Patent Publication Number: US-7910915-B2

Title: Nanowire devices and systems, light-emitting nanowires, and methods of precisely positioning nanoparticles

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a divisional application of application Ser. No. 11,413,375, filed Apr. 28, 2006 now U.S. Pat. No. 7,465,954, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates devices and systems that include nanowires. More particularly, the present invention relates to light-emitting nanowires, devices and systems including nanowires positioned within resonant cavities, and methods for precisely positioning nanoparticles within resonant cavities using nanowires. 
     BACKGROUND OF THE INVENTION 
     Nanotechnology is concerned with the fabrication and application of materials, structures, devices, and systems at the atomic and molecular level. Nanotechnology typically is concerned with structures and devices having elements or features that are less than about 100 nanometers in size. At these dimensions, such structures and devices often exhibit novel and significantly improved physical, chemical, and biological properties, phenomena, and processes due to their extremely small size. The behavior of such structures and devices may not be predictable based on the behavior exhibited by larger, but otherwise identical structures and devices. Nanotechnology is currently making significant contributions to the fields of computer storage, semiconductors, biotechnology, manufacturing and energy. 
     Nanowires are fundamental structures that are often used in nanoscale structures and devices. Nanowires are wire-like structures that typically have diameters of less than about 100 nanometers. In addition to functioning as conventional wires for interconnection applications, nanowires have a wide variety of other potential applications. Recently, devices and systems such as field-effect transistors, radiation detectors, light emitting diodes, lasers, and sensors have been described that employ nanowires in their design. 
     Many nanowires described in the art include conventional semiconductor materials such as silicon-based materials and germanium-based materials. 
     One method of forming such nanowires is the vapor-liquid-solid (VLS) chemical synthesis process. Generally, this method involves depositing particles of a catalyst material such as gold or titanium on a surface of a structure on which it is desired to grow nanowires. The structure is provided within a chamber and heated to temperatures typically ranging between about 500° C. and about 1000° C. Precursor gasses that include elements that will be used to form the nanowires are introduced into the chamber. The particles of catalyst material cause the precursor gasses to at least partially decompose into their respective elements, some of which are transported on or through the particles of catalyst material and deposited on the underlying surface. As this process continues, a nanowire is formed or grown with the catalyst particle remaining on the growing tip or end of the nanowire. 
     Nanowires that include a heterogeneous structure have also been described in the art. For example, longitudinal heterostructure nanowires (LOHN) have been described in which the composition of the nanowire varies along the longitudinal length thereof. Similarly, coaxial heterostructure nanowires (COHN) have been described in which the composition of the nanowire varies in the radial direction. Nanowires that include such heterogeneous structures have been described that include multiple regions of doped semiconductor materials that form pn, pnp, and npn junctions. 
     Many areas of technology, such as optical signal processing for example, employ light-emitting diodes (LED&#39;s) and laser devices such as vertical cavity surface emitting lasers (VCSEL&#39;s) that include active or gain material disposed within a resonant cavity. The resonant cavity may be used to ensure that the spectral line width of the light emitted by the active material is narrow and to provide emitted light having high directivity. Quantum dots that are formed from active material have been provided within resonant cavities to provide such light-emitting diodes and laser devices. One challenge to optimizing the performance of such devices, however, has been the inability to precisely control the position of such quantum dots within the resonant cavity. Therefore, there is a need in the art for methods of precisely positioning quantum dots within resonant cavities. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the present invention includes a radiation-emitting device having at least one nanowire extending between a first electrode and a second electrode. The nanowire is structurally and electrically coupled to the electrodes and includes a double-heterostructure semiconductor device configured to emit electromagnetic radiation when a voltage is applied across the nanowire between the electrodes. 
     In another aspect, the present invention includes a device having a nanowire disposed within a resonant cavity. The nanowire has an active longitudinal segment capable of emitting electromagnetic radiation at a wavelength of electromagnetic radiation within a range extending from about 300 nanometers to about 2,000 nanometers. The active longitudinal segment of the nanowire is selectively disposed at a predetermined location within the resonant cavity, and the resonant cavity is configured to resonate the radiation emitted by the active longitudinal segment of the nanowire. 
     In yet another aspect, the present invention includes a method for precisely positioning an active nanoparticle within a resonant cavity. A structure is provided that includes a resonant cavity configured to resonate at least one wavelength of electromagnetic radiation. A first longitudinal segment of a nanowire having a first composition is grown that extends from a surface of the structure adjacent the resonant cavity. The first longitudinal segment is grown at a known growth rate for a selected amount of time. A second active longitudinal segment of the nanowire that includes an active nanoparticle is grown that extends from an end of the first longitudinal segment. The second active longitudinal segment has an active second composition that is configured to emit electromagnetic radiation at the at least one wavelength upon stimulation. 
     The features, advantages, and alternative aspects of the present invention will be apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view of an embodiment of a device of the present invention that includes a nanowire disposed within a resonant cavity; 
         FIGS. 2-3  are graphs of emission spectra illustrating principles of operation of the device shown in  FIG. 1 ; 
         FIGS. 4A-4I  illustrate a method of fabricating the device illustrated in  FIG. 1 ; 
         FIG. 5  is a side view of another embodiment of a device of the present invention that includes a nanowire disposed within a resonant cavity; 
         FIG. 6A  is a perspective view of another exemplary device of the present invention that includes a nanowire disposed within a resonant cavity; 
         FIG. 6B  is a cross-sectional view of the device shown in  FIG. 6A  taken along section line  6 B- 6 B shown therein; 
         FIG. 7A  is a perspective view of another embodiment of a device of the present invention that includes a nanowire disposed within a resonant cavity; 
         FIG. 7B  is a cross-sectional view of the device shown in  FIG. 7A  taken along section line  7 B- 7 B shown therein; 
         FIG. 7C  is an enlarged partial view of the device shown in  FIGS. 7A-7B ; 
         FIG. 8A  is a perspective view of another embodiment of a device of the present invention; 
         FIG. 8B  is a side view of the device shown in  FIG. 8A ; and 
         FIGS. 9A-9C  are simplified energy band diagrams of a nanowire shown in  FIGS. 8A-8B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein, the term “active material” means material that is capable of emitting electromagnetic radiation upon stimulation. The phrase “III-V type semiconductor material” means any material that is substantially composed of an element or elements from group IIIB of the periodic table (B, Al, Ga, In, and Ti) and an element or elements from group VB of the periodic table (N, P, As, Sb, and Bi). The phrase “II-VI type semiconductor material” means any material that is substantially composed of an element or elements from group IIB of the periodic table (Zn, Cd, and Hg) and an element or elements from group VIB of the periodic table (O, S, Se, Te, and Po). 
     As used herein, the term “heterojunction” means a junction between two regions of semiconductor material that have different bandgap energies. In other words, the bandgap energy on a first side of a heterojunction is different from a bandgap on the opposite, second side of the heterojunction. The phrase “double-heterostructure semiconductor device” means a semiconductor device that includes at least two heterojunctions. 
     An embodiment of a device  110  of the present invention is shown in  FIG. 1 . The device  110  includes a resonant cavity  112  and a plurality of nanowires  120  disposed within the resonant cavity  112 . Each nanowire  120  may include an active longitudinal segment  124  that comprises an active material. The active longitudinal segment  124  of each nanowire  120  may be substantially composed of active material that is capable of emitting electromagnetic radiation at a selected, predetermined wavelength upon stimulation. Such stimulation may be provided by, for example, irradiating the active material with electromagnetic radiation having a particular wavelength (often referred to as “optical pumping”). Alternatively, such stimulation may be provided by electrically stimulating the material. For example, a voltage may be applied across the active material to electrically stimulate the active material. 
