# Patent

## Title
Application of reduced dark current photodetector with a thermoelectric cooler

## Abstract

A IDCA system combining thermo-electric cooler (TEC) and an internal nBn photo-detector having a photo absorbing layer comprising an n-doped semiconductor exhibiting a valence band energy level; and a contact layer comprising a doped semiconductor. A barrier layer is disposed between the photo absorbing layer and the contact layer, the barrier layer exhibiting a valence band energy level substantially equal to the valence band energy level of the doped semiconductor of the photo absorbing layer; the barrier layer exhibiting a thickness and a conductance band gap sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact area and block the flow of thermalized majority carriers from the photo absorbing layer to the contact area. Alternatively, a p-doped semiconductor is utilized, and conductance band energy levels of the barrier and photo absorbing layers are equalized.

## Background
This application is related to, and hereby incorporates by reference the following Patents and Patent Applications in their entirety: U.S. Pat. Nos. 7,687,871; 8,003,434; U.S. patent application Ser. Nos. 13/033,211; 13/622,981; 13/964,883; and 13/167,992.
FIELD OF THE INVENTION
The invention relates generally to the field of semiconductor based photo-detectors and in particular to a photo-detector exhibiting a barrier region between an active semiconductor region and a contact semiconductor region.
BACKGROUND OF THE INVENTION
Photo-detectors are used in a wide variety of applications including imaging. A specific type of photo-detector sensitive to the infra-red wavelengths of light is also known as an infra-red detector. Infra-red covers a broad range of wavelengths, and many materials are only sensitive to a certain range of wavelengths. As a result, the infra-red band is further divided into sub-bands such as near infra-red defined conventionally as 0.75-1.4 μm; short wavelength infra-red defined conventionally as 1.3-3 μm; mid wavelength infra-red defined conventionally as 3-8 μm; and far infra-red defined conventionally as 15-1,000 μm. Infra-red in the range of 5 μm to 8 μm is not well transmitted in the atmosphere and thus for many infra-red detection applications mid-wavelength infra-red is referred to as 3-5 μm.
Infra-red detectors are used in a wide variety of applications, and in particular in the military field where they are used as thermal detectors in night vision equipment, air borne systems, naval systems and missile systems. Highly accurate thermal detectors have been produced using InSb and HgCdTe p-n junction diodes, however these thermal detectors require cooling to cryogenic temperatures of around 77 K which is costly. Examples of these existing technologies are presented in FIG. 5A to FIG. 5F. The cryogenic temperatures primarily are used to reduce the dark current generated in the p-n junction diode by among other effects Shockley Reed Hall (SRH) generation.
There are three main contributions to the dark current, denoted as Idark, of photodiodes based on narrow band gap semiconductors. The fluctuations of the dark current components are a major factor in the noise that limits the device performance. These components are:

    
    
        a) a generation current associated with the Shockley-Reed-Hall (SRH) process in the depletion region, Isrh;
        b) a diffusion current associated with auger or radiative processes in the extrinsic area, Idiff; and
        c) a surface current associated with the surface states in the junction, Isurf. The surface current depends primarily on the passivation process done for the device. Thus, Idark can be expressed as:

Idark=Isrh+Idiff+Isurf  Equation 1

    
    


The SRH generation process is very efficient in the depletion region of photodiodes where the mid-gap traps are highly activated. It is the main source of the dark current in photodiodes operable for mid-wavelength infrared at temperatures below 200K. The current associated with this source is:




  
    
      
        
          J
          SRH
        
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              n
              i
            
            
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              SRH
            
          
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            W
            dep
          
        
      
    
    
      
        Equation
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        2
      
    
  




    
    
        where ni is the intrinsic concentration of the semiconductor, Wdep is the depletion width (typically in the range of 1 μm), and τSRH is the SRH lifetime of minority carriers in the extrinsic area. The SRH lifetime of minority carriers in the extrinsic area depends on the quality of the material, i.e. the trap concentration, and is typically in the range of  ̃1 μsec in low doped material ( ̃1016 cm−3). The dependence of SRH current on ni produces an activation energy of
    
    






  
    
      
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         because the source of this generation process is through mid-gap traps. A secondary source of dark current in photodiodes is thermal generation in the neutral regions and diffusion to the other side of the junction. This thermal generation current depends on the auger or radiative process in this area, and is expressed as:
    
    






  
    
      
        
          
            J
            diff
          
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        where τdiff is the lifetime, and in an n-type material exhibiting a doping concentration, denoted Nd, of  ̃1−2·1016 cm−3 is in the range of  ̃0.5 μsec, depending only slightly on temperature. L is the width of the neutral region of the device or the diffusion length of minority carriers (the smaller of the two) and pn is the hole concentration in the active n type semiconductor in equilibrium and it equal to ni2/Nd. The activation energy of the diffusion current is Eg,
    
    






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         as the process involves band to band excitation.
    
