlayers dislocation densities around 109 cm -2 can be achieved for 2.5 µm thick device structures.

Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [1120] and [1100]. Realistic eight-band k·p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are formed either by the A or the B valence band

We report on the deterministic fabrication of sub-µm mesa-structures containing single quantum dots (QDs) by in-situ electron-beam lithography. The fabrication method is based on a twostep lithography process: After detecting the position and spectral features of single InGaAs QDs by cathodo-liminescence (CL) spectroscopy, circular sub-um mesa-structures are defined by highresolution electron-beam lithography and subsequent etching. Micro-photoluminscence spectroscopy demonstrates the high optical quality of the single-QD mesa-structures with emission linewidths below 15 µeV and g (2) (0) = 0.04. Our lithography method has an alignment precision better than 100 nm which paves the way for a fully-deterministic device technology using in-situ CL lithography.