JP5513659B1 - Photonic crystal photodetector - Google Patents

Abstract

An optical detector having a small size, a high response speed, and a high detection efficiency is provided.
In a photonic crystal photodetector in which a light absorption region is embedded in a photonic crystal in which a first medium and a second medium having different refractive indexes are regularly arranged, the light absorption region An input waveguide is provided in which the second medium on the optical axis is replaced with the first medium, and the width of the input waveguide is wider than the width of the light absorption region.
[Selection] Figure 2

Description

  The present invention relates to a photonic crystal photodetector, and more specifically, a light absorption region is embedded in a photonic crystal in which a first medium and a second medium having different refractive indexes are regularly arranged. The present invention relates to a photonic crystal photodetector.

  In an optical communication system, there is a photodetector as an element for converting an optical signal into an electric signal. In a photodetector to which optical interconnection, which is a technology for connecting silicon chips (electrical integrated circuits) with an optical waveguide, is applied, a small size, improved response speed, high detection efficiency, and the like are desired. As a general structure of the photodetector, there is an optical waveguide type structure having a pin structure. This structure absorbs incident light by forming a light absorption layer in the optical waveguide, and is generated by light absorption by applying a reverse bias voltage to pin junctions formed on both sides of the optical waveguide. The extracted carrier is extracted and converted into an electric current. In recent years, various optical elements processed into sub-micrometer sizes can be realized by improving the manufacturing technique of optical elements, and this is called nanophotonics technology. Even in the photodetector, it is expected to realize minute and high performance by using nanophotonics technology.

  One of the nanophotonic technologies is a fine structure called a photonic crystal. In particular, a structure in which circular holes are periodically formed in a semiconductor thin film (slab) having a thickness of several hundred nm is called a photonic crystal slab structure. Various optical elements are realized in very small sizes on the basis of optical waveguides and optical resonators using this structure. As a photodetector using a photonic crystal slab structure, there is a report example using a photonic crystal optical resonator made of silicon (see, for example, Non-Patent Document 1). This photodetector forms an optical resonator by slightly moving the position of the circular hole of the photonic crystal waveguide locally, and utilizes the nonlinear optical absorption effect in the optical resonator.

  However, since the absorption efficiency is small for light having a wavelength of 1.55 μm, which is often used for optical communication, the detection efficiency is about 10% at the maximum. In addition, since the resonator structure is used, the Q value of the photodetector (an index indicating a small amount of light leakage, which is proportional to the light capture time in the resonator and the light allowed by the resonator) (It is inversely proportional to the wavelength width.) Has a problem that the detectable wavelength range is as narrow as 10 pm or less.

  For light with a wavelength of 1.55 μm, one solution is to use InGaAs with a high absorption coefficient in the absorption layer. In general, a photodetector of the 1.55 μm band is often used. Yes. However, when a photonic crystal slab structure is used, it is necessary to separate and join a waveguide region that does not absorb light to enter and a light absorption region made of InGaAs. In recent years, a technique for embedding a minute InGaAsP in a photonic crystal waveguide using InP as a material has been developed (see, for example, Patent Document 1 and Non-Patent Document 2). In such a buried structure, the effect of confining light in the portion where InGaAsP is buried is compatible with the effect of confining the generated carriers due to the band gap difference due to the difference in refractive index between InP and InGaAsP. Therefore, it is effective to manufacture various optical elements based on the photonic crystal waveguide by using the portion where InGaAsP is embedded as the operation region and the InP portion connected thereto as the optical waveguide region. Such a structure is considered to be effective for manufacturing a minute photodetector.

