Method for producing quantum cascade laser and quantum cascade laser

A method for producing a quantum cascade laser includes the steps of forming a laser structure including a mesa structure and a buried region embedding the mesa structure; forming a mask on the laser structure, the mask including a first pattern that defines a λ/4 period distribution Bragg reflector structure and a second pattern that defines a 3λ/4 period distribution Bragg reflector structure; and forming a first distribution Bragg reflector structure, a second distribution Bragg reflector structure, and a semiconductor waveguide structure by dry-etching the laser structure through the mask, the semiconductor waveguide structure including the mesa structure that has first and second end facets. The first distribution Bragg reflector structure is optically coupled to the first end facet. The second distribution Bragg reflector structure is optically coupled to the second end facet. Here, λ denotes a value of an oscillation wavelength of the quantum cascade laser in vacuum.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a quantum cascade laser and to a quantum cascade laser.

2. Description of the Related Art

Japanese Patent Application No. 2001-136193 discloses a quantum cascade laser.

SUMMARY OF THE INVENTION

A quantum cascade laser emits light in a mid-infrared wavelength band, for example, a 3 to 10 μm wavelength band, by using inter-subband transition. Quantum cascade lasers incorporated in portable devices, for example, gas detectors, are required to operate at low power consumption. Decreasing the cavity length of a quantum cascade laser is effective for decreasing the threshold current and enables low power consumption. Specifically, the threshold current may be effectively decreased by setting the cavity length to, for example, 50 μm to 500 μm. Providing a high-reflection (HR) coating on an end facet of a semiconductor laser is also effective for decreasing the threshold current and the power consumption. The reflectivity of the high-reflection coating is set to be, for example, 90% or higher.

A reflection coating on an end facet of a semiconductor laser is usually formed on an end facet of a bar-shape semiconductor product, for example, a semiconductor laser bar. A semiconductor laser bar that includes an array of semiconductor lasers of short cavity lengths is very small and needs to be handled carefully. Moreover, in order to obtain a high reflectivity, both end facets of this semiconductor laser bar may be coated with thick end facet coatings.

Instead of forming high-reflection coating films on end facets of a semiconductor laser, a distribution Bragg reflector (DBR) may be used as a reflection mirror of a laser cavity. When a distribution Bragg reflector constitutes a laser cavity, the issue of handling of the laser bar in forming high-reflection end facet coatings on both end facets of a short-cavity-length laser bar may be avoided.

A quantum cascade laser includes two reflectors that constitute a laser cavity. One of the reflectors has a high reflectivity and the other one of the reflectors has a lower reflectivity than the other reflector. What is required is to enhance flexibility of setting the reflectivity of each of plural reflectors constituting the laser cavity.

A method for producing a quantum cascade laser according to an aspect of the present invention includes the steps of (a) forming a laser structure on a principal surface of a substrate, the laser structure including a mesa structure and a buried region embedding the mesa structure, the mesa structure including a quantum cascade core layer; (b) forming a mask on the laser structure, the mask including a first pattern that defines a λ/4 period distribution Bragg reflector structure and a second pattern that defines a 3λ/4 period distribution Bragg reflector structure; and (c) forming a first distribution Bragg reflector structure having a λ/4 periodic structure, a second distribution Bragg reflector structure having a 3λ/4 periodic structure, and a semiconductor waveguide structure by dry-etching the laser structure through the mask, the semiconductor waveguide structure including the mesa structure that has a first end facet and a second end facet. The first distribution Bragg reflector structure is optically coupled to the first end facet of the semiconductor waveguide structure. The second distribution Bragg reflector structure is optically coupled to the second end facet of the semiconductor waveguide structure. Here, λ denotes a value of an oscillation wavelength of the quantum cascade laser in vacuum.

A quantum cascade laser according to another aspect of the present invention includes a mesa structure having a first end facet and a second end facet, the mesa structure including a quantum cascade core layer; a first distribution Bragg reflector structure that is optically coupled to the first end facet of the mesa structure, the first distribution Bragg reflector structure having a λ/4 periodic structure; and a second distribution Bragg reflector structure that is optically coupled to the second end facet of the mesa structure, the second distribution Bragg reflector structure having a 3λ/4 periodic structure. The first distribution Bragg reflector structure includes a first semiconductor wall placed a first interval apart from the first end facet of the mesa structure. The second distribution Bragg reflector structure includes a second semiconductor wall placed a second interval apart from the second end facet of the mesa structure. The second semiconductor wall has a height different from that of the first semiconductor wall. In addition, the first semiconductor wall and the second semiconductor wall each have a width larger than a width of the mesa structure. Here, λ denotes a value of an oscillation wavelength of the quantum cascade laser in vacuum.

The above-described object of the present invention and other objects, features, and advantageous effects will be more readily apparent from the detailed description of preferable embodiments of the present invention with reference to the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some specific examples will now be described.

A method for producing a quantum cascade laser according to an embodiment of the present invention includes the steps of (a) forming a laser structure on a principal surface of a substrate, the laser structure including a mesa structure and a buried region embedding the mesa structure, the mesa structure including a quantum cascade core layer; (b) forming a mask on the laser structure, the mask including a first pattern that defines a λ/4 period distribution Bragg reflector structure and a second pattern that defines a 3λ/4 period distribution Bragg reflector structure; and (c) forming a first distribution Bragg reflector structure having a λ/4 periodic structure, a second distribution Bragg reflector structure having a 3λ/4 periodic structure, and a semiconductor waveguide structure by dry-etching the laser structure through the mask, the semiconductor waveguide structure including the mesa structure that has a first end facet and a second end facet. The first distribution Bragg reflector structure is optically coupled to the first end facet of the semiconductor waveguide structure. The second distribution Bragg reflector structure is optically coupled to the second end facet of the semiconductor waveguide structure. Here, λ denotes a value of an oscillation wavelength of the quantum cascade laser in vacuum.

