High-power semiconductor laser device having current confinement structure and index-guided structure, and oscillating in transverse mode

[0002] 1. Field of the Invention

[0003] The present invention relates to a semiconductor laser device having a current confinement structure and an index-guided structure. The present invention also relates to a process for producing a semiconductor laser device having a current confinement structure and an index-guided structure. Further, the present invention relates to a solid-state laser apparatus which includes as an excitation light source a semiconductor laser device having a current confinement structure and an index-guided structure. The solid-state laser apparatus may include provision for generating a second harmonic.

[0004] 2. Description of the Related Art

[0005] (1) In many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 μm band, a current confinement structure and an index-guided structure are provided in crystal layers which constitute the semiconductor laser devices so that each semiconductor laser device oscillates in a fundamental transverse mode. For example, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.102 discloses a semiconductor laser device which emits light in the 0.98 μm band. This semiconductor laser device is formed as follows.

[0006] On an n-type GaAs substrate, an n-type Al_0.48Ga_0.52As lower cladding layer, an undoped Al_0.2Ga_0.8As optical waveguide layer, an Al_0.2Ga_0.8As/In_0.2Ga_0.8As double quantum well active layer, an undoped Al_0.2Ga_0.8AS optical waveguide layer, a p-type AlGaAs first upper cladding layer, a p-type Al_0.67Ga_0.33As etching stop layer, a p-type Al_0.48Ga_0.52As second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type Al_0.67Ga_0.33As etching stop layer by the conventional photolithography and selective etching, and an n-type Al_0.7Ga_0.3As and n-type GaAs materials are embedded in both sides of the ridge structure by selective MOCVD using Cl₂gas. Then, the insulation film is removed, and thereafter a p-type GaAs layer is formed. Thus, a current confinement structure and an index-guided structure are built in the semiconductor laser device.

[0007] However, the above semiconductor laser device has a drawback that it is very difficult to form the AlGaAs second upper cladding layer on the AlGaAs first upper cladding layer, since the AlGaAs first upper cladding layer contains a high Al content and is prone to oxidation, and selective growth of the AlGaAs second upper cladding layer is difficult.

[0008] (2) In addition, IEEE Journal of Selected Topics in Quantum Electronics, vol. 29, No. 6, 1993, pp.1936 discloses a semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 to 1.02 μm band. This semiconductor laser device is formed as follows.

[0009] On an n-type GaAs substrate, an n-type Al_0.4Ga_0.6As lower cladding layer, an undoped Al_0.2Ga_0.8As optical waveguide layer, a GaAs/InGaAs double quantum well active layer, an undoped Al_0.2Ga_0.8As optical waveguide layer, a p-type Al_0.4Ga_0.6As upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above a mid-thickness of the p-type Al_0.4Ga_0.6As upper cladding layer by the conventional photolithography and selective etching, and an n-type In_0.5Ga_0.5P material and an n-type GaAs material are embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and then electrodes are formed. Thus, a current confinement structure and an index-guided structure are realized in the layered construction.

[0010] However, the above semiconductor laser device also has a drawback that it is very difficult to form the InGaP layer on the AlGaAs upper cladding layer, since the AlGaAs upper cladding layer contains a high Al content and is prone to oxidation, and it is difficult to grow an InGaP layer having a different V-group component, on such an upper cladding layer.

[0011] (3) Further, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.189 discloses an all-layer-Al-free semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 μm band. This semiconductor laser device is formed as follows.

[0012] On an n-type GaAs substrate, an n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile strain barrier layer, an InGaAs double quantum well active layer, an InGaAsP tensile strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type InGaP first upper cladding layer by the conventional photolithography and selective etching, and an n-type In_0.5Ga_0.5P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, a current confinement structure and an index-guided structure are realized.

[0013] The reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, the above semiconductor laser device also has a drawback that the kink level is low (about 150 mW) due to poor controllability of the ridge width.

[0014] As in the above example, the conventional current semiconductor laser devices which contain a current confinement structure, oscillate in a fundamental transverse mode, and emit light in the 0.9 to 1.1 μm band, are difficult to produce, or have poor characteristics, and are unreliable in high output power operation.

[0015] (4) High-power semiconductor laser devices having a broad light-emitting area are employed as excitation light sources in conventional solid-state laser apparatuses, in which output laser light is emitted from a solid-state laser crystal. In particular, some solid-state laser apparatuses further include a nonlinear crystal which converts a fundamental wave emitted from the solid-state laser crystal into a second harmonic, and such solid-state laser apparatuses are widely used.

[0016] In the above solid-state laser apparatuses, the semiconductor laser devices as excitation light sources are required to emit laser light with very high output power. In order to achieve the high output power, semiconductor laser devices in which an active layer has a width of 10 μm or greater are used, while the widths of the active layers in single-mode laser devices are usually about 3 μm. Therefore, a number of high-order transverse modes are mixed in oscillated light, and when the oscillation power is increased, the modes of oscillated light are liable to change to different modes due to spatial hole burning of carriers, which is caused by high density distribution of photons in the resonant cavity. At the same time, near-field pattern, far-field pattern, and oscillation spectrum vary. In addition, since efficiencies of current-to-light conversion are different between different transverse modes, the optical output power further varies. This phenomenon is called a kink in the current-optical output power characteristic of a semiconductor laser device.

[0017] Thus, the following problems arise.

[0018] When the above high-power semiconductor laser device is used as an excitation light source in a solid-state laser apparatus, at least one component coupled with an oscillation mode of the solid-state laser resonator is utilized as an excitation light from among oscillated light generated by the semiconductor laser device and condensed by a lens system into a solid-state laser crystal. Therefore, the output intensity varies greatly with changes of the transverse modes. In addition, since the absorption spectrum of the solid-state laser crystal has a fine absorption spectrum structure in a narrow wavelength band, an amount of absorbed light varies in response to the variation of an oscillation spectrum. Thus, the output intensity of the solid-state laser apparatus is further affected by variation of the oscillation spectrum, as well as change of the transverse modes. Furthermore, use of a spatial or spectral portion of the light generated by the solid-state laser device increases high-frequency noise accompanying by switching between the transverse modes.

[0019] As mentioned above, when transverse modes or longitudinal modes in a semiconductor laser device used as an excitation light source in a solid-state laser apparatus change, i.e., when an oscillation spectrum of the semiconductor laser device varies, the excitation efficiency in the solid-state laser apparatus varies, and therefore the optical output also varies. At the same time, high frequency noise is generated. Further, in practice, in order to vary the light intensity of the solid-state laser apparatus, or achieve phase matching with a wavelength conversion element, temperature and excitation current must be varied. Therefore, when oscillation modes change at the same time as the variations of the temperature and the excitation current, the intensity of the output laser light varies strikingly. Although the above variation of the intensity of the output laser light is a variation in the intensity of a DC component of the output laser light, it is probable that AC noise is continuously produced.

[0020] Generally, the intensity and the frequency spectrum of the output laser light of the solid-state laser apparatus depend on the intensity and the spectrum component of the excitation laser light which is emitted from the semiconductor laser device, and actually used for excitation of the solid-state laser crystal. In addition, the intensity and the frequency spectrum of the output laser light of the solid-state laser apparatus also vary with the excitation current and an individual difference in the characteristics of the semiconductor laser device. Thus, the intensity and the frequency spectrum of the output laser light of the solid-state laser apparatus are not uniform. The variation in the intensity of the DC laser light sometimes exceeds 10%, and therefore causes problems in various applications. In particular, in order to generate a high quality image, it is desirable that a noise level is 1% or lower. However, it is impossible to repetitively achieve or maintain the noise level of 1% or lower in the conventional high-power semiconductor laser devices which have an ordinary oscillation region. In addition, when a solid-state laser apparatus and a nonlinear crystal are combined in order to generate a second harmonic, the above noise is amplified by the nonlinear effect, and it is therefore necessary to further suppress the noise.

[0021] Japanese Unexamined Patent Publication, No.11(1999)-74620, which is assigned to the present assignee, discloses that reduction of a strain imposed on the semiconductor laser device by a so-called junction-up type mounting of the semiconductor laser device on a heat sink, as well as prevention of change of the transverse modes by using an index-guided semiconductor laser device, is effective at suppressing noise. However, it is difficult to increase an output power of a semiconductor laser device which is mounted on a heat sink in a junction-up configuration. In addition, photon density in an active layer must be suppressed in order to increase reliability, and the width of the optical waveguide layers must be made broad in order to suppress the photon density in an active layer. However, it is impossible to form an index-guided structure when the optical waveguide layers are broad.

[0022] As mentioned before, the conventional high-power semiconductor laser devices having a broad light-emitting region lack optical stability. That is, the optical output of the conventional high-power semiconductor laser devices having a broad light-emitting region is unstable, and the noise level in the optical output is not sufficiently low. Therefore, it is not desirable to use solid-state laser apparatuses using the conventional high-power semiconductor laser device as an excitation light source, as well as optical fiber laser apparatuses in which the conventional high-power semiconductor laser device is coupled to an optical fiber, in the applications for producing high quality images such as printed images, photographs, and medical images.

[0023] An object of the present invention is to provide a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high.

[0024] Another object of the present invention is to provide a process for producing a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high.

[0025] Still another object of the present invention is to provide a stripe-type index-guided semiconductor laser device which oscillates in multiple transverse modes, and has a stable optical output with low noise.

[0026] A further object of the present invention is to provide a process for producing a stripe-type index-guided semiconductor laser device which oscillates in multiple transverse modes, and has a stable optical output with low noise.

