JP2013191704A - Light receiving element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light receiving element that can be produced by epitaxially growing an absorption layer, a clad layer, and a waveguide layer and the like on a compound semiconductor (e.g., GaAs) substrate, and mainly performing etching processing without depending on bonding and adhesion processes.SOLUTION: A light receiving element has a light receiving part consisting of a semiconductor multilayer structure having a circular-shaped outer periphery. The semiconductor multilayer structure comprises: an active layer 12 consisting of a multiquantum well structure; a light confinement layer 13 provided on both sides of the active layer 12; a first clad layer and a second clad layer provided on both sides of the light confinement layer 13; a first contact layer 15a and a second contact layer 15b provided so as to be connected to the first clad layer and the second clad layer; a lower clad layer connected to the first contact layer 15a and connected to a substrate; and a positive electrode D1 and a negative electrode D2 for applying a bias to the light receiving part. A part of a side wall of the clad layer consists of a low refractive index material 20c.

Description

  The present invention relates to a light receiving element.

  In recent years, internal circuit patterns have been miniaturized as LSI integration density has increased. With this miniaturization, the wiring resistance increases due to the reduction of the wiring cross-sectional area, and the capacitance between the wirings increases due to the narrower spacing between adjacent wirings. As a result, the wiring delay determined by the wiring resistance and the wiring capacitance increases, making it difficult to increase the speed of the LSI. In addition, as multi-core integration and three-dimensional memory integration in LSIs progress, large-capacity transmission between cores and between cores / memory is indispensable, and the transmission speed by electricity is a bottleneck for improving LSI performance. It has become. As a technique for solving such a wiring delay problem associated with high density LSI, an optical wiring technique that replaces an electrical signal with an optical signal has attracted attention.

  Optical wiring technology is a method of transmitting signals using an optical waveguide instead of metal wiring, and does not increase the wiring resistance and inter-wiring capacitance due to the above-mentioned miniaturization, further increasing the operation speed Can be expected. Regarding the semiconductor laser (LD) used as a light source in the optical wiring technology, the element size used in the conventional optical communication is several μm in width and 100 μm in length, which is huge compared to the LSI transistor and wiring pitch. is there. This is a major obstacle to replacing electrical wiring with optical wiring. For this reason, a microring LD using a microring resonator as a small light source has begun to attract attention, and its research and development has become active. To realize optical wiring on an LSI chip, a light source element (transmission unit) and an optical waveguide (transmission unit) as well as a light receiving element (reception unit) are integrated compactly on the same chip together with a drive circuit and an amplifier circuit. Therefore, it is necessary to form an optical transceiver unit.

  Light receiving elements (PD) suitable for such applications include PIN type PDs, MSM type PDs, avalanche type PDs, etc., but when integrated with a microring LD, the same stack is used from the viewpoint of cost reduction. It is strongly desired to be a light receiving element that can be simultaneously fabricated and integrated by processing a wafer having a structure by the same process.

  However, in the wafer having the same laminated structure as the LD as described above, the thickness of the absorption layer is as thin as several tens of nanometers, so that sufficient light absorption efficiency and sufficient light-electric conversion efficiency (or the corresponding conversion efficiency) (or It is very difficult to obtain a photocurrent.

  That is, in order to secure a sufficient light absorption efficiency with an element size of about 10 microns, the thickness of the absorption layer of about several hundreds of nm is usually required. Therefore, when the absorption layer has a thickness of several tens of nm, 100 An absorption length of micron or more is required, and the element size increases, so that integration on an LSI chip becomes difficult. A solution to this problem is to reduce the element size and increase efficiency by connecting a waveguide to the ring-shaped light receiving section and circulating the input light around the ring to increase the effective absorption length. Is expected.

  In fact, there is an example in which a ring-shaped light receiving element based on such a principle is proposed (Patent Document 1). However, such a ring structure is miniaturized and light is efficiently coupled, and the inside of the ring is circulated with low loss so that the light is efficiently absorbed by the active medium and converted into carriers (electron-hole pairs). In addition, it is not easy to convert carriers into current without loss due to recombination. In particular, in a minute ring structure having a diameter of about 10 μm, light loss due to scattering and carrier loss due to surface recombination occur due to processing damage on the ring side wall, which causes further reduction in the photoelectric conversion efficiency. In addition, radiation loss due to light interference is likely to occur at the connection portion between the ring-shaped light receiving portion and the waveguide or inside the ring, and the effect becomes more significant as the size is reduced, making it more difficult to increase efficiency.

  As described above, the realization of a connection structure with a low-loss and fine ring-shaped light-receiving part and waveguide that does not deteriorate the light-receiving characteristics is an extremely important issue in realizing optical wiring on an LSI chip. Become. As a manufacturing method, a method of bonding or bonding a compound semiconductor light absorption layer on a silicon waveguide on a silicon (SOI) substrate is suitable for fine optical wiring in a chip. However, if it can be manufactured without using a bonding or bonding process that is not technically easy to manufacture, the cost can be further reduced. For inter-chip optical wiring that does not require wiring with fine optical waveguides, rather, such a manufacturing method is more suitable, and the realization of the structure of the light receiving element and its integration technology that make this possible is one of the important issues. It has become.

US Pat. No. 6,978,067

  An absorption layer, a cladding layer, a waveguide layer, and the like are epitaxially grown on a compound semiconductor (for example, GaAs) substrate, and a light receiving element that can be manufactured mainly by etching processing regardless of a bonding or adhesion process is provided.

  In order to solve the above-described problems and achieve the object, the light receiving element according to the embodiment has a light receiving portion made of a semiconductor multilayer structure having a circular outer periphery, and the semiconductor multilayer structure is an active having a multiple quantum well structure. A layer, a light confinement layer provided on both sides of the active layer, a first clad layer and a second clad layer provided on both sides of the light confinement layer, and a shape connected to the first clad layer and the second clad layer The first contact layer and the second contact layer provided in the above, a lower cladding layer connected to the first contact layer and connected to the substrate, a positive electrode and a negative electrode for applying a bias to the light receiving portion, A part of the side wall of the cladding layer is made of a low refractive index material.

