Patent Description:
Aspects and embodiments disclosed herein include integration of III-nitride photonic materials (e.g., AlGaN, GaN) on silicon nitride photonics on a silicon platform to enable an active platform working over a broad wavelength range including the visible range. Passive optical circuitry includes silicon nitride waveguides while active components such as electro-optic modulators are made with III-nitride photonic circuitry bonded to the silicon nitride waveguide circuitry. Aspects and embodiments may be particularly beneficial for developing a photonic integrated circuit (PIC) platform operable in the visible range having efficient active and passive functionalities.

The present invention provides a photonic integrated circuit. The photonic integrated circuit comprises a silicon nitride waveguide, silicon nitride support layers, an electro-optic modulator formed of a III-nitride waveguide structure disposed on the silicon nitride waveguide, a dielectric cladding covering the silicon nitride waveguide and electro-optic modulator, and electrical contacts disposed on the dielectric cladding and arranged to apply an electric field to the electro-optic modulator. The III-nitride waveguide structure is bonded to the silicon nitride waveguide and silicon nitride support layers with a III-nitride slab.

In some embodiments, the III-nitride waveguide structure is formed of one of AlxGa<NUM>-xN (<NUM>≤x≤<NUM>), InxGa<NUM>-xN (<NUM>≤x≤<NUM>), or AlxIn<NUM>-xN (<NUM>≤x≤<NUM>).

In some embodiments, the III-nitride waveguide structure includes a stacked layer of quantum wells formed of one of AlxGa<NUM>-xN (<NUM>≤x≤<NUM>), InxGa<NUM>-xN (<NUM>≤x≤<NUM>), or AlxIn<NUM>-xN (<NUM>≤x≤<NUM>).

The III-nitride waveguide structure is bonded to the silicon nitride waveguide and silicon nitride support layers with a III-nitride slab.

In some embodiments, the dielectric cladding is formed of silicon dioxide.

In some embodiments, the silicon nitride waveguide is disposed on an upper surface of a silicon dioxide layer disposed on a silicon substrate.

In some embodiments, the photonic integrated circuit further comprises one or more heterogeneously integrated III-nitride quantum well modulators.

In some embodiments, the photonic integrated circuit further comprises a photodetector.

In some embodiments, the photodetector is optically coupled to the silicon nitride waveguide.

In some embodiments, the photodetector is disposed on a same side of the silicon nitride waveguide as the dielectric cladding.

In some embodiments, the photodetector is disposed on a opposite side of the silicon nitride waveguide as the dielectric cladding.

In some embodiments, the photonic integrated circuit further comprises a conductive via passing through the dielectric cladding and forming an electrical path between a contact pad of the photodetector and an external contact pad disposed on a surface of the dielectric cladding.

In some embodiments, the photonic integrated circuit further comprises a conductive via passing through the dielectric cladding and forming an electrical path between the III-nitride waveguide structure and an external contact pad disposed on a surface of the dielectric cladding.

In some embodiments, the III-nitride waveguide structure has a tapered end portion.

There is also provided a method of forming a photonic integrated circuit. The method comprises forming a silicon nitride waveguide and silicon nitride support layers on a first substrate, forming an electro-optic modulator including a III-nitride waveguide structure on a second substrate, bonding the electro-optic modulator to the silicon nitride waveguide, wherein the III-nitride waveguide structure is bonded to the silicon nitride waveguide and silicon nitride support layers with a III-nitride slab, and removing the second substrate.

In some embodiments, the method further comprises fabricating one or more III-nitride quantum well modulators in the photonic integrated circuit.

In some embodiments, the method further comprises forming a tapered end portion on the III-nitride waveguide structure.

In some embodiments, the method further comprises forming a dielectric cladding over the silicon nitride waveguide and electro-optic modulator.

In some embodiments, the method further comprises forming a conductive via electrically connected to the III-nitride waveguide structure and passing through the dielectric cladding.

In some embodiments, the method further comprises forming a photodetector optically coupled to the silicon nitride waveguide within the photonic integrated circuit.

The figures are included to provide illustration and a further understanding of the various aspects and embodiments and are incorporated in and constitute a part of this specification but are not intended as a definition of the limits of the invention. In the figures:.

It is to be appreciated that embodiments of the methods and apparatuses discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and apparatuses are capable of implementation in other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. The use herein of "including," "comprising," "having," "containing," "involving," and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to "or" may be construed as inclusive so that any terms described using "or" may indicate any of a single, more than one, and all of the described terms. Any references to front and back, left and right, top and bottom, upper and lower, and vertical and horizontal are intended for convenience of description, not to limit the present systems and methods or their components to any one positional or spatial orientation.

The relatively inexpensive silicon platform for photonic integrated circuits (PIC) and the well-understood and inexpensive nature of silicon microfabrication processes motivates the heterogeneous integration of other photonic materials that are transparent in the visible spectrum and have active electro-optic properties on silicon. For example heterogeneous integration of III-V (e.g., indium phosphide compounds) photonics platforms on silicon or heterogeneous integration of lithium niobate on silicon or silicon nitride have been demonstrated successfully but mainly for applications in infrared and have limited functionality at visible wavelengths, or cannot handle large optical powers in the visible wavelengths (for example lithium niobate suffers from photorefractive damage at wavelengths shorter than red wavelengths when laser power goes to mW range.

III-nitride photonics compound materials which include, for example, AlxGa<NUM>-xN (x= <NUM>-<NUM>), InxGa<NUM>-xN (x = <NUM>-<NUM>), and AlxIn<NUM>-xN (x = <NUM>-<NUM>) are example of promising materials that are transparent over a wide wavelength range including the visible range with promising and proven active photonic properties making them useful in devices such as electro-optic modulators and lasers. Some of these platforms may be disposed on a sapphire substrate which does not have the benefits of silicon substrate such as low cost or high-volume production. Other implementations may include depositing aluminum nitride on a SiO<NUM> cladding in which the aluminum nitride is not crystalline and so exhibits large optical propagation loss in the visible range. Some implementations may include bonding GaN on SiO<NUM> to form GaN waveguides on the SiO<NUM> cladding on a Si substrate. Further implementations may involve direct growing of III-nitrides on silicon for making cheaper light-emitting diodes. Such platforms, however, are not suitable for photonic integrated circuits that require appropriate refractive index contrast between different material layers to form optical waveguides with low optical propagation loss. There has been work directed to making photonic integrated circuits of III-nitride on a silicon platform. The problem with such a platform is the higher refractive index of silicon which directly sits under the III-nitride layer and therefor causes the leakage of light from the III-nitride layer to the Si substrate. Some implementations may locally undercut the Si layer under the III-nitrides to make a III-nitride membrane waveguide with air underneath to avoid optical leakage. Undercutting, however, is not a viable solution for long length waveguides.

Aspects and embodiments disclosed herein include methods for heterogeneous integration of III-nitride photonic circuitries on silicon nitride waveguide circuitries which themselves are disposed on a silicon platform. Aspects and embodiments disclosed herein may provide a comprehensive integrated photonic platform that is functional over a wide wavelength range, particularly in the visible and near infrared, wherein the active functions such as high-speed electro-optic modulators are provide by the III-nitride part of the circuit, and the passive optical routing circuitry across entire chip is provided by silicon nitride waveguides and related components (e.g., resonators).

<FIG> illustrates a structure <NUM> that includes a III-nitride (e.g., AlxGa<NUM>-xN (x= <NUM>-<NUM> or <NUM><x<<NUM>), InxGa<NUM>-xN (x = <NUM>-<NUM> or <NUM><x<<NUM>), or AlxIn<NUM>-xN (x = <NUM>-<NUM> or <NUM><x<<NUM>)) waveguide <NUM> bonded to a silicon nitride (SiN) waveguide <NUM>. The silicon nitride waveguide <NUM> is disposed on a layer of SiO<NUM> <NUM> which is itself disposed on a Si substrate <NUM>. In some embodiments, the layer of SiO<NUM> <NUM> may be omitted and the SiN waveguide <NUM> may be disposed directly on the Si substrate <NUM>. The SiN waveguide <NUM> may be, for example, between <NUM> and <NUM> thick. A thin III-nitride slab layer <NUM> is provided to facilitate bonding between the III-nitride waveguide <NUM>, the silicon nitride waveguide <NUM>, and additional silicon nitride support layers <NUM>. Optical intensity after propagating through a SiN waveguide <NUM> reaches the heterogeneous III-nitride on SiN waveguide <NUM>, <NUM> and allows light to penetrate into the III-nitride waveguide <NUM>. The reason for this efficient interaction is that the refractive index of SiN (~<NUM>) is close to that of III-nitride waveguides, allowing these waveguides to be efficiently mode matched.

<FIG> illustrates results of a simulation that shows a III-nitride waveguide layer <NUM> and the SiN waveguide layer <NUM> as illustrated in <FIG> efficiently optically interacting as the optical energy exists in both layers. This simulation is for a wavelength of <NUM> and shows optical intensity interacting with both top and bottom waveguides <NUM>, <NUM>.

