Ordered gallium arsenide (GaAs) nanowires are grown by molecular-beam epitaxy on GaAs (111)B substrates using Au-catalyzed vapor–liquid–solid growth defined by nanochannel alumina (NCA) templates. Field-emission scanning electron microscope images show highly ordered nanowires with a growth direction perpendicular to the substrate. The size (i.e., diameter) distribution of the wires is drastically narrowed by depositing the gold catalyst through an NCA template mask; this narrows the size distribution of the gold dots and arranges them in a well-ordered array, as defined by the NCA template. The nanowire diameter distribution full width at half maximum on the masked substrate is 5.1 nm, compared with 15.7 nm on an unmasked substrate.

GaAs nanowires were grown on GaAs (100) substrates by vapor–liquid–solid growth. About 8% of these nanowires grew in 〈110〉 directions with straight, Y-branched or L-shaped morphologies. The role of strain-induced reduction in surface free energy is discussed as a possible factor contributing to the evolution of 〈110〉 nanowires. Kinking and branching is attributed to growth instabilities resulting from equivalent surface free energies for 〈110〉 growth directions. Transmission electron microscopy verified that 〈110〉 nanowires are defect free.

Highly ordered arrays of nanosized GaAs-based dots were successfully prepared on GaAs (001) substrates by molecular-beam epitaxy using selected area growth. Selected area growth employed alumina nanochannel array (NCA) templates formed by anodic oxidation, bonded to the GaAs substrates. Homogeneous GaAs dots, as well as compositionally modulated heterostructures within the nanosized dots, were demonstrated. In the latter case, multilayer InGaAs/GaAs heterostructured nanodot arrays were fabricated. Dot growth occurred only as defined by the template mask, resulting in a hexagonal lattice of dots with 100 nm period spacing, with dots retaining the circular lateral shape of the pores as determined by the NCA template pore size; dot diameters were adjustable from 45 to 85 nm for a lattice period of 100 nm. Cathodoluminescence spectra from an InGaAs/GaAs 10×10 dot array clearly showed an emission peak at 920 nm (5 K), confirming the formation of a high-quality InGaAs/GaAs quantum dot array.

Growth of high-quality single-crystal AlGaAs nanowires was demonstrated using the vapor–liquid–solid (VLS) mechanism with molecular-beam epitaxy (MBE). Highly ordered AlGaAs nanowire arrays and GaAs∕AlGaAs multilayer nanowires were also prepared. Photoluminescence (PL) from homogeneous AlGaAs and GaAs∕AlGaAs multilayer nanowires was measured. The Al composition of the AlGaAs nanowires was found to be significantly lower than that for planar MBE films grown under the same conditions, as determined from PL and energy-dispersive x-ray spectroscopy measurements. This is explained in terms of the different growth mechanisms for VLS and normal MBE. Such AlGaAs nanowires are expected to have a wide range of applications in electronic and photonic devices.

Comprehensive measurements of the DLG in 0D, 1D, and 2D provide an accurate assessment of DLG value required during TPS commissioning. These DLG measurements can also be used as a quality control tool to quantify changes of the MLC calibration and leaf gap consistency, which is critical for the accurate delivery of dynamically delivered SW IMRT plans.

Most electronic portal imaging devices (EPIDs) developed so far use a Cu plate/phosphor screen to absorb x rays and convert their energies into light, and the light image is then read out. The main problem with this approach is that the Cu plate/phosphor screen must be thin (approximately 2 mm thick) in order to obtain a high spatial resolution, resulting in a low x-ray absorption or low quantum efficiency for megavoltage x rays (typically 2-4%). In addition, the phosphor screen contains high atomic number (high-Z) materials, resulting in an over-response of the detector to low-energy x rays in dosimetric verification. In this paper, we propose a new approach that uses Cerenkov radiation to convert x-ray energy absorbed by the detector into light for portal imaging applications. With our approach, a thick (approximately 10-30 cm) energy conversion layer made of a low-Z dielectric medium, such as a large-area, thick fiber-optic taper consisting of a matrix of optical fibers aligned with the incident x rays, is used to replace the thin Cu plate/phosphor screen. The feasibility of this approach has been investigated using a single optical fiber embedded in a solid material. The spatial resolution expressed by the modulation transfer function (MTF) and the sensitivity of the detector at low doses (approximately one Linac pulse) have been measured. It is predicted that, using this approach, a detective quantum efficiency of an order of magnitude higher at zero frequency can be obtained while maintaining a reasonable MTF, as compared to current EPIDs.

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Highly-ordered GaAs/AlGaAs quantum-dot arrays (QDA) were grown by molecular-beam epitaxy on GaAs (001) using masks of anodic nanochannel alumina (NCA). The QDA replicated the hexagonal lattice pattern of the NCA masks with period spacing of 100 nm. The circular disk-like dots were defined by the nanohole channels of NCA masks with size adjustable between 45 and 85 nm. Both single- and double-well GaAs/AlGaAs QDA exhibited strong photoluminescence. The single-well QDA showed a narrow peak at 1.64 eV with full width at half maximum of only 16 meV, indicating good size uniformity and crystal quality for the QDA. NCA masked epitaxial growth is thus shown to be a promising general approach for fabricating various heterostructure QDA, including both strained and lattice-matched heterostructures.