Abstract:
An apparatus and method pertaining to a perpetual energy harvester. The harvester absorbs ambient infrared radiation and provides continual power regardless of the environment. The device seeks to harvest the largely overlooked blackbody radiation through use of a semiconductor thermal harvester.

Description:
PARENT CASE TEXT 
       [0001]    This is a divisional of application(s) Ser. No. 13/831,840 filed on Mar. 15, 2013. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    This invention was made with United States Government support under W31P4Q10C0034 awarded by US Army Contracting Command and also under W91CRB11C0097 awarded by US Army Contracting Command. As such, the United States Government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention pertains to harvesting ambient radiation from the infrared spectrum to generate power. More particularly this invention is related to a semiconductor perpetual thermal harvesting device that will continually harvest and provide continuous power during both day and night in any environment. 
       BACKGROUND OF INVENTION 
       [0004]    Energy harvesting has been around for many years in the form of windmills and watermills. Modern technology has transformed them into wind turbines, hydro-electric plants, and solar panel arrays. These methods of harvesting energy offers two significant advantages over battery powered solutions: virtually inexhaustible resources and little or no adverse environmental effects. 
         [0005]    However, the various technologies used in large scale energy harvesting all require a connection to the power grid. Given the trend towards wireless systems, the continuously powered device which never needs to be recharged by connecting to the power grid is the ultimate goal. While ultra-low-power technology is developing, current radiation harvesting devices are very limited. The traditional crystalline silicon, cadmium telluride, and copper indium gallium selenide solar cells only provide power during the daytime. None of the current solutions can provide continuous, around the clock energy harvesting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  depicts a thermal harvester cross-section with multiple junctions. 
           [0007]      FIG. 2  depicts a current-voltage characteristic chart for multiple harvester junctions in series. 
           [0008]      FIGS. 3A-F  depicts various compositions of the thermal harvester&#39;s absorption layer. 
           [0009]      FIGS. 4A-C  depicts various cutoff wavelength configurations for the thermal harvester&#39;s junctions. 
           [0010]      FIG. 5A-I  depicts a process for manufacturing a thermal harvester with single or multiple junctions. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Reference is made in detail to the preferred embodiments of the invention. While the invention is described in conjunction with the preferred embodiments, the invention is not intended to be limited by these preferred embodiments. The contemplated embodiments for carrying out the present invention are described in turn with reference to the accompanying figures. 
         [0012]    For the purposes this invention, perpetual is defined to be occurring continuously, independent of time, location, or temperature. Energy harvesting for the purposes of this invention is defined to be absorbing radiation and using it to generate a direct current. The term electrically connected is defined to encompass an electrical current flow, including bidirectional, unidirectional, or any hybrid current flow, such as an uneven current. 
         [0013]    A photovoltaic device is defined to be a device that absorbs photons to generate a current. The absorption of the photons provides the electrons with the energy to jump the bandgap between the valance band and the conduction band, leaving a positive charge called a hole behind. Each bandgap has a minimum amount of energy required by the electrons to jump the bandgap and the amount may vary depending on the material. The minimum amount of energy may be translated to a cutoff wavelength using Plank&#39;s relation: 
         [0000]    
       
         
           
             
               E 
               = 
               
                 hc 
                 λ 
               
             
             , 
           
         
       
     
         [0000]    where h is Plank&#39;s constant, E is energy, and c is the speed of light. Photons with wavelengths longer than the cutoff wavelength will not provide the necessary energy required to surmount the material&#39;s bandgap and will pass through the material. 
         [0014]    Photovoltaic devices typically comprise semiconductors configured in p-n or p-i-n junctions. A p-n junction is formed by joining p-type and n-type semiconductors in close contact. A p-i-n junction is similar to a p-n junction, but includes a very low doped intrinsic layer between the p-type and n-type semiconductors. Due to the electric field generated by the diffusion regions of the p-n or p-i-n junctions the electrons and holes are moved in opposite directions generating a current. 
         [0015]    Radiation is defined to be the electromagnetic spectrum, particularly the near ultraviolet, visible, near infrared, short-wave infrared, mid-wave infrared, long-wave infrared and far infrared bands. The near ultraviolet band is comprised of wavelengths from about 300 to 400 nm. The visible light band is comprised of wavelengths from about 400 to 780 nm. The near infrared band is comprised of wavelengths from about 0.78 to 1 μm. The short-wave infrared band is comprised of wavelengths from about 1 to 3 μm. The mid-wave infrared band is comprised of wavelengths from about 3 to 6 μm. The long-wave infrared band is comprised of wavelengths from 6 to 14 μm. The far infrared band is comprised of wavelengths from about 14 to 40 μm. 
         [0016]    Conventional energy harvesting devices are limited to daytime use, because of their sole focus on the visible spectrum. Blackbody radiation has been largely overlooked. The blackbody radiation spectrum ranges from near ultraviolet to the infrared spectrum. The available blackbody radiation can be calculated through Planck&#39;s blackbody equation, 
         [0000]    
       
