This application is the National Phase of PCT/JP2007/072904, filed Nov. 28, 2007, which is based upon and claims the benefit of priority from Japanese patent application No. 2006-342336, filed on Dec. 20, 2006, the disclosure of which is incorporated herein in its entirety by reference.
The present invention relates to a photodiode. The present invention more particularly relates to a photodiode for converting an optical (including infrared-ray light) signal necessary for an information process and a communication field into an electric signal at a high speed. Further, it relates to an optical communication device and an optical interconnection module employing the foregoing photodiode.
It is very attractive from a viewpoint of a cost and a yield to monolithically integrate and circuitize an optical detector. A silicon photoreceiver monolithically integrated and circuitized on a chip identical to that of a CMOS circuit, i.e. a silicon photodiode, is one of attractive substitutes for a hybrid photoreceiver (for example, an InGaAs photodiode connected to the CMOS circuit or a GaAs circuit). It is expected that the photoreceiver monolithically integrated and circuitized can be manufactured at a lower cost as compared with the hybrid-designed photoreceiver because it can be manufactured by employing a standard silicon process.
By the way, the photodiode is employed in many cases as a means for converting the optical signal into the electric signal at a high speed. A pin type photodiode is representative thereof. The pin type photodiode has a construction in which an i layer of an intrinsic semiconductor has been put between a p layer of a p-type semiconductor and an n layer of an n-type semiconductor. And, when an inverse bias voltage is applied with a bias power source, almost all region of the i layer having a high resistance becomes a depletion layer of an electric charge carrier. A photon of incident light is mainly absorbed in the i layer to generate electron/positive hole pairs. Each of the generated electron and positive hole drifts within the depletion layer in an opposite direction to the other due to the inverse bias voltage, thereby allowing a current to flow, and is detected as a signal voltage with a load resistance. A circuit time constant that is governed by a product of the load resistance and an electric capacity being produced by the depletion layer can be listed as a factor for putting a limit to a response speed of this photoelectric conversion. Further, a carrier drift time necessary for the electron and the positive hole passing through the depletion layer can be listed as the foregoing factor.
Well, there exists a Schottky type photodiode as a photodiode of which the carrier drift time is short. This photodiode has a construction in which a semitransparent metal film is in contact with the n-layer (or n− layer)of the semiconductor. A Schottky junction is formed in the neighborhood of an interface in which the n layer (or n− layer) and the semitransparent metal film contact each other.
Diffusion of the electron from the semitransparent metal film to the n layer (or n− layer) occurs in the neighborhood of this Schottky junction, and this region becomes a depletion layer. When the incident light is radiated in this state, the electron is generated in the n layer (or n− layer). This generated electrons drift within the depletion layer due to the inverse bias voltage. And, the light absorption on the element surface layer can be effectively utilized.
By the way, the pin type photodiode necessitates the i layer, i.e. the depletion layer having a sufficient thickness because of absorption of the photon. On the other hand, the depletion layer of the Schottky type photodiode can be made thin. Thus, the Schottky type photodiode can make the carrier drift time short.
Well, an attempt for adopting a lateral electrode structure and making a gap between the electrodes short so as to thin the depletion layer has been made for the pin type photodiode (see Non-patent document 1). This technique, however, is poor in a light absorption efficiency on the surface layer of the semiconductor. Thus, the high sensitivity is difficult to attain even though the high speediness is enabled.
On the other hand, making a value of the load resistance small so as to make the circuit time constant short causes the voltage of a reproduction signal that is takable to lower. Thus, making an S/N of the reproduction signal large, and yet reducing an error in the reading-off necessitates reducing an electric capacity of the depletion layer. In particular, making the depletion layer thin so as to make the carrier drift time short causes the electric capacity to be increased. Thus, it is necessary to reduce the depletion layer (or an area of the Schottky junction) so as to attain the high speediness. However, reducing the foregoing junction area causes a utilization efficiency of the signal light to lower. As a result, the S/N of the reproduction signal deteriorates.
That is the reason why a development for attaining the high speediness/miniaturization by utilizing a metal surface plasmon or a photonic crystal structure is in progress in the optoelectric conversion device of this type.
For example, a metal/semiconductor/metal (MSM) device (optical detector) in which two electrodes have been mounted on an identical surface of the semiconductor have been proposed (Patent document 1). This MSM type optical detector is one kind of the Schottky type photodiodes having the Schottky junction in the neighborhood of the two electrodes. That is, one part of the light having transmitted through the electrode surface is absorbed in a semiconductor layer (semiconductor absorbing layer) to generate a photocarrier. In such an MSM type optical detector, making the semiconductor thick for a purpose of enhancing a quantum efficiency leads to an increase in a propagation distance of the photocarrier, which causes an operational speed to lower. So as to prevent this operational speed from lowering, the optical detector described in the Patent document 1 has the metal electrode formed along periodic roughness. With this, the incident light is efficiently coupled to the surface plasmon of the metal electrode, and propagates inside the optical detector.
Further, the method has been proposed of manufacturing an MSM type light receiving element having the metal film formed on the semiconductor as a light transmissive insulating film by partially oxidizing it (Patent document 2).