According to the method for producing a quantum cascade laser, both the second distribution Bragg reflector structure having a 3λ/4 periodic structure and the first distribution Bragg reflector structure having a λ/4 periodic structure are formed by the process of dry-etching the laser structure. Etching proceeds faster in the area where the second distribution Bragg reflector structure having a 3λ/4 periodic structure is to be formed than in the area where the first distribution Bragg reflector structure having a λ/4 periodic structure is to be formed. As a result, the etch depth of the 3λ/4 periodic structure is larger than the etch depth of the λ/4 periodic structure. Because of this periodic structure having a large depth, the second distribution Bragg reflector structure has a relatively large reflectivity. Likewise, because of this periodic structure having a small depth, the first distribution Bragg reflector structure has a relatively small reflectivity. This difference in structure makes it possible to generate a difference in reflectivity between the two distribution Bragg reflector structures from a technical viewpoint different from the difference in reflectivity generated by the 3λ/4 period and the λ/4 period.

In the method for producing a quantum cascade laser according to an embodiment, the substrate preferably includes a first region, a second region, and a third region disposed between the first region and the second region. The first distribution Bragg reflector structure includes a first semiconductor wall disposed in the first region to constitute a distribution Bragg reflector structure, the first semiconductor wall being placed a first interval apart from the first end facet of the semiconductor waveguide structure. The second distribution Bragg reflector structure includes a second semiconductor wall disposed in the second region to constitute a distribution Bragg reflector structure, the second semiconductor wall being placed a second interval apart from the second end facet of the semiconductor waveguide structure. In addition, the semiconductor waveguide structure is disposed in the third region.

According to the method for producing a quantum cascade laser, the first semiconductor wall of the first distribution Bragg reflector structure is placed a first interval apart from the first end facet of the semiconductor waveguide structure. The second semiconductor wall of the second distribution Bragg reflector structure is placed a second interval apart from the second end facet of the semiconductor waveguide structure. The first interval and the second interval, both of which are simultaneously produced in the dry etching step for processing the laser structure, are different from each other. A mask used in this dry etching step includes a first pattern for the first distribution Bragg reflector structure and a second pattern for the second distribution Bragg reflector structure. The first pattern includes a first opening that corresponds to the first interval between the first semiconductor wall and the first end facet of the semiconductor waveguide structure. The second pattern includes a second opening that corresponds to the second interval between the second semiconductor wall and the second end facet of the semiconductor waveguide structure. Because of the difference in width between the first opening and the second opening, the difference in local etching rate between individual areas is generated. Specifically, the etch depth obtained by a wide opening is larger than the etch depth obtained by a narrow opening. This difference (difference in width) generates the difference in height between the first semiconductor wall and the second semiconductor wall.

In the method for producing a quantum cascade laser according to an embodiment, preferably, the mesa structure extends in a direction of a first axis. The first semiconductor wall protrudes in a direction of a second axis that intersects the principal surface of the substrate. The first distribution Bragg reflector structure includes a first side wall that connects a first end of the first semiconductor wall to the semiconductor waveguide structure and a second side wall that connects a second end of the first semiconductor wall to the semiconductor waveguide structure. In addition, the first end and the second end of the first semiconductor wall are arranged in a direction of a third axis that intersects the first axis and the second axis.

According to the method for producing a quantum cascade laser, the first and second ends of the first semiconductor wall are respectively connected to the first and second side walls. Thus, etching gas is supplied through the first opening that corresponds to the first interval between the first end facet of the semiconductor waveguide structure and the first semiconductor wall. The area of the first opening changes depending on the interval between the first side wall and the second side wall.

In the method for producing a quantum cascade laser according to an embodiment, preferably, the step of dry-etching the laser structure is performed by using an etchant containing halogen-based gas.

According to the method for producing a quantum cascade laser, the etchant containing halogen gas enables etching of a III-V group compound semiconductor.

A quantum cascade laser according to an embodiment includes (a) a mesa structure having a first end facet and a second end facet, the mesa structure including a quantum cascade core layer; (b) a first distribution Bragg reflector structure that is optically coupled to the first end facet of the mesa structure, the first distribution Bragg reflector structure having a λ/4 periodic structure; and (c) a second distribution Bragg reflector structure that is optically coupled to the second end facet of the mesa structure, the second distribution Bragg reflector structure having a 3λ/4 periodic structure. The first distribution Bragg reflector structure includes a first semiconductor wall placed a first interval apart from the first end facet of the mesa structure. The second distribution Bragg reflector structure includes a second semiconductor wall placed a second interval apart from the second end facet of the mesa structure. The second semiconductor wall has a height different from that of the first semiconductor wall. In addition, the first semiconductor wall and the second semiconductor wall each have a width larger than a width of the mesa structure. Here, λ denotes a value of an oscillation wavelength of the quantum cascade laser in vacuum.

In the quantum cascade laser, the first interval between one side surface of the first semiconductor wall of the first distribution Bragg reflector structure and the first end facet of the mesa structure is correlated to a λ/4 periodic structure. The second interval between one side surface of the second semiconductor wall of the second distribution Bragg reflector structure and the second end facet of the mesa structure is correlated to the 3λ/4 periodic structure. The first distribution Bragg reflector structure and the second distribution Bragg reflector structure respectively have the first semiconductor wall and the second semiconductor wall each having a width larger than a width of the mesa structure. This structure is suitable for reflecting light in the infrared wavelength range. Since the height of the first semiconductor wall is different from the height of the second semiconductor wall, the reflectivity of each of these distribution Bragg reflector structures is adjustable by controlling the height of the semiconductor walls. Specifically, the distance between the lower surface of the quantum cascade core layer and the lower end of the first semiconductor wall is different from the distance between the lower surface of the quantum cascade core layer and the lower end of the second semiconductor wall. This difference in distance generates a difference in reflecting light propagating in the semiconductor portion below the lower surface of the quantum cascade core layer.

DETAILED DESCRIPTION OF EMBODIMENTS

The findings of the present invention can be easily understood from the following detailed description making reference to the attached drawings illustrating examples.

Embodiments of the method for producing a quantum cascade laser and embodiments of the quantum cascade laser will now be described with reference to the attached drawings. If possible, the same parts are represented by the same reference symbols.