  According to the present invention, the light loss and the carrier loss at the light receiving portion and the connection portion between the light receiving portion and the optical waveguide are suppressed, and the light emitting element can be integrated by simultaneous production in the same wafer and the same process. A light receiving element is realized.

It is sectional drawing of the laminated structure which comprises the light receiving element which concerns on 1st Embodiment. It is a typical perspective view showing the patterning shape of the light receiving element concerning a 1st embodiment. It is a typical sectional view of a photo acceptance unit concerning a 1st embodiment. It is the typical sectional view which expanded the principal part of the ring-shaped light-receiving part of the light receiving element concerning a 1st embodiment, and an optical waveguide. It is a figure showing the simulation result which shows the relationship between the applied voltage and light absorption amount of the ring type light receiving element (diameter 10 micrometers) which concerns on 1st Embodiment. It is a figure showing the simulation result which shows the change of the light absorption amount by the width of the waveguide of the ring type light receiving element concerning a 1st embodiment, and the existence of a curved waveguide. It is a figure which represents the connection of the linear waveguide and light-receiving part in the ring type light receiving element which concerns on 1st Embodiment by the presence or absence of a curved waveguide. It is a figure showing the simulation result which shows the change of the light reception sensitivity by the applied voltage of the ring type light receiving element which concerns on 1st Embodiment, a disk type light receiving element, and a spiral type light receiving element. It is a figure which shows the bias dependence of the light absorption characteristic of the ring type light receiving element which concerns on 1st Embodiment, a disk type light receiving element, and a spiral type light receiving element. It is a typical sectional view showing a part of manufacturing process of a ring type photo acceptance unit concerning a 2nd embodiment. It is a typical sectional view showing a part of manufacturing process of a ring type photo acceptance unit concerning a 2nd embodiment. It is a typical sectional view showing a part of manufacturing process of a ring type photo acceptance unit concerning a 2nd embodiment.

  FIG. 1 is a diagram showing a multilayer structure 100 that constitutes a basic laminated structure of a light receiving element and serves as a starting point for manufacturing the light receiving element when the light receiving element according to the embodiment is manufactured. The multilayer structure 100 is a wafer (InGaAs / GaAs) having the same laminated structure as that of a semiconductor laser, and an ultra-small and high-sensitivity light receiving element can be realized by using a laser resonator manufacturing process.

  In the figure, a laminated structure 100 is formed on a substrate S. The laminated structure 100 includes, for example, a lower cladding layer 20, a first contact layer 15a (first semiconductor layer 10), a first cladding layer 14a, a first light confinement layer 13a, an active layer 12, a second light confinement layer 13b, The second cladding layer 14b and the second contact layer 15b are stacked. The laminated structure 100 is continuously formed by, for example, MOCVD (Metal Organic Chemical Vapor Deposition).

Here, the substrate S is, for example, GaAs. The lower cladding layer 20 is, for example, n-type Al _0.96 Ga _0.04 As having a thickness of 1 μm. The first contact layer 15a is, for example, n-type GaAs having a thickness of 0.25 μm. The first cladding layer 14a is, for example, n-type Al _0.92 Ga _0.08 As having a thickness of 0.1 μm. The first optical confinement layer 13a is, for example, n-type GaAs having a thickness of 0.01 μm.

The active layer 12 is formed by alternately stacking, for example, an n-type GaAs quantum well layer having a thickness of 8 nm and a GaAs barrier layer having a thickness of 10 nm, and vertically and vertically (the substrate S side is “down” and the opposite side of the substrate S is “ It referred to above ") a multiple quantum well structure sandwiched by _Al 0.1 _{Ga 0.9} as layers thick 40 nm.

The second optical confinement layer 13b is, for example, p-type GaAs having a thickness of 0.01 μm. The second cladding layer 14b is, for example, p-type Al _0.92 Ga _0.08 As having a thickness of 0.1 μm. The second contact layer 15b is, for example, p-type GaAs having a thickness of 0.01 μm.

  As will be described later, the cladding layer is formed by oxidation. At the time of oxidation, the AL concentration of the base material before oxidation of the first cladding layer 14a and the second cladding layer 14b is the base material before oxidation of the lower cladding layer 20. It is important that the light receiving element according to the present invention is somewhat lower than the Al concentration.

  As will be described later, the ring type light receiving element 120 of the first embodiment as shown in FIGS. 2 and 3 can be obtained by repeating the patterning and etching several times as will be described later.

  FIG. 2 is a schematic perspective view of the ring-type light receiving element 120 according to the first embodiment. In order to easily understand the structure of the light receiving element 120, a cross section obtained by cutting the light receiving element 120 substantially along the center line. Shows the structure. FIG. 2 shows a state before the electrodes D1 and D2 and the insulating films 18, 19, and 80 shown in FIG. As shown in FIG. 2, the light receiving element 120 according to the first embodiment absorbs the light received through the thin line waveguide 70 and the thin line waveguide 70 by the ring-shaped light receiving unit 130 and converts it into an electrical signal. It comprises a ring-shaped light receiving unit 130 that performs the following. The thin-line waveguide 70 and the ring-shaped light receiving unit 130 are composed of the same laminated structure 100 shown in FIG.

  FIG. 3 is a view showing a cross section of the light receiving element 120. FIG. 3 shows a cross sectional view of the light receiving element 120 in the final form in which the insulating films 18, 19, 80 and two upper and lower electrodes D1, D2 are provided. Has been.