Silicon nitride is fully compatible with silicon and so most of the photonic waveguiding routing and circuitry can be done with SiN, and whenever active functionality such as efficient electro-optic modulation is needed, the optical signal is sent to the heterogeneous region including the III-nitride waveguide <NUM> and silicon nitride waveguide <NUM> shown in <FIG> for modulation purposes.

The structure in <FIG> represents a simple schematic to show one main concept of the present disclosure. As illustrated in <FIG>, the structure of <FIG> may be cladded with other optical materials with a lower refractive index than that of SiN and III-nitride. One example of a cladding material which is compatible with silicon platform is SiO<NUM>. <FIG> shows the same structure shown in <FIG>, but with added SiO<NUM> overcladding <NUM>. <FIG> illustrates results of a simulation of a III-nitride waveguide layer <NUM> and SiN waveguide layer <NUM> in a cladded structure as illustrated in <FIG> efficiently optically interacting as the optical energy exists in both layers.

In some embodiments, for example, as illustrated in <FIG> and <FIG>, there are waveguide tapers 105T at the beginning and the end of the III-nitride waveguide <NUM> to allow adiabatic matching of the SiN waveguide layer <NUM> and the III-nitride waveguide layer <NUM>. <FIG> illustrates this structure without SiO<NUM> overcladding <NUM> and <FIG> illustrates this structure with SiO<NUM> overcladding <NUM>.

Two waveguiding electro-optic modulator structures on a heterogeneous III-nitride on silicon nitride waveguiding platform are illustrated in cross sections in <FIG> and <FIG>, respectively. The modulator device shown in <FIG> works based on the known Pockles effect. There are metal electrodes <NUM> (e.g., Cu, Au, or Al) integrated with the III-nitride, e.g., AlGaN waveguide <NUM>. By applying a voltage across the electrodes <NUM> an electric field penetrates into the AlGaN waveguide <NUM> and changes its refractive index resulting in a modulation of light traveling in the AlGaN waveguide <NUM>. The structure shown in <FIG> also works based on Pockels effect but with the inclusion of III-nitride quantum well layers 105B to induce strain and therefore to strongly enhance the Pockels effect. The quantum well layers 105B can be a periodic stack of layers of AlGaN/GaN, or in general AlxGa<NUM>-xN/AlyGa<NUM>-yN wherein x and y are the fraction of Al and Ga in AlGaN and vary between <NUM> and <NUM> and may add to <NUM> and where x≠y. The composition of Al and Ga can be anything between <NUM> and <NUM>, but for sake of simplicity, throughout this disclosure the term AlGaN shall be considered to encompass any of AlxGa<NUM>-xN (x= <NUM>-<NUM> or <NUM><x<<NUM>), InxGa<NUM>-xN (x = <NUM>-<NUM> or <NUM><x<<NUM>), or AlxIn<NUM>-xN (x = <NUM>-<NUM> or <NUM><x<<NUM>). The stack of quantum well layers 105B can be made of any III-nitride family, for example, AlxGa<NUM>-xN or InxGa<NUM>-xN, so long as the stack design provide the desired strain. The thickness of these stack quantum well layers may be, for example, between a few nm to about ten nm. The strain induced in the stack of quantum well layers 105B can strongly enhance the Pockels effect of III-nitride.

