Abstract:
The instant disclosure describes a photodetector that includes at least one portion of a semiconducting layer formed directly on at least a portion of a reflective layer and to be illuminated with a light beam, at least one pad being formed on the portion of the semiconducting layer opposite the reflective layer portion, wherein the pad and the reflective layer portion are made of a metal or of a negative permittivity material, the optical cavity formed between said at least one reflective layer portion and said at least one pad has a thickness strictly lower than a quarter of the ratio of the light beam wavelength to the optical index of the semiconducting layer, and typically representing about one tenth of said ratio.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a photodetector. The present invention relates to such a photodetector comprising a semiconductor layer having a thickness much smaller than the wavelength of the light which is desired to be detected. 
       DISCUSSION OF PRIOR ART 
       [0002]    Current photodetectors generally comprise a semiconductor layer associated with a charge transfer device. When a light beam lights the semiconductor layer, the incident photons form electron/hole pairs inside of it. Such carriers are then transferred by the charge transfer device to an electronic circuit which enables to quantify them. 
         [0003]    The average depth of the forming of electron/hole pairs in a semiconductor layer depends on the wavelength of the incident light beam and on the semiconductor material used: the higher the wavelength, the deeper the electron/hole pairs are likely to form inside of a semiconductor layer. For example, to absorb approximately 99% of red light, it is necessary to have a silicon layer with a thickness of approximately 10 μm. For blue light, a silicon layer having a thickness on the order of 3 μm is sufficient to reach such an absorption rate. It should be noted that these thicknesses are much greater than the wavelength of the light waves which are detected. 
         [0004]    The size of the pixels of a photodetector is generally desired to be decreased. The decrease in the surface area taken up by each pixel on the semiconductor layer essentially poses problems of insulation of the photosensitive regions from one another. Indeed, to avoid interferences between neighboring pixels, it is generally provided to form insulating trenches around each of them. If the semiconductor layer is relatively thick, the insulating trenches take up a non-negligible surface area of the semiconductor layer, which is not compatible with the decrease of the general pixel size. The thickness of semiconductor layers thus has to be decreased. However, as indicated previously, the forming of very thin active layers does not enable to absorb and detect the entire incident light. 
         [0005]    It has thus been provided to increase the equivalent thickness of the semiconductor layer by having photons cross this layer twice or several times. A mirror can thus be provided on one side of the semiconductor layer to ensure a double crossing thereof. Reflective elements may also be provided on either side of the semiconductor layer to form Fabry-Perot type structures. In practice, the reflective elements are Bragg mirrors which are generally thick, formed for example of a stack of from 10 to 30 quarter-wave layers. The advantage of a decrease in the thickness of the active semiconductor layer is thus lost. 
         [0006]    Such devices are especially described in patent application US 2006/151807 where a resonance is created between a lower mirror (Bragg or metal mirror) and an upper metal network forming a mirror. In such a device, the thickness of the cavity between the lower and upper mirrors must be a multiple of one quarter of the ratio between the incident wavelength and the average optical index between the upper and lower mirrors. It is thus impossible to decrease the thickness of this cavity below a given value. 
         [0007]    It has then been provided to use another effect to decrease the thickness of photodetection semiconductor layers. This effect comprises taking advantage of physical collective electron oscillation phenomena (plasmon resonances). 
         [0008]    In  FIG. 1 , on a semiconductor substrate  11 , many metal strips  13  extend to form, at the surface of substrate  11 , a periodic lattice. The photodetector of  FIG. 1  is illuminated by a light beam indicated by arrows  15 . When period P of the lattice formed of strips  13  is equal to the ratio between the wavelength of light beams  15  and the optical index of semiconductor material  11 , surface plasmon resonances appear at the metal-semiconductor interfaces. Such resonances enable to confine photons, and thus the area where electron/hole pairs are formed in substrate  11 , in the vicinity of metal strips  13 . 
         [0009]    However, a structure such as that in  FIG. 1  has the disadvantage of being very selective in terms of frequency. Further, the efficiency of such a structure strongly depends on the angle of incidence and on the polarization of light beam  15 . Finally, period P must be on the order of the ratio of the wavelength of light beam  15  to the optical index of semiconductor material  11 , which does not enable to envisage smaller elementary detectors. 
         [0010]      FIG. 2  illustrates another photodetector comprising very thin semiconductor regions  21  which extend vertically between metal regions  23 . The photodetector is illuminated by a light beam  25 . Surface plasmon resonances coupled between two opposite semiconductor-metal interfaces appear in this metallic photodetector with vertical semiconductor cavities However, in order for coupled surface plasmon resonances to form between two opposite semiconductor-metal interfaces, semiconductor cavities  21  must be very narrow, with a thickness much smaller than the wavelength of the considered light. A problem is then posed for the manufacturing of this structure. 
         [0011]    There thus is a need for a photodetector which is less frequency-selective than known plasmon resonance photodetectors, less dependent on the polarization and on the angle of incidence of the light beam to be detected, and which has a size that can be much smaller than the wavelength of the incident light beam. 
         [0012]    It is further desirable to be able to form photodetectors in which the semiconductor material can be selected from among a wide range of semiconductor compounds, some of which can only be deposited in an extremely thin layer. 
       SUMMARY 
       [0013]    An object of an embodiment of the present invention is to provide a photodetector comprising a very thin semiconductor layer and having at least one of the following features:
       less frequency-selective than known plasmon resonance devices, that is, with an absorption spectrum having a width at mid-height on the order of one third of the wavelength to be detected;   little sensitive to the angle of incidence of the incident light beam;   little sensitive to the polarization of the incident light beam;   capable of selecting one of the two fundamental polarizations; and   having dimensions smaller than the wavelength of a light beam to be detected.       
 