    


Additionally, p-n junction diodes, and particularly those produced for thermal imaging require a passivation layer in the metallurgic junction between the p and n layers. Unfortunately this is often difficult to achieve and significantly adds to the cost of production.
There is thus a long felt need for a thermal imaging device that uses a photo-detector with reduced dark noise. Preferably the photo-detector would be sensitive to the mid wavelength infra-red band and not require expensive passivation in production. Further preferably the photo-detector would be operable at significantly higher temperatures than 77K. Further preferably the thermal imaging device would be able to operate for longer periods, be lighter and require less power, when compared to the existing technology in the art.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present disclosure to overcome the disadvantages of the existing technological deficiencies of photo-detectors and their application within thermal imaging devices, with particular reference to mid and long wavelength infra-red detectors. This is facilitated in the present invention by a photo-detector sensitive to a target waveband comprising a photo absorbing layer, preferably exhibiting a thickness on the order of the optical absorption length. In an exemplary embodiment the photo absorbing layer is deposited to a thickness of between one and two times the optical absorption length. A contact layer is further provided, and a barrier layer is interposed between the photo absorbing layer and the contact layer. The barrier layer exhibits a thickness sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact layer, and a band gap barrier sufficient to block the flow of thermalized majority carriers from the photo absorbing layer to the contact layer. The barrier layer does not significantly block minority carriers.
An infra-red detector in accordance with the principle of the invention can be produced using either an n-doped photo absorbing layer or a p-doped photo absorbing layer, in which the barrier layer is designed to have no offset for minority carriers and a band gap barrier for majority carriers. Current in the detector is thus almost exclusively by minority carriers. In particular, for an n-doped photo absorbing layer the junction between the barrier layer and the absorbing layer is such that there is substantially zero valence band offset, i.e. the band gap difference appears almost exclusively in the conduction band offset. For a p-doped photo absorbing layer the junction between the barrier layer and the absorbing layer is such that there is substantially zero conduction band offset, i.e. the band gap difference appears almost exclusively in the valence band offset.
Advantageously the photo-detector of the subject invention does not exhibit a depletion layer, and thus the dark current is significantly reduced. Furthermore, in an exemplary embodiment passivation is not required as the barrier layer further functions to achieve passivation.
An exemplary photo-detector of the present disclosure comprises: a photo absorbing layer comprising an n-doped semiconductor exhibiting a valence band energy level and a conducting band energy level; a barrier layer, a first side of the barrier layer adjacent a first side of the photo absorbing layer, the barrier layer exhibiting a valence band energy level substantially equal to the valence band energy level of the photo absorbing layer and a conduction band energy level exhibiting a significant band gap in relation to the conduction band of the photo absorbing layer; and a contact area comprising a doped semiconductor, the contact area being adjacent a second side of the barrier layer opposing the first side, the barrier layer exhibiting a thickness, the thickness and the band gap being sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact area and block the flow of thermalized majority carriers from the photo absorbing layer to the contact area.
In one embodiment of the photo detector the barrier layer comprises an undoped semiconductor. In another embodiment the contact area is n-doped. In a further embodiment, the contact area exhibits a valence band energy level substantially equal to the valence band energy level of the n-doped semiconductor of the photo absorbing layer.
In one embodiment of the photo detector the contact area is p-doped. In one further embodiment the contact area exhibits a valence band energy level greater than the valence band energy level of the n-doped semiconductor of the photo absorbing layer. In another further embodiment the barrier layer comprises an undoped semiconductor.
In one embodiment of the photo detector the photo absorbing layer is operable to generate minority carriers in the presence of light energy exhibiting a wavelength of 3-5 microns. In another embodiment the photo-detector further comprises a substrate exhibiting a first side adjacent a second side of the photo absorbing layer, the second side of the photo absorbing layer opposing the first side of the photo absorbing layer, the substrate exhibiting a second side in contact with a metal layer. Preferably, the photo-detector further comprises an additional metal layer in contact with the contact area.
In one embodiment of the photo detector the barrier layer comprises one of AlSb, AlAsSb, GaAlAsSb, AlPSb, AlGaPSb and HgZnTe. In a further embodiment the photo absorbing layer is constituted of one of n-doped InAs, n-doped InAsSb, n-doped InGaAs, n-doped Type II super lattice InAs/InGaSb and n-doped HgCdTe. In a yet further embodiment of the photo detector the contact area is constituted of one of InAs, InGaAs, InAsSb, Type II super lattice InAs/InGaSb, HgCdTe and GaSb. In a yet further embodiment the contact area and the photo absorbing layer exhibit substantially identical compositions.
In one embodiment of the photo detector the photo absorbing layer and the contact area arc constituted of n-doped HgCdTe and the barrier layer is constituted of HgZnTe, and in another embodiment the photo absorbing layer and the contact layer are constituted of n-doped type II super lattice InAs/InGaSb and the barrier layer is constituted of AlGaAsSb.
In another embodiment of the photo detector the photo absorbing layer is constituted of n-doped InAsSb, the barrier layer is constituted of AlGaAsSb and the contact layer is constituted of p-doped GaSb. In one embodiment the photo absorbing layer exhibits a thickness on the order of the optical absorption length.
Another embodiment of a photo-detector comprises: a photo absorbing layer comprising a p-doped semiconductor exhibiting a conduction band energy level and a valence band energy level; a barrier layer, a first side of the barrier layer adjacent a first side of the photo absorbing layer, the barrier layer exhibiting a conduction band energy level substantially equal to the conduction band energy level of the photo absorbing layer and a valence band energy level exhibiting a significant band gap in relation to the valence band of the photo absorbing layer; and a contact area comprising a doped semiconductor, the contact area adjacent a second side of the barrier layer opposing the first side, the barrier layer exhibiting a thickness, the thickness and the band gap being sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact area and to block the flow of thermalized majority carriers from the photo absorbing layer to the contact area.
In one embodiment of a photo-detector the barrier layer comprises an undoped semiconductor. In another embodiment the contact area is p-doped. In one further embodiment of a photo-detector the contact area exhibits a conduction band energy level substantially equal to the conduction band energy level of the p-doped semiconductor of the photo absorbing layer.
In one embodiment of a photo-detector the contact area is n-doped. In one further embodiment the contact area exhibits a conduction band energy level less than the conduction band energy level of the p-doped semiconductor of the photo absorbing layer. In another further embodiment the barrier layer comprises an undoped semiconductor.
In one embodiment of a photo-detector the photo absorbing layer is operable to generate minority carriers in the presence of light energy exhibiting a wavelength of 3-5 microns. In another embodiment the photo-detector further comprises a substrate exhibiting a first side adjacent a second side of the photo absorbing layer, the second side of the photo absorbing layer opposing the first side of the photo absorbing layer, the substrate exhibiting a second side in contact with a metal layer. In a further embodiment the photo-detector further comprises a metal layer in contact with the contact area.
In one embodiment of a photo-detector the barrier layer comprises one of AlSb, AlAsSb, GaAlAsSb, AlPSb, AlGaPSb, InAlAs, InAlAsSb, and HgZnTe. In one further embodiment the photo absorbing layer is constituted of one of p-doped InAs, p-doped InAsSb, p-doped InGaAs, p-doped Type II super lattice InAs/InGaSb and p-doped HgCdTe. In one yet further embodiment the contact area is constituted of one of InAs, InGaAs, InAsSb, Type II super lattice InAs/InGaSb, HgCdTe and GaSb. In one yet further embodiment the contact area and the photo absorbing layer exhibit substantially identical compositions.
An exemplary method of producing a photo-detector, comprises: providing a substrate; depositing on the substrate a photo absorbing layer comprising a doped semiconductor exhibiting an energy level associated with non-conducting majority carriers; depositing on the deposited photo absorbing layer a barrier layer exhibiting a thickness, an energy level associated with minority carriers of the photo absorbing layer substantially equal to the energy level of the photo absorbing layer and a band gap associated with majority carriers of the photo absorbing layer; and depositing on the deposited barrier layer a contact layer comprising a doped semiconductor, the thickness and the band gap of the barrier layer being sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact layer and to block the flow of thermalized majority carriers from the photo absorbing layer to the contact layer.
In one embodiment the method further comprises selectively etching the deposited contact layer to define a plurality of contact areas. In another embodiment at least one of depositing the photo absorbing layer, depositing the barrier layer and depositing the contact layer is done via one of molecular beam epitaxy, metal organic chemical vapor deposition, metal organic phase epitaxy and liquid phase epitaxy.
It is noted that while the photodetector is preferably manufactured on a substrate as described, certain embodiments may remove the substrate or a portion thereof in the final photodetector, and thus in an embodiment of the invention there is provided a photo detector.
Additional features and advantages of the invention will become apparent from the following drawings and description.



