Abstract:
An absorber is disclosed. The disclosed absorber contains a base layer, and a pyramidally shaped absorbing material disposed above the base layer and configured to absorb an incident light and generate minority electrical carriers and majority electrical carrier, wherein the pyramidally shaped absorbing material defines a plurality of holes within it.

Description:
This application is related to co-pending application U.S. application Ser. No. 12/544,218, filed on the same date as the present application, for “Wide Bandwidth Infrared Detector and Imager” by Daniel Yap, Rajesh D. Rajavel, Sarabjit Mehta and Joseph S. Colburn, the disclosure of which is incorporated herein by reference in its entirety. This application is also related to co-pending application U.S. application Ser. No. 12/544,221, filed on the same date as the present application, for “Reduced Volume Infrared Detector” by Daniel Yap, Rajesh D. Rajavel, Sarabjit Mehta and James H. Schaffner, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD 
     The present disclosure relates generally to photon detectors. 
     BACKGROUND AND PRIOR ART 
     According to prior art, optical quantum detectors (which absorb incident photons and generate electrical charge carriers) generally have highest sensitivity over a fairly small bandwidth, generally one octave or less. Contrary to the present disclosure, prior art detectors, used in optical imagers, generally make use of an optical anti-reflective coating to increase the amount of incident light that is coupled into their light-absorbing material. 
     Prior art infrared detectors are described in a Technical Information document (SD-12) by Hamamatsu Photonics K. K. Examples of prior photovoltaic and photoconducting detectors that have sensitivity in MWIR and/or LWIR wavelengths are described in an article by A. Rogalski (Journal of Applied Physics, vol. 93, no. 8, 15 Apr. 2003, pp. 4355-4391). In general, these detectors are formed as 2-dimensional arrays of detector pixels that are connected physically and electrically to a silicon read-out integrated circuit (ROIC). Photovoltaic detectors typically contain P-type semiconductor material, N-type semiconductor material and a PN junction. The incident light can be absorbed primarily in the P-type material, primarily in the N-type material or in substantially both P-type and N-type materials. 
     As depicted in  FIGS. 1   a - 1   d , prior infrared detectors  20 ,  22  and  24  often comprise a substrate  10  on which the array of detector pixels  12  is formed. The incident light  14  illuminates the substrate  10  and passes through the optically transparent substrate  10  to the detector pixels  12 , as depicted in  FIG. 1   a . For these detectors, each detector pixel  12  may be connected to the ROIC  18  by means of a solder bump  16 , depicted in  FIG. 1   b . With the detectors  20  and  22 , the substrate  10  is generally not removed and individual pixels  12  are defined by etching mesa structures that include the PN junction of a detector pixel  12 . For detector  24 , illustrated in  FIG. 1   c , which comprise a thick film of the light-absorbing material  30 , electrical vias  28  are etched through the absorbing film  30  and a PN junction  32  is formed around each of the vias  28 . Metal  34  is then coated over the vias  28  and provide electrical connections between the PN junctions  32  and the ROIC  18 . According to prior art, detectors  20 ,  22  and  24  can be coated with an anti-reflecting film to improve the capture of the light  14 . The anti-reflecting film is normally composed of one or more quarter-wave thickness layers of material that have a value for its refractive index that is between the value of the refractive index of the incident medium (such as air) and the refractive index of the substrate  14 . 
     According to prior art, the infrared detectors  20 ,  22  and  24  can achieve high external quantum efficiency only over a limited optical bandwidth because of their anti-reflective coatings. Because a quarter-wave thickness is achieved exactly for only one specific wavelength of the incident light, the anti-reflective coating is effective for only a small band of wavelengths (nominally less than an octave). 
     To achieve high internal quantum efficiency, the light-absorbing layer (or layers) of infrared detectors in the prior art must have a thickness that is sufficiently large to permit enough of the incident light, coupled in through its front surface, to be absorbed. In fact, the thickness of the absorber must be sufficient to absorb light at the longest wavelength of its desired band of operation. For high efficiency, that thickness is typically several times the value of the longest wavelength of the band, even when the detector has a metal reflector at its back side that enables the overall path-length of the light through the absorber to be doubled. Noisy “dark” current can be generated in the volume of the absorber because of thermal generation of electrical carriers. Thus, having a thick absorber means that the total volume of material contributing to the dark current is large, and the dark current is high. This degrades the detectivity of the detectors  20 ,  22  and  24 . In contrast, a novel infrared detector (imager) presently disclosed, does not require an anti-reflecting film and it provides low reflection for incident light over a large bandwidth of multiple octaves with reduced dark current. 
