Abstract:
A detector of incident infrared radiation has a first region with a first spectral response, and a second region with a second, different spectral response. The second absorption region is stacked on the first and may be separated therefrom by a region in which the chemical composition of the compound semiconductor is graded. Separate contacts are provided to the first and second absorption regions and a further common contact is provided so as to permit the application of either a bias voltage or a skimming voltage across the respective pn junctions. The detector may be operated such that a preselected one of the absorption regions responds to incident infrared radiation of a predetermined waveband while the other absorption region acts as a skimmer of dark current, thereby enhancing the signal to noise ratio of the detector.

Description:
STATEMENT AS TO RIGHTS IN INVENTIONS MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
       [0001]    The present invention was made with governmental support under Contract No. NNL05AB06P awarded by the National Aeronautics and Space Administration. The government has certain rights in the present invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Photodiodes are widely used for sensing light and infrared radiation. The signal-to-noise ratio which can be obtained from photodiodes is limited by the level of thermal noise, which in turn is related to the temperature of the component. The term “dark current” is commonly used in the art to define the current flowing in a photodiode during a totally dark condition. The signal-to-noise ratio in photodiodes is conventionally improved by cooling the component, usually down to a temperature which can be maintained by a liquid nitrogen coolant (77K). The means for cooling the photodiodes down to a low temperature and keeping them there are cumbersome and expensive. It accordingly has been a long-term objective to provide photodetectors which can operate closer to room temperature but at acceptable signal-to-noise ratios. The noisiness of a detector can be quantified by calculating its noise equivalent temperature difference, which is the minimum signal derived from the temperature difference between a target and its background that yields a signal-to-noise ratio of unity. Minimization of the NETD is desirable. 
         [0003]    Detector cells conventionally accumulate charge in respective integration capacitors. A typical well depth for an integrating capacitor is about 10 7  electrons. In infrared detector applications it will typically be filled by background radiation in about 1 ms. Large dark and radiation background currents more quickly saturate the integrating capacitor. If the dark and background current in such devices can be eliminated or reduced, their signal-to-noise ratios will be improved, and the NETDs will be reduced, permitting longer integration times. Further, the size and area of the integration capacitor can then be reduced, decreasing overall pixel size and increasing resolution of an array of such devices, and dynamic range can be enhanced. 
       SUMMARY OF THE INVENTION 
       [0004]    According to one aspect of the invention, an infrared detector is constructed of at least five semiconductor layers. The first layer is highly doped to be of a first conductivity type (such as (n)). A second layer, formed on the first layer, is doped to be of the same conductivity type but at a lower concentration of dopant. This second layer acts as a first absorber region and has a first spectral response to incident radiation. A third layer, also of the first conductivity type, is formed on the second layer, and a portion of this is highly doped to be of a second conductivity type (such as (p)) and forms a contact portion and pn junction diode. 
         [0005]    A fourth layer is formed on the opposite side of the third layer from the second layer, and to be of the first conductivity type. This fourth layer has a second spectral response which is different from the first spectral response; it is sensitive to a different spectral band which may however have some overlap with the first spectral band. A fifth layer is formed on the fourth layer and is highly doped to be of the second conductivity type. The fifth layer is used as a second contact portion spaced from the first contact portion. The fifth and fourth layer form a second pn junction diode spaced from the first pn junction diode. The detector may be operated by using one of the pn junctions as a radiation detector, and the other as a dark current skimmer, with the application of appropriate sensing and skimming voltages on the contacts. 
         [0006]    In another aspect of the invention, a two-waveband radiation detector is provided which has a first absorber layer, adapted to sense a first waveband of incident radiation, and a second absorber layer, adapted to sense a second waveband of incident radiation. The first and second absorber layers are in semiconductive communication with each other and are formed to be of the first conductivity type. Contact regions are formed to be of a second conductivity type opposite that of the first conductivity type. A first of these is formed adjacent the first absorber layer, while a second of these is formed adjacent the second absorber layer. Each contact region and an adjacent absorber layer therefore form a respective pn junction. The second contact region is spaced from the first absorber layer. A common contact is in semiconductive communication with both the first and second absorber layers. Circuitry is provided to impress a predetermined sensing bias voltage across a preselected one of the first and second pn junctions, while a predetermined skimming voltage is impressed across the other pn junction. The predetermined skimming voltage creates a skimming current which will at least partially reduce a dark current component of a readout current read at the common contact, and may remove current generated background radiation as well. 