SHORT DESCRIPTION OF DRAWINGS
For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections throughout.
With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings:
FIG. 1A illustrates a high level schematic view of the layers of a single photo-detector according to an embodiment of the principle of the invention;
FIG. 1B illustrates a side view of a multi-pixel photo-detector according to an embodiment of the principle of the invention;
FIG. 1C illustrates a top level view of the multi-pixel photo-detector of FIG. 1B according to a principle of the invention;
FIG. 2A illustrates the energy band levels of an embodiment of the structure of FIG. 1 according to the principle of the invention in which the photo absorbing layer is n-doped and the contact layer is n-doped;
FIG. 2B illustrates the energy band levels of an embodiment of the structure of FIG. 1 according to the principle of the invention in which the photo absorbing layer is p-doped and the contact layer is p-doped;
FIG. 3A illustrates the energy band levels of an embodiment of the structure of FIG. 1 according to the principle of the invention in which the photo absorbing layer is n-doped and the contact layer is p-doped;
FIG. 3B illustrates the energy band levels of an embodiment of the structure of FIG. 1 according to the principle of the invention in which the photo absorbing layer is p-doped and the contact layer is n-doped; and
FIG. 4 illustrates a high level flow chart of the process of manufacture of the multi pixel photo-detector of FIG. 1B-1C.
FIG. 5a, 5B, 5C, 5D, 5E and 5F illustrate examples of existing applications of the prior-art photo-detector technology.
FIG. 6 illustrates the existing external components of an exemplary thermal imaging device with an integrated dewar cooler assembly, hereafter IDCA.
FIG. 7 illustrates the internal components of an exemplary application of the disclosed subject matter.
FIG. 8 illustrates an exemplary linear micro cooler (Ricor's K527 split) that is utilized in conjunction with the disclosed subject matter.
FIG. 9 illustrates resultant improvement in cooling power requirements.
FIG. 10A illustrates a mother board joined to FPA.
FIG. 10B illustrates a FPA and motherboard located within IDCA arrangement.
FIG. 11 illustrates a schematic application of the disclosed subject matter. FIG. 11A depicts schematically a single stage TEC, and FIG. 11B depicts a multi-stage TEC. FIG. 11C depicts schematically a FPA 1114 disposed on a TEC 1145.
FIG. 12 illustrates an operational flow chart of an exemplary optical imaging device with an IDCA comprising the disclosed FDA.



