Semiconductor optical device apparatus

Details Semiconductor optical device apparatus Semiconductor optical device apparatus

2000-02-23

2004-10-19

Wong, Don (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S044010, C372S045013, C372S046012, C372S049010, C372S049010, C372S050121

Reexamination Certificate

active

06807213

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a semiconductor optical device apparatus such as a semiconductor laser or a semiconductor optical amplifier.
DESCRIPTION OF RELATED ART
A structure so-called as a ridge waveguide type is frequently used to easily produce semiconductor optical device apparatuses.
FIG. 4
shows a manufacturing method for such a structure.
First, an n-type clad layer
402
, an active layer
403
, a p-type clad layer
404
, and a p-type contact layer
405
are formed on a substrate
401
. Subsequently, a photoresist
408
having stripe openings as a pattern made by photolithography is formed on a wafer surface to form a stripe-shaped ridge by a wet etching process using the photoresist as a mask so that the p-clad layer remains with a prescribed thickness. A protection film
409
having insulating property is formed on the whole wafer surface; the protection film at a top of the ridge is removed by photolithography; and a p-side electrode
410
and an n-side electrode
411
are formed. The ridge structure thus formed can make the transverse mode for laser oscillation stabilized and can reduce the threshold currents.
However, with such a conventional manufacturing method for ridge waveguide type semiconductor optical device apparatus, because the ridge portion is formed by an etching, it is difficult to control the thickness of the clad layer in a non-ridge portion
406
with high accuracy. As a result, slight differences in the thickness of the clad layer in the non-ridge portion make the effective refractive index greatly deviated at that portion, thereby making the laser property of the semiconductor optical device apparatus deviated and improvements in product yields not easily obtainable.
To solve such a problem, a method has been proposed in which the thickness of the clad layer of the non-ridge portion is determined using a crystal growth rate during the crystal growth, in which a protection film is formed at the non-ridge portion, and in which the ridge portion is re-grown (see generally, JP-A-5-121,822, JP-A-9-199,791, JP-A-10-326,934, JP-A-326,935, JP-A-10-326,936, JP-A-326,937, JP-A-326,938, JP-A-10-326,945).
FIG. 5
shows producing method and structure for such a laser device. When the ridge portion is formed, a layer is selectively re-grown in using a protection film
506
as a mask on stripe shaped openings
507
, and a p-type second clad layer
508
and a p-type contact layer
509
are sequentially accumulated with trapezoid cross-sectional shapes according to isotropic nature in the growth rate with respect to face orientation. With this method, the thickness of the p-type first clad layer
504
in the non-ridge portion can be controlled with high accuracy, so that the effective refractive index can be controlled easily.
However, the semiconductor optical device thus manufactured by this method also raises a problem. For example, the ridge waveguide type laser as set forth in JP-A-5-121,822 should have a ridge width around one micron at the ridge top if an optical waveguide structure is manufactured to achieve a single fundamental transverse mode. Consequently, because the contact area between the contact layer and the electrode becomes so small, the contact resistance between the contact layer and the electrode may increase, and laser characteristics and reliability may be deteriorated due to oxidized surfaces of the clad layer at the ridge side wall. Therefore, it is difficult to improve the product yield.
In the case of the ridge waveguide type laser as set forth in JP-A-199,791, because the bottommost portion of the ridge becomes in a reversed-mesa shape, the contact layer may not be formed, thereby raising problems such that the device is easily oxidized and that the life time may be adversely affected. Since the electrode is not easily formed at the bottommost portion of the ridge, the interconnection may be cut, thereby creating a problem that the production yield is adversely affected. Therefore, it is demanded to provide a semiconductor optical device apparatus with high reliability and good yield in manufacturing.
Meanwhile, optical discs are made with a higher recording density these days, and according to this, light sources are developed vigorously. To make smaller the condensed spot diameter on a disc plate, practical use of red lasers (635 to 690 nm), instead of near infrared lasers (around 780 nm), begins, and blue semiconductor lasers having wavelength of around 400 to 420 nm, though in a stage of developments, are about to achieve longer lifetime in a CW operation. On the other hand, to focus the spot on the disc plate by condensing the laser beam, the laser beam is preferably formed in a shape closer to a circular shape, but actually, the beam divergence angle in a horizontal direction in a face parallel to the active layer is about one third in comparison with that in the vertical direction. Generally, a widened light intensity profile at the end face of the laser beam emission in the transverse direction causes the divergence angel in the horizontal direction to be small. A beam having an divergence in a shape closer to a circular shape can be obtained by narrowing the width of the stripe-shaped openings and by making the optical intensity profile at the emission end surface small, but the narrowed width of the stripe shaped openings increases current injections density to the active region, thereby promoting bulk deterioration, and raising a problem that the reliability of the device may be lowered. Particularly, in a material for short wavelength light source such as AlGaInP based, AlGaInN based, and MgZnSSe based materials, this problem becomes serious due to larger bulk deterioration caused by current injections in comparison with the conventional AlGaAs based material. If a beam closer to a circular shape is used, there are advantages such that the laser beam can be used with an improved efficiency (i.e., light amount cut by lenses becomes small) and any correction plate for beam shape becomes unnecessary. Therefore, it is demanded to provide a semiconductor optical device apparatus with a smaller beam spot diameter operable in keeping high reliability.
Since media price can be lowered relatively these days, CD-R (recordable), CD-R/W (re-writable), mini-disc (MD), and the like begin to be commercially available, and therefore, the light source is required to have a largely improved light output (70 to 100 mW in CW) in order to correspond to a high speed operation where made of the conventional AlGaAs (wavelength is around 780 nm). With a conventional art, it is hard to adequately suppress the deterioration in laser, particularly, end surface deterioration, during the above high output operation. It is demanded to provide a semiconductor optical device apparatus with high output and high reliability.
Meanwhile, with respect to the compound semiconductor layer containing In in the semiconductor optical device apparatus, the followings have been known. Because lattice matching should be made to the substrate, the In content of the respective layers of the double hetero structure including an n-type clad layer, an active layer, and a p-type clad layer, like InGaAsP/In(AlGa)AsP/InP based and InGaAs/In(AlGa)As/InP based, which are formed on an InP substrate, and InGaP/IN(AlGa)P based and InGaAs/InGaAsP/InGaP based, which are formed on a GaAs substrate, is designed to have 50% or more. In general, the In content is determined to be a necessary composition to match the lattice for the substrate whereas the Al and Ga content is determined to be a necessary composition to adjust the refractive index and the size of the bandgap. For example, for an (AlGa)InP based red visible light laser (600 nm band) produced on a GaAs substrate, the In content is set about 50% of the entire III group as to make the lattice matching of the active layer and the clad layer substantially with the substrate, and the refractive index and the bandgap are adjusted by setting the Al content in the active layer to be small (generally, Al con

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