Semiconductor optical element and its manufacturing method

Abstract

To provide a semiconductor optical device excellent in mass productivity.SOLUTION: A semiconductor optical device is provided, comprising: a substrate of a first conductive type; an optical confinement layer of the first conductive type which is arranged above the substrate of the first conductive type; a multi quantum well layer which is constructed by a plurality of well layers and barrier layers arranged above the optical confinement layer of the first conductive type; an optical confinement layer of a second conductive type that is arranged on the multi quantum well layer; and a PL stabilization layer that is arranged between the optical confinement layer of the first conductive type and the multi quantum well layer. The thickness of the PL stabilization layer is the half or more of the thickness of the multi quantum well layer, and a composition wavelength of the PL stabilization layer is shorter than the composition wavelength of the well layer of the multi quantum well layer, and is longer than the composition wavelength of the optical confinement layer of the first conductive type.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a semiconductor optical device and a method for manufacturing the same. [Background technology]

[0002] In recent years, the Internet society has led to an increasing demand for high-speed, high-capacity semiconductor optical devices. Known examples of semiconductor optical devices include direct-modulation semiconductor lasers and semiconductor lasers that emit continuous light. External modulators that modulate continuous light to generate optical signals include electroabsorption modulators and Mach-Zehnder modulators. The basic structure of these semiconductor optical devices generally consists of a multi-quantum well (MQW) layer sandwiched between p-type and n-type semiconductors. The MQW functions as a light-generating layer when the semiconductor optical device is a semiconductor laser and as an absorption layer when the semiconductor optical device is a modulator. Another known structure sandwiches the MQW between optical confinement layers (SCH layers; Separate Confinement Heterostructure layers). The SCH layers are typically arranged to confine light to the MQW with high efficiency. Therefore, the refractive index of the SCH layers is generally smaller than that of the well and barrier layers that form the MQW. In other words, the compositional wavelength of the SCH layer is generally shorter than that of the well and barrier layers that make up the MQW. A shorter compositional wavelength of the SCH layer than that of the MQW indicates a larger band gap, allowing carriers to move smoothly from the SCH layer to the MQW under forward bias, for example.

[0003] Patent Document 1 discloses a structure in which two n-type SCH layers are provided, and the composition wavelength of the second confinement layer closer to the semiconductor substrate is longer than that of the barrier layer. This structure increases the carrier capture time, thereby increasing the frequency modulation efficiency. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 07-183617 Summary of the Invention [Problem to be solved by the invention]

[0005] The characteristics of semiconductor optical devices depend heavily on the quality of the MQWs. MQWs are fabricated by crystal growth techniques such as MOCVD (Metal Organic Chemical Vapor Deposition). When forming an SCH layer, the upper and lower SCH layers are often grown simultaneously (continuously) so that the MQWs are sandwiched between them. PL measurement (photoluminescence) is a well-known method for evaluating the quality of MQWs. PL measurement is a semiconductor evaluation technique that involves irradiating a semiconductor with external excitation light and measuring the spontaneous emission of carriers generated. A light source that emits light with a wavelength absorbed by the semiconductor layer under evaluation is used as the excitation light source. Spontaneous emission from the MQWs can be measured by using an excitation light source with a wavelength that can be absorbed only by the MQWs. However, because the compositional wavelengths of the MQWs and SCH layers are often close, a light source that emits light with a wavelength that can be absorbed by both the MQWs and the SCH layers is often used. The spontaneous emission spectrum measured here includes both the spontaneous emission from the MQWs and the spontaneous emission from the SCH layers.

[0006] Crystal quality can be inspected by analyzing the spontaneous emission spectrum. However, for example, if the spontaneous emission intensity is low, accurate spectrum analysis is difficult. Furthermore, during mass production, if the spontaneous emission spectrum intensity varies significantly between lots, it is difficult to determine which spectrum is correct. There are several reasons for the low spontaneous emission intensity. One is that the MQW crystal quality is below the desired level, resulting in low spontaneous emission and therefore insufficient absorption of pump light. Another is that the MQW itself has a structure that makes it difficult to absorb pump light (e.g., thin layers), resulting in low absorption of pump light. Furthermore, while the pump light intensity is generally set to ensure sufficient spontaneous emission intensity from the MQW, measurement variations and other factors can sometimes prevent the desired pump light from reaching the semiconductor device, resulting in reduced spontaneous emission intensity from the MQW. Therefore, if the intensity of the evaluation spectrum in PL measurement is low, it may not be possible to determine whether the crystal quality of the MQW is poor, or whether the MQW simply has a structure that cannot absorb sufficient pump light, or whether the MQW was not sufficiently irradiated with pump light, and it may not be possible to evaluate the crystal quality of the MQW.

