Patent Grant
12113333
The present invention relates to a surface emitting laser element and a manufacturing method of the same.
Recently, a development of a surface emitting laser using a photonic crystal has been advanced. For example, Patent Document 1 discloses a semiconductor laser device whose object is to perform manufacturing without fusion process.
Additionally, Patent Document 2 discloses a manufacturing method that manufactures a microstructure of a photonic crystal into a GaN-based semiconductor. Non-Patent Document 1 discloses that a reduced-pressure growth increases a speed of a lateral growth to manufacture a photonic crystal.
Non-Patent Document 2 discloses an in-plane diffraction effect and a threshold gain difference of a photonic crystal laser. Non-Patent Document 3 discloses a three-dimensional coupled wave model of a square lattice photonic crystal laser.
To perform an oscillation operation at a low threshold current density in a surface emitting laser including a photonic crystal, reduction of a resonator loss is necessary. To reduce the resonator loss in the photonic crystal surface emitting laser, increasing a coupling coefficient of a light wave propagating in a direction parallel to a photonic crystal layer (one-dimensional coupling coefficient: κ3) is effective.
The present invention has been made in consideration of the above-described points and an object of which is to provide a surface emitting laser that includes a photonic crystal having a large coupling coefficient with respect to a light wave propagating a photonic crystal layer and allows an oscillation operation at a low threshold current density, and a manufacturing method of the surface emitting laser.
A surface emitting laser element according to a first embodiment of the present invention is a surface emitting laser element formed of a group III nitride semiconductor, comprising:
A manufacturing method according to another embodiment of the present invention is a manufacturing method for manufacturing a surface emitting laser element formed of a group III nitride semiconductor by a MOVPE method, the manufacturing method comprising:
While the following describes preferred embodiments of the present invention, these embodiments may be appropriately modified and combined. In the following description and attached drawings, same reference numerals are given to actually same or equivalent parts for the description.
[Threshold Gain of Photonic Crystal Surface Emitting Laser]
Generally, in the photonic crystal surface emitting laser (hereinafter also simply referred to as a photonic crystal laser), diffraction efficiency of diffraction grating inside a resonator is expressed by a coupling coefficient κ, and the larger the coupling coefficient κ is, the smaller the threshold gain becomes.
In the photonic crystal surface emitting laser, a light wave that propagates inside a surface parallel to the photonic crystal layer is diffracted not only in ±180° direction with respect to a traveling direction of the light wave but also in ±90° direction. Accordingly, in addition to a coupling coefficient κ3 (one-dimensional coupling coefficient) of the light wave that propagates in the ±180° direction, a coupling coefficient κ2D (two-dimensional coupling coefficient) of the light wave that propagates in ±90° direction is present.
That is, in the surface emitting laser including the photonic crystal, the light wave propagating inside the surface is diffracted by the diffraction effect of the photonic crystal, thus forming a two-dimensional resonance mode. Here, when a component of the light not diffracted by the photonic crystal leaks in the in-plane direction, a resonator loss increases, leading to an increase in threshold gain. Accordingly, as long as the resonator loss can be suppressed, the threshold gain can be reduced and an oscillation operation can be performed at a low threshold current density.
To reduce the resonator loss in the photonic crystal surface emitting laser and perform the oscillation operation at the low threshold current density, it is effective to increase the coupling coefficient (κ3) of the light wave propagating inside the photonic crystal layer surface.
[Example of Structure of Photonic Crystal Surface Emitting Laser]
On (a back surface of) the substrate 12, an n-electrode 19A is formed, and on (a top surface of) the p-clad layer 18, a p-electrode 19B is formed. For example, the p-electrode 19B can be formed in an appropriate shape surrounding a photonic crystal region, such as an annular shape.
In this case, the light from the surface emitting laser element 10 is taken out from the top surface of the semiconductor structure layer 11 (namely, the surface of the p-clad layer 18) to the outside in a direction perpendicular to the active layer 15.
A structure in which the light from the surface emitting laser element 10 is taken out from the lower surface side of the semiconductor structure layer 11 to the outside can be employed. In the case, for example, the n-electrode 19A can be formed in the annular shape and the p-electrode 19B can be disposed at a position facing the center of the n-electrode 19A.