FIGS. 1A to 1C, 2A, 2B, 3A, 3B, 4, 5, and 7A to 7Care schematic views of major steps of the method for producing a quantum cascade laser according to one embodiment. In the production process, a laser structure is formed and processed to form first and second distribution Bragg reflector (DBR) structures. In this embodiment, the laser structure is first formed. The method for forming the laser structure will now be described.

A substrate used for crystal growth of semiconductor layers (for example, a substrate11shown inFIG. 1A) is formed. This substrate may include, for example, a semiconductor wafer. Specifically, the substrate includes a III-V compound semiconductor such as InP. As illustrated inFIG. 1A, a stacked semiconductor layer13is grown on a principal surface11aof the substrate11. For example, a metal-organic vapor phase epitaxy method and/or a molecular beam epitaxial growth method may be employed for this growth. The stacked semiconductor layer13includes, for example, a semiconductor layer13afor forming a quantum cascade core layer and a semiconductor layer13bfor forming an upper cladding layer. If needed, the stacked semiconductor layer13may include a semiconductor layer13cfor forming a contact layer or a cap layer. Furthermore, if needed, the stacked semiconductor layer13may include a semiconductor layer for forming a lower cladding layer or a buffer layer.

Example of Substrate11: n-Type InP Wafer

Example of the Stacked Semiconductor Layer13:

Semiconductor layer13afor forming a quantum cascade core layer: InGaAs/AlInAs multi quantum well (MQW) structure, 3 μm in thickness

Semiconductor layer13cfor forming a contact layer: n-type GaInAs, 100 nm in thickness Semiconductor layer for forming a lower cladding layer or a buffer layer: n-type InP

The MQW structure includes InGaAs/AlInAs stacked layer arranged to enable the quantum cascade laser to oscillate in a wavelength range of 3 to 10 μm. The upper cladding layer has the same conductivity type as the semiconductor constituting the substrate11. The lower cladding layer or the buffer layer has the same conductivity type as the semiconductor constituting the substrate11.

An insulator mask (insulator mask15shown inFIG. 1B) is formed on the stacked semiconductor layer13. The insulator mask15has a pattern that defines the shape of a waveguide mesa that includes a core layer (active layer). In this embodiment, the pattern of the insulator mask15has a stripe shape that traverses from one side to the opposite side of one of a plurality of device sections on a wafer. The pattern of the insulator mask15extends in a first axis Ax1direction. The insulator mask15contains, for example, a silicon-based inorganic insulator film. The silicon-based inorganic insulator contains, for example, silicon nitride (SiN), silicon dioxide (SiO2), or silicon oxide nitride (SiON). The SiN film is formed by, for example, a chemical vapor deposition (CVD) method. The thickness of the insulator mask15is about 100 to 1000 nm. As illustrated inFIG. 1B, the stacked semiconductor layer13is etched through the insulator mask15. As a result of etching, the stacked semiconductor layer13and the surface portion of the substrate11are processed to form a mesa structure17. The mesa structure17has a mesa shape and extends in the direction of the first axis Ax1. The mesa structure17is disposed on the principal surface11aof the substrate11.FIG. 1Bis a cross-sectional view taken in a direction intersecting the first axis Ax1. The mesa height HD of the mesa structure17is about 5 to 10 μm. In the embodiment, the mesa height HD is, for example, 7 μm. The mesa width W0of the mesa structure17is about 3 to 10 μm. In the embodiment, the mesa width W0is, for example, 10 μm. The mesa structure17includes a core layer17athat includes a quantum cascade structure, an upper cladding layer17b, and a contact layer17c. Etching is performed by dry-etching using a halogen-based etchant. Examples of the halogen-based etchant include chlorine, hydrogen chloride, silicon tetrachloride, boron trichloride, hydrogen bromide, and hydrogen iodide. The insulator mask15remains unremoved. In the etching step, a first substrate product SP1that includes the mesa structure17is formed. A substrate surface of the first substrate product SP1and side surfaces of the mesa structure17are exposed. The upper surface of the mesa structure17is covered with the insulator mask15.

As illustrated inFIG. 1C, after etching, a semiconductor layer is grown on the side surfaces of the mesa structure17on the first substrate product SP1so as to form a buried region25that embeds the mesa structure17. In this embodiment, a desired semiconductor growth is performed on the first substrate product SP1to bury the mesa structure17. During this crystal growth, a semiconductor layer is selectively grown on the substrate surface of the first substrate product SP1and the side surfaces of the mesa structure17. Meanwhile, the semiconductor layer is not grown on the upper surface of the mesa structure17covered with the insulator mask15. The buried region25is composed of a semi-insulating III-V compound semiconductor, for example, iron-doped InP.

After forming the buried region25, the insulator mask15is removed from the first substrate product SP1to obtain a second substrate product SP2. The insulator mask15made of SiN is etched away with, for example, buffered hydrofluoric acid. After removing the insulator mask15, an upper surface17dof the mesa structure17is exposed at the surface of the second substrate product SP2.

A laser structure19is formed through these steps. The laser structure19includes a substrate11, a mesa structure17on the principal surface11aof the substrate11, and a buried region25that embeds the mesa structure17on the principal surface11aof the substrate11.FIG. 1Cillustrates one of the device sections of the substrate11. The wafer on which the buried region25is to be grown includes not only one mesa structure17shown inFIG. 1Cbut also an array of one or more mesa structures that are substantially identical to the mesa structure17. The buried region25is formed between the mesa structures in this array.

After removal of the insulator mask15, a patterned mask is formed on the mesa structure17and the buried region25. The specific procedure therefor is as follows.FIG. 2Ais a cross-sectional view of the mesa structure17taken along the first axis Ax1. The mesa structure17extends in the direction of the first axis Ax1. An insulating film27used as a mask for forming a reflector is formed on the second substrate product SP2. This insulating film27is composed of, for example, a silicon-based inorganic insulator. Examples of the silicon-based inorganic insulator include silicon nitride, silicon dioxide, and silicon oxide nitride. Silicon nitride, silicon dioxide, and silicon oxide nitride respectively include, for example, SiN, SiO2, and SiON. These silicon-based inorganic insulators are formed by, for example, a chemical vapor deposition (CVD) method. The thickness of the insulating film27is about 100 to 1000 nm. In this embodiment, the thickness of the insulating film27is 500 nm.