     Like optical quantum detectors, solar cells have also been developed to absorb light, however at visible wavelengths. And, solar cells generally do not absorb light at MWIR wavelengths. Solar cells are generally made from material such as silicon. Although both solar cells and infrared imagers have been widely used commercial products for several decades, there does not appear to be any known attempts to combine the features of these two kinds of devices. 
     Surfaces with shallow pyramid-shaped features and the light trapping benefits of such surfaces are known from the field of solar cells. An article by M. A. Green, et al. (“Very High Efficiency Silicon Solar Cells—Science and Technology”, IEEE Transactions on Electron Devices, vol. 46, no. 13, October 1999, pp. 1940-1947) describes solar cells that contain pyramid-shaped surfaces. The light trapping properties of pyramidally textured surfaces is described in an article by P. Campbell and M. A. Green (Journal of Applied Physics, vol. 62, no. 1, 1 Jul. 1987, pp. 243-249). Prior art solar cell  38 , depicted in  FIG. 2 , has pyramids  40  with height that are small compared to the overall thickness of the light-absorbing material  42 . This is because for solar cells, the dark current noise is not a problem of concern. In contrast, in the infrared imager presently disclosed, the height of the pyramid is large compared to the overall thickness of the light absorbing material, with that pyramid height being about one half of the overall thickness of the light absorbing material. 
     Another prior art solar cell  44 , depicted in  FIG. 3 , has pyramids  46  whose height is large compared to the overall thickness of the light absorbing material  48 . This solar cell is described in an article by R. Brendel, et al. (“Ultrathin crystalline silicon solar cells on glass substrates,” Applied Physics Letters, vol. 70, no. 3, 20 Jan. 1997, pp. 390-392). In the solar cell  44 , the PN junctions  50  are located near the sloped faces or sidewalls at the backside of the device. In contrast, for some embodiments of the present invention, the PN junctions may be located near the tips of the pyramid structures at the backside of the device. This allows the infrared imager presently disclosed to achieve reduced area of the junction depletion regions, thereby reducing the dark current from those depletion regions. 
     Another prior art solar cell with back-side electrical contacts whose PN junction area is small is described in an article by R. M. Swanson, et al. (“Point-contact silicon solar cells,” IEEE Transactions on Electron Devices, vol. ED-31, no. 5, May 1984, pp. 661-664). Solar cells having localized PN junctions at their back side as wells as pyramid-shaped texturing of their front side are depicted in  FIG. 4  and are described in an article by R. A. Sinton, et al. (27.5 percent silicon concentrator solar cells,” IEEE Electron Device Letters, vol. EDL-7 no. 10, October 1986, pp. 567-569). Diffusion of the doped regions to create such PN junctions involves subjecting the material to fairly high temperatures, generally &gt;400 degrees-C., and is not compatible with the processes involved in fabricating devices that have thin light absorbing material. 
     As depicted in  FIG. 6 , detectors, such as those in focal-plane array (FPA) imagers, have absorber regions that comprise a thick planar film that is solid (or continuous). It is described in more detail in an article by H. Yuan et al. (“FPA development: from InGaAs, InSb to HgCdTe”, Proceedings of SPIE Vol. 6940, paper 69403C, 2008). According to Yuan, each detector pixel is electrically connected, separately, to the read-out integrated circuit (ROIC)  56  by means of a solder bump  58 . The array of detectors  64  also makes a common electrical connection to the ROIC  56  because they share a contiguous absorber layer  60  composed of n-HgCdTe material. The mesas that define the individual pixels of the detector array  64  are etched through the p+ layer  62  and only partly into the n-HgCdTe layer  60 . The detector array  64  has volume of absorber material that contributes to the thermally generated (diffusion) dark current as defined by the thickness of that n-HgCdTe absorber layer  60  times the total area of the detector array  64 . In this example, a way to reduce the diffusion-current component of the dark current would be to reduce the thickness of the absorber layer  60 . However, such a reduction in layer thickness would also reduce the amount of incident light  66  that is absorbed, thereby reducing the quantum efficiency of the detectors. 
     A known method for reducing the volume of the absorber is shown in  FIG. 7 . This method involves placing the detector at the back end of an optical concentrator  70  such as a Winston cone. This method is more fully described in an article by T. Ashley, et al. (“Epitaxial InSb for elevated temperature operation of large IR focal plane arrays,” Proceedings of SPIE Vol. 5074 (2003), pp. 95-102). An array of such concentrators and detectors would have cones that abut each other at their entrances but those detectors would be physically isolated from each other. Thus, each detector would need to have both of its electrical connections (its P-connection and its N-connection) made to the detector itself. As a consequence, the ROIC would need to provide two electrical connections to each pixel rather than one electrical connection to each pixel, with the other electrical connection being a “common” connection. 