[0007] Furthermore, when evaluating semiconductor layers compatible with the 1.3 μm band used in optical communications, a YAG laser with an oscillation wavelength of 1064 nm is often used as the excitation light source. InP-based semiconductor optical devices often use InP as the substrate. The energy of the excitation light from a YAG laser is smaller than the band gap of InP. Therefore, when MQW compatible with the 1.3 μm band is crystal-grown on an InP substrate, the YAG laser excites the MQW without exciting the InP substrate, making it suitable for evaluating the crystal-grown layer alone.

[0008] The composition wavelength of the SCH layer in a 1.3 μm-band semiconductor optical device is often set to approximately 1.00 μm to 1.1 μm. However, during crystal growth, the composition wavelength of the SCH layer is affected by manufacturing variations. For example, even if the SCH layer is designed to have a set wavelength of 1.05 μm, the actual composition wavelength of the SCH layer may be 1.03 μm or 1.07 μm. In this case, when 1064 nm (1.064 μm) pump light is incident, the pump light may or may not be absorbed by the SCH layer. If the pump light is not absorbed by the SCH layer, it is absorbed only by the MQW, allowing the spontaneous emission from the MQW to be measured, thereby confirming the quality of the MQW. However, if the pump light is absorbed by the SCH layer, the measured spectrum is the spontaneous emission spectrum, including not only the MQW but also the SCH layer. Furthermore, because light is absorbed not only by the MQW but also by the SCH layer, more carriers are excited, and the intensity of spontaneously emitted light from the MQW changes compared to when light is absorbed only by the MQW. If the SCH layer always absorbs light, it would be possible to compare the quality of stacked semiconductor layers based on the fact that the SCH layer absorbs light. However, because manufacturing variations cause variations in the composition wavelength of the SCH layer, if there is a mixture of cases in which the SCH layer absorbs light and cases in which it does not, the results of PL measurements will vary, making it difficult to accurately confirm the quality of the semiconductor multilayer.

[0009] SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a semiconductor optical device that has a structure that allows evaluation of the crystal quality of semiconductor layers, particularly multi-quantum well layers, and that is suitable for mass production. [Means for solving the problem]

[0010] (1) A semiconductor optical device according to the present disclosure comprises a substrate of a first conductivity type, an optical confinement layer of a first conductivity type disposed on the upper side of the substrate of the first conductivity type, a multiple quantum well layer composed of a plurality of well layers and barrier layers disposed on the optical confinement layer of the first conductivity type, an optical confinement layer of a second conductivity type disposed on the multiple quantum well layer, and a PL stabilization layer disposed between the substrate of the first conductivity type and the multiple quantum well layer, wherein the thickness of the PL stabilization layer is at least half the thickness of the multiple quantum well layer, and the compositional wavelength of the PL stabilization layer is shorter than the compositional wavelength of the well layers of the multiple quantum well layer and longer than the compositional wavelength of the optical confinement layer of the first conductivity type. [Effects of the Invention]

[0011] According to the present invention, it becomes possible to stably inspect the crystal quality of the multiple quantum well layer of a semiconductor optical device, thereby realizing stable mass production. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a top view of a semiconductor optical device according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view taken along the line AA shown in FIG. [Figure 3] 3A to 3C are cross-sectional views of the semiconductor optical device according to the first embodiment during manufacture. [Figure 4] FIG. 4 is a top view of a semiconductor optical device according to a second embodiment of the present invention. [Figure 5] FIG. 5 is a cross-sectional view taken along the line AA shown in FIG. [Figure 6] FIG. 6 is an enlarged view of the cross section taken along line AA in FIG. 5. [Figure 7] 5A to 5C are cross-sectional views of the semiconductor laser according to the second embodiment during manufacturing. [Figure 8] FIG. 4 is a schematic diagram of a band diagram of a semiconductor laser according to a second embodiment. [Figure 9] FIG. 2 is a cross-sectional view of the semiconductor optical device according to the third embodiment of the present invention taken along the line AA. [Figure 10]FIG. 2 is a cross-sectional view of a semiconductor optical device according to a fourth embodiment of the present invention taken along the line AA. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In all drawings for explaining the embodiments, components having the same functions are designated by the same reference numerals, and repeated explanations thereof will be omitted. The drawings shown below are merely for explaining examples of the embodiments, and the size of the drawings does not necessarily correspond to the scale described in the examples.