Referring toFIG. 2A, a resist is applied and exposed to form a pattern of a mask for forming a reflector. Specifically, a resist is applied onto the insulating film27and then exposure EXPS for transferring the pattern of a mask for forming a reflector onto the resist is performed so as to form a resist mask29. The width of the reflector to be formed is larger than the width of the mesa structure17; hence, the mask pattern is formed on the mesa structure17and the buried region25. During the exposure EXPS, the pattern of the reflector mask is transferred onto the resist. As a result, a resist mask29is formed. The pattern of the resist mask29is wider than the mesa structure17. InFIG. 2A, the mesa structure17extends in the direction of the first axis Ax1. The resist mask29includes a first pattern portion29a, a second pattern portion29b, and a waveguide pattern portion29cin each of the device sections. The first pattern portion29adefines the distribution Bragg reflector structure having one of the λ/4 period and the 3λ/4 period. The second pattern portion29bdefines the distribution Bragg reflector structure having the other one of the λ/4 period and the 3λ/4 period. The opening size of the pattern portion (one of29aand29b) having the λ/4 period is smaller than the opening size of the pattern portion (the other one of29aand29b) having the 3λ/4 period. Here, λ represents the emission wavelength of the quantum cascade laser (wavelength in vacuum).

The insulating film27is dry-etched through this resist mask29so as to form a mask31for defining the shape of the reflector, as illustrated inFIG. 2B. The mask31includes a first mask portion31a(the pattern that defines the distribution Bragg reflector structure having one of the λ/4 period and the 3λ/4 period) corresponding to the first pattern portion29a, a second mask portion31b(the pattern that defines the distribution Bragg reflector structure having the other one of the λ/4 period and the 3λ/4 period) corresponding to the second pattern portion29b, and a third mask portion31c(the pattern that defines the waveguide length) corresponding to the waveguide pattern portion29cin each of the device sections.

In forming the reflector, as illustrated inFIG. 3A, the semiconductor region below the mask31, namely, the buried region25and the mesa structure17in this embodiment, is dry-etched (ETCH) through the mask31so as to form a third substrate product SP3. Dry-etching ETCH is performed by using, for example, halogen-based gas as an etchant in an inductive coupled plasma reactive ion etching (ICP-RIE) apparatus. The third substrate product SP3includes a first distribution Bragg reflector structure33, a second distribution Bragg reflector structure35, and a semiconductor waveguide structure37that constitute the quantum cascade laser. In the semiconductor waveguide structure37, a mesa structure39has a length in the range of 3 to 10 μm, for example, and is embedded by the buried region25that remains unetched. The mask31has openings derived from the reflector pattern of the resist mask29.

Example of Dry Etching ETCH:

Etching method of etching apparatus: inductive coupled plasma reactive ion etching (ICP-RIE), capacitative coupled plasma reactive ion etching (CCP-RIE), or electron cyclotron resonance plasma reactive ion etching (ECR-RIE).

The first distribution Bragg reflector structure33includes first semiconductor walls34a,34b, and34cformed by etching. The second distribution Bragg reflector structure35includes second semiconductor walls36a,36b, and36cformed by etching. Each of the first semiconductor walls34a,34b, and34chas a width larger than the mesa width W0of the mesa structure39. The first semiconductor walls34a,34b, and34care defined by first openings34d,34e,34f, and34g. The first semiconductor walls34a,34b, and34cand the first openings34d,34e,34f, and34gare alternately arranged to form a periodic change in refractive index. Each of the second semiconductor walls36a,36b, and36chas a width larger than the mesa width W0of the mesa structure39. The second semiconductor walls36a,36b, and36care defined by second openings36d,36e,36f, and36g.

The period of the first distribution Bragg reflector structure33is different from the period of the second distribution Bragg reflector structure35. The height (H1) of the first semiconductor walls34a,34b, and34cof the first distribution Bragg reflector structure33is different from the height (H2) of the second semiconductor walls36a,36b, and36cof the second distribution Bragg reflector structure35. The depth (D1) of the first openings34d,34e, and34fof the first distribution Bragg reflector structure33is different from the depth (D2) of the second openings36d,36e, and36fof the second distribution Bragg reflector structure35.

List of Combinations of Period and Depth (Height)

Structure A: a combination of a shallow distribution Bragg reflector structure having a λ/4 period and a deep distribution Bragg reflector structure having a 3λ/4 period

Structure B: a combination of a deep distribution Bragg reflector structure having a λ/4 period and a shallow distribution Bragg reflector structure having a 3λ/4 period

The period of a distribution Bragg reflector structure is correlated to the reflectivity of the distribution Bragg reflector structure. Assuming that a distribution Bragg reflector structure having a λ/4 period and a distribution Bragg reflector structure having a 3λ/4 are identical except for their period, the reflectivity of the distribution Bragg reflector structure having a λ/4 period is larger than the reflectivity of the distribution Bragg Reflector Structure Having a 3λ/4 Period.

In this embodiment, the structure A (a combination of a shallow distribution Bragg reflector structure having a λ/4 period and a deep distribution Bragg reflector structure having a 3λ/4 period) is formed. This structure is advantageous in that it is formed by conducting dry etching once through a single mask31. A quantum cascade laser generates a long-wavelength laser beam in an infrared region, for example, the mid-infrared wavelength region. A long-wavelength beam not only propagates in the mesa structure39but also spreads to the substrate11as it propagates. Therefore, the depth of the distribution Bragg reflector structure is correlated to the reflection of optical components propagating in the substrate11.

