Patent Publication Number: US-2022231177-A1

Title: Photodiode with improved responsivity

Description:
TECHNICAL FIELD 
     This description relates to semiconductor opto-electronic devices. 
     BACKGROUND 
     A photodiode is a semiconductor device with a p-n junction that converts photons (or light) into electrical current. Photodiodes can be manufactured from a variety of materials including, but not limited to, Silicon, Germanium, and Indium Gallium Arsenide. Each material uses different properties for, for example, cost benefit, increased sensitivity, wavelength range, low noise levels, or even response speed. 
     SUMMARY 
     A device includes a depletion region formed between a p-doped region and a n-doped region in a semiconductor substrate. A light collection region disposed on a top surface of the semiconductor substrate above the depletion region, and an optical scattering element is disposed in the light collection region. The optical scattering element deflects a light photon incident on the light collection region to transit through the depletion region at a non-zero angle to a normal to the light collection region. 
     A method includes disposing an anode terminal and a cathode terminal on a photodiode for applying a bias voltage across the photodiode to maintain a depletion region in the photodiode, and disposing at least one optical scattering element in the photodiode. The at least one optical scattering element scatters light incident on the photodiode to transit the depletion region at a non-zero angle to a normal direction of the photodiode. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates, a cross sectional view, of an example photodiode. 
         FIG. 2A  illustrates an example photodiode that includes an arrangement optical scattering elements. 
         FIG. 2B  illustrates a portion of the photodiode of  FIG. 2A  with optical scattering elements made of polysilicon. 
         FIG. 2C  illustrates an enlarged view of a portion of the photodiode of  FIG. 2B . 
         FIGS. 3A and 3B  illustrate example device layouts of a photodiode including arrangements of optical scattering elements. 
         FIGS. 3C through 3E  illustrate example device layouts of a photodiode including arrangements of optical scattering elements shaped as lines of material. 
         FIG. 3F  illustrates an example device layout of a photodiode in which an optical scattering element is a hole in a dielectric layer. 
         FIG. 4  illustrates an example method for improving the spectral sensitivity or responsivity of a photodiode. 
     
    
    
     DETAILED DESCRIPTION 
     A photodetector (e.g., a photodiode) is a semiconductor device with a p-n junction that converts incident light (photons) into electrical current. The terms light and photons may be used interchangeably herein. The p-n junction is formed between a p-doped region and a n-doped region in a semiconductor substrate. A depletion region in which no free carriers exist is formed about the p-n junction by diffusion of electrons from the n-doped region to the p-doped region and the diffusion of holes from the p-doped region to the n-doped region. A built-in junction voltage creates an electric field across the depletion region allowing current to flow only in one direction (e.g., anode to cathode). across the p-n junction. Light or photons absorbed in the depletion region (or close to it) can create electron hole pairs that will move to opposite ends of the photodiode due to the electric field. Electrons, for example, will move toward a positive potential on a cathode of the photodetector, and the holes will move toward a negative potential on an anode of the photodetector. These moving charge carriers (i.e., electrons and holes) form the current (photocurrent) in the photodetector (e.g., photodiode). A figure of merit of photodetector performance is spectral sensitivity or responsivity R. In case of a photodiode, responsivity R measures the electrical output per optical input. The responsivity of a photodetector is usually expressed in units of either amperes or volts per watt of incident radiant power. Responsivity is a function of the wavelength of the incident radiation and of the photodetector properties, such as the bandgap of the material of which the photodetector is made as well as the externally applied bias. An expression for the responsivity R of a photodetector in which incident light is converted into an electric current (known as a photocurrent) is R=η Q h f=ηλ (μm) 1.23985 (μm×W/A), whereη is the a quantum efficiency (the conversion efficiency of a photon to an electron) of the photodetector for a given wavelength, is the electron charge, is the frequency of the incident light (optical signal), and is Planck&#39;s constant. R has units of amperes per watt (A/W). A responsivity curve of a photodetector may show A/W as a function of wavelength. 