International Publication No. 2011/027555

T. Tanabe, H. Sumikura, H. Taniyama, A. Shinya, and M. Notomi, "All-silicon sub-Gb / s telecom detector with low dark current and high quantum efficiency on chip," Applied Physics Letters, vol. 96, p. 101103, 2010 S. Matsuo, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato, Y. Kawaguchi, and M. Notomi, "High-speed ultracompact buried heterostructure photonic-crystal laser with 13 fJ of energy consumed per bit transmitted, "Nature Photonics, vol. 4, pp. 648-654, 2010

  In general, the response speed of an optical waveguide detector having a pin structure is often limited by a CR time constant and a carrier drift time. As for the former factor, the response speed is limited by the capacitance C and resistance R of the entire photodetector, and it is preferable that one or both of C and R is low. In this structure, since the depletion layer formed in the photodetector serves as a capacitive component, it is desirable to reduce the capacitance C by shortening the total length of the photodetector or expanding the i region. As a result, the CR time constant can be reduced and the response speed can be improved. The latter factor is that the response speed is limited by the carrier drift time when the generated carriers are extracted. In contrast to the CR time constant, by narrowing the i region, the carrier drift time can be reduced and the response speed can be improved. When the buried photonic crystal waveguide described in Patent Document 1 is used, the width of the i region can be reduced due to the confinement effect of light and carriers, so that the carrier drift time is reduced and the response speed is improved. be able to.

  When the buried structure described above is applied to a photodetector, the InGaAs buried region has a higher refractive index than the InP waveguide region. Since the two waveguide wavelength bands are different, a band that cannot be used simply by connecting the waveguides is generated.

  From this, the first problem to be solved is to find a method for matching the waveguide band and absorbing light with high efficiency. Further, when the width of the i region is narrowed, as described above, there is a concern that the response speed may decrease due to an increase in the CR time constant. As a method for preventing this, a method of reducing the capacitance C by shortening the total length of the detector can be considered.

  On the other hand, if the total length is too short, light is not sufficiently absorbed during one round trip, resulting in a trade-off that the light detection efficiency is reduced. Therefore, the second problem to be solved is to find a method for maintaining high absorption efficiency when the total length is shortened.

  An object of the present invention is to provide a photodetector having a small size, a high response speed, and a high detection efficiency.

  In order to achieve such an object, the first embodiment provides a light absorption region in a photonic crystal in which a first medium and a second medium having different refractive indexes are regularly arranged. Embedded in the photonic crystal photodetector, comprising an input waveguide in which the second medium on the optical axis of the light absorption region is replaced with the first medium, and the width of the input waveguide is: It is characterized by being wider than the width of the light absorption region.

  According to a second embodiment, in the first embodiment, at the incident end of the light absorption region of the input waveguide, the sizes of the two second media sandwiching the input waveguide are the second surroundings. It is characterized by being larger than the size of the medium.

  According to a third embodiment, in the first or second embodiment, the second medium on the optical axis opposite to the input waveguide with respect to the light absorption region is used as the first medium. It is further characterized by further comprising a substituted output waveguide.

  As described above, according to the first embodiment, by making the width of the input waveguide wider than the width of the light absorption region, the waveguide band in the input waveguide and the waveguide band in the light absorption region can be reduced. Since the overlapping and the low group velocity band can be used, the optical path length is effectively extended and the absorption efficiency can be increased.

  According to the second embodiment, the size of the two second mediums sandwiching the input waveguide is made larger than the size of the surrounding second medium to constitute a resonator, thereby detecting light. Even when the total length of the vessel is shortened, high detection efficiency and high-speed response are possible.

  According to the third embodiment, generation of return light to the input waveguide can be prevented, so that detection efficiency can be increased.

It is a figure which shows the absorption efficiency with respect to the group refractive index of a light absorption area | region, and the reflectance of a mirror. It is a figure which shows the photodetector using the photonic crystal optical waveguide concerning the 1st Embodiment of this invention. It is a figure which shows the simulation result of the waveguide band structure of a photonic crystal optical waveguide. It is a figure which shows the 1st Example of an electrical junction part. It is a figure which shows the 2nd Example of an electrical junction part. It is a figure which shows the 3rd Example of an electrical junction part. It is a figure which shows the 4th Example of an electrical junction part. It is a figure which shows the evaluation result by the two-dimensional model of 1st Embodiment. It is a figure which shows the manufacturing method of the photodetector of 1st Embodiment. It is a figure which shows the photodetector using the photonic crystal optical waveguide concerning the 2nd Embodiment of this invention. It is a figure which shows the photodetector using the photonic crystal optical waveguide concerning the 3rd Embodiment of this invention. It is a figure which shows the evaluation result by the two-dimensional model of 3rd Embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In order to solve the above-described problem, a low group velocity band in the InGaAs buried region is used as a first means. By utilizing the effect of reducing the group velocity of light in the photonic crystal waveguide, the optical path length can be effectively extended and the absorption efficiency can be increased. As a second means, a resonator structure is used. By forming a reflection mirror at the incident end of the light absorption region made of InGaAs, the light absorption region is used as an optical resonator, and light having a wavelength matching the resonance condition is circulated to increase absorption efficiency. By these means, even when the total length of the photodetector is shortened, high detection efficiency and high-speed response are possible.