     According to  FIG. 8 , a prior-art solar cell  72  has a surface texture with a honeycomb  74  pattern. The solar cell  72  is described in more detail in an article by Zhao, et al. (“A 19.8% efficiency honeycomb multicrystalline silicon solar cell with improved light trapping,” IEEE Transactions on Electron Devices, vol. 46, no. 10, October 1999, pp. 1978-1983). The honeycomb  74  textured surface improves the trapping of the incident light so that the light can be absorbed by the solar cell  72  rather than being reflected away. The honeycombs  74  are a surface texture with a pitch or spacing of 14 μm and a thickness of less than 1 μm. The thickness of the absorber in the solar cell  72  is &gt;200 μm. Thus, the volume represented by the honeycomb is a small fraction of the total volume of the light absorber. There is a need for a detector structure that has reduced volume of absorber material. There also is a need for a reduced-volume detector structure that provides low-resistance lateral flow of the photogenerated carriers, so that those carriers can be collected through the extractor regions or at the ohmic common contacts. 
     An article by Tokranova et al. (Proceedings of SPIE, Vol. 5723, pp. 183-189 (2005)) describes a solar cell comprising a film of porous silicon in which the 16 pores are filled with an organic material. The sunlight is absorbed primarily by the organic material, which in this case is copper phthalocyanine (CuPC), since the absorption efficiency of the porous silicon is poor. Absorption of the light in the CuPC results in generation of electrical charge carrying holes and electrons. The sides of the pores provide a large-area interface between the p-type CuPC and the n-type silicon material that serves to separate the photo-generated holes, which are transported in the porous silicon material, from the photo-generated electrons, which are transported in the CuPC. These prior art devices, however, would not be suitable for use as a low-noise detector. These devices have very large PN junction area and thus the relative contribution of the dark current due to generation in the junction depletion regions is very high. In contrast, the presently disclosed detectors have much smaller PN junction area that can benefit from the reduction of the absorber volume. 
     A novel infrared detector (imager) with low reflection for incident light over a large bandwidth of multiple octaves and with reduced dark current and reduced volume of absorber material is presently disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1   a - 1   d  depict Prior-Art optical imagers. 
         FIG. 2  depicts Prior-Art solar cell with pyramidally textured surface. 
         FIG. 3  depicts Prior-Art solar cell with sloped sidewalls. 
         FIG. 4  depicts Prior-Art solar cell with back side contacts. 
         FIG. 5  depicts Prior-Art solar cell with nanorods with PN junction. 
         FIG. 6  depicts an array of Prior-Art detectors in a focal-plane-array imager. 
         FIG. 7  depicts a reduced diameter photodetector combined with Winston cone optical concentrator according to Prior-Art. 
         FIG. 8  depicts Prior-Art solar cell with honeycomb surface texture. 
         FIG. 9  depicts a honeycomb absorber structure. 
         FIG. 10  depicts a detector array with a honeycomb absorber. 
         FIG. 11  depicts a pyramid-shaped honeycomb absorber structure according to the present disclosure. 
         FIG. 12  depicts a detector pixel with multiple honeycomb pyramids according to the present disclosure. 
         FIG. 13   a  depicts a detector array with pyramid-shaped honeycomb absorber according to the present disclosure. 
         FIG. 13   b  depicts a bottom view of a detector array shown in  FIG. 13   a.    
         FIGS. 14   a - 14   b  depict basic unit-cell used for HFSS simulations of detector array according to the present disclosure. 
         FIG. 15  depicts comparison of simulated absorbance for slab honeycomb absorber and pyramidal honeycomb. 
         FIG. 16  depicts comparison of simulated absorbance for pyramidal honeycomb absorber and for solid pyramid absorber. 
         FIGS. 17   a - 17   b  depict exemplary hole patterns for honeycomb absorbers. 
         FIG. 18  depicts fabrication approach for pyramidal honeycomb. 
         FIGS. 19   a - 19   l  depict a fabrication process for manufacturing detector array shown in  FIG. 13 . 
     
    
    
     In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of every implementation nor relative dimensions of the depicted elements, and are not drawn to scale. 
