Patent Publication Number: US-2023142387-A1

Title: Waveguide dual-depletion region (ddr) photodiodes

Description:
BACKGROUND 
     Optical communication systems are known whereby one or more optical signals, each being modulated to carry information, are transmitted from an optical transmitter to an optical receiver. The optical receivers typically includes, among other things, one or more photodiodes that convert the received optical signals into corresponding electrical signals, which are then further processed. In certain optical communication systems, various components or devices in the receiver are integrated on a common substrate as a photonic integrated circuit (PIC). Such components include optical waveguides and photodiodes that, in some instances, receive light supplied by the optical waveguides. 
     So-called dual-depletion region (DDR) photodiodes are known in which an “absorber” layer is located above an undoped layer of bandgap wider than the absorber, in such a manner that photo-generated holes have a shorter distance to travel to the p-type region anode above the absorber, while photo-generated electrons have a longer distance to travel to the n-type region cathode below the undoped layer. The depletion region extends from above the absorber layer to below the lower undoped layer. Since electron mobility is much higher than hole mobility in InP and related materials, the total transit time of such holes and electrons from the absorber to their respective contacts is comparable or minimized. A similar photodiode without a undoped layer below the absorber would have much higher capacitance, and a similar photodiode with an absorber as thick as the two undoped layers can absorb too much light at the input so that the high speed photocurrent response is nonlinear. Accordingly, carrier lifetime in the DDR photodiode is reduced, and the length and capacitance of the photodiode may be optimized for both high responsivity and high radio frequency (RF) bandwidth. 
     Conventional DDR photodiodes detect light that is incident at a direction that is normal to the substrate upon which the DDR photodiode is provided. PICs, however, include optical waveguides that confine optical signals, whereby the optical signals propagate in the optical waveguides in a direction parallel to the substrate. Accordingly, conventional DDR photodiodes may not be suitable for integration in a PIC, and the associated benefits of such photodiodes may be difficult to obtain in optical receivers including PICs. 
     SUMMARY 
     Consistent with an aspect of the present disclosure, an optical receiver is provided that comprises a substrate and an optical waveguide having a core layer provided on a first region of the substrate. The receiver also includes a photodiode provided on a second region of the substrate, such that an interface between the optical waveguide and the photodiode constitutes a butt joint. The photodiode includes a first semiconductor layer having a p-conductivity type, the first semiconductor layer being a p-type cladding layer. The photodiode also includes a second semiconductor layer having n-conductivity type, the second semiconductor layer being an n-type cladding layer. Further, the photodiode includes an absorber layer provided between the p-type cladding layer and the n-type cladding layer. The absorber layer has a first undoped semiconductor layer, such that the absorber layer is aligned with the core layer of the optical waveguide to receive, via the interface, an optical signal propagating in the optical waveguide. Moreover, the photodiode includes a second undoped semiconductor layer provided between the absorber layer and the second semiconductor layer, such that in an absence of a reverse bias applied to the photodiode, a first depletion region forms in the absorber layer and a second depletion region forms in the second undoped semiconductor layer. 
     Consistent with a further aspect of the present disclosure, an optical receiver is provided that comprises a substrate and an optical waveguide provided on a first region of the substrate. In addition, the optical receiver includes a photodiode provided on a second region of the substrate, such that an interface between the optical waveguide and the photodiode constitutes a butt joint. Further, the photodiode is configured to receive an optical signal supplied by the optical waveguide, wherein the optical signal has a propagation direction in the optical waveguide. The interface between the optical waveguide and the photodiode is provided at a non-orthogonal angle relative to the direction of propagation of the optical signal. 
