Patent Publication Number: US-10777696-B1

Title: High absorption infrared superlattices

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application is a divisional of U.S. application Ser. No. 15/861,825 filed Jan. 4, 2018, which claims the benefit of U.S. Provisional Application No. 62/443,428 filed, Jan. 6, 2017, the contents of which are hereby incorporated by reference in their entries. 
    
    
     GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States for all government purposes without the payment of any royalty. 
    
    
     BACKGROUND 
     Field of the Invention 
     The embodiments herein generally relate to infrared detectors, and more particularly to group III-V semiconductor superlattice infrared detectors built based on diode and barrier device architecture. 
     Background of the Invention 
     A strained layer superlattice (SLS) is a periodic structure of thin layers made out of semiconductor materials. Superlattice structures are grown layer by layer on a substrate wafer. Group III-V arsenide and antimonide based semiconductor superlattice structures are a preferred material choice for infrared detectors for both mid-wave (MWIR: 3-5 μm) and long wave infrared (LWIR: 8-14 μm) bands. Common material systems for these superlattice structures include InAs/GaSb, InAs/InGaSb, and InAs/InAsSb. The choice of the semiconductor materials depends on various factors such as design aspects, properties of the superlattice structure to be achieved, and material growth feasibility. 
     Typically, in a strain-compensated superlattice structure, the tensile strain in one layer (e.g., InAs in an InAs/InAs [1-x] Sb [x]  superlattice structure on GaSb) is compensated by the compressive strain introduced by the other layer (e.g., InAs [1-x] Sb [x]  in an InAs/InAs [1-x] Sb [x]  superlattice structure on GaSb). For MWIR InAs/InAsSb superlattice structures, strain compensation is accomplished by adjusting the thickness of the two layers and the Sb composition. This typically results in a thicker InAs layer compared to the InAsSb layer. 
     A typical superlattice infrared detector structure includes a bottom contact layer, infrared absorber utilizing a superlattice structure, a barrier layer or a depletion region, and a top contact layer, all of which are grown in that order on a preferred substrate. These superlattice detectors are operated as minority carrier devices in which the dark and photo current depend on three basic material properties: minority carrier lifetime, mobility, and absorption coefficient. Although the dark current of today&#39;s InAs/InAsSb superlattice detectors are improving, quantum efficiency (QE) in general suffers mainly due to the device active region thickness being limited by the short diffusion length or due to challenges in growing thick active regions. InAs/InAsSb superlattice structures have easily reached the s range for minority carrier lifetime, but the diffusion length is still short due to the poor hole mobility. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the foregoing, an embodiment herein provides a ternary superlattice structure comprising a substrate and non-changing periodic layer structure on the substrate and comprising alternating infrared absorbing semiconductor materials comprising a first layer of InAs [1-x] Sb [x]  ternary alloy material, and a second layer of In [1-y] Z [y] As ternary alloy material, wherein Z is Ga or Al, wherein x is in a range of greater than zero and less than one, wherein y is in a range of greater than zero and less than one, and wherein a thickness of each of the first and second layers are substantially similar and configured to absorb light in a predetermined spectral band and prevent trapping of carriers in any particular layer. In an example, y is in a range from about 0.05 to about 0.35. In an example, x is in a range of about 0.2 to about 0.8. The first layer of InAs [1-x] Sb [x]  ternary alloy material may comprise at least one monolayer to about 30 monolayers. The second layer of In [1-y] Z [x]  As ternary alloy materials may comprise at least one monolayer to about 30 monolayers. The first layer of InAs [1-x] Sb [x]  ternary alloy material may comprise at least one monolayer to about 20 monolayers. The second layer of In [1-y] Z [y] As ternary alloy materials may comprise at least one monolayer to about 20 monolayers. A total of the InAs [1-x] Sb [x]  ternary alloy material monolayers and the In [1-y] Z [y] As ternary alloy material monolayers may be less than 30. The periodic layer structure may comprise bound holes and unbound electrons whose ground state is at most 10 meV below a highest conduction band minimum. A thickness of the periodic layer structure may be configured based on a predetermined selection of an amount of infrared absorption within the periodic layer structure. 
     Another embodiment provides a method of forming a ternary superlattice structure, the method comprising heating a substrate to a temperature of about 395° C. to about 460° C. in an ultrahigh vacuum ranging from about 10 −8  to 10 −10  Torr in base pressure; depositing at least one monolayer to about 30 monolayers of In [1-y] Z [y] As to form a first semiconductor layer of In [1-y] Z [y] As ternary alloy material on the substrate, wherein Z is selected from the group comprising of Ga and Al; and depositing at least about one monolayer to about 30 monolayers of InAs [1-x] Sb [x]  to form a second semiconductor layer of InAs [1-x] Sb [x]  ternary alloy material on the substrate, wherein x is in a range from greater than zero and less than 1, and y is in a range from greater than zero and less than 1, and wherein a thickness of the first and second semiconductor layers are substantially similar and configured to absorb light in a predetermined spectral band and prevent trapping of electrons in any particular layer. 
     In an example, y is in a range from about 0.05 to about 0.35. In an example, x is in a range from about 0.2 to about 0.8. The rate of deposition to form either the first semiconductor layer of In [1-y] Z [y] As ternary alloy material or the second semiconductor layer of InAs [1-x] Sb [x]  ternary alloy material may be in a range from about 0.4 monolayers per second to about 1.5 monolayers per second. The first semiconductor layer of In [1-y] Z [y] As ternary alloy material may comprise between about 1 to about 20 In [1-y] Z [y] As monolayers, and the second semiconductor layer of InAs [1-x] Sb [x]  ternary alloy material may comprise between about 1 to about 20 InAs [1-x] Sb [x]  monolayers, wherein a sum of the In [1-y] Z [y] As monolayers and the InAs [1-x] Sb [x]  monolayers may be about 30 or less. 
