Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2000-01-20
2001-11-27
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S094000, C372S045013, C372S049010
Reexamination Certificate
active
06323507
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor photonic element such as a semiconductor laser having a multilayer current-constricting structure, a method of fabricating the element, and a semiconductor photonic device using the element.
2. Description of the Prior Art
In recent years, to realize semiconductor lasers having excellent characteristics such as low threshold current, high efficiency, and high output, there has been the strong need to form a high-performance current-constricting structure thereby increasing the injection efficiency of a driving current. To meet the need, conventionally, various improvements have ever been made for the current-constricting structure.
FIG. 1
shows a partial cross-section of a prior-art semiconductor laser of this sort, in which a conventional current-constricting structure is used.
As shown in
FIG. 1
, the prior-art semiconductor laser
100
comprises an n-type InP substrate
101
, and a mesa structure
140
formed on an upper main surface of the substrate
101
, and a current-constricting structure
150
formed on the surface of the substrate
101
at each side of the mesa structure
140
. The mesa structure
140
includes an n-type InP cladding layer
102
formed on the surface of the substrate
101
, a semiconductor active layer
103
formed on the layer
102
, and a p-type InP cladding layer
104
formed on the layer
103
. The current-constricting structure
150
includes a p-type InP current-blocking layer
105
formed on the surface of the substrate
101
, and an n-type InP current-blocking layer
106
formed on the layer
105
.
A p-type InP burying layer
107
is formed to cover the mesa structure
140
and the current-constricting structure
150
. A p-type InGaAs contact layer
108
is formed on the layer
107
. A p-side electrode
109
is formed on the layer
108
. An n-side electrode
110
is formed on a lower main surface of the substrate
101
.
As seen from
FIG. 1
, the current-constricting structure
150
is formed by the p- and n-InP current-blocking layers
105
and
106
, which intervene between the n-type InP substrate
101
and the p-type InP burying layer
107
. Thus, the prior-art semiconductor laser of
FIG. 1
has a pnpn structure (i.e., the thyristor structure), which causes the following problem.
It is supposed that a leakage current flows from the point A in the burying layer
107
to the point B in the substrate
101
. In this case, this leakage current serves as a gate current of the thyristor structure and as a result, a leakage current (which serves as an anode current of the thyristor structure) tends to flow from the point C in the burying layer
107
to the point D in the substrate
101
, as shown in FIG.
1
. Accordingly, the undesired turn-on tends to occur in the thyristor structure, thereby losing the current-constricting function of the current-constricting structure
150
. When the prior-art laser
100
operates in a high-temperature and/or high-output condition where the leakage current becomes large, the undesired turn-on often occurs in particular.
As described above, although the current-constricting structure
150
in the prior-art semiconductor laser
100
of
FIG. 1
is effective for lowering the threshold current, it has the above-identified problem due to the undesired turn-on in the thyristor structure.
FIG. 2
shows a partial cross-section of a prior-art semiconductor laser of this sort using an improved current-constricting structure for suppressing the above-described undesired turn-on problem.
The prior-art semiconductor laser
200
of
FIG. 2
has the same configuration as that of the prior-art semiconductor laser
100
of
FIG. 1
except that a current-constricting structure
250
including two mesa structures
251
is used instead of the current-constricting structure
150
. Therefore, the explanation about the same configuration is omitted here for simplification of description by attaching the same reference symbols as those used in
FIG. 1
to corresponding elements in FIG.
2
.
Each of the mesa structures
251
in the current-constricting structure
250
includes an n-type InP layer
202
formed on the upper main surface of the substrate
101
, an InGaAsP recombination layer
212
formed on the layer
202
, and a p-type InP layer
204
formed on the layer
212
. The structures
251
are located under the p-type InP blocking layer
105
at each side of the mesa structure
140
having the active layer
103
.
The recombination layer
212
serves to cancel the leakage current (which serves as the anode current of the thyristor) due to carrier recombination, thereby decreasing the current gain factor of the thyristor structure. Thus, the unwanted turn-on of the thyristor structure can be suppressed.
Another improved current-constricting structure using a dielectric layer is disclosed in the paper written by N. Iwai et al., Electronics Letters, Vol. 34, No. 14, pp. 1427-1428, Jul. 9, 1998.
FIGS. 3A and 3B
show partial cross-sections showing the fabrication method of a prior-art semiconductor laser of this sort, which has the improved current-constricting structure disclosed in this paper.
First, as shown in
FIG. 3A
, a p-type InP layer
317
with a thickness of 50 nm is formed on an upper main surface of a p-type InP substrate
316
. Then, a p-type InAlAs layer
314
with a thickness of 50 nm is formed on the layer
317
, and a p-type InP layer
321
with a thickness of 100 nm is formed on the layer
314
. A Multiple Quantum Well (MQW) active layer
303
, which is formed by alternately stacking InGaAsP barrier sublayers and InGaAsP well sublayers, is formed on the layer
321
. An n-type InP layer
315
is formed on the layer
303
. An n-type InGaAs contact layer
318
is formed on the layer
315
. These layers
317
,
314
,
321
,
303
,
315
, and
318
are formed by using the Metal Organic Vapor Phase Epitaxy (MOVPE) technique.
Subsequently, channels
320
are formed to reach the underlying InP substrate
316
through the layers
318
,
315
,
303
,
321
,
314
, and
317
, thereby forming a mesa structure
328
, as shown in FIG.
3
A. For example, the pitch of the channels
320
(i.e., the width of the mesa structure
328
) is set as approximately 10 &mgr;m.
Following this, the substrate
316
having the structure of
FIG. 3A
is placed in an oxidation furnace, thereby selectively oxidizing the p-type InAlAs layer
314
to form a dielectric layer
319
while only the strip-shaped middle mart of the layer
314
is not oxidized, as shown in FIG.
3
B. The oxidation of the layer
314
begins at its ends exposed to the channels
320
, and progresses laterally toward the center of the layer
314
. The middle part of the layer
314
serves as the current injection region through which a driving current is injected. The oxidation period is adjusted so that the remaining middle part of the layer
314
has a width of approximately 4.6 &mgr;m. For example, it is set as 150 minutes.
Furthermore, a silicon dioxide (SiO
2
) layer
313
is formed on the n-type InGaAs contact layer
318
and the inner walls of the channels
320
. A strip-shaped window
313
a
is formed in the layer
313
to be overlapped with the remaining InAlAs layer
314
. An n-side electrode
310
is formed on the SiO
2
layer
313
. A p-side electrodes
311
is formed on the lower main surface of the substrate
316
. Thus, the prior-art semiconductor laser
300
is fabricated, as shown in FIG.
3
B.
In the prior-art semiconductor Laser
300
shown in
FIG. 38
, the dielectric layer
319
serves as a current-blocking layer. When the length of the resonator (i.e., the optical waveguide) is set as 300 &mgr;m and high-reflectance coating with a 96% reflectance is applied to the rear end of the waveguide, the obtainable threshold current for continuous oscillation at 25° C. is 18 mA and the obtainable slope efficiency is 0.55 W/A.
A further improved current-constricting structure using a dielectric layer is disclosed in the 16th Semiconductor Laser International C
Kudo Koji
Tsuji Masayoshi
Yokoyama Yoshitaka
NEC Corporation
Ngo Ngan V.
Scully Scott Murphy & Presser
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