FIG. 4is a plan view of a laser structure after etching. Referring toFIGS. 3A and 4, the mask31is left unetched. In each of the device sections, the first mask portion31a, the second mask portion31b, and the third mask portion31cof the mask31respectively correspond to the first pattern portion29a, the second pattern portion29b, and the waveguide pattern portion29cof the resist mask29. In this embodiment, the first distribution Bragg reflector structure33has a λ/4 period and the second distribution Bragg reflector structure35has a 3λ/4 period. The height of the second semiconductor walls36a,36b, and36cof the second distribution Bragg reflector structure35is larger than the height of the first semiconductor walls34a,34b, and34cof the first distribution Bragg reflector structure33. The depth of the second openings36d,36e, and36fof the second distribution Bragg reflector structure35is larger than the depth of the first openings34d,34e, and34fof the first distribution Bragg reflector structure33.

In the third substrate product SP3, the first distribution Bragg reflector structure33, the second distribution Bragg reflector structure35, and the mesa structure39are aligned in a direction of the first axis Ax1. The semiconductor waveguide structure37includes a first end facet37aand a second end facet37b. The first end facet37aand the second end facet37bof the mesa structure39in the semiconductor waveguide structure37are respectively optically coupled to the first distribution Bragg reflector structure33and the second distribution Bragg reflector structure35. As a result, the mesa structure39, the first distribution Bragg reflector structure33, and the second distribution Bragg reflector structure35constitute a laser cavity.

The first openings34dto34gand the second openings36dto36gare each defined by a first width L1(length in the direction of the first axis Ax1) and a second width L2(length in a direction of the third axis Ax3intersecting the first axis Ax1). The first width L1is defined by Bragg conditions. The second width L2is correlated to local supply of the etching gas. The second width L2of the first openings34dto34gand the second openings36dto36gmay be changed in order to adjust the local flow of the etching gas, specifically, local supply and local discharge of the etching gas. The mask31may have a connecting opening that connects the first openings34dto34g(second openings36dto36g) to one another so that the local flow of the etching gas is adjusted by changing the width of the connecting opening.

After completion of dry etching ETCH, the mask31is removed to obtain a third substrate product SP3, as shown inFIG. 3B. The mask31made of SiN is removed by etching using buffered hydrofluoric acid.

According to a method for producing a quantum cascade laser, both the first distribution Bragg reflector structure33having a λ/4 period and the second distribution Bragg reflector structure35having a 3λ/4 are produced by the process of the dry-etching ETCH. Local etching in the area where the second distribution Bragg reflector structure35having a 3λ/4 period is to be formed proceeds faster than local etching in the area where the first distribution Bragg reflector structure33having a λ/4 period is to be formed. Thus, the etch depth (D2) of the 3λ/4 period is larger than the etch depth (D1) of the λ/4 period. Because the depth (D2) of the periodic structure is large, the reflectivity of the second distribution Bragg reflector structure35is increased. Because the depth (D1) of the periodic structure is small, the reflectivity of the first distribution Bragg reflector structure33is decreased. This difference in structure makes it possible to generate a difference in reflectivity between the two distribution Bragg reflector structures from the technical viewpoint different from the difference in optical properties (reflectivity) derived from the structures correlated to the λ/4 period and the 3λ/4 period.

Referring again toFIG. 3B, the substrate11includes a first region11b, a second region11c, and a third region11d. The first region11b, the second region11c, and the third region11dare arranged in the direction of the first axis Ax1. The third region11dof the substrate11is disposed between the first region11band the second region11c. The semiconductor waveguide structure37is disposed in the third region11d. The semiconductor waveguide structure37includes the mesa structure39and the buried region25. The first distribution Bragg reflector structure33is disposed in the first region11b. The first distribution Bragg reflector structure33includes a semiconductor wall34adisposed closest to the first end facet37aof the semiconductor waveguide structure37to constitute a distribution Bragg reflector structure. The semiconductor wall34ais placed a first interval DS1apart from the first end facet37aof the semiconductor waveguide structure37. The second distribution Bragg reflector structure35is disposed in the second region11c. The second distribution Bragg reflector structure35includes a semiconductor wall36adisposed closest to the second end facet37bof the semiconductor waveguide structure37to constitute a distribution Bragg reflector structure. The semiconductor wall36ais placed a second interval DS2apart from the second end facet37bof the semiconductor waveguide structure37. The second interval DS2is larger than the first interval DS1.

According to the method for producing quantum cascade laser, the semiconductor wall34aof the first distribution Bragg reflector structure33is placed a first interval DS1apart from the first end facet37aof the semiconductor waveguide structure37. The semiconductor wall36aof the second distribution Bragg reflector structure35is placed a second interval DS2apart from the second end facet37bof the semiconductor waveguide structure37. A patterned mask31for forming the first interval DS1and the second interval DS2that may be formed in the same dry etching step for processing the laser structure19is used. The patterns corresponding to the two intervals are different from each other. The mask31includes a first mask portion31athat has a pattern for forming the first distribution Bragg reflector structure33and a second mask portion31bthat has a pattern for forming the second distribution Bragg reflector structure35. The first mask portion31ahas a first opening that defines the first interval DS1between the semiconductor wall34aand the first end facet37aof the semiconductor waveguide structure37. The second mask portion31bhas a second opening that defines the second interval DS2between the semiconductor wall36aand the second end facet37bof the semiconductor waveguide structure37. Due to the difference between the width of the first opening and the width of the second opening, the etching rate differs locally between these areas. Specifically, the etch depth formed through a wide opening is larger than the etch depth formed through a narrow opening. Due to this difference, the height H1of the semiconductor wall34ais different from the height H2of the semiconductor wall36a. The height H2of the semiconductor wall36ais larger than the mesa height HD in order to realize a relatively high reflectivity. The height H1of the semiconductor wall34ais smaller than the mesa height HD in order to realize a relatively low reflectivity according to the desired reflectivity.

Referring toFIG. 4in addition toFIG. 3B, the mesa structure39extends in a direction of the first axis Ax1. The first semiconductor walls34a,34b, and34cprotrude in a direction of a second axis Ax2that intersects the principal surface11aof the substrate11. If needed, the first distribution Bragg reflector structure33may include a first side wall33athat connects a first end of the semiconductor wall34aof the first semiconductor walls to the semiconductor waveguide structure37and a second side wall33bthat connects a second end of the semiconductor wall34ato the semiconductor waveguide structure37. The first end and the second end of the semiconductor wall34aare arranged in a direction of a third axis Ax3that intersects the first axis Ax1and the second axis Ax2.

