Patent Description:
A semiconductor laser that emits infrared includes an active layer in which multiple multi-quantum well structures are stacked.

The composition ratios and the thicknesses of the compound semiconductors included in a well layer and a barrier layer are affected by external disturbances (source material flux, fluctuation of the degree of vacuum, etc.) in the epitaxial crystal growth process. Also, as the number of cascade connection periods of the unit multi-quantum well structure increases, the crystal growth time of the active layer lengthens; and the effects of the external disturbances increase even more.

<CIT> relates to a quantum cascade laser including a semiconductor substrate and an active layer which is provided on the semiconductor substrate, and has a cascade structure in which unit laminate structures having quantum well emission layers and injection layers are laminated in multiple stages. The quantum cascade laser is configured such that the unit laminate structure has an emission upper level, an emission lower level and a relaxation miniband MB including an energy level lower than the emission lower level in its subband level structure, and light is generated by an intersubband transition of electrons from the upper level to the lower level, and the electrons after the intersubband transition are relaxed from the lower level to the miniband MB through LO phonon scattering to be injected from the injection layer to the latter stage emission layer via the miniband MB.

The object of the invention is achieved by the subject-matter of the independent claim.

A semiconductor laser according to the present invention is disclosed in claim <NUM>.

Embodiments of the invention will now be described with reference to the drawings.

<FIG> is a schematic cross-sectional view of one chip of a semiconductor laser wafer according to a first embodiment of the invention.

The semiconductor laser wafer <NUM> includes a substrate <NUM>, a first semiconductor layer <NUM>, an active layer <NUM>, a second semiconductor layer <NUM>, and a composition evaluation layer <NUM>.

The first semiconductor layer <NUM> may include, for example, a first contact layer <NUM>, a first cladding layer <NUM>, a first light guide layer <NUM>, etc., in this order on the substrate <NUM>. The second semiconductor layer <NUM> may include, for example, a second light guide layer <NUM>, a second cladding layer <NUM>, a second contact layer <NUM>, etc., in this order on the active layer <NUM>.

The active layer <NUM> is provided on the first semiconductor layer <NUM>. Multiple periods of pairs of a light-emitting multi-quantum well region and an injection multi-quantum well region are stacked in the active layer <NUM>; the light-emitting multi-quantum well region is made of a first compound semiconductor and a second compound semiconductor; and the injection multi-quantum well region is made of the first compound semiconductor and the second compound semiconductor.

The composition evaluation layer <NUM> is provided on the second semiconductor layer <NUM> and includes a first film <NUM> and a second film <NUM>; the first film <NUM> is made of the first compound semiconductor and has a first thickness; and the second film <NUM> is made of a mixed crystal of the second compound semiconductor and has a second thickness. The first compound semiconductor and the second compound semiconductor each may be, for example, ternary compound mixed crystals.

When the semiconductor laser is a quantum cascade laser (QCL) in which the carrier is an electron, the polarities of the first semiconductor layer <NUM> and the second semiconductor layer <NUM> are set to the n-type.

<FIG> is a schematic cross-sectional view of the semiconductor laser according to the first embodiment.

A stacked body that includes the first semiconductor layer <NUM>, the active layer <NUM>, the second semiconductor layer <NUM>, and the composition evaluation layer <NUM> and is epitaxially grown on the substrate <NUM> is patterned into a mesa configuration. The mesa-shaped stacked body is included in a ridge waveguide. In <FIG>, the dug out depth of the mesa reaches partway through the first light guide layer <NUM> of the first semiconductor layer <NUM> provided to be adjacent to the active layer <NUM>. However, the dug out depth of the mesa is not limited to <FIG> and may reach partway through the first cladding layer <NUM> provided below the first light guide layer <NUM>, may reach the lower surface of the active layer <NUM>, or may reach the lower surface of the first semiconductor layer <NUM>.

An insulating film <NUM> that includes a silicon oxide film and/or a silicon nitride film is provided on the side surface of the ridge waveguide and on the bottom surface exposed at the two sides of the ridge waveguide. An upper electrode <NUM> is provided on the upper surface of the ridge waveguide (the front surface of the second film <NUM>); and a lower electrode <NUM> is provided on the back surface of the substrate <NUM>. The ridge waveguide extends in a direction orthogonal to the page surface and is an optical resonator between the two end surfaces. A surface-emitting structure may be used in which a two-dimensional photonic crystal is provided inside the first semiconductor layer or the second semiconductor layer without providing a ridge waveguide.

