1. Field of the Invention
The present invention relates to red and infrared semiconductor laser devices employed as light sources for the pickup units of optical disc devices and as light sources necessary for other electronic devices, information processing devices, etc.
2. Description of Related Art
DVD devices, in which recording and reproduction is carried out using high-capacity digital video discs (DVDs) capable of high density recording, utilize AlGaInP semiconductor lasers with an emission wavelength of 650 nm as laser light sources for recording and reading. For this reason, it has been impossible for the optical pickup units of conventional DVD devices to reproduce compact discs (CD) and mini-discs (MD) reproduced using AlGaAs semiconductor lasers with an emission wavelength of 780 nm.
Consequently, to solve this problem, optical pickup units have been adopted that have laser chips incorporated into separate packages and comprise an AlGaInP semiconductor laser with an emission wavelength of 650 nm and an AlGaAs semiconductor laser with an emission wavelength of 780 nm. However, the problem with such optical pickup units is their increased size due to the installation of the two packages including the AlGaInP semiconductor laser and AlGaAs semiconductor laser, and as a result, the increased size of the DVD devices. Accordingly, in order to solve this problem, JP H11-186651A discloses an integrated semiconductor light-emitting device, which has light-emitting element structures formed of semiconductor layers grown on a common substrate and includes various kinds of semiconductor light-emitting elements with different emission wavelengths.
An exemplary conventional integrated semiconductor light-emitting device is illustrated in FIG. 7. In this integrated semiconductor laser device, an AlGaAs semiconductor laser LD1 with an emission wavelength in the 700-nm band (e.g. 780 nm) and an AlGaInP semiconductor laser LD2 with an emission wavelength in the 600-nm band (e.g. 650 nm) are integrated on a common n-type GaAs substrate 40 in a mutually separated state. A substrate that has, for instance, a (100) crystal plane orientation, or a substrate whose major surface is a surface tilted, for instance, by 5° to 15° from the (100) plane, is used as the n-type GaAs substrate 40.
The AlGaAs semiconductor laser LD1 is made up of an n-type GaAs buffer layer 41, an n-type AlGaAs cladding layer 42, an active layer 43 having a single quantum well (SQW) structure or a multiple quantum well (MQW) structure, a p-type AlGaAs cladding layer 44, and a p-type GaAs capping layer 45, all of them superposed on the substrate 40. The upper portion of the p-type AlGaAs cladding layer 44 and p-type GaAs capping layer 45 have a stripe-like shape extending in one direction. An n-type GaAs current confinement layer 46 is provided on both sides of this stripe-like portion, thereby forming a current confinement structure. A p-electrode 47 is provided on top of the stripe-shaped p-type GaAs capping layer 45 and n-type GaAs current confinement layer 46 in ohmic contact with the p-type GaAs capping layer 45. The p-electrode 47 is, for instance, a Ti/Pt/Au electrode.
The AlGaInP semiconductor laser LD2 is made up of an n-type GaAs buffer layer 51, an n-type AlGaInP cladding layer 52, an active layer 53 having an SQW or MQW structure, a p-type AlGaInP cladding layer 54, a p-type GaInP intermediate layer 55, and a p-type GaAs capping layer 56, all of them superposed on the substrate 40. The upper portion of the p-type AlGaInP cladding layer 54, p-type GaInP intermediate layer 55, and p-type GaAs capping layer 56 have a stripe-like shape extending in one direction. An n-type GaAs current confinement layer 57 is provided on both sides of this stripe-like portion, thereby forming a current confinement structure. A p-electrode 58 is provided on top of the stripe-shaped p-type GaAs capping layer 56 and n-type GaAs current confinement layer 57 in ohmic contact with the p-type GaAs capping layer 56. The p-electrode 58 is, for instance, a Ti/Pt/Au electrode.
An n-electrode 59 is provided on the back side of the n-type GaAs substrate 40 in ohmic contact with the n-type GaAs substrate 40. This n-electrode 59 is, for instance, an AuGe/Ni electrode or an In electrode.
The p-electrode 47 of the AlGaAs semiconductor laser LD1 and p-electrode 58 of the AlGaInP semiconductor laser LD2 are soldered to heat sinks H1, H2 provided on a package base 60 in a mutually electrically isolated state.
In this conventional integrated semiconductor laser device constructed as described above, the AlGaAs semiconductor laser LD1 can be driven by passing electric current between the p-electrode 47 and n-electrode 59, and the AlGaInP semiconductor laser LD2 can be driven by passing electric current between the p-electrode 58 and n-electrode 59. In addition, laser light of the 700-nm wavelength band (e.g. 780 nm) can be extracted by driving the AlGaAs semiconductor laser LD1 and laser light of the 600-nm wavelength band (e.g. 650 nm) can be extracted by driving the AlGaInP semiconductor laser LD2. The selection between driving the AlGaAs semiconductor laser LD1 or driving the AlGaInP semiconductor laser LD2 can be made by switching an external switch.