         
           
             
               
                 W 
                  
                 
                   ( 
                   λ 
                   ) 
                 
               
               = 
               
                 
                   c 
                   1 
                 
                 
                   
                     λ 
                     5 
                   
                    
                   
                     ( 
                     
                       
                         e 
                         
                           c 
                           
                             2 
                             / 
                             
                               λ 
                                
                               T 
                             
                           
                         
                       
                       - 
                       1 
                     
                     ) 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where c 1 =2πc 2 h=37418.32 Wμ 4 cm −2  and c 2 =hc/k=14387.86 μK. In c 1  and c 2 , c is the speed of light, k is Boltzmann&#39;s constant, h is Planck&#39;s constant, k is wavelength, and T is temperature in Kelvin. As temperature increases, power (in watts per meters squared) increases at lower wavelengths. Using Stefan-Boltzmann&#39;s law, 5.67×10 −8 ×T 4 , the total available blackbody radiation at a given temperature can be calculated. At room temperature, about 300 degrees Kelvin, the total available blackbody radiation is estimated to be 500 W/m 2  with a peak between 4 to 10 nm. 
         [0017]    Utilizing Planck&#39;s blackbody equation, the amount of power available at a given wavelength and temperature can be calculated. This aids in determining what bandgap to select for the p-n or p-i-n junction to set the cutoff wavelength. 
         [0018]    A major difficulty to overcome is the fact that intrinsic carriers cause excessive dark current at high temperature. Infrared detectors share the same problem and are normally operated at 77 degrees Kelvin to minimize dark current. In energy harvesting, dark current will subtract from harvested energy. A solution to this problem will be described under  FIG. 2 . 
         [0019]      FIG. 1  illustrates one embodiment of the invention, a cross-section of a semiconductor thermal harvester comprising a single junction. The thermal harvester is a photovoltaic device that absorbs photons with wavelengths in the infrared spectrum. The thermal radiation  160  is comprised of photons with wavelengths in the infrared spectrum comprised of the near infrared, short-wave infrared, mid-wave infrared, long-wave infrared, and far infrared bands. 
         [0020]    The structure of the thermal harvester comprises a substrate  100 . The substrate  100  may be comprised of group IV semiconductors, group III-V semiconductors, or group II-VI semiconductors. Group IV semiconductors are not limited to but may include Si, polysilicon, SiC, Ge, or SiGe alloy. Group III-V semiconductors are not limited to but may include AlN, GaAs, GaN, GaSb, or InSb. Group II-VI semiconductors are not limited to but may include CdZnTe, HgCdTe, HgZnTe, ZnO, ZnS, CdS, or CdTe. Optionally the buffer may be doped to be either n-type or p-type. Alternatively, the substrate  100  may be comprised of silicon on insulator (SOI), metal, dielectric, insulator, or polymer. The material comprising the substrate  100  may be transparent or partially transparent to all or a portion of the spectrum comprising the thermal radiation  160 . 
         [0021]    Optionally, one surface of the substrate  100  may be comprised of grooves  105  which serve an antireflective purpose to focus the thermal radiation  160  and direct it through the substrate  100  and subsequent layers. Optionally, a nanoscaled pattern may be used instead of grooves  105  to accomplish the same purpose. Optionally, either in addition or alternatively to the grooves  105  or pattern, an anti-reflective coating  110  may also be attached to the same surface as the pattern to minimize the amount of thermal radiation  160  reflected and to direct the thermal radiation  160  through the substrate  100  and subsequent layers. The anti-reflective coating  110  may be comprised of a single or multiple layers of anti-reflective material and may be planar or non-planar. Optionally, the anti-reflective coating  110  may be comprised of nanocrystals or photonic crystals to control the direction of light propagation. The anti-reflective coating may be comprised of SiO 2 , SiN x , ZnO, metal oxides, insulators, or semiconductors. 
