Distributed feedback semiconductor laser and manufacturing method of the same

Abstract

To improve a yield of a nitride semiconductor laser device.SOLUTION: A distributed feedback semiconductor laser includes a nitride-based multilayer structure including a first conductivity-type semiconductor layer, an active layer, and a second conductivity-type semiconductor layer sequentially stacked on a substrate. A current confinement part is formed in the second conductivity-type semiconductor layer. A diffraction grating has a periodic refractive index change formed along a waveguide direction of an optical resonator formed by the current confinement part. When a spectrum of an emitted light in the case of being driven with a current 0.9 times as large as the threshold current ITH for starting a laser oscillation at the time of driving at a room temperature is defined as a gain spectrum immediately before the oscillation, a bandwidth at a light intensity 12 dB lower than that of a peak position of the gain spectrum immediately before the oscillation is 70 meV or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to distributed feedback semiconductor lasers. [Background technology]

[0002] Semiconductor Fabry-Perot laser diodes (FP-LDs) made of gallium nitride (GaN)-based materials that oscillate at wavelengths such as 375 nm and 405 nm are used as light sources for exposure machines, etc. Also, GaN-based FP-LDs that oscillate at wavelengths such as 455 nm and 520 nm are used as light sources for laser TVs, etc.

[0003] Conventional FP-LDs are fabricated from a wafer on which a multi-layer structure of crystals that achieves the desired laser characteristics is epitaxially grown.

[0004] To confirm whether a laser oscillating at a desired wavelength can be obtained from the wafer, for example, before starting to fabricate a laser element on the wafer, the photoluminescence (PL) wavelength λ at each position on the wafer surface is measured. PL The PL wavelength λ PL The target wavelength (center value) and its tolerance range are specified for each application, such as 405±1 nm. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-040880 Summary of the Invention [Problem to be solved by the invention]

[0006] However, due to various factors such as the distribution of the off-axis angle of the substrate used for epitaxial crystal growth within the wafer, the stability of the crystal growth temperature, and the spatial distribution of the crystal growth temperature, the PL wavelength λ PL However, there may be areas where the difference is outside the allowable range.

[0007] Empirically, the gain peak wavelength λ Gain and PL wavelength λ PL There is a close relationship between these two. This relationship changes depending on the excitation wavelength of the PL measurement device, the excited carrier density, etc., but is basically expressed by the following formula. λ Gain =λ PL +k k is a constant.

[0008] The Fabry-Perot laser diode (FP-LD) has a gain peak wavelength λ Gain Since the laser oscillates at a wavelength of λ PL However, FP-LDs obtained from positions on the wafer surface that are outside the tolerance range do not oscillate at the desired oscillation wavelength, leading to a decrease in wavelength yield.

[0009] In particular, the PL wavelength λ PL Epi-wafers with a center of gravity outside the allowable range for the in-plane distribution of PL wavelength λ PL It is predicted that epitaxial wafers with a large dispersion of the in-plane distribution of λ will have a poor wavelength yield. Therefore, in the past, such epitaxial wafers were often discarded rather than used for the production of FP-LDs for products.

[0010] The present disclosure has been made in view of the above problems, and one exemplary purpose of an embodiment thereof is to improve the yield of nitride-based semiconductor laser devices. [Means for solving the problem]

[0011] A distributed feedback semiconductor laser according to one embodiment of the present disclosure includes a substrate, a nitride-based multilayer structure including a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer stacked in this order on the substrate, a current confinement portion formed in the second conductivity type semiconductor layer, and a diffraction grating having periodic refractive index changes formed along the waveguiding direction of an optical resonator formed by the current confinement portion. The threshold current I that initiates laser oscillation when operated at room temperature is THWhen the spectrum of emitted light when driven with a current 0.9 times higher than the gain spectrum just before oscillation is taken as the gain spectrum, the bandwidth at an optical intensity 12 dB lower than the peak position of the gain spectrum is 70 meV or more.

[0012] Another aspect of the present disclosure is a method for manufacturing a distributed feedback semiconductor laser, comprising: a preparation step of preparing a semiconductor wafer; a step A of forming a first conductivity type semiconductor layer on the semiconductor wafer; a step B of forming an active layer on the first conductivity type semiconductor layer so that at least one of the In composition and the thickness of the quantum well layer is non-uniform across the semiconductor wafer; a step C of forming a second conductivity type semiconductor layer on the active layer; and a step D of forming a diffraction grating that determines the oscillation wavelength.