Referring to FIG. 1, the absorption efficiency is calculated using a simple model of the light absorption region. FIG. 1A is a simple model of a light absorption region, and mirrors with amplitude reflectances r ₁ and r ₂ are attached to both ends of the light absorption region of the full length L. The absorption efficiency A for light incident from a mirror with an amplitude reflectance r ₂ is expressed by the following equation.

Here, R is the optical power reflectivity, Γ (= 0.5) is the optical confinement coefficient in the InGaAs buried region, α (= 10000 cm ⁻¹ ) is the optical power absorption coefficient of InGaAs, and _ng is the group refraction of the waveguide. The rate, n (= 3.5) is the material refractive index. It was assumed that r ₂ = 1. Φ is a phase change when light propagates through the length L, and is an integral multiple of π under resonance conditions.

FIG. 1B shows the absorption efficiency with respect to the group refractive index _ng . It is assumed that r ₁ = 1 and the total length L is changed to 1, 3, 10 μm. When L = 10 μm is sufficiently long, the absorption efficiency is 100% regardless of the group refractive index _ng . When the total length L becomes shorter, the absorption efficiency decreases when the group refractive index _ng is small. On the other hand, by increasing the group refractive index _ng , the absorption efficiency increases. For example, at L = 1 μm, it approaches 100% when _ng = 20. The above shows the effect of increasing the absorption efficiency even with a short detector by increasing the group refractive index of light (decreasing the group velocity).

FIG. 1C shows the absorption efficiency with respect to the reflectance r ₁ . It is assumed that n _g = n = 3.5 and the total length L is changed to 0.4, 1, 3, 10 μm. From this, when L = 10 μm is sufficiently long, the absorption efficiency is maximized when the reflectance r ₁ is zero. On the other hand, when the total length L is shortened, the absorption efficiency is maximized by providing the resonator with a finite reflectance r ₁ . For example, r ₁ = about 67% when L = 0.4 μm, and r ₁ = about 39% when L = 1 μm. In fact, gamma, alpha, n _g, the value of r ₁ varies depending on the value of _n, in the formula (1), the absorption efficiency of the left side with respect to a given value of each such that 1 r ₁ Need to be set. The above shows the effect of increasing the absorption efficiency even with a short detector by forming an optical resonator.

(First embodiment)
(Photodetector structure)
FIG. 2 shows a photodetector using the photonic crystal optical waveguide according to the first embodiment of the present invention. 2A is a top view, FIG. 2B is a cross-sectional view of the optical waveguide in the optical axis direction, and FIG. 2C is a cross-sectional view including a light absorption region perpendicular to the optical axis. In the photonic crystal photodetector 10, a light absorption region (embedded region) 12 is embedded in a photonic crystal 11. In the photonic crystal 11, a first medium (InP) and a second medium (air) having different refractive indexes are regularly arranged. Specifically, the circular holes 15 are arranged in a triangular lattice pattern in a medium made of InP. In order to extract carriers generated by light absorption in the buried region 12 made of InGaAs, a doped region is formed in the vicinity of the buried region 12. Specifically, an n-doped region 13 is formed in the vicinity of one side parallel to the optical axis of the buried region 12, and a p-doped region 14 is formed in the vicinity of the other opposite side. The upper and lower sides of the photonic crystal 11 are air layers. In addition, on the optical axis of the buried region 12, the second medium is replaced with the first medium to form a photonic crystal optical waveguide (input waveguide) connected to the buried region 12.

  When the period of the circular hole is a [nm], the diameter of the circular hole is 0.48a, the thickness of the InP photonic crystal 11 is 0.6a, the height of the InGaAs buried region 12 is 0.36a, and the width is 0.71a. And The length of the InGaAs buried region 12 is 1a to 20a.