  Here, in the first embodiment, a direction orthogonal to the main surface 10a (FIG. 2) of the first semiconductor layer 10 is referred to as a Z-axis direction, and directions orthogonal to the Z-axis direction are referred to as an X-axis direction and a Y-axis direction. The main surface 10a of the first semiconductor layer 10 is an XY plane orthogonal to the Z-axis direction. Further, the Z-axis direction of the main surface 10a of the first semiconductor layer 10 is referred to as the upper side (upper side), and the opposite -Z direction is referred to as the lower side (lower side).

  In the light receiving element 120, the first semiconductor layer 10 includes a ring-shaped light receiving unit 130 and a thin wire waveguide 70 juxtaposed to the ring-shaped light receiving unit 130. The first semiconductor layer 10 is, for example, a compound semiconductor, and the conductivity type is, for example, the first conductivity type.

  In the light receiving element 120 of the present embodiment, light input from an optical fiber or the like is guided in the thin waveguide 70 and guided to the ring-shaped light receiving unit 130, and the light is confined in the light confinement layer of the light receiving unit 130 to absorb the electric signal. Convert to That is, the ring-shaped light receiving unit 130 functions as an optical / electrical conversion unit.

  In the light receiving element 120, the second semiconductor layer 20 a is in contact with the lower surface of the ring-shaped light receiving part 130 of the first semiconductor layer 10. Further, the low refractive index layer 20 c is in contact with the lower surface of the thin wire waveguide 70. The second semiconductor layer 20a is, for example, a compound semiconductor, and the conductivity type is, for example, the first conductivity type. The second semiconductor layer 20 a and the low refractive index layer 20 c function as the lower cladding layer 20.

  The lower cladding layer 20 is formed on the substrate S, for example. A first electrode D1 is further formed below the substrate S. Note that the first electrode D1 need only be electrically connected to the second semiconductor layer 20a, and thus is not necessarily provided below the substrate S.

  The waveguide 70 is provided along the main surface 10a and guides light emitted from the optical fiber or the like along the main surface of the first semiconductor layer. That is, the waveguide 70 and the ring-shaped light receiving unit 130 having the light absorption layer of the ring-type light receiving element 120 are in a state in which an optical signal can propagate from the waveguide 70 to the ring-shaped light receiving unit 130. The ring-shaped light receiving part 130 and the waveguide 70 having the light absorption layer of the ring type light receiving element are made of, for example, a compound semiconductor.

  When a compound semiconductor is used for the first semiconductor layer 10, the second semiconductor layer 20a, the ring-shaped light receiving unit 130, and the waveguide 70, the compound semiconductor includes a material containing gallium arsenide (GaAs) and gallium phosphide (GaP). Materials, materials containing gallium nitride (GaN), and the like.

  In the light receiving element 120 according to the embodiment, the layer structure of the ring-shaped light receiving unit 130 and the layer structure of the waveguide 70 have the same layer structure. That is, the layer structure of the ring-shaped light receiving unit 130 and the layer structure of the waveguide 70 are formed by performing predetermined patterning by etching on the uniform layer structure formed on the substrate S at the same time. For this reason, the distance between the ring-shaped light receiving unit 130 and the waveguide 70 is determined by the accuracy when the layer structure is patterned by etching.

  Next, a specific laminated structure will be described.

  FIG. 3 is a schematic cross section of a light receiving element completed by forming an insulating layer on the light receiving element shown in FIG. 2 and attaching upper and lower electrodes through a predetermined process.

  FIG. 4 is a schematic cross-sectional view of the displayed ring-shaped light receiving unit 130 and the waveguide 70.

  As shown in FIGS. 3 and 4, the ring-shaped light receiving unit 130 and the waveguide 70 have the same laminated structure, and the first cladding layer 14a, the first light confinement layer 13a, the active layer 12 and the second light confinement layer 13b. And a laminated structure including the second cladding layer 14b.

  The first conductivity type first light confinement layer 13 a is formed below the active layer 12, and the second conductivity type second light confinement layer 13 b is formed above the active layer 12. The first conductivity type first cladding layer 14a is formed below the first light confinement layer 13a, and the second conductivity type second cladding layer 14b is formed above the second light confinement layer 13b.

  A first contact layer 15a of the first conductivity type (first semiconductor layer 10) is formed between the second semiconductor layer 20a and the first cladding layer 14a. In the ring-shaped light receiving unit 130, the first contact layer 15 a is the first semiconductor layer 10.

  A second contact layer 15b of the second conductivity type is formed between the second cladding layer 14b and the second electrode D2.

  The first contact layer 15a is formed in a ring shape at a portion forming a part of the laminated structure, but the portion other than the laminated structure is the first semiconductor layer 10 and has a flat plate shape. Therefore, the first contact layer 15a extends to the inside and the outside of the ring. The active layer 12, the first optical confinement layer 13a, the second optical confinement layer 13b, the first clad layer 14a, the second clad layer 14b, and the second contact layer 15b constitute a laminated structure and are formed in a ring shape.

  The first cladding layer 14a includes a first inner peripheral portion 141a that extends from the ring inner edge to a part of the inside, a first outer peripheral portion 142a that extends from the ring outer edge to a part of the inner portion, a first inner peripheral portion 141a, and a first outer periphery. A first central portion 140a between the portion 142a and the first central portion 140a. The first inner peripheral portion 141a and the first outer peripheral portion 142a are modified into a first modified layer 14c having a refractive index lower than that of the first central portion 140a by thermal oxidation. Accordingly, in the first cladding layer 14a, the first central portion 140a sandwiched between the first inner peripheral portion 141a and the modified layer 14c of the first outer peripheral portion 142a functions as the original first cladding layer.

