AlGaAs semiconductor laser devices are abundantly used for various purposes such as light-emitting devices for pickup in the field of optical discs including CD, MD or the like. In general, the semiconductor laser devices have a structure formed by sequentially stacking, for example, an n-type AlGaAs cladding layer, an AlGaAs active layer, a p-type AlGaAs cladding layer and a p-type GaAs contact layer in this order on an n-type GaAs substrate. However, this structure of the semiconductor laser devices has a serious problem regarding reliability such as Catastrophic Optical Damage (COD) or the short lifetime of the device because the active layer contains aluminum (Al). The reason therefor is that a large number of non-radiative recombination levels are formed in cleavage-formed end faces of a resonator including Al because Al is an easily oxidized substance characteristically. For this reason, there have been attempts for making free from Al the active layer and its peripheral layers in the AlGaAs semiconductor laser devices.
For example, JP 11-220224 A discloses such an aluminum-free semiconductor laser device as sated above. FIG. 9 shows a structure of this semiconductor laser device. On an GaAs substrate 1, there are sequentially stacked a p-type AlGaAs lower cladding layer 2, an i-type InGaAsP lower light-guiding layer 3, a multiquantum well active region 4 composed of one InGaAsP well layer with no stain or a tensile strain of not more than 0.3% (i.e.−0.3%≦strain<0%) and two InGaAsP barrier layers having a tensile strain which are alternately stacked, an i-type InGaAsP upper light-guiding layer 5, an n-type AlGaAs upper first cladding layer 6, an n-type InGaP etching stopper layer 7, an n-type AlGaAs upper second cladding layer 8 and an n-type GaAs contact layer 9. A ridge stripe structure formed of the upper second cladding layer 8 and the contact layer 9 is formed immediately above the etching stopper layer 7.
The ridge stripe structure has a height of 2.2 μm and a width of about 3 μm at the widest portion immediately above the etching stopper layer 7. An SiO2 film is stacked on both lateral sides of the ridge stripe structure as an insulating film 10 to form a current constriction structure wherein a current flows only mainly immediately below the ridge stripe structure. In the semiconductor laser device with the above constitution, materials containing no Al are used in the active region in order to prevent degradation of cleaved end faces due to oxidation of Al, whereby the COD level is improved.
In this conventional semiconductor laser device, no strain or a tensile strain is applied as the strain of the well layer. The reason thereof is as follows.
In InGaAsP of a quaternary compound, there are differences in size and binding energy of constituent atoms. Therefore, there are its compositions and temperatures where phase separation makes the quaternary compound more stable from viewpoint of free energy. This phenomenon of phase separation is referred to as spinodal decomposition. Further, a region causing the phase separation is referred to as a miscibility gap.
FIG. 10 shows spinodal curves where are shown temperatures at which spinodal decomposition of InGaAsP generates. The curves are described in such a literature as Jap. J. Appl. Phys., 21, p797 (1982) etc. and are generally well known. The spinodal decomposition occurs when growing the InGaAsP having a certain composition at a temperature lower than that shown in FIG. 10. Thus, it is required that InGaAsP having a composition on a curve of for example 600° C. should be grown at 600° C. or more. When growing InGaAsP having a composition inside the curve of 600° C., the InGaAsP should be grown at a further higher temperature.
However, when growing the InGaAsP by using the metal organic chemical vapor deposition (MOCVD) method and the like, the growth temperature thereof is about 600° C. to 670° C. in general. This derives from the fact that the growth temperature cannot be raised since an element of phosphorous has a high vapor pressure and is thus easily detached. Therefore, spinodal decomposition generates when growing InGaAsP having a composition inside the spinodal curve of 600° C. to 670° C., so that good InGaAsP crystals are not grown. For this reason, in order to surely obtain good crystals of InGaAsP by growing it in the growth temperature range of 600° C. to 670° C., a composition of InGaAsP existing outside a limitary spinodal curve around 500° C. to 600° C. should be used.
FIG. 10 also shows well known energy bandgap (Eg) lines of InGaAsP etc. that are described in Journal of Electronic Materials, Vol. 3, No. 3, p. 635 (1974). Furthermore, strain amounts (1.0%, 0.25%, 0%, −1.0%) with respect to GaAs are shown in broken lines. It should be noted that the strain is referred to as a “compressive strain” when the strain amount has a positive value, and the strain is referred to as a “tensile strain” when the strain amount has a negative value. The magnitude of the compressive/tensile strain is expressed by an absolute value of the strain amount.
In this case that a semiconductor laser device having an InGaAsP well layer is oscillated at a wavelength of for example 780 nm, Eg required for the InGaAsP well layer is in the vicinity of 1.55 eV to 1.60 eV indicated by a region (a) of FIG. 10. Therefore, intersection between the region (a) and the temperature range of 500° C. to 600° C., which temperature range corresponds to the boundary spinodal curve wherein good InGaAsP crystals are surely obtained, preferably exists just in the vicinity of no strain i.e. the strain amount 0% with respect to GaAs. From the above, in order to obtain good InGaAsP crystals, it is required that the InGaAsP should have a composition existing within the region (a) and toward a right-lower direction than the boundary of the spinodal curve of 500° C. to 600° C. in FIG. 10, namely a composition within the region with no strain or a slight tensile strain. In particular, the spinodal curve designates at least 650° C. within the region (a), at which temperature spinodal decomposition occurs, in a composition having a compressive strain of 0.25% or more wherein the strain effect greatly appears. Thus, it is difficult to surely obtain good InGaAsP crystals under this condition.
In the case where a strain is introduced to a semiconductor laser device, a reduction in the threshold current is expected as the general effect of the strain. The reduction in the threshold current caused by strain varies according to the amount of strain in both compressive and tensile strains. For example, relationship between a strain amount and a threshold current density is described in a literature “P.J.A. Thijs, Proc. 13th IEEE Int. Semiconductor Laser Conf., Takamatsu Japan, p2 (September 1992)” which is related to an InGaAs well layer for oscillating at a long wavelength for communication. FIG. 11 shows relationship between the strain amount and the threshold current density described in this literature. According to this figure, the threshold current density is most lowered around 1.5% in both cases of the compressive strain and the tensile strain. In the case of the tensile strain between 0% exclusive and 1% inclusive, the threshold current density becomes higher compared with the case of no strain. To obtain a lower threshold current density, therefore, it is required that the tensile strain exceed 1%.
However, as mentioned above, the tensile strain of 1% cannot be obtained within the range of the region (a) of the Eg required for the InGaAsP well layer(s) in the case of the semiconductor laser device having the InGaAsP well layer(s) that oscillates laser light at the conventional wavelength of 780 nm. That is, there is a problem that a low threshold current cannot be obtained in the case where a semiconductor laser device having a well layer(s) with a tensile strain is oscillated at a wavelength of 780 nm.