<FIG> and <FIG> show the general fabrication steps for making a heterogeneous integration of a III-nitride waveguide on a silicon nitride waveguide as well as making a Pockels electro-optic modulator as shown in <FIG>. As illustrated in <FIG> a SiO<NUM> layer <NUM> is grown on a Si substrate <NUM> and a SiN waveguide <NUM> and support layers <NUM> are fabricated on the SiO<NUM> layer <NUM> using semiconductor fabrication techniques known in the art that will not be described in detail herein. A thin, for example, about <NUM> thick layer of SiO<NUM> may be grown or deposited on top of the SiN structures <NUM>, <NUM> as illustrated in <FIG>. The thin SiO<NUM> layer may assist in bonding the SiN waveguide <NUM> and support layers <NUM> to the AlGaN waveguide structure as will be described in further detail below. Concurrent with, prior to, or after forming the SiN waveguide structure one may form an AlGaN structure from which the AlGaN waveguide will be formed. As illustrated in <FIG>, a layer of AlGaN <NUM> may be epitaxially grown on a template substrate <NUM>, for example, a SiC substrate or substrate of any other suitable material using known techniques in the semiconductor fabrication arts. The template substrate <NUM> may be coated with a nucleation layer <NUM> and a highly doped AlGaN layer <NUM> disposed on the nucleation layer <NUM> prior to epitaxial growth of the AlGaN layer <NUM> on the highly doped AlGaN layer <NUM>. A thin, for example, about <NUM> thick layer of SiO<NUM> <NUM> may be grown or deposited on the top of the layer of AlGaN <NUM> as illustrated in <FIG>. As illustrated in <FIG>, the AlGaN structure including the AlGaN layer <NUM> is flipped over and the oxide layer <NUM> is brought into contact with the oxide layer <NUM> on the SiN structures <NUM>, <NUM> of the SiN waveguide structure. The SiO<NUM> layers <NUM>, <NUM> are bonded using an oxide-oxide bonding process, for example, anodic boding to form an adhesion layer <NUM> (<FIG>). A wet etching process is then used to remove the substrate <NUM>, nucleation layer <NUM>, and highly-doped AlGaN layer <NUM>. The wet etchant used may selectively etch the highly doped AlGaN layer <NUM> versus the AlGaN layer <NUM> leaving the AlGaN layer <NUM> and resulting in the structure illustrated in <FIG>. Lithography and plasma etching as known in the art of semiconductor fabrication are then utilized to etch portions of the AlGaN layer <NUM> to form the AlGaN waveguide <NUM> and AlGaN slab layer <NUM> as illustrated in <FIG>. A layer of SiO<NUM> <NUM> is then grown or deposited on top of the AlGaN waveguide <NUM> and AlGaN slab layer <NUM> and optionally planarized (<FIG>). A portion of the layer of SiO<NUM> <NUM> is lithographically patterned and etch to expose a portion of the AlGaN slab layer <NUM> (<FIG>). Metal contacts <NUM> are then deposited using, for example, sputtering and photolithographic patterning with one metal contact <NUM> being on top of the SiO<NUM> layer above the AlGaN waveguide <NUM> and the other on the portion of exposed AlGaN slab layer <NUM>, or on a thin SiO<NUM> layer that may remain on the AlGaN slab layer <NUM>.

<FIG> and <FIG> shows the general fabrication steps for making a heterogeneous integration of III-nitride waveguide with layer of stacked AlGaN quantum wells forming a Pockels electro-optic modulator on a silicon nitride waveguide as shown in <FIG>. As illustrated in <FIG> a SiO<NUM> layer <NUM> is gown on a Si substrate <NUM> and a SiN waveguide <NUM> and support layers <NUM> are fabricated on the SiO<NUM> layer <NUM> using semiconductor fabrication techniques known in the art that will not be described in detail herein. A thin, for example, about <NUM> thick layer of SiO<NUM> may be grown or deposited on top of the SiN structures <NUM>, <NUM> as illustrated in <FIG>. The thin SiO<NUM> layer may assist in bonding the SiN waveguide <NUM> and support layers <NUM> to the stacked AlGaN quantum well waveguide structure as will be described in further detail below. Concurrent with, prior to, or after forming the SiN waveguide structure one may form an AlGaN structure from which the AlGaN quantum well waveguide and AlGaN slab layer will be formed. As illustrated in <FIG>, a layer <NUM> including a stack of AlGaN quantum wells may be epitaxially grown on a template substrate <NUM>, for example, a SiC substrate or substrate of any other suitable material using known techniques in the semiconductor fabrication arts. The template substrate <NUM> may be coated with a nucleation layer <NUM> and a highly doped AlGaN layer <NUM> disposed on the nucleation layer <NUM> prior to epitaxial growth of the AlGaN quantum well layer <NUM> on the highly doped AlGaN layer <NUM>. A thin, for example, about <NUM> thick layer of SiO<NUM> <NUM> may be grown or deposited on the top of the layer of AlGaN quantum wells <NUM> as illustrated in <FIG>. As illustrated in <FIG>, the AlGaN structure including the AlGaN quantum well layer <NUM> is flipped over and the oxide layer <NUM> is brought into contact with the oxide layer <NUM> on the SiN structures <NUM>, <NUM> of the SiN waveguide structure. The SiO<NUM> layers <NUM>, <NUM> are bonded using an oxide-oxide bonding process, for example, anodic boding to form an adhesion layer <NUM> (<FIG>). A wet etching process is then used to remove the substrate <NUM>, nucleation layer <NUM>, and highly doped AlGaN layer <NUM>. The wet etchant used may selectively etch the highly doped AlGaN layer <NUM> versus the AlGaN quantum well layer stack <NUM> leaving the AlGaN quantum well layer stack <NUM> and resulting in the structure illustrated in <FIG>. Lithography and plasma etching as known in the art of semiconductor fabrication are then utilized to etch portions of the AlGaN quantum well layer stack <NUM> to form the AlGaN quantum well layer waveguide 105B and AlGaN slab layer <NUM> as illustrated in <FIG>. A layer of SiO<NUM> <NUM> is then grown or deposited on top of the AlGaN quantum well layer waveguide 105B and AlGaN slab layer <NUM> and optionally planarized (<FIG>). A portion of the layer of SiO<NUM> <NUM> is lithographically patterned and etched to expose a portion of the AlGaN slab layer <NUM> (<FIG>). Metal contacts <NUM> are then deposited using, for example, sputtering and photolithographic patterning with one metal contact <NUM> being on top of the SiO<NUM> layer above the AlGaN quantum well layer waveguide 105B and the other on the portion of exposed AlGaN slab layer <NUM>, or on a thin SiO<NUM> layer that may remain on the AlGaN slab layer <NUM>.

