Patent Abstract:
A semiconductor nanowire is coated with a chemical coating layer that comprises a functional material which modulates the quantity of free charge carriers within the semiconductor nanowire. The outer surface of the chemical coating layer includes a chemical group that facilitates bonding with molecules to be detected through electrostatic forces. The bonding between the chemical coating layer and the molecules alters the electrical charge distribution in the chemical coating layer, which alters the amount of the free charge carriers and the conductivity in the semiconductor nanowire. The coated semiconductor nanowire may be employed as a chemical sensor for the type of chemicals that bonds with the functional material in the chemical coating layer. Detection of such chemicals may indicate pH of a solution, a vapor pressure of a reactive material in gas phase, and/or a concentration of a molecule in a solution.

Full Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor devices, and particularly to a semiconductor nanowire charge sensor that detects chemicals 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 but can be as large as 100 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. (The preferred length is less than 1 um) When the lateral dimensions of the semiconductor nanowire is less than tens of nanometers, quantum mechanical effects may 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 layer that modulates the quantity of free charge carriers within the semiconductor nanowire. The chemical coating layer comprises a functional material that affects the amount of free charge carriers in the semiconductor substrate. The inner surface of the chemical coating layer includes a chemical group that facilitates bonding with molecules to be detected through electrostatic forces. The bonding between the chemical coating layer and the molecules alters the electrical charge distribution in the chemical coating layer, which alters the amount of the free charge carriers and the conductivity in the semiconductor nanowire. The coated semiconductor nanowire may be employed as a chemical sensor for a type of chemicals that bonds with the functional material in the chemical coating layer. Detection of such chemicals may indicate pH of a solution, a vapor pressure of a reactive material in gas phase, and/or a concentration of a molecule in a solution. 
     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 chemical coating layer located on the semiconductor nanowire and comprising a functional material that bonds with at least one type of charged molecule, wherein free charge carrier density in the semiconductor nanowire is altered upon bonding of the at least one type of charged molecule and the functional material. 
     According to another aspect of the present invention, a chemical detector is provided, which includes: a semiconductor nanowire located on a substrate and comprising a semiconductor material; and a chemical coating layer located on the semiconductor nanowire and comprising a functional material that bonds with at least one type of chemical molecule, wherein free charge carrier density in the semiconductor nanowire is altered upon bonding of the at least one type of chemical molecule and the functional material, and wherein presence or concentration of the at least one type of chemical molecule is detected by measuring conductivity of the semiconductor nanowire. 
     According to yet another aspect of the present invention, a method of forming a chemical detector is provided, which includes: forming a semiconductor nanowire comprising a semiconductor material on a substrate; and forming a chemical coating layer on the semiconductor nanowire, wherein the chemical coating layer comprises a functional material that bonds with at least one type of chemical molecule, wherein free charge carrier density in the semiconductor nanowire is altered upon bonding of the at least one type of chemical molecule and the functional material. 
     According to still another aspect of the present invention, a method of operating a chemical detector is provided, which includes: providing a chemical detector including a semiconductor nanowire located on a substrate and comprising a semiconductor material and a chemical coating layer located on the semiconductor nanowire and comprising a functional material that bonds with at least one type of chemical molecule; placing the chemical detector in a fluid ambient containing the at least one type of chemical molecule, wherein free charge carrier density in the semiconductor nanowire is altered upon bonding of the at least one type of chemical molecule and the functional material; and measuring a resistance of the semiconductor nanowire. 
    
    
     
       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 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 magnified vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ prior to formation of a chemical coating layer. 
         FIG. 6  is a magnified vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ after formation of the chemical coating layer. 
         FIG. 7  is a schematic view showing atomic arrangement of semiconductor-bound functionalizing molecules of a functional material in the chemical coating layer. 
         FIG. 8  is a schematic view showing atomic arrangement after charged molecules are bound to the semiconductor-functionalizing molecules of the chemical coating layer. 