         [0022]    Optionally, a buffer layer  117  may be included on the opposite surface of the substrate  100  from the coating. The buffer layer  117  may be comprised of group IV semiconductors, group III-V semiconductors, or group II-VI semiconductors. Group IV semiconductors are not limited to but may include Si, polysilicon, SiC, Ge, or SiGe alloy. Group III-V semiconductors are not limited to but may include AlN, GaAs, GaN, InP, GaSb, or InSb. Group II-VI semiconductors are not limited to but may include CdZnTe, HgCdTe, ZnO, ZnS, CdS, or CdTe. 
         [0023]    Optionally, the buffer layer  117  may be a similar material system or a dissimilar material system from the substrate  100 . A similar material system is defined to be where the element or compound comprising one material are the same as the element or compound comprising a second material. A similar material system includes compounds comprised of the same elements but at different ratios. In a similar material system, the materials need not be doped to be the same type. For example, but not to be construed as a limitation, a similar material system may be comprised of a n-type InP substrate and a n-type InP buffer layer. A dissimilar material system is defined to be where the element or compound comprising one material are different from the element or compound comprising a second material. In a dissimilar material system, the materials need not be doped the same type. For example, but not to be construed as a limitation, a dissimilar material system may be comprised of a n-type Si substrate and a n-type CdTe buffer layer. In the present embodiment, for either a similar or dissimilar material system, the buffer layer  117  is doped to be the same type as the substrate  100 . 
         [0024]    Attached to the substrate  100  or the buffer layer  117  is an absorption layer  115  comprising two or more junctions  125  with cutoff wavelengths in the near infrared, short wave infrared, mid wave infrared, long wave infrared, or far infrared bands. The absorption layer  115  may be comprised of HgCdTe, HgZnTe, InSb, InAs, GaSb, PbTe, GaAs or polymer material systems. The absorption layer  115  is comprised of a first material  120  and a second material  130 . The first material  120  nay doped either to be either p-type or n-type. The second material  130  may be located adjacent to the first material  120  and doped to be the opposite type, creating a p-n junction. 
         [0025]    Another junction  125  may be formed by including a third material  135  in the absorption layer  115 . The third material  135  may be located adjacent to the second material  130  and doped such that a p-n junction is created between the second and third materials. Optionally, additional junctions  125  may be formed by incorporating additional materials within the absorption layer  115  and doping the additional materials such that the resulting junctions  125  are p-n junctions. 
         [0026]    For example, but not to be construed as a limitation, in a similar material system absorption layer, if the first material  120  is p-type HgCdTe, the second material  130  is then n-type HgCdTe, and the third material  135  is p-type HgCdTe, creating p-n junctions between each layer. In creating additional junctions  125 , the fourth material  140  is n-type HgCdTe, resulting in an additional p-n junction. 
         [0027]    Optionally, an intrinsic semiconductor material, not shown, may be included between the p-type and n-type materials to create p-i-n junctions. While  FIG. 1  illustrates a three junction thermal harvester, any number of junctions may be used. 
         [0028]    The junctions  125  within the absorption layer  115  are contemplated to be homo-junctions with each junction  125  comprised of similar material systems. Optionally, each junction may be comprised of different materials from the other junctions. For example, but not to serve as a limitation, a two junction thermal harvester may be comprised of a HgCdTe junction and a InSb junction. The HgCdTe junction is comprised of a p-type HgCdTe material and an n-type HgCdTe material. The InSb junction is comprised of a n-type InSb material and a p-type InSb material. 
         [0029]    Optionally hetero-junctions may be used, where the first material  120  and second material  130  composing the junction are comprised of different materials. For example, but not to serve as a limitation, of a hetero-junction configuration of a two junction thermal harvester may be comprised of three materials. The first material  120  may be comprised of p-type HgCdTe, the second material  130  may be comprised of n-type InSb, and the third material  135  may be comprised of p-type PbTe. 