As noted above, this conventional integrated semiconductor laser device has the AlGaAs semiconductor laser LD1 with an emission wavelength in the 700-nm band and the AlGAInP semiconductor laser LD2 with an emission wavelength in the 600-nm band, and, as a result, is capable of producing laser light for DVDs and laser light for CDs or MDs independently of each other. Therefore, installing this integrated semiconductor laser device as a laser light source in the optical pickup units of DVD devices allows for reproduction or recording of DVDs, CDs, and MDs. Moreover, this integrated semiconductor laser device has only one package because the AlGaAs semiconductor laser LD1 and AlGaInP semiconductor laser LD2 have laser structures formed of semiconductor layers grown up on the common n-type GaAs substrate 40. This makes it possible to make the optical pickup smaller and, therefore, permits reduction in the size of DVD devices.
Moreover, rewriting an optical disk at high speed requires the radiant output of the semiconductor laser to be as high as possible. For instance, rewriting a DVD optical disk at a high speed of 4× or more requires a high power of 100 mW or more in terms of the radiant output. In order to obtain such a high power, it is necessary to prevent COD (Catastrophic Optical Damage), during which the facet of the semiconductor laser is melted and destroyed by its own radiant output during high power operation. Decreasing the radiant density at the cavity facet in order to suppress heat generation is effective in preventing COD. For this purpose, it is effective to use a dielectric substance, such as SiO2, Al2O3, or amorphous Si, etc., to coat the front facet of the semiconductor laser, which is used for the extraction of laser light, in order to lower the reflectivity of the front facet.
In general, when the facet is not coated, the reflectivity of the cavity facet in a semiconductor laser made up of an AlGaInP material or AlGaAs material is approximately 30%. In such a case, approximately 30% of the laser light is reflected by the cavity facet and fed back into the resonator and approximately 70% of the light is extracted from the front facet. In contrast, when it is coated with a dielectric film in such a manner that the reflectivity of the front facet is set, for instance, to 10%, 10% of the laser light is reflected by the cavity facet and fed back into the resonator, and 90% of the light is extracted from the front facet. In other words, if the reflectivity of the front facet is reduced while maintaining the same radiant output from the front facet, the radiant density at the cavity facet can be decreased as well. Therefore, decreasing the reflectivity of the front facet leads to an increase in the COD level and is an effective measure for obtaining a high-power laser.
In addition, if the reflectivity of the rear facet, i.e. the facet on the side opposite the cavity facet used for the extraction of laser light, is set to a high level, then the efficiency of light extraction from the front facet of the semiconductor laser can be further improved. Accordingly, facet coating conditions aimed at mining the reflectivity of the front facet and, conversely, at increasing the reflectivity of the rear facet, are employed widely in high-power semiconductor lasers. For the above-mentioned reasons, in order to enable high power operation in a two-wavelength laser integrating semiconductor lasers emitting in the red and infrared region on a common substrate, the facets of the laser cavities are coated with dielectric films capable of simultaneously obtaining both low reflectivity and high reflectivity for red and infrared light.
However, even if the facets are coated as described above, the bandgap energy of the active layer at the laser cavity facets is affected by the surface level and becomes smaller than the bandgap energy of the active layer inside the resonator. Moreover, when current injection is performed in a semiconductor laser, the bandgap energy of the active layer is reduced under the influence of joule heating or non-radiative recombination in the active layer. In particular, because the reflectivity of the front facet in a high power laser is reduced as described above, the radiant density of the active layer in the vicinity of the front facet reaches the highest level in the resonator, making it susceptible to temperature increase. For this reason, the bandgap energy of the active layer in the vicinity of the front facet is further diminished, laser light absorption losses are increased, and more heat is generated. As a result, COD tends to occur at the facet.
To prevent such facet damage, it is known to disorder the quantum well active layer in the vicinity of laser facets by impurity diffusion in order to form facet window structures and increase the bandgap energy of the active layer in the vicinity of the facets in advance. As a result, even if the bandgap of the active layer in the vicinity of the facet is diminished by heat generation, it becomes possible to maintain it in a state that is practically transparent to laser radiation and increase the radiant output level at which COD takes place.