     The selected, predetermined wavelength of electromagnetic radiation emitted by the active material may be, for example, within a range extending from about three-hundred (300) nanometers to about two-thousand (2,000) nanometers. The resonant cavity  112  may be configured to resonate the selected, predetermined wavelength of electromagnetic radiation that is emitted by the active material in the active longitudinal segment  124  of each nanowire  120 . 
     The active longitudinal segment  124  of each nanowire  120  may be selectively disposed at a predetermined location within the resonant cavity  112 . Furthermore, the active longitudinal segment  124  of each nanowire  120  may comprise or behave as an active quantum dot. 
     By way of example and not limitation, the resonant cavity  112  may be formed in a substrate  117 . The substrate  117  may be substantially transparent to the selected, predetermined wavelength of electromagnetic radiation emitted by the active material in the active longitudinal segment  124  of each nanowire  120 . The resonant cavity  112  may include or be formed in an aperture, groove, channel, or trench extending at least partially through the substrate  117 . In the embodiment shown in  FIG. 1 , a trench has been formed in a substantially planar surface  118  of the substrate  117  to provide opposing, substantially parallel vertical sidewalls  119 . The substrate  117  may include silicon (Si). The active material of the active longitudinal segment  124  of each nanowire  120  may include, for example, silicon (Si), a silicon-based material, germanium (Ge), a germanium-based material, a material doped with erbium (Er 3+ ) ions, a III-V type semiconductor material, or a II-VI type semiconductor material. Some III-V type semiconductor materials that may be used as active material include, but are not limited to, GaAs, Al x Ga 1-x As (where x is in a range extending from about 0 to about 0.4), In 1-x Ga x As 1-y P y  (where x is in a range extending from about 0 to about 0.47 and y is equal to about 2.2 times x), InGaN alloys, In 0.49 Al x  Ga 0.51-x P, Ga As 1-y P y  (where y is less than about 0.45), Ga As 1-y P y  doped with N, Zn, or O (where y is greater than about 0.45), GaP doped with Zn or O, and GaP doped with N. Some II-VI type semiconductor materials that may be used as active material include, but are not limited to, ZnO and CdS. 
     The resonant cavity  112  of the device  110  may comprise a Fabry-Perot resonant cavity. A Fabry-Perot resonant cavity may be formed by, for example, providing two parallel reflective members separated from one another by a distance. When electromagnetic radiation is reflecting back and forth between the two reflective members, the reflecting waves may interfere constructively or destructively. When the distance separating the reflective members is equal to an integer multiple of one-half the wavelength of the reflecting electromagnetic radiation (i.e., D=nλ/2, where D is the distance between the reflective members, n is an integer, and A is the wavelength of the electromagnetic radiation), the waves of reflecting electromagnetic radiation may interfere constructively, causing the electromagnetic radiation to resonate within the cavity. When the distance separating the reflective members is not equal to an integer multiple of one-half the wavelength of the radiation (i.e., D≠nλ/2), the waves of reflecting electromagnetic radiation may interfere destructively, thereby dissipating the reflecting electromagnetic radiation within the cavity. 
     By way of example and not limitation, the resonant cavity  112  of the device  110  may include a first reflective member  114  and a second reflective member  116 . The first reflective member  114  and the second reflective member  116  each may be substantially planar. The second reflective member  116  may be oriented substantially parallel to the first reflective member  114 , and the second reflective member  116  may be separated from the first reflective member  114  by a selected distance D. Furthermore, the first reflective member  114  and the second reflective member  116  each may have a reflectivity greater than zero with respect to the selected, predetermined wavelength of electromagnetic radiation that may be emitted by the active material of the active longitudinal segment  124  of each nanowire  120 . One of the first reflective member  114  and the second reflective member  116  may have a reflectivity of about one-hundred (100) percent with respect to the selected, predetermined wavelength of electromagnetic radiation that may be emitted by the active material of the active longitudinal segment  124  of each nanowire  120 . The other reflective member may have a reflectivity of less than one-hundred (100) percent to allow at least some electromagnetic radiation emitted by the active material of the active longitudinal segment  124  of each nanowire  120  to escape from the resonant cavity  112  therethrough. 
     The first reflective member  114  and the second reflective member  116  each may include a substantially planar thin layer of reflective material. By way of example and not limitation, the first reflective member  114  and the second reflective member  116  each may include a substantially planar thin layer of silver disposed on the opposing, substantially parallel vertical sidewalls  119  of the trench formed in the substrate  117 . In one embodiment, the first reflective member  114  and the second reflective member  116  each may have a thickness of less than about fifty (50) nanometers. For example, the first reflective member  114  may have a thickness of about thirty (30) nanometers and a reflectivity of about one-hundred (100) percent with respect to at least one wavelength of electromagnetic radiation emitted by the active material of the active longitudinal segment  124  of each nanowire  120 , and the second reflective member  116  may have a thickness of about five (5) nanometers and a reflectivity of less than one-hundred (100) percent with respect to the selected, predetermined wavelength of electromagnetic radiation that may be emitted by the active material of the active longitudinal segment  124  of each nanowire  120 . In this configuration, when electromagnetic radiation is emitted by the active material of the active longitudinal segment  124  of each nanowire  120 , at least some of the radiation may reflect back and forth between the first reflective member  114  and the second reflective member  116  and resonate within the resonant cavity  112 . At least some of the resonating electromagnetic radiation may pass through the second reflective member  116 . 
     In alternative embodiments, the vertical sidewalls  119  of the trench extending at least partially through the substrate  117  may have a reflectivity greater than zero with respect to the selected, predetermined wavelength of electromagnetic radiation that may be emitted by the active material of the active longitudinal segment  124  of each nanowire  120 . In this configuration, the first reflective member  114  and the second reflective member  116  of the resonant cavity  112  may include the surfaces of the vertical sidewalls  119  of the trench, and no thin layer of reflective material need be applied to the surfaces of the vertical sidewalls  119 . 
     The emission spectrum of the electromagnetic radiation emitted by the active material of the active longitudinal segment  124  of each nanowire  120  within the resonant cavity  112  may be similar to that illustrated in  FIG. 2 . The emission spectrum of the electromagnetic radiation passing through the second reflective member  116  may be similar to that illustrated in  FIG. 3 . The emission peaks shown in  FIG. 3  correspond to the resonant wavelengths of the resonant cavity  112 . The wavelengths of electromagnetic radiation between the peaks may be substantially dissipated within the resonant cavity  112  due to destructive interference, as previously described. 
     The active longitudinal segment  124  of each nanowire  120  may be selectively disposed at a predetermined location within the resonant cavity  112  of the device  110 . For example, the active longitudinal segment  124  of each nanowire  120  may be selectively disposed at a predetermined location within the resonant cavity  112  of the device  110  to maximize the energy within the resonant cavity  112  when the active material of the active longitudinal segment  124  of each nanowire  120  is emitting electromagnetic radiation. 
     In alternative embodiments, at least one of the first reflective member  114  and the second reflective member  116  may be convex. In other embodiments, at least one of the first reflective member  114  and the second reflective member  116  may include a Bragg mirror. 
     Moreover, both the first reflective member  114  and the second reflective member  116  may have a reflectivity of less than one-hundred (100) percent to allow at least some electromagnetic radiation emitted by the active material of the active longitudinal segment  124  of each nanowire  120  to escape from the resonant cavity  112  through both the first reflective member  114  and the second reflective member  116 . Alternatively, both the first reflective member  114  and the second reflective member  116  may have a reflectivity of about one-hundred (100) percent. 