         [0007]    Because the dark current component of the readout current is reduced or eliminated, the detector has an enhanced signal to noise ratio, and may permit longer integration times and/or smaller integration capacitors, all of which are technical advantages over prior art devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Further aspects of the invention and their advantages can be discerned in the following detailed description, in which like characters denote like parts and in which: 
           [0009]      FIG. 1  is a highly magnified schematic sectional view of a photodetector cell according to the invention; 
           [0010]      FIG. 2  is a band diagram for a medium wavelength infrared (MWIR) detector region of the cell illustrated in  FIG. 1 ; 
           [0011]      FIG. 3  is a band diagram for a long wavelength infrared (LWIR) detector region and an n-type Medium Wavelength Infrared (MWIR) detector region of the cell illustrated in  FIG. 1 ; 
           [0012]      FIG. 4  is a graph of responsivity versus wavelength for the LWIR and MWIR detectors in the cell of  FIG. 1 ; 
           [0013]      FIG. 5  is a diagram, based on  FIG. 1 , showing the movement of generated electron-hole pairs under LWIR illumination with in substantial or no bias voltage applied to a MWIR contact; 
           [0014]      FIG. 6  is a diagram, based on  FIG. 1 , showing the movement of electron-hole pairs generated under MWIR illumination; 
           [0015]      FIG. 7  is a view, taken after  FIG. 1 , showing the creation of a depletion region extending to a LWIR absorber region and resultant current skimming, under LWIR illumination and a negative bias voltage applied to the MWIR contact; 
           [0016]      FIG. 8  is a graph, for the structure shown in  FIG. 1 , of LWIR current versus a skimming voltage applied at a MWIR contact, in dark conditions at 77K; 
           [0017]      FIG. 9  is a graph similar to that shown in  FIG. 8 , but with an incident photon flux Φ=0.1 W/cm 2 ; 
           [0018]      FIG. 10  is a graph similar to that shown in  FIGS. 8 and 9 , but with an incident photon flux Φ=1.0 W/cm 2 ; 
           [0019]      FIG. 11  is a graph similar to that shown in  FIGS. 8-10 , but with an incident photon flux Φ=10 W/cm 2 ; 
           [0020]      FIG. 12  is a graph of LWIR current v. skimming voltage applied to a MWIR contact region, where the structure in  FIG. 1  has been altered; 
           [0021]      FIG. 13  is a highly schematic sectional view of a three-contact photodetector cell according to a second embodiment of the invention; 
           [0022]      FIG. 14  is an equivalent circuit for the structure illustrated in  FIG. 13 , illustrating current skimming in this embodiment; 
           [0023]      FIG. 15  is a graph of read current I read  versus skimming voltage V skim  for the photodetector shown in  FIG. 13 , in dark and at a temperature of 77K; 
           [0024]      FIG. 16  is a graph similar to  FIG. 15 , but showing an I read  v. V skim  curve with incident IR radiation flux at 0.1 W/cm 2 ; and 
           [0025]      FIG. 17  is a schematic circuit diagram of the photodetector cell illustrated in  FIG. 8  and a connected readout integrated circuit (ROIC) for operating the cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  is a highly magnified schematic sectional view of a two-color/skimming back-illuminated photodetector cell  100  according to a first embodiment of the invention. This illustrated embodiment is fabricated of an Hg 1-x Cd x Te compound semiconductor (0&lt;x&lt;1; commonly called MCT for mercury cadmium telluride), suitable for detecting radiation in long wave infrared (LWIR) and medium wave infrared (MWIR) bands or colors. The invention has application to other Group II-VI chemistries, to other compound semiconductor detector devices and to radiation detectors generally. 
         [0027]    The cell  100  may be fabricated by successively growing, on a CdTe/Si substrate, HgCdTe layers which are doped differently from each other and/or have different ratios of mercury to cadmium, and then selectively etching back some of these. In particular, an (n) contact layer  102 , MWIR absorber layer  104 , layer  106 , gradient layer  108 , LWIR absorber layer  110  and LWIR (p) contact layer  112  are successively formed on top of each other, preferably using a molecular beam epitaxy (MBE) technique. Regions or layers  102  and  104 , most of region or layer  106 , region or layer  108  and region or layer  110  are doped in situ to be n-type as by the use of an Indium dopant. A MWIR contact region  114  and LWIR contact region  112  are doped to be p-type as by use of an Arsenic implant. Representative thicknesses, semiconductor compositions and dopant concentrations of these layers are presented in the following Table I. Variable x is the relative concentration of cadmium in the mercury-cadmium-telluride (MCT) compound semiconductor Hg 1-x Cd x Te: 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                   
                   
