Optical modulator

A core, constituted by an amorphous undoped semiconductor (i type), which is formed on a lower clad layer, and a p-type layer and an n-type layer which are disposed on the lower clad layer with the core interposed therebetween and are formed in contact with the core are provided. The core is formed to be thicker than the p-type layer and the n-type layer. The p-type layer and the n-type layer are constituted by single crystal silicon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry of PCT Application No. PCT/JP2019/026255, filed on Jul. 2, 2019, which application is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an optical modulator.

BACKGROUND

A technique for manufacturing optical modulation elements (optical modulators) on silicon substrates has attracted much attention in order to reduce the cost of optical integrated circuits. In general, the intensity and phase of light are modulated using a carrier plasma effect in silicon optical modulators. In particular, silicon optical modulators that modulate a light intensity are expected to generate an intensity modulation signal and to be applied to various communication devices such as optical switches and variable optical attenuators. A silicon carrier injection type element known as a representative intensity modulation element is configured as a rib type waveguide including slabs on both sides of a core of which the cross-sectional shape is a rectangular shape.

The element will be described with reference toFIG.7. In the element, a p-type region303and an n-type region304are formed in slabs on both sides of a core302of a rib type waveguide formed on a lower clad layer301, and a p-i-n diode is formed in a horizontal direction. A p-electrode305is connected to the p-type region303, and an n electrode306is connected to the n-type region304. In addition, an upper clad layer307is formed on the core302. It is possible to attenuate the intensity of light guided through an optical waveguide by the core302by applying a forward bias between the p-type region303and the n-type region304of the element and injecting free carriers into the core302.

Power consumption of the above-described intensity modulator is proportional to the square of the amount of current injected, and thus it is necessary to reduce a required amount of current injected in order to achieve low power consumption. The amount of current injected is determined according to a free carrier absorption coefficient that is inversely proportional to the mobility of carriers in a material. An amorphous semiconductor such as amorphous silicon (a-Si) has been proposed as such a type of low mobility material (see NPL 1).

CITATION LIST

Non Patent Literature

SUMMARY

Technical Problem

However, since an amorphous semiconductor has a low mobility of carriers, the manufacture of the above-described diode structure using only an amorphous semiconductor increases element resistance, which results in an increase in power consumption.

Embodiments of the present invention are contrived to solve the above-described problems, and an object thereof is to curb an increase in element resistance of an optical modulator to reduce the amount of current injected.

Means for Solving the Problem

An optical modulator according to embodiments of the present invention includes a core, constituted by an amorphous semiconductor, which is formed on a clad layer, and a p-type layer and an n-type layer which are disposed on the clad layer with the core interposed therebetween and are formed in contact with the core, in which the p-type layer and the n-type layer are constituted by single crystal silicon.

Effects of Embodiments of the Invention

As described above, according to embodiments of the present invention, a core is constituted by an amorphous semiconductor, and a p-type layer and an n-type layer are constituted by single crystal silicon, and thus it is possible to reduce the amount of current injected by curbing an increase in element resistance of an optical modulator.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Hereinafter, an optical modulator according to embodiment of the present invention will be described with reference toFIG.1. The optical modulator includes a core102constituted by an amorphous undoped semiconductor (i type), and a p-type layer103and an n-type layer104which are disposed with the core102interposed therebetween and are formed in contact with the core102, and the core102, the p-type layer103, and the n-type layer104are formed on a lower clad layer101. The core102is formed to be thicker than the p-type layer103and the n-type layer104. The p-type layer103and the n-type layer104are constituted by single crystal silicon.

The core102can be formed of, for example, amorphous silicon. In addition, the core102can also be constituted by amorphous silicon (a-Si: H, a-Si: D) containing silicon with defects terminated with hydrogen or deuterium. These materials have a track record of having been used in applications such as solar cells, and have been reported to have lower mobilities than single crystal silicon.

In addition, the optical modulator includes a p-electrode105formed on the p-type layer103, and an n electrode106formed on the n-type layer104. The p-electrode105is ohmic-connected to the p-type layer103, and the n electrode106is ohmic-connected to the n-type layer104.

Meanwhile, the lower clad layer101is formed on a substrate Iola formed of, for example, silicon. In addition, the upper clad layer107is formed on the core102. For example, this embedded insulating layer can be configured as the lower clad layer101using a well-known silicon-on-insulator (SOI) substrate. In addition, patterns of the p-type layer103and the n-type layer104can be formed by patterning a surface silicon layer of an SOI substrate by a known pattern formation technique. In addition, the p-type layer103and the n-type layer104can be configured by respectively implanting p-type impurities and n-type impurities into the patterns by a known ion implantation technique.

In addition, the core102can be formed by depositing silicon on the lower clad layer101on which the p-type layer103and the n-type layer104are formed by a well-known deposition technique such as a chemical vapor deposition method or a spattering method to form an amorphous silicon film and patterning the amorphous silicon film by a known pattern formation technique. In addition, the p-electrode105and the n electrode106can be formed by a known lithography method or the like.

When a forward bias voltage is applied between the p-electrode105and the n electrode106of the optical modulator according to the above-described embodiment, free carriers are injected into the core102through the p-type layer103and the n-type layer104which are formed of low resistance single crystal silicon. Light being guided is strongly trapped in the core102which constitutes an optical waveguide, and the intensity of the light being guided can be modulated by the injection of the above-described free carriers. Meanwhile, light to be modulated is guided in a direction from the side in front to the side behind the paper ofFIG.1. According to the embodiment, since the core102is constituted by an amorphous semiconductor, the amount of absorption of free carriers in the core102is large, and thus it is possible to reduce the amount of current injected. On the other hand, the p-type layer103and the n-type layer104are constituted by low resistance single crystal silicon, and thus it is possible to curb an increase in element resistance.