     In the embodiment illustrated in  FIG. 1 , the first reflective member  114  and the second reflective member  116  are disposed on the vertical sidewalls  119  of the trench formed in the substrate  117 . In other embodiments, the first reflective member  114  and the second reflective member  116  may be disposed at the opposing longitudinal ends of the trench formed in the substrate  117 . For example, thin layers of reflective material may be provided on the sides of the substrate  117  that are intersected by the trench. The trench may be formed in the substrate  117  between the thin layers of reflective material, such that the thin layers of reflective material provide and define the longitudinal ends of the trench. In such a configuration, the electromagnetic radiation emitted by the active material of the active longitudinal segment  124  of each nanowire  120  may reflect back and forth between the thin layers of reflective material in the longitudinal direction within the trench. 
     A method for fabricating the device  110  shown in  FIG. 1 , and for selectively locating the active longitudinal segment  124  of each nanowire  120  within the resonant cavity  112  will now be described with reference to  FIGS. 4A-4I . 
     Referring to  FIG. 4A , a silicon substrate  117  may be provided. The silicon substrate  117  may include substantially crystalline silicon. The substrate  117  may include a substantially planar surface  118 . The crystal structure of the silicon may be oriented such that the substantially planar surface  118  comprises a (110) plane of the silicon crystal lattice. In this configuration, the (111) planes  134  (represented in  FIG. 4A  by dashed lines) are oriented substantially perpendicular to the substantially planar surface  118 . 
     A trench that extends at least partially through the substrate  117  may be formed in the substantially planar surface  118  of the substrate  117 . In one method of forming the trench, a masking and etching process may be used. As shown in  FIG. 4B , a mask  136  may be formed on or in the surface  118  of the substrate  117 . By way of example and not limitation, the mask  136  may include an oxide layer such as a layer of silica (SiO 2 ). Alternatively, the mask  136  may include a layer of polymer material. A portion of the mask  136  between the dashed lines  138  may be removed to expose a portion of the underlying surface  118  of the silicon substrate  117 , thereby forming the structure shown in  FIG. 4C . Each dashed line  138  ( FIG. 4B ) may be substantially disposed within or oriented parallel to a (111) plane  134  of the silicon crystal lattice. The portion of the mask  136  between the dashed lines  138  may be removed by, for example, electron beam lithography, focused ion beam lithography, or by a masking and etching process. Etching processes include but are not limited to, reactive ion etching processes and wet chemical etching processes. 
     In one particular method, the portion of the mask  136  between the dashed lines  138  may be removed by covering the mask  136  with a second mask (not shown), patterning the second mask to expose the portion of the mask  136  between the dashed lines  138 , and etching the portion of the mask  136  between the dashed lines  138  with a reactive ion etching process using CHF 3  and Ar gases. 
     The exposed portion of the underlying surface  118  of the silicon substrate  117  shown in  FIG. 4C  may be etched to form a trench  140  in the substantially planar surface  118  of the substrate  117  that extends at least partially through the substrate  117 , as shown in  FIG. 4D . The exposed portion of the underlying surface  118  ( FIG. 4C ) of the silicon substrate  117  may be etched using an etchant that will etch the exposed silicon at a rate that is faster than a rate at which the etchant will etch the mask  136 . Therefore, the mask  136  may include any material that will be etched slower than the exposed silicon by a particular etchant. By way of example and not limitation, if the mask  136  includes silica (SiO 2 ), the exposed portion of the underlying surface  118  of the silicon substrate  117  may be etched using a mixture of potassium hydroxide (KOH) and water (about 44% KOH). If the underlying silicon substrate  117  is etched using such a mixture at a temperature of about 110° C. for about one minute, the resulting trench  140  may have a maximum depth of approximately 8 microns. Other etchants, such as, for example, ethylene diamine pyrocatechcol (EDP), may be used to etch the trench  140  in the silicon substrate  117 . 
     Referring to  FIG. 4D , the trench  140  may provide and define opposing, vertical sidewalls  119  that are substantially parallel to one another. Each opposing, vertical sidewall  119  may include or consist of a (111) plane  134  of the silicon crystal lattice. The mask  136  then may be removed to provide the structure shown in  FIG. 4E . 
     By way of example only and not limitation, the figures provided herein show the trench  140  as having a generally V-shaped bottom. The bottom of the trench  140  may have other shapes. The shape of the bottom of the trench  140  may be at least partially dependent on factors including the width of the trench  140  being etched and the method of etching. For example, the trench  140  may have a generally U-shaped bottom or a substantially planar bottom. As such, the illustrations are not intended to limit the scope of any embodiment of the present invention as described herein. 
     The nanowires  120  shown in  FIG. 1  may be grown or formed between the opposing, vertical sidewalls  119  of the trench  140 . By way of example and not limitation, the nanowires  120  may be grown using a vapor-liquid-solid (VLS) reaction or mechanism. A plurality of nanoparticles  146  comprising a catalyst material may be deposited on at least one of the opposing, vertical sidewalls  119  of the trench  140 , as shown in  FIG. 4F . By way of example and not limitation, the catalyst material may include metals such as titanium (Ti), gold (Au), iron (Fe), cobalt (Co), and gallium (Ga), alloys of such metals, and nonmetals such as SiO x  (where x is within a range from about 1 to about 2). 
     Furthermore, the nanoparticles  146  may be selectively deposited on at least one of the opposing, vertical sidewalls  119  of the trench  140 . For example, the nanoparticles  146  may be selectively deposited on one of the sidewalls  119  of the trench  140  but not on the opposing sidewall  119 . Furthermore, the nanoparticles  146  may be selectively deposited at selected, predetermined locations on at least one of the opposing, vertical sidewalls  119  of the trench  140 . The nanowires  120  ( FIG. 1 ) will grow at the locations on the vertical sidewalls  119  where the nanoparticles  146  are disposed. Therefore, the nanowires  120  ( FIG. 1 ) may be attached to selected, predetermined locations on the vertical sidewalls  119  by disposing the nanoparticles  146  at selected, predetermined locations on at least one of the opposing, vertical sidewalls  119  of the trench  140 . 
     With continued reference to  FIG. 4F , the nanoparticles  146  may be deposited on at least one of the opposing, vertical sidewalls  119  of the trench  140  by, for example, depositing a thin film comprising catalyst material that is approximately one nanometer thick on at least one of the opposing, vertical sidewalls  119  of the trench  140 . The thin film may be deposited by, for example, using physical vapor deposition (PVD) techniques including, but not limited to, thermal evaporation techniques, electron-beam evaporation techniques, filament evaporation techniques, and sputtering techniques. Alternatively, the thin film may be deposited by using chemical vapor deposition (CVD) techniques including, but not limited to, atomic layer deposition techniques. 
     The structure then may be annealed at an elevated temperature to form self-assembled nanoparticles from the thin film of catalyst material. The structure may be annealed in a closed, controlled environment. A closed, controlled environment may include, but is not limited to, a reactor chamber in which at least the temperature and the pressure may be selectively controlled. 
     As shown in  FIG. 4F , the thin film may be selectively deposited on one of the sidewalls  119  of the trench  140 , but not on the opposing sidewall  119 , by using an angled deposition process. In an angled deposition process, the substrate  117  may be oriented at an angle with respect to the flow of the catalyst material being deposited. In  FIG. 4F , the catalyst material is flowing in the direction indicated by the directional arrow  148 . The angle of deposition may be defined as the angle between a vector normal to the surface  118  and the direction of the flow of the catalyst material. The angle of deposition may be selected considering the dimensions of the trench  140  such that the catalyst material is deposited on only one of the sidewalls  119  of the trench  140 . The surface  118  of the substrate may shield the opposing sidewall  119  of the trench  140  from deposition of catalyst material. In alternative embodiments, the nanoparticles  146  may be deposited on both of the opposing, vertical sidewalls  119  of the trench  140 . 