[0014] [First embodiment] FIG. 1 is a top view of a semiconductor optical device 1 according to a first embodiment of the present invention. The semiconductor optical device 1 is a semiconductor laser. FIG. 2 is a schematic diagram of an AA cross section perpendicular to the optical axis. The semiconductor optical device 1 includes a structure in which a PL stabilization layer 9, an n-type optical confinement layer (n-type SCH layer 3), a multiple quantum well (MQW) layer 4, a p-type SCH layer 5, and a p-type cladding layer 6 are stacked in this order on an n-type substrate (substrate 2). The semiconductor optical device 1 has a front electrode 7 and a back electrode 8. The front electrode 7 is in contact with the p-type cladding layer 6, and the back electrode 8 is in contact with the n-type (substrate 2). The MQW 4 emits light when a voltage is applied (a current is injected) between these two electrodes. Note that the n-type and p-type may be reversed. A contact layer may be disposed between the p-type cladding layer 6 and the front electrode 7 to reduce electrical resistance.

[0015] Here, the PL stabilization layer 9 is a layer having a longer composition wavelength (smaller band gap) than the n-type SCH layer 3. The PL stabilization layer 9 has a composition wavelength that can absorb the pumping light of a pumping light source, which will be described later.

[0016] The present disclosure is characterized by the PL stabilization layer 9. To explain the effect of the PL stabilization layer 9, the manufacturing procedure for the semiconductor optical device 1 will be described. First, as shown in FIG. 3 , the PL stabilization layer 9, the n-type SCH layer 3, the MQW 4, the p-type SCH layer 5, and the cap layer 10 are crystal-grown over the entire upper surface of the substrate 2. After the crystal growth, PL measurement is performed to confirm the crystal quality of the MQW 4. PL measurement is performed by irradiating excitation light of a wavelength that can be absorbed by the MQW 4 and the PL stabilization layer 9 onto the surface of the cap layer 10 and measuring spontaneous emission (PL) light from the semiconductor optical device 1. While the excitation light can be incident from the substrate 2 side, it is preferable to incident it from the cap layer 10 side for stable evaluation because the substrate 2 is sufficiently thick compared to the other layers, and therefore the excitation light may be absorbed or scattered by the substrate 2. Note that the cap layer 10 is provided to protect the layers below the p-type SCH layer 5 during other manufacturing processes, such as PL measurement, and does not ultimately remain. Therefore, the cap layer 10 is optional.

[0017] PL measurement involves measuring the wavelength spectrum corresponding to the band gap of each layer. PL measurement is a method for assessing the crystalline quality by evaluating the emission intensity, half-width, etc. of this wavelength spectrum. To assess the crystalline quality of MQW4, the wavelength of the excitation light must be shorter than the compositional wavelength of the well layers constituting the MQW4. In this case, the well layers may be too thin to adequately absorb the excitation light. Furthermore, if the compositional wavelengths of the n-type SCH3 and p-type SCH5 are shorter than the wavelength of the excitation light, the two SCH layers may not absorb the excitation light. As a result, the intensity of the PL light output from the MQW4 may be low, making accurate PL measurement impossible. However, the PL stabilization layer 9 in this disclosure enables stable PL measurement. The PL stabilization layer 9 is formed with a compositional wavelength that can absorb the excitation light. Therefore, the excitation light not absorbed by the MQW4 (excitation light that passes through the MQW4) is absorbed by the PL stabilization layer 9. Electrons and holes (carriers) are generated by the excitation light absorbed by the PL stabilization layer 9. Some of the generated carriers migrate to the MQW4 due to energy such as heat. The migrated carriers recombine in the MQW4, generating PL light. This increases the intensity of the PL light from the MQW4, enabling accurate PL measurements. The compositional wavelength of the PL stabilization layer 9 is shorter than that of the well layers of the MQW4. If the compositional wavelength of the PL stabilization layer 9 were longer than that of the well layers, the carriers excited in the PL stabilization layer 9 would recombine there, and the PL light from the semiconductor optical device 1 would mainly be emitted from the PL stabilization layer 9. This would reduce the PL light from the MQW4, making it impossible to evaluate the crystallinity of the MQW4. Furthermore, the compositional wavelength of the PL stabilization layer 9 should preferably be longer than that of the n-type SCH layer 3. If the compositional wavelength of the n-type SCH layer 3 is longer than that of the PL stabilization layer 9, the n-type SCH layer 3 can also absorb the excitation light, allowing stable PL measurements to be performed even without the PL stabilization layer 9. In this case, the PL stabilization layer 9 is unnecessary. However, as will be described later, even if the n-type SCH layer 3 has a composition wavelength that can absorb the excitation light, the PL stabilization layer 9 may be effective.

