Abstract:
A barrier-type photo-detector, such as an infra-red detector, is disclosed. The detector may include an absorber layer having predetermined majority and minority carrier types with corresponding energy bands; and a Barrier made, at least in part, of a semiconductor with a Barrier energy gap and corresponding conduction and valence bands, a first side of said Barrier adjacent a first side of said absorber layer. Metal contact regions may be disposed on the barrier layer, the metal contact regions delineating pixels where image data may be read out from the photo-detector; wherein the Barrier is configured so as to allow minority carrier current flow while blocking majority carrier current flow between the absorber layer and the metal contact regions.

Description:
TECHNICAL FIELD 
     The present disclosure relates to barrier-type photo-detectors based on the structures disclosed in U.S. patent application Ser. No. 11/276,962 filed on Mar. 19, 2006, and issued as U.S. Pat. No. 7,687,871 on Mar. 30, 2010, the entire contents of which are hereby incorporated by reference. The present disclosure also relates to the composition, structure, and production of barrier-type photo-detectors. 
     BACKGROUND 
     This disclosure pertains to a reduced dark current barrier-type photo-detector that is comprised of a doped semiconductor layer, a barrier, and metal contacts disposed on the barrier where the doped semiconductor layer is used for photo-absorption and the effective conduction and valance band alignments for the doped semiconductor layer and barrier are arranged so as to allow photo-generated minority carrier flow to the contacts but filter or block majority carrier flow. Individual elements (e.g., pixels) in the photo-detector array may be defined by the metal contacts disposed on the barrier. The harrier, however, may be preserved such that it extends beyond the extent of the defined pixel areas. Although applicable to a wide range of barrier-type photo-detector, the exemplary embodiments and associated energy band diagrams presented in this disclosure depict an nBm barrier-type photo-detector. The structures and methods discussed herein, however apply as well to pBm (e.g., p-doped) structures. The “m” in this case stands for the metal disposed on the barrier to define readout pixels. 
     An embodiment of an exemplary nBn structure electron band diagram is illustrated in  FIG. 1 . The embodiment represents an embodiment of the concepts described in U.S. Pat. No. 8,044,435. The underlying concept relates to driving minority carriers from a photo absorbing layer  1000  to a contact region  1020  through a barrier  1010  where the compositions of the absorber layer, barrier, and contact layers are such that minority carriers can penetrate the barrier  1010  but majority carriers cannot. As can be seen in  FIG. 2  and as discussed in U.S. Pat. No. 8,044,435,the pixels in the contact layer  2030  are isolated by etching down to, but not through, the barrier  2010  in order to accomplish pixel delineation. In the embodiment shown, each pixel is associated with a contact  2040  and a read-out interconnect point  2080 . Minority carriers generated in the absorber layer  2000  pass through the barrier  2010  and into the contact layer  2030  where they are read-out through the interconnect  2080  via the contact  2040 . This process complicates focal plane array structure and fabrication by requiring additional etch and passivation steps. This fabrication process results in etched mesas, which can contribute excessive dark current for small area devices. 
     It would be an improvement in terms of performance, manufacturability, reliability, versatility and production yield to delineate and isolate pixels in such a photo-detector in ways other than material removal. It would also be an improvement in terms of performance, manufacturability, reliability, versatility, and production yield to delineate and isolate pixels without the need for a contact layer. 
     SUMMARY 
     Aspects of the present disclosure are directed at barrier-type photo-detectors with no or minimal contact layers. Such detectors instead have delineated pixel regions defined by metal contacts disposed directly on or graded I alloyed down to the barrier. 
     Embodiments of techniques and devices disclosed herein may pertain to a photo-detector, such as, for example, an infra-red photo-detector, the photo-detector comprising: an absorber layer having predetermined majority and minority carrier types with corresponding energy bands; a barrier comprising a semiconductor with a barrier energy gap and corresponding conduction and valence bands, a first side of said barrier adjacent a first side of said absorber layer; and metal contact regions disposed on the barrier, the metal contact regions delineating pixels where image data may be read out from the photo-detector; wherein the barrier is configured so as to allow minority carrier current flow while blocking majority carrier current flow between the absorber layer and the metal contact regions. 
