Highly lattice mismatched InP/Si nanowire heterostructures were synthesized using metal organic chemical vapor deposition (MOCVD) process at 450 ºC. The InP nanowire diameter as high as 500 nm is much thicker than the critical diameter (~24 nm for InP/Si) predicted by a recent theoretical work on the coherent growth of nanowire heterostructures. We investigated possible factors that lead to the unusually large diameters in a highly lattice mismatched material system. Dislocations formed at the interfacial plane of the heterostructure due to high lattice mismatch were found to contribute to the growth of nanowires with very large diameters. An extra pair of dislocation lines at the interfacial plane was found to support an increase in nanowire diameter by ~12 nm.
Low temperature metal catalyzed InP nanowires with diameters ranging from 50nm to 500nm using a single step
MOCVD process at 450°C on (111)-oriented silicon substrates have been synthesized. The diameter range is much
higher than the critical limit (~24nm for InP on silicon) reported by a recent theoretical work on coherent growth of
nanowire heterostructures. This article presents the results of our investigation to highlight the possible factors that lead
to the unusually large diameters and help realize stable nanowire heterostructures in a highly lattice mismatched material
system. Our analysis finds dislocations formed at the interfacial plane of the heterostructure due to high lattice mismatch
is the most influential factor contributing to very large diameters. We have simulation results which indicate that each
added pair of orthogonal dislocation lines at the interfacial plane between InP and silicon supports ~12nm increase in the
nanowire diameter. A maximum nanowire density of ~5×108 cm-2 is estimated with growth rates ranging from 0.1
µm/min for the shortest nanowires and 10 μm/min for the longest ones.
We fabricated a photo-conducting device with InP nanowires bridged between phosphorous-doped hydrogenated
amorphous silicon electrodes. Photoresponse of the device with DC bias was characterized with a white light source and
a 630nm He-Ne laser. Experimental results from a large number of devices demonstrate a persistent photoconductivity, a
very unique feature of interest. After the light source is shut off, the photogenerated excess carriers recombine very
slowly over time and the effect is manifested in the form of persistent photocurrent that takes hours to decay to the dark
current level in the range of ~15 nA. Quasi exponential decay of the persistent photocurrent is observed with higher
decay rate at the initial stage just after the light source is turned off. Persistent photocurrent magnitude varies with the
magnitude of bias voltage, intensity and wavelength of the optical illumination. Experimental decay constant is
determined from 0.237/min for -8V bias to 0.174/min for -2V bias. The long recombination time can be attributed to the
carrier trapping in the light-induced traps, defects in nanowires and/or in the interface between the nanowires and the
amorphous silicon electrodes. Slow recombination process may also originate from the spatial separation of
photogenerated electrons and holes by built-in electric fields due to band bending at the heterostructure interfaces
between InP nanowire and amorphous silicon electrodes.
We demonstrate an InP nanowire based photodetector laterally integrated between two (111)-oriented vertical silicon surfaces. The nanowires are grown through a simple single step chemical vapor deposition (CVD) process using gold nanoparticles as catalyst with
in-situ p-doping and have been heteroepitaxially bridged between a pair of prefabricated p-doped Si electrodes. Nonlinear current-voltage characteristics are observed. Although this nonlinearity resembles a back-to-back rectifying profile it originates from space-charge limited conductivity of the nanowires. DC photoelectric characteristics of the device were measured under optical illumination (λ=630 nm) above the bandgap energy (1.34 eV or ~925 nm at room temperature) of InP. The variation in photoconductance with varying input optical power demonstrates high sensitivity of the device to optical illumination.
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