[0018] The thickness of the PL stabilization layer 9 is preferably at least half the thickness of the MQW4. Furthermore, by making the PL stabilization layer 9 at least twice the total thickness of the layers excluding the MQW4 that can absorb the excitation light, stable PL measurements can be achieved. For example, if the n-type SCH layer 3 and the p-type SCH layer 5 (hereinafter, the n-type SCH layer 3 and the p-type SCH layer 5 are collectively referred to as the SCH layers) have a composition wavelength that can absorb the excitation light, carriers generated in the SCH layer migrate to the MQW4 and are output as PL light. However, if the SCH layer is thin, the number of carriers supplied is small, and the PL light intensity remains low. However, if the PL stabilization layer 9 is arranged to be at least twice the total thickness of the n-type SCH layer 3 and the p-type SCH layer 5, sufficient carriers can be supplied.

[0019] Furthermore, manufacturing variations can cause the composition wavelength of the SCH layer to deviate from the design value. For example, when the composition wavelength of the SCH layer and the wavelength of the pump light are close to each other, manufacturing variations can cause the composition wavelength of the SCH layer to absorb the pump light or not. The amount of carriers supplied to the MQW changes depending on whether or not the SCH layer absorbs the pump light, which in turn changes the PL intensity from the MQW. However, the PL stabilization layer 9 can ensure a stable supply of carriers to the MQW, regardless of whether or not the SCH layer absorbs the pump light. In a structure including the PL stabilization layer 9, the intensity of the PL light generated in the MQW can change depending on the presence or absence of pump light in the SCH layer, but the spectral shape of the PL light generated in the MQW is stable, allowing for stable PL measurements.

[0020] [Second embodiment] FIG. 4 is a top view of a semiconductor optical device 201 according to a second embodiment of the present disclosure. The semiconductor optical device 201 is a modulator-integrated semiconductor laser in which a semiconductor laser 211, an electroabsorption modulator (EA modulator) 213, and a waveguide 212 are integrated. While an integrated type is shown in this embodiment, the present disclosure can also achieve the effects of a semiconductor laser alone or an EA modulator alone. The semiconductor optical device 201 is an integrated device in which the semiconductor laser 211, the waveguide 212, and the EA modulator 213 are optically connected to each other in this order. The semiconductor laser 211 emits continuous light, and the waveguide 212 transmits the light emitted from the semiconductor laser 211 to the EA modulator 213. The EA modulator 213 has a multiple quantum well layer that absorbs light corresponding to the oscillation wavelength of the semiconductor laser 211. The continuous light that passes through the waveguide 212 and enters the EA modulator 213 is intensity-modulated by the EA modulator 213 and converted into a modulated optical signal such as a binary or quaternary signal. The modulated optical signal output from the EA modulator 213 is output from the front end facet 221. Note that another structure, for example a window structure in which semi-insulating InP or the like is disposed, may be provided near the front end facet 221. A dielectric non-reflective film (not shown) is formed on the front end facet 221. Furthermore, a dielectric highly reflective film (not shown) is formed on the rear end facet 222, which is the end face on the opposite side of the semiconductor laser 211. As will be described in detail later, the semiconductor optical device 1 has a mesa structure from the semiconductor laser 211 to the EA modulator 213, and is an embedded type semiconductor device in which both sides are embedded with semiconductor burying layers (BH layers 215).

[0021] FIG. 5 is a schematic diagram of an AA cross section perpendicular to the optical axis of the semiconductor laser 211. The mesa structure is composed of a portion of the substrate 202 (n-InP substrate), a laser portion multiple quantum well layer 204, a p-type cladding layer 216, and the p-type contact layer 218 shown in FIG. 6. Here, the p-type cladding layer 216 is composed of a p-InP layer. Both sides of the mesa structure are buried with buried layers (BH layers 215). Details of the mesa structure will be described later. An insulating film 219 is disposed on the upper surface of the BH layer 215, with the exception of a portion. The insulating film 219 is, for example, a SiO2 film. A laser portion electrode 207 is disposed across a portion of the upper surface of the insulating film 219 and the top of the mesa structure. In addition, a backside electrode 208 is disposed on the backside of the substrate 202. The semiconductor laser 211 generates continuous light by applying a voltage (injecting a current) between the laser portion electrode 207 and the backside electrode 208.

[0022] 6 is an enlarged view of the mesa structure and its vicinity of the semiconductor laser 211 shown in FIG. 5. The mesa structure is composed of a part of the substrate 202, a PL stabilization layer 209, an n-type optical confinement layer (SCH layer) 203, a laser portion multiple quantum well layer 204, a p-type SCH layer 205, a p-type cladding layer 216, and a p-type contact layer 218. A diffraction grating is formed in the p-type SCH layer 205, and the semiconductor laser 211 is a DFB (Distributed Feedback) laser that oscillates in the 1.3 μm band. Furthermore, the semiconductor laser 211 is not limited to a DFB laser, and may be an FP (Fabry-Perot) laser, a DBR (Distributed Bragg Reflector) laser, or a DR (Distributed Reflector) laser.