Moreover, the demand for light sources used for optical disc systems capable of high-speed writing, such as CD-Rs intended for 48× recording and DVDs intended for 16× recording, which perform not only reproduction functions, but also recording functions as well, will be increasing in the future. In this case, lasers used as such light sources will have to be capable of high-power operation at levels of at least 250 mW even during high-temperature operation at 80° C. or higher.
As described above, when a semiconductor laser is operated at a high power level, the cavity facet on the side used for the extraction of laser light (front facet) and cavity facet on the opposite side (rear facet) are coated with dielectric films having, respectively, a low reflectivity of not more than 10% (AR, Anti-Reflection) and a high reflectivity of not less than 85% (HR, High-Reflection). Such low reflectivity and high reflectivity coatings bring about improvement in the external differential quantum efficiency (slope efficiency) of the current vs. radiant output characteristic and provide for a high radiant output using a small amount of injection current. In addition, conducting the impurity diffusion-based disordering of the quantum well active layer in the vicinity of the facet prevents COD, which causes the laser facet to be melted and destroyed by radiant output of laser light itself.
When impurity diffusion is boosted, such as by increasing the duration of impurity diffusion or diffusing impurities in high concentration, COD is rendered less likely even at higher radiant output levels because the disordering of the quantum well active layer in the window region becomes more pronounced, the bandgap energy is increased further, and the region becomes more transparent to laser light.
However, if impurity diffusion is boosted excessively, the diffusion of the impurity ends up reaching the vicinity of the substrate. Structures used widely in modern semiconductor lasers intended for CD and DVD optical discs are such that an n-type GaAs substrate is employed as the substrate, an n-type GaAs buffer layer is grown thereon, followed by growing an n-type cladding layer, an active layer, and a p-type cladding layer on top of them, and current injection stripes are formed in a high-resistivity p-type layer. Moreover, a technology is used widely, in which a quantum well active layer is employed as the active layer, with said quantum well active layer disordered using Zn as an impurity that is diffused to form a window region at the facet.
However, with respect to compound semiconductors formed of Group III-V semiconductors, Zn acts as an impurity producing p-type semiconductors, and, therefore, when impurity diffusion intended for window formation is boosted excessively as described above, the n-type cladding layer is rendered p-type and a p-n junction ends up formed between it and the n-type buffer layer in the vicinity of the substrate, which is of the n-type. As a result, if the turn-on voltage (Vw) of the p-n junction between a window region and an n-type buffer layer becomes smaller than the turn-on voltage (Va) of the p-n junction of the active layer in the portion other than the window region, the electric current injected from the electrode flows through the window region, which has a small turn-on voltage.
The above-mentioned phenomenon will now be illustrated specifically with reference to the double heterostructures of red lasers, which are currently widely used. The structure of modern red lasers includes an n-type GaAs buffer layer, an n-type AlGaInP cladding layer, a quantum well active layer composed of AlGaInP/GaInP, and a p-type AlGaInP cladding layer, all formed on an n-type GaAs substrate. In this structure, when the quantum well active layer in the vicinity of the facets is disordered by the Zn diffusion to form window structures, the rate of diffusion of Zn, i.e. the impurity, is higher in AlGAInP materials than in AlGaAs materials including GaAs. For this reason, Zn rapidly diffuses across the n-type AlGaInP cladding layer into the vicinity of the n-type GaAs buffer layer interface, where the rate of diffusion is slower.
As a result of the Zn diffusion, inside the laser element, there appear a p-n junction (T1) formed between the GaAs buffer layer and AlGaInP cladding layer rendered p-type by the diffusion of the impurity and a p-n junction (T2) formed by the active layer in the portion other than the window regions. The turn-on voltage of the p-n junction in the portion other than the window regions is almost equal to the bandgap energy of the quantum well active layer, and the bandgap energy of the GaAs is smaller than the energy of red light with a wavelength of 650 nm. Consequently, the turn-on voltage of the p-n junction Ti becomes smaller than that of the p-n junction T2, and the electric current injected from the p-type AlGaInP cladding layer flows across the p-n junction T1, which has a low turn-on voltage. Therefore, injected electric current is not injected into the active layer that has not been disordered, the radiant efficiency decreases, and the reactive current causes an increase in the operating current value and deterioration in the temperature characteristic. This makes it difficult to obtain a high power characteristic of several hundred mW or more and poses serious obstacles for element reliability.
When the p-n junction T1 is formed between the n-type GaAs buffer layer and AlGaInP cladding layer rendered p-type by the diffusion of the impurity when the disordering of the quantum well active layer is boosted in the above-described manner in order to increase the transparency of the window regions to laser radiation, the loss current component, which does not contribute to the laser oscillation flowing across the junction T1, is increased. Consequently, the radiant efficiency decreases, the operating current value increases, the temperature characteristic decreases, and it becomes more difficult to obtain a high power characteristic of 250 mW or more under high-temperature conditions of 80° C. or higher.
