X. Mei

Growth of Au-catalyzed ordered GaAs nanowire arrays by molecular-beam epitaxy

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.

Growth, branching, and kinking of molecular-beam epitaxial 〈110〉 GaAs nanowires

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.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.

Molecular-beam epitaxial growth of GaAs and InGaAs/GaAs nanodot arrays using anodic Al2O3 nanohole array template masks

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 and photoluminescence characteristics of AlGaAs nanowires

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.

Highly-ordered GaAs/AlGaAs quantum-dot arrays on GaAs (001) substrates grown by molecular-beam epitaxy using nanochannel alumina masks

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.

Fabrication of ZnTe Nanohole Arrays by Reactive Ion Etching Using Anodic Alumina Templates

Highly ordered uniform ZnTe nanohole arrays were successfully fabricated by reactive ion etching using anodic alumina template. These ZnTe nanohole arrays transferred exactly the hexagonal lattice pattern of the original alumina template. Using this method ordered ZnTe nanoholes with pore diameter from 10 nm to several hundred nm can be fabricated. The simplicity, flexibility, and practicality of this technique makes it a prospective method for developing future optical and electronic devices.

Ultrahigh-density, nonlithographic, sub-100 nm pattern transfer by ion implantation and selective chemical etching

A self-assembled array of nanometer-sized holes in alumina has been adapted as a mask for conventional, broad-area, ion implantation. The mask pattern, made up of nanoholes arranged in a two-dimensional triangular array with a 100 nm period and a 55 nm diameter pore size, has been successfully transferred onto single crystal (100) SrTiO3 substrates using 200 and 500 keV energy Pt ion bombardments, at fluences sufficient to amorphize the exposed areas. The amorphized material was removed by selective chemical etching resulting in a periodic array of holes about 55 nm in diameter and 115 nm deep. This parallel, nonlithographic approach is adaptable to submicron depth, variable array geometry and scale, and to any material where a selective etch can be found for the irradiated volume.

Fabrication of Indium Nitride Nanodots Using Anodic Alumina Templates

A highly ordered uniform InN nanodot array was successfully grown by reactive sputtering using an anodic nanohole channel alumina (NCA) template. The nanodot array replicated the hexagonal lattice pattern of the NCA template with a period space of nanometer scale. The simplicity, flexibility, and practicality of this technique make it a prospective method for developing future optical and electronic devices of III-V nitride semiconductors.

Passivation of GaAs(110) with Ga2O3 thin films deposited by electron cyclotron resonance plasma reactive molecular beam epitaxy

Gallium oxide thin films deposited by electron cyclotron resonance plasma molecular beam epitaxy on GaAs(110) surfaces are reported. Room temperature photoluminescence spectra show an enhancement over as-is surfaces by greater than an order of magnitude for semi-insulating wafers. This enhancement is corroborated by low temperature photoluminescence spectra, showing a reduction in AsGa, OAs, and carbon-related emissions. The bonding configuration at the interface to GaAs was investigated by x-ray photoelectron spectroscopy depth profiling and secondary ion mass spectroscopy: Arsenic oxide related compounds were below the sensitivity limits of the former technique, while carbon (both in the film and in the vicinity of the interface) was below the sensitivity limit of the latter technique. Photoluminescence enhancement is also attributed to hydrogen passivation of EL2 defects, which is found to be stable following deposition at temperatures of 400 °C on semi-insulating and p-type wafers.

Cathodoluminescence Study of Highly Ordered Arrays of InGaAs Quantum Dots.

We have examined the optical properties of highly ordered arrays of nanosized InGaAs quantum dots (QDs) with cathodoluminescence (CL) techniques. The InGaAs QDs were formed on GaAs (001) substrates by molecular beam epitaxial growth using alumina nanochannel array (NCA) templates. The behavior of the CL luminescence was studied by temperature and electron beam scan area dependence. The activation energy of the InGaAs QD luminescence was determined to be 43.1 meV. The CL results indicate that the InGaAs QDs are rather uniform with identical electronic structure.