     DETAILED DESCRIPTION 
     The present disclosure relates to photon detectors that may be used in a focal-plane-array optical imagers. The constraints for photon detectors are quite different from the constraints associated with most solar cells described above. The photon detector performance is given in terms of its signal to noise ratio. Although the detected signal is to be maximized, the noise (from thermal generation of electrical carriers rather than from carrier generation accompanying photon absorption) must be kept low. In contrast, a solar cell is concerned with carrier generation but not with noise. Furthermore, the photon detector usually has an absorber material with a direct energy-band gap. Thus, the absorption efficiency is high. Usually the thickness of the absorber is no more than 2 times the longest wavelength of the light to be absorbed, especially when there is a back-side reflector for two-pass absorption. In contrast, most solar cells have an absorber material such as silicon with poor absorption efficiency. Thus, the thickness of a solar cell typically is many times larger (e.g., 10× to 50×) than the wavelength of the light to be absorbed. 
     The novel infrared detector (imager) presently disclosed contains novel quantum detectors having very broad bandwidth, for example, such as having sensitive detection ranging from visible to midwave infrared (MWIR) wavelengths (e.g., 0.4-5.0 μm). The imager may contain an array of multiple detectors, arranged as a collection of pixels, that are electrically coupled to a read-out integrated circuit. Also, the pixel array may lie on a focal plane of imaging optical elements, such as lenses. 
       FIG. 9  depicts a perspective view of an exemplary honeycomb absorber  80  of an infrared detector array. The honeycomb absorber  80  may be a slab of absorber material with holes  82  in it. The remaining absorber material forms a honeycomb-like frame  84 . The honeycomb absorber  80  may absorb the incident light at least as efficiently as a planar, solid absorber. However, the honeycomb absorber  80  accomplishes this efficient absorption with a much smaller volume of absorber material. For temperatures of 200K and higher, the dominant contribution to the dark current noise is thermal generation in the narrow-bandgap absorber material (assuming that the absorber material has sufficiently good quality and is sufficiently free of defects). Thus, the dark current can be reduced and the detectivity of the infrared detector improved by reducing the volume of the absorber material. Also, the honeycomb absorber  80  presents a much lower effective optical refractive index to the incident light. Thus, there is much less reflection of that light from the front surface of the honeycomb absorber. As a result, more of the incident light is coupled into the honeycomb absorber  80  to be absorbed. This low reflectance is achieved over a large range of incident wavelengths. 
       FIG. 10  depicts a detector array  90  with a honeycomb absorber  92 . The honeycomb absorber  92  may extend over multiple detector pixels  94  of the detector array  90 . In one exemplary embodiment, the honeycomb absorber  92  is disposed above a base layer  96 . The base layer  96  may be made of material that absorbs the same incident light  98  as the honeycomb absorber  92 . In another exemplary embodiment, the base layer  96  may be transparent to the incident light  98 . In another exemplary embodiment, the detector array  90  may not contain a base layer. Referring to  FIG. 10 , the detector array  90  also includes extractors  100  disposed on the bottom side of the base layer  96 , if present, or on the bottom side of the honeycomb absorber  92  if there is no base layer. The extractors  100  serve to extract or collect the photo-generated minority carriers from the honeycomb absorber  92  and base layer  96 . The detector array  90  may also include semiconductor/metal contacts  102 ,  104  and  106 . The semiconductor/metal contacts  102 ,  104  and  106  may be Ohmic contacts. The contacts  106  are electrically connected with extractors  100 . The contacts  102  and  104  are electrically connected to the back side of the base layer  96 , when the base layer  96  is present, or to the back side of the honeycomb absorber  92  if the base layer is not present. The base layer  96  provides an alternate path for lateral conduction of the electrical carriers from the honeycomb absorber  92  to the extractors  100 . For a honeycomb absorber  80  depicted in  FIG. 9 , the base layer may be optional as the honeycomb absorber  80  may provide sufficient lateral conduction of the photo-generated electrical carriers. 
     The back side of the honeycomb absorber  92  or base layer  96  may further be covered with a dielectric layer  108  that also serves to passive the bottom surface of the semiconductor base layer  96  or the honeycomb absorber  92 . Furthermore, additional surface-passivating films (not shown) may cover the sidewalls of the honeycomb holes as well as the top side of the honeycomb absorber  92 . Metal bond pads  110 , which also serve as reflective mirrors for the incident light  98 , may cover the back side of the detector pixels  94 . These metal bond pads  110  provide a means to couple the photo-generated current from a detector pixel  94  out to the read-out integrated circuit  116  of an optical imager (not shown), of which the detector array  90  may be part of. The detector array  90  may be bonded to the read-out integrated circuit  116  through solder bumps  118 . Some metal bond pads  110  are in electrical contact with the contacts  106  and provide a low resistance interconnection between the contacts  106  associated with a given detector pixel  94 . Some metal bond pads  110  are in electrical contact with the contact  104  and provide a means to connect to the common electrical contact of the detector array  90 . There is no need for the detector array  90  to have an anti-reflecting film deposited on its top side since the honeycomb absorber  92  achieves anti-reflection performance over a large range of wavelengths for the incident light  98 . 