     The method may comprise providing a flux of arsenic (As) and indium (In) species onto a surface of the first semiconductor layer of In [1-y] Z [y] As, wherein the flux of arsenic (As) species provides an over-pressure of arsenic sufficient to suppress any evaporation of arsenic from the ternary superlattice structure; adjusting between the depositing steps for the first and second semiconductor layers of In [1-y] Z [y] As and InAs [1-x] Sb [x] , wherein the arsenic and antimony fluxes are changed; providing a flux of antimony (Sb), arsenic (As), and indium (In) species onto a surface of the second semiconductor layer of InAs [1-x] Sb [x] , wherein the flux of antimony (Sb) and arsenic (As) species provides an over-pressure sufficient to suppress any evaporation of arsenic and antimony from the ternary superlattice structure; and adjusting between the depositing steps for the first and second semiconductor layers of In [1-y] Z [y] As and InAs [1-x] Sb [x] , wherein the arsenic and antimony fluxes are changed. 
     Another embodiment provides an infrared absorbing detector comprising an infrared absorbing ternary superlattice structure comprising a substrate and alternating semiconductor layers comprising a first layer of InAs [1-x] Sb [x]  ternary alloy material, and a second layer of In [1-y] Z [y] As ternary alloy material, wherein Z is Ga or Al, wherein x is in a range of greater than zero and less than one, wherein y is in a range of greater than zero and less than one. The detector further comprises a unipolar barrier layer on a first side of the ternary superlattice structure and configured to permit transport of one type of carriers across the barrier layer, wherein a bandgap of the barrier layer is greater than the bandgap of the ternary superlattice structure; a first contact layer on a second side of the ternary superlattice structure above the substrate; and a second contact layer adjacent to the unipolar barrier layer. In an example, y is in a range from about 0.05 to about 0.35. In an example, x is in a range of about 0.2 to about 0.8. 
     In an example, the barrier layer may comprise an offset between a conduction band of the ternary superlattice structure and a conduction band of the barrier layer to block a transport of electrons across the barrier layer, wherein valence bands of the ternary superlattice structure, the barrier layer, the first contact layer, and the second contact layer are substantially aligned to allow transport of holes between the ternary superlattice structure, the barrier layer, the first contact layer, and the second contact layer. In another example, the barrier layer may comprise an offset between a valence band of the ternary superlattice structure and a valence band of the barrier layer to block a transport of holes across the barrier layer, wherein conduction bands of the ternary superlattice structure, the barrier layer, the first contact layer, and the second contact layer are substantially aligned to allow transport of electrons between the ternary superlattice structure, the barrier layer, the first contact layer, and the second contact layer. In another example, the barrier layer may comprise a ternary superlattice structure comprising alternating semiconductor layers comprising a first layer of InAs [1-x] Sb [x]  ternary alloy material, and a second layer of In [1-y] Z [y] As ternary alloy material, wherein Z is Ga or Al, wherein x is in a range of greater than zero and less than one, and wherein y is in a range of greater than zero and less than one. 
     These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
         FIG. 1A  is a schematic representation of a ternary superlattice structure grown on a substrate, according to an example. 
         FIG. 1B  is another schematic representation of a ternary superlattice structure grown on a substrate, according to an example. 
         FIG. 2  is a plot of the bandgap (eV) versus lattice constant (A) for exemplary III-V semiconductor base materials and their alloys, according to an example. 
         FIG. 3  is a schematic representation showing band diagrams of an exemplary InGaAs/InAsSb ternary superlattice structure illustrating the conduction and valence bands of bulk layers and the conduction and valence minibands of the superlattice, according to an example. 
         FIG. 4  is a graphical illustration of the absorption coefficient (cm 1 ) versus wavelength (m) for a calculated absorption coefficient at 80K for two InGaAs/InAsSb ternary superlattice structures compared against an InAs/InAsSb superlattice structure, showing a significant improvement in the absorption coefficient for the InGaAs/InAsSb ternary superlattice structures compared to the InAs/InAsSb superlattice structure, according to an example. 
         FIG. 5  is an illustration showing the conduction and valence band profile of an nBn detector structures comprising of n-type InGaAs/InAsSb ternary superlattice structures serving as bottom contact and infrared absorbing layers, a bulk AlGaAsSb quaternary layer serving as the barrier, and another n-type doped ternary superlattice structure layer serving as the top contact layer, according to an example. 
         FIG. 6  is an illustration showing the conduction and valence band profile of an nBp detector structures comprising of n-type InGaAs/InAsSb ternary superlattice structures serving as bottom contact and infrared absorbing layers, a bulk AlGaAsSb quaternary layer serving as the barrier, and a p-type doped ternary superlattice structure layer serving as the top contact layer, according to an example. 
         FIG. 7  is an illustration showing the conduction and valence band profile of a heterojunction pN diode structures comprising of p-type InGaAs/InAsSb ternary superlattice structures serving as bottom contact and infrared absorbing layers, and wider bandgap undoped and n-type doped InAlAs/InAsSb ternary superlattice structures serving as the hole blocking barrier and the top contact layer, respectively, according to an example. 
         FIG. 8  is an illustration showing the conduction and valence band profile of a pBp structures comprising of p-type InGaAs/InAsSb ternary superlattice structures serving as bottom contact and infrared absorbing layers, a wider bandgap undoped InAlAs/InAsSb ternary superlattice structure serving as the hole blocking barrier, and an n-type InGaAs/InAsSb ternary superlattice structure serving as the top contact layer, according to an example. 
         FIG. 9  is a schematic diagram illustrating a system for performing superlattice growth, according to an example. 
         FIG. 10  is an illustration showing (004) X-ray diffraction 2θ-ω scans for three In 1-y Ga y As/InAs 0.65 Sb 0.35  ternary superlattice structures (withy=0, 0.05, and 0.2) grown on a GaSb substrate, according to an example. 
         FIG. 11A  is a graphical plot of experimentally measured quantum efficiency spectra at ˜80 K for three nBn detectors utilizing In [1-y] Ga [y] As/InAs 0.65 Sb 0.35  ternary superlattice structures (with y=0, 0.05, and 0.2), according to an example. 