Here, the width of the optical waveguide, which is an important design point, will be described. FIG. 3 shows a simulation result of the waveguide band structure of the photonic crystal waveguide. FIG. 3A shows a waveguide band when a photonic crystal in which circular holes are uniformly arranged is used as an optical waveguide without forming only one row of circular holes on the optical axis of the buried region 12. The structure is shown. The width of the optical waveguide at this time is defined as W _0. The frequency region indicated by B1 is an even mode waveguide band in the input waveguide, and the frequency region indicated by B2 is an even mode waveguide band in the buried region.

When the input waveguide and the buried region have the same width (W ₀ ), the waveguide band positions of both are shifted. That is, since there are few portions where B1 and B2 overlap, there is a problem that the frequency band in which light cannot enter the embedded region is wide. The low frequency side of the waveguide band in the buried region is a frequency band where the group velocity of light is low, but in this structure, the low frequency side cannot be used.

In order to solve the above problem, the width of the input waveguide in the InP photonic crystal 11 is increased, and conversely, the width of the InGaAs buried region 12 is reduced. In this embodiment, the input width of the waveguide is varied between 1.0 W ₀ of 1.2 W ₀ and varied between the width of the buried region from 0.8 W ₀ of 1.0 W _0. FIG. 3 (b), as an example, the waveguide band structure when expanding the width of the input waveguide 1.1 W _0, by reducing the width of the buried region to 0.95 W _0. The portion where the waveguide band B3 of the input waveguide overlaps with the waveguide band B4 of the buried region becomes wider, and the low frequency side of the buried region can be used. As a result, it is possible to utilize a frequency band in which the group velocity of light is low.

  FIG. 4 shows a first embodiment of the electrical junction. In the photodetector shown in FIG. 2, the electrical junction for extracting carriers from the buried region 12 has a pin structure. In the first embodiment, an electrical junction is formed only by the n-doped regions 13a and 13b, and an n-i-n structure is obtained. Such an electrical junction is called a photoconductor, and performs light detection by changing the conductance of the junction by the incidence of light. Note that the electrical junction may be configured only by the p-doped region element (pip structure), and the same impurity is doped in the two doped regions.

  FIG. 5 shows a second embodiment of the electrical junction. In this structure, the number of circular holes between the n-doped region 13 and the buried region 12 and between the buried region 12 and the p-doped region 14 is about 1-3. That is, a circular hole between the metal electrode 16 and the buried region 12 so that the metal electrodes 16a and 16b (typically Ti / Au) formed in the respective doped regions can be formed near the buried region 12. The number of rows is less than the number of rows of circular holes on both sides of the input waveguide. For this reason, the junction resistance R can be reduced, and the CR time constant can be reduced, that is, the speed can be increased.

  FIG. 6 shows a third embodiment of the electrical junction. As an alternative to the p-i-n structure, that is, instead of forming the n-doped region and the buried region, the metal electrodes 17a and 17b are provided close to the buried region 12. The metal electrodes 17a and 17b are made of a material that is in Schottky contact with InP. Such a Schottky electrode type structure has a problem that the dark current tends to be higher than that of the pin structure, but the junction resistance R can be lowered and the speed can be increased.

  FIG. 7 shows a fourth embodiment of the electrical junction. Similar to the third embodiment, the structure is a Schottky electrode type structure in which comb-shaped metal electrodes 18 a and 18 b are arranged immediately above the buried region 12. In such a comb-shaped electrode structure, by adjusting the period and width of the comb, distributed feedback reflection can be caused with respect to incident light, and the absorption efficiency can be improved. Further, compared to the horizontal electrode arrangement shown in FIG. 6, the distance between the electrodes can be reduced, so that the carrier movement time can be reduced, that is, the speed can be increased.