In some aspects, additional circuitry or active or passive devices may be integrated with or formed on the embodiments of the waveguide structures disclosed herein. For example, as illustrated in <FIG>, in one embodiment, a photodetector such as an on-chip Si photodetector <NUM> may be mounted on the waveguide structure <NUM>. The on-chip Si photodetector <NUM> may optically couple to the SiN waveguide <NUM> and produce an output signal based on the presence or absence of light passing through the SiN waveguide <NUM>. In other embodiment, one or more III-nitride quantum well lasers can also or alternatively be heterogeneously integrated and fabricated in embodiments of the waveguide structures disclosed herein in a similar way to the modulator structure with the stacked AlGaN quantum wells 105B.

<FIG> illustrate an alternative to the photonic integrated circuit of <FIG>. In the embodiment of <FIG>, the on-chip Si detector <NUM> is formed below the SiN waveguide <NUM>, rather than above the SiN waveguide <NUM> as illustrated in <FIG>. Also illustrated in <FIG> and <FIG> are vias <NUM> that may pass through a layer of dielectric <NUM>, for example, SiO<NUM> that covers the on-chip Si detector <NUM>. Layer of dielectric <NUM> may be a portion of the layer of SiO<NUM> <NUM> disposed on the Si substrate <NUM>. The vias <NUM> electrically connect to contact pads <NUM> on the on-chip Si detector <NUM> so that electrical signals from the on-chip Si detector <NUM> may be passed out of the photonic integrated circuit. The vias <NUM> may be formed of a metal, for example, Al or Cu or another suitable conductive material.

As illustrated in <FIG>, an overcladding layer <NUM> formed of, for example SiO<NUM> may be disposed over the AlGaN <NUM> and SiN <NUM> waveguides of the structure of <FIG> and may serve to protect the waveguides. Second layer vias <NUM> may be formed over and in electrical connection with the vias <NUM>. The second layer vias <NUM> provide for electrical signals from the on-chip Si detector <NUM> may be passed through the overcladding layer <NUM> and out of the photonic integrated circuit. An additional via <NUM> may pass through the overcladding layer <NUM> and electrically connect to the AlGaN waveguide <NUM>, for example, to the slab layer <NUM> of the AlGaN waveguide <NUM>. Second layer vias <NUM> and the AlGaN waveguide via <NUM> may be formed of a metal, for example, Al or Cu or another suitable conductive material. Second layer vias <NUM> and the AlGaN waveguide via <NUM> may electrically connect to respective contact pads <NUM>, <NUM> disposed on top of the overcladding layer <NUM>. An additional metal contact <NUM> may be provided on top of the overcladding layer <NUM> to allow one to apply an electric field to the AlGaN waveguide <NUM>.

Claim 1:
A photonic integrated circuit comprising:
a silicon nitride waveguide (<NUM>);
an electro-optic modulator; characterized by further comprising
a dielectric cladding (<NUM>) covering the silicon nitride waveguide (<NUM>) and electro-optic modulator (<NUM>); and silicon nitride support layers (<NUM>); wherein the electrooptical modulator is formed of a III-nitride waveguide structure (<NUM>) disposed on the silicon nitride waveguide; and
electrical contacts (<NUM>) disposed on the dielectric cladding (<NUM>) and arranged to apply an electric field to the electro-optic modulator (<NUM>); and wherein the III-nitride waveguide structure (<NUM>) is bonded to the silicon nitride waveguide (<NUM>) and silicon nitride support layers (<NUM>) with a III-nitride slab (<NUM>).