DETAILED DESCRIPTION
The present embodiments enable a photo-detector sensitive to a target waveband comprising a photo absorbing layer, preferably exhibiting a thickness on the order of an optical absorption length of the target waveband. In an exemplary embodiment the photo absorbing layer is deposited to a thickness of between one and two times the optical absorption length. A contact layer is further provided, and a barrier layer is interposed between the photo absorbing layer and the contact layer. The barrier layer exhibits a thickness sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact layer, and a band gap barrier sufficient to block the flow of thermalized majority carriers from the photo absorbing layer to the contact layer. The barrier layer does not significantly block minority carriers.
An infra-red detector in accordance with the principle of the invention can be produced using either an n-doped photo absorbing layer or a p-doped photo absorbing layer, in which the barrier layer is designed to have substantially no offset for minority carriers and a band gap barrier for majority carriers. Current in the detector is thus almost exclusively by minority carriers. In particular, for an n-doped photo absorbing layer the junction between the barrier layer and the absorbing layer is such that there is substantially zero valence band offset, i.e. the band gap difference appears almost exclusively in the conduction band offset. For a p-doped photo absorbing layer the junction between the barrier layer and the absorbing layer is such that there is substantially zero conduction band offset, i.e. the band gap difference appears almost exclusively in the valence band offset.
Advantageously the photo-detector of the subject invention does not exhibit a depletion layer, and thus the dark current is significantly reduced. Furthermore, in an exemplary embodiment passivation is not required as the barrier layer further functions to achieve passivation.
Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.
FIG. 1A illustrates a high level schematic view of the layers of a photo-detector 10 according to an embodiment of the principle of the invention comprising a substrate 20, a photo absorbing layer 30, a barrier layer 40, a contact layer 50, a metal layer 60 and a metal layer 65. Substrate 20 is provided as a base for deposition and has deposited on one face metal layer 60 for connection to electronic circuitry. In an exemplary embodiment metal layer 60 is constituted of gold. Photo absorbing layer 30 is deposited on the second face of substrate 20 opposing the first face. Photo absorbing layer 30 comprises a doped semiconductor responsive to photons of the object wavelength, and preferably is deposited to a thickness on the order of an optical absorption length. In one embodiment photo absorbing layer 30 is deposited to a thickness of between one and two times the optical absorption length. In an exemplary embodiment photo absorbing layer 30 comprises one of n-doped InAs; n-doped InAsSb; n-doped InGaAs; n-doped type II super lattice of the type InAs/InGaSb; and n-doped HgCdTe. In an alternative embodiment absorbing layer 30 comprises one of p-doped InAs; p-doped InAsSb; p-doped InGaAs; p-doped type II super lattice of the type InAs/InGaSb; and p-doped HgCdTe.
Barrier layer 40 is deposited directly on photo absorbing layer 30 without requiring passivation. Barrier layer 40 is deposited to a thickness sufficient to substantially prevent tunneling of majority carriers from photo absorbing layer 30 to contact layer 50, and in an exemplary embodiment is deposited to a thickness of 50-100 nm. Barrier layer 40 comprises a material selected to exhibit a high band gap barrier for majority carriers from photo absorbing layer 30 and substantially no band gap barrier for minority carriers. Barrier layer 40 is thus sufficient to block both the flow of thermalized majority carriers and the tunneling of majority carriers from photo absorbing layer 30 to contact layer 50. Thus, for an n-type photo absorbing layer 30, the band gap difference appears in the conduction band, whereas substantially no band gap offset appears in the valence band. In one embodiment barrier layer 40 comprises one of AlSb, AlAsSb, GaAlAsSb, AlPSb, AlGaPSb and HgZnTe. In an exemplary embodiment photo absorbing layer 30 comprises n-doped InAs and barrier layer 40 is comprised of AlAsxSb1−x with x ̃0.15, and thus there is  ̃0 valence band offset.
Contact layer 50 is deposited on barrier layer 40. Contact layer 50 functions to absorb the minority carriers diffused from the absorbing layer 30 and is essentially a contact layer. In an exemplary embodiment contact layer 50 is deposited to a thickness of 20-50 nm and is constituted of one of InAs; InAsSb; InGaAs; type II super lattice of the type InAs/InGaSb; HgCdTe and GaSb. Contact layer 50 may be n-doped or p-doped without exceeding the scope of the invention. Advantageously, contact layer 50 may be constituted of the same material as photo absorbing layer 30. Contact layer 50 is etched, preferably by photolithography, to define the detector area. Advantageously etching of barrier layer 40 or absorbing layer 30 is not required. Metal layer 65 is deposited on contact layer 50, and in an exemplary embodiment is constituted of gold. Metal layers 60, 65 enable the connection of an appropriate bias, and a connection to detect a flow of current from photo absorbing layer 30 to contact layer 50.