Meanwhile, as illustrated inFIG.2, an angle formed by a side surface103aof the p-type layer103on the core102side and a side surface104aof the n-type layer104on the core102side with respect to a plane of the lower clad layer101can also be an acute angle.

As known well, the solubility (etching rate) of single crystal silicon in an alkaline etching solution differs depending on a crystal plane, and a (iii) plane is less likely to be etched than a (100) plane. Thus, the side surface103aand the side surface104acan be formed as the above-described inclined surfaces by setting the (100) plane to be a main surface and setting the (iii) plane to be the side surface103aand the side surface104aby wet etching to form the p-type layer103and the n-type layer104. In addition, a surface formed by wet etching is flat and has few defects as compared with a case where dry etching is used. Further, in general, in the deposition of an amorphous semiconductor using a chemical vapor deposition method or a sputtering method, an inclined surface that can be seen from above has a better covering property than a vertical side surface.

As described above, the side surface103aof the p-type layer103and the side surface104aof the n-type layer104which are in contact with the core102are formed as the above-described inclined surfaces, and thus a heterojunction therebetween is formed in better conditions.

In addition, as illustrated inFIG.3, the thickness of the p-type layer103bin which the p-electrode105is formed and the thickness of the n-type layer104bin which the n electrode106is formed can also be made larger than those of the p-type layer103and the n-type layer104in a region in contact with the core102. With such a configuration, the p-electrode105and the n electrode106can be further separated from a light trapping region centered on the core102in a cross-sectional view, and the resistance of the element can be reduced without impairing light trapping.

Incidentally, the optical modulator is used by being optically connected (optical connection) to another optical waveguide. Hereinafter, an optical connection structure between the optical modulator and another optical waveguide will be described.

For example, as illustrated inFIGS.4A,4B, and4C, an optical waveguide131constituted by the core102of the optical modulator and another optical waveguide132can be optically connected through an optical connection portion133.FIG.4Billustrates a cross-section taken along a line aa′ inFIG.4A, andFIG.4Cillustrates a cross-section taken along a line bb′ inFIG.4A. Meanwhile, a cross-section of a line cc′ inFIG.4Ais illustrated inFIG.1.

The other optical waveguide132includes another core111, constituted by single crystal silicon, which is formed on the lower clad layer101. The other core111is constituted by an undoped region of a silicon layer formed to be continuous with the p-type layer103and the n-type layer104. The other core111is formed to have the same thickness and the same width as the core102, and the end face of the core111is connected in the connection portion121. The optical connection portion133includes a slab112which is continuous with the p-type layer103and the n-type layer104. In the optical connection portion133, the other core111is inserted into the slab112. The width of the slab112decreases as a distance from the optical waveguide131increases when seen in a plan view, and there is no width of the slab in the other optical waveguide132.

In addition, as illustrated inFIGS.5A,5B, and5C, the optical waveguide131constituted by the core102of the optical modulator and the other optical waveguide132acan be optically connected to each other through an optical connection portion133a.FIG.5Billustrates a cross-section taken along a line aa′ inFIG.5A, andFIG.5Cillustrates a cross-section taken along a line bb′ inFIG.5A.

The other optical waveguide132aincludes another core113, constituted by single crystal silicon, which is formed on the lower clad layer101. The other core113is constituted by an undoped region of a silicon layer formed to be continuous with the p-type layer103and the n-type layer104. Further, in this example, the other core113, the p-type layer103, and the n-type layer104are formed to have the same thickness. In addition, the width of the other core113decreases as a distance from the optical waveguide131increases in the optical connection portion133awhen seen in a plan view, and the other core113has the same width as a main portion of the core102in the other optical waveguide132awhen seen in a plan view.

In addition, the core102includes an extending portion iota extending to the other optical waveguide132aside and formed on the other core113. In addition, the width of the extending portion iota decreases as a distance from the optical waveguide131increases.

In addition, as illustrated inFIGS.6A and6B, the optical waveguide131constituted by the core102of the optical modulator and the other optical waveguide132bcan be optically connected to each other through an optical connection portion133b.FIG.6Billustrates a cross-section taken along a line aa′ inFIG.6A.

The other optical waveguide132aincludes another core113, constituted by single crystal silicon, which is formed on the lower clad layer101. The other core113is constituted by an undoped region of a silicon layer formed to be continuous with the p-type layer103and the n-type layer104. Further, in this example, the other core113, the p-type layer103, and the n-type layer104are formed to have the same thickness. In addition, the width of the other core113decreases as a distance from the optical waveguide131increases in the optical connection portion133awhen seen in a plan view, and the other core113has the same width as a main portion of the core102in the other optical waveguide132awhen seen in a plan view.

In addition, the core102includes an extending portion102bextending to the other optical waveguide132aside and formed on the other core113. The extending portion102bhas the same width as the core102when seen in a plane view.

As described above, according to embodiments of the present invention, the core is constituted by an amorphous semiconductor, and the p-type layer and the n-type layer are constituted by single crystal silicon, and thus it is possible to reduce the amount of current injected by curbing an increase in element resistance of the optical modulator.

Meanwhile, the present invention is not limited to the above-described embodiment, and it is apparent that various modifications and combinations can be made by one skilled in the art within a technical idea of the present invention. For example, the configuration described usingFIGS.2and3can be adopted in the portions of the optical waveguide131illustrated inFIGS.4A,5A, and6A.

REFERENCE SIGNS LIST