Grigore Moldovan

Clean preparation of nanoparticulate metals in porous supports: a supercritical route

Here we present the synthesis of nanometre sized silver particles which have been trapped within porous substrates; poly(styrene-divinylbenzene) beads and silica aerogels. This is the first time that supercritical carbon dioxide has been used to impregnate such porous materials with silver coordination complexes. In this paper we demonstrate that control over the resultant nanoparticles with respect to size, loading and distribution in the support material has been achieved by simple choice of the precursor complex. The solubility of the precursor complexes in the supercritical solvent is shown to be one of the key parameters in determining the size of the nanoparticles, their distribution and their homogeneity within the support matrix. Moreover, we demonstrate that the same methodology can be applied to two very different substrate materials. In the particular case of aerogels, conventional organic solvents could not be used to prepare nanoparticles because the surface tension of the solvent would lead to fracturing of the aerogel structure.Controlled decomposition of the coordination complexes in situ leads to metallic silver nanoparticles with a narrow size distribution, typically 10–100 nm that are homogeneously dispersed throughout the porous substrate. The whole process is carried out at near ambient temperature and no solvent residues are introduced into the porous media. The silver precursors are specifically designed to be both CO2 soluble and sufficiently labile to ensure facile decomposition to the metal. In-depth characterisation by X-ray diffraction and transmission electron microscopy has been applied to illustrate the homogeneous dispersion of particles throughout the composite material, determine the range and variation in particle size within the solid matrices and fully identify the resultant particles as metallic silver. This enables visualisation of dispersion and concentration, and control over particle size of the fabricated nanocomposite materials.

Structural and electrical characterization of AuTiAlTi/AlGaN/GaN ohmic contacts

AuTiAlTi/AlGaN/GaN ohmic contact structures rapid thermal annealed at 650, 750, 850, and 950 °C have been analyzed using complementary transmission electron microscopy and electrical characterization techniques. The relationship between annealing temperature, interfacial microstructure, and contact resistance is examined. Annealing temperatures of 750 °C or higher are required to produce an ohmic contact. Contacts annealed at 750 and 850 °C show a planar interface between contact and the AlGaN layer, with minimal consumption of the AlGaN and the formation of a thin TiN interfacial layer. Annealing at 950 °C produces the lowest contact resistance, with a structure showing inclusions through the AlGaN/GaN layer. These inclusions are also shown to be a Ti-nitride, having an Al/Au-rich metallurgical barrier layer surrounding them. However, this metallurgical layer does not produce an electrical barrier.

Structural and electrical characterization of AuPdAlTi ohmic contacts to AlGaN∕GaN with varying Ti content

AuPdAlTi∕AlGaN∕GaN ohmic contact structures with varying Ti:Al ratios have been investigated. The relationship between Ti:Al ratio, interfacial microstructure, and contact resistance is examined. Rapid thermal annealing temperatures of 850°C or higher are required to produce an ohmic contact with annealing at 950°C producing the lowest contact resistance in the majority of samples. Samples annealed at 950°C have been analyzed using complementary transmission electron microscopy and electrical characterization techniques. A thin Ti-nitride region is found to form at the contact/semiconductor interface in all samples. Ti-nitride inclusions through the AlGaN∕GaN layer are also observed, surrounded by an Al∕Au rich metallurgical barrier layer, with the size of the inclusions increasing with Ti content. The size of these inclusions does not have any clear effect on the electrical characteristics of the contacts at room temperature, but samples with fewer inclusions show superior electrical characteristics at h...

Effects of KOH etching on the properties of Ga-polar n-GaN surfaces

The effects of a KOH treatment on the properties of n-type GaN surfaces and associated Au/n-GaN contacts have been investigated by X-ray photoelectron spectroscopy, atomic force microscopy, reflection high-energy-electron diffraction, current–voltage and electron-beam-induced current characterization. Ga-polar surfaces grown by molecular beam epitaxy and metal–organic chemical vapour deposition were compared. A decrease in electron barrier height and an increase in non-radiative recombination properties of Au/n-GaN contacts were found with KOH treatment, correlated with an increase of surface Ga vacancies, an increase in surface N–H2 content and a decrease in surface C contamination. A 0.3-eV shift in the Ga3d peak position towards the valence band and a reduction in the dislocation contrast were observed for the case of molecular-beam-epitaxy-grown GaN only, demonstrating that surface Ga vacancies and threading dislocations play only a limited role in defining the resultant metal/GaN contact properties. Accordingly, the surface atomic content and the resulting surface states, following KOH treatment, should be taken into consideration when appraising the electrical properties of n-GaN surfaces and the performance of associated metallic contacts.

