1. Field of the Invention
The present invention relates to an improvement in a semiconductor laser. Especially the present invention relates to a semiconductor laser made by liquid phase epitaxial growth method.
2. Description of Prior Art
Together with remarkable progresses of light communication technology and disk type video recording technology, semiconductor lasers are regarded as most important devices to serve as light sources to be used in such technologies. Accordingly, there are great requirements on reliability and performance of such semiconductor lasers. That is, lasers of longer lifetime and lasing with more stable fundamental mode are required.
Various types of lasers have been hitherto developed, and several inventions on terraced substrate structure type lasers have been also proposed by the inventors which are assigned to the assignee of the present case. Such terraced substrate structure laser intends to provide an active layer which has variation of refractive index in the direction parallel to the junction face of the active layer, thereby to confine its lasing mode in the part of a higher refractive index, and as a result stabilize transverse oscillation mode. FIG. 1 shows one of the representative structure of the terraced substrate structure type transverse mode laser capable of stable oscillation, and comprises the following parts:
______________________________________ a substrate 1 terraced shape of n.sup.+ -GaAs, a first clad layer 2 having a triangular part 21 of n-Ga.sub.1-x Al.sub.x As, an active layer 3 having an oblique lasing region 31 of non-doped Ga.sub.1-y Al.sub.y As, a second clad layer 4 of p-Ga.sub.1-z Al.sub.z As, a contacting layer 5 of p.sup.+ -GaAs, and an isolation layer 6 having stripe-shaped opening 61 of Si.sub.3 N.sub.4, a positive side electrode layer 7 of Ti, Pt, Au layer a negative side electrode layer 8 of Au--Ge--Ni layer. ______________________________________
The layers 2, 3, 4, 5 and 6 are formed by sequential epitaxial growths on the substrate 1, which is formed in a terrace shape having a step part. Therein the first clad layer 2 has a triangular part 21 at the step part, and accordingly the active layer 3 is shaped on the triangular part 21, so as to have an oblique lasing region 31 defined between a higher bending portion 32 and a lower bending portion 33. In this structure, the thickness of the first clad layer 2 under the lasing region 31 is thicker than those under other parts of the active layer 4. That is, the triangular part 21 of the first clad layer 2 at the thickest part is about 1 .mu.m thick, and a lower horizontal part and an upper horizontal part of the first clad layer 2 is only 0.3 .mu.m thick. The emission in the active layer 3 on the upper or lower horizontal part is absorbed by the substrate 1, while the emission in the lasing region 31 on 1 .mu.m thick triangular region 21 is not absorbed by the substrate 1 and makes a single spot lasing. Furthermore, by means of the thickness difference between the lasing region 31 and the upper and the lower horizontal part, a folded ridge type or rib type light waveguide is formed, wherein the effective refractive index is higher in the oblique lasing region 31 in comparison with horizontal parts of the active layer 3, resulting in further stability of lasing mode. Accordingly, by designing the distance between the upper bending portion 32 and the lower bending portion, i.e., the width of the oblique lasing region 31, sufficiently small that only the fundamental transverse mode is confined, a light L by a single spot lasing can be obtained.
In such terraced substrate structure laser, the problem is that, due to oblique disposing of the lasing region with respect to the stripe shaped contacting face of the positive side electrode 7, i.e., the current injection face, the injected current is likely to diverge widely to the lower part of the active region far beyond the lower bending portion 33, since the distance from the stripe shaped injection face of the positive side electrode 7 to active layer 3 is larger in the lower part than that in the upper part. Due to such wasteful divergence of the injected current to the lower part of the active layer 3, the injection efficiency of the current to the lasing region 31 is lowered. Furthermore, the second clad layer 4 and the active layer 3 is likely to be gradually thicker as the position becomes apart from the step part, and at the same time, the lower bending portion 33 of the active layer 3 is likely to be dull. These two phenomena are adverse to the injection efficiency and to the formation of a small spot light beam.