     The quantum efficiency η of a photodetector is the probability that a single light photon incident on the device generates a photocarrier pair (i.e., electron hole pair) that contributes to the detector current. The quantum efficiency r l  depends on the distance in the depletion region traversed by the single photon. The spectral sensitivity or responsivity R of a photodetector depends on a width of the depletion region and a horizontal surface area (i.e., a horizontal size of photodiode) of a light-receiving region of the photodetector over which the light photons are incident as well as the depth of the depletion region below the semiconductor surface. 
     Photodetectors (e.g., photodiodes) are used in a variety of different circuits to convert incident optical signals into electrical current or voltage signals. A photodetector may, for example, be deployed in an opto-coupler to galvanically isolate low-voltage and high-voltage sides of a circuit by using a light emitter (e.g., a light emitting diode (LED)) to transmit a control signal (incident light signal) to the photodetector and reduce electrical noise coupling. In example implementations, a high voltage opto-coupler circuit may use, for example, infrared wavelengths in a range of about 800 to 900 nm for the control signal. A large area photodetector (photodiode) may be needed to generate a required photocurrent for control signals in the infrared range of about 800 to 900 nm, and for the circuit topology used. In some process technologies, a photodiode size may be on the order of 400 μm×400 μm or larger in size. The photodiode area may be 15% or higher of a total die area of the opto-coupler circuit. 
     The disclosure herein describes photodiode structures and techniques for improving or increasing the responsivity of photodiodes. Improving or increasing the responsivity of photodiodes may enable reduction in the size of the photodiodes needed for circuit applications (e.g., for opto-couplers and other types of circuits). 
     Optical scattering elements are integrated in light-receiving regions of the photodiode structures, in accordance with the principles of the present disclosure. The optical scattering elements can deflect (e.g., scatter, refract, or diffract) normally incident light photons to propagate at different scattering angles θ to the normal (e.g., non-zero forward scattering angles θ greater than 0° and up to90°) through the electron-hole pair-generating depletion region of the photodiode. Photons that propagate geometrically at the different angles θ have a longer path length through the electron-hole pair-generating depletion region of the photodiode, and can result in more electron-hole pairs being generated per photon than for the normally incident photons (in other words, the quantum efficiency of the photodetector is increased). 
       FIG. 1  schematically shows, in a cross sectional view, an example photodiode  100  with optical scattering elements  175  disposed in a light-receiving region of the photodiode, in accordance with the principles of the present disclosure. 
     Photodiode  100  may be fabricated on a semiconductor substrate  101  (e.g., a p-type silicon substrate). A n-doped region (e.g., n-doped region  110 ) may be formed in substrate  101 , for example, by thermal diffusion or ion implantation of a n-type dopant (e.g., phosphorus) in substrate  101 . Photodiode  100  may include a p-n junction  115  formed between a n-doped region  110  and a p-doped region  120  (e.g., a region of the bulk substrate). A depletion region  130  may form near p-n junction  115 . A front surface  11  and a back surface  12  of photodiode  100  may be connected to cathode and anode terminals of the photodiode (e.g., an anode terminal  154  and a cathode terminal  152 ). The terminals may be used, for example, to apply a voltage to reverse bias p-n junction  115 . Depletion region  130  may have a width or thickness L (e.g., in a vertical direction  14  along the y axis) perpendicular or normal to front surface  11  of the photodiode). Reverse biasing p-n junction  115  may, for example, increase the width or thickness L of depletion region  130 . 
     Front surface  11  of photodiode  100  may include a light-collection region (e.g., an active area  160 ) over which light photons  180  can be incident on photodiode  100 . Front surface  11  may include dielectric structures  150  (e.g., silicon dioxide) that may be used, for example, as diffusion masks when forming p-doped region  110  below active area  160  or forming other device elements. Further, active area  160  may be covered by a dielectric layer  170  (e.g., a passivating layer, an anti-reflection (AR) coating, etc.). Light photons  180  incident on active area may pass through dielectric layer  170  into depletion region  130 . Light photons  180  absorbed or transiting through depletion region  130  can generate electron-hole pairs for a photocurrent. The generated photocurrent may be proportional to the quantum efficiency of the photons (i.e., a number of electron hole pairs generated by a light photon as the light photon traverses the depletion region) in photodiode  100 . For a photon traversing depletion region  130  normally (e.g. perpendicular to surface  11 ) the quantum efficiency of the photons may be proportional to the width of thickness L of the depletion region. In  FIG. 1 , a photon traversing the depletion region  130  normally is depicted by arrow  181 . 