                 Dopant Concentration, 
               
               
                 Region 
                 Thickness 
                 Composition, x 
                 cm −3   
               
               
                   
               
             
             
               
                 112 
                 1 μm 
                 0.30 
                 N A  = 1 × 10 17   
               
               
                 110 
                 3 μm 
                 0.20 
                 N D  = 1 × 10 15   
               
               
                 Gradient (108) 
                 1 μm 
                 0.20-0.35 
                 N D  = 1 × 10 15   
               
               
                 106 
                 1 μm 
                 0.35 
                 N D  = 1 × 10 15   
               
               
                 104 
                 4 μm 
                 0.30 
                 N D  = 1 × 10 15   
               
               
                 102 
                 1 μm 
                 0.40 
                 N D  = 1 × 10 17   
               
               
                   
               
             
          
         
       
     
         [0028]    The (p+) regions  112  and  114  can be formed by ion implantation and activation annealing. The MWIR contact region  114  can also have a dopant concentration NA of about 1×10 17  cm −3 . 
         [0029]    Region  102  is a wide band-gap buffer layer between the active device and the substrate. Region  104  functions as the MWIR absorber layer with a cutoff wavelength λ c  of about 5.1 μm at 77K. Region  106  is a wide band-gap layer between the MWIR and LWIR absorber regions. Region  108  is a gradient layer in which the chemical composition of the compound semiconductor is gradually altered as a function of distance from region  106 . Region  110  functions as the LWIR absorber layer with a cutoff wavelength λ c  of about 14.5 μm at 77K. Region  112  is a wide band-gap top window or contact region for the LWIR detector. A portion of the stack of layers  108 - 112  is patterned and etched to create the stepped semiconductor shown. This etch may be done by an inductively coupled plasma (ICP) etch process using Argon and methane. The, a (p+) MWIR contact region  114  is formed in layer  106 . 
         [0030]    The cell  100  shown in  FIG. 1  has a smaller (in terms of volume and area) LWIR device to create larger resistance, and a larger MWIR device to accommodate low photon flux. A width w 1  and pixel size of the cell  100  can be about 30 μm, while a width w 2  of the etched portion of the structure can be about 10 μm. 
         [0031]      FIG. 2  is a band diagram of a leftmost portion  200  of the cell  100 , mostly involved with the function of the cell  100  as a MWIR detector, while  FIG. 3  is a band diagram of a rightmost portion  300  of the cell  100 , also involved as an LWIR detector. Areas on these graphs have been identified with the character identifying the semiconductor region to which they correspond. Note the barrier interposed by regions  106  and  108  between the MWIR absorber layer  104  and the LWIR absorber layer  110 . 
         [0032]      FIG. 4  shows a graph of the responsivity (A/W) of this cell at 77 K. Profile  400  is for the MWIR region. Profile  402  is for the LWIR region. There is a relatively small amount of spectral cross-talk as shown by the overlap of the two graphs. When measured as the ratio of the area of the cross-over region to the signal integrated over the entire spectral domain, the cross-talk of the LWIR region as contributed to sensing LWIR wavelengths is about 4.5%, while the cross-talk of the MWIR region as contributing to sensing MWIR wavelengths is about 2.0%. 
         [0033]    Cell  100  may be operated alternatively as an MWIR or as a LWIR device, depending on the voltages impressed on its contacts.  FIG. 5  illustrates LWIR operation under LWIR illumination. No substantial reverse bias is applied to MWIR contact  114 . A depletion region can extend into the LWIR absorber region  110  if the junction between (p+) contact region  114  and (n) LWIR absorber region  104  is placed sufficiently close, such as in the range of one to five microns, to the LWIR absorber region  110 . When LWIR radiation  500  enters the LWIR absorber region  110 , electron-hole pairs  502  will be generated. The electrons will be pulled toward (n) contact  102  while the holes will mostly flow to the LWIR contact  112 . But if a reverse bias is applied between contact  114  an the common contact  102 , some of the holes will diffuse to the MWIR depletion region and are skimmed through the MWIR contact  114 . 
         [0034]      FIG. 7  shows operation of the cell under LWIR illumination and MWIR bias. MWIR depletion region  700  is extended from (p+) contact  114  to join the depletion region formed in the LWIR absorber region  110  in these conditions. A skimming bias voltage of −4V is applied to contact  114 , while a sensing voltage of −0.1V is applied to contact  112 . If the voltages are adjusted such that the holes collected at the MWIR contact  114  account for all or a portion of the dark current, the holes appearing at contact  112  will be more attributable to impinging LWIR radiation and less attributable to dark current, with the objective being to totally skim off all of the dark current to improve the device&#39;s signal to noise ratio. As shown by the graphs of  FIGS. 8-11 , the two-contact embodiment illustrated in  FIGS. 1-7  is capable of skimming between five and ten percent of the LWIR current depending on the flux of the incident radiation. 
         [0035]      FIG. 6  shows operation of this device  100  under MWIR illumination and as a MWIR detector. Here, electron-hole pairs  600  are formed in the MWIR absorber region  104 . The holes are pulled toward MWIR contact  114  while the electrons are pulled toward contact  102 . Contact  112  will skim off some of those holes. 