         FIG. 11B  is a graphical plot of quantum efficiency (%) versus temperature (K) showing the dependence of the experimental quantum efficiency on temperature at −0.5 V bias voltage for the three nBn detectors utilizing In [1-y] Ga [y] As/InAs 0.65 Sb 0.35  ternary superlattice structures (with y=0, 0.05, and 0.2) used in  FIG. 11A , according to an example. 
         FIG. 12  is a graphical plot showing the photoluminescence peak position (bandgap) versus temperature for three sample ternary superlattice structures with a gallium fraction percentage of 0%, 5%, and 20%, according to an example. 
         FIGS. 13A and 13B  are flow diagrams illustrating methods of fabricating a superlattice structure, according to an example. 
         FIG. 14  is a schematic diagram illustrating an infrared absorbing detector, according to an example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the disclosed invention, its various features and the advantageous details thereof, are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted to not unnecessarily obscure what is being disclosed. Examples may be provided and when so provided are intended merely to facilitate an understanding of the ways in which the claimed invention may be practiced and to further enable those of skill in the art to practice its various embodiments. Accordingly, examples should not be construed as limiting the scope of what is disclosed and otherwise claimed. 
     It will be understood that when an element or layer is referred to as being “on”, “connected to”, or “coupled to” another element or layer, it can be directly on, directly connected to, or directly coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly on”, “directly connected to”, or “directly coupled to” another element or layer, there are no intervening elements or layers present. It will be understood that for the purposes of this disclosure, “at least one of X, Y, and Z” can be construed as X only, Y only, Z only, or any combination of two or more items X, Y, and Z (e.g., XYZ, XYY, YZ, ZZ). 
     Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term “including” should be read as meaning “including, without limitation” or the like; the term “example” is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; and adjectives such as “conventional,” “traditional,” “normal,” “standard,” “known” and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. 
     Likewise, a group of items linked with the conjunction “and” should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as “and/or” unless expressly stated otherwise. Similarly, a group of items linked with the conjunction “or” should not be read as requiring mutual exclusivity among that group, but rather should also be read as “and/or” unless expressly stated otherwise. Furthermore, although items, elements or components of the disclosure may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated. The presence of broadening words and phrases such as “one or more,” “at least,” “but not limited to” or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent. 
     As used herein, unless expressly stated otherwise, “operable” refers to able to be used, fit or ready for use or service, usable for a specific purpose, and capable of performing a recited or desired function described herein. In relation to systems and devices, the term “operable” means the system and/or the device is fully functional and calibrated, comprises elements for, and meets applicable operability requirements to perform a recited function when activated. 
     In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. The embodiments herein provide a ternary strained layer superlattice device, infrared detector, and methods of making the same. An infrared absorbing superlattice is defined herein as having an infrared absorbing element that is integrated to form a detector. Referring now to the drawings, and more particularly to  FIGS. 1 through 14 , where similar reference characters denote corresponding features consistently throughout, there are shown exemplary embodiments. 
       FIG. 1A  illustrates a ternary superlattice structure  10  comprising a substrate  12  and a non-changing periodic layer structure  11  on the substrate  12 . In the context of the embodiments herein, “non-changing” refers to a static state of the periodic layer structure  11  in terms of the number of monolayers of the semiconductor materials contained in the periodic layer structure  11  remaining constant. In some examples, the substrate  12  may comprise GaSb (gallium antimonide), InAs (Indium Arsenide), GaAs (Gallium Arsenide), and InP (Indium Phosphide). The non-changing periodic layer structure  11  comprises alternating infrared absorbing semiconductor materials comprising a first layer  14  of InAs [1-x] Sb [x]  ternary alloy material and a second layer  16  of In [1-y] Z [y] As ternary alloy material. Furthermore, either layer  14 ,  16  may be first applied directly to the substrate  12 . According to an example, Z is gallium (Ga) or aluminum (Al), wherein the second layer  16  may be In [1-y] Ga [y] As or In [1-y] Al [y] As. The ternary alloy materials in the first and second layers  14 ,  16  are defined by their alloy fractions, x and y, respectively. In various examples x is in the range of greater than zero and less than one, and y is in the range of greater than zero and less than one. The thickness of each of the first and second layers  14 ,  16  are substantially similar and, in one example, are configured to absorb light in a predetermined spectral band to increase an absorption coefficient, and to increase an electron and hole mobility through the periodic layer structure  11  and prevent trapping of carriers in any particular layer  14 ,  16 . More particularly, the thickness of the periodic layer structure  11  may be configured based on a predetermined selection of an amount of infrared absorption within the periodic layer structure  11 . Moreover, the thickness of each layer  14 ,  16  in the ternary superlattice structure  10 , which may be in the range of approximately 1 to 10 nanometers, depends on the desired optical and electrical properties and growth feasibility of the periodic layer structure  11 . 
     In an example, x is in the range of about 0.2 to about 0.8. In an example, y is in the range from about 0.05 to about 0.35. In another example, y is in the range from about 0.05 to about 0.5, and in another example, y is in the range from about 0.05 to about 0.8. In one example, the first layer  14  of InAs [1-x] Sb [x]  ternary alloy material may comprise at least one monolayer to about 30 monolayers. In another example, the second layer  16  of In [1-y] Z [y] As ternary alloy materials may comprise at least one monolayer to about 30 monolayers. In yet another example, the first layer  14  of InAs [1-x] Sb [x]  ternary alloy material may comprise at least one monolayer to about 20 monolayers. In still another example, the second layer  16  of In [1-y] Z [y] As ternary alloy materials may comprise at least one monolayer to about 20 monolayers. 