     In accordance with the principles of the present disclosure, optical scattering element  175  are disposed in active area  160 , for example, in the path of normally incident light (incident light photons  180 ). Optical scattering elements  175  may, for example, be disposed on, or included in, layer  170  covering active area  160 . Optical scattering elements  175  may be opaque (metallic) or semi-transparent structures (e.g., oxides, nitrides, etc.) that can scatter or diffract normally incident light photons  180  at a scattering angle (e.g., an angle θ greater than 0° and up to 90°) to the normal. Optical scattering elements  175  may be additive structures (e.g., particles) or subtractive structures (e.g., holes) formed in the metallic or non-metallic layers. The metallic or non-metallic layers may be formed of opaque or semi-transparent materials. Example non-metallic materials may include dielectric material (such as silicon oxide or silicon nitride) or semiconductor materials. Optical scattering elements  175  may have dimensions of a few to several hundreds of nanometers (e.g., 100 nm to 7000 nm). In example implementations, optical scattering elements  175  may have dimensions that are larger than the wavelength of incident light photons  180 . 
     In example implementations, for example, for high voltage opto-coupler applications, light photons  180  that are incident on active area  160  of photodiode  100  may, for example, be in an infrared range of wavelengths (e.g., 800 nm−900 nm). 
       FIG. 1  shows, for example, a normally incident photon (e.g., depicted by an arrow  181 ) transiting through depletion region  130  may have a transit path length that corresponds to the width or thickness L of depletion region  130 . Further, incident light photons  180  (depicted by arrows  182  and  183 ) that may be scattered by optical scattering elements  175  (as depicted by arrows  182  and  183 ) to propagate at an angle (e.g., angle θ 2  or θ 3 ) to the normal through depletion region  130 . Incident light photons  180  that propagate at an angle (e.g., angles θ 2  and θ 3 ) to the normal through depletion region  130  have a longer (i.e., increased) transit path in the depletion region compared to the transit path length L of the normally incident photon (e.g., arrow  181 ). The increase in the transit path length for a photon scattered at angle may be geometrically proportional to the inverse cosine function cos −1  (θ) (which always has a numerical value greater than 1). The increase in the transit path length may be visualized, for example, by visual comparison of the length of arrow  181  and the lengths of arrows  182  or  183  shown in  FIG. 1 . 
     The increase in the length of transit path of scattered photons through the depletion region can increase the quantum efficiency of incident photons and the spectral sensitivity or responsivity R of photodiode  100 . 
     In example implementations, optical scattering elements  175  may be formed by patterning a metal layer or a dielectric layer placed over the photodiode active area during device fabrication. While optical scattering elements  175  may scatter light, optical scattering elements  175  may not have any electrical effect or function in photodiode  100 . 
     In example implementations, an arrangement or pattern of optical scattering elements  175  may be disposed in an light-collection region (active region) of a photodiode. In some implementations, the photodiode (like photodiode  100  shown in  FIG. 1 ) may have a p-n junction formed by diffusing a n-region in an p-type substrate. In other implementations, the photodiode may have a p-n junction formed by diffusing a p-region in a n-type substrate. 
       FIG. 2A  shows a cross-sectional view of an example photodiode  200  that includes an arrangement  280  of optical scattering elements (e.g., optical scattering elements  285 ) for intercepting and scattering light photons incident on its light collection region (e.g., active area  260 ), in accordance with the principles of the present disclosure. 
     As seen in  FIG. 2A , example photodiode  200  (having a width WX in the x-direction) may be formed in a p-type semiconductor substrate  201 . Photodiode  200  includes a p-n junction  215  formed in p-type semiconductor substrate  201  between a p-type region  210  and a n-type region  220  (e.g., a diffused n-type region). A top surface  21  of substrate  201  may be covered by one or more passivating, isolating, and antireflection (AR) coating dielectric layers. The passivating, isolating, or antireflection (AR) coating dielectric layers may, for example, include one or more of silicon dioxide, silicon nitride, silicon oxy-nitride (SiON), polyimide, spin on glass, and fluoridated silicon dioxide layers. 
