The present invention relates to a semiconductor laser including (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P or the like wherein the transverse mode is controlled and a manufacturing method thereof.
A semiconductor laser which can emit visible light at a wavelength of 700 nm or less has attracted attention as a light source to be used for an optical disc, a laser printer, a bar code scanner or the like. Among them, a double heterostructure semiconductor laser comprising a GaAs substrate, a Ga.sub.0.5 In.sub.0.5 P or (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P active layer and (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P or Al.sub.y Ga.sub.1-y As cladding layers each making lattice match with adjacent layers is promising as a red-color semiconductor laser material because it can emit light of the shortest wavelength among III-V compound semiconductors which makes lattice match with GaAs.
An inner stripe geometry has been used to restrict the spreading of carriers in the lateral directions and to confine electric current. FIG. 1 shows an example of a semiconductor laser of double heterostructure of inner stripe type, wherein an n-type (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P cladding layer 2, a Ga.sub.0.5 In.sub.0.5 P active layer 3, a p-type (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P cladding layer 4 and a p-type Ga.sub.0.5 In.sub.0.5 P etch stop layer 5 are applied successively to an n-type GaAs substrate 1. Then, an n-type GaAs blocking layer 6 having a stripe-like opening is applied to the etch stop layer 5, and a p-type GaAs cap layer 7 is applied to the blocking layer 6 and the etch stop layer 5. Ohmic contact electrodes 11 and 12 are applied to the cap layer 10 and the substrate 1, respectively. The conductivity type of the layers 1, 2, 4, 5, 6 can be changed to the other type.
In an inner stripe type semiconductor laser explained above, the blocking layer plays a role to confine current. Therefore, the laser action can take place at a low threshold current.
Though the current is confined in an inner stripe type semiconductor laser as explained above, the blocking layer 6 and the cap layer 8 absorb light generated in the active layer 3. Therefore, it is a problem that if the thickness of the cladding layer 4 between the active layer and the cap layer is decreased in order to confine the current more, the light generated in the active layer is absorbed more in the blocking layer and the cap layer. Thus, a part of the light guided in the active layer is lost and the threshold current is increased due to the loss.
It is another problem that the wave front of the guided wave is distorted in a direction parallel to the active layer because the semiconductor laser is a gain guide type. Therefore, astigmatism as large as about forty micrometers arises so that the laser beam cannot be made parallel or focussed into a point with use of a conventional convex lens. Thus, the range of application to optical devices of an inner stripe type semiconductor laser is limited.