[0023] The PL stabilization layer 209 is composed of an undoped InGaAsP layer having a thickness of 100 nm and a composition wavelength of 1.1 μm. The n-type SCH layer 203 is composed of n-type InGaAsP having a thickness of 25 nm and a composition wavelength of 1.05 μm. The p-type SCH layer 205 is composed of p-type InGaAsP having a thickness of 25 nm and a composition wavelength of 1.05 μm. The laser portion multiple quantum well layer 204 has multiple barrier layers and well layers arranged alternately. The laser portion multiple quantum well layer 204 starts and ends with a barrier layer. Here, both the well layer and the barrier layer are 8 nm thick. The barrier layer is undoped InGaAsP having a composition wavelength of 1.08 μm, and the well layer is undoped InGaAsP having a composition wavelength of 1.28 μm. The entire laser portion multiple quantum well layer 204 has a composition corresponding to the 1.3 μm band. The p-type cladding layer 216 is a p-type InP layer with a thickness of 1500 nm. Note that the composition wavelength and thickness described here are merely examples given to explain in detail the structure that provides the effects of the present disclosure, and the effects of the present disclosure can be obtained with different composition wavelengths and layer thicknesses.

[0024] The present disclosure is characterized by the PL stabilization layer 209. To explain the effect of the PL stabilization layer 209, the manufacturing procedure for the semiconductor optical device 201, particularly the manufacturing procedure for the semiconductor laser 211, will be described. First, as shown in FIG. 7 , the PL stabilization layer 209, the n-type SCH layer 203, the laser portion multiple quantum well layer 204, the p-type SCH layer 205, and the p-type InP cap layer 210 are crystal-grown on the entire upper surface of the substrate 202. Note that an n-InP buffer layer may be disposed between the substrate 202 and the PL stabilization layer 209. After the crystal growth, PL measurement is performed to confirm the crystal quality of the laser portion multiple quantum well layer 204. The PL measurement is performed by irradiating the surface side of the p-type InP cap layer 210 with a YAG laser having an oscillation wavelength of 1064 nm and measuring the light emission (PL light) from the semiconductor laser 211.

[0025] The output light (pump light) of the YAG laser is 1064 nm (1.064 μm), so it is absorbed by layers with compositional wavelengths longer than this wavelength and is transmitted through layers with compositional wavelengths shorter than 1.064 μm. In this configuration, the pump light is absorbed by the well layers and the PL stabilization layer 209.

[0026] Figure 8 shows a band diagram that schematically illustrates the optical absorption of the semiconductor laser 211 when a YAG laser beam is incident. The pump light is absorbed in the well layer, generating electron-hole pairs. The electrons and holes recombine to output PL light with a wavelength spectrum corresponding to the band gap of the laser-portion multiple quantum well layer 204. Not all of the pump light is absorbed by the well layer; some reaches the substrate 202. At this time, the pump light is also absorbed by the PL stabilization layer 209. Electrons and holes (carriers) are generated by the pump light absorbed by the PL stabilization layer 209. Some of the generated carriers migrate toward the laser-portion multiple quantum well layer 204 due to energy such as heat. The migrated carriers recombine in the laser-portion multiple quantum well layer 204, resulting in the output of PL light. The PL stabilization layer 209 has a thickness of 100 nm, which sufficiently absorbs the pump light and supplies the laser-portion multiple quantum well layer 204 with sufficient carriers for PL emission. As a result, the PL light output from the laser-portion multiple quantum well layer 204 has sufficient intensity to verify its quality.

[0027] As noted above, the wavelength of the SCH layer in a 1.3 μm-band semiconductor optical device is often close to the wavelength of a YAG laser. In this embodiment, both the n-type SCH layer 203 and the p-type SCH layer 205 are 1.05 μm. However, even if the designed composition wavelength is 1.05 μm, manufacturing variations of approximately ±0.03 μm from the designed wavelength occur. If the variation is toward the longer wavelength side, the composition wavelength of the SCH layer may become a composition wavelength that absorbs light with a wavelength of 1.064 μm. Conversely, if the composition wavelength of the SCH layer is toward the shorter wavelength side, it will deviate from 1.064 μm. This results in variations in the amount of pump light absorbed. If the composition wavelength of the SCH layer shifts toward the longer wavelength side, the pump light is absorbed, carriers are supplied to the laser portion multiple quantum well layer 204, and the PL light intensity increases. On the other hand, if the compositional wavelength of the SCH layer is shifted to the shorter wavelength side, the pump light is not absorbed, but is absorbed only by the well layer, and the PL intensity may be lower than when the SCH layer is shifted to the longer wavelength side. In other words, when multiple semiconductor optical devices with the same structure are manufactured, the PL intensity will vary depending on the compositional wavelength of the SCH layer, making it difficult to determine which one represents the correct PL measurement. This makes stable mass production difficult.