FIG. 1B illustrates a side view of a multi-pixel photo-detector 100 according to an embodiment of the principle of the invention comprising substrate 20, photo absorbing layer 30, barrier layer 40, a first and second contact area 110, a metal layer 6 and a metal layer 65. Substrate 20 is provided as a base for deposition and has deposited on one face metal layer 60 for connection to electronic circuitry. In an exemplary embodiment metal layer 60 is constituted of gold. Photo absorbing layer 30 is deposited on the second face of substrate 20 opposing the first face. Photo absorbing layer 30 comprises a doped semiconductor responsive to photons of the object wavelength, and preferably is deposited to a thickness on the order of an optical absorption length. In one embodiment photo absorbing layer 30 is deposited to between one and two times the optical absorption length. In an exemplary embodiment photo absorbing layer 30 comprises one of n-doped InAs; n-doped InAsSb; n-doped InGaAs; n-doped type II super lattice of the type InAs/InGaSb; and n-doped HgCdTe. In an alternative embodiment absorbing layer 30 comprises one of p-doped InAs; p-doped InAsSb; p-doped InGaAs; p-doped type II super lattice of the type InAs/InGaSb; and p-doped HgCdTe.
The substrate is generally transparent to wavelengths of interest. However as certain substrates may block certain portions of the spectrum, in some embodiments the substrate, or portions thereof, may be removed to allow all the spectrum of interest to be detected by the photodetector array.
Barrier layer 40 is deposited directly on photo absorbing layer 30 without requiring passivation. Barrier layer 40 is deposited to a thickness sufficient to substantially prevent tunneling of majority carriers from photo absorbing layer 30 to first and second contact area 110, and in an exemplary embodiment is deposited to a thickness of 50-100 nm. Barrier layer 40 comprises a material selected to exhibit a high band gap barrier for majority carriers from photo absorbing layer 30 and substantially no band gap barrier for minority carriers. Barrier layer 40 is thus sufficient to block both the flow of thermalized majority carriers and the tunneling of majority carriers from photo absorbing layer 30 to first and second contact area 110. Thus, for an n-type photo absorbing layer 30, the band gap difference appears in the conduction band, whereas substantially no band gap offset appears in the valence band. In one embodiment barrier layer 40 comprises one of AlSb, AlAsSb, GaAlAsSb, AlPSb, AlGaPSb and HgZnTe. In an exemplary embodiment photo absorbing layer 30 comprises n-doped InAs and barrier layer 40 is comprised of AlAsxSb1−x with x ̃0.15, and thus there is  ̃0 valence band offset.
Contact layer 50 as described above in relation to FIG. 1A is deposited on barrier layer 40. Contact layer 50, which as will be described further is etched to define first and second contact area 110, functions to absorb the minority carriers diffused from the absorbing layer 30 and is essentially a contact layer. In an exemplary embodiment contact layer 50 is deposited to a thickness of 20-50 nm and is constituted of one of InAs; InAsSb; InGaAs; type II super lattice of the type InAs/InGaSb; HgCdTe and GaSb. Contact layer 50 may be n-doped or p-doped without exceeding the scope of the invention. Advantageously, contact layer 50 may be constituted of the same material as photo absorbing layer 30. Contact layer 50 is etched, preferably by photolithography, to define first and second contact area 110. Advantageously etching of barrier layer 40 or absorbing layer 30 is not required. In an exemplary embodiment a selective etchant is used which does not etch barrier layer 40. Metal layer 65 is deposited on each of first and second contact area 110, and in an exemplary embodiment is constituted of gold. Thus, a single photo absorbing layer and barrier layer is utilized, with each unetched portion of contact layer 50 defining a pixel or individual detector.
The above has been described in an embodiment in which two pixels, or detectors are defined, however this is not meant to be limiting in any way. A large array of photo-detectors produced as above is specifically included in the invention.
FIG. 1C illustrates a top level view of multi-pixel photo-detector 100 of FIG. 1B according to a principle of the invention showing barrier layer 40, first and second contact area 110 and metal layer 65 defined on each of first and second contact area 110.
FIG. 2A illustrates the energy band levels of an embodiment of the structure of FIG. 1 according to the principle of the invention in which the photo absorbing layer is n-doped and the contact layer is n-doped, in which the x-axis indicates position along the structure of FIG. 1 and the y-axis indicates energy levels in an arbitrary illustrative manner. Three energy band levels are depicted: Ev, the valence band energy band level; Ef, the Fermi energy band level; and Ec the conducting band energy level. Area 100 represents the energy band levels within photo absorbing layer 30, area 110 represents the energy band levels within barrier layer 40 and area 120 represent the energy band levels within contact layer 50.
The valence band energy level is substantially constant throughout areas 100, 110 and 120, and thus minority carriers are not obstructed from flowing from photo absorbing area 100 to contact area 120. It is to be noted that due to the energy levels the minority carriers are captured in contact area 120. Barrier layer 40, represented by area 110, is thick enough so that there is negligible tunneling of majority carriers through it. In an exemplary embodiment barrier layer 40 is deposited to a thickness of 50-100 nm, and the band gap barrier of area 110 is high enough so that there is negligible thermal excitation of majority carriers over it. Area 120 shows energy band levels on a par with that of area 100 however this is not meant to be limiting in any way. In one embodiment Ef in contact layer area 120 is slightly higher than their values in photo absorbing area 100 with the increase being attributed to an increased doping concentration. It is to be noted that no depletion layer is present and therefore there is no SRH current. Photocurrent is a result of optically generated minority carriers which diffuse from photo absorbing area 100 to contact area 120.