     The total of the InAs [1-x] Sb [x]  ternary alloy material monolayers and the In [1-y] Z [y] As ternary alloy material monolayers may be less than 30. The periodic layer structure  11  may comprise bound holes and unbound electrons whose ground state is at most 10 meV below a highest conduction band minimum. The maximum allowed strain, desired bandgap, desired band alignment between conduction bands or valence bands of the layers  14 ,  16 , desired location for the conduction and valence bands of the superlattice structure  10 , optical properties such as absorption coefficient, and electrical properties such as electron and hole mobility may be configured at a predetermined and desired level/degree by changing the thickness of the two layers  14 ,  16  and the alloy fractions, x and y. 
       FIG. 1B , with reference to  FIG. 1A , illustrates another diagram of the ternary superlattice structure  10 . The superlattice structure  10  is formed by a plurality of unit cells  13  grown on the substrate  12  in the direction indicated in  FIG. 1B , where each unit cell  13  comprises the first layer  14  of a ternary alloy InAs [1-x] Sb [x]  and the second layer  16  of a ternary alloy In [1-y] Z [y] As, where Z may be Ga or Al, in various examples. The non-changing periodic layer structure  11 , which is grown on the substrate  12 , is defined as the period of the superlattice structure  10  is defined as a combined thickness  20  of the two ternary alloy layers  14 ,  16 , and the number of periods (also defined as the number of repeats) of the periodic layer structure  11  determines the total thickness  25  of the superlattice structure  10 . While the period of the periodic layer structure  11  is the combined thickness of all the layers in the unit cell  13 , the number of periods is determined based on the desired total thickness  25  of the periodic layer structure  11 . The thicknesses of the first and second layers  14 ,  16  and the corresponding alloy fractions are the parameters used for configuring the superlattice structure  10 . 
     At least one layer  14  or  16  in the unit cell  13  may be doped either n-type or p-type to obtain a desired electron or hole concentration for the superlattice structure  10  thereby making the superlattice structure  10  suitable for building detector, and other, devices. The thickness of the layers  14 ,  16  is determined by the maximum allowed strain of the periodic layer structure  11  grown on the substrate  12 . The alloy fractions (x and y) of the layers  14 ,  16  are varied to configure the conduction and valence bands of the alloy materials in the layers  14 ,  16  thereby obtaining a specific band alignment between conduction bands or valence bands of the layers  14 ,  16 . Accordingly, the thickness  20  and the alloy fractions (x and y) of the layers  14 ,  16  may be varied to obtain desired locations of the conduction and valence bands in the periodic layer structure  11 . Moreover, the thickness  20  and the alloy fractions (x and y) of the layers  14 ,  16  may be varied to engineer the quantum mechanical properties of the electrons and holes in the periodic layer structure  11  thereby obtaining a maximum overlap of the electron and hole wavefunctions. Furthermore, the thickness  20  and the alloy fractions (x and y) of the layers  14 ,  16  may be varied to configure the quantum mechanical properties of the electrons and holes in the periodic layer structure  11  thereby maximizing the absorption coefficient therein. The total thickness  25  of the periodic layer structure  11  may be determined based on the desired amount of infrared absorption in the periodic layer structure  11 . Additionally, the thickness  20  and the alloy fractions (x and y) of the layers  14 ,  16  may be varied to configure the electronic bands of the periodic layer structure  11  to be sensitive by absorbing infrared radiation in any wavelength band. Also, the thickness  20  and the alloy fractions (x and y) of the layers  14 ,  16  may be varied to configure the electronic band of the periodic layer structure  11  to obtain the desired electron and hole effective mass of the periodic layer structure  11 . Further, the thickness  20  and the alloy fractions (x and y) of the layers  14 ,  16  may be varied to configure the electronic band of the periodic layer structure  11  to increase the electron and hole mobility in the periodic layer structure  11 , and thereby improving the sensitivity and carrier transport in the superlattice structure  10 . In an example, the layers  14 ,  16  in the unit cell  13  have their conduction bands aligned, closely aligned, or lined up to yield a type-I band alignment thereby introducing spatially direct optical transitions through excitation of carriers from the valence band to the conduction band of the superlattice structure  10 . 
       FIG. 2 , with reference to  FIGS. 1A and 1B , illustrates the bandgap and lattice constant for the Group III-V semiconductors, which are suitable for use in the superlattice structure  10 . The solid circles represent the binary semiconductors and the lines represent the ternary alloys of the two binary semiconductors connected by each line. For an example, the line  27  connecting GaAs and InAs represents the ternary alloys of GaAs and InAs, which are presented as In [1-y] Ga [y] As. Here, y is the alloy fraction which varies from greater than zero to less than 1. For comparison, the outlined area  29  represents the material space for conventional binary InAs/InAs [1-x] Sb [x]  superlattice structures, while the outlined area  28  shows the extended space for the superlattice structure  10  provided by the embodiments herein. 
     In developing a superlattice structure, such as the superlattice structure  10  provided by the embodiments herein, the alignment of the conduction and valence bands of two constituent materials, such as In [1-y] Ga [y] As and InAs [1-x] Sb [x] , play a critical role. The alloy fractions (x and y) may be adjusted so that the conduction bands of the two constituent materials are aligned. This is one aspect of the embodiments herein which is different than conventional InAs/InAs [1-x] Sb [x]  superlattice structures, and it results in performance enhancement of the superlattice structure  10 . For example, for x=0.35 and y=0.2, the favorable condition for ternary superlattice structures under which only the holes are confined, may be achieved. This leads to an increased electron-hole wavefunction overlap causing an increased absorption coefficient, a parameter that characterizes the absorption properties. Furthermore, a reduction in hole effective mass may also be predicted for the ternary superlattice structure  10  provided by the embodiments herein due to the broadening of its hole bands. This increases the hole mobility (and the diffusion length of n-type ternary superlattice structures) which is highly desired for enhancing performance of the devices that utilize ternary superlattice structure. While ternary superlattice structures may be employed in a broader wavelength spectrum covering short-wave infrared (SWIR: 0.7-2.5 μm), mid-wave infrared (3-5 μm), and long-wave infrared (8-14 μm) bands, they are highly suitable for the MWIR band. 