         FIG. 9  is a graph illustrating the operation of the semiconductor nanowire device of the present invention. Electrical current through the semiconductor nanowire is shown under electrical bias after various types of surface treatment upon the semiconductor nanowire or the surface coating layer. 
         FIG. 10  is a graph of current through the semiconductor nanowire under electrical bias for bare silicon, after application of a chemical coating layer, and after exposure to a fructose solution. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As stated above, the present invention relates to a semiconductor nanowire charge 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 one case, 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 ) can be heavily 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 another case, 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. The first and second semiconductor pads ( 20 ,  30 ) are doped with n-type or p-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 yet another case, 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 ) is not essential 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. 
     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 further thinning of the wire (to get sub-lithographic dimensions). 
     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 utilizing 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  is formed on the exposed surfaces of 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  is formed directly on the semiconductor nanowire  40  and includes a functional material that bonds with at least one type of charged molecule. The charged molecule may be positively charged or negatively charged. Upon bonding of the at least one type of charged molecule and the functional material in the chemical coating layer, the free charge carrier density in the semiconductor nanowire  50  is altered. 
     The chemical coating layer  50  will be formed on the entirety of the semiconductor nanowire  40 . 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. Exemplary molecules include, but are not limited to, pyridine, dipyridine, or terpyridine, a boronic acid. In this case, a functional group in the molecule that will attach to the charged species of interest is exposed on an exterior surface of the chemical coating layer  50 . The functional group bonds with the at least one type of charged molecule. 
     This type of chemical reaction is illustrated in  FIGS. 5-8 , which schematically shows molecular alignment of the functionalizing molecules  50 A and charged molecules  60 .  FIG. 5  shows a magnified vertical cross-sectional view of the semiconductor nanowire  40  along the plane C-C′ at a processing step corresponding to  FIGS. 3A-3C , which is prior to formation of the chemical coating layer  50 .  FIG. 6  shows a magnified vertical cross-sectional view of the exemplary semiconductor structure along the plane C-C′ at a processing step corresponding to  FIGS. 4A-4C , which is after formation of the chemical coating layer  50 . 
       FIG. 7  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 semiconductor-bound 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. 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 pyridine, dipyridyl, terpyridine, porphyrine, organic acids like boronic acid, phosphonic acid, sulfonic acid, unsaturated imides such as maleimide, and fluorinated alcohols such as hexafluoroisopropanol. The second functional group  250 A bonds to the surface of the semiconductor nanowire  40 . Non-limiting examples of the second functional group  150 A include diazonium salts, thiols, alcohols, alkenes, and alkynes. 
     In general, the second functional group  250 A is located in closer proximity to the semiconductor nanowire  40  than the first functional group  150 A in each functionalizing molecule  50 A. If the first functional group  250 A is directly bonded to the surface of the semiconductor nanowire  40  and the second 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  but need not be. 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 . 
       FIG. 8  illustrates the atomic arrangement after molecules  60  are bonded to the functionalizing molecules  50 A of the chemical coating layer  50 . Each charged molecule  50  is bonded to the first functional group  150 A of a functionalizing molecule  50 A. Upon bonding of the at least one type of chemical molecule  60  and the functional material of the chemical coating layer  50 , the electronic charge distribution within the molecule  50 A is altered. For example, the electric dielectric moment of the functionalizing molecule  50 A, which is aligned in the direction perpendicular to the local surface of the semiconductor nanowire  40 , may increase or decrease. Such increase or decrease in the electric dipole moment of the molecule  50 A attracts or repels the charge carriers within the semiconductor nanowire  40 . Because the lateral dimensions of the semiconductor nanowire  40  is typically on the order of 1 nm to 20 nm, such attraction or repulsion of the electrical charges in the semiconductor nanowire alters the free charge carrier density in the semiconductor nanowire  40 . Thus, the bonding of chemical molecules  60  with the first functional group  150 A of the molecule  50 A has a direct impact on the charge carrier density in the semiconductor nanowire  40 , and consequently on the conductivity of the semiconductor nanowire  40 . By measuring the conductivity of the semiconductor nanowire  40 , presence or concentration of the at least one type of chemical molecule is detected. 