[0028] On the other hand, as described above, by making the PL stabilization layer 209 sufficiently thick compared to the other layers and by setting the composition wavelength to reliably absorb the YAG laser light, the PL stabilization layer 209 can reliably absorb the excitation light and supply carriers to the laser portion multiple quantum well layer 204. The intensity of the PL light is stabilized when a certain amount of electrons is supplied. To achieve stable light absorption, the thickness of the PL stabilization layer 209 is preferably 100 nm or greater. Furthermore, for example, if the composition wavelength of the SCH layer varies and the SCH layer absorbs the excitation light and generates carriers, the carriers also migrate to the laser portion multiple quantum well layer 204 and are observed as PL light. Therefore, variation in the composition wavelength of the SCH layer can cause variations in the intensity of the PL light. To avoid this, the influence of light absorption by the SCH layer can be reduced by ensuring that the light absorbed by the PL stabilization layer 209 is sufficiently greater than the light absorbed by the SCH layer. Therefore, the thickness of the PL stabilization layer 209 is preferably thicker than that of the n-type SCH layer 203. Furthermore, the thickness of the PL stabilization layer is preferably at least twice the total thickness of the layers with a composition wavelength capable of absorbing YAG laser light, excluding the laser portion multiple quantum well layer 204. Here, the layer with a composition wavelength capable of absorbing YAG laser light refers to a layer with a composition wavelength shorter than 1.064 μm. If the excitation light source is not a YAG laser, the layer with a composition wavelength capable of absorbing excitation light refers to a layer with a composition wavelength shorter than the wavelength of the excitation light source. In this embodiment, the layers that may absorb YAG laser light due to manufacturing variations are the n-type SCH layer 203 and the p-type SCH layer 205. The total thickness of these two layers is 50 nm. On the other hand, the thickness of the PL stabilization layer 209 is 100 nm, which reduces the effect of composition wavelength variations in the SCH layers on PL light intensity. Therefore, this disclosure is particularly effective when the composition wavelength of the SCH layer is 1.03 μm or more and 1.09 μm or less.

[0029] Furthermore, to avoid the influence of variations in the composition wavelength of the completed PL stabilization layer 209, the composition wavelength of the PL stabilization layer 209 is preferably 1.1 μm or longer. The composition wavelength of the PL stabilization layer 209 is preferably shorter than the composition wavelength of the well layer. If the composition wavelength of the PL stabilization layer 209 is longer than that of the well layer, the rate at which carriers recombine in the PL stabilization layer 209 before recombining in the laser portion multiple quantum well layer 204 increases, making it difficult to supply carriers to the laser portion multiple quantum well layer 204. Furthermore, if the energy barrier between the PL stabilization layer 209 and the adjacent n-type SCH layer 203 is too large, carriers generated in the PL stabilization layer 209 will not easily move toward the laser portion multiple quantum well layer 204. Therefore, the energy barrier between the PL stabilization layer 209 and the adjacent layer is preferably 0.23 μm or shorter in terms of composition wavelength. In this embodiment, the composition wavelength of the PL stabilization layer 209 is preferably 1.28 μm or shorter. Furthermore, the present disclosure is particularly effective when the composition wavelength of PL stabilization layer 209 is longer than that of n-type SCH layer 203. When the composition wavelength of n-type SCH layer 203 is longer than that of PL stabilization layer 209, the YAG laser light is absorbed by n-type SCH layer 203, and the pump light is absorbed even without PL stabilization layer 209, reducing the benefit of providing PL stabilization layer 209. However, from the viewpoint of supplying more carriers to laser portion multiple quantum well layer 204, providing PL stabilization layer 209 does have an advantage.

[0030] The PL stabilization layer 209 is preferably an n-type layer or an undoped layer with a low concentration compared to the substrate 202 and the n-type SCH layer 203. Here, "low concentration" means a carrier concentration of less than 1×10^17 / cm3. The "undoped layer" refers to a layer that is not intentionally doped with impurities, and includes a layer containing impurities at a background level. The PL stabilization layer 209 may be a highly doped layer, for example, 1×10^17 / cm 3 If the n-type layer has a concentration above this level, the effective energy gap will be large, which means that the effective absorption wavelength of the PL stabilization layer 209 will shift to shorter wavelengths. As a result, the composition will be unable to absorb the excitation light, which may reduce the stability of the PL measurement.