     Although superlattice structures are made out of semiconductor materials, once they are properly designed, superlattice structures, in general, are treated as new artificial semiconductors and are characterized by their parameters such as bandgap and electron and hole effective masses. The superlattice structure  10  may also be represented by its conduction-valence band profile, commonly known as the band diagram.  FIG. 3 , with reference to  FIGS. 1A through 2 , illustrates the band diagram for an InGaAs/InAsSb ternary superlattice structure  10 . 
     In general, electrons and holes in the superlattice structure  10  are confined and, as a result, the formation of quantized energy states (or minibands) occurs. As shown in  FIG. 3 , the band diagram includes an electron mini-band  31 , heavy hole-like mini-band  32 , and light hole-like mini-band  33 . The bandgap  34  of the superlattice structure  10  is defined as the energy gap between the electron mini-bands  31  and the heavy hole-like mini-bands  32 . The bandgap  34  has a valence band edge  39   a  and a conduction band edge  39   b . The bandgap  34  determines the longest wavelength of light that can be absorbed by the superlattice structure  10  and it can be tuned by changing the configuration of the superlattice structure  10 . The valence band profile  37  of the InGaAs material and the valance band profile  38  of the InAsSb material are also shown in the band diagram. A distinctive difference between InAs/InAsSb superlattice structures and InGaAs/InAsSb ternary superlattice structures is how the bulk layer conduction band profile varies across the superlattice structure  10 . The InGaAs/InAsSb ternary superlattice structure  10  has a fairly constant conduction band profile  35 ,  36 , while InAs/InAsSb superlattice structures have a discontinuity in the conduction band across the superlattice. Due to this specific conduction band profile  35 ,  36  in the ternary superlattice structure  10 , the absorption coefficient can be significantly enhanced. 
     The thickness  20  of the layers  14 ,  16  (e.g., the period) is one parameter controlling most of the desired band structure parameters and material properties. While the alloy composition, x and y, of the layers  14 ,  16  are determined to achieve the required conduction band profile  35 ,  36  and valence band profile  37 ,  38 , the thickness of the layers  14 ,  16  may be adjusted to balance the strain of the superlattice structure  10 , obtain the required bandgap  34  for the desired wavelength band, hole effective mass of the heavy-hole like mini-bands  32 , hole mobility, and the position of the hole mini-bands  32 ,  33 . When the thickness of the layers  14 ,  16  decreases/increases, the hole mini-bands  32 ,  33  move down/up, respectively, which effectively increases/decreases, respectively, the bandgap  34 . The alloy composition, x and y, of the layers  14 ,  16  can be iteratively tuned to further adjust the bandgap  34  and strain, however, the thickness  20  of the layers  14 ,  16  is used to enhance the material properties such as absorption coefficient and the carrier mobility. Decreasing the thickness  20  of the layers  14 ,  16  also increases the overlap between the electron and hole wavefunctions, enhancing the absorption properties of the superlattice  10  characterized by the absorption coefficient. In addition, reducing the thickness  20  of layers  14 ,  16  broadens the hole mini-bands  32 ,  33  causing a reduction of the hole effective mass, which in turn increases the hole mobility. 
     The calculated absorption coefficient at 80 K for three example superlattice structures are shown in  FIG. 4 , with reference to  FIGS. 1A through 3 . The superlattice structure exhibiting the lowest absorption coefficient spectrum  41  is a conventional InAs/InAsSb superlattice structure (17 ML InAs/5.5 ML InAs[0.65]Sb[0.35]), while the other two are ternary superlattice structures: 8.5 ML In[0.8]Ga[0.2]As/9 ML InAs[0.65]Sb[0.35] structure  42  and 3ML In[0.7]Ga[0.3]As/3 ML InAs[0.65]Sb[0.35] structure  43 . Both ternary superlattice structures  42 ,  43  show significant enhancement in absorption properties and the ternary superlattice structure  43  with 30% gallium shows more than a factor of two enhancement in the 3-5 μm band. This enhancement in the absorption coefficient directly impacts the quantum efficiency of the detectors utilizing these ternary superlattice structures  42 ,  43  as the infrared absorbing material. Band structure calculations also revealed that the optical transitions in the superlattice structure  43  is especially direct and the alignment of the conduction band profile  35 ,  36  is no longer a Type II band alignment. 
     In InGaAs/InAsSb ternary superlattice structures, the gallium composition can be used for “balancing” the strain in the superlattice structure in addition to typical layer thickness balancing performed in binary material systems. This greater flexibility in strain compensation allows for the ternary superlattice structure  10  to contain thin layers  14 ,  16  instead of having relatively thick layered strain balancing layer(s) as in the conventional structures. When the layers  14 ,  16  in the unit cell  13  are thin (e.g., about 5 to about 20 monolayers each), the broadening of the electron and hole wavefunctions increase resulting in reduction in the electron and hole effective masses. Based on band structure calculations for three example In [1-y] Ga [y] As/InAs [0.65] Sb [0.35]  superlattice structures with y=0, 0.05, 0.1, 0.2, and 0.3 show that the hole effective mass decreases with increasing gallium composition in the In [1-y] Ga [y] As layers (e.g., second layer  16 ). The calculated values of the hole effective mass are 2.97, 2.98, 2.14, 1.49, and 0.16 for y=0, 0.05, 0.1, 0.2, and 0.3, respectively. This reduction in hole effective mass for ternary superlattice structures is a predominant advantage as it relates to the hole mobility. With y=0, 0.05, and 0.2, the hole mobility for the ternary superlattice structures with y=0.2 is about an order of magnitude higher than the conventional InAs/InAsSb superlattice structure (y=0). This increase in hole mobility leads to an increase in the minority hole diffusion length in n-type superlattice structures employed in barrier detector structures such as nBn. 
