Pre-patterned GaAs Substrates Research Articles

Bottom-up fabrication of FeSb2 nanowires on crystalline GaAs substrates with ion-induced pre-patterning

In recent decades, nanostructuring has become one of the most important techniques to design and engineer functional materials. The properties of nanostructured materials are influenced by the interplay of its instrinsic bulk properties and the properties of its surface - the relative importance of the latter being enhanced by the increased surface-to-volume ratio in nanostructures. For instance, nanostructuring of a thermoelectric material can reduce the thermal conductivity while maintaining constant electrical conductivity and the Seebeck coefficient, which would improve the thermoelectric properties. For that reason, this study investigated the possibility of preparing nanowires of iron antimonide (FeSb2), a thermoelectric material, on single-crystalline gallium arsenide GaAs (001) substrates with ion-induced surface nanoscale pre-patterning and characterized the structure of the prepared FeSb2 nanowires. The GaAs (001) substrates were pre-patterned using 1 keV Ar+ ion irradiation. By using an ion source with a broad, unfocused ion beam at normal incidence, the patterned area can be scaled to nearly any size. The self-organized surface morphology is formed by reverse epitaxy and is characterized by almost perfectly parallel-aligned ripples at the nanometer scale. For the fabrication of FeSb2 nanowires, iron and antimony were successively deposited on the pre-patterned GaAs substrates at grazing incidence and then annealed. They were characterized using transmission electron microscopy (TEM), in particular high-resolution TEM imaging for structure analysis and spectrum imaging analysis based on energy-dispersive X-ray spectroscopy for element characterization. With the presented fabrication method, FeSb2 nanowires were produced successfully on GaAs(001) substrates with an ion-induced nanopatterned surface. The nanowires have a polycristalline structure and a cross-sectional area which is scalable up to 22 × 22 nm2. Due to the high order nanostructures on the GaAs substrate, the nanowires have a length of several micrometer. This bottom-up nanofabrication process based on ion-induced patterning can be a viable alternative to top-down procedures regarding to efficiency and costs.

Exciton and multiexciton optical properties of single InAs/GaAs site-controlled quantum dots

We have studied the optical properties of InAs site-controlled quantum dots (SCQDs) grown on pre-patterned GaAs substrates. Since InAs nucleates preferentially on the lithography motifs, the location of the resulting QDs is determined by the pattern, which is fabricated by local oxidation nanolithography. Optical characterization has been performed on such SCQDs to study the fundamental and excited states. At the ground state different exciton complex transitions of about 500 μeV linewidth have been identified and the fine structure splitting of the neutral exciton has been determined (≈65 μeV). The observed electronic structure covers the demands of future quantum information technologies.

Magneto-optical properties of single site-controlled InGaAsN quantum wires grown on prepatterned GaAs substrates

The properties of single site-controlled InGaAsN quantum wires (QWRs)—both untreated and irradiated with atomic hydrogen—are probed by micro-magnetophotoluminescence spectroscopy. The strong anisotropy of the diamagnetic shift measured for different orientations of the applied magnetic field confirms the one-dimensional nature of the QWR carrier wave function. In addition, the strain reduction associated with N incorporation is found to promote a larger indium intake in the QWR, enabling the realization of site-controlled QWRs emitting at long (1.3 μm), technologically relevant wavelengths.

Structural and optical properties of InAs quantum dot chains grown on nanoimprint lithography structured GaAs with different pattern orientations

We use large-scale UV nanoimprint lithography prepatterned GaAs substrates for site-controlled growth of InAs quantum dot chains by molecular beam epitaxy. We demonstrate simultaneous fabrication of quantum dot chains with high optical quality along four different crystal orientations, [011], [011¯], [010], and [001]. We show that the [011¯], [010], and [001]-oriented quantum dot chains not only have similar morphology but also experience similar in-plane optical anisotropy, which tends to align along the axis of the quantum dot chain. Our optical and structural results show that InAs quantum dot chains could be a potential platform for nanophotonic waveguiding and integrated circuits.

Molecular beam epitaxial growth of site-controlled InAs quantum dots on pre-patterned GaAs substrates

Ex situ electron-beam lithography followed by conventional wet etching has been used to pattern small holes 60–150 nm wide, ∼13 nm deep in GaAs substrates. These holes act as preferential nucleation sites for InAs dot growth during subsequent overgrowth. By varying either the InAs deposition amount or the thickness of a GaAs buffer layer, the occupancy over the patterned sites can be controlled. Comparison with growth on a planar substrate shows that preferential nucleation occurs due to a reduction in the apparent critical thickness above the pattern site; the magnitude of this reduction is dependent on the dimensions of the initial pattern.