[0013] Any combination of the above elements, or mutual substitution of elements or expressions between methods, devices, systems, etc., are also valid aspects of the present invention or the present disclosure. Furthermore, the description in this section (Means for Solving the Problems) does not explain all essential features of the present invention, and therefore, subcombinations of the described features may also constitute the present invention. [Effects of the Invention]

[0014] According to a semiconductor laser device according to an aspect of the present disclosure, the yield of semiconductor laser devices can be improved. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a perspective view of a nitride semiconductor laser element according to an embodiment. [Figure 2] 10 is a diagram showing the emission spectrum and the oscillation wavelength λDFB of the semiconductor laser element according to the embodiment immediately before oscillation at room temperature. [Figure 3] FIG. 2 is a diagram showing an oscillation spectrum of the semiconductor laser element according to the embodiment. [Figure 4] FIG. 10 is a diagram showing the emission spectra of a plurality of fabricated samples immediately before oscillation. [Figure 5]FIG. 10 is a diagram illustrating the variation in gain spectrum. [Figure 6] 1A and 1B are diagrams showing an example of the distribution of the In composition in the active layer or the thickness of the quantum well layer within the wafer surface. [Figure 7] FIG. 10 is a diagram showing the distribution of the In composition in the active layer and the thickness of the quantum well layer in the stacking direction. [Figure 8] FIG. 10 is a diagram illustrating a stop band. DETAILED DESCRIPTION OF THE INVENTION

[0016] (Outline of the embodiment) A summary of some exemplary embodiments of the present disclosure is provided below. This summary is intended as a prelude to the more detailed description that follows, or to provide a basic understanding of the embodiments. This summary is intended to briefly explain some concepts of one or more embodiments and is not intended to limit the scope of the invention or disclosure. Furthermore, this summary is not an exhaustive overview of all possible embodiments, nor does it limit essential elements of the embodiments. For convenience, the term "one embodiment" may refer to one embodiment (example or variant) or multiple embodiments (examples or variants) disclosed herein.

[0017] (Outline of the embodiment) A distributed feedback laser diode (DFB-LD) according to one embodiment includes a substrate, a nitride-based multilayer structure including a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer stacked in this order on the substrate, a current confinement portion formed in the second conductivity type semiconductor layer, and a diffraction grating having periodic refractive index changes formed along the waveguiding direction of an optical resonator formed by the current confinement portion. The threshold current I that initiates laser oscillation when operated at room temperature is TH When the spectrum of emitted light when driven with a current 0.9 times higher than the gain spectrum just before oscillation is taken as the gain spectrum, the bandwidth at an optical intensity 12 dB lower than the peak position of the gain spectrum (referred to as the 12 dB bandwidth) is 70 meV or more.

[0018] In this specification, the gain spectrum refers to the wavelength dependence of the gain obtained in a quantum well when a current is injected. This corresponds to the spectrum of the emitted light that is observed when the drive current of a semiconductor laser device is increased regardless of laser oscillation, and the spectrum that changes gradually with wavelength. In particular, when the threshold current is increased to I TH When the drive current I OP =0.9×I TH The gain spectrum at this temperature is sometimes referred to as the gain spectrum immediately before oscillation or the radiation spectrum immediately before oscillation. The room temperature can typically be selected to a predetermined value in the range of 25°C to 30°C, such as 25°C, 27°C, or 30°C. The laser oscillation caused by the periodicity of the diffraction grating characteristic of DFB-LD is called DFB oscillation, and the oscillation wavelength at this time is sometimes called the DFB oscillation wavelength, but is also sometimes simply called oscillation or oscillation wavelength.

[0019] The inventors recognized that DFB-LDs with nitride-based multilayer structures can oscillate within a 12 dB bandwidth. Since the peak wavelength variation of the gain spectrum is within approximately 10 nm, if the 12 dB bandwidth is wider than 70 meV, the laser can oscillate at the DFB oscillation wavelength even if the peak wavelength of the gain spectrum varies, improving the yield.

[0020] In one embodiment, the active layer may have a quantum well structure with two or more layers, where the In composition or thickness fluctuations of each well layer are different, thereby making it possible to widen the gain spectrum compared to a single layer.