     The detector array  90  has ohmic contacts  102  formed near each detector pixel  94 . By forming the ohmic contacts  102  at the periphery of each detector pixel  94 , the thickness of the base layer  96  may be reduced or the base layer  96  may not be even used, thereby further reducing the total volume of absorber material. The pattern of ohmic contact metal  102 , and the optional metal interconnect lines (not shown) that contact to and interconnect the ohmic contacts  102 , form a low-resistance path for the photo-generated electrical current to flow from each pixel  94  to the “common” bond pads  104  that are located at the periphery of the detector array  90 . By using the low-resistance metal paths to carry the current instead of requiring the photo-generated majority carriers to flow through the absorber  92 , the thickness of absorber material can also be reduced to further reduce the dark-current generation volume of the detector array, thereby improving its detectivity. 
       FIG. 11  depicts an exemplary embodiment of honeycomb absorber  122  with a pyramidal outline according to the present disclosure. The apex of the pyramid may be pointed toward the incident light. In this exemplary embodiment, the base layer  124  may be disposed on the back side of the honeycomb pyramid  122 . In one exemplary embodiment, a detector pixel may include multiple honeycomb pyramids as shown in  FIG. 12 . The bases of those pyramids in  FIG. 12  may abut each other or they be separated by small gaps. The base layer  126  may provide the necessary lateral conduction for electrical carriers between adjacent pyramids. 
     For a large range of wavelengths of the incident light, the pyramidal honeycomb may provide comparable absorption of that incident light as the bulk honeycomb absorber, even through the volume of the absorber material in the pyramidal honeycomb is ⅓ rd  as great. Also, for a large range of wavelengths of the incident light, the pyramidal honeycomb may provide absorption comparable to that obtained with a solid pyramidal absorber, although the volume of the absorber material again is reduced by at least 2-3×. Thus, the pyramidal honeycomb may have greater detectivity, D*, since its dark current is lower even though its quantum efficiency is comparable. 
       FIG. 13   a  depicts another exemplary embodiment of a detector array  130  according to the present disclosure. The detector array  130  may contain honeycomb pyramidal absorbers  132 . There may be multiple pyramidal absorbers  132  in each of the detector pixel  134 . The pyramidal absorbers  132  may be disposed above a base layer  136 . The base layer  136  may be composed of the same material as the honeycomb pyramidal absorbers  132 . The honeycomb pyramidal absorbers  132  provide lateral conduction for the carriers within a pyramid and the base layer  136  provides lateral conduction between the pyramids of a detector array  130 . The detector array  130  also contains extractor regions  138  disposed on the back side of the base layer  136 . In one exemplary embodiment, if the base layer  136  and honeycomb pyramidal absorbers  132  are composed of n-type semiconductor material (such as, for example, InAlSb material or InAsSb material), the extractor  138  may be composed of p-type semiconductor material (such as, for example, InAlSb material or GaSb material). The extractor regions  138  may be coupled to metal-semiconductor contacts  140 . The metal-semiconductor contacts  140  may be Ohmic contacts and may be disposed on the back side of the extractors  138 . Additional contacts  142  and  143  may be coupled to portions of the base layer  136 . The contacts  142  and  143  may be Ohmic contacts. The contacts  143  may be disposed beneath the valley between two adjacent pyramids. The contacts  142  and  143  may have fairly small area. In one exemplary embodiment, the contacts  142  and  143  may have a width of 1 micrometers and a length of 4 micrometers. The contacts  142  and  143  may be formed near the periphery of the detector pixel  134 . The extractor regions  138  and the contacts  140  may also have small area. In one exemplary embodiment the extractor  138  and the contacts  140  may have an area of 0.5×0.5 micrometers. In another exemplary embodiment, the extractors  138  are located directly below the apex of the honeycomb pyramidal absorbers  132 . 