         [0019]    Thus, an embodiment of the present invention provides a photodetector comprising at least a portion of a semiconductor layer formed directly on at least a portion of a reflective layer and intended to be illuminated by a light beam, at least one pad being formed on the portion of the semiconductor layer, opposite to the reflective layer portion, the pad and the portion of the reflective layer being metallic or made of a material having a negative permittivity, the optical cavity formed between said at least a portion of the reflective layer and said at least one pad has a thickness strictly smaller than one quarter of the ratio of the wavelength of the light beam to the optical index of the semiconductor layer, typically on the order of one tenth of this ratio. 
         [0020]    According to an embodiment of the present invention, at least one dimension of the pad, in a plane parallel to the semiconductor layer, is smaller than the ratio of the wavelength of the light beam to twice the optical index of the semiconductor layer. 
         [0021]    According to an embodiment of the present invention, the portion of the semiconductor layer comprises a horizontal or vertical PN or PIN junction. 
         [0022]    According to an embodiment of the present invention, several pads, of square or rectangular shape, are formed at the surface of the portion of the semiconductor layer, the pads being arranged periodically according to a period smaller than 0.75λ/n, λ being the wavelength of said light beam, n being the optical index of the semiconductor layer. 
         [0023]    According to an embodiment of the present invention, a layer of a transparent material covers the photodetector. 
         [0024]    According to an embodiment of the present invention, the photodetector further comprises a bonding layer, which may also or as an alternative be a layer with an index gradient at the interface between the reflective layer and the semiconductor layer. 
         [0025]    According to an embodiment of the present invention, the pad(s) and the reflective layer are made of a metal capable of creating, when the semiconductor layer is illuminated, a cavity plasmon mode in this semiconductor layer. 
         [0026]    According to an embodiment of the present invention, the reflective layer is made of a metal having a thickness greater than the skin depth of this metal, for the wavelength of said light beam. 
         [0027]    According to an embodiment of the present invention, the metal pad(s) have a thickness ranging between once and three times the skin depth of the metal forming them, for the wavelength of said light beam. 
         [0028]    According to an embodiment of the present invention, the pad(s) and the reflective layer are made of a material having a negative permittivity, for example, an ionic crystal, intended to form, when the semiconductor layer is illuminated, phonons at the interface between said material and the semiconductor layer. 
         [0029]    Another embodiment of the present invention provides a multispectral image sensor comprising several elementary photodetectors such as described hereabove, the dimensions of the pad(s) of each elementary photodetector being adapted to the spectrum to be detected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings: 
           [0031]      FIGS. 1 and 2 , previously described, illustrate two known plasmon resonance photodetectors allowing the absorption of light beams in thin semiconductor layers; 
           [0032]      FIG. 3  illustrates a plasmon resonance photodetector according to an embodiment of the present invention; 
           [0033]      FIGS. 4 ,  5 , and  6  illustrate variations of a photodetector according to an embodiment of the present invention. 
           [0034]      FIGS. 7A and 7B  are curves illustrating advantages of a photodetector according to an embodiment of the present invention; 
           [0035]      FIGS. 8A ,  8 B, and  8 C illustrate three possible shapes of the upper pads of a photodetector according to an embodiment of the present invention; and 
           [0036]      FIG. 9  illustrates a variation of a photodetector according to an embodiment of the present invention. 
       