  Similarly, the second cladding layer 14b includes a second inner peripheral portion 141b that extends from the ring inner edge to a part of the inside, a second outer peripheral portion 142b that extends from the ring outer edge to a part of the inner portion, and a second inner peripheral portion 141b. And a second central portion 140b between the second outer peripheral portion 142b. The second inner peripheral portion 141b and the second outer peripheral portion 142b are modified into a second modified layer 14d having a refractive index lower than the refractive index of the second central portion 140b by thermal oxidation. Therefore, in the second cladding layer 14b, the second central portion 140b sandwiched between the modified layers 14d of the second inner peripheral portion 141b and the second outer peripheral portion 142b functions as the original second cladding layer.

  As shown in FIG. 3, the surface of the ring-shaped light receiving unit 130 and the surface of the waveguide 70 are covered with the insulating film 18. An insulating film 80 is embedded inside the ring of the ring-shaped light receiving unit 130 and between the ring-shaped light receiving unit 130 and the waveguide 70.

  A part of the upper surface of the second contact layer 15b is exposed, and the second electrode D2 is provided in this exposed portion. The second electrode D2 is formed in a disc shape. A part of the second electrode D2 is formed on the insulating films 19 and 80, and the other part is formed on the exposed part of the second contact layer 15b.

The active layer 12 has, for example, an undoped multiple quantum well layer (Multiple Quantum Well). The undoped multiple quantum well layer is, for example, an InGaAs quantum well layer having a thickness of 8 nanometers (nm) and a GaAs barrier layer having a thickness of 10 nm, for example, and the upper and lower layers are formed by 10 nm GaAs barrier layers. In this structure, the upper and lower 10 nm GaAs barrier layers are further sandwiched between, for example, 40 nm thick Al _0.1 Ga _0.9 As layers.

The first optical confinement layer 13a is, for example, an n-type GaAs layer having a thickness of 100 nm. The second optical confinement layer 13b is a p-type GaAs layer having a thickness of 100 nm, for example. The first cladding layer 14a is, for example, an n-type Al _0.92 Ga _0.08 As layer having a thickness of 300 nm. The second cladding layer 14b is a p-type Al _0.92 Ga _0.08 As layer having a thickness of 300 nm, for example. The first contact layer 15a is an n-type GaAs layer having a thickness of 10 nm, for example. The second contact layer 15b is, for example, a p-type GaAs layer having a thickness of 10 nm.

  The first modified layer 14c and the second modified layer 14d are layers obtained by thermally oxidizing the first cladding layer 14a and the second cladding layer 14b, and are more than the first cladding layer 14a and the second cladding layer 14b. It has a low refractive index and electrical insulation properties.

For the substrate S, for example, n-type GaAs is used. Further, for example, n-type Al _0.96 Ga _0.04 As is used for the second semiconductor layer 20a.

For example, a gold-germanium (AuGe) alloy is used for the first electrode D1. The first electrode D1 is, for example, a cathode electrode. For example, a gold-zinc (AuZn) alloy is used for the second electrode D2. The second electrode D2 is, for example, an anode electrode. For example, SiO ₂ is used for the insulating film 18. For the insulating film 80, for example, polyimide is used.

  The ring-shaped light receiving unit 130 is a waveguide of light that circulates along the outer periphery of the ring shape, and incident light incident from the substantial optical waveguide a2 in FIG. 3 is a ring-shaped or helical ring-shaped light receiving unit 130. In the process of turning around, the first semiconductor layer 10 enters the photoactive layer 12 to form electron / hole pairs. The outer diameter of the ring-type light receiving element 120 is, for example, 10 micrometers (μm), and the inner diameter of the ring is, for example, 5 μm. The thickness of the ring type light receiving element 120 is, for example, 1 μm. The first semiconductor layer 10 of the waveguide 70 is a substantial waveguide. Since the first semiconductor layer 10 of the waveguide 70 is sandwiched from above and below by the lower clad layer 20 and the first clad layer 14 a having a lower refractive index than the first semiconductor layer 10, incident light is incident on the first semiconductor layer 10. Then, the light propagates to the adjacent ring-shaped or spiral ring-shaped light receiving portion 130 through the first semiconductor layer 10.

  In the ring-shaped light receiving unit 130, the second semiconductor layer 20a and the low refractive index layer 20b function as the lower cladding layer 20 as described above, so that incident light from the waveguide 70 does not enter the lower cladding layer. The photoactive layer 12 is reached through the first central portion 140a.

  Here, when the second electrode D2 is connected to the cathode of the power source (FIG. 3), the first electrode D1 is connected to the anode of the power source, and a predetermined voltage is applied, electrons and holes generated in the active layer 12 are generated. Each is drawn out from the positive and negative electrodes to an external circuit. That is, a photocurrent flows.

  Here, when the ring-shaped light-receiving part 130 is formed by the RIE (Reactive Ion Etching) method, the side wall of the ring-shaped light-receiving part 130 is roughened, and, for example, a roughness of about 10 nm is generated. Furthermore, damage such as microcracks and crystal defects occurs due to ion impact.

  On the other hand, by forming the first modified layer 14c and the second modified layer 14d by thermally oxidizing the outer peripheral side wall and the inner peripheral side wall of the first cladding layer 14a and the second cladding layer 14b, The roughness and damage of the interface between the second cladding layer 14b, the first modified layer 14c, and the second modified layer 14d can be sufficiently reduced. Thereby, interface scattering of incident light incident on the ring-shaped light receiving unit 130 is suppressed, and sufficient photoelectric conversion characteristics in the ring-shaped light receiving unit 130 are obtained.

  Further, the ring-shaped light receiving part 130 realizes a current confinement structure by the first modified layer 14c and the second modified layer 14d having electrical insulation. With this current confinement structure, photocurrent is selectively extracted from the active layer portion. The photocurrent corresponds to the input light intensity, and linear input / output characteristics are obtained.

