SEM images of epitaxial GaP–GaAs–GaP heterostructure wires before and after a selective bromine etch. (a) Unetched wires grown on a GaP substrate. (b) Wires after in 0.1% bromine in methanol showing tapering of the GaAs segments. Previous to the etch, a GaAs oxide removal was performed using 37% HCl.
(Color online) (a) Illustration of nanostencil gold deposition on trench sidewalls. (b) Substrate and stencil are angled 35° with respect to the direction of the gold deposition.
SEM images of stencil defined gold patterns on Si trench sidewalls. All three images are taken at a 30° angle with respect to the electron beam. (a) Gold deposited through the stencil on a rough RIE Si sidewall. (b) Gold deposited on a KOH etched planarized trench sidewall. Due to the KOH etch, the height of the sidewall is reduced. (c) A zoom of the gold pattern of a RIE etched sidewall planarized with KOH for a short time. The KOH planarized sidewall results in a more well defined gold pattern. Gold particles with diameters as small as are observed.
(Color online) SEM images of wires grown from stencil defined gold patterns. (a) Wire growth within a gold triangle (arrow guide) showing strong epitaxy with very little variation in wire diameters. (b) The trenches are created in a (110)SOI chip having a ⟨111⟩ direction in the lateral plane, though angled 70° with respect to the sidewall. (c) Position control is achieved. (d) A top view of a trench with wires grown from a stencil defined gold pattern. The image illustrates the epitaxy of the wires growing at a 70° angle with respect to the sidewall.
(Color online) SEM images of GaP–GaAs–GaP nanowires grown within a trench in a (111)SOI chip at an angle of 19.5° with respect to the lateral plane. (a) The uniform gold layer results in a high density of wires with very little variation in diameters. An example of a wire bridging the trench is indicated. (b) The same trench wires after of etching in 0.1% bromine in methanol.
(Color online) Summary of the etched structures obtained from different bromine solutions and etching times. Images are taken using SEM. (a) An initial wire. (b) Corrugated surfaces appearing at the GaAs segment when no oxide etch is performed previously to the bromine etch. (c) Bromine etch after oxide removal resulting in a uniform etched GaAs segment. (d) Bromine etch after oxide removal on a tapered wire resulting in a stronger tapered GaAs segment. (e) Tapering of the GaAs segment after longer etch times, resulting in a sharp GaAs tiplike structure. (f) Complete removal of the GaAs segment.
(Color online) Etching rate as function of the bromine in methanol concentration. Assuming that the dependence is linear, a slope of per percent bromine is observed.
(Color online) Selective etching of a GaAs segment resulting in a tapered structure. (a) SEM image of an epitaxial GaP–GaAs–GaP heterostructure nanowire grown on a Si(111) substrate. (b) SEM image of an etched wire. (c) TEM image of a tapered GaAs segment. The tapering angle, which is half the cone angle, is measured to approximately 5° and the etched surface does not adjust to any specific side facet. (d) TEM image of a crystalline tip with an apex radius of . The amorphous layer covering the tip is not only native oxide but also carbon contamination deposited during the TEM analysis.
GaAs segment etch rates at different bromine concentrations.
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