Abstract:
A semiconductor nanowire is coated with a chemical coating layer that selectively attaches to the semiconductor material and which forms a dye in a chemical reaction. The dye layer comprises a material that absorbs electromagnetic radiation. A portion of the absorbed energy induces electronic excitation in the chemical coating layer from which additional free charge carriers are temporarily donated into the semiconductor nanowire. Thus, the conductivity of the semiconductor nanowire increases upon illumination on the dye layer. The semiconductor nanowire, and the resulting dye layer collective operate as a detector for electromagnetic radiation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor devices, and particularly to a semiconductor nanowire electromagnetic radiation sensor that detects electromagnetic radiation through electrical charges induced in a semiconductor wire, methods of manufacturing the same, and methods of operating the same. 
     BACKGROUND OF THE INVENTION 
     A semiconductor nanowire refers to a semiconductor wire having transverse lateral and vertical dimensions of the order of a nanometer (10 −9  meter) or tens of nanometers. Typically, the transverse lateral dimension and the vertical dimension are less than 20 nm. 
     The limitation on the lateral dimension applies to the transverse lateral dimension (the width) and the vertical lateral dimension (the height). The longitudinal lateral dimension (the length) of the semiconductor nanowire is unlimited, and may be, for example, from 1 nm to 1 mm. When the lateral dimensions of the semiconductor nanowire is less than tens of nanometers, quantum mechanical effects become important. As such, semiconductor nanowires are also called semiconductor quantum wires. 
     The transverse lateral dimension of a semiconductor nanowire is currently sublithographic, i.e., may not be printed by a direct image transfer from a photoresist that is patterned by a single exposure. As of 2008, the critical dimension, i.e., the smallest printable dimension that may be printed by lithographic methods, is about 35 nm. Dimensions less than the critical dimension are called sublithographic dimensions. At any given time, the critical dimension and the range of the sublithographic dimension are defined by the best available lithographic tool in the semiconductor industry. In general, the critical dimension and the range of the sublithographic dimension decreases in each successive technology node and established by a manufacturing standard accepted across the semiconductor industry. 
     The charge transport property of a semiconductor nanowire is controlled by the charge carriers present in the semiconductor nanowire. A higher density of free charge carriers in the semiconductor nanowire increases the conductivity of the semiconductor nanowire, while a low density of free charge carriers in the semiconductor nanowire decreases the conductivity of the semiconductor nanowire. 
     SUMMARY OF THE INVENTION 
     A semiconductor nanowire is coated with a chemical coating (functionalizing) layer that modulates the quantity of free charge carriers within the semiconductor nanowire. The chemical coating layer comprises a functional material that modulates the quantity of free charge carriers within the semiconductor nanowire. A metal compound reacts with the functionalizing layer and forms a dye. The dye layer comprises a material that absorbs electromagnetic radiation. A portion of the absorbed energy induces electronic excitation in the chemical coating layer from which additional free charge carriers are temporarily donated into the semiconductor nanowire. Thus, the conductivity of the semiconductor nanowire changes upon illumination on the dye layer. The semiconductor nanowire and the resulting dye layer collective operate as a detector for electromagnetic radiation. 
     According to an aspect of the present invention, a semiconductor device is provided, which includes: a semiconductor nanowire located on a substrate and comprising a semiconductor material; and a dye layer including a chemical coating layer and a metallic element containing layer, wherein the dye layer absorbs electromagnetic radiation, and wherein energy from the absorbed electromagnetic radiation induces electronic excitation in the dye layer and alters free charge carrier density in the semiconductor nanowire, wherein the chemical coating layer is located on the semiconductor nanowire and comprises a functional material that selectively attaches to the semiconductor material and react with the metallic element containing layer to form a dye material. 
     According to another aspect of the present invention, an electromagnetic radiation detector is provided, which includes: a semiconductor nanowire located on a substrate and comprising a semiconductor material; and a dye layer including a chemical coating layer and a metallic element containing layer, wherein the dye layer absorbs electromagnetic radiation, and wherein presence or intensity of electromagnetic radiation is detected by measuring conductivity of the semiconductor nanowire. 
