We report on III-Nitride blue light emitting diodes (LEDs) comprising a GaN-based tunnel junction (TJ) all realized by metalorganic vapor phase epitaxy in a single growth process. The TJ grown atop the LED structures consists of a Mg-doped GaN layer and subsequently grown highly Ge-doped GaN. Long thermal annealing of 60 min at 800 °C is important to reduce the series resistance of the LEDs due to blockage of acceptor-passivating hydrogen diffusion through the n-type doped top layer. Secondary ion mass spectroscopy measurements reveal Mg-incorporation into the topmost GaN:Ge layer, implying a non-abrupt p-n tunnel junction and increased depletion width. Still, significantly improved lateral current spreading as compared to conventional semi-transparent Ni/Au p-contact metallization and consequently a more homogeneous electroluminescence distribution across 1 × 1 mm2 LED structures is achieved. Direct estimation of the depletion width is obtained from electron holography experiments, which allows for a discussion of the possible tunneling mechanism.

In this paper the internal electric fields of nearly lattice matched InAlN/GaN heterostructures were determined. Pin-diodes containing InAlN/GaN heterostructures grown on (0 0 0 1) sapphire substrates by metalorganic vapour phase epitaxy were fabricated by standard lithography and metallization techniques. To determine the polarization fields in the InAlN quantum wells capacitance-voltage-measurements were performed on the pin-diodes. To reduce the measurement error, the heterostructure thicknesses were accurately determined by transmission electron microscopy. Large polarization fields, which correspond mainly to the spontaneous polarizations, for In 0.15 Al 0.85 N (5.9 ± 0.8 MV cm −1 ), In 0.18 Al 0.82 N (5.4 ± 0.9 MV cm −1 ) and In 0.21 Al 0.79 N (5.1 ± 0.8 MV cm −1 ) quantum wells were observed. The results of the internal field strength and field direction are in excellent agreement with values predicted by theory and a CVM-based coupled Poisson/carrier transport simulation approach.

Semi-insulating GaN is a prerequisite for lateral high frequency and high power electronic devices to isolate the device region from parasitic conductive channels. The commonly used dopants for achieving semi-insulating GaN, Fe, and C cause distinct properties of GaN layers since the Fermi-level is located either above (Fe) or below (C) the midgap position. In this study, precursor-based doping of GaN in metalorganic vapor phase epitaxy is used at otherwise identical growth conditions to control the dopant concentrations in the layer. Using electric force microscopy, we have investigated the contact potentials of Fe- and C-doped samples with respect to a cobalt metal probe tip in dependence of on the dopant concentration. While in Fe-doped samples the sign of the contact potential is constant, a change from positive to negative contact potential values is observed at high carbon concentrations, indicating the shift of the Fermi-level below the midgap position. In vertical transport measurements, C-doped GaN layers with a dopant concentration of 4.6 × 1018 cm−3 exhibit up to 5 orders of magnitude lower dark current at room temperature and significantly lower temperature dependence than Fe-doped samples with a similar dopant concentration. Therefore, precursor-based carbon doping is the superior doping technique to achieve semi-insulating GaN.

We report on an over 50% reduction in polarization field strength in c-axis oriented InGaN multi-quantum wells (MQW) by applying quaternary AlGaInN barrier layers with better polarization matching to InGaN than GaN barriers. With the reduction in polarization fields, a strong blue-shift in photoluminescence is observed in agreement with theoretical expectation and simulations. By gracing incidence x-ray diffraction measurements, we demonstrate that partial relaxation already occurs for GaN/InGaN MQWs. As a consequence, the requirement of higher In-content layers for green light emission is in conflict with increasing strain leading to lattice relaxation.

In order to deposit semipolar GaN layers on foreign substrates while still starting the growth along the c-direction, we have etched trenches into r-plane and n-plane sapphire wafers. The GaN MOVPE growth then starts from c-plane-like sidewalls of these trenches, eventually leading to semipolar {1122} and {1011} surfaces with very good properties. GaInN quantum wells grown on such surfaces show very uniform properties on {1011} surfaces, but still reflect the stripe geometry on {1122} surfaces by a slightly larger In uptake at the stripe coalescence regions