    
    
       [0037]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of semiconductor devices, the various drawings are not to scale. 
       DETAILED DESCRIPTION 
       [0038]    The present inventors provide a photodetector comprising a very thin semiconductor layer having an improved absorption. For this purpose, they have a structure of vertical metal-semiconductor-metal type, which further has the advantage of being little sensitive to the angle of incidence and to the optical frequency of the incident light beam, as will be seen hereafter. This structure may also be configured to be little dependent on the polarization of the incident light beam or to select one of the two fundamental polarizations of the incident light beam. 
         [0039]    In the foregoing and in the following description, it is spoken of the creation of surface plasmon resonances or of plasmon modes at the interface between a metal and a semiconductor material. It should be noted that structures similar to those discussed herein, in which the metal regions are replaced with regions of a material having a negative permittivity in the considered spectral field may also be provided. Such materials are for example very heavily-doped semiconductor materials, or again ionic crystals such as silicon carbide. The forming of regions made of one of these materials rather than of a metal enables to form surface phonon resonances rather than surface plasmon resonances. It should be noted that these two resonances cause similar effects on the incident light photons. 
         [0040]      FIG. 3  illustrates an embodiment of a plasmon resonance photodetector. 
         [0041]    On a metal layer  31  is formed a layer of a semiconductor material  33  having a thickness e strictly smaller than one quarter of the ratio between the wavelength of a light beam illuminating the device (arrows  35 ) and the optical index of semiconductor layer  33 . It should be noted that semiconductor layer  33  will preferably have a thickness on the order of one tenth of this ratio. Semiconductor layer  33  is for example made of silicon, of germanium, of a III-V compound or again of a material with a small bandgap such as InSb or CdHgTe-type compounds. The use of one or the other of these materials to form semiconductor layer  33  depends on the wavelength of the light beam which is desired to be detected. Indeed, these different materials enable to cover a wavelength range extending from close ultraviolet (400 nm) to infrared (15 μm). 
         [0042]    On semiconductor layer  33  is formed a metal pad  37 , opposite to metal layer  31 . Thus, semiconductor layer  33 , sandwiched between metal layer  31  and pad  37 , defines an optical cavity. The photodetector is illuminated on the surface comprising semiconductor layer  33  and metal pad  37 , as shown in  FIG. 3  by arrows  35 . Metal layer  31  preferably has a thickness greater than the skin depth of the metal forming it at the frequency of light beam  35 , the skin depth of the metal being the thickness above which a light field no longer propagates into the metal. Metal pad  37  preferably has a thickness ranging between once and three times the skin depth of the metal forming it at this same frequency. As an example, the metal of pad  37  and of layer  31  may be gold, silver, aluminum, or again an alloy of these materials having similar optical properties. In the shown example, pad  37  has a rectangular shape in top view. 
         [0043]    The structure of  FIG. 3  enables to use the electromagnetic resonance appearing within the metal-semiconductor-metal sandwich to reinforce the absorption of semiconductor layer  33 , metal layer  31  and metal pad  37  forming a plasmonic resonator of horizontal cavity type (the cavity plasmons form between pad  37  and layer  31 ). The cavity plasmon resonances enable, in known fashion, to confine the incident photons in semiconductor layer  33 , between metal pad  37  and metal layer  31 . For the photodetector to operate properly and the cavity plasmon modes to form, width L of pad  37  must be such that: 
         [0000]    
       
         
           
             
               
                 
                   
                     L 
                     ≈ 
                     
                       
                         λ 
                         
                           2 
                           · 
                           
                             n 
                             eff 
                           
                         
                       
                       - 
                       
                         Δ 
                          
                         
                           ( 
                           λ 
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    n eff  being the effective optical index of the plasmon mode, greater than the optical index of semiconductor layer  33 , Δ(λ) being a positive term which reflects the spectral shift linked to phase shifts or to absorptions in the photodetector. More simply, the width of pad  37  must be such that: 
         [0000]    
       
         
           
             
               
                 
                   L 
                   &lt; 
                   