     The backside of the base layer  136  may also be covered with a dielectric layer  146  that may serve as a surface-passivation film for the absorber material of the base layer  136 . The contacts  140  of a given detector may be interconnected by means of a metal bond pad  154 . This bond pad may also provide an electrical output for the photo-generated current from the detector pixel  134 . The contacts  142  and  143  may be interconnected by means of metal lines or strips  157  formed on the backside of the base layer  136 , as shown in  FIG. 13   b . The dielectric layer  146  serves to electrically isolate the bond pads (connecting to the contacts  140 ) from the contacts  142  and  143  and their interconnecting metal lines (not shown). In one exemplary embodiment, the metal bond pads (connecting to the contacts  140 ) may substantially cover the back side of a detector pixel  134 . The metal bond pads (connecting to the contacts  140 ) may also serve as reflectors for the incident light  148 . In another exemplary embodiment, additional metal bond pads  155  may be located near the periphery of the detector array  130  and are connected to the contacts  142  to provide electrical common connection to the detector array  130 . 
     In one exemplary embodiment, the sidewalls of the holes in the honeycomb pyramidal absorbers  132  as well as the sloped top-side surfaces of the honeycomb pyramidal absorbers  132  may be covered with a surface-passivating film (not shown). Since the honeycomb pyramidal absorbers  132  have very large surface area compared to its volume, the surface passivation film (not shown) may be used to control the dark current associated with surface states and also to control the surface recombination that would otherwise remove the photo-generated carriers and prevent them from being collected at the contacts  140 ,  142  and  143 . The surface-passivating film may be composed of evaporated silicon dioxide, spin-on polymers (such as, for example, benzo-cyclo-butene or polyimide) or wide-bandgap semiconductor material such as GaAlAsSb. The detector array  130  may be bonded to the read-out integrated circuit  150  through solder bumps  152 . Some metal bond pads  154  are in electrical contact with the contacts  140  and provide a low resistance interconnection between the contacts  140  associated with a given detector pixel  134 . Some metal bond pads  155  are in electrical contact with the contact  142  and provide a means to connect to the common electrical contact of the detector array  130 . There is no need for the detector array  130  to have an anti-reflecting film deposited on its top side since the honeycomb pyramidal absorbers  132  achieves anti-reflection performance over a large range of wavelengths for the incident light  148 . 
     In one exemplary embodiment, HFSS, a 3D electromagnetic field simulation and analysis tool (based on the finite element method) from Ansoft Corporation, was used to determine the absorption efficiency of the honeycomb absorber and of the pyramidal honeycomb.  FIGS. 14   a - 14   b  depict an exemplary unit cell that was used for an HFSS calculation.  FIG. 14   a  depicts a side perspective view of the exemplary one-quarter unit cell that was used for one the HFSS calculations.  FIG. 14   b  depicts a top view of the exemplary one-quarter unit cell that was used for one the HFSS calculations. The honeycomb pyramid may be manufactured to be symmetric about its apex so that only one-fourth of a pyramid needs to be defined for the simulation. In this exemplary embodiment, simulations were preformed under assumption that the collection of pyramids form a periodic array. The  FIGS. 14   a - 14   b  depicts a structure with 16 holes  170  in each quadrant of a pyramid. The holes have a diameter of 0.6 micrometers and the pyramid has a height of 5 micrometers and a base length of 5 micrometers. In this exemplary embodiment, HFSS simulation was performed without a base layer and with a metal reflector located at the base of the pyramids. The model structure shown in  FIGS. 14   a - 14   b  has 1/10 th  the volume of an absorber of comparable 5 micrometer height or thickness. Although a model with 16 holes per quadrant is shown as an example, the same 1/10 th  (or 0.1) volume fill ratio can be achieved with other arrangements of the holes and also with other numbers and sizes of holes. The holes shown in  FIGS. 14   a - 14   b  are placed on a rectangular grid. 
       FIG. 15  depicts the results of the HFSS simulations for a pyramidal honeycomb with 0.1 volume fill ratio and 16 holes in a quadrant as shown in  FIGS. 14   a - 14   b . The absorbance of a pyramidal honeycomb is greater than 70% for incident light with wavelengths between 0.5 and 5 micrometers. In comparison, the absorbance of a honeycomb slab with the same hole-size and pattern also is greater than 70% for wavelengths of 4 micrometers and shorter. However, the pyramidal honeycomb has ⅓ the volume of the honeycomb slab. Thus, the pyramidal honeycomb should have 3× higher detectivity, assuming the dark current can be attributed solely to “diffusion” current from the absorber material. 
       FIG. 16  compares the simulated absorbance of a honeycomb pyramid with a solid pyramid of the same height and base length based on HFSS simulations. The honeycomb pyramid has a volume fill ratio of 0.1 whereas the solid pyramid has a volume fill ratio of 0.33. Note that the honeycomb pyramid has comparable absorbance to that of the solid pyramid for wavelengths of the incident light of 4 micrometers and shorter. This comparable absorbance is obtained even though the volume of absorber material in the honeycomb pyramid is ⅓ rd  as great. Thus, the honeycomb pyramid provides a way to improve the detectivity of the detector array, by reducing the volume of the dark-current producing absorber material, while still achieving high absorption efficiency and high quantum efficiency. 