Further, an MSM type light receiving element utilizing proximity field light that occurs in an edge of the metal film existing in both sides of a light transmissive insulating pattern (of which the pattern width is equal to or less than a wavelength of transmission light) has been proposed (Patent document 3). And, it is described that the response speed of this MSM type light receiving element is fast.
Further, the device structure has been proposed in which a positive polarity (metal electrode) and a negative polarity (metal electrode) have been arranged on the semiconductor in such a manner that electrode fingers are intersected with each the other (nested structure) (Patent document 4). This Patent document 4 discloses the technology for coupling the incident light to each of the transmissive light, the reflection light, surface plasmon polariton, etc. with the resonance. In addition hereto, the Patent document 4 teaches that the photocarrier being generated is reinforced owing to the coupling of the incident light and the surface plasmon. However, reducing an irradiation area of the incident light for a purpose of reducing the electric capacity of the depletion layer in these light receiving elements leads to a decline in the intensity of the detection signal, and a decline in the S/N.
Further, a photovoltaic device (a photovoltaic device utilizing solar energy) having periodically-arranged apertures (or concave portions) formed on one of two electrodes holding a plurality of spherical semiconductors each having a pn junction between them has been proposed (Patent document 5). This photovoltaic device is a device for utilizes the resonance of the incident light and the surface plasmon. This Patent document 5, however, does not teach that the depletion layer is made thin and yet the area is made small for a purpose of attaining the high speediness of the photoelectric conversion.
Further, an optical transmitter having an array of periodic grooves formed around the apertures has been proposed (Patent document 6). And, it is reported that the light that propagates is augmented all the more in the optical transmitter having an array of the periodic grooves formed as compared with the case of the optical transmitter having no array of the periodic grooves. However, it is known that total energy of the light that transmits attenuates as compared with the energy of the incident light (Document: Tineke Thio, H. J. Lezec, T. W. Ebbesen, K. M. Pellerin, G. D. Lewen, A. Nahata, and R. A. Linke, “Giant optical transmission of sub-wavelength apertures: physics and applications”, Nanotechnology, vol. 13, pp.429-432,
Further, an MSM type light receiving element having the light absorption layer as a layer having a multilayer film structure in which a photonic band is formed has been proposed (Patent document 7). It is reported that this MSM type light receiving element is high in the light receiving efficiency. However, the point of reducing the junction area in the MSM junction and making the element capacity small has not been realized also in this structure.
Further, a pin type photodiode (a pin type photodiode employing InGaAs) having a microlens formed on the back side of a substrate, and furthermore, a mirror formed for re-reflecting the light incident from the back side, which has been reflected by the element surface, has been proposed (Patent document 8). It is reported that this photodiode is improved in a tolerance of an optical coupling arraignment with external light, and the element coupling area can be made small. However, also in this structure, a diameter of an optical spot focused by the microlens is in the order of several tens of microns, which puts limits to a reduction in the element capacity and to realization of a high-frequency response.
Non-Patent document 1: S. J. Koester, G. Dehlinger, J. D. Schaub, J. O. Chu, Q. C. Quyang, and A. Grill, “Germanium-on-Insulator Photodetectors”, 2nd International Conference on Group IV Photonics, FBI 2005 (page 172,
Patent document 1: JP-P1984-108376A (page 4-16,
Patent document 2: Patent No. 2666888 (page 3-4,
Patent document 3: Patent No. 2705757 (page 6,
Patent document 4: JP-P1998-509806A (page 26-33,
Patent document 5: JP-P2002-76410A (page 6-9,
Patent document 6: JP-P2000-171763A (page 7-10,
Patent document 7: JP-P2005-150291A (page 5,
Patent document 8: JP-P1994-77518A (page 2,
The metal-semiconductor-metal (MSM) photodiode offers flatness and compatibility with a silicon LSI.
However, the optical detector employing Si (or SiGe), as a rule, exhibits slow responsiveness because of a long carrier life time (1 to 10 μs) and a low light absorption ratio (10 to 100/cm).
Further, the Schottky junction type photodiode, which employs a compound semiconductor, exhibits a fast response.
However, the effective light receiving area becomes small due to the metal electrode. Thus, the sensitivity lowers.
Further, the lateral electrode structure has been proposed for the pin type photodiode for a purpose of layer-thinning the depletion layer.
However, the high sensitivity is difficult to attain even though the high speediness can be attained by making a distance between the electrodes small.
And, making the response of the photodiode fast necessitates thinning the light absorption layer, thereby to make the carrier drift time short, or making the light receiving area, namely, the junction capacity small, thereby to make the circuit time constant small.
However, as a rule, the light receiving sensitivity and the high speediness are in a relation of trade-off with each other.
Thus, the present invention has been accomplished in order to solve the above-mentioned problems, and an object thereof is to provide a device structure capable of making the light receiving sensitivity and the high speediness of the photodiode compatible with each other. In particular, an object thereof is to provide a photodiode that realizes high integration and low power consumption by accomplishing the light absorption layer of which the volume is smaller by two digits or more as compared with that of the conventional light absorption layer.
The foregoing problems are solved by a Schottky junction type photodiode having a conductive layer formed on a surface of a semiconductor layer, which is characterized in configured so that the light can be incident on the back side of the semiconductor layer, wherein a periodic structure in which the light incident from the back side of the semiconductor layer causes a surface plasmon resonance is made around a Shcottky junction of the foregoing photodiode.
Further, the foregoing problems are solved by a p-i-n type photodiode having a p-i-n type junction formed on a surface of a semiconductor layer, which is characterized in configured so that the light can be incident on the back side of the semiconductor layer, wherein a conductive layer is formed, and a periodic structure in which the light incident from the back side of the semiconductor layer causes a surface plasmon resonance is made around the p-i-n junction of the foregoing photodiode.
Further, the foregoing problems are solved by a photodiode having metal-semiconductor-metal junctions intervally arranged on a surface of a semiconductor layer, wherein the foregoing photodiode is configured so that the light can be incident on the back side of the semiconductor layer, and wherein a conductive layer is formed, and a periodic structure in which the light incident from the back side of the semiconductor layer causes a surface plasmon resonance is made around the metal-semiconductor-metal junction of the foregoing photodiode.
Further, the foregoing problems are solved by an optical communication device having the foregoing photodiode formed on a light receiving portion thereof.
Further, the foregoing problems are solved by an optical interconnection module, which includes: a Si substrate having the above-mentioned photodiode configured thereon; and an LSI electronic circuit formed monolithically with the foregoing photodiode on the foregoing Si substrate.
Making the response of the photodiode fast necessitates thinning the electric charge carrier absorption layer, thereby to make the carrier drift time short, and yet making the area of the electric charge carrier absorption layer small, thereby to make the circuit time constant small. However, conventionally, it has been difficult to make the sensitivity and the high speediness compatible.
On the other hand, the present invention focuses the optical energy into a region equal to or less than a wavelength in size, and efficiently converts this into a photocarrier, thereby to gain an electric signal. As a result, compatibility of the sensitivity and the high speediness is realized. And, the fast and high-efficiency photodetector is realized.
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1 semiconductor absorption layer
2 conductive film
3 lower electrode layer
4 buried oxide layer
5 load resistance
6 bias power source
7 oxide film
8 support substrate
9 periodic roughness
10 Schottky contact layer
11 metal film
12 n+ electrode layer
13 MSM electrode
14 forbidden band grating for preventing plasmon resonance from being generated
15 projected shape
16 grooved shape
17 periodic slit array or minute aperture array
18 Schottky contact layer
19 electrode pad
20 optical fiber
21 signal light
22 photodiode of the present invention
23 module frame
24 electric wiring
25 preamp carrier
26 chip carrier
27 VCSEL light source
28 electric wiring via for light source and modulation
29 electric wiring via for photodiode
30 LSI package
31 electric wiring layer for a light source and a modulation
32 electric wiring layer for photodiode
33 optical signal output fiber
34 optical signal input fiber
35 LSI-installed board
36 concave mirror
37 photodiode/light source-installed board
A first photodiode in accordance with the present invention is a Schottky junction type photodiode having a conductive layer formed on a surface of a semiconductor layer. The foregoing photodiode is configured so that the light can be incident on the back side of the foregoing semiconductor layer. The periodic structure in which the light incident from back side of the foregoing semiconductor causes the surface plasmon resonance is made around the Schottky junction of the foregoing photodiode.
A second photodiode in accordance with the present invention is a p-i-n type photodiode having a p-i-n type junction formed on a surface of a semiconductor layer. The foregoing photodiode is configured so that the light can be incident on the back side of the foregoing semiconductor layer. The conductive layer is formed, and the periodic structure in which the light incident from back side of the foregoing semiconductor layer causes the surface plasmon resonance is made around the p-i-n junction of the foregoing photodiode.
A third photodiode in accordance with the present invention is a photodiode having metal-semiconductor-metal junctions intervally arranged on a surface of a semiconductor layer. The foregoing photodiode is configured so that the light can be incident on the back side of the foregoing semiconductor layer. The conductive layer is formed, and the periodic structure in which the light incident from back side of the foregoing semiconductor layer causes the surface plasmon resonance is made around the metal-semiconductor-metal junction of the foregoing photodiode.
A fourth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring the above-mentioned third photodiode as follows. A gap between the metal-semiconductor-metal junctions arranged on the surface of the semiconductor layer is equal to or less than λ/n (where λ is a wavelength of the light incident from the back side of the semiconductor layer, and n is a refractive index of the light in the semiconductor layer). The foregoing metal layer is a layer in which a metal layer forming the Schottky junction with the foregoing semiconductor, and a layer comprised of conductive materials capable of inducing the surface plasmon have been laminated. Or, the foregoing metal layer is a metal layer capable of forming the Schottky junction with the foregoing semiconductor, and yet inducing the surface plasmon.
A fifth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring the above-mentioned third or fourth photodiode as follows. With respect to the foregoing metal-semiconductor-metal junction, at least one of the metal-semiconductor junctions facing each other is a Schottky barrier type junction.
A sixth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to fifth photodiodes as follows. The periodic structure for causing the surface plasmon resonance is a structure in which the conductive layers capable of inducing the surface plasmon have been laminated on the surface of the semiconductor layer having the roughness formed thereon.
A seventh photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to sixth photodiodes as follows. The periodic structure for causing the surface plasmon resonance is a structure in which the conductive layers capable of inducing the surface plasmon have been laminated on the surface of the dielectric layer having the roughness formed thereon.
An eighth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to seventh photodiodes as follows. The periodic structure for preventing the surface plasmon from being generated is made outside the periodic structure for causing the surface plasmon resonance.
A ninth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to eighth photodiodes as follows. The stepped structure for reflecting the surface plasmon is made outside the periodic structure for causing the surface plasmon resonance.
A tenth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring the above-mentioned ninth photodiode as follows. The stepped structure is comprised of a projected shape higher than λ/nd (where nd is a refractive index of the light in the semiconductor layer or the dielectric layer neighboring the conductive film, and λ is a wavelength of the light).
An eleventh photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring the above-mentioned ninth or tenth photodiode as follows. The stepped structure is comprised of a grooved shape deeper than λ/nd (where nd is a refractive index of the light in the semiconductor layer or the dielectric layer neighboring the conductive film, and λ is a wavelength of the light).
A twelfth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned ninth to eleventh photodiodes as follows. The stepped structure is comprised of a shape in which bores formed on the conductive material, of which the diameter is equal to or less than the wavelength of the incident light, have been arrayed.
A thirteenth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned ninth to twelfth photodiodes as follows. The stepped structure is comprised of a shape in which slits formed on the conductive material, of which the width is equal to or less than the wavelength of the incident light, have been arrayed.
A fourteenth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned ninth to thirteenth photodiodes as follows. The conductive layer is made of at least one metal (or alloy) selected from a group consisting of Al, Ag, Au, and Cu.
A fifteenth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to fourteenth photodiodes as follows. The junction area in the photodiode is 100 square microns or less.
A sixteenth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to fourteenth photodiodes as follows. The junction area in the photodiode is 10 square microns or less.
A seventeenth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to fourteenth photodiodes as follows. The junction area in the photodiode is one square micron or less.
An eighteenth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to seventeenth photodiodes as follows. The thickness of the semiconductor absorption layer in the photodiode is 1 μm or less.
A nineteenth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to seventeenth photodiodes as follows. The thickness of the semiconductor absorption layer in the photodiode is 200 nm or less.
A twentieth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to nineteenth photodiodes as follows. The semiconductor absorption layer in the photodiode is made of at least one member selected from a group consisting of Si, SixGel-x (where x is a positive number smaller than 1), Ge, GaN, GaAs, GaInAs, GaInP, and InP.
A twenty-first photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to twentieth photodiodes as follows. The semiconductor absorption layer in the photodiode is made of at least one member selected from a group consisting of Ge and SixGel-x (where x is a positive number smaller than 1). And, the layer that is configured of an alloy of Ni and Ge is formed between the foregoing semiconductor absorption layer and conductive layer.
A twenty-second photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to twenty-first photodiodes as follows. The photodiode is configured on an optical waveguide comprised of the semiconductor.
A twenty-third photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to twenty-first photodiodes as follows. The photodiode is configured so that it can receive the light coming from the optical waveguide formed in the substrate side, which has been reflected by the mirror.
A twenty-fourth photodiode in accordance with the present invention is a photodiode that is obtained by particularly configuring any of the above-mentioned first to twenty-third photodiodes as follows. The substrate of the photodiode is configured of the material transparent to the incident light.
Furthermore, the photodiode in accordance with the present invention will be explained in details.
The first photodiode in accordance with the present invention is a Schottky junction type photodiode having the conductive film formed on the surface of the semiconductor layer. And, the first photodiode is configured so that the light can be incident on the back side of the semiconductor layer. The circumference of the foregoing Schottky junction has the periodic structure in which the light incident from the back side of the foregoing semiconductor layer causes the surface plasmon resonance. This leads to an increase in an efficiency of the optical coupling with the light incident from the back side of the semiconductor layer. This structure is shown in
And, the light incident from the back side of the support substrate 8 is converted into the surface plasmon due to the periodic roughness structure 9 for causing the surface plasmon resonance, and is focused into the Schottky junction existing in the central portion. In addition hereto, the circumference of the semiconductor absorption layer 1 has been established as a semiconductor layer (or dielectric layer) of which the refractive index is lower than that of the semiconductor absorption layer 1. With this refractive index difference, the light is entrapped. That is, this entrapment effect of the light causes the optical power incident in the semiconductor absorption layer 1 to be localized in the minute Schottky junction region. As a result, the efficient photoelectric conversion is attained in the semiconductor absorption layer having a very small volume.
In this photodiode, the periodic roughness structure (the periodic structure in which the light incident from the back side of the semiconductor layer (semiconductor absorption layer) 1 causes the surface plasmon resonance) 9 is made around the Schottky junction.
Herein, in the case of employing Ge or InGaAs compound semiconductor for the material of the semiconductor layer (semiconductor absorption layer) 1, directly laminating Ag and Au as a conductive film does not lead to formation of the Schottky junction. Thus, a leak current becomes large. Thereupon, in the case of employing these materials, for example, a Ge semiconductor absorption layer, the metal layer (or Si layer) such as a Ni layer, in which the junction having high Schottky barrier energy is gained, is desirably inserted into the interface between the conductive film and the semiconductor absorption layer (for example, the Ge layer). In the case of employing InGaAs as a material of the semiconductor absorption layer 1, inserting an InALAs semiconductor layer makes it possible to form the Schottky junction in which a leak current is few.
The second photodiode in accordance with the present invention is a photodiode having a p-i-n type junction formed on the surface of a semiconductor layer. And, the second photodiode is configured so that the light can be incident on the back side of the foregoing semiconductor layer. The conductive layer is formed, and the periodic structure 9 in which the light incident from the back side of the foregoing semiconductor layer causes the surface plasmon resonance is made around the p-i-n junction of the foregoing photodiode. This periodic structure 9 is a periodic roughness structure in which the light incident from the back side (n+ electrode layer 12: support substrate 8 side) of the semiconductor layer (semiconductor absorption layer: i layer) 1 causes the surface plasmon resonance. This leads to an increase in an efficiency of the optical coupling with the light incident from the back side of the semiconductor layer. This structure is shown in
The third photodiode in accordance with the present invention is a photodiode having metal-semiconductor-metal (MSM) junctions intervally arranged on the surface of a semiconductor layer. And, the third photodiode is configured so that the light can be incident on the back side of the semiconductor layer. The conductive layer is formed, and the periodic structure in which the light incident from the back side of the foregoing semiconductor layer causes the surface plasmon resonance is made around the foregoing MSM junction. This leads to an increase in an efficiency of the optical coupling with the light incident from the back side of the semiconductor layer. This structure is shown in
Conventionally, the metal electrode formed on the semiconductor surface causes the light receiving sensitivity to lower because it blocks the light receiving surface of the photodiode. Also even though the electrode interval for causing the surface plasmon resonance is established, the region in which optical electric field intensity is strong exists outside the semiconductor. Thus, the photocarrier cannot be generated efficiently.
On the other hand, by making a configuration so that the light is incident from the back side of the semiconductor absorption layer 1, and the periodic roughness structure 9 exists around the MSM junction (MSM electrode 13), the light is effectively focused into the minute MSM junction. In addition hereto, the circumference of the semiconductor absorption layer 1 has been established as a semiconductor layer (or dielectric layer) of which the refractive index is lower than that of the semiconductor absorption layer 1. With this refractive index difference, the light is entrapped. With this, the very efficient photoelectric conversion can be attained.
Further, when the Schottky junction is formed between the metal and the semiconductor, a depletion layer region of 200 nm or more is formed notwithstanding the zero bias in a situation of the doping concentration being 1×1015 to 1×1016 cm−3. Thus, making the distance between the electrodes small enables a fast and high-sensitivity photodiode operation at a low bias voltage. At this time, it is thinkable that, in the case of having set the thickness of the semiconductor absorption layer to 200 nm or less, the drift time of the photocarrier between the electrodes is several picoseconds also in the semiconductor material (for example, Si) of which a photocarrier mobility is 107 cm/s. And, also in the case of having set the thickness of the semiconductor absorption layer to 1 μm or less, the drift time of 20 ps or less is enabled. Further, when the distance between the MSM electrodes is set to 100 nm or so, the junction electric capacity becomes 10 fF or less in the case of set the MSM junction area to 10 square microns or less. The junction electric capacity becomes 100 fF or less also in the case of set the MSM junction area to 100 square microns or less. That is, when it is assumed that the load resistance is 50 Ω, the circuit time constant is 1 ps with the former, and 10 ps with the latter. Thus, the very fast response is realized.
Each of
Additionally, a phase relation with the resonance mode of the surface plasmon being reflected is of importance. For example, by causing the resonance mode and the reflection phase to coincide with each other, the maximum light receiving sensitivity is attained.
The dispersion relation of the surface plasmon is expressed with the following equation 1.
k
SP
=ω/c{(εm·εd)/(εm+εd)}1/2 [Equation 1]
Where εm and εd are a permittivity of the metal for generating the surface plasmon, and the dielectric substance neighboring it, respectively.
In addition hereto, a propagation length of the surface plasmon is expressed with the following equation 2.
L
SPP
=c/ω{(εm′+εd)/εm′}3/2·εm′2/εm″ [Equation 2]
Where a complex permittivity εm of the metal is expressed as εm′+iεm″.
That is, the optical loss of the surface plasmon largely depends upon a ratio of the square of an imaginary part to an actual part of a permittivity of the metal film. Thus, the conductive layer of the present invention is desirably comprised of at least one metal selected from a group consisting of Al, Ag, Au, and Cu (or, an alloy selected from them). Further, it is very important from a viewpoint of a reduction in a propagation loss of the surface plasmon to make the random roughness of the metal surface small. Thus, the underlayer such as Ta, Cr, Ti, and Zr is desirably formed. Or, the alloying of the metal film by adding the element such as Nb by a very small amount is also effectual.
An intensity distribution of the proximity field light due to the surface plasmon is influenced and changed by the periodic roughness structure, the refractive index of the neighboring dielectric layer, an arrangement of the MSM electrodes, and the refractive index or an absorption coefficient of the semiconductor absorption layer.
And, adopting the structure of the present invention that causes the optical energy to be localized in a very tiny region of the semiconductor makes it possible to generate the electron·positive hole pairs (photocarriers). Thus, causing the depletion region being formed in the semiconductor absorption layer due to the Schottky junction, and the region in which the photocarrier is generated due to the proximity field to coincide with each other enables the efficient photocarrier formation and the localized movement of the photocarrier to be realized. As a result, the photodiode having a high quantum efficiency and a fast response characteristic is obtained.
In the case of utilizing the p-i-n junction structure, the photocarrier can be efficiently generated also in a minute region of 1 μm owing to the focusing of the surface plasmon caused by the periodic roughness formed in the junction circumference, and the light entrapment effect utilizing a difference of the refractive index with the dielectric layer neighboring the semiconductor absorption layer.
At this time, the Schottky junction region or the p-i-n junction region for generating and sweeping the photocarrier can be formed so that its size is 10 square microns or less. As a result, the electric capacity in the junction can be lowered to a very small level. Thus, the circuit time constant in the case of performing a high-frequency operation for the photodiode can be lowered to several picoseconds or less, which realizes a high-frequency operation of several tens of Giga Hertz or more.
An optical waveguide of which a difference of the refractive index between a core and a clad is 5% or more can be utilized in order to entrap the light into a region equal to or less than a wavelength (or 10 square microns or less) in size. Such a channel-type optical waveguide has a structure in which the circumference of the core is surrounded by the medium of which the refractive index is smaller than that of the core. And, a difference of the refractive index between the core and the clad layer causes the light to propagate while repeating total internal reflection. In this case, the larger a difference of the refractive index between the core and the clad layer, the more strongly the light is entrapped into the core. Thus, even though the waveguide is abruptly bent at a small curvature, the light is wave-guided along it. And, with the case that a difference of the refractive index is 5% or more, a light spot diameter of 10 μm or less can be realized. In addition hereto, with the case that a difference of the refractive index between the core and the clad is approximately 10% to 40%, a light spot diameter equal to or less than a wavelength can be realized.
With the structure in which a waveguide core of which the sectional size is approximately 0.3 μm×0.3 μm is made of Si (the refractive index is approximately 3.4), and the circumference (clad layer) of this waveguide core is shrouded in SiO2 (the refractive index is approximately 1.5), a mode size of the light is reduced to a size almost identical to that of the waveguide core. The wave-guiding loss due to the light absorption occurs in the Si waveguide with the case that the wavelength of the light being wave-guided is 850 nm or so. Thereupon, employing, as a waveguide core, SiON etc. exhibiting an optical transmission property that the loss can be ignored over a wide range of wavelengths, and shrouding the circumference thereof in the clad comprised of SiO2 yields the refractive index difference of 5% or more. In this case, the light spot diameter becomes approximately 1 to 4 μm because entrapment of the light becomes weaker as compared with the case of the semiconductor core (Si).
In the case of leaping up an optical path of the optical signal light coming from the optical waveguide having a strong light entrapment property in a vertical direction by utilizing a 45° mirror (or diffraction grating), and optically coupling the optical signal light in the above-mentioned photodiode structure, the optical coupling in a very tiny region is enabled. As a result, a compatibility of the high sensitivity and the high speediness is realized.
The wavelength region of the light in which the present invention is available extends over a wide wavelength range including visible light, near infrared-ray light, and infrared-ray light. A fast photodetector for efficiently generating the photocarrier and gaining the electric signal in the region equal to or smaller than the wavelength is obtained owing to the metal periodic structure for inducing the surface plasmon resonance, the structure in which the semiconductor absorption layers for efficiently entrapping and transmitting the light have been laminated, and besides them, the regulation of the refractive index of the dielectric layer neighboring the semiconductor absorption layer.
Hereinafter, specific examples will be explained while a reference to the accompanied drawings is made.
The Si Schottky type photodiode of the present invention has a metal-semiconductor Schottky junction formed on one part of a semiconductor absorption layer 1 of which the surface has been insulated, for example, SOI (Silicon-on-Insulator). Conductive films 2 for causing the surface plasmon are lamination-formed around this Schottky junction. A periodic roughness structure (periodic roughness structure in which the light incident from the back side (support substrate 8 side) of the semiconductor absorption layer 1 causes the surface plasmon resonance) 9 is made around the Schottky junction and yet in the lower side of the lamination-formed conductive film 2. Additionally, in
The conductive film 2 formed for inducing the surface plasmon is configured of the metals such as Al, Ag, Au and Cu (or the alloy having at least one member of the foregoing metals as an essential component element). So as to form the Schottky junction, a substrate film comprised of the metals such as Cr, Ta, and Ni may be formed.
An n+ Si layer of which a concentration of a dopant such as P is 1×1020 cm−3 or more can be used as a substrate in the lower electrode layer 3. In this case, it is necessary to epitaxial-grow an n− Si layer, being the semiconductor absorption layer (light absorption layer) 1, on an n+ Si layer. However, the dopant concentration of the light absorption layer becomes high due to a thermal diffusion of the dopant element when a growth temperature is raised to 800° C., or more. And, the depleting voltage is augmented, and the thickness of the depletion layer that is obtained at the time of forming the Schottky junction becomes thin. That is, fast driving at a low voltage becomes difficult to attain. Thus, forming the thin n− Silayer (semiconductor absorption layer (light absorption layer) 1 on the n Si layer necessitate a technology of the epitaxial growth at a low temperature equal to or less than 600° C.
In this example, so as to make the light incident from back side of the substrate 8, the Si semiconductor support substrate 8 was layer-thinned to approximately 50 to 100 μm with CMP (chemical mechanical polishing) or the like. In addition hereto, the support substrate 8 in the back side of the photodiode was dissolved, and removed with a mixture solution of a hydrofluoric acid and nitric acid so as to form a window for making the light incident, of which a diameter was approximately 10 to 50 μm.
The light incident from the back side of the support substrate 8 is converted into the surface plasmon due to the periodic roughness structure 9 for causing the surface plasmon resonance, and focused into the Schottky junction existing in the central portion. Further, the circumference of the semiconductor absorption layer 1 is configured of the semiconductor layer (or the dielectric layer (oxide film 7)) of which the refractive index is lower than that of the semiconductor absorption layer 1. Thus, the light power incident on the semiconductor absorption layer 1 is localized in the minute Schottky junction region owing to the entrapment effect caused by the refractive index difference. Thus, the efficient photoelectric conversion is attained in the semiconductor absorption layer having a very small volume.
Next, the manufacturing method will be explained by exemplifying the case of the Si semiconductor.
At first, as shown in
Next, as shown in
Thereafter, as shown in
By the way, the metal layer (conductive film) for forming the Schottky junction is film-formed on the surface of the mesa shape. At this time, as shown in
And, as shown in
The Schottky type photodiode of the present invention has a metal-semiconductor Schottky junction formed on one part of the semiconductor absorption layer 1 of which the surface has been insulated, for example, SOI (Silicon-on-Insulator). An appropriate buffer layer such as Si0.5 Ge0.5, of which the thickness was approximately 10 nm was formed on the SOI layer having a thickness of 100 nm or less with a gas source MBE method because the Ge layer was not lattice-matched to the Si layer. An n− Gelayer was grown on the buffer layer, thereby to form a high-quality Ge semiconductor absorption layer 1 having a low density of penetration transition. Additionally, the Ni substrate layers have been laminated with an evaporation method or the like so as to form the Schottky junction. In
The conductive films 2 for causing the surface Plasmon are lamination-formed around the Schottky junction. And yet, the periodic roughness structure 9 in which the light incident from the back side (support substrate 8 side) of the semiconductor absorption layer 1 causes the surface plasmon is made.
The conductive film 2 formed for inducing the surface plasmon is configured of the metals such as Al, Ag, Au and Cu (or the alloy having at least one member of the foregoing metals as an essential component element). Additionally, the substrate layer formed for forming the Schottky junction may be configured of Cr and Ta. The lower electrode layer 3, which is a layer configured by performing a P (phosphoric)-doping process for the SOI layer, being a substrate of a Ge growth layer, has a sufficient conductivity.
In this example, so as to make the light incident from the back side of the support substrate 8, the Si semiconductor support substrate 8, similarly to the case of the first example 1, is layer-thinned to approximately 50 to 100 μm with the CMP or the like in the case of making the light having a wavelength of 1 μm or less, which was influenced by the light absorption of Si, incident. And, the support substrate 8 in the back side of the photodiode was dissolved, and removed by employing a mixture solution of a hydrofluoric acid and nitric acid so as to form a window for making the light incident, of which the diameter was approximately 10 to 50 μm.
The light incident from the back side of the support substrate 8 is converted into the surface plasmon due to the periodic roughness structure 9 for causing the surface plasmon resonance, and focused into the Schottky junction existing at the central portion. Further, the circumference of the semiconductor absorption layer 1 is configured of the semiconductor layer (or the dielectric layer (oxide film 7)) of which the refractive index is lower than that of the semiconductor absorption layer 1. Thus, the light power incident on the semiconductor absorption layer 1 is localized in the minute Schottky junction region owing to the entrapment effect caused by a difference of the refractive index. With this, the efficient photoelectric conversion is attained in the semiconductor absorption layer having a very small volume.
It is possible in an optical communication wavelength band of which the wavelength is 1.3 to 1.6 μm to handle the Si support substrate 8 as a transparent substrate. Thus, only converting the substrate backside into a mirror surface without performing a working process such as a process of removing the support substrate yields the quantum efficiency of approximately 60%, and the sufficient light receiving sensitivity is attained.
The p-i-n type photodiode of the present invention has a structure in which the p-i-n type junction has been lamination-formed on one part of a semiconductor absorption layer 1 of which the surface has been insulated, for example, the SOI (Silicon-on-Insulator) with CVD (Chemical Vapor Deposition) or the like. A conductive film 2 for causing the surface plasmon is lamination-formed around the p-i-n junction. And yet, the periodic roughness structure (the periodic roughness structure in which the light incident from the back side (support substrate 8 side) of semiconductor absorption layer 1 causes the surface Plasmon resonance) 9 is made. Additionally, in
The conductive film 2 for causing to the surface plasmon is lamination-formed on a p+ electrode layer 11 on the semiconductor absorption layer 1. Thus, the conductive film 2 and the p electrode layer 11 are electrically connected to each other. Further, the conductive film 2 formed for inducing the surface Plasmon is configured of the metals such as Al, Ag, Au and Cu (or the alloy having at least one member of the foregoing metals as an essential component element).