     To prevent the deposition of the nanoparticles  146  on surfaces of the substrate  117  other than the opposing, vertical sidewalls  119  of the trench  140 , the other surfaces of the substrate  117  may be passivated. By way of example and not limitation, a mask material such as silicon nitride (Si 3 N 4 ) may be deposited on all surfaces of the substrate  117 . The silicon nitride then may be removed from only the non-vertical surfaces of the substrate  117  by, for example, using a directional reactive-ion etching process. The exposed, non-vertical surfaces of the substrate  117  then may be passivated by, for example, providing an oxide layer on or in the exposed, non-vertical surfaces of the substrate  117 . The silicon nitride remaining on the opposing, vertical sidewalls  119  of the trench  140  then may be removed using an etchant that will not remove the oxide layer. In alternative methods, the oxide layer may be selectively provided on or in the non-vertical surfaces of the substrate  117  without masking the opposing, vertical sidewalls  119  of the trench  140 . 
     After the nanoparticles  146  of catalyst material have been deposited on the opposing, vertical sidewalls  119  of the trench  140 , the nanowires  120  ( FIG. 1 ) may be formed or grown in the trench  140 . For example, the nanowires  120  may be laterally grown in the trench  140  between the opposing, vertical sidewalls  119 . A first longitudinal segment  122  of each nanowire  120  ( FIG. 1 ) may be grown in a closed, controlled environment within a chemical vapor deposition (CVD) chamber (not shown) to provide the structure shown in  FIG. 4G . The chemical vapor deposition chamber may be heated to an elevated temperature, and a precursor gas or gases may be introduced into the chamber. The precursor gas may include at least one element used to form the first longitudinal segments  122  of the nanowires  120 . 
     In one particular embodiment, the first longitudinal segments  122  may include a silicon-based material. In this embodiment, the nanoparticles  146  of catalyst material may comprise gold-silicon alloy material (formed by depositing a thin film of gold on the silicon material of the opposing, vertical sidewalls  119  of the trench  140  and annealing the thin film, as previously described). The chemical vapor deposition chamber may be heated to a temperature of, for example, about 625° C. and a precursor gas comprising a mixture of SiH 4  and HCl may be introduced into the chamber. Other gases that include silicon atoms such as, for example, dichlorosilane (SiH 2 Cl 2 ) or silicon tetrachloride (SiCl 4 ) also may be used to grow segments of nanowires  120  that include a silicon-based material. 
     The growth rate of the first longitudinal segments  122  of the nanowires  120  may be at least partially a function of the temperature and pressure within the chamber and the flow rate of the precursor gas through the chamber. The particular growth rate of the first longitudinal segments  122  of the nanowires  120  for any set of variables may be determined empirically by growing nanowires from the same precursor gas or gases (using the same set of variables) for a selected amount of time and measuring the average length of the nanowires. Once the growth rate is known for a particular set of variables, the first longitudinal segments  122  may be grown to have a selected, predetermined length by growing the first longitudinal segments  122  for a selected, predetermined amount of time. 
     The growing segments of the nanowires  120  may have a diameter substantially similar to a diameter of the nanoparticles  146  of catalyst material. Therefore, the diameter of the nanowires  120  may be selectively controlled by selectively controlling the size or diameter of the nanoparticles  146 . 
     During formation of the first longitudinal segments  122  of the nanowires  120 , the nanoparticles  146  of catalyst material may at least partially decompose the precursor gas or gases into their respective elements. At least some atoms from a precursor gas (such as Si or Ge for example) may diffuse through or around the nanoparticles  146  of catalyst material where they may be precipitated or deposited on the underlying vertical sidewalls  119  of the substrate  117  or on previously grown portions of the first longitudinal segments  122 . 
     The first longitudinal segments  122  of the nanowires  120  may preferentially grow in a direction substantially normal to the vertical sidewall  119  of the trench  140  when the vertical sidewall  119  comprises a (111) plane in the silicon crystal lattice, as previously described. 
     After the first longitudinal segments  122  of the nanowires  120  have been grown to a selected, predetermined length, the second active longitudinal segments  124  of the nanowires  120  may be grown on or from an end of the first longitudinal segments  122  opposite from the vertical sidewall  119  of the trench  140  on which the first longitudinal segments  122  were grown, as shown in  FIG. 4H . The second active longitudinal segments  124  may be formed in a manner substantially similar to that previously described in relation to the first longitudinal segments  122  of the nanowires  120 . In particular, the composition of the precursor gas or gases flowing through the chamber may be changed to include atoms or elements that will form the second active longitudinal segments  124  of the nanowires  120 . 
     By way of example and not limitation, the second active longitudinal segments  124  may include GaN or ZnO and may be formed by exposing the nanoparticles  146  of catalyst material (which may now be disposed on the ends of the first longitudinal segments  122  of the nanowires  120 ) to a precursor gas or gases comprising the elements to be used to form the second active longitudinal segments  124  of the nanowires  120  in a closed, controlled environment within a reaction chamber. The reaction chamber may be the same reaction chamber or a different reaction chamber used to form the first longitudinal segments  122  of the nanowires  120 . 
     The second active longitudinal segments  124  also may be grown to have a selected, predetermined length by growing the second active longitudinal segments  124  for a selected, predetermined amount of time at a known growth rate in the same manner previously described in relation to the first longitudinal segments  122 . 
     After the second active longitudinal segments  124  of the nanowires  120  have been grown, the third longitudinal segments  126  may be grown on or from an end of the second active longitudinal segments  124  opposite from the first longitudinal segments  122 , as shown in  FIG. 4I . The third longitudinal segments  126  may be formed in a manner substantially similar to that previously described in relation to the first longitudinal segments  122  of the nanowires  120 . In particular, the composition of the precursor gas or gases flowing through the chamber may be changed to include atoms or elements that will form the third longitudinal segments  126  of the nanowires  120 . 
     By way of example and not limitation, the third longitudinal segments  126  of the nanowires  120  may include the same material used to form the first longitudinal segments  122  of the nanowires  120  and may be formed by exposing the nanoparticles  146  of catalyst material (which now may be disposed on the end of the second active longitudinal segments  124  of the nanowires  120 ) to a precursor gas or gases comprising the elements used to form the third longitudinal segments  126  of the nanowires  120  in a reaction chamber. The reaction chamber may be the same reaction chamber or a different reaction chamber used to form the first longitudinal segments  122  and the second active longitudinal segments  124  of the nanowires  120 . 
     The third longitudinal segments  126  may be grown until they impinge on the opposing vertical sidewall  119 . The third longitudinal segments  126  may structurally couple and attach to the opposing vertical sidewall  119  as they grow against and impinge on the sidewall  119 , thereby providing structural rigidity to the nanowires  120 . 
     After the nanowires  120  have been grown between the opposing, vertical sidewalls  119 , a thin layer of reflective material may be applied to each of the sidewalls  119  to form the first reflective member  114  and the second reflective member  116 , as shown in  FIG. 4I . Each thin layer of reflective material may be at least partially reflective with respect to the selected, predetermined wavelength of electromagnetic radiation that may be emitted by the active material of the active longitudinal segment  124  of each nanowire  120 . In this configuration, the first reflective member  114  and the second reflective member  116  each may include a substantially planar thin layer of reflective material, such as, for example, silver or gold. 
     The thin layers of reflective material may be deposited on the sidewalls  119  by, for example, using physical vapor deposition techniques including, but not limited to, thermal evaporation techniques, electron-beam evaporation techniques, filament evaporation techniques, and sputtering techniques. Alternatively, the thin layers of reflective material may be deposited on the sidewalls  119  using chemical vapor deposition techniques, including but not limited to, atomic layer deposition techniques. 