     In some embodiments, the photo-detector may include a passivation layer disposed between the metal contact regions and the barrier; the thickness and composition of the passivation layer being such that the minority carriers passing through the barrier tunnel through the passivation layer to reach the metal contact regions. In some embodiments, the metal contact regions may be alloyed through a passivation layer disposed between the metal contact regions and the barrier, the alloy creating a direct metal contact with the barrier. In some embodiments, the metal contact regions may include molybdenum or a molybdenum alloy. In some embodiments, the photo-detector may include a substrate layer comprising a semiconductor, the absorber layer being disposed between the substrate layer and the barrier. 
     In some embodiments, the absorber layer is an n-doped semiconductor. In some embodiments, the barrier is an un-doped semiconductor. In some embodiments, the barrier has high aluminum content and the detector further comprises a passivation layer disposed between the metal contact regions and the barrier, the passivation layer being configured to prevent oxidation of the barrier by preventing the barrier from being exposed to air. 
     In some embodiments, the barrier includes a mesa structure extending past a furthest extent of the metal contact regions. In some embodiments, the barrier includes a plurality of protruding portions disposed on the mesa structure such that the protruding portions of the harrier are physically separated from each-other and such that each protruding portion of the barrier is physically connected to the mesa structure; and the metal contact regions are disposed on the protruding portions. 
     In some embodiments, the photo-detector may include a plurality of pixel stacks, each pixel stack including a metal contact region, a barrier layer portion, and an absorber layer portion; and each pixel stack being physically separated from the other of said plurality of pixel stacks such that each barrier layer portion in a pixel stack is physically isolated from a barrier layer portion in another pixel stack. 
     Embodiments of techniques and devices disclosed herein may pertain to a method of making a barrier-type photo-detector, such as, for example, an infra-red photo-detector, the method comprising: providing an absorber layer having predetermined majority and minority carrier types with corresponding energy bands; providing a barrier comprising a semiconductor with a harrier energy gap and corresponding conduction and valence bands, a first side of said barrier adjacent a first side of said absorber layer; providing metal contact regions on the second side of said barrier, the metal contact regions delineating pixels where image data may be read out from the photo-detector; and configuring the barrier so as to allow minority carrier current flow while blocking majority carrier current flow between the absorber layer and the metal contact regions. 
     In some embodiments, the method further includes providing a passivation layer on a second side of said barrier opposing said first side; said providing the passivation layer being performed in-situ during photo-detector manufacturing. 
     In some embodiments, the method further includes providing a passivation layer on a second side of said barrier opposing said first side; etching a pattern into the provided passivation layer in-situ; and providing the metal contact regions including providing metal into the etched pattern in the passivation layer. In some embodiments, etching a pattern is performed with a dry-etching technique. 
     In some embodiments, providing metal contact regions includes: providing a continuous layer of metal on the second side of said barrier; and performing in-situ etching of the continuous layer of metal to create the metal contact regions. In some embodiments, the method further includes: providing a passivation layer between the barrier and the metal contact regions; and alloying the metal contact regions through the passivation layer to create a direct metal contact with the barrier. 
     In some embodiments, providing the metal contact regions includes providing the metal contact regions onto the second side of the barrier; and creating a passivation layer on the barrier by allowing those portions of the second side of the barrier not covered by metal contact regions to become oxidized. In some embodiments, the method further includes: etching the barrier layer to create a mesa structure extending past a furthest extent of the metal contact regions. 
     In some embodiments, etching the barrier further includes etching the barrier to create a plurality of protruding portions disposed on the mesa structure such that: the protruding portions of the barrier are physically separated from each-other; each protruding portion of the barrier is physically connected to the mesa structure; and the metal contact regions are disposed on the protruding portions. 
