1. Field of the Invention
This invention relates to a semiconductor laser device that oscillates laser light with an oscillation wavelength in the visible region, especially, a high quality quantum well semiconductor laser device with a superlattice structure that can be readily produced by molecular beam epitaxy.
2. Description of the Prior Art
In recent years, epitaxial growth techniques such as molecular beam epitaxy (MBE) and metal organic-chemical chemical vapor deposition (MOCVD) have been rapidly advanced. By these growth techniques, it is possible to obtain epitaxial growth layers of extreme thinness, on the order of 10 .ANG.. Due to the progress in these crystal growth techniques, it is possible to make laser devices based on device structures having very thin layers, which could not be easily manufactured by conventional liquid phase epitaxy. A typical example of these laser devices is the quantum well (QW) laser, in which the active layer has a thickness of 100 .ANG. or less resulting in the formation of quantum levels therein, whereas the active layer of the conventional double-heterostructure (DH) laser has a thickness of several hundreds of angstroms or more. Thus, this QW laser is advantageous over the conventional DH laser is that the threshold current level is reduced, the temperature characteristics are excellent, and the transient characteristics are excellent. This has been reported by W. T. Tsang, Applied Physics Letters (vol. 39, No. 10, pp. 786, 1981); N. K. Dutta, Journal of Applied Physics (vol. 53, No. 11, pp. 7211 1982); and H. Iwamura et al., Electronics Letters (vol. 19, No. 5, pp. 180, 1983).
As mentioned above, by the use of epitaxial growth techniques such as MBE and MOCVD, it is now possible to put high quality semiconductor laser devices having a new multiple-layered structure into practical use. As an AlGaAs quantum well laser that oscillates at a low threshold current level, the inventors of this invention have proposed a superlattice quantum well semiconductor laser device (for example, T. Hayakawa et al., Applied Physics Letters (vol. 49, No. 11, pp. 636 1986)), which can be readily manufactured. It comprises a superlattice quantum well region, which is composed of alternate layers consisting of GaAs layers and Al.sub.x Ga.sub.1-x As (0&lt;x&lt;1) layers (each of the said layers having a thickness of several mono-layers or less), and optical guiding layers sandwiching the said superlattice quantum well region therebetween. The AlAs mole fraction (i.e., x) of each of the said optical guiding layers is continuously varied.
FIG. 3 shows the AlAs mole fraction (i.e., x) in an Al.sub.x Ga.sub.1-x As mixed crystal in the above-mentioned conventional quantum well semiconductor laser device with a graded-index separate-confinement heterostructure (GRIN-SCH) having an oscillation wavelength in the visible region. The quantum well region 15 has a superlattice structure that is composed of alternate layers consisting of four GaAs layers with a five mono-layer thickness each and three Al.sub.0.28 Ga.sub.0.72 As layers with a two mono-layer thickness each. The thickness of the quantum well region 15 is 73.58 .ANG. (a mono-layer thickness being 2.83 .ANG.). GRIN layers 4 and 6 that sandwich the quantum well region 15 therebetween function as optical guiding layers and are made of non-doped Al.sub.x Ga.sub.1-x As in which the AlAs mole fraction (i.e., x) varies from 0.7 to 0.28 according to the parabolic distribution. Cladding layers 3 and 7 are constituted by n- and p-Al.sub.0.7 Ga.sub.0.3 As crystals, respectively. The semiconductor laser device having the AlAs mole fraction shown in FIG. 3, when it has a cavity length of 250 .mu.m, oscillates laser light with an oscillation wavelength of 785 nm at an extremely low threshold current level of 368 A/cm.sup.2.
The above-mentioned single quantum well laser device having the AlAs mole fraction such as that shown in FIG. 3 is advantageous in that the superlattice structure thereof can be made by MBE and moreover GaAs layers containing no aluminum are used in the superlattice structure, and thus the growth of thin crystal films using MBE can be attained by a single Al cell functioning as a aluminum supplier. However, the above-mentioned laser device is disadvantageous in that the GaAs thin films contained in the single quantum well with a superlattice structure tend to deteriorate resulting in poor reliability.