     The methods described in the preceding paragraphs allow for precise positioning of nanoparticles (such as, for example, active quantum dots) comprising a longitudinal segment of a nanowire that has a selected material composition in a resonant cavity. These methods may be used to precisely position such nanoparticles within various other embodiments of resonant cavities, some of which are described below. 
       FIG. 5  is a side view of another device  210  that embodies teachings of the present invention. The device  210  is similar to the device  110  shown in  FIG. 1  and includes a resonant cavity  212  and a plurality of nanowires  220  disposed within the resonant cavity  212 . Each nanowire  220  may include an active longitudinal segment  224  that comprises an active material. The active longitudinal segment  224  of each nanowire  220  may be substantially composed of active material that is capable of emitting a selected, predetermined wavelength of electromagnetic radiation upon stimulation. The selected, predetermined wavelength may be within a range extending from about three-hundred (300) nanometers to about two-thousand (2,000) nanometers. The resonant cavity  212  may be configured to resonate the selected, predetermined wavelength of electromagnetic radiation that is emitted by the active material of the active longitudinal segment  224  of each nanowire  220 . 
     The active longitudinal segment  224  of each nanowire  220  may be selectively disposed at a predetermined location within the resonant cavity  212 . Furthermore, the active longitudinal segment  224  of each nanowire  220  may comprise or behave as an active quantum dot. 
     With continued reference to  FIG. 5 , the nanowires  220  may be formed or grown in the vertical direction, in contrast to the nanowires  120  shown in  FIG. 1 , which were described as being formed or grown in the lateral direction. 
     The first reflective member  214  and the second reflective member  216  each may include a Bragg mirror (often referred to as a distributed Bragg reflectors or DBR). Bragg mirrors are reflective structures and may have a reflectivity as high as about 99.99%. Bragg mirrors include a multilayer stack of alternating films of high and low refractive index material. As shown in  FIG. 5 , the first reflective member  214  and the second reflective member  216  each may include alternating layers of low-index films  218  and high-index films  219 . The thickness of each low-index film  218  and each high-index film  219  may be selected to be approximately one-fourth the selected, predetermined wavelength of electromagnetic radiation that is emitted by the active material of the active longitudinal segments  224  divided by the refractive index of the material from which the film is formed (λ/4n ri , where λ is the wavelength of the radiation and n ri  is the refractive index of the material). In one particular embodiment of the invention, each low-index film  218  may include AlGaAs and each high-index film  219  may include GaAs. 
     At least one support structure  230  may be provided between at least a portion of the first reflective member  214  and the second reflective member  216  to support the second reflective member  216  relative to the first reflective member  214  and to separate the second reflective member  216  from the first reflective member  214  by a selected distance. The support structure  230  may include a partial layer of material that includes a void formed therethrough in a region comprising the resonant cavity  212 . 
     The first reflective member  214 , the at least one support structure  230 , and the second reflective member  216  may be formed in a layer-by-layer process by, for example, using physical vapor deposition techniques including, but not limited to, thermal evaporation techniques, electron-beam evaporation techniques, filament evaporation techniques, and sputtering techniques. Alternatively, the first reflective member  214  and the second reflective member  216  may be formed in a layer-by-layer process using chemical vapor deposition techniques including, but not limited to, atomic layer deposition techniques. 
     The resonant cavity  212  may behave as a Fabry-Perot resonant cavity in a manner substantially similar to that previously described in relation to the resonant cavity  112  shown in  FIG. 1 . Similarly, each nanowire  220  may be formed or grown between the first reflective member  214  and the second reflective member  216  in a manner substantially similar to that previously described with reference to  FIGS. 4F-4I . In particular, nanoparticles of catalyst material may be deposited on a surface of the first reflective member  214  within the resonant cavity  212 . A first longitudinal segment  222 , second active longitudinal segment  224 , and third longitudinal segment  226  of each nanowire  220  then may be grown using the vapor-liquid-solid reaction or mechanism, as previously described herein. The first longitudinal segment  222  of each nanowire may be structurally coupled to the first reflective member  214 , and the third longitudinal segment  226  may be structurally coupled to the second reflective member  216 . Furthermore, the second active longitudinal segment  224  of each nanowire  220  may be selectively disposed at a predetermined location within the resonant cavity  212  by growing the first longitudinal segment  222  of each nanowire  220  at a known growth rate for a selected, predetermined amount of time as previously described. Moreover, each segment  222 ,  224 ,  226  of each nanowire  220  may have a selected, predetermined length, which may be provided by growing each segment  222 ,  224 ,  226  of each nanowire  220  at a known growth rate for a selected, predetermined amount of time as previously described. 
     The device  110  shown in  FIG. 1  and the device  210  shown in  FIG. 5  each include a Fabry-Perot resonant cavity provided by two substantially planar and parallel reflective members. Other devices that embody teachings of the present invention may include resonant cavities having other configurations. 
     Another device  310  that embodies teachings of the present invention is shown in  FIGS. 6A-6B . The device  310  includes a two-dimensional (2D) photonic crystal  312  that includes a resonant cavity  318  and at least one nanowire  320  disposed within the resonant cavity  318 . The at least one nanowire  320  may include an active longitudinal segment  324  that comprises an active material. The active longitudinal segment  324  of the nanowire  320  may be substantially composed of active material that is capable of emitting electromagnetic radiation at a selected, predetermined wavelength upon stimulation. The selected, predetermined wavelength may be within a range extending from about three-hundred (300) nanometers to about two-thousand (2,000) nanometers. The resonant cavity  318  may be configured to resonate the selected, pre-determined wavelength of electromagnetic radiation that is emitted by the active material of the active longitudinal segment  324  of the at least one nanowire  320 . 
     Photonic crystals are a class of man-made materials that may be formed by dispersing a material of one dielectric constant (or refractive index) periodically within a matrix having a different dielectric constant (or refractive index). A one-dimensional photonic crystal is a three-dimensional structure that exhibits periodicity in dielectric constant in only one dimension. Bragg mirrors, such as those shown in  FIG. 5  and previously described herein, are one example of a one-dimensional photonic crystal. The alternating thin layers have different dielectric constants (and thus, different refractive indices). The combination of several thin layers forms a three-dimensional structure that exhibits periodicity in dielectric constant in only the direction orthogonal to the planes of the thin layers. No periodicity is exhibited in either of the two dimensions contained within the plane of the layers. 
     A two-dimensional photonic crystal can be formed by periodically dispersing rods or columns of a material of one dielectric constant within a matrix having a different dielectric constant. Two-dimensional photonic crystals exhibit periodicity in the directions perpendicular to the length of the rods or columns, but no periodicity is exhibited in the direction parallel to the length of the columns. 
     Finally, a three-dimensional photonic crystal can be formed by periodically dispersing small spheres or other spatially confined areas of a first material having a first dielectric constant within a matrix of a second material having a second, different, dielectric constant. Three-dimensional photonic crystals exhibit periodicity in dielectric constant in all three dimensions within the crystal. 
     Photonic crystals may exhibit a photonic bandgap over a range of wavelengths in directions exhibiting periodicity in dielectric constant. In other words, there may be a range of wavelengths of electromagnetic radiation that will not be transmitted through the photonic crystal in the directions exhibiting periodicity in dielectric constant. This range of wavelengths that are not transmitted is known as a photonic bandgap of the photonic crystal. No photonic bandgap is exhibited in directions that do not exhibit periodicity in dielectric constant. 
     When defects are introduced into the periodic dielectric structure of a photonic crystal, localized electromagnetic modes may be allowed at wavelengths within the photonic bandgap. For example, resonant cavities have been formed in photonic crystals by introducing point defects into the periodic dielectric structure, and waveguides have been formed in photonic crystals by introducing line defects into the periodic dielectric structure. 