     In some embodiments, the method further includes: etching the photo-detector to create a plurality of pixel stacks, each pixel stack including a metal contact region, a barrier layer portion, and an absorber layer portion; said etching being performed to etch completely through the barrier layer portion such that each pixel stack is physically separated from the other of said plurality of pixel stacks and such that each barrier layer portion in a pixel stack is physically isolated from a barrier layer portion in another pixel stack. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred variations of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  shows an energy-band diagram in a barrier-type photo-detector; 
         FIG. 2  shows an embodiment of a barrier-type photo-detection array with pixels delineated by layer material removal; 
         FIG. 3  shows an exemplary electro-optical radiation wavelength spectrum and the relationship with an exemplary compound semiconductor material system; 
         FIG. 4   a  shows an embodiment of a barrier-type photo-detection array with pixels delineated by contacts disposed on a passivation layer; 
         FIG. 4   b  shows an embodiment of a barrier-type photo-detection array with pixels delineated by contacts surrounded by a passivation layer; 
         FIG. 5   a  shows an energy-band diagram in a pixel region of a barrier-type photo-detector as described herein; 
         FIG. 5   b  shows an energy-band diagram in a pixel region of a barrier-type photo-detector as described herein; 
         FIG. 5   c  shows an embodiment of a barrier-type photo-detection array with pixels delineated by contacts disposed on an etched barrier; and 
         FIG. 5   d  shows an embodiment of a barrier-type photo-detection array with pixels delineated by contacts disposed on an etched barrier. 
       The drawings will be described in detail in the course of the detailed description of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents thereof. 
     Improved reliability, radiation hardness, manufacturability, cost savings, and improved production yields can be realized by modifying the structure of a barrier-type detector in ways other than removal of semiconductor material to accomplish pixel isolation. In particular, elimination of a contact layer and application of metal directly to the barrier may make barrier-type photo-detectors more reliable, easier to manufacture, and able to operate at higher temperatures. Further benefits of such modification include drastic reduction or elimination of surface states that would otherwise occur in the mesa sidewalls of the contact layer, which lead to benefits such as improved reliability and radiation hardness. The pixel delineation, according to the teachings of the present application, can be formed by any standard semiconductor metallization technique such as lift-off, blanket vapor deposition or sputtering followed by dry or wet etching. Additionally, the patterned metal can be alloyed with/through the continuous barrier passivation layer. 
     In some embodiments of a device as depicted in  FIG. 2 , the barrier  2010  may be constructed of a material that may rapidly oxidize upon exposure to air. In an nBn device, the contact layer  2030  may prevent barrier oxidation and allow current collection at the contact  2040 . However, charge separation happens at the barrier interface, so in some embodiments a function of the contact layer  2030  is carrier recombination. The contact layer  2030 , because it is a semiconductor, may incur some undesirable side effects. For example, light absorbed by the contact layer  2030  is lost. Also, doping of contact layer  2030  builds in the need for a higher operating voltage and therefore an inherently higher dark current level. 
     In some embodiments, minority carrier recombination can be realized in a metal. interface without a contact layer. However, embodiments using barriers with high aluminum content may require a passivation layer (such as, for example, a GaSb layer) to prevent barrier oxidation. In some embodiments, metal contacts may be deposited onto the passivation layer. 
     An example of a structure with isolated pixels made of metal is illustrated in  FIG. 4   a , Any suitable method for applying metal contact  3050  to a passivated barrier  3020  may be used as long as such methods allow vertical minority carrier transport from the absorber layer  3010  through the barrier  3020 , to the individual pixel regions  3050 , which represent the pixel interconnect points. Such a solution also inherently prevents lateral transport of minority carriers between pixels because the metal contacts  3050  are physically separated from each-other, eliminating or reducing lateral conductivity and making crosstalk virtually impossible. 