FIG. 2B illustrates the energy band levels of an embodiment of the structure of FIG. 1 according to the principle of the invention in which the photo absorbing layer is p-doped and the contact layer is p-doped; in which the x-axis indicates position along the structure of FIG. 1 and the y-axis indicates energy levels in an arbitrary illustrative manner. Three energy band levels are depicted: Ev, the valence band energy level; Ef, the Fermi energy band level; and Ec the conducting band energy level. Area 150 represents the energy band levels within photo absorbing layer 30, area 160 represents the energy band levels within barrier layer 40 and area 170 represent the energy band levels within contact layer 50.
The conduction band energy level is substantially constant throughout areas 150, 160 and 170, and thus minority carriers are not obstructed from flowing from photo absorbing area 150 to contact area 170. It is to be noted that due to the energy levels the minority carriers are captured in contact area 170. Barrier layer 40, represented by area 160, is thick enough so that there is negligible tunneling of majority carriers through it. In an exemplary embodiment barrier layer 40 is deposited to a thickness of 50-100 nm, and the band gap barrier of area 160 is high enough so that there is negligible thermal excitation of majority carriers over it. Area 170 shows energy band levels on a par with that of area 150 however this is not meant to be limiting in any way. In one embodiment Ef in contact layer area 170 is slightly higher than their values in photo absorbing area 150 with the increase being attributed to an increased doping concentration. It is to be noted that no depletion layer is present and therefore there is no SRH current. Photocurrent is a result of optically generated minority carriers which diffuse from photo absorbing area 150 to contact area 170.
FIG. 3A illustrates the energy band levels of an embodiment of the structure of FIG. 1 according to the principle of the invention in which the photo absorbing layer is n-doped and the contact layer is p-doped; in which the x-axis indicates position along the structure of FIG. 1 and the y-axis indicates energy levels in an arbitrary illustrative manner. Three energy band levels are depicted: Ev, the valence band energy level; Ef, the Fermi energy band level; and Ec the conducting band energy level. Area 200 represents the energy band levels within photo absorbing layer 30, area 210 represents the energy band levels within barrier layer 40 and area 220 represent the energy band levels within contact layer 50.
The valence band energy level is substantially constant throughout areas 200 and 210 and is higher in area 220, and thus minority carriers are not obstructed from flowing from photo absorbing area 200 to contact area 220. It is to be noted that due to the energy levels the minority carriers are captured in contact area 220. Barrier layer 40, represented by area 210, is thick enough so that there is negligible tunneling of majority carriers through it. In an exemplary embodiment barrier layer 40 is deposited to a thickness of 50-100 nm, and the band gap barrier of area 210 is high enough so that there is negligible thermal excitation of majority carriers over it. It is to be noted that no depletion layer is present and therefore there is no SRH current. Photocurrent is a result of optically generated minority carriers which diffuse from photo absorbing area 200 to contact area 220.
FIG. 3B illustrates the energy band levels of an embodiment of the structure of FIG. 1 according to the principle of the invention in which the photo absorbing layer is p-doped and the contact layer is n-doped; in which the x-axis indicates position along the structure of FIG. 1 and the y-axis indicates energy levels in an arbitrary illustrative manner. Three energy band levels are depicted: Ev, the valence band energy level; Ef, the Fermi energy band level; and Ec the conducting band energy level. Area 250 represents the energy band levels within photo absorbing layer 30, area 260 represents the energy band levels within barrier layer 40 and area 270 represent the energy band levels within contact layer 50.
The conduction band energy level is substantially constant throughout areas 250 and 260 and it is lower in area 270, and thus minority carriers are not obstructed from flowing from the photo absorbing area 250 to contact area 270. It is to be noted that due to the energy levels the minority carriers are captured in contact area 270. Barrier layer 40, represented by area 260, is thick enough so that there is negligible tunneling of majority carriers through it. In an exemplary embodiment barrier layer 40 is deposited to a thickness of 50-100 nm, and the band gap barrier of area 260 is high enough so that there is negligible thermal excitation of majority carriers over it. It is to be noted that no depletion layer is present and therefore there is no SRH current. Photocurrent is a result of optically generated minority carriers which diffuse from photo absorbing area 250 to contact area 270.
FIG. 4 illustrates a high level flow chart of the process of manufacture of the photo-detector of FIG. 1. In stage 1000 a substrate material is provided as a support for deposition. In stage 1010, a photo absorbing layer is deposited on the substrate. Preferably the photo absorbing layer is deposited to a thickness on the order of the optical absorption length and in an exemplary embodiment is deposited to a thickness of between one and two times the optical absorption length.
In stage 1020, a barrier material is selected such that the flow of thermalized majority carriers from the photo absorbing layer deposited in stage 1010 would be negligible, and the flow of minority carriers is not impeded. In stage 1030, the barrier material selected in stage 1020 is deposited to a thickness sufficient to prevent tunneling of majority carriers through the barrier material. In an exemplary embodiment the thickness is between 50 and 100 nm. Preferably the barrier material is deposited directly on the photo absorbing layer deposited in stage 1010.