     Similar to the In [1-y] Ga [y] As/InAs [1-x] Sb [x]  superlattice structures, which are primarily suited for infrared absorbers, In [1-y] Al [y] As and InAs [1-x] Sb [x]  can also be designed. While the In [1-y] Al [y] As and InAs [1-x] Sb [x]  ternary superlattice structures can also be used as infrared absorbers, they are particularly suitable for hole blocking barriers due to the possibility of wider bandgap compared to InGaAs/InAsSb ternary superlattice structures. For an example, a 4ML In[0.9]Al[0.1]As/3 ML InAs[0.62]Sb[0.38] ternary superlattice structure has a bandgap of 374 meV and its conduction band location is well within the acceptable range to align with the conduction band of the InGaAs/InAsSb ternary superlattice structure absorbers. Similarly, a 9 ML In[0.9]Al[0.1]As/3 ML In[0.5]Al[0.5]Sb ternary superlattice structure has a bandgap of 571 meV and it is suitable for a hole blocking barrier of pBp MWIR structures with p-type InGaAs/InAsSb ternary superlattice structures. 
       FIG. 5 , with reference to  FIGS. 1A through 4 , shows the band diagram of an nBn barrier infrared detector  50  grown on a substrate  51  and comprising of, from bottom to top, a thin n-type doped ternary superlattice as the bottom contact layer  52 , a thick n-type doped ternary superlattice absorber  53 , an electron barrier layer  54 , and another thin n-type ternary superlattice as the top contact  55   a . For proper operation, the superlattices are designed such that their valence bands  56  of all the layers exhibit a reasonable band alignment. 
     Similarly, as shown in  FIG. 6 , with reference to  FIGS. 1A through 5 , the top contact superlattice  55   b  can be doped p-type to form a heterojunction nBp photodiode device  60 . These devices  50 ,  60  are operated as minority carrier devices and the dark and photo current depend on three basic material properties: minority carrier lifetime, mobility, and absorption coefficient. In a conventional n-type superlattice structure absorber, the mobility of the monitory holes is low, thus the diffusion length remains short. Consequently, the detector&#39;s active layer thickness is limited by the diffusion length. Using a ternary superlattice structure  10  as provided by the embodiments herein achieves: (i) a higher absorption coefficient, which increases the absorption, and (ii) a higher hole mobility, which increases the carrier collection efficiency also allowing increasing the absorber thickness. 
     The ternary superlattice structure hole barriers provide some additional detector designs utilizing p-type absorbers. In p-type absorbers, the minority carriers are electrons and the electron diffusion length is a few orders of magnitude higher than the hole mobility.  FIG. 7 , with reference to  FIGS. 1A through 6 , shows the band diagram of apN heterojunction diode structure  70  grown on a substrate  71  and comprising of, from bottom to top, a thin p-type doped ternary superlattice as the bottom contact layer  72 , a thick p-type doped ternary superlattice absorber  73 , a thin wide bandgap undoped ternary superlattice as the hole blocking barrier  74 , and a thin n-type ternary superlattice  75   a  as the top contact. Here, the ternary superlattice structure  70  is configured to obtain a reasonable conduction band alignment  77 . The InGaAs/InAsSb ternary superlattice structure  70  exhibits its conduction band  77  at a relatively higher location compared to that of conventional InAs/InAsSb superlattice structures. This opens up the possibility of designing hole blocking barriers with wider bandgaps. As discussed above, InAlAs/InAsSb and InAlAs/InAlSb superlattice structures are two possible hole blocking barriers suitable for ternary superlattice structure detectors. Similarly, as shown in  FIG. 8 , with reference to  FIGS. 1A through 7 , a pBp structure  80  can be formed by replacing the top contact  75   a  of the pN heterojunction diode  70  (of  FIG. 7 ) with a p-type ternary superlattice structure  75   b  which is same or similar to the absorber  73 . Both of these structures  70 ,  80  utilizing p-type ternary superlattice structures  72 ,  73  have the advantage of the higher absorption coefficient to increase the quantum efficiency. 
     There are two known methods suitable for making the ternary superlattice structure  10 , i.e., molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE). For example, the MBE process begins by inserting a substrate  12  (e.g., GaSb or InAs) into a deposition system  90 , as shown in  FIG. 9 , with reference to  FIGS. 1A through 8 . After removing the surface oxide, the substrate  12  is heated to about 350° C. to about 500° C. and deposition commences. The atomic flux of the InGaAs layer (e.g., second layer  16 ) is carefully calibrated to minimize the amount of excess arsenic in the system (e.g., there is no more than about 50% more arsenic than required to maintain stoichiometric conditions in the InGaAs), and the atomic flux of the InAsSb layer (e.g., first layer  14 ) uses an arsenic flux that is sub-stoichiometric. The ratio of gallium to indium is controlled by the temperature of the effusion cells, and the ratio of arsenic to antimony is controlled by the position of the source valve  91 . Switching between each compound (e.g., InGaAs and InAsSb) is performed using a system of shutters 92, although one of the major aspects of the ternary superlattice structure  10  provided by the embodiments herein is the ability to leave open the arsenic and indium shutters 92 for the entire duration of the deposition. The deposition may occur at the rate of about 0.4 to about 1.0 monolayers per second via MBE, or about 1.2 to about 3 angstroms per second. 
     The main difference between the two deposition processes (MBE and MOVPE) is the use of mass flow controllers (not shown) in MOVPE as opposed to valves and shutters in MBE. MOVPE also use metalorganic precursors (i.e., tertiarybutylarsine (TBA), Arsine, TMSb, TESb, TMGa, etc.) and typically operates at higher substrate temperatures and growth rates. 
     The ternary superlattice structure  10  has been tested experimentally. The samples described below as well as the experimental procedures, test equipment, and materials are examples only, and the embodiments herein are not restricted to these particular samples. Three nBn detector structures that utilize In 1-y Ga y As/InAs 0.65 Sb 0.35  superlattice structures serving as n-type regions and an AlGaAsSb quaternary layer serving as the barrier were experimentally designed. All the detector structures are the same, except for the superlattice structure configuration. The In 1-y Ga y As/InAs 0.65 Sb 0.35  superlattice structures were configured to exhibit the same bandgap and the three ternary superlattice structure configurations have an approximate gallium fraction, y=0, 0.05, and 0.2. Descriptions of each of the samples are described in Table I. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Experimental Sample parameters of 500 nm thick  
               