<FIG> is a conduction band energy level diagram in the vertical direction of the active layer.

The vertical axis is the relative conduction band energy (eV); and the horizontal axis is the vertical-direction position (µm). One period of a multi-quantum well structure <NUM> included in the active layer <NUM> is made of a pair of a light-emitting multi-quantum well region <NUM> and an injection multi-quantum well region <NUM>. The light-emitting multi-quantum well region <NUM> includes multiple well layers and multiple barrier layers. The well layer includes the second compound semiconductor. The barrier layer includes the first compound semiconductor.

When a potential difference is applied to the active layer <NUM> from above and below, an electron undergoes an intersubband transition in the light-emitting multi-quantum well region <NUM>; and a laser oscillation of a wavelength corresponding to the transition level occurs. On the other hand, the energy of the electron after the intersubband transition relaxes as the electron is transported through the injection multi-quantum well region <NUM>; and the electron is injected into the light-emitting multi-quantum well region <NUM> downstream and again contributes to the intersubband transition.

In the first embodiment, the composition evaluation layer <NUM> is provided on the front surface of the semiconductor laser wafer. For example, the first film <NUM> is made of the first compound semiconductor included in the barrier layer. The second film <NUM> is made of the second compound semiconductor included in the well layer.

For example, the first compound semiconductor may be InxAl<NUM>-xAs (<NUM> < x < <NUM>); and the second compound semiconductor may be InyGa<NUM>-yAs (<NUM> < y < <NUM>). The first film <NUM> may be the material included in the well layer; and the second film <NUM> may be the material included in the barrier layer.

Table <NUM> illustrates a configuration example of one period of the set active layer structure included in the active layer <NUM>.

One period includes the pair of the light-emitting multi-quantum well region <NUM> and the injection multi-quantum well region <NUM>. For example, the well layers include In<NUM>Ga<NUM>As which is the second compound semiconductor; and the barrier layers include In<NUM>Al<NUM>As which is the first compound semiconductor. The light-emitting multi-quantum well region <NUM> includes four well layers; and the injection multi-quantum well region <NUM> includes seven well layers. In the active layer <NUM>, for example, <NUM> to <NUM> periods or the like of the quantum well structure are stacked.

In the actual crystal growth process, the composition ratio and/or the film thickness (the growth rate) fluctuates easily due to external disturbances (the source material flux, the degree of vacuum, the growth temperature, etc.). Therefore, characteristic-defect wafers increase if the ridge waveguide formation, the electrode formation, the formation processes of the end surface reflective films, etc., are performed without performing an evaluation sort of the wafer after the crystal growth. Therefore, the overall yield of the semiconductor laser chips decreases. In other words, the chip yield decreases due to the fluctuation of the epitaxial crystal growth process.

<FIG> is a flowchart of an evaluation process of the semiconductor laser wafer according to the first embodiment.

First, X-rays (having a known wavelength λ) are irradiated on the front surface of the composition evaluation layer <NUM> provided on the semiconductor laser wafer <NUM>; and the X-ray diffraction profile is determined by measuring the diffracted light intensity for the diffraction angles (S100).

The diffracted light intensity is the peak at the position where the diffraction angle is <NUM>θ (θ: Bragg angle). Therefore, a lattice constant of the mixed crystal is determined by Formula (<NUM>): <MAT> where n is a natural number. As a result, the composition ratios x and y are determined by utilizing the correlation between the lattice constant of the ternary compound mixed crystal and the composition ratio x (or y).

<FIG> is a graph of the measured X-ray diffraction profile of the first embodiment; and <FIG> is a graph determined by simulating the X-ray diffraction profile of a sample structure in which a composition evaluation layer is formed on an InP substrate.

The vertical axis is the relative diffracted light intensity; and the horizontal axis is the diffraction angle <NUM>θ (θ: Bragg angle). In <FIG>, the peak at the diffraction angle of about <NUM> degrees is InP which is the substrate <NUM>. A subpeak of InyGa<NUM>-yAs (<NUM> < y < <NUM>) occurs at the vicinity of <NUM> degrees at the left side of the peak of the substrate <NUM>. Also, a subpeak of InxAl<NUM>-xAs (<NUM> < x < <NUM>) occurs at the vicinity of <NUM> degrees at the right side of the peak of the substrate.

