The optical absorptance from arrays of GaAs nanowires (NWs) was examined by the finite element method. Absorptance in cylindrical NWs, frustum nanocones (with base wider than the top) and inverted frustum nanocones (with top wider than the base) was compared. The introduction of higher order HE1n modes, the red-shift of the HE1n modes along the NW length due to NW tapering, and the red-shift of the modes due to increase of the overall NW diameter all contribute to a broadening of the absorption spectrum in conical NWs as compared to NWs with a constant diameter. The optical reflectance versus NW top diameter shows a minimum due to a balance between reflectance from the top of the NWs and reflectance from the substrate between NWs. The optimum geometry for photovoltaic energy conversion was determined from the total photocurrent. An optimum photocurrent of 26.5 mAcm-2 was obtained, corresponding to a conical NW morphology with base diameter of 200 nm, top diameter of 110 nm, and length of 2000 nm. An optimized inverse tapered conical morphology gave comparable performance.
InSb nanowire (NW) arrays fabricated by a top-down etching process were investigated for multispectral infrared photodetection. A 2.5 μm thick film of InSb was grown on Si (100) by molecular beam epitaxy using an AlSb buffer layer to alleviate defects associated with lattice mismatch strain, as confirmed by scanning electron microscopy and x-ray diffraction. Using a Ti mask patterned by electron beam lithography, InSb NW arrays with diameters ranging from 300 to 1300 nm (100 nm steps) and pitches ranging from 1000 nm to 3500 (500 nm steps) were reactive ion etched from the thin film. For each 100 nm increase in NW diameter, the peak absorptance wavelength, as measured by Fourier transform infrared spectroscopy, increased by 0.53±0.2 μm. The ability of InSb nanowires to produce highly tunable absorptance from 1.61 to 6.86 μm was demonstrated.
GaAs nanowire (NW) arrays were grown by molecular beam epitaxy using the self-assisted vapor−liquid−solid method with Ga droplets as seed particles. A Ga pre-deposition step is examined to control NW yield and diameter. The NW yield can be increased with suitable duration of a Ga pre-deposition step but is highly dependent on oxide hole diameter and surface conditions. The NW diameter was determined by vapor-solid growth on the NW sidewalls, rather than Ga pre-deposition. The maximum NW yield with a Ga pre-deposition step was very close to 100%, established at shorter Ga deposition durations and for larger holes. This trend was explained within a model where maximum yield is obtained when the Ga droplet volume approximately equals the hole volume.
The droplet contact angle and morphology of the growth interface (vertical, tapered or truncated facets) are known to affect the zincblende or wurtzite crystal phase of III-V nanowires grown by the vapor-liquid-solid method. Here, we present a model which describes the dynamics of the morphological evolution in self-catalyzed III-V nanowires in terms of the time-dependent (or length-dependent) contact angle or top nanowire radius under varying material fluxes. The model fits quite well the contact angle dynamics obtained by in situ growth monitoring of self-catalyzed GaAs nanowires in a transmission electron microscope. These results can be used for modeling the interface dynamics and the related crystal phase switching and for obtaining zincblendewurtzite heterostructures in III-V.
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