         [0036]      FIGS. 8-11  are graphs of LWIR current versus a skimming bias voltage applied to MWIR contact  114  for the cell illustrated in  FIG. 1 .  FIG. 8  shows a graph for dark conditions.  FIG. 9  is a graph for an incident radiation flux Φ of 0.1 W/cm 2 .  FIGS. 10 and 11  are for a flux Φ of 1 W/cm 2  and 10 W/cm 2 , respectively. The given percentages are of skimmed current corresponding to the ratio of the LWIR current at −4V skimming voltage to the LWIR current when no skimming voltage is applied. Under varying illumination conditions, this percentage is between five and ten percent. 
         [0037]    In order to increase the percentage of skimmed current in a two-contact device, one can decrease the background (n) doping of MWIR depletion region  106  to 5×10 14  cm −3  and change the molecular proportion x in the Hg 1-x Cd x Te semiconductor in region  106  to 0.30. This decreases the barrier at the region  106 /region  110  interface and increases the skimming current to about fifteen to twenty percent, as shown by  FIG. 12 . 
         [0038]    Another way to improve operation of this device is to use a three-contact-per-pixel architecture, as illustrated in the highly magnified schematic sectional view of  FIG. 13 . The semiconductor structure of this cell  1300  can be the same as that of the cell illustrated in  FIG. 1 . After the semiconductor structure of the cell  1300  is formed, the cell  1300  can be coated with a suitable passivation layer  1302  on its bottom side adjacent region  102 , and a passivation layer  1304  on its top side adjacent regions  102 - 112 . A contact  1306  is made to n-type region  102 . A contact  1310  is made to the MWIR contact region  114 . A contact  1312  is made to the LWIR contact region  112 . Conductors  1314 ,  1316  and  1318  are then formed to connect these contacts to a top  1320  of the device, where connection can be made to an ROIC circuit and external leads through Indium bumps (not shown). 
         [0039]    The cell  1300  typically will be one of many in a two-dimensional array and its bottom side is presented toward a source of infrared radiation  1322  to be detected. To increase detector efficiency an antireflective coating  1324  may be deposited on passivation layer  116 . The illustrated cell  1300  is capable of single-color LWIR, single-color MWIR, and two-color modes, and in the first two of these a respective filter (one of those schematically represented at  1326 ,  1328 ) is superimposed over the bottom face  1330 . The filters  1326 ,  1328  are either mechanically removable or their filtering characteristics are electronically controlled to permit passage of MWIR wavelengths, LWIR wavelengths or both. 
         [0040]      FIG. 14  is an equivalent circuit model of cell  1300 . An anode of the LWIR detector diode  1400  is connected to the LW contact  1312 , while the cathode thereof is connected to the (n) or common contact  1306 . An anode of the MWIR detector diode  1402  is connected to the MW contact  1310 , while the cathode thereof is connected to the (n) contact  1306 . Using long-wavelength single-color operation as an example, the current I LW  through diode  1400  will be the sum of the dark current I dark  and photogenerated current I photo , and will be negative. A skimming voltage V skim  is applied to the MW contact  1310 . This skimming voltage is so selected that a current I MW  running through the MWIR detector diode  1402  will be equal to but of opposite sign from the dark current I dark . The dark current is thereby cancelled out. The resultant readout current I read  sensed at contact  1306  will then have no dark current component in it at all, completely removing this contribution to the signal to noise ratio. 
         [0041]    Because of this current skimming, less current is available for integration, in turn permitting longer integration times and/or smaller integration capacitors. 
         [0042]      FIGS. 15 and 16  show the simulated operation of cell  1300 . The graph of  FIG. 15  is plotted for a temperature of 77K and in dark conditions. The readout current I read  dips at a skimming voltage V skim  (applied to the MW contact) of about 0.162 V. Since there is no photogenerated current I photo  in these conditions, the objective is to minimize the total readout current. About ninety percent of the dark current is skimmed. 
         [0043]      FIG. 16  is a similar graph in which the incident flux of infrared radiation is 0.1 W/cm 2 . In these conditions, choosing a skimming voltage V skim  of about 0.192 V will yield a maximum amount of skimmed dark and background current. 
         [0044]      FIG. 17  is a schematic circuit diagram of cell  1300  as connected (as by Indium bumps) to related readout integrated circuit (ROIC) circuitry. The cell  1300  includes a MWIR detector/photodiode  1402  and an LWIR detector/photodiode  1400 . A Det_Com — 1 lead  1318  is connected to MWIR photodiode  1402  through contact  1310  (seen in  FIG. 13 ). A Det_Com — 2 lead  1316  is connected to LWIR photodiode  1400  through contact  1312 . The other ends of photodiodes  1400 ,  1402  are connected in common to node  1314  (an Indium bump) via contact  1306 . 
         [0045]    The ROIC circuit  1700  making up the rest of the circuit shown in  FIG. 17  is a direct injection amplifier. A bias adjust signal line  1702  is connected to the gate of a bias adjust transistor  1704 , the current path of which connects node  1314  to a node  1706 . An anti-blooming signal line  1708  is connected to the gate of a transistor  1710 , the current path of which connects node  1712  to ground. The current path of transistor  1710  is parallel to an integration capacitor  1714 . Node  1712  is connected via a transfer gate  1716  to a node  1718 . A reset control line  1720  is connected to the gate of a transistor  1722 , the current path of which connects node  1718  to ground. A S/H capacitor  1724  is in series with the current path of the transistor  1722 . The node  1718  is coupled to a column amplifier (not shown). 
         [0046]    The circuit shown in  FIG. 17  may be operated in any of six different modes, as shown in the following table. 
         [0000]    
       