According to this production method, the first ends and the second ends of the first semiconductor walls34a,34b, and34c, for example, the first end and the second end of the semiconductor wall34a, are respectively connected to the first side wall33aand the second side wall33b. During performance of etching through the mask31, etching gas is supplied from the opening that corresponds to the first interval DS1between the first end facet37aof the semiconductor waveguide structure37and the semiconductor wall34a. The area of the opening changes depending on the interval between the first side wall33aand the second side wall33b. The first ends of the first semiconductor walls34a,34b, and34care connected to one another through the first side wall33aand to the semiconductor waveguide structure37through the first side wall33a. The second ends of the first semiconductor walls34a,34b, and34care connected to one another through the second side wall33band to the semiconductor waveguide structure37through the second side wall33b. During etching by using the mask31, etching gas is supplied from openings that define adjacent two first semiconductor walls among the first semiconductor walls34a,34b, and34c. The area of the openings is changed depending on whether the first side wall33aand the second side wall33bexist. If these side walls exist, the area of the openings is changed depending on the interval between the first side wall33aand the second side wall33b.

As with the structure of the first distribution Bragg reflector structure33, the second distribution Bragg reflector structure35may include a third side wall33cand a fourth side wall33d. The third side wall33cconnects first ends of the second semiconductor walls36a,36b, and36c, for example, a first end of the semiconductor wall36a, to the semiconductor waveguide structure37. The fourth side wall33dconnects the second end of the semiconductor wall36ato the semiconductor waveguide structure37. The first end and the second end of the semiconductor wall36aare arranged in a direction of the third axis Ax3. The local etching rate correlated to the second distribution Bragg reflector structure35changes depending on the interval between the third side wall33cand the fourth side wall33d.

FIG. 5is a graph showing the relationship between the pattern interval and the etching rate obtained by experiments. The data illustrated in the graph and other experimental results show that the etching rate in dry etching for forming a distribution Bragg reflector structure changes depending on the opening size that defines the interval between the two semiconductor walls. In this experiment, the etching rate shows a local maximum at an interval of 5 μm. At an interval exceeding 5 μm, the etching area increases. Therefore, the consumption of the etchant increases. As a result, the local etching rate decreases. At an interval smaller than 5 μm, supply of the etchant is short compared to the consumption of the etchant. As a result, the local etching rate decreases.

Representative examples of the data are as follows:

Pattern Interval, Etching Rate

This difference in etching rate may be associated with the local etching rate for etching for forming a distribution Bragg reflector structure.

FIGS. 6A and 6Bare cross-sectional views of two distribution Bragg reflector structures formed in the same device section.FIG. 6Ashows a cross-section of a distribution Bragg reflector structure having a λ/4 period.FIG. 6Bshows a cross-section of a distribution Bragg reflector structure having a 3λ/4 period. Due to the difference in refractive index between the semiconductor and air, the interval of the semiconductor walls (width: 500 nm) for the distribution Bragg reflector structure having a λ/4 period is about 2000 nm. The interval of the semiconductor walls (width: 1500 nm) for the distribution Bragg reflector structure having a 3λ/4 period is about 5000 nm. By using the difference in interval, the difference in etching rate occurs effectively during etching for forming a distribution Bragg reflector structure having a λ/4 period and a distribution Bragg reflector structure having a 3λ/4 period. In this etching, these different etching rates are realized locally in the single etching step. Because of the different etching rates, semiconductor walls having a height of 5000 nm are formed near the distribution Bragg reflector structure having a λ/4 period and semiconductor walls having a height of 8000 nm are formed near the distribution Bragg reflector structure having a 3λ/4 period. The distribution Bragg reflector structure having a 3λ/4 and containing a portion deeper than the lower surface of the core layer reflects light propagating in the substrate. In contrast, the distribution Bragg reflector structure having a λ/4 period does not effectively reflect light propagating in the substrate since it does not contain a portion deeper than the lower surface of the core layer, or since the depth of the deeper portion than the lower surface of the core layer is small.

The degree of reflecting light propagating in the substrate (hereinafter referred to as “substrate propagating light”) is associated with the reflectivity of the distribution Bragg reflector structures. A larger number of semiconductor walls contributes to increasing reflectivity in a distribution Bragg reflector structure separate from the reflection of the substrate propagating light. Moreover, the difference in the height of semiconductor walls between the two distribution Bragg reflector structures generates a difference in reflecting the substrate propagating light separately from the reflective properties attributable to the periods of the distribution Bragg reflector structures. From this viewpoint, when a semiconductor wall of a distribution Bragg reflector structure is divided into a semiconductor wall upper portion positioned above the core layer and a semiconductor wall lower portion positioned below the core layer, the length of the semiconductor wall lower portion is different between the semiconductor walls of the two distribution Bragg reflector structures. According to this production method, a distribution Bragg reflector structure having a 3λ/4 and a distribution Bragg reflector structure having a λ/4 period that constitute a laser cavity in a single device section are fabricated as a structure that is useful for creating the difference in length of the semiconductor wall lower portion in a depth direction (a direction of the second axis Ax2). The quantum cascade laser includes a cavity in which the length of the semiconductor wall lower portion in a distribution Bragg reflector structure having a 3λ/4 period and the length of the semiconductor wall lower portion in a distribution Bragg reflector structure having a λ/4 period are different from each other. According to this structure, the difference in capacity to reflect the substrate propagating light creates the difference in reflectivity between the two distribution Bragg reflector structures separately from the reflectivity of the distribution Bragg reflector structure derived from the period.