     According to a first embodiment of the present invention, the exemplary semiconductor device of  FIGS. 4A-4C  is a chemical detector that detects the presence or concentration of at least one type of gas phase chemical molecule. The chemical coating layer  50  may comprise at least one functional material such as pyridine, dipyridine, or 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 . The functional material bonds with at least one type of chemical molecule and the free charge carrier density in the semiconductor nanowire  40  is altered upon bonding of the at least one type of chemical molecule and the functional material. For example, the chemical molecule may be trinitrotoluene (TNT) in gas phase, which is present around a solid phase explosive or a liquid phase explosive including trinitrotoluene. The exemplary semiconductor device may thus be employed to detect explosives in a non-invasive manner. 
       FIG. 9  is a graph illustrating the operation of the semiconductor nanowire device of the present invention. A first voltage-current curve  910  represents electrical current through the semiconductor nanowire  40  of  FIGS. 3A-3C , i.e., the semiconductor nanowire device of the present invention without the chemical coating layer  50 . The semiconductor material in this case is single crystalline silicon. To insure that no contaminants or native oxide layer is present on the surface of the semiconductor nanowire, the surface of the semiconductor nanowire  40  may be treated with a hydrofluoric acid (HF) solution. Under such conditions, the semiconductor nanowire device of  FIGS. 3A-3C  display the voltage-current characteristics represented by the first voltage-current curve  910 . 
     A second voltage-current curve  920  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C . 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. In this example, the semiconductor nanowire  40  includes a p-doped single crystalline silicon. The terpyridine layer bound to silicon has a net negative charge and attracts the holes in the semiconductor nanowire  40  so that the free charge carrier density in the semiconductor nanowire decreases. Thus, the electrical current through the semiconductor nanowire  40  decreases upon formation of the chemical coating layer  50  of the monolayer of terpyridine. 
     A third voltage-current curve  930  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C  after dipping in a copper sulfate (CuSO 4 ) solution. The chemical coating layer  50  includes a monolayer of terpyridine, which may be formed by dipping the semiconductor nanowire device of  FIGS. 3A-3C  into a solution of terpyridine. The semiconductor nanowire  40  includes a p-doped single crystalline silicon. After rinsing the surface of the semiconductor nanowire  40  with ammonia to remove any impurities, the surface of the chemical coating layer is dipped in the copper sulfate solution. Weakly bound hydrogen ions are formed on the surface of the chemical coating layer  50  after dipping. The weakly bound hydrogen ions constitute chemical molecules that are bonded to the monolayer of terpyridine. In general, such weak bonding may also be induced by treatment of the chemical coating layer  50  with an ionic salt solution. Each weakly bonded hydrogen ion provides a positive charge on the first functional group  150 A of a molecule  50 A (See  FIG. 8 ). The positive charge reduces the attraction of the holes from the p-doped silicon toward the second functional group  250 A of the functionalizing molecule  50 A (See  FIG. 8 ). Thus, more holes are available in the p-doped silicon, thereby increasing the conductivity of the semiconductor nanowire  40 . 
     A fourth voltage-current curve  940  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C  after dipping in a hydrogen chloride (HCl) solution. The chemical coating layer  50  includes a monolayer of terpyridine, which may be formed by dipping the semiconductor nanowire device of  FIGS. 3A-3C  into a solution of terpyridine. The semiconductor nanowire  40  includes a p-doped single crystalline silicon. A strongly bound hydrogen ions are formed on the surface of the chemical coating layer  50  after dipping in the hydrochloric acid solution. The strongly bound hydrogen ions constitute chemical molecules that are bonded to the monolayer of terpyridine. Each strongly bonded hydrogen ion provides a positive charge on the first functional group  150 A of a functionalizing molecule  50 A (See  FIG. 8 ). The positive charge reduces the attraction of the holes from the p-doped silicon toward the second functional group  250 A of the molecule  50 A (See  FIG. 8 ). Thus, more holes are available in the p-doped silicon, thereby increasing the conductivity of the semiconductor nanowire  40 . Since the strong bonding of the hydrogen atoms effectively provide more positive charge to the chemical coating layer  50 , the more holes are repelled into the semiconductor nanowire than the case of weakly bonded hydrogen atoms. Thus, the increase in the conductivity of the semiconductor nanowire  40  is greater than the case of the weakly bonded hydrogen atoms represented by the third voltage-current curve  930 . 
