Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
1999-03-05
2001-10-09
Mintel, William (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S201000, C257S436000, C257S437000, C438S072000, C438S094000
Reexamination Certificate
active
06300650
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor device that uses a multilayer reflector.
A multilayer reflector is an optical reflector formed of an alternate repetition of first and second layers having respective refractive indices. As a result of such an alternate repetition of the first and second layers, there appears a periodically changing profile of refractive index in the multilayer reflector, while such a periodically changing refractive index profile causes a Bragg reflection in the optical beam incident thereto with a wavelength that satisfies a condition of Bragg reflection. A multilayer reflector is easily formed on a semiconductor structure by an epitaxial process. Thus, the multilayer reflector is used extensively in so-called vertical-cavity surface-emitting laser diode that emits an optical beam perpendicularly to the epitaxial layers.
In a vertical-cavity surface-emitting laser diode, a multilayer reflector is disposed up and below an active layer in which optical radiation is produced as a result of stimulated emission. Thus, a vertical-cavity surface-emitting laser diode is suitable for integration on a semiconductor substrate in the form of two-dimensional array. In relation to this advantageous feature, an extensive application is expected for a vertical-cavity surface-emitting laser diode as an optical source of various optical telecommunication systems, optical information processing systems or optical interconnection switches.
A laser diode for use in optical telecommunication or optical information processing is generally designed to produce an output optical beam in the 1.3 &mgr;m or 1.5 &mgr;m wavelength band, in view of the optical transmission band of the optical fiber used in such conventional optical telecommunication or optical information processing systems as an optical transmission medium. In relation to the foregoing specific wavelength band, the conventional laser diodes for use in optical telecommunication or optical information processing have used GaInPAs for the active layer. Further, in relation to the use of the GaInPAs active layer, the conventional laser diodes, including the vertical-cavity surface-emitting laser diodes, have used InP for the substrate. Thereby, the GaInPAs active layer has a composition such that a lattice matching is achieved to the InP substrate while simultaneously having a bandgap energy corresponding to the foregoing optical wavelength band of 1.3 &mgr;m or 1.5 &mgr;m. Alternatively, the GaInPAs active layer has a composition so as to accumulate a strain therein. In the latter case, the active layer has to be formed to have a thickness not exceeding a critical thickness of the strained heteroepitaxial system formed of the GaInPAs active layer and the InP substrate. When the thickness of the GaInPAs active layer has exceeded the critical thickness, an extensive formation of lattice misfit dislocations would occur in the GaInPAs active layer.
In the foregoing vertical-cavity surface-emitting laser diode for telecommunication applications, a first multilayer reflector is provided between the InP substrate and the GaInPAs active layer, while a second multilayer reflector is provided on the top part of the device. For example, the second multilayer reflector may be provided on a cladding layer covering the GaInPAs active layer.
The multilayer reflector used in the conventional 1.3 &mgr;m or 1.5 &mgr;m band vertical-cavity surface-emitting laser diode is typically formed of an alternate repetition of a first epitaxial layer of InP and a second epitaxial layer of GaInPAs in view of the need of maintaining a lattice matching to the InP substrate. However, the heteroepitaxial system of GaInPAs and InP has a drawback in that the change of the refractive index is very small. In terms of the refractive index difference between the first epitaxial layer and the second epitaxial layer, the change is only in the order of about 0.25. Thus, in order that the multilayer reflector is effective, it has been necessary to increase the number of stacks of the first and second epitaxial layers in the multilayer reflector. For example, it has been necessary to stack the first and second epitaxial layers 40 times or more in order to achieve a desired reflectance of 99.9%. However, such an increased number of stacks of the first and second epitaxial layers increases the time needed for forming the epitaxial structure of the multilayer reflector, and the throughput of production of the laser diode is decreased inevitably. Further, such an increased number of stacks inevitably leads to an increased thickness of the multilayer reflection structure and hence an increased step height. Thereby, the fabrication of the laser diode becomes substantially difficult. For example, a conventional multilayer reflection structure may have a total thickness of as much as about 20 &mgr;m, while the multilayer reflection structure having such a very large thickness tends to suffer from the problem of variation in the thickness of the first and second epitaxial layers in the thickness direction. When this occurs, the desired high reflectance is not achieved. In order that the multilayer reflection structure is to be effective, it is necessary that each of the first and second epitaxial layers in the structure has a thickness corresponding to one-quarter (&lgr;/4) of the wavelength of the optical beam to be reflected.
In order to eliminate the foregoing problem, the Japanese Laid-Open Patent Publication 6-132605 describes a laser diode that uses a strained buffer layer of GaInPAs formed on an InP substrate. The buffer layer has a composition that induces a lattice misfit to the underlying InP substrate and carries thereon an active layer of GaInPAs that achieves a lattice matching to the buffer layer, with a first multilayer reflection structure and a first cladding layer interposed in this order between the buffer layer and the active layer. The first multilayer reflection structure includes an alternate repetition of a first epitaxial layer of AlInAs and a second epitaxial layer of GaInPAs having a composition of Ga
x1
In
1−x1
P
y1
As
1−y1
(0≦x
1
≦1, 0≦y1≦1) each having a thickness corresponding to a quarter of the wavelength (&lgr;/4) of the optical beam to be reflected, wherein the first and second epitaxial layers have respective compositions selected so as to establish a lattice matching to the buffer layer. Further, the first cladding layer is formed of GaInPAs having a composition Ga
x2
Iv
1−x2
P
y2
As
1−y2
, which is set such that the first cladding layer establishes a lattice matching to the buffer layer.
The active layer of GaInPAs is provided on the first cladding layer noted above, wherein the active layer has a composition of Ga
x3
Iv
1−x3
P
y3
As
1−y3
(0≦x3≦1, 0≦y3≦1) selected such that the active layer establishes a lattice matching to the buffer layer. Further, a second cladding layer of GaInPAs having a composition of Ga
x4
Iv
1−x4
P
y4
As
1−y4
(0≦x4≦1, 0≦y4≦1)is provided on the active layer in lattice matching to the foregoing buffer layer, and a second multilayer reflection structure similar to the first multilayer reflection structure is provided on the second cladding layer. Thus, the second multilayer reflection structure is formed of an alternate repetition of a third epitaxial layer of AlInAs and a fourth epitaxial layer of GaInPAs having a composition of Ga
x5
Iv
1−x5
P
y5
As
1−y5
(0≦x5≦1, 0≦y5≦1) both achieving a lattice matching to the buffer layer.
In this prior art laser diode, the epitaxial layers forming the first multilayer reflection structure, the first cladding layer, the active layer, the second cladding layer and the second multilayer reflection structure, all have a composition that achieves a lattice matching to the GaInPAs buffer layer. In other words, all the foregoing layers, including the buffer lay
Dickstein , Shapiro, Morin & Oshinsky, LLP
Mintel William
Ricoh & Company, Ltd.
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