Back to describing the production method, after the distribution Bragg reflector structures (33and35) are formed, a protective film41(for example, a SiON film) is formed on an upper surface39aof the mesa structure39and an upper surface25aof the buried region25, as illustrated inFIG. 7A. The protective film41has an opening41apositioned on the upper surface39aof the mesa structure39so as to allow an electrode to contact a surface of the top semiconductor layer of the mesa structure. After the protective film41is formed, as shown inFIG. 7B, a first electrode43is formed on the surface of the top semiconductor layer of the mesa structure through the opening41aof the protective film41. The first electrode43has, for example, a Ti/Pt/Au structure. If needed, the back surface of the substrate11is polished, and, as illustrated inFIG. 7C, a second electrode45is formed on the back surface of the substrate11. The second electrode45has, for example, a Ti/Pt/Au structure. Through these steps, a quantum cascade laser is produced. According to this embodiment, a quantum cascade laser having a cavity length of 0.5 mm or less at minimum is produced.

An example of a quantum cascade laser having a reflector formed by this method is as follows:

The quantum cascade laser has a buried heterostructure. The reflector has a structure in which semiconductor walls and air gaps are alternately arranged.

Height of semiconductor wall of high-reflection end facet: 8 μm

Height of semiconductor wall of light-emitting end facet: 5 μm

Number of semiconductor walls: 2 pairs

FIG. 8is a schematic diagram of a quantum cascade laser according to an embodiment. A quantum cascade laser51includes a mesa structure53, a first distribution Bragg reflector structure55, and a second distribution Bragg reflector structure57.

The mesa structure53extends in a direction of the first axis Ax1and has a first end facet53aand a second end facet53b. The mesa structure53includes a quantum cascade core layer53c, a cladding layer53d, and a contact layer53e. The first distribution Bragg reflector structure55is optically coupled to the first end facet53aof the mesa structure53and includes one of a λ/4 periodic structure and a 3λ/4 periodic structure. The second distribution Bragg reflector structure57is optically coupled to the second end facet53bof the mesa structure53and has the other one of the λ/4 periodic structure and the 3λ/4 periodic structure. Here, λ denotes the oscillation wavelength of the quantum cascade laser51in vacuum. The first distribution Bragg reflector structure55includes a semiconductor wall54aplaced a first interval DS1apart from the first end facet53aof the mesa structure53. The second distribution Bragg reflector structure57includes a semiconductor wall56aplaced a second interval DS2apart from the second end facet53bof the mesa structure53. The height of the semiconductor wall56ais different from the height of the semiconductor wall54a. The width of the semiconductor wall54aand the width of the semiconductor wall56aare both larger than the mesa width W0of the mesa structure53.

According to the quantum cascade laser51, the first interval DS1between one side surface of the semiconductor wall54ain the first distribution Bragg reflector structure55and the first end facet53aof the mesa structure53is correlated to one of the λ/4 periodic structure and the 3λ/4 periodic structure (for example, the λ/4 periodic structure). The second interval DS2between one side surface of the semiconductor wall56ain the second distribution Bragg reflector structure57and the second end facet53bof the mesa structure53is correlated to the other one of the λ/4 periodic structure and the 3λ/4 periodic structure (for example, the 3λ/4 periodic structure). The first distribution Bragg reflector structure55includes a semiconductor wall54ahaving a width W55larger than the mesa width W0of the mesa structure53. The second distribution Bragg reflector structure57includes a semiconductor wall56ahaving a width W57larger than the mesa width W0of the mesa structure53. This structure is suitable for reflecting light in the infrared wavelength region. The height H55of the semiconductor wall54ais different from the height H57of the semiconductor wall56a. The reflectivity of these distribution Bragg reflector structures may be changed by changing the height of each semiconductor wall. Specifically, the distance between the lower surface of the quantum cascade core layer53cand the lower end of the semiconductor wall54ais different from the distance between the lower surface of the quantum cascade core layer53cand the lower end of the semiconductor wall56a. The difference in interval generates a difference in reflecting the substrate propagating light that propagates in the semiconductor wall lower portion below the lower surface of the quantum cascade core layer53c.

The quantum cascade laser51further includes a substrate59. The substrate59includes a first region59a, a second region59b, and a third region59c. The third region59cof the substrate59is disposed between the first region59aand the second region59b. The first distribution Bragg reflector structure55is disposed on the first region59aand the second distribution Bragg reflector structure57is disposed on the second region59b. The mesa structure53is disposed on the third region59c.

The quantum cascade laser51includes a buried region61that embeds the side surface of the mesa structure53. The upper surfaces of the mesa structure53and the buried region61are covered with an insulating protective film63. If needed, the insulating protective film63covers the surfaces of the first distribution Bragg reflector structure55and the second distribution Bragg reflector structure57. The insulating protective film63has a contact opening63a. A first electrode65makes contact with the upper surface of the mesa structure53through the contact opening63a. A second electrode67makes contact with the back surface of the substrate59.

Each of the first distribution Bragg reflector structure55and the second distribution Bragg reflector structure57preferably includes three or more semiconductor walls. If needed, the quantum cascade laser51may include a first outer wall69athat surrounds the first distribution Bragg reflector structure55. In addition, an end of the first semiconductor wall (54a,54b,54c) of the first distribution Bragg reflector structure55may be connected to the first outer wall69a(this is referred to as a “connected structure”). The quantum cascade laser51may include a second outer wall69bthat surrounds the second distribution Bragg reflector structure57. An end of the second semiconductor wall (56a,56b,56c) of the second distribution Bragg reflector structure57may be distant from the second outer wall69b(this is referred to as an “independent structure”). As mentioned above, the first distribution Bragg reflector structure55is one of the λ/4 periodic structure and the 3λ/4 periodic structure and the second distribution Bragg reflector structure57is the other one of the λ/4 periodic structure and the 3λ/4 periodic structure. The grating period of the first distribution Bragg reflector structure55is different from the grating period of the second distribution Bragg reflector structure57. The difference between the height H55of the first semiconductor wall (54a,54b,54c) and the height H57of the second semiconductor wall (56a,56b,56c) is, for example, 3 μm or more. The difference between the length of the semiconductor wall lower portion of the first semiconductor wall (54a,54b,54c) and the length of the semiconductor wall lower portion of the second semiconductor wall (56a,56b,56c) is, for example, 3 μm or more. These differences contribute to forming a significant difference in reflectivity for the substrate propagating light between the first distribution Bragg reflector structure55and the second distribution Bragg reflector structure57.