The n-guide layer 14 comprises the first embedding layer 14A embedding the photonic crystal layer (PCL) 14P. The first embedding layer 14A is a semiconductor layer (that is, the semiconductor layer with top surfaces of the voids 14C as its bottom surface) on the top surfaces of the two-dimensionally arrayed voids 14C. The n-guide layer 14 also comprises the photonic crystal layer (PCL) 14P, and a base layer 14B as a crystal layer on the substrate side than the photonic crystal layer (PCL) 14P. Additionally, the second embedding layer 21 is formed on the first embedding layer 14A.
[Resonance Effect of Photonic Crystal Surface Emitting Laser]
To obtain the resonate effect in a surface emitting laser including a photonic crystal portion (hereinafter simply referred to as a photonic crystal surface emitting laser in some cases), it is desired to have a high diffraction effect in the photonic crystal portion.
That is, to increase the diffraction effect in the photonic crystal surface emitting laser, the following is preferred. (1) When an oscillation wavelength is denoted as λ and an effective refractive index of the photonic crystal portion is denoted as neff, a two-dimensional refractive index cycle P in the photonic crystal portion meets P=mλ/neff (m is a natural number) in the case of the two-dimensional square lattice photonic crystal, and meets P=mλ×2/(31/2×neff) (m is a natural number) in the case of the two-dimensional triangular lattice photonic crystal. (2) A proportion (FF: filling factor) of a modified refractive index area to a base material in the photonic crystal portion is sufficiently large. (3) In a light intensity distribution in the photonic crystal surface emitting laser, a proportion (ΓPC: confinement factor) of optical intensity distributed in the photonic crystal portion is sufficiently large.
To meet (1), it is necessary to appropriately set a lattice constant (interval PC) of the photonic crystal according to an oscillation wavelength of the photonic crystal laser. Here, since the refractive index cycle P=interval PC, PC can be set based on P meeting (1) described above. For example, in a case of an oscillation at a wavelength of 405 nm using a gallium-nitride-based material, neff is around 2.5. Therefore, with the use of the two-dimensional square lattice photonic crystal, the lattice constant is preferably set to be around 163 nm.
In the two-dimensional square lattice photonic crystal, a direction in which the light wave is diffracted is the same as the array direction of the lattices. Meanwhile, in the two-dimensional triangular lattice photonic crystal, the light wave is diffracted in a direction inclined with respect to the array direction of the lattices by 30°. Accordingly, in (1) described above, P is multiplied by ⅔1/2 in which the two-dimensional triangular lattice photonic crystal satisfies P.
[Growth of Clad Layer and Guide Layer]
Manufacturing steps of the semiconductor structure layer 11 will be described in detail below. As a crystal growth method, a Metalorganic Vapor Phase Epitaxy (MOVPE) method was used, and the semiconductor structure layer 11 was grown on the growth substrate 12 by normal pressure (atmospheric pressure) growth.
As a substrate for growth of the semiconductor structure layer 11, the n-type GaN substrate 12 having +c plane as a growth surface with c-axis inclined with respect to a-axis by 0.4° was employed. The inclination of the c-axis (off angle) may be appropriately changed in a range in which epitaxial growth is possible with the GaN-based semiconductor. On the substrate 12, an n-type AlGaN (layer thickness: 2 μm) having an aluminum (Al) composition of 4% was grown as the n-clad layer 13. As a group III MO (organic metal) material, trimethyl gallium (TMG) and trimethyl aluminum (TMA) were used and ammonia (NH3) was used as a group V material. Additionally, silane (SiH4) was supplied as a doping material. A carrier density at room temperature was approximately 2×1018 cm−3.
Subsequently, TMG and NH3 were supplied and an n-type GaN (layer thickness: 300 nm) was grown as the n-guide layer 14. Silane (SiH4) was supplied simultaneously with the growth, and thus doping was performed. The carrier density was approximately 2×1018 cm−3.