               
                 In 1-y Ga y As/InAs 0.65 S 0.35  superlattice structures 
               
            
           
           
               
               
               
            
               
                   
                   
                 FWHM 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Gallium 
                 In 1-y Ga y As 
                 InAs 1-x S x   
                   
                   
                 Number 
                 Measured 
                 Measured  
                 SL + 1 
               
               
                 fraction 
                 thickness 
                 Thickness 
                 Measured 
                 Measured 
                 of 
                 period 
                 mismatch 
                 (arc 
               
               
                 (%) 
                 (Å) 
                 (Å) 
                 y 
                 x 
                 periods 
                 (Å) 
                 (%) 
                 sec) 
               
               
                   
               
               
                  0 
                 51.73 
                 16.73 
                 0.0000 
                 0.3571 
                 73 
                 66.98 
                 — 
                 81 
               
               
                  5 
                 39.56 
                 19.78 
                 0.0536 
                 0.3558 
                 84 
                 57.94 
                 0.06 
                 53 
               
               
                 20 
                 25.86 
                 27.38 
                 0.1940 
                 0.3571 
                 94 
                 51.35 
                 — 
                 74 
               
               
                   
               
            
           
         
       
     
     These exemplary superlattice structure samples were grown using a Veeco® Gen 930 solid-source molecular beam epitaxy reactor on n-type GaSb (100) wafers. Standard deoxidation techniques and GaSb buffer growth conditions were used prior to superlattice growth. The indium growth rate of the superlattice was set to 0.5 ML/sec, and the gallium growth rate was adjusted according to the desired composition (i.e. the growth rate of the InGaAs layer increased with gallium composition). The arsenic flux was held constant for all three InGaAs compositions, corresponding to an arsenic to Group III atomic flux ratio of approximately 1.1-1.4. The arsenic flux was decreased slightly for the InAs 0.65 Sb 0.35  layers in order to promote antimony incorporation. The growth temperature of the superlattice was approximately 405° C., or about 5° C. above the GaSb surface transition temperature. The number of periods was modified in order to maintain a constant 500 nm thick absorbing region. The surface roughness of the samples was checked post-growth, and verified to be less than 0.5 nm for all three samples. 
     X-ray diffraction measurements were performed on a PANalytical® Empyrean® diffractometer with a hybrid monochromator and a triple crystal. The symmetric measurements around the GaSb (004) reciprocal lattice point are shown in  FIG. 10 , with reference to  FIGS. 1A through 9 . The measured lattice mismatch for all samples was less than 0.1%, which indicates that it should be possible to grow thick absorbing regions without creating strain-induced defect states. Meanwhile, the full-width-half-maximum (FWHM) values of the SL+1 peaks were 50-85 arcsec, and do not exhibit a clear trend. It is possible that the FWHM decreased as the number of periods increased, and then increased again as the gallium affected growth through either intermixing or crystal quality. Finally, the fringe spacing for all three superlattice structure samples was within 4% of their modeled periodicity, which implies that the modeled absorption/emission values should be valid for each sample. 
     Square mesa single element detectors were fabricated and the spectral photoresponse of the experimental devices were also measured. As shown in  FIGS. 11A and 11B , with reference to  FIGS. 1A through 10 , the increase in the absorption for ternary superlattice structures is clearly observed as the highest quantum efficiency is observed for the sample with 20% gallium composition in the experimental InGaAs/InAsSb superlattice structure. Furthermore, the temperature variation of the quantum efficiency confirms that the increase in quantum efficiency is not due to poor carrier collection, but indeed due to the increase in the absorption. 
     To investigate the optical properties of the sample superlattice structures, photoluminescence (PL) and transmission characterization were conducted. The excitation- and temperature-dependent PL measurements were performed using a 532-nm 2-W Coherent VERDI® laser modulated at 60 kHz, a Bruker® 80V Fourier transform infrared (FTIR) spectrometer, a liquid nitrogen cooled HgCdTe detector with a cutoff at approximately 12 μm, and a closed-cycle helium cryostat capable of maintaining temperatures from 4K to 325 K. PL spectra under low excitation (approximately 4 W/cm 2 ) for each of the samples were generated at 80K and 150 K. The sample without any Ga (InAs/InAs 0.65 Sb 0.35  superlattice structure) had the highest PL intensity in the set at both temperatures. The PL intensity decreased monotonically with increased Gallium concentration. Several possibilities exist for this trend, including shorter Shockley-Read-Hall lifetimes caused by defects, increased surface recombination caused by enhanced absorption, or increased carrier escape caused by changes in the conduction band alignment and hole effective mass. As shown in  FIG. 12 , with reference to  FIGS. 1A through 11B , the PL peak locations for the three samples at a given temperature stay within 64 meV over a wide range of temperatures. This result confirms the validity of the sample configuration to predict the material bandgap. The FWHM of the spectra at 80K are 3261 meV. At 150 K, the Ga containing superlattice structures have FWHMs of 35 meV while that of the InAs/InAs 0.65 Sb 0.35  superlattice structure is 5 meV higher. Consistent FWHMs reveal the uniform crystal quality of the entire set of samples, even as additional Ga is added. To determine the bandgap from the PL peak location, ½kT was subtracted, and the resulting bandgaps are shown in Table II for each sample. The measured bandgap according to the PL results is on average 16 meV lower than the calculated (Calc.) bandgap, but the bandgaps are consistent within the set and agree very well with bandgaps determined from absorption (Abs.). 
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 Summary of the results for the bandgap of superlattice structures 
               
            
           
           
               
               
               
            
               
                 Gallium 
                 80K bandgap (eV) 
                 150K bandgap (eV) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 fraction (%) 
                 Calc. 
                 PL 
                 Abs. 
                 Calc. 
                 PL 
                 Abs. 
               
               
                   
               
               
                  0 
                 0.252 
                 0.236 
                 0.234 
                 0.237 
                 0.222 
                 0.223 
               
               
                  5 
                 0.255 
                 0.233 
                 0.234 
                 0.240 
                 0.220 
                 0.219 
               
               
                 20 
                 0.252 
                 0.243 
                 0.240 
                 0.238 
                 0.229 
                 0.223 
               
               
                   
               
            
           
         
       
     
