Current blocking structure to improve semiconductor laser...

Details Current blocking structure to improve semiconductor laser... Current blocking structure to improve semiconductor laser...

2002-11-06

2004-11-02

Wong, Don (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S046012, C372S049010, C372S049010, C372S043010

Reexamination Certificate

active

06813298

ABSTRACT:

BACKGROUND TO THE INVENTION
Buried heterostructure semiconductor lasers have the advantages of operating at low lasing threshold current, high temperature, and providing stable fundamental lateral mode lasing at high optical output. A prior art double-channel planar buried-heterostructure (DC-PBH) laser diode utilizing the p-n-p-n current confinement structure has been demonstrated with GaAs to enhance the current confinement to the active layer [W. T. Tsang, R. A. Logan, and J. P. van der ziel, “A new lateral selective-area growth by liquid-phase epitaxy; The formation of a lateral double-barrier buried-heterostructure laser”, Appl. Phys. Lett., vol. 40, pp. 942-944, 1982.].
FIG. 1
shows the layer structure of a similarly configured DC-PBH laser diode based on InP [T. Numai, M. Fujiwara, N. Shimosaka, K. Kaede, M. Nishio, S. Suzuki, and I. Mito, “1.5 &mgr;m &lgr;/4-shifted DFB LD filter and 100 Mbit/s two-channel wavelength signal switching”, Electron. Lett., vol. 24, pp. 236-237, 1988.]. It consists of an n-InP substrate
11
, an n-InP buffer layer
12
, an undoped-InGaAsP active layer
13
, a p-InP cladding layer
14
, a p-InP current blocking layer
15
, an n-InP current blocking layer
16
, a p-InP cladding layer
17
, and a p-InGaAsP contact layer
18
. Owing to the internal current confinement structure, the DC-PBH laser diode does not require any stripe electrodes and permits a p-type metal contact to be formed on the whole top surface of the contact layer, and thus simplifies the fabrication process and offers high yield. However, the p-n-p-n current confinement structure has a relatively large capacitance, which will lead to a large displacement current flow through the p-n-p-n junction at high frequency operation. Furthermore, a current leakage path exists between the p-InP cladding layer
14
and p-InP current blocking layer
15
to the n-InP buffer layer
12
. This leakage current will also cause optical power saturation when the laser is operated at high temperature or at high power output
U.S. Pat. No. 4,597,085 discloses a technique, whereby an undoped-InP layer
26
a
, as shown in
FIG. 2
, is inserted into the current confinement structure to reduce the capacitance between the p-n junction. In this disclosure the undoped-InP layer
26
a
either forms an n-type layer with an impurity concentration of around 5×10
16
cm
−3
, or a p-type layer with the same impurity concentration due to compensatory dispersion of p-dopant from the p-InP blocking layer
25
during epitaxial growth. With this layer, it reduces the effect of the impurity concentration of n-InP blocking layer
26
on the junction capacitance, and hence lowers the overall capacitance of the p-n-p-n current confinement structure.
Although the undoped-InP layer
26
a
is able to lower the impurity concentration on one side of the p-n junction, the impurity concentration is still in the range of ~10
16
cm
−3
. In addition, this disclosure does not address the issue of the leakage current from the active region to the p-n-p-n confinement layers. As can be seen in
FIG. 2
, the p-InP cladding layer
24
is connected directly to the p-InP blocking layer
25
, which will cause a large leakage of current, resulting in reduction in efficiency and deterioration in high-power performance of semiconductor laser.
Therefore, from the above, the design of the layer structure, especially the p-n-p-n confinement structure, should be improved in order to achieve high frequency, high temperature and high output power operation.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a semiconductor laser having a double-channel planar-buried-heterostructure (DC-PBH) formed on a substrate comprises:
a buffer layer of a first compound semiconductor material of a first conductivity type (n-type) formed on the substrate;
an active layer formed on the buffer layer, the active layer capable of light generation by laser action;
a first cladding layer of a second compound semiconductor material of a second conductivity type (p-type) formed on the active layer;
a first channel and a second channel extending through separate regions of the first cladding layer, the active layer and into the buffer layer, the first and second channels thereby defining an active region disposed between them, each of the first channel and second channel having a structure comprising:
a first current blocking layer of a third compound semiconductor material of the second conductivity type formed on the buffer layer;
an intermediate layer of a fourth compound semiconductor material formed on the first current blocking layer; and,
a second current blocking layer of a fifth compound semiconductor material of the first conductivity type formed on the intermediate layer;
each of the first current blocking layer, intermediate layer and second current blocking layer extending over an upper surface of the first cladding layer except for the portion in the active region;
a second cladding layer of a sixth compound semiconductor material of the second conductivity type formed on the second current blocking layer and covering the upper surface of the first cladding layer in the active region; and,
a contact layer of a seventh compound semiconductor material of the second conductivity type formed on the second cladding layer,
wherein the fourth compound semiconductor, which forms the intermediate layer, is doped with a dopant to reduce junction capacitance and to reduce current leakage through the channel structure.
Preferably, the second compound semiconductor, which forms the first cladding layer, is substantially intrinsic (undoped) to reduce current leakage from the active region.
Preferably, the first and second channels comprise a further current blocking layer of an eighth compound semiconductor interposed between the buffer layer and the first current blocking layer and extending over the same area as the first current blocking layer, said eighth compound semiconductor being doped with a dopant to reduce current leakage from the active region both across a channel and through a channel.
According to another aspect of the present invention, a semiconductor laser having a double-channel planar-buried-heterostructure (DC-PBH) formed on a substrate comprises:
a buffer layer of a first compound semiconductor material of a first conductivity type (n-type) formed on the substrate;
an active layer formed on the buffer layer, the active layer capable of light generation by laser action;
a first cladding layer of a second compound semiconductor material of a second conductivity type (p-type) formed on the active layer;
a first channel and a second channel extending through separate regions of the first cladding layer, the active layer and into the buffer layer, the first and second channels thereby defining an active region disposed between them, each of the first channel and second channel having a structure comprising:
a first current blocking layer of a third compound semiconductor material of the second conductivity type formed above the buffer layer;
an intermediate layer of an undoped fourth compound semiconductor material formed on the first current blocking layer; and,
a second current blocking layer of a fifth compound semiconductor material of the first conductivity type formed on the intermediate layer;
each of the first current blocking layer, intermediate layer and second current blocking layer extending over an upper surface of the first cladding layer except for the portion in the active region;
a second cladding layer of a sixth compound semiconductor material of the second conductivity type formed on the second current blocking layer and covering the upper surface of the first cladding layer in the active region; and,
a contact layer of a seventh compound semiconductor material of the second conductivity type formed on the second cladding layer,
wherein the first and second channels comprise a further current blocking layer of an eighth compound semiconductor interposed between the buff

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