                     
                       λ 
                       
                         2 
                         · 
                         
                           n 
                           eff 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
         [0044]    As will be seen hereafter, a photodetector such as that in  FIG. 3 , having dimensions in accordance with equation (1) or (2) and with an optical cavity between metal layer  31  and metal pad  37  (semiconductor layer  33 ) having a thickness smaller than one quarter of the ratio of the wavelength of light beam  35  to the optical index of semiconductor layer  33 , has the advantage of having a significant absorption, even if the wavelength of the light beam varies slightly. 
         [0045]    A bonding layer  38 , delimited in  FIG. 3  by dotted lines, may be provided between metal layer  31  and semiconductor layer  33  to allow a proper bonding between the two layers. Layer  38  may also be a thin layer with an index gradient or a passivation layer, avoiding parasitic recombinations of photo-carriers at the metal interfaces. It should be noted that, in the following alternative embodiments, this layer, although not shown and described, may be present. 
         [0046]      FIG. 4  illustrates a variation of the photodetector of  FIG. 3 . On metal layer  31  is formed of a pad of semiconductor material  39  covered with a metal pad  37 . As in the case of the structure of  FIG. 3 , surface plasmon modes form in semiconductor pad  39 , at the interface between metal pad  37  and the semiconductor pad and at the interface between metal layer  31  and semiconductor pad  39 , when the photodetector is illuminated by a light beam  41  of adapted wavelength. The forming of the plasmon modes allows a better absorption in semiconductor pad  39 . The plasmon modes in semiconductor pad  39  appear when the width of metal pad  37  and the wavelength of the light beam comply with equation (1) or (2). 
         [0047]      FIG. 5  illustrates an alternative photodetector in which several pads are formed at the surface of a same semiconductor layer. On a metal layer  31  is formed a thin semiconductor layer  33 . On semiconductor layer  33  are formed many metal pads  43 . In the shown example, pads  43  are, in top view, square, and are formed periodically at the surface of semiconductor layer  33 . Each dimension of metal pads  43  at the surface of semiconductor layer  33  complies with equation (1) or (2). The periodization of pads  43  enables to form larger photodetectors. 
         [0048]    In the case of a periodic distribution of pads  43  at the surface of silicon layer  33 , period D of the array must be such that: 
         [0000]    
       
         
           
             
               
                 
                   
                     D 
                     &lt; 
                     
                       
                         3 
                         4 
                       
                        
                       