[0027] (1) According to the first aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of In_x3Ga_1−x3As_1−y3P_y3and formed on the lower optical waveguide layer, where 0<x3≦0.4, 0≦y3≦0.1, and an absolute value of a first product of a strain and a thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; an upper optical waveguide layer formed on the compressive strain quantum well active layer; a first upper cladding layer made of In_x8Ga_1−x8P of a second conductive type, and formed on the upper optical waveguide layer; an etching stop layer made of In_x1Ga_1−x1As_1−y1P_y1of the second conductive type, and formed on the first upper cladding layer other than a stripe area of the first upper cladding layer so as to form a first portion of a stripe groove realizing a current injection window, where 0≦x1≦0.3, 0y1≦0.3, and an absolute value of a second product of a strain and a thickness of the etching stop layer is equal to or smaller than 0.25 nm; a current confinement layer made of In_x8Ga_1−x8P of the first conductive type, and formed on the etching stop layer so as to form a second portion of the stripe groove, where x8=0.49±0.01; a second upper cladding layer made of Al_z4Ga_1−z4As of the second conductive type, and formed over the current confinement layer and the stripe area of the first upper cladding layer so as to cover the stripe groove, where 0.20≦z4≦0.50; and a contact layer of the second conductive type, formed on the second upper cladding layer. In the semiconductor laser device, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.

[0028] The first and second conductive types are opposite to each other in carrier polarity. For example, when the first conductive type is n type, the second conductive type is p type. In addition, the undoped type semiconductor is doped with substantially no impurity.

[0029] The strain Δa of the quantum well active layer is defined as Δa=(ca−cs)/cs, where cs and ca are the lattice constants of the GaAs substrate and the quantum well active layer, respectively, and the strain Δe of the etching stop layer is defined as Δe=(ce−cs)/cs, where ce is the lattice constant of the etching stop layer. That is, in the semiconductor laser device according to the first aspect of the present invention, −0.25 nm≦Δa·da≦0.25 nm, and −0.25 nm≦Δe·de≦0.25 nm, where da and de are the thicknesses of the quantum well active layer and the etching stop layer, respectively. In the first to fourth aspects of the present invention, when a layer grown over the substrate has a lattice constant c, and an absolute value of the amount (c−cs)/cs is equal to or smaller than 0.003, the layer is lattice-matched with the substrate.

[0030] Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii).

[0031] (i) The semiconductor laser device may further include first and second tensile strain barrier layers both made of In_x5Ga_1−x5As_1−y5P_y5, and respectively formed above and below the compressive strain quantum well active layer, where 0≦x5≦0.3 and 0≦y5≦0.6, and an absolute value of a sum of the first product and a third product of a strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.

[0032] The strain Δb of the first and second tensile strain barrier layers is defined as Δb=(cb−cs)/cs, where cb is the lattice constant of the first and second tensile strain barrier layers, and cs is the lattice constant of the substrate. That is, in this semiconductor laser device, −0.25 nm≦Δa·da+Δb·db≦0.25 nm, where db is the total thickness of the first and second tensile strain barrier layers.

[0033] (ii) The semiconductor laser device may further include an additional layer being made of In_x8Ga_1−x8P of the second conductive type, formed below the second upper cladding layer, and having a thickness between 10 to 400 nm, where x8=0.49±0.01. In particular, the optimum thickness of the In_x8Ga_1−x8P layer is 250 to 300 nm.

[0034] (iii) The stripe groove may have a width equal to or greater than 1 μm.

[0035] (2) According to the second aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of In_x3Ga_1−x3As_1−y3P_y3, and formed on the lower optical waveguide layer, where 0≦x3≦0.4, and 0≦y3≦0.1, and an absolute value of a first product of a strain and a thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; an upper optical waveguide layer formed on the compressive strain quantum well active layer; a first upper cladding layer of a second conductive type, formed on the upper optical waveguide layer; a first etching stop layer made of In_x7Ga_1−x7P of the second conductive type, and formed on the first upper cladding layer, where 0≦x7≦1; a second etching stop layer made of In_x1Ga_1−x1As_1−y1P_y1of the second conductive type, and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window, where 0≦x1≦0.3, and 0≦y1≦0.3; a current confinement layer made of In_x8Ga_1−x8P of the first conductive type, and formed on the second etching stop layer so as to form a second portion of the stripe groove, where x8=0.49±0.01; a second upper cladding layer made of Al_z4Ga_1−z4As of the second conductive type, and formed over the current confinement layer and the stripe area of the first upper cladding layer so as to cover the stripe groove, where 0.20≦z4≦0.50; and a contact layer of the second conductive type, formed on the second upper cladding layer. In the semiconductor laser device, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate, and an absolute value of a sum of a second product of a strain and a thickness of the first etching stop layer and a third product of a strain and a thickness of the second etching stop layer is equal to or smaller than 0.25 nm. That is, in the semiconductor laser device according to the second aspect of the present invention, −0.25 nm≦Δe1·de1+Δe2·de2≦0.25 nm, where Δe1 and Δe2 are strains of the first and second etching stop layers, respectively, and de1 and de2 are the thicknesses of the first and second etching stop layers, respectively.

[0036] Preferably, the semiconductor laser device according to the second aspect of the present invention may also have one or any possible combination of the aforementioned additional features (ii) and (iii) and the following additional features (iv) and (v).

[0037] (iv) The semiconductor laser device may further include first and second tensile strain barrier layers both made of In_x5Ga_1−x5As_1−y5P_y5, and respectively formed above and below the compressive strain quantum well active layer, where 0≦x5≦0.3 and 0<y5≦0.6, and an absolute value of a sum of the first product and a fourth product of a strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.

[0038] (v) The first upper cladding layer is made of In_x6Ga_1−x6As_1−y6P_6yor Al_z5Ga_1−z5As, where x6=(0.49±0.01)y6, 0.2<y6<1, and 0.25≦z5≦0.7.

[0039] (3) According to the third aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming a lower cladding layer of a first conductive type, on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer on the lower cladding layer; (c) forming a compressive strain quantum well active layer made of In_x3Ga_1−x3As_1−y3P_y3, on the lower optical waveguide layer, where 0<x3≦0.4, 0≦y3≦0.1, and an absolute value of a first product of a strain and a thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; (d) forming an upper optical waveguide layer on the compressive strain quantum well active layer; (e) forming a first upper cladding layer made of In_x8Ga_1−x8P of a second conductive type, on the upper optical waveguide layer; (f) forming an etching stop layer made of In_x1Ga_1−x1As_1−y1P_y1of the second conductive type, on the first upper cladding layer, where 0≦x1≦0.3, 0≦y1≦0.3, and an absolute value of a second product of a strain and a thickness of the etching stop layer is equal to or smaller than 0.25 nm; (g) forming a current confinement layer made of In_x8Ga_1−x8P of the first conductive type, on the etching stop layer, where x8=0.49±0.01; (h) removing a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (i) removing a stripe area of the etching stop layer so as to form a second portion of the stripe groove; (j) forming a second upper cladding layer made of Al_z4Ga_1−z4As of the second conductive type so that the stripe groove is covered with the second upper cladding layer, where 0.20≦z4≦0.50; and (k) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.

[0040] That is, the semiconductor laser device according to the first aspect of the present invention can be produced by the process according to the third aspect of the present invention.

[0041] Preferably, the process according to the third aspect of the present invention may also have one or any possible combination of the following additional features (vi) to (viii).

[0042] (vi) The process according to the third aspect of the present invention may further comprise the steps of (b1) forming, after the step (b), a first tensile strain barrier layer made of In_x5Ga_1−x5As_1−y5P_y5, on the lower optical waveguide layer, where 0≦x5≦0.3 and 0<y5≦0.6, and (c1) forming, after the step (c), a second tensile strain barrier layer made of In_x5Ga_1−x5As_1−y5P_y5, on the compressive strain quantum well active layer, where an absolute value of a sum of the first product and a third product of a strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.

[0043] (vii) The process according to the third aspect of the present invention may further comprise the step of (j1) forming, before the step (j), an additional layer having a thickness of 10 to 400 nm and being made of In_x8Ga_1−x8P of the second conductive type on the current confinement layer so that the stripe groove is covered with the additional layer, where x8=0.49±0.01. In particular, it is preferable that the thickness of the In_x8Ga_1−x8P layer is 250 to 300 nm.

[0044] (viii) The process according to the third aspect of the present invention may further comprise, after the step (g), the steps of (g1) forming a cap layer made of GaAs, and (g2) removing a stripe area of the cap layer. In addition, in the step (i), a remaining area of the cap layer is also removed.

[0045] The above cap layer may be one of the first and second conductive types and the undoped type.

[0046] (4) According to the fourth aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming a lower cladding layer of a first conductive type on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer on the lower cladding layer; (c) forming a compressive strain quantum well active layer made of In_x3Ga_1−x3As_1−y3P_y3on the lower optical waveguide layer, where 0<x3≦0.4, and 0≦y3≦0.1, and an absolute value of a first product of a strain and a thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; (d) forming an upper optical waveguide layer on the compressive strain quantum well active layer; (e) forming a first upper cladding layer of a second conductive type, on the upper optical waveguide layer; (f) forming a first etching stop layer made of In_x7Ga_1−x7P of the second conductive type, on the first upper cladding layer, where 0≦x7≦1; (g) forming a second etching stop layer made of In_x1Ga_1−x1As_1−y1P_y1of the second conductive type, on the first etching stop layer, where 0≦x1≦0.3, and 0≦y1≦0.3; (h) forming a current confinement layer made of In_x8Ga_1−x8P of the first conductive type, on the second etching stop layer, where x8=0.49±0.01; (i) removing a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (j) removing a stripe area of the second etching stop layer so as to form a second portion of the stripe groove; (k) forming a second upper cladding layer made of Al_z4Ga_1−z4As of the second conductive type so that the stripe groove is covered with the second upper cladding layer, where 0.20≦z4≦0.50; and (1) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate, and an absolute value of a sum of a second product of a strain and a thickness of the first etching stop layer and a third product of a strain and a thickness of the second etching stop layer is equal to or smaller than 0.25 nm.