         [0030]    Each junction  125  is designed to absorb radiation within a targeted or desired range. Each range is defined by the longest wavelength of photons that can be absorbed, called the cutoff wavelength. The cutoff wavelengths for this embodiment of the invention are located within the near infrared, short wave infrared, mid wave infrared, long wave infrared, and far infrared bands. The cutoff wavelength of the junction may be adjusted by varying the concentration of an element of the semiconductor material comprising the absorption layer  115 . For example, but not to serve as a limitation, in a contemplated use of Hg 1-x Cd x Te, adjusting the concentration of Cd in the compound determines the bandgap of the material. The bandgap may range from 0 to 1.5 eV, with higher concentrations of Cd resulting in a larger bandgap. Thus, the desired cutoff wavelengths at each junction may be obtained through selecting the appropriate concentration of Cd for the material comprising the junction. Each material in the absorption layer may comprise either different or similar concentrations of Cd. Optionally, the desired cutoff wavelength may also be adjusted by altering the thickness of the semiconductor materials. In the present embodiment, the thickness of the materials is such that electron tunneling will occur. 
         [0031]    Any intrinsic material in the absorption layer  115  will be as doped as low as possible, generally a concentration of 10 15 /cm 3  or less. The p-type and n-type materials will contain high doping concentrations, typically ranging from 10 17 /cm 3  to 10 18 /cm 3 , but not to the levels of degradation at room temperature. 
         [0032]    Attached to the absorption layer  115 , on the opposite surface from the substrate  100 , is an insulator layer  145 . The insulator layer  145  may be comprised of an oxide layer or metal. A first electrode  150  is electrically connected to the material in the absorption layer  115  adjacent to the insulator layer  145 . Alternatively, in the case that metal is used for the insulator layer  145 , the metal may serve as the first electrode  150 . 
         [0033]    A second electrode  155  is insulated from all other materials and electrically connected to the first material  120 . Alternatively, the second electrode  155  may be electrically connected to either the substrate  100  or buffer layer  117 . Alternatively, in the case that metal is used for a substrate  100 , the substrate  100  may serve as the electrode  155 . The electrodes may be comprised of any suitably conductive material. 
         [0034]    Optionally, while the figure illustrates an electrode configuration that passes through the insulator layer  145 , other electrode arrangements may be used resulting in a circuit comprising the junctions  125  in series between the first and second electrodes. For example, but not to serve as a limitation, in one configuration both electrodes may be configured to enter the device through the substrate  100 ; in another configuration one electrode enters from the insulator layer and the other from the substrate; or in a third configuration the electrodes may be planar layers located on opposite sides of the absorption layer  115 . 
         [0035]    Optionally, in an alternate embodiment the absorption layer  115  may be comprised of an electrolyte layer, not shown. The electrolyte layer may be comprised of any electrolyte material containing free ions. The electrodes electrically connected to the electrolyte layer comprise an anode and a cathode. The anode, the cathode, or both may serve as a photo-electrode. In one embodiment of the electrolyte layer, the cathode may be comprised of metal and the anode may be a photo-anode comprised of an n-type semiconductor. Optionally, if the buffer layer  117  or substrate  100  is n-type, it may serve as a photo-anode. In an alternate embodiment, the anode may be a photo-anode comprised of a n-type semiconductor and the cathode may be a photo-cathode comprised of a p-type semiconductor. Optionally, if the buffer layer  117  or substrate  100  is n-type, it may serve as a photo-anode. Alternatively, if the buffer layer  117  or substrate  100  is p-type, it may serve as a photo-cathode. 
         [0036]    In another alternate embodiment the anode may be comprised of metal and the cathode may be a photo-cathode comprised of a p-type semiconductor. Optionally, if the buffer layer  117  or substrate  100  is p-type, it may serve as a photo-cathode. In all the electrolyte layer embodiments, the electrolyte and the photo-electrodes absorb the incoming radiation and generate electricity. 
         [0037]    Optionally, in an alternate embodiment, the absorption layer  115  may be comprised of a dye-sensitized layer comprising nanoparticles coated in radiation-sensitive dye and an electrolyte material. The nanoparticles may be comprised of various metals or metal oxides. For example, but not to serve as a limitation, the materials comprising the nanoparticles may be TiO 2 , ZnO, etc. The dye-sensitized layer may serve as the anode and a separate electrode may serve as a cathode. The cathode may be comprised of any metal, e.g. platinum, cobalt sulfide, etc. Optionally, the absorption layer may be comprised of a hybridized material system, resulting from a combination of dye, electrolyte, and/or semiconductor materials. 