     The photonic crystal  312  shown in  FIGS. 6A-6B  includes a plurality of elongated cylindrical rods  316  extending from a surface of a substrate  314 . The rods  316  may have a uniform radius and may be disposed in what is referred to in the art as a square lattice. The square lattice may have a lattice constant defined as the distance separating the center of one rod  316  from the center of adjacent rods  316 . The rods  316  exhibit a dielectric constant that differs from a dielectric constant exhibited by the space between the rods  316 . 
     The resonant cavity  318  may be provided in the photonic crystal  312  by providing a point defect in the periodic lattice. Such a defect may be provided by, for example, removing or failing to form at least one rod  316 . As can be seen in  FIG. 6B , one rod  316  is missing in a row of rods  316  to define the resonant cavity  318 . 
     The nanowire  320  may extend from a surface of the substrate  314  at or near the location of the missing rod  316 . The active longitudinal segment  324  of each nanowire  320  may be selectively disposed at a predetermined location within the resonant cavity  318 . Furthermore, the active longitudinal segment  324  of each nanowire  320  may comprise or behave as an active quantum dot. 
     The at least one nanowire  320  may be formed or grown on a surface of the substrate  314  within the resonant cavity  318  in a manner substantially similar to that previously described in relation to the nanowires  120  and  FIGS. 4F-4I . In particular, at least one nanoparticle of catalyst material may be deposited on a surface of the substrate  314  within the resonant cavity  318 . A first longitudinal segment  322  and a second active longitudinal segment  324  of the nanowire  320  then may be grown using the vapor-liquid-solid reaction or mechanism, as previously described. The first longitudinal segment  322  of the nanowire may be structurally coupled to the substrate  314 . Furthermore, the second active longitudinal segment  324  of the nanowire  320  may be selectively disposed at a predetermined location within the resonant cavity  318  by growing the first longitudinal segment  322  of the nanowire  320  at a known growth rate for a selected, predetermined amount of time, as previously described. Moreover, each segment  322 ,  324  of the at least one nanowire  320  may have a selected, predetermined length, which may be provided by growing each segment  322 ,  324  of the nanowire  320  at a known growth rate for a selected, predetermined amount of time. 
     The photonic crystal  312  may exhibit a photonic bandgap over a range of wavelengths of electromagnetic radiation. Certain electromagnetic defect modes at wavelengths within the photonic bandgap may be allowed within the resonant cavity  318 . At least one of the electromagnetic defect modes may resonate within the resonant cavity  318 . Determining the photonic band structure (which may be used to determine the allowed and prohibited electromagnetic modes) of a particular photonic crystal is a complex problem that involves solving the Maxwell equations considering the periodic variation in the dielectric constant through the photonic crystal. Thus, the photonic band structure is at least partially a function of the dielectric constant of the rods  316 , the dielectric constant of the spaces between the rods  316 , the uniform radius of the rods  316 , and the lattice constant of the photonic crystal. Computational methods for computing the band structure of a particular photonic crystal are known in the art. An explanation of these computational methods may be found in, for example, John D. Joannopoulas, Robert D. Meade &amp; Joshua N. Winn,  Photonic Crystals—Molding the Flow of Light , (Princeton University Press 1995), in particular at Appendix D. 
     By way of example and not limitation, the material from which the rods  316  are formed, the uniform radius of the rods  316 , and the lattice constant of the photonic crystal  312  may be selected such that the photonic crystal  312  exhibits a photonic bandgap over a range of wavelengths that includes the wavelength of the electromagnetic radiation that may be emitted by the second active longitudinal section  324  of the nanowire  320  upon stimulation. Furthermore, the resonant cavity  318  may exhibit a defect mode at the selected, predetermined wavelength of the electromagnetic radiation that may be emitted by the second active longitudinal section  324 . In this configuration, electromagnetic radiation emitted by the second active longitudinal section  324  of the nanowire  320  may resonate within the resonant cavity  318 . 
     In alternative embodiments, the resonant cavity  318  may provided by one rod  316  having a radius that is smaller than the uniform radius of the other rods  316 , and the at least one nanowire  320  may be disposed proximate or on the rod  316  having the smaller radius. 
     Devices that embody teachings of the present invention may include photonic crystals having configurations other than that shown in  FIGS. 6B-6C . 
     Another device  410  that embodies teachings of the present invention is shown in  FIGS. 7A-7C . The device  410  includes a two-dimensional (2D) photonic crystal  412  that includes a resonant cavity  418  and a plurality of nanowires  420  ( FIG. 7B ) disposed within the resonant cavity  418 . Each nanowire  420  may include an active longitudinal segment  424  that comprises an active material. The active longitudinal segment  424  of each nanowire  420  may be substantially composed of active material that is capable of emitting electromagnetic radiation at a selected, predetermined wavelength upon stimulation. The selected, predetermined wavelength may be within a range extending from about three-hundred (300) nanometers to about two-thousand (2,000) nanometers. The resonant cavity  418  may be configured to resonate the selected, pre-determined wavelength of electromagnetic radiation that is emitted by the active material of the active longitudinal segment  424  of each nanowire  420 . 
     Referring to  FIG. 7A , the photonic crystal  412  may include a plurality of cylindrical regions  414  dispersed periodically in an array of rows throughout a matrix  416 . A majority of the cylindrical regions  414  have a uniform radius. The matrix  416  exhibits a dielectric constant that differs from a dielectric constant exhibited by the cylindrical regions  414 . In one particular embodiment, the matrix  416  may have a dielectric constant of about 11.4 and the cylindrical regions  414  may have a dielectric constant of about 1. For example, the matrix  416  may be formed from a semiconductor material such as GaAs and the cylindrical regions  414  may include air. Such a structure may be formed by etching the cylindrical regions  414  in a layer of GaAs using known lithographic techniques such as, for example, masking and etching. Dielectric periodicity is exhibited in the photonic crystal in directions perpendicular to a longitudinal axis (not shown) of the cylindrical regions  414 . 
     The cylindrical regions  414  of the photonic crystal  412  may be configured in what is referred to in the art as a triangular lattice. The triangular lattice has a lattice constant defined as the distance separating the center of one cylindrical region  414  from the center of adjacent cylindrical regions  414 . The ratio of the uniform radius of the majority of cylindrical regions  414  to the lattice constant of the photonic crystal  412  may be in a range from about 0.2 to about 0.5. 
     The photonic crystal  412  may include a resonant cavity  418 . The resonant cavity  418  may be provided by including a point defect in the lattice of cylindrical regions  414 . For example, a point defect may be provided by including a cylindrical region  414 ′ having a radius that is less than the uniform radius of the other cylindrical regions  414  in the lattice of the photonic crystal  412 . 
     Referring to  FIG. 7B , each nanowire  420  of the plurality of nanowires  420  may extend laterally from a surface of the matrix  416  at a location within the cylindrical region  414 ′, which has a radius smaller than the uniform radius of the other cylindrical regions  414  and provides the resonant cavity  418 . 
     Referring to  FIG. 7C , each nanowire  420  of the plurality of nanowires  420  may be formed or grown on a surface of the matrix  416  within the resonant cavity  418  (i.e., within the cylindrical region  414 ′) in a manner substantially similar to that previously described in relation to the nanowires  120  and  FIGS. 4F-4I . In particular, a plurality of nanoparticles of catalyst material may be deposited on a surface of the matrix  416  within the cylindrical region  414 ′. A first longitudinal segment  422 , a second active longitudinal segment  424 , and a third longitudinal segment  426  of each nanowire  420  then may be grown using the vapor-liquid-solid reaction or mechanism, as previously described. The first longitudinal segment  422  and the third longitudinal segment  426  of each nanowire  420  may be structurally coupled to the surface of the matrix  416  within the cylindrical region  414 ′. Furthermore, the second active longitudinal segment  424  of each nanowire  420  may be selectively disposed at a predetermined location within the resonant cavity  418  by growing the first longitudinal segment  422  of each nanowire  420  at a known growth rate for a selected, predetermined amount of time, as previously described. Moreover, each segment  422 ,  424 ,  426  of each nanowire  420  may have a selected, predetermined length, which may be provided by growing each segment  422 ,  424 ,  426  of each nanowire  420  at a known growth rate for a selected, predetermined amount of time. The second active longitudinal segment  424  of each nanowire  420  may comprise or behave as an active quantum dot. 