In stage 1040, a contact layer is deposited, preferably directly on the barrier material deposited in stage 1030. In stage 1050, the desired contact areas are defined. Preferably, the contact areas are defined by photolithography and a selective etchant which stops on the top of the barrier layer. Alternatively, the etchant may be controlled to stop once the uncovered portions of contact layer 50 are removed. Thus, the depth of the etch is equivalent to the thickness of the contact layer 50. Advantageously, in an exemplary embodiment no other layer is etched.
In stage 1060 a metal layer is deposited on the contact areas defined in stage 1050 so as to enable electrical connection. Preferably the metal layer is deposited directly on the contact areas defined in stage 1050. In stage 1070, a metal layer is deposited on substrate 20 provided in stage 1000 so as to enable electrical connection.
Deposition of the photo absorbing layer of stage 1010, the barrier layer of stage 1030 and the contact layer of stage 1040 may be accomplished by any means known to those skilled in the art including, without limitation molecular beam epitaxy, metal organic chemical vapor deposition, metal organic phase epitaxy or liquid phase epitaxy.
Thus the present embodiment enable a photo-detector sensitive to a target waveband comprising a photo absorbing layer, preferably exhibiting a thickness on the order of the optical absorption length. In an exemplary embodiment the photo absorbing layer is deposited to a thickness of between one and two times the optical absorption length. A contact layer is further provided, and a barrier layer is interposed between the photo absorbing layer and the contact layer. The barrier layer exhibits a thickness sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact layer, and a band gap barrier sufficient to block the flow of thermalized majority carriers from the photo absorbing layer to the contact layer. The barrier layer does not block minority carriers.
An infra-red detector in accordance with the principle of the invention can be produced using either an n-doped photo absorbing layer or a p-doped photo absorbing layer, in which the barrier layer is designed to have no offset for minority carriers and a band gap barrier for majority carriers. Current in the detector is thus almost exclusively by minority carriers. In particular, for an n-doped photo absorbing layer the junction between the barrier layer and the absorbing layer is such that there is substantially zero valence band offset, i.e. the band gap difference appears almost exclusively in the conduction band offset. For a p-doped photo absorbing layer the junction between the barrier layer and the absorbing layer is such that there is substantially zero conduction band offset, i.e. the band gap difference appears almost exclusively in the valence band offset.
Advantageously the photo-detector of the subject invention does not exhibit a depletion layer, and thus the dark current is significantly reduced. Furthermore, in an exemplary embodiment passivation is not required as the barrier layer further functions to achieve passivation.
An exemplary application of the disclosed subject matter is the inclusion of an array of photo detectors within a focal plan array, hereafter FPA, which form an integral component of optical imaging devices, including thermal imaging devices. Use of the disclosed subject matter within the FPA enables improved thermal imaging device performance, including but not limited to, weight, duration of operation, power requirements, cost, pixel operability and durability.
FIGS. 5A, 5B, 5C and 5D present hand-held imaging systems, which utilize the existing photo detector technology.
FIGS. 5E and 5F present aviation technology, which utilize the existing photo detector technology, including the Lockheed Sniper Pod and the Northrop Grumman EOTS pod.
FIG. 6 presents a further example of the existing technology, wherein light enters the apparatus through the front lens optic 602, interacts with the apparatus internal electroptic componentary 604, where it is converted from infra-red light to a electric signal, which is transmitted and presented on the apparatus' display 606.
FIG. 7 presents in greater detail the essential components of an exemplary integrated dewar cooler system, hereafter IDCS. In one embodiment, light enters the system 700 through the front optic 704, the light than is received by the FPA, which is located on the cold finger of the dewar 702, and is maintained at a cryogenic temperature. In one embodiment of the disclosure, the FPA operates at a temperature of 150K. In other embodiments, the FPA can operate across a temperature spectrum of between 77K and 150K. The IDCS further comprises a micro-cooler 706, which is responsible for the refiguration of the FPA.
FIG. 8 presents an exemplary split linear micro cooler system, comprising a cooler 902 connected via tubing 904 to a cold finger 906, and an external controller 908 connected via wiring 910 to cooler. The exemplary cooler system presented is the Ricor k527 split linear micro cooler. In other embodiment a variety of micro cooler systems can be used.
FIG. 9 illustrates the micro-cooler power consumption per heat load across a spectrum of cold finger temperatures.
FIG. 10A presents the FPA 1002 coupled to a motherboard 1004, which forms a FPA arrangement that can connect to the cold finger.
FIG. 10B presents an IDCS in greater detail, wherein light enters the dewar 1114 via the optical window 1110, with a micro-cooler 1102 providing refrigeration for the FPA.
FIG. 11 illustrates an exemplary schematic IDCA arrangement. The system operates by allowing light through the front element 1110 which is an optical window located behind a fronting lens, not shown. Light enters through the optical window before interacting with the FPA 1114. The FPA arrangement comprises an array of photo-detectors, coupled to a motherboard. The FPA arrangement is contained within the cold finger of a cryogenic vacuum sealed dewar 1116-1112, which is refrigerated by the micro cooler 1102. The dewar additional comprising: a getter unit 1104; a cold shield 1108 contained within the window envelope 1112, which are positioned on the cold finger 1116. Light interacts with the FPA arrangement, which produces a photo-electronic signal in response. The photo-electronic signal is in turn transmitted from the FPA arrangement via wiring to the feed-through pins 1106.