[Void Formation in Guide Layer]
The substrate in which the n-guide layer 14 had been grown, namely, the substrate with the n-guide layer 14 (hereinafter also referred to as a guide layer substrate) was taken out from a MOVPE apparatus and fine voids (holes) were formed in the n-guide layer 14. The following describes the formation of the voids in detail with reference to
The guide layer substrate in which the n-clad layer 13 and the n-guide layer 14 had grown were cleaned to obtain a cleaned surface of the guide layer substrate (
Next, an electron beam (EB) drawing resist RZ was applied over the SiNx film SN at a thickness around 300 nm by a spin coating method, and the resultant product was put in an electron beam drawing device, thus forming a pattern having a two-dimensional periodic structure on the surface of the guide layer substrate (
After developing the patterned resist RZ, the SiNx film SN was selectively dry-etched by an Inductive Coupled Plasma-Reactive Ion Etching (ICP-RIE) apparatus (
Subsequently, the resists RZ were removed, and using the patterned SiNx film SN as a hard mask, the voids CH were formed from the surface through the inside of the n-guide layer 14 (GaN). More specifically, the ICP-RIE apparatus performed a dry etching using a chlorine-based gas to form the two-dimensionally arrayed voids CH in the n-guide layer 14 (
[First Embedding Layer]
The SiNx film SN of the guide layer substrate in which the voids CH having the two-dimensional periodicity were formed in the n-guide layer 14 were removed using hydrofluoric acid (HF) (
In the MOVPE apparatus, the temperature was increased while NH3 was supplied as a nitrogen source without supplying a group III raw material to heat the guide layer substrate up to 1150° C., and annealing was performed, for one minute after the temperature increase, to close the voids CH by mass transport, and the first embedding layer 14A was formed. Nitrogen (N2) was supplied as an atmosphere gas.
At this time, on the top of the n-guide layer 14, the first embedding layer 14A was formed from the planar surface formed by the top surface of the void 14C through the surface of the n-guide layer 14. The first embedding layer 14A had a layer thickness (D1) of 26 nm.
As illustrated in
Note that, in the formation of the first embedding layer 14A, the pits PT do not need to be regularly arrayed in the two-dimensional array on the surface of the first embedding layer 14A. It is only necessary to form the first embedding layer 14A by mass transport so as to have a surface condition in which the pits PT remain on the surface of the first embedding layer 14A.
The first embedding layer 14A is not limited to be GaN. As the first embedding layer 14A, another crystal, for example, a ternary crystal or a quaternary crystal, GaN-based semiconductor crystal layer may be formed, and the first embedding layer 14A preferably has a refractive index higher than that of the n-clad layer 13. For example, AlGaN having an Al composition lower than that of the n-clad layer 13, InGaN, or the like can be used.
[Second Embedding Layer]
After forming the first embedding layer 14A, a group III material gas (TMG) and a group V material gas (NH3) were supplied with a temperature maintained at 1150° C. to form the second embedding layer 21. In the embodiment, hydrogen (H2) was supplied at the same time.
The second embedding layer 21 was able to be formed without changing the shape of the voids 14C. A sum of a thickness (D1=26 nm) of the first embedding layer 14A and a thickness (D2=34 nm) of the second embedding layer 21 in this case was 60 nm.
As illustrated in
As described above, in the formation of the first embedding layer 14A, the embedding by mass transport is ended so as to have the surface condition in which the pits PT appear on the surface of the first embedding layer 14A, thus forming the first embedding layer 14A. The first embedding layer 14A is preferably formed so as to have the state in which the pits PT are regularly arrayed at least partially and remain on the surface of the first embedding layer 14A.
This is because when the pits PT are completely buried by migration atoms and/or molecules formed of Ga and/or GaN in mass transport during the formation of the first embedding layer 14A and the first embedding layer 14A becomes to have the surface condition similar to the other part, the migration atoms and/or molecules aggregate in a defective part or a part with transition remaining on the first embedding layer 14A to form a hillock (projecting portion), and this inhibit flattening of the second embedding layer 21.
Additionally, this is because as long as the pits PT become starting points of growth (regrowth) of the second embedding layer 21 or there is the surface condition in which the pits PT remain and not necessarily to be regular, the uniformity and flattening of the second embedding layer 21 are achieved.
For flattening with the pits PT embedded, the layer thickness of the second embedding layer 21 is preferably larger than the layer thickness of the first embedding layer 14A.
