The present invention relates to a semiconductor laser used in a read-out apparatus such as a DVD (digital video disk) drive or a CD (compact disk) drive for oscillating in a wavelength range of visible red light or infrared ray. Particularly, it relates to a semiconductor laser having a one-chip structure which is easily fabricated by simple steps and emits different wavelength laser beams, which are aligned in the perpendicular direction to the substrate.
Semiconductor laser is commonly used in CD or DVD drive for reading and writing dada, in which laser beam of 780 nm wavelength for CD drive or of 650 nm wavelength for DVD drive is used as a light source. In an apparatus for reading data from both a CD disk and a DVD disk, a light source is desired to emit two different wavelength laser beams, specifically 780 nm and 650 nm. Such a light source for emitting two beams of different wavelength is disclosed as a semiconductor laser of chip type (referred to as an LD hereinafter) in Japanese Patent Laid-open Publication Hei 1-67992 and illustrated in FIG. 4.
There are an n-type cladding layer 32, an active layer 33, a p-type cladding layer 34, and a cap layer 35 of p-type GaAs deposited on an n-type GaAs substrate 31, thus forming a first light emitting region 36. Similarly, an n-type cladding layer 38, an active layer 39, a p-type cladding layer 40, which are made of different materials from those of the previous layers, and a cap layer 41 of p-type GaAs are deposited at the side on the substrate 31, thus forming a second light emitting region 42. And the light emitting regions are shaped of a ridge structure, the upper surfaces of the first and second light emitting regions 36 and 42 and the back surface of the substrate 31 are covered with electrodes 43, 44, and 45 respectively. A resultant semiconductor laser has two light emitting regions developed on the single substrate for emission of two different wavelength laser beams. However, as shown in the Japanese Patent Laid-open Publication 7-67992, the semiconductor laser of such a structure is fabricated by depositing the semiconductor layers.of the first light emitting region 36 on the substrate, and then removing unwanted portions by etching, and selectively growing the second semiconductor layers for the second light emitting region 42 by providing polycrystalline layer so that the second semiconductor layers do not grow on the first light emitting region 36.
Alternatively, another semiconductor light emitting device is disclosed in Japanese Patent Laid-open Publication Hei 7-263752 and illustrated in FIG. 5, which is fabricated by depositing on the semiconductor substrate 51 a stack of semiconductor layers 52, 53, and 54 for light emitting regions, etching the layers to have a step structure, and providing patterns of electrodes 55 to 58 on the back side of the substrate 51 and the upper sides of the stepped layers exposed by etching. This arrangement allows the laminated semiconductor layers to be etched at one time as compared with the above manners in which selectively etching unwanted portions of the previously deposited semiconductor layers and then depositing another group of the semiconductor layers, and its production will be facilitated.
As explained, the semiconductor lasers for emission of two different wavelength beams are implemented with two different light emitting regions developed side by side on the same substrate and with two different light emitting regions developed by depositing the semiconductor layers and etching them to have a step structure.
However, in the former, the second light emitting region is fabricated by etching unwanted portions of the previously deposited layers of the first light emitting region to expose the semiconductor substrate and then depositing another group of the layers by avoiding the first light emitting area to form the second light emitting region on the substrate. Hence, its production requires a significant number of steps and may hardly deposit the semiconductor layers of good crystalline characteristics through selectively growing crystals to form the second light emitting region beside the first light emitting region, thus failing to provide a higher level of illumination.
In the latter, when the semiconductor laser is fabricated in the form of a ridge structure or an SAS structure having current blocking layer embedded as self-aligned, its light emitting regions have to be configured one by one through their respective etching steps and its production will thus be complex. In addition, as shown in FIG. 5, the structure of the semiconductor laser may cause the laser beam from each light emitting region to be not only deflected from the x direction but also dislocated to the z direction. Accordingly, the installation and alignment of this semiconductor laser with focusing lenses will be made much difficult.
It is an object of the present invention to provide a multi-color semiconductor laser which while eliminating the foregoing drawbacks, allows multiple sources of different wavelength laser beams at corresponding light emitting regions to be aligned along the vertical to the substrate and can be fabricated by simple steps of production.
It is another object of the present invention to provide a method of readily fabricating a multi-color semiconductor laser in which multiple sources for emitting different laser beams at corresponding light emitting regions are aligned with each other along the vertical to the substrate.