As general distributed feedback conditions, assuming that the wavelength λ of the incident light, the equivalent refractive index n _eff , the period Λ of the comb, and the integer m,
mλ / n _eff = 2Λ
Λ can be determined from the relational expression. For example, assuming that the wavelength λ of incident light is 1550 nm, the equivalent refractive index n _eff = 2.78 of the InGaAs buried region 12, and the integer m is 1, Λ is about 279 nm. However, since the distributed feedback condition has a finite bandwidth, it is not necessary to set this period strictly. Here, the value is set to Λ = 0 to 2 μm, and at this time, the comb of the metal electrodes 18a and 18b is used. The width is a value between 0 and Λ.

(Photodetector characteristics)
FIG. 8 shows an evaluation result based on the two-dimensional model of the first embodiment. It is the result of having calculated a light absorption characteristic using the finite element method by a two-dimensional model. The length L of the InGaAs buried region 12 was 1a, 2a, 3a, 6a. The width of the buried region 12 was 300 nm, and the imaginary part of the refractive index (that is, the light extinction coefficient) k = 0.123. The equivalent refractive index of the InP photonic crystal 11 was 2.57, and the equivalent refractive index of the buried region 12 was 2.78. Expanding the width of the input waveguide in the photonic crystal 11 to 1.1 W _0, which reduces the width of the buried region 12 to 0.95 W _0.

  FIG. 8 shows the result of calculating the light absorption power with respect to the incident wavelength. The shorter wavelength side than the wavelength of 1.62 μm is the wavegutable band of the photonic crystal waveguide. First, it can be seen that when L = 6a, an absorption efficiency close to 100% is obtained near a wavelength of 1.5 μm. In other words, according to the first embodiment, reflection at the incident end of the buried region 12 is suppressed, good optical coupling is obtained, and sufficient light absorption is obtained in the buried region 12. Is shown.

On the other hand, if the total length L of the buried region 12 is as short as 3a or less, the absorption efficiency is lowered because the light cannot be sufficiently absorbed by one round of light propagation. However, it can be seen that the absorption efficiency is maintained closer to 1.62 μm which is the upper limit of the waveguide band. This means that the closer to 1.62 μm, the lower the group velocity of light (having a high group refractive index _ng ), and even if the total length L is shortened, high absorption efficiency is obtained by the high group refractive index _ng . Because it is.

(Production method)
FIG. 9 shows a method for manufacturing the photodetector of the first embodiment. A substrate is prepared by epitaxially growing an InGaAs sacrificial layer 22 (1 μm), an InP layer 23 (50 nm), an InGaAs active layer 24 (150 nm), and an InP layer 25 (50 nm) on the InP substrate 21 (FIG. 9A). An SiO ₂ layer 26 (300 nm) is deposited as an etching mask, and an InGaAs buried region 24a is formed by electron beam drawing, dry etching, and wet etching (FIG. 9B). The InP layer 27 is regrown around the InGaAs buried region 24a (FIG. 9C).

  An n-doped region 27a is formed by Zn diffusion and a p-doped region 27b is formed by Si ion implantation (FIG. 9D). An array of photonic crystal circular holes 28 is formed by electron beam drawing and dry etching (FIG. 9E). After the metal electrodes 29a and 29b are formed, the InGaAs sacrificial layer 24 under the photonic crystal is removed by wet etching (FIG. 9f). Finally, by separating from the InP substrate 21, it is possible to obtain a photodetector made of a photonic crystal whose upper and lower sides are air layers.

(Second Embodiment)
FIG. 10 shows a photodetector using a photonic crystal optical waveguide according to the second embodiment of the present invention. In the first embodiment shown in FIG. 2, an output waveguide is provided in the InP photonic crystal 11. An output waveguide is formed on the optical axis opposite to the input waveguide with respect to the InGaAs buried region 12 without forming only one row of circular holes (substituting the second medium with the first medium). ing. In the first embodiment, light that cannot be absorbed in the buried region 12 returns to the incident waveguide. When such return light returns to a laser light source or the like connected to the incident waveguide, the oscillation operation of the laser light source becomes unstable. Therefore, an output waveguide can be provided to prevent the generation of return light. Such a structure can be applied to any of the electrical junctions shown in FIGS.