In addition, since the current extracted from the ring-shaped light receiving unit 130 does not flow in the vicinity of the outer peripheral side wall and the inner peripheral side wall of the ring, carrier loss due to the influence of damage on the outer peripheral side wall and the inner peripheral side wall, for example, light emission recombination caused by damage Etc. can be avoided. As a result, it is possible to effectively use the photoelectric conversion current.
The waveguide 70 is optically coupled to the ring-shaped light receiving unit 130. That is, the input light that has propagated through the waveguide 70 is guided to the light receiving unit 130 along the ring. The light is taken into the active layer, and an electric current due to generation of electrons and hole pairs is taken out through the electrodes D1 and D2.

  As illustrated in FIGS. 3 and 4, the waveguide 70 of the light receiving element 120 includes a stacked structure (second stacked structure) including the first cladding layer 14 a, the active layer 12, and the second cladding layer 14 b. That is, the waveguide 70 has the same stacked structure as the ring-shaped light receiving unit 130.

  The waveguide 70 is formed in a part (second part a2) of the first contact layer 15a. The waveguide 70 is provided, for example, extending linearly along the main surface 10a. The waveguide 70 forms an optical waveguide structure with the second portion a2 of the first contact layer 15a as a core and the first cladding layer 14a on the second portion a2 as a cladding. Thereby, light can be guided along the direction of the waveguide 70.

  The lower clad layer 20 includes a low refractive index layer 20c including a portion overlapping the second portion a2 when viewed in the Z-axis direction, and a second semiconductor layer 20a including a portion overlapping the first portion a1. The refractive index of the low refractive index layer 20c is lower than the refractive index of the second portion a2. Here, the refractive index is a refractive index with respect to input light.

  Further, the conductivity of the low refractive index layer 20c is lower than the conductivity of the second semiconductor layer 20a. The low refractive index layer 20c is a modified layer obtained by thermally oxidizing a part of the same material as the second semiconductor layer 20a (material of the lower cladding layer 20), for example. That is, a part of the semiconductor material used when forming the second semiconductor layer 20a is modified by thermal oxidation to form the low refractive index layer 20c. The low refractive index layer 20c is not limited to the one formed by thermally oxidizing the same material as the second semiconductor layer 20a. For example, a layer different from the second semiconductor layer 20a is laminated on the substrate S. May be.

  By providing the low refractive index layer 20c, it is possible to prevent light traveling from the waveguide 70 toward the ring-shaped light receiving unit 130 from leaking to the second semiconductor layer 20c side.

  That is, the waveguide 70 has an optical waveguide structure (like an optical fiber) in which the second portion a2 of the first contact layer 15a is a core, and the upper first cladding layer 14a and the lower low-refractive index layer 20c are cladding. Become. As a result, the input light is confined in the second portion a2 of the first contact layer 15a, and the total reflection is alternately repeated between the first cladding layer 14a and the low refractive index layer 20c, and efficiently proceeds. Waveguided to 130. The waveguide 70 preferably has a structure in which the first contact layer 15b, the second cladding layer 14b, the second optical confinement layer 13b, the active layer 12, and the first optical confinement layer 13a, which are the upper layers, are removed. In other words, from the upper side of the waveguide 70, it is more preferable that the first cladding layer and air only have no light absorption structure.

  Therefore, it is desirable that the boundary position BL between the low refractive index layer 20c and the second semiconductor layer 20a is located on the inner side of the ring than directly below or directly below the ring outer peripheral position of the ring-shaped light receiving unit 130.

  The cladding layer described above is formed by an oxidation treatment. The oxidation treatment is performed at a temperature of 400 ° C. or more and 500 ° C. or less, for example, in a water vapor atmosphere, an oxygen atmosphere, or in the air. At this time, the oxidation rate is determined by the Al composition ratio. That is, oxidation proceeds from the outer peripheral side and inner peripheral side of the cladding layer having a high Al composition ratio toward the inside, and a low refractive index portion is formed. Simultaneously with this oxidation, a current confinement structure is formed. Furthermore, a low refractive index layer 20 c is formed at the end of the lower clad layer 20 by this oxidation.

(Ring type light receiving element)
In the oxidation treatment, the oxidation rate varies depending on the plane orientation of the normal crystal. The (100) plane has the slowest oxidation rate, and the (110) plane and (111) plane have higher oxidation rates in this order. As a result, on the outer peripheral side and inner peripheral side of the ring-shaped first cladding layer 14a and the second cladding layer 14b, the width of the modified layer slightly changes depending on the plane orientation. The smaller the diameter, the greater the effect. Therefore, it is desirable to appropriately determine the ring-shaped outer shape of the light receiving unit 130 before the modification so that the desired ring shape is obtained after the modification by oxidation. By doing so, the low refractive index portion after the oxidation treatment can be formed into a circular shape with a constant curvature radius, and an increase in optical loss can be suppressed. Further, in this oxidation treatment, the optical confinement layer and the multiple quantum well layer with a small Al composition hardly undergo oxidation, so that the semiconductor layer remains as it is.

  The waveguide that introduces light into the light receiving section is a curved waveguide 70C with a slightly larger radius of curvature than the ring diameter, which increases the consistency between the input light and the ring's circular mode, thereby scattering light inside the ring. Loss can be reduced. Moreover, it is desirable to make the waveguide width as thin as possible in order to reduce interference between the input light and the circulating light.

  FIG. 5 shows a simulation result of the light absorption efficiency by the waveguide shape of the ring-type light receiving element (diameter 10 μm) of the first embodiment. When the bias voltage is 2.5 V or more, the absorption efficiency is 90% or more. This is considered because the light distribution is attracted to the GaAS waveguide side. This result shows that the efficiency of light absorption can be improved by narrowing the width of the input waveguide to form a bent waveguide. The bent waveguide may be a spiral bent waveguide. The ring width is 2.5 μm wide to reduce carrier loss due to surface recombination (surface recombination lifetime: 1 ns or longer), reduces light scattering on the side wall, and loss due to light leakage and absorption to the top electrode. Therefore, the oxidation width of the cladding layer is set to 0.6 μm. As a result, even with a minute ring-type light-receiving part having a diameter of about 10 μm, it is possible to reduce the loss of light and photoexcited carriers and realize a small and highly efficient light-receiving element.

(Helix type light receiving element)
Next, a spiral waveguide is patterned on the semiconductor multilayer structure. Here, an oxide structure having a low refractive index that is insulative from the side wall is formed by oxidizing the clad layer of the semiconductor multilayer structure. Even in the helical light-receiving element, if the width of the helical waveguide becomes nonuniform, the light propagation mode is disturbed and the optical loss increases. Therefore, by taking into consideration the difference in the oxidation rate depending on the crystal plane orientation of the semiconductor, the interface after the oxidation treatment can be formed into a spiral shape having a certain width by suppressing the increase in optical loss. Further, in this oxidation treatment, the optical confinement layer and the multiple quantum well layer with a small Al composition hardly undergo oxidation, so that the semiconductor layer remains as it is.

  In the case of the spiral type, the loss at the input portion is significantly reduced as compared with the ring (or disk) type, but the optical loss due to radiation is increased at the termination portion. In order to reduce this, the portion to be processed into a spiral shape is kept at a portion above the GaAs waveguide layer (upper clad layer and absorption layer), and the GaAs waveguide layer has a disk shape (or a thick ring shape). As a result, the radiation loss at the terminal portion can be greatly reduced. Similarly to the ring type of the first embodiment, by oxidizing and constricting the AlGaAs cladding layer, loss of light and carriers can be reduced and highly efficient photoelectric conversion can be achieved.

  FIG. 6 shows the relationship between the width of the input waveguide of the ring-shaped light receiving element and the light absorption amount (relative value). 6 indicates the light absorption characteristic of the ring type light receiving element in which the input waveguide and the ring type light receiving part are connected via the curved waveguide 70C. In the ring type light receiving element including the bent waveguide 70C, when the width of the input waveguide is increased, the light absorption amount tends to be disadvantageous. On the other hand, □ indicates the amount of light absorption when the linear input waveguide 70C is formed in contact with the ring-shaped light receiving portion without passing through the bent waveguide. That is, the linear waveguide 70 on a straight line corresponds to a “tangent” that is in contact with a circle (ring-shaped light receiving portion). When ◯ and □ of the same waveguide width (˜0.4 μm) are seen, it can be seen that the amount of light absorption increases when the curved waveguide 70C is inserted between the linear waveguide and the ring-shaped light receiving portion.

  A curved waveguide having a curvature R of about 7 μm is inserted into a ring-type light receiving portion having a radius of 5 μm. The curvature R can be designed in balance with the element characteristics of the light receiving element.

  Further, in the portion of the input waveguide including the bent waveguide, the portion of the waveguide 70 as shown in FIG. 2 and FIG. 4 also includes the first contact layer 15a, the first cladding layer 14a, and the first light from the substrate side. Although it has a laminated structure including the confinement layer 13a, the active layer 12, the second optical confinement layer 13b, the second cladding layer 14b, and the second contact layer 15b, the first optical confinement layer 13a, the active layer 12, the second layer The optical confinement layer 13b, the second cladding layer 14b, and the second contact layer 15b may be removed, and the first confinement layer 15b and the first cladding layer 14a alone may be used. In this case, the atmosphere covering the first cladding layer 14a also acts as a cladding for the input light propagating through the waveguide 70, and the input light leaks toward the optical confinement layer 13a and the active layer 12 and is absorbed. This is advantageous in preventing this.

  In FIG. 7A, the curved waveguide 70C is not adopted. That is, in FIG. 7A, one side surface (upper side surface in the drawing) of the straight waveguide 70 is substantially in contact with the outermost side surface of the light receiving unit 130 along the tangent line indicated by the broken line. In contrast, in FIG. 7B, a curved waveguide 70C is employed, and one side surface (upper side surface in the figure) of the straight waveguide 70 and the outermost side surface of the light receiving unit 130 are not in direct contact with each other. The bent waveguide 70C is connected along a tangential line substantially indicated by a broken line through the upper side surface in the drawing. That is, in FIG. 7B, one side surface of the straight waveguide 70 is “smoothly connected” to one side surface of the light receiving unit 130 along the tangent, and the curved waveguide 70 </ b> C has a larger radius of curvature than the light receiving unit 130. Therefore, input light guided through the straight waveguide 70 and having various input angles can be efficiently guided to the light receiving unit 130. When the straight waveguide 70, the bent waveguide 70C, and the light receiving unit 130 are actually manufactured, these “one side surfaces” may have irregularities on a microscopic scale. In some cases, on a macroscopic scale, these “one sides” are defined by their envelopes.

  FIG. 8 shows the light receiving sensitivity characteristics of the ring type, the disk type, the spiral type, and the respective light receiving elements. The horizontal axis represents voltage. From the viewpoint of light receiving sensitivity, it can be seen that a spiral or disk-shaped light receiving portion is more advantageous than a ring-shaped light receiving portion. In the ring-shaped and disk-shaped light receiving elements, the waveguide and the light receiving portion are connected via a curved waveguide.

  As described above, according to the present embodiment, an absorption layer, a cladding layer, a waveguide layer, and the like are epitaxially grown on a compound semiconductor (for example, GaAs) substrate, and mainly by etching processing regardless of the bonding or adhesion process. A small and highly efficient light receiving element that can be manufactured can be realized.

  FIG. 9 shows the bias dependence of the light absorption characteristics that are common characteristics of the ring type, disk type, spiral type, and light receiving elements described above. It was confirmed that when the voltage applied to the upper and lower electrodes was increased in any of the light receiving elements, the absorption edge shifted to the long wavelength side.