     Consistent with an additional aspect of the present disclosure, an optical receiver is provided that comprises a substrate and an optical waveguide provided on a first region of the substrate. Further, a photodiode provided on a second region of the substrate, such that an interface between the optical waveguide and the photodiode constitutes a butt joint. The photodiode is configured to receive an optical signal supplied by the optical waveguide, wherein the optical signal propagates in the optical waveguide in a propagation direction, and the optical signal propagates in the photodiode in the same propagation direction. A width of the optical waveguide increases in the propagation direction. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments and together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows an optical receiver employing direct detection consistent with an aspect of the present disclosure; 
         FIG.  2   a    shows an optical receiver employing coherent detection consistent with an additional aspect of the present disclosure; 
         FIG.  2   b    shows an example of a balanced (differential) detector consistent with the present disclosure; 
         FIGS.  3   a - 3   c    show plan view of waveguide-photodiode combinations consistent with aspects of the present disclosure. 
         FIG.  4    shows a perspective view of a waveguide-photodiode configuration shown in  FIG.  3     a;    
         FIG.  5   a    shows a cross-sectional view of photodiode and waveguide consistent with a further aspect of the present disclosure; 
         FIG.  5   b    shows a cross-sectional view of photodiode and waveguide consistent with an additional aspect of the present disclosure; 
         FIG.  5   c    shows a cross-sectional view of photodiode and waveguide consistent with a further aspect of the present disclosure; 
         FIG.  6    shows an example of an energy band diagram consistent with an additional aspect of the present disclosure; 
         FIG.  7    is a plot of mode overlap ratio with a passive waveguide vs. absorber thickness consistent with an aspect of the present disclosure; 
         FIG.  8    shows plots of total harmonic distortion vs. bias voltage consistent with an additional aspect of the present disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Consistent with the present disclosure, a DDR photodiode is provided on a substrate adjacent to a passive waveguide. In order to efficiently capture light output from the waveguide, the photodiode is coupled to the waveguide with a butt-joint. As a result, the photodiode and the waveguide abut one another such that the dominant mode of light propagating in the waveguide parallel to the substrate is supplied directly to a side of the absorber layer of the photodiode without, in one example, evanescent coupling, nor is a resonant coupler required to supply light to the photodiode. Thus, light is absorbed more efficiently in the photodiode such that the photodiode may have a shorter length. In addition, since substantially all light is input to the photodiode, nearly complete absorption and nearly ideal quantum efficiency can be achieved in a relatively short length. Further, the improved linearity associated with DDR photodiodes is preserved with the exemplary butt joint configurations disclosed herein. 
     Reference will now be made in detail to the present exemplary embodiments of the disclosure, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG.  1    shows a high level circuit diagram of receiver  104  consistent with an aspect of the present disclosure. Receiver  104  includes a waveguide  194  that carries optical signals which may be amplitude modulated. The optical signals are supplied to photodiode  192 , which may be appropriately biased. In one example, the photodiode is reversed biased such that a positive reference or bias voltage +Vdd is supplied to cathode  192 - 2  and a negative reference or bias voltage −Vdd is supplied to anode  192 - 1 . 
       FIG.  2   a    shows coherent optical receiver  104   c , which is another example of an optical receiver consistent with the present disclosure. In one example, a polarization multiplexed optical signal is supplied to receiver  104 C. In that case, receiver  104  may include a polarization beam splitter (PBS)  202  operable to receive the input optical signal and to separate the signal into orthogonal polarizations, i.e., vector components of the optical E-field of the incoming optical signal transmitted on optical fiber medium  108 , into signals X and Y. One of the polarizations is parallel to the local oscillator (LO) polarization, and the other is rotated to be parallel to the LO ( 201 ). The LO output is split by an optical coupler ( 203 ). X and Y light are then each mixed with portions of the LO output in their own 90 degree optical hybrid circuits (“hybrid”)  204 - 1  and  204 - 2 . Hybrid  204 - 1  outputs four optical signals O 1   a , O 1   b , O 2   a , and O 2   b , and hybrid  204 - 2  outputs four optical signals O 3   a , O 3   b , O 4   a , and O 4   b , each representing the in-phase and quadrature components of the optical E-field of X and Y signals, and each including light from local oscillator  201  and light from polarization beam splitter  202  or mixing products. Optical signals O 1   a , O 1   b ; O 2   a , O 2   b ; O 3   a , O 3   b ; and O 4   a , O 4   b  are supplied to respective one of photodetector circuits  209 ,  211 ,  213 , and  215 . Each photodetector circuit includes a pair of photodiodes (“2PDs”) configured as single ended or else as balanced detector, as shown in the example  FIG.  2     b.    