     According to yet another as aspect of the present invention, a method of forming an electromagnetic radiation detector is provided, which includes: forming a semiconductor nanowire on a substrate and comprising a semiconductor material; forming a chemical coating layer on the semiconductor nanowire and comprising a functional material that selectively attaches to the semiconductor nanowire; and forming a metallic element containing layer on the chemical coating layer, wherein the metallic element containing layer and the chemical coating layer collectively constitute a dye layer that absorbs electromagnetic radiation, and wherein energy from the absorbed electromagnetic radiation induces electronic excitation in the functional material. 
     According to still another aspect of the present invention, a method of operating an electromagnetic radiation detector is provided, which includes: providing an electromagnetic radiation detector comprising a semiconductor nanowire located on a substrate and comprising a semiconductor material and a dye layer that includes a chemical coating layer and a metallic element containing layer, wherein conductivity of the semiconductor nanowire changes upon exposure of the dye layer to radiation; exposing the dye layer to ambient potentially containing electromagnetic radiation; and measuring conductivity of the semiconductor nanowire while the dye layer is exposed to the electromagnetic radiation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  are various views of an exemplary semiconductor device according to the present invention after patterning of a semiconductor nanowire and first and second semiconductor pads.  FIG. 1A  is a top-down view,  FIG. 1B  is a vertical cross-sectional view along the plane B-B′ in  FIG. 1A , and  FIG. 1C  is a vertical cross-sectional view along the plane C-C′ in  FIG. 1A . 
         FIGS. 2A-2C  are various views of an exemplary semiconductor device according to the present invention after undercutting a buried insulator material from underneath the semiconductor nanowire.  FIG. 2A  is a top-down view,  FIG. 2B  is a vertical cross-sectional view along the plane B-B′ in  FIG. 2A , and  FIG. 2C  is a vertical cross-sectional view along the plane C-C′ in  FIG. 2A . 
         FIGS. 3A-3C  are various views of an exemplary semiconductor device according to the present invention after formation of contact vias.  FIG. 3A  is a top-down view,  FIG. 3B  is a vertical cross-sectional view along the plane B-B′ in  FIG. 3A , and  FIG. 3C  is a vertical cross-sectional view along the plane C-C′ in  FIG. 3A . 
         FIGS. 4A-4C  are various views of an exemplary semiconductor device according to the present invention after formation of a chemical coating and a dye layer.  FIG. 4A  is a top-down view,  FIG. 4B  is a vertical cross-sectional view along the plane B-B′ in  FIG. 4A , and  FIG. 4C  is a vertical cross-sectional view along the plane C-C′ in  FIG. 4A . 
         FIG. 5  is a schematic view showing atomic arrangement of semiconductor-bound functionalizing molecules of a functional material in the chemical coating layer. 
         FIG. 6  is a graph of current through the semiconductor nanowire under electrical bias after formation of a chemical coating layer and exposure to light, and after formation of a dye layer and exposure to light at various intensities. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As stated above, the present invention relates to a semiconductor nanowire electromagnetic radiation sensor that detects chemicals through electrical charges induced in a semiconductor wire, methods of manufacturing the same, and methods of operating the same, which are now described in detail with accompanying figures. It is noted that like and corresponding elements are referred to by like reference numerals. 
     Referring to  FIGS. 1A-1C , an exemplary semiconductor structure according to the present invention includes a substrate  8  which contains a handle substrate  6  and an insulator layer  10 . A semiconductor nanowire  40  comprising a semiconductor material is formed over the substrate  8 . A first semiconductor pad  20  and a second semiconductor pad  30  that comprise a semiconductor material are formed on the substrate  8  such that the first and second semiconductor pads ( 20 ,  30 ) laterally abut a first end portion of the semiconductor nanowire  40  and a second end portion of the semiconductor nanowire  40 , respectively. 
     The semiconductor nanowire  40  and the first and second semiconductor pads ( 20 ,  30 ) may be formed from a semiconductor layer by methods known in the art. For example, a semiconductor-on-insulator (SOI) layer comprising a handle substrate  6 , a buried insulator layer, and a top semiconductor layer may be provided for the purposes of the present invention. The top semiconductor layer may be patterned to form the semiconductor nanowire and the first and second semiconductor pads ( 20 ,  30 ). As the top surface of the buried insulator layer becomes exposed after patterning of the top semiconductor layer, the buried insulator layer becomes the insulator layer  10 . 
