Semiconductor laser element

Details Semiconductor laser element Semiconductor laser element

2000-08-04

2003-09-09

Tran, Minh Loan (Department: 2826)

Coherent light generators

Particular active media

Semiconductor

C372S046012

Reexamination Certificate

active

06618411

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Filed of the Invention
The present invention relates to a ridge waveguide semiconductor laser and, more particularly, to improvements in the optical output characteristic and high-frequency characteristic of a ridge waveguide semiconductor laser based on AlGaInAs/InP semiconductor material.
2. Description of the Related Art
The AlGaInAs/InP laser which has good temperature characteristic, is lately viewed as a promising element as the semiconductor laser used in communications based on optical fibers.
FIG. 18
is a sectional view showing an example of the AlGaInAs/InP laser of the prior art. It is difficult to make the AlGaInAs/InP laser in an embedded structure since Al is included in the active layer as shown in FIG.
18
. Therefore, a ridge waveguide configuration is generally employed. In
FIG. 18
, numeral
1
denotes an n-type InP substrate,
2
denotes an n-type InP cladding layer,
3
denotes an n-type AlInAs cladding layer,
4
denotes an n-type AlGaInAs light confinement layer,
5
denotes an AlGaInAs quantum well layer,
6
denotes a p-type AlGaInAs light confinement layer,
7
denotes a p-type AlInAs cladding layer,
10
b
denotes a p-type InP residue layer,
10
a
denotes a ridge portion comprising the p-type InP layer,
11
denotes a p-type InGaAs contact layer,
12
denotes an SiO
2
insulation layer,
13
denotes a p-type electrode (Au) of the laser, and
14
denotes an n-type electrode (Au/Ge/Ni/Au).
The ridge waveguide semiconductor laser shown in
FIG. 18
has been produced in the process shown in
FIGS. 19A through 19F
in the prior art. First, for example, the MOCVD method is employed to form the n-type InP cladding layer
2
, the n-type AlInAs cladding layer
3
, the n-type AlGaInAs light confinement layer
4
, the AlGaInAs quantum well layer
5
, the p-type AlGaInAs light confinement layer
6
, the p-type AlInAs cladding layer
7
, and the p-type InP layer
10
and the p-type InGaAs contact layer
11
, laminated successively on the n-type InP substrate
1
as shown in FIG.
19
A.
Then, as shown in
FIG. 19B
, a SiO
2
insulating layer
20
is formed that is etched away in a photolithography process while leaving a portion where a ridge is to be formed. The SiO
2
, insulating layer
20
is used as a mask in dry etching or wet etching of the p-type InGaAs contact layer
11
, and the p-type InP layer
10
is partially etched away leaving a part of the thickness, thereby forming the ridge portion
10
a
and the residue layer
10
b
of the InP layer
10
as shown in FIG.
19
C.
Then as shown in
FIG. 19D
, the SiO
2
insulating layer
20
is removed by etching followed by the formation of the SiO
2
insulating layer
12
, and only the portion on top of the ridge
10
a
is etched away in the photolithography process shown in FIG.
19
E. Then the p-type electrode
13
and the n-type electrode
14
of the laser are formed as shown in
FIG. 19F
In the production process described above, since the depth of etching of the p-type InP layer
10
is governed by time, depth of etching is likely to vary among lots, among wafers and among positions in the wafer surfaces. Consequently, there have been significant variations in the semiconductor laser characteristics among lots, among wafers and among positions in the wafer surface.
Variations in the semiconductor laser characteristics caused by the variations in the depth of etching are as follows. In case the p-type InP layer
10
is etched to a small depth, current
22
flowing beside the ridge increases, resulting in an increase in the threshold current, as shown in FIG.
20
A. Also because a light emitting region
23
expands, the angle of spread of light in the horizontal direction decreases. In case the p-type InP layer
10
is etched to a greater depth, on the other hand, the current
22
flowing beside the ridge decreases, resulting in a decrease in the threshold current. Also because the light emitting region
23
becomes narrower, the angle of spread of light in the horizontal direction increases.
To restrict variations in the depth of etching of the p-type InP layer
10
, Japanese Laid-Open Patent Publication No. 11-54837 discloses a method wherein a p-type GaInAsP etching, stopping layer
9
is provided below the InP layer
10
as shown in
FIGS. 21A and 21B
. The p-type GaInAsP etching stopping layer
9
is not etched by common etchants that are used in wet etching of the InP layer
10
. Therefore, depth of etching of the InP layer can be kept constant by stopping the etching of the InP layer
10
at the GaInAsP etching stopping layer when the GaInAsP etching stopping layer is provided below the InP layer
10
.
However, the AlGaInAs/InP laser of the prior art has the following problems.
First, as described in the Japanese Laid-Open Patent Publication No. 11-54837, in case the p-type GaInAsP etching stopper layer is provided on the p-type AlInAs cladding layer, the energy band at the junction of the p-type AlInAs layer and the p-type InGaAsP etching stopping layer has a discontinuous structure, so serial resistance becomes higher and the threshold current of the laser increases. As described in the Japanese Laid-Open Patent Publication No. 11-54837, the serial resistance can be decreased to some extent by optimizing the composition of the InGaAsP etching stopper layer, although it is not possible to completely suppress the increase in the serial resistance due to the discontinuous band structure even if this method is employed.
Light emitted from the AlGaInAs active layer
5
of the AlGaInAs/InP laser is spread around the light emitting region
23
. In the conventional laser structure, the spread of light reaches the p-type electrode
13
as shown in FIG.
4
A. The metal of the p-type electrode has a very high absorption coefficient for light of wavelengths in a range from 1.3 to 1.55 &mgr;m that are the wavelengths of light emitted by the AlGaInAs/InP semiconductor laser. As a result, there occurs an absorption loss due to the p-type electrode which, in turn, causes an increase in the threshold current and a decrease in the light emission efficiency of the laser, thus deteriorating the laser characteristics.
Moreover, since a PN junction region (a region where the p-type layer and the n-type layer adjoin each other: the AlGaInAs quantum well layer
5
in the case of this example) expands laterally to portions where current does not flow, there has been a poor high-frequency characteristic of the semiconductor laser. This problem will be described below with reference to FIG.
22
.
FIG. 22
is a schematic diagram showing an equivalent circuit of the ridge waveguide semiconductor laser of the prior art. Current supplied through the p-type electrode
13
flows through a resistor
25
consisting of the p-type cladding layer
10
, a diode
26
consisting of the quantum well layer
5
and a resistor
27
consisting of the n-type cladding layer
2
, and enters the n-type electrode
14
. A parasitic capacitance is parallel to the main current path, and causes deterioration in the high-frequency characteristic of the laser. The parasitic capacitance consists mainly of a capacitance
28
formed by the SiO
2
insulating layer
12
in a pad portion and a capacitance
29
formed by the PN junction of the AlGaInAs quantum well layer
5
in a region where current does not flow. The capacitance
29
formed by the PN junction is far larger than the capacitance
28
of the pad. Since these components of capacitance are connected in series, total amount of the parasitic capacitance is determined by the capacitance
28
of the pad that has the smaller value. Consequently, roughly speaking, the capacitance
29
formed by the PN junction has a relatively small effect on the high-frequency characteristic of the laser. However, since the capacitance
29
formed by the PN junction is connected to the main current path via the p-type AlInAs cladding layer
7
(which constitutes a resistor component), the smaller the resistance of the p-type AlInAs cladding layer
7
, the greater the effect of the capa

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