WO2017158871A1

WO2017158871A1 - Semiconductor laser device

Info

Publication number: WO2017158871A1
Application number: PCT/JP2016/075677
Authority: WO
Inventors: 理究碇山; 裕之福水; 真司斎藤
Original assignee: 株式会社東芝
Priority date: 2016-03-15
Filing date: 2016-09-01
Publication date: 2017-09-21
Also published as: JP2017168591A

Abstract

A semiconductor laser device of the embodiments has a laminate, which has an active layer capable of emitting a laser light due to intersubband optical transitions, is capable of emitting said laser light, and comprises a compound semiconductor, and a metal oxide film, which covers the active layer which is exposed on at least one side surface of the laminate. The energy barrier for a carrier increases the further from the active layer toward the metal oxide film.

Description

Semiconductor laser device

Embodiments described herein relate generally to a semiconductor laser device.

In the active layer of a unipolar laser in which quantum well structures are cascade-connected, an intersubband optical transition can be generated to obtain an infrared laser beam.

As the number of cascade connections increases, the thickness of the active layer increases and the side area of the active layer increases. If the ratio of the current that flows along the side surface of the active layer without passing through the active layer increases, the effective current that contributes to light emission decreases. For this reason, the luminous efficiency is lowered, the temperature in the vicinity of the side surface is raised, and the element may be deteriorated.

JP 2014-3314 A

Provide a semiconductor laser device with improved luminous efficiency and reliability and easy output.

The semiconductor laser device according to the embodiment has an active layer capable of emitting laser light by intersubband optical transition, and can emit the laser light, and includes a stacked body made of a compound semiconductor, and a side surface of the stacked body And a metal oxide film covering the active layer exposed to at least one of the above. The energy barrier against carriers becomes higher from the active layer toward the metal oxide film.

FIG. 1A is a schematic perspective view of the semiconductor laser device of the first embodiment, FIG. 1B is a schematic cross-sectional view taken along the line AA, and FIG. 1C is a stack, a metal oxide film, and the like. It is a band figure of the interface vicinity of. 2A is a band diagram in which a natural oxide film is formed on the semiconductor surface, FIG. 2B is a band diagram of the semiconductor surface provided with the interface state reducing film, and FIG. 2C is a semiconductor according to a comparative example. It is a model perspective view of a laser apparatus. FIG. 3A is a schematic top view of the semiconductor laser device of the first embodiment, and FIG. 3B is a schematic top view of the first modification. It is a band figure of the interface vicinity of the semiconductor laser apparatus concerning 2nd Embodiment. FIG. 5A is a partial schematic plan view of a semiconductor laser device according to the third embodiment, and FIG. 5B is a partial schematic cross-sectional view along the line BB. FIG. 6A is a partial schematic plan view of a semiconductor laser device according to a modification of the third embodiment, and FIG. 6B is a partial schematic cross-sectional view along the line BB.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1A is a schematic perspective view of the semiconductor laser device of the first embodiment, FIG. 1B is a schematic cross-sectional view taken along the line AA, and FIG. 1C is a stack, a metal oxide film, and the like. It is a band figure of the thermal equilibrium state of the interface vicinity of.
The semiconductor laser device 10 includes a stacked body 20 and a metal oxide film 40.

The laminate 20 has an active layer 22 capable of emitting laser light 80 by optical transition between subbands. Further, the stacked body 20 can emit the laser light 80 along the extending direction of the ridge waveguide 70. The extending direction of the ridge waveguide 70 is parallel to the optical axis 81 of the laser light 80. The metal oxide film 40 covers the active layer 22 exposed on at least one of the four side surfaces of the stacked body 20 parallel to the extending direction of the ridge waveguide 70. The four side surfaces include two surfaces 20a of the stacked body 20 parallel to the extending direction and an end surface 20b of the stacked body 20 perpendicular to the extending direction. In FIG. 1A, the metal oxide film 40 is provided on two surfaces 20a and two end surfaces 20b.

The semiconductor laser device 10 can further include a substrate 30 made of InP or the like and a first electrode 50. The first electrode 50 is provided on the stacked body 20. Further, if the substrate 30 is conductive, for example, the second electrode 60 can be provided on the back surface of the substrate 30.

As shown in FIG. 1B, the stacked body 20 can be provided on a substrate 30 made of a single crystal semiconductor, for example. In this figure, the stacked body 20 includes an active layer 22, a first layer 25, and a second layer 23 from the substrate 30 side. The first layer 25 includes, for example, a first light guide layer 25a, a first cladding layer 25b, and a first contact layer 25c from the active layer 22 side. The second layer 23 includes, for example, a second light guide layer 23a, a second cladding layer 23b, and a buffer layer 23c from the active layer 22 side. The stacked body 20 can include, for example, an n-type layer.