       FIG.  2   b    shows photodetector circuit  209  in greater detail. It is understood that remaining photodetector circuits  211 ,  213 , and  215  have a similar construction and operate in a similar manner as photodetector circuit  209 . As shown in  FIG.  2   b   , optical waveguides  194 - 1  and  194 - 2  supply optical signals O 1   a  and O 1   b , respectively, from optical hybrid  204 - 1  to a corresponding one of photodiodes  292  and  294 . Photodiodes  292  and  294  are connected in a balanced detector configuration, and the output, E 1 , of photodetector circuit  209  is supplied to a transimpedance amplifier (TIA) circuit, whose output then supplies an analog-to-digital conversion (ADC) circuit for further processing. 
     Each of remaining photodetector circuits  211 ,  213 , and  215  generates a corresponding one of electrical signals E 2  to E 4  in a similar manner as that described above with respect to photodetector circuit  209 . Signals E 2  to E 4  are also supplied to respective TIA/ADC circuits. Electrical signals E 1  to E 4  are indicative of data carried by optical signals input to PBS  202 . 
       FIGS.  3   a  to  3   c    show plan views or layouts of examples of passive waveguide-photodiode combinations consistent with the present disclosure. As shown in  FIG.  3   a    an optical signal represented by optical mode  302  propagates in direction  304  in waveguide  194 . Waveguide  194  has a width transverse to direction  304  that increases in direction  304 . For example, at location L 1 , waveguide  194  has a width w 1 , and, at location L 2 , waveguide  194  has a width w 2  that is greater than width w 1 . Location L 1  is farther away from interface  306  than location L 2 . 
     As further shown in  FIG.  3   a   , interface  306  is present between optical waveguide  194  and photodiode  192 . In one example, interface  306  constitutes a butt joint, whereby photodiode  194  is configured to receive optical mode  302  directly from optical waveguide  194 . Moreover, in the example shown in  FIG.  3   a   , interface  306  is oriented or provided at a non-orthogonal angle α relative to direction  304 . Angle α may have a magnitude that is greater than or equal to 5° and less than or equal to 85°. 
     Optical mode or signal  304  next propagates into photodiode  192  and is absorbed along a length of photodiode  192 . In one example, a width of photodiode  192  narrows in a direction corresponding to propagation direction  304 , such that at location L 3  photodiode  192  has a width w 3 , which is greater than a width w 4  of photodiode  192  at location L 4 . Location L 3  is nearer interface  306  than location L 4 . 
     In another example, as shown in  FIG.  3   b   , the width w 5  of photodiode  192  is uniform along a length extending in a propagation direction  304 . However, the width of waveguide  194  increases in a direction corresponding to propagation direction  304 , as shown in  FIG.  3     a.    
     In a further example, as shown in  FIG.  3   c   , both waveguide  194  and photodiode  192  have the same width w 6 , and such width is uniform in propagation direction  304 . 
       FIG.  4    shows a perspective view of waveguide  194  and photodiode  192  corresponding to the configuration shown in  FIG.  3     a.    
       FIG.  5   a    shows a view of waveguide  194  and photodiode  192  taken along cross-section  5  (see  FIG.  3   a   ) adjacent interface  306 . It is understood that, in one example, waveguide  194  and photodiode  192  shown in  FIGS.  3   b  and  3   c    will have a similar construction as that shown  FIG.  5   a   , and, in further examples, waveguide  194  and photodiode  192  have the structures shown in  FIGS.  5   b    and  5   c.    
     As shown in  FIG.  5   a   , waveguide  194  is provided on region  594  of substrate  404 , and photodiode  192  is provided on region  592 . N-type cladding layer  512 , in one example, may be provided on substrate  402  that extends over both regions  594  and  592 . The n-type cladding layer includes, in a further example, indium phosphide (InP). An additional n-type or n-doped layer  511  may be provided on a first cladding layer  512 . Layer  511  may also extend over both region  594  and region  592 . Layer  511  may also include InP. 