(Second Embodiment)
The second embodiment relates to a method for manufacturing the light receiving element 120.

  10A to 10C are schematic cross-sectional views illustrating the method for manufacturing the light receiving element.

  First, as described in FIG. 1, the laminated structure 100 is formed on the substrate S. The laminated structure 100 includes, for example, a lower cladding layer 20, a first contact layer 15a (first semiconductor layer 10), a first cladding layer 14a, a first light confinement layer 13a, an active layer 12, a second light confinement layer 13b, The second cladding layer 14b and the second contact layer 15b are stacked.

  The laminated structure 100 is continuously formed by, for example, the MOCVD (Metal Organic Chemical Vapor Deposition) method.

Here, the substrate S is, for example, GaAs. The lower cladding layer 20 is, for example, n-type Al _0.96 Ga _0.04 As having a thickness of 1 μm. The first contact layer 15a is, for example, n-type GaAs having a thickness of 0.25 μm. The first cladding layer 14a is, for example, n-type Al _0.92 Ga _0.08 As having a thickness of 0.1 μm. The first optical confinement layer 13a is, for example, n-type GaAs having a thickness of 0.01 μm.

The active layer 12 is, for example, an n-type GaAs quantum well layer having a thickness of 8 nm and a GaAs barrier layer having a thickness of 10 nm are alternately stacked, and the upper and lower layers are Al _0.1 Ga _0.9 As having a thickness of 40 nm. It is a multiple quantum well structure sandwiched between layers.

The second optical confinement layer 13b is, for example, p-type GaAs having a thickness of 0.01 μm. The second cladding layer 14b is, for example, p-type Al _0.92 Ga _0.08 As having a thickness of 0.1 μm. The second contact layer 15b is, for example, p-type GaAs having a thickness of 0.01 μm.

  Between the first contact layer 15a and the first cladding layer 14a, between the first cladding layer 14a and the active layer 12, between the active layer 12 and the second cladding layer 14b, and between the second cladding layer 14b and the second cladding layer 14b. An AlGaAs graded layer may be inserted between the contact layer 15b. The AlGaAs graded layer gradually changes the Al composition from 0.1 to 0.92 to relieve the lattice constant mismatch.

  Next, as illustrated in FIG. 10A, the first resist film R <b> 1 corresponding to the ring shape of the light receiving unit 130 and the waveguide 70 connected to the light receiving unit 130 are formed on the laminated structure 100. A second resist film R2 corresponding to the shape is formed, and the laminated structure 100 is etched using the first resist film R1 and the second resist film R2 as a mask.

  The first resist film R1 has, for example, a ring shape with an outer diameter of 10 μm and an inner diameter of 5 μm. The second resist film R2 has a linear shape with a width of 1 μm or more and 2 μm or less, for example. For example, the RIE method is used for the etching. By this etching, the portion of the laminated structure 100 where the first resist film R1 and the second resist film R2 are not formed is removed from the second contact layer 15b until the first contact layer 15a is exposed.

  By this etching, a ring-shaped light receiving portion 130 and a linear waveguide 70 are formed. That is, the light receiving unit 130 and the waveguide 70 are simultaneously formed by a single etching process.

  Further, the light receiving unit 130 and the waveguide 70 are accurately set by the accuracy of photolithography when forming the first resist film R1 and the second resist film R2, and the accuracy of etching such as RIE. Furthermore, the thickness t1 of the first contact layer 15a in the recess RP between the light receiving unit 130 and the waveguide 70 is accurately set by etching such as RIE.

  Next, as shown in FIG. 10-1 (b), an oxidation treatment is performed. In performing the oxidation treatment, a part of the lower clad layer 20 outside the waveguide 70 is removed. The oxidation treatment is performed, for example, at a temperature of 400 ° C. or more and 500 ° C. or less in a water vapor atmosphere, an oxygen atmosphere, or the air. At this time, the oxidation rate is determined by the Al composition ratio. That is, oxidation proceeds from the outer peripheral side and the inner peripheral side of the first cladding layer 14a and the second cladding layer 14b having a high Al composition ratio toward the inside, and the first modified layer 14c and the second modified layer 14d are formed. The Simultaneously with this oxidation, a current confinement structure is formed. Furthermore, a low refractive index layer 20 c is formed at the end of the lower clad layer 20 by this oxidation. Here, the low refractive index layer 20c formed by oxidation spreads from the end of the lower cladding layer 20 to the inside in a direction parallel to the substrate, and is slightly more than the first modified layer 14c and the second modified layer 14d. It is better to extend to the inside. For this purpose, the lower refractive index layer 20c has a slightly higher Al concentration in the lower cladding layer 20 than the Al concentration in the base material of the first modified layer 14c and the second modified layer 14d. It is easy to control the arrangement of the first modified layer 14c and the second modified layer 14d. That is, the oxidation rate is controlled to some extent by the Al concentration.

  Further, the oxidation rate varies depending on the crystal plane orientation. The (100) plane has the slowest oxidation rate, and the oxidation rate increases in the order of the (110) plane and the (111) plane.

  As a result, on the outer peripheral side and inner peripheral side of the ring-shaped first cladding layer 14a and the second cladding layer 14b, the width of the modified layer slightly changes depending on the plane orientation. The smaller the diameter, the greater the effect. Therefore, it is desirable to appropriately determine the ring-shaped outer shape of the light receiving unit 130 before the modification so that the target circular shape is obtained after the modification.

  Next, as illustrated in FIG. 10C, the surface of the light receiving unit 130 and the surface of the waveguide 70 are covered with the insulating film 18.

  Next, as illustrated in FIG. 10D, the insulating film 80 that embeds the light receiving unit 130 and the waveguide 70 is formed. For the insulating film 80, for example, polyimide is used.