In the active layer 22, for example, an intersubband transition region capable of emitting the laser beam 80 and an injection / relaxation region capable of relaxing the energy of carriers (for example, electrons) injected from the intersubband transition region are alternately arranged. It has a stacked cascade structure. The number of stacked layers can be 20 to 100, for example. If the injection / relaxation region, the first cladding layer 25b, the second cladding layer 23b, and the like are n-type layers, electrons cause an intersubband optical transition and laser light is emitted.

That is, the semiconductor laser device of this embodiment operates as a quantum cascade laser (QCL: Quantum Cascade Laser). Increasing the number of stages increases the output because the number of carrier transitions increases. The wavelength of the laser beam 80 can be determined without depending on the band gap energy of the semiconductor. For example, by setting the MQW structure appropriately, it is possible to obtain laser light having a wavelength between infrared and terahertz waves.

The laser beam 80 emitted from the active layer 22 is confined in the vertical direction by the first and second

cladding layers

25b and 23b. The laser beam 80 is guided and emitted in the direction of the optical axis 81 by the metal oxide film 40 provided on the two surfaces 20 a of the stacked body 20.

The first electrode 50 is connected to the contact layer 25 c of the first layer 25 through the opening 40 a provided on the metal oxide film 40.

The relative dielectric constant of the metal oxide film 40 is selected to be higher than the relative dielectric constant of silicon nitride (Si ₃ N ₄ ). The metal oxide film 40 is, for example, an oxide film containing any one metal selected from the group consisting of hafnium (Hf), aluminum (Al), zirconium (Zr), titanium (Ti), and tantalum (Ta), or an oxide thereof. The film is a laminated film. The metal oxide film is not limited to these. That is, the relative dielectric constant of the metal oxide film 40 is selected to be 8 or more, which is the lower limit value of the relative dielectric constant of aluminum oxide.

Next, the dielectric constant of the metal oxide film 40 is exemplified in (Table 1). In the present specification, the relative dielectric constant is represented by a measured value at 1 MHz. In addition, since the relative dielectric constant may change depending on the internal particle structure, manufacturing process, and the like, the numerical values in (Table 1) are an example.

The relative dielectric constant of the metal oxide film 40 of hafnium, aluminum, zirconium, titanium, tantalum or the like is as high as 8 or more. For this reason, the energy level of the conduction band of a compound semiconductor such as InGaAs, InAlAs, GaAs, or AlGaAs is raised toward the metal oxide film 40 by the fixed charge in the metal oxide film 40 (FIG. 1C). That is, the energy barrier against carriers (electrons in this figure) becomes higher as it goes from the active layer 22 to the metal oxide film 40.

The active layer 22 may have a structure in which MQW made of InGaAs (well layer) / InAlAs (barrier layer) or MQW made of GaAs (well layer) / AlGaAs (barrier layer) is cascade-connected. The conduction band lift acts as a barrier for electrons. For this reason, electrons do not flow through the interface between the metal oxide film 40 and the active layer 22, pass through the inside of the active layer 22 in a direction substantially perpendicular to the active layer 22, and can contribute to the intersubband optical transition. A thin natural oxide film may be locally formed between the stacked body 20 and the metal oxide film 40.

2A is a band diagram in a thermal equilibrium state where a natural oxide film is formed on the semiconductor surface, FIG. 2B is a band diagram in a thermal equilibrium state of the semiconductor surface provided with the interface state reducing film, and FIG. FIG. 3 is a schematic perspective view of a semiconductor laser device according to a comparative example.
As shown in FIG. 2A, the end surface and side surface of the laminate 120 have high-density interface states due to cleavage or scribing. Even if the thin natural oxide film 122 exists on the surface of the active layer, for example, carriers such as electrons are likely to flow along the side surface of the stacked body 120, and the current J1 is generated.

Further, as shown in FIG. 2B, the energy level of the conduction band is flat even if the interface state reducing film 124 such as silicon oxide or silicon nitride is provided on the end face or the side face. For this reason, current easily flows along the interface between the stacked body 120 and the interface state reducing film 124. The surface current J1 becomes a reactive current that does not contribute to the intersubband optical transition, and lowers the light emission efficiency. However, in QCL, unlike a pn junction semiconductor laser, there is little COD (catastrophic optical damage) degradation due to non-radiative recombination of holes and electrons at the end face of the active layer.

