News Column

Patent Issued for Semiconductor Light Receiving Element and Optical Communication Device

February 12, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventors Okamoto, Daisuke (Tokyo, JP); Fujikata, Junichi (Tokyo, JP); Nishi, Kenichi (Tokyo, JP), filed on January 9, 2009, was published online on January 28, 2014.

The patent's assignee for patent number 8637951 is NEC Corporation (Tokyo, JP).

News editors obtained the following quote from the background information supplied by the inventors: "In the fields of optical communication, optical information, and optical measurement, semiconductor light receiving elements are often used as photoelectric conversion elements. A principal factor that limits response speed of photoelectrical conversion in the semiconductor light receiving element is the time constant of a circuit, determined by the product of load resistance and electrical capacitance created by a depletion layer, and carrier travel time required for a carrier to pass through the depletion layer, and in order to improve high speed responsiveness of the semiconductor light receiving element, it is desired to decrease the time constant of the circuit and the carrier travel time.

"As a means of decreasing the time constant of the circuit, for example, a means can be noted of reducing light receiving area and decreasing element capacitance. In a semiconductor light receiving element described in Patent Document 1, for the semiconductor light receiving element, which has a structure in which an i-type semiconductor light absorbing layer and a p-type semiconductor layer are stacked in this order on an n-type semiconductor substrate layer, by forming a plurality of opening regions reaching from the p-type semiconductor layer to the i-type semiconductor absorbing layer, effective pn junction area is reduced without reducing light receiving area, and capacitance reduction is aimed at.

"Furthermore, in recent years, there have been many kinds of attempts in order to increase speed and realize high efficiency in the semiconductor light receiving element, using a metal surface plasmon or photonic crystal structure, as compared with the conventional situation (for example, refer to Patent Documents 2 to 4).

"In a description in Patent Document 2, a Schottky photodiode that uses surface plasmons includes: a conductive film having a hole of a diameter smaller than the wavelength of incident light, and a periodic structure that causes a resonance state on a film surface by the surface plasmon that is excited, by the incident light on the film surface of the conductive film provided around the hole; and a semiconductor layer arranged to be in contact with the conductive film in the vicinity of the hole of the conductive film. The light incident on the periodic structure undergoes photoelectric conversion, by exciting surface plasmons, causing the resonance state with the surface plasmons, and generating near-field light at an interface between the conductive film and the semiconductor layer.

"A surface plasmon strengthening photovoltaic device described in Patent Document 3 has a first metallic electrode, having a surface on which incident light is illuminated and a surface which is not illuminated, at least one of the illuminated surface and the non-illuminated surface is provided with apertures having a diameter not more than a wavelength of the incident light, forming a periodic surface topography, where an array of apertures has a strengthening characteristic causing resonance interaction between the incident light and surface plasmons on the surface; a second electrode disposed at a distance from the first metallic electrode; and a plurality of spheres disposed between the first metallic electrode and the second electrode corresponding to the array of apertures; wherein each sphere has a p-doped material first portion and an n-doped material second portion so that a p-n junction is formed at a junction between the first and the second portions, and each of the respective spheres is disposed within the apertures so that one of the first or the second portion is in electrical contact with the first metallic electrode, and the other of the first or the second portion is in electrical contact with the second electrode. The incident light resonates with the surface plasmons on the surface topography.

"Furthermore, in order to increase light sensitivity, a light receiving element described in Patent Document 4 includes a semiconductor substrate, an n-type semiconductor layer disposed above the semiconductor substrate, an i-type semiconductor light absorbing layer disposed above an n-type contact layer and which absorbs light of a given wavelength band, and a p-type semiconductor layer disposed above the i-type semiconductor light absorbing layer, wherein the p-type semiconductor layer has periodically arrayed channels, holes, or columnar protrusions, includes a one dimensional or two dimensional photonic crystal part having a periodic refractive index distribution in a face intersecting the direction of incidence of light, and the photonic crystal part is formed so that at least a part of the incident light is even-number order diffracted light, and is distributed in a face of the light absorbing layer to be propagated.