Size evolution of site-controlled InAs quantum dots grown by molecular beam epitaxy on prepatterned GaAs substrates

Ex situ electron-beam lithography followed by conventional wet or dry etching has been used to pattern small holes of 60–150nm wide, 10–40nm deep in GaAs substrates. These holes then act as preferential nucleation sites for InAs dot growth. Preferential nucleation occurs due to a reduction in the apparent critical thickness above the pattern site, which is dependent on the dimensions of the initial pattern. We demonstrate that subsequent growth of site-controlled dots over the patterned sites varies with InAs deposition amount and growth temperature in a similar way to InAs dot growth on planar substrates. By varying temperature and either InAs deposition amount or pattern size, both the size and position of InAs quantum dots can be controlled.

Strong spatial beryllium doping selectivity on InGaP layers grown on pre-patterned GaAs substrates by chemical beam epitaxy

We present an investigation of beryllium doping selectivity in InGaP layers grown by chemical beam epitaxy on pre-patterned substrates. We observed a resistivity of 3.1×10 −2 and 4.5×10 −2 Ω cm for (1 1 1)A planes with the growth at 500°C and 540°C, respectively. The layers on (0 0 1) planes show a resistivity of 8.9×10 −1 Ω cm with the growth at 500°C, being essentially undoped with the growth at 540°C ⋅ We show how this strong doping selectivity can be explained by Be 3P 2 cluster formation growth, which depends on growth temperature and planar crystalline structure.

Electron transport in modulation-doped GaAs v-groove quantum wires

We report the growth of modulation-doped GaAs/AlxGa1−xAs v-groove quantum wires and structural, electrical and optical investigations of their electronic states and transport properties. By using alternative group III precursors on partially SiO2 masked pre-patterned GaAs substrates, samples have been fabricated which permit electrical measurements of single isolated wire structures without the need for additional electron-beam lithography. Magneto-transport was measured as a function of tilt angle of the incident magnetic field to identify the formation of low-dimensional electron gases in different parts of the structure. Photoluminescence investigations reveal 1D and 2D confined states which show different carrier heating when electric fields are applied along the wire structure.

Systematic growth studies of narrow constrictions formed by molecular beam epitaxy on prepatterned substrates

The overgrowth of prepatterned GaAs substrates by molecular beam epitaxy (MBE) of GaAs/AlGaAs heterostructures is investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). First, mesas with vertical side walls which have a constriction of 1–5 μm, are patterned into a GaAs substrate. During the MBE deposition of GaAs/AlGaAs layers onto this substrate, specific facets form at the boundaries of the patterned area. These facets depend on the shape and orientation of the prepatterning. A detailed study of an overgrown constriction which consists mainly of {110} and intermediate { N11}A ( N=3 or 4) facets shows that the constriction narrows and that a rectangular shape of the prepatterning is maintained in the MBE process. Inter facet diffusion of adatoms modifies the thickness of the MBE layers in the patterned area. The enhanced thickness of layers in the center of the constriction may be utilized to fabricate dot and wire like structures at well-defined positions. AFM measurements in combination with simulations of the surface diffusion clearly demonstrate that the {110} facets yield the main contribution to the enhanced thickness. { N11}A facets can only be employed to obtain the above enhancement, if the prepatterned constriction is round-shaped. This sets a limit for down-scaling to the submicron range.

Luminescence of ZnCdSe/ZnSe ridge quantum wires

Blue light-emitting quantum wire structures fabricated by molecular-beam epitaxial growth on submicrometer prepatterned GaAs substrates were investigated by spatially and time resolved luminescence experiments. The quantum wires are formed due to the different growth rates of ZnCdSe on the (111) and (100) surfaces of the grated substrate. With decreasing wire width, the exciton luminescence splits into two clearly distinguished lines. These lines can be assigned to the emission of the ridge quantum wire and the emission of ZnCdSe quantum wells at the bottom of the grooves. The two-dimensional quantum confinement in the ridge wire is confirmed by a maximum of the decay time at the energy of the ridge luminescence.

Spatial composition dependence in InGaP growth on pre-patterned GaAs substrates by chemical beam epitaxy

We have investigated the spatial composition variation in InGaP layers grown by chemical beam epitaxy (CBE) on pre-patterned substrates. At growth temperature of 540°C, no difference between In and Ga growth properties is observed. At 500°C, we observe the onset of new crystalline planes on the side walls of the pre-patterned structure. Finally, we show how these planes are related to a measured strong spatial composition variation.

In 0.49Ga 0.51P growth on pre-patterned GaAs substrates by chemical beam epitaxy

We present a study on the growth of lattice-matched InGaP on patterned GaAs substrates by chemical beam epitaxy. An experimental analysis of the growth on planes [1 0 0] and [1 1 1]A as a function of growth temperature and pattern dimension is presented. A simple surface kinetics model is proposed allowing the determination of diffusion length, incorporation time and free species lifetime on both planes. Incorporation on planes [1 1 1]A reduces with increase in temperature and there is indication of a relationship between evaporation time and nucleation sites on [1 1 1]A planes