Abstract:
A photodetector includes a detector responsive to incident light to generate an output signal and one or more band gap filters upstream of the broadband detector for absorbing incident photons of predetermined wavelength. The bandgap filters have a bandgap gradient across their width. The photodetector can act as a selective detector without the need for a separate optical filter.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to the field of optoelectronics, and more particularly to an integrated photodetector suitable for use in wavelength division multiplexing applications.  
           [0003]    2. Description of Related Art  
           [0004]    Wavelength division multiplexing (WDM) is becoming an important medium for use in broadband communications. A single fiber can carry multiple wavelengths, each carrying a high speed digital channel. These must be individually detected to extract the signal carried on each wavelength.  
           [0005]    Typically optical filters are used to separate the different wavelengths prior to detection. Sometimes, only a few WDM channels are used, in which case the channels can be quite wide in terms of optical bandwidth. For such systems, bandgap engineered detector chips can be used to obviate the need for optical filters. These solutions are not effective for narrowband channels.  
           [0006]    An alternative solution is to combine a broadband filter with a discrete filter, such as a dichroic mirror, Bragg grating etc. The disadvantage of this solution is the need for extra filter components, and this results in high component cost.  
           [0007]    European patent no. 901,170 discloses a photodetector with filter layer having a given bandgap. Such a photodetector is not capable of filter a wide band of signals and also suffers from re-emission that occurs when charge carriers recombine.  
           [0008]    U.S. Pat. No. 4,213,138 discloses a dual-wavelength photodetector that has two absorption layers that respond to different wavelengths in series.  
           [0009]    In order to provide a practical photodetector, the original photon power outside the detected wavelength should be reduced to between 1 and {fraction (1/10)}% of its original power. This is not possible with prior art proposals. There is a need to provide an efficient selective low pass detector that overcomes these drawbacks of the prior art  
         SUMMARY OF THE INVENTION  
         [0010]    According to the present invention there is provided an integrated photodetector comprising a detector responsive to incident light to generate an output signal; and a bandgap filter arrangement upstream of said detector and integral therewith for absorbing incident photons, said bandgap filter arrangement having a bandgap that varies in the upstream direction.  
           [0011]    In one embodiment the structure includes a plurality of filters with progressively increasing bandgaps. In another embodiment the bandgap forms a gradient through the filter, with the bandgap on the input side being less than on the output side so that photons of gradually higher energy are absorbed as the light passes through the filter.  
           [0012]    The invention is preferably implemented using an InGaAsP system. The filter layers are preferably InGaAsP and the detector InGaAs. The detector is typically a PIN diode.  
           [0013]    In another aspect the invention provides a method of detecting light of a selected wavelength comprising the steps of passing incident light through a bandgap filter arrangement to absorb incident photons, said bandgap filter arrangement having a bandgap that varies in the upstream direction; and detecting light passing through said bandgap filter arrangement with a detector responsive to incident light to generate an output signal, said detector being integral with said bandgap filter arrangement. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The invention will now be described in more detail, by way of example, only with reference to the accompanying drawings, in which:  
         [0015]    [0015]FIG. 1 a  is a schematic diagram of a first embodiment of a detector with a band gap optical filter;  
         [0016]    [0016]FIG. 1 b  is an equivalent circuit of the structure shown in FIG. 1 a;    
         [0017]    [0017]FIG. 2 a  is a schematic diagram of a second embodiment of a detector with a band gap optical filter;  
         [0018]    [0018]FIG. 2 b  is an equivalent circuit of the structure shown in FIG. 2 a;    
         [0019]    [0019]FIG. 3 a  is a schematic diagram of a third embodiment of a detector with a band gap optical filter;  
         [0020]    [0020]FIG. 3 b  is the equivalent circuit of the structure shown in FIG. 3 a;    
         [0021]    [0021]FIG. 4 a  is a schematic diagram of a fourth embodiment of a detector with a band gap optical filter;  
         [0022]    [0022]FIG. 4 b  is the equivalent circuit of the structure shown in FIG. 4 a ; and  
         [0023]    [0023]FIG. 5 shows the complete structure of on embodiment of a detector with a band gap optical filter biased band gap optical filter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 1 a  shows a schematic PIN diode implementation of a detector with a low pass bandgap optical filter. The structure shown can be formed by epitaxial growth techniques in a manner known per se.  
         [0025]    A high band gap, heavily doped n +  substrate  10  of InP has deposited thereon a series of n type filter bandgap layers  12   1  . . .  12   n  of InGaAsP. The band gap of the substrate  10  is sufficiently high to allow photons in the expected wavelength range to pass through the layer without absorption. Each filter layer  12   1  . . .  12   n  has a bandgap n corresponding to a wavelength λ Fn , i.e. the first layer has a bandgap  1  corresponding to a wavelength λ F1 , the second layer has a bandgap  2  corresponding to a wavelength λ F2 , and so on. Photons at wavelength λ Fn  will therefore be absorbed in the layer  12   n .  
         [0026]    The layers  12   1  . . .  12   n  are arranged such that the absorption wavelengths progressively increase, i.e. λ Fn &gt;λFn−1. This means that the bandgaps progressively decrease. Thus, the shorter wavelengths with higher energy are absorbed in the lower layers and the longer wavelengths with less energy are absorbed in the higher layers, where the bandgaps are lower.  