     Delineated pixel regions  3050  of the type illustrated in  FIG. 4   a  can be accomplished by replacing the steps of contact etching, barrier oxidation, barrier passivation otherwise required in an nBn device of the type shown in  FIG. 1  with a mask-less barrier passivation step followed by deposition of metal contacts onto the passivation layer  3030 . The barrier passivation layer  3030  may be deposited epitaxially in-situ. Etching or patterning of the passivation layer  3030  may be avoided in some embodiments where a passivation layer  3030  material is chosen to have a high resistance to lateral transport. This prevents crosstalk or shorting between adjacent pixels without requiring an etched passivation layer  3030 . 
     The semiconductor materials used in the barrier  3020  and I or absorber layers  3010  can be composed of a wide range of semiconductors including Si, InAs, GaSb, GaAs, InSb, AlAs, AISb, HgCdTe, InAsSb, InAsGaSb or any other suitable materials or material combinations that supply the valance and conduction band relationships for passing minority carriers or blocking or suppressing the flow of majority carriers through the barrier. These materials can be formed of suitable amorphous, bulk crystalline, digital alloy, or superlattice configurations. Further improvements in material properties can also be gained through the incorporation of Bi, N, or other materials to the above-listed semiconductors (or combinations thereof). Such alterations in material properties and strain can change the bandgap or conduction or valence band alignments to achieve a desired valence and conduction band configuration such as contemplated in  FIGS. 1 and 5   a . 
     Embodiments of barriers  3020  can be constructed of uniform alloys, superlattices, digital alloys, strain compensation layering or other bandgap-engineered structures. Desirably, the effective conductance and valance band alignments allow the flow of photo-generated minority carriers but block the flow of majority carriers. 
     Embodiments of absorber layer  3010  may include n-doped or p-doped semiconductors. In some variations, the absorber layer may be un-intentionally doped. 
     In the embodiment depicted in  FIG. 4   a , the metal contacts  3050  may be used with conventional Indium type pixel interconnects common for infra-red focal plane arrays. Alternate embodiments can employ a wide range of interconnect methods in combination with embodiments of the present invention. Alternate interconnect embodiments may include methods developed using micro-electrical-mechanical-systems (MEMS) processing and other known methods developed for silicon integrated circuit interconnects. 
     Further variations on the embodiments of the photo-detector described above can include different types of semiconductor (barrier, absorber layer, substrate) having different material combinations or doping types or concentrations. In some embodiments, a bandgap of the photoabsorbing layer (absorber layer  3010 ) can be designed to have a maximum cutoff wavelength that supports the absorption of electromagnetic radiation within the ultraviolet, visible, shortwave (SW), midwave (MW) or longwave (LW) atmospheric transmission bands as shown in  FIG. 3 . Strain can be introduced into an embodiment of a photo-detecting structure to favorably improve the energy band alignments. In embodiments where the barrier  3020  is sufficiently thin so as to not dislocate, often referred to as beneath the critical thickness, the barrier  3020  can be subjected to higher strain than in the absorber layer  3010  due to its thickness. Barrier  3020  embodiments can combine strain with layered materials to bandgap engineer the band alignment favorable to the carrier filtering function (passing minority carriers While blocking majority carriers). In some embodiments, a metamorphic (e.g., strain relaxed) absorber material may be used in the absorber layer  3010 . In such embodiments, the barrier  3020  may exceed the critical thickness. 
     In the embodiment shown, the barrier passivation layer  3030  is sufficiently thin to allow minority carriers to tunnel through from the barrier  3020  to the metal contacts  3050 . The lateral conductivity issue is eliminated because the barrier layer  3020  and the passivation layer  3030  result in high pixel-to-pixel impedance. 
     In alternate embodiments, such as the one shown in  FIG. 4   b , pixel delineation may be accomplished by alloying the metal  3100  into the passivation layer  3110  to create a direct metal contact between the barrier  3120  and the metal  3100 . In some embodiments this may be realized by putting a thin metal layer down over the passivation layer  3110 , patterning the metal layer (using, for example, either wet etch or dry etch in-situ without lift-off), and then alloying the patterned metal  3100  into the passivation layer  3110  to create the direct metal contact. 