     Metal contacts (e.g., anode contact  254 , and cathode contact  252 ) to the device may be disposed on top surface  21  of substrate  201 . A dielectric layer  272  (e.g., a polysilicon layer and or a silicon nitride layer) may be disposed on top surface  21  of substrate  201 . An etch stop layer  274  (e.g., a SiON layer) may be disposed over dielectric layer  272  on top surface  21  of substrate  201 . Further, a dielectric layer  270  (e.g., an oxide layer) may overlay photodiode  200  (enclosing or covering anode contact  254 , cathode contact  252 , dielectric layer  272 , and etch stop layer  274  that may be disposed on top surface  21  of substrate  201 ). 
     An arrangement  280  of optical scattering elements  285  for scattering light photons (e.g., incident light photons  180 ) incident on light collection region (e.g., active area  260 ) may be disposed on surface  21 , on dielectric layer  270 , or included in dielectric layer  270 . 
     In some example implementations, optical scattering elements  285  may be metal or metallic objects embedded in dielectric layer  270  (e.g., an oxide layer). The metal or metallic objects may, for example, be made of copper, aluminum, tungsten, and a metal alloy, etc. In example implementations, as shown in  FIG. 2A , an optical scattering element  285  may be a rectangular metal segment with a height wy, a width wx (and a depth wz (not shown) in a z direction perpendicular to the page of the figure). In example implementations, height wy may, for example, be in a range of 200 nm to 1000 nm, and width wx and depth wz may, for example, each be in a range of 200 nm to 3000 nm. 
     In some example implementations, optical scattering elements  285  may be made of dielectric material (e.g., poly silicon, silicon nitride, etc.). Optical scattering elements  285  may, for example, be polysilicon segments that are patterned and embedded, for example, in dielectric layer  272  disposed on surface  21  of the substrate.  FIG. 2B  shows a portion of photodiode  200  to illustrate optical scattering elements (e.g., optical scattering elements  285 ) made of polysilicon, and  FIG. 2C  shows an exploded view of portion AA of photodiode  200  shown in  FIG. 2B  to further illustrate an individual polysilicon segment used as optical scattering element  285 . 
     In example implementations, as shown in  FIG. 2B  and  FIG. 2C , an optical scattering element  285  may be a rectangular polysilicon segment with a height wy, a width wx (and a depth wz (not shown) in a z direction perpendicular to the page of the figure). In example implementations, height wy may, for example, be in a range of 100 nm to 1000 nm, and each of width wx and depth wz may, for example, be in a range of 200 nm to 3000 nm. 
     Optical scattering elements  285  may be disposed in arrangement  280  in photodiode  200  ( FIG. 2A, 2B and 2C ), for example, with a same inter-element spacing s between each neighboring pair of the elements. In other words, optical scattering elements  285  may be distributed evenly (i.e., evenly spaced or regularly spaced in a periodic pattern) in arrangement  280  (as shown in  FIG. 2A ). In other example implementations, optical scattering elements  285  may be distributed unevenly (e.g., with varying or staggered inter-element spacings s) in arrangement  280 . 
       FIGS. 3A  and  FIG. 3B  show example device layouts  200 L of photodiode  200  (e.g., photodiode  200  with optical scattering elements  285  made of metal as shown in  FIG. 2A , or photodiode  200  with optical scattering elements  285  made of metal as shown in  FIG. 2B ). 
     As shown in  FIGS. 3A and 3B , photodiode  200 , which may be formed on substrate  201 , may have a rectangular (or square) shape with sides having a dimension WX in the x direction and a dimension WZ in the z-direction. 
     Further, anode contact  254  and cathode contact  252  may be disposed as concentric rectangles R 1  and R 2  on substrate  201  in device layout  200 L. The inner concentric rectangle R 2  (cathode contact  252 ) may enclose or define a light collection region (active area  260 ) of photodiode  200 . Further, arrangement  280  of optical scattering elements  285  for scattering light incident on the active area  260  may be placed within the inner concentric rectangle R 2  of cathode contact  252 . 