[0021] In one embodiment, the active layer has a multiple quantum well structure, and the composition of the material constituting at least two well layers may be different, which can further increase the width of the gain spectrum.

[0022] In one embodiment, the active layer has a quantum well structure, and the distribution of the In composition in the stacking direction of the well layers or the thickness of the quantum well layers may vary along the waveguide direction, thereby increasing the width of the gain spectrum compared to when the distribution of the In composition in the stacking direction of the well layers or the thickness of the quantum well layers is uniform along the waveguide direction.

[0023] In one embodiment, the diffraction grating has a stop band width Δλ that appears in the gain spectrum just prior to oscillation. SB but, 1.8×10 -6 λ 2 ≦Δλ SB ≦4.0×10 -6 λ 2 The diffraction grating may be configured so that: If the peak wavelength of the emitted light spectrum varies greatly, the gain at the oscillation wavelength will be significantly lower than the gain at the peak wavelength. Therefore, to achieve stable laser oscillation, it is necessary to increase the coupling coefficient between the guided light and the diffraction grating. The stop band width has a positive correlation with the coupling coefficient, and designing the stop band width to satisfy the above formula makes it easier to obtain the coupling coefficient required for laser oscillation.

[0024] In one embodiment, the distributed feedback semiconductor laser may further include a facet coating film formed on at least one facet and having a reflectivity of 2% or less at the oscillation wavelength, thereby realizing stable oscillation.

[0025] In one embodiment, the diffraction grating may be made of a plurality of projections and recesses formed on the surface of the second conductivity type semiconductor layer along a ridge portion provided on the surface of the second conductivity type semiconductor layer.

[0026] A method for manufacturing a distributed feedback semiconductor laser according to one embodiment includes a preparation step of preparing a semiconductor wafer, a step A of forming a first conductivity type semiconductor layer on the semiconductor wafer, a step B of forming an active layer on the first conductivity type semiconductor layer so that at least one of the In composition and the thickness of the quantum well layer is non-uniform across the entire semiconductor wafer, a step C of forming a second conductivity type semiconductor layer on the active layer, and a step D of forming a diffraction grating that determines the oscillation wavelength.

[0027] In one embodiment, in step B, at least one of the crystal growth temperature, the In ratio in the gas phase, and the V / III ratio may be adjusted to make at least one of the In composition and the thickness of the quantum well layer non-uniform on the semiconductor wafer.

[0028] In one embodiment, the manufacturing method may further include a process for making the off-angle of the semiconductor wafer non-uniform within the plane.

[0029] (Embodiment) Preferred embodiments will be described below with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be designated by the same reference numerals, and redundant descriptions will be omitted where appropriate. Furthermore, the embodiments are merely examples and do not limit the disclosure or invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the disclosure or invention.

[0030] In addition, the dimensions (thickness, length, width, etc.) of each component shown in the drawings may be enlarged or reduced as appropriate for ease of understanding. Furthermore, the dimensions of multiple components do not necessarily represent their relative sizes, and even if a component A is depicted as being thicker than another component B in the drawings, it is possible that component A is thinner than component B.

[0031] (Embodiment) 1 is a perspective view of a nitride semiconductor laser device 100 according to an embodiment. The semiconductor laser device 100 includes an n-type substrate 110, an n-type semiconductor layer 120, an active layer 130, a p-type semiconductor layer 140, and a film made of an insulating material 160.

[0032] The substrate 110 is a nitride semiconductor, and x Al y Ga 1-x-y The substrate 110 may have a composition of GaN (0≦x≦1, 0≦y≦1, 0≦x+y≦1). The material of the substrate 110 may be GaN (x=y=0). The substrate 110 is not limited to this, and any substrate that has a similar effect may be used, for example, a Si substrate or a sapphire substrate.

[0033] An n-type semiconductor layer 120, an active layer 130, and a p-type semiconductor layer 140 are formed in this order on an n-type substrate 110 by epitaxial growth, and form a stacked structure 102.

[0034] The n-type semiconductor layer 120 may include an n-type cladding layer and an n-type guide layer. An active layer (light emitting layer) 130 having a quantum well structure is formed on the n-type semiconductor layer 120. When the oscillation wavelength is set to 405 nm, the material of the quantum well structure is In. 0.09 Ga 0.91 N well layer (4 nm thick) and undoped In 0.02 Ga 0.98 A two-period multiple quantum well (MQW) layer consisting of an N barrier layer (thickness 10 nm) can be selected.

