NOVELTY - Method for direct heteroepitaxial growth of vertical group III-V semiconductor nanowires (14) on silicon substrate (24), involves: treating substrate to remove native oxide and promptly placing into reaction chamber before reformation of native oxide; heating substrate till it reaches thermal equilibrium at growth temperature after stabilization time; providing group III-V precursors into chamber having substrate to establish total flow rate of group III-V precursors, each with respectively chosen molar fraction; and allowing growth of semiconductor nanowires for growth time.    USE - For direct heteroepitaxial growth of vertical group III-V semiconductor nanowires on silicon substrate, to form vertical nanowire structure or flexible vertical nanowire structure; which is useful in photovoltaic device; photoelectrochemical cell; optoelectronic device serving as photovoltaic device, photoelectrochemical cell, photodetector array, light emitting diode, or laser; solar cell or display; and nanowire field effect transistor (FET) and their array (claimed). The photoelectrochemical cells are useful to break down water and CO2 (for hydrogen generation and CO2 conversion to fuel, respectively). The light emitting diode and nanolasers are useful as light sources for Si photonics or intrachip optical communication. The solar cells formed with flexible nanowires include solar tent, curtain, or cloth solar cells.    ADVANTAGE - The method allows for direct epitaxial ground of group III-V semiconductors as nanowires on silicon over large area using metal-organic chemical vapor deposition (MOCVD), with fewer processing steps than prior art methods, which keeps silicon relatively uncontaminated but consistently produces vertical nanowire growth. No pre-deposited material is required to accomplish nanowire growth, which significantly simplifies growth process and eliminates undesirable effects. The method is reliable, allows direct integration of functional III-V-nanowire structures into Si-based microelectronics/photonics, and provides low-cost, high-volume fabrication. In optoelectronic devices, vertically aligned nanowire arrays provide confinement of photons, high carrier mobility, and provide high efficiency light emitting diodes, waveguiding and lasing, and high speed transistor devices and circuits. The nanowire structure has no large base islands surrounding nanowire root. The vertical group III-V nanowires have core-shell structure, which provides radial homojunction or heterojunction, and further provides high optical absorption due to long optical path in axial direction of vertical nanowires and light concentration due to contrast in refractive indices of nanowire materials with surrounding dielectric materials, while reducing distance over which carriers must diffuse. Such structures can also provide electron/hole confinement and high carrier mobilities, which can form 2D free electron gases in 1D cylindrical geometry. In photodiodes, tuning alloy composition of III-V compound nanowires permits bandgap of heterojunction photodiodes to be tuned, which enables broadband photovoltaic cells and photodectors. The heterojunctions have doped nanowire core surrounded by shell of complementary doping, with tandem solar cells structures or with multiple quantum wells and superlattice structures. This enables high optical absorption along long axis of nanowires while considerably reducing carrier collection distance in radial direction. The photoelectrochemical cells formed can directly dissociate water into hydrogen and oxygen. The nanowire arrays allows improved light absorption efficiency, reduced angular dependence, improved electron/hole separation efficiency, and separated electron and hole pathway for collection to provide increased charge collection efficiency compared to planar designs.    DETAILED DESCRIPTION - Method for direct heteroepitaxial growth of vertical group III-V semiconductor nanowires (14) on silicon substrate (24), involves: treating silicon substrate to substantially completely remove native oxide, and promptly handling and placing substrate into reaction chamber before any substantial reformation of native oxide; heating silicon substrate, so that it reaches thermal equilibrium at growth temperature after stabilization time; providing group III-V precursors into chamber having silicon substrate to establish total flow rate of group III-V precursors, each with a respectively chosen molar fraction; and allowing growth of semiconductor nanowires for a growth time. INDEPENDENT CLAIMS are included for the following:    (1) vertical nanowire structure comprising: array of non-tapered group III-V nanowires directly contacting silicon substrate having substantially no non-vertical nanowires;    (2) photovoltaic device (10) comprising: the vertical nanowire structure and electrode to receive power from array;    (3) photoelectrochemical cell comprising: the vertical nanowire structure, in which group III-V nanowires comprises InAs nanowires having core-shell p/n or p-i-n heterojunction group III-V nanowires, disposed in water; and counter electrode disposed in water;    (4) optoelectronic device serving as photovoltaic device, photoelectrochemical cell, photodetector array, light emitting diode, or laser; comprising: substrate; array of ordered vertical core-shell heterojunction group III-V nanowires formed on substrate, where nanowires include multiple quantum well and superlattice structures between p-type and n-type regions of a p/n or p-i-n core/shell junction; and electrode to receive power from or providing power to ordered array;    (5) vertical nanowire structure comprising: array of group III-V nanowires including core and shell layer(s) around core, and further comprising doping or alloy composition creating axial or radial homojunction or heterojunction;    (6) flexible vertical nanowire structure comprising: array of group III-V nanowires embedded in polymer;    (7) solar cell or display comprising: the flexible group III-V nanowire structure, where nanowires include axial or radial heterojunctions or homojunctions;    (8) forming flexible group III-V nanowire array structure, involving: growing group III-V nanowires on substrate; embedding group III-V nanowires in polymer layer; and releasing group III-V nanowires from substrate;    (9) nanowire field effect transistor (FET) comprising: group III-V nanowire source with insulating core formed on silicon substrate; and vertical stack of gate oxide and gate layers surrounding nanowire source; and    (10) array of the FETs comprising: silicon substrate that comprises complementary metal oxide semiconductor (CMOS) integrated circuit.    DESCRIPTION OF DRAWING(S) - The figure shows partial cross section and perspective view of core-shell, radial p-n junction vertical nanowire photovoltaic cell.    Photovoltaic cell (10)    N-type vertical nanowires (14)    P-type shell of nanowire cores (16)    Bottom electrode (22)    Silicon substrate (24)