     In order to obtain the absorption properties of the superlattice structures, IR transmission measurements were performed using a FTIR spectrometer. IR transmission through each sample and their substrates (substrate samples were prepared by etching away the superlattice structures layers) were separately measured at 80K and 150 K. The absorption coefficient of the superlattice structures were obtained by fitting the modeled transmission to the experimentally measured transmission through a regression analysis. 
     These superlattice structures were designed for 150K operation, which is evidenced by the complete coverage of the 3-5 μm spectral band at 150 K, while at 80 K, the absorption coefficient is virtually negligible at 5 μm. While achieving the same bandgap for different lattice-matched designs is remarkable, the most important observation is the trend of the absorption coefficient variation when the Ga composition in the InGaAs layers is increased. 
     The sample with no gallium (InAs/InAs 0.65 Sb 0.35  superlattice structure) has the lowest absorption coefficient, while the sample with the highest Ga fraction (In 0.8 Ga 0.2 As/InAs 0.65 Sb 0.35 ) shows the highest absorption coefficient. It is an increase of 30%-35% over the entire 3-5 μm spectral band. This result clearly proves the advantages of the ternary superlattice structures provided by the embodiments herein, especially in increasing the absorption properties. Furthermore, for the InAs/InAs 0.65 Sb 0.35  sample, the value of the absorption coefficient at 4 μm is about 2017 cm −1  at 80K (2366 cm −1  at 150 K), while the In 0.8 Ga 0.2 As/InAs 0.65 Sb 0.35  superlattice structure sample has an absorption coefficient of 2962 cm −1  at 80K (3256 cm −1  at 150 K). 
     Accordingly, experimental demonstration reveals that an approximately 30%-35% increase in the absorption coefficient may be achieved for an InGaAs/InAsSb ternary superlattice structure  10  compared to conventional InAs/InAsSb superlattice structures. When the ternary superlattice structure  10  is used in infrared sensors, the layer thickness  20  of a unit cell  13  may be reduced by approximately 50% with comparable sensitivity compared to InAs/InAsSb superlattice structures, which is extremely favorable for production. The superlattice structure  10  provided by the embodiments herein achieves an overall system cost-size-weight-and-power (CSWaP) reduction, and increases the reliability and sustainability, all of which are immensely beneficial for military and civilian applications. For example, infrared focal plane array cameras are highly sought for various commercial/civilian applications such as medical imaging, manufacturing, night vision, thermography, imaging in weather satellites, and astronomy, which may utilize the features provided by the embodiments herein. 
       FIG. 13A , with reference to  FIGS. 1A through 12 , illustrates a method  100  of forming a ternary superlattice structure  10 , the method  100  comprising heating ( 101 ) a substrate  12  to a temperature of about 395° C. to about 460° C. in an ultrahigh vacuum ranging from about 10 −8  to 10 −10  Torr in base pressure. Next, the method  100  provides depositing ( 103 ) at least one monolayer to about 30 monolayers of In [1-y] Z [y] As to form a first semiconductor layer  14  of In [1-y] Z [y] As ternary alloy material on the substrate  12 , wherein Z is selected from the group comprising of Ga and Al. The method  100  further includes depositing ( 105 ) at least about one monolayer to about 30 monolayers of InAs [1-x] Sb [x]  to form a second semiconductor layer  16  of InAs [1-x] Sb [x]  ternary alloy material on the substrate  12 , wherein x is in a range from greater than zero and less than 1, and y is in a range from greater than zero and less than 1, and wherein a thickness of the first and second semiconductor layers are substantially similar and configured to absorb light in a predetermined spectral band, to increase an absorption coefficient, and to increase an electron and hole mobility through the ternary superlattice structure  10  and prevent trapping of electrons in any particular layer. 
     In an example, y is in a range from about 0.05 to about 0.35. In an example, x is in a range from about 0.2 to about 0.8. The rate of deposition to form either the first semiconductor layer  14  of In [1-y] Z [y] As ternary alloy material or the second semiconductor layer  16  of InAs [1-x] Sb [x]  ternary alloy material may be in a range from about 0.4 monolayers per second to about 1.5 monolayers per second, according to an example. The first semiconductor layer  14  of In [1-y] Z [y] As ternary alloy material may comprise between about 1 to about 20 In [1-y] Z [y] As monolayers, and the second semiconductor layer  16  of InAs [1-x] Sb [x]  ternary alloy material may comprise between about 1 to about 20 InAs [1-x] Sb [x]  monolayers, wherein a sum of the In [1-y] Z [y] As monolayers and the InAs [1-x] Sb [x]  monolayers may be about 30 or less, according to various examples. 
     As shown in  FIG. 13B , with reference to  FIGS. 1A through 13A , the method  100  may further comprise providing ( 107 ) a flux of arsenic (As) and indium (In) species onto a surface of the first semiconductor layer  14  of In [1-y] Z [y] As, wherein the flux of arsenic (As) species provides an over-pressure of arsenic sufficient to suppress any evaporation of arsenic from the ternary superlattice structure  10 ; adjusting ( 109 ) between the depositing steps for the first and second semiconductor layers  14 ,  16  of In [1-y] Z [y] As and InAs [1-x] Sb [x] , wherein the arsenic and antimony fluxes are changed; providing ( 111 ) a flux of antimony (Sb), arsenic (As), and indium (In) species onto a surface of the second semiconductor layer  16  of InAs [1-x] Sb [x] , wherein the flux of antimony (Sb) and arsenic (As) species provides an over-pressure sufficient to suppress any evaporation of arsenic and antimony from the ternary superlattice structure  10 ; and adjusting ( 113 ) between the depositing steps for the first and second semiconductor layers  14 ,  16  of In [1-y] Z [y] As and InAs [1-x] Sb [x] , wherein the arsenic and antimony fluxes are changed. 
       FIG. 14 , with reference to  FIGS. 1A through 13B , illustrates an infrared absorbing detector  150  comprising an infrared absorbing ternary superlattice structure  10  comprising a substrate  12  and alternating semiconductor layers comprising a first layer  14  of InAs [1-x] Sb [x]  ternary alloy material, and a second layer  16  of In [1-y] Z [y] As ternary alloy material, wherein Z is Ga or Al, wherein x is in a range of greater than zero and less than one, wherein y is in a range of greater than zero and less than one. The detector  150  further comprises a unipolar barrier layer  152  on a first side  154  of the ternary superlattice structure  10  and configured to permit transport of one type of carriers across the barrier layer  152 , wherein a bandgap of the barrier layer  152  is greater than the bandgap of the ternary superlattice structure  10 . A first contact layer  156  is provided on a second side  158  of the ternary superlattice structure  10  above the substrate  12 . A second contact layer  160  is provided adjacent to the unipolar barrier layer  152 . In an example, y is in a range from about 0.05 to about 0.35. In an example, x is in a range of about 0.2 to about 0.8. According to some examples, the ternary superlattice structure  10  is doped n-type to yield a desired electron concentration in the ternary superlattice structure  10  and the first contact layer  156  and the second contact layer are also doped n-type. In an example, the electron transport between the second contact layer  160  and the first contact layer  156  is substantially blocked by the barrier layer  152 . In another example, the second contact layer  160  is a p-type doped layer. 
     In an example, the barrier layer  152  may comprise an offset between a conduction band of the ternary superlattice structure  10  and a conduction band of the barrier layer  152  to block a transport of electrons across the barrier layer  152  (see  FIG. 5  as an example of such an offset in the conduction bands  57 ), wherein valence bands of the ternary superlattice structure  10 , the barrier layer  152 , the first contact layer  156 , and the second contact layer  160  are substantially aligned to allow transport of holes between the ternary superlattice structure  10 , the barrier layer  152 , the first contact layer  156 , and the second contact layer  160  (see  FIG. 5  as an example of such an alignment of the valence bands  56 ). In another example, the barrier layer  152  may comprise an offset between a valence band of the ternary superlattice structure  10  and a valence band of the barrier layer  152  to block a transport of holes across the barrier layer  152  (see  FIG. 8  as an example of such an alignment of the valence bands  76 ), wherein conduction bands of the ternary superlattice structure  10 , the barrier layer  152 , the first contact layer  156 , and the second contact layer  160  are substantially aligned to allow transport of electrons between the ternary superlattice structure  10 , the barrier layer  152 , the first contact layer  156 , and the second contact layer  160 . In another example, the barrier layer  152  may comprise a ternary superlattice structure  10  comprising alternating semiconductor layers comprising a first layer  14  of InAs [1-x] Sb [x]  ternary alloy material, and a second layer  16  of In [1-y] Z [y] As ternary alloy material, wherein Z is Ga or Al, wherein x is in a range of greater than zero and less than one, and wherein y is in a range of greater than zero and less than one. In an example, the barrier layer  152  may be undoped or lightly doped thereby keeping the carrier concentration in the barrier layer  152  minimal. 
     The embodiments herein may be used to construct infrared (IR) detectors and focal plane arrays (FPAs), which are key components in military and civilian applications including medical and imaging devices. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.