     Although the results presented in  FIG. 15  are for honeycomb structures in which the holes are placed on a square grid, other patterns could be used for the holes. In one exemplary embodiment, the holes may be placed on a hexagonal pattern, such as a hexagonal close packed arrangement. 
     The simulation results shown in  FIG. 16  were for a honeycomb pyramid having the hole-pattern depicted in  FIG. 17   a . Approximately similar absorbance results (not shown) were obtained for the hole-pattern shown in  FIG. 17   b .  FIGS. 17   a - 17   b  depict two configurations that have the same number of holes and the same hole-diameter. The diameter of the holes in  FIGS. 17   a - 17   b  is 1.68 micrometers. Holes in  FIGS. 17   a - 17   b  may be formed by methods such as reactive ion etching. 
       FIG. 18  depicts the basic process of fabricating a detector array with honeycomb pyramidal absorbers. In one exemplary embodiment, the honeycomb absorbers may be formed by etching into the epitaxially grown absorber material. In another exemplary embodiment, the honeycomb absorbers may be formed by selective-area metal-organic chemical vapor deposition onto a substrate that has that honeycomb pattern etched into a growth-mask such as silicon dioxide. After the honeycomb absorbers are formed, a pyramidal outline can be formed by, for example, patterning a suitable mask material with a pyramidal shape and then transferring the pyramidal shape of the mask into the honeycomb structure. It may be desirable to temporarily fill the holes of the honeycomb absorbers with a removable material that has approximately the same etching properties as the absorber material of the honeycomb frame. Then, after the pyramids are formed, the substrate wafer with the pyramids can be attached to a handle wafer. This enables the substrate to be removed and thus expose the backside of the honeycomb structure for further processing. 
       FIGS. 19   a - 19   l  depict an exemplary fabrication process for fabricating the detector array and attaching that detector array to the read-out integrated circuit as described above. The following fabrication process is suitable for all of the embodiments disclosed herein of the detector array. 
     Referring to  FIG. 19   a , using epitaxial growth technique such as molecular beam epitaxy or metal-organic chemical vapor deposition, an optional stop etch layer  184 , an extractor layer  186  and then the absorber layer  188  are formed on a substrate wafer  182 . In one exemplary embodiment, for detection of optical wavelengths of 4-5 μm and shorter, the substrate wafer  182  may be composed of GaSb material, the optional stop etch layer  184  may be composed of AlGaSb material, the p-doped extractor layer  186  may be composed of InAsSb material, and an n-doped absorber layer  188  may be composed of InAsSb material. In another exemplary embodiment, the substrate wafer  182  may be composed of GaSb material, the optional stop etch layer  184  may be composed of AlGaSb material, the n-doped extractor layer  186  may be composed of InAsSb material, and the p-doped absorber layer  188  may be composed of InAsSb material. Lattice matched InAs 0.9 Sb 0.1 , for example, provides absorption of wavelengths of 4.7 μm and shorter when operated at 300K temperature and absorption of wavelengths of 4.3 μm and shorter when operated at 200K temperature. 
     For detection of wavelengths as long as 5.0 μm at 200K temperature, material such as InAs 0.8 Sb 0.2 , having substantial lattice mismatch (approximately +0.7%), could be used. Another alternative to InAsSb is to use a Type II superlattice consisting of InAs/GaSb material for the absorber  188 . Another example of a suitable substrate  182 , for detection of &gt;5.0 μm wavelength light, is InSb material. The optional stop etch layer  184  could be a thin layer of InAlSb material. The extractor  186  and absorber  188  could be composed of doped InSb or InAlSb material. 
     Other examples of extractor  186  materials include extractors  186  composed of multiple layers of semiconductor material. In one exemplary embodiment, the extractor  86  for an n-type InAsSb absorber  188  could be composed of layers of n-type GaAlAsSb and p-type GaSb materials. Such a structure facilitates the flow of the photo-generated electrons away from the extractors  186  and to the ohmic contact of the absorber  188 . Such a structure also is effective in reducing the dark current that arises from the depletion layer between the absorber  188  and extractor  186 , as discussed in an article by P. Klipstein (“XBn barrier photodetector for high sensitivity and high operating temperature infrared sensors,” Proceedings of SPIE, vol. 6940, paper 69402U-1), which is incorporated herein by reference. In another exemplary embodiment, the extractor  186  for n-type InAsSb absorber  188  could be composed of layers of n-type GaAlAsSb, p-type GaSb and then n-type InAs materials. The use of the additional n-type InAs layer, which forms a Type II energy-gap alignment with the GaSb, permits the ohmic contact for both the extractor  186  and the absorber  188  to be made to n-type, narrow bandgap materials. 