     The photonic crystal  412  may exhibit a photonic bandgap over a range of wavelengths of electromagnetic radiation. Certain electromagnetic defect modes at wavelengths within the photonic bandgap may be allowed within the resonant cavity  418 . At least one of the electromagnetic defect modes may resonate within the resonant cavity  418 . As previously discussed, the photonic band structure is at least partially a function of the dielectric constant of the matrix  416 , the dielectric constant of the cylindrical regions  414 , the uniform radius of the cylindrical regions  414  and the lattice constant of the photonic crystal  412 . Computational methods for computing the band structure of a particular photonic crystal are known in the art. 
     By way of example and not limitation, the material from which the matrix  416  is formed, the uniform radius of the cylindrical regions  414 , and the lattice constant of the photonic crystal  412  may be selected such that the photonic crystal  412  exhibits a photonic bandgap over a range of wavelengths that includes the selected, predetermined wavelength of electromagnetic radiation that may be emitted by the second active longitudinal section  424  of each nanowire  420  upon stimulation. Furthermore, the resonant cavity  418  may exhibit a defect mode at the wavelength of the selected, predetermined wavelength of electromagnetic radiation that may be emitted by each second active longitudinal section  424 . In this configuration, electromagnetic radiation emitted by the second active longitudinal section  424  of each nanowire  420  may resonate within the resonant cavity  418 . 
     In alternative embodiments, the photonic crystal  412  may be disposed on a substrate (not shown) and each nanowire  420  of the plurality of nanowires  420  may extend vertically from a surface of the substrate within the cylindrical region  414 ′. 
     Each of the device  310  shown in  FIGS. 6A-6B  and the device  410  shown in  FIGS. 7A-7C  may additionally include a waveguide coupled to the resonant cavities provided therein and configured to direct electromagnetic radiation to or from the resonant cavities. By way of example and not limitation, waveguides may be provided in each of the device  310  shown in  FIGS. 6A-6B  and the device  410  shown in  FIGS. 7A-7C  by forming linear defects (missing rows of rods or cylindrical regions) in the lattice structure of the photonic crystal. 
     Each of the previously described devices that embody teachings of the present invention includes a resonant cavity and a nanowire disposed within the resonant cavity. The nanowire includes an active longitudinal segment that is selectively disposed at a predetermined location within the resonant cavity. Each of the active longitudinal segments may include an active material that is capable of emitting electromagnetic radiation upon stimulation by, for example, irradiating the active material with electromagnetic radiation having a particular wavelength (often referred to as “optical pumping”). In alternative embodiments of the present invention, nanowires may include active longitudinal sections that include an active material that is capable of emitting electromagnetic radiation upon electrical stimulation. For example, the nanowires may include a semiconductor diode and the active longitudinal segment may comprise an active layer, which may be selectively disposed at a predetermined location within a resonant cavity. 
     A radiation-emitting device  510  that embodies teachings of the present invention is shown in  FIGS. 8A-8B . The radiation-emitting device  510  includes a first electrode  512 , a second electrode  516 , and at least one nanowire  520  extending between the first electrode  512  and the second electrode  516 . The radiation-emitting device  510  may include a plurality of nanowires  520 . A first end of each nanowire  520  may be structurally and electrically coupled to the first electrode  512 , and a second, opposite end of the at least one nanowire  520  may be structurally and electrically coupled to the second electrode  516 . The at least one nanowire  520  may include a double-heterostructure semiconductor device that is configured to emit electromagnetic radiation when a voltage is applied across the nanowire  520  between the first electrode  512  and the second electrode  516 . 
     The radiation-emitting device  510  may further include a substrate  536 . The substrate  536  may include a semiconductor material, which may be similar to the semiconductor material of the first electrode  512  and the second electrode  516 . Furthermore, a layer of electrical insulator material  538  may be provided between the substrate  536  and the electrodes  512 ,  516 . In alternative embodiments, the substrate  536  may include an electrical insulator material, and no additional layer of electrical insulator material  538  is necessary. Furthermore, the substrate  536  may comprise a portion or region of a higher level device. 
     The first electrode  512  and the second electrode  516  each may include a portion or region of a layer of semiconductor material. The portion or region of the layer of semiconductor material may be doped to provide an n-type semiconductor material or a p-type semiconductor material. The layer of electrical insulator material  538  may electrically insulate the first electrode  512  from the second electrode  516 , and may electrically insulate the substrate  536  from the electrodes  512 ,  516 . 
     By way of example and not limitation, the radiation-emitting device  510  may include three nanowires  520  extending between the first electrode  512  and the second electrode  516 . As shown in  FIG. 8B , each nanowire  520  may include a first longitudinal segment  522 , a second active longitudinal segment  524 , and a third longitudinal segment  526 . An end of each first longitudinal segment  522  may be structurally and electrically coupled to the first electrode  512 , and an end of each third longitudinal segment  526  may be structurally and electrically coupled to the second electrode  516 . A first end of each second active longitudinal segment  524  may be structurally and electrically coupled to an end of a first longitudinal segment  522  opposite the first electrode  512 , and a second end of each second active longitudinal segment  524  may be structurally and electrically coupled to an end of a third longitudinal segment  526  opposite the second electrode  516 . The second active longitudinal segments  524  may have a length that is relatively short compared to a length of the first longitudinal segments  522  and the third longitudinal segments  526 . 
     The first longitudinal segments  522  may comprise an n-type semiconductor material, the second active longitudinal segments  524  may comprise a first p-type material, and the third longitudinal segments  526  may comprise a second p-type material. The first p-type material of the second active longitudinal segments  524  may differ from both the n-type semiconductor material of the first longitudinal segments  522  and the second p-type material of the third longitudinal segments  526 . The bandgap energy in the second active longitudinal segments  524  may be different from the bandgap energy in the first longitudinal segments  522  and the third longitudinal segments  526 . 
     In this configuration, each interface or junction  530  between a first longitudinal segment  522  and a second active longitudinal segment  524  may comprise a heterojunction, and each interface or junction  532  between a second active longitudinal segment  524  and a third longitudinal segment  526  also may comprise a heterojunction, thereby providing a double-heterostructure device. Furthermore, the bandgap energy in the first longitudinal segments  522  may be the same as, or different from, the bandgap energy in the third longitudinal segments  526 . 
     The bandgap energy in the second active longitudinal segments  524  may be less than the bandgap energy in the first longitudinal segments  522  and the third longitudinal segments  526 . 
     The first electrode  512  may include an n-type semiconductor material, which may be identical to the n-type semiconductor material of the first longitudinal segments  522 . The second electrode  516  may include a p-type semiconductor material, which may be identical to the second p-type semiconductor material of the third longitudinal segments  526 . 
     By way of example and not limitation, the n-type semiconductor material of the first longitudinal segments  522 , the first p-type semiconductor material of the second active longitudinal segments  524 , and the second p-type semiconductor material of the third longitudinal segments  526  each may include a silicon-based material or a germanium-based material. 
     In alternative embodiments of the invention, the n-type semiconductor material of the first longitudinal segments  522 , the first p-type semiconductor material of the second active longitudinal segments  524 , and the second p-type semiconductor material of the third longitudinal segments  526  each may include a III-V type semiconductor material or a II-VI type semiconductor material. Furthermore, each nanowire  520  may be doped with or otherwise include erbium ions or boron ions. 