     The top semiconductor layer comprises a semiconductor material, which may be selected from, but is not limited to silicon, germanium, silicon-germanium alloy, silicon carbon alloy, silicon-germanium-carbon alloy, gallium arsenide, indium arsenide, indium phosphide, III-V compound semiconductor materials, II-VI compound semiconductor materials, organic semiconductor materials, and other compound semiconductor materials. In one embodiment, the top semiconductor layer may include a Si-containing semiconductor material such as single crystalline silicon or a single crystalline silicon-germanium alloy. The semiconductor nanowire  40  and the first and second semiconductor pads ( 20 ,  30 ) have the same composition as the top semiconductor layer. If the semiconductor layer comprises a single crystalline semiconductor material, the entirety of the semiconductor nanowire  40  and the first and second semiconductor pads ( 20 ,  30 ) is a single crystalline semiconductor material having an epitaxial alignment throughout. The first and second semiconductor pads ( 20 ,  30 ) have a thickness greater than the vertical dimension of the semiconductor nanowire  40 . 
     The semiconductor nanowire  40  has a free charge carrier density determined by the free charge carrier concentration. In an unperturbed state, the free charge carrier concentration is determined by the dopant concentration in the semiconductor nanowire  40  and the surrounding pads  20  and  30 . 
     In a preferred embodiment, the semiconductor nanowire  40  comprises a substantially intrinsic semiconductor material and the free charge carrier density is the sum of the density of electrons and the density of holes in the substantially intrinsic semiconductor material of the semiconductor nanowire  40 . In this case, the dopant concentration in the semiconductor nanowire  40  is less than 1.0×10 15 /cm 3 , and is typically less than 1.0×10 14 /cm 3 . The first and second semiconductor pads ( 20 ,  30 ) may be doped with n-type dopants or p-type dopants to provide a conductivity that is orders of magnitude higher than the conductivity of the semiconductor nanowire  40 . If the semiconductor nanowire  40  is substantially intrinsic and has a low conductivity, i.e. high resistivity, the type of doping for the first and second semiconductor pads ( 20 ,  30 ) does not matter because the electrical characteristic of the conduction path including the first semiconductor pad  20 , the semiconductor nanowire  40 , and the second semiconductor nanowire  30  is resistive due to the high resistance of the semiconductor nanowire  40 . Typically, the dopant concentration of the first and second semiconductor pads ( 20 ,  30 ) is from 1.0×10 19 /cm 3  to 1.0×10 21 /cm 3  in this case, although lesser and greater dopant concentrations are also contemplated herein. 
     In a first alternate embodiment, the semiconductor nanowire  40  comprises a p-doped semiconductor material and the free charge carrier density is substantially the same as the density of holes in the p-doped semiconductor material of the semiconductor nanowire  40 . This is because the minority charge carrier concentration, i.e., the density of electrons, is many orders of magnitude smaller than the density of holes in this case. In other words, the density of holes in the p-doped semiconductor material is substantially the same as the density of p-type dopant atoms in the p-doped semiconductor material of the semiconductor nanowire  40 . Typically, the dopant concentration in the semiconductor nanowire  40  is from 1.0×10 15 /cm 3  to 1.0×10 19 /cm 3 , although lesser and greater dopant concentrations are also contemplated herein. The first and second semiconductor pads ( 20 ,  30 ) are doped with p-type or n-type dopants. Typically, the dopant concentration of the first and second semiconductor pads ( 20 ,  30 ) is from 1.0×10 19 /cm 3  to 1.0×10 21 /cm 3  in this case, although lesser and greater dopant concentrations are also contemplated herein. 
     In a second alternate embodiment, the semiconductor nanowire  40  comprises an n-doped semiconductor material and the free charge carrier density is substantially the same as the density of electrons in the n-doped semiconductor material of the semiconductor nanowire  40 . This is because the minority charge carrier concentration, i.e., the density of holes, is many orders of magnitude smaller than the density of electrons in this case. In other words, the density of electrons in the n-doped semiconductor material is substantially the same as the density of n-type dopant atoms in the n-doped semiconductor material of the semiconductor nanowire  40 . Typically, the dopant concentration in the semiconductor nanowire  40  is from 1.0×10 15 /cm 3  to 1.0×10 19 /cm 3 , although lesser and greater dopant concentrations are also contemplated herein. Preferably, the first and second semiconductor pads ( 20 ,  30 ) are doped with n-type or p-type. Typically, the dopant concentration of the first and second semiconductor pads ( 20 ,  30 ) is from 1.0×10 19 /cm 3  to 1.0×10 21 /cm 3  in this case, although lesser and greater dopant concentrations are also contemplated herein. 