     Referring to  FIGS. 19   b  and  19   c , pyramidal shapes  190  are formed in the absorber layer  88 . The pyramidal shapes  190  may be formed after the holes for the honeycomb have been formed by etching or by selective area growth as described above. An etching process may be used to form pyramidal shapes  190 . In one exemplary embodiment, the pyramid shapes  190  are etched by depositing and forming an etch mask  192  that has a pyramid shape. The pyramid outline of the masking material  192  may then be transferred into the absorber  188  by dry etching techniques such as, for example, reactive ion etching or ion beam milling. The height of the masking layer  192  may depend on the dry-etch selectivity between the masking material  192  and the material of the absorber  188 . Suitable etch masks  192  may include photoresist, polymers such as benzo-cyclo-butene, and silicon dioxide. A pyramid shape may be formed in photoresist by gray scale lithography. A pyramid shape may be formed in non-photo-definable polymers by means of imprint lithography using a mold that has an inverted pyramid shape. A pyramid shape may be formed in silicon dioxide by using an additional thin mask layer having a square shape and then etching the silicon dioxide with an isotropic wet etchant. 
     Referring to  FIG. 19   d , a material  194  is deposited in the spaces between the pyramids  190 . In one exemplary embodiment, the material  194  may be composed of spin on glasses and various polymers. The material  194  may then be planarized. An optional adhesion layer (not shown) can be deposited on top of the planarized material  194 . Then, a carrier substrate  196  is attached or bonded to the surface of the planarized material  194  or the adhesion layer (not shown). This process is used for wafer transfer. 
     Referring to  FIG. 19   e , the structure shown in  FIG. 19   d  is turned upside-down. Referring to  FIG. 19   f , the substrate  182  is removed or etched away as know in the art. The stop etch layer  184  facilitates this substrate  182 &#39;s removal. The stop etch layer  184  is then also removed by known etching techniques, leaving the extractor layer  186  exposed. The extractor  186  may then be patterned by known photolithography and wet or dry etching methods. In fact, the exposed back side of the absorber layer  188  also can be etched. One exemplary embodiment that makes use of etching the absorber  188  from its back side. The back surface of the back-side etched extractor  186  and absorber  188  could have various shapes, including pyramid shapes, as discussed in detail above. 
     Referring to  FIG. 19   g , regions of ohmic-contact metals  198  and  200  are then deposited and patterned using exemplary techniques such as evaporation and lift-off or sputtering and etching. Both the ohmic-contact metals  198  for the extractor  186  and the ohmic-contact metal  200  for the absorber  188  may be deposited and patterned at the same time. Following the formation of the ohmic-contact metals  198  and  200 , or in some exemplary embodiments, before the formation of the ohmic-contact metals  198  and  200 , a film of dielectric passivation material  202  composed of, for example, silicon dioxide, polyimide or benzo-cyclo-butene material may be deposited on top of the structure. This passivation material  202  may serve to reduce the amount of surface recombination that occurs in the semiconductor material, and especially at the outer edges of PN junctions. 
     Referring to  FIG. 19   h , vias may be patterned and etched through the dielectric passivation material  202  to expose certain regions of or for the ohmic-contact metals  198  and  200 . Metal bonding pads  204  may then be deposited and patterned. These metal bonding pads  204  can then be used for solder bump  205  bonding of the detector array to the read-out integrated circuit  210 , as illustrated in  FIGS. 19   i - 19   l.    
     Referring to  FIGS. 19   i - 19   j , once the detector array, held on the carrier substrate  196 , is bonded to the read-out integrated circuit  210 , the carrier substrate  196  can then be removed. In one exemplary embodiment, a protective layer of material (not shown) such as photoresist may be used to cover and protect the read-out integrated circuit  210 , the solder bumps  206  and the bond pads  204  and  208  from the etchant used to remove the carrier substrate  196 . Referring to  FIG. 19   k , after the carrier substrate  196  is removed, the planarization/fill material  194  may optionally be removed and adhesion material (not shown) over the pyramids  190  can be removed. Referring to  FIG. 19   l , in one exemplary embodiment, ohmic contacts  212  may be formed on the tips of the pyramids  190  and additional metal interconnects can be deposited on patterned onto the now exposed top side of the detector array. 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. Other embodiments are within the scope of the claims. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . ”