Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-02-29
2004-02-24
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S046012
Reexamination Certificate
active
06697404
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to optical semiconductor devices and more particularly to an optical semiconductor device operable in a 1.3 &mgr;m or 1.5 &mgr;m wavelength band.
Today, a telecommunication trunk generally uses an optical telecommunication system in which optical fibers carry information traffic in the form of optical signals. Currently, quartz glass optical fibers having an optical transmission band of 1.3 &mgr;m or 1.5 &mgr;m wavelength are used commonly. In correspondence to the foregoing specific transmission band of the optical fibers, current optical telecommunication systems generally use a GaInAsP double-heterojunction laser diode that includes an active layer of In
1−x
Ga
x
As
y
P
1−y
and a cladding layer of InP. In such a GaInAsP double-heterojunction laser diode, the carriers are accumulated in the active layer by a potential barrier formed in the conduction band and the valence band between the GaInAsP active layer and the InP cladding layer, and stimulated emission of photons is substantially facilitated in the active layer by the carriers thus accumulated therein. In order to obtain a laser oscillation at the wavelength that matches the optical transmission band of the quartz glass optical fibers, the compositional parameter x for Ga and the compositional parameter y for As are adjusted appropriately.
However, such a conventional laser diode that uses a double-heterojunction of GaInAsP and InP has suffered from the problem of relatively large threshold current of laser oscillation and poor temperature characteristic, primarily due to the relatively small band discontinuity (&Dgr;Ec) of the conduction band between the GaInAsP active layer and the InP cladding layer. More specifically, the electrons escape easily from the active layer in such an GaInAsP laser diode because of the small potential barrier &Dgr;Ec formed by the foregoing band discontinuity, and a large drive current has to be supplied in order to sustain a laser oscillation in the active layer. This problem becomes particularly acute at high temperatures in which the carriers experience an increased degree of thermal excitation. Further, the foregoing GaInAsP laser diode has a problem in that the laser oscillation wavelength tends to shift to a longer wavelength side at high temperatures due to the temperature dependence of the bandgap of GaInAsP. It should be noted that the bandgap of GaInAsP decreases with temperature. This shift of the laser oscillation wavelength raises a serious problem particularly in a wavelength multiplex transmission process of optical signals.
In order to avoid the foregoing problems, conventional GaInAsP double-heterojunction laser diodes for use in optical telecommunication trunk or submarine optical cable systems have used a temperature-regulation device, such as a Peltier cooling device, such that the operational temperature of the laser diode is maintained at a predetermined temperature.
On the other hand, there is a strong impetus to expand the use of optical telecommunication technology from the telecommunication trunks to subscriber systems or home systems. In relation to this, there is a demand for an optical semiconductor device suitable for use in home terminals.
When realizing such optical home terminals, it is essential that the optical home terminal is compact and low cost. Further, the optical home terminal should consume little electric power. In order to meet such demands, it is necessary to provide a laser diode that is operable in the 1.3 or 1.5 &mgr;m band with a low threshold current and simultaneously without a temperature regulation.
As long as the foregoing GaInAsP/InP double-heterojunction system is used, the foregoing demand cannot be satisfied. Thus, efforts are being made to construct a laser diode having an active layer of GaInAs on a GaAs substrate such that a large band discontinuity &Dgr;Ec is secured in the conduction band. By increasing the In content in the GaInAs active layer, it is possible to reduce the bandgap energy Eg of the active layer, and the oscillation wavelength of the laser diode approaches the desired 1.3 &mgr;m band. However, such an increase of the oscillation wavelength by increasing the In content in the GaInAs active layer is successful only to the point in which the oscillation wavelength reaches about 1.1 &mgr;m. Beyond that, the lattice misfit between the GaInAs active layer and the GaAs substrate becomes excessive and the epitaxial growth of the GaInAs active layer on the GaAs substrate is no longer possible. It should be noted that the foregoing limit of 1.1 &mgr;m takes into consideration the contribution of compressive strain that acts in the direction to increase the oscillation wavelength of the laser diode.
In view of the foregoing situation, Japanese Laid-Open Patent Publication 7-193327 proposes a laser diode operable in the 1.3 or 1.5 &mgr;m band, in which an active layer of GaInAs is sandwiched by a pair of cladding layers having a composition set such that a large band discontinuity &Dgr;Ec is secured between the active layer and the cladding layer and that the cladding layer has simultaneously a lattice constant close to that of a strained buffer layer provided on a GaAs substrate with a composition of Ga
0.8
In
0.2
As. However, the proposed device is deemed to be unrealistic in view of the large lattice misfit between the active layer and the GaAs substrate. It is believed that the existence of such a large lattice misfit reduces the lifetime of the laser diode substantially.
On the other hand, Japanese Laid-Open Patent Publication 6-37355 describes a compound semiconductor structure that includes a GamnNAs mixed crystal film formed on a GaAs substrate. By adding N to GaInAs, it becomes possible to form the GaInNAs film with a lattice constant that matches the lattice constant of GaAs. The GaInNAs film thus added with N has a reduced bandgap due to a large negative bowing of the bandgap-composition relationship observed in a GaAs-GaN system. Thus, it is expected that a double-heterostructure laser diode having an oscillation wavelength in the 1.3 or 1.5 &mgr;m and simultaneously a large band discontinuity &Dgr;Ec necessary for carrier accumulation, may be obtained by using GaInNAs for the active layer. As the GaInNAs film can have a composition that establishes a lattice matching with GaAs, it is possible to use an AlGaAs or GaAs cladding in combination with the active layer of GaInNAs.
FIG. 1
shows the compositional change of a bandgap Eg for a GaAs-GaN system according to the Japanese Laid-open Patent Publication 6-37355.
Referring to
FIG. 1
, it will be noted that the endmember component GaN has a very large bandgap Eg of about 3.5 eV, contrary to the endmember component GaAs, of which bandgap Eg is only about 1.4 eV. Thus, GaN is expected to be one of the most promising materials of an active layer for an optical semiconductor device that is operable in a blue or ultraviolet wavelength band.
The striking feature of
FIG. 1
is that the compositional change of the bandgap Eg between GaAs and GaN is not linear but there appears a very significant negative bowing. Probably, this large negative bowing of bandgap is related to the existence of a very large difference in the atomic radius between As and N. In fact, there is reported a large miscibility gap in the GaAs-GaN system.
Thus, a bandgap Eg as small as about 1.2 eV is possible for a GaNAs system by incorporating N into a GaAs crystal with a proportion of about 10 mole %. While the GaNAs system of this composition has a small lattice constant due to the small atomic radius of N, a satisfactory lattice matching can be achieved, with respect to a GaAs substrate, by incorporating In.
FIG. 2
shows the construction of a laser diode
1
proposed in the Japanese Laid-Open Patent Publication 7-154023, op cit.
Referring to
FIG. 2
, the laser diode
1
is constructed on a substrate
10
of n-type GaAs and includes a lower cladding layer
11
of n-type GaInP provided on the GaAs substrate
10
Dickstein , Shapiro, Morin & Oshinsky, LLP
Leung Quyen
Ricoh & Company, Ltd.
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