[0031] After verifying the quality of the semiconductor laser 211 through PL measurement using the above procedure, multilayer growth is performed on the EA modulator 213 using known lithography techniques, butt jointing, crystal growth, and other methods. When fabricating the EA modulator 213, the PL stabilization layer 209, similar to the semiconductor laser 211, allows for quality verification of the EA modulator's multiple quantum well layer. Similarly, a waveguide 212 is formed, followed by a p-type cladding layer 216 and a p-type contact layer 218. Before forming the p-type cladding layer 216, a diffraction grating is formed in the p-type SCH layer 205 of the semiconductor laser 211. After the multilayer growth, a mesa structure is formed, and BH layers 215 are formed on both sides of the mesa structure. Furthermore, an insulating film 219, a laser electrode 207, and a modulator electrode 225 are formed on the surface. After the substrate 202 is thinly polished, a back electrode 208 is formed on the back surface. Finally, chipping completes the semiconductor optical device 201.

[0032] The PL stabilization layer 209 has a smaller bandgap than the n-type SCH layer 203. Generally, the bandgap is gradually increased from the barrier layer of the multiple quantum well layer 4 toward the base substrate 202, thereby facilitating smooth carrier migration. Therefore, in the semiconductor optical device 201, the PL stabilization layer 209 may act as a barrier to carrier migration during actual operation. However, because the semiconductor laser 211 oscillates continuous light, the carrier migration speed is not a major concern. Therefore, even with the PL stabilization layer 209 according to the present disclosure, the device is fully suitable for practical use. When the EA modulator 213 or this structure is applied to a directly modulated semiconductor laser, the modulation speed may be limited in some cases. In this case, the reduction in modulation speed can be suppressed by making the PL stabilization layer 209 as thin as possible and narrowing the bandgap with the adjacent layer. Specifically, in the case of a semiconductor optical device compatible with the 1.3 μm band, it is preferable that the composition wavelength of the PL stabilization layer 209 be 1.1 μm or more, which ensures reliable light absorption even with the influence of manufacturing variations, and that the thickness be 300 nm or less. For other wavelength bands, it is preferable to use the thicknesses and the like shown in the first embodiment.

[0033] Furthermore, it is preferable to arrange the PL stabilization layer 209 on the n-side as viewed from the multiple quantum well layer 4. If arranged on the p-side, a large energy barrier will be generated because the PL stabilization layer 209 is undoped, which will have a significant impact on the characteristics of even a semiconductor laser. Note that the PL stabilization layer 209 is not limited to InGaAsP as long as it has a composition wavelength that can absorb YAG laser light. For example, the PL stabilization layer 209 may be InGaAsAl.

[0034] [Third embodiment] 9 is a cross-sectional view of a semiconductor optical device 301 according to the third embodiment. The only difference from the first embodiment is that the positions of the PL stabilization layer 9 and the n-type SCH layer 3 are reversed.

[0035] In the third embodiment, the proximity of the PL stabilization layer 9 to the multiple quantum well layer 4 enables a more stable supply of carriers during PL measurement. However, the optical confinement ratio of the multiple quantum well layer 4 is reduced compared to the first embodiment. When the optical confinement ratio is reduced, the advantage is that the internal loss is reduced, making it possible to oscillate a higher-power laser beam. The disadvantage is that the thick undoped layer (PL stabilization layer 9) is located close to the multiple quantum well layer 4, reducing the electric field strength applied to the multiple quantum well layer 4. For example, if this structure is applied to an EA modulator instead of a semiconductor laser, the disadvantage is that the extinction ratio is reduced.

[0036] [Fourth embodiment] 10 is a cross-sectional view of a semiconductor optical device 401 according to the fourth embodiment. The difference from the first embodiment is that the PL stabilization layer is divided into two layers. From the substrate 2 side, a first PL stabilization layer 409a, an n-type SCH layer 3, a second PL stabilization layer 409b, and a multiple quantum well layer 4 are stacked in this order. Both the first PL stabilization layer 409a and the second PL stabilization layer 409b have the same composition wavelength and are undoped layers.

[0037] In this embodiment, the total thickness of the first PL stabilization layer 409a and the second PL stabilization layer 409b is the same as that of the PL stabilization layer 9 in the first embodiment. Therefore, as described in the first embodiment, it is possible to achieve stable PL measurement. Furthermore, because the PL stabilization layer is divided into two, carrier stagnation during modulation operation can be suppressed compared to when a single PL stabilization layer is provided. This embodiment is particularly effective in, for example, semiconductor lasers and EA modulators that perform modulation operation. Note that, although the PL stabilization layer is composed of two layers in this embodiment, it is not limited to this and may be divided into three layers. An SCH layer or other layer may be sandwiched between the multiple PL stabilization layers. When multiple PL stabilization layers are provided, it is preferable that the composition wavelength of each PL stabilization layer be within the above-mentioned range and that they be undoped. Furthermore, it is preferable that the total thickness of the multiple PL stabilization layers be 100 nm to 300 nm.

