Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-02-06
2004-06-15
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S045013
Reexamination Certificate
active
06751246
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based on French Patent Application No. 01 01 912 filed Feb. 13, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119.
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to buried ribbon semiconductor lasers, especially those used in optical telecommunications.
Prior art buried ribbon laser structures are described on page 60 of the paper by J. C. BOULEY entitled “Evolution et perspectives des structures laser pour télécommunications” [Evolution and perspectives for telecommunication laser structures] published in the review “‘I’Echo des recherches” No. 130, 4th trimester 1987. As described in the upper part of page 60 of the paper, the active portion of a ribbon laser is formed of a rectangular cross section ribbon having a GaInAsP composition, for example, the proportions of the constituents of which depend on the required emission wavelength of the laser. The ribbon is buried in, i.e. entirely surrounded by, a medium of lower refractive index, for example InP. One face of the ribbon, for example the bottom face, is in contact with an n-doped InP layer and the other three lateral faces are in contact with a p-doped InP layer. Burying the active ribbon in the InP medium that completely surrounds it has two purposes. Firstly, it confines the carriers injected into the ribbon, due to the presence of the p-n junction, because of the difference in forbidden band gap E
g
between InP and GaInAsP, for example E
g
=0.35 eV for an active band at 1.3 &mgr;m. It also achieves bidirectional guiding of light because of the large difference in the refractive index n between InP and GaInAsP. Because of the small dimensions of its section (width approximately 1.5 &mgr;m, thickness 0.15 &mgr;m), the waveguide formed by the active layer accepts only one mode of propagation, namely the fundamental mode, whose size is substantially equivalent to the dimensions of the guide. The known benefit of this geometrical configuration is that it produces lasers with a very low threshold current, because the overlapping and therefore interaction of the fundamental mode and the volume of material excited by injecting carriers becomes the optimum. Laser oscillation is then obtained by forming a Fabry-Pérot cavity by cleaving transverse faces of the ribbon.
The various buried ribbon laser structures differ from each other in the methods used to inject electrons into the buried ribbon and confine them there and in the technologies employed.
The theory and fabrication of one buried ribbon structure laser are described hereinafter with reference to
FIGS. 1 and 2
of the accompanying drawings. The structure includes, in the direction of the arrow
1
in FIG.
1
:
an electrical contact layer
2
, for example a layer of TiAu or any other metal alloy,
an n-doped InP layer
3
,
deposited on the latter layer, the active laser material, for example GaInAsP, ribbon
4
, whose composition determines the wavelength obtained; the ribbon has six faces, one face
41
, shown at the bottom in
FIG. 1
, which is in contact with the layer
3
, a top face
42
parallel to the face
41
and two faces
43
,
44
parallel to each other and perpendicular to the faces
41
,
42
, and finally transverse cleaved faces
45
,
46
,
a p-doped InP layer
5
; the layer
5
is in contact with the ribbon
4
on three faces
42
-
44
of the ribbon, the bottom face
41
being in contact with the n-doped InP layer
3
, and
finally, a contact layer
6
, for example a layer of AuPt.
One embodiment of buried structures of this kind is described hereinafter with reference to
FIG. 2
, with additional information thereon.
FIG. 2
has portions A, B, C each representing a cross section of the product obtained in various fabrication steps.
An n-doped InP buffer layer
3
-
2
with an appropriate dopant concentration is grown epitaxially on an InP substrate
3
-
1
which is strongly n-doped, for example doped to a dopant concentration of 5×10
18
. Because of the manner in which it is grown, this layer has better controlled crystal properties than the layer
3
-
1
.
The InGaAsP active layer
4
is then grown epitaxially, followed by an InP layer
35
to protect the active layer
4
. A dielectric mask layer
9
is then deposited to protect the active layer
4
during subsequent fabrication steps, for example a layer of SiO
2
.
The product obtained at the end of this first step is shown in portion A of FIG.
2
.
This is followed by deep etching to form a mesa
11
which emerges above a surviving portion of the layer
3
-
2
. The width of the mesa
11
is the width of the ribbon
4
. The mesa
11
includes from its top portion to its bottom portion what remains of the layers
9
,
35
,
4
after etching and a portion of the thickness of the layer
3
-
2
on either side of the mesa
11
.
The product obtained after deep etching is shown in portion B of FIG.
2
. In portions B and C of
FIG. 2
the faces
43
,
44
are shown perpendicular to the planes of the layers, for example the layers
3
and
4
. It must nevertheless be understood that because of the etching method used these faces are not plane. As a result the angles between planes tangential to the faces
43
,
44
and a plane parallel to the layers can differ from one layer to another.
After removing the mask
9
, the p-doped InP layer
5
is grown epitaxially all around the exposed portion of the ribbon
4
. The ribbon
4
therefore has its lateral faces
42
,
43
and
44
directly adjacent a p-doped InP material, thereby achieving the optical confinement effect, due to the difference in optical index between the material of the layer
4
and the material of the adjacent layers, and the electrical confinement of the ribbon
4
, due to the n-p junction between the layers
3
and
5
. The layer
5
is formed of two superposed portions
5
-
1
and
5
-
2
. The layer
5
-
1
is immediately on top of the layers
3
-
2
and
9
, so that the ribbon is buried in the layer
5
-
1
. The layer
5
-
2
situated immediately on top of the layer
5
-
1
is more strongly p-doped than the layer
5
-
1
to improve the contact with the metal contact layer
6
deposited afterwards, for example a layer of AuPt.
The electrical confinement effect can be increased by ion bombardment using H+ protons of portions of the layer
5
on either side of the mesa of the ribbon
4
and not adjacent the mesa. The bombarded portions
5
-
3
in portion C of
FIG. 2
are therefore rendered non-conductive. This limits current leaks.
SUMMARY OF THE INVENTION
The buried ribbon BRS laser is considered to be one of the simplest buried ribbon heterostructure lasers. However, as the current confinement of BRS lasers is obtained by a forward-biased p-n junction, the current leaks depend greatly on the p-doping level and on the current injected, as shown in FIG.
3
.
FIG. 3
shows curves indicating the percentage of leakage current plotted on the vertical axis relative to the bias current plotted on the horizontal axis for different levels of p-doping of the p-doped InP layer
5
in which the ribbon
4
is buried. The p-doping values are indicated on each curve. The level of n-doping of the ribbon
4
is usually from 10
18
to 2×10
18
/cm
3
. The curves, especially that whose p-doping is 5×10
18
, suggest that a high level of p-doping reduces the leakage current, in particular if the bias current is high.
Moreover, it is known in the art that incorporating a high level of p impurities, for example Zn, in the InP layer
5
during the second stage of epitaxial growth leads to diffusion of the impurities, in particular into the adjacent layer forming the laser active layer
4
of the device. The impurities in the active layer
4
introduced in this way form non-radiating recombination sites and this degrades the performance of the laser.
Also, the concentrations of the impurities, for example Zn, during their incorporation in
Al-Nazer Leith A
Avanex Corporation
Ip Paul
Moser, Patterson & Sheridan L.L.P.
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