     In the examples shown in  FIG. 3A  and  FIG. 3B , arrangement  280  may include optical scattering elements  285  arranged in a rectangular (or square) geometrical pattern in which the optical scattering elements  285  are placed on a rectangular or square x-z lattice. As shown, for example, in  FIG. 3A , the optical scattering elements  285  may be placed evenly in rows R and columns C of the lattice with a same inter-element spacing s in both the x direction and in the z direction. 
     As shown, for example, in  FIG. 3B , optical scattering elements  285  placed in a row (e.g., row RA) on the rectangular or square x-z lattice may be offset in the x direction (e.g., by a distance s/2) relative to the positions of optical scattering elements  285  placed in an alternate row (e.g., row RB). 
     In other example implementations, arrangement  280  may include other shapes and geometrical patterns of the optical scattering elements. 
       FIGS. 3D through 3F  show example device layouts  200 L of photodiode  200  with different shapes and geometrical patterns of the optical scattering elements in arrangement  280  disposed on top surface  21  of substrate  201 . 
     In example implementations, the optical scattering elements may be shaped from lines of material (e.g., additive metal or dielectric material objects, or subtractive holes in dielectric layers)) that are further shaped to form outlines of other shapes (e.g., rectangles, squares, circles, ovals, etc.). In some instances, the lines of material may include additive metal or dielectric material objects. In some instances, the lines of material may be subtractive material objects (e.g., holes in a dielectric layer). 
     In example implementations, the lines of material may be disposed on, or embedded in, dielectric layers (e.g., dielectric layers  270  or  272 ) disposed on top surface  21  of substrate  201 . 
       FIG. 3C  shows an example arrangement  280  including optical scattering elements shaped as lines of material (e.g., optical scattering elements  285 - 1 ,  285 - 2 ,  285 - 3 , etc.). In the example shown in  FIG. 3C , the lines of material (e.g., optical scattering elements  285 - 1 ,  285 - 2 ,  285 - 3 , etc.) may be straight lines extending (e.g., in the x-direction) across inner concentric rectangle R 2  (cathode contact  252 ) that encloses or defines the light collection region (active area  260 ) of photodiode  200 . The straight lines of material (e.g., optical scattering elements  285 - 1 ,  285 - 2 ,  285 - 3 , etc.) may be disposed in a repeating pattern in arrangement  280  with an inter-line spacing GS in a vertical direction (e.g., in the z direction). 
       FIG. 3D , like  FIG. 3C , shows an example arrangement  280  including optical scattering elements shaped as lines of material (e.g., optical scattering elements  285 -r 1 ,  285 -r 2 ,  285 -r 3 , etc.). In the example shown in  FIG. 3D , the lines of material may form rectangles that may be disposed as a pattern of concentric rectangles (with an inter-rectangle spacing GS) within the inner concentric rectangle R 2  (cathode contact  252 ) that encloses or defines the light collection region (active area  260 ) of photodiode  200 . 
       FIG. 3E , like  FIG. 3C , shows an example arrangement  280  including optical scattering elements shaped as lines of material (e.g., optical scattering elements  285 -c 1 ,  285 -c 2 , etc.). In the example shown in  FIG. 3E , the lines of material may form circles that may be disposed as a pattern of concentric circles (with an inter-circle spacing GS) within the inner concentric rectangle R 2  (cathode contact  252 ) that encloses or defines the light collection region (active area  260 ) of photodiode  200 . 
     In example implementations, the optical scattering elements may be shaped from lines of material (e.g., additive metal or dielectric material objects, or subtractive holes in dielectric layers)) that are further shaped to form outlines of other shapes (e.g., rectangles, squares, circles, ovals, etc.). 
     In an example implementation, an optical scattering element may be a subtractive material object (e.g., a hole in a dielectric layer). 