The IDCA arrangements described heretofore for utilize gas displacement based cryogenics system, such as Stirling cycle systems, pulse tube systems, and the like. Such systems are heavy, relatively complex and expensive. However the low dark current of detectors according to the present invention allow utilization of thermo-electric coolers (TEC hereinafter) utilizing the Peltier effect. TEC's are light relatively inexpensive, and require no moving parts which increase system life time. Certain TEC devices are capable of reaching as low as 150 K and below, which allow practical uses of detectors according to the present invention in many applications such as the applications described herein and extending to other devices as well, such as, by way of non-limiting example, heart beat sensing cameras, firefighters heat sensing cameras, health care systems, and potentially even in cellular telephone devices, as well as other heat sensing applications in the military and commercial applications. It is noted that such weight savings would be highly appreciated in the military field, making heat sensing devices light enough to be carried by the individual soldier, without reducing the soldier's fighting ability. Weight saving is also of extreme importance to the aerospace industry, and this aspect of the present invention extends to light sensing equipment in all those fields, while utilizing photodetectors in accordance with the present invention coupled to a TEC.
FIG. 11A depicts schematically a single stage TEC, and FIG. 11B depicts a multi-stage TEC. FIG. 11C depicts schematically a FPA 1114 in accordance with the present invention, disposed on a TEC 1145. A cold shield 1147 surrounds the FPA so as to prevent spurious radiation from the dewar side walls into the sensor. The TEC 1145, the FPA 1114 and the cold shield 1147 are disposed within a dewar 1149 having an optical window. Notably, due to the relatively high operating temperature, the dewar does not have to be a vacuum dewar, and instead may be filled with inert gas. However a vacuum dewar is also explicitly contemplated.
FIG. 12 presents an flow chart diagram of an exemplary operational process of the disclosed subject matter, comprising: light being generated at source 1202; light entering the apparatus through the front lens element 1204; light interacting with the FPA 1206; the FPA generating a electro message 1208; the electro message being transmitted from the FPA to the IDCA's feeder tubes 1210; the electro message being received by the devices electronics 1214; wherein the device either display the image 1216 or transmits the image to an external display device 1218.
In one embodiment, the optical imaging device containing the IDCA has an interchangeable front lens element. In other embodiment, the front lens element may be fixed, may be fixed and variable, and other arrangements as standard in the art.
In one embodiment of the disclosed subject matter, the IDCA with an array of improved photo detector is accommodated within amateur, professional, or commercial optical devices. In other embodiments, the IDCA is located within military equipment. Aviation examples include precision targeting devices, or Electro Optic Targeting Systems (EOTS).
In yet another embodiment, the IDCA can comprise a plurality of FPA.
Examples of the above disclosure included but is not limited to, incorporation of the claimed IDCA within: Lockheed Sniper Pod Technology; Lockheed EOTS pods; AN/AAQ-37 F-35 Distributed Aperture System (DAS made by Northrop Grumman) and other similar technology; hand-held personal cameras; professional cameras; and security optical devices; another example is missile seeker. The disclosed IDCA apparatus could also be incorporated into the existing technologies outlined in the background of the invention.
It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.
Unless otherwise defined, all technical and scientific terms used herein have the same meanings as are commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods are described herein.
All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the patent specification, including definitions, will prevail. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub-combinations of the various features described hereinabove as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description.

## Claims

1. An integrated dewar cooler system comprising:
a light permitting optical window;
a sealed Dewar;
a thermoelectric Cooler; and
a focal plane array,
wherein said focal plane array is an array of photo detectors with a reduced dark current, comprising:
a photo absorbing layer comprising a doped semiconductor exhibiting a valence band energy and a conducting band energy during operation of the photo-detector;
a barrier layer comprising an undoped semiconductor, the barrier layer having a band energy gap and associated conduction and valence band energies, a first side of said barrier layer adjacent a first side of said photo absorbing layer; and
a contact layer comprising a doped semiconductor exhibiting a valence band energy and a conducting band energy during operation of the photo-detector, said contact layer being adjacent a second side of said barrier layer opposing said first side;

wherein the relationship between the photo absorbing layer and contact layer valence and conduction band energies and the barrier layer conduction and valance band energies facilitates minority carrier current flow while inhibiting majority carrier current flow between the contact and photo absorbing layers.

2. An integrated dewar cooler as claimed in claim 1, wherein the dewar is at least partially evacuated.

3. An integrated dewar cooler as claimed in claim 1 wherein the dewar is at least partially filled with an inert gas.