Microcharacterisation of the Defect Structure of
Wide Band Gap Materials
 
(left) Oxford Instruments MonoCL CL Microanalysis system interfaced to an SEM in the EMU, UNSW. (right) Digital Instruments D3000 Scanning Probe Microscope operating in surface potential mode in the EMU, UNSW.

Microcharacterisation of dopants and defects in wide band gap materials is essential for optimising their technologically important physical properties. With the support of the UNSW Electron Microscope Unit (EMU), I have set up specialised atomic force and electron microscopy techniques, to investigate the microscopic defect structure of wide band gap materials.

An Oxford Instruments MonoCL2 Cathodo-luminescence Microanalysis System for imaging and spectroscopy, and a cryogenic specimen stage from the EMU has recently been interfaced to a Scanning Electron Microscope (SEM) in the Atomic Fabrication Facility, CQCT. Cathodoluminescence (CL) is the non-incandescent emission of light from a material resulting from electron irradiation, and is associated with defects and dopants. The detected CL emission is used to modulate the display units of the SEM, thus providing an image or map of the spatial distribution of the associated dopant or defect.

CL microanalysis provides complementary, high spatial resolution and very high sensitivity information about defects and their spatial distribution. For example, CL microanalysis can provide non-destructive depth resolved defect/dopant distributions. This information is not available from Photoluminenscence (PL) techniques. Compared with Electron Spin Resonance (ESR) spectroscopy, CL microanalysis has the advantage of being able to detect with high sensitivity, luminescent diamagnetic as well as paramagnetic defects. In addition, the CL detection limit is up to four orders of magnitude better than achieved by conventional X-ray Microanalysis. Complementary information is also provided using either of two ambient Scanning Probe Microscopes, which I can configure to measure and map electric charge, fields, and potentials associated with the defects/ dopants.

Some of the wide band gap materials currently under investigation include nano-porous GaP, InP, and GaN, AlN/GaN quantum heterostructures, nano-crystalline diamond, MeV ion implanted diamond, nano-crystalline silicon, alkali-borosilicates (for the containment of nuclear waste) and ultra pure silicon dioxide optics in collaboration with the National Ignition Facility, Lawrence Livermore National Laboratory, USA (inertial confinement fusion project).

Marion Stevens-Kalceff

 
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