[0047] That is, the semiconductor laser device according to the second aspect of the present invention can be produced by the process according to the fourth aspect of the present invention.

[0048] Preferably, the process according to the fourth aspect of the present invention may also have one or any possible combination of the aforementioned additional feature (vi) and the following additional features (ix) and (x).

[0049] (ix) The process according to the fourth aspect of the present invention may further comprise the step of (k1) forming, before the step (k), an additional layer having a thickness of 10 to 400 nm and being made of In_x8Ga_1−x8P of the second conductive type on the current confinement layer so that the stripe groove is covered with the additional layer, where x8=0.49±0.01. In particular, it is preferable that the thickness of the In_x8Ga_1−x8P layer is 250 to 300 nm.

[0050] (x) The process according to the fourth aspect of the present invention may further comprise the steps of (h1) forming a cap layer made of GaAs on the current confinement layer, and (h2) removing a stripe area of the cap layer. In addition, in the step (j), a remaining area of the cap layer is also removed.

[0051] The above cap layer may be one of the first and second conductive types and the undoped type.

[0052] (5) The first to fourth aspects of the present invention have the following advantages.

[0053] (a) In the semiconductor laser devices according to the first and second aspects of the present invention, the current confinement layer is made of In_x8Ga_1−x8P, and the second upper cladding layer is made of Al_z4Ga_1−z4As. Therefore, the difference in the refractive indexes between the current confinement layer and the second upper cladding layer realizes, with high accuracy, a difference of about 1.5×10⁻³to 1×10⁻²in the equivalent refractive index between a portion of the active region under the stripe groove and the other portions of the active region under the current confinement layer, and it is possible to cut off oscillation in higher modes. Thus, oscillation in a fundamental transverse mode can be maintained even when the output power becomes high.

[0054] (b) When a base layer on which the second upper cladding layer is formed contains aluminum, the base layer is prone to oxidation, and it is difficult to realize desired characteristics in the semiconductor laser device. However, in the semiconductor laser devices according to the first and second aspects of the present invention, the In_x8Ga_1−x8P first upper cladding layer, the In_x7Ga_1−x7P first etching stop layer, and the In_x8Ga_1−x8P current confinement layer, which can be a base layer of the second upper cladding layer, do not contain aluminum. Therefore, it is easy to form the second upper cladding layer. In addition, since a crystal defect due to oxidation of aluminum does not occur, the characteristics of the semiconductor laser device do not deteriorate, and reliability is improved.

[0055] (c) Since the current confinement layer is arranged within the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced.

[0056] (d) Due to the provision of the current confinement layer, the currents can be confined within a small width during current injection into the active region. Therefore, the transverse mode oscillation is less prone to cause a kink in a current-optical output characteristic. That is, the kink level can be raised.

[0057] (e) Since the etching stop layer is made of InGaAsP, the stripe width can be accurately adjusted by wet etching in which the difference in the etching rate between the etching stop layer and the InGaP current confinement layer is utilized.

[0058] (f) When the InGaP layer having a thickness of 10 to 400 nm is formed before the second upper cladding layer is formed, it is possible to increase the control range of the A1 composition in the Al_z4Ga_1−z4As second upper cladding layer.

[0059] (g) When the In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layers (0≦x5≦0.3 and 0<y5≦0.6) are respectively formed above and below the compressive strain quantum well active layer, various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.

[0060] (h) When the stripe width is equal to or greater than 1 μm, the semiconductor laser devices according to the first and second aspects of present invention are most advantageous since the semiconductor laser devices can oscillate with high output power and low noise, even in multiple modes.

[0061] (i) When a GaAs cap layer is formed on the InGaP current confinement layer, it is possible to prevent formation of a natural oxidation film on the InGaP current confinement layer, as well as metamorphic change in the InGaP current confinement layer, which occurs when a resist layer is formed directly on the InGaP current confinement layer. In addition, since the GaAs cap layer is removed before the second upper optical waveguide layer is formed, it is possible to remove a residue left on the base layer on which the second upper optical waveguide layer is formed, and prevent the occurrence of crystal defects.

[0062] (6) According to the fifth aspect of the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer of an undoped type or the first conductive type, formed on the lower cladding layer; an active layer formed on the lower optical waveguide layer; a first upper optical waveguide layer of an undoped type or a second conductive type, formed on the active layer; an etching stop layer made of In_x1Ga_1−x1As_1−y1P_y1of the second conductive type, and formed on the first upper optical waveguide layer other than a stripe area of the first upper optical waveguide layer so as to form a first portion of a stripe groove realizing a current injection window, where 0≦x1≦0.5, and 0≦y1≦0.8; a current confinement layer made of In_x3(Al_z3Ga_1−z3)_1−x3P of the first conductive type, and formed on the etching stop layer so as to form a second portion of the stripe groove, where 0<z3≦1, and x3=0.49±0.01; a second upper optical waveguide layer of the second conductive type, formed over the current confinement layer and the stripe area of the first upper optical waveguide layer so as to cover the stripe groove; an upper cladding layer of the second conductive type, and formed on the second upper optical waveguide layer; and a contact layer made of GaAs of the second conductive type, and formed on the upper cladding layer. In the semiconductor laser device, a total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 μm, and the active layer is made of one of InGaAs, InGaAsP, and GaAsP.

[0063] Preferably, the semiconductor laser device according to the fifth aspect of the present invention may also have one or any possible combination of the following additional features (xi) and (xvii).

[0064] (xi) The process according to the fifth aspect of the present invention may further comprise a cap layer made of In_0.49Ga_0.51P of the first or second conductive type, and formed between the current confinement layer and the second upper optical waveguide layer.

[0065] (xii) Each of the lower optical waveguide layer and the first and second upper optical waveguide layers is made of In_x2Ga_1−x2p, where x2=0.49±0.01.

[0066] (xiii) Each of the lower optical waveguide layer and the first and second upper optical waveguide layers is made of In_x2Ga_1−x2As_y2P_1−y2, where x2=(0.49±0.01)y2, and 0≦x2≦0.49.

[0067] (xiv) The semiconductor laser device according to the fifth or sixth aspect of the present invention further comprises first and second tensile strain barrier layers made of one of InGaP, InGaAsP, and GaAsP, and respectively formed above and below the active layer.

[0068] (xv) Each of the lower and upper cladding layers is made of one of AlGaAs, InGaAlP, and InGaAlPAs which lattice-match with the GaAs substrate.

[0069] In the fifth to tenth aspects of the present invention, when a layer grown over the substrate has a lattice constant c, and an absolute value of the amount (c−cs)/cs is equal to or smaller than 0.003, the layer is lattice-matched with the substrate, where cs is the lattice constant of the GaAs substrate.

[0070] (xvi) The bottom of the stripe groove has a width of 1 to 5 μm, and a difference in an equivalent refractive index caused by a difference in a refractive index between the current confinement layer and the second upper optical waveguide layer is in a range from 0.0015 to 0.01.

[0071] The difference in the equivalent refractive index is a difference in the equivalent refractive index in propagation modes in the thickness direction, between portions of the active region under the current confinement layer and the other portion of the active region under the stripe groove.

[0072] (xvii) The bottom of the stripe groove has a width equal to or greater than 10 μm.

[0073] (7) According to the sixth aspect of the present invention, there is provided a semiconductor laser device comprising: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer of an undoped type or the first conductive type, formed on the lower cladding layer; an active layer formed on the lower optical waveguide layer; a first upper optical waveguide layer of an undoped type or a second conductive type, formed on the active layer; a first etching stop layer made of In_x9Ga_1−x9P of the second conductive type, and formed on the first upper optical waveguide layer, where 0≦x9≦1; a second etching stop layer made of In_x1Ga_1−x1As_1−y1P_y1of the second conductive type, and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window, where 0≦x1≦0.5, and 0≦y1≦0.8; a current confinement layer made of In_x3(Al_z3Ga_1−z3)_1−x3P of the first conductive type, and formed on the second etching stop layer so as to form a second portion of the stripe groove, where 0<z3≦1, and x3=0.49±0.01; a second upper optical waveguide layer of the second conductive type, formed over the current confinement layer and the stripe area of the first etching stop layer so as to cover the stripe groove; an upper cladding layer of the second conductive type, and formed on the second upper optical waveguide layer; and a contact layer made of GaAs of the second conductive type, and formed on the upper cladding layer. In the semiconductor laser device, a total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 μm, and the active layer is made of one of InGaAs, InGaAsP, and GaAsP.

[0074] Preferably, the semiconductor laser device according to the sixth aspect of the present invention may also have one or any possible combination of the aforementioned additional features (xi) and (xvii).

[0075] (8) According to the seventh aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming a lower cladding layer of a first conductive type, on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer of an undoped type or the first conductive type, on the lower cladding layer; (c) forming an active layer on the lower optical waveguide layer; (d) forming a first upper optical waveguide layer of an undoped type or a second conductive type, on the active layer; (e) forming an etching stop layer made of In_x1Ga_1−x1As_1−y1P_y1of the second conductive type, on the first upper optical waveguide layer, where 0≦x1≦0.5, and 0≦y1≦0.8; (f) forming a current confinement layer made of In_x3(Al_z3Ga_1−z3)_1−x3P of the first conductive type, on the etching stop layer, where 0<z3≦1, and x3=0.49±0.01, (g) removing a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (h) removing a stripe area of the etching stop layer so as to form a second portion of the stripe groove; (i) forming a second upper optical waveguide layer of the second conductive type so that the stripe groove is covered with the second upper optical waveguide layer; (j) forming an upper cladding layer of the second conductive type, on the second upper optical waveguide layer; and (k) forming a contact layer made of GaAs of the second conductive type, on the upper cladding layer. In the process, a total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 μm, and the active layer is made of one of InGaAs, InGaAsP, and GaAsP.