"[Patent Document 1]

"JP Patent Kokai Publication No. JP-P2007-165359A

"[Patent Document 2]

"WO Pamphlet No. 2005/098966

"[Patent Document 3]

"JP Patent No. 3726887

"[Patent Document 4]

"JP Patent Kokai Publication No. JP-P2005-159002A"

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "The entire disclosures of Patent Documents 1 to 4 are incorporated herein by reference thereto. The following analysis is given from a viewpoint of the present invention.

"In a semiconductor light receiving element, if a light receiving area is decreased and element capacitance is lowered, a time constant of a circuit decreases and it is possible to ensure high speed operation. However, in this case, since the light receiving area becomes small, a problem arises in that optical coupling with optic fiber or the like at high efficiency becomes difficult.

"Accordingly, in the semiconductor light receiving element described in Patent Document 1, by forming plural opening regions passing from a p-type semiconductor layer through an i-type semiconductor light absorbing layer, the element capacitance is reduced without reducing the light receiving area. However, there is a trade-off problem in that, in order to get higher speed operation, if carrier travel time is decreased by making the semiconductor light absorbing layer thin, quantum efficiency decreases.

"Devices described in Patent Document 2 and Patent Document 3 have a structure in which light is incident from a surface metal layer (electrode), but in this structure the proportion of the light reflected by the electrode before reaching the absorbing layer is high, and high photoelectric efficiency cannot be obtained. Furthermore, in manufacturing the device described in Patent Document 3, it is difficult to grow high grade crystal on the semiconductor substrate. As a result, it is difficult to apply the device described in Patent Document 3 to usages requiring high speed operation of optical communication or the like.

"In a light receiving element described in Patent Document 4, since channels forming a photonic crystal part are only formed up to a midway portion of the p-type semiconductor layer, it is not possible to obtain an adequate capacitance reduction effect.

"It is an object of the present invention to provide a semiconductor light receiving element with large light receiving area, high speed, and high efficiency, and an optical communication device having the semiconductor light receiving element.

"According to a first aspect of the present invention there is provided a semiconductor light receiving element comprising: a substrate; a semiconductor layer of a first conductivity type formed on the substrate; a non-doped semiconductor light absorbing layer formed on the semiconductor layer of the first conductivity type; a semiconductor layer of a second conductivity type formed on the non-doped semiconductor light absorbing layer; and an electro-conductive layer (may be formed herein as 'conductive layer') formed on the semiconductor layer of the second conductivity type. A plurality of openings, periodically arrayed, are formed in a laminated body composed of the electro-conductive layer, the semiconductor layer of the second conductivity type, and the non-doped semiconductor light absorbing layer. The widths of the openings are less than or equal to incident light wavelength. The openings pass through the electro-conductive layer and the semiconductor layer of the second conductivity type, and reach as far as the non-doped semiconductor light absorbing layer.

"According to a preferable mode of the abovementioned first aspect, the plural openings are periodically arrayed such that surface plasmon resonance is generated at an interface of the electro-conductive layer and the semiconductor layer of the second conductivity type.

"According to a preferable mode of the abovementioned first aspect, when the depth of the openings is d, the thickness of the electro-conductive layer is t.sub.1, the thickness of the semiconductor layer of the second conductivity type is t.sub.2, and the thickness of the non-doped semiconductor light absorbing layer is t.sub.3, the depth d of the openings satisfies a condition: t.sub.1+t.sub.2.ltoreq.d.ltoreq.t.sub.1+t.sub.2+t.sub.3/2

"According to a preferable mode of the abovementioned first aspect, the semiconductor light receiving element is further provided with a first electrode, formed on a semiconductor layer of the first conductivity type, and also electrically connected to the semiconductor layer of the first conductivity type. The electro-conductive layer is electrically connected to the semiconductor layer of the second conductivity type, and functions as a second electrode.

"According to a preferable mode of the abovementioned first aspect, the semiconductor light receiving element is further provided with a first electrode, formed on a semiconductor layer of the first conductivity type, and also electrically connected to the semiconductor layer of the first conductivity type, and a second electrode, formed on a semiconductor of the second conductivity type, and also electrically connected to the semiconductor layer of the second conductivity type. The electro-conductive layer is formed on a semiconductor layer of the second conductivity type via an insulating layer, in a region where the second electrode is not formed, and the electro-conductive layer and the second electrode are not electrically connected.

"According to a preferable mode of the abovementioned first aspect, at least a part of a face of the substrate, on a side opposite a face on which the semiconductor layer of the first conductivity type is formed, is formed in a lens shape.