An exemplary semiconductor laser device configuration provided to address such problems is disclosed in JP 2003-31901A. FIG. 8 shows a partial cutout perspective view of the semiconductor laser device. This semiconductor laser device has a structure in which a buffer layer having a bandgap larger than the bandgap of the quantum well structure active layer is located on a semiconductor substrate and, at the same time, window regions are formed by introducing impurities into the quantum well structure active layer in the vicinity of the facets of the semiconductor laser.
The semiconductor laser of FIG. 8 is a ridge-type semiconductor laser with an emission wavelength of 630 to 690 nm and a pulsed output of not less than 50 mW. A buffer layer 62, which is composed, for instance, of Si-doped n-AlGaAs, and, furthermore, a lower cladding layer 63 made up of n-AlGaInP, are positioned on top of a Si-doped n-GaAs substrate 61. An MQW active layer 64 is arranged on top of the lower cladding layer 63. The MQW active layer has a GaInP/AlGaInP MQW configuration.
N-AlGaAs with an aluminum composition ratio of not less than 0.3, which is a material having a larger bandgap than the MQW active layer 64, is used for the buffer layer 62. In addition, AlGaAs is a material in which Zn exhibits a smaller rate of diffusion than in AlGaInP.
An upper cladding layer 65 composed of p-AlGaInP and, furthermore, a contact layer 66 composed of p-GaAs are positioned on top of the MQW active layer 64. The upper cladding layer 65 and contact layer 66 are formed in the shape of a stripe mesa, thereby forming an optical waveguide 67. With the exception of the contact layer 66, which is located in the top portion of the optical waveguide 67, the side faces of the optical waveguide 67, the upper cladding layer 65, which is exposed when the optical waveguide 67 is formed, as well as a p-GaAs layer 68, which is formed simultaneously with the contact layer 66, are covered with an insulating film 69, which is made up of SiO2 or the like.
A surface p-electrode 70 is arranged on the surface of the insulating film 69 and contact layer 66, thereby providing connection to the contact layer 66 and forming a current path across the optical waveguide 67 to the MQW active layer 64. Window regions 71 are formed in the vicinity of the chip's two facets serving as exit facets on both sides of the optical waveguide 67. The window regions 71 are regions in which the MQW active layer 64 is disordered by the introduction of Zn diffused as an impurity from the surface of the upper cladding layer 65. The window regions 71 include the upper cladding layer 65, MQW active layer 64, and lower cladding layer 63. The contact layer 66 is removed and the insulating film 69 is disposed on the surface of the window region 71. An n-side back electrode 72 is arranged on the back side of the n-GaAs substrate 61.
Because in this structure the lower cladding layer 63 is formed of an n-AlGaInP layer, the rate of diffusion of Zn in the n-AlGaInP layer is high. However, the rate of diffusion of Zn in the n-AlGaAs layer, which constitutes the buffer layer 62, is low, and, as a result, the diffusion of Zn is controlled to stop at the boundary between the lower cladding layer 63 and upper buffer layer 62. However, in some cases Zn is diffused into the buffer layer 62 and a p-n junction ends up being formed in the buffer layer 62.
However, in this conventional example, as a result of using n-AlGaAs having an aluminum composition ratio of not less than 0.3 and a bandgap larger than the bandgap of the MQW active layer 64 for the buffer layer 62, the decrease in the forward turn-on voltage Vf is suppressed and the generation of leak currents is suppressed even when the window regions 71 are formed by Zn diffusion. Therefore, in this semiconductor laser, initial turn-on takes place in the MQW active layer 64 and no initial turn-on occurs at the p-n junction in the buffer layer 62. For this reason, the generation of leak currents that accompanies the decline in the voltage Vf is suppressed.
As noted above, the generation of reactive currents, which do not contribute to the laser oscillation flowing through the window portions, can be suppressed even if the diffusion of the impurity reaches the vicinity of the buffer layer when the impurity diffusion-based disordering of the window quantum well active layer is boosted in order to obtain a laser with a high COD level.
On the other hand, configurations intended for suppressing the generation of the above-described reactive currents flowing through the window portions, which are caused by boosting the impurity diffusion-based disordering aimed at forming the window regions, also are required in case of integrated semiconductor laser devices comprising multiple types of semiconductor lasers with different emission wavelengths. However, the setting of conditions suitable for forming multiple types of semiconductor lasers has not been addressed sufficiently in the past.