       FIG. 3F  shows an example arrangement  280  in which an optical scattering element (e.g., optical scattering element  285 -h 1 ) is a hole in a dielectric layer (e.g., polysilicon layer  285   ps ). The hole may have any shape (e.g., rectangle, square, circle, oval, etc.). The dielectric layer may include materials (e.g., polysilicon, silicon nitride, etc.) which may be transparent, or at least partially transparent, to light, for example, at infrared wavelengths. The dielectric layer (e.g., polysilicon layer  285   ps ) may be disposed on top surface  21  of substrate  201 . The hole (e.g., optical scattering element  285 -h 1 ) may have any shape (e.g., rectangle, square, circle, oval, etc.). 
     In the example shown in  FIG. 3G , arrangement  280  may include optical scattering elements  285 -h 1  (holes) arranged in a rectangular or square geometrical pattern (like the optical scattering elements  285  in  FIG. 3A ). Optical scattering elements  285 -h 1  may be placed in rows and columns on a rectangular or square x-z lattice. The optical scattering elements  285 -h 1  may be placed evenly in rows R and columns C of the lattice with a same inter-element spacing s in both the x direction and in the z direction. 
       FIG. 4  illustrates an example method  400  for improving the spectral sensitivity or responsivity of a photodiode. The photodiode may be a semiconductor device that includes a light collection region (i.e., an active area) on which light photons can be incident for conversion into a photocurrent. The light photons transiting through a depletion region in the photodiode can generate electron-hole pairs for the photocurrent. 
     Method  400  includes disposing an anode and a cathode for applying a bias voltage to maintain a depletion region in the diode ( 410 ); and disposing at least one optical scattering element in the diode, the at least one optical scattering element scattering light incident on the diode in a normal direction to transit the depletion region at an angle to the normal direction. 
     It will be understood that, in the foregoing description, when an element, such as a layer, a region, a substrate, or component is referred to as being on, connected to, electrically connected to, coupled to, or electrically coupled to another element, it may be directly on, connected or coupled to the other element, or one or more intervening elements may be present. In contrast, when an element is referred to as being directly on, directly connected to or directly coupled to another element or layer, there are no intervening elements or layers present. Although the terms directly on, directly connected to, or directly coupled to may not be used throughout the detailed description, elements that are shown as being directly on, directly connected or directly coupled can be referred to as such. The claims of the application, if any, may be amended to recite exemplary relationships described in the specification or shown in the figures. 
     As used in the specification and claims, a singular form may, unless definitely indicating a particular case in terms of the context, include a plural form. Spatially relative terms (e.g., over, above, upper, under, beneath, below, lower, and so forth) are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. In some implementations, the relative terms above and below can, respectively, include vertically above and vertically below. In some implementations, the term adjacent can include laterally adjacent to or horizontally adjacent to. 
     Some implementations may be implemented using various semiconductor processing and/or packaging techniques. Some implementations may be implemented using various types of semiconductor processing techniques associated with semiconductor substrates including, but not limited to, for example, Silicon (Si), Silicon Germaniums (SiGe), Germanium (Ge), Gallium Arsenide (GaAs), Gallium Nitride (GaN), Silicon Carbide (SiC) and/or so forth. 
     While certain features of the described implementations have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the scope of the implementations. It should be understood that they have been presented by way of example only, not limitation, and various changes in form and details may be made. Any portion of the apparatus and/or methods described herein may be combined in any combination, except mutually exclusive combinations. The  7 implementations described herein can include various combinations and/or sub-combinations of the functions, components and/or features of the different implementations described. 
     For example, a photodiode fabricated on a n-type substate will have opposite types of doped regions than a photodiode fabricated on a p-type substrate (in other words, n-type and p-type regions in the photodiode fabricated on the n-type substate may respectively correspond to p-type and n-type regions in the photodiode fabricated on the p-type substrate). While incorporation of optical scattering elements in the light collection regions of photodiodes has been described herein using, for example, photodiodes fabricated on p-type substrates (e.g., p-type substate  101 ,  FIG. 1 , and p-type substrate  210 ,  FIGS. 2A through 3F ), the disclosed principles of incorporating optical scattering elements in the light collection region to scatter incident light through the photodiode&#39;s depletion region are also applicable to photodiodes fabricated on n-type substrates.