While the description has been given with the example of growing GaN as the second embedding layer 21, the crystal composition of the second embedding layer 21 may differ from that of the first embedding layer 14A. As the second embedding layer 21, another crystal, for example, a ternary crystal or quaternary crystal, GaN-based semiconductor crystal layer may be formed. For example, as the second embedding layer 21, a crystal layer that contains In in the composition, such as an lnGaN layer, can be used. Additionally, formation of a crystal layer having a refractive index with respect to an emission wavelength higher than that of the first embedding layer as the second embedding layer 21 allows increasing the coupling coefficient κ3 compared with the case of the GaN crystal layer.
The temperature at which the second embedding layer 21 is formed is preferably in a range of from 750° C. to 1150° C. in a case where the constituent material is GaN and in a range of from 750° C. to 900° C. in a case where the constituent material is InGaN.
[Growth of Active Layer, p-Side Semiconductor Layer]
Subsequently, as the active layer 15, a multi-quantum well (MQW) layer was grown. A barrier layer and a well layer of the MQW were GaN and InGaN, respectively. After the temperature of the substrate was decreased down to 800° C., triethyl gallium (TEG) was supplied as a supply source of group III atoms and NH3 was supplied as a nitrogen source, thus growing the barrier layer. The temperature same as that of the barrier layer was set, TEG and trimethyl indium (TMI) were supplied as supply sources of the group III atom and NH3 was supplied as a nitrogen source, thus growing the well layer. A center wavelength of PhotoLuminescence (PL) light from the active layer 15 in the embodiment was 410 nm.
After the growth of the active layer 15, the substrate was maintained at 800° C., and the p-side guide layer 16 was grown at the layer thickness of 100 nm. TEG and NH3 were supplied, and the p-side guide layer 16 was grown as undoped GaN (u-GaN) to which a dopant was not doped.
After the growth of the p-side guide layer 16, the substrate temperature was increased up to 1050° C. at a speed of 100° C./min to grow the electron blocking layer (EB L) 17 and the p-clad layer 18. TMG and TMA were supplied as group III atom sources and NH3 was supplied as a nitrogen source, thus growing the EBL 17. TMG and TMA were supplied as group III atom sources and NH3 was supplied as a nitrogen source, thus growing the p-clad layer 18. Al concentrations of the EBL 17 and the p-clad layer 18 were 18% and 6%, and layer thicknesses were 17 nm and 600 nm, respectively. In the respective growths of the EBL 17 and the p-clad layer 18, Bis-cyclopentadienyl magnesium (Cp2Mg) was suppled as a dopant simultaneously with a group III atom source gas and the nitrogen source. The p-clad layer (p-AlGaN clad layer) 18 activated in the air at 700° C. for five minutes had a carrier density of 4×1017 cm−3.
[Coupling Coefficient κ3]
As described above, to reduce the resonator loss in the photonic crystal surface emitting laser and perform the oscillation operation at the low threshold current density, it is effective to increase the coupling coefficient κ3 of the light wave propagating inside the photonic crystal layer 14P surface.
To examine this point, by solving the coupled wave equation of the GaN-based photonic crystal surface emitting laser element having the structure illustrated in
The photonic crystal surface emitting laser used to calculate the light coupling coefficient κ3 illustrated in
As illustrated in
As described above, in the embodiment, (i) annealing is performed on the guide layer substrate in which the voids CH having the two-dimensional periodicity are formed in the n-guide layer 14 while the gas containing the nitrogen source is supplied to close the openings of the voids by mass transport, thus forming the first embedding layer, and (ii) the group III raw material and the gas containing the nitrogen source are supplied to form the first embedding layer.
When the temperature increases up to the temperature at which mass transport occurs, the void shape changes such that surface energy becomes the smallest. That is, for example, side surfaces of the void of the C-plane GaN deform to the {10-10} planes where a dangling bond density is small. The voids are closed (obstructed) by annealing, and the first embedding layer having the surface formed of the (0001) plane is formed. At this time, the surface of the first embedding layer becomes the step-terrace structure in which bunching is performed at the width same as the cyclic interval of the voids. Since the annealing temperature is a temperature at which the mass transport occurs, the annealing temperature when the voids CH are closed in a case where the n-guide layer is formed of GaN is preferably in a range of from 1000° C. to 1200° C.
Additionally, the first embedding layer is formed of the same elements as that of the n-guide layer forming the voids.