[0076] That is, the semiconductor laser device according to the fifth aspect of the present invention can be produced by the process according to the seventh aspect of the present invention.

[0077] Preferably, the process according to the seventh aspect of the present invention may also have one or any possible combination of the following features (xviii) and (xix).

[0078] (xviii) The process according to the seventh aspect of the present invention may further comprise the steps of (b1) forming, after the step (b), a first tensile strain barrier layer made of one of InGaP, InGaAsP, and GaAsP, on the lower optical waveguide layer, and (c1) forming, after the step (c), a second tensile strain barrier layer made of one of InGaP, InGaAsP, and GaAsP, on the active layer.

[0079] (xix) The process according to the seventh aspect of the present invention may further comprise, after the step (f), the steps of (f1) forming a cap layer made of In_0.49Ga_0.51P of the first or second conductive type, and (f2) removing a stripe area of the cap layer. In addition, in the step (h), a remaining area of the cap layer is also removed.

[0080] (9) According to the eighth aspect of the present invention, there is provided a process for producing a semiconductor laser device, comprising the steps of: (a) forming a lower cladding layer of a first conductive type, on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer of an undoped type or the first conductive type, on the lower cladding layer; (c) forming an active layer on the lower optical waveguide layer; (d) forming a first upper optical waveguide layer of an undoped type or a second conductive type, on the active layer; (e) forming a first etching stop layer made of In_x9Ga_1−x9P of the second conductive type, on the first upper optical waveguide layer, where 0≦x9≦1; (f) forming a second etching stop layer made of In_x1Ga_1−x1As_1−y1P_y1of the second conductive type, on the first etching stop layer, where 0≦x1≦0.5, and 0≦y1≦0.8; (g) forming a current confinement layer made of In_x3(Al_z3Ga_1−z3)_1−x3P of the first conductive type, on the second etching stop layer, where 0<z3≦1, and x3=0.49±0.01; (h) removing a stripe area of the current confinement layer so as to form a first portion of a stripe groove for realizing a current injection window; (i) removing a stripe area of the second etching stop layer so as to form a second portion of the stripe groove; (j) forming a second upper optical waveguide layer of the second conductive type so that the stripe groove is covered with the second upper optical waveguide layer; (k) forming an upper cladding layer of the second conductive type, on the second upper optical waveguide layer; and (1) forming a contact layer made of GaAs of the second conductive type, on the upper cladding layer. In the process, a total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 μm, and the active layer is made of one of InGaAs, InGaAsP, and GaAsP.

[0081] That is, the semiconductor laser device according to the sixth aspect of the present invention can be produced by the process according to the eighth aspect of the present invention.

[0082] Preferably, the process according to the eighth aspect of the present invention may also have one or any possible combination of the aforementioned feature (xviii) and the following feature (xx).

[0083] (xx) The process according to the eighth aspect of the present invention may further comprise, after the step (g), the steps of (g1) forming a cap layer made of In_0.49Ga_0.51P of the first or second conductive type, and (g2) removing a stripe area of the cap layer. In addition, in the step (i), a remaining area of the cap layer is also removed.

[0084] (10) According to the ninth aspect of the present invention, there is provided a solid-state laser apparatus having as an exciting light source the semiconductor laser device according to the fifth aspect of the present invention.

[0085] Preferably, the solid-state laser apparatus according to the ninth aspect of the present invention may also have one or any possible combination of the following additional feature (xxi) and the aforementioned additional features (xi) to (xvii).

[0086] (xxi) The solid-state laser apparatus according to the ninth aspect of the present invention may further comprise a solid-state laser crystal which is excited with first laser light emitted from the excitation light source, and emits second laser light, and a wavelength conversion crystal which converts the second laser light into a second harmonic.

[0087] (11) According to the tenth aspect of the present invention, there is provided a solid-state laser apparatus having as an exciting light source the semiconductor laser device according to the sixth aspect of the present invention.

[0088] Preferably, the solid-state laser apparatus according to the tenth aspect of the present invention may also have one or any possible combination of the aforementioned additional features (xi) to (xvii), and (xxi).

[0089] (12) The fifth to tenth aspects of the present invention have the following advantages.

[0090] (a) Since the current confinement structure is formed, the strain caused by the junction-down type mounting can be reduced, and the fluctuation in the transverse modes during oscillation can be suppressed. Thus, the variation in the optical output of the solid-state laser apparatus can be suppressed. In addition, when the junction-down type mounting is employed, the heat dissipation characteristics of the semiconductor laser device are improved. Therefore, the amount of variation in the wavelength, which is caused by increase in a driving current, can be reduced. Thus, it is possible to realize a solid-state laser apparatus which is highly reliable for a long time.

[0091] (b) In the semiconductor laser devices according to the fifth and sixth aspects of the present invention, the active layers do not contain aluminum. Therefore, the semiconductor laser devices are free from deterioration caused by oxidation of aluminum, and the semiconductor laser devices are reliable even when the semiconductor laser devices operate with high output power.

[0092] (c) In the semiconductor laser devices according to the fifth and sixth aspects of the present invention, the current confinement layer is made of In_x3(Al_z3Ga_1−z3)_1−x3P. Therefore, when the second upper optical waveguide layer is made of InGaP or InGaAsP, the difference in the refractive indexes between the current confinement layer and the second upper cladding layer realizes a difference of about 1.5×10⁻³to 1×10⁻²in the equivalent refractive index between the portion of the active region under the stripe groove and the other portions of the active region under the current confinement layer. In particular, when the width of the oscillation region is narrow, e.g., 1 μm to 5 μm, and the difference in the equivalent refractive index is too great, the transverse modes become unstable. However, when the difference in the equivalent refractive index is about 1.5×10⁻³to 1×10⁻², the semiconductor laser devices can operate in a basic transverse mode even when the output power is high, and the instability of the transverse mode due to occurrence of higher modes can be prevented.

[0093] (d) Since the current confinement layer is arranged within the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced, and it is possible to operate the semiconductor laser devices with high output power.

[0094] (e) In order to prevent deterioration of a light-emitting end surface caused by high photon density in a high-power semiconductor laser device, it is effective to increase a thickness of an optical waveguide layer so as to reduce a peak photon density in an active layer. However, in conventional semiconductor laser devices having a current confinement structure and an index-guided structure, the thickness of the optical waveguide layer located between the active layer and the current confinement layer is limited since the distance between the active layer and the current confinement layer cannot be increased in order to realize a basic transverse mode by the effect of the index-guided structure. According to the fifth to tenth aspects of the present invention, the second upper optical waveguide layer is arranged over the current confinement layer, and the total thickness of the lower optical waveguide layer and the first and second upper optical waveguide layers is equal to or greater than 0.6 μm. Therefore, it is possible to substantially increase the thickness of the optical waveguide layer, and reduce the peak photon density in the active layer. In addition, it is possible to prevent the temperature rise at a light-emitting end surface caused by increase in non-emission currents, and therefore deterioration of the light-emitting end surface due to high photon density can be prevented, thus increasing reliability in a high power operation.

[0095] (f) In the sixth, eighth, and tenth aspects of the present invention, the first etching stop layer is made of InGaP, and the second etching stop layer made of InGaAsP is arranged over the first etching stop layer. Therefore, when a sulfuric acid etchant is used, only the InGaAsP second etching stop layer is etched, and the InGaP first etching stop layer is not etched. That is, it is possible to stop the etching accurately on the upper boundary of the first etching stop layer, and thus the index-guided structure and the stripe width can be accurately formed by etching.

[0096] (g) When the InGaP cap layer is formed on the In_x3(Al_z3Ga_1−z3P current confinement layer, it is possible to prevent formation of a natural oxidation film on the In_x3(Al_z3Ga_1−z3)_1−x3P current confinement layer, as well as metamorphic change in the In_x3(Al_z3Ga_1−z3)_1−x3P current confinement layer, which occurs when a resist layer is formed directly on the In_x3(Al_z3Ga_1−z3)_1−x3P current confinement layer.

[0097] (h) When each of the lower optical waveguide layer and the first and second upper optical waveguide layers is made of In_x2Ga_1−x2p or In_x2Ga_1−x2As_y2P_1−y2, the band gap difference between the active layer and the optical waveguide layers can be made greater than the band gap differences in the conventional semiconductor laser devices. Therefore, the leakage current can be prevented, carriers can be efficiently confined, and thus the threshold current can be lowered.

[0098] (i) When the tensile strain barrier layers made of InGaP, InGaAsP, or GaAsP are formed above and below the compressive strain quantum well active layer, respectively, various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.

[0099] (j) When each of the lower and upper cladding layers is made of one of AlGaAs, InGaAlP, and InGaAlPAs which lattice-match with the GaAs substrate, carriers and light can be effectively confined within the active layer, and the efficiency can be increased, since the band gaps of the cladding layers made of such materials are greater than the band gaps of the optical waveguide layers, and the refractive indexes of the cladding layers are smaller than the refractive indexes of the optical waveguide layers.

[0100] (k) Since the solid-state laser apparatuses according to the ninth and tenth aspects of the present invention use as excitation light sources the reliable, high power semiconductor laser devices according to the fifth and sixth aspects of the present invention, reliable, high power solid-state laser apparatuses are realized.

[0101] (1) In particular, when the semiconductor laser devices used in the solid-state laser apparatuses according to the ninth or tenth aspect of the present invention have an oscillation region (i.e., a stripe groove) with a width of 10 μm or broader, reliable, high power laser light can be obtained.