     As further shown in  FIG.  5   a   , waveguide  194  provided over region  594  includes, in one example, a core layer  610 , which includes a quaternary semiconductor material, such as, indium gallium arsenide phosphide (InGaAsP) or other suitable quaternary semiconductor material, such as AlGaInAs. Layer  610  is typically undoped or else lightly doped (1E17) n-type. As used herein, undoped means unintentionally doped or a doping concentration that is less than or equal to 1×1016. Waveguide  194  may also include undoped or n− layer  508 , which is further provided on region  594 , and a layer  506  provided on layer  508 . Layer  508  may be undoped or lightly doped n−. Waveguide  194  provided over region  594  further includes a second cladding layer  504 , which, in one example, is doped p-type or implanted with H or He to be semi-insulating, and, in further example includes InP. 
     Photodiode  192 , as noted above, is formed over region  592  of substrate  404 . Photodiode  192  may be a DDR photodiode including an absorber layer. In the example shown in  FIG.  5   a   , photodiode  192  includes absorber layer  522 , which includes, for example, indium gallium arsenide (InGaAs). In a further example, absorber layer  522  is undoped or undoped. Photodiode  192  further includes cladding layer  520 , which may be p-type and includes InP. Cladding layer  520  is provided on absorber layer  522 . In addition, contact layer  518 , which is also p-type and includes InGaAs, may be provided on p-type cladding  520 . As further shown in  FIG.  5   a   , an additional contact layer  516  including a conductor or metal may be provided on contact layer  518 . 
     As noted above, the absorber layer, such as layer  522  of photodiode  192  in configured with undoped layer  508  below or has a thickness or electron/hole mobility combination such that photo-generated holes have a shorter distance to travel to the p-type anode ( 520 ) while photo-generated electrons have a longer distance to travel to the n-type cathode ( 511 ) or photodiode  192 . As further noted above, since the electron mobility is higher than the hole mobility for InP and related materials, the transit time of such holes and electrons is substantially the same. Accordingly, carrier lifetime in the photodiode  192  is reduced, and radio frequency (RF) bandwidth is increased. 
     Moreover, absorber layer  522  is provided in a manner to be aligned with and abuts core layer  610  of waveguide  192 , such that light is efficiently input to absorber layer  522  via interface  306  with minimal loss. 
       FIG.  5   b    shows an example similar to that shown in  FIG.  5   a   . In  FIG.  5   b   , however, optical core layer  510  extends over region  592  and constitutes part of photodiode  592 . In the example shown in  FIG.  5   b    an optical mode or optical signal  503  propagates in a direction indicated by arrow  514  in core  510 . In photodiode  592 , a tail portion  503 - 1  or mode  503  extends into absorber layer  522 , and this generates electron-hole pairs in a manner similar to that described above. In  FIG.  5   b   , mode  503  is evanescently coupled to absorber  522 . 
       FIG.  5   c    shows an example similar to that shown in  FIG.  5   b   . In  FIG.  5   c   , however, photodiode  192  includes an undoped layer  524  and an undoped or n-doped layer  526  provided between layer  508  and absorber layer  522 . Layer  524  is provided on quaternary layer  526 , in this example. In this example, layer  524  includes, for example, InP that is undoped and layer  526  includes n-type InP. 
     In the examples shown in  FIG.  5   c   , optical mode or optical signal  503  propagates in waveguide  194  in the direction indicated by arrow  514 . Layers  524  and  526  are configured to facilitate a quaternary mode transfer from core  510  to absorber  522  by way of resonate coupling. As a result, mode  503  generates electron-hole pairs in absorber layer  522  in a manner similar to that described above. 
     In each of the examples shown in  FIGS.  5   a - 5   c   , appropriate biases are applied to contact  516  and a contact, for example, to substrate  402  so that the above-described electron-hole pairs generate a photocurrent indicative of the optical signal  503 . 