The sample structure of <FIG> is a structure in which an In<NUM>Al<NUM>As film (set thickness: <NUM>) and an In<NUM>Ga<NUM>As film (set thickness: <NUM>) are provided in this order on an InP substrate. In the X-ray diffraction profile according to the simulation, a subpeak of In<NUM>Ga<NUM>As occurs at a diffraction angle of about <NUM> degrees; and a subpeak of In<NUM>Al<NUM>As occurs at a diffraction angle of about <NUM> degrees.

A simulation of the sample structure having the composition ratios x and y as variables also is performed. Thus, the composition ratios x and y can be determined so that the X-ray diffraction profile that is obtained according to the simulation matches the measured X-ray diffraction profile of <FIG> (S102).

<FIG> is a schematic cross-sectional view of a semiconductor laser wafer according to a comparative example.

An active layer <NUM> has the same structure as that of Table <NUM>; but a composition evaluation layer is not provided. In the X-ray diffraction profile of the comparative example, the peak of the diffracted light intensity of InxAl<NUM>-xAs and the peak of the diffracted light intensity of InyGa<NUM>-yAs of the composition evaluation layer are weak. Therefore, the accuracy of the composition ratios is insufficient when determining the composition ratios x and y from the measurement of the measured X-ray diffraction profile.

Also, if the film thicknesses of the first film <NUM> and the second film <NUM> are less than <NUM>, the diffracted light intensity decreases; and the detection sensitivity decreases. On the other hand, if the film thicknesses are greater than <NUM>, the crystallinity of the entire wafer decreases because the critical film thickness is approached. Therefore, the film thicknesses of the first film <NUM> and the second film <NUM> are not less than <NUM> and not more than <NUM>. By providing the composition evaluation layer <NUM> further toward the wafer front surface side than the active layer <NUM>, the attenuation of the X-rays in the wafer interior can be reduced.

Then, when the differences between the determined composition ratios x and y and the setting values of the composition ratios illustrated in Table <NUM> each are not more than the prescribed values, the composition ratios x and y are taken to be in the tolerance ranges (S104); and the flow proceeds to the next evaluation process. On the other hand, when the differences between the measured composition ratios x and y and the setting values of the composition ratios illustrated in Table <NUM> are greater than the prescribed values, the wafer is determined to be a defective wafer because the composition ratios x and y do not satisfy the tolerance ranges (S106). For example, the reference prescribed value can be set so that the absolute value of the difference between the determined composition ratio and the setting value (Table <NUM>) is <NUM>% of the setting value, etc. The chip yield can be increased by performing the evaluation process of the composition ratios x and y described above after the crystal growth process. To further increase the chip yield, it is favorable to add an evaluation process of the film thickness.

<FIG> is a graph of the measured X-ray diffraction profile of the first embodiment; and <FIG> is a graph of an X-ray diffraction profile of the structure of the first embodiment determined by a simulation.

<FIG> is the measured X-ray diffraction profile of the first embodiment and is the profile obtained in step S100 (the same as <FIG>).

On the other hand, the X-ray diffraction profile is simulated by setting the composition ratios of In to the composition ratios x and y determined in step S102 and by using the film thickness of the well layer and the film thickness of the barrier layer as variables (S108). In such a case, for example, film thicknesses corresponding to the last two layers (the InAlAs layer having a thickness of <NUM> and the InGaAs layer having a film thickness of <NUM>) of the MQW setting values of Table <NUM> can be used as the two variables. The assumptions of the simulation are that the growth rates of InGaAs and InAlAs are constant over the entire active layer <NUM>, and the composition ratios x and y are the values determined in step S102.

The two film thicknesses are fit so that the profile obtained by the simulation matches the measured X-ray profile (<FIG>) (S110). In such a case, for example, the correlation between the satellite peak and subpeak intensities, the diffraction angles, the film thicknesses, etc., can be utilized. The positions of the two film thicknesses used as variables can be selected from the configuration of Table <NUM>.

The film thicknesses (two) of the X-ray diffraction profile obtained by the fitting are determined to be in tolerance ranges when the differences between the film thicknesses (two) and the film thicknesses of the setting values of Table <NUM> are not more than the prescribed values (S112); and the wafer is determined to be a non-defective wafer. On the other hand, the wafer is determined to be a defective wafer when the film thickness differences are greater than the prescribed values (S114). The prescribed value can be set so that, for example, the absolute value of the film thickness difference is <NUM>% of the setting value, etc..