         [0038]    The device is oriented to absorb thermal radiation  160  arriving from the indicated direction, first passing through the substrate  100 . In a separate embodiment, not shown, if the thermal radiation  160  is arriving from the direction of the insulator layer  145 , the insulator layer  145  may be comprised of a material that is transparent to the thermal radiation  150 . If metal is being used as an insulator layer  135 , the metal may be transparent or partially transparent to the thermal radiation  160  and conductive. For example, but not to serve as a limitation, the metal may be comprised of indium tin oxide (ITO) or SnO 2 . Optionally, an antireflective coating may be applied on the surface of the insulator layer facing the incoming thermal radiation  160 . The anti-reflective coating  110  may be comprised of a single or multiple layers of anti-reflective material and may be planar or non-planar. Optionally, the anti-reflective coating  110  may be comprised of nanocrystals, photonic crystals, or a mixture of both to control the direction of light propagation. The anti-reflective coating may be comprised of SiO 2 , SiN x , ZnO, metal oxides, insulators, or semiconductors. 
         [0039]      FIG. 2  depicts the current-voltage characteristic of junctions in a multiple junction energy harvester, both individually and connected in series. Each individual junction may be represented as a diode with its own current-voltage (I-V) characteristic. The diodes A, B, and C illustrate that each junction generates a current with a different magnitude. Multiple junctions stacked together may be represented as diodes connected in series. The result is increased open circuit voltage but the magnitude of the current is equal to the lowest magnitude current generated by the diodes, leading to increased power generation. For example, but not to serve as a limitation, when diodes A, B, and C are connected in series, the voltage, V d , is increased from under 0.1 V, for each individual diode, to over 0.2 V. The current of the diodes in series is then equal to the current generated by diode A, the lowest magnitude current. This increased open circuit voltage minimizes the problem excessive dark current or reverse bias leakage current causes in intrinsic carriers at high temperatures because dark current subtracts from harvested energy. With no limitation to low operating temperatures to minimize the dark current, the perpetual energy harvester has the ability to operate in any environment. 
         [0040]      FIG. 3A-F  illustrates various embodiments of the absorption layer  115  for the thermal harvester. The electrodes in these embodiments are not shown, but are configured as described in  FIG. 1 .  FIG. 3A  illustrates planarized or level materials on top of the substrate  100 . The material comprising the absorption layer  115  are created with controlled thickness and density. The materials comprising the absorption layer  115  are as described in  FIG. 1 . While the figure shows materials of even thickness or width, the width of the material may be altered depending on the desired cutoff wavelength. 
         [0041]      FIG. 3B  illustrates an alternative embodiment of the absorption layer  115 . The materials comprising the absorption layer  115  are patterned using three-dimensional structures to increase the surface area of the junctions. The thickness and density of the material comprising the junctions are controlled. This may be done through a top down fabrication approach where each layer is planarized and etched prior to the creation of the subsequent layers. Alternatively, the patterns may be formed through a controlled epitaxial growth process. While the figure depicts the three-dimensional structure patterns as rectangular waves, other patterns may be used that increase the surface area while controlling thickness and density of the materials. For example, but not to act as a limitation, sinusoidal waves, triangular waves, arc-shaped waves, columns, cylinders, cones, pyramids, polygonal structures, etc. 
         [0042]      FIG. 3C  illustrates another embodiment of a patterned absorption layer  115 . The materials comprising the absorption layer  115  are not controlled in their density or thickness and may be created using a bottom-up approach, where each material utilizes a self assembled monolayer. Alternatively, other self-assembly techniques may be used. While the figure depicts a three-dimensional structure wave pattern, other three-dimensional structure patterns may be used which increase the surface area. For example, but not to act as a limitation, sinusoidal waves, triangular waves, rectangular waves, arc-shaped waves, columns, cylinders, cones, pyramids, polygonal structures, etc. 