[0102] (m) In particular, when the solid-state laser apparatuses according to the ninth or tenth aspect of the present invention comprise a wavelength conversion crystal which converts the solid-state laser light into a second harmonic, reliable, high power second harmonic laser light can be obtained.

[0103] FIGS. 1A to 2D are cross-sectional views of representative stages in a process for producing a semiconductor laser device as the first embodiment of the present invention.

[0104] FIGS. 2A to 2D are cross-sectional views of representative stages in a process for producing a semiconductor laser device as the second embodiment of the present invention.

[0105] FIG. 3 is a cross-sectional view of a semiconductor laser device as the third embodiment of the present invention.

[0106] FIG. 4 is a cross-sectional view of a semiconductor laser device as the fourth embodiment of the present invention.

[0107] FIGS. 5A to 5D are cross-sectional views of representative stages in a process for producing a semiconductor laser device as the fifth embodiment of the present invention.

[0108] FIGS. 6A to 6D are cross-sectional views of representative stages in a process for producing a semiconductor laser device as the sixth embodiment of the present invention.

[0109] FIG. 7 is a cross-sectional view of a semiconductor laser device as the seventh embodiment of the present invention.

[0110] FIG. 8 is a cross-sectional view of a semiconductor laser device as the eighth embodiment of the present invention.

[0111] FIGS. 9A to 9D are cross-sectional views of representative stages in a process for producing a semiconductor laser device as the ninth embodiment of the present invention.

[0112] FIG. 10 is a perspective view of a conventional semiconductor laser device.

[0113] FIG. 11 is a diagram schematically illustrating a configuration of a system for measuring noise.

[0114] FIG. 12 is a graph indicating results of the noise measurement in the conventional semiconductor laser device and a semiconductor laser device according to the present invention.

[0115] FIGS. 13A to 13D are cross-sectional views of representative stages in a process for producing a semiconductor laser device as the tenth embodiment of the present invention.

[0116] FIGS. 14A to 14D are cross-sectional views of representative stages in a process for producing a semiconductor laser device as the eleventh embodiment of the present invention.

[0117] FIG. 15 is a diagram schematically illustrating the construction of a solid-state laser apparatus which uses as an excitation light source a semiconductor laser device according to the present invention.

[0118] FIG. 16 is a diagram schematically illustrating the construction of another solid-state laser apparatus which uses as an excitation light source a semiconductor laser device according to the present invention.

[0119] Embodiments of the present invention are explained in detail below with reference to drawings.

[0120] FIGS. 1A to 1D show cross sections of the representative stages in the process for producing a semiconductor laser device as the first embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device.

[0121] First, as illustrated in FIG. 1A, an n-type In_x8Ga_1−x8P lower cladding layer 2 (x8=0.49±0.01), an n-type or i-type (intrinsic) In_x2Ga_1−x2As_1−y2P_y2optical waveguide layer 3 (x2=(0.49±0.01)y2, 023 x2≦0.3), an In_x3Ga_1−x3As_1−y3P_y3compressive strain quantum well active layer 4 (0<x3≦0.4, 0≦y3≦0.1), a p-type or i-type (intrinsic) In_x2Ga_1−x2As_1−y2P_y2optical waveguide layer 5 (x2=(0.49±0.01)y2, 0≦x2≦0.3), a p-type In_x8Ga_1−x8P first upper cladding layer 6 (x8=0.49±0.01), an n-type or p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 7 (0≦x1≦0.3, 0≦y1≦0.3) having a thickness of, for example, 20 nm, an n-type In_x8Ga_1−x8P current confinement layer 8 having a thickness of, for example, 1 μm, and a GaAs cap layer 9 having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate 1 by organometallic vapor phase epitaxy. Then, a SiO₂film 10 is formed over the GaAs cap layer 9, and a stripe area of the SiO₂film 10 having a width of about 3 μm and extending in the <011> direction is removed by conventional lithography so that a stripe area of the GaAs cap layer 9 is exposed.

[0122] Next, as illustrated in FIG. 1B, the exposed stripe area of the GaAs cap layer 9 is etched with a sulfuric acid etchant by using the SiO₂film 10 as a mask. Then, a stripe area of the n-type In_x8Ga_1−x8P current confinement layer 8 is etched with a hydrochloric acid etchant until a stripe area of the n-type or p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 7 is exposed.

[0123] Thereafter, as illustrated in FIG. 1C, the remaining areas of the SiO₂film 10 are removed by etching with a fluoric acid etchant. Then, the remaining areas of the GaAs cap layer 9 and the exposed area of the n-type or p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 7 are removed by etching with a sulfuric acid etchant.

[0124] Finally, as illustrated in FIG. 1D, a p-type Al_z4Ga_1−z4As second upper cladding layer 11 (0.2≦z4≦0.5) and a p-type GaAs contact layer 12 are formed over the layered structure of FIG. 1C. Then, a p electrode 13 is formed on the p-type GaAs contact layer 12. In addition, the exposed surface of the substrate 1 is polished, and an n electrode 14 is formed on the polished surface of the substrate 1. Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device.

[0125] In the above construction, the p-type In_x8Ga_1−x8P first upper cladding layer 6 has such a thickness that oscillation in a fundamental transverse mode can be maintained even when output power becomes high. In addition, since a current confinement structure and a real refractive index structure are realized by the p-type Al_z4Ga_1−z4As second upper cladding layer 11 and the n-type In_x8Ga_1−x8P current confinement layer 8, it is possible to realize a difference of about 1.5×10⁻³to 1×10⁻²in the equivalent refractive index between the portions of the active region under the current confinement layer and the other portion of the active region under the stripe groove. Therefore, oscillation in a fundamental transverse mode can be maintained even when the output power becomes high, and thus a reliable semiconductor laser device can be realized.

[0126] The GaAs cap layer may be one of the n type, p type, and the undoped type. In addition, since a GaAs cap layer is formed on the InGaP current confinement layer, it is possible to prevent formation of a natural oxidation film on the InGaP current confinement layer, and metamorphic change in the InGaP current confinement layer, which occurs when a resist layer is formed directly on the InGaP current confinement layer. In addition, since the GaAs cap layer is removed before the second upper cladding layer is formed, it is possible to remove a residue left on the base layer on which the second upper cladding layer is formed, and prevent the occurrence of crystal defects.

[0127] FIGS. 2A to 2D show cross sections of the representative stages in the process for producing a semiconductor laser device as the second embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device.

[0128] The construction of the semiconductor laser device as the second embodiment is different from the first embodiment in that the GaAs cap layer 9 in the first embodiment is not formed. That is, a SiO₂film 10 is formed directly on the InGaP current confinement layer 8, as illustrated in FIG. 2A. Then, as illustrated in FIG. 2B, a stripe area of the In_x8Ga_1−x8P current confinement layer 8 is etched with a hydrochloric acid etchant by using the SiO₂film 10 as a mask until the In_x1Ga_1−x1As_1−y1P_y1etching stop layer 7 is exposed. Next, as illustrated in FIG. 2C, the remaining areas of the SiO₂film 10 are removed by etching with a fluoric acid etchant. Then, the exposed area of the In_x1Ga_1−x1As_1−y1P_y1etching stop layer 7 is removed by etching with a sulfuric acid etchant. Thereafter, through the same process as the first embodiment, the semiconductor laser device as the second embodiment is formed as illustrated in FIG. 2D.

[0129] FIG. 3 shows a cross section of a semiconductor laser device as the third embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. The construction of the semiconductor laser device as the third embodiment is different from the first embodiment in that an InGaP layer is formed before the second upper cladding layer is formed.

[0130] That is, after the SiO₂film 10 is removed by etching with a fluoric acid etchant, and a stripe area of the GaAs cap layer 9 and the exposed stripe area of the In_x1Ga_1−x1As_1−y1P_y1etching stop layer 7 are removed by etching with a sulfuric acid etchant in the same manner as the first embodiment, a p-type In_x8Ga_1−x8P layer 15 (x8=0.49±0.01) having a thickness of, for example, about 50 nm is formed as illustrated in FIG. 3. Then, the p-type Al_z4Ga_1−z4As second upper cladding layer 11 (0.20≦z4≦0.50) and the p-type GaAs contact layer 12 are formed on the p-type In_x8Ga_1−x8P layer 15. Thereafter, through the same process as the first embodiment, the semiconductor laser device as the third embodiment is formed into a chip.

[0131] Due to the provision of the p-type In_x8Ga_1−x8P layer 15, the control range of the Al composition in the Al_z4Ga_1−z4As second upper cladding layer can be increased. When the thickness of the cladding layer is about 100 nm, it is preferable that the thickness of the In_x8Ga_1−x8P layer 15 is 10 to 400 nm. In particular, the optimum thickness of the In_x8Ga_1−x8P layer is 250 to 300 nm.

[0132] FIG. 4 shows a cross section of a semiconductor laser device as the fourth embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. The construction of the semiconductor laser device as the fourth embodiment is different from the first embodiment in that an In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 16 (0≦x5≦0.3, 0<y5≦0.6) and an In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 17 are formed below and above the In_x3Ga_1−x3As_1−y3P_y3compressive strain quantum well active layer 4 (0<x3≦0.4, 0≦y3≦0.1), respectively. Due to the provision of the In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 16 (0≦x5≦0.3, 0<y5≦0.6) and the In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 17, various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.

[0133] FIGS. 5A to 5D show cross sections of the representative stages in the process for producing a semiconductor laser device as the fifth embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device.