Cryogenic FIB Lift-out as a Preparation Method for Damage-Free Soft Matter TEM Imaging

The removal of a thinned lamella from a bulk sample for Transmission Electron Microscopy (TEM) analysis has been possible in the Focused Ion Beam Scanning Electron Microscope (FIB-SEM) for over 20 years either via in situ (by use of a micromanipulator) or ex situ lift-out approaches [1]. Both offer swift, site specific preparation for TEM analysis, particularly in light of advancements in corrected TEM. These techniques, however, are currently only applied to samples at room temperature, typically from the materials sector. The majority of biological samples contain a high degree of water, which will be removed under vacuum, leading to the shrinking and rearrangement of the sample. To overcome this, samples can be prepared by either critical point drying, fixation and resin impregnation (often combined with heavy metal staining) or cryogenic fixation. For both fixed and cryo-preserved samples, the preparation of thin-sections has always typically been prepared with a microtome, which yield samples of 60100 nm. However, these commonly suffer from compression artifacts and/ or knife marks, which distort data. There is also a desire to move away from staining and methods which dehydrate or allow / permit structural or chemical re-arrangement.

Application of the Taguchi method for assessment of surface treatment procedures for Ti/n-type GaN contacts

Abstract A Taguchi approach was used to appraise the relative importance of gallium nitride surface preparation on the processing of Ti/n-type GaN contacts. It is shown that a potassium hydroxide etch, silicon tetrachloride etch or hydrochloric acid clean prior to metal deposition can significantly improve the performance of the contact for the criteria of metal adhesion, contact resistance, electrical behaviour and across wafer uniformity.

Noise deconvolution for area fraction measurements

Abstract A novel approach to area fraction measurement is developed to deal with the limitations of the segmentation method. The new approach takes advantage of the statistical nature of the noise within an image to deconvolute the image histogram and calculate, by means of fitting, the area fractions. Both segmentation and noise deconvolution approaches are critically discussed, and their limitations and advantages tested using two extreme backscattered electron images of a metal matrix composite. The deconvolution approach is shown to produce accurate results in a situation in which the segmentation approach fails.

Live Quantitative BSE Acquisition with Standard-less Calibration

Whilst successive new generations of electronics are being deployed in Scanning Electron Microscopy (SEM) for faster and lower noise imaging, most of electron signals acquired today still remain purely qualitative. Conventional SEM is indeed based purely on contrast, e.g. distances or angles, however knowledge of absolute intensity provides a deeper level of analysis, e.g. concentration or gradients. Quantitative analysis has become the norm for a few techniques, such as Energy Dispersive X-ray Spectroscopy (EDS) and Electron Beam Induced Current (EBIC), however Backscattered Electron (BSE) imaging is rarely quantitative (qBSE) because it still requires calibration samples [1]. This work formulates an automated calibration method for standard-less BSE quantification.

New Possibilities for State-of-the-Art Electron Microscopy with Fast Backscattered Electron Detectors

Today’s Scanning Electron Microscopes are aiming for fast sample processing from sample insertion to the final picture or information needed. TV speed imaging at high resolutions helps to search and identify sample regions as well as to acquire high quality pictures without the need for a search in reduced image resolution mode. However, not all types of detectors are able to support the capabilities offered by new SEMs. For compositional examination, Secondary Electron (SE) detectors are often used for coarse searching before switching to mostly slower Backscattered Electron (BSE) detectors. This costs time and leads to superfluous electron exposure to the specimen. Furthermore, the two different detector types provide different sample information, and thus hinder finding the region of interest on the specimen. In this paper, we show the advantages of exclusively using BSE detection with a high speed segmented BSD module in the context of work flow and fast image acquisition.

A Robust 3D Scanning Technique for SEM

3D scanning is a popular technique in light-based instruments, driven primarily by the need to model and analyze shapes in 3D, however electron beam-based instruments have not yet been able to claim a significant role in this new advance. This work proposes and demonstrates a robust SEM technique that uses the same principles, but covers the micron range that is beyond the limit of optical 3D scanners.