The second embedding layer is formed by supplying the group III raw material to newly grow a layer. Accordingly, like the first embedding layer, the second embedding layer needs not to be formed of the element same as the n-guide layer forming the voids. However, from an aspect of light confinement, the second embedding layer is preferably made of a material having a refractive index to the same extent or more than that of the first embedding layer.
With the second embedding layer, the bunched surface structure of the first embedding layer is recovered, and the surface of the step-terrace structure with the step height equivalent to the height of one bilayer is obtained. Accordingly, the surface of the second embedding layer becomes the atomically flat (0001) plane.
The above-described method allows the distance between the photonic crystal layer and the active layer in the GaN-based photonic crystal surface emitting laser to be 100 nm or less. That is, in the GaN-based material photonic crystal surface emitting laser, the photonic crystal surface emitting laser in which the light coupling coefficient κ3 can be increased and the oscillation operation can be performed at the low threshold current density can be achieved.
[Void Embedding Method and Structure of Comparative Example]
In Comparative Example, the guide layer substrate was heated to 1050° C. in the MOVPE apparatus, the group III material gas (TMG) and the group V material gas (NH3) were supplied to perform crystal growth so as to form a depressed portion having the {10-11} facets on the substrate surface, and the opening of the void CH was closed (
At this time, a distance E1 between the (000-1) plane as the top surface of the embedded void CH and the (0001) plane as the outermost surface of the n-guide layer 14 was approximately 140 nm (
After the void CH was closed by the {10-11} facets, the supply of the group III material gas was stopped, the temperature was increased up to 1150° C. at a temperature increase speed of 100° C./min while the V group material gas (NH3) was supplied, and the temperature was held for one minute. This eliminates the {10-11} facets formed on the surface of the n-guide layer 14, and the surface became the flat (0001) plane. That is, the surface was flattened by mass transport.
At this time, a distance E2 between the surface (the top surface, the (000-1) plane) on the active layer 15 side of the void (cavity) 14C formed to be embedded and the surface (that is, the (0001) plane) of the n-guide layer 14 was approximately 105 nm.
That is, the void embedding method of Comparative Example preferentially grows the {10-11} facets to close the void, and subsequently forms the (0001) plane on the surface by annealing. The manufacturing method can thin the distance between the photonic crystal layer 14P and the active layer 15 only around 100 nm. That is, this method cannot increase the light coupling coefficient κ3 in the GaN-based photonic crystal surface emitting laser, and the oscillation operation at the low threshold current density is difficult.
[Crystallinity: SIMS Analysis]
Since oxygen is taken in during the formation of the void CH in the n-guide layer 14 by etching, oxygen is unintentionally taken into the first embedding layer as well.
In the above-described embodiment, since the group III material gas (TMG), the group V material gas (NH3), and hydrogen (H2) are supplied to form the second embedding layer 21, the oxygen unintentionally taken into the second embedding layer 21 significantly decreases. For example, the oxygen taken into the second embedding layer 21 becomes 5×1016 cm−3 or less.
Thus, it was confirmed that, in the second embedding layer 21, generation of group III atom voids in the crystal due to doping of high concentration oxygen was suppressed, and the film with satisfactory crystalline was able to be obtained. Therefore, the crystal layer growing on the second embedding layer 21 also has satisfactory crystallinity.
Various kinds of values and the like in the embodiment are merely examples and are appropriately changeable within the range not departing from the scope of the present invention.
While the description has been given in the embodiment with the example of the semiconductor structure layer 11 including the electron blocking layer 17, the electron blocking layer 17 may be omitted. Alternatively, the semiconductor structure layer 11 may include any semiconductor layer including a contact layer and a current diffusion layer.
Additionally, while this Specification has given the description with an example in which the first conductivity type semiconductor (the n-type semiconductor), the active layer, and the second conductivity type semiconductor (the p-type semiconductor as a conductivity type opposite to the first conductivity type) are grown in this order, the first conductivity type may be a p-type and the second conductivity type may be an n-type.
