Although initially developed as an imaging tool, the helium ion microscope (HIM) is finding applications in nanofabrication as its focused ion beam is capable of highly-localized material modification. In this study, an external pattern generator is used to explore the capabilities of the HIM for localized milling of a ∼7 nm thick layer of silicon-on-insulator, with atomic force microscopy (AFM) used to characterize the resulting patterns. The dose and patterned area size are varied and milling to depths >7 nm is demonstrated. At high doses and large areas, protuberances form, primarily due to sub-surface swelling caused by the implanted helium. The results suggest this technique could enable the rapid prototyping of next-generation nanoelectronic devices in thin silicon.

Epidemiological research on respiratory syncytial virus (RSV) infections in children was carried out at the Virology Laboratory, University Teaching Hospital (UTH), in Lusaka, Zambia, from January–December 1996. Specimens including 736 nasal washings and 2424 throat swabs were collected from children with acute respiratory infections (ARI) and tested for RSV by enzyme immunoassay and by virus isolation. RSV was isolated in 62 (4·1%) of 1496 throat swabs collected from March to September and was detected in 99 (16·3%) of 609 nasal washings from March to November. The average RSV isolation rate was 2·6% and the average RSV detection rate was 13·5%. The highest RSV isolation (8·1%) and detection (30·5%) rates were in June 1996. RSV antibody in the 278 serum specimens collected from Zambian children, who were hospitalized in the paediatric ward, UTH, was detected using a standard neutralization test. The antibody positive rate was 60–80% in children >4 years. It is evident that RSV is one of the main causal agents of ARI in children in Zambia.

A novel source of α-amylase has been identified in Pycnoporus sanguineus. Spores incubated in the submerged system using wheat bran as a carbon source showed α-amylase activity. Induction of α-amylase was also observed from mycelium incubated in submerged fermentation (LM4) using starch or wheat bran as the carbon source. The effect of the inhibitors on the production of enzyme by mycelium incubated in the presence of glucose, suggested the existence of catabolic repression. A comparative study showed that in a solid state fermentation system (SSM), even in the presence of high levels of glucose, the production of α-amylase was 4-fold greater than that in a submerged system. High levels of β-glucosidase and xylanase activities were also found in SSM after 72 h incubation. P. sanguineus produces at least three important hydrolytic enzymes using economical procedures.

Electrically detected magnetic resonance (EDMR) has been used to study carrier recombination in GaAs/AlGaAs quantum well structures grown by molecular beam epitaxy. The spin dependent photoconductivity signals depend strongly on the electrical contact properties. Using silver paste contacts a narrow (18 G) resonance located at g = 2.001 is observed. It has been previously attributed to surface defects on GaAs. Using alloyed In-contacts other signals are detected. The dominant resonance observed at 9 GHz has an isotropic g-value of g = 1.99 with a halfwidth of 200 G and is therefore assigned to Cr4+. Other signals of considerably lower intensity are explained by the well known electron paramagnetic resonance (EPR) properties of the Gai-interstitial and the AsGa-antisite defects. EDMR performed at 34 GHz allows the experimental separation of the two sets of hyperfine lines.

DX centers are semiconductor dopants which form deep states due to a large lattice relaxation. At low temperature, the DX centers exhibit persistent photoconductivity. When exposed to light in a spatial pattern, the photocarriers are confined to the illuminated regions by Coulomb interaction with the localised DX centers. The resulting spatial modulation of the free carrier density gives rise to a modulation of both the electrical conductivity and the dielectric constant. We demonstrate both effects by measurements of the conductance anisotropy and optical diffraction of samples exposed to excitation in a striped pattern. Erasure is achieved by thermal annealing. The constrast ratio of the conductivity modulation is greater than 108; in our experiment it is limited to ∼100 by light scattering. We estimate that 100nm resolution is feasible. Optical diffraction efficiencies up to 40% have been demonstrated in a stripe-illuminated thick sample. The persistence of the written patterns at low temperature is potentially useful in high-density data storage applications and the fabrication of erasable submicron devices.

We investigated the reason of the (imbalanced) accumulation of electrons in AIGaSb/lnAs/AIGaSb QW system in spite of the p-type conduction of undoped AIGaSb. It was found that the concentration of the accumulated electrons negligibly depended on the number of the interfaces, but increased linearly with the effective AlSb thickness. These results indicate that donor levels in AIGaSb are the dominant electron sources. We propose a model that the deep acceptors with larger concentration and donors coexist, and the electron accumulation depends on the energy position of the acceptor in AIGaSb with respect to the quantum level in the InAs well. Acceptor levels obtained experimentally are about 100 meV higher than the bottom of the InAs conduction band, and we succeeded in eliminating the electron accumulation by making the quantum level of the InAs well higher than this acceptor level. The origins of the donors and acceptors are also discussed.

The influence of chemically different donor species on the nature of shallow donor states in Al0.6Ga0.4As/GaAs heterostructures has been investigated by optically detected magnetic resonance (ODMR). Previous theoretical work by Morgan predicts a triplet state for group IV donors and a singlet state for group VI donors. ODMR experiments were performed on as-grown and implanted Si-, Se-, and S-doped epitaxial layers of Al0.6Ga0.4As grown on (001) GaAs substrates. The effective-mass states are modified by the heter-oepitaxial strain in these layers. The Si donors are characterized as quasi-independent valley states. The Se and S donors have valley-orbit splitting energies (i.e. chemical shifts) of 19-20 meV . The results indicate that Si, Se, and S donors are on the lattice sites in the metastable state of DX.

The effects of specific local environments on DX centers were investigated for two different substitutional-site-donors of Si and Te in selectively-doped ordered-alloy samples. Capacitance transient technique under hydrostatic pressure was utilized for the determination of the DX energy-level structure. Downward movement of the energy levels for both Si- and Te-DX centers was observed when Al occupies the 2nd or 1st nearest neighbor site, respectively. A very large shift of the electron-emission activation-energy with pressure was found for the Te-DX centers surrounded by specific Al and Ga mixed-environments, whereas negligible shift for the Si-DX centers was observed regardless of the environment. These effects were discussed in relation to the lattice relaxation of the relevant DX centers.

EBIC has been used to investigate the beam-induced damage in As+ -implanted Si subjected to scanned laser or electronbeam annealing. Comparison is made between these annealing techniques; in general, electron-beam annealing is found to give superior results. In addition, several different experiments combining these techniques are described. Samples were pre-annealed prior to laser annealing, using either thermal or electron-beam annealing. Other samples were given a thermal post-anneal after laser-annealing. The effect of these treatments on defect formation is discussed.