[0038] As described above, by placing a PL stabilization layer between the multiple quantum well layer and the n-type semiconductor substrate, it is possible to stabilize the PL intensity during PL measurement, making it possible to accurately inspect the crystal quality.

[0039] Although the example has been shown in which only a PL stabilization layer and an SCH layer are disposed between the multiple quantum well layer 4 and the substrate 2, other layers may be present between the multiple quantum well layer 4 and the substrate 2. Furthermore, the ideas shown in the third and fourth embodiments may be combined with the second embodiment. [Explanation of symbols]

[0040] 1, semiconductor optical element, 2 substrate, 3 n-type SCH layer, 4 multiple quantum well layer, 5 p-type SCH layer, 6 cladding layer, 7 front electrode, 8 back electrode, 9 PL stabilization layer, 10 cap layer, 201 semiconductor optical element, 202 substrate, 203 n-type SCH layer, 204 laser portion multiple quantum well layer, 205 p-type SCH layer, 207 laser portion electrode, 208 back electrode, 209 PL stabilization layer, 210 p-type InP cap layer, 211 semiconductor laser, 212 waveguide, 213 EA modulator, 215 BH layer, 216 p-type cladding layer, 218 p-type contact layer, 219 insulating film, 221 front end facet, 222 rear end facet, 225 modulator portion electrode, 301 semiconductor optical element, 401 semiconductor optical element, 409a first PL stabilization layer, 409b 2nd PL stabilization layer.

Claims

1. a substrate of a first conductivity type; an optical confinement layer of a first conductivity type disposed above the substrate of the first conductivity type; a multi-quantum well layer composed of a plurality of well layers and barrier layers disposed above the first conductivity type optical confinement layer; an optical confinement layer of a second conductivity type disposed on the multiple quantum well layer; a PL stabilization layer disposed between the first conductivity type substrate and the multiple quantum well layer; the thickness of the PL stabilization layer is at least half the thickness of the multiple quantum well layer; a composition wavelength of the PL stabilization layer is shorter than a composition wavelength of the well layer of the multiple quantum well layer and longer than a composition wavelength of the first conductivity type optical confinement layer; the thickness of the PL stabilization layer is at least twice the total thickness of the first conductivity type light confinement layer and the second conductivity type light confinement layer; Semiconductor optical element.

2. 2. The semiconductor optical device according to claim 1, A semiconductor optical device, wherein the PL stabilization layer has a carrier concentration that is undoped or less than 1×10^17 / cm3.

3. 3. The semiconductor optical device according to claim 1, A semiconductor optical device, wherein the composition wavelength of the PL stabilization layer is longer than the wavelength of the excitation light used in measuring the PL of the multiple quantum well layer.

4. 4. The semiconductor optical device according to claim 3, a composition wavelength of the first conductivity type optical confinement layer and the second conductivity type optical confinement layer is longer than a wavelength of the excitation light;

5. 5. The semiconductor optical device according to claim 1, The PL stabilization layer is a multi-layer structure, A semiconductor optical device, wherein the thickness of the PL stabilization layer is the total thickness of the plurality of layers.

6. 6. The semiconductor optical device according to claim 1, The PL stabilization layer is disposed between the substrate of the first conductivity type and the optical confinement layer of the first conductivity type.

7. 7. The semiconductor optical device according to claim 1, The PL stabilization layer is disposed between the multiple quantum well layer and the first conductivity type optical confinement layer.

8. 8. The semiconductor optical device according to claim 1, The first conductivity type is n-type, and the second conductivity type is p-type.

9. 8. The semiconductor optical device according to claim 1, The multiple quantum well layer emits and absorbs light in the 1.3 μm band, the composition wavelength of the PL stabilization layer is 1.1 μm or more; The PL stabilization layer has a thickness greater than that of the first conductivity type light confinement layer.

10. 10. The semiconductor optical device according to claim 9, The PL stabilization layer has a composition wavelength of 1.28 μm or less.

11. 11. The semiconductor optical device according to claim 9, a bandgap wavelength of the first conductivity type optical confinement layer and the second conductivity type optical confinement layer is 1.03 μm or more and 1.09 μm or less.

12. 12. The semiconductor optical device according to claim 1, The PL stabilization layer has a thickness of 100 nm or more and 300 nm or less.

13. 6. The semiconductor optical device according to claim 5, The semiconductor optical device, wherein the total thickness of the plurality of layers is 100 nm or more and 300 nm or less.

14. 14. The semiconductor optical device according to claim 1, The semiconductor optical element is a semiconductor laser.

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