Porosity in Nitride Semiconductors

Abstract

This thesis presents an exploration of how electrochemical etching (ECE) can be used to create pores in nitride semiconductors in order to engineer their properties. ECE offers a simple, flexible approach to pore formation, driven by an electric field between the material surface and a suitable electrolyte. This means that the material must be conductive in order to form pores. This makes doping density and potential the two key parameters for controlling the process. A particular focus of this work has been to understand how pores can form in a subsurface doped layer, below a non-porous non-intentionally doped capping layer. It is found that this is enabled by nanopipes formed at dislocations, which allow the electrolyte to reach the doped layer with minimal damage to the capping layer. Large birefringence of 0.14 has been produced in porous GaN structures and this has been thoroughly studied through optical measurements, finite element modelling and 3D structural data obtained using focussed ion beam milling combined with scanning electron microscopy. Characterisation of structures with periodic layers of porous GaN has also been demonstrated, using x-ray diffraction measurements. This has been used to measure the layer thickness and porosity of these samples and predicts their optical reflectivity with higher accuracy than the standard method for such characterisation. Forming pores in nitride semiconductors other than GaN has also been demonstrated. Porous AlGaN UV reflectors have been created, with a peak reflectivity of 89% at 324 nm and ECE has been applied to InGaN quantum wells with the aim of enhancing the luminescence. Measurements indicate that ECE creates both enhancement and a blueshift in the emission peak. Controlling and characterising pore formation at this scale is a huge challenge and further study of the pore morphology is required to verify the mechanism causing this.