Optical semiconductor device and method of fabricating the same

Details Optical semiconductor device and method of fabricating the same Optical semiconductor device and method of fabricating the same

2002-02-04

2004-09-14

Thomas, Tom (Department: 2815)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Groove formation

C438S245000, C438S360000, C438S388000

Reexamination Certificate

active

06790697

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical semiconductor device and a method of fabricating the same and, more particularly, to an optical semiconductor device having a semi-insulating buried heterostructure wherein an optical integrated circuit consisting of a plurality of waveguides or an optical integrated circuit obtained by adding an electronic device thereto is formed by using a high resistive semi-insulating semiconductor, the optical semiconductor device being represented by an optical cross switch such as a side-light injection type bistable laser or a directional coupler, and a method of fabricating the same.
GaInAsP/InP semiconductor lasers have a basic arrangement in which a layer structure near a light emission region is a double heterostructure consisting of a Ga
1-x
In
x
As
y
P
1-y
active layer about 100 nm thick and p- and n-type InP layers (cladding layers). These cladding layers vertically sandwich the active layer and have a larger band gap than that of the active layer.
Due to effective carrier confinement by this double heterostructure, in the active layer of the double heterostructure, it is possible to form excitation carriers at a high density (up to 10
18
cm
−3
) upon energization at a relatively low current density (1 to 10 kA/cm
2
).
Also, the refractive index of a cladding layer having a large forbidden band is generally smaller than that of an active layer. The double heterostructure uses this refractive index difference to form an optical waveguide in the direction of thickness, confining laser light in the vicinity of the active layer.
A semiconductor laser having this double heterostructure with the above properties can continuously oscillate at room temperature, so the double heterostructure is used as a common basic structure of practical semiconductor lasers.
Furthermore, in practical semiconductor lasers, various stripe structures are used in the horizontal direction parallel to the p-n junction surface to give the lasers the waveguide properties of confining a current or injected carriers into a stripe region in that direction, thereby stabilizing the transverse mode of oscillated laser light.
This stripe active region is sometimes buried in a cladding layer region having a larger forbidden band. The result is a buried heterostructure (BH) in which the double heterostructure is also formed in the horizontal direction.
In this buried heterostructure, carriers are also confined in the horizontal direction, resulting in an increased current injection efficiency.
Furthermore, a two-dimensional optical waveguide is formed by the refractive index difference between the active layer and the cladding region in which the active layer is buried. Consequently, it is possible to obtain a fundamental transverse mode semiconductor laser with a high efficiency and a stable oscillation mode.
One example of the means of forming a buried layer to obtain a buried heterostructure is HVPE (Hydride (Chloride) Vapor Phase Epitaxy) which makes use of a difference from thermal equilibrium by using a nitrogen- or hydrogen-diluted gas mixture of a Group V gas, such as PH
3
or PCl
3
, and a Group III gas of, e.g., a metal halide (InCl) formed by a thermo-chemical reaction with HCl.
MOVPE (Metal Organic Vapor Phase Epitaxy) is also available in which a gasified (diluted) organic metal (primarily of Group III) is crystal-grown together with a Group V gas (PH
3
) by substrate heating.
Alternatively, the temperature of a solution containing a semiconductor material is decreased by bringing the solution into contact with the substrate surface. Consequently, the liquid phase in the boundary region supersaturates and precipitates (crystal-grows) on the substrate. This method is called LPE (Liquid Phase Epitaxy).
FIGS. 29A and 29B
are schematic cross-sectional views showing the arrangements of semiconductor lasers having an SIBH (Semi-Insulating Buried Hetero) structure formed by selectively burying semi-insulating InP using HVPE.
This semiconductor laser is fabricated as follows.
First, an n-type InP cladding layer
2
is crystal-grown on an n-type InP substrate
1
of (001) orientation by using MOCVD or MBE (Molecular Beam Epitaxy). Examples of the dopant for obtaining n-type are Se, Si, and S.
Subsequently, an active layer
3
is formed on top of the structure.
This active layer
3
consists of a guide layer (light confining layer) made from, e.g., undoped or n-type-doped InGaAsP, an active layer formed on the guide layer and made from undoped InGaAsP, and a guide layer formed on the active layer and made from undoped or p-type-doped InGaAs.
Subsequently, a p-type InP overcladding layer
4
is formed on the active layer
3
. Examples of the dopant for obtaining a p-type layer are Zn and Be.
A p-type InGaAs or InGaAsP electrode contacting layer
5
is then formed on the overcladding layer
4
. This electrode contacting layer
5
is formed to obtain an ohmic contact (and to decrease the contact resistance) with an electrode (to be described later).
An InP layer is sometimes formed on the electrode contacting layer
5
to protect the electrode layer or increase the adhesion of a mask material. However, no such layer is used in this structure.
Subsequently, a stripe pattern (not shown) made from silicon oxide is formed on the electrode contacting layer
5
by photolithography and etching. This stripe pattern is used as a mask to perform etching to a portion below the active layer, forming a stripe etching mesa.
A semi-insulating InP buried layer
6
is then formed to bury the both sides of the etching mesa by using Fe as a dopant. This formation is done by HVPE as described above.
A p-type electrode
7
consisting of an Au—Zn—Ni alloy is formed on the buried layer
6
, and an n-type electrode consisting of an Au—Ge—Ni alloy is formed on the lower surface of the substrate
1
. Consequently, a semiconductor laser with the structure shown in
FIG. 29A
is formed.
Note that as the p-type electrode
7
, a Ti—Pt—Au alloy can also be Schottky-connected in some instances.
FIG. 29B
is a sectional view showing the arrangement of a semiconductor laser formed using an overcladding layer
4
a
and an electrode contacting layer
5
a
both increased in area to increase the injection efficiency of carriers into an active layer
3
.
In
FIG. 29B
, reference numeral
9
denotes an n-type InP current blocking layer for suppressing a recombination current with captured electrons resulting from injection of holes from the overcladding layer
4
a
into a semi-insulating layer
6
; and
10
, an insulating layer made from silicon oxide or silicon nitride. The rest of the arrangement is similar to that in FIG.
29
A.
Note that the current blocking layer
9
is usually formed using the same growth apparatus (e.g., an MOCVD or MOVPE apparatus) as for the p-type overcladding layer
4
a.
On the other hand, as a semiconductor laser in which a buried layer is formed by LPE or MOCVD (MOVPE), semiconductor lasers having a p-n buried structure are available in which, as illustrated in
FIGS. 30A and 30B
, conductive carriers are confined by forming a p-n junction barrier.
In the structure shown in
FIG. 30A
, the both sides of a cladding layer
2
and an active layer
3
, as an etching mesa, are buried with a p-type InP current blocking layer
11
and an n-type InP current blocking layer
12
by using LPE or MOCVD. Thereafter, a p-type overcladding layer
4
a
is grown.
FIG. 30B
is a sectional view showing the arrangement of a semiconductor laser with a DCPBH (Double Channel Planar Buried Hetero) structure.
In this structure, an etching mesa is not singly formed; that is, an etching mesa consisting of a buffer layer
2
and an active layer
3
is formed by forming trenches.
These trenches are buried with current blocking layers
11
a
and
12
a.
Note that the same reference numerals as in
FIGS. 29A and 29B
denote the same portions in
FIGS. 30A and 30B
.
Also, as with the waveguide lasers described above, vertical resonator type surface emission lasers which vertic

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