[0035] The active layer 130 has non-uniform In composition and quantum well layer thicknesses in the optical guiding direction and stacking direction of the resonator. Specifically, the active layer 130 has quantum well structure in which the In composition and quantum well layer thicknesses vary along the guiding direction and stacking direction. This non-uniformity provides a broadband gain spectrum, which will be described later.

[0036] A ridge portion (also called a mesa portion) 142 is formed in the p-type semiconductor layer 140 (p-type cladding layer) as a current confinement structure 141. The height of the ridge portion 142 can be several hundred nm, and the width of the ridge portion 142 (mesa width) can be several microns. For example, the height may be 500 nm, and the width may be 1 μm to 2 μm.

[0037] The p-type cladding layer 140 having the ridge portion 142, together with the active layer 130 and the n-type cladding layer 120, forms a ridge-type waveguide 144. The ridge-type waveguide 144 extends in a first direction (the z-axis direction in the figure). For example, the waveguide length may be 400 μm to 3000 μm.

[0038] The n-side electrode 171 is formed on the back surface of the substrate 110 , and the p-side electrode 172 is formed on the top surface of the ridge portion 142 .

[0039] Diffraction gratings 150_1 and 150_2 are formed on both sides of the mesa adjacent to the ridge-type waveguide 144 in the second direction (x-axis direction). The diffraction gratings 150_1 and 150_2 have a periodic refractive index change along the first direction (z-axis direction), which is the waveguiding direction of the optical resonator. The diffraction gratings 150_1 and 150_2 each have a plurality of grooves 152 formed in the p-type semiconductor layer 140 on the sides of the mesa. The plurality of grooves 152 are adjacent to each other in the first direction, and each groove extends in the second direction. The diffraction gratings 150 adjust the oscillation wavelength λ of the DFB-LD. DFB is prescribed.

[0040] A film made of an insulating material 160 is formed on the p-type semiconductor layer 140. The insulating material 160 is formed so as to fill the grooves 152 of the diffraction grating 150.

[0041] The pitch of the diffraction grating 150 is set to the desired oscillation wavelength λ of the DFB-LD. DFB The order of the diffraction grating 150 is not limited to a first-order diffraction grating, but may be, for example, a third-order diffraction grating.

[0042] FIG. 2 shows the emission spectrum and the oscillation wavelength λ of the semiconductor laser device 100 according to the embodiment at room temperature (for example, 25° C.) immediately before oscillation. DFB FIG.

[0043] The spectrum of emitted light just before oscillation is the threshold current I TH 0.9 times the operating current I OPIn this embodiment, the width of the band (hereinafter referred to as 12 dB bandwidth) at an optical intensity 12 dB lower than the peak position of the gain spectrum (hereinafter referred to as 12 dB bandwidth) Δ BAND The peak position of the gain spectrum in Figure 2 is about 410 nm (actually 412 nm), and the 12 dB bandwidth Δ BAND is an energy width of 70 meV or more, which corresponds to a wavelength of 10 nm.

[0044] This wide 12 dB bandwidth Δ BAND is determined by the inhomogeneity of the In composition in the z-axis and y-axis directions of the active layer 130 or the inhomogeneity of the thickness of the quantum well layer. Gain is 412 nm, and the oscillation wavelength λ DFB is 417 nm, and the oscillation wavelength λ DFB and peak wavelength λ Gain The difference (detuning amount = λ DFB -λ Gain )ΔG is 5 nm. Also, the oscillation wavelength λ DFB Adjacent to this, there is a dip (called a stop band).

[0045] 3 is a diagram showing the spectrum (oscillation spectrum) of emitted light when the semiconductor laser device 100 according to the embodiment oscillates. The oscillation spectrum is obtained by dividing the threshold current I TH Larger current I OP is the spectrum obtained when the oscillation wavelength λ DFB It has a strong peak at

[0046] The above is the configuration of the semiconductor laser device 100. Next, the advantages of the semiconductor laser device 100 will be described.

[0047] As a result of the investigation of nitride DFB-LDs, the inventors have found the following: BANDThe bandwidth of the semiconductor chips was almost the same, but the peak wavelength (gain peak) λ of the gain spectrum was different due to manufacturing variations. Gain is scattered.