     The insulator layer  10  is a dielectric material layer, i.e., a layer including a dielectric material. The dielectric material of the insulator layer  10  may be, for example, silicon oxide, silicon nitride, silicon oxynitride, quartz, a ceramic material, or a combination thereof. The thickness of the insulator layer  10  may be from 50 nm to 1,000 nm, although lesser and greater thicknesses are also contemplated herein. The handle substrate  6  may comprise a semiconductor material, an insulator material, or a conductive material. In some cases, the handle substrate  6  and the insulator layer  10  may comprise the same dielectric material and may be of unitary and integral construction. 
     The semiconductor nanowire  40  and the first and second semiconductor pads ( 20 ,  30 ) may be formed, for example, by lithographic patterning employing photolithography and an anisotropic etch. For example, shapes corresponding to the semiconductor nanowire  40  and the first and second semiconductor pads ( 20 ,  30 ) may be patterned in a photoresist (not shown). The pattern in the photoresist is subsequently transferred by an anisotropic etch into the top semiconductor layer to form the semiconductor nanowire  40  and the first and second semiconductor pads ( 20 ,  30 ). Optionally, the edges of the semiconductor nanowire  40  may be rounded during the anisotropic etch. 
     The cross-sectional area of the semiconductor nanowire  40  in the C-C′ plane may be rectangular, circular, elliptical, or a shape that may be formed by at least one curvilinear shape and/or at least one polygonal shape. Preferably, the cross-sectional area of the semiconductor nanowire  40  is substantially circular, and has a dimension less than 20 nm. 
     Referring to  FIGS. 2A-2C , a substantially isotropic etch is performed on the dielectric material of the insulator layer  10  selective to the semiconductor material of the semiconductor nanowire  40  and the first and second semiconductor pads ( 20 ,  30 ). The semiconductor nanowire  40  and the first and second semiconductor pads ( 20 ,  30 ) are employed as an etch mask for the substantially isotropic etch. The substantially isotropic etch may be a wet etch or a dry etch. Because the etch is substantially isotropic, the semiconductor nanowire  40  and the edges of the first and second semiconductor pads ( 20 ,  30 ) are undercut as the etch progresses. The etch proceeds at least until the portion of the insulator layer  10  located directly underneath semiconductor nanowire  40  is removed. The semiconductor nanowire becomes suspended over the substrate  8  and does not directly contact the substrate  8 . In other words, the semiconductor nanowire  40  does not have direct physical contact with the remaining portions of the insulator layer  10 . 
     The etch also removes the dielectric material of the insulator layer  10  from underneath the peripheral portions of the first semiconductor pad  20  and the second semiconductor pad  30 . A first insulator pedestal  12  comprising a remaining portion of the insulator layer  10  is formed directly underneath a center portion of the first semiconductor pad  20 . Likewise, a second insulator pedestal  13  comprising another remaining portion of the insulator layer  10  is formed directly underneath a center portion of the second semiconductor pad  30 . The first insulator pedestal  12  and the second insulator pedestal  13  are of integral construction with the insulator layer  10  and have the same composition as the insulator layer  10 . 
     The semiconductor nanowire  40  may be thinned to reduce the lateral width to a sublithographic dimension, i.e., a dimension that may not be printed directly by lithographic techniques. In case the width of the semiconductor nanowire  40  is a sublithographic dimension, which may be, for example, from 1 nm to 20 nm. 
     If additional thinning is employed to provide a sublithographic dimension for the lateral width of the semiconductor nanowire  40 , the additional thinning may employ thermal conversion of the surface of the semiconductor nanowire  40  into a dielectric material such as a semiconductor oxide, a semiconductor nitride, or a semiconductor oxynitride, followed by removal of the dielectric material. For example, if the semiconductor nanowire  40  includes silicon, the semiconductor oxide material may be silicon oxide, which may be removed by hydrofluoric acid (HF). Alternately or in conjunction, a substantially isotropic etch may be employed to thin the semiconductor nanowire  40  to a sublithographic dimension. 