       FIG.  6    shows an example of an energy band diagram  600  consistent with a further aspect of the present disclosure. As shown in  FIG.  6   , energy band diagram  600  includes regions  712 ,  711 ,  722 , and  720  corresponding to the energy bands present in layers  511 ,  508 ,  522  and  520  of  FIG.  5   a   . Energy band diagram  600  further includes “band smoothing” regions  602  and  604 , which, in one example, includes a layer or layers comprising a quaternary semiconductor alloy, such as InxGa1−xAsyP1−y, with a bandgap intermediate between that of the InGaAs absorber and InP that facilitates carrier transit. Region  602  may be provided between InP layer  508  and absorber layer  522 , and region  604  may be provided between absorber layer  522  and InP layer  520  in  FIG.  5   a   . In one example, the concentration of phosphorus in the quaternary alloy InxGa1−xAsyP1−y varies continuously along a thickness of region  602  and varies along a thickness of region  604 , e.g., in a direction away from the substrate (along the x-axis in  FIG.  6   ). Alternatively, the smoothing region  602  and  604  includes one or more layers, including a ternary semiconductor alloy or composition, and each layer within the band smoothing region may have a discrete composition and corresponding bandgap. Thus, band smoothing region  602  may include either a quaternary or ternary semiconductor alloy and band smoothing region  604  may include either a quaternary or ternary semiconductor alloy. 
     As further shown in  FIG.  6   , band smoothing region  604  is provided between region  720  (corresponding to layer  520 ) and region  722  (corresponding to absorber layer  522 ). A valence band edge Ev within region  604  has an associated first energy that is between a second energy associated with a valence band edge Ev within region  722  and a third energy associated with a valance band edge Ev within region  720 . In addition, band smoothing region  602  is provided between region  722  and region  711  (corresponding to layer  508 ). A conduction band edge Ec within region  602  has an associated fourth energy that is between a fifth energy associated with a conduction band edge Ec within region  722  and a sixth energy associated with a conduction band edge Ec within region  711 . 
     In each of the above example, the concentration of phosphorus in the band smoothing regions  602  and  604  may vary along a thickness of such regions, e.g., in a direction along the x-axis in  FIG.  6   . 
     In a further example, each of band smoothing regions  602  and  604  includes AlGaInAs. 
       FIG.  7    is a plot  700  of the overlap ratio of the optical mode in photodiode  192  to the optical mode in waveguide  194  vs absorber thickness for  FIG.  5   a   .  FIG.  7    shows an example in which an absorber thickness of 0.1 microns results in a maximum amount of overlap of the optical modes propagating in waveguide  192  and photodiode  194 , nearly 100%. 
       FIG.  8    illustrates plots  800  of total harmonic distortion (THD) for two DDR photodiodes consistent with aspects of the present disclosure. These plots were generated in connection with an example in which light output from two lasers was mixed to obtain a beating signal having a 1 GHz beating tone. The beating signal was received by a photodiode consistent with the present disclosure, such as photodiode  192 . The total harmonic distortion may be defined as the ratio between the total power of the harmonics (2 GHz, 3 GHz, etc.) over the RF power at 1 GHz. Optical signals incident from the two lasers provide photocurrents of 4.6 mA and 0.4 mA, respectively. The dashed curved in  FIG.  8    represents the THD as a function of bias voltage of a photodiode having band smoothing layers similar to those described above. The solid curve represent the THD as a function of bias voltage of a photodiode without smoothing layers. The dashed curve indicates that for a given bias voltage, the THD, and thus the linearity is greater for the photodiode having smoothing layers, such as a band smooth layer between the p-type core layer  520  and absorber layer  522 , and between  522  and  508  below, than that of the photodiode without such smoothing layers. The measurements depicted in  FIG.  8    were obtained from photodiodes with a 3 micron input width. 
     In a further example, a responsivity of 1.1. A/W was measured in connection with a 25 micron long photodiode. 
     Other embodiments will be apparent to those skilled in the art from consideration of the specification. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.