[0048] Then, multiple semiconductor chips are installed with an oscillation wavelength λ DFB A number of samples were fabricated by forming diffraction gratings with different sizes.

[0049] FIG. 4 shows the oscillation spectra of the fabricated samples. The sample on the left (first sample) has a peak wavelength λ Gain is 412 nm, and the oscillation wavelength λ DFB is 410.7 nm, and the detuning width ΔG is -1.3 nm.

[0050] The central sample (second sample) has a peak wavelength λ Gain is 412.4 nm, and the oscillation wavelength λ DFB is 413.2 nm, and the detuning width ΔG=+0.8 nm.

[0051] The sample on the right (third sample) has a peak wavelength λ Gain is 413.3 nm, and the oscillation wavelength λ DFB is 415.6 nm, and the detuning width ΔG=+2.3 nm.

[0052] Stable oscillation was confirmed in multiple samples. Further investigation led to the conclusion that laser oscillation is possible within a range of 12 dB from the peak of the gain spectrum. Initially, the inventors thought that increasing the absolute value of the detuning amount ΔG would prevent oscillation. However, the experimental results were contrary to this expectation, and showed that oscillation occurred within the 12 dB band regardless of the detuning amount ΔG.

[0053] The semiconductor laser device 100 according to the embodiment is based on this finding.

[0054] 5 is a diagram illustrating the variation in the gain spectrum. The peak wavelength λ of the gain spectrum varies depending on the position on the wafer due to the influence of manufacturing variations. Gain In this embodiment, the peak wavelength λ Gain is 12dB bandwidth Δ BAND Even if the oscillation wavelength λ DFB The gain at the peak ratio is guaranteed to be higher than -12 dB, and therefore the oscillation wavelength λ DFB It is possible to oscillate with

[0055] In general semiconductor manufacturing processes, it is empirically known that the peak wavelength λ of the gain spectrum is near the lasing wavelength of 410 nm. Gain The variation of is within 10 nm. If the 12 dB bandwidth of the gain spectrum is kept wider than 70 meV, the peak wavelength λ Gain Even if there is variation, a good product can be obtained.

[0056] For example, consider manufacturing a laser with a wavelength of 410 nm. If the wavelength tolerance is 410±1 nm, the gain peak λ Gain Only elements that fall within this tolerance range will meet the wavelength specification.

[0057] In this embodiment, the gain peak λ of each element Gain is within the range of 410±5 nm, the pitch of the diffraction grating 150 is set to the oscillation wavelength λ DFB By appropriately designing the wavelength to be 410 nm, it will fall within the wavelength tolerance range, and the yield can be significantly improved.

[0058] Furthermore, the PL wavelength λ within the wafer surface PL By understanding the distribution of the gain peaks to some extent in advance and varying the pitch of the diffraction grating 150 in the range of 409 to 411 nm, converted into the DFB oscillation wavelength, in accordance with the distribution, even elements with gain peaks in the range of 410±6 nm will fall within the wavelength tolerance range. This will further improve the wavelength yield.

[0059] Next, an example of a method for manufacturing the semiconductor laser device 100 having a wide 12 dB bandwidth gain spectrum will be described.

[0060] In one embodiment, the substrate 110 has an n-type carrier concentration of 1×10 18 cm -3 An n-type GaN (0001) substrate (wafer) can be used.

[0061] To intentionally change the off-angle of the substrate 110 within each chip on the wafer, the surface of the substrate 110 may be processed by dry etching or the like (referred to as Process 1). It is preferable to change the off-angle within each chip within a range of approximately ±0.1°. By changing the off-angle within a chip, it is possible to intentionally introduce fluctuations in the In composition into the InGaN active layer 130, thereby expanding the 12 dB bandwidth of the gain spectrum.

[0062] The n-type semiconductor layer 120, the active layer 130, and the p-type semiconductor layer 140 are formed in this order by epitaxial growth on the n-type substrate 110. In one embodiment, a MOVPE (Metalorganic Vapor Phase Epitaxy) apparatus is used for the epitaxial growth.

[0063] A mixture of hydrogen and nitrogen can be used as the carrier gas. The Ga, Al, and In sources can be trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), or trimethylindium (TMIn), respectively. Silane (SiH4) can be used as the n-type dopant, and biscyclopentadienylmagnesium (Cp2Mg) can be used as the p-type dopant.