FIGS. 9 to 14show several structures of the reflectors. The combination of the connecting structure and the independent structure and the λ/4 periodic structure and the 3λ/4 periodic structure is as follows. The outer wall is related to the amount of etchant locally supplied in forming the distribution Bragg reflector structures by etching. The connecting structure and the independent structure are both related to supply of the etchant during formation of the distribution Bragg reflector structures by etching.

A combination of a λ/4 periodic structure with a connecting structure and a 3λ/4 periodic structure with a connecting structure

A combination of a λ/4 periodic structure with a connecting structure and a 3λ/4 periodic structure having an independent structure

A combination of a λ/4 periodic structure having an independent structure and a 3λ/4 periodic structure with a connecting structure

A combination of a λ/4 periodic structure having an independent structure and a 3λ/4 periodic structure having an independent structure

FIG. 9is a plan view of a distribution Bragg reflector structure DB9R having a 3λ/4 period. The distribution Bragg reflector structure DB9R includes multiple semiconductor walls, for example, three semiconductor walls. The thickness of each semiconductor wall is, for example, 1.5 μm. The interval between the emitting end facet (53a,53b) and the semiconductor wall and the interval between the semiconductor walls are, for example, 5 μm to define the air gap. Light emitted from the emission end facet (53a,53b) spreads at an angle of about 45 degrees, for example. Each of the semiconductor walls have first and second extended portions respectively on the left and right sides of the edges of the mesa structure53so that the semiconductor walls may receive and reflect the emitted light from the emission end facet (53a,53b). The first and second extended portions each have a predetermined length (for example, 25 μm or more). The outer wall of the distribution Bragg reflector structure DB9R connects ends of the semiconductor walls.

FIG. 10is a plan view of a distribution Bragg reflector structure DB10R having a 3λ/4 period. The distribution Bragg reflector structure DB10R includes multiple semiconductor walls, for example, three semiconductor walls. The thickness of each semiconductor wall is, for example, 1.5 μm. The interval between the emitting end facet (53a,53b) and the semiconductor wall and the interval between the semiconductor walls are, for example, 5 μm to define the air gap. Light emitted from the emission end facet (53a,53b) spreads at an angle of about 45 degrees. Each of the semiconductor walls have first and second extended portions respectively on the left and right sides of the edges of the mesa structure53so that the semiconductor walls may receive and reflect the emitted light from the emission end facet (53a,53b). The first and second extended portions each have a predetermined length (for example, 25 μm or more). The ends of the semiconductor walls are distant from the outer wall of the distribution Bragg reflector structure DB10R.

FIG. 11is a plan view of a distribution Bragg reflector structure DB11R having a λ/4 period. The distribution Bragg reflector structure DB11R includes multiple semiconductor walls, for example, three semiconductor walls. The thickness of each semiconductor wall is 0.5 μm. The interval between the emitting end facet (53a,53b) and the semiconductor wall and the interval between the semiconductor walls are, for example, 2 μm to define the air gap. Light emitted from the emission end facet (53a,53b) spreads at an angle of about 45 degrees, for example. Each of the semiconductor walls have first and second extended portions respectively on the left and right sides of the edges of the mesa structure53so that the semiconductor wall may receive and reflect the emitted light from the emission end facet (53a,53b). The first and second extended portions each have a length of 10 μm. The outer wall of the distribution Bragg reflector structure DB11R supports the ends of the mesa structure53.

FIG. 12is a plan view of a distribution Bragg reflector structure DB12R having a λ/4 period. The distribution Bragg reflector structure DB12R includes three semiconductor walls. The thickness of each semiconductor wall is 0.5 μm. The interval between the semiconductor walls and the interval between the emitting end facet (53a,53b) and the semiconductor wall are, for example, 2 μm to define the air gap. Light emitted from the emission end facet (53a,53b) spreads at an angle of about 45 degrees, for example. Each of the semiconductor walls have first and second extended portions respectively on the left and right sides of the edges of the mesa structure53so that the semiconductor wall may receive and reflect the spreading light. The first and second extended portions each have a particular length (for example, 10 μm or more). The ends of the mesa structure53are distant from the outer wall of the distribution Bragg reflector structure DB12R.

FIG. 13is a plan view of a distribution Bragg reflector structure DB13R having a λ/4 period. The distribution Bragg reflector structure DB13R includes three semiconductor walls. The thickness of each semiconductor wall is 0.5 μm. The interval between the semiconductor walls and the interval between the semiconductor wall and the emission end facet (53a,53b) are, for example, 2 μm to define the air gap. Light emitted from the emission end facet (53a,53b) spreads at an angle of about 45 degrees. In order for the semiconductor walls to receive and reflect the spreading light, each of the semiconductor walls has 10-μm long extended portions respectively on the left and right sides of the edges of the mesa structure53. The outer wall of the distribution Bragg reflector structure DB13R supports the ends of the mesa structure53.

FIG. 14is a plan view of a distribution Bragg reflector structure DB14R having a λ/4 period. The distribution Bragg reflector structure DB14R includes three semiconductor walls. The thickness of each semiconductor wall is, for example, 0.5 μm. The interval between the semiconductor walls and the interval between the emitting end facet (53a,53b) and the semiconductor wall are, for example, 2 μm to define the air gap. Light emitted from the emission end facet (53a,53b) spreads at an angle of about 45 degrees. In order for the semiconductor walls to receive and reflect the spreading light, each of the semiconductor walls has extended portions respectively on the left and right sides of the edges of the mesa structure53. The extended portions each have a particular length (for example, 10 μm or more). The ends of the mesa structure53are distant from the outer wall of the distribution Bragg reflector structure DB14R.

While the principle of the present invention has been described above through preferable embodiments with reference to the drawings, a person skilled in the art would naturally recognize that various modifications and alterations of arrangement and details are possible without stepping out of the scope of the principle. The present invention is not limited to specific structures disclosed in the embodiments. All modifications and alterations within the scope and spirit of the claims are entitled to protection.