In QCL, for example, the active layer width is 5 to 20 μm, and the thickness of the active layer 22 is several μm. Therefore, the area of the active layer exposed on the side surface is wide. On the other hand, the pn junction laser diode has, for example, an active layer width of several μm and an active layer thickness of 0.1 μm. That is, even with the same planar size, the area exposed on the side surface of the active layer of the QCL is as wide as 50 to 200 times that of the pn junction laser diode.

In the semiconductor laser device according to the comparative example of FIG. 2C, the side surface (including the end surface) of the stacked body 120 is covered with an interface state reducing film 124 such as a covalent bond insulating film. Therefore, a current J1 that does not contribute to light emission flows from the well layer exposed on the wide side surface (including the end surface) along the interface. Since this current J1 does not flow through an internal multi-quantum well structure (MQW), it becomes a reactive current without contributing to the intersubband optical transition. Further, when an excessive current flows in a narrow region near the interface, a temperature rise occurs. In the vicinity of the interface where many crystal defects and interface states exist due to the temperature rise, dislocations are proliferated and the device is likely to deteriorate. Furthermore, if there is an interface state near the center of the band gap, the lifetime of electrons is short and the light emission transition is reduced. For this reason, luminous efficiency may further decrease.

On the other hand, in the first embodiment, the current flowing through the interface between the stacked body 20 including the active layer 22 and the metal oxide film 40 is reduced by the rise of the conduction band. For this reason, as shown in FIG. 1A, the current Is flowing through the side surface (including the end surface) is sufficiently reduced with respect to the current Io flowing through the intersubband transition region, and the output of the semiconductor laser device 10 is increased. In addition, high reliability can be achieved.

FIG. 3A is a schematic top view of the semiconductor laser device of the first embodiment, and FIG. 3B is a schematic top view of the first modification.
In FIG. 3A, the metal oxide film 40 covers two (side) surfaces and two end surfaces 20 b among the four side surfaces of the stacked body 20. Thus, if all the two surfaces 20a and two end surfaces 20b of the laminated body 20 are covered, the surface current can be reduced over the entire side surface of the laminated body 20. Note that the two surfaces and two end surfaces of the active layer 22 may be further covered. In the first modification shown in FIG. 3B, the metal oxide film 40 covers the two surfaces 20 a of the stacked body 20 parallel to the ridge waveguide 70. Note that the two surfaces of the active layer 22 parallel to the ridge waveguide 70 may be covered.

In the case of an edge-emitting laser, the process of forming the metal oxide film 40 on the side surface of the mesa-shaped ridge waveguide 70 can be performed in a wafer state. After the cleavage process, a low reflection film or a high reflection film can be formed on the end face 20b. However, in the structure of FIG. 3A, a process of further forming the metal oxide film 40 on the end face 20b exposed by the cleavage. Becomes more necessary and the number of processes increases.

FIG. 4 is a band diagram of a thermal equilibrium state in the vicinity of the interface of the semiconductor laser device according to the second embodiment.
In the second embodiment, a covalent bond insulating film 42 is provided on the surface of the stacked body 20 including a compound semiconductor, and a metal oxide film 40 is further provided on the surface of the covalent bond insulating film 42. The covalent bond insulating film 42 is, for example, silicon oxide (SiO ₂ ), silicon nitride (Si ₃ N ₄ ), silicon carbide (SiC), boron nitride (BN), aluminum nitride (AlN), or the like. The thicknesses of the metal oxide film 40 and the covalent bond insulating film 42 can be set to 2 nm or more, for example. The metal oxide film 40 is made of the same material as that in the first embodiment. The metal oxide film 40 functions as an ion bond insulator. Table 2 is an example of the relative dielectric constant of the covalent bond insulator.

When the covalent bond insulating film 42 is silicon oxide, the direction of the polarity of the dipole generated at the interface varies depending on the oxygen surface density of the metal oxide 40. For example, Al ₂ in oxygen surface dense _Al ₂ _O 3 / SiO 2 / semiconductor or _HfO ₂ / SiO 2 / semiconductor structure, at the interface between the covalent insulating film 42 and the metal oxide film 40, a high dielectric constant Negative fixed charges are generated on the O ₃ and HfO ₂ sides. Further, in the configuration of Y ₂ O ₃ / SiO ₂ / semiconductor or La ₂ O ₃ / SiO ₂ / semiconductor having a low oxygen surface density, positive fixed charges are generated on the Y ₂ O ₃ or La ₂ O ₃ side. Y ₂ O _{3 has} a relative dielectric constant of about 11.

When the stacked body 20 includes InGaAs, InAlAs, GaAs, and AlGaAs, the metal oxide film 40 of the second embodiment acts as a negative fixed charge film, and the energy level of the conduction band of the stacked body 20 is a covalent bond. It lifts toward the interface with the insulating film 42. For this reason, it can suppress that the unnecessary electric current which does not contribute to the optical transition between subbands flows into the orthogonal | vertical direction along the interface vicinity.