         [0027]    On top of the layer  12   n  is grown a high bandgap InP n type or n +  type buffer layer  14 . Low band gap InGaAs detector layer  16 , of n −  conductivity type, is formed on buffer layer  14 , which serves to separate the detector layer  16  from the filter layers  12   n . This detector layer  16  has a bandgap suitable for absorbing photons of wavelength λ D , that is the detector layer  16  has a bandgap equal to the target wavelength for detection λ D , which is greater than λ Fn . Thus photons passing through the filter layers  12   n  pass through the high band gap buffer layer  14  to be absorbed by the detector layer  16 .  
         [0028]    On top of layer  16  is deposited a contact layer  18  with a heavily doped p +  region  20  providing an anode for the detector layer  16 . The equivalent circuit of this arrangement is shown in FIG. 1 b.    
         [0029]    In operation, incident photons pass through filter layers  12   n . Photons having an energy less than a certain value such that their wavelength λ&lt;λ Fn  are absorbed, leaving only photons of wavelength λ&gt;λ Fn  to reach the detector layer  16 .  
         [0030]    The detector layer  16 , which does not have to be highly discriminating due to the presence of the upstream filters, develops an output signal developed across the structure that depends on the intensity of incident light the substrate  10 .  
         [0031]    In an alternative arrangement shown in FIG. 2 a , instead of arranging the layers in a stack, as shown in FIG. 1 a , the single InGaAsP filter layer  12  has a bandgap that progressively decreases across its thickness. The bandgap on the entry side is greater than that on the exit side. A gradient is formed between the entry and exit side so that so that photons of gradually decreasing energy are absorbed as they move through the layer. The higher energy photons of shorter wavelength are absorbed on the entry side. The equivalent circuit for FIG. 2 a  is shown in FIG. 2 b.    
         [0032]    While the above described embodiments represent an improvement over the prior art, charge carriers liberated by the absorbed photons in the filter layer can combine to cause photon re-emission, which can impact on efficiency.  
         [0033]    This problem is addressed in the embodiments of FIGS. 3 a  and  4   a , where a pn junction is associated with each filter layer to remove any liberated charge carriers before than can recombine to cause re-emission.  
         [0034]    In FIG. 3 a  the same reference numerals are employed as in FIG. 1 a . The structure is similar to that shown in FIG. 1 a , except that a heavily doped p +  type high band gap anode layer  14   n  is grown on top of each filter layer  12   n . In addition to serving as a buffer layer, this p +  type layer creates a pn junction with the underlying n −  type filter layer  12   n . In operation, this pn junction is reverse biased to create an electric field in the bandgap filter that removes the liberated charge carriers before they have time to recombine.  
         [0035]    The equivalent circuit for FIG. 3 a  is shown in FIG. 3 b.    
         [0036]    [0036]FIG. 4 a  shows a gradient structure similar to that shown in FIG. 2 a , but with a single heavily doped p+ anode layer  14  on top of the filter layer  12  with the bandgap gradient. The equivalent circuit is shown in FIG. 4 b . This embodiment works in a similar manner to that shown in FIG. 2 b  except that the pn junction created by the layers  12  and  14  creates an electric field when reverse biased that removes the liberated charge carriers before recombination can occur.  
         [0037]    A practical example of the embodiment of FIG. 4 a  is shown in FIG. 5. This embodiment is implemented using an InGaAsP (Indium Gallium Arsenic Phosphorus) semiconductor material system, although it will be apparent to one skilled in the art that other semiconductor material systems can be used. The various layers are formed by doping semiconductor materials in a manner known per se. The structure is epitaxially grown on the InP substrate  10 . The filter layers are quaternary mixtures (InGaAsP) and the detector is a ternary mixture of (InGaAs). The quaternary mixture of InGaAsP makes it possible to design a range of energy bandgaps, while still maintaining the same lattice constant as for InP.  
         [0038]    The filter layer  12  has a variable bandgap across its width as described with reference to FIG. 4 a , although it will be appreciated that it can also consist of a stack of alternate layers as described with reference to FIG. 3 a.    
         [0039]    The top contact layer  18  is formed on the detector layer  16  and has p+ contact region  20 .  
         [0040]    A via  26  is etched into the detector layer to reach the anode filter layer  14 . An insulating layer  28  is then deposited over the contact layer  18  and the sidewalls of the via  16 . Metal contacts  24  and  30  are then added to reach the contact region  20  and the anode layer  14  forming the p layer of the pn junction. Contact layer  24  provides the anode for the detector layer  16 . Contact layer  28  serves as the cathode for the detector layer  16  and the anode for the pn junction of the filter layer. Contact layer  22  serves as a cathode contact for the filter. This has a window  32  for the admission of photons into the device.  
         [0041]    The described photodetector is effective at removing short wavelength components, and as a result the detector layer  16  with a low band gap does not need to be highly discriminating.  
         [0042]    It will be appreciated that the invention makes extra filter components unnecessary in WDM applications since the filter layer(s) absorb photons below a certain cut-off wavelength. The structure attenuates low wavelength photonic power while over a certain wavelength range the device will exhibit high responsivity.