US-12620780-B2 - Surface-emitting quantum cascade laser

US12620780B2US 12620780 B2US12620780 B2US 12620780B2US-12620780-B2

Abstract

According to one embodiment, a surface-emitting quantum cascade laser includes a substrate; a mesa portion of a semiconductor stacked body located on the substrate, and a reflective film located at a sidewall of the mesa portion. The mesa portion includes a light-emitting layer emitting light due to an intersubband transition of a carrier, and a photonic crystal layer including a two-dimensional diffraction grating.

Inventors

Rei Hashimoto
Tsutomu Kakuno
Kei Kaneko
Shinji Saito

Assignees

KABUSHIKI KAISHA TOSHIBA

Dates

Publication Date: 20260505
Application Date: 20210428
Priority Date: 20200619

Claims (14)

1 . A surface-emitting quantum cascade laser, comprising: a substrate; a first cladding layer provided on the substrate; a mesa portion of a semiconductor stacked body stacked in a stacking direction and located on the first cladding layer, the mesa portion including a light-emitting layer emitting light due to an intersubband transition of a carrier, a photonic crystal layer including a plurality of pits either arranged as or forming a two-dimensional diffraction grating, and a second cladding layer located on the photonic crystal layer and located in the plurality of pits; a reflective film located at opposing sidewalls of the mesa portion; a Fabry-Perot resonator located in the mesa portion; and an insulating film, a width direction between the opposing sidewalls being perpendicular to the stacking direction, and a width of the mesa portion in the width direction being less than a width of the first cladding layer in the width direction, wherein the mesa portion includes a guide layer located between the first cladding layer and the light-emitting layer, a width of the guide layer is smaller than the width of the first cladding layer in the width direction, the width of the first cladding layer in the width direction is the same as a width of the substrate in the width direction, the first cladding layer protrudes in both directions in the width direction with respect to the mesa portion, wherein the light-emitting layer, photonic crystal layer, second cladding layer and guide layer each have the same width in the width direction, the insulating film is located between the reflective film and the sidewall of the mesa portion, the first cladding layer has an uppermost planar surface extending under the mesa, the insulating film and the reflective film, and the guide layer and the insulating film are formed directly on the uppermost surface of the first cladding layer.
2 . The laser according to claim 1 , wherein the mesa portion has a rectangular prism shape including four sidewalls, and the reflective film is located on at least two opposite sidewalls of the four sidewalls.
3 . The laser according to claim 1 , wherein the mesa portion has a circular columnar shape.
4 . The laser according to claim 1 , wherein a reflectance of the reflective film for light emitted by the light-emitting layer is not less than 40%.
5 . The laser according to claim 1 , wherein the reflective film is a metal film.
6 . The laser according to claim 5 , wherein the metal film includes gold.
7 . The laser according to claim 5 , further comprising: a first electrode connected with the reflective film at the mesa portion; and an insulating film located between the reflective film and the sidewall of the mesa portion.
8 . The laser according to claim 1 , wherein the reflective film is a dielectric multilayer film.
9 . The laser according to claim 1 , further comprising: a first electrode connected with the reflective film at the mesa portion; and a second electrode located at a back surface of the substrate.
10 . The laser according to claim 9 , wherein the second electrode has a frame shape.
11 . The laser according to claim 1 , wherein the light-emitting layer includes an n-type Group III-V compound semiconductor doped with silicon.
12 . The laser according to claim 1 , wherein the first cladding layer does not extend into the mesa portion.
13 . The laser according to claim 7 , wherein a lowermost surface of the insulating film and a lowermost surface of the guide layer are coplanar.
14 . A surface-emitting quantum cascade laser, comprising: a substrate; a first cladding layer provided on the substrate; a mesa portion of a semiconductor stacked body stacked in a stacking direction and located on the first cladding layer, the mesa portion including a light-emitting layer emitting light due to an intersubband transition of a carrier, a photonic crystal layer including a plurality of pits either arranged as or forming a two-dimensional diffraction grating, and a second cladding layer located on the photonic crystal layer and located in the plurality of pits; a reflective film located at opposing sidewalls of the mesa portion; a Fabry-Perot resonator located in the mesa portion; a first electrode; and an insulating film, a width direction between the opposing sidewalls being perpendicular to the stacking direction, and a width of the mesa portion in the width direction being less than a width of the first cladding layer in the width direction, wherein the mesa portion includes a guide layer located between the first cladding layer and the light-emitting layer, a width of the guide layer is smaller than the width of the first cladding layer in the width direction, the width of the first cladding layer in the width direction is the same as a width of the substrate in the width direction, the reflective film is a metal film, the first electrode is connected with the reflective film at the mesa portion, the insulating film is located between the reflective film and the sidewall of the mesa portion, and a lowermost surface of the insulating film and a lowermost surface of the guide layer are coplanar.