  Further, as shown in FIG. 10-2 (e), the insulating film 80 is removed from the insulating layer 18 and the insulating film 80 (polyimide) covering the light receiving unit 130 and the waveguide 70 by, for example, CMP (Chemical Mechanical Polishing). To do. Due to the difference in hardness between the insulating film 80 (polyimide) and the insulating layer 18, the peripheral portion in contact with the light receiving unit 130 and the waveguide 70 is polished a little more. As a result, the insulating layer 18 on the light receiving unit 130. The insulating film 80 (polyimide) is removed, and the second contact layer 15b is exposed. After this CMP treatment, removal of CMP abrasive residue and removal of damage of GaAs constituting the second contact layer 15b are performed.

  Thereafter, as shown in FIG. 10-2 (f), the insulating film 19 is formed again using a metal mask or the like so as to cover the periphery of the light receiving unit 130 again. The insulating film 19 is formed so as to cover the outer peripheral surfaces of the ring-shaped first cladding layer 14a and the second cladding layer 14b.

  Finally, as shown in FIG. 10-3 (g), the positive electrode D1 is first formed on the entire light receiving portion using a metal mask or the like again. In the case of a ring type or disk type light receiving element, the positive electrode D1 may be formed only on the second contact layer 15b. After forming the positive electrode D1, the negative electrode D2 is formed on the back surface of the substrate S as shown in FIG. 10-3 (h).

  By the manufacturing method as described above, the light receiving element 120 in which the light receiving portion 130 and the waveguide 70 are integrally formed on the lower cladding layer 20 can be manufactured. That is, an absorption layer, a cladding layer, a waveguide layer, and the like are epitaxially grown on a compound semiconductor (for example, GaAs) substrate, and a light receiving element can be manufactured mainly by etching processing regardless of bonding or adhesion processes.

  In addition, although this Embodiment and its modification were demonstrated above, this invention is not limited to these examples. For example, for each of the above-described embodiments or modifications thereof, those in which those skilled in the art appropriately added, deleted, and changed the design, and those in which the features of each embodiment are appropriately combined, As long as the gist of the present invention is provided, it is included in the scope of the present invention. For example, in each of the above-described embodiments and modifications, the first conductivity type has been described as n-type, and the second conductivity type has been described as p-type. However, in the present invention, the first conductivity type is p-type, It is also possible to implement the second conductivity type as an n-type.

  As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 10 ... 1st semiconductor layer, 10a ... Main surface, 10c ... Modified layer, 12 ... Active layer, 13a ... 1st optical confinement layer, 13b ... 2nd optical confinement layer, 14a ... 1st clad layer, 14b ... 1st Cladding layer, 14c ... first modified layer, 14d ... second modified layer, 15a ... first contact layer, 15b ... second contact layer, 18, 19, 80 ... insulating film, 20 ... lower cladding layer, 20a ... 2nd semiconductor layer, 20c ... low refractive index layer, 70 ... optical waveguide, 93 ... wiring pattern, 95 ... groove, 100 ... laminated structure, 120 ... semiconductor light emitting element, 130 ... light receiving part, 140a ... 1st center part, 140b ... second central portion, 141a ... first inner peripheral portion, 141b ... second inner peripheral portion, 142a ... first outer peripheral portion, 142b ... second outer peripheral portion, D1 ... first electrode, D2 ... second electrode, S …substrate

Claims (13)

A light-receiving part having a circular semiconductor multilayer structure and a waveguide connected to the light-receiving part;
The semiconductor multilayer structure includes an active layer having a multiple quantum well structure, a light confinement layer provided on both sides of the active layer, a first cladding layer and a second cladding layer provided on both sides of the light confinement layer, A first contact layer and a second contact layer provided so as to be connected to the first clad layer and the second clad layer, and a lower clad layer connected to the first contact layer and connected to the substrate,
A part of the side wall of the light receiving portion is oxidized and becomes a material having a lower refractive index than the first cladding layer, the second cladding layer, and the lower cladding layer,
The waveguide includes the first clad layer, the second clad layer, and the lower clad layer formed by the oxidation of the first clad layer, the second clad layer, and the lower clad layer that constitute the waveguide. Has become a material with a low refractive index,
A light receiving element.   The light receiving element according to claim 1, wherein the light receiving unit has a ring shape or a disk shape.   The light receiving element according to claim 1, wherein a part of a side surface of the light receiving portion is smoothly connected to one side surface of the waveguide.   The light receiving element according to claim 3, wherein the waveguide forms a curved waveguide at a connection portion with the light receiving portion.   The light receiving element according to claim 1, wherein at least a part of the light receiving unit is formed of a spiral bent waveguide.   6. The waveguide according to claim 1, wherein an active layer, an optical confinement layer, a second cladding layer, and a second contact layer constituting the waveguide are removed from the waveguide. Light receiving element.   The light receiving element according to claim 1, wherein the low refractive index material has an electrical insulation characteristic. The first clad layer and the second clad layer are electrically insulated by dry etching or ion implantation at a central portion of the disc-shaped light receiving portion. Light receiving element. 4. The light receiving element according to claim 2, wherein a side wall of the light receiving portion after oxidation has a ring shape with a substantially constant curvature radius. 5.   10. The light receiving element according to claim 1, wherein the light receiving element includes the multiple quantum well structure constituting the light receiving unit and a portion where the light confinement layer is not oxidized. 11.   The light receiving element according to claim 1, wherein a part of the light receiving unit is optically coupled to the waveguide.   6. The light receiving element according to claim 1, wherein an electrode is formed so as to be in contact with a part of the ring-shaped contact layer.   The wavelength of the absorption edge of the multiple quantum well structure is shifted to the longer wavelength side by 10 nm or more by increasing the bias applied to the light receiving unit. Light receiving element.

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