[0064] After placing the n-type GaN substrate in the MOVPE apparatus, the temperature of the n-type GaN substrate is raised while supplying NH3, and the temperature is raised to the growth temperature to cause epitaxial growth.

[0065] The n-type semiconductor layer 120 may include an n-type cladding layer and an n-type guide layer. For example, the material of the n-type cladding layer may be a Si-doped n-type GaN layer (Si concentration 1×10 18cm -3 , thickness 1 μm), Si-doped n-type Al 0.04 Ga 0.96 N(Si concentration 1×10 18 cm -3 The n-type guide layer is made of Si-doped n-type GaN (Si concentration 5×10 17 cm -3 , thickness 0.1 μm).

[0066] GaN growth can be performed at a substrate temperature of 1100°C, with a TMG supply rate of 58 μmol / min and an NH3 supply rate of 0.23 mol / min.AlGaN growth of the n-type cladding layer can be performed at a substrate temperature of 1120°C, with a TMA supply rate of 3 μmol / min, a TMG supply rate of 58 μmol / min, and an NH3 supply rate of 0.36 mol / min.

[0067] An active layer (light emitting layer) 130 having a quantum well structure is formed on the n-type semiconductor layer 120. DFB When the wavelength is set to 405 nm, the quantum well structure material is In 0.09 Ga 0.91 N well layer (4 nm thick) and undoped In 0.02 Ga 0.98 A two-period multiple quantum well (MQW) layer consisting of an N barrier layer (thickness 10 nm) can be selected.

[0068] In this case, the growth conditions for InGaN-MQW can be set as follows: substrate temperature: approximately 840°C, TEG supply rate: 18 μmol / min, NH3 supply rate: 0.29 mol / min. The TMIn supply rate can be set to approximately 12 μmol / min for the well layer and 2.4 μmol / min for the barrier layer.

[0069] Here, one or more of the following processes may be carried out to intentionally introduce fluctuations in the In composition into the InGaN active layer and expand the 12 dB bandwidth of the gain spectrum. Process 2 During the growth of the InGaN-MQW well layer, the growth temperature is varied over time within a range of about ±5°C.

[0070] Step 3 The amount of TMI supplied may be changed over time within a range of about ±5 μmol.

[0071] These treatments can introduce non-uniformity, i.e., fluctuations or variations, into the In composition in the active layer in the stacking direction of the nitride-based multilayer structure and within the wafer surface, and / or into the thickness of the quantum well layer due to differences in the crystal growth rate. In general semiconductor laser manufacturing processes, it is considered necessary to suppress concentration variations within the wafer surface, but the present embodiment can be said to be performing treatments that are the exact opposite of conventional wisdom.

[0072] Furthermore, to expand the 12 dB bandwidth of the gain spectrum, it is advisable to use two or more well layers. Each well layer will have its own independent fluctuations and distributions in the In composition and quantum well layer thickness, which makes it possible to widen the gain spectrum compared to a single layer.

[0073] The p-type semiconductor layer 140 is made of Mg-doped p-type Al 0.2 Ga 0.8 A carrier block (electron block EB) layer made of N, Mg-doped p-type GaN (Mg concentration 2×10 19 cm -3 , 0.1 μm thick), a p-type guide layer consisting of Mg-doped p-type Al 0.04 Ga 0.96 N(Mg concentration 1×10 19 cm -3 , thickness 0.5 μm), a p-type cladding layer consisting of Mg-doped p-type GaN (Mg concentration 1×10 20 cm -3 The p-type contact layer may be formed of a p-type silicon nitride layer (0.03 μm thick).

[0074] The AlGaN growth of the p-type electron barrier layer can be performed at a substrate temperature of 1050°C with a TMA supply rate of 5 μmol / min, a TMG supply rate of 15 μmol / min, and an NH3 supply rate of 0.23 mol / min. The AlGaN growth of the p-type cladding layer can be performed at a substrate temperature of 1050°C with a TMA supply rate of 1.3 μmol / min, a TMG supply rate of 24 μmol / min, and an NH3 supply rate of 0.23 mol / min. The GaN growth of the p-type contact layer can be performed at a growth temperature of 1050°C with a TMG supply rate of 10 μmol / min, and an NH3 supply rate of 0.23 mol / min.