     In another embodiment, barrier oxidation may be employed as a passivation technique. In such an embodiment, the passivation layer  3110  may be removed after metal (such as, for example, molybdenum or a molybdenum alloy) is patterned onto the barrier  3120 . A wet or dry in-situ etch may then be performed to pattern the metal into contacts  3100  that are directly contacting the barrier. The barrier may then be allowed to oxidize in those regions not covered with contacts  3100 , effectively creating a passivation layer  3110  in the oxidized portions. In some embodiments, such as, for example, a technique using SF 6 /Ar dry etching of blanket deposited TiW contact metal, pixels with spacing of between 80 and 120 nm can be realized. 
     By eliminating the contact layer altogether, fabrication of a detector as disclosed herein can be simplified. Defects and opportunities for patterning error or contamination associated with masking and lift-off operations can be eliminated. By eliminating the existence of etched surfaces on side walls of delineated pixels surface recombination and surface dark current generation are further reduced, thereby allowing for further improvements in quantum efficiency, sensitivity and I or higher temperature operation. 
     In some embodiments, valence band alignment issues that would otherwise exist between the barrier and a contact layer are removed. As shown in  FIG. 1 , close (and, in some cases, near-perfect) band alignment is required. In addition, valence band barriers can exist even for perfectly aligned barrier interfaces, causing increased operating voltage. Decreasing the operating bias reduces the required absorber depletion region, resulting in reduction in G-R. current. Eliminating the contact layer relaxes the band gap alignment requirement somewhat by removing the contact layer related valance band barriers. The voltage required for minority carrier collection is then reduced because impediments to hole flow on the contact side of the barrier are removed, thereby reducing the dark current levels in the detector. 
     The specific energy-band properties of a barrier-type detector having isolated pixel regions created without a contact layer is explained with reference to the embodiment shown in  FIG. 5   a .  FIG. 5   a  depicts an energy band configured for carrier transport through the barrier  5100  for photo-detection in the pixel region  5120  at the contact. A voltage bias for operation is applied between the contact  5120  and absorber layer  5130  terminals. 
     As can be seen from the diagram in  FIG. 5   a , a detector as discussed herein performs similarly to barrier-type photo-detectors having a contact layer. The barrier  5100  conduction E c and valence E v  band alignments are designed to allow the flow of photo-generated minority carriers from the absorber layer  5130  and block the flow of majority carriers. Replacing the doped contact layer otherwise found in nBn-type devices with metal contact  5120  reduces the voltage required for minority carrier collection. Furthermore, this reduced voltage reduces the depletion zone in the absorber layer, resulting in reduced levels of parasitic dark current. This can be realized because suppression of majority carrier flow through the barrier  5100  prevents these carriers from being depleted in the absorber layer  5130 . Furthermore, in the embodiment shown, the work. function of the metal contact  5120  is selected so that it is aligned to the Fermi level  5110  of the absorber layer  5130 . By preventing the Fermi level  5110  of the absorber layer from passing through the middle of the band gap, the activation energy for dark current generation of the device is further increased, thereby reducing carrier generation and collection. 
     In an embodiment as depicted in  FIG. 5   b , the barrier  5200  may be covered with a passivation layer  5210  that is in contact with metal contact  5220 . Such an embodiment may have the energy band properties shown. In the embodiment shown, the passivation layer  5210  disposed on the barrier  5200  is non-conductive, thereby passivating majority or minority carrier flow and reducing or minimizing lateral carrier transport between metal contact  5220 . The passivation layer  5210  portions not covered with a metal contact  5220  delineate pixels while eliminating lateral conduction and crosstalk, and allow at least part of the barrier  5200  to extend underneath as grown, maintaining the passivating properties at the exposed surfaces. The passivation layer  5210  portions covered with a metal contact  5220  allow for minority carrier transport to the metal contact  5220 . In some embodiments, this may be realized by having the minority carriers tunnel through the passivation layer  5201 . In other embodiments, this may be realized by alloying the metal contact  5220  into the passivation layer to create a more direct metal contact with the barrier. In yet further embodiments, the metal contact  5220  may be partially alloyed into the passivation layer  5210  to reduce the distance that a carrier must tunnel through before being collected for recombination and readout. 