                         λ 
                         n 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    for the absorption of semiconductor layer  33  to be improved. 
         [0049]      FIG. 6  illustrates a variation of a photodetector comprising a thin semiconductor layer  33  at the surface of which is formed an array of rectangular pads  45  having at least its width L complying with equation (1) or (2). Period D of pads  45 , at the surface of layer  33 , complies with equation (3). On the lower surface of semiconductor  33  is formed an array of strips  47  which are located at least in front of pads  45 . Plasmon modes thus appear at the semiconductor-metal interfaces. 
         [0050]    It should be noted that the forming of rectangular pads  45  favors the absorption of a light beam polarized perpendicularly to the main direction of strips  45  (TM polarization). The forming of square pads such as pads  43  of  FIG. 5  enables to have a photodetector which is little dependent on the polarization of the incident light beam. A photodetector intended to select one or the other of fundamental polarizations TE or TM can thus be formed. 
         [0051]      FIGS. 7A and 7B  are curves illustrating advantages of the photodetectors discussed herein. 
         [0052]      FIG. 7A  shows, according to the wavelength of an incident beam, two curves  51  and  53 , respectively of reflective power R of a photodetector based on cadmium telluride mercury (CdHgTe or CMT) such as those discussed hereabove, and of absorption A of the semiconductor. The photodetector considered herein is sized to optimize the detection of a magnetic transverse polarized light beam having a 5,000-nm wavelength. To achieve this, the dimensions of the photodetector are selected such that equation (1) or (2), and if need be equation (3), are satisfied. 
         [0053]    For a light beam having a 5,000-nm wavelength, the absorption in the CdHgTe layer of the photodetector is on the order of 0.77 and the reflective power is almost zero. Conversely to known plasmon photodetectors which have the disadvantage of having a very small detection band, the photodetector considered herein has an absorption which only relatively slightly decreases, and a reflective power which only relatively slightly increases, when the wavelength of the light beam varies by 200 nm around the 5,000-nm wavelength. The width at mid-height of the absorption peak is on the order of 1,700 nm, which is on the order of one third of the wavelength of the incident beam. 
         [0054]      FIG. 7B  shows two curves  55  and  57 , respectively of reflective power R of a CdHgTe-based photodetector and of absorption A of the semiconductor in the case of a photodetector such as shown herein, according to the angle of incidence of the light beam, the photodetector being sized to correctly absorb the considered light beam. It should be noted, in curves  55  and  57 , that a variation of angle of incidence θ of the light beam on the order of 50° almost does not vary the absorption and the reflective power of the photodetector. 
         [0055]    Thus, the photodetectors of  FIGS. 3 to 6  allow a wide-band detection, little dependent on the angle of incidence of the light beam. Further, the forming of a photodetector comprising a thin semiconductor layer enables to form small pixels properly insulated from one another. 
         [0056]    Semiconductor layer  33 ,  39  may be provided to comprise different regions enabling to collect the photogenerated carriers. For example, it may be provided to locally form a vertical or horizontal PN junction in semiconductor layer  33 ,  39  or again a structure of vertical or horizontal PIN diode type (not shown). In this case, the photogenerated electrons may be stored in the N-type doped regions of the semiconductor layer and the detection is performed by transferring these electrons to an electronic circuit, for example, by means of a transfer transistor. 
         [0057]    Once the charges have been formed in semiconductor portion  33  or  39 , several solutions are possible to read these charges. 
         [0058]    A first solution comprises polarizing one or several ones of metal pads  37 ,  43 ,  45  formed at the surface of the semiconductor layer to a first voltage and of polarizing the layer or the lower pads  31 ,  47  to a second voltage. The flowing of a current resulting from this polarization is then representative of the illumination of the photodetector. A variation comprises polarizing one or several metal pads  43 ,  45 , at the surface of semiconductor layer  33 ,  39  to a first voltage and polarizing other metal pads  43 ,  45  to a second voltage. The detection is then only performed from the upper surface of the photodetector. 
         [0059]      FIGS. 8A ,  8 B, and  8 C show three possible variations of this last case, and more specifically three possible structures of the metal pads formed at the surface of a semiconductor layer  33  and allowing the photodetection. 
         [0060]    In the example of  FIG. 8A , the metal pads formed at the surface of the semiconductor layer are separated in two assemblies  61  and  63  having a basic comb shape, combs  61  and  63  being interdigited. Each comb  61  and  63  comprises a main strip from which secondary strips extend. Combs  61  and  63  are provided so that the width of the main strip and of the secondary strips complies with equation (1) or (2). The secondary strips have a period complying with equation (3). 
         [0061]    In the example of  FIG. 8B , the metal pads formed at the surface of semiconductor layer  33  are separated in two sets of fingers  65  and  67 . Each set of fingers is formed of a main strip and of secondary fingers which extend from the main strip with a substantially circular shape. The main strips and the secondary fingers comply with equations (1) or (2) and (3). 
         [0062]    In the example of  FIG. 8C , square pads  68 , distributed in an array at the surface of a semiconductor layer  33 , are not polarized. The dimensions and the period of pads  68  comply with equations (1) or (2) and (3). Pads  68  are surrounded with two electrodes  69 A and  69 B enabling the collection. 
         [0063]      FIG. 9  illustrates an alternative embodiment of a photodetector. The photodetector comprises a semiconductor layer  33  on which are formed pads  73 ,  75 , and  77  made of metal or of a material having a negative permittivity over the considered frequency band. Under layer  33 , at least in front of pads  73 ,  75 , and  77 , are formed metal portions  71 . Pads  73 ,  75 , and  77  have different widths at the surface of semiconductor layer  33 . Since the pad width is linked to the wavelength of the photo-detected light, pads  73 ,  75 , and  77  enable to detect light having different wavelengths. Thus, the photodetector of  FIG. 9  enables to selectively detect three light beams of different wavelengths. 
         [0064]    In  FIG. 9 , a transparent layer  79  is formed on the photodetector. This layer enables to protect the photodetector. It should be noted that such a layer may be provided on each of the previously-described variations. This layer may also have a shape capable of focusing the incident light beams towards each elementary detector (semi-spherical lens shape above each pad  73 ,  75 ,  77 , for example). 
         [0065]    The previously-described photodetectors may be used, for example, to form a color image sensor. Each elementary photodetector of the image sensor then comprises pads  73 ,  75 ,  77  sized to detect a given color (red, green, blue). These photo-detectors are assembled in an array of color pixels. As an example of dimensions, the largest elementary photodetector, intended to detect red light waves (λ≈600 nm), comprising a silicon layer (n  4 ), will comprise a pad having at least one dimension on the order of 50 nm. This dimension is particularly small as compared with current dimensions of conventional photodetectors where electron/hole pairs are directly created in a semiconductor layer. Further, the “insulation” between elementary photodetectors is simple to achieve and takes up a minimum surface area due to the fact that an extremely thin semiconductor layer is difficult to interrupt. A multispectral image sensor other than a color sensor may similarly be provided. 
         [0066]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, although photodetectors comprising upper pads and a lower layer made of a metal have been described herein, it should be understood that the variation in which the photodetector comprises upper pads and a lower layer of a material having a negative permittivity at the considered frequency may be applied to each of the variations described herein.