[0134] First, as illustrated in FIG. 5A, an n-type In_x6Ga_1−x6As_1−y6P_y6lower cladding layer 82 (x6=(0.49±0.01)y6, 0.2<y6<1), an n-type or i-type In_x2Ga_1−x2As_1−y2P_y2optical waveguide layer 83 (x2=(0.49±0.01)y2, 0≦x2<x6), an In_x3Ga_1−x3As_1−y3P_y3compressive strain quantum well active layer 84 (0<x3≦0.4, 0≦y3≦0.1), a p-type or i-type In_x2Ga_1−x2As_1−y2P_y2optical waveguide layer 85, a p-type In_x6Ga_1−x6As_1−y6P_y6first upper cladding layer 86, a p-type In_x7Ga_1−x7P first etching stop layer 87 (0≦x7≦1), a p-type In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 88 (0≦x1≦0.3, 0≦y1≦0.3) having a thickness of, for example, 20 nm, an n-type In_x8Ga_1−x8P current confinement layer 89 (x8=0.49±0.01) having a thickness of, for example, 1 μm, and a GaAs cap layer 90 having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate 81 by organometallic vapor phase epitaxy. Then, a SiO₂film 91 is formed over the n-type GaAs cap layer 90, and a stripe area of the SiO₂film 91 having a width of about 3 μm and extending in the <011> direction is removed by conventional lithography.

[0135] Next, as illustrated in FIG. 5B, a stripe area of the n-type GaAs cap layer 90 is etched with a sulfuric acid etchant by using the SiO₂film 91 as a mask. Then, a stripe area of the n-type In_x8Ga_1−x8P current confinement layer 89 is etched with a hydrochloric acid etchant until a stripe area of the p-type In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 88 is exposed.

[0136] Thereafter, as illustrated in FIG. 5C, the remaining areas of the SiO₂film 91 are removed by etching with a fluoric acid etchant and the remaining areas of the n-type GaAs cap layer 90 and a stripe area of the p-type In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 88 are them removed by etching with a sulfuric acid etchant so that a stripe area of the p-type In_x7Ga_1−x7P first etching stop layer 87 is exposed.

[0137] Finally, as illustrated in FIG. 5D, a p-type Al_z4Ga_1−z4As second upper cladding layer 92 (0.20≦z4≦0.50) and a p-type GaAs contact layer 93 are formed over the layered structure of FIG. 6C. Then, a p electrode 94 is formed on the p-type GaAs contact layer 93. In addition, the exposed surface of the substrate 81 is polished, and an n electrode 95 is formed on the polished surface of the substrate 81. Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device.

[0138] In the construction of the fifth embodiment, the GaAs cap layer is the n type, p type, or undoped type.

[0139] In the construction of the fifth embodiment, the n-type GaAs cap layer may be dispensed with. FIGS. 6A to 6D show cross sections of the representative stages in the process for producing a semiconductor laser device as the sixth embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device.

[0140] The construction of the semiconductor laser device as the sixth embodiment is different from the fifth embodiment in that the GaAs cap layer 90 in the fifth embodiment is not formed. That is, the SiO₂film 91 is formed directly on the InGaP current confinement layer 89, as illustrated in FIG. 6A. Then, as illustrated in FIG. 6B, a stripe area of the In_x8Ga_1−x8P current confinement layer 89 is etched with a hydrochloric acid etchant by using the SiO₂film 91 as a mask until a stripe area of the In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 88 is exposed. Next, as illustrated in FIG. 6C, the remaining areas of the SiO₂film 91 are removed by etching with a fluoric acid etchant. Then, the exposed stripe area of the In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 88 is removed by etching with a sulfuric acid etchant. Thereafter, through the same process as the fifth embodiment, the semiconductor laser device as the sixth embodiment is formed as illustrated in FIG. 6D.

[0141] FIG. 7 shows a cross section of a semiconductor laser device as the seventh embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. The construction of the semiconductor laser device as the seventh embodiment is different from the fifth embodiment in that an InGaP layer is formed before the second upper cladding layer is formed.

[0142] That is, after the SiO₂film 91 is removed by etching with a fluoric acid etchant, and the GaAs cap layer 90 and the stripe area of the p-type In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 88 are removed by etching with a sulfuric acid etchant in the same manner as the fifth embodiment, a p-type In_x8Ga_1−x8P layer 96 (x8=0.49±0.01) having a thickness of, for example, about 50 nm is formed as illustrated in FIG. 7. Then, the p-type Al_z4Ga_1−z4As second upper cladding layer 92 (0.20≦z4≦0.50) and the p-type GaAs contact layer 93 are formed on the p-type In_x8Ga_1−x8P layer 96. Thereafter, through the same process as the fifth embodiment, the semiconductor laser device as the seventh embodiment is formed into a chip.

[0143] Similar to the third embodiment, when the thickness of the cladding layer is about 100 nm, it is preferable that the thickness of the In_x8Ga_1−x8P layer 96 is 10 to 400 nm. In particular, the optimum thickness of the In_x8Ga_1−x8P layer is 250 to 300 nm.

[0144] FIG. 8 shows a cross section of a semiconductor laser device as the eighth embodiment, where the cross section is perpendicular to the direction of light emitted from the semiconductor laser device. The construction of the semiconductor laser device as the eighth embodiment is different from the fifth embodiment in that an In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 97 (0≦x5≦0.3, 0<y5≦0.6) and an In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 98 are formed below and above the In_x3Ga_1−x3As_1−y3P_y3compressive strain quantum well active layer 84 (0<x3≦0.4, 0≦y3≦0.1), respectively. Due to the provision of the In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 97 (0≦x5≦0.3, 0<y5≦0.6) and the In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 98, various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.

[0145] (i) In the semiconductor laser devices as the first to fourth embodiments, the lower cladding layer may be made of Al_z5Ga_1−z5As, where 0.25≦z5≦0.7, and the optical waveguide layers may be made of Al_z2Ga_1−z2As, where 0≦z2≦0.2.

[0146] (ii) In the semiconductor laser devices as the fifth to eighth embodiments, the lower cladding layer and the first upper cladding layer may be made of Al_z5Ga_1−z5As, where 0.25≦z5≦0.7.

[0147] (iii) Due to the In_x3Ga_1−x3As_1−y3P_y3compressive strain quantum well active layers (0<x3≦0.4, 0≦y3≦0.1), the oscillation wavelengths of the semiconductor laser devices as the first to eighth embodiments can be controlled in the range of 900 to 1,200 nm.

[0148] (iv) The constructions of the first to eighth embodiments can be used not only in an index-guided structure semiconductor laser devices, but also in other semiconductor laser devices having a diffraction lattice, as well as in optical integrated circuits.

[0149] (v) Although n-type GaAs substrates are used in the constructions of the first to eighth embodiments, instead, p-type GaAs substrates may be used. When the GaAs substrate is a p-type, the conductivity types of all of the other layers in the constructions of the first to eighth embodiments should be inverted.

[0150] (vi) The constructions of the first to eighth embodiments can be used not only in semiconductor laser devices which oscillate in a fundamental transverse mode, but also in index-guided broad-stripe semiconductor laser devices which are designed for realizing high output power, have a stripe width of 3 μm or greater, and oscillate in multiple modes.

[0151] (vii) Although the constructions of the first to eighth embodiments have a so-called single-quantum-well separate-confinement heterostructure (SQW−SCH) which includes a single quantum well and an optical waveguide made of a material having a fixed composition, instead, a multiple quantum well structure made of a plurality of quantum wells may be used.

[0152] (viii) Each layer in the constructions of the first to eighth embodiments may be formed by molecular beam epitaxy using solid or gas raw material.

[0153] (ix) In addition, all of the contents of the Japanese Patent Application, No. 2000-031733 are incorporated into this specification by reference.

[0154] FIGS. 9A to 9D show cross sections of the representative stages in the process for producing a semiconductor laser device as the ninth embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device.

[0155] First, as illustrated in FIG. 9A, an n-type Ga_0.37Al_0.63As lower cladding layer 212, an n-type or i-type (intrinsic) In_0.49Ga_0.51P lower optical waveguide layer 213 having a thickness of, for example, 400 nm, an In_0.12Ga_0.88As_0.75P_0.25quantum well active layer 214 having a thickness of, for example, 8 nm, a p-type or i-type (intrinsic) In_0.49Ga_0.5P first upper optical waveguide layer 215 having a thickness of, for example, 200 nm, a p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 216 (0≦x1≦0.5, 0≦y1≦0.8) having a thickness of, for example, 20 nm, an n-type In_0.49(Al_0.1Ga_0.9)_0.5P current confinement layer 217, an n-type In_0.49Ga_0.51P cap layer 218 having a thickness of, for example, 20 nm, and an n-type GaAs cap layer 219 having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate 211 by organometallic vapor phase epitaxy. Then, a SiO₂film 220 is formed over the In_0.49Ga_0.51P cap layer 219, and a stripe area having a width of about 50 μm of the SiO₂film 220 is removed by conventional lithography so that a stripe area of the n-type GaAs cap layer 219 is exposed.

[0156] Next, as illustrated in FIG. 9B, the exposed stripe area of the n-type GaAs cap layer 219 is etched with a sulfuric acid etchant by using the SiO₂film 220 as a mask. Then, an exposed stripe area of the n-type In_0.49Ga_0.51P cap layer 218 and a corresponding stripe area of the n-type In_0.49(Al_0.1Ga0.9)_0.51P current confinement layer 217 are etched with a hydrochloric acid etchant until a stripe area of the p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 216 is exposed.

[0157] Thereafter, as illustrated in FIG. 9C, the remaining areas of the SiO₂film 220 are removed by etching with a fluoric acid etchant. Then, the remaining areas of the n-type GaAs cap layer 219 and the exposed stripe area of the p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 216 are removed by etching with a sulfuric acid etchant.