In the semiconductor laser wafer <NUM> of the embodiment, the composition evaluation layer <NUM> is provided between the active layer <NUM> and the wafer front surface or between the active layer <NUM> and the wafer front surface. The In composition ratio y of the well layer and the In composition ratio x of the barrier layer included in the active layer <NUM> are determined by the X-ray diffraction measurement in the wafer state. Also, the X-ray diffraction profile is simulated by using the composition ratios x and y determined by the measurements and by using the film thickness of the well layer and the film thickness of the barrier layer as variables. The external disturbances that occur in the crystal growth process include not only the fluctuation of the source material flux but also the fluctuation of the degree of vacuum, the fluctuation of the growth temperature, etc. Therefore, the simulation accuracy of the film thickness fluctuation can be increased by performing the X-ray diffraction profile simulation of the wafer by using the four variables. As a result, the semiconductor laser chip yield can be increased.

In the simulation of the X-ray diffraction profile, the diffraction angle where the diffracted light intensity is the peak is proximal to the InP diffraction angle for the cladding layers (e.g., InP) <NUM> and <NUM>, the light guide layers (e.g., InGaAs) <NUM> and <NUM>, the contact layers (e.g., InGaAs) <NUM> and <NUM>, etc. Therefore, the effects on the X-ray diffraction profile can be small. Of course, the composition ratios of these layers can be determined individually.

<FIG> is a schematic cross-sectional view of a semiconductor laser wafer according to a modification of the first embodiment.

The composition evaluation layer <NUM> may be provided between the active layer <NUM> and the front surface of the semiconductor laser wafer <NUM>. If the composition evaluation layer <NUM> is provided lower than the active layer <NUM>, the optical path of the X-rays lengthens; attenuation and the like occurs; and the detection sensitivity decreases. In the modification of the first embodiment, a guide layer, a cladding layer, a contact layer, etc., are provided between the active layer <NUM> and the composition evaluation layer <NUM>.

The thickness of the cladding layer is large, i.e., <NUM> to <NUM>, etc. The impurity concentration of the contact layer is higher than the impurity concentration of the cladding layer. Therefore, there is a possibility that the growth conditions may be changed between the crystal growth process of the active layer <NUM> and the crystal growth process of the composition evaluation layer <NUM>. Conversely, in the modification, the change of the growth conditions can be small between the crystal growth process of the active layer <NUM> and the crystal growth process of the composition evaluation layer <NUM>. Therefore, the simulation accuracy can be high.

The semiconductor laser may be an interband cascade laser (ICL). In such a case, the active layer is provided on the first semiconductor layer and includes multiple periods of the pair of the light-emitting multi-quantum well region made of the first compound semiconductor and the second compound semiconductor and the injection multi-quantum well region made of the first compound semiconductor and the second compound semiconductor. Electrons are injected into the light-emitting multi-quantum well region from the electron injection layer of the injection multi-quantum well region on the upstream side; and holes are injected into the light-emitting multi-quantum well region from the injection multi-quantum well region on the downstream side. As a result, the electrons and the holes recombine in the light-emitting multi-quantum well region; and infrared laser light that corresponds to the interband transition level formed in the light-emitting multi-quantum well region is emitted.

According to the embodiment, a semiconductor laser is provided in which the productivity of the semiconductor laser chip is high and infrared can be emitted. The semiconductor laser is utilized in environmental measurements, infrared detection, the detection of designated substances, etc..

Claim 1:
A semiconductor laser, comprising:
a substrate (<NUM>);
a first semiconductor layer (<NUM>) provided on the substrate (<NUM>);
an active layer (<NUM>) provided on the first semiconductor layer (<NUM>), multiple periods of pairs of a light-emitting multi-quantum well region (<NUM>) and an injection multi-quantum well region (<NUM>) being stacked in the active layer (<NUM>), the light-emitting multi-quantum well region (<NUM>) being made of a first compound semiconductor and a second compound semiconductor, the injection multi-quantum well region (<NUM>) being made of the first compound semiconductor and the second compound semiconductor;
a second semiconductor layer (<NUM>) provided on the active layer (<NUM>); and
a composition evaluation layer (<NUM>) provided above the active layer (<NUM>), the composition evaluation layer (<NUM>) including a first film (<NUM>) and a second film (<NUM>) and being provided above the active layer (<NUM>), the first film (<NUM>) being made of the first compound semiconductor and having a first thickness, the second film (<NUM>) being made of the second compound semiconductor and having a second thickness;
wherein:
the first compound semiconductor and the second compound semiconductor each are ternary crystals;
the first thickness is not less than <NUM> and not more than <NUM>, and
the second thickness is not less than <NUM> and not more than <NUM>.