Synchrotron X-ray based characterization of technologically relevant III-V surfaces and nanostructures

Sammanfattning: Innovative design and materials are needed to satisfy the demand for efficient and scalable devices for electronic and opto-electronic applications, such as transistors, LEDs, and solar cells. Nanostructured III-V semiconductors are an appealing solution, combining the excellent functional properties of III-V materials with the flexibility typical of nanostructures, such as the nanowires (NWs) studied here. However, there are a number of open challenges, that currently hinder the performance of III-V nanostructure devices: first, the surface quality of III-V materials is still one of their main limiting factors. Other problems specific of III-V NWs are the control of dopant incorporation - crucial for their functionalization -, and of their structural inhomogeneity (e.g. lattice strain and tilt), that can affect opto-electronic performance. These problematics require a set of non-trivial cutting-edge characterization tools: here an approach based on a combination of X-ray synchrotron techniques is demonstrated.Synchrotron based X-ray photoelectron spectroscopy (XPS) has been used to study the surface chemistry of III-V model systems and to monitor industrially relevant processing on them. A new passivation process improving the surface quality of InAs substrates used for electronics has been investigated: the surface structure and composition resulting from thermal oxidation followed by ex situ deposition of a high-k material via atomic layer deposition (ALD) has been assessed with XPS. The implementation of this passivation approach in gate stacks showed improvements in performance, that were attributed to the specific stoichiometry of the thermal oxide. The dynamics of the ALD process on InAs was also studied in situ with ambient pressure XPS: it was observed that the chemisorption of the precursor is an important step to ensure a good quality of the high-k oxide deposition.Dopant evaluation in NWs is challenging due to their small dimensions. Here, a first approach to this problem was to perform XPS to study the effects of Zn dopant incorporation on the surface of GaAs NWs, used for solar cells. High doping conditions during growth were found to form a Zn layer on the outside of the NW that suppresses the native oxides, which are generally a cause of poor passivation of III-V surfaces. In another experiment, XPS scanning microscopy was used to study surface Zn doping in an InP NW with an axial pn junction, also used for solar cells. The surface potential drop along the junction was monitored in operando, while applying a bias to the NW device, and it was found smaller than what expected for the bulk. Finally, a quantitative evaluation of Zn dopants incorporation in III-V NWs was studied for the first time with nano-focused X-ray fluorescence, due to the excellent combination of low detection limits and spatial resolution. Dopant gradients and memory effects were noted along InP and InGaP NWs, showing complex dopant incorporation mechanisms during the growth. The structural inhomogeneity in InGaN nano-pyramids for next generation LEDs was also investigated. The influence of different processing parameters on lattice and strain were studied with full field X-ray diffraction microscopy. This imaging technique uses Bragg diffraction intensity as contrast mechanism and has a large field of view, useful for imaging at once large areas patterned with pyramids, giving valuable statistical consistency. The growth parameters providing the best lattice quality and homogeneity were assessed.This thesis shows how cutting edge synchrotron characterization methods can provide useful information for improving III-V surfaces and nanostructures for next generation devices. Moreover, in most cases advances in the characterization methods are achieved, that can be relevant also in other and broader scientific fields.