[0158] Finally, as illustrated in FIG. 9D, a p-type In_0.49Ga_0.51P second upper optical waveguide layer 221 having a thickness of, for example, 200 nm, a p-type Ga_0.37Al_0.63As upper cladding layer 222 having a thickness of, for example, 1 μm, and a p-type GaAs contact layer 223 having a thickness of, for example, 3 μm are formed over the layered structure of FIG. 9C. Then, a p electrode 224 is formed on the p-type GaAs contact layer 223. In addition, the exposed surface of the substrate 211 is polished, and an n electrode 225 is formed on the polished surface of the substrate 211. Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating 226 and a low reflectance coating 227 are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device.

[0159] Finally, the p electrode side of the laser chip is bonded to a heat sink or the like with conductive indium solder. Then, the semiconductor laser device is bonded to a heat sink or the like at a surface of the semiconductor laser device with conductive indium solder, where the surface is located nearer to the active layer than the opposite surface of the semiconductor laser device. That is, the semiconductor laser device is bonded to the heat sink or the like so as to form the so-called junction-down configuration.

[0160] Noise generated in the semiconductor laser device as the ninth embodiment of the present invention is compared with noise generated in a conventional semiconductor laser device.

[0161] FIG. 10 is a perspective view of a conventional semiconductor laser device. The semiconductor laser device of FIG. 10 is formed as follows.

[0162] First, an n-type Ga_0.37Al_0.63As lower cladding layer 232, an n-type In_0.49Ga_0.51P lower optical waveguide layer 233 having a thickness of, for example, 400 nm, an In_0.12Ga_0.88AS_0.75P_0.25quantum well active layer 234 having a thickness of, for example, 8 nm, a p-type or i-type (intrinsic) In_0.49Ga_0.51P upper optical waveguide layer 235 having a thickness of, for example, 400 nm, a p-type Ga_0.37Al_0.63As cladding layer 236, and a p-type GaAs contact layer 237 are formed on an n-type GaAs substrate 231 by organometallic vapor phase epitaxy. Then, a first insulation film (not shown) is formed on p-type GaAs contact layer 237, and parallel stripe areas of the first insulation film, each having a width of about 50 μm, are removed by conventional lithography. Next, the parallel stripe areas of the above layered structure are etched to the depth of the upper surface of the p-type or i-type In_0.49Ga_0.51P upper optical waveguide layer 235 by wet etching using the remaining areas of the first insulation film as a mask so as to form a ridge stripe structure. When a solution of sulfuric acid and hydrogen peroxide is used as an etchant, the etching automatically stops at the upper boundary of the p-type or i-type In_0.49Ga_0.51P upper optical waveguide layer 235. Thereafter, the remaining areas of the first insulation film are removed, and then a second insulation film 239 is formed over the ridge stripe structure. Next, a stripe portion of the second insulation film 239 on the top of the ridge stripe is removed by etching by conventional lithography. Then, a p electrode 240 is formed over the above structure. In addition, the exposed surface of the substrate 231 is polished, and an n electrode 241 is formed on the polished surface of the substrate 231. Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating 243 and a low reflectance coating 244 are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device. Finally, the above semiconductor laser device is bonded to a heat sink or the like at a surface of the semiconductor laser device with indium solder, where the surface is located nearer to the active layer than the opposite surface of the semiconductor laser device. That is, the semiconductor laser device is bonded to the heat sink or the like so as to form the so-called junction-down configuration.

[0163] FIG. 11 is a diagram schematically illustrating a configuration of a system for measuring noise. In the system of FIG. 11, a portion of laser light emitted from a semiconductor laser device 251, which is under temperature control, is received by a light receiving element 252, and noise (stability) of the laser light is measured by a noise measurement device 253. In the measurement, the composition of the etching stop layer in the semiconductor laser devices as the ninth embodiment of the present invention is adjusted as x1=0.34 and y1=0.7, and the resonator lengths of the conventional semiconductor laser devices and the semiconductor laser devices as the ninth embodiment are 0.9 mm. The measurement is made in the optical output range of 100 to 500 mW.

[0164] The result of the noise measurement is indicated as cumulative distributions in FIG. 12. As illustrated in FIG. 12, when the noise intensity of 1% or less is required, the yield rate of the conventional semiconductor laser devices is 75%, and the yield rate of the semiconductor laser devices as the ninth embodiment of the present invention is 95%. This result indicates that noise reduction in the semiconductor laser device according to the original image is achieved by the junction-down mounting.

[0165] FIGS. 13A to 13D show cross sections of the representative stages in the process for producing a semiconductor laser device as the tenth embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device.

[0166] First, as illustrated in FIG. 13A, an n-type In_0.49(Al_z1Ga_1−z1)_0.51P lower cladding layer 262, an n-type In_0.49Ga_0.51P lower optical waveguide layer 263, an In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 264 (0≦x5≦0.3, x5<0.49y5), an In_x3Ga_1−x3As_1−y3P_y3compressive strain quantum well active layer 265 (0<x3≦0.4, x3>0.49y3), an In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 266 (0≦x5≦0.3, x5<0.49y5), a p-type In_0.49Ga_0.51P first upper optical waveguide layer 267, a p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 268 (0≦x1≦0.5, 0≦y1≦0.8) having a thickness of, for example, 20 nm, an n-type In_0.49(Al_z3Ga_1−z3Ga_1−z3)_0.51P current confinement layer 269 (0<z1≦z3≦1) having a thickness of, for example, 1 μm, an n-type In_0.49Ga_0.51P cap layer 270 having a thickness of, for example, 10 nm, and an n-type GaAs cap layer 271 having a thickness of, for example, 10 nm are formed on an n-type GaAs substrate 261 by organometallic vapor phase epitaxy. Then, a SiO₂film 272 is formed over the n-type GaAs cap layer 271, and a stripe area of the SiO₂film 271 having a width of about 50 μm and extending in the <011> direction is removed by conventional lithography so that a stripe area of the n-type GaAs cap layer 271 is exposed.

[0167] As described before, the strain Δa of the active layer is expressed as Δa=(ca−cs)/cs, where cs and ca are the lattice constants of the GaAs substrate and the active layer, respectively, and the strain Δb of the tensile strain barrier layers is expressed as Δb=(cb−cs)/cs, where cb is the lattice constant of the tensile strain barrier layers. That is, the aforementioned sum of the first product and the third product is expressed as Δa·da+Δb·db, where da is the thickness of the active layer, db is the total thickness of the first and second tensile strain barrier layers, the first product is the product of the strain and the thickness of the active layer, and the third product is the product of the strain of the first and second tensile strain barrier layers and the total thickness of the first and second tensile strain barrier layers. Thus, the semiconductor laser device of the tenth embodiment satisfies the requirement, −0.25 nm≦Δa·da+Δb·db≦0.25 nm.

[0168] Next, as illustrated in FIG. 13B, the exposed stripe area of the n-type GaAs cap layer 271 is etched with a sulfuric acid etchant by using the SiO₂film 272 as a mask. Then, an exposed stripe area of the n-type In_0.49Ga_0.51P cap layer 270 and a corresponding stripe area of the n-type In_0.49(Al_z3Ga_1−z3)_0.51P current confinement layer 269 are etched with a hydrochloric acid etchant until a stripe area of the p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 268 is exposed.

[0169] Thereafter, as illustrated in FIG. 13C, the remaining areas of the SiO₂film 272 are removed by etching with a fluoric acid etchant. Then, the remaining areas of the n-type GaAs cap layer 271 and the exposed area of the p-type In_x1Ga_1−x1As_1−y1P_y1etching stop layer 268 are removed by etching with a sulfuric acid etchant so that a stripe area of the p-type In_0.49Ga_0.51P first upper optical waveguide layer 267 is exposed.

[0170] Finally, as illustrated in FIG. 13D, a p-type In_0.49Ga_0.51P second upper optical waveguide layer 273, a p-type In_0.49(Al_z1Ga_1−z1)_0.51P upper cladding layer 274, and a p-type GaAs contact layer 275 are formed over the layered structure of FIG. 13C. Then, a p electrode 276 is formed on the p-type GaAs contact layer 275. In addition, the exposed surface of the substrate 261 is polished, and an n electrode 277 is formed on the polished surface of the substrate 261. Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device.

[0171] In the tenth embodiment, the lower cladding layer and the first upper cladding layer may be made of Al_x2Ga_1−z2As, where 0.55≦z2≦0.8.

[0172] The In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layers (0≦x5≦0.3, x5<0.49y5) may be dispensed with. In this case, the active layer may be an In_x7Ga_1−x7As_1−y7P_y7tensile strain quantum well active layer, where 0≦x7≦0.4, and x7<0.49y7.

[0173] When the stripe width of the semiconductor laser device as the tenth embodiment of the present invention is 1 to 5 μm, the semiconductor laser device can oscillate in a basic transverse mode.

[0174] FIGS. 14A to 14D show cross sections of the representative stages in the process for producing a semiconductor laser device as the eleventh embodiment, where the cross sections are perpendicular to the direction of light emitted from the semiconductor laser device.