[0075] The above is the method for manufacturing the semiconductor laser device 100.

[0076] FIG. 6 shows an example of the distribution of the In composition in the active layer across the wafer. In the figure, shading represents the In composition and the thickness of the quantum well layer. This non-uniform distribution of the In composition, which is sufficiently small compared to the chip size, or the thickness of the quantum well layer, is intentionally introduced by the above-mentioned processes 1 to 3. The wafer 600 includes multiple chips (devices) 602. Positions 602A and 602B indicate different positions within the chip 602. Coordinate axes Z and X correspond to coordinate axes z and x in the semiconductor laser device 100. Coordinate axis Y corresponds to the stacking direction in the semiconductor laser device 100.

[0077] Figure 7 shows the distribution of In composition in the y-axis (stacking direction) in the active layer. The upper row shows the In composition at position 602A, and the lower row shows the In composition at position 602B. The dashed lines represent the averages. This example uses a case where the design center of the In composition in the well layer is 9%, the design center of the well layer thickness is 4 nm, the design center of the In composition in the barrier layer is 2%, and the design center of the barrier layer thickness is 10 nm. The In composition distribution and thickness in the stacking direction of the well layer at positions 602A and 602B are different, centered around 9% and 4 nm, respectively. The average composition and thickness of well layer 1 and well layer 2 at each position are also different, centered around 9% and 4 nm, respectively. Furthermore, the average composition and thickness of well layer 1 at position 602A and well layer 1 at position 602B are also different, centered around 9% and 4 nm, respectively. Furthermore, the average composition and thickness of well layer 2 at position 602A and well layer 2 at position 602B are also different, centered around 9% and 4 nm, respectively. The distribution of the In composition and the thickness of the quantum well layer is, for example, ±1% and ±1 nm, respectively, which allows a 12 dB bandwidth of the gain spectrum exceeding 70 meV to be obtained.

[0078] In this embodiment, the pitch of the diffraction grating 150 is determined so as to obtain a desired oscillation wavelength, and therefore the oscillation wavelength λ DFB The gain at the peak wavelength may be significantly lower than the gain at room temperature. Therefore, in order to achieve stable laser oscillation, it is necessary to increase the coupling coefficient. However, since the coupling coefficient is an index that is difficult to evaluate directly and quantitatively, the stop band width Δλ, which has a positive correlation with the coupling coefficient, is used. SB It is convenient to use as a parameter for design. SB can be easily and directly measured from the spectrum of emitted light just before oscillation.

[0079] Figure 8 is a diagram explaining the stop band. The left side of Figure 8 shows the spectrum of emitted light just before oscillation in a DFB-LD with a uniform diffraction grating with no phase shift. In a DFB-LD, a region where the emitted light is minimal is formed near a wavelength determined by the pitch (period) of the diffraction grating and the effective refractive index, and this region is called the stop band. Stop band width Δλ SB =λ2-λ1, and the greater the coupling between the diffraction grating and the propagating light, the smaller Δλ SB In a DFB-LD without phase shift, DFB oscillation generally occurs at either λ1 or λ2, but in this example, λ2 = λ DFB It is as follows.

[0080] The right side of Figure 8 shows the spectrum of emitted light just before oscillation from a DFB-LD with a diffraction grating with a phase shift, such as a λ / 4 shift. In the case of a diffraction grating with a phase shift, laser oscillation occurs at a wavelength near the center of the stop band. The stop band width in such a case is also Δλ SB =λ2-λ1, where λ2≠λ DFB It is as follows.

[0081] If the reflectivity of both facets of a semiconductor laser is not zero, the stop band width λ SB It is known that the magnitude of changes depending on the phase of the grating at the end face (end face phase). The inventors have investigated such cases and have concluded that stable oscillation is possible as long as the reflectivity of either end face is 2% or less. Therefore, the stop band width Δλ SB The appropriate range is expressed by formula (1). 1.8×10 -6 λ 2 ≦Δλ SB ≦4.0×10 -6 λ 2 …(1) Stop band width Δλ SB If we design within the range of equation (1), we can obtain an appropriate coupling coefficient, and λ DFB Even if the gain is far from the peak, stable oscillation is possible.