       FIG. 9A  illustrates a simplified exemplary energy band diagram of a nanowire  520  in the absence of a bias across the nanowire  520  (i.e., there is no voltage applied between the first electrode  512  and the second electrode  516 ). The energy band diagram illustrates an example of the general relationship which may be provided between the energy levels of the conduction bands E c  and the energy levels of the valence bands E v  in each of the first longitudinal segments  522 , the second active longitudinal segments  524 , and the third longitudinal segments  526 . In the diagram, increasing energy is represented in the positive (vertically upwards) direction along the Y axis and the relative position along the nanowire  520  is represented along the X axis. The bandgap energy in each segment is the difference between the energy level of the conduction band E c  and the energy level of the valence band E v  in that segment (i.e., bandgap energy=E c −E v ). 
       FIG. 9B  is substantially similar to  FIG. 9A , but illustrates an example of the general relationship which may be provided between the energy levels of the conduction bands E c  and the energy levels of the valence bands E v  in each of the first longitudinal segments  522 , the second active longitudinal segments  524 , and the third longitudinal segments  526  in the presence of an applied forward bias (i.e., when a voltage difference is applied between the first electrode  512  and the second electrode  516 ). 
     As can be seen by comparing the energy band diagram shown in  FIG. 9A  with the energy band diagram shown in  FIG. 9B , the difference between the energy level of the conduction band E c  in the first longitudinal segment  522  and the energy level of the conduction band E c  in the second active longitudinal segment  524  may be reduced in the presence of a forward bias. This difference between the energy level of the conduction band E c  in the first longitudinal segment  522  and the energy level of the conduction band E c  in the second active longitudinal segment  524  may provide an energy barrier that must be overcome by electrons passing from the first longitudinal segment  522  into the second active longitudinal segment  524 . When no voltage is applied between the first electrode  512  and the second electrode  516 , this energy barrier may be too high for the electrons in the conduction band of the first longitudinal segment  522  to overcome, and the electrons may be prevented from being injected into the second active longitudinal segment  524  of the nanowire  520  across the junction  530 . In the presence of a forward bias, however, the energy barrier may be significantly reduced, and electrons may be injected into the second active longitudinal segment  524  of the nanowire  520  across the junction  530 . These electrons may recombine with holes in the second active longitudinal segment  524  of the nanowire  520 . Photons of electromagnetic radiation may be emitted by the nanowire  520  upon recombination of the electrons with the holes in the second active longitudinal segment  524  of the nanowire  520 . 
     As can be seen with reference to  FIG. 9B , the difference between the energy level of the conduction band E c  in the third longitudinal segment  526  and the energy level of the conduction band E c  in the second active longitudinal segment  524  may be large enough (even in the presence of a forward bias) to prevent electrons in the conduction band of the second active longitudinal segment  524  from being injected into the third longitudinal segment  526  of the nanowire  520  across the junction  532 . In other words, electrons in the conduction band may be confined to the conduction band of the second active longitudinal segment  524  due to the energy barrier between the conduction band of the second active longitudinal segment  524  and the conduction band of the third longitudinal segment  526 . By confining the electrons in the conduction band to the relatively short second active longitudinal segment  524  of the nanowire  520 , a relatively high concentration of charge carriers (electrons and holes) may be provided, which may enhance recombination and emission of electromagnetic radiation. 
     In alternative embodiments of the invention, the general relationship between the energy band structures of the longitudinal segments  522 ,  524 ,  526  of each nanowire  520  may be as depicted in  FIG. 9C . The general relationship between the energy band structures of the longitudinal segments  522 ,  524 ,  526  as depicted in  FIG. 9C  is substantially similar to the general relationship between the energy band structures of the longitudinal segments  522 ,  524 ,  526  as depicted in  FIG. 9B . As seen in  FIG. 9C , however, the energy level of the conduction band E c  in the second active longitudinal segments  524  may be lower than the energy level of the conduction band E c  in the first active longitudinal segments  522  when a forward bias is applied across the nanowires  520 . In such a configuration, electrons in the conduction band may be further confined to the conduction band of the second active longitudinal segments  524  due to the energy barrier between the conduction band of the second active longitudinal segments  524  and the conduction band of the first longitudinal segments  522 . By further confining the electrons in the conduction band to the relatively short second active longitudinal segments  524  of the nanowires  520 , a concentration of charge carriers (electrons and holes) may be further increased, which may further enhance recombination and emission of electromagnetic radiation. 
     As is known in the art, the energy difference between the energy of the electrons before recombination (the energy of the electron in the conduction band) and the energy of the electrons after recombination (the energy of the electron in the valence band) may define the wavelengths of electromagnetic radiation that are emitted by the nanowire  520 . In some embodiments of the invention, the second p-type material of the second active longitudinal segment  524  of the nanowire  520  may be selected to exhibit a bandgap energy in a range extending from about 1.5 electron volts (eV) to about 3.0 electron volts (eV). In this configuration, the nanowire  520  may be configured to emit electromagnetic radiation having a wavelength in a range extending from about 400 nanometers to about 800 nanometers when a voltage is applied across the nanowire  520  between the first electrode  512  and the second electrode  516 . 
     Optionally, the exterior surfaces of the nanowires  520  and, in particular, the second active longitudinal segments  524  of the nanowires  520  may be passivated to minimize surface recombination between electrons and holes. By way of example and not limitation, if the second active longitudinal segments  524  of the nanowires  520  comprises a germanium-based material, the surfaces of the second active longitudinal segments  524  of the nanowires  520  may be passivated by depositing or growing a thin layer of a silicon-based material on the surfaces of the second active longitudinal segments  524  of the nanowires  520 . 
     The radiation device  510  may further include a resonant cavity. By way of example and not limitation, the first electrode  512  may include a substantially vertical sidewall  514  and the second electrode  516  may include an opposing, substantially vertical, sidewall  518 , as shown in  FIG. 8B . The substantially vertical sidewall  514  and the substantially vertical sidewall  518  may be substantially parallel to one another, and the materials of the first electrode  512  and the second electrode  516  may be at least partly reflective with respect to the electromagnetic radiation emitted by the nanowires  520 . In this configuration, the first electrode  512  and the second electrode  516  may define a Fabry-Perot resonant cavity that behaves in a manner similar to that previously described in relation to the Fabry-Perot resonant cavity  112  of the device  110  shown in  FIG. 1 . 
     Furthermore, the distance separating the first electrode  512  and the second electrode  516  may be selected to be equal to an integer multiple of one-half of a wavelength of electromagnetic radiation that is emitted by the nanowires  520 , which are disposed within the resonant cavity. In this configuration, the resonant cavity may be configured to resonate at least one wavelength of electromagnetic radiation that is emitted by the nanowires  520 . Optionally, a thin layer of reflective material may be applied to at least one of the substantially vertical sidewall  514  and the substantially vertical sidewall  518 . 
     As will be obvious to those of ordinary skill in the art, the light-emitting device  510  shown in  FIGS. 8A-8B  may be fabricated using the methods previously described herein in relation to the device  110  and  FIGS. 4A-4I . 
     The devices and methods described herein provide improved devices that may be used in a wide variety of devices and applications. By way of example and not limitation, such devices and methods may be used in light-emitting diodes, optical amplifiers, lasers, optical signal processing devices, and molecular sensors. 
     Although the foregoing description contains many specifics, these are not to be construed as limiting the scope of the present invention, but merely as providing certain representative embodiments. Similarly, other embodiments of the invention can be devised which do not depart from the spirit or scope of the present invention. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions, and modifications to the invention, as disclosed herein, which fall within the meaning and scope of the claims, are encompassed by the present invention.