MBE-LEEM study of growth mechanisms of half metallic semiconductors

The goal of this PhD project is to further develop the Molecular Beam Epitaxy capabilities of the Low Energy Electron Microscope (LEEM) system at Cardiff University, to investigate nanostructure nucleation and half-metallic MnAs growth.

Participation in the project will enable the PhD student to work and develop a world-wide unique research facility, collaborate with academic and industrial partners, increase her/his global contact network, carry out state of the art research in electron microscopy, semiconductor epitaxy and spintronics and gather technical knowledge on the inner workings of a UHV system and develop an international scientific profile.

LEEM provides the capability to image surface dynamics at video rates, while simultaneously depositing atoms via techniques such as MBE. A movie with 5 nm resolution in XY plane and atomic resolution in Z-axis is formed in the projector column of the microscope and since a low energy electron diffraction (LEED) pattern is also obtained, various diffraction and phase contrast imaging modes are possible.

The capability of LEEM to image growth in real-time under deposition flux at elevated temperatures has resolved many important growth issues including the behaviour of Si(100) surface steps, the evolution of dislocations and nanostructure formation during droplet epitaxy. However, despite these outstanding achievements, the integration of LEEM with the precise temperature measurement, flux control and deposition source versatility associated with state-of-the-art MBE is yet to be developed. Transfer of optimal growth parameters determined from real-space imaging to larger MBE systems has therefore been restricted.

The key instrument development component of this project will enhance the III-V growth capabilities of the existing LEEM system at Cardiff to match the high levels of control attainable with current MBE technology. The MBE-LEEM system will directly interface with other growth and analysis laboratories to drive academic and industrial progress in photonics, nanoelectronics and spintronics.

This project aims to combine the latest developments in these areas with LEEM to develop a state-of-the-art MBE-LEEM system.

To illustrate the application of the resulting III-V MBE-LEEM we will apply the system to study an important and topical problem in thin film growth and spintronics namely the MBE growth of zinc-blende (ZB) MnAs. Epitaxy of MnAs needs to be carried out at low temperatures to avoid decomposition, which makes MBE more suitable than MOVPE.

MnAs thin films grown by MBE usually adopts one of three phases: a hexagonal ferromagnetic metallic phase, an orthorhombic paramagnetic phase or a hexagonal paramagnetic phase. The recent theoretical prediction that the ZB form of MnAs should exhibit half metallic properties has received significant interest for potential spintronic applications and has stimulated a desire to grow ZB-MnAs. A potentially major breakthrough in this area was made by Kim et al. who successfully grew the ZB phase using an ultrathin InAs buffer layer at 400˚C. Subsequently the half metallic character of the samples was confirmed.

The growth of ZB-MnAs is however very sensitive to the morphology of the underlying InAs layer and thicknesses above 2 ML produce metallic MnAs. It is well known that InAs on GaAs substrates occurs in Stranski-Krastanov (SK) growth where a 2D wetting layer is formed before 3D island growth occurs.

During this project we will image MnAs nucleation in real time using InAs buffer layers with thicknesses between 1 and 3 MLs.

MBE-LEEM enables in-situ characterization of the growth dynamics, offering real-time imaging of morphology and surface structure. The resulting videos will identify relaxation processes and surface structure evolution during InAs buffer layer growth and MnAs nucleation.

Work utilising the Cardiff University’s MBE-LEEM will be complemented with cross-sectional atomic resolution transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) measurements at Warwick University of as-grown samples. Warwick will also perform MnAs MBE growth, using optimal growth conditions established in the MBE-LEEM system to demonstrate scalability.

The project will carry out video imaging of state-of-the-art MBE nucleation dynamics to optimise InAs QD nucleation and ZB-MnAs growth. The improved control of the developed MBE-LEEM system will allow us to interact and transfer growth conditions to our partners in Sheffield National Centre for III-V Technologies, Warwick University and IQE ltd. (world leader in semiconductor wafer manufacturing).ations.