FIG. 5A is a partial schematic plan view of a semiconductor laser device according to the third embodiment, and FIG. 5B is a partial schematic cross-sectional view along the line BB.
The semiconductor laser device 11 includes a stacked body 20, a first electrode 80, and a metal oxide film 46. The stacked body 20 includes a first active layer 22 capable of emitting laser light by intersubband optical transition, and a plurality of pits 29 provided on the active layer 22 so as to form a two-dimensional lattice. The first layer 25 having the surface 25 c, the active layer 22, and the substrate 30 are included. The pit 29 is a region in which a triangular prism region is cut out from the first surface 25 c in the depth direction of the first layer 25.

The first electrode 80 is provided on the flat surface 25 a of the first surface 25 c of the first layer 25, and has a frame portion 81 and a stripe portion 82. A region surrounded by the frame portion 81 and the stripe portion 82 becomes a periodic opening, and the pit portion 25b constituting the first surface 25c is exposed. The metal oxide film 46 covers at least the active layer 22 exposed on the side surface of the stacked body 20.

Each pit 29 is asymmetric with respect to a line parallel to the grid. Laser light is emitted in a direction perpendicular to the active layer 22 from the pits 29 exposed at the openings. That is, the semiconductor laser device 11 of the third embodiment is a surface emitting type.

In this embodiment, metal oxide films 46 are provided on the four side surfaces of the stacked body 20. The energy level of the conduction band of the exposed active layer 22 is raised toward the interface with the metal oxide film 46. For this reason, it is suppressed that an unnecessary electric current flows along an interface, and luminous efficiency and reliability are improved.

FIG. 6A is a partial schematic plan view of a semiconductor laser device according to a modification of the third embodiment, and FIG. 6B is a partial schematic cross-sectional view along the line BB.
The inner walls of the plurality of pits 29 can be further covered with the metal oxide film 47. In this way, the current flowing along the inner wall of the pit 29 can be suppressed. For this reason, it is possible to suppress unnecessary current from flowing in the two-dimensional lattice and to make the current uniform over the entire surface of the two-dimensional lattice.

According to the first to third embodiments and the modifications associated therewith, a semiconductor laser device with improved luminous efficiency and reliability and easy output can be provided. These semiconductor laser devices can be widely applied to laser processing, environmental measurement, exhalation measurement, and the like.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

Claims

An active layer capable of emitting laser light by optical transition between subbands, capable of emitting the laser light, and a laminate made of a compound semiconductor;
A metal oxide film covering the active layer exposed on at least one of the side surfaces of the laminate;
With
The semiconductor laser device, wherein an energy barrier against carriers increases as going from the active layer to the metal oxide film.
The semiconductor laser device according to claim 1, wherein the stacked body includes an n-type layer.
2. The semiconductor laser device according to claim 1, wherein the metal oxide film has a relative dielectric constant of 8 or more.
3. The semiconductor laser device according to claim 2, wherein the metal oxide film has a relative dielectric constant of 8 or more.
4. The semiconductor laser device according to claim 3, wherein the metal oxide film is an oxide film including any one of a group consisting of hafnium, aluminum, zirconium, titanium, and tantalum, or a stack of the oxide films.
The semiconductor laser device according to claim 4, wherein the metal oxide film is an oxide film including any one of the group consisting of hafnium, aluminum, zirconium, titanium, and tantalum, or a stack of the oxide films.
7. The semiconductor laser device according to claim 1, further comprising a covalent bond insulating film provided between the stacked body and the metal oxide film.
The semiconductor laser device according to claim 7, wherein the covalent bond insulating film is one of a group consisting of SiO 2 , Si 3 N 4 , SiC, BN, and AlN.
The laminate includes a ridge waveguide;
The laser light is emitted along the extending direction of the ridge waveguide;
The side surface of the laminate includes a surface parallel to the extending direction and an end surface perpendicular to the extending direction.
The semiconductor laser device according to claim 1.
A first electrode provided on the laminate and having a periodic opening;
The laminate includes a first layer provided on the active layer,
The first surface of the first layer includes a flat region provided with the first electrode, and a pit region provided such that a plurality of pits form a two-dimensional lattice,
Each pit is asymmetric with respect to a line parallel to the sides of the grid,
The semiconductor laser device according to claim 1, wherein the laser light is emitted in a direction perpendicular to the active layer from a pit exposed in the opening.
11. The semiconductor laser device according to claim 10, wherein the metal oxide film further covers inner walls of the plurality of pits.