1. Field of the Invention
This invention relates to a semiconductor laser device. This invention particularly relates to a composition of a semiconductor layer, which constitutes a semiconductor laser device.
2. Description of the Prior Art
As a semiconductor laser, which produces a laser beam having a wavelength in the band of 0.7 .mu.m to 0.85 .mu.m, there has heretofore been known a semiconductor laser comprising an n-GaAs substrate, an n-AlGaAs cladding layer, an n- or i-AlGaAs optical waveguide layer, an i-AlGaAs active layer, a p- or i-AlGaAs optical waveguide layer, a p-AlGaAs cladding layer, and a p-GaAs capping layer, which layers are formed on the substrate. However, such a structure has the drawbacks in that Al contained in the active layer is chemically active and apt to be oxidized, and therefore the end face of a resonator is readily deteriorated due to cleavage. Accordingly, the reliability of the semiconductor laser cannot be kept high.
Therefore, as a semiconductor laser, which produces a laser beam having a wavelength of a 875 nm band and is of the all Al-free type, there has heretofore been proposed a semiconductor laser comprising an n-GaAs substrate, an n-InGaP cladding layer, an undoped InGaAsP optical waveguide layer, a GaAs quantum well active layer, an undoped InGaAsP optical waveguide layer, a p-InGaP cladding layer, and a p-GaAs capping layer, which layers are formed on the substrate. Such a semiconductor laser is described in, for example, "IEEE Photonics Technology Letters," Vol. 6, No. 4, 1994, p. 465. However, the proposed Al-free semiconductor laser has the drawbacks in that the dependence of device characteristics upon temperatures is large and in that, though the maximum light output power is as high as 4.2W, the light emission efficiency becomes low due to the occurrence of leak current at light output power of above 1 W. The proposed Al-free semiconductor laser is not suitable as a high-output-power semiconductor laser for producing a laser beam having wavelengths of a short-wavelength band in the vicinity of 0.8 .mu.m.
Also, as a semiconductor laser, which produces a laser beam having a wavelength of a 0.8 .mu.m band and in which an active layer is of the Al-free type, there has heretofore been reported a semiconductor laser comprising an n-GaAs substrate, an n-AlGaAs cladding layer, an i-InGaP optical waveguide layer, an InGaAsP quantum well active layer, an i-InGaP optical waveguide layer, a p-AlGaAs cladding layer, and a p-GaAs capping layer, which layers are formed on the substrate. Such a semiconductor laser is described in, for example, "Jpn. J. Appl. Phys.," Vol. 34, 1995, pp. L1175-1177. However, the reported semiconductor laser has the drawbacks in that the temperature characteristics of the device characteristics are high due to carrier over flow. Therefore, the drive current cannot be kept small under the conditions for radiation a laser beam having a high intensity, and the reliability cannot be kept high due to an increase in the device temperature due to heat generation.
Further, a semiconductor laser having an enhanced reliability has been proposed in, for example, "IEEE Journal of Selected Topics in Quantum Electronics," Vol. 3, No. 2, 1997, p. 180. In the proposed semiconductor laser, a GaInP semiconductor of a composition ratio having a compressive strain with respect to a substrate is employed as an active layer, and an AlGaInP layer having a tensile strain of at least a level that cancels the compressive strain of the active layer is employed as a side barrier layer. Crystal structure relaxation is caused to occur in the vicinity of a radiating end face of the laser device, and the band gap at the end face is kept large. In this manner, light absorption at the time of laser beam radiation is kept low, and deterioration of the device due to light absorption at the end face is reduced. The reliability is there by enhanced. However, as described in "Jpn. J. Appl. Phys.," Vol. 21, 1982, p. L323, in cases where a semiconductor laser for producing a laser beam having a wavelength of an 800 nm band is to be constituted of an InGaAsP type of active layer, in the relationship between the InGaAsP type of composition ratio and the band gap, the composition region, in which phase separation is caused to occur, overlaps upon the composition ratio having a compressive strain. Therefore, it is difficult to form an active layer, which has a large compressive strain, with the InGaAsP type of semiconductor layer. Accordingly, it is difficult to constitute the semiconductor laser having an enhanced reliability, which is described in the literature shown above, as a semiconductor laser for producing a laser beam having a wavelength of the 800 nm band.