     According to a second embodiment of the present invention, the exemplary semiconductor device of  FIGS. 4A-4C  is a chemical detector that detects the presence or concentration of at least one type of liquid phase chemical molecule. The chemical coating layer  50  may comprise at least one functional material such as a boronic acid. In general, an acid molecule may be employed as the functionalizing molecule of the chemical coating layer  50 . As in the first embodiment, 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 . The functional material bonds with at least one type of chemical molecule and the free charge carrier density in the semiconductor nanowire  40  is altered upon bonding of the at least one type of chemical molecule and the functional material. For example, the chemical molecule may be saccharide (such as glucose or fructose) or a glycoprotein (such as a cancer marker) which is present in blood. The exemplary semiconductor device may thus be employed to detect the presence or quantity of glucose or fructose in blood or for cancer detection. 
       FIG. 10  is another graph of current through the semiconductor nanowire under electrical bias for bare silicon, after application of a chemical coating layer, and after exposure to a fructose solution. A first voltage-current curve  1010  represents electrical current through the semiconductor nanowire  40  of  FIGS. 3A-3C , i.e., the semiconductor nanowire device of the present invention without the chemical coating layer  50 . The semiconductor material in this case is single crystalline silicon. To insure that no contaminants or native oxide layer is present on the surface of the semiconductor nanowire, the surface of the semiconductor nanowire  40  may be treated with a hydrofluoric acid (HF) solution. Under such conditions, the semiconductor nanowire device of  FIGS. 3A-3C  display the voltage-current characteristics represented by the first voltage-current curve  1010 . 
     A second voltage-current curve  1020  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C . In this case, the chemical coating layer  50  includes a layer of a boronic acid, which may be formed by dipping the semiconductor nanowire device of  FIGS. 3A-3C  into a solution containing a boronic acid. Alternately, the chemical coating layer  50  of a boronic acid. In this example, the semiconductor nanowire  40  includes a intrinsic single crystalline silicon with heavily p-doped pads  20  and  30 . A boronic acid layer attracts the electrons in the semiconductor nanowire  40  so that the free charge carrier density in the semiconductor nanowire decreases. Thus, the electrical current through the semiconductor nanowire  40  decreases upon formation of the chemical coating layer  50  of the monolayer of boronic acid. 
     A third voltage-current curve  1030  represents electrical current through the semiconductor nanowire  40  of  FIGS. 4A-4C  after dipping in a fructose solution. The chemical coating layer  50  includes a layer of a boronic acid, which may be formed by dipping the semiconductor nanowire device of  FIGS. 3A-3C  into a solution containing a boronic acid. The semiconductor nanowire  40  includes a p-doped single crystalline silicon. A strongly bound fructose molecules are formed on the surface of the chemical coating layer  50  after dipping in a sample solution including fructose. In one case, the sample solution may be blood. The strongly bound fructose ions constitute chemical molecules that are bonded to the chemical coating layer  50 . Each fructose ion provides a negative charge on the first functional group  150 A of a molecule  50 A (See  FIG. 8 ). The negative charge reduces the attraction of the electrons from the effectively n-doped silicon toward the second functional group  250 A of the molecule  50 A (See  FIG. 8 ). Thus, more electrons are available in the n-doped silicon, thereby increasing the conductivity of the semiconductor nanowire  40 . The increase in the conductivity of the semiconductor nanowire  40  may be measured to determine the amount of fructose in the sample solution. 
     While the present invention is described employing an embodiment in which the chemical coating layer  50  is formed after the formation of the first and second contact structures ( 28 ,  38 ), embodiments in which the chemical coating layer  50  is 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.

Technology Classification (CPC): 6