A semiconductor laser according to the present invention comprises: a semiconductor substrate; a first light emitting region of semiconductor layers provided on the semiconductor substrate, the semiconductor layers having an active layer, determined by a first wavelength of laser beam, sandwiched between two cladding layers of which the band gap is greater than that of the active layer; and a second light emitting region of semiconductor layers provided on the first light emitting region, the semiconductor layers having an active layer, determined by a second wavelength of laser beam, sandwiched between two cladding layers of which the band gap is greater than that of the active layer, wherein the first light emitting region and the second light emitting region are aligned with each other along the perpendicular to the semiconductor substrate. Three or more of the light emitting regions may be added with equal success.
As the first and second light emitting regions are aligned vertically, they can simply be fabricated as compared with the conventional steps of etching a portion of a group of semiconductor layers and then depositing another group of semiconductor layers. Also, the laser beam generating points are aligned along the vertical to the semiconductor substrate and their different wavelength laser beams can be focused with a single lens thus contributing to the compactness and precision of an optical pickup apparatus.
The first and second light emitting regions may be formed of a ridge structure. This allows the current injection area to be defined in a desired range thus permitting a resultant semiconductor laser to have a low threshold and provide a higher level of performance. The two ridge structures of the first and second light emitting regions can readily be fabricated by a single action of etching, hence shortening the procedure of production.
More specifically, a semiconductor laser comprises:
a semiconductor substrate of a first conductivity type;
a first light emitting region provided on the semiconductor substrate, the first light emitting region comprising a cladding layer of the first conductivity type, an active layer, and a cladding layer of a second conductivity type; a first contact layer of the second conductivity type provided on the first light emitting region; a second light emitting region provided on the first contact layer, the second light emitting region comprising a cladding layer of the second conductivity type, an active layer, and a cladding layer of the first conductivity type; a recess arranged to define a ridge structure of the semiconductor layers which include the cladding layer of the second conductivity type in the first light emitting region, the first contact layer, and all the semiconductor layers in the second light emitting region; a first electrode forming layer of the second conductivity type embedded over a current blocking layer of the first light emitting region in the recess so as to electrically connect with the first contact layer; and a second contact layer of the first conductivity type electrically connected with the cladding layer of the first conductivity type in the second light emitting region and provided over a current blocking layer which is seated on a part of the first electrode layer, embedded in the recess to cover the sides of the ridge structure of the second light emitting region, and forms a hetero barrier blocking structure with the cladding layer of the second conductivity type and the active layer in the second light emitting region.
It may be modified in which each of the first and second light emitting regions includes a current blocking layer which has a stripe groove provided therein, the two stripe grooves in the first and second light emitting regions aligned along the perpendicular to the semiconductor substrate.
It may also be modified in which the first and second light emitting regions are joined with each other by a semiconductor layer of the same conductivity type while an upper electrode of the first light emitting region and a lower electrode of the second light emitting region are formed of a common layer.
A method for manufacturing a semiconductor laser according to the present invention comprises the steps of:
(a) depositing on a semiconductor substrate a group of semiconductor layers incorporating a first light emitting region, a first contact layer, and another group of semiconductor layers incorporating a second light emitting region in a sequence; (b) etching from the surface of the semiconductor layers to a depth in the first light emitting region to shape the first and second light emitting regions to a ridge structure which is a current injection area; (c) depositing on both sides of the current injection area of the ridge structure shaped by the etching, a first current blocking layer for blocking a flow of current in the first light emitting region, a first electrode forming layer connected to the first contact layer, and a second current blocking layer for blocking a flow of current in the second light emitting region in a sequence; (d) depositing a second contact layer on the another group of semiconductor layers and the second current blocking layer; and (e) etching partially the second contact layer and the second current blocking layer to expose a portion of the first electrode layer.
It may be modified in which depositing the another group of semiconductor layers with the structure of at least an active layer sandwiched between an n-type cladding layer and a p-type cladding layer, and depositing the second current blocking layer with a semiconductor having a hetero barrier blocking structure with the active layer and n-type and p-type cladding layers, directly on both sides of the active layer and the cladding layers. This facilitates embedding the current blocking layer. The hetero barrier blocking structure means such a specific combination of semiconductor layers that a difference in the band gap is great enough to inhibit the flow of current at a common operating voltage.