         [0043]      FIG. 3D  illustrates another embodiment of the absorption layer  115  with trap structures  300 . The trap structures  300  may include any type of formation which traps radiation by reflecting, refracting, or scattering photons within the formation until the photons are absorbed by a junction. The formations may be comprised of gaps, as illustrated in  FIG. 3D , or patterned junctions, as illustrated in  FIGS. 3B  and C. While the trap structures  300  depicted in the embodiment are gaps in the form of columns, other shapes or configurations may be used which trap radiation by reflecting, refracting, or scattering photons. The materials comprising the trap structures  300  may be air, which results in gaps in the insulator layer between the formations. 
         [0044]      FIG. 3E  illustrates an alternate embodiment where the trap structures  300  may be comprised of nanocrystals or nanostructures  305 , materials transparent to the wavelengths of radiation to be trapped, and/or materials comprising insulating properties. The nanocrystals or nanostructures  305  may be comprised of nanoparticles, nanowires, nanodots, nanorods, nanotubes, branched nanostructures, nanobipods, nanotripods, nanotetrapods, quantum dots, nanopillars, H shaped structures, cavity structures, crescent structures, chiral or asymmetric structures, or other structural configurations or any combination of the aforementioned. The materials comprising the nanocrystals or nanostructures  305  may include semiconductors, metals, dielectrics, ferroelectrics, insulators, or a combination of materials. Optionally, the nanocrystals or nanostructures  305  may be the same size or varying sizes. The nanocrystals or nanostructures  305  may till the trap structure and/or be embedded into a polymer or insulating material. The material in which the nanocrystals or nanostructures  305  are embedded is transparent to the wavelengths of radiation to be trapped. The nanocrystals or nanostructures  305  assist in enhancing the scattering of photons within the trap structure  300 . 
         [0045]    The materials comprising insulating properties may include compounds, nanocrystals, embedded nanostructures, or any nanostructure having insulating properties. For example, but not to serve as a limitation, compounds comprising insulating properties may include SiO 2 , AlN, AlO 3 , or SiN X .  FIG. 4F  illustrates an embodiment of nanocrystals, embedded nanostructors, or nanostructures having insulating properties. The nanocrystals may be comprised solely of an insulator material  305   a.  Alternatively, the nanostructure may be comprised of a core-shell configuration. The core may be comprised of a metal, semiconductor, dielectric, insulator, or ferroelectric material. The shell surrounding the core may be an insulator coating  305   b.  Alternatively, the trap structure may be filled with insulating material and the nanostructure may be comprised of metal embedded into the insulating material  305   c.  In addition to aiding the reflection, refracting, or scattering of the trap structure, utilizing material with insulating properties also prevents potential short circuits. 
         [0046]    The trapping, reflecting, refracting, or scattering of the radiation within the trap structures  300  until the radiation is absorbed by a junction increases the efficiency of the thermal harvester. In addition, the trap structures  300  slightly increase the cutoff wavelength for each junction, allowing each junction to absorb a slightly wider range of radiation. Moreover, while the figure illustrates an absorption layer  115  comprised of planar materials, trap structures may also be made in absorption layers  115  not comprised of planar materials. 
         [0047]      FIG. 4A-C  depicts alternative embodiments for configuring the junctions  125  in a multi-layered harvester&#39;s absorption layer  115 , which are described above in  FIG. 1 . The electrodes in these embodiments are not shown, but are configured as described in  FIG. 1 . For clarity of explanation, the shaded layers in this figure represent junctions  125 , not materials. While each figure depicts five junctions, any number of junctions play be used. 
         [0048]      FIG. 4A  illustrates an embodiment of the absorption layer  115  where the junctions  125  are arranged in ascending order of cutoff wavelengths in relation to the direction of the incoming radiation  160 . In the depicted embodiment, the radiation  160  approaches the device from the direction of the substrate  100 . The junction with the shortest cutoff wavelength  400  is located closest to the substrate  100 . The junction with the second shortest cutoff wavelength  405  is adjacent to the junction with the shortest cutoff wavelength  400 . The sequence continues until the junction with the longest cutoff wavelength  420  is located furthest from the substrate  100  and adjacent to the insulator layer  145 . 