     Referring to  FIGS. 3A-3C , a first contact structure  28 , a second contact structure  38 , a first lead wire  29 , and a second lead wire  39  are formed by methods known in the art. The first contact structure  28  is formed directly on the first semiconductor pad  20 . The second contact structure  38  is formed directly on the second semiconductor pad  30 . The first lead wire  29  is formed directly on the first semiconductor pad  20 . The second lead wire  39  is formed directly on the second semiconductor pad  30 . The first contact structure  28 , the second contact structure  38 , the first lead wire  29 , and the second lead wire  39  collectively enable the measurement of the resistance of the conductive path including the first semiconductor pad  20 , the semiconductor nanowire  40 , and the second semiconductor pad  30 . For example, the first contact structure  28  and the second contract structure  38  may be solder balls, which typically have a lateral dimension from 40 microns to 200 microns. Alternately, the first contact structure  28  and the second contract structure  38  may be a metal-semiconductor alloy pad or a metallic pad having a lateral dimension on the order of 100 microns to enable contact with a test probe. The first lead wire  29  and the second lead wire  39  may be replaced with electrical probes. 
     Referring to  FIGS. 4A-4C , a chemical coating layer  50  and a metallic element containing layer  60  are formed on the exposed surfaces of the semiconductor nanowire  40 . 
     The chemical coating layer  50  is formed directly on the semiconductor nanowire  40 . The chemical coating layer  50  may be formed, for example, by dipping the exemplary semiconductor structure in a solution containing the chemical for the chemical coating layer  50 . Alternately, the chemical coating layer  50  may be formed by vapor deposition or by a spray. The chemical coating layer  50  includes a functional material that selectively binds to the semiconductor nanowire  40 . 
     The metallic element containing layer  60  is formed directly on the chemical coating layer  50 . The metallic element containing layer  60  may be formed, for example, by dipping the exemplary semiconductor structure with a chemical coating in a solution containing a metallic element containing solution. A molecule including a metal element of the metallic element containing solution react with a molecule of the chemical coating layer to form a dye material, which may be a metal organic dye material. The chemical coating layer  50  and the metallic element containing layer  60  are combined to form a dye layer  70 . The dye material is formed by complexation of first functional group  150 A of the polar molecules in the chemical coating layer with a transition metals of the metallic element containing layer  60 . Alternately, the dye layer  70  may be formed by vapor deposition or by a spray with the chemical for the metallic element containing layer  60 . The dye layer  70  surrounds and encircles the semiconductor nanowire  40 . 
     The chemical coating layer  50  may be formed on the entirety of the semiconductor nanowire  40 . Preferably, the chemical property of the chemical coating layer  50  is such that the chemical coating layer  50  will not be formed on the exposed surfaces of the insulator layer  20 . The chemical coating layer  50 , the first semiconductor pad  20 , and the second semiconductor pad  30  encapsulate the semiconductor nanowire  40 . 
     For example, the functional material of the chemical coating layer  50  may include a semiconductor-bound functionalizing molecule that is self-aligned in the chemical coating layer  50 . A semiconductor-bound functionalizing molecule, as defined in the present invention, is a molecule that selectively binds to only to the semiconductor material that the nanowire is made from (such as hydrogen-terminated silicon). It may have a non-zero electrical dipole moment. In other words, the center of positive charge distribution weighted by the probability of the presence of the positive charge is not the same as the center of negative charge distribution weighted by the probability of the presence of the negative charge in such molecule. 
       FIG. 5  illustrates molecular alignment of the functionalizing molecules  50 A within the chemical coating layer  50 . In this illustrative example, the chemical coating layer  50  is a monolayer of the functionalizing molecules  50 A. The functional material is the molecules  50 A. 
     In this illustrative example, each functionalizing molecule  50 A includes a first functional group  150 A and a second functional group  250 A. A first functional group  150 A in each functionalizing molecule  50 A is exposed on an exterior surface of the chemical coating layer  50 . In this case, each functionalizing molecule  50 A may be self-aligned on the surface of the semiconductor nanowire  40  so that the first functional group  150 A is on an exterior surface of the chemical coating layer  50 . Non-limiting examples of the first functional group  150 A include dipyridyl, terpyrydine, and porphyrine. The second functional group  250 A bonds to the surface of the semiconductor nanowire  40 . Non-limiting examples of the second functional group  250 A include diazonium salt, mercapto, phenol, alcohol, alkene, and alkyne. 