[0175] First, as illustrated in FIG. 14A, an n-type In_0.49(Al_z1Ga_1−z1)0.51P lower cladding layer 282 (0≦z1≦1), an n-type or i-type In_x2Ga_1−x2As_1−y2P_y2lower optical waveguide layer 283 (x2=(0.49±0.01)y2, 0≦x2≦0.3), an In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 284 (0≦x5≦0.3, x5<0.49y5), an In_x3Ga_1−x3As_1−y3P_y3compressive strain quantum well active layer 285 (0<x3≦0.4, x3>0.49y3), an In_x5Ga_1−x5As_1−y5P_y5tensile strain barrier layer 286 (0≦x5≦0.3, x5<0.49y5), a p-type or i-type In_x2Ga_1−x2As_1−y2P_y2first upper optical waveguide layer 287, a p-type In_x9Ga_1−x9P first etching stop layer 288 (0≦x9≦1) having a thickness of, for example, 10 nm, a p-type In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 289 (0≦x1≦0.5, 0≦y1≦0.8) having a thickness of, for example, 10 μm, an n-type In_0.49(Al_z2Ga_1−z2)_0.51P current confinement layer 290 (0<z2≦0.1) having a thickness of, for example, 1 μm, and an n-type GaAs cap layer 291 are formed on an n-type GaAs substrate 281 by organometallic vapor phase epitaxy. Then, a SiO₂film 292 is formed over the n-type GaAs cap layer 291, and a stripe area of the SiO₂film 292 having a width of about 3 μm and extending in the <011> direction is removed by conventional lithography so that a stripe area of the n-type GaAs cap layer 291 is exposed.

[0176] Next, as illustrated in FIG. 14B, the exposed stripe area of the n-type GaAs cap layer 291 is etched with a sulfuric acid etchant by using the SiO₂film 292 as a mask. Then, a stripe area of the n-type In_0.49(Al_z2Ga_1−z2)_0.51P current confinement layer 290 is etched with a hydrochloric acid etchant until a stripe area of the p-type In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 289 is exposed.

[0177] Thereafter, as illustrated in FIG. 14C, the remaining areas of the SiO₂film 292 are removed by etching with a fluoric acid etchant. Then, the remaining areas of the n-type GaAs cap layer 291 and the exposed area of the p-type In_x1Ga_1−x1As_1−y1P_y1second etching stop layer 289 are removed by etching with a sulfuric acid etchant so that a stripe area of the p-type In_x9Ga_1−x9P first etching stop layer 288 is exposed.

[0178] Finally, as illustrated in FIG. 14D, a p-type In_x2Ga_1−x2As_1−y2P_y2second upper optical waveguide layer 293, a p-type In_0.49(Al_z1Ga_1−z1)_0.51P upper cladding layer 294, and a p-type GaAs contact layer 295 are formed over the layered structure of FIG. 14C. Then, a p electrode 296 is formed on the p-type GaAs contact layer 295. In addition, the exposed surface of the substrate 281 is polished, and an n electrode 297 is formed on the polished surface of the substrate 281. Next, both end surfaces of the layered construction are cleaved, and a high reflectance coating and a low reflectance coating are provided on the respective end surfaces so as to form a resonator. Then, the above construction is formed into a chip of a semiconductor laser device.

[0179] Each of the lower and upper cladding layers of the semiconductor laser device as the eleventh embodiment of the present invention may be made of an Al_z2Ga_1−z2As material (0.3≦z2≦0.8) which lattice-matches with the GaAs substrate, or an In_x8Ga_1−x8As_1−y8P_y8material (x8=0.49y8) which has a band gap greater than the band gaps of the lower, first upper, and second upper optical waveguide layers, or an In_x(Al_zGa_1−z)_1−xAs_1−yP_ymaterial, where x=(0.49±0.01)y, and z and y are determined so that the band gap of each of the lower and upper cladding layers is greater than the band gaps of the lower, first upper, and second upper optical waveguide layers.

[0180] When the stripe width of the semiconductor laser device as the eleventh embodiment of the present invention is 1 to 5 μm, the semiconductor laser device can oscillate in a basic transverse mode.

[0181] When the active layer is made of a compressive strain type or tensile strain type In_x3Ga_1−x3As_1−y3P_y3material, or an In_x3Ga_1−x3As_1−y3P_y3material which lattice-matches with the GaAs substrate, the oscillation wavelength of the semiconductor laser device as the eleventh embodiment can be controlled in the range of 700 to 1,200 nm.

[0182] In the solid-state laser apparatuses as the twelfth and thirteenth embodiments of the present invention, the semiconductor laser devices as the ninth to eleventh embodiments of the present invention can be used as excitation light sources which excite solid-state laser crystals.

[0183] FIG. 15 is a diagram schematically illustrating the construction of a solid-state laser apparatus which uses as an excitation light source a semiconductor laser device according to the present invention.

[0184] The solid-state laser apparatus of FIG. 15 comprises a high-power semiconductor laser device 101 which has a broad oscillation region, and emits excitation laser light, a lens 102 which collects the excitation laser light, a solid-state laser crystal 103 which is excited by the collected excitation laser light, and emits laser light, and an output mirror 104 which is a concave mirror arranged on the light exiting side of the solid-state laser crystal 103. The light exiting end surface of the semiconductor laser device 101 has a reflectance of 15 to 50% in order to reflect light returned from the solid-state laser crystal 103. The solid-state laser crystal 103 has a coating 105 at an end surface on the side of the semiconductor laser device 101, where the coating 105 has a high reflectance at the wavelength of the laser light emitted from the solid-state laser crystal 103, and a low reflectance at the wavelength of the excitation laser light from the semiconductor laser device 101.

[0185] The output mirror 104 and the coating 105 realize a resonator of the solid-state laser apparatus. The excitation laser light emitted from the semiconductor laser device 101 has a wavelength of 809 nm, and is collected by the lens 102 into the solid-state laser crystal 103. Only at least one component of the collected excitation laser light which can be coupled to oscillation modes of the resonator of the solid-state laser apparatus contributes to excitation of the solid-state laser crystal 103. Laser light having an oscillation wavelength of 946 nm is emitted from the solid-state laser crystal 103 through the output mirror 104. The semiconductor laser device 101 and the solid-state laser crystal 103 are temperature controlled using a Peltier element (not shown). A portion of the laser light which exits through the output mirror 104 branches off at a beam splitter 106 to a light receiving element 107 for APC (Automatic Power Control). That is, the intensity of the branched portion of the laser light is fed back to the semiconductor laser device 101 in order to maintain the intensity of the laser light output from the solid-state laser crystal 103 constant.

[0186] Since solid-state laser apparatus as the twelfth embodiment of the present invention uses the semiconductor laser device 101 according to the present invention, which generates low noise, and does not cause wavelength change, it is possible to obtain stable laser light without intensity variation from the solid-state laser apparatus of FIG. 15.

[0187] FIG. 16 is a diagram schematically illustrating the construction of another solid-state laser apparatus using as an excitation light source a semiconductor laser device according to the present invention. The solid-state laser apparatus of FIG. 16 generates a second harmonic of laser light emitted from a solid-state laser crystal.

[0188] The solid-state laser apparatus of FIG. 16 comprises a high-power semiconductor laser device 111 which has a broad oscillation region, and emits excitation laser light, a lens 112 which collects the excitation laser light, a solid-state laser crystal 113 which is excited by the collected excitation laser light, and emits laser light, and an output mirror 114 which is a concave mirror arranged on the light exiting side of the solid-state laser crystal 113. The light exiting end surface of the semiconductor laser device 111 has a reflectance of 15 to 50% in order to reflect light returned from the solid-state laser crystal 113. The solid-state laser crystal 113 has a coating 116 at an end surface on the side of the semiconductor laser device 111, where the coating 116 has a high reflectance at the wavelength of the laser light emitted from the solid-state laser crystal 113, and a low reflectance at the wavelength of the excitation laser light from the semiconductor laser device 111.

[0189] The output mirror 114 and the coating 116 realize a resonator of the solid-state laser apparatus. In the resonator, a KNbO₃nonlinear crystal 115 is arranged in order to convert the laser light emitted from the solid-state laser crystal 113 into a second harmonic, which has a wavelength one-half the wavelength of the laser light emitted from the solid-state laser crystal 113. The output mirror 114 has a high reflectance at the wavelength of the laser light emitted from the solid-state laser crystal 113, and passes the second harmonic generated by the nonlinear crystal 115. The solid-state laser crystal 113 may be Nd:YVO₄or the like, and the KNbO₃nonlinear crystal 115 may be replaced with a KTP crystal or the like. The semiconductor laser device 111, the solid-state laser crystal 113, and the KNbO₃nonlinear crystal 115 are temperature controlled using a Peltier element (not shown). A portion of the laser light which exits through the output mirror 114 branches off at a beam splitter 117 to a light receiving element 118 for APC (Automatic Power Control). That is, the intensity of the branched portion of the laser light is fed back to the semiconductor laser device 111 in order to maintain the intensity of the laser light output from the solid-state laser crystal 113 constant.

[0190] The excitation laser light emitted from the semiconductor laser device 111 has a wavelength of 809 nm, and is collected by the lens 112 into the solid-state laser crystal 113. Only at least one component of the collected excitation laser light which can be coupled to oscillation modes of the resonator of the solid-state laser apparatus contributes to excitation of the solid-state laser crystal 113. Laser light having an oscillation wavelength of 946 nm is emitted from the solid-state laser crystal 113 through the output mirror 114, and converted into a second harmonic (blue laser light) having a wavelength of 473 nm by the nonlinear crystal 115. The second harmonic exits through the output mirror 114.

[0191] For the same reason as the twelfth embodiment, it is also possible to obtain stable laser light without intensity variation from the solid-state laser apparatus of FIG. 16.

[0192] (i) Each layer in the semiconductor laser devices as the ninth to eleventh embodiments may be formed by molecular beam epitaxy using solid or gas raw material.

[0193] (ii) Although n-type GaAs substrates are used in the constructions of the ninth to eleventh embodiments, instead, p-type GaAs substrates may be used. When the GaAs substrate is a p-type, the conductivity types of all of the other layers in the constructions of the ninth to eleventh embodiments should be inverted.

[0194] (iii) In addition, all of the contents of the Japanese patent application, Nos. 11(1999)-329866 and 2000-031733 are incorporated into this specification by reference.