(Third embodiment)
(Photodetector structure)
FIG. 11 shows a photodetector using a photonic crystal optical waveguide according to the third embodiment of the present invention. In the first embodiment shown in FIG. 2, the length of the InGaAs buried region 12 is shortened to 3a or less. The width of the input waveguide and the buried region 12 in the InP photonic crystal 11 is the same as that in the first embodiment. Here, in order to form a resonator, it is necessary to form a reflection mirror. Here, at the incident end of the buried region 12 of the input waveguide, the diameters of the two circular holes 15a and 15b sandwiched between the input waveguides are expanded within a range of 1 to 2 times the diameter of the surrounding circular holes. Circular hole 15a, by adjusting the size of the 15b, to adjust the reflectance _{r 1.}

  In the third embodiment, an output waveguide can be provided as in the second embodiment.

(Photodetector characteristics)
In FIG. 12, the evaluation result by the two-dimensional model of 3rd Embodiment is shown. As in the first embodiment, the light absorption characteristics are calculated using a finite element method based on a two-dimensional model. The length L of the InGaAs buried region 12 is fixed to 1a, and other parameters are the same as those in the first embodiment. The width of the buried region 12 is 300 nm, the imaginary part of the refractive index (ie, the light extinction coefficient) k = 0.123, the equivalent refractive index of the InP photonic crystal 11 is 2.57, and the equivalent refractive index of the buried region 12 is 2. 78. Expanding the width of the input waveguide 1.1 W _0, which reduces the width of the buried region 12 to 0.95 W _0.

In order to compare the structures of the resonators, the circular holes 15a and 15b on both sides of the input waveguide at the incident end of the buried region 12 are 1 time, 1.4 times, 1.7 times and 2 times as large as the surrounding circular holes. did.
It can be seen that the absorption efficiency near 1.5 μm is high when the enlargement ratio is 1.7 times. This reflects the effect of increasing the absorption rate due to the resonance of light. The bandwidth of the absorption spectrum is 40 nm or more, and light detection in a very wide wavelength region can be performed as compared with a photodetector using a conventional photonic crystal optical resonator.

10 Photonic crystal photodetector 11 Photonic crystal 12 Light absorption region (embedded region)
13, 27a n doped region 14, 27b p doped region 15 circular hole 16, 17, 18, 29 metal electrode 21 InP substrate 22 InGaAs sacrificial layer 23, 25, 27 InP layer 24 InGaAs active layer 26 SiO ₂ layer

Claims (8)

In a photonic crystal photodetector in which a light absorption region is embedded in a photonic crystal in which a first medium and a second medium having different refractive indexes are regularly arranged,
An input waveguide obtained by replacing the second medium on the optical axis of the light absorption region with the first medium;
The photonic crystal photodetector, wherein the width of the input waveguide is wider than the width of the light absorption region.   In the incident end of the light absorption region of the input waveguide, the size of the two second media sandwiching the input waveguide is larger than the size of the surrounding second medium. The photonic crystal photodetector according to claim 1.   2. The optical waveguide according to claim 1, further comprising an output waveguide obtained by replacing the second medium on the optical axis opposite to the input waveguide with respect to the light absorption region by the first medium. Or the photonic crystal photodetector of 2.   4. The photonic crystal photodetector according to claim 1, wherein the first medium is made of InP, and the light absorption region is made of InGaAs.   In order to extract carriers from the light absorption region, a first doped region adjacent to one side parallel to the optical axis of the light absorption region and a second doped region adjacent to the opposite side are provided. 5. The photonic crystal photodetector according to claim 1, wherein the first and second doped regions are doped with the same impurity. 6. In order to extract carriers from the light absorption region, a first doped region adjacent to one side parallel to the optical axis of the light absorption region and a second doped region adjacent to the opposite side are provided. Formed,
A metal electrode is formed on each of the first and second doped regions;
A region where the second medium is formed between the metal electrode and the light absorption region is narrower than a region where the second medium is formed on both sides of the input waveguide. The photonic crystal photodetector according to any one of claims 1 to 4.   A first Schottky electrode adjacent to one side parallel to the optical axis of the light absorption region and a second Schottky electrode adjacent to the opposite side for extracting carriers from the light absorption region The photonic crystal photodetector according to claim 1, wherein the photonic crystal photodetector is formed.   The photonic crystal photodetector according to claim 7, wherein the first and the Schottky electrodes have a comb-shaped structure in the light absorption region.

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