Speaker: Tongtong Zhu Research Fellow Department of Materials Science and Metallurgy, University of Cambridge
Time: Thursday, April 6, 2017, 13:30 p.m.
Site: Material room 210 room conference room

Gallium nitride (GaN) is widely regarded as the most important new semiconductor material of our times. III-nitride materials have not only achieved commercial success in solid-state lighting and power electronics, but also exhibit particular advantages for the development of quantum light sources and the exploration of quantum information science and light-matter interactions. Looking to the future, what new opportunities could the widebandgap III-nitride semiconductors offer, and new functionalities could one introduce to complement existing materials and technologies?

Theme 1: Nitride quantum dots for efficient photonic and quantum photonic devices

Light emitters for producing single photons on demand are essential building blocks for many quantum photonic applications, such as optical quantum computing, integrated quantum photonics and quantum cryptography. The need for an efficient electrically driven device with deterministically polarised individual photons on demand at room temperature is a major obstacle. Most current materials are only usable at liquid helium temperatures and/or are extremely difficult to integrate with optical cavities and conventional electronic/photonic circuits.

I have invented two novel growth methods for non-polar InGaN QDs with remarkable optical properties and temperature stability due to the reduced internal electric fields of non-polar planes and the in-plane anisotropy thus inherent high degrees of linear polarisation1,2. These QDs enabled the very first semiconductor quantum dot to concurrently achieve single-photon generation, linearly polarised emission, a fixed polarisation axis predefined by the material crystallography, and an ultrafast GHz repetition rate all at 220 K3. These achievements illustrate great potential for generating polarised single photons on demand from the ground state and entangled photon pairs with III-nitride systems, a material system that may allow us to harness solid-state quantum information processing at room temperature.

Moreover, most researchers either grow nanowires/nanorods, or focus solely on quantum dots on planar surfaces. I have recently demonstrated for the first time, quantum emission from non-polar m-plane InGaN QDs embedded in GaN nanorod systems4. Such QD-in-nanorod structures can efficiently generate ultra-fast (~250 ps) linearly polarized single photons up to 100 K, which can potentially also be coupled into the photonic modes of the hexagonal nanorods, thus opening an entirely new avenue to the fabrication of highly efficient and highly polarized single photon sources.

Theme 2: Porous III-nitride semiconductors – new directions for multifunctional energy-efficient devices

GaN epitaxy has been much explored over the last two decades, but, strikingly, no simple wet-etching technique is available at room temperature due to the chemical inertness of GaN. Recently, I have developed a facile conductivity based selective electrochemical process for porosification of GaN at room temperature without UV illumination or extra processing steps5, providing new opportunities in processing and crafting III-nitrides (i.e. porous etching, undercutting, texturing). I was the first to demonstrate wafer-scale fabrication of mesoporous GaN distributed Bragg reflectors (DBRs) with high reflectance (>96%) on both polar and non-polar crystal orientations. Due to the good vertical electrical conductivity and the ability to tune the spectral response across the entire visible spectrum, these porous GaN DBRs can be straightforwardly integrated into a range of existing and emerging optoelectronic devices. The porous nitrides represent a new and almost entirely unexplored family of semiconductor materials which may exhibit many interesting properties that conventional bulk GaN does not have, whilst the study of porous nitrides is unique in opening up an entirely new field of tailored, multifunctional materials for applications in the photonics, energy, and sustainable development sectors.