         [0049]    For example, but not to serve as a limitation, each layer may be designed to absorb radiation in different bands. The shortest cutoff wavelength junction  400  may be designed to absorb radiation in the near infrared band. The next junction  405  may be designed to absorb radiation in the short-wave infrared band. The following junction  410  may be designed to absorb radiation in the mid-wave infrared band. The subsequent junction  415  may be designed to absorb radiation in the long-wave infrared band. The last junction  420  may be designed to absorb radiation in the far infrared band. 
         [0050]    In an alternate embodiment where the radiation  160  approaches the device from the insulator layer  145 , not shown, the junction with shortest cutoff wavelength  400  is located adjacent to the insulator layer  145 . The subsequent junctions are arranged in order of ascending cutoff wavelengths with the junction with the longest cutoff wavelength  420  adjacent to the substrate  100 . 
         [0051]      FIG. 4B  illustrates an embodiment of the absorption layer  115  with junctions  125  in a long central wavelength configuration, where the longest cutoff wavelength junction  435  is located between junctions that have shorter cutoff wavelengths. In the depicted embodiment, the junction with the longest cutoff wavelength  435  is in the center and is surrounded by the junctions with the next longest cutoff wavelengths  430  and  440 . The junctions with the shortest cutoff wavelengths  425  and  445  are located furthest away from the center layer  435 . The junctions not located in the center may have the same or different cutoff wavelengths. Alternatively, the junctions may be arranged in ascending cutoff wavelengths until reaching the longest cutoff wavelength junction  435  and begin ascending again starting with the junctions with the shortest cutoff wavelength. 
         [0052]    The reflective layer  450  is comprised of a material which reflects the radiation back through the absorption layer  115 . Examples of this material may include various types of metals or metamaterials, including electromagnetic bandgap metamaterials such as photonic crystals or left handed materials which control the direction of light propagation. This configuration increases efficiency by allowing energy to be absorbed from radiation coming from multiple directions. For example, but not to serve as a limitation, the radiation to be harvested may arrive by passing through the insulator layer  145 , be reflected after passing unabsorbed through the junctions  125 , or pass through the substrate  100 . 
         [0053]      FIG. 4C  illustrates an embodiment for the absorption layer  115  in a short central wavelength configuration, where the longest cutoff wavelength junctions  455  and  475  are located adjacent to the substrate  100  and reflective layer  450 , with the shorter cutoff wavelength junctions  460 ,  465 , and  470  located in between. In the present depiction, the junction with the shortest cutoff wavelength  465  is located in the center. Optionally, this embodiment may apply to any configuration where a junction with a shorter cutoff wavelength is between junctions with longer cutoff wavelengths. This configuration has similar capabilities as the configuration described in  FIG. 4B . 
         [0054]      FIG. 5A-I  illustrates a process for creating both single and multiple layered thermal harvesters. The electrodes in this process are not shown, but are configured as described in  FIG. 1 .  FIG. 5A  illustrates the beginning of the process with a substrate  500 . The substrate  500  may be comprised of group IV semiconductors, group III-V semiconductors, group II-VI semiconductors, SOI, dielectric, insulator, polymer, or metal. The group IV semiconductors may include, but are not limited to, Si, polysilicon, SiC, Ge, or Si—Ge alloy. Group compounds may be comprised of AlN, GaAs, GaN, InP, GaSb, or InSb. Group II-VI compounds may be comprised of CdZnTe, HgCdTe, mercury HgZnTe, ZnO, ZnS, CdS, or CdTe. The substrate  500  is subject to an oxidization process as shown in  FIG. 5B . The process creates an oxidized layer  505  on both the top and bottom surfaces of the substrate. 
         [0055]    Optionally,  FIG. 5C  one surface of the substrate is patterned and etched using a lithographic process, creating grooves  510  or a pattern that acts as an anti-reflective surface to direct the radiation. Alternatively, instead of etching the pattern into the substrate  500 , the pattern may be created by attaching a layer of material comprising the pattern to the substrate  500 . The addition of a layer comprising a pattern may be done through wafer bonding, growth, or deposition techniques.  FIG. 5D  shows the removal of the remaining oxide material prior to proceeding with additional steps.  FIG. 5E  illustrates an optional step of depositing an antireflective coating  515  on the same side as the grooves  510  to function as an antireflective surface. The antireflective coating  515  may be comprised of multiple layers or a nano-crystal layer. Alternatively, an oxide layer may be used instead of the antireflective coating  515 . 