     In general, the second functional group  250 A is attached to the semiconductor nanowire  40 . If the second functional group  250 A is directly bonded to the surface of the semiconductor nanowire  40  and the first functional group  150 A is located on the exterior surface of the chemical coating layer  50 , the direction along the first functional group  150 A and the second functional group  250 A in the functionalizing molecule  50 A may be a radial direction from a coaxial center line of the semiconductor nanowire  40 . The coaxial center line runs along the direction connecting the first semiconductor pad  20  and the second semiconductor pad  30 . If the semiconductor nanowire  40  has a circular cross-sectional area, the radial directions converge on the coaxial center line of the semiconductor nanowire  40 . 
     The chemical coating layer  50  may comprises at least one functional material such as porphyrine, dipyridine, terpyridine. Such functional materials for the chemical coating layer  50  are self-aligned on the surface of the semiconductor nanowire such that the dipole moment of the functionalizing molecules are aligned perpendicular to the nearest surface of the semiconductor nanowire  40 . These molecules also form a self-assembled monolayer on the semiconductor surface. 
     The dye layer  70  absorbs electromagnetic radiation. The electromagnetic radiation may be gamma ray, X-ray, ultraviolet radiation, radiation in visible spectrum, or infrared radiation. Preferably, the electromagnetic radiation is ultraviolet radiation, radiation in visible spectrum, or infrared radiation. More preferably, the electromagnetic radiation is ultraviolet radiation or radiation in visible spectrum. Upon absorption of the electromagnetic radiation by the dye layer  70 , the energy from the absorbed electromagnetic radiation induces electronic excitation in the functional material of the dye layer  70 . 
     The dye layer  70  may comprise a metal organic dye material. For example, the dye layer  70  may comprise a metal-organic dye material including at least one of a transition metal, Cu, Ru, ft, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi, and Po. As defined herein, transition metals refer to elements in Group 1B, 2B, 3B, 4B, 5B, 6B, 7B, and 8B of the Periodic Table of Elements including Lanthanides and Actinides. 
     In one example, the dye layer  70  includes a metal organic die containing iridium and has a peak absorption wavelength around 380 nm. In another example, the dye layer  70  includes a metal organic die containing zinc and has a peak absorption wavelength between 320 nm and 350 nm. In yet another example, the dye layer  70  includes a metal organic die containing copper and has a peak absorption wavelength around 300 nm. 
     The exemplary semiconductor device of the present invention operates as an electromagnetic radiation detector in which the presence or intensity of electromagnetic radiation is detected by measuring conductivity of the semiconductor nanowire  40 . The conductivity of the semiconductor nanowire  40  increases upon exposure of the dye layer  70  to electromagnetic radiation of the wavelength that may be absorbed by the dye layer  70 . 
     To determine whether an ambient contains electromagnetic radiation within the wavelength range of the absorption band of the material of the dye layer  70 , the dye layer  70  is exposed to an ambient potentially containing such electromagnetic radiation. The conductivity of the semiconductor nanowire  40  is measured while the dye layer  70  is exposed to the electromagnetic radiation. If the conductivity of the semiconductor nanowire  60  increases upon exposure to the testing ambient, the testing ambient has a detectable level of electromagnetic radiation within the wavelength range of the absorption band of the material of the dye layer  70 . If the conductivity of the semiconductor nanowire  60  does not change upon exposure to the testing ambient, the testing ambient does not have a detectable level of electromagnetic radiation within the wavelength range of the absorption band of the material of the dye layer  70 . 
     In case the electromagnetic radiation detector detects presence of electromagnetic radiation, the intensity of the electromagnetic radiation may be determined based on the degree of the increase in the conductivity of the semiconductor nanowire  40 . 
     Preferably, the semiconductor nanowire has a dopant concentration less than 1.0×10 15 /cm 3  so that the background level conductivity of the semiconductor nanowire in the absence of electromagnetic radiation may be minimized and the signal-to-noise ratio of the electromagnetic radiation detector may be increased. 
