NOVELTY - A semiconductor structure comprises: a substrate (100) and II-VI alloy layer (101) having elemental composition formed over surface of substrate, where elemental composition of II-VI alloy layer laterally continuously varies between first II-VI compound or alloy at first position (A) over surface of substrate and second II-VI compound or alloy at second position (B, C, D) over surface of substrate.    USE - As semiconductor structure and nanowires, useful for manufacturing laser, solar cell, multispectral detector (claimed) light emitting diodes, color engineered display and lighting, and spectrometer on-a-chip. The multiwavelength laser chip is useful for wide variety of applications such as on-chip wavelength division multiplexing (WDM) source, multiwavelength sensor, optimizable white-light source, red-green-blue (RGB) display, and nanophotonics integrated circuits.    ADVANTAGE - The semiconductor structure provides semiconductor structure comprising laterally varying II-VI alloys including nanowires, suitable for forming spatially wavelength varying lasers and light emitting diodes, multi-color detectors and laterally distributed solar cells where each cell absorbs one given wavelength range of solar radiation. The semiconductor comprising II-VI alloy layer with elemental composition of II-VI alloy layer laterally varying upon substrate provides semiconductor composition and structural assemblies of nanowires which enable continuously tunable bandgap on single substrate, and achieves full-range composition variation for desired wavelength range within single substrate in single run of growth i.e. fully tunable lasing within entire composition range on single substrate; which is advantageous over various prior art semiconductor alloy nanowires of different compositions obtained under separate growth conditions. Further, method for preparing semiconductor structure enables straightforward preparation of semiconductor structures in single step. The semiconductor structure includes growth of ternary and quaternary semiconductor alloy nanowires and growth of alloys having controllable and variable compositions, leading to tunable bandgap change in entire visible spectrum. Such unique nanostructured materials with controllable alloy composition can be used in wide range of applications. The laser formed provides laser chip with wide wavelength tuning range, continuously tunable over length of chip, such as exhibits continuously tunable emission wavelength of (350-840, preferably 500-070) nm. The solar cell formed shows different wavelength bands of solar radiation directed onto, and absorbed by, different cells, with bandgap ranges related to wavelength bands. The multispectral detector formed shows different wavelength bands of a multispectral-component signal directed onto, and absorbed by, different cells, with bandgap ranges related to wavelength bands.    DETAILED DESCRIPTION - A semiconductor structure comprises: substrate (100) and II-VI alloy layer (101) having elemental composition formed over surface of substrate, where elemental composition of II-VI alloy layer laterally continuously varies between first II-VI compound or alloy at first position (A) over surface of substrate and second II-VI compound or alloy at second position (B, C, D) over surface of substrate. The II-VI alloy layer laterally varies with respect to first pair of II-VI compounds selected from constituent compounds of first and second II-VI compounds or alloys, where from first to second positions, one of first pair of II-VI compounds continuously increases in compositional abundance of II-VI alloy layer and other II-VI compound of first pair continuously decreases in compositional abundance of II-VI alloy layer. Each II-VI compound or alloy of first and second II-VI compounds or alloys comprises (CdZnMgHg)(SSeTe) compound or alloy (preferably CdS1-zSez compound or alloy, or Cd1-aZnaS compound or alloy (where z, a = 0 at first position and 1 at second position)). The II-VI alloy layer further varies between second II-VI compound or alloy at second position and third II-VI compound or alloy at third position over surface of substrate; where from second to third position, II-VI alloy layer varies with respect to a second pair of II-VI compounds selected from constituent compounds of second and third II-VI compounds or alloys, where one II-VI compound of second pair continuously increases in compositional abundance of II-VI alloy layer and other II-VI compound of second pair continuously decreases in compositional abundance of II-VI alloy layer, where each II-VI compound or alloy is Cd1-xZnxS1-ySey alloy (where y is 1 at first position and 0 at second position and x is constant between first and second positions; x is 0 at second position and 0.6 at third position and y is constant between second and third positions; or x is 0 and y is 1 at first position and x is 1 and y is 0 at second position). The laterally varying II-VI alloy layer comprises nanowires. The first and second positions are separated by less than or equal to 2 or 0.5-5 cm. The semiconductor further comprises patterned catalyst layer of Au formed directly on substrate, and II-VI alloy layer is formed directly on catalyst layer. INDEPENDENT CLAIMS are included for the following:    (1) preparing laterally composition-varying II-VI alloy layer over at least portion of surface of substrate involving: contacting at least portion of surface of substrate within reaction zone with chemical vapor to form a laterally composition-varying II-VI alloy layer over portion of surface of substrate, where chemical vapor is generated by heating at least two II-VI binary compounds; and reaction zone has temperature gradient of at least 50-100 degrees C along extent of reaction zone, where extent is 0.5-5 cm in length, such that entire laterally composition-varying II-VI alloy layer is formed in single step;    (2) Znx'Cd1-x'Sy'Se1-y' nanowire (where x' and y' are 0-1); and    (3) preparing Znx'Cd1-x'Sy'Se1-y' nanowires involving: contacting at least portion of surface of substrate with chemical vapor to form at least one Znx'Cd1-x'Sy'Se1-y' nanowire over portion of surface of substrate, where chemical vapor is generated by heating ZnS to first temperature and CdSe to second temperature that is less than first temperature.    DESCRIPTION OF DRAWING(S) - The figure shows the semiconductor structure having II-VI alloy layer with laterally varying elemental composition of the layer, on substrate surface.    Substrate (100)    Laterally varying II-VI alloy layer (101)    First position having first II-VI compound or alloy (A)    Second position having second II-VI compound or alloy (B, C, D)