Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-02-28
2004-03-23
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S050121
Reexamination Certificate
active
06711195
ABSTRACT:
BACKGROUND OF THE INVENTION
Photonic devices include semiconductor lasers, e.g., vertical cavity surface-emitting lasers (VCSELs) and edge-emitting lasers (EELs), and semiconductor light-emitting diodes (LEDs). Applications for photonic devices are many and include optical communications, optical measuring instruments and optical storage devices.
Photonic devices that generate long-wavelength infra-red light are of great interest in the optical communications industry since existing optical fibers have a relatively low loss in this wavelength range. Wavelengths in the wavelength range that extends from about 1.5 to about 1.6 micrometers (&mgr;m), commonly referred to as the 1.55 &mgr;m wavelength range, are typically used in optical communications applications, since semiconductor lasers and other components that operate in this wavelength range are relatively low in cost and are widely available. However, optical fibers have a lower optical dispersion in a wavelength range that extends from about 1.25 &mgr;m to about 1.35 &mgr;m, commonly referred to as the 1.3 &mgr;m wavelength range. This wavelength range is less commonly used for optical communications because lasers that operate in this wavelength range are based on an indium phosphide (InP) substrate and so are substantially more expensive that lasers based on a gallium arsenide (GaAs) substrate. Moreover, it is difficult to make VCSELS that operate in the 1.3 &mgr;m wavelength range due to the lack of suitable mirror materials compatible with InP.
The active layer of a photonic device is the layer in which electrons and holes combine to generate light. Although it is possible to make photonic devices with a homogeneous active layer, an active layer that includes a quantum-well structure provides the photonic device with a lower threshold current, a higher efficiency and a greater flexibility in choice of emission wavelength.
A quantum-well structure is composed of at least one (n) quantum-well layer interleaved with a corresponding number (n+1) of barrier layers. Each of the quantum well layers has a thickness in the range from about one nanometer to about ten nanometers. The barrier layers are typically thicker than the quantum well layers. The semiconductor materials of the layers of the quantum-well structure depend on the desired emission wavelength of the photonic device. The semiconductor material of the barrier layers differs from that of the quantum-well layer, and has a larger bandgap energy and a lower refractive index than that of the quantum well layer.
The active layer is composed of the quantum-well structure sandwiched between two cladding layers. The semiconductor materials constituting the quantum-well structure are typically undoped. One of the cladding layers is doped n-type, the other of the cladding layers is doped p-type. Thus, the active layer has a p-i-n structure.
A quantum-well structure composed of gallium arsenide antimonide (GaAsSb) quantum-well layers with gallium arsenide (GaAs) barrier layers has been proposed for the active region of VCSELS structured to generate light with a wavelength of 1.3 &mgr;m.
FIG. 1
is an energy-band diagram of an exemplary active layer
10
incorporating such a quantum-well structure having one quantum-well layer. Band energy is plotted as ordinate and distance from the substrate is plotted as abscissa.
The active layer
10
is composed of the substrate-side cladding layer
12
, the substrate-side barrier layer
14
of GaAs, the quantum-well layer
16
of GaAsSb, the remote-side barrier layer
18
of GaAs and the remote-side cladding layer
20
. The energy-band diagram of
FIG. 1
shows the energies of the conduction band
22
and the valence band
24
of the semiconductor material of each of the layers just described.
The quantum-well structure composed of the barrier layers
14
and
18
of GaAs and the quantum-well layer
16
of GaAsSb has what is known as a Type II heterostructure. In a Type II heterostructure, the energy of the valance band
24
of the GaAsSb of the quantum-well layer
16
is greater than the energy of the valance band of the GaAs of the barrier layers
14
and
18
and the energy of the conduction band
22
of the GaAsSb of the quantum-well layer is also greater than the energy of the conduction band of the GaAs of the barrier layers.
The line-up of the band energies in a quantum-well structure having a Type II heterostructure confines electrons
26
to the conduction band
22
of the barrier layers
14
and
18
and confines holes
28
to the valance band
24
of the quantum-well layer
16
. As a result, the electron-hole recombination process occurs between carriers confined in physically-different layers and is called spatially indirect. An active layer incorporating a quantum-well structure having a Type-II heterostructure can emit and absorb photons with energies well below the bandgap energy of the material of either the quantum-well layer or the barrier layers. Photonic devices incorporating such an active layer operate at wavelengths much longer than those corresponding to the bandgap energies of the materials of the quantum-well structure. However, active layers incorporating a quantum-well structure having a Type-II heterostructure have a relatively low gain due to the low overlap between the electron and hole wave functions.
Another disadvantage of active layers incorporating a quantum-well structure having a Type II heterostructure is that edge-emitting lasers incorporating such an active layer have a threshold current density that depends on the device dimensions and an operating wavelength that depends on the operating current. These variations in threshold current density and operating wavelength can lead to problems in lasers used in optical communications applications where channel spacings of a few hundred GHz impose strict wavelength stability requirements.
In addition, for the active region to generate light at 1.3 &mgr;m, the GaAsSb of the quantum-well layer
16
has an antimony (Sb) fraction of about 0.35, i.e., x=~0.35 in GaAs
1-x
Sb
x
. With this antimony fraction, the GaAsSb has a lattice constant substantially larger than that of GaAs, so that the quantum-well layer is under substantial compressive strain when grown on GaAs. It is therefore difficult to fabricate active regions having more than one or two quantum wells without an unacceptably high defect density occurring as a result of relaxation of the strain. Barrier layers of GaAs are incapable of providing strain compensation for quantum-well layers of GaAsSb grown on a substrate of GaAs.
An alternative quantum-well structure that has been proposed for the active region of VCSELs structured to generate light at 1.3 &mgr;m is composed of gallium arsenide antimonide (GaAsSb) quantum-well layers with aluminum gallium arsenide (AlGaAs) barrier layers.
FIG. 2
is an energy-band diagram of an exemplary active layer
40
incorporating such a quantum-well structure having one quantum-well layer. As in the energy-band diagram of
FIG. 1
, band energy is plotted as ordinate and distance from the substrate is plotted as abscissa.
The active layer
40
is composed of the substrate-side cladding layer
42
, the substrate-side barrier layer
44
of AlGaAs, the quantum-well layer
46
of GaAsSb, the remote-side barrier layer
48
of AlGaAs and the remote-side cladding layer
50
. The energy-band diagram shows the energies of the conduction band
22
and the valence band
24
of the semiconductor materials of the layers just described.
The active layer composed of the barrier layers
44
and
48
of AlGaAs and the quantum-well layer
46
of GaAsSb has what is known as a Type I heterostructure. In a Type I heterostructure composed of GaAsSb and AlGaAs, the energy of the valance band
24
of the GaAsSb of the quantum-well layer
46
is greater than the energy of the valance band of the AlGaAs of the barrier layers
44
and
48
, but the energy of the conduction band
22
of the GaAsSb of the quantum-well layer is less than the energy of the conduction band of
Chang Ying-Lan
Corzine Scott W.
Dupuis Russell D.
Noh Min Soo
Ryou Jae Hyun
Hardcastle Ian
Ip Paul
Nguyen Tuan
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