'Norwegian University of Science and Technology (NTNU) Library'
Abstract
Hybrid integration of defect-free III-nitride semiconductor nanocolumns and two-dimensional graphene as their substrate is an extremely promising route towards the development of ultraviolet light emitters, as graphene can be simultaneously utilized as a transparent conductive electrode. Nevertheless, a proof-of-concept of such hybrid device system has not been achieved before this work, and the study of highly dense vertical nanocolumns on graphene is also inadequately discussed. This PhD dissertation presents the investigation on the molecular beam epitaxial growth and the associated structural, optical and electrical properties of GaN nanocolumns and GaN/AlGaN nanocolumn ultraviolet light-emitting diode structures formed on graphene.
Self-organized GaN nanocolumns are grown firstly on amorphous fused silica, and then on graphene substrates by employing AlN buffer layer. High density of vertical nanocolumns characterized with excellent crystalline quality is achieved on these substrates. Particularly for the growth on graphene possessing no dangling bonds in its surface, additional study is carried out to clarify the role of the thin AlN as an intermediate layer between the formation of self-assembled GaN nanocolumns and graphene. Besides leading to the distinct arrangements of AlN that can affect the growth orientation of GaN nanocolumns, different AlN growth conditions unintentionally alter the structural properties of graphene.
Based on the understandings gained through the studies mentioned above, vertical growth of heterostructured GaN/AlGaN self-organized nanocolumns is subsequently realized on graphene. This growth orientation of the nanocolumns on graphene is essential for the light-emitting diode fabrication from as-grown nanocolumn samples. Here, graphene is employed as the growth substrate and simultaneously as the transparent conducting electrode for wurtzite GaN/AlGaN nanocolumns. In spite of high sheet resistance of graphene after the nanocolumn growth, a single excitonic emission peak can be observed at 365 and ~350 nm (ultraviolet-A region) for the devices grown on double-layer graphene and single-layer graphene, respectively.
This PhD thesis shows a vivid example on the development of nitride nanocolumn/graphene-based device technology. In this regard, the combination between these two materials provides a new approach in designing the ultraviolet light-emitting diodes, owing to the unique graphene properties