"According to a preferable mode of the abovementioned first aspect, the plurality of openings are periodically arrayed in a triangular grid form or a square grid form, and the surface shape of the openings is a circular form, an elliptical form, or a polygonal form.

"According to a preferable mode of the abovementioned first aspect, the proportion of total area of the plurality of openings with regard to area (including area of the openings) of an upper face of the electro-conductive layer, is 50% or more, and 80% or less.

"According to a preferable mode of the abovementioned first aspect, when the wavelength of incident light is .lamda., the refractive index of the semiconductor layer of the second conductivity type is n, and the period of the openings, which is the distance between centers of openings that are adjacent, is period P, nP/.lamda. satisfies either of the conditions of 0.9
"According to a preferable mode of the abovementioned first aspect, the semiconductor light receiving element is further provided with a Bragg reflection multi-layer film in a layer below the semiconductor layer of the first conductivity type.

"According to a preferable mode of the abovementioned first aspect, the electro-conductive layer includes at least one metallic material (selected) from among Al, Ag, Au, and Cu.

"According to a preferable mode of the abovementioned first aspect, the non-doped semiconductor light absorbing layer is formed from at least one selected from the group of Si, Si.sub.x Ge.sub.1-x (note that x is a positive number less than 1), Ge, GaN, GaAs, GaInAs, GaInP, and InP.

"According to a preferable mode of the abovementioned first aspect, the semiconductor light receiving element is further provided with an antireflective film or a protective film on the electro-conductive layer.

"According to a second aspect of the present invention, an optical communication device having a semiconductor light receiving element is provided. The semiconductor light receiving element is provided with a substrate; a semiconductor layer of a first conductivity type formed on the substrate; a non-doped semiconductor light absorbing layer formed on the semiconductor layer of the first conductivity type; a semiconductor layer of a second conductivity type formed on the non-doped semiconductor light absorbing layer; and an electro-conductive layer formed on the semiconductor layer of the second conductivity type. A plurality of openings, periodically arrayed, are formed in a laminated body composed of the electro-conductive layer, the semiconductor layer of the second conductivity type, and the non-doped semiconductor light absorbing layer. The widths of the openings are less than or equal to the wavelength of incident light. The openings pass through the electro-conductive layer and the semiconductor layer of the second conductivity type, and reach as far as the non-doped semiconductor light absorbing layer.

"According to a preferable mode of the abovementioned second aspect, the semiconductor light receiving element is disposed so that light is incident from the substrate side.

"The present invention possesses at least one among the following effects.

"In the present invention, light that is incident on a conductive layer (or a second electrode) having periodic openings excites surface plasmons at a semiconductor interface with the conductive layer, and forms a resonance state. As a result, a strong electrical field is generated inside a non-doped semiconductor light absorbing layer, and absorption is effectively performed with regard to a thin non-doped semiconductor light absorbing layer. In this way, it is possible to get high quantum efficiency even for a case where the light absorbing layer is thin. Furthermore, since plural openings are provided, it is possible to decrease element capacitance per unit area. In this way, it is possible to realize higher speeds. In addition, since it is possible to maintain light receiving area even when element capacitance is lowered, it is possible to facilitate alignment with optic fiber and the like. Therefore, according to the present invention, a semiconductor light receiving element and an optical communication device having high efficiency, high speed, and high applicability are realized.

"Furthermore, according to the present invention, since the semiconductor light receiving element that has high speed and high efficiency can be realized using Ge having good compatibility with a Si CMOS process, high integration and mass production are facilitated and it is possible to reduce manufacturing cost."

For additional information on this patent, see: Okamoto, Daisuke; Fujikata, Junichi; Nishi, Kenichi. Semiconductor Light Receiving Element and Optical Communication Device. U.S. Patent Number 8637951, filed January 9, 2009, and published online on January 28, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=55&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=2719&f=G&l=50&co1=AND&d=PTXT&s1=20140128.PD.&OS=ISD/20140128&RS=ISD/20140128

Keywords for this news article include: Electronics, Semiconductor, NEC Corporation.

Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC


For more stories covering the world of technology, please see HispanicBusiness' Tech Channel



Source: Electronics Newsweekly


Story Tools