Index-antiguiding in narrow-ridge GaN-based laser diodes investigated by measurements of the current-dependent gain and index spectra and by self-consistent simulation

The threshold current density of narrow (1.5 {\mu}m) ridge-waveguide InGaN multi-quantum-well laser diodes, as well as the shape of their lateral far-field patterns, strongly depend on the etch depth of the ridge waveguide. Both effects can be attributed to strong index-antiguiding. A value of the antiguiding factor R = 10 is experimentally determined near threshold by measurements of the current-dependent gain and refractive index spectra. The device performances are simulated self-consistently solving the Schr\"odinger-Poisson equations and the equations for charge transport and waveguiding. Assuming a carrier-induced index change which matches the experimentally determined antiguiding factor, both the measured high threshold current and the shape of the far-field pattern of lasers with shallow ridges can be reproduced theoretically.Comment: This is an author-created, un-copyedited version of an article accepted for publication in the IEEE Journal of Quantum Electronics. IEEE is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

Normally-off transistor topologies in gallium nitride technology

Devices based on group-III-nitride compound semiconductors (AlN, InN, GaN) are gaining more and more momentum in modern electronics. Based on their excellent material properties like high critical electric fields, carrier concentrations and mobilities, these devices enable increasingly compact and efficient power supplies. Even today, there are notebook power supplies in the market whose size could be reduced by 40\% using GaN technology. For this application, normally-off transistors are required which are non-conducting without applied gate voltage, thus enabling fail-safe operation. But efficient AlGaN/GaN heterostructure field-effect transistors (HFET) are inherently normally-on. Therefore, a challenge in the past decade was fabricating normally-off HFET while retaining the performance of the normally-on types. In the presented thesis, three different promising concepts for high and stable threshold voltages will be investigated, while considering theoretical and experimental limitations. First, the electrostatics of conventional GaN devices are explained. Afterwards, a gate dielectric is included in this model and the influence of interface charge between the dielectric and a recessed AlGaN barrier on the threshold voltage will be discussed. Methods to improve the interface properties will be then introduced, for which a positive shift of the threshold voltage can be observed. These characterizations also include hysteresis measurements to estimate the threshold voltage stability of the devices. Based on the electrostatic model, the physical limitations of this threshold voltage shift will be elaborated. HFETs with a locally completely-removed AlGaN barrier can achieve even higher threshold voltages. However, a drawback of these devices is a reduced electron mobility in the channel. In order to mitigate this effect, the impact of an amorphous AlN interlayer will be investigated. Additionally, a novel PEALD process is developed for the deposition of this interlayer. In the third part, the concept of the p-GaN-gated HFET is discussed, which achieves normally-off operation without the challenging gate dielectric and its interface. Therefore, the electrostatic model is adapted and the impact of the relevant parameters is shown. A trade-off between threshold voltage and device performance is revealed, which is adjusted through the AlGaN barrier thickness and composition. Additionally, the removal of the p-GaN layer outside of the gate area is critical but required. A novel fabrication scheme will be introduced, which enables self-aligned and highly selective removal of p-GaN. This renders the entire manufacturing more robust and reproducible. Also, approaches for even further improvement of the p-GaN-gated HFET are shown. Finally, favorable applications for the different device topologies presented throughout this work will be explored