As details have been described above, the present invention allows providing the surface emitting laser that allows increasing the coupling coefficient κ3 of the light wave propagating in the direction parallel to the photonic crystal layer, reducing the resonator loss, and can perform the oscillation operation at the low threshold current density, and the manufacturing method of the surface emitting laser.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-164927 | Sep 2018 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2019/033938 | 8/29/2019 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2020/050130 | 3/12/2020 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 7206488 | Altug | Apr 2007 | B1 |
| 20070036189 | Hori | Feb 2007 | A1 |
| 20070109639 | Trevor et al. | May 2007 | A1 |
| 20070201526 | Hori | Aug 2007 | A1 |
| 20070280318 | Yoshimoto et al. | Dec 2007 | A1 |
| 20100246625 | Kawashima et al. | Sep 2010 | A1 |
| 20110039364 | Kawashima et al. | Feb 2011 | A1 |
| 20110237077 | Kawashima | Sep 2011 | A1 |
| 20120018758 | Matioli | Jan 2012 | A1 |
| 20120269224 | Nagatomo | Oct 2012 | A1 |
| 20130121358 | Hirose | May 2013 | A1 |
| 20140097456 | Kawashima | Apr 2014 | A1 |
| 20140327015 | Kawashima et al. | Nov 2014 | A1 |
| 20190067911 | Caër | Feb 2019 | A1 |
| 20200251887 | Noda et al. | Aug 2020 | A1 |
| 20210013700 | Noda et al. | Jan 2021 | A1 |
| Number | Date | Country |
|---|---|---|
| 2004111766 | Apr 2004 | JP |
| 2007234835 | Sep 2007 | JP |
| 2009130110 | Jun 2009 | JP |
| 2009302250 | Dec 2009 | JP |
| 2010109223 | May 2010 | JP |
| 4818464 | Jul 2010 | JP |
| 2010232488 | Oct 2010 | JP |
| 2011035078 | Feb 2011 | JP |
| 2011119349 | Jun 2011 | JP |
| 5082447 | Sep 2012 | JP |
| 2013135001 | Jul 2013 | JP |
| 2006062084 | Jun 2006 | WO |
| 2011013363 | Feb 2011 | WO |
| 2018155710 | Aug 2018 | WO |
| Entry |
|---|
| International Search Report (ISR) (and English translation thereof) dated Nov. 26, 2019 issued in International Application No. PCT/JP2019/033938. |
| Written Opinion dated Nov. 26, 2019 issued in International Application No. PCT/JP2019/033938. |
| Kawashima, et al., “GaN-based surface-emitting laser with two-dimensional photonic crystal acting as distributed-feedback grating and optical cladding”, Applied Physics Letters, Dec. 22, 2010, vo. 97, p. 251112. |
| Liang, et al., “Three-dimensional coupled-wave model for square-lattice photonic crystal lasers with transverse electric polarization: A general approach”, Phys. Rev. B vol. 84 (2011) 195119. |
| Miyake, et al., “Effects of Reactor Pressure on Epitaxial Lateral Overgrowth of GaN via Low-Pressure Metalorganic Vapor Phase Epitaxy”, 1999 Jpn. J. Appl. Phys. 38 L 1000. |
| Tanaka, et al., “Discussion of In-plane Diffraction and Threshold Gain Difference in PCSEL”, Preprints of Autumn Meeting of 2016 of Japan Society of Applied Physics. |
| Yoshimoto, et al., “GaN Photonic-Crystal Surface-Emitting Laser Operating at Blue-Violet Wavelengths”, Laser and Electro-Optics Society (LEOS) 2008, Nov. 2008, 21st Annual Meeting of the IEEE. |
| Tanaka, et al., “Discussion of In-plane Diffraction and Threshold Gain Difference in PCSEL”, Preprints of Autumn Meeting of 2016 of the Japan Society of Applied Physics. |
| Extended European Search Report (EESR) dated May 10, 2022, issued in counterpart European Application No. 19858088.8. |
| Matioli, et al., “Growth of embedded photonic crystals for GaN-based optoelectronic devices”, Journal of Applied Physics, American Institute of Physics. |
| Japanese Office Action (and English language translation thereof) dated Oct. 25, 2022, issued in counterpart Japanese Application No. 2018-164927. |
| Japanese Office Action (and English language translation thereof) dated Feb. 20, 2024, issued in counterpart Japanese Application No. 2023-072364. |
| Number | Date | Country | |
|---|---|---|---|
| 20210328406 A1 | Oct 2021 | US |