     In yet another embodiment, the barrier interfaces can be compositionally graded to provide for minority carrier transport while avoiding charge trapping and undesirable carrier recombination at the barrier interfaces. Embodiments having graded interfaces can reduce the reverse bias needed to extract photo-generated carriers while reducing or eliminating charge storing behavior. In some embodiments, grading of the barrier composition at the absorber interface can reduce or eliminate notches, which can arise in ungraded interfaces that may result in higher required bias voltages, charge storage effects, or reductions in quantum efficiency due to carrier recombination. Such embodiments may entail a more complex or involved growth process for the barrier than the structure of  FIG. 5   a . Such graded barrier or graded interface embodiments may be realized by applying the techniques and structures disclosed in U.S. Pat. No. 8,044,435, the entire contents of which are hereby incorporated by reference. 
     In yet another embodiment, doping profiles of the absorber layer and/or barrier can be adjusted to ensure zero-bias detectivity. In some embodiments, the barrier interface and/or bulk are doped such that the Femi-level of the absorber layer and the contact layer are aligned with no built-in potential across the valance band. 
     In yet further embodiments, such as the one depicted in  FIG. 5   c , a direct-metal barrier-type detector may include a fully or partially etched barrier  5410 . In such an embodiment, a mesa structure may be etched into the barrier  5410  by etching individual pixel regions such that each metal contact  5420  is disposed on a portion of barrier  5410  protruding above an overall mesa structure in the barrier  5410  that extends beyond the individual pixel regions. In some embodiments, the absorber layer  5401  may be co-extensive with the barrier mesa. In other embodiments, the absorber layer may be fully covered by the barrier (not shown). Also, in some embodiments, the barrier mesa and exposed sides of the protruding portions  5430  may be covered with a passivation layer (not shown) to prevent oxidation of the barrier. In some embodiments, such a passivation layer may also be disposed between the metal contact  5420  and the barrier  5410  in the manner described above in  FIG. 4   a . 
     In yet another embodiment, such as the one depicted in  FIG. 5   d , an nBm (semiconductor-barrier-metal) photo-detector may include a fully or partially etched absorber layer  5301  in addition to a fully or partially etched barrier  5310 . On some embodiments, as pixel pitches become ever-smaller, the lateral diffusion component of cross-talk (modulation transfer function) becomes more punitive. One approach to address this is to etch partially or fully through the absorber layer  5301 . Quantum efficiency (QE) and I or fill factor may be traded off for modulation bandwidth in such an embodiment. In the embodiment shown in  FIG. 5   d , the metal contact  5320  may be disposed directly onto the barrier  5310  or may be disposed onto a thin passivation layer  5330  disposed on the barrier  5310  as described previously. The barrier (as well as, in some embodiments, the passivation layer  5330  and I or the metal contact  5320 ) may be etched through down to the absorber layer  5301 . In some embodiments, the absorber layer  5301  may also be partially etched  5350  or fully etched (not shown) down to a substrate material (not shown). In some such embodiments, the individual pixel stacks  5360  including the metal contact  5320 , barrier  5310 , and etched absorber layer  5301  portion may have the exposed portions of the barrier  5310  and absorber layer  5301  covered with a passivation layer  5350 . In some embodiments, this may be the same passivation layer  5330  as that disposed between the metal  5330  and the barrier  5310 . In other embodiments, there may be no passivation layer  5330  between the barrier  5310  and the metal contact  5320 , allowing for direct metal contact with the barrier. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.