Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-05-30
2003-11-25
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S044010, C372S045013
Reexamination Certificate
active
06654396
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device and a method for fabricating thereof, and in more detail a semiconductor laser device having a structure capable of preventing non-emissive failure due to short circuit and a method for fabricating such device.
2. Description of the Related Art
A visible light semiconductor laser device having a stacked structure on a GaAs substrate, wherein an active layer is sandwiched by cladding layers made of AlGaInP or GaInP, has an oscillation wavelength between 630 nm and 690 nm, and attracts a good deal of attention as a light source for an optical pickup used in an optical disc drive.
A structure and fabrication method of a conventional AlGaInP-base visible light semiconductor laser device will be explained hereinafter referring to FIG.
6
.
FIG. 6
so shows a cross-sectional view of the substrate showing a structure of an AlGaInP-base semiconductor laser device.
An AlGaInP-base semiconductor laser device
10
has on a GaAs substrate
12
a stacked structure comprises a lower cladding layer
14
made of n-AlGaInP, an active layer
16
, an upper cladding layer
18
made of p-AlGaInP, and a contact layer
20
made of p-GaAs, and all layers are epitaxially grown in this order.
An additional semiconductor layer such as light confining layer may optionally be provided between the upper cladding layer
18
and the contact layer
20
. Also a buffer layer made of compound semiconductor may optionally be provided between the GaAs substrate
12
and the lower cladding layer
14
.
Of such stacked structure, the upper cladding layer
18
and the contact layer
20
are formed as a mesa-structured portion having a ridge stripe pattern.
The both sides of the upper cladding layer
18
and the contact layer
20
composing the mesa-structured portion, and the upper cladding layer
18
are buried with an n-GaAs layer
22
provided as a current blocking layer to ensure current constriction, thereby a central portion of the active layer becomes an oscillation area
15
of laser light.
A metal layer made of Au, Ni and the like, or a metal stacked film is provided as a p-side electrode
24
on the n-GaAs layer
22
and the contact layer
20
, and as an n-side electrode
26
on the rear surface of the GaAs substrate
12
, respectively.
In order to fabricate such semiconductor laser device
10
, at first the lower cladding layer
14
, active layer
16
, upper cladding layer
18
and contact layer
20
are epitaxially grown in this order on the GaAs substrate
12
by the metal-organic chemical vapor deposition (MOCVD) process.
The contact layer
20
and the upper cladding layer
18
are then etched to form the mesa-structured portion, and the n-GaAs layer
22
is then selectively grown on the both sides of the mesa-structured portion and on the upper cladding layer
18
.
Next, the p-side electrode
24
and n-side electrode
26
are formed by, for example, the sputtering process on the outermost surface and on the rear surface of the GaAs substrate
12
.
In the process of epitaxially growing the AlGaInP layer and the like to form the stack-structured portion, there has, however, been a problem of generating a growth defect in the epitaxially grown layer(s) if fine particles of GaAs or so adhere thereon, or foreign intermediate products are formed on the substrate during the epitaxial growth.
In the process of etching the stack-structured portion to form the mesa-structured portion after the epitaxial growth, etching with an acid of such epitaxially grown layer having the growth defect will result in formation of a pit-like shape defect portion
28
of several to tens tm diameter reaching the GaAs substrate
12
as shown in
FIG. 7
, since the portion of the growth defect is labile to acid and shows a high etch rate.
If the electrode layer
24
is formed in this situation, the electrode layer
24
intruded into the shape defect portion
28
will come into contact with the GaAs substrate
12
to cause short circuit. Such shape defect portion
28
can be produced in the stack-structured portion made of compound semiconductor layers not only during the wet etching but also during acid cleaning or alkali cleaning based on the same mechanism as described above.
As a result, short circuit will occur between currents injected to the both electrodes, thereby current which essentially has to be injected to the oscillation area in the active layer responsible for laser oscillation is reduced, and it causes non-emissive failures such that no laser oscillation occurs or the laser oscillation does not continue.
It is, however, quite difficult in practice in fabricating the semiconductor laser device to epitaxially grow the compound semiconductor layer after thoroughly cleaning the GaAs substrate and confirming that no particles adhering thereon. Thus so long as the semiconductor laser device is fabricated according to the conventional process, those suffering from non-emissive failures will be more or less produced to degrade the production yield.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a semiconductor laser device having a structure capable of preventing non-emissive failure and a method for fabricating such device.
To accomplish such object, a semiconductor laser device comprises: a compound semiconductor substrate; a lower cladding layer; an active layer; an upper cladding layer and a contact layer respectively formed on the compound semiconductor substrate, wherein an upper part of the upper cladding layer and the contact layer are formed as a mesa-structured portion having a ridge stripe pattern,; and a current blocking layer having a pit-like recess penetrating thereof and extending towards the compound semiconductor substrate, the both sides of the mesa structured portion are buried with the current blocking layer, and a portion of the recess other than that penetrating the current blocking layer being covered or buried with an insulating film or a compound semiconductor layer with a high resistivity.
In the present invention, of the pit-like recess, a portion of which other than that penetrating the current blocking layer is covered or buried with an insulating film or a compound semiconductor layer with a high resistivity, so that the compound semiconductor substrate and the electrode layers other than the a current injection area are kept insulated, thereby the non-emissive failures as observed for the conventional semiconductor laser device is avoided.
The pit-like recess may not necessarily reach the compound semiconductor substrate and may be such that penetrating the current blocking layer to reach the upper cladding layer, active layer or lower cladding layer. It is also allowable that not only a portion of the recess other than that penetrating the current blocking layer, but also the entire part of the recess is covered or buried with an insulating film or a compound semiconductor layer with a high resistivity.
The current blocking layer is made of a compound semiconductor layer with a high resistivity, or a current blocking layer using a p-n junction isolation.
The present invention is applicable irrespective of compositions of the compound semiconductor substrate or compound semiconductor layers and, for example, preferably applicable to a semiconductor laser device with a laser oscillating structure composed of an AlGaInP-base or GaInP-base compound semiconductor layer formed on a GaAs substrate. The present invention is applicable to both semiconductor laser devices of edge-emitting type and surface-emitting type.
A structure responsible for the laser emission is not necessarily of the stacked structure comprising the lower cladding layer, active layer, upper cladding layer and contact layer, but also may be such structure that having a buffer layer between the substrate and the under cladding layer, or also may be such structure that having another layer such as a light confining layer between the contact layer and upper cladding layer.
In
Hoshi Nozomu
Nagasaki Hiroki
Flores Ruiz Delma R.
Ip Paul
Sonnenschein Nath & Rosenthal LLP
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