The MSM type photodiode of the present invention has a structure in which the metal-semiconductor-metal (MSM) junction has been formed on one part of the semiconductor absorption layer 1 of which the surface has been insulated, for example, the SOI (Silicon-on-Insulator). And, a gap between the metal electrodes is defined to be a distance smaller than λ/n (λ: a wavelength of the incident light, and n: an optical refractive index of the semiconductor layer). This yields a structure in which the light incident from the back side of the semiconductor absorption layer 1 is entrapped into the semiconductor absorption layer 1. Additionally, in
An MSM electrode 13 is configured of the metals such as Al, Ag, Au and Cu (or the alloy having at least one member of the foregoing metals as an essential component element) so as to induce the surface plasmon. Additionally, so as to form the Schottky junction, a substrate film comprised of the metals such as Cr, Ta, and Ni may be formed. Further, it is also possible to form an Ohmic junction by using Ti etc. as an electrode film facing the MSM electrode that is employed for a substrate film.
The conductive film 2, which can cause the surface plasmon, is formed adjacently to the circumference of the MSM junction. And, the periodic roughness structure 9 is made so as to causes the surface plasmon resonance.
The quantum efficiency in this case is shown in Table-1.
The quantum efficiency that is two times to three times or so as large as that of the case of only the periodic roughness structure 9 is gained in any structure. And, it can be seen from it that the surface plasmon is efficiently reflected, which realizes the focusing into the Schottky junction, and furthermore the localization of the light energy into the semiconductor absorption layer.
In this example, the photodiode is a Schottky type photodiode that uses the substrate obtained by epitaxial-growing the Ge film on the SOI substrate, and has a Ni/Au electrode formed thereon. It is a photodiode having the conductive film having the roughness structure (the roughness structure comprised of Ag (or Au), which enables the light coupling and the focusing owing to the surface plasmon), formed around this photodiode. And, with the case of employing the conductive film (metal film) for transmission by the near infrared-ray light having a wavelength of 1.55 μm, the roughness period of the roughness structure thereof becomes approximately 1.2 μm, and with the case of employing the conductive film having the eight-period roughness existing on a concentric circle, the diameter of its periphery becomes approximately 20 μm. The depth of the roughness at this time was set to 0.1 to 0.4 μm or so. The diameter of the Schottky junction was set to 0.3 to 0.7 μm or so. The photodiode is installed in a chip carrier 26. And, it is optically coupled by an optical fiber 20 and a lens, and further is electrically connected to a following preamp IC 25.
Commonly, in the optical receiver module of 40 G bps, a side incidence waveguide type photodiode is, in many cases, employed for the photodiode being installed therein. The reason is that, when the absorption layer is made thin so as to reduce an electric charge carrier drift time, the high quantum efficiency cannot be attained in a surface incidence type photodiode in which the light is made incident on the surface of the semiconductor. On the other hand, the waveguide type photodiode absorbs the light in an intra-plane direction of the absorption layer, thereby allowing the high quantum efficiency to be attained with the electric charge carrier drift time kept short. However, the thickness of the semiconductor absorption layer, commonly, is 1 μm or less in a waveguide type element for 40 G bps. A coupling tolerance of a position with the optical fiber in this case needs to be kept at a level of ±1 μm or so, which poses a big problem with both of a package design and a manufacturing cost.
On the other hand, the photodiode in accordance with the present invention has a an active effective-diameter of 20 μm. For this, the coupling tolerance can be enlarged to ±2 μm or more. As a result, the light coupling can be carried out only with simple lens coupling. With this, a low manufacturing cost of the receiver module for optical transmission is enabled.
Incidentally, in the optical receiver module of 40 G bps in accordance with the present invention shown in
In
A photodiode wiring layer 32 is electrically connected to the photodiode wiring via 29 of the LSI. Herein, the other well-known methods in which a planar optical waveguide is used instead of the optical fiber can be employed for inputting the optical signal. Further, a focusing mechanism such as a convex lens can be employed instead of the concave mirror. Further, the preamp for amplifying the electric signal can be placed in the way to the photodiode wiring layer immediately after the photodiode.
The electric signal coming from the LSI, which goes through an electric wiring layer 31 for a light source and a modulation from an electric signal via 28 for a light source and a modulation, is converted into the optical signal by a VCSEL (Vertical-Cavity Surface-emitting Laser) optical source 27 provided, with an electric modulation mechanism. The optical signal is reflected at the concave mirror 36, and sent to an optical signal output fiber 33. The VCSEL optical source 27 provided with the electric modulation mechanism can be replaced with the other well-known mechanisms for modulating the light by electricity, for example, a Mach-Zehnder type modulator for modulating the light coming from an external light source with an electro-optical effect or a thermo-optical effect.
Herein, in the general LSI intra-chip interconnect, in the case of aiming at a fast operation of 20 GHz or more, the compound semiconductor material such as InGaAs grown on the InP substrate is employed for the photodiode being installed therein so as to make the response fast. However, the compound semiconductor has poor matching to the Si semiconductor element in terms of the manufacturing process, and hence becomes costly.
On the other hand, the manufacturing cost of the photodiode of the present invention can be reduced because Si can be employed therefor. And, the fast photoelectric conversion operation of approximately 40 GHz was confirmed in the optical interconnect in accordance with the present invention shown in
Number | Date | Country | Kind |
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2006-342336 | Dec 2006 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2007/072904 | 11/28/2007 | WO | 00 | 6/17/2009 |