[0082] In this embodiment, the pitch of the diffraction grating 150 is determined so as to obtain a desired oscillation wavelength. DFB The gain at the DFB oscillation wavelength may be significantly lower than the gain at the peak wavelength at room temperature. Therefore, for stable DFB oscillation, at least one facet must have a reflectivity of 2% or less at the oscillation wavelength. This makes the threshold gain required for Fabry-Perot (FP) mode oscillation sufficiently higher than the threshold gain required for DFB oscillation. Therefore, even if the gain at the DFB oscillation wavelength is significantly lower than the gain at the gain peak wavelength, oscillation in the FP mode (FP oscillation) can be suppressed, enabling stable DFB oscillation.

[0083] The methods for increasing the indium content of the active layer or the thickness non-uniformity of the quantum well layers within the chip are not limited to those described above. For example, the active layer 130 may have a multiple quantum well structure, with the composition of the material constituting at least two well layers being different. This can increase the width of the gain spectrum.

[0084] The embodiments merely illustrate the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the spirit of the present invention as defined in the claims. [Explanation of symbols]

[0085] 100 Semiconductor laser element 110 Substrate 120 n-type semiconductor layer 130 Active layer 140 p-type semiconductor layer 142 Ridge 144 Ridge Waveguide 150 Diffraction Grating 152 Groove 160 Insulating Materials

Claims

1. A substrate; a nitride-based multilayer structure including a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer stacked in this order on the substrate; a current confinement portion formed in the second conductivity type semiconductor layer; a diffraction grating having a periodic refractive index change formed along the waveguiding direction of the optical resonator formed by the current confinement portion; Equipped with The threshold current I that initiates laser oscillation when driven at room temperature ＴＨ a bandwidth of 70 meV or more at an optical intensity 12 dB lower than the peak position of the gain spectrum immediately before oscillation, when the spectrum of emitted light when driven with a current 0.9 times the peak position of the gain spectrum immediately before oscillation is taken as the gain spectrum immediately before oscillation.

2. 2. The distributed feedback semiconductor laser according to claim 1, wherein the active layer has a quantum well structure of two or more layers.

3. 2. The distributed feedback semiconductor laser according to claim 1, wherein the active layer has a multiple quantum well structure, and at least two well layers have different compositions of materials.

4. 2. The distributed feedback semiconductor laser according to claim 1, wherein the active layer has a multiple quantum well structure, and the thickness of at least two well layers is different.

5. 2. The distributed feedback semiconductor laser according to claim 1, wherein the active layer has a quantum well structure, and the In composition of the well layer varies along the waveguide direction.

6. 2. The distributed feedback semiconductor laser according to claim 1, wherein the active layer has a quantum well structure, and the thickness of the well layer varies along the waveguide direction.

7. The diffraction grating has a stop band width Δλ that appears in the gain spectrum immediately before oscillation. ＳＢ but, 1.8×10 －６ ・l ２ ≦Dl ＳＢ ≦4.0×10 －６ ・l ２ 7. The distributed feedback semiconductor laser device according to claim 1, wherein the semiconductor laser device is configured such that:

8. 7. The distributed feedback semiconductor laser device according to claim 1, further comprising a facet coating film formed on at least one facet and having a reflectance of 2% or less at the oscillation wavelength.

9. 7. The distributed feedback semiconductor laser according to claim 1, wherein the diffraction grating is made up of a plurality of projections and recesses formed on the surface of the second conductivity type semiconductor layer along a ridge portion provided on the surface of the second conductivity type semiconductor layer.

10. a preparation step of preparing a semiconductor wafer; A step A of forming a first conductivity type semiconductor layer on the semiconductor wafer; a step B of forming an active layer on the first conductivity type semiconductor layer so that at least one of an In composition and a thickness of a quantum well layer is non-uniform across the entire semiconductor wafer; a step C of forming a second conductivity type semiconductor layer on the active layer; Step D: forming a diffraction grating that determines the oscillation wavelength; 1. A method for manufacturing a distributed feedback semiconductor laser, comprising:

11. 11. The manufacturing method according to claim 10, wherein in step B, at least one of the In composition and the thickness of the quantum well layer on the semiconductor wafer is made non-uniform by adjusting at least one of the crystal growth temperature, the In ratio in the gas phase, and the V / III ratio.

12. 12. The manufacturing method according to claim 10, further comprising a process for making the off-angle of the semiconductor wafer non-uniform within the plane.

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