         [0056]      FIG. 5F  depicts the removal of the oxide layer  505  from the non-etched surface. Optionally after the removal of the oxide layer  505 , a buffer layer  520  may be added. The buffer layer  520  may be comprised of group IV semiconductors, group III-V semiconductors, or group II-VI semiconductors. Group IV semiconductors are not limited to but may include Si, polysilicon, SiC, Ge, or SiGe alloy. Group III-V semiconductors are not limited to but may include AlN, GaAs, GaN, InP, GaSb, or InSb. Group II-VI semiconductors are not limited to but may include CdZnTe, HgCdTe, HgZnTe, ZnO, ZnS, CdS, or CdTe. Optionally, the buffer layer  520  may be a similar material system or a dissimilar material system from the substrate  500 . In either material system, the butler layer is doped to be the same type as the substrate  500 . 
         [0057]    The buffer layer  520  may be epitaxially grown, deposited, or wafer bonded to the substrate  500 . A absorption layer  525 , comprising at least one junction, is created either directly on the substrate  500  or the buffer layer  525 . The absorption layer  525  may be comprised of p-n or p-i-n junctions. The absorption layer  525  may be created by growing a semiconductor material  530 , which may be doped either p-type or n-type. A second semiconductor material  540  may be grown on and doped to be the opposing type from the first semiconductor material  530  to create a p-n junction. Optionally, an intrinsic material  535  may be included between the p-type and n-type semiconductor materials to create a p-i-n junction. The growth of the materials may be accomplished through various techniques such as Molecular Beam Epitaxy (MBE), vapor phase epitaxy (VPE), liquid-phase epitaxy (LPE), metal-organic chemical vapor deposition (MO-CVD), or other chemical vapor deposition techniques. Alternatively, a p-type or n-type semiconductor material  530  may be grown and the upper portion may be doped to the opposite type creating the second semiconductor material  540 . 
         [0058]    Optionally, to create multiple junctions, additional semiconductor materials may be added.  FIG. 5F  illustrates a third semiconductor material  545 , doped to be the opposite type of  540 . Optionally, an intrinsic material  535  may be included between the second and third semiconductor materials to create a p-i-n junction. The semiconductor materials  530 ,  540 ,  545  and intrinsic material  535  may be comprised of HgCdTe, HgZnTe, InSb, InAs, GaSb, GaAs, or PbTe material systems. Optionally, each individual junction may be comprised of a different material. 
         [0059]    The junctions within the absorption layer  525  are contemplated to be homo-junctions comprised of a single material. Optionally, hetero-junctions may be used. For example, but not to serve as a limitation, in a two junction thermal harvester comprised of three materials, the first material may be comprised of HgCdTe and doped to be p-type, the second material may be comprised of InSb and doped n-type, and the third material may be comprised of PbTe and doped p-type. While  FIG. 5F  depicts a absorption layer  525  with two junctions, any number of junctions may be created using this process. The absorption layer  525  may be arranged in a variety of configurations as illustrated in  FIGS. 3 and 4 . 
         [0060]      FIG. 5G  illustrates an optional step to create trap structures  550  within the absorption layer  525  through an etching process, such as photolithography, to create the desired shape and configuration of the trap structures. While the trap structures  550  depicted in the process are columns, other shapes or configurations may be used which reflect the radiation until it is absorbed by a junction. 
         [0061]    The etched trap structures  550  may be filled with a material that is transparent to the wavelength of radiation desired to be trapped  555 , as shown in  FIG. 5H . Filling the trap structures with transparent material and the nanostructures may be accomplished through spin coating, MBE, VPE, LPE, MO-CVD, other chemical vapor deposition, or printing or ink-jettable techniques. The material  555  is then planatized to be level with absorption layer  525 .  FIG. 5I  shows the addition of an insulator layer  560  over the absorption layer  525 . Alternatively, air may be used for the etched trap structures, not shown, resulting in gaps in the insulator layer  560  between the columns. 
         [0062]    The embodiments were chosen and described in order to explain the principles and applications of the invention, thereby allowing others skilled in the art to utilize the invention in its various embodiments and modifications according to the particular purpose contemplated. The scope of the invention is intended to be defined by the claims appended hereto and their equivalents.