Description

CROSS-REFERENCE TO RELATED APPLICATION This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-106268, filed on Jun. 19, 2020; the entire contents of which are incorporated herein by reference. FIELD Embodiments described herein relate generally to a surface-emitting quantum cascade laser. BACKGROUND A quantum cascade laser has been proposed in which surface emission made possible by utilizing a photonic crystal layer that includes a two-dimensional diffraction grating. The oscillation threshold current of such a surface-emitting quantum cascade laser undesirably increases when the oscillation wavelength that is controlled by the photonic crystal layer shifts from the peak wavelength of the gain curve of the light-emitting layer due to manufacturing fluctuation or the like. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view of a surface-emitting quantum cascade laser according to an embodiment; FIGS. 2A and 2B are schematic plan views of the surface-emitting quantum cascade laser according to the embodiment; FIG. 3 is a schematic plan view of a photonic crystal layer of the surface-emitting quantum cascade laser according to the embodiment; FIG. 4A shows a gain spectrum of an oscillation of the surface-emitting quantum cascade laser according to the embodiment; FIG. 4B shows current-output characteristics of the surface-emitting quantum cascade laser according to the embodiment; and FIG. 5 is a schematic cross-sectional view showing another example of a reflective film of the surface-emitting quantum cascade laser according to the embodiment. DETAILED DESCRIPTION According to one embodiment, a surface-emitting quantum cascade laser includes a substrate; a mesa portion of a semiconductor stacked body located on the substrate, and a reflective film located at a sidewall of the mesa portion. The mesa portion includes a light-emitting layer emitting light due to an intersubband transition of a carrier, and a photonic crystal layer including a two-dimensional diffraction grating. Embodiments will now be described with reference to the drawings. The same components in the drawings are marked with the same reference numerals. FIG. 1 is a schematic cross-sectional view of a surface-emitting quantum cascade laser according to the embodiment. The surface-emitting quantum cascade laser according to the embodiment includes a substrate 10, a mesa portion 20 of a semiconductor stacked body, a first electrode 31, a second electrode 32, and a reflective film 51. A first cladding layer 11 is located on the substrate 10; and the mesa portion 20 is located on the first cladding layer 11. The mesa portion 20 protrudes in a columnar shape on the first cladding layer 11 and includes a confinement structure for a current supplied to a light-emitting layer 13 via the first electrode 31 and the second electrode 32. FIG. 2A is a schematic plan view of the surface-emitting quantum cascade laser according to the embodiment when viewed from the light-emitting surface side (the lower surface side of FIG. 1). An X-direction and a Y-direction are two directions that are orthogonal to each other in a plane parallel to a light-emitting surface 10a. The mesa portion 20 has a rectangular prism shape that includes four sidewalls 20a, 20b, 20c, and 20d. In the example shown in FIG. 2A, the upper surface or lower surface of the mesa portion 20 is square. As shown in FIG. 1, the mesa portion 20 includes a first guide layer 12 located on the first cladding layer 11, the light-emitting layer 13 located on the first guide layer 12, a photonic crystal layer 14 located on the light-emitting layer 13, and a second cladding layer 15 located on the photonic crystal layer 14. The light-emitting layer 13 includes a quantum well structure that generates intersubband transitions of carriers. For example, the light-emitting layer 13 includes an n-type Group III-V compound semiconductor layer doped with silicon and emits light due to intersubband transitions of electrons. The refractive index of the first cladding layer 11 and the refractive index of the second cladding layer 15 are less than the refractive index of the first guide layer 12, the refractive index of the light-emitting layer 13, and the refractive index of the photonic crystal layer 14. The photonic crystal layer 14 includes a two-dimensional diffraction grating. The light that is emitted by the light-emitting layer 13 resonates in directions along the front surface of the light-emitting layer 13; and modes are selected by the two-dimensional diffraction grating and emitted in a substantially normal direction with respect to the front surface of the light-emitting layer 13. The substantially normal direction means a direction having an angle with respect to the front surface of the light-emitting layer 13 that is not less than 81° and not more than 99°. FIG. 3 is a schematic plan view of the photonic crystal layer 14.