     The conductivity of the semiconductor nanowire  40  may be measured by measuring the resistance of the semiconductor nanowire  40 . For example, the resistance of the semiconductor nanowire  40  may be measured by applying an electrical bias between the first and second semiconductor pads ( 20 ,  30 ) and measuring current through the first semiconductor pad  20 , the semiconductor nanowire  40 , and the second semiconductor pad  30 . The first contact structure  28 , the second contact structure  38 , the first lead wire  29 , and the second lead wire  39  may be employed as the path of electrical current for measurement of the resistance of the semiconductor nanowire  40 . 
       FIG. 6  is a graph illustrating the operation of the semiconductor nanowire device of the present invention. A first voltage-current curve  610  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C  without the metallic element containing layer  60 , i.e., the semiconductor nanowire device of the present invention including the chemical coating layer  50  but not including the metallic element containing layer  60 . In this case, the chemical coating layer  50  includes a monolayer of terpyridine formed by dipping the semiconductor nanowire device of  FIGS. 3A-3C  into a solution of terpyridine. Alternately, the chemical coating layer  50  of terpyridine may be formed by any other method of applying a thin coating of terpyridine. The semiconductor nanowire  40  includes substantially intrinsic single crystalline silicon having a dopant concentration less than 1.0×10 15 /cm 3 . 
     A second voltage-current curve  620  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C  measured in the absence of any electromagnetic radiation. The semiconductor nanowire  40  is surrounded by the dye layer  70 , which contains the dye layer  70  including the chemical coating layer  50  and the metallic element containing layer  60 . The semiconductor nanowire  40  includes substantially intrinsic single crystalline silicon having a dopant concentration less than 1.0×10 15 /cm 3 . The dye layer  70  includes a metal organic dye including ruthenium. The presence of the dye layer  70  shifts the baseline response curve for the electrical current under electrical bias relative to the first voltage-current curve  610 , which is a curve in the absence of the dye layer  70 . 
     A third voltage-current curve  630  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C  while the dye layer  70  is exposed to a low level of electromagnetic radiation. The semiconductor nanowire  40  includes substantially intrinsic single crystalline silicon having a dopant concentration less than 1.0×10 15 /cm 3 . The dye layer  70  includes the same metal organic dye that includes ruthenium. The low level of radiation shifts the response curve for the electrical current under electrical bias relative to the second voltage-current curve  620 . Specifically, the additional free charge carriers are introduced from the dye layer  70  as the electromagnetic radiation is absorbed in the dye layer  70 . Thus, more current flows through the semiconductor nanowire  40  due to the additional free charge carriers in the semiconductor nanowire  40 , and the third voltage-current curve  630  shows higher level of electrical current relative to the second voltage-current curve  620 . 
     A fourth voltage-current curve  640  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C  while the dye layer  70  is exposed to a high level of electromagnetic radiation. The semiconductor nanowire  40  includes substantially intrinsic single crystalline silicon having a dopant concentration less than 1.0×10 15 /cm 3 . The dye layer  70  includes the same metal organic dye that includes ruthenium. The high level of radiation further shifts the response curve for the electrical current under electrical bias relative to the third voltage-current curve  630 . The number of additional free charge carriers that are introduced from the dye layer  70  into the semiconductor nanowire  40  increases as more electromagnetic radiation is absorbed in the dye layer. Thus, even more current flows through the semiconductor nanowire  40  due to the additional free charge carriers in the semiconductor nanowire  40  relative to the case of the low level of electromagnetic radiation, and the fourth voltage-current curve  640  shows higher level of electrical current relative to the third voltage-current curve  630 . 
     Thus, the electromagnetic radiation detector of the present invention may detect not only presence or absence of electromagnetic radiation within the absorption wavelength range of the dye layer  70  but also the intensity of the electromagnetic radiation in a testing ambient. 
     While the present invention is described employing an embodiment in which the chemical coating layer  50  and the metallic element containing layer  60  are formed after the formation of the first and second contact structures ( 28 ,  38 ), embodiments in which the chemical coating layer  50  and the metallic element containing layer  60  are formed prior to the formation of the first and second contact structures ( 28 ,  38 ) are explicitly contemplated herein. Further, instead of employing a single semiconductor nanowire  40 , the present invention may be practiced with a plurality of semiconductor nanowires  40  that are connected in parallel to increase signal strength as a detector. Such variations are explicitly contemplated herein. 
     While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.