         
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
                   
                   
                   
                 Operating 
                   
               
               
                 MWIR 
                   
                 LWIR 
                   
                 Color 
                 Comment 
               
               
                   
               
             
             
               
                 Det_Com_1 
                   
                 Det_Com_2 
                   
                   
                   
               
               
                 (1318) 
                   
                 (1316) 
               
               
                 With Filter 
                 No 
                 With Filter 
                 No 
               
               
                   
                 Filter 
                   
                 Filter 
               
               
                   
                 OFF 
                   
                   
                   
                 Both colors 
               
               
                   
                   
                   
                   
                   
                 off 
               
               
                   
                 OFF 
                   
                 ON 
                 LWIR 
                 Single color 
               
               
                   
                   
                   
                   
                 only 
               
               
                   
                 ON 
                   
                 OFF 
                 MWIR 
                 Single color 
               
               
                   
                   
                   
                   
                 only 
               
               
                   
                 ON 
                   
                 ON 
                 LWIR and 
                 Both colors 
               
               
                   
                   
                   
                   
                 MWIR 
               
               
                 ON 
                   
                   
                 ON 
                 LWIR 
                 MWIR used 
               
               
                   
                   
                   
                   
                   
                 to skim 
               
               
                   
                 ON 
                 ON 
                   
                 MWIR 
                 LWIR used 
               
               
                   
                   
                   
                   
                   
                 to skim 
               
               
                   
               
             
          
         
       
     
         [0047]    In summary, a photodetector has been presented which has two photodiodes. The detector may be operated such that one of the photodiodes is used as the sensor and the other of the photodiodes is used to skim off a portion or all of the dark current of the device. The signal to noise ratio and utility of the device as a detector therefore show improvement over prior art devices. 
         [0048]    While illustrated embodiments of the present invention have been described and illustrated in the appended drawings, the present invention is not limited thereto but only by the scope and spirit of the appended claims.